U.S. patent application number 10/429013 was filed with the patent office on 2003-11-20 for methods of manufacturing cold-rolled and hot-dip galvanized steel sheet excellent in strain age hardening property.
This patent application is currently assigned to Kawasaki Steel Corporation, a corporation of Japan. Invention is credited to Furukimi, Osamu, Matsuoka, Saiji, Sakata, Kei, Shimizu, Tetsuo.
Application Number | 20030213535 10/429013 |
Document ID | / |
Family ID | 27554759 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030213535 |
Kind Code |
A1 |
Matsuoka, Saiji ; et
al. |
November 20, 2003 |
Methods of manufacturing cold-rolled and hot-dip galvanized steel
sheet excellent in strain age hardening property
Abstract
The present invention provides a steel sheet having a chemical
composition comprising 0.15% or less C, 2.0% or less Si, 3.0% or
less Mn, P, S, Al and N in adjusted amounts, from 0.5 to 3.0% Cu,
or one or more of Cr, Mo and W in a total amount of 2.0% or less,
and having a composite structure comprising ferrite and martensite
having an area ratio of 2% or more. The steel sheet is in the form
of a high-strength hot-rolled steel sheet, a high-strength
cold-rolled steel sheet, or a hot-dip galvanized steel sheet. There
is thus available a steel sheet excellent in press-formability and
in strain age hardening property as represented by a .DELTA.TS of
80 MPa or more.
Inventors: |
Matsuoka, Saiji; (Okayama,
JP) ; Shimizu, Tetsuo; (Okayama, JP) ; Sakata,
Kei; (Chiba, JP) ; Furukimi, Osamu; (Chiba,
JP) |
Correspondence
Address: |
IP DEPARTMENT OF PIPER RUDNICK LLP
3400 TWO LOGAN SQUARE
18TH AND ARCH STREETS
PHILADELPHIA
PA
19103
US
|
Assignee: |
Kawasaki Steel Corporation, a
corporation of Japan
Hyogo
JP
|
Family ID: |
27554759 |
Appl. No.: |
10/429013 |
Filed: |
May 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10429013 |
May 2, 2003 |
|
|
|
09980300 |
Dec 31, 2001 |
|
|
|
Current U.S.
Class: |
148/602 ;
148/332 |
Current CPC
Class: |
C21D 8/0236 20130101;
C22C 38/12 20130101; C23C 2/06 20130101; C22C 38/04 20130101; C22C
38/16 20130101; Y10T 428/12799 20150115; C21D 8/0226 20130101; C21D
8/0278 20130101; C21D 1/185 20130101; C22C 38/06 20130101; C22C
38/02 20130101; C23C 2/40 20130101; C21D 2211/008 20130101; C21D
8/0273 20130101; C23C 2/02 20130101; C21D 2211/005 20130101 |
Class at
Publication: |
148/602 ;
148/332 |
International
Class: |
C21D 008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2000 |
JP |
2000-106340 |
Apr 10, 2000 |
JP |
2000-107870 |
Apr 17, 2000 |
JP |
2000-114933 |
Sep 20, 2000 |
JP |
2000-286008 |
Sep 20, 2000 |
JP |
2000-286009 |
Sep 29, 2000 |
JP |
2000-299640 |
Claims
1. A steel sheet excellent in press-formability and in strain age
hardening property as typically represented by a .DELTA.TS of 80
MPa or more, comprising a structure having ferrite phase as a main
phase forming a composite structure with a secondary phase
containing martensite phase in an area ratio of 2% or more.
2. A steel sheet according to claim 1, which is a hot-rolled steel
sheet.
3. A steel sheet according to claim 2, comprising, in weight
percentage:
19 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, Cu:
from 0.5 to 3.0%,
and the balance Fe and incidental impurities.
4. A steel sheet according to claim 3, containing, in weight
percentage, one or more selected from the following groups A to C,
in addition to the above-mentioned chemical composition: group A:
Ni: 2.0% or less; group B: one or two of Cr and Mo: 2.0% or less in
total; and group C: one or more of Nb, Ti and V: 0.2% or less in
total.
5. A steel sheet according to claim 2, having a chemical
composition comprising, in weight percentage:
20 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02%, or less Al: 0.1% or less, N: 0.02% or less,
one or more selected from the group consisting of from 0.05 to 2.0%
Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, 2.0% or less in
total, and the balance Fe and incidental impurities.
6. A steel sheet according to claim 5, further comprising, in
addition to the above-mentioned chemical composition, in weight
percentage, one or more selected from the group consisting of Nb,
Ti, and V, 2.0% or less in total.
7. A manufacturing method of a steel sheet excellent in
press-formability and in strain age hardening property as typically
represented by a .DELTA.TS of 80 MPa or more, comprising the steps,
when hot-rolling a steel slab having a chemical composition
comprising, in weight percentage:
21 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, and Cu:
from 0.5 to 3.0%,
into a hot-rolled steel sheet having a prescribed thickness,
carrying out said hot rolling with a finish rolling end temperature
FDT of the Ar.sub.3 transformation point or more, then after the
completion of the finish rolling, cooling the hot-rolled steel
sheet to a temperature region from the (Ar.sub.3 transformation
point) to the (Ar.sub.1 transformation point) at a cooling rate of
5.degree. C./second or more, air-cooling or slowly cooling the
sheet within said temperature region for a period of from 1 to 20
seconds, then cooling the sheet again at a cooling rate of
5.degree. C./second or more, and coiling the sheet at a temperature
of 550.degree. C. or below.
8. A manufacturing method of a hot-rolled steel sheet according to
claim 7, containing, in addition to said chemical composition in
weight percentage, one or more selected from the following groups A
to C: group A: Ni: 2.0% or less; group B: one or two of Cr and Mo:
2.0% or less in total; and group C: one or more of Nb, Ti and V:
0.2% or less in total.
9. A manufacturing method of a hot-rolled steel sheet, according to
claim 7, wherein said steel slab has a chemical composition
containing, in weight percentage:
22 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less,
and further containing one or more selected from the group
consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr, and from
0.05 to 2.0% W, 2.0% or less in total.
10. A manufacturing method of a hot-rolled steel sheet according to
any one of claims 7 to 9, wherein all or part of said finish
rolling comprises lubrication rolling.
11. A steel sheet according to claim 1, which is a cold-rolled
steel sheet.
12. A steel sheet according to claim 11, comprising, in weight
percentage:
23 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, Cu:
from 0.5 to 3.0%,
and the balance Fe and incidental impurities.
13. A steel sheet according to claim 12, containing, in weight
percentage, one or more selected from the following groups A to C,
in addition to the above-mentioned chemical composition: group: Ni:
2.0% or less; group B: one or two of Cr and Mo: 2.0% or less in
total; and group C: one or more of Nb, Ti and V: 0.2% or less in
total.
14. A steel sheet according to claim 11, having a chemical
composition comprising, in weight percentage:
24 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less,
one or more selected from the group consisting of from 0.05 to 2.0%
Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, 2.0% or less in
total, and the balance Fe and incidental impurities.
15. A steel sheet according to claim 14, further comprising, in
addition to the above-mentioned chemical composition, in weight
percentage, one or more selected from the group consisting of Nb,
Ti and V, 2.0% or less in total.
16. A manufacturing method of a cold-rolled steel sheet excellent
in press-formability and in strain age hardening property typically
represented by a .DELTA.TS of 80 MPa or more, comprising the steps
of using a steel slab having a chemical composition containing, in
weight percentage:
25 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, and Cu:
from 0.5 to 3.0%
as a material; a hot rolling step of applying hot rolling to said
material into a hot-rolled steel sheet; a cold rolling step of
applying cold rolling to said hot-rolled steel sheet into a
cold-rolled steel sheet; and a recrystallization annealing step of
applying recrystallization annealing into a cold-rolled annealed
steel sheet; these steps being sequentially applied; wherein said
recrystallization annealing is conducted in a ferrite+austenite
dual phase region within a temperature range of from Ac.sub.1
transformation point to Ac.sub.3 transformation point.
17. A manufacturing method of a cold-rolled steel sheet according
to claim 16, containing, in addition to said chemical composition
in weight percentage, one or more selected from the following
groups A to C: group A: Ni: 2.0% or less; group B: one or two of Cr
and Mo: 2.0% or less in total; and group C: one or more of Nb, Ti
and V: 0.2% or less in total.
18. A manufacturing method of a cold-rolled steel sheet according
to claim 16, wherein said steel slab has a chemical composition
containing, in weight percentage:
26 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less,
and further containing one or more selected from the group
consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr, and from
0.05 to 2.0% W.
19. A manufacturing method of a cold-rolled steel sheet according
to any one of claims 16 to 18, wherein said hot rolling is
conducted under conditions including a heating temperature of said
material of 900.degree. C. or more, a finish rolling end
temperature of 700.degree. C. or more, and a coiling temperature of
800.degree. C. or below.
20. A manufacturing method of a cold-rolled steel sheet according
to any one of claims 16 to 19, wherein all or part of said hot
rolling comprises lubrication rolling.
21. A hot-dip galvanized steel sheet comprising a hot-dip
galvanizing layer or an alloyed hot-dip galvanizing layer formed on
the surface of the steel sheet according to any one of claims 2 to
6.
22. A hot-dip galvanized steel sheet comprising a hot-dip
galvanizing layer or an alloyed hot-dip galvanizing layer formed on
the surface of the steel sheet according to any one of claims 11 to
15.
23. A manufacturing method of a hot-dip galvanized steel sheet
excellent in press-formability, and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
comprising the steps of using a steel sheet having a chemical
composition containing, in weight percentage:
27 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, and Cu:
from 0.5 to 3.0%,
applying annealing comprising heating to a dual phase region of
ferrite+austenite within a temperature range of from Ac.sub.3
transformation point to Ac.sub.1 transformation point to said steel
sheet on a line for conducting continuous hot-dip galvanizing, and
then, performing a hot-dip galvanizing treatment thereby forming a
hot-dip galvanizing layer on the surface of said steel sheet.
24. A manufacturing method of a hot-dip galvanized steel sheet
according to claim 23, further containing, in weight percentage, in
addition to said chemical composition, one or more of the following
groups A to C: group A: Ni: 2.0% or less; group B: one or two of Cr
and Mo, 0.2% or less in total; and group C: one or more of Nb, Ti
and V, 0.2% or less in total.
25. A manufacturing method of a hot-dip galvanized steel sheet
according to claim 23, wherein said steel sheet is replaced by a
steel sheet having a chemical composition containing, in weight
percentage:
28 C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or
less, S: 0.02% or less, Al: 0.1% or less, and N: 0.02% or less,
and further comprising one or more selected from the group
consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr and from
0.05 to 2.0% W, 2.0% or less in total.
26. A manufacturing method of a hot-dip galvanized steel sheet
according to any one of claims 23 to 25, wherein, prior to said
annealing, a preheating treatment of heating the sheet at a
temperature of 700.degree. C. or more on a continuous annealing
line, and then applying a pretreatment comprising a pickling
treatment.
27. A manufacturing method of a hot-dip galvanized steel sheet
according to any one of claims 23 to 26, comprising the steps of
conducting said hot-dip galvanizing treatment to form a hot-dip
galvanizing layer on the surface of the steel sheet, and then,
performing an alloying treatment of said hot-dip galvanizing
layer.
28. A manufacturing method of a hot-dip galvanized steel sheet
excellent in press-formability, and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more
according to any one of claims 23 to 27, wherein said steel sheet
is a hot-rolled steel sheet manufactured by hot-rolling the
material having said chemical composition under conditions
including a heating temperature of 900.degree. C. or more, a finish
rolling end temperature of 700.degree. C. or more and a coiling
temperature of 800.degree. C. or below, or a cold-rolled steel
sheet obtained by cold-rolling said hot-rolled steel sheet.
29. A manufacturing method of a hot-dip galvanized steel sheet
excellent in press-formability and in strain age hardening property
as typically represented by a .DELTA.TS of 80 MPa or more, further
comprising a step of applying a hot-dip galvanizing treatment to
the hot-rolled steel sheet resulting from the manufacturing method
of a hot-rolled steel sheet according to any one of claims 7 to 10
to form a hot-dip galvanizing layer on the surface of said
hot-rolled steel sheet.
30. A manufacturing method of a hot-dip galvanized steel sheet
excellent in press-formability and in strain age hardening property
as typically represented by a .DELTA.TS of 80 MPa or more, further
comprising a step of applying a hot-dip galvanizing treatment to
the cold-rolled steel sheet resulting from the manufacturing method
of a cold-rolled steel sheet according to any one of claims 16 to
20 to form a hot-dip galvanizing layer on the surface of said
cold-rolled steel sheet.
31. A manufacturing method of a hot-dip galvanized steel sheet
according to any one of claims 29 and 30, further comprising the
step of carrying out an alloying treatment after said hot-dip
galvanizing treatment.
Description
TECHNICAL FIELD
[0001] The present invention relates mainly to steel sheets for
automobile, and more particularly, to steel sheets having a very
high strain age hardening property, excellent in press-formability
such as bending workability, stretch-flanging workability, and
drawing workability, in which tensile strength increases
considerably through a heat treatment after press forming, and
manufacturing methods thereof. The term "steel sheets" as herein
used shall include hot-rolled steel sheets, cold-rolled steel
sheets, and plated steel sheets.
BACKGROUND ART
[0002] Weight reduction of automobile bodies has become in recent
years a very important issue in relation to emission control for
the purpose of preserving global environments. More recently,
efforts are made to achieve a higher strength of automotive steel
sheets and reduce steel sheet thickness.
[0003] Because many of the body parts of automobile made of steel
sheets are formed by press-working, steel sheets used are required
to have an excellent press-formability. In order to achieve an
excellent press-formability, it is necessary to ensure a low yield
strength and a high elongation. Stretch-flanging may be frequently
applied in some cases, so that it is also necessary to have a high
hole-expanding ratio. In general, however, a higher strength of
steel sheet leads to an increase in yield strength and
deterioration of shape freezability, and tends to result in a lower
elongation and a poorer hole-expanding ratio, thus leading to a
lower press-formability. As a result, there as conventionally been
an increasing demand for high-strength hot-rolled steel sheets,
high-strength cold-rolled steel sheets and high-strength plated
steel sheets having high elongation and excellent in
press-formability.
[0004] Importance is now placed on safety of automobile body to
protect a driver and passengers upon collision, and for this
purpose, steel sheets are demanded to have an improved impact
resistance as a standard of safety upon collision. For the purpose
of improving impact resistance, a higher strength in a completed
automobile is more favorable. There has therefore been the
strongest demand for high-strength hot-rolled steel sheets,
high-strength cold-rolled steel sheets and high-strength plated
steel sheets having a low strength and a high elongation and
excellent in press-formability upon forming automobile parts, and
having a high strength and excellent in impact resistance in
completed products.
[0005] To satisfy such a demand, a steel sheet high both in
press-formability and strength was developed. This is a baking
hardening type steel sheet of which yield stress increases by
applying a baking treatment usually including holding at a high
temperature of 100 to 200.degree. C. after press forming. This
steel sheet is based on a process comprising the steps of
controlling the content of C remaining finally in a solid-solution
state (solute C content) within an appropriate range, keeping
mildness, satisfactory shape freezability and elongation during
press forming, preventing movement of dislocation introduced during
press forming by the residual solute C fixed to it during the
baking treatment after press forming, thereby causing an increase
in yield stress. However, in this baking hardening type automotive
steel sheet, while yield stress can be increased, it was impossible
to increase tensile strength.
[0006] Japanese Examined Patent Application Publication No. 5-24979
discloses a baking hardening high-strength cold-rolled steel sheet
having a chemical composition comprising from 0.08 to 0.20% C, from
1.5 to 3.5% Mn and the balance Fe and incidental impurities, and
having a structure composed of uniform bainite containing up to 5%
ferrite or bainite partially containing martensite. The cold-rolled
steel sheet disclosed in Japanese Examined Patent Application
Publication No. 5-24979 has an object to achieve a high baking
hardening amount conventionally unavailable through conversion of
structure from the conventional structure mainly comprising ferrite
into a structure mainly comprising bainite, by rapidly cooling the
steel sheet after continuous annealing within a temperature range
of from 400 to 200.degree. C. in the cooling step and then slowly
cooling the same. In the steel sheet disclosed in Japanese Examined
Patent Application Publication No. 5-24979, however, while a high
baking hardening amount conventionally unavailable is obtained
through an increase in yield strength after baking, it is yet
impossible to increase tensile strength, and there still remains a
problem in that improvement of impact resistance cannot be
expected.
[0007] On the other hand, several hot-rolled steel sheets are
proposed with a view to increasing not only yield stress but also
tensile strength by applying a heat treatment after press
forming.
[0008] For example, Japanese Examined Patent Application
Publication No. 8-23048 proposes a manufacturing method of a
hot-rolled steel sheet, comprising the steps of reheating a steel
containing from 0.02 to 0.13% C, up to 2.0% Si, from 0.6 to 2.5%
Mn, up to 0.10% sol. Al, and from 0.0080% to 0.0250% N to a
temperature of at least 1,100.degree. C., applying a hot rolling
end finish rolling at a temperature of from 850 to 950.degree. C.,
then cooling the hot-rolled steel sheet at a cooling rate of at
least 15.degree. C./second to a temperature of under 150.degree.
C., and coiling the same, thereby achieving a composite structure
mainly comprising ferrite and martensite. In the steel sheet
manufactured by the technique disclosed in Japanese Examined Patent
Application Publication No. 8-23048, however, while tensile
strength is increased, together with yield stress, by strain age
hardening, a serious problem is posed in that coiling of the steel
sheet at a very low coiling temperature as under 150.degree. C.
results in large dispersions of mechanical properties. Another
problems include large dispersions of increment of yield stress
after press forming and baking treatments, as well as an
insufficient press-formability resulting from a low hole-expanding
ratio (.lambda.) and a decreased stretch-flanging workability.
[0009] On the other hand, for some portions, automotive parts are
required to have a high corrosion resistance. A hot-dip galvanized
steel sheet is suitable as a material applied to portions required
to have a high corrosion resistance, and a particular demand exists
for hot-dip galvanized steel sheets excellent in press-formability
during forming, and is considerably hardened by a heat treatment
after forming.
[0010] To respond to such a demand, for example Japanese Patent
Publication No. 2802513 proposes a manufacturing method of a
hot-dip galvanized steel sheet using a hot-rolled steel sheet as a
substrate. The patented method comprises the steps of hot-rolling a
steel slab containing up to 0.05% C, from 0.05 to 0.5% Mn, up to
0.1% Al and from 0.8 to 2.0% Cu under conditions including a
coiling temperature of up to 530.degree. C., reducing the steel
sheet surface by heating the hot-rolled steel sheet to a
temperature of up to 530.degree. C., and hot-dip-galvanizing the
sheet, whereby a remarkable hardening is available through a heat
treatment after forming. In the steel sheet manufactured by this
method, however, in order to obtain a remarkable hardening from the
heat treatment after forming, the heat treatment temperature must
be at least 500.degree. C., and this has posed a problem in
practice.
[0011] Japanese Unexamined Patent Application Publication No.
10-310824 proposes a manufacturing method of an alloyed hot-dip
galvanized steel sheet permitting expectation of an increase in
strength through a heat treatment after forming, using a hot-rolled
or cold-rolled steel sheet as a substrate. This method comprises
the steps of hot-rolling a steel containing from 0.01 to 0.08% C,
appropriate amounts of Si, Mn, P, S, Al and N, and one or more of
Cr, W and Mo in a total amount of from 0.05 to 3.0%, or
cold-rolling or temper-rolling the sheet and annealing the same,
applying hot-dip galvanizing the sheet, and then, conducting a
heating/alloying treatment. The Publication asserts that, after
forming, tensile strength is increased by heating the sheet at a
temperature within a range of from 200 to 450.degree. C. However,
the resultant steel sheet involves a problem in that, because the
microstructure comprises a ferrite single phase, a
ferrite+pearlite, or a ferrite+bainite structure, a high elongation
and a low yield strength are unavailable, resulting in a low
press-formability.
[0012] Japanese Unexamined Patent Application Publication No.
11-199975 proposes a hot-rolled steel sheet for working excellent
in fatigue property, containing from 0.03 to 2.0% C, appropriate
amounts of Si, Mn, P, S and Al, from 0.2 to 2.0% Cu, and from
0.0002 to 0.002% B, of which the microstructure is a composite
structure having ferrite as a main phase and martensite as the
second phase, and the state of presence of Cu in the ferrite phase
in a solid-solution state and/or precipitation of up to 2 nm. The
proposed steel sheet has an object based on a fact that fatigue
limit ratio is remarkably improved only when compositely adding Cu
and B, and achieving the finest state of Cu as up to 2 nm. For this
purpose, it is essential to end hot finish rolling at a temperature
of at least the Ar.sub.3 transformation point, air-cool the sheet
within a temperature region of from Ar.sub.3 to Ar.sub.1 in cooling
for a period of from 1 to 10 seconds, then cool the sheet at a
cooling rate of at least 20.degree. C./second, and coil the cooled
sheet at a temperature of up to 350.degree. C. A low coiling
temperature of up to 350.degree. C. poses a problem of causing a
serious deformation of the shape of the hot-rolled steel sheet,
thus preventing industrially stable manufacture.
DISCLOSURE OF INVENTION
[0013] The present invention was developed in view of the fact
that, in spite of the strong demand as described above, a technique
for industrially stably manufacturing a steel sheet satisfying
these properties has never been proposed, and has an object to
favorably solve the problems described above and to provide a
high-strength steel sheet suitable as an automotive steel sheet,
having an excellent press-formability, and excellent in strain age
hardening property causing tensile strength to increase
considerably through a heat treatment at a relatively low
temperature after press-forming, and a manufacturing method
permitting stable production of such a high-strength steel sheet.
The term "steel sheets" as herein used shall include hot-rolled
steel sheets, cold-rolled steel sheets and plated steel sheets.
[0014] To achieve the above-mentioned object of the invention, the
present inventors carried out extensive studies on the effect of
the steel sheet structure and alloying elements on strain age
hardening property. As a result, the following findings were
obtained. It is possible to obtain a high strain age hardening
bringing about an increase in yield stress, and in addition, a
remarkable increase in tensile strength, after application of a
pre-strain treatment of an amount of prestrain of 5% or more and a
heat treatment at a relatively low temperature within a range of
from 150 to 350.degree. C. There is thus available a steel sheet
having a satisfactory elongation, a low yield strength and a high
hole expanding ratio, and excellent in press-formability.
[0015] On the basis of the novel findings as described above, the
present inventors carried out further extensive studies and found
that the above-mentioned phenomenon occurred in steel sheets not
containing Cu as well. When a prestrain is imparted by using a
steel sheet containing one or more of Mo, Cr and W in place of Cu,
and achieving a ferrite+martensite composite structure, and a heat
treatment was applied at a low temperature, very fine carbides were
formed to strain-induced-precipitate in martensite, resulting in an
increase in tensile strength. The strain-induced precipitation upon
heating to a low temperature was found to become more remarkable by
containing one or more of Nb, V and Ti, in addition to one or more
of Mo, Cr and W.
[0016] The present invention was completed through further studies
on the basis of the aforementioned findings. The gist of the
invention is as follows:
[0017] (1) A steel sheet excellent in press-formability and in
strain age hardening property as typically represented by a
.DELTA.TS of 80 MPa or more, comprising a structure having ferrite
phase as a main phase forming a composite structure with a
secondary phase containing martensite phase in an area ratio of 2%
or more.
[0018] (2) A steel sheet excellent in press-formability and in
strain age hardening property as typically represented by a
.DELTA.TS of 80 MPa or more as in (1) above, wherein the steel
sheet is a hot-rolled steel sheet.
[0019] (3), A steel sheet according to (2) above, excellent in
press-formability and in strain age hardening property as typically
represented by a .DELTA.TS of 80 MPa or more, comprising, in weight
percentage: 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1%
or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, from
0.5 to 3.0% Cu and the balance Fe and incidental impurities.
[0020] (4) A steel sheet according to (3) above, containing, in
weight percentage, one or more selected from the following groups A
to C, in addition to the above-mentioned chemical composition:
[0021] group A: Ni: 2.0% or less;
[0022] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0023] group C: one or more of Nb, Ti and V: 0.2% or less in
total.
[0024] (5) A steel sheet according to (2) above, excellent in
press-formability and in strain age hardening property as typically
represented by a .DELTA.TS of 80 MPa or more, having a chemical
composition comprising, in weight percentage: 0.15% or less C, 2.0%
or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1%
or less Al, 0.02% or less N, one or more selected from the group
consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr and from
0.05 to 2.0% W, 2.0% or less in total, and the balance Fe and
incidental impurities.
[0025] (6) A steel sheet according to (5) above, excellent in
press-formability and in strain age hardening property as typically
represented by a .DELTA.TS of 80 MPa or more, further comprising,
in addition to the above-mentioned chemical composition, in weight
percentage, one or more selected from the group consisting of Nb,
Ti, and V, 2.0% or less in total.
[0026] (7) A manufacturing method of a steel sheet excellent in
press-formability and in strain age hardening property as typically
represented by a .DELTA.TS of 80 MPa or more, comprising the steps,
when hot-rolling a steel slab having a chemical composition
comprising, in weight percentage, 0.15% or less C, 2.0% or less Si,
3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al,
0.02% or less N, and from 0.5 to 3.0% Cu, or additionally
containing one or more selected from the following groups A to
C:
[0027] group A: Ni: 2.0% or less;
[0028] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0029] group C: one or more of Nb, Ti and V: 0.2% or less in
total,
[0030] and preferably the balance Fe and incidental impurities,
into a hot-rolled steel sheet having a prescribed thickness,
carrying out the hot rolling with a finish rolling end temperature
FDT of the Ar.sub.3 transformation point or more, then after the
completion of the finish rolling, cooling the hot-rolled steel
sheet to a temperature region from the (Ar.sub.3 transformation
point) to the (Ar.sub.1 transformation point) at a cooling rate of
5.degree. C./second or more, air-cooling or slowly cooling the
sheet within the temperature region for a period of from 1 to 20
seconds, then cooling the sheet again at a cooling rate of
5.degree. C./second or more, and coiling the sheet at a temperature
of 550.degree. C. or below.
[0031] (8) A manufacturing method of a hot-rolled steel sheet
excellent in press-formability and in strain age hardening property
as typical represented by a .DELTA.TS of 80 MPa or more, according
to (6) above, wherein the steel slab has a chemical composition
containing, in weight percentage, 0.15% or less C, 2.0% or less Si,
3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al,
0.02% or less N, and further containing one or more selected from
the group consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr,
and from 0.05 to 2.0% W, 2.0% or less in total, or further
containing one or more selected from the group consisting of Nb, Ti
and V, in an amount of 2.0% or less in total, and preferably, the
balance Fe and incidental impurities.
[0032] (9) A manufacturing method of a hot-rolled steel sheet
excellent in press-formability and in strain age hardening property
as typically represented by a .DELTA.TS of 80 MPa or more,
according to (7) or (8) above, wherein all or part of the finish
rolling comprises lubrication rolling.
[0033] (10) A steel sheet excellent in press-formability and in
strain age hardening property as typically represented by a
.DELTA.TS of 80 MPa or more, according to (1) above, which is a
cold-rolled steel sheet.
[0034] (11) A steel sheet excellent in press-formability and in
strain age hardening property as typically represented by a
.DELTA.TS of 80 MPa or more, according to (10) above, comprising,
in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or
less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or
less N, from 0.5 to 3.0% Cu, and the balance Fe and incidental
impurities.
[0035] (12) A steel sheet excellent in press-formability and in
strain age hardening property as typically represented by a
.DELTA.TS of 80 MPa or more, according to (11) above, containing,
in weight percentage, one or more selected from the following
groups A to C, in addition to the above-mentioned chemical
composition:
[0036] group A: Ni: 2.0% or less;
[0037] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0038] group C: one or more of Nb, Ti and V: 0.2% or less in
total.
[0039] (13) A steel sheet excellent in press-formability and in
strain age hardening property as typically represented by a
.DELTA.TS of 80 MPa or more, according to (10) above, having a
chemical composition comprising, in weight percentage, in addition
to the above-mentioned chemical composition, 0.15% or less C, 2.0%
or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1%
or less Al, 0.02% or less N, one or more selected from the group
consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr and from
0.05 to 2.0% W, 2.0% or less in total, and the balance Fe and
incidental impurities.
[0040] (14) A steel sheet excellent in press-formability and in
strain age hardening property as typically represented by a
.DELTA.TS of 80 MPa or more, according to (13) above, further
comprising, in addition to the above-mentioned chemical
composition, in weight percentage, one or more selected from the
group consisting of Nb, Ti and V, 2.0% or less in total.
[0041] (15) A manufacturing method of a cold-rolled steel sheet
excellent in press-formability and in strain age hardening property
as typically represented by a .DELTA.TS of 80 MPa or more,
comprising the steps of using a steel slab having a chemical
composition containing, in weight percentage, 0.15% or less C, 2.0%
or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1%
or less Al, 0.02% or less N, and from 0.5 to 3.0% Cu, or further
containing one or more selected from the following groups A to
C:
[0042] group A: Ni: 2.0% or less;
[0043] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0044] group C: one or more of Nb, Ti and V: 0.2% or less in total,
and preferably, the balance Fe and incidental impurities as a
material; a hot rolling step of applying hot rolling to the
material into a hot-rolled steel sheet; a cold rolling step of
applying cold rolling to the hot-rolled steel sheet into a
cold-rolled steel sheet; and a
[0045] recrystallization annealing step of applying
recrystallization annealing into a cold-rolled annealed steel
sheet; these steps being sequentially applied; wherein the
recrystallization annealing is conducted in a ferrite+austenite
dual phase region within a temperature range of from Ac.sub.1
transformation point to Ac.sub.3 transformation point.
[0046] (16) A manufacturing method of a cold-rolled steel sheet
excellent in press-formability and in strain age hardening property
as typically represented by a .DELTA.TS of 80 MPa or more,
according to (15) above, wherein the steel slab has a chemical
composition containing, in weight percentage, 0.15% or less C, 2.0%
or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1%
or less Al, 0.02% or less N, and further containing one or more
selected from the group consisting of from 0.05 to 2.0% Mo, from
0.05 to 2.0% Cr, and from 0.05 to 2.0% W, or further containing one
or more of Nb, Ti and V, 2.0% or less in total, and preferably, the
balance Fe and incidental impurities.
[0047] (17) A manufacturing method of a cold-rolled steel sheet
excellent in press-formability and in strain age hardening property
as typically represented by a .DELTA.TS of 80 MPa or more,
according to (15) or (16) above, wherein the hot rolling is
conducted under conditions including a heating temperature of the
material of 900.degree. C. or more, a finish rolling end
temperature of 700.degree. C. or more, and a coiling temperature of
800.degree. C. or below.
[0048] (18) A manufacturing method of a cold-rolled steel sheet
excellent in press-formability and in strain age hardening property
as typically represented by a .DELTA.TS of 80 MPa or more,
according to any one of (15) to (17) above, wherein all or part of
the hot rolling comprises lubrication rolling.
[0049] (19) A hot-dip galvanized steel sheet excellent in
press-formability and in strain age hardening property as typically
represented by a .DELTA.TS of 80 MPa or more, comprising a hot-dip
galvanizing layer or an alloyed hot-dip galvanizing layer formed on
the surface of the hot-rolled steel sheet according to any one of
(2) to (6) above.
[0050] (20) A hot-dip galvanized steel sheet excellent in
press-formability and in strain age hardening property as typically
represented by a .DELTA.TS of 80 MPa or more, comprising a hot-dip
galvanizing layer or an alloyed hot-dip galvanizing layer formed on
the surface of the cold-rolled steel sheet according to any one of
(10) to (14) above.
[0051] (21) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
comprising the steps of using a steel sheet having a chemical
composition containing, in weight percentage, 0.15% or less C, 2.0%
or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1%
or less Al, 0.02% or less N, and from 0.5 to 3.0% Cu, or further
containing one or more selected from the following groups:
[0052] group A: 2.0% or less Ni;
[0053] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0054] group C: one or more of Nb, Ti and V: 0.2% or less in
total,
[0055] preferably the balance Fe and incidental impurities,
[0056] applying annealing comprising heating to a dual phase region
of ferrite+austenite within a temperature range of from Ac.sub.3
transformation point to Ac.sub.1 transformation point to the steel
sheet on a line for conducting continuous hot-dip galvanizing, and
then, performing a hot-dip galvanizing treatment, thereby forming a
hot-dip galvanizing layer on the surface of the steel sheet.
[0057] (22) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
according to (21) above, wherein the steel sheet is replaced by a
steel sheet having a chemical composition containing, in weight
percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1%
or less P, 0.02% or less S, 0.1% or less Al, and 0.02% or less N,
and further comprising one or more selected from the group
consisting of from 0.05 to-2.0% Mo, from 0.05 to 2.0% Cr and from
0.05 to 2.0% W, 2.0% or less in total, or further containing one or
more of Nb, Ti and V in an amount of 2.0% or less in total,
preferably the balance Fe and incidental impurities.
[0058] (23) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by as .DELTA.TS of 80 MPa or
more, according to (21) or (22) above, wherein, prior to the
annealing, a preheating treatment of heating the sheet at a
temperature of 700.degree. C. or more on a continuous annealing
line, and then applying a pretreatment comprising a pickling
treatment.
[0059] (24) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
according to any one of (21) to (23) above, comprising the steps of
conducting the hot-dip galvanizing treatment to form a hot-dip
galvanizing layer on the surface of the steel sheet, and then,
performing an alloying treatment of the hot-dip galvanizing
layer.
[0060] (25) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
according to any one of (21) to (24) above, wherein the steel sheet
is a hot-rolled steel sheet manufactured by hot-rolling the
material having the chemical composition under conditions including
a heating temperature of 900.degree. C. or more, a finish rolling
end temperature of 700.degree. C. or more and a coiling temperature
of 800.degree. C. or below, or a cold-rolled steel sheet obtained
by cold-rolling the hot-rolled steel sheet.
[0061] (26) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
further comprising a step of applying a hot-dip galvanizing
treatment to the hot-rolled steel sheet resulting from the
manufacturing method of a hot-rolled steel sheet according to any
one of (7) to (9) above to form a hot-dip galvanizing layer on the
surface of the hot-rolled steel sheet.
[0062] (27) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
further comprising a step of applying a hot-dip galvanizing
treatment to the cold-rolled steel sheet resulting from the
manufacturing method of a cold-rolled steel sheet according to any
one of (15) to (18) above to form a hot-dip galvanizing layer on
the surface of the cold-rolled steel sheet.
[0063] (28) A manufacturing method of a hot-dip galvanized steel
sheet excellent in press-formability and in strain age hardening
property as typically represented by a .DELTA.TS of 80 MPa or more,
according to any one of (26) and (27) above, further comprising the
step of carrying out an alloying treatment after the hot-dip
galvanizing treatment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] FIG. 1 is a graph illustrating the effect of the Cu content
on the relationship between .DELTA.TS and the (hot-rolled) steel
sheet structure after a pre-strain--heat treatment;
[0065] FIG. 2 is a graph illustrating the effect of the Cu content
on the relationship between .DELTA.TS and the heat treatment
temperature after a pre-strain--heat treatment of a hot-rolled
steel sheet;
[0066] FIG. 3 is a graph illustrating the effect of the Cu content
on the relationship between .lambda. and YR of a hot-rolled steel
sheet;
[0067] FIG. 4 is a graph illustrating the effect of the Cu content
on the relationship between .DELTA.TS and the recrystallization
temperature after pre-strain--heat treatment of a cold-rolled steel
sheet;
[0068] FIG. 5 is a graph illustrating the effect of the Cu content
on the relationship between .DELTA.TS and the heat treatment
temperature after pre-strain--heat treatment of a cold-rolled steel
sheet;
[0069] FIG. 6 is a graph illustrating the effect of the Cu content
on the relationship between .lambda. and YR of a cold-rolled steel
sheet;
[0070] FIG. 7 is a graph illustrating the effect of the Cu content
on the relationship between .DELTA.TS and the recrystallization
annealing temperature after a pre-strain--heat treatment of a
hot-dip galvanized steel sheet;
[0071] FIG. 8 is a graph illustrating the effect of the Cu content
on the relationship between .DELTA.TS and the heat treatment
temperature after a pre-strain-heat treatment of a hot-dip
galvanized steel sheet; and
[0072] FIG. 9 is a graph illustrating the effect of the Cu content
on the relationship between .lambda. and YR of a hot-dip galvanized
steel sheet.
BEST MODE FOR CARRYING OUT THE INVENTION
[0073] The term "being excellent in strain age hardening property"
shall mean that, when a steel sheet is subjected to a pre-strain
treatment of an amount of tensile plastic strain of 5% or more, and
then, to a heat treatment at a temperature within a range of from
150 to 350.degree. C. for a holding time of 30 seconds or more, the
increment .DELTA.TS in tensile strength between before and after
the heat treatment {=(tensile strength after heat
treatment)--(tensile strength before pre-strain treatment)} is 80
MPa or more, or .DELTA.TS should preferably be 100 MPa or more. It
is needless to mention that the heat treatment causes an increase
in yield stress, bringing about a .DELTA.YS of 80 MPa or more. The
term .DELTA.YS means an increment of yield strength from before to
after the heat treatment, and is defined as .DELTA.YS ={(yield
strength after heat treatment)-(yield strength before pre-strain
treatment)}.
[0074] When regulating the strain age hardening property, the
amount of pre-strain plays an important role. The present inventors
investigated the effect of the amount of prestrain on the
subsequent strain age hardening property by assuming types of
deformation to which automotive steel sheets are subjected. The
resultant findings included the possibility to arrange data in
terms of uniaxial equivalent strain (tensile strain) except for a
very deep drawing, that the uniaxial equivalent strain amount
substantially accounts for more than 5% for actual parts, and that
the parts strength exhibits a good agreement with the strength
available after a strain aging treatment of a prestrain of 5%.
Considering these findings, the prestrain (deformation) of a strain
aging treatment is assumed to give a tensile plastic strain of 5%
or more in the present invention.
[0075] The conventional baking treatment conditions include
170.degree. C..times.20 minutes as standards. When using
precipitation strengthening of very fine Cu as in the present
invention, a heat treatment temperature of 150.degree. C. or more
is necessary. Under conditions including a temperature of over
350.degree. C., on the other hand, the effect is saturated, and
even a tendency toward softening is exhibited. Heating to a
temperature of over 350.degree. C. causes marked occurrence of
thermal strain or temper color. For these reasons, a heat treatment
temperature range of from 150 to 350.degree. C. is adopted for
strain age hardening in the invention. The holding time of the heat
treatment temperature should be 30 seconds or more. Holding a heat
treatment temperature within a range of from 150 to 350.degree. C.
for about 30 seconds permits achievement of substantially
sufficient strain age hardening. When desiring a more stable strain
age hardening, the holding time should preferably be 60 seconds or
more, or more preferably, 300 seconds or more.
[0076] While no particular restriction is imposed on the
aforementioned heating method in the heat treatment, atmospheric
heating in a furnace, as well as induction heating, and heating by
non-oxidizing flame, a laser or plasma are suitably applicable.
So-called hot pressing for pressing a steel sheet while heating the
same is very effective means in the present invention.
[0077] The result of a fundamental experiment carried out by the
present inventors on hot-rolled steel sheets will first be
described.
[0078] A sheet bar having a chemical composition containing, in
weight percentage, 0.04% C, 0.82% Si, 1.6% Mn, 0.01% P, 0.005% S,
0.04% Al and 0.002% N, with Cu varying to 0.3% and 1.3% was heated
to 1,150.degree. C. and soaked at this temperature, subjected to
three-pass rolling to a thickness of 2.0 mm so as to achieve a
finish rolling end temperature of 850.degree. C., and converted
from a single ferrite structure steel sheet into a hot-rolled steel
sheet having a composite ferrite +martensite structure by changing
cooling conditions and the coiling temperature.
[0079] Tensile property was investigated through a tensile test on
these hot-rolled steel sheets. A pre-strain treatment of a tensile
prestrain of 5% was applied to test pieces sampled from these
hot-rolled steel sheets. Then, after applying a heat treatment at
50 to 350.degree. C. for 20 minutes, a tensile test was carried out
to determine tensile property, and the strain age hardening
property was evaluated.
[0080] The strain age hardening property was evaluated in terms of
the increment .DELTA.TS of tensile strength from before to after
the heat treatment. The term .DELTA.TS is herein defined as a
difference between tensile strength TS.sub.HT after heat treatment
and tensile strength TS when no heat treatment is applied
{=(tensile strength TS.sub.HT after heat treatment)--(tensile
strength TS before pre-strain treatment)}. The tensile test was
carried out by using JIS #5 tensile test pieces.
[0081] FIG. 1 illustrates the effect of the Cu content on the
relationship between .DELTA.TS and the steel sheet (hot-rolled
steel sheet) structure. The value of .DELTA.TS was determined by
conducting a pre-strain treatment of a tensile prestrain of 5% on
the test pieces, and then, applying a heat treatment of 250.degree.
C..times.20 minutes. It is suggested from FIG. 1 that, for a Cu
content of 1.3 wt. %, a high strain age hardening property as
represented by a .DELTA.TS of 80 MPa or more is available by
achieving a composite ferrite+martensite steel sheet structure. In
the case of a Cu content of 0.3 wt. %, .DELTA.TS is under 80 MPa,
and a high strain age hardening property cannot be obtained even by
achieving a composite ferrite+martensite steel sheet structure.
[0082] It is possible to manufacture a hot-rolled steel sheet
having a high strain age hardening property by limiting the Cu
content within an appropriate range, and achieving a composite
ferrite+martensite structure.
[0083] FIG. 2 illustrates the effect of the Cu content on the
relationship between .DELTA.TS and the heat treatment temperature
after pre-strain treatment. The hot-rolled sheet used was prepared
by cooling the sheet after hot rolling at a cooling rate of
20.degree. C./second to 700.degree. C., then, after air-cooling for
5 seconds, cooling the sheet at a cooling.rate of 30.degree.
C./second to 450.degree. C., and then, applying a coiling
equivalent treatment at 450.degree. C. for one hour. The thus
obtained hot-rolled steel sheet had a composite microstructure
comprising ferrite as a main phase and martensite of an area ratio
of 8%. After applying a pre-strain treatment to these hot-rolled
steel sheets, a heat treatment was carried out to determine
.DELTA.TS.
[0084] As is known from FIG. 2, .DELTA.TS increases along with an
increase in the heat treatment temperature, and this increment is
largely dependent upon the Cu content. When the Cu content is 1.3
wt. %, a high strain age hardening property can be obtained at a
heat treatment temperature of 150.degree. C. or more and a
.DELTA.TS of 80 MPa or more. With a Cu content of 0.3 wt. %,
.DELTA.TS is under 80 MPa, and a high strain age hardening property
is unavailable at any heat treatment temperature.
[0085] From steel sheets having Cu contents of 0.3 wt. % and 1.3
wt. %, respectively, materials (hot-rolled steel sheets) having a
yield ratio YR (=(yield strength YS/tensile strength
TS).times.100%) of within a range of from 50 to 90% were prepared
by changing the cooling rate after hot rolling to various levels
with a structure converted from ferrite +martensite into single
ferrite phase. The hole expanding ratio (.lambda.) was determined
by carrying out a hole expanding test on these materials
(hot-rolled steel sheets). In the hole expanding test, the hole
expanding ratio .lambda. was determined by forming punch holes in
test pieces through punching with a punch having a diameter of 10
mm, and conducting hole expansion until occurrence of cracks
running through the thickness, so that the burr is outside, by
means of a conical punch having a vertical angle of 60.degree.. The
hole expanding ratio .lambda. was determined by using a formula:
.lambda.(%)={(d-d.sub.0)/d.sub.0}.time- s.100, where d.sub.0:
initial hole diameter, and d: hole inside diameter upon occurrence
of cracks.
[0086] These result are arranged in terms of the relationship
between the hole expanding ratio .lambda. and yield ratio YR, and
the derived effect of the Cu content on the relationship between
the hole expanding ratio .lambda. and yield ratio YR is illustrated
in FIG. 3.
[0087] FIG. 3 suggests that a steel sheet having a Cu content of
0.3 wt. % has a composite ferrite (.alpha.)+martensite structure,
and with a YR of under 70%, the decreasing YR results in a decrease
in .lambda.. A steel sheet having a Cu content of 1.3 wt. % has a
composite ferrite (.alpha.)+martensite structure and keeps a high
.lambda.-value even with a decreasing YR. In a steel sheet having a
Cu content of 0.3 wt.%, a low YR and a high .lambda. cannot
simultaneously be obtained.
[0088] This suggests the possibility to manufacture a hot-rolled
steel sheet satisfying requirements of both a low yield ratio and a
high hole expanding ratio by limiting the Cu content within an
appropriate range and achieving a composite ferrite
(.alpha.)+martensite structure.
[0089] In the hot-rolled steel sheet of the invention, very fine Cu
precipitates in the steel sheet as a result of a pre-strain with an
amount of strain of 2% or more as measured upon measuring the
increment of deformation stress from before to after a usual heat
treatment and the heat treatment carried out at a relatively low
temperature as within a range of from 150 to 350.degree. C.
According to an investigation conducted by the present inventors, a
high strain age hardening property leading to an increase in yield
stress and a remarkable increase in tensile strength is considered
to have been obtained through this precipitation of very fine Cu.
Precipitation of very fine Cu by a heat treatment in a relatively
low temperature region has never been observed in ultra-low carbon
steel or low-carbon steel in reports so far released. A reason of
precipitation of very fine Cu in a heat treatment at a relatively
low temperature has not as yet been clarified to date, but it is
conceivable that, during holding in the dual phase region of
ferrite (.alpha.)+austenite (.gamma.), Cu is largely distributed in
the .gamma.-phase, distributed Cu remaining even after cooling
being converted into an super-saturated solid-solution state in
martensite, and very finely precipitates through imparting of a
prestrain of 5% or more and a low-temperature heat treatment.
[0090] The hole expanding ratio is increased in a steel sheet to
which Cu is added and in which a composite ferrite+martensite
structure is achieved. A detailed mechanism of this increase has
not as yet been clarified. It is however considered attributable to
the fact that addition of Cu reduces the difference in hardness
between ferrite and martensite.
[0091] The hot-rolled steel sheet of the invention is a
high-strength hot-rolled steel sheet having a tensile strength TS
of 440 MPa or more and excellent in press-formability, of which
tensile strength remarkably increases as a result of a heat
treatment at a relatively low temperature after press forming,
leading to an excellent strain age hardening property with a
.DELTA.TS of 80 MPa or more.
[0092] The structure of the hot-rolled steel sheet of the invention
will now be described.
[0093] The hot-rolled steel sheet of the invention has a composite
structure comprising a ferrite phase and a secondary phase
containing martensite phase having an area ratio of 2% or more
relative to the entire structure.
[0094] In order to obtain a steel sheet having a low yield strength
YS and a high elongation El, and excellent in press-formability, in
the invention, it is necessary to convert the structure of the
hot-rolled steel sheet of the invention into a composite structure
comprising a ferrite phase which is the main phase and a secondary
phase containing martensite. Ferrite serving as the main phase
should preferably have an area ratio of 50% or more. With ferrite
of under 50%, it is difficult to keep a high elongation, resulting
in a lower press-formability. When a satisfactory elongation is
required, the area ratio of the ferrite phase should preferably be
80% or more. For the purpose of making full use of advantages of
the composite structure, the ferrite phase should preferably be 98%
or less.
[0095] In the invention, steel must contain martensite as the
secondary phase in an area ratio of 2% or more relative to the
entire structure. An area ratio of martensite of under 2% cannot
simultaneously satisfy a low YS and a high El. The secondary phase
may be a single martensite phase having an area ratio of 2% or
more, or may be a mixture of a martensite phase of an area ratio of
2% or more and a secondary phase comprising a pearlite phase, a
bainite phase, or a retained austenite phase.
[0096] The hot-rolled steel sheet having the above-mentioned
structure thus becomes a steel sheet excellent in
press-formability, with a low yield strength and a high elongation,
and in strain age hardening property.
[0097] The reasons of limiting the chemical composition of the
hot-rolled steel sheet of the invention will now be described. The
weight percentage, wt. %, will hereafter be denoted simply as
%.
[0098] C: 0.15% or less:
[0099] C is an element which improves strength of a steel sheet,
and promotes formation of a composite structure of ferrite and
martensite, and should preferably be contained in an amount of
0.01% or more for forming a composite structure in the invention. A
C content of over 0.15% on the other hand causes an increase in
partial ratio of carbides in steel, resulting in a decrease in
elongation, and hence a decrease in press-formability. A more
important problem is that a C content of over 0.15% leads to a
serious decrease in spot weldability and arc weldability. For these
reasons, in the invention, the C content is limited to 0.15% or
less. From the point of view of formability, the C content should
more preferably be 0.10% or less.
[0100] Si: 2.0% or less:
[0101] Si is a useful strengthening element which can improve
strength of a steel sheet without causing a marked decrease in
elongation of the steel sheet, and is effective for accelerating
ferrite transformation and promoting martensite formation through C
concentration into non-transformed austenite. A Si content of over
2.0% however leads to deterioration of press-formability and
deteriorates the surface quality. The Si content is therefore
limited to 2.0% or less. With a view to forming martensite, Si
should preferably be contained in an amount of 0.1% or more.
[0102] Mn: 3.0% or less:
[0103] Mn has a function of strengthening steel, and of
accelerating formation of a composite ferrite+martensite structure.
Mn is an element effective for preventing hot cracking caused by S,
and should therefore be contained in an amount dependent upon S
content. These effects are particularly remarkable at a Mn content
of 0.5% or more. On the other hand, a Mn content of over 3.0%
results in deterioration of press-formability and weldabillity. The
Mn content is therefore limited to 3.0% or less, and more
preferably, to 1.0% or more.
[0104] P: 0.10% or less:
[0105] P has a function of strengthening steel, and can be
contained in an amount necessary for a desired strength. An
excessive P content however causes deterioration of
press-formability. The P content is therefore limited to 0.10% or
less. When a further higher press-formability is required, the P
content should preferably be 0.08% or less.
[0106] S: 0.02% or less:
[0107] S is an element which is present as inclusions in steel and
causes deterioration of elongation, formability, and particularly
stretch flanging formability of a steel sheet. It should therefore
be the lowest possible. A S content reduced to 0.02% or less does
not exert much adverse effect. In the invention, therefore, the S
content is limited to 0.02% or less. When an excellent stretch
flanging formability is required, the S content should preferably
be 0.010% or less.
[0108] Al: 0.10% or less:
[0109] Al is an element which is added as a deoxidizing element of
steel, and is useful for improving cleanliness of steel. However,
an Al content of over 0.10% cannot give a further deoxidizing
effect, but causes in contrast deterioration of press-formability.
The Al content is therefore limited to 0.10% or less, and
preferably, 0.01% or more. The invention does not exclude a
steelmaking process based on a deoxidation by means of a deoxidizer
other than Al. For example, Ti deoxidation or Si deoxidation may be
used, and steel sheets produced by such deoxidation methods are
also included in the scope of the invention.
[0110] N: 0.02% or less:
[0111] N is an element which increases strength of a steel sheet
through solid-solution strengthing or strain age hardening. A N
content of over 0.02% however causes an increase in the content of
nitrides in the steel sheet, which in turn causes a serious
deterioration of elongation, and furthermore, of press-formability.
The N content is therefore limited to 0.02% or less. When further
improvement of press-formability is required, the N content should
suitably be 0.01% or less.
[0112] Cu: from 0.5 to 3.0%:
[0113] Cu is an element which remarkably increases strain age
hardening of a steel sheet (increase in strength after
pre-strain--heat treatment), and is one of the most important
elements in the invention. With a Cu content of under 0.5%, an
increase in tensile strength of over .DELTA.TS: 80 MPa even by
using different pre-strain--heat treatment conditions cannot be
obtained. In the invention, therefore, Cu should be contained in an
amount of 0.5% or more. With a Cu content of over 3.0%, on the
other hand, the effect is saturated so that an effect corresponding
to the content cannot be expected, leading to unfavorable economic
effects. Deterioration of press-formability results, and the
surface quality of the steel sheet degrades. The Cu content is
therefore limited within a range of from 0.5 to 3.0%. In order to
simultaneously achieve a higher .DELTA.TS and an excellent
press-formability, the Cu content should preferably be within a
range of from 1.0 to 2.5%.
[0114] In the invention, in addition to the chemical composition
containing Cu as described above, it is desirable to contain, in
weight percentage, one or more of the following groups A to C:
[0115] group A: Ni: 2.0% or less;
[0116] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0117] group C: one or more of Nb, Ti and V: 0.2% or less in
total.
[0118] Group A: Ni: 2.0% or less:
[0119] Group A: Ni is an element effective for preventing surface
defects produced on the steel sheet surface upon adding Cu, and can
be contained as required. If contained, the Ni content, depending
upon the Cu content, should preferably be about a half the Cu
content. An Ni content of over 2.0% cannot give a corresponding
effect because of saturation of the effect, leading to economic
disadvantages, and causes deterioration of press-formability. The
Ni content should preferably be limited to 2.0% or less.
[0120] Group B: one or two of Cr and Mo: 2.0% or less in total:
[0121] Group B: As in Mn, both Cr and Mo have a function of
promoting formation of a composite ferrite+martensite structure,
and can be contained as required. If one or two of Cr and Mo are
contained in an amount of over 2.0% in total, there occurs a
decrease in press-formability. It is therefore desirable to limit
the total content of one or two of Cr and Mo forming group B to
2.0% or less.
[0122] Group C: one or more of Nb, Ti and V: 0.2% or less in
total:
[0123] Group C: Nb, Ti and V are carbide-forming elements which
effectively act to increase strength through fine dispersion of
carbides, and can be selected and contained as required. However,
if the total content of one or more of Nb, Ti and V is over 0.2%,
there occurs deterioration of press-formability. The total content
of Nb, Ti and/or V should therefore preferably be limited to 0.2%
or less.
[0124] In the invention, in place of the aforementioned Cu, or
further one or more of the above-mentioned groups A to C, one or
more selected from the group consisting of from 0.05 to 2.0% Mo,
from 0.05 to 2.0% Cr, and from 0.05 to 2.0% W may be contained in
an amount of 2.0% or less in total, or further one or more selected
from the group consisting of Nb, Ti and V in an amount of 2.0% or
less in total.
[0125] One or more selected from the group consisting of from 0.05
to 2.0% Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, in an
amount of 2.0% in total:
[0126] Mo, Cr and W are elements which cause a remarkable increase
in strain age hardening of a steel sheet, are the most important
elements in the invention, and can be selected and contained.
Containing one or more of Mo, Cr and W, and achievement of a
composite ferrite+martensite structure cause strain-induced fine
precipitation of fine carbides during pre-strain--heat treatment,
thus making it possible to obtain a tensile strength as represented
by a .DELTA.TS of 80 MPa or more. With a content of each of these
elements of under 0.05%, changing of pre-strain--heat treatment
conditions or the steel sheet structure does not give an increase
in tensile strength represented by a .DELTA.TS of 80 MPa or more.
On the other hand, even if the content of each of these elements is
over 2.0%, an effect corresponding to the content cannot be
expected as a result of saturation of the effect, leading to
economic disadvantages, and this results in deterioration of
press-formability. The contents of Mo, Cr and W are therefore
limited within a range of from 0.05 to 2.0% for Mo, from 0.05 to
2.0% for Cr, and from 0.05 to 2.0% for W. From the point of view of
press-formability, the total content of Mo, Cr and/or W is limited
to 2.0% or less.
[0127] One or more of Nb, Ti and V: 2.0% or less in total: Nb, Ti
and V are carbide-forming elements, and can be selected and
contained as required. Containing one or more of Nb, Ti and V, and
achievement of a composite ferrite+martensite structure cause
strain-induced fine precipitation of fine carbides during
pre-strain--heat treatment, thus making it possible to obtain a
tensile strength as represented by a .DELTA.TS of 80 MPa or more.
However, a total content of one or more of Nb, Ti and V of over
2.0% causes deterioration of press-formability. The total content
of Nb, Ti and/or V should therefore preferably be limited to 2.0%
or less.
[0128] Apart from the above-mentioned elements, one or two of 0.1%
or less Cu and 0.1% or less REM may be contained. Ca and REM are
elements contributing to improvement of elongation through shape
control of inclusions. If the Ca content is over 0.1% and the REM
content is over 0.1%, however, there would be a decrease in
cleanliness, and a decrease in elongation.
[0129] From the point of view of forming martensite, one or two of
up to 0.1% B and up to 0.1% Zr may be contained.
[0130] The balance except for the above-mentioned constituents
comprises Fe and incidental impurities. Allowable incidental
impurities include 0.01% or less Sb, 0.01% or less Pb, 0.1% or less
Sn, 0.01% or less Zn, and 0.1% or less Co.
[0131] The hot-rolled steel sheet having the aforementioned
chemical composition and structure has a low yield strength and a
high elongation, excellent in press-formability and in strain age
hardening property.
[0132] A manufacturing method of the hot-rolled steel sheet of the
present invention will now be described.
[0133] The hot-rolled steel sheet of the invention is made from a
steel slab, as a material, having a chemical composition within the
ranges described above, and by hot-rolling such a material into a
prescribed thickness.
[0134] While the steel slab used should preferably be manufactured
by the continuous casting process to prevent macro-segregation of
the constituents, or may be manufactured by the ingot casting
process or the thin continuous casting process. An energy-saving
process such as direct-hot-charge rolling or direct rolling is
applicable with no problem, which comprises the steps of
manufacturing a steel slab, then once cooling the slab to room
temperature, then reheating as in the conventional art, and
charging the same into a reheating furnace as a hot slab without
cooling, or immediately rolling the slab after slight holding.
[0135] It is not necessary to impose a particular restriction on
the reheating temperature of the material (steel slab), but it
should preferably be 900.degree. C. or more.
[0136] Slab reheating temperature: 900.degree. C. or more:
[0137] The slab reheating temperature SRT should preferably be the
lowest possible with a view to preventing surface defects caused by
Cu when the chemical composition contains Cu. However, with a
reheating temperature of under 900.degree. C., there is an increase
in the rolling load, thus increasing the risk of occurrence of a
trouble during hot rolling. Considering the increase in scale loss
caused along with the increase in weight loss of oxidation, the
slab reheating temperature should preferably be 1,300.degree. C. or
below.
[0138] From the point of view of reducing the slab reheating
temperature and preventing occurrence of a trouble during hot
rolling, use of a so-called sheet bar heater based on heating a
sheet bar is of course an effective method.
[0139] The reheated slab is then hot-rolled. Hot rolling should
preferably be performed at a finish rolling end temperature FDT of
the Ar.sub.3 transformation point or more.
[0140] Finish rolling end temperature: Ar.sub.3 transformation
point or more:
[0141] By adopting a finish rolling end temperature FDT of the
Ar.sub.3 transformation point or more, it is possible to obtain a
uniform structure of the hot-rolled mother sheet, and a composite
ferrite+martensite structure through cooling after hot rolling.
This ensures maintenance of an excellent press-formability. On the
other hand, a finish rolling end temperature of under the Ar.sub.3
transformation point leads to a non-uniform structure of the
hot-rolled mother sheet, and the remaining deformation structure
causes deterioration of press-formability. Furthermore, a finish
rolling end temperature of under the Ar.sub.3 transformation point
results in a higher rolling load during hot rolling, and a higher
risk of occurrence of troubles during hot rolling. The FDT of hot
rolling should therefore preferably be Ar.sub.3 transformation
point or more.
[0142] After the completion of finish rolling, cooling should
preferably be carried out at a cooling rate of 5.degree. C./second
or more to a temperature region from Ar.sub.3 transformation point
to Ar.sub.1 transformation point.
[0143] By cooling the sheet after hot rolling as described above,
it is possible to accelerate ferrite transformation through the
subsequent cooling step. With a cooling rate of under 5.degree.
C./second, ferrite transformation is not promoted in subsequent
cooling, thus leading to deterioration of press-formability.
[0144] Then, it is desirable to air-cool or slowly cool the sheet
for a period from 1 to 20 seconds within a temperature region of
from (Ar.sub.3 transformation point) to (Ar.sub.1 transformation
point). By conducting air cooling or slow cooling within the
temperature region of from (Ar.sub.3 transformation point) to
(Ar.sub.1 transformation point) transformation from austenite to
ferrite is promoted, and furthermore, C is concentrated in
non-transformed austenite, which is transformed into martensite
through subsequent cooling, thus forming a composite
ferrite+martensite structure. An air cooling or slow cooling of
under 1 second within the temperature region of from (Ar.sub.3
transformation point) to (Ar.sub.1 transformation point) leads to
only a slight amount of transformation from austenite into ferrite,
resulting in a slight amount of concentration of C into
non-transformed austenite, and hence in only a small amount of
formation of martensite. On the other hand, a cooling time of over
20 seconds causes transformation of austenite to pearlite, thus
making it impossible to obtain a composite ferrite+martensite
structure.
[0145] After air cooling or slow cooling, the rolled sheet is
cooled again at a cooling rate of 5.degree. C./second or more, and
coiled at a coiling temperature of 550.degree. C. or below.
[0146] By cooling the sheet at a cooling rate of 5.degree.
C./second or more, non-transformed austenite is transformed into
martensite. This converts the structure into a composite
ferrite+martensite structure. When the cooling rate is under
5.degree. C./second or the coiling temperature CT is higher than
550.degree. C., non-transformed austenite is transformed into
pearlite or bainite, and martensite is not formed, thus leading to
a decrease in press-formability. The cooling rate should more
preferably be 10.degree. C./second or more, or still more
preferably, 100.degree. C./second or less from the point of view of
hot-rolled sheet shape. The coiling temperature CT should be under
500.degree. C., and preferably, 350.degree. C. or more from the
point of view of the hot-rolled sheet shape. A coiling temperature
of under 350.degree. C. causes serious disorder of the steel sheet
shape, and an increase in the risk of occurrence of inconveniences
during practical use.
[0147] In hot rolling in the present invention, all or part of
finish rolling may be lubrication rolling to reduce the rolling
load during hot rolling. Application of lubrication rolling is
effective with a view to achieving a uniform steel sheet shape and
a uniform material quality. The frictional coefficient during
lubrication rolling should preferably be within a range of from
0.25 to 0.10. It is desirable to adopt a continuous rolling process
comprising connecting sheet bars in succession and rolling the same
continuously. Application of the continuous rolling process is
desirable also from the point of view of operational stability of
hot rolling.
[0148] After the completion of hot rolling, temper rolling of 10%
or less may be applied for adjustment such as shape correction or
surface roughness control.
[0149] The hot-rolled steel sheet of the invention is applicable
not only for working but also as an mother sheet for surface
treatment. Applicable surface treatments include galvanizing
(including alloying), tin-plating and enameling.
[0150] After annealing or a surface treatment such as galvanizing,
the hot-rolled steel sheet of the invention may be subjected to a
special treatment to improve chemical conversion treatment
property, weldability, press-formability and corrosion
resistance.
[0151] The cold-rolled steel sheet will now be described.
[0152] First, the result of a fundamental experiment carried out by
the present inventors on the cold-rolled steel sheet will be
presented.
[0153] A sheet bar having a chemical composition comprising, in
weight percentage, 0.04% C, 0.02% Si, 1.7% Mn, 0.01% P, 0.005% S,
0.04% Al, 0.002% N and 0.3 or 1.3% Cu was heated to 1,150.degree.
C., soaked and subjected to three-pass rolling into a thickness of
4.0 mm so that the finish rolling end temperature was 900.degree.
C. After the completion of finish rolling and coiling, a
temperature holding equivalent treatment of 600.degree. C..times.1
h was applied. Thereafter, the sheet was cold-rolled at a reduction
of 70% into a cold-rolled steel sheet having a thickness of 1.2 mm.
Then, recrystallization annealing was applied to cold-rolled sheets
under various conditions.
[0154] Tensile properties were investigated by conducting a tensile
test on the resultant cold-rolled steel sheets. Strain age
hardening properties of these cold-rolled steel sheets were
investigated.
[0155] Tensile properties were determined by first sampling test
pieces from these cold-rolled steel sheets, applying a pre-strain
treatment with a tensile prestrain of 5% to these test pieces, then
performing a heat treatment of 50 to 350.degree. C..times.20
minutes, and then conducting a tensile test. The strain age
hardening properties were evaluated in terms of the tensile
strength increment .DELTA.TS from before to after the heat
treatment, as described in the section of hot-rolled steel
sheet.
[0156] FIG. 4 illustrates the effect of the Cu content on the
relationship between .DELTA.TS of the cold-rolled steel sheet and
the recrystallization annealing temperature. The value of .DELTA.TS
was determined by applying a pre-strain treatment with a tensile
prestrain of 5% to test pieces sampled from the resultant
cold-rolled steel sheets, conducting a heat treatment of
250.degree. C..times.20 minutes, and carrying out a tensile
test.
[0157] FIG. 4 suggests that a high strain age hardening property as
represented by a .DELTA.TS of 80 MPa or more is available, in the
case of a Cu content of 1.3 wt. %, by using a recrystallization
annealing temperature of 700.degree. C. or more to convert the
steel sheet structure into a composite ferrite+martensite
structure. On the other hand, in the case of a Cu content of 0.3
wt. %, a high strain age hardening property is unavailable because
.DELTA.TS is under 80 MPa at any redrystallization annealing
temperature. FIG. 4 suggests the possibility to manufacture a
cold-rolled steel sheet having a high strain age hardening property
by optimizing the Cu content and achieving a composite
ferrite+martensite structure.
[0158] FIG. 5 illustrates the effect of the Cu content on the
relationship between .DELTA.TS of the cold-rolled steel sheet and
the heat treatment temperature after a pre-strain treatment. The
steel sheet used was annealed at 800.degree. C. which was the dual
phase region of ferrite (.alpha.)+austenite (.gamma.) for a holding
time of 40 seconds after cold rolling, and cooled from a holding
temperature (800.degree. C.) at a cooling rate of 30.degree.
C./second to room temperature. The steel sheets had a composite
ferrite+martensite (secondary phase) microstructure, with a
martensite structural partial ratio represented by an area ratio of
8%.
[0159] It is known from FIG. 5 that .DELTA.TS increases according
as the heat treatment temperature increases, and the increment
thereof largely depends upon the Cu content. With a Cu content of
1.3 wt. %, a high strain age hardening property as represented by a
.DELTA.TS of 80 MPa or more is available at a heat treatment
temperature of 150.degree. C. or more. For a Cu content of 0.3 wt.
%, .DELTA.TS is under 80 MPa at any heat treatment temperature, and
a high strain age hardening property cannot be obtained.
[0160] For steel sheets as cold-rolled having a Cu content of 0.3
or 1.3 wt. %, materials (steel sheets) were prepared under various
recrystallization annealing conditions, with a composite
ferrite+martensite structure or a single ferrite structure, of
which the yield ratio YR (=(yield strength YS/tensile strength
TS).times.100%) ranged from 50 to 90%. For these materials (steel
sheets) a hole expanding test was carried out to determine the hole
expanding ratio (.lambda.). In the hole expanding test, the hole
expanding ratio .lambda. was determined by forming a punch hole in
a test piece by punching with a punch having a diameter of 10 mm,
expanding the hole until production of cracks running through the
thickness so that burs were produced on the outside by means of a
conical punch having a vertical angle of 60.degree.. The
hole-expanding ratio .lambda. was calculated by a formula:
.lambda.(%)={(d-d.sub.0)/d.sub.0}.times.100, where d.sub.0: initial
hole diameter, and d: inner hole diameter upon occurrence of
cracks.
[0161] These results, arranged in terms of the relationship between
the hole expanding ratio .lambda. and the yield ratio YR, to serve
as the effect of the Cu content on the relationship between the
hole expanding ratio .lambda. and the yield ratio YR of the
cold-rolled steel sheet are illustrated in FIG. 6.
[0162] According to FIG. 6, in a steel sheet having a Cu content of
0.3 wt. %, achievement of a composite ferrite+martensite structure
and a YR of under 70% lead to a decrease in .lambda. along with a
decrease in YR. In a steel sheet having a Cu content of 1.3 wt. %,
a high .lambda.-value is maintained even when a composite
ferrite+martensite structure is achieved and a low YR is kept. On
the other hand, a low YR and a high .lambda. cannot simultaneously
be obtained in the steel sheet having a Cu content of 0.3 wt.
%.
[0163] It is known from FIG. 6 that a cold-rolled steel sheet
satisfying both a low yield ratio and a high hole expanding ratio
can be manufactured by using a Cu content within an appropriate
range and achieving a composite ferrite+martensite structure.
[0164] In the cold-rolled steel sheet of the invention, very fine
Cu precipitates in the steel sheet as a result of a pre-strain with
an amount of strain larger than 2% which is the amount of prestrain
upon measuring the deformation stress increment from before to
after a usual heat treatment, and a heat treatment within a
relatively low temperature region as from 150 to 350.degree. C.
According to a study carried out by the present inventors, a high
strain age hardening property bringing about an increase in yield
stress and a remarkable increase in tensile strength is considered
to have been obtained from this precipitation of very fine Cu. Such
precipitation of very fine Cu by a heat treatment in a
low-temperature region has never been observed in ultra-low carbon
steel or low-carbon steel in reports so far released. The reason of
precipitation of very fine Cu by a heat treatment in a
low-temperature region has not as yet been clarified to date. A
conceivable reason is that, during annealing in the dual phase
region of .alpha.+.gamma. phase, much Cu is distributed in the
.gamma.-phase, and the distributed Cu is kept even after cooling in
an super-saturated solid-solution state (of Cu) in martensite,
which precipitates in a very fine form as a result of imparting of
a prestrain of at least 5% and a low-temperature heat
treatment.
[0165] A detailed mechanism which gives a high hole expanding ratio
of the steel sheet added with Cu and having a composite
ferrite+martensite structure is not clearly known at present, but
it is considered to be due to the fact that addition of Cu reduced
the difference in hardness between ferrite and martensite.
[0166] The cold-rolled steel sheet of the invention is a
high-strength cold-rolled steel sheet having a tensile strength TS
of 440 MPa or more and excellent in press-formability, of which
tensile strength is remarkably increased by a heat treatment at a
relatively low temperature after press forming, and having an
excellent strain age hardening property typically represented by a
.DELTA.TS 80 MPa or more.
[0167] The structure of the cold-rolled steel sheet of the
invention will now be described.
[0168] The cold-rolled steel sheet of the invention has a composite
structure comprising a ferrite phase and a secondary phase
containing a martensite phase of an area ratio of 2% or more.
[0169] For the purpose of achieving a cold-rolled steel sheet
having a low yield strength YS and a high elongation El and
excellent in press-formability, in the invention, it is necessary
to achieve a composite structure comprising a ferrite phase which
is the main phase and a secondary phase containing martensite.
Ferrite, the main phase, should preferably have an area ratio of
50% or more. If ferrite is under 50% in area ratio, it is difficult
to keep a high elongation, leading to a lower press-formability.
When a better elongation is required, the ferrite phase should
preferably have an area ratio of 80% or more. For making use of the
composite structure, the ferrite phase should preferably have an
area ratio of 98% or less.
[0170] In the present invention, martensite as the secondary phase
must be contained in an area ratio of 2% or more. When the area
ratio of martensite is under 2%, a low YS and a high El cannot
simultaneously be satisfied. The secondary phase may be a single
martensite phase having an area ratio of 2% or more, or a mixture
of a martensite phase having an area ratio of 2% or more with any
of the other pearlite phase, bainite phase and retained austenite
phase. There is imposed no particular restriction in this
respect.
[0171] The cold-rolled steel sheet having the structure as
described above has a low yield strength and a high elongation, is
excellent in press-formability, and excellent in strain age
hardening property.
[0172] The reasons of limiting the chemical composition of the
cold-rolled steel sheet of the invention to the aforementioned
ranges will now be described. The weight percentage will simply be
denoted hereafter as %.
[0173] C: 0.15% or less:
[0174] C is an element which improves strength of a steel sheet,
and promotes formation of a composite structure of ferrite and
martensite, and should preferably be contained in an amount of
0.01% or more for forming a composite structure in the invention. A
C content of over 0.15% on the other hand causes an increase in
partial ratio of carbides in steel, resulting in a decrease in
elongation, and hence a decrease in press-formability. A more
important problem is that a C content of over 0.15% leads to a
serious decrease in spot weldability and arc weldability. For these
reasons, in the invention, the C content is limited to 0.15% or
less. From the point of view of formability, the C content should
more preferably be 0.10% or less.
[0175] Si: 2.0% or less:
[0176] Si is a useful strengthening element which can improve
strength of a steel sheet without causing a marked decrease in
elongation of the steel sheet. A Si content of over 2.0% however
leads to deterioration of press-formability and degrades the
surface quality. The Si content is therefore limited to 2.0% or
less, and preferably, to 0.1% or more.
[0177] Mn: 3.0% or less:
[0178] Mn has a function of strengthening steel, reducing the
critical cooling rate for obtaining a composite ferrite+martensite
structure, and accelerating formation of the composite
ferrite+martensite structure. The Mn content should preferably
correspond to the cooling rate after recrystallization annealing.
Mn is an element effective for preventing hot cracking caused by S,
and should therefore be contained in an amount dependent upon the S
content. These effects are particularly remarkable at a Mn content
of 0.5% or more. On the other hand, a Mn content of over 3.0%
results in deterioration of press-formability and weldability. The
Mn content is therefore limited to 3.0% or less, and more
preferably, to 1.0% or more.
[0179] P: 0.10% or less:
[0180] P has a function of strengthening steel, and can be
contained in an amount necessary for a desired strength. An
excessive P content however causes deterioration of
press-formability. The P content is therefore limited to 0.10% or
less. When a further higher press-formability is required, the P
content should preferably be 0.08% or less.
[0181] S: 0.02% or less:
[0182] S is an element which is present as inclusions in steel and
causes deterioration of elongation, formability, and particularly
stretch flanging formability of a steel sheet. It should therefore
be the lowest possible. A S content reduced to up to 0.02% does not
exert much adverse effect. In the invention, therefore, the S
content is limited to 0.02% or less. When an excellent stretch
flanging formability is required, the S content should preferably
be 0.010% or less.
[0183] Al: 0.10% or less:
[0184] Al is an element which is added as a deoxidizing element of
steel, and is useful for improving cleanliness of steel. However,
an Al content of over 0.10% cannot give a further deoxidizing
effect, but causes in contrast deterioration of press-formability.
The Al content is therefore limited to 0.10% or less. The invention
does not exclude a steelmaking process based on a deoxidation by
means of a deoxidizer other than Al. For example, Ti deoxidation or
Si deoxidation may be used, and steel sheets produced by such
deoxidation methods are also included in the scope of the
invention. In this case, addition of Ca or REM to molten steel does
not impair the features of the steel sheet of the invention at all.
It is needless to mention that steel sheets containing Ca or REM
are also included within the scope of the invention.
[0185] N: 0.02% or less:
[0186] N is an element which increases strength of a steel sheet
through solid-solution strengthing or strain age hardening. A N
content of over 0.02% however causes an increase in the content of
nitrides in the steel sheet, which in turn causes a serious
deterioration of elongation, and furthermore, of press-formability.
The N content is therefore limited to 0.02% or less. When further
improvement of press-formability is required, the N content should
suitably be 0.01% or less.
[0187] Cu: from 0.5 to 3.0%:
[0188] Cu is an element which remarkably increase strain age
hardening of a steel sheet (increase in strength after
pre-strain--heat treatment), and is one of the most important
elements in the invention. With a Cu content of under 0.5%, an
increase in tensile strength of over .DELTA.TS: 80 MPa cannot be
obtained even by using different pre-strain--heat treatment
conditions. In the invention, therefore, Cu should be contained in
an amount of 0.5% or more. With a Cu content of over 3.0%, on the
other hand, the effect is saturated so that an effect corresponding
to the content cannot be expected, leading to unfavorable economic
effects. Deterioration of press-formability results, and the
surface quality of the steel sheet is degraded. The Cu content is
therefore limited within a range of from 0.5 to 3.0%. In order to
simultaneously achieve a higher .DELTA.TS and an excellent
press-formability, the Cu content should preferably be within a
range of from 1.0 to 2.5%.
[0189] In the invention, in addition to the chemical composition
containing Cu as described above, it is desirable to contain, in
weight percentage, one or more of the following groups A to C:
[0190] group A: Ni: 2.0% or less;
[0191] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0192] group C: one or more of Nb, Ti and V: 0.2% or less in
total.
[0193] Group A: Ni: 2.0% or less:
[0194] Group A: Ni is an element effective for preventing surface
defects produced on the steel sheet surface upon adding Cu, and can
be contained as required. If contained, the Ni content, depending
upon the Cu content, should preferably be about a half the Cu
content. A Ni content of over 2.0% cannot give a corresponding
effect because of saturation of the effect, leading to economic
disadvantages, and causes deterioration of press-formability. The
Ni content should preferably be limited to 2.0% or less.
[0195] Group B: one or two of Cr and Mo: 2.0% or less in total:
[0196] Group B: As in Mn, both Cr and Mo have a function of
promoting formation of a composite ferrite+martensite structure,
and can be contained as required. If one or two of Cr and Mo are
contained in an amount of over 2.0% in total, there occurs a
decrease in press-formability. It is therefore desirable to limit
the total content of one or two of Cr and Mo forming group B to
2.0% or less.
[0197] Group C: one or more of Nb, Ti and V: 0.2% or less in
total:
[0198] Group C: Nb, Ti and V are carbide-forming elements which
effectively act to increase strength through fine dispersion of
carbides, and can be selected and contained as required. However,
if the total content of one or more of Nb, Ti and V is over 0.2%,
there occurs deterioration of press-formability. The total content
of Nb, Ti and/or V should therefore preferably be limited to 0.2%
or less.
[0199] In the invention, in place of the aforementioned Cu, one or
more selected from the group consisting of from 0.05 to 2.0% Mo,
from 0.05 to 2.0% Cr, and from 0.05 to 2.0% W may be contained in
an amount of 2.0% or less in total, or further one or more selected
from the group consisting of Nb, Ti and V in an amount of 2.0% or
less in total.
[0200] One or more selected from the group consisting of from 0.05
to 2.0% Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, in an
amount of 2.0% or less in total:
[0201] Mo, Cr and W are elements which cause a remarkable increase
in strain age hardening of a steel sheet, are the most important
elements in the invention, and can be selected and contained as
required. Containing one or more of Mo, Cr and W and achievement of
a composite ferrite+martensite structure cause strain-induced fine
precipitation of fine carbides during pre-strain--heat treatment,
thus making it possible to obtain a tensile strength as represented
by a .DELTA.TS of 80 MPa or more. With a content of each of these
elements of under 0.05%, changing of pre-strain--heat treatment
conditions or the steel sheet structure does not give an increase
in tensile strength as represented by a .DELTA.TS of 80 MPa or
more. On the other hand, even if the content of each of these
elements is over 2.0%, an effect corresponding to the content
cannot be expected as a result of saturation of the effect, leading
to economic disadvantages, and this results in deterioration of
press-formability. The contents of Mo, Cr and W are therefore
limited within a range of from 0.05 to 2.0% for Mo, from 0.05 to
2.0% for Cr, and from 0.05 to 2.0% for W. From the point of view of
press-formability, the total content of Mo, Cr and W is limited to
2.0% or less.
[0202] One or more of Nb, Ti and V: 2.0% or less in total:
[0203] Nb, Ti and V are carbide-forming elements, and, when
containing one or more of Mo, Cr and W, can be selected and
contained as required. Containing one or more of Nb, Ti and V, and
achievement of a composite ferrite+martensite structure cause
strain-induced fine precipitation of fine carbides during
pre-strain--heat treatment, thus making it possible to obtain a
tensile strength as represented by a .DELTA.TS of 80 MPa or more.
However, a total content of one or more of Nb, Ti and V of over
2.0% causes deterioration of press-formability. The total content
of Nb, Ti and/or V should therefore preferably be limited to 2.0%
or less.
[0204] Apart from the above-mentioned elements, one or two of 0.1%
or less Ca and 0.1% or less REM may be contained. Ca and REM are
elements contributing to improvement of elongation through shape
control of inclusions. If the Ca content is over 0.1% and the REM
content is over 0.1%, however, there would be a decrease in
cleanliness, and a decrease in elongation.
[0205] From the point of view of forming martensite, one or two of
0.1% or less B and 0.1% or less Zr may be contained.
[0206] The balance-except for the above-mentioned elements
comprises Fe and incidental impurities. Allowable incidental
impurities include 0.01% or less Sb, 0.01% or less Pb, 0.1% or less
Sn, 0.01% or less Zn, and 0.1% or less Co.
[0207] The manufacturing method of the cold-rolled steel sheet of
the invention will now be described.
[0208] The cold-rolled steel sheet of the invention is manufactured
by using, as a material, a steel slab having the chemical
composition within the aforementioned ranges, and sequentially
carrying out a hot rolling step of hot-rolling the steel slab into
a hot-rolled steel sheet, a cold rolling step of cold-rolling the
hot-rolled steel sheet into a cold-rolled steel sheet, and a
recrystallization annealing step of applying recrystallization
annealing to the cold-rolled steel sheet into a cold-rolled
annealed steel sheet.
[0209] While the steel slab used should preferably be manufactured
by the continuous casting process to prevent macro-segregation of
the elements, it may be manufactured by the ingot casting process
or the thin-slab continuous casting process. An energy-saving
process such as direct-hot-charge rolling or direct rolling is
applicable with no problem, which comprises the steps of
manufacturing a steel slab, then once cooling the slab to room
temperature, then reheating the slab as in the conventional art,
and charging the same into a reheating furnace as a hot slab
without cooling, or immediately rolling the slab after slight
holding.
[0210] The above-mentioned material (steel slab) is reheated, and
subjected to the hot rolling step of applying hot rolling to make a
hot-rolled steel sheet. Usual known conditions for the hot rolling
step pose no problem only so far as these conditions permit
manufacture of a hot-rolled steel sheet having a desired thickness.
Preferable hot rolling conditions are as follows:
[0211] Slab reheating temperature: 900.degree. C. or more.
[0212] The slab reheating temperature SRT should preferably be the
lowest possible with a view to preventing surface defects caused by
Cu when the chemical composition contains Cu. However, with a
reheating temperature of under 900.degree. C., there is an increase
in the rolling load, thus increasing the risk of occurrence of a
trouble during hot rolling. Considering the increase in scale loss
caused along with the increase in weight loss of oxidation, the
slab reheating temperature should preferably be 1,300.degree. C. or
less.
[0213] From the point of view of reducing the slab reheating
temperature and preventing occurrence of a trouble during hot
rolling, use of a so-called sheet bar heater based on heating a
sheet bar is of course an effective method.
[0214] Finish rolling end temperature: 700.degree. C. or more:
[0215] By adopting a finish rolling end temperature FDT of
700.degree. C. or more, it is possible to obtain a uniform
hot-rolled mother sheet structure which can give an excellent
formability after cold rolling and recrystallization annealing. On
the other hand, a finish rolling end temperature of under
700.degree. C. results in a non-uniform hot-rolled mother sheet
structure, and a higher rolling load during hot rolling, leading to
an increased risk of occurrence of troubles during hot rolling. For
these reasons, the FDT in the hot rolling step should preferably be
700.degree. C. or more.
[0216] Coiling temperature: 800.degree. C. or below:
[0217] The coiling temperature CT should preferably be 800.degree.
C. or below, and more preferably, 200.degree. C. or more. A coiling
temperature of over 800.degree. C. tends to cause a decrease in
yield as a result of increase of scale causing a scale loss. With a
coiling temperature of under 200.degree. C., the steel sheet shape
is in marked disorder, and there is an increasing risk of
occurrence of inconveniences in practical use.
[0218] In the hot rolling step in the invention, as described
above, it is desirable to reheat the slab to a temperature of
900.degree. C. or more, hot-roll the reheated slab at a finish
rolling end temperature of 700.degree. C. or more, and coil the
hot-rolled steel sheet at a coiling temperature of 800.degree. C.
or below, and preferably 200.degree. C. or more.
[0219] In hot rolling in the present invention, all or part of
finish rolling may be lubrication rolling to reduce the rolling
load during hot rolling. Application of lubrication rolling is
effective with a view to achieving a uniform steel sheet shape and
a uniform material quality. The frictional coefficient during
lubrication rolling should preferably be within a range of from
0.25 to 0.10. It is desirable to adopt a continuous rolling process
comprising connecting sheet bars in succession and rolling the same
continuously. Application of the continuous rolling process is
desirable also from the point of view of operational stability of
hot rolling.
[0220] Then, the cold rolling step is conducted on the hot-rolled
steel sheet. In the cold rolling step, the hot-rolled steel sheet
is cold-rolled into a cold-rolled steel sheet. The cold rolling
conditions suffice to permit production of a cold-rolled steel
sheet having a desired dimensions, and no particular restriction is
imposed. The cold rolling reduction should preferably be 40% or
more. With a reduction of under 40%, it becomes difficult for
recrystallization to take place uniformly during the
recrystallization annealing that follows.
[0221] Then, the cold-rolled steel sheet is subjected to a
recrystallization annealing step to convert the sheet into a
cold-rolled annealed steel sheet. Recrystallization annealing
should preferably be carried out on a continuous annealing line, or
on a continuous hot-dip galvanizing line. The annealing temperature
for recrystallization annealing should preferably be within an
(.alpha.+.gamma.) dual phase region in a temperature range of from
the Ac.sub.1 transformation point to the Ac.sub.3 transformation
point. An annealing temperature of under the Ac.sub.1
transformation point leads to a single ferrite phase. At a high
temperature of over Ac.sub.3 transformation point results in
coarsening of crystal grains, a single austenite phase, and a
serious deterioration of press-formability. By annealing the sheet
in the (.alpha.+.gamma.) dual phase region, it is possible to
obtain a composite ferrite+martensite structure and a high
.DELTA.TS.
[0222] The cooling rate for cooling the sheet during
recrystallization annealing should preferably be 1.degree.
C./second or more with a view to forming martensite.
[0223] After the completion of hot rolling, temper rolling of 10%
or less may be applied for adjustment such as shape correction or
surface roughness control.
[0224] The cold-rolled steel sheet of the invention is applicable
not only for working but also as an mother sheet for surface
treatment. Applicable surface treatments include galvanizing
(including alloying), tin-plating and enameling.
[0225] After annealing or a surface treatment such as galvanizing,
the cold-rolled steel sheet of the invention may be subjected to a
special treatment to improve chemical conversion treatment
property, weldability, press-formability and corrosion
resistance.
[0226] The hot-dip galvanized steel sheet will now be
described.
[0227] First, the result of a fundamental experiment carried out by
the present inventors on the hot-dip galvanized steel sheet will be
presented.
[0228] A sheet bar having a chemical composition comprising, in
weight percentage, 0.04% C, 0.02% Si, 1.7% Mn, 0.01% P, 0.004% S,
0.04% Al, 0.002% N and 0.3 or 1.3% Cu was heated to 1,150.degree.
C., soaked and subjected to three-pass rolling into a thickness of
4.0 mm so that the finish rolling end temperature was 900.degree.
C. After the completion of finish rolling and coiling, a
temperature holding equivalent treatment of 600.degree. C..times.1
h was applied. Thereafter, the sheet was cold-rolled at a reduction
of 70% into a cold-rolled steel sheet having a thickness of 1.2
mm.
[0229] These cold-rolled steel sheets were subjected to
recrystallization annealing under various conditions, then rapidly
cooled to a temperature region of from 450 to 500.degree. C., and
immersed in a hot-dip galvanizing bath (0.13 wt. % Al--Zn bath),
thereby forming a hot-dip galvanizing layer on the surface. Then,
the galvanized steel sheet was reheated to a temperature range of
from 450 to 550.degree. C. to apply an alloying treatment of the
hot-dip galvanizing layer (Fe content in the galvanizing layer:
about 10%).
[0230] For the resultant hot-dip galvanized steel sheet, tensile
properties were investigated through a tensile test. An
investigation was conducted on strain age hardening properties of
these galvanized steel sheets.
[0231] Tensile properties were determined by first sampling test
pieces from these hot-dip galvanized steel sheets, applying a
pre-strain treatment with a tensile prestrain of 5% to these test
pieces, then performing a heat treatment of 50 to 350.degree.
C..times.20 minutes, and then conducting a tensile test. The strain
age hardening properties were evaluated in terms of the tensile
strength increment .DELTA.TS from before to after heat treatment,
as described in the section of hot-rolled steel sheet.
[0232] FIG. 7 illustrates the effect of the Cu content on the
relationship between .DELTA.TS of the hot-dip galvanized steel
sheet and the recrystallization annealing temperature. The value of
.DELTA.TS was determined by applying a pre-strain treatment with a
tensile prestrain of 5% to test pieces sampled from the resultant
hot-dip galvanized steel sheets, conducting a heat treatment of
250.degree. C..times.20 minutes, and carrying out a tensile
test.
[0233] FIG. 7 suggests that a high strain age hardening property as
represented by a .DELTA.TS of 80 MPa or more is available, in the
case of a Cu content of 1.3 wt. %, by using a recrystallization
annealing temperature of 700.degree. C. or more to convert the
steel sheet structure into a composite ferrite+martensite
structure. On the other hand, in the case of a Cu content of 0.3
wt. %, a high strain age hardening property is unavailable because
.DELTA.TS is under 80 MPa at any recrystallization annealing
temperature. FIG. 7 suggests the possibility to manufacture a
hot-dip galvanized steel sheet having a high strain age hardening
property by optimizing the Cu content and achieving a composite
ferrite+martensite structure.
[0234] FIG. 8 illustrates the effect of the Cu content on the
relationship between .DELTA.TS of the hot-dip galvanized steel
sheet and the heat treatment temperature after a pre-strain
treatment. The value of .DELTA.TS was determined on hot-dip
galvanized steel sheets manufactured by applying annealing at
800.degree. C. for a holding time of 40 seconds in the
ferrite+austenite dual phase region as recrystallization annealing
conditions to cold-rolled steel sheet, at various heat treatment
temperatures after pre-strain treatment. The microstructure after
annealing was a composite ferrite+martensite structure having a
martensite area ratio of 7%.
[0235] It is known from FIG. 8 that .DELTA.TS increases according
as the heat treatment temperature increases, and the increment
thereof largely depends upon the Cu content. With a Cu content of
1.3 wt. %, a high strain age hardening property as represented by a
.DELTA.TS of 80 MPa or more is available at a heat treatment
temperature of 150.degree. C. or more. For a Cu content of 0.3 wt.
%, .DELTA.TS is under 80 MPa at any heat treatment temperature, and
a high strain age hardening property cannot be obtained.
[0236] For steel sheets as cold-rolled having a Cu content of 0.3
or 1.3 wt. % recrystallization annealing was performed under
various recrystallization annealing conditions after cold rolling.
The sheets were then rapidly cooled to a temperature region of from
450 to 500.degree. C., then immersed in a hot-dip galvanizing bath
(0.13 wt. % Al--Zn bath) to form a hot-dip galvanizing layer on the
surface thereof, and the structure was converted from
ferrite+martensite to a single ferrite phase. Then, the sheet was
reheated to a temperature range of from 450 to 550.degree. C. to
apply an alloying treatment (Fe content in the galvanizing layer:
about 10%) to the hot-dip galvanizing layer. Materials (steel
sheet) limiting the yield ratio YR (=(yield strength YS/tensile
strength TS).times.100%) within a range of from 50 to 90% were thus
obtained.
[0237] For these materials (steel sheets), a hole expanding test
was carried out to determine the hole expanding ratio (.lambda.).
In the hole expanding test, the hole expanding ratio .lambda. was
determined by forming a punch hole in a test piece by punching with
a punch having a diameter of 10 mm, expanding the hole until
production of cracks running through the thickness so that burs are
produced on the outside by means of a conical punch having a
vertical angle of 60.degree.. The hole expanding ratio .lambda. was
calculated by a formula:
.lambda.(%)={(d-d.sub.0)/d.sub.0}.times.100, where do: initial hole
diameter, and d: inner hole diameter upon occurrence of cracks.
[0238] These results on the hot-dip galvanized steel sheet,
arranged in terms of the relationship between the hole expanding
ratio .lambda. and the yield ratio YR, to serve as the effect of
the Cu content on the relationship between the hole expanding ratio
YR of the cold-rolled steel sheet are illustrated in FIG. 9.
[0239] According to FIG. 9, in a steel sheet having a Cu content of
0.3 wt. %, achievement of a composite ferrite+martensite structure
and a YR of under 70% lead to a decrease in .lambda. along with a
decrease in YR. In a steel sheet having a Cu content of 1.3 wt. %,
a high .lambda.-value is maintained even when a composite
ferrite+martensite structure is achieved and a low YR is kept. On
the other hand, a low YR and a high .lambda. cannot simultaneously
be obtained in the steel sheet having a Cu content of 0.3 wt.
%.
[0240] It is known from FIG. 9 that a hot-dip galvanized steel
sheet satisfying both a low yield ratio and a high hole expanding
ratio can be manufactured by using a Cu content within an
appropriate range and achieving a composite ferrite+martensite
structure.
[0241] In the hot-dip galvanized steel sheet of the invention, very
fine Cu precipitates in the steel sheet as a result of a pre-strain
with an amount of strain larger than 2% which is the amount of
prestrain upon measuring the deformation stress increment from
before to after a usual heat treatment, and a heat treatment within
a relatively low temperature region as from 150 to 350.degree. C.
According to a study carried out by the present inventors, a high
strain age hardening property bringing about an increase in yield
stress and a remarkable increase in tensile strength is considered
to have been obtained from this precipitation of very fine Cu. Such
precipitation of very fine Cu by a heat treatment in a
low-temperature region has never been observed in ultra-low carbon
steel or low-carbon steel in reports so far released. The reason of
precipitation of very fine Cu by a heat treatment in a
low-temperature region has not as yet been clarified to date. A
conceivable reason is that, during annealing in the .alpha.+.beta.
dual phase, much Cu is distributed in the .gamma.-phase, and the
distributed Cu is kept even after cooling in an super-saturated
solid-solution state of Cu in martensite, which precipitates in a
very fine form as a result of imparting of a prestrain of 5% or
more and a low-temperature heat treatment.
[0242] A detailed mechanism which give a high hole expanding ratio
of the steel sheet added with Cu and having a composite
ferrite+martensite structure is not clearly known at present, but
it is considered to be due to the fact that addition of Cu reduced
the difference in hardness between ferrite and martensite.
[0243] On the basis of the novel findings described above, the
present inventors carried out further studies and obtained findings
that the aforementioned phenomenon could take place also in a
hot-dip galvanized steel sheet not containing Cu. According to
these new findings, imparting of a prestrain and application of a
heat treatment at a low temperature causes strain-induced
precipitation of very fine carbides in martensite by adding one or
more of Mo, Cr and W in place of Cu and converting the structure
into a composite ferrite+martensite structure. Strain-induced fine
precipitation upon heating at a low temperature is more remarkable
by further adding one or more of Nb, V and Ti in addition to one or
more of Mo, Cr and W.
[0244] The hot-dip galvanized steel sheet of the invention has a
hot-dip galvanizing layer or an alloying hot-galvanizing layer
formed on the surface thereof, and is a high-strength hot-dip
galvanized steel sheet having a tensile strength TS of 440 MPa or
more, and excellent in press-formability. Tensile strength thereof
remarkably increases through a heat treatment applied at a
relatively low temperature after press-forming to have an excellent
strain age hardening property as represented by a .DELTA.TS of 80
MPa or more. The steel sheet may be a hot-rolled steel sheet or a
cold-rolled steel sheet.
[0245] The structure of the hot-dip galvanized steel sheet of the
invention will now be described.
[0246] The hot-dip galvanized steel sheet of the invention has a
composite structure comprising a ferrite phase and a secondary
phase containing martensite phase having an area ratio of 2% or
more relative to the entire structure.
[0247] In order to obtain a hot-dip galvanized steel sheet having a
low yield strength YS and a high elongation El, and excellent in
press-formability, in the invention, it is necessary to convert the
structure of the hot-dip galvanized steel sheet of the invention
into a composite structure comprising a ferrite phase which is the
main phase and a secondary phase containing martensite. Ferrite
serving as the main phase should preferably have an area ratio of
50% or more. With ferrite of under 50%, it is difficult to keep a
high elongation, resulting in a lower press-formability. When a
satisfactory elongation is required, the area ratio of the ferrite
phase should preferably be 80% or more. For the purpose of making
full use of advantages of the composite structure, the ferrite
phase should preferably be 98% or less.
[0248] In the hot-dip galvanized steel sheet of the invention,
steel must contain martensite as the secondary phase in an area
ratio of 2% or more. An area ratio of martensite of under 2% cannot
simultaneously satisfy a low YS and a high El. The secondary phase
may be a single martensite phase having an area ratio of 2% or
more, or may be a mixture of a martensite phase of an area ratio of
2% or more and a sub phase comprising a pearlite phase, a bainite
phase, or a residual austenite phase.
[0249] The hot-dip galvanized steel sheet having the
above-mentioned structure thus becomes a steel sheet excellent in
press-formability, with a low yield strength and a high elongation,
and in strain age hardening property.
[0250] The reasons of limiting the chemical composition of the
hot-dip galvanized steel sheet of the invention will now be
described. The weight percentage, wt. %, will hereafter be denoted
simply as %.
[0251] C: 0.15% or less:
[0252] C is an element which improves strength of a steel sheet,
and promotes formation of a composite structure of ferrite and
martensite, and should preferably be contained in an amount of
0.01% or more for forming a composite ferrite+martensite structure
in the invention. A C content of over 0.15% on the other hand
causes an increase in partial ratio of carbides in steel, resulting
in a decrease in elongation, and hence a decrease in
press-formability. A more important problem is that a C content of
over 0.15% leads to a serious decrease in spot weldability and arc
weldability. For these reasons, in the invention, the C content is
limited to 0.15% or less. From the point of view of formability,
the C content should more preferably be 0.10% or less.
[0253] Si: 2.0% or less:
[0254] Si is a useful strengthening element which can improve
strength of a steel sheet without causing a marked decrease in
elongation of the steel sheet. A Si content of over 2.0% however
leads to deterioration of press-formability and degrades
platability. The Si content is therefore limited to 2.0% or less,
and preferably, 0.1% or more.
[0255] Mn: 3.0% or less:
[0256] Mn has a function of strengthening steel, reducing the
critical cooling rate for obtaining a composite ferrite +martensite
structure, and of accelerating formation of the composite
ferrite+martensite structure. Mn is an element effective for
preventing hot cracking caused by S, and should therefore be
contained in an amount dependent upon the S content. These effects
are particularly remarkable at an Mn content of 0.5% or more. On
the other hand, an Mn content of over 3.0% results in deterioration
of press-formability and weldability. The Mn content is therefore
limited to 3.0% or less, and more preferably, to 1.0% or more.
[0257] P: 0.10% or less:
[0258] P has a function of strengthening steel, and can be
contained in an amount necessary for a desired strength. An
excessive P content however causes deterioration of
press-formability. The P content is therefore limited to 0.10% or
less. When a further higher press-formability is required, the P
content should preferably be 0.08% or less.
[0259] S: 0.02% or less:
[0260] S is an element which is present as inclusions in steel and
causes deterioration of elongation, formability, and particularly
stretch flanging formability of a steel sheet. It should therefore
be the lowest possible. A S content reduced to 0.02% or less does
not exert much adverse effect. In the invention, therefore, the S
content is limited to 0.02% or less. When an excellent stretch
flanging formability is required, the S content should preferably
be 0.010% or less.
[0261] Al: 0.10% or less:
[0262] Al is an element which is added as a deoxidizing element of
steel, and is useful for improving cleanliness of steel. However,
an Al content of over 0.10% cannot give a further deoxidizing
effect, but causes in contrast deterioration of press-formability.
The Al content is therefore limited to 0.10% or less. The invention
does not exclude a steelmaking process based on a deoxidation by
means of a deoxidizer other than Al. For example, Ti deoxidation or
Si deoxidation may be used, and steel sheets produced by such
deoxidation methods are also included in the scope of the
invention.
[0263] N: 0.02% or less:
[0264] N is an element which increases strength of a steel sheet
through solid-solution strengthing or strain age hardening. A N
content of over 0.02% however causes an increase in the content of
nitrides in the steel sheet, which in turn causes a serious
deterioration of elongation, and furthermore, of press-formability.
The N content is therefore limited to 0.02% or less. When further
improvement of press-formability is required, the N content should
suitably be 0.01% or less, and preferably 0.0005% or more.
[0265] Cu: from 0.5 to 3.0%:
[0266] Cu is an element which remarkably increases strain age
hardening of the hot-dip galvanized steel sheet of the invention
(increase in strength after pre-strain--heat treatment), and is one
of the most important elements in the invention. With a Cu content
of under. 0.5%, an increase in tensile strength of over .DELTA.TS:
80 MPa cannot be obtained even by using different
pre-determination--heat treatment conditions. In the invention,
therefore, Cu should be contained in an amount of 0.5% or more.
With a Cu content of over 3.0%, on the other hand, the effect is
saturated so that an effect corresponding to the content cannot be
expected, leading to unfavorable economic effects. Deterioration of
press-formability results, and the surface quality of the steel
sheet is degraded. The Cu content is therefore limited within a
range of from 0.5 to 3.0%. In order to simultaneously achieve a
higher .DELTA.TS and an excellent press-formability, the Cu content
should preferably be within a range of from 1.0 to 2.5%.
[0267] In the hot-dip galvanized steel sheet of the invention, in
addition to the chemical composition containing Cu as described
above, it is desirable to contain one or more of the following
groups A to C:
[0268] group A: Ni: 2.0% or less;
[0269] group B: one or two of Cr and Mo: 2.0% or less in total;
and
[0270] group C: one or more of Nb, Ti and V: 0.2% or less in
total.
[0271] Group A: Ni: 2.0% or less:
[0272] Group A: Ni is an element effective for preventing surface
defects produced on the steel sheet surface upon adding Cu, and can
be contained as required. If contained, the Ni content, depending
upon the Cu content, should preferably be about a half the Cu
content. A Ni content of over 2.0% cannot give a corresponding
effect because of saturation of the effect, leading to economic
disadvantages, and causes deterioration of press-formability. The
Ni content should preferably be limited to 2.0% or less.
[0273] Group B: one or two of Cr and Mo: 2.0% or less in total:
[0274] Group B: As in Mn, both Cr and Mo have a function of
reducing the critical cooling rate for obtaining a composite
ferrite+martensite structure and promoting formation of a composite
ferrite+martensite structure, and can be contained as required. If
one or two of Cr and Mo are contained in an amount of over 2.0% in
total, there occurs a decrease in press-formability. It is
therefore desirable to limit the total content of one or two of Cr
and Mo forming group B to 2.0% or less.
[0275] Group C: one or more of Nb, Ti and V: 0.2% or less in
total:
[0276] Group C: Nb, Ti and v are carbide-forming elements which
effectively act to increase strength through fine dispersion of
carbides, and can be selected and contained as required. However,
if the total content of one or more of Nb, Ti and V is over 0.2%,
there occurs deterioration of press-formability. The total content
of Nb, Ti and/or V should therefore preferably be limited to 0.2%
or less.
[0277] In the hot-dip galvanized steel sheet of the invention, in
place of the aforementioned Cu, one or more selected from the group
consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr. and from
0.05 to 2.0% W may be contained in an amount of 2.0% or less in
total, or further one or more selected from the group consisting of
Nb, Ti and V in an amount of 2.0% or less in total.
[0278] One or more selected from the group consisting of from 0.05
to 2.0% Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, in an
amount of 2.0% or less in total:
[0279] Mo, Cr and W are elements which cause a remarkable increase
in strain age hardening of a steel sheet, are the most important
elements in the invention, and can be selected and contained as
required. Containing one or more of Mo, Cr and W, and achievement
of a composite ferrite+martensite structure cause strain-induced
fine precipitation of fine carbides during pre-strain--heat
treatment, thus making it possible to obtain a tensile strength as
represented by a .DELTA.TS of 80 MPa or more. With a content of
each of these elements of under 0.05%, changing of pre-strain--heat
treatment conditions or the steel sheet structure does not give an
increase in tensile strength represented by a .DELTA.TS of 80 MPa
or more. On the other hand, even if the content of each of these
elements is over 2.0%, an effect corresponding to the content
cannot be expected as a result of saturation of the effect, leading
to economic disadvantages, and this results in deterioration of
press-formability. The contents of Mo, Cr and W are therefore
limited within a range of from 0.05 to 2.0% for Mo, from 0.05 to
2.0% for Cr, and from 0.05 to 2.0% for W. From the point of view of
press-formability, the total content of Mo, Cr and W is limited to
2.0% or less One or more of Nb, Ti and V: 2.0% or less in
total:
[0280] Nb, Ti and V are carbide-forming elements, and, when
containing one or more of Mo, Cr and W, can be selected and
contained as required. Containing one or more of Nb, Ti and V, and
achievement of a composite ferrite+martensite structure cause
strain-induced fine precipitation of fine carbides during
pre-strain--heat treatment, thus making it possible to obtain a
tensile strength as represented by a .DELTA.TS of 80 MPa or more.
However, a total content of one or more of Nb, Ti and V of over
2.0% causes deterioration of press-formability. The total content
of Nb, Ti and/or V should therefore preferably be limited to 2.0%
or less.
[0281] Apart from the above-mentioned elements, one or two of 0.1%
or less Ca and 0.1% or less REM may be contained. Ca and REM are
elements contributing to improvement of elongation through shape
control of inclusions. If the Ca content is over 0.1% and the REM
content is over 0.1%, however, there would be a decrease in
cleanliness, and a decrease in elongation.
[0282] From the point of view of forming martensite, one or two of
0.1% or less B and 0.1% or less Zr may be contained.
[0283] The balance except for the above-mentioned elements
comprises Fe and incidental impurities. Allowable incidental
impurities include 0.01% or less Sb, 0.01% or less Pb, 0.1% or less
Sn, 0.01% or less Zn, and 0.1% or less Co.
[0284] The manufacturing method of the hot-dip galvanized steel
sheet of the invention will now be described.
[0285] The hot-dip galvanized steel sheet of the invention is
manufactured by annealing the steel sheet having the aforementioned
chemical composition through heating to ferrite+austenite dual
phase region within a temperature region of from Ac.sub.3
transformation point to Ac.sub.1 transformation point on a line for
continuous hot-dip galvanizing, and applying a hot-dip galvanizing
treatment, thereby forming a hot-dip galvanizing layer on the
surface of the steel sheet.
[0286] A hot-rolled steel sheet or a cold-rolled steel sheet may be
used.
[0287] A preferable manufacturing method of the steel sheet used
will be described. It is needless to mention that the manufacturing
method of the hot-dip galvanized steel sheet of the invention is
not limited to the described one.
[0288] First, the manufacturing method suitable for the hot-rolled
steel sheet used as a galvanizing substrate will be described.
[0289] The material used (steel slab) should preferably be prepared
by making molten steel having the aforementioned chemical
composition by a conventionally known process, and for preventing
macro-segregation of the elements, a steel slab should preferably
be manufactured by the continuous casting process. The ingot making
process or the thin-slab continuous casting process is applicable.
Apart from the conventional process comprising the steps of
manufacturing a steel slab, the cooling the steel slab once to room
temperature, and the reheating the slab, an energy-saving process
of charging the hot steel slab into a reheating furnace without
cooling the same, or after a slight temperature holding,
immediately rolling as in direct-hot-charge rolling or direct
rolling is applicable with no problem.
[0290] The above-mentioned material (steel slab) is reheated, and
rolled into a hot-rolled sheet through application of the hot
rolling step. No particular problem is encountered as to
conventionally known conditions so far as such conditions permit
manufacture of a hot-rolled steel sheet having a desired thickness
in the hot rolling step. Preferable conditions for hot rolling are
as follows:
[0291] Slab reheating temperature: 900.degree. C. or more
[0292] With a reheating temperature of under 900.degree. C., there
is an increase in the rolling load, thus increasing the risk of
occurrence of troubles during hot rolling. When Cu is contained,
the slab reheating temperature should preferably be the lowest
possible to prevent surface defects caused by Cu. Considering the
increase in scale loss caused along with the increase in weight
loss of oxidation, the slab reheating temperature should preferably
be 1,300.degree. C. or below.
[0293] From the point of view of reducing the slab reheating
temperature and preventing occurrence of troubles during hot
rolling, use of a so-called sheet bar heater based on heating a
sheet bar is of course an effective method.
[0294] Finish rolling end temperature: 700.degree. C. or more:
[0295] By adopting a finish rolling end temperature FDT of
700.degree. C. or more, it is possible to obtain a uniform
structure of the hot-rolled mother sheet. On the other hand, a
finish rolling end temperature of under 700.degree. C. leads to a
non-uniform structure of the hot-rolled mother sheet and a higher
rolling load during hot rolling, thus increasing the risk of
occurrence of troubles during hot rolling. The FDT for the hot
rolling step should therefore preferably be 700.degree. C. or
more.
[0296] Coiling temperature: 800.degree. C. or below:
[0297] The coiling temperature CT should preferably be 800.degree.
C. or below, and more preferably, 200.degree. C. or more. A coiling
temperature of over 800.degree. C. tends to cause a decrease in
yield as a result of scale loss due to an increase of scale. With a
coiling temperature of under 200.degree. C., the steel sheet shape
is seriously disturbed, and there is an increasing risk of
occurrence of inconveniences in practical use.
[0298] The hot-rolled steel sheet suitably applicable in the
invention should preferably be prepared by reheating the slab
having the aforementioned chemical composition to 900.degree. C. or
more, subjecting the same to hot rolling so that the finish rolling
end temperature becomes 700.degree. C. or more and coiling the same
at a coiling temperature of 800.degree. C. or more, and preferably,
200.degree. C. or more.
[0299] In the hot rolling step, all or part of finish rolling may
comprise lubrication rolling to reduce the rolling load during hot
rolling. Application of lubrication rolling is effective also from
the point of view of achieving a uniform steel sheet shape and a
uniform material quality. The frictional coefficient upon
lubrication rolling should preferably be within a range of from
0.25 to 0.10. It is desirable to convert neighboring sheet bars to
form a continuous rolling process for continuously carrying out
finish rolling. Application of the continuous rolling process is
desirable also from the point of view of operational stability of
hot rolling.
[0300] The hot-rolled sheet with scale adhering thereto may be
subjected to hot-rolled sheet annealing to form an internal oxide
film in the surface layer of the steel sheet. Formation of the
internal oxide layer improves hot-dip galvanizing property for
preventing surface concentration of Si, Mn and P.
[0301] The hot-rolled sheet manufactured by the above-mentioned
method may be used as an mother sheet for plating, and moreover,
the cold-rolled sheet manufactured by applying cold rolling step to
the above-mentioned hot-rolled sheet.
[0302] In the cold rolling step, cold rolling is applied to the
hot-rolled sheet. Any cold rolling conditions may be used so far as
such conditions permit production of cold-rolled steel sheets of
desired dimensions and shape, and no particular restriction is
imposed. The reduction in cold rolling should preferably be 40% or
more. A reduction of under 40% makes it difficult for
recrystallization to take place uniformly during annealing, the
next step.
[0303] In the present invention, the above-mentioned hot-rolled or
cold-rolled (steel) sheet should preferably be subjected to
annealing of heating the sheet to a ferrite (.alpha.)+austenite
(.gamma.) dual-phase region within a temperature range of from
Ac.sub.1 transformation point to Ac.sub.3 transformation point on a
continuous hot-dip galvanizing line.
[0304] A heating temperature of under Ac.sub.1 transformation point
leads to a ferrite single-phase structure. A heating temperature of
over Ac.sub.3 transformation point results in coarsening of crystal
grains and in an austenite single-phase structure, causing serious
deterioration of press-formability. Annealing in the
(.alpha.+.gamma.) dual-phase region makes it possible to obtain a
composite ferrite+martensite structure and a high .DELTA.TS.
[0305] In order to obtain a composite ferrite+martensite structure,
cooling should preferably be carried out from the dual-phase region
heating temperature to the hot-dip galvanizing treatment
temperature at a cooling rate of 5.degree. C./second or more. With
a cooling rate of under 5.degree. C./second, it becomes difficult
for martensite transformation to take place and to achieve a
composite ferrite+martensite structure.
[0306] The hot-dip galvanizing treatment may be carried out under
treatment conditions (galvanizing bath temperature: 450 to
500.degree. C.) commonly used in a usual continuous hot-dip
galvanizing line, and it is not necessary to impose a particular
restriction. Because galvanizing at an excessively high temperature
leads to a poor platability, galvanizing should preferably be
conducted at a temperature of 500.degree. C. or below. Galvanizing
at a temperature of under 450.degree. C. poses a problem of
deterioration of platability.
[0307] With a view to forming martensite, the cooling rate from the
hot-dip galvanizing temperature to 300.degree. C. should preferably
be 5.degree. C./second or more.
[0308] For the purpose of adjusting the galvanizing weight as
required after galvanizing, wiping may be performed.
[0309] After hot-dip galvanizing, an alloying treatment of the
hot-dip galvanizing layer may be applied. The alloying treatment of
the hot-dip galvanizing layer should preferably be carried out by
reheating the sheet to a temperature region of from 460 to
560.degree. C. after the hot-dip galvanizing treatment. An alloying
treatment at a temperature of over 560.degree. C. causes
deterioration of platability. On the other hand, an alloying
treatment at a temperature of under 460.degree. C. causes a slower
progress of alloying, hence deterioration of productivity.
[0310] In the manufacturing method of the hot-dip galvanized steel
sheet of the invention, application of a preheating treatment for
heating the sheet to a temperature of 700.degree. C. or more on the
continuous annealing line, and then, a pretreatment step of
pickling for removing a concentrated layer of the elements in steel
formed during the preheating treatment is desirable for improving
platability.
[0311] On the surface of the steel sheet preheated on the
continuous annealing line, P in steel is concentrated, and oxides
of Si, Mn and Cr are concentrated, forming a surface concentration
layer. It is favorable for improving platability to remove this
surface concentration layer through pickling and to conduct
annealing in a reducing atmosphere subsequently on the continuous
hot-dip galvanizing line. With a preheating treatment temperature
of under 700.degree. C., formation of a surface concentration layer
is not promoted, and improvement of platability is not accelerated.
At preheating temperature of 1,000.degree. C. or below is desirable
from the point of view of press-formability.
[0312] After the hot-dip galvanizing or the alloying treatment,
temper rolling of 10% or less may be applied for adjustments such
as shape correction and surface roughness adjustment.
[0313] To the steel sheet of the invention, a special treatment may
be applied after the hot-dip galvanizing, for improving chemical
conversion treatment property, weldability, press-formability and
corrosion resistance.
EXAMPLES
Example 1
[0314] Molten steel having the chemical composition as shown in
Table 1 was made in a converter, and cast into steel slabs by the
continuous casting process. These steel slabs were heated, and
hot-rolled under the conditions shown in Table 2 into hot-rolled
steel strips having a thickness of 2.0 mm (hot-rolled steel
sheets), followed by temper rolling of 1.0%. Steel sheet No. 2 was
rolled by lubrication rolling on latter four stands of finish
rolling.
[0315] For the thus obtained hot-rolled steel strips (hot-rolled
steel sheets), the microstructure, tensile properties, strain age
hardening property and hole expanding ratio were determined.
Press-formability was evaluated in terms of elongation El and yield
strength.
[0316] (1) Microstructure
[0317] Test pieces were sampled from the resultant steel strips,
and for the cross-section (section C) perpendicular to the rolling
direction, microstructure was shot by means of an optical
microscope or a scanning type electron microscope, and the
structural partial ratio of ferrite, the main phase, and the kind
and structural partial ratio of the secondary phase were determined
by use of an image analyzer.
[0318] (2) Tensile Properties
[0319] JIS #5 tensile test pieces were sampled from the resultant
steel strips (hot-rolled sheets), and a tensile test was carried
out in accordance with JIS Z2241 to determine yield strength YS,
tensile strength TS, elongation El and yield ratio YR.
[0320] (3) Strain Age Hardening Property
[0321] JIS #5 tensile test pieces were sampled in the rolling
direction from the resultant steel strips (hot-rolled steel
sheets). A plastic deformation of 5% was applied as a pre-strain
(tensile prestrain), and then, after conducting a heat treatment of
250.degree. C..times.20 min., a tensile test was carried out to
determine tensile properties (yield stress YS.sub.HT, and tensile
strength TS.sub.HT) and to calculate .DELTA.YS=YS.sub.HT-YS, and
.DELTA.TS=TS.sub.HT-TS. YS.sub.HT and TS.sub.HT are yield stress
and tensile strength after the pre-strain -heat treatment, and YS
and TS are yield stress and tensile strength of the steel strips
(hot-rolled steel sheets).
[0322] (4) Hole Expanding Ratio
[0323] A hole was formed by punching a test piece sampled from the
resultant steel strip (hot-rolled sheet) by means of a punch having
a diameter of 10 mm. Then, The hole was expanded until occurrence
of cracks running through the thickness by use of a conical punch
having a vertical angle of 60.degree. so that burrs were produced
on the outside, thereby determining the hole expanding ratio
.lambda.. The hole expanding ratio X was calculated by a formula:
.lambda.(%)={(d-d.sub.0)/d.sub.0}.times.100, where, d.sub.0:
initial hole diameter, and d: inner hole diameter upon occurrence
of cracks.
[0324] These results are shown in Table 3.
1TABLE 1 TRANSFORMATION STEEL CHEMICAL COMPOSITION (wt. %) POINT
(.degree. C.) NO. C Si Mn P S Al N Cu Ni Cr Mo Nb Ti V A.sub.c3
A.sub.c1 A 0.035 0.76 1.72 0.01 0.004 0.035 0.002 1.72 -- -- -- --
-- -- 840 704 B 0.038 0.52 1.58 0.01 0.001 0.032 0.002 1.44 0.62 --
0.31 -- -- -- 843 712 C 0.042 0.88 1.48 0.01 0.005 0.028 0.002 1.21
0.53 0.52 -- -- -- -- 841 713 D 0.039 1.05 1.61 0.01 0.005 0.033
0.002 1.38 0.42 -- -- 0.01 0.01 0.01 842 706 E 0.036 0.88 1.82 0.01
0.006 0.033 0.002 0.15 -- -- -- -- -- -- 830 705 F 0.036 0.62 1.75
0.01 0.004 0.032 0.002 0.72 -- -- -- -- -- -- 840 706 G 0.039 0.71
1.66 0.01 0.003 0.033 0.002 0.95 -- -- -- -- -- -- 843 705
[0325]
2 TABLE 2 HOT ROLLING - COOLING AFTER ROLLING FINISH AIR SLAB
ROLLING COOLING COOLING/SLOW COOLING REHEATING END RATE COOLING
RATE COILING STEEL TEMP. TEMP. FROM A.sub.r3 BETWEEN A.sub.r3
BEFORE TEMP. SHEET STEEL SRT FDT TO A.sub.r1 AND A.sub.r1 COILING
CT NO. NO. .degree. C. .degree. C. .degree. C./s s .degree. C.
.degree. C. 1 A 1150 850 30 5 30 450 2 B 1150 850 30 5 30 450 3 B
1150 850 10 0 20 600 4 B 1150 700 10 0 10 450 5 C 1150 850 30 5 30
450 6 D 1150 850 30 5 30 450 7 E 1150 850 30 5 30 450 8 F 1150 850
30 5 30 450 9 G 1150 850 30 5 30 450
[0326]
3 TABLE 3 STRAIN HOLE PROPERTIES AGE EXPAN- MICROSTRUCTURE AFTER
PRE- HARD- SION SECONDARY PHASE HOT-ROLLED SHEET STRAIN - ENING
HOLE FERRITE MAR- PROPERTIES HEAT PROPER- EXPAND- STEEL AREA TEN-
AREA TENSILE PROPERTIES TREATMENT TIES ING SHEET STEEL RATIO SITE
RATIO YS TS E1 YR YS.sub.HT TS.sub.HT .DELTA.YS .DELTA.TS RATIO
.lambda. NO. NO. % KIND % % (MPa) (MPa) (%) % MPa MPa MPa MPa %
REMARKS 1 A 93 M 7 7 350 630 31 56 700 780 350 150 145 EXAMPLE 2 B
90 M 10 10 365 660 29 55 740 820 375 160 140 EXAMPLE 3 B 80 P 0 20
670 730 13 92 720 760 50 30 70 COMPARA- TIVE EXAMPLE 4 B 100 -- 0 0
470 670 12 70 580 695 110 25 60 COMPARA- TIVE EXAMPLE 5 C 92 M 8 8
355 650 30 55 720 800 365 150 140 EXAMPLE 6 D 91 M 9 9 365 670 29
54 730 815 365 145 135 EXAMPLE 7 E 92 M 8 8 300 530 36 57 480 550
180 20 60 COMPARA- TIVE EXAMPLE 8 F 90 M 10 10 335 610 32 55 660
740 325 130 140 EXAMPLE 9 G 92 M 8 8 340 620 31 55 680 755 340 135
135 EXAMPLE M: MARTENSITE; P: PEARLITE; B: BAINITE
[0327] All Examples of the invention showed a low yield strength
YS, a high elongation El, a low yield ratio YR, and a high hole
expanding ratio .lambda., suggesting that these hot-rolled steel
sheets have an excellent press-formability including stretch
flanging formability, and showed high .DELTA.YS, and a very large
.DELTA.TS, suggesting to have an excellent strain age hardening
property. Comparative Examples outside the scope of the invention,
in contrast, suggest that the samples are hot-rolled steel sheets
having decreased press-formability and strain age hardening
property as having a high yield strength YS, a low elongation El, a
small hole expanding ratio .lambda., or a low .DELTA.TS.
Example 2
[0328] Molten steel having the chemical composition as shown in
Table 4 was made in a converter and cast into steel slabs by the
continuous casting process. These steel slabs were reheated, and
hot-rolled under conditions shown in Table 5 into hot-rolled steel
strips (hot-rolled sheets) having a thickness of 2.0 mm, followed
by temper rolling of a reduction of 1.0%.
[0329] For the resultant hot-rolled steel strips (hot-rolled steel
sheets), microstructure, tensile properties, strain age hardening
property and hole expanding ratio were determined as in Example
1.
[0330] The results are shown in Table 6.
4TABLE 4 TRANSFORMATION STEEL CHEMICAL COMPOSITION (wt. %) POINT
(.degree. C.) NO. C Si Mn P S Al N Cr Mo W Nb Ti V A.sub.c3
A.sub.c1 H 0.056 0.29 1.52 0.01 0.004 0.033 0.002 0.13 0.45 -- --
-- -- 820 705 I 0.058 0.68 1.58 0.01 0.003 0.032 0.002 -- 0.31 --
0.04 -- 0.05 830 715 J 0.053 0.58 1.48 0.01 0.005 0.029 0.002 --
0.45 -- 0.04 0.03 -- 835 710 K 0.049 0.72 1.88 0.01 0.001 0.033
0.002 -- -- 0.52 -- -- -- 825 710 L 0.051 1.02 1.62 0.01 0.004
0.031 0.002 -- 0.35 -- -- 0.04 -- 820 705 M 0.052 0.88 1.55 0.01
0.003 0.031 0.002 0.48 -- -- 0.05 -- -- 835 705 N 0.055 0.62 1.88
0.01 0.004 0.029 0.002 -- -- -- -- -- -- 835 705 P 0.053 0.59 1.66
0.01 0.003 0.029 0.002 0.48 -- -- -- -- -- 830 710 Q 0.052 0.62
1.78 0.01 0.004 0.038 0.002 -- 0.58 -- -- -- -- 825 705 R 0.055
0.61 1.62 0.01 0.003 0.033 0.002 0.19 -- 0.28 -- -- -- 815 715 S
0.054 0.58 1.82 0.01 0.004 0.036 0.002 0.33 0.22 0.15 0.04 0.02
0.05 820 720
[0331]
5 TABLE 5 HOT ROLLING - COOLING AFTER ROLLING FINISH AIR SLAB
ROLLING COOLING COOLING/SLOW COOLING REHEATING END RATE COOLING
RATE COILING STEEL TEMP. TEMP. FROM A.sub.r3 BETWEEN A.sub.r3
BEFORE TEMP. SHEET STEEL SRT FDT TO A.sub.r1 AND A.sub.r1 COILING
CT NO. NO. .degree. C. .degree. C. .degree. C./s S .degree. C.
.degree. C. 10 H 1150 850 30 5 30 450 11 I 1150 850 30 5 30 450 12
I 1150 850 10 0 20 600 13 I 1150 850 10 0 10 450 14 J 1150 850 30 5
30 450 15 K 1150 850 30 5 30 450 16 L 1150 850 30 5 30 450 17 M
1150 850 30 5 30 450 18 N 1150 850 30 5 30 450 19 P 1150 850 30 5
30 450 20 Q 1150 850 30 5 30 450 21 R 1150 850 30 5 30 450 22 S
1150 850 30 5 30 450
[0332]
6 TABLE 6 STRAIN HOLE PROPERTIES AGE EXPAN- MICROSTRUCTURE AFTER
PRE- HARD- SION SECONDARY PHASE HOT-ROLLED SHEET STRAIN - ENING
HOLE FERRITE MAR- PROPERTIES HEAT PROPER- EXPAND- STEEL AREA TEN-
AREA TENSILE PROPERTIES TREATMENT TIES ING SHEET STEEL RATIO SITE
RATIO YS TS E1 YR YS.sub.HT TS.sub.HT .DELTA.YS .DELTA.TS RATIO
.lambda. NO. NO. % KIND % % (MPa) (MPa) (%) % MPa MPa MPa MPa %
REMARKS 10 H 92 M 8 8 345 620 31 56 690 770 345 150 125 EXAMPLE 11
I 90 M 10 10 360 650 30 55 730 810 370 160 145 EXAMPLE 12 I 78 P 0
22 670 720 12 93 730 740 60 20 60 COMPARA- TIVE EXAMPLE 13 I 100 --
0 0 465 660 11 70 660 675 195 15 70 COMPARA- TIVE EXAMPLE 14 J 91 M
9 9 350 640 30 55 710 790 360 150 140 EXAMPLE 15 K 91 M 9 9 360 660
30 55 725 805 365 145 125 EXAMPLE 16 L 93 M 7 7 300 520 37 58 630
650 330 130 140 EXAMPLE 17 M 90 M 10 10 330 600 33 55 660 730 330
130 140 EXAMPLE 18 N 92 M 8 8 335 610 32 55 550 640 215 30 70
COMPARA- TIVE EXAMPLE 19 P 93 M 7 7 325 590 33 55 650 730 325 130
125 EXAMPLE 20 Q 92 M 8 8 330 600 33 55 660 735 330 135 130 EXAMPLE
21 R 94 M 6 6 345 620 31 56 680 765 335 145 125 EXAMPLE 22 S 93 M 7
7 360 660 30 55 720 800 360 140 150 EXAMPLE M: MARTENSITE; P:
PEARLITE; B: BAINITE
[0333] All Examples of the invention showed a low yield strength
YS, a high elongation El, a low yield ratio YR, and a high hole
expanding ratio .lambda., suggesting that these hot-rolled steel
sheets have an excellent press-formability including stretch
flanging formability, and showed a high .DELTA.YS and a very large
:.DELTA.TS, suggesting to have an excellent strain age hardening
property. Comparative Examples outside the scope of the invention,
in contrast, suggest that the samples are hot-rolled steel sheets
having decreased press-formability and strain age hardening
property as having a high yield strength YS, a low elongation El, a
small hole-expanding ratio .lambda. or a low .DELTA.TS.
Example 3
[0334] Molten steel having the chemical composition as shown in
Table 7 was made in a converter and cast into steel slabs by the
continuous casting process. These steel slabs were reheated to
1,150.degree. C. as shown in Table 8, and then hot-rolled in a hot
rolling step with a finish rolling end temperature of 900.degree.
C. and a coiling temperature of 600.degree. C. into hot-rolled
steel strips (hot-rolled steel sheets) having a thickness of 4.0
mm. The steel sheet No. 2-2 was lubrication-rolled through the
latter four stands of finish rolling. Then, these hot-rolled steel
strips (hot-rolled sheets) were subjected to a cold rolling step
for cold pickling and cold rolling into cold-rolled steel strips
(cold-rolled sheets) having a thickness of 1.2 mm. Then,
recrystallization annealing was applied to these cold-rolled steel
strips (cold-rolled sheet) on a continuous annealing line, at an
annealing temperature shown in Table 8. The resultant steel strips
(cold-rolled annealed sheets) were subjected to temper rolling at
an elongation of 0.8%.
[0335] Test pieces were sampled from the resultant steel strips,
and microstructure, tensile properties, strain age hardening
property and hole expanding property were investigated as in
Example 1. Press-formability was evaluated in terms of elongation
El, yield strength and hole expanding ratio.
[0336] The results are shown in Table 9.
7TABLE 7 TRANSFORMATION STEEL CHEMICAL COMPOSITION (wt. %) POINT
(.degree. C.) NO. C Si Mn P S Al N Cu Ni Cr Mo Nb Ti V A.sub.c1
A.sub.c3 2A 0.035 0.02 1.72 0.01 0.004 0.035 0.002 1.52 -- -- -- --
-- -- 705 850 2B 0.038 0.02 1.58 0.01 0.001 0.032 0.002 1.44 0.62
-- 0.11 -- -- -- 710 850 2C 0.042 0.03 1.48 0.01 0.005 0.028 0.002
1.21 0.53 0.12 -- -- -- -- 710 855 2D 0.039 0.02 1.61 0.01 0.005
0.033 0.002 1.38 0.42 -- -- 0.01 0.01 0.01 705 845 2E 0.036 0.02
1.82 0.01 0.006 0.033 0.002 0.25 -- -- -- -- -- -- 705 835 2F 0.032
0.02 1.72 0.01 0.003 0.031 0.002 0.72 -- -- -- -- -- -- 705 855 2G
0.033 0.02 1.65 0.01 0.004 0.032 0.002 0.95 -- -- -- -- -- -- 706
850
[0337]
8 TABLE 8 HOT ROLLING STEP FINISH ROLLING COLD ROLLING SLAB END
COILING STEP RECRYSTALLIZATION STEEL REHEATING TEMP. TEMP. COLD
ROLLING ANNEALING SHEET STEEL TEMP. FDT CT REDUCTION ANNEALING
TEMP. NO. NO. (.degree. C.) .degree. C. .degree. C. % (.degree. C.)
2-1 2A 1150 900 600 70 800 2-2 2B 800 2-3 2B 980 2-4 2B 680 2-5 2C
800 2-6 2D 800 2-7 2E 800 2-8 2F 1150 900 600 70 800 2-9 2G 1150
900 600 70 800
[0338]
9 TABLE 9 MICROSTRUCTURE COLD-ROLLED SHEET FERRITE SECONDARY PHASE
PROPERTIES STEEL AREA MARTENSITE AREA TENSILE PROPERTIES SHEET
STEEL RATIO AREA RATIO RATIO YS TS E1 YR NO. NO. % KIND % % (MPa)
(MPa) (%) % 2-1 2A 93 M 7 7 345 620 31 56 2-2 2B 90 M 10 10 355 650
29 55 2-3 2B 0 P,B,M 7 100 670 720 11 93 2-4 2B 100 -- 0 0 650 660
11 98 2-5 2C 92 M 8 8 350 640 30 55 2-6 2D 91 M 9 9 360 660 28 55
2-7 2E 92 M 8 8 290 520 36 56 2-8 2F 97 M 3 3 320 580 33 55 2-9 2G
97 M 3 3 330 600 32 55 PROPERTIES AFTER PRE- HOLE STRAIN - STRAIN
AGE EXPANSION HEAT HARDENING HOLE STEEL TREATMENT PROPERTIES
EXPANDING SHEET STEEL YS.sub.HT TS.sub.HT .DELTA.YS .DELTA.TS RATIO
.lambda. NO. NO. MPa MPa MPa MPa % REMARKS 2-1 2A 690 770 345 150
145 EXAMPLE 2-2 2B 730 810 375 160 140 EXAMPLE 2-3 2B 730 750 60 30
70 COMPARATIVE EXAMPLE 2-4 2B 680 685 30 25 60 COMPARATIVE EXAMPLE
2-5 2C 710 790 360 150 140 EXAMPLE 2-6 2D 730 805 370 145 135
EXAMPLE 2-7 2E 480 540 190 20 60 COMPARATIVE EXAMPLE 2-8 2F 650 720
330 140 150 EXAMPLE 2-9 2G 670 745 340 145 145 EXAMPLE F: FERRITE
M: MARTENSITE P: PEARLITE B: BAINITE
[0339] All Examples of the invention showed a low yield strength
YS, a high elongation El, a low yield ratio YR, and a high hole
expanding ratio .lambda., suggesting that the hot-rolled steel
sheets have an excellent press-formability including stretch
flanging formability, and showed a very large .DELTA.TS, suggesting
to have an excellent strain age hardening property. Comparative
Examples outside the scope of the invention, in contrast, suggest
that the samples are hot-rolled steel sheets having decreased
press-formability and strain age hardening property as having a
high yield strength YS, a low elongation El, a small hole-expanding
ratio .lambda., or a low .DELTA.TS.
Example 4
[0340] Molten steel having the chemical composition as shown in
Table 10 was made in a converter and cast into steel slabs by the
continuous casting process. These steel slabs were reheated to
1,250.degree. C., and hot-rolled in a hot rolling step for hot
rolling with a finish rolling end temperature of 900.degree. C. and
a coiling temperature of 600.degree. C. into hot-rolled steel
strips (hot-rolled sheets) having a thickness of 4.0 mm. Then,
these hot-rolled steel strips (hot-rolled sheets) were subjected to
a cold rolling step of pickling and cold-rolling into cold rolled
steel strips (cold-rolled sheets) having a thickness of 1.2 mm.
Then, recrystallization annealing was applied to these cold-rolled
steel strips (cold-rolled sheets) on a continuous annealing line at
an annealing temperature shown in Table 11. The resultant steel
strips (cold-rolled annealed sheets) were further subjected to
temper rolling of an elongation of 0.8%.
[0341] Test pieces were sampled from the resultant steel strips,
and microstructure, tensile properties, strain age hardening
property and hole expanding property were investigated, as Example
1. Press-formability was evaluated in terms of elongation, yield
strength and hole expanding ratio.
[0342] The results are shown in Table 12.
10TABLE 10 TRANSFORMATION STEEL CHEMICAL COMPOSITION (wt. %) POINT
(.degree. C.) NO. C Si Mn P S Al N Cr Mo W Nb Ti V A.sub.c1
A.sub.c3 2H 0.055 0.02 1.52 0.01 0.004 0.032 0.002 0.15 0.45 -- --
-- -- 720 880 2I 0.058 0.02 1.56 0.01 0.002 0.032 0.002 -- 0.32 --
0.04 -- 0.05 715 875 2J 0.052 0.03 1.48 0.01 0.005 0.028 0.002 --
0.48 -- 0.05 0.03 -- 720 885 2K 0.049 0.02 1.86 0.01 0.005 0.033
0.002 -- -- 0.54 -- -- -- 715 875 2L 0.052 0.02 1.62 0.01 0.004
0.032 0.002 -- 0.35 -- -- 0.05 -- 715 880 2M 0.052 0.02 1.52 0.01
0.003 0.031 0.002 0.50 -- -- 0.05 -- -- 710 885 2N 0.053 0.02 1.88
0.01 0.004 0.032 0.002 -- -- -- -- -- -- 705 830 2P 0.052 0.02 1.66
0.01 0.004 0.033 0.00 0.55 -- -- -- -- -- 705 880 2Q 0.055 0.02
1.49 0.01 0.003 0.031 0.00 -- 0.55 -- -- -- -- 710 880 2R 0.049
0.02 1.73 0.01 0.002 0.032 0.00 -- 0.38 0.11 -- -- -- 710 885 2S
0.032 0.02 1.72 0.01 0.003 0.031 0.002 0.45 -- 0.15 0.04 -- -- 705
855 2T 0.033 0.02 1.65 0.01 0.004 0.032 0.002 0.52 -- 0.25 0.03
0.05 0.04 706 850
[0343]
11 TABLE 11 HOT ROLLING STEP FINISH ROLLING COLD ROLLING SLAB END
COILING STEP RECRYSTALLIZATION STEEL REHEATING TEMP. TEMP. COLD
ROLLING ANNEALING SHEET STEEL TEMP. FDT CT REDUCTION ANNEALING
TEMP. NO. NO. (.degree. C.) .degree. C. .degree. C. % (.degree. C.)
2-10 2H 1250 900 600 70 800 2-11 2I 800 2-12 2I 980 2-13 2I 680
2-14 2J 800 2-15 2K 800 2-16 2L 800 2-17 2M 800 2-18 2N 800 2-19 2P
800 2-20 2Q 800 2-21 2R 800 2-22 2S 800 2-23 2T 800
[0344]
12 TABLE 12 MICROSTRUCTURE STRAIN HOLE SECONDARY PHASE PROPERTIES
AGE EXPAN- MAR- AFTER PRE- HARD- SION TEN- COLD-ROLLED SHEET STRAIN
- ENING HOLE FERRITE SITE PROPERTIES HEAT PROPER- EXPAND- STEEL
AREA AREA AREA TENSILE PROPERTIES TREATMENT TIES ING SHEET STEEL
RATIO RATIO RATIO YS TS E1 YR YS.sub.HT TS.sub.HT .DELTA.YS
.DELTA.TS RATIO .lambda. NO. NO. % KIND % % (MPa) (MPa) (%) % MPa
MPa MPa MPa % REMARKS 2-10 2H 92 M 8 8 335 610 31 55 675 750 340
140 125 EXAMPLE 2-11 2I 90 M 10 10 355 640 30 55 710 790 355 150
140 EXAMPLE 2-12 2I 0 P,B, 8 100 670 720 11 93 680 740 10 20 70
COMPARA- TIVE M EXAMPLE 2-13 2I 100 -- 0 0 620 640 12 97 640 655 20
15 60 COMPARA- TIVE EXAMPLE 2-14 2J 92 M 8 8 340 620 31 55 680 760
340 140 135 EXAMPLE 2-15 2K 90 M 10 10 345 610 30 57 670 745 325
135 120 EXAMPLE 2-16 2L 92 M 8 8 350 630 30 56 670 740 320 110 130
EXAMPLE 2-17 2M 94 M 6 6 330 600 32 55 660 730 330 130 130 EXAMPLE
2-18 2N 93 M 7 7 330 600 31 55 550 610 220 10 70 COMPARA- TIVE
EXAMPLE 2-19 2P 93 M 7 7 340 620 31 55 660 740 320 120 120 EXAMPLE
2-20 2Q 95 M 5 5 350 630 30 56 680 750 330 120 125 EXAMPLE 2-21 2R
92 M 8 8 335 610 31 55 665 745 330 135 120 EXAMPLE 2-22 2S 94 M 6 6
355 640 30 55 690 770 335 130 140 EXAMPLE 2-23 2T 93 M 7 7 340 620
30 55 665 750 325 130 130 EXAMPLE F: FERRITE M: MARTENSITE P:
PEARLITE B: BAINITE
[0345] All Examples of the invention showed a low yield strength
YS, a high elongation El, a low yield ratio YR, and a high hole
expanding ratio .lambda., suggesting that these hot-rolled steel
sheets have an excellent press-formability including stretch
flanging formability, and showed a very large .DELTA.TS, suggesting
to have an excellent strain age hardening property. Comparative
Examples outside the scope of the invention, in contrast, suggest
that the samples are hot-rolled steel sheets having a low
.DELTA.TS, decreased press-formability and strain age hardening
property as having a high yield strength YS, a low elongation El, a
small hole expanding ratio .lambda..
Example 5
[0346] Molten steel having the chemical composition as shown in
Table 13 was made in a converter and cast into steel slabs by the
continuous casting process. These steel slabs were hot-rolled under
the conditions shown in Table 14 into hot-rolled steel strips
(hot-rolled sheets). Steel sheet No. 3-3 was lubrication-rolled on
the latter four stands of finish rolling. After pickling, these
hot-rolled steel strips (hot-rolled sheet) were annealed on a
continuous hot-dip galvanizing line (CGL) under the conditions
shown in Table 14, and then subjected to a hot-dip galvanizing
treatment, thereby forming a hot-dip galvanizing layer on the
surface of the steel sheet. Then, an alloying treatment of the
hot-dip galvanizing layer was applied under the conditions shown in
Table 14. Some of the steel sheets were left as hot-dip
galvanized.
[0347] After further pickling, the hot-rolled steel strips
(hot-rolled sheets) were subjected to a cold rolling step under the
conditions shown in Table 14 into cold-rolled steel strips
(cold-rolled sheets). These cold-rolled steel strips (cold-rolled
sheets) were annealed under the conditions shown in Table 14 on a
continuous hot-dip galvanizing line (CGL), and then subjected to a
hot-dip galvanizing treatment to form a hot-dip galvanizing layer
on the surface of the steel sheets. Then, an alloying treatment of
the hot-dip galvanizing layer was applied under the conditions
shown in Table 14. Some of the steel sheets were left as
hot-dip-galvanized.
[0348] Prior to annealing on the continuous hot-dip galvanizing
line (CGL), some of the steel sheets were subjected to a preheating
treatment under the conditions shown in Table 14, and then to a
pretreatment steel for pickling. Pickling in the pretreatment step
was conducted in a pickling tank on the entry side of CGL.
[0349] The galvanizing bath temperature was within a range of from
460 to 480.degree. C., and the temperature of the steel sheets to
be dipped was within a range of from the galvanizing bath
temperature to (bath temperature +10.degree. C.). In the alloying
treatment, the sheets were reheated to the alloying temperature,
and held at the temperature for a period of from 15 to 28 seconds.
These steel sheets were further subjected to temper rolling of an
elongation of 1.0%.
[0350] For the hot-dip galvanized steel sheets (steel strips)
obtained through the above-mentioned steps, microstructure, tensile
properties, strain age hardening property, and hole expanding ratio
were determined as in Example 1. Press-formability was evaluated in
terms of elongation El, yield strength and hole-expanding
ratio.
[0351] The results are shown in Table 15.
13TABLE 13 TRANSFORMATION STEEL CHEMICAL COMPOSITION (wt. %) POINT
(.degree. C.) NO. C Si Mn P S Al N Cu Ni Cr Mo Nb Ti V A.sub.c1
A.sub.c3 3A 0.034 0.02 1.70 0.01 0.004 0.034 0.002 1.50 -- -- -- --
-- -- 705 842 3B 0.037 0.02 1.56 0.01 0.001 0.033 0.002 1.45 0.60
-- 0.12 -- -- -- 711 848 3C 0.041 0.03 1.45 0.01 0.005 0.029 0.002
1.28 0.51 0.13 -- -- -- -- 711 847 3D 0.038 0.02 1.60 0.01 0.005
0.032 0.002 1.35 0.43 -- -- 0.01 0.01 0.01 707 845 3E 0.037 0.02
1.80 0.01 0.006 0.034 0.002 0.14 -- -- -- -- -- -- 706 835 3F 0.035
0.02 1.66 0.01 0.003 0.033 0.002 0.72 -- -- -- -- -- -- 706 844 3G
0.036 0.02 1.68 0.01 0.005 0.036 0.002 0.96 -- -- -- -- -- -- 706
843
[0352]
14 TABLE 14 HOT ROLLING STEP COLD ROLLING FINISH STEP SLAB ROLLING
COILING COLD STEEL REHEATING END TEMP. TEMP. FINAL ROLLING FINAL
SHEET STEEL TEMP. FDT CT THICKNESS REDUCTION THICKNESS NO. NO.
(.degree. C.) .degree. C. .degree. C. mm % mm 3-1 3A 1150 850 600
1.6 -- -- 3-2 3B 1150 850 600 1.6 -- -- 3-3 3B 3-4 3B 3-5 3B 3-6 3C
1150 850 600 1.6 -- -- 3-7 3D 1150 850 600 1.6 -- -- 3-8 3E 1150
850 600 1.6 -- -- 3-9 3F 1150 850 600 1.6 -- -- 3-10 3G 1150 850
600 1.6 -- -- 3-11 3A 1150 850 600 4.0 70 1.2 3-12 3B 1150 850 600
4.0 70 1.2 3-13 3B 3-14 3B 3-15 3B 3-16 3C 1150 850 600 4.0 70 1.2
3-17 3D 1150 850 600 4.0 70 1.2 3-18 3E 1150 850 600 4.0 70 1.2
3-19 3F 1150 850 600 4.0 70 1.2 3-20 3G 1150 850 600 4.0 70 1.2
PRETREATMENT STEP PREHEATING ANNEALING TEMPER STEEL TREATMENT KIND
HEATING ALLOYING ROLLING SHEET STEEL TEMP. PICKLING OF TEMP. TEMP.
REDUCTION NO. NO. LINE .degree. C. YES/NO LINE .degree. C. PLATING
.degree. C. % 3-1 3A -- -- -- CGL 800 ALLOYING 510 1.0 3-2 3B -- --
-- CGL 800 1.0 3-3 3B CAL 800 YES CGL 780 1.0 3-4 3B -- -- -- CGL
980 1.0 3-5 3B -- -- -- CGL 680 1.0 3-6 3C -- -- -- CGL 800
NON-ALLOYING -- 1.0 3-7 3D -- -- -- CGL 800 ALLOYING 520 1.0 3-8 3E
-- -- -- CGL 800 1.0 3-9 3F -- -- -- CGL 800 1.0 3-10 3G -- -- --
CGL 800 ALLOYING 510 1.0 3-11 3A -- -- -- CGL 800 1.0 3-12 3B -- --
-- CGL 800 1.0 3-13 3B CAL 800 YES CGL 780 1.0 3-14 3B -- -- -- CGL
980 1.0 3-15 3B -- -- -- CGL 680 1.0 3-16 3C -- -- -- CGL 800 1.0
3-17 3D -- -- -- CGL 800 1.0 3-18 3E -- -- -- CGL 800 1.0 3-19 3F
-- -- -- CGL 800 NON-ALLOYING -- 1.0 3-20 3G -- -- -- CGL 800
NON-ALLOYING -- 1.0
[0353]
15 TABLE 15 STRAIN HOLE PROPERTIES AGE EXPAN- MICROSTRUCTURE AFTER
PRE- HARD- SION SECONDARY PHASE PLATED SHEET STRAIN - ENING HOLE
FERRITE MAR- PROPERTIES HEAT PROPER- EXPAND- STEEL AREA TEN- AREA
TENSILE PROPERTIES TREATMENT TIES ING SHEET STEEL RATIO SITE RATIO
YS TS E1 YR YS.sub.HT TS.sub.HT .DELTA.YS .DELTA.TS RATIO .lambda.
NO. NO. % KIND % % (MPa) (MPa) (%) % MPa MPa MPa MPa % REMARKS 3-1
3A 94 M 6 6 340 620 30 55 690 765 350 145 140 EXAMPLE 3-2 3B 91 M 9
9 355 640 29 55 720 795 365 155 135 EXAMPLE 3-3 3B 91 M 9 9 340 620
30 55 690 775 350 155 135 EXAMPLE 3-4 3B 0 M,P,B 6 100 670 710 12
94 720 740 50 30 65 COMPARA- TIVE EXAMPLE 3-5 3B 100 -- 0 0 630 650
11 97 670 675 40 25 55 COMPARA- TIVE EXAMPLE 3-6 3C 93 M 7 7 350
630 29 56 680 775 330 145 135 EXAMPLE 3-7 3D 92 M 8 8 360 650 28 55
710 795 350 145 130 EXAMPLE 3-8 3E 93 M 7 7 290 510 36 57 470 530
180 20 60 COMPARA- TIVE EXAMPLE 3-9 3F 96 M 4 4 310 570 33 54 640
710 330 140 140 EXAMPLE 3-10 3G 95 M 5 5 320 590 32 54 660 735 340
145 135 EXAMPLE 3-11 3A 92 M 8 8 345 630 31 55 700 780 355 150 145
EXAMPLE 3-12 3B 90 M 10 10 360 660 29 55 730 820 370 160 140
EXAMPLE 3-13 3B 90 M 10 10 350 640 30 55 720 800 370 160 140
EXAMPLE 3-14 3B 0 M,P,B 8 100 680 720 12 94 730 750 50 30 70
COMPARA- TIVE EXAMPLE 3-15 3B 100 -- 0 0 640 660 11 97 660 685 20
25 60 COMPARA- TIVE EXAMPLE 3-16 3C 91 M 9 9 355 650 30 55 720 800
365 150 140 EXAMPLE 3-17 3D 91 M 9 9 360 660 29 55 720 805 360 145
135 EXAMPLE 3-18 3E 93 M 7 7 290 520 36 56 480 540 190 20 60
COMPARA- TIVE EXAMPLE 3-19 3F 97 M 3 3 320 580 34 55 640 715 320
135 135 EXAMPLE 3-20 3G 96 M 4 4 330 600 33 55 670 740 70 140 140
EXAMPLE *) M: MARTENSITE, P: PEARLITE, B: BAINITE
[0354] All Examples of the invention showed a low yield strength
YS, a high elongation El, a low yield ratio YR, and a high
hole-expanding ratio .lambda., suggesting that these hot-rolled
steel sheets have an excellent press-formability including stretch
flanging formability, and showed a high .DELTA.YS, and a very large
.DELTA.TS, suggesting to have an excellent strain age hardening
property. Comparative Examples outside the scope of the invention,
in contrast, suggest that the samples are hot-rolled steel sheets
having decreased press-formability and strain age hardening
property as having a high yield strength YS, a low elongation El, a
small hole expanding ratio .lambda., or a low .DELTA.TS.
Example 6
[0355] Molten steel having the chemical composition as shown in
Table 16 was made in a converter and cast into steel slabs by the
continuous casting process. These steel slabs were hot-rolled under
the conditions shown in Table 17 into hot-rolled steel strips
(hot-rolled sheets) having a thickness of 1.6 or 4.0 mm. After
pickling, the hot-rolled steel strips having a thickness of 1.6 mm
were annealed under the conditions shown in Table 17 on a
continuous hot-dip galvanizing line (CGL), and the subjected to a
hot-dip galvanizing treatment, thereby forming a hot-dip
galvanizing layer on the surface of each steel sheet. Then, an
alloying treatment of the hot-dip galvanizing layer was applied
under the conditions shown in Table 17. Some of the steel sheets
were left as hot-dip galvanized.
[0356] After further pickling, the hot-rolled steel strips
(hot-rolled sheets) were cold-rolled under the conditions shown in
Table 17 into cold-rolled steel strips (cold-rolled sheets). These
cold-rolled steel strips (cold-rolled sheets) were annealed under
the conditions shown in Table 17 on a continuous hot-dip
galvanizing line (CGL), and then, subjected to a hot-dip
galvanizing treatment, thereby forming a hot-dip galvanizing layer
on the surface of each steel sheet. Then, an alloying treatment of
the hot-dip galvanizing layer was applied. Some of the steel sheets
were left as hot-dip galvanized.
[0357] Prior to annealing of the continuous hot-dip galvanizing
line (CGL), some of the steel sheets were subjected to a preheating
treatment under the conditions shown in Table 17 on a continuous
annealing line (CAL), and a pretreatment step for pickling.
Pickling in the pretreatment step was accomplished in a pickling
tank on the entry side of CGL.
[0358] The galvanizing bath temperature was within a range of from
460 to 480.degree. C., and the temperature of the steel sheets to
be dipped was within a range of from the galvanizing bath
temperature to (bath temperature +10.degree. C.). In the alloying
treatment, the sheets were reheated to the alloying temperature,
and held at the temperature for a period of from 15 to 28 seconds.
These steel sheets were further subjected to temper rolling of an
elongation of 1.0%.
[0359] For the hot-dip galvanized steel sheets (steel strips)
obtained through the above-mentioned steps, microstructure, tensile
properties, strain age hardening property, and hole expanding ratio
were determined as in Example 1. Press-formability was evaluated in
terms of elongation El, yield strength and hole expanding
ratio.
[0360] The results are shown in Table 18.
16TABLE 16 TRANSFORMATION STEEL CHEMICAL COMPOSITION (wt. %) POINT
(.degree. C.) NO. C Si Mn P S Al N Cr Mo W Nb Ti V A.sub.c1
A.sub.c3 3H 0.054 0.02 1.56 0.01 0.004 0.034 0.002 0.15 0.43 -- --
-- -- 715 870 3I 0.048 0.02 1.52 0.01 0.002 0.033 0.002 -- 0.32 --
0.04 -- 0.05 715 875 3J 0.051 0.03 1.55 0.01 0.005 0.029 0.002 --
0.48 -- 0.05 0.03 -- 715 885 3K 0.055 0.02 1.86 0.01 0.005 0.033
0.002 -- -- 0.51 -- -- -- 715 870 3L 0.056 0.02 1.61 0.01 0.001
0.034 0.002 -- 0.33 -- -- 0.05 -- 710 880 3M 0.052 0.02 1.52 0.01
0.003 0.033 0.002 0.50 -- -- 0.05 -- -- 710 875 3N 0.054 0.02 1.88
0.01 0.005 0.032 0.002 -- -- -- -- -- -- 705 830 3P 0.052 0.02 1.66
0.01 0.005 0.031 0.002 0.52 -- -- -- -- -- 705 870 3Q 0.051 0.02
1.63 0.01 0.004 0.032 0.002 -- 0.53 -- -- -- -- 710 870 3R 0.055
0.02 1.81 0.01 0.003 0.029 0.002 -- 0.33 0.22 -- -- -- 715 875 3S
0.053 0.02 1.74 0.01 0.005 0.033 0.002 0.42 -- 0.12 0.04 -- -- 715
870 3T 0.053 0.02 1.62 0.01 0.002 0.034 0.002 0.29 -- 0.22 0.03
0.02 0.04 715 875
[0361]
17 TABLE 17 HOT ROLLING STEP COLD ROLLING FINISH STEP SLAB ROLLING
COILING COLD STEEL REHEATING END TEMP. TEMP. FINAL ROLLING FINAL
SHEET STEEL TEMP. FDT CT THICKNESS REDUCTION THICKNESS NO. NO.
(.degree. C.) .degree. C. .degree. C. mm % mm 3-21 3H 1250 850 600
1.6 -- -- 3-22 3I 1250 850 600 1.6 -- -- 3-23 3-24 3-25 3-26 3J
1250 850 600 1.6 -- -- 3-27 3K 1250 850 600 1.6 -- -- 3-28 3L 1250
850 600 1.6 -- -- 3-29 3M 1250 850 600 1.6 -- -- 3-30 3N 1250 850
600 1.6 -- -- 3-31 3H 1250 850 600 4.0 70 1.2 3-32 3I 1250 850 600
4.0 70 1.2 3-33 3-34 3-35 3-36 3J 1250 850 600 4.0 70 1.2 3-37 3K
1250 850 600 4.0 70 1.2 3-38 3L 1250 850 600 4.0 70 1.2 3-39 3M
1250 850 600 4.0 70 1.2 3-40 3N 1250 850 600 4.0 70 1.2 3-41 3P
1250 850 600 4.0 70 1.2 3-42 3Q 1250 850 600 4.0 70 1.2 3-43 3R
1250 850 600 4.0 70 1.2 3-44 3S 1250 850 600 4.0 70 1.2 3-45 3T
1250 850 600 4.0 70 1.2 PRETREATMENT STEP PREHEATING ANNEALING
TEMPER STEEL TREATMENT KIND HEATING ALLOYING ROLLING SHEET STEEL
TEMP. PICKLING OF TEMP. TEMP. REDUCTION NO. NO. LINE .degree. C.
YES/NO LINE .degree. C. PLATING .degree. C. % 3-21 3H -- -- -- CGL
800 ALLOYING 510 1.0 3-22 3I -- -- -- CGL 800 1.0 3-23 CAL 800 YES
CGL 780 1.0 3-24 -- -- -- CGL 980 1.0 3-25 -- -- -- CGL 680 1.0
3-26 3J -- -- -- CGL 800 NON-ALLOYING -- 1.0 3-27 3K -- -- -- CGL
800 NON-ALLOYING -- 1.0 3-28 3L -- -- -- CGL 800 ALLOYING 520 1.0
3-29 3M -- -- -- CGL 800 1.0 3-30 3N -- -- -- CGL 800 1.0 3-31 3H
-- -- -- CGL 800 ALLOYING 510 1.0 3-32 3I -- -- -- CGL 800 1.0 3-33
CAL 800 YES CGL 780 1.0 3-34 -- -- -- CGL 980 1.0 3-35 -- -- -- CGL
680 1.0 3-36 3J -- -- -- CGL 800 1.0 3-37 3K -- -- -- CGL 800
ALLOYING 520 1.0 3-38 3L -- -- -- CGL 800 1.0 3-39 3M -- -- -- CGL
800 1.0 3-40 3N -- -- -- CGL 800 1.0 3-41 3P -- -- -- CGL 800 1.0
3-42 3Q -- -- -- CGL 800 1.0 3-43 3R -- -- -- CGL 800 NON-ALLOYING
1.0 3-44 3S -- -- -- CGL 800 NON-ALLOYING 1.0 3-45 3T -- -- -- CGL
800 ALLOYING 520 1.0
[0362]
18 TABLE 18 STRAIN HOLE PROPERTIES AGE EXPAN- MICROSTRUCTURE AFTER
PRE- HARD- SION SECONDARY PHASE* PLATED SHEET STRAIN - ENING HOLE
FERRITE MAR- PROPERTIES HEAT PROPER- EXPAND- STEEL AREA TEN- AREA
TENSILE PROPERTIES TREATMENT TIES ING SHEET STEEL RATIO SITE RATIO
YS TS E1 YR YS.sub.HT TS.sub.HT .DELTA.YS .DELTA.TS RATIO .lambda.
NO. NO. % KIND % % (MPa) (MPa) (%) % MPa MPa MPa MPa % REMARKS 3-21
3H 93 M 7 7 335 610 30 55 671 745 336 135 120 EXAMPLE 3-22 3I 90 M
10 10 350 640 29 55 707 785 357 145 140 EXAMPLE 3-23 3I 90 M 10 10
340 620 30 55 689 765 349 145 140 EXAMPLE 3-24 3I 0 M,P,B 7 100 665
710 12 94 710 730 45 20 60 COMPARA- TIVE EXAMPLE 3-25 3I 100 -- 0 0
560 580 11 97 590 595 30 15 70 COMPARA- TIVE EXAMPLE 3-26 3J 92 M 8
8 350 620 29 56 680 755 330 135 135 EXAMPLE 3-27 3K 91 M 9 9 335
610 28 55 671 745 336 135 120 EXAMPLE 3-28 3L 92 M 8 8 360 630 36
57 681 745 321 115 135 EXAMPLE 3-29 3M 95 M 5 5 325 600 33 54 657
730 332 130 140 EXAMPLE 3-30 3N 94 M 6 6 325 600 32 54 554 615 229
15 70 COMPARA- TIVE EXAMPLE 3-31 3H 91 M 9 9 340 620 31 55 684 760
344 140 120 EXAMPLE 3-32 3I 90 M 10 10 360 650 29 55 720 800 360
150 135 EXAMPLE 3-33 3I 90 M 10 10 345 630 30 55 702 780 357 150
130 EXAMPLE 3-34 3I 0 M,P,B 8 100 675 720 12 94 720 740 45 20 70
COMPARA- TIVE EXAMPLE 3-35 3I 100 -- 0 0 570 590 11 97 590 605 20
15 70 COMPARA- TIVE EXAMPLE 3-36 3J 90 M 10 10 345 630 30 55 693
770 348 140 120 EXAMPLE 3-37 3K 91 M 9 9 360 620 29 56 680 755 335
135 125 EXAMPLE 3-38 3L 92 M 8 8 360 640 36 56 685 770 325 130 135
EXAMPLE 3-39 3M 96 M 4 4 335 610 34 55 671 745 336 135 140 EXAMPLE
3-40 3N 95 M 5 5 340 610 33 56 567 630 227 20 70 COMPARA- TIVE
EXAMPLE 3-41 3P 96 M 4 4 335 610 30 55 670 745 335 135 125 EXAMPLE
3-42 3Q 94 M 6 6 340 620 30 55 690 770 350 150 120 EXAMPLE 3-43 3R
93 M 7 7 350 640 29 55 705 785 355 145 120 EXAMPLE 3-44 3S 95 M 5 5
360 650 29 55 680 780 320 130 135 EXAMPLE 3-45 3T 94 M 6 6 340 620
30 55 690 775 340 140 120 EXAMPLE *) M: MARTENSITE, P: PEARLITE, B:
BAINITE
[0363] All Examples of the invention showed a low yield strength
YS, a high elongation El, a low yield ratio YR, and a high hole
expanding ratio .lambda., suggesting that these galvanized steel
sheets have an excellent press-formability including stretch
flanging formability, and showed a high .DELTA.YS, and a very large
.DELTA.TS, suggesting to have an excellent strain age hardening
property. Comparative Examples outside the scope of the invention,
in contrast, suggest that the samples are galvanized steel sheets
having decreased press-formability and strain age hardening
property as having a high yield strength YS, a low elongation El, a
small hole expanding ratio .lambda., or a low .DELTA.TS.
INDUSTRIAL APPLICABILITY
[0364] According to the present invention, it is possible to stably
manufacture hot-rolled steel sheets, cold-rolled steel sheets and
plated steel sheets in which tensile strength remarkably increased
through a heat treatment applied after press forming while
maintaining an excellent press-formability, giving industrially
remarkable effects. When applying a steel sheet of the invention to
automotive parts, there are available advantages of easy press
forming, high and stable parts properties after completion, and
sufficient contribution to the weight reduction of the automobile
body.
* * * * *