U.S. patent application number 09/937889 was filed with the patent office on 2003-03-06 for high tensile hot-rolled steel sheet having excellent strain aging hardening properties and method for producing the same.
Invention is credited to Furukimi, Osamu, Kaneko, Sinjiro, Katayama, Noriyuki, Kurosawa, Nobutaka, Sakata, Kei, Tominaga, Yoichi, Tosaka, Akio.
Application Number | 20030041932 09/937889 |
Document ID | / |
Family ID | 27342459 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030041932 |
Kind Code |
A1 |
Tosaka, Akio ; et
al. |
March 6, 2003 |
High tensile hot-rolled steel sheet having excellent strain aging
hardening properties and method for producing the same
Abstract
The present invention provides a high tensile strength
hot-rolled steel sheet having superior strain aging hardenability,
which has high formability and stable quality characteristics, and
in which satisfactory strength is obtained when the steel sheet is
formed into automotive components, thus enabling the reduction in
weight of automobile bodies. Specifically, a method for producing a
high tensile strength hot-rolled steel sheet having superior strain
aging hardenability with a BH of 80 MPa or more, a .DELTA.TS of 40
MPa or more, and a tensile strength of 440 MPa or more includes the
steps of heating a steel slab to 1,000.degree. C. or more, the
steel slab containing, in percent by mass, 0.15% or less of C, 2.0%
or less of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or
less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N, and
optionally 0.1% or less in total of at least one of more than 0.02%
to 0.1% of Nb and more than 0.02% to 0.1% of V, the ratio N (mass
%)/Al (mass %) being 0.3 or more; rough-rolling the steel slab to
form a sheet bar; finish-rolling the sheet bar at a finishing
temperature of 800.degree. C. or more; cooling at a cooling rate of
20.degree. C. to 40.degree. C./s or more within 0.5 second after
the finish-rolling; and coiling at a temperature of 650.degree. C.
to 450.degree. C. or less.
Inventors: |
Tosaka, Akio; (Chiba,
JP) ; Kaneko, Sinjiro; (Chiba, JP) ; Tominaga,
Yoichi; (Chiba, JP) ; Katayama, Noriyuki;
(Chiba, JP) ; Kurosawa, Nobutaka; (Chiba, JP)
; Sakata, Kei; (Chiba, JP) ; Furukimi, Osamu;
(Chiba, JP) |
Correspondence
Address: |
SCHNADER HARRISON SEGAL & LEWIS, LLP
1600 MARKET STREET
SUITE 3600
PHILADELPHIA
PA
19103
|
Family ID: |
27342459 |
Appl. No.: |
09/937889 |
Filed: |
October 2, 2001 |
PCT Filed: |
February 14, 2001 |
PCT NO: |
PCT/JP01/01005 |
Current U.S.
Class: |
148/602 ;
148/320 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/02 20130101; C22C 38/04 20130101; C21D 8/0236 20130101;
C22C 38/002 20130101; C22C 38/12 20130101; C22C 38/06 20130101;
C21D 8/0226 20130101; C21D 2221/02 20130101 |
Class at
Publication: |
148/602 ;
148/320 |
International
Class: |
C21D 008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2000 |
JP |
2000-046335 |
Feb 29, 2000 |
JP |
2000-053439 |
May 26, 2000 |
JP |
2000-156272 |
Claims
1. A high tensile strength hot-rolled steel sheet having superior
strain aging hardenability comprising: in percent by mass, 0.15% or
less of C; 2.0% or less of Si; 3.0% or less of Mn; 0.08% or less of
P; 0.02% or less of S; 0.02% or less of Al; 0.0050% to 0.0250% of
N; and the balance being Fe and incidental impurities, the ratio N
(mass %)/Al (mass %) being 0.3 or more, N in the dissolved state
being 0.0010% or more.
2. A high tensile strength hot-rolled steel sheet having superior
strain aging hardenability with a tensile strength of 440 MPa or
more comprising: in percent by mass, 0.15% or less of C; 2.0% or
less of Si; 3.0% or less of Mn; 0.08% or less of P; 0.02% or less
of S; 0.02% or less of Al; 0.0050% to 0.0250% of N; and the balance
being Fe and incidental impurities, the ratio N (mass %)/Al (mass
%) being 0.3 or more, N in the dissolved state being 0.0010% or
more, wherein the hot-rolled steel sheet has a structure in which
the areal rate of the ferrite phase having an average grain size of
10 .mu.m or less is 50% or more.
3. A steel sheet according to claim 2 further comprising at least
one selected from the group consisting of the following Group a to
Group d: Group a: 1.0% or less in total of at least one of Cu, Ni,
Cr, and Mo Group b: 0.1% or less in total of at least one of Nb,
Ti, and V Group c: 0.0030% or less of B Group d: 0.0010% to 0.010%
in total of at least one of Ca and REM.
4. A steel sheet according to either claim 2 or 3, wherein the high
tensile strength hot-rolled sheet has a thickness of 4.0 mm or
less.
5. A high tensile strength hot-rolled plated steel sheet produced
by electroplating or hot-dip plating a steel sheet according to any
one of claims 2 to 4.
6. A method for producing a high tensile strength hot-rolled steel
sheet having superior strain aging hardenability with a tensile
strength of 440 MPa or more comprising the steps of: heating a
steel slab to 1,000.degree. C. or more, the steel slab comprising:
in percent by mass, 0.15% or less of C; 2.0% or less of Si; 3.0% or
less of Mn; 0.08% or less of P; 0.02% or less of S; 0.02% or less
of Al; 0.0050% to 0.0250% of N; and optionally further comprising
at least one selected from the group consisting of the following
Group a to Group d, the ratio N (mass %)/Al (mass %) being 0.3 or
more: Group a: 1.0% or less in total of at least one of Cu, Ni, Cr,
and Mo Group b: 0.1% or less in total of at least one of Nb, Ti,
and V Group c: 0.0030% or less of B Group d: 0.0010% to 0.010% in
total of at least one of Ca and REM; rough-rolling the steel slab
to form a sheet bar; finish-rolling the sheet bar at a finishing
temperature of 800.degree. C. or more; cooling at a cooling rate of
20.degree. C./s or more within 0.5 second after the finish-rolling;
and coiling at a temperature of 650.degree. C. or less.
7. A method according to according to claim 6, further comprising
the step of performing at least one of skin pass rolling and
leveling with an elongation of 1.5% to 10% after the coiling step
is performed.
8. A method according to either claim 6 or 7, further comprising
the step of joining consecutive sheet bars to each other between
the steps of rough-rolling and finish-rolling.
9. A method according to any one of claims 6 to 8, further
comprising the step of using at least one of a sheet bar edge
heater for heating a widthwise end of the sheet bar and a sheet bar
heater for heating a lengthwise end of the sheet bar between the
steps of rough-rolling and finish-rolling.
10. A high tensile strength hot-rolled steel sheet having superior
strain aging hardenability with a BH of 80 MPa or more, a .DELTA.TS
of 40 MPa or more, and a tensile strength of 440 MPa or more
comprising, in percent by mass, 0.15% or less of C; 2.0% or less of
Si; 3.0% or less of Mn; 0.08% or less of P; 0.02% or less of S;
0.02% or less of Al; 0.0050% to 0.0250% of N; and the balance being
Fe and incidental impurities, the ratio N (mass %)/Al (mass %)
being 0.3 or more, N in the dissolved state being 0.0010% or more,
wherein the hot-rolled steel sheet has a structure in which the
areal rate of the ferrite phase having an average grain size of 10
.mu.m or less is 70% or more, and the areal rate of the martensite
phase is 5% or more.
11. A method for producing a high tensile strength hot-rolled steel
sheet having superior strain aging hardenability with a BH of 80
MPa or more, a .DELTA.TS of 40 MPa or more, and a tensile strength
of 440 MPa or more comprising the steps of: heating a steel slab to
1,000.degree. C. or more, the steel slab comprising: in percent by
mass, 0.15% or less of C; 2.0% or less of Si; 3.0% or less of Mn;
0.08% or less of P; 0.02% or less of S; 0.02% or less of Al;
0.0050% to 0.0250% of N; and optionally further comprising at least
one selected from the group consisting of the following Group a to
Group d, the ratio N (mass %)/Al (mass %) being 0.3 or more: Group
a: 1.0% or less in total of at least one of Cu, Ni, Cr, and Mo
Group b: 0.1% or less in total of at least one of Nb, Ti, and V
Group c: 0.0030% or less of B Group d: 0.0010% to 0.010% in total
of at least one of Ca and REM; rough-rolling the steel slab to form
a sheet bar; finish-rolling the sheet bar at a finishing
temperature of 800.degree. C. or more; cooling at a cooling rate of
20.degree. C./s or more within 0.5 second after the finish-rolling;
and coiling at a temperature of 450.degree. C. or less.
12. A high tensile strength hot-rolled steel sheet having superior
strain aging hardenability comprising: in percent by mass, 0.03% to
0.1% of C; 2.0% or less of Si; 1.0% to 3.0% of Mn; 0.08% or less of
P; 0.02% or less of S; 0.02% or less of Al; 0.0050% to 0.0250% of
N; 0.1% or less in total of at least one of more than 0.02% to 0.1%
of Nb and more than 0.02% to 0.1% of V; and the balance being Fe
and incidental impurities, the ratio N (mass %)/Al (mass %) being
0.3 or more, N in the dissolved state being 0.0010% or more, the
total of precipitated Nb and precipitated V being 0.015% or more,
wherein the hot-rolled steel sheet has a structure in which the
areal rate of the ferrite phase having an average grain size of 10
.mu.m or less is 80% or more, and the average grain size of a
precipitate comprising a Nb carbonitride or a V carbonitride is
0.05 .mu.m or less.
13. A method for producing a high tensile strength hot-rolled steel
sheet having superior strain aging hardenability comprising the
steps of: heating a steel slab to 1,100.degree. C. or more, the
steel slab comprising: in percent by mass, 0.03% to 0.1% of C; 2.0%
or less of Si; 1.0% to 3.0% of Mn; 0.08% or less of P; 0.02% or
less of S; 0.02% or less of Al; 0.0050% to 0.0250% of N: 0.1% or
less in total of at least one of more than 0.02% to 0.1% of Nb and
more than 0.02% to 0.1% of V; and the balance being Fe and
incidental impurities; rough-rolling the steel slab to form a sheet
bar; finish-rolling the sheet bar at a finishing temperature of
800.degree. C. or more; cooling at a cooling rate of 40.degree.
C./s or more within 0.5 second after the finish-rolling; and
coiling in the temperature range of 550 to 650.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to high tensile strength
hot-rolled steel sheets having superior strain aging hardenability.
More particularly, the invention relates to a high tensile strength
hot-rolled steel sheet having a TS (tensile strength) of 440 MPa or
more, and relates to a method for producing the same. The high
tensile strength hot-rolled steel sheet is mainly used for
automobiles as a thin hot-rolled steel sheet having high
workability. Furthermore, the high tensile strength hot-rolled
steel sheet is-used as a replacement for a thin cold-rolled steel
sheet having a thickness of approximately 4.0 mm or less and which
was employed because it was difficult to produce a steel sheet with
such a small thickness by hot rolling. The applications of the
steel sheet in accordance with the present invention extend over a
wide range from use for relatively light working, such as slight
bending and forming of pipes by roll forming, to relatively heavy
working, such as drawing by a press.
[0002] The present invention concerns not only hot-rolled steel
sheets but also electroplated steel sheets and hot-dip plated steel
sheets using the hot-rolled steel sheets as mother plates.
[0003] In the present invention, "having superior strain aging
hardenability" means to have the following characteristics:
[0004] 1) when a steel sheet is subjected to predeformation with a
tensile strain of 5% and then aging treatment by retaining the
steel sheet at 170.degree. C. for 20 minutes, an increase in
deformation stress before and after the aging treatment
(hereinafter referred to as BH; BH=Yield stress after aging
treatment--Predeformation stress before aging treatment) is 80 MPa
or more; and
[0005] 2) an increase in tensile strength before and after strain
aging treatment (the predeformation +the aging treatment) (herein
after referred to as .DELTA.TS; .DELTA.TS=Tensile strength after
aging treatment--Tensile strength before predeformation) is 40 MPa
or more.
BACKGROUND ART
[0006] Many thin steel sheets are used as materials for automobile
bodies. Cold-rolled steel sheets used to be used for applications
in which superior formability is required. However, owing to
adjustment of steel compositions (chemical constituents) and
optimization of hot rolling conditions, it has become possible to
produce hot-rolled steel sheets having high formability (high
workability), and therefore, the hot-rolled steel sheets are
increasingly used as materials for automobile bodies.
[0007] In order to meet restrictions on exhaust gas in view of the
global environment, reductions in automobile body weight are very
important. In order to reduce the automobile body weight, it is
effective to increase the tensile strength of steel sheets and
decrease the thickness of the steel sheets. Automotive components
to which higher tensile strength and thinner steel sheets are
applied must have various characteristics. For example, the
required characteristics include static strength to bending and
torsional deformation, fatigue strength, and impact resistance.
Therefore, the high tensile strength steel sheets used for the
automotive components must have such characteristics after
formation and working are performed.
[0008] On the other hand, press forming is performed to steel
sheets when automotive components are manufactured. Excessively
high strength of the steel sheets gives rise to problems; for
example, shape fixability is degraded, and defects, such as
cracking and necking, are caused during formation due to a decrease
in ductility. Such problems have hindered the expansion of the
application of high tensile strength steel sheets to automobile
bodies.
[0009] In order to overcome the difficulties described above, for
example, with respect to cold-rolled steel sheets for outer panels,
a steel sheet production technique is known in which an ultra low
carbon steel is used as a raw material and the C amount ultimately
remaining in the dissolved state is restricted within an
appropriate range. In this technique, a strain aging hardening
phenomenon, which occurs in a paint baking step performed at
170.degree. C..times.approximately 20 minutes after press forming,
is used. Shape fixability and ductility are secured during
formation by maintaining the softness, and dent resistance is
secured after formation by an increase in YS (yield stress) due to
strain aging hardening. However, in this technique, in order to
avoid stretcher strain leading to surface defects, an amount of the
increase in YS cannot be increased sufficiently, and since
.DELTA.TS is as small as several Mpa, the thickness of the steel
sheet cannot be decreased sufficiently.
[0010] On the other hand, in the applications in which appearance
is not a great problem, a steel sheet in which the bake hardening
amount is further increased by using dissolved N (Japanese Examined
Patent Application Publication No. 7-30408), and a steel sheet in
which bake hardenability is further improved by using a dual-phase
structure composed of ferrite and martensite (Japanese Examined
Patent Application Publication No. 8-23048) have been
disclosed.
[0011] However, in such steel sheets, although a higher bake
hardening amount can be obtained because YS (yield stress) is
increased to a certain extent after paint baking, it is not
possible to increase TS (tensile strength), and no great
improvement in fatigue resistance and impact resistance after
formation is expected. Therefore, the steel sheets cannot be used
for components in which fatigue resistance, impact resistance,
etc., are required, which is disadvantageous. Since the amount of
the increase in the yield stress YS is unstable, it is not possible
to decrease the thickness of the steel sheets in such a way as to
contribute to lightening of automotive components, which is also
disadvantageous.
[0012] Moreover, when a thin steel sheet with a thickness of 2.0 mm
or less is produced, since the shape of the steel sheet becomes
unsatisfactory in the hot rolling process, it is considerably
difficult to press-form the steel sheet.
[0013] It is an object of the present invention to provide a high
tensile strength hot-rolled steel sheet having superior strain
aging hardenability which overcomes the limitations of the
conventional techniques described above, which has high formability
and stable quality characteristics, and in which satisfactory
strength is obtained when the steel sheet is formed into automotive
components, thus greatly contributing to lightening of automobile
bodies. It is another object of the present invention to provide a
method for industrially producing such a steel sheet at low costs
and without disturbing the shape thereof.
DISCLOSURE OF INVENTION
[0014] In order to solve the problems described above, the present
inventors have produced various steel sheets by changing
compositions and production methods and have conducted many
material evaluation tests. As a result, it has been found that an
improvement in formability and an increase in strength after
formation are easily made compatible with each other by using N,
which has not been used positively in the field where high
workability is required, as a strengthening element, and by
effectively using a large strain aging hardening phenomenon
exhibited by the action of N as the strengthening element. In order
to effectively use the strain aging hardening phenomenon by N, the
strain aging hardening phenomenon by N must be effectively combined
with paint baking conditions for automobiles and heat-treating
conditions after formation. The present inventors have found that
it is effective to adjust the microstructure and the amount of
dissolved N in a steel sheet within predetermined ranges by
optimizing the hot rolling conditions. It has also been found that
in order to stably cause the strain aging hardening phenomenon by
N, it is particularly important to control the Al content according
to the N content in terms of compositions.
[0015] That is, by using N as the strengthening element, by
adjusting the content of Al which is a key element in an
appropriate range, and by properly setting the hot rolling
conditions so that the-microstructure and the dissolved N are
optimized, it is possible to obtain a steel sheet (steel sheet of
the present invention) having significantly superior formability
and strain aging hardenability compared to a conventional
solid-solution strengthening type C-Mn steel sheet and a
precipitation strengthening steel sheet (conventional steel
sheets).
[0016] In general, in order to evaluate bake hardenability, a
tensile test is used. Since large variations in strength occurred
when the conventional steel sheets were subjected to plastic
deformation under the actual press conditions, the conventional
steel sheets could not be applied to components in which high
reliability was required even if the conventional steel sheets were
evaluated as having desired bake hardenability in the tensile test.
In contrast, variations in strength are small when the steel sheet
of the present invention is subjected to plastic deformation under
the actual press conditions. Furthermore, the steel sheet of the
present invention has a higher evaluation of bake hardenability
according to the tensile test compare to the conventional steel
sheets. It has been found that stable component strength
characteristics are obtained by using the steel sheet of the
present invention.
[0017] The thin hot-rolled steel sheet used for automobile bodies
must have very accurate shape and dimension. It has been found that
accuracy of shape and dimension is greatly improved by employing a
continuous rolling technique which has recently been put into
practical use in the hot rolling process for producing the steel
sheet of the present invention. Furthermore, it has been found that
variations in material properties can be greatly decreased by
partially heating or cooling the rolled material so that the
temperature profiles in the width direction and in the lengthwise
direction become uniform.
[0018] The present invention has been achieved based on the
findings described above and are summarized as follows.
[0019] (1) A high tensile strength hot-rolled steel sheet having
superior strain aging hardenability contains, in percent by mass,
0.15% or less of C, 2.0% or less of Si, 3.0% or less of Mn, 0.08%
or less of P, 0.02% or less of S, 0.02% or less of Al, 0.0050% to
0.0250% of N, and the balance being Fe and incidental impurities,
the ratio N (mass %)/Al (mass %) being 0.3 or more, N in the
dissolved state being 0.0010% or more.
[0020] (2) A high tensile strength hot-rolled steel sheet having
superior strain aging hardenability with a tensile strength of 440
MPa or more contains, in percent by mass, 0.15% or less of C, 2.0%
or less of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or
less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N, and the
balance being Fe and incidental impurities, the ratio N (mass %)/Al
(mass %) being 0.3 or more, N in the dissolved state being 0.0010%
or more, and also has a structure in which the areal rate of the
ferrite phase having an average grain size of 10 .mu.m or less is
50% or more.
[0021] (3) A steel sheet according to (2) further contains at least
one selected from the group consisting of the following Group a to
Group d:
[0022] Group a: 1.0% or less in total of at least one of Cu, Ni,
Cr, and Mo
[0023] Group b: 0.1% or less in total of at least one of Nb, Ti,
and V
[0024] Group c: 0.0030% or less of B
[0025] Group d: 0.0010% to 0.010% in total of at least one of Ca
and REM.
[0026] (4) A steel sheet according to either (2) or (3), wherein
the thickness of the high tensile strength hot-rolled sheet is 4.0
mm or less.
[0027] (5) A high tensile strength hot-rolled plated steel sheet
produced by electroplating or hot-dip plating a steel sheet
according to any one of (2) to (4).
[0028] (6) A method for producing a high tensile strength
hot-rolled steel sheet having superior strain aging hardenability
with a tensile strength of 440 MPa or more includes the steps of
heating a steel slab to 1,000.degree. C. or more, the steel slab
containing, in percent by mass, 0.15% or less of C, 2.0% or less of
Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S,
0.02% or less of Al, 0.0050% to 0.0250% of N, and optionally
further containing at least one selected from the group consisting
of the following Group a to Group d, the ratio N (mass %)/Al (mass
%) being 0.3 or more; rough-rolling the steel slab to form a sheet
bar; finish-rolling the sheet bar at a finishing temperature of
800.degree. C. or more; cooling at a cooling rate of 20.degree.
C./s or more within 0.5 second after the finish-rolling; and
coiling at a temperature of 650.degree. C. or less:
[0029] Group a: 1.0% or less in total of at least one of Cu, Ni,
Cr, and Mo
[0030] Group b: 0.1% or less in total of at least one of Nb, Ti,
and V
[0031] Group c: 0.0030% or less of B
[0032] Group d: 0.0010% to 0.010% in total of at least one of Ca
and REM.
[0033] (7) A method according to (6) further includes the step of
performing at least one of skin pass rolling and leveling with an
elongation of 1.5% to 10% after the coiling step is performed.
[0034] (8) A method according to either (6) or (7) further includes
the step of joining consecutive sheet bars to each other between
the steps of rough-rolling and finish-rolling.
[0035] (9) A method according to any one of (6) to (8) further
includes the step of using at least one of a sheet bar edge heater
for heating a widthwise end of the sheet bar and a sheet bar heater
for heating a lengthwise end of the sheet bar between the steps of
rough-rolling and finish-rolling.
[0036] (10) A high tensile strength hot-rolled steel sheet having
superior strain aging hardenability with a BH of 80 MPa or more, a
.DELTA.TS of 40 MPa or more, and a tensile strength of 440 MPa or
more contains, in percent by mass, 0.15% or less of C, 2.0% or less
of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S,
0.02% or less of Al, 0.0050% to 0.0250% of N, and the balance being
Fe and incidental impurities, the ratio N (mass %)/Al (mass %)
being 0.3 or more, N in the dissolved state being 0.0010% or more,
and also has a structure in which the areal rate of the ferrite
phase having an average grain size of 10 .mu.m or less is 70% or
more, and the areal rate of the martensite phase is 5% or more.
[0037] (11) A method for producing a high tensile strength
hot-rolled steel sheet having superior strain aging hardenability
with a BH of 80 MPa or more, a .DELTA.TS of 40 MPa or more, and a
tensile strength of 440 MPa or more includes the steps of heating a
steel slab to 1,000.degree. C. or more, the steel slab containing,
in percent by mass, 0.15% or less of C, 2.0% or less of Si, 3.0% or
less of Mn, 0.08% or less of P, 0.02% or less of S, 0.02% or less
of Al, 0.0050% to 0.0250% of N, and optionally further containing
at least one selected from the group consisting of the following
Group a to Group d, the ratio N (mass %)/Al (mass %) being 0.3 or
more; rough-rolling the steel slab to form a sheet bar;
finish-rolling the sheet bar at a finishing temperature of
800.degree. C. or more; cooling at a cooling rate of 20.degree.
C./s or more within 0.5 second after the finish-rolling; and
coiling at a temperature of 450.degree. C. or less:
[0038] Group a: 1.0% or less in total of at least one of Cu, Ni,
Cr, and Mo
[0039] Group b: 0.1% or less in total of at least one of Nb, Ti,
and V
[0040] Group c: 0.0030% or less of B
[0041] Group d: 0.0010% to 0.010% in total of at least one of Ca
and REM.
[0042] (12) A high tensile strength hot-rolled steel sheet having
superior strain aging hardenability contains, in percent by mass,
0.03% to 0.1% of C, 2.0% or less of Si, 1.0% to 3.0% of Mn, 0.08%
or less of P, 0.02% or less of S, 0.02% or less of Al, 0.0050% to
0.0250% of N, 0.1% or less in total of at least one of more than
0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V, and the
balance being Fe and incidental impurities, the ratio N (mass %)/Al
(mass %) being 0.3 or more, N in the dissolved state being 0.0010%
or more, the total of precipitated Nb and precipitated V being
0.015% or more, and also has a structure in which the areal rate of
the ferrite phase having an average grain size of 10 .mu.m or less
is 80% or more, and the average grain size of a precipitate
composed of a Nb carbonitride or a V carbonitride is 0.05 .mu.m or
less.
[0043] (13) A method for producing a high tensile strength
hot-rolled steel sheet having superior strain aging hardenability
includes the steps of heating a steel slab to 1,100.degree. C. or
more, the steel slab containing, in percent by mass, 0.03% to 0.1%
of C, 2.0% or less of Si, 1.0% to 3.0% of Mn, 0.08% or less of P,
0.02% or less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N,
0.1% or less in total of at least one of more than 0.02% to 0.1% of
Nb and more than 0.02% to 0.1% of V, and the balance being Fe and
incidental impurities; rough-rolling the steel slab to form a sheet
bar; finish-rolling the sheet bar at a finishing temperature of
800.degree. C. or more; cooling at a cooling rate of 40.degree.
C./s or more within 0.5 second after the finish-rolling; and
coiling in the temperature range of 550 to 650.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] FIG. 1 is a graph which shows BH (an increase in deformation
stress) with respect to examples of the present invention and
comparative examples.
[0045] FIG. 2 is a graph which shows .DELTA.TS (an increase in
tensile strength) with respect to examples of the present invention
and comparative examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0046] First, the chemical compositions of steel in the present
invention will be described. The content (%) of each constituent
element is shown in percent by mass.
[0047] C: 0.15% or less
[0048] C is an element which increases the strength of steel
sheets, and in order to ensure desired strength, the C content is
preferably set at 0.005% or more. The C content is also preferably
set at 0.005% or more in order to suppress grain coarsening. If the
C content exceeds 0.15%, the following problems arise. (1) Since
the percentage of carbides in steel becomes excessive and the
ductility of steel sheets is greatly decreased, formability is
degraded. (2) Spot weldability and arc weldability are greatly
degraded. (3) With respect to hot rolling of a steel sheet with a
large width and a small thickness, deformation resistance greatly
increases below the austenite low temperature range, and the
rolling force rises suddenly, resulting in a difficulty in rolling.
Therefore, the C content is set at 0.15% or less. Additionally, in
view of an improvement in formability, the C content is preferably
0.08% or less, and in applications where good ductility is
particularly important, the C content is more preferably 0.05% or
less.
[0049] However, with respect to a steel sheet of the present
invention containing 0.1% or less in total of at least one of more
than 0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V, the C
content is preferably set at 0.03% to 0.1%. C is an element which
increases the strength of steel sheets and ensures desired strength
by formation of carbonitrides with Nb and V (precipitates), and
thus the C content is preferably set at 0.03% or more. In order to
suppress grain coarsening, preferably, the C content is also set at
0.03% or more. On the other hand, as will be described below, in
order to finely precipitate carbonitrides of Nb and V, after hot
rolling is completed, the carbonitrides must be precipitated in the
low-temperature ferrite phase. If the C content exceeds 0.1% at
this stage, coarse carbonitrides are formed during hot rolling,
resulting in a decrease in the strength of the steel sheet.
Therefore, the C content is set at 0.1% or less.
[0050] Si: 2.0% or less
[0051] Si is an effective element which increases the strength of
steel sheets without greatly decreasing the ductility of steel. On
the other hand, since Si greatly increases the Ar.sub.3
transformation temperature, a large amount of the ferrite phase
tends to be generated during finish rolling. Si also adversely
affects steel sheets, for example, degrading of surface properties
and glossy surface. In order to obtain the strength-increasing
effect significantly, the Si content is preferably set at 0.1% or
more. If the Si content is 2.0% or less, it is possible to inhibit
a large increase of the transformation temperature by adjusting the
amount of Mn which is added to steel in combination with Si, and
satisfactory surface properties are also ensured. Therefore, the Si
content is set at 2.0% or less. Additionally, in order to ensure
high ductility with a TS of more than 500 MPa, in view of the
balance between strength and ductility, the Si content is
preferably set at 0.3% or more.
[0052] Mn: 3.0% or less
[0053] Mn decreases the Ar.sub.3 transformation temperature, and it
is possible to make Mn counter the action of Si for increasing the
transformation temperature. Mn is an element which is effective in
preventing hot brittleness due to S, and in view of preventing hot
brittleness, Mn is preferably added according to the amount of S.
Since Mn has a grain refining effect, it is desirable that Mn be
actively added so that Mn is used for improving material
properties. In view of stably fixing S, the Mn content is
preferably set at approximately 0.2% or more, and in order to meet
the strength requirement of TS 500 MPa class, the Mn content is
preferably set at 1.2% or more, and more preferably, at 1.5% or
more. By increasing the Mn content to such a level, variations of
mechanical properties and strain aging hardenability of steel
sheets are reduced with respect to the change in hot rolling
conditions, thus being effective in stabilizing the quality.
[0054] However, if the Mn content exceeds 3.0%, the following
problems arise. (1) Although the detailed mechanism is unknown, the
deformation resistance at elevated temperatures of steel sheets
tends to be increased. (2) Weldability and formability at the
welding zone tend to be degraded. (3) Since the generation of
ferrite is greatly suppressed, ductility is degraded. Therefore,
the Mn content is preferably limited to 3.0% or less. Additionally,
in applications where more satisfactory corrosion resistance and
formability are required, the Mn content is preferably set at 2.5%
or less.
[0055] With respect to a product with particularly small thickness,
since the quality and shape are minutely changed due to the
variation of the transformation temperature, it is important to
more strictly balance between the action of Mn for decreasing the
transformation temperature and the action of Si for increasing the
transformation temperature. From such a viewpoint, in the steel
sheet used for automobile bodies with a thickness of approximately
4.0 mm or less, the ratio Mn/Si (ratio between the Mn amount and
the Si amount) is preferably set at 3 or more.
[0056] However, with respect to a steel sheet of the present
invention containing 0.1% or less in total of at least one of more
than 0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V, the Mn
content is preferably set at 1.0% to 3.0%. If the Mn content is
less than 1.0%, the Ar.sub.3 transformation temperature increases,
and carbonitrides are remarkably formed in the high-temperature
ferrite phase, and since the carbonitrides coarsen, it becomes
difficult to ensure desired strength. Therefore, the Mn content
must be 1.0% or more.
[0057] P: 0.08% or less
[0058] Although P is effective as a solid-solution strengthening
element, if the P content is excessive, steel is embrittled and the
stretch-flanging property of the steel sheet is degraded. P also
tends to segregate in steel, resulting in embrittlement at the
welding zone. Therefore, the P content is set at 0.08% or less.
Additionally, when the stretch-flanging property and toughness at
the welding zone are regarded as particularly important, the P
content is preferably set at 0.04% or less.
[0059] S: 0.02% or less
[0060] S is an element which is present as an inclusion, degrades
the ductility of the steel sheet, and also degrades the corrosion
resistance. Therefore, the S content is limited to 0.02% or less.
In applications where particularly good workability is required,
the S content is preferably set at 0.015%. When the required level
of the stretch-flanging property, which is particularly susceptible
to the S amount, is high, the S content is preferably 0.008% or
less. Although the detailed mechanism is unknown, if the S content
is decreased to 0.008% or less, the strain aging hardenability of
the hot-rolled steel sheet tends to be stabilized at a higher
level. For this reason, the S content is also preferably 0.008% or
less.
[0061] Al: 0.02% or less
[0062] Al is added to steel as a deoxidizing element, which is
effective in improving the cleanness of the steel, and Al is also
preferably added to the steel in order to achieve texture
refinement. However, if the Al content is excessive, the following
problems arise. (1) The surface properties of steel sheets are
degraded. (2) The amount of dissolved N which is important in the
present invention is decreased. (3) Even if dissolved N is ensured,
if the Al content exceeds 0.02%, variations in strain aging
hardenability due to the change in production conditions are
increased. Therefore, the Al content is limited to 0.02% or less.
Additionally, in view of material stability, the Al content is more
preferably set at 0.001% to 0.016%.
[0063] N: 0.0050% to 0.0250%
[0064] N is the most important constituent element in the present
invention. That is, by the addition of an appropriate amount of N
to control the production conditions, it is possible to secure a
necessary and sufficient amount of N in the dissolved state in the
mother plate (as hot rolled). Thereby, the effect of an increase in
strength (YS, TS) due to solid-solution strengthening and strain
aging hardening is satisfactorily exhibited, and it is possible to
stably satisfy the mechanical property conditions of the steel
sheet of the present invention, i.e., TS of 440 MPa or more, BH of
80 MPa ore more, and .DELTA.TS of 40 MPa or more. N also decreases
the Ar.sub.3 transformation temperature. Since it is possible to
prevent a thin steel sheet, whose temperature is easily decreased
during hot rolling, from being rolled at a temperature lower than
the Ar.sub.3 transformation temperature, N is effective in
stabilizing operation.
[0065] If the N content is less than 0.0050%, it is not possible to
obtain the strength-increasing effect. On the other hand, if the N
content exceeds 0.0250%, the rate of occurrence of internal defects
of the steel sheet increases, and also slab cracking during
continuous casting, etc., often occurs. Therefore, the N content is
set at 0.0050% to 0.0250%. In view of material stability and
improvements in yield in consideration of the whole manufacturing
process, the N content is preferably set at 0.0070% to 0.0170%.
Additionally, if the N content is in the range of the present
invention, there are no adverse effects on weldability.
[0066] Even if N is added, if the N content is in the range of the
present invention, there is substantially no increase in
deformation resistance at elevated temperatures during the
production of steel sheets. It has been found that use of
strengthening due to N is significantly advantageous to the
production of high tensile strength thin hot-rolled steel
sheets.
[0067] N in the dissolved state: 0.0010% or more
[0068] In order to ensure sufficient strength in the mother plate
and to exhibit satisfactory strain aging hardenability due to N,
i.e., to set the BH at 80 MPa or more and the .DELTA.TS at 40 MPa
or more, 0.0010% or more of N in the dissolved state (hereinafter
referred to as "dissolved N") must be present in steel. Herein, the
amount of dissolved N is found by subtracting the amount of
precipitated N from the total amount of N in steel. As a method for
extracting precipitated N, i.e., as a method for dissolving
ferrite, an acidolysis, a halogen process, or an electrolytic
process may be used. As a result of comparative study among these
methods for dissolving ferrite, the present inventors have found
that the electrolytic process is most superior. In the electrolytic
process, only ferrite can be stably dissolved without decomposing
significantly unstable precipitates, such as carbides and nitrides.
Accordingly, in the present invention, precipitated N is extracted
by dissolving ferrite using the electrolytic process. As an
electrolytic solution, an acetylacetone-based solution is used, and
electrolysis is performed at a constant potential. The residue
extracted by the electrolytic process is chemically analyzed to
find the N amount in the residue, which is defined as the amount of
precipitated N.
[0069] Additionally, in order to achieve large BH and .DELTA.TS,
the amount of dissolved N is preferably set at 0.0020% or more, and
in order to achieve larger BH and .DELTA.TS, the amount of
dissolved N is preferably set at 0.0030% or more.
[0070] N/Al (ratio between the N amount and the Al amount): 0.3 or
more
[0071] As described above, in order to keep 0.0010% or more of
dissolved N stably without being affected by the production
conditions, the amount of Al, which is an element for strongly
fixing N, must be limited to 0.02% or less. As a result of
searching for the conditions in which the amount of dissolved N
after hot rolling is 0.0010% or more with respect to steels in
which the combination of the N amount and the Al amount is widely
changed within the compositional range of the present invention, it
has been found that the ratio N/Al must be 0.3 or more.
Furthermore, cooling conditions and the coiling temperature
condition after finish-rolling must be set in the ranges described
below. Therefore, the Al amount is limited to N/0.3 or less.
[0072] Group a: 1.0% or less in total of at least one of Cu, Ni,
Cr, and Mo
[0073] Since all of the elements Cu, Ni, Cr, and Mo in Group a
contribute to an increase in the strength of steel sheets, they may
be added alone or in combination. However, if it is an excessive
amount, deformation resistance at elevated temperatures is
increased, chemical conversion properties and surface treatment
properties in a broad sense are degraded, formability at the
welding zone is degraded due to hardening of the welding zone, and
so on. Therefore, the total amount of Group a is preferably 1.0% or
less.
[0074] Group b: 0.1% or less in total of Nb, Ti, and V
[0075] Since all of the elements Nb, Ti, and V in Group b
contribute to refinement and uniformization of the grain size, they
may be added alone or in combination. However, if the amount is
excessive, deformation resistance at elevated temperatures is
increased, chemical conversion properties and surface treatment
properties in a broad sense, such as paintability, are degraded,
formability at the welding zone is degraded due to hardening of the
welding zone, and so on. Therefore, the total amount of Group b is
preferably 0.1% or less.
[0076] Group c: 0.0030% or less of B
[0077] The element B in Group c improve the hardenability of steel.
B is appropriately added to steel in order to increase the strength
of the steel by changing the structure phases other than ferrite to
low-temperature transformation phases. However, if the amount is
excessive, since B precipitates as BN, it is not possible to secure
the dissolved N. Therefore, the B content must be limited to
0.0030% or less.
[0078] Group d: 0.0010% to 0.010% in total of at least one of Ca
and REM
[0079] The elements Ca and REM in Group d control the shapes of
inclusions, and, in particular, when the stretch-flanging property
is required, they are added alone or in combination. In such a
case, if the total amount is less than 0.0010%, the control effect
is insufficient. On the other hand, if the total amount exceeds
0.010%, the occurrence of surface defects becomes conspicuous.
Therefore, the total amount of Group d to be added is preferably
set in the range of 0.0010% to 0.010%.
[0080] When Nb and V are added in the present invention,
preferably, 0.1% in total of at least one of more than 0.02% to
0.1% of Nb and more than 0.02% to 0.1% of V is contained.
[0081] Nb and V are important constituent elements in the present
invention. By adding appropriate amounts of Nb and V and by
controlling the production conditions as described below, it is
possible to form an appropriate amount of significantly fine
carbonitrides, and desired strength is ensured and the yield ratio
can be greatly increased. Thereby, fatigue resistance and impact
resistance are remarkably improved. Furthermore, the fine
carbonitrides of Nb and V improve the strain aging hardenability
and contribute to refinement and uniformization of the ferrite
grain size. If the content of Nb or V (i.e., the concentration of
the additive constituent in steel) is 0.02% or less, the effect
thereof is small, and therefore, the content of Nb or V is set at
more than 0.02%.
[0082] On the other hand, the content of Nb and V (total content
when both elements are added in combination) exceeding 0.1% gives
rise to problems; for example, (1) an increase in deformation
resistance at elevated temperatures, (2) degradation of chemical
conversion properties and surface treatment properties, such as
paintability, and (3) degradation of formability at the welding
zone due to hardening at the welding zone. Therefore, the content
of Nb and V (total content when both elements are added in
combination) is set at 0.1% or less.
[0083] Total amount of precipitated Nb and precipitated V: 0.015%
or more
[0084] Nb and V are precipitated as fine carbonitrides, thus
increasing strength and improving strain aging hardenability. If
the amount of Nb or V present as carbonitrides, or the total amount
of these when Nb and V are added in combination, is less than
0.015%, the strength increasing effect and the strain aging
hardenability improving effect are not exhibited sufficiently. In
the composition of steel of the present invention, since
substantially all the precipitation of Nb and V are precipitated as
carbonitrides, the amount of Nb and the amount of V present as
carbonitrides of Nb and V are determined by measuring the amount of
precipitated Nb and the amount of precipitated V, respectively.
Therefore, the total amount of precipitated Nb and precipitated V
is limited to 0.015% or more. Herein, in order to measure the
amount of precipitated Nb and the amount of precipitated V,
extraction is performed by the electrolysis process described
above, and the amount of Nb and the amount of V in the residue are
determined as precipitated Nb and precipitated V.
[0085] Next, the structure and mechanical properties of steel
sheets will be described.
[0086] Areal Rate of Ferrite Phase:
[0087] Steel sheets used for automobiles must have satisfactory
workability. In order to ensure ductility necessary as steel sheets
used for automobiles, the areal rate of the ferrite phase is
preferably 50% or more.
[0088] Additionally, when high strength is required, the areal rate
of the ferrite phase is set at less than 50%, and the areal rate of
the bainite phase or the martensite phase is set at 35% or more, or
the total areal rate thereof is set at 35% or more. By using such a
structural composition, the steel sheet having a tensile strength
of 780 Mpa or more, as steel sheet tensile characteristics, is
easily obtained. In such a case, the steel sheet is preferably
applied to a section in which an emphasis is placed on strength
rather than on ductility in the automotive application.
[0089] When satisfactory ductility is required, the areal rate of
the ferrite phase is preferably set at 70% or more, and when more
satisfactory ductility is required, the areal rate of the ferrite
phase is more preferably set at 80% or more. Herein, examples of
ferrite also include bainitic ferrite and acicular ferrite which do
not contain carbides, in addition to so-called ferrite (polygonal
ferrite).
[0090] Additionally, although phases other than the ferrite phase
are not specifically limited, in view of increasing strength, each
single phase of bainite, martensite, and retained austenite or a
mixed phase thereof is preferred.
[0091] Average Grain Size of Ferrite Phase: 10 .mu.m or less
[0092] In the present invention, the average grain size is
determined by the value which is larger when compared between the
value measured by mensuration according to ASTM based on a
photograph of the sectional structure and the nominal grain size
measured by an intercept method (for example, refer to "Thermal
Treatment" 24 (1984) 334 by Umemoto, et al.).
[0093] In the present invention, although dissolved N is secured in
the mother plate, according to the experiment and analysis results
by the present inventors, even if the amount of dissolved N is kept
at a certain level, if the average grain size of the ferrite phase
exceeds 10 .mu.m, variations in strain aging hardenability are
increased. Although the detailed mechanism for the above is
unknown, the segregation and precipitation of alloying elements in
the grain boundaries, and working and heat treatment applied
thereto are considered to be related to the variations. Independent
of the reasons, in order to stabilize strain aging hardenability,
the average grain size of the ferrite phase must be set at 10 .mu.m
or less. Additionally, in order to further improve and stabilize BH
and .DELTA.TS, the average grain size is preferably set at 8 .mu.m
or less.
[0094] When the martensite phase (M phase) is contained in the
structure in the present invention, the areal rate of the M phase
is preferably 5% or more. The M phase contained in the structure at
the areal rate of 5% or more is effective in the present invention.
Thereby, the steel sheet has satisfactory ductility in spite of
high strength and high BH and .DELTA.TS. If the areal rate of the M
phase is less than 5%, the effect thereof is not obtained
sufficiently. Due to the presence of the martensite phase at the
areal rate of 5% or more, in addition to the improvement in
ductility, the yield ratio =YS/TS is decreased, and the shape
fixability improving effect is remarkably exhibited particularly
when working is performed in the minute strain range.
[0095] In view of ductility and the low yield ratio, the areal rate
of the M phase is preferably less than 35%, and more preferably, 7%
to 20%. In such a case, in the steel sheet of the present
invention, in addition to ferrite and martensite, the bainite
phase, the pearlite phase, etc., may be contained in the structure
if the areal rate thereof is several percent.
[0096] On the other hand, in view of an increase in strength, the
areal rate of the M phase is preferably 35% or more, or the total
area rate of the M phase and the bainite phase is preferably 35% or
more. In such a case, the structure may contain the pearlite phase
and the retained austenite phase at the areal rate of several
percent, in addition to the ferrite, bainite, and martensite
phases.
[0097] In the present invention, when Nb and V are added, the
average grain size of the precipitate comprising Nb or V
carbonitrides is preferably 0.05 .mu.m or less. In order for the
carbonitrides of Nb or V to increase strength and to improve strain
aging hardenability, the carbonitrides must be precipitated finely.
If the average grain size of the carbonitrides is coarser than 0.05
.mu.m, the effects thereof are not exhibited. Therefore, the
average grain size of the carbonitrides is set at 0.05 .mu.m or
less.
[0098] Additionally, in order to measure the grain size of the
carbonitrides of Nb and V, at least 20 visual fields are observed
by a transmission electron microscope with a magnifying power of
100,000 using thin films. With respect to the precipitates
observed, carbonitrides of Nb and V are identified using an
energy-dispersive X-ray analyzer (EDX). The grain size is defined
as 1/2 of the sum of the determined breadth and length of the
carbonitride of Nb and V. The grain size is measured for all the
carbonitrides of Nb and V in the visual field, and the average of
the total sum is defined as the average grain size.
[0099] Tensile Strength (TS): 440 MPa or more
[0100] A steel sheet used for structural members of automobile
bodies must have a TS of 440 MPa or more. A steel sheet used for
structural members in which further strength is required must have
a TS of 540 MPa or more.
[0101] Strain Aging Hardenability
[0102] In the present invention, as described above, "having
superior strain aging hardenability" means to have the following
characteristics:
[0103] 1) when a steel sheet is subjected to predeformation with a
tensile strain of 5% and then aging treatment by retaining the
steel sheet at 170.degree. C. for 20 minutes, an increase in
deformation stress before and after the aging treatment
(hereinafter referred to as BH; BH=Yield stress after aging
treatment--Predeformation stress before aging treatment) is 80 MPa
or more; and
[0104] 2) an increase in tensile strength before and after strain
aging treatment (the predeformation +the aging treatment) (herein
after referred to as .DELTA.TS; .DELTA.TS Tensile strength after
aging treatment--Tensile strength before predeformation) is 40 MPa
or more.
[0105] Predeformation with a Tensile Strain of 5%
[0106] When strain aging hardenability is defined, a prestrain
(predeformation) is an important factor. The present inventors have
studied the influence of the prestrain on strain aging
hardenability, assuming the deformation mode applied to steel
sheets used for automobiles. As a result, it has been found that
(1) the deformation stress in the deformation mode described above
can be substantially integrated into a uniaxial stress (tensile
strain) except for extremely deep drawing; (2) in a real component,
the uniaxial stress generally exceeds 5%; and (3) component
strength (strength of a real component) well corresponds to the
strength obtained after strain aging treatment with a prestrain of
5% is performed. Based on the knowledge described above, the
predeformation for the strain aging treatment is defined as a
tensile strain of 5%.
[0107] Aging Treatment Conditions: (Heating Temperature)
170.degree. C..times.(Retention Time) 20 minutes
[0108] In the conventional paint baking treatment conditions,
170.degree. C..times.20 minutes is adopted as the standard.
Therefore, 170.degree. C..times.20 minutes is defined as the aging
treatment conditions. Additionally, when a strain of 5% or more is
applied to a steel sheet of the present invention containing a
large amount of dissolved N, hardening is performed by treatment at
a lower temperature. In other words, the aging conditions may be
set more widely. In general, in order to increase the amount of
hardening, retention at a higher temperature for a longer time is
advantageous as long as softening is prevented.
[0109] Specifically, in the steel sheet of the present invention,
the lower limit of the heating temperature in which hardening is
noticeable after predeformation is approximately 100.degree. C. On
the other hand, if the heating temperature exceeds 300.degree. C.,
hardening hits the peak, and if the heating temperature is
400.degree. C. or more, a tendency toward slightly softening
appears, and also thermal strain and temper color become
conspicuous. As for the retention time, hardening is satisfactorily
achieved if the retention time is set at approximately 30 seconds
at a heating temperature of approximately 200.degree. C. In order
to achieve the larger amount of hardening and stable hardening, the
retention time is preferably set at 60 seconds or more. However,
even if retention is performed for more than 20 minutes, no further
hardening is achieved, and production efficiency is reduced,
resulting in no practical benefits.
[0110] For the reasons described above, when the steel sheet of the
present invention is used, after working is performed, preferably,
the heating temperature is set at 100 to 300.degree. C. and the
retention time is set at 30 seconds to 20 minutes as the aging
treatment conditions. In the present invention, even under the
aging conditions of low-temperature heating and short-time
retention in which sufficient hardening is not achieved in the
conventional paint baking type steel sheet, a large amount of
hardening can be obtained. Additionally, the method for heating is
not specifically limited, and in addition to atmospheric heating
using a furnace which is employed for general paint baking,
induction heating, heating by non-oxidizing flame, laser beam, or
plasma, or the like may be preferably used.
[0111] H: 80 MPa or more, .DELTA.TS: 40 MPa or more
[0112] Automobile components must have strength which can cope with
complex stress loading from outside. Therefore, it is important for
the material steel sheet to have a strength characteristic in the
small strain range as well as a strength characteristic in the
large strain range. From this viewpoint, the present inventors have
limited BH to 80 MPa or more and TS to 40 MPa or more with respect
to the steel sheet of the present invention to be used as a
material for automobile components. More preferably, BH is set at
100 MPa or more and .DELTA.TS is set at 50 MPa or more. It is to be
understood that the above limitations define BH and .DELTA.TS under
the conditions of aging treatment of 170.degree. C..times.20
minutes after a prestrain of 5% is applied. BH and .DELTA.TS may be
increased also by setting the heating temperature higher and/or by
setting the retention time longer.
[0113] In the steel sheet of the present invention, even if
accelerated aging by heating (artificial heating) is not performed
after forming and working, only by leaving the steel sheet at room
temperature, an increase in strength corresponding to at least
approximately 40% of full aging is expected. Moreover, on the other
hand, in the state in which forming and working are not performed,
even if the steel sheet is left at room temperature for a long
time, aging degradation, i.e., a phenomenon in which YS increases
and El (elongation) decreases, does not occur, which is a superior
characteristic not observed in the known art.
[0114] When the thickness of the produced steel sheet exceeds 4.0
mm, the advantages of the present invention are lost because even
the conventional steel sheet having large deformation resistance at
elevated temperatures can be easily hot-rolled and because steel
sheets having a thickness of more than 4.0 mm are not substantially
used for automobiles. Therefore, the steel sheet of the present
invention preferably has a thickness of 4.0 mm or less.
[0115] A plated steel sheet obtained by electroplating or hot-dip
plating the steel sheet of the present invention also has TS, BH,
and .DELTA.TS which are substantially the same as those before
plating. As the type of plating, any one of electro-galvanizing,
hot-dip galvanizing, hot-dip galvannealing, electrotinning,
electrolytic chromium plating, and electrolytic nickel plating may
be preferably used.
[0116] Next, the method for producing the steel sheet of the
present invention will be described.
[0117] The steel sheet of the present invention is produced
basically by a hot-rolling process in which a steel slab having the
composition within the ranges of the present invention is heated,
the steel slab is rough-rolled to form a sheet bar, the sheet bar
is finish-rolled, and coiling is performed after cooling. Although
the slab is preferably formed by continuous casting in order to
avoid macroscopic segregation of constituents, the slab may be
formed by an ingot-making method, or a thin slab continuous casting
method. Instead of the ordinary process in which the produced slab
is cooled to room temperature and heating is performed again, an
energy-saving process, such as a process in which a hot slab
without cooling is inserted into a furnace or a direct rolling
process in which a produced slab is directly rolled after slight
retention of heat, may be used. In particular, in order to
efficiently secure N in the dissolved state, direct rolling is one
of the effective techniques.
[0118] Hot-rolling conditions are defined as follows.
[0119] Slab Heating Temperature: 1,000.degree. C. or more
[0120] In order to secure the initial amount of dissolved N and to
meet the target (0.0010% or more) of dissolved N in the product,
the slab heating temperature (hereinafter referred to as "SRT") is
set at 1,000.degree. C. or more. Additionally, in order to avoid an
increase in loss due to oxidation weight gain, the SRT is
preferably 1,280.degree. C. or less. Rough-rolling of the heated
slab may be performed in a known method.
[0121] After rough-rolling is performed, the sheet bar is subjected
to finish-rolling. In the present invention, finish-rolling is
preferably performed continuously by joining consecutive sheet bars
to each other between rough-rolling and finish-rolling. As the
joining means, fusion-pressure welding, laser beam welding,
electron beam welding, or the like may be appropriately used.
[0122] Thereby, the proportion of non-steady sections (front ends
and back ends of the processed member) in which the shape is easily
disturbed during finish-rolling and subsequent cooling is
decreased, and the stable rolling length (the continuous length
which can be rolled under the same conditions) and the stable
cooling length (the continuous length which can be cooled under
tension) are extended, and thereby accuracy of shape and dimension
and the yield of the product are improved. Lubrication-rolling,
which was difficult to perform due to stability in continuous
rolling and biting properties in the conventional single-shot
rolling for each sheet bar, can be easily performed to thin, wide
sheet bars, and the rolling force and the bearing stress are
reduced, resulting in an extension of the roller life.
[0123] In the present invention, preferably, at least one of a
sheet bar edge heater for heating a widthwise end of the sheet bar
and a sheet bar heater for heating a lengthwise end of the sheet
bar is used between the steps of rough-rolling and finish-rolling
so that the temperature profiles in the width direction and in the
lengthwise direction become uniform. Thereby, the variations in
material properties within the steel sheet can be further
decreased. A sheet bar edge heater or sheet bar heater of induction
heating type is preferably used.
[0124] First, the temperature variation in the width direction is
compensated for by the sheet bar edge heater. At this stage,
heating is preferably adjusted so that the temperature range in the
width direction at the finishing side in finish-rolling is within
approximately 20.degree. C., although it depends on the steel
composition, etc. Next, the temperature variation in the
longitudinal direction is compensated for by the sheet bar heater.
At this stage, heating is preferably adjusted so that the
temperature in the lengthwise end is higher than the temperature in
the center by approximately 20.degree. C.
[0125] Finishing Temperature in Finish-rolling: 800.degree. C. or
more
[0126] In finish-rolling, in order to adjust the texture of the
steel sheet uniformly and finely, the finishing temperature in
finish-rolling (hereinafter referred to as "FDT") is set at
800.degree. C. or more. If the FDT is less than 800.degree. C., the
finish-rolling temperature is too low and the texture becomes
nonuniform, and deformation textures partially remain, which may
result in various problems during press forming. Although the
remaining of such deformation textures may be avoided by
high-temperature coiling, if high-temperature coiling is performed,
coarse grains are generated and strength is decreased, and also the
amount of dissolved N is also greatly decreased. Therefore, it
becomes difficult to obtain a target TS of 440 MPa. Additionally,
in order to further improve the mechanical properties, the FDT is
preferably set at 820.degree. C. or more.
[0127] In finish-rolling, to perform lubrication-rolling to reduce
the load during hot-rolling is effective in uniformizing the shape
and material properties. In such a case, the coefficient of
friction is preferably in the range of 0.25 to 0.10, and it is
desirable that the lubrication-rolling be performed in combination
with the continuous rolling in view of the operational stability in
hot-rolling.
[0128] Cooling after Rolling: Water-cooling at a cooling rate of
20.degree. C./s or more started within 0.5 second after rolling
[0129] After rolling is completed, cooling is started immediately
(within approximately 0.5 second), and the cooling must be
performed rapidly at an average cooling rate of 20.degree. C./s or
more. If these conditions are not satisfied, since grains grow
excessively, refinement of the grain size is not achieved, and
also, since AlN precipitates excessively due to strain energy
introduced by rolling, the amount of dissolved N becomes
insufficient. Additionally, in order to ensure uniformity in the
material properties and shape, the average cooling rate is
preferably set at 300.degree. C./s or less.
[0130] In the present invention, with respect to the cooling
pattern when the M phase is contained in the structure at the areal
rate of 5% or more, cooling may be performed continuously as is
usually done, or in order to control the .gamma. to .alpha.
transformation during cooling and to achieve the phase separation
in the structure advantageously, it is also effective to perform
slow cooling (interruption of rapid cooling) for approximately 1 to
5 seconds at a rate of 10.degree. C./s or less in the temperature
range of 700 to 800.degree. C. However, after the slow cooling,
rapid cooling must be performed again at a rate of 20.degree. C./s
or more.
[0131] Coiling Temperature: 650.degree. C. or less
[0132] As the coiling temperature (hereinafter referred to as "CT")
decreases, the strength of the steel sheet increases, and in order
to achieve the target TS of 440 MPa or more at CT 650.degree. C. or
less, the CT is set at 650.degree. C. or less. Additionally, if the
CT is less than 200.degree. C., the shape of the steel sheet is
easily disturbed and problems may arise in practical use, and
therefore, CT is preferably 200.degree. C. or more. In view of
material uniformity, CT is preferably 300.degree. C. or more, and
more preferably, more than 450.degree. C.
[0133] In the present invention, when the M phase is contained in
the structure at the areal rate of 5% or more, the coiling
temperature is preferably set at 450.degree. C. or less. The
strength of the steel sheet increases as the coiling temperature
decreases. At a CT of 450.degree. C. or less, the texture is
refined and the areal rate of the M phase reaches 5% or more, and
thereby the target TS of 440 MPa or more is achieved. Therefore,
the CT is set at 450.degree. C. or less. Furthermore, in order to
obtain the M phase stably, 40.degree. C./s or more is preferable.
Additionally, if the CT is less than 100.degree. C., the shape of
the steel sheet is easily disturbed and the possibility of causing
problems in practical use increases. Therefore, the CT is
preferably 100.degree. C. or more. In view of material uniformity,
the CT is preferably 150.degree. C. or more.
[0134] In the present invention, when Nb and V are contained, the
coiling temperature is preferably set at 550 to 650.degree. C. In
such a case, if the coiling temperature is higher than 650.degree.
C., since carbonitrides of Nb and V are coarsened, it becomes
difficult to adjust the grain size thereof to 0.05 .mu.m or less
and the strength of the steel sheet is also decreased. If the CT is
lower than 550.degree. C., since precipitation of carbonitrides of
Nb and V is suppressed, the predetermined amount of carbonitrides
cannot be secured. Therefore, the CT is set at 550 to 650.degree.
C.
[0135] Furthermore, in the present invention, preferably, working
(working after hot-rolling) is performed by at least one of skin
pass rolling and leveling with an elongation of 1.5% to 10% after
coiling is performed. Additionally, the elongation of skin pass
rolling is equal to the reduction rate of skin pass rolling.
[0136] Skin pass rolling and leveling are usually performed to
adjust roughness and to correct shape. In the present invention, in
addition thereto, skin pass rolling and leveling are effective in
increasing and stabilizing the BH and .DELTA.TS. Such an effect is
remarkably caused at an elongation of 1.5% or more. However, if the
elongation exceeds 10%, ductility is decreased. Therefore, working
after hot-rolling is preferably performed with an elongation of
1.5% to 10%. Additionally, although the working mode is different
between skin pass rolling and leveling (the former is rolling and
the latter is repeated bending and stretching), the effects of the
elongation on the strain aging hardenability of the steel sheet of
the present invention in both workings are substantially the same.
In the present invention, acid pickling may be performed before or
after the working after hot-rolling.
EXAMPLE 1
[0137] Each of the steels having the compositions shown in Table 1
was melted in a converter, and a slab was formed by continuous
casting. The slab was hot-rolled under the conditions shown in
Table 2 to produce a hot-rolled steel sheet. In finish-rolling,
sheet bars were not joined to each other and tandem rolling was
performed for the individual sheet bars. With respect to the
resultant hot-rolled steel sheet, the dissolved N, the
microstructure, the tensile characteristics, the strain aging
hardenability, and improvements in fatigue resistance and impact
resistance due to strain aging treatment were investigated.
[0138] The amount of dissolved N was measured by the method
described above.
[0139] In order to observe the microstructure, with respect to the
C cross section (the cross section perpendicular to the rolling
direction) excluding the portions 10% from the surfaces in the
thickness direction, the enlarged image of the structure appearing
due to corrosion was analyzed.
[0140] The tensile tests for checking the tensile characteristics
and the strain aging hardenability were performed according to JIS
Z 2241 using JIS No. 5 test pieces.
[0141] The strain aging treatment was performed with a prestrain of
5% under the aging treatment conditions: 170.degree. C..times.20
minutes.
[0142] The fatigue resistance was evaluated by the fatigue limit
obtained by a tensile fatigue test according to JIS Z 2273.
[0143] The impact resistance was evaluated by the absorbed energy
found by integrating stress in the strain range of 0 to 30% with
respect to the stress-strain curve measured at a strain rate of
2,000/s according to a high-speed tensile test method described in
"Journal of the Society of Materials Science Japan.
47,10(1998)1058".
[0144] The results thereof are shown in Table 3. In the examples of
the present invention, significantly higher BH and .DELTA.TS were
observed compared to the comparative examples, and the improvements
in fatigue resistance and impact resistance due to the strain aging
treatment were larger compared to the comparative examples.
[0145] Additionally, the characteristics of plated steel sheets
obtained by hot-dip galvanizing the steel Nos. C and D were
substantially the same as those of the steel sheets before plating.
In order to perform plating treatment, the steel sheet was immersed
in a galvanizing bath and after the immersed steel sheet was
retrieved, the-areal weight was adjusted by gas-wiping. The plating
treatment was performed under the conditions of sheet temperature:
475.degree. C., plating bath: 0.13% Al-Zn, bath temperature:
475.degree. C., immersion time: 3 seconds, and areal weight: 45
g/m.sup.2.
EXAMPLE 2
[0146] The steel having the composition shown in Table 4 was cast
into a slab in the same manner as Example 1, and the slab was
hot-rolled under the conditions shown in Table 5. Thereby,
hot-rolled steel sheets (with a thickness of 1.6 mm) in which the
average cooling rates were greatly varied were obtained. In such a
case, when finish-rolling was performed, consecutive sheet bars
with a thickness of 25 mm were joined to each other by
fusion-pressure welding at the initial stand, and tandem rolling
was performed continuously. Between rough-rolling and
finish-rolling, the temperature of the sheet bar was adjusted using
a sheet bar edge heater and a sheet bar heater of induction heating
type. The resultant hot-rolled steel sheets were investigated in
the same manner as Example 1.
[0147] The results thereof are shown in Table 6. In all the steel
sheets, it is clear that the strain aging hardenability was stable
at a high level. In Example 2, due to the continuous rolling and
the temperature adjustment of the sheet bar, the thickness accuracy
and the shape were improved compared to Example 1. Furthermore,
since finish-rolling was continuously performed by joining
consecutive sheet bars to each other, the rolling conditions and
cooling conditions for one sheet bar were uniformly set in the
entire length in the longitudinal direction. As a result, stable
strain aging hardenability was confirmed over the entire length of
the steel sheet.
EXAMPLE 3
[0148] With respect to the steel sheet Nos. A, N, and J shown in
Table 3, the BH (increase in deformation stress) and the .DELTA.TS
(increase in tensile strength) were investigated with varied aging
treatment conditions. The results thereof are shown in FIGS. 1 and
2. In the examples of the present invention (A and N),
significantly greater hardening was observed compared to the
comparative example (J) in the low-temperature, short-time aging
treatment. Thereby, it is obvious that the steel sheet of the
present invention has superior strain aging hardenability. It is
also clear that the examples A and N of the present invention
exhibit superior strain aging hardenability under the strain aging
treatment conditions in the wide ranges of 100 to 300.degree.
C..times.30 seconds to 20 minutes.
EXAMPLE 4
[0149] Each of the steels having the compositions shown in Tables 7
and 8 was melted in a converter, and a slab was formed by
continuous casting. The slab was hot-rolled under the conditions
shown in Tables 9 and 10 to produce a hot-rolled steel sheet. With
respect to the resultant hot-rolled steel sheet, the dissolved N,
the microstructure, the tensile characteristics, strain aging
hardenability, and improvements in fatigue resistance and impact
resistance due to strain aging treatment were investigated.
[0150] The amount of dissolved N was measured by the method
described above.
[0151] In order to observe the microstructure, with respect to the
C cross section (the cross section perpendicular to the rolling
direction) in the center in the thickness direction, the enlarged
image of the structure appearing due to corrosion was analyzed.
[0152] The tensile tests for checking the tensile characteristics
and the strain aging hardenability were performed according to JIS
Z 2241 using JIS No. 5 test pieces.
[0153] The strain aging treatment was performed with a prestrain of
5% under the aging treatment conditions: 170.degree. C..times.20
minutes.
[0154] The fatigue resistance and the impact resistance were
evaluated in the same manner as Example 1.
[0155] The results thereof are shown in Tables 11 and 12. In the
examples of the present invention, significantly higher BH and
.DELTA.TS were observed compared to the comparative examples, and
the improvements in fatigue resistance and impact resistance due to
the strain aging treatment were larger compared to the comparative
examples.
[0156] Additionally, the characteristics of plated steel sheets
obtained by hot-dip galvanizing the steel Nos. C and D were
substantially the same as those of the steel sheets before plating.
In order to perform plating treatment, the steel sheet was immersed
in a galvanizing bath and after the immersed steel sheet was
retrieved, the areal weight was adjusted by gas-wiping. The plating
treatment was performed under the conditions of sheet temperature:
475.degree. C., plating bath: 0.13% Al-Zn, bath temperature:
475.degree. C., immersion time: 3 seconds, and areal weight 45
g/m.sup.2.
[0157] With respect to the steel sheet No. A (steel of the present
invention) and the steel sheet No. 0 (comparative steel) shown in
Tables 11 and 12, BH and .DELTA.TS were investigated with a
prestrain of 5% under the aging treatment conditions shown in Table
13. Table 13 also shows the results thereof.
[0158] As is obvious from Table 13, the steel No. A of the present
invention exhibits high values of BH and .DELTA.TS even under the
relatively low-temperature, short-time aging treatment conditions
of 100.degree. C..times.30 seconds.
EXAMPLE 5
[0159] Each of the steels having the compositions shown in Table 14
was melted in a converter, and a slab was formed by continuous
casting. The slab was hot-rolled under the conditions shown in
Table 15 to produce a hot-rolled steel sheet. In finish-rolling,
sheet bars were not joined to each other and tandem rolling was
performed for the individual sheet bars. With respect to the
resultant hot-rolled steel sheet, the dissolved N, the
microstructure, the tensile characteristics, the strain aging
hardenability, and improvements in fatigue resistance and impact
resistance due to strain aging treatment were investigated.
[0160] The amount of dissolved N, the amount of precipitated Nb*,
and the amount of precipitated V were measured by the methods
described above.
[0161] In order to observe the microstructure, with respect to the
C cross section (the cross section perpendicular to the rolling
direction) excluding the portions 10% from the surfaces in the
thickness direction, the enlarged image of the structure appearing
due to corrosion was analyzed. The average grain size of Nb and V
carbonitrides was obtained using a transmission electron microscope
and an energy-dispersive X-ray analyzer.
[0162] The tensile tests for checking the tensile characteristics
and the strain aging hardenability were performed according to JIS
Z 2241 using JIS No. 5 test pieces.
[0163] The strain aging treatment was performed with a prestrain of
5% under the aging treatment conditions: 170.degree. C..times.20
minutes.
[0164] The fatigue resistance and the impact resistance were
evaluated by the methods described in Example 1. Furthermore, in
order to evaluate the impact resistance and the fatigue resistance
relative to the strength level of the steel sheet (strain aged
steel), the ratio of absorbed energy En (MJ/) to the tensile
strength TS (MPa) of the strain aged steel, En/TS (MJ/(MPa)) and
the ratio of the fatigue limit .sigma.w (MPa) to the tensile
strength TS (MPa) of the strain aged steel, .sigma.w/TS were
obtained.
[0165] The results thereof are shown in Table 16. In the examples
of the present invention, the values of BH and .DELTA.TS are large,
and also high fatigue resistance and impact resistance are
exhibited. The values of En/TS and .sigma.w/TS are also large, and
superior fatigue resistance and impact resistance are exhibited
compared to the comparative steels having the same strength
level.
[0166] Additionally, the characteristics of a plated steel sheet
obtained by hot-dip galvanizing the steel sheet No. C1 were
substantially the same as those of the steel sheet before plating.
In order to perform plating treatment, the steel sheet was immersed
in a galvanizing bath and after the immersed steel sheet was
retrieved, the areal weight was adjusted by gas-wiping. The plating
treatment was performed under the conditions of sheet temperature:
475.degree. C., plating bath: 0.13% Al-Zn, bath temperature:
475.degree. C., immersion time: 3 seconds, and areal weight 45
g/m.sup.2.
INDUSTRIAL APPLICABILITY
[0167] With respect to the high tensile strength hot-rolled steel
sheet of the present invention, since dissolved N is appropriately
used, the strength of the mother plate with a TS of 440 MPa or more
is exhibited, and superior strain aging hardenability with a BH of
80 MPa or more and a .DELTA.TS of 40 MPa or more is exhibited after
strain aging treatment is performed. The same characteristics are
exhibited after plating is performed, and moreover, it is possible
to perform hot-rolling inexpensively without disturbing the shape.
The thickness of the steel sheet used for automotive components can
be decreased, for example, from approximately 2.0 mm to
approximately 1.6 mm, thus greatly contributing to lightening of
automobile bodies.
1TABLE 1 Steel C Si Mn P S Al N Others No. % % % % % % % N/Al % 1
0.07 0.25 1.80 0.015 0.003 0.012 0.0105 0.88 -- 2 0.05 0.50 1.60
0.008 0.002 0.008 0.0150 1.88 -- 3 0.08 0.15 2.00 0.010 0.002 0.011
0.0095 0.86 -- 4 0.05 0.35 1.75 0.005 0.002 0.011 0.0120 1.09
Mo:0.15 5 0.05 0.45 1.65 0.045 0.001 0.007 0.0123 1.76 -- 6 0.05
0.15 2.00 0.008 0.001 0.004 0.0140 3.50 Ti:0.015 7 0.03 0.15 2.00
0.008 0.001 0.011 0.0140 1.27 Nb:0.015,B:0.0008 8 0.05 0.15 1.55
0.004 0.003 0.011 0.0121 1.10 Ni:0.05 9 0.05 0.15 1.61 0.008 0.002
0.005 0.0118 2.36 Cu:0.10,Ni:0.05 10 0.07 0.25 1.80 0.015 0.003
0.004 0.0042 0.08 -- 11 0.05 0.15 1.80 0.007 0.002 0.004 0.0140
3.50 Cu:0.15 12 0.05 0.15 1.80 0.007 0.002 0.004 0.0145 3.63
V:0.015 13 0.05 0.15 1.77 0.007 0.002 0.004 0.0142 3.55
Cr:0.15,Ti:0.015 14 0.06 0.15 1.78 0.005 0.002 0.004 0.0141 3.53
Nb:0.015,V:0.015 15 0.04 0.15 1.82 0.004 0.002 0.004 0.0139 3.48
Ni:0.05,Ti:0.015 16 0.05 0.15 1.81 0.005 0.002 0.004 0.0141 3.53
Cu:0.10,B:0.003 17 0.05 0.15 1.80 0.007 0.002 0.004 0.0140 3.50
Ca:0.0015 18 0.04 0.15 1.78 0.007 0.002 0.004 0.0141 3.53
Cu:0.10,Ca:0.002 19 0.05 0.15 1.77 0.005 0.002 0.004 0.0140 3.53
Nb:0.020,REM:0.002 20 0.05 0.15 1.81 0.006 0.002 0.004 0.0140 3.50
B:0.0003 21 0.05 0.15 1.80 0.007 0.002 0.004 0.0140 3.50
B:0.0002,REM:0.002 22 0.04 0.15 1.79 0.007 0.002 0.004 0.0141 3.53
Cr:0.10,Nb:0.02 B:0.0003,Ca:0.0015 23 0.08 0.15 2.00 0.010 0.002
0.016 0.0050 0.31 -- (The balance being Fe and incidental
impurities)
[0168]
2TABLE 2 Steel Thick- Sheet Steel. SRT FDT ness .DELTA.t V CT No.
No. .degree. C. .degree. C. mm s .degree. C./s .degree. C. Others A
1 1,220 880 1.6 0.2 80 520 -- B 2 1,200 890 1.8 0.2 65 540 -- C 3
1,150 890 1.4 0.1 75 520 -- D 4 1,220 850 1.6 0.1 75 570 -- E 5
1,270 850 1.8 0.2 65 580 -- F 6 1,200 890 1.8 0.3 65 520 -- G 7
1,100 840 2.3 0.2 55 530 -- H 8 1,100 845 2.0 0.3 60 540 -- I 9
1,100 850 1.8 0.4 70 530 HCR J 10 1,100 880 1.8 0.3 70 530 -- K 1
1,130 840 1.8 1.5 70 540 -- L 1 1,220 850 1.8 0.3 70 680 -- M 1
1,220 880 1.8 0.2 70 600 -- N 1 1,220 890 1.8 0.2 70 250 LV O 1
1,230 880 1.4 0.2 73 420 SK P 11 1,200 890 1.8 0.3 65 530 -- Q 12
1,200 890 1.8 0.3 65 530 -- R 13 1,200 890 1.8 0.3 65 530 -- S 14
1,200 890 1.8 0.3 65 530 -- T 15 1,200 890 1.8 0.3 65 530 -- U 16
1,200 890 1.8 0.3 65 530 -- V 17 1,200 890 1.8 0.3 65 530 -- W 18
1,200 890 1.8 0.3 65 530 -- X 19 1,200 890 1.8 0.3 65 530 -- Y 20
1,200 890 1.8 0.3 65 530 -- Z 21 1,200 890 1.8 0.3 65 530 -- AA 22
1,200 890 1.8 0.3 65 530 -- AB 23 1,150 890 1.4 0.5 40 646 -- SRT:
Slab heating temperature FDT: Finishing temperature in
finish-rolling CT: coiling temperature .DELTA.t: Cooling delay time
V: Average cooling rate HCR: Hot slab (900.degree. C. or more) was
inserted into furnace. LV: Leveling after coiling (Elongation 1.5%)
SK: Skin pass rolling after coiling (Reduction rate 2.0%)
[0169]
3 TABLE 3 Remarks (PI: Steel sheet Strain Example of Dis- Steel
sheet tensile aging present solved N structure character- harden-
Fatigue invention Steel in steel Phase istics ability resis- Impact
CE: Sheet sheet compo- V.alpha. d YS TS El BH .DELTA.TS tance
resis- Comparative No. % sition % .mu.m MPa MPa % MPa MPa MPa tance
example) A 0.0071 F,P,B 85 8.2 351 474 38 113 55 95 1.18 PI B
0.0121 F,P,B 90 8.4 368 469 36 110 52 90 1.15 PI C 0.0060 F,B 85
7.9 355 512 35 115 61 97 1.19 PI D 0.0082 F,B 87 7.8 365 532 34 115
63 98 1.18 PI E 0.0112 F,P,B 92 8.1 338 485 37 108 55 94 1.16 PI F
0.0075 F,B 85 7.4 353 508 36 92 62 98 1.19 PI G 0.0088 F,B 83 5.9
411 610 31 112 74 101 1.19 PI H 0.0084 F,P 93 7.8 326 465 37 108 52
88 1.15 PI I 0.0102 F,B 88 8.3 331 475 38 105 55 89 1.13 PI J
0.0002 F,P,B 85 8.4 334 454 37 22 5 0 1.00 CE K 0.0008 F,P,B 90
10.8 332 434 38 32 15 20 1.01 CE L 0.0005 F,P 95 11.0 295 411 38 10
12 18 0.99 CE M 0.0065 R,P,B 86 8.3 348 468 38 110 50 93 1.13 PI N
0.0100 F,M 83 7.9 363 605 34 155 105 125 1.25 PI O 0.0105 F,M,B 86
7.6 355 481 37 118 63 112 1.20 PI P 0.0095 F,B 85 7.7 361 485 38
120 69 105 1.21 PI Q 0.0093 F,B 87 7.4 371 480 36 118 59 98 1.18 PI
R 0.0082 F,B,M 82 6.5 365 505 38 119 71 102 1.18 PI S 0.0075 F,B 82
6.3 381 485 37 119 69 103 1.20 PI T 0.0085 F,B 85 6.5 359 479 38
115 56 99 1.19 PI U 0.0072 F,B 84 7.2 358 480 38 115 57 98 1.18 PI
V 0.0098 F,B 85 8.1 355 475 39 102 65 101 1.19 PI W 0.0101 F,B 83
8.0 365 480 38 113 69 104 1.18 PI X 0.0095 F,B 81 5.9 480 510 36
119 75 102 1.19 PI Y 0.0120 F,B 85 7.1 355 475 39 115 59 99 1.19 PI
Z 0.0115 F,B 85 7.2 360 479 38 115 61 102 1.18 PI AA 0.0115 F,B 82
5.8 369 525 37 118 65 109 1.19 PI AB 0.0011 F,P,B 85 9.5 368 471 36
99 53 88 1.18 PI F: Ferrite P: Pearlite B: Bainite M: Martensite
V.alpha.: Areal rate of ferrite phase d: Average grain size of
ferrite phase Fatigue resistance = (Fatigue limit of strain aged
steel) - (Fatigue limit of steel as hot-rolled) Impact resistance =
(Absorbed energy of strain aged steel) / (Absorbed energy of steel
as hot-rolled)
[0170]
4 TABLE 4 Oth- Steel C Si Mn P S Al N ers No. % % % % % % % N/Al %
24 0.08 0.35 1.55 0.009 0.002 0.012 00135 1.11 -- (The balance
being Fe and incidental impurities)
[0171]
5TABLE 5 Steel Thick- Sheet Steel SRT FDT ness .DELTA.t V CT Oth-
Re- No. No. .degree. C. .degree. C. mm s .degree. C./s .degree. C.
ers marks AC 11 1,280 920 1.6 0.2 95 480 Con- PI tin- uous roll-
ing AD 11 1,220 890 1.6 0.2 65 520 Con- PI tin- uous roll- ing AE
11 1,180 925 1.6 0.1 100 520 Con- PI tin- uous roll- ing
[0172]
6 TABLE 6 Remarks (PI: Steel sheet Strain Example of Dis- Steel
sheet tensile aging present solved N structure character- harden-
Fatigue invention Steel in steel Phase istics ability resis- Impact
CE: Sheet sheet compo- V.alpha. d YS TS El BH .DELTA.TS tance
resis- Comparative No. % sition % .mu.m MPa MPa % MPa MPa MPa tance
example) AC 0.0095 F,P,B 88 8.1 351 474 38 115 58 95 1.19 PI AD
0.0092 F,P,B 89 8.3 368 469 37 110 52 90 1.15 PI AE 0.0088 F,P,B 85
7.6 364 495 37 115 65 100 1.18 PI
[0173]
7TABLE 7 Steel C Si Mn P S Al N Others No. % % % % % % % N/Al % 1
0.07 0.25 1.80 0.015 0.003 0.012 0.0105 0.88 -- 2 0.05 0.50 1.60
0.008 0.002 0.008 0.0150 1.88 -- 3 0.08 0.15 2.00 0.010 0.002 0.011
0.0095 0.86 -- 4 0.05 0.35 1.75 0.005 0.002 0.011 0.0120 1.09
Mo:0.15 5 0.05 0.45 1.65 0.045 0.001 0.007 0.0123 1.76 -- 6 0.05
0.15 2.00 0.008 0.001 0.004 0.0140 3.50 Ti:0.015 7 0.03 0.15 2.00
0.008 0.001 0.011 0.0140 1.27 Nb:0.015,B:0.0008 8 0.05 0.15 1.55
0.004 0.003 0.011 0.0121 1.10 Ni:0.05 9 0.05 0.15 1.61 0.008 0.002
0.005 0.0118 2.36 Cu:0.10,Ni:0.05 10 0.07 0.25 1.80 0.015 0.003
0.055 0.0042 0.08 -- 11 0.08 0.35 1.55 0.009 0.002 0.012 0.0135
1.12 Mo:0.50 12 0.05 0.15 1.80 0.007 0.002 0.004 0.0140 3.50
Cu:0.15 (The balance being Fe and incidental impurities)
[0174]
8TABLE 8 Steel C Si Mn P S Al N Others No. % % % % % % % N/Al % 13
0.05 0.15 1.80 0.007 0.002 0.004 0.0145 3.63 V:0.015 14 0.05 0.15
1.77 0.007 0.002 0.004 0.0142 3.55 Cr:0.15,Ti:0.015 15 0.06 0.15
1.78 0.005 0.002 0.004 0.0141 3.53 Nb:0.015,V:0.015 16 0.04 0.15
1.82 0.004 0.002 0.004 0.0139 3.48 Ni:0.05,Ti:0.015 17 0.05 0.15
1.81 0.005 0.002 0.004 0.0141 3.53 Cu:0.10,B:0.0030 18 0.05 0.15
1.80 0.007 0.002 0.004 0.0140 3.50 Ca:0.0015 19 0.04 0.15 1.78
0.007 0.002 0.004 0.0141 3.53 Cu:0.10,Ca:0.0020 20 0.05 0.15 1.77
0.005 0.002 0.004 0.0140 3.53 Nb:0.020,REM:0.0020 21 0.05 0.15 1.81
0.006 0.002 0.004 0.0140 3.50 B:0.0003 22 0.05 0.15 1.80 0.007
0.002 0.004 0.0140 3.50 B:0.0002,REM:0.0020 23 0.04 0.15 1.79 0.007
0.002 0.004 0.0141 3.53 Cr:0.10,Nb:0.02 B:0.003,Ca:0.0015 24 0.08
0.15 2.00 0.010 0.002 0.016 0.0050 0.31 -- 25 0.06 0.15 2.65 0.015
0.002 0.012 0.0142 1.18 Nb:0.008,Ti:0.005 26 0.08 0.15 2.95 0.015
0.002 0.005 0.0180 3.60 -- 27 0.08 0.45 2.90 0.011 0.002 0.011
0.0175 1.59 Nb:0.038 (The balance being Fe and incidental
impurities)
[0175]
9TABLE 9 Steel Thick- Sheet Steel. SRT FDT ness .DELTA.t V CT No.
No. .degree. C. .degree. C. mm s .degree. C./s .degree. C. Others A
1 1,180 880 2.3 0.3 55 280 -- B 2 1,180 880 2.3 0.3 55 400 -- C 3
1,170 880 2.3 0.3 55 380 -- D 4 1,200 890 1.6 0.3 60 380 -- E 5
1,220 890 1.6 0.3 60 400 JCR F 6 1,200 890 1.6 0.3 60 325 -- G 7
1,220 870 1.6 0.3 60 280 -- H 8 1,270 870 1.6 0.3 60 250 -- I 9
1,250 850 1.8 0.2 60 320 HCR J 10 1,250 850 1.8 0.2 60 350 -- K 1
1,270 850 1.8 0.2 60 350 -- L 1 1,250 850 1.4 0.2 70 290 LV M 1
1,250 850 1.4 0.2 70 320 -- N 1 1,250 850 1.4 0.2 70 560 -- O 1 950
720 1.4 0.2 70 350 -- P 11 1,180 880 2.0 0.2 50 350 SK SRT: Slab
heating temperature FDT: Finishing temperature in finish-rolling
CT: coiling temperature .DELTA.t: Cooling delay time V: Average
cooling rate HCR: Hot slab (900.degree. C. or more) was inserted
into furnace. JCR: Sheet bar joining and continuous rolling LV:
Leveling after coiling (Elongation 2%) SK: Skin pass rolling after
coiling (Reduction rate 1.0%)
[0176]
10TABLE 10 Steel Thick- Sheet Steel SRT FDT ness .DELTA.t V CT No.
No. .degree. C. .degree. C. mm s .degree. C./s .degree. C. Others Q
11 1,180 880 2.0 2.0 55 360 -- R 11 1,180 880 2.0 0.2 10 350 -- S
12 1,200 885 1.6 0.3 55 250 -- T 13 1,220 890 1.6 0.3 60 350 -- U
14 1,220 900 1.6 0.2 55 300 -- V 15 1,220 885 1.6 0.3 55 300 -- W
16 1,200 895 1.6 0.3 55 300 -- X 17 1,200 890 1.6 0.3 55 280 -- Y
18 1,220 900 1.6 0.3 60 250 -- Z 19 1,200 905 1.6 0.3 55 280 -- AA
20 1,220 910 1.6 0.3 50 250 -- AB 21 1,180 910 1.6 0.2 55 250 -- AC
22 1,180 910 1.6 0.3 60 280 -- AD 23 1,200 900 1.6 0.2 65 250 -- AE
24 1,210 890 1.6 0.4 40 320 -- AF 25 1,170 870 1.6 0.4 45 380 -- AG
26 1,200 890 1.6 0.4 85 400 -- AH 27 1,250 910 1.6 0.3 65 420 --
SRT: Slab heating temperature FDT: Finishing temperature in
finish-rolling CT: coiling temperature .DELTA.t: Cooling delay time
V: Average cooling rate HCR: Hot slab (900.degree. C. or more) was
inserted into furnace. JCR: Sheet bar joining and continuous
rolling LV: Leveling after coiling (Elongation 2%) SK: Skin pass
rolling after coiling (Reduction rate 1.0%)
[0177]
11 TABLE 11 Remarks (PI: Example of Dis- Strain aging present
solved N Steel sheet structure Steel sheet tensile harden- Fatigue
invention Steel in steel Phase characteristics ability resis-
Impact CE: Sheet sheet compo- V.alpha. d VM YS TS El BH .DELTA.TS
tance resis- Comparative No. % sition % .mu.m % MPa MPa YR % MPa
MPa MPa tance example) A 0.0080 F,M,B 81 6.9 17 403 620 0.65 32 151
85 125 1.29 PI B 0.0120 F,M,B 87 6.9 12 385 598 0.64 33 150 95 119
1.28 PI C 0.0072 F,M 79 5.7 21 415 645 0.64 30 165 90 118 1.28 PI D
0.0097 F,M 82 6.8 18 402 625 0.64 31 150 101 121 1.31 PI E 0.0105
F,M,B 86 6.8 12 395 605 0.65 31 150 92 115 1.28 PI F 0.0110 F,M 79
6.1 21 420 650 0.65 29 161 90 122 1.27 PI G 0.0085 F,M 89 6.7 11
367 565 0.65 34 150 102 119 1.29 PI H 0.0095 F,M,B 86 6.8 12 370
570 0.65 33 151 88 125 1.28 PI I 0.0085 F,M,B 85 6.6 14 391 605
0.65 32 155 105 115 1.31 PI J 0.0008 F,M,B 81 6.9 13 385 595 0.65
28 75 42 45 1.10 CE K 0.0085 F,M,B 82 6.9 16 401 620 0.65 31 159 87
115 1.27 PI L 0.0087 F,M 83 6.6 17 420 630 0.67 31 160 85 120 1.28
PI M 0.0087 F,M 83 6.6 17 405 620 0.65 32 150 92 115 1.29 PI N
0.0085 F,P,B 90 8.0 0 415 530 0.78 29 72 15 51 1.09 CE O 0.0045
F,B,M 97 10.9 3 395 505 0.78 34 40 10 57 1.08 CE P 0.0082 F,M 85
6.8 15 342 598 0.57 32 145 88 115 1.27 PI F: Ferrite, P: Pearlite,
B: Bainite, M: Martensite V.alpha.: Areal rate of ferrite phase, d:
Average grain size of ferrite phase, VM: Areal rate of martensite
phase Fatigue resistance = (Fatigue limit of strain aged steel) -
(Fatigue limit of steel as hot-rolled) Impact resistance =
(Absorbed energy of strain aged steel) / (Absorbed energy of steel
as hot-rolled)
[0178]
12 TABLE 12 Remarks (PI: Example of Dis- Strain aging present
solved N Steel sheet structure Steel sheet tensile harden- Fatigue
invention Steel in steel Phase characteristics ability resis-
Impact CE: Sheet sheet compo- V.alpha. d VM YS TS El BH .DELTA.TS
tance resis- Comparative No. % sition % .mu.m % MPa MPa YR % MPa
MPa MPa tance example) Q 0.0042 F,P,B,M 95 10.5 3 392 520 0.78 33
70 15 53 1.08 CE R 0.0032 F,P,B,M 97 10.5 2 406 520 0.78 33 65 18
55 1.09 CE S 0.0115 F,M 82 6.7 18 404 628 0.64 31 152 102 122 1.30
PI T 0.0125 F,M,B 83 6.8 16 400 630 0.63 31 138 105 118 1.29 PI U
0.0110 F,M 82 6.6 18 415 640 0.67 31 152 105 120 1.31 PI V 0.0120
F,M 84 5.9 16 410 645 0.63 30 155 105 125 1.30 PI W 0.0105 F,M 84
6.4 16 395 625 0.63 31 145 102 120 1.28 PI X 0.0105 F,M 83 6.4 17
390 615 0.63 32 140 95 105 1.25 PI Y 0.0120 F,M 84 6.2 16 370 615
0.60 31 150 98 110 1.28 PI Z 0.0115 F,M 85 6.1 16 365 619 0.58 31
155 102 115 1.25 PI AA 0.0120 F,M 85 5.2 15 445 649 0.68 31 168 95
125 1.32 PI AB 0.0120 F,M 82 6.7 18 385 620 0.62 32 151 105 115
1.28 PI AC 0.0110 F,M 83 6.8 17 380 620 0.61 32 145 105 110 1.25 PI
AD 0.0105 F,M 80 6.4 20 405 669 0.60 30 140 108 105 1.24 PI AE
0.0010 F,M,B 87 6.9 10 365 595 0.61 33 105 72 95 1.18 PI AF 0.0086
F,M 49 7.0 51 540 795 0.68 19 95 71 95 1.15 PI AG 0.0135 F,M,B 45
5.1 42 600 997 0.60 14 153 102 98 1.05 PI AH 0.0131 F,B,M 45 5.3 12
650 1,080 0.60 13 145 98 94 1.06 PI F: Ferrite, P: Pearlite, B:
Bainite, M: Martensite V.alpha.: Areal rate of ferrite phase, d:
Average grain size of ferrite phase, VM: Areal rate of martensite
phase Fatigue resistance = (Fatigue limit of strain aged steel) -
(Fatigue limit of steel as hot-rolled) Impact resistance =
(Absorbed energy of strain aged steel) / (Absorbed energy of steel
as hot-rolled)
[0179]
13TABLE 13 Aging treatment conditions A (Steel of present Heat-
Heat- invention) O (Comparative steel) treating treating BH
.DELTA.TS BH .DELTA.TS temperature time (MPa) (MPa) (MPa) (MPa)
100.degree. C. 30 sec 120 60 20 3 100.degree. C. 10 min 130 70 24 3
100.degree. C. 20 min 135 75 25 4 300.degree. C. 30 sec 140 65 30 5
300.degree. C. 10 min 155 70 35 5 300.degree. C. 20 min 160 70 40
10 170.degree. C. 20 min 151 85 40 10
[0180]
14TABLE 14 Steel C Si Mn P S Al N Nb V N/Al No. % % % % % % % % %
-- A 0.06 0.02 1.2 0.012 0.0030 0.015 0.015 0.2 -- 1.0 B 0.08 0.02
1.0 0.010 0.0050 0.015 0.015 0.040 -- 1.0 C 0.05 0.02 1.4 0.010
0.0040 0.012 0.015 0.070 -- 1.25 D 0.08 0.4 1.7 0.015 0.0040 0.015
0.015 0.050 -- 1.0 E 0.05 0.2 1.2 0.010 0.0050 0.011 0.015 0.010 --
1.36 F 0.04 0.1 1.3 0.012 0.0030 0.015 0.017 -- 0.15 1.13 G 0.08
0.02 1.4 0.015 0.0040 0.015 0.015 -- 0.05 1.0 H 0.06 0.7 0.9 0.010
0.0030 0.017 0.020 -- 0.08 1.18 I 0.08 0.8 1.8 0.007 0.0020 0.004
0.014 -- 0.010 3.5 J 0.05 0.1 1.2 0.010 0.0040 0.010 0.018 0.03
0.03 1.8 K 0.03 0.2 1.8 0.010 0.0030 0.012 0.0010 0.04 -- 0.08 L
0.06 0.01 1.5 0.015 0.0050 0.010 0.004 -- 0.05 0.4 (The balance
being Fe and incidental impurities)
[0181]
15 TABLE 15 Steel Thick- Steel sheet SRT FDT ness .DELTA.t V CT No.
No. .degree. C. .degree. C. mm s .degree. C./s .degree. C. A A1
1,220 820 1.6 0.2 50 600 B B1 1,250 850 1.8 0.1 50 550 B2 1,250 850
1.8 0.1 50 700 B3 1,250 850 1.8 0.1 50 450 B4 1,050 850 1.8 0.1 50
600 C C1 1,250 880 1.4 0.1 80 550 D D1 1,220 880 2.9 0.3 50 600 E
E1 1,220 850 1.8 0.2 50 600 F F1 1,250 850 1.6 0.2 60 640 G G1
1,220 850 1.4 0.1 100 550 G2 1,220 850 1.4 0.1 100 720 G3 1,220 850
1.4 0.1 100 450 G4 1,220 850 1.4 1.0 100 600 H H1 1,250 880 2.3 0.2
50 600 I I1 1,250 850 1.6 0.2 50 540 J J1 1,230 880 2.0 0.2 50 560
J2 1,250 880 2.0 0.2 10 640 K K1 1,250 880 1.8 0.1 60 580 L L1
1,250 850 1.6 0.3 50 600 SRT: Slab heating temperature FDT:
Finishing temperature in finish-rolling CT: coiling temperature
.DELTA.t: Cooling delay time V: Average cooling rate
[0182]
16 TABLE 16 Remarks (PI: Dis- Example of solved Steel Strain aging
present N in sheet Steel sheet structure Steel sheet Tensile
harden- Fatigue invention Steel steel Nb* + Phase characteristics
ability resis- Impact CE: Sheet sheet V* compo- V.alpha. d dp YS TS
El BH .DELTA.TS tance resis- Comparative No. % % sition % .mu.m
.mu.m MPa MPa % MPa MPa MPa tance En/TS .sigma..sub.w/TS example)
A1 0.0020 0.080 F + B 92 10.2 0.5 405 581 26 82 35 35 1.02 0.29
0.82 CE B1 0.0120 0.032 F + B 90 6.8 0.03 515 624 28 88 46 103 1.19
0.33 1.05 PI B2 0.0009 0.038 F + B 97 12.4 0.19 402 583 29 32 8 18
1.04 0.28 0.80 CE B3 0.0140 0.008 F + B 78 6.2 0.02 467 649 24 91
42 106 1.22 0.30 0.96 CE B4 0.0015 0.031 F + B 95 9.8 0.8 410 592
25 81 40 88 1.13 0.29 0.98 CE C1 0.0142 0.057 F + B 92 6.8 0.03 515
617 27 84 44 105 1.18 0.34 1.03 PI D1 0.0090 0.041 F + P + B 82 5.9
0.02 652 804 19 87 42 95 1.15 0.33 1.07 PI E1 0.0092 0.008 F + B 94
6.5 0.03 390 566 30 88 42 99 1.16 0.31 0.90 CE F1 0.0030 0.071 F +
B 92 11.8 0.3 451 610 24 81 20 38 1.03 0.30 0.84 CE G1 0.0125 0.041
F + B 95 6.9 0.02 521 622 27 84 44 102 1.18 0.34 1.07 PI G2 0.0008
0.045 F + B 98 11.6 0.28 392 571 29 25 4 21 1.02 0.29 0.79 CE G3
0.0139 0.009 F + B 82 5.5 0.02 450 655 22 87 42 104 1.19 0.30 0.94
CE G4 0.0009 0.030 F + B 94 10.3 0.04 396 577 29 31 5 19 1.01 0.29
0.81 CE H1 0.0182 0.060 F + B 90 5.9 0.02 655 811 18 88 40 98 1.15
0.33 1.04 PI I1 0.0125 0.009 F + P + B 85 6.2 0.02 559 804 17 81 42
100 1.18 0.31 0.96 CE J1 0.0155 0.048 F + B 92 6.8 0.02 529 621 28
84 45 102 1.21 0.34 1.06 PI J2 0.0008 0.021 F + B 97 10.9 0.07 381
560 29 27 7 24 1.02 0.30 0.83 CE K1 0.0002 0.017 F + B 90 6.2 0.03
467 599 28 11 2 19 1.00 0.29 0.80 CE L1 0.0009 0.026 F + B 93 6.9
0.04 472 602 29 20 5 21 1.04 0.29 0.81 CE F: Ferrite, P: Pearlite,
B: Bainite V.alpha.: Areal rate of ferrite phase, d: Average grain
size of ferrite phase Fatigue resistance = (Fatigue limit of strain
aged steel) - (Fatigue limit of steel as hot-rolled) Impact
resistance = (Absorbed energy of strain aged steel) / (Absorbed
energy of steel as hot-rolled) En: Absorbed energy of strain aged
steel, .sigma..sub.w: Fatigue limit of strain aged steel Nb*:
Amount of Nb precipitated as Nb carbonitride V*: Amount of V
precipitated as V carbonitride dp: Average grain size of Nb
carbonitride or V carbonitride (Average grain size of both Nb
carbonitride and V carbonitride when Nb and V are added in
combination)
* * * * *