U.S. patent application number 12/927331 was filed with the patent office on 2011-08-11 for method for producing a galvanized steel sheet.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Takeshi Fujita, Takayuki Futatsuka, Hideyuki Kimura, Saiji Matsuoka, Yoshihiko Ono.
Application Number | 20110192504 12/927331 |
Document ID | / |
Family ID | 38256219 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110192504 |
Kind Code |
A1 |
Kimura; Hideyuki ; et
al. |
August 11, 2011 |
Method for producing a galvanized steel sheet
Abstract
A method for producing a galvanized steel sheet including (a)
melting a steel having a steel composition comprising 0.005 to 0.04
mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass %
or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less
than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass
%)+1.29 Cr (mass %) is 2.1 to 2.8, and the balance being iron and
unavoidable impurities, (b) hot rolling and cold rolling the steel
from step (a) to provide a steel sheet, and (c) annealing the steel
sheet from step (c) at an annealing temperature of at least the Ac1
point and not more than the Ac3 point, wherein the galvanized steel
sheet has a structure which includes a ferrite phase and a
martensite phase with a volume fraction being at least 3.0% and
less than 10%, the average particle diameter of the ferrite is
larger than 6 .mu.m and not more than 15 .mu.m, and 90% or more of
the martensite phase exists in a ferrite grain boundary.
Inventors: |
Kimura; Hideyuki; (Fukuyama,
JP) ; Ono; Yoshihiko; (Fukuyama, JP) ; Fujita;
Takeshi; (Fukuyama, JP) ; Futatsuka; Takayuki;
(Fukuyama, JP) ; Matsuoka; Saiji; (Tokyo,
JP) |
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
38256219 |
Appl. No.: |
12/927331 |
Filed: |
November 12, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12084173 |
Apr 25, 2008 |
|
|
|
PCT/JP2006/326320 |
Dec 25, 2006 |
|
|
|
12927331 |
|
|
|
|
Current U.S.
Class: |
148/546 |
Current CPC
Class: |
C23C 2/28 20130101; C22C
38/06 20130101; C22C 38/38 20130101; C21D 2211/005 20130101; C21D
9/46 20130101; C22C 38/001 20130101; C21D 8/0205 20130101; C23C
2/02 20130101; C22C 38/02 20130101 |
Class at
Publication: |
148/546 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2006 |
JP |
2006-003137 |
Dec 8, 2006 |
JP |
2006-331782 |
Claims
1. A method for producing a galvanized steel sheet comprising: (a)
melting a steel having a steel composition comprising 0.005 to 0.04
mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass %
or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less
than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass
%)+1.29 Cr (mass %) is in a range of 2.1 to 2.8, and the balance
being iron and unavoidable impurities, (b) hot rolling in,a hot
rolling step and cold rolling in a cold rolling step the steel from
step (a) to provide a steel sheet, and (c) annealing the steel
sheet from step (c) at an annealing temperature of at least the Ac1
point and not more than the Ac3 point, wherein the galvanized steel
sheet has a structure which consists of a ferrite phase and a
martensite phase with a volume fraction being at least 3.0% and
less than 10%, the average particle diameter of the ferrite is
larger than 6 .mu.m and not more than 15 .mu.m, and 90% or more of
the martensite phase exists in a ferrite grain boundary.
2. A method for producing a galvanized steel sheet comprising: (a)
melting a steel having a steel composition comprising 0.005 to 0.04
mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass %
or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less
than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass
%)+1.29 Cr (mass %) is in a range of 2.2 to 2.8, and the balance
being iron and unavoidable impurities, (b) hot rolling in a hot
rolling step and cold rolling in a cold rolling step the steel from
step (a) to provide a steel sheet, and (c) annealing the steel
sheet from step (c) at an annealing temperature of at least the Ac1
point and not more than the Ac3 point, wherein the galvanized steel
sheet has a structure which consists of a ferrite phase and a
martensite phase with a volume fraction being at least 3.0% and
less than 10%, the average particle diameter of the ferrite is
larger than 6 .mu.m and not more than 15 .mu.m, and 90% or more of
the martensite phase exists in a ferrite grain boundary.
3. A method for producing a galvanized steel sheet comprising: (a)
melting a steel having a steel composition comprising 0.005 to 0.04
mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass %
or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less
than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass
%)+1.29 Cr (mass %) is in a range of 2.3 to 2.8, and the balance
being iron and unavoidable impurities, (b) hot rolling in a hot
rolling step and cold rolling in a cold rolling step the steel from
step (a) to provide a steel sheet, and (c) annealing the steel
sheet from step (c) at an annealing temperature of at least the Ac1
point and not more than the Ac3 point, wherein the galvanized steel
sheet has a structure which consists of a ferrite phase and a
martensite phase with a volume fraction being at least 3.0% and
less than 10%, the average particle diameter of the ferrite is
larger than 6 .mu.m and not more than 15 .mu.m, and 90% or more of
the martensite phase exists in a ferrite grain boundary.
4. A method for producing a galvanized steel sheet comprising: (a)
melting a steel having a steel composition comprising 0.005 to 0.04
mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass %
or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less
than 0.008 mass % N and 0.35 to 0.8 mass % Cr, wherein Mn (mass
%)+1.29 Cr (mass %) is in a range of 2.3 to 2.8, and the balance
being iron and unavoidable impurities, (b) hot rolling in a hot
rolling step and cold rolling in a cold rolling step the steel from
step (a) to provide a steel sheet, and (c) annealing the steel
sheet from step (c) at an annealing temperature of at least the Ac1
point and not more than the Ac3 point, wherein the galvanized steel
sheet has a structure which consists of a ferrite phase and a
martensite phase with a volume fraction being at least 3.0% and
less than 10%, the average particle diameter of the ferrite is
larger than 6 .mu.m and not more than 15 .mu.m, and 90% or more of
the martensite phase exists in a ferrite grain boundary.
5. The method of producing a galvanized steel sheet according to
claim 1, wherein the steel composition further comprises one or
more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass %
or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb.
6. The method of producing a galvanized steel sheet according to
claim 2, wherein the steel composition further comprises one or
more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass %
or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb.
7. The method of producing a galvanized steel sheet according to
claim 3, wherein the steel composition further comprises one or
more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass %
or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb.
8. The method of producing a galvanized steel sheet according to
claim 4, wherein the steel composition further comprises one or
more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass %
or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb.
9. The method for producing a galvanized steel sheet according to
claim 1, wherein zinc which is used to plate the steel sheet is
alloyed.
10. The method for producing a galvanized steel sheet according to
claim 2, wherein zinc which is used to plate the steel sheet is
alloyed.
11. The method for producing a galvanized steel sheet according to
claim 3, wherein zinc which is used to plate the steel sheet is
alloyed.
12. The method for producing a galvanized steel sheet according to
claim 4, wherein zinc which is used to plate the steel sheet is
alloyed.
13. The method for producing a galvanized steel sheet according to
claim 1, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
14. The method for producing a galvanized steel sheet according to
claim 2, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
15. The method for producing a galvanized steel sheet according to
claim 3, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
16. The method for producing a galvanized steel sheet according to
claim 4, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
17. The method for producing a galvanized steel sheet according to
claim 5, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
18. The method for producing a galvanized steel sheet according to
claim 6, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
19. The method for producing a galvanized steel sheet according to
claim 7, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
20. The method for producing a galvanized steel sheet according to
claim 8, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
21. The method for producing a galvanized steel sheet according to
claim 9, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
22. The method for producing a galvanized steel sheet according to
claim 10, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
23. The method for producing a galvanized steel sheet according to
claim 11, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
24. The method for producing a galvanized steel sheet according to
claim 12, wherein a steel sheet after the hot rolling step has a
structure containing a low-temperature transformation phase at a
volume fraction of 60% or higher.
25. The method for producing a galvanized steel sheet according to
claim 1, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
26. The method for producing a galvanized steel sheet according to
claim 2, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
27. The method for producing a galvanized steel sheet according to
claim 3, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
28. The method for producing a galvanized steel sheet according to
claim 4, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
29. The method for producing a galvanized steel sheet according to
claim 5, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
30. The method for producing a galvanized steel sheet according to
claim 6, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
31. The method for producing a galvanized steel sheet according to
claim 7, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
32. The method for producing a galvanized steel sheet according to
claim 8, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
33. The method for producing a galvanized steel sheet according to
claim 13, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
34. The method for producing a galvanized steel sheet according to
claim 14, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
35. The method for producing a galvanized steel sheet according to
claim 15, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
36. The method for producing a galvanized steel sheet according to
claim 16, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
37. The method for producing a galvanized steel sheet according to
claim 17, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
38. The method for producing a galvanized steel sheet according to
claim 18, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
39. The method for producing a galvanized steel sheet according to
claim 19, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
40. The method for producing a galvanized steel sheet according to
claim 20, wherein zinc which is used to plate the steel sheet is
alloyed after galvanization.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional application of U.S.
application Ser. No. 12/084,173 filed Apr. 25, 2008, which is the
United States national phase application of International
application PCT/JP2006/326320 filed Dec. 25, 2006. The entire
contents of each of U.S. application Ser. No. 12/084,173 and
International application PCT/JP2006/326320 are hereby incorporated
by reference herein.
[0002] The present invention relates to galvanized steel sheets
that are applicable in fields including automobiles and home
appliances, have favorable press-formability and are-excellent in
terms of strength-ductility balance and bake-hardenability, as well
as methods for producing such galvanized steel sheets.
BACKGROUND ART
[0003] Recently, improvement in fuel efficiency of automobiles has
been demanded from the perspective of global environment
sustainability, and safety improvement of automobile bodies has
also been desired from the perspective of protecting persons on
board from accidental damage. To meet these demands, positive
research for weight reduction of automobile bodies along with
reinforcement thereof has been conducted. It is said that enhancing
the strength of materials of components is effective to meet these
demands, weight reduction of automobile bodies along with
reinforcement thereof. However, enhancement in the strength often
leads to deterioration in formability, and thus not only improved
strength but also excellent press-formability is necessary to
produce steel sheets for automobiles requiring complicated
forming.
[0004] Several approaches have thus been proposed to raise the
strength of steel sheets while maintaining processability thereof.
In a representative approach, large quantities of solid solution
strengthening elements, Si and P, are added into interstitial free
steel as a base material to achieve a tensile strength in the range
of 340 to 490 MPa. For instance, Patent Document 1 discloses an
example of methods for producing high-tensile stress steel sheets
with a tensile strength of 490 MPa grade by adding P into
Ti-containing extra-low-carbon steel.
[0005] Also investigated aiming high formability of steel sheets
along with high strength thereof are dual-phase steel sheets, which
are including a second hard phase, such as martensite or bainite,
in the structure of ferrite main. For example, Patent Document 2
discloses a method for producing a steel sheet, wherein the
structure of the steel sheet consists of ferrite and a second
phase, recovery of the processed structure of ferrite is delayed by
using a heating rate of at least 10.degree. C./s for heating from
500 to 700.degree. C. during heating to the annealing temperature,
fine particles of ferrite measuring 2 to 6 .mu.m in diameter are
used to finely disperse the second hard phase to act as the
starting points of fracture, and thereby the steel sheet acquires
favorable strength-ductility balance of approximately 17000 MPa*%,
the product of strength and ductility. Furthermore, Patent
Documents 3 and 4 disclose methods for producing a steel sheet,
wherein the structure of the steel sheet consists of ferrite and a
second phase containing martensite, the rate of cooling after
recrystallization is predetermined, the fraction of the second
phase and the content ratio of martensite in the second phase are
controlled, and thereby the steel sheet acquires a strength of 500
MPa or lower and favorable strength-ductility balance of
approximately 17000 MPa*%.
[0006] Moreover, being developed as steel sheets achieving
favorable press-formability along with post-forming high strength,
steel sheets with bake-hardenability (hereinafter sometimes
referred to as BH) are relatively soft and easily press-formed in
press-forming, and then can be hardened by BH process to improve
the strength as a component. These BH steel sheets are based on a
hardening technique utilizing strain aging that occurs in the
presence of C and N dispersed in steel. For example, Patent
Document 5 discloses a steel sheet wherein solid C of approximately
30 ppm is dispersed in ferrite structure to fix dislocations,
thereby enhancing bake-hardenability. Additionally, steel sheets
described in Patent Document 5 are usually used as outer panels for
automobiles. However, such steel sheets originally contain solid C
at a small amount and thus BH is approximately in the range of 30
to 50 MPa at most. Also, extra-low-carbon steel used as a base
material makes it difficult to improve the strength as a component
to 440 MPa or higher. In response to this, research has been
conducted on Dual Phase steel sheets wherein martensitic
transformation induces dislocations in the mother phase, ferrite,
and solid C dispersed in the ferrite fixes the dislocations,
thereby improving BH. For example, Patent Document 6 discloses a
method for producing a steel sheet, wherein steel contains Mn, Cr
and Mo so that the total content ratio thereof (Mn+1.29Cr+3.29Mo),
a index of BH, is in the range of 1.3 to 2.1%, the structure of the
steel sheet contains at least 70% in volume fraction of ferrite and
1 to 15% in volume fraction of martensite, and thereby the steel
sheet acquires a strength in the range of 440 to 640 MPa and BH
equal to or higher than 60 MPa.
[0007] Patent Document 1: Japanese Examined Patent Application
Publication No. S57-57945
[0008] Patent Document 2: Japanese Unexamined Patent Application
Publication No. 2002-235145
[0009] Patent Document 3: Japanese Unexamined Patent Application
Publication No. 2002-322537
[0010] Patent Document 4: Japanese Unexamined Patent Application
Publication No. 2001-207237
[0011] Patent Document 5: Japanese Unexamined Patent Application
Publication No. S59-31827
[0012] Patent Document 6: Japanese Unexamined Patent Application
Publication No. 2006-233294
DISCLOSURE OF INVENTION
[0013] However, the background arts described above have the
following problems.
[0014] For example, techniques described in Patent Documents 1 and
5 involve solid solution hardening as an indispensable
strengthening mechanism to enhance the strength. In the case of a
strength being equal to or higher than 440 MPa, large quantities of
Si and P should be added and thus issues deterioration of on the
surface characteristics, such as difficulties in alloying, red
scales or plating failures, are significant. It is therefore
difficult to apply these techniques to outer panels of automobiles
requiring stringent control of surface quality.
[0015] The technique described in Patent Document 2 uses ferrite
particles with an average diameter being in the range of 2 to 6
.mu.m, although reduction in the diameter of each ferrite particle
leads to decreases in n value and uniform elongation. So this
technique cannot be easily applied to outer panels of automobiles
mainly formed by stretch forming, such as doors and hoods. Patent
Documents 3 and 4 state that, in the techniques described therein,
the primary cooling rate used in the production process thereof for
cooling from the annealing temperature to the plating temperature
is set in the range of 1 to 10.degree. C./s so as to improve the
content ratio of martensite in the second phase, and preferably it
is set in the range of 1 to 3.degree. C./s so as to reduce the
volume fraction of the second phase to 10% or lower. However, for
example, in the example where a primary cooling rate of 3.degree.
C./s is used for cooling from the annealing temperature of
800.degree. C. to the plating temperature of 460.degree. C., it
takes approximately 113 seconds to complete the cooling step. This
may affect the productivity. Moreover, the inventors actually
cooled steel with Mn+1.3Cr of 2.15 at a primary cooling rate of
3.degree. C./s according to the examples described in Patent
Documents 3 and 4 (Sample 43, Examples, DESCRIPTION of Patent
Document 3; Sample 29, Examples, DESCRIPTION of Patent Document 4),
and evaluated the resulting microstructure. As a result, pearlitic
or bainitic transformation progressed during the cooling step and
it was difficult to achieve 90% or higher content ratio of
martensite in the second phase consistently. This result indicates
that steel sheets with excellent strength-ductility balance cannot
be easily obtained by using the components and production methods
described in Patent Document 3 or 4 because the ductility may be
decreased as the result of pearlite or bainite generation in the
second phase.
[0016] As for the techniques described in Patent Documents 2 to 4,
the inventors actually prepared 0.6 to 0.8 mmt GA materials for
panels according to the examples thereof and conducted a press test
of the materials at the door model. As a result, portions like the
vicinity of embossed areas, forming of which was rather difficult,
cracked. In response to this, representative characteristics of the
materials were measured and then TS was 443 MPa, El was 35.5%, and
TS.times.El was 15727 MPa*%, suggesting that the strength-ductility
balance was not so good. This may be due to the fact that the
thickness of steel sheets used in the examples described in Patent
Documents 2 to 4 was 1.2 mm and this large thickness probably
contributed to the favorable balance between strength and
ductility. Therefore, the inventors verified this assumption using
Formula (2) derived from Oliver formula represented by Formula (1)
(source: Puresu Seikei Nanni Handobukku (Handbook on Difficulties
in Press-forming) 2nd Ed., P. 458, Usukouhan Seikei Gijutsu Kai),
which is commonly used by those skilled in the art for evaluating
ductility of thin steel sheets with different thicknesses.
El=.lamda.( A/L).sup.m (1).
[0017] In Formula (1), .lamda. and m are material constants, and in
general, m for iron is 0.4. The parameter A represents the
cross-section area and L represents the gauge length.
El.sub.2/El.sub.1=(t.sub.2/t.sub.1).sup.0.2 (2)
[0018] In Formula (2), El.sub.1 and El.sub.2 represent the
elongation (%) where the sheet thickness is t.sub.1 (mm) and
t.sub.2 (mm), respectively.
[0019] In this verification, the sheet thickness was assumed to be
0.75 mm, which is the thickness often used in the application of
outer panels for automobiles, and the strength-ductility balance
was not so good in any of the examples tested. More specifically,
the example described in Patent Document 2 (Sample 35, Example,
DESCRIPTION) exhibited TS of 446 MPa, El of 35.7% and TS.times.El
of 15922 MPa*%, the example described in Patent Document 3 (Sample
43, Example, DESCRIPTION) exhibited TS of 441 MPa, El of 35.6% and
TS.times.El of 15700 MPa*%, and the example described in Patent
Document 4 (Sample 29', Example, DESCRIPTION) exhibited TS of 442
MPa, El of 35.5% and TS.times.El of 15691 MPa*%. In addition,
considering press-formability, steel sheets having TS.times.El
equal to or higher than 16000 MPa*% can be used in practical using
without any problems, and TS.times.El is preferably 16500 MPa*% and
more preferably 17000 MPa*%. Consequently, it is difficult to apply
the technique described in Patent Documents 2 to 4 to outer panels
of automobiles, such as doors and hoods.
[0020] Furthermore, in the technique described in Patent Document
6, a second cooling rate is conducted under the conditions where
the cooling rate is 100.degree. C./s or higher and the cooling stop
temperature is 200.degree. C. or lower, for the purpose of
controlling the martensite volume fraction and the quantity of
dispersed solid C in ferrite as well as ensuring high BH. However,
these cooling conditions can be satisfied only in an extraordinary
method like water jet described in Patent Document 6, so that, in
practice, the industrial manufacturing using this technique is
difficult. In addition, Patent Document 6 discusses only
formability with reference to the results of a cylinder forming
test, omitting descriptions of ductility-related parameters such as
total elongation, uniform elongation and local elongation.
Therefore, steel sheets obtained using this technique may be
insufficient in terms of the strength-ductility balance, and thus
cannot be easily applied to outer panels of automobiles, such as
doors and hoods.
[0021] The present invention was made to solve these problems and
provides a galvanized steel sheet having a tensile strength in the
range of 340 to 590 MPa, TS.times.El being equal to or higher than
16000 MPa*% considering press-formability, and the yield stress
difference between a value measured after the application of 2%
prestrain and a value measured after subsequent bake-hardening by
heating at 170.degree. C. for 20 minutes being equal to or higher
than 50 MPa, in other words, a galvanized steel sheet that has high
formability and is excellent in strength-ductility balance and
bake-hardenability, as well as a method for producing the same.
[0022] To solve the problems described above, the inventors focused
on a dual-phase steel consisting of a ferrite phase and a
martensite phase. As a result, the following findings were
obtained.
[0023] First, transformation strengthening is utilized as a
strengthening mechanism and the volume fraction of the martensite
phase is reduced as much as possible, and thereby the strength
range of 340 to 590 MPa, which was difficult to achieve using
interstitial free steel as a base material, is obtained.
[0024] Furthermore, the particle diameter of ferrite and the
position of the martensite phase are controlled so as to enhance
the deformability of ferrite, and thereby the uniform elongation is
improved.
[0025] Moreover, the second phase is uniformly dispersed to improve
local elongation, and thus a galvanized steel sheet having
excellent balance between strength and ductility can be
obtained.
[0026] Additionally, the content ratio of Mn and Cr, a index of
bake-hardenability, is appropriately controlled so as to obtain
high BH.
[0027] The present invention was made based on these findings, and
is summarized as follows.
[0028] [1] A galvanized steel sheet that contains C, Si, Mn, P, 5,
Al, N and Cr at content ratios in mass % of 0.005 to 0.04%, 1.5% or
lower, 1.0 to 2.0%, 0.10% or lower, 0.03% or lower, 0.01 to 0.1%,
less than 0.008% and 0.2 to 1.0%, respectively, with Mn (mass
%)+1.29Cr (mass %) being in the range of 2.1 to 2.8, and contains
iron and unavoidable impurities as the balance, wherein the
structure thereof consists of a ferrite phase and a martensite
phase with a volume fraction being at least 3.0% and less than 10%,
the average particle diameter of the ferrite is larger than 6 .mu.m
and not more than 15 .mu.m, and 90% or more of the martensite phase
exists in a ferrite grain boundary.
[0029] [2] A galvanized steel sheet that contains C, Si, Mn, P, S,
Al, N and Cr at content ratios in mass % of 0.005 to 0.04%, 1.5% or
lower, 1.0 to 2.0%, 0.10% or lower, 0.03% or lower, 0.01 to 0.1%,
less than 0.008% and 0.2 to 1.0%, respectively, with Mn (mass
%)+1.29Cr (mass %) being in the range of 2.2 to 2.8, and contains
iron and unavoidable impurities as the balance, wherein the
structure thereof consists of a ferrite phase and a martensite
phase with a volume fraction being at least 3.0% and less than 10%,
the average particle diameter of the ferrite is larger than 6 .mu.m
and not more than 15 .mu.m, and 90% or more of the martensite phase
exists in a ferrite grain boundary.
[0030] [3] A galvanized steel sheet that contains C, Si, Mn, P, S,
Al, N and Cr at content ratios in mass % of 0.005 to 0.04%, 1.5% or
lower, 1.0 to 2.0%, 0.10% or lower, 0.03% or lower, 0.01 to 0.1%,
less than 0.008% and 0.2 to 1.0%, respectively, with Mn (mass
%)+1.29Cr (mass %) being in the range of 2.3 to 2.8, and contains
iron and unavoidable impurities as the balance, wherein the
structure thereof consists of a ferrite phase and a martensite
phase with a volume fraction being at least 3.0% and less,than 10%,
the average particle diameter of the ferrite is larger than 6 .mu.m
and not more than 15 .mu.m, and 90% or more of the martensite phase
exists in a ferrite grain boundary.
[0031] [4] A galvanized steel sheet that contains C, Si, Mn, P, S,
Al, N and Cr at content ratios in mass % of 0.005 to 0.04%, 1.5% or
lower, 1.0 to 2.0%, 0.10% or lower, 0.03% or lower, 0.01 to 0.1%,
less than 0.008% and 0.35 to 0.8%, respectively, with Mn (mass
%)+1.29Cr (mass %) being in the range of 2.3 to 2.8, and contains
iron and unavoidable impurities as the balance, wherein the
structure thereof consists of a ferrite phase and a martensite
phase with a volume fraction being at least 3.0% and less than 10%,
the average particle diameter of the ferrite is larger than 6 .mu.m
and not more than 15 .mu.m, and 90% or more of the martensite phase
exists in a ferrite grain boundary.
[0032] [5] The galvanized steel sheet according to any one of [1]
to [4] described above, further containing one or more of No, V, Ti
and Nb at content ratios in mass % of 0.5% or lower, 0.5% or lower,
0.01% or lower, 0.1% or lower and 0.1% or lower, respectively.
[0033] [6] The galvanized steel sheet according to any one of [1]
to [5] described above, wherein zinc used to plate the steel sheet
is alloyed.
[0034] [7] A method for producing a galvanized steel sheet
including a step of melting steel having the chemical composition
described in any one of [1] to [5] above, subsequent hot and cold
rolling steps, and a step of annealing the obtained steel sheet at
an annealing temperature being at least the Ac1 point and not more
than the Ac3 point.
[0035] [8] A method for producing a galvanized steel sheet
including a cold rolling step for rolling a hot-rolled steel sheet
that has the chemical composition described in any one of [1] to
[5] above and further contains a low-temperature transformation
phase at a volume fraction of 60% or higher, and a step of
annealing the obtained steel sheet at an annealing temperature
being at least the Ac1 point and not more than the Ac3 point.
[0036] [9] The method for producing a galvanized steel sheet
according to [7] or [8] described above, wherein zinc used to plate
the steel sheet is alloyed after galvanization.
[0037] In addition, percentages representing components contained
in steel in this description are all mass percentages.
[0038] The present invention provides a galvanized steel sheet
excellent in strength-ductility balance and bake-hardenability by
appropriately controlling the content ratio of Mn and Cr, the
average particle diameter of ferrite, and the position,
distribution profile and volume fraction of a martensite phase.
Furthermore, galvanized steel sheets according to the present
invention have such excellent characteristics and are applicable in
fields of home appliance, steel sheets for automobiles and others,
thus being beneficial to industry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a diagram that shows the relationship between the
content of Mn and Cr and TS.times.El.
[0040] FIG. 2 is a diagram that shows the relationship between the
content ratio of Mn and Cr and the bake-hardenability (BH).
BEST MODE FOR CARRYING OUT THE INVENTION
[0041] The present invention is explained in detail below.
[0042] First, the reason why the chemical composition of steel is
limited as described above in the present invention is stated.
[0043] C: 0.005 to 0.04%
[0044] In the present invention, C is one of very important
elements and is highly effective in forming a martensite phase to
enhance the strength. However, a content of C exceeding 0.04% would
lead to significant deterioration in formability and decreases in
weldability. Therefore, the content of C should not exceed 0.04%.
On the other hand, the martensite phase is required to account for
at least a volume fraction needed to ensure the strength and high
BH, and therefore C should be contained to some extent.
Consequently, the content of C should be at least 0.005%, and
preferably higher than 0.010%. [0045] Si: 1.5% or lower
[0046] Si is an element effective in raising the strength and
consistently producing a composite structure. However, a content of
Si exceeding 1.5% would lead to significant deterioration in
surface characteristics and phosphatability. Therefore, the content
of Si should be 1.5% or lower, and preferably 1.0% or lower. [0047]
Mn: 1.0 to 2.0%
[0048] Mn is one of important elements used in the present
invention. Mn has a very important role in the formation of a
martensite phase and an ability to improve EH, and acts to prevent
slabs from cracking during a hot rolling step because of the grain
boundary-embrittling effect of S by fixing S contained in steel in
the form of MnS. Therefore, the content of Mn should be at least
1.0%. However, a content of Mn exceeding 2.0% would lead to
significant increases in the cost for slabs, and adding a large
quantity of Mn would promote the formation of band-shaped
structures, thereby deteriorating the formability. Therefore, the
content ratio of Mn should not exceed 2.0%. [0049] P: 0.10% or
lower
[0050] P is an element effective in raising the strength. However,
a content of P exceeding 0.10% would lead to decreases in the
alloying rate of a zinc coating layer, thereby causing insufficient
plating or a failure of plating, and resistance to secondary
working embrittlement of a steel sheet. Therefore, the content of P
should not exceed 0.1%. [0051] S: 0.03% or lower
[0052] S deteriorates the hot formability and raises the
susceptibility of slabs to cracks due to heating, and fine
precipitation of MnS that form when the content ratio of S exceeds
0.03% degrade the formability. Therefore, the content of S should
not exceed 0.03%. [0053] Al: 0.01 to 0.1%.
[0054] Al is a deoxidizing element having the effect of removing
inclusions in steel. However, Al contained at a content less than
0.01% cannot provide this effect consistently. On the other hand, a
content of Al exceeding 0.1% would result in the increased amount
of alumina inclusion clusters, which affect formability.
Consequently, the content of Al should be in the range of 0.01 to
0.1%. [0055] N: less than 0.008%
[0056] To improve the processability and aging characteristics, the
lower the content of N is better. A content ratio of N being equal
to or higher than 0.008% would result in the formation of an
excessive amount of nitrides, thereby degrading the ductility and
strength. Therefore, the content of N should be less than 0.008%.
[0057] Cr: 0.2 to 1.0%
[0058] Cr is one of important elements used in the present
invention. Cr is an element that improves BH and is added to form a
stable martensite phase. It improves BH more effectively than Mn
and helps a martensite phase exist in a grain boundary, and thus is
an element advantageous to the structure formation according to the
present invention. Furthermore, in the present invention, Cr is an
indispensable element since it strengthens solid solutions to only
a slight extent and is suitable for low-strength DP steel, and thus
is added at a content of 0.2% or more, preferably 0.35% or more,
and more preferably more than 0.5%, so as to achieve these
advantageous effects. However, a content of Cr exceeding 1.0% would
result in not only the saturation of such advantageous effects but
also deterioration the ductility due to the formation of carbides.
Consequently, the content ratio of Cr should be in the range of 0.2
to 1.0%, and preferably 0.35 to 0.8% to ensure the sufficient
strength and ductility. [0059] Content ratio of Mn and Cr: Mn (mass
%)+1.29Cr (mass %) in the range of 2.1 to 2.8
[0060] Mn and Cr are elements that improve BH, and it is extremely
important to control them to the optimum content ratios for the
formation of a martensite phase. A total content ratio of Mn and Cr
being less than 2.1% would result in difficulties in the formation
of a DP structure and make it impossible to achieve desired BH,
thereby leading to decrease in the strength as a component.
Furthermore, an increased yield ratio makes it difficult to carry
out a press-forming step and causes defective shape. Also, pearlite
and bainite would be likely to form in a cooling step following a
crystallization annealing step, thereby reducing BH. On the other
hand, a weighted total content ratio of Mn and Cr exceeding 2.8%
would result in not only the saturation of the advantageous effects
described above but also decreases in formability because of
residual martensite in ferrite particles increasing with the rise
of the martensite volume fraction. Moreover, increases in the yield
point associated with the rise of strength also reduce the
press-formability significantly, and cause the rise of
manufacturing cost by necessitating the addition of excessive
amounts of alloy elements. Consequently, the weighted content ratio
of Mn and Cr, Mn+1.29Cr, should be in the range of 2.1 to 2.8%. To
achieve high BH, the lower limit thereof is preferably 2.2%., and
more preferably 2.3%. Also, to ensure favorable formability, the
upper limit thereof is preferably 2.6%.
[0061] The above-mentioned essential elements provide steel
according to the present invention with desired characteristics,
but one or more of the following elements may be added in addition
to the above-mentioned essential elements, as needed:
[0062] Mo (0.5% or lower), V (0.5% or lower), B (0.01% or lower),
Ti (0.1% or lower) and Nb (0.1% or lower). Mo: 0.5% or lower, V:
0.5% or lower
[0063] Mo and V are elements that each improve BH, and may be added
to form a stable martensite phase. However, content of Mo and/or V
exceeding 0.5% each would reduce the ductility and increase the
cost. Therefore, the content of Mo and/or V should not exceed 0.5%
each, if applicable. [0064] B: 0.01% or lower
[0065] B is an element effective in improving BH, and may be added
to form a stable martensite phase. However, a content ratio of B
exceeding 0.01% would not provide an effect worth the cost.
Therefore, the content of B should not exceed 0.01%, if applicable.
[0066] Ti: 0.1% or lower, Nb: 0.1% or lower
[0067] Ti and Nb are elements that effectively improve the
deep-drawing characteristics by decreasing the quantities of
dispersed solid C and N through the formation of carbonitrides.
However, content of Ti and/or Nb exceeding 0.1% each would result
in the saturation of such an advantageous effect and the rise of
the recrystallization temperature for annealing, thereby
deteriorating the productivity. Therefore, the content of Ti and/or
Nb should not exceed 0.1% each, if applicable.
[0068] In addition, the chemical components excluding the
above-described elements are Fe and unavoidable impurities. As an
example of such unavoidable impurities, O forms nonmetal inclusions
affecting the product quality, so it is preferably removed so as to
account for a content of 0.003% or lower.
[0069] Next, the structure of the galvanized steel sheet according
to the present invention is described below.
[0070] The galvanized steel sheet according to the present
invention consists of a ferrite phase and a martensite phase with a
volume fraction being at least 3.0% and less than 10%, the average
particle diameter of the ferrite is larger than 6 .mu.m and not
more than 15 .mu.m, and 90% or more of the martensite phase exists
in a ferrite grain boundary. These are essential requirements of
the present invention and a structure satisfying these requirements
would provide a galvanized steel sheet with excellent
strength-ductility balance according to the present invention.
[0071] Volume fraction of the martensite phase: at least 3.0% and
less than 10%
[0072] A two-phase structure consisting of a ferrite phase and a
martensite phase with a volume fraction being at least 3.0% and
less than 10%.constitutes the galvanized steel sheet according to
the present invention. A volume fraction of the martensite phase
being 10% or higher would make a steel sheet for outer panels of
automobiles, an intended product of the present invention,
insufficient in the press-formability. Therefore, the volume
fraction of the martensite phase should not exceed 10% and, to
ensure sufficient formability, the volume fraction of the
martensite phase is preferably less than 8%. On the other hand, a
volume fraction of the martensite phase being less than 3.0% would
cause the mobile dislocation density, introduced with
transformation, to be insufficient, thereby decreasing BH and
reducing the dent resistance. Furthermore, it increases YP and
makes YPEl more likely to remain, hereby decreasing the
press-formability and the surface regularity of obtained panels,
respectively. Therefore, the volume fraction of the martensite
phase should be at least 3.0%.
[0073] In addition, the steel sheet according to the present
invention may contain a pearlite phase, a bainite phase, a residual
.gamma. phase and unavoidable carbides to the maximum extent of
approximately 3% besides the above-mentioned two phases, ferrite
and martensite phases. However, a pearlite or bainite phase formed
near the martensite phase would often provide the origins of voids
and promote the growth of such voids. Therefore, to ensure
sufficient formability, such a pearlite phase, a bainite phase, a
residual .gamma. phase and unavoidable carbides are contained
preferably at less than 1.5%, and more preferably at 1.0% or less.
[0074] Average particle .diameter of ferrite: larger than 6 .mu.m
and not more than 15 .mu.m
[0075] The smaller the particle diameters of crystals are, the more
reduced n value and uniform elongation contributing to the
stretch-formability are. In the case where the average particle
diameter of ferrite is 6 .mu.m or lower, the decrease in n value
and uniform elongation is more significant. However, an average
particle diameter of ferrite exceeding 15 .mu.m would cause the
surface roughness to be introduced during a press-forming step and
deteriorate the surface characteristics, and thus is not
recommended. Consequently, the average particle diameter of ferrite
should be larger than 6 .mu.m and not exceed 15 .mu.m. [0076]
Position of the martensite phase: 90% in the ferrite grain
boundary
[0077] The position of the martensite phase is a very important
factor of the present invention and is an essential requirement of
the advantageous effects of the present invention. A martensite
phase existing in a ferrite particle reduces the deformability of
the ferrite, and a percentage of such a martensite phase in a
ferrite particle being 10% or higher would make this tendency
stronger. Therefore, to achieve excellent strength-ductility
balance intended by the present invention, 90% or more of the
martensite phase should be in the ferrite grain boundary. In
addition, to further improve- the strength-ductility balance, it is
preferable that 95% or more of the martensite phase exists in the
ferrite grain boundary.
[0078] Next, manufacturing conditions for the galvanized steel
sheet according to the present invention, which is excellent in the
strength-ductility balance and BH, are explained.
[0079] The galvanized steel sheet according to the present
invention is produced by melting steel the content ratios of whose
chemical components are adjusted so as to fall within the ranges
described above, rolling the steel in hot and subsequent cold
rolling steps, and annealing the obtained steel sheet at an
annealing temperature being at least the Ac1 point and not more
than the Ac3 point. In the cold rolling step, the hot-rolled steel
sheet preferably contains a low-temperature transformation phase at
a volume fraction of 60% or higher.
[0080] Furthermore, it is more preferable that, during a
galvanization step following the annealing step, the galvanized
steel sheet according to the present invention is subjected to
recrystallization annealing at an annealing temperature being at
least the Ac1 point and not more than the Ac3 point, primary
cooling from the annealing temperature to a galvanization
temperature with an average cooling rate exceeding 3.degree. C./s
and being not more than 15.degree. C./s, and then secondary cooling
with an average cooling rate being not less than 5.degree. C./s.
The step of alloying the plating may be added after the
galvanization step. Such a process of galvanizing annealed steel
sheets can be carried out using a continuous galvanization
line.
[0081] Preferred conditions and manufacturing conditions of the
structure of the hot-rolled steel sheet are described in detail
below. [0082] Structure of a hot-rolled steel sheet:
low-temperature transformation phase having a volume fraction of
60% or more (preferred range)
[0083] In the above-mentioned process, the hot-rolled steel sheet
obtained in the hot rolling step preferably has a structure
containing a low-temperature transformation phase at a volume
fraction of 60% or higher. A known hot-rolled steel sheet having a
structure that consists of ferrite and pearlite phases would be
likely to hold insoluble carbides while .alpha.+.gamma. biphasic
regions are being annealed. This problem and uneven distribution of
the pearlite phase in the hot-rolled steel sheet result in uneven
distribution of large .gamma. phases. As a result, a structure
consisting of rather large and unevenly distributed martensite
phases is formed. On the other hand, in the case of a hot-rolled
steel sheet containing a low-temperature transformation phase at a
volume fraction of 60% or higher, such as the hot-rolled steel
sheet according to the present invention, fine carbides are
dissolved once in a ferrite phase during a heating stage of an
annealing step, and then uniform and fine .gamma. phases are
generated from the ferrite grain boundary while .alpha.+.gamma.
biphasic regions are being annealed. As a result, uniform
distribution of the martensite phase in the ferrite grain boundary,
which is intended by the present invention, is achieved and local
elongation is improved. In addition, such a low-temperature
transformation phase contained in the hot-rolled steel sheet is an
acicular ferrite phase, a bainitic ferrite phase, a bainite phase,
a martensite phase or a mixed phase thereof. Meanwhile, a
hot-rolled steel sheet containing a low-temperature transformation
phase at a volume fraction of 60% or higher can be obtained by
suppressing the transformation or growth of ferrite that occurs
after a finish rolling step. For example, it can be obtained by
cooling the steel sheet at a cooling rate of 50.degree. C./s or
higher after a finish rolling step to suppress the transformation
of ferrite and then taking up the steel sheet at a temperature of
600.degree. C. or lower. More preferably, the taking-up temperature
is less than 550.degree. C. [0084] Heating rate: less than
10.degree. C./s for the temperature range from the Ac1
transformation point, -50.degree. C., to the annealing temperature
(preferred range)
[0085] The heating rate for recrystallization annealing is not
particularly limited. However, to facilitate the production of the
steel sheet structure (with the preferred average particle diameter
of ferrite and the preferred position of the martensite phase)
intended by the present invention, it is preferable that
recrystallization is fully completed before the temperature exceeds
the Ac1 transformation point. Therefore, for example, the heating
rate for the temperature range from the Ac1 transformation point,
-50.degree. C., to the annealing temperature is preferably less
than 10.degree. C./s. In addition, at temperatures lower than this
temperature range, the heating rate does not always have to be
lower than 10.degree. C./s and may be much higher. Of course, a
hot-rolled steel sheet containing a low-temperature transformation
phase at a volume fraction of 60% or higher would provide the
structure according to the present invention more efficiently.
[0086] Annealing temperature: at least the Ac1 point and not More
than the Ac3 point
[0087] To obtain a microstructure consisting of ferrite and
martensite phases, the annealing temperature should be adequately
high. If an annealing temperature is less than the Ac1 point, no
austenite phase forms and accordingly no martensite phase forms. In
such a situation, the particle diameter of ferrite is so small that
the press-formability is reduced in association with decreases in n
value and uniform elongation. On the other hand, an annealing
temperature exceeding the Ac3 point would result in that the
ferrite phase is fully austenitized, thereby deteriorating
characteristics such as formability obtained by recrystallization.
The particle diameter of ferrite is so large in this situation that
surface characteristics are also worsened. Furthermore, C is
contained at a low content ratio in the steel according to the
present invention, so that annealing at a high temperature would
result in insufficient concentration of C in the .gamma. phase.
This makes it difficult to form a DP structure and accordingly
reduces the strength and EH. Furthermore, even if a DP structure is
formed by raising quenching characteristic to a sufficient level, a
large amount of martensite precipitates in the particles and thus
the ductility is deteriorated. Consequently, the annealing
temperature should be at least the Ac1 point and not exceed the Ac3
point. To ensure sufficient formability, the annealing temperature
is preferably at least the Ac1 point and not more than a
temperature 100.degree. C. higher than the Ac1 point. As for the
annealing time, to achieve a favorable average particle diameter of
ferrite and promote the concentration of component elements in an
austenite phase, the duration thereof is preferably at least 15
seconds and shorter than 60 seconds. In addition, the Ac1 and Ac3
points may be determined by actual measurement or calculated using
the following formula ("Leslie Tekkou Zairyou Gaku" (The Physical
Metallurgy of Steels), P. 273, MARUZEN Co., Ltd.): [0088]
Ac1=723-10.7Mn+29.1Si+16.9Cr [0089] Ac3=910-203C
0.5+44.7Si+104V+31.5Mo-30Mn-11Cr+700P+400Al+400Ti [0090] Primary
cooling rate: higher than 3.degree. C./s and not more than
15.degree. C./s (preferred range)
[0091] In the production process of the galvanized steel sheet, the
primary cooling rate for cooling from the annealing temperature to
the galvanization temperature is not particularly limited. However,
to form martensite, the average cooling rate is preferably higher
than 3.degree. C./s and not more than 15.degree. C./s. The cooling
rate exceeding 3.degree. C./s would prevent austenite from
transforming into pearlite in the cooling step, thereby helping a
martensite phase intended by the present invention form. This
improves the strength-ductility balance and BH. On the other hand,
the cooling rate is preferably 15.degree. C./s or lower because in
this range the steel sheet structure intended by the present
invention can be consistently formed extending in both lateral
direction and longitudinal direction (running direction) of a steel
sheet. Therefore, the average cooling rate for cooling from the
annealing temperature to the galvanization temperature is
preferably higher than 3.degree. C./s and not more than 15.degree.
C./s, and a more effective average cooling rate is in the range of
5 to 15.degree. C./s. In addition, the galvanization temperature is
in the normal range, i.e., approximately in the range of 400 to
480.degree. C. [0092] Secondary cooling rate: 5.degree. C./s or
higher (preferred range)
[0093] The secondary cooling rate after the galvanization step or
the additional step of alloying the plating layer is not
particularly limited. However, the cooling rate being 5.degree.
C./s or higher would prevent austenite from transforming into
pearlite or other phases, thereby helping a martensite phase form.
Therefore, the secondary cooling rate is preferably 5.degree. C./s
or higher. On the other hand, the upper limit of the second cooling
rate is not particularly limited as well, although it is preferably
less than 100.degree. C./s for such purposes as preventing the
deformation of the steel sheets. In addition, the plating layer is
alloyed by continuously heating it typically at a temperature
approximately in the range of 500 to 700.degree. C., and preferably
approximately in the range of 550 to 600.degree. C., for a few
seconds to several tens of seconds.
[0094] Conditions not described above are as follows. A method for
melting steel is not particularly limited, and examples of such a
method may include an electric furnace, a converter or the like.
Also, a method for casting molten steel may be continuous casting
to form cast slabs or ingot casting to form steel ingots.
Continuously cast slabs may be reheated using a heating furnace
before being hot-roiled or directly sent to the hot rolling step.
Steel ingots may be rough rolling before being hot-rolled. The
finish temperature of hot-rolling is preferably the Ar3 point or
higher. The cold-rolling ratio is in the range of 50 to 85% of the
value used in normal operations.
[0095] As for galvanization conditions, the plating weight is
preferably in the range of 20 to 70 g/m.sup.2, and Fe % in a
plating layer is preferably in the range of 6 to 15%.
[0096] In addition, the present invention may include the step of
temper-rolling steel sheets according to the present invention to
reform the steel sheets after a heat treatment step. Also, in the
present invention, it is intended that steel materials are
subjected to ordinary steelmaking, casting and hot-rolling steps to
produce steel sheets. However, the hot-rolling step may be partly
or completely omitted, for example, with the use of thin slab
casting.
[0097] Of course, electrogalvanization of steel sheets obtained in
the above-mentioned processes also provides the intended
advantageous effects. Such electrogalvanized steel sheets may be
coated with an organic layer thereafter.
EXAMPLES
[0098] The present invention is described in more detail below with
reference to examples.
[0099] Steels A to Y each having a distinct-chemical composition
listed in Table 1 were molten by vacuum melting and then shaped
into slabs by continuous casting. Steels A to S are examples of the
present invention. As comparative examples, each of Steels T and U
has the content of C deviating from its range according to the
present invention, each of Steels V, X and Y has the content ratio
of Mn and Cr deviating from its range according to the present
invention, and Steel W has the contents of Mn and Cr each deviating
from the range according to the present invention.
[0100] Each of the slabs obtained in the above-mentioned steps was
heated at 1200.degree. C., subjected to finish rolling at a
temperature equal to or higher than the Ar3 point, cooled in water,
and then taken up at a temperature exceeding 500.degree. C. and
being less than 650.degree. C. In this way, hot-rolled steel sheets
having volume fractions of a low-temperature transformation phase
varying in the range of 5 to 100% were produced.
[0101] Each of these hot-rolled steel sheets was pickled and then
subjected to cold rolling at a rolling ratio of 75%, so that
cold-rolled steel sheets each having a thickness of 0.75 mm were
obtained.
[0102] In an infrared furnace, samples cut out of these cold-rolled
steel sheets were each heated from the Ac1 transformation point,
-50.degree. C., to the annealing temperature at a heating rate in
the range of 5 to 20.degree. C./s as shown in Table 2, maintained
at the annealing temperature indicated in Table 2 for 30 seconds,
cooled at a primary cooling rate in the range of 3 to 20.degree.
C./s, and then galvanized in a plating bath adjusted to 460.degree.
C. Thereafter, the samples were each alloyed at 550.degree. C. for
15 seconds, and then cooled at a secondary cooling rate in the
range of 4 to 20.degree. C./s. In this way, alloyed galvanized
steel sheets were obtained.
[0103] Subsequently, samples were taken from these alloyed
galvanized steel sheets. These samples were evaluated for the
average particle diameter of ferrite, the volume fraction of a
martensite phase, the volume fraction of a second phase excluding
the martensite phase and the percentage of the martensite phase in
the grain boundary, and mechanical properties and BH thereof were
measured as performance characteristics.
[0104] Each sample was cut in the direction of thickness at the
middle thereof, and then, in accordance with the method described
in JIS G 0552, the average particle diameter of ferrite of each
sample was measured using an optical microscope image (with a
magnitude of 400) showing structure of the section.
[0105] The section of each cut sample was polished and corroded
with nital, and then the volume fraction of a martensite phase, the
volume fraction of a second phase excluding the martensite phase
and the percentage of the martensite phase in the grain boundary
were measured using an SEM (scanning electron microscope) image of
the microstructure of the section. It should be noted that, in
these measurement steps, fields within the central area of the
section, each having a size of 100 .mu.m in length and 200 .mu.m in
width, were continuously imaged with a magnitude of 2000 and then
the average values of the above-mentioned parameters were
calculated from the obtained images.
[0106] As mechanical properties, the YP (yield point), TS (tensile
strength), T-El (total elongation), U-El (uniform elongation) and
L-El (local elongation) of JIS-5 test pieces taken from the samples
were measured in a tensile test according to the test method
specified in JIS Z 2241.
[0107] BH of each sample was also measured using JIS-5 test pieces
taken from the samples in accordance with the method specified in
JIS G 3135, where the increase in the yield point was measured as
BH the tensile test performed after the application of 2% prestrain
and subsequent heating at 170.degree. C. for 20 minutes.
[0108] In the present invention, TS.times.El should be 16000 MPa*%
or higher, and it is preferably 16500 MPa*% or higher and more
preferably 17000 MPa*% or higher. On the other hand, BH should be
50 MPa or higher, and it is preferably 55 MPa or higher and more
preferably 60 MPa or higher. This lower limit of BH is the value
necessary to achieve the dent resistance required in the process of
making steel sheets for automobile outer panels thinner and
lighter.
[0109] The results of the above-mentioned tests and the
manufacturing conditions used are listed in Table 2.
[0110] In Table 2, Samples 1, 4, 5, 7 to 13, 15, 17 to 35, 37 and
38 are the examples of the present invention, each of which has the
chemical composition and the manufacturing conditions according to
the present invention, and has a structure where the volume
fraction of a martensite phase is at least 3.0% and less than 10%,
the average particle diameter of ferrite exceeds 6 .mu.m and is not
more than 15 .mu.m, and 90% or more of the martensite phase in the
ferrite grain boundary. These examples of the present invention
exhibited TS.times.El of at least 16000 MPa*% and BH of at least 50
MPa, thereby demonstrating that the obtained galvanized steel
sheets are excellent in the strength-ductility balance and BH.
[0111] On the other hand, as comparative examples, each of Samples
39 and 40 has the content of C deviating from its range according
to the present invention, each of Samples 41, 43 and 44 has the
content ratio of Mn and Cr deviating from its range according to
the present invention, and Sample 42 has the contents of Mn and Cr
each deviating from the range according to the present invention.
Also, each of the other comparative examples, Samples 2, 3, 6, 14,
16 and 36, was annealed at a temperature deviating from, the range
of annealing temperature according to the present invention, and in
these samples, at least one of the volume fraction of a martensite
phase, the average particle diameter of ferrite and the percentage
of the martensite phase in the ferrite grain boundary are out of
the corresponding range according to the present invention. Each
comparative example exhibited substandard TS.times.El and BH
values, and thus these comparative examples are considered
insufficient in the press-formability and difficult to make thinner
than existing steel sheets.
[0112] Furthermore, comparison between or among the examples of the
present invention having the same chemical composition and
different structures of the hot-rolled sheet, i.e., comparison
between Samples 1 and 4, 5 and 7, 10 and 11, and among Samples 25
to 27, suggested that Samples 1, 5, 7, 10, 25 and 26, in which the
content ratio of a low-temperature transformation phase in the
structure of the hot-rolled steel sheet is in the preferred range,
60% or higher, is better in terms of the strength-ductility balance
than Samples 4, 11 and 27. Moreover, under the same chemical
composition, comparison between Samples 5 and 9, and 10 and 12
heated at different heating rates, comparison between Samples 5 and
8, and 32 and 35 annealed at different temperatures, comparison
among Samples 32 to 34 cooled at different primary cooling rates,
and comparison among Samples 25, 28 and 29 cooled at different
secondary cooling rates were made. As a result, Samples 7 and 10
each heated at a heating rate in the preferred range, less than
10.degree. C./s, Samples 5 and 32 each annealed at a temperature in
the preferred range, not more than 100.degree. C. higher than the
Ac.sub.1 point, Sample 32 cooled at a primary cooling rate in the
preferred range, higher than 3.degree. C./s and not more than
15.degree. C./s, Samples 25 and 29 each cooled at a secondary
cooling rate in the preferred range, 5.degree. C./s or higher, were
better in terms of the strength-ductility balance than Samples 9,
12, 8, 35, 33, 34 and 28.
[0113] Excluding Samples 39 and 40 whose content of C deviates from
the range according to the present invention, FIG. 1 shows the
summary of relationship among the content ratios of Mn and Cr and
the TS.times.El values for Samples 1, 5, 10, 13, 15, 17 to 25, 30
to 32, 37,38 and 41 to 44 based on the results listed in Table 2.
These examples of the present invention and comparative examples
each have a low-temperature transformation phase in the structure
of the hot-rolled steel sheet at a percentage of 100% and contain
Mn and Cr at different content ratios, and the heating temperature,
annealing temperature, primary cooling rate and secondary cooling
rate of these samples were in the preferred ranges according to the
present invention. As seen in FIG. 1, TS.times.El was higher than
16000 MPa*% for all the examples of the present invention, and
higher than 16500 MPa*% for the examples under the preferred
conditions, i.e., examples containing Mn and Cr at a content ratio
in the range of 2.2 to 2.6%, confirming the favorable
strength-ductility balance. This drawing also shows that the
examples under the more preferred conditions, i.e., samples in
which the content of Cr was in the range of 0.35 to 0.8% and the
content ratio of Mn and Cr was in the range of 2.3 to 2.6%, had
TS.times.El being 17000 MPa*% or higher, thereby suggesting that
these conditions resulted in more favorable strength-ductility
balance than the other conditions.
[0114] FIG. 2 shows the summary of relationship between the content
ratio of Mn and Cr and the BH of the above-mentioned steel samples.
As is obvious in FIG. 2, BH was higher than 50 MPa in the examples
of the present invention under the condition where the content
ratio of Mn and Cr was 2.1% or higher, higher than 55 MPa in some
of the examples under the condition where the content ratio of Mn
and Cr was 2.2% or higher, and 60 MPa or higher in some of the
examples under the condition where the content ratio of Mn and Cr
was 2.3% or higher. This suggests that BH is favorable as well.
INDUSTRIAL APPLICABILITY
[0115] The galvanized steel sheets according to the present
invention are excellent in the strength-ductility balance and BH,
and thus can be used as components having high formability and are
suitably used in the production of inner and outer panels for
automobiles and other applications requiring high formability.
Furthermore, inner and outer panels for automobiles using the
galvanized steel sheets according to the present invention can be
made thinner and lighter than those using known steel sheets.
TABLE-US-00001 TABLE 1 Si Mn P S Sol. Al N Cr C mass (mass (mass
(mass (mass (mass (mass Others Mn + 1.29Cr Steel (mass %) ( %) %)
%) %) %) %) %) (mass %) (mass %) Remarks A 0.013 0.24 1.70 0.028
0.003 0.034 0.0036 0.40 -- 2.22 Composition according to the
present invention B 0.027 0.03 1.90 0.011 0.008 0.038 0.0020 0.60
-- 2.67 Composition according to the present invention C 0.025 0.02
1.80 0.016 0.006 0.034 0.0032 0.40 -- 2.32 Composition according to
the present invention D 0.018 0.01 2.00 0.001 0.011 0.029 0.0029
0.30 -- 2.39 Composition according to the present invention E 0.031
0.28 1.50 0.030 0.009 0.048 0.0022 0.50 -- 2.15 Composition
according to the present invention F 0.028 0.01 1.60 0.010 0.012
0.042 0.0029 0.80 -- 2.63 Composition according to the present
invention G 0.010 0.17 1.80 0.018 0.006 0.054 0.0055 0.25 -- 2.12
Composition according to the present invention H 0.029 0.05 1.90
0.065 0.009 0.021 0.0039 0.40 -- 2.42 Composition according to the
present invention I 0.023 0.03 1.80 0.010 0.006 0.034 0.0032 0.35
Mo: 0.2 2.25 Composition according to the present invention V: 0.1
J 0.025 0.05 1.80 0.018 0.011 0.029 0.0029 0.60 Ti: 0.02 2.57
Composition according to the present invention Nb: 0.03 K 0.028
0.09 1.65 0.022 0.009 0.048 0.0022 0.40 B: 0.002 2.17 Composition
according to the present invention L 0.019 0.01 1.65 0.031 0.012
0.042 0.0029 0.40 -- 2.17 Composition according to the present
invention M 0.022 0.03 1.65 0.018 0.006 0.054 0.0055 0.45 -- 2.23
Composition according to the present invention N 0.033 0.02 1.65
0.026 0.009 0.021 0.0039 0.65 -- 2.49 Composition according to the
present invention O 0.038 0.21 1.65 0.032 0.007 0.032 0.0033 0.70
-- 2.55 Composition according to the present invention P 0.021 0.06
1.50 0.035 0.009 0.033 0.0044 0.60 -- 2.27 Composition according to
the present invention Q 0.016 0.03 1.50 0.020 0.009 0.041 0.0048
0.65 -- 2.34 Composition according to the present invention R 0.016
0.08 1.50 0.011 0.015 0.035 0.0041 0.90 -- 2.66 Composition
according to the present invention S 0.033 0.01 1.40 0.018 0.008
0.033 0.0028 0.75 -- 2.37 Composition according to the present
invention T 0.002 0.02 1.60 0.020 0.005 0.0500 0.0040 0.60 -- 2.37
Comparative composition U 0.046 0.21 1.80 0.037 0.015 0.044 0.0032
0.40 -- 2.32 Comparative composition V 0.018 0.06 1.70 0.075 0.007
0.041 0.0013 0.20 -- 1.96 Comparative composition W 0.026 0.01 2.10
0.011 0.005 0.045 0.0038 0.10 -- 2.23 Comparative composition X
0.017 0.25 1.50 0.075 0.009 0.039 0.0038 0.30 -- 1.89 Comparative
composition Y 0.033 0.05 1.80 0.011 0.028 0.057 0.0034 0.90 -- 2.96
Comparative composition
TABLE-US-00002 TABLE 2 Average Content ratio of a low- Primary
Secondary particle Volume temperature transformation Heating Ac1
Ac3 Annealing cooling cooling diameter fraction of Sample phase in
the structure of the rate* point point temperature rate rate of
ferrite martensite No. Steel hot-rolled steel sheet (%) (.degree.
C./s) (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C./s)
(.degree. C./s) (.mu.m) (%) 1 A 100 5 719 875 770 5 8 8.7 3.5 2 A
100 5 719 875 680 5 8 5.7 0 3 A 100 5 719 875 890 5 8 15.4 1.6 4 A
50 5 719 875 770 5 8 8.3 3.7 5 B 100 5 714 837 770 5 8 8.0 8.8 6 B
100 5 714 837 840 5 8 9.2 2.6 7 B 70 5 714 837 770 5 8 8.4 8.3 8 B
100 5 714 837 820 5 8 8.8 9.0 9 B 100 20 714 837 770 5 8 7.8 8.7 10
C 100 5 711 845 770 5 8 7.3 6.4 11 C 5 5 711 845 770 5 8 7.9 6.6 12
C 100 20 711 845 770 5 8 6.8 6.5 13 D 100 5 707 832 770 5 8 8.2 5.0
14 D 100 5 707 832 860 5 8 10.9 1.9 15 E 100 5 724 876 770 5 8 7.7
7.1 16 E 100 5 724 876 700 5 8 8.3 0 17 F 100 5 720 843 770 5 8
10.5 7.4 18 G 100 5 713 875 770 5 8 8.5 3.9 19 H 100 5 711 870 770
5 8 7.4 7.6 20 I 100 5 711 860 740 5 8 7.5 4.7 21 J 100 5 715 858
750 5 8 6.9 7.4 22 K 100 5 715 869 800 5 8 13 4.2 23 L 100 5 712
867 790 5 8 12.3 4.0 24 M 100 5 714 861 780 5 8 11.3 4.6 25 N 100 5
717 844 780 5 8 10.9 6.7 26 N 80 5 717 844 800 5 8 11.5 6.9 27 N 30
5 717 844 790 5 8 10.4 6.5 28 N 100 5 717 844 760 5 4 10.2 6.3 29 N
100 5 717 844 800 5 20 11.3 7.0 30 O 100 5 723 858 810 5 8 11.7 7.2
31 P 100 5 719 869 770 5 8 7.5 4.9 32 Q 100 5 719 864 750 5 8 7.3
5.5 33 Q 100 5 719 864 750 3 8 7.3 5.4 34 Q 100 5 719 864 780 20 8
7.6 5.8 35 Q 100 5 719 864 850 5 8 7.8 5.8 36 Q 100 5 719 864 880 5
8 8.4 1.5 37 R 100 5 724 855 740 5 8 6.8 8.1 38 S 100 5 721 849 750
5 8 7.2 5.7 39 T 100 5 717 881 770 5 8 13.1 0 40 U 100 5 717 861
770 5 8 7.4 11.6 41 V 100 5 710 901 770 5 8 7.9 2.5 42 W 100 5 703
839 770 5 8 7.5 4.7 43 X 100 5 719 915 770 5 8 8.1 2.3 44 Y 100 5
720 842 770 5 8 7.8 10.7 Volume Percentage of fraction of a
martensite second phase crystallizing in Sample excluding the grain
YP TS T-El U-El L-El TS .times. El BH No. martensite (%) boundary
(%) (MPa) (MPa) (%) (%) (%) (MPa %) (MPa) Remarks 1 1.2 95 227 419
39.7 22.8 16.9 16634 57 Example of the present invention 2 0 -- 282
380 40.0 21.4 18.6 15200 22 Comparative Example 3 2.8 78 236 409
38.4 21.8 16.6 15720 34 Comparative Exampl 4 1.6 95 231 414 39.0
22.8 16.2 16146 54 Example of the present invention 5 0.5 94 267
507 32.1 20.0 12.1 16275 66 Example of the present invention 6 2.3
81 292 479 31.9 19.9 12.0 15280 41 Comparative Example 7 0.6 93 265
509 31.9 19.9 12.0 16237 67 Example of the present invention 8 0.7
94 270 510 31.4 19.4 12.0 16030 66 Example of the present invention
9 0.6 91 266 505 31.8 19.7 12.1 16050 64 Example of the present
invention 10 0.9 98 224 458 37.6 22 2 15.4 17221 61 Example of the
present invention 11 1.2 97 227 450 36.8 22.2 14.6 16560 61 Example
of the present invention 12 1.1 95 226 460 36.7 22.2 14.5 16900 60
Example of the present invention 13 1.2 97 239 466 35.5 21.5 14.0
16543 62 Example of the present invention 14 2.6 84 274 457 34.3
20.5 13.8 15675 38 Comparative Example 15 1.9 94 237 474 33.8 20.7
13 1 16021 51 Example of the present invention 16 0 -- 283 415 38.5
24.1 14.4 15978 23 Comparative Example 17 0.5 92 262 478 34.3 20.5
13.8 16395 66 Example of the present invention 18 1.6 91 226 422
38.5 22.3 16.2 16247 52 Example of the present invention 19 0.7 97
238 485 35.6 21.5 14.1 17266 62 Example of the present invention 20
1.1 93 245 444 38.0 21.8 16.2 16850 56 Example of the present
invention 21 0.6 98 248 478 35.8 22.3 13.5 17120 65 Example of the
present invention 22 1.7 92 211 438 37.3 23.1 14.2 16320 53 Example
of the present invention 23 1.6 93 216 435 37.7 23.3 14.4 16400 51
Example of the present invention 24 1.1 94 222 441 38.1 23.8 14.3
16840 56 Example of the present invention 25 0.7 98 255 469 37.3
22.8 14.5 17500 64 Example of the present invention 26 0.7 98 252
466 37.6 22.9 14.7 17540 62 Example of the present invention 27 1.1
98 258 471 35.7 22.2 13.5 16800 62 Example of the present invention
28 1.1 97 258 469 36.0 22.6 13.4 16900 63 Example of the present
inventionn 29 0.5 98 249 460 38.5 23.6 14.9 17700 65 Example of the
present invention 30 0.6 99 255 476 36.0 21.4 14.6 17100 63 Example
of the present invention 31 1.1 94 232 446 38.0 22.9 15.1 16950 57
Example of the present invention 32 0.8 97 228 453 37.8 23.5 14.2
17100 63 Example of the present invention 33 1.3 98 228 455 37.1
23.2 13.9 16120 61 Example of the present invention 34 0.5 94 224
449 37.6 23.3 14.3 16880 61 Example of the present inve thention 35
1.0 95 238 455 36.7 23.0 13.7 16680 62 Example of the present
invention 36 2.5 82 277 446 35.6 22.5 13.1 15860 45 Comparative
Example 37 0.5 93 260 487 33.3 19.6 13.6 16200 64 Example of the
present invention 38 0.8 98 232 456 37.7 22.8 14.9 17200 61 Example
of the present invention 39 -- -- 236 346 42.9 24.1 18.8 14843 10
Comparative Example 40 2.3 96 303 577 27.4 16.6 10.8 15810 63
Comparative Example 41 2.8 97 271 401 39.7 23.0 16.7 15920 45
Comparative Example 42 2.1 86 235 463 33.9 20.3 13.6 15696 55
Comparative Example 43 2.7 99 288 406 39.2 22.7 16.5 15915 39
Comparative Example 44 0.4 91 281 492 31.3 20.4 10.9 15400 63
Comparative Example *Heating rate for heating from Ac1
transformation point, -50.degree. C., to a constant annealing
temperature
* * * * *