U.S. patent application number 12/682801 was filed with the patent office on 2010-09-02 for high tensile strength galvanized steel sheet excellent in formability and method for manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Shinjiro Kaneko, Saiji Matsuoka, Tatsuya Nakagaito, Shusaku Takagi.
Application Number | 20100218857 12/682801 |
Document ID | / |
Family ID | 40579639 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100218857 |
Kind Code |
A1 |
Nakagaito; Tatsuya ; et
al. |
September 2, 2010 |
HIGH TENSILE STRENGTH GALVANIZED STEEL SHEET EXCELLENT IN
FORMABILITY AND METHOD FOR MANUFACTURING THE SAME
Abstract
A high tensile strength galvanized steel sheet with excellent
formability and anti-crush properties contains, in terms of % by
mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to 3.5% of Mn,
0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5% of Al,
0.007% or less of N, in addition, 0.01 to 0.2% in total of at least
one element selected from Ti, Nb, and V, the remainder being Fe and
unavoidable impurities, the steel sheet having a microstructure
composed of, in terms of area fraction, 20 to 87% of ferrite, 3 to
10% in total of martensite and residual austenite, and 10 to 60% of
tempered martensite, and a second phase composed of the martensite,
residual austenite, and tempered martensite having an average
crystal grain diameter of 3 .mu.m or less.
Inventors: |
Nakagaito; Tatsuya; (Tokyo,
JP) ; Takagi; Shusaku; (Tokyo, JP) ; Matsuoka;
Saiji; (Tokyo, JP) ; Kaneko; Shinjiro; (Tokyo,
JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER LLP (US)
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
40579639 |
Appl. No.: |
12/682801 |
Filed: |
October 23, 2008 |
PCT Filed: |
October 23, 2008 |
PCT NO: |
PCT/JP2008/069699 |
371 Date: |
April 13, 2010 |
Current U.S.
Class: |
148/533 ;
148/320; 148/330; 148/331; 148/332; 148/333; 148/336; 148/337 |
Current CPC
Class: |
C22C 38/12 20130101;
C21D 8/04 20130101; C21D 2211/009 20130101; C22C 38/28 20130101;
C22C 38/001 20130101; C22C 38/38 20130101; C21D 8/0405 20130101;
Y10T 428/12799 20150115; C21D 9/46 20130101; C21D 9/48 20130101;
C22C 38/06 20130101; C22C 38/14 20130101; C22C 38/08 20130101; C21D
2211/002 20130101; C22C 38/002 20130101; C21D 2211/005 20130101;
C22C 38/04 20130101; C22C 38/24 20130101; C21D 8/0263 20130101;
C21D 2211/001 20130101; C22C 38/18 20130101; C21D 2211/008
20130101; C22C 38/02 20130101; C22C 38/32 20130101; C22C 38/26
20130101; C21D 8/0426 20130101; C22C 38/16 20130101; C21D 8/0436
20130101; C23C 30/00 20130101; B32B 15/013 20130101; C22C 38/005
20130101; C21D 1/26 20130101; C23C 2/06 20130101 |
Class at
Publication: |
148/533 ;
148/320; 148/337; 148/333; 148/332; 148/336; 148/330; 148/331 |
International
Class: |
C21D 8/02 20060101
C21D008/02; B32B 15/18 20060101 B32B015/18; B32B 15/01 20060101
B32B015/01; C23C 2/28 20060101 C23C002/28 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 25, 2007 |
JP |
2007-277039 |
Oct 25, 2007 |
JP |
2007-277040 |
Claims
1. A high tensile strength galvanized steel sheet with excellent
formability, comprising, in terms of % by mass, 0.05 to 0.3% of C,
0.01 to 2.5% of Si, 0.5 to 3.5% of Mn, 0.003 to 0.100% of P, 0.02%
or less of S, 0.010 to 1.5% of Al, and 0.007% or less of N, the
remainder being Fe and unavoidable impurities, and having a
microstructure composed of, in terms of area fraction, 20 to 87% of
ferrite, 3 to 10% in total of martensite and residual austenite,
and 10 to 60% of tempered martensite.
2. The high tensile strength galvanized steel sheet of claim 1,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.005 to 2.00% of Cr, 0.005
to 2.00% of Mo, 0.005 to 2.00% of V, 0.005 to 2.00% of Ni, and
0.005 to 2.00% of Cu.
3. The high tensile strength galvanized steel sheet of claim 1,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.01 to 0.20% of Ti and 0.01
to 0.20% of Nb.
4. The high tensile strength galvanized steel sheet of claim 1,
further comprising, in terms of % by mass, 0.0002 to 0.005% of
B.
5. The high tensile strength galvanized steel sheet of claim 1,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and
0.001 to 0.005% of REM.
6. The high tensile strength galvanized steel sheet of claim 1,
wherein the galvanized steel sheet is a galvannealed steel
sheet.
7. A method for making a high tensile strength galvanized steel
sheet with excellent formability, comprising: subjecting a slab
having an elemental composition of claim 1 to hot rolling and cold
rolling thereby making a cold rolled steel sheet; subjecting the
cold rolled steel sheet to annealing including heating and
maintaining the steel sheet in a temperature range from 750 to
950.degree. C. for 10 seconds or more; cooling the steel sheet from
750.degree. C. to a temperature range from (Ms point -100.degree.
C.) to (Ms point -200.degree. C.) at an average cooling rate of
10.degree. C./s or more; reheating and maintaining the steel sheet
in a temperature range from 350 to 600.degree. C. for 1 to 600
seconds; and subjecting the annealed steel sheet to galvanizing
treatment.
8. The method of claim 7, wherein the galvanizing treatment is
followed by galvannealing treatment.
9. A high tensile strength galvanized steel sheet with excellent
formability and anti-crush properties, comprising, in terms of % by
mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to 3.5% of Mn,
0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5% of Al, and
0.01 to 0.2% in total of at least one element selected from the
group consisting of Ti, Nb, and V, the remainder being Fe and
unavoidable impurities, and having a microstructure composed of, in
terms of area fraction, 20 to 87% of ferrite, 3 to 10% in total of
martensite and residual austenite, and 10 to 60% of tempered
martensite, and a second phase composed of the martensite, residual
austenite, and tempered martensite having an average crystal grain
diameter of 3 .mu.m or less.
10. The high tensile strength galvanized steel sheet of claim 9,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.005 to 2.00% of Cr, 0.005
to 2.00% of Mo, 0.005 to 2.00% of Ni, and 0.005 to 2.00% of Cu.
11. The high tensile strength galvanized steel sheet of claim 9,
further comprising, in terms of % by mass, 0.0002 to 0.005% of
B.
12. The high tensile strength galvanized steel sheet of claim 9,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and
0.001 to 0.005% of REM.
13. The high tensile strength galvanized steel sheet of claim 9,
wherein the galvanized steel sheet is a galvannealed steel
sheet.
14. A method for making a high tensile strength galvanized steel
sheet with excellent formability and anti-crush properties,
comprising: subjecting a slab having an elemental composition of
claim 9 to hot rolling and cold rolling thereby making a cold
rolled steel sheet; subjecting the cold rolled steel sheet to
annealing including heating the steel sheet in a temperature range
from 500.degree. C. to the Ac.sub.1 transformation point at an
average temperature rising rate of 10.degree. C./s or more; heating
and maintaining the steel sheet in a temperature range from the
Ac.sub.1 transformation point to (Ac.sub.3 transformation point
+30.degree. C.) for 10 seconds or more; cooling the steel sheet to
a temperature range from (Ms point -100.degree. C.) to (Ms point
-200.degree. C.) at an average cooling rate of 10.degree. C./s or
more; reheating and maintaining the steel sheet in a temperature
range from 350 to 600.degree. C. for 1 to 600 seconds; and
subjecting the annealed steel sheet to galvanizing treatment.
15. The method of claim 14, wherein the galvanizing treatment is
followed by galvannealing treatment.
16. The high tensile strength galvanized steel sheet of claim 2,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.01 to 0.20% of Ti and 0.01
to 0.20% of Nb.
17. The high tensile strength galvanized steel sheet of claim 2,
further comprising, in terms of % by mass, 0.0002 to 0.005% of
B.
18. The high tensile strength galvanized steel sheet of claim 3,
further comprising, in terms of % by mass, 0.0002 to 0.005% of
B.
19. The high tensile strength galvanized steel sheet of claim 2,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and
0.001 to 0.005% of REM.
20. The high tensile strength galvanized steel sheet of claim 3,
further comprising at least one element selected from the group
consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and
0.001 to 0.005% of REM.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2008/069699, with an international filing date of Oct. 23,
2008 (WO 2009/054539 A1, published Apr. 30, 2009), which is based
on Japanese Patent Application Nos. 2007-277039, filed Oct. 25,
2007, and 2007-277040, filed Oct. 25, 2007, the subject matter of
which is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a high tensile strength
galvanized steel sheet and a method for making the same, the
galvanized steel sheet being excellent in formability and
anti-crush properties and used in industrial fields such as
automobile and electrical industries.
BACKGROUND
[0003] In recent years, from the standpoint of global environment
protection, improvement of fuel efficiency of automobiles is an
important issue. Therefore, there is a growing tendency to reduce
car body weight by increasing the tensile strength and reducing the
thickness of the steel sheet composing a car. The increase of the
tensile strength of the car body material contributes to the
improvement of crush safety, so that high tensile strength steel
sheets are increasingly used as car body materials. However, in
general cases, the increase of the tensile strength of a steel
sheet can result in the deterioration of the ductility of the steel
sheet, or the deterioration of formability. Therefore, there is a
demand for a galvanized steel sheet having high tensile strength
and high formability, and excellent corrosion resistance.
[0004] High tensile strength galvanized steel sheets of the
multiphase type such as DP (Dual Phase) steel composed of ferrite
and martensite, and TRIP (Transformation Induced Plasticity) steel
utilizing transformation induced plasticity of residual austenite
have been developed to satisfy such a demand.
[0005] "Tetsu To Hagane (Irong and Steel)," Vol. 83 (1997), p. 748
describes that ferrite-martensite two-phase steel exhibits
excellent anti-crush properties. However, the ferrite-martensite
two-phase steel has an r value of less than 1.0, and low deep
drawability, so that its applicability is limited.
[0006] Japanese Unexamined Patent Application Publication No.
11-279691 proposes a high tensile strength galvannealed steel sheet
with good formability, the steel sheet containing, in terms of % by
mass, 0.05 to 0.15% of C, 0.3 to 1.5% of Si, 1.5 to 2.8% of Mn,
0.03% or less of P, 0.02% or less of S, 0.005 to 0.5% of Al,
0.0060% or less of N, the remainder being Fe and unavoidable
impurities, the elemental composition satisfying (Mn %)/(C
%).gtoreq.15 and (Si %)/(C %).gtoreq.4, and the steel sheet being
composed of ferrite containing, in terms of volume fraction, 3 to
20% of martensite and residual austenite. However, these high
tensile strength galvanized steel sheets of the composite structure
type exhibit high elongation E1 as determined by uniaxial
stretching, but have poor stretch-flangeability required for a hole
expansion process and the like.
[0007] Therefore, Japanese Unexamined Patent Application
Publication No. 6-93340 discloses a method for making a high
tensile strength galvanized steel sheet with excellent
stretch-flangeability. Under the method, a steel sheet composed of,
in terms of % by mass, 0.02 to 0.30% of C, 1.5% or less of Si, 0.60
to 3.0% of Mn, 0.20% or less of P, 0.05% or less of S, 0.01 to
0.10% of Al, the remainder being Fe and unavoidable impurities, is
subjected to hot rolling at a temperature not lower than the
Ac.sub.3 transformation point, pickled and cold-rolled, and the
steel sheet is heated and maintained at a temperature not lower
than the recrystallization temperature and Ac.sub.1 transformation
point on a continuous annealing galvanizing line. Subsequently,
before immersion in a galvanizing bath, the steel sheet is rapidly
cooled to the Ms point or lower thereby forming martensite
partially or wholly in the steel sheet, and then heated to a
temperature not lower than the Ms point and at least equal to the
galvanizing bath temperature and galvannealing furnace temperature
thereby forming partially or completely tempered martensite.
[0008] The high tensile strength galvanized steel sheet described
in JP '340 provides excellent stretch-flangeability. However, the
product of tensile strength TS and E1 as determined by uniaxial
stretching, or the TS-E1 balance of the steel sheet is low. The
ratio of yield strength YS to TS, or yield ratio YR (YS/TS) is
high, which results in poor formability. In addition, the steel
sheet has poor anti-crush properties which are necessary for
securing crush safety.
[0009] It could therefore be helpful to provide a high tensile
strength galvanized steel sheet with excellent formability and a
method for making the same, the steel sheet providing a high TS-E1
balance, excellent stretch-flangeability, and a low YR. It could
also be helpful to provide a high tensile strength galvanized steel
sheet with excellent anti-crush properties and a method for making
the same, the steel sheet providing a high TS-E1 balance and
excellent stretch-flangeability.
SUMMARY
[0010] As a result of dedicated research on a high tensile strength
galvanized steel sheet with excellent formability providing a high
TS-E1 balance (specifically TS.times.E1.gtoreq.19000 MPa%),
excellent stretch-flangeability (specifically the below-described
hole expansion ratio satisfies .lamda..gtoreq.70%), and a low YR
(specifically YR<75%), we discovered: [0011] i) excellent
stretch-flangeability, a high TS-E1 balance, and a low YR are
achieved with a microstructure having an optimized elemental
composition and containing, in terms of area fraction, 20 to 87% of
ferrite, 3 to 10% in total of martensite and residual austenite,
and 10 to 60% of tempered martensite; and [0012] ii) the
microstructure is produced through annealing including forced
cooling from 750 to 950.degree. C. to a temperature range from (Ms
point -100.degree. C.) to (Ms point -200.degree. C.), and then
reheating, followed by galvanizing treatment. The Ms point is the
temperature at which transformation from austenite to martensite
begins, and can be determined from the variation of the coefficient
of linear expansion of the steel during cooling.
[0013] We thus provide a high tensile strength galvanized steel
sheet with excellent formability, the steel sheet containing, in
terms of % by mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to
3.5% of Mn, 0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5%
of Al, and 0.007% or less of N, the remainder being Fe and
unavoidable impurities, and the steel sheet having a microstructure
composed of, in terms of area fraction, 20 to 87% of ferrite, 3 to
10% in total of martensite and residual austenite, and 10 to 60% of
tempered martensite.
[0014] The high tensile strength galvanized steel sheet may further
contain one or more elements selected from, in terms of % by mass,
0.005 to 2.00% of Cr, 0.005 to 2.00% of Mo, 0.005 to 2.00% of V,
0.005 to 2.00% of Ni, and 0.005 to 2.00% of Cu. In addition, the
steel sheet may further contain one or two elements selected from,
in terms of % by mass, 0.01 to 0.20% of Ti and 0.01 to 0.20% of Nb,
and one or more elements selected from 0.0002 to 0.005% of B, 0.001
to 0.005% of Ca, and 0.001 to 0.005% of REM.
[0015] The high tensile strength galvanized steel sheet may be a
galvanized or galvannealed steel sheet.
[0016] The high tensile strength galvanized steel sheet may be
produced by, for example, a method for making a high tensile
strength galvanized steel sheet with excellent formability,
including steps of subjecting a slab having the above-described
elemental composition to hot rolling and cold rolling thereby
making a cold rolled steel sheet, subjecting the cold rolled steel
sheet to annealing including steps of heating and maintaining the
steel sheet in a temperature range from 750 to 950.degree. C. for
10 seconds or more, cooling the steel sheet from 750.degree. C. to
a temperature range from (Ms point -100.degree. C.) to (Ms point
-200.degree. C.) at an average cooling rate of 10.degree. C./s or
more, and reheating and maintaining the steel sheet in a
temperature range from 350 to 600.degree. C. for 1 to 600 seconds,
and then subjecting the annealed steel sheet to galvanizing
treatment.
[0017] Under the method for making a high tensile strength
galvanized steel sheet, the galvanizing treatment may be followed
by galvannealing treatment.
[0018] We produce a high tensile strength galvanized steel sheet
with excellent formability providing a high TS-E1 balance,
excellent stretch-flangeability, and a low YR. Through the use of
the high tensile strength galvanized steel sheet as a car body, the
car has a decreased weight, improved corrosion resistance, and
improved crush safety.
[0019] As a result of dedicated research on a high tensile strength
galvanized steel sheet providing a high TS-E1 balance (specifically
TS.times.E1.gtoreq.19000 MPa%), excellent stretch-flangeability
(specifically the below-described hole expansion ratio,
.lamda..gtoreq.70%), and excellent anti-crush properties
(specifically the below-described ratio of the absorption energy AE
and TS, AE/TS.gtoreq.0.063), we discovered: [0020] iii) excellent
stretch-flangeability, a high TS-E1 balance, and excellent
anti-crush properties are achieved with a microstructure having an
optimized elemental composition and containing, in terms of area
fraction, 20 to 87% of ferrite, 3 to 10% in total of martensite and
residual austenite, and 10 to 60% of tempered martensite, the
second phase composed of the martensite, residual austenite, and
tempered martensite having an average crystal grain diameter of 3
.mu.m or less; and [0021] iv) the microstructure is produced
through annealing including steps of heating the steel sheet in a
temperature range from 500.degree. C. to the Ac.sub.1
transformation point at an average temperature rising rate of
10.degree. C./s or more, heating and maintaining the steel sheet in
a temperature range from the Ac.sub.1 transformation point to
(Ac.sub.3 transformation point+30.degree. C.) for 10 seconds or
more thereby forming more fine austenite through transformation,
forcedly cooling the steel sheet to a temperature range from (Ms
point -100.degree. C.) to (Ms point -200.degree. C.) at an average
cooling rate of 10.degree. C./s or more, and reheating the steel
sheet, and then subjecting the annealed steel sheet to galvanizing
treatment. The Ms point is the temperature at which transformation
from austenite to martensite begins, and can be determined from the
variation of the coefficient of linear expansion of the steel
during cooling.
[0022] We thus provide a high tensile strength galvanized steel
sheet with excellent formability and anti-crush properties, the
steel sheet containing, in terms of % by mass, 0.05 to 0.3% of C,
0.01 to 2.5% of Si, 0.5 to 3.5% of Mn, 0.003 to 0.100% of P, 0.02%
or less of S, 0.010 to 1.5% of Al, in addition, 0.01 to 0.2% in
total of at least one element selected from Ti, Nb, and V, the
remainder being Fe and unavoidable impurities, the steel sheet
having a microstructure composed of, in terms of area fraction, 20
to 87% of ferrite, 3 to 10% in total of martensite and residual
austenite, and 10 to 60% of tempered martensite, and the second
phase composed of the martensite, residual austenite, and tempered
martensite having an average crystal grain diameter of 3 .mu.m or
less.
[0023] The high tensile strength galvanized steel sheet may further
contain one or more elements selected from, in terms of % by mass,
0.005 to 2.00% of Cr, 0.005 to 2.00% of Mo, 0.005 to 2.00% of Ni,
and 0.005 to 2.00% of Cu. In addition, as necessary, the steel
sheet may further contain one or more elements selected from, in
terms of % by mass, 0.0002 to 0.005% of B, 0.001 to 0.005% of Ca,
and 0.001 to 0.005% of REM.
[0024] The high tensile strength galvanized steel sheet may be a
galvanized or galvannealed steel sheet.
[0025] The high tensile strength galvanized steel sheet may be
produced by, for example, a making method including steps of
subjecting a slab having the above-described elemental composition
to hot rolling and cold rolling thereby making a cold rolled steel
sheet, subjecting the cold rolled steel sheet to annealing
including steps of heating the steel sheet in a temperature range
from 500.degree. C. to the Ac.sub.1 transformation point at an
average temperature rising rate of 10.degree. C./s or more, heating
and maintaining the steel sheet in a temperature range from the
Ac.sub.1 transformation point to (Ac.sub.3 transformation point
+30.degree. C.) for 10 seconds or more, cooling the steel sheet to
a temperature range from (Ms point -100.degree. C.) to (Ms point
-200.degree. C.) at an average cooling rate of 10.degree. C./s or
more, and reheating and maintaining the steel sheet in a
temperature range from 350 to 600.degree. C. for 1 to 600 seconds,
and then subjecting the annealed steel sheet to galvanizing
treatment.
[0026] The galvanizing treatment may be followed by galvannealing
treatment.
[0027] We produce a high tensile strength providing a high TS-E1
balance, excellent stretch-flangeability, and excellent anti-crush
properties. Through the use of the high tensile strength galvanized
steel sheet as an car body, the car has a reduced weight, improved
corrosion resistance, and improved crush safety.
DETAILED DESCRIPTION
[0028] Details are described below. The "%" expressing the content
of an element means "% by mass" unless otherwise stated.
1) Elemental Composition
C: 0.05 to 0.3%
[0029] C is an element stabilizing austenite, and necessary for
forming the second phase such as martensite other than ferrite
thereby increasing the TS and improving the TS-E1 balance. If the C
content is less than 0.05%, formation of the second phase other
than ferrite is inhibited, and thus the TS-E1 balance deteriorates.
On the other hand, if the C content is more than 0.3%, the
weldability deteriorates. Accordingly, the C content is from 0.05
to 0.3%, preferably from 0.08 to 0.15%.
Si: 0.01 to 2.5%
[0030] Si is an element effective at solute strengthening steel
thereby improving the TS-E1 balance. The Si content must be 0.01%
or more to achieve this. If the Si content is more than 2.5%, E1
deteriorates and the surface quality and weldability deteriorate.
Accordingly, the Si content is from 0.01 to 2.5%, preferably from
0.7 to 2.0%.
Mn: 0.5 to 3.5%
[0031] Mn is an element effective at strengthening steel, and
promoting the formation of the second phase such as martensite. The
Mn content must be 0.01% or more to achieve this. On the other
hand, if the Mn content is more than 3.5%, the ductility of ferrite
markedly deteriorates due to the excessive increase in the size of
the second phase and solute strengthening, which results in the
deterioration of formability. Accordingly, the Mn content is from
0.5 to 3.5%, preferably from 1.5 to 3.0%.
P: 0.003 to 0.100%
[0032] P is an element effective at strengthening steel. The P
content must be 0.003 or more to achieve this. On the other hand,
if the P content is more than 0.100%, the steel is embrittled by
grain boundary segregation, which results in the deterioration of
the anti-crush properties. Accordingly, the P content is from 0.003
to 0.100%.
S: 0.02% or less
[0033] S occurs as an intervening substance such as MnS, and
deteriorates the anti-crush properties and weldability. Therefore,
the Si content is preferably as low as possible. However, from the
viewpoint of production cost, the S content is 0.02% or less.
Al: 0.010 to 1.5%
[0034] Al is an element effective at forming ferrite thereby
improving the TS-E1 balance. The Al content must be 0.010% or more
to achieve this. On the other hand, if the Al content is more than
1.5%, slab cracking tends to occur during continuous casting.
Accordingly, the Al content is from 0.010 to 1.5%.
N: 0.007% or less
[0035] N is an element deteriorating the aging resistance of the
steel. If the N content is more than 0.007%, the aging resistance
markedly deteriorates. Accordingly, the N content is 0.007% or
less, and is preferably as low as possible.
At least one selected from Ti, Nb, and V: 0.01 to 0.2% in total
[0036] Ti, Nb, and V are elements which precipitate in the forms
of, for example, TiC, NbC, and VC, and are effective at refining
the steel structure. The total content of the at least one element
selected from Ti, Nb, and V must be 0.01% or more to achieve this.
On the other hand, if the total content of the at least one element
selected from Ti, Nb, and V is more than 0.2%, excessive
precipitation occurs, which results in the deterioration of the
ductility of ferrite. Accordingly, the total content of the at
least one element selected from Ti, Nb, and V is from 0.01 to
0.2%.
[0037] The remainder is composed of Fe and unavoidable impurities,
and as necessary may further contain, for the below-described
reason, 0.005 to 2.00% of Cr, 0.005 to 2.00% of Mo, 0.005 to 2.00%
of V, 0.005 to 2.00% of Ni, 0.005 to 2.00% of Cu, 0.01 to 0.20% of
Ti, 0.01 to 0.20% of Nb, 0.0002 to 0.005% of B, 0.001 to 0.005% of
Ca, and 0.001 to 0.005% of REM. Cr, Mo, V, Ni, and Cu: 0.005 to
2.00% each
[0038] Cr, Mo, V, Ni, and Cu are elements effective at inhibiting
the formation of perlite during cooling from the heating
temperature in annealing, and promoting the formation of martensite
and other phases thereby reinforcing the steel. The content of the
at least one element selected from Cr, Mo, V, Ni, and Cu must be
0.005% to achieve this. On the other hand, if the respective
contents of Cr, Mo, V, Ni, and Cu are more than 2.00%, the effect
is saturated, which results in an increase in cost. Accordingly,
the respective contents of Cr, Mo, V, Ni, and Cu are from 0.005 to
2.00%.
Ti and Nb: 0.01 to 0.20% each
[0039] Ti and Nb are elements effective at forming carbonitrides,
and increasing the tensile strength of the steel through
precipitation strengthening. The content of the at least one
element selected from Ti and Nb must be 0.01% or more to achieve
this. On the other hand, if the respective contents of Ti and Nb
are more than 0.20%, the tensile strength is excessively increased,
which results in the deterioration of ductility. Accordingly, the
respective contents of Ti and Nb are from 0.01 to 0.20%.
B: 0.0002 to 0.005%
[0040] B is an element effective at inhibiting the formation of
ferrite from the austenite grain boundary, and forming a second
phase such as martensite to increase the tensile strength of the
steel. The B content must be 0.0002% or more to achieve this. On
the other hand, if the B content is more than 0.005%, the effect is
saturated, which results in an increase in cost. Accordingly, the B
content is from 0.0002 to 0.005%.
Ca, REM: 0.001 to 0.005% each
[0041] Ca and REM are elements effective at improving formability
through the control of the sulfide form. The content of the at
least one element selected from Ca and REM must be 0.001% or more
to achieve this. On the other hand, if the respective contents of
Ca and REM are more than 0.005%, steel cleanness may be affected.
Accordingly, the respective contents of Ca and REM are from 0.001
to 0.005%.
2) Microstructure
[0042] Area fraction of ferrite: 20 to 87%
[0043] Ferrite improves the TS-E1 balance.
[0044] To satisfy TS.times.E1.gtoreq.19000 MPa%, the area fraction
of ferrite must be 20% or more, preferably 50% or more. As
described below, the total area fraction of martensite and residual
austenite is 3% or more, and the area fraction of tempered
martensite is 10% or more, so that the upper limit of the area
fraction of ferrite is 87%.
Total area fraction of martensite and residual austenite: 3 to
10%
[0045] Martensite and residual austenite contribute to
reinforcement of the steel, improve the TS-E1 balance, and decrease
the YR. The total area fraction of martensite and residual
austenite must be 3% or more to achieve this. However, if the total
area fraction of martensite and residual austenite is more than
10%, the stretch-flangeability deteriorates. Therefore, the total
area fraction of martensite and residual austenite is from 3 to
10%.
Area fraction of tempered martensite: 10 to 60%
[0046] Tempered martensite affects the stretch-flangeability less
than martensite before tempering or residual austenite, so that an
effective second phase is formed achieving high tensile strength
while maintaining excellent stretch-flangeability satisfying
.lamda..gtoreq.50%. The area fraction of tempered martensite must
be 10% or more to achieve this. However, if the area fraction of
tempered martensite is more than 60%, TS.times.E1.gtoreq.19000 MPa%
is not satisfied. Accordingly, the area fraction of tempered
martensite is from 10 to 60%.
Average crystal grain diameter of second phase composed of
martensite, residual austenite, and tempered martensite: 3 .mu.m or
less
[0047] The presence of the second phase composed of martensite,
residual austenite, and tempered martensite effectively improves
the anti-crush properties. In particular, when the average crystal
grain diameter of the second phase is 3 .mu.m or less,
AE/TS.gtoreq.0.063 is satisfied. Accordingly, the average crystal
grain diameter of the second phase composed of martensite, residual
austenite, and tempered martensite is preferably 3 .mu.m or
less.
[0048] In addition to martensite, residual austenite, and tempered
martensite, the second phase may further contain perlite and
bainite. Good results are achieved as long as the above-described
area fractions of ferrite, martensite, residual austenite, and
tempered martensite, and the average crystal grain diameter of the
second phase are satisfied. From the viewpoint of
stretch-flangeability, the area fraction of perlite is preferably
3% or less.
[0049] The area fractions of ferrite, martensite, residual
austenite, and tempered martensite refer to the proportions of the
respective phases in the observed area, and were determined as
follows: a section of a steel sheet in the thickness direction was
polished, corroded with 3% nital, the quarter-thickness position
was observed with an SEM (scanning electron microscope) under a
magnification of 1000.times. to 3000.times., and the area fraction
was calculated using commercial image processing software. The
total area of the second phase composed of martensite, residual
austenite, and tempered martensite was divided by the total number
of second phase grains to calculate the average area of one second
phase grain, and its square root was used as the average crystal
grain diameter of the second phase.
3) Production Conditions 1
[0050] The high tensile strength galvanized steel sheet may be
produced by, for example, a method including steps of subjecting a
slab having the above-described elemental composition to hot
rolling and cold rolling thereby making a cold rolled steel sheet,
subjecting the cold rolled steel sheet to annealing including steps
of heating and maintaining the steel sheet in a temperature range
from 750 to 950.degree. C. for 10 seconds or more, cooling the
steel sheet from 750.degree. C. to a temperature range from (Ms
point -100.degree. C.) to (Ms point -200.degree. C.) at an average
cooling rate of 10.degree. C./s or more, and reheating and
maintaining the steel sheet in a temperature range from 350 to
600.degree. C. for 1 to 600 seconds, and then subjecting the
annealed steel sheet to galvanizing treatment.
Heating conditions during annealing: temperature range from 750 to
950.degree. C. for 10 seconds or more
[0051] If the heating temperature during annealing is lower than
750.degree. C., or the maintaining period is less than 10 seconds,
austenite is insufficiently formed, so that the second phase such
as martensite is insufficiently formed by subsequent cooling. On
the other hand, if the heating temperature is higher than
950.degree. C., austenite is coarsened, whereby formation of
ferrite during cooling is inhibited, and the area fraction of
ferrite falls short of 20%. Accordingly, heating temperature during
annealing is maintained in a temperature range from 750 to
950.degree. C. for 10 seconds or more. The upper limit of the
maintaining period is not particularly defined. However, even if
the heating temperature is maintained for 600 seconds or more, the
effect is saturated, which results in an increase in cost.
Accordingly, the maintaining period is preferably less than 600
seconds.
Cooling conditions during annealing: from 750.degree. C. to a
temperature range from (Ms point -100.degree. C.) to (Ms point
-200.degree. C.) at an average cooling rate of 10.degree. C./s or
more
[0052] After heating, the steel sheet must be cooled from
750.degree. C. at an average cooling rate of 10.degree. C./s or
more. If the average cooling rate is less than 10.degree. C./s,
perlite is formed in large amounts, so that necessary amounts of
tempered martensite, martensite, and residual austenite cannot be
obtained. The upper limit of the cooling rate is not particularly
defined, but is preferably 200.degree. C./s or less to prevent
deterioration of the shape of the steel sheet, and to avoid
difficulty regarding stopping of cooling within the temperature
range from (Ms point -100.degree. C.) to (Ms point -200.degree.
C.). The temperature at which cooling is stopped is one of the most
important factors for controlling the amounts of martensite,
residual austenite, and tempered martensite formed by the
subsequent reheating, galvanizing, and galvannealing of the coated
phase. More specifically, the amounts of martensite and
untransformed martensite are determined when cooling is stopped,
and the subsequent heat treatment transforms martensite into
tempered martensite, and untransformed austenite into martensite or
residual austenite, whereby the strength, TS-E1 balance,
stretch-flangeability, and YR of the steel are determined. If the
temperature at which cooling is stopped is higher than (Ms point
-100.degree. C.), martensite is insufficiently transformed, so that
the amount of untransformed austenite increases, and the total area
fraction of martensite and residual austenite exceeds 10%, which
results in the deterioration of the stretch-flangeability. On the
other hand, if the temperature at which cooling is stopped is lower
than (Ms point -200.degree. C.), most of austenite is transformed
into martensite, the amount of untransformed austenite decreases,
and the total area fraction of martensite and residual austenite is
below 3%, which results in the deterioration of the TS-E1 balance
and increase of the YR. Accordingly, the cooling treatment during
annealing must be carried out from 750.degree. C. to a temperature
range from (Ms point -100.degree. C.) to (Ms point -200.degree. C.)
at an average cooling rate of 10.degree. C./s or more.
Reheating conditions during annealing: temperature range from 350
to 600.degree. C. for 1 to 600 seconds
[0053] After cooling to the temperature range from (Ms point
-100.degree. C.) to (Ms point -200.degree. C.) at an average
cooling rate of 10.degree. C./s or more, reheating is carried out,
and a temperature range from 350 to 600.degree. C. is maintained
for 1 second or more to temper the martensite formed during cooling
thereby forming tempered martensite at an area fraction of 10 to
60%. As a result of this, high tensile strength is achieved and
excellent stretch-flangeability is maintained. If the reheating
temperature is below 350.degree. C. or the maintaining period is
less than 1 second, the area fraction of the tempered martensite is
less than 10%, which results in the deterioration of the
stretch-flangeability. On the other hand, if the reheating
temperature is higher than 600.degree. C. or the maintaining period
is more than 600 seconds, the untransformed austenite formed during
cooling is transformed into perlite or bainite, and finally the
total area fraction of martensite and residual austenite is less
than 3%, which results in the deterioration of the TS-E1 balance or
the increase of the YR. Accordingly, the reheating temperature
during annealing must be maintained within a temperature range from
350 to 600.degree. C. for 1 to 600 seconds.
[0054] Other conditions of the production method are not
particularly limited, but are preferably the following
conditions.
[0055] The slab is preferably produced by a continuous casting
process to prevent macro segregation, and may be produced by an
ingot casting or thin slab casting process. Hot rolling of the slab
may be carried out by once cooling the slab to room temperature,
followed by reheating, or by charging the slab into a heating
furnace without cooling the slab to room temperature.
Alternatively, an energy saving process may be used, wherein the
slab is slightly insulated, and then subjected to hot rolling. When
the slab is heated, the heating temperature is preferably
1100.degree. C. or higher to dissolve the carbide and prevent the
increase of the rolling load. Further, to prevent the increase of
scale loss, the heating temperature for the slab is preferably
1300.degree. C. or lower.
[0056] During hot rolling of the slab, from the viewpoint of
securing the rolling temperature, the rough bar after rough rolling
may be heated. Alternatively, a so-called "continuous rolling"
process may be used, wherein two rough bars are joined together,
and subjected to continuous finish rolling. To prevent the
deterioration of formability after cold rolling and annealing, and
the formation of a band structure which can increase the
anisotropy, the finish rolling is carried out at a temperature not
lower than the Ar.sub.3 transformation point. Further, to reduce
the rolling load and improve the uniformity of the shape and
material, lubrication rolling is preferably carried out in the
whole or partial path of finish rolling thereby giving a
coefficient of friction of 0.10 to 0.25.
[0057] From the viewpoints of temperature control and prevention of
decarbonization, the steel sheet after hot rolling is preferably
wound up at a temperature of 450 to 700.degree. C.
[0058] The wound steel sheet is subjected to pickling thereby
removing scales, and then cold rolling at a rolling ratio of
preferably 40% or more. Subsequently, the steel sheet is annealed
under the above-described conditions, and then galvanized.
[0059] The galvanizing treatment is carried out by immersing the
steel sheet in a galvanizing bath at 440 to 500.degree. C.
containing 0.12 to 0.22% of Al (when no galvannealing is involved)
or 0.08 to 0.18% of Al (when followed by galvannealing), and then
the coating weight is adjusted by, for example, gas wiping. The
galvanizing treatment may be followed by galvannealing treatment at
450 to 600.degree. C. for 1 to 30 seconds.
[0060] The galvanized steel sheet or galvannealed steel sheet may
be subjected to temper rolling for the purpose of shape correction
or adjustment of surface roughness. Further, various coating
treatments such as resin or oil coating may be applied.
4) Production Conditions 2
[0061] The high tensile strength galvanized steel sheet may be
produced by, for example, a method including steps of subjecting a
slab having the above-described elemental composition to hot
rolling and cold rolling thereby making a cold rolled steel sheet,
subjecting the cold rolled steel sheet to annealing including steps
of heating the steel sheet to a temperature range from 500.degree.
C. to Ac.sub.1 transformation point at an average temperature
rising rate of 10.degree. C./s or more, heating and maintaining the
steel sheet in a temperature range from AC.sub.1 transformation
point to (Ac.sub.3 transformation point +30.degree. C.) for 10
seconds or more, cooling the steel sheet from 750.degree. C. to a
temperature range from (Ms point -100.degree. C.) to (Ms point
-200.degree. C.) at an average cooling rate of 10.degree. C./s or
more, and reheating and maintaining the steel sheet in a
temperature range from 350 to 600.degree. C. for 1 to 600 seconds,
and then subjecting the annealed steel sheet to galvanizing
treatment.
Temperature rising conditions during annealing: temperature rising
in a temperature range from 500.degree. C. to Ac.sub.1
transformation point at a temperature rising rate of 10.degree.
C./s or more
[0062] The temperature rising rate during annealing is an important
factor for refining the average crystal grain diameter of the
second phase composed of martensite, residual austenite, and
tempered martensite. In the steel having the elemental composition,
fine carbides of Ti, Nb, and V inhibit recrystallization. When the
temperature is risen in a temperature range from 500.degree. C. to
Ac.sub.1 transformation point at an average temperature rising rate
of 10.degree. C./s or more, the steel sheet is heated to the
subsequent temperature range from the Ac.sub.1 transformation point
with little recrystallization. Therefore, during heating, the
uncrystallized ferrite causes austenite transformation to form fine
austenite. As a result, the second phase after cooling and
reheating has an average crystal grain diameter of 3 .mu.m or less,
whereby excellent anti-crush properties satisfying
AE/TS.gtoreq.0.063 are achieved. On the other hand, if the average
temperature rising rate in the temperature range from 500.degree.
C. to Ac.sub.1 transformation point is less than 10.degree. C./s,
recrystallization occurs during temperature rising in the
temperature range from 500.degree. C. to Ac.sub.1 transformation
point, and the recrystallized ferrite causes austenite
transformation after grain growth to a degree. As a result,
austenite is not refined, and the average crystal grain diameter of
the second phase cannot be 3 .mu.m or less. Accordingly, it is
necessary to rise the temperature in the temperature range from
500.degree. C. to Ac.sub.1 transformation point at an average
temperature rising rate of 10.degree. C./s or more, preferably
20.degree. C./s or more.
Heating conditions during annealing: temperature range from
Ac.sub.1 transformation point to (Ac.sub.3 transformation point
+30.degree. C.) for 10 seconds or more
[0063] If the heating temperature during annealing is below the
Ac.sub.1 transformation point, or the maintaining period is less
than 10 seconds, formation of austenite does not occur, or
insufficiently occurs, so that a sufficient amount of second phase
such as martensite cannot be secured by subsequent cooling. On the
other hand, if the heating temperature is higher than (Ac.sub.3
trans-formation point +30.degree. C.), austenite grains markedly
grow, whereby refinement of austenite is inhibited. In addition,
the growth of austenite grains inhibits formation of ferrite during
cooling, so that the area fraction of ferrite cannot be 20% or
more. Accordingly, the heating treatment during annealing must be
carried out in a temperature range from Ac.sub.1 transformation
point to (Ac.sub.3 transformation point +30.degree. C.) for 10
seconds or more. From the viewpoints of inhibition of austenite
coarsening and energy cost, the maintaining period is preferably
300 seconds or less.
Cooling conditions during annealing: cooling from the heating
temperature to a temperature range from (Ms point -100.degree. C.)
to (Ms point -200.degree. C.) at an average cooling rate of
10.degree. C./s or more
[0064] After the heating treatment, the steel sheet must be cooled
from the heating temperature at an average cooling rate of
10.degree. C./s or more. If the average cooling rate is below
10.degree. C./s, perlite is heavily formed, so that necessary
amounts of tempered martensite, martensite, and residual austenite
cannot be obtained. The upper limit of the cooling rate is not
particularly defined, but is preferably 200.degree. C./s or less to
prevent deterioration of the shape of the steel sheet, and avoid
difficulty at stopping cooling within the temperature range from
(Ms point -100.degree. C.) to (Ms point -200.degree. C.).
[0065] The temperature at which cooling is stopped is one of the
most important factors for controlling the amounts of martensite,
residual austenite, and tempered martensite formed by the
subsequent reheating, galvanizing, and galvannealing of the coated
phase. More specifically, the amounts of martensite and
untransformed martensite are determined when cooling is stopped,
and the subsequent heat treatment transforms martensite into
tempered martensite, and untransformed austenite into martensite or
residual austenite, whereby the strength, TS-E1 balance,
stretch-flangeability, and YR of the steel are determined. If the
cooling treatment is stopped at a temperature higher than (Ms point
-100.degree. C.), martensite is insufficiently transformed, so that
the amount of untransformed austenite increases, and the total area
fraction of martensite and residual austenite exceeds 10%, which
results in the deterioration of the stretch-flangeability. On the
other hand, if the cooling temperature is stopped at a temperature
lower than (Ms point -200.degree. C.), most of austenite is
transformed into martensite, the amount of untransformed austenite
decreases, and the total area fraction of martensite and residual
austenite is below 3%, which results in the deterioration of the
TS-E1 balance. Accordingly, the cooling treatment during annealing
must be carried out from the heating temperature to a temperature
range from (Ms point -100.degree. C.) to (Ms point -200.degree. C.)
at an average cooling rate of 10.degree. C./s or more.
Reheating conditions during annealing: temperature range from 350
to 600.degree. C. for 1 to 600 seconds
[0066] After cooling to the temperature range from (Ms point
-100.degree. C.) to (Ms point -200.degree. C.) at an average
cooling rate of 10.degree. C./s or more, reheating is carried out,
and a temperature range from 350 to 600.degree. C. is maintained
for 1 second or more to temper the martensite formed during cooling
thereby forming tempered martensite at an area fraction of 10 to
60%. As a result, high tensile strength is achieved with excellent
stretch-flangeability maintained. If the reheating temperature is
below 350.degree. C. or the maintaining period is less than 1
second, the area fraction of the tempered martensite is less than
10%, which results in the deterioration of the
stretch-flangeability. On the other hand, if the reheating
temperature is higher than 600.degree. C. or the maintaining period
is more than 600 seconds, the untransformed austenite formed during
cooling is transformed into perlite or bainite, and finally the
total area fraction of martensite and residual austenite is less
than 3%, which results in the deterioration of the TS-E1 balance.
Accordingly, the reheating temperature during annealing must be
maintained within a temperature range from 350 to 600.degree. C.
for 1 to 600 seconds.
[0067] Other conditions of the production method are not
particularly limited, but are preferably the following
conditions.
[0068] The slab is preferably produced by a continuous casting
process to prevent macro segregation, and may be produced by an
ingot casting or thin slab casting process. Hot rolling of the slab
may be carried out by once cooling the slab to room temperature,
followed by reheating, or by charging the slab into a heating
furnace without cooling the slab to room temperature.
Alternatively, an energy saving process may be used, wherein the
slab is slightly insulated, and then subjected to hot rolling. When
the slab is heated, the heating temperature is preferably
1100.degree. C. or higher to dissolve the carbide and prevent the
increase of the rolling load. Further, the heating temperature for
the slab is preferably 1300.degree. C. or lower to prevent the
increase of scale loss.
[0069] During hot rolling of the slab, from the viewpoint of
securing the rolling temperature, the rough bar after rough rolling
may be heated. Alternatively, a so-called "continuous rolling"
process may be used, wherein two rough bars are joined together,
and subjected to continuous finish rolling. To prevent the
deterioration of formability after cold rolling and annealing, and
the formation of a band structure which can increase the
anisotropy, the finish rolling is carried out at a temperature not
lower than the Ar.sub.3 transformation point. Further, to reduce
the rolling load and improve the uniformity of the shape and
material, lubrication rolling is preferably carried out in the
whole or partial path of finish rolling thereby giving a
coefficient of friction of 0.10 to 0.25.
[0070] From the viewpoints of temperature control and prevention of
decarbonization, the steel sheet after hot rolling is preferably
wound up at a temperature of 450 to 700.degree. C.
[0071] The wound steel sheet is subjected to pickling thereby
removing scales, and then cold rolling at a rolling ratio of
preferably 40% or more. Subsequently, the steel sheet is annealed
under the above-described conditions, and then galvanized.
[0072] The galvanizing treatment is carried out by immersing the
steel sheet in a galvanizing bath at 440 to 500.degree. C.
containing 0.12 to 0.22% of Al (when no galvannealing is involved)
or 0.08 to 0.18% of Al (when followed by galvannealing), and then
the coating weight is adjusted by, for example, gas wiping. The
galvanizing treatment may be followed by galvannealing treatment at
450 to 600.degree. C. for 1 to 30 seconds.
[0073] The galvanized steel sheet or galvannealed steel sheet may
be subjected to temper rolling for the purpose of shape correction
or adjustment of surface roughness. Further, various coating
treatments such as resin or oil coating may be applied.
EXAMPLES
Example 1
[0074] The steels A to S having the elemental compositions shown in
Table 1 were ingoted by a converter, made into slabs by a
continuous casting process. Subsequently, the slabs were subjected
to hot rolling at a finish temperature of 900.degree. C. to give a
thickness of 3.0 mm, cooled at a cooling rate of 10.degree. C./s,
and then wound up at a temperature of 600.degree. C. Subsequently,
after pickling, the slabs were subjected to cold rolling to give a
thickness of 1.2 mm, and annealed on a continuous galvanizing line
under the conditions shown in Tables 2 and 3. Thereafter, the steel
sheets were immersed in a galvanizing bath at 460.degree. C. to
form a coating at a coating weight of 35 to 45 g/m.sup.2, subjected
to galvannealing treatment at 520.degree. C., and cooled at a
cooling rate of 10.degree. C./s to make galvanized steel sheets 1
to 44. As shown in Tables 2 and 3, some galvanized steel sheets
were not subjected to galvannealing treatment. The galvanized steel
sheets thus obtained were measured for the area fractions of
ferrite, martensite, residual austenite, and tempered martensite by
the above-described method. Further, JIS No. 5 tensile test
specimens were cut out along and perpendicular to the rolling
direction, and subjected to tensile test according to JIS Z 2241.
Further, test specimens of 150 mm.times.150 mm were cut out, and
subjected to hole expansion test three times according to JFS T
1001 (Japan Iron and Steel Federation standard) to determine the
average hole expansion ratio .lamda. (%), whereby the
stretch-flangeability was evaluated.
[0075] The results are shown in Tables 4 and 5, indicating that all
of our galvanized steel sheets satisfied TSE1.gtoreq.19000 MPa%,
hole expansion ratio .lamda..gtoreq.70, and YR<75%, representing
their high TS-E1 balance, excellent stretch-flangeability, and low
YR.
TABLE-US-00001 TABLE 1 Elemental composition (% by mass) Steel C Si
Mn P S Al N Cr Mo V Ni Cu Ti Nb B Ca REM Note A 0.06 1.0 2.3 0.020
0.003 0.035 0.003 -- -- -- -- -- -- -- -- -- -- Beyond the scope of
the invention B 0.12 1.5 2.0 0.015 0.002 0.037 0.002 -- -- -- -- --
-- -- -- -- -- Beyond the scope of the invention C 0.16 0.7 1.4
0.017 0.004 0.700 0.005 -- -- -- -- -- -- -- -- -- -- Beyond the
scope of the invention D 0.25 0.02 1.8 0.019 0.002 0.041 0.004 --
-- -- -- -- -- -- -- -- -- Beyond the scope of the invention E 0.10
1.3 2.1 0.025 0.003 0.036 0.004 -- -- -- -- -- -- -- -- -- --
Beyond the scope of the invention F 0.20 0.3 1.6 0.013 0.005 0.028
0.005 -- -- -- -- -- -- -- -- -- -- Beyond the scope of the
invention G 0.13 1.3 1.2 0.008 0.006 0.031 0.003 0.60 -- -- -- --
-- -- -- -- -- Beyond the scope of the invention H 0.16 0.6 2.7
0.014 0.002 0.033 0.004 -- 0.3 -- -- -- -- -- -- -- -- Beyond the
scope of the invention I 0.08 1.0 2.2 0.007 0.003 0.025 0.002 -- --
0.1 -- -- -- -- -- -- -- Beyond the scope of the invention J 0.12
1.1 1.9 0.007 0.002 0.033 0.001 -- -- -- 0.5 -- -- -- -- -- --
Beyond the scope of the invention K 0.10 1.5 2.7 0.014 0.001 0.042
0.003 -- -- -- -- 0.3 -- -- -- -- -- Beyond the scope of the
invention L 0.10 0.6 1.9 0.021 0.005 0.015 0.004 -- -- -- -- --
0.05 -- -- -- -- Beyond the scope of the invention M 0.16 1.2 2.9
0.006 0.004 0.026 0.002 -- -- -- -- -- -- 0.03 -- -- -- Beyond the
scope of the invention N 0.09 2.0 2.1 0.012 0.003 0.028 0.005 -- --
-- -- -- 0.02 -- 0.001 -- -- Beyond the scope of the invention O
0.08 1.0 2.2 0.010 0.002 0.046 0.001 0.3 -- -- -- -- -- -- -- 0.003
-- Beyond the scope of the invention P 0.07 1.3 2.9 0.019 0.004
0.036 0.003 -- -- -- -- -- -- 0.04 -- -- 0.002 Beyond the scope of
the invention Q 0.04 1.4 1.6 0.013 0.002 0.022 0.002 -- -- -- -- --
-- -- -- -- -- Beyond the scope of the invention R 0.15 0.5 3.6
0.022 0.001 0.036 0.002 -- -- -- -- -- -- -- -- -- -- Beyond the
scope of the invention S 0.08 1.2 0.4 0.007 0.003 0.029 0.002 -- --
-- -- -- -- -- -- -- -- Beyond the scope of the invention
TABLE-US-00002 TABLE 2 Annealing conditions Heating temperature
Heating Reheating Galvanized (end-point maintaining Cooling Cooling
Reheating maintaining steel temperature) period rate end-point
temperature period Ms point sheet No. Steel (.degree. C.) (s)
(.degree. C./s) (.degree. C.) (.degree. C.) (s) (.degree. C.)
Galvanealing Note 1 A 830 60 50 200 400 40 353 Treated Example 2
720 60 50 120 400 30 245 Treated Comparative Example 3 810 60 50
100 420 30 341 Treated Comparative Example 4 B 780 90 80 180 430 60
318 Treated Example 5 780 5 80 70 430 60 184 Treated Comparative
Example 6 800 60 80 50 400 60 329 Treated Comparative Example 7 C
880 90 30 150 450 45 265 Untreated Example 8 880 90 5 120 450 45
196 Untreated Comparative Example 9 880 90 30 30 450 45 265
Untreated Comparative Example 10 D 780 150 70 140 450 60 261
Treated Example 11 780 60 150 20 450 60 237 Treated Comparative
Example 12 780 90 100 200 450 50 250 Treated Comparative Example 13
E 850 75 80 170 400 30 297 Treated Example 14 850 60 80 160 300 60
279 Treated Comparative Example 15 830 75 80 160 650 60 279 Treated
Comparative Example 16 850 75 80 40 400 30 297 Treated Comparative
Example 17 F 800 240 90 100 400 90 248 Treated Example 18 820 240
90 100 400 0 270 Treated Comparative Example 19 800 240 90 100 450
900 282 Treated Comparative Example 20 800 240 90 220 400 90 248
Treated Comparative Example 21 G 850 60 100 150 500 30 279 Treated
Example 22 850 60 100 20 500 30 279 Treated Comparative Example
TABLE-US-00003 TABLE 3 Annealing conditions Heating temperature
Heating Reheating Galvanized (end-point maintaining Cooling Cooling
Reheating maintaining steel temperature) period rate end-point
temperature period Ms point sheet No. Steel (.degree. C.) (s)
(.degree. C./s) (.degree. C.) (.degree. C.) (s) (.degree. C.)
Galvanealing Note 23 H 840 120 90 190 400 30 316 Treated Example 24
840 120 90 50 400 30 316 Treated Comparative Example 25 1000 120
150 200 350 30 380 Treated Comparative Example 26 I 830 75 150 250
500 45 380 Treated Example 27 830 75 150 300 500 45 380 Treated
Comparative Example 28 J 800 45 80 180 400 20 319 Untreated Example
29 800 45 80 50 400 20 319 Untreated Comparative Example 30 K 750
200 100 210 550 10 348 Treated Example 31 750 200 100 50 550 10 348
Treated Comparative Example 32 L 780 120 150 230 400 60 342 Treated
Example 33 780 120 150 300 400 60 342 Treated Comparative Example
34 M 840 90 150 180 400 20 341 Untreated Example 35 840 90 150 280
400 20 341 Untreated Comparative Example 36 N 820 60 50 160 450 90
308 Treated Example 37 820 60 50 50 450 90 308 Treated Comparative
Example 38 O 800 45 1000 220 450 150 389 Treated Example 39 800 45
1000 20 450 150 389 Treated Comparative Example 40 P 860 30 30 200
450 30 377 Treated Example 41 860 30 30 320 450 30 377 Treated
Comparative Example 42 Q 800 60 30 200 400 60 328 Treated
Comparative Example 43 R 820 90 80 180 400 30 347 Treated
Comparative Example 44 S 820 75 80 20 400 120 121 Treated
Comparative Example
TABLE-US-00004 TABLE 4 Microstructure* F Galvanized Area M +
residual .gamma. Tempered M Tensile characteristic values steel
fraction Area fraction Area fraction YS TS El YR TS .times. El
.lamda. sheet No. (%) (%) (%) Other (MPa) (MPa) (%) (%) (MPa %) (%)
Note 1 80 4 16 -- 389 670 32 58 21440 74 Example 2 90 2 2 P 502 605
25 83 15125 50 Comparative Example 3 82 1 17 -- 518 682 26 76 17732
81 Comparative Example 4 72 5 18 B 553 801 27 69 21627 85 Example 5
85 4 5 P 628 730 22 86 16060 45 Comparative Example 6 70 1 29 --
632 810 23 78 18630 84 Comparative Example 7 77 7 14 P 715 979 21
73 20559 74 Example 8 82 2 8 P 767 913 17 84 15521 56 Comparative
Example 9 77 1 22 -- 792 990 18 80 17820 77 Comparative Example 10
59 9 32 -- 788 1065 18 74 19170 71 Example 11 63 1 36 -- 874 1040
15 84 15600 90 Comparative Example 12 61 23 12 B 783 1103 14 71
15442 40 Comparative Example 13 79 6 15 -- 481 697 30 69 20910 75
Example 14 81 13 6 -- 508 782 27 65 21114 46 Comparative Example 15
81 1 13 P 584 704 21 83 14784 72 Comparative Example 16 77 1 22 --
544 706 25 77 17650 80 Comparative Example 17 70 6 24 -- 618 858 23
72 19734 78 Example 18 67 14 19 -- 593 885 22 67 19470 56
Comparative Example 19 65 1 34 -- 697 820 20 85 16400 85
Comparative Example 20 72 17 6 B 546 910 21 60 19110 27 Comparative
Example 21 78 5 17 -- 418 686 32 61 21952 78 Example 22 75 1 24 --
537 698 25 77 17450 79 Comparative Example *F ferrite, M
martensite, .gamma. austenite, P pearlite, B bainite
TABLE-US-00005 TABLE 5 Microstructure* Galvanized F M + residual
.gamma. Tempered M Tensile characteristic values steel Area
fraction Area fraction Area ratio YS TS El YR TS .times. El .lamda.
sheet No. (%) (%) (%) Other (MPa) (MPa) (%) (%) (MPa %) (%) Note 23
55 10 35 -- 812 1113 18 73 20034 72 Example 24 56 2 42 -- 926 1129
15 82 16935 80 Comparative Example 25 5 8 87 -- 1085 1220 9 89
10980 85 Comparative Example 26 64 9 27 -- 629 861 24 73 20664 78
Example 27 64 23 11 B 563 923 22 61 20306 27 Comparative Example 28
71 6 23 -- 594 825 25 72 20625 75 Example 29 74 2 24 -- 676 834 21
81 17514 76 Comparative Example 30 62 8 30 -- 671 958 23 70 22034
79 Example 31 60 1 39 -- 804 980 19 82 18620 85 Comparative Example
32 73 8 19 -- 589 830 23 71 19090 74 Example 33 74 14 7 B 566 885
22 64 19470 43 Comparative Example 34 40 7 53 -- 935 1299 16 72
20784 82 Example 35 40 31 29 -- 863 1370 15 63 20550 38 Comparative
Example 36 80 4 16 -- 587 839 26 70 21814 83 Example 37 77 1 22 --
715 851 22 84 18722 78 Comparative Example 38 60 6 34 B 583 845 24
69 20280 81 Example 39 60 0 40 -- 680 861 20 79 17220 85
Comparative Example 40 58 6 36 -- 801 1112 19 72 21128 83 Example
41 60 22 18 -- 761 1171 18 65 21078 25 Comparative Example 42 91 1
8 -- 363 471 35 77 16485 62 Comparative Example 43 15 9 76 -- 1028
1224 12 84 14688 75 Comparative Example 44 93 1 4 P 245 335 45 73
15075 65 Comparative Example *F ferrite, M martensite, .gamma.
austenite, P pearlite, B bainite
Example 2
[0076] The steels AA to AL having the elemental compositions shown
in Table 6 were ingoted by a converter, made into slabs by a
continuous casting process. Subsequently, the slabs were subjected
to hot rolling at a finish temperature of 900.degree. C. to give a
thickness of 3.0 mm, cooled at a cooling rate of 10.degree. C./s,
and then wound up at a temperature of 600.degree. C. Subsequently,
after pickling, the slabs were subjected to cold rolling to give a
thickness of 1.2 mm, and annealed on a continuous galvanizing line
under the conditions shown in Tables 7. Thereafter, the steel
sheets were immersed in a galvanizing bath at 460.degree. C. to
form a coating layer at a coating weight of 35 to 45 g/m.sup.2,
subjected to galvannealing treatment at 520.degree. C., and cooled
at a cooling rate of 10.degree. C./s to make galvanized steel
sheets 101 to 130. As shown in Table 7, some galvanized steel
sheets were not subjected to galvannealing treatment. The
galvanized steel sheets thus obtained were measured for the area
fractions of ferrite, martensite, residual austenite, and tempered
martensite, and the average crystal grain diameter of the second
phase composed of martensite, residual austenite, and tempered
martensite by the above-described methods. Further, JIS No. 5
tensile test specimens were cut out along and perpendicular to the
rolling direction, and subjected to tensile test according to JIS Z
2241 to determine TS.times.E1. Further, test specimens of 150
mm.times.150 mm were cut out, and subjected to hole expansion test
three times according to JFS T 1001 (Japan Iron and Steel
Federation standard) to determine the average hole expansion ratio
.lamda. (%), whereby the stretch-flangeability was evaluated.
Further, according to the method described in "Tetsu To Hagane
(Iron and Steel)," Vol. 83 (1997), p. 748, test specimens having a
width of 5 mm and a length of 7 mm were cut out along and
perpendicular to the rolling direction, and subjected to tensile
test at a strain rate of 2000/s. The stress-true strain curve was
integrated over the strain amount of 0 to 10% to calculate the
absorption energy AE and AE/TS, whereby the anti-crush properties
were evaluated.
[0077] The results are shown in Tables 8 and 9, indicating that all
of our galvanized steel sheets satisfied TSE1.gtoreq.19000 MPa%,
hole expansion ratio .lamda..gtoreq.50, and AE/TS.gtoreq.0.063,
representing their high TS-E1 balance, excellent
stretch-flangeability, and excellent anti-crush properties.
TABLE-US-00006 TABLE 6 Elemental composition (% by mass) Steel C Si
Mn P S Al Ti Nb V Cr Mo Ni Cu AA 0.10 1.0 2.0 0.011 0.005 0.03 0.04
-- -- -- -- -- -- AB 0.08 0.8 2.5 0.010 0.002 0.04 -- 0.02 -- -- --
-- -- AC 0.21 1.4 1.6 0.009 0.010 0.03 -- -- 0.05 0.2 -- -- -- AD
0.14 2.0 1.8 0.008 0.004 0.60 0.10 -- -- -- 0.3 -- -- AE 0.18 0.2
2.2 0.012 0.003 0.04 0.02 0.03 -- -- -- 0.3 -- AF 0.09 1.2 1.4
0.009 0.001 0.30 -- 0.02 0.02 -- -- -- 0.2 AG 0.12 1.5 1.9 0.007
0.007 0.05 0.05 -- 0.03 0.3 -- -- -- AH 0.08 0.9 2.3 0.012 0.004
0.03 0.10 0.03 -- 0.03 0.1 -- -- AI 0.11 1.8 2.0 0.021 0.005 1.20
0.01 0.01 -- -- -- -- -- AJ 0.03 0.5 1.4 0.008 0.006 0.04 0.02 --
-- -- -- -- -- AK 0.07 0.2 0.2 0.009 0.004 0.05 -- 0.03 -- -- -- --
-- AL 0.12 1.0 1.8 0.011 0.003 0.03 -- -- -- -- -- -- -- Elemental
composition Ac.sub.1 Ac.sub.3 (% by mass) Transformation
Transformation Steel B Ca REM point (.degree. C.) point (.degree.
C.) Note AA -- -- -- 721 875 Beyond the scope of the invention AB
-- -- -- 710 851 Beyond the scope of the invention AC -- -- -- 735
876 Beyond the scope of the invention AD -- -- -- 684 896 Beyond
the scope of the invention AE 0.001 -- -- 706 805 Beyond the scope
of the invention AF -- 0.003 -- 742 920 Beyond the scope of the
invention AG -- -- 0.005 737 892 Beyond the scope of the invention
AH 0.002 -- -- 718 863 Beyond the scope of the invention AI --
0.002 0.001 734 886 Beyond the scope of the invention AJ -- -- --
719 883 Beyond the scope of the invention AK -- -- -- 728 899
Beyond the scope of the invention AL -- -- -- 723 876 Beyond the
scope of the invention
TABLE-US-00007 TABLE 7 Annealing conditions Heating temperature
Heating Reheating Galvanized Heating (end-point main- Cooling
Cooling Reheating maintaining steel rate temperature) taining rate
end-point temperature period Galvan- sheet No. Steel (.degree.
C./s) (.degree. C.) period (s) (.degree. C./s) (.degree. C.)
(.degree. C.) (s) Ms point (.degree. C.) nealing Note 101 AA 25 820
60 50 180 400 60 329 Treated Example 102 5 820 60 50 160 400 60 309
Treated Comparative Example 103 25 820 60 50 260 400 60 329 Treated
Comparative Example 104 AB 25 780 90 80 200 450 90 354 Untreated
Example 105 23 680 90 80 140 450 90 261 Untreated Comparative
Example 106 15 920 90 80 220 450 90 378 Untreated Comparative
Example 107 AC 73 840 40 30 120 360 40 266 Treated Example 108 70
800 5 30 100 360 40 221 Treated Comparative Example 109 70 780 30
30 30 360 40 246 Treated Comparative Example 110 AD 33 820 20 40
190 500 20 328 Treated Example 111 12 780 40 5 170 500 20 294
Treated Comparative Example 112 20 800 40 30 300 500 20 343 Treated
Comparative Example 113 AE 26 740 80 50 220 400 120 352 Treated
Example 114 2 760 50 50 200 400 120 344 Treated Comparative Example
115 20 780 60 50 220 250 30 355 Treated Comparative Example 116 20
780 60 50 220 650 60 346 Treated Comparative Example 117 20 1000 60
150 200 350 60 390 Treated Comparative Example 118 AF 30 790 30 25
150 420 60 285 Treated Example 119 7 810 30 25 100 420 60 285
Treated Comparative Example 120 20 800 40 25 160 450 900 285
Treated Comparative Example 121 25 780 40 25 140 400 0 270 Treated
Comparative Example 122 AG 100 810 40 30 190 470 20 316 Treated
Example 123 5 800 40 30 160 470 20 297 Treated Comparative Example
124 AH 30 820 60 150 240 380 60 378 Untreated Example 125 12 830 60
150 320 380 60 367 Untreated Comparative Example 126 AI 34 820 90
70 250 440 60 409 Treated Example 127 20 840 90 70 100 440 60 398
Treated Comparative Example 128 AJ 25 820 60 50 230 420 60 388
Treated Comparative Example 129 AK 32 840 80 50 150 400 60 291
Treated Comparative Example 130 AL 30 800 90 50 180 400 60 307
Treated Comparative Example
TABLE-US-00008 TABLE 8 Microstructure* F Crystal grain Galvanized
Area diameter of Tensile characteristic values steel fraction M +
residual .gamma. Tempered M second phase TS .times. El AE sheet No.
(%) area fraction (%) Area fraction (%) (.mu.m) TS (MPa) El (%)
(MPa %) .lamda. (%) (MJ/m.sup.3) AE/TS Note 101 75 5 20 2.2 845 24
20280 80 53 0.063 Example 102 78 4 18 4.5 830 25 20750 75 40 0.048
Comparative Example 103 75 12 13 2.4 870 24 20880 45 54 0.062
Comparative Example 104 71 5 24 2.0 882 22 19404 87 59 0.067
Example 105 85 1 1 1.8 750 20 15000 65 32 0.042 Comparative Example
106 62 7 31 4.2 856 22 18832 72 40 0.047 Comparative Example 107 65
7 28 1.4 1046 20 20920 82 72 0.069 Example 108 71 4 8 1.8 972 17
16524 70 52 0.053 Comparative Example 109 68 1 31 1.5 1010 17 17170
87 73 0.072 Comparative Example 110 70 7 23 2.1 1208 18 21744 74 79
0.065 Example 111 75 2 8 2.4 1070 16 17120 43 54 0.050 Comparative
Example 112 67 21 12 2.4 1270 17 21590 24 89 0.070 Comparative
Example 113 40 9 51 2.1 1228 16 19648 55 82 0.067 Example 114 44 8
48 5.5 1180 16 18880 58 53 0.045 Comparative Example 115 38 14 48
2.3 1340 14 18760 29 84 0.063 Comparative Example 116 43 2 55 1.9
1023 14 14322 36 62 0.061 Comparative Example 117 4 8 88 7 1280 8
10240 75 65 0.051 Comparative Example *F ferrite, M martensite,
.gamma. austenite
TABLE-US-00009 TABLE 9 Microstructure* F Crystal grain Galvanized
Area diameter of Tensile characteristic values steel fraction M +
residual .gamma. Tempered M second phase TS .times. El AE sheet No.
(%) area fraction (%) Area fraction (%) (.mu.m) TS (MPa) El (%)
(MPa %) .lamda. (%) (MJ/m.sup.3) AE/TS Note 118 85 4 11 2.0 742 28
20776 78 51 0.069 Example 119 85 4 11 3.4 725 29 21025 82 40 0.055
Comparative Example 120 85 1 14 2.3 693 25 17325 92 45 0.065
Comparative Example 121 86 12 2 2.1 773 26 20098 43 53 0.069
Comparative Example 122 72 7 21 1.2 1024 21 21504 65 73 0.071
Example 123 75 6 19 3.3 995 22 21890 70 53 0.053 Comparative
Example 124 65 8 27 1.7 1286 15 19290 63 91 0.071 Example 125 69 18
13 2.5 1332 15 19980 23 97 0.073 Comparative Example 126 60 7 33
2.7 946 23 21758 82 61 0.064 Example 127 64 1 35 2.5 898 19 17062
95 55 0.061 Comparative Example 128 90 2 8 1.5 398 38 15124 68 19
0.047 Comparative Example 129 90 2 8 2.3 309 43 13287 63 13 0.042
Comparative Example 130 75 6 19 3.4 717 27 19359 70 35 0.049
Comparative Example *F ferrite, M martensite, .gamma. austenite
* * * * *