U.S. patent application number 16/076098 was filed with the patent office on 2020-05-28 for high-strength galvanized steel sheet and method of producing the same.
The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Yoshimasa Funakawa, Hiroshi Hasegawa, Katsutoshi Takashima, Hiromi Yoshitomi.
Application Number | 20200165708 16/076098 |
Document ID | / |
Family ID | 59563943 |
Filed Date | 2020-05-28 |
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United States Patent
Application |
20200165708 |
Kind Code |
A1 |
Hasegawa; Hiroshi ; et
al. |
May 28, 2020 |
HIGH-STRENGTH GALVANIZED STEEL SHEET AND METHOD OF PRODUCING THE
SAME
Abstract
A high-strength galvanized steel sheet includes a steel sheet
and a particular galvanized layer. The steel sheet has a particular
composition and a steel microstructure. The steel microstructure
contains ferrite and carbide-free bainite constituting 25% or less
(including 0%) by area fraction in total, tempered martensite and
carbide-containing bainite constituting 70% to 97% by area fraction
in total, martensite and retained austenite constituting 3% to 20%
by area fraction in total, and retained austenite constituting 1%
to 5% by area fraction. The ferrite, the carbide-free bainite, the
martensite, and the retained austenite constitute 3% to 30% by area
fraction in total. The retained austenite has a C content of 0.10%
to 0.50%. The carbide-containing bainite and the tempered
martensite have an average grain size of 5 to 20 .mu.m.
Inventors: |
Hasegawa; Hiroshi; (Tokyo,
JP) ; Funakawa; Yoshimasa; (Tokyo, JP) ;
Yoshitomi; Hiromi; (Tokyo, JP) ; Takashima;
Katsutoshi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
59563943 |
Appl. No.: |
16/076098 |
Filed: |
January 30, 2017 |
PCT Filed: |
January 30, 2017 |
PCT NO: |
PCT/JP2017/003151 |
371 Date: |
August 7, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/02 20130101;
Y02P 10/212 20151101; C23C 2/02 20130101; C21D 2211/001 20130101;
C22C 38/00 20130101; C23C 2/28 20130101; C22C 38/14 20130101; C21D
2211/002 20130101; C21D 2211/005 20130101; C22C 38/16 20130101;
C21D 8/0226 20130101; C22C 38/60 20130101; C22C 38/38 20130101;
C23C 2/40 20130101; C22C 38/12 20130101; C21D 9/46 20130101; C22C
38/32 20130101; C22C 38/04 20130101; C23C 18/54 20130101; C22C
38/002 20130101; C22C 38/001 20130101; C23C 2/06 20130101; C22C
38/06 20130101; C21D 2211/008 20130101; C21D 8/0236 20130101; C22C
38/08 20130101; C22C 38/005 20130101 |
International
Class: |
C22C 38/04 20060101
C22C038/04; C22C 38/14 20060101 C22C038/14; C21D 8/02 20060101
C21D008/02; C23C 18/54 20060101 C23C018/54; C23C 2/40 20060101
C23C002/40; C22C 38/08 20060101 C22C038/08; C22C 38/02 20060101
C22C038/02; C22C 38/06 20060101 C22C038/06; C22C 38/00 20060101
C22C038/00; C22C 38/12 20060101 C22C038/12; C23C 2/02 20060101
C23C002/02; C22C 38/32 20060101 C22C038/32; C21D 9/46 20060101
C21D009/46; C22C 38/16 20060101 C22C038/16; C22C 38/60 20060101
C22C038/60 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 10, 2016 |
JP |
2016-023403 |
Claims
1.-3. (canceled)
4. A high-strength galvanized steel sheet comprising: a steel sheet
and a galvanized layer, wherein the steel sheet has a composition
and a steel microstructure, the composition including, on a mass
percent basis, C: 0.08% to 0.15%, Si: 0.1% to 1.0%, Mn: 2.0% to
3.0%, P: 0.100% or less (excluding 0%), S: 0.02% or less (excluding
0%), Al: 0.01% to 1.0%, and N: 0.010% or less, and one or more
selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005%
to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%, Cr+Ti+Nb+Mo+V
being greater than or equal to Si, the remainder being Fe and
incidental impurities, the steel microstructure containing ferrite
and carbide-free bainite constituting 25% or less (including 0%) by
area fraction in total, tempered martensite and carbide-containing
bainite constituting 70% to 97% by area fraction in total,
martensite and retained austenite constituting 3% to 20% by area
fraction in total, and retained austenite constituting 1% to 5% by
area fraction, the ferrite, the carbide-free bainite, the
martensite, and the retained austenite constituting 3% to 30% by
area fraction in total, the retained austenite having a C content
of 0.10% to 0.50%, the carbide-containing bainite and the tempered
martensite having an average grain size of 5 to 20 .mu.m, and the
galvanized layer is disposed on the steel sheet and has an Fe
content of 6% or more by mass.
5. The high-strength galvanized steel sheet according to claim 4,
wherein the composition further includes, on a mass percent basis,
one or more selected from Ni: 0.005% to 2.0%, Cu: 0.005% to 2.0%,
B: 0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, REM: 0.0001% to
0.0050%, Sn: 0.01% to 0.50%, and Sb: 0.0010% to 0.10%.
6. A method of producing a high-strength galvanized steel sheet,
comprising: an annealing step of hot-rolling and pickling a steel
having the composition according to claim 4 to form a hot-rolled
sheet, optionally cold-rolling the hot-rolled sheet at a rolling
reduction of 20% or more to form a cold-rolled sheet, heating the
hot-rolled sheet or the cold-rolled sheet to a heating temperature
of 780.degree. C. to 950.degree. C., a heating rate of 600.degree.
C. to 720.degree. C. being 3.0.degree. C./s or less, cooling the
sheet to a temperature of 150.degree. C. to 400.degree. C., an
average cooling rate of 450.degree. C. to 720.degree. C. being
10.degree. C./s or more, a residence time at 780.degree. C. to
950.degree. C. in the heating and cooling being 30 seconds or more,
and after the cooling, reheating the sheet at a heating temperature
of more than 150.degree. C. and 600.degree. C. or less, a residence
time at a temperature of more than 150.degree. C. and 600.degree.
C. or less being 1000 seconds or less, or after the cooling,
holding the sheet at a temperature of more than 150.degree. C. and
400.degree. C. or less for a residence time of 1000 seconds or
less; and a galvanizing step of galvanizing the annealed sheet
after the annealing step, then heating the sheet to a temperature
of 460.degree. C. to 600.degree. C. for plating and alloying
treatment, and cooling the sheet to 50.degree. C. or less.
7. A method of producing a high-strength galvanized steel sheet,
comprising: an annealing step of hot-rolling and pickling a steel
having the composition according to claim 5 to form a hot-rolled
sheet, optionally cold-rolling the hot-rolled sheet at a rolling
reduction of 20% or more to faun a cold-rolled sheet, heating the
hot-rolled sheet or the cold-rolled sheet to a heating temperature
of 780.degree. C. to 950.degree. C., a heating rate of 600.degree.
C. to 720.degree. C. being 3.0.degree. C./s or less, cooling the
sheet to a temperature of 150.degree. C. to 400.degree. C., an
average cooling rate of 450.degree. C. to 720.degree. C. being
10.degree. C./s or more, a residence time at 780.degree. C. to
950.degree. C. in the heating and cooling being 30 seconds or more,
and after the cooling, reheating the sheet at a heating temperature
of more than 150.degree. C. and 600.degree. C. or less, a residence
time at a temperature of more than 150.degree. C. and 600.degree.
C. or less being 1000 seconds or less, or after the cooling,
holding the sheet at a temperature of more than 150.degree. C. and
400.degree. C. or less for a residence time of 1000 seconds or
less; and a galvanizing step of galvanizing the annealed sheet
after the annealing step, then heating the sheet to a temperature
of 460.degree. C. to 600.degree. C. for plating and alloying
treatment, and cooling the sheet to 50.degree. C. or less.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a high-strength galvanized steel
sheet suitable for automotive parts and having good bendability,
spot weld crack resistance, stretch-flangeability, and axial
compression stability (which means that a sheet can be stably
pleated without cracks or fractures in axial compression) and to a
method of producing the high-strength galvanized steel sheet.
BACKGROUND
[0002] From the perspective of improved crash safety, fuel
efficiency, and rust prevention of automobiles, there is a demand
for anticorrosive high-strength steel sheets for use in automotive
parts. However, higher-strength materials often have poorer
workability. Thus, there is a demand for steel sheets with high
workability as well as high strength and rust prevention.
Furthermore, processed parts should have high performance. For
example, parts such as crash boxes having a bent portion or a
punched surface should have good bendability and
stretch-flangeability from the perspective of workability. From the
perspective of assembly and performance of parts, parts should have
good spot welds and stable axial compression characteristics. Those
characteristics are desirably totally satisfied. Japanese
Unexamined Patent Application Publication No. 2015-193907 discloses
a technique related to a galvannealed steel sheet with good
bendability and stretch-flangeability. However, Japanese Unexamined
Patent Application Publication No. 2015-193907 does not describe
spot weld cracking due to galvanizing or axial compression
stability, and there is room for improvement in those
characteristics.
[0003] Japanese Unexamined Patent Application Publication No.
2009-138222 discloses a technique for decreasing the size of
ferrite grains to improve axial compression stability. However,
Japanese Unexamined Patent Application Publication No. 2009-138222
does not consider stretch-flangeability, leaving room for
improvement of stretch-flangeability. Furthermore, Japanese
Unexamined Patent Application Publication No. 2009-138222 has no
findings in galvanized steel sheets and does not consider spot weld
cracking due to galvanizing.
[0004] Thus, no steel sheets totally satisfy high strength,
bendability, spot weld crack resistance, stretch-flangeability, and
axial compression stability.
[0005] It could therefore be helpful to provide a high-strength
galvanized steel sheet with good bendability, spot weld crack
resistance, stretch-flangeability, and axial compression stability
and a method of producing the high-strength galvanized steel
sheet.
SUMMARY
[0006] We thus provide a high-strength hot-dip galvanized steel
sheet that includes a steel sheet and a galvanized layer disposed
on the steel sheet, wherein the steel sheet has a composition and a
steel microstructure, the composition including, on a mass percent
basis, C: 0.08% to 0.15%, Si: 0.1% to 1.0%, Mn: 2.0% to 3.0%, P:
0.100% or less (excluding 0%), S: 0.02% or less (excluding 0%), Al:
0.01% to 1.0%, and N: 0.010% or less, and one or more selected from
Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005% to 0.20%, Mo:
0.005% to 2.0%, and V: 0.005% to 2.0%, Cr+Ti+Nb+Mo+V being greater
than or equal to Si, the remainder being Fe and incidental
impurities, the steel microstructure containing ferrite and
carbide-free bainite constituting 25% or less (including 0%) by
area fraction in total, tempered martensite and carbide-containing
bainite constituting 70% to 97% by area fraction in total,
martensite and retained austenite constituting 3% to 20% by area
fraction in total, and retained austenite constituting 1% to 5% by
area fraction, the ferrite, the carbide-free bainite, the
martensite, and the retained austenite constituting 3% to 30% by
area fraction in total, the retained austenite having a C content
of 0.10% to 0.50%, the carbide-containing bainite and the tempered
martensite having an average grain size of 5 to 20 .mu.m.
[0007] We further provide:
[1] A high-strength galvanized steel sheet including a steel sheet
and a galvanized layer, wherein the steel sheet has a composition
and a steel microstructure, the composition including, on a mass
percent basis, C: 0.08% to 0.15%, Si: 0.1% to 1.0%, Mn: 2.0% to
3.0%, P: 0.100% or less (excluding 0%), S: 0.02% or less (excluding
0%), Al: 0.01% to 1.0%, and N: 0.010% or less, and one or more
selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005%
to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%, Cr+Ti+Nb+Mo+V
being greater than or equal to Si, the remainder being Fe and
incidental impurities, the steel microstructure containing ferrite
and carbide-free bainite constituting 25% or less (including 0%) by
area fraction in total, tempered martensite and carbide-containing
bainite constituting 70% to 97% by area fraction in total,
martensite and retained austenite constituting 3% to 20% by area
fraction in total, and retained austenite constituting 1% to 5% by
area fraction, the ferrite, the carbide-free bainite, the
martensite, and the retained austenite constituting 3% to 30% by
area fraction in total, the retained austenite having a C content
of 0.10% to 0.50%, the carbide-containing bainite and the tempered
martensite having an average grain size of 5 to 20 .mu.m, and the
galvanized layer is disposed on the steel sheet and has an Fe
content of 6% or more by mass. [2] The high-strength galvanized
steel sheet according to [1], wherein the composition further
includes, on a mass percent basis, one or more selected from Ni:
0.005% to 2.0%, Cu: 0.005% to 2.0%, B: 0.0001% to 0.0050%, Ca:
0.0001% to 0.0050%, REM: 0.0001% to 0.0050%, Sn: 0.01% to 0.50%,
and Sb: 0.0010% to 0.10%. [3] A method of producing a high-strength
galvanized steel sheet, including: an annealing step of hot-rolling
and pickling a steel having a composition according to [1] or [2]
to form a hot-rolled sheet, optionally cold-rolling the hot-rolled
sheet at a rolling reduction of 20% or more to form a cold-rolled
sheet, heating the hot-rolled sheet or the cold-rolled sheet to a
heating temperature in the range of 780.degree. C. to 950.degree.
C., a heating rate in a temperature range of 600.degree. C. to
720.degree. C. being 3.0.degree. C./s or less, cooling the sheet to
a temperature in the range of 150.degree. C. to 400.degree. C., an
average cooling rate in a temperature range of 450.degree. C. to
720.degree. C. being 10.degree. C./s or more, a residence time in
the temperature range of 780.degree. C. to 950.degree. C. in the
heating and cooling being 30 seconds or more and, after the
cooling, reheating the sheet at a heating temperature of more than
150.degree. C. and 600.degree. C. or less, a residence time at a
temperature of more than 150.degree. C. and 600.degree. C. or less
being 1000 seconds or less, or after the cooling, holding the sheet
at a temperature of more than 150.degree. C. and 400.degree. C. or
less for a residence time of 1000 seconds or less; and a
galvanizing step of galvanizing the annealed sheet after the
annealing step, then heating the sheet to a temperature in the
range of 460.degree. C. to 600.degree. C. for plating and alloying
treatment, and cooling the sheet to 50.degree. C. or less.
[0008] We provide a high-strength galvanized steel sheet with good
bendability, spot weld crack resistance, stretch-flangeability, and
axial compression stability. The high-strength galvanized steel
sheet is suitable as a material for automotive parts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is an explanatory view of carbide-containing bainite
(a) and carbide-free bainite (b).
[0010] FIG. 2 is a schematic view of an axial compression component
used in the evaluation of axial compression stability.
[0011] FIG. 3 is a schematic view illustrating a method of forming
a compression test specimen used in the evaluation of axial
compression stability.
[0012] FIG. 4 is a schematic view illustrating a method of
evaluating spot weld cracking.
REFERENCE SIGNS LIST
[0013] 1 spot weld [0014] 2 base plate [0015] 3 TIG weld [0016] 4
electrode
DETAILED DESCRIPTION
[0017] Examples of our steel sheets and methods will be described
below. This disclosure is not limited to the examples.
[0018] Our high-strength galvanized steel sheet includes a steel
sheet and a galvanized layer. First, the steel sheet will be
described below. The steel sheet has a particular composition and a
particular steel microstructure. The composition and the steel
microstructure are described below in this order.
[0019] The steel sheet has a composition including, on a mass
percent basis, C: 0.08% to 0.15%, Si: 0.1% to 1.0%, Mn: 2.0% to
3.0%, P: 0.100% or less (excluding 0%), S: 0.02% or less (excluding
0%), Al: 0.01% to 1.0%, and N: 0.010% or less, and one or more
selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005%
to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%, Cr+Ti+Nb+Mo+V
being greater than or equal to Si, the remainder being Fe and
incidental impurities.
[0020] The composition may further include, on a mass percent
basis, one or more selected from Ni: 0.005% to 2.0%, Cu: 0.005% to
2.0%, B: 0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, REM: 0.0001%
to 0.0050%, Sn: 0.01% to 0.50%, and Sb: 0.0010% to 0.10%. Each of
the components will be described below. The symbol "%" in the
component content refers to "% by mass".
C: 0.08% to 0.15%
[0021] C is an effective element that contributes to an increased
area fraction, strength, and axial compression stability of
martensite or retained austenite. C content of less than 0.08% is
insufficient for such effects. C content of more than 0.15% results
in deteriorated bendability or spot weld cracking. Thus, C content
is 0.08% to 0.15%. The lower limit is preferably 0.09% or more. The
upper limit is preferably 0.13% or less.
Si: 0.1% to 1.0%
[0022] Si reduces formation of carbide and promotes concentration
of C in austenite, thereby inducing formation of retained
austenite. Thus, Si is an effective element that contributes to
improved axial compression stability. Such effects require a Si
content of 0.1% or more. Si content of more than 1.0% results in
excessive stabilization of retained austenite and low axial
compression stability. Thus, Si content is 0.1% to 1.0%. The lower
limit is preferably 0.15% or more, more preferably more than 0.3%.
The upper limit is preferably 0.7% or less.
Mn: 2.0% to 3.0%
[0023] Mn is an element that is effective in inhibiting formation
of ferrite or carbide-free bainite and forming tempered martensite
or retained austenite. Mn content of 2.0% or less is insufficient
for such effects. Mn content of more than 3.0% results in spot weld
cracking. Thus, Mn content is 2.0% to 3.0%. The lower limit is
preferably 2.3% or more. The upper limit is preferably 2.8% or
less.
P: 0.100% or Less (Excluding 0%)
[0024] P lowers hole expandability, and the P content is desirably
minimized. The P content may be 0.100% or less. Although P content
has no lower limit, P content of less than 0.001% results in low
production efficiency. Thus, P content is preferably 0.001% or
more.
S: 0.02% or Less (Excluding 0%)
[0025] S lowers hole expandability, and S content is preferably
minimized. S content may be 0.02% or less. Although S content has
no lower limit, S content of less than 0.0002% results in low
production efficiency. Thus, S content is preferably 0.0002% or
more.
Al: 0.01% to 1.0%
[0026] Al functions as a deoxidizer and is preferably added in a
deoxidation step. Such an effect requires Al content of 0.01% or
more. Al content of more than 1.0% results in a large amount of
ferrite or carbide-free bainite, which is unfavorable. Thus, Al
content is 0.01% to 1.0%. The lower limit is preferably 0.02% or
more. The upper limit is preferably 0.50% or less.
N: 0.010% or Less
[0027] An excessive amount of N results in spot weld cracking or
deteriorated bendability. Thus, N content is 0.010% or less. The
lower limit is preferably 0.0010% or more. The upper limit is
preferably 0.0060% or less.
At Least One Selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%,
Nb: 0.005% to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%
[0028] Cr, Nb, Ti, Mo, and V form carbides and decrease the C
content of retained austenite. Thus, these elements are important
components that contribute to improved axial compression stability.
Cr, Nb, Ti, Mo, or V content lower than the lower limit described
above results in low axial compression stability. Cr, Nb, Ti, Mo,
or V content higher than the upper limit results in an excessively
low C content of retained austenite and a low hole expanding ratio.
Thus, at least one selected from Cr, Nb, Ti, Mo, and V contents is
Cr: 0.005% to 2.0%, Nb: 0.005% to 0.20%, Ti: 0.005% to 0.20%, Mo:
0.005% to 2.0%, or V: 0.005% to 2.0%. The preferred lowest contents
are 0.10% or more for Cr, 0.01% or more for Nb, 0.01% or more for
Ti, 0.05% or more for Mo, and 0.03% or more for V. The preferred
highest contents are 0.80% or less for Cr, 0.05% or less for Nb,
0.05% or less for Ti, 0.50% or less for Mo, and 0.50% or less for
V.
[0029] For two or more elements selected from Cr, Nb, Ti, Mo, and
V, the amount of at least one of the elements is in the range
described above. This is because the amounts of the other elements
below the lower limits do not reduce the desired effects.
Components in amounts below the lower limits are considered to be
incidental impurities.
Cr+Ti+Nb+Mo+V.gtoreq.Si
[0030] The amount of Cr, Ti, Nb, Mo, and V, which are
carbide-forming elements, greater than or equal to the amount of
Si, which is a carbide-inhibiting element, results in a low C
content of retained austenite and desired axial compression
stability. Thus, the sum of the Cr, Ti, Nb, Mo, and V contents is
greater than or equal to the Si content.
[0031] The remainder includes Fe and incidental impurities. If
necessary, the composition may include an appropriate amount of one
or more of the following elements.
[0032] As described above, the composition may further include one
or more selected from Ni: 0.005% to 2.0%, Cu: 0.005% to 2.0%, B:
0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, REM: 0.0001% to
0.0050%, Sn: 0.01% to 0.50%, and Sb: 0.0010% to 0.10%.
[0033] Ni, Cu, and B are effective elements that induce formation
of martensite and that contribute to high strength. Ni, Cu, and B
contents are above the lower limits to effectively produce such
effects. Ni, Cu, or B content above the upper limit results in spot
weld cracking or deteriorated axial compression stability. The
preferred lowest contents are 0.050% or more for Ni, 0.050% or more
for Cu, and 0.0005% or more for B. The preferred highest contents
are 0.5% or less for Ni, 0.5% or less for Cu, and 0.0040% or less
for B.
[0034] Ca and REM are effective elements that contribute to
improved hole expandability due to the morphology control of
inclusions. The amounts of these elements are above the lower
limits to effectively produce such an effect. Ca content or a REM
content of more than 0.0050% may result in poor hole expandability
due to an increased amount of inclusions. The preferred lowest
contents are 0.0005% or more for Ca and 0.0005% or more for REM.
The preferred highest contents are 0.0030% or less for Ca and
0.0030% or less for REM.
[0035] Sn and Sb are elements that inhibit, for example,
decarbonization, denitrification, or deboronization and are
effective in inhibiting strength reduction of steel. Sn and Sb are
also effective in inhibiting spot weld cracking. The amounts of
these elements are above the lower limits to effectively produce
such effects. Sn content or Sb content above the upper limit may
result in poor hole expandability. The preferred lowest content is
0.0050% or more for each of Sn and Sb. The preferred highest
contents are 0.10% or less for Sn and 0.05% or less for Sb.
[0036] Ni, Cu, B, Ca, REM, Sn, and Sb contents below their
respective lower limits are permissible because these do not reduce
the desired effects. Below the lower limits, these components are
considered to be incidental impurities.
[0037] Impurity elements such as Zr, Mg, La, and Ce may constitute
up to 0.002% as incidental impurities.
[0038] The steel microstructure is described below. The steel
microstructure contains ferrite and carbide-free bainite
constituting 25% or less (including 0%) by area fraction in total,
tempered martensite and carbide-containing bainite constituting 70%
to 97% by area fraction in total, martensite and retained austenite
constituting 3% to 20% by area fraction in total, and retained
austenite constituting 1% to 5% by area fraction. The ferrite, the
carbide-free bainite, the martensite, and the retained austenite
constitute 3% to 30% by area fraction in total. The retained
austenite has a C content of 0.10% to 0.50%. The carbide-containing
bainite and the tempered martensite have an average grain size of 5
to 20 .mu.m. Bainite includes tempered bainite.
Ferrite and Carbide-Free Bainite: 25% or Less (Including 0%) in
Total
[0039] Ferrite and carbide-free bainite are unfavorable in terms of
the balance between strength and stretch-flangeability and in terms
of axial compression stability. Ferrite and carbide-free bainite
can constitute up to 25% by area fraction in total. Thus, the total
area fraction of ferrite and carbide-free bainite is 25% or less,
preferably 15% or less. The area fraction is determined by the
measurement method described in the examples.
[0040] Carbide-free bainite contains no carbide in an image data,
which is taken with a scanning electron microscope (SEM) at a
magnification of 1500 at one fourth the thickness of a sheet from a
surface of the sheet in a cross section of the sheet parallel to
the rolling direction and parallel to the thickness direction
polished and then subjected to corrosion with 3% nital. In image
data shown in FIG. 1, carbide, which appears as white dots or
lines, can be distinguished from island martensite or retained
austenite, which does not appear as dots or lines. Dots or lines
have a minor axis length of 100 nm or less. Carbide-free bainite
may include island martensite that appears as white or light gray
portions in image data. Carbide may be an iron carbide such as
cementite, Ti carbide, or Nb carbide.
Tempered Martensite and Carbide-Containing Bainite: 70% to 97% in
Total
[0041] Tempered martensite and carbide-containing bainite are
important for the balance between strength and
stretch-flangeability. A total area fraction of these components
below 70% results in an undesired tensile strength (TS) or hole
expanding ratio. A total area fraction of more than 97% results in
low axial compression stability due to a decreased amount of
retained austenite or martensite. Thus, the total area fraction of
tempered martensite and carbide-containing bainite is 70% to 97%.
The lower limit is preferably 75% or more, more preferably 80% or
more. The upper limit is preferably 95% or less, more preferably
93% or less. The area fraction is determined by the measurement
method described in the examples.
[0042] Carbide-containing bainite contains carbide in an image
data, which is taken with a scanning electron microscope (SEM) at a
magnification of 1500 at one fourth the thickness of a sheet from a
surface of the sheet in a cross section of the sheet parallel to
the rolling direction and parallel to the thickness direction
polished and then subjected to corrosion with 3% nital. The area
fraction is determined by the method described in the examples.
Martensite and Retained Austenite: 3% to 20% in Total
[0043] A total area fraction of martensite (which means as-quenched
martensite and also includes autotempered martensite) and retained
austenite below 3% results in low axial compression stability. A
total area fraction of more than 20% results in a low hole
expanding ratio. Thus, the total area fraction of martensite and
retained austenite is 3% to 20%. The lower limit is preferably 5%
or more. The upper limit is preferably 15% or less. The area
fraction is determined by the measurement method described in the
examples. Retained austenite has a C content of 0.10% to 0.50%, as
described below.
Retained Austenite: 1% to 5%
[0044] A minute amount of retained austenite is important for axial
compression stability. Although the mechanism is not entirely
clear, we believe that unstable retained austenite in a deformation
stress concentrated portion changes immediately into martensite in
axial compression and promotes strain propagation. Less than 1% of
unstable retained austenite is insufficient for such an effect.
More than 5% of unstable retained austenite lowers bendability or
stretch-flangeability. Thus, the area fraction of retained
austenite is 1% to 5%. The area fraction is determined by the
measurement method described in the examples.
C Content of Retained Austenite: 0.10% to 0.50%
[0045] C content of retained austenite above 0.50% results in
excessively stable retained austenite and low axial compression
stability. C content of less than 0.10% results in a low hole
expanding ratio. Thus, C content of retained austenite is 0.10% to
0.50%. The lower limit is preferably 0.2% or more. The upper limit
is preferably 0.45% or less. The C content is determined by the
measurement method described in the examples.
Ferrite, Carbide-Free Bainite, Martensite, and Retained Austenite:
3% to 30% in Total
[0046] The total area fraction of ferrite, carbide-free bainite,
martensite, and retained austenite, if present at all, is 3% to 30%
in terms of the balance between bendability, axial compression
stability, and stretch-flangeability. A total area fraction of less
than 3% results in undesired amounts of martensite and retained
austenite and low axial compression stability. A total area
fraction of more than 30% results in low bendability and a low hole
expanding ratio. Thus, the total area fraction of ferrite,
carbide-free bainite, martensite, and retained austenite is 3% to
30%. The lower limit is preferably 4% or more, more preferably 5%
or more. The upper limit is preferably 15% or less, more preferably
12% or less.
Average Grain Sizes of Tempered Martensite and Carbide-Containing
Bainite: 5 to 20 .mu.m
[0047] An average grain size of tempered martensite or
carbide-containing bainite below 5 .mu.m results in an increased
amount of retained austenite at grain boundaries and results in low
bendability or a low hole expanding ratio. An average grain size of
tempered martensite or carbide-containing bainite above 20 .mu.m
results in low bendability or axial compression stability. Thus,
each of tempered martensite and carbide-containing bainite has an
average grain size of 5 to 20 .mu.m. The lower limit is preferably
6 .mu.m or more. The upper limit is preferably 16 .mu.m or less.
The average grain size is determined by the method described in the
examples.
[0048] The steel microstructure may include other microstructures
such as pearlite. Other microstructures may constitute up to 10% by
area fraction without reducing the desired effects. In other words,
the phases described above preferably constitute 90% or more.
[0049] Next, the galvanized layer is described below. The
galvanized layer has an Fe content of 6% or more by mass. Fe
content of 6% or more by mass can result in improved spot weld
crack resistance. The Fe content has no upper limit but is
preferably 15% or less by mass in consideration of powdering.
Method of Producing High-Strength Galvanized Steel Sheet
[0050] A method of producing a high-strength galvanized steel sheet
includes an annealing step and a galvanizing step.
[0051] The annealing step includes hot-rolling and pickling a steel
having the composition described above to form a hot-rolled sheet,
optionally cold-rolling the hot-rolled sheet at a rolling reduction
of 20% or more to form a cold-rolled sheet, heating the hot-rolled
sheet or the cold-rolled sheet to a heating temperature of
780.degree. C. to 950.degree. C., the heating rate in the
temperature range of 600.degree. C. to 720.degree. C. being
3.0.degree. C./s or less, cooling the sheet to a temperature of
150.degree. C. to 400.degree. C., the average cooling rate in the
temperature range of 450.degree. C. to 720.degree. C. being
10.degree. C./s or more, the residence time in the temperature
range of 780.degree. C. to 950.degree. C. in the heating and
cooling being 30 seconds or more, and after the cooling, reheating
the sheet at a heating temperature of more than 150.degree. C. and
600.degree. C. or less, the residence time at a temperature of more
than 150.degree. C. and 600.degree. C. or less being 1000 seconds
or less, or after the cooling, holding the sheet at a temperature
of more than 150.degree. C. and 400.degree. C. or less for a
residence time of 1000 seconds or less.
[0052] First, a hot-rolled sheet for use in a production method
will be described below. The hot-rolled sheet may be produced by
any method. A slab for use in the production of the hot-rolled
sheet is preferably formed by a continuous casting process to
prevent macrosegregation or may be formed by an ingot casting
process or a thin slab casting process. Prior to hot-rolling the
slab, the slab may be cooled to room temperature and subsequently
reheated for hot rolling or may be placed in a furnace for hot
rolling without being cooled to room temperature. Alternatively, an
energy-saving process may also be performed in which the slab is
immediately hot-rolled after lightly conserving heat. The slab is
preferably heated to 1100.degree. C. or more to melt carbide or
prevent an increase in rolling force. The slab is preferably heated
to 1300.degree. C. or less to prevent an increase in scale loss.
The slab heating temperature is the surface temperature of the
slab. Prior to hot-rolling the slab, a rough bar after rough
rolling may be heated. A continuous rolling process may also be
performed in which rough bars are joined together and continuously
subjected to finish rolling. Finish rolling may increase anisotropy
and reduce workability after cold rolling and annealing. Thus, the
finishing temperature is preferably higher than the Ara
transformation temperature. To reduce rolling force or make the
shape and material quality uniform, lubrication rolling to achieve
a friction coefficient of 0.10 to 0.25 is preferably performed in
all or part of the finish rolling passes.
[0053] A steel sheet after coiling is pickled to remove scales and,
if necessary, is subjected to heat treatment and/or cold rolling at
a rolling reduction of 20% or more.
Rolling Reduction in Cold Rolling: 20% or More
[0054] A rolling reduction in cold rolling below 20% results in the
absence of the steel microstructure due to coarse grains formed by
annealing. Thus, the rolling reduction in cold rolling is 20% or
more, preferably 30% or more. Although the rolling reduction has no
upper limit, a rolling reduction of more than 90% results in an
undesired shape of the rolled sheet. Thus, a rolling reduction of
90% or less is preferred.
Heating Rate in the Temperature Range of 600.degree. C. to
720.degree. C.: 3.0.degree. C./s or Less
[0055] A heating rate of more than 3.0.degree. C./s in the
temperature range of 600.degree. C. to 720.degree. C. results in
the absence of a steel microstructure due to a decrease in grain
size. Thus, the heating rate in the temperature range of
600.degree. C. to 720.degree. C. is 3.0.degree. C./s or less. The
heating rate has no lower limit but is preferably 0.1.degree. C./s
or more in terms of, for example, production efficiency. Outside
this temperature range, any heating rate is permissible. The
heating rate is typically 0.5.degree. C./s to 3.0.degree. C./s. The
heating rate refers to the average heating rate.
Heating Temperature: 780.degree. C. to 950.degree. C.
[0056] A heating temperature (annealing temperature) of less than
780.degree. C. results in an undesired steel microstructure due to
excessive formation of ferrite. A heating temperature of more than
950.degree. C. results in an undesired steel microstructure due to
coarse austenite. Thus, the heating temperature is 780.degree. C.
to 950.degree. C., preferably 780.degree. C. to 900.degree. C.
Average Cooling Rate in the Temperature Range of 450.degree. C. to
720.degree. C.: 10.degree. C./s or More
[0057] An average cooling rate of less than 10.degree. C./s in the
temperature range of 450.degree. C. to 720.degree. C. results in an
undesired steel microstructure due to formation of a large amount
of ferrite or carbide-free bainite. Thus, the average cooling rate
in the temperature range of 450.degree. C. to 720.degree. C. is
10.degree. C./s or more, preferably 15.degree. C./s or more.
Although the average cooling rate has no upper limit, an average
cooling rate of more than 100.degree. C./s tends to result in
considerable temperature variations. Thus, the average cooling rate
is preferably 100.degree. C./s or less from the viewpoint of
operation. Outside this temperature range, any average cooling rate
is permissible. The average cooling rate is typically 15.degree.
C./s to 40.degree. C./s.
Cooling Stop Temperature: 150.degree. C. to 400.degree. C.
[0058] A cooling stop temperature of less than 150.degree. C.
results in an undesired steel microstructure due to formation of a
decreased amount of retained austenite. A cooling stop temperature
of more than 400.degree. C. results in an undesired steel
microstructure due to formation of an increased amount of
martensite or retained austenite. Thus, the cooling stop
temperature is 150.degree. C. to 400.degree. C., preferably
150.degree. C. to 350.degree. C. Cooling stop and retention hardly
affects the final characteristics and may be performed as
required.
Residence Time in the Temperature Range of 780.degree. C. to
950.degree. C.: 30 Seconds or More
[0059] A residence time of less than 30 seconds in the temperature
range of 780.degree. C. to 950.degree. C. in the heating and
cooling results in an undesired steel microstructure due to
insufficient formation of austenite and due to formation of an
increased amount of ferrite. Thus, the residence time in the
temperature range of 780.degree. C. to 950.degree. C. is 30 seconds
or more. The residence time has no upper limit but is preferably
1000 seconds or less in terms of, for example, production
efficiency.
Reheating Temperature: More than 150.degree. C. and 600.degree. C.
or Less
[0060] A reheating temperature of more than 600.degree. C. results
in an undesired amount of retained austenite due to the
decomposition of austenite into pearlite or the like. This also
results in softening of tempered martensite or carbide-containing
bainite, thus resulting in low strength. Because heating to a
temperature lower than the cooling stop temperature is impossible,
the lower limit is more than 150.degree. C. Thus, the reheating
temperature is more than 150.degree. C. and 600.degree. C. or less,
preferably more than 150.degree. C. and 500.degree. C. or less.
Residence Time at a Temperature of More than 150.degree. C. and
600.degree. C. or Less: 1000 Seconds or Less
[0061] A residence time of more than 1000 seconds results in an
undesired amount of retained austenite due to decomposition of
austenite into pearlite or the like. Thus, the residence time is
1000 seconds or less, preferably 500 seconds or less. The residence
time is preferably 5 seconds or more in terms of operational
stability.
[0062] Instead of reheating, after cooling, the sheet may be held
at a temperature of more than 150.degree. C. and 400.degree. C. or
less for a residence time of 1000 seconds or less.
[0063] The galvanizing step after the annealing step includes
galvanizing an annealed sheet, then heating the sheet to a
temperature of 460.degree. C. to 600.degree. C. for plating and
alloying treatment, and cooling the sheet to 50.degree. C. or less.
For example, a coated layer formed on the annealed sheet contains,
on a mass percent basis, Fe: 5.0% to 20.0%, Al: 0.001% to 1.0%, and
one or two or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr,
Co, Ca, Cu, Li, Ti, Be, Bi, and REM: 0% to 30% in total, the
remainder being Zn and incidental impurities. The plating treatment
method can be hot-dip galvanizing under appropriate conditions.
Plating and Alloying Treatment Temperature: 460.degree. C. to
600.degree. C.
[0064] In the alloying treatment after the galvanizing treatment,
an alloying treatment temperature of less than 460.degree. C.
results in spot weld cracking due to insufficient alloying. An
alloying temperature of more than 600.degree. C. results in an
undesired amount of retained austenite due to the decomposition of
austenite into pearlite or the like. Thus, the alloying treatment
temperature is 460.degree. C. to 600.degree. C., preferably
470.degree. C. to 580.degree. C. The alloying time is typically,
but not limited to, 1 to 120 seconds.
Cooling Stop Temperature: 50.degree. C. or Less
[0065] After the alloying treatment, the sheet is cooled to
50.degree. C. or less. Cooling to 50.degree. C. or less changes
austenite partly into retained austenite and partly into
martensite, thus forming the desired microstructure.
Examples
[0066] A steel with a composition listed in Table 1 was melted in a
vacuum melting furnace in a laboratory and was rolled to form a
steel slab (in Table 1, N is an incidental impurity). The steel
slab was heated to 1200.degree. C., rough-rolled, and finish-rolled
to form a hot-rolled sheet 3.0 mm in thickness. The finish-rolling
temperature in the hot rolling was 900.degree. C., and the coiling
temperature was 500.degree. C. The hot-rolled sheet was then
pickled and optionally cold-rolled to form a cold-rolled sheet 1.4
mm in thickness (the rolling reduction is listed in Table 2). The
hot-rolled sheet or cold-rolled sheet was annealed. The annealing
was performed in a heat treatment and plating treatment apparatus
in a laboratory under the conditions shown in Table 2. Thus,
galvannealed steel sheets 1 to 31 were formed. In galvanizing
treatment, the sheet was immersed in a plating bath at 460.degree.
C. and plated at 35 to 45 g/m.sup.2. In alloying treatment, the
plated sheet was held at a temperature of 450.degree. C. to
650.degree. C. for 30 seconds and cooled to 50.degree. C. or less.
The galvannealed steel sheet was subjected to temper rolling at an
elongation percentage of 0.3% and subjected to microstructure
observation by the following test method. The galvannealed steel
sheet was examined for tensile properties, bendability, hole
expandability, spot weld crack resistance, and axial compression
stability.
Microstructure Observation
[0067] The area fractions of ferrite, carbide-free bainite,
martensite, tempered martensite, and carbide-containing bainite
refer to the area percentages of respective microstructures based
on the observed area. These area fractions were determined by
cutting a sample from the annealed steel sheet, taking image data
in three visual fields with a scanning electron microscope (SEM) at
a magnification of 1500 at one fourth the thickness of the sample
in a cross section of the sample parallel to the rolling direction
and parallel to the thickness direction polished and then subjected
to corrosion with 3% nital, determining the area fraction of each
microstructure using Image-Pro available from Media Cybernetics,
and taking the average area fraction of the visual fields as the
area fraction of each microstructure. In image data, ferrite is
black, martensite and retained austenite are white or light gray,
and bainite, including aligned carbide and/or island martensite, is
black or dark gray (Observation of grain boundaries between bainite
grains can differentiate between carbide-free bainite and
carbide-free bainite. Island martensite is white or light gray in
image data, as shown in FIG. 1.). The area fraction of bainite
refers to the area fraction of black or dark gray portions in the
bainite except white or light gray portions. The area fraction of
martensite was determined by subtracting the area fraction (the
volume fraction was considered to be the area fraction) of retained
austenite described later from the area fraction of the white or
light gray microstructures. Martensite may be autotempered
martensite containing carbide or tempered martensite.
Carbide-containing martensite has different carbide orientations
and is different from bainite. Island martensite is also martensite
having any of the characteristics described above. A white portion
that does not appear as a dot or line was considered to be the
martensite or retained austenite. Pearlite can be seen as a black
and white layered microstructure. The grain sizes of tempered
martensite and carbide-containing bainite were measured by the
intercept method in an image in which the area fraction was
determined. The grain sizes were averaged to determine the average
grain size of the microstructure. The number of lines used in the
intercept method were 10 in the longitudinal direction and 10 in
the transverse direction. Thus, an image was divided into 11 equal
parts in each direction. Grain boundaries include no packet
boundaries or no block boundaries.
[0068] The volume fraction of retained austenite was determined by
grinding an annealed steel sheet by one fourth the thickness of the
sheet, chemically polishing the surface by 0.1 mm, measuring the
integrated reflection intensities of the (200), (220), and (311)
planes of fcc iron (austenite) and the (200), (211), and (220)
planes of bcc iron (ferrite) with an X-ray diffractometer using Mo
K.alpha. radiation, and calculating the volume fraction from the
intensity ratio of the integrated reflection intensities of the
planes of the fcc iron to the integrated reflection intensities of
the planes of the bcc iron. The volume fraction was considered to
be the area fraction of retained austenite.
[0069] The lattice constant of retained austenite is calculated
using formula (1) from the diffraction peak shift of the (220)
plane with an X-ray diffractometer using Co K.alpha. radiation. The
C content of retained austenite is calculated using formula
(2).
.alpha.=0.17889 {square root over (2)}/sin .theta. (1)
a=0.3578+0.0033[C]+0.000095[Mn]+0.00006[Cr]+0.0022[N]+0.00056[Al]+0.0001-
5[Cu]+0.00031[Mo] (2)
[0070] In the formulae, a denotes the lattice constant (nm) of
retained austenite phase, .theta. denotes the diffraction peak
angle of the (220) plane divided by 2 (rad), and [M] denotes the
mass percent of the element M in the retained austenite phase. The
mass percent of the element M other than C in the retained
austenite phase is based on the total mass of steel.
Fe Content
[0071] The Fe content of the galvanized layer was determined by
dissolving the galvanized layer in hydrochloric acid containing an
inhibitor and measuring the Fe content of the hydrochloric acid by
ICP analysis.
Tensile Test
[0072] A JIS 5 tensile test piece (JIS Z 2201) was taken from an
annealed sheet in a direction perpendicular to the rolling
direction. TS was measured at a strain rate of 10.sup.-3/s in a
tensile test according to JIS Z 2241. 980 MPa or more is
acceptable.
Bending Test
[0073] A strip of test specimen 30 mm in width and 100 mm in length
was taken from an annealed sheet. A bending test was performed in
the direction parallel to the rolling direction. A 90-degree V
bending test was performed at a stroke rate of 10 mm/s, an
indentation load of 10 tons, a press holding time of 5 seconds, and
a bend radius of 3.5 mm for a test specimen 1.4 mm in thickness or
7.5 mm for a test specimen 3.0 mm in thickness. A bend line was
observed with a 10.times. magnifying lens. A test specimen without
0.5-mm or larger cracks was rated "good".
Axial Compression Stability
[0074] A test specimen 120 mm in width and 78 mm in length and a
test specimen 120 mm in width and 150 mm in length were taken from
an annealed sheet. The width direction is perpendicular to the
rolling direction. The bend radius was 4 mm for a test specimen 1.4
mm in thickness or 8 mm for a test specimen 3.0 mm in thickness. An
axial compression component illustrated in FIG. 2 was formed by
bending and spot welding.
[0075] Subsequently, as illustrated in FIG. 3, the axial
compression component was joined to a base plate by TIG welding,
thus forming a compression test specimen. In a compression test,
the compression test specimen was compressed by 80 mm with an
impactor at a uniform impact speed of 10 m/s from above in FIG. 3.
A compressed specimen pleated without cracks or fractures was rated
"good".
Spot Weld Cracking
[0076] Two test specimens 50 mm in width and 150 mm in length were
taken from an annealed sheet in the direction parallel to the
rolling direction. As illustrated in FIG. 4, two test specimens
were clamped together. The clearance between a lower electrode
(fixed) and a steel sheet was 2 mm. The welding pressure was 3.5
kN. The welding nugget diameter was 5.9 mm for test specimens 1.4
mm in thickness or 8.7 mm for test specimens 3.0 mm in thickness.
An upper electrode was pushed against the test specimens under
one-step constant current conditions to perform spot welding. After
welding, the test specimens were cut in half. A cross section
parallel to the thickness direction was observed with an optical
microscope. Test specimens without 0.2-mm or larger cracks were
considered to have good spot weld crack resistance.
Hole-Expanding Test
[0077] Stretch-flangeability was evaluated by subjecting a 150
mm.times.150 mm test specimen three times to a hole-expanding test
according to JFS T 1001 (the Japan Iron and Steel Federation
standard, 2008) with a 60-degree conical punch to determine the
average hole expanding ratio (%). A hole expanding ratio of 40% or
more was good.
[0078] Table 3 shows the results.
TABLE-US-00001 TABLE 1 Composition (mass %) Cr + Nb + Ti + Mo +
Steel C Si Mn P S Al N Cr Nb Ti Mo V Others V - Si Notes A 0.10
0.20 2.7 0.015 0.001 0.030 0.003 0.80 0.000 0.000 0.00 0.00 --
0.600 Example B 0.11 0.50 2.6 0.015 0.001 0.030 0.003 0.60 0.015
0.035 0.00 0.00 -- 0.150 Example C 0.12 0.60 2.3 0.015 0.001 0.030
0.003 0.40 0.025 0.025 0.15 0.03 -- 0.030 Example D 0.09 0.70 2.9
0.015 0.001 0.030 0.003 0.60 0.035 0.015 0.30 0.10 Ni: 0.1 0.350
Example E 0.13 0.15 2.3 0.015 0.001 0.030 0.003 0.00 0.045 0.000
0.30 0.00 Cu: 0.1 0.195 Example F 0.11 0.45 2.5 0.015 0.001 0.030
0.003 0.60 0.035 0.015 0.00 0.00 B: 0.0011 0.200 Example G 0.14
0.20 2.8 0.015 0.001 0.030 0.003 0.10 0.025 0.025 0.20 0.00 Ca:
0.0010 0.150 Example H 0.10 0.20 2.8 0.015 0.001 0.030 0.003 0.20
0.015 0.015 0.20 0.00 REM: 0.0020 0.230 Example I 0.10 0.20 2.8
0.015 0.001 0.030 0.003 0.00 0.000 0.015 0.20 0.00 Sn: 0.01 0.015
Example J 0.10 0.20 2.8 0.015 0.001 0.030 0.003 0.20 0.025 0.015
0.00 0.00 Sb: 0.01 0.040 Example K 0.10 0.60 2.8 0.015 0.001 0.030
0.003 0.30 0.000 0.015 0.10 0.00 -- -0.185 Comparative example L
0.05 0.20 2.8 0.015 0.001 0.030 0.003 0.20 0.015 0.015 0.20 0.00 --
0.230 Comparative example M 0.16 0.20 2.8 0.015 0.001 0.030 0.003
0.20 0.015 0.015 0.20 0.00 -- 0.230 Comparative example N 0.10 1.10
2.8 0.015 0.001 0.030 0.003 1.00 0.000 0.015 0.20 0.00 -- 0.115
Comparative example O 0.10 0.00 2.8 0.015 0.001 0.030 0.003 0.20
0.015 0.015 0.20 0.00 -- 0.430 Comparative example P 0.10 6.20 1.5
0.015 0.001 0.030 0.003 0.20 0.015 0.015 0.20 0.00 -- 0.230
Comparative example Q 0.10 0.20 3.2 0.015 0.001 0.030 0.003 0.20
0.015 0.015 0.20 0.00 -- 0.230 Comparative example R 0.12 0.20 2.7
0.015 0.001 0.030 0.003 0.00 0.000 0.000 0.30 0.00 -- 0.100 Example
* The underline indicates the outside of the scope of this
disclosure.
TABLE-US-00002 TABLE 2 Conditions Average Average Reheating heating
Residence cooling Cooling temperature Residence Cold- rate at
Heating time at rate at stop or holding time in Plating and Steel
rolling 600.degree. C.- temper- 780.degree. C.- 450.degree. C.-
temper- temperature reheating alloying sheet reduction 720.degree.
C. ature 950.degree. C. 720.degree. C. ature after cooling or
holding temperature No. Steel (%) (.degree. C./s) (.degree. C.) (s)
(.degree. C./s) (.degree. C.) (.degree. C.) (s) (.degree. C.) Notes
1 A 50 1 830 120 20 250 450 100 530 Example 2 10 1 830 120 20 250
450 100 530 Comparative example 3 50 1 770 120 20 250 450 100 530
Comparative example 4 B 50 1 830 120 20 250 450 100 530 Example 5
50 1 980 120 20 250 450 100 530 Comparative example 6 50 1 830 10
20 250 450 100 530 Comparative example 7 C -- 1 830 120 50 250 450
100 530 Example 8 -- 1 830 120 50 250 550 100 530 Example 9 -- 1
830 120 3 250 450 100 530 Comparative example 10 D 50 1 930 120 20
250 450 100 530 Example 11 50 1 930 120 20 500 500 100 530
Comparative example 12 50 1 930 120 20 25 450 100 530 Comparative
example 13 E 50 1 830 120 20 250 450 100 530 Example 14 50 1 830
120 20 250 650 100 530 Comparative example 15 50 1 830 120 20 250
450 1200 530 Comparative example 16 50 7 830 120 20 250 450 100 530
Comparative example 17 F 50 1 830 120 20 250 450 100 530 Example 18
50 1 830 120 20 250 450 100 450 Comparative example 19 50 1 830 120
20 250 450 100 620 Comparative example 20 G 50 1 830 120 20 250 450
100 530 Example 21 H 50 1 830 120 20 250 450 100 530 Example 22 I
50 1 830 120 20 250 450 100 530 Example 23 J 50 1 830 120 20 250
450 100 530 Example 24 K 50 1 830 120 20 250 450 100 530
Comparative example 25 L 50 1 830 120 20 250 450 100 530
Comparative example 26 M 50 1 830 120 20 250 450 100 530
Comparative example 27 N 50 1 830 120 20 250 450 100 530
Comparative example 28 O 50 1 830 120 20 250 450 100 530
Comparative example 29 P 50 1 830 120 20 250 450 100 530
Comparative example 30 Q 50 1 830 120 20 250 450 100 530
Comparative example 31 R 50 1 830 120 20 250 450 100 530 Example *
The underline indicates the outside of the scope of this
disclosure.
TABLE-US-00003 TABLE 3 Steel microstructure *1 Steel V(F + B1) +
sheet V(F + B1) V(TM + B2) V(M + .gamma.) V(.gamma.) V(M + .gamma.)
Others d(B2) d(TM) C(.gamma.) No. (%) (%) (%) (%) (%) (%) (.mu.m)
(.mu.m) (%) 1 0 94 6 3 6 0 8.6 8.4 0.37 2 0 93 7 3 7 0 20.7 20.9
0.35 3 41 23 36 1 77 0 5.8 6.0 0.12 4 1 90 9 4 10 0 7.3 7.1 0.39 5
0 92 8 2 8 0 23.1 21.9 0.41 6 42 25 33 3 75 0 5.9 5.9 0.41 7 3 89 8
4 11 0 7.2 7.0 0.48 8 2 90 8 2 10 0 7.3 7.6 0.45 9 47 18 35 2 82 0
5.4 5.9 0.40 10 9 88 3 3 12 0 13.7 14.0 0.35 11 33 0 67 1 100 0 7.9
7.7 0.10 12 0 100 0 0 0 0 13.9 12.6 -- 13 0 97 3 1 3 0 5.9 6.1 0.40
14 0 97 0 0 0 3 6.1 6.0 -- 15 0 96 1 0 1 3 6.0 6.5 -- 16 0 91 7 6 7
2 4.3 3.9 0.37 17 0 95 5 2 5 0 7.0 7.3 0.37 18 0 95 5 3 5 0 7.0 6.6
0.39 19 0 96 0 0 0 4 7.2 6.8 -- 20 0 93 7 2 7 0 8.0 8.2 0.35 21 0
95 4 1 4 1 9.1 8.8 0.28 22 0 96 4 2 4 0 8.8 8.0 0.31 23 0 95 5 1 5
0 9.3 9.8 0.29 24 4 93 3 2 7 0 9.1 7.9 0.54 25 1 96 3 1 4 0 8.7 8.2
0.30 26 0 90 10 3 10 0 8.3 8.6 0.36 27 6 92 2 1 8 0 8.8 9.0 0.53 28
0 95 5 0 5 0 9.0 9.3 -- 29 30 59 11 3 41 0 5.7 6.2 0.48 30 0 93 7 3
7 0 7.5 7.1 0.30 31 2 93 5 4 7 0 6.6 7.0 0.32 Coated Mechanical
characteristics Steel layer Hole Spot Axial sheet Fe content TS
expandability weld crack compression No. (mass %) (MPa) Bendability
(%) resistance stability Notes 1 12 1024 Good 67 Good Good Example
2 11 1030 Poor 51 Good Poor Comparative example 3 12 1129 Good 23
Good Poor Comparative example 4 10 1063 Good 59 Good Good Example 5
10 1056 Poor 55 Good Poor Comparative example 6 11 1133 Good 24
Good Poor Comparative example 7 10 1125 Good 60 Good Good Example 8
10 1084 Good 57 Good Good Example 9 10 1041 Good 34 Good Poor
Comparative example 10 9 998 Good 76 Good Good Example 11 10 1193
Good 38 Good Poor Comparative example 12 10 991 Good 82 Good Poor
Comparative example 13 13 1093 Good 46 Good Good Example 14 12 827
Good 44 Good Poor Comparative example 15 12 1022 Good 46 Good Poor
Comparative example 16 12 1107 Poor 38 Good Good Comparative
example 17 10 1028 Good 55 Good Good Example 18 1 1135 Good 59 Poor
Good Comparative example 19 16 866 Good 50 Good Poor Comparative
example 20 12 1106 Good 47 Good Good Example 21 12 1014 Good 58
Good Good Example 22 11 1009 Good 62 Good Good Example 23 12 1005
Good 63 Good Good Example 24 9 1034 Good 60 Good Poor Comparative
example 25 12 912 Good 84 Good Good Comparative example 26 11 1120
Poor 41 Poor Good Comparative example 27 8 1128 Good 63 Good Poor
Comparative example 28 13 1016 Good 49 Good Poor Comparative
example 29 11 920 Good 35 Good Good Comparative example 30 10 1119
Good 59 Poor Good Comparative example 31 11 1139 Good 60 Good Good
Example *1 V(F + B1): total area fraction of ferrite and
carbide-free bainite, V(TM + B2): total area fraction of tempered
martensite and carbide-containing bainite, V(M + .gamma.): total
area fraction of martensite and retained austenite, V(.gamma.):
area fraction of retained austenite, C(.gamma.): C content of
retained austenite, d(B2): average grain size of carbide-containing
bainite, d(TM): average grain size of tempered martensite *2 The
underline indicates the outside of the scope of this
disclosure.
[0079] The galvanized steel sheets according to the examples had TS
of 980 MPa or more and had good bendability, spot weld crack
resistance, stretch-flangeability, and axial compression stability.
The Comparative examples had undesired strength, poor bendability,
poor spot weld crack resistance, poor stretch-flangeability, or
poor axial compression stability.
INDUSTRIAL APPLICABILITY
[0080] We provide a galvanized steel sheet with TS of 980 MPa or
more, good bendability, spot weld crack resistance,
stretch-flangeability, and axial compression stability. The use of
a high-strength galvanized steel sheet in automotive parts can
contribute greatly to improved crash safety and fuel efficiency of
automobiles.
* * * * *