U.S. patent application number 14/784341 was filed with the patent office on 2016-03-03 for high strength hot-rolled steel sheet and method of producing the same.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Chikara Kami, Katsumi Nakajima, Kazuhiko Yamazaki.
Application Number | 20160060723 14/784341 |
Document ID | / |
Family ID | 51731024 |
Filed Date | 2016-03-03 |
United States Patent
Application |
20160060723 |
Kind Code |
A1 |
Yamazaki; Kazuhiko ; et
al. |
March 3, 2016 |
HIGH STRENGTH HOT-ROLLED STEEL SHEET AND METHOD OF PRODUCING THE
SAME
Abstract
A high-strength hot-rolled steel sheet has a tensile strength TS
of 980 MPa or more and is manufactured by controlling the steel
sheet to have a chemical composition containing C: more than 0.1%
and 0.2% or less, Si: 0.5% or more and 3.0% or less, Mn: 1.0% or
more and 3.5% or less, P: 0.05% or less, S: 0.004% or less, Al:
0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.15% or
less, V: more than 0.10% and 0.30% or less, and the balance being
Fe and inevitable impurities. Surface regions have a microstructure
including mainly a ferrite phase, and an inner region has a
microstructure including mainly a bainite phase. The proportions of
the surface regions in the thickness direction of the steel sheet
are 1.0% or more and 5.0% or less of the whole thickness
respectively from the upper and lower surfaces of the steel
sheet.
Inventors: |
Yamazaki; Kazuhiko;
(Kawasaki, JP) ; Nakajima; Katsumi; (Kawasaki,
JP) ; Kami; Chikara; (Chiba, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
51731024 |
Appl. No.: |
14/784341 |
Filed: |
March 11, 2014 |
PCT Filed: |
March 11, 2014 |
PCT NO: |
PCT/JP2014/001380 |
371 Date: |
October 14, 2015 |
Current U.S.
Class: |
148/602 ;
148/330 |
Current CPC
Class: |
C22C 38/14 20130101;
C22C 38/50 20130101; C21D 9/46 20130101; C22C 38/58 20130101; C22C
38/46 20130101; C21D 2211/005 20130101; C21D 8/0263 20130101; C22C
38/04 20130101; C21D 8/0226 20130101; C22C 38/12 20130101; C21D
2211/002 20130101; C22C 38/005 20130101; C22C 38/06 20130101; C21D
6/008 20130101; C21D 6/004 20130101; C22C 38/02 20130101; C22C
38/001 20130101; C21D 6/005 20130101; C21D 8/02 20130101; C22C
38/002 20130101; C22C 38/16 20130101; C21D 8/0205 20130101 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/58 20060101 C22C038/58; C22C 38/50 20060101
C22C038/50; C22C 38/46 20060101 C22C038/46; C22C 38/00 20060101
C22C038/00; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 6/00 20060101
C21D006/00; C22C 38/16 20060101 C22C038/16 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 15, 2013 |
JP |
2013-084449 |
Claims
1.-4. (canceled)
5. A high-strength hot-rolled steel sheet, comprising: a chemical
composition containing, by mass %, C: more than 0.1% and 0.2% or
less, Si: 0.5% or more and 3.0% or less, Mn: 1.0% or more and 3.5%
or less, P: 0.05% or less, S: 0.004% or less, Al: 0.10% or less, N:
0.008% or less, Ti: 0.05% or more and 0.15% or less, V: more than
0.10% and 0.30% or less, and the balance being Fe and inevitable
impurities, surface regions including mainly a ferrite phase, and
an inner region including mainly a bainite phase, the surface
regions having a microstructure including mainly a ferrite phase in
an amount of 80% or more in terms of area fraction and the balance
being at least one selected from a bainite phase, a martensite
phase, and a retained austenite phase in an amount of 0% or more
and 20% or less in terms of area fraction, the inner region having
a microstructure including mainly a bainite phase in an amount of
more than 90% in terms of area fraction and the balance being at
least one selected from a ferrite phase, a martensite phase, and a
retained austenite phase in an amount of 0% or more and less than
10% in terms of area fraction, and proportions of the surface
regions in a thickness direction of the steel sheet being 1.0% or
more and 5.0% or less of a whole thickness, respectively, from
upper and lower surfaces of the steel sheet.
6. The high-strength hot-rolled steel sheet according to claim 5,
wherein the chemical composition further contains, by mass %, at
least one, selected from the group consisting of Nb: 0.003% or more
and 0.2% or less, B: 0.0002% or more and 0.0015% or less, Cu:
0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or
less, Cr: 0.005% or more and 0.2% or less, and Mo: 0.005% or more
and 0.2% or less.
7. The high-strength hot-rolled steel sheet according to claim 5,
wherein the chemical composition further contains, by mass %, at
least one selected from the group consisting of Ca: 0.0002% or more
and 0.01% or less and REM: 0.0002% or more and 0.01% or less.
8. The high-strength hot-rolled steel sheet according to claim 6,
wherein the chemical composition further contains, by mass %, at
least one selected from the group consisting of Ca: 0.0002% or more
and 0.01% or less and REM: 0.0002% or more and 0.01% or less.
9. A method of manufacturing a high-strength hot-rolled steel sheet
comprising: heating a steel slab having the chemical composition
according to claim 5 in a temperature range of 1250.degree. C. or
higher, holding the heated steel slab in the temperature range for
3600 seconds or more, performing hot rolling including rough
rolling and finish rolling under conditions that a finish delivery
temperature is 840.degree. C. or higher and 940.degree. C. or
lower, starting cooling immediately after hot rolling has been
performed, performing cooling at an average cooling rate of
25.degree. C./s or more, and performing coiling at a coiling
temperature of 350.degree. C. or higher and 500.degree. C. or
lower.
10. A method of manufacturing a high-strength hot-rolled steel
sheet comprising: heating a steel slab having the chemical
composition according to claim 6 in a temperature range of
1250.degree. C. or higher, holding the heated steel slab in the
temperature range for 3600 seconds or more, performing hot rolling
including rough rolling and finish rolling under conditions that a
finish delivery temperature is 840.degree. C. or higher and
940.degree. C. or lower, starting cooling immediately after hot
rolling has been performed, performing cooling at an average
cooling rate of 25.degree. C./s or more, and performing coiling at
a coiling temperature of 350.degree. C. or higher and 500.degree.
C. or lower.
11. A method of manufacturing a high-strength hot-rolled steel
sheet comprising: heating a steel slab having the chemical
composition according to claim 7 in a temperature range of
1250.degree. C. or higher, holding the heated steel slab in the
temperature range for 3600 seconds or more, performing hot rolling
including rough rolling and finish rolling under conditions that a
finish delivery temperature is 840.degree. C. or higher and
940.degree. C. or lower, starting cooling immediately after hot
rolling has been performed, performing cooling at an average
cooling rate of 25.degree. C./s or more, and performing coiling at
a coiling temperature of 350.degree. C. or higher and 500.degree.
C. or lower.
12. The method according to claim 9, wherein the chemical
composition further contains: 1) at least one, selected from the
group consisting of Nb: 0.003% or more and 0.2% or less, B: 0.0002%
or more and 0.0015% or less, Cu: 0.005% or more and 0.2% or less,
Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.2% or
less, and Mo: 0.005% or more and 0.2% or less, and 2) at least one
selected from the group consisting of Ca: 0.0002% or more and 0.01%
or less and REM: 0.0002% or more and 0.01% or less.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a high-strength hot-rolled steel
sheet having a tensile strength of 980 MPa or more which can be
preferably used as a material for structural parts and skeleton
members of automobiles or frames of trucks and, in particular,
relates to improvement of bending workability.
BACKGROUND
[0002] Nowadays, automobile exhaust gas regulations are being
strengthened from the viewpoint of global environment conservation.
In such a situation, since the improvement of the fuel efficiency
of automobiles such as trucks is an important problem to be solved,
there is a growing demand for the strengthening and thickness
reduction of materials used for automobiles. Therefore, there is a
tendency toward actively using high-strength hot-rolled steel
sheets as materials for automotive parts, and demands for
high-strength hot-rolled steel sheets are increasing year by year.
In particular, a high-strength hot-rolled steel sheet having a
tensile strength of 980 MPa or more is highly anticipated to serve
as a material which significantly improves the fuel efficiency of
automobiles.
[0003] Generally, however, an increase in the strength of a steel
sheet is accompanied by a decrease in bending workability.
Therefore, various investigations have been conducted to provide
bending workability required for a material for automotive parts to
a high-strength hot-rolled steel sheet.
[0004] For example, Japanese Unexamined Patent Application
Publication No. 2012-062558 proposes a technology to manufacture a
hot-rolled steel sheet, the technology including heating and
holding a steel material having a chemical composition containing,
by mass %, C: 0.05% to 0.15%, Si: 0.2% to 1.2%, Mn; 1.0% to 2.0%,
P: 0.04% or less, S: 0.0030% or less, Al: 0.005% to 0.10%, N:
0.005% or less, Ti: 0.03% to 0.13%, and the balance being Fe and
inevitable impurities at a temperature of 1200.degree. C. to
1350.degree. C. for 1200 seconds or more, then hot-rolling the
heated material under the conditions that the finishing temperature
of rough rolling is 1050.degree. C. or higher and the finishing
temperature of finish rolling is 830.degree. C. to 930.degree. C.,
and thereafter cooling the hot-rolled steel sheet at an average
cooling rate of 35.degree. C./s or more to a coiling temperature of
350.degree. C. to 550.degree. C.
[0005] According to the technology proposed in Japanese Unexamined
Patent Application Publication No. 2012-062558, it is possible to
obtain a hot-rolled steel sheet having a microstructure including,
in terms of area fraction, less than 80% of a bainite phase and 10%
or more of a ferrite phase having a grain diameter of 2 to 15 .mu.m
in surface regions which are respectively within 1.5 to 3.0% of the
whole thickness of the steel sheet from both surfaces of the steel
sheet and including, in terms of area fraction, more than 95% of a
bainite phase in an inner region which is a region other than the
surface regions. According to the technique proposed in Japanese
Unexamined Patent Application Publication No. 2012-062558, by
forming the soft microstructure described above in the surface
regions, it is possible to obtain a high-strength hot-rolled steel
sheet excellent in bending workability having a tensile strength of
780 MPa or more.
[0006] Japanese Unexamined Patent Application Publication No.
2009-270142 proposes a technology to manufacture a hot-rolled steel
sheet, the technique including heating a steel slab having a
chemical composition containing, by mass %, C: 0.05% to 0.19%, Si:
0.05% to 1.0%, Mn: 0.3% to 2.5%, P: 0.03% or less, S: 0.025% or
less, Ti: 0.005% to 0.1%, Cr: 0.03% to 1.0%, Sol.Al: 0.005% to
0.1%, N: 0.0005% to 0.01%, B: 0.0001% to 0.01%, and the balance
being Fe and inevitable impurities, in which the relationship
3C.ltoreq.0.27Mn+0.2Cr+0.05Cu+0.11Ni+0.25Mo.ltoreq.3C+0.3 (C, Mn,
Cr, Cu, Ni, Mo are represented in units of mass %), at a
temperature of 1070.degree. C. or higher and 1300.degree. C. or
lower, then hot-rolling the heated slab under the condition that
the finishing temperature is 850.degree. C. or higher and
1070.degree. C. or lower, and thereafter cooling the hot-rolled
steel sheet to a temperature of 300.degree. C. or lower at a
cooling rate V.sub.C (.degree. C./sec) which satisfies the
relationship 1.2/C.ltoreq.V.sub.C.ltoreq.1.8/C (C is represented in
units of mass %).
[0007] According to the technology proposed in Japanese Unexamined
Patent Application Publication No. 2009-270142, it is possible to
obtain a hot-rolled steel sheet composed of a surface layer thereof
having a microstructure including, in terms of area fraction, 80%
or more of a bainite phase, a Vickers hardness Hv of 210 or more
and 300 or less, an average value of the lengths of the major axis
of bainite grains of 5 .mu.m or less, and an average grain boundary
carbide diameter of 0.5 .mu.m or less. According to the technology
proposed in Japanese Unexamined Patent Application Publication No.
2009-270142, it is possible to obtain a hot-rolled steel sheet for
a steel pipe used for a machinery structure excellent in terms of
fatigue resistance and bending workability by controlling the
microstructure in the surface layer of the steel sheet to be
uniform and fine and to include mainly a bainite structure to have
an intermediate hardness and by decreasing the size of grain
boundary precipitated carbides.
[0008] However, in Japanese Unexamined Patent Application
Publication No. 2012-062558, since it is not always possible to
obtain a high-strength hot-rolled steel sheet having a tensile
strength of 980 MPa or more, there is room for improvement.
[0009] In Japanese Unexamined Patent Application Publication No.
2009-270142, since the microstructure of a steel sheet is
controlled by setting a cooling stop temperature to be 300.degree.
C. or lower and by adjusting mainly a cooling rate, it can be
difficult to stably achieve the desired microstructure of a steel
sheet due to a variation in cooling rate in mass production.
Moreover, in the technique proposed in Japanese Unexamined Patent
Application Publication No. 2009-270142, since the cooling stop
temperature is set to 300.degree. C. or lower, there is a tendency
for a hard microstructure to be formed in the surface regions of a
steel sheet, which results in a situation where it is not possible
to provide a hot-rolled steel sheet with sufficient bending
workability.
[0010] It could thus be helpful to provide a high-strength
hot-rolled steel sheet having a high tensile strength of 980 MPa or
more and excellent bending workability which has, in particular, a
thickness of 3.2 mm or more and 14 mm or less and a method of
manufacturing the steel sheet.
SUMMARY
[0011] We thus provide:
[0012] [1] A high-strength hot-rolled steel sheet excellent in
bending workability, the steel sheet comprising:
[0013] a chemical composition containing, by mass %, C: more than
0.1% and 0.2% or less, Si: 0.5% or more and 3.0% or less, Mn: 1.0%
or more and 3.5% or less, P: 0.05% or less, S: 0.004% or less, Al:
0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.15% or
less, V: more than 0.10% and 0.30% or less, and the balance being
Fe and inevitable impurities,
[0014] surface regions including mainly a ferrite phase, and an
inner region including mainly a bainite phase,
[0015] the surface regions having a microstructure including mainly
a ferrite phase in an amount of 80% or more in terms of area
fraction and the balance being at least one selected from among a
bainite phase, a martensite phase, and a retained austenite phase
in an amount of 0% or more and 20% or less in terms of area
fraction,
[0016] the inner region having a microstructure including mainly a
bainite phase in an amount of more than 90% in terms of area
fraction and the balance being at least one selected from among a
ferrite phase, a martensite phase, and a retained austenite phase
in an amount of 0% or more and less than 10% in terms of area
fraction, and
[0017] the proportions of the surface regions in the thickness
direction of the steel sheet being 1.0% or more and 5.0% or less of
the whole thickness respectively from the upper and lower surfaces
of the steel sheet.
[0018] [2] The high-strength hot-rolled steel sheet excellent in
bending workability according to item [1], wherein the chemical
composition further contains, by mass %, at least one selected from
the group consisting of Nb: 0.003% or more and 0.2% or less, B:
0.0002% or more and 0.0015% or less, Cu: 0.005% or more and 0.2% or
less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and
0.2% or less, and Mo: 0.005% or more and 0.2% or less.
[0019] [3] The high-strength hot-rolled steel sheet excellent in
terms of bending workability according to item [1] or [2], wherein
the chemical composition further contains, by mass %, at least one
selected from the group consisting of Ca: 0.0002% or more and 0.01%
or less and REM: 0.0002% or more and 0.01% or less.
[0020] [4] A method of manufacturing a high-strength hot-rolled
steel sheet excellent in bending workability, the method
including:
[0021] heating a steel slab having the chemical composition
according to any one of items [1] to [3] in a temperature range of
1250.degree. C. or higher,
[0022] holding the heated steel slab in the temperature range for
3600 seconds or more,
[0023] performing hot rolling including rough rolling and finish
rolling under the condition that the finish delivery temperature is
840.degree. C. or higher and 940.degree. C. or lower,
[0024] starting cooling immediately after hot rolling has been
performed,
[0025] performing cooling at an average cooling rate of 25.degree.
C./s or more, and
[0026] performing coiling at a coiling temperature of 350.degree.
C. or higher and 500.degree. C. or lower.
[0027] It is possible to obtain a high-strength hot-rolled steel
sheet having a tensile strength of 980 MPa or more and excellent in
bending workability. Therefore, when our steel sheets are applied
to the structural parts and skeleton members of automobiles, the
frames of trucks and the like, it is possible to reduce auto weight
while maintaining automotive safety and it is possible to reduce
environmental loads. In addition, it is possible to stably
manufacture a hot-rolled steel sheet having increased bending
workability while maintaining a high strength of 980 MPa or more in
terms of tensile strength, which has a marked effect on
industry.
DETAILED DESCRIPTION
[0028] We conducted investigations for the purpose of increasing
the bending workability of a hot-rolled steel sheet while
maintaining a high strength of 980 MPa or more in terms of tensile
strength TS. We found that it is very effective to form a
microstructure including mainly a ferrite phase in the surface
regions of a hot-rolled steel sheet and a microstructure including
mainly a bainite phase in the region other than the surface regions
(inner region) of the hot-rolled steel sheet to achieve a
satisfactory strength-bending workability balance of the hot-rolled
steel sheet. In addition, we found that there is a significant
increase in the bending workability of a hot-rolled steel sheet
while maintaining a high strength TS of 980 MPa or more in terms of
tensile strength by controlling the area fraction of a ferrite
phase in surface regions and a bainite phase in an inner region and
by controlling the proportions of surface regions in the thickness
direction of a steel sheet.
[0029] We also conducted investigations regarding a method of
forming the desired microstructure for a hot-rolled steel sheet
described above, that is, a method of forming a microstructure
including mainly a bainite phase with a specified amount of ferrite
phase being formed in surface regions of the hot-rolled steel
sheet. We conceived of heating a steel material at a temperature of
1250.degree. C. or higher and holding the heated steel material at
the heating temperature for 3600 seconds or more when a hot-rolled
steel sheet is manufactured by heating steel material with a
specified chemical composition and then by hot-rolling the heated
steel material.
[0030] When a steel material is held in a high temperature range
for a long time, the surface of the steel material is decarburized.
In addition, the lower the C content of steel, the more the ferrite
phase is likely to be formed. Therefore, we conducted experiments
to make the amount of a ferrite phase formed in the surface regions
of a hot-rolled steel sheet larger than that in a region other than
the surface regions of the hot-rolled steel sheet in cooling and
coiling processes after hot rolling has been performed by using the
decarburizing mentioned above to decarburize the surface regions of
the steel material in a heating process of the steel material. As a
result, we found that, by heating a steel material at a temperature
of 1250.degree. C. or higher and by holding the heated steel
material at the heating temperature for 3600 seconds or more, it is
possible to obtain a hot-rolled steel sheet having the desired
microstructure after cooling and coiling have been performed after
hot rolling has been performed. As described above, we found that,
by utilizing decarburization, it is possible to stably achieve the
desired microstructure in the surface regions of a hot-rolled steel
sheet.
[0031] Our steel sheets and methods will be described in detail
hereafter.
[0032] First, the reasons for the limitations on the chemical
composition of the hot-rolled steel sheet will be described.
Hereinafter, % used when describing a chemical composition always
represents mass %, unless otherwise noted.
C: More than 0.1% and 0.2% or Less
[0033] C increases the strength of steel and promotes formation of
a bainite phase. Accordingly, it is necessary that the C content be
more than 0.1%. On the other hand, when the C content is more than
0.2%, since it is difficult to control formation of a bainite
phase, there is an increase in the amount of a martensite phase,
which is a hard phase, resulting in a decrease in bending
workability of the hot-rolled steel sheet. Therefore, the C content
is more than 0.1% and 0.2% or less, or preferably 0.12% or more and
0.18% or less.
Si: 0.5% or More and 3.0% or Less
[0034] Since Si suppresses formation of large-size oxides and
cementite, which decrease bending workability, and since Si
facilitates formation of a ferrite phase, which improves bending
workability, it is effective to add Si in an amount of 0.5% or
more. In addition, Si is a chemical element which contributes to
solute strengthening. On the other hand, when the Si content is
more than 3.0%, since there is a significant decrease in the
surface quality of a hot-rolled steel sheet, there is a decrease in
phosphatability and corrosion resistance. Therefore, the Si content
is 0.5% or more and 3.0% or less, preferably 0.5% or more and 2.5%
or less, more preferably 0.6% or more and 2.0% or less, or further
more preferably 0.7% or more.
Mn: 1.0% or More and 3.5% or Less
[0035] Mn is one of the most important constituent chemical
elements. Mn is a chemical element which increases the strength of
steel by forming a solid solution and promotes formation of a
bainite phase through increasing hardenability. It is necessary
that the Mn content be 1.0% or more to realize such effects. Since
Mn is a chemical element which tends to be concentrated in the
central portion of a slab when slab casting is performed, it is
possible to realize an increase in strength due to this
concentration while keeping satisfactory bending workability for
the surface regions of a hot-rolled steel sheet. On the other hand,
when the Mn content is more than 3.5%, since there is also an
increase in the Mn concentration in the surface regions, there is a
deterioration in bending workability of the hot-rolled steel sheet.
Therefore, the Mn content is 1.0% or more and 3.5% or less,
preferably 1.5% or more and 3.0% or less, or more preferably more
than 2.0% and 2.5% or less.
P: 0.05% or Less
[0036] Although P is a chemical element which contributes to an
increase in the strength of steel by forming a solid solution, P is
a chemical element which also causes a decrease in low-temperature
toughness and workability as a result of being segregated at grain
boundaries, in particular, at prior-austenite grain boundaries.
Therefore, it is preferable that the P content be as small as
possible, but it is acceptable that the P content be 0.05% or less.
Therefore, the P content is 0.05% or less. However, since it is not
possible to realize an effect corresponding to an increase in the
refining costs when the P content is excessively decreased, it is
preferable that the P content be 0.003% or more and 0.03% or less,
or more preferably 0.005% or more and 0.02% or less.
S: 0.004% or Less
[0037] S decreases the workability of a hot-rolled steel sheet as a
result of combining with Ti and Mn to form large-size sulfides.
Therefore, it is preferable that the S content be as small as
possible, but it is acceptable that the S content be 0.004% or
less. Therefore, the S content is 0.004% or less. However, since it
is not possible to realize an effect corresponding to an increase
in the refining costs when the S content is excessively decreased,
it is preferable that the S content be 0.0003% or more and 0.004%
or less.
Al: 0.10% or Less
[0038] Al is a chemical element effective to increase the cleanness
of steel by functioning as a deoxidizing agent. On the other hand,
when the Al content is excessively large, there is an increase in
the amount of oxide inclusions, which results in a decrease in the
toughness of a hot-rolled steel sheet and results in the occurrence
of defects. Therefore, the Al content is 0.10% or less, preferably
0.005% or more and 0.08% or less, or more preferably 0.01% or more
and 0.05% or less.
N: 0.008% or Less
[0039] N contributes to make a crystal grain size fine by being
precipitated as the (form of) nitrides combining with
nitride-forming elements. However, since N tends to combine with Ti
to form large-size nitrides at a high temperature, there is a
decrease in the bending workability of a hot-rolled steel sheet.
Therefore, it is preferable that the N content be as small as
possible. Therefore, the N content is 0.008% or less, preferably
0.001% or more and 0.006% or less, or more preferably 0.002% or
more and 0.005% or less.
[0040] Ti: 0.05% or More and 0.15% or Less
[0041] Ti is one of the most important constituent chemical
elements. Ti contributes to an increase in the strength of steel
through making a crystal grain size fine as a result of forming
carbonitrides and through precipitation strengthening. In addition,
Ti has a role in improving the bending workability of a hot-rolled
steel sheet by decreasing the amount of cementite in steel as a
result of forming a large number of fine clusters of (Ti, V)C at a
low temperature of 300.degree. C. or higher and 450.degree. C. or
lower. It is necessary that the Ti content be 0.05% or more to
realize such effects. On the other hand, when the Ti content is
more than 0.15%, the effects described above become saturated. In
addition, when the Ti content is more than 0.15%, since large-size
TiCs are left undissolved in steel when a slab is heated, there is
a decrease in the bending workability of a hot-rolled steel sheet,
and there is a decrease in the amount of solid solute C before
bainite transformation occurs, which results in a decrease in the
strength of a steel sheet. Therefore, the Ti content is 0.05% or
more and 0.15% or less, or preferably 0.08% or more and 0.13% or
less.
V: More than 0.10% and 0.30% or Less
[0042] V is one of the most important constituent chemical
elements. V contributes to an increase in the strength of steel
through making a crystal grain size fine as a result of forming
carbonitrides and through precipitation strengthening. V increases
hardenability, and V contributes to formation of a bainite phase
and to refine a bainite phase. V has a role in improving the
bending workability of a hot-rolled steel sheet by decreasing the
amount of cementite in steel as a result of forming a large number
of fine clusters of (Ti, V)C at a low temperature of 300.degree. C.
or higher and 450.degree. C. or lower. It is necessary that the V
content be more than 0.10% to realize such effects. On the other
hand, when the V content is more than 0.30%, there is a decrease in
the ductility of a hot-rolled steel sheet, and there is an increase
in cost. Therefore, the V content is more than 0.10% and 0.30% or
less, or preferably 0.15% or more and 0.25% or less.
[0043] Although the chemical composition described above is the
basic chemical composition of the hot-rolled steel sheet, the
hot-rolled steel sheet may contain, as needed, at least one
selected from the group consisting of Nb: 0.003% or more and 0.2%
or less, B: 0.0002% or more and 0.0015% or less, Cu: 0.005% or more
and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005%
or more and 0.2% or less, and Mo: 0.005% or more and 0.2% or less,
for example, to increase hole expansion formability or to increase
strength.
Nb: 0.003% or More and 0.2% or Less
[0044] Nb is a chemical element which contributes to an increase in
the strength of steel through formation of carbonitrides. It is
preferable that the Nb content be 0.003% or more to realize such an
effect. On the other hand, when the Nb content is more than 0.2%,
since there is an increase in deformation resistance, there is an
increase in rolling force in hot rolling when a hot-rolled steel
sheet is manufactured, which results in concern that the rolling
operation may be difficult to perform due to an excessive increase
in the load placed on a rolling mill. In addition, when the Nb
content is more than 0.2%, there is a tendency for the workability
of a hot-rolled steel sheet to decrease due to formation of
large-size precipitates. Therefore, it is preferable that the Nb
content be 0.003% or more and 0.2% or less, more preferably 0.01%
or more and 0.15% or less, or further more preferably 0.015% or
more and 0.1% or less.
B: 0.0002% or More and 0.0015% or Less
[0045] B is a chemical element which inhibits formation and growth
of a ferrite phase as a result of being segregated at austenite
grain boundaries. Also, B is a chemical element which contributes
to improve hardenability and forming a bainite phase and refining a
bainite phase. It is preferable that the B content be 0.0002% or
more to realize such effects. However, when the B content is more
than 0.0015%, since formation of a martensite phase is promoted,
there is a possibility that there may be a significant
deterioration in the bending workability of a hot-rolled steel
sheet. Therefore, when B is added, it is preferable that the B
content be 0.0002% or more and 0.0015% or less, or more preferably
0.0004% or more and 0.0012% or less.
Cu: 0.005% or More and 0.2% or Less
[0046] Cu is a chemical element which contributes to an increase in
the strength of steel by forming a solid solution. In addition, Cu
is a chemical element having a function of increasing
hardenability, which, in particular, decreases the bainite
transformation temperature, and which contributes to decreasing the
grain size of a bainite phase. It is preferable that the Cu content
be 0.005% or more to realize such effects. However, when the Cu
content is more than 0.2%, there is a decrease in the surface
quality of a hot-rolled steel sheet. Therefore, it is preferable
that the Cu content be 0.005% or more and 0.2% or less, or more
preferably 0.01% or more and 0.15% or less.
Ni: 0.005% or More and 0.2% or Less
[0047] Ni is a chemical element which contributes to an increase in
the strength of steel by forming a solid solution. In addition, Ni
has a function of increasing hardenability and facilitates
formation of a bainite phase. It is preferable that the Ni content
be 0.005% or more to realize such effects. However, when the Ni
content is more than 0.2%, since a martensite phase tends to be
formed, there is a possibility that there may be a significant
deterioration in the hole expansion formability of a hot-rolled
steel sheet. Therefore, it is preferable that the Ni content be
0.005% or more and 0.2% or less, or more preferably 0.01% or more
and 0.15% or less.
Cr: 0.005% or More and 0.2% or Less
[0048] Cr contributes to an increase in the strength of a
hot-rolled steel sheet by forming carbides. It is preferable that
the Cr content be 0.005% or more to realize such an effect. On the
other hand, when the Cr content is more than 0.2%, there is concern
that there may be a decrease in the corrosion resistance of a
hot-rolled steel sheet. Therefore, it is preferable that the Cr
content be 0.005% or more and 0.2% or less, or more preferably
0.01% or more and 0.15% or less.
Mo: 0.005% or More and 0.2% or Less
[0049] Mo contributes to an increase in the hole expansion
formability and strength of a hot-rolled steel sheet by promoting
formation of a bainite phase through increasing hardenability. It
is preferable that the Mo content be 0.005% or more to realize such
effects. However, when the Mo content is more than 0.2%, since a
martensite phase tends to be formed, there is a possibility that
there may be a deterioration in the bending workability of a
hot-rolled steel sheet. Therefore, it is preferable that the Mo
content be 0.005% or more and 0.2% or less, or more preferably
0.01% or more and 0.15% or less.
[0050] In addition, the hot-rolled steel sheet may contain, as
needed, at least one selected from the group consisting of Ca:
0.0002% or more and 0.01% or less and REM: 0.0002% or more and
0.01% or less.
Ca: 0.0002% or More and 0.01% or Less
[0051] Since Ca is a chemical element which controls the shape of
sulfide inclusions, Ca is effective in increasing the bending
workability of a hot-rolled steel sheet. It is preferable that the
Ca content be 0.0002% or more to realize such an effect. However,
when the Ca content is more than 0.01%, there is a possibility that
surface defects may occur in a hot-rolled steel sheet. Therefore,
it is preferable that the Ca content be 0.0002% or more and 0.01%
or less, or more preferably 0.0004% or more and 0.005% or less.
REM: 0.0002% or More and 0.01% or Less
[0052] Since REM, like Ca, controls the shape of sulfide
inclusions, REM reduces the negative effect of sulfide inclusions
on the bending workability of a hot-rolled steel sheet. It is
preferable that REM content be 0.0002% or more to realize such an
effect. However, when REM content is more than 0.01%, since there
is a decrease in the cleanness of steel, there is a tendency for
the hole expansion formability of a hot-rolled steel sheet to
decrease. Therefore, when REM is added, it is preferable that REM
content be 0.0002% or more and 0.01% or less, or more preferably
0.0004% or more and 0.005% or less.
[0053] The balance other than the chemical elements described above
includes Fe and inevitable impurities. Examples of the inevitable
impurities include Sb, Sn, and Zn. It is acceptable that the
contents of Sb, Sn, and Zn be respectively 0.01% or less, 0.1% or
less, and 0.01% or less.
[0054] Hereafter, the reasons for the limitations on the
microstructure of the hot-rolled steel sheet will be described.
[0055] The microstructure of the hot-rolled steel sheet has a
structure that a microstructure in the surface layers placed on the
upper and lower surface sides of the steel sheet (hereinafter,
called surface regions) and a microstructure in the inside of the
steel sheet other than the surface regions (hereinafter, called
inner region), where the microstructure in the inner region is
interposed between the microstructures of the surface regions on
the upper and lower surface sides of the steel sheet. While the
microstructure in the inner region is controlled to be
substantially a bainite single phase, the microstructures in the
surface regions are controlled to include mainly a ferrite phase
such that the microstructures in the surface regions are softer
than that in the inner region to increase the bending workability
of a hot-rolled steel sheet while keeping a high strength of 980
MPa or more in terms of tensile strength TS.
[0056] The microstructure in the surface regions is controlled to
include mainly a ferrite phase in an amount of 80% or more in terms
of area fraction and the balance being at least one selected from a
bainite phase, a martensite phase, and a retained austenite phase
in an amount of 0% or more and 20% or less in terms of area
fraction.
Area Fraction of a Ferrite Phase in the Surface Regions: 80% or
More
[0057] When the area fraction of a ferrite phase, which is a main
phase, in the surface regions is less than 80%, since there is a
decrease in the bending workability of a hot-rolled steel sheet, it
is difficult to achieve a tensile strength of 980 MPa or more and
satisfactory bending workability. Therefore, the area fraction of a
ferrite phase in the surface regions is 80% or more, preferably 90%
or more, or it may be 100% (which means a ferrite single
phase).
Total Area Fraction of a Bainite Phase, a Martensite Phase, and a
Retained Austenite Phase in the Surface Regions: 0% or More and 20%
or Less
[0058] In the surface regions, microstructures other than a ferrite
phase are at least one of a bainite phase, a martensite phase, and
a retained austenite phase, and the total area fraction of these
phases is 0% or more and 20% or less, or preferably 10% or
less.
[0059] On the other hand, the microstructure of the inner region is
controlled to include mainly a bainite phase in an amount of more
than 90% in terms of area fraction and the balance being at least
one selected from the group of a ferrite phase, a martensite phase,
and a retained austenite phase in an amount of 0% or more and less
than 10% in terms of area fraction.
Area fraction of a bainite phase in the inner region: more than
90%
[0060] When the area fraction of a bainite phase, which is a main
phase, in the inner region is 90% or less, since there is a
decrease in the strength and bending workability of a hot-rolled
steel sheet, it is not possible to stably achieve the desired high
strength and satisfactory bending workability. Therefore, the area
fraction of a bainite phase in the inner region is more than 90%,
preferably 95% or more, and most preferably 100% (which means a
bainite single phase).
Total Area Fraction of a Ferrite Phase, a Martensite Phase, and a
Retained Austenite Phase in the Inner Region: 0% or More and Less
than 10%
[0061] In the inner region, microstructures other than a bainite
phase are one or more of a ferrite phase, a martensite phase, and a
retained austenite phase, and the total area fraction of these
phases is 0% or more and less than 10%, or preferably 5% or
less.
The Proportions of the Surface Regions in the Thickness Direction
of the Steel Sheet: 1.0% or More and 5.0% or Less of the Whole
Thickness Respectively from the Upper and Lower Surfaces of the
Steel Sheet
[0062] By controlling the proportions of the surface regions to the
whole hot-rolled steel sheet to achieve a satisfactory
strength-bending workability balance of the hot-rolled steel sheet,
a hot-rolled steel sheet having a tensile strength of 980 MPa or
more and being excellent in terms of bending workability is
obtained. When the depths of the surface regions in the thickness
direction of the steel sheet are less than 1.0% of the whole
thickness respectively from the upper and lower surfaces of the
steel sheet, it is difficult to provide a hot-rolled steel sheet
with satisfactory bending workability, and a crack tends to occur
when the hot-rolled steel sheet is subjected to bending work. On
the other hand, when the depth of the soft surface regions, which
increases bending workability, in the thickness direction of the
steel sheet is more than 5.0% of the whole thickness respectively
from the upper and lower surfaces of the steel sheet, since there
is a decrease in the strength of the whole hot-rolled steel sheet,
it is not possible to achieve the desired tensile strength of 980
MPa or more. Therefore, the proportions of the surface regions in
the thickness direction of the steel sheet are 1.0% or more and
5.0% or less, or preferably 1.5% or more and 3.5% or less, of the
whole thickness respectively from the upper and lower surfaces of
the steel sheet.
[0063] As described above, by specifying a chemical composition and
a microstructure, it is possible to obtain a high-strength
hot-rolled steel sheet having a tensile strength TS of 980 MPa or
more and bending workability which is required for materials for
automotive parts. Although there is no particular limitation on the
thickness of the hot-rolled steel sheet, it is preferable that the
thickness be about 3.2 mm or more and 14 mm or less.
[0064] Hereafter, the preferable method of manufacturing the
hot-rolled steel sheet will be described.
[0065] Our methods include heating a steel material having the
chemical composition described above in a temperature range of
1250.degree. C. or higher, then holding the heated steel material
in the temperature range for 3600 seconds or more, performing hot
rolling including rough rolling and finish rolling under the
condition that the finish delivery temperature is 840.degree. C. or
higher and 940.degree. C. or lower, starting cooling immediately
after hot rolling has been finished, performing cooling at an
average cooling rate of 25.degree. C./s or more, and performing
coiling at a coiling temperature of 350.degree. C. or higher and
500.degree. C. or lower. ".degree. C." represents a "surface
temperature", unless otherwise noted.
[0066] There is no particular limitation on what method is used to
manufacture a steel material, and any of the common methods can be
used to manufacture molten steel having the chemical composition
described above, using a converter and the like, and forming a
steel material such as a slab, using a casting method such as a
continuous casting method. Also, an ingot casting and blooming
method may be used.
Heating Temperature of a Steel Material: 1250.degree. C. or
Higher
Holding Time of the Steel Material at the Heating Temperature: 3600
Seconds or More
[0067] The heating temperature of the steel material and the
holding time of the steel material at the heating temperature are
important factors in the manufacturing conditions. By heating a
steel material such as a slab at a heating temperature of
1250.degree. C. or higher, and by holding the heated steel material
in the heating temperature range of 1250.degree. C. or higher for
3600 seconds or more, since the surface layers of the steel
material are decarburized, there is a decrease in the carbon
concentration in the surface layers. As a result, since a ferrite
phase is more likely to be formed in the surface regions of the
steel material than in the inner region, it is possible to stably
achieve the microstructure for a hot-rolled steel sheet described
above.
[0068] In the steel material, almost all the carbonitride-forming
elements such as Ti are present in the form of large-size
carbonitrides. The presence of such large-size non-uniform
carbonitrides causes a deterioration in the bending workability of
a hot-rolled steel sheet. To make such large-size precipitates
solid solutions formed before hot rolling process, it is necessary
to limit the heating temperature of the steel material to
1250.degree. C. or higher, or preferably 1260.degree. C. or higher
and 1350.degree. C. or lower. In addition, the holding time of the
steel material in a temperature range of 1250.degree. C. or higher
is 3600 seconds or more, or preferably 4000 seconds or more.
However, when the holding time of the steel material in a
temperature range of 1250.degree. C. or higher is excessively long,
since there is an increase in scale generation amount, for example,
biting of scale tends to occur in the subsequent hot rolling
process, which results in concern that there may be a deterioration
in the surface quality of a hot-rolled steel sheet. Therefore, it
is preferable that the holding time be 7200 seconds or less.
[0069] Following heating and holding the steel material, hot
rolling including rough rolling and finish rolling is performed on
the steel material. There is no particular limitation on what
conditions are used for the rough rolling as long as a desired size
for a sheet bar is achieved. Following the rough rolling, finish
rolling is performed. It is preferable to perform descaling before
finish rolling is performed or between the rolling stands in the
middle of finish rolling.
Finish Delivery Temperature: 840.degree. C. or Higher and
940.degree. C. or Lower
[0070] The finish delivery temperature is one of the important
factors in the manufacturing conditions. When the finish delivery
temperature is lower than 840.degree. C., since there is an
increase in the area fraction of a ferrite phase when rolling is
performed for forming a dual phase of a ferrite phase and an
austenite phase, it is difficult to achieve the desired
microstructure described above for a hot-rolled steel sheet. On the
other hand, when the finish delivery temperature is excessively
higher than 940.degree. C., since there is an increase in
hardenability due to the growth of austenite grains in the surface
regions of the steel sheet, phases other than a ferrite phase such
as a bainite phase tend to be formed, which makes it difficult to
achieve the desired microstructure described above for a hot-rolled
steel sheet. Therefore, the finish delivery temperature is
840.degree. C. or higher and 940.degree. C. or lower, preferably
850.degree. C. or higher and 930.degree. C. or lower, or more
preferably 860.degree. C. or higher and 920.degree. C. or
lower.
Average Cooling Rate: 25.degree. C./s or More
[0071] Immediately after finish rolling has been performed,
preferably within 1.5 seconds, accelerated cooling is started,
cooling is stopped at a coiling temperature and, then, the
hot-rolled steel sheet is coiled in a coil shape. When the average
cooling rate from the finish delivery temperature to the coiling
temperature is less than 25.degree. C./s, it is not possible to
form sufficient amount of bainite phase in the inner region of a
hot-rolled steel sheet. Therefore, the average cooling rate
described above is 25.degree. C./s or more, or preferably
30.degree. C./s or more. Although there is no particular limitation
on the upper limit of the average cooling rate, since it is
difficult to achieve the desired microstructure in the surface
regions due to bainite transformation being promoted when the
average cooling rate is excessively large, it is preferable that
the average cooling rate be 120.degree. C./s or less. The average
cooling rate described above is the average cooling rate in terms
of the surface temperature of a steel sheet.
Coiling Temperature: 350.degree. C. or Higher and 500.degree. C. or
Lower
[0072] When the coiling temperature is lower than 350.degree. C., a
martensite phase and a retained austenite phase, which are hard,
tend to be formed in the inner region of a hot-rolled steel sheet.
As a result, since it is not possible to achieve the desired
microstructure, it is difficult to provide a hot-rolled
microstructure the desired bending workability. On the other hand,
when the coiling temperature is higher than 500.degree. C., since a
ferrite phase tends to be formed in the inner region of a
hot-rolled steel sheet, large-size pearlite phase and large-size
martensite phase are formed, which results in a decrease in the
strength and bending workability of a hot-rolled steel sheet.
Considering the reasons described above, the coiling temperature is
350.degree. C. or higher and 500.degree. C. or lower, or preferably
350.degree. C. or higher and 450.degree. C. or lower.
[0073] To reduce segregation of the composition in steel at a
continuous casting, for example, electromagnetic stirring (EMS) or
light-reduction casting (IBSR) may be used. By performing an
electromagnetic stirring treatment, since equiaxial crystals are
formed in the central portion in the thickness direction in a steel
sheet, it is possible to reduce the segregation. In addition, when
light-reduction casting is performed, since the flow of molten
steel in a non-solidified portion in a continuous cast slab is
prevented, it is possible to reduce the segregation in the central
portion in the thickness direction in a steel sheet. By using at
least one of these treatments to reduce segregation, it is possible
to further raise the level of bending workability described
below.
[0074] After coiling has been performed, as with common methods of
manufacturing a hot-rolled steel sheet, temper rolling may be
performed, or scale formed on the surface of a steel sheet may be
removed by performing pickling. Moreover, a plating treatment such
as hot dip galvanizing or electrogalvanizing or chemical conversion
coating may be performed.
Examples
[0075] Molten steels having the chemical compositions given in
Table 1 were manufactured using a converter, and then slabs (steel
materials) were manufactured using a continuous casting method. In
hot-rolled steel sheets other than hot-rolled steel sheet No. 1'
composed of steel A given in Tables 1 through 3 described below,
electromagnetic stirring (EMS) was performed to reduce the
segregation of composition when continuous casting was performed.
Subsequently, by heating and holding these steel materials under
the conditions given in Table 2, by performing hot rolling
including rough rolling and finish rolling under the conditions
given in Table 2, by performing cooling under the conditions given
in Table 2 after finish rolling had been performed, and by
performing coiling at the coiling temperatures given in Table 2,
hot-rolled steel sheets having the thicknesses given in Table 2
were manufactured.
[0076] Microstructure observation, a tensile test, and a bending
test were performed on test pieces taken from the obtained
hot-rolled steel sheets. Microstructure observation and the tests
were performed using the following methods.
(i) Microstructure Observation
[0077] By taking a test piece for a scanning electron microscope
(SEM) from the obtained hot-rolled steel sheet, by polishing a
cross section in the thickness direction parallel to the rolling
direction, by emergence of a microstructure using an etching
solution (3%-nital solution), and by observing the vicinity of
surface layers using a scanning electron microscope (SEM) at a
magnification of 800 to 1500 times, surface regions were
identified. Since a ferrite phase is observed more in surface
regions than in an inner region, it is possible to distinguish
surface regions from an inner region.
[0078] After the surface regions had been identified, by
determining the depth of the region in which the surface region was
formed on each of the upper and lower surface sides of the test
piece (depth in the thickness direction from each of the upper and
lower surfaces of the test piece), the proportion of the surface
region which was formed on each of the upper and lower surfaces of
the test piece in the thickness direction of the test piece (the
thickness direction of the steel sheet) was determined.
Specifically, by determining the depth d.sub.1 of the region in
which the surface region was formed on the upper surface side of
the test piece (depth in the thickness direction from the upper
surface of the test piece) and the depth d.sub.2 of the region in
which the surface region was formed on the lower surface side of
the test piece (depth in the thickness direction from the lower
surface of the test piece), by calculating the average value d
(=(d.sub.1+d.sub.2)/2) of these depths, and by calculating the
ratio (d/t.times.100(%)) of the average value d to the total
thickness t of the test piece (that is, the thickness of the
hot-rolled steel sheet), the proportion of the surface region which
had been formed on each of the upper and lower surface sides in the
thickness direction of the test piece (the thickness direction of
the steel sheet) was determined. The depths (d.sub.1 and d.sub.2)
described above were determined based on the scale bars in the
images obtained using a SEM at a magnification of 800 to 1500
times.
[0079] The area fractions of the each constituent phases of the
surface region were determined by taking photographs using a SEM at
a magnification of 3000 times in 5 fields of view at the central
position in the depth direction of the surface region and the
vicinity of the central position and by performing image analysis
within the range of .+-.20 .mu.m in the depth direction from the
central position to quantify the area fractions of the constituent
phases. The area fractions of the constituent phases of the inner
region were determined by taking photographs using a SEM (scanning
electron microscope) at a magnification of 3000 times in 5 fields
of view each at a position located at 1/4 of the thickness and at a
position located at 1/2 of the thickness and by performing image
analysis to quantify the area fractions of the constituent
phases.
(ii) Tensile Test
[0080] A tensile test was performed in accordance with a method
prescribed in JIS Z 2241 (2011) on a JIS No. 5 tensile test piece
(having a GL of 50 mm) which had been taken from the obtained
hot-rolled steel sheet so that the tensile direction was at a right
angle to the rolling direction to obtain yield strength (yield
point) YP, tensile strength TS, and total elongation El.
(iii) Bending Test
[0081] After performing shearing work on the obtained hot-rolled
steel sheet, a bending test piece of 20 mm.times.150 mm was taken
so that the longitudinal direction of the test piece was at a right
angle to the rolling direction. A 180.degree. bending test was
performed on the test piece having a shear plane in accordance with
a pressing bend method prescribed in JIS Z 2248 (2006). By
performing the test on three test pieces for each case, by defining
a limit bending radius R (mm) as the minimum bending radius with
which a crack did not occur, and by calculating an R/t value by
dividing R by the thickness t (mm) of the hot-rolled steel sheet,
bending workability of the hot-rolled steel sheet was evaluated
based on the R/t value. An R/t value of 0.50 or less was evaluated
as a case of excellent bending workability.
[0082] Obtained results are given in Table 3.
TABLE-US-00001 TABLE 1 Chemical Composition (mass %) Balance: Fe
and Inevitable Impurities Steel C Si Mn P S Al N Ti V Other Note A
0.11 0.6 2.4 0.021 0.0015 0.030 0.0031 0.15 0.24 -- Example Steel B
0.18 0.8 2.0 0.014 0.0031 0.065 0.0078 0.12 0.20 -- Example Steel C
0.07 0.8 2.2 0.015 0.0034 0.062 0.0034 0.10 0.15 -- Comparative
Steel D 0.14 1.0 2.4 0.015 0.0021 0.041 0.0043 0.09 0.15 -- Example
Steel E 0.13 1.2 1.8 0.023 0.0019 0.043 0.0032 0.08 0.15 -- Example
Steel F 0.14 0.2 1.9 0.022 0.0020 0.044 0.0053 0.09 0.25 --
Comparative Steel G 0.15 0.7 2.1 0.046 0.0014 0.015 0.0047 0.11
0.20 -- Example Steel H 0.12 0.9 3.3 0.023 0.0038 0.082 0.0040 0.15
0.22 -- Example Steel I 0.13 0.6 4.0 0.016 0.0012 0.019 0.0057 0.15
0.15 -- Comparative Steel J 0.14 2.5 1.1 0.012 0.0018 0.058 0.0061
0.06 0.25 -- Example Steel K 0.13 0.5 1.9 0.021 0.0033 0.061 0.0049
0.20 0.20 -- Comparative Steel L 0.16 0.8 2.3 0.037 0.0009 0.047
0.0060 0.09 0.05 -- Comparative Steel M 0.11 0.7 1.8 0.014 0.0026
0.066 0.0052 0.10 0.15 Nb: 0.08 Example Steel N 0.11 0.6 2.2 0.011
0.0022 0.023 0.0043 0.10 0.11 Ni: 0.15, Cr: 0.2 Example Steel O
0.14 0.7 1.8 0.013 0.0032 0.071 0.0021 0.12 0.20 Mo: 0.1 Example
Steel P 0.16 0.7 2.1 0.014 0.0021 0.038 0.0049 0.15 0.15 B: 0.0007
Example Steel Q 0.15 0.5 1.9 0.015 0.0009 0.023 0.0054 0.11 0.22
Ca: 0.005 Example Steel R 0.11 0.9 2.4 0.016 0.0011 0.033 0.0047
0.11 0.28 REM: 0.003 Example Steel S 0.15 0.7 2.3 0.028 0.0008
0.050 0.0061 0.09 0.18 Cu: 0.1 Example Steel
TABLE-US-00002 TABLE 2 Hot- rolled Hot Rolling Condition Steel Slab
Heating Holding Finish Delivery Average Coiling Thickness Sheet
Temperature Time Temperature Cooling Rate Temperature t No. Steel
(.degree. C.) *1 (s) (.degree. C.) (.degree. C./s) (.degree. C.)
(mm) Note 1 A 1280 3800 910 35 420 6 Example Steel 1' 1280 3800 910
35 420 6 Example Steel 2 1260 4000 910 40 380 6 Example Steel 3 B
1260 3800 970 45 380 3.2 Comparative Steel 4 1280 3800 920 45 400
3.2 Example Steel 5 C 1260 3800 920 35 400 8 Comparative Steel 6 D
1280 4000 890 40 390 4 Example Steel 7 1200 3000 890 40 380 4
Comparative Steel 8 E 1260 4400 940 35 360 8 Example Steel 9 1280
4200 800 35 410 8 Comparative Steel 10 F 1260 3800 910 40 370 6
Comparative Steel 11 G 1260 4000 920 45 390 3.2 Example Steel 12
1280 3800 850 45 400 3.2 Example Steel 13 H 1260 4600 900 25 380 12
Example Steel 14 1280 3800 930 25 520 12 Comparative Steel 15 I
1260 3600 870 30 410 8 Comparative Steel 16 J 1300 4000 900 25 440
10 Example Steel 17 1280 3800 890 25 390 10 Example Steel 18 K 1250
4200 940 35 420 6 Comparative Steel 19 L 1260 4200 900 45 360 3.2
Comparative Steel 20 M 1280 3800 880 40 400 4 Example Steel 21 N
1260 4200 890 40 380 4 Example Steel 22 O 1260 4400 870 35 360 6
Example Steel 23 P 1280 4200 920 35 420 6 Example Steel 24 Q 1300
4000 920 35 370 6 Example Steel 25 R 1280 4600 940 30 400 8 Example
Steel 26 S 1260 4600 890 30 390 8 Example Steel *1 Holding time at
the slab heating temperature (s)
TABLE-US-00003 TABLE 3 Microstructure of Hot-rolled Steel Sheet *2
Inner Region Position Position Located Located at 1/4 of at 1/2 of
Surface Region Thickness Thickness Mechanical Property of
Hot-rolled Steel Sheet Hot- Propor- Area Area Area Total rolled
tion of Fraction Fraction Fraction Yield Tensile Elon- Steel
Surface of F of B of B Stress Strength gation Sheet Region Phase
Phase Phase YP TS EL R/t No. Steel *3 (%) (%) Other (%) Other (%)
Other (MPa) (MPa) (%) *4 Note 1 A 2.0 83 B 92 M, .gamma. 94 M,
.gamma. 864 1034 15.7 0.45 Example Steel 1' 2.1 84 B 92 M, .gamma.
95 M, .gamma. 865 1035 14.5 0.50 Example Steel 2 1.6 85 B, M 94 M,
.gamma. 95 M, .gamma. 920 1094 13.7 0.32 Example Steel 3 B 0.5 61
B, M 95 M 97 M 895 1042 11.2 0.65 Comparative Steel 4 1.9 82 B 93 M
94 M 853 1012 13.0 0.39 Example Steel 5 C 2.3 86 B 92 F 94 F 805
958 17.3 0.43 Comparative Steel 6 D 2.6 84 B 93 M 94 M 837 997 14.1
0.32 Example Steel 7 1.5 53 B, M 94 M 97 M 866 1012 12.5 0.54
Comparative Steel 8 E 1.4 82 B 95 M, .gamma. 97 M, .gamma. 867 1016
14.5 0.40 Example Steel 9 3.6 94 B 79 F 83 F 761 941 20.5 0.35
Comparative Steel 10 F 0.3 33 B, M 94 M 97 M 856 992 13.2 0.56
Comparative Steel 11 G 1.3 81 B, M 93 M 96 M 852 1003 12.6 0.43
Example Steel 12 3.9 88 B 91 M 93 M 818 988 14.5 0.25 Example Steel
13 H 1.8 92 B 100 -- 100 -- 993 1207 16.1 0.42 Example Steel 14 1.4
53 B, M, P 77 F, M, P 81 F, M, P 879 1072 20.0 0.57 Comparative
Steel 15 I 0.7 36 B, M 93 M 100 M 928 1096 14.7 0.67 Comparative
Steel 16 J 3.6 85 B 91 F, M 92 F, M 823 1002 19.7 0.40 Example
Steel 17 2.8 87 B 92 F, M 94 F, M 896 1077 16.6 0.32 Example Steel
18 K 1.6 84 B, M 82 F 85 F 821 973 15.7 0.48 Comparative Steel 19 L
1.6 83 B, M 95 F, M 96 M 805 968 12.3 0.32 Comparative Steel 20 M
2.9 85 B 93 F, M 96 M 912 1093 13.3 0.30 Example Steel 21 N 2.0 81
B, M 94 M 94 M 869 1022 12.8 0.40 Example Steel 22 O 2.5 100 -- 95
M 95 M 863 1037 14.9 0.22 Example Steel 23 P 2.3 83 B 92 M 92 M 891
1069 15.3 0.41 Example Steel 24 Q 2.2 81 B 94 M 94 M 863 1011 13.8
0.44 Example Steel 25 R 2.2 82 B 93 M 93 M 885 1050 15.5 0.46
Example Steel 26 S 1.6 88 B, M 93 M, .gamma. 93 M, .gamma. 836 1005
17.0 0.30 Example Steel *2 B: bainite, F: ferrite, M: martensite,
.gamma.: retained austenite *3 The proportion of a surface region
formed on each of the upper and lower surface sides of a steel
sheet in the thickness direction. *4 R: limit bending radius (mm),
t: thickness (mm)
[0083] The hot-rolled steel sheets of the examples were all
hot-rolled steel sheets having the desired strength (980 MPa or
more in terms of TS) and excellent bending workability (R/t value
of 0.50 or less). On the other hand, in the hot-rolled steel sheets
of the comparative examples, which were out of our range, the
desired strength or satisfactory bending workability was not
achieved.
* * * * *