U.S. patent application number 15/574838 was filed with the patent office on 2018-12-27 for high-strength thin steel sheet and method for manufacturing 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 Akimasa KIDO, Taro KIZU, Tetsushi TADANI, Shunsuke TOYODA.
Application Number | 20180371574 15/574838 |
Document ID | / |
Family ID | 57686171 |
Filed Date | 2018-12-27 |
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United States Patent
Application |
20180371574 |
Kind Code |
A9 |
KIZU; Taro ; et al. |
December 27, 2018 |
HIGH-STRENGTH THIN STEEL SHEET AND METHOD FOR MANUFACTURING THE
SAME
Abstract
This disclosure provides a predetermined composition, where a
conversion value C* of total carbon contents in Ti, Nb and V
precipitates whose grain sizes are less than 20 nm is 0.010 mass %
to 0.100 mass %, Fe content in Fe precipitates is 0.03 mass % to
0.50 mass %, and an average grain size of ferrite grains whose
grain sizes are top 5 % large in ferrite grain size distribution of
rolling direction cross section is (4000/TS).sup.2 .mu.m or less,
the TS indicating tensile strength in unit of MPa.
Inventors: |
KIZU; Taro; (Chiyoda-ku,
Tokyo, JP) ; TOYODA; Shunsuke; (Chiyoda-ku, Tokyo,
JP) ; KIDO; Akimasa; (Chiyoda-ku, Tokyo, JP) ;
TADANI; Tetsushi; (Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku, Tokyo
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20180155806 A1 |
June 7, 2018 |
|
|
Family ID: |
57686171 |
Appl. No.: |
15/574838 |
Filed: |
July 5, 2016 |
PCT Filed: |
July 5, 2016 |
PCT NO: |
PCT/JP2016/003207 PCKC 00 |
371 Date: |
November 17, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B21B 2001/225 20130101;
C22C 38/24 20130101; C22C 38/002 20130101; C21D 8/0205 20130101;
C22C 38/04 20130101; C22C 38/001 20130101; C22C 38/005 20130101;
C22C 38/06 20130101; C21D 7/13 20130101; C22C 38/42 20130101; C21D
8/0226 20130101; C22C 38/12 20130101; C22C 38/38 20130101; B21B
1/22 20130101; C22C 38/46 20130101; C22C 38/28 20130101; C22C 38/60
20130101; C22C 38/58 20130101; C21D 2211/004 20130101; C21D 6/008
20130101; C21D 1/18 20130101; C22C 38/02 20130101; C21D 2211/005
20130101; C22C 38/50 20130101; C21D 9/46 20130101; C22C 38/14
20130101; C22C 38/48 20130101; B21B 3/00 20130101; C21D 8/0263
20130101; C22C 38/44 20130101; C21D 6/005 20130101; C22C 38/00
20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C21D 7/13 20060101
C21D007/13; C21D 6/00 20060101 C21D006/00; C21D 1/18 20060101
C21D001/18; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/38 20060101 C22C038/38; C22C 38/28 20060101
C22C038/28; C22C 38/24 20060101 C22C038/24; C22C 38/00 20060101
C22C038/00; B21B 1/22 20060101 B21B001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 2015 |
JP |
2015-135432 |
Claims
1. A high-strength thin steel sheet comprising a chemical
composition containing, in mass %, C: 0.05% to 0.20%, Si: 0.6% to
1.5%, Mn: 1.3% to 3.0%, P: 0.10% or less, S: 0.030% or less, Al:
0.10% or less, N: 0.010% or less, and at least one selected from
Ti: 0.01% to 1.00%, Nb: 0.01% to 1.00%, and V: 0.01% to 1.00%, the
balance consisting of Fe and inevitable impurities, wherein a
conversion value C* of total carbon contents in Ti, Nb and V
precipitates whose grain sizes are less than 20 nm, defined by the
following formula (1), is 0.010 mass % to 0.100 mass %, Fe content
in Fe precipitates is 0.03 mass % to 0.50 mass %, and an average
grain size of ferrite grains whose grain sizes are top 5% large in
ferrite grain size distribution of rolling direction cross section
is (4000/TS).sup.2 .mu.m or less, the TS indicating tensile
strength in unit of MPa, C*=([Ti]/48+[Nb]/93+[V]/51).times.12 (1)
where [Ti], [Nb] and [V] each indicate contents of Ti, Nb and V in
Ti, Nb and V precipitates whose grain sizes are less than 20
nm.
2. The high-strength thin steel sheet according to claim 1, wherein
the composition further comprises, in mass %, at least one selected
from Mo: 0.005% to 0.50%, Ta: 0.005% to 0.50%, and W: 0.005% to
0.50%, a conversion value C** of total carbon contents in Ti, Nb,
V, Mo, Ta and W precipitates whose grain sizes are less than 20 nm,
defined by the following formula (2), is 0.010 mass % to 0.100 mass
%, C**=([Ti]/48+[Nb]/93+[V]/51+[Mo]/96+[Ta]/181+[W]/184).times.12
(2) where [Ti], [Nb], [V], [Mo], [Ta] and [W] each indicate
contents of Ti, Nb, V, Mo, Ta and W in Ti, Nb, V, Mo, Ta and W
precipitates whose grain sizes are less than 20 nm.
3-7. (canceled)
8. The high-strength thin steel sheet according to claim 1, wherein
the composition further comprises, in mass %, at least one selected
from groups (a) to (c): (a) at least one selected from Cr: 0.01% to
1.00%, Ni: 0.01% to 1.00%, and Cu: 0.01% to 1.00%; (b) Sb: 0.005%
to 0.050%; and (c) one or both selected from Ca: 0.0005% to 0.0100%
and REM: 0.0005% to 0.0100%.
9. The high-strength thin steel sheet according to claim 2, wherein
the composition further comprises, in mass %, at least one selected
from groups (a) to (c): (a) at least one selected from Cr: 0.01% to
1.00%, Ni: 0.01% to 1.00%, and Cu: 0.01% to 1.00%; (b) Sb: 0.005%
to 0.050%; and (c) one or both selected from Ca: 0.0005% to 0.0100%
and REM: 0.0005% to 0.0100%.
10. A method for manufacturing the high-strength thin steel sheet
according to claim 1, comprising: hot rolling a steel slab having
the composition according to claim 1 to obtain a steel sheet, the
hot rolling comprising rough rolling and finish rolling; and
cooling and coiling the steel sheet after completing the finish
rolling, wherein cumulative strain R.sub.t defined by the following
formula (3) in the finish rolling is 1.3 or more and finisher
delivery temperature is 820.degree. C. or higher and lower than
930.degree. C., the steel sheet is cooled down from the finisher
delivery temperature to a temperature where slow cooling starts at
an average cooling rate of 30.degree. C./s or higher after
completing the finish rolling, then slow cooling is started at a
temperature of 750.degree. C. to 600.degree. C. where an average
cooling rate is lower than 10.degree. C./s and cooling time is 1
second to 10 seconds during the slow cooling, and the steel sheet
is cooled down to a coiling temperature of 350.degree. C. or higher
and lower than 530.degree. C. at an average cooling rate of
10.degree. C./s or higher after completing the slow cooling, R t =
R 1 + R 2 + + R m ( = n = 1 m R n ) ( 3 ) ##EQU00003## where
R.sub.n is strain accumulated at an n.sup.th stand from upstream
side when finish rolling is performed with m stands and is defined
by the following formula,
R.sub.n=-1n1-0.01.times.r.sub.n.times.[1-0.01.times.exp{-(11800+2.times.1-
0.sup.3.times.[C])/T.sub.n.times.273)+13.1-0.1.times.[C]}] where
r.sub.n is rolling reduction rate (%) at an n.sup.th stand from
upstream side, T.sub.n is entry temperature (.degree. C.) at an
n.sup.th stand from upstream side, [C] is C content in mass % in
steel, and n is an integer from 1 to m, provided that when
exp{-(11800+2.times.10.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}
exceeds 100, a value thereof is set to be 100.
11. The method for manufacturing a high-strength thin steel sheet
according to claim 10, wherein an additional work is performed with
a sheet thickness reduction rate being 0.1% to 3.0% after the hot
rolling.
12. A method for manufacturing the high-strength thin steel sheet
according to claim 2, comprising: hot rolling a steel slab having
the composition according to claim 2 to obtain a steel sheet, the
hot rolling comprising rough rolling and finish rolling; and
cooling and coiling the steel sheet after completing the finish
rolling, wherein cumulative strain R.sub.t defined by the following
formula (3) in the finish rolling is 1.3 or more and finisher
delivery temperature is 820.degree. C. or higher and lower than
930.degree. C., the steel sheet is cooled down from the finisher
delivery temperature to a temperature where slow cooling starts at
an average cooling rate of 30.degree. C./s or higher after
completing the finish rolling, then slow cooling is started at a
temperature of 750.degree. C. to 600.degree. C. where an average
cooling rate is lower than 10.degree. C./s and cooling time is 1
second to 10 seconds during the slow cooling, and the steel sheet
is cooled down to a coiling temperature of 350.degree. C. or higher
and lower than 530.degree. C. at an average cooling rate of
10.degree. C./s or higher after completing the slow cooling, R t =
R 1 + R 2 + + R m ( = n = 1 m R n ) ( 3 ) ##EQU00004## where
R.sub.n is strain accumulated at an n.sup.th stand from upstream
side when finish rolling is performed with m stands and is defined
by the following formula,
R.sub.n=-1n1-0.01.times.r.sub.n.times.[1-0.01.times.exp{-(11800+2.times.1-
0.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}] where r.sub.n
is rolling reduction rate (%) at an n.sup.th stand from upstream
side, T.sub.n is entry temperature (.degree. C.) at an n.sup.th
stand from upstream side, [C] is C content in mass % in steel, and
n is an integer from 1 to m, provided that when
exp{-(11800+2.times.10.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}
exceeds 100, a value thereof is set to be 100.
13. The method for manufacturing a high-strength thin steel sheet
according to claim 12, wherein an additional work is performed with
a sheet thickness reduction rate being 0.1% to 3.0% after the hot
rolling.
14. A method for manufacturing the high-strength thin steel sheet
according to claim 8, comprising: hot rolling a steel slab having
the composition according to claim 8 to obtain a steel sheet, the
hot rolling comprising rough rolling and finish rolling; and
cooling and coiling the steel sheet after completing the finish
rolling, wherein cumulative strain R.sub.t defined by the following
formula (3) in the finish rolling is 1.3 or more and finisher
delivery temperature is 820.degree. C. or higher and lower than
930.degree. C., the steel sheet is cooled down from the finisher
delivery temperature to a temperature where slow cooling starts at
an average cooling rate of 30.degree. C./s or higher after
completing the finish rolling, then slow cooling is started at a
temperature of 750.degree. C. to 600.degree. C. where an average
cooling rate is lower than 10.degree. C./s and cooling time is 1
second to 10 seconds during the slow cooling, and the steel sheet
is cooled down to a coiling temperature of 350.degree. C. or higher
and lower than 530.degree. C. at an average cooling rate of
10.degree. C./s or higher after completing the slow cooling, R t =
R 1 + R 2 + + R m ( = n = 1 m R n ) ( 3 ) ##EQU00005## where
R.sub.n is strain accumulated at an n.sup.th stand from upstream
side when finish rolling is performed with m stands and is defined
by the following formula,
R.sub.n=1n1-0.01.times.r.sub.n.times.[1-0.01.times.exp{-(11800+2.times.10-
.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}] where r.sub.n
is rolling reduction rate (%) at an n.sup.th stand from upstream
side, T.sub.n is entry temperature (.degree. C.) at an n.sup.th
stand from upstream side, [C] is C content in mass % in steel, and
n is an integer from 1 to m, provided that when
exp{-(11800+2.times.10.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}
exceeds 100, a value thereof is set to be 100.
15. The method for manufacturing a high-strength thin steel sheet
according to claim 14, wherein an additional work is performed with
a sheet thickness reduction rate being 0.1% to 3.0% after the hot
rolling.
16. A method for manufacturing the high-strength thin steel sheet
according to claim 9, comprising: hot rolling a steel slab having
the composition according to claim 9 to obtain a steel sheet, the
hot rolling comprising rough rolling and finish rolling; and
cooling and coiling the steel sheet after completing the finish
rolling, wherein cumulative strain R.sub.t defined by the following
formula (3) in the finish rolling is 1.3 or more and finisher
delivery temperature is 820.degree. C. or higher and lower than
930.degree. C., the steel sheet is cooled down from the finisher
delivery temperature to a temperature where slow cooling starts at
an average cooling rate of 30.degree. C./s or higher after
completing the finish rolling, then slow cooling is started at a
temperature of 750.degree. C. to 600.degree. C. where an average
cooling rate is lower than 10.degree. C./s and cooling time is 1
second to 10 seconds during the slow cooling, and the steel sheet
is cooled down to a coiling temperature of 350.degree. C. or higher
and lower than 530.degree. C. at an average cooling rate of
10.degree. C./s or higher after completing the slow cooling, R t =
R 1 + R 2 + + R m ( = n = 1 m R n ) ( 3 ) ##EQU00006## where
R.sub.n is strain accumulated at an n.sup.th stand from upstream
side when finish rolling is performed with m stands and is defined
by the following formula,
R.sub.n=-1n1-0.01.times.r.sub.n.times.[1-0.01.times.exp{-(11800+2.times.1-
0.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}] where r.sub.n
is rolling reduction rate (%) at an n.sup.th stand from upstream
side, T.sub.n is entry temperature (.degree. C.) at an n.sup.th
stand from upstream side, [C] is C content in mass % in steel, and
n is an integer from 1 to m, provided that when
exp{-(11800+2.times.10.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}
exceeds 100, a value thereof is set to be 100.
17. The method for manufacturing a high-strength thin steel sheet
according to claim 16, wherein an additional work is performed with
a sheet thickness reduction rate being 0.1% to 3.0% after the hot
rolling.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a high-strength thin steel sheet
having excellent blanking workability and toughness which are
suitable for applications, for example, suspension parts such as
lower arms and frames, frameworks such as pillars and members as
well as their reinforcing members, door impact beams, and seat
members of automobiles, and structural members for vending
machines, desks, consumer electrical appliances, office automation
equipment, building materials, and the like. This disclosure also
relates to a method for manufacturing the high-strength thin steel
sheet.
BACKGROUND
[0002] In recent years, responding to increasing public concern
about global environment issues, there has been a growing demand
for, for example, curbing use of thick steel sheets which
necessitate relatively large CO.sub.2 emission during manufacturing
of the steel sheets. Furthermore, in the automobile industry, there
has been a growing demand for, for example, lighter-weight vehicles
which improve a fuel consumption rate while reducing exhaust gas.
For these reasons, steel sheets have been made stronger and
thinner.
[0003] High-strength steel sheets generally have poor blanking
workability and toughness. Therefore, it is desired to develop a
high-strength thin which can be used for parts molded by press
blanking or for parts requiring toughness or, particularly, for
parts that are molded by press punching and require toughness at
the same time.
[0004] For example, JP 2008-261029 A (PTL 1) describes a steel
sheet excellent in blanking workability, which is "a high-strength
hot rolled steel sheet excellent in blanking workability,
comprising, in mass %, C: 0.010% to 0.200%, Si: 0.01% to 1.5%, Mn:
0.25% to 3%, controlling P to 0.05% or less, further comprising at
least one of Ti: 0.03% to 0.2%, Nb: 0.01% to 0.2%, V: 0.01% to
0.2%, and Mo: 0.01% to 0.2%, the balance consisting of Fe and
inevitable impurities, and a segregation amount of C at large-angle
crystal grain boundaries of ferrite being 4 atms/nm.sup.2 to 10
atms/nm.sup.2".
[0005] Additionally, WO 2013/022043 (PTL 2) describes a steel sheet
excellent in toughness, which is a "high yield ratio hot rolled
steel sheet which has an excellent low temperature impact energy
absorption and HAZ softening resistance characterized by
comprising, by mass %, C: 0.04% to 0.09%, Si: 0.4% or less, Mn:
1.2% to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less,
Nb: 0.02% to 0.09%, Ti: 0.02% to 0.07%, and N: 0.005% or less, a
balance of Fe and unavoidable impurities, where 2.0.ltoreq.Mn+8[%
Ti]+12[% Nb]2.6, and having a metal structure which comprises an
area percentage of pearlite of 5% or less, a total area percentage
of martensite and retained austenite of 0.5% or less, and a balance
of one or both of ferrite and bainite, having an average grain size
of ferrite and bainite of 10 .mu.m or less, having an average grain
size of alloy carbonitrides with incoherent interfaces which
contain Ti and Nb of 20 nm or less, having a yield ratio of 0.85 or
more, and having a maximum tensile strength of 600 MPa or
more".
CITATION LIST
Patent Literature
[0006] PTL 1: JP 2008-261029 A
[0007] PTL 2: WO 2013/022043
SUMMARY
Technical Problem
[0008] However, for the steel sheet described in PTL 1, conditions
required for excellent toughness such as the grain size of
precipitates were not taken into consideration, and there was a
problem that excellent blanking workability and toughness could not
be compatibly attained.
[0009] Additionally, for the steel sheet described in PTL 2,
conditions required for excellent blanking workability were not
taken into consideration, and there was also a problem that
excellent blanking workability and toughness could not be
compatibly attained.
[0010] To solve the above problems, it could be helpful to provide
a high-strength thin steel sheet having both of excellent blanking
workability and excellent toughness, as well as an advantageous
manufacturing method thereof.
[0011] The high-strength thin steel sheet in this disclosure is
intended for a steel sheet having a thickness of 1 mm to 4 mm. In
addition to a hot rolled steel sheet, the high-strength thin steel
sheet in this disclosure also includes a steel sheet which has been
subjected to surface treatment such as hot-dip galvanizing,
galvannealing and electrogalvanization. Steel sheets obtained by
subjecting the above-mentioned steel sheets to, for example,
chemical conversion treatment to form a layer thereon are also
included. Note that the sheet thickness does not include the
thickness of planting or layer.
Solution to Problem
[0012] As a result of a keen study to solve the above problems, we
discovered the following.
(1) Blanking workability can be significantly improved by having a
certain composition and simultaneously precipitating fine
precipitates of Ti, Nb, V and the like whose grain sizes are less
than 20 nm and Fe precipitates such as cementite in an appropriate
amount.
[0013] Regarding this mechanism, our consideration is as follows.
Fe precipitates are precipitated, and these Fe precipitates serve
as origins of cracks during blanking. Additionally, fine
precipitates of Ti, Nb, V and the like promote propagation of the
cracks. Therefore, it is considered that by precipitating Fe
precipitates and fine precipitates of Ti, Nb, V and the like in an
appropriate amount, end face cracking during blanking is
suppressed, and accordingly, blanking workability is significantly
improved.
[0014] Examples of fine precipitates of Ti, Nb, V and the like
include carbide, composite carbide, carbonitride and composite
carbonitride of Ti, Nb and V. Depending on the composition, it is
Ti, Nb, V, Mo, Ta and W in some cases. Examples of Fe precipitates
include cementite i.e. .theta. carbide and carbide.
[0015] (2) The ferrite grain size in the rolling direction of a
steel sheet has a great influence on toughness. Particularly, the
average grain size of top 5% large grain sizes greatly influences
toughness. By appropriately controlling the average grain size of
ferrite whose grain size is top 5% large according to tensile
strength TS (MPa), toughness can be significantly improved.
[0016] Furthermore, since the above-mentioned fine precipitates of
Ti, Nb, V and the like serve as origins of transition, toughness is
further improved.
[0017] This disclosure is based on the aforementioned discoveries
and further studies.
[0018] Specifically, the primary features of this disclosure are as
described below.
1. A high-strength thin steel sheet comprising a chemical
composition containing (consisting of), in mass %, C: 0.05% to
0.20%, Si: 0.6% to 1.5%, Mn: 1.3% to 3.0%, P: 0.10% or less, S:
0.030% or less, Al: 0.10% or less, N: 0.010% or less, and at least
one selected from Ti: 0.01% to 1.00%, Nb: 0.01% to 1.00%, and V:
0.01% to 1.00%, the balance consisting of Fe and inevitable
impurities, where
[0019] a conversion value C* of total carbon contents in Ti, Nb and
V precipitates whose grain sizes are less than 20 nm, defined by
the following formula (1), is 0.010 mass % to 0.100 mass %,
[0020] Fe content in Fe precipitates is 0.03 mass % to 0.50 mass %,
and an average grain size of ferrite grains whose grain sizes are
top 5% large in ferrite grain size distribution of rolling
direction cross section is (4000/TS).sup.2 .mu.m or less, the TS
indicating tensile strength in unit of MPa,
C*=([Ti]/48+[Nb]/93+[V]/51).times.12 (1)
where [Ti], [Nb] and [V] each indicate contents of Ti, Nb and V in
Ti, Nb and V precipitates whose grain sizes are less than 20
nm.
[0021] 2. The high-strength thin steel sheet according to 1., where
the composition further contains, in mass %, at least one selected
from Mo: 0.005% to 0.50%, Ta: 0.005% to 0.50%, and W: 0.005% to
0.50%,
[0022] a conversion value C** of total carbon contents in Ti, Nb,
V, Mo, Ta and W precipitates whose grain sizes are less than 20 nm,
defined by the following formula (2), is 0.010 mass % to 0.100 mass
%,
C**=([Ti]/48+[Nb]/93+[V]/51+[Mo]/96+[Ta]/181+[W]/184).times.12
(2)
where [Ti], [Nb], [V], [Mo], [Ta] and [W] each indicate contents of
Ti, Nb, V, Mo, Ta and W in Ti, Nb, V, Mo, Ta and W precipitates
whose grain sizes are less than 20 nm.
[0023] 3. The high-strength thin steel sheet according to 1. or 2.,
where the composition further contains, in mass %, at least one
selected from Cr: 0.01% to 1.00%, Ni: 0.01% to 1.00%, and Cu: 0.01%
to 1.00%.
[0024] 4. The high-strength thin steel sheet according to any one
of 1. to 3., where the composition further contains, in mass %, Sb:
0.005% to 0.050%.
[0025] 5. The high-strength thin steel sheet according to any one
of 1. to 4., where the composition further contains, in mass %, one
or both selected from Ca: 0.0005% to 0.0100% and REM: 0.0005% to
0.0100%.
[0026] 6. A method for manufacturing the high-strength thin steel
sheet according to any one of 1. to 5., including:
[0027] hot rolling a steel slab having the composition according to
any one of 1. to 5. to obtain a steel sheet, the hot rolling
comprising rough rolling and finish rolling; and
[0028] cooling and coiling the steel sheet after completing the
finish rolling, where
[0029] cumulative strain R.sub.t defined by the following formula
(3) in the finish rolling is 1.3 or more and finisher delivery
temperature is 820.degree. C. or higher and lower than 930.degree.
C.,
[0030] the steel sheet is cooled down from the finisher delivery
temperature to a temperature where slow cooling starts at an
average cooling rate of 30.degree. C./s or higher after completing
the finish rolling, then slow cooling is started at a temperature
of 750.degree. C. to 600.degree. C. where an average cooling rate
is lower than 10.degree. C./s and cooling time is 1 second to 10
seconds during the slow cooling, and the steel sheet is cooled down
to a coiling temperature of 350.degree. C. or higher and lower than
530.degree. C. at an average cooling rate of 10.degree. C./s or
higher after completing the slow cooling,
R t = R 1 + R 2 + + R m ( = n = 1 m R n ) ( 3 ) ##EQU00001##
where R.sub.n is strain accumulated at an n.sup.th stand from
upstream side when finish rolling is performed with m stands and is
defined by the following formula,
R.sub.n=-1n1-0.01.times.r.sub.n.times.[1-0.01.times.exp{-(11800+2.times.-
10.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}]
[0031] where r.sub.n is rolling reduction rate (%) at an n.sup.th
stand from upstream side, T.sub.n is entry temperature (.degree.
C.) at an n.sup.th stand from upstream side, [C] is C content in
mass % in steel, and n is an integer from 1 to m,
[0032] provided that when
exp{-(11800+2.times.10.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}
exceeds 100, a value thereof is set to be 100.
[0033] 7. The method for manufacturing a high-strength thin steel
sheet according to 6., where an additional work is performed with a
sheet thickness reduction rate being 0.1% to 3.0% after the hot
rolling.
Advantageous Effect
[0034] This disclosure provides a high-strength thin steel sheet
having excellent blanking workability and toughness which are
suitable for applications such as members for automobiles and
various structural members, and therefore has an industrially
significant advantageous effect.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The disclosure will be further described below with
reference to the accompanying drawings, where
[0036] FIG. 1 illustrates the relationship between carbon content
conversion value C* or C** and blanking cracking length ratio in
examples and comparative examples where the carbon content
conversion value C* or C** is outside an appropriate range,
[0037] FIG. 2 illustrates the relationship between carbon content
conversion value C* or C** and DBTT in examples and comparative
examples where the carbon content conversion value C* or C** is
outside an appropriate range,
[0038] FIG. 3 illustrates the relationship between Fe content in Fe
precipitates and blanking cracking length ratio in examples and
comparative examples where the Fe content in Fe precipitates is
outside an appropriate range, and
[0039] FIG. 4 illustrates the relationship between (an average
grain size of top 5% ferrite grains in ferrite grain size
distribution of rolling direction cross section)/(4000/TS).sup.2
and DBTT in examples and comparative examples where the average
grain size of top 5% ferrite grains in ferrite grain size
distribution of rolling direction is outside an appropriate
range.
DETAILED DESCRIPTION
[0040] The following describes this disclosure in detail.
[0041] First, the chemical composition of the high-strength thin
steel sheet of this disclosure will be described. Hereinafter, the
unit "%" relating to the content of elements in the chemical
composition refers to "mass %" unless specified otherwise.
[0042] C: 0.05% to 0.20%
[0043] C forms fine carbide, composite carbide, carbonitride and
composite carbonitride of Ti, Nb, V and the like, which will be
simply referred to as precipitates hereinafter, and contributes to
improvement in strength, blanking workability and toughness.
Additionally, C forms cementite with Fe, which also contributes to
improvement in blanking workability. Therefore, C content should be
0.05% or more. On the other hand, C suppresses ferrite
transformation, and accordingly an excessive amount of C suppresses
formation of fine precipitates of Ti, Nb, V and the like.
Additionally, an excessive amount of C forms too much cementite,
leading to deterioration of toughness. Therefore, C content should
be 0.20% or less. C content is preferably 0.15% or less. C content
is more preferably 0.12% or less.
[0044] Si: 0.6% to 1.5%
[0045] Si accelerates ferrite transformation and promotes formation
of fine precipitates of Ti, Nb, V and the like which precipitate
simultaneously with the transformation during slow cooling
performed in the cooling after hot rolling when manufacturing the
steel sheet. Si also contributes to improvement in strength as a
solid-solution-strengthening element without greatly deteriorating
formability. To obtain these effects, Si content should be 0.6% or
more. On the other hand, an excessive amount of Si accelerates the
above-mentioned ferrite transformation too much. As a result, the
precipitates of Ti, Nb, V and the like coarsen and eventually an
appropriate amount of these fine precipitates cannot be obtained.
Furthermore, not only toughness is deteriorated but also oxides of
Si are likely to be formed on the surface of steel sheet, which
accordingly tend to cause problems such as poor chemical conversion
treatment on hot rolled steel sheets and non-coating on coated
steel sheets. From this point of view, Si content should be 1.5% or
less. Si content is preferably 1.2% or less.
[0046] Mn: 1.3% to 3.0%
[0047] Mn suppresses ferrite transformation before the start of
slow cooling and suppresses coarsening of precipitates of Ti, Nb, V
and the like during the cooling after hot rolling when
manufacturing the steel sheet. Mn also contributes to improvement
in strength by solid solution strengthening. Furthermore, M is
bonded to harmful S in the steel to form MnS, thereby rendering the
S harmless. To obtain these effects, Mn content should be 1.3% or
more. Mn content is preferably 1.5% or more. On the other hand, an
excessive amount of Mn leads to slab cracking, suppresses ferrite
transformation, and suppresses formation of fine precipitates of
Ti, Nb, V and the like. Therefore, Mn content should be 3.0% or
less. Mn content is preferably 2.5% or less. Mn content is more
preferably 2.0% or less.
[0048] P: 0.10% or less
[0049] P segregates at grain boundaries, deteriorating ductility
and toughness. Additionally, a large amount of P accelerates
ferrite transformation before the start of slow cooling and
coarsens precipitates of Ti, Nb, V and the like during the cooling
after hot rolling when manufacturing the steel sheet. Therefore, P
content should be 0.10% or less. P content is preferably 0.05% or
less. P content is more preferably 0.03% or less. P content is
still more preferably 0.01% or less. The lower limit of P content
is not particularly limited. However, since excessive removal of P
leads to an increase in cost, the lower limit of P content is
preferably 0.003%.
[0050] S: 0.030% or less
[0051] S decreases ductility during hot rolling, thereby inducing
hot cracking and deteriorating surface characteristics.
Additionally, S contributes little to strength, and, as an impurity
element, leads to formation of coarse sulfide, thereby
deteriorating ductility and stretch flangeability. For these
reason, it is desirable to reduce S as much as possible. Therefore,
S content should be 0.030% or less. S content is preferably 0.010%
or less. S content is more preferably 0.003% or less. S content is
still more preferably 0.001% or less. The lower limit of S content
is not particularly limited. However, since excessive removal of S
leads to an increase in cost, the lower limit of S content is
preferably 0.0003%.
[0052] Al: 0.10% or less
[0053] When Al content exceeds 0.10%, toughness and weldability are
greatly deteriorated. Additionally, Al oxide is likely to be formed
on the surface, which may accordingly cause problems such as poor
chemical conversion treatment on hot rolled steel sheets and
non-coating on coated steel sheets. Therefore, Al content should be
0.10% or less. Al content is preferably 0.06% or less. Although the
lower limit of Al content is not particularly limited, there is no
problem if Al is contained in an amount of 0.01% or more as
Al-killed steel.
[0054] N: 0.010% or less
[0055] Although N forms coarse nitrides at a high temperature with
Ti, Nb, V and the like, these nitrides contribute little to
strength. Therefore, a large amount of N lowers the effect of
increasing strength of Ti, Nb, and V and deteriorates toughness.
Additionally, since N causes slab cracking during hot rolling,
surface flaws may occur. Thus, N content should be 0.010% or less.
N content is preferably 0.005% or less. N content is more
preferably 0.003% or less. N content is still more preferably
0.002% or less. The lower limit of N content is not particularly
limited. However, since excessive removal of N leads to an increase
in cost, the lower limit of N content is preferably 0.0010%.
[0056] At least one selected from Ti: 0.01% to 1.00%, Nb: 0.01% to
1.00% and V: 0.01% to 1.00%
[0057] Ti, Nb and V form fine precipitates with C, increasing
strength and contributing to improvement in blanking workability
and toughness. To obtain such effect, it is necessary to contain at
least one selected from Ti, Nb and V, each at an amount of 0.01% or
more. The amount is preferably 0.05% or more. On the other hand,
even Ti, Nb and V are contained each at an amount of more than
1.00%, the effect of increasing strength will not be improved more.
On the contrary, their fine precipitates excessively precipitate,
deteriorating toughness and blanking workability. Therefore,
contents of Ti, V and Nb should be each 1.00% or less. Contents of
Ti, V and Nb are preferably each 0.80% or less.
[0058] In addition to the basic components described above, the
high-strength thin steel sheet of this disclosure may also contain
appropriate amounts of following elements in order to further
improve the strength, blanking workability and toughness.
[0059] At least one selected from Mo: 0.005% to 0.50%, Ta: 0.005%
to 0.50%, and W: 0.005% to 0.50%
[0060] Similar to Ti, Nb and V, Mo, Ta and W form fine precipitates
with C, increasing strength and contributing to improvement in
blanking workability and toughness. Therefore, when containing Mo,
Ta and W, contents of Mo, Ta and W are preferably each 0.005% or
more. Contents of Mo, Ta and W are more preferably each 0.01% or
more. On the other hand, even Mo, Ta and W are contained each at an
amount of more than 0.50%, the effect of increasing strength will
not be improved more. On the contrary, their fine precipitates
excessively precipitate, deteriorating toughness and blanking
workability. Thus, when containing Mo, Ta and W, contents of Mo, Ta
and W are preferably each 0.50% or less. Contents of Mo, Ta and W
are more preferably each 0.40% or less.
[0061] At least one selected from Cr: 0.01% to 1.00%, Ni: 0.01% to
1.00% and Cu: 0.01% to 1.00%
[0062] Cr, Ni and Cu improve strength and toughness by refining the
structure. Therefore, when containing Cr, Ni and Cu, contents of
Cr, Ni and Cu are preferably each 0.01% or more. On the other hand,
containing Cr, Ni and Cu each at an amount of more than 1.00%
saturates the effect and increases cost. Thus, when containing Cr,
Ni and Cu, contents of Cr, Ni and Cu are preferably each 1.00% or
less.
[0063] Sb: 0.005% to 0.050%
[0064] Sb segregates on the surface during hot rolling, thereby
preventing the slab from being nitrided and suppressing formation
of coarse nitrides. Therefore, when containing Sb, Sb content is
preferably 0.005% or more. On the other hand, containing Sb at an
amount of more than 0.050% saturates the effect and increases cost.
Thus, when containing Sb, Sb content is preferably 0.050% or
less.
[0065] At least one or both selected from Ca: 0.0005% to 0.0100%
and REM: 0.0005% to 0.0100%
[0066] Ca and REM improve ductility and stretch flangeability by
controlling formation of sulfide. Therefore, when containing Ca and
REM, contents of Ca and REM are preferably each 0.0005% or more. On
the other hand, containing Ca and REM at an amount of more than
0.0100% saturates the effect and increases cost. Thus, when
containing Ca and REM, Ca content and REM content are preferably
each 0.0100% or less.
[0067] The balance other than the above components is Fe and
inevitable impurities.
[0068] Next, the reason why the structure of the high-strength thin
steel sheet of this disclosure is limited will be described.
conversion value C* of total carbon contents in Ti, Nb and V
precipitates whose grain sizes are less than 20 nm: 0.010 mass % to
0.100 mass %, or, conversion value C** of total carbon contents in
Ti, Nb, V, Mo, Ta and W precipitates whose grain sizes are less
than 20 nm: 0.010 mass % to 0.100 mass %
[0069] Ti, Nb and V precipitates whose grain sizes are less than 20
nm contribute to improvement in blanking workability and toughness.
To obtain such effect, conversion value C* of total carbon contents
in Ti, Nb and V precipitates whose grain sizes are less than 20 nm
(hereinafter simply referred to as carbon content conversion value
C*) should be 0.010 mass % or more. Carbon content conversion value
C* is preferably 0.015 mass %.
[0070] On the other hand, an excessive amount of such precipitates
deteriorates blanking workability and toughness because of the
internal stress around the precipitates. Therefore, carbon content
conversion value C* should be 0.100 mass % or less. Carbon content
conversion value C* is preferably 0.080 mass % or less. Carbon
content conversion value C* is more preferably 0.050 mass % or
less.
[0071] Here, C* is calculated by the following formula (1).
C*=([Ti]/48+[Nb]/93+[V]/51).times.12 (1)
[0072] where [Ti], [Nb] and [V] each indicate the contents of Ti,
Nb and V in Ti, Nb and V precipitates whose grain sizes are less
than 20 nm. In a case where Ti, Nb or V is not contained, [Ti],
[Nb] or [V] is zero.
[0073] When the high-strength thin steel sheet of this disclosure
contains Mo, Ta and W in addition to at least one selected from Ti,
Nb and V, conversion value C** of total carbon contents in Ti, Nb,
V, Mo, Ta and W precipitates whose grain sizes are less than 20 nm
(hereinafter simply referred to as carbon content conversion value
C**) defined by the following formula (2) is 0.010 mass % to 0.100
mass %. The preferred range of C** and its reason are similar to
that of C*.
C**=([Ti]/48+[Nb]/93+[V]/51+[Mo]/96+[Ta]/181+[W]/184).times.12
(2)
where [Ti], [Nb], [V], [Mo], [Ta], and [W] each indicate the
contents of Ti, Nb, V, Mo, Ta and W in Ti, Nb, V, Mo, Ta and W
precipitates whose grain sizes are less than 20 nm. In a case where
Ti, Nb, V, Mo, Ta or W is not contained, [Ti], [Nb], [V], [Mo],
[Ta] or [W] is zero. Note that when calculating C**, it is a
prerequisite to satisfy the provision of C*.
[0074] Since Ti, Nb and V precipitates and the like whose grain
sizes are 20 nm or more contribute little to improvement in
blanking workability and toughness, this disclosure chooses Ti, Nb
and V precipitates and the like whose grain sizes are less than 20
nm.
[0075] Fe content in Fe precipitates: 0.03 mass % to 0.50 mass
%
[0076] Fe precipitates, particularly cementite, serve as origins of
cracks during blanking and contribute to improvement in blanking
workability. To obtain such effect, Fe content in Fe precipitates
should be 0.03 mass % or more. Fe content in Fe precipitates is
preferably 0.05 mass % or more. Fe content in Fe precipitates is
more preferably 0.10 mass % or more. On the other hand, when Fe
precipitates is excessive, the Fe precipitates may become origins
of brittle fracture. Therefore, Fe content in Fe precipitates
should be 0.50 mass % or less. Fe content in Fe precipitates is
preferably 0.40 mass % or less. Fe content in Fe precipitates is
more preferably 0.30 mass % or less.
[0077] Average grain size of ferrite grains whose grain sizes are
top 5% large in ferrite grain size distribution of rolling
direction cross section: (4000/TS).sup.2 .mu.m less, the TS
indicating tensile strength in unit of MPa
[0078] A large average grain size of ferrite grains whose grain
sizes are top 5% large in ferrite grain size distribution of
rolling direction cross section greatly deteriorates toughness.
Particularly, since toughness tends to decrease as tensile strength
TS (MPa) increases, it is important to reduce the grain size
according to tensile strength. Therefore, the average grain size of
grain sizes that are top 5% large in ferrite grain size
distribution of rolling direction cross section (hereinafter simply
referred to as average grain size of top 5%) should be (4000/TS
(MPa)).sup.2 m or less. The TS here is tensile strength of steel
sheet in unit of MPa. The average grain size of top 5% is
preferably (3500/TS (MPa)).sup.2 .mu.m or less. Note that TS is
expressed in unit of MPa. When calculating the above
(4000/TS).sup.2 and (3500/TS).sup.2, M is only used as Mantissa
part rather than M (=10.sup.6). For example, when TS is 780 MPa,
values of (4000/TS).sup.2 and (3500/TS).sup.2 can be calculated
with TS=780. Although the lower limit of the average grain size is
not particularly limited, the lower limit is usually 5.0 .mu.m.
[0079] The high-strength thin steel sheet of this disclosure
preferably has a tensile strength TS of 780 MPa or more.
[0080] The structure of the high-strength thin steel sheet of this
disclosure is preferably a structure mainly composed of ferrite,
specifically, a structure composed of ferrite whose area ratio is
50% or more with respect to the entire structure and the balance.
Structure other than ferrite may be bainite and martensite.
[0081] The following describes a method for manufacturing the
high-strength thin steel sheet of this disclosure.
[0082] The method for manufacturing the high-strength thin steel
sheet of this disclosure includes hot rolling a steel slab having
the above-mentioned composition to obtain a steel sheet, the hot
rolling comprising rough rolling and finish rolling, and cooling
and coiling the steel sheet after completing the finish
rolling.
[0083] When using this method, cumulative strain R.sub.t in the
finish rolling is 1.3 or more, and finisher delivery temperature is
820.degree. C. or higher and lower than 930.degree. C. The steel
sheet is cooled down from the finisher delivery temperature to a
temperature where slow cooling starts at an average cooling rate of
30.degree. C./s or higher after completing the finish rolling, then
slow cooling is started at a temperature of 750.degree. C. to
600.degree. C. where an average cooling rate is lower than
10.degree. C./s and cooling time is 1 second to 10 seconds during
the slow cooling. After completing the slow cooling, the steel
sheet is cooled down to a coiling temperature of 350.degree. C. or
higher and lower than 530.degree. C. at an average cooling rate of
10.degree. C./s or higher.
[0084] The reasons for limiting the manufacturing conditions will
be described below. Note that the smelting method for obtaining a
steel slab is not particularly limited and a publicly-known
smelting method such as a converter, an electric heating furnace or
the like can be adopted. After smelting, it is preferable to form
steel slabs by a continuous casting method from the perspective of,
for example, productivity, but adopting publicly-known casting
methods such as ingot casting-blooming or thin slab continuous
casting to form steel slabs is also acceptable.
[0085] Cumulative strain R.sub.t in finish rolling: 1.3 or more
[0086] By increasing cumulative strain R.sub.t during finish
rolling, ferrite grain size of the hot rolled steel sheet obtained
after hot rolling, cooling, and coiling can be reduced.
Particularly, by setting the cumulative strain during finish
rolling to 1.3 or more, it is possible to introduce uniform strain
into the hot rolled steel sheet by finish rolling. As a result, it
is possible to reduce variations in the grain size of ferrite
grains in the rolling direction and reduce the average grain size
of the top 5% ferrite grains. Therefore, cumulative strain R.sub.t
during finish rolling should be 1.3 or more. Cumulative strain
R.sub.t during finish rolling is preferably 1.5 or more. The upper
limit of cumulative strain R.sub.t during finish rolling is not
particularly limited. However, a too large cumulative strain may
excessively accelerate ferrite transformation during the cooling
after hot rolling and lead to coarsening of precipitates of Ti, Nb,
V and the like. Therefore, cumulative strain R.sub.t during finish
rolling is preferably 2.2 or less. Cumulative strain R.sub.t during
finish rolling is more preferably 2.0 or less.
[0087] The cumulative strain R.sub.t during finish rolling is
defined by the following formula (3),
R t = R 1 + R 2 + + R m ( = n = 1 m R n ) ( 3 ) ##EQU00002##
[0088] where R.sub.n is strain accumulated at an n.sup.th stand
from upstream side when finish rolling is performed with m stands,
and R.sub.n is defined by the following formula,
R.sub.n=1n1-0.01.times.r.sub.n.times.[1-0.01.times.exp{-(11800+2.times.1-
0.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}]
[0089] where r.sub.n is rolling reduction rate (%) at an n.sup.th
stand from upstream side, T.sub.n is entry temperature (.degree.
C.) at an n.sup.th stand from upstream side, and [C] is C content
in mass % in steel. Additionally, n is an integer from 1 to m, and
m is usually 7. The rolling reduction rate r.sub.n(%) is
represented by r.sub.n=(t.sub.an-t.sub.bn)/t.sub.an.times.100 where
t.sub.an is the entrance side sheet thickness of n.sup.th stand and
t.sub.bn is the exit side sheet thickness.
[0090] However, when
exp{-(11800-2.times.10.sup.3.times.[C])/(T.sub.n+273)+13.1-0.1.times.[C]}
exceeds 100, the value is set to be 100.
[0091] Finisher delivery temperature: 820.degree. C. or higher and
lower than 930.degree. C.
[0092] When finisher delivery temperature is lower than 820.degree.
C., ferrite transformation is accelerated before the start of slow
cooling and precipitates of Ti, Nb, V and the like coarsen during
the cooling after hot rolling. In a case where the finisher
delivery temperature is in ferrite region, the precipitates of Ti,
Nb, V and the like become coarser because of strain-induced
precipitation. Additionally, ferrite crystal grains become
elongated with a low temperature and cracks develop along the
elongated grains, leading to significant deterioration of blanking
workability. Therefore, finisher delivery temperature should be
820.degree. C. or higher. Finisher delivery temperature is
preferably 850.degree. C. or higher. On the other hand, when
finisher delivery temperature is 930.degree. C. or higher, ferrite
transformation is suppressed during the cooling after hot rolling,
and formation of fine precipitates of Ti, Nb, V and the like is
suppressed. Therefore, finisher delivery temperature should be
lower than 930.degree. C. Finisher delivery temperature is
preferably lower than 900.degree. C.
[0093] The finisher delivery temperature here is the exit side
temperature (.degree. C.) at an m.sup.th stand from upstream side
when finish rolling is performed with m stands.
[0094] Average cooling rate from finisher delivery temperature to
starting temperature of slow cooling: 30.degree. C./s or higher
[0095] When the average cooling rate from finisher delivery
temperature to starting temperature of slow cooling is lower than
30.degree. C./s, ferrite transformation is accelerated and
precipitates of Ti, Nb, V and the like coarsen. Therefore, the
average cooling rate from finisher delivery temperature to starting
temperature of slow cooling should be 30.degree. C./s or higher.
The average cooling rate is preferably 50.degree. C./s or higher.
The average cooling rate is more preferably 80.degree. C./s or
higher. Although the upper limit of the average cooling rate is not
particularly limited, it is about 200.degree. C./s from the
perspective of temperature control.
[0096] Starting temperature of slow cooling: 750.degree. C. to
600.degree. C.
[0097] When starting temperature of slow cooling exceeds
750.degree. C., ferrite transformation takes place at a high
temperature and ferrite crystal grains coarsen. Precipitates of Ti,
Nb, V and the like also coarsen. Therefore, starting temperature of
slow cooling should be 750.degree. C. or lower. On the other hand,
when starting temperature of slow cooling is lower than 600.degree.
C., precipitates of Ti, Nb, V and the like are not sufficient.
Therefore, starting temperature of slow cooling should be
600.degree. C. or higher.
[0098] Average cooling rate during slow cooling: lower than
10.degree. C./s
[0099] When the average cooling rate during slow cooling is
10.degree. C./s or higher, ferrite transformation is not sufficient
and the amount of fine precipitates of Ti, Nb, V and the like
decreases. Therefore, the average cooling rate during slow cooling
should be lower than 10.degree. C./s. The average cooling rate
during slow cooling is preferably lower than 6.degree. C./s.
Although the lower limit of average cooling rate during slow
cooling is not particularly limited, it can be about 2.degree.
C./s. The average cooling rate during slow cooling is preferably
4.degree. C./s or higher.
[0100] Cooling time of slow cooling: 1 second to 10 seconds
[0101] When cooling time of slow cooling is less than 1 second,
ferrite transformation is not sufficient and the amount of fine
precipitates of Ti, Nb, V and the like decreases. Therefore,
cooling time of slow cooling should be 1 second or more. Cooling
time of slow cooling is preferably 2 seconds or more. Cooling time
of slow cooling is more preferably 3 seconds or more. On the other
hand, when cooling time of slow cooling exceeds 10 seconds,
precipitates of Ti, Nb, V and the like coarsen. Ferrite crystal
grains also coarsen. Therefore, cooling time of slow cooling should
be 10 seconds or less. Cooling time of slow cooling is preferably 6
seconds or less.
[0102] Average cooling rate down to coiling temperature after slow
cooling: 10.degree. C./s or higher
[0103] When the average cooling rate down to coiling temperature
after slow cooling is lower than 10.degree. C./s, precipitates of
Ti, Nb, V and the like coarsen. Ferrite crystal grains also
coarsen. Therefore, the average cooling rate down to coiling
temperature after slow cooling should be 10.degree. C./s or higher.
The average cooling rate is preferably 30.degree. C./s or higher.
The average cooling rate is more preferably 50.degree. C./s or
higher. Although the upper limit of the average cooling rate is not
particularly limited, it is about 100.degree. C./s from the
perspective of temperature control.
[0104] Coiling temperature: 350.degree. C. or higher and less than
530.degree. C.
[0105] When coiling temperature is 530.degree. C. or higher,
precipitates of Ti, Nb, V and the like coarsen. Ferrite crystal
grains also coarsen. Therefore, coiling temperature should be lower
than 530.degree. C. Coiling temperature is preferably lower than
480.degree. C. On the other hand, when coiling temperature is lower
than 350.degree. C., the generation of cementite, which is a
precipitate of Fe and C, is suppressed. Therefore, coilng
temperature should be 350.degree. C. or higher.
[0106] Note that the above finisher delivery temperature, starting
temperature of slow cooling and coiling temperature are all
temperatures at the surface of steel sheet and that the average
cooling rate is also specified based on the temperature at the
surface of steel sheet.
[0107] After the hot rolling as described above, it is possible to
perform an additional work with a sheet thickness reduction rate
being 0.1% or higher to increase the number of mobile dislocations
and to further improve blanking workability. The sheet thickness
reduction rate is preferably 0.3% or higher. When the sheet
thickness reduction rate exceeds 3.0%, however, dislocations are
difficult to move because of the interaction between the
dislocations, and blanking workability deteriorates. Therefore, the
sheet thickness reduction rate is preferably 3.0% or lower when an
additional work is performed after the hot rolling. The sheet
thickness reduction rate is more preferably 2.0% or lower. The
sheet thickness reduction rate is still more preferably 1.0% or
lower.
[0108] The above-mentioned work may be a process of rolling by
rolls or applying tensile to a steel sheet, or a combination of
both.
[0109] Furthermore, composite plating of zinc plating and Al or
composite plating of zinc and Al, composite plating of zinc and Ni,
Al plating, composite plating of Al and Si, and the like may be
applied to the steel sheet obtained as described above. A layer
formed by chemical conversion treatment or the like is also
acceptable.
EXAMPLES
[0110] Molten steel having the composition listed in Table 1 was
obtained by a publicly-known smelting method and continuously cast
to obtain steel slabs. These slabs were heated and subjected to
rough rolling, and then finish rolling was performed under the
conditions listed in Table 2. After the finish rolling, cooling and
coiling were performed to obtain hot rolled steel sheets. The
finish rolling was carried out by a hot rolling mill consisting of
7 stands. Additionally, some of the steel sheets were further
subjected to reduction rolling at room temperature by a rolling
roll.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) No. C Si Mn P
S Al N Ti Nb V Mo Ta W Others Remarks 1 0.10 1.5 1.6 0.07 0.008
0.09 0.005 0.15 0.06 0.17 -- -- -- Sb: 0.008 Conforming steel 2
0.14 0.7 1.7 0.01 0.001 0.06 0.003 0.10 -- 0.21 0.42 -- -- Sb:
0.012 Conforming steel 3 0.07 1.0 2.5 0.02 0.023 0.05 0.003 0.11
0.03 0.05 0.03 0.02 0.03 -- Conforming steel 4 0.17 1.0 2.1 0.02
0.002 0.04 0.006 0.06 -- 0.55 -- -- -- -- Conforming steel 5 0.06
0.7 1.5 0.01 0.001 0.05 0.003 0.25 -- -- -- -- -- -- Conforming
steel 6 0.15 0.5 1.9 0.01 0.001 0.04 0.007 0.05 -- 0.55 -- -- -- --
Comparative steel 7 0.06 1.0 1.7 0.01 0.003 0.03 0.004 0.21 0.05 --
-- -- -- -- Conforming steel 8 0.15 1.6 1.5 0.03 0.021 0.04 0.005
0.06 -- 0.52 -- -- -- -- Comparative steel 9 0.11 0.8 1.7 0.02
0.001 0.03 0.004 0.05 -- 0.25 -- -- -- -- Conforming steel 10 0.19
1.2 1.6 0.01 0.002 0.04 0.005 -- -- 0.77 -- -- -- -- Conforming
steel 11 0.12 1.0 1.4 0.11 0.001 0.04 0.008 0.09 -- 0.35 -- -- --
-- Comparative steel 12 0.15 0.7 1.9 0.09 0.007 0.05 0.004 0.09 --
0.54 -- -- 0.05 Ca: 0.0040 Conforming steel 13 0.08 1.2 2.8 0.04
0.018 0.06 0.005 0.15 -- 0.15 -- -- -- Cr: 0.03 Conforming steel 14
0.08 1.2 1.2 0.01 0.004 0.08 0.006 0.07 -- 0.15 -- -- -- --
Comparative steel 15 0.05 1.3 1.4 0.02 0.001 0.06 0.005 0.19 -- --
-- -- -- -- Conforming steel 16 0.09 1.2 1.2 0.02 0.011 0.02 0.005
0.12 -- 0.21 -- -- -- -- Comparative steel 17 0.12 1.1 1.4 0.01
0.002 0.03 0.005 0.05 -- 0.22 0.35 -- -- -- Conforming steel 18
0.11 1.1 1.6 0.01 0.002 0.03 0.005 0.11 -- 0.25 -- -- -- --
Conforming steel 19 0.18 1.1 1.7 0.01 0.001 0.05 0.004 0.05 -- 0.65
-- -- -- -- Conforming steel 20 0.11 1.0 1.5 0.01 0.001 0.04 0.004
0.14 -- 0.27 -- -- -- -- Conforming steel 21 0.06 0.8 2.0 0.05
0.003 0.06 0.005 0.15 -- -- 0.05 -- -- -- Conforming steel 22 0.12
1.1 1.5 0.01 0.003 0.04 0.004 0.19 -- 0.28 -- -- -- Ca: 0.0060,
REM: 0.0070 Conforming steel 23 0.16 0.8 2.1 0.03 0.015 0.06 0.005
0.07 -- 0.41 0.34 0.03 0.06 Cr: 0.06, Ni: 0.08, Conforming steel
Cu: 0.07, Sb: 0.010, Ca: 0.0030, REM: 0.0050 24 0.12 1.2 3.1 0.01
0.003 0.05 0.004 0.08 0.05 -- 0.32 -- -- -- Comparative steel 25
0.11 1.5 1.5 0.01 0.001 0.05 0.004 0.11 -- 0.25 -- -- -- Ca: 0.0080
Conforming steel 26 0.12 1.7 1.4 0.01 0.001 0.07 0.004 0.07 0.05
0.35 -- -- -- -- Comparative steel 27 0.09 0.9 2.0 0.01 0.001 0.04
0.003 0.11 -- 0.22 -- -- -- -- Conforming steel 28 0.13 1.6 1.5
0.03 0.003 0.03 0.005 -- -- 0.51 -- -- -- -- Comparative steel 29
0.07 0.8 1.8 0.01 0.001 0.04 0.003 0.15 -- 0.15 -- -- -- --
Conforming steel 30 0.08 0.8 1.8 0.01 0.002 0.05 0.006 0.09 -- 0.21
-- -- -- Cr: 0.05 Conforming steel 31 0.20 1.0 1.4 0.01 0.001 0.06
0.005 -- -- 0.95 -- -- -- -- Conforming steel 32 0.05 0.6 1.7 0.02
0.028 0.03 0.004 0.05 0.02 0.05 -- -- -- -- Conforming steel 33
0.22 0.9 1.6 0.02 0.002 0.06 0.006 0.06 0.05 0.89 0.22 -- -- --
Comparative steel 34 0.09 1.4 2.2 0.05 0.013 0.07 0.008 0.12 --
0.25 -- -- -- Cr: 0.05, Ni: 0.06, Cu: 0.05 Conforming steel 35 0.04
1.1 1.5 0.01 0.001 0.05 0.004 0.16 -- -- -- -- -- Cr: 0.04
Comparative steel 36 0.13 0.9 1.6 0.01 0.002 0.03 0.005 0.09 --
0.21 0.31 -- -- Cr: 0.05 Conforming steel 37 0.11 1.3 1.3 0.08
0.005 0.05 0.003 0.14 -- 0.31 -- -- -- Ca: 0.0080 Conforming steel
38 0.19 1.2 1.8 0.01 0.001 0.05 0.003 -- -- 1.10 -- -- -- --
Comparative steel Underline indicates that it is outside an
appropriate range.
TABLE-US-00002 TABLE 2 Conditions of hot rolling, cooling and
coiling r.sub.1 T.sub.1 r.sub.2 T.sub.2 r.sub.3 T.sub.3 r.sub.4
T.sub.4 r.sub.5 T.sub.5 r.sub.6 T.sub.6 r.sub.7 T.sub.7 No. (%)
(.degree. C.) R.sub.1 (%) (.degree. C.) R.sub.2 (%) (.degree. C.)
R.sub.3 (%) (.degree. C.) R.sub.4 (%) (.degree. C.) R.sub.5 (%)
(.degree. C.) R.sub.6 (%) (.degree. C.) 1 41 1040 0.22 41 1020 0.25
38 1000 0.26 35 980 0.26 31 960 0.25 30 950 0.25 22 940 2 52 990
0.42 41 980 0.33 37 970 0.3 26 960 0.21 29 940 0.25 27 920 0.25 21
910 3 49 1050 0.23 46 1030 0.26 40 1020 0.24 25 1000 0.16 25 970
0.18 22 960 0.17 15 940 4 47 1020 0.33 38 1010 0.27 35 990 0.27 24
980 0.18 27 970 0.22 25 950 0.21 17 940 5 48 950 0.42 40 940 0.35
40 930 0.36 28 920 0.25 25 910 0.22 25 900 0.23 16 890 6 49 980
0.41 41 960 0.36 37 940 0.34 27 930 0.24 27 910 0.25 25 880 0.24 15
870 7 50 1030 0.28 42 1010 0.26 38 1000 0.25 27 980 0.19 23 970
0.17 22 950 0.17 16 940 8 48 950 0.45 45 940 0.43 38 930 0.36 25
910 0.23 23 880 0.22 25 870 0.25 17 850 9 48 1000 0.35 40 990 0.3
36 980 0.28 25 960 0.2 22 950 0.18 22 930 0.19 19 910 10 51 950
0.50 45 930 0.45 43 900 0.46 27 880 0.27 23 870 0.23 25 850 0.26 21
840 11 47 980 0.38 38 970 0.31 34 960 0.28 27 940 0.23 25 930 0.22
24 920 0.21 15 910 12 41 980 0.33 40 970 0.34 37 960 0.32 21 940
0.18 25 930 0.22 26 920 0.24 15 900 13 40 1010 0.26 40 990 0.29 41
980 0.31 34 970 0.26 35 950 0.29 24 940 0.2 21 930 14 47 1000 0.33
41 980 0.31 35 970 0.27 31 950 0.25 28 930 0.24 22 910 0.19 18 890
15 47 980 0.35 41 960 0.33 38 950 0.31 26 930 0.22 25 910 0.22 24
890 0.22 19 880 16 52 990 0.40 40 980 0.31 35 960 0.28 29 940 0.25
25 930 0.21 21 920 0.18 16 900 17 49 980 0.40 39 960 0.33 37 940
0.33 24 920 0.21 27 910 0.25 24 890 0.23 18 880 18 49 950 0.45 39
930 0.36 38 910 0.37 27 880 0.26 25 860 0.25 23 840 0.23 19 830 19
48 980 0.41 42 960 0.38 44 940 0.43 26 920 0.24 24 900 0.23 26 880
0.26 19 860 20 49 960 0.43 38 950 0.33 38 930 0.35 30 910 0.28 27
910 0.25 22 890 0.21 17 880 21 51 1030 0.28 39 1020 0.23 41 1010
0.26 25 990 0.17 27 980 0.19 24 960 0.18 15 940 22 49 970 0.42 43
960 0.37 39 950 0.34 31 940 0.27 26 930 0.23 25 920 0.22 20 910 23
46 1060 0.24 41 1050 0.23 39 1030 0.25 20 1000 0.14 25 980 0.19 26
960 0.21 16 940 24 45 1010 0.31 40 990 0.3 33 980 0.25 26 970 0.20
24 950 0.20 25 930 0.22 16 920 25 48 1020 0.31 39 1010 0.26 40 990
0.3 27 980 0.20 24 970 0.18 23 960 0.18 18 940 26 51 1010 0.36 41
1000 0.29 37 980 0.29 28 970 0.22 26 950 0.21 23 940 0.19 19 920 27
46 1010 0.31 41 990 0.30 35 970 0.27 27 960 0.21 23 940 0.19 23 920
0.20 16 900 28 51 960 0.46 42 950 0.38 36 930 0.33 28 920 0.25 24
900 0.22 23 880 0.22 16 860 29 50 1040 0.26 45 1030 0.25 32 1020
0.18 24 1010 0.14 22 990 0.15 20 970 0.14 15 950 30 46 970 0.37 42
950 0.36 41 930 0.38 28 920 0.25 24 900 0.22 25 890 0.23 18 870 31
45 1000 0.36 41 990 0.33 38 970 0.33 29 960 0.25 26 950 0.23 25 930
0.23 18 920 32 45 1000 0.30 42 980 0.31 35 970 0.26 33 960 0.26 25
950 0.2 25 940 0.2 20 930 33 48 1020 0.36 39 1000 0.31 35 980 0.29
28 960 0.24 24 940 0.21 25 920 0.23 20 900 34 45 980 0.35 41 970
0.33 42 960 0.35 38 940 0.34 32 930 0.28 30 920 0.27 22 910 35 50
1050 0.21 38 1030 0.19 38 1010 0.23 30 1000 0.19 25 980 0.17 24 960
0.18 19 940 36 46 1000 0.34 41 990 0.31 38 970 0.31 33 950 0.28 27
940 0.23 26 930 0.23 19 920 37 55 1020 0.37 40 1010 0.27 36 990
0.26 35 980 0.27 32 970 0.25 26 960 0.21 17 940 38 49 950 0.48 39
940 0.37 40 930 0.39 27 920 0.25 24 910 0.22 25 900 0.24 17 880
Conditions of hot rolling, cooling and coiling Average cooling
Average rate Average cooling Additional down cooling rate work to
slow Slow rate Cooling down Sheet Finisher cooling cooling during
time of to thickness delivery starting starting slow slow coiling
Coiling reduction temperature temperature temperature cooling
cooling temperature temperature rate No. R.sub.7 R.sub.t (.degree.
C.) (.degree. C./s) (.degree. C.) (.degree. C./s) (s) (.degree.
C./s) (.degree. C.) (%) Remarks 1 0.18 1.7 920 200 600 5 7 20 490
-- Example 2 0.19 2.0 890 100 640 5 3 40 450 2.5 Example 3 0.12 1.3
920 100 620 2 5 70 450 -- Example 4 0.14 1.6 920 40 700 5 3 50 380
-- Example 5 0.14 2.0 880 70 650 4 4 35 450 -- Example 6 0.14 2.0
860 80 650 6 6 40 440 0.3 Comparative Example 7 0.13 1.4 930 80 670
5 4 25 480 -- Comparative Example 8 0.17 2.1 830 80 650 10 4 40 380
-- Comparative Example 9 0.17 1.7 890 85 640 7 5 10 530 --
Comparative Example 10 0.21 2.4 820 60 640 4 4 60 420 0.3 Example
11 0.13 1.8 890 80 640 4 5 30 460 -- Comparative Example 12 0.13
1.8 890 90 630 5 6 35 440 -- Example 13 0.18 1.8 915 120 630 7 10
25 500 0.5 Example 14 0.16 1.8 875 75 630 7 6 20 490 -- Comparative
Example 15 0.17 1.8 860 35 760 8 6 40 460 -- Comparative Example 16
0.14 1.8 885 70 660 5 0.4 25 470 -- Comparative Example 17 0.17 1.9
860 70 650 4 5 9 510 0.2 Comparative Example 18 0.19 2.1 810 75 620
7 5 35 430 0.5 Comparative Example 19 0.19 2.1 840 70 680 6 3 20
490 -- Example 20 0.16 2.0 870 75 660 4 3 30 460 -- Example 21 0.12
1.4 925 30 750 5 2 50 470 -- Example 22 0.18 2.0 880 90 650 5 4 15
510 -- Example 23 0.13 1.4 925 80 650 4 4 40 580 -- Example 24 0.14
1.6 905 55 700 3 4 25 480 0.1 Comparative Example 25 0.15 1.6 920
25 740 4 6 30 480 -- Comparative Example 26 0.16 1.7 900 75 660 4
11 35 400 -- Comparative Example 27 0.14 1.6 880 55 670 5 4 90 340
-- Comparative Example 28 0.15 2.0 855 65 630 5 5 30 450 --
Comparative Example 29 0.11 1.2 925 70 650 5 4 30 450 --
Comparative Example 30 0.17 2.0 855 150 590 3 5 45 360 0.1
Comparative Example 31 0.16 1.9 900 50 680 5 6 45 400 -- Example 32
0.16 1.7 910 50 720 3 1 10 520 0.1 Example 33 0.19 1.8 880 70 640 4
4 35 460 -- Comparative Example 34 0.20 2.1 895 150 610 9 8 100 350
-- Example 35 0.15 1.3 920 80 650 3 3 25 480 -- Comparative Example
36 0.17 1.9 900 70 650 3 3 25 470 -- Example 37 0.14 1.8 920 80 670
4 5 35 480 1.5 Example 38 0.16 2.1 870 60 650 5 4 35 450 --
Comparative Example Underline indicates that it is outside an
appropriate range.
[0111] Test pieces were taken from the resulting steel sheets and
subjected to the following evaluations (i) to (vi), [0112] (i)
measurement of conversion value C* of total carbon contents in Ti,
Nb and V precipitates whose grain sizes are less than 20 nm or
conversion value C** of total carbon contents in Ti, Nb, V, Mo, Ta
and W precipitates whose grain sizes are less than 20 nm, [0113]
(ii) measurement of Fe content in Fe precipitates, [0114] (iii)
measurement of average grain size of ferrite grains whose grain
sizes are top 5% large in ferrite grain size distribution of
rolling direction cross section, [0115] (iv) tensile test, [0116]
(v) blanking test, and [0117] (vi) evaluation of toughness.
[0118] The evaluation results are listed in Table 3. Evaluation
methods are as stated below.
[0119] (i) measurement of conversion value C* of total carbon
contents in Ti, Nb and V precipitates whose grain sizes are less
than 20 nm or conversion value C** of total carbon contents in Ti,
Nb, V, Mo, Ta and W precipitates whose grain sizes are less than 20
nm
[0120] As described in JP 4737278 B, constant current electrolysis
was carried out in a 10% AA electrolytic solution, which was a 10
vol % electrolytic solution of acetylacetone-1 mass % of
tetramethylammonium chloride-methanol, using a test piece taken
from the steel sheet as the anode, and the electrolytic solution
was filtered with a filter whose pore size is 20 nm after a certain
amount of the test piece was dissolved. Subsequently, contents of
Ti, Nb and B as well as contents of Mo, Ta and W in the resulting
filtrate were obtained by ICP emission spectroscopy analysis, and
carbon content conversion value C* or carbon content conversion
value C** was calculated by the above formula (1) or (2) with the
obtained results.
[0121] (ii) measurement of Fe content in Fe precipitates
[0122] Constant current electrolysis was carried out in a 10% AA
electrolytic solution using a test piece taken from the steel sheet
as the anode, and a certain amount of the test piece was dissolved.
Subsequently, extraction residue obtained by the electrolysis was
filtered with a filter whose pore size is 0.2 .mu.m to recover Fe
precipitates. After dissolving the obtained Fe precipitates with
mixed acid, Fe was quantified by ICP emission spectroscopy
analysis, and Fe content in the Fe precipitates was calculated with
the measurement result.
[0123] Since the Fe precipitates are in an agglomerated state, Fe
precipitates whose grain sizes are less than 0.2 .mu.m also can be
recovered by filtering the Fe precipitates with a filter having a
pore size of 0.2 .mu.m.
[0124] (iii) measurement of average grain size of ferrite grains
whose grain sizes are top 5% large in ferrite grain size
distribution of rolling direction
[0125] A cross section of rolling direction--sheet thickness
direction was embedded in resin and polished. After subjecting the
cross section to nital etching, EBSD (Electron Backscatter
Diffraction) measurement was made at three locations with a step
size of 0.1 .mu.m in an area of 100 .mu.m.times.100 .mu.m where the
center is the 1/4 sheet thickness position, a position
corresponding to 1/4 of the sheet thickness in the depth direction
from the surface of the steel sheet, and ferrite grain size
distribution in the rolling direction was obtained with a setting
where an orientation difference of 15.degree. or more is the grain
boundary.
[0126] All of the steel sheets obtained as described above had a
structure mainly composed of ferrite, which means the area ratio of
ferrite is 50% or more. The area ratio of ferrite can be obtained
by embedding the cross section of rolling direction--sheet
thickness direction in resin, polishing the cross section,
subjecting the cross section to nital etching, observing three
visual fields at 3000 times magnification under an SEM (Scanning
Electron Microscope) on the 1/4 sheet thickness position,
calculating the area ratio of constituent phase in the obtained
structure micrograph for three visual fields, and averaging the
values. Ferrite appears as a gray structure i.e. base steel
structure in the above-mentioned structure micrograph.
[0127] Additionally, ferrite grain size distribution in the rolling
direction cross section was obtained by the so-called section
method, in which nine lines are drawn at equal intervals parallel
to the rolling direction for each measurement location in the EBSD
measurement and the section length of each ferrite grain in the
rolling direction is measured. The average value of the measured
section lengths was taken as the average grain size of ferrite
grains in the rolling direction. The average value of grain sizes
of ferrite grains up to 5% in an order from the largest grain size
was taken as the average grain size of top 5% large grain sizes.
When selecting the ferrite grains whose grain sizes are top 5%
large, ferrite grains having a grain size of less than 0.1 .mu.m
were excluded. Additionally, in order to obtain the ferrite grain
size distribution, 200 or more ferrite grains were measured to
obtain their grain sizes.
[0128] (vi) tensile test
[0129] In tensile test, a JIS No. 5 tensile test piece was cut out
with the longitudinal direction being the direction orthogonal to
the rolling direction. The tensile test was carried out according
to JIS Z 2241, and yield strength YP, tensile strength TS, and
total elongation El were evaluated.
[0130] (v) blanking test
[0131] Blanking workability was evaluated by blanking a hole having
a diameter of 10 mm three times at a time with a clearance of 20%,
observing the blanked end face all around and calculating the
average value of perimeter ratio of the portion where cracking had
occurred (hereinafter also referred to as blanking cracking length
ratio). When the blanking cracking length ratio is 10% or less,
blanking workability can be considered as excellent.
[0132] (iv) evaluation of toughness
[0133] The evaluation conditions were set according to JIS Z 2242
except the sheet thickness, which was the original thickness as
listed in Table 3, and a DBTT (Ductile-brittle Transition
Temperature) was obtained by Charpy impact test. The V-notch test
piece here was made so that the longitudinal direction was in the
direction orthogonal to the rolling direction. When the DBTT
(Ductile-brittle Transition Temperature) is lower than -40.degree.
C., toughness can be considered as excellent.
TABLE-US-00003 TABLE 3 Steel structure Average grain size Average
grain size of ferrite Sheet Fe content in Fe of ferrite whose grain
size is top 5% Tensile test thickness C* or C** precipitates in
rolling direction large in rolling direction YP TS No. (mm) (mass
%) (mass %) (.mu.m) (.mu.m) (MPa) (MPa) 1 2.9 0.055 0.13 6.9 14.6
760 860 2 2.4 0.038 0.22 5.2 12.8 880 1010 3 2.0 0.025 0.08 10.8
23.1 720 820 4 2.3 0.058 0.31 5.2 10.1 1020 1190 5 2.9 0.018 0.05
8.6 17.6 770 840 6 3.2 0.008 0.25 4.6 8.6 1060 1210 7 2.6 0.005
0.06 11.0 23.5 730 810 8 2.9 0.008 0.21 5.3 10.1 1050 1180 9 2.3
0.009 0.11 7.2 20.5 800 900 10 2.6 0.090 0.35 4.5 8.1 1100 1280 11
2.6 0.009 0.18 6.9 12.5 920 1040 12 2.6 0.071 0.26 5.3 10.7 950
1200 13 4.0 0.035 0.07 8.8 18.3 730 850 14 2.6 0.008 0.09 7.6 19.8
720 820 15 2.3 0.008 0.03 11.8 27.8 750 810 16 2.6 0.007 0.13 7.9
18.3 780 890 17 2.4 0.009 0.12 7.2 17.6 802 990 18 2.5 0.009 0.16
8.2 14.3 820 990 19 2.1 0.071 0.33 4.8 9.5 1060 1220 20 2.6 0.051
0.15 6.8 13.2 850 1020 21 2.6 0.015 0.04 11.2 22.5 720 810 22 2.3
0.046 0.16 5.2 10.9 920 1080 23 2.8 0.062 0.28 5.3 9.5 1050 1230 24
2.5 0.007 0.19 8.1 16.8 760 910 25 2.9 0.008 0.17 9.8 14.6 830 950
26 2.5 0.009 0.16 7.1 16.3 880 1030 27 2.8 0.025 0.02 7.6 17.9 790
890 28 2.9 0.006 0.20 4.8 8.9 1020 1170 29 2.2 0.021 0.07 10.9 20.1
800 920 30 3.2 0.005 0.09 7.2 15.6 780 900 31 2.9 0.095 0.45 3.9
8.2 1160 1350 32 3.2 0.010 0.03 9.6 19.3 710 780 33 3.2 0.009 0.55
4.3 8.5 1080 1320 34 3.6 0.057 0.10 7.5 15.9 710 840 35 2.9 0.008
0.02 9.8 22.3 710 790 36 2.3 0.042 0.18 5.6 11.9 1000 1100 37 2.6
0.042 0.15 5.3 12.8 880 1060 38 2.5 0.110 0.35 3.9 7.9 1250 1320
Blanking test Evaluation of Tensile test Blanking cracking
toughness El length ratio DBTT No. (%) (4000/TS).sup.2 (%)
(.degree. C.) Remarks 1 18 21.6 0 -80 Example 2 17 15.7 0 -80
Example 3 19 23.8 0 -100 Example 4 16 11.3 0 -40 Example 5 18 22.7
0 -120 Example 6 14 10.9 15 -20 Comparative Example 7 18 24.4 15
-30 Comparative Example 8 15 11.5 15 -30 Comparative Example 9 17
19.8 20 0 Comparative Example 10 14 9.8 5 -40 Example 11 16 14.8 15
-30 Comparative Example 12 15 11.1 5 -40 Example 13 18 22.1 0 -80
Example 14 18 23.8 20 -20 Comparative Example 15 18 24.4 20 10
Comparative Example 16 17 20.2 15 -20 Comparative Example 17 17
16.3 20 -10 Comparative Example 18 16 16.3 35 -20 Comparative
Example 19 15 10.7 5 -50 Example 20 17 15.4 0 -80 Example 21 19
24.4 5 -90 Example 22 17 13.7 0 -60 Example 23 15 10.6 5 -40
Example 24 17 19.3 20 -20 Comparative Example 25 17 17.7 20 -20
Comparative Example 26 16 15.1 20 0 Comparative Example 27 16 20.2
15 -40 Comparative Example 28 14 11.7 25 -30 Comparative Example 29
17 18.9 5 -30 Comparative Example 30 17 19.8 25 -20 Comparative
Example 31 13 8.8 10 -40 Example 32 20 26.3 10 -80 Example 33 13
9.2 20 10 Comparative Example 34 19 22.7 0 -90 Example 35 18 25.6
30 20 Comparative Example 36 16 13.2 0 -50 Example 37 17 14.2 0 -50
Example 38 14 9.2 20 -10 Comparative Example Underline indicates
that it is outside an appropriate range.
[0134] According to Table 3, it is understood that a high-strength
thin steel sheet having excellent blanking workability and
toughness as well as a high strength where the tensile strength TS
is 780 MPa or more can be obtained in all examples.
[0135] Additionally, FIGS. 1 and 2 each illustrate the relationship
between carbon content conversion value C* or C** and DBTT, and the
relationship between carbon content conversion value C* or C** and
blanking cracking length ratio in examples and comparative examples
where the carbon content conversion value C* or C** is outside an
appropriate range.
[0136] According to FIGS. 1 and 2, it is understood that DBTT is
-40.degree. C. or lower and blanking cracking length ratio is 10%
or less when content conversion value C* or C** is in a range of
0.010 mass % to 0.100 mass %.
[0137] Furthermore, FIG. 3 illustrates the relationship between Fe
content in Fe precipitates and blanking cracking length ratio in
examples and comparative examples where the Fe content in Fe
precipitates is outside an appropriate range.
[0138] According to FIG. 3, it is understood that by controlling Fe
content in Fe precipitates to a range of 0.03 mass % to 0.50 mass
%, blanking cracking length ratio can be 10% or less.
[0139] Moreover, FIG. 4 illustrates the relationship between (an
average grain size of top 5% ferrite grains in ferrite grain size
distribution of rolling direction)/(4000/TS).sup.2 and DBTT in
examples and comparative examples where the average grain size of
top 5% ferrite grains in ferrite grain size distribution of rolling
direction cross section is outside an appropriate range.
[0140] According to FIG. 4, it is understood that DBTT is
-40.degree. C. or lower when (an average grain size of top 5%
ferrite grains in ferrite grain size distribution of rolling
direction cross section)/(4000/TS).sup.2 is 1.0 or less, in other
words, DBTT is -40.degree. C. or lower when an average grain size
of top 5% ferrite grains in ferrite grain size distribution of
rolling direction cross section is (4000/TS).sup.2 .mu.m or less in
relation to tensile strength TS in unit of MPa.
* * * * *