U.S. patent application number 12/670153 was filed with the patent office on 2010-08-05 for high-strength steel sheet.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Satoshi Kinoshiro, Koichi Nakagawa, Nobuyuki Nakamura, Kazuhiro Seto, Katsumi Yamada, Takeshi Yokota.
Application Number | 20100196189 12/670153 |
Document ID | / |
Family ID | 40304476 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100196189 |
Kind Code |
A1 |
Nakagawa; Koichi ; et
al. |
August 5, 2010 |
HIGH-STRENGTH STEEL SHEET
Abstract
A high-strength steel sheet has high stretch flangeability after
working and corrosion resistance after painting. The steel sheet
contains, on the basis of mass percent, C: 0.02% to 0.20%, Si: 0.3%
or less, Mn: 0.5% to 2.5%, P: 0.06% or less, S: 0.01% or less, Al:
0.1% or less, Ti: 0.05% to 0.25%, and V: 0.05% to 0.25%, the
remainder being Fe and incidental impurities. The steel sheet has a
substantially ferritic single phase, the ferritic single phase
containing precipitates having a size of less than 20 nm, the
precipitates containing 200 to 1750 mass ppm Ti and 150 to 1750
mass ppm V, V dissolved in solid solution being 200 or more but
less than 1750 mass ppm.
Inventors: |
Nakagawa; Koichi; (Tokyo,
JP) ; Yokota; Takeshi; (Tokyo, JP) ; Nakamura;
Nobuyuki; (Tokyo, JP) ; Seto; Kazuhiro;
(Tokyo, JP) ; Kinoshiro; Satoshi; (Tokyo, JP)
; Yamada; Katsumi; (Tokyo, JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER LLP (US)
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
40304476 |
Appl. No.: |
12/670153 |
Filed: |
July 31, 2008 |
PCT Filed: |
July 31, 2008 |
PCT NO: |
PCT/JP2008/064175 |
371 Date: |
March 11, 2010 |
Current U.S.
Class: |
420/114 ;
420/104; 420/120; 420/122; 420/125; 420/126 |
Current CPC
Class: |
C22C 38/04 20130101;
C21D 2211/005 20130101; C21D 9/48 20130101; C22C 38/24 20130101;
C22C 38/06 20130101; C22C 38/14 20130101; C22C 38/28 20130101; C22C
38/12 20130101; C21D 8/0426 20130101; C21D 2211/004 20130101 |
Class at
Publication: |
420/114 ;
420/120; 420/126; 420/104; 420/122; 420/125 |
International
Class: |
C22C 38/22 20060101
C22C038/22; C22C 38/04 20060101 C22C038/04; C22C 38/14 20060101
C22C038/14; C22C 38/18 20060101 C22C038/18; C22C 38/12 20060101
C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2007 |
JP |
2007-198944 |
Claims
1. A high-strength steel sheet comprising, on the basis of mass
percent, C: 0.02% to 0.20%, Si: 0.3% or less, Mn: 0.5% to 2.5%, P:
0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti: 0.05% to
0.25%, and V: 0.05% to 0.25%, the remainder being Fe and incidental
impurities, having a substantially ferritic single phase,
containing precipitates having a size of less than 20 nm, the
precipitates containing 200 to 1750 mass ppm Ti and 150 to 1750
mass ppm V, V dissolved in solid solution being 200 or more but
less than 1750 mass ppm.
2. The high-strength steel sheet according to claim 1, further
comprising, on the basis of mass percent, any one or two or more of
Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to 0.05%.
3. The high-strength steel sheet according to claim 1, having a
tensile strength TS of 780 MPa or more.
4. The high-strength steel sheet according to claim 1, having a
one-side maximum peel width of 3.0 mm or less after a tape peel
test in a warm salt water immersion test.
5. The high-strength steel sheet according to claim 3, having a
one-side maximum peel width of 3.0 mm or less after a tape peel
test in a warm salt water immersion test.
6. The high-strength steel sheet according to claim 1, having a
stretch flangeability .lamda..sub.10 of 60% or more after rolling
at an elongation percentage of 10%.
7. The high-strength steel sheet according to claim 3, having a
stretch flangeability .lamda..sub.10 of 60% or more after rolling
at an elongation percentage of 10%.
8. The high-strength steel sheet according to claim 2, having a
tensile strength TS of 780 MPa or more.
9. The high-strength steel sheet according to claim 2, having a
one-side maximum peel width of 3.0 mm or less after a tape peel
test in a warm salt water immersion test.
10. The high-strength steel sheet according to claim 8, having a
one-side maximum peel width of 3.0 mm or less after a tape peel
test in a warm salt water immersion test.
11. The high-strength steel sheet according to claim 2, having a
stretch flangeability .lamda..sub.10 of 60% or more after rolling
at an elongation percentage of 10%.
12. The high-strength steel sheet according to claim 3, having a
stretch flangeability .lamda..sub.10 of 60% or more after rolling
at an elongation percentage of 10%.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2008/064175, with an international filing date of Jul. 31,
2008 (WO 2009/017256 A1, published Feb. 5, 2009), which is based on
Japanese Patent Application No. 2007-198944, filed Jul. 31, 2007,
the subject matter of which is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a high-strength steel sheet
having high stretch flange-ability after working and corrosion
resistance after painting.
BACKGROUND
[0003] Automobile parts, such as chassis and truck frames, require
formability (mainly elongation and stretch flangeability), and
steel having a tensile strength on the order of 590 MPa has been
used for such applications. However, to reduce the effects of
automobiles on the environment and to improve crashworthiness of
automobiles, use of higher-strength automotive steel sheets has
been promoted in recent years, and use of steel having a tensile
strength on the order of 780 MPa is being investigated.
[0004] In general, steel materials having higher strength have
lower workability. High-strength high-workability steel sheets have
therefore been studied. For example, Japanese Patent No. 3591502 B2
and Japanese Unexamined Patent Application Publication Nos.
2006-161112 A, 2004-143518 A, 2003-321740 A, 2003-293083 A and
2003-160836 A describe techniques for improving elongation and
stretch flangeability.
[0005] Japanese Patent No. 3591502 B2 discloses a technique
relating to high-workability high-strength steel sheet having a
tensile strength of 590 MPa or more, wherein the steel sheet has a
substantially ferritic single phase in which carbide containing Ti
and Mo having an average particle size of less than 10 nm is
dispersedly precipitated.
[0006] Japanese Unexamined Patent Application Publication No.
2006-161112 A discloses a technique relating to a high-strength
hot-rolled steel sheet having a strength of 880 MPa or more and a
yield ratio of 0.80 or more. The steel sheet has a steel structure
that contains, on the basis of mass, C: 0.08% to 0.20%, Si: 0.001%
or more but less than 0.2%, Mn: more than 1.0% but not more than
3.0%, Al: 0.001% to 0.5%, V: more than 0.1% but not more than 0.5%,
Ti: 0.05% or more but less than 0.2%, and Nb: 0.005% to 0.5%,
provided that the following three formulae are satisfied, the
remainder being Fe and incidental impurities, and that contains 70%
by volume or more ferrite having an average particle size of 5
.mu.m or less and a hardness of 250 Hv or more.
(Ti/48+Nb/93).times.C/12.ltoreq.4.5.times.10.sup.-5 (Formula 1)
0.5.ltoreq.(V/51+Ti/48+Nb/93)/(C/12).ltoreq.5 1.5 (Formula 2)
V+Ti.times.2+Nb.times.1.4+C.times.2+Mn.times.0.1.gtoreq.0.80
(Formula 3)
[0007] Japanese Unexamined Patent Application Publicatio No.
2004-143518 A discloses a technique relating to a hot-rolled steel
.sheet that contains, on the basis of mass, C: 0.05% to 0.2%, Si:
0.001% to 3.0%, Mn: 0.5 to 3.0, P: 0.001% to 0.2%, Al: 0.001% to
3%, and V: more than 0.1% but not more than 1.5%, the remainder
being Fe and impurities, and has a structure mainly composed of
ferrite phase having an average particle size in the range of 1 to
5 .mu.m, the ferrite particles containing carbonitride of V having
an average particle size of 50 nm or less.
[0008] Japanese Unexamined Patent Application Publication No.
2003-321740 A discloses a thermally stable high-strength thin steel
sheet that contains precipitated carbide in the steel structure. In
the thin steel sheet, carbide has a NaCl-type crystal structure
represented by MC wherein M denotes a metallic element composed of
at least two metals, and the at least two metals are regularly
spaced in a crystal lattice, forming a superlattice.
[0009] Japanese Unexamined Patent Application Publication No.
2003-293083 A discloses the following hot-rolled steel sheet. The
steel sheet has a composition of C: 0.0002% to 0.25%, Si: 0.003% to
3.0%, Mn: 0.003% to 3.0%, and Al: 0.002% to 2.0% on the basis of
mass percent, the remainder being Fe and incidental impurities, the
impurities containing 0.15% or less P, 0.05% or less S, and 0.01%
or less N. A ferrite phase accounts for 70% by area or more of the
metal structure and has an average grain size of 20 .mu.m or less
and an aspect ratio of 3 or less. Seventy percent or more of
ferrite grain boundaries are high-angle grain boundaries. Among
ferrite phases defined by high-angle grain boundaries, the area
percentage of precipitates having a maximum diameter of 30 .mu.m or
less and a minimum diameter of 5 nm or more is 2% or less of the
metal structure. Second phases having the largest area percentage
among phases other than the ferrite phases and the precipitates
have an average grain size of 20 .mu.m or less. High-angle grain
boundaries of ferrite phases are disposed between the nearest
second phases.
[0010] Japanese Unexamined Patent Application Publication No.
2003-160836 A discloses a drawable high-strength thin steel sheet
that has excellent shape fixability and burring characteristics,
wherein the thin steel sheet contains, on the basis of mass
percent, C: 0.01% to 0.1%, S.ltoreq.0.03%, N.ltoreq.0.005%, and Ti:
0.05% to 0.5%, the Ti content satisfying
Ti-48/12C-48/14N-48/32S.gtoreq.0%, the remainder being Fe and
incidental impurities, at least the mean values of X-ray random
intensity ratios in a plane at half the thickness of the steel
sheet are 3 or more for {100}<011> to {223}<110>
orientations and 3:5 or less for three orientations of
{554}<225>, {111}<112>, and {111}<110>, the
arithmetical mean rough Ra of at least one of the surfaces of the
steel sheet ranges from 1 to 3.5, and the steel sheet is coated
with a lubricating composition.
[0011] However, the art described above has the following
problems.
[0012] Because the steel sheet contains Mo in Japanese Patent No.
3591502 B2 and Japanese Unexamined Patent Application Publication
No. 2003-321740 A, a recent increase in the cost of Mo has resulted
in a marked increase in the cost of the steel sheet.
[0013] With the increasing globalization of the automobile
industry, automotive steel sheets are being used under severe
corrosion conditions and, therefore, steel sheets require higher
corrosion resistance after painting. However, the addition of Mo
prevents the formation or growth of crystals during chemical
conversion, thereby lowering the corrosion resistance of a steel
sheet after painting. The addition of Mo therefore cannot satisfy
this requirement. Thus, the steel described in Japanese Patent No.
3591502 B2 and Japanese Unexamined Patent Application Publication
No. 2003-321740 A does not have corrosion resistance after painting
that satisfies recent requirements of the automobile industry.
[0014] With recent advances in pressing techniques, processing such
as drawing or stretch forming.fwdarw.piercing.fwdarw.flange forming
is increasingly employed. Flanges of steel sheets formed by such
processing require stretch flangeability after drawing or stretch
forming and piercing, that is, stretch flangeability after working.
However, in Japanese Unexamined Patent Application Publication Nos.
2006-161112 A, 2004-143518 A and 2003-321740 A, a TS of 780 MPa or
more is not always compatible with sufficient stretch flangeability
after working. The addition of Nb in Japanese Unexamined Patent
Application Publication No. 2004-143518 A significantly retards the
recrystallization of austenite after hot rolling. Deformed
austenite therefore remains in a steel sheet, thereby lowering
workability. The addition of Nb also disadvantageously increases
rolling load in hot rolling.
[0015] Japanese Unexamined Patent Application Publication No.
2003-293083 A discloses single-phase ferritic steel sheets having a
tensile strength TS of 422 MPa or less (for example, test numbers 1
to 5 in Table. 6 and test number 45 in Table 8 in Examples) and
multiphase steel sheets composed of a ferrite phase and a second
phase and having a tensile strength TS of 780 MPa or more (for
example, test numbers 33 to 36 in Table 6 and test number 49 in
Table 8 in Examples). These steel sheets described in Japanese
Unexamined Patent Application Publication No. 2003-293083 A mainly
take advantage of solid-solution strengthening due to Si or Mn and
transformation hardening utilizing a hard second phase. These steel
sheets must therefore be cooled to a temperature in the range of
600.degree. C. to 800.degree. C. at an average cooling rate of
30.degree. C./s or more within two seconds after finish rolling,
air-cooled for 3 to 15 seconds, and then water-cooled at an average
cooling rate of 30.degree. C./s or more before coiling. This
promotes two-phase separation during ferrite transformation,
allowing the steel sheets to have a mixed structure of the ferrite
phase and the second phase. The finish-rolling temperature ranges
from (Ae3 point+100.degree. C.) to Ae3 point, which is lower than
the temperature range suitable for manufacture described below. For
example, the finish-rolling temperature for multiphase steel sheets
having a tensile strength TS of 780 MPa or more (test numbers 33 to
36 in Table 6 in Examples) ranged from 871.degree. C. to
800.degree. C. A low finish-rolling temperature results in a
decrease in the solubility limit of a carbide-forming element, such
as Ti, in an austenite phase. Furthermore, because rolling
introduces precipitation sites, precipitates having a size of 20 nm
or more are formed. This phenomenon is referred to as
strain-induced precipitation. In the steel sheets and the method
for manufacturing the steel sheets described in Japanese Unexamined
Patent Application Publication No. 2003-293083, strain-induced
precipitation increases the amount of precipitates having a size of
20 run or more.
[0016] Japanese Unexamined Patent Application Publication No.
2003-293083 A also discloses a technique in which a ferritic single
phase can be manufactured by greatly decreasing the C content and
decreasing the amount of austenite forming element, Mn, in a steel
composition (see steel numbers AA to AE in Table 2 in Examples).
However, a decrease in the amount of Mn, which is also a
solid-solution strengthening element, lowers the solid-solution
strengthening level. A decrease in C content results in a decrease
in the amount of precipitated carbide, for example, of Ti or Nb,
which has precipitation hardening effects, thereby lowering the
precipitation hardening level. Thus, even with a combination of the
solid-solution strengthening level and the precipitation hardening
level, a single-phase ferritic steel sheet cannot have a strength
of 780 MPa or more (see test numbers 1 to 5 in Table 6 and test
number 45 in Table 8 in Examples). For those reasons, a steel sheet
that has a substantially ferritic single phase, a tensile strength
of 780 MPa or more, and other characteristics cannot be
manufactured by the technique described in Japanese Unexamined
Patent Application Publication No. 2003-293083 A.
[0017] Japanese Unexamined Patent Application Publication No.
2003-160836 A discloses steel sheets having a tensile strength
.sigma..sub.B of 780 MPa or more (for example, steel symbols A-4,
A-8, A-10, C, E, and H in Table 2 in Examples). The YRs of these
steel sheets (YR represents .sigma..sub.Y/.sigma..sub.B.times.100
(%)) are as low as 69% to 74%, indicating that these steel sheets
contain a hard second phase, such as a martensite phase.
[0018] As in Japanese Unexamined Patent Application Publication No.
2003-293083 A, the possible basic ideas behind the design of a
steel sheet having a strength of 780 MPa or more according to
Japanese Unexamined Patent Application Publication No. 2003-160836
A mainly take advantage of solid-solution strengthening due to Si
or Mn and transformation hardening utilizing a hard second phase.
As described in Japanese Unexamined Patent Application Publication
No. 2003-293083 A, therefore, rolling at a total reduction of 25%
or more must be performed at a finish-rolling temperature (Ar3
point+100.degree. C. or less) lower than the temperature range
suitable for manufacture. For example, according to an example of
Japanese Unexamined Patent Application Publication No. 2003-160836
A, the finish-rolling temperature for a steel sheet having a
tensile strength .sigma..sub.B of 780 MPa or more ranged from
800.degree. C. to 890.degree. C. In the steel sheets and the method
for manufacturing the steel sheets described in Japanese Unexamined
Patent Application Publication No. 2003-160836 A, as described in
Japanese Unexamined Patent Application Publication No. 2003-293083
A, strain-induced precipitation increases the amount of
precipitates having a size of 20 nm or more. Consequently, a steel
sheet that has a substantially ferritic single phase, a tensile
strength of 780 MPa or more, and other characteristics cannot be
manufactured.
[0019] In view of the situations described above, it could be
helpful to provide a high-strength steel sheet having high stretch
flangeability after working and corrosion resistance after
painting.
SUMMARY
[0020] As a result of investigations to develop a high-strength
hot-rolled steel sheet that has high stretch flangeability after
working, corrosion resistance after painting, and a tensile
strength of 780 MPa or more, we found as follows: [0021] i) To
manufacture a high-strength steel sheet having high corrosion
resistance after painting, precipitates must remain fine (less than
20 nm), and the percentage of fine precipitates (having a size less
than 20 nm) must be increased. Although precipitates containing
Ti--Mo or Ti--V remain fine, mixed precipitation of Ti and V is
useful in improving corrosion resistance after painting; and [0022]
ii) Solid solution of V is important in improving stretch
flangeability after working. There is an optimum V content of solid
solution for an improvement in characteristics.
[0023] We thus provide: [0024] [1] A high-strength steel sheet
containing, on the basis of mass percent, C: 0.02% to 0.20%, Si:
0.3% or less, Mn: 0.5% to 2.5%, P: 0.06% or less, S: 0.01% or less,
Al: 0.1% or less, Ti: 0.05% to 0.25%, and V: 0.05% to 0.25%, the
remainder being Fe and incidental impurities, wherein the steel
sheet has a substantially ferritic single phase, the ferritic
single phase containing precipitates having a size of less than 20
nm, the precipitates containing 200 to 1750 mass ppm Ti and 150 to
1750 mass ppm V, V dissolved in solid solution being 200 or more
but less than 1750 mass ppm. [0025] [2] In [1], the steel sheet
further contains, on the basis of mass percent, any one or two or
more of Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to
0.05%. [0026] [3] In [1] or [2], the high-strength steel sheet has
a tensile strength TS of 780 MPa or more. [0027] [4] In [1] or [2],
the high-strength steel sheet has a one-side maximum peel width of
3.0 mm or less after a tape peel test in a warm salt water
immersion test. [0028] [5] In [3], the high-strength steel sheet
has a one-side maximum peel width of 3.0 mm or less after a tape
peel test in a warm salt water immersion test. [0029] [6] In [1] or
[2], the high-strength steel sheet has a stretch flangeability
.lamda..sub.10 of 60% or more after rolling at an elongation
percentage of 10%. [0030] [7] In [3], the high-strength steel sheet
has a stretch flangeability .lamda..sub.10 of 60% or more after
rolling at an elongation percentage of 10%.
DETAILED DESCRIPTION
[0031] The percentages and ppm of components of steel are based on
mass percent and mass ppm. Our high-strength steel sheets have a
tensile strength (hereinafter also referred to as TS) of 780 MPa or
more and include hot-rolled steel sheets and surface-treated steel
sheets, which are high-strength steel sheets subjected to surface
treatment, such as plating.
[0032] Target characteristics include a stretch flangeability
(.lamda..sub.10) of 60% or more after rolling at an elongation
percentage of 10% and a one-side maximum peel width of 3.0 mm or
less after a tape peel test in a warm salt water immersion test
(SDT) described below.
[0033] We provide a high-strength hot-rolled steel sheet that has
high stretch flange-ability after working, corrosion resistance
after painting, and a TS of 780 MPa or more. Our steel sheets have
these advantages without the addition of Mo and can therefore
reduce costs.
[0034] For example, use of a high-strength hot-rolled steel sheet
in automobile chassis and truck frames should allow thickness
reduction, reduce the effects of automobiles on the environment,
and markedly improve crashworthiness of automobiles.
[0035] Our steel sheets and methods will be described in detail
below.
[0036] (1) First, the reason to specify the chemical components
(composition) of steel will be described below.
C: 0.02% to 0.20%
[0037] C can be precipitated in ferrite as carbide with Ti or V,
thereby contributing to high strength of a steel sheet. 0.02% or
more C is required to achieve a TS of 780 MPa or more. However,
more than 0.20% C results in coarsening of precipitates and the
formation of a second phase, lowering stretch flangeability after
working. Thus, the C content ranges from 0.02% to 0.20%, preferably
0.03% to 0.15%.
Si: 0.3% or Less
[0038] Although Si can contribute to solid-solution strengthening,
the addition of more than 0.3% Si results in the formation of
cementite at grain boundaries, lowering stretch flangeability after
working. Thus, the Si content is 0.3% or less, preferably 0.001% to
0.2%.
Mn: 0.5% to 2.5%
[0039] Mn can contribute to solid-solution strengthening. However,
the TS is less than 780 MPa at a Mn content of less than 0.5%. The
addition of more than 2.5% Mn markedly lowers weldability. Thus,
the Mn content ranges from 0.5% to 2.5%, preferably 0.6% to
2.0%.
P: 0.06% or Less
[0040] P can segregate at prior austenite grain boundaries,
lowering workability and low-temperature toughness. Thus, the P
content is preferably minimized and is 0.06% or less, preferably in
the range of 0.001% to 0.055%.
S: 0.01% or Less
[0041] S can segregate at prior austenite grain boundaries or can
be precipitated as MnS. The segregation or a large amount of MnS
lowers low-temperature toughness. S also markedly lowers stretch
flangeability, regardless of the presence or absence of working.
Thus, the S content is preferably minimized and is 0.01% or less,
preferably in the range of 0.0001% to 0.005%.
Al: 0.1% or Less
[0042] Al can be added to steel as a deoxidizer and effectively
improves the cleanliness of the steel. Preferably, 0.001% or more
Al is added to steel to produce this effect. However, more than
0.1% Al results in the generation of a large number of inclusions,
causing flaws in a steel sheet. Thus, the Al content is 0.1% or
less, preferably 0.01% to 0.04%.
Ti: 0.05% to 0.25%
[0043] Ti is very important for the precipitation hardening of
ferrite and is an important factor in our steel sheets. A required
strength is difficult to achieve at a Ti content of less than
0.05%. However, the effects of Ti become saturated at a Ti content
of more than 0.25%, and more than 0.25% Ti only increases costs.
Thus, the Ti content ranges from 0.05% to 0.25%, preferably 0.08%
to 0.20%.
V: 0.05% to 0.25%
[0044] V can contribute to an improvement in strength by
precipitation hardening or solid-solution strengthening. Like Ti, V
is therefore an important factor in our steel sheets. A proper
amount of V, together with Ti, tends to be precipitated as fine
Ti-V carbide having a particle size (hereinafter also referred to
as "size") of less than 20 nm. Unlike Mo, V does not lower
corrosion resistance after painting. Less than 0.05% V is
insufficient for the effects described above. However, the effects
of V become saturated at a V content of more than 0.25%, and more
than 0.25% V only increases costs. Thus, the V content ranges from
0.05% to 0.25%, preferably 0.06% to 0.20%.
[0045] With these essential additive elements, the steels can have
target characteristics. In addition to the essential additive
elements, any one or two or more of Cr: 0.01% to 0.5%, W: 0.005% to
0.2%, and Zr: 0.0005% to 0.05% may be added for the following
reasons. Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to
0.05%
[0046] Like V, Cr, W, and Zr can strengthen ferrite as a
precipitate or solid solution. Less than 0.01% Cr, less than 0.005%
W, or less than 0.0005% Zr makes a negligible contribution to high
strength of steel. However, more than 0.5% Cr, more than 0.2% W, or
more than 0.05% Zr lowers workability. Thus, when any one or two or
more of Cr, W, and Zr are added, their amounts are Cr: 0.01% to
0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to 0.05%, preferably Cr:
0.03% to 0.3%, W: 0.01% to 0.18%, and Zr: 0.001% to 0.04%.
[0047] The remainder consists of Fe and incidental impurities. As
an incidental impurity, for example, .largecircle. forms a
non-metallic inclusion and has adverse effects on the quality of
steel. .largecircle. is therefore desirably decreased to 0.003% or
less. 0.1% or less Cu, Ni, Sn, and/or Sb may be contained as a
trace element without compromising the operational advantages of
our steel sheets.
[0048] (2) The structure of a high-strength steel sheet will be
described below.
Substantially Ferritic Single Phase
[0049] To achieve a TS of 780 MPa or more and improve stretch
flangeability after working, ferrite having a low dislocation
density is effective, and a single phase is effective. In
particular, a highly ductile ferritic single phase has a marked
improving effect on stretch flangeability after working. However, a
completely ferritic single phase is not necessary, and even a
substantially ferritic single phase can sufficiently produce the
effect. A substantially ferritic single phase, as used herein,
refers to allowance for a minute amount of another phase or
precipitate other than carbide, and the volume percentage of
ferrite is preferably 95% or more. A substantially ferritic single
phase may contain up to 5% by volume of cementite, pearlite, and/or
bainite without affecting the characteristics of the steel
sheets.
[0050] The volume percentage of ferrite can be determined by
exposing a microstructure in the vertical cross-section parallel to
the rolling direction using 3% nital, observing the microstructure
at a quarter thickness in the depth direction with a scanning
electron microscope (SEM) at a magnification of 1500, and
determining the ferrite area ratio, for example, using an
image-processing software "Ryusi Kaiseki (particle analysis) II"
from Sumitomo Metal Technology, Inc.
200 to 1750 ppm Ti and 150 to 1750 ppm V in Precipitates Having a
Size below 20 nm in a Ferritic Single Phase
[0051] In a high-strength steel sheet, precipitates containing Ti
and/or V exist in ferrite mainly as carbides. This is probably
because the solubility limit of C in ferrite is low, and
supersaturated C is therefore easily precipitated in ferrite as
carbide. Such a precipitate increases the hardness (strength) of
soft ferrite, thereby achieving a TS of 780 MPa or more. Such a
precipitate also increases YS, achieving YR (.dbd.YS/YR) of 83% or
more.
[0052] As described above, to manufacture a high-strength steel
sheet, it is important that precipitates remain fine (less than 20
nm), and the percentage of fine precipitates (having a size less
than 20 nm) is increased. A precipitate having a size of 20 nm or
more has a small effect in preventing dislocation movement and
cannot sufficiently increase the hardness of ferrite, sometimes
resulting in low strength.
[0053] A further investigation revealed that a fine precipitate
size is important for corrosion resistance after painting. In
conventional Ti (addition of Ti alone) HSLA steel, a precipitate
have a tendency to become coarse with increasing Ti content. In
such a steel sheet, therefore, corrosion resistance after painting
also has a tendency to decrease with decreasing strength. Although
the reason for a deterioration in corrosion resistance after
painting associated with coarsening of a precipitate is not clear,
a coarse precipitate should prevent the formation or growth of
crystals during chemical conversion.
[0054] Thus, a precipitate preferably has a size of less than 20
nm. A fine precipitate having a size of less than 20 nm can be
formed by the addition of both Ti and V. V forms a complex carbide
mainly with Ti. Although there is no clear reason, these
precipitates remain stable and fine at high temperatures within the
coiling temperature for a long period of time.
[0055] It is important to control the Ti content and the V content
of precipitates having a size of less than 20 nm. When the Ti
content and the V content of precipitates having a size of less
than 20 nm are less than 200 ppm and less than 150 ppm,
respectively, the number density of the precipitates is small, and
the distance between precipitates increases. The precipitates
therefore have a small effect in preventing dislocation movement.
Thus, the precipitates cannot sufficiently increase the hardness of
ferrite, and therefore the TS cannot be 780 MPa or more. When the
Ti content and the V content of precipitates having a size of less
than 20 nm are 200 ppm or more and less than 150 ppm, respectively,
the precipitates have a tendency to become coarse, and therefore
the TS may be less than 780 MPa. When the Ti content and the V
content of precipitates having a size of less than 20 nm are less
than 200 ppm and 150 ppm or more, respectively, the precipitation
efficiency of V decreases, and therefore the TS may be less than
780 MPa. When the Ti content or the V content of precipitates
having a size of less than 20 nm is more than 1750 ppm, the
corrosion resistance after painting decreases, and therefore the
target characteristics cannot be achieved. This is probably because
a large number of fine precipitates prevent the formation or growth
of crystals on the surface of a steel sheet during chemical
conversion. Thus, the amounts of precipitated Ti and V in
precipitates having a size of less than 20 nm must be
satisfactorily controlled.
[0056] When the ratio of the Ti content to the V content of
precipitates having a size of less than 20 nm satisfies
0.4.ltoreq.(Ti/48)/(V/51).ltoreq.2.5, the TS can be 785 MPa or
more, thus achieving more suitable conditions. Although there is no
clear reason, optimization of the ratio of Ti to V should improve
heat stability.
[0057] Thus, the Ti content and the V content of precipitates
having a size of less than 20 nm range from 200 to 1750 ppm and 150
to 1750 ppm, respectively. Furthermore, the ratio of the Ti content
to the V content of precipitates having a size of less than 20 nm
preferably satisfies 0.4.ltoreq.(Ti/48)/(V/51).ltoreq.2.5.
[0058] A precipitate and/or an inclusion is hereinafter also
collectively referred to as a precipitate or the like.
[0059] The Ti content and the V content can be controlled by the
coiling temperature. The coiling temperature preferably ranges from
500.degree. C. to 700.degree. C. At a coiling temperature above
700.degree. C., precipitates become coarse, and the amounts of
precipitated Ti and V in precipitates having a size of less than 20
nm are less than 200 ppm and less than 150 ppm, respectively, and
the TS cannot be 780 MPa or more. At a coiling temperature below
500.degree. C., the amounts of precipitated Ti and V in
precipitates having a size of less than 20 nm are also less than
200 ppm and less than 150 ppm, respectively. Such a low coiling
temperature should result in insufficient diffusion of Ti and
V.
[0060] The Ti content and the V content of precipitates having a
size of less than 20 nm can be determined by the following
method.
[0061] After a predetermined amount of sample is electrolyzed in an
electrolyte, the sample is removed from the electrolyte and is
immersed in a dispersive solution. Precipitates in the solution is
filtered with a filter having a pore size of 20 nm. Precipitates in
filtrate passing through the filter having a pore size of 20 nm
have a size of less than 20 nm. The filtrate after filtration is
appropriately analyzed by inductively coupled plasma (ICP) emission
spectroscopic analysis, ICP mass spectrometry, atomic absorption
spectrometry, or the like to determine the Ti content and the V
content of precipitates having a size of less than 20 nm. Structure
Containing 200 ppm or More but Less Than 1750 ppm V in Solid
Solution.
[0062] V in solid solution is the most important factor. Solid
solution of V is important in improving stretch flangeability after
working. Less than 200 ppm V in solid solution has an insufficient
effect, and 200 ppm or more V in solid solution is required to
produce the effect described above. 1750 ppm or more V in solid
solution exhibits a saturated effect and is considered as an upper
limit.
[0063] Thus, the amount of V in solid solution is 200 ppm or more
but less than 1750 ppm. Although the workability of steel slightly
deteriorates with increasing strength, when the Ti content and the
V content of precipitates having a size of less than 20 nm are both
1750 ppm or less, 200 ppm or more V in solid solution can
sufficiently ensure target stretch flangeability after working.
[0064] 200 ppm or more but less than 1750 ppm V in solid solution
can be measured, for example, by the following method.
[0065] After a predetermined amount of sample is electrolyzed in a
nonaqueous solvent electrolyte, the electrolyte is subjected to
elementary analysis. The analysis method may be inductively coupled
plasma (ICP) emission spectroscopic analysis, ICP mass
spectrometry, or atomic absorption spectrometry.
[0066] (3) A method for manufacturing a high-strength steel sheet
will be described below.
[0067] For example, a high-strength steel sheet can be manufactured
by heating a steel slab adjusted within the chemical component
ranges described above at a temperature in the range of
1150.degree. C. to 1350.degree. C., hot-rolling the steel slab at a
finish-rolling temperature in the range of 850.degree. C. to
1100.degree. C., and coiling the rolled steel at a temperature in
the range of 500.degree. C. to 700.degree. C. Conditions suitable
for these processes will be described in detail below.
Steel Slab Heating Temperature: 1150.degree. C. to 1350.degree.
C.
[0068] A carbide-forming element, such as Ti or V, is mostly
present as a precipitate in a steel slab. To be precipitated as
desired in a ferrite phase after hot rolling, a precipitate in the
form of carbide must be temporarily dissolved before hot rolling. A
precipitate must therefore be heated at 1150.degree. C. or
more.
[0069] At a temperature below 1150.degree. C., carbide having a
size of 20 nm or more, which does not contribute to precipitation
hardening or corrosion resistance after painting, remains. This
reduces the amount of Ti and V involved in the formation of fine
precipitates having a size of less than 20 nm required. A target
amount of precipitates having a size of less than 20 nm cannot
therefore be obtained in coiling described below. In a method for
manufacturing a steel sheet, most desirably, carbide containing Ti
or V remains dissolved during slab heating and finish rolling, and
is precipitated as fine carbide containing Ti or V during coiling
after finish rolling. The heating temperature is therefore more
preferably 1170.degree. C. or more so that carbide can be dissolved
almost completely.
[0070] However, heating at a temperature above 1350.degree. C.
excessively increases the crystal grain size, lowering stretch
flangeability and elongation after working. Taking subsequent
heat-treatment conditions into consideration, an increase in
crystal grain size can be almost completely prevented at a heating
temperature of 1300.degree. C. or less.
[0071] Thus, the slab heating temperature preferably ranges from
1150.degree. C. to 1350.degree. C., more preferably 1170.degree. C.
to 1300.degree. C.
Finish-Rolling Temperature in Hot Rolling: 850.degree. C. to
1100.degree. C.
[0072] The control of finish-rolling temperature is important in
ensuring the Ti content and the V content of precipitates having a
size of less than 20 nm. Preferably, a steel slab after working is
hot-rolled at a finish-rolling temperature in the range of
850.degree. C. to 1100.degree. C., which is the final temperature
of hot rolling. At a finish-rolling temperature below 850.degree.
C., a steel slab is rolled in a ferrite+austenite region and has an
elongated ferrite phase. This may lower stretch flangeability or
elongation after working. Even if a steel slab is heated at a
temperature of 1150.degree. C. or more to temporarily dissolve a
carbide precipitate before rolling, carbide containing Ti or V is
precipitated at a finish-rolling temperature below 850.degree. C.
because of strain-induced precipitation. This reduces the amount of
Ti and V involved in the formation of fine precipitates having a
size of less than 20 nm required. A target amount of precipitates
having a size of less than 20 nm cannot therefore be obtained in
coiling described below. Thus, it is important to perform the
subsequent coiling process while carbide containing Ti or V
temporarily dissolved during the slab heating described above
remains dissolved in finish rolling as much as possible. The
finish-rolling temperature is more preferably 935.degree. C. or
more such that carbide remains dissolved.
[0073] A finish-rolling temperature above 1100.degree. C. may
result in coarsening of ferrite particles and a TS below 780 MPa.
The finish-rolling temperature is more preferably 990.degree. C. or
less to prevent coarsening of ferrite particles.
[0074] Thus, the finish-rolling temperature preferably ranges from
850.degree. C. to 1100.degree. C., more preferably 935.degree. C.
to 990.degree. C.
Coiling Temperature: 500.degree. C. to 700.degree. C.
[0075] The control of coiling temperature is important in ensuring
the Ti content and the V content of precipitates having a size of
less than 20 nm. As described above, this is because, in the most
desirable manufacturing form, this coiling process yields a large
number of precipitation sites from which carbide is precipitated,
thus preventing carbide grains from growing to 20 nm or more. The
coiling temperature preferably ranges from 500.degree. C. to
700.degree. C. so that steel has a substantially ferritic single
phase and the characteristics can be achieved.
[0076] A coiling temperature below 500.degree. C. may result in an
insufficient amount of precipitated carbide containing Ti and/or V
and reduced strength. Furthermore, a bainite phase may be formed in
place of a ferritic single phase.
[0077] To form a large number of precipitation sites and produce
carbide from these precipitation sites, the coiling temperature is
preferably 500.degree. C. or more, more preferably 550.degree. C.
or more.
[0078] A coiling temperature above 700.degree. C. may result in
coarsening of precipitated carbide and reduced strength. A coiling
temperature above 700.degree. C. may also promote the formation of
a pearlite phase, lowering stretch flangeability after working. The
coiling temperature is more preferably 650.degree. C. or less to
prevent coarsening of precipitated carbide without fail.
[0079] Thus, the coiling temperature preferably ranges from
500.degree. C. to 700.degree. C., more preferably 550.degree. C. to
650.degree. C.
[0080] The steel sheets include surface-treated steel sheets and
surface-coated steel sheets. In particular, a steel sheet may be
subjected to hot-dip galvanizing to form a galvanized steel sheet,
and this disclosure can be suitably applied to such a galvanized
steel sheet. Because our steel sheets have excellent workability,
such a galvanized steel sheet can also have excellent workability.
Hot-dip galvanizing is zinc and zinc-based (approximately 90% or
more) hot dipping and includes hot dipping including an alloying
element, such as Al or Cr, as well as zinc. Hot-dip galvanizing may
be performed alone or followed by alloying.
[0081] A steel melting method is not particularly limited, and any
known melting method may be suitable. For example, a suitable
melting method involves melting in a converter or an electric
furnace and secondary refining in a vacuum degassing furnace. A
casting method is preferably continuous casting in terms of
productivity and quality. After casting, hot direct rolling may be
performed immediately or after concurrent heating, without
compromising the advantages of our steel sheets. Furthermore, a
hot-rolled material may be heated after rough rolling and before
finish rolling, continuous hot rolling in which rolled materials
are joined may be performed after rough rolling, or heating and
continuous rolling of a heating material of a rolled material may
be performed simultaneously. These do not compromise the advantages
of our steel sheets.
Examples
Example 1
[0082] Steel having a composition shown in Table 1 was melted in a
converter and was formed into a steel slab by continuous casting.
The steel slab was subjected to heating, hot rolling, and coiling
under conditions shown in Table 2 to form a hot-rolled steel sheet
having a thickness of 2.0 mm.
TABLE-US-00001 TABLE 1 Type of Composition (mass %) steel C Si Mn P
S Al Ti V Note A 0.040 0.01 1.45 0.01 0.0015 0.03 0.105 0.120
Conforming steel B 0.120 0.02 1.20 0.02 0.0008 0.03 0.240 0.100
Conforming steel C 0.100 0.02 1.20 0.01 0.0080 0.03 0.110 0.245
Conforming steel D 0.150 0.02 1.40 0.03 0.0020 0.03 0.230 0.224
Conforming steel E 0.050 0.01 2.02 0.01 0.0020 0.03 0.120 0.120
Conforming steel F 0.050 0.01 0.65 0.01 0.0015 0.03 0.110 0.136
Conforming steel G 0.045 0.02 1.34 0.02 0.0007 0.02 0.060 0.110
Conforming steel H 0.050 0.02 1.30 0.01 0.0008 0.02 0.110 0.052
Conforming steel I 0.030 0.01 1.32 0.01 0.0007 0.02 0.080 0.070
Conforming steel J 0.040 0.01 1.40 0.02 0.0015 0.03 0.126 0.152
Conforming steel K 0.250 0.01 1.20 0.02 0.0020 0.03 0.120 0.130
Nonconforming L 0.001 0.01 1.19 0.02 0.0020 0.03 0.120 0.130
Nonconforming M 0.080 0.50 1.30 0.01 0.0012 0.03 0.070 0.070
Nonconforming N 0.050 0.01 0.35 0.02 0.0015 0.03 0.080 0.080
Nonconforming O 0.050 0.01 3.00 0.02 0.0014 0.03 0.080 0.080
Nonconforming P 0.150 0.01 1.60 0.02 0.0015 0.03 0.040 0.120
Nonconforming Q 0.160 0.01 1.60 0.02 0.0016 0.02 0.070 0.032
Nonconforming R 0.152 0.01 1.62 0.02 0.0015 0.03 0.280 0.120
Nonconforming S 0.161 0.01 1.61 0.02 0.0014 0.03 0.150 0.300
Nonconforming X 0.090 0.06 1.35 0.04 0.0014 0.05 0.150 0.160
Conforming steel
[0083] The microstructure of the hot-rolled steel sheet was
analyzed by the following method to determine the Ti content and
the V content of precipitates having a size of less than 20 nm and
the amount of V in solid solution. The tensile strength TS, the
stretch flange-ability after working .lamda..sub.10, and the
corrosion resistance after painting (SDT one-side maximum peel
width) were measured.
Analysis of Microstructure
[0084] The hot-rolled steel sheet thus formed was cut into an
appropriate size. Approximately 0.2 g of hot-rolled steel sheet was
subjected to constant-current electrolysis at an electric current
density of 20 mA/cm.sup.2 in 10% AA electrolyte (10% by volume
acetylacetone-1% by mass tetramethylammonium
chloride-methanol).
Measurement of the Ti Content and the V Content of Precipitates
Having a Size of Less Than 20 nm
[0085] After electrolysis, a test piece on which a precipitate was
deposited was removed from the electrolyte and was immersed in
aqueous sodium hexametaphosphate (500 mg/l) (hereinafter referred
to as aqueous SHMP). Ultrasonic vibration was applied to the test
piece to detach and extract the precipitate from the test piece in
aqueous SHMP. The aqueous SHMP containing the precipitate was then
passed through a filter having a pore size of 20 nm. The filtrate
was analyzed with an ICP spectrometer to measure the absolute
amounts of Ti and V in the filtrate. The absolute amounts of Ti and
V were divided by the weight of the electrolyzed sample to
calculate the Ti content and the V content of precipitates having a
size of less than 20 nm. The weight of electrolyzed sample was
calculated by subtracting the sample weight after the detachment of
the precipitate from the sample weight before electrolysis.
Measurement of the Amount of V in Solid Solution
[0086] After electrolysis, the concentrations of V and a
comparative element Fe in the electrolyte were measured by ICP mass
spectrometry. On the Basis of the concentrations thus measured, the
ratio of the concentration of V to the concentration of Fe was
calculated. The ratio was multiplied by the Fe content of the
sample to calculate the amount of V in solid solution. The Fe
content of the sample can be calculated by subtracting the
summation of compositions other than Fe from 100%.
TS
[0087] A tensile test according to JIS Z 2241 was performed with a
JIS No. 5 specimen in the tensile direction parallel to the rolling
direction to measure TS.
Stretch Flangeability after Working: .lamda..sub.10
[0088] After rolling at an elongation percentage of 10%, a hole
expanding test according to the Japan Iron and Steel Federation
Standard JFS T 1001 was performed to measure .lamda..sub.10.
Corrosion Resistance after Painting: SDT One-Side Maximum Peel
Width
[0089] A chemical conversion treatment was performed under more
adverse temperature and concentration conditions than the standard
conditions using a degreasing agent, Surf-cleaner ECO90, a surface
conditioner, Surffine 5N-10, and a chemical conversion treatment
agent, Surfdine SD2800, all manufactured by Nippon Paint Co., Ltd.
As an example of standard conditions, a degreasing process included
a concentration of 16 g/l, a treatment temperature in the range of
42.degree. C. to 44.degree. C., a treatment time of 120 s, and
spray degreasing, and a surface conditioning process included a
total alkalinity in the range of 1.5 to 2.5 points, a free acidity
in the range of 0.7 to 0.9 points, an accelerator concentration in
the range of 2.8 to 3.5 points, a treatment temperature of
44.degree. C., and a treatment time of 120 s. Under adverse
conditions, a treatment temperature in a chemical conversion
treatment process was decreased to 38.degree. C. Subsequently,
electrodeposition coating was performed using an electrodeposition
paint, V-50, manufactured by Nippon Paint Co., Ltd. The target
amount of deposited chemical conversion film ranged from 2 to 2.5
g/m.sup.2, and the target film thickness in electrodeposition
coating was 25 .mu.m.
[0090] Corrosion resistance after painting was determined in a warm
salt water immersion test (SDT). A crosscut was formed with a
cutter in a sample subjected to chemical conversion treatment and
electrodeposition coating. The sample was immersed in warm salt
water (5% NaCl at 55.degree. C.) for 10 days, was then washed with
water, and was dried. Tape peeling on the crosscut was performed to
measure the maximum peel width on the left and right sides of the
crosscut. A one-side maximum peel width of 3.0 mm or less was
considered as high corrosion resistance after painting.
[0091] Table 2 shows the results, together with manufacturing
conditions.
TABLE-US-00002 TABLE 2 Elonga- Stretch Precip- Precip- Amount One-
Slab Finish- tion flange- itated itated of V side heating rolling
Coiling after ability Ti content V content in solid maximum Type
temper- temper- temper- pre- after for <20 for <20 solution
peel of ature ature ature TS straining working: nm (mass nm (mass
(mass width No steel (.degree. C.) (.degree. C.) (.degree. C.)
(MPa) (%) .lamda.10(%) ppm) ppm) ppm) (mm) Phase Note 1 A 1250 920
630 812 20 87 752 818 340 1.7 Ferrite: Example 100% 2 B 1300 926
632 952 18 79 1580 765 231 2.5 Ferrite: Example 100% 3 C 1270 911
650 966 17 81 703 1700 380 2.2 Ferrite: Example 100% 4 C 1270 900
580 865 17 95 635 657 1350 1.2 Ferrite: Example 99%, Remainder
Cementite 1% 5 D 1270 917 603 1190 16 61 1700 1682 213 2.2 Ferrite:
Example 98%, Remainder: Bainite 2% 6 E 1250 921 611 940 18 92 808
658 476 1.2 Ferrite: Example 100% 7 F 1250 900 590 834 20 98 727
735 540 1.4 Ferrite: Example 100% 8 G 1250 918 670 815 19 82 230
450 420 1.4 Ferrite: Example 100% 9 H 1250 920 580 802 18 93 352
167 272 1.2 Ferrite: Example 100% 10 I 1160 905 625 785 22 97 532
372 306 1.2 Ferrite: Example 100% 11 J 1250 920 630 936 18 83 863
1129 274 2.0 Ferrite: Example 100% 12 A 1250 920 480 760 19 63 150
121 934 2.0 Ferrite: Comparative 100% Example 13 G 1250 920 720 765
18 90 220 98 330 5.2 Ferrite: Comparative 100% Example 14 G 1250
915 750 760 15 78 140 80 908 5.1 Ferrite: Comparative 100% Example
15 K 1250 923 590 851 20 45 821 702 568 0.8 Ferrote" Comparative
90%, Example Remainder: Pearlite 10% 16 L 1250 918 585 659 25 60 50
45 568 1.1 Ferrite: Comparative 100% Example 17 M 1250 918 595 850
17 40 480 353 330 0.8 Ferrite: Comparative 92%, Example Remainder:
Cementite 8% 18 N 1250 920 575 765 18 75 560 540 247 1.0 Ferrite:
Comparative 100% Example 19 O 1250 916 565 851 14 43 560 432 350
1.1 Ferrite: Comparative 100% Example 20 P 1160 921 575 653 23 75
180 324 832 1.2 Ferrite: Comparative 100% Example 21 Q 1160 922 650
765 16 73 490 14 223 1.2 Ferrite: Comparative 100% Example 22 Q
1160 920 510 782 16 50 502 220 90 1.1 Ferrite: Comparative 100%
Example 23 R 1250 910 605 1280 13 93 2065 602 580 5.5 Ferrite:
Comparative 100% Example 24 S 1250 900 610 1290 14 91 971 1890 530
5.3 Ferrite: Comparative 100% Example 29 A 1250 935 600 825 19 70
800 825 340 2.0 Ferrite: Example 100% 30 A 1260 980 580 820 19 68
802 830 355 2.1 Ferrite: Example 100% 31 A 1260 1020 630 826 18 73
801 824 349 2.1 Ferrite: Example 100% 32 J 1260 940 620 982 17 63
923 1120 270 2.6 Ferrite: Example 100% 33 C 1260 960 600 983 17 65
812 1702 375 2.5 Ferrite: Example 100% 34 X 1300 965 600 1005 16 62
1205 1108 305 2.8 Ferrite: Example 100%
[0092] Table 2 shows that the working examples had a TS of 780 MPa
or more, .lamda..sub.10 of 60% or more, and an SDT one-side maximum
peel width of 3.0 mm or less, indicating that the hot-rolled steel
sheets had high stretch flangeability after working and corrosion
resistance after painting.
[0093] In contrast, the comparative examples had a low TS
(strength), small .lamda..sub.10 (stretch flangeability after
working), and/or a large SDT one-side maximum peel width (corrosion
resistance after painting). Example 2
[0094] Steel having a composition shown in Table 3 was melted in a
converter and was formed into a steel slab by continuous casting.
The steel slab was subjected to heating, hot rolling, and coiling
under conditions shown in Table 4 to form a hot-rolled steel sheet
having a thickness of 2.0 mm.
TABLE-US-00003 TABLE 3 Type of Composition (mass %) steel C Si Mn P
S Al Ti V Cr W Zr Note T 0.040 0.01 1.40 0.01 0.0014 0.03 0.100
0.115 0.10 -- -- Conforming steel U 0.040 0.02 1.43 0.01 0.0015
0.03 0.104 0.105 -- 0.150 -- Conforming steel V 0.041 0.01 1.42
0.01 0.0014 0.03 0.102 0.105 -- -- 0.0030 Conforming steel W 0.040
0.02 1.40 0.01 0.0014 0.03 0.101 0.115 0.20 0.140 0.0050 Conforming
steel
[0095] In the same way as in Example 1, the microstructure of the
hot-rolled steel sheet thus formed was analyzed to determine the Ti
content and the V content of precipitates having a size of less
than 20 nm and the amount of V in solid solution. In the same way
as in Example 1, the tensile strength TS, the stretch flangeability
after working .lamda..sub.10, and the corrosion resistance after
painting (SDT one-side maximum peel width) were measured.
[0096] Table 4 shows the results.
TABLE-US-00004 TABLE 4 Elonga- Stretch Precip- Precip- Amount One-
Slab Finish- tion flange- itated itated of V side heating rolling
Coiling after ability Ti content V content in solid maximum Type
temper- temper- temper- pre- after for <20 for <20 solution
peel of ature ature ature TS straining working: nm (mass nm (mass
(mass width No steel (.degree. C.) (.degree. C.) (.degree. C.)
(MPa) (%) .lamda.10(%) ppm) ppm) ppm) (mm) Phase Note 25 T 1250 921
625 832 17 99 750 815 250 2.5 Ferrite: Example 100% 26 U 1250 918
620 830 18 90 753 760 252 2.2 Ferrite: Example 100% 27 V 1250 920
621 829 17 93 753 770 250 2.0 Ferrite: Example 100% 28 W 1250 921
620 842 18 98 760 823 251 2.6 Ferrite: Example 100% 35 T 1250 940
600 835 18 92 780 820 240 2.2 Ferrite: Example 100% 36 T 1270 960
630 840 17 93 782 823 244 2.1 Ferrite: Example 100% 37 T 1300 980
620 837 18 95 788 830 245 2.3 Ferrite: Example 100%
[0097] Table 4 shows that the working examples had a TS of 780 MPa
or more, .lamda..sub.10 of 60% or more, and an SDT one-side maximum
peel width of 3.0 mm or less, indicating that the hot-rolled steel
sheets had high stretch flangeability after working and corrosion
resistance after painting.
[0098] As compared with the steel sheet No. 1 (Table 2), the steel
sheets Nos. 25 to 28 and 35 to 37, which further contained Cr, W,
or Zr, had an improved TS.
INDUSTRIAL APPLICABILITY
[0099] Our steel sheets have high strength, high stretch
flangeability after working, and high corrosion resistance after
painting, and are therefore most suitable for, for example,
automobile and truck frames, and components that require elongation
and stretch flangeability.
* * * * *