U.S. patent application number 12/299407 was filed with the patent office on 2009-09-24 for ultrahigh strength steel sheet and strength part for automobile utilizing the same.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Tadanobu Inoue, Yuuji Kimura, Kotobu Nagai, Eizaburou Nakanishi, Yoshio Okada, Masamoto Ono, Hideyuki Sasaoka.
Application Number | 20090236015 12/299407 |
Document ID | / |
Family ID | 38693701 |
Filed Date | 2009-09-24 |
United States Patent
Application |
20090236015 |
Kind Code |
A1 |
Sasaoka; Hideyuki ; et
al. |
September 24, 2009 |
ULTRAHIGH STRENGTH STEEL SHEET AND STRENGTH PART FOR AUTOMOBILE
UTILIZING THE SAME
Abstract
There is provided an ultrahigh strength steel sheet containing
0.10 to 0.40 mass % of C, 0.01 to 3.5 mass % of Cr, at least one
selected from the group consisting of 0.10 to 2.0 mass % of Mo,
0.20 to 1.5 mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0
mass % of Ti and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of
P and 0.01 mass % or less of S as impurities and the balance being
Fe and unavoidable impurities based on the total mass of the steel
sheet and having a base structure of lower bainite, a prior
austenite grain size of 30 .mu.m or smaller and a tensile strength
of 980 MPa or higher. There is also provided an automotive strength
part using the ultrahigh strength steel sheet.
Inventors: |
Sasaoka; Hideyuki;
(Kanagawa, JP) ; Ono; Masamoto; (Kanagawa, JP)
; Nakanishi; Eizaburou; (Kanagawa, JP) ; Okada;
Yoshio; (Kanagawa, JP) ; Inoue; Tadanobu;
(Ibaraki, JP) ; Kimura; Yuuji; (Ibaraki, JP)
; Nagai; Kotobu; (Ibaraki, JP) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
Kanagawa
JP
|
Family ID: |
38693701 |
Appl. No.: |
12/299407 |
Filed: |
April 3, 2007 |
PCT Filed: |
April 3, 2007 |
PCT NO: |
PCT/JP2007/057424 |
371 Date: |
November 3, 2008 |
Current U.S.
Class: |
148/332 ;
148/334; 148/335 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/44 20130101; C22C 38/04 20130101; C22C 38/42 20130101; C22C
38/22 20130101 |
Class at
Publication: |
148/332 ;
148/334; 148/335 |
International
Class: |
C22C 38/20 20060101
C22C038/20; C22C 38/22 20060101 C22C038/22; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2006 |
JP |
2006-137225 |
May 17, 2006 |
JP |
2006-137226 |
May 17, 2006 |
JP |
2006-137227 |
Claims
1. An ultrahigh strength steel sheet comprising 0.10 to 0.40 mass %
of C, 0.01 to 3.5 mass % of Cr. at least one selected from the
group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5 mass % of
W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti and 0.005 to
1.0 mass % of Nb, 0.02 mass % or less of P and 0.01 mass % or less
of S as impurities and the balance being Fe and unavoidable
impurities based on the total mass of the steel sheet and having a
base structure of either lower bainite, tempered lower bainite or
tempered martensite, a prior austenite grain size of 30 .mu.m or
smaller and a tensile strength of 980 MPa or higher.
2-3. (canceled)
4. The ultrahigh strength steel sheet according to claim 1, wherein
the steel sheet further comprises at least one of 0.1 to 3.0 mass %
of Cu and 0.1 to 3.0 mass % of Ni based on the total mass of the
steel sheet.
5. The ultrahigh strength steel sheet according to claim 1, wherein
the steel sheet further comprises at least one of 0.01 to 2.5 mass
% of Si and 0.1 to 1.0 mass % of Mn based on the total mass of the
steel sheet.
6. The ultrahigh strength steel sheet according to claim 1, wherein
the steel sheet further comprises 0.001 to 0.1 mass % of Al based
on the total mass of the steel sheet.
7. The ultrahigh strength steel sheet according to claim 1, wherein
the steel sheet has an average prior austenite grain size of 3 to
10 .mu.m.
8. The ultrahigh strength steel sheet according to claim 1, wherein
the steel sheet is either a hot rolled steel sheet or a cold rolled
steel sheet.
9. The ultrahigh strength steel sheet according to claim 1, wherein
the steel sheet is surface treated by zinc plating.
10. The ultrahigh strength steel sheet according to claim 1,
wherein the steel sheet is treated by film lamination.
11. An automotive strength part using the ultrahigh strength steel
sheet according to claim 1.
12. The automotive strength part according to claim 11, wherein the
automotive strength part is formed by subjecting the ultrahigh
strength steel sheet to any of press molding process, hydroform
process and blow molding process
13. The automotive strength part according to claim 11, wherein the
automotive strength part has a cut-processed portion.
Description
TECHNICAL FIELD
[0001] The present invention relates to an ultrahigh strength steel
sheet having good moldability and high delayed fracture resistance
and an automotive strength part using the ultrahigh strength steel
sheet.
BACKGROUND OF THE INVENTION
[0002] In order to achieve vehicle body weight reductions for
compatibility between vehicle collision safety and environmental
concern, there has recently been a growing attempt to apply
ultrahigh strength steel sheets to intricate press-molded parts
such as front side members, rear side members, rockers and pillars.
It is thus desired to improve the moldability of the ultrahigh
strength steel sheets.
[0003] Although the material strength of the ultrahigh strength
steel sheets can be secured by various strengthening techniques,
the workability of the ultrahigh strength steel sheets
significantly decreases with increase in strength due to structural
heterogeneity, local hard/soft phase coexistence and the like so
that it is difficult for the ultrahigh strength steel sheets to
combine both of high strength and moldability under the current
circumstances. Further, the ultrahigh strength steel sheets face
another problem of delayed fracture due to hydrogen embrittlement
when the strength of the ultrahigh strength steel sheets becomes
1180 MPa or higher.
[0004] Against the above backdrop, attention is being given to TRIP
(Transformation Induced Plasticity) steel sheets as high strength
steel sheets having good moldability and showing a large elongation
as a consequence of induced transformation from reduced austenite
to martensite by forming deformation.
[0005] However, Non-Patent Document 1 reports that the delayed
fracture of the TRIP steel sheet gets promoted by deformation
induced transformation of retained austenite.
[0006] Patent Document 1 proposes a high strength steel sheet
having improved delayed facture resistance by the formation of a
deposit of niobium (Nb), but provides no findings about the
moldability of the high strength steel sheet. There has been a
demand for the ultrahigh strength steel sheets to combine both of
moldability and delayed fracture resistance.
[0007] Non-Patent Document 1: [0008] Yamazaki et al., "Effects of
Retained Austenite and deformation on Delayed Fracture Properties
of Ultrahigh Strength Steel Sheets", Iron and Steel, 1997, Vol. 83,
No. 11, P. 66-71
[0009] Patent Document 1: [0010] Japanese Laid-Open Patent
Publication No. 2005-68548
SUMMARY OF THE INVENTION
[0011] The present invention has been made to solve the above prior
art problems. It is an object of the present invention to provide
an ultrahigh strength steel sheet having good moldability and high
delayed fracture resistance and an automotive strength part using
the ultrahigh strength steel sheet.
[0012] As a result of extensive researches, it has been found by
the present inventors that the above problems can be solved by
forming the base structure of the steel sheet from either lower
bainite, tempered lower bainite or tempered martensite and by
decreasing the prior austenite grain size of the steel sheet. The
present invention is based on this finding.
[0013] According to a first aspect of the present invention, there
is provided an ultrahigh strength steel sheet comprising 0.10 to
0.40 mass % of C, 0.01 to 3.5 mass % of Cr, at least one selected
from the group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5
mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti
and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of P and 0.01
mass % or less of S as impurities and the balance being Fe and
unavoidable impurities based on the total mass of the steel sheet
and having a base structure of either lower bainite, tempered lower
bainite or tempered martensite, a prior austenite grain size of 30
.mu.m or smaller and a tensile strength of 980 MPa or higher.
[0014] According to a second aspect of the present invention, there
is provided an automotive strength part using the ultrahigh
strength steel sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a stress-strain diagram of a plate-shaped test
piece under tensile test.
[0016] FIG. 2 is a schematic diagram outlining a deep drawing test
and a method of determining a limiting drawing ratio as a deep
drawability factor.
[0017] FIG. 3 is a schematic diagram outlining a stretch forming
test.
[0018] FIG. 4 is a schematic diagram outlining a hat bending
test.
[0019] FIG. 5 is a schematic diagram showing a wall warp amount
(curvature) as a shape fixability factor.
DETAILED DESCRIPTION
[0020] An ultrahigh strength steel sheet of the present invention
will be first described below. In the following description, all
percentages (%) in concentrations, contents, filling amounts and
the like are by mass unless otherwise specified.
[0021] The ultrahigh strength steel sheet of the present invention
contains molybdenum (Mo), tungsten (W), vanadium (V), titanium
(Ti), niobium (Nb) or any combination thereof. Further, the
ultrahigh strength steel sheet of the present invention has a base
structure of lower bainite, tempered lower bainite or tempered
martensite and a prior austenite grain size of 30 .mu.m or
smaller.
[0022] The steel sheet attains a tensile strength of 980 MPa or
higher when the base structure of the steel sheet is formed by a
hard phase of lower bainite, tempered lower bainite or tempered
martensite. It is more preferable that the steel sheet has a
tensile strength of 1180 MPa or higher. Generally, the tempered
lower bainite phase is formed, after heating to 1100.degree. C. or
higher, under the manufacturing conditions of a finishing
temperature of 850.degree. C. or higher, a rolling draft of 30% or
higher and a holding temperature of 300 to 500.degree. C. and under
the tempering conditions of a tempering temperature of 400 to
700.degree. C. The tempered martensite phase is generally formed,
after heating to 1100.degree. C. or higher, under the manufacturing
conditions of a finishing temperature of 850.degree. C. or higher,
a rolling draft of 30% or higher and a holding temperature of 150
to 300.degree. C. and under the tempering conditions of a tempering
temperature of 550 to 700.degree. C.
[0023] The prior austenite grain size of the steel sheet is
controlled to within a small grain size range of 1 to 30 .mu.m. The
steel sheet cannot expect improvements in deep drawability, stretch
formability and shape fixability when the prior austenite grain
size exceeds 30 .mu.m. When the prior austenite grain size is less
than 1 .mu.m, the steel sheet is likely to deteriorate in
mechanical properties and to be difficult to manufacture. It is
particularly desirable to control the prior austenite grain size of
the steel sheet to within 3 to 10 .mu.m in order that the steel
sheet obtains further improvements in deep drawability, stretch
formability and shape fixability and thereby attains sufficient
moldability required for automotive part molding.
[0024] The composition of the ultrahigh strength steel sheet will
be now described below in more detail.
[0025] The ultrahigh strength steel sheet of the present invention
comprises 0.10 to 0.40% carbon (C), 0.01 to 3.5% chromium (Cr), at
least one selected from the group consisting of 0.10 to 2.0%
molybdenum (Mo), 0.20 to 1.5% tungsten (W), 0.002 to 1.0% vanadium
(V), 0.002 to 1.0% titanium (Ti) and 0.005 to 1.0% niobium (Nb),
0.02% or less phosphorus (P) and 0.01% or less sulfur (S) as
impurities and the balance substantially being iron (Fe) and
unavoidable impurities based on the total mass of the steel sheet.
It is preferable that the ultrahigh strength steel sheet contains
either one or both of 0.1 to 3.0% copper (Cu) and 0.1 to 3.0%
nickel (Ni) as an additive component or components. It is also
preferable that the ultrahigh strength steel sheet contains either
one or both of 0.01 to 2.5% silicon (Si) and 0.1 to 1.0% manganese
(Mn) as an additive component or components. Preferably, the
ultrahigh strength steel sheet further contains 0.001 to 0.1%
aluminum (Al) as an additive component.
[0026] With the above composition, the steel sheet is able to not
only secure good moldability but also attain high delayed fracture
resistance by formation of fine alloy carbide.
[0027] Carbon (C) is the most effective element for increasing the
strength of the steel sheet. In order for the steel sheet to attain
a strength of 980 MPa or higher, it is desirable that the C content
of the steel sheet is 0.10% or higher. When the C content of the
steel sheet exceeds 0.4%, however, the steel sheet is likely to
decrease in toughness. The C content of the steel sheet is thus
controlled to within 0.10 to 0.40%.
[0028] Chromium (Cr) is an effective element for improving the
hardenability of the steel sheet and increasing the strength of the
steel sheet by dissolving in cementite. It is desirable that the Cr
content of the steel sheet is at least 0.01% or higher, more
desirably 1% or higher. When an excessive amount of Cr is added to
the steel sheet, however, it turns out that the effect of the Cr
element becomes saturated and that the steel sheet decreases in
toughness. The upper limit of the Cr content of the steel sheet is
thus set to 3.5%.
[0029] Molybdenum (Mo) is one of the most critical elements to the
ultrahigh strength steel sheet of the present invention and is
effective for not only improving the hardenability of the steel
sheet but also decreasing the grain size of the steel sheet by
formation of alloy carbide and promoting substitution of hydrogen
in the steel sheet. When the Mo content of the steel sheet is less
than 0.10%, the alloy carbide is unlikely to be formed. On the
other hand, Mo is an expensive alloying element. The Mo content of
the steel sheet is thus controlled to within 0.1 to 2.0%.
[0030] As Tungsten (W), vanadium (V), titanium (Ti) and niobium
(Nb) produce the same additive effect as Mo, it suffices that the
steel sheet contains at least one element selected from Mo, W, V,
Ti and Nb in order to secure not only good moldability but also
high delayed fracture resistance. The W content, V content, Ti
content and Nb content of the steel sheet are controlled to within
0.20 to 1.5%, 0.002 to 1.0%, 0.002 to 1.0% and 0.005 to 1.0%,
respectively.
[0031] Phosphorus (P) causes a decrease in the grain boundary
strength of the steel sheet. It is thus desirable to minimize the P
content of the steel sheet. The upper limit of the P content of the
steel sheet is set to 0.02%
[0032] Sulfur (S) also causes a decrease in the grain boundary
strength of the steel sheet. It is thus desirable to minimize the S
content of the steel sheet. The upper limit of the S content of the
steel sheet is set to 0.01%.
[0033] Copper (Cu) is an effective element for strengthening the
steel sheet and contributes to prevention of delayed fracture by
fine deposit thereof. It is desirable that the Cu content of the
steel sheet is 0.1% or more. However, the excessive addition of Cu
results in workability deterioration. The upper limit of the Cu
content of the steel sheet is thus preferably set to 3.0%.
[0034] Nickel (Ni) is an effective element for improving the
hardenability of the steel sheet for sufficient steel sheet
strength and increasing the corrosion resistance of the steel
sheet. When the Ni content of the steel sheet is less than 1%, the
Ni element does not produce a desired effect. When the Ni content
of the steel sheet exceeds 3.0%, the steel sheet deteriorates in
workability. It is thus desirable to control the Ni content of the
steel sheet to within 0.1 to 3.0%.
[0035] Silicon (Si) is an effective element for deoxidation and
strength improvement. It is desirable that the steel sheet contains
0.2% or more Si including some added as a deoxidant and remaining
in the steel sheet. However, the excessive addition of Si results
in toughness deterioration. The upper limit of the Si content of
the steel sheet is thus preferably set to 2.5%.
[0036] Manganese (Mn) is an effective element for strength
improvement. When the Mn content of the steel sheet is less than
0.1, the Mn element is unlikely to produce a desired effect. By
contrast, it turns out that the cosegregation of P and S becomes
promoted and that the steel sheet decreases in toughness when an
excessive amount of Mn is added to the steel sheet. The Mn content
of the steel sheet is thus preferably controlled to within 0.1 to
1.0%.
[0037] Aluminum (Al) is added for deoxidation. When an excessive
amount of Al is added to the steel sheet, however, the amount of
inclusions in the steel sheet increases to cause a deterioration in
workability. The Al content of the steel sheet is thus preferably
controlled to within 0.001 to 0.1%.
[0038] The ultrahigh strength steel sheet of the present invention
can be processed by either hot rolling or cold rolling because of
its good moldability. The thickness of the ultrahigh strength steel
sheet is generally 0.5 to 2.3 mm. In view of the material design,
the ultrahigh strength steel sheet may be surface treated by zinc
plating or treated by film lamination.
[0039] Next, an automotive strength part of the present invention
will be described below.
[0040] The automotive strength part of the present invention is
produced from the above-explained ultrahigh strength steel sheet
and thus combines good moldability and high delayed fracture
resistance. More specifically, the automotive strength part is
produced by subjecting the high strength steel sheet to any of
press forming process (cold press forming, warm press forming, hot
press forming), hydroform process and blow molding process.
[0041] In general, a component part has a high risk of delayed
fracture due to a large residual stress when processed by cutting
e.g. piercing or trimming. Even with such a cut-processed portion,
the automotive strength part of the present invention has less
delayed fracture and thus can be used effectively.
EXAMPLES
[0042] The present invention will be described in more detail with
reference to the following examples. It should be however noted
that the following examples are only illustrative and are not
intended to limit the invention thereto.
Examples 1 to 5 and Comparative Examples 1 to 6
[0043] Steel sheets of Examples 1 to 5 and Comparative Examples 1
to 6 were formed from various steel materials. The compositions of
the steel materials and the manufacturing conditions of the steel
sheets are indicated in TABLES 1 and 2. Each of the steel sheets
was tested for mechanical properties such as tensile strength and
SD (stress decrease after uniform elongation), structure,
moldability and delayed fracture susceptibility by the following
procedures.
TABLE-US-00001 TABLE 1 Steel type Material composition (mass %)
number C Si Mn P S Cu Ni Cr Mo V Ti Nb Al A 0.35 0.2 0.7 0.019
0.013 0.05 0.25 1 0.2 -- -- -- -- B 0.2 0.25 0.45 0.015 0.003 0.05
1 2 0.65 -- -- -- -- C 0.38 0.24 0.45 0.015 0.003 0.05 1 2 0.65 --
-- -- 0.01 D 0.38 0.24 0.45 0.015 0.003 0.05 1 2 0.65 -- -- 0.03
0.01 E 0.18 1.52 0.35 0.008 0.002 0.1 0.05 3.2 0.3 -- -- -- -- F
0.07 0.545 2.415 0.008 0.002 -- -- 0.2 -- -- -- -- 0.038 G 0.07
0.54 2.22 0.007 0.001 -- -- 0.217 -- -- -- -- 0.038 H 0.18 0.2 1.82
0.009 0.002 -- -- -- 0.02 0.05 -- -- -- I 0.08 0.01 0.8 0.015 0.001
-- -- -- 0.02 -- 0.05 -- --
TABLE-US-00002 TABLE 2 Manufacturing conditions Manu- Heating
facturing tempera- Finishing Rolling Cooling Holding condition ture
temperature draft rate temperature number (.degree. C.) (.degree.
C.) (%) (.degree. C./sec) (.degree. C.) 1a 1200 920 0 30 400 2a
1200 920 35 30 400 3a 1200 920 35 30 550
[0044] 1. Mechanical Properties
[0045] (1) Tensile Strength
[0046] The tensile strength was evaluated by preparing a No. 5 test
piece according to JIS Z 2201 and carrying out a tensile test on
the test piece according to JIS Z 2241.
[0047] (2) Stress Decrease (SD)
[0048] FIG. 1 shows a schematic stress-strain diagram of a
plate-shaped test piece, such as a No. 5 test piece or No. 13 test
piece according to JIS Z 2201, under tensile test. The
toughness/ductility was evaluated as "good" when the test piece had
a stress decrease (SD) of 180 MPa or greater on the definition of
the stress decrease (SD) as a difference between tensile strength
(TS) and breaking strength.
[0049] 2. Structure
[0050] (1) Base Structure
[0051] The base structure was evaluated by preparing a test piece,
grinding a cross section of the test piece, etching the cross
section of the test piece with a nital solution and then observing
the cross section of the test piece with a magnification of 100 to
1000 times by optical microscope and with a magnification of 1000
to 5000 times by scanning electron microscope.
[0052] (2) Prior Austenite Grain Size
[0053] The prior austenite grain size was evaluated according to
JIS G0551. The evaluation of the prior austenite grain size was
herein made on the test piece having a base structure of lower
bainite.
[0054] 3. Moldability
[0055] The moldability was rated in three levels: ".largecircle.
(good)", ".DELTA. (ordinary)" and "X (bad)" based on the deep
drawability, stretch formability and shape fixability in view of
the application to intricate press-molded automotive parts. The
deep drawability, stretch formability and shape fixability were
evaluated by the following procedures.
[0056] (1) Deep Drawability
[0057] FIG. 2 outlines a deep drawing test. In the deep drawing
test, the ratio D/d.sub.p between the maximum blank diameter to the
punch diameter was defined as a limiting drawing ratio LDR where D
was the maximum blank diameter at which cylindrical drawing was
accomplished with no fracture and d.sub.p was the punch diameter.
Herein, a test tool unit was used including a cylindrical punch 4
of 5 mm in punch shoulder radius and 50 mm in diameter d.sub.p, a
die 1 of 7 mm in die shoulder radius and a wrinkle suppressor 2 in
such a manner as to move the punch 4 at a speed of 3 mm/sec with 50
kN of pressure applied to the wrinkle suppressor 2. The maximum
blank diameter D was measured by preparing test pieces 3 from the
steel sheet of each example, subjecting the test pieces 3 to deep
drawing with increasing blank diameters, and then, determining the
blank diameter at which the test piece was completely drawn with no
fracture as the maximum blank diameter D. The limiting drawing
ratio LDR was calculated as the ratio D/50 between the maximum
blank diameter and the punch diameter. The deep drawability was
evaluated as "good" for a larger LDR value.
[0058] (2) Stretch Formability
[0059] FIG. 3 outlines a stretch forming test. In the stretch
forming test, the drawing height immediately before the occurrence
of fracture during spherical-head stretch forming was defined as a
limiting drawing height LDH. Herein, a test tool unit was used
including a spherical-headed punch 4 of 50 mm in radius, a beaded
die 1 of 5 mm in die shoulder radius and a wrinkle suppressor 2 in
such a manner as to move the punch 4 at a speed of 10 mm/sec with
high pressure applied to the wrinkle suppressor 2 to avoid material
inflow from the surroundings. A test piece 3 of 200 mm.times.200 mm
was prepared from the steel sheet of each example. The moving
distance from the point of contact between the test piece 3 and the
punch 4 to the point immediately before the fracture was measured
as the limiting drawing height LDH. The stretch formability was
evaluated as "good" for a larger LDH value.
[0060] (3) Shape Fixability
[0061] FIG. 4 outlines a hat bending test for evaluating a shape
fixability factor. Herein, a test tool unit was used including a
punch 4 of 75 mm in width and 5 mm in punch shoulder radius, a die
1 of 5 mm in die shoulder radius and a wrinkle suppressor 2 in such
a manner as to move the punch 4 by 80 mm at a speed of 10 mm/sec
with 200 kN of pressure applied to the wrinkle suppressor 2. A test
piece 3 of 300 mm.times.50 mm was prepared from the steel sheet of
each example. After subjecting the test piece 3 to hat bending, the
test piece 3 was taken out of the test unit. The curvature of the
test piece 3 was then measured in the manner shown in FIG. 5. The
shape fixability was evaluated as "good" for a larger curvature
value.
[0062] 4. Delayed Fracture Susceptibility
[0063] The delayed fracture resistance was evaluated as
".largecircle. (not cracked)" or "X (cracked)" by preparing a strip
test piece of 100 mm.times.50 mm from the steel sheet of each
example, bending the test piece by a hat bending test machine,
unbending the test piece, subjecting a wall section of the test
piece to piercing, immersing the test piece in a 0.1 mol/m.sup.3
aqueous hydrochloric acid solution for 100 hours, and then,
examining the occurrence or nonoccurrence of a crack in the test
piece.
[0064] The evaluation results are indicated in TABLE 3.
TABLE-US-00003 TABLE 3 Mechanical properties Structure Steel type -
Tensile Prior .gamma. Moldability Delayed Manufacturing strength SD
Base grain size TS .times. LDR TS .times. LDH Curvature/TS Total
fracture conditions (Mpa) (MPa) structure (.mu.m) (MPa) (MPa mm)
(mm.sup.-1 MPa.sup.-1) evaluation resistance Example 1 A-2a 1202
284 LB 6.3 2212 46878 2.53 .times. 10.sup.-6 .largecircle.
.largecircle. Example 2 B-2a 1247 372 LB 5.2 2469 47635 3.06
.times. 10.sup.-6 .largecircle. .largecircle. Example 3 C-2a 1385
218 LB 9.5 2078 50137 3.48 .times. 10.sup.-6 .largecircle.
.largecircle. Example 4 D-2a 1423 255 LB 7.3 2191 52366 3.14
.times. 10.sup.-6 .largecircle. .largecircle. Example 5 E-2a 1329
208 LB 10 2445 50143 3.42 .times. 10.sup.-6 .largecircle.
.largecircle. Comparative C-1a 1201 69 LB 80.3 1561 37351 4.59
.times. 10.sup.-6 X .largecircle. Example 1 Comparative C-3a 1480
58 UB -- 1776 44548 5.45 .times. 10.sup.-6 X X Example 2
Comparative F 1041 149 F -- 1915 38309 4.95 .times. 10.sup.-6
.DELTA. .largecircle. Example 3 Comparative G 832 122 F -- 1514
28704 3.63 .times. 10.sup.-6 X .largecircle. Example 4 Comparative
H 968 93 F -- 1897 35235 5.01 .times. 10.sup.-6 X .largecircle.
Example 5 Comparative I 1019 156 F -- 1956 37805 1.96 .times.
10.sup.-6 .DELTA. .largecircle. Example 6 [Note] F: Ferrite, UB:
Upper bainite, LB: Lower bainite
[0065] As indicated in TABLE 3, the ultrahigh strength steel sheets
of Examples 1 to 5 had a tensile strength of 980 MPa or higher and
showed sufficient deep drawability, stretch formability and shape
fixability to satisfy the requirements for automotive parts. No
crack occurred in the ultrahigh strength steel sheets of Examples 1
to 5 in the delayed fracture test. It can be thus concluded that
the ultrahigh strength steel sheets of Examples 1 to 5 combined
moldability and delayed fracture resistance. By contrast, the steel
sheets of Comparative Examples 1 and 2 had a tensile strength of
980 MPa but did not combine moldability and delayed fracture
resistance as the prior austenite grain size of the steel sheet of
Comparative Example 1 and the base structure of the steel sheet of
Comparative Example 2 were out of the scope of the present
invention. The base structures and compositions of the steel sheets
of Comparative Examples 3 to 6 (commercial products) were out of
the scope of the present invention. Some of the steel sheets of
Comparative Examples 3 to 6 had a tensile strength of less than 980
MPa. The steel sheets of Comparative Examples 3 to 6 had no problem
in delayed fracture resistance but were inferior in moldability to
those of Examples 1 to 5.
Examples 6 to 10 and Comparative Examples 7 and 8
[0066] Steel sheets of Examples 6 to 10 and Comparative Examples 7
and 8 were formed using various steel materials of TABLE 1 under
the manufacturing/tempering conditions of TABLE 4. Each of the
steel sheets was tested for mechanical properties such as tensile
strength and SD (stress decrease after uniform elongation),
structure, moldability and delayed fracture resistance in the same
manners as above. The evaluation of the prior austenite grain size
was herein made on the steel sheet having a base structure of
tempered lower bainite.
TABLE-US-00004 TABLE 4 Manufacturing conditions Manufacturing
Heating Finishing Rolling Cooling Holding condition temperature
temperature draft rate temperature Tempering number (.degree. C.)
(.degree. C.) (%) (.degree. C./sec) (.degree. C.) conditions 1b
1200 920 0 30 400 500.degree. C. .times. 1 hr 2b 1200 920 35 30 400
500.degree. C. .times. 1 hr 3b 1200 920 35 30 550 500.degree. C.
.times. 1 hr
[0067] The evaluation results are indicated in TABLE 5.
TABLE-US-00005 TABLE 5 Mechanical properties Structure Steel type -
Tensile Prior .gamma. Moldability Delayed Manufacturing strength SD
Base grain size TS .times. LDR TS .times. LDH Curvature/TS Total
fracture conditions (Mpa) (MPa) structure (.mu.m) (MPa) (MPa mm)
(mm.sup.-1 MPa.sup.-1) evaluation resistance Example 6 A-2b 1185
280 YLB 5.9 2252 47400 2.13 .times. 10.sup.-6 .largecircle.
.largecircle. Example 7 B-2b 1222 332 YLB 6.1 2493 48269 2.48
.times. 10.sup.-6 .largecircle. .largecircle. Example 8 C-2b 1337
287 YLB 8.8 2567 51033 2.65 .times. 10.sup.-6 .largecircle.
.largecircle. Example 9 D-2b 1365 295 YLB 7.1 2648 53235 2.56
.times. 10.sup.-6 .largecircle. .largecircle. Example 10 E-2b 1275
223 YLB 10 2346 48106 3.12 .times. 10.sup.-6 .largecircle.
.largecircle. Comparative C-1b 1130 93 YLB 88.5 1537 36160 4.83
.times. 10.sup.-6 X .largecircle. Example 7 Comparative C-3b 1413
69 YUB -- 1766 43803 5.18 .times. 10.sup.-6 X X Example 8 [Note] F:
Ferrite, YUB: Tempered upper bainite, YLB: Tempered lower
bainite
[0068] As indicated in FIG. 5, the ultrahigh strength steel sheets
of Examples 6 to 10 had a tensile strength of 980 MPa or higher and
showed sufficient deep drawability, stretch formability and shape
fixability to satisfy the requirements for automotive parts. No
crack occurred in the ultrahigh strength steel sheets of Examples 6
to 10 in the delayed fracture test. It can be thus concluded that
the ultrahigh strength steel sheets of Examples 6 to 10 combined
moldability and delayed fracture resistance. By contrast, the steel
sheets of Comparative Examples 7 and 8 had a tensile strength of
980 MPa but did not combine moldability and delayed fracture
resistance as the prior austenite grain size of the steel sheet of
Comparative Example 7 and the base structure of the steel sheet of
Comparative Example 8 were out of the scope of the present
invention. The above-mentioned steel sheets of Comparative Examples
3 to 6 (commercial products) were also inferior in moldability to
the steel sheets of Examples 6 to 10.
Examples 11 to 15 and Comparative Example 9
[0069] Steel sheets of Examples 11 to 15 and Comparative Example 9
were formed using various steel materials of TABLE 1 under the
manufacturing/tempering conditions of TABLE 6. Each of the steel
sheets was tested for mechanical properties such as tensile
strength and SD (stress decrease after uniform elongation),
structure, moldability and delayed fracture resistance in the same
manners as above. The evaluation of the prior austenite grain size
was herein made on the steel sheet having a base structure of
tempered martensite.
TABLE-US-00006 TABLE 6 Manufacturing conditions Manufacturing
Heating Finishing Rolling Cooling Holding condition temperature
temperature draft rate temperature Tempering number (.degree. C.)
(.degree. C.) (%) (.degree. C./sec) (.degree. C.) conditions 1c
1200 920 0 30 250 600.degree. C. .times. 1 hr 2c 1200 920 35 30 250
600.degree. C. .times. 1 hr
[0070] The evaluation results are indicated in TABLE 7.
TABLE-US-00007 TABLE 7 Mechanical properties Structure Steel type -
Tensile Prior .gamma. Moldability Delayed Manufacturing strength SD
Base grain size TS .times. LDR TS .times. LDH Curvature/TS Total
fracture conditions (Mpa) (MPa) structure (.mu.m) (MPa) (MPa mm)
(mm.sup.-1 MPa.sup.-1) evaluation resistance Example 11 A-2c 1212
219 YM 4.9 2097 47268 1.77 .times. 10.sup.-6 .largecircle.
.largecircle. Example 12 B-2c 1193 297 YM 4.5 2434 42948 1.63
.times. 10.sup.-6 .largecircle. .largecircle. Example 13 C-2c 1266
372 YM 6.9 2481 43614 1.66 .times. 10.sup.-6 .largecircle.
.largecircle. Example 14 D-2c 1284 329 YM 7.5 2542 44940 1.62
.times. 10.sup.-6 .largecircle. .largecircle. Example 15 E-2c 1254
223 YM 10 2307 47313 2.53 .times. 10.sup.-6 .largecircle.
.largecircle. Comparative C-1c 1230 263 YM 82.5 1845 39360 4.98
.times. 10.sup.-6 X X Example 9 [Note] F: Ferrite, YM: Tempered
martensite
[0071] As indicated in FIG. 7, the ultrahigh strength steel sheets
of Examples 11 to 15 had a tensile strength of 980 MPa or higher
and showed sufficient deep drawability, stretch formability and
shape fixability to satisfy the requirements for automotive parts.
No crack occurred in the ultrahigh strength steel sheets of
Examples 11 to 15 in the delayed fracture test. It can be thus
concluded that the ultrahigh strength steel sheets of Examples 11
to 15 combined moldability and delayed fracture resistance. By
contrast, the steel sheet of Comparative Example 9 had a tensile
strength of 980 MPa but did not combine moldability and delayed
fracture resistance as the prior austenite grain size of the steel
sheet of Comparative Example 9 was out of the scope of the present
invention. The above-mentioned steel sheets of Comparative Examples
3 to 6 (commercial products) were also inferior in moldability to
the steel sheets of Examples 11 to 15.
[0072] As described above, the steel sheet has the excellent
effects of attaining not only sufficient moldability required for
automotive parts as compared to conventional high strength steel
sheets but also improved delayed fracture resistance, while
securing a tensile strength of 980 MPa, by forming the base
structure of the steel sheet from either lower bainite, tempered
lower bainite or tempered martensite and reducing the prior
austenite grain size of the steel sheet. It is therefore possible
to provide the industrially useful ultrahigh strength steel sheet
having both of moldability and delayed fracture resistance and the
automotive strength part using the ultrahigh strength steel
sheet.
[0073] Although the present invention has been described with
reference to the above specific embodiments of the invention, the
present invention is not limited to the above-described
embodiments. Various modifications and variations of the
embodiments described above will occur to those skilled in the art
in light of the above teaching.
* * * * *