U.S. patent application number 10/505575 was filed with the patent office on 2005-06-09 for steel plate subjected to heat treatment and process for producing the same.
This patent application is currently assigned to JFE Steel Corporation. Invention is credited to Fujita, Takeshi, Hasegawa, Kohei, Urabe, Toshiaki.
Application Number | 20050121119 10/505575 |
Document ID | / |
Family ID | 27800192 |
Filed Date | 2005-06-09 |
United States Patent
Application |
20050121119 |
Kind Code |
A1 |
Hasegawa, Kohei ; et
al. |
June 9, 2005 |
Steel plate subjected to heat treatment and process for producing
the same
Abstract
The present invention provides a steel sheet for heat treatment
consisting essentially of, by mass %, 0.05-0.09% C, below 1% Si,
1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1% sol. Al,
0.005% N or less, 0.0003-0.003% B, Ti in a range satisfying
(48/32)S+(48/14)N.ltoreq.Ti.lto- req.2[(48/32)S+(48/14)N], and the
rest of Fe, wherein the average diameter of iron carbides
precipitating in steel is 2 .mu.m or smaller. The steel sheet for
heat treatment according to the present invention has high strength
and excellent hydrogen embrittlement resistance after being press
formed and quenched, so that the steel sheet is well suited for
automobile construction members.
Inventors: |
Hasegawa, Kohei; (Hiroshima,
JP) ; Fujita, Takeshi; (Chiba, JP) ; Urabe,
Toshiaki; (Chiba, JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER RUDNICK GRAY CARY US LLP
1650 MARKET ST
SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE Steel Corporation
2-3, Uchisaiwai-cho 2-chome
Chiyoda-ku, Tokyo
JP
100-0011
|
Family ID: |
27800192 |
Appl. No.: |
10/505575 |
Filed: |
September 30, 2004 |
PCT Filed: |
February 28, 2003 |
PCT NO: |
PCT/JP03/02300 |
Current U.S.
Class: |
148/602 ;
420/106; 420/120 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/04 20130101; C22C 38/14 20130101; C21D 9/46 20130101 |
Class at
Publication: |
148/602 ;
420/106; 420/120 |
International
Class: |
C22C 038/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2002 |
JP |
2002-63488 |
Claims
1. A steel sheet for heat treatment consisting essentially of, by
mass %, 0.05-0.09% C, below 1% Si, 1.6-2.4% Mn, 0.02% P or less,
0.02% S or less, 0.01-0.1% sol.Al, 0.005% N or less, 0.0003-0.003%
B, Ti satisfying formula (1), and the rest of Fe:
(48/32)S+(48/14)N.ltoreq.Ti.ltoreq.2[(48- /32)S+(48/14)N] (1) (the
content of each element shown by the element symbol in formula (1)
is represented by mass %), wherein the average diameter of iron
carbides precipitating in steel is 2 .mu.m or smaller.
2. The steel sheet for heat treatment according to claim 1, further
comprising at least one element selected from 0.1-2% Cr and 0.1-2%
Mo.
3. A method for manufacturing a steel sheet for heat treatment
comprising the steps of: hot rolling a steel slab having
composition as defined in claim 1 or 2 into a steel sheet; cooling
the hot rolled steel sheet at an average cooling rate of 30.degree.
C./s or less; and coiling the cooled steel sheet at a coiling
temperature of 500.degree. C. or above.
4. A method for manufacturing a steel sheet for heat treatment
comprising the steps of: hot rolling a steel slab containing
composition defined in claim 1 or 2 into sa steel sheet; cold
rolling the hot rolled steel sheet; and annealing the cold rolled
steel sheet for recrystallization, wherein the annealed cold rolled
steel sheet is cooled at an average cooling rate of 30.degree. C./s
or less to 400.degree. C. after annealing.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel sheet for heat
treatment that can be steadily imparted with high strength and
excellent hydrogen embrittlement resistance by heat treatment
conducted after forming process such as press forming, and a
manufacturing method thereof.
BACKGROUND ART
[0002] In view of weight reduction for fuel economy and safety for
cabin passengers/crews against an accident and the like, high
strength steel sheets are used in a car as body construction
members, reinforcing members, and various other mechanical
construction components. However, the use of high strength steel
sheets causes various problems such as difficulty of forming
intricately shaped components, and high frequency of occurrence of
brittle fracture, that is, so-called hydrogen embrittlement
(delayed fracture), which is caused by hydrogen-absorption into
steel from an environment.
[0003] Generally, to meet the requirement of formability and high
strength, such a method as described hereunder is employed. After
being formed by, for example, press forming, a steel sheet such as
a cold rolled steel sheet or a hot rolled steel sheet is heated by
induction-heating method or furnace-heating method, and then
subjected to quenching such as water quenching, oil quenching, or
press quenching. Steel sheets of several types suitable for this
method have been developed in the following prior arts.
[0004] According to Japanese Examined Patent Publication No.
3-2942, there is proposed a steel sheet for precise punching which
is excellent in formability in various forming modes and
quench-hardenability after short time and rapid heating. The steel
sheet is made of a Cr and B-added steel containing 0.10-0.19% C and
0.7-1.5% Mn.
[0005] According to Japanese Patent No. 2713382, there is proposed
a method for manufacturing a high strength automobile member
excellent in hydrogen embrittlement resistance. The method is
characterized in that a steel containing 0.2-0.5% C, 0.5-1.6% Mn
and 0.5-1.5% Cr is treated with a lubricant film forming agent,
then formed, and finally subjected to quenching and tempering
treatment.
[0006] According to Japanese Examined Patent Publication No.
7-103420, there is proposed a method for manufacturing a member
using a B-added steel. The method is characterized in that a
B-added steel containing 0.15-0.40% C and 0.60-1.50% Mn is
subjected to cold press forming, then heated at a quenching
temperature of 850.degree. C. to below 950.degree. C., and water
quenched at a quenching intensity of 0.35 cm.sup.-1 to below
1.50cm.sup.-1.
[0007] According to each of Japanese Unexamined Patent Application
Publications No. 5-98356 and No. 5-98357, there is proposed a
method for manufacturing a Ti and B-based high carbon steel sheet
having excellent formability and toughness without tempering
treatment. This method is characterized in that a (Ti) and B-added
steel containing 0.15-0.40% C and 0.6-1.50% Mn is used to inhibit
the precipitation of cementite. Concurrently, B is added to secure
hardenability, and the precipitation of AlN (and TiN) is performed
to inhibit abnormal growth of austenite grains.
[0008] According to Japanese Unexamined Patent Application
Publication No. 6-116679, there is proposed a method for
manufacturing a steel sheet and a safety component thereof against
car collision, the safety component being fabricated by press
quenching. The steel sheet is manufactured in such a manner that a
Ti, Nb and B-added steel containing 0.20-0.40% C and 0.20-0.40% Mn
is heated at a temperature of Ac.sub.1 to (Ac.sub.1+30).degree. C.
for 1 to 20 hours after being hot rolled, and then cooled to a
temperature below (Ac.sub.1+30).degree. C. at a cooling rate of
20.degree. C./s or less. The safety component is fabricated by
press quenching in the following manner. After being formed into a
predetermined shape, the steel sheet is then heated to 850.degree.
C. or above, cooled at a cooling rate of 80 to 150.degree. C./s to
a temperature range between 450 and 500.degree. C. while being kept
in a metal die, and further cooled at a cooling rate of 20 to
100.degree. C./s to an ordinary temperature not exceeding
100.degree. C. Consequently, the component has a tensile strength
of 1150 N/mm.sup.2.
[0009] According to Japanese Unexamined Patent Application
Publication No. 8-269615, there is proposed a hot rolled steel
sheet that can be imparted with wear resistance after being formed
without impairing stretch flangeability. This is accomplished in
such a manner that after being formed, the steel sheet is subjected
to rapid heating such as induction quenching whereby to harden the
surface without cracks. The hot rolled steel sheet consists of, by
weight, 0.18-0.30% C, 0.01-1.0% Si, 0.2-1.5% Mn, 0.1-0.5% Cr,
0.0006-0.0040% B, 0.03% P or less, 0.02% S or less, 0.08% sol.Al or
less, and 0.01% N or less, and balance Fe with inevitable
impurities. The steel sheet has a mixed structure of ferrite and
bainite.
[0010] According to Japanese Unexamined Patent Application
Publication No. 10-96031, there is proposed a method for
manufacturing a high carbon hot rolled steel sheet and a high
carbon cold rolled steel sheet that are each excellent in ductility
before being quenched and that are each capable of having a
predetermined hardness and toughness after being quenched.
According to the method, a Cr, Ti and B-added hot rolled steel
sheet containing 0.25-0.65% C and 0.20-0.40% Mn is heated at a
temperature of 650.degree. C. to below Ac.sub.1 for 10 to 30 hours,
or is slowly cooled to (Ac.sub.1-30).degree. C. at a cooling rate
of 3 to 20.degree. C./h or to (Ac.sub.1-20).degree. C. at a cooling
rate of 3 to 10.degree. C./h after being heated at a temperature of
Ac.sub.1 to (Ac.sub.1+30).degree. C. for 1 to 20 hours, followed,
by necessary, by being cold rolled at a reduction rate of 30 to 70%
and heated at a temperature of 650.degree. C. to below Ac.sub.1 for
20 seconds or more.
[0011] According to Japanese Unexamined Patent Application
Publication No. 10-147816, there is proposed a method for
manufacturing a high carbon steel sheet that is excellent in
formability and that is capable of having sufficient strength after
being formed and heat treated. The method is characterized in that
a Cr, Ti and B-added steel containing 0.25-0.45% C and 0.2-0.5% Mn
is hot rolled, coiled at a temperature between 550 and 600.degree.
C., pickled, heated in an atmosphere of 95vol.% or more of hydrogen
at a temperature between Ac.sub.1 and (Ac.sub.1+30).degree. C. for
1 to 10 hours, slowly cooled to (Ar.sub.1-50).degree. C. or less at
a cooling rate of 3 to 20.degree. C./h. Alternatively, the steel
sheet is further cold rolled and annealed at a temperature of
(Ac.sub.1-10).degree. C. or less.
[0012] According to Japanese Unexamined Patent Application
Publication No. 10-251757, there is proposed a method for
manufacturing a high carbon steel sheet that is excellent in
formability and that is capable of having sufficient strength
through a post forming heat treatment. The method is characterized
in that a Cr, Ti and B-added steel containing 0.25-0.45% C and
0.2-0.5% Mn is hot rolled at a finishing temperature between
(Ar.sub.3+20) and (Ar.sub.3+50).degree. C., coiled at a temperature
between 550 and 600.degree. C., pickled, heated in an atmosphere of
95vol.% or more of hydrogen at a temperature between Ac.sub.1 and
(Ac.sub.1+30).degree. C. for 1 to 10 hours, and slowly cooled to
(Ar.sub.1-50).degree. C. or less at a cooling rate of 3 to
20.degree. C./h.
[0013] According to Japanese Unexamined Patent Application
Publication No. 10-60522, there is proposed a steel sheet having
excellent formability that can be imparted with sufficiently high
strength through melting and rapid solidification using high
density energy irradiation such as laser irradiation. The steel
sheet is characterized by comprising 0.04-0.3% C and 3% Mn or less,
and by receiving high density energy irradiation for a time that
satisfies a predetermined formula, the bead pitch of high density
energy irradiation being larger than 1 mm.
[0014] According to Japanese Unexamined Patent Application
Publication No. 2000-144319, a steel sheet and a manufacturing
method therefor are proposed, the steel sheet having sufficient
formability adaptable for body construction members of car and high
strength through post forming quenching. The steel sheet is made of
a Ti and B-added steel containing 0.05-0.20% C, 0.8-2.0% Mn, and Ti
in a range of 3.4.times.N(%) or less. The steel sheet is
manufactured in such a manner that a steel slab having the
aforementioned composition is hot rolled, and coiled at a coiling
temperature of 600.degree. C. or above. Alternatively, the hot
rolled steel sheet is coiled at a coiling temperature of
480.degree. C. or above, followed by being cold rolled and
annealed.
[0015] However, the above prior arts have problems described
hereunder.
[0016] In Patent Publication No. 3-2942, since the C content is
high, hydrogen embrittlement resistance after quenching is not
excellent. In addition, since the Mn content is as low as 0.7-1.5%,
high strength after quenching can not be steadily obtained.
[0017] In Patent No. 2713382, since the C content is high, hydrogen
embrittlement resistance after quenching is not excellent. Since
the Mn content is as low as 0.5-1.6.%, high strength after
quenching can not be steadily obtained. Further, since tempering is
essentially required, manufacturing cost (heat treatment cost) is
increased.
[0018] Similar problems arise in any of Japanese Examined Patent
Publication No. 7-103420 and Japanese Unexamined Patent Application
Publications No. 5-98356, No. 6-116679, No. 8-269615, No. 10-96031,
No. 10-147816, and No. 10-251757. That is, since the C content is
high, hydrogen embrittlement resistance is not sufficient, and
since the Mn content is low, high strength after quenching can not
be steadily obtained.
[0019] In Japanese Examined Patent Publication No. 10-60522, in the
quenching by high density energy irradiation such as laser
irradiation, heating is performed restrictively to a narrow linear
range. As such, high strength is given to a limited portion, and
thus high strength of the overall component can not be obtained.
Further, since the bead pitch in the linearly heated portion is
required to be larger than 1 mm, the entire surface cannot be
uniformly quenched.
[0020] In Patent Publication No. 2000-144319, the C and Mn content
ranges are so wide that high strength after quenching can not be
steadily obtained. Excellent hydrogen embrittlement resistance can
not be obtained either.
DISCLOSURE OF THE INVENTION
[0021] An object of the present invention is to provide a steel
sheet for heat treatment that can be steadily imparted with high
strength and excellent hydrogen embrittlement resistance by heat
treatment conducted after forming process such as press forming,
and a manufacturing method thereof.
[0022] The object is achieved by providing a steel sheet for heat
treatment consisting essentially of, by mass %, 0.05-0.09% C, 1% Si
or less, 1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1%
sol.Al, 0.005% N or less, 0.0003-0.003% B, Ti satisfying formula
(1), and the balance of Fe, wherein the average diameter of iron
carbides precipitating in the steel is 2 .mu.m or smaller.
(48/32)S+(48/14)N.ltoreq.Ti.ltoreq.2[(48/32)S+(48/14)N] (1)
[0023] In formula (1), each element symbol represents the content
of each element, by mass %.
[0024] A steel sheet for heat treatment according to the present
invention can be manufactured by a method comprising the steps of:
hot rolling a steel slab having the aforementioned composition into
a steel sheet; cooling the hot rolled steel sheet at an average
cooling rate of 30.degree. C./s or less; and coiling the cooled hot
rolled steel sheet at a coiling temperature of 500.degree. C. or
above.
EMBODIMENTS OF THE INVENTION
[0025] The present inventors conducted study and research on a
steel sheet that can be steadily imparted with high strength and
excellent high hydrogen embrittlement resistance by heat treatment
conducted after forming process such as press forming.
Consequently, it is found that reduction in C content, addition of
B, and control of iron carbides are effective. This will be
described in more detail below.
[0026] 1. Composition
[0027] C: C is an important element to enhance strength of steel
sheet by heat treatment. C should be added in an amount of 0.05% or
more to impart sufficiently high strength to steel sheet. On the
other hand, however, when C content exceeds 0.09%, hydrogen
embrittlement resistance after heat treatment is deteriorated. In
view of these facts, C content is specified to 0.05-0.09%.
[0028] Si: Si can be appropriately added by necessity. However, Si
content exceeding 1% not only deteriorates chemical conversion
treatability, but also leads to an increase in manufacturing cost.
In view of these facts, Si content is specified to 1% or less.
[0029] Mn: Mn is an essential element to steadily impart high
strength independently of heat treatment conditions such as soaking
temperature, holding time and cooling rate. Mn content less than
1.6% can not sufficiently stabilize hardenability of steel sheet;
and on the other hand, Mn content exceeding 2.4% deteriorates press
formability of steel sheet. For these reasons, Mn content is
specified to 1.6-2.4%.
[0030] P: P is an impurity in steel. P content exceeding 0.02%
deteriorates formability and weldability of steel sheet, so that P
content is specified to 0.02% or less. Although P should be
preferably removed as much as possible in steelmaking process, too
much reduction of P content leads to an increase in manufacturing
cost.
[0031] S: S is an impurity in steel. S content exceeding 0.02%
deteriorates formability and weldability of steel sheet, so that S
content is specified to 0.02% or less. Although S should be
preferably removed as much as possible in steelmaking process, too
much reduction of S content leads to an increase in manufacturing
cost.
[0032] Sol.Al: Al is added as a deoxidizing agent and for
precipitating N in the form of AlN. While sol.Al content less than
0.01% is not sufficiently effective, sol.Al content exceeding 0.1%
saturates the effect, leading to an increase in manufacturing cost.
For these reasons, sol.Al content is specified to 0.01-0.1%.
[0033] N: N is an impurity in steel. N content exceeding 0.005%
deteriorates formability of steel sheet, so that N content is
specified to 0.005% or less. Although N should be preferably
removed as much as possible in steelmaking process, too much
reduction of N content leads to an increase in manufacturing
cost.
[0034] Ti: Ti combines with N in the form of TiN and thus prevents
B from precipitating in the form of BN whereby enhancing the effect
of B. Ti generates sulfides while a steel slab is being cooled
after heating in advance of nitrides generation, so that, to
completely delete solute N, Ti should be added in an amount greater
than or equal to the atomic equivalents of N and S, that is,
greater than or equal to (48/32)S+(48/14)N. On the other hand,
however, when Ti is excessively added in an amount exceeding twice
the atomic equivalents, TiC precipitates, so that the formability
of steel sheet is deteriorated. For these reasons, Ti content is
specified to the range of from (48/32)S+(48/14)N to
2[(48/32)S+(48/14)N]%.
[0035] B: B should exist in the form of solute B in steel so as to
steadily obtain high strength independently of heat treatment
conditions such as soaking temperature, holding time and cooling
rate. B content less than 0.0003% does not sufficiently exhibit
this effect. On the other hand, B content exceeding 0.003% not only
saturates the effect of B, but also reduces productivity in steel
sheet manufacturing process. For these reasons, B content is
controlled to 0.0003 to 0.003%.
[0036] The strength enhancement of steel sheet can be more steadily
implemented when at least one element selected from 0.1-2% Cr and
0.1-2% Mo is added in addition to the composition described above.
The content of Cr and Mo is each specified to 0.1-2% for the reason
that the content of 0.1% or less is insufficient to steadily
implement the strength enhancement, whereas the content exceeding
2% deteriorates the formability of steel sheet.
[0037] While the rest is essentially Fe, small amounts of
inevitable impurities and other elements may be included within a
range that does not disturb the advantage of the present
invention.
[0038] 2. Iron Carbides
[0039] The average diameter of iron carbides precipitating in steel
influences the dissolution of the iron carbides at heat treatment.
The average diameter should be controlled to 2 .mu.m or smaller so
that the iron carbides can be dissolved into austenite in a very
short time, leading to high strength after quenching.
[0040] 3. Manufacturing Method
[0041] The steel sheet for heat treatment of the present invention
can be manufactured by a method for manufacturing a steel sheet for
heat treatment comprising the steps of: hot rolling a steel slab
having the above-described composition into a steel sheet; cooling
the hot rolled steel sheet at an average cooling rate of 30.degree.
C./s or less; and coiling the cooled steel sheet at a coiling
temperature of 500.degree. C. or higher.
[0042] In the above, the hot rolled steel sheet is cooled at the
average cooling rate of 30.degree. C./s or less for the reason that
an average cooling rate exceeding 30.degree. C./s generates second
phases that deteriorate the formability of steel sheet. For the
same reason, the coiling temperature is set to 500.degree. C. or
higher.
[0043] Alternatively, the steel sheet for heat treatment of the
present invention can be manufactured by a method for manufacturing
a steel sheet for heat treatment comprising the steps of: hot
rolling a steel slab having the above-described composition into a
steel sheet; cold rolling the hot rolled steel sheet; and annealing
the cold rolled steel sheet for recrystallization, wherein the
annealed steel sheet is cooled at an average cooling rate of
30.degree. C./s or less to 400.degree. C.
[0044] In the above, the annealed steel sheet is cooled at the
average cooling rate of 30.degree. C./s or less to 400.degree. C.
for the reason that generation of second phases is inhibited not to
deteriorate the formability of steel sheet.
[0045] In the present invention, heating temperature of steel slab
prior to hot rolling should be preferably controlled to
1200-1250.degree. C. from the viewpoint of enhancing the
formability. Finishing temperature at hot rolling should be
preferably controlled to Ar.sub.3-890.degree. C. from the viewpoint
of making the ferrite structure to be uniform and fine. When
improving flatness of hot rolled steel sheet and deleting yield
point elongation to enhance formability of hot rolled steel sheet,
temper rolling should be preferably conducted at an elongation rate
of 0.3-1.5% after coiling.
[0046] Even when manufacturing a cold rolled steel sheet, the
above-described hot rolling conditions, that is, the slab heating
temperature controlled to 1200-1250.degree. C. and the finishing
temperature controlled to Ar.sub.3-890.degree. C., should be
preferably taken from the same viewpoint. Further, also the cooling
rate after hot rolling should be preferably controlled to
30.degree. C./s or less for a reason that when the average cooling
rate from hot rolling final pass to coiling exceeds 30.degree.
C./s, second phases are generated whereby to reduce
manufacturability.
[0047] Reduction rate at cold rolling should be preferably
controlled to 60% or greater to obtain fine iron carbides having an
average diameter of 2 .mu.m or smaller, which are essential to the
present invention. From the viewpoint of formability, annealing
temperature should be preferably controlled to 670-720.degree. C.
at box annealing and to 690-730.degree. C. or 800-850.degree. C. at
continuous annealing. When improving flatness of cold rolled steel
sheet and deleting yield point elongation to enhance formability of
cold rolled steel sheet, temper rolling should be preferably
conducted at an elongation rate of 0.3-1.5% after annealing.
EXAMPLE 1
[0048] Slabs were cast after vacuum melting of steels 1-14 having
composition shown in Table 1. After reheated at 1250.degree. C.,
the slabs were each hot rolled at a finishing temperature of
870.degree. C. into hot rolled steel sheets. The hot rolled steel
sheets were each cold rolled to 1.2 mm, and subjected to
720.degree. C..times.2 min. annealing simulating continuous
annealing. Thus produced cold rolled steel sheets 1-14 were cooled
at an average cooling rate of 10.degree. C./s, and temper rolled at
an elongation rate of 1.5%. Further, the cold rolled steel sheets
13 and 14 were each heat treated at 600.degree. C. to control the
carbide diameter.
[0049] JIS No. 5 tensile test pieces were taken from the cold
rolled steel sheets in the direction rectangular to the rolling
direction (i.e., width direction) to measure mechanical
properties.
[0050] Then, for these cold rolled steel sheets, tensile strengths
were measured after quenching performed under the following three
conditions:
[0051] Condition 1: Water quenching after 1000.degree. C..times.5
min. heating
[0052] Condition 2: Water quenching after 1000.degree. C..times.5
min. heating and air cooling to 800.degree. C.
[0053] Condition 3: Water quenching after 900.degree. C..times.5
sec. heating
[0054] Condition 1 is an ideal solution treatment and quenching
condition. Condition 2 is a condition for delayed quenching after
solution treatment. Condition 3 is a condition simulating low
temperature and short time solution treatment such as quenching
after induction-heating. For a steel sheet for heat treatment
according to the present invention, it is preferable that high
strength can be steadily obtained after quenching under any of the
conditions 1 to 3.
[0055] Further, 30.times.100 mm rectangular test pieces were cut
out from the cold rolled steel sheets quenched under the condition
1, and bent to 180.degree. at a radius of 10 mmR, being U-shaped.
Then, the U-shaped test pieces were tightened with bolts at both
ends of the test piece by a force corresponding to spring back, and
immersed into a 0.1 N hydrochloric acid solution to measure time
until cracking occurs. In this manner, the hydrogen embrittlement
resistance was investigated. A criterion for excellent hydrogen
embrittlement resistance is no crack occurring in at least 30 days
(delayed fracture time: at least 30 days).
[0056] Table 2 shows mechanical properties, tensile strengths after
quenching, and delayed fracture time.
[0057] Any of steel sheets 2, 7, and 11 to 13 has a high ductility
(El) leading to excellent formability, a tensile strength of 1200
MPa or higher after quenching independently of the quenching
conditions, and 30 days or longer delayed fracture time leading to
excellent hydrogen embrittlement resistance.
[0058] In comparison, steel sheet 1 of comparative example has C
content less than the present invention range, so that the tensile
strength after quenching is insufficient. Steel sheet 3 has C
content greater than the invention range, so that the delayed
fracture time is as short as three days resulting in poor hydrogen
embrittlement resistance. In addition, the sheet 3 has an average
carbide diameter exceeding 2 .mu.m, so that the tensile strength
after quenching is insufficient at low temperature and short time
solution treatment under the condition 3. Steel sheet 4 has Mn
content less than the invention range, so that the tensile strength
after quenching is insufficient under the condition 2. Steel sheet
5 has Mn content greater than the invention range, so that the
ductility is low, hence offering poor formability. Steel sheet 6
has Ti content less than the invention range, so that the tensile
strength after quenching is insufficient under the condition 2.
Steel sheet 8 has Ti content greater than the invention range, so
that the steel sheet has low ductility, hence offering poor
formability. Steel sheet 9 has B content less than the invention
range, so that the steel sheet has insufficient tensile strength
after quenching under the condition 2. Steel sheet 10 has B content
greater than the invention range, so that the steel sheet has low
ductility, hence offering poor formability. Steel sheet 14 has an
average carbide diameter exceeding 2 .mu.m, so that the steel sheet
has insufficient tensile strength at low temperature and short time
heat treatment under the condition 3.
1 TABLE 1 Chemical composition (mass %) Steel C Si Mn P S sol. Al N
Ti B Cr Mo (48/32)S + (48/14)N 2[(48/32)S + (48/14)N] Remarks 1
0.035 0.03 2.0 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01
<0.01 0.021 0.041 Comparative example 2 0.065 0.03 1.9 0.015
0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041
Inventive example 3 0.125 0.03 1.6 0.015 0.008 0.045 0.025 0.025
0.0010 <0.01 <0.01 0.021 0.041 Comparative example 4 0.085
0.03 1.5 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01
0.021 0.041 Comparative example 5 0.065 0.03 2.7 0.015 0.008 0.045
0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Comparative
example 6 0.065 0.03 1.8 0.015 0.008 0.045 0.025 0.012 0.0010
<0.01 <0.01 0.021 0.041 Comparative example 7 0.075 0.03 1.7
0.015 0.008 0.045 0.025 0.031 0.0010 <0.01 <0.01 0.021 0.041
Inventive example 8 0.065 0.03 2.1 0.015 0.008 0.045 0.025 0.050
0.0010 <0.01 <0.01 0.021 0.041 Comparative example 9 0.073
0.03 1.8 0.015 0.008 0.045 0.025 0.025 0.0002 <0.01 <0.01
0.021 0.041 Comparative example 10 0.063 0.03 2.0 0.015 0.008 0.045
0.025 0.025 0.0050 <0.01 <0.01 0.021 0.041 Comparative
example 11 0.065 0.03 1.7 0.015 0.008 0.045 0.025 0.022 0.0010 0.50
<0.01 0.021 0.041 Inventive example 12 0.065 0.03 1.8 0.015
0.008 0.045 0.025 0.027 0.0010 <0.01 0.30 0.021 0.041 Inventive
example 13 0.065 0.03 2.0 0.015 0.008 0.045 0.025 0.025 0.0010
<0.01 <0.01 0.021 0.041 Inventive example 14 0.073 0.03 1.9
0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041
Comparative example Underline: Outside of Present Invention
Range
[0059]
2 TABLE 2 Delayed fracture Average Mechanical properties (hydrogen
Steel carbide YP TS El Tensile strength after quenching (MPa)
embrittlement) sheet Steel diameter (.mu.m) (MPa) (MPa) (%)
Condition 1 Condition 2 Condition 3 time Remark 1 1 0.5 301 429
34.9 965 936 946 30 days or longer Comparative example 2 2 0.8 309
442 33.9 1295 1256 1269 30 days or longer Inventive example 3 3 3.5
314 448 33.5 1432 1389 960 3 days Comparative example 4 4 0.9 299
427 35.1 1288 940 1262 30 days or longer Comparative example 5 5
0.8 349 498 28.0 1525 1521 1494 30 days or longer Comparative
example 6 6 0.7 305 435 34.5 1266 960 1241 30 days or longer
Comparative example 7 7 0.5 305 435 34.5 1291 1253 1266 30 days or
longer Inventive example 8 8 0.8 340 485 29.0 1352 1312 1325 30
days or longer Comparative example 9 9 0.7 308 441 34.0 1239 955
1214 30 days or longer Comparative example 10 10 0.7 322 460 29.5
1320 1280 1294 30 days or longer Comparative example 11 11 0.7 300
428 35.0 1358 1317 1331 30 days or longer Inventive example 12 12
0.8 305 435 34.5 1320 1280 1294 30 days or longer Inventive example
13 13 1.5 314 449 33.4 1324 1284 1234 30 days or longer Inventive
example 14 14 3.1 313 447 33.5 1338 1298 975 30 days or longer
Comparative example Underline: Outside of Present Invention
Range
EXAMPLE 2
[0060] By using the steels 2 and 7 shown in Table 1, steel sheets A
to G were manufactured under manufacturing conditions shown in
Table 3. For these steel sheets, quenching under the conditions
similar to those in EXAMPLE 1 was conducted, and measurements
similar thereto were conducted. The results are shown in Table
4.
[0061] Any of steel sheets A, D, E, and F has high ductility (El)
leading to excellent formability, tensile strength of 1200 MPa or
higher after quenching independently of the conditions, and 30 days
or longer delayed fracture time leading to excellent hydrogen
embrittlement resistance.
[0062] In comparison, steel sheets B and C (comparative examples)
each have low ductility, hence offering poor formability. This is
because the steel sheet B received rapid cooling not only after hot
rolling but also after continuous annealing, and the steel sheet C
received low temperature coiling after hot rolling.
3 TABLE 3 Hot rolling conditions Annealing conditions Heating
Finishing Coiling Cold Soaking Cooling Steel temperature
temperature Cooling rate temperature reduction temperature Holding
rate sheet Steel (.degree. C.) (.degree. C.) (.degree. C./s)
(.degree. C.) rate (%) (.degree. C.) time (.degree. C./s) Remarks A
2 1230 880 20 600 -- -- -- -- Inventive example B 2 1230 870 35 600
-- -- -- -- Comparative example C 2 1230 860 20 450 -- -- -- --
Comparative example D 7 1230 870 20 600 60 700 2 minutes 15
Inventive example E 7 1230 870 20 600 60 830 2 minutes 20 Inventive
example F 7 1230 870 20 600 60 690 2 hours 0.01 Inventive example G
7 1230 870 20 600 60 720 2 minutes 35 Comparative example
Underline: Outside of Present Invention Range
[0063]
4 TABLE 4 Delayed fracture Mechanical properties Tensile strength
after (hydrogen Steel YP TS El quenching (MPa) embrittlement) sheet
(MPa) (MPa) (%) Condition 1 Condition 2 Condition 3 time Remark A
309 442 33.9 1295 1256 1269 30 days or longer Inventive example B
357 510 29.4 1320 1280 1294 30 days or longer Comparative example C
387 553 27.1 1273 1235 1248 30 days or longer Comparative example D
302 432 34.7 1280 1242 1254 30 days or longer Inventive example E
295 421 35.6 1260 1521 1235 30 days or longer Inventive example F
279 399 37.6 1265 1227 1240 30 days or longer Inventive example G
352 503 29.8 1291 1253 1266 30 days or longer Comparative
example
* * * * *