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°C to below 950°C,
and water quenched at a quenching intensity of 0.35cm
-1 to below 1.50cm
-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
1 to (Ac
1+30)°C for 1 to 20 hours after being hot rolled, and then cooled to a temperature
below (Ac
1+30)°C at a cooling rate of 20°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°C or above, cooled at a cooling rate of 80 to
150°C/s to a temperature range between 450 and 500°C while being kept in a metal die,
and further cooled at a cooling rate of 20 to 100°C/s to an ordinary temperature not
exceeding 100°C. Consequently, the component has a tensile strength of 1150N/mm
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°C
to below Ac
1 for 10 to 30 hours, or is slowly cooled to (Ac
1-30)°C at a cooling rate of 3 to 20°C/h or to (Ac
1-20)°C at a cooling rate of 3 to 10°C/h after being heated at a temperature of Ac
1 to (Ac
1+30)°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°C to below Ac
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°C, pickled, heated in an atmosphere of 95vol.% or more of hydrogen at a temperature
between Ac
1 and (Ac
1+30)°C for 1 to 10 hours, slowly cooled to (Ar
1-50)°C or less at a cooling rate of 3 to 20°C/h. Alternatively, the steel sheet is
further cold rolled and annealed at a temperature of (Ac
1-10)°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
3+20) and (Ar
3+50)°C, coiled at a temperature between 550 and 600°C, pickled, heated in an atmosphere
of 95vol.% or more of hydrogen at a temperature between Ac
1 and (Ac
1+30)°C for 1 to 10 hours, and slowly cooled to (Ar
1-50)°C or less at a cooling rate of 3 to 20°/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 1mm.
[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×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°C or above.
Alternatively, the hot rolled steel sheet is coiled at a coiling temperature of 480°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 1mm, 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 2µm or smaller.
[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°C/s or less; and coiling the cooled hot rolled steel sheet at a coiling
temperature of 500°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.
1. Composition
[0026] 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%.
[0027] 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.
[0028] 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%.
[0029] 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.
[0030] 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.
[0031] 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%.
[0032] 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.
[0033] 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]%.
[0034] 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%.
[0035] 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.
[0036] 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.
2. Iron carbides
[0037] 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 2µm or smaller so that the iron carbides can be dissolved into austenite in a very
short time, leading to high strength after quenching.
3. Manufacturing method
[0038] 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°C/s or less; and
coiling the cooled steel sheet at a coiling temperature of 500°C or higher.
[0039] In the above, the hot rolled steel sheet is cooled at the average cooling rate of
30°C/s or less for the reason that an average cooling rate exceeding 30°C/s generates
second phases that deteriorate the formability of steel sheet. For the same reason,
the coiling temperature is set to 500°C or higher.
[0040] 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°C/s or less to 400°C.
[0041] In the above, the annealed steel sheet is cooled at the average cooling rate of 30°C/s
or less to 400°C for the reason that generation of second phases is inhibited not
to deteriorate the formability of steel sheet.
[0042] In the present invention, heating temperature of steel slab prior to hot rolling
should be preferably controlled to 1200-1250°C from the viewpoint of enhancing the
formability. Finishing temperature at hot rolling should be preferably controlled
to Ar
3-890°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.
[0043] Even when manufacturing a cold rolled steel sheet, the above-described hot rolling
conditions, that is, the slab heating temperature controlled to 1200-1250°C and the
finishing temperature controlled to Ar
3-890°C, should be preferably taken from the same viewpoint. Further, also the cooling
rate after hot rolling should be preferably controlled to 30°C/s or less for a reason
that when the average cooling rate from hot rolling final pass to coiling exceeds
30°C/s, second phases are generated whereby to reduce manufacturability.
[0044] Reduction rate at cold rolling should be preferably controlled to 60% or greater
to obtain fine iron carbides having an average diameter of 2µm or smaller, which are
essential to the present invention. From the viewpoint of formability, annealing temperature
should be preferably controlled to 670-720°C at box annealing and to 690-730°C or
800-850°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
[0045] Slabs were cast after vacuum melting of steels 1-14 having composition shown in Table
1. After reheated at 1250°C, the slabs were each hot rolled at a finishing temperature
of 870°C into hot rolled steel sheets. The hot rolled steel sheets were each cold
rolled to 1.2mm, and subjected to 720°C × 2min. annealing simulating continuous annealing.
Thus produced cold rolled steel sheets 1-14 were cooled at an average cooling rate
of 10°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°C to control the carbide diameter.
[0046] 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.
[0047] Then, for these cold rolled steel sheets, tensile strengths were measured after quenching
performed under the following three conditions:
Condition 1: Water quenching after 1000°C × 5min. heating
Condition 2: Water quenching after 1000°C × 5min. heating and air cooling to 800°C
Condition 3: Water quenching after 900°C × 5sec. heating
[0048] 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.
[0049] Further, 30×100mm rectangular test pieces were cut out from the cold rolled steel
sheets quenched under the condition 1, and bent to 180° at a radius of 10mmR, 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 0.1N 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).
[0050] Table 2 shows mechanical properties, tensile strengths after quenching, and delayed
fracture time.
[0051] Any of steel sheets 2, 7, and 11 to 13 has a high ductility (El) leading to excellent
formability, a tensile strength of 1200MPa or higher after quenching independently
of the quenching conditions, and 30 days or longer delayed fracture time leading to
excellent hydrogen embrittlement resistance.
[0052] 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 2µm, 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
2µm, so that the steel sheet has insufficient tensile strength at low temperature
and short time heat treatment under the condition 3.
EXAMPLE 2
[0053] 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.
[0054] Any of steel sheets A, D, E, and F has high ductility (El) leading to excellent formability,
tensile strength of 1200MPa or higher after quenching independently of the conditions,
and 30 days or longer delayed fracture time leading to excellent hydrogen embrittlement
resistance.
[0055] In comparison, steel sheets Band 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.
TABLE 4
Steel sheet |
Mechanical properties |
Tensile strength after quenching (MPa) |
Delayed fracture (hydrogen embrittlement) time |
Remark |
|
YP
(MPa) |
TS
(MPa) |
El
(%) |
Condition 1 |
Condition 2 |
Condition 3 |
|
|
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 |