SPECIFICATION
TITLE OF INVENTION
HOT-STAMPED STEEL, METHOD OF PRODUCING OF STEEL SHEET FOR HOT
STAMPING, AND METHOD OF PRODUCING HOT-STAMPED STEEL
5
Field of the Invention
[0001]
The present invention relates to a hot-stamped steel that is excellent in terms of
balance between strength and toughness. Particularly, the present invention relates to a
10 hot-stamped steel having a strength of 1470 MPa or more and a sufficient energy
absorption capability. In addition, the present invention relates to a method of
producing a steel sheet for hot stamping that is applied to parts manufactured through hot
stamping, and a method of producing a hot-stamped steel in which this steel sheet for hot
stamping is used.
15 Priority is claimed on Japanese Patent Application No. 2010-135217, filed June
14, 2010, and Japanese Patent Application No. 2011-092811, filed April 19, 2011, the
contents of which are incorporated herein by reference.
Description of Related Art
20 [0002]
The weight reduction of a vehicle body is an urgent issue from the viewpoint of
global environmental protection so that, in recent years, active studies have been being
made regarding application of a high-strength steel sheet to a vehicle body, and the
strength of the steel has also been increasing. However, since the formability of a steel
25 sheet deteriorates as the strength of the steel sheet increases, the shape-freezing
2
properties need to be considered. Meanwhile, in ordinarily-used pressing, the forming
loads gradually increase, and thus there is a huge problem with the pressing capability of
the steel sheet in terms of being put into practical use (use of a high-strength steel sheet).
[0003]
5 From the above viewpoint, hot stamping techniques are used. In hot stamping
techniques, a steel sheet is heated to a high temperature in an austenite range, and then
pressed. Therefore, compared to ordinary pressing performed at room temperature,
forming loads significantly decrease. In addition, since quenching is substantially
performed in a die at the same time as pressing, it is possible to obtain a strength that
10 corresponds to the amount of C included in steel, and hot stamping techniques are
attracting attention as a technique that satisfies both shape-freezing properties and
strength. Patent Citations Ito 3 disclose a method in which a strength of 1000 MPa to
2000 MPa is obtained using hot stamping techniques. Patent Citation 1 discloses a steel
sheet for hot stamping which has a predetermined average grain size of prior austenite
15 grains and a predetermined amount of martensite after hot stamping, has a strength of
1770 MPa to 1940 MPa, and is excellent in terms of ductility, but does not evaluate
toughness. In addition, Patent Citation 2 discloses a technique in which cleanness and
the segregation degree of P and S are controlled so as to significantly improve toughness
after hot stamping. However, Patent Citation 2 does not describe the average grain size
20 of prior austenite grains. Furthermore, Patent Citation 3 discloses a technique in which
toughness is improved by controlling the average grain size of prior austenite grains and
using auto-tempered martensite. However, Patent Citation 3 does not disclose the shape
of prior austenite (for example, a grain size ratio of prior austenite which will be
described below) and the controlling method regarding microstructures formed after hot
25 stamping, and there is a possibility that the microstructures cannot be sufficiently
3
controlled, and the balance between strength and toughness cannot be sufficiently
secured. Meanwhile, Patent Citation 4 discloses a high-strength hot-rolled steel sheet
which has a predetermined aspect ratio of a prior-austenite grain size and is excellent in
terms of low-temperature toughness. However, in Patent Citation 4, since the aspect
5 ratio of prior austenite grain sizes before hot stamping is extremely high, there is a
possibility that microstructures cannot be sufficiently controlled, and the balance between
strength and toughness cannot be sufficiently secured after hot stamping.
Patent Citation
10 [0004]
[Patent Citation 1] Japanese Unexamined Patent Application, First Publication
No. 2010-174282
[Patent Citation 2] Japanese Unexamined Patent Application, First Publication
No. 2007-314817
15 [Patent Citation 3] Japanese Unexamined Patent Application, First Publication
No. 2006-152427
[Patent Citation 4] Japanese Unexamined Patent Application, First Publication
No. 2011-52321
20 SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0005]
Vehicle components, particularly, parts, such as a frame, members, and
reinforcement, are classified into parts that efficiently absorb energy in case of collision
25 and parts that have a sufficient proof strength and transmit energy without deformation in
4
case of collision according to the functions. Particularly, since there is a demand for a
reinforcement having a higher strength, in cold pressing, the capability of a pressing
machine is lacking, or shape-freezing properties deteriorate. Therefore, the number of
parts to which hot stamping is applied (hot-stamped steel) is increasing among parts that
5 need to have a strength of 1470 MPa or more. In addition, in order to realize additional
weight reduction, there is a demand for a member having a strength of particularly 1770
MPa or more.
Methods for Solving the Problem
10 [0006]
Here, the present inventors manufactured a part having sufficient toughness and
a tensile strength of 1470 MPa or more using hot stamping in consideration of the above
circumstances, and completed the present invention.
[0007]
15 The summery are as follows.
(1) A hot-stamped steel according to an aspect of the present invention includes,
by mass%, C: 0.20% to 0.35%, Si: 0.1% to 0.5%, the total of at least one selected from
Mn and Cr: 1% to 3%, Al: 0.005% to 0.06%, Ti: 0.002% to 0.1%, Nb: 0.002% to 0.1%,
0: 0.003% to 0.007%, and a balance of iron and inevitable impurities, wherein P is
20 limited to 0.015% or less, S is limited to 0.01% or less, and N is limited to 0.004% or less,
the dimensional ratio of the lengths of prior austenite grains in a rolling direction to the
lengths of the prior austenite in the sheet thickness direction is 1.3 to 2.5, the average
grain size of the prior austenite grains is 6 μm or less, the microstructure includes 98% or
more of martensite, and the tensile strength is 1470 MPa or more.
25 (2) The high-strength steel sheet according to the above (1) may further include,
5
by mass%, one or more of B: 0.005% or less, V: 0.1% or less, Mo: 0.5% or less, Ca:
0.03% or less, Mg: 0.03% or less, REM: 0.03% or less, Cu: 0.5% or less, Sri: 0.1% or
less, Ni: 0.5% or less, and W: 1% or less.
[0008]
5 (3) The high-strength steel sheet according to the above (1) or (2) may further
comprise a coating layer formed by solidification of molten metal on the surface.
(4) A method of producing a steel sheet for a hot-stamped steel according to an
aspect of the present invention includes a first process in which a slab is heated to a
temperature range of 1270°C or lower; a second process in which finish rolling is
10 performed in a temperature range of 800°C to 900°C so that the total reduction from a
third last stand to a last stand becomes 60% or more; a third process in which cooling
begins within 1 second from the end of the second process; and a fourth process in which
coiling is performed in a temperature of 600°C or lower. The slab includes: by mass%, C:
0.20% to 0.35%, Si: 0.1% to 0.5%, the total of at least one selected from Mn and Cr: 1%
15 to 3%, Al: 0.005% to 0.06%, Ti: 0.002% to 0.1%, Nb: 0.002% to 0.1%, 0: 0.003% to
0.007%, and a balance of iron and inevitable impurities, wherein P is limited to 0.015%
or less, S is limited to 0.01% or less, and N is limited to 0.004% or less.
(5) In the method of producing a steel sheet for a hot-stamped steel according to
the above (4), the slab may further include, by mass%, one or more of B: 0.005% or less,
20 V: 0.1% or less, Mo: 0.5% or less, Ca: 0.03% or less, Mg: 0.03% or less, REM: 0.03% or
less, Cu: 0.5% or less, Sri: 0.1% or less, Ni: 0.5% or less, and W: 1% or less.
(6) The method of producing a steel sheet for a hot-stamped steel according to
the above (4) or (5) may further include, after the fourth process, a process in which cold
rolling is performed.
25 (7) The method of producing a steel sheet for a hot-stamped steel according to
6
the above (4) or (5) may further include, after the fourth process, a process in which cold
rolling and continuous annealing is performed.
(8) The method of producing a steel sheet for a hot-stamped steel according to
the above (4) or (5) may further include, after the fourth process, a process in which
5 coating of molten metal is performed.
(9) The method of producing a steel sheet for a hot-stamped steel according to
the above (4) or (5) may further include, after the fourth process, a process in which cold
rolling is performed, and coating of molten metal is performed.
(10) The method of producing a steel sheet for a hot-stamped steel according to
10 the above (4) or (5) may further include, after the fourth process, a process in which cold
rolling and continuous annealing are performed, and coating of molten metal is
performed.
[0009]
(11) In a method of producing a hot-stamped steel according to an aspect of the
15 present invention includes hot-stamping a steel sheet obtained using the method of
producing a steel sheet for a hot-stamped steel according to the above (4) under a
condition in which the steel sheet is heated to a temperature range of an Ac3 point to
900°C at a heating rate of 3 °C/s or more, and then the steel sheet is cooled at a cooling
rate of 150 °C/s or more in a temperature range of 300°C to an Ar3 point.
20 (12) In a method of producing a hot-stamped steel according to an aspect of the
present invention includes hot-stamping a steel sheet obtained using the method of
producing a steel sheet for a hot-stamped steel according to the above (5) under a
condition in which the steel sheet is heated to a temperature range of an Ac3 point to
900°C at a heating rate of 3 °C/s or more, and then the steel sheet is cooled at a cooling
25 rate of 150 °C/s or more in a temperature range of 300°C to an Ara point..
7
Effects of the Invention
[0010]
According to the hot-pressed steel according to the above (1) and (2), after hot
5 stamping, the prior austenite grain size and the shape of prior austenite are appropriately
controlled while a strength of 1470 MPa or more is secured so that the balance between
strength and toughness improves, energy absorption properties can be increased in case
of collision, and the weight of a part can be reduced at a higher degree.
In the method of producing a steel sheet for a hot-pressed steel according to the
10 above (3) to (10), it is possible to provide a steel sheet for a hot-pressed steel for which
the prior austenite grain size and the shape of prior austenite can be appropriately
controlled while a strength of 1470 MPa or more is secured after hot stamping.
In the method of producing a hot-pressed steel according to the above (11) and
(12), it is possible to provide a hot-pressed steel that is excellent in terms of the balance
15 between strength and toughness and energy absorption properties in case of collision.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011]
FIG 1 is a view showing the relationship between the amount of C and the
20 strength of a hot-rolled steel sheet after hot stamping.
FIG 2 is a view showing the relationship between the grain size of prior
austenite and the absorbed energy of a hot-rolled steel sheet after hot stamping.
FIG 3 is a view showing the relationship between the grain size ratio of prior
austenite and the absorbed energy of a hot-rolled steel sheet after hot stamping.
25 FIG 4 is a view showing the relationship between the finishing temperature
8
during hot rolling and the grain size of prior austenite after hot stamping.
FIG 5 is a view showing the relationship between the finishing temperature
during hot rolling and the grain size ratio of prior austenite after hot stamping.
FIG 6 is a view showing the relationship between the cooling-start time after
5 finish rolling and the grain size of prior austenite after hot stamping.
FIG 7 is a view showing the relationship between the cooling-start time after
finish rolling and the grain size ratio of prior austenite after hot stamping.
FIG. 8 is a view showing the relationship between the grain size of prior
austenite after hot stamping and the absorbed energy of a cold-rolled steel sheet.
10 FIG. 9 is a view showing the relationship between the grain size ratio of prior
austenite after hot stamping and the absorbed energy of a cold-rolled steel sheet.
FIG 10 is a view showing a V-notch specimen used in the tests of
delayed-fracture resistance in examples according to the present invention.
FIG 11 is a flowchart showing a method of producing a steel sheet for hot
15 stamping according to an embodiment of the present invention and a method of
producing a hot-stamped steel according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0012]
20 Firstly, tests through which the present invention has been completed will be
described.
[0013]
The inventors melted steels including the chemical components shown in Table
I into a laboratory size, heated obtained ingots to 1250°C, then, performed hot rolling in
25 which the total reduction at the final rolling and the rolling immediately before the final
9
rolling was controlled to be 60%, the finishing temperature was controlled to be 880°C,
and the sheet thickness was controlled to be 1.4 mm, began cooling at a cooling rate of
200 °C/s or less 1 second (1 s) after the end of the hot rolling, and performed coiling at
600°C. The obtained hot-rolled steel sheets were pickled, heated to 850°C at a heating
5 rate of 10 °C/s, then, the temperature was maintained for 20 s, immediately, the steel
sheets were cooled to room temperature at a cooling rate of 150 °C/s, and thereby the
steel sheets having the thermal history were obtained. In this case, steel sheets
including 98% or more of martensite as a microstructure were obtained. After that, No.
5 specimens described in JIS Z 2201 were prepared from the hot-rolled steel sheets
10 having the thermal history, and tensile tests were performed according to the testing
method described in JIS Z 2241. The obtained results of the tensile tests are shown in
FIG. 1. That is, it was found that it is necessary to add 0.20% or more of C by mass% to
steel in order to obtain a tensile strength of 1470 MPa or more after hot stamping which
is an object of the present invention.
15 [0014]
Furthermore, hot-rolling was performed under a variety of conditions using the
steel including the No. 2 chemical components in Table 1 so as to manufacture 3.2
mm-thick hot-rolled steel sheets and 1.6 mm-thick hot-rolled steel sheets. Here. cold
rolling was subsequently performed on the 3.2 mm-thick hot-rolled steel sheets so as to
20 manufacture 0.8 mm-thick cold-rolled steel sheets.
Firstly, the tensile strength and toughness of the 1.6 mm-thick hot-rolled steel
sheets were investigated when hot stamping was performed on the steel sheets under heat
treatment conditions (thermal history) in which the steel sheets were heated to 900°C at a
heating rate of 10 °C/s and cooled to room temperature at a cooling rate of 200 °C/s.
10
Steel sheets including 98% or more of martensite were obtained as a microstructure in all
of the hot rolling conditions. In addition, the martensite was not tempered martensite.
As a result of tensile tests according to the same testing methods as above, a tensile
strength of 1470 MPa or more was obtained in all of the hot rolling conditions. For the
5 toughness, V-notch specimens (width: 10 mm) were prepared, Charpy impact tests were
performed, and the absorbed energies (in terms of a sheet thickness of 10 mm) were
evaluated at -40°C. Furthermore, the prior austenite grain size (average value) after hot
stamping (thermal history) and the prior austenite grain size ratio (the dimensional ratio
of the length of prior austenite in a rolling direction to the length of prior austenite in the
10 sheet thickness direction) were evaluated by a method described later, and the
relationship between the above and the absorbed energy was investigated. The obtained
results are shown in FIGS. 2 and 3. That is, it was found that, from the viewpoint of
securing toughness after hot stamping, it is important to control the prior austenite grain
size in the steel sheet after hot stamping to be 6 .tm or less and control the prior austenite
15 grain size ratio (the length in the rolling direction/ the length in the sheet thickness
direction) to be 1.3 or more.
[0015]
Furthermore, the inventors found that the prior austenite grain size was 6 μm or
less, and the prior austenite grain size ratio (the length in the rolling direction/ the length
20 in the sheet thickness direction) was 1.3 or more even in hot-stamped steel sheets (steels)
in a case in which the prior austenite grain size was 6 μm or less, and the prior austenite
grain size ratio (the length in the rolling direction/ the length in the sheet thickness
direction) is 1.3 or more in hot-rolled steel sheets.
The mechanism is considered to be as follows. For example, in a case in which
11
the prior austenite grain size is as small as 6 μm or less, and the prior austenite grain size
ratio (the length in the rolling direction/ the length in the sheet thickness direction) is 1.3
or more, as high a proportion as almost 100% of the microstructure transforms from
austenite to ferrite and cementite in the cooling and coiling process after hot rolling,, and,
5 furthermore, as high a proportion as almost 100% of the microstructure transforms ferrite
and cementite to austenite during heating before hot stamping. Therefore, it is
considered that, in this case, prior austenite grains for which the grain size is 6 μm or less,
and the prior austenite grain size ratio (the length in the rolling direction/ the length in the
sheet thickness direction) is 1.3 or more can be secured even after hot stamping even
10 when the transformation from austenite to ferrite and cementite and transformation of
ferrite and cementite to austenite are repeated.
[0016]
Therefore, in order to secure the above prior austenite grain size and the above
prior austenite grain size ratio after hot stamping, the finishing temperature in hot rolling
15 and the cooling-start time after finishing rolling are important as shown in FIGS. 4 to 7.
That is, it is necessary to end hot rolling (finish rolling) at 900°C or lower and begin
cooling within 1 second after the end of the finish rolling (the cooling-start time is 1 s or
less). While the cooling rate from the beginning of the cooling after the hot rolling to
coiling was controlled to be 200 °C/s or less in the above tests, the prior austenite grain
20 size after hot pressing can be controlled to be 6 μm or less, and the prior austenite grain
size ratio (the length in the rolling direction/ the length in the sheet thickness direction)
can be controlled to be 1.3 or more even when the cooling rate exceeds 200 °C/s.
[0017]
Meanwhile, for the above 0.8 mm-thick cold-rolled steel sheets, the tensile
12
strength and toughness were investigated in the same manner as for the hot-rolled steel
sheets after the steel sheets were heated to 850°C at a heating rate of 10 °C/s, and then
cooled to room temperature at a cooling rate of 150 °C/s. For the cold-rolled steel
sheets as well, a tensile strength of 1470 MPa or more could be obtained in all of the hot
5 rolling conditions. FIGS. 8 and 9 show the results of Charpy impact tests performed in
the same manner as above. It is considered that the characteristics of the cold-rolled
steel sheets also have a correlation with the conditions of hot rolling, and it was found
that the characteristics of the cold-rolled steel sheets showed a favorable correlation with
the prior austenite grain size and the prior austenite grain size ratio (the length in the
10 rolling direction/ the length in the sheet thickness direction) after hot stamping.
Meanwhile, here, for the measurement of the prior austenite grain size and the prior
austenite grain size ratio, etching was performed using an aqueous solution including
sodium dodecylbenzene sulfonate, picric acid, oxalic acid and chloric acid, and a 1/8 t
portion (or 7/8 t portion) of the sheet thickness was observed using an optical
15 microscope.
[0018]
[Table 1]
0
0
N
C.lh;nica1 cmpcwrits (mass%) Ac3
o.
Si Nh Cr F S AI Ti Nb REM N 0 (0Q
0.18 0.14 1.24 0.22 0.012 0.0018 0.035 0.031 0.034 0.011 0.0028 0.0034
0.20 0.15 1.35 0.18 0.014 0.0025 0.027 0.081 0.058 0.017 0.0034 0.0047 838
3 0.23 0.13 1.27 0.25 0.015 0.0027 0.034 0.061 0.072 0.009 0.0029 0.0039 829
4 0.26 0.14 1.08 0.37 0.015 0.0021 0.032 0.025 0.052 0.006 0.0032 0.0031 814
5 0.28 0.1s 1.35 0.19 0.012 0.0018 0.026 0.031 0.051 0.007 0.0022 0.0042 800
0.30 0.15 1.23 an 0.016 0.0019 0.033 0.028 0.032 0.005 0.0033 0.0033 804
0.32 0.13 1.32 0.31 0.014 0.0023 0.028 0.031 0.025 0.018 0.0029 0.0045 795
8 0.35 0.16 1.05 0.72 0.015 0.0015 0.025 0.033 0.048 0.027 0.0033 0.0051 799
9 0.37 0.15 1.22 0.24 0.015 0.0018 0.031 0.024 0.018 0.008 0.0029 0.0064 789
10 0.40 0.14 1.19 0.36 0.014 0.0021 0.033 0.029 0.055 0.012 0.0031 0.0039 787
14
The present invention has been completed based on the above testing
circumstances.
[0020]
Hereinafter, a hot-stamped steel according to an embodiment of the present
5 invention will be described. Firstly, the chemical composition of the hot-stamped steels
of the embodiment and steel sheets that will be used for the hot-stamped steels will be
described. Meanwhile, here, "%" indicates "mass%."
[0021]
C is an element that plays an important role in the embodiment, and particularly
10 has a large influence on strength after quenching. Therefore, in order to obtain a tensile
strength of 1470 MPa or more, the amount of C needs to be 0.20% or more. On the
other hand, when the amount of C exceeds 0.35%, fracture becomes liable to occur
during impact deformation, weldability deteriorates, and the strength of a weld degrades.
Therefore, the upper limit of the amount of Cis 0.35%. Ina case in w rich a tensile
15 strength needs to be secured more reliably, the amount of C is preferably 0.21 % or more.
In addition, in a case in which weldability is further enhanced, the amount of C is
preferably 0.32% or less, and is more preferably 0.30% or less.
[0022]
Since Si is a solid solution strengthening element and an element that suppresses
20 precipitation of cementite, the amount of Si needs to be 0.1% or more. On the other
hand, when Si is excessively added to steel, coatability on the surface of a steel sheet
deteriorate in a case in which metal coating is performed as described below. Therefore,
the upper limit of the amount of Si is 0.5%.
[0023]
25 Mn and Cr are important elements for securing hardenability, and the total of at
15
least one selected from Mn and Cr needs to be 1 % or more in a case in which hot
stamping is performed. On the other hand, when the total of at least one selected from
Mn and Cr exceeds 3%, hardenability is enhanced, and the strength of the hot-rolled steel
sheet becomes excessively large. Therefore, in this case, since the load becomes
5 excessively large in a case in which cold forming, such as cold rolling, is performed, the
upper limit of the total of at least one selected from Mn and Cr needs to be 3%, and is
preferably 2.7%.
Here, for example, in a case in which Mn is included in steel, in order to further
secure hardenability, the amount of Mn is preferably 1.0% or more, is more preferably
10 1.1% or more, and is most preferably 1.2% or more. In addition, in order to sufficiently
secure cold formability, the amount of Mn is preferably 3.0% or less, is more preferably
2.8% or less, and is most preferably 2.7% or less.
In addition, for example, in a case in which Cr is included in steel, the amount of
Cr may be 0.005% or more, and is preferably 0.15% or more in order to further secure
15 hardenability. In addition, in order to more sufficiently secure cold formability, the
amount of Cr is preferably 1.0% or less.
[0024]
Ti and Nb are also important elements in the embodiment. In order to control
the dimensional ratio of the lengths of prior austenite grains in a rolling direction to the
20 lengths of prior austenite in the sheet thickness direction after hot stamping to be 1.3 or
more and control the average grain size of the prior austenite grains to be 6 μm or less,
the amount of Ti and the amount of Nb each need to be 0.002% or more, are preferably
0.005% or more, are more preferably 0.010% or more, and are most preferably 0.015%
or more. On the other hand, since the effects reach an upper limit even when the
25 amount of Ti or the amount of Nb exceeds 0.1 %, the upper limits of the amount of Ti and
16
the amount of Nb each are 0.1%.
[0025]
0 is an element necessary to form oxides. When the amount of 0 is less than
0.003%, the number of fine oxides is small, and therefore a prior austenite grain size of 6
5 μm or less is not obtained. Therefore, the lower limit of the amount of 0 needs to be
0.003%. On the other hand, when the amount of 0 exceeds 0.007%, the amount of
oxides being formed becomes too large, and therefore formability and toughness
deteriorate. Therefore, the upper limit of the amount of 0 is 0.007%, is preferably
0.006%, and is more preferably 0.005%.
10 [0026]
P is a solid solution strengthening element, and can enhance the strength of a
steel sheet at relatively low cost. However, P is liable to segregate at grain boundaries,
and there is a problem of low-temperature embrittlement in the case of a high strength,
and therefore the upper limit of the amount of P is 0.015%, and is preferably 0.010%.
15 On the other hand, the amount of P may be 0%; however, when the amount of P is lower
than 0.001%, the costs for removing P increase extremely. Therefore, regarding P
included as an inevitable impurity, the lower limit of the amount of P is preferably
0.001%, and more preferably 0.005%.
[0027]
20 Since S is an inevitable impurity, has an influence on the hot embrittlement of
steel, and deteriorates formability, particularly, hot formability, the amount of S is
preferably lower. Therefore, the upper limit of the amount of S is 0.01%, and is
preferably 0.009%. However, while the amount of S may be 0%, in a case in which the
amount of S is reduced to less than 0.001%, the desulfurization costs increase extremely,
25 and therefore the lower limit of the amount of S is preferably 0.001%, and is more
17
preferably 0.002%.
[0028]
Al is added for deoxidization and inevitably included in steel. When the
amount of Al is less than 0.005%, deoxidization is not sufficient, and a large amount of
5 oxides remain in steel. Therefore, local deformability deteriorates, and physical
properties significantly vary. Therefore, the lower limit of the amount of Al is 0.005%
or more, and is preferably 0.20% or more. On the other hand, when the amount of Al
exceeds 0.06%, a large amount of oxides mainly including alumina remains in steel, and
local deformability deteriorates. Therefore, the upper limit of the amount of Al is
10 0.06%, and is preferably 0.05%.
[0029]
N is also inevitably included in steel. The amount of N may be 0%; however,
when the amount of N is extremely reduced, the costs increase, and therefore the lower
limit of the amount of Nis preferably 0.001%, and is more preferably ft0015%. On the
15 other hand, when the amount of N exceeds 0.004%, inclusions are formed, and toughness
after quenching deteriorates. Therefore, the upper limit of the amount of N is 0.004%,
and is preferably 0.0035%.
[0030]
Meanwhile, a chemical composition consisting of the above basic chemical
20 components (basic elements) and a balance of Fe and inevitable impurities is the basic
composition of the embodiment. However, in addition to the basic composition, at least
one selected from the following chemical components (optional elements) can be
included in steel (instead of some of Fe in the balance). In addition, even in a case in
which the optional elements are not included in steel, the effects of the embodiment are
25 not impaired, and therefore, the lower limit of the optional elements may be 0%.
18
Meanwhile, the effects of the embodiment are not impaired even when the optional
elements are inevitably mixed into steel.
[0031]
B is an effective element for securing hardenability ; however, when the amount
5 of B is less than 0.0005 %, the effect is not easily exhibited . Therefore, in a case in
which more favorable hardenability is secured , the amount of B is preferably 0.0005% or
more. On the other hand, when the amount of B exceeds 0.005%, the effect reaches an
upper limit, and therefore the upper limit of the amount of B is 0.005%, and is preferably
0.002%.
10 [0032]
Ca and Mg are deoxidizing elements, and are effective elements for refining of
the grain size of prior austenite since Ca and Mg form fine oxides. Therefore, in a case
in which prior austenite is refined using Ca or Mg, the amount of Ca or the amount of
Mg is preferably 0.005% or more. However, when the amount of Ca or the amount of
15 Mg exceeds 0.03%, the effect reaches an upper limit, and therefore the upper limits of the
amount of Ca and the amount of Mg are 0.03%, are preferably 0.02%, and are more
preferably 0.015%.
Rare earth metals (REM) including Ce and the like are deoxidizing elements,
and are effective elements for refining of the grain size of prior austenite since REM
20 form fine oxides. Therefore, in a case in which prior austenite is refined using REM,
the amount of REM is preferably 0.005% or more. However, when the amount of REM
exceeds 0 . 03%, the effect reaches an upper limit, and therefore the upper limit of the
amount of REM is 0.03%, is preferably 0.028 %, and is more preferably 0.025%.
[0033]
25 V is an element that is added to steel for refining of a microstructure from the
19
viewpoint of toughness securement. That is, in a case in which a steel sheet is heated to
Ac3 point or higher, V forms fine carbides so as to supperss recrystallization and grain
growth and thus refines austenite grains, and therefore an effect of improving toughness
is obtained. When the amount of V is less than 0.005%, the effect cannot be obtained,
5 and therefore, in a case in which more favorable toughness is secured, the amount of V is
preferably 0.005% or more, is more preferably 0.010% or more, and is most preferably
0.03 0% or more. On the other hand, when the amount of V exceeds 0.1%, the effect
reaches an upper limit, and the costs increases, and therefore the upper limit of the
amount of V is 0.1%, is preferably 0.09%, and is more preferably 0.08%.
10 [0034]
Similarly to Ti, Nb, and V, in a case in which a steel sheet is heated to Ac3 point
or higher, Mo also forms fine carbides so as to suppress recrystallization and grain
growth and thus refines austenite grains, and therefore an effect of improving toughness
is obtained. Therefore, in a case in which more favorable toughness is aecured, the
15 lower limit of the amount of Mo is preferably 0.05%, is more preferably 0.08%, and is
most preferably 0.10%. On the other hand, when the amount of Mo exceeds 0.5%, the
effect reaches an upper limit, and the costs increases, and therefore the upper limit of the
amount of Mo is 0.5%, and is preferably 0.45%.
[0035]
20 W is added to steel in a case in which martensite is formed more stably in a hot
stamping process. When the amount of W is less than 0.1%, the effect is not sufficient,
and therefore the lower limit of the amount of W is preferably 0.1 % in a case in which
the effect is sufficiently obtained. When the amount of W exceeds 1%, the effect
reaches an upper limit, and therefore the upper limit of the amount of W is 1 %.
25 [0036]
20
Meanwhile, for example, in a case in which scraps are used in a steel-making
process, there is a case in which elements, such as Cu, Sri, and Ni, are included in steel.
Even in this case, the effects according to the embodiment are not directly influenced.
However, when the elements are included excessively in steel, cracking occurs during hot
5 rolling. Therefore, the upper limit of the amount of Cu is 0.5%, is preferably 0.3%, and
is more preferably 0.2%. Similarly, the upper limit of the amount of Sri is 0.1%, is
preferably 0.05%, and is more preferably 0.02%. In addition, the upper limit of the
amount ofNi is 0.5%, is preferably 0.3%, and is more preferably 0.1%. Meanwhile, the
lower limits of the elements are not particularly limited, the lower limits of the amount of
10 Cu, the amount of Sri, and the amount of Ni are preferably 0.01%, 0.005%, and 0.01%
respectively in consideration of refining costs in a case in which the elements are
inevitably mixed into steel.
[0037]
As described above, the hot-stamped steel of the embodiment and the steel sheet
15 used for the hot-stamped steel have a chemical composition consisting of the above basic
elements and the balance of Fe and inevitable impurities or a chemical composition
consisting of the basic elements, one or more of the above optional elements, and the
balance of Fe and inevitable impurities.
[0038]
20 Furthermore, as described above, the hot-stamped steel according to the
embodiment includes 98% or more of martensite in terms of area percentage. Some or
all of the martensite may be tempered martensite. Meanwhile, the microstructure of the
balance of the martensite is not particularly limited, and may be at least one selected
from bainite and residual austenite. Meanwhile, the upper limit of the amount of the
25 martensite maybe 100%.
21
Additionally, in the embodiment, the dimensional ratio (prior austenite grain size
ratio) of the lengths of prior austenite grains in the rolling direction to the lengths of prior
austenite grains in the sheet thickness direction is 1.3 or more, and the average grain size
of prior austenite grains is 6 .tm or less in terms of equivalent circle diameter. The
5 lower limit of the average grain size of prior austenite grains is not particularly limited,
and may be 3.0 μm in consideration of measurement resolution. Here, when the prior
austenite grain size ratio of hot-stamped prior austenite grains exceeds 2.5, the anisotropy
of the steel sheet becomes excessively large, and thus there is a concern of deterioration
of toughness. Therefore, the prior austenite grain size ratio needs to be 2.5 or less. In
10 a case in which it is necessary to further suppress the anisotropy of the steel sheet, the
prior austenite grain size ratio is preferably 2.0 or less.
Meanwhile, the amount of the martensite, the prior austenite gain size, and the
prior austenite grain size ratio are measured by observing the microstructure of a cross
section of a specimen using an optical microscope.
15 [0039]
In addition, the hot-stamped steel of the embodiment and the steel sheet used for
the hot-stamped steel has a tensile strength of 1470 MPa or more as described above. In
addition, the upper limit of the tensile strength is not particularly limited; however, for
example, the tensile strength is preferably 2450 MPa or less. Meanwhile, the dimension
20 (size) is not particularly limited, and can be appropriately selected according to use.
[0040]
Hereinafter, a method of producing the steel sheet for hot stamping according to
an embodiment of the present invention will be described.
[0041]
25 In the embodiment, steel having a chemical composition that consists of the
22
above basic elements, furthermore, the above optional elements according to necessity,
and the balance of Fe and inevitable impurities are used. The steel is continuously cast
so as to manufacture a slab, and the slab is heated to a temperature range of 1250°C or
lower (first process). The heated slab is hot-rolled, during which finish rolling is
5 performed in a temperature range of 800°C to 900°C (finishing temperature) so that the
total reduction of 3 passes from rolling at the third last stand to rolling at the last stand
becomes 60% or more (second process). Cooling begins within 1 second from the end
of hot rolling (finish rolling) for a steel sheet obtained through the hot rolling (third
process). Furthermore, coiling is performed on the steel sheet in a temperature of
10 600°C or lower so as to manufacture a hot-rolled steel sheet (fourth process).
[0042]
Here, the continuous casting method is not particularly limited, and may be an
ordinary continuous casting method or a thin slab casting method in which the thickness
of a slab is 100 mm or less. The effects of the embodiment do not change due to the
15 type of the continuous casting method.
[0043]
In the embodiment, hot rolling conditions are extremely important particularly
for toughness after hot stamping. That is, in order to control the dimensional ratio of
the lengths of the prior austenite grains in the rolling direction to the lengths of prior
20 austenite grains in the sheet thickness direction (grain size ratio of prior austenite) after
hot stamping to be 1.3 or more and control the average grain size to be 6 μm or less, the
heating temperature during hot rolling is preferably lower. For this, the heating
temperature is controlled to be 1270°C or lower, and preferably to be 1250°C or lower.
Meanwhile, when the heating temperature is too low, deformation resistance becomes
23
extremely large during hot rolling, and therefore rolling properties degrade. Therefore,
the lower limit of the heating temperature is preferably 1050°C. In addition, the
finishing temperature is also preferably as low as possible, but a finishing temperature of
800°C or higher and preferably 850°C or higher is secured in consideration of rolling
5 properties. On the other hand, when the finishing temperature exceeds 900°C, the prior
austenite grain size ratio becomes smaller than 1.3, and toughness deteriorates, and
therefore the upper limit of the finishing temperature is 900°C. At this time, the total
reduction from the third last stand to the last stand (the total amount of the reduction at
the third last stand, the reduction at the second last stand, and the reduction at the last
10 stand) is controlled to be 60% or more, and preferably to be 70% or more. Meanwhile,
the upper limit of the total reduction from the third last stand to the last stand is not
particularly limited, and may be 95% in consideration of the sheet thickness of a
hot-rolled steel sheet. Furthermore, cooling rapidly begins after the end of the finish
rolling, and, specifically, cooling beings within 1 second from the end of the finish
15 rolling, and preferably within 0.5 seconds from the end of the finish rolling. Meanwhile,
the cooling rate from the beginning of the cooling after the hot rolling to coiling may be
200 °C/s or less or more than 200 °C/s. After that, coiling is performed in a temperature
range of 600°C or lower so that the prior austenite grain size ratio can be controlled to be
1.3 or more, and the average grain size of the prior austenite grains can be controlled to
20 be 6 μm or less after hot stamping. When the coiling temperature exceeds 600°C, the
total reduction (3 passes) is less than 60%, or the cooling-start time after the finish rolling
exceeds 1 second, it is not possible to control the prior austenite grain size ratio to be 1.3
or more, and control the average grain size of the prior austenite grains to be 6 μm or less
after hot stamping. Meanwhile, when the coiling is performed in a temperature of lower
24
than 400°C, the strength of the hot-rolled steel sheet becomes too large, and therefore the
lower limit of the coiling temperature is preferably 400°C. Particularly, in order to
obtain a microstructure including ferrite and pearlite, the coiling temperature is
preferably 500°C or higher. On the other hand, in a case in which the coiling is
5 performed in a temperature of lower than 400°C, a reheating treatment intended for
softening may be performed after the coiling. Meanwhile, the cooling-end temperature
of cooling that begins within 1 second from the end of the finish rolling is not
particularly limited as long as austenite is sufficiently transformed to ferrite and
cementite, and, for example, in a case in which cooling is controlled in a single step, the
10 cooling-end temperature is 400°C or higher. In addition, the lower limit of the
cooling-start time after the finish rolling is not particularly limited, and may be 0.01
seconds in consideration of the capability of a cooling facility.
[0044]
Furthermore, processes, such as cold rolling, continuous annealing, and a variety
15 of coating or plating, can be performed on the obtained hot-rolled steel sheet according to
necessity. For example, cold rolling can be performed on the hot-rolled steel sheet so as
to manufacture a cold-rolled steel sheet. Continuous annealing may also be performed
on the cold-rolled steel sheet according to necessity. In addition, a variety of coating or
plating (for example, coating of molten metal) can be performed on the hot-rolled steel
20 sheet and the cold-rolled steel sheet (including the cold-rolled steel sheet that has
undergone continuous annealing) so as to manufacture coated steel sheets.
[0045]
Here, cold rolling conditions, continuous annealing conditions, and coating
conditions are not particularly limited, and cold rolling, continuous annealing, and
25
coating may be performed in an ordinary range. That is, the cold rolling is performed in
a reduction range of ordinarily performed cold rolling, and, specifically, the cold rolling
can be performed at a reduction of 40% to 80%. The coating is performed immediately
after the hot rolling, immediately after the cold rolling, or after recrystallization
5 annealing, but heating conditions or cooling conditions are not particularly limited.
Furthermore, Zn or Al is ordinarily used as a coating metal, but whether or not the Zn
coating is alloyed is not limited . In addition, for Al coating, the coating may include Si,
and the effects of the embodiment are not influenced.
[0046]
10 Skin pass may be performed on the hot-rolled steel sheet, the cold-rolled steel
sheet, and the coated steel sheet. The skin pass is not particularly limited, and the skin
pass can be performed at an appropriate timing according to necessity in order to
appropriately adjust the shape.
[0047]
15 Hereinafter, a method of producing the hot-stamped steel according to the
embodiment of the present invention will be described.
[0048]
In the embodiment, hot stamping is performed on the hot-rolled steel sheet, the
cold-rolled steel sheet, and the coated steel sheet which are manufactured under the
20 conditions of the embodiment under conditions in which the steel sheets are heated to a
temperature range of Ac3 point to 900°C at a heating rate of 3 °C/s or more, and then are
cooled at a cooling rate of 150 °C/s or more in the temperature range of 300°C to an Ar3
point so as to produce hot-stamped steels.
[0049]
25 Regarding the heat treatment conditions when hot stamping is performed on the
26
hot-rolled steel sheet, the cold-rolled steel sheet, and the coated steel sheet, in a case in
which the heating rate is less than 3 °C/s or the steel sheets are heated to higher than
900°C, the prior austenite grain size of 6 μm or less cannot be obtained, and the
dimensional ratio of the lengths of the prior austenite grains in the rolling direction to the
5 lengths of prior austenite grains in the sheet thickness direction becomes less than 1.3
after hot stamping. In addition, since the thermal holding time is preferably shorter
from the viewpoint of suppressing grain growth, the thermal holding time is set to 180
seconds or less. In addition, when the cooling rate is less than 150 °C/s during cooling
in the temperature range of 300°C to the Ar3 point, the strength in a part is liable to
10 change, and there is a concern that toughness may deteriorate due to precipitation of
coarse carbides. Therefore, the cooling rate in the temperature range of 300°C to the
Ar3 point is controlled to be 150 °C/s or more. Meanwhile, the upper limit of the
cooling rate in the temperature range is not particularly limited, and may be 500 °C/s in
consideration of the fact that the effect of transformation control reaches the upper limit.
15 On the other hand, when the heating temperature becomes lower than Ac3 point, some
areas are not transformed to austenite, and therefore martensite is not formed in the areas,
and a sufficient strength cannot be obtained. Furthermore, the effects of the
embodiment are not influenced even when cementite is precipitated due to auto
tempering during cooling or after cooling in hot stamping. Meanwhile, in order to more
20 reliably control the morphology of the prior austenite grains, the heating rate is
preferably 5 °C/s or more. The upper limit of the heating rate is not particularly limited,
and maybe 100 °C/s in consideration of the capability of a heating facility. In addition,
in a case in which the Ac3 point exceeds 870°C, the heating temperature is preferably
870°C or lower.
27
[Examples]
[Example 1]
[0050]
Steels having the chemical components shown in Table 2 (Steels A to Y) were
5 supplied from a converter, cast into slabs, and hot-rolled under predetermined hot rolling
conditions (heating temperature: 1220°C, finishing temperature: 870°C, total reduction
applied from the third last stand to the last stand: 65%, time from the end of finish rolling
to the beginning of cooling: 0.5 seconds, coiling temperature: 600°C), thereby
manufacturing 3 mm-thick hot-rolled steel sheets. For Steels A to L and Steels U to Y,
10 the prior austenite grain sizes in the hot-rolled steel sheet were 6 μm or less, and the
dimensional ratios of the length of prior austenite in a rolling direction to the length of
prior austenite in the sheet thickness direction were 1.3 or more. After the hot-rolled
steel sheets were cold-rolled so as to obtain 1.4 nun-thick cold-rolled steel sheets,
continuous annealing was performed under the conditions shown in Tabie 3, and a
15 coating was performed after the annealing according to necessity. The coating at this
time is galvanizing (GI (with no alloying)), galvannealing (GA (with an alloying)) or
alumizing (Al) including Al and 10% of Si. The steel sheets were heated to 900°C in a
heating furnace in a laboratory at a heating rate of 15 °C/s, temperature was maintained
for 60 seconds, then, the steel sheets were inserted between dies having a water supply
20 inlet through which water was supplied from the surface and a water drain outlet through
which the water was discharged, and was cooled to room temperature through spraying
of water (cooling at 150 °C/s to 500 °C/s), thereby simulating the thermal history during
hot stamping. Meanwhile, as a result of observing the microstructure of a cross section
using an optical microscope, the steel sheet subjected to the thermal history included
28
98% or more of martensite in terms of area percentage. Furthermore, in order to
evaluate the strength after the heat treatment, No. 5 specimens described in JIS Z 2201
were prepared from the steel sheets subjected to the thermal history, and tensile tests
were performed according to the testing method described in JIS Z 2241. The obtained
5 results are shown in Table 2 in the same manner. In addition, delayed-fracture
resistance and low-temperature toughness were also evaluated. For delayed-fracture
resistance, specimens having a V notch as shown in FIG 10 were used, the specimens
were immersed in an aqueous solution of 3 g/l of ammonium thiocyanate dissolved in a
3% saline solution at room temperature for 24 hours, and the presence of fracture was
10 determined (no fracture: A, fracture present: B). Meanwhile, for low-temperature
toughness, Charpy tests were performed at -40°C, and steel sheets (after being subjected
to the thermal history) for which an absorbed energy of 100 J/cm2 to 150 J/cm2 and a
percentage ductile fracture of 50% or more were obtained in a case in which evaluation
was made on a converted thickness of 10 mm were determined to be `pass (A)'. Steels
15 according to the present invention (Steels A to K and Steels U to Y) had a tensile strength
TS of 1470 MPa or more, and had sufficient delayed-fracture resistance and
low-temperature toughness. Meanwhile, for Steel Lin which the amount of C was less
than 0.20%, the tensile strength TS failed to reach 1470 MPa. In addition, for Steel M
in which the amount of C exceeded 0.35%, the tensile strength TS was 2230 MPa,
20 delayed-fracture resistance and low-temperature toughness degraded. Furthermore, for
Steels N, 0, R, S, and T to which Ti or Nb was not added, since the dimensional ratio of
the lengths of the prior austenite grains in the rolling direction to the lengths of the prior
austenite in the sheet thickness direction did not reach 1.3, and the average grain size was
larger than 6 μm after hot stamping of the thermal history, toughness was low.
25 Meanwhile, for Steel P in which the amount of Si exceeded 0.5%, delayed-fracture
29
resistance was not sufficient, and the coatability was poor. Furthermore, for Steel Q in
which the amount of 0 was less than 0.003%, since the prior austenite grains having an
average grain size of 6 μm or less were not obtained, the delayed-fracture resistance was
poor.
5 [0051]
[Table 2]
Chemical components (mass%) Ac3 Ar3
Steel
C Si Mn Cr P S t-Al Ti Nb V Mo B REM 0 N Otheres (°C) (°C)
A 0.21 0.14 1.27 0.25 0.006 0.0028 0.029 0.031 0.038 - - - - 0.0037 0.0021 - 813 712
B 0.25 0.28 1.34 0.18 0.008 0.0034 0.038 0.019 0.057 - 0. 15 0.0011 0.009 0.0041 0.0022 Cu:0.09,Ni:0.04,Sn0.013 809 588
C 0.28 0.12 1 .37 0.21 0.008 0.0051 0.034 0.008 0.042 - - - 0.009 0.0032 0.0015 - 789 682
D 0.28 0.15 1.77 0.23 0.005 0.0032 0.028 0.014 0.071 - - - 0. 018 0.0048 0.0029 - 776 626
E 0.28 0.18 2.67 0.24 0.007 0.0027 0.031 0.072 0.054 - - - 0.016 0.0038 0.0018 Cu0.11,Ni:0.05,Sn0.013 777 557
F 0.28 0.15 1 .23 0.79 0.011 0.0037 0.041 0.015 0.085 0.07 - - - 0.0045 0.0023 Cu0. 08,Ni:0.05,Sn:r 0.011 802 603
O 0.29 0.12 1 . 58 0.18 0.013 0.0033 0.028 0.037 0.052 - 0.11 - - 0.0035 0.0018 Ca0.008 794 648
H 0.29 0.17 1.32 0.32 0.014 0.0024 0.022 0.045 0.076 - - 0.0007 0.025 0.0038 0.0022 Mg0.011 805 568
I 0.30 0.15 1 .28 0.87 0.007 0.0093 0.028 0.015 0.015 0.05 0.21 0.0015 0.024 0.0057 0.0024 - 789 556
J 0.32 0.23 1.31 0.56 0.011 0.0035 0.038 0.024 0.037 - - - 0.008 0.0035 0.0018 Cu:0. 10,Ni:0.04,Sn:0.012 799 659
K 0.35 0.49 1.22 0.19 0.009 0.0021 0.047 0.011 0.007 - 0.42 0.0011 0.011 0.0061 0.0018 W0.52 805 598
L 0.19 0.35 1 .56 0.22 0.007 0.0023 0.029 0.022 0.015 - - - 0.009 0.0035 0.0022 - 816 721
M 0.36 0.21 2. 11 0.24 0.005 0.0077 0.039 0.027 0.009 - - - 0.011 0.0041 0.0015 - 764 624
N 0.31 0.19 1 .23 0.87 0.002 0.0051 0.028 0.009 - 0.012 - - 0.009 0.0038 0.0021 W0.15 786 667
O 0.34 0.22 1.15 0.26 0.004 0.0029 0.031 0.024 - - 0.14 - 0. 018 0.0048 0.0023 - 796 713
P 0.33 0.67 1.32 0.25 0.011 0.0087 0.026 0.011 0.045 0.091 0.42 0.0009 - 0.0038 0.0018 - 816 565
Q 0.31 0.26 1.33 0.22 0.009 0.0042 0.008 0.075 0.012 0.054 - - 0.015 0.0015 0.0013 Cu:0.12,Ni0.08,Sn:0.018 814 690
R 0.28 0.35 1 .28 0.56 0.013 0.0051 0.015 - 0.038 - - - 0.008 0.0038 0.0022 - 795 670
S 0.30 0.15 1 . 11 0.33 0.008 0.0029 0.026 0.072 - 0.072 - - - 0.0035 0.0018 Ca:0.013 796 722
T 0.30 0.23 2.21 0.18 0.007 0.0023 0.029 - - - 0.0011 0.011 0.0038 0.0023 - 767 568
U 0.30 0.25 1. 88 0.20 0.003 0.0018 0.026 0.042 0.012 - - - 0.009 0.0042 0.0022 Cu0.12,Nii0.08 791 660
V 0.31 0.14 1.57 0.21 0.004 0.0025 0.033 0.008 0.009 - - - 0.018 0.0036 0.0031 Cu0.12,Ni0.06 782 679
W 0.35 0.13 1. 15 0.75 0.007 0.0037 0.017 0.089 0.015 - - 0.0009 0.015 0.0041 0.0017 Cu:0.15,Ni0.07,Sn0.008 813 562
X 0.35 0.35 1 . 74
1
0.23 0.003 0.0018 0.028 0.045 0.055 0.062 - 0.0014 0.009 0.0035 0.0014 Cu:0.09,Ni0.07 ,Sn:0.011 791 536
Y 0.30 0. 3 1.15 0.68 0.003 0.0034 0.037 0.020 0.005 0.035 - 0.0008 0.010 0.0051 0.0021 Cu:0.09,Ni:0.04,Sn0.091 809 600
* Cells underlined in this Table do not satisfy the conditions according to the present invention.
teel
Annealing
temperature
(°C)
oating
Skin
pass
(%)
TS
before
heat
treatment
tea)
TS
after
cooling
(MPa)
El
CO)
Grain size
of
prior
austenite
(!im)
Grain size ratio
of prior austenite
(rolling d rectionisheet
thickness direction)
O
oatability
Delayedfracture
resistance
oughness
Percentage
ductile
fracture
o
(/o)
A 750 None 0.5 695 1524 11.6 5.7 1.34 - A A 100
B 770 At 0.5 750 1684 10.9 5.9 1.38 A A A 100
C 780 None 1.0 782 1785 11.5 5.8 1.46 - A A 90
D 750 None 1.0 792 1781 12.3 5.7 1.51 - A A 90
E 750 None 1.0 812 1801 11.5 5.9 1.45 - A A 100
F 780 None 1.0 814 1795 11.5 5.5 1.48 - A A 90
G 780 None 1.0 811 1795 11.2 5.3 1.52 - A A 95
H 800 None 1.0 795 1792 11.4 5.7 1.38 - A A 85
I 780 Al 0.7 812 1821 11.3 4.5 1.34 A A A 100
J 790 Z n 6A) 1.2 832 1922 10.7 5.8 1.32 A A A 90
K 780 Zn(GI) 1.5 887 2014 10.2 4.8 1.38 A A A 80
L 780 Al 1.5 742 1458 14.1 5.8 1.37 A A A 100
M 750 None 0.8 912 2470 7-8 5.2 1.41 - B B 20
N 780 Al 1.2 825 1878 10.9 10.1 1.02 A A B 30
0 790 None 2.1 869 1995 10.3 11.8 1.11 - A B 30
P 780 Zn(GI) 1.5 857 1972 10.4 8.4 1.34 B B A 70
Q 800 Zn GGA) 1.1 822 1946 10.5 9.4 1.37 A B A 60
R 780 7.n(G1) 0.8 788 1805 11.4 8.9 1.19 A A B 40
S 780 Al 1.2 815 1835 11.2 9.7 1.02 A A B 35
T 750 None 1.2 820 1826 1L2 154 0.98 A B B 20
U 780 Al 1.0 802 1833 11.2 5.7 1.31 A A A 75
V 770 None 0.7 810 1857 11.1 5.7 1.39 A A A 80
W 800 Zn(GI) 1.5 894 2101 9.8 5.2 1.31 A A A 80
X 780 Al 0.8 905 2191 9.4 5.2 1.34 A A A 75
Y 790 Al 1.2 797 1895 10.8 5.7 1.33 A A A 85
* Cells underlined in this Table do not satisfy the conditions according to the present invention.
32
[Example 2]
[0053]
For Steels I, U, and Y in Table 2, 2 mm-thick hot-rolled steel sheets were
obtained under predetermined hot rolling conditions (heating temperature: 1250°C,
5 finishing temperature: 880°C, total reduction applied from the third last stand to the last
stand: 60%, time from the end of finish rolling to the beginning of cooling: 0.8 seconds,
coiling temperature: 550°C), and then pickled. A heating and cooling treatment was
performed in which the hot-rolled steel sheets were heated to 880°C in a heating furnace
as they were, the temperature was maintained for 120 seconds, then, the steel sheets were
10 inserted between dies having a water supply inlet through which water was supplied from
the surface and a water drain outlet through which the water was discharged, and was
cooled to room temperature through spraying of water. Furthermore, the same heating
and cooling treatment was performed on the hot-rolled steel sheets on which galvanizing
(GI), galvannealing (GA), or aluminizing including Al and 10% of Si had been
15 performed after the pickling. Meanwhile, 3.2 mm-thick hot-rolled steel sheets were
obtained under predetermined hot rolling conditions (heating temperature: 1250°C,
finishing temperature: 890°C, total reduction applied from the third last stand to the last
stand: 70%, time from the end of finish rolling to the beginning of cooling: 0.5 seconds,
coiling temperature: 500°C), pickled in the same manner, and cold-rolled at a reduction
20 of 50%, thereby producing 1.6 mm-thick cold-rolled steel sheets. The cold-rolled steel
sheets were put into a heating furnace heated to 900°C in the laboratory, the temperature
was maintained for 60 seconds, and the steel sheets were cooled in the same manner as in
Example 1. Meanwhile, as a result of observing the microstructure of a cross section
using an optical microscope, the steel sheet subjected to the thermal history included
teel
Cold
rolling
oating
Skin
pass
(%)
TS
before
heat
treatment
(MPa)
TS
after
ccooook
(MPa)
El
41.)
Grain size
of
prior austeni[e
(μm)
Gram size ratio
off prior austenite
(rolling direction/sheet
thickness direction)
(-)
Delayedfracture
resistance
oughness
No None 1.0 795 1845 11.1 5.8 1.42 A A
No GI 12 808 1851 11.1 5.4 1.46 A A
No GA 1.5 818 1848 11.1 5.3 1.42 A A
I No Al 0.8 821 1838 11.2 5.4 1.44 A A
Yes None 1.0 812 1847 13.1 5.6 1.42 A A
Yes GI 1.2 822 1852 12.5 5.3 1.45 A A
Yes GA 1.5 830 1851 12.4 5.2 1.43 A A
Yes Al 0.8 835 1842 12.8 5.7 1.41 A A
No None 1.0 812 1849 11 .1 5.8 1.32 A A
No GI 1.2 821 1852 11.1 5.5 1.33 A A
No GA 1.5 829 1855 11.1 5.4 1.34 A A
U No Al 0.8 835 1841 11.2 5.8 1.32 A A
Yes None 1.0 828 1844 12.8 5.2 1.31 A A
Yes GI 1.2 834 1848 12.5 5.6 1.32 A A
Yes GA 1.5 839 1854 12.4 5.7 1.33 A A
Yes Al 0.8 842 1852 12.9 5.6 1.32 A A
No None 1.0 829 1849 11 .8 5.6 1.37 A A
No GI 1.2 835 1857 11.5 5.7 1.39 A A
No GA 1.5 842 1848 11 .2 5.4 1.38 A A
Y
No Al 0.8 851 1849 11 .2 5.6 1.37 A A
Yes None 1 .0 836 1852 13.5 5.7 1.38 A A
Yes GI 1.2 842 1849 13.7 5.6 1.39 A A
Yes GA 1.5 848 1856 13.6 5.7 1.37 A A
Yes Al 0.8 852 1856 13.7 5.8 1.38 A A
H
N
P
34
[Example 3]
[0055]
Steel I in Table 2 was subjected to hot rolling under the hot rolling conditions
shown in Table 5 and, subsequently, cold rolling at a reduction of 50%. The steel sheet
5 was heated to 850°C at the heating rate shown in Table 5, then, inserted between dies
having a water supply inlet through which water was supplied from the surface and a
water drain outlet through which the water was discharged, and was cooled to room
temperature through spraying of water. Meanwhile, as a result of observing the
microstructure of a cross section using an optical microscope, the steel sheet subjected to
10 the thermal history included 98% or more of martensite in terms of area percentage.
For the obtained steel sheets, the same material properties as in Example 1 were
evaluated, and the obtained results are shown in Table 5. For toughness, Charpy tests
were performed at -120°C, and steel sheets (after being subjected to the thermal history)
for which an absorbed energy of 85 J/cm2 or more was obtained in a case in which
15 evaluation was made on a converted thickness of 10 mm were determined to be `pass
(A)'. In addition, cracking of the edge portions of the cold-rolled steel sheet after the
cold rolling was checked, and a case in which cracking was not confirmed was evaluated
to be "A," and a case in which cracking was confirmed was evaluated to be "B." For
Nos. 1 to 5 according to the present invention, a tensile strength TS of a 1770 MPa level,
20 sufficient delayed-fracture resistance and toughness could be obtained. Meanwhile, for
No. 6 in which the heating temperature was higher than 1250°C, No. 7 in which the total
reduction from the third last stand to the last stand was less than 60%, and No. 10 in
which the heating rate was lower than 3 °C/s when hot stamping was performed, since
the dimensional ratio of the lengths of prior austenite grains in a rolling direction to the
25 lengths of prior austenite grains in the sheet thickness direction after hot stamping of the
35
thermal history was smaller than 1.3, toughness was poor. Meanwhile, for No. 8 in
which the finishing temperature was lower than 800°C (a temperature near the Ara point)
in the hot rolling, since the dimensional ratio of the lengths of prior austenite grains in
the rolling direction to the lengths of prior austenite in the sheet thickness direction
5 exceeded 2.5, toughness was not sufficient. In addition, for No. 9 in which the coiling
temperature (cooling-end temperature) was 400°C, and a reheating treatment was not
performed, since the strength of the hot-rolled steel sheet was too high, cold rolling
properties were poor. However, the No. 9 cold-rolled steel sheet had sufficient
delayed-fracture resistance and toughness. Further, separately from the examples, even
10 in a case in which the prior austenite grain size exceeded 6 μm, which was, for example,
15 μm, in a hot-rolled steel sheet, there was a case in which the prior austenite grain size
became 6 μm or less in a hot-stamped steel sheet (steel). However, in this case, it was
difficult to secure a dimensional ratio of the length of prior austenite in the rolling
direction to the length of prior austenite in the sheet thickness direction of 1.3 or more
15 through austenite transformation (re-austenite transformation) during heating in hot
stamping, and it was not possible to satisfy the value of toughness in a hot-stamped steel.
[0056]
[Table 5]
No.
Heating
temperature
(°C)
Total
reduction
(%)q
finishing
temperature
(°C)
Coolingstart
time
Coding
temperature
(°C)
Heating rate
during
hot stamping
(°C/s
TS
before
heat
treatment
(MPa)
TS
after
cooling
(NWa)
El )
(%
Grain sine
of
prior
austenite
(pm)
Grain size ratio
of prior austenite
(rolling direction/sheet
thickness direction)
l-)
Cold
rolling
properties'
Delayedfracture
resistance
Toughness
1 1200 60 880 0.5 600 5 752 1775 12.6 5.7 1.34 A A A
2 1250 65 900 0.8 500 10 785 1791 12.7 5.8 1.41 A A A
3 1250 70 850 1.0 550 10 771 1786 11.9 5.8 1.44 A A A
4 1250 70 870 0.1 580 50 783 1825 12.8 5.7 1.53 A A A
5 1270 60 900 0.5 550 100 791 1808 11.2 5.5 1.63 A A A
6 1300 60 900 0.1 600 10 789 1792 12.8 7 1.09 A A B
7 1200 50 880 0.5 650 50 775 1780 12.1 j 1.23 A A B
8 1230 60 55700 0.5 600 10 784 1795 9.8 5.8 111 A A B
9 1250 70 900 0.3 400 5 923 1785 10.9 5.5 1.37 B A A
10 1250 60 890 0.5 580 1 781 1787 12.2 5.8 0.98 A A B
1) Total reduction from the third last stand to the last stand
2) Cracking of edge portions after cold rolling
* Cells underlined in this Table do not satisfy the conditions according to the present invention.
37
Industrial Applicability
[0057]
According to the present invention, it is possible to provide a hot-stamped steel
having a strength of 1470 MPa or more and ductility in a part, to produce an
5 ultrahigh-strength steel sheet for hot stamping which is excellent in terms of the balance
of strength and toughness after hot stamping, and to producing a hot-stamped steel
having the above characteristics by controlling heating conditions and subsequent
cooling conditions when hot stamping is performed.
38
What is claimed is:
1. A hot-stamped steel comprising, by mass%:
C: 0.20% to 0.35%;
5 Si: 0.1%to 0.5%;
a total of at least one selected from Mn and Cr: 1% to 3%;
Al: 0.005% to 0.06%;
Ti: 0.002% to 0.1 %;
Nb: 0.002% to 0.1%;
10 0: 0.003% to 0.007%; and
a balance of iron and inevitable impurities, wherein
an amount of P is limited to 0.015% or less,
an amount of S is limited to 0.01% or less,
an amount of N is limited to 0.004% or less,
15 a dimensional ratio of lengths of prior austenite grains in a rolling direction to
the lengths of the prior austenite grains in a sheet thickness direction is 1.3 to 2.5,
an average grain size of the prior austenite grains is 6 μm or less,
a microstructure includes 98% or more of martensite, and a tensile strength is
1470 MPa or more.
20
2. The hot-stamped steel according to claim 1, further comprising, by mass%, one or
more of:
B: 0.005% or less;
V: 0.1% or less;
25 Me: 0.5% or less;
39
Ca: 0.03% or less;
Mg: 0.03% or less;
REM: 0.03% or less;
Cu: 0.5% or less;
5 Sri: 0.1% or less;
Ni: 0.5% or less; and
W: 1 % or less.
3. The hot-stamped steel according to claim 1 or 2, further comprising, a coating layer
10 formed by solidification of molten metal on a surface.
4. A method of producing a steel sheet for a hot-stamped steel, the method comprising:
a first process in which a slab is heated to a temperature range of 1270°C or
lower;
15 a second process in which a finish rolling is performed in a temperature range of
800°C to 900°C so that a total reduction from a third last stand to a last stand becomes
60% or more;
a third process in which a cooling begins within 1 second from an end of the
second process; and
20 a fourth process in which a coiling is performed in a temperature of 600°C or
lower,
the slab comprising: by mass%,
C: 0.20% to 0.35%,
Si: 0.1%to 0.5%,
25 a total of at least one selected from Mn and Cr: I% to 3%,
40
Al: 0.005% to 0.06%,
Ti: 0.002% to 0.1 %,
Nb: 0.002% to 0.1 %,
0: 0.003% to 0.007%, and
5 a balance of iron and inevitable impurities, wherein
P is limited to 0.015% or less,
S is limited to 0.01% or less, and
N is limited to 0.004% or less.
10 5. The method of producing a steel sheet for a hot-stamped steel according to claim 4,
wherein the slab further includes, by mass%, one or more of
B: 0.005% or less,
V: 0.1 % or less,
Me: 0.5% or less,
15 Ca: 0.03% or less,
Mg: 0.03% or less,
REM: 0.03% or less,
Cu: 0.5% or less,
Sn: 0.1 % or less,
20 Ni: 0.5% or less, and
W: 1% or less.
6. The method of producing a steel sheet for a hot-stamped steel according to claim 4
or 5; further comprising, after the fourth process,
25 a process in which a cold rolling is performed.
41
7. The method of producing a steel sheet for a hot-stamped steel according to claim 4
or 5, further comprising, after the fourth process,
a process in which a cold rolling and a continuous annealing is performed.
5
8. The method of producing a steel sheet for a hot-stamped steel according to claim 4
or 5, further comprising , after the fourth process,
a process in which a coating of molten metal is performed.
10 9. The method of producing a steel sheet for a hot-stamped steel according to claim 4
or 5, further comprising, after the fourth process,
a process in which a cold rolling is performed , and a coating of molten metal is
performed.
15 10. The method of producing a steel sheet for a hot-stamped steel according to claim 4
or 5, further comprising, after the fourth process,
a process in which a cold rolling and a continuous annealing are performed, and
a coating of molten metal is performed.
20 it. A method of producing a hot-stamped steel, the method comprising,
hot-stamping a, steel sheet obtained using the method of producing a steel sheet
for a hot-stamped steel according to claim 4 under a condition in which the steel sheet is
heated to a temperature range of an Ac3 point to 900 ° C at a heating rate of 3 °C/s or
more, and then the steel sheet is cooled at a cooling rate of 150 °C/s or more in a
25 temperature range of 300 °C to an Ar3 point.
42
12. A method of producing a hot-stamped steel, the method comprising,
hot-stamping a steel sheet obtained using the method of producing a steel sheet
for a hot-stamped steel according to claim 5 under a condition in which the steel sheet is
5 heated to a temperature range of an Ac3 point to 900°C at a heating rate of 3 °C/s or,
more, and then the steel sheet is cooled at a cooling rate of 150 °C/s or more in a
temperature range of 300°C to an Ara point.