TECHNICAL FIELD
[0001] The present invention relates to a heattreated
steel material used for an automobile and the
like, and a method of manufacturing the same.
BACKGROUND ART
[00021 A steel sheet for automobile is required to
improve fuel efficiency and crashworthiness.
Accordingly, attempts are being made to increase
strength of the steel sheet for automobile. However,
ductility such as press formability generally
decreases in accordance with the improvement of
strength, so that it is difficult to manufacture a
component having a complicated shape. For example,
in accordance with the decrease in ductility, a
portion with a high working degree fractures, or
springback and wall warp become large to deteriorate
accuracy in size. Therefore, it is not easy to
manufacture a component by press-forming a highstrength
steel sheet, particularly, a steel sheet
having tensile strength of 780 MPa or more.
[00031 Patent Literatures 1 and 2 describe a forming
method called as a hot stamping method having an
object to obtain high formability in a high-strength
steel sheet. According to the hot stamping method,
it is possible to form a high-strength steel sheet
with high accuracy, and a steel materlal obtained
through the hot stamping method also has high
strength. Further, a microstructure of the steel
material obtained through the hot stamping method is
substantially made of a martensite single phase, and
has excellent local deformability and toughness
compared to a steel material obtained by performing
cold forming on a high-strength steel sheet with
multi-phase structure.
[0004] Generally, crushing strength when collision
of an automobile occurs greatly depends on material
strength. For this reason, in recent years, a demand
regarding a steel material having tensile strength of
1.800 GPa or more, for example, has been increasing,
and Patent Literature 3 describes a method having an
object to obtain a steel material having tensile
strength of 2.0 GPa or more.
[0005] According to the method described in Patent
Literature 3, although it is possible to achieve the
desired object, sufficient toughness and weldability
cannot be obtained. Even with the use of the other
conventional techniques such as steel sheets
described in Patent literatures 4 to 6, and the like,
it is not possible to obtain tensile strength of
1.800 GPa or more while achieving excellent toughness
and weldability.
CITATION LIST
PATENT LITERATURE
[0006] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2002-102980
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2012-180594
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2012-1802
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2013-104081
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2006-152427
Patent Literature 6: International Publication
Pamphlet No. WO 2013/105631
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007] The present invention has an object to
provide a heat-treated steel material capable of
obtaining tensile strength of 1.800 GPa or more while
achieving excellent toughness and weldability, and a
method of manufacturing the same.
SOLUTION TO PROBLEM
[0008] As a result of earnest studies to solve the
above problems, the present inventors found out that
when a heat-treated steel material contains specific
amounts of C and Mn, it is possible to obtain
strength of 1.800 GPa or more with obtaining
excellent toughness and weldability, although details
thereof will be described later.
(00091 The higher a C content, the higher a
dislocation density in martensite and finer
substructures (lath, block, packet) in a prior
austenite grain. Based on the above description, it
is considered that a factor other than solid-solution
strengtheninq of C also greatly contributes to the
strength of martensite. The mechanism by which
dislocation occurs in the martensite and the
mechanism by which the substructures become fine, is
estimated as follows. Transformation from austenite
to martensite is accompanied by expansion, so that in
accordance with martensite transformation, strain
(transformation strain) is introduced into
surrounding nori-transformed austenite, and in order
to lessen the transformation strain, the martensite
right after the transformation undergoes supplemental
deformation. On this occasion, since the
transformation strainin austenite strengthened by C
is large, fine lath and block are generated to reduce
the transformation strain, and the martensite
undergoes supplemental deformation while being
subjected to introduction of a large number of
dislocations. It is estimated that, because of such
mechanisms, the dislocation density in the martensite
is high, and the substructures in the prior austenite
grain become fine.
[0010] The present inventors found out, based on the
above-described estimation, that the dislocation
density increases, crystal grains become fine, and
the tensile strength dramatically increases, in
accordance with quenching, also when a steel sheet
contains Mn, which introduces a compressive strain
into a surrounding lattice similarly to C.
Specifically, the present inventors found out that
when a heat-treated steel material including
martensite as its main structure contains a specific
amount of Mn, the steel material is affected by
indirect strengthening such as dislocation
strengthening and grain refinement strengthening, in
addition to solid-solution strengthening of Mn,
resulting in that desired tensile strength can be
obtained. Further, it has been clarified by the
present inventors that in a heat-treated steel
material including martensite as its main structure,
Mn has strengthening property of about 100 MPa/mass%
including the above-described indirect strengthening.
[OOll] It has been conventionally considered that
the strength of martensite mainly depends on the
solid-solution strengthening property of C, and there
is no influence of an alloying element almost at all
(for example, Leslie et al., Iron & Steel Material
Science, Maruzen, 1985), so that it has not been
known that Mn exerts large influence on the
improvement of strength of the heat-treated steel
material.
[00121 Then, based on these findings, the inventors
of the present application reached the following
various embodiments of the invention.
[00131 (1)
A heat-treated steel material, including:
a chemical composition represented by, in mass%:
C: 0.05% to 0.30%;
Mn: 2.0% to 10.0%;
Cr: 0.01% to 1.00%;
Ti: 0.010% to 0.100%;
B: 0.0010% to 0.0100%;
Si: 0.08% or less;
P: 0.050% or less;
S: 0.0500% or less;
N: 0.0100% or less;
Ni: 0.0% to 2.0%;
Cu: 0.0% Lo 1.0%;
Mo: 0.0% to 1.0%;
v: 0.0% to 1.0%;
Al: 0.00% to 1.00%;
~ b :0. 00% to 1.00%; and
the balance: Fe and impurities, and
a microstructure represented by
martensite: 90 volume% or more,
wherein an "Expression 1" is satisried where [Cl
denotes a C content (mass%) and [Mn] denoLes a Mn
content (mass%),
4612 x [C] + 102 x /Mnl + 605 1800 ...
"Expression 1";
wherein a dislocation density in the martensite
is equal to or more than 9.0 x 1015 m-2; and
wherein a tensile strength is 1.800 GPa or more.
[00141 (2)
The heat-treated steel material according to (I),
wherein in the chemical composition,
Ni: 0.1% to 2.0%,
Cu: 0.1% to 1.0%,
Mo: 0.1% to 1.0%,
V: 0.1% to 1.0%,
Al: 0.01% to 1.00%, or
Nb: 0.01% to 1.00%, or
any combination thereof is satisfied.
[00151 (3)
A method of manufacturing a heat-treated steel
material, including:
heating a steel sheet to a temperature zone of
not less than an Ac3 point nor more than "the Ac3
point + 200°C" at an average heating rate of 1O0C/s or
more;
next, cooling the steel sheet from the
temperature zone to an Ms point at a rate equal to or
more than an upper critical cooling rate; and
next, cooling the steel sheet from the Ms point
to 100°C at an average cooling rate of 50°C/s or more,
wherein the steel sheet includes a chemical
composition represented by, in mass%:
C: 0.05% to 0.30%;
Mn: 2.0% to 10.0%;
Cr: 0.01% to 1.00%;
Ti: 0.010% to 0.100%;
B: 0.0010% to 0.0100%;
Si: 0.08% or less;
P: 0.050% or less;
S: 0.0500% or less;
N: 0.0100% or less;
Ni: 0.0% to 2.0%;
Cu: 0.0% to 1.0%;
Mo: 0.0% to 1.0%;
V: 0.0% to 1.0%;
Al: 0.00% to 1.00%;
~ b :0. 00% to 1.00%; and
the balance: Fe and impurities,
wherein an "Expression 1" is satisfied where [CI
denotes a C content (mass%) and [Mn] denotes a Mn
content (mass%),
4612 x [C] + 102 x [Mn] + 605 2 1800 ...
"Expression 1".
100161 (4)
The method of manufacturing the heat-treated
steel material according to ( 3 ) , wherein in the
chemical composition,
Ni: 0.1% to 2.0%,
Cu: 0.1% to 1.09,
Mo: 0.1% to 1.0%,
V: 0.1% to 1.0%,
Al: 0.01% to 1.00%, or
Nb: 0.01% to 1.00% or
any combination thereof is satisfied.
[00171 (5)
The method of manufacturing the heat-treated
steel material according to (3) or (4), wherein the
steel sheet is subjected to forming before the
temperature of the steel sheet reaches the Ms point
after the heatiny the steel sheet to the temperature
zone of not less than the Ac3 point nor more than "the
Ac3 point t 200°C".
ADVANTAGEOUS EFFECTS OF INVENTION
[0018] According to the present invention, it is
possible to obtain strength of 1.800 GPa or more with
obtaining excellent toughness and weldability.
DESCRIPTION OF EMBODIMENTS
[0019] Hereinafter, an embodiment .of the present
invention will be described. Although details will
be described later, a heat-treated steel material
according to the embodiment of the present invention
is manufactured by quenching a specific steel sheet
for heat treatment. Therefore, hardenability of the
steel sheet for heat treatment and a quenching
condition exert influence on the heat-treated steel
material.
[0020] First, a chemical composition of the heattreated
steel material according to the embodiment of
the present invention and the steel sheet for heat
treatment used for manufacturing the heat-treated
steel material will be described. In the following
description, " s " being a unit of content of each
element contained in the heat-treated steel material
and the steel sheet used for manufacturing the heattreated
steel material means 'mass%" unless otherwise
mentioned. The heat-treated steel material according
to the present embodiment and the steel sheet used
for manufacturing the heat-treated steel material
includes a chemical composition represented by C:
0.05% to 0.30%, Mn: 2.0% to 10.08, Cr: 0.01% to
1.00%, Ti: 0.010% to 0.100%, B: 0.0010% to 0.0100%,
Si: 0.08% or less, P: 0.050% or less, S: 0.0500% or
less, N: 0.0100% or less, Ni: 0.0% to 2,0%, Cu: 0.0%
to 1.0%, Mo: 0.0% to 1.O%, V : 0.0% to 1.08, Al: 0.00%
to 1.00%, Nb: 0.00% to 1.00%, and the balance: Fe and
impurities, and an "Expression 1" is satisfied where
[C] denotes a C content (mass%) and [Mn] denotes a Mn
content (mass%). Examples of the impurities are
Chose contained in a raw maLerial s u c h as an ore or
scrap, and those contained during manufacturing
processes.
4612 x [C] + 102 x [Mn] + 605 1800 ...
"Expression 1";
[0021] (C: 0.05% to 0.30%)
C is an element that enhances hardenability of
the steel sheet for heat treatment and improves
strength of the heat-treated steel material. IC the
C content is less than 0.05%, the strength of the
heat-treated steel material is not sufficient. Thus,
the C content is 0.05% or more. The C content is
preferably 0.08% or more. On the other hand, if the
C content exceeds 0.30%, the strength of the heattreated
steel material is too high, resulting in that
toughness and weldability significantly deteriorate.
Thus, the C content is 0.30% or less. The C content
is preferably 0.28% or less, and more preferably
0.25% or less.
[0022] (Mn: 2.0% to 10.0%)
Mn is an element which enhances the hardenability
of the steel sheet for heat treatment. Mn
strengthens martensite through not only solidsolution
strengthening but also facilitation of
introduction of a large number of dislocations during
martensite transformation, which occurs when
manufacturing the heat-treated steel material.
Specjfically, Mn has an effect of facilitating the
dislocation strengthening. Mn refines substructures
in a prior austenite grain after the martensite
transformation through the introduction of
dislocations, to thereby strer~gthen the martensite.
Specifically, Mn also has an effect of facilitating
grain refinement strengthening. Therefore, Mn is a
particularly important element. If the Mn content is
less than 2.0% where the C content is 0.05% to 0.30%,
the effect by the above function cannot be
sufficiently obtained, resulting in that the strength
of the heat-treated steel material is not sufficient.
Thus, the Mn content is 2.0% or more. The Mn content
is preferably 2.5% or more, and more preferably 3.6%
or more. On the other hand, if the Mn content
exceeds 10.0%, the strength of the heat-treated steel
material is too high, resulting in that toughness and
hydrogen embrittlement resistance significantly
deteriorate. Thus, the Mn content is 10.0% or less.
The Mn content is preferably 9.0% or less. A
strengthening property of Mn in the heat-treated
steel material including martensite as its main
structure is about 100 MPa/mass%, which is about 2.5
times a strengthening property of Mn in a steel
material including ferrite as its main structure
(about 40 MPa/mass%) .
[0023] (Cr: 0.01% to 1.00%)
Cr is an element which enhances the hardenability
of the steel sheet for heat treatment, thereby
enabling to stably obtain the strength of the heattreated
steel material. If the Cr content is less
than 0.01%, there is a case where the effect by the
above function cannot be sufficiently- obtaine'd.
Thus, the Cr content is 0.01% or more. The Cr
content is preferably 0.02% or more. On the other
hand, if the Cr content exceeds 1.00%, Cr
concentrates in carbides in the steel sheet for heat
treatment, resulting in that the hardenability
lowers. This is because, as Cr concentrates, the
carbides are more stabilized, and the carbides are
less solid-soluble during heating for quenching.
Thus, the Cr content is 1.00% or less. The Cr
content is preferably 0.80% or less.
[0024] (Ti: 0.010% to 0.100%)
Ti has an effect of greatly improving the
toughness of the heat-treated steel material.
Namely, Ti suppresses recrystallization and further
forms fine carbides to suppress' grain growth of
austenite during heat treatment for quenching at a
temperature of an Ac3 point or higher. Fine austenite
yrains are obtained by the suppressi.ur1 01 Lhe grain
growth, resulting in that the toughness greatly
improves. Ti also has an effect of preferentially
bonding with N in the steel sheet for heat treatment,
thereby suppressing R from being consumed by the
precipitation of BN. As will be described later, B
has an effect of improving the hardenability, so that
it is possible to securely obtain the effect of
improving the hardenability by B through suppressing
the consumption of B. If the Ti content is less than
0.010%, there is a case where the effect by the above
function cannot be sufficiently obtained. Thus, the
Ti content is 0.010% or more. The Ti content is
preferably 0.015% or more. On the other hand, if the
Ti content exceeds 0.100%, a precipitation amount of
Tic increases so that C is consumed, and accordingly,
there is a case where the heat-treated steel material
cannot obtain sufficient strength. Thus, the Ti
content is 0.100% or less. The Ti content is
preferably 0.080% or less.
[0025] (B: 0.0010% to 0.0100%)
B is a very important element having an effect of
significantly enhancing the hardenability of the
steel sheet for heat treatment. B also has an effect
of strengthening a grain boundary to increase the
toughness by segregating in the grain boundary. B
also has an effect of improving the toughness by
suppressing the grain growth of austenite during
heating of the steel sheet for heat treatment. If
the B content is less than 0.0010%. there is a case
where the effect by the above function cannot be
sufficiently obtained. Thus, the B content is
0.0010% or more. The B content is preferably 0.0012%
or more. On the other hand, if the B content exceeds
0.0100%, a large amount of coarse compounds
precipitate to deteriorate the toughness of the heattreated
steel material. Thus, the B content is
0.0100% or less. The B content is preferably 0.0080%
or less.
[GO261 (Si: 0.08% or less)
Si is not an essential element, but is contained
in the steel as impurities, for example. The higher
the Si content, the higher a temperature at which
austenite transformation occurs. As this temperature
is high, a cost required for heating for quenching
increases, or quenching is likely to be insufficient
due to insufficient heating. Besides, as the Si
content is high, wettability and alloying
processability of the steel sheet for heat Lreatment
are lowered, and therefore stability of hot-dip
process and alloying process deteriorates.
Therefore, the lower the Si content, the better. In
particular, when the Si content exceeds 0.08%, the
temperature at which austenite transformation occurs
is noticeably high. Thus, the Si content is 0.08% or
less. The Si content is preferably 0.05% or less.
[0027] (P: 0.050% or less)
P is not an essential element, but is contained
in the steel as impurities, for example. P
deteriorates the toughness of the heat-treated steel
material. Therefore, the lower the P content, the
better. In particular, when the P content exceeds
0.050%, the toughness noticeably lowers. Thus, the P
content is 0.050% or less. The P content is
preferably 0.005% or less. It requires a
considerable cost to decrease the P content to less
than 0.001%, and it sometimes requires a more
enormous cost to decrease the P content to less than
0.001%. Thus, there is no need to decrease the P
content to less than 0.001%.
[0028] (S: 0.0500% or less)
S is not an essential element, but is contained
in the steel as impurities, for example. S
deteriorates the toughness of the heat-treated steel
material. Therefore, the lower the S content, the
better. In particular, when the S content exceeds
0.0500%, the toughness noticeably lowers. Thus, the
S content is 0.0500% or less. The S content is
preferably 0.0300% or less. It requires a
considerable cost to decrease the S content to less
than 0.0002%, and it sometimes requires a more
enormous cost to decrease the S content to less than
0.0002%. Thus, there is no need to decrease the S
content to less than 0.0002%.
[0029] (N: 0.0100% or less)
N is not an essential element, but is contained
in the steel as impurities, for example. N
contributes to the formation of a coarse nitride and
deteriorates local deformability and the toughness of
the heat-treated steel material. Therefore, the
lower the N content, the better. In particular, when
the N content exceeds 0.0100%, the local
deformability and the toughness noticeably lower.
Thus, the N content is 0.0100% or less. It requires
a considerable cost to decrease the
N content to less than 0.0008%. Thus, there is no
need to decrease the N content to less than 0.0008%.
It sometimes requires a more enormous cost to
decrease the N content to less than 0.0002%.
[00301 Ni, Cu, Mo, V, Al, and Nb are not essential
elements, but are optional elements which may be
appropriately contained, up to a specific amount as a
limit, in the steel sheet for heat treatment and the
heat-treated steel material.
[0031] (Ni: 0.0% to 2.0%, Cu: 0.0% to 1.0%, Mo: 0.0%
Lo 1.0%, V: 0.0% LO 1.08, Al: 0.00% to 1.001, Nb:
0.00% to 1.00%)
Ni, Cu, Mo, V, Al, and Nb are elements which
enhance the hardenability of the steel sheet for heat
treatment, thereby enabling to stably' obtain the
strength of the heat-treated steel material. Thus,
one or any combination selected from the group
consisting of these elements may be contained.
However, if the Ni content exceeds 2.0%, the effect
by the above function saturates, which only increases
a wasteful cost. Thus, the Ni content is 2.0% or
less. If the Cu content exceeds 1.0%, the effect by
the above function saturates, which only increases a
wasteful cost. Thus, the Cu content is 1.0% or less.
If the Mo content exceeds 1.0%, the effect by the
above function saturates, which only increases a
wasteful cost. Thus, the Mo content is 1.0% or less.
If the V content exceeds 1.0%, the effect by the
above function saturates, which only increases a
wasteful cost. Thus, the V content is 1.0% or less.
If the A1 content exceeds 1.00%, the effect by the
above function saturates, which only increases a
wasteful cost. Thus, the Al content is 1.00% or
less. If the Nb content exceeds 1.00%, the effect by
the above function saturates, which only increases a
wasteful cost. Thus, the Nb content is 1.00% or
less. In order to securely obtain the effect by the
above function, each of the Ni content, the Cu
content, the Mo content, and the V content is
preferably 0.1% or more, and each of the A1 content
and the Nb content is preferably 0.01% or more.
Namely, it is preferable to satisfy one or any
combination of the following: "Ni: 0.1% to 2.0%",
"Cu: 0.1% to 1.0%", "Mo: 0.1% to 1.0%", "V: 0.1% to
1.0%", "Al: 0.01% to 1.00%", or "Nb: 0.01% to 1.00%".
[0032] As described above, C and Mn increase the
strength of the heat-treated steel material mainly by
increasing the strength of martensite. However, it
is not possible to obtain tensile strength of 1.800
GPa or more, if the "Expression 1" is not satisfied
where [C] denotes a C content (mass%) and [Mn]
- 17 -
denotes a Mn content (mass%). Accordingly, the
"Expression I" should be satisfied.
4612 x [C] + 102 x [Mnl + 605 2 1800 ...
"Expression 1";
[0033] Next, a microstructure of the heat-treated
steel material according to the present embodiment
will be described. The heat-treated steel material
according to the present embodiment includes a
microstructure represented by martensite: 90 volume%
or more. The balance of the microstructure is, for
example, retained austenite. When the microstructure
is formed of martensite and retained austenite, a
volume fraction (volume%) of the martensite may be
measured through an X-ray diffraction method with
high accuracy. Specifically, diffracted X-rays
obtained by the martensite and the retained austenite
are detected, and the volume fraction may be measured
based on an area raLiu uT the diCLracLion curve.
When the microstructure includes another phase such
as ferrite, an area ratio (area%) of the other phase
is measured through microscopic observation, for
example. The structure of the heat-treated steel
material is isotropic, so that a value of an area
ratio of a phase obtained at a certain cross section
may be regarded to be equivalent to a volume fraction
in the heat-treated steel material. Thus, the value
of the area ratio measured through the microscopic
observation may be regarded as the volume fraction
(volume%).
to0341 Next, a dislocation density in martensite in
the heat-treated steel material according to the
present embodiment will be described. The
dislocation density in the martensite contributes to
the improvement of tensile strength. When the
dislocation density in the martensite is less than
9.0 x 1015 m-', it is not possible to obtain the
tensile strength of 1.800 GPa or more. Thus, the
dislocation density in the martensite is 9.0 x 10'' m-'
or more.
to0351 The dislocation density may be calculated
through an evaluation method based on the Williamson-
Hall method, for example. The Williamson-Hall method
is described in 'G. K. Williamson and W. H. Hall:
Acta Metallurgica, 1(1953), 22", "G. K. Williamson
and R. E. Smallman: Philosophical Magazine, 8(1956),
34", and others, for example. Concretely, peak
fitting of respective diffraction spectra of a {200)
plane, a (211) plane, and a (220) plane of bodycentered
cubic structure is carried out, and p x
cos0/A is plotted on a horizontal axis, and sin0/A is
plotted on a vertical axis based on each peak
position (0) and half-width (6). An inclination
obtained from the plotting corresponds to local
strain E, and the dislocation density p (m-') is
determined based on a following "Expression 2"
proposed by Wlliamson, Smallman, et al. Here, b
denotes a magnitude of Burgers vector (nm).
p = 14.4 x z2/b2 . "Expression 2"
[00361 Further, the heat-treated steel material
according to the present embodiment has the tensile
strength of 1.800 GPa or more. The tensile strength
mayb be measured based on rules of ASTM standard E8,
for example. In this case, when producing test
pieces, soaked portions are polished until their
thicknesses become 1.2 mm, to be worked into halfsize
plate-shaped test pieces of ASTM standard E8, so
that a tensile direction is parallel to the rolling
direction. A length of a parallel portion of each of
the half-size plate-shaped test pieces.is 32 mm, and
a width of the parallel portion is 6 . 2 5 mm. Then, a
strain gage is attached to each of the test pieces,
and a tensile test is conducted at a strain rate of 3
mm/min at room temperature.
[00371 Next, a method of manufacturing the heattreated
steel material, namely, a method of treating
the steel sheet for heat treatment, will be
described. In the treatment of the steel sheet for
heat treatment, the steel sheet for heat treatment is
heated to a temperature zone of not less than an A c ~
point nor more than "the A c ~po int + 200°C'' at an
average heating rate of 10°C/s or more, the steel
sheet is then cooled from the temperature zone to an
Ms point at a rate equal to or more than an upper
critical cooling rate, and thereafter, the steel
sheet is cooled from the Ms point to 100°C at an
average cooling rate of 50°C/s or more.
[0038] If the steel sheet for heat treatment is
heated to the temperature zone of the Ac3 point or
more, the structure becomes an austenite single
phase. If the average heating rate is less than
10°C/s, there is a case that an austenite grain
becomes excessively coarse, or the dislocation
density lowers due to recovery, thereby deteriorating
the strength and the toughness of the heat-treated
steel material. Thus, the average heating rate is
10°C/s or more. The average heating rate is
preferably 20°C/s or more, and more preferably 50°C/s
or more. When the reaching temperature of the
heating exceeds "the Ac3 point + 200°C", there is a
case that an austenite grain becomes excessively
coarse, or the dislocation density lowers, thereby
deteriorating the strength and the toughness of the
heat-treated steel material. Thus, the reaching
temperature is "the Ac3 point + 200°C" or less.
[00391 The above-described series of heating and
cooling may also be carried out by, for example, a
hot stamping method, in which heat treatment and hot
forming are conducted concurrently, or high-frequency
heating and quenching. The period of time of
retention of the steel sheet in the temperature zone
of not less than the Ac3 point nor more than "the Ac3
point + 200°C" is preferably 30 seconds or more, from
a viewpoint of increasing the hardenability of steel
by accelerating the austenite transformation to
dissolve carbides. The retention time is preferably
600 seconds or less, from a viewpoint of
productivity.
[0040] If the steel sheet is cooled from the
temperature zone to the Ms point at the rate equal to
or mere than the upper critical cooling rate after
being subjected to the above-described heating, the
structure of the austenite single phase is
maintained, without occurrence of diffusion
transformation. If the cooling rate is less than the
upper critical cooling rate, the diffusion
transformation occurs so that ferrite is easily
generated, resulting in that the microstructure in
which the volume fraction of martensite is 90 volume%
or more is not be obtained. Thus, the cooling rate
to the Ms point is equal to or more than the upper
critical cooling rate.
[00411 If the steel sheet is cooled from the Ms
point to lOO0C at the average cooling rate of 5OoC/s
or more after the cooling to the Ms point, the
transformation from austenite to martensite occurs,
resulting in that the microstructure in which the
volume fraction of martensite is 90 volume% or more
can be obtained. As described above, the
transformation from austenite to martensite is
accompanied by expansion, so that in accordance with
the martensite transformation, strain (transformation
strain) is introduced into surrounding nontransformed
austenite, and in order to lessen the
transformation strain, the martensite right after the
transformation undergoes supplemental deformation.
Concretely, the martensite undergoes slip deformation
while being subjected to introduction of
dislocations. Consequently, the martensite includes
high-density dislocations. In the present
embodiment, the specific amounts of C and Mn are
contained, so that the dislocations are generated in
the martensite at extremely high density, and the
dislocation density becomes 9.0 x 1015 m-' or more-. If
the average cooling rate from the Ms point to 100°C is
less than 50°C/s, recovery of dislocations easily
occurs in accordance with auto-tempering, resulting
in that the dislocation density becomes insufficient
and the sufficient tensile strength cannot be
obtained. Thus, the average cooling rate is 50°C/s or
more. The average cooling rate is preferably 100°C/s
or more, and more preferably 500°C/s or more.
[00421 In the manner as described above, the heattreated
steel material according to the present
embodiment provided with the excellent toughness and
weldability, and the tensile strength of 1.800 GPa or
more, can be manufactured. An average grain diameter
of prior austenite grains in the heat-treated steel
material is about 10 vm to 20 vm.
100431 A cooling rate from less than 100°C to the
room temperature is preferably a rate of air cooling
or more. If the cooling rate is less than the air
cooling rate, there is a case that the tensile
strength lowers due to the influence of autotempering.
[0044] It is also possible to perform hot forming
such as the hot stamping described above, durinq the
above-described series of heating and cooling.
Specifically, the steel sheet for hezt treatment may
be subjected to forming in a die before the
temperature of the steel sheet reaches the Ms point
after the heating to the temperature zone of not less
than the Ac3 point nor more than "the Ac3 point +
200°C". Bending, drawing, bulging, hole expansion,
and flanging may be cited as examples of the hot
forming. These belong to press forming, but, as long
as it is possible to cool the steel sheet in parallel
with the hot forming or right after the hot forming,
hot forming other than the press forming, such as
roll forming, may also be performed.
[0045] The steel sheet for heat treatment may be a
hot-rolled steel sheet or a cold-rolled steel sheet.
An annealed hot-rolled steel sheet or an annealed
cold-rolled steel sheet obtained by performing
annealing on a hot-rolled steel sheet or a coldrolled
steel sheet may also be used as the steel
sheet for heat treatment.
[00461 The steel sheet for heat treatment may be a
surface-treated steel sheet such as a plated steel
sheet. Namely, a plating layer may be provided on
the steel sheet for heat treatment. The plating
layer contributes to improvement of corrosion
resistance and the like, for example. The plating
layer may be an eleclroplatiny layer or a hot-dip
plating layer. An electrogalvanizing layer and a Zn-
Ni alloy electroplating layer may be cited as
examples of the electroplating layer. A hot-dip
galvanizing layer, an alloyed hot-dip galvanizing
layer, a hot-dip aluminum plating layer, a hot-dip
Zn-A1 alloy plating layer, a hot-dip Zn-Al-Mg alloy
plating layer, and a hot-dip Zn-Al-Mg-Si alloy
plating layer may be cited as examples of the hot-dip
plating layer. A coating amount of the plating layer
is not particularly limited, and may be a coating
amount within an ordinary range, for example.
Similarly to the steel sheet for heat treatment, the
heat-treated steel material may be provided with a
plating layer.
[ 0 0 4 7 ] Note that any one of the above-described
embodiments only presents concrete examples in
carrying out the present invention, and the technical
scope of the present invention should not be
construed in a limited manner by these. That is, the
present invention may be embodied in various forms
without departing from its technical idea or its main
feature.
EXAMPLES
[OD481 Next, experiments conducted by the inventors
of the present application will be described.
[ 0 0 4 9 ] In the experiment, slabs each including a
chemical composition presented in Table 1 were
subjected to hot-rolling and cold-rolling, to thereby
manufacture cold-rolled steel sheets each including a
thickness of 1.4 mm, as steel sheets for heat
treatment. Blank columns in Table 1 indicate that
contents of elements in the blank columns are less
than detection limits, and the balance is Fe and
impurities. Underlines in Table 1 indicate that the
underlined numerical values are out of the ranges of
the present invention.
[0050] [Table 11
LEFT SIDE OF
[ 0 0 5 1 ] Then, samples each including a thickness of
1 . 4 mm, a width of 30 mm, and a length of 200 mm were
produced from the respective cold-rolled steel
sheets, and the samples were heated and cooled under
conditions presented in Table 2 . The heating and
cooling imitate heat treatment in hot forming. The
heating in the experiment was performed by
energization heating. After the cooling, soaked
portions were cut out from the samples, and the
soaked portions were subjected to a tensile test and
an X-ray diffraction test.
[ 0 0 5 2 ] The tensile test was conducted based on rules
of ASTM standard E 8 . In the tensile test, a tensile
tester made by Instron corporation was used. When
preparing test pieces, soaking portions were polished
until their thicknesses became 1 . 2 mm, to be worked
into half-size plate-shaped test pieces of ASTM
standard E 8 , so that a tensile direction was parallel
to the rolling direction. A length of a parallel
portion of each of the half-size plate-shaped test
pieces was 32 mm, and a width of the parallel portion
was 6 . 2 5 mm. Then, a strain gage was attached to
each of the test pieces, and a tensile test was
conducted at a strain rate of 3 mm/min at room
temperature. As the strain gage, KFG-5 (gage length:
5 mm) made by KYOWA ELECTRONIC INSTRUMENTS CO., LTD.
was used.
[ 0 0 5 3 ] In the X-ray diffraction test, portions up to
a depth of 0 . 1 mm from surfaces of the soaked
portions were chemically polished by using
hydrofluoric acid and a hydrogen peroxide solution,
thereby preparing test pieces for the X-ray
diffraction test each having a thickness of 1.1 mm.
Then, a Co tube was used to obtain an X-ray
diffraction spectrum of each of the test pieces in a
range of 28 from 45' to 130°, and a dislocation
density was determined from the X-ray diffraction
spectrum. Further, volume fractions of martensite
were also determined based on the detection results
of the diffracted X-rays and results of observation
by optical microscope according to need in addition
to the results of the diffracted X-rays.
[0054] The dislocation density was calculated
through the evaluation method based on the abovedescribed
Williamson-Hall method. Concretely, in
this experiment, peak fitting of respective
diffraction spectra of a {200) plane, a {211) plane,
and a {220) plane of body-centered cubic structure
was carried out, and p x cos8/A was plotted on a
horizontal axis and sin8/A was plotted on a vertical
axis based on each peak position ( 0 ) and half-width
( p ) . Then, the dislocation density p (m-') was
determined based on the "Expression 2".
[0055] Results of these are presented in Table 2.
Underlines in Table 2 indicate that the underlined
numerical values are out of the ranges of the present
invention.
[0056] [Table 21

[0057] As presented in Table 2, in the samples No. 1
to No. 6, No. 10 to No. 13, and No. 16 to No. 20,
since the chemical compositions were within the
ranges of the present invention, and the
manufacturing conditions were also within the ranges
of the present in~vention, desired microstructures and
dislocation densities were obtained in the heattreated
steel materials. Further,. since the chemical
compositions, the microstructures, and the
dislocation densities were within the ranges of the
present invention, the tensile strengths of 1.800 GPa
or more were obtained.
[0058] In the samples No. 7 to No. 9, No. 14, No.
15, No. 21, and No. 22, although the chemical
compositions were within the ranges of the present
invention, the manufacturing conditions were out of
the ranges of the present invention, and thus it was
not possible to obtain desired dislocation densities.
Further, since the dislocation densities were out of
the ranges of the present invention, the tensile
strengths were low to be less than 1.800 GPa.
[0059] In the samples No. 23 and No. 24, since 'the
Mn contents were out of the ranges of the present
invention, even though the manufacturing conditions
were within the ranges of the present invention, the
dislocation densities were less than 9.0 x 1015 m-',
and the'tensile strengths were low to be' less than
1.800 GPa.
[0060] In the sample No. 25, since the C content was
out of the range of the present invention, even
though the manufacturing condition was within the
range of the present invention, the dislocation
density was less than 9 . 0 x 1015 m-', and the tensile
strength was low to be less than 1 . 8 0 0 GPa.
[ 0 0 6 1 ] In the sample No. 26, the "Expression 1" was
not satisfied, so that even when the manufacturing
condition was within the range of the present
invention, the dislocation density was less than 9.0
x 1015 m-', and the tensile strength was low to be less
than 1 . 8 0 0 GPa.
[ 0 0 6 2 ] From these results, it is understood that it
is possible to obtain a high-strength heat-treated
steel material according to the present invention.
Further, according to the present invention, it is
not required that C is contained to such an extent as
to deteriorate the toughness and the weldability in
order to obtain the high streriyth, so that it is also
possible to obtain excellent toughness and
weldability.
INDUSTRIAL APPLICABILITY
LO0631 The present invention may be used in the
industries of manufacturing heat-treated materials
and the like used for automobiles, for example, and
in the industries of using them. The present
invention may also be used in the industries of
manufacturing other mechanical structural components,
the industries of using them, and the like.

CLAIMS
[Claim 11 A heat-treated steel material, comprising:
a chemical composition representcd by, in mass%:
c: 0.05% to 0.30%;
Mn: 2.0% to 10.0%;
Cr: 0.01% to 1.00%;
Ti: 0.010% to 0.100%;
B: 0.0010% to 0.0100%;
Si: 0.08% or less;
P: 0.050% or less;
S: 0.0500% or less;
N: 0.0100% or less;
Ni: 0.0% to 2.0%;
Cu: 0.0% to 1.0%;
Mo: 0.0% to 1.0%;
v: 0.0% to 1.0%;
Al: 0.00% to 1.00%;
~ b :0. 00% to 1.00%; and
the balance: Fe and impurities, and
a microstructure represented by
martensite: 90 volume% or more,
wherein an "Expression 1" is satisfied where [C]
denotes a C content (mass%) and [Mn] denotes a Mn
content (mass%),
4612 x [C] + 102 x [Mn] + 605 1800 ...
"Expression 1";
wherein a dislocation density in the martensite
is equal to or more than 9.0 x 1015 m-7; and
wherein a tensile strength is 1.800 GPa or more.
[Claim 21 The heat-treated steel material according
to claim 1, wherein in the chemical composition,
Ni: 0.1% to 2.0%,
Cu: 0.1% to 1.0%,
Mo: 0.1% to 1.01,
V: 0.1% to 1.0%,
Al: 0.01% to 1.00%, or
Nb: 0.01% to 1.001, or
any combination thereof is satisfied.
[Claim 31 A method of manufacturing a heat-treated
steel material, comprising:
heating a steel sheet to a temperature zone of
not less than an Ac3 point nor more than "the Ac3
point 4- 200°C" at an average heating rate of 10°C/s or
more;
next, cooling the steel sheet from the
temperature zone to an Ms point at a rate equal to or
more khan an upper critical cooling rate; and
next, cooling the steel sheet from the Ms point
to 100°C at an average cooling rate of 50°C/s or more,
wherein the steel sheet comprises a chemical
composition represented by, in mass%:
c: 0.05% to 0.30%;
Mn: 2.0% to 10.0%;
Cr: 0.01% to 1.00%;
Ti: 0.010% to 0.100%;
B: 0.0010% to 0.0100%;
Si: 0.08% or less;
P: 0.050% or less:
S: 0.0500% or less;
N: 0.0100% or less;
Ni: 0.0% to 2.0%;
Cu: 0.0% to 1.0%;
Mo: 0.0% to 1.0%;
V: 0.0% to 1.0%;
Al: 0.00% to 1.00%;
~ b :0. 00% to 1.00%; and
the balance: Fe and impurities,
wherein an "Expression 1" is satisfied where [C]
denotes a C content (mass%) and [Mn] denotes a Mn
content (mass%),
4612 x [C] + 102 x [Mn] f 605 2 1800 ...
"Expression 1".
[Claim 41 The method of manufacturing the heattreated
steel material according to claim 3, wherein
in the chemical composition,
Ni: 0.1% to 2.0%,
Cu: 0.1% to 1.0%,
Mo: 0.1% to 1.0%,
V: 0.1% to 1.0%,
Al: 0.01% to 1.00%, or
Nb: 0.01% to 1.00% or
any combination thereof is satisfied.
[Claim 51 The method of manufacturing the heattreated
steel material according to claim 3 or 4,
wherein the steel sheet is subjected to forming
before the temperature of the steel sheet reaches the
Ms point after the heating the steel sheet to the
temperature zone of not less than the A c ~ point nor
more than "the A c 3 point + 2 0 0 ° C "