TECHNICAL FIELD
[0001] The present invention relates to a heattreated
steel material used for automobiles and so on
and a method of manufacturing the same.
BACKGROUND ART
[0002] Steel sheets for automobiles are required to
realize improvement of fuel efficiency and collision
resistance. Accordingly, attempts are being made to
increase strength of the steel sheets for
automobiles. However, the improvement of strength is
generally accompanied by a decrease of ductility such
as press formability and accordingly makes it
difficult to manufacture a component having a
complicated shape, For example, in accordance with
the decrease of ductility, a highly-worked portion
fractures, or springback and wall warping are become
significant to deteriorate dimensional accuracy.
Therefore, it is not easy to manufacture a component
by press-forming a high-strength steel sheet,
especially, a steel sheet having 780 MPa tensile
strength or more. Roll forming instead of the press
forming can easily work a high-strength steel sheet,
but its application is only limited to a component
having a uniform cross section in a longitudinal
direction.
[000 3] Patent Literature 1 discloses a method called
- 1 -
hot pressing having an obj ect to obtain high
for inability in a high-strength steel sheet. By the
hot pressing, it is possible to form a high-strength
steel sheet highly accurately to obtain a hot-pressed
steel sheet material having high strength.
[0004] Patent Literature 2 discloses a hot forming
method having an object to obtain stable strength and
toughness, and Patent Literature 3 discloses a steel
sheet aiming at improved formability and
hardenability. Patent Literature A discloses a steel
sheet aiming at realizing both strength and
formability, Patent Literature 5 discloses an art
having an object to manufacture steel sheets having a
plurality of strength levels from the same steel
type, and Patent Literature 6 discloses a method of
manufacturing a steel pipe having an object to
improve formability and torsional fatigue resistance.
Patent Literature 7 discloses an art to improve a
cooling rate in hot forming. Non-patent Literature 1
discloses a relation between a cooling rate in
quenching, and hardness and a structure of a hotpressed
steel material.
[0005] Incidentally, collision resistance of an
automobile depends on not only tensile strength but
also yield strength and toughness suitable for the
tensile strength. For example, for a bumper
reinforce, a center pillar, and the like, it is
required that plastic deformation is suppressed as
much as possible so as not to fracture soon even if
- 2 -
they deform.
[0006] However, it is difficult to obtain excellent
collision resistance by the aforesaid conventional
arts .
CITATION LIST
PATENT LITERATURE
[0007] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2002-102980
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2004-353026
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2002-180186
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2009-203549
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2007-291464
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2010-242164
Patent Literature 7: Japanese Laid-open Patent
Publication No. 2005-169394
NON-PATENT LITERATURE
[0008] Non-Patent Literature 1: Tetsu-to-Haganef
Vol. 96 (2010) No. 6 378
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0009] It is an object of the present invention to
provide a heat-treated steel material possible to
obtain excellent collision resistance, and a method
of manufacturing the same.
- 3 -
SOLUTION TO PROBLEM
[0010] The present inventors have conducted studious
studies in order to investigate a cause that makes it
difficult to obtain sufficient tensile strength, and
yield strength and toughness suitable for the tensile
strength in a conventional heat-treated steel
material manufactured through heat treatment such as
hot stamping. As a result, it has been found out
that the heat-treated steel material, even if
appropriately heat-treated, unavoidably contains
retained austenite in its structure, that yield
strength decreases as a volume fraction of the
retained austenite is higher, and that the decrease
of the yield strength is mainly caused by the
retained austenite.
[0011] The present inventors have also found out
that, in order to reduce retained austenite, a
cooling rate in quenching, especially, a cooling rate
in a temperature range of a martensitic
transformation point (Ms point) or lower is
important.
[0012] The present inventors have also found out
that, even if a steel sheet for heat treatment used
for manufacturing a heat-treated steel material
contains Cr and B, which greatly contribute to an
improvement of hardenability, toughness of the heattreated
steel material manufactured from this steel
sheet does not deteriorate. A conventional heattreated
steel material contains Mn for the purpose of
~ 4 _
improving hardenability, but Mn causes deterioration
of toughness. If a steel sheet for heat treatment
contains Cr and B, it is possible to ensure
hardenability even if the Mn content is suppressed to
low, so that toughness of the heat-treated steel
material can be improved.
[0013] Then, the inventors of the present
application have reached the follov/ing various forms
of the invention based on these findings.
[0014] (1)
A heat-treated steel material including:
a chemical composition expressed by, in mass%:
C: 0.16% to 0.38%;
Mn: 0.6% to 1.5%;
Cr: 0.4% to 2.0%;
Ti: 0.01% to 0.10%;
B: 0.001% to 0.010%;
Si: 0.20% or less;
P: 0.05% or less;
S: 0.05% or less;
N: 0.01% or less;
Ni: 0% to 2.0%;
Cu: 0% to 1.0%;
Mo: 0% to 1.0%;
V: 0% to 1.0%;
Al: 0% to 1.0%;
Nb: 0% to 1.0%;
REM: 0% to 0.1%; and
the balance: Fe and impurities; and
_ 5 _
a structure expressed by:
retained austenite: 1.5 volume% or less; and
the balance: martensite.
[0015] (2)
The heat-treated steel material according to (1),
wherein C: 0.16 to 0.25% in the chemical composition.
[0016] (3)
The heat-treated steel material according to (1)
or (2) , including a mechanical property expressed by
a yield ratio: 0.70 or more.
[0017] (4)
The heat-treated steel material according to any
one of (1) to (3), wherein the chemical composition
satisfies:
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 1.0%;
Nb: 0.01% to 1.0%; or
REM: 0.001% to 0.1%; or
any combination thereof.
[0018] (5)
A method of manufacturing a heat-treated steel
material, including:
heating a steel sheet to a temperature of an AC3
point or higher;
next cooling the steel sheet to a Ms point at a
cooling rate equal to a critical cooling rate or
- 6 -
more; and
next cooling the steel sheet from the Ms point
100°C at a 3 5°C/second average cooling rate or more,
wherein
the steel sheet includes a chemical composition
expressed by, in mass%:
C: 0.16% to 0.38%;
Mn: 0.6% to 1.5%;
Cr: 0.4% to 2.0%;
Ti: 0.01% to 0.10%;
B: 0.001% to 0.010%;
Si: 0.20% or less;
P: 0.05% or less;
S: 0,05% or less;
N: 0.01% or less;
Ni: 0% to 2.0%
Cu: 0% to 1.0%
Mo: 0% to 1.0%
V: 0% to 1.0%;
Al: 0% to 1.0%;
Nb: 0% to 1.0%;
REM: 0% to 0.1%; and
the balance: Fe and impurities.
[0019] (6)
The method of manufacturing the heat-treated
steel material according to (5), wherein C: 0.16 to
0.2 5% in the chemical composition.
[0020] (7)
The method of manufacturing the heat-treated
- 7 -
steel material according to (5) or (6), wherein the
chemical composition satisfies:
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 1.0%;
Nb: 0.01% to 1.0%; or
REM: 0.001% to 0.1%; or
any combination thereof.
[0021] (8)
The method of manufacturing the heat-treated
steel material according to any one of (5) to (7),
including forming the steel sheet after the heating
the steel sheet to the temperature of the Ac3 point or
higher before a temperature of the steel sheet
reaches the Ms point.
ADVANTAGEOUS EFFECTS OF INVENTION
[0022] According to the present invention, it is
possible to obtain excellent collision resistance.
DESCRIPTION OF EMBODIMENTS
[0023] Hereinafter, an embodiment of the present
invention will be described. A heat-treated steel
material according to the embodiment of the present
invention, which will be described in detail later,
is manufactured by quenching a predetermined steel
sheet for heat treatment. Therefore, hardenability
and a quenching condition of the steel sheet for heat
treatment influence the heat-treated steel material.
[0024] 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 same will be
described. In the following description, "%" being a
unit of the content of each element contained in the
heat-treated steel material and the steel sheet used
for manufacturing the same means "mass%" unless
otherwise mentioned. The heat-treated steel material
according to the embodiment and the steel sheet used
for manufacturing the same include a chemical
composition expressed by C: 0.16% to 0.38%, Mn: 0.6%
to 1.5%, Cr: 0.4% to 2.0%, Ti: 0.01% to 0.10%, B:
0.001% to 0.010%, Si: 0.20% or less, P: 0.05% or
less, S: 0.05% or less, N: 0.01% or less, Ni: 0% to
2.0%, Cu: 0% to 1.0%, Mo: 0% to 1,0%, V: 0% or 1.0%,
Al: 0% to 1.0%, Nb: 0% to 1.0%, REM (rare earth
metal): 0% to 0.1%, and the balance: Fe and
impurities. Examples of the impurities are those
contained in a raw material such as an ore and scrap
iron, and those contained during manufacturing steps.
[0025] (C: 0.16% to 0.38%)
C is a very important element that enhances
hardenability of the steel sheet for heat treatment
and mainly affects strength of the heat-treated steel
material. When the C content is less than 0.16%,
strength of the heat-treated steel material is not
sufficient. Therefore, the C content is 0.16% or
more. When the C content is over 0.38%, the strength
- 9 -
of the heat-treated steel material is too high,
leading to a great deterioration of toughness.
Therefore, the C content is 0.36% or less. The C
content is preferably 0.36% or less.
[0026] The C content is preferably 0.16% to 0.25% to
obtain tensile strength of not less than 1400 MPa nor
more than 1700 MPa, and the C content is preferably
over 0.25% and 0.38% or less to obtain tensile
strength of over 1700 MPa and 2200 MPa or less.
[0027] (Mn: 0.6% to 1.5%)
Mn has a function to improve hardenability of the
steel sheet for heat treatment, and enable to stably
ensure strength of the heat-treated steel material.
When the Mn content is less than 0.6%, it is not
sometimes possible to sufficiently obtain the effect
by the above function. Therefore, the Mn content is
0.6% or more. When the Mn content is over 1.5%,
segregation is significant, resulting in
deterioration of uniformity of a mechanical property
to deteriorate toughness. Therefore, the Mn content
is 1.5% or less. The Mn content is preferably 1.3%
or less.
[0028] (Cr: 0.4% to 2.0%)
Cr has a function to improve hardenability of the
steel sheet for heat treatment, and enable to stably
ensure strength of the heat-treated steel material.
When the Cr content is less than 0.4%, it is not
sometimes possible to sufficiently obtain the effect
by the above function. Therefore, the Cr content is
- 10 -
0.4% or more. When the Cr content is over 2.0%, Cr
concentrates in carbide in the steel sheet for heat
treatment, so that hardenability lowers. This is
because, as Cr concentrates, solid dissolving of the
carbide becomes slow during heating for quenching.
Therefore, the Cr content is 2.0% or less. The Cr
content is preferably 1.0% or less.
[0029] (Ti: 0.01% to 0.10%)
Ti has a function to greatly improve toughness of
the heat-treated steel material. That is Ti
suppresses recrystallization and forms further fine
carbide so as to suppress grain growth of austenite
in heat treatment at a temperature equal to an Ac3
point or higher for quenching. Owing to the
suppression of the grain growth, fine austenite
grains are obtained, leading to a great improvement
of toughness. Ti also has a function to
preferentially bond with N in the steel sheet for
heat treatment, thereby suppressing B from being
consumed by the precipitation of BN. As will be
described later, B also has a function to improve
hardenability, and therefore, by suppressing the
consumption of B, it is possible to surely obtain the
effect of improving hardenability by B. When the Ti
content is less than 0.01%, it is not sometimes
possible to sufficiently obtain the effect by the
above function. Therefore, the Ti content is 0.01%
or more. When the Ti content is over 0.10%, a
precipitation amount of TiC increases, so that C is
- 11 -
consumed, and therefore, it is not sometimes possible
to obtain sufficient strength. Therefore, the Ti
content is 0.10% or less. The Ti content is
preferably 0.08% or less.
[0030] (B: 0.001% to 0.010%)
B is a very important element having a function
to greatly enhance hardenability of the steel sheet
for heat treatment. B also has a function to
strengthen a grain boundary to increase toughness by
segregating at the grain boundary. B also has a
function to suppress the grain growth of the
austenite to improve toughness, similarly to Ti.
When the B content is less than 0.001%, it is not
sometimes possible to sufficiently obtain the effect
by the above function. Therefore, the B content is
0.001% or more. When the B content is over 0.010%, a
large amount of a coarse boride precipitates to
deteriorate toughness. Therefore, the B content is
0.010% or less. The B content is preferably 0.006%
or less.
[0031] (Si: 0.20% or less)
Si is not an essential element but is contained
as impurities in steel, for instance. Si causes
deterioration of yield strength accompanying an
increase of retained austenite. Further, the higher
the Si content is, the higher a temperature at which
austenite transformation occurs. As this temperature
is higher, a cost required for heating for quenching
increases, or quenching is likely to be insufficient
- 12 -
due to insufficient heating. Further, as the Si
content is higher, wettability and alloying
treatability of the steel sheet for heat treatment
become lower, which lowers stability of hot dipping
and alloying treatment. Therefore, the lower the Si
content, the better. Especially when the Si content
is over 0.20%, the decrease of yield strength is
significant. Therefore, the Si content is 0.20% or
less. The Si content is preferably 0.15% or less.
[0032] (P: 0.05% or less)
P is not an essential element but is contained as
impurities in steel, for instance. P deteriorates
toughness of the heat-treated steel material.
Therefore, the lower the P content, the better.
Especially when the P content is over 0.05%, the
decrease of toughness is significant. Therefore, the
P content is 0.05% or less. The P content is
preferably 0.005% or less.
[0033] (S: 0.05% or less)
S is not an essential element but is contained as
impurities in steel, for instance. S deteriorates
toughness of the heat-treated steel material.
Therefore, the lower the S content, the better.
Especially when the S content is over 0.05%, the
decrease of toughness is significant. Therefore, the
S content is 0.05% or less. The S content is
preferably 0.02% or less.
[0034] (N: 0.01% or less)
N is not an essential element but is contained as
- 13 -
impurities in steel, for instance. N contributes to
formation of coarse nitride and deteriorates local
deformability and toughness of the heat-treated steel
material. Therefore, the lower the N content, the
better. Especially when the N content is over 0.01%,
the decrease of local deformability and toughness is
significant. Therefore, the N content is 0.01% or
less. It requires a considerable cost to decrease
the N content to less than 0.0008%, and it sometimes
requires a more enormous cost to decrease it to less
than 0.0002%.
[0035] Ni, Cu, Mo, V, Al, Nb, and REM are not
essential elements but are optional elements that may
be appropriately contained in the steel sheet for
heat treatment and the heat-treated steel material,
within ranges of predetermined limit amounts.
[0036] (Ni: 0% to 2.0%, Cu: 0% to 1.0%, Mo: 0% to
1.0%, V: 0% to 1.0%, Al: 0% to 1.0%, Nb: 0% to 1.0%,
REM: 0% to 0.1%)
Ni, Cu, Mo, V, Al, Nb, and REM have a function to
improve hardenability and/or toughness of the steel
sheet for heat treatment. Therefore, one or any
combination selected from the group consisting of
these elements may be contained. However, when the
Ni content is over 2.0%, the effect by the above
function is saturated, only resulting in a wasteful
cost increase. Therefore, the Ni content is 2.0% or
less. When the Cu content is over 1.0%, the effect
by the above function is saturated, only resulting in
- 14 -
a wasteful cost increase. Therefore, the Cu content
is 1.0% or less. When the Mo content is over 1.0%,
the effect by the above function is saturated, only
resulting in a wasteful cost increase. Therefore,
the Mo content is 1.0% or less. When the V content
is over 1.0%, the effect by the above function is
saturated, only resulting in a wasteful cost
increase. Therefore, the V content is 1.0% or less.
When the Al content is over 1.0%, the effect by the
above function is saturated, only resulting in a
wasteful cost increase. Therefore, the Al content is
1.0% or less. When the Nb content is over 1.0%, the
effect by the above function is saturated, only
resulting in a wasteful cost increase. Therefore,
the Nb content is 1.0% or less. When the REM content
is over 0.1%, the effect by the above function is
saturated, only resulting in a wasteful cost
increase. Therefore, the REM content is 0.1% or
less. To surely obtain the effect by the above
function, the Ni content, the Cu content, the Mo
content, and the V content all are preferably 0.1% or
more, the Al content and the Nb content both are
preferably 0.01% or more, and the REM content is
preferably 0.001% or more. That is, it is preferable
that "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
1.0%", "Nb: 0.01% to 1.0%", or "REM: 0.001% to 0.1%",
or any combination thereof be satisfied. REM is
added to molten steel using a Fe~Si-REM alloy, for
- 15 -
instance, and this alloy contains Ce, La, Nd, and Pr,
for instance.
[0037] Next, a structure of the heat-treated steel
material according to the embodiment will be
described. The heat-treated steel material according
to the embodiment includes a structure expressed by:
retained austenite: 1.5 volume% or less; and the
balance: martensite. The martensite is, for example,
auto-tempered martensite, but is not limited to the
auto-tempered martensite .
[0038] {Retained austenite: 1.5 volume% or less)
The retained austenite is not an essential
structure but is unavoidably contained in the
structure of the heat-treated steel material. The
retained austenite causes a decrease of yield
strength as described above, and accordingly as a
volume fraction of the retained austenite is higher,
yield strength is lower. Especially when the volume
fraction of the retained austenite is over 1.5
volume!, the decrease of yield strength is
significant, which makes it difficult to apply the
heat-treated steel material to a bumper reinforce, a
center pillar, and the like. Therefore, the volume
fraction of the retained austenite is 1.5 volume% or
less .
[0039] Next, the mechanical property of the heattreated
steel material according to the embodiment
will be described. The heat-treated steel material
according to the embodiment preferably includes a
- 16 -
mechanical property expressed by a yield ratio: 0.70
or more. Collision resistance may be evaluated based
on tensile strength, and yield strength and toughness
suitable for tensile strength, and yield strength
suitable for tensile strength is expressed by the
yield ratio. Then, under a condition where tensile
strength or yield strength is comparable, the higher
the yield ratio, the better. When the yield ratio is
less than 0.70, it is not sometimes possible to
obtain sufficient collision resistance in the
application in a bumper reinforce or a center pillar.
Therefore, the yield ratio is preferably 0.70 or
more,
[004 0] Next, a method of manufacturing the heattreated
steel material, that is, 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 range of the Ac3 point or
higher, thereafter is cooled to a Ms point at a
cooling rate equal to a critical cooling rate or
more, and thereafter is cooled from the Ms point to
100°C at an average cooling rate of 35°C/second or
more,
[0041] Once the steel sheet for heat treatment is
heated to the temperature range of the AC3 point or
higher, the structure becomes an austenite single
phase. If it is thereafter cooled to the Ms point at
the cooling rate equal to the critical cooling rate
- 17 -
or more, the structure of the austenite single phase
is maintained without any occurrence of diffusion
transformation. Once it is thereafter cooled from
the Ms point to 100°C at the average cooling rate of
the 35°C/second or more, the structure in which the
volume fraction of the retained austenite is 1.5
volume% or less and the balance is the martensite is
obtained.
[0042] In the above-described manner, it is possible
to manufacture the heat-treated steel material
according to this embodiment having excellent
collision resistance.
[004 3] During the series of heating and cooling, hot
forming such as hot stamping may be performed.
Specifically, the steel sheet for heat treatment may
be subjected to the forming in a die until the
temperature reaches the Ms point after being heated
to the temperature range of the AC3 point or higher,
Bending, drawing, bulging, hole expansion, and
flanging may be exemplified as the hot forming.
These belong to press forming, but if it is possible
to cool the steel sheet in parallel with the hot
forming or immediately after the hot forming, hot
forming such as roll forming, other than the press
forming, may be performed.
[0044] In a case where the hot forming is performed,
it is preferable that a pipe and an injection hole
for a cooling medium are provided in the die, and the
cooling medium is directly sprayed to the steel sheet
- 18 -
for heat treatment during the cooling from the Ms
point to 100°C, or while it is kept at a press bottom
dead center, for example. Water, polyhydric
alcohols, aqueous solutions of polyhydric alcohols,
polyglycol, mineral oil whose flash point is 120°C or
higher, synthetic ester, silicone oil, fluorine oil,
grease whose dropping point is 120°C or higher,
mineral oil, and water emulsion in which a surface
active agent is compounded with synthetic ester may
be exemplified as the cooling medium. One or any
combination thereof may be usable. Using the die and
the cooling medium as described above makes it
possible to easily realize the 35°C/second cooling
rate or more. Such a cooling method is described in
Patent Literature 7, for example. As the series of
heating and cooling, hardening by high-freguency
heating may be performed.
[004 5] The retention time in the temperature range
of the AC3 point or higher is preferably one minute or
more in order to sufficiently cause the
transformation to the austenite. Generally, by the
ten-minute retention, the structure becomes the
austenite single phase, and the retention for more
than ten minutes lowers productivity. Therefore, in
view of productivity, the retention time is
preferably ten minutes or less.
[0046] The steel sheet for heat treatment may be a
hot-rolled steel sheet, or may be a cold-rolled steel
sheet. An annealed hot-rolled steel sheet or an
- 19 -
annealed cold-rolled steel sheet which is a hotrolled
steel sheet or a cold-rolled steel sheet
having been subjected to annealing may be used as the
steel sheet for heat treatment.
[0047] The steel sheet for heat treatment may be a
surface-treated steel sheet such as a plated steel
sheet. That is, a plating layer may be provided on
the steel sheet for heat treatment. The plating
layer contributes to an improvement of corrosion
resistance, for instance. The plating layer may be
an electroplating layer, or may be a hot-dip plating
layer. An electrogalvanizing layer and a Zn-Ni alloy
electroplating layer may be exemplified as the
electroplating layer. A hot-dip galvanizing layer,
an alloyed hot-dip galvanizing layer, a hot-dip
aluminum plating layer, a hot-dip Zn-Al 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
exemplified as the hot-dip plating layer. A coating
weight of the plating layer is not particularly
limited, and is a coating weight within an ordinary
range, for instance. Similarly to the steel sheet
for heat treatment, the heat-treated steel material
may be provided with a plating layer.
[0048] Next, an example of a method of manufacturing
the steel sheet for heat treatment will be described.
In this manufacturing method, hot rolling, pickling,
cold rolling, annealing, and plating are performed,
for instance.
- 20 -
[0049] In the hot rolling, a steel ingot or a steel
billet having the aforesaid chemical composition
whose temperature is 1050°C or higher is hot-rolled,
and thereafter, is coiled in a temperature range of
not lower than 400°C nor higher than 700°C.
[0050] The steel ingot or steel billet sometimes
contains non-metal inclusions which are a cause to
deteriorate toughness and local deformability of the
heat-treated steel material obtained by the quenching
of the steel sheet for heat treatment. Therefore,
when the steel ingot or steel billet is subjected to
the hot rolling, it is preferable to sufficiently
solid-dissolve these non-metal inclusions. The solid
dissolving of the aforesaid non-metal inclusions is
promoted when the temperature of the steel ingot or
steel billet having the above chemical composition
has reached 1050°C or higher when it is subjected to
the hot rolling. Therefore, the temperature of the
steel ingot or steel billet that is to be subjected
to the hot rolling is preferably 1050°C or higher. It
is sufficient that the temperature of the steel ingot
or steel billet is 1050°C or higher when it is
subjected to the hot rolling. That is, after
continuous casting or the like, the steel ingot or
steel billet whose temperature has become lower than
1050°C may be heated to 1050°C or higher, or the steel
ingot after the continuous casting or the steel
billet after bloom rolling may be subjected to the
hot rolling without decreasing its temperature to
- 21 -
lower than 1050°C.
[0051] By setting the coiling temperature to 400°C or
higher, it is possible to obtain a high ferrite area
ratio. As the ferrite area ratio is higher, the
strength of the hot-rolled steel sheet obtained by
the hot rolling is reduced, and therefore, it becomes
easy to control a load and control flatness and
thickness of the steel sheet at the time of the later
cold rolling, which improves manufacturing
efficiency. Therefore, the coiling temperature is
preferably 400°C or higher.
[0052] When the coiling temperature is 700°C or
lower, it is possible to suppress scale growth after
the coiling to suppress the generation of a scale
flaw. When the coiling temperature is 700°C or lower,
the deformation due to the self-weight of a coil
after the coiling is also suppressed, and the
generation of a scratch flaw on a surface of the coil
due to this deformation is suppressed. Therefore,
the coiling temperature is preferably 700°C or lower.
The aforesaid deformation occurs because, when nontransformed
austenite remains after the coiling of
the hot rolling and this non-transformed austenite
transforms to ferrite after the coiling, coiling
tension of the coil is lost in accordance with volume
expansion due to the ferrite transformation and the
following thermal contraction.
[0053] The pickling may be performed by a common
procedure. Skin pass rolling may be performed before
- 22 -
the pickling or after the pickling. The skin pass
rolling corrects flatness or promotes the peeling of
scales, for instance. An elongation percentage when
the skin pass rolling is performed is not
particularly limited and, for example, is not less
than 0.3% nor more than 3.0%.
[0054] In a case where a cold-rolled steel sheet is
manufactured as the steel sheet for heat treatment,
cold rolling of the pickled steel sheet obtained by
the pickling is performed. The cold rolling may be
performed by a common procedure. A reduction ratio
of the cold rolling is not particularly limited, and
is a reduction ratio within an ordinary range, for
example, not less than 30% nor more than 8 0%.
[0055] In a case where an annealed hot-rolled steel
sheet or an annealed cold-rolled steel sheet is
manufactured as the steel sheet for heat treatment,
the hot-rolled steel sheet or the cold-rolled steel
sheet is annealed. In the annealing, the hot-rolled
steel sheet or the cold-rolled steel sheet is kept in
a temperature range of not lower than 550°C nor higher
than 950°C, for instance.
[005 6] When the retention temperature in the
annealing is 550°C or higher, it is possible to reduce
a difference in property accompanying a difference in
a hot-rolling condition to further stabilize the
property after the quenching, whichever of the
annealed hot-rolled steel sheet or the annealed coldrolled
steel sheet is manufactured, Further, in a
- 23 -
case where the cold-rolled steel sheet is annealed at
5 50°C or higher, the cold-rolled steel sheet softens
due to recrystallization, which can improve
workability. That is, it is possible to obtain the
annealed cold-rolled steel sheet having good
workability. Therefore, the retention temperature in
the annealing is preferably 550°C or higher.
[0057] When the retention temperature in the
annealing is higher than 950°C, the structure
sometimes becomes coarse. When the structure becomes
coarse, toughness after the quenching sometimes
deteriorates. Further, even if the retention
temperature in the annealing is higher than 950°C, the
effect worth the temperature increase is not
obtained, only resulting in cost increase and
productivity decrease. Therefore, the retention
temperature in the annealing is preferably 950°C or
lower.
[0058] After the annealing, cooling is preferably
performed to 550°C at an average cooling rate of not
less than 3°C/second nor more than 20°C/second, When
the above average cooling rate is 3°C/second or more,
the generation of coarse pearlite and coarse
cementite are suppressed, and it is possible to
improve the property after the quenching. When the
aforesaid average cooling rate is 20°C/second or less,
the occurrence of strength unevenness or the like is
suppressed, and stabilizing the quality of the
annealed hot-rolled steel sheet or the annealed cold-
- 24 -
rolled steel sheet is facilitated.
[0059] In a case where a plated steel sheet is
manufactured as the steel sheet for heat treatment,
electroplating or hot dipping is performed, for
instance. The electroplating and the hot dipping
both may be performed by a common procedure. For
example, in a case where the hot-dip galvanizing is
performed, a continuous hot-dip galvanizing facility
may be used to continuously perform the plating
subsequently to the aforesaid annealing.
Alternatively, the plating may be performed
independently of the aforesaid annealing. In the
hot-dip galvanizing, alloying treatment may be
performed to form an alloyed hot-dip galvanizing
layer. In a case where the alloying treatment is
performed, an alloying temperature is preferably not
lower than 480°C nor higher than 600°C. When the
alloying temperature is 480°C or higher, unevenness of
the alloying treatment can be suppressed. When the
alloying temperature is 600°C or lower, it is possible
to ensure high productivity as well as suppress a
manufacturing cost. Skin pass rolling may be
performed after the hot-dip galvanizing. The skin
pass rolling corrects flatness, for instance. An
elongation percentage when the skin pass rolling is
performed is not particularly limited and may be an
elongation percentage similar to that in a common
procedure.
[0060] Note that the above-described embodiments all
- 25 -
only present 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
[0061] Next, experiments conducted by the inventors
of the present application will be described.
[00 62] (First Experiment)
In a first experiment, cold-rolled steel sheets
each with a 1.4 mm thickness including the chemical
compositions listed in Table 1 were manufactured as
steel sheets for heat treatment. These steel sheets
v/ere manufactured by hot rolling and cold rolling of
slabs prepared by melting in a laboratory. The
underlines in Table 1 indicate that the numerical
values were out of the ranges of the present
invention.
[0063] [Table 1]
- 26 -
3 0 mm width, and a 200 mm length were taken from the
cold-rolled steel sheets, and the samples were heattreated
(heated and cooled) under the conditions
listed in Table 1. This heat treatment imitates heat
treatment in hot forming. The heating in this
experiment was performed by ohmic heating. After the
heat treatment, soaking portions were taken from the
samples, and these soaking portions were subjected to
an X-ray diffraction test, a tensile test, and a
Charpy impact test. A cooling rate (8 0°C/second) to
an Ms point was egual to a critical cooling rate or
more .
[0065] In the X-ray diffraction test, portions up to
1/8 depth of the thickness from surfaces of the
soaking portions were chemically polished using
hydrofluoric acid or a hydrogen peroxide solution,
whereby specimens for the X-ray diffraction test were
prepared, and volume fractions (volume%) of retained
austenite (retained y) in these specimens were found.
Note that the remainder other than the retained
austenite was martensite.
[0066] In the tensile test, the soaking portions
were worked into half-size piate-shaped specimens of
ASTM E8 having a 1.2 mm thickness, and the tensile
test of the specimens was conducted, and their
tensile strengths and yield strengths were measured.
A length of a parallel portion of each of the halfsize
plate-shaped specimens is 32 mm and a width of
the parallel portion is 6.25 mm. Further, yield
- 28 -
ratios were calculated from the tensile strengths and
the yield strengths.
[0067] In the Charpy impact test, the soaking
portions were polished until their thickness became
1.2 mm, and V-notched specimens in v/hich three pieces
of them were stacked were prepared, and these
specimens were subjected to the Charpy impact test,
and impact values at -8 0°C were found.
[0068] The results of these are listed in Table 2.
The underlines in Table 2 indicate that the numerical
values were out of the ranges of the present
invention or were out of the preferable ranges.
[0069] [Table 2]
- 29 -
No. 16, and No. 17, since their chemical compositions
and structures were within the ranges of the present
invention, tensile strengths equal to or more than
1400 MPa v/ere obtained, excellent yield ratios equal
to or higher than 0.70 were obtained, and impact
values equal to or more than 5 0 J/cm2, which is
preferable for the tensile strength of 1400 MPa or
more, were obtained.
[0071] In the samples Mo. 3, No. 4, No. 7, No. 8,
No. 11, No. 12, and No. 15, though their chemical
compositions v/ere within the range of the present
invention, their structures v/ere out of the range of
the present invention, and therefore their yield
ratios were low as less than 0.70.
[0072] In the samples No. 1, No. 2, No. 5, No. 6,
No. 9, No. 10, No. 13, No. 14, No. 16, and No. 17,
since their average cooling rates from Ms points to
100°C were all 35°C/s or more and their manufacturing
conditions were within the range of the present
invention, desired structures were obtained. On the
other hand, in the samples No. 3, No. 4, No. 7, No.
8, No. 11, No. 12, and No. 15, since their average
cooling rates from Ms points to 100°C v/ere all less
than 35°C/s and their manufacturing conditions were
out of the range of the present invention, the
desired structure could not be obtained.
[0073] In the samples No. 18 and No. 19, since their
Si contents were out of the range of the present
invention, their volume fractions of retained
- 31 -
austenite were over 1.5 volume% and their yield
ratios were less than 0.70 even though their average
cooling rates from Ms points to 100°C were 35°C/s or
more .
[0074] In the samples No. 20 and No. 21, since their
Mn contents were out of the range of the present
invention, their impact values were less than 50 J/cm2
and the desired toughness could not be obtained.
[0075] (Second Experiment)
In a second experiment, cold-rolled steel sheets
each with a 1.4 mm thickness having the chemical
compositions listed in Table 3 were manufactured as
steel sheets for heat treatment. These steel sheets
were manufactured by hot rolling and cold rolling of
slabs prepared by melting in a laboratory. The
underlines in Table 3 indicate that the numerical
values were out of the ranges of the present
invention.
[0076] [Table 3]
- 32 -
[0077] Then, heat treatment and evaluation tests
similar to those of the first experiment were
conducted. The results of these are listed in Table
4. The underlines in Table A indicate that the
numerical values were out of the ranges of the
present invention or were out of the preferable
ranges.
[0078] [Table 4]
- 34 -
No. 32, No. 34, No. 35, No. 37, No. 38, No. 40, No.
41, No. 43, and No. 44, since their chemical
compositions and structures were within the ranges of
the present invention, tensile strengths equal to or
more than 1800 MPa were obtained, excellent yield
ratios equal to or higher than 0.70 were also
obtained, and when the tensile strength was 1800 MPa
or more, impact values equal to or more than 40 J/cm2,
which is preferable for the tensile strength of 18 00
MPa or more, were obtained.
[0080] In the samples No. 33, No. 36, No. 39, and
No. 42, though their chemical compositions were
within the range of the present invention, their
structures were out of the range of the present
invention, and therefore their yield ratios were low
as less than 0.70.
[0081] In the samples No. 31, No. 32, No. 34, No.
35, No. 37, No. 38, No. 40, No. 41, No. 43, and No.
44, since their average cooling rates from Ms points
to 100°C were all 35°C/s or more and their
manufacturing conditions were all within the range of
the present invention, desired structures were
obtained. On the other hand, in the samples No. 33,
No. 36, No. 39, and No. 42, since their average
cooling rates from Ms points to 100°C were all less
than 35°C/s and their manufacturing conditions were
out of the range of the present invention, the
desired structure could not be obtained.
[0082] In the samples No. 45 and No. 46, since their
- 36 ~
Si contents v/ere out of the range of the present
invention, volume fractions of retained austenite
were over 1.5 vol%, and yield ratios were less than
0.70 even though their average cooling rates from Ms
points to 100°C v/ere 35°C/s or more.
[0083] In the samples No. 4 7 and No. 48, since their
Mn contents were out of the range of the present
invention, their impact values v/ere less than 40 J/cm2
and the desired toughness could not be obtained.
INDUSTRIAL APPLICABILITY
[0084] The present invention may be utilized for the
industries manufacturing heat-treated members and the
like used for automobiles, such as, for example, a
bumper reinforce and a center pillar, and in the
industries using these. The present invention may
also be utilized for the industries manufacturing
other mechanical structural components, the
industries using them, and the like.

CLAIMS
[Claim 1] A heat-treated steel material comprising:
a chemical composition expressed by, in mass%:
C: 0.16% to 0.38%;
Mn: 0.6% to 1.5%;
Cr; 0.4% to 2.0%;
Ti; 0.01% to 0.10%;
B: 0.001% to 0.010%;
Si: 0.20% or less;
P: 0.05% or less;
S: 0.05% or less;
N: 0.01% or less;
Ni: 0% to 2.0%;
Cu: 0% to 1.0%;
Mo: 0% to 1.0%;
V: 0% to 1,0%;
Al: 0% to 1.0%;
Nb: 0% to 1.0%;
REM: 0% to 0.1%; and
the balance: Fe and impurities; and
a structure expressed by:
retained austenite: 1.5 volume% or less; and
the balance: martensite.
[Claim 2] The heat-treated steel material according
to claim 1, wherein C: 0.16 to 0.25% in the chemical
composition.
[Claim 3] The heat-treated steel material according
to claim 1 or 2, comprising a mechanical property
expressed by
- 38 -
a yield ratio: 0.70 or more.
[Claim 4] The heat-treated steel material according
to any one of claims 1 to 3, wherein the chemical
composition satisfies:
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 1.0%;
Nb: 0.01% to 1.0%; or
REM: 0.001% to 0.1%; or
any combination thereof.
[Claim 5] A method of manufacturing a heat-treated
steel material, comprising:
heating a steel sheet to a temperature of an Ac3
point or higher;
next cooling the steel sheet to a Ms point at a
cooling rate equal to a critical cooling rate or
more; and
next cooling the steel sheet from the Ms point to
100°C at a 35°C/second average cooling rate or more,
wherein
the steel sheet comprises a chemical composition
expressed by, in mass%:
C: 0.16% to 0.38%;
Mn: 0.6% to 1.5%;
Cr: 0.4% to 2.0%;
Ti: 0.01% to 0.10%;
B: 0.001% to 0.010%;
- 39 -
Si: 0.20% or less;
P: 0.05% or less;
S: 0.05% or less;
N: 0.01% or less;
Ni: 0% to 2.0%;
Cu: 0% to 1.0%;
Mo: 0% to 1.0%;
V: 0% to 1.0%;
Al: 0% to 1.0%;
Nb: 0% to 1.0%;
REM: 0% to 0.1%; and
the balance: Fe and impurities.
[Claim 6] The method of manufacturing the heattreated
steel material according to claim 5, wherein
C: 0.16 to 0.25% in the chemical composition.
[Claim 7] The method of manufacturing the heattreated
steel material according to claim 5 or 6,
wherein the chemical composition satisfies:
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 1.0%;
Nb: 0.01% to 1.0%; or
REM: 0.001% to 0.1%; or
any combination thereof.
[Claim 8] The method of manufacturing the heattreated
steel material according to any one of claims
5 to 7, comprising forming the steel sheet after the
- 40 -
heating the steel sheet to the temperature of the
point or higher before a temperature of the steel
sheet reaches the Ms point.