TECHNICAL FIELD
[0001] The present invention relates to a steel
product and a manufacturing method thereof, and
relates particularly to a steel product whose tensile
strength is 980 MPa or more and which has excellent
ductility and impact property, and a manufacturing
method thereof.
BACKGROUND ART
[0002) In recent years, development of a steel
product which contributes to energy conservation has
been demanded in view of protecting global
environment. In fields of an automobile steel
product, an oil well pipe steel product, a building
construction steel product and so on, a super-highstrength
steel product which is light weighted and
applicable to severe use environment is increasingly
demanded and its scope of application is broadened.
Consequently, securing not only a strength property
but also safety in use environment is important in
the super-high-strength steel product used in these
fields. Concretely, it is important to raise a
tolerance to external plastic deformation by
increasing ductility of the steel product.
[0003) For example, in a case where an automobile
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ll
collides with a structure, in order to alleviate its
impact sufficiently by an anti-collision member of a
vehicle, it is desired that tensile strength of a
steel product may be 980 MPa or more and a value of a
product (TS x EL) of the tensile strength (TS) and a
total elongation (EL) may be 16000 Mpa·% or more.
However, since ductility decreases considerably as
the tensile strength rises, there has been no superhigh-
strength steel product which satisfies the
above-described property and is capable of being
industrially mass-produced. Thus, various research
and development has been done to improve ductility of
the super-high-strength steel product and structure
control methods to materialize the research and
development have been suggested (See Patent
References 1 to 4) .
[0004] However, by conventional techniques, it is
impossible to obtain sufficient ductility and impact
property while securing the tensile strength of 980
MPa or more.
CITATION LIST
PATENT REFERENCE
[0005] Patent Reference 1: Japanese Laid-open Patent
Publication No. 2004-269920
Patent Reference 2: Japanese Laid-open Patent
Publication No. 2010-90475
Patent Reference 3: Japanese Laid-open Patent
Publication No. 2003-138345
- 2 -
Patent Reference 4: Japanese Laid-open Patent
Publication No. 2014-25091
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006] An object of the present invention is to
provide a steel product and a manufacturing method
thereof, the steel product having excellent ductility
and impact property while having tensile strength of
980 MPa or more.
SOLUTION TO PROBLEM
[0007] The present inventors have conducted keen
study to solve the above-described problem. As a
result, the present inventors have reached the
following finding.
[0008] When a steel material is heated to a twophase
region of ferrite and austenite, a surface is
decarburized, whereby a structure (hereinafter,
referred to as a "decarburized ferrite layeru) made
of a soft ferrite phase is formed. When
decarburization becomes prominent, the decarburized
ferrite layer is formed thick in a surface of a steel
product.
[0009] When a thickness of the decarburized ferrite
layer becomes 5 Mm or more, coarse ferrite comes to
be generated, resulting in that ductility and impact
property may be deteriorated.
[0010] Thus, in order to manufacture a high-strength
steel product, a proper heat treatment is applied to
- 3 -
a steel material which contains particularly Si and
Mn more positively than normal to thereby suppress
decarburization in a surface. It has become obvious
that the above enables stably obtaining a steel
product having excellent ductility and impact
property while having tensile strength of 980 MPa or
more, such a steel product having not been able to be
manufactured by a conventional technique.
[0011] The present invention is made based on the
above-described finding and its basic gist is a steel
product and a manufacturing method thereof described
below.
[0012] (1) A steel product which has:
a chemical composition represented by, in mass%,
C: 0.050% to 0.35%,
Si: 0.50% to 3.0%,
Mn: exceeding 3.0% to 7.5% or less,
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%
Ti: 0% to 1.0%
Nb: 0% to 1.0%
Cr: 0% to 1. 0%
Mo: 0% to 1.0%
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
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ll
ll
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
REM: 0% to 0.01%,
Zr: 0% to 0.01%,
B: 0% to 0.01%,
Bi: 0% to 0.01%, and
the balance: Fe and impurities; and
a metal structure in which a thickness of a
decarburized ferrite layer is 5 ~m or less and a
volume ratio of retained austenite is 10% to 40%,
wherein tensile strength is 980 MPa or more,
[0013]
( 1 ) '
(2) The steel product according to the above
wherein, in the metal structure, a number density
of cementite is less than 2/~m 2 •
[0014] (3) The steel product according to the above
(1) or (2),
wherein, in the chemical composition,
V: 0.05% to 1.0%
is satisfied.
[0015] (4) The steel product according to any one of
the above ( 1 ) to ( 3) '
wherein, in the chemical composition,
Ti: 0.003% to 1.0%,
Nb: 0.003% to 1.0%,
Cr: 0.01% to 1 . 0%'
~lo: 0.01% to 1 . 0% f
Cu: 0.01% to 1. 0%, or
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II
Ni: 0.01% to 1.0%,
or arbitrary combination of the above is
satisfied.
[0016] (5) The steel product according to any one of
the above (1) to (4),
wherein, in the chemical composition,
Ca: 0.0003% to 0.01%,
Mg: 0.0003% to 0.01%,
REM: 0.0003% to 0.01%,
Zr: 0.0003% to 0.01%,
B: 0.0003% to 0.01%, or
Bi: 0.0003% to 0.01%,
or arbitrary combination of the above is
satisfied.
[0017] (6) The steel product according to any one of
the above ( 1) to ( 5) ,
wherein an average C concentration in the
retained austenite is 0.6% or less in mass%.
[0018] (7) A manufacturing method of a steel product
which has the steps of:
heating a steel material to a temperature of
670°C or more in a manner that an average heating
speed between 500°C to 670°C is 1°C/s to 5°C/s, which
steel material has a chemical composition represented
by, in mass%,
C: 0.050% to 0.35%,
Si: 0.50% to 3.0%,
Mn: exceeding 3.0% to 7.5% or less,
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!i !,
p: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%,
Ti: 0% to 1.0%,
Nb: 0% to 1. Q% 1
Cr: 0% to 1.0%,
Mo: 0% to 1 • 0% 1
Cu: 0% to 1 o 0% 1
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
REM: 0% to 0.01%,
Zr: 0% to 0.01%,
B: 0% to 0.01%,
Bi: 0% to 0.01%, and
the balance: Fe and impurities, and has a metal
structure in which volume ratios of bainite and
martensite are 90% or more in total and an average
value of aspect ratios of bainite and martensite is
1.5 or more;
holding the temperature in a temperature range of
670°C to 780°C for 60 s to 1200 s after the heating;
and
performing cooling to a temperature of 150°C or
less in a manner that an average cooling speed
between the temperature range and 150°C is 5°C/s to
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~~ ,l
500°C/s, after the holding.
[0019) (8) The manufacturing method of the steel
product according to the above (7),
wherein, in the chemical composition,
V: 0.05% to 1.0%
is satisfied, and
wherein 70% or more of V contained in the steel
material is solid-solved.
ADVANTAGEOUS EFFECTS OF INVENTION
[0020) According to the present invention, since a
chemical composition and a metal composition are
appropriate, it is possible to obtain tensile
strength of 980 MPa or more and excellent ductility
and impact property.
DESCRIPTION OF EMBODIMENTS
[0021) 1. Chemical Composition
First, a chemical composition of a steel product
according to an embodiment of the present invention
and a steel material used for its manufacturing will
be described. In the following description, •1"
being a unit of a content of each element contained
in the steel product and a steel sheet used for its
manufacturing means "mass%" unless otherwise
specified. The steel product according to the
present embodiment and the steel material used for
its manufacturing has a chemical composition
represented by C: 0.050% to 0.35%, Si: 0.50% to 3.0%,
Mn: exceeding 3.0% to 7.5% or less, P: 0.05% or less,
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ll
S: 0.01% or less, sol. A1: 0.001% to 3.0%, N: 0.01%
or less, V: 0% to 1.0%, Ti: 0% to 1.0%, Nb: 0% to
1.0%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 1.0%,
Ni: 0% to 1.0%, Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM:
0% to 0.01%, Zr: 0% to 0.01%, B: 0% to 0.01%, Bi: 0%
to 0.01%, and the balance: Fe and impurities. As
impurities, there are exemplified what is contained
in raw materials such as ore and scrap iron, and what
is contained in a manufacturing process.
[0022] C: 0.050% to 0.35%
C is an element which contributes to strength
increase and ductility improvement. In order to
obtain a steel product which has tensile strength of
980 MPa or more and, further, in which a value of a
product (TS x EL) of tensile strength (TS) and total
elongation (EL) is 16000 MPa·% or more, a C content is
required to be 0.050% or more. However, containing C
exceeding 0.35% deteriorates an impact property.
Therefore, the C content is required to be 0.35% or
less and is preferable to be 0.25% or less. Note
that in order to obtain tensile strength of 1000 MPa
or more, the C content is preferable to be 0.080% or
more.
[0023] Si: 0.50% to 3.0%
Si is an element which contributes to strength
increase and ductility improvement by enhancing
generation of austenite. In order to make the value
of the product (TS x EL) 16000 MPa·% or more, an Si
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ii
content is required to be 0.50% or more. However,
containing Si exceeding 3.0% deteriorates the impact
property. Therefore, the Si content is set to be
3.0% or less. Note that in order to improve
weldability, the Si content is preferable to be 1.0%
or more.
[0024] Mn: exceeding 3.0% to 7.5% or less
Mn, similarly to Si, is an element which
contributes to strength increase and ductility
improvement by enhancing generation of austenite. In.
order to make the tensile strength of the steel
product 980 MPa or more and to make the value of the
product (TS x EL) 16000 MPa·% or more, Mn is required
to be contained exceeding 3.0%. However, containing
Mn exceeding 7.5% makes refining and casting in a
steel converter considerably difficult. Therefore,
an Mn content is required to be 7.5% or less and is
preferable to be 6.5% or less. Note that in order to
obtain tensile strength of 1000 MPa or more, the Mn
content is preferable to be 4.0% or more.
[0025] P: 0. 05% or less
Though P is an element contained as an impurity,
since being also the element which contributes to
strength increase, P may be positively contained.
However, containing P exceeding 0.05% considerably
deteriorates weldability. Thus, a P content is set
to be 0.05% or less. The P content is preferable to
be 0.02% or less. When the above-described effect is
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desired, the P content is preferable to be 0.005% or
more.
[0026] S: 0.01% or less
Since S is contained inevitably as an impurity,
an S content is better as low as possible. In
particular, the S content exceeding 0.01% brings
about considerable deterioration of weldability.
Thus, the S content is set to be 0.01% or less. The
S content is preferable to be 0.005% or less, and is
more preferable to be 0.0015% or less.
[0027] sol. Al: 0.001% to 3.0%
Al is an element which has an action to deoxidize
steel. In order to achieve soundness of a steel
product, sol. Al is contained 0.001% or more.
Meanwhile, if a sol. Al content exceeding 3.0%,
casting becomes considerably difficult. Thus, the
sol. Al content is set to be 3.0% or less. The sol.
Al content is preferable to be 0.010% or more and is
preferable to be 1.2% or less. Note that the sol. Al
content means a content of acid-soluble Al in the
steel product.
[0028] N: 0.01% or less
Since N is contained inevitably as the impurity,
an N content is better as low as possible. In
particular, the N content exceeding 0.01% brings
about considerable deterioration of an anti-aging
property. Thus, the N content is set to be 0.01% or
less. The N content is preferable to be 0.006% or
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"I'I,
less, and is more preferable to be 0.004% or less.
[0029] V, Ti, Nb, Cr, Mo, Ni, Ca, Mg, REM, Zr, and Bi
are not essential elements but arbitrary elements
which may be contained appropriately to the extent of
a predetermined amount in a steel material used for
the steel product according to the present embodiment
and for manufacturing thereof.
[0030] V: 0% to 1.0%
V is an element which considerably increases
yield strength of a steel product and prevents
decarburization. Therefore, V may be contained.
However, containing V exceeding 1.0% makes hot
working considerably difficult. Therefore, a V
content is set to be 1.0% or less. Further, in order
to make the yield strength of the steel product 900
MPa or more, it is preferable that V is contained
0.05% or more. Note that if tensile strength of 1100
MPa or more is desired, the V content is further
preferable to be 0.15% or more. Further, if V is
contained in a steel material, it becomes easy to
adjust an average value of aspect ratios of bainite
and martensite to be 1.5 or more in the steel
material.
[0031] Ti: 0% to 1.0%
Nb: 0% to 1.0%
Cr: 0% to 1.0%
Mo: 0% to 1. 0%
Cu: 0% to 1. 0%
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ij
Ni: 0% to 1.0%
These elements are elements effective for stably
securing strength of a steel product. Therefore, one
kind or more selected from the above-described
elements may be contained. However, regarding every
element, being contained exceeding 1.0% makes hot
working difficult. Thus, a content of each element
is required to be 1% or less respectively. When the
above-described effect is desired, it is preferable
to satisfy Ti: 0.003% or more, Nb: .0.003% or more,
Cr: 0.01% or more, Mo: 0.01% or more, Cu: 0.01% or
more, or Ni: 0.01% or more, or arbitrary combination
of the above. Note that when two kinds or more of
the above-described elements are contained complexly,
the total content thereof is preferable to be 3% or
less.
[0032] Ca: 0% to 0.01%
Mg: 0% to 0.01%
REM: 0% to 0.01%
Zr: 0% to 0.01%
B: 0% to 0.01%
Bi: 0% to 0.01%
These elements are elements which have an action
to increase low temperature toughness. Therefore,
one kind or more selected from the above-described
elements may be contained. However, regarding every
element, being contained exceeding 0.01% deteriorates
a surface property. Thus, the content of each
- 13 -
element is required to be 0.01% or less respectively.
When the above-described effect is desired, the
content of one kind or more selected from these
elements is preferable to be 0.0003% or more. Note
that when two kinds or more of the above-described
elements are contained complexly, the total content
thereof is preferable to be 0.05% or less. Here, REM
indicates a total of 17 elements of Sc, Y, and
lanthanoid, and the above-described content of REM
means the total content of these elements.
Lanthanoid is added in a form of misch metal
industrially.
[0033] 2. Metal Structure
Thickness of decarburized ferrite layer: 5 ~m or
less
As described above, a decarburized ferrite layer
is a structure made of a soft ferrite phase which is
formed as a result that a surface of a steel product
is decarburized during a heat treatment. Further,
the decarburized ferrite layer is a structure which
includes a ferrite phase exhibiting a columnar shape
or a multangular shape 90% or more in terms of area
ratio. In order to maintain an excellent impact
property while having tensile strength as high as 980
MPa or more and to, it is necessary to suppress
decarburization in a surface layer portion. When a
thickness of the decarburized ferrite layer exceeds 5
~m, not only a fatigue property of the steel product
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but also an impact property is reduced, and thus the
thickness of the decarburized ferrite layer is set to
be 5 ~m or less.
[0034] Volume ratio of retained austenite: 10% to
40%
In the steel product according to the embodiment
of the present invention, in order to considerably
improve ductility of the steel product while the
steel product has the tensile strength of 980 MPa or
more, a volume ratio of retained austenite is
required to be 10% or more. Meanwhile, the volume
ratio of the retained austenite exceeding 40% brings
about deterioration of anti-delayed fracture property.
Thus, the volume ratio of the retained austenite is
set to be 40% or less.
[0035] Number density of cementite: less than 2/~m 2
In the steel product according to the embodiment
of the present invention, in order to considerably
improve the impact property, it is preferable to set
a number density of cementite to be less than 2/~m 2 •
Note that the number density of cementite is better
as low as possible, thus a lower limit is not set in
particular.
[0036] Average C concentration in retained
austenite: 0.60% or less
Further, setting an average C concentration in
retained austenite to be 0.60% or less in terms of
mass% makes martensite generated with a TRIP
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phenomenon soft, to thereby suppress generation of a
microcrack, resulting in considerable improvement of
the impact property of the steel property. Thus, it
is preferable to set the average C concentration in
the retained austenite to be 0.60% or less in terms
of mass%. The average C concentration of the
retained austenite is more preferable as low as
possible, so that a lower limit is not set in
particular.
[0037] 3. Mechanical Property
The steel product according to the embodiment of
the present invention has tensile strength of 980 MPa
or more. The tensile strength of the steel product
is preferable to be 1000 MPa or more. Further,
according to the steel product according to the
embodiment of the present invention, excellent
ductility and impact property can be obtained. For
example, it is possible to obtain ductility of 16000
MPa·% or more in terms of value of a product of
tensile strength and total elongation. For example,
it is possible to obtain the impact property of 30
J/cm2 or more in terms of impact value of a Charpy
test at O"C. Further, when V is contained in the
steel product, it is possible to obtain, for example,
0.2% proof stress (yield strength) in which yield
strength is 900 MPa or more.
[0038] 4. Manufacturing Method
A manufacturing method of the steel product
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according to the present invention is not limited in
particular, and the steel product can be manufactured,
for example, by applying a heat treatment described
below to a steel material having the above-described
chemical composition.
[0039] 4-1 Steel Material
As a steel material to be subjected to the heat
treatment, there is used one having a metal structure
in which, for example, volume ratios of bainite and
martensite are 90% or more in total and an average
value of aspect ratios of bainite and martensite is
1.5 or more. Further, the volume ratios of bainite
and martensite are preferable to be 95% or more in
total. Further, when the V content of the steel
material is 0.05% to 1.0%, 70% or more of V contained
in the steel material is preferable to be solidsolved.
[0040] If the volume ratios of bainite and
martensite in the steel material are less than 90% in
total, it becomes difficult to make the tensile
strength of the steel product 980 MPa or more.
Further, a volume ratio of retained austenite becomes
low, resulting in that ductility may be deteriorated.
Further, when the aspect ratios of bainite and
martensite become large, cementite precipitates in
parallel to a steel sheet surface, to thereby shield
decarburization. When an average value of the aspect
ratios of bainite and martensite is less than 1.5,
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shielding of decarburization is insufficient, so that
a decarburized ferrite layer is generated. Further,
when the average value of the aspect ratios of
bainite and martensite is less than 1.5, nucleation
of cementite is promoted and cementite is finely
dispersed, bringing about a high number density.
Note that the aspect ratio is a value obtained as a
result of dividing a major axis by a minor axis of
each grain of bainite and martensite when observed
from a cross-section (hereinafter, L cross-section)
perpendicular to a rolling direction in relation to
prior austenite grain. Further, adopted is an
average value of the aspect ratios obtained for all
the grains in the observed surface.
[0041] Further, when solid-solved V among V
contained in the steel is less than 70%, desired
yield strength cannot be obtained after the heat
treatment. Further, since austenite growth during
the heat treatment is delayed, the volume ratio of
retained austenite may become low. Therefore, it is
preferable that 70% or more V among V contained in a
steel material is solid-solved. A solid solution
amount of V can be measured by analyzing residue by
using an ICP-OES (Inductively Coupled Plasma Optical
Emission Spectrometry) after the steel material is
subjected to electroextraction, for example.
[0042] The above-described steel material can be
manufactured, for example, by hot rolling at a
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II
comparatively low temperature. Concretely; hot
rolling is carried out so that a finishing
temperature may be 800°C or less and a reduction ratio
of a final pass may be 10% or more, and within 3 s
after the end of finish rolling, rapid cooling to a
temperature of 600°C or less is carried out at an
average cooling speed of 20°C/s or more. Hot rolling
at a comparative low temperature as above is normally
avoided since a non-recrystallized grain is generated.
Further, when the steel material contains V 0.05% or
more, hot rolling is carried out so that the
finishing temperature may be 950°C or less and the
reduction ratio of the final pass may be 10% or more,
and rapid cooling to the temperature of 600°C or less
is carried out at the average cooling speed of 20°C/s
or more within 3 s after the end of finish rolling.
When V is contained in particular, the average value
of the aspect ratios of bainite and martensite is
easy to become 1.5 or more. Further, in a case of a
steel structure in which an average value of aspect
ratios of bainite and martensite is 1.5 or more, a
steel material thereof may be tempered.
[0043] 4-2 Heat Treatment
As described above, the steel product according
to the present invention can be manufactured by
applying following processings to the above-described
steel materials.
detail below.
Each step will be described in
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[0044] a) Heating Step
First, the above-described steel material is
heated to a temperature of 670"C or more in a manner
that the average heating speed between SOO"C and 670"C
becomes l"C/s to 5"C/s. Though cementite has an
action to suppress decarburization during the heat
treatment, coarse cementite, if remaining in the
steel product, deteriorates an impact property
considerably. Therefore, a grain diameter of
cementite and temperature control between soo•c to
670"C where a precipitation reaction is easy to be
controlled are quite important.
[0045] The average heating speed less than l"C/s
brings about coarse cementite to thereby suppress
decarburization. However, coarse cementite remains
in the steel product after the heat treatment to
thereby deteriorate the impact property. Further,
generation of austenite becomes insufficient, which
may deteriorate ductility. Meanwhile, the average
heating speed exceeding S"C/s brings about easy
melting of cementite during the heat treatment,
resulting in that a decarburization reaction during
the heat treatment cannot be suppressed.
[0046] Note that in heating to 500"C, the average
heating speed is preferable to be set at 0.2"C/s to
500"C/s. The average heating speed less than 0.2"C/s
decreases productivity. On the other hand, the
average heating speed exceeding SOO"C/s may bring
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about difficulty in temperature control between soo•c
to 670"C due to overshoot or the like.
[004 7] b) Holding Step
After the above-described heating, the
temperature is held in a temperature range of 670"C to
780"C for 60 s to 1200 s. A holding temperature of
less than 670"C not only leads to deterioration of
ductility but also may bring about difficulty in
making the tensile strength of the steel product 980
MPa or more. On the other hand, when the holding
temperature exceeds 780"C, it is not possible to make
the volume ratio of retained austenite of the steel
product 10% or more, resulting in that deterioration
of ductility may become prominent.
[0048] Further, when a holding time is less than 60
s, a generated structure and tensile strength are not
stable, and thus securing the tensile strength of 980
MPa or more may become difficult. On the other hand,
when the holding time exceeds 1200 s, internal
oxidation becomes prominent, resulting in that not
only the impact property is deteriorated but also a
decarburized ferrite layer becomes easy to be
generated. The holding time is preferable to be 120
s or more and is preferable to be 900 s or less.
[0049] c) Cooling Step
After the aforementioned heating holding, cooling
is carried out to a temperature of 150"C or less in a
manner that an average cooling speed between the
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above-described temperature range and 150"C becomes
5"C/s to 500"C/s. An average cooling speed of less
than 5"C/s brings about excessive generation of soft
ferrite and pearlite, which may result in difficulty
in making the tensile strength of the steel product
980 MPa or more. On the other hand, the average
cooling speed exceeding 500"C/s leads to easy
generation of a quenching crack.
[0050] The average cooling speed is preferable to be
8"C/s or more, and is preferable to be 100°C/s or less.
When the average cool·ing speed to 150"C is set to be
5°C/s to 500"/s, the cooling speed at 150°C or less
may be the same or different as/from the abovedescribed
range.
[0051] Further, in the temperature range of 350"C to
150"C during cooling, C becomes easy to be unevenly
distributed in austenite. Therefore, in order to
make an average C concentration in retained austenite
of a steel product 0.60% or less, cooling is
preferable to be carried out in a manner that a
residence time in the above-described temperature
range is 40 s or less.
[0052] Hereinafter, the present invention will be
described in more detail by way of examples, but the
present invention is not limited to these examples.
EXAMPLES
[0053] Steel materials which have chemical
compositions shown in Table 1 and metal structures
- 22 -
shown in Table 2 were subjected to heat treatments
under conditions shown in Table 3.
[0054] [Table 1]
STEEL CHEMICAL COMPOSITION (HASS%, REMAINDER: Fe AND IMPURITIES)
KIND c Si Mn p s sol. AI N OTHERS
A 0.23 1.68 3.31 0.012 0.0013 0.035 0.0042
B 0.074 1.76 5.25 0.012 0.0013 0.029 0.0043 Ca: 0.0013
c 0.14 1.73 4.21 0.010 0.0011 0.034 0.0035 REM: 0.0021
D 0.095 1.87 3.64 0.012 0.0014 0.035 0.0042 Ni: 0.87
E 0.092 2.05 4.95 0.012 0.0013 0.028 0.0041 Mg: 0.0014, Bi: 0.0016
F 0.10 3.25. 6.31 0.012 0.0013 0.028 0.0042
G 0.098 1.43 4.26 0.009 0.0012 0.028 0.0046 Cu: 0.32, Ni: 0.45, Zr: 0.0012
H 0.52. 1.26 3.13 0.011 0.0011 0.028 0.0045
I 0.15 1.89 4.64 0.012 0.0014 0.031 0.0045 Ti: 0.015, Nb: 0.022, Cr: 0.43
J 0.10 1.98 4.97 0.010 0.0011 0.028 0.0041
K 0.23 1.43 1.02. 0.012 0.0012 0.037 0.0041
L 0.11 1.52 4.42 0.011 0.0009 0.230 0.0042 Mo: 0.12
M 0.12 0.75 4.63 0.013 0.0012 0.032 0.0042
N 0.15 1.93 4.89 0.009 0.0009 0.028 0.0039 Ca: 0.001, Mo: 0.15, V: 0.47
0 0.12 1.93 4.11 0.010 0.0009 0.034 0.0043 Mg: 0.001, Cr: 0.72. V: 0.37
p 0.030. 1.91 5.05 0.011 0.0010 0.026 0.0043 V: 0.16
Q 0.10 1.92 4.91 0.011 0.0012 0.028 0.0032 V: 0.30
R 0.10 2.03 2.53 * 0.012 0.0012 0.029 0.0045 V: 0.16
s 0.16 1.52 4.78 0.005 0.0012 0.024 0.0041 Ti: 0.05, Bi: 0.002, V: 0.25
T 0.20 1.94 4.88 0.012 0.0011 0.032 0.0042 V: 0.60
u 0.072 0.30 * 4.92 0.010 0.0011 0.027 0.0037 V: 0.10
v 0.10 1.97 4.89 0.013 0.0013 0.032 0.0043 V:0.07
w 0.10 1.94 5.01 0.011 0.0014 0.028 0.0046 V: 0.03
X 0.10 1.95 4.97 0.013 0.0011 0.026 0.0045 Zr: 0.002, B: 0.001, V: 0.30
y 0.30 1.87 5.02 0.013 0.0011 0.024 0.0048 REM: 0.002, V: 0.85
z 0.10 0.80 4.93 0.012 0.0010 0.314 0.0049 B: 0.001, V: 0.20
AA 0.084 2.42 6.63 0.012 0.0013 0.041 0.0035 V: 0.10
BB 0.11 1.98 3.20 0.013 0.0009 0.041 0.0047 Ni: 0.9, Cu: 0.6, V: 0.20
cc 0.16 1.54 4.78 0.012 0.0011 0.034 0.0038 Nb: 0.03, V: 0.25
DO 0.25 1.93 4.85 0.009 0.0011 0.028 0.0036 V: 0.16
* NEANING THAT IT IS OUT OF A RANGE PRESCRIBED BY THE PRESEN'f INVENTION.
- 23 -
I'I,
[0055] [Table 2]
HOT ROLLH~G H;CCf.SS !;'1EEL H.UERrAL
7ESl StEEL fHIISHINC C~~~JU,'CIVE COOLII;G Ce>:H>UIC-:1 A>"iER
I'.'\RIE'M wo 0 wo 1.8 - iEIIfH•A1UflE II? 40'C/s
2 A '"" " AELE.R 1 s, tO A l'lX~I "" 0 wo H - 7ll':Pf.PAWRE J...'r 40'C/~
3 A 700 " MIE!\ 2 s, 'i"O A !'!:<).~ "' 0 wo 1.6 - UHP<:P./\.101\E 1\1: 40'C/s
4 A 700 " AfTER 2 3, '10 A ~ 45 "' 95 " - 7D!f·f.P.Al:ORE A? ~"C/"'
5 B 700 " Afi!:R I s, TO A f;O<;..'J; 100 0 wo 1.6 - l:UifEPA1URE AT: (Q'C/s
6 c 700 " AIT'£1\ 2 s, ro A I'>Zll "" 0 "' 18 - flliPU'AiVRE 1-.'i: 40\:/s
7 0 700 " AriU1 1 a, ro A F«>:t "" 0 "' 1.6 - 7UlftPA7URr. A? tO'cls
8 0 700 " AF"l.ER l a, 10 A ~...:»! wo 0 "' 1.6 ffHH.R'.71JRE h'I 40'i:;:/s
B 0 7W " MTER 1':> s, 70 A F--Xff " 0 95 H -
iUHEP.A7URE A? 40\:/s
" E 700 " Arru< 1 s, ro A 1100.'1 100 0 100 1.9 - HXP.EFA'i'URE A? 40"c/s
11 F 780 " Af'iUt. 2 6, ro A~ 100 0 100 1.6 -
YUIU"~A7U!l!: lo.T tO'C/s
12 G 700 15
J.IY£R 1 s, YO A FI:XlH 100 0 100 1.7 - TD{~£AA7URF. J,T 40'C/.s
13 G 700 15
Af'1f.ft l s, 1'0 1o. F>X>H 100 0 100 1.6 -
H2!~U'ATUR& AT tO'C/s
14 H 700 15 to.FY&R 1 s, ro A FtX>H 100 0 100 1.9 - H~t?U'A~\fft& .... ,. ~O'ct:;
16 ' 700 15
lo.FTI'.R l s, TO 1o. 1'0:)!( 100 0 100 1.7 - 'i:l:K?U'Annu: I..Y 40'C/s
16 J 700 15
lo.ITER l .s, YO A BXm 100 0 100 1.6 - TmPEPAYUR£ M {Q'(;fs
H J "" 15
At"Y£R 2 .s, 7'0 A f£XlM 100 0 100 1.7
1EM1'£PA'r.WZ "' tO'Cf.s
16 K . 700 15
Af1ER 1 s, TO A AX't( 100 0 100 1.8 - TDiCER.'I.T\lR& J..Y 40'Cf.s
19 l 700 15 to.ITY.R 1 s, YO A ~ 100 0 100 1.6 - TEM~EPATURE J..Y 40'C/s
20 " 700 15 M'T£R 1 .o., TO A AX>:-! "' 0 100 18 -
n:xnAAruR£ I.Y tot/ ...
21 " ""' 15 Af-r£R 2 "'• 'iO I. flO<:« 100 0 100 1.6 0.47 YD'!N;t>AT\/R& 1>3 40'C/.«
22 0 ""' 15
JU"ii:R.ls, "lOA~ 100 0 100 16 0,7 "l'Do!PEPAY\..J;ttJRt I.T 40'C/,.
" p "" 15 Af'l£-R l .s, TO A nx«
100 0 100 1.7 0.16 T£KPFI'AWRE AT 40'C/s
" Q ""' 15 AFU:R 1 s, '£0 J.. !»>N: 100 0 100 1.6 0.30 YD{Pfl'ATURF. AY tO'C/s
20 R OJ() 15 An£R 2 -"• TO 1'. FoX« 100 0 100 1.7 O.t6 W:Xf'<:!'AWP.<: I'.T {O"(:!:;
27 s 830 15 .>.F'i£R l .s, 'IQ 500'c M 0 100 100 1.6 025
(O'C s
26 T 630 15
.MTDI 1 s, TO A ~ 100 0 100 1.6 060 TD!Pf.Pxti.IR£ AT tO'C/s
29 u 630 15
AtYP< l s, TO A f>Xlt{ 100 0 100 1.6 0.10 'ID-!PU>ATURE J.t 40'C/$
30 v "" 15
.>.f7£R Z s, YO A f>Xll'l 100 0 100 1.7 om TlKPU'Ali)RE At {0"Cfs
31 v 830 15
AFJER 1 s, TO 1\. 1<001-l 100 0 "" 1.6 0.07 'llllPER .. l\JRt AT {O'CJ,.
32 v ""' 15
Af7£R 1 s, TO A F>C>O« 100 0 100 16 O.Q7 TUlfu>ATUR& At 40'Cfs
33 w ""' 15
AF"li>R 1 s, ro A ro;.H 100 0 100 1.7 0.03 '7I1W£Ml\IRE /..Y ~O'C/s
34 w 860 15
Af'1ER 2 s, '£0 11. F00M 100 0 100 11 o.ro 1UlPEI'Al\IIU: l·."f 40'Ch<
35 X 630 15 AfYER l .s, 70 11. R(>.)H 100 0 100 1.6 0.30 1£MPEI'AtvRE A? ~O'C/s
36 y "" 15 i'lfY!.:R 2 5 1 YC> A ROCIH 100 0 100 1.7 0.65 TDIPEI'AYVIU: 1."1 40'C/s.
37 y "" 15
JU""iER l s, 10 A fiOO!f
100 0 100 1.6 0.65 TUU'Ei'An!Rl: AT 40'C/s
36 z 630 15 AlYER l s, '70 A F>XlM 100 0 100 1.7 0-"l 1lh'O'E:AAY\JRE AT 40'C/s
39 z 630 15 AFYER Z "• 70 A F:OCfl 100 0 100 1.6 0.20
'ilXl'U'ATOR£ "T 40'C/~
40 z B30 15 l\f1£ft 2 s, 10 620 ., ., 0 ., 40"C/s 1.6 0.20
41 M 830 15 ltF"l£.'<. l s,fOAf~ 100 0 100 " 0.10 7lX~£PAl'UIU: "t 40'C/s
42 96 ""' 15
Af'1E..'I.ls, 10A~ 6S 0 65 1.7 0.20 101P£PA11JR& AT 25"G/s
43 BB .., 15 Af1£.AttrJU: Nr z~·~~s
44 EB "' 5 AME.Rl s, WIIP.C.:« 100 0 100 1.3 020 1U!F£PAI\IR£ AT 40"t/s
45 oc 630 15 Af7&R 1 s, 10 " ROC« 100 0 100 u 025 YEMPEI'Ai\IIU; /.1 40'C/s ... 00 "' 15 AIT£R 1 s, 10 11. RiXM 100 0 100 1.6 0.16 TlHPEI'AYI!RE /..7 40'C/,;
47 00 630 15
/l.fTER 1 s, '70 11. P.OOH 100 0 100 1.6 0.16 ., • - TP«'EI'..UUIU: A7 40'C/s HEM Ul •IIA"f l"f lS 001 OF A f\M· ~ E OF A C ~· c~·· "' SI"i"IO!i PRESCIHBED n 7 !E: Fi<
' <
IT liNE1~ r 10!.1 .
24
SOLlD·SOLV£0 SDLID·S.7!W
{!V.SS.I "' - -
- -
- -
- -
- -
- -
- -
-
- -
- -
- -
- -
- -
- -
- -
- -
- -
- -
- -
- -
0.42 89
033 89
0.32 ..
0.14 66
027 "' 0.13 81
0.22 "" 0.49 82
0.06 so
0.00 ..
0.05 71
0.00 " 0.03 100
0.03 100
0.25 63
0.71 64
0.69 "
0.17 65
0.19 6S
0.17 65
ow 90
0.16 90
0.17 65
0.17 65
0.21 64
0.13 61
0.14 66
[0056) [Table 3)
HEATING STEP STEP ~~'""g STEP
TEST STEEL AVERAGE HOLDING HOLDING AVERAGE
KIND HEATING TUI8
32 COOLING
RESIDENCE
TEN PERATURE
TIMEH (s}
SPEED81 (C/s) (C) {s) SPEED13 ('Cis)
1 A 3 700 400 50 5
2 A 3 700 300 50 5
3 A 10 700 350 50 5
4 A 3 700 300 50 5
5 B 3 710 350 50 5
6 c 3 720 350 50 5
7 D 3 720 250 50 6
8 D 3 680 200 3 67
9 D 3 710 400 50 5
10 E 3 700 400 50 5
11 F • 3 700 300 50 6
12 G 3 700 350 50 5
13 G 3 800 400 50 5
14 H • 3 700 200 50 5
15 I 3 700 300 50 5
16 J 3 700 200 50 5
17 J 3 700 2000 50 5
18 K • 3 730 250 50 5
19 L 3 700 300 50 5
20 M 3 700 250 50 5
21 N 3 700 400 40 6
22 0 3 710 500 2!j 10
23 0 0.2 680 200 40 5
24 p • 3 700 500 30 7
25 Q 3 700 500 40 5
26 R • 3 690 500 20 11
27 s 3 700 350 10 22
28 T 3 700 700 40 5
29 u • 3 675 5oo 30 7
30 v 3 700 500 20 10
31 v 3 675 30 20 10
32 v 3 800 500 20 10
33 w 3 700 500 40 5
34 w 3 700 500 40 5
~ X 3 700 360 8 25
y 3 700 500 40 5
37 y 3 750 300 40 5
38 z 3 700 450 40 5
39 z 3 690 400 3 67
40 z 3 685 500 30 7
41 AA 3 685 600 30 7
42 -* 3 705 540 40 5
43 3 650 500 40 5
44 BB 3 700 700 40 5
45 cc 3 700 500 40 5
46 DO 3 680 500 15 13
47 DD 3 680 500 10 20
THAT IT IS OIIT OF A RANGE BY THE
#1 HEANING AN AVERAGE HEATING SPEED BETNEEN SOO"C AND 670°C.
#2 HEANING A TIHE TO HOLD A TE!·fPERATURE AFTER A HOI,DING TENPERATURE IS REACHED.
#3 HEAt liNG AN AVERAGE COOLING SPEED BETI'lEEN THE HOLDING TENPERATURE AND 150 °C.
#4 BEANING A RESIDENCE TINE IN A TEHPERATURE RANGE OF 350°C TO 150"C DURING COOLING.
- 25 -
l
[0057] The steel material having been used was
manufactured by hot-working slab which has been
smelted in a laboratory under the condition shown in
Table 2. This steel material was cut into a size of
1.6 mm in thickness, 100 mm in width, and 200 mm in
length, and was heated, held, and cooled in
accordance with the condition of Table 3. A
thermocouple was attached to a steel material surface,
and temperature measurement during the heat treatment
was carried out. An average heating speed shown in
Table 3 is a value in a temperature range between
500°C to 670°C, and a holding time is a time during
which a temperature is held, after a holding
temperature is reached, at that temperature. Further,
an average cooling speed is a value in a temperature
range between the holding temperature and 150°C, and a
residence time is a residence time in a temperature
range from 350°C to 150°C during cooling.
[0058] Regarding the metal structure of the steel
material before the heat treatment, as well as a
metal structure and a mechanical property of a steel
product obtained by the heat treatment, investigation
were carried out by metal structure observation, Xray
diffraction measurement, tensile test, and Charpy
impact test as will be described below.
[0059]
An L cross-section of the steel material was
- 26 -
observed and photographed by an electron microscope
and a region of 0.04 mm2 in total was analyzed,
whereby area ratios and aspect ratios of bainite and
martensite were measured. Since the structure of the
steel material was isotropic, a value of the abovedescribed
area ratio was regarded as a volume ratio
of bainite and martensite. Note that the aspect
ratio was obtained as a result of dividing a major
axis by a minor axis of each grain of bainite and
martensite in relation to prior austenite grain, and
its average value was calculated.
[0060] An observation position was set to be a
position of about one fourth a plate thickness
(position of l/4t), avoiding a central segregation
portion. The reason to avoid the central segregation
portion will be described below. The central
segregation portion sometimes has a metal structure
partially different from a representative metal
structure of a steel product. However, the central
segregation portion, being a minute region in
relation to the entire plate thickness, hardly
influences the property of the steel product. In
other words, the metal structure of the central
segregation portion cannot be referred to as
representing the metal structure of the steel product.
Thus, in identification of the metal structure, it is
preferable to avoid the central segregation portion.
[0061]
- 27 -
i,I'
An amount of V solid-solved in the steel material
was measured, after the steel material was subjected
to electroextraction, by analyzing residue by using
ICP-OES (Inductively Coupled Plasma Optical Emission
Spectrometry).
[0062]
A test piece of 20 mm in width and 20 mm in
length was taken from each steel product, chemical
polishing was applied to this test piece to reduce a
thickness by 0.4 mm, and X-ray diffraction was
performed three times to a surface of the test piece
after chemical polishing. Obtained profiles were
analyzed and respectively averaged, to thereby
calculate a volume ratio of retained austenite.
[0063]
The profile obtained by X-ray diffraction was
analyzed, a lattice constant of austenite was
calculated, and an average C concentration in the
retained austenite was determined based on the
formula below.
c = (a- 3.572) I 0.033
Each symbol in the above formula means the
following.
a: lattice constant of austenite (A)
c: average C concentration in retained austenite
(mass%)
[0064]
- 28 -
An L cross-section of a steel product was
observed and photographed by an electron microscope
and a l mm region of a steel sheet surface was
analyzed, whereby a thickness of a decarburized
ferrite layer was measured.
[0065]
Regarding a number density of cementite, a region
of 2500 ~m 2 in total was analyzed to measure the
number density of cementite.
[0066]
A JIS No. 5 tensile test piece of 1.6 mm in
thickness was taken from each steel product, a
tensile test was carried out based on JIS Z 2241
(2011), and TS (tensile strength), YS (yield strength,
0.2% proof strength), and EL (total elongation) were
measured. Further, a value of TS x EL was calculated
from the above TS and EL.
[0067)
Front and rear surfaces of each steel product was
ground to be 1.2 mm in thickness to thereby fabricate
a V notch test piece. Four such test pieces were
stacked and screwed and then subjected to a Charpy
impact test based on JIS Z 2242 (2005). The impact
property was rated as good (0) when an impact value
at o•c was 30 J/cm2 or more, and was rated as
defective (x) when the impact value at o•c was less
than 30 J/cm2
•
[0068) Results of the metal structure observation of
- 29 -
,'
h
the steel material are shown in Table 2, and results
of X-ray diffraction measurement, tensile tests, and
Charpy impact tests are shown together in Table 4.
[0069] [Table 4]
- 30 -
ll
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31
fl
[0070] As shown in Tables 2 to 4, regarding each of
comparative examples of test numbers 2, 4, 9, 34, and
44, since the aspect ratios of bainite and martensite
of the steel material were less than 1.5, a thickness
of the decarburized ferrite layer was over 5 ~m,
resulting in a bad impact property. Regarding test
numbers 8 and 39, a low average cooling speed
resulted in excessive generation of pearlite, so that
the tensile strength of 980 MPa or more could not be
obtained. Regarding a test number 3, a high average
heating speed in the heat treatment caused a
thickness of the decarburized ferrite layer to be 5
~m or more, resulting in a bad impact property.
[0071] Regarding a test number 11, since an Si
content was higher than a prescribed range, an impact
property was inferior; Regarding a test number 14,
since a C content was higher than a prescribed range,
an impact property was inferior. Regarding each of
test numbers 13 and 32, a high holding temperature in
the heat treatment lowered a volume ratio of retained
austenite, resulting in bad ductility. Regarding a
test number 17, a long holding time in the heat
treatment caused a thickness of a decarburized
ferrite layer to be 5 ~m or more, resulting in a bad
impact property.
[0072] Regarding each of test numbers 18 and 26, an
Mn content was lower than a prescribed range,
regarding a test number 24, a C content was lower
- 32 -
than a prescribed range, and regarding a test number
29, an Si content was lower than a prescribed range,
and thus, ductility was bad and, in addition, tensile
strength of 980 MPa or more could not be obtained.
Regarding a test number 23, a low heating speed in
the heat treatment lowered a volume ratio of retained
austenite, resulting in bad ductility and, further, a
bad impact property. Regarding a test number 31,
since a holding time in the heat treatment was short,
a structure to be generated and tensile strength were
not stabilized, so that tensile strength of 980 MPa
or more could not be obtained. Regarding a test
number 40, volume ratios of bainite and martensite
were less than 90% in total, and regarding a test
number 43, a holding temperature in the heat
treatment was low, whereby a volume ratio of retained
austenite was low, resulting in that ductility is bad
and further that tensile strength of 980 MPa or more
could not be obtained.
[0073] On the other hand, regarding each of examples
of the present invention of test numbers 1, 5 to 7,
10 , 12, 15, 16, 19 to 22, 25, 27, 28, 30, 33, 35 to
38, 41, 42, and 45 to 47, tensile strength of 980 MPa
or more was obtained, ductility was excellent with a
value of a product (TS x EL) of tensile strength and
total elongation being 16000 MPa·% or more, and an
impact property was also good with an impact value of
a Charpy test at o•c being 30 J/cm2 or more.
- 33 -
INDUSTRIAL APPLICABILITY
[0074] The present invention is usable, for example,
in an automobile-related industry, an energy-related
industry, and a construction-related industry.

CLAIMS
[Claim 1] A steel product comprising:
a chemical composition represented by, in mass%,
C: 0.050% to 0.35%,
Si: 0.50% to 3.0%,
Mn: exceeding 3.0% to 7.5% or less,
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%,
Ti: 0% to 1.0%,
Nb: 0% to l. 0%,
Cr: 0% to 1.0%,
t-1o: 0% to 1.0%,
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
REM: 0% to 0. 01%,
Zr: 0% to 0.01%,
B: 0% to 0.01%,
Bi: 0% to 0.01%, and
the balance: Fe and impurities; and
a metal structure in which a thickness of a
decarburized ferrite layer is 5 ~m or less and a
volume ratio of retained austenite is 10% to 40%,
wherein tensile strength is 980 MPa or more.
- 35 -
,,
H
[Claim 2] The steel product according to claim 1,
wherein, in the metal structure, a number density
of cementite is less than 2/~m 2 •
[Claim 3] The steel product according to claim 1 or
2'
wherein, in the chemical composition,
V: 0.05% to 1.0%
is satisfied.
[Claim 4] The steel product according to any one of
claims 1 to 3,
wherein, in the chemical composition,
Ti: 0.003% to 1.0%,
Nb: 0 . 0 0 3% to 1 . 0%,
Cr: 0.01% to 1.0%,
Mo: 0.01% to 1.0%,
Cu: 0.01% to 1.0%, or
Ni: 0.01% to 1.0%,
or arbitrary combination of the above is
satisfied.
[Claim 5] The steel product according to any one of
claims 1 to 4,
wherein, in the chemical composition,
Ca: 0.0003% to 0.01%,
Mg: 0.0003% to 0.01%,
REM: 0.0003% to 0.01%,
Zr: 0.0003% to 0.01%,
B: 0.0003% to 0.01%, or
Bi: 0.0003% to 0.01%,
- 36 -
or arbitrary combination of the above is
satisfied.
[Claim 6] The steel product according to any one of
claims 1 to 5,
wherein an average C concentration in the
retained austenite is 0.6% or less in mass%.
[Claim 7] A manufacturing method of a steel product
comprising the steps of:
heating a steel material to a temperature of
670"C or more in a manner that an average heating
speed between 500"C to 670"C is l"C/s to 5"C/s, which
steel material has a chemical composition represented
by' in mass%,
C: 0.050% to 0~35%,
Si: 0.50% to 3.0%,
Mn: exceeding 3.0% to 7.5% or less,
P: 0.05% or less,
s: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%,
Ti: 0% to 1 • 0% 1
Nb: 0% to 1.0%,
Cr: 0% to 1.0%/
Mo: 0% to 1 • 0% 1
Cu: 0% to 1 o Q% 1
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
- 37 -
ll
Mg: 0% to 0.01%,
REM: 0% to 0. 01%,
Zr: 0% to 0.01%,
B: 0% to 0.01%,
Bi: 0% to 0.01%, and
the balance: Fe and. impurities, and has a metal
structure in which volume ratios of bainite and
martensite are 90% or more in total and aspect ratios
of bainite and martensite are 1.5 or more;
holding the temperatute in a temperature range
of 670°C to 780°C for 60 s to 1200 s after the
heating; and
performing cooling to a temperature of 150°C or
less in a manner that an average cooling speed
between the temperature range and 150°C is 5°C/s to
500°C/s, after the holding.
[Claim 8) The manufacturing method of the steel
product according to claim 7,
wherein, in the chemical composition,
V: 0.05% to 1.0%
is satisfied, and
wherein 70% or more of V contained in the steel
material is solid-solved.