[Technical Field of the Invention]
[0001]
The present invention relates to a hot rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2020-180729, filed in
Japan on October 28, 2020, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, from the viewpoint of protecting the global environment,
efforts have been made to reduce the amount of carbon dioxide gas emitted in many
fields. Vehicle manufacturers are also actively developing techniques for reducing
the weight of vehicle bodies for the purpose of reducing fuel consumption. However,
it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on
improvement in collision resistance to secure the safety of the occupants.
[0003]
In order to achieve both vehicle body weight reduction and collision
resistance, an investigation has been conducted to make a member thin by using a high
strength steel sheet. Therefore, there is a strong demand for a steel sheet having both
high strength and excellent workability, and several techniques have been
conventionally proposed to meet this demand. Since there are various working
methods for vehicle members, the required formability differs depending on members
to which the working methods are applied, but among these, ductility and bendability
are placed as important indices for workability.
[0004]
- 1 -
As steel sheets having both a high strength and excellent workability, dual
phase steel sheets (DP steel sheets) composed of a composite structure of soft ferrite
and full hard martensite and TRIP steel sheets for which transformation induced
plasticity (TRIP) is used have been conventionally proposed.
[0005]
For example, Patent Document 1 discloses a hot rolled steel sheet having a
microstructure containing ferrite and martensite and being excellent in terms of
strength, elongation, and hole expansibility, in which, in the microstructure, by area%,
ferrite is 90% to 98%, martensite is 2% to 10%, bainite is 0% to 3%, and pearlite is
0% to 3%. DP steel sheets and TRIP steel sheets have a low yield ratio and thus may
not be applicable to automobile suspension parts where higher impact strength and
fatigue strength are required.
[0006]
In general, to automobile suspension parts, steel sheets composed of a
composite structure of ferrite and bainite, for which precipitation hardening is used are
applied. For example, Patent Document 2 discloses a high-burring workability and
high-strength composite structure steel sheet having a tensile strength of 540 MPa or
more and being excellent in terms of surface properties and notch fatigue properties, in
which a primary phase of the microstructure is composed of polygonal ferrite
precipitation-hardened by a Ti carbide, a second phase is a composite structure
composed of a low temperature transformation product that is 1% to 10% in terms of
an area fraction (fsd (% )) and dispersed as a plurality of structures.
[0007]
However, in the steel sheets as described above, sufficient toughness may not
be obtained in a case where the tensile strength is set to 780 MPa or more. In
- 2 -
addition, in steel sheets where the Si content is increased in order for highstrengthening,
a scale pattern may remain even in a case where scale has been removed,
and the external appearance of the steel sheets may deteriorate.
[0008]
In addition, Patent Document 3 discloses a hot rolled steel sheet in which a
microstructure contains ferrite as a primary phase, at least one of martensite and
residual austenite as a second phase, and a plurality of inclusions, and the sum of the
lengths in the rolling direction of a group of inclusions having a length of 30 )liD or
more in the rolling direction and independent inclusions having a length of 30 )liD or
more in the rolling direction is 0 mm or more and 0.25 mm or less per 1 mm2
.
[0009]
However, in the technique described in Patent Document 3, the toughness at
low temperatures is insufficient, and there is a need to further improve the toughness at
low temperatures in order to make it possible to sufficiently suppress fracture during
use in cold regions and during impact.
[Prior Art Document]
[Patent Document]
[0010]
[Patent Document 1] PCT International Publication No. W02018/033990
[Patent Document 2] PCT International Publication No. WO 2014/051005
[Patent Document 3] PCT International Publication No. WO 2012/128228
[Non-Patent Document]
[0011]
[Non-Patent Document 1] J. Webel, J. Gola, D. Britz, F. Mucklich,
Materials Characterization 144 (2018) 584-596
- 3 -
[Non-Patent Document 2] D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9,
546
[Non-Patent Document 3] K. Zuiderveld, Contrast Limited Adaptive
Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.),
Cambridge, MA, Academic Press, 1994, pp. 474-485
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0012]
The present invention has been made in view of the above circumstances.
An object of the present invention is to provide a hot rolled steel sheet having high
strength and yield ratio and being excellent in terms of ductility, bendability, toughness,
and external appearance.
[Means for Solving the Problem]
[0013]
The gist of the present invention is as described below.
[0014]
(1) A hot rolled steel sheet according to an aspect of the present invention
containing, by mass%, as a chemical composition:
C: 0.025% to 0.055%,
Mn: 1.00% to 2.00%,
sol. Al: 0.200% or more and less than 0.500%,
Ti: 0.030% to 0.200%,
Si: 0.100% or less,
P: 0.100% or less,
S: 0.030% or less,
- 4 -
N: 0.100% or less,
0: 0.010% or less,
Nb: 0% to 0.050%,
V: 0% to 0.050%,
Cu: 0% to 2.00%,
Cr: 0% to 2.00%'
Mo: 0% to 1.000%,
Ni: 0% to 2.00%,
B: 0% to 0.0100%,
Ca: 0% to 0.0200%,
Mg: 0% to 0.0200%,
REM: 0% to 0.1000%,
Bi: 0% to 0.0200%,
Zr: 0% to 1.000%,
Co: 0% to 1.000%,
Zn: 0% to 1.000%,
W: 0% to 1.000%,
Sn: 0% to 0.050%, and
a remainder: Fe and impurities,
in which a microstructure contains, by area%,
polygonal ferrite: 2.0% or more and less than 10.0%, and
a remainder in the microstructure: more than 90.0% and 98.0% or less, and
a correlation value represented by the following formula (1), which is
obtained by analyzing the remainder in the microstructure in a SEM image of the
microstructure by a gray-level co-occurrence matrix method, is 0.82 to 0.95, and a
- 5 -
maximum probability value represented by the following formula (2) is 0.0040 to
0.0200.
. L L P(i,J)[(i-JLx)·(J-JLy)]
Correlatzon = i j axay
( 1 )
Maxilnun1 Probability= Max( P(i,J)) • • • ( 2 )
where P(i, j) in the formula (1) and the formula (2) is a gray-level cooccurrence
matrix, and J.lx, J.ly, Gx, and Gy are represented by the following formulas (3)
to (6).
flx = 2: i 2: j i ( P ( i, 1)) . . . ( 3)
. • . ( 4)
()X = L i L j p ( i' j) ( i - JL,~ )
2
• • . ( 5 )
. • • ( 6)
(2) The hot rolled steel sheet according to (1) may further contain, as the
chemical composition, by mass%, one or more among the group consisting of
Nb: 0.001% to 0.050%,
V: 0.001% to 0.050%,
Cu: 0.01% to 2.00%,
Cr: 0.01% to 2.00%,
Mo: 0.001% to 1.000%,
- 6 -
Ni: 0.01% to 2.00%,
B: 0.0001% to 0.0100%,
Ca: 0.0001% to 0.0200%,
Mg: 0.0001% to 0.0200%,
REM: 0.0001% to 0.1000%,
Bi: 0.0001% to 0.0200%,
Zr: 0.001% to 1.000%,
Co: 0.001% to 1.000%,
Zn: 0.001% to 1.000%,
W: 0.001% to 1.000%, and
Sn: 0.001% to 0.050%.
(3) In the hot rolled steel sheet according to (1) or (2), the maximum
probability value of the microstructure may be 0.0080 to 0.0200.
(4) In the hot rolled steel sheet according to any one of (1) to (3), the chemical
composition may satisfy Si +T-Al< 0.500% when a Si content by mass% is
represented by Si, and anAl content by mass% is represented by T - Al.
(5) In the hot rolled steel sheet according to any one of (1) to (4), a tensile
strength may be 780 MPa or more, and
a yield ratio that is obtained by dividing a yield stress by the tensile strength
may be 0.86 or more.
[Effects ofthe Invention]
[0015]
According to the above-described aspect of the present invention, it is
possible to provide a hot rolled steel sheet having high strength and yield ratio and
being excellent in terms of ductility, bendability, toughnes s, and external appearance.
- 7 -
In addition, according to the above-described preferable aspect of the present invention,
it is possible to provide a hot rolled steel sheet having superior bendability.
[Embodiments of the Invention]
[0016]
In view of the above problems, the present inventors repeated intensive
studies on the chemical compositions of a hot rolled steel sheet and the relationship
between a microstructure and mechanical properties. As a result, the present
inventors found that a hot rolled steel sheet having high strength and yield ratio and
being excellent in terms of ductility, bendability, toughness, and external appearance
can be obtained by decreasing the Si content and providing a microstructure having a
low temperature transformation structure with specific characteristics (bainitic ferrite).
[0017]
Hereinafter, the chemical composition and microstructure of a hot rolled steel
sheet according to the present embodiment will be more specifically described.
However, the present invention is not limited only to a configuration disclosed in the
present embodiment and can be modified in a variety of manners within the scope of
the gist of the present invention.
The numerical limit range described below with "to" in between includes the
lower limit and the upper limit. Numerical values expressed with "less than" or
"more than" are not included in numerical ranges. In the following description, %
regarding the chemical composition of the hot rolled steel sheet is mass% unless
particularly otherwise specified.
[0018]
1. Chemical Composition
The hot rolled steel sheet according to the present embodiment contains, by
- 8 -
mass%, C: 0.025% to 0.055%, Mn: 1.00% to 2.00%, sol. Al: 0.200% or more and less
than 0.500%, Ti: 0.030% to 0.200%, Si: 0.100% or less, P: 0.100% or less, S: 0.030%
or less, N: 0.100% or less, 0: 0.010% or less, and a remainder of Fe and impurities.
Each element will be described in detail below.
[0019]
C: 0.025% to 0.055%
C is an element required to obtain a desired strength. When the C content is
less than 0.025%, a desired tensile strength cannot be obtained. Therefore, the C
content is set to 0.025% or more. The C content is preferably 0.027% or more and
more preferably 0.030% or more.
On the other hand, when the C content is more than 0.055%, the bendability
and toughness of the hot rolled steel sheet deteriorate. Therefore, the C content is set
to 0.055% or less. The C content is preferably 0.052% or less and more preferably
0.050% or less.
[0020]
Mn: 1.00% to 2.00%
Mn is an element that suppresses ferritic transformation to increase the
strength of the hot rolled steel sheet. When the Mn content is less than 1.00%, a
desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.00%
or more. The Mn content is preferably 1.20% or more and more preferably 1.30% or
more.
On the other hand, when the Mn content is more than 2.00%, the bendability
and toughness of the hot rolled steel sheet deteriorate. Therefore, the Mn content is
set to 2.00% or less. The Mn content is preferably 1.90% or less and more preferably
1.70% or less or 1.60% or less.
- 9 -
[0021]
sol. Al: 0.200% or more and less than 0.500%
Al has an action of deoxidizing steel to make the steel sound (suppressing the
generation of a defect such as blowholes in the steel) and also has an action of
promoting the formation of a low temperature transformation structure with specific
characteristics (bainitic ferrite) and enhancing the bendability and toughness of the hot
rolled steel sheet. When the sol. Al content is less than 0.200%, an effect by the
action cannot be obtained. Therefore, the soL Al content is set to 0.200% or more.
The soL Al content is preferably 0.250% or more and more preferably 0.300% or more.
On the other hand, when the soL Al content is 0.500% or more, the above
effects are saturated, which is not economically preferable. In addition, when the sol.
Al content is 0.500% or more, polygonal ferrite is excessively precipitated. Therefore,
the soL Al content is set to less than 0.500%. The sol. Al content is preferably
0.450% or less and more preferably 0.400% or less or 0.350% or less.
The sol. Al means acid-soluble Al and refers to solid solution Al present in
steel in a solid solution state.
[0022]
In the chemical composition of the hot rolled steel sheet according to the
present embodiment, when the Si content by mass% is represented by Si, and the Al
content by mass% is represented by T - Al, Si + T - Al < 0.500% may be satisfied.
When Si +T -Al < 0.500% is satisfied, the area ratio of polygonal ferrite can be stably
set to 10% or less. In addition, the occurrence of slab cracking can be further reduced.
T-Al mentioned herein refers to the total content (mass%) of Al that is
contained in the hot rolled steel sheet and is the sum of the acid-soluble Al (soL Al)
content and the content of a relatively small amount of acid-insoluble Al (insoL Al).
- 10 -
The T- Al content may be set to 0.200% to 0.500% as necessary. The upper
limit thereof may be set to 0.450%, 0.400% or 0.350%, and the lower limit thereof may
be set to 0.250% or 0.300%.
[0023]
Ti: 0.030% to 0.200%
Ti is precipitated in steel as a carbide or a nitride and has an action of refining
the microstructure by an austenite pinning effect and increasing the tensile strength of
the hot rolled steel sheet by precipitation hardening. When the Ti content is less than
0.030%, a desired tensile strength cannot be obtained. Therefore, the Ti content is set
to 0.030% or more. The Ti content is preferably 0.050% or more and more preferably
0.100% or more.
On the other hand, when the Ti content is more than 0.200%, the tensile
strength of the hot rolled steel sheet deteriorates due to the excessive precipitation of
polygonal ferrite. Therefore, the Ti content is set to 0.200% or less. The Ti content
is preferably 0.180% or less and more preferably 0.150% or less.
[0024]
Si: 0.100% or less
Si has an action of improving the ductility of the hot rolled steel sheet by
promoting the formation of ferrite and an action of increasing the strength of the hot
rolled steel sheet by the solid solution strengthening of ferrite. In addition, Si has an
action of making steel sound by deoxidation. However, when the Si content is more
than 0.100%, scale is generated on the surface of the hot rolled steel sheet, and a scale
pattern remains on the surface of the hot rolled steel sheet even in a case where the
scale has been removed. As a result, the external appearance of the hot rolled steel
sheet deteriorates. Therefore, the Si content is set to 0.100% or less. The Si content
- 11 -
is preferably 0.080% or less and more preferably 0.050% or less.
The lower limit of the Si content does not need to be particularly specified,
and the S content may be set to 0.010% or more.
[0025]
P: 0.100% or less
Pis an element that is generally contained in steel as an impurity and has an
action of increasing the strength of the hot rolled steel sheet by solid solution
strengthening. Therefore, P may be positively contained. However, Pis an element
that is easily segregated, and, when the P content exceeds 0.100%, the deterioration of
the bendability of the hot rolled steel sheet attributed to boundary segregation becomes
significant. Therefore, the P content is set to 0.100% or less. The P content is
preferably 0.050% or less and more preferably 0.030% or less.
The lower limit of the P content does not need to be particularly specified, and
the P content may be set to 0.001% from the viewpoint of refining cost.
[0026]
S: 0.030% orless
S is an element that is contained in steel as an impurity. In addition, S is an
element that forms a sulfide-based inclusion in steel to degrade the bendability of the
hot rolled steel sheet. When the S content exceeds 0.030%, the bendability of the hot
rolled steel sheet significantly deteriorates. Therefore, the S content is set to 0.030%
or less. The S content is preferably 0.010% or less and more preferably 0.005% or
less.
The lower limit of the S content does not need to be particularly specified, and
the S content may be set to 0.0001% from the viewpoint of refining cost.
[0027]
- 12 -
N: 0.100% or less
N is an element that is contained in steel as an impurity and has an action of
degrading the bendability of the hot rolled steel sheet. When the N content is more
than 0.100%, the bendability of the hot rolled steel sheet significantly deteriorates.
Therefore, theN content is set to 0.100% or less. TheN content is preferably 0.080%
or less, more preferably 0.070% or less, and still more preferably 0.010% or less or
0.006% or less.
The lower limit of theN content does not need to be particularly specified,
and theN content may be set to 0.001% or more.
[0028]
0: 0.010% or less
When contained in steel in large quantities, 0 is an element that forms a
coarse oxide that becomes the starting point of fracture and causes brittle fractures and
hydrogen-induced cracks. When the 0 content is more than 0.010%, brittle fractures
and hydrogen-induced cracks are likely to be initiated. Therefore, the 0 content is set
to 0.010% or less. The 0 content is preferably 0.008% or less and more preferably
0.005% or less or 0.003% or less.
The 0 content may be set to 0.0005% or more or 0.001% or more in order to
disperse a large number of fine oxides during the deoxidation of molten steel.
[0029]
The hot rolled steel sheet according to the present embodiment may contain
the following elements as optional elements instead of some of Fe. In a case where
the above optional elements are not contained, the lower limit of the content thereof is
0%. Hereinafter, the optional elements will be described in detail.
[0030]
- 13 -
Nb: 0% to 0.050%
Nb is an element that is finely precipitated in steel as a carbide and a nitride
and improves the strength of steel by precipitation hardening. In order to reliably
obtain this effect, the Nb content is preferably set to 0.001% or more.
However, when the Nb content is more than 0.050%, the bendability of the
hot rolled steel sheet deteriorates. Therefore, the Nb content is set to 0.050% or less.
The Nb content is preferably 0.030% or less and more preferably 0.020% or less or
0.010% or less. In order to cut the alloy cost, the Nb content may be set to 0.005% or
less, 0.003% or less, or 0.001% or less as necessary.
[0031]
V: 0% to 0.050%
Vis, similar to Nb, an element that is finely precipitated in steel as a carbide
and a nitride and improves the strength of steel by precipitation hardening. In order
to reliably obtain this effect, the V content is preferably set to 0.001% or more.
However, when the V content is more than 0.050%, the bendability of the hot
rolled steel sheet deteriorates. Therefore, the V content is set to 0.050% or less. The
V content is preferably 0.030% or less and more preferably 0.020% or less or 0.010%
or less. In order to cut the alloy cost, the V content may be set to 0.005% or less,
0.003% or less, or 0.001% or less as necessary.
[0032]
Cu: 0% to 2.00%
Cu has an action of enhancing the hardenability of the hot rolled steel sheet
and an action of being precipitated as a carbide in steel at a low temperature to increase
the strength of the hot rolled steel sheet. In order to more reliably obtain the effect by
these actions, the Cu content is preferably set to 0.01% or more.
- 14 -
However, when the Cu content is more than 2.00%, grain boundary cracking
may occur in the slab. Therefore, the Cu content is set to 2.00% or less. The Cu
content is preferably 1.00% or less and more preferably 0.60% or less or 0.30% or les s.
In order to cut the alloy cost, the Cu content may be set to 0.10% or less, 0.03% or less,
or 0.01% or less as necessary.
[0033]
Cr: 0% to 2.00%
Cr has an action of enhancing the hardenability of the hot rolled steel sheet.
In order to more reliably obtain the effect by this action, the Cr content is preferably
set to 0.01% or more.
However, when the Cr content is more than 2.00%, the chemical
convertibility of the hot rolled steel sheet significantly deteriorates. Therefore, the Cr
content is set to 2.00% or les s. The Cu content is preferably 1.00% or less and more
preferably 0.60% or less or 0.30% or less. In order to cut the alloy cost, the Cu
content may be set to 0.10% or less, 0.03% or less , or 0.01% or less as necessary.
[0034]
Mo: 0% to 1.000%
Mo has an action of enhancing the hardenability of the hot rolled steel sheet
and an action of being precipitated as a carbide in steel to increase the strength of the
hot rolled steel sheet. In order to more reliably obtain the effect by these actions, the
Mo content is preferably set to 0.001% or more.
However, even when the Mo content is set to more than 1.000%, the effect by
the actions is saturated, which is not economically preferable. Therefore, the Mo
content is set to 1.000% or less. The Mo content is preferably 0.600% or less and
more preferably 0.400% or less, 0.200% or less, 0.100% or less, or 0.030% or less. In
- 15 -
order to cut the alloy cost, the Mo content may be set to 0.010% or less, 0.003% or less,
or 0.001% or less as necessary.
[0035]
Ni: 0% to 2.00%
Ni has an action of enhancing the hardenability of the hot rolled steel sheet.
In order to more reliably obtain the effect by this action, the Ni content is preferably
set to 0.01% or more and more preferably set to 0.02% or more.
However, since Ni is an expensive element, it is not economically preferable
to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.
The Ni content is preferably 1.00% or less and more preferably 0.60% or less or 0.30%
or less. In order to cut the alloy cost, the Ni content may be set to 0.10% or less,
0.03% or less, or 0.01% or less as necessary.
[0036]
B: 0% to 0.0100%
B has an action of enhancing the hardenability of the hot rolled steel sheet.
In order to more reliably obtain the effect by this action, the B content is preferably set
to 0. 0001% or more.
However, when the B content is more than 0.0100%, the bendability of the
hot rolled steel sheet significantly deteriorates. Therefore, the B content is set to
0.0100% or less. The B content is preferably 0.0050% or less and more preferably
0.0030% or less or 0.0020% or less. In order to cut the alloy cost, the B content may
be set to 0.0010% or les s, 0.0003% or less, or 0.0001% or less as necessary.
[0037]
Ca: 0% to 0.0200%
Ca has an action of enhancing the bendability of the hot rolled steel sheet by
- 16 -
adjusting the shape of an inclusion in steel to a preferable shape. In order to more
reliably obtain the effect by this action, theCa content is preferably set to 0.0001% or
more and more preferably set to 0.0005% or more.
However, when theCa content is more than 0.0200%, an inclusion is
excessively formed in steel, and the bendability of the hot rolled steel sheet deteriorates.
Therefore, theCa content is set to 0.0200% or less. TheCa content is preferably
0.0100% or less and more preferably 0.0050% or less or 0.0020% or less. In order to
cut the alloy cost, the B content may be set to 0.0010% or less, 0.0003% or less, or
0.0001% or less as necessary.
[0038]
Mg: 0% to 0.0200%
Mg has an action of enhancing the bendability of the hot rolled steel sheet by
adjusting the shape of an inclusion in steel to a preferable shape. In order to more
reliably obtain the effect by this action, the Mg content is preferably set to 0.0001% or
more and more preferably set to 0.0005% or more.
However, when the Mg content is more than 0.0200%, an inclusion is
excessively formed in steel, and the bendability of the hot rolled steel sheet deteriorates.
Therefore, the Mg content is set to 0.0200% or less. The Mg content is preferably
0.0100% or less and more preferably 0.0050% or less or 0.0020% or les s. In order to
cut the alloy cost, the B content may be set to 0.0010% or less, 0.0003% or les s, or
0.0001% or less as necessary.
[0039]
REM: 0% to 0.1000%
REM has an action of enhancing the bendability of the hot rolled steel sheet
by adjusting the shape of an inclusion in steel to a preferable shape. In order to more
- 17 -
reliably obtain the effect by this action, the REM content is preferably set to 0.0001%
or more and more preferably set to 0.0005% or more.
However, when the REM content is more than 0.1000%, an inclusion is
excessively formed in steel, and the bendability of the hot rolled steel sheet deteriorates.
Therefore, the REM content is set to 0.1000% or less. The REM content is preferably
0.0100% or less and more preferably 0.0050% or less or 0.0020% or less. In order to
cut the alloy cost, the REM content may be set to 0.0010% or less, 0.0003% or less, or
0.0001% or less as necessary.
Here, REM refers to a total of 17 elements consisting of Sc, Y, and
lanthanoids, and the REM content refers to the total amount of these elements.
[0040]
Bi: 0% to 0.0200%
In addition, Bi has an action of enhancing the bendability of the hot-rolled
steel sheet by refining the solidification structure. In order to more reliably obtain the
effect by this action, the Bi content is preferably set to 0.0001% or more and more
preferably set to 0.0005% or more.
However, even when the Bi content is more than 0.0200%, the effect by the
action is saturated, which is not economically preferable. Therefore, the Bi content is
set to 0.0200% or less. The Bi content is preferably 0.0100% or less and more
preferably 0.0050% or less or 0.0020% or less. In order to cut the alloy cost, the Bi
content may be set to 0.0010% or les s, 0.0003% or less, or 0.0001% or less as
necessary.
[0041]
Zr: 0% to 1.000%
Co: 0% to 1.000%
- 18 -
Zn: 0% to 1.000%
W: 0% to 1.000%
Sn: 0% to 0.050%
Regarding Zr, Co, Zn, and W, the present inventors have confirmed that, even
when 1.000% or less of each of these elements is contained, the effect of the hot rolled
steel sheet according to the present embodiment is not impaired. Therefore, the
content of each of Zr, Co, Zn, and W may be set to 1.000% or less. The upper limit
of the content of each of Zr, Co, Zn, and W is preferably 0.600% or less and more
preferably 0.400% or less, 0.200% or less, 0.100% or less, or 0.030% or les s. In
order to cut the alloy cost, the content of each of Zr, Co, Zn, and W may be set to
0.010% or less, 0.003% or less, or 0.001% or less as necessary. The total content of
Zr, Co, Zn, and W may be set to 1.000% or less, 0.100% or less, or 0.010% or less.
In addition, the present inventors have confirmed that, even when a small
amount of Sn is contained, the effect of the hot rolled steel sheet according to the
present embodiment is not impaired. However, when a large amount of Sn is
contained, a defect may be generated during hot rolling, and thus the Sn content is set
to 0.050% or less. The Sn content is preferably 0.030% or less and more preferably
0.020% or less. In order to cut the alloy cost, the Sn content may be set to 0.010% or
less, 0.003% or less, or 0.001% or less as necessary.
[0042]
The remainder of the chemical composition of the hot rolled steel sheet
according to the present embodiment may be Fe and an impurity. In the present
embodiment, the impurity means a substance that is incorporated from ore as a raw
material, a scrap, manufacturing environment, or the like and/or a substance that is
permitted to an extent that the hot rolled steel sheet according to the present
- 19 -
embodiment is not adversely affected.
[0043]
The chemical composition of the above hot-rolled steel sheet may be
measured by a general analytical method. For example, the chemical composition
may be measured using inductively coupled plasma-atomic emission spectrometry
(ICP-AES). sol. Al may be measured by the ICP-AES using a filtrate after a sample
is decomposed with an acid by heating. C and S may be measured by using a
combustion-infrared absorption method, N may be measured by using the inert gas
melting-thermal conductivity method, and 0 may be measured using an inert gas
melting-non-dispersive infrared absorption method.
[0044]
Microstructure of hot rolled steel sheet
Next, the microstructure of the hot rolled steel sheet according to the present
embodiment will be described.
In the hot rolled steel sheet according to the present embodiment, the
microstructure contains, by area%, polygonal ferrite: 2.0% or more and less than
10.0% and the remainder in the microstructure: more than 90.0% and 98.0% or less,
and a correlation value represented by the following formula (1 ), which is obtained by
analyzing the remainder in the microstructure in a SEM image of the microstructure by
a gray-level co-occurrence matrix (GLCM) method, is 0.82 to 0.95, and a maximum
probability value represented by the following formula (2) is 0.0040 to 0.0200.
[0045]
In the present embodiment, the microstructural fractions, the correlation value,
and the maximum probability value in the microstructure at a 114 position of the sheet
thickness and the center position in the sheet width direction in a cross section parallel
- 20 -
to the rolling direction are specified. The reason therefor is that the microstructure at
this position indicates a typical microstructure of the steel sheet. "114 position of the
sheet thickness" means a position separated from the surface by 1/4 of the sheet
thickness, which will be true below. The distance from the surface may slightly differ
depending on the circumstances of test piece sampling as necessary, but is set within a
range of a region from a 1/8 depth from the surface to a 3/8 depth from the surface.
[0046]
Area ratio of polygonal ferrite: 2.0% or more and less than 10.0%
Polygonal ferrite is a structure formed when fcc transforms into bee at a
relatively high temperature. Since polygonal ferrite has a low strength and is likely to
deteriorate in toughness, when the area ratio thereof is excessive, desired tensile
strength and toughness cannot be obtained. Therefore, the area ratio of polygonal
ferrite is set to less than 10.0%. The area ratio of polygonal ferrite is preferably 9.0%
or less or 8.0% or less and more preferably 7.0% or less or 6.0% or less.
In order to increase the yield ratio, the area ratio of polygonal ferrite is set to
2.0% or more. The area ratio of polygonal ferrite is preferably 3.0% or more and
more preferably 4.0% or more or 4.5% or more.
[0047]
Remainder in the microstructure: more than 90.0% and 98.0% or less
In the hot rolled steel sheet according to the present embodiment, in addition
to polygonal ferrite, more than 90.0% and 98.0% or less of the remainder in the
microstructure is contained. A specific remainder in the microstructure is 87.0% to
98.0% of bainitic ferrite and a total of 0% to 3.0% of "cementite, pearlite, fresh
martensite, tempered martensite, and residual austenite" in terms of area ratio. The
remainder in the microstructure formed of one or more structures of of bainitic ferrite,
- 21 -
cementite, pearlite, fresh martensite, tempered martensite, and residual austenite has,
unlike polygonal ferrite, a relatively high crystal orientation difference therein, and
thus the GAM value to be described below becomes more than 0.4 o. On the other
hand, polygonal ferrite has a GAM value of 0.4 o or less. Therefore, it is possible to
easily distinguish polygonal ferrite and the remainder in the microstructure using the
GAM value.
In the microstructure according to the present embodiment, it is also possible
to set the area ratio of polygonal ferrite to 2. 0% or more and less than 10. 0%, the area
ratio of bainitic ferrite to 87.0 to 98.0%, and the area ratio of other structures to 0% to
3.0%. In this case, the lower limit of the area ratio of bainitic ferrite may be set to
88.0%, 89.0%, 90.0%, or 91.0%, and the upper limit may be set to 97.0%, 96.0%,
95.0%, or 93.0%. The other structures are formed of one or more structures of
bainitic ferrite, cementite, pearlite, fresh martensite, tempered martensite, and residual
austenite. The upper limit of the area ratio of the other structures may be set to 2.5%,
2.0%, or 1.5%. The lower limit of the area ratio of the other structures is 0%, but
may be set to 0.1 %, 0.3%, or 0.6%.
[0048]
The area ratio of each structure is obtained by the following method.
A sample is sampled from the hot rolled steel sheet such that a cross section
parallel to a rolling direction at the 1/4 position of the sheet thickness and the center
position in the sheet width direction becomes an observed section. While also
depending on a measurement device, the sample is set to a size where about 10 mm in
the rolling direction can be observed. The cut-out cross section of the sample is
polished using silicon carbide paper having a grit of #600 to #1500 and then finished
as a mirror surface using liquid in which diamond powder having a grain size in a
- 22 -
range of 1 )lm to 6 )lm is dispersed in a dilution solution, such as an alcohol, and pure
water. Next, the cross section is polished for eight minutes at room temperature using
colloidal silica containing no alkaline solution to remove strain introduced into the
surface layer of the sample.
[0049]
At the 1/4 position of the sheet thickness from the surface of the cross section
of the sample, a region that is 100 )lm in the rolling direction and 100 )lm in the sheet
thickness direction is measured by the electron backscatter diffraction method at
measurement intervals of 0.1 )lm, thereby obtaining crystal orientation information.
For the measurement, an EBSD analyzer composed of a thermal field emission
scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD
detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree
of vacuum in the EBSD analyzer is set to 9.6 x 10·5 Pa or less, the accelerating voltage
is set to 15 kV, the irradiation current level is set to 13, and the irradiation time of the
electron beam is set to 0.01 seconds/point.
[0050]
In the obtained crystal orientation information, grains having an fcc crystal
structure are determined as residual austenite using a "Phase Map" function installed in
software "OIM Analysis (registered trademark)" included in the EBSD analyzer. In
addition, in regions where the crystal structure is determined to be bee, crystal grains
surrounded by grain boundaries with an orientation difference of 15° or more are
specified. For each of the specified crystal grains, whether the grain average
misorientation (GAM value) is 0.4° or less or more than 0.4° is determined. The
above-described operation is performed in at least five regions. Crystal grains with a
GAM value of 0.4° or less are determined to be polygonal ferrite. The area ratio of
- 23 -
polygonal ferrite is calculated using the total observed area as the denominator and the
total area of polygonal ferrite as the numerator.
[0051]
In addition, the area ratio of residual austenite is obtained by calculating the
average value of the area ratios of regions determined to be residual austenite. For
crystal grains with a GAM value of more than 0.4°, the correlation value (C value) and
the maximum probability value are measured by a method to be described below.
[0052]
For regions determined to be other than polygonal ferrite among the regions
where the crystal structure is determined to be bee, under a condition that a grain
boundary having an orientation difference of 15° or more is defined as a grain
boundary, the "grain average IQ" of the polygonal ferrite regions is calculated using a
"Grain Average IQ" function installed in software "OIM Analysis (registered
trademark)" included in the EBSD analyzer. When the maximum value is indicated
by Ia, regions where the "grain average IQ" becomes IaJ2 or less are determined to be
"cementite, pearlite, fresh martensite, and tempered martensite". The area ratio of
these regions is calculated, thereby obtaining the total of the area ratios of "cementite,
pearlite, fresh martensite, and tempered martensite".
[0053]
The area ratio of bainitic ferrite is obtained by subtracting the area ratios of
polygonal ferrite, residual austenite, and "cementite, pearlite, fresh martensite, and
tempered martensite" obtained by the above-described method from 100%.
The area ratio of the remainder in the microstructure is obtained by
calculating the sum of the area ratio of residual austenite, the area ratio of bainitic
ferrite, and "cementite, pearlite, fresh martensite, and tempered martensite" obtained
- 24 -
by the above-described method.
[0054]
Bainitic ferrite is a structure almost all of which is determined to be bainite in
a case where the structure is observed with an optical microscope. In a case where
the microstructure of the hot rolled steel sheet according to the present embodiment is
observed with an optical microscope, at least 80% or more of bainite is observed in
terms of area ratio. The structure is observed with an optical microscope by, for
example, the following method. A sample for structure observation is cut out such
that a sheet thickness cross section parallel to the rolling direction becomes an
observed section, and the observed section is mirror-polished. Nital etching is
performed on the mirror-polished sample, and then the structure is observed.
[0055]
Correlation value (C value): 0.82 to 0.95
Maximum probability value (M value): 0.0040 to 0.0200
In order to obtain high strength and yield ratio and excellent ductility,
bendability, and toughness, it is important to make the microstructure low in nonuniformity
and high in uniformity between crystal grains. In the present embodiment,
a correlation value (hereinafter, also referred to as C value) is adopted as an index of
non-uniformity between extremely small regions of the microstructure, and a
maximum probability value (hereinafter, also referred to as M value) is adopted as an
index of uniformity of the entire microstructure.
[0056]
The C value represents the non-uniformity within a crystal grain of the
microstructure. In a case where points separated in the submicron order in a crystal
grain are non-uniform, the C value improves. In the present embodiment, since there
- 25 -
is a need to make the microstructure have bainitic ferrite having fine subgrain
boundaries or precipitates in crystal grains, it is necessary to control the C value to a
desired value. When the C value is less than 0.82, high strength and yield ratio
cannot be obtained. Therefore, the C value is set to 0.82 or more. The C value is
preferably 0.83 or more and more preferably 0.85 or more.
In a case where the C value is more than 0.95, a substructure excessively
develops in the microstructure, and it becomes difficult to obtain a high yield ratio in
order to introduce moving dislocation during cooling. Therefore, the C value is set to
0.95 or less. The C value is preferably 0.90 or less and more preferably 0.88 or less.
[0057]
TheM value represents the uniformity of the entire microstructure and
increases as the area of regions having a certain brightness difference increases. A
high M value means that the uniformity of the microstructure is high. In the present
embodiment, since there is a need to make the microstructure mainly contain highly
uniform bainitic ferrite, it is necessary to increase the M value. In a case where the M
value is less than 0.0040, bainitic ferrite in which fine cementite or MA (a mixture of
fresh martensite and residual austenite) is dispersed in the structure is formed, and
excellent bendability and toughness can be obtained. Therefore, theM value is set to
0.0040 or more. TheM value is preferably 0.0060 or more and more preferably
0.0080 or more. When the M value is set to 0.0080 or more, the bendability of the
hot rolled steel sheet can be further enhanced.

What is claimed is:
CLAIMS
1. A hot rolled steel sheet comprising, by mass%, as a chemical
composition:
C: 0.025% to 0.055%,
Mn: 1.00% to 2.00%,
sol. Al: 0.200% or more and less than 0.500%,
Ti: 0.030% to 0.200%,
Si: 0.100% or less,
P: 0.100% or less,
S: 0.030% or less,
N: 0.100% or less,
0: 0.010% or les s,
Nb: 0% to 0.050%,
V: 0% to 0.050%,
Cu: 0% to 2.00%,
Cr: 0% to 2.00%'
Mo: 0% to 1.000%,
Ni: 0% to 2.00%,
B: 0% to 0.0100%,
Ca: 0% to 0.0200%,
Mg: 0% to 0.0200%,
REM: 0% to 0.1000%,
Bi: 0% to 0.0200%,
Zr: 0% to 1.000%,
- 60 -
Co: 0% to 1.000%,
Zn: 0% to 1.000%,
W: 0% to 1.000%,
Sn: 0% to 0.050%, and
a remainder: Fe and impurities,
wherein a microstructure contains, by area%,
polygonal ferrite: 2.0% or more and less than 10.0%, and
a remainder in the microstructure: more than 90.0% and 98.0% or less, and
a correlation value represented by the following formula (1), which is
obtained by analyzing the remainder in the microstructure in a SEM image of the
microstructure by a gray-level co-occurrence matrix method, is 0.82 to 0.95, and a
maximum probability value represented by the following formula (2) is 0.0040 to
0.0200,
. L L P(i,J)[(i-JLx)·(J-JLy)]
Correlation =
i j (jx(jy
( 1 )
Maximum Probability= Max(P(i,J)) • • • ( 2)
where P(i, j) in the formula (1) and the formula (2) is a gray-level cooccurrence
matrix, and )lX, )ly, ax, and ay are represented by the following formulas
(3) to (6),
• • • ( 3)
• • • ( 4)
- 61 -
ax = Li L1P(i,J)(i- flx )
2
(Jy = LiLjP(i,J)(i- Jly r
. • • ( 5)
• • 0 ( 6)
2. The hot rolled steel sheet according to claim 1, comprising, as the
chemical composition, by mass%, one or more among the group consisting of:
0.0200.
Nb: 0.001% to 0.050%,
V: 0.001% to 0.050%,
Cu: 0.01% to 2.00%,
Cr: 0.01% to 2.00%,
Mo: 0.001% to 1.000%,
Ni: 0.01% to 2.00%,
B: 0.0001% to 0.0100%,
Ca: 0.0001% to 0.0200%,
Mg: 0.0001% to 0.0200%,
REM: 0.0001% to 0.1000%,
Bi: 0.0001% to 0.0200%,
Zr: 0.001% to 1.000%,
Co: 0.001% to 1.000%,
Zn: 0.001% to 1.000%,
W: 0.001% to 1.000%, and
Sn: 0.001% to 0.050%.
3. The hot rolled steel sheet according to claim 1 or 2,
wherein the maximum probability value of the microstructure is 0.0080 to
- 62 -
4. The hot rolled steel sheet according to any one of claims 1 to 3,
wherein the chemical composition satisfies Si +T-Al< 0.500% when a Si
content by mass% is represented by Si, and anAl content by mass% is represented by
T-AL
5. The hot rolled steel sheet according to any one of claims 1 to 4,
wherein a tensile strength is 780 MPa or more, and
a yield ratio that is obtained by dividing a yield stress by the tensile strength is
0.86 or more.