[Technical Field of the Invention]
[0001]
The present invention relates to a hot-rolled steel sheet and a manufacturing
method thereof.
Priority is claimed on Japanese Patent Application No. 2020-082656, filed
May 8, 2020, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, weight reduction of automobiles and each machine
component has been underway. Designing an optimum shape as the component
shape ensures stiffness and thereby makes it possible to reduce the weights of
automobiles and each machine component. Furthermore, in blank-formed
components such as a press-formed component, the weights can be reduced by
reducing the sheet thicknesses of component materials. However, in the case of
attempting to ensure the static fracture strength and the yield strength while reducing
the sheet thicknesses, it becomes necessary to use high-strength materials. In
particular, for automobile suspension components such as lower control arms, trailing
arms, or knuckles, studies have begun about the application of higher than 780 MPa
class steel sheets. Since these automobile suspension components are manufactured
by performing bending forming and the like on steel sheets, steel sheets that are
applied to these automobile suspension components are required to have excellent
formability.
- 1 -
[0003]
For example, Patent Document 1 discloses a hot-rolled steel sheet in which, in
a hot rolling step, the finish rolling temperature and the rolling reduction are set within
predetermined ranges, thereby controlling the grain sizes and aspect ratios of prior
austenite and reducing anisotropy.
[0004]
Patent Document 2 discloses a cold-rolled steel sheet in which, in a hot rolling
step, the rolling reduction and the average strain rate are set within appropriate ranges
in a predetermined finish rolling temperature range, thereby improving the toughness.
[0005]
In order to further reduce the weights of automobiles, each machine
component, or the like, it is also expected to apply steel sheets having a sheet thickness
premised on a cold-rolled steel sheet to automobile suspension components. The
techniques described in Patent Document 1 and Patent Document 2 are effective in the
manufacturing of automobile suspension components to which a high strength steel
sheet is applied.
[0006]
However, the present inventors found that, even in steel sheets to which the
techniques of Patent Document 1 and Patent Document 2 are applied, there are cases
where the fatigue properties (durability and impact resistance) after the steel sheets are
formed into component shapes are not sufficient. This is considered to be because a
sharpened recessed part such as a fine crack is formed in the cross section of the inside
of a bend (hereinafter, simply referred to as "inside bend") in a bending forming part
even when a load simulating the operation environment is not imparted after bending
forming. It is considered that this recessed part brings about an effect of a notch such
- 2 -
as a fine crack and degrades the durability of components. The inventors found that
the formation of a sharpened recessed part such as a fine crack at the inside bend
becomes easier as the strength of a steel sheet increases.
[Prior Art Document]
[Patent Document]
[0007]
[Patent Document 1] Japanese Patent No. 5068688
[Patent Document 2] Japanese Patent No. 3858146
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0008]
The inventors investigated recessed parts that are formed at the inside bend in
order to enable the provision of a steel sheet that is a high strength steel sheet and has
improved in terms of a sharpened recessed part at an inside bend that is initiated during
bending forming. As a result, the present inventors found that the sharpened recessed
part such as a fine crack at the inside bend (hereinafter, a sharpened recessed part such
as a fine crack that is formed at a inside bend will be referred to as "inside bend
recessed part") is not a fine crack and is attributed to unevenness formed by the plastic
buckling of the surface layer of the steel sheet toward the outside of the plane in a
microscopic region during bending forming. In addition, the present inventors found
that, in a case where the depth of an inside bend recessed part exceed a certain value,
the fatigue properties of hot-rolled steel sheets significantly deteriorate.
[0009]
An object of the present invention is to provide a hot-rolled steel sheet having
a high strength and excellent formability and enabling reduction in the depth of an
- 3 -
inside bend recessed part that is formed during bending forming and a manufacturing
method thereof.
[Means for Solving the Problem]
[0010]
As a result of inventive studies, the present inventors found that the depth of
an inside bend recessed part formed during bending forming can be reduced to an
extent that component performance is not degraded by setting a chemical composition
and a metallographic structure appropriate for obtaining a high strength and,
furthermore, particularly controlling the rotation angle of a specific crystal orientation
in the sheet thickness direction. A high strength in the present embodiment means
that the tensile (maximum) strength is 880 MPa or more. In addition, excellent
formability means that the hole expansion rate is 35% or more.
[0011]
The gist of the present invention made based on the above-described findings
is as follows.
(1) A hot-rolled steel sheet according to an aspect of the present invention
contains, as a chemical composition, by mass%:
C: 0.060% to 0.170%,
Si: 0.030% to 1.700%,
Mn: 1.20% to 3.00%,
Al: 0.010% to 0.700%,
Nb: 0.005% to 0.050%,
P: 0.0800% or less,
S: 0.0100% or less,
N: 0.0050% or less,
- 4 -
Ti: 0% to 0.1800%,
Mo: 0% to 0.150%,
V: 0% to 0.3000%,
Cr: 0% to 0.500%,
B: 0% to 0.0030%, and
a remainder consisting of Fe and an impurity,
in which, in metallographic structures at a 1/4 position in a sheet thickness
direction from a surface and at a 1/2 position in the sheet thickness direction from the
surface, by vol%,
bainite and martensite are a total of 80.0% or more,
ferrite is 20.0% or less, and
cementite and residual austenite are a total of 0% to 1 0.0%,
in a metallographic structure of a region from the surface to a 100 11m position
in the sheet thickness direction from the surface,
an average grain diameter of prior austenite grains is less than 30.00 11m,
a region, where a rotation angle between a normal line of the surface and a
(0 11) pole near the normal line is 5° or less, is 0.150 or less from the surface in terms
of a sheet thickness direction position standardized by a sheet thickness,
a region, where the rotation angle between the normal line of the surface and
the (011) pole near the normal line becomes 20° or more, is 0.250 or more from the
surface in terms of the sheet thickness direction position standardized by the sheet
thickness, and
a tensile strength is 880 MPa or more.
(2) The hot-rolled steel sheet according to (1) may further contain, as the
chemical composition, by mass%, one or more selected from the group consisting of
- 5 -
Ti: 0.0200% to 0.1800%,
Mo: 0.030% to 0.150%,
V: 0.0500% to 0.3000%,
Cr: 0.050% to 0.500%, and
B: 0.0001% to 0.0030%.
(3) A manufacturing method of a hot-rolled steel sheet according to another
aspect of the present invention is a manufacturing method of the hot-rolled steel sheet
according to (1) or (2), including
a casting step of, in continuous casting of a slab having the chemical
composition according to ( 1 ), performing the continuous casting in a manner that an
average surface temperature gradient in a region from a meniscus to 1.0 m from the
meniscus becomes 300 to 650 °Cim to obtain the slab,
a heating step of heating the slab to 1200°C or higher and holding the slab for
30 minutes or longer,
a hot rolling step of performing rough rolling on the slab, and performing
finish rolling in a manner that a total rolling reduction in a temperature range of 870ac
to 980°C becomes 80% or larger, an elapsed time between rolling stands in the
temperature range of 870ac to 980°C becomes 0.3 to 5.0 seconds, and a total rolling
reduction in a temperature range of lower than 870°C becomes smaller than 10%,
a cooling step of cooling for 30.0 seconds or shorter to cool to a temperature
range of lower than 300°C after the finish rolling, and
a coiling step of coiling in a manner that a coiling temperature becomes lower
than 300°C after the cooling.
(4) The manufacturing method of the hot-rolled steel sheet according to (3)
may further include a heat treatment step of holding in a temperature range of 200ac or
- 6 -
higher and lower than 4Sooc for 90 to 80000 seconds after the coiling.
[Effects ofthe Invention]
[0012]
According to the aspects of the present invention, it is possible to provide a
hot-rolled steel sheet having a high strength and excellent formability and enabling
reduction in the depth of an inside bend recessed part that is formed during bending
forming and a manufacturing method thereof.
[Brief Description of the Drawings]
[0013]
FIG. 1 is a view showing a relationship between a sheet thickness direction
position standardized by a sheet thickness of a region where a rotation angle between a
normal line of a surface of a steel sheet and a (0 11) pole near the normal line becomes
so or less and a depth of an inside bend recessed part in an example.
FIG. 2 is a view showing a relationship between a sheet thickness direction
position standardized by the sheet thickness of a region where the rotation angle
between the normal line of the surface of the steel sheet and the (0 11) pole near the
normal line becomes 20° or more and the depth of the inside bend recessed part in the
example.
FIG. 3 is a view showing a relationship among the sheet thickness direction
position standardized by the sheet thickness of the region where the rotation angle
between the normal line of the surface of the steel sheet and the (0 11) pole near the
normal line becomes so or less, the sheet thickness direction position standardized by
the sheet thickness of the region where the rotation angle between the normal line of
the surface of the steel sheet and the (011) pole near the normal line becomes 20° or
more, and the evaluation result of the inside bend recessed part in the example.
- 7 -
[Embodiments of the Invention]
[0014]
Hereinafter, a hot-rolled steel sheet according to the present embodiment
(hereinafter, simply referred to as the steel sheet in some cases) will be described in
detaiL 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.
Numerical limiting ranges expressed below using "to" include the lower limit
and the upper limit in the ranges. Numerical values expressed with "more than" and
"less than" are not included in numerical ranges. "%" regarding chemical
compositions all indicates "mass%".
[0015]
The hot-rolled steel sheet according to the present embodiment contains, by
mass%, C: 0.060% to 0.170%, Si: 0.030% to 1.700%, Mn: 1.20% to 3.00%, Al:
0.010% to 0.700%, Nb: 0.005% to 0.050%, P: 0.0800% or less, S: 0.0100% or less, N:
0.0050% or less, and a remainder of Fe and an impurity. Hereinafter, each element
will be described in detaiL
[0016]
C: 0.060% to 0.170%
Cis one element that determines the strength of the hot-rolled steel sheet.
When the C content is less than 0.060%, it is not possible to obtain a tensile strength of
880 MPa or more. Therefore, the C content is set to 0.060% or more. The C content
is preferably 0.080% or more.
On the other hand, when the C content is more than 0.170%, the hole
expansibility of the hot-rolled steel sheet deteriorates, and it is not possible to obtain a
- 8 -
hole expansion rate of 35% or more. Hot-rolled steel sheets having a hole expansion
rate of less than 35% are not applicable to components. Therefore, the C content is
set to 0.170% or less. The C content is preferably 0.150% or less.
[0017]
Si: 0.030% to 1.700%
Si is an element that improves the strength of the hot-rolled steel sheet by
solid solution strengthening. In addition, Si is also an element that has an effect on
suppressing the formation of a carbide and suppresses softening during a heat
treatment. In order to obtain these effects, the Si content is set to 0.030% or more.
The Si content is preferably 0.050% or more.
On the other hand, since Si has a high oxide-forming capability, when the Si
content is excessive, an oxide is formed in a weld or the volume percentage of residual
austenite becomes more than 10%, and the hole expansibility of the hot-rolled steel
sheet deteriorates. Therefore, the Si content is set to 1.700% or less. In order to
further suppress softening during tempering, the Si content is preferably set to 1.300%
or less.
[0018]
Mn: 1.20% to 3.00%
Mn is an element necessary to improve the strength of the hot-rolled steel
sheet. When the Mn content is less than 1.20%, it is not possible to obtain a tensile
strength of 880 MPa or more. Therefore, the Mn content is set to 1.20% or more.
The Mn content is preferably 1.50% or more.
On the other hand, when the Mn content exceeds 3.00%, the toughness of a
cast slab deteriorates, and hot rolling is not possible. Therefore, the Mn content is set
to 3.00% or less. The Mn content is preferably 2.70% or less.
- 9 -
[0019]
Al: 0.010% to 0.700%
Al is an element that acts as a deoxidizing agent and improves the cleanliness
of steeL In order to obtain this effect, the Al content is set to 0.010% or more. The
Al content is preferably 0.100% or more.
On the other hand, when the Al content is more than 0. 700%, casting becomes
difficult. Therefore, the Al content is set to 0.700% or less. Al is an oxidizing
element, and the Al content is preferably 0.300% or less in order to obtain an effect on
additional improvement in continuous castability and a cost reduction effect.
[0020]
Nb: 0.005% to 0.050%
In order to obtain an average grain diameter of prior austenite grains of less
than 30.00 J.lm in a hot rolling step, the Nb content needs to be set to 0.005% or more.
When the Nb content is less than 0.005%, it is not possible to obtain an average grain
diameter of the prior austenite grains of less than 30.00 J.lm in the hot rolling step, and
a desired metallographic structure cannot be obtained in the end. Therefore, the Nb
content is set to 0.005% or more. The Nb content is preferably 0.010% or more or
0.020% or more.
On the other hand, when the Nb content is more than 0.050%, the toughness
of the cast slab deteriorates, and hot rolling is not possible. Therefore, the Nb content
is set to 0.050% or less. The Nb content is preferably 0.040% or les s.
[0021]
P: 0.0800% or less
Pis an impurity element that is inevitably incorporated into the hot-rolled
steel sheet in a manufacturing process of the hot-rolled steel sheet. The higher the P
- 10 -
content, the more the hot-rolled steel sheet embrittles. In a case where the hot-rolled
steel sheet is applied to automobile suspension components, a P content of up to
0.0800% is acceptable. Therefore, the P content is set to 0.0800% or less. The P
content is preferably 0.0500% or less. When the P content is reduced to less than
0.0005%, the dephosphorization cost significantly increases, and thus the P content
may be set to 0.0005% or more.
[0022]
S: 0.0100% or less
In a case when a large amount of S is contained in molten steel, MnS is
formed, and the hole expansibility and toughness of the hot -rolled steel sheet are
degraded. Therefore, the S content is set to 0.0100% or less. The S content is
preferably 0.0080% or less. When the S content is reduced to less than 0.0001%, the
desulfurization cost significantly increases, and thus the S content may be set to
0.0001% or more.
[0023]
N: 0.0050% or less
N is an impurity element that is inevitably incorporated into the hot-rolled
steel sheet in the manufacturing process of the hot-rolled steel sheet. When theN
content becomes more than 0.0050%, the amount of residual austenite in the hot-rolled
steel sheet increases, and there are cases where the hole expansibility of the hot-rolled
steel sheet deteriorates and the slab toughness deteriorates. Therefore, the N content
is set to 0.0050% or less. TheN content is preferably 0.0040% or less. When theN
content is reduced to less than 0.0001%, the steelmaking cost significantly increases,
and thus the N content may be set to 0.0001% or more.
[0024]
- 11 -
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, a manufacturing environment, or the like and is allowed to an extent
that the hot-rolled steel sheet according to the present embodiment is not adversely
affected.
[0025]
The hot-rolled steel sheet according to the present embodiment may contain
one or more of the group consisting of Ti, Mo, V, Cr, and B as an arbitrary element
instead of some of Fe. In a case where the arbitrary element is not contained, the
lower limit of the content is 0%. Hereinafter, each arbitrary element will be described.
[0026]
Ti: 0% to 0.1800%,
Ti is an element that increases the strength of the hot-rolled steel sheet by
being precipitated as a fine carbide in steel and thus may be contained. In order to
reliably obtain the effect, the Ti content is preferably set to 0.0200% or more. On the
other hand, even when more than 0.1800% ofTi is contained, the above-described
effect is saturated. Therefore, the Ti content is preferably set to 0.1800% or less.
[0027]
Mo: 0% to 0.150%
Mo is an element that enhances the hardenability of steel and may be
contained as an element that adjusts the strength of the hot-rolled steel sheet. In order
to reliably obtain the above-described effect, the Mo content is preferably set to
0.030% or more. On the other hand, even when more than 0.150% ofMo is
contained, the above-described effect is saturated. Therefore, the Mo content is
- 12 -
preferably set to 0.150% or less.
[0028]
V: 0% to 0.3000%
Vis an element that develops an effect similar to that of Ti. In order to
reliably obtain an effect of precipitation hardening by the formation of a fine carbide,
the V content is preferably set to 0.0500% or more. However, when Vis excessively
contained, a nitride is formed in steel, which degrades the slab toughness and makes
threading difficult. Therefore, the V content is preferably set to 0.3000% or less.
[0029]
Cr: 0% to 0.500%
Cr is an element that develops an effect similar to that of Mn. In order to
reliably obtain a strength improvement effect of the hot-rolled steel sheet, the Cr
content is preferably set to 0.050% or more. On the other hand, even when more than
0.500% of Cr is contained, the above-described effect is saturated. Therefore, the Cr
content is preferably set to 0.500% or less.
[0030]
B: 0% to 0.0030%
B is an element that develops an effect similar to that of Mo and is an element
that has an effect on improvement in hardenability and increases the strength of the
hot-rolled steel sheet. In order to reliably obtain the effect, the B content is preferably
set to 0.0001% or more. On the other hand, even when more than 0.0030% of B is
contained, the above-described effect is saturated, and thus the B content is preferably
set to 0.0030% or less.
[0031]
The above-described chemical composition of the hot-rolled steel sheet may
- 13 -
be analyzed using a spark discharge emission spectrophotometer or the like. For C
and S, values identified by combusting the hot-rolled steel sheet in an oxygen stream
using a gas component analyzer or the like and measuring C and S by an infrared
absorption method are adopted. In addition, for N, a value identified by melting a test
piece collected from the hot-rolled steel sheet in a helium stream and measuring N by a
thermal conductivity method is adopted.
[0032]
Next, the metallographic structure of the hot-rolled steel sheet according to
the present embodiment will be described. The characteristics of the metallographic
structure are limited to an extent that not only an effect on improvement in the strength
and formability of the hot-rolled steel sheet but also an effect on reduction in the
depths of inside bend recessed parts can be obtained.
[0033]
In the hot-rolled steel sheet according to the present embodiment, in the
metallographic structures at a 114 position in the sheet thickness direction from the
surface and at a 1/2 position in the sheet thickness direction from the surface, by vol %,
bainite and martensite are a total of 80.0% or more, ferrite is 20.0% or less, cementite
and residual austenite are a total of 0% to 1 0.0%, in the metallographic structure in a
region from the surface to a 100 11m position in the sheet thickness direction from the
surface, the average grain diameter of prior austenite grains is less than 30.00 11m, a
region, where the rotation angle between the normal line of the surface and a (011)
pole near the normal line becomes 5° or less, is 0.150 or less from the surface in terms
of the sheet thickness direction position standardized by the sheet thickness, a region,
where the rotation angle between the normal line of the surface and the (0 11) pole near
the normal line becomes 20a or more, is 0.250 or more from the surface in terms of the
- 14 -
sheet thickness direction position standardized by the sheet thickness.
Hereinafter, each regulation will be described.
[0034]
Bainite and martensite: Total of 80.0% or more
In a case where the volume percentage of bainite and martensite is less than
80% in total, it is not possible to obtain a tensile strength of 880 MPa or more and/or a
hole expansion rate of 35% or more. Therefore, the volume percentage of the bainite
and the martensite is set to a total of 80.0% or more. The volume percentage of the
bainite and the martensite is preferably 83.0% or more.
The martensite may be tempered, and the martensite may contain cementite
and residual austenite. The volume percentage of the cementite and the residual
austenite may be set to a total of 10.0% or les s.
[0035]
Ferrite: 20.0% or less
When the volume percentage of ferrite is more than 20.0%, the volume
percentage of the bainite and the martensite does not become a total of 80.0% or more,
and it is not possible to obtain a desired tensile strength. Therefore, the volume
percentage of the ferrite is set to 20.0% or less. In order to further improve the
strength, the volume percentage of the ferrite is preferably 17.0% or less and more
preferably 15.0% or less. The volume percentage of the ferrite may be set to 10.0%
or more from the viewpoint of ensuring hole expansibility.
[0036]
Cementite and residual austenite: 0% to 10.0%
As described above, there are cases where martensite contain cementite and
residual austenite. When the volume percentage of the cementite and the residual
- 15 -
austenite is more than a total of 10.0%, the hole expansibility of the hot-rolled steel
sheet deteriorates due to the deterioration of local deformability. Therefore, the
volume percentage of the cementite and the residual austenite is set to 10.0% or less.
The volume percentage of the cementite and the residual austenite is preferably 7.0%
or less and more preferably 5.0% or less. The volume percentage of the cementite
and the residual austenite is preferably as small as possible, and thus the lower limit is
0%.
[0037]
Measuring method of volume percentage of ferrite
As the volume percentage of the ferrite, the area ratio of crystal grains in
which an iron-based carbide is not formed, which are obtained by observing the
structure on a metallographic structure photograph, is used. A sample is collected
such that a sheet thickness cross section that intersects the rolling direction of the hotrolled
steel sheet at right angles can be observed, the cross section is corroded using a
nital etching solution having a concentration of 3% to 5% to make the ferrite visible,
and the structure is observed using metallographic structure photographs each captured
at a magnification of 500 to 1000 times at the 1/4 position in the sheet thickness
direction from the surface of the hot-rolled steel sheet and at the 1/2 position in the
sheet thickness direction from the surface. For one kind of steel, the metallographic
structure photographs are prepared at 3 or more visual fields in each of the 1/4 position
in the sheet thickness direction from the surface and the 1/2 position in the sheet
thickness direction from the surface. The area ratio of the ferrite that is observed in
each metallographic structure photograph is obtained, and the average value thereof is
calculated, thereby obtaining the volume percentage of the ferrite. The iron-based
carbide is recognized as black granular contrast having a circle equivalent diameter of
- 16 -
1 )lm or less in the metallographic structure photograph and is observed in the crystal
gram.
[0038]
Measuring method of volume percentage of bainite and martensite
As the total of the volume percentages of the bainite and the martensite in the
present embodiment, a value obtained by subtracting the volume percentage of the
ferrite and the total of the volume percentages of the cementite and the residual
austenite that are measured by a method to be described below from 100.0% is used.
[0039]
Measuring method of volume percentage of residual austenite
The volume percentage of the residual austenite is measured by EBSP.
Analysis by EBSP is performed using a sample collected from the same position as the
sample collection position at the time of measuring the volume percentage of the
ferrite at the 1/4 position in the sheet thickness direction from the surface of the hotrolled
steel sheet and at the 112 position in the sheet thickness direction from the
surface. The sample needs to be polished using silicon carbide paper #600 to #1500,
then, finished into a mirror surface using a liquid containing a diamond powder having
grain sizes of 1 to 6 )lm dispersed in a diluted solution such as an alcohol or pure water,
and then finished by electrolytic polishing for the purpose of sufficiently removing
strain in a cross section to be measured. In the electrolytic polishing, in order to
remove mechanical polishing strain on an observed section, the sample needs to be
polished a minimum of 20 )lm and polished a maximum of 50 )lm. The sample is
preferably polished 30 )lm or less in consideration of rollover at the end portion.
In the measurement by EBSP, the accelerating voltage is set to 15 to 25 kV,
the measurement is performed at intervals of at least 0.25 )lm or less, and the crystal
- 17 -
orientation information at each measurement point in a range that is 150 11m or more in
the sheet thickness direction and 250 11m or more in a rolling direction is obtained.
Out of the obtained crystal structures, grains having an fcc crystal structure are
determined as the residual austenite using a "Phase Map" function installed in software
"OIM Analysis (registered trademark)" included in an EBSP analyzer. The ratio of
measurement points determined as the residual austenite is obtained, thereby obtaining
the area ratio of the residual austenite. The obtained area ratio of the residual
austenite is regarded as the volume percentage of the residual austenite.
Here, the larger the number of the measurement points, the more preferable,
and thus it is preferable that the measurement intervals are narrow and the
measurement range is wide. However, in a case where the measurement intervals are
less than 0.01 11m, adjacent points interfere with the spreading width of an electron
beam. Therefore, the measurement intervals are set to 0.01 11m or more. In addition,
the measurement range needs to be set to 200 11m in the sheet thickness direction and
400 11m in the sheet width direction at a maximum. In addition, in the measurement,
an instrument including a thermal field emission-type scanning electron microscope
(JSM-7001F manufactured by JEOLLtd.) and an EBSD detector (DVC 5-type detector
manufactured by TSL) is used. At this time, the degree of vacuum in the instrument
is set to 9.6 x 10·5 Pa or less, the irradiation current level is set to 13, and the
irradiation level of the electron beam is set to 62.
[0040]
Measuring method of volume percentage of cementite
The volume percentage of the cementite is measured using a sample collected
from the same position as the sample collection position at the time of measuring the
volume percentage of the ferrite at the 1/4 position in the sheet thickness direction
- 18 -
from the surface of the hot-rolled steel sheet and at the 112 position in the sheet
thickness direction from the surface. The sheet thickness cross section is polished
with abrasive paper or alumina abrasive grains to be finished into a mirror surface,
then, corroded with a 3% nital solution and picral, and observed using a scanning
electron microscope (SEM). Subsequently, a plurality of visual fields are captured
using a photograph device attached to the SEM at a magnification of 2000 times such
that the total observed visual field area becomes 1.6 x 107 1-1m2 or more, and the area
ratio of the cementite is measured using image analysis software such as particle
analysis software. Therefore, the area ratio of the cementite is obtained. The
obtained area ratio of the cementite is regarded as the volume percentage of the
cementite.
[0041]
Average grain diameter of prior austenite grains: Less than 30.00 J.lm
The inside bend recessed part is caused by the plastic buckling of crystal
grains in the surface layer of the hot-rolled steel sheet and is affected by the sizes of the
structures of the bainite and the martensite, which have low deformability. For the
sizes of these structures, the size of the prior austenite grain becomes the maximum
unit (that is, there is no case where the bainite and the martensite become larger than
the prior austenite grain). As a characteristic, the bainite and the martensite are in a
form of being divided into several structural units called blocks. In order to make the
depths of the inside bend recessed parts less than 30.0 J.lm, the average grain diameter
of the prior austenite grains, which becomes the maximum size of the structural units
of the bainite and the martensite, which are primary phases (volume percentage of
80.0% or more) of the hot-rolled steel sheet according to the present embodiment, is
set to less than 30.00 J.lm. In order to further suppress the deterioration of the fatigue
- 19 -
properties attributed to the inside bend recessed parts, the average grain diameter of the
prior austenite grains is preferably set to less than 20.00 J.lm. In addition, since the
deterioration of the fatigue properties attributed to the inside bend recessed parts is
affected by the average grain diameter of the prior austenite grains in the surface layer
region, it is in a surface layer region (a region from the surface of the hot-rolled steel
sheet to a 100 J.lm position in the sheet thickness direction from the surface) that the
average grain diameter of the prior austenite grains is set to less than 30.00 J.lm.
[0042]
Measuring method of average grain diameter of prior austenite grains
In order to measure the average grain diameter of the prior austenite grains, a
sample is collected such that a sheet thickness cross section that intersects the rolling
direction of the hot-rolled steel sheet at right angles can be observed, and the sample is
used after the structure on the sheet thickness cross section is made visible with a
saturated aqueous solution of picric acid and an etching solution of sodium
dodecylbenzene sulfonate. In a surface layer region (a region from the surface of the
hot-rolled steel sheet to a 100 J.lm position in the sheet thickness direction from the
surface) of this sample, the circle equivalent diameters of the prior austenite grains are
measured using a structure photograph captured at a magnification of 500 times using
a scanning electron microscope. The scanning electron microscope needs to be
equipped with a two-electron detector. Regarding the capturing of the structure
photograph, the sample is irradiated with an electron beam in a vacuum at 9.6 x 10·5 Pa
or less, an accelerating voltage of 15 kV, and an irradiation current level of 13, and a
secondary electron image of the surface layer region (the region from the surface of the
hot-rolled steel sheet to the 100 J.lm position in the sheet thickness direction from the
surface) is captured. The number of visual fields captured is set to 10 or more visual
- 20 -
fields. In the captured secondary electron image, the prior austenite grain boundaries
are captured as bright contrast. The circle equivalent diameter is calculated for one of
the prior austenite grains that is included in the observed visual field. The abovedescribed
operation is performed on all of the prior austenite grains that are included in
the observed visual field except for prior austenite grains that are not fully included in
the captured visual field, such as prior austenite grains in the end portion of the
captured visual field, and the circle equivalent diameters of all of the prior austenite
grains in the captured visual field are obtained. The average grain diameter of the
prior austenite grains is obtained by calculating the average value of the circle
equivalent diameters of the prior austenite grains obtained in the individual captured
visual fields.
[0043]
Region where rotation angle between normal line of surface and (011) pole
near normal line becomes so or less: O.lSO or less from surface in terms of sheet
thickness direction position standardized by sheet thickness, and
region where rotation angle between normal line of surface and (0 11) pole
near normal line becomes 20° or more: 0.2SO or more from surface in terms of sheet
thickness direction position standardized by sheet thickness
The present inventors found that, when a region where the rotation angle
between the normal line of the surface of the hot-rolled steel sheet and the (011) pole
near the normal line becomes so or less is made present at 0.1SO or less from the
surface in terms of the sheet thickness direction position standardized by the sheet
thickness, and a region where the rotation angle becomes 20° or more is made present
at 0.2SO or more from the surface in terms of the sheet thickness direction position
standardized by the sheet thickness, it is possible to reduce the depths of inside bend
- 21 -
recessed parts in an arbitrary sheet surface direction. The sheet thickness direction
position standardized by the sheet thickness is expressed as d/t where d represents the
sheet thickness direction depth and t represents the sheet thickness.
[0044]
As described above, inside bend recessed parts are attributed to a microscopic
plastic buckling phenomenon in the surface layer of the hot-rolled steel sheet. The
present inventors considered this plastic buckling phenomenon as a microscopic plastic
flow and understood that the plastic buckling phenomenon results from a basic
behavior that is caused by the rotation of crystal grains. In the case of bending
distortion, the amount of crystal grains rotated depends on the distortion gradient from
the neutral axis toward the sheet thickness surface. The present inventors considered
that the distribution of orientation groups having different crystal rotation behaviors in
the sheet thickness direction causes an imbalance in local distortion and promotes
buckling on the surface layer of the hot-rolled steel sheet.
[0045]
Therefore, the inventors paid attention to and investigated the relationship
between the depths of inside bend recessed parts and crystal orientations in the sheet
thickness direction. As a typical crystal orientation, a (011) pole is drawn in the sheet
thickness direction and divided into a region where the rotation angle is 5° or less and
the crystal orientation does not change and a region where the rotation angle is 20° or
more and the crystal orientation does not change. The present inventors considered
that the thickness in a range where the crystal orientation does not change causes
distortion unevenness in the sheet thickness direction and investigated the relationship
between the proportions of the depths in the sheet thickness direction in the individual
ranges and the depths of inside bend recessed parts. As a result, as shown in FIG. 1
- 22 -
and FIG. 2, when the region where the rotation angle between the normal line of the
surface of the hot-rolled steel sheet and the (011) pole near the normal line becomes so
or less is present at more than 0.1SO in terms of the sheet thickness direction position
(sheet thickness direction depth d/sheet thickness t) standardized by the sheet thickness,
the depths of inside bend recessed parts become 30.0 J.lm or more. In addition, it was
found that, even when the region where the rotation angle between the normal line of
the surface of the hot -rolled steel sheet and the (0 11) pole near the normal line
becomes 20° or more is present at less than 0.2SO in terms of the sheet thickness
direction position standardized by the sheet thickness, similarly, the depths of inside
bend recessed parts become 30.0 J.lm or more. FIG. 1 is a view obtained from an
example to be described below and a view showing the relationship between the sheet
thickness direction position standardized by the sheet thickness of the region where the
rotation angle between the normal line of the surface of the steel sheet and the (011)
pole near the normal line becomes so or less and the depth of the inside bend recessed
part. FIG. 2 is a view obtained from an example to be described below and a view
showing the relationship between the sheet thickness direction position standardized by
the sheet thickness of the region where the rotation angle between the normal line of
the surface and the (011) pole near the normal line becomes 20o or more and the depth
of the inside bend recessed part.
[0046]
From the above-described investigation, the present inventors found that, in
order to reduce the depth of the inside bend recessed part, there is the most favorable
range of the depth proportions of the region where the angle formed between the
normal line of the surface of the hot-rolled steel sheet and the (011) pole becomes so or
less and the region where the rotation angle becomes 20° or more. As shown in FIG.
- 23 -
3, when the region where the rotation angle between the normal line of the surface of
the hot-rolled steel sheet and the (011) pole near the normal line becomes 5° or less is
made present at 0.150 or less from the surface in terms of the sheet thickness direction
position standardized by the sheet thickness, and the region where the rotation angle
becomes 20° or more is made present at 0.250 or more from the surface in terms of the
sheet thickness direction position standardized by the sheet thickness, it is possible to
make the depths of inside bend recessed parts less than 30.0 11m. FIG 3 is a view
obtained from the example to be described below and a view showing the relationship
among the sheet thickness direction position standardized by the sheet thickness of the
region where the rotation angle between the normal line of the surface and the (0 11)
pole near the normal line becomes 5° or less, the sheet thickness direction position
standardized by the sheet thickness of the region where the rotation angle between the
normal line of the surface and the (0 11) pole near the normal line becomes 20° or more,
and the evaluation result of the inside bend recessed part in the example.
[0047]
Hereinafter, a measuring method of the region having a predetermined
rotation angle between the normal line of the surface of the steel sheet and the (011)
pole near the normal line will be described.
Measurement is performed by EBSP using a sample having a cross section
finished into a mirror surface by the same method as for the sample used for the
measurement of the volume percentage of the prior austenite grains. The sample
needs to be finished by electrolytic polishing for the purpose of sufficiently removing
strain in the cross section to be measured. In the electrolytic polishing, in order to
remove mechanical polishing strain on an observed section, the sample needs to be
polished a minimum of 20 11m and polished a maximum of 50 11m. The sample is
- 24 -
preferably polished 30 11m or less in consideration of rollover at the end portion.
In the measurement by EBSP, the accelerating voltage is set to 15 to 25 kV,
and the measurement range is set to a measurement range that covers the overall sheet
thickness. The measurement range needs to be 1000 11m or more in the rolling
direction. In addition, since the purpose is to measure the average characteristics of
crystal orientations, the measurement intervals may be 5 11m or more. The
measurement intervals are set to 30 11m or less in order to avoid an increase in the
number of crystal grains that are not measured by mistake. Crystal orientation data
need to be recorded along with the measurement coordinate system. From the
obtained crystal orientation data, the rotation angle between the normal line of the
surface of the steel sheet and the (011) pole near the normal line is measured by the
following method.
[0048]
The rotation angle between the normal line of the surface of the hot-rolled
steel sheet and the (0 11) pole near the normal line is a value that is measured by
plotting the crystal orientation data obtained by the EBSP measurement on a positive
pole figure. At the time of plotting the crystal orientations on the positive pole figure,
in the coordinate system of the positive pole figure, poles of the (0 11) orientation are
displayed such that normal lines (origin: ND) become the normal lines to the sheet
surface of the hot-rolled steel sheet, the horizontal axis TD becomes the sheet width
direction, and the axis RD orthogonal to the horizontal axis becomes the rolling
direction.
As described above, the crystal orientation is a group of points measured at
predetermined intervals in a measurement range that is 1000 11m or more in the rolling
direction and covers the overall sheet thickness range. This group of points is divided
- 25 -
into 20 sections in the sheet thickness direction, and a (011) pole figure is drawn. In
the (011) pole figure at each depth direction position from the surface of the steel sheet
drawn as described above, the angle between the origin ND (normal line of the surface
of the hot-rolled steel sheet) and the nearest (011) pole is measured. This
measurement value is defined as the rotation angle between the normal line of the
surface and the (011) pole near the normal line. A value obtained by dividing each
depth direction position by the sheet thickness is defined as the sheet thickness
direction position (sheet thickness direction depth d/sheet thickness t) standardized by
the sheet thickness, and the region where the rotation angle becomes 5° or less and the
region where the rotation angle becomes 20° or more are obtained at this sheet
thickness direction position standardized by the sheet thickness.
[0049]
Tensile strength: 880 MPa or more
In the hot-rolled steel sheet according to the present embodiment, the tensile
strength is 880 MPa or more. When the tensile strength is less than 880 MPa, it
becomes difficult to apply the hot-rolled steel sheet to suspension components of
automobiles. The tensile strength may be 900 MPa or more. The tensile strength is
preferably as high as possible, but may be 1500 MPa or less from the viewpoint of a
weight reduction effect of the high-strengthening of the hot-rolled steel sheet.
The tensile strength is measured by performing a tensile test in accordance
with JIS Z 2241: 2011 using a No. 5 test piece of JIS Z 2241: 2011. A position where
the tensile test piece is collected is the central position in the sheet width direction, and
a direction perpendicular to the rolling direction is the longitudinal direction.
[0050]
Hole expansion rate: 35% or more
- 26 -
In the hot-rolled steel sheet according to the present embodiment, the hole
expansion rate is 35% or more. When the hole expansion rate is less than 35%,
forming-induced fracture occurs in a burring portion, and it becomes difficult to apply
the hot-rolled steel sheet to suspension components of automobiles. The hole
expansion rate may be set to 50% or more in order to reduce the ironing rate of the
burring portion and reduce the load on a die in a pressing step. In a case where the
hole expansion rate is set to 80% or more, it is possible to eliminate ironing and to
enhance the stiffness of components by obtaining a sufficient burring height.
Therefore, the hole expansion rate may be set to 80% or more.
The hole expansion rate is measured by performing a hole expansion test in
accordance with JIS Z 2256: 2010.
[0051]
Next, a preferable manufacturing method of the hot-rolled steel sheet
according to the present embodiment will be described. A casting step and a hot
rolling step to be described below are important steps for controlling the crystal
orientation distribution in the sheet thickness direction and the average grain diameter
of the prior austenite grains, which are requirements necessary to reduce the depths of
the inside bend recessed parts.
[0052]
The preferable manufacturing method of the hot-rolled steel sheet according
to the present embodiment includes the following steps.
A casting step of, in continuous casting of a slab having a predetermined
chemical composition, performing the continuous casting in a manner that an average
surface temperature gradient in a region from a meniscus to 1.0 m from the meniscus
becomes 300 to 650 °Cim to obtain the slab,
- 27 -
a heating step of heating the slab to 1200oc or higher and holding the slab for
30 minutes or longer,
a hot rolling step of performing rough rolling on the slab, and then performing
finish rolling in a manner that a total rolling reduction in a temperature range of 870°C
to 980°C becomes 80% or larger, an elapsed time between rolling stands in the
temperature range of 870°C to 980°C becomes 0.3 to 5.0 seconds, and a total rolling
reduction in a temperature range of lower than 870°C becomes smaller than 10%,
a cooling step of, after the finish rolling, cooling a hot-rolled steel sheet for
30.0 seconds or shorter to cool the hot-rolled steel sheet to a temperature range of
lower than 300°C, and
a coiling step of, after the cooling, coiling the hot-rolled steel sheet in a
manner that a coiling temperature becomes lower than 300°C.
The preferable manufacturing method of the hot -rolled steel sheet according
to the present embodiment may further include a heat treatment step of, after the
coiling, holding the hot-rolled steel sheet in a temperature range of 200°C or higher
and lower than 450°C for 90 to 80000 seconds.
Hereinafter, each step will be described.
[0053]
Casting step
In the continuous casting of a slab having the above-described chemical
composition, the average surface temperature gradient in a region from the meniscus to
1.0 m from the meniscus is set to 300 to 650 °C/m. The surface temperature gradient
in the early stage of solidification affects the rotation angle between the normal line of
the surface of the hot-rolled steel sheet and the (011) pole near the normal line. In the
present embodiment, the average surface temperature gradient refers to a temperature
- 28 -
gradient obtained by dividing the temperature in a mold in contact with a solidified
shell by the distance from the meniscus. The temperature is measured with
thermocouples embedded in the mold. The thermocouples are embedded at a 0 mm
position below the meniscus that is 0.010 mm or less from the outer surface (solidified
shell) of the mold and a 1.0 mm below the meniscus that is 0.010 mm or less from the
outer surface (solidified shell) of the mold in the center portion of the long side surface
of the slab in the width direction. The thermocouple that is embedded at the 0 mm
position below the meniscus needs to be 0.040 mm or less and preferably needs to be
0.005 mm or less distant from the meniscus (in a casting direction). A value obtained
by dividing each measured temperature by the section distance is regarded as the
average surface temperature gradient.
[0054]
When the average surface temperature gradient in the region from the
meniscus to 1.0 m from the meniscus is less than 300 °Cim, the region where the
rotation angle between the normal line of the surface of the hot-rolled steel sheet and
the (011) pole near the normal line is 5o or less is present at more than 0.150 from the
surface in terms of the sheet thickness direction position standardized by the sheet
thickness. On the other hand, when the average temperature gradient in the abovedescribed
region is more than 650 °C/m, the region where the rotation angle between
the normal line of the surface of the hot-rolled steel sheet and the (011) pole near the
normal line is 20° or more is present at less than 0.250 from the surface in terms of the
sheet thickness direction position standardized by the sheet thickness. Therefore, the
average surface temperature gradient in the region from the meniscus to 1.0 m from the
meniscus is set to 300 to 650 °Cim, and the slab is manufactured. The lower limit of
the average surface temperature gradient is preferably 350 °Cim or 400 °Cim, and the
- 29 -
upper limit of the average surface temperature gradient is preferably 600 °C/m or
550 °Cim.
[0055]
The average casting velocity in the casting step may be in an ordinary range,
may be 0.8 rnlmin or faster, or may be 1.2 rnlmin or faster. From the viewpoint of
cost reduction, the average casting velocity in the casting step is preferably set to 1.2
rnlmin or faster. On the other hand, when the average casting velocity is faster than
2.5 rnlmin, the cooling temperature gradient in the slab thickness direction increases
due to the increase in the casting velocity, and the slab internal stress in a solidification
process increases, which makes it easy for a defect to be initiated. Therefore, the
average casting velocity is preferably 2.5 rnlmin or slower. In addition, when the
average casting velocity is 0.6 rnlmin or slower, the cooling temperature gradient in the
slab thickness direction decreases, but the economic efficiency is significantly
impaired. Therefore, the average casting velocity is preferably 0.6 to 2.5 rnlmin.
[0056]
Heating step
The slab obtained by the continuous casting is heated such that the slab
surface temperature becomes 1200oc or higher and is held in a temperature range of
1200°C or higher for 30 minutes or longer, thereby solutionizing the slab. When the
heating temperature is lower than 1200°C, homogenization and carbide dissolution by
a solutionizing treatment does not proceed, and ferritic transformation proceeds,
whereby the strength of the hot-rolled steel sheet decreases. In a case where the slab
contains Ti, the heating temperature is preferably set to 1230°C or higher in order to
more reliably form a solid solution of Ti. In addition, regarding the slab temperature
before heating, the slab may be cooled to room temperature or may remain at a high
- 30 -
temperature after the continuous casting in a case where there is a concern of cracking
caused by thermal stress or the like. The slab is heated in the heating step by
charging the slab into a furnace controlled to a predetermined temperature, and a time
taken for the slab surface temperature to become 1200ac or higher needs to be set to
30 minutes or longer, which is sufficient. When the holding time in the temperature
range of 1200ac or higher is shorter than 30 minutes, it is not possible to obtain a
desired amount of bainite and martensite. The holding time is preferably 40 minutes
or longer, 60 minutes or longer, or 100 minutes or longer. For example, the heating
temperature needs to be 1400°C or lower, and the heating time needs to be 300 minutes
or shorter.
In addition, in a case where the slab contains Ti, a time for the slab surface
temperature to becomes 1230°C or higher needs to be set to 60 minutes or longer,
which is sufficient. In the furnace, the slab is disposed on an inorganic substance skid,
and the slab may be solutionized by being heated to equal to or lower than a
temperature at which the slab heated by a reaction between the inorganic substance and
iron at this time does not dissolve.

CLAIMS
mass%:
1. A hot-rolled steel sheet comprising, as a chemical composition, by
C: 0.060% to 0.170%;
Si: 0.030% to 1.700%;
Mn: 1.20% to 3.00%;
Al: 0.010% to 0.700%;
Nb: 0.005% to 0.050%;
P: 0.0800% or less;
S: 0.0100% or less;
N: 0.0050% or less;
Ti: 0% to 0.1800%;
Mo: 0% to 0.150%;
V: 0% to 0.3000%;
Cr: 0% to 0.500%;
B: 0% to 0.0030%; and
a remainder consisting of Fe and an impurity,
wherein, in metallographic structures at a 1/4 position in a sheet thickness
direction from a surface and at a 1/2 position in the sheet thickness direction from the
surface, by vol%,
bainite and martensite are a total of 80.0% or more,
ferrite is 20.0% or less, and
cementite and residual au stenite are a total of 0% to 10.0%,
in a metallographic structure of a region from the surface to a 100 11m position
in the sheet thickness direction from the surface,
- 49 -
an average grain diameter of prior austenite grains is less than 30.00 11m,
a region, where a rotation angle between a normal line of the surface and a
(0 11) pole near the normal line is sa or less, is 0.150 or less from the surface in terms
of a sheet thickness direction position standardized by a sheet thickness,
a region, where the rotation angle between the normal line of the surface and
the (011) pole near the normal line becomes 20a or more, is 0.250 or more from the
surface in terms of the sheet thickness direction position standardized by the sheet
thickness, and
a tensile strength is 880 MPa or more.
2. The hot-rolled steel sheet according to claim 1, further comprising, as the
chemical composition, by mass%, one or more selected from the group consisting of:
Ti: 0.0200% to 0.1800%;
Mo: 0.030% to 0.150%;
V: 0.0500% to 0.3000%,
Cr: 0.050% to 0.500%; and
B: 0.0001% to 0.0030%.
3. A manufacturing method of the hot-rolled steel sheet according to claim 1
or 2, comprising:
a casting step of, in continuous casting of a slab having the chemical
composition according to claim 1, performing the continuous casting in a manner that
an average surface temperature gradient in a region from a meniscus to 1.0 m from the
meniscus becomes 300 to 650 °C/m to obtain the slab;
a heating step of heating the slab to 1200ac or higher and holding the slab for
30 minutes or longer;
a hot rolling step of performing rough rolling on the slab, and performing
- 50 -
finish rolling in a manner that a total rolling reduction in a temperature range of 870°C
to 980°C becomes 80% or larger, an elapsed time between rolling stands in the
temperature range of 870°C to 980°C becomes 0.3 to 5.0 seconds, and a total rolling
reduction in a temperature range of lower than 870oc becomes smaller than 10%;
a cooling step of cooling for 30.0 seconds or shorter to cool to a temperature
range of lower than 300°C after the finish rolling; and
a coiling step of, coiling in a manner that a coiling temperature becomes lower
than 300°C after the cooling.
4. The manufacturing method of the hot-rolled steel sheet according to
claim 3, further comprising:
a heat treatment step of holding in a temperature range of 200°C or higher and
lower than 450°C for 90 to 80000 seconds after the coiling.