[0001]The present invention relates to a steel sheet and a method for manufacturing
the same.
Priority is claimed on Japanese Patent Application No. 2019-025635, filed on
February 15, 2019, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Recently, in order to protect the global environment, it is desired to improve
the fuel consumption of a vehicle. Regarding the improvement of the fuel
consumption of a vehicle, high-strengthening is further required for a steel sheet for a
vehicle in order to reduce the weight of a vehicle body while securing safety. This
high-strengthening is required not only for a structural member such as a member or a
pillar but also for an exterior component (for example, a roof, a hood, a fender, or a
door) of a vehicle. For this requirement, a material has been developed in order to
simultaneously achieve strength and elongation (formability).
[0003]
On the other hand, the forming of an exterior panel component of a vehicle
tends to become more complicated. When the strength of a steel sheet increases in
order to reduce the weight, it is difficult to process the steel sheet in a complicated
shape. When the thickness of a steel sheet is reduced in order to reduce the weight, a
surface of the steel sheet is likely to be uneven during forming into a complicated
shape. When the surface is uneven, the external appearance after forming deteriorates.
- 1 -
Regarding an exterior panel component, not only properties such as strength but also
design and surface quality are important. Therefore, the external appearance after
forming is required to be excellent. The unevenness occurring after forming
described herein refers to unevenness occurring on a surface of a formed component
even when the steel sheet surface after manufacturing is not uneven. Even when the
formability of the steel sheet is improved, the occurrence is not necessarily suppressed.
Therefore, when a high strength steel sheet is applied to an exterior panel, there is a
large problem.
[0004]
Regarding a relationship between the external appearance after forming and
material properties in a steel sheet to be applied to an exterior panel component, for
example, Patent Document 1 discloses a ferritic steel sheet in which, in order to
improve surface properties after stretching, an area fraction of crystal having a crystal
orientation ofless than ±15° from { 001} plane parallel to a steel sheet surface is 0.25
or less and an average grain size of the crystal is 25 J..Lm or less.
However, Patent Document 1 relates to a ferritic steel sheet in which a C
content is 0.0060% or less. However, as a result of an investigation by the present
inventors, it was found that, in the case of a steel sheet having a C content more than
that of the steel sheet described in Patent Document 1, it is difficult to reduce the area
fraction of crystal having a crystal orientation of less than ±15° from { 001 } plane
parallel to a steel sheet surface. That is, with the method described in Patent
Document 1, high-strengthening and improvement of surface properties after
processing cannot be satisfied simultaneously.
[0005]
For example, Patent Document 2 discloses a steel sheet including ferrite as a
- 2 -
primary phase and having an excellent Young's modulus in an orthogonal-to-rolling
direction in which an X-ray random intensity ratio in a thickness 114layer is controlled.
However, Patent Document 2 does not disclose a relationship between the external
appearance after forming and a structure from the viewpoint of a countermeasure
against surface roughness or pattern defects.
[0006]
That is, in the related art, a high strength steel sheet having excellent
formability in which surface roughness or pattern defects after forming is improved is
not disclosed.
[Prior Art Document]
[Patent Document]
[0007]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2016-156079
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2012-233229
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0008]
The present invention has been made in consideration of the above-described
problems. An object of the present invention is to provide: a high strength steel sheet
in which formability is excellent and the occurrence of surface unevenness during
forming is suppressed; and a method for manufacturing the same.
[Means for Solving the Problem]
- 3 -
[0009]
The present inventors conducted an investigation on a method for achieving
the object.
As a result, it was found that the surface unevenness during forming occurs
due to inhomogeneous deformation occurs due to inhomogeneous deformation during
forming caused by inhomogeneity in strength in a microscopic region.
As a result of further thorough investigation by the present inventors, by
controlling a metallographic structure such that ferrite is a primary phase in order to
improve formability and by controlling an average grain size of ferrite and a texture of
ferrite in a metallographic structure in a surface layer region to be different from those
in an internal region of the steel sheet, a steel sheet in which the occurrence of surface
unevenness after forming is suppressed such that the external appearance (surface
appearance quality) after forming is excellent can be obtained.
[0010]
In addition, as a result of investigation, the present inventors found that, in
order to control the metallographic structure in the surface layer region, it is effective
to apply strain after hot rolling instead of after cold rolling and to set a cold-rolling
reduction and heat treatment conditions after the strain application depending on the
working amount.
[0011]
The present invention has been made based on the above findings , and the
scope thereof is as follows .
[ 1] According to one aspect of the present invention, there is provided a
steel sheet includes, as a chemical composition, by mass%: C: 0.0015% to 0.0400%;
Mn: 0.20% to 1.50%; P: 0.010% to 0.100%; Cr: 0.001% to 0.500%;Si: 0.200% or less;
- 4 -
S: 0.020% or less; sol. Al: 0.200% or less; N: 0.0150% or less; Mo: 0% to 0.500%; B:
0% to 0.0100%; Nb: 0% to 0.200%; Ti: 0% to 0.200%; Ni: 0% to 0.200%; Cu: 0% to
0.100%; and a remainder including iron and impurities, in which a metallographic
structure in a surface layer region includes ferrite having a volume fraction of 90% or
more, and in the surface layer region, an average grain size of the ferrite is 1.0 J..Lm to
15.0 J..Lm, and a texture in which an XoDF{OOl}/{111), s as a ratio of an intensity of { 001}
orientation to an intensity of { 111} orientation in the ferrite is 0.30 or more and less
than 3.50 is included.
[2] In the steel sheet according to [1], the chemical composition may include,
by mass%, one or more selected from the group consisting of: Mo: 0.001% to 0.500%;
B: 0.0001% to 0.0100%; Nb: 0.001% to 0.200%; Ti: 0.001% to 0.200%; Ni: 0.001% to
0.200%; and Cu: 0.001% to 0.100%.
[3] In the steel sheet according to [1] or [2], a texture in which an
XoDF{OOl}/{111}, I as a ratio of an intensity of { 001} orientation to an intensity of { 111}
orientation in ferrite is 0.001 or more and less than 1.0 may be included in an internal
regiOn.
[ 4] In the steel sheet according to one of [ 1] to [3], the intensity ratio
XoDF{OOlJ/{111}, sin the surface layer region and an XoDF{OOlJ/{111 }, I as a ratio of an
intensity of { 001} orientation to an intensity of { 111} orientation in ferrite in an
internal region may satisfy the following Expression (1), and
the average grain size of the ferrite in the surface layer region may be less
than an average grain size of the ferrite in the internal region,
-0.20 < X o DF{OOl}/{ 111 J, s - X o DF{OOl}/{111), I< 0.40 (1).
[5] In the steel sheet according to one of [1] to [4], a plating layer may be
provided on a surface.
- 5 -
[6] According to another aspect of the present invention, there is provided a
method for manufacturing a steel sheet including: a heating process of heating a slab
having the chemical composition according to [1] to 1000°C or higher; a hot-rolling
process of hot-rolling the slab such that a rolling fini shing temperature is 950°C or
lower to obtain a hot-rolled steel sheet; a stress application process of applying a stress
to the hot-rolled steel sheet after the hot-rolling process such that an absolute value of a
residual stress as on a surface is 100 MPa to 250 MPa; a cold-rolling process of coldrolling
the hot-rolled steel sheet after the stress application process such that a
cumulative rolling reduction RCR is 70% to 90% to obtain a cold-rolled steel sheet; an
annealing process of heating the cold-rolled steel sheet such that an average heating
rate in a range from 300°C to a soaking temperature T1 oc that satisfies the following
Expression (2) is 1.5 °C/sec to 10.0 °C/sec and holding the heated steel sheet at the
soaking temperature T1 °C for 30 seconds to 150 seconds for annealing; and a cooling
process of cooling the cold-rolled steel sheet after the annealing process to a
temperature range of 550°C to 650°C such that an average cooling rate in a range from
the soaking temperature T1 oc to 650°C is 1.0 °C/sec to 10.0 °C/sec and cooling the
cooled steel sheet to a temperature range of 200°C to 490°C such that the average
cooling rate is 5 °C/sec to 500 °C/sec.
Ac1 + 550- 25 x ln(as) - 4.5 x RcR 5 T1 5 Ac1 + 550 - 25 x ln(a s)- 4 x RcR
(2)
Ac1 in Expression (2) is represented by the following Expression (3). An
element symbol in the following Expression (3) represents an amount of the
corresponding element by mass%, and when the corresponding element is not included,
0 is substituted into the corresponding element symbol.
Ac1 = 723 - 10.7 x Mn - 16.9 x Ni + 29.1 x Si + 16.9 x Cr (3)
- 6 -
[7] In the method for manufacturing a steel sheet according to [6], the stress
application process may be performed at 40°C to 500°C.
[8] In the method for manufacturing a steel sheet according to [6] or [7] , in
the hot-rolling process, a finish rolling start temperature may be 900°C or lower.
[9] The method for manufacturing a steel sheet according to one of [6] to [8],
may further include a holding process of holding the cold-rolled steel sheet after the
cooling process in a temperature range of 200°C to 490°C for 30 seconds to 600
seconds.
[Effects of the Invention]
[0012]
In the steel sheet according to the aspect of the present invention, the
occurrence of surface unevenness is suppressed even after various deformation during
press forming as compared to a material in the related art. Therefore, the steel sheet
according to the aspect of the present invention has excellent appearance quality of the
surface and can contribute to improvement of the vividness and design of coating. In
addition, the steel sheet according to the present invention has high strength, and thus
can contribute to further reduction in the weight of a vehicle. In addition, since
formability is excellent, the steel sheet according to the present invention is also
applicable to an exterior component having a complicated shape. In the present
invention, the high strength represents a tensile strength of 340 MPa or higher.
In addition, with the method for manufacturing a steel sheet according to the
aspect of the present invention, a high strength steel sheet in which formability is
excellent and the occurrence of surface unevenness is suppressed even after various
deformation during press forming can be manufactured.
[Brief Description of the Drawings]
- 7 -
[0013]
FIG. 1 is a diagram showing a relationship between surface properties after
forming and a texture parameter.
[Embodiments ofthe Invention]
[0014]
A steel sheet according to an embodiment of the present invention (the steel
sheet according to the embodiment) includes, as a chemical composition, by mass%: C:
0.0015% to 0.0400%; Mn: 0.20% to 1.50%; P: 0.010% to 0.100%; Cr: 0.001% to
0.500%; Si: 0.200% or less; S: 0.020% or less; sol. Al: 0.200% or less; N: 0.0150% or
less; Mo: 0% to 0.500%; B: 0% to 0.0100%; Nb: 0% to 0.200%; Ti: 0% to 0.200%; Ni:
0% to 0.200%; Cu: 0% to 0.100%; and a remainder including iron and impurities.
In addition, in the steel sheet according to the embodiment, a metallographic
structure in a surface layer region includes ferrite having a volume fraction of 90% or
more, and in the surface layer region, an average grain size of the ferrite is 1.0 )liD to
15.0 )liD, and a texture in which an XoDF{OOl)/{111}, s as a ratio of an intensity of { 001}
orientation to an intensity of { 111} orientation in the ferrite is 0.30 or more and less
than 3.50 is included.
[0015]
In the steel sheet according to the embodiment, it is preferable that a texture in
which an XoDF{ oo1 Jl{ 111), I as a ratio of an intensity of { 001 } orientation to an intensity
of { 111} orientation in ferrite is 0.001 or more and less than 1.00 is included in an
internal region.
In addition, in the steel sheet according to the embodiment, it is preferable
that the intensity ratio XoDF{OOlJI{lllJ, sand an XoDF{OOlJ/{111], I as a ratio of an intensity
of { 001} orientation to an intensity of { 111} orientation in ferrite in an internal region
- 8 -
satisfies the following Expression (1), and it is preferable that the average grain size of
the ferrite in the surface layer region is less than an average grain size of the ferrite in
the internal region.
-0.20 < XoDF{OOlJ/1 111), s - XoDF{OOl}/{111), I< 0.40 (1).
[0016]
Hereinafter, the steel sheet according to the embodiment will be described in
detail. The present invention is not limited only to the configuration disclosed in the
embodiment and can be modified within a range not departing from the scope of the
present invention. A limited numerical range described below includes a lower limit
value and an upper limit value. A numerical value shown together with "more than"
or "less than" is not included in a numerical range. All the "%" in the chemical
composition represents "mass%". First, the reason for limiting the chemical
composition of the steel sheet according to the embodiment will be described.
[0017]

[C: 0.0015% to 0.0400%]
C (carbon) is an element that increases the strength of the steel sheet. In
addition, as the C content decreases, a { 111} texture is likely to be developed. In
order to obtain a desired strength and a desired texture, the C content is set to be
0.0015% or more. The C content is preferably 0.0030% or more and more preferably
0.0060% or more.
On the other hand, when the C content is more than 0.0400%, the formability
of the steel sheet deteriorates. Therefore, the C content is set to be 0.0400% or less.
The C content is preferably 0.0300% or less and more preferably 0.0200% or less.
[0018]
- 9 -
[Mn: 0.20% to 1.50%]
Mn (manganese) is an element that increases the strength of the steel sheet.
In addition, Mn is an element that immobilizes S (sulfur) in the steel as MnS or the like
to prevent cracking during hot rolling. In order to obtain the effects, the Mn content
is set to be 0.20% or more. The Mn content is preferably 0.30% or more.
On the other hand, when the Mn content is more than 1.50%, a cold rolling
force during cold rolling at a high rolling reduction increases, and the productivity
decreases. In addition, segregation of Mn is likely to occur. Therefore, the hard
phase aggregates after annealing such that pattern defects are likely to be formed on
the surface after forming. Therefore, the Mn content is set to be 1.50% or less. The
Mn content is preferably 1.30% or less and more preferably 1.10% or less.
[0019]
[P: 0.010% to 0.100%]
P (phosphorus) is an element that improves the strength of the steel sheet. In
order to obtain a desired strength, the P content is set to be 0.010% or more. The P
content is preferably 0.015% or more and more preferably 0.020% or more.
On the other hand, when an excess amount of Pis included in the steel,
cracking is promoted during hot rolling or cold rolling, and the ductility or weldability
of the steel sheet deteriorates. Therefore, the P content is set to be 0.100% or less.
It is preferable that the P content is set to be 0.080% or less.
[0020]
[Cr: 0.001% to 0.500%]
Cr (chromium) is an element that improves the strength of the steel sheet. In
order to obtain a desired strength, the Cr content is set to be 0.001% or more. The Cr
content is preferably 0.050% or more.
- 10 -
On the other hand, when the Cr content is more than 0.500%, the strength of
the steel sheet provided for cold rolling increases, and a cold rolling force during cold
rolling at a high rolling reduction increases. In addition, the alloy cost increases.
Therefore, the Cr content is set to be 0.500% or less. The Cr content is preferably
0.350% or less.
[0021]
[Si: 0.200% or less]
Si (silicon) is a deoxidizing element of steel that is effective for increasing the
strength of the steel sheet. However, when the Si content is more than 0.200%, scale
peelability during production deteriorates, and surface defects are likely to be formed
on the product. In addition, a cold rolling force during cold rolling at a high rolling
reduction increases, and the productivity decreases. Further, the weldability and the
deformability of the steel sheet deteriorates. Therefore, the Si content is limited to
0.200% or less. The Si content is preferably 0.150% or less.
In addition, in order to reliably obtain the deoxidizing effect of steel and the
effect of improving the strength, the Si content may be 0.005% or more.
[0022]
[S: 0.020% or less]
S (sulfur) is an impurity. When an excess amount of S is included in the
steel, MnS stretched by hot rolling is formed, and the deformability of the steel sheet
deteriorates. Therefore, the S content is limited to 0.020% or less. The S content is
preferably small and may be 0%. In consideration of existing general refining
(including secondary refining), the S content may be set to be 0.002% or more.
[0023]
[sol. Al: 0.200% or less]
- 11 -
Al (aluminum) is a deoxidizing element of steel. However, when the sol. Al
content is more than 0.200%, scale peelability during production deteriorates, and
surface defects are likely to be formed on the product. In addition, the weldability of
the steel sheet deteriorates. Therefore, the sol. Al content is set to be 0.200% or less.
The sol. Al content is preferably 0.150% or less.
In addition, in order to reliably obtain the deoxidizing effect of steel, the sol.
Al content may be 0.020% or more.
[0024]
[N: 0.0150% or less]
N (nitrogen) is an impurity and is an element that deteriorates the
deformability of the steel sheet. Accordingly, theN content is limited to 0.0150% or
less. TheN content is preferably small and may be 0%. However, in consideration
of existing general refining (including secondary refining), theN content may be
0.0005% or more.
[0025]
That is, the steel sheet according to the embodiment may include the abovedescribed
elements and a remainder including Fe and impurities. However, in order
to improve various properties, the following elements (optional elements) may be
included instead of a part of Fe. From the viewpoint of reducing the alloy cost, it is
not necessary to add the optional elements to the steel on purpose. Therefore, the
lower limit of the amount of each of the optional elements is 0%. The impurities
refer to components that are unintentionally included from raw materials or other
manufacturing processes in the process of manufacturing the steel sheet.
[0026]
[Mo: 0% to 0.500%]
- 12 -
Mo (molybdenum) is an element that improves the strength of the steel sheet.
In order to obtain a desired strength, Mo is optionally included. In order to obtain the
effect, the Mo content is preferably 0.001% or more. The Mo content is more
preferably 0.010% or more.
On the other hand, when the Mo content is more than 0.500%, the
deformability of the steel sheet may deteriorate. In addition, the alloy cost increases.
Therefore, the Mo content is set to be 0.500% or less. The Mo content is preferably
0.350% or less.
[0027]
[B: 0% to 0.0100%]
B (boron) is an element that immobilizes carbon and nitrogen in the steel to
form a fine carbonitride. The fine carbonitride contributes to precipitation hardening,
microstructure control, grain refinement strengthening, and the like of the steel.
Therefore, B may be optionally included. In order to obtain the effect, the B content
is preferably 0.0001% or more.
On the other hand, when the B content is more than 0.0100%, the effect is
saturated, and the workability (deformability) of the steel sheet may deteriorate. In
addition, the strength of the steel sheet provided for cold rolling increases by including
B. Therefore, a cold rolling force during cold rolling at a high rolling reduction
increases. Therefore, when B is included, the B content is set to be 0.0100% or less.
[0028]
[Nb: 0% to 0.200%]
Nb (niobium) is an element that immobilizes carbon and nitrogen in the steel
to form a fine carbonitride. The fine Nb carbonitride contributes to precipitation
hardening, microstructure control, grain refinement strengthening, and the like of the
- 13 -
steel. Therefore, Nb may be optionally included. In order to obtain the effect, the
Nb content is preferably 0.001% or more.
On the other hand, when the Nb content is more than 0.200%, the effect is
saturated, the strength of the steel sheet provided for cold rolling increases, and a cold
rolling force during cold rolling at a high rolling reduction increases. Therefore, even
when Nb is included, the Nb content is 0.200% or less.
[0029]
[Ti: 0% to 0.200%]
Ti (titanium) is an element that immobilizes carbon and nitrogen in the steel to
form a fine carbonitride. The fine carbonitride contributes to precipitation hardening,
microstructure control, grain refinement strengthening, and the like of the steel.
Therefore, Ti may be optionally included. In order to obtain this effect, the Ti content
is preferably 0.001% or more.
On the other hand, when the Ti content is more than 0.200%, the effect is
saturated, the strength of the steel sheet provided for cold rolling increases, and a cold
rolling force during cold rolling at a high rolling reduction increases. Therefore, even
when Ti is included, the Ti content is 0.200% or less.
[0030]
[Ni: 0% to 0.200%]
Ni (nickel) is an element that contributes to the improvement of the strength
of the steel sheet. Therefore, Ni may be optionally included. In order to obtain the
effect, the Ni content is preferably 0.001% or more.
On the other hand, when the Ni content is excessively large, the strength of
the steel sheet provided for cold rolling increases, and a cold rolling force during cold
rolling at a high rolling reduction increases. In addition, inclusion of an excess
- 14 -
amount of Ni causes an increase in alloy cost. Therefore, even when Ni is included,
the Ni content is 0.200% or less.
[0031]
[Cu: 0% to 0.100%]
Cu (copper) is an element that stabilizes austenite. By delaying
transformation from austenite to ferrite, crystal grains are refined, which contributes to
improvement of the strength. Therefore, Cu may be optionally included. In order to
obtain the effect, the Cu content is preferably 0.001% or more.
On the other hand, when the Cu content is more than 0.100%, the effect is
saturated, the strength of the steel sheet provided for cold rolling increases, and a cold
rolling force during cold rolling at a high rolling reduction increases. Therefore, even
when Cu is included, the Cu content is 0.100% or less.
[0032]
The above-described chemical composition of the steel sheet may be
measured using a general analysis method. For example, the chemical composition
may be measured using Inductively Coupled Plasma-Atomic Emission Spectrometry
(ICP-AES). C and S may be measured using an infrared absorption method after
combustion, and N may be measured using an inert gas fusion-thermal conductivity
method. In a case where the steel sheet includes a plating layer on the surface, the
chemical composition may be analyzed after removing the plating layer from the
surface by mechanical grinding.
[0033]

In the steel sheet according to the embodiment, when the sheet thickness is
represented by t, a depth range from the surface to t/4 in a sheet thickness direction is
- 15 -
divided into two regions, a depth range from the surface as a starting point to a depth
position of 50 11m in a depth direction is represented by a surface layer region, and a
range from the surface layer region to a center side of the steel sheet is represented by
an internal region. When the thickness of the steel sheet is 0.20 mm or less, a region
from the surface to a depth of t/4 in the sheet thickness direction is defined as a surface
layer region, and a region with a depth of t/4 to t/2 is defined as an internal region.
When the sheet thickness of the steel sheet is more than 0.40 mm, it is preferable that
the internal region is a range from a position of more than 50 f.!m from the surface in
the sheet thickness direction to a position of 100 f.!m from the surface in the sheet
thickness direction.
As a result of a thorough investigation by the present inventors, it was found
that the surface unevenness during forming occurs due to inhomogeneous deformation
occurs during forming caused by inhomogeneity in strength in a microscopic region.
In particular, it was found that the occurrence of surface roughness is largely affected
by the metallographic structure in the surface layer region. Therefore, in the steel
sheet according to the embodiment, the metallographic structure in the surface layer
region is controlled as follows .
[0034]
[Including Ferrite having Volume Fraction of 90% or More]
When the volume fraction of the ferrite in the surface layer region is less than
90%, the surface appearance quality of the steel sheet after forming is likely to
deteriorate. Therefore, the volume fraction of the ferrite is 90% or more. The
volume fraction is preferably 95% or more or 98% or more. Since all the
metallographic structures in the surface layer region may be formed of ferrite, the
upper limit may be 100%.
- 16 -
[0035]
The remainder in microstructure in the surface layer region includes, for
example, one or more selected from the group consisting of pearlite, bainite, martensite,
and tempered martensite. When the volume fraction of ferrite in the surface layer
region is 100%, the volume fraction of the remainder in microstructure is 0%.
[0036]
The volume fraction of ferrite in the surface layer region is obtained using the
following method.
A sample (the size is substantially 20 mm in the rolling direction x 20 mm in
the width direction x the thickness of the steel sheet) for metallographic structure
(microstructure) observation is collected from a W/4 position or a 3W/4 position of a
sheet width W of the steel sheet (that is, a W/4 position in the width direction from any
end portion of the steel sheet in the width direction), and a metallographic structure
(microstructure) in a range from the surface to the 1/4 thickness position is observed
using an optical microscope to calculate the area fraction of ferrite in a range from the
surface of the steel sheet (in a case where a plating layer is present, the surface
excluding the plating layer) to 50 f.!m. In order to prepare the sample, a sheet
thickness cross section in the orthogonal-to-rolling direction (direction perpendicular
to the rolling direction) is polished as an observed section and is etched with the
LePera reagent.
[0037]
"Microstructures" are classified based on an optical microscope image at a
magnification by 500-times. When the optical microscope observation is performed
after the LePera corrosion, the respective structures are observed with different colors,
for example, bainite is observed to be black, martensite (tempered martensite) is
- 17 -
observed to be white, and ferrite is observed to be gray. Therefore, ferrite and other
hard structures can be easily distinguished from each other.
[0038]
A region ranging from the surface to a 1/4 thickness position in the sheet
thickness direction from the surface in the steel sheet etched with the LePera reagent is
observed in 10 viewing fields at a magnification by 500-times, a region from the
surface to a position of 50 1-1m of the steel sheet in the obtained optical microscope
image is designated, and the image is analyzed using image analysis software
"Photoshop CS5" (manufactured by Adobe Inc.) to obtain the area fraction of ferrite.
In an image analysis method, for example, a maximum luminosity value Lmax and a
minimum luminosity value Lmm of the image are acquired from the image, a portion
that has pixels having a luminosity of Lmax - 0.3 x (Lmax - Lmm) to Lmax is defined as a
white region, a portion that has pixels having a luminosity of Lmm to Lmin + 0.3 x (Lmax
- Lmm) is defined as a black region, a portion other than the white and black regions is
defined as a gray region, and the area fraction of ferrite that is the gray region is
calculated. When the ferrite area ratio is 100%, the white region is not observed.
Therefore, when the entire region is the gray region, the ferrite fraction is 100%. By
performing the image analysis as described above in 10 observed viewing field in total,
the area fraction of ferrite is measured. Further, the area fraction values are averaged
to calculate the average value. This average value is set as the volume fraction of
ferrite in the surface layer region.
When the thickness of the steel sheet is 0.20 mm or less, the above-described
structure observation is performed on a region from the surface to a depth of t/4 in the
sheet thickness direction.
[0039]
- 18 -
[Average Grain Size of Ferrite being 1.0 )..lm to 15.0 )..lm]
When the average grain size of ferrite is more than 15.0 )..lm, the external
appearance after forming deteriorates. Therefore, the average grain size of ferrite in
the surface layer region is set to be 15.0 ~Jm or less. The average grain size is
preferably 12.0 ~Jm or less.
On the other hand, when the average grain size of ferrite is less than 1.0 )..lm,
ferrite grains having {001} orientation are likely to be formed in a state where they
aggregate. Even in a case where each of the ferrite grains having { 001} orientation is
small, when the grains are formed in a state where they aggregate, deformation
concentrates on the aggregated portion. Therefore, the external appearance after
forming deteriorates. Therefore, the average grain size of ferrite in the surface layer
region is set to be 1.0 !Jm or more. The average grain size is preferably 3.0 !Jm or
more and more preferably 6.0 ~Jm or more.
[0040]
The average grain size of ferrite in the surface layer region can be obtained
using the following method.
Using the same method as described above, a region ranging from the surface
to a 114 thickness position in the sheet thickness direction from the surface in the steel
sheet etched with the LePera reagent is observed in 10 viewing fields at a
magnification by 500-times, a region from the surface to a position of 50 )..lm x 200 )..lm
of the steel sheet in the optical microscope image is selected, and the image is analyzed
using image analysis software "Photoshop CS5" (manufactured by Adobe Inc.) as
described above to calculate the area fraction of ferrite and the number of ferrite grains,
respectively. By adding up the values and dividing the area fraction of ferrite by the
number of ferrite grains, the average area fraction per ferrite grain is calculated. The
- 19 -
circle equivalent diameter is calculated based on the average area fraction and the
number of grains, and the obtained circle equivalent diameter is set as the average
grain size of ferrite. When the thickness of the steel sheet is 0.20 mm or less, a region
of a depth from the surface of the steel sheet to t/4 in the optical microscope image x
200 J.lm is selected, and the image is analyzed.
[0041]
[Texture in which XoDF{OOIJ/{111}, s as Ratio of Intensity of { 001 } Orientation to
Intensity of { 111 } Orientation in Ferrite is 0.30 or more and less than 3.50 being
included]
A texture in which an XoDF{OOIJ/{111}, s as a ratio of an intensity of { 001}
orientation to an intensity of { 111 } orientation in the ferrite (ratio between maximum
values of X-ray random intensity ratios) is 0.30 or more and less than 3.50 is included
in the surface layer region such that the external appearance of the steel sheet after
forming is improved. The reason for this is not clear but is presumed to be that the
inhomogeneous deformation on the surface is suppressed due to an interaction between
the existence form of ferrite and the crystal orientation distribution of ferrite.
When XoDF{OOl}/{111], s is less than 0.30, inhomogeneous deformation caused
by an orientation distribution and a difference in intensity of each crystal of the
material is likely to occur, and deformation concentration on the orientation in the
vicinity of { 001 } in ferrite is significant. On the other hand, when XoDF{ 001 Jl{ 111 J, s is
more than 3.50, inhomogeneous deformation caused by an orientation distribution and
a difference in intensity of each crystal of the material is likely to occur, and
unevenness of the steel sheet surface is likely to occur.
[0042]
XoDF{OOl }/{ 111 J, s as the ratio of the intensity of { 001 } orientation to the
- 20 -
intensity of { 111 } orientation in ferrite of the surface layer region can be obtained in
the following method using Electron Backscattering Diffraction (EBSD) method.
Regarding a sample provided for EBSD method, the steel sheet is polished by
mechanical grinding, strain is removed by chemical polishing or electrolytic polishing,
the sample is prepared such that the cross section in the sheet thickness direction
including the range from the surface to the 1/4 thickness position from the surface in
the sheet thickness direction is a measurement surface, and the texture is measured.
Regarding a sample collection position in the sheet width direction, the sample is
collected in the vicinity of a sheet width position ofW/4 or 3W/4 (position at a
distance of 1/4 of the sheet width from an end surface of the steel sheet).
[0043]
In the region of the sample ranging from the surface of the steel sheet to 50
J..Lm from the surface in the sheet thickness direction, a crystal orientation distribution is
measured by EBSD method at a pitch of 0.5 J..Lm or less. When the thickness of the
steel sheet is 0.20 mm or less, the measurement is performed on a region from the
surface to a depth of t/4 in the sheet thickness direction. Ferrite is extracted using an
Image Quality (IQ) map that is analyzable by EBSP-OIM (registered trade name,
Electron Backscattering Diffraction Pattern-Orientation Image Microscopy). Ferrite
has a characteristic in that the IQ value is high, and thus can be simply classified from
other metallographic structures using this method. A threshold of the IQ value is set
such that the area fraction of ferrite that is calculated by the observation of the
microstructure obtained by the LePera corrosion matches the area fraction of ferrite
calculated based on the IQ value.
[0044]
In a cross section of ~2 = 45o in a three-dimensional texture (ODF:
- 21 -
Orientation Distribution Function) calculated using crystal orientations of the extracted
ferrite, a ratio of a maximum value of X-ray random intensity ratios of a { 001}
orientation group to a maximum value of X-ray random intensity ratios of a { 111}
orientation group (y-fiber) (the maximum value of X-ray random intensity ratios of
{001} orientation group I the maximum value of X-ray random intensity ratios of { 111}
orientation group (y-fiber)) is obtained as XoDF{OOl)/{111), s. The X-ray random
intensity ratio is a numerical value obtained by measuring a diffraction intensity of a
standard sample having no pile-up in a specific orientation and a diffraction intensity
of a sample material by X-ray diffraction under the same conditions and dividing the
obtained diffraction intensity of the sample material by the diffraction intensity of the
standard sample. For example, in a case where the steel sheet is rolled at a high
rolling reduction of 70% or higher and annealed, the texture is developed, and the Xray
random intensity of the { 111} orientation group (y-fiber) increases.
[0045]
Here, { hkl} represents that, when a sample is collected using the abovedescribed
method, the normal direction of a sheet surface is parallel to .
Regarding the crystal orientation, typically, an orientation perpendicular to a sheet
surface is represented by (hkl) or {hkl}. {hkl} is a generic term for equivalent planes,
and (hkl) represents each of crystal planes. That is, in the embodiment, a bodycentered
cubic structure (bee structure) is targeted. For example, the respective
planes (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1), and (-1-1-1) are equivalent
and cannot be distinguished from each other. In this case, these orientations are
collectively referred to as " { 111 } orientation group". The ODF representation is used
for representing other orientations of a crystal structure having low symmetry.
Therefore, in the ODF representation, each of orientations is generally represented by
- 22 -
(hkl)[uvw]. However, in the embodiment, attention is paid to the normal direction
orientation { hkl} from which the finding that the normal direction orientation of a
sheet surface has a large effect on the development of unevenness was obtained. {hkl}
and (hkl) have the same definition.
[0046]
In a case where the product is a steel sheet including a plating layer, the
surface of the steel sheet excluding the plating layer is defined as an origin of the
surface layer region.
[0047]

In the steel sheet according to the embodiment, it is preferable that, in a state
where the metallographic structure in the surface layer region is controlled as described
above, a metallographic structure in an internal region (in a case where the thickness of
the steel sheet is 0.20 mm or less, a range from a t/4 position to a t/2 position) ranging
from a position of more than 50 J.lm from the surface in the sheet thickness direction to
a 1/4 thickness position (in a case where the sheet thickness is represented by t: t/4)
from the surface in the sheet thickness direction is also controlled.
[0048]
[Texture in which XoDF{OOlJI{l11J, I as Ratio of Intensity of { 001} Orientation to
Intensity of { 111} Orientation in Ferrite is 0.001 or more and less than 1.00 being
included]
A texture in which an XoDF{OOlJ/{111}, I as a ratio of an intensity of {001 }
orientation to an intensity of { 111 } orientation in the ferrite (ratio between maximum
values of X-ray random intensity ratios) is 0.001 or more and less than 1.00 is included
in the internal region such that the external appearance of the steel sheet after forming
- 23 -
can be further improved, which is preferable.
[0049]
[Intensity Ratio XoDF{OOIJ/{111}, sand Intensity Ratio XoDF{OOIJ/{111 }, I Satisfying
Expression (1) (-0.20 < XoDF{OOIJ/{111], s- XODF{oOIJ/{111], I< 0.40) and Average Grain
size of Ferrite in Surface Layer Region being less than Average Grain Size of Ferrite in
Internal Region]
When the intensity ratio XoDF{OOIJ/{111], sin ferrite in the surface layer region
and the intensity ratio XoDF{OOl }/{ 111 J, I in ferrite in the internal region satisfies the
following Expression ( 1) and the average grain size of ferrite in the surface layer
region is less than an average grain size of ferrite in the internal region,
inhomogeneous deformation in the surface layer region is suppressed, which is
preferable.
-0.20 < XoDF{OOIJI{ 111 J, s - XoDF{OOIJ/{111}, I< 0.40
[0050]
(1)
The average grain size in the internal region can be obtained by using a steel
sheet etched with the LePera reagent, selecting a range from a position of more than 50
)lm from the surface of the sample in the sheet thickness direction to a 1/4 thickness
position from the surface in the sheet thickness direction, and analyzing the range with
the same method as that of the surface layer region.
In addition, a texture of ferrite in the internal region can be obtained by
designating a range from a position of more than 50 )lm from the surface of the sample
in the sheet thickness direction to a 1/4 thickness position from the surface in the sheet
thickness direction by the above-described EBSD method and analyzing the range with
the same method as that of the surface layer region.
When the thickness of the steel sheet is 0.20 mm or less, a range from a t/4
- 24 -
position to a t/2 position is selected and analyzed.
[0051]

The thickness of the steel sheet according to the embodiment is not
particularly limited. However, in a case where the steel sheet is applied to an exterior
member, when the sheet thickness is more than 0.55 mm, the contribution to a
reduction in the weight of the member is small. In addition, when the sheet thickness
is less than 0.12 mm, there may be a problem in rigidity. Therefore, the sheet
thickness is preferably 0.12 mm to 0.55 mm.
In addition, the thickness of the steel sheet can be obtained by sampling a
sheet from an end portion of a steel sheet coil in a longitudinal direction, collecting a
sample for sheet thickness measurement from a position of 300 mm from the end
portion in the sheet width direction, and measuring the thickness of the sample using a
micrometer.
[0052]

The steel sheet according to the embodiment may include a plating layer on
the surface (on the surface of the steel sheet). By including the plating layer on the
surface, corrosion resistance is improved, which is preferable.
A plating to be applied is not particularly limited, and examples thereof
include hot-dip galvanizing, hot-dip galvannealing, electrogalvanizing, Zn-Ni plating
(alloy electrogalvanizing), Sn plating, Al-Si plating, electrogalvannealing, hot-dip zincaluminum
alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dip zincaluminum-
magnesium alloy-Si plated steel sheet, and zinc-Al alloy deposition.
[0053]
- 25 -

Next, a preferable method for manufacturing the steel sheet according to the
embodiment will be described. The effects can be obtained as long as the steel sheet
according to the embodiment has the above-described properties irrespective of the
manufacturing method. However, with the following method, the steel sheet can be
stably manufactured, which is preferable.
[0054]
Specifically, the steel sheet according to the embodiment can be manufactured
with a manufacturing method including the following processes (i) to (vi).
(i) A heating process of heating a slab having the above-described chemical
composition to 1 ooooc or higher.
(ii) A hot-rolling process of hot-rolling the slab such that a rolling finishing
temperature is 950oc or lower to obtain a hot-rolled steel sheet.
(iii) A stress application process of applying a stress to the hot-rolled steel
sheet after the hot-rolling process such that an absolute value of a residual stress os on
a surface is 100 MPa to 250 MPa.
(iv) A cold-rolling process of cold-rolling the hot-rolled steel sheet after the
stress application process such that a cumulative rolling reduction RcR is 70% to 90%
to obtain a cold-rolled steel sheet.
(v) An annealing process of heating the cold-rolled steel sheet such that an
average heating rate in a range from 300°C to a soaking temperature T1 oc that satisfies
the following Expression (2) is 1.5 °C/sec to 10.0 °C/sec and holding the heated steel
sheet at the soaking temperature T1 oc for 30 seconds to 150 seconds for annealing.
Acr + 550- 25 x ln(os) - 4.5 x RcR :S T l ::; Acr + 550- 25 x ln(os)- 4 x RcR
(2)
- 26 -
(Note that Act in Expression (2) is represented by Expression (3) Act = 723 -
10.7 x Mn- 16.9 x Ni + 29.1 x Si + 16.9 x Cr)
(vi) A cooling process of cooling the cold-rolled steel sheet after the annealing
process to a temperature range of 55ooc to 65ooc such that an average cooling rate in
a range from the soaking temperature T1 octo 650°C is 1.0 °C/sec to 10.0 °C/sec and
cooling the cooled steel sheet to a temperature range of 200°C to 490°C such that the
average cooling rate is 5 °C/sec to 500 °C/sec.
In addition, in order to obtain the effect of tempering the hard phase that is
present in a small amount, the manufacturing method may further the following
process.
(vii) A holding process of holding the cold-rolled steel sheet after the cooling
process in a temperature range of 200oc to 490°C for 30 seconds to 600 seconds.
Hereinafter, the each process will be described.
[0055]
[Heating Process]
In the heating process, a slab having the predetermined chemical composition
is heated to 1 000°C or higher before rolling. When the heating temperature is lower
than 1 000°C, a rolling reaction force during hot rolling increases, sufficient hot rolling
cannot be performed, and there may be a case where the desired thickness of the
product cannot be obtained. Alternatively, there may a case where the steel sheet
cannot be coiled due to deterioration in the sheet shape.
It is not necessary to limit the upper limit of the heating temperature, and it is
not preferable that the heating temperature is excessively high from the viewpoint of
economy. Due to this reason, it is preferable that the slab heating temperature is
lower than 1300°C. In addition, the slab provided for the heating process is not
- 27 -
limited. For example, a slab that is manufactured using a continuous casting method
after melting molten steel having the above-descried chemical composition using a
converter or an electric furnace can be used. For example, an ingot-making method
or a thin slab casting method may be adopted instead of the continuous casting method.
[0056]
[Hot-Rolling Process]
In the hot-rolling process, the slab heated to 1000°C or higher in the heating
process is hot-rolled and coiled to obtain a hot-rolled steel sheet.
When the rolling finishing temperature is higher than 950°C, the average
grain size of the hot-rolled steel sheet excessively increases. In this case, the average
grain size of the final product sheet increases, and an increase in average grain size
causes a decrease in yield strength and deterioration in the surface appearance quality
after forming, which is not preferable. Therefore, the rolling finishing temperature is
set to be preferably 950°C or lower.
In addition, in order to reduce the grain size of the steel sheet and to improve
the surface appearance quality, the finish rolling start temperature is preferably 900°C
or lower. The finish rolling start temperature is more preferably 850°C or lower. In
addition, from the viewpoint of reducing a rolling force during hot rolling, the rolling
start temperature is preferably 700°C or higher and more preferably 750°C or higher.
[0057]
When a temperature change (finish rolling finishing temperature- fini sh
rolling start temperature) in the hot -rolling process is + soc or higher, recrystallization
is promoted by deformation heating in the hot-rolling process, and crystal grains are
refined, which is preferable.
In addition, in order to refine crystal grains, the coiling temperature in the
- 28 -
coiling process is preferably 75ooc or lower and more preferably 65ooc or lower. In
addition, from the viewpoint of reducing the strength of the steel sheet provided for
cold rolling, the coiling temperature is preferably 450°C or higher and more preferably
5oooc or higher.
[0058]
[Stress Application Process]
In the stress application process, a stress is applied to the hot-rolled steel sheet
after the hot-rolling process such that an absolute value of a residual stress as on a
surface is 100 MPa to 250 MPa. For example, a stress can be applied by grinding the
hot-rolled steel sheet using a surface grinding brush after hot rolling or pickling. At
that time, while changing a contact pressure of the grinding brush on the steel sheet
surface, a surface layer residual stress is measured on-line using a portable X-ray
residual stress analyzer and may be controlled to be in the above-described range. By
performing cold rolling, annealing, and cooling in a state where the residual stress is
applied to the surface to be in the above-described range, a steel sheet including ferrite
having a desired texture can be obtained.
[0059]
When the residual stress Gs is lower than 100 MPa or higher than 250 MPa,
the desired texture can be obtained after cold rolling, annealing, and cooling to be
performed after the stress application. In addition, in a case where the residual stress
is applied after cold rolling instead of after hot rolling, the residual stress is widely
distributed in the sheet thickness direction. Therefore, a desired metallographic
structure cannot be obtained only on the surface layer of the material.
A method of applying the residual stress to the surface of the hot-rolled steel
sheet is not limited to the above-described grinding brush. For example, a method of
- 29 -
performing surface grinding such as shot blasting or machining may also be used. In
the case of shot blasting, fine unevenness may occur on the surface due to collision
with shot media, or shot media may be trapped to form defects during the next cold
rolling or the like. Therefore, the method of applying the stress by grinding using a
brush is preferable.
In addition, during rolling using a roll such as a skin pass, a stress is applied to
the entire steel sheet in the thickness direction and the desired hard phase distribution
and the texture cannot be obtained only on the surface layer of the material.
[0060]
It is preferable that the stress application process is performed at a steel sheet
temperature of 40°C to 500°C. By performing the stress application process in this
temperature range, the residual stress can be efficiently applied to the range
corresponding to the surface layer region, and the cracking caused by the residual
stress of the hot-rolled steel sheet can be suppressed.
[0061]
[Cold-Rolling Process]
In the cold-rolling process, the hot-rolled steel sheet is cold-rolled after the
stress application process such that a cumulative rolling reduction RcR is 70% to 90%
to obtain a cold-rolled steel sheet. By cold-rolling the hot-rolled steel sheet to which
the predetermined residual stress is applied at the above-described cumulative rolling
reduction, ferrite having the desired texture can be obtained after annealing and
cooling.
When the cumulative rolling reduction RcR is less than 70%, the texture of the
cold-rolled steel sheet is not sufficiently developed. Therefore, the desired texture
cannot be obtained after annealing. In addition, when the cumulative rolling
- 30 -
reduction RcR is more than 90%, the textme of the cold-rolled steel sheet is excessively
developed. Therefore, the desired texture cannot be obtained after annealing. In
addition, the rolling force increases, and the homogeneity of the material in the sheet
width direction deteriorates. Fwther, the production stability also deteriorates.
Therefore, the cumulative rolling reduction RcR during cold rolling is set to be 70% to
90%.
[0062]
[Annealing Process]
In the annealing process, the cold-rolled steel sheet is heated to the soaking
temperatme Tl oc at the average heating rate corresponding to Act, the residual stress
applied in the stress application process, and the cumulative rolling reduction RcR in
the cold-rolling process, and is held at the soaking temperature corresponding to Act,
the residual stress applied in the stress application process, and the cumulative rolling
reduction RcR in the cold-rolling process.
Specifically, in the annealing process, the cold-rolled steel sheet is heated such
that an average heating rate in a range from 300°C to a soaking temperature T1 oc that
sati sfies the following Expression (2) is 1.5 °C/sec to 10.0 °C/sec and holding the
heated steel sheet at the soaking temperature T1 °C for 30 seconds to 150 seconds for
annealing.
[0063]
Act+ 550 - 25 x ln(as)- 4.5 x RcR ~ Tl ~Act+ 550- 25 x ln(as) - 4 x RcR
(2)
Act in Expression (2) is represented by the following Expression (3). An
element symbol in the following Expression (3) represents an amount of the
corresponding element by mass%, and when the corresponding element is not included,
- 31 -
0 is substituted into the corresponding element symbol.
Act= 723- 10.7 x Mn- 16.9 x Ni + 29.1 x Si + 16.9 x Cr
[0064]
(3)
When the average heating rate is slower than 1.5 °C/sec, a long period of time
is required for heating, and the productivity deteriorates, which is not preferable. In
addition, when the average heating rate is faster than 10.0 °C/sec, the homogeneity of
the temperature in the sheet width direction deteriorates, which is not preferable.
In addition, when the soaking temperature T1 is lower than the left side of
Expression (2), recrystallization of ferrite and reversible transformation from ferrite to
austenite do not sufficiently progress, and the desired texture cannot be obtained. In
addition, inhomogeneous deformation during forming is promoted due t o a difference
in intensity between non-recrystallized crystal grains and recrystallized crystal grains,
which is not preferable. In addition, when the soaking temperature T1 is higher than
the right side of Expression (2), recrystallization of ferrite and reversible
transformation from ferrite to austenite sufficiently progresses, cr ystal grains are
coarsened, and the desired texture cannot be obtained, which is not preferable.
The average heating rate can be obtained from (Heating End Temperature -
Heating Start Temperature) I (Heating Time).
[0065]
[Cooling Process]
In the cooling process, the cold-rolled steel sheet after soaking in the
annealing process is cooled. During cooling, the cold-rolled steel sheet is cooled to a
temperature range of 550°C to 650°C such that an average cooling rate in a range from
the soaking temperature Tl octo 650°C is 1.0 °C/sec to 10.0 oc /sec, and is further
cooled to a temperature range of 200°C to 490°C such that the average cooling rate is
- 32 -
5 °C/sec to 500 °C/sec.
When the average cooling rate in a range from T1 octo 650°C is slower than
1.0 oc/sec, the desired metallographic structure in the surface layer region cannot be
obtained. On the other hand, when the average cooling rate is faster than 1 0.0°C,
ferritic transformation does not sufficiently progress, and the desired volume fraction
of ferrite cannot be obtained.
In addition, when the average cooling rate from this temperature range to a
temperature range of 200oc to 490°C after cooling is performed in a temperature range
of 550°C to 650°C is slower than 5 °C/sec, the desired texture of ferrite cannot be
obtained. On the other hand, it is difficult to set the average cooling rate to be faster
than 500 °C/sec due to the facility restriction. Therefore, the upper limit is set to be
500 °C/sec.
The average cooling rate can be obtained from (Cooling Start TemperatureCooling
End Temperature) I (Cooling Time).
[0066]
[Holding Process]
The cold-rolled steel sheet that is cooled to 200°C to 490°C may be held in
the temperature range of for 30 to 600 seconds.
By holding the cold-rolled steel sheet in the temperature range for the
predetermined time, the effect of tempering the hard phase that is present in a small
amount can be obtained, which is preferable.
The cold-rolled steel sheet that is cooled to 200°C to 490°C or the cold-rolled
steel sheet after the holding process may be cooled to room temperature at 10 oc /sec or
faster.
[0067]
- 33 -
A plating process of forming a plating layer on the surface may be further
performed on the cold-rolled steel sheet obtained using the above-described method.
Examples of the plating process include the following process.
[0068]
[Electroplating Process]
[Galvannealing process]
The cold-rolled steel sheet after the cooling process or the holding process
may be electroplated to form an electroplating layer on the surface. An electroplating
method is not particularly limited. Conditions may be determined depending on
required properties (for example, corrosion resistance or adhesion).
In addition, after electroplating, the cold-rolled steel sheet may be heated to
alloy plating metaL
[0069]
[Hot-Dip Galvanizing Process]
[Gal vannealing process]
The cold-rolled steel sheet after the cooling process or the holding process
may be hot-dip galvanized to form a hot-dip galvanized layer on the surface. A hotdip
galvanizing method is not particularly limited. Conditions may be determined
depending on required properties (for example, corrosion resistance or adhesion).
In addition, the cold-rolled steel sheet after hot-dip galvanizing may be heattreated
to alloy a plating layer. In a case where alloying is performed, it is preferable
that the cold-rolled steel sheet is heat-treated in a temperature range of 400°C to 600°C
for 3 to 60 seconds.
[0070]
With the above-described manufacturing method, the steel sheet according to
- 34 -
the embodiment can be obtained.
[Examples]
[0071]
Next, examples of the present invention will be described. However,
conditions of the examples are merely exemplary to confirm the operability and the
effects of the present invention, and the present invention is not limited to these
condition examples. The present invention can adopt various conditions within a
range not departing from the scope of the present invention as long as the object of the
present invention can be achieved under the conditions.
[0072]
Steels having chemical compositions shown in "Steel Pieces No. A toT' of
Table 1 were melted, and slabs having a thickness of 240 to 300 mm were
manufactured by continuous casting. The obtained slabs were heated at a temperature
shown in the tables. The heated slabs were hot-rolled under conditions shown in
Table 2 and were coiled.
Next, the coil was uncoiled and a stress was applied to the hot-rolled steel
sheet. At this time, while measuring the surface layer residual stress on-line using a
portable X-ray residual stress analyzer at a working temperature (steel sheet
temperature) shown in Table 2, a contact pressure of a grinding brush on the steel sheet
surface was changed such that the residual stress as was as shown in Table 2. Next,
by performing cold rolling at a cumulative rolling reduction RcR shown in Table 2,
steel sheets Al to Tl were obtained.
"Temperature Change in Hot-Rolling Process" in Table 2 shows a temperature
change (finish rolling finishing temperature- fini sh rolling start temperature) in the
hot-rolling process. In addition, in Table 2, the residual stress as is shown in the
- 35 -
example (example where "* 1" is shown in the field "Steel Sheet Temperature") where
the stress application process was not performed. It is considered that this residual
stress as was generated by inhomogeneity in cooling rate during steel sheet cooling.
[0073]
Next, by performing annealing and cooling under conditions shown in Tables
3A and 3B, some steel sheets were held at 200°C to 490°C for 30 to 600 seconds.
After cooling or holding, the steel sheets were air-cooled to room temperature. Next,
some steel sheets were plated in various ways to form a plating layer on the surface.
In Tables 3A and 3B, CR represents that no plating was performed, GI represents that
hot-dip galvanizing was performed, GA represents that hot-dip galvannealing was
performed, EG represents that electroplating was performed, EGA represents that
electrogalvannealing was performed, and Sn, Zn-Al-Mg, Al-Si or the like represents
that plating including these elements was performed. In addition, in Tables 3A and
3B, phosphate coating EG represents that phosphate coating electrogalvanizing was
performed, and lubricant GArepresents lubricant hot-dip galvannealing.
[0074]
Regarding each of the product sheets No. Ala to Tla, the observation of the
metallographic structures in the surface layer region and the internal region and the
measurement ofXoDF{OOIJI{111J,S, XoDF{OOlJI{l11J,I, and the sheet thickness were
performed using the above-described method. The results are shown in Tables 4A
and 4B.
[0075]
[Evaluation of Tensile Strength]
The tensile strength of the obtained product sheet was obtained in a tensile
test that was performed according to JIS Z 2241 using a JIS No. 5 test piece cut from
- 36 -
the direction perpendicular to the rolling direction. As a result, the tensile strengths
of all the product sheets according to the present invention were 340 MPa or higher.
[0076]
[Evaluation of Surface Properties of Steel Sheet]
In addition, regarding each of the manufactured product sheets, the surface
properties of the steel sheet were evaluated.
Specifically, the surface of the manufactured steel sheet was observed by
visual inspection to evaluate the surface properties. The evaluation criteria of the
surface properties of the steel sheet were as follows.
A: no pattern was formed (more desirably, can be used as an exterior material)
B: an acceptable small pattern was formed (can be used as an exterior material)
C: an unacceptable pattern was formed (can be used as a component but
cannot be used as an exterior material)
D: a significant pattern defect was formed (cannot be used as a component)
[0077]
[Forming Test of Steel Sheet]
Regarding each of the manufactured product sheets, a forming test was
performed.
Regarding forming, plastic strain of 10% in the rolling width direction was
applied to the steel sheet of which the surface properties was measured in a cylinder
drawing forming test with the Marciniak method using a deep drawing tester, a
cylindrical punch of <)>50 mm, and a cylindrical die of <)>54 mm.
A test piece of 100 mm in the rolling width direction x 50 mm in the rolling
direction was prepared from a portion deformed by forming, and an arithmetic mean
height Pa of a profile curve defined by JIS B0601 (2001) was measured in the direction
- 37 -
perpendicular to the rolling direction according to JIS B0633 (2001). The evaluation
was performed in the portion deformed by forming, and the evaluation length was 30
mm.
A test piece of 100 mm in the rolling width direction x 50 mm in the rolling
direction was prepared from a flat portion of the formed article, and an arithmetic
mean height Pa of a profile curve defined by JIS B060 1 (200 1) was measured in the
direction perpendicular to the rolling direction according to JIS B0633 (2001). The
evaluation length was 30 mm.
The amount .llPa of increase in roughness (.llPa = Pa of Formed Article - Pa of
Steel Sheet) was calculated using Pa of the formed article and Pa of the steel sheet
obtained in the above-described measurement test.
[0078]
The surface properties of the steel sheet after forming were evaluated based
on the .llPa. The evaluation criteria were as follows.
A: .llPa ::; 0.25 J.lm (more desirably, can be used as an exterior material)
B: 0.25 J.lm < .llPa::; 0.35 J.lm (can be used as an exterior material)
C: 0.35 J.lm < .llPa ::; 0.55 J.lm (can be used as a component but cannot be used
as an exterior material)
D: 0.55 J.lm < .llPa (cannot be used as a component)
[0079]
[Comprehensive Evaluation]
Regarding evaluation criteria of the surface properties, among the abovedescribed
two evaluation results (the evaluation of the surface properties of the steel
sheet and the evaluation of the surface properties after forming), an evaluation result
having a lower score was obtained as the comprehensive evaluation. In a case where
- 38 -
the result of the comprehensive evaluation was C or D, the steel sheet was not able to
be used as an exterior material or a component and was determined to be unacceptable.
A: more desirably, the material can be used as an exterior material
B: the material can be used as an exterior material
C: the material cannot be used as an exterior material
D: the material cannot be used as a component
[0080]
The above-described test results are shown in Tables 4A and 4B.
- 39 -
[0081]
[Table 1]
Slab Chemical Composition mass% (Remainder: Fe+ Impurities)
Ac1
No. c Si Mn p s sol.Al N Cr Mo B Nb Ti Ni Cu
A 0.0110 0.020 0.40 0.080 0.003 0.042 0.0030 0.100 0.010 0.0000 0.003 0.000 0.001 0.000 721
B 0.0400 0.010 0.26 0.030 0.007 0.030 0.0030 0.005 0.300 0.0000 0.003 0.000 0.000 0.010 721
c 0.0015 0.010 0.58 0.015 0.006 0.050 0.0025 0.400 0.100 0.0013 0.013 0.001 0.013 0.000 724
D 0.0025 0.012 0.84 0.024 0.010 0.050 0.0022 0.032 0.001 0.0009 0.003 0.004 0.000 0.000 715
E 0.0320 0.010 1.12 0.020 0.003 0.045 0.0028 0.002 0.001 0. 0005 0.002 0.000 0.010 0.010 7 11
F 0.0070 0.080 0.25 0.050 0.005 0.195 0.0040 0.001 0.001 0. 0000 0.002 0.005 0.000 0.000 723
G 0.0100 0.200 0.20 0.010 0.006 0.030 0.0033 0.004 0.010 0.0000 0.000 0.003 0.010 0.000 727
H 0.0080 0.030 1.50 0.050 0.005 0.050 0.0040 0.005 0.001 0. 0000 0.013 0.000 0.000 0.010 708
I 0.0100 0.020 1.20 0.060 0.004 0.045 0.0040 0.002 0.000 0.0000 0.004 0.000 0.010 0.000 7 11
I 0.0014 0.200 0.20 0.050 0.006 0.030 0.0020 0.001 0.000 0.0000 0.000 0.002 0.000 0.000 727
K 0.0080 0.030 1.65 0.020 0.006 0.030 0.0033 0.004 0.010 0.0000 0.000 0.002 0.004 0.000 706
1 0.0100 0.400 0.20 0.020 0.006 0.030 0.0033 0.004 0.000 0.0000 0.000 0.007 0.000 0.000 733
M 0.0200 0.050 0.40 0.015 0.001 0.025 0.0110 0. 600 0.550 0.0002 0.020 0.000 0.010 0.000 730
N 0.0370 0.010 0.15 0.010 0.005 0.029 0.0028 0.001 0.001 0.0000 0.000 0.000 0.000 0.000 722
Q 0.0500 0.070 0.40 0.015 0.001 0.025 0.0090 0.020 0.001 0.0000 0.002 0.030 0.000 0.000 721
£ 0.0035 0.130 1.53 0.030 0.006 0.040 0.0030 0.023 0.000 0.0000 0.001 0.000 0.040 0.020 710
Q 0.0100 0.020 0.20 0.020 0.006 0.210 0.0033 0.004 0.001 0.0000 0.000 0.001 0.000 0.000 722
R 0.0065 0.010 1.37 0.104 0.005 0.125 0.0040 0.010 0.000 0.0000 0.000 0.010 0.000 0.000 709
~ 0.0300 0.011 1.33 0.005 0.002 0.120 0.0045 0.001 0.001 0.0000 0.000 0.000 0.000 0.000 709
T 0.0090 0.030 0.60 0.050 0.005 0.150 0.0040 0.020 0.000 0. 0000 0.000 0.000 0.000 0.000 718
The underline represents that the value is outside of the range of the present invention.
- 40 -
[0082]
[Table 2]
Heating Process Hot-Rol1ing Process Coiling Process Stress Application Process Co1d-Ro11ing Process
Slab Stee] Heating Finish Ro11ing Ro11ing Finishing Change in Temperature Coiling Residua] Stee] Sheet Cumulative Ro11ing
No. Sheet No. Temperature Start Temperature Temperature ofHot-Ro11ing Process Temperature Stress as Temperature Reduction RcR oc oc oc oc oc MPa oc %
A A1 1200 950 890 -60 500 167 112 78
A A2 1200 950 890 -60 600 120 40 85
A A3 1200 950 890 -60 530 101 112 92
A A4 1220 990 910 -80 600 34 162 85
B B1 1200 930 880 -50 460 47 30 77
B B2 1100 850 865 15 460 111 242 85
B B3 1200 810 840 30 600 159 43 85
B B4 1200 930 880 -50 550 201 30 80
c C1 1200 910 890 -20 640 189 30 85
c C2 1200 845 870 25 640 129 103 80
c C3 1050 800 850 50 680 29 ?.::1 90
c C4 1050 800 850 50 680 108 30 66
c C5 1050 800 850 50 700 252 20 72
c C6 1200 1010 900 -110 640 20 120 85
c C7 1230 1030 930 -100 660 16 130 85
D D1 1100 850 885 35 700 107 30 90
D D2 1100 850 885 35 700 144 30 87
D D3 1100 850 885 35 560 32 30 92
E E1 1300 1080 950 -130 560 105 120 85
E E2 1280 1050 930 -120 700 101 50 85
E E3 1100 860 875 15 680 108 40 87
E E4 1100 860 875 15 680 24 40 81
F F1 1200 950 900 -50 700 266 27 92
F F2 1200 950 900 -50 700 108 25 90
G G1 1100 850 885 35 750 146 30 85
G G2 1100 850 885 35 750 276 503 87
G G3 1100 850 885 35 600 66 75 68
H H1 1200 930 890 -40 550 94 45 85
H H2 1200 930 890 -40 680 106 45 77
H H3 1300 1090 960 -130 600 111 45 85
I Il 1200 850 890 40 600 111 30 82
- 41 -
I 12 1200 850 890
J J1 1200 910 890
J 12 1200 910 890
K K1 1200 920 890
K K2 1200 820 845
L L1 1250 850 880
L L2 1250 850 880
M M1 1200 925 895
M M2 1200 925 895
N N1 1250 960 910
0 01 1200 925 870
p P1 1100 860 865
p P2 1200 950 890
Q Q_1 1200 950 905
R R1 1200 920 890
s S1 1200 930 880
s S2 1200 930 880
T T1 1100 850 865
The underhne represents that the va]ue 1s outs1de of the range of the present mventwn.
*1 represents that the stress application process was not performed.
40 650 15 01 82
-20 700 141 35 83
-20 580 104 so 83
-30 520 127 20 88
25 480 111 25 82
30 670 26 so 85
30 700 232 25 82
-30 600 103 110 87
-30 580 55 110 92
-SO 600 138 30 87
-55 650 156 45 72
5 550 34 0 1 80
-60 760 112 40 80
-45 650 113 40 72
-30 550 37 01 80
-SO 500 133 300 80
-SO 500 267 25 92
15 550 105 45 80
- 42 -
[0083]
[Table 3A]
Annealing Process Cooling Process Surface Treatment
Steel Product Average Left Side Right Side Soaking !Average Cooling Cooling
Average
Cooling Sheet
Sheet Sheet Heating of of [Temperature
Annealing
Rate in Tl °C to Stop
Cooling Rate
Stop
Holding
Thickness
Time in 200°C to Process Type of Plating
No. No. Rate Expression ~xpression Tl 650°C Temperature
490°C !Temperature mm
oCJs (2) (2) oc sec oCJs oc oc
°C/s
Al Ala 5.8 792 831 810 60 6.2 570 40 400 Not GA 0.50
Provided
Al Alb 3.3 792 831 852 100 3.8 570 15 400 Provided GA 0.50
A2 A2a 2.7 769 811 800 60 3.1 570 8 350
Not
Provided GI 0.35
A2 A2b 3.7 769 811 750 90 4.3 570 10 350 Provided GI 0.35
A2 A2c 3.7 769 811 800 90 4 .3 570 10 400 Provided GA 0.35
A3 A3a 2.7 742 788 785 120 3.1 570 8 450
Not
GA 0.25
Provided
A4 A4a 3.2 800 843 810 80 2.6 580 9 450
Not
GA 0.45
Provided
Bl Bla 2.7 828 866 820 120 3.1 570 8 450
Not
GA 0.65
Provided
B2 B2a 3.7 771 813 800 90 4.3 600 10 450
Not
GA 0.40
Provided
B3 B3a 5.8 762 804 800 60 6.2 560 16 400
Not
GA 0.40
Provided
B4 B4a 2.7 778 8 18 790 120 3.1 570 8 400 Provided Zn-Al-Mg-Si 0.45
B4 B4b 3.7 778 818 790 90 4.3 580 10 450
Not
Lubricant GA 0.45
Provided
C l Cla 2.0 760 803 790 140 2.9 570 7 420
Not
GI 0.40
Provided
C2 C2a 2.0 792 832 810 140 2.9 610 7 450
Not
A1-Si 0.45
Provided
C3 C3a 2.0 784 829 8 10 140 2.9 620 7 450
Not Zn-A1 0.20 Provided
C4 C4a 5.4 860 893 840 60 6.2 600 40 200 Provided CR 0.70
C5 C5a 1.7 8 11 847 820 150 3.0 570 10 300 Provided GA 0.57
C6 C6a 2.9 816 859 820 4 2.8 570 11 460
Not
GA 0.40
Provided
- 43 -
C7 C7a 3.2 822 864 830 90 3.2 580 8 470
Not
GA 0.40
Provided
D1 D1a 4.2 743 788 770 80 4.8 550 14 400
Not
Zn-Al-Mg 0.25
Provided
D2 D2a 3.2 749 793 780 110 3.6 570 9 400
Not
GA 0.40
Provided
D3 D3a 2.5 764 810 800 130 2.9 560 7 490
Not
GA 0.18
Provided
E1 E1a 5.8 762 805 800 60 6.2 580 16 470
Not
GA 0.35
Provided
E2 E2a 2.7 763 806 800 120 3.1 590 8 460
Not
GA 0.35
Provided
E3 E3a 9.5 753 796 780 30 9.9 580 80 400
Not
Provided GA 0.30
E3 E3b 3.7 753 796 780 90 4.3 570 20 450
Not
GA 0.30
Provided
E4 E4a 3.7 817 858 820 90 4.3 570 20 460 Not GA 0.55
Provided
F1 F1a 3.8 719 765 740 70 6.7 570 80 250 Provided EG 0.11
F2 F2a 2.7 751 796 790 120 3.1 580 8 450
Not
Provided Sn 0.14
The nnderhne represents that the value 1s outs1de of the range of the present mventwn.
- 44 -
[0084]
[Table 3B]
Annealing Process Cooling Process Surface Treatment
Steel Product Average
!Left Side o
Right Side Soaking
Annealing
Average Coolin~ Cooling Average Cooling Holding
Sheet
Sheet Sheet Heating of [Temperature Rate in T1 °C to Stop Cooling Rate Stop Thickness
No. No. Rate
Expression
Expression T1
Time 650°C [Temperature in 200°C to Temperature
Process Type of Plating
(2)
mm
°C/s (2) oc sec oC/s oc 490°C oc
G1 G1a 2.7 769 812 780 120 3.1 570 8 350 Not GI 0.40
G2 G2a 5.8 745 788 770 60 6.2 570 16 350 Provided GI 0.35
G3 G3a 2.5 866 900 830 110 4.3 570 20 250 Provided CR 0.77
H1 H1a 1.7 762 804 800 150 3.0 550 10 300 Provided CR 0.35
H2 H2a 3.4 795 833 820 80 6.0 610 20 300 Provided CR 0.50
H3 H3a 1. 7 758 800 800 150 3.0 580 10 300 Provided CR 0.38
I1 Ita 6.6 774 815 810 50 7.6 570 100 300 Provided CR 0.45
12 I2a 6.6 824 865 825 50 7.6 570 100 200 Provided CR 0.45
J1 Jla 6.6 780 821 780 50 7.6 590 100 250 Provided Phosphate Coating EG 0.50
12 J2a 2.2 787 829 790 120 3.9 560 50 300 Provided CR 0.50
K1 K1a 2.9 739 783 750 90 5.2 560 24 400 Provided CR 0.30
K2 K2a 2.5 769 810 770 110 4.3 550 20 200 Not CR 0.40
L1 Ll a 2.2 8 19 861 810 120 3.9 570 18 200 Not EG 0.35
L2 L2a 2.2 777 818 780 120 3.9 570 18 350 Provided CR 0.55
L2 L2b 3.7 777 818 780 90 4.3 570 20 250 Not GA 0.55
L2 L2c 9.3 777 818 780 150 9.9 550 500 200 Not EGA 0.55
M1 M1a 2.0 773 816 775 150 1.1 580 50 460 Not GA 0.40
M2 M2a 11 .5 766 812 770 30 9.9 550 500 460 Not GA 0.11
N1 N1a 4.4 757 801 760 60 7.7 560 35 400 Provided CR 0.30
01 01a 4.2 821 857 830 80 4.8 570 20 430 Not GA 0.65
P1 P1a 5.4 8 12 852 820 60 6.2 570 32 430 Not GA 0.60
P2 P2a 3.8 782 822 800 100 5.4 560 25 450 Not GA 0.60
ill .Q.l1! 2.7 829 865 830 120 3.1 570 16 450 Not GA 0.85
R1 R1a 2.7 809 849 800 120 3.1 600 16 400 Not GI 0.60
S1 S1a 3.7 777 817 800 90 4.4 550 19 350 Provided CR 0.60
S2 S2a 3.7 705 751 800 90 4.3 570 18 445 Not GA 0.15
T1 T1a 2.7 791 831 820 120 3.1 570 8 450 Not GA 0.45
The underlme represents that the value 1s outSide of the range of the present mventlon.
- 45 -
[0085]
[Table 4A]
Surface Layer Region Internal Region
Evaluation
Forming Test
Product
Average Ferrite Average
XoDF{001)1{111). s- of Surface
Amount L'.Pa Evaluation o
Comprehensi vc
Sheet No.
Grain Size Volume IXoDF{001)1{111l Grain Size XoDF{001)1{111l
XoDF{001)1{111), 1 Properties [f.!m] of Surface
Evaluation
Note
of Ferrite Fraction sin Ferrite of Ferrite 1in Ferrite (Steel Sheet) Increase in Properties
flill % flill Roughness after F orrnin~
Ala 10.2 99 1.30 12.0 0.93 0.37 A 0.11 A A Example
Alb 16.7 98 0.72 16.8 0.18 0.54 B 0.37 c c Comparative Example
A2a 13.1 94 1.70 13.4 1.31 0.39 A 0.27 B B Example
A2b 9.8 100 0.25 12.6 0.32 -0.07 B 0.71 D D Comparative Example
A2c 11.1 97 1.25 13.4 0.88 0.37 A 0.25 A A Example
A3a 8.1 99 0.18 8.0 0.17 0.01 B 0.63 D D Comparative Example
A4a 10.2 98 4.20 10.5 1.60 2.60 A 0.66 D D Comparative Example
Bla 10.2 97 0.24 9.5 0.66 -0.42 B 0.62 D D Comparative Example
B2a 5.0 95 1.30 9.9 0.44 0.86 A 0.24 A A Example
B3a 4.6 95 1.38 10.8 0.98 0.40 A 0.21 A A Example
B4a 3.2 97 2.96 10.2 2.15 0.81 A 0.29 B B Example
B4b 3.5 97 3.03 10.9 1.24 1.79 A 0.28 B B Example
Cla 10.8 100 1.80 12.8 1.40 0.40 A 0.25 A A Example
C2a 13.6 100 0.65 13.9 0.35 0.30 A 0.23 A A Example
C3a 16.1 100 0.35 13.5 0.92 -0.57 B 0.54 c c Comparative Example
C4a 14.5 99 3.52 15.9 0.60 2.92 A 0.40 c c Comparative Example
C5a 8.9 99 4.08 10.8 1.24 2.84 A 0.64 D D Comparative Example
C6a 10.9 99 3.52 11.2 1.50 2.02 A 0.69 D D Comparative Example
C7a 16.0 100 1.42 14.2 1.59 -0.17 A 0.56 D D Comparative Example
Dla 8.1 99 0.66 12.7 0.30 0.36 A 0.14 A A Example
D2a 9.8 100 0.55 15.0 0.25 0.30 A 0.16 A A Example
D3a 12.8 100 4.50 12.4 2.70 1.80 A 1.00 D D Comparative Example
Ela 7.7 97 1.12 9.6 0.84 0 .28 A 0.17 A A Example
E2a 8.9 98 1.34 10.0 1.10 0.24 A 0.19 A A Example
E3a 6.0 98 0.81 8.9 0.75 0.06 B 0.13 A B Example
E3b 6.6 100 0.63 9.6 0.45 0.18 A 0.17 A A Example
E4a 8.3 99 0.24 11.8 0.85 -0.61 B 0.51 c c Comparative Example
Fla 2.5 99 3.60 6.7 2.27 1.33 A 0.36 c c Comparative Example
F2a 9.2 100 2.33 10.5 0.77 1.56 A 0.10 A A Example
The underhne represents that the value 1s outs1de of the range of the present mventwn.
- 46 -
[0086]
[Table 4B]
Surface Layer Region Internal Region Forming Test
Average Ferrite Average
Evaluation of Amount
Evaluation of
Product
Grain Size Volume IXoDF(001JI(111J Grain Size IXoDF(001JI(111),
XoDF(OOl)/(111), s- Surface L'I.Pa [f.!m] Surface ~omprehensive Note
Sheet No.
of Ferrite Fraction sin Ferrite of Ferrite rin Ferrite
XoDF(001)1( 111 ), r Properties ofincrease
Properties after
Evaluation
%
(Steel Sheet) ln
flill flill Roughness Forming
G1a 8.8 97 3.24 9.0 0.88 2.36 B O.D7 A B Example
G2a 8.0 96 3.55 11.2 2.14 1.41 B 0.37 c c Comparative Example
G3a 10.1 98 0.27 10.0 0.22 0.05 B 0.36 c c Comparative Example
H1a 13.9 99 0.29 12.9 0.34 -0.05 A 0.39 c c Comparative Example
H2a 9.7 98 0.33 13.5 0.15 0.18 A 0.30 B B Example
H3a 15.3 99 0.41 17.6 0.32 0.09 A 0.37 c c Comparative Example
Ila 11.0 99 0.88 13.8 0.79 0.09 B 0.18 A B Example
I2a 16.9 99 0.24 12.6 0.40 -0.16 c 0.47 c c Comparative Example
Jla 13.8 99 0.11 16.8 0.05 0.06 A 0.92 D D Comparative Example
J2a 12.2 99 0.08 16.0 0.09 -0.01 A 0.68 D D Comparative Example
K1a 7.1 98 0.15 11.0 0.09 0.06 A 0.45 c c Comparative Example
K2a 9.8 98 0.09 12.4 0.06 0.03 A 0.38 c c Comparative Example
Lla 12.1 100 0.19 11.9 0.78 -0.59 c 0.67 D D Comparative Example
L2a 5.8 98 0.27 9.8 0.08 0.19 B 0.44 c c Comparative Example
L2b 5.2 99 0.28 11.4 0.08 0.20 c 0.39 c c Comparative Example
L2c 3.2 100 0.26 7.6 0.09 0.17 c 0.36 c c Comparative Example
M1a 12.6 100 0.27 12.0 0.25 0.02 c 0.56 D D Comparative Example
M2a 9.4 99 0.26 9.6 0.62 -0.36 B 0.42 c c Comparative Example
N1a 12.7 100 0.08 13.4 0.12 -0.04 A 0.85 D D Comparative Example
01a 7.9 89 4.55 10.5 3.10 1.45 A 0.53 c c Comparative Example
P1a 17.4 97 0.55 14.3 1.08 -0.53 D 0.39 c D Comparative Example
P2a 11.4 97 3.57 12.3 3.47 0.10 B 0.38 c c Comparative Example
Q_1a 10.0 99 0.24 12.6 0.14 0.10 B 0.72 D D Comparative Example
R1a 15.5 99 0.23 12.3 0.73 -0.50 c 1.03 D D Comparative Example
S1a 12.8 98 3.60 13.0 3.34 0.26 A 0.50 c c Comparative Example
S2a 10.6 98 6.10 14.2 4.76 1.34 B 0.65 D D Comparative Example
Tla 13.9 100 0.90 14.4 0.68 0.22 B 0.33 B B Example
The underhne represents that the value 1s outs1de of the range of the present mventwn.
- 47 -
[0087]
As shown in Tables 1 to 4B, in the examples (Examples) where the chemical
composition, the metallographic structure in the surface layer region, and
XoDF{OOIJ/{111), s were in the ranges of the present invention, the result of the
comprehensive evaluation was A orB, the formation of the surface unevenness was
suppressed in the stage of the steel sheet and after working. On the other hand, in the
examples (Comparative Examples) where one or more of the chemical composition,
the metallographic structure in the surface layer region, and XoDF{OOIJ/{111}, s were
outside of the ranges of the present invention, a pattern was formed or unevenness
occurred in the stage of the steel sheet or after forming such that the material was not
able to be used as an exterior material or a component.
[0088]
FIG. 1 is a diagram showing a relationship between surface properties after
forming and a texture parameter obtained in Examples. The • plot of FIG. 1 shows
an example where the average grain size of ferrite in the surface layer region was more
than 15.0 )lm.
Referring to FIG. 1, it can be seen that the surface properties after forming
were excellent in the examples where the texture parameter was in the range of the
present invention (the ratio XoDF{OOI }/{111), s of the intensity of { 001} orientation to the
intensity of { 111 } orientation in ferrite was 0.30 or more and less than 3.50).
[Industrial Applicability]
[0089]
With the above-described aspect of the present invention, a high strength steel
sheet in which formability is excellent and the occurrence of surface unevenness is
suppressed even after various deformation during press forming can be manufactured.
- 48 -
Therefore, the industrial applicability is high.

WE CLAIMS

1. A steel sheet comprising, as a chemical composition, by mass%:
C: 0.0015% to 0.0400%;
Mn: 0.20% to 1.50%;
P: 0.010% to 0.100%;
Cr: 0.001% to 0.500%;
Si: 0.200% or less;
S: 0.020% or less;
sol. Al: 0.200% or less;
N: 0.0150% or less;
Mo: 0% to 0.500%;
B: 0% to 0.0100%;
Nb: 0% to 0.200%;
Ti: 0% to 0.200%;
Ni: 0% to 0.200%;
Cu: 0% to 0.100%; and
a remainder including iron and impurities,
wherein a metallographic structure in a surface layer region includes ferrite
having a volume fraction of 90% or more, and
in the surface layer region,
an average grain size of the ferrite is 1.0 1-Lm to 15.0 ,....m, and
a texture in which an XoDFJOOl}ll111 J, s as a ratio of an intensity of { 001}
orientation to an intensity of { 111 } orientation in the ferrite is 0.30 or more and less
than 3.50 is included.
2. The steel sheet according to claim 1,
- 50 -
wherein the chemical composition includes, by mass%, one or more selected
from the group consisting of:
Mo: 0.001% to 0.500%;
B: 0.0001% to 0.0100%;
Nb: 0.001% to 0.200%;
Ti: 0.001% to 0.200%;
Ni: 0.001% to 0.200%; and
Cu: 0.001% to 0.100%.
3. The steel sheet according to claim 1 or 2,
wherein a texture in which an XoDF{OOIJ/1 111 J, I as a ratio of an intensity of { 001}
orientation to an intensity of { 111} orientation in ferrite is 0.001 or more and less than
1.00 is included in an internal region.
4. The steel sheet according to one of claims 1 to 3,
wherein the intensity ratio XoDF{OOIJ/{111], sand an XoDF{ OOIJ/{ 111], I as a ratio of
an intensity of { 001} orientation to an intensity of { 111} orientation in ferrite in an
internal region satisfy the following Expression (1), and
the average grain size of the ferrite in the surface layer region is less than an
average grain size of the ferrite in the internal region,
-0.20 < XoDF{OOl}/{ 111 J, s - XoDF{OOl}/{111), I< 0.40 (1 ).
5. The steel sheet according to one of claims 1 to 4,
wherein a plating layer is provided on a surface.
6. A method for manufacturing a steel sheet comprising:
a heating process of heating a slab having the chemical composition according
to claim 1 to 1 000°C or higher;
a hot-rolling process of hot-rolling the slab such that a rolling finishing
- 51 -
temperature is 950°C or lower to obtain a hot-rolled steel sheet;
a stress application process of applying a stress to the hot-rolled steel sheet
after the hot-rolling process such that an absolute value of a residual stress as on a
surface is 100 MPa to 250 MPa;
a cold-rolling process of cold-rolling the hot-rolled steel sheet after the stress
application process such that a cumulative rolling reduction RCR is 70% to 90% to
obtain a cold-rolled steel sheet;
an annealing process of heating the cold-rolled steel sheet such that an
average heating rate in a range from 300°C to a soaking temperature Tl oc that satisfies
the following Expression (2) is 1.5 °C/sec to 10.0 °C/sec and holding the heated steel
sheet at the soaking temperature Tl °C for 30 seconds to 150 seconds for annealing;
and
a cooling process of cooling the cold-rolled steel sheet after the annealing
process to a temperature range of 55ooc to 65ooc such that an average cooling rate in
a range from the soaking temperature T1 octo 650°C is 1.0 °C/sec to 10.0 °C/sec and
cooling the cooled steel sheet to a temperature range of 200°C to 490°C such that the
average cooling rate is 5 °C/sec to 500 °C/sec,
Ac1 + 550- 25 x ln(a s) - 4.5 x RcR:::; T1 :::; Ac1 + 550- 25 x ln(os) - 4 x RCR
(2)
Ac1 in Expression (2) is represented by the following Expression (3),
an element symbol in the following Expression (3) represents an amount of
the corresponding element by mass%, and when the corresponding element is not
included, 0 is substituted into the corresponding element symbol, and
Ac1 = 723- 10.7 x Mn- 16.9 x Ni + 29.1 x Si + 16.9 x Cr (3).
7. The method for manufacturing a steel sheet according to claim 6,
- 52 -
or lower.
wherein the stress application process is performed at 40°C to 500°C.
8. The method for manufacturing a steel sheet according to claim 6 or 7,
wherein in the hot-rolling process, a fini sh rolling start temperature is 900°C
9. The method for manufacturing a steel sheet according to one of claims 6
to 8, the method further comprising:
a holding process of holding the cold-rolled steel sheet after the cooling
process in a temperature range of 200°C to 490°C for 30 seconds to 600 seconds.