[0001]The present invention relates to a high-strength steel sheet and a method for
manufacturing the same.
Priority is claimed on Japanese Patent Application No. 2019-055471, filed
March 22, 2019, the content of which is incorporated herein by reference.
[Related Art]
[0002]
In recent years, in order to protect the global environment, the fuel efficiency
of vehicles has needed to be improved. Regarding the improvement of fuel efficiency
of vehicles, high-strength steel sheets are being applied to steel sheets used for vehicle
components (steel sheets for vehicles), in order to reduce a weight of a vehicle body
while ensuring collision resistance, and the high-strength steel sheets are also being
developed for undercarriage compartments. It is necessary that steel sheets applied to
undercarriage compartments of vehicles have excellent fatigue resistance, in addition
to high tensile strength and high proof stress (high YP).
[0003]
For example, Patent Documents 1 and 2 disclose steel sheets which are highly
strengthened by annealing a hot-rolled steel sheet and performing skin pass rolling
before and after the annealing. In addition, Patent Documents 1 and 2 disclose that
these steel sheets are excellent in fatigue resistance.
However, for both the high-strength steel sheets disclosed in Patent
- 2 -
Documents 1 and 2, the tensile strength is less than 1,180 MPa.
In recent years, when further reducing weight of the vehicle, the tensile
strength of 1,180 MPa or more is required for a steel sheet for a vehicle, and such
demands cannot be answered in the technologies disclosed in Patent Documents 1 and
2.
[0004]
As described above, in the related art, a steel sheet having high tensile
strength of 1,180 MPa or more, high proof stress, and excellent fatigue resistance has
not been proposed.
[Prior Art Document]
[Patent Document]
[0005]
[Patent Document 1] PCT International Publication No. WO 2018/026013
[Patent Document 2] PCT International Publication No. WO 2010/137317
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0006]
The present invention has been made in view of the above problems. An
object of the present invention is to provide a high-strength steel sheet having high
proof stress, and excellent fatigue resistance and having tensile strength of 1,180 MPa
or more, suitable for undercarriage compartments of vehicles, and a method for
manufacturing the same.
[Means for Solving the Problem]
[0007]
The present inventors have intensively studied a method for solving the above
- 3 -
problems. As a result, it is found that, in a steel sheet having a predetermined
chemical composition, a microstructure is set as a structure in which the primary phase
is tempered martensite and the remainder consists of ferrite and bainite, the
microstructure contains 5.0 × 1011 pieces/mm3 or more of precipitate, per unit volume,
containing Ti and having an equivalent circle diameter of 5.0 nm or less, and Hvs/Hvc
which is a ratio of an average hardness Hvs at a position of a depth of 20 μm from a
surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from the surface is
0.85 or more, and accordingly, a steel sheet having high proof stress, and excellent
fatigue resistance, and tensile strength of 1,180 MPa or more can be manufactured.
In addition, the inventors found that in order to obtain such a steel sheet it is
effective that a slab to be subjected to hot rolling is heated to higher than 1,280°C to
dissolve Ti or Nb contained in a large amount, a coiling temperature after the hot
rolling is set to lower than 300°C to obtain 80% or more of martensite fraction,
precipitation of precipitate during the coiling after the hot rolling is suppressed, and the
hot-rolled steel sheet after the coiling is lightly reduced to introduce dislocation, the
dislocation is set as a nucleation site of the precipitate of Ti or Nb, and a heat treatment
is performed in a temperature range of 450°C to Ac1°C for a short period of time, and
thereby the precipitate containing fine Ti is precipitated by a predetermined amount or
more.
[0008]
The present invention has been made based on the above findings, and a gist
thereof is as below.
(1) A high-strength steel sheet according to one aspect of the present
invention, including, as a chemical composition, by mass%: C: 0.020 to 0.120%; Si:
0.01 to 2.00%; Mn: 1.00 to 3.00%; Ti: 0.010 to 0.200%; Nb: 0 to 0.100%; V: 0 to
- 4 -
0.200%; Al: 0.005 to 1.000%; P: 0.100% or less; S: 0.0100% or less; N: 0.0100% or
less; Ni: 0 to 2.00%; Cu: 0 to 2.00%; Cr: 0 to 2.00%; Mo: 0 to 2.00%; W: 0 to 0.100%;
B: 0 to 0.0100%; REM: 0 to 0.0300%; Ca: 0 to 0.0300%; Mg: 0 to 0.0300%; and a
remainder of Fe and impurities, in which 0.100 ≤ Ti + Nb + V ≤ 0.450 is satisfied, a
microstructure contains, by volume percentage, 80% or more of tempered martensite,
and a remainder consists of ferrite and bainite, the microstructure contains 5.0 × 1011
pieces/mm3 or more of, per unit volume, precipitate containing Ti and having an
equivalent circle diameter of 5.0 nm or less, Hvs/Hvc which is a ratio of an average
hardness Hvs at a position of a depth of 20 μm from a surface to an average hardness
Hvc at a position of 0.20 to 0.50 mm from the surface is 0.85 or more, and a tensile
strength is 1,180 MPa or more.
[0009]
(2) The high-strength steel sheet according to (1), may include, as the
chemical composition, by mass%, at least one or two or more selected from the group
consisting of: Ni: 0.01 to 2.00%; Cu: 0.01 to 2.00%; Cr: 0. 01 to 2.00%; Mo: 0.01 to
2.00%; W: 0.005 to 0.100%; B: 0.0005 to 0.0100%; REM: 0.0003 to 0.0300%; Ca:
0.0003 to 0.0300%; and Mg: 0.0003 to 0.0300%.
[0010]
(3) The high-strength steel sheet according to (1) or (2) may include a hot-dip
galvanized layer on the surface.
[0011]
(4) In the high-strength steel sheet according to (3), the hot-dip galvanized
layer may be a hot-dip galvannealed layer.
[0012]
(5) A method for manufacturing the high-strength steel sheet according to
- 5 -
another aspect of the present invention is a method for manufacturing the high-strength
steel sheet according to (1) or (2), the method including: a heating step of heating a
slab including, as a chemical composition, by mass%: C: 0.020 to 0.120%; Si: 0.01 to
2.00%; Mn: 1.00 to 3.00%; Ti: 0.010 to 0.200%; Nb: 0 to 0.100%; V: 0 to 0.200%; Al:
0.005 to 1.000%; P: 0.100% or less; S: 0.0100% or less; N: 0.0100% or less; Ni: 0 to
2.00%; Cu: 0 to 2.00%; Cr: 0 to 2.00%; Mo: 0 to 2.00%; W: 0 to 0.100%; B: 0 to
0.0100%; REM: 0 to 0.0300%; Ca: 0 to 0.0300%; Mg: 0 to 0.0300%; and a remainder
of Fe and impurities, to higher than 1,280°C; a hot rolling step of performing hot
rolling with respect to the slab such that a finish rolling temperature is 930°C or higher
to obtain a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet at a
temperature lower than 300°C and cooling the hot-rolled steel sheet to room
temperature; a pickling step of pickling the hot-rolled steel sheet after the coiling; a
light reduction step of performing light reduction with respect to the hot-rolled steel
sheet after the pickling step at rolling reduction of 1% to 30%; and a reheating step of
reheating the hot-rolled steel sheet after the light reduction step in a temperature range
of 450°C to Ac 1°C and holding for 10 to 1,500 seconds.
[0013]
(6) The method for manufacturing a high-strength steel sheet according to (5)
may further include a plating step of hot-dip galvanizing the hot-rolled steel sheet after
the reheating step.
[0014]
(7) The method for manufacturing a high-strength steel sheet according to (6)
may further include performing a galvannealing step of galvannealing by heating the
hot-rolled steel sheet after the hot-dip galvanizing step to 460°C to 600°C.
[Effects of the Invention]
- 6 -
[0015]
According to the above aspects of the present invention, it is possible to
provide a high-strength steel sheet having a tensile strength of 1,180 MPa or more,
which has high proof stress, and excellent fatigue resistance. This steel sheet has
great industrial value, because it contributes to weight reduction of vehicle components.
In addition, this steel sheet is suitable for undercarriage compartments of vehicles,
since it has high strength (high tensile strength), high proof stress, and excellent fatigue
resistance.
The high-strength steel sheet of the present invention includes a plated steel
sheet such as a high-strength hot-dip galvanized steel sheet and a high-strength
galvanized steel sheet including a galvanized layer on a surface.
[Brief Description of the Drawings]
[0016]
FIG. 1A is a diagram showing the number density of precipitate containing Ti
in each particle diameter for a comparative steel (steel number C9 of examples).
FIG. 1B is a diagram showing the number density of precipitate containing Ti
in each particle diameter for a steel of the present invention (steel number C5 of
examples).
FIG. 2 is an example showing the hardness distribution of a surface layer of a
steel sheet of the steel of the present invention (steel number C16 in the examples) and
the comparative steel (steel number 17 in the examples).
FIG. 3A is a diagram showing the relationship between the heat treatment
temperature and the number density of precipitate containing Ti and having an
equivalent circle diameter of 5.0 nm or less, of the steel having the chemical
composition C shown in Table 1 after light reduction and heat treatment.
- 7 -
FIG. 3B is a diagram showing the relationship between the heat treatment
temperature and tensile strength, of the steel having the chemical composition C shown
in Table 1 after light reduction and heat treatment.
FIG. 3C is a diagram showing the relationship between the heat treatment
temperature and the Hvs/Hvc (hardness ratio between surface layer to inside), of the
steel having the chemical composition C shown in Table 1 after light reduction and
heat treatment .
FIG. 3D is a diagram showing the relationship between the heat treatment
temperature and the fatigue limit/TS (ratio of fatigue limit to TS), of the steel having
the chemical composition C shown in Table 1after light reduction and heat treatment.
FIG. 4A is a diagram showing the relationship between rolling reduction
under light reduction and the number density of precipitate containing Ti and having
equivalent circle diameter of 5.0 nm or less, of the steel having the chemical
composition C shown in Table 1 after performing hot rolling under the conditions of
C5, C14 to C17, and the light reduction is applied.
FIG. 4B is a diagram showing the relationship between rolling reduction
under light reduction and tensile strength, of the steel of the chemical composition C
shown in Table 1 after performing hot rolling under the conditions of C5, C14 to C17,
and the light reduction is applied.
FIG. 4C is a diagram showing the relationship between rolling reduction
under light reduction and Hvs/Hvc (hardness ratio of surface layer to inside), of the
steel of the chemical composition C shown in Table 1 after performing hot rolling
under the conditions of C5, C14 to C17, and the light reduction is applied.
FIG. 4D is a diagram showing the relationship between rolling reduction
under light reduction and the fatigue limit/TS (ratio of fatigue limit to TS), of the steel
- 8 -
of the chemical composition C shown in Table 1 after performing hot rolling under the
conditions of C5, C14 to C17, and the light reduction is applied.
FIG. 5A is a diagram showing the relationship between the heat treatment
time and the number density of precipitate containing Ti and having an equivalent
circle diameter of 5.0 nm or less after heat treatment, of the steel having the chemical
composition C shown in Table 1 after light reduction and heat treatment.
FIG. 5B is a diagram showing the relationship between the heat treatment
time and the tensile strength after heat treatment, of the steel having the chemical
composition C shown in Table 1 after light reduction and heat treatment.
FIG. 5C is a diagram showing the relationship between the heat treatment
time and the Hvs/Hvc after heat treatment (hardness ratio of surface layer to inside), of
the steel having the chemical composition C shown in Table 1 after light reduction and
heat treatment.
FIG. 5D is a diagram showing the relationship between the heat treatment
time and the fatigue limit/TS (ratio of fatigue limit to TS) after heat treatment, of the
steel having the chemical composition C shown in Table 1 after light reduction and
heat treatment.
[Embodiments of the Invention]
[0017]
A high-strength steel sheet according to one embodiment of the present
invention (hereinafter, steel sheet according to the present embodiment) includes a
predetermined chemical composition, a microstructure contains, by volume percentage,
tempered martensite of 80% or more, a remainder consists of ferrite and bainite, the
tempered martensite contains 5.0 × 1011 pieces/mm3 or more of, per unit volume,
precipitate having an equivalent circle diameter of 5.0 nm or less and containing Ti,
- 9 -
and Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth of 20
μm from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from
the surface is 0.85 or more. In addition, the steel sheet according to the present
embodiment has a tensile strength of 1,180 MPa or more.
[0018]
Hereinafter, the steel sheet according to the present embodiment will be
described in detail.
[0019]

First, reasons for limiting the microstructure (metallographic structure) will be
described.
In the steel sheet according to the present embodiment, primary phases of the
microstructure are tempered martensite of 80% or more by volume percentage.
As will be described later, the steel sheet according to the present embodiment
is controlled so that the precipitate, having an equivalent circle diameter of 5.0 nm or
less and containing Ti, has a number density of 5.0 × 1011 pieces/mm3 or more, by
using hot rolling, subsequent dislocation introduction under light reduction and heat
treatment. Accordingly, it is necessary to set the primary phases of the microstructure
before the heat treatment, as martensite containing many dislocations as precipitation
sites of precipitate during the heat treatment. By performing the heat treatment with
respect to the martensite containing many dislocations, tempered martensite containing
fine precipitate becomes the primary phase.
In addition, since ferrite and bainite are formed at a high temperature, in a
case where these structures are formed, precipitate containing Ti precipitated therein
- 10 -
also tends to coarsen. In this case, it is not possible to ensure 5.0 × 1011 pieces/mm3
or more of the precipitate containing Ti and having an equivalent circle diameter of 5.0
nm or less. This also implies that, it is necessary that the microstructure contains, by
volume percentage, tempered martensite of 80% or more in total, and the remainder is
20% or less. Preferably, the volume percentage of tempered martensite is 90% or
more. In the present embodiment, the tempered martensite means martensite
containing precipitate containing cementite and/or Ti.
[0020]
For the microstructure, the steel sheet is cut out in parallel to a rolling
direction, polished and etched with a nital reagent so that a sheet thickness direction is
an observed section, and then a 1/4 position of the sheet thickness from the surface in
the sheet thickness direction is observed using a SEM at magnification of 1,000 to
30,000 times. Accordingly, ferrite, bainite, pearlite, and martensite can be identified.
That is, the determination can be performed based on microstructural morphology,
such that, the ferrite is an equiaxed grain that does not contain iron-based carbides, the
pearlite is a layered structure of ferrite and cementite, and the bainite is a lath-shaped
structure and is a structure containing cementite or residual austenite between laths.
The area ratio of each structure identified from the SEM observation image is obtained,
and this is defined as the volume percentage.
The martensite includes both tempered martensites containing carbide in lath
and as quenched martensite not containing carbide (fresh martensite), and these can be
identified by observing with a SEM and a TEM, and confirming the presence or
absence of carbide. In general, the tempered martensite often refers to those
containing iron-based carbides such as cementite, but in the present embodiment, the
martensite containing fine precipitate containing Ti is also defined as the tempered
- 11 -
martensite. Each volume percentage is obtained by observing 5 or more viewing
fields (for example, 5 to 10 viewing fields) at the above magnification and averaging
the fractions of each structure obtained in each viewing field.
[0021]

Next, a reason why the present inventors focused on a size and a number
density of the precipitate will be described. The present inventors conducted
intensive studies about the relationship between the size and the number density of
precipitate for ensuring a tensile strength of 1,180 MPa or more. As a result, it was
found that, the size (equivalent circle diameter) of the precipitate contained in the hotrolled
steel sheet of the related art and steel sheets in Patent Documents 1 and 2 could
not be controlled to 5.0 nm or less, and the number density was also small.
Accordingly, the tensile strength of 1,180 MPa or more cannot be ensured. As a
result of further studies by the present inventors, it is found that, the reason of this is
that, the number density of the precipitate having an equivalent circle diameter of 5.0
nm or less is less than 5.0 × 1011 pieces/mm3 in Patent Documents 1 and 2, since the
amount of Ti and the like forming the precipitate is small or it exists as coarse
precipitate at a stage of the slab and is not dissolved even during the slab heating even
when the Ti and the like are contained, and the TiC precipitated in the heat treatment
during a long period of time such as coiling after hot rolling are coarsened.
In the steel sheet according to the present embodiment, by setting the
tempered martensite containing precipitate containing Ti and having an equivalent
circle diameter of 5.0 nm or less in a number density of 5.0 × 1011 pieces/mm3 or more
- 12 -
as a primary phase, the tensile strength of 1,180 MPa or more can be ensured and the
fatigue resistance is also excellent.
[0022]
The reasons for limiting the size and number density of the precipitate will be
described.
The number density, per unit volume, of the precipitate containing Ti and
having an equivalent circle diameter of 5.0 nm or less is set to 5.0 × 1011 pieces/mm3
or more, in order to ensure the tensile strength of 1,180 MPa or more. In a case
where the number density is less than 5.0 × 1011 pieces/mm3, it is difficult to ensure the
tensile strength of 1,180 MPa or more. Therefore, the number density of precipitate
containing Ti and having an equivalent circle diameter of 5.0 nm or less needs to be
5.0 × 1011 pieces/mm3 or more.
The precipitate is set as the precipitate containing Ti, since a large amount of
the precipitate containing Ti is easily dissolved in the heating stage of the slab before
hot rolling, and the precipitate is precipitated as a fine precipitate having the equivalent
circle diameter of 5.0 nm or less. The type of precipitate such as carbide, nitride,
carbonitride, and the like is not limited, and particularly, the carbide is preferable,
since it is precipitated as a fine precipitate having a diameter of 5.0 nm or less and
contributes to improvement of strength. The precipitate of Ti is mainly contained in
the tempered martensite, which is the primary phase.
Although Nb has an effect similar to that of Ti, the amount of Nb carbide that
can be dissolved in the heating stage of the slab is small, and even in a case where Nb
is contained alone, the tensile strength of 1,180 MPa or more cannot be ensured. In
addition, although V can be dissolved in a large amount in the heating stage of the slab,
the size of the precipitate is relatively large, and even in a case where V is contained
- 13 -
alone, it is difficult to ensure 5.0 × 1011 pieces/mm3 or more of the precipitate having a
diameter of 5.0 nm or less. Accordingly, it is necessary to use the precipitate
containing Ti. However, as long as 5.0 × 1011 pieces/mm3 or more of the precipitate
having a diameter of 5.0 nm or less can be ensured, a composite precipitate ((Ti, Nb,
V) C, and the like) having a structure in which a part of Ti is substituted with Nb, V,
and/or Mo may be used.
The reason why the size of the precipitate is controlled to 5.0 nm or less in
terms of the equivalent circle diameter together with the number density described
above is to ensure the tensile strength of 1,180 MPa or more. For the precipitate
having the equivalent circle diameter more than 5.0 nm, the number density cannot be
set to 5.0 × 1011 pieces/mm3 or more, and the tensile strength of 1,180 MPa or more
cannot be ensured.
The equivalent circle diameter is a value in a case where observed shape of
the precipitate is assumed as a circle, and converted into a diameter of a circle whose
area is equivalent. Specifically, the precipitate of Ti may have a plate shape or needle
shape, in addition to the spherical shape. However, the area of the observed
precipitate is measured, the precipitate is assumed to be a circle, and a value converted
into a diameter of the circle whose area is equivalent is defined as the equivalent circle
diameter.
The steel sheet according to the present embodiment ensures the strength of
the steel sheet by utilizing precipitation hardening. Accordingly, the softening in a
heat-affected zone, which has been a problem during welding such as arc welding, can
be suppressed, and the fatigue strength of the weld is also excellent. In addition, the
steel sheet according to the present embodiment has increased strength due to the
precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less.
- 14 -
In such a case, a yield ratio (= YP/TS), which is a ratio of yield stress (YP) to tensile
strength (TS), is extremely high at 0.90 or more. By using the steel sheet according
to the present embodiment having a high yield ratio, it is possible to provide
undercarriage compartments for vehicles that are not easily deformed during riding on
a curb or colliding.
[0023]
Regarding the number density of the precipitate containing Ti, the number
density, per equivalent circle diameter at a pitch of 1.0 nm, of the precipitate contained
per unit volume of the steel sheet (for example, the number density of the equivalent
circle diameter more than 0 nm and 1.0 nm or less, the number density of the
equivalent circle diameter more than 1.0 nm and 2.0 nm or less, the number density of
the equivalent circle diameter more than 2.0 nm and 3.0 nm or less, and the like) is
measured using an electrolytic extraction residual method. The number density of the
precipitate is desirably measured at from a thickness position of 0.20 mm to 3/8 in a
depth direction from the surface where a typical structure of the steel sheet is obtained,
for example, from the position in the vicinity of 1/4 of the sheet thickness from the
surface. The sheet thickness center is not preferable as a measurement position,
because coarse carbides may be present due to the influence of center segregation and
a local chemical composition differs due to the influence of segregation. The position
less than 0.20 mm in the depth direction from the surface is not preferable as the
measurement position, because it is affected by high-density dislocation introduced
under light reduction or decarburization during the heating, and the number density of
carbides may differ from the inside.
In the measurement, composition analysis of a carbide is performed with a
transmission electron microscope (TEM) and an EDS, and it may be confirmed that
- 15 -
fine precipitate is the precipitate containing Ti. Specifically, the steel sheet is
polished from the surface to a 1/4 position of the sheet thickness, about 1 g of the steel
sheet is dissolved according to the electrolytic extraction residual method, the obtained
solution containing Ti precipitate is filtered with a filter paper, and the obtained
precipitate is attached to C replica and observed with the TEM. In the observation,
the magnification is set to 50,000 to 100,000 times, the viewing field is set to 20 to 30,
and the chemical composition of the obtained precipitate is specified with the EDS.
Then, the image obtained by the TEM observation is subjected to image analysis, and
the equivalent circle diameter and the number density of each precipitate are calculated.
The lower limit of the size of the precipitate which is a measurement target is
not particularly determined, and the effect can be obtained by setting the number
density of the precipitate having the equivalent circle diameter of 5.0 nm or less to 5.0
× 1011 pieces/mm3 or more per unit volume. However, in the hot-rolled steel sheet
according to the present embodiment, since it is considered that the amount of the
precipitate having the equivalent circle diameter less than 0.4 nm is small, the
precipitate having the equivalent circle diameter of 0.4 nm or more may be set as a
substantial target.
[0024]

In the steel sheet according to the present embodiment, it is necessary to set
Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth of 20 μm
from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from the
surface (in the sheet thickness direction, range from the position of 0.20 mm from the
- 16 -
surface to the position of 0.50 mm from the surface) to 0.85 or more.
The ratio Hvs/Hvc of the average hardness Hvs at the position of 20 μm in the
sheet thickness direction from the surface to the average hardness Hvc at the position
of 0.20 to 0.50 mm in the sheet thickness direction from the surface is set to 0.85 or
more, in order to increase the Hvs/Hvc and largely improve fatigue resistance.
In general, fatigue fracture occurs from the surface. Accordingly, it is
effective to harden a surface layer, in order to suppress occurrence of fatigue crack.
Meanwhile, the hot-rolled steel sheet is exposed to an oxidizing atmosphere during
slab heating and hot rolling. Accordingly, decarburization and the like easily occur
and the surface layer hardness is easily reduced. In a case where the surface layer
hardness is reduced, the fatigue resistance is deteriorated.
As a result of intensive studies by the present inventors, it is found that, by
combining the light reduction and subsequent heat treatment, the surface layer can be
preferentially hardened, thereby improving fatigue resistance.
The hardness at the position of 20 μm in the depth direction from the surface
(sheet thickness direction) is defined as the hardness of the surface layer, because the
fatigue resistance can be improved by increasing the hardness at this position. In
addition, it is because that, it is difficult to measure the hardness at a position less than
20 μm from the surface, because it is affected by the surface, and on the other hand, the
increase in hardness at the position inside the steel sheet than at the position 20 μm
from the surface has slight correlation with the fatigue resistance.
The average hardness Hvc at the position of 0.20 to 0.50 mm from the surface
is set as an average hardness in this range. The hardness of the sheet thickness center
may not be stable due to the influence of segregation such as Mn. Therefore, it is
desirable to avoid the measurement of the hardness of the sheet thickness center, that is,
- 17 -
a segregation portion.
The reason why Hvs/Hvc is set to 0.85 or more is that a great effect of
improving the fatigue resistance is exhibited by setting the hardness ratio to 0.85 or
more. Since this effect is more greatly exhibited at 0.87 or more, Hvs/Hvc is
preferably 0.87 or more. Hvs/Hvc is more preferably 0.90 or more.
[0025]
The average hardness Hvs at the position of a depth of 20 μm from the surface
and the average hardness Hvc at the position of 0.20 to 0.50 mm from the surface are
obtained by the following method.
For the average hardness Hvs at the position of a depth of 20 μm from the
surface, a sample is cut out from the 1/4 position in the width direction of the steel
sheet so that a cross section parallel to the rolling direction is a measurement surface,
embedding polishing is performed, a Vickers hardness at the position of 20 μm from
the surface is measured at 10 points with a load of 10 gf based on JIS Z 2244: 2009,
and an average value thereof is set as Hvs. For the Hvc, a sample is cut out from the
1/4 position in the width direction of the steel sheet so that a cross section parallel to
the rolling direction is a measurement surface, embedding polishing is performed, a
Vickers hardness at a pitch of about 0.05 mm in the sheet thickness direction from the
position of 0.20 to 0.50 mm from the surface is measured at 7 points with a load of 10
gf (for example, measured at the positions of 0.20 mm, 0.25 mm, 0.30 mm, 0.35 mm,
0.40 mm, 0.45 mm, and 0.50 mm from the surface), and an average value thereof is set
as Hvc.
[0026]

The steel sheet according to the present embodiment has a tensile strength of
- 18 -
1,180 MPa or more, in consideration of the strength required when further reducing the
weight of vehicle.
[0027]
The tensile strength (TS) was obtained by a tensile test performed based on
JIS Z 2241:2011 using JIS No. 5 test piece cut out in the direction perpendicular to the
rolling direction.
[0028]
The sheet thickness of the steel sheet according to the present embodiment is
not particularly limited, but is, for example, 1.0 to 4.0 mm, in consideration of
manufacturing stability and the like. It is preferably 1.5 to 3.0 mm.
[0029]
Next, the reason for limiting the chemical composition of the steel sheet
according to the present embodiment will be described. % of the content indicates
mass%.
[0030]
C: 0.020 to 0.120%;
C is an element effective for increasing the strength of the steel sheet. In
addition, C is an element that forms carbide containing Ti. In a case where the C
content is less than 0.020%, it is not possible to ensure the number density of the
carbide of 5.0 × 1011 pieces/mm3 or more. Therefore, the C content is set to 0.020%
or more.
On the other hand, in a case where the C content exceeds 0.120%, not only the
effect is saturated, but also the carbide is difficult to be dissolved during the slab
heating. Therefore, the C content is set to 0.120% or less. It is preferably 0.090% or
less.
- 19 -
[0031]
Si: 0.01 to 2.00%
Si is an element that contributes to high-strengthening of a steel sheet by solid
solution strengthening. Therefore, the Si content is set to 0.01% or more.
On the other hand, in a case where the Si content exceeds 2.00%, not only the
effect is saturated, but also strong scale is generated on the hot-rolled steel sheet, and
the external appearance and pickling property are deteriorated. Therefore, the Si
content is set to 2.00% or less.
[0032]
Mn: 1.00 to 3.00%
Mn is an element effective for increasing the volume percentage of tempered
martensite in the microstructure of the steel sheet and increasing the strength of the
steel sheet. In order to set the volume percentage of tempered martensite to 80% or
more, the Mn content is set to 1.00% or more. In a case where the Mn content is less
than 1.00%, the volume percentage of tempered martensite decreases, and sufficient
strengthening cannot be performed.
On the other hand, in a case where the Mn content exceeds 3.00%, the effect
is saturated and the economic efficiency is lowered. Therefore, the Mn content is set
to 3.00% or less.
[0033]
Al: 0.005 to 1.000%
Al is an element effective for microstructure control by hot rolling and
deoxidation. In order to obtain these effects, the Al content is set to 0.005% or more.
In a case where the Al content is less than 0.005%, a sufficient deoxidation effect
cannot be obtained, and a large amount of inclusions (oxide) is formed in the steel
- 20 -
sheet.
On the other hand, in a case where the Al content exceeds 1.000%, the slab is
embrittled, which is not preferable. Therefore, the Al content is set to 1.000% or less.
[0034]
Ti: 0.010 to 0.200%
Nb: 0 to 0.100%
V: 0 to 0.200%;
0.100 ≤ Ti + Nb + V ≤ 0.450 (Ti, Nb, and V represent the Ti content, the Nb
content, the V content in mass%, respectively)
Ti, Nb, and V are elements that form precipitates (carbide, nitride,
carbonitride, and the like) by bonding with C or N, and contribute to improvement of
steel sheet strength through precipitation hardening by these precipitates. In order to
obtain 5.0 × 1011 pieces/mm3 or more of fine precipitate containing Ti and having the
equivalent circle diameter of 5.0 nm or less through the manufacturing method which
will be described later, the total content of Ti, Nb, and V (Ti + Nb + V) is set to
0.100% or more, while setting the Ti content to 0.010% or more. The total amount of
Ti, Nb, and V is desirably 0.130% or more, and more desirably 0.150% or more.
On the other hand, containing excessive content of total amount of Ti, Nb, and
V (Ti + Nb + V) which exceeds 0.450%, the slab or the steel sheet is embrittled, which
causes troubles during manufacturing. Therefore, the total amount of Ti, Nb, and V is
set to 0.450% or less.
In addition, the upper limit of the Ti content is set to 0.200%, the upper limit
of the Nb content is set to 0.100%, and the upper limit of the V content is set to
0.200%, because, in a case where the amounts thereof exceed these upper limits, it is
difficult to dissolve the coarse precipitates precipitated at a casting stage, even in a
- 21 -
case where a lower limit of a slab heating temperature is set to higher than 1,280°C.
Further, the excessive content of Ti, Nb, and V causes embrittlement of the slab and
the steel sheet. Therefore, it is desirable that Ti content has an upper limit of 0.200%,
Nb content has an upper limit of 0.100%, and V content has an upper limit of 0.200%.
Any combination of Ti, Nb, and V may be used for ensuring 5.0 × 1011
pieces/mm3 or more of fine carbide containing Ti and having the equivalent circle
diameter of 5.0 nm or less, however, in order to dissolve the carbide during the heating
of the hot-rolled slab, the content of Ti, which is easy to contain in a larger amount and
is inexpensive, is at least 0.010% or more.
[0035]
P: 0.100% or less
P is an element that segregates in a sheet thickness center portion of the steel
sheet and is also an element that embrittles the weld. In a case where the P content
exceeds 0.100%, the characteristics are greatly deteriorated. Therefore, the P content
is set to 0.100% or less. It is preferably 0.050% or less. It is preferable that the P
content is low, and the effect is exhibited without particularly determining the lower
limit (may be 0%), but reducing the P content to less than 0.001% is economically
disadvantageous. Therefore, the lower limit of the P content may be 0.001%.
[0036]
S: 0.0100% or less
S is an element that causes slab embrittlement by being present as a sulfide.
In addition, S is an element that deteriorates formability of the steel sheet. Therefore,
the S content is limited. In a case where the S content exceeds 0.0100%, the
characteristics are greatly deteriorated. Therefore, the S content is set to 0.0100% or
less. On the other hand, the effect is exhibited without particularly determining the
- 22 -
lower limit (may be 0%), but reducing the S content to less than 0.0001% is
economically disadvantageous. Therefore, the lower limit of the S content may be
0.0001%.
[0037]
N: 0.0100% or less
N is an element that forms coarse nitride and deteriorates bendability and hole
expansibility. In a case where the N content exceeds 0.0100%, the bendability and
the hole expansibility are greatly deteriorated. Therefore, the N content is set to
0.0100% or less. In addition, N becomes coarse TiN by bonding with Ti, and in a
case where a large amount of N is contained, the number density of precipitate
containing Ti and having the equivalent circle diameter of 5.0 nm or less is less than
5.0 × 1011 pieces/mm3. Therefore, it is preferable that the N content is low.
On the other hand, it is not necessary to particularly determine the lower limit
of the N content (may be 0%), but in a case where the N content is reduced to less than
0.0001%, the manufacturing cost increases greatly. Therefore, the substantial lower
limit of the N content is 0.0001%. From a viewpoint of manufacturing cost, the N
content may be 0.0005% or more.
[0038]
The above elements are the basic chemical composition of the steel sheet
according to the present embodiment, the chemical composition of the steel sheet
according to the present embodiment contains the above elements, the remainder may
consist of Fe and impurities. However, for the purpose of improving various
properties, the following compositions can be further contained. Since the following
elements do not necessarily have to be contained, the lower limit of the content is 0%.
[0039]
- 23 -
Ni: 0 to 2.00%
Cu: 0 to 2.00%
Cr: 0 to 2.00%
Mo: 0 to 2.00%
Ni, Cu, Cr, and Mo are elements that contribute to the high-strengthening of
the steel sheet through microstructure control by hot rolling. When obtaining this
effect, the effect is exhibited greatly, by containing one or two or more of Ni, Cu, Cr,
and Mo in an amount of 0.01% or more, respectively. Therefore, when obtaining the
effect, each content is preferably 0.01% or more.
On the other hand, in a case where the content of each element exceeds 2.00%,
weldability, hot workability, and the like are deteriorated. Therefore, even when
these are contained, the upper limit of each content of Ni, Cu, Cr, and Mo is set to
2.00%.
[0040]
W: 0 to 0.100%
W is an element that contributes to the improvement of the strength of the
steel sheet through precipitation hardening. When obtaining this effect, the W
content is preferably set to 0.005% or more.
On the other hand, in a case where the W content exceeds 0.100%, not only
the effect is saturated but also the hot workability is deteriorated. Therefore, even
when this is contained, the W content is set to 0.100% or less.
[0041]
B: 0 to 0.0100%
B is an element effective for controlling the transformation during hot rolling
and improving the strength of the steel sheet through the structure strengthening.
- 24 -
When obtaining this effect, the B content is preferably set to 0.0005% or more.
On the other hand, in a case where the B content exceeds 0.0100%, not only
the effect is saturated, but also iron-based boride is precipitated, and an effect of
improving hardenability by a solid solution B is lost. Therefore, the B content is
preferably set to 0.0100% or less. The B content is more preferably 0.0080% or less,
and even more preferably 0.0050% or less.
[0042]
REM: 0 to 0.0300%
Ca: 0 to 0.0300%
Mg: 0 to 0.0300%
REM, Ca, and Mg are elements that affect the strength of the steel sheet and
contribute to improvement of a material properties. In a case where a total of one or
two or more of REM, Ca, and Mg is less than 0.0003%, a sufficient effect cannot be
obtained. Therefore, when obtaining the effect, the total content of REM, Ca, and Mg
is preferably set to 0.0003% or more.
On the other hand, in a case where each content of REM, Ca, and Mg exceeds
0.0300%, castability or hot workability is deteriorated. Therefore, even when these
are contained, the amount of each is set to 0.0300% or less.
In the present embodiment, REM is an abbreviation for Rare Earth Metal and
refers to an element belonging to the lanthanoid series, and the REM content is the
total amount of these elements. REM is often added as mischmetal, and in addition to
Ce, REM may contain elements of the lanthanoid series in a complex manner. Even
in a case where the steel sheet according to the present embodiment contains elements
of the lanthanoid series other than La or Ce as impurities, the effect is exhibited. In
addition, although a metal is added, the effect is exhibited.
- 25 -
[0043]
As described above, the steel sheet according to the present embodiment
contains basic elements, contains optional elements, as necessary, and the remainder is
Fe and impurities. The impurities refer to compositions that are unintentionally
contained from a raw material in the manufacturing process of the steel sheet, or in
other manufacturing steps. For example, as the impurities, O may be contained in a
trace amount, in addition to P, S, and N. O may form oxide and may be present as
inclusions.
[0044]
The steel sheet according to the present embodiment may further include a
hot-dip galvanizing on its surface. In addition, the hot-dip galvanized layer may be
hot-dip galvannealed layer subjected to a galvannealing treatment.
Since the galvanizing contributes to the improvement of corrosion resistance,
it is desirable to use a galvanized hot-dip galvanized steel sheet or hot-dip
galvannealed steel sheet, in a case where the steel sheet is applied for the usage where
corrosion resistance is expected.
Since there is a concern that the undercarriage compartments of a vehicle may
be pitted due to corrosion, it may not be possible to thin the undercarriage
compartments a certain sheet thickness or less, even in a case where the highstrengthening
is performed. One object of high-strengthening of the steel sheet is
reducing weight by thinning. Accordingly, although the high-strength steel sheet is
developed, the application site is limited, in a case where the corrosion resistance is
low. As a method for solving these problems, it is considered that the steel sheet is
subjected to plating such as hot-dip galvanizing with high corrosion resistance. Since
the steel sheet compositions are controlled as described above, the steel sheet
- 26 -
according to the present embodiment can be subjected to the hot-dip galvanizing.
The plating layer may be electrogalvanized layer, or may be a plating
containing Al and/or Mg, in addition to Zn.
[0045]
Next, a preferable method for manufacturing the steel sheet according to the
present embodiment will be described. The effect can be obtained, as long as the
steel sheet according to the present embodiment has the above-mentioned
characteristics, regardless of the manufacturing method. However, the following
method is preferable, because it can be stably manufactured.
[0046]
Specifically, the steel sheet according to the present embodiment can be
manufactured by a manufacturing method including the following steps (I) to (VI).
(I) A heating step of heating a slab having a predetermined chemical
composition to higher than 1,280°C
(II) A hot rolling step of performing hot rolling with respect to the slab so that
a finish rolling temperature is 930°C or higher to obtain a hot-rolled steel sheet
(III) A coiling step of coiling the hot-rolled steel sheet at lower than 300°C
and cooling it to room temperature
(IV) A pickling step of pickling the hot-rolled steel sheet after the coiling step
(V) A light reduction step of performing reduction the hot-rolled steel sheet
after the pickling step with a rolling reduction of 1% to 30%.
(VI) A reheating step of reheating the hot-rolled steel sheet after the light
reduction step to a temperature range of 450°C to Ac1°C and holding it for 10 to 1500
seconds
Hereinafter, preferable conditions for each step will be described.
- 27 -
[0047]

In the heating step, the slab having the above-mentioned chemical
composition to be subjected to the hot rolling step is heated to higher than 1,280°C.
The reason for setting the heating temperature to higher than 1,280°C is for dissolving
elements such as Ti, Nb, and V contained in the slab that contribute to precipitation
hardening (in many cases, they are present as coarse precipitate of more than 5.0 nm in
the slab), and precipitating 5.0 × 1011 pieces/mm3 or more of the precipitate containing
Ti and having the equivalent circle diameter of 5.0 nm or less in the subsequent heat
treatment step. In order to ensure the precipitate having a predetermined number
density, it is necessary to use a large amount of Ti, Nb, and V. Accordingly, it is
necessary to heat the slab at a temperature equal to or higher than those in the
invention of the related art (Patent Documents 1 and 2). In a case where the heating
temperature is 1,280°C or lower, Ti, Nb, and V are not sufficiently dissolved.
The upper limit of the heating temperature is not particularly limited, but in a
case where it exceeds 1,400°C, not only the effect is saturated, but also the scale
formed on the slab surface is dissolved, and the dissolved oxide damages a refractory
in a heating furnace, which is not preferable. Therefore, the heating temperature is
preferably 1,400°C or lower.
[0048]

The hot rolling is performed with respect to the heated slab. In the hot
rolling, rough rolling is performed as necessary, and then finish rolling is performed.
A finish rolling temperature (finish rolling completion temperature) is set to 930°C or
higher.
- 28 -
Since the steel sheet according to the present embodiment contains a large
amount of Ti, Nb, and V, in a case where the temperature of the slab or the roughly
rolled hot-rolled steel sheet before the finish rolling is decreased, precipitate containing
Ti is formed. The carbide containing Ti which is precipitated at this stage has a large
size. Accordingly, it is necessary to carry out the finish rolling and the coiling while
suppressing the precipitate containing Ti before the finish rolling. In a case where the
finish rolling temperature is lower than 930°C, the formation of precipitate containing
Ti is remarkable. Accordingly, the finish rolling temperature is set to 930°C or
higher. It is not necessary to particularly limit the upper limit of the finish rolling
temperature.
[0049]

The steel sheet after the hot rolling step (hot-rolled steel sheet) is cooled and
then coiled. A coiling temperature of the hot-rolled steel sheet is set to lower than
300°C, and after the coiling, the hot-rolled steel sheet is cooled to room temperature in
a state of a coil.
Any method can be used for cooling to the coiling temperature, as long as it
can be cooled, but a method for cooling using water from a nozzle is generally used,
and productivity is also excellent. In addition, the steel sheet according to the present
embodiment needs to have martensite as the primary phase, and it is necessary to
suppress the formation of ferrite, pearlite, and bainite structures before coiling.
Accordingly, it is desirable to perform water cooling with a high cooling rate. A
cooling rate for water cooling is, for example, 20°C/sec or higher. In addition, since
the temperature lower than 300°C is lower than the martensitic transformation start
temperature, ferrite, pearlite, and bainite are hardly generated, in a case where water
- 29 -
cooling is performed to this temperature range.
In a case where the coiling temperature is 300°C or higher, bainite is formed
and the volume percentage of martensite is less than 80%. In addition, during coiling,
precipitate containing Ti is formed in the ferrite and bainite structures, and the
precipitate is held at a high temperature for a long time. Accordingly, the number of
precipitates having the equivalent circle diameter of more than 5.0 nm increases due to
grain growth. As a result, the number density of fine precipitate is less than 5.0 ×
1011 pieces/mm3, even in a case where reduction and heat treatment are performed in a
subsequent step. Therefore, the coiling temperature is set to less than 300°C.
The martensite after the coiling step may be either as-quenched martensite
(fresh martensite) containing almost no iron-based carbide, or auto-tempered
martensite in which iron-based carbide is precipitated in the martensite, in a case where
it is cooled to room temperature after coiling.
The cooling conditions during cooling the coil coiled at lower than 300°C to
room temperature are not necessary to be particularly limited, but for example, the coil
may be left to cool to room temperature.
[0050]

The hot-rolled steel sheet after the coiling step is pickled. By performing the
pickling, it is possible to improve the plating property in the subsequent manufacturing
step and increase the chemical convertibility in the vehicle manufacturing step. In
addition, in a case where the hot-rolled steel sheet with a scale is lightly reduced, the
scale is peeled off and is pushed in, which may cause a defect. Therefore, the hotrolled
steel sheet is first pickled before the light reduction.
The pickling conditions are not particularly limited, but the pickling is
- 30 -
generally performed with hydrochloric acid or sulfuric acid containing an inhibitor.
[0051]

In the light reduction step, reduction is applied to the hot-rolled steel sheet
after the pickling step at rolling reduction of 1% to 30%.
By perfoming the reduction to the hot-rolled steel sheet, a precipitation site
for precipitation of the precipitate in the heat treatment in the subsequent step is
introduced. By introducing the precipitation site, the fine carbide containing Ti and
having the equivalent circle diameter of 5.0 nm or less can be precipitated by 5.0 ×
1011 pieces/mm3 or more by the heat treatment. In addition, as shown in FIGS. 4A to
4D, the TS, the Hvs/Hvc, and fatigue limit can be increased by setting the rolling
reduction to 1% or more. Therefore, the reduction at rolling reduction of 1% or more
is applied.
Meanwhile, in a case where the rolling reduction exceeds 30%, not only the
effect is saturated, but also the recovery of the introduced dislocation becomes
insufficient, resulting in a great deterioration in elongation. In addition, in the
reheating step, which is a subsequent step, recrystallization may occur depending on
the heating temperature and heating time, and the consistency between the Ti
precipitate and a primary phase (here, recrystallized ferrite) is lost, and the amount of
precipitation strengthening is reduced. In this case, it is difficult to ensure a tensile
strength of 1,180 MPa or more. Therefore, the rolling reduction is set to 30% or less.
The rolling reduction is preferably less than 20%, more preferably 15% or less, and
even more preferably less than 15%.
As long as the dislocation that acts as a nucleation site of the precipitate can
be introduced, the reduction may be performed by applying the reduction pf 1% to
- 31 -
30% in one pass, or by dividing into a plurality of times and the reduction may be
performed so that the cumulative rolling reduction is 1% and 30%.
In the method for manufacturing the steel sheet according to the present
embodiment, the light reduction step is the most important step and is a step with a role
different from a so-called cold rolling. That is, the cold rolling is usually performed
for controlling the sheet thickness of the steel sheet, controlling a texture or controlling
the grain diameter by using recrystallization. However, the light reduction step in the
present embodiment is performed, in order to promote the fine carbide precipitation
due to the introduction of dislocation, as described above.
[0052]

The hot-rolled steel sheet after the light reduction step is subjected to the heat
treatment in which the hot-rolled steel sheet is reheated to a temperature range of
450°C to Ac1°C, and held at this temperature range for 10 to 1,500 seconds. By
performing the heat treatment by reheating the hot-rolled steel sheet after the light
reduction step, the precipitate containing Ti and having the equivalent circle diameter
of 5.0 nm or less can be precipitated by 5.0 × 1011 pieces/mm3 or more. In a case
where the heat treatment temperature (reheating temperature) in the reheating step is
lower than 450°C, the diffusion of atoms is insufficient and a sufficient amount of the
precipitate cannot be obtained. Considering the heat treatment in a short period of
time, the heat treatment temperature is desirably 500°C or higher. In a case where the
heat treatment temperature exceeds Ac1°C, the precipitate is coarsened and austenite
formed in the heat treatment transform into ferrite or bainite during the cooling.
Accordingly, the volume percentage of tempered martensite may not be 80% or more,
and a consistent relationship between the Ti precipitate and the primary phase (here,
- 32 -
martensite transformed from the austenite in the cooling process) may collapse due to
the transformation to the austenite, and the amount of precipitation hardening is
reduced. As a result, it is difficult to ensure the tensile strength of 1,180 MPa or more,
although the number density of the precipitate is within the above range. Therefore,
the heat treatment temperature is set to Ac1°C or lower and desirably 700°C or lower.
Ac1 (Ac1 transformation point) (°C) can be specified by measuring the expansion
curve during the heating. Specifically, the Ac1 transformation point can be specified
by measuring the transformation curve during the heating at 5°C/sec.
FIGS. 1A and 1B are diagrams showing number density of the precipitate
containing Ti for each particle diameter (equivalent circle diameter) of steel number
C9 (without reheating) and steel number C5 (reheated to 640°C) in the examples.
As shown in FIG. 1B, by performing suitable reheating (heat treatment) after
light reduction, it is found that, the number density (number density on the left side of
a broken line in the drawing) of the precipitate containing Ti and having the particle
diameter (equivalent circle diameter) of 5.0 nm or less increases.
In addition, as shown in FIGS. 3A to 3D, by setting the reheating temperature
(heat treatment temperature) to 450°C to Ac1°C, the number density of the precipitate
containing Ti and having the particle diameter (equivalent circle diameter) of 5.0 nm or
less, TS, Hvs/Hvc, and the fatigue limit after heat treatment and light reduction
increase.
In a case where the heat treatment time (holding time) in the reheating step is
shorter than 10 seconds, the diffusion of atoms is insufficient, and the precipitate
containing Ti and having the equivalent circle diameter of 5.0 nm or less cannot be
precipitated by 5.0 × 1011 pieces/mm3 or more. In a case where the heat treatment
time exceeds 1,500 seconds, the precipitate becomes coarse, and the number of
- 33 -
precipitates containing Ti and having the equivalent circle diameter of 5.0 nm or less is
less than 5.0 × 1011 pieces/mm3. For this reason, it is necessary to set the heat
treatment time to 10 and 1,500 seconds. The heat treatment in the temperature range
of 450°C to Ac1°C also includes heating and slow cooling in this temperature range.
That is, the heat treatment time means the time during which the steel sheet is in the
temperature range of 450°C to Ac1°C after the reheating, and in a case where the steel
sheet is held in this temperature range for a predetermined time, the temperature may
change in the middle.
As shown in FIGS. 5A to 5D, by setting the heat treatment time in a range of
10 to 1,500 seconds, the number density, TS, Hvs/Hvc, and the fatigue limit of the
precipitate containing Ti and having the particle diameter (equivalent circle diameter)
of 5.0 nm or less after heat treatment and light reduction increase.
In addition, as shown in FIG. 2, the implementation of light reduction and
reheating preferentially increases the surface layer hardness.
The cooling after the holding step is not particularly limited.
[0053]
The steel sheet according to the present embodiment can be obtained by the
manufacturing method including the above steps. However, in a case where the steel
sheet according to the present embodiment is a hot-dip galvanized steel sheet or a hotdip
galvannealed steel sheet, in order to improve corrosion resistance, it is preferable
that the following steps are further included.
[0054]

The hot-rolled steel sheet after the reheating step is subjected to hot-dip
galvanizing. Since the galvanizing contributes to the improvement of corrosion
- 34 -
resistance, it is desirable to perform galvanizing, in a case where the steel sheet is
applied for the usage where corrosion resistance is expected. The galvanizing is
preferably hot-dip galvanizing. The conditions for hot-dip galvanizing are not
particularly limited, and well-known conditions may be used.
When the hot-rolled steel sheet after hot-dip galvanizing (hot-dip galvanized
steel sheet) is heated to 460°C to 600°C to galvannealing plating, it is possible to
manufacture a hot-dip galvannealed steel sheet in which a hot-dip galvanized layer is a
hot-dip galvannealed layer. The hot-dip galvannealed steel sheet may be subjected to
galvannealing in the galvannealing step according to the usage, since the effect of
improving spot weldability or improving sliding ability during drawing can be applied,
in addition to the improvement of corrosion resistance.
Even in a case where Al plating, plating containing Mg, and electroplating are
performed, instead of galvanizing, it is possible to manufacture the steel sheet
according to the present embodiment having the tensile strength of 1,180 MPa or more
and excellent fatigue resistance.
[Examples]
[0055]
Steels having chemical compositions shown in kinds of steel A to P and a to f
in Table 1 were dissolved, and a slab having a thickness of 240 to 300 mm was
manufactured by continuous casting.
The obtained slab was heated and subjected to finish rolling under conditions
shown in Tables 2-1 and 2-2 to obtain a hot-rolled steel sheet having a thickness of 2.3
mm, and the hot-rolled steel sheet was subjected to water cooling to the coiling
temperature, coiled as a coil, and air-cooled to room temperature.
After the coil was uncoiled, the pickling was performed, and the hot-rolled
- 35 -
steel sheet after the pickling was lightly reduced at the rolling reductions shown in
Tables 2-1 and 2-2. However, in Tables 2-1 and 2-2, the light reduction was not
performed for examples in which the rolling reduction was 0%.
The hot-rolled steel sheet after the light reduction (hot-rolled steel sheet after
the pickling, in a case where the light reduction is not performed) was subjected to heat
treatment by reheating at a temperature shown in Tables 2-1 and 2-2, to manufacture
hot-rolled steel sheets having steel numbers A1 to f1.
With respect to the hot-rolled steel sheet after the heat treatment, the plating
was performed, as necessary, and in some examples, further galvannealing treatment
was performed. In Tables 2-1 and 2-2, HR indicates a hot-rolled steel sheet not
subjected to plating, GI indicates a hot-dip galvanized steel sheet, and GA indicates a
hot-dip galvannealed steel sheet.
- 36 -
[0056]
[Table 1]
- 37 -
Kind
of steel
Mass% Remainder Fe and impurities Ti + Nb +
V
(%)
Ac1
(°C)
Note
C Si Mn Ti Nb V Al P S N Ni Cu Cr Mo W B REM Ca Mg
A 0.08 0.78 2.56 0.172 0.00 0.00 0.026 0.007 0.0016 0.0019 0.172 718
Steel of present
invention
B 0.07 0.47 2.19 0.155 0.00 0.00 0.029 0.009 0.0021 0.0024 0.0019 0.155 713
Steel of present
invention
C 0.08 1.20 2.49 0.161 0.05 0.00 0.029 0.009 0.0021 0.0024 0.211 726
Steel of present
invention
D 0.07 0.65 1.97 0.142 0.05 0.00 0.013 0.007 0.0019 0.0021 0.0034 0.192 716
Steel of present
invention
E 0.09 0.29 2.21 0.131 0.03 0.09 0.026 0.006 0.0005 0.0046 0.251 710
Steel of present
invention
F 0.08 0.42 1.84 0.156 0.03 0.00 0.045 0.009 0.0016 0.0027 0.23 0.186 712
Steel of present
invention
G 0.07 0.39 1.81 0.154 0.02 0.00 0.026 0.008 0.0021 0.0023 0.78 0.174 712
Steel of present
invention
H 0.08 0.45 1.88 0.161 0.03 0.00 0.024 0.007 0.0016 0.0016 0.19 0.11 0.191 713
Steel of present
invention
I 0.08 0.65 1.54 0.149 0.05 0.13 0.008 0.005 0.0020 0.0009 0.73 0.0024 0.329 716
Steel of present
invention
J 0.09 0.03 2.09 0.159 0.03 0.00 0.293 0.009 0.0014 0.0049 0.0016 0.189 705
Steel of present
invention
K 0.07 0.49 2.29 0.161 0.02 0.04 0.029 0.005 0.0009 0.0028 0.029 0.221 714
Steel of present
invention
L 0.08 0.54 2.34 0.155 0.03 0.00 0.016 0.011 0.0015 0.0023 0.0013 0.185 714
Steel of present
invention
M 0.08 0.57 2.41 0.152 0.03 0.00 0.024 0.008 0.0023 0.0021 0.0016 0.182 715
Steel of present
invention
N 0.11 0.43 2.35 0.156 0.09 0.15 0.021 0.006 0.0017 0.0020 0.396 712
Steel of present
invention
O 0.07 0.46 2.43 0.159 0.04 0.00 0.028 0.003 0.0014 0.0019 0.0011 0.199 713
Steel of present
invention
P 0.11 0.99 2.03 0.164 0.06 0.16 0.036 0.010 0.0030 0.0024 0.384 722
Steel of present
invention
a 0.01 1.56 2.98 0.162 0.03 0.00 0.019 0.006 0.0019 0.0025 0.192 734 Comparative steel
b 0.16 1.19 2.08 0.148 0.00 0.00 0.023 0.008 0.0024 0.0021 0.0047 0.148 723 Comparative steel
c 0.09 0.71 0.68 0.152 0.03 0.00 0.013 0.009 0.0016 0.0019 0.182 717 Comparative steel
d 0.08 0.42 2.39 0.000 0.00 0.00 0.038 0.013 0.0011 0.0023 0.0027 0.000 712 Comparative steel
e 0.09 0.45 2.44 0.005 0.05 0.04 0.009 0.011 0.0004 0.0016 0.0019 0.095 712 Comparative steel
f 0.09 0.31 2.26 0.044 0.03 0.00 0.033 0.014 0.0026 0.0034 0.074 710 Comparative steel
The underlined numbers are beyond the range of the present invention.
- 38 -
[0057]
[Table 2-1]
Steel
number
Kind of
steel
Slab heating
temperature (°C)
Finish rolling
temperature (°C)
Coiling
temperature (°C)
Pickling
Rolling reduction
under light reduction
(%)
Heat treatment
temperature (°C)
Holding
time(sec)
Plating and
the like
Galvannealing
temperature(°C) Note
A1 A 1290 970 25 Performed 5 660 40 GA 590 Steel of present invention
B1 B 1290 950 160 Performed 3 650 60 GA 590 Steel of present invention
C1 C 1300 960 25 Performed 2 650 120 HR - Steel of present invention
C2 C 1290 940 25 Performed 3 650 120 GI - Steel of present invention
C3 C 1310 970 25 Performed 5 740 100 GA 580 Comparative steel
C4 C 1300 960 25 Performed 5 680 100 GA 580 Steel of present invention
C5 C 1310 940 25 Performed 5 640 160 GA 590 Steel of present invention
C6 C 1300 950 25 Performed 5 610 120 GA 580 Steel of present invention
C7 C 1290 960 25 Performed 5 560 60 GA 590 Steel of present invention
C8 C 1300 950 25 Performed 5 490 80 GA 590 Steel of present invention
C9 C 1320 960 25 Performed 5 - 100 GA 590 Comparative steel
C10 C 1230 970 25 Performed 1 630 100 GA 580 Comparative steel
C11 C 1290 840 80 Performed 1 650 120 GA 580 Comparative steel
C12 C 1300 960 490 Performed 2 650 360 GA 590 Comparative steel
C13 C 1320 1000 670 Performed 3 640 40 GA 590 Comparative steel
C14 C 1290 970 25 Performed 1 650 60 GA 590 Steel of present invention
C15 C 1300 960 80 Performed 7 640 80 GA 590 Steel of present invention
C16 C 1290 980 25 Performed 10 640 80 GA 590 Steel of present invention
C17 C 1310 950 25 Performed 0 650 120 GA 580 Comparative steel
C18 C 1290 980 120 Performed 5 430 100 GA 590 Comparative steel
C19 C 1300 990 80 Performed 5 760 100 GA 590 Comparative steel
C20 C 1300 960 25 Performed 5 620 3 GA 560 Comparative steel
C21 C 1300 970 25 Performed 5 680 1700 GA 600 Comparative steel
C22 C 1320 990 270 Performed 5 800 120 GA 590 Comparative steel
C23 C 1300 1000 25 Performed 35 650 360 GA 600 Comparative steel
C24 C 1290 980 25 Performed 3 660 60 GA 580 Steel of present invention
C25 C 1310 960 25 Performed 3 650 60 GA 590 Steel of present invention
D1 D 1300 960 80 Performed 3 650 120 HR - Steel of present invention
D2 D 1290 960 25 Performed 4 640 60 GI - Steel of present invention
D3 D 1300 950 25 Performed 5 650 80 GA 550 Steel of present invention
D4 D 1250 980 80 Performed 3 660 80 GA 540 Comparative steel
D5 D 1300 870 130 Performed 3 630 100 GA 540 Comparative steel
D6 D 1290 980 25 Performed 5 650 100 GA 550 Steel of present invention
D7 D 1290 960 25 Performed 5 640 120 GA 550 Steel of present invention
D8 D 1300 980 120 Performed 5 650 40 GA 540 Steel of present invention
D9 D 1290 960 520 Performed 5 650 60 GA 540 Comparative steel
D10 D 1290 970 630 Performed 2 660 80 GA 540 Comparative steel
D11 D 1300 950 80 Performed 0 640 120 GA 550 Comparative steel
D12 D 1300 980 25 Performed 5 420 80 GA 550 Comparative steel
The underlined numbers are beyond the range of the present invention.
- 39 -
[0058]
[Table 2-2]
Steel
number
Kind of
steel
Slab heating
temperature (°C)
Finish rolling
temperature (°C)
Coiling
temperature (°C)
Pickling
Rolling reduction under
light reduction (%)
Heat treatment
temperature (°C)
Holding
time(sec)
Plating and
the like
Galvannealing
temperature(°C) Note
D13 D 1290 980 25 Performed 5 730 40 GA 550 Comparative steel
D14 D 1310 980 25 Performed 5 620 6 GA 550 Comparative steel
D15 D 1290 960 25 Performed 5 690 2100 GA 580 Comparative steel
E1 E 1290 990 130 Performed 3 630 40 HR - Steel of present invention
E2 E 1290 970 140 Performed 5 650 60 GI - Steel of present invention
E3 E 1300 980 80 Performed 3 640 120 GA 530 Steel of present invention
E4 E 1260 980 220 Performed 5 670 100 GA 540 Comparative steel
E5 E 1290 880 25 Performed 5 630 80 GA 530 Comparative steel
E6 E 1290 980 440 Performed 3 650 60 GA 530 Comparative steel
E7 E 1290 970 590 Performed 5 650 60 GA 530 Comparative steel
E8 E 1290 970 80 Performed 5 650 120 GA 540 Steel of present invention
E9 E 1300 980 25 Performed 7 630 100 GA 530 Steel of present invention
E10 E 1320 960 80 Performed 3 620 100 GA 540 Steel of present invention
E11 E 1290 960 150 Performed 0 670 80 GA 530 Comparative steel
E12 E 1330 980 25 Performed 5 410 100 GA 530 Comparative steel
E13 E 1310 970 80 Performed 5 740 100 GA 530 Comparative steel
E14 E 1290 980 25 Performed 5 640 8 GA 540 Comparative steel
E15 E 1300 980 25 Performed 5 700 1600 GA 580 Comparative steel
E16 E 1320 1020 25 Performed 5 780 120 GA 600 Comparative steel
E17 E 1310 990 25 Performed 53 640 540 GA 620 Comparative steel
F1 F 1290 970 90 Performed 5 640 120 GA 540 Steel of present invention
G1 G 1290 960 180 Performed 4 640 60 GA 530 Steel of present invention
H1 H 1310 990 180 Performed 6 650 80 GA 540 Steel of present invention
I1 I 1290 980 25 Performed 5 650 40 GA 560 Steel of present invention
J1 J 1290 960 25 Performed 7 660 80 GA 540 Steel of present invention
K1 K 1320 970 25 Performed 5 650 100 GA 530 Steel of present invention
L1 L 1290 970 25 Performed 5 650 100 GA 540 Steel of present invention
M1 M 1300 990 190 Performed 4 670 60 GA 540 Steel of present invention
N1 N 1330 970 25 Performed 12 660 40 GA 550 Steel of present invention
O1 O 1300 960 25 Performed 4 650 60 GA 540 Steel of present invention
P1 P 1320 980 25 Performed 15 640 120 GA 580 Steel of present invention
a1 a 1290 980 25 Performed 7 650 120 GA 620 Comparative steel
b1 b 1290 970 25 Performed 5 650 40 GA 590 Comparative steel
c1 c 1290 960 25 Performed 4 670 60 GA 580 Comparative steel
d1 d 1290 970 25 Performed 5 650 100 GA 550 Comparative steel
e1 e 1290 970 25 Performed 5 660 100 GA 550 Comparative steel
f1 f 1300 960 25 Performed 10 650 100 GA 560 Comparative steel
The underlined numbers are beyond the range of the present invention.
- 40 -
[0059]
For the obtained hot-rolled steel sheet, microstructure observation,
measurement of the number density of precipitate containing Ti and having the
equivalent circle diameter of 5.0 nm or less, measurement of Hvs/Hvc, evaluation of
tensile properties, evaluation of hole expansibility, and evaluation of fatigue resistance
were performed.
[0060]

Regarding the microstructure, the obtained hot-rolled steel sheet was cut out
in parallel to the rolling direction, and polished and etched with a nital reagent, and the
1/4 position with a thickness from the surface in the sheet thickness direction is
observed in 5 viewing fields with the SEM at the magnification of 3,000 times.
Thereby, ferrite, bainite, pearlite, fresh martensite, and tempered martensite were
identified, and an area ratio of the tempered martensite, and other structures is
determined as the volume percentage.
[0061]

For the number density of the precipitate containing Ti, the number density of
the precipitate included per unit volume of the steel sheet for each equivalent circle
diameter at 1 nm pitch was measured, by using the electrolytic extraction residual
method with respect to the sample collected from the 1/4 position from the surface.
In this case, composition analysis of a carbide was performed with a transmission
electron microscope (TEM) and an EDS, and it was confirmed that fine precipitate was
the precipitate containing Ti.
- 41 -
[0062]

For the average hardness Hvs at the position of a depth of 20 μm from the
surface, a sample was cut out from the 1/4 position in the width direction of the steel
sheet so that a cross section parallel to the rolling direction was a measurement surface,
embedding polishing was performed, a Vickers hardness at the position of 20 μm from
the surface was measured at 10 points with a load of 10 gf based on JIS Z 2244: 2009,
and an average value thereof was set as Hvs. For the Hvc, a sample was cut out from
the 1/4 position in the width direction of the steel sheet so that a cross section parallel
to the rolling direction was a measurement surface, embedding polishing was
performed, a Vickers hardness at a pitch of about 0.05 mm in the sheet thickness
direction from the position of 0.20 to 0.50 mm from the surface was measured at 7
points with a load of 10 gf based on JIS Z 2244:2009, and an average value thereof
was set as Hvc. Hvs/Hvc was obtained from these Hvs and Hvc.
[0063]

The tensile properties (YP, TS, and El) were obtained by a tensile test
performed based on JIS Z 2241:2011 using JIS No. 5 test piece cut out in the direction
perpendicular to the rolling direction.
It was determined that a preferable proof stress and tensile strength were
obtained (high proof stress and high strength) when YP/TS was 0.90 or more and TS
was 1,180 MPa or more.
[0064]

A hole expansion rate was determined by a hole expansion test method based
- 42 -
on JIS Z 2256:2010. Specifically, the test piece was cut out from a 1/4 width position
in the width direction of the steel sheet, and punched using a punch having a diameter
of 10 mm and a die having an inner diameter of 10.6 mm, a burr of the punched part
was set to on the opposite side of the punch using a 60° Conical punch, hole expansion
was performed, the test was stopped at the timing when crack generated on the
punched part penetrates the sheet thickness, and a hole diameter after the hole
expansion test was measured to obtain the hole expansion rate. In a case where the
hole expansion rate is 20% or more, it is determined that the hole expansibility is
excellent. In a case where the hole expansion rate is 20% or more, it is suitable for
undercarriage compartments having a burring portion and a stretch flange portion.
[0065]

The fatigue resistance was measured and evaluated by a plane bending fatigue
test at stress ratio, R=-1 described in JIS Z 2275:1978. Specifically, after obtaining a
relationship between the applied stress and the number of repetitions, the stress without
fracture, even in a case where the stress is repeatedly applied 107 times, was defined as
a fatigue limit (FS), and the fatigue resistance was adjusted with a value obtained by
dividing the fatigue limit by TS. In a case where this value exceeds 0.40, the fatigue
resistance was determined to be excellent.
The results are shown in Tables 3-1 and 3-2.
- 43 -
[0066]
[Table 3-1]
- 44 -
Steel
number
Microstructure volume percentage
(%)
Number density of precipitate having
equivalent circle diameter of 5.0 nm
or less
(pieces/mm3)
Hvs
/
Hvc
Tensile properties
Hole
expansion
rate
(%)
Fatigue limit
ratio
Note
Tempered
martensite
Other structure
YP
(MPa)
TS
(MPa)
Yield ratio
El
(%)
A1 100 - 8.9× 1011 0.89 1154 1189 0.97 13 47 0.44 Steel of present invention
B1 100 - 9.5 × 1011 0.87 1152 1203 0.96 13 43 0.42 Steel of present invention
C1 100 - 1.0 × 1012 0.86 1182 1223 0.97 12 47 0.41 Steel of present invention
C2 100 - 9.3 × 1011 0.87 1172 1216 0.96 13 44 0.42 Steel of present invention
C3 89 Fresh martensite 1.6 × 1011 0.82 1113 1162 0.96 14 22 0.39 Comparative steel
C4 100 - 6.4 × 1011 0.87 1186 1219 0.97 13 52 0.42 Steel of present invention
C5 100 - 9.8 × 1011 0.89 1234 1279 0.96 12 49 0.44 Steel of present invention
C6 100 - 7.2 × 1011 0.87 1201 1246 0.96 12 50 0.44 Steel of present invention
C7 100 - 6.5 × 1011 0.86 1163 1213 0.96 13 55 0.44 Steel of present invention
C8 100 - 5.6 × 1011 0.85 1129 1182 0.96 13 53 0.42 Steel of present invention
C9 100 - 6.2 × 1010 0.79 986 1161 0.85 4 45 0.37 Comparative steel
C10 100 - 2.5 × 1011 0.85 1078 1116 0.97 14 51 0.4 Comparative steel
C11 100 - 3.8 × 1011 0.85 1106 1154 0.96 13 52 0.4 Comparative steel
C12 23 Bainite 3.9 × 1011 0.86 1026 1078 0.95 14 55 0.41 Comparative steel
C13 6 Ferrite, pearlite 1.8 × 1010 0.87 938 986 0.95 15 58 0.42 Comparative steel
C14 100 - 5.3 × 1011 0.86 1154 1192 0.97 13 46 0.41 Steel of present invention
C15 100 - 8.8 × 1011 0.91 1251 1299 0.96 12 42 0.46 Steel of present invention
C16 100 - 9.0 × 1011 0.94 1268 1313 0.97 12 46 0.49 Steel of present invention
C17 100 - 2.6 × 1011 0.82 1118 1159 0.96 13 51 0.35 Comparative steel
C18 100 - 9.1 × 1010 0.81 947 1087 0.87 13 56 0.37 Comparative steel
C19 100 - 4.2 × 1010 0.84 872 1024 0.85 14 23 0.38 Comparative steel
C20 100 - 1.9 × 1010 0.82 967 1145 0.84 13 46 0.36 Comparative steel
C21 100 - 2.6 × 1011 0.84 921 1103 0.83 14 40 0.38 Comparative steel
C22 45 Ferrite 1.8 × 1011 0.81 703 934 0.75 15 31 0.34 Comparative steel
C23 68
Ferrite
(recrystallization)
5.3 × 1011 0.75 684 884 0.77 16 43 0.33 Comparative steel
C24 88 Bainite 7.6 × 1011 0.88 1106 1191 0.93 12 51 0.41 Steel of present invention
C25 96 Ferrite 6.9 × 1011 0.86 1076 1182 0.91 12 46 0.41 Steel of present invention
D1 100 - 9.5 × 1011 0.87 1186 1234 0.96 13 44 0.42 Steel of present invention
D2 100 - 1.3 × 1012 0.88 1185 1218 0.97 13 46 0.43 Steel of present invention
D3 100 - 9.1 × 1012 0.89 1209 1242 0.97 11 43 0.44 Steel of present invention
D4 100 - 1.6 × 1011 0.87 1062 1099 0.97 14 51 0.42 Comparative steel
D5 100 - 3.9 × 1011 0.87 1102 1146 0.96 12 47 0.42 Comparative steel
D6 100 - 9.0 × 1011 0.89 1182 1234 0.96 13 42 0.44 Steel of present invention
D7 100 - 9.5 × 1011 0.89 1181 1229 0.96 12 46 0.44 Steel of present invention
D8 100 - 1.0 × 1012 0.89 1201 1240 0.97 12 47 0.44 Steel of present invention
D9 23 Bainite, pearlite 2.8 × 1011 0.89 985 1042 0.95 14 56 0.44 Comparative steel
D10 9
Ferrite, bainite,
pearlite
4.8 × 108 0.86 921 962 0.96 15 53 0.41 Comparative steel
D11 100 - 4.1 × 1011 0.81 1095 1143 0.96 14 52 0.36 Comparative steel
The underlined numbers are beyond the range of the present invention.
- 45 -
[0067]
[Table 3-2]
Steel
number
Microstructure volume percentage
(%)
Number density of precipitate having
equivalent circle diameter of 5.0 nm or
less
(pieces/mm3)
Hvs
/
Hvc
Tensile properties Hole expansion
rate
(%)
Fatigue limit
ratio
Note
Tempered
martensite
Other structure
YP
(MPa)
TS
(MPa)
Yield ratio
El
(%)
D12 100 - 7.9 × 1010 0.83 919 1062 0.87 15 58 0.38 Comparative steel
D13 100 - 3.9 × 1010 0.84 861 1011 0.85 15 27 0.37 Comparative steel
D14 100 - 2.1 × 1011 0.81 936 1123 0.83 14 48 0.35 Comparative steel
D15 100 - 2.9 × 1011 0.84 925 1098 0.84 15 39 0.38 Comparative steel
E1 100 - 7.9 × 1011 0.87 1172 1214 0.97 14 42 0.42 Steel of present invention
E2 100 - 9.5 × 1011 0.89 1149 1196 0.96 14 45 0.44 Steel of present invention
E3 100 - 9.0 × 1011 0.87 1162 1208 0.96 13 43 0.42 Steel of present invention
E4 100 - 9.4 × 1010 0.89 1119 1164 0.96 12 45 0.44 Comparative steel
E5 100 - 2.5 × 1011 0.89 1104 1124 0.98 13 49 0.44 Comparative steel
E6 39 Bainite 3.1 × 1011 0.87 926 1026 0.90 14 51 0.42 Comparative steel
E7 4 Ferrite, pearlite 6.4 × 109 0.89 872 912 0.96 15 54 0.44 Comparative steel
E8 100 - 1.4 × 1012 0.89 1211 1261 0.96 11 44 0.44 Steel of present invention
E9 100 - 8.6 × 1011 0.91 1172 1221 0.96 12 43 0.46 Steel of present invention
E10 100 - 7.2 × 1011 0.87 1156 1208 0.96 13 47 0.42 Steel of present invention
E11 100 - 4.2 × 1011 0.82 1117 1137 0.98 14 48 0.36 Comparative steel
E12 100 - 5.2 × 109 0.82 951 1108 0.86 14 52 0.35 Comparative steel
E13 100 - 2.6 × 1010 0.84 864 998 0.87 16 26 0.38 Comparative steel
E14 100 - 3.6 × 1011 0.83 986 1157 0.85 13 46 0.36 Comparative steel
E15 100 - 1.8 × 1011 0.84 876 1046 0.84 15 39 0.37 Comparative steel
E16 46 Ferrite 2.4 × 1011 0.80 684 927 0.74 15 35 0.32 Comparative steel
E17 23
Ferrite
(recrystallization)
5.9 × 1011 0.77 638 842 0.76 15 46 0.34 Comparative steel
F1 100 - 9.1 × 1011 0.89 1182 1216 0.97 13 46 0.44 Steel of present invention
G1 100 - 8.0 × 1011 0.88 1160 1192 0.97 13 47 0.43 Steel of present invention
H1 100 - 7.7 × 1011 0.9 1148 1182 0.97 13 46 0.45 Steel of present invention
I1 100 - 9.4 × 1011 0.89 1172 1199 0.98 13 50 0.44 Steel of present invention
J1 100 - 8.8 × 1011 0.91 1169 1204 0.97 12 43 0.46 Steel of present invention
K1 100 - 8.1 × 1011 0.89 1184 1208 0.98 13 46 0.44 Steel of present invention
L1 100 - 7.2 × 1011 0.89 1142 1194 0.96 14 45 0.44 Steel of present invention
M1 100 - 7.8 × 1011 0.88 1149 1198 0.96 13 42 0.43 Steel of present invention
N1 100 - 2.3 × 1012 0.87 1402 1475 0.95 11 32 0.47 Steel of present invention
O1 100 - 9.1 × 1011 0.88 1186 1230 0.96 11 46 0.43 Steel of present invention
P1 100 - 3.4 × 1012 0.91 1419 1489 0.95 12 35 0.47 Steel of present invention
a1 16 Bainite 1.4 × 1011 0.86 712 845 0.84 17 57 0.46 Comparative steel
b1 100 - 3.3 × 1011 0.84 984 1112 0.88 12 48 0.37 Comparative steel
c1 0 Ferrite, pearlite 1.6 × 1010 0.82 795 892 0.89 15 56 0.38 Comparative steel
d1 100 - 0 0.84 566 716 0.79 18 89 0.36 Comparative steel
e1 100 - 1.2 × 108 0.84 723 849 0.85 16 76 0.35 Comparative steel
f1 100 - 2.6 × 109 0.82 631 761 0.83 20 73 0.32 Comparative steel
- 46 -
The underlined numbers are beyond the range of the present invention.
- 47 -
[0068]
As can be seen from Tables 1 to 3-2, in the example (steel of the present
invention) having the chemical composition of the present invention and satisfying the
hot rolling conditions, the rolling reduction, and the heat treatment conditions of the
present invention, the number density of the precipitate containing Ti and having the
equivalent circle diameter of 5.0 nm or less was 5.0 × 1011 pieces/mm3 or more. In
addition, in these examples, a tensile strength of 1,180 MPa or more, a high yield ratio
(YP/TS) of 0.90 or more, and excellent fatigue resistance were achieved.
[0069]
On the other hand, in the steel numbers C3, C19, D13, and E13 in which the
heat treatment temperature in the reheating step exceeds Ac1°C, the precipitate became
coarse, the number density of the precipitate containing Ti and having an equivalent
circle diameter of 5.0 nm or less could not be ensured as 5.0 × 1011 pieces/mm3 or
more, and tensile strength of 1,180 MPa or more could not be ensured. In addition, in
the C3 in which the heat treatment temperature was high, since the austenite formed
during heating was transformed into martensite, a structure containing a large amount
of fresh martensite was obtained, and a value with a low hole expansion rate was
shown.
In the C22 and E16 in which the heat treatment temperature was high, since
the austenite formed during heating was transformed into ferrite, the precipitate
containing Ti was coarsened and the consistency between the precipitate and ferrite
was lost due to the transformation. Therefore, it was not possible to ensure a tensile
strength of 1,180 MPa or more.
In the steel numbers C9, C18, D12, and E12 in which the heat treatment
temperature was lower than 450°C, the formation of precipitate containing Ti was
- 48 -
insufficient. As a result, the number density was less than 5.0 × 1011 pieces/mm3, and
it was not possible to ensure a tensile strength of 1,180 MPa or more.
In the steel numbers C10, D4, and E4 in which the slab heating temperature
was 1,280°C or lower, the coarse precipitate formed during casting could not be
dissolved at hot rolling, and even in a case where the subsequent reduction and heat
treatment were performed, the number density of precipitate containing Ti and having
an equivalent circle diameter of 5.0 nm or less could not be ensured as 5.0 × 1011
pieces/mm3 or more, and it was not possible to ensure a tensile strength of 1,180 MPa
or more.
In the steel numbers C11, D5, and E5 in which the hot-rolled finish rolling
temperature was lower than 930°C, coarse precipitate was formed until the finish
rolling, and even in a case where the subsequent reduction and heat treatment were
performed, the number density of the precipitate containing Ti and having an
equivalent circle diameter of 5.0 nm or less could not be ensured as 5.0 × 1011
pieces/mm3 or more, and a tensile strength of 1,180 MPa or more could not be ensured.
In the steel numbers C12, C13, D9, D10, E6, and E7 in which the coiling
temperature after hot rolling was 300°C or higher, ferrite, bainite, and pearlite were
generated in an amount of 20 volume% or more after hot rolling and coiling, and
martensite volume percentage could not be 80% or more. Therefore, even in a case
where the subsequent reduction and heat treatment are performed, the number density
of precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less
could not be ensured as 5.0 × 1011 pieces/mm3 or more, and it was not possible to
ensure a tensile strength of 1,180 MPa or more.
In the steel numbers C17, D11, and E11 in which the light reduction is less
than 1%, the dislocation as a nucleation site of precipitate is not introduced, the
- 49 -
number density of precipitate containing Ti and having an equivalent circle diameter of
5.0 nm or less could not be ensured as 5.0 × 1011 pieces/mm3 or more, and it was not
possible to ensure a tensile strength of 1,180 MPa or more.
In the steel numbers C23 and E17 in which the light reduction is more than
30%, recrystallization was occurred during the heat treatment. As a result, the
consistency between the ferrite as the primary phase and the precipitate containing Ti
was lost. Accordingly, the amount of hardening due to the precipitate was decreased,
and it was not possible to ensure a tensile strength of 1,180 MPa or more.
In the steel numbers C20, D14, and E14 in which a heat treatment time was
shorter than 10 seconds, the heat treatment time was too short. Accordingly, the
number density of precipitate containing Ti and having an equivalent circle diameter of
5.0 nm or less could not be ensured as 5.0 × 1011 pieces/mm3 or more, and it was not
possible to ensure a tensile strength of 1,180 MPa or more.
In the steel numbers C21, D15, and E15 in which a heat treatment time was
longer than 1,500 seconds, the precipitate was coarsened during the heat treatment.
Accordingly, the number density of precipitate containing Ti and having an equivalent
circle diameter of 5.0 nm or less could not be ensured as 5.0 × 1011 pieces/mm3 or more,
and it was not possible to ensure tensile strength of 1,180 MPa or more.
In the steel number a1, since the C content was too low, the number density of
precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less
could not be ensured as 5.0 × 1011 pieces/mm3 or more, and it was not possible to
ensure a tensile strength of 1,180 MPa or more.
In the steel number b1, since the C content was too high, the coarse precipitate
of Ti could not be sufficiently dissolved under the present slab heating conditions, and
even in a case where reduction and heat treatment were performed in the subsequent
- 50 -
step, the number density of precipitate containing Ti and having an equivalent circle
diameter of 5.0 nm or less could not be ensured as 5.0 × 1011 pieces/mm3 or more, and
it was not possible to ensure a tensile strength of 1,180 MPa or more.
In the steel number c1, since the Mn content was too low, ferrite and pearlite
were formed between the finishing of hot rolling to coiling, and the tempered
martensite could not be 80% or more. As a result, the number density of precipitate
containing Ti and having an equivalent circle diameter of 5.0 nm or less could not be
ensured as 5.0 × 1011 pieces/mm3 or more, and it was not possible to ensure a tensile
strength of 1,180 MPa or more.
In the steel numbers d1 and e1, since the amounts of Ti, Nb, and V were too
low, the number density of precipitate containing Ti and having an equivalent circle
diameter of 5.0 nm or less could not be ensured as 5.0 × 1011 pieces/mm3 or more, and
it was not possible to ensure a tensile strength of 1,180 MPa or more.
In the steel number f1, since the amounts of Ti, Nb, and V were too low, the
number density of precipitate containing Ti and having an equivalent circle diameter of
5.0 nm or less could not be ensured as 5.0 × 1011 pieces/mm3 or more and it was not
possible to ensure a tensile strength of 1,180 MPa or more.
[Industrial Applicability]
[0070]
According to the present invention, it is possible to provide a high-strength
steel sheet having a tensile strength of 1,180 MPa or more, which has high proof stress,
and excellent fatigue resistance. This steel sheet has great industrial value, because it
contributes to weight reduction of vehicle components. In addition, this steel sheet is
suitable for undercarriage compartments of vehicles, since it has high strength (high
tensile strength), high proof stress, and excellent fatigue resistance.

WE CLAIMS

1. A high-strength steel sheet, comprising,
as a chemical composition, by mass%:
C: 0.020 to 0.120%;
Si: 0.01 to 2.00%;
Mn: 1.00 to 3.00%;
Ti: 0.010 to 0.200%;
Nb: 0 to 0.100%;
V: 0 to 0.200%;
Al: 0.005 to 1.000%;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0100% or less;
Ni: 0 to 2.00%;
Cu: 0 to 2.00%;
Cr: 0 to 2.00%;
Mo: 0 to 2.00%;
W: 0 to 0.100%;
B: 0 to 0.0100%;
REM: 0 to 0.0300%;
Ca: 0 to 0.0300%;
Mg: 0 to 0.0300%; and
a remainder of Fe and impurities,
wherein 0.100 ≤ Ti + Nb + V ≤ 0.450 is satisfied,
- 52 -
a microstructure contains, by volume percentage, 80% or more of tempered
martensite, and a remainder consists of ferrite and bainite,
the microstructure contains 5.0 × 1011 pieces/mm3 or more of, per unit volume,
precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less,
Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth
of 20 μm from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm
from the surface is 0.85 or more, and
a tensile strength is 1,180 MPa or more.
2. The high-strength steel sheet according to claim 1, comprising:
as the chemical composition, by mass%, at least one or two or more selected
from the group consisting of:
Ni: 0.01 to 2.00%;
Cu: 0.01 to 2.00%;
Cr: 0. 01 to 2.00%;
Mo: 0.01 to 2.00%;
W: 0.005 to 0.100%;
B: 0.0005 to 0.0100%;
REM: 0.0003 to 0.0300%;
Ca: 0.0003 to 0.0300%; and
Mg: 0.0003 to 0.0300%.
3. The high-strength steel sheet according to claim 1 or 2 further comprising
a hot-dip galvanized layer on the surface.
4. The high-strength steel sheet according to claim 3,
wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.
5. A method for manufacturing the high-strength steel sheet according to
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claim 1 or 2, the method comprising:
a heating step of heating a slab including, as a chemical composition, by
mass%: C: 0.020 to 0.120%, Si: 0.01 to 2.00%, Mn: 1.00 to 3.00%, Ti: 0.010 to
0.200%, Nb: 0 to 0.100%, V: 0 to 0.200%, Al: 0.005 to 1.000%, P: 0.100% or less, S:
0.0100% or less, N: 0.0100% or less, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Cr: 0 to 2.00%,
Mo: 0 to 2.00%, W: 0 to 0.100%, B: 0 to 0.0100%, REM: 0 to 0.0300%, Ca: 0 to
0.0300%, Mg: 0 to 0.0300%, and a remainder of Fe and impurities, to higher than
1,280°C;
a hot rolling step of performing hot rolling with respect to the slab such that a
finish rolling temperature is 930°C or higher to obtain a hot-rolled steel sheet;
a coiling step of coiling the hot-rolled steel sheet at a temperature lower than
300°C and cooling the hot-rolled steel sheet to room temperature;
a pickling step of pickling the hot-rolled steel sheet after the coiling step;
a light reduction step of performing light reduction with respect to the hotrolled
steel sheet after the pickling step at rolling reduction of 1% to 30%; and
a reheating step of reheating the hot-rolled steel sheet after the light reduction
step in a temperature range of 450°C to Ac1°C and holding for 10 to 1,500 seconds.
6. The method for manufacturing a high-strength steel sheet according to
claim 5, further comprising a plating step of hot-dip galvanizing the hot-rolled steel
sheet after the reheating step.
7. The method for manufacturing a high-strength steel sheet according to
claim 6 further comprising performing a galvannealing step of galvannealing by
heating the hot-rolled steel sheet after the hot-dip galvanizing step to 460°C to 600°C.