[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. 2019-055469, 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 utilized as steel sheets used for vehicle
components (steel sheets for vehicles), in order to reduce the 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 for the steel sheets applied
to undercarriage compartments of vehicles to have excellent fatigue resistance, in
addition to high tensile strength, high proof stress (high YP), and high ductility.
[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, the high-strength steel sheet disclosed in Patent Document 1 does
- 2 -
not have a tensile strength of 980 MPa or more. In addition, although the highstrength
steel sheet disclosed in Patent Document 2 can ensure a tensile strength of 980
MPa or more, it is necessary to improve ductility (particularly elongation), in order to
further expand application to undercarriage compartments.
[0004]
As described above, in the related art, a steel sheet having high tensile
strength of 980 MPa or more, high proof stress, high ductility, 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, high ductility, and excellent fatigue resistance and having tensile strength
of 980 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
problems. As a result, it is found that, in a steel sheet having a predetermined
- 3 -
chemical composition, a microstructure is set as a structure containing 95% or more of
tempered martensite and bainite in total, the microstructure contains 5.0 × 109
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, high ductility (high elongation), and
excellent fatigue resistance, tensile strength of 980 MPa or more, and a product (TS ×
El) of tensile strength and ductility (elongation) of 12,000 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 1,230°C or higher to
dissolve Ti or Nb contained in a large amount,, a coiling temperature after the hot
rolling is set to 300°C or higher and 600°C or lower to obtain a mixed structure of
martensite and bainite, 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 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 0.090%; Si:
- 4 -
0.01 to 2.00%; Mn: 1.00 to 3.00%; Ti: 0.010 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%; Nb: 0 to 0.100%; V: 0 to 0.100%; 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, 95% or more of tempered
martensite and bainite in total, and a remainder consists of ferrite and pearlite, the
microstructure contains 5.0 × 109 pieces/mm3 or more of, per unit volume, precipitate
having an equivalent circle diameter of 5.0 nm or less and containing Ti, 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, a tensile strength is 980 MPa or more, and a product of the tensile
strength and elongation is 12,000 MPa×% or more.
(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%; Nb: 0.005 to 0.100%; V: 0.005 to 0.100%; 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 (1) or (2) may include a hot-dip
galvanized layer on the surface.
(4) In the high-strength steel sheet according to (3), the hot-dip galvanized
layer may be a hot-dip galvannealed layer.
(5) A method for manufacturing the high-strength steel sheet according to
another aspect of the present invention is a method for manufacturing the high-strength
- 5 -
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 0.090%; Si: 0.01 to
2.00%; Mn: 1.00 to 3.00%; Ti: 0.010 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%; Nb: 0 to 0.100%; V: 0 to 0.100%; 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 1,230°C or higher; 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
300°C or higher and 600°C or lower and then 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 hot-rolled steel
sheet after the pickling step at rolling reduction higher than 5% and 30% or less; 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 (5)
may further include 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 (6)
may further include a galvannealing step of performing galvannealing by heating the
hot-rolled steel sheet after the hot-dip galvanizing step to 460°C to 600°C.
[Effects of the Invention]
[0009]
According to the above aspects of the present invention, it is possible to
provide a high-strength steel sheet having a tensile strength of 980 MPa or more,
- 6 -
which has high proof stress, high ductility, 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, high
ductility, 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]
[0010]
FIG. 1A is a diagram showing the number density of precipitate containing Ti
of a steel of the present invention in each particle diameter.
FIG. 1B is a diagram showing the number density of precipitate containing Ti
of a comparative steel in each particle diameter.
FIG. 2A is a diagram showing the relationship between the coiling
temperature after hot rolling and YP (proof stress).
FIG. 2B is a diagram showing the relationship between the coiling
temperature after hot rolling and TS (tensile strength).
FIG. 2C is a diagram showing the relationship between the coiling
temperature after hot rolling and TS × El (elongation).
FIG. 2D is a diagram showing the relationship between the coiling
temperature after hot rolling and λ (hole expansion ratio).
FIG. 3A is a diagram showing the relationship between the rolling reduction
under light reduction and YP (proof stress).
FIG. 3B is a diagram showing the relationship between the rolling reduction
- 7 -
under light reduction and TS (tensile strength).
FIG. 3C is a diagram showing the relationship between the rolling reduction
under light reduction and TS × El (elongation).
FIG. 3D is a diagram showing the relationship between the rolling reduction
under light reduction and λ (hole expansion ratio).
FIG. 4A is a diagram showing the relationship between the heat treatment
temperature in a reheating step and YP (proof stress).
FIG. 4B is a diagram showing the relationship between the heat treatment
temperature in the reheating step and TS (tensile strength).
FIG. 4C is a diagram showing the relationship between the heat treatment
temperature in the reheating step and TS × El (elongation).
FIG. 4D is a diagram showing the relationship between the heat treatment
temperature in the reheating step and λ (hole expansion ratio).
FIG. 5A is a diagram showing the relationship between the heat treatment
time in the reheating step and YP (proof stress).
FIG. 5B is a diagram showing the relationship between the heat treatment
time in the reheating step and TS (tensile strength).
FIG. 5C is a diagram showing the relationship between the heat treatment
time in the reheating step and TS × El (elongation).
FIG. 5D is a diagram showing the relationship between the heat treatment
time in the reheating step and λ (hole expansion ratio).
[Embodiments of the Invention]
[0011]
A high-strength steel sheet according to one embodiment of the present
invention (hereinafter, steel sheet according to the present embodiment) includes a
- 8 -
predetermined chemical composition, a microstructure contains 95% or more of
tempered martensite and bainite in total, a remainder consists of ferrite and pearlite, the
microstructure contains 5.0 × 109 pieces/mm3 or more of, per unit volume, precipitate
having an equivalent circle diameter of 5.0 nm or less and containing Ti, 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 980 MPa or more, and a product of tensile strength and elongation
of 12,000 MPa×% or more.
In the steel sheet according to the present embodiment, high strength, high
ductility, high proof stress satisfying YP/TS ≥ 0.90, and excellent fatigue resistance
satisfying fatigue limit/TS ≥ 0.40 or more are obtained. In addition, a hole expansion
ratio of 40% or more can be ensured.
[0012]
Hereinafter, the steel sheet according to the present embodiment will be
described in detail.
[0013]

First, reasons for limiting the microstructure will be described.
In the steel sheet according to the present embodiment, primary phases of the
microstructure are 95% or more of tempered martensite and bainite in total 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
- 9 -
less and containing Ti, has a number density of 5.0 × 109 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 or bainite containing many dislocations as
precipitation sites of precipitate during the heat treatment. By performing the heat
treatment with respect to the martensite or the bainite containing many dislocations,
tempered martensite and/or bainite containing fine precipitate becomes the primary
phase. In addition, the dislocations in the martensite and the bainite existing before
the heat treatment or the dislocation introduced during processing are recovered and
rearranged by the heat treatment. Accordingly, the heat treatment also causes the
improvement of ductility. In particular, since the bainite has a higher elongation than
the martensite, it is preferable to set the volume percentage of the bainite to 50% or
more, when particularly excellent ductility is required.
In addition, since ferrite and pearlite are formed at a high temperature, in a
case where these structures are formed, precipitate containing Ti precipitated therein
also tends to coarsen. In this case, it is not possible to ensure 5.0 × 109 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, 95% or more of tempered martensite and/or bainite in total, and
the remainder is 5% or less. In the present embodiment, the tempered martensite
means martensite containing precipitate containing cementite and/or Ti.
[0014]
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
- 10 -
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.
An area ratio of each structure identified from the SEM observation image is obtained,
and this is defined as the volume percentage. 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.
The martensite includes both tempered martensites containing carbide in lath
and as quenched martensite not containing carbide (fresh martensite), and these are
observed with an SEM and a TEM, and the presence or absence of carbide can be
confirmed and identified. 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
martensite.
[0015]

Next, a reason why the present inventors focused on the size and the number
density of the precipitate will be described. The present inventors conducted
intensive studies about a relationship between the size and the number density of
- 11 -
precipitate in which a tensile strength of 980 MPa or more can be ensured. As a
result, it was found that, the size (equivalent circle diameter) of the precipitate
contained in the hot-rolled 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. 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 × 109 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 × 109 pieces/mm3 or more
as a primary phase, the tensile strength of 980 MPa or more can be ensured and the
fatigue resistance is also excellent.
[0016]
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 × 109 pieces/mm3 or
more, in order to ensure the tensile strength of 980 MPa or more. In a case where the
number density is less than 5.0 × 109 pieces/mm3, it is difficult to ensure the tensile
strength of 980 MPa or more. Therefore, it is necessary that the number density of
- 12 -
the precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less
is 5.0 × 109 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 an equivalent
circle diameter of 5.0 nm or less. 5.0 nm here is the equivalent circle diameter. The
type of precipitate such as carbide, nitride, carbonitride, and the like is not limited, and
particularly, the carbide is preferable, since the carbide 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 and
bainite, which is the primary phase.
Although Nb has an effect similar to that of Ti, the amount of Nb carbide Nb
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 980 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
alone, it is difficult to ensure 5.0 × 109 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 × 109 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 980 MPa or more. For the precipitate having
- 13 -
an equivalent circle diameter more than 5.0 nm, the number density cannot be set to
5.0 × 109 pieces/mm3 or more, and the tensile strength of 980 MPa or more cannot be
ensured.
The equivalent circle diameter is a value in a case where observed shape of
the precipitate is assumed to be 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.
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.
[0017]
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 equivalent circle
- 14 -
diameter more than 0 nm and 1.0 nm or less, the number density of equivalent circle
diameter more than 1.0 nm and 2.0 nm or less, the number density of 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 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
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.
- 15 -
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 an equivalent circle diameter of 5.0 nm or less to 5.0 ×
109 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 an equivalent circle diameter less than 0.4 nm is small, the
precipitate having an equivalent circle diameter of 0.4 nm or more may be set as a
substantial target.
[0018]

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
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
- 16 -
and the surface layer hardness is easily reduced. In a case where the surface layer
hardness is reduced, the fatigue resistance is also 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, 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 the 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,
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 (Hvs/Hvc) to
0.85 or more. Since this effect is more greatly exhibited at 0.87 or more, it is
preferably 0.87 or more. It is more preferably 0.90 or more.
[0019]
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.
- 17 -
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.
[0020]

From a viewpoint of improving the fuel efficiency of vehicles by applying the
steel sheet to the undercarriage compartments, in the steel sheet according to the
present embodiment, a tensile strength is 980 MPa or more and a product of tensile
strength and elongation is 12,000 MPa×% or more.
It is not necessary to limit the upper limit of the tensile strength, but the
tensile strength may be less than 1,180 MPa, in order to ensure the elongation at a
certain level or more. The tensile strength may be 1,179 MPa or less, or 1,170 MPa
or less.
[0021]
- 18 -
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.
[0022]
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%.
[0023]
C: 0.020 to 0.090%
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 × 109 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 0.090%, 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 0.090% or less. It is preferably 0.080% or
less.
[0024]
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
- 19 -
content is set to 2.00% or less.
[0025]
Mn: 1.00 to 3.00%
Mn is an element effective for increasing the volume percentage of martensite
and bainite in the microstructure of the steel sheet and increasing the strength of the
steel sheet. In order to set the total volume percentage of martensite and bainite to
95% 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 martensite and bainite 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. The Mn content is preferably 2.65% or less, and more preferably
2.30% or less.
[0026]
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
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.
[0027]
Ti: 0.010 to 0.200%
Nb: 0 to 0.100%
- 20 -
V: 0 to 0.100%
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 × 109 pieces/mm3 or more of fine precipitate containing Ti and having a
equivalent circle diameter of 5.0 nm or less through the manufacturing method which
will be described later, the total amount 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 0.105% or more, and more desirably 0.110% or more.
On the other hand, in a case where the total amount of Ti, Nb, and V (Ti + Nb
+ V) exceeds 0.450%, these precipitates are excessively precipitated on the slab or the
hot-rolled sheet, causing embrittlement, resulting in poor manufacturability.
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
0.100%, because, in a case where the contents thereof exceed these upper limits, it is
difficult to dissolve the coarse precipitates precipitated at a casting stage, even in a
case where a lower limit of a slab heating temperature is set to 1,230°C or higher.
Further, the excessive amount of Ti, Nb, and V causes embrittlement of the slab and
the steel sheet. Therefore, it is desirable that the Ti content has an upper limit of
0.200%, the Nb content has an upper limit of 0.100%, and the V content has an upper
limit of 0.100%.
Any combination of Ti, Nb, and V may be used for ensuring 5.0 × 109
- 21 -
pieces/mm3 or more of fine carbide containing Ti and having an 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 amount of Ti, which is easy to contain in large amounts and
is inexpensive, is at least 0.010% or more.
[0028]
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%.
[0029]
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
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%.
[0030]
N: 0.0100% or less
- 22 -
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 an equivalent circle diameter of 5.0 nm or less is less than 5.0
× 109 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.
[0031]
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 amount thereof
is 0%.
[0032]
Ni: 0 to 2.00%
Cu: 0 to 2.00%
Cr: 0 to 2.00%
Mo: 0 to 2.00%
- 23 -
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, the amount of each is preferably 0.01% or more.
On the other hand, in a case where the amount of each element exceeds 2.00%,
weldability, hot workability, and the like are deteriorated. Therefore, even when
these are contained, the amount of each of Ni, Cu, Cr, and Mo is set to 2.00% or less.
[0033]
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.
[0034]
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.
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, even when this is
contained, the B content is set to 0.0100% or less. The B content is preferably
- 24 -
0.0080% or less, and more preferably 0.0050% or less.
[0035]
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 amount of REM, Ca, and
Mg is preferably set to 0.0003% or more.
On the other hand, in a case where the amount of each 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.
[0036]
As described above, the steel sheet according to the present embodiment
contains basic elements, contains any elements, as necessary, and the remainder
consists of Fe and impurities. The impurities refer to compositions that are
unintentionally contained from a raw material in the manufacturing process of the steel
- 25 -
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.
[0037]
The steel sheet according to the present embodiment may further include a
hot-dip galvanized layer on its surface. In addition, the hot-dip galvanizing may be
hot-dip galvannealing subjected to a galvannealing treatment.
Since the galvanizing contributes to the improvement of corrosion resistance,
it is desirable to use a 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
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.
[0038]
Next, a preferable method for manufacturing the steel sheet according to the
- 26 -
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.
[0039]
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 1,230°C or higher
(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 300°C or higher and
600°C or lower 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 reducing the hot-rolled steel sheet after the
pickling step with a rolling reduction of more than 5% and 30% or less
(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.
[0040]

In the heating step, the slab having the above-mentioned chemical
composition to be subjected to the hot rolling step is heated to 1,230°C or higher.
The reason for setting the heating temperature to 1,230°C or higher is to dissolve
- 27 -
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) to precipitate 5.0 × 109 pieces/mm3 or more of the precipitate containing Ti
and having an 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 less than 1,230°C, 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.
[0041]

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.
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
- 28 -
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.
[0042]

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 300°C or
higher and 600°C or lower, 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. A cooling rate for water cooling is, for example,
20°C/sec or higher.
In a case where the coiling temperature exceeds 600°C, ferrite is formed, and
the volume percentage of tempered martensite and bainite cannot be 95% or more,
resulting in an inferior balance between strength and formability. In addition, in a
case where the coiling temperature exceeds 600°C, precipitate having an equivalent
circle diameter more than 5.0nm is formed in martensite and bainite, the number
density of the precipitate having an equivalent circle diameter of 5.0 nm or less to be
precipitated in the subsequent heat treatment step decreases, and the number density of
the precipitate may be lower than 5.0 × 109 pieces/mm3. On the other hand, in a case
where the coiling temperature is lower than 300°C, the structure has martensite as the
primary phase, and the high-strengthening is easily performed, but the ductility is
- 29 -
lowered. Therefore, in order to satisfy both high ductility and high strength, it is
necessary to set the coiling temperature to 300°C or higher.
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 to room temperature are not
particularly limited, and for example, the coil may be left to cool to room temperature.
Alternatively, even if the water cooling is performed for the purpose of shortening the
cooling period, the desired hot-rolled coil can be obtained.
FIGS. 1A and 1B show diagrams showing the number density of each particle
diameter (equivalent circle diameter) of the precipitate containing Ti, in an example in
which the coiling temperature is 500°C and the rolling reduction under light reduction
is 7% and an example in which the coiling temperature is 650°C and the rolling
reduction under light reduction is 7%.
In addition, as shown in FIGS. 2A to 2D, the characteristics change depending
on the coiling temperature.
It is considered that, it is because, as shown in FIG. 1A, by coiling at an
appropriate temperature, the number density (number density on the left side of a
broken line in the drawing) of the precipitate containing Ti and the particle diameter
(equivalent circle diameter) of 5.0 nm or less increases.
[0043]

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
- 30 -
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 pickled before the light reduction. The pickling conditions are not
particularly limited, but the pickling is generally performed with hydrochloric acid or
sulfuric acid containing an inhibitor.
[0044]

In the light reduction step, reduction is applied to the hot-rolled steel sheet
after the pickling step at rolling reduction of more than 5% and 30% or less.
By applying the reduction to the hot-rolled steel sheet, a precipitation site for
precipitation of the precipitate in the heat treatment which is the subsequent step is
introduced. By introducing the precipitation site, the fine carbide containing Ti and
having an equivalent circle diameter of 5.0 nm or less can be precipitated by 5.0 × 109
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 more than 5%. Therefore, the reduction at rolling reduction of more than 5% 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 hardening is reduced. In this case, it is difficult to ensure a tensile
- 31 -
strength of 980 MPa or more. Therefore, the rolling reduction is set to 30% or less.
The rolling reduction is preferably less than 20%, and more preferably less than 15%.
In a case where the dislocation that acts as a nucleation site of the precipitate
can be introduced, the reduction may be performed by reducing the pressure by more
than 5% and 30% or less in one pass, or by dividing into a plurality of times and the
reduction may be performed so that the cumulative rolling reduction is more than 5%
and 30% or less.
As shown in FIGS. 3A to 3D, high strength and high ductility can be obtained
by setting the rolling reduction under light reduction to more than 5%.
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.
[0045]

The hot-rolled steel sheet after the light reduction step is performed heat
treatments 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 carbide containing Ti and having an equivalent circle diameter of
5.0 nm or less can be precipitated by 5.0 × 109 pieces/mm3 or more. In a case where
the heat treatment temperature (reheating temperature) is lower than 450°C, the
- 32 -
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 ferrite is formed from the
austenite formed in the heat treatment during the cooling. Accordingly, the total
volume percentage of tempered martensite and bainite may not be 95% or more, and a
consistent relationship between the Ti precipitate and the primary phase (here, bainite
or 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 980 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.
In addition, as shown in FIGS. 4A to 4D, high strength and high ductility can
be obtained by setting the reheating temperature (heat treatment temperature) to 450 to
Ac1°C.
In a case where the heat treatment time in the reheating step is shorter than 10
seconds, the diffusion of atoms is insufficient, and the carbide containing Ti and
having an equivalent circle diameter of 5.0 nm or less cannot be precipitated by 5.0 ×
109 pieces/mm3 or more. In a case where the heat treatment time exceeds 1,500
seconds, the precipitate becomes coarse, and the number of precipitates containing Ti
and having an equivalent circle diameter of 5.0 nm or less is less than 5.0 × 109
pieces/mm3. For this reason, it is necessary to set the heat treatment time to 10 to
- 33 -
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, high strength and high ductility can be obtained
by setting the reheating time (heat treatment time) to be in the range of 10 to 1,500
seconds.
The cooling after the holding step is not particularly limited.
[0046]
The steel sheet according to the present embodiment can be obtained by a
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.
[0047]

The hot-rolled steel sheet after the reheating step is subjected to hot-dip
galvanizing. Since the galvanizing contributes to the improvement of corrosion
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.
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 (galvannealing step).
- 34 -
Accordingly, 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 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, other than galvanizing, it is possible to manufacture a steel sheet according
to the present embodiment having a tensile strength of 980 MPa or more and excellent
fatigue resistance.
[Examples]
[0048]
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
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
- 35 -
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 -
[0049]
[Table 1]
Kind
of steel
Mass% Remainder Fe and impurities Ti
+ Nb + V
(%)
Ac1
(°C)
Ms
(°C)
Note
C Si Mn Ti Nb V Al P S N Ni Cu Cr Mo W B REM Ca Mg
A 0.06 1.36 2.63 0.112 0.000 0.000 0.032 0.008 0.003 0.004 0.112 737 426 Steel of present invention
B 0.05 0.23 2.03 0.121 0.000 0.000 0.019 0.008 0.002 0.003 0.0019 0.121 727 453 Steel of present invention
C 0.07 0.47 2.13 0.113 0.020 0.000 0.023 0.009 0.002 0.003 0.0024 0.133 729 442 Steel of present invention
D 0.05 0.45 1.86 0.082 0.050 0.000 0.025 0.007 0.002 0.003 0.0026 0.132 732 459 Steel of present invention
E 0.06 0.50 2.06 0.076 0.040 0.050 0.029 0.011 0.002 0.003 0.0026 0.166 729 448 Steel of present invention
F 0.07 0.39 2.06 0.105 0.020 0.000 0.038 0.009 0.000 0.002 0.19 0.125 725 444 Steel of present invention
G 0.06 0.42 1.45 0.110 0.010 0.000 0.026 0.009 0.003 0.001 0.88 0.0026 0.120 735 454 Steel of present invention
H 0.06 0.36 1.74 0.109 0.020 0.000 0.027 0.009 0.003 0.004 0.26 0.18 0.0021 0.129 731 460 Steel of present invention
I 0.07 0.41 1.96 0.096 0.030 0.060 0.034 0.008 0.004 0.003 0.73 0.186 728 434 Steel of present invention
J 0.06 0.03 2.03 0.089 0.030 0.000 0.319 0.010 0.002 0.003 0.0024 0.119 672 449 Steel of present invention
K 0.05 0.42 1.89 0.113 0.010 0.030 0.032 0.009 0.002 0.003 0.029 0.0016 0.153 730 458 Steel of present invention
L 0.06 0.53 2.04 0.106 0.010 0.000 0.006 0.008 0.002 0.002 0.0027 0.0034 0.116 734 449 Steel of present invention
M 0.07 0.51 2.24 0.094 0.020 0.000 0.029 0.009 0.003 0.003 0.0026 0.0039 0.114 727 437 Steel of present invention
N 0.07 0.42 1.89 0.089 0.020 0.080 0.035 0.008 0.002 0.003 0.0019 0.189 728 451 Steel of present invention
O 0.08 0.43 2.13 0.156 0.030 0.000 0.008 0.010 0.002 0.003 0.0022 0.0026 0.186 730 438 Steel of present invention
P 0.08 1.06 2.06 0.159 0.010 0.140 0.056 0.011 0.001 0.004 0.0026 0.309 734 441 Steel of present invention
a 0.01 1.03 1.94 0.162 0.030 0.000 0.061 0.006 0.001 0.003 0.0028 0.192 736 471 Comparative steel
b 0.18 0.42 2.12 0.148 0.000 0.000 0.019 0.016 0.006 0.003 0.0041 0.148 726 402 Comparative steel
c 0.06 0.56 0.46 0.152 0.030 0.000 0.034 0.009 0.004 0.002 0.0039 0.182 748 510 Comparative steel
d 0.07 0.76 2.53 0.000 0.000 0.000 0.049 0.012 0.003 0.003 0.0024 0.000 725 426 Comparative steel
e 0.06 0.43 2.16 0.005 0.050 0.020 0.038 0.012 0.004 0.002 0.0022 0.075 725 444 Comparative steel
f 0.05 0.29 2.26 0.044 0.030 0.000 0.029 0.015 0.003 0.002 0.0019 0.074 723 444 Comparative steel
The underlined numbers are outside of the range of the present invention.
- 37 -
[0050]
[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 970 380 Performed 7.0 650 120 GA 580
Steel of present
invention
B1 B 1280 960 470 Performed 7.0 640 120 HR -
Steel of present
invention
B2 B 1270 950 470 Performed 7.0 670 120 GI -
Steel of present
invention
B3 B 1260 970 460 Performed 7.0 750 120 GA 560 Comparative steel
B4 B 1270 950 430 Performed 7.0 650 160 GA 560
Steel of present
invention
B5 B 1260 960 390 Performed 5.5 660 100 GA 570
Steel of present
invention
B6 B 1260 950 420 Performed 10.0 680 100 GA 560
Steel of present
invention
B7 B 1270 960 530 Performed 7.0 620 80 GA 570
Steel of present
invention
B8 B 1280 960 330 Performed 7.0 640 60 GA 570
Steel of present
invention
B9 B 1260 980 390 Performed 5.5 - - HR - Comparative steel
B10 B 1210 960 420 Performed 7.0 650 120 GA 560 Comparative steel
B11 B 1280 850 430 Performed 7.0 670 120 GA 570 Comparative steel
B12 B 1270 950 25 Performed 7.0 680 480 GA 570 Comparative steel
B13 B 1280 990 650 Performed 7.0 650 360 GA 570 Comparative steel
B14 B 1280 960 430 Performed 7.0 650 40 GA 560
Steel of present
invention
B15 B 1290 970 440 Performed 7.0 600 60 GA 560
Steel of present
invention
B16 B 1270 980 430 Performed 7.0 550 80 GA 560
Steel of present
invention
B17 B 1270 1000 520 Performed 0.0 640 120 GA 560 Comparative steel
B18 B 1280 970 490 Performed 2.0 650 120 GA 560 Comparative steel
B19 B 1280 960 480 Performed 7.0 430 120 GA 570 Comparative steel
B20 B 1280 960 550 Performed 9.0 760 120 GA 560 Comparative steel
B21 B 1290 970 420 Performed 7.0 640 3 GA 560 Comparative steel
B22 B 1270 970 490 Performed 7.0 690 1800 GA 590 Comparative steel
B23 B 1280 990 470 Performed 7.0 800 120 GA 600 Comparative steel
B24 B 1270 970 460 Performed 35.0 660 360 GA 590 Comparative steel
B25 B 1260 960 500 Performed 6.0 640 120 GA 560
Steel of present
invention
C1 C 1280 960 440 Performed 7.0 650 120 GA 540
Steel of present
invention
D1 D 1270 970 430 Performed 7.0 680 100 HR -
Steel of present
invention
D2 D 1280 960 450 Performed 7.0 660 100 GI -
Steel of present
invention
D3 D 1280 970 450 Performed 7.0 760 120 GA 560 Comparative steel
D4 D 1270 970 420 Performed 7.0 650 160 GA 560
Steel of present
invention
D5 D 1270 970 470 Performed 5.5 640 120 GA 570
Steel of present
invention
D6 D 1260 960 430 Performed 10.0 650 120 GA 560
Steel of present
invention
D7 D 1280 970 460 Performed 7.0 660 160 GA 570
Steel of present
invention
D8 D 1280 970 480 Performed 7.0 650 60 GA 570
Steel of present
invention
D9 D 1270 970 380 Performed 7.0 - - HR - Comparative steel
D10 D 1220 970 520 Performed 7.0 660 120 GA 560 Comparative steel
D11 D 1290 830 390 Performed 7.0 680 120 GA 570 Comparative steel
D12 D 1280 970 25 Performed 7.0 700 120 GA 570 Comparative steel
D13 D 1260 960 680 Performed 7.0 690 480 GA 570 Comparative steel
The underlined numbers are outside of the range of the present invention.
- 38 -
[0051]
[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
D14 D 1270 970 430 Performed 7.0 650 40 GA 560
Steel of present
invention
D15 D 1280 970 450 Performed 8.0 630 60 GA 560
Steel of present
invention
D16 D 1270 970 450 Performed 7.0 660 100 GA 560
Steel of present
invention
D17 D 1270 960 490 Performed 0.0 640 120 GA 560
Comparative
steel
D18 D 1270 990 480 Performed 7.0 430 120 GA 570
Comparative
steel
D19 D 1290 990 480 Performed 6.0 770 120 GA 560
Comparative
steel
D20 D 1290 980 560 Performed 7.0 630 6 GA 560
Comparative
steel
D21 D 1290 960 460 Performed 6.0 680 2200 GA 590
Comparative
steel
D22 D 1270 990 450 Performed 7.0 820 120 GA 600
Comparative
steel
D23 D 1280 1020 420 Performed 50.0 650 360 GA 600
Comparative
steel
E1 E 1280 980 400 Performed 7.0 650 120 GA 560
Steel of present
invention
F1 F 1270 960 420 Performed 7.0 660 120 GA 550
Steel of present
invention
G1 G 1280 980 420 Performed 7.0 650 120 GA 540
Steel of present
invention
H1 H 1280 980 440 Performed 7.0 650 120 GA 560
Steel of present
invention
I1 I 1290 980 390 Performed 7.0 670 120 GA 560
Steel of present
invention
J1 J 1280 960 450 Performed 7.0 630 120 GA 570
Steel of present
invention
K1 K 1280 950 430 Performed 7.0 690 120 GA 550
Steel of present
invention
L1 L 1300 970 440 Performed 7.0 700 120 GA 560
Steel of present
invention
M1 M 1300 980 430 Performed 7.0 680 120 GA 550
Steel of present
invention
N1 N 1290 980 440 Performed 7.0 670 120 GA 560
Steel of present
invention
O1 O 1300 980 440 Performed 7.0 660 120 HR -
Steel of present
invention
O2 O 1310 980 440 Performed 7.0 670 120 GI -
Steel of present
invention
O3 O 1290 970 430 Performed 7.0 750 160 GA 560
Comparative
steel
O4 O 1300 980 430 Performed 7.0 640 120 GA 560
Steel of present
invention
O5 O 1300 960 450 Performed 6.0 640 120 GA 570
Steel of present
invention
O6 O 1310 960 390 Performed 9.0 630 80 GA 560
Steel of present
invention
O7 O 1290 960 390 Performed 7.0 680 160 GA 570
Steel of present
invention
O8 O 1290 980 420 Performed 7.0 670 40 GA 570
Steel of present
invention
O9 O 1290 980 430 Performed 7.0 - - HR -
Comparative
steel
O10 O 1200 950 510 Performed 7.0 650 160 GA 560
Comparative
steel
O11 O 1290 840 460 Performed 7.0 670 160 GA 570
Comparative
steel
O12 O 1300 980 25 Performed 7.0 680 120 GA 570
Comparative
steel
O13 O 1310 1000 650 Performed 7.0 700 360 GA 570
Comparative
steel
P1 P 1320 970 390 Performed 6.0 670 120 GA 570
Steel of present
invention
- 39 -
a1 a 1260 960 420 Performed 7.0 650 120 GA 560
Comparative
steel
b1 b 1270 970 390 Performed 7.0 670 120 GA 580
Comparative
steel
c1 c 1250 960 460 Performed 7.0 650 120 GA 560
Comparative
steel
d1 d 1260 960 450 Performed 7.0 680 120 GA 580
Comparative
steel
e1 e 1260 980 480 Performed 7.0 660 120 GA 560
Comparative
steel
f1 f 1270 980 460 Performed 7.0 660 120 GA 570
Comparative
steel
The underlined numbers are outside of the range of the present invention.
- 40 -
[0052]
For the obtained hot-rolled steel sheet, microstructure observation,
measurement of the number density of precipitate containing Ti and having an
equivalent circle diameter of 5.0 nm or less, measurement of Hvs/Hvc, evaluation of
tensile properties, evaluation of hole expansibility, and evaluation of the fatigue
resistance were performed.
[0053]

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
position with a thickness of 1/4 from the surface in the sheet thickness direction is
observed with the SEM at the magnification of 1,000 to 30,000 times. Accordingly,
ferrite, bainite, pearlite, fresh martensite, and tempered martensite were identified, and
an area ratio of the tempered martensite, the bainite, and other structures are obtained
defined as the volume percentage.
[0054]

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 -
[0055]

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 to 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 to Hvc. Hvs/Hvc was obtained from these Hvs and Hvc.
[0056]

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. In a case where the tensile strength was 980
MPa or more and the product of strength and elongation (TS × El) was 12,000 MPa×%
or more, it was determined that the strength and ductility were excellent. In addition,
in a case where YP/TS was 0.90 or more, it was determined that the proof stress was
high.
[0057]

- 42 -
The hole expansion ratio was determined by a hole expansion test method
based 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 the hole diameter
after the hole expansion test was measured to obtain the hole expansion ratio. In a
case where the hole expansion ratio is 40% or more, it is determined that the hole
expansibility is excellent. In a case where the hole expansion ratio is 40% or more, it
is suitable for undercarriage compartments having a burring portion and a stretch
flange portion.
[0058]

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, it was
determined that fatigue resistance is excellent.
The results are shown in Tables 3-1 to 3-3.
- 43 -
[0059]
[Table 3-1]
Steel
number
Kind of
steel
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
ratio
(%)
Fatigue
limit ratio
Tempered Note
martensite
Bainite Total Other structure
YP
(MPa)
TS
(MPa)
Yield
ratio
El
(%)
TS × El
(MPa × %)
A1 GA 42 58 100 - 2.6 × 1010 0.89 992 1053 0.94 14 14742 81 0.46 Steel of present invention
B1 HR 16 84 100 - 2.5 × 1010 0.88 964 1018 0.95 15 15270 76 0.45 Steel of present invention
B2 GI 13 87 100 - 1.6 × 1010 0.89 956 1006 0.95 15 15090 85 0.44 Steel of present invention
B3 GA 17 83 100 - 3.8 × 109 0.84 842 923 0.91 15 13845 32 0.37 Comparative steel
B4 GA 22 78 100 - 2.8 × 1010 0.89 979 1023 0.96 15 15345 82 0.44 Steel of present invention
B5 GA 50 50 100 - 2.1 × 1010 0.88 950 994 0.96 15 14910 78 0.42 Steel of present invention
B6 GA 34 66 100 - 6.9 × 1010 0.87 982 1049 0.94 14 14686 76 0.45 Steel of present invention
B7 GA 28 72 100 - 2.7 × 1010 0.88 905 982 0.92 15 14730 80 0.47 Steel of present invention
B8 GA 26 74 100 - 5.6 × 1010 0.90 1007 1056 0.95 14 14784 75 0.45 Steel of present invention
B9 GA 53 47 100 - 4.1 × 109 0.81 642 854 0.75 6 5124 62 0.38 Comparative steel
B10 GA 36 64 100 - 3.6 × 109 0.84 810 903 0.90 15 13545 83 0.44 Comparative steel
B11 GA 28 72 100 - 4.1 × 109 0.83 824 943 0.87 15 14145 80 0.43 Comparative steel
B12 GA 100 0 100 - 5.9 × 1011 0.92 879 1043 0.84 11 11473 62 0.47 Comparative steel
B13 GA 0 18 18 Ferrite, pearlite 1.6 × 109 0.83 567 741 0.77 18 13338 79 0.42 Comparative steel
B14 GA 28 72 100 - 3.2 × 1010 0.91 970 1022 0.95 15 15330 82 0.46 Steel of present invention
B15 GA 19 81 100 - 2.6 × 1010 0.89 964 1019 0.95 15 15285 83 0.47 Steel of present invention
B16 GA 28 72 100 - 1.3 × 1010 0.90 956 992 0.96 14 13888 81 0.48 Steel of present invention
B17 GA 16 84 100 - 2.9 × 109 0.77 822 953 0.86 15 14295 79 0.38 Comparative steel
B18 GA 22 78 100 - 4.1 × 109 0.79 832 968 0.86 15 14520 80 0.39 Comparative steel
B19 GA 16 84 100 - 4.1 × 109 0.83 726 892 0.81 8 7136 64 0.37 Comparative steel
B20 GA 17 83 100 - 2.8 × 109 0.84 762 842 0.90 16 13472 35 0.36 Comparative steel
B21 GA 36 64 100 - 3.9 × 109 0.84 742 906 0.82 9 8154 71 0.38 Comparative steel
B22 GA 13 87 100 - 2.7 × 109 0.82 872 968 0.90 14 13552 89 0.45 Comparative steel
B23 GA 22 56 78 Ferrite 2.2 × 109 0.81 762 916 0.83 15 13740 31 0.39 Comparative steel
B24 GA 6 33 39
Ferrite
(recrystallization)
2.2 × 1010 0.74 616 842 0.73 16 13472 46 0.35 Comparative steel
B25 GA 26 70 96 Ferrite 5.7 × 109 0.88 902 992 0.91 15 14880 66 0.41 Steel of present invention
C1 GA 17 83 100 - 4.1 × 1010 0.89 980 1034 0.95 14 14476 82 0.46 Steel of present invention
- 44 -
[0060]
[Table 3-2]
Steel
number
Kind of
steel
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
ratio
(%)
Fatigue
limit ratio
Tempered Note
martensite
Bainite Total Other structure
YP
(MPa)
TS
(MPa)
Yield
ratio
El
(%)
TS × El
(MPa × %)
D1 HR 35 65 100 - 2.9 × 1010 0.92 974 1038 0.94 15 15570 83 0.48
Steel of present
invention
D2 GI 25 75 100 - 3.3 × 1010 0.91 969 1034 0.94 14 14476 78 0.47
Steel of present
invention
D3 GA 26 74 100 - 2.4 × 109 0.84 912 937 0.97 14 13118 29 0.38 Comparative steel
D4 GA 43 57 100 - 3.7 × 1010 0.91 982 1033 0.95 14 14462 85 0.46
Steel of present
invention
D5 GA 14 86 100 - 1.6 × 1010 0.89 956 1010 0.95 16 16160 78 0.45
Steel of present
invention
D6 GA 35 65 100 - 6.9 × 1010 0.90 988 1054 0.94 14 14756 82 0.47
Steel of present
invention
D7 GA 13 87 100 - 2.6 × 1010 0.92 982 1035 0.95 15 15525 84 0.48
Steel of present
invention
D8 GA 14 86 100 - 2.8 × 1010 0.91 991 1037 0.96 15 15555 80 0.47
Steel of present
invention
D9 GA 49 51 100 - 4.3 × 109 0.82 673 876 0.77 5 4380 62 0.37 Comparative steel
D10 GA 13 87 100 - 3.7 × 109 0.83 832 910 0.91 15 13650 76 0.42 Comparative steel
D11 GA 46 54 100 - 3.9 × 109 0.84 816 937 0.87 15 14055 78 0.43 Comparative steel
D12 GA 100 0 100 - 5.6 × 1011 0.94 991 1068 0.93 11 11748 80 0.48 Comparative steel
D13 GA 0 13 13 Ferrite, pearlite 2.8 × 109 0.79 682 786 0.87 18 14148 76 0.43 Comparative steel
D14 GA 35 65 100 - 3.3 × 1010 0.91 986 1039 0.95 14 14546 83 0.46
Steel of present
invention
D15 GA 14 86 100 - 4.9 × 1010 0.89 977 1035 0.94 14 14490 84 0.45
Steel of present
invention
D16 GA 15 85 100 - 3.5 × 1010 0.88 980 1037 0.95 13 13481 81 0.47
Steel of present
invention
D17 GA 13 87 100 - 4.0 × 109 0.80 812 967 0.84 14 13538 86 0.37 Comparative steel
D18 GA 15 85 100 - 4.5 × 109 0.83 726 909 0.80 7 6363 59 0.38 Comparative steel
D19 GA 13 87 100 - 2.6 × 109 0.84 762 868 0.88 16 13888 32 0.36 Comparative steel
D20 GA 14 86 100 - 3.8 × 109 0.81 829 1052 0.79 8 8416 68 0.44 Comparative steel
D21 GA 13 87 100 - 2.2 × 109 0.79 862 958 0.90 14 13412 73 0.46 Comparative steel
D22 GA 19 81 100 - 2.6 × 109 0.82 749 938 0.80 13 12194 29 0.42 Comparative steel
D23 GA 7 42 49
Ferrite
(recrystallization)
1.9 × 1010 0.73 598 864 0.69 16 13824 46 0.38 Comparative steel
E1 GA 46 54 100 - 5.6 × 1010 0.91 954 1016 0.94 15 15240 78 0.47 Steel of present
- 45 -
invention
F1 GA 29 71 100 - 3.8 × 1010 0.90 964 1027 0.94 14 14378 80 0.48
Steel of present
invention
G1 GA 37 63 100 - 3.9 × 1010 0.92 970 1033 0.94 14 14462 83 0.45
Steel of present
invention
H1 GA 26 74 100 - 4.3 × 1010 0.91 955 1028 0.93 14 14392 79 0.47
Steel of present
invention
- 46 -
[0061]
[Table 3-3]
Steel
number
Kind of
steel
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
ratio
(%)
Fatigue
limit ratio
Tempered Note
martensite
Bainite Total Other structure
YP
(MPa)
TS
(MPa)
Yield ratio
El
(%)
TS × El
(MPa × %)
I1 GA 45 55 100 - 5.1 × 1010 0.92 963 1032 0.93 15 15480 81 0.46 Steel of present invention
J1 GA 11 89 100 - 5.6 × 1010 0.90 960 1032 0.93 15 15480 80 0.46 Steel of present invention
K1 GA 32 68 100 - 5.6 × 1010 0.89 958 1024 0.94 14 14336 83 0.45 Steel of present invention
L1 GA 15 85 100 - 4.3 × 1010 0.91 961 1019 0.94 16 16304 82 0.47 Steel of present invention
M1 GA 16 84 100 - 5.1 × 1010 0.90 963 1028 0.94 14 14392 78 0.48 Steel of present invention
N1 GA 17 83 100 - 3.7 × 1010 0.91 940 1009 0.93 15 15135 77 0.46 Steel of present invention
O1 HR 13 87 100 - 6.8 × 1011 0.92 1166 1196 0.97 12 14352 52 0.45 Steel of present invention
O2 GI 12 88 100 - 5.5 × 1011 0.92 1182 1201 0.98 13 15613 54 0.46 Steel of present invention
O3 GA 21 79 100 - 4.3 × 109 0.84 842 945 0.89 15 14175 19 0.38 Comparative steel
O4 GA 22 78 100 - 5.6 × 1011 0.90 1161 1196 0.97 12 14352 56 0.46 Steel of present invention
O5 GA 6 94 100 - 5.2 × 1011 0.92 1139 1186 0.96 13 15418 60 0.47 Steel of present invention
O6 GA 46 54 100 - 6.3 × 1011 0.91 1179 1223 0.96 12 14676 54 0.45 Steel of present invention
O7 GA 48 52 100 - 5.3 × 1011 0.92 1164 1202 0.97 12 14424 58 0.46 Steel of present invention
O8 GA 29 71 100 - 5.5 × 1011 0.91 1176 1208 0.97 12 14496 55 0.46 Steel of present invention
O9 GA 22 78 100 - 3.8 × 109 0.83 672 862 0.78 4 3448 43 0.39 Comparative steel
O10 GA 13 87 100 - 4.3 × 109 0.84 842 962 0.88 14 13468 51 0.43 Comparative steel
O11 GA 9 91 100 - 4.8 × 109 0.83 853 971 0.88 14 13594 48 0.42 Comparative steel
O12 GA 100 0 100 - 6.7 × 1011 0.94 1156 1209 0.96 9 10881 55 0.48 Comparative steel
O13 GA 0 9 9
Ferrite,
pearlite
3.6 × 109 0.79 745 854 0.87 17 14518 59 0.42 Comparative steel
P1 GA 46 54 100 - 5.9 × 1011 0.92 1176 1216 0.97 12 14592 52 0.46 Steel of present invention
a1 GA 0 39 39 Ferrite 3.4 × 109 0.78 642 756 0.85 22 16632 72 0.39 Comparative steel
b1 GA 29 71 100 - 2.4 × 109 0.76 763 896 0.85 17 15232 56 0.38 Comparative steel
c1 GA 0 33 33
Ferrite,
pearlite
8.4 × 108 0.75 682 786 0.87 19 14934 56 0.38 Comparative steel
d1 GA 12 88 100 - 0 0.77 576 675 0.85 27 18225 63 0.36 Comparative steel
- 47 -
e1 GA 13 87 100 - 2.2 × 108 0.74 613 721 0.85 24 17304 70 0.37 Comparative steel
f1 GA 6 94 100 - 1.9 × 109 0.76 598 689 0.87 26 17914 65 0.36 Comparative steel I I I= I I= I
- 48 -
[0062]
As can be seen from Tables 1 to 3-3, 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 an
equivalent circle diameter of 5.0 nm or less was 5.0 × 109 pieces/mm3 or more. In
addition, in these examples, a tensile strength of 980 MPa or more, a high yield ratio of
0.90 or more, TS × El of 12,000 MPa×% or more, and excellent fatigue resistance were
achieved.
[0063]
In contrast, in comparative examples in which any one or more of the
chemical composition, the slab heating temperature, the finish temperature, the coiling
temperature, the light reduction conditions, and the heat treatment conditions are
outside of the range of the present invention, the microstructure of the steel sheet,
Hvs/Hvc, and the number density or the tensile strength of the precipitate containing Ti
and having an equivalent circle diameter of 5.0 nm or less was low. As a result,
tensile strength of 980 MPa or more, high proof stress of YP/TS ≥ 0.90, high ductility
of TS × El ≥ 12,000 MPa×%, and excellent fatigue resistance of fatigue limit/TS ≥
0.40 or more could not be obtained at the same time.
[Industrial Applicability]
[0064]
According to the present invention, it is possible to provide a high-strength
steel sheet having a tensile strength of 980 MPa or more, which has high proof stress,
high ductility, and excellent fatigue resistance. This steel sheet has great industrial
value, because it contributes to weight reduction of vehicle components. In addition,
- 49 -
this steel sheet is suitable for undercarriage compartments of vehicles, since it has high
strength (high tensile strength), high proof stress, high ductility, and excellent fatigue
resistance.

WE CLAIMS

1. A high-strength steel sheet, comprising,
as a chemical composition, by mass%:
C: 0.020 to 0.090%;
Si: 0.01 to 2.00%;
Mn: 1.00 to 3.00%;
Ti: 0.010 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%;
Nb: 0 to 0.100%;
V: 0 to 0.100%;
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,
- 51 -
wherein 0.100 ≤ Ti + Nb + V ≤ 0.450 is satisfied,
a microstructure contains, by volume percentage, 95% or more of tempered
martensite and bainite in total, and a remainder consists of ferrite and pearlite,
the microstructure contains 5.0 × 109 pieces/mm3 or more of, per unit volume,
precipitate having an equivalent circle diameter of 5.0 nm or less and containing Ti,
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,
a tensile strength is 980 MPa or more, and
a product of the tensile strength and elongation is 12,000 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%;
Nb: 0.005 to 0.100%;
V: 0.005 to 0.100%;
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
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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
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 0.090%; Si: 0.01 to 2.00%; Mn: 1.00 to 3.00%; Ti: 0.010 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%; Nb: 0 to
0.100%; V: 0 to 0.100%; 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 1,230°C or
higher;
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 300°C or higher and
600°C or lower and then 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 higher than 5% and 30% or
less; 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.
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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.