[Technical Field]
5 [0001]
The present invention relates to a steel sheet and a manufacturing method
thereof.
Priority is claimed on Japanese Patent Application No. 2020-123531, filed July
20, 2020, the content of which is incorporated herein by reference.
10 [Background Art]
[0002]
In order to suppress the amount of carbon dioxide exhausted from automobiles,
attempts are underway to reduce the weights of automobile vehicle bodies while ensuring
safety by using high strength steel sheets. However, generally, an increase in the
15 strength of steel sheets degrades formability. In high strength steel sheets, it is difficult
to satisfy both strength and formability, and several measures have been proposed to
solve this problem.
For steel sheets that are used for components for vehicles, not only strength but
also a variety of workability that is required at the time of forming components such as
20 press workability and weldability are required. Specifically, from the viewpoint of
press workability, there are many cases where excellent elongation (total elongation in
tensile tests; El) and excellent hole expansibility (hole expansion rate; A.) are required for
steel sheets.
[0003]
25 For example, Patent Document 1 discloses a thin steel sheet having a
1
composition of, by mass%, C: 0.07% or more and 0.20% or less, Si: 0.01% or more and
2.0% or less, Mn: 1.8% or more and 3.5% or less, P: 0.05% or less, S: 0 .005% or less,
Al: 0.01% or more and 2.0% or less, N: 0.0060% or less, Si + Al: 0.7% or more and a
remainder including Fe and unavoidable impurities and a metallurgical structure in which
5 a ferrite area ratio is 30% or less (including 0% ), a tempered martensite area ratio is 70%
or more (including 100% ), a residual austenite area ratio is 4.5% or less (including 0% ),
and an average aspect ratio of iron-based carbides, precipitated in tempered martensite
grains, having a grain size in the largest 10% is 3.5 or more.
[Citation List]
10 [Patent Document]
[0004]
[Patent Document 1]
PTC International Publication No. WO 2018/043453
[Summary of the Invention]
15 [Problems to be Solved by the Invention]
[0005]
However, when a full hard structure is present in a soft structure as in Patent
Document 1, voids are likely to be generated at the interface between these structures,
and, as a result, there is a possibility that hole expansibility may be impaired.
20 In addition, generally, the structures of steel sheets are originally anisotropic.
Therefore, when pressing or the like is performed on a steel sheet, the distortability also
becomes anisotropic due to the anisotropy of the structures. However, in Patent
Document 1, attention is not paid to such anisotropy of the distortability of the steel
sheet.
25 In consideration of the above-described circumstance, an objective of the present
2
invention is to provide a steel sheet having a high strength and being excellent in terms of
elongation, hole expansibility and the anisotropy of distortability and a manufacturing
method thereof.
[Means for Solving the Problem]
5 [0006]
The present invention has been made based on the above-described finding, and
the gist of the present invention is as described below.
[0007]
(1) A steel sheet according to one aspect of the present invention, in which a
10 chemical composition contains, by mass%:
15
20
25
C: 0.20% to 0.40%,
Si: 0.5% to 2.0%,
Al: 0.001% to 1.0%,
Mn: 0.1% to 4.0%,
V: 0.150% or less,
Ti: 0.10% or less,
Nb: 0.10% or less,
P: 0.0200% or less,
S: 0.0200% or less,
N: 0.0200% or less,
0: 0.0200% or less,
Ni: 0% to 1.00%,
Mo: 0% to 1.00%,
Cr: 0% to 2.000%,
B: 0% to 0.0100%,
3
5
10
15
20
Cu: 0% to 0.500%,
W: 0% to 0.10%,
Ta: 0% to 0.10%,
Sn: 0% to 0.050%,
Co: 0% to 0.50%,
Sb: 0% to 0.050%,
As: 0% to 0.050%,
Mg: 0% to 0.050%,
Ca: 0% to 0.040%,
Y: 0% to 0.050%,
Zr: 0% to 0.050%, and
La: 0% to 0.050%
with a remainder consistinf of iron and impurities,
a total amount ofV, Ti and Nb is 0.030% to 0.150%,
a metallurgical structure at a thickness 1/4 portion is, by volume percentage,
martensite: 40% to 97%,
ferrite and bainite: 50% or less,
residual austenite: 3% to 20%, and
a remainder in microstructure: 5% or less,
an area ratio of residual austenite having an aspect ratio of 3 or more is 80% or
more with respect to a total area of the residual austenite, and
the number of carbides having a grain diameter of 8 to 40 nm per square
micrometer is five or more in the residual austenite.
(2) The steel sheet according to (1), in which the chemical composition may
25 contain, by mass%, one or more selected from the group consisting of
4
5
10
15
Ni: 0.01% to 1.00%,
Mo: 0.01% to 1.00%,
Cr: 0.001% to 2.000%,
B: 0.0001% to 0.0100%,
Cu: 0.001% to 0.500%,
W: 0.001% to 0.10%,
Ta: 0.001% to 0.1 0%,
Sn: 0.001% to 0.050%,
Co: 0.001% to 0.50%,
Sb: 0.001% to 0.050%,
As: 0.001% to 0.050%,
Mg: 0.0001% to 0.050%,
Ca: 0.001% to 0.040%,
Y: 0.001% to 0.050%,
Zr: 0.001% to 0.050%, and
La: 0.001% to 0.050%.
(3) The steel sheet according to (1) or (2) may have a hot-dip galvanized layer
on a surface.
(4) The steel sheet according to (1) or (2) may have a hot-dip galvannealed layer
20 on a surface.
( 5) A manufacturing method of a steel sheet according to an aspect of the present
invention has a hot rolling step of heating a slab having the chemical composition
according to (1) or (2) at 1150°C or higher for one hour or longer and hot-rolling the slab
to produce a hot-rolled steel sheet in which prior austenite grain diameters are less than
25 30 !-!ill,
5
a first cooling step of cooling the hot-rolled steel sheet to a temperature range of
800°C or lower within three seconds from an end of the hot rolling step,
a coiling step of cooling the hot-rolled steel sheet after the first cooling step to a
temperature range of 300°C or lower at an average cooling rate of 30 °C/s or faster and
5 coiling the hot-rolled steel sheet,
a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step
at a rolling reduction of 0.1% to 30% to produce a cold-rolled steel sheet,
an annealing step of heating the cold-rolled steel sheet in a temperature range of
480°C to Ac 1 at an average heating rate of 0.5 to 1.5 °C/s and soaking the cold-rolled
10 steel sheet in a temperature range of Ac1 to Ac3,
a second cooling step of cooling the cold-rolled steel sheet after the annealing
step at an average cooling rate of 4 °C/s or faster, and
a temperature retention step of retaining the cold-rolled steel sheet after the
second cooling step at 300°C to 480°C for 10 seconds or longer.
15 (6) The manufacturing method of a steel sheet according to (5), in which the hot
rolling step may have a finish rolling step of continuous! y passing the slab through a
plurality of rolling stands to perform rolling,
in the finish rolling step:
a rolling start temperature in the rolling stand third from a final of the rolling
20 stands may be 850°C to l 000°C;
25
in each of the three last rolling stands in the finish rolling, the slab may be rolled
at a rolling reduction of larger than l 0%;
an interpass time between the individual rolling stands in the three last rolling
stands in the finish rolling may be three seconds or shorter; and
(Tn- Tn+l) that is a difference between an exit temperature Tn of the n1
h rolling
6
stand and an entrance temperature Tn+l of the (n + 1)1h rolling stand toward the
downstream side of the four last rolling stands in the finish rolling may be 1 ooc or more.
(7) The manufacturing method of a steel sheet according to (5) or (6), in which
the cold-rolled steel sheet after the annealing step may be controlled to be in a
5 temperature range of (zinc plating bath temperature- 40)°C to (zinc plating bath
temperature+ 50)°C and immersed in a hot-dip galvanizing bath, thereby forming hotdip
galvanized layer.
(8) The manufacturing method of a steel sheet according to (7), in which the hotdip
galvanized layer may be alloyed in a temperature range of 300°C to 500°C.
10 [Effects of the Invention]
[0008]
According to the aspects of the present invention, it is possible to provide a steel
sheet having a high strength and being excellent in terms of elongation, hole
expansibility and the anisotropy of distortability and a manufacturing method thereof.
15 [Embodiment(s) for implementing the Invention]
[0009]
Hereinafter, a steel sheet according to the present embodiment and a
manufacturing method thereof will be described. However, the present invention is not
limited only to a configuration disclosed in the present embodiment and can be modified
20 in a variety of manners within the scope of the gist of the present invention. In addition,
numerical limiting ranges expressed below using "to" include the lower limit value and
the upper limit value in the ranges. Numerical values expressed with "less than" or
"more than" are not included in numerical ranges. In the following description, "%"
regarding the chemical composition of the steel sheet indicates "mass%" unless
25 particular! y otherwise designated.
7
5
10
15
20
25
[0010]
In a steel sheet according to the present embodiment, the chemical composition
contains, by mass%,
C: 0.20% to 0.40%,
Si: 0.50% to 2.0%,
Al: 0.0010% to 1.0%,
Mn: 0.1% to 4.0%,
V: 0.150% or less,
Ti: 0.10% or less,
Nb: 0.10% or less,
P: 0.0200% or less,
S: 0.0200% or less,
N: 0.0200% or less,
0: 0.0200% or less,
Ni: 0% to 1.00%,
Mo: 0% to 1.00%,
Cr: 0% to 2.000%,
B: 0% to 0.0100%,
Cu: 0% to 0.500%,
W: 0% to 0.10%,
Ta: 0% to 0.10%,
Sn: 0% to 0.050%,
Co: 0% to 0.50%,
Sb: 0% to 0.050%,
As: 0% to 0.050%,
8
5
10
15
Mg: 0% to 0.050%,
Ca: 0% to 0.040%,
Y: 0% to 0.050%,
Zr: 0% to 0.050%, and
La: 0% to 0.050%
with a remainder consisting of iron and impurities,
a total amount ofV, Ti and Nb is 0.030% to 0.150%,
the metallurgical structure at a thickness 1/4 portion is, by volume percentage,
martensite: 40% to 97%,
ferrite and bainite: 50% or less,
residual austenite: 3% to 20%, and
a remainder in microstructure: 5% or less,
residual austenite having an aspect ratio of 3 or more occupies 80% or more of
all residual austenite, and
the number of carbides having a grain diameter of 8 to 40 nm per square
micrometer is five or more in the residual austenite.
[0011]
Hereinafter, the steel sheet according to one aspect of the present invention will
be described.
20 [0012]
First, the metallurgical structure of the steel sheet according to the present
embodiment will be described. Hereinafter, microstructural fractions will be expressed
by volume percentages, and thus the unit "%" of the microstructural fractions means
"vol%". For structures for which the microstructural fractions are identified by image
25 processing, area ratios are regarded as volume percentages. The metallurgical structure
9
of the steel sheet according to the present embodiment represents the metallurgical
structure at a thickness 1/4 portion unless particularly otherwise described.
[0013]
Metallurgical structure
5 Ferrite and bainite: 50% or less in total
Ferrite is a soft structure and is thus a structure that is easily distorted and
contributes to improvement in elongation. However, in order to obtain a preferable
strength, the volume percentage of ferrite needs to be limited.
In addition, bainite is soft compared with martensite and thus has an effect on
10 improvement in ductility. However, in order to obtain a preferable strength, similar to
ferrite, the volume percentage of bainite needs to be limited.
In the steel sheet according to the present embodiment, the volume percentages
of ferrite and bainite are 50% or less in total. In order to increase the strength of the
steel sheet, the total volume percentage of ferrite and bainite may be set to 40% or less or
15 may be set to 35% or less in total. In order to obtain the effect of the present
embodiment, the lower limit of the total volume percentage of ferrite and bainite is 0%
since ferrite and bainite are not essential metallurgical structures.
20
25
[0014]
Residual austenite: 3% to 20%
Residual austenite has an effect on improving ductility by the transformationinduced
plasticity effect (TRIP effect) to contribute to improvement in uniform
elongation. In order to obtain the effect, the volume percentage of residual austenite is
set to 3% or more. The volume percentage of residual austenite is preferably 5% or
more and more preferably 7% or more.
On the other hand, when the volume percentage of residual austenite becomes
10
excessive, the grain diameters of residual austenite become large. Residual austenite
having such large grain diameters becomes coarse and full hard martensite after
distortion. Martensite having a high C concentration that is formed from such residual
austenite is likely to act as a starting point of cracking and degrades hole expansibility,
5 which is not preferable. Therefore, the volume percentage of residual austenite is set to
20% or less. The volume percentage of residual austenite is preferably 18% or less and
more preferably 16% or less.
[0015]
In addition, in the present embodiment, as described below, it is possible to
10 enhance the stability of residual austenite by controlling the aspect ratios of residual
austenite and the number density of fine carbides in residual austenite. When the
stability of residual austenite is high, it is possible to suppress strain-induced
transformation to a fresh martensite phase having a high C concentration, which is a full
hard phase. That is, when the stability of residual austenite is enhanced, fresh
15 martensite having a high C concentration is less likely to be formed even when high
strain is applied thereto. As a result, it is possible to suppress the occurrence of
cracking in the formed fresh martensite having a high C concentration and the
propagation of fractures, which leads to improvement in hole expansibility.
Furthermore, the high stability of residual austenite makes it possible to suppress the
20 localization of distortion and thus makes it possible to degrade the anisotropy of
distortability.
[0016]
Martensite: 40% to 97%
The metallurgical structure of the steel sheet according to the present
25 embodiment includes 40% to 97% of martensite. Here, "martensite" is a general term
11
5
10
of so-called fresh martensite and tempered martensite that are formed in the process of
cooling and temperature rising in a heat treatment of steel.
Fresh martensite is a full hard structure having a high dislocation density and is
thus a structure that contributes to improvement in tensile strength.
Tempered martensite is, similar to fresh martensite, a collection of lath-shaped
crystal grains. On the other hand, unlike fresh martensite, tempered martensite is a full
hard structure containing fine iron-based carbides inside due to tempering. Tempered
martensite can be obtained by tempering fresh martensite formed by cooling or the like
after annealing through a heat treatment or the like.
Bainite is also a structure containing fine iron-based carbides, but can be
distinguished from tempered martensite due to the fact that there is a plurality of variants
of the iron-based carbide in tempered martensite, but there is a single variant of the ironbased
carbide in bainite.
In the steel sheet of the present embodiment, the volume percentage of
15 martensite is set to 40% or more. The volume percentage of martensite is preferably set
to 45% or more and more preferably set to 50% or more. In addition, the volume
percentage of martensite is set to 97% or less, preferably set to 95% or less and more
preferably set to 90% or less.
[0017]
20 Remainder in microstructure: 5% or less
As the structures of the remainder other than the above-described ferrite, bainite,
residual austenite and martensite, pearlite and the like are exemplary examples. In order
to obtain a desired characteristic of the steel sheet according to the present embodiment,
the volume percentage of the remainder in microstructure is set to 5% or less, preferably
25 set to 4% or less and more preferably set to 3% or less. The remainder in
12
5
microstructure may not be present, and the lower limit thereof is 0%.
[0018]
Proportion of residual austenite having aspect ratio of 3 or more: 80% or more of all
residual austenite
When residual austenite is given a needle-like shape, the stability of residual
austenite at the time of receiving strain improves. That is, when the stability of residual
austenite is high, it is possible to suppress strain-induced transformation to fresh
martensite having a high C concentration, which is a full hard phase, even when residual
austenite receives strain due to working. This mechanism will be described below.
10 [0019]
Residual austenite transforms to martensite stepwise from grain boundaries, and
this transformation induces strain. When this martensitic transformation proceeds,
dislocations initiated in the vicinity of a grain boundary move through the inside of the
grain to the grain boundary on the opposite side, and the dislocations are accumulated.
15 In a case where residual austenite has a needle-like shape, the distance from the vicinity
of the grain boundary where the dislocations are initiated to the grain boundary where the
dislocations are accumulated is short. Therefore, a repulsive force is generated between
the accumulated dislocation and a dislocation that is newly initiated, and strain that is
induced by martensitic transformation is not allowed. Since martensitic transformation
20 is inhibited by the above-described mechanism, the stability of residual austenite
Improves.
[0020]
Residual austenite that is formed with no control over the shape does not
become a needle-like structure, and thus the stability varies with individual residual
25 austenite. Therefore, the anisotropy of distortability is large in residual austenite that is
13
not a needle-like structure. On the other hand, in the steel sheet according to the present
embodiment, residual austenite is stabilized by giving a needle-like shape, and thus it is
possible to reduce the anisotropy of distortability.
In the present embodiment, "residual austenite given a needle-like shape" is
5 defined as "residual austenite having an aspect ratio of 3 or more". In the steel sheet of
the present embodiment, the area ratio of residual austenite having an aspect ratio of 3 or
more is 80% or more with respect to the total area of residual austenite. When residual
austenite having an aspect ratio of 3 or more is 80% or more of all residual austenite, the
anisotropy of distortability is reduced. Residual austenite having an aspect ratio of 3 or
10 more is preferably 83% or more and more preferably 85% or more of all residual
austenite. The upper limit of the proportion of residual austenite having an aspect ratio
of 3 or more in all residual austenite is not particularly determined, but is ideally 100%.
[0021]
Number density of carbides having grain diameter of 8 to 40 nm in residual austenite:
15 Five carbides/~m2 or more
In the present embodiment, residual austenite is stabilized not only by giving a
needle-like shape to residual austenite but also by controlling the amount of fine
precipitates in residual austenite. This is because the fine precipitates in residual
austenite stop the propagation of strain-induced transformation from grain boundaries.
20 In the steel sheet of the present embodiment, fine carbides are precipitated in residual
austenite. The scope of the carbide in the present embodiment includes a carbonitride.
[0022]
In the present embodiment, the grain diameters of the carbides effective for
suppressing the propagation of strain-induced transformation is 8 nm or more. That is,
25 carbides having a grain diameter of less than 8 nm do not effectively act for the
14
suppression of the propagation of strain-induced transformation. Therefore, in the
present embodiment, carbides having a grain diameter of less than 8 nm are not
considered as precipitates that need to be controlled. On the other hand, an excessive
increase in the grain diameter of the carbide means a concern of the carbide itself being
5 likely to crack and likely to cause breakage. Therefore, the grain diameters of the
carbides are preferably 40 nm or less. From the above-described viewpoints, in the
present embodiment, the number density of carbides having a grain diameter of 8 to 40
nm is controlled.
10
15
20
[0023]
In the steel sheet according to the present embodiment, in order to suppress the
propagation of strain-induced transformation, the number density of carbides having a
grain diameter of 8 to 40 nm in residual austenite is 5 carbides/~m2 or more. The
number density of the carbides is preferably 6 carbides/~m2 or more and more preferably
7 carbides/~m2 or more.
In such a case, preferable characteristics can be obtained by stabilizing residual
austenite. The upper limit of the number density of carbides having a grain diameter of
8 to 40 nm in residual austenite is not particularly determined, but is approximately 100
carbides/~m2 in the composition and the heat treatment of the present embodiment.
[0024]
Next, the identification of ferrite, bainite, residual austenite and martensite and
the calculation of the volume percentages will be described. As described above, the
values of area ratios calculated by image processing are regarded as volume percentages.
In addition, in the present embodiment, the metallurgical structure of a cross section
parallel to a rolling direction at a 1/4 depth of the sheet thickness (thickness 1/4 portion)
25 from the surface of the steel sheet is regulated. The reason therefor is the metallurgical
15
structure at this position shows a typical metallurgical structure of the steel sheet. In the
present embodiment, the "1/4 depth position" of the sheet thickness is an observation
position for specifying the metallurgical structure and is not strictly limited to the 1/4
depth. A metallurgical structure that is obtained by observing somewhere in a range of
5 1/8 to 3/8 depth of the sheet thickness can be regarded as the metallurgical structure at
the 1/4 depth position.
[0025]
The volume percentage of residual austenite can be calculated by measuring
diffraction intensities using X-rays.
10 [0026]
In the measurement using X-rays, a portion from the sheet surface of a sample to
a depth 1/4 position is removed by mechanical polishing and chemical polishing, and the
microstructural fraction of residual austenite can be calculated from the integrated
intensity ratio of the diffraction peaks of (200) and (211) of a bee phase and (200), (220)
15 and (311) of an fcc phase using MoKa rays at a sheet thickness 1/4 position. As a
general calculation method, a five-peak method is used.
20
[0027]
The volume percentage of fresh martensite is obtained by the following
procedure.
An observed section of the sample is etched with a LePera solution, and a
secondary electron image of a 100 ~m x 100 ~m region in the sheet thickness 1/8 to 3/8
range, in which the sheet thickness 1/4 is centered, obtained with a field emission
scanning electron microscope (FE-SEM) is observed at a magnification of 3000 times.
Since fresh martensite and residual austenite are not corroded by LePera corrosion, the
25 area ratio of uncorroded regions is the total area ratio of fresh martensite and residual
16
austenite. In the present embodiment, the total area ratio of these fresh martensite and
residual austenite is regarded as the "total volume percentage" of these structures. The
volume percentage of fresh martensite can be calculated by subtracting the volume
percentage of residual austenite measured with X-rays from the area ratio of the
5 uncorroded regions.
[0028]
The volume percentages of ferrite, bainite and tempered martensite can be
determined from a secondary electron image obtained by observing the 1/8 to 3/8 sheet
thickness range (that is, the sheet thickness range in which the 1/4 sheet thickness
10 position is centered) with FE-SEM. The observed section is a sheet thickness cross
section of the steel sheet parallel to the rolling direction. Polishing and Nital etching are
performed on the observed section, and a 100 ~m x 100 ~m region in the sheet thickness
1/8 to 3/8 range, in which the sheet thickness 1/4 is centered, on the observed section is
observed at a magnification of 3000 times. The same region as the region observed by
15 the LePera corrosion can be confirmed by leaving a plurality of indentations around the
region observed by the LePera corrosion. There is a case where the microstructure
(configuration elements) of the steel sheet significant! y differs in each of the vicinity of
the steel sheet surface and the vicinity of the steel sheet center in the sheet thickness
direction from the microstructure in other portions due to decarburization and Mn
20 segregation. Therefore, in the present embodiment, the microstructure is observed at
the 1/4 sheet thickness position as a base. Ferrite is a structure in which the insides of
grain boundaries appear in uniform contrast. Bainite is a collection of lath-shaped
crystal grains and is a structure in which iron-based carbides having a major axis of 20
nm or more are not contained or a structure in which iron-based carbides having a major
25 axis of 20 nm or more are contained and the carbides belong to a single variant, that is, a
17
group of iron-based carbides elongated in the same direction. Here, the group of ironbased
carbides elongated in the same direction refers to a group in which the difference
in the elongation direction in the group of iron-based carbides is 5° or less. Tempered
martensite can be distinguished from bainite due to the fact that cementite in the structure
5 has a plurality of variants.
[0029]
The volume percentage of martensite can be obtained by combining the volume
percentage of fresh martensite and the volume percentage of tempered martensite
specified by the above-described method.
10 [0030]
The proportion of residual austenite having an aspect ratio of 3 or more in all
residual austenite is determined by an EBSD analysis method in which FE-SEM is used.
Specifically, a test piece in which a sheet thickness cross section of the steel
sheet parallel to the rolling direction is used as an observed section is collected, the
15 observed section of the test piece is polished, then, a strain-influenced layer is removed
by electrolytic polishing, and EBSD analysis is performed on a 100 ~m x 100 ~m region
in a sheet thickness 1/8 to 3/8 range, in which the sheet thickness 1/4 is centered, at
measurement steps set to 0.05 ~m. The measurement magnification in the present
embodiment is set to 3000 times.
20
25
A residual austenite map is produced from measured data, residual austenite
having an aspect ratio of 3 or more is extracted, and the area fraction is obtained.
[0031]
The number density of the carbides having a grain diameter of 8 to 40 nm in
residual austenite is measured as described below.
In a thin film sample of a round region having a diameter of 3.0 mm at the 1/4
18
position from the surface of the steel sheet, three visual fields in a region containing
austenite phase are observed using a transmission electron microscope (TEM) at a
magnification of 500,000 times. A precipitate from which the corresponding alloy
carbide-forming elements (Ti, Nb and V) are detected by the energy-dispersive X-ray
5 spectroscopy (EDX) in each visual field is determined as a carbide. The area of each
precipitate in austenite phase is obtained using an image analysis apparatus and
converted into a circle-converted diameter.
A value obtained by calculating the number of carbides having a circleconverted
diameter of 8 nm or more and 40 nm or less and dividing this by the area of the
10 observed visual field is regarded as the number density of carbides in each visual field.
15
The same operation is performed on the three visual fields, and the obtained arithmetic
mean is determined as the number density of the carbides having a circle-converted
diameter of 8 to 40 nm.
[0032]
Next, the reasons for limiting the chemical composition of the steel sheet
according to the present embodiment will be described. Hereinafter, "%"relating to the
composition means "mass%".
[0033]
Chemical composition
20 C: 0.20% to 0.40%
25
C is an element that ensures a predetermined amount of martensite and improves
the strength of the steel sheet. When the C content is 0.20% or more, it is possible to
obtain a predetermined amount of martensite and to ensure a desired tensile strength.
The C content is preferably 0.25% or more.
In addition, in order to ensure the hole expansibility, the C content is set to
19
0.40% or less. The C content is preferably 0.35% or less.
[0034]
Si: 0.5% to 2.0%
Si is a useful element for increasing the strength of the steel sheet by solid
5 solution strengthening. In addition, Si suppresses the formation of cementite and is thus
an effective element for forming residual austenite after annealing by accelerating the
concentration of carbon (C) in austenite. In addition, Si has an effect on segregating
carbon (C) on y grain boundaries in an annealing step to be described below. When the
Si content is set to 0.5% or more, it is possible to obtain the effect of the above-described
10 action. The Si content is preferably 0.6% or more and more preferably 0.7% or more.
In addition, the Si content is set to 2.0% or less in consideration of the chemical
convertibility and the plating property during welding. The Si content is preferably
1.9% or less and more preferably 1.8% or less.
[0035]
15 Al: 0.001% or more and 1.0% or less
Al is an element that acts as a deoxidizing agent, suppresses the precipitation of
cementite and contributes to the stabilization of residual austenite. In order to obtain the
above-described effect of Al being contained, the Al content is set to 0.001% or more.
The Al content is preferably 0.005% or more and more preferably 0.010% or more.
20 However, from the viewpoint of suppressing the workability of the steel sheet being
degraded by a coarse Al oxide, the Al content is set to 1.0% or less. The Al content is
preferably 0.9% or less and more preferably 0.8% or less.
[0036]
Mn: 0.1% to 4.0%
25 Mn has an action of improving the hardenability of steel and is an effective
20
5
element for obtaining the metallurgical structure of the present embodiment. When the
Mn content is set to 0.1% or more, it is possible to obtain the metallurgical structure of
the present embodiment. The Mn content is preferably 1.0% or more and more
preferably 1.5% or more.
On the other hand, in consideration of the hardenability improvement effect
being degraded by the segregation of Mn and an increase in the material cost, the Mn
content is set to 4.0% or less. The Mn content is preferably 3.5% or less.
[0037]
V: 0.150% or less, Ti: 0.10% or less, Nb: 0.10% or less and V + Ti + Nb: 0.030% to
10 0.150%
V, Ti and Nb are all important elements for controlling the form of a carbide, and
there is a need to appropriately control the total amount ofV, Ti and Nb in order to
homogeneously and finely precipitate in residual austenite. Since not all of V, Ti and
Nb need to be contained, the lower limit of the amount of each element of V, Ti and Nb is
15 zero, but the total content ofV, Ti and Nb is 0.030% or more in order to form carbides in
the present embodiment. The total amount ofV, Ti and Nb is preferably 0.040% or
more and more preferably 0.050% or more. On the other hand, when the total amount
of V, Ti and Nb is too large, complex carbides are precipitated during hot rolling, and
these complex carbides become coarse in the following steps. From the viewpoint of
20 appropriate! y controlling the form of a carbide, The total amount of V, Ti and Nb is
0.030% or more and 0.150% or less. The total amount ofV, Ti and Nb is preferably
0.120% or less and more preferably 0.100% or less.
On the other hand, when the V content is too large, the growth of V carbides is
accelerated, a large number of coarse V carbides are precipitated to cause an increase in
25 the strength and deterioration of the ductility of steel, and there is a concern that the
21
formability of the steel sheet may deteriorate. In addition, Ti is an element that may
form a coarse Ti oxide or TiN to degrade the formability of the steel sheet.
Furthermore, when the Ti content is too large, Ti carbides are precipitated in a hot rolling
process and become coarse in the following steps. When the Nb content is too large,
5 Nb carbides are precipitated in the hot rolling process and become coarse in the
following steps. Therefore, significant deterioration of the ductility is caused together
with an increase in the strength of the steel sheet, and there is a concern that the
formability of the steel sheet may deteriorate. For the above-described reasons, the
upper limit of each element is set to V: 0.150% or less, Ti: 0.10% or less and Nb: 0.10%
10 or less.
[0038]
P: 0.0200% or less
P is an impurity element and is an element that is segregated in the sheet
thickness center portion of the steel sheet to impair the toughness and embrittles a welded
15 part. When the P content exceeds 0.0200%, the weld strength or the hole expansibility
significantly deteriorates. Therefore, the P content is set to 0.0200% or less. The P
content is preferably 0.0100% or less.
[0039]
The P content is preferably as small as possible; however, when the P content is
20 reduced to less than 0.0001% in practical steel sheets, the manufacturing cost
significantly increases, which becomes economically disadvantageous. Therefore, the
lower limit value of the P content may be set to 0.0001% or more.
[0040]
S: 0.0200% or less
25 S is an impurity element and is an element that impairs the weldability and
22
impairs the manufacturability during casting and during hot rolling. In addition, S is
also an element that forms coarse MnS to impair the hole expansibility. When the S
content exceeds 0.0200%, the weldability, the manufacturability, and the hole
expansibility significantly deteriorate. Therefore, the S content is set to 0.0200% or
5 less.
[0041]
The S content is preferably as small as possible; however, when the S content is
reduced to less than 0.0001% in practical steel sheets, the manufacturing cost
significantly increases, which becomes economically disadvantageous. Therefore, the
10 lower limit value of the S content may be set to 0.0001% or more.
[0042]
N: 0.0200% or less
N is an element that forms a coarse nitride, impairs the bendability or the hole
expansibility and causes the generation of a blowhole during welding. When the N
15 content exceeds 0.0200%, the deterioration of the hole expansibility or the generation of
a blowhole becomes significant. Therefore, theN content is set to 0.0200% or less.
[0043]
TheN content is preferably as small as possible; however, when theN content is
reduced to less than 0.0001% in practical steel sheets, the manufacturing cost
20 significantly increases, which becomes economically disadvantageous. Therefore, the
lower limit value of theN content may be set to 0.0001% or more.
[0044]
0: 0.0200% or less
0 is an element that forms a coarse oxide, impairs the bendability or the hole
25 expansibility and causes the generation of a blowhole during welding. When the 0
23
content exceeds 0.0200%, the deterioration of the hole expansibility or the generation of
a blowhole becomes significant. Therefore, the 0 content is set to 0.0200% or less.
[0045]
The 0 content is preferably as small as possible; however, when the 0 content is
5 reduced to less than 0.0005% in practical steel sheets, the manufacturing cost
significantly increases, which becomes economically disadvantageous. Therefore, the
lower limit value of the 0 content may be set to 0.0005% or more.
[0046]
The steel sheet according to the present embodiment may contain one or more
10 selected from the group consisting ofNi: 0.01% to 1.00%, Mo: 0.01% to 1.00%, Cr:
0.001% to 2.000%, B: 0.0001% to 0.0100%, Cu: 0.001% to 0.500%, W: 0.001% to
0.10%, Ta: 0.001% to 0.10%, Sn: 0.001% to 0.050%, Co: 0.001% to 0.50%, Sb: 0.001%
to 0.050%, As: 0.001% to 0.050%, Mg: 0.0001% to 0.050%, Ca: 0.001% to 0.040%, Y:
0.001% to 0.050%, Zr: 0.001% to 0.050% and La: 0.001% to 0.050%. Since these
15 elements may not be contained, the lower limit of each of these elements is 0%.
[0047]
Ni: 0% to 1.00%
Ni is an effective element for improving the strength of the steel sheet. The Ni
content may be 0%, but the Ni content is preferably 0.001% or more in order to obtain
20 the above-described effect. On the other hand, when the Ni content is too large, there is
a concern that the ductility of the steel sheet may deteriorate to cause the deterioration of
the formability. Therefore, the Ni content is preferably 1.00% or less.
[0048]
Mo: 0% to 1.00%
25 Similar to Cr, Mo is an element that contributes to the high-strengthening of the
24
steel sheet. This effect can be obtained even when the Mo content is small. The Mo
content may be 0%, but the Mo content is preferably 0.01% or more in order to obtain the
above-described effect. On the other hand, when the Mo content exceeds 1.00%, coarse
Mo carbides are formed and there is a concern that the cold formability of the steel sheet
5 may deteriorate. Therefore, the Mo content is preferably 1.00% or less.
[0049]
Cr: 0% to 2.000%
Cr is an element that improves the hardenability of steel and contributes to highstrengthening
and is an effective element for obtaining the above-described metallurgical
10 structure. Therefore, Cr may be contained. In order to sufficiently obtain the abovedescribed
effect, the Cr content is preferably set to 0.01% or more.
However, even when Cr is excessively contained, the effect of the above-described action
is saturated, which makes it economically disadvantageous to excessively contain Cr.
Therefore, even in a case where Cr is contained, the Cr content is set to 2.000% or less.
15 [0050]
B: 0% to 0.0100%
B is an element that suppresses the formation of ferrite and pearlite in a cooling
process from austenite and accelerates the formation of a low temperature transformation
structure such as bainite or martensite. In addition, B is a helpful element for the high-
20 strengthening of steel. This effect can be obtained even when the B content is small.
The B content may be 0%, but the B content is preferably set to 0.0001% or more in
order to obtain the above-described effect. However, when the B content is too large, a
coarse B oxide is formed, as a result, the B oxide acts as a starting point for the
generation of voids during press forming, and there is a concern that the formability of
25 the steel sheet may deteriorate. Therefore, the B content is preferably 0.0100% or less.
25
For the identification of less than 0.0001% of B, close attention needs to be paid to
analysis. In a case where the B content is below the lower detection limit of an
analyzer, the B content may be regarded as 0% in some cases.
[0051]
5 Cu: 0% to 0.500%
Cu is an element that contributes to improvement in the strength of the steel
sheet. This effect can be obtained even when the Cu content is small. The Cu content
may be 0%, but the Cu content is preferably 0.001% or more in order to obtain the
above-described effect. However, when the Cu content is too large, surface hot
10 shortness is caused and there is a concern that productivity in hot rolling may be
degraded. Therefore, the Cu content is preferably 0.500% or less.
[0052]
W: 0% to 0.10%
W is an effective element for improving the strength of the steel sheet. TheW
15 content may be 0%, but theW content is preferably 0.001% or more in order to obtain
the above-described effect. On the other hand, when the W content is too large, a large
number of fine W carbides are precipitated, an excessive increase in the strength and the
deterioration of the ductility of the steel sheet are caused, and there is a concern that the
cold workability of the steel sheet may be degraded. Therefore, the W content is
20 preferably set to 0.10% or less.

[CLAIMS]
What is claimed is:
1. A steel sheet,
wherein a chemical composition contains, by mass%:
C: 0.20% to 0.40%,
Si: 0.5% to 2.0%,
Al: 0.001% to 1.0%,
Mn: 0.1% to 4.0%,
V: 0.150% or less,
Ti: 0.10% or less,
Nb: 0.10% or less,
P: 0.0200% or less,
S: 0.0200% or less,
N: 0.0200% or less,
0: 0.0200% or less,
Ni: 0% to 1.00%,
Mo: 0% to 1.00%,
Cr: 0% to 2.000%,
B: 0% to 0.0100%,
Cu: 0% to 0.500%,
W: 0% to 0.10%,
Ta: 0% to 0.10%,
Sn: 0% to 0.050%,
Co: 0% to 0.50%,
Sb: 0% to 0.050%,
71
5
10
As: 0% to 0.050%,
Mg: 0% to 0.050%,
Ca: 0% to 0.040%,
Y: 0% to 0.050%,
Zr: 0% to 0.050%, and
La: 0% to 0.050%
with a remainder consisting of iron and impurities,
a total amount ofV, Ti and Nb is 0.030% to 0.150%,
the metallurgical structure at a thickness 1/4 portion is, by volume percentage,
martensite: 40% to 97%,
ferrite and bainite: 50% or less,
residual austenite: 3% to 20%, and
a remainder in microstructure: 5% or less,
an area ratio of residual austenite having an aspect ratio of 3 or more is 80% or
15 more with respect to a total area of the residual austenite, and
the number of carbides having a grain diameter of 8 to 40 nm per square
micrometer is five or more in the residual austenite.
2. The steel sheet according to claim 1,
20 wherein the chemical composition contains, by mass%, one or more selected
25
from the group consisting of
Ni: 0.01% to 1.00%,
Mo: 0.01% to 1.00%,
Cr: 0.001% to 2.000%,
B: 0.0001% to 0.0100%,
72
5
10
15
Cu: 0.001% to 0.500%,
W: 0.001% to 0.10%,
Ta: 0.001% to 0.1 0%,
Sn: 0.001% to 0.050%,
Co: 0.001% to 0.50%,
Sb: 0.001% to 0.050%,
As: 0.001% to 0.050%,
Mg: 0.0001% to 0.050%,
Ca: 0.001% to 0.040%,
Y: 0.001% to 0.050%,
Zr: 0.001% to 0.050%, and
La: 0.001% to 0.050%.
3. The steel sheet according to claim 1 or 2, further comprising:
a hot-dip galvanized layer on a surface.
4. The steel sheet according to claim 1 or 2, further comprising:
a hot-dip galvannealed layer on a surface.
20 5. A manufacturing method of a steel sheet, comprising:
25
a hot rolling step of heating a slab having the chemical composition according to
claim 1 or 2 at 1150°C or higher for one hour or longer and hot-rolling the slab to
produce a hot-rolled steel sheet in which prior austenite grain diameters are less than 30
~m;
a first cooling step of cooling the hot-rolled steel sheet to a temperature range of
73
5
10
800°C or lower within three seconds from an end of the hot rolling step;
a coiling step of cooling the hot-rolled steel sheet after the first cooling step to a
temperature range of 300°C or lower at an average cooling rate of 30 °C/s or faster and
coiling the hot-rolled steel sheet;
a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step
at a rolling reduction of 0.1% to 30% to produce a cold-rolled steel sheet;
an annealing step of heating the cold-rolled steel sheet in a temperature range of
480°C to Ac 1 at an average heating rate of 0.5 to 1.5 °C/s and soaking the cold-rolled
steel sheet in a temperature range of Ac1 to Ac3,
a second cooling step of cooling the cold-rolled steel sheet after the annealing
step at an average cooling rate of 4 °C/s or faster; and
a temperature retention step of retaining the cold-rolled steel sheet after the
second cooling step at 300°C to 480°C for 10 seconds or longer.
15 6. The manufacturing method of a steel sheet according to claim 5,
wherein the hot rolling step has a finish rolling step of continuously passing the
slab through a plurality of rolling stands to perform rolling,
in the finish rolling step:
a rolling start temperature in the rolling stand third from a final of the rolling
20 stands is 850°C to l 000°C;
25
in each of the three last rolling stands in the finish rolling, the slab is rolled at a
rolling reduction of larger than l 0%;
an interpass time between the individual rolling stands in the three last rolling
stands in the finish rolling is three seconds or shorter; and
(Tn- Tn+l) that is a difference between an exit temperature Tn of the n1
h rolling
74
stand and an entrance temperature Tn+l of the (n + 1)1h rolling stand on the downstream
side of the four last rolling stands in the finish rolling is 1 ooc or more.
7. The manufacturing method of a steel sheet according to claim 5 or 6,
5 wherein the cold-rolled steel sheet after the annealing step is controlled to be in
a temperature range of (zinc plating bath temperature- 40)°C to (zinc plating bath
temperature+ 50)°C and immersed in a hot-dip galvanizing bath, thereby forming hotdip
galvanized layer.
10 8. The manufacturing method of a steel sheet according to claim 7,
wherein the hot-dip galvanized layer is alloyed in a temperature range of 300°C
to 500°C.