[Technical Field of the Invention]
[0001]
The present invention relates to a steel sheet and a method for manufacturing a
steel sheet.
Priority is claimed on Japanese Patent Application No. 2020-165790, filed on
September 30, 2020, the content of which is incorporated herein by reference.
[Related 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, in general, when the
strength of a steel sheet is increased, delayed fracture is likely to occur. Delayed
fracture is a phenomenon in which hydrogen that intrudes into steel from the
environment due to corrosion or the like degrades the strength and fracture properties of
the steel to cause cracking and fracture. The higher the strength of the steel sheet, the
higher the susceptibility to delayed fracture. From the viewpoint of further increasing
the strength of mechanical parts, high strength steel sheets that are applied to
mechanical parts are required to have excellent delayed fracture resistance properties.
Here, "delayed fracture resistance properties" are an index of resistance to delayed
fracture. A steel sheet that does not easily allow the occurrence of delayed fracture is
judged to have favorable delayed fracture resistance properties.
In addition, high strength steel sheets that are used for mechanical parts are
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also required to have an excellent balance between strength and ductility in order to
ensure both the stiffness of the mechanical parts and the easiness of manufacture.
Here, "balance between strength and ductility" is a value that is evaluated by a value
obtained by multiplying the tensile strength TS and the elongation EL of the steel sheet.
In addition, from the viewpoint of prolonging the service lives of mechanical
parts, high strength steel sheets that are applied to mechanical parts are also required to
have excellent fatigue resistance properties. The fatigue resistance properties are a
value that is evaluated by, for example, a yield ratio. The yield ratio is a value
obtained by dividing the yield stress by the tensile strength.
[0003]
Examples of the prior arts of high strength steel sheets include the followings.
[0004]
Patent Document 1 discloses a high strength hot rolled steel sheet having
excellent external appearance and excellent isotropy of toughness and yield strength,
having a chemical composition including, by mass%, C: 0.04% or more and 0.15% or
less, Si: 0.01% or more and 0.25% or less, Mn: 0.1% or more and 2.5% or less, P: 0.1%
or less, S: 0.01% or less, Al: 0.005% or more and 0.05% or less, N: 0.01% or les s, Ti:
0.01% or more and 0.12% or less, B: 0.0003% or more and 0.0050% or less, and a
remainder: Fe and unavoidable impurities, in which 90% or more of the structure is
martensite, the amount of TiC precipitated is 0.05% or less, and the cleanliness of an Abased
inclusion that is defined in JIS G 0202 is 0.010% or less.
[0005]
However, in Patent Document 1, no studies are made on delayed fracture. In
addition, in the steel sheet described in Patent Document 1, the C content is 0.15% or
less, and the tensile strength is roughly 1300 MPa or less. Patent Document 1 does not
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suggest a method for improving the delayed fracture resistance properties of a high
strength steel sheet having a C content of 0.20% or more.
[0006]
Patent Document 2 discloses a high strength steel sheet, in which the
composition contains, by mass%, C: 0.20% or more and less than 0.45%, Si: 0.50% or
more and 2.50% or less, Mn: 1.5% or more and 4.0% or less, P: 0.050% or less, S:
0.0050% or less, Al: 0.01% or more and 0.10% or less, Ti: 0.020% or more and 0.150%
or less, N: 0.0005% or more and 0.0070% or less, 0: 0.0050% or less, and a remainder
consisting of iron and unavoidable impurities, the structure includes, in terms of area
ratio, 30% or more and 70% or less of ferrite and bainite in total, 15% or more of
residual austenite, and 5% or more and 35% or less of martensite, an average circle
equivalent diameter of the residual austenite is 3.0 f.!m or less, in the structure, the total
number of TiC and a composite precipitates containing TiC, which have a major axis of
5 nm or more and 100 nm or less, is 2 x 105 or more per 1 mm2
, and the total number of
carbides, nitrides, and oxides all containing Ti and composite precipitates containing
them, which have a major axis of250 nm or more, is 8x103 or less per 1 mm2.
[0007]
However, in the steel sheet described in Patent Document 2, as a way for
detoxifying hydrogen that has intruded into steel, only the control of the Mn content and
the P content is provided. Therefore, even in the steel sheet described in Patent
Document 2, there is room for further improving the delayed fracture resistance
properties.
[0008]
Patent Document 3 discloses a wear-resistant steel sheet, in which the
composition contains, by mass%, C: 0.20% to 0.45%, Si: 0.01% to 1.0%, Mn: 0.3% to
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2.5%, P: 0.020% or less, S: 0.01% or less, Cr: 0.01% to 2.0%, Ti: 0.10% to 1.00%, B:
0.0001% to 0.0100%, Al: 0.1% or less, N: 0.01% or less, and a remainder consisting of
Fe and unavoidable impurities, in the structure, the volume fraction of martensite at a
depth of 1 mm from the surface of the wear-resistant steel sheet is 90% or more, and the
prior austenite grain size at the thickness middle portion of the wear-resistant steel sheet
is 80 f.!m or less, the number density of TiC precipitates having a size of 0.5 f.!m or more
at the depth of 1 mm from the surface of the wear-resistant steel sheet is 400
precipitates/mm2 or more, and the concentration ofMn [Mn] (mass%) and the
concentration of P [P] (mass%) at the sheet thickness center segregation portion satisfy
0.04[Mn] + [P] < 0.50.
[0009]
However, in the steel sheet described in Patent Document 3, coarse TiC is used
to improve the wear resistance. According to the present inventors' findings, the
coarsening of TiC is considered to impair the delayed fracture resistance properties.
[Prior Art Document]
[Patent Document]
[0010]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2014-47414
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2018-3114
[Patent Document 3] PCT International Publication No. WO 2017/183057
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0011]
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An object of the present invention is to provide a steel sheet having a high
strength, an excellent balance between strength and ductility, excellent delayed fracture
resistance properties, and, furthermore, excellent fatigue resistance properties, and a
method for manufacturing the same.
[Means for Solving the Problem]
[0012]
The gist of the present invention is as described below.
(1) A steel sheet according to one aspect of the present invention includes, as a
chemical composition, in a unit of mass%, C: 0.20% or more and 0.45% or less, Si:
0.01% or more and 2.50% or less, Mn: 1.20% or more and 3.50% or les s, P: 0.040% or
less, S: 0.010% or less, Al: 0.001% or more and 0.100% or less, N: 0.0001% or more
and 0.0100% or less, Ti: 0.005% or more and 0.100% or less, B: 0% or more and
0.010% or less, 0: 0.006% or less, Mo: 0% or more and 0.50 % or less, Nb: 0% or more
and 0.20% or less, Cr: 0% or more and 0.50% or less, V: 0% or more and 0.50% or les s,
Cu: 0% or more and 1.00% or less, W: 0% or more and 0.100% or less, Ta: 0% or more
and 0.10% or less, Ni: 0% or more and 1.00% or less Sn: 0% or more and 0.050% or
less, Co: 0% or more and 0.50% or less, Sb: 0% or more and 0.050% or less, As: 0% or
more and 0.050% or less, Mg: 0% or more and 0.050% or less, Ca: 0% or more and
0.040% or less, Y: 0% or more and 0.050% or less, Zr: 0% or more and 0.050% or less,
La: 0% or more and 0.050% or less, Ce: 0% or more and 0.050% or less, and a
remainder consisting of Fe and impurities, in which a Ti content and aN content satisfy
the following formula 1, at a sheet thickness 1/4 position, a metallographic structure
includes 90% or more of martensite in terms of volume fraction, at the sheet thickness
114 position, a number density of TiC having a circle equivalent diameter of 1 to 500 nm
is 3.5 x 104 particles/mm2 or more, at the sheet thickness 1/4 position, a value of a
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median value of a Mn concentration+ 3a is 5.00% or less, a hardness measured at the
sheet thickness 1/4 position is 1.30 times or more a hardness measured at a position 50
)lm deep from a surface of the steel sheet, and a tensile strength is 1310 MPa or more.
Ti- 3.5 x N ~ 0.003 (formula 1)
Here, element symbols Ti and N in the formula 1 mean the Ti content and the
N content of the steel sheet.
(2) The steel sheet according to (1) may include hot-dip galvanizing, hot-dip
gal vannealing, electro plating, or aluminum plating.
(3) A method for manufacturing a steel sheet according to another aspect of the
present invention includes hot-rolling a cast piece having the chemical composition
according to (1) with a finish rolling end temperature set to an Ac3 point or higher to
obtain a steel sheet, coiling the steel sheet at a coiling temperature set to 5oooc or
lower, cold-rolling the steel sheet at a rolling reduction set to 0% to 20%, and annealing
the steel sheet in a temperature range of the Ac3 point or higher with an oxygen
potential in a temperature range of 700°C or higher set to -1.2 or higher and 0 or lower,
in which, when the steel sheet is heated up to the temperature range of the Ac3 point or
higher in the annealing, the steel sheet is held in a temperature range of 500°C to 700°C
for 70 to 130 seconds, and, when the steel sheet is cooled from the temperature range of
the Ac3 point or higher in the annealing, the steel sheet is held in a temperature range of
700°C to soooc for 4 to 25 seconds.
(4) The method for manufacturing a steel sheet according to (3) may further
include tempering the annealed steel sheet.
(5) The method for manufacturing a steel sheet according to (3) or (4) may
further include performing hot-dip galvanizing, hot-dip galvannealing, electro plating,
or aluminum plating on the annealed steel sheet.
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[Effects of the Invention]
[0013]
According to the present invention, it is possible to provide a steel sheet having
a high strength, an excellent balance between strength and ductility, excellent delayed
fracture resistance properties, and, furthermore, excellent fatigue resistance properties,
and a method for manufacturing the same.
[Embodiments ofthe Invention]
[0014]
The present inventors paid attention to TiC as a way for improving the delayed
fracture resistance properties. TiC acts as a hydrogen-trapping site and is thus capable
of detoxifying hydrogen that has intruded into steel.
[0015]
However, from coarse TiC having a circle equivalent diameter of more than
500 nm, the above-described effect cannot be sufficiently obtained. In order to
improve the delayed fracture resistance properties through TiC, it is necessary to
disperse a large amount of fine TiC having a circle equivalent diameter of 1 to 500 nm
in the steel sheet. The present inventors repeated studies regarding a way for finely
dispersing TiC. As a result, the present inventors found that annealing a steel sheet
manufactured as described below is extremely effective for finely dispersing TiC.
(A) The structure of the steel sheet before annealing is made to include mainly
bainite and/or martensite.
(B) Ti is contained in a solid solution state in the steel sheet before annealing.
(C) The amount of dislocation introduced into the steel sheet before annealing
by cold rolling is controlled.
(D) The temperature of the steel sheet is held within a temperature range of
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sooac to 700°C during heating for annealing and cooling after annealing.
[0016]
(A) First, it is preferable that the structure of the steel sheet before annealing is
made to include mainly bainite and/or martensite. Such a low temperature
transformation structure includes a number of dislocations. The use of these
dislocations as TiC precipitation sites makes it possible to finely precipitate TiC in the
steel sheet when the temperature is raised to anneal the steel sheet.

What is claimed is:
CLAIMS
1. A steel sheet comprising, as a chemical composition, in a unit of mass%:
C: 0.20% or more and 0.45% or less;
Si: 0.01% or more and 2.50% or less;
Mn: 1.20% or more and 3.50% or less;
P: 0.040% or less;
S: 0.010% or less;
Al: 0.001% or more and 0.100% or less;
N: 0.0001% or more and 0.0100% or less;
Ti: 0.005% or more and 0.100% or less;
B: 0% or more and 0.010% or less;
0: 0.006% or less;
Mo: 0% or more and 0.50% or less;
Nb: 0% or more and 0.20% or less;
Cr: 0% or more and 0.50% or less;
V: 0% or more and 0.50% or less;
Cu: 0% or more and 1.00% or less;
W: 0% or more and 0.100% or less;
Ta: 0% or more and 0.10% or less;
Ni: 0% or more and 1.00% or less;
Sn: 0% or more and 0.050% or less;
Co: 0% or more and 0.50% or less;
Sb: 0% or more and 0.050% or less;
As: 0% or more and 0.050% or less;
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Mg: 0% or more and 0.050% or less;
Ca: 0% or more and 0.040% or less;
Y: 0% or more and 0.050% or less;
Zr: 0% or more and 0.050% or less;
La: 0% or more and 0.050% or less;
Ce: 0% or more and 0.050% or less; and
a remainder consisting of Fe and impurities,
wherein a Ti content and a N content satisfy the following formula 1,
at a sheet thickness 1/4 position, a metallographic structure includes 90% or
more of martensite in terms of volume fraction,
at the sheet thickness 1/4 position, a number density of TiC having a circle
equivalent diameter of 1 to 500 nm is 3.5 x 104 particles/mm2 or more,
at the sheet thickness 1/4 position, a value of a median value of a Mn
concentration+ 3a is 5.00% or less,
a hardness measured at the sheet thickness 1/4 position is 1.30 times or more a
hardness measured at a position 50 JJ.m deep from a surface of the steel sheet, and
a tensile strength is 1310 MPa or more,
Ti- 3.5 x N ~ 0.003 (formula 1)
here, element symbols Ti and N in the formula 1 mean the Ti content and theN
content of the steel sheet.
2. The steel sheet according to claim 1, comprising:
hot-dip galvanizing, hot-dip galvannealing, electro plating, or aluminum
plating.
3. A method for manufacturing a steel sheet comprising:
hot-rolling a cast piece having the chemical composition according to claim 1
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with a finish rolling end temperature set to an Ac3 point or higher to obtain a steel sheet;
coiling the steel sheet at a coiling temperature set to 500°C or lower;
cold-rolling the steel sheet at a rolling reduction set to 0% to 20%; and
annealing the steel sheet in a temperature range of the Ac3 point or higher with
an oxygen potential in a temperature range of 700°C or higher set to -1.2 or higher and
0 or lower,
wherein, when the steel sheet is heated up to the temperature range of the Ac3
point or higher in the annealing, the steel sheet is held in a temperature range of 5oooc
to 700°C for 70 to 130 seconds, and
when the steel sheet is cooled from the temperature range of the Ac3 point or
higher in the annealing, the steel sheet is held in a temperature range of 700°C to 500°C
for 4 to 25 seconds.
4. The method for manufacturing a steel sheet according to claim 3, further
compnsmg:
tempering the annealed steel sheet.
5. The method for manufacturing a steel sheet according to claim 3 or 4,
further comprising:
performing hot-dip galvanizing, hot-dip galvannealing, electro plating, or
aluminum plating on the annealed steel sheet.