Title of the Invention] HOT-ROLLED STEEL SHEET
[Technical Field]
[0001]
The present invention relates to a hot-rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2021-005008, filed
January 15, 2021, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, the strength of steel sheets has been increasing to ensure the
collision safety of automobiles and reduce the environmental load. In order to increase
the strength of steel sheets, making microstructures have a martensite single phase is
effective. Since steel sheets in which microstructures have a martensite single phase
have poor ductility compared to composite structure steel sheets such as dual phase (DP)
15 steel sheets and transformation induced plasticity (TRIP) steel sheets, the steel sheets
20
may be processed to have desired shapes mainly through bending in many cases. For
this reason, steel sheets having a martensite single phase are required to have excellent
bendability.
[0003]
Patent Document 1 discloses a high-strength hot-rolled steel sheet which has a
martensite phase or tempered martensite phase as a main phase, has a structure in which
a volume ratio of the main phase with respect to the entire structure is 90% or more, an
average grain size of prior austenite grains is 20 J.lm or less in a cross section parallel to a
rolling direction and 15 J.lm or less in a cross section perpendicular to the rolling
25 direction, and an aspect ratio of the prior austenite grains in the cross section parallel to
1
the rolling direction is 18 or less, and has excellent low temperature toughness.
[0004]
Patent Document 2 discloses a high-strength hot-rolled steel sheet in which a
steel structure is composed of at least one of a martensite phase and a tempered
5 martensite phase, which has a main phase in which an area ratio with respect to the entire
steel structure is 95% or more, which contains cementite with an average grain size of 0.5
11m or less in a lath of the martensite phase and/or the tempered martensite phase, and in
which the content of cementite is, in % by mass, 0.01 to 0.08%.
10
[0005]
However, the inventors of the present invention have found that the steel sheets
described in Patent Documents 1 and 2 do not provide sufficient bendability.
[Citation List]
[Patent Document]
[0006]
15 [Patent Document 1]
Japanese Unexamined Patent Application, First Publication No. 2016-211073
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. 2018-188675
[Summary of the Invention]
20 [Problems to be Solved by the Invention]
[0007]
An object of the present invention which has been made in view of the above
circumstances is to provide a hot-rolled steel sheet having high strength and excellent
bendability.
25 [Means for Solving the Problem]
2
[0008]
The inventors of the present invention obtained the following findings as results
of creative research and conceived the present invention.
It was found that a hot-rolled steel sheet having high strength and excellent
5 bendability can be obtained by setting a total area ratio of martensite and tempered
martensite to 90% or more in a microstructure at a location at 1/4 a sheet thickness from
the surface and setting a total area ratio of martensite and tempered martensite to 70% or
more and less than 90% in a microstructure at a location of 200 11m from the surface.
10
[0009]
Also, the inventors of the present invention have found that controlling fini sh
rolling conditions and cooling conditions after finish rolling are particularly effective
when obtaining the hot-rolled steel sheet.
[0010]
The gist of the present invention made on the basis of the above findings is as
15 follows.
20
25
(1) A hot-rolled steel sheet according to an aspect of the present invention has a chemical
composition which contains, in % by mass,
C: 0.050 to 0.150%,
Si: 0.01 to 1.00%,
Mn: 1.00 to 2.50%,
P: 0.100% or less,
S: 0.020% or less,
N: 0.0050% or less,
Al: 0.001 to 0.300%,
Ti: 0.001 to 0.100%,
3
5
10
15
20
25
B: 0.0005 to 0.0050%,
Nb: 0 to 0.100%,
Cr: 0 to 1.00%,
V: 0 to 0.30%,
Cu: 0 to 0.30%,
Ni: 0 to 0.30%, and
Ca: 0 to 0.0050%,
wherein the remainder is composed of Fe and impurities,
Vc is represented by the following Expressions (1) to (3) is 10 to 40,
a microstructure contains, in area ratio,
at a location at 114 a sheet thickness,
a total amount of 90% or more of one or more of martensite and tempered
martensite and
a total amount of 10% or less of one or more ferrite, bainite, and pearlite,
at a location of 200 11m from a surface,
a total amount of 70% or more and less than 90% of one or more of martensite
and tempered martensite and
a total amount of more than 10% and 30% or less of one of more of ferrite,
bainite, and pearlite:
when an effective amount of B2:0.0005% by mass is satisfied,
V C= 1 02.94-0.75x(2.7xC+0.4xSi+Mn+0.45xNi+0.8xCr)
when an effective amount of B<0.0005% by mass is satisfied,
V c= 1 0 3.69- 0.75x(2.7xC+0.4xSi+Mn+0.45xNi+0.8xCr) (1 ),
an effective amount of B=l0.8lx(B/10.81-solid solution N amount/14.01)
(2), and
4
5
solid solution N amount=l4.0lx(N/14.01-Ti/47.88) (3),
here, each element symbol in the above Expression (1) is the content of the
element in % by mass, and when the element is not contained, 0 is substituted,
B in the above Expression (2) is, in % by mass, a B content,
when the effective amount of B is a negative value, the effective amount of B is
set to 0,
Nand Ti in the above Expression (3) are, in% by mass, contents, and
when the solid solution N amount is a negative value, the solid solution N
amount is 0.
10 (2) The hot-rolled steel sheet according to the above (1) may have the chemical
composition which contains, in% by mass,
15
one or two or more of the group including
Nb: 0.005 to 0.100%,
Cr: 0.005 to 1.00%,
V: 0.005 to 0.30%,
Cu: 0.005 to 0.30%,
Ni: 0.005 to 0.30%, and
Ca: 0.0010 to 0.0050%.
(3) In the hot-rolled steel sheet according to the above (1) or (2), the microstructure may
20 have, at the location at 1/4 a sheet thickness, an average aspect ratio of prior austenite
grains of 1.0 to 3.0.
(4) In the hot-rolled steel sheet according to any one of the above (1) to (3), the
microstructure may have, at the location at 1/4 a sheet thickness, an average grain size of
prior austenite grains of 5 to 40 )liD.
25 [Effects of the Invention]
5
[0011]
According to the aspect associated with the present invention, it is possible to
provide a hot-rolled steel sheet having high strength and excellent bendability.
[Embodiment(s) for implementing the Invention]
5 [0012]
A hot-rolled steel sheet according to an embodiment will be described in detail
below. Here, the present invention is not limited only to the constitution disclosed in
the embodiment and various modifications are possible without departing from the gist of
the present invention.
10 [0013]
In a numerical limitation range which will be described below having numerical
values having the term "to" written therebetween, a lower limit value and a higher limit
value are included in the range. Numerical values written with the terms "less than"
and "more than" are not included in the numerical value range. All "%" in chemical
15 compositions refer to"% by mass."
[0014]
In a hot-rolled steel sheet according to an embodiment, a chemical composition
contains, in % by mass, C: 0.050 to 0.150%, Si: 0.01 to 1.00%, Mn: 1.00 to 2.50%, P:
0.100% or less, S: 0.020% or less, N: 0.0050% or less, Al: 0.001 to 0.300%, Ti: 0.001 to
20 0.100%, B: 0.0005 to 0.0050%, and the remainder: Fe and impurities. Each element
will be described below.

[CLAIMS]
1. A hot-rolled steel sheet having a chemical composition which contains, in % by
mass,
C: 0.050 to 0.150%,
Si: 0.01 to 1.00%,
Mn: 1.00 to 2.50%,
P: 0.100% or less,
S: 0.020% or less,
N: 0.0050% or less,
Al: 0.001 to 0.300%,
Ti: 0.001 to 0.100%,
B: 0.0005 to 0.0050%,
Nb: 0 to 0.100%,
Cr: 0 to 1.00%,
V: 0 to 0.30%,
Cu: 0 to 0.30%,
Ni: 0 to 0.30%, and
Ca: 0 to 0.0050%,
wherein the remainder is composed of Fe and impurities,
Vc represented by the following Expressions (1) to (3) is 10 to 40,
a microstructure contains, in area ratio,
at a location at 114 a sheet thickness,
a total amount of 90% or more of one or more of martensite and tempered
martensite and
a total amount of 10% or less of one or more ferrite, bainite, and pearlite,
42
at a location of 200 11m from a surface,
a total amount of 70% or more and less than 90% of one or more of martensite
and tempered martensite and
a total amount of more than 10% and 30% or less of one of more of ferrite,
5 bainite, and pearlite:
10
15
when an effective amount of B ~0.0005% by mass is satisfied,
V c= 1 02.94-0.75x(2.7xC+0.4xSi+Mn+0.45xNi+0.8xCr)
when an effective amount of B<0.0005% by mass is satisfied,
V c= 1 03.69-0.75x(2.7xC+0.4xSi+Mn+0.45xNi+0.8xCr) (1),
an effective amount of B=10.81x(B/10.81-solid solution N amount/14.01)
(2), and
solid solution N amount=14.01x(N/14.01-Ti/47.88) (3),
here, each element symbol in the above Expression (1) is the content of the
element in % by mass, and when the element is not contained, 0 is substituted,
set to 0,
B in the above Expression (2) is, in % by mass, a B content,
when the effective amount of B is a negative value, the effective amount of B is
Nand Ti in the above Expression (3) are, in % by mass, contents, and
when the solid solution N amount is a negative value, the solid solution N
20 amount is 0.
2. The hot-rolled steel sheet according to claim 1 having the chemical composition
which contains, in % by mass,
one or two or more of the group including
43
5
Nb: 0.005 to 0.100%,
Cr: 0.005 to 1.00%,
V: 0.005 to 0.30%,
Cu: 0.005 to 0.30%,
Ni: 0.005 to 0.30%, and
Ca: 0.0010 to 0.0050%.
3. The hot-rolled steel sheet according to claim 1 or 2, wherein the microstructure has,
at the location at 1/4 a sheet thickness,
10 an average aspect ratio of prior austenite grains of 1.0 to 3.0.
15
4. The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the
microstructure has, at the location at 1/4 a sheet thickness,
an average grain size of prior austenite grains of 5 to 40 J.lm.