In recent years, improvement of the fuel economy of automobiles has been sought from the
viewpoint of restrictions on emission of hothouse effect gases accompanying measures against
global warming. High strength steel sheet is being increasingly used for lightening the weight of
15 car bodies and securing safety in collision. In particular, recently, the need for ultra-high strength
steel sheet with a tensile strength of 980 MPa or more has been rising.
[0003]
Hot dip galvanized steel sheet used for automobile parts is being asked to be improved in not
only strength, but also press formability, weldability, and various other aspects of workability
20 required for forming parts. Specifically, excellent bendability is being sought from steel sheet from
the viewpoint of press formability.
[0004]
PTL 1 discloses steel sheet in which B is contained at the steel sheet surface layer part
mainly in a precipitated state and inside of the steel sheet mainly in a solid solution state so as to
25 improve the bendability.
[0005]
PTL 2 discloses high strength steel sheet excellent in delayed fracture resistance of a cut
end face and steel sheet base material having a martensite single phase structure, having a region
with a KAM value (kernel average misorientation value) of a value of 1ﾟ or more comprising
30 50% or more of the total, and having a maximum tensile residual stress in the surface layer
region from the surface down to the 1/4 depth position of sheet thickness of 80 MPa or less.
[0006]
As art for the improvement of the bendability of high strength steel sheet, for example, PTL
3 describes high strength cold rolled steel sheet with a surface layer part mainly comprised of
35 ferrite which is produced by decarburization of the steel sheet. Further, PTL 4 describes ultrahigh strength cold rolled steel sheet having a soft layer at the surface layer part which is
2
produced by annealing for decarburization of steel sheet.
[CITATION LIST]
[PATENT LITERATURE]
5 [0007]
[PTL 1] WO2017/002883
[PTL 2] Japanese Unexamined Patent Publication No. 2015-155572
[PTL 3] Japanese Unexamined Patent Publication No. 10-130782
[PTL 4] Japanese Unexamined Patent Publication No. 5-195149
10
SUMMARY
[TECHNICAL FIELD]
[0008]
Further, high strength steel sheet used for auto parts is being required to not break due to
15 deformation by collision after being formed into a part. In particular, the steel sheet used for auto
parts has to be excellent in not only bendability before press forming, but also bendability after
plastic strain is introduced by press forming. However, improvement of the bendability after
plastic strain is introduced has not necessarily been sufficiently studied up to now.
[0009]
20 Therefore, the present invention has as its object the provision of steel sheet excellent in
tensile strength and improved in bendability after plastic working and a method of production
thereof.
[SOLUTION TO PROBLEM]
25 [0010]
The inventors engaged in repeated intensive studies for solving the above problem and as a
result discovered that it is possible to improve the bendability after plastic working by forming a
suitable deboronized layer at the surface layer part. The present invention was perfected based
such a finding. The present invention includes the following aspects.
30 [0011]
(Aspect 1)
Steel sheet, in which steel sheet,
a chemical composition of the steel sheet contains, by mass%,
C: 0.06 to 0.30%,
35 Si: 0.01 to 2.50%,
Mn: 1.00 to 3.50%,
3
Ti: 0.001 to 0.100%,
B: 0.0005 to 0.0050%,
P: 0.050% or less,
S: 0.0100% or less,
5 Al: 1.500% or less,
N: 0.010% or less,
O: 0.0100% or less,
Cr: 0 to 1.00%,
Mo: 0 to 1.00%,
10 Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Co: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 1.00%,
15 Sb: 0 to 0.50%,
Nb: 0 to 0.200%,
V: 0 to 1.00%,
As: 0 to 0.10%,
Zn: 0 to 1.00%,
20 Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Ce: 0 to 0.0150%,
Zr: 0 to 0.0100%,
La: 0 to 0.0150%,
25 Hf: 0 to 0.0100%,
Bi: 0 to 0.0100%,
an REM other than Ce and La: 0 to 0.0100%, and
bal.: Fe and impurities,
a steel structure in a range of a 1/8 depth position to 3/8 depth position of sheet thickness of
30 the steel sheet comprises, by area%,
ferrite: 30% or less,
tempered martensite: 40% or more,
a total of retained austenite and fresh martensite: 15% or less,
total of pearlite and cementite: 5% or less, and
35 bal.: bainite,
a surface layer part of the steel sheet has a deboronized layer with an emission intensity of
4
B, measured by high frequency glow discharge spectrometry in a depth direction from the steel
sheet surface, satisfying the following formula (1) and formula (2), and
a tensile strength is 1180 MPa or more:
B30/B150<0.90 ... (1)
5 0.90B140/B1501.10... (2)
where,
B30: emission intensity of B at depth position of 30 m from the steel sheet surface
B140: emission intensity of B at depth position of 140 m from the steel sheet surface
B150: emission intensity of B at depth position of 150 m from the steel sheet surface
10 [0012]
(Aspect 2)
The steel sheet according to the aspect 1 wherein a surface layer part of the steel sheet has
an emission intensity of C, measured by high frequency glow discharge spectrometry in a depth
direction from the steel sheet surface, satisfying the following formula (3) and formula (4),
15 C40/C150>0.50 ... (3)
0.90C140/C1501.10... (4)
where,
C40: emission intensity of C at depth position of 40 m from the steel sheet surface
C140: emission intensity of C at depth position of 140 m from the steel sheet surface
20 C150: emission intensity of C at depth position of 150 m from the steel sheet surface
[0013]
(Aspect 3)
The steel sheet according to the aspect 1 or 2, wherein a tensile residual stress acting in
rolling perpendicular direction at the steel sheet surface is 200 MPa or less.
25 [0014]
(Aspect 4)
The steel sheet according to any one of the aspects 1 to 3, wherein the steel sheet surface
has a hot dip galvanized layer or a hot dip galvannealed layer.
[0015]
30 (Aspect 5)
A method of production of steel sheet, which method of production of steel sheet
comprising
a hot rolling step (a) of hot rolling a slab having a chemical composition comprising, by
mass%,
35 C: 0.06 to 0.30%,
Si: 0.01 to 2.50%,
5
Mn: 1.00 to 3.50%,
Ti: 0.001 to 0.100%,
B: 0.0005 to 0.0050%,
P: 0.050% or less,
5 S: 0.0100% or less,
Al: 1.500% or less,
N: 0.010% or less,
O: 0.0100% or less,
Cr: 0 to 1.00%,
10 Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Co: 0 to 1.00%,
W: 0 to 1.00%,
15 Sn: 0 to 1.00%,
Sb: 0 to 0.50%,
Nb: 0 to 0.200%,
V: 0 to 1.00%,
As: 0 to 0.10%,
20 Zn: 0 to 1.00%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Ce: 0 to 0.0150%,
Zr: 0 to 0.0100%,
25 La: 0 to 0.0150%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.0100%,
an REM other than Ce and La: 0 to 0.0100%, and
bal.: Fe and impurities at a 850 to 950C finish rolling end temperature to obtain hot rolled
30 steel sheet, then cooling the hot rolled steel sheet down to 450 to 650C and coiling the hot
rolled steel sheet ,
a pickling step (b) of pickling the steel sheet obtained at the hot rolling step (a),
a cold rolling step (c) of cold rolling the steel sheet obtained by the pickling step (b) by a 30
to 75% rolling reduction to obtain a cold rolled steel sheet,
35 a heat treatment step (d) of heat treating the steel sheet obtained at the cold rolling step (c),
and
6
a grinding step (e), before or after the pickling step (b), of using a rotary type grinding brush
containing an abrasive to grind the front and back surfaces of the steel sheet obtained at the hot
rolling step (a) or the steel sheet obtained at the pickling step (b),
in the hot rolling step (a), finish rolling comprises three passes or more, the rolling
5 reduction of the respective passes of the final three passes of the finish rolling is 20% or more,
the time between passes is within 1 second, the entry side steel sheet temperature before the final
three passes is 1000C or less, and the time from the completion of the final pass to the start of
cooling is within 3 seconds,
in the grinding step (e), a rotational speed R (rpm) of the grinding brush, a diameter D (m)
10 of the grinding brush, and a running speed V (m/min) of the steel sheet satisfy the following
formula (5),
the heat treatment step (d) further provided with
a step (d-1) of heating the steel sheet obtained at the cold rolling step (c) from 650C to a
maximum heating temperature of the Ac1+50C or more and 950C or less by an average
15 heating speed of 0.5 to 500C/s,
a step (d-2) of holding the steel sheet obtained at the cold rolling step (c) at the maximum
heating temperature for 1 second to 300 seconds,
a step (d-3) of cooling the steel sheet obtained at the cold rolling step (c) down to the Ms
point-100C or less in temperature, at which step, cooling from 700C to 500C by a 10C/s or
20 more average cooling speed, and
a step (d-4) of holding the steel sheet obtained at the cold rolling step (c) at 200 to 350C
for 50 to 600 seconds,
at step (d-1), the atmosphere in the surroundings of the steel sheet obtained at the step (c)
having a steam partial pressure pH2O and hydrogen partial pressure pH2 satisfying the following
25 formula (6):
[Mathematical 1]
[0016]
(Aspect 6)
30 The method of production of steel sheet according to the aspect 5, wherein
the hot rolling step (a) further comprises a step of retaining the heat of the hot rolled steel
sheet after coiling within 30 minutes in a heat insulating vessel with inside walls covered by a
heat insulating material, wherein
7
a peak temperature of an atmospheric temperature inside of the heat insulating vessel is 500
to 650C, and the time from the atmospheric temperature to the peak temperature is 1 to 8 hours.
[0017]
(Aspect 7)
5 The method of production of steel sheet according to the aspect 5 or 6, wherein,
in the above step (d-1), the atmosphere of the surroundings of the steel sheet obtained at the
above cold rolling step (c) has a steam partial pressure pH2 O and hydrogen partial pressure pH2
satisfying the following formula (7).
-4.0log(pH2 O/pH2 )-1.0 ...(7)
10 [0018]
(Aspect 8)
The method of production of steel sheet according to any one of the aspects 5 to 7, wherein,
the above step (d-3) stops the cooling while the temperature of the steel sheet obtained at
the above cold rolling step (c) is between the Ms point and 650C and performs natural cooling
15 for 0.1 to 3.0 seconds.
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0019]
According to the present invention, it is possible to obtain steel sheet excellent in tensile
20 strength and excellent in bendability after plastic working.
BRIEF DESCRIPTION OF DRAWINGS
[0020]
FIG. 1 is a view schematically showing a cross-section of a plated steel sheet 1 including a
25 base steel sheet 2 according to one embodiment of the present invention sliced in the sheet
thickness direction.
DESCRIPTION OF EMBODIMENTS
[0021]
30 Below, a plated steel sheet including a steel sheet of one embodiment of the present
invention as a base steel sheet will be explained in detail while referring to FIG. 1. Note that,
FIG. 1 is a view schematically showing a cross-section of a plated steel sheet 1 including a base
steel sheet 2 according to one embodiment of the present invention sliced in the sheet thickness
direction.
35 [0022]
The present invention prescribes the features of a specific position of the steel sheet in the
8
sheet thickness direction. In the following explanation, these features will sometimes be
explained using a position of the steel sheet in the sheet thickness direction based on the steel
sheet surface.
[0023]
5 Note that, the “sheet thickness direction” and the “depth direction” of the steel sheet are
synonymous, and therefore in this Description, a position of the steel sheet in the sheet thickness
direction based on the steel sheet surface will sometimes be called the “depth position”.
In relation to this, in this Description, the “x/y depth position of sheet thickness (in this
case, 'x' and 'y' are natural numbers satisfying x
As shown in FIG. 1, the plated steel sheet 1 is plated steel sheet having the base steel sheet
2 of the present embodiment and a plating layer 3 provided on both surfaces of the base steel
sheet 2. It should be noted that, the plating layer 3 may also be provided on one surface of the
base steel sheet 2.
15 [0028]
Further, the plated steel sheet 1, as shown in FIG. 1, has a surface layer part PS defined as a
region in the sheet thickness direction from the steel sheet surface Sd to a depth position P150 of
150 m.
[0029]
20
Further, in the present embodiment, the base steel sheet 2 has the following features:
First, the chemical composition of the base steel sheet 2 contains, by mass%,
C: 0.06 to 0.30%,
Si: 0.01 to 2.50%,
25 Mn: 1.00 to 3.50%,
Ti: 0.001 to 0.100%,
B: 0.0005 to 0.0050%,
P: 0.050% or less,
S: 0.0100% or less,
30 Al: 1.500% or less,
N: 0.010% or less,
O: 0.0100% or less,
Cr: 0 to 1.00%,
Mo: 0 to 1.00%,
35 Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
10
Co: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 1.00%,
Sb: 0 to 0.50%,
5 Nb: 0 to 0.200%,
V: 0 to 1.00%,
As: 0 to 0.10%,
Zn: 0 to 1.00%,
Ca: 0 to 0.0100%,
10 Mg: 0 to 0.0100%,
Ce: 0 to 0.0150%,
Zr: 0 to 0.0100%,
La: 0 to 0.0150%,
Hf: 0 to 0.0100%,
15 Bi: 0 to 0.0100%,
an REM other than Ce and La: 0 to 0.0100%, and
bal.: Fe and impurities.
[0030]
Further, the steel structure in the range of the 1/8 depth position to the 3/8 depth position of
20 sheet thickness of the base steel sheet 2 comprises, by area%, ferrite: 30% or less, tempered
martensite: 40% or more, total of retained austenite and fresh martensite: 15% or less, total of
pearlite and cementite: 5% or less, and bal.: bainite.
[0031]
Further, the surface layer part PS of the base steel sheet 2 has a deboronized layer PB with
25 an emission intensity of B, measured by high frequency glow discharge spectrometry in the
depth direction from the steel sheet surface Sd, satisfying the following formula (1) and formula
(2):
B30/B150<0.90 ... (1)
0.90B140/B1501.10... (2)
30 where,
B30: emission intensity of B at depth position of 30 m from steel sheet surface Sd
B140: emission intensity of B at depth position of 140 m from steel sheet surface Sd
B150: emission intensity of B at depth position of 150 m from steel sheet surface Sd
[0032]
35 Further, the tensile strength of the base steel sheet 2 is 1180 MPa or more.
[0033]
11
Below, these features in the base steel sheet 2 will be explained in detail.
[0034]
(Chemical Composition)
First, the reasons for limiting the chemical composition of the base steel sheet according to
5 the present embodiment (below, sometimes simply referred to as the “steel sheet”) in the abovementioned way will be explained. It should be noted that, in this Description, the “%”s
prescribing the chemical composition, unless particularly indicated otherwise, are all “mass%”.
Further, in this Description, the “to” indicating a numerical range, unless particularly indicated
otherwise, is used in the sense including the numbers described before and after it as the lower
10 limit value and upper limit value.
Steel sheet, in which steel sheet,
a chemical composition of the steel sheet contains, by mass%,
5 C: 0.06 to 0.30%,
Si: 0.01 to 2.50%,
Mn: 1.00 to 3.50%,
Ti: 0.001 to 0.100%,
B: 0.0005 to 0.0050%,
10 P: 0.050% or less,
S: 0.0100% or less,
Al: 1.500% or less,
N: 0.010% or less,
O: 0.0100% or less,
15 Cr: 0 to 1.00%,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Co: 0 to 1.00%,
20 W: 0 to 1.00%,
Sn: 0 to 1.00%,
Sb: 0 to 0.50%,
Nb: 0 to 0.200%,
V: 0 to 1.00%,
25 As: 0 to 0.10%,
Zn: 0 to 1.00%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Ce: 0 to 0.0150%,
30 Zr: 0 to 0.0100%,
La: 0 to 0.0150%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.0100%,
an REM other than Ce and La: 0 to 0.0100%, and
35 bal.: Fe and impurities,
a steel structure in a range of a 1/8 depth position to 3/8 depth position of sheet thickness of
47
the steel sheet comprises, by area%,
ferrite: 30% or less,
tempered martensite: 40% or more,
a total of retained austenite and fresh martensite: 15% or less,
5 total of pearlite and cementite: 5% or less, and
bal.: bainite,
a surface layer part of the steel sheet has a deboronized layer with an emission intensity of
B, measured by high frequency glow discharge spectrometry in a depth direction from a steel
sheet surface, satisfying following formula (1) and formula (2), and
10 a tensile strength is 1180 MPa or more:
B30/B150<0.90 ... (1)
0.90B140/B1501.10... (2)
where,
B30: emission intensity of B at depth position of 30 m from the steel sheet surface
15 B140: emission intensity of B at depth position of 140 m from the steel sheet surface
B150: emission intensity of B at depth position of 150 m from the steel sheet surface
[Claim 2]
The steel sheet according to claim 1 wherein the surface layer part of the steel sheet has an
20 emission intensity of C, measured by high frequency glow discharge spectrometry in the depth
direction from the steel sheet surface, satisfying following formula (3) and formula (4),
C40/C150>0.50 ... (3)
0.90C140/C1501.10... (4)
where,
25 C40: emission intensity of C at depth position of 40 m from the steel sheet surface
C140: emission intensity of C at depth position of 140 m from the steel sheet surface
C150: emission intensity of C at depth position of 150 m from the steel sheet surface
[Claim 3]
30 The steel sheet according to claim 1 or 2, wherein a tensile residual stress acting in rolling
perpendicular direction at the steel sheet surface is 200 MPa or less.
[Claim 4]
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet surface has a
35 hot dip galvanized layer or a hot dip galvannealed layer.
48
[Claim 5]
A method of production of steel sheet, which method of production of steel sheet
comprising
a hot rolling step (a) of hot rolling a slab having a chemical composition comprising, by
5 mass%,
C: 0.06 to 0.30%,
Si: 0.01 to 2.50%,
Mn: 1.00 to 3.50%,
Ti: 0.001 to 0.100%,
10 B: 0.0005 to 0.0050%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 1.500% or less,
N: 0.010% or less,
15 O: 0.0100% or less,
Cr: 0 to 1.00%,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
20 Co: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 1.00%,
Sb: 0 to 0.50%,
Nb: 0 to 0.200%,
25 V: 0 to 1.00%,
As: 0 to 0.10%,
Zn: 0 to 1.00%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
30 Ce: 0 to 0.0150%,
Zr: 0 to 0.0100%,
La: 0 to 0.0150%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.0100%,
35 an REM other than Ce and La: 0 to 0.0100%, and
bal.: Fe and impurities at a 850 to 950C finish rolling end temperature to obtain hot rolled
49
steel sheet, then cooling the hot rolled steel sheet down to 450 to 650C and coiling the hot
rolled steel sheet,
a pickling step (b) of pickling the steel sheet obtained at the hot rolling step (a),
a cold rolling step (c) of cold rolling the steel sheet obtained by the pickling step (b) by a 30
5 to 75% rolling reduction to obtain a cold rolled steel sheet,
a heat treatment step (d) of heat treating the steel sheet obtained at the cold rolling step (c),
and
a grinding step (e), before or after the pickling step (b), of using a rotary type grinding brush
containing an abrasive to grind front and back surfaces of the steel sheet obtained at the hot
10 rolling step (a) or the steel sheet obtained at the pickling step (b),
in the hot rolling step (a), finish rolling comprises three passes or more, a rolling reduction
of respective passes of final three passes of the finish rolling is 20% or more, a time between
passes is within 1 second, an entry side steel sheet temperature before the final three passes is
1000C or less, and a time from completion of a final pass to start of cooling is within 3 seconds,
15 in the grinding step (e), a rotational speed R (rpm) of the grinding brush, a diameter D (m)
of the grinding brush, and a running speed V (m/min) of the steel sheet satisfy the following
formula (5),
the heat treatment step (d) further provided with
a step (d-1) of heating the steel sheet obtained at the cold rolling step (c) from 650C to a
20 maximum heating temperature of Ac1+50C or more and 950C or less by an average heating
speed of 0.5 to 500C/s,
a step (d-2) of holding the steel sheet obtained at the cold rolling step (c) at the maximum
heating temperature for 1 second to 300 seconds,
a step (d-3) of cooling the steel sheet obtained at the cold rolling step (c) down to Ms point25 100C or less in temperature, at which step, cooling from 700C to 500C by a 10C/s or more
average cooling speed, and
a step (d-4) of holding the steel sheet obtained at the cold rolling step (c) at 200 to 350C
for 50 to 600 seconds,
at step (d-1), an atmosphere in surroundings of the steel sheet obtained at the cold rolling
30 step (c) having a steam partial pressure pH2O and hydrogen partial pressure pH2 satisfying a
following formula (6):
[Mathematical 1]
50
[Claim 6]
The method of production of steel sheet according to claim 5, wherein
the hot rolling step (a) further comprises a step of retaining a heat of the hot rolled steel
sheet after coiling within 30 minutes in a heat insulating vessel with inside walls covered by a
5 heat insulating material, wherein
a peak temperature of an atmospheric temperature inside of the heat insulating vessel is 500
to 650C, and a time from the atmospheric temperature to the peak temperature is 1 to 8 hours.
[Claim 7]
10 The method of production of steel sheet according to claim 5 or 6, wherein,
in the above step (d-1), the atmosphere of the surroundings of the steel sheet obtained at the
above cold rolling step (c) has a steam partial pressure pH2 O and hydrogen partial pressure pH2
satisfying a following formula (7).
-4.0log(pH2 O/pH2 )-1.0 ...(7)
15
[Claim 8]
The method of production of steel sheet according to any one of claims 5 to 7, wherein,
the above step (d-3) stops the cooling while the temperature of the steel sheet obtained at
the above cold rolling step (c) is between the Ms point and 650C and performs natural cooling
20 for 0.1 to 3.0 seconds.