FIELD
[0001]
The present invention relates to a steel sheet high in strength and excellent in weldability
and a method for producing the same.
10 [0002]
When using a spot welder to weld galvanized steel sheet, sometimes the melted zinc causes
the steel sheet to crack. Such a crack is called an “LME crack (liquid metal embrittlement
crack)” and occurs due to molten zinc penetrating to the inside of the steel sheet along the grain
boundaries of the steel.
15 [0003]
Up until now, numerous inventions have been disclosed relating to DP steel (dual phase
steel) and high strength steel sheet, but among them, there have been few examples of
disclosures of art relating to the suppression of spot welding LME cracks. (For example, see
PTLs 1 to 3.)
20 [0004]
PTL 1 discloses a steel member provided with a surface layer and a first martensite layer
arranged at a layer below the surface layer and having a concentration of nitrogen element of 0.2
to 1.0 mass%, the surface layer having at least one of any of lithium-iron composite oxides, FeO,
and Fe3O4 as a main constituent and containing at least one selected from the group consisting
25 of dissolved silicon, silicon oxides, and silicon nitrides, the first martensite layer including an
area ratio of 30% or less of  phases and an area ratio of 10% or less of  phases. In PTL 1, the
scope of disclosure is limited to art advantageous to increasing strength by high frequency
heating hardening. Art of improving weldability is not disclosed.
[0005]
30 PTL 2 discloses a method of producing a hot rolled steel sheet comprising hot rolling a slab
having a chemical composition containing, by mass%, C: 0.05 to 0.25%, Si: 1.0% or less, Mn:
2.0 to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 1.0% or less, N: 0.001 to 0.015%, and one
or more selected from Ti: 0.003 to 0.030%, Nb: 0.010 to 0.050%, and Mo: 0.005 to 0.100% and
a balance of Fe and unavoidable impurities during which, in the finish rolling, making the
35 temperature from the second pass counting back from the final pass to the final pass 800 to
950C, making the cumulative rolling reduction from the second pass counting back from the
2
final pass to the final pass 10 to 40%, making the rolling reduction of the final pass 8 to 25%,
starting cooling at 0.5 to 3.0 s after the end of final rolling, cooling in the temperature region of
600 to 720C by an average cooling rate of 30C/s or more, and coiling at 590C or less.
However, PTL 2 does not in any way disclose the art of suppressing LME.
5 [0006]
PTL 3 discloses a high strength steel sheet containing, by mass%, C: 0.075% to 0.350%, Si:
0.30% to 2.50%, Mn: 1.20% to 3.50%, P: 0.001% to 0.100%, S: 0.0001% to 0.0100%, Al:
0.005% to 2.500%, and N: 0.0001% to 0.0100% and a balance of iron and unavoidable
impurities, wherein a region, in which oxide particles containing Si and/or Mn and having a
10 particle size of 20 nm or more are dispersed at an average distance between particles of 2.5 m
or less, is present in a range of an average depth from the surface of 0.3 m to 15 m, an average
particle size of oxide particles in the region is 0.3 m or less, and an average hardness at a
location of a depth from the interface with the region of 30 m is Hv250 or more. However, PTL
3 does not disclose the art of suppressing LME.
15
[CITATION LIST]
[PATENT LITERATURE]
[0007]
[PTL 1] Japanese Unexamined Patent Publication No. 2019-35111
20 [PTL 2] Japanese Unexamined Patent Publication No. 2018-90894
[PTL 3] Japanese Unexamined Patent Publication No. 2013-60630
SUMMARY
[TECHNICAL PROBLEM]
25 [0008]
The present invention, in consideration of the above situation, has as its object the provision
of a steel sheet high in strength and excellent in weldability and a method for producing the
same.
30 [SOLUTION TO PROBLEM]
[0009]
The inventors engaged in intensive research on the solution to the above problem and
clarified that “strain” has a great effect on the occurrence of LME cracks. For example, even in
the same current application cycle (heat history), LME cracks remarkably occur if spot welding
35 so as to increase the amount of plastic deformation of steel sheet. It is believed that the reason
why LME cracks more easily occur along with an increase in “strain” is that “penetration of
3
molten zinc to the inside of the steel sheet” as stated above more easily occurs. Therefore, by
preventing an increase of strain at the surface layer of the steel sheet, it becomes possible to
suppress the occurrence of spot welding LME cracks. The inventors discovered the method of
imparting a difference in strength in a thickness direction so as to prevent an increase in strain at
the surface layer of steel sheet. Specifically, they strongly 5 controlled the surface-most layer (first
depth region) by precipitation strengthening, imparted a soft layer (second depth region) reduced
in concentration of carbon at the inside of the thickness of the hard surface-most layer, and
provided a layer (third depth region) harder than this soft layer at the further inside of the
thickness. By providing this three-layer structure with a gradient of characteristics from the
10 surface layer of thickness toward the center layer of thickness, the soft layer (second depth
region) receives the strain when receiving deformation and excessive increase of the strain at the
surface-most layer (first depth region) can be suppressed.
[0010]
Further, the inventors learned through an accumulation of various research that steel sheet
15 of a layer structure having such a suitable difference in hardness in the thickness direction is
difficult to produce if just slightly changing the hot rolling conditions, annealing conditions, etc.,
and can only be produced by optimizing the conditions in the integrated steps of the hot rolling
and annealing steps, etc., and thereby completed the present invention.
[0011]
20 The gist of the present invention is as follows.
[0012]
(1) A steel sheet having a chemical composition comprising, by mass%,
C: 0.20 to 0.40%,
Si: 0.01 to 2.00%,
25 Mn: 0.10% to 4.00%,
P: 0.0200% or less,
S: 0.0200% or less,
Al: 1.500% or less,
N: 0.0200% or less,
30 Ti: 0.005 to 0.500%,
Co: 0 to 0.5000%,
Ni: 0 to 1.0000%,
Mo: 0 to 1.0000%,
Cr: 0 to 2.0000%,
35 O: 0 to 0.0200%,
B: 0 to 0.0100%,
4
Nb: 0 to 0.5000%,
V: 0 to 0.5000%,
Cu: 0 to 0.5000%,
W: 0 to 0.1000%,
5 Ta: 0 to 0.1000%,
Sn: 0 to 0.0500%,
Sb: 0 to 0.0500%,
As: 0 to 0.0500%,
Mg: 0 to 0.0500%,
10 Ca: 0 to 0.0500%,
Y: 0 to 0.0500%,
Zr: 0 to 0.0500%,
La: 0 to 0.0500%,
Ce: 0 to 0.0500%, and
15 a balance of Fe and impurities,
wherein precipitates having a diameter of less than 0.1 m are present in a number density
of 10 to 200/m2 in a depth region of 1 to 10 m from a surface,
an amount of dissolved C in a depth region of 10 to 60 m from the surface is less than 0.20
mass%, and
20 a tensile strength is 1200 MPa or more.
(2) The steel sheet according to the above (1) wherein the chemical composition comprises,
by mass%, one or more selected from the group consisting of
Co: 0.0001 to 0.5000%,
Ni: 0.0001 to 1.0000%,
25 Mo: 0.0001 to 1.0000%,
Cr: 0.0001 to 2.0000%,
O: 0.0001 to 0.0200%,
B: 0.0001 to 0.0100%,
Nb: 0.0001 to 0.5000%,
30 V: 0.0001 to 0.5000%,
Cu: 0.0001 to 0.5000%,
W: 0.0001 to 0.1000%,
Ta: 0.0001 to 0.1000%,
Sn: 0.0001 to 0.0500%,
35 Sb: 0.0001 to 0.0500%,
As: 0.0001 to 0.0500%,
5
Mg: 0.0001 to 0.0500%,
Ca: 0.0001 to 0.0500%,
Y: 0.0001 to 0.0500%,
Zr: 0.0001 to 0.0500%,
5 La: 0.0001 to 0.0500%, and
Ce: 0.0001 to 0.0500%.
(3) The steel sheet according to the above (1) or (2), wherein a plating layer containing zinc,
aluminum, magnesium, an alloy consisting of any combination thereof, or an alloy of at least one
of these elements and iron is formed on at least one surface of the steel sheet.
10 (4) A method for producing a steel sheet comprising
a step of hot rolling a steel slab having a chemical composition according to the above (1)
or (2), then coiling it at 580C or less,
a step of pickling the obtained hot rolled steel sheet to remove oxide scale present on the
surface of the hot rolled steel sheet and remove the surface layer of the hot rolled steel sheet
15 down to at least 5 m, and
a step of cold rolling the hot rolled steel sheet, then annealing it, wherein the annealing
comprises holding the obtained cold rolled steel sheet in an atmosphere of a dew point of -20 to
20C at a temperature region of 200 to 400C for 20 to 180 seconds, then holding it in an
atmosphere of a dew point of -20 to 20C at a temperature region of 740 to 900C for 45 to 300
20 seconds.
(5) The method for producing the steel sheet according to the above (4), wherein, in the
annealing, a plating layer containing zinc, aluminum, magnesium, an alloy consisting of any
combination thereof, or an alloy of at least one of these elements and iron is formed on at least
one surface of the cold rolled steel sheet.
25
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0013]
According to the present invention, it is possible to provide steel sheet high in strength and
excellent in weldability and a method for producing the same.
30
DESCRIPTION OF EMBODIMENTS
[0014]
Below, embodiments of the present invention will be explained. These explanations are
intended to simply illustrate the embodiments of the present invention. The present invention is
35 not limited to the following embodiments.
[0015]
6

The steel sheet according to an embodiment of the present invention has a chemical
composition comprising, by mass%,
C: 0.20 to 0.40%,
5 Si: 0.01 to 2.00%,
Mn: 0.10% to 4.00%,
P: 0.0200% or less,
S: 0.0200% or less,
Al: 1.500% or less,
10 N: 0.0200% or less,
Ti: 0.005 to 0.500%,
Co: 0 to 0.5000%,
Ni: 0 to 1.0000%,
Mo: 0 to 1.0000%,
15 Cr: 0 to 2.0000%,
O: 0 to 0.0200%,
B: 0 to 0.0100%,
Nb: 0 to 0.5000%,
V: 0 to 0.5000%,
20 Cu: 0 to 0.5000%,
W: 0 to 0.1000%,
Ta: 0 to 0.1000%,
Sn: 0 to 0.0500%,
Sb: 0 to 0.0500%,
25 As: 0 to 0.0500%,
Mg: 0 to 0.0500%,
Ca: 0 to 0.0500%,
Y: 0 to 0.0500%,
Zr: 0 to 0.0500%,
30 La: 0 to 0.0500%,
Ce: 0 to 0.0500%, and
a balance of Fe and impurities,
wherein precipitates having a diameter of less than 0.1 m are present in a number density
of 10 to 200/m2 in a depth region of 1 to 10 m from a surface,
35 an amount of dissolved C in a depth region of 10 to 60 m from the surface is less than 0.20
mass%, and
7
a tensile strength is 1200 MPa or more.
[0016]
First, the reasons for limiting the chemical composition of the steel sheet according to an
embodiment of the present invention will be explained. The “%” of the constituents here means
mass%. Further, in this Description, the “5 to” showing a range of numerical values is used in the
sense including the numerical values before and after it as lower limit values and upper limit
values unless otherwise indicated.
[0017]
(C: 0.20 to 0.40%)
10 C is an element making the tensile strength increase inexpensively and is an extremely
important element for control of the strength of the steel. To sufficiently obtain such an effect,
the C content is 0.20% or more. The C content may also be 0.22% or more, 0.25% or more, or
0.28% or more. On the other hand, if excessively including C, sometimes the occurrence of LME
is promoted. For this reason, the C content is 0.40% or less. The C content may also be 0.38% or
15 less, 0.36% or less, or 0.34% or less.
[0018]
(Si: 0.01 to 2.00%)
Si is an element acting as a deoxidizer and suppressing the precipitation of carbides in a
cooling process during cold rolled annealing. To sufficiently obtain such an effect, the Si content
20 is 0.01% or more. The Si content may also be 0.10% or more, 0.30% or more, or 0.80% or more.
On the other hand, if excessively including Si, an increase in the steel strength and a drop in the
elongation are invited and further sometimes cracking of the steel sheet by LME at the time of
spot welding is invited. For this reason, the Si content is 2.00% or less. The Si content may also
be 1.80% or less, 1.50% or less, or 1.20% or less.
25 [0019]
(Mn: 0.10 to 4.00%)
Mn is a factor affecting the ferrite transformation of steel and is an element effective for
raising the strength. To sufficiently obtain such an effect, the Mn content is 0.10% or more. The
Mn content may also be 0.50% or more, 1.00% or more, or 1.50% or more. On the other hand, if
30 excessively including Mn, an increase in the steel strength and a drop in the elongation are
invited and sometimes cracking of the steel sheet due to LME at the time of spot welding is
invited. For this reason, the Mn content is 4.00% or less. The Mn content may also be 3.30% or
less, 3.00% or less, or 2.70% or less.
[0020]
35 (P: 0.0200% or Less)
P is an element strongly segregating at the ferrite grain boundaries and prompting
8
embrittlement of the grain boundaries. The P content is preferably as small as possible, therefore
ideally is 0%. However, excessive reduction of the P content would invite a major increase in
costs, therefore the P content may also be 0.0001% or more and may be 0.0010% or more or
0.0050% or more. On the other hand, if excessively including P, embrittlement of the steel is
invited and further sometimes cracking 5 of the steel sheet due to LME is invited. For this reason,
the P content is 0.0200% or less. The P content may also be 0.0180% or less, 0.0150% or less, or
0.0100% or less.
[0021]
(S: 0.0200% or Less)
10 S is an element forming MnS and other nonmetallic inclusions in the steel and inviting a
drop in ductility of steel parts. The S content is preferably as small as possible, therefore ideally
is 0%. However, excessive reduction of the S content would invite a major increase in costs,
therefore the S content may also be 0.0001% or more and may be 0.0002% or more, 0.0010% or
more, or 0.0050% or more. On the other hand, if excessively including S, occurrence of cracks
15 starting from nonmetallic inclusions at the time of cold forming is invited and further sometimes
cracking of the steel sheet due to LME at the time of spot welding is invited. For this reason, the
S content is 0.0200% or less. The S content may also be 0.0180% or less, 0.0150% or less, or
0.0100% or less.
[0022]
20 (Al: 1.500% or Less)
Al is an element acting as a deoxidizer of steel and stabilizing ferrite and may be included
in accordance with need. Al need not be included, therefore the lower limit of the Al content is
0%. To sufficiently obtain this effect, the Al content is preferably 0.001% or more and may also
be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively
25 including Al, ferrite transformation and bainite transformation are excessively promoted in the
cooling process in cold rolled annealing, therefore the strength of the steel sheet sometimes falls.
For this reason, the Al content is 1.500% or less. The Al content may also be 1.400% or less,
1.200% or less, or 1.000% or less.
[0023]
30 (N: 0.0200% or Less)
N is an element forming coarse nitrides in the steel sheet and causing a drop in the
workability of the steel sheet. Further, N is an element becoming a cause of formation of blow
holes at the time of welding. The N content is preferably as small as possible, therefore ideally is
0%. However, excessive reduction of the N content would invite a major increase in production
35 costs, therefore the N may be 0.0001% or more and may be 0.0005% or more, 0.0010% or more,
or 0.0050% or more. On the other hand, if excessively including N, it will bond with Ti to form
9
large amounts of TiN, therefore the amount of dissolved Ti in the steel sheet will become smaller
and sometimes it will become no longer possible to control the formation of precipitates (for
example, Ti oxides) at the steel sheet surface layer. Therefore, the N content is 0.0200% or less.
The N content may be 0.0150% or less, 0.0100% or less, or 0.0080% or less.
5 [0024]
(Ti: 0.005% to 0.500%)
Ti is an element required for bonding with the oxygen penetrating the surface layer of the
steel from the annealing atmosphere and forming fine precipitates at the steel sheet surface layer
(for example, Ti oxides) in the steps of heating and soaking in the cold rolled annealing. To
10 make the precipitate sufficiently form, the Ti content is 0.005% or more. The Ti content may
also be 0.010% or more, 0.050% or more, 0.100% or more, or 0.150% or more. On the other
hand, if excessively containing Ti, sometimes excessive formation of precipitates is caused or
ferrite transformation is promoted and a drop in strength is caused in the cooling process during
the cold rolled annealing. For this reason, the Ti content is 0.500% or less. The Ti content may
15 also be 0.450% or less, 0.400% or less, 0.350% or less, or 0.300% or less.
[0025]
The basic chemical composition of the steel sheet in the present embodiment is as explained
above. Furthermore, the steel sheet in the present embodiment may contain at least one element
among the following optional elements in place of part of the balance of Fe in accordance with
20 need. These elements need not be included, therefore the lower limits are 0%.
[0026]
(Co: 0 to 0.5000%)
Co is an element effective for control of the morphology of the carbides and increase of
strength and may be included for control of the dissolved carbon in accordance with need. To
25 sufficiently obtain these effects, the Co content is preferably 0.0001% or more. The Co content
may also be 0.0010% or more, 0.0100% or more, or 0.0400% or more. On the other hand, if
excessively including Co, a large amount of fine Co carbides precipitate and sometimes an
excessive rise of the strength of the steel material and/or a drop in the ductility is invited. For this
reason, the Co content is preferably 0.5000% or less. The Co content may also be 0.4000% or
30 less, 0.3000% or less, or 0.2000% or less.
[0027]
(Ni: 0 to 1.0000%)
Ni is a strengthening element and is effective for improvement of the hardenability. In
addition, it improves the wettability and promotes an alloying reaction, therefore may be
35 included in accordance with need. To sufficiently obtain these effects, the Ni content is
preferably 0.0001% or more. The Ni content may also be 0.0010% or more, 0.0100% or more, or
10
0.0500% or more. On the other hand, if excessively including Ni, it sometimes has a detrimental
effect on the productivity at the time of production and hot rolling and causes a drop in the
elongation. For this reason, the Ni content is preferably 1.0000% or less. The Ni content may
also be 0.9000% or less, 0.5000% or less, or 0.200% or less.
5 [0028]
(Mo: 0 to 1.0000%)
Mo is an element effective for improving the strength of steel sheet. Further, Mo is an
element having the effect of inhibiting the ferrite transformation which occurs at the time of heat
treatment in continuous annealing facilities or continuous hot dip galvanization facilities. To
10 sufficiently obtain these effects, the Mo content is preferably 0.0001% or more. The Mo content
may also be 0.0010% or more, 0.0100% or more, or 0.0500% or more. On the other hand, if
excessively including Mo, a large amount of fine Mo carbides precipitates and sometimes invite
an excessive rise in strength of the steel material and/or a drop in ductility. For this reason, the
Mo content is preferably 1.0000% or less. The Mo content may also be 0.9000% or less,
15 0.8000% or less, or 0.700% or less.
[0029]
(Cr: 0 to 2.0000%)
Cr, like Mn, is an element suppressing pearlite transformation and effective for increasing
the strength of steel and may be included as needed. To sufficiently obtain such an effect, the Cr
20 content is preferably 0.0001% or more. The Cr content may also be 0.0010% or more, 0.0100%
or more, or 0.0500% or more. On the other hand, if excessively including Cr, this sometimes
invites an excessive rise in strength of the steel material and/or a drop in ductility. For this
reason, the Cr content is preferably 2.0000% or less. The Cr content may also be 1.7000% or
less, 1.5000% or less, or 1.000% or less.
25 [0030]
(O: 0 to 0.0200%)
O forms oxides and causes the workability to deteriorate, therefore has to be kept down in
content. In particular, oxides are often present as inclusions. If present at the stamped end faces
or cut surfaces, they form notch like defects and coarse dimples at the end faces, therefore invite
30 stress concentration at the time of stretch forming and strong working. These become starting
points of crack formation and cause a major deterioration of the workability. For this reason, the
O content may also be 0%, but excessive reduction invites a major increase in costs and is not
economically preferable. For this reason, the O content is preferably 0.0001% or more. The O
content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other
35 hand, if excessively including O, the above tendency becomes remarkable. In addition,
sometimes excessive formation of precipitates is caused. For this reason, the O content is
11
preferably 0.0200% or less. The O content may also be 0.0150% or less, 0.0100% or less, or
0.0050% or less.
[0031]
(B: 0 to 0.0100%)
B is an element suppressing the formation of 5 ferrite and pearlite in the cooling process from
austenite and promotes the formation of bainite or martensite and other low temperature
transformed structures. Further, B is an element beneficial for increasing the strength of steel and
may be included as needed. However, if the B content is too low, sometimes the effect of
increasing the strength and other improvements are not sufficiently obtained. Furthermore,
10 identification of less than 0.0001% requires careful attention in analysis. Depending on the
analytical apparatus, the lower limit of detection will be reached. For this reason, the B content is
preferably 0.0001% or more. The B content may also be 0.0005% or more, 0.0010% or more, or
0.0015% or more. On the other hand, if excessively including B, formation of coarse B oxides in
the steel is invited. These become starting points of formation of voids at the time of cold
15 forming, whereby the hole expandability and other cold workability sometimes deteriorate. For
this reason, the B content is preferably 0.0100% or less. The B content may also be 0.0080% or
less, 0.0060% or less, or 0.0040% or less.
[0032]
(Nb: 0 to 0.5000%)
20 Nb is an element effective for control of the morphology of carbides and an element also
effective for improving the toughness since its addition refines the structure. To sufficiently
obtain these effects, the Nb content is preferably 0.0001% or more. The Nb content may also be
0.0010% or more, 0.0100% or more, or 0.0200% or more. On the other hand, if excessively
including Nb, a large number of fine, hard Nb carbides precipitate and invite remarkable
25 deterioration of the ductility and sometimes cause a drop in the cold workability other cold
workability. For this reason, the Nb content is preferably 0.5000% or less. The Nb content may
also be 0.4000% or less, 0.2000% or less, or 0.1000% or less.
[0033]
(V: 0 to 0.5000%)
30 V is a strengthening element and contributes to a rise in strength of the steel sheet by
precipitation strengthening, fine grain strengthening by inhibiting growth of crystal grains, and
dislocation strengthening through inhibiting recrystallization. To sufficiently obtain such an
effect, the V content is preferably 0.0001% or more. The V content may also be 0.0010% or
more, 0.0100% or more, or 0.0200% or more. On the other hand, if excessively including V, the
35 precipitation of carbonitrides becomes greater and sometimes the hole expandability and other
cold workability deteriorate. For this reason, the V content is preferably 0.5000% or less. The V
12
content may also be 0.4000% or less, 0.2000% or less, or 0.1000% or less.
[0034]
(Cu: 0 to 0.5000%)
Cu is an element effective for improvement of the strength of the steel sheet. To sufficiently
obtain such an effect, the 5 Cu content is preferably 0.0001% or more. The Cu content may also be
0.0010% or more, 0.0100% or more, or 0.0200% or more. On the other hand, if excessively
including Cu, during the hot rolling, the steel material becomes brittle and sometimes hot rolling
becomes impossible. Furthermore, the strength of the steel remarkably rises and sometimes the
hole expandability and other cold workability deteriorates. For this reason, the Cu content is
10 preferably 0.5000% or less. The Cu content may also be 0.4000% or less, 0.2000% or less, or
0.1000% or less.
[0035]
(W: 0 to 0.1000%)
W is effective for raising the strength of the steel sheet and is an extremely important
15 element since precipitates and crystals containing W become hydrogen trapping sites. To
sufficiently obtain these effects, the W content is preferably 0.0001% or more. The W content
may also be 0.0010% or more, 0.0050% or more, or 0.0100% or more. On the other hand, if
excessively including W, the workability sometimes falls. For this reason, the W content is
preferably 0.1000% or less. The W content may also be 0.0800% or less, 0.0600% or less, or
20 0.0400% or less.
[0036]
(Ta: 0 to 0.1000%)
Ta, like Co, is an element effective for control of the morphology of the carbides and
increase of strength and may be included in accordance with need. To sufficiently obtain these
25 effects, the Ta content is preferably 0.0001% or more. The Ta content may also be 0.0010% or
more, 0.0050% or more, or 0.0100% or more. On the other hand, if excessively including Ta, a
large number of fine Ta carbides precipitate and sometimes a rise in strength of the steel sheet
and drop in ductility are invited and the hole expandability and other cold workability are made
to drop. For this reason, the Ta content is preferably 0.1000% or less. The Ta content may also
30 be 0.0800% or less, 0.0600% or less, or 0.0400% or less.
[0037]
(Sn: 0 to 0.0500%)
Sn is an element included in steel when using scrap as a raw material. The less the better.
Therefore, the Sn content may also be 0%, but excessive reduction invites an increase in refining
35 costs. For this reason, the Sn content is preferably 0.0001% or more. The Sn content may also be
0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if excessively
13
including Sn, sometimes a drop in hole expandability and other cold workability is caused due to
embrittlement of the ferrite. For this reason, the Sn content is preferably 0.0500% or less. The Sn
content may also be 0.0400% or less, 0.0200% or less, or 0.0100% or less.
[0038]
5 (Sb: 0 to 0.0500%)
Sb, like Sn, is an element included when using scrap as a steel raw material. Sb strongly
segregates at the grain boundaries and invites embrittlement of the grain boundaries and a drop
in ductility, therefore the less the better. 0% is also possible. However, excessive reduction
invites an increase in refining costs. For this reason, the Sb content is preferably 0.0001% or
10 more. The Sb content may also be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On
the other hand, if excessively including Sb, sometimes a drop in the hole expandability and other
cold workability is caused. For this reason, the Sb content is preferably 0.0500% or less. The Sb
content may also be 0.0400% or less, 0.0200% or less, or 0.0100% or less.
[0039]
15 (As: 0 to 0.0500%)
As, like Sn and Sb, is an element included when using scrap as a steel raw material. It is an
element which strongly segregates at the grain boundaries. The less the better. Therefore, the As
content may be 0%, but excessive reduction invites an increase in the refining costs. For this
reason, the As content is preferably 0.0001% or more. The As content may also be 0.0005% or
20 more, 0.0010% or more, or 0.0020% or more. On the other hand, if excessively including As, a
drop in cold workability is sometimes invited. For this reason, the As content is preferably
0.0500% or less. The As content may also be 0.0400% or less, 0.0300% or less, or 0.0200% or
less.
[0040]
25 (Mg: 0 to 0.0500%)
Mg is an element enabling control of the morphology of sulfides with trace addition and
may be included in accordance with need. To sufficiently obtain such an effect, the Mg content
is preferably 0.0001% or more. The Mg content may also be 0.0005% or more, 0.0010% or
more, or 0.0020% or more. On the other hand, if excessively including Mg, sometimes a drop in
30 the hole expandability and other cold workability is caused due to the formation of coarse
inclusions. For this reason, the Mg content is preferably 0.0500% or less. Mg content may also
be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0041]
(Ca: 0 to 0.0500%)
35 Ca is useful as a deoxidizing element and also has an effect on control of the morphology of
sulfides. To sufficiently obtain these effects, the Ca content is preferably 0.0001% or more. The
14
Ca content may also be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other
hand, if excessively including Ca, sometimes the hole expandability and other cold workability
deteriorate. For this reason, the Ca content is preferably 0.0500% or less. The Ca content may
also be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
5 [0042]
(Y: 0 to 0.0500%)
Y, like Mg and Ca, is an element enabling control of the morphology of sulfides with trace
addition and may be included in accordance with need. To sufficiently obtain such an effect, the
Y content is preferably 0.0001% or more. The Y content may also be 0.0005% or more, 0.0010%
10 or more, or 0.0020% or more. On the other hand, if excessively including Y, coarse Y oxides are
formed and sometimes the hole expandability and other cold workability fall. For this reason, the
Y content is preferably 0.0500% or less. The Y content may also be 0.0400% or less, 0.0300% or
less, or 0.0200% or less.
[0043]
15 (Zr: 0 to 0.0500%)
Zr, like Mg, Ca, and Y, is an element enabling control of the morphology of sulfides with
trace addition and may be included in accordance with need. To sufficiently obtain such an
effect, the Zr content is preferably 0.0001% or more. The Zr content may also be 0.0005% or
more, 0.0010% or more, or 0.0020% or more. On the other hand, if excessively including Zr,
20 coarse Zr oxides are formed and sometimes the hole expandability and other cold workability
fall. For this reason, the Zr content is preferably 0.0500% or less. The Zr content may also be
0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0044]
(La: 0 to 0.0500%)
25 La is an element enabling control of the morphology of sulfides with trace addition and may
be included in accordance with need. To sufficiently obtain such an effect, the La content is
preferably 0.0001% or more. The La content may also be 0.0005% or more, 0.0010% or more, or
0.0020% or more. On the other hand, if excessively including La, La oxides are formed and a
drop in hole expandability and other cold workability is sometimes invited. For this reason, the
30 La content is preferably 0.0500% or less. The La content may also be 0.0400% or less, 0.0300%
or less, or 0.0200% or less.
[0045]
(Ce: 0 to 0.0500%)
Ce, like La, is an element enabling control of the morphology of sulfides with trace addition
35 and may be included in accordance with need. To sufficiently obtain such an effect, the Ce
content is preferably 0.0001% or more. The Ce content may also be 0.0005% or more, 0.0010%
15
or more, or 0.0020% or more. On the other hand, if excessively including Ce, Ce oxides are
formed and a drop in hole expandability and other cold workability is sometimes invited. For this
reason, the Ce content is preferably 0.0500% or less. The Ce content may also be 0.0400% or
less, 0.0300% or less, or 0.0200% or less.
5 [0046]
In the steel sheet in the present embodiment, the balance other than the constituents
explained above is Fe and impurities. The “impurities” are constituents, etc., entering due to
various factors in the producing process, first and foremost the raw materials such as the ores and
scraps, etc., when industrially producing the steel sheet according to the present embodiment.
10 [0047]
Next, the features of the structure and characteristics of the steel sheet according to an
embodiment of the present invention will be explained.
[0048]
(Number Density of Precipitates Having Diameter of Less Than 0.1 m in First Depth Region of
1 to 10 m From Steel Sheet Surface: 10 to 200/m215 )
The steel sheet in the present embodiment contains precipitates having a diameter of less
than 0.1 m in the first depth region of 1 to 10 m from steel sheet surface in a number density
of 10 to 200/m2. By fine precipitates being present in a large number in such a way, the steel
sheet structure of the first depth region becomes finer and as a result the strength and hardness in
20 the first depth region of the steel sheet becomes higher than the strength and hardness of the
second depth region explained later. For this reason, it is possible to increase the deformation
resistance of the steel sheet in the first depth region at the stage heated to a high temperature at
the time of spot welding. Therefore, when pressing the electrodes against steel sheets at the time
of spot welding and applying current and applying a load while holding them at a high
25 temperature, it is possible to keep down an increase in the plastic strain in the surface layer
region of the steel sheet (first depth region). If the number density of the precipitates is low, it is
not possible to raise the deformation resistance at the time of welding and becomes difficult to
keep down LME crack. For this reason, the lower limit value of the number density of
precipitates having a diameter of less than 0.1 m in the first depth region is 10/m2 or more and
may be 15/m2 or more or 30/m2 30 or more. On the other hand, if the number density of the
precipitates is too great, oxides are present in a high density, whereby the electrical resistance of
the steel sheet surface increases and the amount of heat emission at the steel sheet surface layer
becomes higher. For this reason, sometimes a drop in the weldability is caused. For this reason,
the upper limit value of the number density of precipitates having a diameter of less than 0.1 m
in the first depth region is 200/m2 or less and may be 150/m2 or less or 120/m2 35 or less. The
above precipitates may be any precipitates and are not particularly limited, but for example
16
include Ti precipitates and W precipitates, more specifically include Ti oxides and Ti carbides.
The “precipitates” in the present invention are for example particles of oxides or carbides such as
TiO, TiO2, Ti2O3, Ti3O5, and TiC.
[0049]
(Amount of Dissolved C in Second Depth Region of 10 to 60 5 m From Steel Sheet Surface: Less
Than 0.20 Mass%)
In general, the amount of dissolved C has an effect on the strength of steel. The greater the
amount of dissolved C, the more the deformation resistance increases. On the other hand, the
smaller the amount of dissolved C, the more the strength of the steel falls, i.e., the relatively
10 softer the steel becomes. As explained before, to suppress the occurrence of LME crack at the
time of spot welding, it is important to prevent an increase of strain at the steel sheet surface
layer. Therefore, in the steel sheet according to an embodiment of the present invention, by
making the amount of dissolved C in a second depth region of 10 to 60 m from the steel sheet
surface relatively smaller than the C content of the steel sheet as a whole, it is possible to reduce
15 the strength of the depth region. For this reason, the strength of the second depth region becomes
lower than the first depth region. As a result, the second depth region can absorb more of the
strain introduced to the steel sheet by hot deformation at the time of spot welding than the first
depth region and LME crack can be suppressed. If the amount of dissolved C at second depth
region is 0.20 mass% or higher, the strength of the steel at the second depth region increases. For
20 this reason, the increase of the strain occurring at the first depth region cannot be sufficiently
suppressed at the second depth region and it becomes difficult to suppress LME crack. For this
reason, the amount of dissolved C at the depth region is less than 0.20 mass%, preferably is 0.15
mass% or less, more preferably 0.10 mass% or less. The lower limit value of the amount of
dissolved C is not particularly limited and may be 0 mass%, but in general is 0.01 mass% and
25 may be 0.02 mass% or 0.03 mass%.
[0050]
In the steel sheet according to an embodiment of the present invention, at the inner part
(center in depth direction of steel sheet) side from the second depth region of 10 to 60 m from
the steel sheet surface, the average carbon concentration becomes substantially the same as or
30 completely the same as the carbon concentration of the base material. For this reason, compared
with the second depth region of 10 to 60 m from the steel sheet surface, a harder layer becomes
present in a region 60 m or more deeper than the steel sheet surface. More specifically, the
amount of dissolved C at the depth region of 60 m to ¼ thickness from the steel sheet surface
(below, referred to as the “third depth region”) is higher than the amount of dissolved C at the
35 second depth region of 10 to 60 m from the steel sheet surface. For the purpose of obtaining the
effect of the hardness at the third depth region becoming sufficiently higher than the hardness at
17
the second depth region, the amount of dissolved C at the third depth region may be, for
example, 1.10 times or more of the amount of dissolved C at the second depth region, 1.15 times
or more, or 1.20 times or more and/or 0.40 mass% or less, 0.38 mass% or less, 0.36 mass% or
less, or 0.34 mass% or less.
5 [0051]
(Plating Layer)
The steel sheet according to an embodiment of the present invention may include a plating
layer at least at one surface, preferably at both surfaces, for the purpose of improving the
corrosion resistance, etc. This plating layer may be a plating layer having any composition
10 known to persons skilled in the art. It is not particularly limited, but, for example, may include
zinc, aluminum, magnesium, or an alloy consisting of any combination thereof. Further, the
plating layer may be subjected to alloying treatment or need not be subjected to alloying
treatment. If performing the alloying treatment, the plating layer may include an alloy of at least
one of the above elements and the iron diffused from the steel sheet. Further, the amount of
15 deposition of the plating layer is not particularly limited and may be a general amount of
deposition.
[0052]
(Tensile Strength: TS)
For lightening the weight of a structural member using steel as its material and for
20 improving the resistance of the structural member in plastic deformation, the steel material
preferably has a large work hardening ability and exhibits its maximum strength, specifically
preferably has a tensile strength of 1200 MPa or more. If the tensile strength is low, the effect of
lightening the weight of the structural member using steel as its material and improving the
deformation resistance becomes smaller. For this reason, the tensile strength of the steel sheet is
25 1200 MPa or more and may also be 1280 MPa or more, 1400 MPa or more, or 1500 MPa or
more. On the other hand, if the tensile strength is too high, the material easily becomes brittle
and fractures during plastic deformation and falls in formability. For this reason, the tensile
strength of the steel sheet is generally 2300 MPa or less and may be 2100 MPa or less, 2000
MPa or less, or 1900 MPa or less. The tensile strength is measured by obtaining a JIS No. 5 test
30 piece from a direction in which a longitudinal direction of the test piece becomes parallel to the
direction perpendicular to rolling of the steel sheet and performing a tensile test based on JIS Z
2241(2011).
[0053]
(Total Elongation: t-El)
35 According to a specific embodiment of the present invention, in addition to a high strength
and excellent weldability, improvement of the total elongation is also possible. For example, a
18
total elongation of 5.0% or more, 7.0% or more, or 10.0% or more can be achieved. The upper
limit value is not particularly prescribed, but, for example, the total elongation may be 25.0% or
less or 20.0% or less. When working the steel sheet material cold to produce a structural
member, elongation becomes required for finishing it to a complicated shape. Therefore, steel
sheet able to achieve such a high tot 5 al elongation is extremely useful in producing a structural
member. The total elongation is measured by obtaining a JIS No. 5 test piece from a direction in
which a longitudinal direction of the test piece becomes parallel to the direction perpendicular to
rolling of the steel sheet and performing a tensile test based on JIS Z 2241(2011).
[0054]
10 (Hole Expansion Value: )
According to a specific embodiment of the present invention, in addition to a high strength
and excellent weldability, improvement of the hole expandability is also possible. For example, a
hole expansion value of 10.0% or more, 15.0% or more, or 20.0% or more can be achieved. The
upper limit value is not particularly prescribed, but, for example, the hole expansion value may
15 be 90.0% or less or 80.0% or less. When working the steel sheet material cold to produce a
structural member, hole expandability becomes required in addition to elongation for finishing it
to a complicated shape. Therefore, steel sheet able to realize such a high hole expansion value is
extremely useful in producing a structural member. The hole expansion value is determined in
the following way. First, a test piece is punched to give a circular hole of a diameter of 10 mm
20 (initial hole: hole diameter d010 mm) under conditions giving a clearance of 12.5%. The piece
is set so that the burr becomes the die side and the initial hole is expanded by an apex angle 60
conical punch until a crack is formed passing through the sheet thickness. The hole diameter
d1mm at the time of cracking is measured, and the hole expansion value  (%) of each test piece
is found by the following formula. This hole expansion test is performed five times and the
25 average value of these is determined as the hole expansion value .
100(d1-d0)/d0
[0055]
(Sheet Thickness)
The thickness of the steel sheet is a factor affecting the rigidity of the steel member after
30 shaping. The greater the thickness, the higher the rigidity of the member. Therefore, from the
viewpoint of raising the rigidity, a thickness of 0.2 mm or more is preferable. The thickness may
be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. On the other hand, if
the thickness is too great, the shaping load at the time of bulging increases and sometimes wear
of the die or a drop in productivity is invited. For this reason, a thickness of 6.0 mm or less is
35 preferable. The thickness may also be 5.0 mm or less or 4.0 mm or less.
[0056]
19
Next, the methods of examination and measurement of the structure prescribed above will
be explained.
[0057]
(Method of Measurement of Number Density of Precipitates Having Diameter of Less Than 0.1
m in Depth Region of 5 1 to 10 m From Sheet Surface)
The diameter and number density of precipitates in the depth region of 1 to 10 m from the
steel sheet surface were measured by observing the structure at a cross-section of the steel. The
dispersed state of the precipitates remains unchanged in the direction of observation in the RD
direction (rolling direction of steel sheet) or TD direction (transverse direction of steel sheet),
10 therefore it is sufficient to observe the structure in a plane vertical to the ND plane (steel sheet
surface). The material is preliminarily treated by mechanical polishing to finish the polished
surface to a mirror surface. From the surface layer part, a focused ion beam (FIB) processing
device is used to cut out a sample for observation use and was observed by field emission
transmission electron microscopy (FE-TEM) by a magnification of 50,000X and analyzed for
15 composition by energy dispersive X-ray spectrometry (EDX) together to identify the precipitates
and find the diameters of the individual precipitate particles. The field of observation is a region
of 10 m in the thickness direction and, when making the thickness direction the height direction
in the observed image, which is a two-dimensional diagram, a length in the horizontal direction
perpendicular to that height direction of 5 m, i.e., 50 m2. The total number of precipitates
20 having a diameter of less than 0.1 m obtained by observation and analysis of composition was
divided by this area to thereby find the number of precipitates per unit area (number density).
Further, if dimensions in which such regions are included, there is no limit on the area of the
sample used for observation, but for measuring the amount of dissolved C of the surface layer
part explained later, the height of the sample is preferably more than 60 m. Furthermore, the
25 total number of carbides measured can change if the thickness of the sample changes, therefore
the thickness of the sample is 10 to 30 nm. A sample is preferably fabricated to a thickness of 15
to 25 nm.
[0058]
(Method of Measurement of Amount of Dissolved C in Depth Region of 10 to 60 m From Steel
30 Sheet Surface)
The amount of dissolved C in the depth region is found by cutting out a sample for
evaluation use in the same way as the procedure described above and by observing it by FETEM
and analyzing it by EDX. To find the composition at the depth region of 10 to 60 m from
the steel sheet surface, the height of the sample has to be at least more than 60 m. If C is
35 present not as dissolved C, but as precipitates, the form of presence is limited to the two types of
nonmetallic inclusions containing oxides and of carbides. The concentration of C at the
20
nonmetallic inclusions containing oxides and the carbides has a value of more than 2 times the
average value of the constituent of the steel sheet. For this reason, in the map analyzed values by
FE-TEM and EDX in the depth region of 10 to 60 m from the steel sheet surface, a region of 2
times or less of the average composition of the steel sheet is deemed the steel base phase and the
average amount of C of that re 5 gion is the amount of dissolved C. If measuring the amount of
dissolved C of the third depth region, the height of the sample is at least more than 90 m. In the
map analyzed values by FE-TEM and EDX in the depth region of 60 to 90 m from the steel
sheet surface, the region of 2 times or less of the average composition of the steel sheet is
deemed the steel base phase, and the average amount of C of that region is the amount of
10 dissolved C at the third depth region (depth region of 60 m to ¼ thickness from steel sheet
surface).
[0059]

The method for producing a steel sheet according to an embodiment of the present
15 invention is characterized by using a material having the above-mentioned ranges of constituents
and integrally managing the hot rolling and cold rolling and annealing conditions. Specifically,
the method for producing a steel sheet according to an embodiment of the present invention
comprises
a step of hot rolling a steel slab having the same chemical composition as the chemical
20 composition explained above relating to the steel sheet, then coiling it at 580C or less,
a step of pickling the obtained hot rolled steel sheet to remove oxide scale present on the
surface of the hot rolled steel sheet and remove the surface layer of the hot rolled steel sheet
down to at least 5 m, and
a step of cold rolling the hot rolled steel sheet, then annealing it, wherein the annealing
25 comprises holding the obtained cold rolled steel sheet in an atmosphere of a dew point of -20 to
20C at a temperature region of 200 to 400C for 20 to 180 seconds, then holding it in an
atmosphere of a dew point of -20 to 20C at a temperature region of 740 to 900C for 45 to 300
seconds.
[0060]
30 (Hot Rolling and Coiling Step)
In this step, a steel slab having the same chemical composition as the chemical composition
explained above in relation to the steel sheet is supplied to the hot rolling operation. The steel
slab used is preferably cast by a continuous casting method from the viewpoint of productivity,
but may also be produced by an ingot making method or thin slab casting method. Further, the
35 cast steel slab may also be optionally roughly rolled before finish rolling so as to adjust the
thickness, etc. Such rough rolling need only secure the desired sheet bar dimensions. The
21
conditions are not particularly limited. The hot rolling is not particularly limited, but in general is
performed under conditions giving a temperature of completion of finish rolling of 650C or
more. This is because if the completion temperature of finish rolling is too low, the rolling
reaction force will rise and the desired thickness will be difficult to stably obtain. The upper limit
is not particularly limited, but in general t 5 he completion temperature of finish rolling is 950C or
less.
[0061]
(Coiling Temperature)
After the hot rolling, the obtained hot rolled steel sheet is coiled at a coiling temperature of
10 580C or less. The coiling temperature is an important factor controlling the deformation
behavior of the steel structure from austenite to ferrite, pearlite, bainite, and martensite and
controlling the precipitation behavior of Ti. If coiling at a relatively high temperature, after
coiling, sometimes coarse Ti precipitates are formed in the steel structure. In such a case, it
becomes no long possible to impart a sufficient gradient of the characteristics (strength,
15 hardness, etc.) to the surface layer structure of the steel sheet after the cold rolled annealing
explained in detail later. Therefore, to suppress formation of such coarse Ti precipitates, the
coiling temperature is preferably as low as possible, specifically, is 580C or less. The coiling
temperature is preferably 550C or less. For example, the coiling temperature may be room
temperature or less, but for coiling at a temperature of room temperature or less, it is necessary to
20 lower the temperature of the water cooling the steel sheet to room temperature or less. This
causes an increase of the production costs. Further, due to rapid cooling, the residual stress in the
steel sheet rises, therefore for example if coiling the steel sheet at a temperature of less than
10C, in the later pickling step, when uncoiling the sheet, cracking of the steel sheet is invited
and the productivity falls. For this reason, while not limited to this, the lower limit value of the
25 coiling temperature is generally 10C or more, preferably is 50C or more.
[0062]
(Pickling Step)
The coiled hot rolled steel sheet is uncoiled and supplied for pickling. By pickling, it is
possible to remove oxide scale present on the surface of the hot rolled steel sheet and to improve
30 the chemical convertibility or plateability of the cold rolled steel sheet. “Oxide scale” means the
layer of oxides formed on the surface of the steel sheet (external oxide layer) and includes
fayalite (Fe2 SiO4 ) of the complex oxide of FeO and SiO2 formed at the interface with steel
sheet, etc. In addition, pickling causes promotion of the dissolution of the surface layer of the
steel sheet. The oxides formed below the oxide scale at the surface layer of the hot rolled steel
35 sheet, i.e., formed inside the steel sheet (internal oxides), are also completely removed. By
completely removing the oxides formed inside the steel sheet, i.e., by making the thickness of the
22
internal oxide layer formed inside the steel sheet 0 ｍ, it becomes possible to suppress bonding
of the Ti in the steel with oxygen to enable Ti to be present in a dissolved state. Here, the
thickness of the internal oxide layer means the distance from the surface of the steel sheet in the
case advancing in the thickness direction of the steel sheet (direction vertical to surface of steel
sheet) to the furthest position where the internal o 5 xide layer is present. By leaving dissolved Ti at
the inside of thickness from the newly formed surface appearing after pickling, it is possible to
form large numbers of fine Ti precipitates at the surface-most layer of the steel sheet after cold
rolled annealing and as a result impart a sufficient gradient of characteristics to the surface layer
structure. The pickling may be performed one time, but may be performed divided into a
10 plurality of times or may be mechanically polished by a grinding brush, etc., before or after
pickling for more reliably removing the oxides in the steel formed below the oxide scale of the
hot rolled steel sheet. Further, instead of measuring the change of thickness before and after
pickling, it is also possible to find the amount of removal of the steel sheet surface layer from the
change of the coil weight before and after pickling. If the amount of removal of the steel sheet
15 surface layer is less than 5 m, the oxides below the oxide scale are not completely removed,
i.e., the thickness of the internal oxide layer becomes more than 0 m. In the heating step at the
time of cold rolled annealing, oxygen is supplied from the internal oxides remaining at the steel
sheet surface layer, Ti oxides precipitate and coarsen at the steel sheet surface layer, and it
becomes no longer possible to impart a sufficient gradient of characteristics to the surface layer
20 structure of the steel sheet after cold rolled annealing. For this reason, the amount of removal of
the steel sheet surface layer is 5 m or more, more specifically 5 m or more per side, preferably
7 m or more, more preferably 10 m or more. The greater the amount of removal of the steel
sheet surface layer by the pickling, the better, but excessive melt loss of steel causes a drop in the
pickling speed and yield and resultant drop in productivity. Therefore, the upper limit value is
25 generally 150 m or less and may be 120 m or less, 100 m or less, 70 m or less, 50 m or
less, or 30 m or less.
[0063]
(Cold Rolling and Annealing Step)
Finally, the obtained hot rolled steel sheet is cold rolled, then annealed under predetermined
30 conditions (below, referred to as the “cold rolled annealing”) whereby the steel sheet according
to an embodiment of the present invention is obtained. The rolling reduction in the cold rolling is
not limited and may be any suitable value. For example, the rolling reduction may be 5% or
more, 10% or more, or 30% or more and/or may be 90% or less, 75% or less, or 50% or less.
Below, the cold rolled annealing will be explained in detail.
35 [0064]
(Cold Rolled Annealing)
23
(Dew Point at Temperature Region of 200 to 400C)
To raise the dew point of the gas atmosphere in the furnace in the heating step during the
cold rolled annealing, specifically by controlling the dew point to a range of -20 to 20C, it is
possible to promote penetration of oxygen to the inside of the steel sheet and form fine Ti
precipitates in the surface-most layer pa 5 rt of the steel sheet. These fine Ti precipitates can be
used as nuclei for the formation of 10/m2 precipitates with a diameter of less than 0.1 m at the
depth region of 1 to 10 m from the steel sheet surface in the soaking treatment after the heat
treatment and increase of the hardness of the surface-most layer at the steel sheet after cold
rolled annealing. If the dew point is too low, the amount of oxygen penetrating inside the steel
10 sheet becomes insufficient and the nuclei of fine Ti precipitates become scarcer, therefore it
becomes impossible to cause precipitates to form in a sufficient amount at the surface-most layer
of the steel sheet after the cold rolled annealing. For this reason, the lower limit value of the dew
point is -20C or more, preferably -15C or more. On the other hand, if the dew point is high, the
amount of oxygen penetrating inside the steel sheet becomes excessive and coarse Ti precipitates
15 are formed in a low number density. For this reason, the upper limit value of the dew point is
20C or less, preferably 15C or less.
[0065]
(Holding Time at Temperature Region of 200 to 400C)
To form fine Ti precipitates at the surface-most layer part of the steel sheet in the heating
20 step in the cold rolled annealing, along with the dew point, it is effective to control the holding
time in the temperature region of 200 to 400C. Here, the “holding time” means the time of
dwelling at the temperature region of 200 to 400C and accordingly includes the time when the
temperature is being gradually raised between 200 to 400C. If the holding time is short, the
amount of oxygen penetrating inside the steel sheet becomes insufficient and the nuclei of fine Ti
25 precipitates become scarcer, therefore it becomes impossible to cause precipitates to form in a
sufficient amount at the surface-most layer of the steel sheet after the cold rolled annealing. For
this reason, the lower limit value of the holding time is 20 seconds or more, preferably 30
seconds or more. On the other hand, if the holding time is long, the amount of oxygen
penetrating inside the steel sheet becomes excessive and coarse Ti precipitates are formed in a
30 low number density. For this reason, the upper limit value of the holding time is 180 seconds or
less, preferably 150 seconds or less.
[0066]
(Dew Point at Temperature Region of 740 to 900C)
By optimizing the dew point and holding time in the temperature region of 200 to 400C in
35 cold rolled annealing to cause the formation of fine Ti precipitates at the surface-most layer part
of the steel sheet, then using these fine Ti precipitates as nuclei to control the dew point at 740 to
24
900C, it is possible to cause the formation of a sufficient amount of precipitates at the surfacemost
layer of the steel sheet. Further, with holding at 740 to 900C, dispersion of alloy elements
in the steel is promoted more compared with holding at 200 to 400C, therefore the C dissolved
in the steel bonds with the oxygen to be removed in the atmosphere (decarburization reaction),
resulting in a drop in the 5 amount of dissolved C. According to this effect, it is possible to
decrease the amount of dissolved C in the region of 10 to 60 ｍ from the steel sheet surface to
less than 0.20 mass% and possible to newly form a soft layer at this region. If the dew point is
too low, the amount of oxygen penetrating inside the steel sheet becomes insufficient, therefore
coarsening of the Ti precipitates and oxides including Si and Mn having these Ti precipitates as
10 nuclei becomes insufficient and it becomes impossible to make precipitates form in a sufficient
amount at the surface-most layer after cold rolled annealing. In addition, it becomes no longer
possible to decrease the amount of dissolved C at the depth region of 10 to 60 ｍ from the steel
sheet surface. For this reason, the lower limit of the dew point is -20C or more, preferably -
15C or more. On the other hand, if the dew point is high, the amount of oxygen penetrating
15 inside the steel sheet becomes excessive and it becomes no longer possible to suppress the
coarsening and merging of Ti precipitates and oxides including Si and Mn having the Ti
precipitates as nuclei and the number density of precipitates falls. For this reason, the upper limit
of the dew point is 20C or less, preferably 15C or less.

CLAIMS
[Claim 1]
A steel sheet having a chemical composition comprising, by mass%,
5 C: 0.20 to 0.40%,
Si: 0.01 to 2.00%,
Mn: 0.10% to 4.00%,
P: 0.0200% or less,
S: 0.0200% or less,
10 Al: 1.500% or less,
N: 0.0200% or less,
Ti: 0.005 to 0.500%,
Co: 0 to 0.5000%,
Ni: 0 to 1.0000%,
15 Mo: 0 to 1.0000%,
Cr: 0 to 2.0000%,
O: 0 to 0.0200%,
B: 0 to 0.0100%,
Nb: 0 to 0.5000%,
20 V: 0 to 0.5000%,
Cu: 0 to 0.5000%,
W: 0 to 0.1000%,
Ta: 0 to 0.1000%,
Sn: 0 to 0.0500%,
25 Sb: 0 to 0.0500%,
As: 0 to 0.0500%,
Mg: 0 to 0.0500%,
Ca: 0 to 0.0500%,
Y: 0 to 0.0500%,
30 Zr: 0 to 0.0500%,
La: 0 to 0.0500%,
Ce: 0 to 0.0500%, and
a balance of Fe and impurities,
wherein precipitates having a diameter of less than 0.1 m are present in a number density
of 10 to 200/m2 35 in a depth region of 1 to 10 m from a surface,
an amount of dissolved C in a depth region of 10 to 60 m from the surface is less than 0.20
50
mass%, and
a tensile strength is 1200 MPa or more.
[Claim 2]
The steel sheet according to cla 5 im 1 wherein the chemical composition comprises, by
mass%, one or more selected from the group consisting of
Co: 0.0001 to 0.5000%,
Ni: 0.0001 to 1.0000%,
Mo: 0.0001 to 1.0000%,
10 Cr: 0.0001 to 2.0000%,
O: 0.0001 to 0.0200%,
B: 0.0001 to 0.0100%,
Nb: 0.0001 to 0.5000%,
V: 0.0001 to 0.5000%,
15 Cu: 0.0001 to 0.5000%,
W: 0.0001 to 0.1000%,
Ta: 0.0001 to 0.1000%,
Sn: 0.0001 to 0.0500%,
Sb: 0.0001 to 0.0500%,
20 As: 0.0001 to 0.0500%,
Mg: 0.0001 to 0.0500%,
Ca: 0.0001 to 0.0500%,
Y: 0.0001 to 0.0500%,
Zr: 0.0001 to 0.0500%,
25 La: 0.0001 to 0.0500%, and
Ce: 0.0001 to 0.0500%.
[Claim 3]
The steel sheet according to claim 1 or 2, wherein a plating layer containing zinc,
30 aluminum, magnesium, an alloy consisting of any combination thereof, or an alloy of at least one
of these elements and iron is formed on at least one surface of the steel sheet.
[Claim 4]
A method for producing a steel sheet comprising
35 a step of hot rolling a steel slab having a chemical composition according to claim 1 or 2,
then coiling it at 580C or less,
51
a step of pickling the obtained hot rolled steel sheet to remove oxide scale present on the
surface of the hot rolled steel sheet and remove the surface layer of the hot rolled steel sheet
down to at least 5 m, and
a step of cold rolling the hot rolled steel sheet, then annealing it, wherein the annealing
comprises holding the obtained cold roll 5 ed steel sheet in an atmosphere of a dew point of -20 to
20C at a temperature region of 200 to 400C for 20 to 180 seconds, then holding it in an
atmosphere of a dew point of -20 to 20C at a temperature region of 740 to 900C for 45 to 300
seconds.
10 [Claim 5]
The method for producing the steel sheet according to claim 4, wherein, in the annealing, a
plating layer containing zinc, aluminum, magnesium, an alloy consisting of any combination
thereof, or an alloy of at least one of these elements and iron is formed on at least one surface of
the cold rolled steel sheet.