[0001]The present invention relates to steel sheet.
10 BACKGROUND
[0002]In recent years, from the viewpoint of improvement of the fuel efficiency etc. of
automobiles for the purpose of environmental conservation, steel sheet for automobile use is
being made higher in strength and thinner in thickness so as to lighten the weight of automobiles.
15 Further, the steel sheet used for auto parts is formed into various shapes, so excellent formability
is demanded. Furthermore, in the process of assembly of automobiles, the formed parts are
welded together, so good weldability is also important as a criteria for selection of the steel sheet
used for the structural parts of automobiles.
[0003]
20 In this regard, in welding steel sheet which has been galvanized, in particular high strength
steel sheet, for example, as described in the following PTL 1, sometimes the drop in weldability
due to liquid metal embrittlement (LME) cracking becomes a problem. LME cracking is
believed to occur due to the molten zinc penetrating the austenite grain boundaries at the surface
layer part of the steel sheet at the time of welding and causing the steel sheet to become brittle
25 and further tensile stress being applied to the steel sheet at the time of welding.
[CITATIONS LIST]
[PATENT LITERATURE]
[0004]
30 [PTL 1] Japanese Patent No. 6388099
SUMMARY
[TECHNICAL PROBLEM]
[0005]
35 As explained above, LME cracking is believed to occur due to molten zinc penetrating the
austenite grain boundaries of the surface layer part of the steel sheet. For this reason, the
2
weldability of steel sheet is believed to be greatly affected by the state of the surface layer part.
Therefore, the present invention has as its object the provision of steel sheet realizing both
formability and weldability.
5 [SOLUTION TO PROBLEM]
[0006]
The inventors thought that by suppressing the penetration of molten zinc into the austenite
grain boundaries of the surface layer part of steel sheet, it would be possible to suppress LME
cracking and improve the weldability of steel sheet.
10 [0007]
First, the inventors prepared several steel sheets with large numbers of Si oxide grains
distributed at the surface layer part and investigated the weldability of these steel sheets. As a
result, these steel sheets were all improved in weldability. The mechanism of this effect is not
completely clear, but is believed to be as follows:
15 [0008]
The Si oxide grains distributed at the surface layer part of steel sheet can promote ferrite
transformation by forming nucleation sites of ferrite during cooling at the time of welding.
Further, even if Si oxide grains dissolve at a high temperature during welding, the solute Si acts
as a ferrite former and therefore ferrite transformation can be promoted. If ferrite transformation
20 is promoted in this way, a large amount of ferrite is formed at the austenite grain boundaries.
Further, at the surface layer part of steel sheet, it is believed that penetration of molten zinc into
the austenite grain boundaries will be suppressed by the ferrite formed at the austenite grain
boundaries. As a result, it is believed that LME cracking will be suppressed and the weldability
of the steel sheet will be improved.
25 [0009]
However, it is known that depending on the state of distribution of Si oxide grains, the
weldability will be improved while the formability will deteriorate. The inventors continued with
more detailed investigation. As a result, the inventors discovered that suitably controlling the
distribution of Si oxide grains at the surface layer part of steel sheet is important for preventing
30 deterioration of the formability.
[0010]
The gist of the present invention obtained in the above way is as follows:
[0011]
(1) A steel sheet with a tensile strength of 600 MPa or more, a region down to 30 m from
35 a surface of the steel sheet in a sheet thickness direction being defined as a “surface layer part”,
at the surface layer part of at least one side, particle size 20 nm or more Si oxide grains being
3
present in a 3000 to 6000/mm2 or less number density, an average of natural logarithms of the
particle sizes of the Si oxide grains measured in m units being -2.0 to -1.2, a standard error of
the natural logarithms of the particle sizes being 0.6 or less, a number of Si oxide grains with
deviations of the natural logarithms of the particle sizes from the average larger than 2 times the
standard error being 5% 5 or less of the total number of Si oxide grains with particle sizes of 20
nm or more, a chemical composition at a position of 1/4 of the thickness from the surface of the
steel sheet comprising, by mass%, C: 0.050 to 0.800%, Si: 0.01 to 2.50%, Mn: 0.01 to 8.0%, P:
0.1000% or less, S: 0.0500% or less, Al: 0.050% or less, N: 0.0100% or less, O: 0 to 0.020%, Cr:
0 to 3.00%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.20%,
10 Cu: 0 to 1.000%, Ni: 0 to 1.000%, and bal.: Fe and impurities, and a number density of Si oxide
grains at a position of 1/2 of the thickness of the steel sheet being 1000/mm2 or less.
[0012]
(2) The steel sheet of (1), wherein the chemical composition further comprises at least one
element selected from the group consisting of Cr: 0.01 to 3.00%, Mo: 0.01 to 1.00%, B: 0.001 to
15 0.0100%, Ti: 0.010 to 0.200%, Nb: 0.010 to 0.200%, V: 0.01 to 0.20%, Cu: 0.010 to 1.000%,
and Ni: 0.010 to 1.000%.
[0013]
(3) The steel sheet of (1) or (2), wherein the steel sheet further comprises a hot dip
galvanized layer, a hot dip galvannealed layer, or an electrogalvanized layer at the surface of the
20 surface layer part.
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0014]
According to the present invention, it is possible to provide steel sheet able to realize both
25 formability and weldability. Such steel sheet of the present invention is suitable as a material for
auto parts etc.
BRIEF DESCRIPTION OF DRAWINGS
[0015]
30 FIG. 1 is a schematic view showing one example of a method of adding iron oxide to a
surface layer part of molten steel in a continuous casting process of a method for manufacturing
steel sheet of the present invention.
DESCRIPTION OF EMBODIMENTS
35 [0016]
Below, embodiments of the present invention will be explained. Note that, the embodiments
4
shown below are for facilitating understanding of the present invention and must not be
interpreted as limiting the present invention. The present invention can be changed and improved
from the following embodiments without departing from the gist. Note that, the “to” in
expressions of the numerical ranges means inclusion of the numerical value at the left side as the
lower lim 5 it and the numerical value of the right side as the upper limit.
[0017]
The tensile strength of the steel sheet according to the present embodiment is made 600
MPa or more, preferably is made 900 MPa or more. In the steel sheet according to the present
embodiment, the region down to 30 m from the surface of the steel sheet in the sheet thickness
10 direction is defined as the surface layer part and the region at the inside from the surface layer
part in the sheet thickness direction is defined as the center part of the steel sheet. The surface
layer part of steel sheet according to the present embodiment satisfies the conditions explained
below:
[0018]
15 Si oxide grains are present at the surface layer part on at least one side of the steel sheet.
The surface layer part may be formed at only one side of the steel sheet or may be formed at both
sides of the steel sheet. In the present embodiment, the number density of the Si oxide grains is
3000 to 6000/mm2 . Furthermore, the particle sizes of the Si oxide grains in the present
embodiment respectively fall in line with the following particle size distribution. That is, the
20 natural logarithms of the particle sizes of the Si oxide grains measured in m units are on the
average -2.0 to -1.2 and the standard error is 0.6 or less. Further, among the Si oxide grains, the
number of ones with deviations of the natural logarithms of the particle sizes from the average of
more than 2 times the standard error is 5% or less of the number of all Si oxide grains to be
measured at the surface layer part. By satisfying the condition of the above Si oxides, it becomes
25 possible to realize both formability and weldability of high strength steel sheet having a 600 MPa
or more, preferably 900 MPa or more tensile strength.
[0019]
The average of the natural logarithms of the particle sizes of the Si oxide grains measured in
m units is -1.2 or less. Due to this, coarse Si oxide grains are kept from becoming starting
30 points for cracking at the time of shaping of the steel sheet and the formability of the steel sheet
is kept from deteriorating. From this viewpoint, this average is more preferably -1.3 or less, still
more preferably -1.5 or less. Note that, in the present invention, the “formability” indicates the
total elongation obtained by taking a Japan Industrial Standard JIS No. 5 test piece perpendicular
to the rolling direction from the steel sheet and subjecting the test piece to a tensile test based on
35 JIS Z 2241 (2011). On the other hand, by the average of the natural logarithms of the Si oxide
grains measured in m units being made -2.0 or more, the weldability of the steel sheet is
5
improved. From this viewpoint, the average is preferably -1.9 or more, more preferably -1.7 or
more.
[0020]
Further, the standard error of the natural logarithms of the particle sizes of the Si oxide
grains measured in m units is 5 0.6 or less. Due to this, it is possible to keep coarse Si oxide
grains from becoming starting points for cracking at the time of shaping of the steel sheet and
keep the formability of the steel sheet from deteriorating. The lower limit of the standard error is
ideally 0. However, making the lower limit of the standard error less than 0.1 is difficult
technically. For this reason, the lower limit of the standard error may be made 0.1.
10 [0021]
Further, in the above particle size distribution, the ratio of the Si oxide grains with natural
logarithms of the particle sizes more than 2 times the standard error from the average is 5% or
less of the total Si oxide grains. By the particle sizes of the Si oxide grains being controlled in
this way, it is possible to keep coarse Si oxide grains from becoming starting points for cracking
15 at the time of shaping of the steel sheet and keep the formability of the steel sheet from
deteriorating. The lower limit of the above ratio is ideally 0. However, making the above ratio
less than 1% is difficult technically, so the lower limit may be made 1%.
[0022]
The number density of the large number of Si oxide grains present at the surface layer part
of at least one side of the steel sheet is 3000 to 6000/mm2 20 . By making the number density of Si
oxide grains present at the surface layer part of the steel sheet 3000/mm2 or more, LME
cracking is suppressed and the weldability is improved. On the other hand, by making the
number density of Si oxide grains present at the surface layer part of the steel sheet 6000/mm2
or less, the formability of the steel sheet is kept from deteriorating. From this viewpoint, the
number density of the Si oxide grains is more preferably 5500/mm2 25 or less, still more preferably
5000/mm2 or less.
[0023]
Further, the above such large number of Si oxide grains may be present beyond the surface
layer part of the range down to 30 m from the surface. If a large number of Si oxide grains are
30 present at both sides of the steel sheet, the respective surface layer parts may be formed under
similar conditions or the respective surface layer parts may be formed under mutually different
conditions. At that time, if one of the surface layer parts satisfies the above conditions, the other
of the surface layer parts need not satisfy the above conditions, but preferably both of the surface
layer parts satisfy the above conditions. If making the surface layer part satisfying the above
35 conditions the joined surface, LME cracking can be suppressed.
[0024]
6
In the present invention, “the number density of Si oxide grains at the surface layer part”
and “the particle size distribution of Si oxide grains at the surface layer part” are determined in
the following way.
[0025]
The number and particle sizes of the Si oxide g 5 rains can be identified by examination of a
cross-section parallel to the rolling direction of the steel sheet and the sheet thickness direction
by a power of 10000X. Specifically, first, the cross-section of the cutout steel sheet is finished to
a mirror surface by mechanical polishing, then a Nital etchant is used to bring out the steel
structures. After that, the steel structures are examined using a scanning electron microscope
(SEM) at a 0.04 mm2 10 region (region included in surface layer part) centered about a position of
15 m from the surface of the steel sheet in the sheet thickness direction (position of 1/2 of
thickness of the surface layer part). The value of the thus observed and counted number of Si
oxide grains converted to the number per mm2 is made the “number density of Si oxide grains of
the surface layer part”. Further, the particle sizes of Si oxide grains obtained by measurement in
15 that region (circumscribing circle equivalent diameter) and the frequency at which the values of
the particle sizes are measured are made the “particle size distribution of Si oxide grains of the
surface layer part”. Here, the Si oxide grains included in the above number of Si oxide grains are
made ones with particle sizes of 20 nm or more.
[0026]
20 The mechanism of this effect is not completely clear, but is believed to be as follows: If the
particle sizes of the Si oxide grains present at the surface layer part do not fall in line with the
above particle size distribution, there may tend to be many coarse Si oxide grains present at the
surface layer part. These coarse Si oxide grains become causes of concentration of strain and
stress at the time of shaping the steel sheet and may become starting points for cracking by
25 formation of voids. On the other hand, by the particle sizes of the large number of Si oxide
grains distributed at the surface layer part falling in line with the above particle size distribution,
in this way, ferrite transformation can be promoted while cracking due to coarse Si oxide grains
can be kept from occurring. For this reason, it is believed the formability of the steel sheet can be
kept from deteriorating while the weldability can be improved.
30 [0027]
Note that, no notable effect on the weldability of steel sheet could be recognized due to the
number density or particle sizes of Si oxide grains at portions other than the surface layer part of
the steel sheet. From this, even if using sheets which had been considered disadvantageous for
weldability in the past such as the excellent ductility (Dual Phase) steel or TRIP (transformation
35 induced plasticity) steel for the center part of the steel sheet, it is possible to obtain steel sheet
excellent in weldability by forming the surface layer part in the above way. This point is one of
7
the superior points of the present invention.
[0028]
However, from the viewpoint of suppressing deterioration of the formability of steel sheet,
the number density of the Si oxide grains present at the center part of the steel sheet is preferably
small. Specifically, the number density of Si 5 oxide grains at the center of the steel sheet in the
sheet thickness direction (position of 1/2 of the thickness) is made 1000/mm2 or less. In this
region, the region of 0.04 mm2 centered at the position of 1/2 of the thickness is examined for
steel structures by a power of 10000X in the same way as above. The value of the number of Si
oxide grains counted by observation in this way converted to the number per mm2 is made the
10 number density of the Si oxide grains at the center part of the steel sheet.
[0029]
By modifying the surface layer part in this way, it is possible to obtain steel sheet realizing
both formability and weldability.
[0030]
15 Next, the chemical composition of the steel sheet of the present invention will be explained.
In the steel sheet of the present invention, the chemical composition at the center part of the steel
sheet preferably satisfies the following conditions. The chemical composition of the center part
of the steel sheet shall mean the chemical composition measured the position of 1/4 of the
thickness from the surface of the steel sheet. The “%” regarding the content of elements mean
20 “mass%” unless otherwise indicated.
[0031]
“C: 0.050 to 0.800%”
C is an element raising the strength of steel sheet. To obtain the effect of sufficiently raising
the strength of steel sheet, the content of C is made 0.050% or more. Further, by the content of C
25 being 0.800% or less, the toughness of the steel sheet can be kept from dropping. From this
viewpoint, the content of C is preferably 0.600% or less, more preferably is 0.500% or less.
[0032]
“Si: 0.01 to 2.50%”
Si is an element stabilizing the ferrite. That is, Si raises the Ac3 transformation point, so it
30 is possible to form a large amount of ferrite in a broad range of annealing temperatures. This is
added from the viewpoint of improvement of the controllability of the structures of the steel
sheet. To obtain such an effect by Si, the content of Si is made 0.01% or more. In addition, Si is
an element suppressing the coarsening of iron-based carbides and raising the strength and
formability of steel sheet. Further, Si is added as a solution strengthening element for
35 contributing to higher strength of steel sheet. From these viewpoints, the content of Si is more
preferably 1.00% or more, still more preferably 1.20% or more. However, if the content of Si
8
becomes greater, sometimes the steel sheet becomes brittle and the ductility deteriorates. For this
reason, the content of Si is made 2.50% or less. The content of Si is preferably 2.20% or less,
more preferably 2.00% or less.
[0033]
5 “Mn: 0.1 to 8.0%”
Mn is an element raising the hardenability of steel. To obtain such an effect by Mn, the
content of Mn is made 0.1% or more. From the viewpoint of keeping Mn from segregating and
the difference in hardness from becoming too great, the content of Mn is preferably 8.0% or less,
more preferably 5.0% or less, still more preferably 4.0%, still more preferably 3.0% or less.
10 [0034]
“P: 0.1000% or less”
P is an element sometimes segregating and causing embrittlement of the weld zone. For this
reason, the content of P is preferably small. Specifically, the content of P is made 0.1000% or
less. The lower limit of the content of P is 0. Making the content of P less than 0.0010% is
15 economically disadvantageous, so 0.0010% may be made the lower limit.
[0035]
“S: 0.0500% or less”
S is an element sometimes having a detrimental effect on the weldability of steel sheet and
the manufacturability at the time of casting and the time of hot rolling. For this reason, the
20 content of S is preferably small. Specifically, the content of S is made 0.0500% or less. The
lower limit of the content of S is 0. Making the content of S less than 0.0001% is economically
disadvantageous, so 0.0001% may be made the lower limit. The content of S may also be
0.0010% or more.
[0036]
25 “Al: 0.050% or less”
Al is an element acting as a deoxidizer and is added in the deoxidation process according to
need. If using Al as a deoxidizer, Al sometimes remains in the steel sheet, so 0.050% or less of
Al may also be contained. There is no need for Al to be contained, so the lower limit is 0. From
the viewpoint of manufacturing ability, the lower limit may be made 0.0001%.
30 [0037]
“N: 0.0100% or less”
N sometimes forms coarse nitrides and causes deterioration of the bendability of steel sheet.
Further, N sometimes causes formation of blowholes at the time of welding. Accordingly, the
content of N is preferably small. Specifically, the content of N is made 0.0100% or less. The
35 lower limit of the content of N is 0. Making the content of N less than 0.0005% is economically
disadvantageous, so 0.0005% may be made the lower limit.
9
[0038]
“O: 0 to 0.020%”
O is an element required for forming Si oxides at the surface layer. However, if Si oxides
are formed at the surface layer part, O need not be present at a position of 1/4 of the thickness
from the surface. Therefore, 5 the lower limit of content of O at the position of 1/4 of the thickness
from the surface may be 0. However, to efficiently form Si oxides at the surface layer part,
0.001% may be made the lower limit. If the amount of O of the position of 1/4 of the thickness
from the surface becomes greater, the elongation falls, so the upper limit is made 0.020%.
[0039]
10 The balance of the chemical composition of the center part of the steel sheet is comprised of
Fe and impurities. However, instead of part of Fe, the following contents may also be contained.
[0040]
“Cr: 0 to 3.00%, Mo: 0 to 1.00%, and B: 0 to 0.0100%”
Cr, Mo, and B are respectively elements raising the hardenability of steel to contribute to
15 improvement of the strength of the steel sheet. The effect by these elements being included is
obtained even if the contents of these elements are small amounts. The contents of these
elements may be 0% as well, but to sufficiently obtain the above effects, preferably the content
of Cr is 0.01% or more, the content of Mo is 0.01% or more, and the content of B is 0.0001% or
more. On the other hand, from the viewpoint of keeping the pickling ability, weldability, hot
20 formability, etc. of the steel sheet from deteriorating, the content of Cr is made 3.00% or less, the
content of Mo is made 1.00% or less, and the content of B is made 0.0100% or less.
[0041]
“Ti: 0 to 0.200%, Nb: 0 to 0.200%, and V: 0 to 0.20%”
Ti, Nb, and V are respectively elements contributing to improvement of the strength of steel
25 sheet. These elements contribute to improvement of the strength of steel sheet by precipitation
strengthening and suppression of growth of ferrite crystal grains for strengthening fine grains and
suppression of recrystallization for dislocation strengthening. The effect by these elements being
included is obtained by the contents of these elements even in small amounts. The contents of
these elements may be 0% as well, but to sufficiently obtain this effect, preferably the respective
30 contents of Ti and Nb are 0.010% or more and the content of V is 0.01% or more. However,
from the viewpoint of keeping the formability of the steel sheet from deteriorating due to the
precipitation of carbonitrides increasing, the respective contents of Ti and Nb are made 0.200%
or less and the content of V is made 0.20% or less.
[0042]
35 “Cu: 0 to 1.000% and Ni: 0 to 1.000%”
Cu and Ni are respectively elements contributing to improvement of the strength of steel
10
sheet. The effect by these elements being included is obtained by the contents of these elements
even in small amounts. The contents of these elements may be 0% as well, but to sufficiently
obtain this effect, preferably the respective contents of Cu and Ni are 0.010% or more. On the
other hand, from the viewpoint of keeping the pickling ability, weldability, hot formability, etc.
of the steel sheet from deteriorating, the respective 5 contents of Cu and Ni are made 1.000% or
less.
[0043]
Furthermore, at the center part of the steel sheet, the following elements may be
intentionally or unavoidably included in place of part of the Fe in a range where the effect of the
10 present invention is obtained. That is, the steel sheet of the present embodiment may contain W:
0 to 0.1%, Ta: 0 to 0.1%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, As: 0 to 0.05%, Mg: 0 to 0.05%, Ca: 0
to 0.05%, Zr: 00.05%, and Y: 0 to 0.05%, La: 0 to 0.05%, Ce: 0 to 0.05% or less, and other
REMs (rare earth metals).
[0044]
15 Note that, the steel sheet of the present invention may further include a hot dip galvanized
layer, hot dip galvannealed layer, or electrogalvanized layer on the surface of the surface layer
part. In this way, even if a coating layer is formed, the steel sheet of the present invention can
realize both formability and weldability by modification of the surface layer part in the above
way.
20 [0045]
Next, one example of the method for manufacture for obtaining the steel sheet of the
present invention will be explained.
[0046]
The present invention lies in the point of controlling the particle sizes of the large number
25 of Si oxide grains contained in the surface layer part so as to fall in line with the above particle
size distribution. Below, the method for manufacturing the steel sheet of the present invention
controlling the number density and particle sizes of the Si oxide grains will be explained. Note
that, the steel sheet of the present invention includes hot rolled steel sheet, cold rolled steel sheet,
plated steel sheet, etc.
30 [0047]
Hot rolled steel sheet
[0048]
In the present embodiment, the method for manufacturing hot rolled steel sheet is not
particularly limited. For example, the method of adding wire to control the distribution of Si
35 oxide grains at the surface layer part, the method of separately manufacturing steel sheet
corresponding to the center part of steel sheet and steel sheet corresponding to the surface layer
11
part in which a large number of Si oxide grains are distributed and stacking and joining these
steel sheets to obtain a multi-layer steel sheet, and the method of adjusting the descaling before
rough rolling to leave scale containing Si oxides at the surface of the steel sheet may be
illustrated.
5 [0049]
Below, the method of adding wire to control the distribution of Si oxide grains at the
surface layer part will be explained.
[0050]
In the process of pouring molten steel satisfying the chemical composition of the center part
10 of steel sheet from a tundish to a continuous casting machine, wire shaped iron oxide is added to
a portion corresponding to the surface layer part of the steel sheet to obtain a slab. FIG. 1 is a
schematic view showing a method of adding wire shaped iron oxide to the surface layer part of
molten steel in a continuous casting process of a method for manufacturing the steel sheet of the
present invention. If iron oxide is added to the surface layer part of molten steel in this way,
15 oxygen contained in the iron oxide bonds with Si whereby Si oxide grains are formed.
[0051]
When adding wire shaped iron oxide to molten steel, 1 mm or more and 50 mm or less
diameter wire shaped iron oxide is used. By the diameter of the wire (wire diameter) being made
1 mm or more, the average of the above particle size distribution which the particle sizes of the
20 Si oxide grains should fall in line with may be made -2.0 or more. On the other hand, by the wire
diameter being made 50 mm or less, the average of the above particle size distribution may be
made -1.2 or less. Further, by the wire diameter being made 50 mm or less, the standard error of
the above particle size distribution may be made 0.6 or less.
[0052]
25 Further, the wire shaped iron oxide is added to the molten steel by arranging a plurality of
wires in the width direction of the molten steel so that the centers of the wires pass through
positions away from the surface of the molten steel by the wire diameters (mm units) or more
and the wire diameters (mm units)+30 mm or less and so that the distances between the centers
of the wires become the wire diameters (mm units) or more and the wire diameters (mm
30 units)+30 mm or less. By the positions of the centers of the wires and the distances of the wires
being controlled in this way, the distribution of Si oxides of the surface layers of the steel sheet
and the number density of the Si oxides at the inner layer of the steel sheet can be suitably
adjusted.
[0053]
35 Further, wire shaped iron oxide is added to the molten steel so that the difference in the feed
rate of the wire and the flow rate of molten steel becomes -500 mm/min or more and 500
12
mm/min or less. By the iron oxide being added to the molten steel in this way, the number
density of the Si oxide grains at the surface layer part can be controlled to 3000/mm2 or more
and 6000/mm2 or less. The faster the feed rate of the wire, the smaller the number density of the
Si oxide grains tends to become. The slower the feed rate of the wire, the larger the number
5 density of Si oxide grains tends to become.
[0054]
After forming a slab having a layer forming the center part of the steel sheet and a surface
layer part in which Si oxide grains are distributed in the above way, the slab is heated by a
1100C or more and 1350C or less, preferably more than 1150C and 1350C or less heating
10 temperature. By the heating temperature of the slab being made 1100C or more, anisotropy of
the crystal orientations due to casting can be suppressed. On the other hand, by making the
heating temperature of the slab 1350C or less, a major increase in the manufacturing costs can
be suppressed.
[0055]
15 After heating the slab in the above way, the slab is supplied to the hot rolling. This hot
rolling process includes a rough rolling process and a finish rolling processing of a finishing
temperature of 800C or more and 980C or less. By making the finishing temperature of the hot
rolling 800C or more, it is easy to keep the rolling reaction force from becoming higher and
stably obtain the desired sheet thickness. On the other hand, by making the finishing temperature
20 of the hot rolling 980C or less, it is possible to end the hot rolling even without using a separate
heating apparatus in the process from ending the heating of the slab to the completion of the hot
rolling and to suppress any major increase in the manufacturing costs of the steel sheet.
[0056]
After that, the steel sheet hot rolled in the above way is cooled until 550C or more and
25 750C or less in temperature by an average cooling rate of 2.5C/s or more. This cooling process
is a process required for rendering the majority of the steel sheet low temperature transformed
structures and making the steel sheet higher in strength. By the average cooling rate being made
2.5C/s or more, ferrite transformation or pearlite transformation is suppressed and a drop in
strength of the steel sheet can be suppressed. The average cooling rate is preferably 5C/s or
30 more, more preferably 10C/s or more. However, at a temperature higher than 750C, ferrite
transformation or pearlite transformation becomes hard to occur, so the average cooling rate is
not limited. Further, at a temperature lower than 550C, the structures transform to low
temperature transformed structures, so the average cooling rate is not limited.
[0057]
35 Next, the steel sheet cooled in this cooling process is coiled up. In this coiling process, the
coiling temperature is made 550C or less. By making the coiling temperature 550C or less,
13
ferrite transformation or pearlite transformation at the surface layer part of the steel sheet is
suppressed. The coiling temperature is preferably 500C or less, more preferably 300C or less.
In this way, it is possible to obtain the coiled up hot rolled steel sheet of the present invention.
[0058]
5 Cold rolled steel sheet
[0059]
Next, an example of the method for manufacturing cold rolled steel sheet in the steel sheet
encompassed by the present invention will be explained.
[0060]
10 First, a slab is obtained in the same way as the above example of hot rolled steel sheet, then
the slab is heated in the same way as the example of manufacture of hot rolled steel sheet and hot
rolled. After that, in the same way as the example of manufacture of hot rolled steel, the steel
sheet which was hot rolled is cooled and coiled. However, in the coiling process, the coiling
temperature is made 20C or more and 700C or less.
15 [0061]
Next, the hot rolled steel sheet coiled up in the above way is uncoiled and pickled. This
pickling process removes oxides (scale) at the surface of the hot rolled steel sheet. This may be
performed one time or divided into several times.
[0062]
20 Next, the steel sheet is cold rolled. In this cold rolling process, preferably the total reduction
rate is 85% or less. By the total rolling reduction being made 85% or less, the ductility of the
center part of the steel sheet is kept from dropping and the center part of the steel sheet is kept
from fracturing during cold rolling. On the other hand, to sufficiently promote recrystallization in
the next annealing process, the total rolling reduction in the cold rolling process is preferably
25 20% or more, more preferably 30% or more. Before cold rolling, to reduce the cold rolling load,
the steel sheet may be annealed at 700C or less in temperature.
[0063]
After the cold rolling process, the steel sheet is annealed. In this annealing process, to make
the steel sheet higher in strength, it is important to make the majority of the structures of the steel
30 sheet low temperature transformed structures and suppress ferrite transformation and pearlite
transformation. In the annealing process, first, the steel sheet is held at the Ac3 point-50C or
more and 900C or less in temperature at the center part of the steel sheet for 5 seconds or more.
The reason for making the heating temperature the Ac3 point-50C or more at the center part of
the steel sheet is that by heating the center part of the steel sheet to the dual-phase region of
35 ferrite and austenite or the single phase region of austenite, by subsequent heat treatment,
transformed structures are obtained and steel sheet having the desired strength is obtained. On
14
the other hand, by making the heating temperature in the annealing process 900C or less, it is
possible to keep the prior austenite grains at the center part of the steel sheet from becoming
coarser and to keep the toughness of the steel sheet from deteriorating.
[0064]
Note 5 that, the Ac3 point is found by the following formula:
Ac3 (C)910-203C+44.7Si-30Mn+700P-20Cu-15.2Ni-11Cr+31.5Mo+400Ti+104V+
400Al
Here, C, Si, Mn, P, Cu, Ni, Cr, Mo, Ti, V, and Al are the contents (mass%) of the elements
contained in the slab.
10 [0065]
After the above annealing process, the annealed steel sheet can be cooled from 550C or
more to 750C or less in temperature by an average cooling rate of 2.5C/s or more to obtain the
cold rolled steel sheet of the present invention. This cooling process is a process required for
making the steel sheet higher in strength. By the average cooling rate being made 2.5C/s or
15 more, ferrite transformation or pearlite transformation is suppressed and a drop in strength of the
steel sheet can be suppressed. The average cooling rate is preferably 5C/s or more, more
preferably 10C/s or more. However, at a temperature higher than 750C, ferrite transformation
or pearlite transformation becomes hard to occur, so the average cooling rate is not limited.
Further, at a temperature lower than 550C, the structures transform to low temperature
20 transformed structures, so the average cooling rate is not limited. At a temperature of 550C or
less, the steel sheet may be cooled by a constant cooling rate down to room temperature. The
steel sheet may be held at a temperature of 200C or more and 550C or less to promote bainite
transformation or temper the martensite. However, if holding steel sheet at 300C or more and
550C or less for a long period of time, the strength of the steel sheet may fall, so if holding steel
25 sheet at that temperature region, the holding time is preferably 600 seconds or less.
[0066]
The above explanation is meant to just illustrate the method of manufacture for obtaining
the steel sheet of the present invention. As explained above, the method of manufacture of the
steel sheet of the present invention is not limited to method of using the addition of wire for
30 controlling the distribution of Si oxide grains of the surface layer part.
[0067]
Plated steel sheet
[0068]
Next, an example of the method for manufacturing plated steel encompassed by the present
35 invention will be explained.
[0069]
15
The surface of the surface layer part of the cold rolled steel sheet manufactured in this way
is hot dip galvanized to obtain hot dip galvanized steel sheet. If hot dip galvanizing it, the coating
bath temperature may be the conventionally used condition. That is, the coating bath temperature
is, for example, made 440C or more and 550C or less.
5 [0070]
Further, as explained above, after performing hot dip galvanization, hot dip galvannealed
steel sheet is obtained by performing heating and alloying treatment. As the heating temperature
in the alloying in the case of heating and alloying treatment, the condition applied in the past
may be used. That is, the heating temperature of the alloying is, for example, made 400C or
10 more and 600C or less. The heating method in the alloying is not particularly limited. Direct
heating by combustion gas, induction heating, direct resistance heating, and other heating
systems suitable for hot dip coating facilities in the past can be used. After the alloying
treatment, the steel sheet is cooled to 200C or less and is skin-pass rolled in accordance with
need.
15 [0071]
Further, as the method for manufacturing electrogalvanized steel sheet, the following
example may be mentioned. For example, the cold rolled steel sheet is treated before coating by
alkali degreasing, rinsing, pickling, and rinsing in that order. After that, the pretreated steel sheet
is, for example, electrolytically treated using a liquid circulation type electroplating apparatus,
20 using a plating bath comprised of zinc sulfate, sodium sulfate, and sulfuric acid at a current
density of 100A/dm2 or so until a predetermined plating thickness..
[0072]
The present invention was made with the object of suppressing liquid metal embrittlement
(LME) cracking caused by welding steel sheet which has been galvanized. LME cracking
25 sometimes occurs even if one of the steel sheets to be welded is galvanized steel sheet and the
other is steel sheet which has not been galvanized. Even if the steel sheet of the present invention
is not galvanized, if what it is welded with is galvanized steel sheet, LME cracking can be
suppressed. Therefore, the above-mentioned steel sheet solves the problem of suppression of
LME cracking not only in steel sheet which has been galvanized, but also steel sheet which has
30 not been galvanized.
EXAMPLES
[0073]
Thickness 250 mm continuously cast slabs having the chemical compositions shown in
35 Table 1 were manufactured. In the process, wire shaped iron oxide was added under the
conditions shown in Tables 2-1 to 2-3 to obtain slabs having Si oxides distributed at the surface
16
layer parts. Whether the wire shaped iron oxide was added to the surface layer parts at one side
or the wire shaped iron oxide was added to the surface layer parts at two sides is shown as the
“position of surface layer part” in Tables 4-1 to 4-3. The slabs were supplied for hot rolling
under conditions of the heating temperatures, finishing temperatures, and coiling temperatures
shown in Tables 2-1 to 2-3 to obtain hot rolled st 5 eel sheets. For steel sheets to be used as cold
rolled steel sheets, after obtaining hot rolled steel sheets as explained above, the sheets were
pickled and cold rolled by total reduction rates of 50% then annealed under the conditions shown
in Tables 2-1 to 2-3. Further, some of the steel sheets were coated by an ordinary method to
obtain plated steel sheets as shown in Tables 2-1 to 2-3.
10 [0074]
The results of evaluation of the obtained steel sheets are shown in Tables 3-1 to 4-3. Details
of the “number density” of Si oxides at the surface layer parts shown in Tables 4-1 to 4-3,
“average”, “standard error”, and “ratio deviated from average” are as explained above. Further,
the chemical compositions at the positions of 1/4 of the thickness from the surface of the
15 obtained steel sheet are shown in Tables 3-1 to 3-3. Furthermore, the obtained steel sheets were
subjected to tensile tests and welding tests as follows.
[0075]
The tensile strength (MPa) and total elongation (%) were measured by preparing a JIS No. 5
test piece in accordance with JIS Z 2241 (2011) having a direction perpendicular to the rolling
20 direction as the long axis and subjecting it to a tensile test. In the present embodiment, a case
where the tensile strength was 600 MPa or more and the value of the tensile strengthtotal
elongation was 10000 MPa% or more was deemed passing.
[0076]
Further, the welding test was performed as follows. A 50 mm80 mm test piece was taken
25 from the steel sheet. A servo motor pressure type single-phase AC spot welder (power frequency
50 Hz) was used to weld plated steel sheet to this test piece. After that, the steel structures in the
region of the center part of a nugget of the steel sheet were examined by an optical microscope.
In the present embodiment, as a result of this observation, a case where no cracking could be
found was deemed passing.
30 [0077]
[Table 1]
17
Table 1
Steel type Slab composition (mass%) Ac3
(C)
Remarks
No. C Si Mn P S N O Al B Ti Nb V Mo Cr Ni Cu
A 0.527 0.06 0.9 0.0823 0.0030 0.0006 0.002 0.005 0.0074 0.063 0.025 0.03 0.07 0.25 0.095 0.160 821 Inv. steel
B 0.680 1.10 1.0 0.0081 0.0083 0.0052 0.008 0.004 0.0006 0.018 0.110 0.03 0.53 0.76 0.108 0.543 775 Inv. steel
C 0.367 1.82 2.8 0.0106 0.0043 0.0075 0.014 0.006
794 Inv. steel
D 0.084 2.08 4.3 0.0728 0.0257 0.0011 0.003 0.005 0.0011 0.024 0.024 0.15 0.12 0.15 0.073 0.080 893 Inv. steel
E 0.319 0.79 5.4 0.0110 0.0026 0.0008 0.001 0.039
692 Inv. steel
F 0.574 2.46 0.7 0.0074 0.0047 0.0005 0.001 0.043 0.0009 0.021 0.037 0.03 0.19 1.58 0.049 0.078 865 Inv. steel
G 0.704 1.64 0.7 0.0057 0.0062 0.0009 0.004 0.006 0.0056 0.026 0.033 0.02 0.82 0.19 0.259 0.284 825 Inv. steel
H 0.632 0.99 1.2 0.0095 0.0137 0.0011 0.008 0.004
765 Inv. steel
I 0.496 0.21 0.6 0.0193 0.0369 0.0081 0.001 0.005 0.0005 0.113 0.157 0.02 0.06 0.37 0.048 0.094 816 Inv. steel
J 0.169 1.49 6.2 0.0061 0.0037 0.0013 0.001 0.012 0.0005 0.026 0.025 0.02 0.12 0.54 0.162 0.745 709 Inv. steel
K 0.790 2.29 0.8 0.0080 0.0033 0.0007 0.003 0.005 0.0011 0.148 0.047 0.12 0.07 2.26 0.846 0.130 849 Inv. steel
L 0.139 0.89 1.8 0.0157 0.0419 0.0010 0.001 0.009
835 Inv. steel
M 0.277 0.62 1.9 0.0258 0.0054 0.0005 0.009 0.003 0.0010 0.028 0.026 0.04 0.07 0.17 0.061 0.110 806 Inv. steel
N 0.227 1.64 2.5 0.0518 0.0050 0.0030 0.005 0.027
859 Inv. steel
O 0.436 0.37 0.6 0.0038 0.0035 0.0018 0.001 0.004 0.0010 0.025 0.027 0.03 0.06 2.47 0.105 0.075 764 Inv. steel
P 0.034 0.45 1.1 0.0100 0.0030 0.0041 0.002 0.003 0.0008 0.028 0.113 0.02 0.05 0.19 0.080 0.103 877 Comp.steel
Q 0.818 1.00 6.3 0.0070 0.0023 0.0010 0.001 0.022 0.0009 0.034 0.023 0.03 0.12 0.36 0.103 0.129 608 Comp.steel
R 0.306 1.79 2.7 0.0113 0.0028 0.0008 0.009 0.037
0.12 0.36
819 Inv. steel
S 0.166 1.49 2.2 0.0063 0.0034 0.0018 0.003 0.013 0.0018 0.024
847 Inv. steel
T 0.232 1.54 2.5 0.0512 0.0042 0.0033 0.001 0.028
0.006 0.005 0.01
857 Inv. steel
[0078]
[Table 2]
5
18
Table 2-1
Steel type No. Steel sheet Classification
Continuous casting Hot rolling Annealing
Iron oxide
wire
diameter
(mm)
Position of
center of
iron oxide
wire (mm)
Distance of
iron oxide
wire (mm)
Difference of feed
rate of iron oxide
wire and flow rate
of molten steel
(mm/min)
Heating
temp.
(C)
Finish
temp. of
hot rolling
(C)
750C to
550C
average
cooling
rate (C/s)
Coiling
temp.
(C)
Heating
temp.
(C)
750C to
550C
average
cooling
rate (C/s)
A 1 Cold rolled sheet Ex. 40 45 61 394 1281 882 46.3 455 850 48.1
A 2 Cold rolled sheet Comp. ex. 0.5 5 28 89 1309 839 44.5 352 803 43.8
A 3 Hot rolled sheet Ex. 30 33 59 75 1343 847 47.7 393 - -
A 4 Cold rolled sheet Ex. 42 72 53 -95 1155 949 44.8 57 846 9.8
B 5 Hot rolled sheet Ex. 11 31 26 282 1110 972 42.8 463 - -
B 6 Cold rolled sheet Comp. ex. 51 66 65 30 1263 883 42.9 471 762 15.7
B 7 Hot dip galvanized steel sheet Ex. 22 32 45 250 1111 868 46.9 370 803 47.0
B 8 Cold rolled sheet Ex. 4 23 23 100 1106 902 47.7 379 751 42.1
C 9 Cold rolled sheet Ex. 31 53 49 144 1163 812 45.6 462 822 11.5
C 10 Cold rolled sheet Ex. 40 62 54 134 1282 898 43.0 460 791 47.5
C 11 Hot rolled sheet Ex. 30 50 55 -471 1260 875 28.9 98 - -
D 12 Cold rolled sheet Ex. 43 49 72 -184 1272 897 28.1 473 873 45.6
D 13 Cold rolled sheet Ex. 36 45 53 290 1235 967 46.7 64 892 45.9
D 14 Hot rolled sheet Ex. 16 46 43 477 1123 934 47.2 465 - -
E 15 Cold rolled sheet Ex. 28 39 58 -461 1143 814 47.0 446 738 11.0
E 16 Cold rolled sheet Comp. ex. 23 32 14 189 1134 806 47.2 79 749 45.6
E 17 Hot rolled sheet Ex. 46 75 48 -435 1310 844 44.4 165 - -
E 18 Cold rolled sheet Ex. 30 37 55 -359 1289 805 12.4 444 785 45.5
F 19 Hot rolled sheet Ex. 47 75 77 -379 1127 843 10.5 417 - -
F 20 Cold rolled sheet Ex. 9 39 33 -26 1166 881 12.1 77 837 45.4
F 21 Cold rolled sheet Ex. 34 62 64 -101 1286 864 45.6 469 857 29.8
G 22 Cold rolled sheet Ex. 19 26 48 -242 1340 934 45.0 422 847 46.4
G 23 Cold rolled sheet Comp. ex. 5 22 10 -530 1190 860 43.0 466 824 45.8
G 24 Cold rolled sheet Ex. 31 50 45 -467 1322 912 39.9 474 810 46.0
[0079]
[Table 3]
5
19
Table 2-2
Steel type No. Steel sheet Classification
Continuous casting Hot rolling Annealing
Iron oxide
wire
diameter
(mm)
Position of
center of
iron oxide
wire (mm)
Distance of
iron oxide
wire (mm)
Difference of feed
rate of iron oxide
wire and flow rate
of molten steel
(mm/min)
Heating
temp.
(C)
Finish
temp. of
hot
rolling
(C)
750C to
550C
average
cooling rate
(C/s)
Coiling
temp.
(C)
Heating
temp.
(C)
750C to
550C
average
cooling rate
(C/s)
G 25 Hot rolled sheet Ex. 12 21 38 -41 1225 890 35.8 475 - -
H 26 Cold rolled sheet Ex. 14 16 15 3 1238 950 46.7 369 760 41.9
H 27 Cold rolled sheet Comp. ex. 14 44 42 521 1124 893 46.4 42 776 45.9
H 28 Cold rolled sheet Comp. ex. 40 65 64 -10 1227 933 9.6 64 780 1.0
H 29 Cold rolled sheet Ex. 17 41 40 21 1190 824 47.0 439 780 45.3
H 30 Hot rolled sheet Ex. 11 31 38 -219 1308 863 44.5 275 - -
I 31 Hot rolled sheet Ex. 2 15 28 75 1213 836 43.4 66 - -
I 32 Cold rolled sheet Ex. 42 51 50 464 1213 874 10.5 106 838 42.7
I 33 Cold rolled sheet Ex. 39 52 50 -198 1169 837 45.5 81 896 45.6
J 34 Cold rolled sheet Ex. 21 37 51 230 1307 961 13.2 463 763 47.3
J 35 Cold rolled sheet Ex. 40 55 42 -312 1266 930 35.9 64 801 19.2
J 36 Hot dip galvannealed steel sheet Ex. 49 77 77 -336 1199 893 48.3 468 772 45.3
K 37 Cold rolled sheet Ex. 35 65 63 -438 1202 905 46.6 290 827 28.7
K 38 Cold rolled sheet Ex. 23 23 33 -139 1153 959 46.2 416 835 46.4
K 39 Cold rolled sheet Ex. 26 54 46 218 1202 907 18.4 437 857 44.3
L 40 Hot dip galvannealed steel sheet Ex. 11 39 21 -210 1314 929 44.6 446 846 38.0
L 41 Cold rolled sheet Ex. 3 33 7 144 1141 893 27.7 450 847 44.3
L 42 Cold rolled sheet Ex. 6 36 16 -266 1133 854 42.6 65 846 44.8
M 43 Cold rolled sheet Ex. 9 28 38 -286 1321 918 19.2 454 885 42.2
M 44 Cold rolled sheet Comp. ex. 21 28 36 -430 1247 917 1.3 270 885 45.1
M 45 Hot dip galvanized steel sheet Ex. 49 60 68 -358 1176 975 19.3 469 792 29.2
M 46 Cold rolled sheet Ex. 15 37 30 27 1337 915 9.1 450 863 45.7
N 47 Cold rolled sheet Ex. 34 40 55 339 1292 880 42.1 83 857 44.3
N 48 Electroplated steel sheet Ex. 9 38 19 -82 1297 814 46.2 458 878 46.8
[0080]
[Table 4]
5
20
Table 2-3
Steel type No. Steel sheet Classification
Continuous casting Hot rolling Annealing
Iron oxide
wire
diameter
(mm)
Position of
center of
iron oxide
wire (mm)
Distance of
iron oxide
wire (mm)
Difference of feed
rate of iron oxide
wire and flow rate
of molten steel
(mm/min)
Heating
temp.
(C)
Finish
temp. of
hot rolling
(C)
750C to
550C
average
cooling
rate (C/s)
Coiling
temp.
(C)
Heating
temp.
(C)
750C to
550C
average
cooling
rate (C/s)
N 49 Cold rolled sheet Ex. 23 42 46 170 1190 939 46.0 171 886 46.3
O 50 Cold rolled sheet Ex. 18 47 25 458 1128 853 36.2 162 751 45.7
O 51 Cold rolled sheet Ex. 26 54 46 -148 1249 938 45.5 283 770 46.5
O 52 Cold rolled sheet Ex. 19 48 43 429 1237 815 47.2 470 832 47.2
P 53 Cold rolled sheet Comp. ex. 47 48 76 289 1206 951 46.1 443 824 17.9
Q 54 Cold rolled sheet Comp. ex. 6 21 35 432 1288 836 47.1 477 729 35.8
C 55 Cold rolled sheet Comp. ex. 34 26 48 -42 1306 978 46.9 441 835 41.7
D 56 Cold rolled sheet Comp. ex. 8 17 37 -124 1345 962 39.8 480 762 38.9
F 57 Cold rolled sheet Comp. ex. 47 47 106 7 1288 919 10.7 442 864 7.5
R 58 Cold rolled sheet Ex. 32 51 36 111 1179 897 22.7 523 808 9.8
R 59 Cold rolled sheet Ex. 29 37 41 -52 1310 934 49.9 517 774 41.0
S 60 Cold rolled sheet Ex. 41 45 44 -44 1307 893 44.9 513 841 29.8
S 61 Cold rolled sheet Ex. 39 55 52 -186 1266 933 33.8 542 824 19.2
T 62 Cold rolled sheet Ex. 33 36 35 17 1297 915 43.8 483 857 44.8
T 63 Cold rolled sheet Ex. 29 40 41 -144 1190 880 32.5 494 843 44.3
D 64 Cold rolled sheet Comp. ex. 27 62 45 414 1213 914 42.4 418 862 41.4
A 65 Hot rolled sheet Comp. ex. 0.5 29 20 -10 1273 912 43.0 528 - -
B 66 Hot rolled sheet Comp. ex. 55 59 56 21 1229 886 39.9 490 - -
C 67 Hot rolled sheet Comp. ex. 31 24 40 -219 1279 901 35.8 516 - -
D 68 Hot rolled sheet Comp. ex. 40 78 43 75 1190 939 46.7 533 - -
E 69 Hot rolled sheet Comp. ex. 30 40 26 464 1182 953 46.4 446 - -
F 70 Hot rolled sheet Comp. ex. 43 55 78 -198 1294 908 9.6 450 - -
G 71 Hot rolled sheet Comp. ex. 36 42 49 -544 1217 885 47.0 510 - -
H 72 Hot rolled sheet Comp. ex. 16 26 23 516 1206 815 44.5 544 - -
[0081]
[Table 5]
5
21
Table 3-1
Steel type No.
Composition at thickness 1/4 position (mass%)
C Si Mn P S N O Al B Ti Nb V Mo Cr Ni Cu
A 1 0.518 0.06 0.9 0.0823 0.0030 0.0006 0.002 0.005 0.0074 0.062 0.025 0.03 0.07 0.25 0.095 0.160
A 2 0.518 0.06 0.9 0.0823 0.0030 0.0006 0.002 0.005 0.0074 0.063 0.025 0.03 0.07 0.25 0.095 0.160
A 3 0.516 0.06 0.9 0.0823 0.0030 0.0006 0.002 0.005 0.0074 0.062 0.025 0.03 0.07 0.25 0.095 0.160
A 4 0.521 0.06 0.9 0.0823 0.0030 0.0006 0.002 0.005 0.0074 0.063 0.025 0.03 0.07 0.25 0.095 0.160
B 5 0.670 1.09 1.0 0.0081 0.0083 0.0052 0.007 0.004 0.0006 0.018 0.110 0.03 0.53 0.76 0.108 0.543
B 6 0.667 1.10 1.0 0.0081 0.0083 0.0052 0.008 0.004 0.0006 0.018 0.110 0.03 0.53 0.76 0.108 0.543
B 7 0.679 1.10 1.0 0.0081 0.0083 0.0052 0.008 0.004 0.0006 0.018 0.110 0.03 0.53 0.76 0.108 0.543
B 8 0.672 1.10 1.0 0.0081 0.0083 0.0052 0.008 0.004 0.0006 0.018 0.110 0.03 0.53 0.76 0.108 0.543
C 9 0.363 1.82 2.8 0.0106 0.0043 0.0075 0.014 0.006 - - - - - - - -
C 10 0.360 1.82 2.8 0.0106 0.0043 0.0075 0.014 0.006 - - - - - - - -
C 11 0.367 1.80 2.8 0.0106 0.0043 0.0075 0.011 0.006 - - - - - - - -
D 12 0.083 2.05 4.2 0.0728 0.0257 0.0011 0.003 0.005 0.0011 0.024 0.024 0.15 0.12 0.15 0.073 0.080
D 13 0.084 2.04 4.3 0.0728 0.0257 0.0011 0.002 0.005 0.0011 0.024 0.024 0.15 0.12 0.15 0.073 0.080
D 14 0.082 2.07 4.3 0.0728 0.0257 0.0011 0.003 0.005 0.0011 0.024 0.024 0.15 0.12 0.15 0.073 0.080
E 15 0.314 0.79 5.3 0.0110 0.0026 0.0008 0.001 0.039 - - - - - - - -
E 16 0.313 0.78 5.3 0.0110 0.0026 0.0008 0.001 0.039 - - - - - - - -
E 17 0.313 0.79 5.3 0.0110 0.0026 0.0008 0.001 0.039 - - - - - - - -
E 18 0.313 0.78 5.3 0.0110 0.0026 0.0008 0.001 0.039 - - - - - - - -
F 19 0.567 2.42 0.7 0.0074 0.0047 0.0005 0.001 0.043 0.0009 0.021 0.037 0.03 0.19 1.58 0.049 0.078
F 20 0.572 2.42 0.7 0.0074 0.0047 0.0005 0.001 0.043 0.0009 0.021 0.037 0.03 0.19 1.58 0.049 0.078
F 21 0.572 2.46 0.7 0.0074 0.0047 0.0005 0.001 0.042 0.0009 0.021 0.037 0.03 0.19 1.58 0.049 0.078
G 22 0.691 1.61 0.7 0.0057 0.0062 0.0009 0.003 0.006 0.0056 0.026 0.033 0.02 0.82 0.19 0.259 0.284
G 23 0.704 1.62 0.7 0.0057 0.0062 0.0009 0.004 0.006 0.0056 0.026 0.033 0.02 0.82 0.19 0.259 0.284
G 24 0.691 1.64 0.7 0.0057 0.0062 0.0009 0.004 0.006 0.0056 0.026 0.033 0.02 0.82 0.19 0.259 0.284
[0082]
[Table 6]
5
22
Table 3-2
Steel type No.
Composition at thickness 1/4 position (mass%)
C Si Mn P S N O Al B Ti Nb V Mo Cr Ni Cu
G 25 0.699 1.61 0.7 0.0057 0.0062 0.0009 0.004 0.006 0.0056 0.026 0.033 0.02 0.82 0.19 0.259 0.284
H 26 0.632 0.98 1.2 0.0095 0.0137 0.0011 0.008 0.004 - - - - - - - -
H 27 0.632 0.99 1.2 0.0095 0.0137 0.0011 0.008 0.004 - - - - - - - -
H 28 0.631 0.99 1.2 0.0095 0.0137 0.0011 0.008 0.004 - - - - - - - -
H 29 0.621 0.99 1.2 0.0095 0.0137 0.0011 0.008 0.004 - - - - - - - -
H 30 0.628 0.99 1.2 0.0095 0.0137 0.0011 0.008 0.004 - - - - - - - -
I 31 0.495 0.21 0.6 0.0193 0.0369 0.0081 0.001 0.005 0.0005 0.112 0.157 0.02 0.06 0.37 0.048 0.094
I 32 0.496 0.21 0.6 0.0193 0.0369 0.0081 0.001 0.005 0.0005 0.111 0.157 0.02 0.06 0.37 0.048 0.094
I 33 0.494 0.21 0.6 0.0193 0.0369 0.0081 0.001 0.005 0.0005 0.111 0.157 0.02 0.06 0.37 0.048 0.094
J 34 0.166 1.48 6.1 0.0061 0.0037 0.0013 0.001 0.012 0.0005 0.026 0.025 0.02 0.12 0.54 0.162 0.745
J 35 0.166 1.47 6.1 0.0061 0.0037 0.0013 0.001 0.012 0.0005 0.026 0.025 0.02 0.12 0.54 0.162 0.745
J 36 0.166 1.48 6.1 0.0061 0.0037 0.0013 0.001 0.012 0.0005 0.026 0.025 0.02 0.12 0.54 0.162 0.745
K 37 0.785 2.29 0.8 0.0080 0.0033 0.0007 0.003 0.005 0.0011 0.147 0.047 0.12 0.07 2.26 0.846 0.130
K 38 0.781 2.28 0.8 0.0080 0.0033 0.0007 0.003 0.005 0.0011 0.146 0.047 0.12 0.07 2.26 0.846 0.130
K 39 0.775 2.26 0.8 0.0080 0.0033 0.0007 0.002 0.005 0.0011 0.146 0.047 0.12 0.07 2.26 0.846 0.130
L 40 0.136 0.87 1.8 0.0157 0.0419 0.0010 0.001 0.009 - - - - - - - -
L 41 0.138 0.89 1.8 0.0157 0.0419 0.0010 0.001 0.009 - - - - - - - -
L 42 0.136 0.88 1.8 0.0157 0.0419 0.0010 0.001 0.009 - - - - - - - -
M 43 0.273 0.61 1.9 0.0258 0.0054 0.0005 0.009 0.003 0.0010 0.028 0.026 0.04 0.07 0.17 0.061 0.110
M 44 0.277 0.62 1.9 0.0258 0.0054 0.0005 0.009 0.003 0.0010 0.028 0.026 0.04 0.07 0.17 0.061 0.110
M 45 0.277 0.61 1.9 0.0258 0.0054 0.0005 0.009 0.003 0.0010 0.028 0.026 0.04 0.07 0.17 0.061 0.110
M 46 0.277 0.61 1.9 0.0258 0.0054 0.0005 0.009 0.003 0.0010 0.028 0.026 0.04 0.07 0.17 0.061 0.110
N 47 0.223 1.63 2.5 0.0518 0.0050 0.0030 0.005 0.027 - - - - - - - -
N 48 0.225 1.62 2.5 0.0518 0.0050 0.0030 0.005 0.027 - - - - - - - -
[0083]
[Table 7]
5
23
Table 3-3
Steel type No.
Composition at thickness 1/4 position (mass%)
C Si Mn P S N O Al B Ti Nb V Mo Cr Ni Cu
N 49 0.227 1.63 2.5 0.0518 0.0050 0.0030 0.005 0.027 - - - - - - - -
O 50 0.435 0.37 0.6 0.0038 0.0035 0.0018 0.001 0.004 0.0010 0.025 0.027 0.03 0.06 2.47 0.105 0.075
O 51 0.436 0.36 0.6 0.0038 0.0035 0.0018 0.001 0.004 0.0010 0.025 0.027 0.03 0.06 2.47 0.105 0.075
O 52 0.427 0.37 0.6 0.0038 0.0035 0.0018 0.001 0.004 0.0010 0.025 0.027 0.03 0.06 2.47 0.105 0.075
P 53 0.034 0.44 1.1 0.0100 0.0030 0.0041 0.002 0.003 0.0008 0.028 0.113 0.02 0.05 0.19 0.080 0.103
Q 54 0.810 0.98 6.3 0.0070 0.0023 0.0010 0.001 0.022 0.0009 0.033 0.023 0.03 0.12 0.36 0.103 0.129
C 55 0.366 1.81 2.8 0.0106 0.0043 0.0075 0.013 0.006 - - - - - - - -
D 56 0.083 2.04 4.3 0.0728 0.0257 0.0011 0.003 0.005 0.0011 0.024 0.024 0.15 0.12 0.15 0.073 0.080
F 57 0.567 2.43 0.7 0.0074 0.0047 0.0005 0.001 0.042 0.0009 0.021 0.037 0.03 0.19 1.58 0.049 0.078
R 58 0.301 1.79 5.3 0.0111 0.0028 0.0008 0.009 0.036
0.12 0.36
R 59 0.304 1.79 5.3 0.0111 0.0028 0.0008 0.009 0.036
0.12 0.36
S 60 0.165 1.49 6.0 0.0062 0.0034 0.0018 0.003 0.013 0.0018 0.024
S 61 0.165 1.49 6.0 0.0062 0.0034 0.0018 0.003 0.013 0.0018 0.024
T 62 0.230 1.54 2.4 0.0503 0.0042 0.0033 0.001 0.028
0.006 0.005 0.01
T 63 0.230 1.54 2.4 0.0503 0.0042 0.0033 0.001 0.028
0.006 0.005 0.01
D 64 0.080 1.88 4.0 0.0728 0.0257 0.0011 0.003 0.004 0.0011 0.022 0.024 0.15 0.12 0.15 0.073 0.080
A 65 0.527 0.06 0.9 0.0823 0.0030 0.0006 0.002 0.005 0.0074 0.063 0.025 0.03 0.07 0.25 0.095 0.160
B 66 0.680 1.09 1.0 0.0081 0.0083 0.0052 0.008 0.004 0.0006 0.018 0.110 0.03 0.53 0.76 0.108 0.543
C 67 0.367 1.82 2.8 0.0106 0.0043 0.0075 0.013 0.006
D 68 0.084 2.06 4.3 0.0728 0.0257 0.0011 0.003 0.005 0.0011 0.024 0.024 0.15 0.12 0.15 0.073 0.080
E 69 0.319 0.79 5.4 0.0110 0.0026 0.0008 0.001 0.039
F 70 0.574 2.45 0.7 0.0074 0.0047 0.0005 0.001 0.043 0.0009 0.021 0.037 0.03 0.19 1.58 0.049 0.078
G 71 0.704 1.64 0.7 0.0057 0.0062 0.0009 0.004 0.006 0.0056 0.026 0.033 0.02 0.82 0.19 0.259 0.284
H 72 0.632 0.99 1.2 0.0095 0.0137 0.0011 0.008 0.004
[0084]
[Table 8]
5
24
Table 4-1
Steel type No. Classification
Si oxides at surface layer part
Number density of Si
oxides at thickness 1/2
position (/mm
2
)
Mechanical properties
Sheet
thickness
(mm)
Position of
surface
layer part
Number
density
(/mm
2
)
Average
Standard
error
Ratio of
deviation from
average (%)
Tensile
strength
(MPa)
Total
elongation
(%)
Tensile strength
Total elongation
(MPa%)
Weld
cracking
A 1 Ex. 5075 -1.2 0.5 3 500 1240 11 13640 No 2.4 2 sides
A 2 Comp. ex. 3550 -2.4 0.5 5 525 660 27 17820 Yes 1.6 2 sides
A 3 Ex. 5000 -1.3 0.5 5 900 960 17 16320 No 1.8 2 sides
A 4 Ex. 5975 -1.2 0.6 1 500 700 19 13300 No 1.4 2 sides
B 5 Ex. 4400 -1.3 0.4 5 675 2100 8 16800 No 0.8 2 sides
B 6 Comp. ex. 6025 -0.8 0.8 2 625 1200 8 9600 Yes 2.2 1 side
B 7 Ex. 5200 -1.2 0.4 5 625 1690 10 16900 No 1.4 2 sides
B 8 Ex. 5575 -1.3 0.4 4 500 740 16 11840 No 1.4 2 sides
C 9 Ex. 5700 -1.4 0.6 3 850 940 21 19740 No 0.8 1 side
C 10 Ex. 5825 -1.2 0.6 5 750 1030 18 18540 No 0.8 2 sides
C 11 Ex. 5250 -1.3 0.6 5 500 1890 6 11340 No 1.8 2 sides
D 12 Ex. 5575 -1.2 0.6 5 500 660 30 19800 No 1.0 2 sides
D 13 Ex. 5600 -1.4 0.6 5 500 910 22 20020 No 1.2 2 sides
D 14 Ex. 5325 -1.6 0.3 1 725 1190 14 16660 No 2.0 2 sides
E 15 Ex. 5600 -1.2 0.4 1 500 930 14 13020 No 2.0 1 side
E 16 Comp. ex. 7000 -0.9 0.5 1 500 1030 5 5150 Yes 2.2 2 sides
E 17 Ex. 5950 -1.2 0.6 4 525 1920 7 13440 No 2.2 2 sides
E 18 Ex. 5275 -1.4 0.4 2 550 1200 15 18000 No 1.6 2 sides
F 19 Ex. 5850 -1.2 0.6 2 525 710 18 12780 No 1.4 2 sides
F 20 Ex. 5900 -1.4 0.4 1 500 670 19 12730 No 2.2 2 sides
F 21 Ex. 5725 -1.4 0.5 1 525 1030 17 17510 No 2.4 2 sides
G 22 Ex. 5275 -1.2 0.6 1 900 1570 7 10990 No 1.6 2 sides
G 23 Comp. ex. 6950 -1.7 0.5 3 500 2210 3 6630 Yes 1.2 2 sides
G 24 Ex. 5125 -1.5 0.6 5 875 900 25 22500 No 1.2 2 sides
[0085]
[Table 9]
5
25
Table 4-2
Steel type No. Classification
Si oxides at surface layer part
Number density of Si
oxides at thickness 1/2
position (/mm
2
)
Mechanical properties
Sheet
thickness
(mm)
Position of
surface
layer part
Number
density
(/mm
2
)
Average
Standard
error
Ratio of
deviation from
average (%)
Tensile
strength
(MPa)
Total
elongation
(%)
Tensile strength
Total elongation
(MPa%)
Weld
cracking
G 25 Ex. 5775 -1.8 0.5 5 500 1980 11 21780 No 2.2 2 sides
H 26 Ex. 3900 -1.2 0.5 3 775 1040 18 18720 No 1.4 2 sides
H 27 Comp. ex. 2025 -1.5 0.6 4 575 2090 5 10450 Yes 1.4 2 sides
H 28 Comp. ex. 5850 -1.3 0.6 1 525 480 35 16800 No 2.2 1 side
H 29 Ex. 4775 -1.5 0.3 1 500 1380 8 11040 No 2.0 2 sides
H 30 Ex. 5150 -1.2 0.4 4 525 1980 11 21780 No 1.0 2 sides
I 31 Ex. 4525 -1.3 0.4 2 575 1330 9 11970 No 1.0 2 sides
I 32 Ex. 5325 -1.2 0.6 4 725 1370 9 12330 No 2.2 2 sides
I 33 Ex. 5725 -1.3 0.5 1 725 1800 8 14400 No 1.4 2 sides
J 34 Ex. 4950 -1.2 0.3 1 650 1480 12 17760 No 2.4 1 side
J 35 Ex. 5275 -1.3 0.6 5 500 1110 14 15540 No 1.6 2 sides
J 36 Ex. 5975 -1.2 0.6 5 625 1480 9 13320 No 2.0 2 sides
K 37 Ex. 5725 -1.4 0.5 3 500 810 30 24300 No 1.0 1 side
K 38 Ex. 5650 -1.4 0.6 4 500 970 24 23280 No 1.2 2 sides
K 39 Ex. 5400 -1.5 0.6 3 550 1400 13 18200 No 0.8 2 sides
L 40 Ex. 4775 -1.5 0.4 1 725 1100 24 26400 No 1.8 1 side
L 41 Ex. 5275 -1.3 0.4 3 500 1110 15 16650 No 2.0 2 sides
L 42 Ex. 5825 -1.6 0.4 2 750 1100 25 27500 No 1.6 2 sides
M 43 Ex. 5575 -1.8 0.5 1 900 1560 10 15600 No 2.4 2 sides
M 44 Comp. ex. 4575 -1.3 0.6 1 625 470 50 23500 No 0.8 2 sides
M 45 Ex. 5825 -1.2 0.6 4 500 740 14 10360 No 1.6 1 side
M 46 Ex. 4575 -1.5 0.3 3 500 1560 16 24960 No 2.4 2 sides
N 47 Ex. 5000 -1.2 0.6 1 525 920 15 13800 No 1.2 2 sides
N 48 Ex. 5125 -1.3 0.5 2 625 1210 13 15730 No 1.8 2 sides
[0086]
[Table 10]
5
26
Table 4-3
Steel type No. Classification
Si oxides at surface layer part
Number density of Si
oxides at thickness 1/2
position (/mm
2
)
Mechanical properties
Sheet
thickness
(mm)
Position of
surface
layer part
Number
density
(/mm
2
)
Average
Standard
error
Ratio of
deviation from
average (%)
Tensile
strength
(MPa)
Total
elongation
(%)
Tensile strength
Total elongation
(MPa%)
Weld
cracking
N 49 Ex. 5400 -1.4 0.5 1 675 1310 9 11790 No 0.8 2 sides
O 50 Ex. 5850 -1.5 0.4 1 500 780 27 21060 No 1.0 2 sides
O 51 Ex. 5450 -1.5 0.5 5 700 1030 16 16480 No 2.4 2 sides
O 52 Ex. 5000 -1.3 0.3 1 900 1620 15 24300 No 1.4 1 side
P 53 Comp. ex. 5125 -1.2 0.6 1 500 240 50 12000 No 1.2 2 sides
Q 54 Comp. ex. 4325 -1.9 0.6 5 500 2460 4 9840 No 0.8 1 side
C 55 Comp. ex. 2675 -1.3 0.5 4 650 1190 10 11900 Yes 1.8 2 sides
D 56 Comp. ex. 3950 -1.5 0.6 2 500 490 37 18130 No 1.2 1 side
F 57 Comp. ex. 1875 -1.2 0.6 5 550 1650 11 18150 Yes 1.6 2 sides
R 58 Ex. 5125 -1.2 0.5 5 500 1290 9 11610 No 1.4 2 sides
R 59 Ex. 4775 -1.3 0.6 2 500 1520 13 19760 No 1.6 2 sides
S 60 Ex. 5975 -1.4 0.6 5 550 960 17 16320 No 1.6 2 sides
S 61 Ex. 5900 -1.5 0.3 4 625 700 19 13300 No 1.6 2 sides
T 62 Ex. 4400 -1.5 0.4 3 825 1310 11 14410 No 1.2 2 sides
T 63 Ex. 5775 -1.3 0.4 4 525 990 23 22770 No 1.4 2 sides
D 64 Comp. ex. 2525 -1.6 0.4 1 1050 870 11 9570 Yes 2.0 2 sides
A 65 Comp. ex. 4550 -2.3 0.6 4 550 1310 10 13100 Yes 3.6 2 sides
B 66 Comp. ex. 6100 -0.9 0.7 4 650 2200 4 8800 Yes 2.8 1 side
C 67 Comp. ex. 2225 -1.2 0.6 2 625 1250 9 11250 Yes 3.4 2 sides
D 68 Comp. ex. 2600 -1.4 0.5 3 1075 790 12 9480 Yes 3.0 1 side
E 69 Comp. ex. 7075 -0.8 0.6 4 825 1130 8 9040 Yes 3.0 2 sides
F 70 Comp. ex. 1925 -1.3 0.5 2 525 1950 6 11700 Yes 3.2 2 sides
G 71 Comp. ex. 6550 -1.6 0.5 1 775 2330 3 6990 Yes 3.2 2 sides
H 72 Comp. ex. 2075 -1.5 0.6 3 550 1990 6 11940 Yes 2.8 2 sides
27
[0087]
In the steel sheets of Comparative Examples 2 and 65, the diameters of the wires added in
the continuous casting process are smaller than 1 mm and the averages of the logarithmic values
of the particle sizes of the Si oxides of the surface layer parts are smaller than -2.0. As a result,
in the steel sheets of these comparative examples, the 5 tensile strengthtotal elongation’s are
10000 MPa% or more, but the weldabilities are not improved.
[0088]
In the steel sheets of Comparative Examples 6 and 66, the diameters of the wires added in
the continuous casting process are larger than 50 mm,the averages of the logarithmic values of
10 the particle sizes of the Si oxides of the surface layer parts are larger than -1.2, and the standard
errors are larger than 0.6. As a result, in the steel sheets of these comparative examples, the
tensile strengthtotal elongation’s are less than 10000 MPa% and the weldabilities are not
improved.
[0089]
15 In the steel sheets of Comparative Examples 16 and 69, the distances between wires added
in the continuous casting process are smaller than the wire diameters, the number densities of Si
oxides of the surface layer parts are larger than 6000/mm2 , and the averages of the logarithmic
values of the particle sizes are larger than -1.2. As a result, the tensile strengthtotal elongation’s
are less than 10000 MPa% and but the weldabilities are not improved.
20 [0090]
In the steel sheets of Comparative Examples 23 and 71, the differences of the feed rates of
the wires added in the continuous casting process and the flow rates of molten steels are smaller
than -500 mm/min and the number densities of the Si oxides of the surface layer parts are larger
than 6000/mm2 . As a result, the tensile strengthtotal elongation’s are less than 10000 MPa%
25 and the weldabilities are not improved.
[0091]
Further, in the steel sheets of Comparative Examples 27 and 72, the differences of the feed
rates of the wires added in the continuous casting process and the flow rates of molten steels are
larger than 500 mm/min and the number densities of the Si oxides of the surface layer parts are
smaller than 3000/mm2 30 . As a result, the tensile strengthtotal elongation’s are 10000 MPa% or
more and the weldabilities are not improved. In contrast to this, in the steel sheets in the
examples of the present invention satisfying the requirement of the difference of the feed rate of
the wire and the flow rate of molten being -500 mm/min or more and 500 mm/min or less, the
number density of the Si oxides satisfies the above requirement and the weldability is improved
35 without lowering the tensile strengthtotal elongation.
[0092]
28
In the steel sheets of Comparative Examples 28 and 44, the average cooling rates at 750C
to 550C in the hot rolling process or annealing process are smaller than 2.5C/s. As a result, in
these steel sheets, the tensile strengths are lower than 600 MPa.
[0093]
In the steel sheets of Comparative 5 Examples 53 and 54, the concentrations of C do not
satisfy the requirements of the present embodiment. As a result, in these steel sheets, the
requirements of the tensile strength and tensile strength total elongation are not satisfied.
[0094]
In the steel sheets of Comparative Examples 55 and 67, the distances between the center
10 positions of the wires added in the continuous casting process and the surfaces of the molten
steels are smaller than the wire diameters and the number densities of the Si oxides of the
surface layer parts fall below 3000/mm2 . As a result, the weldabilities are low.
[0095]
In the steel sheet of Comparative Examples 56, the heating temperature in the annealing
15 process is lower than the Ac3 point-50C, so the tensile strength is lower than 600 MPa.
[0096]
In the steel sheets of Comparative Examples 57 and 70, the distances between wires added
in the continuous casting process are larger than the wire diameters+30 mm and the number
densities of Si oxides of the surface layer parts are smaller than 3000/mm2 . As a result, the
20 tensile strengthtotal elongation’s are 10000 MPa% or more, but the weldabilities are not
improved.
[0097]
In the steel sheets of Comparative Examples 64 and 68, the distances between the center
positions of the wires added in the continuous casting process and the surfaces of the molten
25 steels are larger than the wire diameters+30 mm, so the number densities of the Si oxides of the
surface layer parts fall below 3000/mm2 and the number densities of the Si oxides of the
positions of 1/2 of the thickness of the steel sheets rise above 1000/mm2 . As a result, the tensile
strengthtotal elongation’s are less than 10000 MPa% and the weldabilities are not improved.
[0098]
30 In contrast to this, the steel sheets of the examples satisfying the chemical constituents and
method of manufacture of the present invention have number densities of Si oxides satisfying
predetermined requirements and could be confirmed as being improved in weldability without
lowering the tensile strength  total elongation.
[0099]
35 In the above way, according to the present invention, it was confirmed that steel sheet
obtaining both formability and weldability by modification of the surface layer part was
29
obtained.
INDUSTRIAL APPLICABILITY
[0100]
According to the present 5 invention, it is possible to provide steel sheet excellent in
formability and weldability. Such steel sheet of the present invention is suitable, for example, for
a structural material in the field of automobiles and other transport machinery.
REFERENCE SIGNS LIST
10 [0101]
1 molten steel
2 wire
11 tundish
12 continuous casting machine

WE CLAIMS

[Claim 1]A steel sheet with a tensile strength of 600 MPa or more,
a region down to 30 m from a surface of the 5 steel sheet in a sheet thickness direction being
defined as a “surface layer part”,
at the surface layer part of at least one side,
particle size 20 nm or more Si oxide grains being present in a 3000 to 6000/mm2 or less
number density,
10 an average of natural logarithms of the particle sizes of the Si oxide grains measured in m
units being -2.0 to -1.2, a standard error of the natural logarithms of the particle sizes being 0.6
or less,
a number of Si oxide grains with deviations of the natural logarithms of the particle sizes
from the average of larger than 2 times the standard error being 5% or less of the total number of
15 Si oxide grains with particle sizes of 20 nm or more
a chemical composition at a position of 1/4 of the thickness from the surface of the steel
sheet comprising, by mass%,
C: 0.050 to 0.800%,
Si: 0.01 to 2.50%,
20 Mn: 0.01 to 8.0%,
P: 0.1000% or less,
S: 0.0500% or less,
Al: 0.050% or less,
N: 0.0100% or less,
25 O: 0 to 0.020%,
Cr: 0 to 3.00%,
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
Ti: 0 to 0.200%,
30 Nb: 0 to 0.200%,
V: 0 to 0.20%,
Cu: 0 to 1.000%,
Ni: 0 to 1.000%, and
bal.: Fe and impurities, and
35 a number density of Si oxide grains at a position of 1/2 of the thickness of the steel sheet
being 1000/mm2 or less.
31
[Claim 2]
The steel sheet according to claim 1, wherein the chemical composition further comprises at
least one element selected from the group consisting of
5 Cr: 0.01 to 3.00%,
Mo: 0.01 to 1.00%,
B: 0.001 to 0.0100%,
Ti: 0.010 to 0.200%,
Nb: 0.010 to 0.200%,
10 V: 0.01 to 0.20%,
Cu: 0.010 to 1.000%, and
Ni: 0.010 to 1.000%.
[Claim 3]
15 The steel sheet according to claim 1 or 2, wherein the steel sheet further comprises a hot dip
galvanized layer, a hot dip galvannealed layer, or an electrogalvanized layer at the surface of the
surface layer part.