TECHNICAL FIELD
[0001] The present invention relates to a highstrength
steel sheet suitable for a comparatively
long structural member such as a frame of a truck.
BACKGROUND ART
[0002] Weight reduction of transportation machines
such as an automobile and a railway vehicle is
desired in order to curtail exhaust gas by
improvement of fuel consumption. Though usage of a
thin steel sheet for a member of the transportation
machine is effective in reducing weight of the
transportation machine, it is desired that the steel
sheet itself has high strength in order to secure
desired strength while using the thin steel sheet.
[0003] For a member of a transportation machine such
as a side frame of a truck, a steel sheet in which a
scale (black scale) generated during hot rolling
remains is sometimes used in view of a cost or the
like. However, in a conventional steel sheet in
which a scale remains, the scale may exfoliate in
finishing such as passing in leveler equipment or
working such as bending and pressing carried out by a
user. Exfoliation of a scale necessitates care for a
roll or a mold to which the scale attaches. Further,
when the scale remains after the care, the scale may
be pushed into a steel sheet processed thereafter, to
generate a depression pattern in the steel sheet.
- 1 -
i
Therefore, excellent scale adhesion is required of a
steel sheet in which a scale remains in order to
suppress exfoliation of the scale from a base iron.
[ 0004] Though a steel sheet aiming at improvement of
scale adhesion is known, a conventional steel sheet
cannot achieve both good mechanical property and
excellent scale adhesion.
CITATION LIST
PATENT LITERATURE
[0005] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2014-31537
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2012-162778
Patent Literature 3: Japanese Patent No. 5459028
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2004-244 680
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2000-87185
Patent Literature 6: Japanese Laid-open Patent
Publication No. 7-34137
Patent Literature 7: Japanese Laid-open Patent
Publication No. 2014-51683
Patent Literature 8: Japanese Laid-open Patent
Publication No. 7-118792
Patent Literature 9: Japanese Laid-open Patent
Publication No. 2014-118592
NON-PATENT LITERATURE
[0006] Non-Patent Literature 1: Kobe Steel
Engineering Reports Vol. 56 No. 32 (Dec. 2006) P.22
- 2 -
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0007] An object of the present invention is to
provide a steel sheet capable of achieving both good
mechanical property and excellent scale adhesion.
SOLUTION TO PROBLEM
[0008] The present inventors conducted keen study in
order to solve the above-described problem.
Consequently, it has become obvious that forms of a
scale and a subscale substantially affect improvement
of scale adhesion. Further, it has also become
obvious that the forms of the scale and the subscale
are affected by a condition of hot rolling in
particular.
[0009] The present inventors further conducted keen
study based on the above observation and reached
modes of the invention described below.
[0010] (1) A steel sheet including:
a base iron;
a scale of 10.0 ~m or less in thickness on a
surface of the base iron; and
a subscale between the base iron and the scale,
wherein the base iron comprises a chemical
composition represented by, in mass%,
C: 0.05% to 0.20%,
Si: 0.01% to 1.50%,
Mn: 1.50% to 2.50%,
P: 0.05% or less,
S: 0.03% or less,
- 3 -
Al: 0.005% to 0.10%,
N: 0.008% or less,
Cr: 0.30% to 1.00%,
Ti: 0.06% to 0.20%,
Nb: 0.00% to 0.10%,
V: 0.00% to 0.20%,
B: 0.0000% to 0.0050%,
Cu: 0.00% to 0.50%,
Ni: 0.00% to 0.50%,
Mo: 0.00% to 0.50%,
W: 0.00% to 0.50%,
Ca: 0.0000% to 0.0050%,
Mg: 0.0000% to 0.0050%,
REM: 0.000% to 0.010%, and
the balance: Fe and impurities,
wherein, in the subscale,
an average value of Cr concentrations is
1.50 mass% to 5.00 mass%, and
one part or more exist(s) where a ratio of
Cr concentrations between two adjacent
measurement regions separate by 1 ~m is 0.90 or
less or 1.11 or more in a range of 50 ~m in
length in a rolling direction, and
wherein a percentage of an amount of Ti contained
in carbide or carbonitride of 100 nm or more and 1 ~m
or less in grain diameter to a parameter Tietf
represented by a following formula 1 is 30% or less,
[Til denoting a Ti content (mass%) and [N] denoting a
N content (mass%) in the following formula 1,
- 4 -
Tiett ~ [Til - 48/14 [N] (formula 1).
[0011] {2) The steel sheet according to (1),
wherein, in the chemical composition,
Nb: 0.001% to 0.10%,
V: 0.001% to 0.20%,
B: 0.0001% to 0.0050%,
Cu: 0.01% to 0.50%,
Ni: 0.01% to 0.50%,
Mo: 0.01% to 0.50%, or
W: 0.01% to 0.50%,
or any combination thereof is satisfied.
[0012] (3) The steel sheet according to (1) or {2),
wherein, in the chemical composition,
Ca: 0.0005% to 0.0050%,
Mg: 0.0005% to 0.0050%, or
REM: 0.0005% to 0.010%,
or any combination thereof is satisfied.
ADVANTAGEOUS EFFECTS OF INVENTION
[0013] According to the present invention, both good
mechanical property and excellent scale adhesion can
be achieved, since forms of a scale and a subscale
are appropriate.
BRIEF DESCRIPTION OF DRAWINGS
[0014] [Fig. 1] Fig. 1 is a chart illustrating an
example of a result of Cr concentration mapping; and
[Fig. 2] Fig. 2 is a chart illustrating a
relation between form of scale and scale adhesion.
DESCRIPTION OF EMBODIMENTS
[0015] The present inventors studied influence of a
- 5 -
i
thickness of a scale and a form of a subscale upon
scale adhesion.
[0016] In measuring the thicknesses of the scales,
samples in which surfaces parallel to a rolling
direction and a thickness direction were observation
surfaces were taken from various steel sheets, the
observation surfaces were mirror polished, and
observation by using an optical microscope was
carried out at a magnification of 1000 times. Then,
an average value of the thicknesses of the scales
obtained in 10 or more visual fields was defined as
the thickness of the scale of the steel sheet.
[0017] In analysis of the form of the subscale,
samples in which surfaces parallel to the rolling
direction and the thickness direction were
observation surfaces were taken from various steel
sheets, the observation surfaces were mirror
polished, and Cr concentrations (mass%) of the
subscales were analyzed by using an electron probe
micro analyzer (EPMA). Concretely, mapping of the Cr
concentrations was carried out in a region which
includes the scale and the base iron in 50 pm or more
in length in the rolling direction, at an
acceleration voltage of 15.0 kV and at an irradiation
current of 50 nA, with a measurement time per point
being 20 msec. In this mapping, an interval between
measurement points was set to 0.1 pm in both the
rolling direction and the thickness direction.
[0018] Fig. 1 illustrates an example of a result of
- 6 -
the mapping. A Cr content of the base iron of the
sample used in this example was 3.9 mass%, and an
analysis object was a region whose length in a
rolling direction was 60 pm and which included the
scale and the base iron. In Fig. 1, a part in which
the Cr concentration is particularly high is a
subscale, a part thereunder is the base iron and a
part thereabove is the scale. As is obvious from
Fig. 1, the Cr concentration of the subscale is
higher than that of the base iron.
[0019] The present inventors carried out following
analysis about the result of the mapping of the Cr
concentrations. In this analysis, a measurement
region was defined as a region made of 10 measurement
points continually lining up in the rolling
direction. Since an interval between the measurement
points was 0.1 pm, a dimension in the rolling
direction of the measurement region was 1 pm.
Further, since a length in the rolling direction of
an object region of the mapping of the Cr
concentrations was 50 pm or more, there were 50 or
more measurement regions. An average value and a
maximum value Cmax of the Cr concentrations were
found for every measurement region, an average value
Ave of the maximum values Cmax among the 50 or more
measurement regions were calculated, and the average
value Ave was defined as an average value of the Cr
concentrations in the subscale.
[0020] Further, regarding the 50 or more measurement
- 7 -
regions, a concentration ratio Rcr of one maximum
value Cmax to the other maximum value Cmax between
the two adjacent measurement regions was found. In
other words, a quotient obtained as a result of
dividing one maximum value Cmax by the other maximum
value Cmax was found. At this time, either one of
the maximum values Cmax was arbitrarily chosen as a
numerator. For example, in a case where the maximum
value Cmax of the two measurement regions are 3.90%
and 3.30%, the concentration ratio Rcr is 1.18 or 0.85
and in a case where the maximum values Cmax of the
two measurement regions are 1.70% and 1.62%, the
concentration ratio Rcr is 1.05 or 0.95. Further, in
a case where the maximum values Cmax of the two
measurement regions are equal, the concentration
ratio Rcr is 1.00, and if the maximum values Cmax of
the Cr concentrations in the subscale are uniform,
the concentration ratio Rcr is 1.00 in any measurement
region. As described above, the concentration ratio
Rcr reflects variation of the maxim values Cmax of the
Cr concentrations in the subscale, and as the
concentration ratio Rcr is closer to 1. 00, the
variation of the maximum values Cmax of the Cr
concentrations in the subscale is small.
[0021] The scale adhesion was evaluated by taking a
strip test piece in a manner that a longitudinal
direction was parallel to a width direction of the
steel sheet, assuming press working of a side frame
of a truck, by a V-block method described in JIS
- 8 -
Z2248. A size of the test piece was 30 mm in width
(rolling direction) and 200 mm in length (width
direction) . A bending angle was set to 90 degrees
and an inside radius was set to two times a sheet
thickness.
[0022] After bending, adhesive cellophane tape of 18
mm in width was applied in a width center part of
bend outside along the longitudinal direction of the
test piece and then peeled, and an area ratio of a
scale attached to the adhesive cellophane tape was
calculated in a region where the steel sheet and a Vblock
were not in contact.
[0023] The test piece with the area ratio of the
scale attached to the adhesive cellophane tape, that
is, the area ratio of the scale exfoliated from the
steel sheet, was 10% or less was judged good and one
with the area ratio of over 10% was judged bad. The
present inventors made sure that when the area ratio
of the scale exfoliated from the steel sheet is 10%
or less in this experiment, exfoliation in a
processing in practical use does not substantially
occur.
[0024] Relation between the thickness of the scale
and the scale adhesion was sorted out and it was
found that when the thickness of the scale exceeded
10.0 pm, good scale adhesion was not able to be
obtained regardless of the Cr concentration of the
scale. Meanwhile, when the thickness of the scale
was 10.0 pm or less, good scale adhesion was
- 9 -
sometimes able to be obtained or not obtained,
depending on the form of the subscale.
[0025) Thus, regarding the steel sheet of 10.0 pm or
less in thickness of the scale, the present inventors
sorted out relation between an average Ave of the Cr
concentrations as well as a value Rd, which is the
farthest value from 1.00 among concentration ratios
Rcr, and the scale adhesion. Fig. 2 illustrates the
result. A horizontal axis in Fig. 2 indicates the
average value Ave of the Cr concentrations and a
vertical axis indicates the value Rd, which is the
farthest value from 1.00 among the concentration
ratios Rcr·
[0026) As illustrated in Fig. 2, in the sample in
which the average value Ave of the Cr concentrations
was less than 1.50 mass% or over 5.00 mass%, the
scale adhesion was bad. Further, in the sample in
which the value Rd, which is the farthest value from
1.00 among the concentration ratios Rcrr is over 0.90
and less than 1.11, the scale adhesion was bad even
if the average value Ave of the Cr concentrations was
1.50 mass% to 5.00 mass%.
[0027) From the above, it became obvious that, as
for subscale, it is important that the average value
Ave of the Cr concentrations is 1.50 mass% to 5.00
mass% and that one part or more exist(s) where a
concentration ratio(s) Rcr between two adjacent
measurement regions separate by 1 pm is 0.90 or less
or 1.11 or more in a range of 50 pm in length in the
- 10 -
rolling direction in order to obtain excellent scale
adhesion.
[0028] Further, as a mechanical property suitable
for application to a side frame of a truck, there may
be cited that a yield strength in the rolling
direction is 700 MPa or more and less than 800 MPa
and that a yield ratio is 85% or more, and in order
to achieve the above, precipitation strengthening by
carbide containing Ti and carbonitride containing Ti
with a grain diameter of less than 100 nm is quite
effective. Hereinafter, the carbide containing Ti
and the carbonitride containing Ti may be
collectively referred to as Ti carbide.
[0029] Hereinafter, an embodiment of the present
invention will be described.
[0030] First, a chemical composition of a steel
sheet according to the embodiment of the present
invention and a steel used for manufacturing thereof
will be described. Details being described later,
the steel sheet according to the embodiment of the
present invention is manufactured through casting of
the steel, slab heating, hot rolling, first cooling,
coiling, and second cooling. Therefore, the chemical
composition of the steel sheet and the steel is one
in consideration of not only a property of the steel
sheet but also the above processing. In the
following explanation, "%• being a unit of a content
of each element contained in the steel sheet and the
steel means "mass%• as long as not otherwise
- 11 -
specified. The steel sheet according to the
embodiment and the steel used for manufacturing
thereof have a chemical composition represented by,
in mass%, C: 0.05% to 0.20%, Si: 0.01% to 1.50%, Mn:
1.50% to 2.50%, P: 0.05% or less, S: 0.03% or less,
Al: 0.005% to 0.10%, N: 0.008% or less, Cr: 0.30% to
1.00%, Ti: 0.06% to 0.20%, Nb: 0.00% to 0.10%, V:
0.00% to 0.20%, B: 0.0000% to 0.0050%, Cu: 0.00% to
0.50%, Ni: 0.00% to 0.50%, Mo: 0.00% to 0.50%, W:
0.00% to 0.50%, Ca: 0.0000% to 0.0050%, Mg: 0.0000%
to 0. 0050%, REM: 0. 000% to 0. 010%, and the balance:
Fe and impurities. As the impurities, ones included
in a raw materials, such as ore and scrap, and ones
included in a manufacturing process are exemplified.
Sn and As may be cited as examples of the impurities.
[0031] (C: 0.05% to 0.20%)
C contributes to improvement of strength. A C
content of less than 0.05% cannot attain sufficient
strength, for example, yield strength of 700 MPa or
more in the rolling direction or a yield ratio of 85%
or more, or both thereof. Therefore, the C content
is 0.05% or more and preferably 0.08% or more.
Meanwhile, a C content of over 0.20% brings about
excessive strength, to reduce ductility or to reduce
weldability and toughness. Therefore, the C content
is 0.20% or less, preferably 0.15% or less, and more
preferably 0.14% or less.
[0032] (Si: 0.01% to 1.50%)
Si contributes to improvement of strength and
- 12 -
acts as a deoxidizer. Si also contributes to
improvement of a shape of a welded part in arc
welding. A Si content of less than 0.01% cannot
attain such effects sufficiently. Therefore, the Si
content is 0.01% or more, and preferably 0.02% or
more. Meanwhile, a Si content of over 1.50% makes a
large amount of Si scales occur in a surface of a
steel sheet so as to deteriorate a surface property,
or reduces toughness. Therefore, the Si content is
1.50% or less and preferably 1.20% or less. When the
Si content is 1.50% or less, influence of Si to scale
adhesion can be ignored in the present embodiment.
[0033] (Mn: 1.50% to 2.50%)
Mn contributes to improvement of strength through
strengthening of a structure. A Mn content of less
than 1.50% cannot attain such an effect sufficiently.
For example, it is impossible to obtain yield
strength of 700 MPa or more in the rolling direction
or a yield ratio of 85%, or both thereof. Therefore,
the Mn content is 1.50% or more and preferably 1.60%
or more. Meanwhile, a Mn content of over 2.50%
brings about excessive strength so as to reduce
ductility, or reduces weldability and toughness.
Therefore, the Mn content is 2.50% or less,
preferably 2.40% or less, and more preferably 2.30%
or less.
[0034] (P: 0.05% or less)
P is not an essential element, and is contained
in steel as an impurity, for example. Since P
- 13 -
deteriorates ductility and toughness, a P content is
better as low as possible. In particular, a P
content of over 0.05% notably reduces ductility and
toughness. Therefore, the P content is 0.05% or
less, preferably 0.04% or less, and more preferably
0.03% or less. It is costly to decrease the P
content, and in order to decrease the P content to
less than 0.0005%, a cost increases notably. Thus,
the P content may be 0.0005% or more, and may be
0.0010% or more in view of the cost.
[0035] (S: 0.03% or less)
S is not an essential element, and is contained
in steel as an impurity, for example. Since S
generates MnS and deteriorates ductility,
weldability, and toughness, an S content is better as
low as possible. In particular, the S content of
over 0.03% notably reduces ductility, weldability,
and toughness. Therefore, the S content is 0.03% or
less, preferably 0.01% or less, and more preferably
0.007% or less. It is costly to decrease the S
content, and in order to decrease the S content to
less than 0.0005%, a cost increases notably. Thus,
the S content may be 0.0005% or more, may be 0.0010%
or more in view of the cost, and may be 0.0010% or
more in view of the cost.
[0036] (Al: 0.005% to 0.10%)
Al acts as a deoxidizer. An Al content of less
than 0.005% cannot attain such an effect. Therefore,
the Al content is 0.005% or more and preferably
- 14 -
0.015% or more. Meanwhile, anAl content of over
0.10% reduces toughness and weldability. Therefore,
the Al content is 0.10% or less and preferably 0.08%
or less.
[0037] (N: 0. 008% or less)
N is not an essential element, and is contained
in steel as an impurity, for example. N forms TiN
and consumes Ti so as to impede generation of fine Ti
carbide suitable for precipitation strengthening.
Thus, the N content is better as low as possible. In
particular, the N content of over 0.008% notably
reduces precipitation strengthening capability.
Therefore, the N content is 0.008% or less and
preferably 0.007% or less. It is costly to decrease
the N content, and in order to decrease the N content
to less than 0.0005%, a cost increases notably.
Thus, the N content may be 0.0005% or more, may be
0.0010% or more in view of the cost, and may be
0.0010% or more in view of the cost.
[0038] (Cr: 0.30% to 1.00%)
Cr contributes to improvement of strength and
increases scale adhesion through formation of a
subscale. A Cr content of less than 0.30% cannot
attain such effects. Therefore, the Cr content is
0.30% or more and preferably 0.25% or more.
Meanwhile, if the Cr content is over 1.00%, Cr
contained in the subscale becomes excessive,
resulting in that the scale adhesion is reduced.
Therefore, the Cr content is 1.00% or less and
- 15 -
preferably 0.80% or less.
[0039] (Ti: 0.06% to 0.20%)
Ti contributes to improvement of yield strength
by suppressing recrystallization to thereby suppress
coarsening of a grain, and contributes to improvement
of yield strength and a yield ratio through
precipitation strengthening by precipitating as Ti
carbide. A Ti content of less than 0.06% cannot
attain such effects sufficiently. Therefore, the Ti
content is 0.06% or more and preferably 0.07% or
more. Meanwhile, a Ti content of over 0.20% reduces
toughness, weldability, and ductility, or makes Ti
carbide not able to be solid-solved sufficiently
during slab heating, resulting in shortage of an
amount of Ti effective for precipitation
strengthening, to cause reduction of the yield
strength and the yield ratio. Therefore, the Ti
content is 0.20% or less and preferably 0.16% or
less.
[0040] Nb, V, B, Cu, Ni, Mo, W, Ca, Mg, and REM are
not essential elements but are arbitrary elements
which may be appropriately contained in a steel sheet
and steel to the extent of a specific amount.
[0041] (Nb: 0.00% to 0.10%, V: 0.00% to 0.20%)
Nb and V precipitate as carbonitride to thereby
contribute to improvement of strength, or contribute
to suppression of coarsening of a grain. Suppression
of coarsening of the grain contributes to improvement
of yield strength and improvement of toughness.
- 16 -
Therefore, Nb or V, or both thereof may be contained.
In order to obtain such effects sufficiently, a Nb
content is preferably 0.001% or more and more
preferably 0.010% or more, and a V content is
preferably 0.001% or more and more preferably 0.010%
or more. Meanwhile, a Nb content of over 0.10%
reduces toughness and ductility, to make Nb
carbonitride not able to be solid-solved sufficiently
during slab heating, resulting in shortage of solidsolution
C effective for securing strength, to cause
reduction of the yield strength and the yield ratio.
Therefore, the Nb content is 0.10% or less and
preferably 0.08% or less. A V content of over 0.2%
reduces toughness and ductility. Therefore, the V
content is 0.20% or less and preferably 0.16% or
less.
[0042) (B: 0.0000% to 0.0050%)
B contributes to improvement of strength through
strengthening of a structure. Therefore, B may be
contained. In order to obtain such an effect
sufficiently, a B content is preferably 0.0001% or
more and more preferably 0.0005% or more. Mean1-1hile,
a B content of over 0.0050% reduces toughness or
saturates an improvement effect of strength.
Therefore, the B content is 0.0050% or less and
preferably 0.0030% or less.
[0043] (Cu: 0.00% to 0.50%)
Cu contributes to improvement of strength.
Therefore, Cu may be contained. In order to obtain
- 17 -
such an effect sufficiently, a Cu content is
preferably 0.01% or more and more preferably 0.03% or
more. Meanwhile, a Cu content of over 0.50% reduces
toughness and weldability, or increases apprehension
of a hot tear of slab. Therefore, the Cu content is
0.50% or less and preferably 0.30% or less.
[0044] (Ni: 0.00% to 0.50%)
Ni contributes to improvement of strength or
contributes to improvement of toughness and
suppression of a hot tear of slab. Therefore, Ni may
be contained. In order to obtain such effects
sufficiently, aNi content is preferably 0.01% or
more and more preferably 0.03% or more. Meanwhile, a
Ni content of over 0.50% unnecessarily increases a
cost. Therefore, the Ni content is 0.50% or less and
preferably 0.30% or less.
[0045] (Mo: 0.00% to 0.50%, W: 0.00% to 0.50%)
Mo and W contribute to improvement of strength.
Therefore, Mo or W, or both thereof may be contained.
In order to obtain such effects sufficiently, a Mo
content is preferably 0.01% or more and more
preferably 0.03% or more, and a W content is
preferably 0.01% or more and more preferably 0.03% or
more. Meanwhile, a Mo content of over 0.50%
unnecessarily increases a cost. Therefore, the Mo
content is 0.50% or less and preferably 0.35% or
less. A W content of over 0.50% unnecessarily
increases a cost. Therefore, the W content is 0.50%
or less and preferably 0.35% or less.
- 18 -
[0046] From the above, regarding Nb, V, B, Cu, Ni,
Mo, and W, it is preferable that "Nb: 0.001% to
0. 10%", "V: 0. 001% to 0. 20%", "B: 0. 0001% to
0.0050%", "Cu: 0.01% to 0.50%", "Ni: 0.01% to 0.50%",
"Mo: 0.01% to 0.50%", or "W: 0.01% to 0.50%", or any
combination thereof is satisfied.
[0047] (Ca: 0. 0000% to 0. 0050%, Mg: 0. 0000% to
0.0050%, REM: 0.000% to 0.010%)
Ca, Mg, and REM contribute to improvement of
toughness and suppression of reduction of ductility
by spheroidizing a non-metal inclusion. Therefore,
Ca, Mg, or REM, or any combination thereof may be
contained. In order to obtain such effects
sufficiently, a Ca content is preferably 0.0005% or
more and more preferably 0.0010% or more, an Mg
content is preferably 0.0005% or more and more
preferably 0.0010% or more, and a REM content is
preferably 0.0005% or more and more preferably
0.0010% or more. Meanwhile, a Ca content of over
0.0050% prominently coarsens the inclusion and
increases the number of the inclusions, to reduce
toughness. Therefore, the Ca content is 0.0050% or
less and preferably 0.0035% or less. A Mg content of
over 0.0050% prominently coarsens the inclusion and
increases the number of the inclusions, to reduce
toughness. Therefore, the Mg content is 0.0050% or
less and preferably 0.0035% or less. A REM content
of over 0.010% prominently coarsens the inclusion and
increases the number of the inclusions, to reduce
- 19 -
toughness. Therefore, the REM content is 0.010% or
less and preferably 0.007% or less.
[0048l From the above, regarding Ca, Mg, and REM,
it is preferable that "Ca: 0.0005% to 0.0050%", "Mg:
0. 0005% to 0. 0050%", or "REM: 0. 0005% to 0. 010%", or
any combination thereof is satisfied.
[0049l REM (rare earth metal) indicates elements of
17 kinds in total of Sc, Y, and lanthanoid, and a
"REM content" means a total content of these elements
of 17 kinds. Lanthanoid is industrially added as a
form of misch metal, for example.
[0050l Next, form of Ti in the steel sheet according
to the embodiment of the present invention will be
described. In the steel sheet according to the
embodiment of the present invention, when [Til
denotes a Ti content (mass%) and [Nl denotes a N
content (mass%), a ratio RTi of an amount (mass%) of
Ti contained in Ti carbide of 100 nm or more and 1 pm
or less in grain diameter to a parameter Tieff
(effective Ti amount) represented by the following
formula 1 is 30% or less.
Tietf = [Til - 48/14 [Nl (formula 1)
[0051l While Ti carbide contributes to improvement
of yield stress and a yield ratio through
precipitation strengthening, an amount of Ti
contained in Ti carbide whose grain diameter is 100
nm or more, particularly 100 pm or more and 1 pm or
less in relation to an effective Ti amount, largely
influences formation of fine Ti carbide in coiling.
- 20 -
A ratio RTi of over 30% makes consumption of Ti by
coarse Ti carbide excessive, and as a result that
driving force to formation of the fine Ti carbide in
coiling is reduced, it is impossible to obtain
sufficient yield strength and yield ratio in the
rolling direction. Therefore, the ratio RTi is 30% or
less.
[0052] A method of measurement of precipitated Ti is
not limited as long as highly accurate measurement is
possible. For example, precipitated Ti can be
calculated as a result of carrying out random
observation until at least 50 precipitates are
observed with a transmission electron microscope,
deriving a size distribution of the precipitates from
a size of the individual precipitate and a size of
the whole visual field, and obtaining a Ti
concentration in the precipitate by means of energy
dispersive X-ray spectroscopy (EDS) .
[0053] Next, forms of a scale and a subscale in the
steel sheet according to the embodiment of the
present invention will be described. In the steel
sheet according to the embodiment of the present
invention, the thickness of the scale is 10.0 pm or
less, and in the subscale, the average value Ave of
the Cr concentrations is 1.50 mass% to 5.00 mass% and
one part or more exist(s) where the concentration
ratio Rcr between two adjacent measurement regions
separate by 1 pm is 0.90 or less or 1.11 or more in a
range of 50 pm in length in a rolling direction.
- 21 -
[0054] (Thickness of scale: 10.0 ~m or less)
As the scale is thicker, distortion occurring in
the scale during a processing of the steel sheet is
larger, so that a crack occurs in the scale and that
exfoliation is likely to occur. Further, as is
obvious from the above-described experiment, when the
thickness of the scale is over 10.0 ~m, good scale
adhesion cannot be obtained. Therefore, the
thickness of the scale is 10.0 pm or less and
preferably 8.0 pm or less.

[Claim 1) (After amendment)
A steel sheet comprising:
a base iron;
a scale of lOoO vm or less in thickness on a
surface of the base iron; and
a subscale between the base iron and the scale,
wherein the base iron comprises a chemical
composition represented by, in mass%,
C: Oo05% to Oo20%,
Si: OoOl% to 1.50%,
Mn: 1.50% to 2o50%,
P: Oo 05% or less,
S: Oo03% or less,
Al: Oo005% to OolO%,
N: Oo008% or less,
Cr: 0030% to 1.00%,
Ti: Oo06% to Oo20%,
Nb: 0000% to OolO%,
V: OoOO% to 0020%,
B: OoOOOO% to Oo0050%,
Cu: OoOO% to Oo50%,
Ni: OoOO% to 0050%,
Mo: OoOO% to Oo50%,
W: OoOO% to 0050%,
Ca: 000000% to Oo0050%,
English translation of Amendments under PCT Article 19
Mg: 0.0000% to 0.0050%,
REM: 0.000% to 0.010%, and
the balance: Fe and impurities,
wherein, in the base iron, a percentage of an
amount of Ti contained in carbide or carbonitride of
100 nm or more and 1 pm or less in grain diameter to
a parameter Tietf represented by a following formula 1
is 30% or less, [Til denoting a Ti content (mass%)
and [Nl denoting a N content (mass%) in the following
formula 1,
wherein, in the subscale,
an average value of Cr concentrations is
1.50 mass% to 5.00 mass%, wherein the average
value of Cr concentrations is an average value
Ave of maximum values Cmax among 50 or more
measurement regions, each of the 50 or more
measurement regions is made of 10 measurement
points of Cr concentration continually lining up
in a rolling direction, and an interval between
the measurement points is 0.1 pm, and
one part or more exist(s) where a ratio of
one's maximum value Cmax to the other's maximum
value Cmax is 0.90 or less or 1.11 or more
between two adjacent measurement regions among
the 50 or more measurement regions,
T ieff [Til, - 48/14 [Nl (formula 1).
[Claim 2l The steel sheet according to claim 1,
wherein, in the chemical composition,
Nb: 0.001% to 0.10%,
English translation of Amendments under PCT Article 19
V: 0.001% to 0.20%,
B: 0.0001% to 0.0050%,
Cu: 0.01% to 0.50%,
Ni: 0. 01% to 0.50%,
Mo: 0.01% to 0.50%, or
W: 0.01% to 0.50%,
or any combination of the above is satisfied.
[Claim 3] The steel sheet according to claim 1 or
claim 2, wherein, in the chemical composition,
Ca: 0.0005% to 0.0050%,
Mg: 0 . 0 0 0 5% to 0 . 0 0 50%, or
REM: 0.0005% to 0.010%,
or any combination of the above is satisfied.