Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
TITLE OF INVENTION: STEEL SHEET FOR HOT STAMPING AND
METHOD FOR PRODUCING STEEL SHEET FOR HOT STAMPING,
AND HOT STAMP FORMED BODY
TECHNICAL FIELD
[0001] The present invention relates to a steel
sheet for hot stamping excellent in scale adhesion at
the time of hot stamping and a method for producing
the steel sheet for hot stamping, and a hot stamp
formed body that is a formed body of the steel sheet
for hot stamping.
BACKGROUND ART
[0002]
Weight reduction of the members such as door
guard bars and side members of automobiles are being
studied to cope with the recent trend of improvement
in fuel efficiency, and in terms of a material,
increase in strength of a steel sheet is promoted
from the viewpoint of strength and crash safety that
should be ensured even when the thickness is reduced.
Hereinafter, strength means both tensile strength and
yield strength.
However, formability of a material
deteriorates as the strength increases, and therefore
in order to realize reduction in weight of the above
described members, it is necessary to produce a steel
sheet that satisfies both formability and high
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strength. As a method for obtaining high formability
simultaneously with high strength, there are TRIP
(TRansformation Induced Plasticity) steels taking
advantage of martensitic transformation of retained
austenite that are described in Patent Literature 1
and Patent Literature 2, and application of TRIP
steels has been expanding in recent years. In
the
steel, however, although deep drawability and
elongation are improved at the time of forming, due
to a high steel sheet strength, it has a problem of
low shape fixability of a member after press forming.
[0003] In
order to form a high strength steel sheet,
which is inferior in formability, with good shape
fixability, there is a method called hot press that
is described in Patent Literature 3 and Patent
Literature 4. The
method performs forming at a
temperature of 200 C to about 500 C at which the steel
sheet strength reduces. However, when forming of the
high strength steel sheet of 780 MPa or more is
considered, the method has problems in that even when
the forming temperature is increased, the steel sheet
strength may still be high in some cases and thus
forming is difficult, and in that the steel sheet
strength after forming is reduced by heating, and
thus predetermined strength cannot be obtained in
some cases.
[0004] As a
method for solving the problems, there
exists a method called hot stamping that cuts a soft
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steel sheet in a predetermined size, thereafter,
heats the steel sheet to an austenite single phase
region at 800 C or higher, thereafter performs press
forming in the austenite single phase region as
disclosed in Patent Literature 5, and thereafter
performs hardening. As a result, it is possible to
manufacture a member that has high strength of 980
MPa or more and is excellent in shape fixability.
[0005] However, in hot stamping, a steel sheet is
inserted into a heating furnace, or is heated to a
high temperature exceeding 800 C by electrical heating
or far-infrared heating in the atmosphere, and thus
hot stamping has a problem of scale generated on a
steel sheet surface. A die
may be worn out due to
the generated scale released at the time of hot
stamping, and therefore it is required that scale
adhesion should be excellent at the time of hot
stamping. As a technique that solves these problems,
there is known a technique of restraining generation
of scale by making an atmosphere in the heating
furnace a non-oxidation atmosphere in Patent
Literature 6, for example. However, it is necessary
to implement atmosphere control in the heating
furnace strictly, and thus facility cost increases,
and productivity is reduced.
Further, the steel
sheet which is taken out is exposed to the atmosphere,
and thus the technique has a problem of unavoidable
formation of scale. In
addition, in recent years,
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for the purpose of enhancing productivity of hot
stamping, the method for electrically heating a steel
sheet in the atmosphere has been developed. At the
time of heating in the atmosphere, avoidance of
oxidation of the steel sheet is difficult, and thus a
problem of die wear due to loose scale at the time of
hot stamping easily becomes evident. As a
result,
regular repair of the die is essential.
[0006] There
is known a technique of restraining
wear of a die caused by loose scale by using, in hot
stamping, a steel sheet with zinc plating or Al
plating applied to a steel sheet surface as the steel
sheet that solves these problems. However, since
zinc plating or Al plating are melted into a liquid
phase at the time of heating, the technique has a
problem of zinc or Al adhering to the inside of the
heating furnace and the die at the time of conveyance
of the steel sheet or the time of pressing. A
deposit of adhering zinc or Al has a problem of
causing indentation flaws of a hot stamp formed body,
and adhering to the formed body to worsen the outer
appearance.
Consequently, it is necessary to repair
the die regularly.
[0007]
Consequently, it is required to develop a
steel sheet for hot stamping in which scale does not
detach at the time of hot stamping, and adhesion of a
molten metal to a die does not occur.
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CITATION LIST
PATENT LITERATURES
[0008]
Patent Literature 1: Japanese Laid-open Patent
Publication No. 01-230715
Patent Literature 2: Japanese Laid-open Patent
Publication No. 02-217425
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2002-143935
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2003-154413
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2002-18531
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2004-106034
Patent Literature 7: Japanese Laid-open Patent
Publication No. 2002-18531
Patent Literature 8: Japanese Laid-open Patent
Publication No. 2008-240046
Patent Literature 9: Japanese Laid-open Patent
Publication No. 2010-174302
Patent literature 10: Japanese Laid-open Patent
Publication No. 2008-214650
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0009] In
the light of the aforementioned problems,
the present invention has an object to provide a
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steel sheet for hot stamping that is excellent in
scale adhesion at the time of hot stamping, without
an occurrence of adhesion of a molten metal to a die,
a method for manufacturing the steel sheet for hot
stamping, and a hot stamp formed body.
SOLUTION TO PROBLEM
[0010] The
present inventors have studied earnestly
on methods to solve the above described problems. As
a result, with the intention to improve scale
adhesion of a steel sheet, 0.50 mass% to 3.00 mass%
of Si is contained in the steel sheet, the amount of
rust inhibiting oil that is applied to the steel
sheet is set to be within a range of 50 mg/m2 to 1500
mg/m2, and surface roughness of the steel sheet is set
as Rz>2.5 m.
Further, an S content included in the
rust inhibiting oil is preferably set at 5 mass% or
less.
Thereby, it has been found that scale adhesion
at the time of heating and at the time of hot
stamping is improved. In
general, enclosures in the
coating oil concentrate into an interface between a
base iron and scale, and thereby deteriorate scale
adhesion. However, it has been found out that it is
possible to ensure scale adhesion by using
restriction on an enclosure amount, and an anchor
effect using irregularities on the steel sheet
surface in combination.
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[0011] The
present invention is made based on the
above described knowledge, and the gist of the
present invention is as follows.
(1) A steel sheet for hot stamping including a
composition containing,
in mass%,
C: 0.100% to 0.600%;
Si: 0.50% to 3.00%;
Mn: 1.20% to 4.00%;
Ti: 0.005% to 0.100%;
B: 0.0005% to 0.0100%;
P: 0.100% or less;
S: 0.0001% to 0.0100%;
Al: 0.005% to 1.000%;
N: 0.0100% or less;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
Cr: 0% to 2.00%;
Mo: 0% to 2.00%;
Nb: 0% to 0.100%;
/: 0% to 0.100%;
W: 0% to 0.100%, and
a total of one kind or two or more kinds selected
from a group consisting of REM, Ca, Ce and Mg: 0% to
0.0300%,
with a balance being Fe and impurities,
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wherein surface roughness of the steel sheet
satisfies Rz>2.5 m, and coating oil in an amount of
50 mg/m2 to 1500 mg/m2 is applied onto a surface.
[0012]
(2) The steel sheet for hot stamping according to (1)
described above,
wherein an amount of S contained in the coating
oil which is applied onto the steel sheet is 5% or
less in mass%.
[0013]
(3) The steel sheet for hot stamping according to (1)
or (2) described above,
wherein the composition of the steel sheet
contains, in mass%,
one kind or two or more kinds selected from a
group consisting of
Ni: 0.01% to 2.00%,
Cu: 0.01% to 2.00%,
Cr: 0.01% to 2.00%,
Mo: 0.01% to 2.00%,
Nb: 0.005% to 0.100%,
V: 0.005% to 0.100%, and
W: 0.005% to 0.100%.
[0014]
(4) The steel sheet for hot stamping according to any
one of (1) to (3) described above,
wherein the composition of the steel sheet
contains, in mass %,
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a total of 0.0003% to 0.0300% of one kind or two
or more kinds selected from the group consisting of
REM, Ca, Ce and Mg.
[0015]
(5) A method for producing a steel sheet for hot
stamping, including
a step of casting a slab containing,
in mass%,
C: 0.100% to 0.600%,
Si: 0.50% to 3.00%,
Mn: 1.20% to 4.00%,
Ti: 0.005% to 0.100%,
B: 0.0005% to 0.0100%,
P: 0.100% or less,
S: 0.0001% to 0.0100%,
Al: 0.005% to 1.000%,
N: 0.0100% or less,
Ni: 0% to 2.00%,
Cu: 0% to 2.00%,
Cr: 0% to 2.00%,
Mo: 0% to 2.00%,
Nb: 0% to 0.100%,
V: 0% to 0.100%,
W: 0% to 0.100%, and
a total of one kind or two or more kinds selected
from a group consisting of REM, Ca, Ce and Mg: 0% to
0.0300%,
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with a balance being Fe and impurities, and hot
rolling the slab directly or by allowing the slab to
cool and heating the slab to obtain a hot-rolled
steel sheet,
a step of pickling the hot-rolled steel sheet for
30 seconds or more in an aqueous solution having a
temperature of 80 C to lower than 100 C and including
an inhibitor with a concentration of an acid being 3
mass% to 20 mass%, and
a step of applying a rust inhibiting oil to the
steel sheet after carrying out the pickling,
wherein a rust inhibiting oil remaining amount on
a steel sheet surface is limited to 50 mg/m2 to 1500
mg/m2.
[0016]
(6) The method for producing a steel sheet for hot
stamping according to (5) described above, wherein
the rust inhibiting oil is applied to the hot-rolled
steel sheet which has been pickled.
[0017]
(7) The method for producing a steel sheet for hot
stamping according to (5) described above, further
including a step of cold rolling the hot-rolled steel
sheet which has been pickled to obtain a cold-rolled
steel sheet,
wherein the rust inhibiting oil is applied to the
cold-rolled steel sheet.
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[0018]
(8) The method for producing a steel sheet for hot
stamping according to (5) described above, further
including a step of cold rolling the hot-rolled steel
sheet which has been pickled, and further performing
thermal treatment in a continuous annealing facility
or a box type annealing furnace to obtain a cold-
rolled steel sheet,
wherein the rust inhibiting oil is applied to the
cold-rolled steel sheet.
[0019]
(9) The method for producing a steel sheet for hot
stamping according to any one of (5) to (8) described
above,
wherein an amount of S in the rust inhibiting oil
that is applied to the steel sheet is 5% or less in
mass%.
[0020]
(10) The method for producing a steel sheet for hot
stamping according to any one of (5) to (9),
wherein a composition of the slab contains, in
mass %,
one kind or two or more kinds selected from a
group consisting of
Ni: 0.01% to 2.00%,
Cu: 0.01% to 2.00%,
Cr: 0.01% to 2.00%,
Mo: 0.01% to 2.00%,
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Nb: 0.005% to 0.100%,
V: 0.005% to 0.100%, and
W: 0.005% to 0.100%.
[0021]
(11) The method for producing a steel sheet for hot
stamping according to any one of (5) to (10)
described above,
wherein a composition of the slab contains, in
mass %,
a total of 0.0003% to 0.0300% of one kind or two
or more kinds selected from the group consisting of
REM, Ca, Ce and Mg.
[0022]
(12) A hot stamp formed body, including a composition
containing,
in mass%,
C: 0.100% to 0.600%,
Si: 0.50% to 3.00%,
Mn: 1.20% to 4.00%,
Ti: 0.005% to 0.100%,
B: 0.0005% to 0.0100%,
P: 0.100% or less,
S: 0.0001% to 0.0100%,
Al: 0.005% to 1.000%,
N: 0.0100% or less,
Ni: 0% to 2.00%,
Cu: 0% to 2.00%,
Cr: 0% to 2.00%,
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Mo: 0% to 2.00%,
Nb: 0% to 0.100%,
V: 0% to 0.100%,
W: 0% to 0.100%, and
a total of one kind or two or more kinds selected
from a group consisting of REM, Ca, Ce and Mg: 0% to
0.0300%,
with a balance being Fe and impurities,
wherein three or more irregularities in a range
of 0.2 m to 8.0 m in depth are present per 100 m in
an interface between scale and a base iron, and
tensile strength is 1180 MPa or more.
[0023]
(13) The hot stamp formed body according to (12)
described above,
wherein an Si oxide, FeO, Fe304 and Fe203 are
included in a surface of the hot stamp formed body,
and a thickness of the scale is 10 m or less.
[0024]
(14) The hot stamp formed body according to (12) or
(13) described above,
wherein the composition of the hot stamp formed
body contains, in mass%,
one kind or two or more kinds selected from a
group consisting of
Ni: 0.01% to 2.00%,
Cu: 0.01% to 2.00%,
Cr: 0.01% to 2.00%,
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MO: 0.01% to 2.00%,
Nb: 0.005% to 0.100%,
V: 0.005% to 0.100%, and
W: 0.005% to 0.100%.
[0025]
(15) The hot stamp formed body according to any one
of (12) to (14) described above,
wherein the composition of the hot stamp formed
body contains, in mass %,
a total of 0.0003% to 0.0300% of one kind or two
or more kinds selected from the group consisting of
REM, Ca, Ce and Mg.
ADVANTAGEOUS EFFECTS OF INVENTION
[0026] According to the present invention, the steel
sheet for hot stamping excellent in scale adhesion at
the time of hot stamping, in which adhesion of a
molten metal to the die does not occur, the method
for producing the steel sheet for hot stamping and
the hot stamp formed body can be provided.
BRIEF DESCRIPTION OF DRAWINGS
[0027] [Fig. 1] Fig. 1 is a diagram illustrating a
relationship between a coating oil amount on a steel
sheet and surface roughness Rz of the steel sheet.
[Fig. 2] Fig. 2 is a diagram for explaining
that when an S concentration in coating oil increases,
scale easily detaches.
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[Fig. 3] Fig. 3 is
a diagram illustrating a
relationship between a pickling time period and the
surface roughness Rz of the steel sheet.
[Fig. 4A] Fig. 4A
is a photograph showing a
microstructure of a surface layer of a hot-rolled
steel sheet before pickling.
[Fig. 4B] Fig. 48 is a photograph showing the
surface layer microstructure after pickling.
[Fig. 5] Fig. 5 is
a diagram illustrating a
relationship between an coating oil amount and a
thickness of scale.
[Fig. 6A] Fig. 6A
is a photograph showing a
section of a hot stamp formed body surface of an
example of the present invention.
[Fig. 6B] Fig. 6B
is a photograph showing a
section of a hot stamp formed body surface of a
comparative example.
[Fig. 7] Fig. 7 is
a diagram for explaining
that when the surface roughness Rz before hot stamp
thermal treatment is less than 2.5, a number density
of irregularities after hot stamp thermal treatment
is less than 3.
DESCRIPTION OF EMBODIMENTS
[0028] A steel sheet for hot stamping of the present
invention contains from 0.5 mass% to 3.0 mass% of Si
in the steel sheet, an amount of rust inhibiting oil
applied to the steel sheet is in a range of 50 mg/m2
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to 1500 mg/m2, and surface roughness of the steel
sheet is Rz>2.5 m. It is
preferable that an S
content contained in the rust inhibiting oil be 5
mass% or less.
First of all, the reason why the present
inventors paid attention to the coating oil will be
described.
[0029] With an objective of improving scale adhesion
of the steel sheets to which no plating is applied
(cold-rolled steel sheets or hot-rolled steel sheets),
the present inventors have investigated the surface
properties of the steel sheets, and influences of
various kinds of treatment. As a result, the present
inventors have found that although the steel sheets
after degreasing show excellent scale adhesion, scale
adhesion significantly deteriorates after rust
inhibiting oil is applied. When the present
inventors investigated the relationship between scale
adhesion and rust inhibiting oil in more detail, it
has been found that when an amount of S contained as
impurities in the rust inhibiting oil increases,
scale tends to detach easily. It is
conceivable that
S in the rust inhibiting oil has an influence on
scale adhesion, although the detailed reason is
unclear.
[0030] On
the other hand, it is necessary to apply
rust inhibiting oil such as mineral oil to a pickled
hot-rolled steel sheet for hot stamping, and a cold-
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rolled steel sheet for hot stamping after cold
rolling or annealing in order to restrain rust from
occurring in the period from production to use. In
particular, a steel sheet after pickling has been
generally coated with oil of more than 1500 mg/m2,
assuming that the period from delivery to a customer
to use is long. When the present inventors
investigated the influence of the coating oil amount
for the purpose of making scale adhesion and rust
inhibition properties compatible, the
present
inventors have found that as illustrated in Fig. 1,
scale adhesion is enhanced by strictly controlling
the range of the coating oil amount and the surface
roughness of a steel sheet. The effect is exhibited
by setting the coating oil amount at 50 mg/m2 to 1500
mg/m2. A lower limit of the coating oil amount is set
at 50 mg/m2, because it is difficult to ensure
excellent rust inhibition properties with the coating
oil amount less than the coating oil amount of 50
mg/m2. The lower limit of the coating oil amount is
preferably 100 mg/m2 or more, and more preferably 200
mg/m2 or more. An upper limit is set at 1500 mg/m2 to
obtain an effect of excellent scale adhesion. The
upper limit of the coating oil amount is set at 1500
mg/m2 because when the coating oil amount exceeds 1500
mg/m2, scale adhesion deteriorates. The upper limit
is preferably 1000 mg/m2, is more preferably 900 mg/m2,
and far more preferably is 800 mg/m2. Further, coated
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oil on the steel sheet surface burns at the time of
heating, and therefore becomes the cause of
generating soot. From
this, a smaller coating oil
amount is more preferable.
[0031] Scale adhesion illustrated in Fig. 1 was
evaluated by a hot shallow drawing test in a
cylindrical die of (100 mm and a depth of 20 mm. After
a steel sheet was heated to a temperature range of
800 C to 1100 C at 50 C/s in an electrical heater, and
was retained for 0 seconds to 120 seconds,
energization was stopped, the steel sheet was cooled
to 650 C by standing to cool, and hot shallow drawing
was performed in the above described die.
Specimens
after forming were visually observed, and specimens
in which an area where scale was detached accounted
for 5% or less were determined as having good
(circle) scale adhesion, specimens in which the area
where scale was detached accounted for 5 to 15% were
determined as poor (triangle), and specimens in which
the area where scale was detached accounted for more
than 15% were determined as very poor (X). The
specimens in which the area where scale was detached
accounted for 5% or less were determined as within
the range of the present invention.
[0032] It is
possible to evaluate scale adhesion
without particularly limiting the heating method.
For example, conditions of any of a heating furnace,
far-infrared rays, near-infrared rays and electrical
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heating may be adopted.
Further, when a steel sheet
is heated in a heating furnace, more excellent scale
adhesion can be obtained by thinning scale by
controlling the atmosphere in the heating furnace and
restraining oxidation of the steel sheet.
[0033] Note
that a shallow drawing test temperature
may be in any temperature region as long as a steel
sheet can be processed, but in general, a steel sheet
for hot stamping has high strength and excellent
shape fixability by processing in an austenite region
and subsequent die hardening. From
this,
characteristics evaluation was carried out by hot
shallow drawing at 650 C exceeding Ar3.
[0034] As an
oil coating method, electrostatic oil
coating, spray, a roll coater and the like are
generally used, but the oil coating method is not
limited as long as the coating oil amount can be
ensured.
[0035]
Although the kind of oil is not specified,
NOX-RUST530F (made by PARKER INDUSTRIES, INC.) or the
like is generally used if the oil is mineral oil, for
example, and if the coating oil amount satisfies the
range of the present invention, the kind of oil is
not limited.
[0036]
Although the coating oil amount may be
measured by any method as long as the coating oil
amount can be measured, the present inventors
measured the coating oil amount by the following
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method. The
steel sheet coated with rust inhibiting
oil was cut into 150 mm square first, and thereafter,
a tape was applied so that a 100 mm by 100 mm region
is exposed.
Subsequently, the weights of the coating
oil and the steel sheet to which seal was carried out
(including the weight of the tape) were measured in
advance.
Subsequently, degreasing was performed by
wiping off the rust inhibiting oil on the steel sheet
surface with cloth containing acetone, the weight of
the degreased steel sheet was measured, the weights
before and after degreasing were compared, and
thereby the coating oil amount per unit area was
calculated. Measurement was carried out at three
spots in each of the steel sheets, and an average
value of the attached amounts was determined as a
coating oil attaching amount of each of the steel
sheets.
[0037] It is preferable to restrict the S content
contained in the rust inhibiting oil to 5 mass% or
less. When
the present inventors investigated the
relationship between the S content in the coating oil
and a scale detached area ratio as illustrated in Fig.
2, the present inventors have found that as the S
content in the coating oil becomes smaller, the scale
adhesion increases, and especially when the S content
in the coating oil is 5 mass% or less, the scale
detached area becomes substantially 0%. It is
conceivable that while the oil contained in the rust
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inhibiting oil is burned and eliminated during
heating, S contained as an impurity remains on the
steel sheet surface to concentrate into scale, and
thereby deteriorates scale adhesion,
although
detailed mechanism is unclear.
Hence, it is
preferable to reduce the content of S contained in
the rust inhibiting oil. The S content is preferably
4 mass% or less, and is more preferably 3 mass% or
less. Although analysis of S in the rust inhibiting
oil may be performed by any method as long as S can
be analyzed, the present inventors extracted 5 mL of
the rust inhibiting oil which is applied to the steel
sheet, and carried out analysis by fluorescence X-
rays (C-ray Fluorescence Sulfur-in-Oil Analyzer SLFA-
2800/HORIBA). In
measurement, measurement was
carried out with n=3, and an average value thereof
was defined as the S content.
[0038] The
surface roughness of the steel sheet will
be described next. In order to ensure scale adhesion,
the surface roughness of the steel sheet needs to
satisfy Rz>2.5 m. A
result obtained by
investigating a relationship between the surface
roughness Rz of the steel sheet and scale adhesion is
as illustrated in Fig. 1 described above. By
providing irregularities on an interface between
scale that is generated at the time of hot stamping
thermal treatment and a base iron, the irregularities
are formed on the interface between the base iron and
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scale, and further increase in adhesion is brought
about. The
effect is generally referred to as an
anchor effect. In
particular, scale that is
generated at the time of heating in the present steel
sheet is thin. As a result, in the present steel
sheet in which the thickness of the scale is thin,
scale having irregularities is formed by receiving an
influence of the base iron surface state.
Hence, the
surface roughness of the steel sheet before hot
stamping needs to satisfy Rz>2.5 m. When
Rz2.5 m,
the surface roughness of the steel sheet is small,
and the anchor effect is insufficient, and thus
excellent scale adhesion at the time of hot stamping
cannot be ensured. Although the effect of the
excellent scale adhesion of the present invention can
be obtained without particularly providing the upper
limit, if scale adhesion is excessively increased, it
becomes difficult to remove scale in a downstream
process such as shot blast, for example. Thus,
it is
preferable to set Rz<8.0 m. It is
more preferable
to set Rz<7.0 m. However, even if Rz8.0 m is set,
it is possible to ensure excellent scale adhesion
that is the effect of the present invention. Note
that in the steel sheet in which an Si content is
less than 0.50 mass%, even if the surface roughness
of Rz>2.5 m is set, thick Fe scale is formed at the
time of heating, and thus even when the
irregularities are on the steel sheet surface, the
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interface between the base iron and the scale becomes
flat by excessive oxidation. As a result, the
irregularities in the interface between the scale and
the base iron are eliminated, and the effect of the
excellent scale adhesion that is the effect of the
present invention is not exhibited.
[0039]
Although measurement of the surface roughness
Rz may be performed by any method, the present
inventors measured the region of a length of 10 mm
with n=3, with use of a contact surface roughness
measuring instrument (SURFC0M2000DX/SD3 made by TOKYO
SEIMITSU CO., LTD) with a probe point angle of 600,
and a point R of 2 m, and determined the average
value as the surface roughness Rz of each of the
steel sheets.
[0040] Next, a scale structure of the hot stamp
formed body will be described. The
steel sheet for
hot stamping of the present invention ensures scale
adhesion by control of the irregularities in the
interface between the scale and the base iron. Hence,
the scale can be scale mainly composed of an Si oxide,
Fe304, Fe203 and FeO. An Si oxide exists in the
interface between base iron and iron scale (FeO, Fe2031
Fe203), and thereby controls a thickness of the iron
scale. Hence, the scale needs to contain an Si oxide.
Since the main object is to control the thickness of
the iron oxide, even if the Si oxide is very thin, it
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is sufficient if the Si oxide exists, and even with 1
nm, the Si oxide exhibits the effect.
[0041] Composition analysis of the scale of the
formed body was carried out by X-ray diffraction by
cutting out the sheet from a bottom of the
cylindrical portion of a shallow drawn specimen piece.
From a peak intensity ratio of the respective oxides,
volume ratios of the respective Fe oxides were
measured. The
Si oxide existed very thinly, and the
volume ratio was less than 1%, and thus quantitative
evaluation in X-ray diffraction was difficult.
However, it is possible to confirm that an Si oxide
exists in the interface between the scale and the
base iron by line analysis of EPMA (Electron Probe
Micro Analyzer).
[0042] The thickness of the scale is preferably 10
m or less. When
the thickness of the scale is 10 m
or less, scale adhesion is enhanced more. When
the
thickness of the scale exceeds 10 m, the scale tends
to detach easily due to a thermal stress that works
at the time of cooling at the time of hot stamping.
Thereafter, in a scale removing process such as shot
blast or wet blast, fractures occur among Fe scales,
and a scale existing on an outer side detaches. As a
result, the scale also has a problem of being
inferior in scale removability. Hence, the thickness
of the scale is preferably 10 m or less. The
thickness of the scale is more preferably 7 m or
24
CA 02979978 2017-09-15
less, and is more preferably 5 gm or less. The
thickness of the scale is achieved by controlling the
coating oil amount within the predetermined range
simultaneously with controlling the Si content of the
steel sheet within a predetermined range. Fig.
5
illustrates a relationship between the coating oil
amount and the scale thickness.
[0043] In
the interface between the base iron and
the scale in the hot stamp formed body of the present
invention, three or more irregularities of 0.2 gm to
8.0 gm are present per 100 gm. Fig.
6A shows a
photograph of an interface between a base iron and
scale of a formed body excellent in scale adhesion,
and Fig. 6B shows a photograph of an interface
between a base iron and scale inferior in scale
adhesion. Since
the irregularities contribute to
enhancement in scale adhesion at the time of hot
stamping, and thus excellent scale adhesion can be
ensured by controlling the irregularities within the
above described range.
Irregularities of less than
0.2 gm provide an insufficient anchor effect, and
provide inferior scale adhesion. With irregularities
of 8.0 gm or more, scale adhesion is so strong that
scale is difficult to remove in the subsequent scale
removal process, for example, by shot blast or wet
blast, and therefore it is preferable to make the
irregularities in the interface between scale and the
base iron 8.0 gm or less. The
irregularities are
CA 02979978 2017-09-15
more preferably 6.0 Rm or less, and more preferably
4.0 Rm or less. Note
that even if the irregularities
exceed 8.0 Rm, excellent scale adhesiveness that is
the effect of the present invention can be ensured.
[0044] When the number of irregularities of 0.2 Rm
to 8.0 Rm per 100 Rm is less than three, an
improvement effect of scale adhesion is not
sufficient, and thus the number of irregularities per
100 Rm is set at three or more. It is
possible to
ensure excellent scale adhesion which is the effect
of the present invention without particularly setting
an upper limit of the number of irregularities per
100 Rm. Note that the irregularities of the formed
body are correlated with the surface roughness Rz of
the steel sheet as illustrated in Fig. 7, and are
controllable by setting the steel sheet surface
roughness as Rz>2.5 Rm.
[0045] Next, chemical compositions of the steel
sheet and the hot stamp formed body of the present
invention will be described. Note
that hereunder %
means mass%.
C: 0.100% to 0.600%
C represents an element that is contained to
enhance the strength of the steel sheet. If a
C
content is less than 0.100%, tensile strength of 1180
MPa or more cannot be ensured, and a formed body with
high strength which is the object of hot stamp cannot
be ensured. When the C content exceeds 0.600%,
26
CA 02979978 2017-09-15
weldability and processibility become insufficient,
and thus the C content is set at 0.100% to 0.600%.
The C content is preferably 0.100% to 0.550%, and is
more preferably 0.150% to 0.500%.
However, if the
strength of the formed body is not required,
excellent scale adhesion can be ensured even if the C
content is less than 0.150%.
[0046] Si: 0.50% to 3.00%
Si enhances scale adhesion by controlling the
scale composition at the time of hot stamping, and
therefore Si is an essential element. If
the Si
content is less than 0.50%, the thickness of Fe scale
cannot be controlled, and excellent scale adhesion
cannot be ensured.
Consequently, it is necessary to
set the Si content at 0.50% or more.
Further, when
application to a member which is difficult to form at
the time of hot stamping is considered, it is
preferable to increase the Si content. Accordingly,
the Si content is preferably 0.70% or more, and is
more preferably 0.90% or more.
Meanwhile, Si
increases an Ae3 point, and the heating temperature
necessary to make martensite a main phase, and thus
if the Si is excessively contained, productivity and
economic efficiency are reduced. Hence, an upper
limit of the Si content is set as 3.00%. The upper
limit of the Si content is preferably 2.5%, and the
upper limit is more preferably 2.0%.
However, it is
27
CA 02979978 2017-09-15
possible to ensure excellent scale adhesion excepting
productivity and economic efficiency.
[0047] Mn: 1.20% to 4.00%
Mn delays ferrite transformation in a cooling
process at the time of hot stamping, and makes a hot
stamp formed body into a structure having a
martensite main phase, and thus it is necessary to
contain 1.20% or more of Mn. If
the Mn content is
less than 1.20%, martensite cannot be made a main
phase, and it is difficult to ensure high strength
which is an object of the hot stamp formed body, and
thus a lower limit of the Mn content is set as 1.20%.
However, if the strength of the formed body is not
required, excellent scale adhesion can be ensured
even if the Mn content is less than 1.20%. When
the
Mn content exceeds 4.00%, the effect is saturated,
embrittlement is caused, and a fracture is caused at
the time of casting, cold rolling or hot rolling, and
thus an upper limit of the Mn content is set as 4.00%.
The Mn content is preferably within a range of 1.50%
to 3.50%, and is more preferably within a range of
2.00% to 3.00%.
[0048] Ti: 0.005% to 0.100%
Ti is an element that combines with N to form TiN,
and thereby restrains B from being a nitride to
enhance hardenability. The effect becomes remarkable
when a Ti content is 0.005% or more, and thus the Ti
content is set as 0.005% or more.
However, when the
28
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Ti content exceeds 0.100%, a Ti carbide is formed, an
amount of C that contributes to strengthening
martensite is reduced, and reduction in strength is
caused, and thus an upper limit of the Ti content is
set as 0.100%. The C
content is preferably within a
range of 0.005% to 0.080%, and is more preferably
within a range of 0.005% to 0.060%.
[0049] B: 0.0005% to 0.0100%
B enhances hardenability at the time of hot
stamping, and contributes to making a main phase of
martensite. The
effect is remarkable when a B
content is 0.0005% or more, and thus it is necessary
to set the B content at 0.0005% or more. When
the B
content exceeds 0.0100%, the effect is saturated, an
iron boride is precipitated, and the effect of
hardenability of B is lost, and thus an upper limit
of the B content is set at 0.0100%. The B content is
preferably within a range of 0.0005% to 0.0080%, and
is more preferably within a range of 0.0005% to
0.0050%.
[0050]
P: 0.100% or less
P is an element that segregates in a central
portion of a sheet thickness of the steel sheet, and
is an element that embrittles a welded portion.
Accordingly, an upper limit of a P content is set at
0.100%. A more preferable upper limit is 0.050%.
The lower the P content, the more preferable, and
29
CA 02979978 2017-09-15
although the effect of the present invention is
exhibited without particularly setting the lower
limit, but it is economically disadvantageous to
reduce P to less than 0.001% from the viewpoint of
productivity and cost of dephosphorization, and thus
the lower limit is preferably set at 0.001%.
[0051] S: 0.0001% to 0.0100%
S exerts a large influence on scale adhesion, and
thus it is necessary to restrict a content in the
steel sheet. Accordingly, an upper limit of an S
content is set at 0.0100%. A lower limit of the S
content is set at 0.0001% because it is economically
disadvantageous from the viewpoint of productivity
and cost of dephosphorization. The S content is
preferably within a range of 0.0001% to 0.0070%, and
is more preferably within a range of 0.0003% to
0.0050%.
[0052] Al: 0.005% to 1.000%
Al acts as a deoxidizer, and thus an Al content
is set as 0.005% or more. When
the Al content is
less than 0.005%, a sufficient deoxidization effect
cannot be obtained, and a large amount of enclosure
(oxide) exist in the steel sheet. These enclosures
become starting points of destruction at the time of
hot stamping, and the causes of breakage, and
therefore are not preferable. The effect becomes
remarkable when the Al content reaches 0.005% or more,
and thus it is necessary to set the Al content at
CA 02979978 2017-09-15
0.005% or more. When
the Al content exceeds 1.000%,
the Ac3 point is increased and a heating temperature
at the time of hot stamping is increased. That
is,
hot stamp is a technique of obtaining a formed body
with high strength having a complicated shape by
heating a steel sheet to an austenite single phase
region, and subjecting the steel sheet to hot die
press excellent in formability, and rapidly cooling
by using a die. As a
result, when a large amount of
Al is contained, the Ac3 point is significantly
increased, increase in the heating temperature
required for austenite single phase region heating is
caused, and productivity is reduced.
Consequently,
it is necessary to set an upper limit of the Al
content at 1.000%. The
Al content is preferably
within a range of 0.005% to 0.500%, and is more
preferably within a range of 0.005% to 0.300%.
[0053] N: 0.0100% or less
N is an element that forms coarse nitrides and
deteriorates bendability and hole-expandability.
When an N content exceeds 0.0100%, bendability and
hole-expandability are significantly deteriorated,
and thus an upper limit of the N content is set at
0.0100%. Note that N becomes a cause of generating a
blowhole at the time of welding, and thus the smaller
the N content is, the more preferable. Accordingly,
the N content is preferably 0.0070 or less, and is
more preferably 0.0050% or less. Although it is not
31
CA 02979978 2017-09-15
necessary to particularly set a lower limit of the N
content, manufacturing cost increases significantly
when the N content is reduced to less than 0.0001%,
and thus a practical lower limit is 0.0001%. From
the viewpoint of manufacturing cost, the N content is
more preferably 0.0005% or more.
[0054] Note
that other unavoidable elements may be
contained in extremely small amounts. For
example, 0
forms an oxide and exists as an enclosure.
The steel sheet of the present invention further
contains the following elements in accordance with
necessity.
[0055] Ni: 0.01% to 2.00%
Cu: 0.01% to 2.00%
Cr: 0.01% to 2.00%
Mo: 0.01% to 2.00%
Ni, Cu, Cr and Mo are elements that contribute to
increase in strength by enhancing hardenability at
the time of hot stamping, and making a main phase of
martensite. The effect becomes remarkable by
containing 0.01% or more of each one kind or two or
more kinds selected from a group consisting of Ni, Cu,
Cr and Mo, and thus contents of the elements are
preferably 0.01% respectively. When the content of
each of the elements exceeds a predetermined amount,
weldability, hot workability and the like are
deteriorated, or the strength of the steel sheet for
hot stamping is so high as to be likely to cause a
32
CA 02979978 2017-09-15
manufacturing trouble, and thus upper limits of the
contents of these elements are preferably set at
2.00%.
[0056] Nb: 0.005 to 0.100%
V: 0.005 to 0.100%
W: 0.005 to 0.100%
Nb, V and W are elements that strengthen fine
grains by inhibiting growth of austenite at the time
of hot stamping, and contribute to increase in
strength and enhancement in tenacity.
Hence, one
kind or two or more kinds selected from a group
consisting of these elements may be contained. The
effect becomes more remarkable when 0.005% or more of
each of the elements are contained, and thus it is
preferable that 0.005% or more of each of the
elements be contained. Note
that when more than
0.100% of each of these elements is contained, it is
not preferable because Nb, V and W carbides are
formed, an amount of C that contributes to
strengthening martensite is reduced, and reduction in
strength is caused. Each
of the elements is
preferably in a range of 0.005% to 0.090%.
[0057] A
total of one kind or two or more kinds
selected from a group consisting of REM, Ca, Ce and
Mg: 0.0003% to 0.0300%
In the present invention, 0.0003% to 0.0300% of
one kind or two or more kinds selected from a group
33
CA 02979978 2017-09-15
consisting of REM, Ca, Ce and Mg may be further
contained in total.
REM, Ca, Ce and Mg are elements that enhance
strength and contribute to improvement of the
material. When
the total of one kind or two or more
kinds selected from the group consisting of REM, Ca,
Ce and Mg is less than 0.0003%, a sufficient effect
cannot be obtained, and thus it is preferable to set
a lower limit of the total at 0.0003%. When the
total of one kind or two or more kinds selected from
the group consisting of REM, Ca, Ce and Mg exceeds
0.0300%, castability and hot workability are likely
to be deteriorated, and thus it is preferable to set
an upper limit of the total at 0.0300%. Note that
REM is an abbreviation of Rare Earth Metal, and
refers to an element belonging to a lanthanoid system.
In the present invention, REM is often added in misch
metal, and besides Ce, elements of a lanthanoid
system are sometimes contained in combination.
[0058] In
the present invention, the effect of the
present invention becomes apparent even when elements
of a lanthanoid system other than La and Ce are
contained as unavoidable impurities, and the effect
of the present invention becomes apparent even when
the other elements such as metals are contained as
impurities.
34
CA 029799782017-09-15
[0059] Next,
features of microstructures of the
steel sheet for hot stamping and hot stamp formed
body of the present invention will be described.
Provided that the chemical composition, the
surface roughness of the steel sheet, and the coating
oil amount satisfy the ranges of the present
invention, the effect of the present invention can be
exhibited by any of a pickled hot-rolled steel sheet,
a cold-rolled steel sheet obtained by cold-rolling a
hot-rolled steel sheet, or a cold-rolled steel sheet
to which annealing is applied after cold rolling.
[0060] These
steel sheets are heated to an austenite
region exceeding 800 C at the time of hot stamping,
and therefore exhibit performance as steel sheets for
hot stamping having excellent scale adhesion that is
the effect of the present invention without
particularly limiting the microstructure.
However,
when mechanical cutting of the steel sheets and cold
punching are carried out prior to hot stamping, the
strength of the steel sheets is preferably as low as
possible in order to reduce wear and tear of dies,
cutting edges of cutters, or punching dies.
Consequently, the microstructure of the steel sheet
for hot stamping is preferably ferrite and pearlite
structures, or a bainite structure and a structure
obtained by tempering martensite.
However, if wear
and tear of a punch and dies at the time of
mechanical cutting and cold punching do not become a
CA 02979978 2017-09-15
problem, it is possible to ensure excellent scale
adhesion which is the effect of the present invention,
even if one kind or two or more kinds of retained
austenite, martensite in a hardened state, and
bainite are contained.
Further, in order to reduce
the strength of the steel sheet, thermal treatment in
a box type annealing furnace or a continuous
annealing facility may be carried out.
Alternatively,
even when cold rolling is carried out after the above
softening treatment, and the sheet thickness is
controlled to a predetermined sheet thickness,
excellent scale adhesion which is the effect of the
present invention is ensured.
[0061] When
formed body strength after hot stamping
is enhanced, and high component strength is obtained,
the microstructure of the formed body preferably has
a martensite main phase. In
particular, in order to
ensure tensile strength of 1180 MPa or more, a volume
ratio of martensite that is a main phase is
preferably made 60% or more.
Martensite may be
subjected to tempering after hot stamping, and made
tempered martensite. As
the structure other than
martensite, bainite, ferrite, pearlite, cementite and
retained austenite may be contained.
Further, even
if the martensite volume rate is less than 60%, it is
possible to ensure the excellent scale adhesion of
the present invention.
36
CA 029799782017-09-15
[0062] The
following methods are used in
identification of the microstructures (tempered
martensite, martensite, bainite, ferrite, pearlite,
retained austenite and a remaining structure)
composing the steel sheet structure, confirmation of
existence positions, and measurement of area ratios.
For example, it is possible to corrode a section in a
steel sheet rolling direction or a section in a
direction perpendicular to the rolling direction with
a nital reagent and the reagent disclosed in Japanese
Laid-open Patent Publication No. 59-219473, and
observe the structure with a 1000 to 100000-power
scanning electron microscope (SEM: Scanning Electron
Microscope) and transmission electron microscope
(TEM: Transmission Electron Microscope). The
present
inventors determined the sheet thickness section
parallel with the rolling direction of the steel
sheet as an observation surface, extracted a specimen,
polished the observation surface, performed nital
etching, observed a range of thickness of 1/8 to 3/8
with 1/4 of the sheet thickness as a center with a
field emission scanning electron microscope (FE-SEM:
Field Emission Scanning Electron Microscope),
measured an area fraction, and the area fraction was
taken as a volume fraction. As for the volume
fraction of the retained austenite, the volume
fraction was measured by performing X-ray diffraction
with the surface which was parallel with the sheet
37
CA 02979978 2017-09-15
surface of the parent steel sheet and had a 1/4 of
thickness, used as the observation surface.
[0063] Next,
a method for producing the steel sheet
for hot stamping of the present invention will be
described.
Although the other operation conditions are based
on a usual method, the following conditions are
preferable in terms of productivity.
[0064] In
order to produce the steel sheet in the
present invention, a slab having the same component
composition as the component composition of the
aforementioned steel sheet is cast first. As the
slab provided for hot rolling, a continuously cast
slab, the slab produced by a thin slab caster or the
like can be used. The
method for manufacturing the
steel sheet of the present invention is adapted to a
process like continuous casting-direct rolling (CC-
DR) that performs hot rolling immediately after
casting.
- Slab heating temperature: 1100 C or higher
- Hot-rolling completion
temperature: Ar3
transformation point or higher
- Coiling temperature: 700 C or lower
- Cold rolling ratio: 30 to 70%
[0065] The slab heating temperature is preferably
set at 1100 C or higher. The slab heating temperature
in a temperature region of lower than 1100 C causes
reduction in the finishing rolling temperature, and
38
CA 02979978 2017-09-15
thus strength at the time of finishing rolling tends
to be high. As a result, there is the possibility
that rolling becomes difficult, a poor shape of the
steel sheet after rolling is caused, and thus the
slab heating temperature is preferably set at 1100 C
or higher.
[0066] The
finishing rolling temperature is
preferably set at the Ar3 transformation point or
higher. When the finishing rolling temperature
becomes lower than the Ar3 transformation point, a
rolling load becomes high, and there is the
possibility that rolling becomes difficult, and a
poor shape of the steel sheet after rolling is caused,
and thus a lower limit of the finishing rolling
temperature is preferably set at the Ar3
transformation point. An
upper limit of the
finishing rolling temperature does not have to be
particularly set, but if the finishing rolling
temperature is set to be excessively high, the slab
heating temperature has to be made excessively high
in order to ensure the temperature, and thus the
upper limit of the finishing rolling temperature is
preferably 1100 C.
[0067] The coiling temperature is preferably set at
700 C or lower. When the coiling temperature exceeds
700 C, the thickness of the oxides formed on the steel
sheet surface is excessively increased, and the
pickling property is deteriorated, and thus the
39
CA 02979978 2017-09-15
coiling temperature higher than 700 C is not
preferable. When cold rolling is performed
thereafter, a lower limit of the coiling temperature
is preferably set at 40000. When the coiling
temperature is lower than 400 C, the strength of the
hot-rolled steel sheet extremely increases, and a
sheet fracture and a poor shape at the time of cold
rolling are easily caused, and thus the lower limit
of the coiling temperature is preferably set at 400 C.
However, if the hot-rolled steel sheet which is
coiled is intended to be softened by heating the
coiled hot-rolled steel sheet in the box type
annealing furnace or the continuous annealing
facility, the steel sheet may be coiled at a low
temperature of lower than 400 C. Note that at the
time of hot-rolling, rough-rolled sheet's may be
bonded to one another and finishing rolling may be
continuously performed.
Further, the rough-rolled
sheet may be coiled temporarily.
[0068] Next,
pickling is applied to the hot-rolled
steel sheet which is produced in this way for 30
seconds or more in an aqueous solution with an
temperature of 80 C to 100 C in which a concentration
of acid is 3 mass% to 20 mass% and an inhibitor is
included. In
the present invention, pickling under
the present conditions is extremely important, and in
order to control the surface roughness Rz of the
steel sheet to more than 2.5 m, pickling under the
CA 02979978 2017-09-15
above described conditions is necessary. Note
that
an aqueous solution of a hydrochloric acid, a
sulfuric acid or the like as an acid is generally
used, and an aqua regia or the like may be used.
[0069] The temperature of the aqueous solution is
set at 80 C to lower than 100 C, because with a
temperature lower than 80 C, a reaction rate is low,
and it takes a long time to bring the surface
roughness of the hot-rolled steel sheet into a proper
range. Meanwhile, heating at a temperature of 100 C
or higher is dangerous and is not preferable because
the solution boils and splashes although the reaction
of pickling has no problem.
[0070]
Further, the reason why the concentration of
the acid is set at 3 mass% to 20 mass% is to control
the surface roughness Rz of the hot-rolled steel
sheet within the proper range. When
the
concentration of the acid is less than 3 mass%, it
takes a long time to control the irregularities on
the surface by pickling. When
the concentration of
the acid exceeds 20 mass%, a pickling tank is damaged
significantly and facility management becomes
difficult, and thus it is not preferable. A
preferable range of the concentration of the acid is
a range of 5 mass% to 15 mass%.
[0071]
Further, the reason why the pickling time
period is set at 30 seconds or more is to stably give
predetermined irregularities (irregularities of
41
CA 02979978 2017-09-15
Rz>2.5 m) to the steel sheet surface by pickling.
When the pickling tank is divided into a plurality of
tanks, if a pickling time period of some of the
pickling tanks or a total pickling time period
satisfies the above described conditions, the surface
roughness Rz of the hot-rolled steel sheet can be
brought into the range of the present invention, even
if concentrations or temperatures of the individual
pickling tanks differ from one another.
Further,
pickling may be carried out by being divided into a
plurality of times. Note that in the experiment by
the present inventors, a hydrochloric acid including
an inhibitor was used, but the effect of the present
invention can be obtained by using another acid such
as hydrochloric acid using no inhibitor, a sulfuric
acid, and a nitric acid, or a composite of these
acids, as long as the surface roughness Rz can be
controlled by pickling.
[0072]
Further, the irregularities formed by
pickling of the hot-rolled steel sheet also remain
even after temper rolling, cold rolling or annealing
is carried out, and thus it is extremely important to
control the pickling conditions, and give
irregularities to the sheet surface after pickling.
Consequently, temper rolling may be carried out to
the hot-rolled steel sheet after pickling.
[0073]
Further, even with a cold-rolled steel sheet
to which only cold rolling is performed, or a cold-
42
CA 02979978 2017-09-15
rolled steel sheet thermally treated in a continuous
annealing facility or a box type annealing furnace
after cold rolling, irregularities are formed on the
surface by performing pickling before cold rolling,
and the predetermined effect can be obtained. Note
that cold rolling is preferably performed with roll
roughness Rz for cold rolling within a range of 1.0
m to 20.0 m, and the cold rolling roll also
includes temper rolling roll.
[0074] Cold
rolling is applied to the hot-rolled
steel sheet pickled under the conditions as above at
a draft of 30% to 80%, and the steel sheet may be
passed through a continuous annealing facility. When
the draft is less than 30%, it becomes difficult to
keep the shape of the steel sheet flat, and ductility
of the finished product deteriorates, and thus a
lower limit of the draft is preferably set at 30%.
When the draft exceeds 80%, a rolling load becomes
excessively large, and cold rolling becomes difficult,
and thus an upper limit of the draft is preferably
set at 80%. The
draft is more preferably 40% to 70%.
The effect of the present invention becomes apparent
even without particularly specifying the number of
times of rolling pass and the draft of each pass, and
thus it is not necessary to specify the number of
times of rolling pass, and the draft at each pass.
[0075]
Thereafter, the cold-rolled steel sheet may
be passed through the continuous annealing line. An
43
CA 02979978 2017-09-15
object of the treatment is to soften the steel sheet
which is highly strengthened by cold-rolling, and
thus any conditions may be adopted as long as the
condition is such that the steel sheet is softened.
For example, when the annealing temperature is in a
range of 550 C to 750 C, dislocation introduced at the
time of cold rolling is released by recovery,
recrystalization, or phase transformation, and thus
annealing is preferably performed in this temperature
region.
[0076] By
performing annealing by a box type furnace
for the similar purpose, the steel sheet for hot
stamping excellent in scale adhesion of the present
invention can be obtained.
[0077] Thereafter, oil coating is carried out. As
an oil coating method, electrostatic oiling, spray, a
roll coater and the like are generally used, and as
long as a coating oil amount in a range of 50 mg/m2 to
1500 mg/m2 can be ensured, the method is not limited.
In the present invention, coating of a predetermined
amount of oil was carried out by an electrostatic
oiling machine.
Further, as long as the coating oil
amount in the range of 50 mg /m2 to 1500 mg/m2 can be .
ensured, a rust inhibitor in an amount equal to or
larger than the coating oil amount may be applied,
and degreasing may be performed.
[0078] The
excellent scale adhesion that is the
effect of the present invention and a rust inhibition
44
CA 02979978 2017-09-15
property can be made compatible without particularly
limiting the hot stamping conditions. For
example,
by producing by the production method shown as
follows, compatibility of excellent performance of
the tensile strength of 1180 MPa or more and
productivity is achieved. At the time of performing
hot stamping, heating is preferably performed to a
temperature region of 800 C to 1100 C at a heating
rate of 2 C/second or more. By
heating at a rate of
2 C/second or more, scale generation at the time of
heating can be restrained, and the effect of
improvement in scale adhesion is provided. The
heating rate is preferably 5 C/second or more, and is
more preferably 10 C/second or more.
Further,
increase of the heating rate is also effective for
the purpose of enhancing productivity.
[0079] The
annealing temperature at the time of
performing hot stamping is preferably within the
range of 800 C to 1100 C. By
performing annealing in
this temperature region, it is possible to make the
structure into an austenite single phase structure,
and the structure can be made into a structure having
martensite as a main phase by cooling that is
performed subsequently. When the
annealing
temperature at this time is lower than 800 C, the
structure at the time of annealing is made into a
ferrite and austenite structures, the ferrite grows
in the cooling process, the ferrite volume ratio
CA 02979978 2017-09-15
exceeds 10%, and the tensile strength of the hot
stamp formed body becomes lower than 1180 MPa.
Consequently, a lower limit of the annealing
temperature is preferably set at 800 C. When the
annealing temperature exceeds 1100 C, not only the
effect is saturated, but also the scale thickness is
significantly increased, and there arises the fear
that scale adhesion is reduced. Consequently, it is
preferable to perform annealing at 1100 C or lower.
The annealing temperature is more preferably in a
range of 830 C to 1050 C.
[0080] After
heating, retention may be performed in
the temperature region of 800 C to 1100 C. When
retention is carried out at a high temperature,
melting of carbides included in the steel sheet is
possible, and contribution is made to increase in the
strength of the steel sheet and enhancement in
hardenability. Retention includes residence, heating
removal and cooling removal in the present
temperature region. Since
the object is to melt the
carbides, the object is achieved as long as the
residence time period in the present temperature
region is ensured. Although the limitation on the
retention time period is not particularly provided,
1000 seconds is preferably set as an upper limit,
because when the retention time period is 1000
seconds or more, the scale thickness becomes
excessively large, and scale adhesion is deteriorated.
46
CA 02979978 2017-09-15
[0081] Thereafter, a temperature of 800 C to 700 C is
preferably reduced at an average cooling rate of
C/second or more. Here,
700 C is a die cooling
start temperature, and the reason why the temperature
of 800 C to 700 C is reduced at 5 C/second or more is
to avoid ferrite transformation,
bainite
transformation and pearlite transformation, and make
the structure into a martensite main phase. When the
cooling rate is less than 5 C/second, these soft
structures are formed, and it is difficult to ensure
the tensile strength of 1180 MPa or more.
Meanwhile,
the effect of the present invention is exhibited
without particularly setting the upper limit of the
cooling rate. The
reason why the temperature range
which is reduced at 5 C/second or more is set from
800 C to 700 C is that in this temperature range, the
structure of ferrite or the like that causes
reduction in strength is likely to be formed.
Cooling at this time is not limited to continuous
cooling, and even when retention and heating in the
temperature region are performed, the effect of the
present invention is exhibited as long as the average
cooling rate is 5 C/second or more. The effect of the
present invention can be exhibited
without
particularly limiting the cooling method. That
is,
the effect of the present invention can be exhibited
by either one of cooling using a die or die cooling
using water cooling in combination.
47
CA 02979978 2017-09-15
EXAMPLES
[0082] Next, examples of the present invention will
be described, and conditions in the examples are only
one example of the conditions adopted to confirm
implementability and the effect of the present
invention, and the present invention is not limited
to the one condition example. The present invention
can adopt various conditions as long as the
conditions achieve the object of the present
invention without departing from the gist of the
present invention.
[0083] First, slabs of the component compositions of
A to S and a to n shown in Table I were cast, and
after the slabs were temporarily cooled to a room
temperature, heating was carried out for 220 minutes
in a heating furnace with a furnace temperature =
1230 C, hot rolling was carried out with the finishing
rolling temperature = 920 C to 960 C, and coiling was
carried out under the temperature conditions shown in
Table 2.
48
Table.1 Chemical component (mass%)
1-3 0
I C Si Mn P S Ti B N
Al Others a) 0
tr CO
A
0.211 1.04 2.29 0.011 0.0009 0.025 0.0028 0.0023 0.023 - H 06
B
0.207 0.67 2.09 0.009 0.0012 0.023 0.0014 0.0027 0.019 - (i) .....,
C 0.189 1.83 2.55 0.007 0.0016 0.028 0.0029 0.0026 0.035 - HI
D
0.207 1.21 1.44 0.012 0.0018 0.035 0.0009 0.0031 0.056 Cr=0.68
E 0.208 1.19 1.82 0.013 0.0022 0.045 0.0022 0.0024 0.045 Mo=0.13
F 0.221 1.11 1.74 0.008 0.0019 0.029
0.0023 0.0022 0.024 Ni=0.44, Cu=0.12
G
0.203 1.05 2.27 0.009 0.0017 0.024 0.0020 0.0026 0.029 Nb=0.068
H
0.219 0.98 2.35 0.010 0.0033 0.021 0.0024 0.0037 0.018 V=0.054
I
0.228 1.24 2.19 0.011 0.0027 0.022 0.0021 0.0024 0.033 W=0.033
J
0.218 1.24 2.31 0.015 0.0045 0.026 0.0034 0.0034 0.027 REM=0.0046
K
0.234 1.05 2.37 0.016 0.0039 0.024 0.0025 0.0021 0.025 Ca=0.0033
L 0.219 1.03 2.19 0.009 0.0048 0.025 0.0021 0.0038 0.011 Ce=0.0029
M 0.246 1.11 2.27 0.013 0.0052 0.019 0.0021 0.0029 0.007 Mg=0.0019
N 0.309 1.09 2.19 0.008
0.0024 0.016 0.0026 , 0.0019 0.022 - P
O
0.314 0.78 2.11 0.013 0.0028 0.025 0.0018 0.0024 0.030 - 0
N,
u,
,N
P 0.311 1.32 1.87 0.011 0.0030 0.028 0.0020 0.0023 0.035 Cr=0.18 -]
u,
u,
Lc)
Q 0.356 1.24 2.09 0.015 0.0034 0.022 0.0034 0.0025 0.029 - -]
.3
R 0.349 1.06 1.43 0.016 0.0019 0.021 0.0032 0.0022 0.027 Cr=0.46 N,
0
,
-]
1
S
0.412 0.99 2.22 0.007 0.0014 0.028 0.0023 0.0020 0.024 - .
u,
'
a
0.097 0.98 2.03 0.022 0.0022 0.033 0.0028 0.0023 0.021 - ,
u,
b
0.698 1.49 1.68 0.007 0.0006 0.024 0.0026 0.0029 0.089 -
c
0.203 0.34 2.11 0.015 0.0027 0.029 0.0026 0.0029 0.019 -
d
0.194 3.25 2.09 0.009 0.0018 0.021 0.0031 0.0025 0.023 -
e
0.211 1.03 1.12 0.014 0.0016 0.026 0.0018 0.0021 0.026 -
f
0.199 1.23 7.89 0.024 0.0039 0.025 0.0049 0.0042 0.038 -
g
0.205 0.78 2.31 0.009 0.0148 0.021 0.0012 0.0031 0.022 -
h
0.209 1.16 2.31 0.007 0.0013 - 0.0018 0.0034 0.031 -
i
0.184 1.08 1.42 0.006 0.0037 0.139 0.0034 0.0037 0.022 -
j 0.210 1.05 1.89 0.016 0.0035 0.022 -
_
0.0028 0.027 -
k 0.201 0.98 1.45 0.011 0.0055 0.027 0.1180 0.0033 0.024 -
I
0.198 1.21 1.52 0.008 0.0042 0.033 0.0027 0.0191 0.046 - _
m
0.205 0.87 2.42 0.011 0.0057 0.021 0.0016 0.0082 0.001 -
n 0.213 0.94 1.98 0.012 0.0019 0.023 0.0024 0.0024 1.285 -
The underlined part means being outside the range of the present invention.
"-" means that each element is not added.
CA 02979978 2017-09-15
[ 0 0 8 5 ]
[Table 2]
Table.2
Coating
Coiling Acid Acid Pickling Kind of oil
Steel Steel temperature concentration temperature
ti me coating amount S content
number grade*1 (CC) ( %) (CC) period (s) oil
(mg/m2) (mass%) Remarks
Al FH 580 8 83 160 NOX503F- 0
Comparative steel
A2 FH 600 6 87 200 NOX503F 60 0 Present
invention steel
A3 FH 590 8 85 160 NOX503F 140 1 Present
invention steel
A4 FH 680 7 90 680 NOX503F 270 1 Present
invention steel
AS FH 590 9 89 160 NO/GI/3F 490 1 Present
invention steel
A6 FI-1 600 9 86 160 NOX503F 780 1 Present
invention steel
A7 FH 580 8 84 240 NOX503F 1020 1 Present
invention steel
A8 FH 550 9 86 40 NOX503F 1480 5 Present
Invention steel
A9 FH 510 6 82 lia NOX503F 1000 2 Compa
rative steel
A10 FH 520 7 85 24 NOX503F 970 1
comparative steel
All FH 510 6 85 18 NOX503F 820 1
fnmnarative steel
Al2 FH 620 10 94 180 NOX503F 1790 a
Comparative steel
A13 FH 560 8 89 65 NOX503F 2050 ¾
Comoarative steel
A14 FH 600 9 85 200 NOX503F 3720 Z
Comparative steel
A15 FH 570 8 87 240 NOX504F 4890 .2
Comparative steel
131 HR 580 12 85 230 NOX503F 490 1 Present
invention steel
Cl CR 590 6 86 160 NOX503F 420 1 Present
invention steel
DI. FH 560 8 85 100 NOX503F 550 1 Present
invention steel
El FH 560 7 83 100 NOX503F 1030 2 Present
invention steel
Fl FH 570 6 88 140 NOX503 F 1200 3 Present
invention steel
Gl. FH 610 8 93 200 NOX503F 820 1 Present
invention steel
H1 FH 600 10 89 130 NOX503 F 670 1 Present
invention steel
II FH 580 8 86 240 NOX503F 980 0 Present
invention steel
J1 FH 550 9 90 80 NOX503F 1180 2 Present
invention steel
K1 FH 570 8 84 160 NOX503F 630 1 Present
invention steel
Li FH 590 9 88 220 NOX503F 940 0 Present
invention steel
M1 FH 600 6 90 200 NOX503F 430 1 Present
invention steel
N1 FH 590 8 83 200 NOX503F 570 1 Present
invention steel
N2 FH 560 9 89 80 NOX503F 690 1 Present
invention steel
N3 FH 550 11 92 70 NOX503F 700 1 Present
invention steel
N4 FH 600 10 94 120 NOX503F 800 1 Present
invention steel
N5 FH 520 7 82 a NOX503F 760 1
Comparative steel
N6 FH 530 7 83 18 NOX503F 80 1 f
omnarative steel
N7 FH 590 8 86 210 NOX503F- 0 Comparative
steel
N8 FH 570 9 87 190 NOX504F 3560 a
fomnarative steel
N9 FH 600 9 92 200 NOX503F 4820 a COMDa
rative steel
N10 FH 590 8 88 240 NOX503F 6090 Z
Comparative steel
01 HR 590 8 94 240 NOX503F 1220 2 Present
invention steel
P1 CR 580 7 86 200 NOX503F 890 1 Present
invention steel
QI FH 580 9 86 190 NOX503F 800 I Present
invention steel
R1 HR 590 8 83 200 NOX503F 1370 3 Present
invention steel
Si FH 560 9 89 200 NOX503F 680 1 Present
invention steel
al FH 590 8 85 240 NOX503F 5901 Comparative
steel
bl_,2:2. -..2. ....2, 22 .l'a ....'2. 22 _..z.
onmnarative steel
cl FH 560 8 84 240 NOX503F 1260 2
Comparative steel
dl FH 480 7 86 a NOX503F 2450 2
Comparative steel
el FH 570 9 90 270 NOX503F 990 1
Comparative steel
fl_L.2. ,L.2. 22 _4,z Comparative
steel
gl FH 580 9 88 210 NOX503F 1210 1
Comparative steel
hl FH 560 8 92 180 NOX503F 1040 0
Comnarativeteel
il FH 590 7 89 220 NOX503F 1300 1 f
omparative steel
11 FH 570 8 88 200 NOX503F 1230 2
Comparative steel
kl FH 640 8 85 190 NOX503F 840 1
comparative steel
II FH 610 9 82 80 NOX503F 900 2
Comparative steel
ml FH 560 9 93 280 NOX503F 1000 1
Comparative steel
n1 FH 560 9 86 180 NOX503F 570 1
Comparative steel
*1 means that FH: left as cold rolled, HR: hot-rolled steel sheet, and CR:
cold-rolled steel sheet annealed after cold rolling.
*2 means that Mn is excessively high, many fractures occur in casti ng and hot
rol I i ng ti me, and no hot-rolled steel sheet was able to be
produced.
CA 02979978 2017-09-15
[0086] The finished sheet thickness of the hot-
rolled steel sheet provided for hot stamping as the
hot-rolled steel sheet was made 1.6 mm. The
sheet
thickness of the hot-rolled steel sheet provided for
cold rolling was made 3.2 mm. When
pickling was
carried out under the conditions in Table 2
thereafter, and cold rolling was performed, the sheet
thickness was made 50% (3.2 mm -* 1.6 mm).
Thereafter,
annealing was performed for some of the steel sheets
in a continuous annealing facility, and the steel
sheets were made into cold-rolled steel sheets.
Thereafter, by using NOX-RUST503F (made by PARKER
INDUSTRIES, INC.), N0X503F (made by PARKER INDUSTRIES,
INC.) was applied to the hot-rolled steel sheets and
the cold-rolled steel sheets by an electrostatic
oiling machine, in a range of no coating oil to 6090
mg /m2.
[0087] Thereafter, the steel sheets were cut into a
predetermined size, after which, electrical heating
was performed to 900 C at 50 C/second, retention for
seconds at 900 C was carried out, thereafter,
standing to cool for 10 seconds was performed, and
hardening was performed in the above described hot
shallow drawing dies at a temperature of 650 C or
higher. Visual observation of the obtained hot stamp
formed bodies was performed, and the steel sheets
without detachment of scale were determined as the
steel sheets excellent in scale adhesion.
51
CA 02979978 2()17-09-15
[0088] Concerning the rust inhibition property,
retention for 30 days was carried out at a room
temperature, and the steel sheets with no rust
generated on the steel sheet surfaces were defined as
the steel sheets excellent in rust inhibition
property. In
combination, with use of flat sheet
test pieces, hot stamping was performed under the
aforementioned conditions, and
tensile
characteristics were evaluated. The
evaluation
result is shown in Table 3.
52
CA 02979978 2017-09-15
[0089]
[Table 3]
Ta ble.3
Scale Irregularities in Scale Presence or
Steel Steel Rz thickness scale/base iron detached
Scale absence of rust TS of formed
number 8rade*1 (pm (urn) interface area (%) adhesion
generation body (MPa) Remarks
Al FH 3.7 4 6 0 0 Presence 1555 Comparative
steel
A2 FH 3.6 5 5 o o Absence 1562 Present
invention steel
A3 FH 4.0 4 8 0 0 Absence 1560 Present
invention steel
A4 FH 4.4 5 9 0 o Absence 1559 Present
invention steel
AS FH 4.2 4 6 0 o Absence 1555 Present
invention steel
A6 FH 4.5 6 8 0 o Absence 1550 Present
invention steel
A7 FH 3.8 6 7 0 o Absence 1557 Present
invention steel
AS FH 2.6 5 4 3 o Absence 1562 Present
invention steel
AS FH 2.2 2 0 11 A Absence 1562 Comparative
steel
, A10 FH 1.9 3 1 16 8 Absence 1554
Comparative steel
All F1-1 2.4 2 2 8 A Absence 1564 Comparative
steel
Al2 FH 4.8 12 7 14 A Absence 1549 Comparative
steel
A13 FH 2.6 14 4 26 8 Absence 1556 c omna rative
steel
A14 FH 3.9 15 8 44 A Absence 1560 comnarative
steel
A1.5 FH 3.6 18 7 68 8 Absence 1567 comnarative
steel
B1 HR 5.9 4 13 0 o Absence 1549 Present
invention steel
Cl CR 4.1 4 6 0 0 Absence 1483 Present
invention steel
D1 FH 3.7 3 5 0 o Absence 1529 Present
invention steel
El FH 3.8 3 8 0 o Absence 1550 Present
invention steel
Fl FH 3.7 7 7 o o Absence 1625 Present
invention steel
G1 FH 4.5 4 8 o o Absence 1572 Present
invention steel
H1 FH 4.6 4 6 0 o Absence 1645 Present
invention steel
11 FH 4.4 5 6 0 o Absence 1687
Present invention steel
.11 FH 3.8 5 8 0 o Absence 1639
Present invention steel
K1 FH 4.0 3 7 0 o Absence 1752 Present
invention steel
Li FH 4.5 4 8 0 o Absence 1624
Present invention steel
M1 FH 4.3 4 7 0 o Absence 1715 Present
invention steel
Ni FH 4.4 4 7 a o Absence 1834 Present
invention steel
N2 FH 3.9 3 6 0 o Absence 1828 Present
invention steel
N3 FH 3.2 3 5 0 o Absence 1833 Present
invention steel
N4 FH 4.5 4 10 0 o Absence 1829 Present
invention steel
N5 FH 2.3 2 0 9 A Absence 1830 Comparative
steel
N6 FH 1.8 3 1 13 A Absence 1826 Comparative
steel
N7 FH 3.9 4 8 0 o Presence 1834 comparative
*,13.1.
N8 FH 4.6 13 8 39 8 Absence 1823 Coma rative
steel
N9 FH 4.3 13 7 47 8 Absence 1835 Comparative
steel
NIO FH 4.5 21 8 45 8. Absence 1830 comnarative
steel
01 HR 5.3 4 13 0 o Absence 1854 Present
invention steel
P1 CR 4.4 4 8 0 o Absence 1847 Present
invention steel
QI FH 4.7 4 9 0 o Absence 2108 Present
invention steel
R1 HR 6.0 4 12 0 o Absence 2138
Present invention steel
Si FH 3.9 3 7 0 o Absence 2505 Present
invention steel
al FH 4.3 4 8 o o Absence 1054 Comnarative
steel
bl FH -*2 -*2 -*2 -*2 -*2 -*2 -*2 comnarative steel
Ci FH 2.4 16 0 89 8 Absence 1483 c omnarative
steel
dl FH 1.8 1 o 8_4 8 Absence 1598 Comparative
steel
el FH 3.8 4 9 0 o Absence 987 comparative steel
fl FH -*2 -*2 -*2 -*2 -*2 -*2 -*2
comparative steel
gl FH 4.3 5 9 9.2 8 Absence 1604 Comparative
steel
hl FH 4.1 4 8 0 o Absence 1156 Compa rative
steel
il FH 4.0 5 7 0 o Absence 1095 comparative
s_teel
.11 FH 4.6 4 7 0 o Absence 1023
Comparative steel
kl FH 4.4 7 11 0 o Absence 1154 Comoa rative
steel
11 FH 4.1 5 9 0 o Absence 1072
comnarative steel
M1 FH 3.9 4 s 0 o Absence _*3 co mcia ratiVe
steel
n1 FH 4.5 4 7 0 o Absence 1008 Comparative
steel
*1 means that FH: left as cold rolled. HR: hot-rolled steel sheet, and CR:
cold-rolled steel sheet annealed after cold rolling.
*2 means that Mn was excessively high, many fractures occurred in casting and
hot rolling time, and no hot-rolled steel sheet was able to be
produced.
*3 means that at the time of hot stamping, a fracture with the enclosure as
the starting point occurred, and the tensile test was not be a ble to
be carried out with the formed body.
53
CA 029799782017-09-15
[0090] As for the tensile characteristics, the
tensile test pieces which were in conformity with JIS
Z 2201 were extracted, the tensile test was performed
in conformity with JIS Z 2241, and the maximum
tensile strength was measured. The
formed bodies
having the maximum tensile strength of 1180 MPa or
more were determined as the formed bodies of the
present invention.
[0091]
Composition analyses of the scales of the
formed bodies were carried out by X-ray diffraction
by cutting out sheets from the bottoms of the
cylindrical portions of the shallow drawing test
pieces. From
the peak strength ratios of the
respective oxides, the volume ratios of the
respective Fe oxides were measured. The
Si oxides
were present very thinly and the volume ratio was
less than 1%, and thus quantitative evaluation by X-
ray diffraction was difficult.
However, it could be
confirmed that the Si oxides were present in the
interface between the scale and the base iron by the
line analysis of EPMA.
As for evaluation of the irregularities in the
interfaces of the scales and the base irons formed in
the formed bodies, embedded polishing was carried out
for the steel sheets cut out from the above described
position, and thereafter, SEM observation was
performed by power of 3000 from the section
perpendicular to the rolling direction. Five
visual
54
CA 02979978 2017-09-15
fields were observed in each of the test pieces, and
the number density of the irregularities in the range
of 0.2 gm to 1.0 gm per length of 100 gm was measured.
[0092] The
formed bodies satisfying the conditions
of the present invention were able to make excellent
rust inhibition properties and excellent scale
adhesion compatible. The
formed bodies that do not
satisfy the conditions of the invention were inferior
in scale adhesion, or inferior in corrosion
resistance.
INDUSTRIAL APPLICABILITY
[0093] According to the present invention, the steel
sheet excellent in scale adhesion at the time of hot
stamping can be provided, the problems of wear and
tear of the die at the time of hot stamping, plating
adhesion to the die, and
indentation . flaws
accompanying it can be solved, and thus the present
invention can bring about significant enhancement in
productivity, and has an industrially large value.
