COLD-ROLLED STEEL SHEET AND METHOD FOR
MANUFACTURING SAME, AND HOT-STAMP FORMED BODY
[Technical Field of the Invention]
The present invention relates to a cold-rolled steel sheet, a hot-dip galvanized
cold-rolled steel sheet, a galvam~ealedc old-rolled steel sheet, an electrogalvanized
cold-rolled steel sheet, or an aluminized cold-rolled steel sheet, a hot-stamp formed
body obtained by using the same, and a method for manufacturing the same.
Priority is claimed on Japanese Patent Application No. 2012-174215, filed on
August 6,2012 and Japanese Patent Application No. 2012-174216, filed on August 6,
2012, the contents of which are incorporated herein by reference.
[Related Art]
[0002] I
Currently, there is demand for improved collision safety and reduced weight
in steel sheets used for vehicles. In order to achieve both improvement of collision
safety and reduction in weight, a high-strength steel sheet has been developed in which
the strength represented by tensile strength and the like is high. However, demand for
a high-strength steel sheet has been increasing.
Given such circumstances, hot-stamping (also referred to as hot-pressing, hotstamping,
diequenching, or press-quenching) has recently attracted attention as a
method for obtaining higher strength. Hot-stamping is a forming method in which a
steel sheet is subjected to hot-forming after being heated at a high temperature, for
example, 700°C or higher, to improve the formability of the steel sheet and to quench
the steel sheet by cooling, after the formation thereof, to obtain desired material
properties. As a steel sheet having both press formability and high strength, a steel
sheet having a martensite single-phase structure and a steel sheet having a multi-phase
structure such as a ferrite-martensite structure or a ferrite-bainite structure are known.
Among these, a composite-structure steel sheet in which martensite is dispersed in a
ferrite matrix has a low yield ratio and superior ductility.
However, in recent steel sheets for a vehicle, the strength has increased, and
complex forming has been performed. Therefore, when the strength of a recent
composite-structurc steel sheet is high, the ductility thereof is insufficient. Further,
there is a case where an additional process may be performed on a formed body afier
hot-stamping or a case where impact absorbing ability may be expected from a formed
body after hot-stamping. Therefore, recently, it has become necessary that the
ductility of a formed body after hot-stamping be maintained to be high.
[OOO3]
Such complex-structure stc?el,sheets are disclosed in, for example, Patent
Documents 1 to 3.
[0004]
However, even with these techniques ofthe related art, it is difficult to satisfy
the demands including further reduction in weight and complication of shapes of parts
in recent vehicles. Further, in the steel sheets of the related art, it is difficult to
maintain the ductility after hot-stamping to be high.
[Prior Art Document]
[Patent Document]
[OOOS]
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. 1-16-128688
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2000-3 19756
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2005-120436
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
An object of the invention is to provide a cold-rolled steel sheet for hotstamping,
a hot-dip galvanized cold-rolled steel sheet for hot-stamping, a galvannealed
cold-rolled steel sheet for hot-stamping, an electrogalvanized cold-rolled steel sheet for
hot-stamping, or an aluminized cold-rolled steel sheet for hot-stamping, and a method
for manufacturing the same, in which the strength can be secured before and after hotstamping,
superior ductility can be obtained, and the formability is superior before and
after hot-stamping. Further, anothbr ,object of the invention is to provide a hot-stamp
lormed body having superior formability.
[Means for Solving the Problem]
[0007]
The prcsent inventors have thoroughly researched cold-rolled steel sheets,
hot-dip galvanized cold-rolled steel sheets, galvannealed cold-rolled steel sheets,
electrogalvanized cold-rolled steel sheets, and aluminized cold-rolled stecl sheets, in
which the strength can be secured belore hot-stamping (before heating to perform
quenching in hot-stamping) and after hot-stamping (after quenching in the hotstamping),
and the formability (ductility) is superior. As a result, the present
inventors have found that a cold-rolled steel sheet capable of securing higher
formability than the related at before and after hot-stamping can be industrially
manufactured by controlling fractions of ferrite, bainite and retained austenite of a steel
sheet before hot-stamping to be predetermined values, controlling a difference in the
fraction of the retained austenite between a thickness surface portion and a thickness
center portion of the steel sheet to be a specific range, and controlling a distribution of
the retained austenite in the thickness center portion to be a specific range. This coldrolled
steel sheet may also be used as a galvanized cold-rolled steel sheet or an
aluminized cold-rolled steel sheet. Further, the present inventors have found that, in
order to obtain the above-described steel shcet, it is necessary to control the time
before and after rough-rolling and the thickness of the steel sheet in a hot-rolling, and
to control a rolling reduction in a cold rolling. Here, securing higher formability than
the related art represents a product TSxEl of tensile strength TS and ductility El being
20000 MPa% or more. In addition, the present inventors have found that, in order to
control the fraction of the retained austenite, it is effective to control the relationship
between adulation fiom a time at Whi~hth e steel sheet exits from a furnace to a time at
which rough-rolling of the steel sheet starts and a dulation from a time at which the
rough-rolling of the steel sheet is finished to a time at which finish-rolling of the steel
sheet starts to be in a spccific range; and to control the relationship between Llre
thiclcnesses of the steel sheet before and after rough-rolling to be in a specific range.
Further, the present inventors have also found that a formed body obtained by forming
such a steel sheet by hot-stamping is superior in ductility, and an additional working
such as drawing is easily performed on the formed body. Based on the above findings,
the present inventors have conceived the following various aspects of the invention
[OOOS]
(1) In a cold-rolled steel sheet according to an aspect of the invention, a chemical
composition includes, in terms of mass%: C: 0.1% to 0.3%; Si: 0.01% to 2.0%; Mn:
I 1.5% to 2.5%; P: 0.001% to 0.06%; S: 0.001% to 0.01%; N: 0.0005% to O.Ol%;Al:
0.01% to 0.05%; B: 0% to 0.002%; Mo: 0% to 0.5%; Cr: 0% to 0.5%; V: 0% to 0.1%;
i Ti: 0% to 0.1%; Nb: 0% to 0.05%; Ni: 0% to 1.0%; Cu: 0% to 1.0%; Ca: 0% to
0.005%; E M : 0% to 0.005%; and rest including Fe and impurities, a structure before
and after a hot-stamping includes a ferrite: 30 area% to 90 area%, a martensite: 0
area% or more and less than 20 area%, a pearlite: 0 area% to 10 area%, a retained
austenite: 5 volume% to 20 volume%, and rest structure: a bainite, a hardness of the
retained austenite measured with a nano indenter before and after the hot-stamping
satisfies Expression 1 and Expression 2, a tensile strength and a ductility satisfy a
relation of TSxE1>20000MPa%,
H2 / H1 < 1.1 : Expression 1,
i oHM < 20: Expression 2, and
the "HI" represents the hardness of the retained austenite existing within a
thiclmess surface portion before and after the hot-stamping, the thickness surface
portion being an area within 200 pm in the thiclmess direction of a surlace of the coldrolled
steel sheet, the "H2" represents the hardness ofthe retained austenite existing
within a thickness center portion before and after the hot-stamping, the thickness
center portion being an area within & 100pm along the thickness direction of a center
plane of the cold-rolled steel sheet in the thickness direction, the "oHM" represents a
variance of the hardness of the retained austenite within the thickness center portion
before or after the hot-stamping, "TS" represents the tensile strength of the cold-rolled
steel sheet in terms of MPa, and "El" represents the ductility of the cold-rolled steel
sheet in terms of%
(2) In the cold-rolled steel sheet according to (I), the chemical con~positionm ay
include one or more elements selected from a group consisting of, in terms of mass%:
B: 0.0005% to 0.002%; Mo: 0.01% to 0.5%; Cr: 0.01% to 0.5%; V: 0.001% to 0.1%;
Ti: 0.001% to 0.1%; Nb: 0.001% to 0.05%; Ni: 0.01% to 1.0%; Cu: 0.01% to 1.0%;
Ca: 0.0005% to 0.005%; and REM: 0.0005% to 0.005%.
[OO lo]
(3) In the cold-rolled steel sheet according to (1) or (2), a hot-dip galvanized plating
may be formed on the surface of the cold-rolled steel sheet.
[OOll]
(4) In the cold-rolled steel sheet according to (1) or (2), a galvannealed plating may
be formed on the surface of the cold-rolled steel sheet.
[OO 121
(5) In the cold-rolled steel sheet according to (1) or (2), an electrogalvanized plating
may be formed on the surface of thdcold-rolled steel sheet.
[00 131
(6) In the cold-rolled steel sheet according to (1) or (2), an aluminum plating may be
formed on the surface ot'the cold-rolled steel sheet.
[00 141
(7) Amethod according to another aspect of the invention for manufacturing the
cold-rolled steel sheet according to (1) or (2) including: casting a molten steel having
the chemical composition into a steel; hot-rolling after the casting, in which heating is
performed on the steel in a furnace, and then rough-rolling and finish-rolling are
performed on the steel under a condition satisfiing Expression 3; coiling the steel after
the hot-rolling; pickling the steel after the coiling; cold-rolling the steel after the
picltling with a cold-rolling mill having a plurality of stands under a condition
satisfying Expression 4; annealing after the cold-rolling, in which annealing is
performed on the steel at 700°C to 850DC, and then the steel is cooled; temper-rolling
the steel after the annealing, in which
2 < (115) x (hlih2) x (1i10) x (tl+t2)03< 6: Expression 3,
1.5 x (rllr) + 1.2 x (r2ir) + (r3lr) > 1: Expression 4, and
"hl" represents a thickness of the steel before the rough-rolling in terms of
mm, "h2" represents the thickness of the steel after the rough-rolling in terms of mm,
"tl" represents a dulation from a time at which the steel exits the furnace to a time at
which the rough-rolling of the steel starts in terms of seconds, "t2" represents a
dulation from a time at which the rough-rolling is finished to a time at which the
finish-rolling starts in terms of seconds, and "ri" represents an individual target coldrolling
reduction of an i,h stand (i = 1,2, 3......) from a first stand along the plurality of
the stands in terms of %, and "r" represents a total target cold-rolling reduction in the
cold-rolling in terms of %. b,
[00 151
(8) The method according to (7) for manufacturing the cold-rolled steel sheet
according to (3), may include hot-dip galvanizing between the annealing and the
temper-rolling, in which the hot-dip galvanized plating is formed on the steel.
[0016]
(9) The method according to (8) for manufacturing the cold-rolled steel sheet
according to (4), may include galvannealing between the hot-dip galvanizing and the
temper-rolling, in which the steel is galvannealed.
[00 171
(10) The method according to (7) for manufacturing the cold-rolled steel sheet
according to (5), may include electrogalvanizing after the temper-rolling, in which the
electrogalvanized plating is lormed on the steel.
[OOlS]
(1 1) The method according to (7) for manufacturing the cold-rolled steel sheet
according to (6), may include aluminizing between the annealing and the temperrolling,
in which the aluminum plating is formed on the steel.
[00 191
(12) A hot-stamp formed body according to still another aspect of the invention is
obtained by using the cold-rolled steel sheet according to any one of (1) to (6).
[Effects of the Invention]
[0020]
According to the invention, a hardness distribution (a hardness distribution at
the thickness center portion, and a aifcerence in hardness between the thickness surface
portion and the thickness center portion) of the retained austenite measured with a
nano indenter is controlled to be appropriate before and after hot-stamping.
Therefore, superior ductility can be obtained. Accordingly, in a formed body
manufactured by hot-stamping, superior ductility can be obtained before and after hotstamping.
[Brief Description of the Drawing]
[0021]
FIG. 1A is a graph showing a relationship between H2IHI and OHM before
and after hot-stamping.
FIG. 1B is a graph showing a relationship between OHM and TSxEl before
and after hot-stamping and showing a basis for Expressions 1 and 2.
FIG. 2 is a graph showing a relationship between Expression 3 relating to hotrolling
conditions and TSxEl before and after hot-stamping and showing a basis for
Expression 3.
FIG. 3 is a graph showing a relationship between Expression 4 relating to a
cold-rolling reduction and hardness H2lHl before and after hot-stamping and showing
a basis for Expression 4.
FIG. 4 is a perspective view showing a hot-stamp formed body according to
an example of the invention.
FIG. 5 is a flowchart showing a cold-rolled steel sheet according to the
invention and a method for manufacturing various plated cold-rolled steel sheets.
[Embodiments of the Invention]
[0022]
It was found that, as described above, in order to improve the formability of
the steel sheet before and after hot-hamping, it is important to control the hardness
distribution of the retained austenite at a predctermined position ofthe steel sheet (the
hardness distribution at the thickness center portion, and the difference in hardness
between the thiclmess surface portion and the thickness center portion) to be
appropriate. Research regarding the relationship between the formability of the steel
sheet before and after hot-stamping and the hardness of the retained austenite has yet to
be conducted.
[0023]
Here, a steel sheet according to an embodiment of the invention and the
reason for limiting the chemical components of steel used for manufacturing the steel
sheet will be described. Hereinafter, "%" which is the unit indicating the content of
each component represents "mass%.
[0024]
(C: 0.1% to 0.3%)
C is an important element to stably remain austenite. When the C content is
less than 0.1%, it is not possible to to sufficiently remain the austenite. On the other
hand, when the C content is more than 0.3%, the weldability of the steel sheet
decreases. Accordingly, the range of the C content is set to 0.1% to 0.3%. When the
demand for weldability is high, the C content is preferably set to 0.25% or less.
(Si: 0.01% to 2.0%)
Si is an important element for suppressing the formation of harmful carbides
and lor deoxidation. However, when the Si content is more than 2.096, the ductility
of the steel sheet decreases, and the chemical conversion treatability of the steel sheet
also decreases. Accordingly, the upper limit of the Si content is set to 2.0%. In
addition, when the Si content is lessY4an 0.01%, the deoxidation effect cannot be
sufficiently obtained. Accordingly, the lower limit of the Si content is set to 0.01%.
[0026]
(Al: 0.01% to 0.05%)
A1 is important as a deoxidizer. For this purpose, the lower limit of the A1
content is set to 0.01%. On the other hand, even when the A1 content is excessively
large, the above effect is saturated, and conversely, the steel is embrittled.
Accordingly, the upper limit of the A1 content is set to 0.05%.
[0027]
(Mn: 1.5% to 2.5%)
Mn is an important element for improving hardenability to strcngthen the steel
sheet. When the Mn content is less than 1.5%, the strength of the steel sheet cannot
be sufficiently improved. However, when the Mn content is more than 2.5%, the
hardenability of the steel sheet is increased more than necessary, and the strength is
increased in an unfavorable amount, which leads to a decrease in ductility.
Accordingly, the Mn content is set to 1.5% to 2.5%. When the demand for ductility is
high, the upper limit of the Mn content is preferably set to 2.0%.
[0028]
(P: 0.001% to 0.06%)
When the I' content is large, P is segregated in a grain boundary, and the local
ductility and the weldability of the steel sheet deteriorate. Accordingly, the upper
limit of the P content is set to 0.06%. On the other hand, an unnecessary decrease in
the P content causes an increase in the cost during refinement. Therefore, the lower
limit of the P content is set to 0.001%.
[0029]
(S: 0.001% to 0.01%) P
S is an element which forms MnS with Mn and significantly decreases the
local ductility and the weldability of the steel sheet. Accordingly, the upper limit of
the S content is set to 0.01%. In addition, due to the refinement cost, the lower limit
of the S content is preferably set to 0.001%.
[0030]
(N: 0.0005% to 0.01%)
N is important for precipitating nitrides such as AlN to refine crystal grains of
a structure of the steel shect. However, when N content is more than 0.01%, solidsolution
N remains and thus the ductility olthe steel sheet decreases. Accordingly,
the upper limit of the N content is set to 0.01%. In addition, in order to refine the
crystal grains of the structure and to reduce the cost during refinement, the lower limit
of the N content is preferably set to 0.0005%.
[0031]
(Nb: 0% to 0.05%, Ti: 0% to 0.1%, V: 0% to 0.1%, Mo: 0% to 0.5%, Cr: 0% to 0.5%)
The steel sheet according to the embodiment does not necessarily include Nb,
Ti, V, Mo, and Cr. Accordingly, the lower limits of the contents of these elements are
0%. However, Nb, Ti, and V are precipitated as fine carbonitrides and strengthen the
steel. In addition, Mo and Cr improve the hardenability of the steel sheet and
strengthen the steel. In order to obtain these effects, the steel sheet may include one
element or two or more elements selected from among Nb: 0.001% or more, Ti:
0.001% or more, V: 0.001% or more, Mo: 0.01% or more, and Cr: 0.01% or more.
On the other hand, it is necessary that the upper limit of the Nb content be set to 0.05%,
the upper limit of the Ti content be set to 0.1%, the upper limit of the V content be set
to 0.1%, the upper limit of the Mo cbqtent be set to 0.5%, and the upper limit of the Cr
content be set to 0.5%. When the steel sheet contains these elements in amounts
more than the upper limits, the effect of increasing the strength is saturated, and the
ductility decreases
[0032]
(Ca: 0% to 0.005%, REM: 0% to 0.005%)
The steel sheet according to the embodiment does not necessarily include Ca
and REM (rare earth element). Accordingly, the lower limits of the contents of these
elements are 0%. However, Ca controls the shapes of sulfides and nitrides to improve
the local ductility and the hole expansibility of the steel. In order to obtain this effect,
the Ca content may be set to 0.0005% or more. However, when the Ca content is
excessively large, the workability of the steel deteriorates. Accordingly, the upper
limit of the Ca content is set to 0.005%. When being included in the steel, REM
exhibits the same effect as Ca. For the same reason as Ca, the lower limit of the REM
content may be set to 0.0005%, and the upper limit thereof is necessarily set to 0.005%.
REM refers to a collective term for 17 elements including 15 lanthanoid
elements, Y, and Sc. Among these elements, one element or two or more elements
may be included in the steel sheet. The REM content refers to the total content of
these elements.
[0033]
(Cu: 0% to 1.0%, Ni: 0% to 1.0%, B: 0% to 0.002%)
The steel sheet according to the embodiment does not necessarily include Cu,
Ni, and B. Accordingly, the lower limits of the contents of these elements are 0%.
However, these elements also improve hardenability and increase the strength of the
steel. Accordingly, in order to obtain these effects, the lower limit of the Cu content
may be set to 0.01%, the lower limihfthe Ni content may be set to 0.01%, and the
lower limit of the B content may be set to 0.0005%. On the other hand, it is
necessary that the upper limit of the Cu content be set to 1.0%, the upper limit of the
Ni content be set to 1.0%, and the upper limit of the B content be set to 0.002'%.
When the steel sheet includes these elements in amounts more than the upper limits,
the effect of increasing the strength is saturated, and the ductility of the steel decreases.
[0034]
(Rest: Fe and Impurity)
The rest of the steel according to the embodiment includes Fe and impurity.
Here, the impurity refers to elements which are, when the steel is industrially
manufactured, incorporated from raw materials such as ore or scrap or incorporated by
various factors of the manufacturing process, and the impurity is allowed to be
included in the steel in a range not adversely affecting the steel. Examples of the
impurity elements include Sn and As.
[0035]
(Hardness Distribution of Retained Austenite in Thickness center portion of Steel
Sheet: oHM<20 Before and After Hot-Stamping)
(Difference in Hardness of Retained Austenite between Thiclmess center portion and
Thickness surface portion of Steel Sheet: H2A31<1.1)
When the steel includes the retained austenite, the ductility of the steel is
improved. However, in order to stably retain the retained austenite, it is necessary
that the C content be optimum. However, when the steel sheet is actually
manufactured, the C content in the steel sheet varies between the thickness surface
portion and the thickness center portion in many cases. Specifically, when the C
content in the thickness surface portion is small, the amount of the retained austenite is
insuficient, and when the C content1 iq the thickness center portion is large, the
retained austenite is decomposed, and cementite is precipitated. As a result, the
ductility of the steel sheet deteriorates. In order to improve the ductility of the steel
sheet by the steel sheet including the retained austenite, it is necessary to solve such a
problem.
The present inventors have thoroughly studied the hardness of retained
austenite and have found the following: as shown in FIGS. lA and lB, when a
difference in the hardness of the retained austenite between the thiclcness surface
portion and the thickness center portion (hardness ratio) before and after hot-stamping
is in a predetermined state, and when a hardness distribution of the retained austenite
of the thickness center portion (variance) is in a predetermined state, the formability
such as ductility is superior. Further, the present inventors have lound that, even after
quenching in hot-stamping, the C distribution included in the steel is substantially
maintained, and the formability such as ductility is maintained to be high. The reason
is considered to be as follows: the hardness distribution of the retained austenite
formed before hot-stamping has a large influence even after hot-stamping such that C,
which is rich in the thickness center portion, maintains its rich state in the center part
even after hot-stamping. Therefore, when a difference in the hardness of the retained
austenite between the thickness surface portion and the thickness center portion is large
and a variance thereof is large in the steel sheet before hot-stamping, the steel sheet
after hot-stamping shows the same tendency.
[0036]
The present inventors have found that, in the embodiment, when the hardness
of the retained austenite measured with a nano indenter (manufactured by Hysitron
Corporation) at a magnification of 1000 times before and after the hot-stamping
satisfies Expression 1 and Expressidn 2, a steel sheet and a hot-stamp formed body
having superior formability can be obtained. Here, "H1" represents the hardness of
the retained austenite existing within the thickness surface portion which is the area
within 200 pm in the thickness direction of the outerrnost surface of the steel sheet,
"H2" represents the hardness of the retained austenite existing within the thiclmess
center portion which is the area within *I00 pm along the thickness direction of a
center plane of the steel sheet in the thickness direction, and "OHM represents a
variance of the hardness of the retained austenite within the thickness center portion.
The hardness of the retained austenite at each of 300 points is measured. Since the
hardness measured by the nano indenter is a dimensionless parameter, a unit is not
assigned to this hardness measurement.
H2 / H1 I . 1 : Expression 1
OHM < 20: Expression 2
[0037]
FIG. 1A is a graph showing a relationship between H2lHl and OHM before
and after hot-stamping, and FIG. 1B is a graph showing a relationship between OHM
and TSxEl before and afler hot-stamping. FIG. 1A and FIG 1B show a basis for
Expressions 1 and 2. It can be seen from FIG. 1A that, in the steel sheet in which
H2lH1 is less than 12, OHM is less than 20. Further, it also can be seen from FIG. 1A
that the values of H2IHl and OHM do not largely change before and after hot-stamping.
Furthermore, it can be seen from FIG. 1B that, when OHM is less than 20, TSxEl is
more than 20000 MPa% which is the target of the embodiment.
The value of H21H1 being 1.1 or more represents the hardness of the retaincd
austenite of the thiclmess center portion being 1.1 times or more of the hardness of the
retained austenite of the thickness surface portion. When I-I2lH1 is 1.1 or more
before and after hot-stamping, as shbyn in FIG. 1 A, OHM is 20 or more before and
after hot-stamping. In this case, the hardness of the thickness center portion
excessively increases, TSxE1<20000 MPa%, and sufficient formability cannot be
obtained before and aflcr hot-stamping.
In the invention, "H2IH1 being 1.1 or more before and afler hot-stamping"
represents H2IH1 being 1.1 or more before hot-stamping and H2lH1 being 1.1 or more
after hot-stamping. A ratio of H2 after hot-stamping to H1 before hot-stamping, or a
ratio of H2 before hot-stamping to H1 after hot-stamping is not calculated.
[0038]
The variance OHM being 20 or more represents a large variation in the
hardness of the retained austenite. That is, when the variance OHM is 20 or more, a
portion having a locally excessively high hardness is present in the steel sheet. In this
case, TSxE1<20000 MPa%, and sufficient formability cannot be obtained before and
after hot-stamping.
[0039]
In the embodiment, the hardness of the retained austenite is measured with the
nano indenter at a magnification of 1000 times. Since an indentation formed in a
Vicltcrs hardness test is larger than the retained austenite, the hardness (Vickers
hardness) obtained in the Vickers hardness test indicates the macroscopic hardness of
the retained austenite and peripheral structures thereof (for example, ferrite)
Accordingly, in the Vickers hardness test, the hardness of the retained austenite itself
cannot be obtained. Since formability (ductility) is largely affected by the hardness of
the retained austenite itself, the Vickers hardness is insufficient as an index for
evaluating formability. On the other hand, in the hardness measurement by the nano
indenter, the hardness of the retained austenite itself can be measured. This is
because, by adjusting settings during (be hardness measurement, the size of an
indentation formed in the hardness measurement by the nano indenter can be reduced
to be smaller than the size of the retained austenite. By using the hardness obtained
by the nano indenter as an evaluation index, the formability of the steel sheet can be
evaluated more accurately. In the embodiment, since the relationship regarding the
hardness of the retained austenite before and after hot-stamping which is measured by
the nano indenter is appropriate, extremely superior formability can be obtained.
[0040]
(Area Ratio of Ferrite: 30% to 90% Before and After Hot-Stamping, Area Ratio of
Pearlite: 0% to 10% Before and After Hot-Stamping, Area Ratio of Martensite: 0% or
More and Less than 20% Before and Arier Hot-Stamping, Volume Ratio of Retained
austenite: 5% to 20% Before and After Hot-Stamping, and Rest Structure: Bainite
Before and After Hot-Stamping)
In the embodiment, an area ratio of ferrite in the structure before and after
hot-stamping is 30% to 90%. When the area ratio of ferrite is less than 30%,
sui3cient ductility cannot be obtained. On the other hand, when the area ratio of
ferrite is more than 90%, the bard phase is insufficient, and sufficient strength cannot
be obtained. Accordingly, the area ratio of ferrite is set to 30% to 90%.
The structure before and after hot-stamping also includes the retaincd
austenite. In the embodiment, the volume ratio of the retained austenite is set to 5%
to 20%. By 5% or more of the retained austenite being present in the steel sheet,
ductility is secured. It is not necessary to define the upper limit of the volume ratio of
the retained austenite. However, the upper limit of the area ratio of the retained
austenite is set to about 20% in consideration of, for example, the capacity of an actual
manufacturing facility. \T
The structure before and after hot-stamping may include martensite. In this
case, the area ratio of martensite is less than 20%. This is because, when a steel sheet
is manufactured under a manufacturing condition where the structure includcs 5
volume% to 20 volume% of the retained austenite, the structure cannot include 20
area% or more of martensite.
As described above, in the embodiment, major portions of the structure before
and after hot-stamping are occupied by ferrite and the retained austenite, and the
structure may further include martensite. In addition, in the embodiment, it is
preferable that the structure before and after hot-stamping does not include pearlite.
Pearlite is a hard and brittle structure. Therefore, when the structure includes more
than 10 area% of pearlite, the tensile strength and the ductility of the steel sheet may
decrease. Accordingly, the area ratio of pearlite is set to 0% to 10%.
In the embodiment, the rest (rest structure) of the structure before and after
hot-stamping mainly includes bainite.
The content of each structure is measured using the following method. The
area ratios offerrite, bainite, and pearlite can be obtained by cutting the steel sheet in a
direction perpendicular to a rolling direction, polishing the cut surface, exposing the
structure of the cut surface by nital etching, and observing a thickness 114 portion of
the cut surface at a magnification of 1000 times. The area ratio of martensite can be
obtained by cutting the steel sheet in a direction perpendicular to a rolling direction,
polishing the cut surface, exposing the structure of the cut surface by Le Pera etching,
and observing a thickness 114 portion of the cut surface at a magnification of 1000
times. The volume ratio of the retained austenite can be obtained by polishing a
region of the steel sheet from the sukfqce to the thickness 114 portion and measuring
the steel sheet using an X-ray diffractometer. The thickness 114 portion refers to a
part of the steel sheet at a distance of 114 of the thickness of the steel sheet from the
surface in the thickness direction of the steel sheet
When hot-stamping is performed on such a steel sheet, a tensile strength of
500 MPa to 1500 MPa can be realized in the steel sheet after hot-stamping. In
addition, by satisfying the above-described conditions, an effect of significantly
improving formability can be obtained, particularly, in a steel sheet having a tensile
strength of about 550 MPa to 1200 MPa
On the surface of the cold-rolled steel sheet according to the embodiment, a
galvanized plating, for example, a hot-dip galvanized plating, a galvannealed plating,
an electrogalvanized plating, or an aluminum plating may be formed to obtain a hotdip
galvanized cold-rolled steel sheet, a galvannealed cold-rolled steel sheet, an
electrogalvanized cold-rolled steel sheet, or an aluminized cold-rolled steel sheet.
The formation of such a plating is preferable from the viewpoint of corrosion
resistance.
[0043]
Hereinafier, a method for manufacturing the steel sheet according to the
embodiment will be described.
[0044]
(Casting S 1)
When the steel sheet according to the embodiment is manufactured, a molten
steel melted in a converter is continha~slyc ast into a slab (steel) under ordinary
conditions. During the continuous casting, when the casting speed is fast, a
precipitate such as Ti is too fine. On the other hand, when the casting speed is slow,
the productivity is poor, the above-described precipitate is coarsened, and the number
of particles is small. As a result, properties such as delayed fracture resistance may
not be controlled. Therefore, the casting speed is preferably set to 1.0 dmin to 2.5
mlmin.
100451
(Hot-Rolling S2)
The slab after the castingmay be heated in a furnace such as a tunnel kiln and
then may be provided for hot-rolling, directly. Alternatively, when the slab is cooled
to lower than 1 10OoC, the slab can bc reheated in a furnace such as a tunnel kiln.
Irrespective of whether or not the slab is cooled, the temperature of the slab exited
from the furnace is preferably set to 11 OO°C to 1300°C. When the hot-rolling starts at
a temperature of lower than 1 10O0C, it is diff~cultto secure a finishing temperature,
which may cause a decrease in the ductility of the steel sheet. In addition, in a stcel
sheet to which Ti andlor Nb is added, the melting of the precipitate during heating is
insufficient, which may cause a decrease in the strength of the steel sheet. On the
other hand, when the heating temperature is higher than 130OoC, the amount of scale
formed is large, and surface properties of the steel sheet may not be superior.
Next, rough-rolling and finish-rolling are performed under a condition by
which the following Expression 3 is established. In the embodiment, "hot-rolling"
includes the heating, the rough-rolling, and the finish-rolling. As shown in FIG. 2, in
a steel sheet which is manufactured by performing the rolling under the condition
satisfying Expression 3, TSxE1)2000Q MPa% is satisfied before and after hotstamping.
Here, "tl" represents a dulation from a time at which the slab exits from
the furnace to a time at which rough-rolling of the slab starts in the hot-rolling in terms
of seconds, "t2" represents adulation from a time at which rough-rolling is iinished to
a time at which finish-rolling starts in terms of seconds, "hl" represents a thickness of
the slab before the rough-rolling, and "h2" represents a thickness of the slab after the
rough-rolling.
2 < (115) x (hllh2) x (1110) x (tl+t2)03< 6: Expression 3
[0047]
When the hot-rolling is performed under the condition satisfying Expression 3,
a band where a laminated concentration difference of alloy elements such as C and Mn
is generated is divided. As a result, in the steel sheet after annealing, a bias of the C
concentration of the retained austenite is eliminated. As described above, it is lcnown
that, when the C concentration octhe retained austenite is not uniform and varies, the
ductility is poor. When the hot-rolling is performed such that Expression 3 is
established, the C concentration of the retained austenite is uniform, and a steel sheet
having superior ductility can be obtained.
This Expression 3 is experimentally obtained to evaluate a relation between
the sheet thickness before and after the rolling and the time from the rough-rolling to
the finish-rolling; and a rolling load during the finish-rolling and the division
(fragmentation) of pearlite caused by the rolling load.
[0048]
Following the rough-rolling, the finish-rolling of the hot-rolling is performed
at a finishing temperature of, preferably, Ar3 to 970°C. When the finishing
temperature is lower than Ar3, (a*) dual-phase rolling is performed, which may cause
a decreasc in ductility. In addition:xyhen the finishing temperature is higher than
97OoC, the austenite grain size is large, and the fraction of ferrite decreases. As a
result, the ductility may decrease.
[0049]
(Coiling S3)
After the hot-rolling, the steel is cooled at an average cooling rate of,
preferably, 20 "Clsec or faster and is coiled at a predetermined coiling temperature C%
When the average cooling rate is slower than 20 "Clsec, pearlite which causes a
decrease in ductility may be excessively formed.
[0050]
(Pickling S4 and Cold-Rolling S5)
After [he coiling, the steel sheet is picltled. After the pickling, in cold-rolling,
the steel sheet is cold-rolled with a cold-rolling mill having multiple stands. When
the steel sheet is cold-rolled under a condition satisfying the following Expression 4 as
shown in FIG. 3, a steel sheet which satisfies the above-described Expression 1 can be
obtained. The steel sheet satisfying Expression 1 leads to not only the steel sheet
before hot-stamping satisfying TSxE1>20000 MPa% but also the steel sheet even after
hot-stamping satisfying TSxE1>20000 MPa%. Here, "ri" represents an individual
target cold-rolling reduction of ith stand (i = 1, 2, 3......) from a first stand along the
plural stands in the cold-rolling, and "r" represents a total target cold-rolling reduction
(%) in the cold-rolling. The total rolling reduction refers to percentage of a
cumulative rolling reduction amount (a difference between an inlet sheet thickness
before an initial pass and an outlet sheet thickness after a final pass) to an inlet sheet
thickness of the first stand and may also be referred to as a cumulative rolling
reduction. ?
1.5 x (rllr) + 1.2 x (r2lr) + (r3lr) > 1: Expression 4
[005 11
When the cold-rolling is performed under the condition satisfying Expression
4, even if large pearlite is present in the steel sheet before the cold-rolling, pearlite can
be sufficiently divided (fragmented) in the cold-rolling. As a result, by the annealing
after the cold-rolling, pearlite can be eliminated, or the amount of pearlite can be
suppressed to the minimum. Therefore, a structure which satisfies Expressions 1 and
2 can be easily obtained. On the other hand, when Expression 4 is not established, a
cold-rolling reduction in an upstream stand is insufficient, coarse pearlite is likely to
remain, and the desired retained austenite cannot be formed in subsequent annealing.
In addition, the present inventors have found that, by satisfying Expression 4, the form
of the obtained retained austenite structure after annealing can be maintained to be the
same even after hot-stamping, and the ductility is superior even alter hot-stamping.
When the steel sheet according to the embodiment is heated until the dual-phase in hotstamping,
the retained austenite and the hard phase before the hot-stamping are
transformed into an austenite structure, and feirite before the hot-stamping is
maintained as it is. C (carbon) in austenite does not move to peripheral ferrite. Next,
after cooling, austenite is transformed into the hard phase including martensite. That
is, when Expression 4 is satisfied and the above-described H2/H1 is in the
predetermined range, this state is maintained as it is even after hot-stamping, and thus
formability afier hot-stamping is superior.
[0052]
As described, r, rl, r2, and r3 are target cold-rolling reductions, and typically
the cold-rolling is performed with controlling an actual cold-rolling reduction to be the
same as the target cold-rolling redudtipn. It is not preferable that the cold-rolling is
performed with a difference between the target cold-rolling reduction and the actual
cold-rolling reduction. Accordingly, when the actual cold-rolling reduction satisfies
the above-described Expression 4, it should be seen that the invention is embodied:
The actual cold-rolling reduction is preferably within *lo% of the target cold-rolling
reduction.
[0053]
(Annealing S6 and Temper-Rolling S7)
After the cold-rolling, the steel sheet is annealed to be recrystallized. By this
annealing, desired martensite is formed. An annealing temperature is preferably in a
range of 700°C to 850°C. By annealing the steel sheet in this range, ferrite and
martensite have a desired area ratio, which can contribute to the improvement of
TSxEl. In the subsequent temper-rolling, the steel sheet is temper-rolled using a
known technique.
[0054]
Further, in order to improve corrosion resistance, the manufacturing method
according to the embodiment may include a hot-dip galvanizing, in which the hot-dip
galvanized plating is formed on the surface of the cold-rolled steel sheet, between the
annealing and the temper-rolling. Further, the manufacturing method may include a
galvannealing after the hot-dip galvanized plating, in which the cold-rolled steel sheet
is galvannealed. When the steel sheet is galvannealed, a treatment of increasing the
thickness of an oxide film on the plating surface may be further performed, in which
the galvannealed plating surface is brought into contact with a material, such as water
vapor, for oxidizing the plating surfacc.
[0055]
In addition to the hot-dip ghlyanizing and the galvannealing, the
manufacturing method according to the embodiment may include, for example, an
electrogalvanizing after the temper-rolling, in which the electrogalvanized plating is
formed on the surface of the cold-rolled steel sheet. In addition, instead of the hot-dip
galvanizing, the manufacturing method according to the embodiment may include an
aluminizing between the annealing and the temper-rolling, in which the aluminum
plating is formed on the surface of the cold-rolled steel sheet. As a method for
aluminizing, a hot-dip aluminizing is typically used and is preferable.
[0056]
By hot-stamping the steel sheet after such a series of processing, a hot-stamp
formed body (steel sheet after hot-stamping) can be obtained. The hot-stamping is
preferably performed, for example, under the following conditions. First, the steel
sheet is heated to 700°C or higher at a temperature increase rate of 5 "Clsec or faster.
In order to improve the formability of the steel sheet, the heating temperature is
preferably 1000°C or lower and particularly preferably ACJ point or lower. After the
heating is finished, hot-stamping is performed after a predetermined holding time.
Next, the steel sheet is cooled to be in a range of 300°C to 500°C at a cooling rate of
50 "Cisec or faster. Next, the temperature of the steel sheet is held in a range of
300°C to 500°C for 100 seconds or longer (quenching in the hot-stamping).
When the heating temperature before the hot-stamping is lower than 700°C,
the strength of the steel sheet may not he secured due to insufficient quenching. On
the other hand, when the heating temperature is higher than 100O0C, the steel sheet
may be excessively softened. In addition, when a plating is formed on the surface of
the steel sheet, particularly, when a galvanized plating is formed thereon, it is not
preferable that ihe heating temperature be higher than 1000°C because zinc may be
evaporated and disappears. Accorhli~glyt,h e heating temperature of the hot-stamping
is preferably 700°C or higher and 1000°C or lower. In order to further suppress
undesirable changes of the structure, it is particularly preferable that the upper limit of
the heating temperature be set to ACJ point. Typically, the ACJ point is calculated
from the following Expression 5.
Ac3=937.2-436.5x[C]+56x[Si]-19.7x[Mn]-16.3x[Cu]-26.6x[Ni]-
4.9x[Cr]+38.1x[Mo]+124.8x[V]+136.3x[Ti]-19.1x[Nb]+198.4x[A1]+3315x[B]:
Expression 5
[Cl, [Sil, [Mnl, [Cul, [Nil, [Crl, [Mol, [Vl, [Ti], [Nbl, [All, and [Bl indicate
the contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, and B in the steel, respectively,
in terms of mass%.
When the temperature increase rate before the hot-stamping is slower than
5 "Clsec, it is difficult to control the tcmperature increase rate, and the productivity
significantly decreases. Therefore, it is preferable that the heating is performed at a
temperature increase rate of 5 "Clsec or faster. When the cooling rate is slower than
50 "Clsec, it is difficult to control the cooling rate, and the productivity significantly
decreases. Therefore, it is preferable that the cooling is performed at a cooling rate of
50 "Clsec or faster. The upper limits of the temperature increase rate and the cooling
rate are not particularly defined. In a range of the typical capacity of the existing
facility, the upper limits of both the temperature increase rate and the cooling rate are
about 500 "Clsec. However, in the future, the development of a facility where heating
andlor cooling can be performed at a high speed is estimated. In this case, heating
andlor cooling may be performed at a rate of 500 "Clsec or faster.
The reason why the temperature of the steel sheet is held in a range of 300°C
to 500°C for 100 seconds or longer is $hat austenite is made to remain even after
cooling to normal temperature by promoting bainite transformation to concentrate C in
untransformed austenite.
LO0571
As a result, by satisfying the above-described conditions, a high-strength steel
sheet can be manufactured in which the hardness distribution and the structure can be
maintained before and after hot-stamping, the strength can be secured, and superior
ductility can be obtained.
[Examples]
[005S]
Steels having various components shown in Table 1 were continuously cast
into slabs at a casting speed of 1.0 mlmin to 2.5 mlmin. These slabs after casting
were directly heated in a furnace using an ordinary method or were temporarily cooled
and then heated in a furnace using an ordinary method. In Table 2, Symbol "-"
represents that the corresponding element is not intentionally included in the
corresponding steel. These slabs were hot-rolled under conditions shown in Table 3
to obtain hot-rolled steel sheets, and the hot-rolled steel sheets were coiled. Next, the
hot-rolled steel sheets were pickled to remove scale from the surface of the steel sheet,
and the thicknesses of the hot-rolled steel sheets were reduced by cold-rolling, thereby
obtaining cold-rolled stccl sheets having a thickness of 1.2 mm to 1.4 mm. Further,
these cold-rolled steel sheets were annealed in a continuous annealing furnace at an
annealing temperature shown in Table 4. On some of the cold-rolled steel sheets after
the annealing, a hot-dip galvanized plating was formed during the cooling after
soaking in the continuous annealing furnace. Further, some of the steel sheets on
which the hot-dip galvanized plating was formed wcre galvannealed to form a
galvannealed plating thereon. In addition, on some of the cold-rolled steel sheets
after the annealing on which a hot-dip galvanized plating was not formed, an
aluminum plating was formed. The steel sheets (both of the plated steel sheets and
the non-plated steel shecls) obtained as above were temper-rolled. On sonlc of the
temper-rolled and non-plated steel sheets, an electrogalvanized plating was formed.
Samples for evaluating material properties were collected from the various
steel sheets obtained as above, and a material test, the hardness measurement of the
retained austenite, and the measurement of the content of the structure were performed.
As a result, regarding the steel sheets before hot-stamping, the tensile strength (TS),
the ductility (El), the content of each structure, the hardness distribution of the retained
austenite (the ratio H2IH1 of the average hardness of the thickness center portion to the
average hardness of the thickness surface portion, and the hardness distribution OHM
of the thickness center portion) were obtained. Next, in ordcr to obtain a hot-stamp
formed body having the form shown in FIG. 4, hot-stamping was performed in which
each of the various steel sheets was heated at a temperature increase rate of 10 "Clsec,
was held at a heating temperature of 780°C for 10 seconds, was hot-stamped, was
cooled to 300°C to 500°C at a cooling rate of 100 "Clsec, and was held at the
temperature for 100 seconds or longer. From the formed body obtained as above, a
sample was cut out at a position shown in FIG. 4 to perform the material test, the
hardness measurement of the retained austenite, and the measurement of the content of
the structure. As a result, regarding the steel sheets after hot-stamping (hot-stamp
formed bodies), the tensile strength (TS), the ductility (El), the content of each
structure, the hardness distribution ofthe retained austenite (the ratio H2lH1 of the
average hardness of the thickness center portion to the average hardness of the
thiclcness surface portion, and the hardness distribution OHM of the thickness center
portion) were obtained. The results ?re shown in Tables 5 to 8. In "Type of Plating"
shown in Tables 5 and 6, GI represents that a hot-dip galvanized plating was formed,
GA represents that a galvannealed plating was formed, EG represents that an
electrogalvanized plating was formed, and A1 represents that an aluminum plating was
formed. CR represents a non-plated steel sheet, that is, a cold-rolled steel sheet. "In
Range" and "Out of Range" which are shown in the tables for the determination of
numerical values (determination of pass-fail) indicate that tne numerical value were in
the range and were out of the range defined in the invention, respectively.
[0059]
[Table I]
[0060]
[Table 21
[0061]
[Table 31
[0062]
[Table 41
[0063]
[Table 51
[0064]
[Table 61
[0065]
[Table 71
[0066]
[Table 81
[0067]
It can be seen from the aboVe,examples that, when the requirements of the
invention are satisfied, a high-strength cold-rolled steel sheet, a high-strength hot-dip
galvanized cold-rolled steel sheet, a high-strength galvannealed cold-rolled steel sheet,
an electrogalvanized cold-rolled steel sheet, or an aluminized cold-rolled steel sheet for
hot-stamping which is superior can be obtained in which TSxEL120000 MPa% is
satisfied before and after hot-stamping.
[Brief Description of the Reference Symbols]
[0068]
S1: Casting
S2: Hot-Rolling
S3: Coiling
S4: Pickling
S5: Cold-Rolling
S6: Annealing
S7: Temper-Rolling
A 11
SYMBOL "-" REPRESENTS THAT THE ELEMENT THEREOF IS NOT INTENTIONALLY IN1
-2-2
. ~
UDED.
SYMBOL "-" REPRESENTS THAT THE ELEMENT THEREOF IS NOT INTENTIONALLY INCLUDED.

DETERMINATIO DETERMINATION
(TABLE 81
[Document TypeICLAIMS
1. A cold-rolled steel sheet having a chemical composition comprising, in terms
of mass%:
C: 0.1% to 0.3%;
Si: 0.01% to 2.0%:
Mn: 1.5% to 2.5%;
N: 0.0005% to 0.01%;
Al: 0.01% to 0.05%;
B: 0% to 0.002%;
Mo: 0% to 0.5%;
Cr: 0% to 0.5%;
V: 0% to 0.1%; b," ,
Ti: 0% to 0.1%;
Nb: 0% to 0.05%;
Ni: 0% to 1.0%;
Cu: 0% to 1.0%;
Ca: 0% to 0.005%;
REM: 0% to 0.005%; and
rest including Fe and impurities, wherein
a structure before and after a hot-stamping includes
a ferrite: 30 area% to 90 area%,
a martensite: 0 area% or more and less than 20 area%,
a pearlite: 0 area% to 10 area%,
a retained austenite: 5 volume% to 20 volume%, and
a rest structure: a bainite,
a hardness of the retained austenite measured with a nano indenter before and
after the hot-stamping satisfies Expression 1 and Expression 2,
a tensile strength and a ductility satisfy a relation of TSxE1>20000MPa.%,
H2 I H1 < 1.1: Expression 1,
OHM < 20: Expression 2, and
the "HI" represents the hardness of the retained austenite existing wilhin a
thickness surface portion before and after the hot-stamping, the thiclcness surface
portion being an area within 200 pm in the thickness direction of a surface of the coldrolled
steel sheet, the "HY represents the hardness of the retained austenite existing
within a thiclmess center portion before and after the hot-stamping, the thickness
center portion being an area within + 100pm along the thickness direction of a center
plane of the cold-rolled steel sheet ih the thickness direction, the "OHM" represents a
variance of the hardness of the retained austenite within the thiclcness center portion
before or after the hot-stamping, "TS" represents the tensile strength of the cold-rollcd
steel sheet in terms of MPa, and "El" represents the ductility of the cold-rolled steel
sheet in terms of %.
2. The cold-rolled steel sheet according to claim 1, wherein
the chemical composition includes one or more elements selected from a
group consisting of, in terms of mass%:
B: 0.0005% to 0.002%;
Mo: 0.01% to 0.5%;
Cr: 0.01% to 0.5%;
V: 0.001% to 0.1%;
Ti: 0.001% to 0.1%;
Nb: 0.001% to 0.05%;
Ni: 0.01% to 1.0%;
Cu: 0.01% to 1.0%;
Ca: 0.0005% to 0.005%; and
3. The cold-rolled steel sheet according to claim 1 or 2, wherein
a hot-dip galvanized plating is formed on the surface of the cold-rolled steel
sheet.
4. The cold-rolled steel sheet according to claim 1 or 2, wherein
a galvannealed plating is fortped on the surface of the cold-rolled steel sheet.
5. The cold-rolled steel shect according to claim 1 or 2, wherein
an electrogalvanized plating is formed on the surface of the cold-rollcd steel
sheet.
6. The cold-rolled steel sheet according to claim 1 or 2, wherein
an aluminum plating is formed on the surface of the cold-rolled steel sheet.
7. A method for manufacturing the cold-rolled steel sheet according to claim 1 or
2, comprising:
casting a molten steel having the chemical composition into a steel;
hot-rolling after the casting, in which heating is performed on the steel in a
furnace, and then rough-rolling and finish-rolling are performed on the steel under a
condition satisfying Expression 3;
coiling the steel after the hot-rolling;
pickling the steel after the coiling;
cold-rolling the steel after the pickling with a cold-rolling mill having a
plurality of stands under a condition satisfying Expression 4;
annealing after the cold-rolling, in which annealing is performed on the steel
at 700°C to 850°C, and then the steel is cooled;
temper-rolling the steel after the annealing, wherein
2 < (115) x (hllh2) x (1110) x (tl+t21'.~< 6: Expression 3;
1.5 x (rllr) + 1.2 x (r2lr) + (r3lr) > 1 : Expression 4, and
"hl" represents a thickness of the steel before the rough-rolling in terms of
mm, "h2" represents the thickness &the steel after the rough-rolling in terms of mm,
"tl" represents a dulation from a time at which the steel exits the furnace to a time at
which the rough-rolling of the steel starts in terms of seconds, "t2" represents a
dulation from a time at which the rough-rolling is finished to a time at which ihe
finish-rolling starts in terms of seconds, and "ri" represents an individual target coldrolling
reduction of an ith stand (i = 1, 2, 3......) from a first stand along the plurality of
the stands in terms of %, and "r" represents a total target cold-rolling reduction in the
cold-rolling in terms of %.
8. The method according to claim 7 for manufacturing the cold-rolled steel sheet
according to claim 3, further comprising
-"' -
hot-dip galvanizing between the annealing and the temper-rolling, in which
the hot-dip galvanized plating is formed on the steel.
9. The method according to claim 8 for manufacturing the cold-rolled steel sheet
according to claim 4, further comprising
galvannealing between the hot-dip galvanizing and the temper-rolling, in
which the steel is galvannealed.
10. The method according to claim 7 for manufacturing the cold-rolled steel sheet
according to claim 5, further comprising
electrogalvanizing after the temper-rolling, in which the electrogalvanized
plating is formed on the steel.
11. The method according to Claim 7 for manufacturing the cold-rolled steel sheet " according to claim 6, further comprising
aluminizing between the annealing and the temper-rolling, in which the
aluminum plating is formed on the steel.
12. A hot-stamp formed body obtained by using the cold-rolled steel sheet
according to any one of claims 1 to 6.