Specification
Title of Invention
HOT-STAMPED STEEL, COLD-ROLLED STEEL SHEET AND METHOD FOR
5 PRODUCING HOT-STAMPED STEEL
Technical Field of the Invention
[0001]
The present invention relates to a hot-stamped steel having an excellent
10 formability (hole expansibility), an excellent chemical conversion treatment property, and
an excellent plating adhesion after hot stamping, a cold-rolled steel sheet which is used as
a material for the hot-stamped steel, and a method for producing a hot-stamped steel
sheet.
Priority is claimed on Japanese Patent Application No. 2013-076835, filed April
15 2, 2013, the content of which is incorporated herein by reference.
Related Art
[0002]
At the moment, a steel sheet for a vehicle is required to be improved in terms of
20 collision safety and to have a reduced weight. In such a situation, hot stamping (also
called hot pressing, hot stamping, diequenching, press quenching or the like) is drawing
attention as a method for obtaining a high strength. The hot stamping refers to a
forming method in which a steel sheet is heated at a high temperature of, for example,
700°C or more, then hot-formed so as to improve the formability of the steel sheet, and
25 quenched by cooling after forming, thereby obtaining desired material qualities. As
2
described above, a steel sheet used for a body structure of a vehicle is required to have a
high press workability and a high strength. A steel sheet having a ferrite and martensite
structure, a steel sheet having a ferrite and bainite structure, a steel sheet containing
retained austenite in a structure or the like is known as a steel sheet having both press
5 workability and high strength. Among these steel sheets, a multi-phase steel sheet
having martensite dispersed in a ferrite base has a low yield ratio and a high tensile
strength, and furthermore, has excellent elongation characteristics. However, the
multi-phase steel sheet has a poor hole expansibility since stress concentrates at the
interface between the ferrite and the martensite, and cracking is likely to initiate from the
1 0 interface.
15
[0003]
For example, Patent Documents 1 to 3 disclose the multi-phase steel sheet. In
addition, Patent Documents 4 to 6 describe relationships between the hardness and
formability of a steel sheet.
[0004]
However, even with these techniques of the related art, it is difficult to obtain a
steel sheet which satisfies the current requirements for a vehicle such as an additional
reduction of the weight and more complicated shapes of a components. Various types
of strength can be improved by adding elements such as Si and Mn as well as by
20 changing the microstructure. However, when the amount of Si exceeds a constant
an1ount as described below by adding Si, elongation or hole expansibility of steel may
degrade. Furthermore, when the amount ofSi or the amount ofMn increases, that
chemical conversion treatment property or plating adhesion after hot stamping may
degrade, which is not preferable.
25
Prior Art Document
Patent Document
[0005]
3
[Patent Document 1] Japanese Unexamined PatentApp1ication, First
5 Publication No. H6-128688
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2000-319756
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2005-120436
10 [Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2005-256141
[Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. 2001-355044
[Patent Document 6] Japanese Unexamined Patent Application, First
15 Publication No. Hll-189842
20
25
Disclosure of the Invention
Problems to be Solved by the Invention
[0006]
An object of the present invention is to provide a cold-rolled steel sheet capable
of ensuring a strength and having a more favorable hole expansibility, an excellent
chemical conversion treatment property, and an excellent plating adhesion when
produced into a hot-stamped steel, a hot-stamped steel, and a method for producing the
same hot -stamped steel.
4
Means for Solving the Problem
[0007]
The present inventors carried out intensive studies regarding a cold-rolled steel
sheet for hot stamping that ensured a strength after hot stamping (after quenching in a hot
5 stamping), had an excellent formability (hole expansibility), and had an excellent
chemical conversion treatment property and an excellent plating adhesion after hot
stamping. As a result, it was found that, when an appropriate relationship is established
among the amount of Si, the amount of Mn and the amount of C, a fraction of a ferrite
and a fraction of a martensite in the steel sheet are set to predetermined fractions, and the
10 hardness ratio (difference of a hardness) of the martensite between a surface portion of a
sheet thickness and a central portion of the sheet thickness and the hardness distribution
of the martensite in the central portion of the sheet thickness are set in specific ranges, it
is possible to industrially produce a cold-rolled steel sheet for hot stamping capable of
ensuring a formability, that is, a characteristic ofTS x .A :C:50000 MPa·% that is a larger
15 value than ever in terms ofTS x A that is a product of a tensile strength TS and a hole
expansion ratio A. Furthermore, it was found that, when this cold-rolled steel sheet is
used for hot stamping, a hot-stamped steel having an excellent hole expansibility even
after the hot stamping is obtained. In addition, it was also clarified that the limitation of
segregation ofMnS in the central portion of the sheet thickness of the cold-rolled steel
20 sheet for hot stamping is also effective in improving the hole expansibility of the
hot-stamped steel. In particular, it was found that, when the amount ofMn which is a
main element for improving hardenability is reduced and the fraction or hardness of
martensite decreases, hole expandability is maximized by the limitation of segregation of
MnS and chemical conversion treatment property and plating adhesion are excellent after
25 hot stamping. In addition, it was also found that, in cold-rolling, an adjustment of a
5
fraction of a cold-rolling reduction to a total cold-rolling reduction (cumulative rolling
reduction) from an uppermost stand to a third stand based on the uppermost stand within
a specific range is effective in controlling a hardness of the martensite. Furthermore,
the inventors have found a variety of aspects of the present invention as described below.
5 In addition, it was found that the effects are not impaired even when a hot-dip galvanized
layer, a galvannealed layer, an electro galvanized layer and an aluminized layer are
formed on the cold-rolled steel sheet.
[0008]
(1) That is, according to a first aspect of the present invention, a hot-stamped
10 steel includes, by mass%, C: 0.030% to 0.150%, Si: 0.010% to 1.000%, Mn: 0.50% or
more and less than 1.50%, P: 0.001% to 0.060%, S: 0.001% to 0.010%, N: 0.0005% to
0.0100%, AI: 0.010% to 0.050%, and optionally at least one ofB: 0.0005% to 0.0020%,
Mo: 0.01% to 0.50%, Cr: 0.01% to 0.50%, V: 0.001% to 0.100%, Ti: 0.001% to 0.100%,
Nb: 0.001% to 0.050%, Ni: 0.01% to 1.00%, Cu: 0.01% to 1.00%, Ca: 0.0005% to
15 0.0050%, REM: 0.00050% to 0.0050%, and a balance of Fe and impurities, in which,
when [C] is the amount ofC by mass%, [Si] is the amount ofSi by mass%, and [Mn] is
the amount ofMn by mass%, the following expression (A) is satisfied, the area fraction
of a ferrite is 40% to 95% and the area fraction of a martensite is 5% to 60%, the total of
the area fraction of the ferrite and the area fraction of the martensite is 60% or more, the
20 hot-stamped steel optionally further includes one or more of a pearlite, a retained
austenite, and a bainite, the area fraction of the pearlite is 10% or less, the volume
fraction of the retained austenite is 5% or less, and the area fraction of the bainite is less
than 40%, the hardness of the martensite measured with a nanoindenter satisfies the
following expression (B) and the following expression (C), TS x A which is a product of
25 a tensile strength TS and a hole expansion ratio A is 50000 MPa·% or more,
6
(5 x [Si] + [Mn]) I [C] > 10 (A),
H2/ HI< 1.10 (B),
aHM < 20 (C), and
the H 1 is the average hardness of the martensite in a surface portion of a sheet
5 thickness of the hot-stamped steel, the surface portion is an area having a width of200
rtm in a thickness direction from an outermost layer, the H2 is the average hardness of the
martensite in a central portion of the sheet thickness of the hot-stamped steel, the central
portion is an area having a width of 200 rtm in the thickness direction at a center of the
sheet thiclmess, and the a HM is the variance of the average hardness of the martensite in
10 the central portion of the sheet thickness of the hot-stamped steel.
15
[0009]
(2) In the hot-stamped steel according to the above (1 ), the area fraction of MnS
existing in the hot-stamped steel and having an equivalent circle diameter ofO.lrtm to 10
[till may be 0.01% or less, and the following expression (D) may be satisfied,
n2/ nl < 1.5 (D), and
the nl is an average number density per 10000 rtm2 of the MnS having an
equivalent circle dian1eter of 0.1 rtm to 10 rtm in a 1/4 portion of the sheet thickness of
the hot-stamped steel, and the n2 is the average number density per 10000 rtm2 of the
MnS having an equivalent circle diameter ofO.lrtm to 10 rtm in the central portion of the
20 sheet thickness of the hot -stamped steel.
25
[0010]
(3) In the hot-stamped steel according to the above(!) or (2), a hot-dip
galvanized layer may be formed on a surface thereof.
[0011]
(4) In the hot-stamped steel according to the above (3), the hot-dip galvanized
5
layer may be alloyed.
[0012]
7
(5) In the hot-stamped steel according to the above (1) or (2), an
electro galvanized layer may be formed on a surface thereof.
[0013]
(6) In the hot-stamped steel according to the above (1) or (2), an aluminized
layer may be formed on a surface thereof.
[0014]
(7) According to another aspect of the present invention, there is provided a
10 method for producing a hot-stamped steel including casting a molten steel having a
chemical composition according to the above (1) and obtaining a steel, heating the steel,
hot-rolling the steel with a hot-rolling mill including a plurality of siands, coiling the
steel after the hot-rolling, pickling the steel after the coiling, cold-rolling the steel with a
cold-rolling mill including a plurality of stands after the pickling under a condition
15 satisfying the following expression (E), annealing in which the steel is annealed under
700°C to 850°C after the cold-rolling and is cooled, temper-rolling the steel after the
annealing, and hot stamping in which the steel is heated to a temperature range of 700°C
to 1 000°C after the temper-rolling, is hot stamped within the temperature range, and
thereafter is cooled to a room temperature or more and 300°C or less,
20
25
1.5 x r1 I r + 1.2 x r2 I r +r3 I r > 1.00 (E), and
the ri (i = 1, 2, 3) is an individual target cold-rolling reduction at an ith stand (i =
1, 2, 3) based on an uppermost stand in the plurality of stands in the cold-rolling in
unit %, and the r is the total cold-rolling reduction in the cold-rolling in unit%.
[0015]
(8) In the method for producing the hot-stamped steel according to the above (7),
8
the cold-rolling may be carried out under a condition satisfYing the following expression
(E'),
1.20?: 1.5 x r1 I r + 1.2 x r2 I r +r3 I r > 1.00 (E'), and
the ri (i = 1, 2, 3) is the individual target cold-rolling reduction at the ith stand (i
5 = 1, 2, 3) based on the uppermost stand in the plurality of stands in the cold-rolling in
unit %, and the r is the total cold-rolling reduction in the cold-rolling in unit%.
10
[0016]
(9) In the method for producing the hot -stamped steel according to the above (7)
or (8),
when CT is a coiling temperature in the coiling in unit °C, [C] is the amount of
C in the steel by mass%, [Mn] is the amount ofMn in the steel by mass%, [Si] is the
amount of Si in the steel by mass%, and [Mo] is the amount of Mo in the steel by mass%,
the following expression (F) may be satisfied,
560-474 x [C]- 90 x [Mn]-20 x [Cr]-20 x [Mo] 1500 (G).
[0018]
(11) The method for producing the hot-stamped steel according to any one of the
25 above (7) to (1 0) may further include galvanizing the steel between the annealing and the
5
9
temper-rolling.
[0019]
(12) The method for producing the hot-stamped steel according to the above (11)
may further include alloying the steel between the galvanizing and the temper-rolling.
[0020]
(13) The method for producing the hot-stamped steel according to any one of the
above (7) to (1 0) may further include electrogalvanizing the steel after the temper-rolling.
[0021]
(14) The method for producing the hot-stamped steel according to any one of the
10 above (7) to (1 0) may further include aluminizing the steel between the annealing and the
temper-rolling.
[0022]
(15) According to another aspect of the present invention, a cold-rolled steel
sheet includes, by mass%, C: 0.030% to 0.150%; Si: 0.010% to 1.000%; Mn: 0.50% or
15 more and less than 1.50%; P: 0.001% to 0.060%; S: 0.001% to 0.010%; N: 0.0005% to
0.0100%; AI: 0.010% to 0.050%, and optionally at least one ofB: 0.0005% to 0.0020%;
Mo: 0.01% to 0.50%; Cr: 0.01% to 0.50%; V: 0.001% to 0.100%; Ti: 0.001%, to 0.100%;
Nb: 0.001% to 0.050%; Ni: 0.01% to 1.00%; Cu: 0.01% to 1.00%; Ca: 0.0005% to
0.0050%; REM: 0.0005% to 0.0050%, and a balance of Fe and unavoidable impurities,
20 in which, when [C] is the amount of C by mass%, [Si] is the amount of Si by mass%, and
[Mn] is the amount of Mn by mass%, the following expression (A) is satisfied, the area
fraction of a ferrite is 40% to 95% and the area fraction of a martensite is 5% to 60%, the
total of the area fraction of the ferrite and the area fraction of the martensite is 60% or
more, the cold-rolled steel sheet optionally further includes one or more of a pearlite, a
25 retained austenite, and a bainite, the area fraction of the pearlite is 1 0% or less, the
5
10
volume fraction of the retained austenite is 5% or less, and the area fraction of the bainite
is less than 40%, the hardness of the martensite measured with a nanoindenter satisfies
the following expression (H) and the following expression (I), TS x A which is a product
ofthe tensile strength TS and the hole expansion ratio A is 50000 MPa·% or more,
(5 x [Si] + [Mn]) I [C] > 10 (A),
H20 I HlO < 1.10 (H),
o"HMO <20 (I), and
the Hl 0 is the average hardness of the martensite in a surface portion of a sheet
thiclmess, the surface portion is an area having a width of 200 !lm in a thickness direction
1 0 from an outermost layer, the H20 is the average hardness of the martensite in a central
portion of the sheet thickness, the central portion is an area having a width of200 !lm in
the thickness direction at a center of the sheet thickness, and the a HMO is the variance of
the average hardness of the martensite in the central portion of the sheet thickness.
15
20
25
[0023]
(16) In the cold-rolled steel sheet according to the above (15), the area fraction
ofMnS existing in the cold-rolled steel sheet and having an equivalent circle diameter of
0.1 flm to 10 f1mmay be: 0.01% or less,
the following expression (J) is satisfied,
n20 I n1 0 < 1.5 (J), and
the n10 is an average number density per 10000 f1m2 of the MnS having an
equivalent circle diameter of 0.1 flill to 10 flm in a 114 portion of the sheet thickness, and
the n20 is an average number density per 10000 f1m2 of the MnS having an equivalent
circle diameter of 0.1 flm to 1 0 !lm in the central portion of the sheet thiclmess.
[0024]
(1 7) In the cold-rolled steel sheet according to the above (15) or (16), a hot-dip
5
11
galvanized layer may be formed on a surface thereof.
[0025]
(18) In the cold-rolled steel sheet according to the above (17), the hot-dip
galvanized layer may be alloyed.
[0026]
(19) In the cold-rolled steel sheet according to the above (15) or (16), an
electrogalvanized layer may be formed on a surface thereof.
[0027]
(20) In the cold-rolled steel sheet according to the above (15) or (16), an
10 aluminized layer may be formed on a surface thereof.
Effects of the Invention
[0028]
According to the above-described aspect of the present invention, since an
15 appropriate relationship is established among the amount of C, the amount of Mn and the
amount of Si, and the hardness of the martensite measured with a nanoindenter is set to
an appropriate value in the cold-rolled steel sheet before hot stamping and hot-stamped
steel after hot stamping, it is possible to obtain a more favorable hole expansibility in the
hot-stamped steel and chemical conversion treatment property and plating adhesion are
20 favorable even after hot stamping.
Brief Description of the Drawings
[0029]
FIG. 1 is a graph showing the relationship between (5 x [Si] + [Mn]) I [C] and
25 TS x 'A in a cold-rolled steel sheet for hot stamping before quenching in the hot stamping
12
and a hot -stamped steel.
FIG. 2A is a graph showing the foundation of an expression (B) and is a graph
showing the relationship between an H20 I HI 0 and a a HMO in the cold-rolled steel sheet
for hot stamping before quenching in the hot stamping and the relationship between H2 I
5 HI and aHM in the hot-stamped steel.
10
FIG. 2B is a graph showing the foundation of an expression (C) and is a graph
showing the relationship between aHMO and TS x A in the cold-rolled steel sheet for hot
stamping before quenching in the hot stamping and the relationship between aHM and
TS x A in the hot -stamped steel.
FIG. 3 is a graph showing the relationship between n20 In 10 and TS x A in the
cold-rolled steel sheet for hot stamping before quenching in the hot stamping and the
relationship between n21 nl and TS x A in the hot-stamped steel and showing the
foundation of an expression (D).
FIG. 4 is a graph showing the relationship between 1.5 x rll r + 1.2 x r21 r + r3
15 I r and H20 I H 1 0 in the cold-rolled steel sheet for hot stamping before quenching in the
hot stamping and the relationship between 1.5 x r1 I r + I .2 x r2 I r + r3 I rand H2 I Hl
in the hot-stamped steel, and showing the foundation of an expression (E).
20
25
FIG. SA is a graph showing the relationship between an expression (F) and a
fraction of a martensite.
FIG. 5B is a graph showing the relationship between the expression (F) and a
fraction of a pearlite.
FIG. 6 is a graph showing the relationship between T x ln(t) I (1.7 x [Mn] + [S])
and TS x A, and showing the foundation of an expression (G).
FIG. 7 is a perspective view of a hot-stamped steel used in an example.
FIG. 8 is a flowchart showing a method for producing the hot-stamped steel for
5
13
which a cold-rolled steel sheet for hot stamping is used according to an embodiment of
the present invention.
Embodiments of the Invention
[0030]
As described above, it is important to establish an appropriate relationship
among the amount of Si, the amount ofMn and the amount of C and provide an
appropriate hardness to martensite in a predetermined position in a hot-stamped steel (or
a cold-rolled steel sheet) in order to improve hole expansibility of the hot-stamped steel.
10 Thus far, there have been no studies regarding the relationship between the hole
expansibility or the hardness of the martensite in a hot-stamped steel.
[0031]
Herein, reasons for limiting a chemical composition of a hot-stamped steel
according to an embodiment of the present invention (in some cases, also referred to as a
15 hot-stamped steel according to the present embodiment) and steel used for manufacture
thereof will be described. Hereinafter, "%" that is the units of the amount of an
individual component indicates "mass%".
20
[0032]
C: 0.030% to 0.150%
C is an important element to strengthen the martensite and increase the strength
of the steel. When the amount of Cis less than 0.030%, it is not possible to sufficiently
increase the strength of the steel. On the other hand, when the amount of C exceeds
0.150%, degradation of the ductility (elongation) of the steel becomes significant.
Therefore, the range of the amount ofC is set to 0.030% to 0.150%. In a case in which
25 there is a demand for high hole expansibility, the amount of C is desirably set to 0.100%
14
or less.
[0033]
Si: 0.010% to 1.000%
Si is an important element for suppressing a formation of harmful carbide and
5 obtaining a multi-phase structure mainly including a ferrite structure and a balance of the
matiensite. However, in a case in which the atnount of Si exceeds 1. 000%, the
elongation or hole expansibility of the steel degrades, and a chemical conversion
treatment property or plating adhesion after hot statnping also degrades. Therefore, the
amount of Si is set to 1.000% or less. In addition, while Si is added for deoxidation, a
10 deoxidation effect is not sufficient when the atnount of Si is less than 0.010%.
Therefore, the amount of Si is set to 0.010% or more.
[0034]
AI: 0.010% to 0.050%
AI is an important element as a deoxidizing agent. To obtain the deoxidation
15 effect, the atnount of AI is set to 0.010% or more. On the other hand, even when AI is
excessively added, the above-described effect is saturated, and conversely, the steel
becomes brittle. Therefore, the amount of AI is set to be in a range of 0. 0 I 0% to
0.050%.
20
[0035]
Mn: 0.50% or more and less than 1.50%
Mn is an important element for increasing a hardenability of the steel and
strengthening the steel. However, when the amount ofMn is less than 0.50%, it is not
possible to sufficiently increase the strength of the steel. On the other hand, Mn is
selectively oxidized on a surface in a similar manner with Si, and thereby chemical
25 conversion treatment property or plating adhesion after hot statnping degrades. As a
15
result of studies by the inventors, it was found that when the amount of Mn is 1.50% or
more, plating adhesion degrades. Therefore, in the embodiment, the amount of Mn is
set to less than 1.5%. It is more preferable that the upper limit of the amount of Mn be
1.45%. Therefore, the amount ofMn is set to be in a range of 0.50% to less than 1.50%.
5 In a case in which there is a demand for high elongation, the amount of Mn is desirably
set to 1.00% or less.
[0036]
P: 0.001% to 0.060%
In a case in which the amount is large, P segregates at a grain boundary, and
10 deteriorates the local ductility and weldability of the steel. Therefore, the amount ofP
is set to 0.060% or less. On the other hand, since an unnecessary decrease of Pleads to
an increase in the cost of refining, the amount ofP is desirably set to 0.00 I% or more.
15
20
[0037]
S: 0.001% to 0.010%
S is an element that forms MnS and significantly deteriorates the local ductility
or weldability of the steel. Therefore, the upper limit of the amount of S is set to
0.0 I 0%. In addition, in order to reduce refining costs, the lower limit of the amount of
S is desirably set to 0.001%.
[0038]
N: 0.0005% to 0.0100%
N is an important element to precipitate AlN and the like and to refine crystal
grams. However, when the amount ofN exceeds 0.0100%, a solute N (a solute
nitrogen) remains and the ductility of the steel is degraded. Therefore, the amount ofN
is set to 0.0100% or less. Due to a problem of refining costs, the lower limit of the
25 amount ofN is desirably set to 0.0005%.
16
[0039]
The hot -stamped steel according to the embodiment has a basic composition
including the above-described elements, Fe and unavoidable impurities as a balance, but
may further contain any one or more elements selected from Nb, Ti, V, Mo, Cr, Ca, REM
5 (rare earth metal), Cu, Ni and B as elements that have thus far been used in amounts that
are within the below-described ranges to improve the strength, to control a shape of a
sulfide or an oxide, and the like. Even when the hot-stamped steel or cold-rolled steel
sheet does not include Nb, Ti, V, Mo, Cr, Ca, REM, Cu, Ni, and B, various properties of
the hot-stamped steel or cold-rolled steel sheet can be improved sufficiently. Therefore,
10 the lower limits of the amounts ofNb, Ti, V, Mo, Cr, Ca, REM, Cu, Ni, and Bare 0%.
[0040]
Nb, Ti and V are elements that precipitate fine carbonitrideand strengthen the
steeL In addition, Mo and Cr are elements that increase hardenability and strengthen
the steel. To obtain these effects, the steel desirably contains Nb: 0.001% or more, Ti:
15 0.001% or more, V: 0.001% or more, Mo: 0.01% or more, and Cr: 0.01% or more.
20
However, even when Nb: more than 0.050%, Ti: more than 0.100%, V: more than
0.100%, Mo: more than 0.50%, or Cr: more than 0.50% are contained, the
strength-increasing effect is saturated, and there is a concern that the degradation of the
elongation or the hole expansibility may be caused.
[0041]
The steel may further contain Cain a range of 0.0005% to 0.0050%. Ca and
rare earth metal (REM) control the shape of sulfides or oxides and improve the local
ductility or the hole expansibility. To obtain this effect using the Ca, it is preferable to
add 0.0005% or more Ca. However, since there is a concern that an excessive addition
25 may deteriorate workability, the upper limit of the amount ofCa is set to 0.0050%. For
17
the same reason, for the rare earth metal (REM) as well, it is preferable to set the lower
limit of the amount to 0.0005% and the upper limit of the amount to 0.0050%.
[0042]
The steel may further contain Cu: 0.01% to 1.00%, Ni: 0.01% to 1.00% arid B:
5 0.0005% to 0.0020%. These elements also can improve the hardenability and increase
the strength of the steel. However, to obtain the effect, it is preferable to contain Cu:
0.01% or more, Ni: 0.01% or more and B: 0.0005% or more. In a case in which the
amounts are equal to or less than the above-described values, the effect that strengthens
the steel is small. On the other hand, even when Cu: more than 1.00%, Ni: more than
10 1.00% and B: more than 0.0020% are added, the strength-increasing effect is saturated,
and there is a concern that the ductility may degrade.
[0043]
In a case in which the steel contains B, Mo, Cr, V, Ti, Nb, Ni, Cu, Ca and REM,
one or more elements are contained. The balance of the steel is composed ofF e and
15 unavoidable impurities. Elements other than the above-described elements (for example,
Sn, As and the like) may be further contained as unavoidable impurities as long as the
elements do not impair characteristics. Furthermore, when B, Mo, Cr, V, Ti, Nb, Ni, Cu,
Ca and REM are contained in amounts that are less than the above-described lower limits,
the elements are treated as unavoidable impurities.
20
25
[0044]
In addition, in the hot-stamped steel according to the embodiment, as shown in
FIG. I, when the amount ofC (mass%), the amount ofSi (mass%) and the amount ofMn
(mass%) are represented by [C], [Si] and [Mn] respectively, it is important to satisfy the
following expression (A).
(5 x [Si] + [Mn]) I [C] > 10 (A)
18
To satisfy a condition ofTS x A" 50000 MPa·%, the above expression (A) is
preferably satisfied. When the value of(5 x [Si] + [Mn]) I [C] is 10 or less, it is not
possible to obtain a sufficient hole expansibility. This is because, when the amount of C
is large, the hardness of a hard phase becomes too high, a hardness difference (ratio of
5 the hardness) between the hard phase and a soft phase becomes great, and therefore the A
value deteriorates, and, when the amount of Si or the amount of Mn is small, TS becomes
low. Regarding the value of (5 x [Si] + [Mn]) I [C], since the value does not change
even after hot stamping as described above, the expression is preferably satisfied when
the cold-rolled steel sheet is produced.
10 [0045]
Generally, it is the martensite rather than the ferrite to dominate the formability
(hole expansibility) in a dual-phase steel (DP steel). As a result of intensive studies by
the inventors regarding the hardness of martensite, it was clarified that, when the
hardness difference (the ratio of the hardness) of the martensite between a surface portion
15 of a sheet thickness and a central portion of the sheet thickness, and the hardness
distribution of the martensite in the central portion of the sheet thickness are in a
predetermined state in a phase before quenching in the hot stamping, the stale is almost
maintained even after hot stamping as shown in FIGS. 2A and 2B, and the formability
such as elongation or hole expansibility becomes favorable. This is considered to be
20 because the hardness distribution of the martensite formed before quenching in the hot
stamping still has a significant effect even after hot stamping, and alloy elements
concentrated in the central portion ?fthe sheet thickness still hold a state of being
concentrated in the central portion of the sheet thickness even after hot stamping. That
is, in the cold-rolled steel sheet before quenching in the hot stamping, in a case in which
25 the hardness ratio between the martensite in the surface portion of the sheet thickness and
19
the martensite in the central portion of the sheet thickness is great, or a variance of the
hardness of the martensite is great, the same tendency is exhibited even after hot
stamping. As shown in FIGS. 2A and 2B, the hardness ratio between the surface
portion of the sheet thickness and the central portion of the sheet thickness in the
5 cold-rolled steel sheet according to the embodiment before quenching in the hot stamping
and the hardness ratio between the surface portion of the sheet thickness and the central
portion of the sheet thickness in the hot -stamped steel according to the embodiment are
almost the same. In addition, similarly, the variance of the hardness of the martensite in
the central portion of the sheet thickness in the cold-rolled steel sheet according to the
10 embodiment before quenching in the hot stamping and the variance of the hardness of the
martensite in the central portion of the sheet thickness in the hot-stamped steel according
to the embodiment are almost the same. Therefore, the formability of the cold-rolled
steel sheet according to the embodiment is similarly excellent to the formability of the
hot -stamped steel according to the embodiment.
15 [0046]
In addition, regarding the hardness of the martensite measured with an
nanoindenter manufactured by Hysitron Corporation, the inventors found that the
fulfillments of the following expression (B) and the following expression (C) are
advantageous to the hole expansibility of the hot-stamped steel. The fulfillments of the
20 expression (H) and the expression (I) are also advantageous in the same manner. Here,
"HI" is the average hardness of the martensite in the surface portion of the sheet
thickness that is within an area having a width of 200 [liD in a thickness direction from an
outermost layer of the hot-stamped steel, "I-!2" is the average hardness of the martensite
in an area having a width of± 1 00 [liD in the thickness direction from the central portion
25 of the sheet thickness in the central pmiion of the sheet thickness in the hot-stamped steel,
20
and "aHM" is the variance of the hardness of the martensite in an area having a width of
±100 f1m in the thickness direction from the central portion of the sheet thickness in the
hot -stamped steel. In addition, "H 1 0" is the hardness of the martensite in the surface
portion of the sheet thickness in the cold-rolled steel sheet before quenching in the hot
5 stamping, "H20" is the hardness of the martensite in the central portion of the sheet
thickness, that is, in an area having a width of 200 f1m in the thickness direction in a
center of the sheet thickness in the cold-rolled steel sheet before quenching in the hot
stamping, and "aHMO" is the variance of the hardness of the martensite in the central
portion of the sheet thickness in cold-rolled steel sheet before quenching in the hot
10 stamping. The Hl, HlO, H2, H20, aHM and aHMO are obtained from 300-point
measurements for each. An area having a width of± 100 f1m in the thickness direction
from the central portion of the sheet thickness refers to an area having a center at the
center of the sheet thiclmess and having a width of200 f1m in the thickness direction.
15
H2/ HI< 1.10 (B)
aHM <20 (C)
H20 I HlO < 1.10 (H)
a HMO < 20 (I)
In addition, here, the variance is a value obtained using the following expression
(K) and indicating a distribution of the hardness of the martensite.
20 aHM = (1/ n) x ~ [n, i=l] (Xave- x;i (K)
X ave is the average value of the hardness, and Xi is an ith hardness.
[0047]
A value ofH2/Hl of 1.10 or more represents that the hardness of the martensite
in the central portion of the sheet thiclmess is 1.10 or more times the hardness of the
25 martensite in the surface portion of the sheet thickness, and, in this case, aHM becomes
21
20 or more even after hot stamping as shown in FIG. 2A. When the valne of the H2 I
HI is 1.1 0 or more, the hardness of the central portion of the sheet thiclmess becomes too
high, TS x A becomes less than 50000 MPa·% as shown in FIG. 2B, and a sufficient
formability cannot be obtained both before quenching (that is, before hot stamping) and
5 after quenching (that is, after hot stamping). Furthermore, theoretically, there is a case
in which the lower limit of the H2 I HI becomes the same in the central portion of the
sheet thickness and in the surface portion of the sheet thickness unless a special thermal
treatment is carried out; however, in an actual production process, when considering
productivity, the lower limit is, for example, approximately 1.005. What has been
10 described above regarding the value ofH2 I HI shall also apply in a similar manner to
the value of H20 I HIO.
[0048]
In addition, the variance aHM being 20 or more even after hot stamping
indicates that a scattering of the hardness of the martensite is large, and portions in which
15 the hardness is too high locally exist. In this case, TS x A becomes less than 50000
MPa·% as shown in FIG. 2B, and a sufficient hole expansibility of the hot-stamped steel
cannot be obtained. What has been described above regarding the value of the aHM
shall also apply in a similar manner to the value of the a HMO.
20
[0049]
In the hot-stamped steel according to the embodiment, the area fraction of ferrite
is 40% to 95%. When the area fraction of ferrite is less than 40%, a sufficient
elongation or a sufficient hole expansibility cannot be obtained. On the other hand,
when the area fraction of the ferrite exceeds 95%, the martensite becomes insufficient,
and a sufficient strength cannot be obtained. Therefore, the area fraction of ferrite in the
25 hot-stamped steel is set to 40% to 95%. In addition, the hot-stamped steel also includes
22
martensite, the area fraction of martensite is 5% to 60%, and the total of the area fraction
of ferrite and the area fraction of martensite is 60% or more. All or principal portions of
the hot -stamped steel are occupied by ferrite and martensite, and furthermore, one or
more of bainite and retained austenite may be included in the hot-stamped steel.
5 However, when retained austenite remains in the hot-stamped steel, a secondary working
brittleness and a delayed fracture characteristic are likely to degrade. Therefore, it is
preferable that retained austenite is substantially not included; however, unavoidably, 5%
or less of retained austenite in a volume fraction may be included. Since pearlite is a
hard and brittle structure, it is preferable not to include pearlite in the hot-stamped steel;
10 however, unavoidably, up to 10% of pearlite in an area fraction may be included.
Furthermore, the amount of bainite may be 40% at most in an area fraction with respect
to a region excluding ferrite and martensite. Here, ferrite, bainite and pearlite were
observed through Nita! etching, and martensite was observed through Le pera etching.
In both cases, a 114 portion of the sheet thickness was observed at a magnification of
15 1000 times. The volume fraction of retained austenite was measured with an X-ray
diffraction apparatus after polishing the steel sheet up to the 114 portion of the sheet
'
thickness. The 1/4 portion of the sheet thickness refers to a portion 1/4 of lhe thickness
of the steel sheet away from a surface of the steel sheet in a thickness direction of the
steel sheet in the steel sheet.
20 [0050]
In the embodiment, the hardness of the martensite is specified by a hardness
obtained using a nanoindenter under the following conditions.
·Magnification for observing indentation: x1000
·Visual field for observation: height of90 f.Lm and width of 120 f.Lm
25 · Indenter shape: Berkovich-type three-sided pyramid diamond indenter
5
23
·Compression load: 500 [.tN (50 mgf)
· Loading time for indenter compression: 10 seconds
·Unloading time period for indenter compression: 10 seconds (the indenter is
not kept at a position of the maximum load.)
A relationship between compression depth and load is obtained under the above
condition, and hardness is calculated from the relationship. The hardness can be
calculated by a conventional method. The hardness is measured at 1 0 positions, the
hardness of martensite is obtained by an arithmetic average for the 10 hardness values.
The individual positions for measurement are not particularly limited as long as the
10 positions are within martensite grains. However, the distance between positons for
measurement must be 5 [.tm or longer
Since an indentation formed in an ordinary Vickers hardness test is larger than
the martensite, according to the Vickers hardness test, while a macroscopic hardness of
the martensite and peripheral structures thereof (ferrite and the like) can be obtained, it is
15 not possible to obtain the hardness of the martensite itself. Since the formability (hole
expansibility) is significantly affected by the hardness of the martensite itself, it is
difficult to sufficiently evaluate the formability only with a Vickers hardness. On the
contrary, in the embodiment, since the distribution state of hardness is given based on the
hardness of the martensite in the hot-stamped steel measured with the nanoindenter, it is
20 possible to obtain an extremely favorable formability.
[0051]
In addition, in the cold-rolled steel sheet before quenching in the hot stamping
and the hot-stamped steel, as a result of observing MnS at a location of 1/4 of the sheet
thickness and in the central portion of the sheet thickness, it was found that it is
25 preferable that the area fraction of the MnS having an equivalent circle diameter of 0.1
24
~tm to 10 J.!m is 0.01% or less, and, as shown in FIG. 3, the following expression (D) ((J)
as well) is satisfied in order to favorably and stably satisfy the condition ofTS x A ::0>
50000 MPa·%. When the MnS having an equivalent circle diameter of 0.1 J.!m or more
exists during a hole expansibility test, since stress concentrates in the vicinity thereof,
5 cracking is likely to occur. A reason for not counting the MnS having an equivalent
circle diameter of less than 0.1 J.!m is that the effect on the stress concentration is small.
In addition, a reason for not counting the MnS having an equivalent circle diameter of
more than 1 0 J.!m is that, when the MnS having the above-described particle size is
included in the hot-stamped steel or the cold-rolled steel sheet, the particle size is too
10 large, and the hot-stamped steel or the cold-rolled steel sheet becomes unsuitable for
working. Furthermore, when the area fraction of the MnS having an equivalent circle
diameter ofO.l ~tm to 10 J.!m exceeds 0.01%, since it becomes easy for fine cracks
generated due to the stress concentration to propagate, the hole expansibility further
deteriorates, and there is a case in which the condition ofTS x A ::0> 50000 MPa·% is not
15 satisfied. Here, "nl" and "nlO" are number densities of the MnS having an equivalent
circle diameter ofO.l J.lm to 10 J.lm at the 1/4 portion of the sheet thickness in the
hot-stamped steel and the cold-rolled steel sheet before quenching in the hot stamping,
respectively, and "n2" and "n20" are number densities ofthe MnS having an equivalent
circle diameter of 0.1 J.lm to 10 J.!m at the central portion of the sheet thickness in the
20 hot-stamped steel and the cold-rolled steel sheet before quenching in the hot stamping,
respectively.
n2 I nl < 1.5 (D)
n20 I nlO < 1.5 (J)
These relationships are all identical to the steel sheet before quenching in the hot
25 stamping, the steel sheet after hot stamping, and the hot-stamped steel.
25
[0052]
When the area fraction of the MnS having an equivalent circle diameter of 0.1
11m to 10 11m is more than 0.01% after hot stamping, the hole expansibility is likely to
degrade. The lower limit of the area fraction of the MnS is not particularly specified,
5 however, 0.0001% or more of the MnS is present due to a below-described measurement
method, a limitation of a magnification and a visual field, and an original amount of Mn
or the S. In addition, a value of an n2/nl (or an n20/nl 0) of 1.5 or more indicates that a
number density of the MnS having an equivalent circle diameter of 0.1 11m to 10 11m in
the central portion of the sheet thickness of the hot-stamped steel (or the cold-rolled steel
10 sheet before hot stamping) is 1.5 or more times the number density of the MnS having an
equivalent circle diameter of 0.1 11m or more in the 114 portion of the sheet thickness of
the hot-stamped steel (or the cold-rolled steel sheet before hot stamping). In this case,
the formability is likely to degrade due to a segregation of the MnS in the central portion
of the sheet thickness of the hot -stamped steel (or the cold-rolled steel sheet before hot
15 stamping). In the embodiment, the equivalent circle diameter and number density of the
MnS having an equivalent circle diameter of 0.1 11m to 10 11m were measured with a field
emission scanning electron microscope (Fe-SEM) manufactured by JEOL Ltd. At a
measurement, a magnification was 1000 times, and a measurement area of the visual
field was set to 0.12 x 0.09 mm2 (= 10800 11m2
"' 10000 11m\ Ten visual fields were
20 observed in the 1/4 portion of the sheet thiclmess, and ten visual fields were observed in
the central portion of the sheet thickness. The area fraction of the MnS having an
equivalent circle diameter of 0.1 11m to 10 11m was computed with particle analysis
software. In the hot-stamped steel according to the embodiment, the form (shape and
number) of the MnS formed before hot stamping is the same before and after hot
25 stamping. FIG. 3 is a view showing a relationship between the n2/ nl and TS x A after
26
hot stamping and a relationship between an n20 I n1 0 and TS x lc before quenching in the
hot stamping, and, according to FIG. 3, the n20 I n10 of the cold-rolled steel sheet before
quenching in the hot stamping and the n21 n1 of the hot-stamped steel are almost the
same. This is because the form of the MnS does not change at a typical heating
5 temperature of hot stamping.
(0053)
When the hot stamping is carried out on the cold-rolled steel sheet having the
above-described configuration, it is possible to obtain a hot-stamped steel having a
tensile strength of 400 MPa to 1000 MPa, and hole expansibility is significantly
10 improved in the hot -stamped steel having a tensile strength of approximately 400 MPa to
800 MPa.
(0054)
Furthermore, a hot-dip galvanized layer, a galvannealed layer, an
electro galvanized layer or an aluminized layer may be formed on a surface of the
15 hot-stamped steel according to the embodiment. It is preferable to form the
above-described plating in terms of rust prevention. A formation of the above-described
platings does not impair the effects of the embodiment. The above-described platings
can be carried out with a well-known method.
20
(0055)
A cold-rolled steel sheet according to another embodiment of the present
invention includes, by mass%, C: 0.030% to 0.150%; Si: 0.010% to 1.000%; Mn: 0.50%
or more and less than 1.50%; P: 0.001% to 0.060%; S: 0.001% to 0.010%; N: 0.0005% to
0.0100%; AI: 0.010% to 0.050%, and optionally at least one ofB: 0.0005% to 0.0020%;
Mo: 0.01% to 0.50%; Cr: 0.01% to 0.50%; V: 0.001% to 0.100%; Ti: 0.001% to 0.100%;
25 Nb: 0.001% to 0.050%; Ni: 0.01% to 1.00%; Cu: 0.01% to 1.00%; Ca: 0.0005% to
27
0.0050%; REM: 0.0005% to 0.0050%, and a balance of Fe and impurities, in which,
when [CJ is the amount ofC by mass%, [Si] is the amount ofSi by mass%, and [Mn] is
the amount ofMn by mass%, the following expression (A) is satisfied, the area fraction
offerrite is 40% to 95% and the area fraction of martensite is 5% to 60%, the totalofthe
5 area fraction of ferrite and the area fraction of martensite is 60% or more, the cold-rolled
steel sheet optionally further can include one or more of pearlite, retained austenite, and
bainite, the area fraction of pearlite is 1 0% or less, the volume fraction of retained
austenite is 5% or less, and the area fraction of bainite is less than 40%, the hardness of
the martensite measured with a nanoindenter satisfies the following expression (H) and
10 the following expression (I), TS x A which is a product of tensile strength TS and hole
expansion ratio A is 50000 MPa·% or more.
15
20
(5 x [Si] + [Mn]) I [C] > 10 (A)
H20 I H10 < 1.10 (H)
aHMO < 20 (I)
The H10 is the average hardness of the martensite in a surface portion of a sheet
thickness, the H20 is the average hardness of the martensite in a central portion of the
sheet thickness, the central portion is an area having a width of 200 J.lm in the thickness
direction at a center of the sheet thickness, and the aHMO is the variance of the average
hardness of the martensite in the central portion of the sheet thickness.
The above hot-stamped steel is obtained by hot-stamping the cold-rolled steel
sheet according to the embodiment as described below. Even when the cold-rolled steel
sheet is hot stamped, the chemical composition of the cold-rolled steel sheet does not
change. In addition, as described above, when the hardness ratio of the martensite
between the surface portion of the sheet thickness, and the central portion of the sheet
25 thickness and the hardness distribution of the martensite in the central portion of the
28
sheet thickness are in the above predetermined state in a phase before quenching in the
hot stamping, the state is almost maintained even after hot stamping (see also FIG. 2A
and FIG. 2B). Furthermore, when the state of ferrite, martensite, pearlite, retained
austenite, and bainite is in the above predetermined state in a phase before quenching in
5 the hot stamping, the state is almost maintained even after hot stamping. Accordingly,
the features of the cold-rolled steel sheet according to the embodiment are substantially
the same as the features of the above hot-stamped steel.
[0056]
In the cold-rolled steel sheet according to the embodiment, the area fraction of
10 MnS existing in the cold-rolled steel sheet and having an equivalent circle diameter of
0.1 [lm to 10 [lm may be 0.01% or less, and the following expression (J) may be satisfied
n20 I nlO < 1.5 (J)
The nl 0 is the average number density per 10000 [lm2 of the MnS having an
equivalent circle diameter ofO.l [lm to 10 [lm in a 114 portion of the sheet thickness, and
15 the n20 is the average number density per 10000 ~Lm2 of the MnS having an equivalent
circle diameter of 0.1 [lm to I 0 [lm in the central portion of the sheet thickness.
As described above, the ratio ofn20 to n!O having the cold-rolled steel sheet
before hot stamping is almost maintained even after hot-stamping the cold-rolled steel
sheet (see also FIG. 3). In addition, the area fraction ofMnS is almost the same before
20 and after hot stamping. Accordingly, features having the cold-rolled steel sheet
according to the embodiment are substantially the same as features having the above
hot -stamped steel.
[0057]
A hot-dip galvanized layer may be formed on a surface of the cold-rolled steel
25 sheet according to the embodiment in a similar manner with the above-described
5
29
hot-stamped steel. In addition, the hot-dip galvanized layer may be alloyed in the
cold-rolled steel sheet according to the embodiment. Furthermore, an electrogalvanized
layer or aluminized layer may be formed on the surface of the cold-rolled steel sheet
according to the embodiment.
[0058]
Hereinafter, a method for producing the cold-rolled steel sheet (a cold-rolled
steel sheet, a galvanized cold-rolled steel sheet, a galvannealed cold-rolled steel sheet, an
electro galvanized cold-rolled steel sheet and an aluminized cold-rolled steel sheet) and a
method for producing the hot-stamped steel for which the cold-rolled steel sheet is used
1 0 according to the embodiments will be described.
[0059]
When producing the cold-rolled steel sheet and the hot-stamped steel for which
the cold-rolled steel sheet is used according to the embodiments, as an ordinary condition,
a molten steel from a converter is continuously cast, thereby producing a steel. In the
15 continuous casting, when a casting rate is fast, precipitates ofTi and the like become too
fine, and, when the casting rate is slow, productivity deteriorates, and consequently, the
above-described precipitates coarsen and the number of grains (for example, ferrite,
martensite and the like) in the microstructure decreases, the grains coarsen in the
microstructure, and thus, there is a case other characteristics such as a delayed fracture
20 cannot be controlled. Therefore, the casting rate is desirably 1.0 m/minute to 2.5
m/minute.
[0060]
The steel after the casting can be subjected to hot-rolling as it is. Alternatively,
in a case in which the steel after cooling has been cooled to less than 11 00°C, it is
25 possible to reheat the steel after cooling to II 00°C to 1300°C in a tunnel furnace or the
30
like and subject the steel to hot-rolling. When the heating temperature is less than
1100°C, it is difficult to ensure a finishing temperature in the hot-rolling, which causes a
degradation of the elongation. In addition, in the hot-stamped steel for which a
cold-rolled steel sheet to which Ti and Nb are added is used, since the dissolution of the
5 precipitates becomes insufficient during the heating, which causes a decrease in strength.
10
15
On the other hand, when the heating temperature is more than 1300°C, the amount of
scale formed increases, and there is a case in which it is not possible to make surface
property of the hot-stamped steel favorable.
[0061]
In addition, to decrease the area fraction of the MnS having an equivalent circle
diameter of 0.1 J.!m to 10 J.!m, when the amount of Mn and the amount of S in the steel
are respectively represented by [Mn] and [S] by mass%, it is preferable for a temperature
T (0 C) of a heating furnace before carrying out hot-rolling, an in-furnace timet (minutes),
[Mn] and [S] to satisfy a following expression (G) as shown in FIG. 6.
T x ln(t) I (1.7 x [Mn] + [S]) > 1500 (G)
When T x ln(t) I (1.7 x [Mn] + [S]) is equal to or less than 1500, the area
fraction of the MnS having an equivalent circle diameter of 0.1 J.!m to 10 ~tm becomes
large, and there is a case in which a difference between the number density of the MnS
having an equivalent circle diameter ofO.l J.!m to 10 J.!m in the 114 portion of the sheet
20 thickness and the number density of the MnS having an equivalent circle diameter ofO.l
J.!m to 10 J.!m in the central portion of the sheet thickness becomes large. The
temperature of the heating furnace before carrying out hot-rolling refers to an extraction
temperature at an outlet side of the heating furnace, and the in-furnace time refers to a
time elapsed from a placement of the steel into the hot heating frnnace to an extraction of
25 the steel from the heating furnace. Since the MnS does not change even after hot
31
stamping as described above, it is preferable to satisfy tbe expression (G) in a heating
step before hot-rolling.
[0062]
Next, the hot-rolling is carried out according to a conventional method. At this
5 time, it is desirable to carry out hot-rolling on the steel at the finishing temperature (the
hot-rolling end temperature) which is set to be in a range of an Ar3 temperature to 970°C.
When the finishing temperature is less than the Ar3 temperature, the hot-rolling includes
a (a+ y) two-phase region rolling (two-phase region rolling of the ferrite+ the
martensite), and there is a concern that the elongation may degrade. On the other hand,
I 0 when the finishing temperature exceeds 970°C, the austenite grain size coarsens, and the
fraction of the ferrite becomes small, and thus, there is a concern that the elongation may
degrade. A hot-rolling facility may have a plurality of stands.
15
Here, the Ar3 temperature was estimated from an inflection point of a length of a
test specimen after carrying out a formastor test.
[0063)
After the hot-rolling, the steel is cooled at an average cooling rate of
20 °C/second to 500 °C/second, and is coiled at a predetermined coiling temperature CT.
In a case in which the average cooling rate is less than 20 °C/second, the pearlite that
causes the degradation of the ductility is likely to be formed. On the other hand, the
20 upper limit of the cooling rate is not particularly specified and is set to approximately
500 °C/second in consideration of a facility specification, but is not limited thereto.
[0064]
After coiling the steel, pickling is carried out, and cold-rolling is carried out.
At this time, to obtain a range satisfying the above-described expression (C) as shown in
25 FIG. 4, the cold-rolling is carried out under a condition in which the following expression
32
(E) is satisfied. When conditions for annealing, cooling and the like described below
are further satisfied after the above-described rolling, TS x 'A 2 50000 MPa·% is ensured
in the cold-rolled steel sheet before hot stamping and/or the hot-stamped steel. From
the viewpoint of the productivity, the cold-rolling is desirably carried out with a tandem
5 rolling mill in which a plurality of rolling mills are linearly disposed, and the steel sheet
is continuously rolled in a single direction, thereby obtaining a predetermined thickness.
1.5 x rl I r + 1.2 x r2/ r +r3 I r > 1.00 (E)
Here, the "ri" is an individual target cold-rolling reduction(%) at an ith stand (i
= 1, 2, 3) from an uppermost stand in the cold-rolling, and the "r" is a total target
10 cold-rolling reduction(%) in the cold-rolling. The total cold-rolling reduction is a
so-called cumulative reduction, and on a basis of the sheet thickness at an inlet of a first
stand, is a percentage of the cumulative reduction (the difference between the sheet
thickness at the inlet before a first pass and the sheet thickness at an outlet after a final
pass) with respect to the above-described basis.
15 [0065]
When the steel is cold-rolled under the conditions in which the expression (E) is
satisfied, it is possible to sufficiently divide pearlite in the cold-rolling even when a large
pearlite exists before the cold-rolling. As a result, it is possible to eliminate pearlite or
limit the area fraction of pearlite to a minimum through the annealing carried out after
20 cold-rolling, and therefore it becomes easy to obtain a structure in which the expression
(B) and the expression (C) (or the expression (H) and the expression (I)) are satisfied.
On the other hand, in a case in which the expression (E) is not satisfied, the cold-rolling
reductions in upper stream stands are not sufficient, the large pemlite is likely to remain,
and it is not possible to form a desired mmtensite in the following annealing. Therefore,
25 it is not possible to obtain a structure in which the expression (B) and the expression (C)
33
(or the expression (H) and the expression (I)) are satisfied. That is, in the case in which
the expression (E) is not satisfied, it is not possible to obtain a feature of H2/Hl < 1.10
(or H20/H1 0 < 1.1 0), and a feature of crHM < 20 (or crHMO < 20). In addition, the
inventors found that, when the expression (E) is satisfied, an obtained form of the ·
5 martensite structure after the annealing is maintained in almost the same state even after
hot stamping is carried out, and therefore the hot -stamped steel according to the
embodiment becomes advantageous in terms of the elongation or the hole expansibility
even after hot stamping. In a case in which the hot-stamped steel according to the
embodiment is heated up to the two-phase region in the hot stamping, a hard phase
10 including martensite before quenching in the hot stamping turns into an austenite
structure, and ferrite before quenching in the hot stamping remains as it is. Carbon (C)
in austenite does not move to the peripheral ferrite. After that, when cooled, austenite
turns into a hard phase including martensite. That is, when the expression (E) is
satisfied, the expression (H) is satisfied before hot stamping and the expression (B) is
15 satisfied after hot stamping, and thereby the hot-stamped steel becomes excellent in terms
of the formability.
[0066]
r, r1, r2 and r3 are the target cold-rolling reductions. Generally, the cold-rolling
is carried out while controlling the target cold-rolling reduction and an actual cold-rolling
20 reduction to become substantially the same value. It is not preferable to carry out the
cold-rolling in a state in which the actual cold-rolling reduction is unnecessarily made to
be different from the target cold-rolling reduction. However, in a case in which there is
a large difference between a target rolling reduction and an actual rolling reduction, it is
possible to consider that the embodiment is carried out when the actual cold-rolling
25 reductions satisfy the expression (E). Furthermore, the actual cold-rolling reduction is
5
34
preferably within ±I 0% of the target cold-rolling reduction.
In addition, it is more preferable that the actual cold-rolling reductions satisfy
the following expression.
1.20 ::> 1.5 x rl I r + 1.2 x r2/ r + r3 I r > 1.00 (E')
When "1.5 x rl/ r + 1.2 x r2/ r + r3 I r" exceeds 1.20, a heavy load is applied to
a cold rolling mill, productivity is degraded. Tensile strength of the steel sheet
according to the above-described embodiment is a range of 400 MPa to 1000 MPa, and is
much larger than the tensile strength of typical cold-rolled steel sheets. It is necessary
to apply a rolling load of 1800 ton or more per a stand in order to carry out the
10 cold-rolling under a condition that "1.5 x r1 I r + 1.2 x r2 I r + r3 I r" exceeds 1.20 in the
steel sheet having such tensile strength. It is difficult to apply such heavy rolling load in
15
consideration of rigidity of stands and/or rolling facility capability. · Furthermore, when
such heavy rolling load is applied, there is a concern that production efficiency is
degraded.
[0067]
After cold-rolling, a recrystallization is caused in the steel sheet by annealing the
steel. The annealing forms a desired martensite. Furthermore, regarding an annealing
temperature, it is preferable to carry out the annealing by heating the steel sheet to 700°C
to 850°C, and cool the steel sheet to a room temperature or a temperature at which a
20 surface treatment such as the galvanizing is carried out. When the annealing is carried
out in the above-described range, it is possible to stably ensure a predetermined area
fraction of the ferrite and a predetermined area fraction of the martensite, to stably set the
total of the area fraction of the ferrite and the area fraction of the mmiensite to 60% or
more, and to contribute to an improvement ofTS x A. A holding time at 700°C to
25 850°C is preferably 1 second or more as long as the productivity is not impaired (for
35
example, 300 second) to reliably obtain a predetermined structure. The
temperature-increase rate is preferable in a range of 1 °C/second to an upper limit of a
facility capacity, and the cooling rate is preferable in a range of I °C/second to the upper
limit of the facility capacity. In a temper-rolling step, temper-rolling is carried out with
5 a conventional method. The elongation ratio of the temper-rolling is, generally,
approximately 0.2% to 5%, and is preferable within a range in which a yield point
elongation is avoided and the shape of the steel sheet can be corrected.
[0068]
As a still more preferable condition of the embodiment, when the amount of C
10 (mass%), the amount ofMn (mass%), the amount ofSi (mass%) and the amount ofMo
15
(mass%) of the steel are represented by [C], [Mn], [Si] and [Mo], respectively, regarding
the coiling temperature CT, it is preferable to satisfy the following expression (F).
560-474 x [C]- 90 x [Mn]- 20 x [Cr]- 20 x [Mo] < CT < 830-270 x [C]- 90
x [Mn]- 70 x [Cr]- 80 x [Mo]
[0069]
(F)
As shown in FIG. SA, when the coiling temperature CT is less than "560- 474 x
[C]- 90 x [Mn]- 20 x [Cr]- 20 x [Mo]", the martensite is excessively formed, the steel
sheet becomes too hard, and there is a case in which the following cold-rolling becomes
difficult. On the other hand, as shown in FIG. 5B, when the coiling temperature CT
20 exceeds "830- 270 x [C]- 90 x [Mn]- 70 x [Cr]- 80 x [Mo]", a banded structure of the
ferrite and the pearlite is likely to be formed, and furthermore, a fraction of the pearlite in
the central portion of the sheet thickness is likely to increase. Therefore, the uniformity
of a distribution of the martensite formed in the following annealing degrades, and it
becomes difficult to satisfy the above-described expression (C). In addition, there is a
25 case in which it becomes difficult for the martensite to be formed in a sufficient amount.
36
[0070]
When the expression (F) is satisfied, the ferrite and the hard phase have an ideal
distribution form before hot stamping as described above. In this case, when a
two-phase region heating is carried out in the hot stamping, the distribution form is
5 maintained as described above. If it is possible to more reliably ensure a microstructure
having the above-described feature by satisfying the expression (F), the microstructure is
maintained even after hot stamping, and the hot-stamped steel becomes excellent in terms
of formability.
10
[0071]
Furthermore, to improve the rust-preventing capability, it is also preferable to
include a galvanizing step in which a galvanized layer is formed on the steel between an
annealing step and the temper-rolling step, and to form the galvanized layer on a surface
of the cold-rolled steel sheet. Furthermore, it is also preferable that the method for
producing according to the embodiment include an alloying step in which an alloying
15 treatment is performed after galvanizing the steel. In a case in which the alloying
20
treatment is performed, a treatment in which a galvannealed surface is brought into
contact with a substance oxidizing the galvannealed surface such as water vapor, thereby
thickening of an oxidized film may be further carried out on the surface.
[0072]
It is also preferable to include, for example, an electrogalvanizing step in which
an electro galvanized layer is formed on the steel after the temper-rolling step as well as
the galvanizing step and the galvannealing step and to form an electro galvanized layer on
the surface of the cold-rolled steel sheet. In addition, it is also preferable to include,
instead of the galvanizing step, an aluminizing step in which an aluminized layer is
25 formed on the steel between the annealing step and the temper-rolling step. The
37
aluminizing is generally hot-dip aluminizing, which is preferable.
[0073]
After a series of the above-described treatments, the steel is heated to a
temperature range of 700°C to 1000°C, and is hot stamped in the temperature range. In
5 the hot stamping step, the hot stamping is desirably canied out, for example, under the
following conditions. First, the steel sheet is heated up to 700°C to 1 000°C at the
temperature-increase rate of 5 °C/second to 500 °C/second, and the hot stamping (a hot
stamping step) is canied out after the holding time of 1 second to 120 seconds. To
improve the formability, the heating temperature is preferably an Ac3 temperature or less.
10 Subsequently, the steel sheet is cooled, for example, to the room temperature to 300°C at
the cooling rate of 10 °C/second to 1000 °C/second (quenching in the hot stamping).
The Ac3 temperature was calculated from the inflection point of the length of the test
specimen after carrying out the formastor test and measuring the infection point.
15
[0074]
When the heating temperature in the hot stamping step is less than 700°C, the
quenching is not sufficient, and consequently, the strength cannot be ensured, which is
not preferable. When the heating temperature is more than 1 000°C, the steel sheet
becomes too soft, and, in a case in which a plating, particularly zinc plating, is formed on
the surface of the steel sheet, there is a concern that the zinc may be evaporated and
20 burned, which is not preferable. Therefore, the heating temperature in the hot stamping
is preferably 700°C to 1000°C. When the temperature-increase rate is less than
5 °C/second, since it is difficult to control heating in the hot stamping, and the
productivity significantly degrades, it is preferable to cany out the heating at the
temperature-increase rate of 5 °C/second or more. On the other hand, the upper limit of
25 the temperature-increase rate of 500 °C/second depends on a current heating capability,
38
but is not necessary to limit thereto. At a cooling rate of less than 10 °C/second, since
the rate control of the cooling after the hot stamping step is difficult, and the productivity
also significantly degrades, it is preferable to carry out the cooling at the cooling rate of
10 °C/second or more. The upper limit of the cooling rate of 1000 °C/second depends
5 on a current cooling capability, but is not necessary to limit thereto. A reason for setting
a time until the hot stamping after an increase in the temperature to 1 second or more is a
current process control capability (a lower limit of a facility capability), and a reason for
setting the time until the hot stamping after the increase in the temperature to 120
seconds or less is to avoid an evaporation of the zinc or the like in a case in which the
10 galvanized layer or the like is formed on the surface of the steel sheet. The reason for
setting the cooling temperature to the room temperature to 300°C is to sufficiently ensure
the martensite and ensure the strength of the hot-stamped steel.
FIG 8 is a flowchart showing the method for producing the hot-stamped steel
according to the embodiment of the present invention. Each of reference signs S 1 to
15 S 13 in the drawing corresponds to individual step described above.
[0075]
In the hot-stamped steel of the embodiment, the expression (B) and the
expression (C) are satisfied even after hot stamping is carried out under the
above-described condition. In addition, consequently, it is possible to satisfy the
20 condition ofTS x A 2: 50000 MPa·% even after hot stamping is carried out.
[0076]
As described above, when the above-described conditions are satisfied, it is
possible to manufacture the hot-stamped steel in which the hardness distribution or the
structure is maintained even after hot stamping, and consequently the strength is ensured
25 and a more favorable hole expansibility can be obtained.
39
Examples
[0077]
Steel having a composition described in Table 1-1 and Table 1-2 was
continuously cast at a casting rate of 1.0 rnlminute to 2.5 m/minute, a slab was heated in
5 a heating furnace under a conditions shown in Table 5-1 and Table 5-2 with a
conventional method as it is or after cooling the slab once, and hot-rolling was carried
out at a finishing temperature of9!0°C to 930°C, thereby producing a hot rolled steel
sheet. After that, the hot rolled steel sheet was coiled at a coiling temperature CT
described in Table 5-l and Table 5-2. After that, pickling was carried out so as to
I 0 remove a scale on a surface of the steel sheet, and a sheet thickness was made to be 1.2
mm to I .4 mm through cold-rolling. At this time, the cold-rolling was carried out so
that the value of the expression (E) became a value described in Table 5-1 and Table 5-2.
Aftei' cold-rolling, annealing was carried out in a continuous annealing furnace at an
annealing temperature described in Table 2-1 and Table 2-2. On a part of the steel
15 sheets, a galvanized layer was further formed in the middle of cooling after a soaking in
the continuous annealing furnace, and then an alloying treatment was further performed
on a part of the part of the steel sheets, thereby forming a galvannealed layer. In
addition, an electrogalvanized layer or an aluminized layer was formed on another part of
the steel sheets. Furthermore, temper-rolling was carried out at an elongation ratio of
20 I% according to a conventional method. In this state, a sample was taken to evaluate
material qualities and the like before quenching in the hot stamping, and a material
quality test or the like was carried out. After that, to obtain a hot-stamped steel having a
form as shown in FIG. 7, hot stamping was carried out. In the hot stamping, a
temperature was increased at a temperature-increase rate of 10 °C/second to
25 I 00 °C/second, the steel sheet was held at a heating temperature of 800°C for I 0 seconds,
40
and was cooled at a cooling rate of I 00 °C/second to 200°C or less. A sample was cut
from a location of FIG. 7 in an obtained hot-stamped steel, the material quality test and
the like were carried out, and the tensile strength (TS), the elongation (El), the hole
expansion ratio (A.) and the like were obtained. The results are described in Table2-l to
5 Table 5-2. The hole expansion ratios A. in the tables were obtained from the following
expression (L).
10
15
A.(%)= {(d'-d)/d} X 100 (L)
d': a hole diameter when a crack penetrates the sheet thickness
d: an initial hole diameter
Furthermore, regarding plating types in Table 3-1 and Table 3-2, CR represents a
non-plated cold-rolled steel sheet, GI represents that the galvanized layer is formed, GA
represents that the galvannealed layer is formed, EG represents that the electrogalvanized
layer is formed, and AI represents that the aluminized layer is formed.
Furthermore, determinations G and B in the tables have the following meanings.
G: a target condition expression is satisfied.
B: the target condition expression is not satisfied.
[0078]
The chemical conversion treatment property after hot stamping was evaluated as
a surface property after hot stamping in a hot-stamped steel produced from a non-plated
20 cold-rolled steel sheet. The plating adhesion of hot-stamped steel was evaluated as a
surface property after hot stamping when zinc, aluminum, or the like was plated on a
cold-rolled steel sheet from which a hot-stamped steel was produced.
The chemical conversion treatment property was evaluated through the
following procedure. First, a chemical conversion treatment was applied to each sample
25 under a condition that the bath temperature was 43°C and the time period for chemical
41
conversion treatment was 120 seconds using a commercial chemical conversion
treatment agent (Palbond PB-L3020 system manufactured by Nihon Parkerizing Co.
Ltd.). Second, the crystal uniformity of a conversion coating was evaluated by SEM
observation on the surface of each sample to which the chemical conversion treatment is
5 applied. The crystal uniformity of a conversion coating was classified by the following
valuation standards. Good (G) was given to a sample without lack of hiding in crystals
of the conversion coating, bad (B) was given to a sample with a lack of hiding in an area
of crystals of the conversion coating, and very bad (VB) was given to a sample with a
conspicuous lack of hiding in crystals of the conversion coating.
10 The plating adhesion was evaluated through the following procedure. First, a
sheet specimen for testing having a height of 100 rnrn, a width of 200 mm, and a
thickness of 2 mm was taken from a plated cold-rolled steel sheet. The plating adhesion
was evaluated by applying a V bending and straightening test to the sheet specimen. In
the V bending and straightening test, the above sheet specimen was bent using a die for
15 the V bending test (a bending angle of 60°), and then the sheet specimen after the V
bending was straightened again by a press working. A cellophane tape
("CELLOTAPE™ CT405AP-24" manufactured by Nichiban Co. Ltd.) was stuck on a
portion (deformed portion) which was located in the inside of a bent portion during V
bending in the straightened sheet specimen, and then the cellophane tape was taken off
20 by hand. Next, the width of a detached plating layer which is stuck on the cellophane
tape was measured. In the Examples, good (G) was given to a sheet specimen in which
the width was 5 rnrn or less, bad (B) was given to a sheet specimen in which the width
was more than 5 mm and 10 mm or less, and very bad (VB) was given to a sheet
specimen in which the width was more than 10 mm.
25 [0079]
[ ..
~
0
0
00
0
~
STEEL
TYPE
REfERENCE
SYMBOL
A
B
c
D
E
F
G
H
I
J
K
L
M
N
0
p
Q
R
s
T
u
w
X
'{
z
c Si
EXAMPlE 0.045 0.143
" 0,061 0.224
" 0.149 0.970
" 0.075 0.520
" 0.082 0.072
" 0,098 0.212
n 0.102 0.372
" 0.085 0.473
n 0.095 0.720
" 0.071 0.777
u 0.091 0.185
" 0.102 0.632
" 0.105 0.301
" 0.105 0.253
'' 0.144 0,945
n 0.095 0.243
If 0.115 0.342
n OJ2i 0.175
If 0.129 0.57'1
" 0.141 0.150
" 0.129 0.105
tl 0.143 0.652
" 0.141 0.922
If 0.131 0.155
tl 0.149 0.105
Mn p s N AI Cr
0,55 0,002 0.007 0,0033 0.031 0
0.63 0.025 0.005 0.0054 0.025 0
1.45 0.006 0.009 0.0055 0.035 0.22
o.sg 0.007 0.006 0.0025 0.020 0
0.51 0.006 0.009 0.0032 0.045 0.40
U5 0.007 0.009 0.0075 0.035 0
0.82 0.013 0.008 0.0035 0.037 0
0,53 0,056 0,001 0,0029 0.041 0.39
0.72 0.008 0.002 0.0055 0.032 0
0.82 0.006 0.008 0.0014 0.015 0
1.2 i 0.006 0.009 0.0035 0.041 0
1.11 0.015 0.007 0.0041 0.032 0
1.22 0.012 0.009 0.0015 0.035 0
1.44 0.008 0.005 0.0032 0.042 0
0.89 0.008 0,006 0,0043 O,Q35 0
1.45 0.009 0.007 0.0025 O.o39 0.49
1,03 0.015 0,004 0.0038 0.037 0
0.78 0.008 0.003 0.0038 0.036 0
0.93 0,016 0.006 0.0024 0.039 0
1.40 O.D18 0.003 0.0029 0.031 0
1.35 0.018 0.007 0.0064 0.019 0
1.17 0,012 0.006 0.0019 0,038 Q
1.02 0,015 0.004 0.0066 0.026 0.25
1.47 0,008 0.006 0.0065 0.043 0.37
1.32 0.009 0.003 0.0061 0.031 0
Mo v Ti Nb Ni
0 0 0 0 0
0 0 0 0 0.5
0 0 0 0 0
0.25 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0.15 0 0 0 0
0 0.05 0 0 0
0.45 0 0 0 0
a 0 0 0 0
0.37 0 O.!Jl 0 0
0 0 0 0 0
0.35 0 0 0 0
0.21 0 0 0 0
0 0 0 0 0
0,15 0 0 0.!)3 0
0 G 0 O.D3 0
0.19 0 0 0 0
0.21 0 0.03 0 0
0.29 0 0 0 0
0 0 0 {) 0
0.16 0 0.07 0 0
0 0 0 0 0
0.25 0.04 0 0 0
Cu Ca B
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0.7 0.005 0
0 0 0
0 0,004 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0.0019
0 0 0
0 0 0
0 0 0.0011
0 0 0
0 0 0
0 0 0
0 0 0.0009
0 0.003 0
Q Q 0.0015
0 0 0.0013
a 0 0
REM
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.0025
0
0
EXrRESSlON
(A)
28.1
28.7
42.3
43.9
10.6
22,5
26.3
34,1
45.5
66.3
22.4
41.9
26.0
25.8
39,0
28.1
23.8
13.7
Z9.3
15.2
14.5
31;0
39;9
17.1
12.4
~
2::
(--D'
.,.
tv
'
f-':
STEEL
TYPE REfERENCE c Si Mn p s N AI Cr
SY!IBOL
AA COMPARATIVE EXAMPLE 0.079 0.2.05 0.89 0.012 0.006 0.0021 0.029 0
AB " 0.092 0.219 0.913 O.o10 0.004 0.0029 0.041 0
AC u 0.105 0.103 1.22 0.008 0.002 0.0041 O.Q39 0
AD II 0.0)6 0;355 0.98 D.Oi3 0.005 0.0039 0.033 0
AE " 0.142 0.246 0.69 0.009 0.003 0.0030 0.031 0
AF " 0.129 0.363 1.28 0.007 0.003 0.0040 0.042 0
AG COEMXPAAMRPALTEIV E 0.118 0.563 1.13 0.008 0.004 0.0039 0.041 0
AH " Q.Q21 0.323 1.49 0.006 0,002 0.0031 0.032 0
AI " 0.231 0.602 1.39 0.004 0.005 0.0013 O.Q40 0
AJ " 0.093 Q&!l.1 1.01 0.006 0.008 0.0039 0.036 0
f:. AK N 0.098 1.493 O.rl 0.007 0.003 0.0041 0.036 0.38
AL " 0.126 0.780 !J.Z1 0.011 0.003 0.0035 0.032 0
AM u 0.136 o,o4o .2.Zli 0.008 0.003 0.0044 0.039 0
AN " 0.103 0.265 1.12 0095 0.004 0.0025 0.042 0.36
AO u 0.072 0.223 1.41 0.002 .Q.Q2.ii 0.0052 0.036 0
AP " 0.051 0,281 1.03 0.012 0.001 0.1630 0,032 0
AQ " 0.141 0;011 1.3B 0.019 0.008 0.0045 Q.QQl 0
AR If 0.149 0.150 1.23 0.005 0.003 0.0035 0.065 0
AS " 0.133 0.030 1.1G 0.012 0.004 0.0020 0.035 0
AT If 0.135 0.170 1.24 omo 0.004 0.0023 0.03.5 0
AU u 0.139 o.:l31 1.43 0.013 0,002 0.0044 0.030 0
AV " 0.137 0.192 ~ 0.011 0.002 0.0041 0.033 0
AW 0/ 0.136 0,040 2.]5 o.oos 0.003 0.0044 0,039 0
AX " 0.137 0.192 150 o.onl 0.002 0.0041 0.033 0
Mo v Ti Nb ~j i Cu
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 e 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
02.3 0 0 0 0 0
0.33 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0.12 0 0 0.03 0 0
0 0 0 0 OA 0
0 l) 0 0.04 0 0
0.23 0 0 0 0 0
0.37 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0.02 0 0
0 0 0 0.00 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
Ca B REM
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0.0050
0 0 0
0 0.0011 0
0 0.0013 0
0 0 0
0 0 0
0 0 0
0 0 0
0,003 0 0
0 0 0
0 0 0
0 0.001 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
EXPRESSiO~
(A)
24.2
22.3
16.5
36.3
13.5
24.0
33.4
115,0
19.0
11.1
83.4
32.6
21.7
23.7
35.1
47.7
10.2
13.3
M
15.5
22.2
18.0
21,7
1B.O ----------
~ cr" "- N'
~
.,.
w
STEEl
TYPE TEST ANNEALING
REFERENCE REFERENCE TEMPERATURE TS EL
SYMBOL SYMBOl (Mpa) (%)
{"Cj
A 1 790 445 35Ji
8 2 BOO 468 36;2
c 3 750 502 31;2
D 4 790 542 33.1
E 5 795 542 34,8
F 6 790 585 2!t5
G 7 745 552 27.2
H 8 792 622 29.1
l 9 782 598 28,3
J 10 771 565 29.2
K II 811 635 27.1
L 12 752 672 30.6
M 13 782 612 31.4
N 14 821 631 29.6 I
r
0 15 769 629 28.7
p 16 781 692 27,1
Q l7 781 678 25.8
R 18 782 672 21.5
s 19 771 729 23.1
T 20 785 745 28.5
u 21 813 761 21.6
w 22 831 796 19.2
X 23 8'15 862 18.2
y 24 802 911 19.2
z 25 841 !021 ns
AFTER ANl>EALING AND TEMPER-ROLLING AND BEFORE HOT STAMPING
I'ERRITE FERRITE RES !DUAL
lc MARTENSITE + AUSTENil'E TS xEL rsx ~c
AREA AREA MARTENS !IE VOLUME N'~ FRACTION FRACTION(%) AREA FRACTION ''~/ (%) FRACTION(%) (%)
121 15798 53845 92 7 99 1
115 16942 53B2.0 87 6 93 3
132 15662 86264 82 10 92 2
105 17940 56910 84 8 92 3
98 18862 53116 78 7 85 4
86 15503 50310 78 6 84 2
92 15014 50784 65 8 73 4
87 18100 54114 88 6 94 3
93 16923 55614 82 9 91 4
105 16498 59325 75 9 84 3
79 17209 50165 78 10 88 2
89 20563 59808 87 7 94 0
82 19217 50164 56 27 83 2
87 . 18678 54897 58 27 85 5
89 18052 559a·1 78 13 9~ 4
77 18753 53284 71 24 95 2
76 17492 52884 56 32 88 3
89 14448 59808 63 27 90 3
79 16840 57591 55 32 87 4
71 21233 52895 44 41 86 3
68 1B438 51748 44 39 83 5
65 15283 51740 46 37 83 4
51 15688 52582 47 40 87 2
59 17491 53749 4o · 38 83 2
55 13784 56155 43 41 64 4
!>AJNITE PEARLITE
AREA AREA
FRACTION FRACTION
(%) (i'i i,)
0 0
4 0
5 1
5 0
11 0
7 7
15 8
3 a
5 0
7 6
6 4
5 1
6 9
4 6
3 2
2 l
5 7
7 0
9 0
12 0
9 3
10 3
5 5
1f) 0
12 0
PEARLiTE
AREA
FRACTION
BEFORE
COLD
ROLLING(%)
25
25
34
28
42
6
72
35
4
29
34
15
8
42
33
25
28
53
46
23
23
18
51
43
15
~ p;l
0"
(0
N
' ~
~
~
0
0
00
~
~
..,,..
AFTER ANNEALING AND TEMPER-ROLLING AND BEFORE HOT STAMPING
STEEL fEST ANNEALING FERRllE RESWUAL TYPE REFEREI4CE TE:~PERA TURE ?c
FE.RRITE MARTENS HE + AUSTENITE REFERENCE SYMBOL Cc) TS EL TS xEL TSx A AREA AREA ¥;ART ENS !TE VOLUl!E SYMBOL (Mpa) (%) (%) FRACTION fRACTION(%) AREA FRACTION (%) FRACT!QN(%; (~)
AA 26 804 582 27.2 76 1ii830 4423? 62 8 70 2
AS 27 797 606 27.5 68 16665 .:!122ll 58 13 71 1
AC 28 769 581 27.6 79 16036 45899 51 9 60 3
AD 29 756 611 21.3 65 13014 Mill.ilJ .11 15 ~ 1
AE 30 792 598 24.1 75 14412 44850 52 9 61 2
AF 31 742 643 27.2 71 17490 ~ 59 21 so 2
AG 3Z 772 602 29,1 62 17518 37324 72 17 89 2
AH 33 761 372 40.8 117 15178 4J:i.Z:! 95 Q 96 1
AI 34 J.89 1493 9.1 29 13586 43297 9 77 86 3
AJ 35 7!;8 682 21.6 66 14731 45012 69 17 86 2
Al< 36 802 602 30.3 59 18241 .a.:lill 71l 20 96 2
Al 37 789 362 42.1 127 15240 45974 so
'" 88 !
AM 38 76Jl 832 15.7 42 13062 ~ .:lli 42 77 3
AN 39 802 802 19.6 46 15719 38892 56 32 88 3
AO 40 816 598 24.1 38 14412 illZj 69 19 88 4
AP 41 779 496 33.2 72 16467 35712 79 12 91 2
AQ 42 840 829 20.2 32 16746 26528 lJl 61 89 0
AR 43 778 968 14.2 39 13746 .ill!l2. 2.1 lU 90 0
AS 45 778 912 16.2 45 14774 41040 4li 32 78 0
AT 4li m 713 15.9 51 11337 ~ ;JQ 10 ~ I
AU 47 889 1023 11,3 32 11560 32736 2 56 58 1
AV 48 832 956 18.1 55 17304 52580 44 39 83 2
AW 38 766 832 15,7 42 13062 34944 35 42 77 3
~ 48 832 956 18.! 55 17304 52530 44 39 83 2
BAINITE PEARLITE
AREA AREA
FRACTION FRACTION
f~>~·~/ (%)
13 15
14 1.1
17 20
29 2i
7 ;JQ
8 11
8 11
3 0
I 10
4 8
2 0
0 11
13 1
9 a
5 3
6 1
11 0
0 10
18 4
16 ;l;l
33 8
13 2
13 7
13 2
PEARLITE
AREA
fRACTION
BEFORE
COLD
ROLLING(%)
25
31
17
42
28
41
21
3
9
26
7
15
14
16
16
11
22
·11
!3
40
7
45
14
45
~ o" c;
tv
t'v
~
~
0
0
00
tv
~
-!""'
V>
~ g:
"w
t'v
~
~
0
0
00
-"" ~
STEEL
TYPE
REFERENCE
SYMBOL
A
8
c
D
E
F
G
H
r
J
K
L
M
N
0
p
Q
R
s
T
u
w
X
y
z
TS EL ),
(Mpa) (%) (%)
462 40.2
447 41.2
512 36.2
553 32.7
589 31.9
589 32.1
561 30.9
632 30.0
698 28,3
755 25.9
7.21 24,5
752 24.2
789 20.9
768 19.8
802 21.2
835 18,8
872 22 .. 5
852 21,5
912 20.1
965 18.5
989 n.o
1025 15.9
1049 17.2
1102 14.5
1189 13.1
AfTER HOT STAMPING
FERRITE
AREA MARTENSITE
TS X EL TSx A FRACTION AHEA (%) FRACTION(%)
135 1857Z 62370 92 6
125 18416 55875 85 7
ns 18534 58880 83 10
115 18083 63595 82 7
99 19378 58311 81 6
87 18907 51243 82 7
90 17335 50490 66 10
89 18960 56248 86 8
75 19753 52350 &5 7
87 19555 65685 59 12
72 17665 51912 52 22
78 18198 58656 53 23
69 16490 54441 57 35
72 15206 55296 59 27
65 17002 52130 41 35
75 15698 62525 45 2~
61 19620 53192 41 39
69 18318 58788 47 31
56 18331 51072 56 3l
62 11853 59830 41 41
55 16813 54395 49 37
53 16296 54325 46 38
49 18043 $1401 46 31
51 15979 56202 43 40
55 15576 65395 45 48
FERRITE RESIDUAL BAINITE + AUSTENITE AREA MARTENSITE VOLUME
AREA FRACTION FRA,C,T, ION
FRACTION(~) (%) '"
98 1 0
92 3 4
93 1 5
89 3 8
87 1 12
89 2 4
76 2 14
94 4 0
72 4 23
71 1 25
74 1 19
76 2 21
92 2 6
86 5 4
76 4 11
68 1 31
80 4 10
78 4 13
as 4 2
82 3 12
86 1 13
84 4 12
83 J 11
83 1 16
93 2 5
PEARLITE
AREA
FRACTION
(%)
I
I
1
0
0
5
8
2
I
3
B
1
0
5
9
0
6
5
6
3
0
0
3
0
0
PlATlNB
TYPE
•)
GA
Gl
GA.
GA
OR
GA
Gl
EG
GA
AI
GA
OR
C'R
GA
Cl
EC
AI
CR
CR
(',A
GA
GA
GA
Gl
GA
~ p;l
cr'
(D'
w
-' ~
~
0
0
00 w
~
-"" ~
~ cr"
(;"
.j>.
' ~
~
~
0
0
00
V>
~
STEEL
TYPE
REFERENCE
SYMBOL
AA
AS
AC
AD
AE
AF
AG
AH
AI
AJ
AK
AL
AM
AN
AC
AP
AQ
AR
AS
AT
AU
AV
AW
AX
TS EL },
(Mpa) (%) (%)
756 19,2
821 18.3
891 17.6
922 16.8
1021 15.8
1152 13.8
723 19.1
412 42.1
1513 8.3
821 16.9
912 18.9
398 41.2
1023 14.2
923 17.6
)36 19.2
543 31.0
1128 14,3
1062 12.9
1109 13.8
1021 11.9
1236 9.9
1151 13.1
1023 14.2
1151 13.1
AfTER HOT STAMP !NB
FERRITE
AREA MARTE~SlTF
TSxEL TSx ;t FRACTION AREA
FRACTION(%) (%)
63 14515 47628 37 39
57 15024 i'illl ~ 42
51 15682 45441 32 41
41 15490 37802 29 38
31 16132 ,'ill.ll 49 J.1
38 15898 illli 37 42
61 13809 .ill.illl. 72 16
109 17345 44908 97 Q
27 12558 =ill.llll !2 .!l!l.
52 13875 42692 57 25
43 17237 39216 85 32
113 16398 .1!1Jill. 86 2.
43 14527 ~ 45 43
46 16245 ±llil!. 57 31
41 14131 ;!lU1!l. 63 26
68 1683c~ ~ 78 14
34 16130 38352 29 63
35 13700 37170 29 65
41 15304 .:!.:i.:!.!l!l. 46 32
38 12150 .ilJll.!l!l. 30 28
34 '12236 ~ l ill!
46 15078 52946 41 44
43 14527 .:l.:llli!ll. 45 43
4$ 15078 52946 41 t;t,.
FERRITE RESIDUAL • AUSTENITE BAlN!TE MARTENSITE VOL'UME AREA AREA FRACTJDN FRACTION
FRACTION(%) (%) (%)
76 2 11
81 1 6
73 2 10
67 1 14
so 2 7
79 . 2 1
88 2 a
97 0 3
94 3 2
82 2 13
97 2 1
88 0 I
88 3 8
88 3 9
89 4 7
92 1 6
. 92 0 6
94 0 0
78 3 14
58 1 11
76 4 18
85 4 10
88 3 8
85 4 10
PEARLITE
AREA
FRACTION
(%}
11
ll
15
18
11
18
ll
0
I
3
0
11
1
0
0
I
2
6
5
30
2
1
1
1
PLATING
TYPE
*)
GA
CR
GA
EG
Gl
N
Gl
EG
N
GA
GA
GA
GA
Gl
OR
GA
GA
G.A
GA
GI
Gl
Gl
CR
OR
...j.>.,.
I 0
0
00
0~
STEEL
TYPE
REFERENCE
SYMBOL
A
8
c
D
E
F
G
H
l
J
K
L
M
N
0
p
0
R
s
T
u
w
X
y
z
LEFT SIDE OF
EXPRESSION
(B) BEfORE
HOT STAMPING
1.01
1.04
1.05
W8
1.07
1.08
!,08
1.02
1.05
1.05
1,03
!.03
1.08
1.06
1,07
!.04
1.04
1.02
1.0ii
1.09
1.07
1.09
1.06
!.Q4
1.0~
LEFT SIDE OF
DETERMINATION EXPRESSION (B) AfTER DETERMINATION
HOT STAMPING
G 1.02 G
G 1.02 G
G 1.07 G
G 1.07 G
G 1,05 G
0 1.09 0
G 1,09 G
G 1.03 G
G 1.04 G
G 1.01 G
G 1,Q4 G
G 1.02 G
G 1.06 G
G 1.08 G
G 1,08 G
G 1.05 G
G 1.06 G
G 1.04 G
G 1,05 G
G 1.08 G
G 1.08 G
G 1.08 G
G 1,08 G
G 1.05 G
G 1.05 G -- ------
l.EFT SID~ OF LEFT SIDE OF
EXPRESSION DETERMlNATJON EXPRESSION
(G) BEFORE
HOT ST A~P I NG 110tC) sf~JPEI~l13 G 15
l7 G 16
5 G 3
17 G 15
18 G 17
12 G 13
15 G 12
7 G 9
a G 9
15 G 14
19 G 18
14 G 13
14 G 15
12 G 13
13 G 12
11 G 10
12 G 12
15 G 15
16 G 18
10 G 15
6 G 5
7 G 9
11 G 16
12 G 11
10 G 9 .
AREA FRACTION OF
DETERMINATION MnS OF D. 1 ,u m
OR MORE BEFORE
IIOT STAMPING
G 0.004
G 0.006
G 00!6
G 0.006
G 0.006
G 0015
G 0.008
G 0.006
G 0.005
G 0.005
G 0.005
G 0.006
G 0.01?
G 0.003
G 0,003
G 0.006
G 0.005
G 0.006
G 0.008
G 0.003
G \l.!l.1.4
G 0.006
G 0.006
G 0.006
_G 0.006
'REA FRACT! ON OF
MnS OF 0.1 ,um
OR MORE AFTER
IIOT STAMP 1 NG
0,004
0.005
0014
0.006
0.007
0015
0.007
0.005
0.006
0.006
0.006
0.007
.ll..Qll
0.003
0,004
0.005
0.006
0.007
0,008
0.004
Jl.lill.
0.007
0,006
0.004
0.007
.j>.
00
STFEL LEFT SIDE OF U'H SWE OF
TYPE EXPRESSION DETERMINATION EXPRESSION
REFERENCE (B) BEFORE (B) AfTER
SYMBDI. HOT STAMPING HOT STMIPlNG
AA U3 B 1 Hi
AB 115 B 11'6
AC 113 B i t5
AD .1.1Ji B lJ..a
AE .1U a .1U
AF 1 11 B 1 10
AO 116 B !17
AH = B = AJ w B 1..1.9
AJ 123 8 1;tz,
AK 1 19 8 1 18
AL = 8 -
AM 1.41 B 1 39
AN 1 28 8 1..22.
AO 1 29 B 1..;JJ
AP 1.06 G . 1.05
AQ .1.1Ji B 1.21
AR 1,09 G 1.07
AS 1 23 B l.2J
AT ill B l.2.!l.
AU 1.06 G 1.07
AV 1.06 0 1.07
AW 1 41 B 1 39
AX 1.06 G 1.07
LEFT SIDE OF LEFT SIDE OF AREA FRAGTJON OF AREA FRACTION OF
DETERM INA T JON EXPRESSION DETER!Ilt'AT IDN EXPRESSiml DETERMINATION MnS OF 0. 1 ;Im (C) BEFORE (C) AfTER OR MilRE BEFORE ~off ~&R?}FT~~
HOT STAMPING HOT STAMPING HOT STAMPING HOT STAMP !NG
B 2.,1 s ll B 0.011 OQL3
6 22 8 21 B 0.008 0,007
a 21 8 20 B 0050 0.006
B 2.!l. B 2.!l. B 0"006 0.007
8 ll s 21 B 0.009 0.009
8 19 G 18 G 0.003 0,003
B 25 B 24 B 0.003 0.003
B - B = B 0.004 0.004
B ZJ 8 2:J. B 0.006 0.006
B 21 B 23 B 0.007 0.008
8 23 B 22 B 0.007 0.006
8 = B = B 0.006 0.006
B 3! B ;1!) B 0.006 0.007
8 26 8 29 B 0.008 0.009
6 28 8 33 B 0.005 0.004
G II G 12 G 0.005 0.007
B 2.l 8 2.!l. B 0.003 0.003
G 17 G 17 G 0.002 0.002
8 23 8 23 B 0.006 0.00"1
8 2.1 B 2.!l. B 0.005 0.006
G I 18 G 19 G 0.006 0.005
G 18 G 19 G 0.006 0.005
8 31 B 30 B o.. a o6 0.007
G 18 G _. __ 19 G 0.006 0.005
- HARDNESS WAS NOT MEASURED BECAUSE THE AREA fRACTION OF MARTENSITE IS S!D;~IFICANTLY SI\ALL.
~ 0" " ~
t0 ~
~ ""
[0087]
[Table 5-1]
50
~
0
0
00
00
~
STEEL
TYPE
REFERENCE
SYMBOL
A
B
c
0
E
F
G
H
l
J
K
L
M
N
0
p
0
R
s
T
u
w
X
y
z
BEFORE HOT STAMPI~G
"0 " LEFT
~ nl n2 SIDE OF
EXPRESSlON ~ (D) ~ >-
l.W
0
10 12 1.2 G
6 7 1.2 G
3 5 17 B
7 6 0.9 G
2 2 1.0 G
2 2 1.0 G
1 1 1,0 G
5 6 1.2 G
3 4 1.3 G
4 4 1.0 G
6 7 1.2 G
5 7 1.4 G
II 20 LJ! B
5 6 1.2 G
3 3 1.0 G
5 6 1.2 G
a 9 1.1 G
16 16 1.1 G
11 12 1.1 G
6 7 1.2 G
7 15 2.1 B
16 20 1.3 G
22 26 1,2 G
22 29 1.3 G
27 32 1.2 G
AFTER HOT STAMPING
:z
0 SURfACE
LEFT !< PROPERTY
n1 n2 EXSPIDREES SO JFO N z AFTER HOT ~ STAMPING
(D) w,_
w
0
8 11 1.4 G 0
6 5 0.8 G 0
3 5 1 7 B 0
6 6 1.0 G 0
2 2 1.0 G 0
2 2 1.0 G 0
1 I 1.0 G 0
5 5 1.0 G 0
4 4 1.0 G 0
4 5 1.3 G 0
7 9 1.3 G 0
5 6 1.2 G 0
II 19 1.1 8 0
6 7 1.2 G 0
3 3 1,0 G 0
5 5 l.O G 0
7 8 1.1 G 0
15 18 1.2 G 0
10 12 1,2 G 0
6 6 1.0 G 0
7 14 2.0 B 0
15 19 1.3 G 0
22 23 1,0 G 0
21 28 1.3 G 0
26 32 1.2 G 0
"s" LEFT ><- LEFT RIGHT SlOE Of z SIDE OF SHlE OF
EXPRESSION "" EXPRESSION CT EXPRESSION
(E) "w' {F) (!:">w- "
C>
1.32 G 489 580 768
1.13 VG 474 650 757
1.23 0 354 510 644
1.29 G 457 580 728
1.51 G 487 615 734
1.23 G 410 12.l 700
1.43 G 438 741 729
1.10 VG 461 585 720
1.38 G 450 542 740
1.34 G 444 5\12 701
1.22 G 408 715 o97
1.42 G 404 482 673
1.24 G 400 463 692
1.33 G 374 502 644
1.36 G 407 631 694
1.52 G 375 5.2.7 640
1.61 G 410 526 694
1.40 G 432 543 727
1,28 G 411 554 696
1.20 VG 363 523 649
1.41 G 372 621 650
1.07 VG 387 521 6Bli
1.26 G 393 682 670
1.24 G 358 482 838
1.55 G 366 451 651
5 IN-FURNACE
~ IEMPERATUHE Tli!E OF
"'
OF HEATING HEATING
es FURNACE FURNACE
>- (MINUTES) w
"'
G 11 so 65
G 1250 72
G 1154 68
G 1260 n
G 1215 116
B 1322 135
B 1173 123
G 1205 95
G 1189 87
G 1221 89
8 12ll2 95
G 1212 165
G 1105 25
G 1295 i95
G 1240 135
G 1298 201
(l 1192 120
G 1250 179
G 1232 122
G 1232 162
G 1113 20
G 12UO 12o
B 1180 141
G 1280 162
G 1260 181
LEFT
SIDE OF_,,
EXPRESSION
(G)
0229
4968
~968
4570
6593
3302
402£
6084
4331
3909
2649
3267
1708
2784
4004
2785
3252
4879
3729
26~~0
l4:!.!l
3049
3380
2600
2915
s"' >-
~
"e"s >-
"a J
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
B
G
G
G
G
-""
~-~-~-----'- -·-·-'-----'---------"-'=-'-~'-'- --=-~,---=----=--o.-=o..-==~-'-'"'------
BEFORE HOT STAMPING AfTeR HOT S1AMP!~G STEEL a a SURFACE LEFT
TYPE LEFT ;: LEFl ;:: PROPERTY SIDE OF
REFERENCE sIDE OF ;1' SIDE OF ;1'
"1 "2 EXPRESSION ;;; n 1 "2 EXPRESS ION ;;;
AFTER HOT EXPRESSION
SYMBOL (0) ffi (D) ffi STA!IPIMG (E)
,...
1!: ~
AA 12 Ll u G 12 14 1.2 G 0 w
AB 10 12 1.2 G JG 13 1.3 G 0 ll.!ll
AC 15 18 L2 G 16 19 1'.2 G 0 ~
AD 6 8 1,3 G 6 7 1.2 G 0 2&1
AE 12 16 1.3 G 12 !5 1.3 G 0 0.12
AF 18 22 1.2 G 17 22 1.3 G 0 w
AG 6 7 1.2 G 5 7 1.4 G 0 ill
AH 4 5 1.3 G 4 4 1.0 c 0 1.18
i'' AI 12 15 !.3 G 12 14 1.2 G 0 1.16
AJ 17 21 1.2 G 1:> 21 1.4 G 0 1.26
AK 12 14 1.2 G 12 13 1.1 G 0 1.25
AL 2 2 1,0 G 2 2 1.0 G 0 1,16
AM 16 22 1.4 G 15 21 1.4 G X 1.26
AN 10 12 1.2 G 10 11 IJ G 0 1.!9
AO 1 '! 12 1.1 G 10 11 1.1 G 0 1.08
AP 7 9 1.3 G 7 8 1.1 G 0 1.17
AQ 13 14 1.1 G 14 16 1.1 G 0 1.06
AR 21 26 1.2 G 22 25 1.1 G 0 1.36
AS 18 19 'l.1 G 18 18 1.0 G 0 1.16
AT 15 17 1.1 G 16 16 1.0 G 0 1.17
AU 17 19 1,1 G 16 18 1,1 G 0 1.39
AV 17 19 1.1 G 16 18 1.1 G /.;), 1.42
AW 16 22 1.4 G 15 21 1.4 G X 1.25
AX 17 19 1.1 G 1S 18 1.1 G /.;), 1.43
"Q " "' "'
~ LEFT RIGHT ~
SIDE OF SIPE OF ill CT ~ EXPRESSION EXPRESSION ~
tlJ
(f) (F)
tJ 0 "'
B 442 582 729 G
8 430 535 719 G
B 400 426 692 G
B 436 623 721 G
B 431 611 730 G
B 384 396 680 G
B 402 557 696 c
vc 413 462 689 c
VG 325 476 643 G
G 4.20 543 696 G
c 435 558 687 G
vc 481 m 117 c
G 248 539 546 G
VG 401 560 667 G
VG 39li 523 673 G
VG 443 551 124 G
VG 363 402 648 G
G 371 432 649 G
VG 398 630 695 G
vc 384 669 682 G
G 365 456 664 G
G 3£0 456 658 G
G 248 539 546 G
G 360 456 658 G
IN-fURfiACE
TEMPERATURE TIME OF
OF HEAHNG HEATING
FURNACE FURNACE
(MINUTES)
1210 !28
1236 116
1210 12.5
1210 145
ns2 152
1198 86
1209 147
1209 135
1260 !65
1230 98
1211 156
1180 161
1291 332
1219 135
1266 173
1230 125
1250 140
1241 192
1263 191
1203 203
1248 192
1248 192
1291 332
1248 192
LEfT
SIDE OF
EXPRESSION
{G)
3865
3o9l
2814
3604
4S2!
2449
3134
2339
2717
3269
5054
16656
1602
3134
2694
3378
2£05
3115
3540
3026
2697
2571
1602
2571
=
2
:;;:
""
,f_f_i
UJ
0
G
G
G
G
G
G
c
c
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
0
~ cr
<'D
u,
N'
~
u,
N
53
[0089]
Based on the above-described examples and comparative examples, it is found
that, as long as the conditions of the present invention are satisfied, it is possible to obtain
a cold-rolled steel sheet, a galvanized cold-rolled steel sheet, a galvannealed cold-rolled
5 steel sheet, a electro galvanized cold-rolled steel sheet, or a alluminized cold-rolled steel
sheet all of which satisfY TS x A :2: 50000 MPa·% even after hot stamping, and a
hot-stamped steel manufactnred from the obtained cold-rolled steel sheet.
10
Industrial Applicability
[0090]
Since the cold-rolled steel sheet and the hot-stamped steel which are obtained in
the present invention can satisfy TS x A :2: 50000 MPa·% after hot stamping, the
cold-rolled steel sheet and the hot-stamped steel have a high press workability and a high
strength, and satisfies the current requirements for a vehicle such as an additional
15 reduction of the weight and a more complicated shape of a component.
Brief Description of the Reference Symbols
[0091]
Sl: MELTING STEP
20 S2: CASTING STEP
S3: HEATING STEP
S4: HOT-ROLLING STEP
SS: COILING STEP
S6: PICKLING STEP
25 S7: COLD-ROLLING STEP
54
S8: ANNEALING STEP
S9: TEMPER-ROLLING STEP
S!O: GALVANIZING STEP
Sll: ALLOYING STEP
5 S12: ALUMINIZING STEP
Sl3: ELECTROGALVANIZING STEP

What is claimed is:
CLAIMS
1. A hot-stamped steel comprising, by mass%:
C: 0.030% to 0.150%;
Si: 0.010% to 1.000%;
Mn: 0.50% or more and less than 1.50%;
P: 0.001% to 0.060%;
S: 0.001% to 0.010%;
N: 0.0005% to 0.0100%;
AI: 0.010% to 0.050%, and
optionally at least one of
B: 0.0005% to 0.0020%;
Mo: 0.01% to 0.50%;
Cr: 0.01% to 0.50%;
V: 0.001% to 0.100%;
Ti: 0.001% to 0.100%;
Nb: 0.001% to 0.050%;
Ni: 0.01% to 1.00%;
Cu: 0.01% to 1.00%;
Ca: 0.0005% to 0.0050%;
REM: 0.0005% to 0.0050%, and
a balance of Fe and impurities, wherein
when [C] is an amount of C by mass%, [Si] is an amount of Si by mass%, and
25 [Mn] is an amount ofMn by mass%, a following expression (A) is satisfied,
5
56
an area fraction of a ferrite is 40% to 95% and an area fraction of a martensite is
5%to 60%,
a total of the area fraction of the ferrite and the area fraction of the martensite is
60% or more,
the hot-stamped steel optionally further includes one or more of a pearlite, a
retained austenite, and a bainite, an area fraction of the pearlite is 10% or less, a volume
fraction of the retained austenite is 5% or less, and an area fraction of the bainite is less
than40%,
a hardness of the martensite measured with a nanoindenter satisfies a following
10 expression (B) and a following expression (C),
15
TS x A which is a product of a tensile strength TS and a hole expansion ratio A is
50000 MPa·% or more,
(5 x [Si] + [Mn]) I [C] > 10 (A),
H2/ HI< 1.10 (B),
crHM <20 (C), and
the HI is an average hardness of the martensite in a surface portion of a sheet
thickness of the hot-stamped steel, the surface portion is an area having a width of 200
1-1m in a thickness direction from an outermost layer, the H2 is an average hardness of the
martensite in a central portion of the sheet thickness of the hot-stamped steel, the central
20 portion is an area having a width of200 f!ID in the thickness direction at a center of the
sheet thickness, and the crHM is a variance of the average hardness of the martensite in
the central portion of the sheet thiclmess of the hot-stamped steel.
2. The hot-stamped steel according to claim 1, wherein
25 an area fraction of MnS existing in the hot-stamped steel and having an
57
equivalent circle diameter of 0.1 J.Lm to 10 J.Lm is 0.01% or less,
a following expression (D) is satisfied,
n2/nl 1.00 (E), and
the ri (i =I, 2, 3) is an individual target cold-rolling reduction at an ith stand (i =
I, 2, 3) based on an uppermost stand in the plurality of stands in the cold-rolling in
15 unit %, and the r is a total cold-rolling reduction in the cold-rolling in unit %.
8. The method for producing the hot-stamped steel according to claim 7,
wherein
the cold-rolling is carried out under a condition satisfying a following
20 expression (E'),
25
1.20 2: 1.5 x rl I r + 1.2 x r2 I r +r3 I r > 1.00 (E'), and
the ri (i = I, 2, 3) is the individual target cold-rolling reduction at the ith stand (i
= 1, 2, 3) based on the uppermost stand in the plurality of stands in the cold-rolling in
unit%, and the r is the total cold-rolling reduction in the cold-rolling in unit%.
59
9. The method for producing the hot -stamped steel according to claim 7 or 8,
wherein
when CT is a coiling temperature in the coiling in unit oc, [C] is the amount of
C in the steel by mass%, [Mn] is the amount ofMn in the steel by mass%, [Si] is the
5 amount of Si in the steel by mass%, and [Mo] is the amount of Mo in the steel by mass%,
a following expression (F) is satisfied,
10
15
20
25
560-474 x [C]- 90 x [Mn]- 20 x [Cr]- 20 x [Mo] < CT < 830-270 x [C]- 90
x [Mn]- 70 x [Cr]- 80 x [Mo] (F).
I 0. The method for producing the hot-stamped steel according to any one of
claims 7 to 9, wherein
when Tis a heating temperature in the heating in unit °C, tis an in-furnace time
in the heating in unit minute, [Mn] is the amount ofMn in the steel by mass%, and [S] is
an amount of Sin the steel by mass%, a following expression (G) is satisfied.
T x ln(t) I (1.7 x [Mn] + [S]) > 1500 (G)
11. The method for producing the hot -stamped steel according to any one of
claims 7 to 10, further comprising:
galvanizing the steel between the annealing and the temper-rolling.
12. The method for producing the hot-stamped steel according to claim 11,
further comprising:
alloying the steel between the galvanizing and the temper-rolling.
13. The method for producing the hot -stamped steel according to any one of
60
claims 7 to I 0, further comprising:
electro galvanizing the steel after the temper-rolling.
14. · The method for producing the hot-stamped steel according to any orie of
5 claims 7 to 10, further comprising:
10
15
20
25
aluminizing the steel between the annealing and the temper-rolling.
15. A cold-rolled steel sheet comprising, by mass%:
C: 0.030% to 0.150%;
Si: 0.010% to 1.000%;
Mn: 0.50% or more and less than 1.50%;
P: 0.001% to 0.060%;
S: 0.001% to 0.010%;
N: 0.0005% to 0.0100%;
AI: 0.010% to 0.050%, and
optionally at least one of
B: 0.0005% to 0.0020%;
Mo: 0.01% to 0.50%;
Cr: 0.01% to 0.50%;
V: O.OOI%to 0.100%;
Ti: 0.00 I% to 0.1 00%;
Nb: 0.001% to 0.050%;
Ni: 0.01% to 1.00%;
Cu: 0.01% to 1.00%;
Ca: 0.0005% to 0.0050%;
5
61
REM: 0.0005% to 0.0050%, and
a balance of Fe and unavoidable impurities, wherein
when [C] is an amount of C by mass%, [Si] is an amount of Si by mass%, and
[Mn] is an amount ofMn by mass%, a following expression (A) is satisfied,
an area fraction of a ferrite is 40% to 95% and art area fraction of a martensite is
5%to 60%,
a total of the area fraction of the ferrite and the area fraction of the martensite is
60% or more,
the cold-rolled steel sheet optionally further includes one or more of a pearlite, a
10 retained austenite, and a bainite, an area fraction of the pearlite is 10% or less, a volume
15
20
fraction of the retained austenite is 5% or less, and an area fraction of the bainite is less
than40%,
a hardness of the martensite measured with a nanoindenter satisfies a following
expression (H) and a following expression (I),
TS x A which is a product of a tensile strength TS and a hole expansion ratio A is
50000 MPa·% or more,
(5 x [Si] + [Mn]) I [C] > 10 (A),
H20 I HlO < 1.10 (H),
aHMO < 20 (I), and
the HI 0 is an average hardness of the martensite in a surface portion of a sheet
thickness, the surface portion is an area having a width of 200 J.Lm in a thickness direction
from an outermost layer, the H20 is an average hardness of the martensite in a central
portion of the sheet thickness, the central portion is an area having a width of 200 J.llll in
the thickness direction at a center of the sheet thickness, and the a HMO is a variance of
25 the average hardness of the martensite in the central portion of the sheet thickness.
62
i
16. The cold-rolled steel sheet according to claim 15, wherein
an area fraction of MnS existing in the cold-rolled steel sheet and having an
equivalent circle diameter of 0.1 !J.m to 10 !J.m is 0.01% or less,
5 a following expression (J) is satisfied,
n20 I nlO < 1.5 (J), and 1
, I
the nl 0 is an average number density per 10000 !J.m2 oft\1e MuS having an
!
equivalent circle diameter of 0 .l !J.m to 1 0 !J.111 in a l /4 pmiion of the sheet thickness, and
!
the n20 is an average number density per 10000 !J.m2 of the MnS liaving an equivalent
10 circle diameter ofO.l !J.m to 10 ~min the central portion of the sheet thickness.
15
20
17. The cold-rolled steel sheet according to claim 15 16, wherein a hot'dip
galvanized layer is fmmed on a surface thereof.
18. The cold-rolled steel sheet according to claim 17, ~herein the hot-dip
I
galvanized layer is alloyed.
19. The cold-rolled steel sheet according to claim 15 or 16, wherein an
electro galvanized layer is formed on a surface thereof.
20. The cold-rolled steel sheet according to claim 15 oi 16, wherein an
aluminized layer is formed on a surface thereof.