1
Specification
Title of Invention
HOT STAMPED STEEL AND METHOD FOR PRODUCING HOT STAMPED STEEL
Technical Field of the Invention
[OOOI]
The present invention relates to a hot stamped steel for which a cold rolled steel
sheet for hot stamping having an excellent formability after hot stamping is used, and a
method for producing the same.
Priority is claimed on Japanese Patent Application No. 2012-004550, filed
January 13,2012, 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
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 ofthe steel sheet, and
quenched by cooling after forming, thereby obtaining desired material qualities. As
described above, a steel sheet used for a body structure of a vehicle is required to have a
high press wol-lability 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
2
retained ausienite in a structure or the like is known as a steel sheet having both press
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
5 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
interface.
[0003]
For example, patent Documents 1 to 3 disclose the multi-phase steel sheet. In
10 addition, Patent Documents 4 to 6 describe relationships between the hardness and
formability ofa 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
I 15 reduction of the weight and more complicated shapes of a components.
Prior Art Document
Patent Document
[0005]
20 [Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. H6-128688
[Patent Document 21 Japanese {Jnexamined Patent Application, First
Publication No. 2000-3 19756
[Patent Document 31 Japanese Unexamined Patent Application, First
25 Publication No. 2005-120436
3
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. 2005-2561 41
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 2001-355044
[Patent Document 61 Japanese Unexamined Patent Application, First
Publication No. H11-189842
Disclosure of the Invention
Problems to be Solved by the Invention
[0006]
An object of the present invention is to provide a hot stamped steel, for which a
cold rolled steel sheet capable of ensuring a strength and have a more favorable hole
expansibility when produced into a hot stamped steel is used, and a method for producing
the same hot stamped steel.
15
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
20 stamping process) and had an excellent formability (hole expansibility). As a result, it
was found that, regarding the steel composition, when an appropriate relationship is
established among the amount of Si, the amount of Mn and the amount of C, a fraction of
a rerrite and a fiaction of a martensite in the steel sheet are set to predetermined fractions,
and the hardness ratio (difference of a hardness) of the mai-tensite between a surface part
25 of a sheet thickness and a central part of the shcet thickness of the steel sheet and the
4
hardness distribution of the martensite in the central part of the sheet thiclcness are set in
specific ranges, it is possible to industrially produce a cold rolled steel sheet for hot
stamping capable of ensuring, in the steel sheet, a formability, that is, a characteristic of
TS x h 2 50000MPa- % that is a larger value than ever in terms of TS x h that is a
5 product of a tensile strength TS and a hole expansion ratio h. Furthermore, it was found
that, when this cold rolled steel sheet is used for hot stamping, a hot stamped steel having
excellent formability even after the hot stamping is obtained. In addition, it was also
clarified that the suppression of a segregation of MnS in the central part of the sheet
thickness of the cold rolled steel sheet for hot stamping is also effective in improving the
10 formability (hole expansibility) of the hot stamped steel. In addition, it was also found
that, in cold-rolling, an adjustment of a 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
15 of the present invention as described below. In addition, it was found that the effects are
not impaired even when a hot dip galvanized layer, a galvannealed layer, an
electrogalvanized layer and an aluminized layer are formed on the cold rollcll steel sheet.
[OOOS]
(1) That is, according to a first aspect of the present invention, a hot stamped
20 steel includes, by mass%, C: 0.030% to 0.150%, Si: 0.010% to 1.00%, Mn: 1.50% to
2.70%, P: 0.001% to 0.060%, S: 0.001% to 0.010%, N: 0.0005% to 0.0100%,Al: 0.010%
to 0.050%, and optionally one or more 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 O.lOO%,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:
25 0.00050% to 0.0050%, and a balance including Fe and unavoidable impurities, in which,
5
when [C] represents an amount of C by mass%, [Si] represents an amount of Si by
mass%, and [Mn] represents an amount of Mn by mass%, a following expression (A) is
satisfied, a metallographic structure after a hot stamping includes 40% to 90% of a ferrite
and 10% to 60% of a martensite in an area fraction, a total of an area fraction of the
ferrite and an area fraction of the martensite is 60% or more, the metallographic structure
may optionally further includes one or more of 10% or less of a perlite in an area fraction,
5% or less of a retained austenite in a volume ratio, and less than 40% of a bainite as a
remainder in an area fraction, a hardness of the martensite measured with a nanoindenter
satisfies a following expression (B) and a following expression(C), TS x h which is a
product of a tensile strength TS and a hole expansion ratio his 50000MPa.% or more,
(5 x [Si] + [Mn]) / [C] > 11 (A),
132/H1<1.10 (B),
OHM < 20 (C), and
the H1 is an average hardness of the martensite in a surface part of a sheet
thiclcness after the hot stamping, the H2 is an average hardness of the martensite in a
central part of the sheet thickness which is an area having a width of 200 pm in a
thickness direction at a center of the sheet thickness after the hot stamping, ;111cl the OHM
is a variance of the average hardness of the martensite in the central part of the sheet
thickness after the hot stamping.
[0009]
(2) In the hot stamped steel according to the above (I), an area fraction of MnS
existing in the hot stamped steel and having an equivalent circle diameter of 0.1 pm to 10
pm may be 0.01% or less, and a following expression (D) may be satisfied,
n2/n1<1.5 (D),and
the nl is an average number density per 10000 pm2 of the MnS having the
6
equivalent circle diameter of 0.1 pm to 10 pm in a 114 part of the sheet thickness after the
hot stamping, and the n2 is an average number density per 10000 pm2 of the MnS having
I the equivalent circle diameter of 0.1 pm to 10 pm in the central part of the sheet
thickness after the hot stamping.
5 [OO 101
I
i (3) In the hot stamped steel according to the above (1) or (2), a hot dip
galvanizing may be formed on a surface thereof.
[OOll]
(4) In the hot stamped steel according to the above (3), a galvannealing may be
10 formed on a surface of the hot dip galvanizing.
1001 21
( 5 ) In the hot stamped steel according to the above (1) or (2), an
electrogalvanizing may be formed on a surface thereof.
[OO 131
15 (6) In the hot stamped steel according to the above (1) or (2), an aluminizing
may be formed on a surface thereof.
[OO 141
(7) According to another aspect of the present invention, there is provided a
method for producing a hot stamped steel including casting a molten steel having a
20 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 stands, 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
satisfying a following expression (E), annealing in which the steel is annealed under
25 700°C to 850°C and cooled after the cold-rolling, temper-rolling the steel after cooled
7
following annealed, and hot stamping in which the steel is heated to a temperature range
of 700°C to 1000°C after the temper-rolling, hot-stamped within the temperature range,
and thereafter cooled to a room temperature or more and 300°C or less,
1.5 x r l / r + 1 . 2 x r 2 / r + r 3 / r > 1.0 (El, and
5 the ri (i = 1,2, 3) represents 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 represents a total cold-rolling reduction in the
cold-rolling in unit %.
[OO 151
10 (8) In the method for producing the hot stamped steel according to the above (7),
when CT represents a coiling temperature in the coiling in unit OC, [C] represents the
I
! amount of C by mass%, [Mn] represents the amount of Mn by mass%, [Si] represents the
I amount of Si by mass%, and [Mo] represents the amount of Mo by mass% in the steel
sheet, a following expression (F) may he satisfied,
I I 15 560.474~ [C]-90x [Mnl-20x [Cr]-20x [Mo] 1500 (GI.
[00 171
25 (10) The method for producing the hot stamped steel according to any one of the
8
above (7) to (9) may further include galvanizing the steel between the annealing and the
temper-rolling.
[OO 181
(1 1) The method for producing the hot stamped steel according to the above (10)
5 may further include alloying the steel between the galvanizing and the temper-rolling.
[OO 191
(12) The method for producing the hot stamped steel according to any one of the
above (7) to (9) may further include electrogalvanizing the steel after the temper-rolling.
[0020]
10 (13) The method for producing the hot stamped steel according to any one of the
above (7) to (9) may further include aluminizing the steel between the annealing and the
I
temper-rolling.
Effects of the Invention
15 [002 11
According to the above-described aspect of the present invention, since an
appropriate relationship is established among the amount of C, the amount or Mn and the
amount of Si, and, even in the hot stamped steel, the hardness of the matensite measured
with a nanoindenter is set to an appropriate value, it is possible to obtain a more
20 favorable hole expansibility in the hot stamped steel.
Brief Description of the Drawings
to0221
FIG. 1 is a graph illustrating the relationship between (5 x [Si] + [Mn]) / LC] and
25 TS x h in a cold rolled steel sheet for hot stamping before hot stamping and a hot
stamped steel.
FIG. 2A is a graph illustrating a foundation of an expression (B) and is a graph
illustrating the relationship between an H20 / HI0 and a oHMO in the cold rolled steel
I i sheet for hot stamping before hot stamping and the relationship between H2 / HI and
5 OHM in the hot stamped steel.
FIG. 2B is a graph illustrating a foundation of an expression (C) and is a graph
illustrating the relationship between oHMO and TS x h in the cold rolled steel sheet for
hot stamping before hot stamping and the relationship between OHM and TS x h in the
hot stamped steel.
10 FIG. 3 is a graph illustrating the relationship between n20 / n10 and TS x h in the
cold rolled steel sheet for hot stamping before hot stamping and the relationship between
I
n2 I nl and TS x h in the hot stamped steel and illustrating a foundation of an expression
(Dl.
FIG. 4 is a graph illustrating the relationship between 1.5 x rl I r + 1.2 x r2 I r +
15 r3 I r and H20 / H10 in the cold rolled steel sheet for hot stamping before hot stamping
and the relationship between 1.5 x rl I r + 1.2 x 1-2 / 2 + r3 1 r and H2 I HI in the hot
stamped steel, and illustrating a foundation of an expression (E).
FIG. 5A is a graph illustrating the relationship between an expression (F) and a
fraction of a martensite.
20 FIG. 5B is a graph illustrating the relationship between the expression (F) and a
fraction of a pearlite.
FIG. 6 is a graph illustrating the relationship between T x ln(t) / (1.7 x [Mn] t
[S]) and TS x h, and illustrating a foundation of an expression (G).
FIG. 7 is a perspective view ofa hot stamped steel used in an example.
25 FIG. 8 is a flowchart illustrating a method for producing the hot stamped steel
10
for which a cold rolled steel sheet for hot stamping is used according to an embodiment
ofthe present invention.
Embodiments of the Invention
5 [0023]
As described above, it is important to establish an appropriate relationship
among the amount of Si, the amount of Mn and the amount of C and provide an
appropriate hardness to a martensite in a predetermined position in a steel sheet in order
to improve formability (hole expansibility). Thus far, there have been no studies
10 regarding the relationship between the formability or the hardness of the martensite in a
hot stamped steel.
[0024]
Herein, reasons for limiting a chemical composition of a hot stamped steel for
which a cold rolled steel sheet for hot stamping is used according to an embodiment of
15 the present invention (in some cases, also referred to as a hot stamped steel for which a
cold rolled steel sheet for hot stamping is used according to the present embodiment) and
steel used for manufactr~reth ereof will be described. Hereinafter, " %t hat is a unit of
an amount of an individual component indicates "mass%.
[0025]
20 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 ofC is 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.
25 Therefore, the range of the amount of C is set to 0.030% to 0.150%. In a case in which
11
there is a demand for high hole expansibility, the amount of C is desirably set to 0.100%
or less.
[0026]
Si: 0.010% to 1.000%
5 Si is an important element for suppressing a formation of a harmful carbide and
obtaining a multi-phase structure mainly including a ferrite structure and a balance of the
martensite. However, in a case in which the amount of Si exceeds 1.0%, the elongation
or hole expansibility of the steel degrades, and a chemical conversion treatment property
also degrades. Therefore, the amount of Si is set to 1.000% or less. In addition, while
10 the Si is added for deoxidation, a deoxidation effect is not sufficient when the amount of
I
I ,i Si is less than 0.010%. Therefore, the amount of Si is set to 0.010% or more.
'i
.i [0027]
I
Al: 0.010% to 0.050%
! A1 is an important element as a deoxidizing agent. To obtain the deoxidation
15 effect, the amount ofAl is set to 0.010% or more. On the other hand, even when the A1
is excessively added, the above-described effect is saturated, and conversely, the steel
becomes brittle. TherrTore, the amount ofAl is set in a range of 0.010% to 0.050%.
[0028]
Mn: 1.50% to 2.70%
20 Mn is an important element for increasing a hardenability of the steel and
strengthening the steel. However, when the amount of Mn is less than 1.50%, it is not
possible to sufficiently increase the strength of the steel. On the other hand, when the
amount of Mn exceeds 2.70%, since the hardenability increases more than necessary, an
increase in the strength of the steel is caused, and consequently, the elongation or hole
25 expansibility of the steel degrades. Therefore, the amount of Mn is set in a range of
1.50% to 2.70%. In a case in which there is a demand for high elongation, the amount
of Mn is desirably set to 2.00% or less.
[0029]
P: 0.001% to 0.060%
5 In a case in which the amount is large, P segregates at a grain boundary, and
deteriorates the local ductility and weldability of the steel. Therefore, the amount of P
is set to 0.060% or less. On the other hand, since an unnecessary decrease of P leads to
!'
an increasing in the cost of refining, the amount of P is desirably set to 0.001% or more.
[0030]
10 S: 0.001% to 0.010%
S is an element that forms MnS and significantly deteriorates the local ductility
or weldability. Therefore, the upper limit of the amount of S is set to 0.010%. In
!
addition, in order to reduce refining costs, a lower limit of the amount of S is desirably
set to 0.001%.
15 [0031]
N: 0.0005% to 0.0100%
N is an important element to precipitate A1N and the likc and minialr~rizec rystal
grains. However, when the amount of N exceeds 0.0100%, a N solid solution (nitrogen
solid solution) remains and the ductility of the steel is degraded. Therefore, the amount
20 of N is set to 0.0100% or less. Due to a problem of refining costs, the lower limit of the
amount of N is desirably set to 0.0005%.
[0032]
The hot stamped steel for which the cold rolled steel sheet for hot stamping is
used according to the embodiment has a basic composition including the above-described
25 components, Fe as a balance and unavoidable impurities, but may further contain any one
13
or more elements of Nb, Ti, V, Mo, Cr, Ca, REM (rare earth metal), Cu, Ni and B as
elements that have thus far been used in amounts that are equal to or less than the
below-described upper limits to improve the strength, to control a shape of a sulfide or an
i oxide, and the like. Since these chemical elements are not necessarily added to the steel
5 sheet, the lower limits thereof are 0%.
[0033]
Nb, Ti and V are elements that precipitate a fine carbonitride and strengthen the
steel. In addition, Mo and Cr are elements that increase hardenability and strengthen
the steel. To obtain these effects, it is desirable to contain Nb: 0.001% or more, Ti:
10 0.001% or more, V: 0.001% or more, Mo: 0.01% or more, and Cr: 0.01% or more.
However, even when Nb: more than 0.050%, Ti: more than 0.100%, V: more than
0.100%, Mo: more than 0.50%, and Cr: more than 0.50% are contained, the
strength-increasing effect is saturated, and there is a concern that the degradation of thc
elongation or the hole expansibility may be caused.
15 [0034]
The steel may further contain Ca in a range of 0.0005% to 0.0050%. Ca and
rare earth metal (EM) control the shape of the sulfide or the oxide and implove the
local ductility or the hole expansibility. To obtain this effect using the Ca, it is
preferable to add 0.0005% or more of the Ca. However, since there is a concern that an
20 excessive addition may deteriorate workability, an upper limit of the amount of Ca is set
to 0.0050%. For 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%.
[0035]
25 The steel may further contain Cu: 0.01% to 1.00%, Ni: 0.01% to 1.00% and B:
14
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
1 I 5 the steel is small. On the other hand, even when Cu: more than 1.00%, Ni: more than
I
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.
[0036]
In a case in which the steel contains B, Mo, Cr, V, Ti, Nb, Ni, Cu, Ca and REM,
10 one or more elements are contained. The balance of the steel is composed of Fe and
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,
15 the elements are treated as unavoidable impurities.
[0037]
In addition, in the hot stamped steel lor which the cold rolled steel sheet for hot
stamping is used according to the embodiment, as illustrated in FIG. 1, when the amount
of C (mass%), the amount of Si (mass%) and the amount of Mn (mass%) are represented
20 by [C], [Si] and [Mn] respectively, it is important to satisfy a following expression (A).
(5 x [Si] + [Mn]) / [C] > 11 (A)
To satisfy a condition of TS x h 2 50000MPa.%, the above expression (A) is
preferably satisfied. When the value of (5 x [Si] + [Mn]) / [C] is 11 or less, it is not
possible to obtain a sufficient hole expansibility. This is because, when the amount of C
25 is large, the hardness of a hard phase becomes too high, a hardness difference (ratio of
15
the hardness) between the hard phase and a soft phase becomes great, and therefore the h
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]) / [C], since the value does not change
I
even after hot stamping as described above, the expression is preferably satisfied during a
5 production of the metal sheet.
[0038]
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
10 hardness difference (the ratio ofthe hardness) of the martensite between a surface part of
a sheet thickness and a central part of the sheet thickness, and the hardness distribution of
the martensite in the central part of the sheet thicltness are in a predetermined state in a
phase of before hot stamping (before heating for carrying out quenching in a hot
stamping process), the state is almost maintained even after hot stamping as illustrated in
15 FIGS. 2A and 2B, and the formability such as elongation or hole expansibility becomes
favorable. This is considered to be because the hardness distribution of the martensite
formed before hot stamping still has a significant effect even after hot stamplclg, and
alloy elements concentrated in the central part of the sheet thickness still hold a state of
being concentrated in the central part of the sheet thickness even after hot stamping.
20 That is, in the steel sheet before hot stamping, in a case in which the hardness ratio
between the martensite in the surface part of the sheet thickness and the martensite in the
central part of the sheet thiclcness is great, or a variance of the hardness of the martensite
is great, the same tendency is exhibited even after hot stamping. As illustrated in FIGS.
2A and 2B, the hardness ratio between the surface part of the sheet thickness and the
25 central part of the sheet thickness in the cold rolled steel sheet for hot stamping for the
16
hot stamped steel according to the embodiment before hot stamping and the hardness
ratio between the surface part of the sheet thickness and the central part of the sheet
thickness in the hot stamped steel, for which the cold rolled steel sheet for hot stamping
is used according to the embodiment, are almost the same. In addition, similarly, the
5 variance of the hardness of the martensite in the central pail of the sheet thickness in the
cold rolled steel sheet for hot stamping for the hot stamped steel according to the
embodiment before hot stamping and the variance of the hardness of the martensite in the
central part of the sheet thickness in the hot stamped steel, for which the cold rolled steel
sheet for hot stamping is used according to the embodiment, are almost the same.
10 Therefore, the formability of the cold rolled steel sheet for a hot stamping for the hot
stamped steel according to the embodiment is similarly excellent to the formability of the
hot stamped steel for which the cold rolled steel sheet for hot stamping is used according
to the embodiment.
[0039]
15 In addition, regarding the hardness of the martensite measured with an
nanoindenter manufactured by Hysitron Corporation at a magnification of 1000 times,
the inventors found that a following expression (B) and a following expressicitr (C) ((H)
and (1) as well) being satisfied arc advantageous to the formability of the hot stamped
steel. Here, "HI" is the average hardness of the martensite in the surface part of the
20 sheet thickness that is within an area having a width of 200 pm in a thickness direction
from an outermost layer of the steel sheet in the thickness direction in the hot stamped
steel, "H2" is the average hardness of the martensite in an area having a width of +lo0
pm in the thickness direction kom the central part of the sheet thicltness in the central
part of the sheet thicltness in the hot stamped steel, and "OHM is the variance of the
25 hardness of the martensite in an area having a width of *I00 pm in the thielmess
17
direction from the central part of the sheet thickness in the hot stamped steel. In
addition, "HIP is the hardness ofthe martensite in the surface part of the sheet thickness
in the cold rolled steel sheet for hot stamping before hot stamping, "H20" is the hardness
of the martensite in the central part of the sheet thickness, that is, in an area having a
5 width of 200 pm in the thickness direction in a center of the sheet thickness in the cold
rolled steel sheet for hot stamping before hot stamping, and "oHMO" is the variance of
the hardness of the martensite in the central part of the sheet thickness in cold rolled steel
sheet for hot stamping before hot stamping. The H1, HI 0, H2, H20, OHM and oHMO
are obtained respectively from 300-point measurements for each. An area lraving a
10 width of 1100 pm in the thickness direction from the central part of the sheet thickness
refers to an area having a center at the center of the sheet thickness and having a
dimension of200 pm in the thickness direction.
H2 I H1 < 1.10 (B)
OHM < 20 (c)
15 H20/H10< 1.10 (HI
oHMO i 2 0 (I)
In addition, hc~et,h e variance is a value obtained using a following cxpression
(K) and indicating a distribution of the hardness of the martensite.
[0040]
20 [Expression 11
x,, represents the average value of the hardness, and x, represents an i'h
hardness.
[0041]
A value of H21131 of 1.10 or more represents that the hardness of the martensite
in the central part of the sheet thickness is 1.10 or more times the hardness of the
martensite in the surface part of the sheet thickness, and, in this case, OHM becomes 20
or more even after hot stamping as illustrated in FIG. 2A. When the value of the H2 1
HI is 1.10 or more, the hardness of the central part of the sheet thickness becomes too
high, TS x h becomes less than 50000MPa.% as illustrated in FIG. 2B, and a sufficient
formability cannot be obtained both before quenching (that is, before hot stamping) and
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 part of the sheet
thiclcness and in the surface part ofthe 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, up to approximately 1.005. What has been described
above regarding the value of H2 I H1 shall also apply in a similar manner to the value of
H20 I H10.
[0042]
In addition, thc variance OHM being 20 or more even after hot stamping
indicates that a scattering of the hardness oftlie martensite is large, and parts in which the
hardness is too high locally exist. Jn this case, TS x h becomes less than 50000MPa.%
as illustrated in FIG. 2B, and a sufficient folmability of the hot stamped steel cannot be
obtained. What has been described above regarding the value of the OHM shall also
apply in a similar manner to the value of the oHMO.
[0043]
In the hot stamped steel according to the embodiment, the area fiaction ofthe
19
ferrite in a metallographic structure after hot stamping is 40% to 90%. When the area
fraction of the 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 90%, the martensite becomes insufficient, and a sufficient strength cannot
5 be obtained. Therefore, the area fraction of the ferrite in the hot stamped steel is set to
40% to 90%. In addition, the metallographic structure of the hot stamped steel also
includes the martensite, an area fraction of the martensite is 10% to 60%, and a total of
the area fraction of the ferrite and the area fraction of the martensite is 60% or more.
All or principal parts orthe metallographic structure of the hot stamped steel are
10 occupied by the ferrite and the martensite, and furthermore, one or more of a pearlite, a
I
I
bainite as remainder and a retained austenite may be included in the metallographic
structure. However, when the retained austenite remains in the metallographic structure,
a secondary working brittleness and a delayed fracture characteristic are likely to degrade.
Therefore, it is preferable that the retained austenite is substantially not included;
15 however, unavoidably, 5% or less of the retained austenite in a volume ratio may be
included. Since the pearlite is a hard and brittle structure, it is preferable not to include
the pearlite in the metallographic structure; however, unavoidably, up to 10% of the
pearlite in an area fraction may be included. Furthermore, the amount of the bainite as
remainder is preferably 40% or less in an area fraction with respect to a region excluding
20 the ferrite and the martensite. Here, the metallographic structures of the ferrite, the
bainite as remainder and the pearlite were observed through Nital etching, and the
metallographic structure of the martensite was observed through Le pera etching. In
both cases, a 114 part ofthe sheet thickness was observcd at a magnification of 1000
times. The volume ratio of the retained austenite was measured with an X-ray
25 diffraction apparatus after polishing the stccl sheet up to the 114 part of the sheet
20
thickness. The 114 part of the sheet thickness refers to a part 114 of the 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.
[0044]
i 5 Ln the embodiment, the hardness of the martensite measured at a magnification
of 1000 times is specified by using a nanoindenter. 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 not possible to obtain the hardness of
10 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 an
appropriate relationship of the hardness of the martensite in the hot stamped steel
I
measured with the nanoindenter is provided, it is possible to obtain an extremely
15 favorable formability.
[0045]
In addition, in the cold rolled steel sheet for hot stamping before hot stamping
and the hot stamped steel, as a result of observing MnS at a location of 114 of the sheet
thickness and in the central part of the sheet thickness, it was found that it is preferable
20 that an area fraction of the MnS having an equivalent circle diameter of 0.1 pm to 10 pm
is 0.01% or less, and, as illustrated in FIG. 3, a following expression (D) ((J) as well) is
satisfied in order to favorably and stably satisfy the condition of TS x h? 50000MPa.%.
When the MnS having an equivalent circle diameter of 0.1 pm or more exists during a
hole expansibility test, since stress concentrates in the vicinity thereof, cracking is likely
25 to occur. A reason for not counting the MnS having the equivalent circle diameter of
2 1
less than 0.1 pm is that an effect on the stress concentration is small. In addition, a
reason for not counting the MnS having the equivalent circle diameter of more than 10
pm is that, the MnS having the above-described grain size is included in a latter half, the
I
grain size is too large, and the steel sheet becomes unsuitable for working. Furthermore,
5 when the area fraction of the MnS having the equivalent circle diameter of0.1 pm to 10
pm 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 of TS x h 1 50000MPa.% is not satisfied. Here, "nl" and "n1O"
are number densities of the MnS having the equivalent circle diameter of 0.1 pin to 10
10 pm at the 114 part of the sheet thickness in the hot stamped steel and the cold rollcd steel
sheet before hot stamping respectively, and "n2" and "n20" are number densities of the
MnS having the equivalent circle diameter of 0.1 pm to 10 pm at the central part of the
sheet thickness in the hot stamped steel and the cold rolled steel sheet before hot
stamping respectively.
15 n2/n1<1.5 (D)
n20ln10<1.5 (J)
These relationships are all identical to the steel sheet before hot stau~pingt,h e
steel sheet after hot stamping, and the hot stamped steel.
[0046]
20 When the area fraction of the MnS having the equivalent circle diameter of 0.1
pm to 10 pm is more than 0.01% alter hot stamping, the formability is likely to degrade.
The lower limit of the area fraction of the MnS is not pai-ticularly specified, however,
0.0001% or more of the MnS is present due to a below-described measurement method, a
limitation ofa magnification and a visual field, and an original amount of Mn or the S.
25 In addition, a value of an n2lnl (or an n20ln10) being 1.5 or more represents that a
22
number density of the MnS having the equivalent circle diameter of 0.1 pm to 10 pm in
the central part of the sheet thickness of the hot stamped steel (or the cold rolled steel
sheet for hot stamping before hot stamping) is 1.5 or more times the number density of
the MnS having the equivalent circle diameter of 0.1 pm to 10 pm in the 114 part of the
5 sheet thickness of the hot stamped steel (or the cold rolled steel sheet for hot stamping
before hot stamping). In this case, the formability is likely to degrade due to a
segregation of the MnS in the central part of the sheet thickness of the hot stamped steel
(or the cold rolled steel sheet for hot stamping before hot stamping). In the embodiment,
the equivalent circle diameter and number density of the MnS having the equivalent
10 circle diameter of 0.1 pm to 10 pm 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 pm2 - 10000 pm2). Ten visual fields were observed in the 114
part of the sheet thickness, and ten visual fields were observed in the central part of the
15 sheet thickness. The area fraction of the MnS having the equivalent circle diameter of
0.1 pm to 10 pm was computed with particle analysis software. In the hot stamped steel
for which the cold rolled steel sheet for hot stamping is used according to thc
embodiment, the form (shape and number) of the MnS formed before hot stamping is the
same before and after hot stamping. FIG. 3 is a view illustrating a relationship between
20 the n2 I nl and TS x h after hot stamping and a relationship between an n20 In10 and TS
x h before hot stamping, and, according to FIG. 3, the n20 / n10 of the cold rolled steel
sheet before hot stamping and the n2 I nl of the hot stamped steel are almost the same.
This is because the form of the MnS does not change at a heating temperature of a hot
stamping, generally.
25 [0047]
23
When the hot stamping is carried out on the steel sheet having the
above-described configuration, it is possible to realize a tensile strength of 500 MPa to
1500 MPa, and a significant formability-improving effect is obtained in the hot stamped
steel having the tensile strength of approximately 550 MPa to 1200 MPa.
5 [0048]
Furthermore, it is preferable to form a galvanizing, a galvannealing, an
electrogalvanizing or an aluminizing on a surface of the hot stamped steel for which the
cold rolled steel sheet for hot stamping is used according to the embodiment in terms of
rust prevention. A forlnation of the above-described platings does not impair the effects
10 of the embodiment. The above-described platings can be carried out with a well-known
method.
[0049]
Hereinafter, a method for producing the hot stamped steel for which the cold
rolled steel sheet (a cold rolled steel sheet, a galvanized cold rolled steel sheet, a
15 galvannealed cold rolled steel sheet, an electrogalvannealed cold rolled steel sheet and an
aluminized cold rolled steel sheet) for hot stamping is used according to the embodiment
will be described.
[00501
When producing the hot stamped steel for which the cold rolled steel sheet for
20 hot stamping is used according to the embodiment, as an ordinary condition, a molten
steel from a melting process in a converter is continuously cast, thereby producing a slab.
In the continuous casting, when a casting rate is fast, a precipitate of Ti and the like
becomes too fine, and, when the casting rate is slow, a productivity deteriorates, and
consequently, a metallographic structure of the above-described precipitate coarsens and
25 the number of particles in the metallographic structure decreases, and thus, there is a case
24
other characteristics such as a delayed fracture cannot be controlled. Therefore, the
casting rate is desirably 1.0 mlminute to 2.5 mlminute.
[0051]
The slab after the casting can be subjected to hot-rolling as it is. Alternatively,
in a case in which the slab after cooling has been cooled to less than 1 10O0C, it is
possible to reheat the slab after cooling to 1 l0O0C to 1300°C in a tunnel furnace or the
like and subject the slab to hot-rolling. When a slab temperature is less than 1 10O0C, 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 steel
sheet for hot stamping to which Ti and Nb are added is used, since a dissolution of the
precipitate becomes insufficient during the heating, which causes a decrease in a strength.
On the other hand, when the heating temperature is more than 1300°C, a generation of a
scale becomes great, and there is a case in which it is not possible to make favorable a
surface property of the hot stamped steel for which the cold rollcd steel sheet for hot
stamping is used.
[0052]
In addition, to decrease the area fraction of the MnS having the equivalent circle
diameter of 0.1 pm to 10 pm, when the amount of Mn and the amount of S in the steel
arc respectively represented by [Mn] and [S] by mass%, it is preferable for a temperature
T ("C) of a heating furnace before carrying out hot-rolling, an in-furnace time t (minutes),
[Mn] and [S] to satisfy a following expression (G) as illustrated in FIG. 6.
T x ln(t) 1 (1.7 x [Mn] + [S]) > 1500 (GI
When T x ln(1) /(I .7 x [Mn] + [S]) is equal to or less than 1500, the area
fraction of the MnS having the equivalent circle diameter of 0.1 pm to 10 pm becomes
large, and there is a case in which a difference between the number density of the MnS
25
having the equivalent circle diameter of 0.1 pn to 10 pm in the 114 part ofthe sheet
thickness and the number density of the MnS having the equivalent circle diameter of 0.1
pm to 10 pm in the central part of the sheet thickness becomes large. The temperature
of the heating furnace before carrying out hot-rolling refers to an extraction temperature
5 at an outlet side of the heating furnace, and the in-furnace time refers to a time elapsed
from an insertion of the slab into the hot heating furnace to an extraction of the slab from
the heating furnace. Since the MnS does not change even after hot stamping as
described above, it is preferable to satisfy the expression (G) in a heating process before
hot-rolling.
10 [0053]
Next, the hot-rolling is carried out according to a conventional method. At this
time, it is desirable to carry out hot-rolling on the slab at the finishing temperature (the
hot-rolling end temperature) which is set in a range of an iqT3 temperature to 970°C.
When the finishing temperature is less than the Ar3 temperature, the hot-rolling becomes
15 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,
when the finishing temperature exceeds 97OoC, an austenite grain sizc 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.
20 Here, the Ar3 temperature was estimated from an inflection point of a length of a
test specimen alier carrying out a formastor test.
[0054]
After the hot-rolling, the steel is coolcd at an average cooling rate of 20
"Clsecond to 500 OCIsecond, and is coilcd at a predetermined coiling temperature CT.
25 In a case in which the average cooling rate is less than 20 "Clsecotld, the pearlite that
26
causes the degradation of the ductility is likely to be formed. On the other hand, an
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.
[OOSS]
After the coiling, pickling is carried out, and cold-rolling is carried out. At this
time, to obtain a range satisfying the above-described expression (C) as illustrated in FIG.
4, the cold-rolling is carried out under a condition in which a following expression (E) is
satisfied. When conditions for annealing, cooling and the like described below are
further satisfied after the above-described rolling, TS x h 2 50000 MPa% is ensured in
the cold rolled steel sheet for hot stamping before hot stamping andlor the hot stamped
steel. From the viewpoint ofthe productivity, the cold-rolling is desirably carried out
with a tandem 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 / r + 1.2 x r 2 / r + r 3 / r > 1.0 (El
Here, the "ri" represents 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" represents a
total target 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 cun~ulativere duction (a 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.
[0056]
When the cold-rolling is carried out under the conditions in which the
expression (E) is satisfied, it is possible to sufficiently divide the pearlite in the
27
cold-rolling even when a large pearlite exists before the cold-rolling. As a result, it is
possible to burn the pearlite or suppress the area fraction of the pearlite to a minimum
through the annealing carried out after cold-rolling, and therefore it becomes easy to
obtain a structure in which an expression (B) and an expression (C) are satisfied. On
5 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 pearlite is likely to remain,
and it is not possible to form a desired martensite in the following annealing. In
addition, the inventors found that, when the expression (E) is satisfied, an obtained form
of the martensite structure after the annealing is maintained in almost the same state even
10 after hot stamping is carried out, and therefore the hot stamped steel for which the cold
rolled steel sheet for hot stamping is used 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 for which the cold rolled steel sheet for hot
stamping is used according to the embodiment is heated up to the two-phase region in the
15 hot stamping, a hard phase including the martensite before hot stamping turns into an
austenite structure, and the ferrite before hot stamping remains as it is. Carbon (C) in
the austenite does not move to the peripheral ferrite. After that, when cooicd, the
austenite turns into a hard phase including the martensite. That is, when the expression
(E) is satisfied and the above-described H2 I HI (or H20 I H10) is in a predetermined
20 range, the H2 / H1 is maintained even after hot stamping and hot stamped steel becomes
excellent in terms of the formability.
[0057]
In the embodiment, r, rl, r2 and r3 are the target cold-rolling reductions.
Generally, the cold-rolling is carried out while controlling the target cold-rolling
25 reduction and an actual cold-rolling reduction to become substantially the same valuc.
28
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.
I-Iowever, 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 reduction satisfies the expression (E). Furthermore, the
actual cold-rolling reduction is preferably within 110% of the cold-rolling reduction.
[0058]
After cold-rolling, a recrystallization is caused in the steel sheet by carrying out
the annealing. 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 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 a total ofthe area fraction of the ferrite and the area fraction of the martensite to 60%
or more, and to contribute to an improvement of TS x h. Other annealing temperature
conditions are not particularly specified, but a holding time at 700°C to 850°C is
preferably 1 second or more as long as the productivity is not impaired to reliably obtain
a predetermined structure, and it is also preferable to appropriately determine a
temperature-increase rate in a range of 1 "C/sccond to an upper limit of a facility capacity,
and to appropriately determine the cooling rate in a range of 1 "C/second to the upper
limit of the facility capacity. In a temper-rolling process, temper-rolling is carried out
with a conventional method. An 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.
[0059]
As a still more preferable condition of the embodiment, when the amount of C
(mass%), the amount of Mn (mass%), the amount of Si (mass%) and the amount of Mo
(mass%) of the steel are represented by IC], [Mn], [Si] and [Mo] respectively, regarding
5 the coiling temperature CT, it is preferable to satisfy a following expression (I;).
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)
[0060]
As illustrated in FIG. 5A, when the coiling temperature CT is less than "560 -
10 474 x [C] - 90 x [Mn] - 20 x [Cr] - 20 x [Ma]", 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 illustrated in FIG. 5B, when the coiling
temperature CT 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
15 of the pearlite in the central part of the sheet thickness is likely to increase. Therefore, a
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 case in which it becomes difficult for the martensite to bc foimed in a sufficient
amount.
20 [006 l]
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 casc, when a
two-phase region heating is carried out in the hot stamping, the distribution form is
maintained as described above. If it is possible to more reliably ensure the
25 above-described metallographic structnre by satisfying the expression (F), the
3 0
metallographic structure is maintained even after hot stamping, and the hot stamped steel
becomes excellent in terms of the formability.
[0062]
Furthermore, to improve the rust-preventing capability, it is also preferable to
5 include a galvanizing process in which a galvanizing is formed between an annealing
process and the temper-rolling process, and to form the galvanizing on a surface of the
cold rolled steel sheet. Furthermore, it is also preferable to include an alloying process
in which an alloying treatment is performed after galvanizing. In a case in which the
alloying treatment is performed, a treatment in which a galvannealed surface is brought
10 into contact with a substance oxidizing a sheet surface such as water vapor, thereby
thickening an oxidized filin may be further carried out on the surface.
[0063]
It is also preferable to include, for example, an electrogalvanizing process in
which an electrogalvanizing is formed after the temper-rolling process as well as the
15 galvanizing and the galvannealing and to form an electrogalvanizing on the surface of the
cold rolled steel sheet. In addition, it is also preferable to include, instead of the
galvanizing, an aluminiling process in which an aluminizing is formed between the
annealing process and the temper-rolling process, and to form the aluminizing on the
surface of the cold rolled steel sheet. The aluminizing is generally hot dip aluminizing,
20 which is preferable.
[0064]
After a series of the above-described treatments, the hot stamping is cai~iedo ut
by heating the steel sheet to 700°C to 1 OOO°C. In the hot stamping process, the hot
stamping is desirably carried out, for example, under thc following conditions. First,
25 the steel sheet is heated up to 700°C to 1000°C at the temperature-increase rate of 5
3 1
"Clsecond to 500 OCIsecond, and the hot stamping (a hot stamping process) is carried out
after the holding time of 1 second to 120 seconds. To improve the formability, the
heating temperature is preferably an ACJ temperature or less. The ACJ temperature was
estimated from the inflection point of the length of the test specimen after carrying out
5 the formastor test. Subsequently, the steel sheet is cooled, for example, to the room
temperature to 300°C at the cooling rate of 10 "Clsecond to 1000 OCIsecond (quenching
in the hot stamping).
[0065]
When the heating temperature in the hot stamping process is less than 700°C,
10 the quenching is not sufficient, and consequently, the strength cannot be ensured, which
is not preferable. When the heating temperature is more than 100O0C, the steel sheet
bccomes too soft, and, in a case in which a plating, particularly zinc plating, is formed on
the surface of the steel sheet, and the sheet, there is a concern that the zinc may be
evaporated and burned, which is not preferable. Therefore, the heating temperature in
15 the hot stamping is preferably 700°C to 1000°C. When the temperature-increase rate is
less than 5 "Clsecond, since it is difficult to control heating in the hot stamping, and the
productivity significantly deglades, it is preferable to carry out the heating al the
temperature-increase rate of 5 "Clsecond or more. On the other hand, an upper limit of
the temperature-increase rate of 500 "Clsccond is depends on a current heating capability,
20 but is not necessary to limit thereto. At the cooling rate of less than 10 "Clsecond, since
the rate control of the cooling after hot stamping process is difficult, and the productivity
also significantly degrades, it is prel'erable to cany out the cooling at the cooling rate of
10 "Clsecond or more. An upper limit of the cooling rate of 1000 "Clsecond depends
on a current cooling capability, but is not necessary to limit thereto. A reason for setting
25 a time until the hot stamping after an increase in the temperature to 1 second or more is a
jL
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
galvanizing or the like is formed on the surface of the steel sheet. A reason for setting
5 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 illustrating the method for producing the cold rolled steel
sheet according to the embodiment of the present invention. Reference signs S1 to S13
in the drawing respectively correspond to individual process described above.
10 [0066]
In the hot stamped steel of the embodiment, the expression (B) and the
!
:I 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
condition of TS x h > 50000MPa.% even after hot sta'mping is carried out.
15 [0067]
As described above, when the above-described conditions are satisfied, it is
possible to manufacturc the hot stamped steel in which the hardness distribr~liono r the
structure is maintained even after hot stamping, and consequently the strength is ensured
and a more favorable hole expansibility can be obtained.
20 Examples
[0068]
Steel having a composition described in Table 1 was continuously cast at a
casting rate of 1.0 mlnlinute to 2.5 mlminute, a slab was heated in a heating furnace
under a conditions shown in Table 2 with a conventional method as it is or after cooling
25 the steel once, and hot-rolling was cai~icdo ut at a finishing temperature of 910°C to
33
93OoC, thereby producing a hot rolled steel sheet. After that, the hot rolled steel sheet
was coiled at a coiling temperature CT described in Table 1. After that, pickling was
carried out so as to remove a scale on a surface of the steel sheet, and a sheet thickness
was made to be 1.2 mm to 1.4 mm through cold-rolling. At this time, the cold-rolling
5 was carried out so that the value of the expression (E) became a value described in Table
5. After cold-rolling, annealing was carried out in a continuous annealing furnace at an
annealing temperature described in Table 2. On a part of the steel sheets, a galvanizing
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 the part of the steel
10 sheets, thereby forming a galvannealing. In addition, an electrogalvanizing or an
aluminizing was formed on the part of the steel sheets. Fw-thermore, temper-rolling was
canied out at an elongation ratio of 1% according to a convcntional method. In this
state, a sample was taken to evaluate material qualities and the like before hot stamping,
and a material quality test or the lilcc was carried out. After that, to obtain a hot
15 stamped steel having a form as illustrated in FIG. 7, hot stamping in which a temperature
was increased at a temperature-increase rate of 10 "Clsecond to 100 'Clsecond, the steel
sheet was held at a hcating temperature of 780°C for 10 seconds, and was cooled at a
cooling rate of 100 "Clsecond to 200°C or less was carried out. A sample was cut from
a location of FIG. 7 in an obtained hot stamped steel, the material quality test and the like
20 were carried out, and the tensile strength (TS), the elongation (El), the hole expansion
ratio (h) and the like were obtained. The results arc described in Table 2, Table 3
(continuation of Table 2), Table 4 and Table 5 (continuation of Table 4). The hole
expansion ratios h in the tables were obtained from a following expression (L).
h(%)= {(d'- d)/d} x 100 (L)
25 d': a hole diameter whcn a crack penetrates the sheet thiclu~ess
d: an initial hole diameter
Furthermore, regarding plating types in Table 2, CR represents a non-plated cold
rolled steel sheet, GI represents that the galvanizing is formed, GA represents that the
galvannealing is formed, EG represents that the electrogalvanizing is formed, and A1
5 represents that the aluminizing 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.
In addition, since the expression (H), the expression (I) and the expicssion (J)
10 are substantially the same as the expression (B), the expression (C) and the expression
I
I I
(D) respectively, in headings of the respective tables, the expression (B), the expression
i~ (C) and the expression (D) are described as representatives.
! [0069]
i ~ [Table 11
15
[0070]
[Table 21
[0071]
20 [Table 31
~00721
[Table 41
[Table 51
[0074]
[Table 61
5
[0075]
[Table 71
[0076]
10 [Table 81
[0077]
[Table 91
15 [0078]
Based on the above-described examples, as long as the conditions of the present
invention are satisfied, it is possible to obtain a hot stamped steel for which an excellent
cold rolled steel sheet for hot stamping, an excellent galvanized cold rolled steel sheet for
hot stamping, an excellent galvannealed cold rolled steel sheet for hot stamping, an
20 excellent electrogalvanized cold rolled steel sheet for hot stamping or an excellent
alluminized cold rolled steel sheet for hot stamping all of which satisfy TS x h > 50000
MPa% is used, even after hot stamping.
Industrial Applicability
2 5 100791
36
Since the hot stamped steel, which are obtained in the present invention and for
which the cold rolled steel sheet for hot stamping is used, can satisfy TS x h 2 50000
MPa% after hot stamping, the hot stamped steel has a high press workability and a high
strength, and satisfies the current requirements for a vehicle such as an additional
5 reduction of the weight and a more complicated shape of 1I a component.
Brief Description of the Reference Symbols
[OOSO]
S 1 : MELTING PROCESS
10 S2: CASTING PROCESS
S3: I-IEATING PROCESS
S4: HOT-ROLLING PROCESS
S5: COILING PROCESS
S6: PICKLING PROCESS
15 S7: COLD-ROLLING PROCESS
S8: ANNEALING PROCESS
SO: TEMPER-ROLLING PROCESS
S 10: GALVANIZING PROCESS
S 11 : ALLOYING PROCESS
20 S 12: ALUMINIZING PROCESS
S 13: ELECTROGALVANIZING PROCESS

Table 2
1 - . I 1 1 After annealing and temper-rolling and before hot stamping I Pearlite /
'ype
reference
symbol
~~~t
reference
symbol
Annealing
temperature
("C)
area
fraction
before cold
rolling(%)
TS (Mpa)
Martensite
area
fraction(%)
EL (%)
mFaerrterintes it'e
area
fraction(%)
A (%)
aRuestseindiutea l
area
fraction(%)
TS X EL
Bainite
area
fraction(%)
Pearlite
area
fraction(%)
TS X A
Ferrite
area
fraction(%)

I 1 1 After hot stam~ina 1 1
I - . . I

J- Y6 -
R
CLAIMS
1. A hot stamped steel comprising, by mass%:
C: 0.030% to 0.150%;
5 Si: 0.010% to 1.00%;
Mn: 1.50% to 2.70%;
P: 0.001% to 0.060%;
S: 0.001% to 0.010%;
N: 0.0005% to 0.0100%;
10 Al: 0.001% to 0.050%, and
optionally one or more of
B: 0.0005% to 0.0020%;
Mo: 0.01% to 0.50%;
Cr: 0.01% to 0.50%;
15 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%;
20 Ca: 0.0005% to 0.0050%;
REM: 0.0005% to 0.0050%, and
a balance including Fe and unavoidable impurities, whcrein
when [C] represents an amount of C by mass%, [Si] represents an amount of Si
by mass%, and [Mn] represents an amount of Mil by mass%, a following expression (A)
25 is satisfied,
a metallographic structure after a hot stamping includes 40% to 90% of a ferrite
and 10% to 60% of a martensite in an area fraction,
a total of an area fraction of the ferrite and an area fraction of the martensite is
60% or more,
5 the metallographic structure optionally further includes one or more of 10% or
less of a perlite in an area fraction, 5% or less of a retained austenite in a volume ratio,
and less than 40% of a bainite as a remainder in an area fraction,
a hardness of the martensite measured with a nanoindenter satisfies a following
expression (B) and a following expression(C),
10 TS x h which is a product of a tensile strength TS and a hole expansion ratio h is
50000MPa-% or more,
(5 x [Si] + [Mn]) / [C] > 1 I (A),
H2 / H1 1.10 (B),
OHM < 20 (C), and
15 the H1 is an average hardness of the martensite in a surface part of a sheet
thickness after the hot stamping, the H2 is an average hardness of the martensite in a
central part ofthe sheet thickness which is an area having a width of 200 pni in a
thickness direction at a center of the sheet thickness after the hot stamping, and the OHM
is a variance of the average hardness of the martensite in the central part of the sheet
20 thickness after the hot stamping.
2. The hot stamped steel according to claim 1, wherein
an area fraction of MnS existing in the hot stamped steel and having an
equivalent circle diameter of 0.1 pm to 10 pm is 0.01% or less,
2 5 a following expression (D) is satisfied,
n2/n1<1.5 (D),and
the nl is an average number density per 10000 pm2 of the MnS haying the
equivalent circle diameter of 0.1 pm to 10 pm in a 114 part of the sheet thickness after the
hot stamping, and the n2 is an average number density per 10000 pm2 of the MnS having
5 the equivalent circle diameter of 0.1 pm to 10 pm in the central part of the sheet
thickness after the hot stamping.
3. The hot stamped steel according to claim 1 or 2, wherein a hot dip
galvanizing is formed on a surface thereof.
10
I
I 4. The hot stamped steel according to claim 3, wherein a galvannealing is
formed on a surface of the hot dip galvanizing
5. The hot stamped steel according to claim 1 or 2, wherein an
15 electrogalvanizing is formed on a surface thereof.
6. The hot stamped steel according to claim 1 or 2, wherein an aluminizing is
formed on a surface thereof.
20 7. A method for producing a hot stamped steel, the method comprising:
casting a molten steel having a chemical composition according to claim 1 and
obtaining a steel;
heating the steel;
hot-rolling the steel with a hot-rolling mill including a plurality of stands;
25 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 satisfying a following expression (E);
annealing in which the steel is annealed under 700°C to 850°C and cooled after
1
1 5 the cold-rolling;
temper-rolling the steel after cooled following annealed;
hot stamping in which the steel is heated to a temperature range of 700°C to
1000°C after the temper-rolling, hot-stamped within the temperature range, and thereafter
cooled to a room temperature or more and 300°C or less,
10 1.5 x r l / r + 1 . 2 x r 2 / r + r 3 / r > l . O (El, and
the ri (i = 1,2, 3) represents 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 represents a total cold-rolling reduction in the
cold-rolling in unit %.
15
8. The method for producing the hot stamped steel according to claim 7,
wherein
when CT represents a coiling temperature in the coiling in unit OC, [C]
represents the amount of C by mass%, [Mn] represents the amount of Mn by mass%, [Si]
20 represents the amount of Si by inass%, and [Mo] represents the amount of Mo by mass%
in the steel sheet, a following expression (F) is satisfied,
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).
25 9. The method for producing the hot stamped steel according to claim 8,
wherein
when T represents a heating temperature in thc heating in unit OC, t represents an
in-Sumace time in the heating in unit minute, [Mn] represents the amount of Mn by
mass%, and [S] represents an amount of S by mass% in the steel sheet, a following
5 expiession (G) is satisfied.
T x ln(t) l(1.7 x [Mn] 4- [S]) > 1500 (G)
10. The method for producing the hot stamped steel according to any one o l
claims 7 to 9, further comprising:
10 galvanizing the steel between the annealing and the temper-rolling.
11. The mcthod for producing the hot stamped steel according to claim 10.
further comprising:
alloying the steel between the galvanizing and the temper-rolling
12. The method for producing the hot stamped steel according to any one 01
claims 7 to 9, further comprising:
electrogalvanizing the steel alter the temper-rolling.
20 13. The method for producing the hot stamped steel according to any one of
claims 7 to 9, further comprising:
aluminizing the steel between the a~inealinga nd the temper-rolling.
Dated this 0110712014
[RANJNA MJB II'A-DUTT I
OF REMFRY & SAGAR
ATI'ORNEY FOR THll APPLICN\I'T[S]