Specification
[Title of the Invention] HOT STAMPED STEEL AND METHOD FOR
PRODUCING THE SAME
[Technical Field of the Invention]
[OOOl]
The present invention relates to a hot stamped steel having an excellent
formability for which a cold rolled steel sheet for lot stamping is used, and a method
for producing the same. The cold rolled steel sheet of the present invention includes a
cold rolled steel sheet, a hot dip galvanized cold rolled steel sheet, a galvannealed cold
rolled steel sheet, an electrogalvanized cold rolled steel sheet and an aluminized cold
rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2012-004552, filed on
January 13,2012, the' content of which is incorporated herein by reference.
[Related Art]
[0002]
Cut~entlya, steel sheet for a vehicle is required to be improved in terms of
collision safety and have a reduced weight. Currently, there is demand for a higherstrength
steel sheet in addition to a 980 MPa (980 MPa or higher)-class steel sheet and
an 1180 MPa (1180 MPa or higher)-class steel sheet in terms of a tensile strength.
For example, there is a demand for a steel sheet having a tensile strength of more than
1.5 GPa. In the above-described circumstance, hot stanlping (also called hot pressing,
diequenching, press quenching or the like) is drawing attention as a method for
obtaining a high strength. The hot stanlping refers to a for~ningm ethod in which a
steel sheet is heated at a temperature of 750°C or more, hot-formed (worked) so as to
improve a formability of a high-strength steel sheet, and then cooled so as to quench
the steel sheet, thereby obtaining desired material qualities.
A steel sheet having a ferrite and maltensite, a steel sheet having a ferrite and
bainite, a steel sheet containing retained austenite in the structure or the like is known
as a steel sheet having both a press workability and a high strength. Among the
above-described steel sheets, a multi-phase steel sheet having a martensite dispersed in
a ferrite base (a steel sheet including a ferrite and the martensite, that is, a so-called DP
steel sheet) 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 an interface between the ferrite and the
tna~tensite,a nd cracking is likely to originate froom the interface. In addition, a steel
sheet having the above-described multi-phases is not capable of exhibiting a 1.5 GPaclass
tensile strength.
[0003]
For example, Patent Documents 1 to 3 disclose the above-described multiphase
steel sheets. In addition, Patent Docunlents 4 to 6 describe a relationship
between a hardness and the formability of the high-strength steel sheet.
[0004]
However, even with the above-described techniques of the related art, it is
difficult to satisfy cu~xellrte quirements for a vehicle such as an additional reduction of
a weight, an additional increase in a strength and a more complicated cotnponent shape
and a working perfor~nances uch as the hole expansibility after the hot stamping.
[Prior Art Document]
[Patent Document]
[OOOS]
[Patent Docunlent 11 Japanese Unexanlined Patent Application, First
Publication No. H6-128688
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2000-319756
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2005-120436
[Patent Document 41 Japanese Unexamined Patent Application, First
PublicationNo. 2005-256141
[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]
The present invention has been made in consideration of the above-described
problem. That is, an object of the present invention is to provide a hot stamped steel
for wliich a cold rolled steel sheet for hot stamping (including a galvanized steel sheet
or an aluminized steel sheet as described below) is used and which ensures a strength
of 1.5 GPa or more, preferably 1.8 GPa or more, and more preferably 2.0 GPa or Illore
and has a more favorable hole expansibility, and a method for producing the same.
Here, the hot stamped steel refers to a molded article obtained by using the abovedescribed
cold rolled steel sheet for hot stanlping as a niaterial and fornling the
material through hot stanlping.
[Means for Solving the Problel~l]
[0007]
The present inventors first carried out intensive studies regarding a cold rolled
steel sheet for hot stamping used for a hot stamped steel which ensures a strength of
1.5 GPa or more, preferably 1.8 GPa or more, and more preferably 2.0 GPa or more
and has an excellent formability (hole expansibility), and hot stamping conditions.
As a result, it was found that, in the cold rolled steel sheet for hot stanlping (the cold
rolled steel sheet before the hot stamping), a more favorable forn~abilityth an ever, that
is, a product of a tensile strength TS and a hole expansion ratio h (TS x h) of 50000
MPa% or more can be ensured by (i), with regard to a steel composition, establishing
an appropriate relationship anlong an a~llounto f Si, an amount ofM n and an amount of
C, (ii) adjusting a fraction (area fraction) of a ferrite and a fraction (area fiaction) of a
nlartensite to predetermined fractions, and (iii) adjusting a rolling reduction of coldrolling
so as to set a hardness ratio (a difference of a hardness) of tlle martensite
between a surface portion of a sheet thickness (surface part) and a center portion of the
sheet tl~ickness(c entral part) of the steel sheet and a hardness distribution of the
martensite in the central part in a specific range. The cold rolled steel sheet before
the hot stamping refers to a cold rolled steel sheet in a state in which a heating in a hot
stamping process in which the steel sheet is heated to 750°C to 1000°C, worked and
cooled is about to be carried out. In addition, it was found that, when the hot
sta~nping is carlied out on the cold rolled steel sheet for hot stamping under the hot
sta~npingc onditions described below, the hardness ratio oft he ~nartensiteb etween the
surface pottion of the sheet thickness and the central part of the steel sheet and the
hardness distribution of the nlartensite in thc central part are almost maintained even
after the hot stamping, and a hot stamped steel having a high strength and an excellent
formability in which TS x h reaches 50000 MPa.% or more can be obtained. In
addition, it was also clarified that it is also effective to suppress a segregation of MnS
in the central part of the sheet thickness of the cold rolled steel sheet for hot stamping
to improve the formability (hole expansibility) of the hot stamped steel.
In addition, it was also found that, in cold-rolling, it is also effective to adjust
a fraction of a cold-rolling reduction in each stand from an uppermost stand to a third
stand in a total cold-rolling reduction (cumulative rolling reduction) to a specific range
to control the hardness of the martensite. Based on the above-described finding, the
inventors have found a variety of aspects of the present invention described below. In
addition, it was found that the effects are not impaired even when hot dip galvanizing,
galvannealing, electrogalvanizing and alu~ninizing are carried out on the cold rolled
steel sheet for hot stamping.
[0008]
( 1 ) That is, according to a first aspect of the present invention, there is
provided a hot stamped steel including, by mass%, C: more than 0.150% to 0.300%,
Si: 0.010% to 1.000%, 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 tnore
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
including Fe and unavoidable impurities, in \vl~ichw, hen [C]r epresents 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 iincludes 80% or more of a martensite in an area fraction, and optionally,
ful-her includes one or more of 10% or less of a pearlite in an area fraction, 5% or less
of a retained austenite in a volun~era tio, 20% or less of a ferrite in an area fraction, and
less than 20% of a bainite in an area fraction, TS x h \vlnich is a product of TS that is a
tensile strength and h that is a hole expansion ratio is 50000MPa.% or more, and a
hardness of the martensite measured with a nanoindellter satisfies a following
expression-b and a following expression-c.
5 x [Si] + [Mn]) / [C] > 10 (a)
H2/H1 4.10 (b)
OHM < 20 (c)
Here, the H1 represents an average hardness of the maltensite in a surface
portion, tlie H2 represents the average hardness of the martensite in a center part of a
sheet thickness that is an area having a width of &I00 p n ~in a thickness direction from
a center of the sheet thickness, and the OHM represents a variance of the hardness of
tlie martensite existing in the central part of the sheet thickness.
[0009]
(2) In the hot stanlped steel according to the above (I), an area fraction of a
MnS existing in the metallographic structure 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)
Here, the nl represents an average 11u11lber density per 10000 p2 of the MnS
in a 1/4 part of the sheet thickness, and the 112 represents an average number density
per 10000 pm2 of the MnS in the central part of the sheet thickness.
[OO lo]
(3) In the hot stamped steel according to tlie above (1) or (2), a hot dip
galvanizing may be formed on a surface thereof.
[OOIl]
(4) In the llot stanlped steel according to tie above (3), the hot dip galvanized
layer Inay include galvannealing.
[0012]
(5) In the hot stamped steel according to the above (1) or (2), an
electrogalvanizing may be fi~rtl~feorr med on a surface thereof.
[00 1 31
(6) In the hot stamped steel according to the above (1) or (2), an aluminizing
may be further formed on a surface thereof.
[0014]
(7) According to another aspect of the present invention, there is provided a
~netl~ofodr producing a hot stamped steel including casting a molten steel having a
chemical composition according to the above (1) and obtain a steel; heating the steel;
hot-rolling the steel with a hot-rolling facility having a plurality of stands; coiling the
steel after the hot-rolling; pickling the steel after the coiling; cold-rolling the steel after
the pickling with a cold rolling 1nil1 having a plurality of stands under a condition
satisfying a following expression-e; annealing in which the steel is heated under 700°C
to 850°C and cooled after the cold-rolling; temper-rolling the steel after the annealing;
and hot stamping in which the steel is heated to a temperature range of 750°C or more
at a temperature-increase rate of 5 OC/second or more, formed within the temperature
range, and cooled to 20°C to 300°C at a cooling rate of 10 "C/second or more after the
tempel:rolling.
1.5 x r l / r + 1 , 2 x r 2 / r + r 3 / r > 1 (e)
Here, ri represents an individual cold-rolling reduction (%) at an i"' stand
based on an uppernlost stand among a plurality of the stands in the cold-rolling process
where i is 1,2 or 3, and r represents a total cold-rolling reduction (%) in the coldrolling.
[00 151
(8) In the method for producing the hot stamped steel according to the above
(7), when CT (OC) represents a coiling temperature in the coiling; [C] represents an
amount of C by mass%, [Si] represents an amount of Si by mass%, [Mn] represents an
amount of Ma by mass% in the steel; and [Mo] represents an amount of Mo by mass%
in the steel, a following expression-f may be satisfied.
560-474 x [C]-90 x [Mn-20 x [Crl-20 x [Mo] 1500 (9)
[0017]
(10) The method for producing the hot stamped steel according to any one of
the above (7) to (9) may further include galvanizing between the atulealing and the
temper-rolling.
[OO 1 81
(11) The inethod for producing the hot stamped steel according to the above
(10) rnay further include alloying between the hot dip galvanizing and the temperrolling.
[0019]
(12) The method for producing the hot stanlped steel according to any one of
the above (7) to (9) may further include electrogalvanizing between the temper-rolling
and the hot stamping.
[0020]
(13) The method for producing the hot stamped steel according to any one of
the above (7) to (9) may further include aluminizing between tlie annealing and the
temper-rolling.
[Effects of the Invention]
[0021]
According to the present invention, since an appropriate relationship is
established among the amount of the C, tlie amount of the Mn and the amount of the Si,
and the hardness of the maltensite measured with a nanoindenter is set to an
appropriate value in the molded article after the hot stamping, it is possible to obtain a
hot stamped steel having a favorable hole expansibility.
[Brief Description of the Drawing]
roo221
FIG. 1 is a graph illustrating a relationship between (5 x [Si] + [Mn]) / [C] and
TS x h.
FIG. 2A is a graph illustrating a foundation of an expression b and an
expression c, and is a graph illustrating a relationship between H2 1 H1 and DHM of a
hot stamped steel.
FIG. 2B is a grapli illustrating a foundation of the expressio~ci , and is a grapli
illustrating a relationship between tlie OHM and TS x h.
FIG. 3 is a grapli illustrating a relationsliip between n2 1 nl and TS x h before
and after hot stamping, and illustrating a foundation of expression d.
FIG. 4 is a graph illustrating a relationship between 1.5 x rl / r + 1.2 x 1.2 / r +
r3 / r, and the H2 /HI, and illustrating a foundation of an expression e.
FIG. 5A is a graph illustrating a relationship between an expression f and a
fraction of a maltensite.
FIG. 5B is a graph illustrating a relationship between the expression f and a
fraction of a pearlite.
FIG. 6 is a graph illustrating a relationship bet\veen T x ln(t) /(I .7 x [Mn] +
[S]) and TS x h, and illustrating a foundation of expression g.
FIG. 7 is a perspective view of a hot starnped steel used in an example.
FIG. 8 is a flowchart illustrating a lnetllod for producing the hot stanlped steel
according to an embodiment of the present invention.
[Embodiments of the Invention]
[0023]
As described above, it is imnpol-tant to establish an appropriate relationship
among an amount of Si, an amount of Mn and an amount of C, and furthermore, to set
an appropriate hardness of a martensite at a predeternlined position to improve a
formability (hole expansibility) of a hot stainped steel. Thus far, there have been no
studies regarding a relationship between the forn~abilityo f the hot stamped steel and
the hardness of the martensite.
[0024]
Hereinafter, an enlbodi~nenot f the present invention will be described in
detail.
First, reasons for linliting a chemical colnposition of a cold rolled steel sheet
lor hot stamping (including a hot dip galvanized cold rolled steel sheet or an
aluminized cold rolled steel sheet and, in some cases, referred to as a cold rolled steel
shcet according to the embodiment or simply as a cold rolled steel sheet for hot
stamping) used for a hot stamped steel according to an embodiment of the present
invention (the hot stamped steel according to the present enlbodiment or, in some cases,
referred to simply as the hot stanlped steel) will be described. Hereinafter, " % that is
a unit of an amount of an individual component indicates "mass%. Since a
component ainount of a chemical co~npositiono f the steel sheet does not change in the
hot stamping, the chemical coinposition is identical in both the cold rolled steel sheet
and the hot stamped steel for which the cold rolled steel sheet is used.
[0025]
C: more than 0.150% to 0.300%
C is an important element to strengthen a fei~itean d the ma^-tensite and
increase a strength ora steel. However, when a11 amount of the C is 0.150% or less, a
sufficient amount of a martensite cannot be obtained, and it is not possible to
sufficiently increase the strength. On the other hand, when the amount of the C
exceeds 0.300%, an elongation and the hole expansibility significantly degrades.
Therefore, a range of the amount of the C is set to more than 0.150% and 0.300% or
less.
[0026]
Si: 0.010% to 1.000%
Si is an inlportant element to suppress a generation of a harmfill carbide and
to obtain multi-phases mainly including the ferrite and the martensite. However,
when an amount of the Si exceeds 1.000%, elongation or hole expausibility degrades,
and a chemical conversion property also degrades. Therefore, the alnount of the Si is
set to 1.000% or less. I11 addition, the Si is added for deoxidation, but a deoxidation
effect is not sufficient at the amount of the Si of less than 0.010%. Therefore, the
a~nounot f the Si is set to 0.010% or more.
[0027]
Al: 0.010% to 0.050%
A1 is an important element as a deoxidizing agent. To obtain the deoxidation
effect, an a~nounot f the A1 is set to 0.010% or more. On tlie other hand, even when
the A1 is excessively added, the above-described effect is saturated, and conversely, the
steel becomes brittle, and TS x h is decreased. Therefore, the amount of the A1 is set
in a range of 0.010% to 0.050%.
[0028]
Mn: 1.50% to 2.70%
Mn is an ilnportant elernellt to improve a hardenability and strengthen the
steel. However, when an amount of the Mn is less than 1.50%, it is not possible to
sufficiently increase the strength. On the other hand, when the amount of the Mn
exceeds 2.70%, the hardenability becotnes excessive, and the elongation or the hole
expansibility degrades. Therefore, the amount of the Mn is set to 1.50% to 2.70%.
In a case in which higher elongation is required, the amount of the Mn is desirably set
to 2.00% or less.
[0029]
P: 0.001% to 0.060%
At a large amount, P segregates at grain boundaries, and deteriorates a local
elongation and a weldability. Therefore, an amount of the P is set to 0.060% or less.
Tile amount of the P is desirably smaller, but an extreme decrease in the amount of the
P leads to a cost increase for refining, and therefore the amount of the P is desirably set
to 0.001% or more.
[0030]
S: 0.001% to 0.010%
S is an elemelit that forms MuS and significantly deteriorates the local
elongation or the weldabilitp. Therefore, an upper limit of an amount of the S is set to
0.010%. 111 addition, the anlonut of tlie S is desirably smaller; however, due to a
problem of a refiniug cost, a lower limit of the amount of the S is desirably set to
0.001%.
[003 I]
N: 0.0005% to 0.0100%
N is an important element to precipitate AlN and the like and miniaturize
crystal grains. However, ~vliena n amoutit of the N exceeds 0.0100%, a nitrogen solid
solutio~rle mains and elongation or hole expansibility is degraded. Therefore, an
amount of the N is set to 0.0100% or less. The amount of the N is desirably smaller;
however, due to a problem of a refining cost, a lower limit of the amount of the N is
desirably set to 0.0005%.
[0032]
The cold rolled steel sheet according to the embodiment has a basic
composition including the above-described elements and a balance including iron and
unavoidable impurities, however, it1 some cases, includes at least one element of Nb, Ti,
V, Mo, Cr, Ca, REM (rare earth metal), Cu, Ni and B as elements that have thus far
been used in an alilount tliat is equal to or less than an upper limit described below to
i~nprovetl ie strength, to control a shape of a sulfide or an oxide, and the like. The
above-described chemical elements are not necessarily added to the steel sheet, and
therefore a lower limit thereof is 0%.
[0033]
Nb, Ti and V are elements tliat precipitate a fine carbonitride and strengthen
the steel. In addition, Mo and Cr are elements that increase the hardenability and
strengthen the stcel. To obtain the above-described effects, it is desirable to include
Nb: 0.001% or more, Ti: 0.001% or more, V: 0.001% or more, Mo: 0.01% or more and
Cr: 0.01% or more. 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, a strength-increasing effect is saturated, and tlle degradation of the
elongation or the hole expansibility is caused. Therefore, upper linlits of Nb, Ti, V,
Mo and Cr are set to 0.050%, 0.100%, 0.100%, 0.50% and 0.50%, respectively.
[0034]
Ca controls the shape of the sulfide or the oxide and improves the local
elongation or the hole expansibility. To obtain the above-described effect, it is
desirable to contain 0.0005% or more of the Ca. However, since an excessive
addition deteriorates a workability, an upper limit of an amount of the Ca is set to
0.0050%.
Similarly to Ca, rare earth metal (EM) controls the shape of the sulfide and
the oxide and improves the local elongation or the hole expansibility. To obtain the
above-described effect, it is desirable to contain 0.0005% or more of the REM.
Ho\veve~; since an excessive addition deteriorates the workability, an upper limit of an
antount of the E M is set to 0.0050%.
[0035]
The steel can further include Cn: 0.01% to 1.00%, Ni: 0.01% to 1.00% and B:
0.0005% to 0.0020%. The above-described elements also can improve the
liardenability and increase the strength of the steel. However, to obtain the abovedescribed
effect, it is desirable to contain Cu: 0.01% or more, Ni: 0.01% or more and
B: 0.0005% or more. In amounts that are equal to or less than tlie above-described
amounts, tlie effect that strengthens tlie steel is small. On the other hand, even when
Cu: more than 1.00%, Ni: more than 1.00% and B: more than 0.0020% are added, the
strength-increasing effect is saturated, and the elongation or the hole expansibility
degrades. Therefore, an upper limit of an amount of the Cu is set to 1.00%, an upper
limit of an amount of the Ni is set to 1.00%, a~ida n upper limit of an amount of B is set
to 0.0020%.
[0036]
In a case in which B, Mo, Cr, V, Ti, Nb, Ni, Cu, Ca and REM are included, at
least one element is included. Tile balance of the steel includes Fe and ut~avoidable
impurities. As the unavoidable impurities, elements other than the above-described
elements (for example, Sn, As and the like) may be further included as long as
characteristics are not impaired. When B, Mo, Cr, V, Ti, Nb, Ni, Cu, Ca and REM are
included in atnounts that is less than the above-described lower limits, the elements are
treated as tlie unavoidable impurities.
[0037]
Furthermore, in the hot stamped steel according to the embodiment, when [C]
rcpresents the amount of the C (massYo), [Si] represents the amount of Si (mass%) and
[Mn] represents the a~ilounot f Mn (mass%), it is important to sat is^ tlie following
expression a to obtain the sufficient hole expansibility as illustrated in FIG. 1.
(5 x [Si] + [MI]]) / [C] > 10 (a)
When a value of (5 x [Si] + [Mn]) / [C] is 10 or less, TS x h becomes less
than 50000 MPa%, and it is not possible to obtain the sufficient hole expaasibility.
This is because, when the amount of tlie C is high, a hardness of a hard phase becomes
too high and a difference between a hardness of a hard phase and a hardness of a soft
phase becomes great, and thereby, a value of h is deteriorated, and, whe~t the amount of
the Si or tlie amount of the Mn is small, TS becomes low. Therefore, it is necessary
to set the each ele~nenitn the above-described ranges, and furthermore, to control a
balance among the amounts thereof. Since the value of (5 x [Si] + [Mn]) / [C] does
not change even after hot stamping as described above, the value is preferably satisfied
when producing the cold rolled steel sheet. However, even when (5 x [Si] + [Mn]) /
[C] > 10 is satisfied, in a case in which the H2 / H1 or the oHM described below does
not satisfy the conditions, the sufficient hole expansibility cannot be obtained. In FIG.
1, a reference sign for after the hot stamping indicates the hot stamped steel, and a
reference sign for before the hot stamping indicates the cold rolled steel sheet for hot
stamping.
[0038]
Generally, it is tlie Inartensite rather than the ferrite to dominate the
formability (hole expansibility) in the cold rolled steel sheet having the metallographic
structure mainly including the ferrite and the martensite. The inventors carried out
intensive studies regarding a relationship between the liardness and the formability
such as the elongation or the hole expansibility of tlie martensite. As a result, it was
found that, wl~ena llardness ratio (a difference of the hardness) of tlie martensite
between a surface portion of a sheet thickness and a central part of the sheet thickness,
and a hardness distribution of the martensite in the central part of the sheet thickness
are in a predetermined state regarding a hot stamp formability according to the
embodinlent as illustrated in FIGS. 2A and 2B, the formability such as the elongation
or the hole expansibility becotnes favorable. In addition, it was clarified that, when
the liardness ratio and the hardness distribution are in a predetermined range in the cold
rolled steel sheet for hot stamping used for the hot stamp formability according to tlie
embodiment, the hardness ratio and the hardness distribution are ahnost maintained in
the hot staniped steel as well, and the formability such as the elongation or the hole
expansibility becomes favorable. This is because the hardness distribution of the
martensite formed in the cold rolled steel sheet for hot stamping also has a significant
effect on the hot stamped steel after the hot stamping. Specifically, this is considered
to be because alloy elentents condensed in the central part of the sheet thickness still
hold a state of being condensed in the central part even after the hot stamping is carried
out. That is, in the cold rolled steel sheet for hot stamping, in a case in which the
hardness difference of the tnaitensite between the surface portion of the sheet thickness
and the central part of the sheet thickness is great or a case in which a variance of the
hardness of the maltensite is great in the central part of the sheet thickness, the siriiilar
hardness ratio and the similar variance are obtained in the hot stamped steel as well.
In FIGS. 2A and 2B, a reference sign for after the hot stamping indicates the hot
stamped steel, and a reference sign for before the hot stamping indicates the cold rolled
steel sheet for hot stamping.
[0039]
The inventors also found that, regarding a hardness measurement of the
martensite measured with a nanoindenter manufactured by I-Iysitron Corporation at
1000 times, when the following expression b and the following expression c are
satisfied, the formability of the hot stamped steel inlproves. Here, an "HI" is the
hardness of the maitensite in the surface portion of the sheet thickness that is within an
area having a width of 200 pm in a thickness direction from an outermost layer of the
hot stamped steel. An "H2" is the hardness of the martensite in the central part of the
sheet thickness of the hot stamped steel, that is, in an area having a width of *lo0 lun
in the thickness direction from the central part of the sheet thickness. A "OHM is the
variance of the hardness of the maltensite existing in an area having a width of 200 pin
in the thickness direction in the central part of the sheet thickness of the hot stantped
steel. The H1, the H2 and the (rHM are each obtained from 300-point measurements.
The area having a width of 200 pm in the thickness direction in the central part of the
sheet thickness refers to an area having a center at a center of the sheet thickness and
having a dimension of 200 pm in the thickness direction.
H2/H1<1.10 (b)
OHM < 20 (c)
In addition, here, the variance is a value obtained using the following
expression h and indicating a distribution of the hardness of the martensite.
[0040]
[Expression 11
(h)
[0041]
An X, represents an average value of the measured hardness of the
martensite, and Xi represents the hardness of an i"' martensite.
FIG. 2A illustrates the ratios between the hardness of the nlartensite in the
surface portion and the hardness of the martensite in the central part of the sheet
tllickness of the hot stamped steel and the cold rolled steel sheet for hot stamping. In
addition, FIG. 2B collectively illustrates the variance of the hardness of the martensite
existing in the width ofh100 pm in the sheet thickness direction from the center of the
sheet thickness of the hot stamped steel and the cold rolled steel sheet for hot stamping.
As illustrated in FIGS. 2A and 2B, the hardness ratio of the cold rolled steel sheet
before the llot stanlping and the hardness ratio of the cold rolled steel sheet after the
hot stamping are almost the same. In addition, the variances of the hardness of the
maltensite in the central part of the sheet thickness are also altnost the same both in the
cold rolled steel sheet before the hot stamping and in the cold rolled steel sheet after
tlie hot stamping.
[0042]
In the hot stamped steel, a value of the H2 / H1 being 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 portion of the sheet thickness.
That is, this indicates that the hardness in the central part of the sheet thickness
becomes too high. As illustrated in FIG. 2A, \vhen the H2 1 H1 is 1 .I0 or Inore, the
GHM reaches 20 or more. In this case, TS x h becomes less than 50000MPa%, and a
sufficient formability cannot be obtained after quenching, that is, in the liot stamped
steel. Theoretically, there is a case in which a lower limit of the H2 / H1 becomes the
same in tlie central part 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 in consideration of a productivity, the lower limit is, for example,
up to approximately 1.005.
[0043]
The variance OHM of the hot stamped steel being 20 or more indicates that a
variation of the hardness of the martensite is large, and parts in which the hardness is
too high locally exist. In this case, TS x h becomes less than 50000MPa%. That is,
a sufficient formability cannot be obtained in the liot stamped steel.
[0044]
Next, the metallographic structure of the hot stamped steel according to the
enlbodiment will be described. hi area fiaction of the martensite is 80% or more in
the hot stamped steel according to the embodiment. When the area fraction of the
~nartensiteis less than SO%, a sufficient strength that has been recently required for the
hot stamped steel (for example, 1.5 GPA) cannot be obtained. Therefore, the area
fraction of the martensite is set to 80% or more. All or principal parts of the
~netallographics tructure of the hot stamped steel are occupied by the inartensite, and
may further include one or more of 0% to 10% of a pearlite in an area fraction, 0% to
5% of a retained austenite in a volunle ratio, 0% to 20% of the ferrite it1 an area
fraction, and 0% to less than 20% of a bainite in an area fraction. While there is a
case in which 0% to 20% of the ferrite exists depending on a hot stamping condition,
there is no problem with the strength after the hot stamping within the above-described
range. When the retained austenite remains in the metallographic structure, a
secondary working brittleness and a delayed fiacture characteristic are likely to
degrade. Therefore, it is preferable that the residual austenite is substantially not
included; however, unavoidably, 5% or less of the residual austenite in a volume ratio
may be included. Since the pearlite is a hard and brittle stn~cturei,t is preferable not
to include the pearlite; ho\vevel; unavoidably, up to 10% of the pearlite in an area
fraction may be included. The baitiite is a structure that can be formed as a residual
structure, and is an intermediate structure in terms of the strength or the formability,
may be included. The baitiite may be included up to less than 20% in terms of an area
fraction. In the elnbodiment, the nletallographic structures of the ferrite, the bainite
and the pearlite were observed through Nital etching, and the metallographic structure
of the martensite was observed tlirough Le pera etching. All the metallographic
stl~~cturwese re observed in a 114 part of the sheet thickness with an optical microscope
at 1000 times. The volunle ratio of the retained austenite was measured with an Xray
diffraction apparatus after polishing the steel sheet up to the 114 part of the sheet
tliich~css.
[0045]
Next, the desirable nletallographic structure of the cold rolled steel sheet for
hot stamping for which the hot stamped steel according to the emboditnet~ti s used will
be described. The metallograpliic structure of the hot stamped steel is affected by the
metallographic structure of the cold rolled steel sheet for hot stamping. Tlierefore,
when the metallograpl~cs tructure of the cold rolled steel sheet for hot stamping is
controlled, it becomes easy to obtain the above-described metallographic structure in
the hot stamped steel. In the cold rolled steel sheet according to the embodiment, the
area fraction of the ferrite is desirably 40% to 90%. When the area fiaction of the
ferrite is less than 40%, the strength becomes too high even before the hot stamping
and there is a case in which the shape of the hot stamped steel deteriorates or cutting
becomes difficult. Therefore, the area fraction of the ferrite before the hot stamping
is desirably set to 40% or more. In addition, in the cold rolled steel sheet according to
the embodiment, since an amount of alloy elements is great, it is difficult to set the area
fraction of the ferrite to more than 90%. In the metallographic structure, in addition
to the ferrite, the martensite is included, and the area fraction thereof is desirably 10%
to 60%. A total of the area fraction of the ferrite and the area fraction of the
martensite is desirably 60% or more before the hot stamping. The ~iletallograpllic
structure nmay further include one or more of the pearlite, tlie bainite and the retained
austenite. However, when the retained austenite remai~lsin the metallographic
structure, the secondary working brittleness and the delayed fracture characteristics are
likely to degrade, and therefore it is preferable that the retained austenite be
substantially not included. However, unavoidably, 5% or less of the retained
austenite may be included in a volunie ratio. Since the pearlite is a hard and brittle
structure, the pearlite is preferably not included; however, unavoidably, up to 10% of
the pearlite may be included in an area fiaction. Up to 20% or less of tlie bainite as
the residual structure can be included in an area fraction for the same reason as
described above. Similarly to the cold rolled steel sheet before the hot stamping, tlie
nletallographic structures of the ferrite, the bainite and the pearlite were observed
tlxough Nital etching, and tlie metallographic structure of tlie martensite was observed
through Le pera etching. All tlie metallographic structures were observed in a 114
part of the sheet thickness with an optical microscope at 1000 times. The volume
ratio of the retained austenite was measured with an X-ray diffraction apparatus after
polishing the steel sheet up to the 114 part of the sheet thickness.
[0046]
In addition, in the hot stamped steel according to the embodiment, the
hardness of the marteasite measured with a llanoindenter at 1000 times (indentation
hardness (GPa or ~ l m t no~r )a value obtained by converting the indentation hardness
to a Vickers hardness (Hv)) is specified. In an ordinary Vickers hardness test, a
formed indentation becomes larger than the niartensite. Therefore, a macroscopic
hardness of the martensite and peripheral structures thereof (the ferrite and the like)
can be obtained, but it is not possible to obtain tlie hardness of the maltensite itself.
Since the formability such as the liole expansibility is significantly affected by the
hardness of the martensite itself, it is difficult to sufficiently evaluate tlie formability
only with the Vickers hardness. On the contrary, in tlie hot stamped steel according to
the embodiment, since the hardness ratio of the hardness of the maltensite nieasured
with the nanoindenter and a dispersion state are controlled in an appropriate range, it is
possible to obtain an extremely favorable formability.
[0047]
The MnS was observed at a location of 114 of the sheet thickness (a location
that is 114 of the sheet thickness deep fiom the surface) and the central part of the sheet
thickness of the hot stan~peds teel. As a result, it was found that an area fraction of
the MnS having an equivalent circle diameter of 0.1 pm to 10 pm of 0.01% or less and ,
as illustrated in FIG. 3, the following expression d being satisfied are preferable for
favorably and stably obtaining TS x h 2 50000 MPa.%.
112/i11<1.5 (d)
Here, the nl represents a number density (average number density)
(grains110000 pm2) of the MnS having the equivalent circle diameter of 0.1 p111 to 10
pm per unit area in the 114 pait of the sheet thickness of the hot stanlped steel, and the
112 represents a number density (average nun~berd ensity) (grains110000 pi11~)o f the
MnS having the equivalent circle dian~etero f 0.1 pm to 10 pm per unit area in the
central part of the sheet thickness of the hot stamped steel.
Areason for the forinability in~provingin a case in which the area fraction of
MnS of 0.1 ptn to 10 pm is 0.01% or less is considered that, when a hole expansion
test is carried out, if there is MnS having the equivalent circle diameter of 0.1 pm or
more, since stress concentrates in a vicinity thereof, cracking is likely to occur. A
reason for not counting the MnS having the equivalent circle diameter of less than 0.1
pm is that an effect on the stress concentration is small, and a reason for not counting
the MnS having the equivalent circle dianleter of more than 10 pm is that the MnS
having the equivalent circle diameter of more than 10 pin is originally not suitable for
working. Furthermore, when the area fiaction of the MnS having the equivalent
circle diameter of 0.1 pn~to 10 Inn exceeds 0.01%, since it becomes easy for fine
cracks generated due to the stress concentration to propagate. Therefore, there is a
case in which the hole expansibility degrades. Furthermore, a lower limit of the area
fiaction of the MnS is not pai-ticularly specified, but it is reasonable to set the lower
limit to 0.0001% or Inore since setting the lower limit to less than 0.0001% in
consideration of a n~easuren~etmite thod described below, litnitations of a
~iiagnificationa nd a visual field, the amount of the Mn or the S, and a desulfurization
treatment capability has an effect on a productivity and a cost.
[0048]
When tlie area fraction of MnS having the equivalent circle diameter of 0.1
kutn to 10 ~11i1n the hot stamped steel is more than 0.01%, as described above, the
formability is likely to degrade due to the stress concentration. Avalue of the n2 I nl
being 1.5 or more in the hot stamped steel indicates that the number density of tlie
MnS in the central part of the sheet thickness of the hot stamped steel is 1.5 or more
times the number density of the MnS in the 114 pait of the sheet thickness of the hot
stamped steel. In this case, the fornlability is likely to degrade due to a segregation of
the MnS in the central part of tlie sheet thickness. In the embodiment, the equivalent
circle diameter and the number density of the MnS were measured with a field
e~nissions canning electron microscope (Fe-SEM) ~iianufacturedb y JEOL Ltd. The
magnification was 1000 times, and a measurement area of the visual field was set to
0.12~0.09m tn2 (=lo800 ~~nt2~10p0n020). 10 visual fields were observed at the
location of 114 of the sheet thickness from the surface (the 114 part of the sheet
thickness), and 10 visual fields were observed in tlie central part of tlie sheet thickness.
The area fraction of the MnS was computed with particle analysis software. In tlie
embodiment, the MnS was observed in the cold rolled steel sheet for hot stamping in
addition to the hot stamped steel. As a result, it was found that a form of the MnS
fortned before the hot stamping (in the cold rolled steel sheet for hot stamping) did not
change even in tlie hot stamped steel (after the hot stamping). FIG. 3 is a view
illustrating a relationship between the n2 / nl and TS x h of tlie hot stamped steel, and
also illustrates an evaluation of measure~iienrte sults of the number density of the MnS
in the 114 pai-t of the sheet thickness and in the central pai-t of the sheet thickness of the
cold rolled steel sheet for hot stalnping using the same index as for the hot stamped
steel. In FIG. 3, a reference sign for after the hot stamping indicates the hot stamped
steel, and a reference sign for before the hot stanlping indicates the cold rolled steel
sheet for hot stamping. As illustrated in FIG. 3, the n2 / nl (a ratio of the MnS
between the 114 part of the sheet thickness and the central part of the sheet thickness) ,
of the cold rolled steel sheet for hot stamping and the hot stamped steel is almost the
same. This is because the form of the MnS does not change at a heating teinperature
of the hot stamping.
[0049]
The hot stainped steel according to the embodiment is obtained, for example,
by heating the cold rolled steel sheet according to the embodiment to 750°C to 1000°C
at a temperature-increase rate of, 5 OC/second to 500 OC/second, forming (working) the
steel sheet for 1 second to 120 seconds, and cooling the steel sheet to a temperature
range of 20°C to 300°C at a cooling rate of 10 "Clsecond to 1000 oC/secoi~d. An
obtained hot stamped steel has a tensile strength of 1500 MPa to 2200 MPa, and can
obtain a significant formability-improving effect, particularly, in a steel sheet having a
high strength of approximately 1800 MPa to 2000 MPa.
[0050]
It is preferable to form a galvanizing, for example, a hot dip galvanizing, a
galvalmealing, an electrogalvanizing, or an aluminizing on the hot starnped steel
according to the etnboditnei~itn terms of rust prevention. In a case in which a plating
is formed on the hot stamped steel, a plated layer does not change under the abovedescribed
hot stamping condition, and therefore a plating may be formed on the cold
rolled steel sheet for hot sta~nping. Even lvhen the above-described plating is formed
on the hot stamped steel, the effects of the e~nbodi~neanrte not impaired. The abovedescribed
platings can be formed with a well-luiown method.
too5 11
Hereinafter, a ~netl~ofodr producing the cold rolled steel sheet accordi~igto
tlie embodiment and tlie hot stamped steel according to tlie embodiment obtained by
hot-stamping the cold rolled steel sheet will be described.
[0052]
When producing the cold rolled steel sheet according to the embodiment, as
an ordinary condition, a nlolten steel nlelted so as to have the above-described
cl~emicacl omposition is continuously cast after a convertel; thereby producing a slab.
In the continuous casting, when a casting rate is fast, a precipitate of Ti and the like
becomes too fine. On the otl~ehr and, when the casting rate is slow, productivity
deteriorates, and consequently, the above-described precipitate coarsens so as to
decrease the number of particles, and there is a case in which other characteristics such
as a delayed fracture cannot be controlled appears. Therefore, the casti~igra te is
desirably 1.0 &minute to 2.5 nilminute.
[0053]
The slab after the melting and the casting can be subjected to hot-rolling as
cast. Alternatively, in a case in which the slab is cooled to less than 110O0C, it is
possible to reheat tlie slab to llOO°C to 1300°C in a tunnel furnace or the like and
subject the slab to tlie hot-rolling. When a temperature of the slab during the hotrolling
is less than 11 OO°C, it is difficult to ensure a finishing teniperature in tlie liotrolling,
which causes a degradation of the elongation. In addition, in the steel sheet to
which Ti or Nb is added, a dissolution of the precipitate becomes insufficient during
the heating, which causes a decrease in the strength. On the other hand, when the
temperature of the slab is more than 130OoC, a generation of a scale becomes great,
and there is a concern that it may be inlpossible to make the surface quality of the steel
sheet favorable.
In addition, to decrease the area fraction of the MnS, when [Mn] represents
the amount of the Mn (mass%) and [S] represent the amount of the S (mass%) in the
steel, it is preferable for a temperature T (OC) of a heating filmace before carrying out
the hot-rolling, an in-fi~rnaceti me t (minutes), [Mn] and [S] to satisfy the following
expression g as illustrated in FIG. 6.
T x ln(t) / (1.7 x [Mn] + [S]) > 1500 (6)
When a value of T x ln(t) / (1.7 x [Mn] + [S]) is equal to or less than 1500, the
area fraction of the MnS becomes large, and there is a case in which a difference
between the nunlber of the MnS in the 114 part of the sheet thickness and the number
of the MnS in the central part of the sheet thickness becomes large. The temperature
of the heating furnace before carrying out the 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 an insertion of the slab into the hot heating filrnace to an extraction
of the slab fro111 the heating furnace. Since the MnS does not change with the hotrolling
or the hot stamping as described above, it is preferable to satisfy the expression
g during heating of the slab. The above-described In represents a natural logarithnl.
[0054]
Next, the hot-rolling is carried out according to a conventional method. At
this time, it is desirable to set the finishing temperature (a hot-rolling end temperature)
to anAr3 temperature to 970°C and carry out the hot-rolling on the slab. When the
finishing tenlperature is less than the Ar3 temperature, there is a concern that the
rolling nlay become a two-phase region rolling of the ferrite (a) and the austenite (y),
and the elongation may degrade. On the other hand, when the finishing temperature
is inore than 97OoC, an austenite grain size coarsens, a fraction of the ferrite becomes
small, and there is a concern that the elongation rimy degrade.
The Ar3 temperature can be estimated from an inflection point after carrying
out a forn~astotre st and ~neasuringa change in a length of a test specimen in response
to a temperature change.
[0055]
After the hot-rolling, the steel is cooled at an average cooling rate of
20 "C/second to 500 oC/second, and is coiled at the predeterlnined coiling temperature
CT°C. In a case in which the cooling rate is less than 20 "Clsecond, the pearlite
causing the degradation of the elongation is likely to be fornled, which is not
preferable.
On the other hand, an upper limit of the cooling rate is not particularly
specified, but the upper litnit ofthe cooling rate is desirably set to approximately
500 oC/second from a viewpoint of a facility specification, but is not limited thereto.
[0056]
After the coiling, pickling is carried out, and cold-rolling is carried out. At
this time, as illustrated in FIG 4, the cold-rolling is ca~xiedo ut under a condition in
which the following expression e is satisfied to obtain a range satisfying the abovedescribed
expression b. When the above-described rolling is carried out, and then
annealing, cooling and the like are performed in below-described conditions, TS x h>
50000 MPa% can be obtained in the cold rolled steel sheet before hot stanlping, and
furthe~lnorei,t is possible to ensure TS x h > 50000 MPa% in the hot stanlped steel for
which the cold rolled steel sheet is used. Meanu~hilet, he cold-rolling is desirably
carried out with a tandem rolling Inill in which a plurality of rolling mills is linearly
disposed, and the steel sheet is continuously rolled in a single direction, thereby
obtaining a predetermined thickness.
1.5 x r l / r + 1 . 2 x r 2 / r + r 3 / r > 1.0 (el
Here, the "ri (i=l, 2 or 3)" represents an individual target cold-rolling
reduction (%) at an it" stand (i = 1,2,3) based on an uppernlost stand in the coldrolling,
and the "r" represents a total target cold-rolling reduction (%) in the coldrolling.
The total cold-rolling reduction is a so-called cumulative reduction, is based
on the sheet thickness at an inlet of a first stand, and is a percentage of the cumulative
reduction (a difference between the sheet thickness at the inlet of a first pass and the
sheet thickness at an outlet after a final pass) with respect to the above-described basis.
[0057]
When the cold-rolling is carried out under a condition in which the abovedescribed
expression e is satisfied, it is possible to sufficiently divide the pearlite in the
cold-rolling even when the large pearlite exists before the cold-rolling. As a result, it
is possible to burn the pearlite or suppress tlie area fraction of the pearlite to the
minimum extent through annealing carried out after the cold-rolling. Therefore, it
becomes easy to obtain a structure satisfying the expression b and the expression c.
On the other hand, in a case in xvl~ichth e expression e is not satisfied, the cold-rolling
reductions in the upper stream stands are not sufficient, and the large pearlite is likely
to remain. As a result, it is not possible to form the martensite having a desired form
in an annealing process.
In addition, the inventors found that, in the cold rolled steel sheet that had
been subjected to a rolling satisfying the expression e, it was possible to maintain the
form of tlie ma~tensites tructure obtained after the annealing in almost the same state
even when the hot sta~npingis carried out afterwards, and the elongation or the hole
expansibility became advantageous. In a case it1 whictt the cold rolled steel sheet for
hot stamping according to the embodiment is heated up to an austenite region through
the hot stamping, the llard phase includitlg the martellsite turns into an austenite having
a high C concentratioa, and the ferrite phase turns into the austenite having a low C
concentration. When the austenite is cooled afterwards, the austenite forms a hard
phase including martensite. That is, wl~et~hel hot stamping is carried out on the steel
sheet for hot stamping having the hardness of the maltensite so as to satisfy the
expression e (so as to make the above-described H2 I H1 in a predeternlined range), the
above-described H2 I H1 reaches in a predetermined range even after the hot stamping,
and the formability after the hot stamping becomes excellent.
[OOSS]
In the embodiment, the 1; the rl, the r2 and the r3 are the target cold-rolling
reductions. Generally, the target cold-rolling reduction and an actual cold-rolling
reduction are cotltrolled so as to become substantially the same value, and the coldrolling
is carried out. It is not preferable to carry out the target cold-rolling after
unnecessarily making the actual cold-rolling reduction different from the cold-rollitig
reduction. In a case in which there is a large difference between a target rolling
reduction and an actual rollitlg reduction, it is possible to consider that the embodiment
is carried out when the actual cold-rolling reduction satisfies the expression e. The
actual cold-rolling reduction is preferably converged witlun *lo% of the cold-rolling
reduction.
[0059]
After the cold-rolling, the annealing is carried out. When the annealing is
carried out, a recrystallization is caused in the steel sheet, and the desired martensite is
formed. Regarding an annealing tenlperature, it is preferable to carry out the
anuealing by heating the steel sheet to a range of 700°C to 850°C according to a
conventional method, and to cool the steel sheet to 20°C or a temperature at wl~icha
surface treattnent such as the hot dip galvanizing is carried out. When the annealing
is carried out in the above-described range, it is possible to ensure a desirable fraction
of the ferrite and a desirable area fraction of the maltensite and to obtain a total of the
area fraction of the ferrite and the area fraction of the lnartet~siteo f 60% or more, TS x
h improves.
Conditions other than the annealing tenlperature are not particularly specified,
but a lower limit of a holding time at 70OoC to 850°C is preferably set to 1 second or
more to reliably obtain a predeternlined structure, for example, approximately 10
minntes as loug as the productivity is not impaired. It is preferable to appropriately
determine the temperature-increase rate to 1 OC/secot~tdo an upper limit of a facility
capacity, for example, 1000 "Clsecond, and to appropriately determine the cooling rate
to 1 OCIsecond to the upper limit of the facility capacity, for example, 500 'Clsecond.
Temper-rolling may be carried out with a conventional method. An elongation ratio
of the temper-rolling is, generally, approximately 0.2% to 596, and is preferable when a
yield point elongation is avoided and the shape of the steel sheet can be corrected.
[0060]
As a still more preferable condition of the present invention, when [Cj
represents the amount of the C (mass%), [Mnj represents the amount of Mn (mass%),
[Si] represents the amount of Si (mass%), and [Moj represents the anlount of Mo
(mass%) in steel, the coiling temperature CT in a coiling process preferably satisfies
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] ( f )
[0061]
Wlien the coiling temperature CT is less than 560 - 474 x [C] - 90 x [Mn] -
20 x [Cr] - 20 x [Mo], that is, CT - 560 - 474 x [C] - 90 x [Mn] - 20 x [Cr] - 20 x
[Mo] is less than zero as illustrated in FIG. 5A, the martensite is excessively formed,
and the steel sheet becomes too hard such that there is a case in which the subsequent
cold-rolling becon~esd ifficult. On the otller hand, when the coiling temperature CT is
more than 830 - 270 x [C] - 90 x [Mn] - 70 x [Cr] - 80 x [Mo], that is, 830 - 270 x
[C] - 90 x [Mn] - 70 x [Cr] - 80 x [Mo] is more than zero as illustrated in FIG. 5B, a
banded structure including the ferrite and the pearlite is likely to be formed. In
addition, a fraction of the pearlite in the central part of the sheet thickness is likely to
become high. Therefore, a uniformity of a distribution of the tnartensite being
formed in the subsequent annealing process degrades, and it becomes difficult to
satisfy tlie above-described expression b. In addition, there is a case in which it
becomes difficult for a sufficient amount of the martensite to be formed.
Wlien the expression f is satisfied, the ferrite and the hard phase have an ideal
distribution form before the hot stamping as described above. Furthermore, in this
case, the C and the like are likely to diffuse in a unifoilil manner after heating is carried
out in the hot stamping. Therefore, the distribution for111 of the hardness of the
martensite in the hot stamped steel becomes approxin~atelyid eal. When it is possible
to more reliably ensure the above-described nietallographic structure by satisfying the
expression f, the for~i~abiliotyf the hot stamped steel becomes excellent.
[0062]
Furthennore, to improve a rust-preventing capability, it is also preferable to
include a hot dip galvanizing process in which a hot dip galvanizing is formed between
the annealing process and the temper-rolling process and to form the hot dip
galvanizing on a surface of the cold rolled steel sheet. Further~norei,t is also
preferable to include an alloying process in which an alloying is folmed between the
hot dip galvanizing process and the temper-rolling process to obtain a galvannealing by
alloying the hot dip galvanizing. In a case in which the alloying is carried out, a
treatment in which a galvannealed surface is brought into contact with a substance
oxidizing a plated surface such as water vapor, thereby thickening an oxidized film
may be further carried out 011 the surface.
[0063]
It is also preferable to include, for example, an electrogalvanizing process in
which an electrogalvanizing is forlned on the surface of the cold rolled steel sheet after
the teniper-rolling process other than the hot dip galvanizing process and the
galvannealing process. In addition, it is also preferable to include, instead of the hot
dip galvanizing, an aluminizing process in which an aluminizing is forlned between the
amlealing 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, which is preferable.
[0064]
After a series of the above-described treatments, the hot stamping is carried
out on the obtained cold rolled steel sheet for hot stanlping, thereby producing a hot
stamped steel. In a hot stamping process, tlie hot sta~npingis desirably carried out
under, for cxample, tlie following conditions. First, the steel sheet is heated up to
750°C to 1000°C at the teniperature-increase rate of 5 "C/second to 500 OC/second.
After the heating, working (forming) is carried out for 1 second to 120 seconds. To
obtain a high strength; the heating teniperature is preferably more than an Ac3
temperature. The Ac3 temperature was estimated fro111 the inflection point of tlie
length of the test specimen after carrying out the formastor test.
Subseqoently, it is preferable to cool the steel sheet to 20°C to 300°C at the
cooling rate of, for example, 10 "Clsecond to 1000 "C/second. When the heating
temperature is less than 75OoC, in the liot stamped steel, the fraction of the martensite
is not sufficient, and tlie strength cannot be ensured. When the heating temperature is
more than 1000°C, the steel sheet beco~nesto o soft, and, in a case in whicli a plating is
formed on the surface of the steel sheet, particularly, in a case in wvl~ichz inc is plated,
tllere is a concern that the zinc may be evaporated and burned, which is not preferable.
Therefore, the heating tetnperature in the hot stamping process is preferably 750°C to
1000°C. When the temperature-increase rate is less than 5 "C/secoad, since a control
thereof is difficult, and the productivity significantly degrades, it is preferable to heat
the steel sheet at the temperature-increase rate of 5 "Clsecond or more. On tlie other
hand, an upper litnit of the teniperature-increase rate of 500 "C/second is from a
current heating capability, but is not liriiited thereto. At the cooling rate of less than
10 OC/second, since the rate control tl~ereoifs difficult,a nd the productivity also
significantly degrades, it is preferable to cool the steel slieet at the cooling rate of
10 "C/second or inore. An upper linlit of the cooling rate is not particularly specified,
but beconies 1000 "Clsecond or less in consideration of a current cooli~igc apability.
A reason for carrying out the temperature increasing and the forming working within 1
second to 120 seconds is to avoid tlie evaporation of tlie zinc and the like in a case in
which the hot dip galvanizing and the like are fornled on the surface of the steel slieet.
A reason for setting the cooling temperature to 20°C (the room tenlperature) to 30OoC
is to sufficiently ensure the martensite so as to ensure the strength after the liot
staillping.
[0065]
According to what has been described above, when tlie above-described
conditions are satisfied, it is possible to produce the hot stamped steel in which the
hardness distribution or tlie structure is ahnost maintained even after the hot stamping,
and consequently the strength is ensured and the more favorable hole expansibility can
be obtained.
FIG. 8 illustrates a flowchart (processes S1 to S14) of an example of the
production method described above.
[Example]
[0066]
A steel having a coinposition described in Table 1 was continuously cast at a
casting rate of 1.0 dininute to 2.5 mlminute, then, a slab was heated in a heating
furnace under a condition of Table 2 according to a conventional method as cast or
after cooling tlie steel once, and hot rolling was carried out at a finishing temperature
of 910°C to 930°C, thereby producing a liot rolled steel sliect. After that, the liot
rolled steel sheet was coiled at a coiling temperature CT described in Table 2. After
that, scales on a surface of the steel sheet were re~noved by carrying out pickling, and a
sheet thickness was set to 1.2 mm to 1.4 mnl through cold-rolling. At this time, tlie
cold rolling was carried out so that tlie value of the expression e became the value
described in Table 2. After tlie cold-rolling, annealing was carried out in a continuous
annealing fi~rnacea t an annealing temperature described in Tables 3 and 4. On a part
of tlie steel sheets, a hot dip galvanizing was formed in the nmiddle of cooling after
soaking in the continuous annealing furnace, and then alloying was fui-ther carried out
on the part thereof, thereby for~ning a galvannealing. In addition, an
electrogalvanizing or an alu~ninizingw as formed on the part of the steel sheets.
Temper rolling was carried out at an elongation ratio of 1% according to a
conventional method. In this state, a sample was taken to evaluate material qualities
and the like of the cold rolled steel sheet for hot stamping, and a material quality test or
the like was carried out. After that, to obtain a hot stamped steel havi~tga form
illustrated in FIG. 7, hot stamping in which a temperature was increased at a
temperature-increase of 10 "C/second, the steel sheet was held at a heating temperature
of 850°C for 10 seconds, and cooled to 200°C or less at a cooling rate of
100 "C/second was carried out. A sample was cut out from a location of FIG. 7 in an
obtained molded article, a material quality test and a structure observation were carried
out, and fractions of individual structures, a number density of MnS, a hardness, a
tensile strength (TS), an elongation (El), a hole expansion ratio (A) and the like were
obtained. The results are described in Tables 3 to 8. The hole expansion ratios h in
Tables 3 to 6 are obtained with the following expression i.
A(%)= {(d'-d)/d) x 100 (9
d': a hole diameter when a crack penetrates a sheet thickness
d: an initial hole diameter
Regarding plating types in Tables 5 and 6, CR represents a non-plated cold
rolled steel sheet, GI represents a formation of the hot dip galvanizing, GArepresents a
formation of the galvalmealing, EG represents a formati011 of the electrogalvanizing,
and A1 represents a formation of the aluminizing.
An amount of "0" in Table 1 indicates that an amount is equal to or less than a
measurement lower limit.
Determinations G and B in Tables 2.7 and 8 are defined as follows.
G: a target condition expression is satisfied.
B: the target condition expression is not satisfied.
[0067]
[Table 11

Table 1-2
(mass%)
[0068]
[Table 21
Heating
Test furnace furnace
resfyemrebnocle temperature in-ftuimrnea ce ") (minutes)
[0069]
[Table 31

[0070]
[Table 41

[0071]
[Table 51

[0072]
[Table 61

[0073]
[Table 71

[0074]
[Table 81

[0075]
It is found fro111 Tables 1 to 8 that, when the conditions of the present
invention are satisfied, it is possible to obtain the hot stamped steel for which the highstrength
cold rolled steel sheet satisfying TS x h? 50000 MPa% is used.
[Industrial Applicability]
[0076]
According to the present inventioa, since an appropriate relationship is
established among the amount of the C, the amount of the Mn and the amount of the Si,
and an appropriate hardness measured with a nanoindenter is provided to the
martensite, it is possible to provide the hot stamped steel which ensures the strength of
1.5 GPa or more, and has a niore favorable hole expansibility.
[Brief Description of the Reference Symbols]
[0077]
S1: MELTING PROCESS
S2: CASTING PROCESS
S3:. HEATING PROCESS
S4: HOT-ROLLING PROCESS
S5: COILING PROCESS
S6: PICKLING PROCESS
S7: COLD-ROLLING PROCESS
S8: ANNEALING PROCESS
S9: TEMPER-ROLLING PROCESS
S10: HOT STAMPING PROCESS
S11: GALVANIZING PROCESS
S 12: ALLOYING PROCESS
,513: ALUMINIZING PROCESS
S14: ELECTROGALVANIZING PROCESS
[Document Type] CLAIMS
[Claim 11
A hot stamped steel comprising, by mass%:
C: inore than 0.150% to 0.300%;
Si: 0.010% to 1.000%;
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 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 including Fe and unavoidable impurities,
wherein, ~vllen[ 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 satisficd,
a metallographic structure includes 80% or more of a martensite in an area
fraction, and optionally, further includes one or more of 10% or less of a pearlite in an
area fraction, 5% or less of a retained austenite in a volun~era tio, 20% or less of a
ferrite in an area fraction, and less than 20% of a bainite in an area fraction,
TS x h which is a product of TS that is a tensile strength and h that is a hole
expansion ratio is 50000MPa% or more, and
a hardness of the maitensite measured with a nanoindenter satisfies a
following expression-b and a following expression-c,
5 x [Si] + [Mn]) / [C] > 10 ( 4
H2IH1 4.10 (b)
OHM < 20 (c)
here, the HI represents an average hardness of the martensite in a surface
portion, the H2 represents the average hardness of the martensite in a center part of a
sheet thickness that is an area having a width of *I00 pm in a thickness direction frolorn
a center of the sheet thickness, and the OHM represents a variance of the hardness of
the martensite existing in the central part of the sheet tluckness.
[Claim 21
The hot stamped steel according to claim 1,
wherein an area fraction of a MnS existing in the rnetallographic structure aud
having an equivalent circle diameter of 0.1 pm to 10 pm is 0.01% or less, and
a following expression-d is satisfied,
n2/n1<1.5 (d)
here, the nl represents an average number density per 10000 pm2 of the MnS
in a 114 part of the sheet thickness, and the n2 represents an average number density
per 10000 pm2 of the MnS in the central part of the sheet tliickness.
[Claitn 31
The hot stamped steel according to clainl 1 or 2,
wherein a hot dip galvanizing is formed on a surface thereof.
[Claitn 41
The hot statnped steel according to claim 3,
wherein the hot dip galvanized layer includes a galva~mealing,
[Claim 51
The hot statnped steel according to claim 1 or 2,
wherein an electrogalvanizing is further formed on a surface thereof.
[Claim 61
The hot stamped steel according to claim 1 or 2,
wherein an alutninizitlg is fi~rtherfo rmed on a surface thereof.
[Claim 71
A method for producing a hot stamped steel, the method colllprising:
casting a moltell steel having a chemical compositiotl according to claim 1
and obtain a steel;
heating the steel;
hot-rolling the steel with a hot-rolling facility having a plurality of stands;
coiling the steel after the hot-rolling;
pickling the steel after the coiling;
cold-rolling the steel after the pickling with a cold rolling Inill having a
plurality of stands under a condition satisfying a following expression-e;
annealing in which the steel is heated under 700°C to 850°C and cooled after
the cold-rolling;
temper-rolling the steel after the anllealing; and
hot stamping in which the steel is heated to a temperatnre range of 750°C or
more at a tenlperature-increase rate of 5 OC/second or more, formed within the
temperature range, and cooled to 20°C to 300°C at a cooling rate of 10 "Clsecond or
Inore after the temper-rolling,
1 . 5 x r l / r + 1 . 2 x r 2 / r + r 3 / r > 1 (el
here, ri represents an individual cold-rolling reduction in unit % at an it" stand
based on an uppermost stand among a plurality of the stands in the cold-rolling where i
is 1,2 or 3, and r represents a total cold-rolling reduction in unit % in the cold-rolling.
[Claim 81
The method for producing a hot stamped steel according to claim 7,
wherein, when CT in unit OC represents a coiling temperature in the coiling;
[C] represents an amount of C by mass%, [Mn] represents an amount of Mn
by mass%, [Si] represents an amount of Si by mass%, and [Mo] represents an a~nount
of Mo by mass% in the steel;
a following expression-f is satisfied;
560 - 474 x [C] 9 0 x [Mn] - 20 x [Cr] - 20 x [Mo] < CT < 830 - 270 x [C]
- 90 x [Mn] - 70 x [Cr] - 80 x [Mo] (fl
[Claim 91
The method for producing a hot stamped steel according to claim 7 or 8,
wherein, when T in unit OC represents a heating temperature in the heating, r
in unit minutes represents an in-furnace time; and
[Mn] represents an amount of Mn by mass%, and [S] represents an amount
of S by mass% in the steel,
a follo\ving expression-g is satisfied,
T x ln(t) / (1.7 x [Mn] + [S]) > 1500 (g).
[Claim 101
The tnethod for producing a hot stamped steel according to claim 7 or 8,
fi~rtllecr omprisitlg:
galvanizing the steel between the au~ealinga nd the tenlper-l.ollitlg.
[Clainl 111
Tlie mctllod for producing a hot sta~llpeds teel according to claim 10, fu~tller
comprising:
alloying the steel between the hot dip galvanizi~lga nd tlte tenlper-rollit~g.
[Claim 121
The nlethod for producitlg a hot stamped steel according to claim 7 or 8,
filrther coml~rising:
electrogalvatuzing the steel between the temper-rolling and the hot sta~llpi~~g.
[Clailn 131
The method for producing a hot stamped steel according to claitn 7 or 8,
fi~rthecr otllprisillg:
aluminizing the steel between the annealing and the temper-rolling.
Dated this July 01,2014
[RANJNA MEFITA-DUTT]
OF REMFI 10 is satisfied, a nletallographic structure
includes 80% or more of a martensite in an area fraction, and optionally, fui-ther
includes one or more of 10% or less of a pearlite in an area fraction, 5% or less of a
retained austenite in a volume ratio, 20% or less of a ferrite in an area fraction, and less
than 20% of a bainite in an area fraction, TS x ?L which is a product of TS that is a
tensile strength and lb that is a hole expansion ratio is 50000MPa% or more, and a
hardness of the martensite measured with a nanoindenter satisfies H2 1 HI 4.10 and
OHM i 20.
FIG. 1
65000
60000
55000
S 50000
2 45000
ZE -
-< 40000
x AFTER HOT STAMPING
35000 I-- BEFORE HOT STAMPING
30000 (OTHER CONDITIONS NOT SATISFIED)
25000
20000
0 1 0 2 0 30 40 50 60 70
(5x [SiI+IMnl)/[Cl
FIG. 2A
40
35
30
25
ZE = 20 b
15
10
5
0
1.00 1.05 1.10 1.15 1.20 1.25 1.30
H2/H1
AFTER HOT STAMPING
FIG. 2B
- B
- OK @ I3 a NG
-
- la
-
- AFTER HOT STAMPING
AFTER HOT STAMPING
FIG. 3
90000
80000
- 70000 60000
-2= 50000 40000
X
v, 30000
F-
20000
10000
0
1 1.2 1.4 1. 6 1.8 2 2.2 2.4
n2/nl
FIG. 4
4/6
FIG. 5A
FRACTION OF MARTENS I TE
FIG. 5B
100
@@I 90- ?5
8 70- V53
+ BEFORE HOT STAMPING
80 AFTER HOT STAMPING
a
fi 60-
&I 50-
40-
+
2
g
z
El
I I I 0 ~ m t
-400 -300 -200 -100 w- 0 100 200 300 400