DESCRIPTION
TITLE OF INVENTION: STEEL SHEET FOR HOT STAMPING,
METHOD OF MANUFACTURING THE SAME, AND
HOT STAMPED S'I'hhL SHhE'T MEMBER
TECHNICAL FIELD
[0001] The present invention relates to a steel
sheet for hot stamping, a method of manufacturing the
same, and a hot stamped steel sheet member suitable
for manufacturing a mechanical structure component
and so on.
BACKGROUND ART
[0002] Efforts to reduce a weight of steel used for
a vehicle body by high-strengthening of the steel
have been continued for reduction in weight of a
vehicle. High-strengthening of steel contributes to
improvement in a collision resistant property of a
vehicle. As for a thin steel sheet widely used for a
vehicle, press formability generally deteriorates and
it becomes more difficult to manufacture a component
having a complicated shape, in accordance with
increase in strength. For example, a portion with
high degree of processing fractures and dimensional
accuracy deteriorates due to larger springback in
accordance with lowering of ductility. Accordingly,
it is not easy to manufacture components by prcssforming
of a high-strength steel sheet, in
particular, a steel sheet having tensile-strength of
780 MPa or more. It is easy to process a highstrength
steel sheet not by press-forming but by
roll-forming, but an application target is limited to
a component having a uniform cross section in a
longitudinal direction.
[0003] A method called as hot stamping aimed for
obtaining high formability for a high-strength steel
sheet is described in Patent Literature 1. According
to the hot stamping, it is possible to form a highstrength
steel sheet with high accuracy, and obtain a
high-strength hot stamped steel sheet member.
[0004] A material whose strength after quenching is
1500 MPa class is described in Patent Literatures 2
and 3. A steel sheet for hot stamping aimed for
improvement in ducLi1it.y while obtaining highstrength
is described in Patent Literature 4. A
steel sheet aimed for improvement in hardenability is
described in Patent Literatures 5 and 6. However,
there are problems in these arts in which it is
necessary to start the quenching at high temperature,
it is difficult to improve toughness and local
deformability, and others.
CITATION LIST
PATENT LITERATURE
[0005] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2002-102980
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2011-236483
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2004-323944
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2010-65292
Patent Literature 5: Japanese Laid-open Patent
Publlcat~on No. 2011-195958
Patent Literature 6: Japanese Patent Publication
No. 3879459
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006] An object of the present invention is to
provide a steel sheet for hot stamping, a method of
manufacturing the same, and a hot stamped steel sheet
member capable of obtaining good toughness and local
deformability even when quenching is started at a
relatively low temperature.
SOLUTION TO PROBLEM
[0007] The present inventors studied hard from a
point of view in which it is important to enable to
generate martensite with high area ratio by quenching
to obtain fine toughness and local deformability
after the quenching, and a special chemical
composition is important to enable to start quenching
at a relatively low temperature. As a result, it is
found that good toughness and local deformability can
be obtained even if quenching is started at a
relatively low temperature owing to a combination of
a chemical composition and a steel structure within a
certain range. The present inventors have come to
think of various embodiments of invention described
below.
[00081 (1)
A steel sheet for hot stamping, including
a chemical composition expressed by, in mass%:
C: U.UH% or more and less than U.iO#;
Si: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N
content is expressed by [N];
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0.05%;
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%;
W: 0% to 1%;
Nb: 0% to 1%;
v: 0% to 1%;
balance: Fe and impurities,
wherein :
an expression (1) is satisfied when an Mn content
is represented by [Mn] and a C content is represented
by [Cl,
where [Mn] + 6. 67 x LC] - 2.73 2 0 (1); and
the steel sheet includes a steel structure
expressed by, in an area ratio:
balnlte: 1% to 95%;
ferrite: 5% to 94%; and
balance: one or more selected from the group
consisting of pearlite, martensite and retained
austenite.
[00091 (2)
The steel sheet for hot stamping according to
(I), wherein
an area ratio of bainite is 20% to 95%, and
an area ratio of ferrite is 5% to 80%.
iOOl01 (3)
The steel sheet for hot stamping according to
(I), wherein
an area ratio of bainite is 1% or more and less
than 20%, and
an area ratio of ferrite is 40% to 94%.
[OOllI ( 4 )
The steel sheet for hot stamping according to any
one of (1) to ( 3 ) , wherein the chemical content
contains :
Ti: 48/14 x [N] % to 0.04 + 48/14 x [N] %.
[00121 (5)
The steel sheet for hot .stamping according to any
one of (1) to (4), wherein
a Cr content is 0.01% to 1.00%, and
when the Mn content is represented by [Mn] and
t h e C r c o n t e n t i s r e p r e s e n t e d by [ C r ] , an e x p r e s s i o n
( 2 ) is s a t i s f i e d ,
1 . 6 I [Mn] + [ C r ] 5 3 . 5 (2).
[00131 ( 6 )
The s t e e l s h e e t f o r h o t s t a m p i n g a c c o r d i n g t o any
one of (1) t o ( 5 ) , w h e r e i n t h e c h e m i c a l c o n t e n t
c o n t a i n s :
B1: 0.0001% t o 0 . 5 % .
[00141 (7)
The s t e e l s h e e t f o r h o t s t a m p i n g a c c o r d i n g t o any
one of (1) t o ( 6 ) , r i h e r e i n t h e c h e m i c a l c o m p o s i t i o n
c o n t a i n s one o r more s e l e c t e d from t h e g r o u p
c o n s i s t i n g o f :
Ca: 0.0005% t o 0 . 0 5 % ;
Mg: 0.0005% t o 0 . 0 5 % ; a n d
REM: 0.0005% t o 0 . 0 5 % .
[00151 ( 8 )
The s t e e l s h e e t f o r h o t s t a m p i n g a c c o r d i n g t o any
one of (1) t o ( 7 ) , w h e r e i n t h e c h e m i c a l c o m p o s i t i o n
c o n t a i n s one o r more s e l e c t e d from t h e g r o u p
c o n s i s t i n g o f :
Mo: 0.03% t o 1%;
Cu: 0.01% t o 1%;
N i : 0.01% t o 1%; and
W: 0.01% t o 1%.
[00161 ( 9 )
The s t e e l s h e e t f o r h o t s t a m p i n g a c c o r d i n g t o any
one o f (1) t o ( E ) , w h e r e i n t h e c h e m i c a l c o m p o s i t i o n
c o n t a i n s one o r two s e l e c t e d from t h e g r o u p
consisting of:
Nb: 0.005% to 1%; and
V: 0.005% to 1%.
l U U l / ] (10)
A method of manufacturing a steel sheet for hot
stamping, including:
hot-rolling a steel ingot or a steel billet;
acid-pickling a steel sheet obtained by the hotrolling;
cold-rolling a steel sheet obtained by the acid
pickling; and '
performing a heat treatment of a steel sheet
obtained by the cold-rolling,
wherein:
the steel ingot or the steel billet includes a
chemical composition expressed by, in mass%:
C: 0.08% or more and less than 0.20%;
Si: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N
content is represented by [Nl;
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0.05%;
M0: 0% to 1%;
cu: U% to 1%;
Ni: 0% to 1%;
W: 0% to 1%;
Nb: 0% to 1%;
V: 0% to 1%;
balance: Fe and impurities;
an expression (1) is satisfied when an Mn content
is represented by [Mn] and a C content is represented
by [Cl,
where [Mn] + 6.67 x [C] - 2.73 2 0 (1) ;
the hot-rolling includes:
starting a rolling at a temperature of 1050°C
or more; and
thereafter coiling at a temperature zone of
400°C to 700°C; and
the performing the heat treatment includes:
heating to a temperature zone of 700°C to
840°C;
thereafter cooling to a temperature of 50OoC
or less at an average cooling rate of 5OC/sec to
10O0C/sec; and
thereafter retaining at a temperature zone
of 300°C to 500°C for 5 seconds to 600 seconds.
[0018] (11)
A method of manufacturing a steel sheet for hot
stamping, including:
hot-rolling a steel ingot or a steel billet;
acid-pickling a steel sheet obtained by the hotroiling;
cold-roiling a steel sheet obtained by the acid
pickling;
performing a heat treatment of a steel sheet
obtained by the cold-rolling, and
plating a steel sheet obtained by the heat
treatment,
wherein:
the steel ingot or the steel billet includes a
chemical composition expressed by, in mass%:
C: 0.08% or more and less than 0.20%;
si: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 t 48/14 x [N] % when an N
content is represented by [Nl;
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0.05%;
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%;
W: '0% to 1%;
Nb: 0% to 1%;
. . v: 0% to 1%;
balance: Fe and impurities;
an expression (1) is satisfied when an Mn content
is represented by [Mn] and a C content is represented
by [Cl,
where [Mn] t 6.67 x [C] -2.73 2 0 (1);
the hot-rolling includes:
starting a rolling at a temperature of 1050°C
or more; and
thereafter coiling at a temperature zone of
400°C to 700°C;
the performing the heat treatment includes:
heating to a temperature zone of 700°C to
84OoC;
thereafter cooling to a temperature of 58OoC
or less at an average cooling rate of 3OC/sec to
20°C/sec; and
thereafter retaining at a temperature zone
of 500°C to 570°C for 5 seconds to 600 seconds; and
the plating includes:
performing a hot-dip galvanizing treatment;
and
thcrcaftcr performing an alloying treatment
at a temperature zone of 500°C to 650°C.
[0019] (12)
The method of manufacturing the steel sheet for
hot stamping according to (10) or, (11))
wherein the chemical composition contains:
Ti: 48/14 x [N] % to 0.04 + 48/14 x [N] %.
[0020] (13)
The method of manufacturing the steel sheet for
hot stamping according to any one of (10) to (12),
wherein a Cr content is 0.01% to 1.00%, and when
the Mn content is represented by [Mn] and the Cr
content is represented by [Cr], an expression (2) is
satisfied,
1.6 I [Mn] + [Crl I 3.5 (2).
[0021] (14)
The method of manufacturing the steel sheet for
hot stamping according to any one of (10) to (13),
wherein the chemical composition contains:
Bi: 0.0001% to 0.5%.
[a0221 (15)
The method of manufacturing the steel sheet for
hot stamping according to any one of (10) to (14),
wherein the chemical composition contains one
kind or two or more kinds selected from a group made
up of:
Ca: 0.0005% to 0.05%;
Mg: 0.0005% to 0.05%; and
REM: 0.0005% to 0.05%.
' [00231 (16)
The method of manufacturing the steel sheet for
hot stamping according to any one of (10) to (15),
wherein the chemical composition contains one
kind or two or more kinds selected from a group made
up of:
Mo: 0% to 1%;
Cu: 0% to i%;
Ni: 0% to 1%; and
W: 0% to 1%.
[0024] (17)
The method of manufacturing the steel sheet for
hot stamping according to any one of (10) to (16),
wherein the chemical composition contains one
kind or two kinds selected from a group made up of:
Nb: 0% to 1%; and
V: 0% to 1%.
i00251 (18)
A hot stamped steel sheet member, having a
chemical composition represented by, in mass%:
C: 0.08% or more and less than 0.20%;
Si: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N content
is represented by [N] ;
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0.05%;
Mo: 0% to 1%;
cu: U% to 1%;
Ni: 0% to 1%;
w: 0% to 1%;
Nb: 0% to 1%;
v: 0% to 1%;
balance: Fe and impurities,
wherein an expression (1) is satisfied when an Mn
content is represented by [Mn], a C content is
represented by [Cl,
where [Mn] + 6.67 x [C] - 2.73 2 0 (1)
and having a steel structure in which an area
ratio of martensite is 90% or more, and a tensile
strength is 1600 MPa or less.
[0026] (19)
The hot stamped steel sheet member according to
(18) t
wherein the chemical composition contains:
Ti: 48/14 x [N] % to 0.04 + 48/14 x [N] %.
[0027] (20)
The hot stamped steel sheet member according to
(18) or (191,
wherein a Cr content is 0.01% to 1.00%, and when
the Mn content is represented by [Mn] and the Cr
content is represented by [Cr], an expression (2) is
satisfied,
I. 6 5 [Mn] + [Cr] 1 3.5 (2).
[00281 (21)
The hot stamped steel sheet member according to
any one of (18) to (20),
whereln the chemlcai composition contains:
Bi: 0.0001% to 0.5%.
[0029] (22)
The hot stamped steel sheet member according to
any one of (18) to (21),
wherein the chemical composition contains one
kind or two or more kinds selected from a group made
up of:
Ca: 0.0005% to 0.05%;
Mg: 0.0005% to 0.05%; and
REM: 0.0005% to 0.05%.
[0030] (23)
The hot stamped steel sheet member according to
any one of (18) to (22),
wherein the chemical composition contains one
kind or two or more kinds selected from a group made
up of:
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%; and
W: 0% to 1%.
[0031] (24)
The hot stamped steel sheet member according to
any one of (18) to (23),
wherein the chemical composition contains one
kind or two kinds selected from a group made up of:
Nb: 0% to 1%; and
V: 0% to 1%.
ADVANTAGEOUS EFFECTS OF INVENTION
[ U U 3 Z J According to the present Invention, ~t IS
possible to obtain good toughness and local
deformability even if quenching is started at a
relatively low temperature.
DESCRIPTION OF EMBODIMENTS
[0033] Hereinafter, embodiments of the present
invention are described. The embodiments of the
present invention relate to a steel sheet for hot
stamping used for manufacturing a hot stamped steel
sheet member by hot stamping. In hot stamping, a
steel sheet for hot stamping is generally heated to
be formed and then quenched. According to the steel
sheet for hot stamping of the present embodiment, it
is possible to obtain an enough amount of martensite
even if the quenching after the heating is started at
a relatively low temperature with, for example, a
combination of a chemical composition and a steel
structure described below.
[0034] First, the chemical composition of the steel
sheet for hot stamping according to the present
embodiment is described. In the following
being a unit of a content description, "5" of each
element contained in the steel sheet for hot stamping
means 'mass%" unless otherwise specified.
[0035] The chemical composition of the steel sheet
for hot stamping according to the present embodiment
is expressed by, in mass%, C: 0.08% or more and less
than 0.20%; Si: 0.003% to 0.2%; Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%; B: 0.0003% to 0.01%; P:
U.I% or less; S : 0.004% or less; N: 0.0i% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N content is
represented by [N]; Cr: 0% to 1.00%; Bi: 0% to 0.5%;
Ca: 0% to 0.05%; Mg: 0% to 0.05%; REM: 0% to 0.05%;
MO: 0% to 1%; Cu: 0% to 1%; Ni: 0% to 1%; W: 0% to
1%; Nb: 0% to 1%; V: 0% to 1%; balance: Fe and
impurities. Besides, an expression (1) is satisfied
when an Mn content is represented by [Mn], a C
content is represented by [C]. As the impurities,
ones contained in raw materials such as ore and
scrap, and ones contained in a manufacturing process
are exemplified.
[Mn] + 6.67 x [C] - 2.73 2 0 (1)
[0036] (C: 0.08% or more and less than 0.20%)
C is a very important element which increases
hardenability and mainly affects strength after
quenching. C is also a very important element which
is effective for controlling local deformability
(local ductility) and toughness (impact absorption
characteristic) after hot stamping. When a C content
is less than 0.08%, the strength after quenching may
be insufficient, or enough hardenability cannot be
obtained. Accordingly, the C content is 0.08% or
more. When the C content is 0.20% or more, the
strength after quenching is excessively high, and
enough local deformability and toughness cannot be
obtained. Therefore, the C content is less than
0.20%. The C content is preferably 0.18% or less,
and more preferably 0.16% or less.
[0037] (Si: 0.003% to 0.2%)
Si is a very effective element to improve the
hardenability and to stably secure the strength after
quenching. Si has a function to suppress generation
of scale in a high-temperature heating in the hot
stamping. When an Si content is less than 0.003%, it
is difficult to obtain the above-stated functions.
Accordingly, the Si content is 0.003% or more. To
obtain the above-stated functions more certainly, the
Si content is preferably 0.01% or more. When the Si
content exceeds 0.2%, a temperature in which an
austenite transformation occurs is remarkably high.
Accordingly, a cost required for the heating of the
hot stamping may increase, or the quenching is
insufficient caused by heating shortage. Besides,
when a hot-dip plating treatment is performed,
wettability is lowered. Therefore, the Si content is
0.2% or less.
[0038] (Mn: 1.6% to 3.5%)
Mn is a very effective element to improve the
hardenability and to stably secure the strength after
quenching. When an Mn content is less than 1.6%, it
is difficult to obtain the above-stated functions.
Accordingly, the Mn content is 1.6% or more. To
obtain the above-stated functions more certainly, the
Mn content is preferably 2.0% or more. When the Mn
content exceeds 3.5%, effects by the above-stated
functions are saturated, and the toughness after
quenching is lowered. Accordingly, the Mn content is
3.5% or less.
[0039] Besides, when the Mn content is represented
by [Mn], and the C content is represented by [Cl, the
expression (1) is satisfied.
[Mn] t 6.67 x [C] - 2.73 2 0 (1)
When the expression (1) is not satisfied, the
hardenability is insufficient, and desired mechanical
properties cannot be obtained after the hot stamping.
[0040] (sol. A1 (acid-soluble Al) : 0.0002% to 2.0%)
A1 is an element having a function to enable
soundness of steel by deoxidizing molten steel. When
an sol. A1 content is less than 0.0002%, it is
difficult to obtain the above-stated function.
Accordingly, the sol. A1 content is 0.0002% or more.
A1 is also a very effective element to improve the
hardenability and to stably secure the strength after
quenching. However, when the sol. A1 content exceeds
2.0%, effects by the above-stated functions are
saturated, and a cost unnecessarily increases.
Accordingly, the sol. A1 content is 2.0% or less.
[0041] (B: 0.0003% to 0.01%)
B is a very effective element to improve the
hardenability and to stably secure the strength after
quenching. When a B content is less than 0.0003%, it
is difficult to obtain the above-stated functions.
Accordingly, the B content is 0.0003% or more. When
the B content exceeds 0.01%, effects by the abovestated
functions are saturated, and the toughness
after quenching is lowered. Accordingly, the B
content is 0.01% or less.
[0042] (P: 0.1% or less)
P is not an essential element, and may be
contained as an impurity in steel, for example. From
a point of view of toughness, the less a P content
is, the better it is. In particular, when the P
content exceeds 0.1%, the lowering of the toughness
is remarkable. Accordingly, the P content is 0.1% or
less. To secure better toughness, the P content is
preferably 0.05% or less, and more preferably 0.03%
or less. P is able to contribute to improve the
hardenability and to stably secure the strength after
quenching. P may be intentionally contained to
obtain the functions. It costs a lot to reduce the P
content, and the cost remarkably increases to reduce
the P content to less than 0.0002%. Accordingly, the
P content may be 0.0002% or more.
[0043] ( S : 0.004% or less)
S is not an essential element, and may be
contained as an impurity in steel, for example. From
a point of view of toughness, the less an S content
is, the better it is. In particular, when the S
content exceeds 0.004%, the lowering of the toughness
is remarkable. Accordingly, the S content is 0.004%
or less. It costs a lot to reduce the S content, and
the cost remarkably increases to reduce the S content
to less than 0.0002%. Accordingly, the S content may
be 0.0002% or more.
[0044] (N: 0.01% or less)
N is not an essential element, and may be
contained as an impurity in steel, for example. From
a point of view of toughness, the less an N content
is, the better it is. In particular, when the N
content exceeds 0.01%, the local deformability and
the toughness are remarkably lowered in accordance
with formation of coarse nitrides. Accordingly, the
N content is 0.01% or less. It costs a lot to reduce
the N content, and the cost remarkably increases to
reduce the N content to less than 0.0002%.
Accordingly, the N content may be 0.0002% or more, or
to be 0.0008% or more to further reduce the cost.
[0045] Ti, Cr, Bi, Ca, Mg, REM, Mo, Cu, Ni, W, Nb,
and V are not essential elements, and are arbitrary
elements, which may be appropriately contained in the
steel sheet for hot stamping up to a predetermined
amount as a limit.
[0046] (Ti: 0% to 0.04 + 48/14 x [N] % when the N
content is represented by [N] % )
Ti suppresses recrystallizatlon of austenite,
suppresses a grain growth by forming further fine
carbides to refine austenite grains when the steel
sheet for hot stamping is heated to the Ac3 point or
more in the hot stamping. As a result, the toughness
of the hot stamped steel sheet member largely
improves. Accordingly, Ti may be contained. To
surely obtain the effect, a Ti content is preferably
48/14 x [N] % or more. However, when the Ti content
exceeds 48/14 x [N] + 0.04, the effect by the abovestated
tunctlon is saturated, and the cost
unnecessarily increases. Accordingly, the Tl content
is 0.04 + 48/14 x [N] or less. Namely, it is
preferable that 'Ti: 48/14 x [N] % to 0.04 + 48/14 x
[N] % " is satisfied.
(00471 (Cr: 0% to 1.00%)
Cr is a very effective element to improve the
hardenability and to stably secure the strength after
quenching as same as Mn. Accordingly, Cr may be
contained. To surely obtain the effect, a Cr content
is preferably 0.01% or more, and more preferably 0.1%
or more. However, when the Cr content exceeds 1.00%,
the above-stated effects are saturated, and the cost
unnecessarily increases. Accordingly, the Cr content
is 1.00% or less. Namely, it is preferable that "Cr:
0.01% to 1.00%" is satisfied. Besides, when a sum of
the Mn content and the Cr content exceeds 3.5%, the
effects by the above-stated functions are saturated,
and the toughness after quenching is lowered.
Accordingly, the sum of the Mn content and the Cr
content is 3.5% or less. Namely, when the Mn content
is represented by [Mn], the Cr content is represented
by [Cr], it is preferable that an expression (2) is
satisfied. The sum of the Mn content and the Cr
content is preferably 2.8% or less.
1.6 I [Mn] + [Cr] I 3.5 (2)
- 21 -
[0048] (Bi: 0% to 0.5%)
Bl is an element which has a function to become a
solidification nucleus in a solidification process of
molten steel and make a secondary arm spacing of
dendrite small so as to suppress segregation of Mn
and others within the secondary arm spacing of
dendrite. In particular, in the present embodiment,
Mn of 1.6% or more is contained, and therefore, Bi is
effective to suppress the lowering of the toughness
resulting from the segregation of Mn. Accordingly,
Bi may be containe'd. To surely obtain the effect, a
Bi content is preferably 0.0001% or more. However,
when the Bi content exceeds 0.5%, the effect by the
above-stated function is saturated, and the cost
unnecessarily increases. Accordingly, the Bi content
is 0.5% or less. Namely, it is preferable that "Bi:
0.0001% to 0.5%" is satisfied. From a point of view
of suppression of the segregation of Mn and others,
the Bi content is more preferably 0.0002% or more,
and further preferably 0.0005% or more.
[0049] (Ca: 0% to 0.05%, Mg: 0% to 0.05%, REM: 0% to
0.05%)
Each of Ca, Mg, and REM is an element which has a
function to refine inclusions in the steel and
prevent cracks resulting from the inclusions in the
hot stamping. Accordingly, one or more selected from
the group consisting of these elements may be
contained. To surely obtain the effect, each of a Ca
content, an Mg content, and a REM content is
preferably 0.0005% or more. However, when any of the
contents exceed 0.05%, the effect by the above-stated
function is saturated, and the cost unnecessarily
increases. Accordingly, each of the Ca content, the
Mg content, and the REM content is 0.05% or less.
Namely, it is preferable that at least one of "Ca:
0.0005% to 0.05%", "Mg: 0.0005% to 0.05%", and "REM:
0.0005% to 0.05%" is satisfied.
[0050] REM (rare-earth metal) indicates 17 kinds of
elements as a total of Sc, Y, and lanthanoid, and the
"REM content" means a total content of these 17 kinds
of elements. Lanthanoid may be industrially added as
a form of, for example, misch metal.
[0051] (Mo: 0% to 1%, Cu: 0% to 1 , Ni: 0% to 1 % W:
0% to 1%)
Each of Mo, Cu, Ni and W is an effective element
to improve the hardenability and to stably secure the
strength after quenching. Accordingly, one or more
selected from the group consisting of these elements
may be contained. To surely obtain the effect, an Mo
content is preferably 0.03% or more, and each of a Cu
content, an Ni content, and a W content is preferably
0.01% or more. However, when any of the contents
exceeds 1%, the effects by the above-stated functions
are saturated and the cost unnecessarily increases.
Accordingly, each of the Mo content, the Cu content,
the Ni content, and the W content is 1% or less.
Namely, it is preferable that at least one of "Mo:
0.03% to I%", "Cu: 0. 01% to I%", "Ni: 0.01% to I%",
and W: "0.01% to 1%" is satisfied.
[0052] (Nb: 0% to 1%, V: 0% to 1%)
Each of Nb and V suppresses the recrystallization
of austenite, suppresses the graln growth by formlng
further fine carbides to refine austen~te grains when
the steel sheet for hot stamping is heated to the Ac3
point or more in the hot stamping. Accordingly, one
or two selected from the group consisting of these
elements may be contained. To surely obtain the
effect, each of an Nb content and a V content is
preferably 0.005% or more. However, when any of the
contents exceeds 1%, the effect by the above-stated
function is saturated and the cost unnecessarily
increases. Accordingly, each of the Nb content and
the V content is 1% or less. Namely, it is
preferable that at least one of "Nb: 0.005% to 1%"
and 'V: 0.005% to 1%" is satisfied.
[0053] Next, a steel structure of the steel sheet
for hot stamping according to the present embodiment
is described. The steel sheet for hot stamping has
the steel structure expressed by bainite: 1% to 95%;
ferrite: 5% to 94%; and the balance: one or more
selected from the group consisting of pearlite,
martensite and retained austenite. Each of numerical
values relating to the steel structure is, for
example, an average value of a whole of the steel
sheet for hot stamping, but it can be represented by
a numerical value relating to the steel structure at
a point whose depth from a surface of the steel sheet
for hot stamping is 1/4 of a thickness of the steel
sheet (hereinafter, this point may be described as a
"1/4 depth position"). For example, when the
thickness of the steel sheet tor hot stamplng is 2 . U
mm, it can be represented by a numerical value at a
point whose depth from the surface is 0.50 mm. This
is because the steel structure at the 1/4 depth
position represents an average steel structure in a
thickness direction of a hot stamped steel sheet
member. In the present invention, an area ratio of
each phase or a structure measured at the 1/4 depth
position is regarded as the area ratio of each.
[0054] In general, in the hot stamping, C is solidsolved
through austenite transformation by heating
The hardenability and the hardness after quenching
become high by the solid-solution of C. A phase or a
structure whose C concentration is high such as
bainite, pearlite, martensite, cementite, and
retained austenite becomes an initial point of
austenitization in the heating. Accordingly, the
more these area ratios are, the more the
austenitization is accelerated. On the other hand,
ferrite, whose C concentration is low, is difficult
to be the initial point of austenitization, and the
more the area ratio of ferrite is, the more the grain
growth of austenite is suppressed.
100551 (Area Ratio of Ferrite: 5% to 94%)
When the area ratio of ferrite is 5% or more, the
initial point of austenitization is appropriately
suppressed. Accordingly, it is possible to suppress
the excessive grain growth of austenite, and to
further improve the toughness after hot stamping.
Accordingly, the area ratio of ferrlte is 5% or more.
To surely obtain the effect, the area ratio of
ferrite is preferably 15% or more, more preferably
30% or more, and further preferably 40% or more.
[0056] When the area ratio of ferrite is 94% or
less, the initial point of austenitization is
appropriately secured. Accordingly, it is possible
to promptly progress austenitization, and
productivity improves. Therefore, the area ratio of
ferrite is 94% or less.
[0057] Respec~ive areas of polygonal ferrite,
acicular ferrite, and bainitic ferrite are included
in the area of ferrite, but an area of ferrite
existing in pearlite is not included therein.
[0058] (Area Ratio of Bainite: 1% to 95%)
From a point of view of acceleration of austenite
transformation in the heating, it is preferable that
the phases or structures each of whose C
concentration is high such as bainlte, pearlite,
martensite, cementite, and retained austenite are
included. However, when the phase or the structure
whose C content is excessively high is contained in
Che steel sheet for hot stamping, C is concentrated
in austenite transformed from these phases or
structures, and therefore, when the quenching is
performed after that, a portion whose strength is
locally high is generated in a microscopic point of
view, and the local deformability and the toughness
are easy to be lowered. When the C concentration is
focused, the phase or the structure whose C
concentration is the lowest is bainite among bainite,
pearlite, martensite, cementite, and retained
austenite. Therefore, the phase or the structure
where the local concentration of C is difficult to
occur is bainite. When the area ratio of bainite is
1% or more, the improvements in the local
deformability and the toughness are remarkable.
Accordingly, the area ratio of bainite is 1% or more.
Besides, it is preferable that the structure other
than ferrite is bainite. The area ratio of ferrite
is 5% or more, and therefore, the area ratio of
bainite is 95% or less.
[0059] A sum of the area ratio of ferrite and the
area ratio of bainite is preferably 40% or more, more
preferably 45% or more, further preferably 50% or
more, and still further preferably 55% or more. The
smaller the sum of the area ratio of ferrite and the
area ratio of bainite is, the more the area ratios of
pearlite, martensite, cementite, and retained
austenite are. Therefore, C is difficult to diffuse,
uneven quenching is easy to occur, and a hardness
disLributior~ is easy to be large. Accordingly,
sufficient local deformability and toughness are
difficult to be obtained. The sum of the area ratio
of ferrite and the area ratio of bainite is further
preferably 90% or more, and still further preferably
100%.
[0060] When the area ratio of bainite is 20% to 95%,
it is preterable that the area ratio of ferrite is 5%
to 80%. The more the area ratio of bainite is, the
shorter the time required for austenitization in the
quenching is. Accordingly, it is preferable to
reduce the cost and the time.
[0061] When the area ratio of bainite is 1% or more
and less than 20%, it is preferable that the area
ratio of ferrite is 40% to 94%. The more the area
ratio of ferrite is, the more excellent preformability
can be obtained. In particular, when the
area ratio of ferrite is 40% or more, improvement in
the pre-formability is remarkable.
[0062] The balance other than ferrite and bainite
is, for example, one or more of pearlite, martensite,
cementite, or retained austenite. The smaller these
contents are, the more preferable it is.
[0063] It is preferable that a plating layer
containing Zn is formed at the surface of the steel
sheet for hot stamping. Namely, a surface treated
steel sheet is preferable as the steel sheet for hot
stamping. Owing to the plating layer, an effect such
as improvement in corrosion resistance of the hot
stamped steel sheet member obtained by the hot
stamping can be obtained. Kinds of the plating layer
are not particularly limited, and an electroplated
layer and a hot-dip plating layer are exemplified.
As the electroplated layer, an electrogalvanized
layer and an electroplated Zn-Ni alloy layer are
exemplified. As the hot-dip plating layer, a hot-dip
galvanized iayer, an alloyed hot-dip galvanized
layer, a hot-dip aluminum plating layer, a hot-dip
Zn-A1 alloy plating layer, a hot-dip Zn-Al-Mg alloy
plating layer, and a hot-dip Zn-Al-Mg-Si alloy
plating layer are exemplified.
[ 0 0 6 4 ] A plating deposition amount is also not
particularly limited, and may be a general one.
Particularly when the plating layer is a pure
galvanizing layer, the plating layer may be partially
evaporated when the steel sheet for hot stamping is
heated to the temperature of the Ac3 point or more in
the hot stamping. Accordingly, in particular, it is
preferable to set the plating deposition amount to be
large in case of electrogalvanizing, whose deposition
amount tends to be smaller than the hot-dip plating
layer. Due to reasons as stated above, the hot-dip
galvanized based layer is preferable as the plating
layer, and in particular, a hot-dip galvanized alloy
layer (including the alloyed hot-dip galvanizing
layer), whose melting point is more than that of pure
zinc is preferable.
[0065] Next, a method of manufacturing the steel
sheet for hot stamping is described. The steel sheet
for hot stamping according to the present embodiment
may obtain the desired effect as long as it includes
the above-stated chemical composition and steel
structure regardless of the method of manufacturing
the same. Accordingly, the manufacturing method
thereof is not particularly limited. However,
according to the manufacturing method described
below, it is possible to surely manufacture the steel
sheet for hot stamping according to the present
embodiment.
[0066] The manufacturing method of the steel sheet
for hot stamping according to the present embodiment
may be classified broadly into two kinds. One is a
manufacturing method which does not include the
plating treatment, and the other is a manufacturing
method which includes the plating treatment.
[0067] First, the manufacturing method which does
not include the plating treatment is described. In
the manufacturing method, a steel ingot or a steel
billet is hot-rolled, a steel sheet obtained by the
hot-rolling is acid-pickled, a steel sheet obtained
by the acid pickling treatment is cold-rolled, and a
heat treatment of a steel sheet obtained by the coldrolling
is performed. A chemical composition of the
steel ingot or the steel billet is substantially
matched with the chemical composition of the steel
sheet for hot stamping to be manufactured.
[0068] In the hot-rolling, the rolling is started at
a temperature of 1050°C or more, and thereafter, it is
coiled at a temperature zone of 400°C to 700°C.
[0069] The steel ingot or the steel billet may
contain non-metal inclusions to be a cause to
deteriorate the local deformability and the toughness
of the hot stamped steel sheet member obtained by the
quenching of the steel sheet for hot stamping.
Accordingly, it is preferable to enable enough solidsolution
of these non-metal inclusions in the hotrolling
of the steel ingot or the steel billet. When
the steel ingot or the steel billet having the abovestated
chemical composition is hot-rolled, the solidsolution
of the non-metal inclusions is accelerated
if the rolling is started at the temperature of 1050°C
or more. Therefore, the start temperature of the
rolling is 1050°C or more. For example, the rolling
may be started after heating a steel ingot or a steel
billet whose temperature is less than 1050°C to the
temperature of 1050°C or more, or the rolling may be
started at the temperature of 1050°C or more without
lowering the temperature of the steel ingot after
continuous casting or the steel billet after bloom
rolling to the temperature of less than 1050°C. A
finish temperature of the hot-rolling is not
particularly limited, but it is preferably 820°C or
more.
[0070] When a coiling temperature after the rolling
is less than 400°C, ferrite cannot be enough secured,
and therefore, strength of the hot-rolled steel sheet
becomes excessively high. According~ly, it is
difficult to obtain the above-stated steel structure.
Besides, in the cold-rolling, control of a load and
control of flatness and thickness of the steel sheet
become difficult, and as a result, proper coldrolling
cannot be performed or manufacturing
efficiency is lowered. Accordingly, the coiling
temperature is 400°C or more.
[0071] When the coiling temperature after the
rolling is 700°C or more, the steel structure becomes
any of only ferrite, a combination of ferrite and
cementite, or perlite, and the steel structure
containing bainite cannot be obtained. Accordingly,
the coiling temperature is 70OoC or less. In general,
when the coiling temperature is too high, a scale
grows after the coiling, and therefore, scale flaws
are easy to be generated. Besides, when the coiling
temperature is too high, a steel sheet coil in hightemperature
state is easy to be deformed by its own
weight after the coiling, and scratches may be
generated at a surface of the steel sheet coil caused
by the deformation. A reason for the deformation 1s
because non-transformed austenite remains after the
coiling, and ferrite transformation may occur. If
the ferrite transformation occurs after the coiling,
the volume of the steel sheet coil expands in
accordance with the ferrite transformation, and heat
shrinkage occurs after that, and therefore, the steel
sheet coil loses coiling tension.
[0072] Conditions of the acid pickling treatment are
not particularly limited, and for example, it may be
performed based on a typical condition. Skin pass
rolling may be performed before or after the acid
pickling treatment. For example, the flatness may be
corrected and peeling of the scale may be accelerated
owing to the skin pass rolling. An elongation
percentage of the skin pass roll~ng is not
particularly limited, and for example, it may be 0.3%
or more and less than 3.0%.
[0073] Conditions of the cold-rolling are not
particularly limited, and for example, it may be
performed based on a typical condition. A reduction
ratio is not particularly limited, and the reduction
ratio is preferably 80% or less.
[a0741 In the heat treatment (recrystallization
annealing), the steel sheet is heated to a
temperature zone of 700°C to 840°C, cooled to a
temperature of 500°C or less at an average cooling
rate of S°C/sec to 100°C/sec, and thereafter, is
retained at a temperature zone of 300°C to 50OoC for 5
seconds to 600 seconds.
[0075] The heat treatment causes recrystallization.
When a heating temperature of the heat treatment is
less than 700°C, the recrystallization is not enough
caused, the steel structure after the hot-rolling is
easy to remain, and it is difficult to obtain the
steel sheet for hot stamping having a desired steel
structure. Accordingly, when the steel sheet for hot
starciping is quenched, it is difficult to obtain
stable characteristics. Therefore, the heating
temperature of the heat treatment is 700°C or more.
When the heating temperature of the heat treatment
exceeds 840°C, heat quantity to secure the temperature
increases to ralse a manufacturing cost, or to lower
the manufacturing efficiency. Therefore the heating
temperature of the heat treatment is 840°C or less.
[0076] When the average cooling rate to the
temperature of 500°C or less is less than 5OC/sec,
perlite or coarse cementite is excessively generated,
and the desired steel structure cannot be obtained.
Accordingly, enough hardenability cannot be obtained,
and characteristics after the quenching may
deteriorate. Accordingly, the average cooling rate
to the temperature of 500°C or less is S0C/sec or
more. When the average cooling rate to the
temperature of 500°C or less exceeds 10O0C/sec,
martensite or retained austenite is excessively
generated, and the desired steel structure cannot be
obtained. Therefore, the enough hardenability cannot
be obtained, and the characteristics after the
quenching may deteriorate. Accordingly, the average
cooling rate to the temperature of 500°C or less is
10O0C/sec or less.
[0077] When the retention time at the temperature
zone of 300°C to 500°C is less than 5 seconds, the
desired steel structure cannot be obtained.
Therefore, the retention time is 5 seconds or more.
When the retention time cxcccds 600 seconds, the
manufacturing efficiency is remarkably lowered
Accordingly, the retention time is 600 seconds or
less.
[0078] It is possible to manufacture the steel sheet
for hot stamping according to the embodiment by a
series of processes as stated above.
L U U I Y J Next, the manufacturing method including the
plating treatment is described. In the manufacturing
method, a steel ingot or a steel billet is hotrolled,
a steel sheet obtained by the hot-rolling is
acid-pickled, a steel sheet obtained by the acld
pickling treatment is cold-rolled, a heat treatment
of a steel sheet obtained by the cold-rolling is
performed, and a plating treatment is performed. A
chemical composition of the steel ingot or the steel
billet is substantially matched with the chemical
composition of the steel sheet for hot stamping to be
manufactured. The hot-rolling, the acid pickling
treatment, and the cold-rolling may be performed
under the similar conditions as the manufacturing
method which does not include the plating treatment
[0080] In the heat treatment, the steel sheet is
heated to a temperature zone of 700°C to 840°C, is
cooled to a temperature of 580°C or less at an average
cooling rate of 3OC/sec to 20°C/sec, and thereafter,
is retained at a temperature zone of 500°C to 570°C
for 5 seconds to 600 seconds.
[0081] The heat treatment causes recrystallization,
but in this manufacturing method, the plating
treatment is performed later, and therefore,
conditions are different from those of the
manufacturing method which does not include the
plating treatment. The heating temperature of the
heat treatment is 700°C to 840°C from the similar
reason as that of the manufacturing method which does
not include the platlng treatment.
[0082] When the average cooling rate to the
temperature of 58OoC or less is less than 3'C/sec, the
desired steel structure cannot be obtained even if
the plating treatment is performed. Accordingly, the
average cooling rate to the temperature of 58OoC or
less is 3"C/sec or more. When the average cooling
rate to the temperature of 580°C or less exceeds
20°C/sec, the desired steel structure cannot be
obtained even if the plating treatment is performed
Therefore, the average cooling rate to the
temperature of 58OoC or less is 2OoC/sec or less.
[0083] When the retention time at the temperature
zone of 500°C to 570°C is less than 5 seconds, the
desired steel structure cannot be obtained.
Therefore, the retention time is 5 seconds or more.
When the retention time exceeds 600 seconds, the
manufacturing efficiency is remarkably lowered.
Accordingly, the retention time is 600 seconds or
less.
[0084] In the plating treatment, the hot-dip
galvanizing treatment is performed, and thereafter,
an alloying treatment is performed at a temperature
zone of 500°C to 650°C.
[0085] Conditions of the hot-dip galvanizing
treatment are not particularly limited, and for
example, it may be performed based on a typical
condition. A continuous hot-dlp galvanizing
equipment may be disposed to continue to an equipment
of che above-scared heac creacmenc, and che hear
treatment and the hot-dip galvanizing treatment may
be continuously performed. Besides, the hot-dip
galvanizing equipment may be independently disposed
from the equipment of the heat treatment.
[0086] When a temperature of the alloying treatment
is less than 500°C, the alloying may be insufficient,
and an Fe content of an galvanized layer after the
alloying treatment may be less than 8%. Accordingly,
the temperature of the alloying treatment is 500°C or
more. When the temperature of the alloying treatment
exceeds 650°C, the Fe content of the galvanized layer
after the alloying treatment may exceed 20%.
Therefore, the temperature of the alloying treatment
is 650°C or less. The skin pass rolling may be
performed after the alloying treatment, and the
effects are not affected by the process. For
example, the flatness is corrected by the skin pass
rolling. The elongation percentage of the skin pass
rolling is not particularly limited, and may be
similar to a typical elongation percentage.
[0087] The Fe content of the galvanized layer is
preferably 8% or more. When the Fe content is 8% or
more, an Fe-Zn solid-solution formation rate is
increased and it is possible to more surely suppress
a generation of a hot-dip galvanized phase in the hot
stamping. The more the Fe content is, the more the
effect is, on the other hand, the manufacturing
efficiency is lowered. Accordingly, the Fe content
is preferably LU% or less.
[0088] When the plating deposition amount is less
than 35 g/m2 per one side, a desired corrosion
resistance of the hot stamped steel sheet member may
not be able to be obtained. Besides, the
manufacturing efficiency is easy to be lowered.
Adcordingly, the plating deposition amount is
preferably 35 g/m2 or more per one side.
[0089] (Hot stamped steel sheet member)
The hot stamping of the steel sheet for hot
stamping according to the embodiment is performed,
and thereby, the hot stamped steel sheet member is
obtained. Accordingly, the chemical composition of
the hot stamped steel sheet member substantially
matches with that of the steel sheet for hot
stamping. A steel structure of the hot stamped steel
sheet member depends on conditions of the hot
stamping, and in the embodiment of the present
invention, the area ratio of martensite is 90% or
more. When the area rat10 of martensite is less than
90%, the fine local deformability and toughness
cannot be obtained even if the chemical composition
is a proper one. When the area ratio of martensite
is 90% or more, it is possible to obtain the fine
local deformability and toughness. Besides, a
tensile-strength of the hot stamped steel sheet
member may be 1600 MPa or less due to a relationship
with the desired local deformability, toughness, and
others.
EXAiviPLE
[0090] Next, experiments performed by the present
inventors are described.
[0091] (First Experiment)
In a first experiment, slabs having chemical
compositions listed in Table 1 were used, and steel
sheets for hot stamping having steel structures
listed in Table 2 were manufactured. A part of the
steel sheets for hot stamping was made to be a plated
steel sheet by performing a plating treatment. "GA"
~n a field of a "steel sheet type" represents an
alloyed hot-dip galvanized steel sheet, "EG"
represents an electrogalvanized steel sheet, "GI"
represents a hot-dip galvanized steel sheet, and "CR"
represents a cold-rolled steel sheet.
[0092] In the manufacturing of the steel sheet for
hot stamping, the hot-rolling was performed for the
slabs having the chemical compositions listed in
Table 1, and hot-rolled steel sheets with a thickness
of 3.2 mm each were obtained. Then, the hot-rolled
steel sheets were acid-pickled so as to obtain acid
pickling steel sheets. A part of the acid pickling
steel sheets was cold-rolled so as to obtain coldrolled
steel sheets with a thickness of 1.6 mm.
Thereafter, the heat treatment and the plating
treatment of the cold-rolled steel sheets were
performed. In a part of tests, a part of the coldrolling,
heat treatment, and plating treatment was or
were not performed. Conditions of the hot-rolling (a
slab heatlng temperature and a coiling temperature),
presence/absence of the cold-rolling,
presence/absence and conditions of the heat treatment
(a heating temperature, an average cooling rate to a
temperature of 550°C or less, a cooling stop
temperature, and a retention time at the cooling stop
temperature), and presence/absence of the plating
treatment are listed in Table 2.
[0093] A temperature of a hot-dip galvanizing bath
when the hot-dip galvanized steel sheet or the
alloyed hot-dip galvanized steel sheet was
manufactured was set to be 460°C. The plating
deposition amount was set to be 45 g/m2, and the Fe
content of the plating was set to be 13%. The
alloying treatment when the alloyed hot-dip
galvanized steel sheet was manufactured was performed
at 550°C for 20 seconds.
[0094] The steel structure of each of the steel
sheets for hot stamping was found as described below.
Namely, nital etching was performed for a cross
section which was in parallel to a rolling direction
of the steel sheet for hot stamping, and the steel
structures of 10 visual fields were observed at a 114
depth position using a scanning electron microscope.
A measurement magnification was set to be 1000 times
in this observation.
[0095] (Table 1)
[ 0 0 9 6 ] (Table 2 )
[0097] A hot stamping test was performed using a hot
stamping tester. In this test, a test piece was
heated until a surface temperature reached 900°C in a
heating furnace, tias retained at 900°C for two
minutes, then was pulled out of the heating furnace.
Thereafter, the test piece was naturally cooled to a
predetermined temperature (a quenching start
temperature), and a rapid quenching treatment was
performed at the predetermined temperature. The
predetermined temperatures were 800°C or 500°C. A
shape of the hot stamped steel sheet member obtained
by the hot stamping was set to be a flat sheet. A
size of the test piece for the hot stamping test was
set to be 200 mm in width and 80 mm in length.
[0098] A measurement of tensile strength after the
quenching was also performed. In this measurement, a
JIS No. 5 tensile test piece was obtained in a
direction perpendicular to the rolling direction, the
tensile test was performed based on JIS Z 2241, and
the tensile strength (TS) was measured.
[0099] A hardness test after the quenching was also
performed. In the hardness test, Vickers hardness of
a cross section in parallel to the rolling direction
was measured. This measurement was performed based
on JIS Z 2244, and a measurement load was set to be
98 kN. In this measurement, the hardness was
measured at 10 points each kept off for 200 pm at the
same depth position from a surface of a sample, and
an average value was found. The Vickers hardness
when the quenching was started at 800°C (Hv800) and
the Vickers hardness when the quenching was started
at 500°C (Hv500) were found, and a difference between
these values was also found. Bes~des, an area ratio
of martensite of the steel sheet in which the
quenching was started at 800°C was also found.
[0100] An evaluation of the local deformability
after the quenching was also performed. In this
evaluation, a hole expansion ratio was measured
according to a "JFST1001 hole expansion test method"
of the Japan Iron and Steel Federation Standard.
[OlOl] An evaluation of the toughness after the
quenching was also performed. In this evaluation, an
absorbed energy of the steel sheet in which the
quenching treatment was performed from 800°C was
measured by a Charpy impact test. In the measurement
of the absorbed energy, three pieces were superposed
as for one in which the cold-rolling was not
performed, and six pieces were superposed as for one
in which the cold-rolling was performed, they were
each fastened with screws to manufacture test pieces
with a total thickness of 9.6 mm. The test piece was
a V-notch Charpy test piece described in JIS Z 2202.
A test method was based on a method described in JIS
2242, and the absorbed energy at -40°C was
investigated. The absorbed energies obtained from
the Charpy test pieces in which notches were made in
a 0' direction or a 90' direction relative to the
rolling direction were represented by JO, J90,
respectively.
[ 0 1 0 2 ] These results are listed in Table 3. In the
evaluation of the hardenability, ones in which the
difference between HvBUU and HvSOU was 13 Hv or less,
and the area ratio of martensite was 90% or more were
evaluated as "superior", and the others were
evaluated as "inferior". In the evaluation of the
local deformability, ones in which the hole expansion
ratio was 40% or more were evaluated as "superior",
and the others were evaluated as "inferior". In the
evaluation of the toughness, ones in which both J O
and J90 were 3 0 5 or more and a value of J O / J 9 0 was
0 . 5 or more were evaluated as "superior", and the
others were evaluated ds "inferior". The value of
J O / J 9 0 reflects anisotropy of the toughness.
[ 0 1 0 3 ] [Table 31
VIOEZXUE W STEEL- Ilr OXW-WU~WICATESO UT OF PAIICE OFMSVWUMlITlWL
U , O L ~ E UOI m RW L~'1lSIIDICATESW TOF DESIRED WlOE
- 46 -
[ 0 1 0 4 1 As it can be seen from Table 3, the steel
sheets for hot stamping of the example which were
within a range of the present invention had the good
hai-denabi:ity, local defoxma;ility, and LL UUyllLl lt:>>.
Besides, these had an appropriate tensile strength of
1 6 0 0 MPa or less after the quenching. On the other
hand, at least one characteristic was deteriorated in
the comparative examples.
[ 0 1 0 5 ] In each of tests No. 67 and No. 6 9 , in which
the coiling temperature was set at 35OoC, the proper
cold-rolling could not be performed, as listed in
Table 2. In each of tests No. 68 and No. 7 0 , in
which the coiling temperature was set at 750°C,
deformation was generated after the coiling, and
therefore, post processes could not be performed.
[ 0 1 0 6 ] (Second Experiment)
In a second experiment, an evaluation of a
heating time required for the quenching of the steel
sheet was performed. In this evaluation, the steel
sheet was heated up to 900°C before the quenching, and
both of the Vickers hardness after retaining at 900°C
for four minutes and quenching and the Vickers
hardness after retaining at 900°C for 1 . 5 minutes and
quenching were measured. Then, a difference between
the two kinds of Vickers hardness was found. One in
which the difference was 15 or less was evaluated as
" 0 " , and one in which the difference was over 1 5 and
30 or less was evaluated as '0" . The results were
listed in Table 4.
[0107] [Table 41
TABLE 4
[0108] As listed in Table 4, enough austenitization
was caused by retaining for 1.5 minutes and the
difference of the Vickers hardness was 15 or less in
the tests No. 30 to No. 32, though enough
austenitization was not caused by retaining for 1.5
minutes and the difference of the Vickers hardness
was over 15 in the test No. 33, in which the area
ratio of bainite was less than 20%.
[0109] (Third Experiment)
In a third experiment, an evaluation of preformability
of steel sheets each of whose area ratio
of bainite is 1% or more and less than 20% was
performed. In this evaluation, the tensile test of
the steel sheet for hot stamping before the quenching
was performed. In this measurement, a JIS No. 5
tensile test piece was prepared in a direction
perpendicular to the rolling direction, the tensile
test was performed based on JIS Z 2241, and a total
elongation (T-EL) was measured. One in which the
total elongation (T-EL) was 10% or more was evaluated
a s \' @) sf , and the others were evaluated as "0". The
results are listed in Table 5.
[0110] [Table 51
TABLE 5
[Olll] As listed in Table 5, the good preformability
could be obtained in the test No. 33, in
which the area ratio of ferrite was less than 40%,
and excellent pre-formability could be obtained'in
each of tests No. 3, No. 6, and No. 8, in which the
area ratio of ferrite was 40% or more.
[0112] (Fourth Experiment)
In a fourth experiment, evaluations of the local
deformability and the toughness as same as the first
experiment were performed as for the hot stamped
steel sheet members. The results were listed in
Table 6. The heating temperature before the
quenching was set to be 90OoC, the retention time at
90OoC was set to be four minutes, and the hot stamped
steel sheet members was naturally cooled in
atmosphere at a room temperature to 200°C in the
quenching, in manufacturing of the hot stamped steel
sheet members of the tests No. 71, No. 72, and No.
73.
[0113] [Table 61
TABLE 6
UNDERLINE INDICATES OUT OF RANGE OF PRESENT INVENTION, OR OUT OF DESIRED RANGE
[0114] As listed in Table 6, when the area ratio of
martensite was 90% or more, the fine local
deformability and toughness could be obtained, but
the enough local deformability and toughness could
not be obtained in the comparative examples, in which
the area ratio of martensite was less than 90%.
INDUSTRIAL APPLICABILITY
[0115] The present invention may be used for, for
example, manufacturing industries and using
industries of a steel sheet for hot stamping used for
vehicle body components such as a door beam and a
center pillar and others. The present invention may
be used also for manufacturing industries and using
industries of other machine structural components and
others.
CLAIMS
[Clam 11 A steel sheet for hot stamping, comprising
a chemical composition expressed by, in mass%:
C: 0.08% or more and less than 0.20%;
Si: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N
content is expressed by [Nl;
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0.05%;
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%;
W: 0% to 1%;
Nb: 0% to 1%;
v: 0% to 1%;
balance: Fe and impurities,
wherein:
an expression (1) is satisfied when an Mn content
is represented by [Mn] and a C content is represented
by [Cl,
where [Mn] + 6.67 x [C] - 2.73 2 0 (1); and
the steel sheet comprises a steel structure
expressed by, in an area ratio:
bainlte: 1% to 95%;
ferrite: 5% to 94%; and
balance: one or more selected from the group
consisting of pearlite, martensite and retalned
austenite
[Claim 21 The steel sheet for hot stamping according
to claim 1, wherein
an area ratio of bainite is 20% to 95%, and
an area ratio of ferrite is 5% to 80%.
[Claim 33 The steel sheet for hot stamping according
to claim 1, wherein
an area ratio of bainite is 1% or more and less
than 20%, and
an area ratio of ferrite is 40% to 94%.
[Claim 41 The steel sheet for hot stamping according
to any one of claims 1 to 3, wherein the chemical
content contains:
Ti: 48/14 x [N] % to 0.04 + 48/14 x [N] %.
[Claim 53 The steel sheet for hot stamping according
to any one of claims 1 to 4, wherein
a Cr content is 0.01% to 1.00%, and
when the Mn content is represented by [Mn] and
the Cr content is represented by [Cr], an expression
(2) is satisfied,
1.6 < [Mn] + [Cr] < 3.5 (2).
[Claim 61 The steel sheet for hot stamping according
to any one of claims 1 to 5, wherein the chemical
content contains:
Bi: 0.0001% to 0.5%.
[Claim 71 The steel sheet for hot stamplng according
to any one of claims 1 to 6, wherein the chemical
composition contains one or more selected from the
group consisting of:
Ca: 0.0005% to 0.05%;
Mg: 0.0005% to 0.05%; and
REM: 0.0005% to 0.05%.
[Claim 81 The steel sheet for hot stamping according
to any one of claims 1 to 7, wherein the chemical
composition contains one or more selected from the
group consisting of:
Mo: 0.03% to 1%;
Cu: 0.01% to 1%;
Ni: 0.01% to 1%; and
W: 0.01% to 1%.
[Claim 91 The steel sheet for hot stamping according
to any one of claims 1 to 8, wherein the chemical
composition contains one or two selected from the
group consisting of:
Nb: 0.005% to 1%; and
V: 0.005% to 1%.
[Claim lo] A method of manufacturing a steel sheet
for hot stamping, comprising:
hot-rolling a steel ingot or a steel billet;
acid-pickling a steel sheet obtained by the hotrolling;
cold-rolling a steel sheet obtained by the acid
pickling; and
performing a heat treatment of a steel sheet
obtained by the cold-rolling,
wherein :
the steel ingot or the steel billet comprises a
chemical composition expressed by, in mass%:
C: 0.08% or more and less than 0.20%;
Si: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N
content is represented by [N];
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0.05%;
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%;
W: 0% to 1%;
Nb: 0% to 1%;
v: 0% to 1%;
balance: Fe and impurities;
an expression (1) is satisfied when an Mn content
is represented by [Mn] and a C content is represented
by [Clr
where [Mn] + 6.67 x [C] - 2.73 t 0 (1) ;
the hot-rolling comprises:
starting a rolling at a temperature of 1050°C
or more; and
thereafter coiling at a temperature zone of
400°C to 700°C; and
the performing the heat treatment comprises:
heating to a temperature zone of 700°C to
840°C;
thereafter cooling to a temperature of 500°C
or less at an average cooling rate of 5"C/sec to
100°C/sec; and
thereafter retaining at a temperature zone
of 300°C to 500°C for 5 seconds to 600 seconds.
[Claim 111 A method of manufacturing a steel sheet
for hot stamping, comprising:
hot-rolling a steel ingot or a steel billet;
acid-pickling a steel sheet obtained by the hotrolling;
cold-rolling a steel sheet obtained by the acid
pickling;
performing a heat treatment of a steel sheet
obtained by the cold-rolling, and
plating a steel sheet obtained by the heat
treatment,
wherein :
the steel ingot or the steel billet comprises a
chemical composition expressed by, in mass%:
C: 0.08% or more and less than 0.20%;
s1: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N
content is represented by [ N l ;
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0.05%;
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%;
W: 0% to 1%;
Nb: 0% to 1%;
V: 0% to 1%;
balance: Fe and impurities;
an expression (1) is satisfied when an Mn content
is represented by [Mn] and a C content is represented
by [Cl,
where [Mn] + 6.67 x [C] - 2.73 2 0 (1) ;
the hot-rolling comprises:
starting a rolling at a temperature of 1050°C
or more; and
thereafter coiling at a temperature zone of
~ U U " C to 700°C;
the performing the heat treatment comprises:
heating to a temperature zone of 70OoC to
840°C;
thereafter cooling to a temperature of 580°C
or less at an average cooling rate of 3OC/sec to
20°C/sec; and
thereafter retaining at a temperature zone
of 50G°C to 570°C for 5 seconds to 600 seconds; and
the plating comprises:
performing a hot-dip galvanizing treatment;
and
thereafter performing an alloying treatment
at a temperature zone of 500°C to 650°C.
[Claim 121 The method of manufacturing the steel
sheet for hot stamping according to claim 10 or claim
11,
wherein the chemical composition contains:
Ti: 48/14 x [N] % to 0.04 + 48/14 x [N] %.
[Claim 131 The method of manufacturing the steel
sheet for hot stamping according to any one of claims
10 to 12,
wherein a Cr content is 0.01% to 1.00%, and when
the Mn content is represented by [Mn] and the Cr
content is represented by [Cr], an expression (2) is
satisfied,
1.6 5 [Mn] + [Cr] 2 3.5 (2).
[Claim 141 The method of manufacturing the steel
sheet for hot stamping according to any one of claims
10 to 13,
wherein the chemical composition contains:
Bi: 0.0001% to 0.5%.
[Claim 151 The method of manufacturing the steel
sheet for hot stamping according to any one of claims
10 to 14,
wherein the chemical composition contains one
kind or two or more kinds selected from a group made
up of:
Ca: 0.0005% to 0.05%;
Mg: 0.0005% to 0.05%; and
REM: 0.0005% to 0.05%.
[Claim 161 The method of manufacturing the steel
sheet for hot stamping according to any one of claims
10 to 15,
wherein the chemical composition contains one
kind or two or more kinds selected from a group made
up of:
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%; and
W: 0% to 1%.
[Claim 171 The method of manufacturing the steel
sheet for hot stamping according to any one of claims
10 to 16,
wherein the chemical composition contains one
kind or two kinds selected from a group made up of:
Nb: 0% to 1%; and
V: 0% to 1%.
[ C l a i m 181 A hnt stamped steel sheet m.emher, having
a chemical composition expressed by, in mass%:
C: 0.08% or more and less than 0.20%;
Si: 0.003% to 0.2%;
Mn: 1.6% to 3.5%;
sol. Al: 0.0002% to 2.0%;
B: 0.0003% to 0.01%;
P: 0.1% or less;
S: 0.004% or less;
N: 0.01% or less;
Ti: 0% to 0.04 + 48/14 x [N] % when an N content
is represented by [N];
Cr: 0% to 1.00%;
Bi: 0% to 0.5%;
Ca: 0% to 0.05%;
Mg: 0% to 0.05%;
REM: 0% to 0. 05%;
Mo: 0% to 1%;
Cu: 0% to 1%;
Ni: 0% to 1%;
W: 0% to 1%;
Nb: 0% to 1%;
V: 0% to 1%;
balance: Fe and impurities,
wherein an expression (1) is satisfied when an Mn
content is represented by [Mnl, a C content is
represented by [C],
where [Mn] + 6. 67 x [C] - 2.73 2 0 (1)
and having a steel structure in which an area
ratio of martensite is 90% or more; and a tensile
strength is 1600 MPa or less.
[Claim 191 The hot stamped steel sheet member
according to claim 18,
wherein the chemical composition contains:
Ti: 48/14 x [N] % to 0.04 + 48/14 x [N] %.
[Claim 201 The hot stamped steel sheet member
according to claim 18 or claim 19,
wherein a Cr content is 0.01% to 1.00%, and when
the Mn content is represented by [Mn] and the Cr
content is represented by [Cr], an expression (2) is
satisfied,
1.6 I [Mn] t [Cr] 53.5 (2).
[Claim 211 The hot stamped steel sheet member
according to any one of claims 18 to 20,
wherein the chemical composition contains:
Bi: 0.0001% to 0.5%.
[Claim 221 The hot stamped steel sheet member
according to any one of claims 18 to 21,
wherein the chemical composition contains one
kind or two or more kinds selected from a group made
up of:
Ca: 0.0005% to 0.05%;
Mg: 0.0005% to 0.05%; and
REM: 0.0005% to 0.05%.
[Claim 231 The hot stamped steel sheet member
according to any one of claims 18 to 22,
wherein the chemical' composition contains one
kind or two or more kinds selected from a group made
up. of :
Mo: 0% to- 1%;.
Cu: 0% to 1%;
Ni: 0| to 1%; and
W: 0% to 1%.. _
[Claim 24] The hot stamped steel sheet member
according to any one of claims 18 to 23,
wherein the chemical composition contains one
kind or two kinds selected from a group made up of:
Nb: 0% to 1%; and
V: 0% to 1%