DESCRIPTION
TITLE OF INVENTION: HOT-PRESSED STEEL SHEET MEMBER,
METHOD OF MANUFACTURING THE SAME, AND
STEEL SHEET FOR HOT PRESSING
TECHNICAL FIELD
[0001] The present invention relates to a hotpressed
steel sheet member used for a machine
structural component and the like, a method for
manufacturing the same, and a steel sheet for hot
pressing.
BACKGROUND ART
100021 For reduction in weight of an automobile,
efforts are advanced to increase the strength of a
steel material used for an automobile body and to
reduce the weight of steel material used. In a thin
steel sheet widely used for the automobile, press
formability thereof generally decreases with an
increase in strength, making it difficult to
manufacture a component having a complicated shape.
For example, a highly processed portion fractures
with a decrease in ductility, and springback becomes
prominent to deteriorate dimensional accuracy.
Accordingly, it is difficult to manufacture
components by performing press-forming on a highstrength
steel sheet, in particular, a steel sheet
having a tensile strength of 980 MPa or more. It is
easy to process the high-strength steel sheet not by
press-forming but by roll-forming, but its
application target is limited to a component having a
uniform c r o s s s e c t i o n i n a l o n g i t u d i n a l d i r e c t i o n .
[00031 Methods c a l l e d h o t p r e s s i n g i n t e n d e d t o
o b t a i n high f o r m a b i l i t y i n t h e h i g h - s t r e n g t h steel
s h e e t a r e d e s c r i b e d i n P a t e n t L i t e r a t u r e s 1 and 2.
By t h e h o t p r e s s i n g , it is p o s s i b l e t o form t h e h i g h -
s t r e n g t h s t e e l s h e e t w i t h h i g h a c c u r a c y t o o b t a i n a
h i g h - s t r e n g - t h h o t - p r e s s e d s t e e l s h e e t member.
[00041 On t h e o t h e r hand, t h e h o t - p r e s s e d s t e e l
s h e e t member i s r e q u i r e d t o be improved a l s o i n
d u c t i l i t y . However, steel s t r u c t u r e of t h e steel
s h e e t o b t a i n e d by t h e methods d e s c r i b e d i n P a t e n t
L i t e r a t u r e s 1 and 2 is s u b s t a n t i a l l y a m a r t e n s i t e
s i n g l e p h a s e , a n d t h u s it is d i f f i c u l t f o r t h e
methods t o improve i n d u c t i l i t y .
[0005] ~ i ~ h - s t r e n g thho t - p r e s s e d steel s h e e t members
i n t e n d e d t o improve i n d u c t i l i t y a r e d e s c r i b e d i n
P a t e n t L i t e r a t u r e s 3 ,and 4 , b u t i n t h e s e c o n v e n t i o n a l
h o t - p r e s s e d steel s h e e t members, it h a s a n o t h e r
problem of a d e c r e a s e i n t o u g h n e s s . The d e c r e a s e i n
t o u g h n e s s c a u s e s a problem n o t o n l y i n t h e c a s e of
t h e use f o r an a u t o m o b i l e b u t a l s o i n t h e c a s e of t h e
u s e f o r a machine s t r u c t u r a l component. P a t e n t
L i t e r a t u r e s 5 and 6 each d e s c r i b e a t e c h n i q u e
i n t e n d e d t o improve a f a t i g u e p r o p e r t y , b u t even
t h e s e have d i f f i c u l t y i n o b t a i n i n g s u f f i c i e n t
d u c t i l i t y and t o u g h n e s s .
CITATION LIST
PATENT LITERATURE
[0006] P a t e n t L i t e r a t u r e 1: U.K. P a t e n t No. 1490535
P a t e n t L i t e r a t u r e 2: J a p a n e s e Laid-open P a t e n t
P u b l i c a t i o n No. 10-96031
P a t e n t L i t e r a t u r e 3: J a p a n e s e Laid-open P a t e n t
P u b l i c a t i o n No. 2010-65292
P a t e n t L i t e r a t u r e 4 : J a p a n e s e Laid-open P a t e n t
P u b l i c a t i o n No. 2007-16296
P a t e n t L i t e r a t u r e 5: J a p a n e s e Laid-open P a t e n t
P u b l i c a t i o n No. 2007-247001
P a t e n t L i t e r a t u r e 6: J a p a n e s e Laid-open P a t e n t
P u b l i c a t i o n No. 2005-298957
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007] An o b j e c t of t h e p r e s e n t i n v e n t i o n is t o
p r o v i d e a h o t - p r e s s e d steel s h e e t member h a v i n g
e x c e l l e n t d u c t i l i t y and t o u g h n e s s w i t h a h i g h
s t r e n g t h , a method o f m a n u f a c t u r i n g t h e same, and a
s t e e l s h e e t f o r h o t p r e s s i n g .
SOLUTION TO PROBLEM
[0008] The i n v e n t o r s of t h e p r e s e n t a p p l i c a t i o n
s t u d i e d t h e r e a s o n why t h e d e c r e a s e i n t o u g h n e s s is
c a u s e d by t h e c o n v e n t i o n a l h i g h - s t r e n g t h h o t - p r e s s e d
steel s h e e t member i n t e n d e d t o improve d u c t i l i t y . A s
a r e s u l t , it became c l e a r t h a t when a m u l t i - p h a s e
s t r u c t u r e c o n t a i n i n g f e r r i t e and m a r t e n s i t e is t o be
made as t h e s t e e l s t r u c t u r e of t h e h o t - p r e s s e d s t e e l
s h e e t member f o r t h e p u r p o s e of improving d u c t i l i t y ,
d e c a r b u r i z a t i o n is l i k e l y t o p r o g r e s s and a d e c r e a s e
i n t o u g h n e s s by t h e d e c a r b u r i z a t i o n is caused d u r i n g
h e a t i n g and a i r c o o l i n g i n h o t p r e s s i n g f o r o b t a i n i n g
the hot-pressed steel sheet member. That is, it
became clear that the ferrite ratio increases in a
region ranging from the surface of the hot-pressed
steel sheet member to 15 pm or so in depth due to the
decarburization, and a lamellar structure
substantially made of a ferrite single phase
(hereinafter, to be sometimes referred to as a
"ferrite layer") sometimes appears, for example, and
vulnerability of ferrite grain boundaries in the
region induces significant deterioration of
toughness. The decarburization is significant
particularly when obtaining a multi-phase structure,
but the decarburization has not been recognized
before.
[0009] As a result of earnest studies based on such
findings, the inventors of the present application
have found that a hot-pressed steel sheet member
having a steel structure being a multi-phase
structure containing ferrite and martensite, and
having a surface layer portion in which
decarburization is suppressed can be obtained by
treating a steel sheet for hot pressing having a
chemical composition containing specific amounts of C
and Mn and relatively large amount of Si, and having
a specific steel structure including hot pressing
under specific conditions. Further, the inventors of
the present application also have found that this
hot-pressed steel sheet member has a high tensile
strength of 980 MPa or more and also has excellent
- 4 -
ductility and toughness. The inventors of the
present application also have found that this hotpressed
steel sheet member also has an excellent
fatigue property beyond expectation. Then, the
inventors of the present application has reached the
following various aspects of the invention.
[0010] (1) A hot-pressed steel sheet member,
including:
a chemical composition represented by, in mass%:
c: 0.10% to 0.34%;
Si: 0.5% to 2.0%;
Mn: 1.0% to 3.0%;
sol. Al: 0.001% to 1.0%;
P: 0.05% or ,less;
S: 0.01% or less;
N: 0.01% or less;
Ti: 0% to 0.20%;
Nb: 0% to 0.20%;
v: 0% to 0.20%;
Cr: 0% to 1.0%;
Mo: 0% to 1.0%;
cu: 0% to 1.0%;
Ni: 0% to 1.0%;
Ca: 0% to 0.01%;
Mg: 0% to 0.01%;
REM: 0% to 0.01%;
Zr: 0% to 0.01%;
B: 0% to 0.01%;
Bi: 0% to 0.01%; and
balance: Fe and impurities; and
a steel structure in which:
an area ratio of ferrite in a surface layer
portion ranging from a surface to 15 pm in depth is
equal to or less than 1.20 times an area ratio of
ferrite in an inner layer portion being a portion
excluding the surface layer portion; and
the inner layer portion includes a steel
structure represcntcd, in area%:
ferrite: 10% to 70%;
martensite: 30% to 90%; and
a total area ratio of ferrite and
martensite: 90% to loo%,
wherein a concentration of Mn in the martensite
is equal to or more than 1.20 times a concentration
of Mn in the ferrite, in the inner layer portion, and
wherein a tensile strength is 980 MPa or more.
[0011] (2) The hot-pressed steel sheet member
according to (I), wherein the chemical composition
contains one or more selected from the group
consisting of, in mass%:
Ti: 0.003% to 0.20%;
Nb: 0.003% to 0.20%;
V: 0.003% to 0.20%;
Cr: 0.005% to 1.0%;'
Mo: 0.005% to 1.0%;
Cu: 0.005% to 1.0%; and
Ni: 0.005% to 1.0%.
[0012] (3) The hot-pressed steel sheet member
according to (1) or (2), wherein the chemical
composition contains one or more selected from the
group consisting of, in mass%:
Ca: 0.0003% to 0.01%;
Mg: 0.0003% to 0.01%;
REM: 0.0003% to 0.01%; and
Zr: 0.0003% to 0.01%.
[0013] (4) The hot-pressed steel sheet member
according to any one of (1) to (3), wherein the
chemical composition contains, in mass%, B: 0.0003%
to 0.01%.
[0014] (5) The hot-pressed steel sheet member
according to any one of (1) to ( 4 ) , wherein the
chemical composition contains, in mass%, Bi: 0.0003%
to 0.01%.
[0015] (6) A steel sheet for hot pressing,
including:
a chemical composition represented by, in mass%:
C: 0.10% to 0.34%;
Si: 0.5% to 2.0%;
Mn: 1.0% to 3.0%;
sol. Al: 0.001% to 1.0% or less;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Ti: 0% to 0.20%;
Nb: 0% to 0.20%;
V: 0% to 0.20%;
Cr: 0% to 1.0%;
M 0 : 0% t o 1 . 0 % ;
cu: 0% t o 1 . 0 % ;
N i : 0% t o 1.0%;
Ca: 0% t o 0.01%;
Mg: 0% t o 0.01%;
REM: 0% t o 0.01%;
Z r : 0% t o 0.01%;
B: 0% t o 0.01%;
B i : 0% t o 0 . 0 1 % ; a n d
b a l a n c e : Fe and i m p u r i t i e s ; and
a steel s t r u c t u r e c o n t a i n i n g ferrite and
c e m e n t i t e , r e p r e s e n t e d , i n a r e a % :
a t o t a l a r e a r a t i o of b a i n i t e and
m a r t e n s i t e : 0% t o 10%; and
an a r e a r a t i o of c e m e n t i t e : 1% o r more, and
wherein a c o n c e n t r a t i o n of Mn i n t h e c e m e n t i t e is
5% o r more.
[0016] ( 7 ) The steel s h e e t f o r h o t p r e s s i n g
a c c o r d i n g t o ( 6 ) , wherein 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 group
c o n s i s t i n g of, i n mass%:
T i : 0.003% t o 0.20%;
N b : 0.003% t o 0.20%;
V: 0.003% t o 0.20%;
C r : 0.005% t o 1 . 0 % ;
Mo: 0.005% t o 1 . 0 % ;
Cu: 0.005% t o 1 . 0 % ; and
N i : 0.005% t o 1 . 0 % .
[0017] (8) The steel s h e e t f o r h o t p r e s s i n g
according to (6) or ( 7 ) , wherein the chemical
composition contains one or more selected from the
group consisting of, in mass%:
Ca: 0.0003% to 0.01%;
Mg: 0.0003% to 0.01%;
REM: 0.0003% to 0.01%; and
Zr: 0.0003% to 0.01%.
[0018] (9) The steel sheet for hot pressing
according to any one of (6) to (D), wherein the
chemical composition contains, in mass%, B: 0.0003%
to 0.01%.
[0019] (10) The steel sheet for hot pressing
according to any one of (6) to ( 9 1 , wherein the
chemical composition contains, in mass%, Bi: 0.0003%
to 0.01%.
[0020] (11) A method of manufacturing a hot-pressed
steel sheet member, including:
a step of heating the steel sheet for hot
pressing according to any one of (6) to (10) in a
temperature zone of 720°C to an ACJ point so as to
cause a concentration of Mn in austenite to be equal
to or more than 1.20 times a concentration of Mn in
the ferrite; and
a step of hot pressing and cooling down to an Ms
point at an average cooling rate of 10 'C/second to
500 OC/second after the heating,
wherein a reduced C content on a surface of the
steel sheet for hot pressing during a time period
from completion of the step of heating to start of
- 9 -
t h e s t e p of h o t p r e s s i n g is l e s s t h a n 0.0005 mass%.
[0021] (12) The method of m a n u f a c t u r i n g t h e h o t -
p r e s s e d s t e e l s h e e t member a c c o r d i n g t o (11), wherein
a t i m e p e r i o d f o r which t h e s t e e l s h e e t f o r h o t
p r e s s i n g is exposed t o t h e atmosphere d u r i n g t h e t i m e
p e r i o d from c o m p l e t i o n of t h e s t e p of h e a t i n g t o
s t a r t of t h e s t e p of h o t p r e s s i n g is less t h a n 15
s e c o n d s .
ADVANTAGEOUS EFFECTS OF INVENTION
[00221 According t o t h e p r e s e n t i n v e n t i o n , it is
p o s s i b l e t o o b t a i n e x c e l l e n t d u c t i l i t y and t o u g h n e s s
w h i l e o b t a i n i n g a high t e n s i l e s t r e n g t h .
DESCRIPTION OF EMBODIMENTS
[a0231 H e r e i n a f t e r , embodiments of t h e p r e s e n t
i n v e n t i o n w i l l be d e s c r i b e d . The embodiments o f t h e
p r e s e n t i n v e n t i o n r e l a t e t o a h o t - p r e s s e d s t e e l s h e e t
member h a v i n g a t e n s i l e s t r e n g t h of 980 MPa o r more.
LO0241 F i r s t , c h e m i c a l c o m p o s i t i o n s o f t h e h o t -
p r e s s e d s t e e l s h e e t member ( h e r e i n a f t e r , sometimes
r e f e r r e d t o as a " s t e e l s h e e t member") a c c o r d i n g t o
t h e embodiment of t h e p r e s e n t i n v e n t i o n and a s t e e l
s h e e t f o r h o t p r e s s i n g used f o r m a n u f a c t u r i n g t h e
same w i l l be d e s c r i b e d . I n t h e f o l l o w i n g
d e s c r i p t i o n , " % " b e i n g a u n i t of a c o n t e n t o f e a c h
element c o n t a i n e d i n t h e steel s h e e t member o r t h e
steel s h e e t f o r h o t p r e s s i n g means "mass%" u n l e s s
o t h e r w i s e s p e c i f i e d .
[0025] The c h e m i c a l c o m p o s i t i o n of t h e s t e e l s h e e t
member a c c o r d i n g t o t h e embodiment and t h e steel
s h e e t f o r h o t p r e s s i n g used f o r m a n u f a c t u r i n g t h e
s a m e is r e p r e s e n t e d by, i n mass%, C: 0.10% t o 0.34%,
S i : 0.5% t o 2 . 0 % , Mn: 1.0% t o 3 . 0 % , s o l . Al: 0.001%
t o 1 . 0 % , P: 0.05% o r l e s s , S: 0.01% o r less, N : 0.01%
o r less, T i : 0% t o 0.20%, Nb: 0% t o 0.20%, V: 0% t o
0.20%, Cr: 0% t o 1 . 0 % , Mo: 0% t o 1 . 0 8 , Cu: 0% t o
1 . 0 % , N i : 0% t o 1 . 0 % , Ca: 0% t o 0.01%, Mg: 0% t o
0.01%, REM: 0% t o 0.01%, Z r : 0% t o 0.01%, B: 0% t o
0.01%, B i : 0% t o 0.018, and b a l a n c e : Fe and
i m p u r i t i e s . Examples of t h e i m p u r i t i e s i n c l u d e o n e s
c o n t a i n e d i n raw m a t e r i a l s s u c h a s o r e and s c r a p , and
ones mixed i n d u r i n g a m a n u f a c t u r i n g process.
[0026] (C: 0.10% t o 0.34%)
C is a v e r y i m p o r t a n t e l e m e n t which i n c r e a s e s
h a r d e n a b i l i t y o f t h e s t e e l s h e e t f o r h o t p r e s s i n g and
m a i n l y d e t e r m i n e s t h e s t r e n g t h o f t h e steel s h e e t
member. When t h e C c o n t e n t o f t h e steel s h e e t member
is less t h a n 0.10%, it may be d i f f i c u l t t o s e c u r e t h e
t e n s i l e s t r e n g t h of 980 MPa o r more. A c c o r d i n g l y ,
t h e C c o n t e n t of t h e steel s h e e t member is 0.10% o r
more. The C c o n t e n t of t h e s t e e l s h e e t member is
p r e f e r a b l y 0.12% o r more. When t h e C c o n t e n t of t h e
s t e e l s h e e t member is g r e a t e r t h a n 0.34%, m a r t e n s i t e
i n t h e s t e e l s h e e t member may become h a r d and
d e t e r i o r a t i o n o f t o u g h n e s s may be s i g n i f i c a n t . Thus,
t h e C c o n t e n t o f t h e s t e e l s h e e t member is 0.34% o r
less. I n t e r m s o f i m p r o v i n g w e l d a b i l i t y , t h e C
c o n t e n t of t h e s t e e l s h e e t member is p r e f e r a b l y 0.30%
o r less, and more p r e f e r a b l y 0.25% o r l e s s . A s w i l l
be d e s c r i b e d l a t e r , d e c a r b u r i z a t i o n sometimes o c c u r s
i n m a n u f a c t u r i n g of t h e h o t - p r e s s e d s t e e l s h e e t
member, b u t t h e amount of t h e d e c a r b u r i z a t i o n is
n e g l i g i b l y s m a l l , and t h e r e f o r e t h e C c o n t e n t of t h e
steel s h e e t f o r h o t p r e s s i n g s u b s t a n t i a l l y
c o r r e s p o n d s t o t h e C c o n t e n t of t h e steel s h e e t
member.
[0027] ( S i : 0.5% t o 2 . 0 % )
S i is a v e r y e f f e c t i v e e l e m e n t f o r improving
d u c t i l i t y of t h e steel s h e e t member and s t a b l y
s e c u r i n g s t r e n g t h of t h e s t e e l s h e e t member. When
t h e S i c o n t e n t is less t h a n 0.5%, it may be d i f f i c u l t
t o o b t a i n t h e a b o v e - d e s c r i b e d e f f e c t s . Thus, t h e S i
c o n t e n t is 0.5% o r more. When t h e S i c o n t e n t is
g r e a t e r t h a n 2.0%, t h e a b o v e - d e s c r i b e d e f f e c t may be
s a t u r a t e d t o r e s u l t i n economical d i s a d v a n t a g e , a n d
p l a t i n g w e t t a b i l i t y s i g n i f i c a n t l y d e c r e a s e s t o
f r e q u e n t l y c a u s e u n p l a t i n g . Thus, t h e S i c o n t e n t is
2.0% o r less. I n terms of improving w e l d a b i l i t y , t h e
S i c o n t e n t is p r e f e r a b l y 0.7% o r more, and more
p r e f e r a b l y 1.1% o r more. I n t e r m s of s u p p r e s s i n g
s u r f a c e d e f e c t s of t h e steel s h e e t member, t h e S i
c o n t e n t is p r e f e r a b l y 1 . 8 % or less, and more
p r e f e r a b l y 1.35% o r less.
[0028] (Mn: 1.0% t o 3 . 0 % )
Mn is a v e r y e f f e c t i v e e l e m e n t f o r improving
h a r d e n a b i l i t y of t h e steel s h e e t f o r h o t p r e s s i n g and
s e c u r i n g s t r e n g t h of t h e steel s h e e t member. When
t h e Mn c o n t e n t is less t h a n 1 . 0 % , it may be v e r y
d i f f i c u l t t o s e c u r e a t e n s i l e s t r e n g t h of 980 MPa o r
more i n t h e s t e e l s h e e t member. Thus, t h e Mn c o n t e n t
is 1.0% o r more. For more s e c u r e l y o b t a i n i n g t h e
a b o v e - d e s c r i b e d e f f e c t s , t h e Mn c o n t e n t is p r e f e r a b l y
1.1% o r more, and more p r e f e r a b l y 1.15% o r more.
When t h e Mn c o n t e n t is g r e a t e r t h a n 3 . 0 % , t h e steel
s t r u c t u r e of t h e s t e e l s h e e t member may become a
s i g n i f i c a n t band s t r u c t u r e and d e t e r i o r a t i o n of
b e n d a b i l i t y and c r a s h w o r t h i n e s s may become
s i g n i f i c a n t . Thus, t h e Mn c o n t e n t is 3.0% o r less.
I n t e r m s o f p r o d u c t i v i t y i n h o t - r o l l i n g and c o l d -
r o l l i n g f o r o b t a i n i n g t h e steel s h e e t f o r h o t
p r e s s i n g , t h e Mn c o n t e n t is p r e f e r a b l y 2.5% o r less,
and more p r e f e r a b l y 2 . 4 5 % o r less.
[0029] ( s o l . A 1 ( a c i d - s o l u b l e A l ) : 0.001% t o 1.0%)
A 1 is an e l e m e n t h a v i n g an e f f e c t o f d e o x i d i z i n g
s t e e l t o m a k e steel m a t e r i a l b e t t e r . When t h e s o l .
A 1 c o n t e n t is less t h a n 0.001%, it may be d i f f i c u l t
t o o b t a i n t h e a b o v e - d e s c r i b e d e f f e c t . Thus, t h e s o l .
A 1 c o n t e n t is 0.001% o r more. I n o r d e r t o more
s e c u r e l y o b t a i n t h e a b o v e - d e s c r i b e d e f f e c t , t h e s o l .
A 1 c o n t e n t is p r e f e r a b l y 0.015% o r more. When t h e
s o l . A 1 c o n t e n t is g r e a t e r t h a n 1 . 0 % , t h e w e l d a b i l i t y
s i g n i f i c a n t l y may d e c r e a s e , o x i d e - b a s e d i n c l u s i o n s
may i n c r e a s e , and t h e s u r f a c e p r o p e r t y may
s i g n i f i c a n t l y d e t e r i o r a t e . Thus, t h e s o l . A 1 c o n t e n t
is 1.0% o r less. I n o r d e r t o o b t a i n b e t t e r s u r f a c e
p r o p e r t y , t h e s o l . A 1 c o n t e n t is p r e f e r a b l y 0.080% o r
less.
[0030] ( P : 0.05% or less)
P is not an essential element and is contained,
for example, as an impurity in steel. In terms of
weldability, a lower P content is better. In
particular, when the P content is more than 0.05%,
the weldability may significantly decrease. Thus,
the P content is 0.05% or less. In order to secure
better weldability, the P content is preferably
0.018% or less. On the sther hand, P has an effect
of enhancing the strength of the steel by solid
solution strengthening. To obtain the effect, 0.003%
or more of P may be contained.
[0031] ( S : 0.01% or less)
S is not an essential element and is contained,
for example, as an impurity in steel. In terms of
the weldability, a lower S content is better. In
particular; when the S content is more than 0.01%,
the weldability may significantly decrease. Thus,
the S content is 0.01% or less. In order to secure
better weldability, the S content is preferably
0.003% or less, and more preferably 0.0015% or less.
[0032] (N: 0.01% or less)
N is not an essential element and is contained,
for example, as an impurity in steel. In terms of
the weldability, a lower N content is better. In
particular, when the N content is more than 0.01%,
the weldability may significantly decrease. Thus,
the N content is 0.01% or less. In order to secure
better weldability, the N content is preferably
0.006% o r l e s s .
[0033] T i , Nb, V, C r , Mo, Cu, N i , Ca, Mg, REM, Z r ,
B, and B i a r e n o t e s s e n t i a l e l e m e n t s , and a r e
a r b i t r a r y e l e m e n t s which may be a p p r o p r i a t e l y
c o n t a i n e d , up t o a s p e c i f i c amount a s a l i m i t , i n t h e
s t e e l s h e e t member and t h e s t e e l s h e e t f o r h o t
p r e s s i n g .
[0034] ( T i : 0% t o 0.20%, Nb: 0% t o 0.20%, V: 0% t o
0.20%, C r : 0% t o 1 . 0 % , Mo: 0% t o 1 . 0 % , Cu: 0% t o
1 . 0 % , and N i : 0% t o 1 . 0 % )
Each of T i , Nb, V, C r , Mo, Cu, and N i is an
e l e m e n t e f f e c t i v e f o r s t a b l y s e c u r i n g s t r e n g t h of t h e
steel s h e e t member. Thus, one o r more s e l e c t e d from
t h e group c o n s i s t i n g of t h e s e e l e m e n t s may a l s o be
c o n t a i n e d . However, when t h e c o n t e n t of one o f T i ,
Nb, and V is more t h a n 0.20%, h o t - r o l l i n g and c o l d -
r o l l i n g f o r o b t a i n i n g t h e s t e e l s h e e t f o r h o t
p r e s s i n g may become d i f f i c u l t t o be p e r f o r m e d , a n d
f u r t h e r it may become d i f f i c u l t t o s t a b l y s e c u r e
s t r e n g t h . Thus, t h e T i c o n t e n t , t h e Nb c o n t e n t , and
t h e V c o n t e n t a r e e a c h 0.20% o r less. When t h e C r
c o n t e n t is g r e a t e r t h a n 1 . 0 % , it may become d i f f i c u l t
t o s t a b l y s e c u r e s t r e n g t h . Thus, t h e C r c o n t e n t is
1.0% o r less. When t h e Mo c o n t e n t is g r e a t e r t h a n
1 . 0 % , h o t - r o l l i n g and c o l d - r o l l i n g f o r o b t a i n i n g t h e
s t e e l s h e e t f o r h o t p r e s s i n g may become d i f f i c u l t t o
be p e r f o r m e d . Thus, t h e Mo c o n t e n t is 1.0% o r less.
When t h e c o n t e n t of one of Cu and N i is 1 . 0 % , t h e
a b o v e - d e s c r i b e d e f f e c t s may be s a t u r a t e d t o r e s u l t i n
economical d i s a d v a n t a g e , and h o t - r o l l i n g and c o l d -
r o l l i n g f o r o b t a i n i n g t h e steel s h e e t f o r h o t
p r e s s i n g may become d i f f i c u l t t o be performed. Thus,
t h e Cu c o n t e n t and t h e N i c o n t e n t a r e each 1.0% o r
less. I n o r d e r t o s t a b l y s e c u r e t h e s t r e n g t h of t h e
s t e e l s h e e t member, each of t h e T i c o n t e n t , t h e Nb
c o n t e n t , and t h e V c o n t e n t is p r e f e r a b l y 0.003% o r
more, and each of t h e C r c o n t e n t , t h e Mo c o n t e n t , t h e
CLI c o n t e n t i and t h e Ni c o n t e n t is p r e f e r a b l y 0.005%
o r more. That is, a t l e a s t one of " T i . . 0.003% t o
0 . 2 0 % , " "Nb: 0.003% t o 0 . 2 0 % , " "V: 0.003% t o 0 . 2 0 % , "
" C r : 0.005% t o 1 . 0 % , " "Mo: 0.005% t o 1 . 0 % , " "Cu:
0.005% t o 1 . 0 % , " and " N i : 0.005% t o 1.0%" is
p r e f e r a b l y s a t i s f i e d .
[0035] (Ca: 0% t o 0.01%, Mg: 0% t o 0.01%, REM: 0% t o
0.01%, and Z r : 0% t o 0.01%)
Each of Ca, Mg, REM, and Z r is an element which
h a s an e f f e c t of c o n t r i b u t i n g t o c o n t r o l of
i n c l u s i o n s , i n p a r t i c u l a r , f i n e d i s p e r s i o n of
i n c l u s i o n s t o enhance t o u g h n e s s . Thus, one o r more
s e l e c t e d from t h e group c o n s i s t i n g of t h e s e e l e m e n t s
may be c o n t a i n e d . However, when t h e c o n t e n t of any
one o f them is more t h a n 0.01%, t h e d e t e r i o r a t i o n i n
s u r f a c e p r o p e r t y may become o b v i o u s . Thus, each o f
t h e Ca c o n t e n t , t h e Mg c o n t e n t , t h e REM c o n t e n t , and
t h e Z r c o n t e n t is 0.01% o r less. I n o r d e r t o improve
t h e t o u g h n e s s , each of t h e Ca c o n t e n t , t h e Mg
c o n t e n t , t h e REM c o n t e n t , and t h e Z r c o n t e n t is
p r e f e r a b l y 0.0003% o r more. That is, a t l e a s t one of
"Ca: 0.0003% t o 0 . 0 1 % , " "Mg: 0.0003% t o 0 . 0 1 % , " "REM:
0.0003% t o 0 . 0 1 % , " and " Z r : 0.0003% t o 0.01%" is
p r e f e r a b l y s a t i s f i e d .
[0036] REP4 ( r a r e - e a r t h m e t a l ) i n d i c a t e s 17 k i n d s o f
e l e m e n t s i n t o t a l of Sc, Y, and l a n t h a n o i d , and t h e
"REM c o n t e n t " means a t o t a l c o n t e n t of t h e s e 17 k i n d s
of e l e m e n t s . L a n t h a n o i d is i n d u s t r i a l l y added a s a
form o f , f o r example, misch m e t a l .
[00371 (8: 0% t o 0 . 0 1 % )
B i s an e l e m e n t h a v i n g an e f f e c t t o enhance
t o u g h n e s s of t h e s t e e l s h e e t . Thus, B may be
c o n t a i n e d . However, when t h e B c o n t e n t is more t h a n
0.01%, h o t w o r k a b i l i t y may d e t e r i o r a t e , and h o t -
r o l l i n g f o r o b t a i n i n g t h e s t e e l s h e e t f o r h o t
p r e s s i n g may become d i f f i c u l t . Thus, t h e B c o n t e n t
is 0.01% o r less. I n o r d e r t o improve t h e t o u g h n e s s ,
t h e B c o n t e n t is p r e f e r a b l y 0.0003% o r more. That
is, t h e B c o n t e n t is p r e f e r a b l y 0.0003% t o 0.01%.
[0038] ( B i : 0% t o 0 . 0 1 % )
B i is an e l e m e n t h a v i n g an e f f e c t t o u n i f o r m i z e
t h e steel s t r u c t u r e a n d e n h a n c e c r a s h w o r t h i n e s s .
Thus, B i may be c o n t a i n e d . However, when t h e B i
c o n t e n t is more t h a n 0.01%, h o t w o r k a b i l i t y may
d e t e r i o r a t e , and h o t - r o l l i n g f o r o b t a i n i n g t h e s t e e l
s h e e t f o r h o t p r e s s i n g may become d i f f i c u l t . Thus,
t h e B i c o n t e n t is 0.01% o r less. I n o r d e r t o improve
t h e c r a s h w o r t h i n e s s , t h e B i c o n t e n t is p r e f e r a b l y
0.0003% o r more. That i s , t h e B i c o n t e n t is
p r e f e r a b l y 0.0003% t o 0.01%.
[ 0 0 3 9 ] Next, t h e s t e e l s t r u c t u r e of t h e s t e e l s h e e t
member a c c o r d i n g t o t h e embodiment w i l l be d e s c r i b e d .
T h i s steel s h e e t member i n c l u d e s a steel s t r u c t u r e i n
which an a r e a r a t i o of f e r r i t e i n a s u r f a c e l a y e r
p o r t i o n r a n g i n g from t h e s u r f a c e t o 15 urn i n d e p t h i s
e q u a l t o o r less t h a n 1 . 2 0 t i m e s an a r e a r a t i o o f
f e r r i t e i n an i n n e r l a y e r p o r t i o n b e i n g a p o r t i o n
e x c l u d i n g t h e s u r f a c e l a y e r p o r t i o n , and t h e i n n e r
l a y e r p o r t i o n i n c l u d e s t h e steel s t r u c t u r e
r e p r e s e n t e d , i n a r e a % , f e r r i t e : 10% t o 70% and
m a r t e n s i t e : 30% t o 90%, a t o t a l a r e a r a t i o of f e r r i t e
and m a r t e n s i t e : 90% t o 100%. I n t h e i n n e r l a y e r
p o r t i o n , t h e c o n c e n t r a t i a n of Mn i n t h e m a r t e n s i t e is
e q u a l t o o r more t h a n 1.20 t i m e s t h e c o n c e n t r a t i o n of
Mn i n t h e f e r r i t e i n t h e i n n e r l a y e r p o r t i o n . The
s u r f a c e l a y e r p o r t i o n of t h e s t e e l s h e e t member means
a s u r f a c e p o r t i o n r a n g i n g from t h e s u r f a c e to 15 pm
i n d e p t h , and t h e i n n e r l a y e r p o r t i o n means a p o r t i o n
e x c l u d i n g t h i s s u r f a c e l a y e r p o r t i o n . That is, t h e
i n n e r l a y e r p o r t i o n is a p o r t i o n o t h e r t h a n t h e
s u r f a c e l a y e r p o r t i o n of t h e steel s h e e t member.
Each of n u m e r i c a l v a l u e s r e l a t i n g t o t h e s t e e l
s t r u c t u r e o f t h e i n n e r l a y e r p o r t i o n is, f o r example,
an a v e r a g e v a l u e of t h e whole of t h e i n n e r l a y e r
p o r t i o n i n a t h i c k n e s s d i r e c t i o n , b u t it may be
r e p r e s e n t e d by a n u m e r i c a l v a l u e r e l a t i n g t o t h e
s t e e l s t r u c t u r e a t a p o i n t where t h e d e p t h from t h e
s u r f a c e of t h e s t e e l s h e e t member is 1 / 4 of t h e
t h i c k n e s s of t h e steel s h e e t member ( h e r e i n a f t e r ,
t h i s p o i n t is sometimes r e f e r r e d t o a s a " 1 / 4 d e p t h
p o s i t i o n " ) . For example, when t h e t h i c k n e s s o f t h e
s t e e l s h e e t member is 2 . 0 mm, it may b e r e p r e s e n t e d
by a n u m e r i c a l v a l u e a t a p o i n t p o s i t i o n e d a t 0.50 mm
i n d e p t h from t h e s u r f a c e . T h i s is b e c a u s e t h e steel
s t r u c t u r e a t t h e 1 / 4 d e p t h p o s i t i o n i n d i c a t e s an
a v e r a g e s t e e l s t r u c t u r e i n t h e t h i c k n e s s d i r e c t i o n of
t h e s t e e l s h e e t member. Thus, i n t h e p r e s e n t
i n v e n t i o n , t h e a r e a r a t i o of f e r r i t e and t h e a r e a
r a t i o o f m a r t e n s i t e measured a t t h e 1 / 4 d e p t h
p o s i t i o n a r e r e g a r d e d as an a r e a r a t i o of f e r r i t e and
an a r e a r a t i o of m a r t e n s i t e i n t h e i n n e r l a y e r
p o r t i o n r e s p e c t i v e l y . The r e a s o n why t h e s u r f a c e
l a y e r p o r t i o n is d e t e r m i n e d as a s u r f a c e p o r t i o n
r a n g i n g from t h e s u r f a c e t o 15 pm i n d e p t h is b e c a u s e
t h e maximum d e p t h i n a r a n g e where d e c a r b u r i z a t i o n
o c c u r s is n e a r l y 15 pm w i t h i n t h e s t u d i e s by t h e
i n v e n t o r s of t h e p r e s e n t a p p l i c a t i o n .
[0040] (Area r a t i o of f e r r i t e i n t h e s u r f a c e l a y e r
p o r t i o n : e q u a l t o o r less t h a n 1.20 t i m e s t h e a r e a
r a t i o of f e r r i t e i n t h e i n n e r l a y e r p o r t i o n )
When t h e a r e a r a t i o o f f e r r i t e i n t h e s u r f a c e
l a y e r p o r t i o n i s g r e a t e r t h a n 1 . 2 0 t i m e s t h e a r e a
r a t i o o f f e r r i t e i n t h e i n n e r l a y e r p o r t i o n , f e r r i t e
g r a i n b o u n d a r i e s i n t h e s u r f a c e l a y e r p o r t i o n may be
v u l n e r a b l e and t h e t o u g h n e s s may be s i g n i f i c a n t l y
l o w . T h u s , t h e a r e a r a t i o o f f e r r i t e i n t h e s u r f a c e
l a y e r p o r t i o n is e q u a l t o o r less t h a n 1 . 2 0 t i m e s t h e
a r e a r a t i o o f f e r r i t e i n t h e i n n e r l a y e r p o r t i o n .
- 19 -
The a r e a r a t i o of f e r r i t e i n t h e s u r f a c e l a y e r
p o r t i o n i s p r e f e r a b l y e q u a l t o o r l e s s t h a n 1.18 t h e
a r e a r a t i o of f e r r i t e i n t h e i n n e r l a y e r p o r t i o n .
When t h e steel s h e e t f o r h o t p r e s s i n g a c c o r d i n g t o
t h e embodiment of t h e p r e s e n t i n v e n t i o n i s used t o be
s u b j e c t e d t o h o t p r e s s i n g under a l a t e r - d e s c r i b e d
c o n d i t i o n , d e c a r b u r i z a t i o n does n o t e a s i l y o c c u r , and
t h e r e f o r e t h e a r e a r a t i o of f e r r i t e i n t h e s u r f a c e
l a y e r p o r t i o n of t h e steel s h e e t member is liLLc-ly t o
be e q u a l t o or less t h a n 1 . 1 6 t h e a r e a r a t i o o f
f e r r i t e i n t h e i n n e r l a y e r p o r t i o n .
[00411 A t r e a t m e n t t o i n c r e a s e t h e c o n c e n t r a t i o n of
C i n t h e v i c i n i t y of t h e s u r f a c e of t h e s t e e l s h e e t
such a s a c a r b u r i z a t i o n t r e a t m e n t is n o t p e r f o r m e d i n
h e a t i n g i n c o n v e n t i o n a l h o t p r e s s i n g . Thus, t h e a r e a
r a t i o of f e r r i t e i n t h e s u r f a c e l a y e r p o r t i o n d o e s
n o t n o r m a l l y become less t h a n t h e a r e a r a t i o of
f e r r i t e i n t h e i n n e r l a y e r p o r t i o n , and t h e a r e a
r a t i o of f e r r i t e i n t h e s u r f a c e l a y e r p o r t i o n i s
e q u a l t o o r more t h a n 1 . 0 t i m e t h e a r e a r a t i o of
f e r r i t e i n t h e i n n e r l a y e r p o r t i o n .
[0042] (Area r a t i o o f f e r r i t e i n t h e i n n e r l a y e r
p o r t i o n : 10% t o 70%)
A s p e c i f i c amount o f f e r r i t e is made t o e x i s t i n
t h e i n n e r l a y e r p o r t i o n , t h e r e b y making it p o s s i b l e
t o o b t a i n good d u c t i l i t y . When t h e a r e a r a t i o o f
f e r r i t e i n t h e i n n e r l a y e r p o r t i o n is l e s s t h a n l o % ,
most of t h e f e r r i t e may be i s o l a t e d , t o make it
d i f f i c u l t t o o b t a i n good d u c t i l i t y . Thus, t h e area
r a t i o o f f e r r i t e i n t h e i n n e r l a y e r p o r t i o n is 10% o r
more. When t h e a r e a r a t i o o f f e r r i t e i n t h e i n n e r
l a y e r p o r t i o n is g r e a t e r t h a n 708, m a r t e n s i t e b e i n g a
s t r e n g t h e n i n g phase may n o t be s u f f i c i e n t l y s e c u r e d
and it may be d i f f i c u l t t o s e c u r e a t e n s i l e s t r e n g t h
of 980 MPa o r more. Thus, t h e a r e a r a t i o of f e r r i t e
i n t h e i n n e r l a y e r p o r t i o n is 70% o r l e s s . For
s e c u r i n g b e t t e r d u c t i l i t y , t h e a r e a r a t i o of f e r r i t e
i n t h e i n n e r l a y e r p o r t i o n is p r e f e r a b l y 30% o r more.
[0043] ( A r e a r a t i o of m a r t e n s i t e i n t h e i n n e r l a y e r
p o r t i o n : 30% t o 90%)
A specific amount of m a r t e n s i t e is made to e x i s t
i n t h e i n n e r l a y e r p o r t i o n , t h e r e b y making it
p o s s i b l e t o o b t a i n a h i g h s t r e n g t h . When t h e a r e a
r a t i o of m a r t e n s i t e i n t h e i n n e r l a y e r p o r t i o n is
less t h a n 30%, it may be d i f f i c u l t t o s e c u r e a
t e n s i l e s t r e n g t h of 980 MPa o r more. Thus, t h e a r e a
r a t i o of m a r t e n s i t e i n t h e i n n e r l a y e r p o r t i o n is 30%
o r more. When t h e area r a t i o of m a r t e n s i t e i n t h e
i n n e r l a y e r p o r t i o n is g r e a t e r t h a n 90%, t h e a r e a
r a t i o of f e r r i t e becomes less t h a n l o % , r e s u l t i n g i n
t h a t it may be d i f f i c u l t t o o b t a i n good d u c t i l i t y a s
d e s c r i b e d a b o v e . Thus, t h e a r e a r a t i o of m a r t e n s i t e
i n t h e i n n e r l a y e r p o r t i o n i s 90% o r less. For
s e c u r i n g b e t t e r d u c t i l i t y , t h e a r e a r a t i o of
m a r t e n s i t e i n t h e i n n e r l a y e r p o r t i o n is p r e f e r a b l y
70% o r less.
I00441 ( T o t a l a r e a r a t i o o f f e r r i t e and m a r t e n s i t e
i n t h e i n n e r l a y e r p o r t i o n : 90% t o 1008)
The inner layer portion of the hot-pressed steel
sheet member according to the embodiment is
preferably composed of ferrite and martensite,
namely, the total area ratio of ferrite and
martensite is preferably 100%. However, depending on
the manufacturing conditions, one or more selected
from the group consisting of bainite, retained
austenite, cementite, and pearlite may be contained
as a phase or a structure other than ferrite and
martensite. In this case, when the area ratio of the
phase or the structure other than ferrite and
martensite is greater than lo%, target properties may
not be obtained in some cases due to the influence of
the phase or the structure. Accordingly, the area
ratio of the phase or the structure other than
ferrite and martensite in the inner layer portion is
10% or less. That is, the total area ratio of
ferrite and martensite in the inner layer portion is
90% or more.
[00451 As a method of measuring the area ratio of
each phase in the above steel structure, a method
well-known to the skilled person in the art may be
employed. Each of the area ratios is obtained, for
example, as an average value of a value measured in a
cross section perpendicular to a rolling direction
and a value measured in a cross section perpendicular
to a sheet width direction (a direction perpendicular
to the rolling direction). In other words, the area
ratio is obtained, for example, as an average value
of area ratios measured in two cross sections.
[0046] (Concentration of Mn in the martensite in the
inner layer portion: equal to or more than 1.20 times
the concentration of Mn in the ferrite in the inner
layer portion)
When the concentration of Mn in the martensite in
the inner layer portion is less than 1.20 times the
concentration of Mn in the ferrite in the inner layer
portion, the area ratio of ferrite in the surface
layer portion is high inevitably, resulting in that
good toughness may not be obtained. Thus, the
concentration of Mn in the martensite in the inner
layer portion is equal to or more than 1.20 times the
concentration of Mn in the ferrite in the inner layer
portion. The upper limit of this ratio is not
limited in particular, but the ratio does not exceed
3.0.
[0047] The steel sheet member can be manufactured by
treating a specific steel sheet for hot pressing
under specific conditions.
[0048] Here, a steel structure and the like in the
steel sheet for hot pressing used for manufacturing
the steel sheet member according to the embodiment
will be described. This steel sheet for hot pressing
includes a steel structure containing ferrite and
cementite with the total area ratio of bainite and
martensite of 0% to 10% and an area ratio of
cementite of 1% or more. The concentration of Mn in
the cementite is 5% or more.
- 23 -
[0049] (Ferrite and Cementite)
Ferrite and cementite may exist in a manner to be
contained in pearlite, or may also exist
independently of pearlite. As an example of the
steel structure of the steel sheet for hot pressing,
a multi-phase structure of ferrite and pearlite, and
a multi-phase structure of ferrite, pearlite, and
spheroidized cementite are cited. The steel
s t r u t u r e of the steel sheet for hot pressing may
also further contain martensite. When the total area
ratio of ferrite and cementite is less than 90%,
decarburization may be likely to occur during hot
pressing. Thus, the total area ratio of ferrite and
cementite is preferably 90% or more including the
ferrite and cementite contained in pearlite.
[0050] (Area ratio of cementite: 1% or more)
When the area ratio of cementite is less than 1%,
decarburization may be likely to occur during hot
pressing, resulting in that good toughness may not be
easily obtained in the hot-pressed steel sheet member
obtained from this steel sheet for hot pressing.
Thus, the area ratio of cementite is 1% or more.
[0051] (Total area ratio of bainite and martensite:
0% to 10%)
When the total area ratio of bainite and
martensite is greater than lo%, decarburization may
be very likely to occur during hot pressing,
resulting in that good toughness may not be obtained
in the hot-pressed steel sheet member obtained from
t h i s s t e e l s h e e t f o r h o t p r e s s i n g . Thus, t h e t o t a l
a r e a r a t i o of b a i n i t e and m a r t e n s i t e i s 10% o r less.
B a i n i t e and m a r t e n s i t e need n o t t o be c o n t a i n e d .
Then, when t h e t o t a l a r e a r a t i o of b a i n i t e and
m a r t e n s i t e is 10% o r less, good t o u g h n e s s may be
o b t a i n e d i n t h e h o t - p r e s s e d s t e e l s h e e t member a s
l o n g a s f e r r i t e and c e m e n t i t e a r e c o n t a i n e d .
[0052] ( C o n c e n t r a t i o n of Mn i n t h e c e m e n t i t e : 5% o r
more)
When t h e c o n c e n t r a t i o n of Mn i n t h e c e m e n t i t e is
less t h a n 5%, d e c a r b u r i z a t i o n may be l i k e l y t o o c c u r
d u r i n g h o t p r e s s i n g , r e s u l t i n g i n t h a t good t o u g h n e s s
may n o t b e o b t a i n e d i n t h e h o t - p r e s s e d steel s h e e t
member o b t a i n e d from t h i s s t e e l s h e e t f o r h o t
p r e s s i n g . Thus, t h e c o n c e n t r a t i o n of Mn i n t h e
c e m e n t i t e is 5% o r more.
[0053] Next, a method of m a n u f a c t u r i n g t h e s t e e l
s h e e t member a c c o r d i n g t o t h e embodiment, namely, a
method of t r e a t i n g t h e s t e e l s h e e t f o r h o t p r e s s i n g
w i l l be d e s c r i b e d . I n t h e t r e a t m e n t o f t h e s t e e l
s h e e t f o r h o t p r e s s i n g , t h e s t e e l s h e e t f o r h o t
p r e s s i n g is h e a t e d i n a t e m p e r a t u r e zone of 720°C t o
an A c 3 p o i n t , t h e c o n c e n t r a t i o n of Mn i n a u s t e n i t e i s
c a u s e d t o b e e q u a l t o o r more t h a n 1 . 2 0 t i m e s t h e
c o n c e n t r a t i o n of Mn i n t h e f e r r i t e , h o t p r e s s i n g and
c o o l i n g down t o an M s p o i n t at an a v e r a g e c o o l i n g
r a t e o f 1 0 "C/second t o 500 "C/second is p e r f o r m e d
a f t e r t h e h e a t i n g . A r e d u c e d C c o n t e n t on a s u r f a c e
of t h e s t e e l s h e e t f o r h o t p r e s s i n g d u r i n g a t i m e
period from completion of the heating to start of the
hot pressing is less than 0 . 0 0 0 5 mass%.
[0054] (Heating temperature of the steel sheet for
hot pressing: a temperature zone of 720°C to an A c ~
point)
The steel sheet to be subjected to hot pressing,
namely, the steel sheet for hot pressing is heated in
a temperature zone of 720°C to the Ac3 point. The Ac3
point is a temperature (unit: OC) at which Lhe steel
structure becomes an austenite single phase, which is
calculated by the following empirical formula (i).
[0055] Ac3 = 910 - 203 x ( c ' . ~ ) - 1 5 . 2 x Ni + 44.7 x
Si + 104 x V + 3 1 . 5 x Mo - 30 x Mn - 11 w Cr - 20 w
cu + 700 x P + 400 x ~1 + 50 x Ti , . (i)
Here, the element symbol in the above formula
indicates the content (unit: mass%) of each element
in a chemical composition of the steel sheet.
[ 0 0 5 6 ] When the heating temperature is less than
720°C, formation of austenite accompanying solid
solution of cementite may be difficult or
insufficient, resulting in a difficulty in making the
tensile strength of the steel sheet member become 980
MPa or more. Thus, the heating temperature is 720°C
or more. When the heating temperature is greater
than the Ac3 point, the steel structure of the steel
sheet member may become a martensite single phasei
resulting in significant deterioration of ductility.
Thus, the heating temperature is the Ac3 point or
less.
- 26 -
[0057] The h e a t i n g r a t e up t o t h e t e m p e r a t u r e zone
o f 720°C t o t h e Ac3 p o i n t and t h e h e a t i n g t i m e f o r
h o l d i n g a t t h e a b o v e - d e s c r i b e d t e m p e r a t u r e zone a r e
n o t l i m i t e d i n p a r t i c u l a r , b u t t h e y a r e e a c h
p r e f e r a b l y w i t h i n t h e f o l l o w i n g r a n g e .
[0058] An a v e r a g e h e a t i n g r a t e i n t h e h e a t i n g up t o
t h e t e m p e r a t u r e zone of 720°C t o t h e A c 3 p o i n t is
p r e f e r a b l y 0 . 2 OC/second t o 100 OC/second. S e t t i n g
t h e a v e r a g e h e a t i n g r a t e t o 0 . 2 "C/second o r more
makes it p o s s i b l e t o s e c u r e h i g h e r p r o d u c t i v i t y .
F u r t h e r , s e t t i n g t h e a v e r a g e h e a t i n g r a t e t o 100
OC/second o r less makes it e a s y t o c o n t r o l t h e h e a t i n g
t e m p e r a t u r e when it is h e a t e d by u s i n g a normal
f u r n a c e .
[0059] P a r t i c u l a r l y , t h e a v e r a g e h e a t i n g r a t e i n a
t e m p e r a t u r e zone of 600°C t o 720°C is p r e f e r a b l y 0.2
OC/second t o 10 OC/second. T h i s is t o more promote
d i s t r i b u t i o n of Mn between t h e f e r r i t e and t h e
a u s t e n i t e , more promote c o n c e n t r a t i o n of Mn i n t h e
a u s t e n i t e , and t o s u p p r e s s d e c a r b u r i z a t i o n more
s e c u r e l y .
[0060] The h e a t i n g t i m e i n t h e t e m p e r a t u r e zone of
720°C t o t h e Ac3 p o i n t i s p r e f e r a b l y 3 m i n u t e s t o 10
m i n u t e s . The h e a t i n g t i m e is a t i m e p e r i o d from t h e
t i m e which t h e t e m p e r a t u r e of t h e steel s h e e t r e a c h e s
720°C t o a t i m e o f c o m p l e t i o n o f t h e h e a t i n g . The
t i m e of t h e c o m p l e t i o n of t h e h e a t i n g , s p e c i f i c a l l y ,
is t h e t i m e which t h e s t e e l s h e e t is t a k e n o u t of t h e
h e a t i n g f u r n a c e i n t h e c a s e o f f u r n a c e h e a t i n g , and
is the time which energization or the like is turned
off in the case of energization heating or induction
heating. The heating time is 3 minutes or more, and
thereby the distribution of Mn between the ferrite
and the austenite is promoted more securely and the
concentrating of Mn in the austenite is more
promoted, resulting in that decarburization is
further suppressed. Therefore, the area ratio of
ferrite in the surface layer portion of the steel
sheet member becomes likely to be equal to or less
than 1.20 times the area ratio of ferrite in the
inner layer portion. The heating time is 10 minutes
or less, and thereby the steel structure of the steel
sheet member can be made finer, resulting in a
further improvement in impact resistance of the steel
sheet member.
[0061] (Concentration of Mn in the austenite: equal
to or more than 1.20 times the concentration of Mn in
the ferrite)
The concentration of Mn in the austenite is
caused to be equal to or more than 1.2 times the
concentration of Mn in the ferrite by the completion
of the heating. The austenite is more stabilized and
decarburization becomes very unlikely to occur in hot
pressing by causing the concentration of Mn in the
austenite to be equal to or more than 1 . 2 t-imes the
concentration of Mn in the ferrite. When the
concentration of Mn in the austenite is not caused to
be equal to or more than 1.2 times the concentration
of Mn i n t h e f e r r i t e , namely when t h e c o n c e n t r a t i o n
of Mn i n t h e a u s t e n i t e is less t h a n 1 . 2 times t h e
c o n c e n t r a t i o n of Mn i n t h e f e r r i t e a t t h e h e a t i n g end
t i m e , t h e d i s t r i b u t i o n o f Mn between t h e f e r r i t e and
t h e a u s t e n i t e may n o t b e promoted s u f f i c i e n t l y , and
t h e r e f o r e t h e a u s t e n i t e is l i k e l y t o be decomposed,
and d e c a r b u r i z a t i o n may p r o g r e s s e a s i l y w h i l e t h e
sgeel s h e e t is exposed t o t h e a t m o s p h e r e d u r i n g a
t i m e p e r i o d from t h e c o m p l e t i o n of t h e h e a t i n g t o
s t a r t o f t h e h o t p r e s s i n g . Thus, t h e c o n c e n t r a t i o n
o f Mn i n t h e a u s t e n i t e is c a u s e d t o be e q u a l t o o r
more t h a n 1 . 2 t i m e s t h e c o n c e n t r a t i o n of Mn i n t h e
f e r r i t e by t h e c o m p l e t i o n o f t h e h e a t i n g . The upper
l i m i t of t h i s r a t i o is n o t l i m i t e d i n p a r t i c u l a r , b u t
t h e r a t i o d o e s n o t e x c e e d 3 . 0 . The c o n c e n t r a t i o n of
Mn i n t h e a u s t e n i t e and t h e c o n c e n t r a t i o n of Mn i n
t h e f e r r i t e may be a d j u s t e d by t h e c h e m i c a l
c o m p o s i t i o n and t h e s t e e l s t r u c t u r e of t h e steel
s h e e t f o r h o t p r e s s i n g and t h e h e a t i n g c o n d i t i o n .
For example, t h e h e a t i n g t i m e i n t h e t e m p e r a t u r e zone
of 720°C t o t h e Ac3 p o i n t is p r o l o n g e d , t h e r e b y making
it p o s s i b l e t o promote c o n c e n t r a t i n g of Mn i n t h e
a u s t e n i t e .
[ 0 0 6 2 ] (A r e d u c e d C c o n t e n t on t h e s u r f a c e of t h e
s t e e l s h e e t f o r h o t p r e s s i n g d u r i n g t h e t i m e p e r i o d
from t h e c o m p l e t i o n o f t h e h e a t i n g t o s t a r t of t h e
h o t p r e s s i n g : less t h a n 0 . 0 0 0 5 % )
When t h e r e d u c e d C c o n t e n t on t h e s u r f a c e of t h e
s t e e l s h e e t f o r h o t p r e s s i n g d u r i n g t h i s t i m e p e r i o d
is 0.0005% o r more, it may be d i f f i c u l t t o make t h e
a r e a r a t i o of f e r r i t e i n t h e s u r f a c e l a y e r p o r t i o n of
t h e steel s h e e t member become e q u a l t o o r less t h a n
1 . 2 0 t i m e s t h e area r a t i o o f f e r r i t e i n t h e i n n e r
l a y e r p o r t i o n due t o an i n f l u e n c e of d e c a r b u r i z a t i o n .
T h e r e f o r e , it may be d i f f i c u l t t o o b t a i n s u f f i c i e n t
t o u g h n e s s i n t h e steel s h e e t member. Thus, t h i s
reduced C c o n t e n t is less t h a n 0.0005%. The r e d u c e d
C c o n t e n t can be measured by u s i n g a glow d i s c h a r g e
s p e c t r o s c o p e (GDS) o r an e l e c t r o n p r o b e m i c r o
a n a l y z e r (EPMA), f o r example. That is, a s u r f a c e o f
t h e steel s h e e t f o r h o t p r e s s i n g i s a n a l y z e d a t t h e
t i m e of t h e c o m p l e t i o n of t h e h e a t i n g and a t t h e h o t
p r e s s i n g s t a r t t i m e and r e s u l t s of t h e a n a l y s e s a r e
compared, and t h e r e b y t h e reduced C c o n t e n t can be
found.
[0063] A method of a d j u s t i n g t h e reduced C c o n t e n t
is n o t l i m i t e d i n p a r t i c u l a r . For example, t h e steel
s h e e t is sometimes exposed t o t h e a t m o s p h e r e between
e x t r a c t i o n from a h e a t i n g a p p a r a t u s such a s a h e a t i n g
f u r n a c e used f o r t h e a b o v e - d e s c r i b e d h e a t i n g and
i n p u t i n t o a h o t p r e s s i n g a p p a r a t u s , b u t t h i s t i m e
p e r i o d i s p r e f e r a b l y a s s h o r t a s p o s s i b l e and is
p r e f e r a b l y less t h a n 15 seconds a t l o n g e s t , and is
more p r e f e r a b l y 1 0 s e c o n d s o r l e s s . T h i s is b e c a u s e
when t h i s t i m e p e r i o d is 15 seconds or more,
d e c a r b u r i z a t i o n may p r o g r e s s and t h e a r e a r a t i o o f
f e r r i t e i n t h e s u r f a c e l a y e r p o r t i o n of t h e s t e e l
s h e e t member may i n c r e a s e .
[0064] A d j u s t m e n t of t h i s t i m e p e r i o d can be
p e r f o r m e d by c o n t r o l l i n g a t r a n s f e r t i m e from
e x t r a c t i o n from t h e h e a t i n g a p p a r a t u s t o a p r e s s d i e
of t h e h o t p r e s s i n g a p p a r a t u s , f o r example.
[0065] (Average c o o l i n g r a t e down t o t h e M s p o i n t :
n o t less t h a n 10 OC/second n o r more t h a n 500
OC/second)
A f t e r t h e h e a t i n g , h o t p r e s s i n g and c o o l i n g down
t o t h e M s p o i n t a t a n a v e r a g e c o o l i n g r a t e of 10
OC/second t o 500 OC/second is p e r f o r m e d . When t h e
a v e r a g e c o o l i n g r a t e is less t h a n 10 OC/second,
d i f f u s i o n a l t r a n s f o r m a t i o n such a s b a i n i t e
t r a n s f o r m a t i o n may p r o g r e s s e x c e s s i v e l y t o t h e r e b y
make it d i f f i c u l t t o s e c u r e t h e a r e a r a t i o of
m a r t e n s i t e b e i n g a s t r e n g t h e n i n g p h a s e , r e s u l t i n g i n
a d i f f i c u l t y i n making t h e t e n s i l e s t r e n g t h of t h e
steel s h e e t member become 980 MPa o r more. Thus, t h e
a v e r a g e c o o l i n g r a t e is 10 OC/second o r more. When
t h e a v e r a g e c o o l i n g r a t e is g r e a t e r t h a n 500
OC/second, it may become v e r y d i f f i c u l t t o hold
s o a k i n g of t h e member, r e s u l t i n g i n t h a t s t r e n g t h is
no l o n g e r s t a b i l i z e d . Thus, t h e a v e r a g e c o o l i n g r a t e
is 500 OC/second o r l e s s .
[0066] I n t h i s c o o l i n g , h e a t g e n e r a t i o n by phase
t r a n s f o r m a t i o n is l i k e l y t o e x t r e m e l y i n c r e a s e a f t e r
t h e t e m p e r a t u r e r e a c h e s 400°C. T h e r e f o r e , when t h e
c o o l i n g i n a low t e m p e r a t u r e zone of less t h a n 40OoC
is p e r f o r m e d by t h e same method as t h e c o o l i n g i n a
t e m p e r a t u r e zone o f 400°C o r more, it may be d i f f i c u l t
to secure a sufficient average cooling rate in some
cases. It is preferable to perform the cooling down
to the Ms point from 400°C more forcibly than the
cooling down to 400°C. For example, it is preferable
to employ the following method.
LO0671 Generally, the cooling in the hot pressing is
performed by setting a die made of steel used for
forming a heated steel sheet to normal temperature or
a temperature of about several tcns of degrees
centigrade in advance and bringing the steel sheet
into contact with the die. Accordingly, the average
cooling rate can be controlled, for example, by
change in heat capacity with the change in dimension
of the die. The average cooling rate can be also
controlled by changing the material of the die to a
different metal (for example, Cu or the like). The
average cooling rate can be also controlled by using
a water-cooling die and changing the amount of
cooling water flowing through the die. The average
cooling rate can be also controlled by forming a
plurality of grooves in the die in advance and
passing water through the grooves during hot
pressing. The average cooling rate can be also
controlled by raising a hot pressing machine in the
middle of hot pressing and passing water through its
space. The average cooling rate can be also
controlled by adjusting a die clearance and changing
a contact area of the die with the steel sheet.
[ 0 0 6 8 ] Examples of the method of increasing the
cooling rate at around 400°C and below include the
following three kinds.
(a) Immediately after reaching 400°C, the steel
sheet is moved to a die different in heat capacity or
a die at room temperature.
(b) A water-cooling die is used and the water
f10.w rate through the die is increased immediately
after reaching 400°C.
(c) Immediately after reaching 400°c, water is
passed between the die and the steel sheet. In this
method, the cooling rate may be further increased by
increasing the quantity of water according to
temperature.
[ 0 0 6 9 ] The mode of the forming in the hot pressing
in the embodiment is not particularly limited.
Examples of the mode of the forming include bending,
drawing, bulging, hole expansion, and flanging. The
mode of the forming may be appropriately selected
depending on the kind of a target steel sheet member.
Representative examples of the steel sheet member
include a door guard bar, a bumper reinforcement and
the like which are automobile reinforcing components.
The hot forming is not limited to the hot pressing as
long as the steel sheet can be cooled simultaneously
with forming or immediately after forming. For
example, roll forming may be performed as the hot
forming.
[ 0 0 7 0 ] Such a series of treatments are performed on
the above-described steel sheet for hot pressing,
thereby the steel sheet member according to the
embodiment can be manufactured. In other words, it
is possible to obtain a hot-pressed steel sheet
member having a desired steel structure, a tensile
strength of 980 MPa or more, and excellent ductility
and ~oughness.
[00711 For example, the ductility can be evaluated
by a total elongation (EL) in a tensile test, and the
total elongation in the tensile test is preferably
12% or more in the embodiment. The total elongation
is more preferably 14% or more.
[00721 After the hot pressing and cooling, shot
blasting may be performed. By the shot blasting,
scale can be removed. The shot blasting also has an
effect of introducing a compressive stress into the
surface of the steel sheet member, and therefore
effects of suppressing delayed fracture and improving
a fatigue strength can be also obtained.
(00731 In the above-described method of
manufacturing the steel sheet member, the hot
pressing is not accompanied by preforming, the steel
sheet for hot pressing is heated to the temperature
zone of 720°C to the Ac3 point to cause austenite
transformation to some extent, and then is formed.
Thus, the mechanical properties of the steel sheet
for hot pressing at room temperature before heating
are not important. Therefore, as the steel sheet for
hot pressing, for example, a hot-rolled steel sheet,
a cold-rolled steel sheet, a plated steel sheet and
the like may be used. Examples of the hot-rolled
steel sheet include one containing a multi-phase
structure of ferrite and pearlite and one containing
spheroidized cementite after spheroidizing annealing
at a temperature of 650°C to 700°C. Examples of the
cold-rolled steel sheet include a full hard material
and an annealed material. Examples of the plated
steel sheet include an aluminum plated steel sheet
and a zinc plated steel sheet. Their manufacturing
methods are not particularly limited. When the hotrolled
steel sheet or the full hard material is used,
the distribution of Mn during heating of the hot
pressing is more likely to be promoted in the case of
the steel structure being a multi-phase structure of
ferrite and pearlite. When the annealed material is
used, the distribution of Mn during heating of the
hot pressing is more likely to be promoted when an
annealing temperature is in a ferrite and austenite
two-phase temperature zone.
[ 0 0 7 4 ] The steel sheet member according to the
embodiment can also be manufactured by going through
hot pressing with preforming. For example, in a
range where the above-described conditions of the
heating, the decarburization treatment, and the
cooling are satisfied, the hot-pressed steel sheet
member may be manufactured by preforming by press
working of the steel sheet for hot pressing using a
die in a specific shape, putting it into the same
type of die, applying a pressing force thereto, and
- 35 -
rapidly cooling it. Also in this case, the kind of
the steel sheet for hot pressing and its stccl
structure are not limited, but it is preferable to
use a steel sheet that has a strength as low as
possible and has ductility. For example, the tensile
strength is preferably 700 MPa or less. ' A coiling
temperature after the hot-rolling of the hot-rolled
steel sheet is preferably 450°C or higher in order to
obtain a soft steel sheet, and is preferably 700°C or
lower in order to reduce scale loss. In the coldrolled
steel sheet, annealing is preferable to obtain
a soft steel sheet, and the annealing temperature is
preferably an Acl point to an Ac3 point. The average
cooling rate down to room temperature after annealing
is preferably an upper critical cooling rate or
lower.
[ 0 0 7 5 ] It should be noted that the above-described
embodiment merely illustrates a concrete example of
implementing the present invention, and the technical
scope of the present invention is not to be construed
in a restrictive manner by the embodiment. That is,
the present invention may be implemented in various
forms without departing from the technical spirit or
main features thereof.
EXAMPLE
100761 Next, the experiment performed by the
inventors of the present application will be
described. In this experiment, first, 17 kinds of
steel materials having chemical compositions listed
i n T a b l e 1 were used t o f a b r i c a t e 24 k i n d s o f s t e e l
s h e e t s f o r h o t p r e s s i n g ( s t e e l s h e e t s t o b e s u b j e c t e d
t o a h e a t t r e a t m e n t ) h a v i n g s t e e l s t r u c t u r e s l i s t e d
i n T a b l e 2. The b a l a n c e o f e a c h s t e e l m a t e r i a l was
Fe and i m p u r i t i e s . F u r t h e r , a r e a r a t i o s of f e r r i t e
and c e m e n t i t e c o n t a i n e d i n p e a r l i t e a r e a l s o i n c l u d e d
i n t h e t o t a l a r e a r a t i o o f f e r r i t e and c e m e n t i t e i n
T a b l e 2 . I n t h e f a b r i c a t i o n of t h e s t e e l s h e e t t o be
s u b j e c t e d t o a h e a t t r e a t m e n t , f i r s t , s l a b s p r e p a r e d
i n a l a b o r a t o r y were e a c h h e a t e d a t 1250°C f o r 30
m i n u t e s and h o t r o l l e d t o 2.6 mm i n t h i c k n e s s a t a
t e m p e r a t u r e o f 900°C o r more. Then, t h e r e s u l t a n t
p r o d u c t s were each c o o l e d down t o 60OoC u s i n g a water
s p r a y and c h a r g e d i n t o a f u r n a c e t o be h e l d f o r 30
m i n u t e s a t 600°C. T h e r e a f t e r , slow c o o l i n g was
p e r f o r m e d down t o t h e room t e m p e r a t u r e a t 20 OC/hour.
T h i s c o o l i n g p r o c e s s is one s i m u l a t i n g a c o i l i n g s t e p
i n h o t r o l l i n g . The s t e e l s t r u c t u r e s of h o t - r o l l e d
s t e e l s h e e t s o b t a i n e d as above each were a m u l t i -
p h a s e s t r u c t u r e of f e r r i t e and p e a r l i t e .
COO771 Next, s c a l e s were removed from e a c h of t h e
h o t - r o l l e d s t e e l s h e e t s , and t h e n t h e h o t - r o l l e d
s t e e l . s h e e t s were e a c h c o l d r o l l e d t o 1 . 2 mm i n
t h i c k n e s s , e x c l u d i n g a sample m a t e r i a l No. 2 1 by
p i c k l i n g . As f o r a sample m a t e r i a l No. 6, a c o l d -
r o l l e d s t e e l . s h e e t o b t a i n e d by t h e c o l d r o l l i n g was
a n n e a l e d i n an a u s t e n i t e s i n g l e - p h a s e r e g i o n a f t e r
t h e c o l d r o l l i n g . A s f o r a sample m a t e r i a l N o . 19, a
c o l d - r o l l e d s t e e l s h e e t o b t a i n e d by t h e c o l d r o l l i n g
was a n n e a l e d i n a f e r r i t e and a u s t e n i t e two-phase
r e g i o n a f t e r t h e c o l d r o l l i n g , and f u r t h e r was
s u b j e c t e d t o h o t - d i p g a l v a n i z i n g w i t h a c o a t i n g
weight p e r one s i d e of 60 g / m 2 .
100781 A s f o r t h e sample m a t e r i a l No. 21, s c a l e s
were removed from t h e h o t - r o l l e d steel s h e e t by
p i c k l i n g , and t h e r e a f t e r s p h e r o i d i z i n g a n n e a l i n g was
performed. I n t h i s s p h e r o i d i z i n g a n n e a l i n g , t h e h o t -
r o l l e d s t e e l s h e e t was h e l d a t 650°C f u r 5 k u u r s .
[0079] A f t e r t h e f a b r i c a t i o n of t h e s t e e l s h e e t s t o
be s u b j e c t e d t o a h e a t t r e a t m e n t , t h e steel s h e e t s
were h e a t e d i n a g a s h e a t i n g f u r n a c e w i t h an a i r - f u e l
r a t i o of 0.85 under c o n d i t i o n s l i s t e d i n T a b l e 2. I n
T a b l e 2, "HEATING TIME'' i n d i c a t e s a t i m e p e r i o d from
when t h e steel s h e e t is charged i n t o t h e g a s h e a t i n g
f u r n a c e and t h e n t h e t e m p e r a t u r e of t h e steel s h e e t
r e a c h e s 720°C t o when t h e steel s h e e t is t a k e n o u t of
t h e g a s h e a t i n g f u r n a c e . I n Table 2, "HEATING
TEMPERATURE" i n d i c a t e s n o t t h e t e m p e r a t u r e of t h e
s t e e l s h e e t b u t t h e t e m p e r a t u r e i n s i d e t h e g a s
h e a t i n g f u r n a c e . Then, t h e steel s h e e t s were each
t a k e n o u t of t h e g a s h e a t i n g f u r n a c e , a i r c o o l i n g w a s
performed f o r v a r i o u s t i m e p e r i o d s , h o t p r e s s i n g of
each of t h e steel s h e e t s was p e r f o r m e d , a n d t h e s t e e l
s h e e t s were each c o o l e d a f t e r t h e h o t p r e s s i n g . I n
t h e h o t p r e s s i n g , a f l a t d i e made of steel was u s e d ,
That i s , forming was n o t performed. When c o o l i n g t h e
steel s h e e t , t h e s t e e l s h e e t was c o o l e d down t o t h e
M s p o i n t a t an a v e r a g e c o o l i n g r a t e l i s t e d i n Table 2
with leaving the steel sheet in contact with the die,
and further cooled down to 150°C, and then the steel
sheet was taken out of the die to let the steel sheet
cool. When cooling down to 150°C, the periphery of
the die was cooled by cooling water until the
temperature of the steel sheet became 150°C, or a die
adjusted to the normal temperature was prepared, and
then the steel sheet was held in the die until the
temperature of the steel sheet became 150°C. In a
measurement of the average cooling rate down to 150°C,
a thermocouple was attached to the steel sheet in
advance, and temperature history of the steel sheet
was analyzed. In this manner, 24 types of sample
materials (sample steel sheets) were fabricated. The
sample material (sample steel sheet) is sometimes
referred to as a "hot-pressed steel sheet" below.
[0080] [Table 11
[0081] [Table 21
[00821 A f t e r t h e h o t - p r e s s e d steel s h e e t s were
o b t a i n e d , r e g a r d i n g each of t h e s e steel s h e e t s , an
area ratio of ferrite in the surface layer portion,
an area ratio of ferrite in the inner layer portion,
and an area ratio of martensite in the inner layer
portion were found. These area ratios each are an
average value of values calculated by performing an
image analysis of optical microscope observation
images or electron microscope observation images of
two cross sections: a cross section perpendicular to
the rolling direction; and a cross section
perpendicular to the sheet width direction (direction
perpendicular to the rolling direction). In an
observation of the steel structure of the surface
layer portion, the region ranging from the surface of
the steel sheet to 15 pm in depth was observed. In
an observation of the steel structure of the inner
layer portion, it was observed at the l/4 depth
position. The ratio of the area ratio of ferrite in
the surface layer portion to the area ratio of
ferrite in the inner layer portion, and the area
ratio of ferrite and the area ratio of martensite in
the inner layer portion are listed in Table 3.
[0003] The mechanical properties of the hot-pressed
steel sheets were also examined. In this
examination, measurements of a tensile strength ( T S )
and total elongation (EL), and evaluation of
toughness were performed. In the measurements of the
tensile strength and the total elongation, a JIS No.
5 tensile test piece was taken from each of the steel
sheets in a direction perpendicular to the rolling
d i r e c t i o n t o be s u b j e c t e d t o a t e n s i l e t e s t . I n t h e
e v a l u a t i o n o f t o u g h n e s s , a Charpy impact t e s t was
performed a t O°C t o measure a p e r c e n t a g e b r i t t l e
f r a c t u r e . I n a f a b r i c a t i o n of samples f o r t h e Charpy
impact t e s t , f o u r V-notch t e s t p i e c e s were t a k e n from
each of t h e s t e e l s h e e t s , and t h e s e were s t a c k e d t o
be s c r e w e d - t o g e t h e r . These e x a m i n a t i o n r e s u l t s a r e
a l s o l i s t e d i n T a b l e 3 . Regarding e a c h o f t h e h o t -
p r e s s e d s t e e l s h e e t s , h o t p r e s s i n g u s i n g a f l a t d i e
made of s t e e l w a s performed, b u t forming was n o t
p e r f o r m e d a t t h e t i m e of h o t p r e s s i n g . However, t h e
m e c h a n i c a l p r o p e r t i e s of each of t h e s e h o t - p r e s s e d
steel s h e e t s r e f l e c t m e c h a n i c a l p r o p ' e r t i e s of t h e
h o t - p r e s s e d s t e e l s h e e t member f a b r i c a t e d by b e i n g
s u b j e c t e d t o t h e same t h e r m a l h i s t o r y as t h a t of t h e
h o t p r e s s i n g i n t h i s e x p e r i m e n t a t t h e t i m e of
f o r m i n g . That is, a s l o n g a s t h e t h e r m a l h i s t o r y is
s u b s t a n t i a l l y t h e s a m e r e g a r d l e s s o f whether o r n o t
forming is p e r f o r m e d a t t h e t i m e of h o t p r e s s i n g , t h e
m e c h a n i c a l p r o p e r t i e s t h e r e a f t e r become s u b s t a n t i a l l y
t h e same.
[0084] The c o n c e n t r a t i o n of Mn i n f e r r i t e and t h e
c o n c e n t r a t i o n of Mn i n a u s t e n i t e i m m e d i a t e l y a f t e r
t h e h e a t i n g were measured by u s i n g an e l e c t r o n probe
micro a n a l y z e r (EPMA). I n t h i s measurement, h e a t i n g
under t h e c o n d i t i o n s l i s t e d i n T a b l e 2 was performed
i n t h e g a s h e a t i n g f u r n a c e and w a t e r c o o l i n g was
performed i m m e d i a t e l y a f t e r b e i n g t a k e n o u t of t h e
g a s h e a t i n g f u r n a c e i n o r d e r t o h o l d t h e steel
structure immediately after the heating. By this
water cooling, the austenite was transformed into
martensite without diffusion and the ferrite was held
as it was. Thus, the concentration of Mn in the
ferrite after the water cooling corresponded to the
concentration of Mn in the ferrite immediately after
the heating, and-the concentration of Mn in the
martensite after the water cooling corresponded to
the concentration of Mn in the austeniLe i~it~nediately
after the heating. Then, the ratio of the
concentration of Mn in the austenite to the
concentration of Mn in the ferrite (Mn ratio) was
calculated. This result is also listed in Table 3.
[00851 [Table 31
[ 0 0 8 6 ] As listed in Table 3, the sample materials
No. 1, No. 3, No. 5, No. 8 to No. 10, No. 12, No. 13,
- 45 -
No. 15, No. 17 t o No. 19, No. 21, and No. 22 each
b e i n g a p r e s e n t i n v e n t i o n example e x h i b i t e d e x c e l l e n t
d u c t i 1 , i t y and t o u g h n e s s . That is, a t e n s i l e s t r e n g t h
of 980 MPa o r more ( T S ) , t o t a l e l o n g a t i o n of 12% o r
more ( E L ) , and a p e r c e n t a g e b r i t t l e f r a c t u r e of 10%
o r less were o b t a i n e d .
100871 On t h e o t h e r hand, i n t h e sample m a t e r i a l No.
2, a t e n s i l e s t r e n g t h of 980 MPa o r more was n o t
o b t a i n e d a f t e r c o o l i n g ( a f t e r a n l l e d l i n g ) b e c a u s e t h e
c h e m i c a l c o m p o s i t i o n was o u t s i d e t h e r a n g e of t h e
p r e s e n t i n v e n t i o n . I n t h e sample m a t e r i a l s No. 4 and
No. 7, a d e s i r e d s t e e l s t r u c t u r e was n o t o b t a i n e d and
a t e n s i l e s t r e n g t h of 980 MPa o r more was n o t
o b t a i n e d a f t e r c o o l i n g ( a f t e r a n n e a l i n g ) b e c a u s e t h e
m a n u f a c t u r i n g c o n d i t i o n was o u t s i d e t h e r a n g e o f t h e
p r e s e n t i n v e n t i o n and t h e steel s t r u c t u r e a f t e r h o t
p r e s s i n g w a s a l s o o u t s i d e t h e r a n g e of t h e p r e s e n t
i n v e n t i o n . I n t h e sample m a t e r i a l No. 6, e x c e s s i v e
d e c a r b u r i z a t i o n o c c u r r e d b e c a u s e t h e steel s t r u c t u r e
of t h e steel s h e e t t o be s u b j e c t e d t o a h e a t
t r e a t m e n t was o u t s i d e t h e r a n g e o f t h e p r e s e n t
i n v e n t i o n . That is, t h e m a n u f a c t u r i n g c o n d i t i o n was
o u t s i d e t h e r a n g e o f t h e p r e s e n t i n v e n t i o n . The
s t e e l s t r u c t u r e a f t e r h o t p r e s s i n g was a l s o o u t s i d e
t h e r a n g e of t h e p r e s e n t i n v e n t i o n . T h e r e f o r e , a
d e s i r e d steel s t r u c t u r e was n o t o b t a i n e d and t h e
p e r c e n t a g e b r i t t l e f r a c t u r e was g r e a t e r t h a n 10%. I n
t h e sample m a t e r i a l 11, t h e t o t a l e l o n g a t i o n was le'ss
t h a n 12% b e c a u s e t h e c h e m i c a l c o m p o s i t i o n was o u t s i d e
t h e r a n g e o f t h e p r e s e n t i n v e n t i o n . I n t h e sample
m a t e r i a l No. 1 4 , t h e t o t a l e l o n g a t i o n was less t h a n
12% b e c a u s e t h e m a n u f a c t u r i n g c o n d i t i o n was o u t s i d e
t h e r a n g e o f t h e p r e s e n t i n v e n t i o n and t h e s t e e l
s t r u c t u r e a f t e r h o t p r e s s i n g was a l s o o u t s i d e t h e
r a n g e o f t h e p r e s e n t i n v e n t i o n . I n t h e sample
m a t e r i a l No.. 16, a d e s i r e d steel s t r u c t u r e was n o t
o b t a i n e d and t h e p e r c e n t a g e b r i t t l e f r a c t u r e was
g r e a t e r t h a n 10% b e c a u s e t h e m a n u f a c t u r i n g c o n d i t i o n
was o u t s i d e t h e r a n g e of t h e p r e s e n t i n v e n t i o n and
t h e s t e e l s t r u c t u r e a f t e r h o t p r e s s i n g was a l s o
o u t s i d e t h e r a n g e o f t h e p r e s e n t i n v e n t i o n . I n t h e
sample m a t e r i a l No. 20, a t e n s i l e s t r e n g t h of 980 MPa
o r more was n o t o b t a i n e d ' a f t e r c o o l i n g ( a f t e r
a n n e a l i n g ) b e c a u s e t h e c h e m i c a l c o m p o s i t i o n was
o u t s i d e t h e r a n g e o f t h e p r e s e n t i n v e n t i o n . F u r t h e r ,
e x c e s s i v e d e c a r b u r i z a t i o n o c c u r r e d b e c a u s e t h e steel
s t r u c t u r e of t h e steel s h e e t t o be s u b j e c t e d t o a
h e a t t r e a t m e n t was o u t s i d e t h e r a n g e o f t h e p r e s e n t
i n v e n t i o n . That is, t h e m a n u f a c t u r i n g c o n d i t i o n was
o u t s i d e t h e r a n g e of t h e p r e s e n t i n v e n t i o n .
T h e r e f o r e , a d e s i r e d s t e e l s t r u c t u r e was n o t o b t a i n e d
and t h e p e r c e n t a g e b r i t t l e f r a c t u r e was g r e a t e r t h a n
10%. I n t h e sample m a t e r i a l No. 23, e x c e s s i v e
d e c a r b u r i z a t i o n o c c u r r e d b e c a u s e t h e s t e e l s t r u c t u r e
of t h e s t e e l s h e e t t o be s u b j e c t e d t o a h e a t
t r e a t m e n t was o u t s i d e t h e r a n g e o f t h e p r e s e n t
i n v e n t i o n . That i s , t h e m a n u f a c t u r i n g c o n d i t i o n was
o u t s i d e t h e r a n g e o f t h e p r e s e n t i n v e n t i o n .
Therefore, a desired steel structure was not obtained
and the percentage brittle fracture was greater than
10%. In the sample material No. 24, excessive
decarburization occurred because the concentration of
Mn in the cementite of the steel sheet to be
subjected to a heat treatment was outside the range
of the present invention. That is, the manufact-uring
condition was outside the range of the present
invention. Therefore, a desired steel structure was
not obtained and the percentage brittle fracture was
greater than 10%.
INDUSTRIAL APPLICABILITY
100881 The present invention may be used for, for
example, industries of manufacturing and using
automobile body structural components and so on in
which imporLance is placed on excellent ductility and
toughness. The present invention may be used also
for industries of manufacturing and using other
machine structural components, and so on.
CLAIMS
[Claim I] A hot-pressed steel sheet member,
comprising:
a chemical composition represented by, in mass%:
C: 0.10% to 0.34%;
Si: 0.5% to 2.0%;
Mn: 1.0% to 3.0%;
sol. Al: 0.001% to 1.0%;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Ti: 0% to 0.20%;
Nb: 0% to 0.20%;
V: 0% to 0.20%;
Cr: 0% to 1.0%;
Mo: 0% to 1.0%;
cu: 0% to 1.0%;
Ni: 0% to 1.0%;
Ca: 0% to 0.01%;
Mg: 0% to 0.01%;
REM: 0% to 0.01%;
Zr: 0% to 0.01%;
B: 0% to 0.01%;
Bi: 0% to 0.01%; and
balance: Fe and impurities; and
a steel structure in which:
an area ratio of ferrite in a surface layer
portion ranging from a surface to 15 pm in depth is
equal to or less than 1.20 times an area ratio of
ferrite in an inner layer portion being a portion
excluding the surface layer portion; and
the inner layer portion comprises a steel
structure represented, in area%:
ferrite: 10% to 70%;
martensite: 30% to 90%; and
a total area ratio of ferrite and
martensite: 90% to loo%,
wherein a concentraLion of Mn in the martensite
is equal to or more than 1.20 times a concentration
of Mn in the ferrite, in the inner layer portion, and
wherein a tensile strength of the hot-pressed
steel sheet member is 980 MPa or more.
[Claim 21 The hot-pressed steel sheet member
according to claim 1, wherein the chemical
composition comprises one or more selected from the
group consisting of, in mass%:
Ti: 0.003% to 0.20%;
Nb: 0.003% to 0.20%;
V: 0.003% to 0.20%;
Cr: 0.005% to 1.0%;
Mo: 0.005% to 1.0%;
Cu: 0.005% to 1.0%; and
Ni: 0.005% to 1.0%.
[Claim 31 The hot-pressed steel sheet member
according to claim 1 or 2, wherein the chemical
composition comprises one or more selected from the
group consisting of, in mass%:
Ca: 0.0003% to 0.01%;
Kg: 0.0003% to 0.01%;
REM: 0.0003% to 0.01%; and
Zr: 0.0003% to 0.01%.
[Claim 41 The hot-pressed steel sheet member
according to any one of claims 1 to 3, wherein the
chemical composition comprises, in mass%, B: 0.0003%
to 0.01%.
[Claim 51 The hot-pressed steel sheet member
according to any one of claims 1 to 4, wherein the
chemical composition comprises, in mass%, Bi: 0.0003%
to 0.01%.
[Claim 61 A steel sheet for hot pressing,
comprising:
a chemical composition represented by, in mass%:
c: 0.10% to 0.34%;
Si: 0.5% to 2.0%;
Mn: 1.0% to 3.0%;
sol. Al: 0.001% to 1.0% or less;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Ti: 0% to 0.20%;
Nb: 0% to 0.20%;
v: 0% to 0.20%;
Cr: 0% to 1.0%;
Mo: 0% to 1.0%;
Cu: 0% to 1.0%;
Ni: 0% to 1.0%;
Ca: 0% to 0.01%:
Mg: 0% to 0.01%;
REM,: 0% to 0.01%;
Zr: 0% to 0.01%;
B: 0% to 0.01%;
Bi: 0% to 0.01%; and
balance: Fe and impurities; and
a steel structure comprising ferrite and
cementite, represented, in area%:
a total area ratio of bainite and
martensite: 0% to 10%; and
an area ratio of cementite: 1% or more, and
wherein a concentration of Mn in the cementite is
5% or more.
[Claim 71 The steel sheet for hot pressing according
to claim 6, wherein the chemical composition
comprises one or more selected from the group
consisting of, in mass%:
Ti: 0.003% to 0.20%;
Nb: 0.003% to 0.20%;
V: 0.003% to 0.20%;
Cr: 0.005% to 1.0%;
Mo: 0.005% to 1.0%;
Cu: 0.005% to 1.0%; and
Ni: 0.005% to 1.0%.
[Claim 81 The steel sheet for hot pressing according
to claim 6 or 7, wherein the chemical composition
comprises one or more selected from the group
consisting of, in mass%:
Ca: 0.0003% to 0.01%;
Mg: 0.0003% t o 0.01%;
REM: 0.0003% t o 0.01%; and
Z r : 0.0003% t o 0.01%.
[Claim 91 The s t e e l s h e e t f o r h o t p r e s s i n g a c c o r d i n g
t o any one of claims 6 t o 8, wherein t h e c h e m i c a l
c o m p o s i t i o n c o m p r i s e s , i n mass%, 8 : 0.0003% t o 0.01%.
[Claim 101 The s t e e l s h e e t f o r h o t p r e s s i n g
a c c o r d i n g t o any one o f c l a i m s 6 t o 9, wherein t h e
c h e m i c a l c o m p o s i t i o n c o m p r i s e s , i n mass%, B i : 0.0003%
t o 0.01%.
[Claim 111 A method of m a n u f a c t u r i n g a h o t - p r e s s e d
s t e e l s h e e t member, c o m p r i s i n g :
a s t e p of h e a t i n g t h e steel s h e e t f o r h o t
p r e s s i n g a c c o r d i n g t o any one o f claims 6 t o 10 i n a
t e m p e r a t u r e zone o f 720°C t o an Ac3 p o i n t f o r c a u s i n g
a c o n c e n t r a t i o n of Mn i n a u s t e n i t e t o be e q u a l t o o r
more t h a n 1 . 2 0 t i m e s a c o n c e n t r a t i o n of Mn i n t h e
f e r r i t e ; and
a s t e p o f h o t p r e s s i n g and c o o l i n g down t o an M s
p o i n t a t an a v e r a g e c o o l i n g r a t e o f 1 0 OC/second t o
500 OC/second a f t e r t h e h e a t i n g ,
wherein a reduced C c o n t e n t on a s u r f a c e o f t h e
s t e e l s h e e t f o r h o t p r e s s i n g d u r i n g a t i m e p e r i o d
from c o m p l e t i o n of t h e s t e p o f h e a t i n g t o s t a r t of
t h e s t e p o f h o t p r e s s i n g is less t h a n 0 . 0 0 0 5 m a s s % .
[Claim 121 The method o f m a n u f a c t u r i n g t h e h o t -
p r e s s e d steel s h e e t member a c c o r d i n g t o c l a i m 11,
wherein a t i m e p e r i o d f o r which t h e steel s h e e t f o r
h o t p r e s s i n g is exposed t o t h e a t m o s p h e r e d u r i n g t h e
time period from completion of the step of heating to
start of the step of hot pressing is less than 15
seconds.