Manufacturing method for hot press formed steel member
Provided is a method for manufacturing a hot press formed steel member that has high strengths, an excellent balance between strength and ductility, and good deformation properties upon crush on collision (crashworthiness). The manufacturing method is highly efficient and allows a high degree of freedom with respect to the shape to be formed. The method manufactures a steel member by heating a steel sheet having a specific chemical composition and subjecting the steel sheet to at least one time of hot press forming. In the method, the heating temperature is equal to or higher than the Ac 3 transformation temperature, and a starting temperature of the hot press forming is in the range from the heating temperature to martensite start (Ms) temperature. Cooling from [(Ms temperature)−150° C.] down to 80° C. is performed so that a tempering parameter (λ) specified by Expression (1) is in the range from 7100 to 8030.
1. A method for manufacturing a hot press formed steel member, the method comprising heating a steel sheet and subjecting the steel sheet to at least one time of hot press forming, the steel sheet comprising, as a chemical composition in mass percent,
C in a content of 0.15% to 0.4%;
Si in a content of greater than 1.0% to 1.65%;
Al in a content of 0.5% or less, excluding 0%;
Mn in a content of 1% to 3.5%;
Ti in a content of 0.10% or less, excluding 0%;
B in a content of 0.005% or less, excluding 0%; and
iron,
wherein:
a heating temperature is equal to or higher than Ac 3 transformation temperature,
a starting temperature of the hot press forming is in a range from the heating temperature to a martensite start (Ms) temperature, and
cooling from [(Ms temperature)−150° C.] down to 80° C. is performed so that a tempering parameter (λ) as specified by Expression (1) is in a range from 7100 to 8030:
λ
=
293
×
{
log
(
∑
n
=
1
5000
t
n
′
3600
)
+
20
}
-
430
×
[
Si
]
(
1
)
where λ represents the tempering parameter; [Si] represents the content, in mass percent, of Si in the steel; and t n ′ represents a value as specified by Expression (2):
t
n
′
=
3600
×
10
^
{
T
n
-
1
+
273
293
×
(
log
t
n
-
t
n
-
1
3600
+
20
)
-
20
}
(
2
)
where t n represents an n-th time, in seconds, when a total cooling time from [(Ms temperature)−150° C.] down to 80° C. is divided into 5000 equal parts; T n represents a temperature (° C.) at the n-th time t n , where t 0 is 0 second, and T 0 is [(Ms temperature)−150° C.]; and 10^{ } represents { }-th power of 10.
2. The manufacturing method according to claim 1 ,
wherein the cooling from [(Ms temperature)−150° C.] down to 80° C. is performed at an average cooling rate of 5° C./s to 20° C./s.
3. The manufacturing method according to claim 1 ,
wherein a finishing temperature of final hot press forming in the at least one time of hot press forming is equal to or lower than the Ms temperature.
4. The manufacturing method according to claim 1 ,
wherein the heating of the steel sheet up to the heating temperature is performed at an average rate of temperature rise of 5° C./s or more in a temperature range from 100° C. up to the heating temperature.
5. The manufacturing method according to claim 1 , comprising
tempering at a temperature from 100° C. to lower than 600° C. after the hot press forming.
6. The manufacturing method according to claim 1 ,
wherein the steel sheet further comprises, in the chemical composition, at least one element selected from the group consisting of:
(a) Cr in a content of 5% or less, excluding 0%;
(b) at least one element selected from the group consisting of Ni and Cu in a total content of 0.5% or less, excluding 0%;
(c) Mo in a content of 1% or less, excluding 0%; and
(d) Nb in a content of 0.1% or less, excluding 0%.