Method of manufacturing hot press-formed member having excellent productivity, weldability and formability
An method of manufacturing a hot press-formed member comprises heating a blank of an aluminum-based plated steel sheet in a heating furnace, removing the heated blank from the heating furnace and conveying the removed blank between an upper mold portion and a lower mold portion of a mold, mounted on a press, to be seated; and performing a forming process after the upper mold portion of the mold is in contact with the seated blank.
1. A method of manufacturing a hot press-formed member, the method comprising:
heating a blank of an aluminum-based plated steel sheet in a heating furnace;
removing the heated blank from the heating furnace and conveying the removed blank between an upper mold portion and a lower mold portion of a mold, mounted on a press, to be seated; and
performing a forming process after the upper mold portion of the mold is in contact with the seated blank,
wherein the heating furnace is a continuous heating furnace comprising section A, section B, and section C provided in a conveying direction of a blank,
heating in section A satisfies conditions specified by a figure ‘abcde’ having cumulative in-furnace maintaining times and in-furnace atmospheric temperature coordinates of approximately a(0.2 min, 750° C.), b(1.0 min, 750° C.), c(1.0 min, 800° C.), d(1.5 min, 900° C.), and e(0.2 min, 900° C.),
heating in section B satisfies conditions specified by a figure ‘fghi’ having cumulative in-furnace maintaining times and atmospheric temperature coordinates of approximately
f
(
1.3
+
t
-
1.2
0.6
×
0.5
min
,
930
°
C
)
,
g
(
3.8
+
t
-
1.2
0.6
×
0.5
min
,
930
°
C
)
,
h
(
3.3
+
t
-
1.2
0.6
×
0.5
min
,
960
°
C
)
,
and
i
(
0.8
+
t
-
1.2
0.6
×
0.5
min
,
960
°
C
)
,
and
heating in section C satisfies conditions specified by a figure ‘jklm’ having cumulative in-furnace maintaining times and atmospheric temperature coordinates of approximately
j
(
3.7
+
t
-
1.2
0.6
min
,
870
°
C
)
,
k
(
11.7
+
t
-
1.2
0.6
×
2
min
,
870
°
C
)
,
1
(
7.03
+
t
-
1.2
0.6
×
2
min
,
940
°
C
)
,
and
m
(
2.53
+
t
-
1.2
0.6
min
,
940
°
C
)
,
a highest atmospheric temperature of section C is lower than a highest atmospheric temperature of section B,
Relational Expression 1 is satisfied, and
a time required before the forming process is performed after the blank is seated is two seconds or less,
T≤ 8.2× t +(temp−900)/30 Relation Expression 1:
where T denotes a sum of time required for conveying and seating a blank and time required before forming is performed after the blank is seated and a unit thereof second (s), t denotes a thickness of a blank and a unit thereof is millimeters (mm), and temp denotes a furnace extraction temperature and a unit thereof is degrees Celsius (° C.).
2. The method of claim 1 , wherein heating in section C is performed at an atmospheric temperature of 930° C. or less.
3. The method of claim 1 , wherein a highest atmospheric temperature of section C is Tb-20° C. or less, based on a highest atmospheric temperature (Tb) of section B.
4. The method of claim 1 , wherein the thickness t is 1.5 mm or less, and an atmospheric temperature of section B is more than 930° C. to less than 940° C.
5. The method of claim 1 , wherein the thickness t is 1.5 mm or less, and an atmospheric temperature of section C is 870° C. or more to less than 880° C.
6. The method of claim 1 , wherein the thickness t is greater than 1.5 mm, and an atmospheric temperature of section C is 870° C. or more to less than 900° C.
7. The method of claim 1 , wherein the thickness t is greater than 1.5 mm, and an atmospheric temperature of section B is more than 940° C. to 960° C. or less.
8. The method of claim 1 , wherein the sum of time required for conveying and seating a blank and time required before forming is performed after the blank is seated is greater than 10 seconds.
9. The method of claim 1 , wherein a plating layer of the blank has a thickness of 25 μm or more.
10. The method of claim 1 , after the performing of the forming process, further comprising:
an in-mold cooling step in which the upper mold portion of the mold reaches a press bottom dead center and is then maintained to quench a formed material; and
a take-out step of removing the cooled formed member,
wherein a diffusion layer of the formed member has a thickness of 15 μm or less, and
an alloy layer of the formed member has a thickness of 35 μm to 50 μm.
11. The method of claim 1 , after the performing of the forming process, the method further comprising:
an in-mold cooling step in which the upper mold portion of the mold reaches a press bottom dead center and is then maintained to quench a formed material; and
a take-out step of removing a cooled formed member,
wherein a ratio of a thickness of a diffusion layer of the formed member to a thickness of an alloy layer of the formed member (the thickness of the diffusion layer/the thickness of the alloy layer) was 0.33 or less.
12. The method of claim 1 , wherein the heating is performed to a condition that a value of Relational Expression below is 2 or more,
Relational Expression 2:
∑
n
=
1
x
[
{
(
T
n
-
870
)
×
t
n
t
total
×
0.1334
}
×
k
]
÷
t
where T n denotes a heating furnace atmospheric temperature in an n-th section in a conveying direction of a blank and a unit thereof is degrees Celsius (° C.), t n denotes a heating furnace maintaining time in the n-th section in the conveying direction of the blank and a unit thereof is minute, t total denotes total maintaining time in the heating furnace and a unit thereof is minute, x denotes the number of sections maintained at a specific atmospheric temperature in the heating furnace, k is an integer of 3 in the case of a final section in section B, an integer of −1 in the case in which a section subsequent to section B, and an integer of 1 in the other cases, and t denotes a thickness of the blank and a unit thereof is millimeters (mm).