Galvannealed steel sheet and manufacturing method thereof
View Patent ↗A galvannealed steel sheet includes a plating layer containing 7.2-10.6 mass % of Fe, 0.2-0.4 mass % of Al, and 0.1 mass % or more of at least one of Ni, Co, Cu, and In, and the balance of Zn and impurities. In a vertical cross-section of the plating layer, an average thickness of a ζ phase is 0.2 μm or less, and an average thickness of a Γ phase is 0.5 μm or less. In the Γ phase, at least one of Ni, Co, Cu, and In are contained at a ratio in the Γ phase of 0.5 mass % or more. A phase existing in contact with the Γ phase is a mixed phase of Γ 1 phase and δ phase, and a δ phase percentage defined by “(δ phase/Γ phase contact interface length)/(δ phase/Γ phase contact interface length+Γ 1 phase/Γ phase contact interface length)×100” is 10% or more.
1. A galvannealed steel sheet, comprising:
a base iron; and
a plating layer containing 7.2 to 10.6 mass % of Fe, 0.2 to 0.4 mass % of Al, and 0.1 mass % or more in total of one type or more types selected from the group consisting of Ni, Co, Cu, and In, and the balance comprising Zn and impurities,
wherein
the plating layer is formed on a surface of the base iron,
wherein
in a vertical cross-section of the plating layer, an average thickness of a ζ phase is 0.2 μm or less, and an average thickness of a Γ phase existing in contact with the base iron is 0.5 μm or less,
wherein
the one type or more types selected from the group consisting of Ni, Co, Cu, and In contained in the plating layer are contained in the Γ phase at 0.5 mass % or more in total, and
wherein
a phase existing in contact with the Γ phase is a mixed phase of Γ 1 phase and δ phase, and a δ phase percentage defined by Expression (1) below is 10% or more,
δ phase percentage=(δ phase/Γ phase contact interface length)/(δ phase/Γ phase contact interface length+Γ 1 phase/Γ phase contact interface length)×100 (1),
wherein the δ phase/Γ phase contact interface length is the length of the interface at which the δ phase and the Γ phase are in contact, and the Γ 1 phase/Γ phase contact interface length is the length of the interface at which the Γ 1 phase and the Γ phase are in contact.
2. A manufacturing method of the galvannealed steel sheet of claim 1 to manufacture the galvannealed steel sheet that is to be subjected to press working with a sheet thickness reduction percentage (%) of 5% or more, the method comprising:
calculating a necessary pre-plating weight (g/m 2 ) based on the sheet thickness reduction percentage (%) during press working of the galvannealed steel sheet according to Expression (2) below;
pre-plating with the calculated necessary pre-plating weight (g/m 2 ) or more in total of one type or more types selected from the group consisting of Ni, Co, Cu, and In on a base iron;
dipping the pre-plated base iron into a plating bath containing 0.1 to 0.3 mass % of Al to perform galvanizing thereon to obtain a galvanized steel sheet;
heating the galvanized steel sheet in a heating furnace;
and after the temperature of the galvanized steel sheet reaching a maximum temperature at the exit of the heating furnace, performing slow cooling of the galvanized steel sheet in a soaking furnace adjusting a temperature integral value S calculated by Expression (3) below in a range of 300 or more to less than 800 and performing an alloying treatment of the galvanized steel sheet,
necessary pre-plating weight (g/m 2 )=0.0222×sheet thickness reduction percentage (%)−0.0625 (2), and
S
=
(
T
11
-
T
0
)
×
t
1
/
2
+
(
(
T
11
-
T
0
)
+
(
T
12
-
T
0
)
)
×
t
2
/
2
+
(
(
T
12
-
T
0
)
+
(
T
21
-
T
0
)
)
×
Δ
t
/
2
+
(
(
T
21
-
T
0
)
+
(
T
22
-
T
0
)
)
×
t
3
/
2
+
(
T
22
-
T
0
)
×
t
4
/
2
(
3
)
wherein T 0 =420 (° C.),
T 11 =steel sheet temperature at the exit of the heating furnace (° C.),
T 12 =steel sheet temperature at the entry of a cooling zone of the soaking furnace (° C.),
T 21 =steel sheet temperature at the exit of the cooling zone (° C.),
T 22 =steel sheet temperature at the exit of the soaking furnace (° C.),
t 1 =treatment time from T 0 to the exit of the heating furnace (second),
t 2 =treatment time from the exit of the heating furnace to the entry of the cooling zone of the soaking furnace (second),
Δt=treatment time from the entry of the cooling zone of the soaking furnace to the exit of the cooling zone (second),
t 3 =treatment time from the exit of the cooling zone of the soaking furnace to the exit of the soaking furnace (second), and
t 4 =treatment time from the entry of a rapid cooling zone to T 0 (second).