Hot-dip galvanized steel sheet and manufacturing method of the same

1. A hot-dip galvanized steel sheet including a base steel sheet and a hot-dip galvanizing layer formed on at least one surface of the base steel sheet, wherein

the base steel sheet contains, in mass %,

C: not less than 0.05% nor more than 0.50%,

Si: not less than 0.1% nor more than 3.0%,

Mn: not less than 0.5% nor more than 5.0%,

P: not less than 0.001% nor more than 0.5%,

S: not less than 0.001% nor more than 0.03%,

Al: not less than 0.005% nor more than 1.0%, and

one or two or more of elements selected from Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and a rare-earth element REM: 0% to not more than 1% each, and

a balance being composed of Fe and inevitable impurities, and

in the base steel sheet, a value of H A representing average Vickers hardness in a surface layer ranging from an interface between the base steel sheet and the hot-dip galvanizing layer to 50 μm in depth and a value of H B representing average Vickers hardness in a deep portion ranging from the interface to greater than 50 μm in depth satisfy all the following relational expressions (1) to (3),

50≤H A ≤500  (1),

50≤H B ≤500  (2),

0.5 ≤H A /H B ≤0.9  (3).

2. The hot-dip galvanized steel sheet according to claim 1 , wherein

W C(A) , W Si(A) , and W Mn(A) representing content percentages of C, Si, and Mn in mass % in the surface layer of the base steel sheet respectively and W C(B) , W Si(B) , and W Mn (B) representing content percentages of C, Si, and Mn in mass % in the deep portion of the base steel sheet respectively satisfy all the following relational expressions (4) to (6),

0.1 ≤W C(A) /W C(B) ≤0.5  (4),

0.1 ≤W Si(A) /W Si(B) ≤0.5  (5),

0.1 ≤W Mn(A) /W Mn(B) ≤0.5  (6).

3. The hot-dip galvanized steel sheet according to claim 1 , wherein

the base steel sheet contains one or two or more of elements selected from Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and a rare-earth element REM in not less than 0.0001% nor more than 1% each.

4. The hot-dip galvanized steel sheet according to claim 1 , wherein

the hot-dip galvanizing layer has a thickness in the range of not less than 1 μm nor more than 30 μm and contains not less than 4 mass % nor more than 14 mass % of Fe, not less than 0.1 mass % nor more than 1 mass % of Al, and a balance being composed of Zn and inevitable impurities.

5. The hot-dip galvanized steel sheet according to claim 2 , wherein

the base steel sheet contains one or two or more of elements selected from Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and a rare-earth element REM in not less than 0.0001% nor more than 1% each.

6. The hot-dip galvanized steel sheet according to claim 5 , wherein

the hot-dip galvanizing layer has a thickness in the range of not less than 1 μm nor more than 30 μm, and contains not less than 4 mass % nor more than 14 mass % of Fe, not less than 0.1 mass % nor more than 1 mass % of Al, and a balance being composed of Zn and inevitable impurities.

7. The hot-dip galvanized steel sheet according to claim 2 , wherein

the hot-dip galvanizing layer has a thickness in the range of not less than 1 μm nor more than 30 μm, and contains not less than 4 mass % nor more than 14 mass % of Fe, not less than 0.1 mass % nor more than 1 mass % of Al, and a balance being composed of Zn and inevitable impurities.

8. The hot-dip galvanized steel sheet according to claim 3 , wherein

the hot-dip galvanizing layer has a thickness in the range of not less than 1 μm nor more than 30 μm, and contains not less than 4 mass % nor more than 14 mass % of Fe, not less than 0.1 mass % nor more than 1 mass % of Al, and a balance being composed of Zn and inevitable impurities.

9. A method for manufacturing the hot-dip galvanized steel sheet of claim 1 by performing a hot-dip galvanizing treatment on the base steel sheet, wherein

the base steel sheet is obtained after undergoing a casting step, a hot rolling step, a pickling step, a cold rolling step, an annealing step, and a soaking and holding step, and contains, in mass %,

C: not less than 0.05% nor more than 0.50%,

Si: not less than 0.1% nor more than 3.0%,

Mn: not less than 0.5% nor more than 5.0%,

P: not less than 0.001% nor more than 0.5%,

S: not less than 0.001% nor more than 0.03%,

Al: not less than 0.005% nor more than 1.0%, and

one or two or more of elements selected from Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and a rare-earth element REM: 0% to not more than 1% each, and

a balance being composed of Fe and inevitable impurities,

the annealing step and the soaking and holding step are performed in a continuous hot-dip galvanizing facility equipped with an all radiant tube type heating furnace as a heating furnace and a soaking furnace,

the annealing step is performed so as to satisfy the following heating furnace conditions:

heating temperature: a sheet temperature T 0 [° C.] representing the maximum temperature that, when a cold-rolled steel sheet obtained after undergoing the cold rolling step is heated in the heating furnace, the cold-rolled steel sheet reaches is in the range of not lower than a temperature T 1 [° C.] nor higher than a temperature T 2 [° C.];

heating time period: a heating time period S 0 [second] in the heating furnace is in the range of not shorter than a time period S 1 [second] nor longer than a time period S 2 [second]; and

atmosphere gas: a nitrogen atmosphere containing carbon dioxide and carbon monoxide in which log(PCO 2 /PCO) being a logarithmic value of a value of, in the heating furnace, a partial pressure value of carbon dioxide divided by a partial pressure value of carbon monoxide exhibits a value in the range of not less than −2 nor more than 1,

here, the temperatures T 1 and T 2 and the time periods S 1 and S 2 are defined as follows:

T 1 : a temperature [° C.] satisfying the following relational expression (7) using W Si (B) and W Mn (B) representing content percentages of Si and Mn in mass % in a deep portion ranging from a surface of the cold-rolled steel sheet to greater than 50 μm in depth respectively;

T 1 =500−50 ×W Si(B) −20 ×W Mn(B)   (7)

T 2 : a temperature [° C.] satisfying the following relational expression (8) using a temperature T Ac3 [° C.] corresponding to a transformation point A c3 of the cold-rolled steel sheet;

T 2 =T Ac3 +40  (8)

S 1 : a time period satisfying the following relational expression (9) using W Si (B) [mass %] representing the content percentage of Si and W Mn(B) [mass %] representing the content percentage of Mn in the deep portion of the cold-rolled steel sheet; and

S 1 =50 +20 ×W Si(B) +10 ×W Mn(B)   (9)

S 2 : a time period satisfying the following relational expression (10) using W C (B) [mass %] representing content percentage of C in the deep portion of the cold-rolled steel sheet,

S 2 =200 +1000 ×W C(B)   (10)

the soaking and holding step is performed so as to satisfy the following soaking furnace conditions:

soaking and holding time period: a time period during which the cold-rolled steel sheet is held in the soaking furnace is in the range of not shorter than 100 seconds nor longer than 600 seconds; and

atmosphere gas: a nitrogen atmosphere containing carbon dioxide and carbon monoxide in which a value of log(PCO 2 /PCO) in the soaking furnace is in the range of −5 or more to less than −2, and

in the plating step, a hot-dip galvanizing layer containing not less than 4 mass % nor more than 14 mass % of Fe, not less than 0.1 mass % nor more than 1 mass % of Al, and a balance being composed of Zn and inevitable impurities is formed on the surface of the base steel sheet so as to have a thickness of not less than 1 μm nor more than 30 μm.

10. The method according to claim 9 , wherein

on the occasion of performing the hot-dip galvanizing treatment, the base steel sheet obtained after undergoing the soaking and holding step is immersed in a hot-dip galvanizing bath containing not less than 0.05 mass % nor more than 0.20 mass % of Al, and then is subjected to an alloying treatment in which heating is performed to a heating temperature in the range of not lower than 450° C. nor higher than 560° C.