Method for producing a galvannealed steel sheet

1. A method for producing a galvannealed steel sheet, the method comprising:

casting a molten steel which comprises, as a chemical composition, by mass %,

0.0005% to 0.01% of C,

0.001% to 1.0% of Si,

0.01% to 2.0% of Mn,

0.005% to 0.1% of P,

0.01% to 0.10% of Al,

0.02% or less of S,

0.1% or less of Ni,

0.1% or less of Cu,

0.01% or less of N, and

a balance consisting of Fe and unavoidable impurities in order to obtain a slab;

heating the slab in 1100° C. to 1300° C.;

hot-rolling the slab after the heating under conditions such that a finishing temperature is in 800° C. to 1050° C. and a coiling temperature is in 500° C. to 800° C. in order to obtain a hot-rolled steel sheet;

surface-removing the hot-rolled steel sheet within a range in μm of GL expressed by a following Expression 1 or more and GU expressed by a following Expression 2 or less from an interface toward a steel along a thickness direction in order to obtain a scale-removed rolled steel sheet, when ten measurement points of the hot-rolled steel sheet are set in a transverse direction by equally dividing a line-segment having a reference length of 50 mm by 10, when a steel surface part of the hot-rolled steel sheet is from the interface between a scale and the steel to 2 μm in depth toward the steel along the thickness direction, and when a Ni max and a Cu max are respectively a maximum Ni content and a maximum Cu content in mass % of the steel surface part in the ten measurement points of the hot-rolled steel sheet;

galvanizing the scale-removed rolled steel sheet after the surface-removing in order to obtain a galvanized steel sheet; and

galvannealing the galvanized steel sheet after the galvanizing in order to obtain a galvannealed steel sheet,

GL=(Ni max +0.8×Cu max )×0.2  (Expression 1),

GU=(Ni max +0.8×Cu max )×4  (Expression 2).

2. The method for producing the galvannealed steel sheet according to claim 1 ,

wherein the molten steel further comprises, as the chemical composition, by mass %, at least one selected from

0. 0001% to 0.0050% of B,

0.001% to 0.1% of Nb,

0.001% to 0.1% of Ti, and

0.001% to 0.1% of Mo.

3. The method for producing the galvannealed steel sheet according to claim 2 , the method further comprising

pickling at least a surface of the hot-rolled steel sheet before the surface-removing or the scale-removed rolled steel sheet after the surface-removing.

4. The method for producing the galvannealed steel sheet according to claim 1 , the method further comprising

pickling at least a surface of the hot-rolled steel sheet before the surface-removing or the scale-removed rolled steel sheet after the surface-removing.

5. The method for producing the galvannealed steel sheet according to claim 1 , the method further comprising:

cold-rolling the scale-removed rolled steel sheet before the galvanizing under a cold-reduction of 50% to 95%; and

annealing the scale-removed rolled steel sheet after the cold-rolling in a temperature which is a recrystallization temperature or higher.

6. The method for producing the galvannealed steel sheet according to claim 2 , the method further comprising:

cold-rolling the scale-removed rolled steel sheet before the galvanizing under a cold-reduction of 50% to 95%; and

annealing the scale-removed rolled steel sheet after the cold-rolling in a temperature which is a recrystallization temperature or higher.

7. The method for producing the galvannealed steel sheet according to claim 3 , the method further comprising:

cold-rolling the scale-removed rolled steel sheet before the galvanizing under a cold-reduction of 50% to 95%; and

annealing the scale-removed rolled steel sheet after the cold-rolling in a temperature which is a recrystallization temperature or higher.

8. The method for producing the galvannealed steel sheet according to claim 4 , the method further comprising:

cold-rolling the scale-removed rolled steel sheet before the galvanizing under a cold-reduction of 50% to 95%; and

annealing the scale-removed rolled steel sheet after the cold-rolling in a temperature which is a recrystallization temperature or higher.