High-strength hot-dip galvannealed steel sheet and method for producing same

1. A method for producing a high-strength hot-dip galvannealed steel sheet, in which a high-strength steel sheet is used as a base material, the method comprising:

a rolling step (x) of rolling a hot-dip galvannealed steel sheet with a coating layer having an Fe concentration of 8% to 17% by mass; and

a heat treatment step (y) of heating the coated steel sheet which has been subjected to the rolling step (x) under the conditions satisfying the following formulae (1) and (3):

(273+ T )×(20+2×log 10 ( t ))≥8000  (1)

60≤ T≤ 120  (3)

where T: heating temperature (° C.) of the coated steel sheet, and t: holding time (hr) at the heating temperature T.

2. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 1 , further comprising: before the rolling step (x),

an annealing step (a) of annealing the steel sheet;

a coating treatment step (b) of hot-dip galvanizing the steel sheet which has been subjected to the annealing step (a); and

an alloying treatment step (c) of subjecting a coating layer obtained in the coating treatment step (b) to an alloying treatment to obtain the coating layer having an Fe concentration of 8% to 17% by mass.

3. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 1 , wherein, in the rolling step (x), the coated steel sheet is rolled with light reduction at a rolling reduction of 0.10% to 1%.

4. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 1 , wherein the high-strength steel sheet has a composition containing, in percent by mass,

C: 0.03% to 0.35%,

Si: 0.01% to 2.00%,

Mn: 2.0% to 10.0%,

Al: 0.001% to 1.000%,

P: 0.10% or less, and

S: 0.01% or less

with the balance being Fe and unavoidable impurities, and has a tensile strength of 980 MPa or more, and a product (TS×EL) of tensile strength (TS) and total elongation (EL) of 16,000 MPa·% or more; and

a coating weight per one side of the coating layer is 20 to 120 g/m 2 .

5. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 4 , wherein the steel sheet further contains, in percent by mass, one or more selected from

B: 0.001% to 0.005%,

Nb: 0.005% to 0.050%,

Ti: 0.005% to 0.080%,

Cr: 0.001% to 1.000%,

Mo: 0.05% to 1.00%,

Cu: 0.05% to 1.00%,

Ni: 0.05% to 1.00%, and

Sb: 0.001% to 0.200%.

6. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 2 , wherein, in the annealing step (a), in accordance with an Ac 1 temperature and an Ac 3 temperature of the steel sheet, a steel sheet temperature (° C.) is set to be [Ac 1 +(Ac 3 −Ac 1 )/6] to 950° C., and a holding time at the steel sheet temperature is set to be 60 to 600 seconds; and

in the alloying treatment step (c), the alloying treatment temperature is set to be 460° C. to 650° C.

7. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 2 , wherein, in the annealing step (a), a region where a steel sheet temperature is 600° C. to 900° C. is set in an atmosphere having a H 2 concentration of 3% to 20% by volume, and a dew point of −60° C. to −30° C.

8. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 5 , wherein, in the annealing step (a), in accordance with an Ac 1 temperature and an Ac 3 temperature of the steel sheet, a steel sheet temperature (° C.) is set to be [Ac 1 +(Ac 3 −Ac 1 )/6] to 950° C., and a holding time at the steel sheet temperature is set to be 60 to 600 seconds; and

in the alloying treatment step (c), the alloying treatment temperature is set to be 460° C. to 650° C.

9. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 5 , wherein, in the annealing step (a), a region where a steel sheet temperature is 600° C. to 900° C. is set in an atmosphere having a H 2 concentration of 3% to 20% by volume, and a dew point of −60° C. to −30° C.

10. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 6 , wherein, in the annealing step (a), a region where the steel sheet temperature is 600° C. to 900° C. is set in an atmosphere having a H 2 concentration of 3% to 20% by volume, and a dew point of −60° C. to −30° C.

11. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 8 , wherein, in the annealing step (a), a region where the steel sheet temperature is 600° C. to 900° C. is set in an atmosphere having a H 2 concentration of 3% to 20% by volume, and a dew point of −60° C. to −30° C.

12. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 1 , wherein the holding time t at the heating temperature T is from 1 to 500 hours.

13. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 1 , wherein a surface roughness of a work roll used in the rolling is 1.5 μm or less.

14. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 1 ,

wherein the high-strength hot-dip galvannealed steel sheet includes a high-strength steel sheet serving as a base material, wherein a coating layer has an Fe concentration of 8% to 17% by mass, and out of hydrogen being present in the steel sheet, an amount of hydrogen that is released when a temperature of the steel sheet is increased to 200° C. is 0.35 mass ppm or less,

and wherein an average length (L) per unit area of microcracks introduced into the coating layer at a surface of the steel sheet is 0.010 μm/μm 2 or more and 0.070 μm/μm 2 or less, in which a percentage of microcracks that extend in a direction orthogonal to a rolling direction is 60% or less relative to a total length of all the microcracks.

15. The method for producing a high-strength hot-dip galvannealed steel sheet according to claim 14 , wherein the high-strength steel sheet has a product (TS×EL) of tensile strength (TS) and total elongation (EL) of 16,000 MPa·% or more.