High-strength galvanized steel sheet and method for manufacturing same

1. A high-strength galvanized steel sheet comprising:

a steel sheet having a steel composition having a component composition containing, in mass %,

C: 0.10% or more and 0.30% or less,

Si: less than 1.2%,

Mn: 2.0% or more and 3.5% or less,

P: 0.010% or less,

S: 0.002% or less,

Al: 1% or less,

N: 0.006% or less, and

the balance including Fe and unavoidable impurities, and

the steel sheet having a steel structure containing 50% or more of martensite, 30% or less, including 0%, of ferrite, and 10 to 50% of bainite, and further containing less than 5%, including 0%, of residual austenite, in terms of area ratio,

30% or more of the martensite being tempered martensite, including self-tempered martensite,

the steel sheet having an amount of diffusible hydrogen in the steel sheet being 0.20 mass ppm or less; and

a galvanizing layer provided on a surface of the steel sheet, the galvanizing layer having a content amount of Fe of 8 to 15% in mass %, and an attachment amount of plating per one surface of 20 to 120 g/m 2 ,

wherein an amount of Mn oxides contained in the galvanizing layer is 0.050 g/m 2 or more, and

the high-strength galvanized steel sheet has a yield strength of 700 MPa or more and a yield strength ratio of 65% or more and less than 85%.

2. The high-strength galvanized steel sheet according to claim 1 , wherein the component composition further contains one or more Groups A to C,

Group A: any one or more selected from, in mass %, one or more of Ti, Nb, V, and Zr: 0.005 to 0.1% in total, one or more of Mo, Cr, Cu, and Ni: 0.005 to 0.5% in total, and B: 0.0003 to 0.005%

Group B: any one or two selected from, in mass %, Sb: 0.001 to 0.1% and Sn: 0.001 to 0.1%

Group C: in mass %, Ca: 0.0010% or less.

3. A method for manufacturing a high-strength galvanized steel sheet according to claim 1 comprising:

an annealing step of heating a cold rolled material having the component composition according to claim 1 in an in-annealing-furnace atmosphere with a hydrogen concentration H of 1 vol % or more and 13 vol % or less, at an in-annealing-furnace temperature T of (an Ac3 point- 20° C.) to 900° C. or less for 5 sec or more, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for 10 sec or more;

a plating step of subjecting a steel sheet after the annealing step to plating treatment and alloying treatment, and performing cooling up to 100° C. or less at an average cooling rate of 3° C./s or more; and

a later heat treatment step of allowing a plated steel sheet after the plating step to stay in an in-furnace atmosphere with a hydrogen concentration H of 10 vol % or less and a dew point Dp of 50° C. or less, at a temperature T in ° C. of 200° C. or less for a time tin hr or more that is 0.01 hr or more and satisfies a (1) formula:

130−18.3 ×ln ( t )≤ T   (1)

wherein Ac3=910−203√C+44.7×Si−30Mn−11P+700S+400×Al+400×Ti,

wherein the atomic symbols in the Ac3 equation denote the contents of the corresponding chemical elements in mass %, and the atomic symbol of the corresponding chemical element which is not contained is assigned a value of 0.

4. A method for manufacturing a high-strength galvanized steel sheet according to claim 2 comprising:

an annealing step of heating a cold rolled material having the component composition according to claim 2 in an in-annealing-furnace atmosphere with a hydrogen concentration H of 1 vol % or more and 13 vol % or less, at an in-annealing-furnace temperature T of (an Ac3 point- 20° C.) to 900° C. or less for 5 sec or more, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for 10 sec or more;

a plating step of subjecting a steel sheet after the annealing step to plating treatment and alloying treatment, and performing cooling up to 100° C. or less at an average cooling rate of 3° C./s or more; and

a later heat treatment step of allowing a plated steel sheet after the plating step to stay in an in-furnace atmosphere with a hydrogen concentration H of 10 vol % or less and a dew point Dp of 50° C. or less, at a temperature T in ° C. of 200° C. or less for a time tin hr or more that is 0.01 hr or more and satisfies a (1) formula:

130−18.3 ×ln ( t )≤ T   (1)

wherein Ac3=910−203√C+44.7×Si−30Mn−11P+700S+400×Al+400×Ti,

wherein the atomic symbols in the Ac3 equation denote the contents of the corresponding chemical elements in mass %, and the atomic symbol of the chemical element which is not contained is assigned a value of 0.

5. The method for manufacturing a high-strength galvanized steel sheet according to claim 3 , comprising, before the annealing step, an earlier treatment step of heating the cold rolled material up to an Ac1 point to the Ac3 point+50° C. and performing pickling,

wherein Ac1=751−27C+18Si−12Mn −23Cu−23Ni+24Cr+23Mo−40V−6Ti+32Zr+233Nb−169Al−895B,

wherein the atomic symbols in the Ac1 equation denote the contents of the corresponding chemical elements in mass %, and the atomic symbol of the chemical element which is not contained is assigned a value of 0.

6. The method for manufacturing a high-strength galvanized steel sheet according to claim 4 , comprising, before the annealing step, an earlier treatment step of heating the cold rolled material up to an Ac1 point to the Ac3 point+50° C. and performing pickling,

wherein Ac1=751−27C+18Si−12Mn −23Cu−23Ni+24Cr+23Mo−40V−6Ti+32Zr+233Nb−169Al−895B,

wherein the atomic symbols in the Ac1 equation denote the contents of the corresponding chemical elements in mass %, and the atomic symbol of the chemical element which is not contained is assigned a value of 0.

7. The method for manufacturing a high-strength galvanized steel sheet according to claim 3 , wherein, after the plating step, temper rolling is performed at an extension rate of 0.1% or more.

8. The method for manufacturing a high-strength galvanized steel sheet according to claim 4 , wherein, after the plating step, temper rolling is performed at an extension rate of 0.1% or more.

9. The method for manufacturing a high-strength galvanized steel sheet according to claim 5 , wherein, after the plating step, temper rolling is performed at an extension rate of 0.1% or more.

10. The method for manufacturing a high-strength galvanized steel sheet according to claim 6 , wherein, after the plating step, temper rolling is performed at an extension rate of 0.1% or more.

11. The method for manufacturing a high-strength galvanized steel sheet according to claim 7 , wherein width trimming is performed after the later heat treatment step.

12. The method for manufacturing a high-strength galvanized steel sheet according to claim 8 , wherein width trimming is performed after the later heat treatment step.

13. The method for manufacturing a high-strength galvanized steel sheet according to claim 9 , wherein width trimming is performed after the later heat treatment step.

14. The method for manufacturing a high-strength galvanized steel sheet according to claim 10 , wherein width trimming is performed after the later heat treatment step.

15. The method for manufacturing a high-strength galvanized steel sheet according to claim 7 ,

wherein width trimming is performed before the later heat treatment step, and

a staying time t in hr for staying at a temperature T in ° C. of 200° C. or less in the later heat treatment step is 0.01 hr or more and satisfies a (2) formula:

115−18.3 ×ln ( t )≤ T   (2).

16. The method for manufacturing a high-strength galvanized steel sheet according to claim 8 ,

wherein width trimming is performed before the later heat treatment step, and

a staying time t in hr for staying at a temperature T in ° C. of 200° C. or less in the later heat treatment step is 0.01 hr or more and satisfies a (2) formula:

115−18.3 ×ln ( t )≤ T   (2).

17. The method for manufacturing a high-strength galvanized steel sheet according to claim 9 ,

wherein width trimming is performed before the later heat treatment step, and

a staying time t in hr for staying at a temperature T in ° C. of 200° C. or less in the later heat treatment step is 0.01 hr or more and satisfies a (2) formula:

115−18.3 ×ln ( t )≤ T   (2).

18. The method for manufacturing a high-strength galvanized steel sheet according to claim 10 ,

wherein width trimming is performed before the later heat treatment step, and

a staying time t in hr for staying at a temperature T in ° C. of 200° C. or less in the later heat treatment step is 0.01 hr or more and satisfies a (2) formula:

115−18.3 ×ln ( t )≤ T   (2).