High-strength galvanized steel sheet and method for manufacturing same
A high-strength galvanized steel sheet includes a steel sheet having a steel composition having a specific component composition, a steel structure containing martensite and bainite at more than or equal to 70% (including 100%), ferrite at less than 20% (including 0%), and retained austenite at less than 5% (including 0%) in terms of area ratio, the amount of diffusible hydrogen in steel being less than or equal to 0.20 mass ppm; and a galvanizing layer provided on a surface of the steel sheet, having a content amount of Fe of 8 to 15% in mass %, and having an coating weight per one surface of 20 to 120 g/m 2 , wherein the amount of Mn oxides contained in the galvanizing layer is less than or equal to 0.050 g/m 2 , and a tensile strength is more than or equal to 1100 MPa and a yield ratio is more than or equal to 0.85.
1. A high-strength galvanized steel sheet comprising:
a steel sheet having a steel composition having
a component composition containing, in mass %,
C: more than or equal to 0.10% and less than or equal to 0.30%,
Si: less than 1.2%,
Mn: more than or equal to 2.0% and less than or equal to 3.5%,
P: less than or equal to 0.010%,
S: less than or equal to 0.002%,
Al: less than or equal to 1%,
N: less than or equal to 0.006%, and
the balance including Fe and unavoidable impurities, and
a steel structure containing martensite and bainite at more than or equal to 70% (including 100%), ferrite at less than 20% (including 0%), and retained austenite at less than 5% (including 0%) in terms of area ratio,
an amount of diffusible hydrogen in steel being less than or equal to 0.20 mass ppm of molecular hydrogen; and a galvanizing layer provided on a surface of the steel sheet, having a content amount of Fe of 8 to 15% in mass %, and having an coating weight per one surface of 20 to 120 g/m 2 ,
wherein an amount of Mn oxides contained in the galvanizing layer is less than or equal to 0.050 g/m 2 , and
a tensile strength is more than or equal to 1100 MPa and a yield ratio is more than or equal to 0.85.
2. The high-strength galvanized steel sheet according to claim 1 , wherein the component composition further contains at least one selected from the groups of A to C, in mass %,
group A:
at least one selected from, 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 Sb: 0.001 to 0.1% and Sn: 0.001 to 0.1%; and
group C:
Ca: less than or equal to 0.0010%.
3. A method for manufacturing the high-strength galvanized steel sheet according to claim 1 , comprising:
an annealing step of allowing a cold rolled material having the component composition according to claim 1 to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within the annealing furnace T of an A c3 point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds;
a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and
a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1)
130−18.3× In ( t )≤ T (1).
4. The method for manufacturing a high-strength galvanized steel sheet according to claim 3 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%.
5. The method for manufacturing a high-strength galvanized steel sheet according to claim 4 , wherein width trimming is performed after the post heat treatment step.
6. The method for manufacturing a high-strength galvanized steel sheet according to claim 4 ,
wherein width trimming is performed before the post heat treatment step, and
a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
115−18.3× In ( t )≤ T (2).
7. A method for manufacturing a high-strength galvanized steel sheet according to claim 1 , comprising:
a pretreatment step of heating a cold rolled material, having the component composition according to claim 1 , up to an A c3 point to the A c3 point+50° C. in a temperature region and performing pickling after said heating to form a pickled sheet;
an annealing step of allowing the pickled sheet to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within an annealing furnaces T of an A c3 point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds;
a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and
a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1):
130−18.3× In ( t )≤ T (1).
8. The method for manufacturing a high-strength galvanized steel sheet according to claim 7 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%.
9. The method for manufacturing a high-strength galvanized steel sheet according to claim 8 , wherein width trimming is performed after the post heat treatment step.
10. The method for manufacturing a high-strength galvanized steel sheet according to claim 8 ,
wherein width trimming is performed before the post heat treatment step, and
a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
115−18.3× In ( t )≤ T (2).
11. A method for manufacturing the high-strength galvanized steel sheet according to claim 2 , comprising:
an annealing step of allowing a cold rolled material having the component composition according to claim 2 to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within the annealing furnace T of an A c3 point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds;
a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and
a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1)
130−18.3× In ( t )≤ T (1).
12. The method for manufacturing a high-strength galvanized steel sheet according to claim 11 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%.
13. The method for manufacturing a high-strength galvanized steel sheet according to claim 12 , wherein width trimming is performed after the post heat treatment step.
14. The method for manufacturing a high-strength galvanized steel sheet according to claim 12 ,
wherein width trimming is performed before the post heat treatment step, and
a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
115−18.3× In ( t )≤ T (2).
15. A method for manufacturing a high-strength galvanized steel sheet according to claim 2 , comprising:
a pretreatment step of heating a cold rolled material, having the component composition according to claim 2 , up to an A c3 point to the A c3 point+50° C. in a temperature region and performing pickling after said heating to form a pickled sheet;
an annealing step of allowing the pickled sheet to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within an annealing furnaces T of an A c3 point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds;
a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and
a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1):
130−18.3× In ( t )≤ T (1).
16. The method for manufacturing a high-strength galvanized steel sheet according to claim 15 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%.
17. The method for manufacturing a high-strength galvanized steel sheet according to claim 16 , wherein width trimming is performed after the post heat treatment step.
18. The method for manufacturing a high-strength galvanized steel sheet according to claim 16 ,
wherein width trimming is performed before the post heat treatment step, and
a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
115−18.3× In ( t )≤ T (2).