High-strength galvannealed steel sheet having excellent formability and fatigue resistance and method for manufacturing the same

1. A method for manufacturing a 700 MPa or more high-strength galvannealed steel sheet having excellent formability and fatigue resistance, the method comprising:

hot-rolling a slab to produce a hot-rolled sheet having a microstructure in which a total area ratio of bainite and martensite is 80% or more, the slab having, by % by mass, C: 0.05% to 0.3%, Si: 0.5% to 2.5%, Mn: 1.0% to 3.5%, P: 0.003% to 0.100%, S: 0.02% or less, Al: 0.010% to 0.1%, and the balance including iron and unavoidable impurities;

cold-rolling the hot-rolled sheet to produce a cold-rolled steel sheet;

continuously annealing the cold-rolled steel sheet by heating to 750° C. to 900° C. at an average heating rate of 8° C./s or more from 500° C. to an A 1 transformation point, holding the steel sheet for 10 seconds or more, and then cooling the steel sheet to a temperature region of 300° C. to 530° C. at an average cooling rate of 3° C. is or more from 750° C. to 530° C.;

galvanizing the steel sheet; and

further coating-alloying the steel sheet in a temperature region of 540° C. to 600° C. for 5 to 60 seconds.

2. The method according to claim 1 , wherein the continuously annealing step includes, after cooling, holding the steel sheet in a temperature region of 300° C. to 530° C. for 20 to 900 seconds.

3. The method according to claim 2 , wherein the slab contains at least one group selected from the group A to D consisting of:

group A: at least one element selected from, by % by mass, Cr: 0.005% to 2.00%, Mo: 0.005% to 2.00%, V: 0.005% to 2.00%, Ni: 0.005% to 2.00%, and Cu: 0.005% to 2.00%;

group B: at least one element selected from, by % by mass, Ti: 0.01% to 0.20% and Nb: 0.01% to 0.20%;

group C: by % by mass, B: 0.0002% to 0.005%; and

group D: one or two elements selected from, by % by mass, Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.

4. The method according to claim 2 , wherein the steel sheet has a hole expansion rate λ of 40% or more.

5. The method according to claim 2 , wherein the steel sheet has a microstructure containing 50% or more of ferrite, 5% to 35% of martensite and 2% to 15% of pearlite in terms of an area ratio, the martensite having an average grain size of 3 μm or less and an average distance of 5 μm or less between adjacent martensite grains.

6. The method according to claim 1 , wherein the slab contains at least one group selected from the group A to D consisting of:

group A: at least one element selected from, by % by mass, Cr: 0.005% to 2.00%, Mo: 0.005% to 2.00%, V: 0.005% to 2.00%, Ni: 0.005% to 2.00%, and Cu: 0.005% to 2.00%;

group B: at least one element selected from, by % by mass, Ti: 0.01% to 0.20% and Nb: 0.01% to 0.20%;

group C: by % mass, B: 0.0002% to 0.005%; and

group D: one or two elements selected from, by % by mass, Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.

7. The method according to claim 1 , wherein the steel sheet has a hole expansion rate λ of 40% or more.

8. The method according to claim 1 , wherein the steel sheet has a microstructure containing 50% or more of ferrite, 5% to 35% of martensite, and 2% to 15% of pearlite in terms of an area ratio, the martensite having an average grain size of 3 μm or less and an average distance of 5 μm or less between adjacent martensite grains.

9. A method for manufacturing a 700 MPa or more high-strength galvannealed steel sheet having excellent formability and fatigue resistance, the method comprising:

hot-rolling, in a hot-rolling step, a slab at a finish rolling temperature equal to or higher than an A 3 transformation point, cooling at an average cooling rate of 50° C./s or more, and then coiling at a temperature of 300° C. or more and 550° C. or less to produce a hot-rolled sheet having a microstructure in which a total area ratio of bainite and martensite is 80% or more, the slab having, by % by mass, C: 0.05% to 0.3%, Si: 0.5% to 2.5%, Mn: 1.0% to 3.5%, P: 0.003% to 0.100%, S: 0.02% or less, Al: 0.010% to 0.1%, and the balance including iron and unavoidable impurities;

cold-rolling the hot-rolled sheet to produce a cold-rolled steel sheet;

continuously annealing the cold-rolled steel sheet by heating to 750° C. to 900° C. at an average heating rate of 8° C./s or more from 500° C. to an A 1 transformation point, holding the steel sheet for 10 seconds or more, and then cooling the steel sheet to a temperature region of 300° C. to 530° C. at an average cooling rate of 3° C./s or more from 750° C. to 530° C.;

galvanizing the steel sheet; and

further coating-alloying the steel sheet in a temperature region of 540° C. to 600° C. for 5 to 60 seconds.

10. The method according to claim 9 , wherein the slab contains at least one group selected from the group A to D consisting of:

group A: at least one element selected from, by % by mass, Cr: 0.005% to 2.00%, Mo: 0.005% to 2.00%, V: 0.005% to 2.00%, Ni: 0.005% to 2.00%, and Cu: 0.005% to 2.00%;

group B: at least one element selected from, by % by mass, Ti: 0.01% to 0.20% and Nb: 0.01% to 0.20%;

group C: by % by mass, B: 0.0002% to 0.005%; and

group D: one or two elements selected from, by % by mass, Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.

11. The method according to claim 9 , wherein the steel sheet has a hole expansion rate λ of 40% or more.

12. The method according to claim 9 , wherein the steel sheet has a microstructure containing 50% or more of ferrite, 5% to 35% of martensite, and 2% to 15% of pearlite in terms of an area ratio, the martensite having an average grain size of 3 μm or less and an average distance of 5μm or less between adjacent martensite grains.

13. A method for manufacturing a 700 MPa or more high-strength galvannealed steel sheet having excellent formability and fatigue resistance, the method comprising:

hot-rolling, in a hot-rolling step, a slab at a finish rolling temperature equal to or higher than an A 3 transformation point, cooling at an average cooling rate of 50° C./s or more, and then coiling at a temperature of 300° C. or more and 550° C. or less to produce a hot-rolled sheet having a microstructure in which a total area ratio of bainite and martensite is 80% or more, the slab having, by % by mass, C: 0,05% to 0.3%, Si: 0.5% to 2.5%, Mn: 1.0% to 3.5%, P: 0.003% to 0.100%, S: 0.02% or less, Al: 0.010% to 0.1%, and the balance including iron and unavoidable impurities;

cold-rolling the hot-rolled sheet to produce a cold-rolled steel sheet;

continuously annealing the cold-rolled steel sheet by heating to 750° C. to 900° C. at an average heating rate of 8° C./s or more from 500° C. to an A 1 transformation point, holding the steel sheet for 10 seconds or more, cooling the steel sheet to a temperature region of 300° C. to 530° C. at an average cooling rate of 3° C./s or more from 750° C. to 530° C., and then holding the steel sheet for 20 to 900 seconds in a temperature region of 300° C. to 530° C.;

galvanizing the steel sheet; and

further coating-alloying the steel sheet in a temperature region of 540° C. to 600° C. for 5 to 60 seconds.

14. The method according to claim 13 , wherein the slab contains at least one group selected from the group A to D consisting of:

group A: at least one element selected from, by % by mass, Cr: 0.005% to 2.00%, Mo: 0.005% to 2.00%, V: 0.005% to 2.00%, Ni: 0.005% to 2.00%, and Cu: 0.005% to 2.00%;

group B: at least one element selected from, by % by mass, Ti: 0.01% to 0.20% and Nb: 0.01% to 0.20%;

group C: by % by mass, B: 0.0002% to 0.005%; and

group D: one or two elements selected from, by % by mass, Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.

15. The method according to claim 13 , wherein the steel sheet has a hole expansion rate λ of 40% or more.

16. The method according to claim 13 , wherein the steel sheet has a microstructure containing 50% or more of ferrite, 5% to 35% of martensite, and 2% to 15% of pearlite in terms of an area ratio, the martensite having an average grain size of 3 μm or less and an average distance of 5 μm or less between adjacent martensite grains.