High-strength steel sheet having excellent processability and paint bake hardenability, and method for producing of high-strength steel sheet

1. A high-strength steel sheet comprising, by mass %:

C: 0.01% to 0.10%;

Si: 0.15% or less;

Mn: 0.80% to 1.50%;

P: 0.10% or less;

S: 0.015% or less;

Al: more than 0.20% to 0.80%;

Cr: 0.01% to 0.40%;

N: 0.0100% or less; and

a balance consisting of iron and unavoidable impurities, wherein:

a metallic structure is composed of ferrite and a hard second phase;

an area fraction of the ferrite is 80% or more;

an area fraction of the hard second phase is 1% to 20%;

a fraction of unrecrystallized ferrite in the ferrite is less than 10%;

ferrite grain sizes are 5 μm to 20 μm; and

a fraction of ferrite crystal grains having an aspect ratio of 1.2 or less in entire ferrite crystal grains is 60% or more.

2. The high-strength steel sheet according to claim 1 , wherein a component composition of the high-strength steel sheet satisfies

Mn/Cr is 3.0 or less, and

Cr/(Si+Al) is 3.0 or less.

3. The high-strength steel sheet according to claim 1 , further comprising, by mass %, one or more of:

Nb: 0.0005% to 0.0500%;

Ti: 0.0005% to 0.0500%;

Mo: 0.005% to 1.500%;

W: 0.005% to 1.500%;

B: 0.0001% to 0.0100%;

Ni: 0.005% to 1.500%;

Cu: 0.005% to 1.500%; and

V: 0.005% to 1.500%.

4. The high-strength steel sheet according to claim 1 , wherein a zinc galvanization coat or a zinc alloy galvanization coat is provided at a surface of the high-strength steel sheet.

5. The high-strength steel sheet according to claim 4 ,

wherein the high-strength steel sheet further comprises Cr: 0.20% to 0.40%, and P: less than 0.015%.

6. A method for producing a high-strength steel sheet,

wherein a billet having chemical components of the high-strength steel sheet according to claim 1 is hot rolled, pickled, cold rolled at a percentage reduction in a thickness of more than 60% so as to obtain a steel sheet,

then, the steel sheet is heated to a temperature range of 720° C. to 850° C. at a heating rate controlled to 1° C./s to 10° C./s in a temperature range of an Ac 1 transformation point to an Ac 3 transformation point, and subjected to an annealing for a retention time of 10 seconds to 200 seconds during which a temperature of the steel sheet is 720° C. to 850° C., and

after the annealing, the steel sheet is subjected to a first cooling to 500° C. or lower at a cooling rate of 3° C./s or more, and then subjected to a skin pass rolling of 2.0% or less.

7. The method for producing the high-strength steel sheet according to claim 6 , wherein a heat treatment is carried out in a temperature range of 200° C. to 450° C. for 30 seconds or more before the skin pass rolling, and

a second cooling is carried out at a cooling rate of 1° C./s to 3° C./s to 100° C. or lower after the heat treatment.

8. The method for producing the high-strength steel sheet according to claim 6 , wherein galvanization is carried out on the steel sheet after the first cooling and before the skin pass rolling.

9. The method for producing the high-strength steel sheet according to claim 8 , wherein a heat treatment for alloying is carried out for 10 seconds or longer in a temperature range of 450° C. to 600° C. at a timing after the galvanization and before the skin pass rolling.

10. The method for producing the high-strength steel sheet according to claim 7 , wherein the galvanization is carried out on the steel sheet at a timing after the heat treatment and before the second cooling.

11. The method for producing the high-strength steel sheet according to claim 10 , wherein the heat treatment for alloying is carried out for 10 seconds or longer in a temperature range of 450° C. to 600° C. at a timing after the galvanization and before the second cooling.

12. The high-strength steel sheet according to claim 2 , further comprising, by mass %, one or more of:

Nb: 0.0005% to 0.0500%;

Ti: 0.0005% to 0.0500%;

Mo: 0.005% to 1.500%;

W: 0.005% to 1.500%;

B: 0.0001% to 0.0100%;

Ni: 0.005% to 1.500%;

Cu: 0.005% to 1.500%; and

V: 0.005% to 1.500%.

13. The high-strength steel sheet according to claim 2 , wherein a zinc galvanization coat or a zinc alloy galvanization coat is provided at a surface of the high-strength steel sheet.

14. The high-strength steel sheet according to claim 3 , wherein a zinc galvanization coat or a zinc alloy galvanization coat is provided at a surface of the high-strength steel sheet.

15. The high-strength steel sheet according to claim 12 , wherein a zinc galvanization coat or a zinc alloy galvanization coat is provided at a surface of the high-strength steel sheet.

16. The high-strength steel sheet according to claim 13 ,

wherein the high-strength steel sheet further comprises Cr: 0.20% to 0.40%, and P: less than 0.015%.

17. The high-strength steel sheet according to claim 14 ,

wherein the high-strength steel sheet further comprises Cr: 0.20% to 0.40%, and P: less than 0.015%.

18. The high-strength steel sheet according to claim 15 ,

wherein the high-strength steel sheet further comprises Cr: 0.20% to 0.40%, and P: less than 0.015%.

19. A method for producing a high-strength steel sheet,

wherein a billet having chemical components of the high-strength steel sheet according to claim 2 is hot rolled, pickled, cold rolled at a percentage reduction in a thickness of more than 60% so as to obtain a steel sheet,

then, the steel sheet is heated to a temperature range of 720° C. to 850° C. at a heating rate controlled to 1° C./s to 10° C./s in a temperature range of an Ac 1 transformation point to an Ac 3 transformation point, and subjected to an annealing for a retention time of 10 seconds to 200 seconds during which a temperature of the steel sheet is 720° C. to 850° C., and

after the annealing, the steel sheet is subjected to a first cooling to 500° C. or lower at a cooling rate of 3° C./s or more, and then subjected to a skin pass rolling of 2.0% or less.

20. A method for producing a high-strength steel sheet,

wherein a billet having chemical components of the high-strength steel sheet according to claim 3 is hot rolled, pickled, cold rolled at a percentage reduction in a thickness of more than 60% so as to obtain a steel sheet,

then, the steel sheet is heated to a temperature range of 720° C. to 850° C. at a heating rate controlled to 1° C./s to 10° C./s in a temperature range of an Ac 1 transformation point to an Ac 3 transformation point, and subjected to an annealing for a retention time of 10 seconds to 200 seconds during which a temperature of the steel sheet is 720° C. to 850° C., and

after the annealing, the steel sheet is subjected to a first cooling to 500° C. or lower at a cooling rate of 3° C./s or more, and then subjected to a skin pass rolling of 2.0% or less.

21. A method for producing a high-strength steel sheet,

wherein a billet having chemical components of the high-strength steel sheet according to claim 12 is hot rolled, pickled, cold rolled at a percentage reduction in a thickness of more than 60% so as to obtain a steel sheet,

then, the steel sheet is heated to a temperature range of 720° C. to 850° C. at a heating rate controlled to 1° C./s to 10° C./s in a temperature range of an Ac 1 transformation point to an Ac 3 transformation point, and subjected to an annealing for a retention time of 10 seconds to 200 seconds during which a temperature of the steel sheet is 720° C. to 850° C., and

after the annealing, the steel sheet is subjected to a first cooling to 500° C. or lower at a cooling rate of 3° C./s or more, and then subjected to a skin pass rolling of 2.0% or less.

22. The method for producing the high-strength steel sheet according to claim 19 , wherein a heat treatment is carried out in a temperature range of 200° C. to 450° C. for 30 seconds or more before the skin pass rolling, and

a second cooling is carried out at a cooling rate of 1° C./s to 3° C./s to 100° C. or lower after the heat treatment.

23. The method for producing the high-strength steel sheet according to claim 20 , wherein a heat treatment is carried out in a temperature range of 200° C. to 450° C. for 30 seconds or more before the skin pass rolling, and

a second cooling is carried out at a cooling rate of 1° C./s to 3° C./s to 100° C. or lower after the heat treatment.

24. The method for producing the high-strength steel sheet according to claim 21 , wherein a heat treatment is carried out in a temperature range of 200° C. to 450° C. for 30 seconds or more before the skin pass rolling, and

a second cooling is carried out at a cooling rate of 1° C./s to 3° C./s to 100° C. or lower after the heat treatment.

25. The method for producing the high-strength steel sheet according to claim 19 , wherein galvanization is carried out on the steel sheet after the first cooling and before the skin pass rolling.

26. The method for producing the high-strength steel sheet according to claim 20 , wherein galvanization is carried out on the steel sheet after the first cooling and before the skin pass rolling.

27. The method for producing the high-strength steel sheet according to claim 21 , wherein galvanization is carried out on the steel sheet after the first cooling and before the skin pass rolling.