Method for producing cold-rolled steel sheet

1. A method for manufacturing a cold-rolled steel sheet having a metallic structure such that the main phase is a low-temperature transformation producing phase, and the secondary phase contains retained austenite, which is more than 6.0% and less than 25.0% in volume ratio with respect to a total structure, characterized by comprising the following steps (C) to (E):

(C) a hot-rolling step in which a slab having a chemical composition consisting, in mass percent, of C: more than 0.020% and less than 0.30%, Si: more than 0.10% and at most 3.00%, Mn: more than 1.00% and at most 3.50%, P: at most 0.10%, S: at most 0.010%, sol.Al: at least 0% and at most 2.00%, N: at most 0.010%, Ti: at least 0% and less than 0.050%, Nb: at least 0% and less than 0.050%, V: at least 0% and at most 0.50%, Cr: at least 0% and at most 1.0%, Mo: at least 0% and at most 0.50%, B: at least 0% and at most 0.010%, Ca: at least 0% and at most 0.010%, Mg: at least 0% and at most 0.010%, REM: at least 0% and at most 0.050%, and Bi: at least 0% and at most 0.050%, the remainder of Fe and impurities, is subjected to hot rolling such that the roll draft of the final one pass is higher than 15%, and rolling is finished in the temperature region of Ar 3 point or higher to form a hot-rolled steel sheet, and the hot-rolled steel sheet is cooled to the temperature region of 780° C. or lower within 0.4 seconds after the completion of the rolling, and is coiled in the temperature region of higher than 400° C.;

(D) a cold-rolling step in which the hot-rolled steel sheet obtained by the step (C) is subjected to cold rolling in which a cold rolling ratio is 40% or higher, to form a cold-rolled steel sheet; and

(E) an annealing step in which the cold-rolled steel sheet is subjected to soaking treatment in the temperature region of (Ac 3 point−40° C.) or higher, thereafter cooled to the temperature region of 500° C. or lower and 300° C. or higher, and is held in that temperature region for 30 seconds or longer.

2. A method for manufacturing a cold-rolled steel sheet having a metallic structure such that the main phase is a low-temperature transformation producing phase, and the secondary phase contains retained austenite, which is more than 6.0% and less than 25.0% in volume ratio with respect to a total structure, characterized by comprising the following steps (F) to (I):

(F) a hot-rolling step in which a slab having a chemical composition consisting, in mass percent, of C: more than 0.020% and less than 0.30%, Si: more than 0.10% and at most 3.00%, Mn: more than 1.00% and at most 3.50%, P: at most 0.10%, S: at most 0.010%, sol.Al: at least 0% and at most 2.00%, N: at most 0.010%, Ti: at least 0% and less than 0.050%, Nb: at least 0% and less than 0.050%, V: at least 0% and at most 0.50%, Cr: at least 0% and at most 1.0%, Mo: at least 0% and at most 0.50%, B: at least 0% and at most 0.010%, Ca: at least 0% and at most 0.010%, Mg: at least 0% and at most 0.010%, REM: at least 0% and at most 0.050%, and Bi: at least 0% and at most 0.050%, the remainder of Fe and impurities, is subjected to hot rolling such that the rolling is finished in the temperature region of Ar 3 point or higher to form a hot-rolled steel sheet, and the hot-rolled steel sheet is cooled to the temperature region of 780° C. or lower within 0.4 seconds after the completion of the rolling, and is coiled in the temperature region of lower than 400° C.;

(G) a hot-rolled sheet annealing step in which the hot-rolled steel sheet obtained by the step (F) is subjected to annealing such that the hot-rolled steel sheet is heated to the temperature region of 300° C. or higher to form a hot-rolled and annealed steel sheet;

(H) a cold-rolling step in which the hot-rolled and annealed steel sheet is subjected to cold rolling in which a cold rolling ratio is 40% or higher, to form a cold-rolled steel sheet; and

(I) an annealing step in which the cold-rolled steel sheet is subjected to soaking treatment in the temperature region of (Ac 3 point−40° C.) or higher, thereafter cooled to the temperature region of 500° C. or lower and 300° C. or higher, and is held in that temperature region for 30 seconds or longer.

3. The method for manufacturing a cold-rolled steel sheet as set forth in claim 1 , wherein, in the annealing step (E), the soaking treatment is performed in the temperature region of (Ac 3 point−40° C.) or higher and lower than (Ac 3 point+50° C.).

4. The method for manufacturing a cold-rolled steel sheet as set forth in claim 2 , wherein, in the annealing step (I), the soaking treatment is performed in the temperature region of (Ac 3 point−40° C.) or higher and lower than (Ac 3 point+50° C.).

5. The method for manufacturing a cold-rolled steel sheet as set forth in claim 1 , wherein, in the annealing step (E), the cooling is performed by 50° C. or more at a cooling rate of lower than 10.0° C./s after the soaking treatment.

6. The method for manufacturing a cold-rolled steel sheet as set forth in claim 2 , wherein, in the annealing step (I), the cooling is performed by 50° C. or more at a cooling rate of lower than 10.0° C./s after the soaking treatment.

7. The method for manufacturing a cold-rolled steel sheet as set forth in claim 1 , wherein, in the metallic structure of the cold-rolled steel sheet, the secondary phase contains retained austenite and polygonal ferrite.

8. The method for manufacturing a cold-rolled steel sheet as set forth in claim 2 , wherein, in the metallic structure of the cold-rolled steel sheet, the secondary phase contains retained austenite and polygonal ferrite.

9. The method for manufacturing a cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition contains, in mass percent, one kind or two or more kinds selected from a group consisting of Ti: at least 0.005% and less than 0.050%, Nb: at least 0.005% and less than 0.050%, and V: at least 0.010% and at most 0.50%.

10. The method for manufacturing a cold-rolled steel sheet as set forth in claim 2 , wherein the chemical composition contains, in mass percent, one kind or two or more kinds selected from a group consisting of Ti: at least 0.005% and less than 0.050%, Nb: at least 0.005% and less than 0.050%, and V: at least 0.010% and at most 0.50%.

11. The method for manufacturing a cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition contains, in mass percent, one kind or two or more kinds selected from a group consisting of Cr: at least 0.20% and at most 1.0%, Mo: at least 0.05% and at most 0.50%, and B: at least 0.0010% and at most 0.010%.

12. The method for manufacturing a cold-rolled steel sheet as set forth in claim 2 , wherein the chemical composition contains, in mass percent, one kind or two or more kinds selected from a group consisting of Cr: at least 0.20% and at most 1.0%, Mo: at least 0.05% and at most 0.50%, and B: at least 0.0010% and at most 0.010%.

13. The method for manufacturing a cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition contains, in mass percent, one kind or two or more kinds selected from a group consisting of Ca: at least 0.0005% and at most 0.010%, Mg: at least 0.0005% and at most 0.010%, REM: at least 0.0005% and at most 0.050%, and Bi: at least 0.0010% and at most 0.050%.

14. The method for manufacturing a cold-rolled steel sheet as set forth in claim 2 , wherein the chemical composition contains, in mass percent, one kind or two or more kinds selected from a group consisting of Ca: at least 0.0005% and at most 0.010%, Mg: at least 0.0005% and at most 0.010%, REM: at least 0.0005% and at most 0.050%, and Bi: at least 0.0010% and at most 0.050%.