High-strength cold-rolled steel sheet and method for manufacturing same

1. A high-strength cold-rolled steel sheet comprising:

a composition containing, in mass %,

C: more than 0.18% and 0.45% or less,

Si: 0.5% or more and 2.5% or less,

Mn: more than 2.5% and 4.0% or less,

P: 0.05% or less,

S: 0.01% or less,

Al: 0.01% or more and 0.1% or less, and

N: 0.01% or less,

with a balance consisting of Fe and inevitable impurities, C content and Mn content satisfying the following formula (1)

7.5C+Mn≥5.0  (1)

where C and Mn respectively denote the C content and the Mn content in mass %; and

a microstructure in which:

a total area ratio of ferrite and bainitic ferrite is 10% or more and 50% or less;

an area ratio of retained austenite is more than 15% and 50% or less;

an area ratio of tempered martensite is 31% or more and 60% or less;

a ratio of retained austenite with an aspect ratio of 0.6 or less to the retained austenite is 70% or more in area ratio;

a ratio of retained austenite with an aspect ratio of 0.6 or less existing in ferrite grain boundaries with an orientation difference of 40° or more to the retained austenite with an aspect ratio of 0.6 or less is 50% or more in area ratio; and

an average KAM value of bcc phase is 1° or less, and

the high-strength cold-rolled steel sheet has a tensile strength of 1320 MPa or more.

2. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains, in mass %, at least one selected from the group consisting of

Ti: 0.005% or more and 0.035% or less,

Nb: 0.005% or more and 0.035% or less,

V: 0.005% or more and 0.035% or less,

Mo: 0.005% or more and 0.035% or less,

B: 0.0003% or more and 0.01% or less,

Cr: 0.05% or more and 1.0% or less,

Ni: 0.05% or more and 1.0% or less,

Cu: 0.05% or more and 1.0% or less,

Sb: 0.002% or more and 0.05% or less,

Sn: 0.002% or more and 0.05% or less,

Ca: 0.0005% or more and 0.005% or less,

Mg: 0.0005% or more and 0.005% or less, and

REM: 0.0005% or more and 0.005% or less.

3. The high-strength cold-rolled steel sheet according to claim 1 , comprising

a coated or plated layer at a surface thereof.

4. The high-strength cold-rolled steel sheet according to claim 2 , comprising

a coated or plated layer at a surface thereof.

5. A method for manufacturing the high-strength cold-rolled steel sheet according to claim 1 , the method comprising: hot rolling a steel material having the composition according to claim 1 , to obtain a hot-rolled steel sheet; pickling the hot-rolled steel sheet; cold rolling the hot-rolled steel sheet that has been pickled at a rolling reduction of 30% or more, to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet at an annealing temperature T 1 of A c3 point or more and 950° C. or less, cooling the cold-rolled steel sheet from the annealing temperature T 1 to a cooling stop temperature T 2 of 250° C. or more and less than 350° C. at an average cooling rate of more than 10° C./sec, and holding the cold-rolled steel sheet at the cooling stop temperature T 2 for 10 sec or more, to obtain a first cold-rolled and annealed sheet; heating the first cold-rolled and annealed sheet at an annealing temperature T 3 of 680° C. or more and 820° C. or less, cooling the first cold-rolled and annealed sheet from the annealing temperature T 3 to a cooling stop temperature T 4 of 300° C. or more and 550° C. or less, holding the first cold-rolled and annealed sheet at the cooling stop temperature T 4 for 10 sec or more, and cooling the first cold-rolled and annealed sheet to a room temperature, to obtain a second cold-rolled and annealed sheet; and heating the second cold-rolled and annealed sheet at an annealing temperature T S of 100° C. or more and 550° C. or less, to obtain a third cold-rolled and annealed sheet; thereby producing the high-strength cold-rolled steel sheet according to claim 1 .

6. A method for manufacturing the high-strength cold-rolled steel sheet according to claim 2 , the method comprising: hot rolling a steel material having the composition according to claim 2 , to obtain a hot-rolled steel sheet; pickling the hot-rolled steel sheet; cold rolling the hot-rolled steel sheet that has been pickled at a rolling reduction of 30% or more, to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet at an annealing temperature T 1 of A c3 point or more and 950° C. or less, cooling the cold-rolled steel sheet from the annealing temperature T 1 to a cooling stop temperature T 2 of 250° C. or more and less than 350° C. at an average cooling rate of more than 10° C./sec, and holding the cold-rolled steel sheet at the cooling stop temperature T 2 for 10 sec or more, to obtain a first cold-rolled and annealed sheet; heating the first cold-rolled and annealed sheet at an annealing temperature T 3 of 680° C. or more and 820° C. or less, cooling the first cold-rolled and annealed sheet from the annealing temperature T 3 to a cooling stop temperature T 4 of 300° C. or more and 550° C. or less, holding the first cold-rolled and annealed sheet at the cooling stop temperature T 4 for 10 sec or more, and cooling the first cold-rolled and annealed sheet to a room temperature, to obtain a second cold-rolled and annealed sheet; and heating the second cold-rolled and annealed sheet at an annealing temperature T S of 100° C. or more and 550° C. or less, to obtain a third cold-rolled and annealed sheet; thereby producing the high-strength cold-rolled steel sheet according to claim 2 .

7. The method for manufacturing the high-strength cold-rolled steel sheet according to claim 5 , further comprising

subjecting the second cold-rolled and annealed sheet or the third cold-rolled and annealed sheet to a coating or plating treatment.

8. The method for manufacturing the high-strength cold-rolled steel sheet according to claim 6 , further comprising

subjecting the second cold-rolled and annealed sheet or the third cold-rolled and annealed sheet to a coating or plating treatment.