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 0.15% and 0.45% or less,

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

Mn: 1.5% or more and 3.0% 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; and

a microstructure in which:

a total area ratio of ferrite and bainitic ferrite is 20% or more and 80% or less;

an area ratio of retained austenite is more than 10% and 40% or less;

an area ratio of tempered martensite is more than 0% and 50% or less;

a ratio of retained austenite with an aspect ratio of 0.5 or less to the retained austenite is 75% or more in area ratio;

a ratio of retained austenite with an aspect ratio of 0.5 or less existing in ferrite grain boundaries with an orientation difference of 40° or more to the retained austenite with an aspect ratio of 0.5 or less is 50% or more in area ratio; and

an average Kernel Average Misorientation value of bcc phase is 1° or less, and

the high-strength cold rolled steel sheet has a tensile strength of 980 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 2 , comprising

a coated or plated layer at a surface thereof.

4. 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 Ac 3 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 having a microstructure mainly composed of lower bainite;

heating the first cold rolled and annealed sheet at an annealing temperature T 3 of 700° C. or more and 850° 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 5 of 100° C. or more and 550° C. or less, to obtain a third cold rolled and annealed sheet.

5. The method for manufacturing the high-strength cold rolled steel sheet according to claim 4 , further comprising

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

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

a coated or plated layer at a surface thereof.

7. 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 Ac 3 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 having a microstructure mainly composed of lower bainite;

heating the first cold rolled and annealed sheet at an annealing temperature T 3 of 700° C. or more and 850° 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 5 of 100° C. or more and 550° C. or less, to obtain a third cold rolled and annealed sheet.

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

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