High strength cold rolled steel sheet with low yield ratio and method of manufacturing the same

1. A high strength cold rolled steel sheet with a low yield ratio comprising a chemical composition containing C: 0.05% to 0.10%, Si: 0.6% to 1.3%, Mn: 1.4% to 2.2%, P: 0.08% or less, S: 0.010% or less, Al: 0.01% to 0.08%, N: 0.010% or less, and the remainder being Fe and incidental impurities, on a percent by mass basis, and a microstructure in which average grain size of martensite is 5 μm or less, and average grain size of ferrite is 15 μm or less, the microstructure consisting of a volume fraction of ferrite of 70% or more, a volume fraction of bainite of 3% or more, a volume fraction of retained austenite of 4% to 7%, a volume fraction of martensite of 1% to 6% and, optionally, spheroidal cementite,

wherein average C concentration (percent by mass) in the retained austenite is 0.30% to 0.70%, yield ratio is 64% or less and tensile strength is 590 MPa or more.

2. The high strength cold rolled steel sheet with a low yield ratio according to claim 1 , the chemical composition further containing, on a percent by mass basis, at least one element selected from Group A to Group E:

Group A

V: 0.10% or less, Ti: 0.10% or less, and Nb: 0.10% or less;

Group B

Cr: 0.50% or less and Mo: 0.50% or less;

Group C

Cu: 0.50% or less and Ni: 0.50% or less;

Group D

B: 0.0030% or less;

Group E

0.0050% or less in total of at least one of Ca and REM.

3. A method of manufacturing the high strength cold rolled steel sheet according to claim 1 , the method comprising:

preparing a steel slab,

performing hot rolling to produce a steel sheet,

performing pickling,

subjecting the pickled steel sheet to cold rolling, and

performing annealing under conditions of performing heating to an annealing temperature at 780° C. to 900° C. at an average heating rate of 3° C./s to 30° C./s,

performing holding at the annealing temperature for 30 s to 500 s,

subsequently performing cooling to a first cooling temperature within a temperature range of (annealing temperature—10° C.) to (annealing temperature—30° C.) at a first average cooling rate of 5° C./s or less,

subsequently performing cooling to a second cooling temperature at 350° C. to 450° C. at a second average cooling rate of 5° C./s to 30° C./s, and

subsequently performing cooling to room temperature at a third average cooling rate of 5° C./s or less.

4. A method of manufacturing the high strength cold rolled steel sheet according to claim 1 , the method comprising:

preparing a steel slab,

performing hot rolling under the conditions of steel slab temperature: 1,150° C. to 1,300° C. and finishing delivery temperature: 850° C. to 950° C.,

starting cooling within 1 second after finishing of the hot rolling,

performing cooling to 550° C. or lower at an average cooling rate of 50° C./s or more,

performing coiling after the cooling to produce a hot rolled steel sheet,

performing pickling,

subjecting the pickled hot rolled steel sheet to cold rolling,

performing annealing under the conditions of performing heating to an annealing temperature of 780° C. to 900° C. at an average heating rate of 3° C./s to 30° C./s,

performing holding at the annealing temperature for 30 s to 500 s,

subsequently performing cooling to a first cooling temperature within a temperature range of (annealing temperature—10° C.) to (annealing temperature—30° C.) at a first average cooling rate of 5° C./s or less,

subsequently performing cooling to a second cooling temperature at 350° C. to 450° C. at a second average cooling rate of 5° C./s to 30° C./s, and

subsequently performing cooling to room temperature at a third average cooling rate of 5° C./s or less.

5. A method of manufacturing the high strength cold rolled steel sheet according to claim 2 , the method comprising:

preparing a steel slab,

performing hot rolling to produce a steel sheet,

performing pickling,

subjecting the pickled steel sheet to cold rolling, and

performing annealing under conditions of performing heating to an annealing temperature at 780° C. to 900° C. at an average heating rate of 3° C./s to 30° C./s,

performing holding at the annealing temperature for 30 s to 500 s,

subsequently performing cooling to a first cooling temperature within a temperature range of (annealing temperature—10° C.) to (annealing temperature—30° C.) at a first average cooling rate of 5° C./s or less,

subsequently performing cooling to a second cooling temperature at 350° C. to 450° C. at a second average cooling rate of 5° C./s to 30° C./s, and

subsequently performing cooling to room temperature at a third average cooling rate of 5° C./s or less.

6. A method of manufacturing the high strength cold rolled steel sheet according to claim 2 , the method comprising:

preparing a steel slab,

performing hot rolling under the conditions of steel slab temperature: 1,150° C. to 1,300° C. and finishing delivery temperature: 850° C. to 950° C.,

starting cooling within 1 second after finishing of the hot rolling,

performing cooling to 550° C. or lower at an average cooling rate of 50° C./s or more,

performing coiling after the cooling to produce a hot rolled steel sheet,

performing pickling,

subjecting the pickled hot rolled steel sheet to cold rolling,

performing annealing under the conditions of performing heating to an annealing temperature of 780° C. to 900° C. at an average heating rate of 3° C./s to 30° C./s,

performing holding at the annealing temperature for 30 s to 500 s,

subsequently performing cooling to a first cooling temperature within a temperature range of (annealing temperature—10° C.) to (annealing temperature—30° C.) at a first average cooling rate of 5° C./s or less,

subsequently performing cooling to a second cooling temperature at 350° C. to 450° C. at a second average cooling rate of 5° C./s to 30° C./s, and

subsequently performing cooling to room temperature at a third average cooling rate of 5° C./s or less.