High-yield-ratio, high-strength cold rolled steel sheet and production method therefor

1. A high-yield-ratio, high-strength cold rolled steel sheet, comprising a composition and a microstructure,

the composition comprising, in terms of percent by mass, C: 0.05% to 0.15%, Si: 0.6% to 2.5%, Mn: 2.2% to 3.5%, P: 0.08% or less, S: 0.010% or less, Al: 0.01% to 0.08%, N: 0.010% or less, Ti: 0.002% to 0.05%, B: 0.0002% to 0.0050%, and the balance being Fe and unavoidable impurities,

the microstructure comprising a volume fraction of 20% to 55% of ferrite having an average grain size of 7 μm or less, a volume fraction of 5% to 15% of retained austenite, a volume fraction of 0.5% to 7% of martensite having an average grain size of 4 μm or less, and a structure composed of a balance of bainite and tempered martensite and having an average grain size of 6 μm or less, the bainite having a volume fraction of 10 to 25% and the tempered martensite having a volume fraction of 20 to 50%, and

a difference in nano-hardness between the ferrite and the structure composed of bainite and/or tempered martensite being 3.5 GPa or less, and

a difference in nano-hardness between the structure composed of bainite and/or tempered martensite and the martensite being 2.5 GPa or less,

wherein the steel sheet has a yield ratio of 80% or more.

2. The steel sheet according to claim 1 , wherein the composition further comprises, in terms of percent by mass, at least one selected from V: 0.10% or less and Nb: 0.10% or less.

3. The steel sheet according to claim 1 , wherein the composition further comprises, in terms of percent by mass, at least one selected from Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, and Ni: 0.50% or less.

4. The steel sheet according to claim 1 , wherein the composition further comprises, in terms of percent by mass, at least one selected from Ca: 0.0050% or less and REM: 0.0050% or less.

5. A method of producing a high-yield-ratio, high-strength cold rolled steel sheet comprising:

providing a steel slab having a chemical composition described in claim 1 ,

hot-rolling the steel slab under conditions of hot rolling start temperature: 1150° C. to 1300° C. and finishing delivery temperature: 850° C. to 950° C.,

starting cooling within 1 s after completion of hot rolling,

performing cooling to 650° C. or lower at a first average cooling rate of 80° C./s or more as first cooling,

performing cooling to 550° C. or lower at a second average cooling rate of 5° C./s or more as second cooling,

performing coiling at a coiling temperature: 550° C. or lower,

performing pickling and cold-rolling,

performing heating to a temperature zone of 750° C. or higher at an average heating rate of 3 to 30° C./s,

holding a first soaking temperature of 750° C. or higher for 30 s or longer,

performing cooling from the first soaking temperature to a cooling end temperature in a temperature zone of 150° C. to 350° C. at a third average cooling rate of 3° C./s or more,

performing heating to a second soaking temperature in a temperature zone of 350° C. to 500° C.,

holding the second soaking temperature for 20 s or longer, and

performing cooling to room temperature.

6. The steel sheet according to claim 2 , wherein the composition further comprises, in terms of percent by mass, at least one selected from Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, and Ni: 0.50% or less.

7. The steel sheet according to claim 2 , wherein the composition further comprises, in terms of percent by mass, at least one selected from Ca: 0.0050% or less and REM: 0.0050% or less.

8. The steel sheet according to claim 3 , wherein the composition further comprises, in terms of percent by mass, at least one selected from Ca: 0.0050% or less and REM: 0.0050% or less.

9. A method of producing a high-yield-ratio, high-strength cold rolled steel sheet comprising:

providing a steel slab having a chemical composition described in claim 2 ,

hot-rolling the steel slab under conditions of hot rolling start temperature: 1150° C. to 1300° C. and finishing delivery temperature: 850° C. to 950° C.,

starting cooling within 1 s after completion of hot rolling,

performing cooling to 650° C. or lower at a first average cooling rate of 80° C./s or more as first cooling,

performing cooling to 550° C. or lower at a second average cooling rate of 5° C./s or more as second cooling,

performing coiling at a coiling temperature: 550° C. or lower,

performing pickling and cold-rolling,

performing heating to a temperature zone of 750° C. or higher at an average heating rate of 3 to 30° C./s,

holding a first soaking temperature of 750° C. or higher for 30 s or longer,

performing cooling from the first soaking temperature to a cooling end temperature in a temperature zone of 150° C. to 350° C. at a third average cooling rate of 3° C./s or more,

performing heating to a second soaking temperature in a temperature zone of 350° C. to 500° C.,

holding the second soaking temperature for 20 s or longer, and

performing cooling to room temperature.

10. A method of producing a high-yield-ratio, high-strength cold rolled steel sheet comprising:

providing a steel slab having a chemical composition described in claim 3 ,

hot-rolling the steel slab under conditions of hot rolling start temperature: 1150° C. to 1300° C. and finishing delivery temperature: 850° C. to 950° C.,

starting cooling within 1 s after completion of hot rolling,

performing cooling to 650° C. or lower at a first average cooling rate of 80° C./s or more as first cooling,

performing cooling to 550° C. or lower at a second average cooling rate of 5° C./s or more as second cooling,

performing coiling at a coiling temperature: 550° C. or lower,

performing pickling and cold-rolling,

performing heating to a temperature zone of 750° C. or higher at an average heating rate of 3 to 30° C./s,

holding a first soaking temperature of 750° C. or higher for 30 s or longer,

performing cooling from the first soaking temperature to a cooling end temperature in a temperature zone of 150° C. to 350° C. at a third average cooling rate of 3° C./s or more,

performing heating to a second soaking temperature in a temperature zone of 350° C. to 500° C.,

holding the second soaking temperature for 20 s or longer, and

performing cooling to room temperature.

11. A method of producing a high-yield-ratio, high-strength cold rolled steel sheet comprising:

providing a steel slab having a chemical composition described in claim 4 ,

hot-rolling the steel slab under conditions of hot rolling start temperature: 1150° C. to 1300° C. and finishing delivery temperature: 850° C. to 950° C.,

starting cooling within 1 s after completion of hot rolling,

performing cooling to 650° C. or lower at a first average cooling rate of 80° C./s or more as first cooling,

performing cooling to 550° C. or lower at a second average cooling rate of 5° C./s or more as second cooling,

performing coiling at a coiling temperature: 550° C. or lower,

performing pickling and cold-rolling,

performing heating to a temperature zone of 750° C. or higher at an average heating rate of 3 to 30° C./s,

holding a first soaking temperature of 750° C. or higher for 30 s or longer,

performing cooling from the first soaking temperature to a cooling end temperature in a temperature zone of 150° C. to 350° C. at a third average cooling rate of 3° C./s or more,

performing heating to a second soaking temperature in a temperature zone of 350° C. to 500° C.,

holding the second soaking temperature for 20 s or longer, and

performing cooling to room temperature.

12. The steel sheet according to claim 6 , wherein the composition further comprises, in terms of percent by mass, at least one selected from Ca: 0.0050% or less and REM: 0.0050% or less.