High-strength cold-rolled steel sheet and method for manufacturing the same (as amended)

1. A high-strength cold-rolled steel sheet having a chemical composition comprising: by mass %, C: 0.15% or more and 0.30% or less, Si: 0.8% or more and 2.4% or less, Mn: 2.4% or more and 3.5% or less, P: 0.08% or less, S: 0.005% or less, Al: 0.01% or more and 0.08% or less, N: 0.010% or less, Ti: 0.002% or more and 0.05% or less, B: 0.0002% or more and 0.0050% or less, and the balance being Fe and inevitable impurities, a microstructure including ferrite having an average grain diameter of 3 μm or less and a volume fraction of 5% or less (including 0%), retained austenite having a volume fraction of 10% or more and 20% or less, martensite having an average grain diameter of 4 μm or less and a volume fraction of 20% or less (including 0%), and the balance including bainite and/or tempered martensite, wherein an average number of cementite grains having a grain diameter of 0.1 μm or more per 100 firm in a cross section in the thickness direction parallel to the rolling direction of the steel sheet is 30 or more.

2. The high-strength cold-rolled steel sheet according to claim 1 , the steel sheet having the chemical composition further comprising: at least one of the following groups A to C,

A: by mass %, one or more selected from V: 0.10% or less and Nb: 0.10% or less;

B: by mass %, one or more selected from Cr: 0.50% or less, Mo: 0.50% car less, Cu: 0.50% or less, and Ni: 0.50% or less; and

C: by mass %, Ca and/or REM in an amount of 0.0050% or less in total.

3. A method for manufacturing a high-strength cold-rolled steel sheet, the method comprising:

performing hot rolling on a steel slab with a hot rolling start temperature of 1150° C. or higher and 1300° C. or lower and a finishing delivery temperature of 850° C. or higher and 950° C. or lower;

starting cooling within one second after hot rolling has been, performed;

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

subsequently performing second cooling to a temperature of 550° C. or lower at a second average cooling rate of 5° C./s or more, then coiling the cooled steel sheet at a coiling temperature of 550° C. or lower;

then performing a first heat treatment in which the coiled steel sheet is held in a temperature range of 400° C. or higher and 750° C. or lower for 30 seconds or more;

subsequently performing cold rolling; and

performing continuous annealing as a second heat treatment, in which the cold-roiled steel sheet is heated to a temperature range of 830° C. or higher at an average heating rate of 3° C./s or more and 30° C./s or less, in which the heated steel sheet is held at a first soaking temperature of 830° C. or higher for 30 seconds or more, in which the held steel sheet is then cooled from the first soaking temperature to a cooling stop temperature range expressed by Ta° C., which satisfies relational expression (1) below, at an average cooling rate of 3° C./s or more, in which the cooled steel sheet is subsequently heated to a temperature range expressed by Tb° C., which satisfies relational expression (2) below, in which the heated steel sheet is held at a second soaking temperature in a temperature range expressed by Tb° C., which satisfies relational expression (2) below, for 20 seconds or more, and in which the held steel sheet is then cooled to room temperature to produce the high-strength cold-rolled steel sheet of claim 1 :

0.35≤1−exp{−0.011×(561−[C]×474−[Mn]×33−[Ni]×17−[Cr]×17−[Mo]×21−Ta)}≤0.95,  Relational expression (1):

−3.0≤1−exp{−0.011×(561−[C]×474−[Mn]×33−[Ni]×17−[Cr]×17−[Mo]×21−Tb)}<0.35,  Relational expression (2):

where, symbol [M] in each relational expression denotes the content (mass %) of the chemical element denoted by M.

4. A method for manufacturing a high-strength old-rolled steel sheet, the method comprising:

performing hot rolling on a steel slab with a hot rolling start temperature of 1150° C. or higher and 1300° C. or lower and a finishing delivery temperature of 850° C. or higher and 950° C. or lower;

starting cooling within one second after hot rolling has been performed;

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

subsequently performing second cooling to a temperature of 550° C. or lower at a second average cooling rate of 5° C./s or more, then coiling the cooled steel sheet at a coiling temperature of 550° C. or lower;

then performing a first heat treatment in which the coiled steel sheet is held temperature range of 400° C. or higher and 750° C. or lower for 30 seconds or more;

subsequently performing cold rolling; and

performing continuous annealing as a second heat treatment, in which the cold-rolled steel sheet is heated to a temperature range of 830° C. or higher at an average heating rate of 3° C./s or more and 30° C./s or less, in which the heated steel sheet is held at a first soaking temperature of 830° C. or higher for 30 seconds or more, in which the held steel sheet is then cooled from the first soaking temperature to a cooling stop temperature range expressed by Ta° C., which satisfies relational expression (1) below, at an average cooling rate of 3° C./s or more, in which the cooled steel sheet is subsequently heated to a temperature range expressed by Tb° C., which satisfies relational expression (2) below, in which the heated steel sheet is held at a second soaking temperature in a temperature range expressed by Tb° C., which satisfies relational expression (2) below, for 20 seconds or more, and in which the held steel sheet is then cooled to room temperature to produce the high-strength cold-rolled steel sheet of claim 2 :

0.35≤1−exp{−0.011×(561−[C]×474−[Mn]×33−[Ni]×17−[Cr]×17−[Mo]×21−Ta)}≤0.95,  Relational expression (1):

−3.0≤1−exp{−0.011×(561−[C]×474−[Mn]×33−[Ni]×17−[Cr]×17−[Mo]×21−Tb)}<0.35,  Relational expression (2):

where, symbol [M] in each relational expression notes the content (mass %) of the chemical element denoted by M.