Cold-rolled steel sheet, plated steel sheet, and method for manufacturing the same

1. A method for manufacturing a cold-rolled steel sheet, the cold-rolled steel sheet consisting of, by mass %:

C: 0.020% or more and 0.080% or less;

Si: 0.20% or more and 1.00% or less;

Mn: 0.80% or more and 2.30% or less;

P: 0.0050% or more and 0.1500% or less;

S: 0.0020% or more and 0.0150% or less;

Al: 0.010% or more and 0.100% or less;

N: 0.0010% or more and 0.0100% or less;

one or more of Nb and Ti which satisfy a requirement of 0.005%≤Nb+Ti<0.030%; and

a remainder including Fe and unavoidable impurities,

wherein a structure consists of, a ferrite, a bainite, and an other phase, the other phase includes one or more of a pearlite, a residual austenite, and a martensite, an area ratio of the ferrite is 80% or more and less than 95%, an area ratio of a non-recrystallization ferrite in the ferrite is 1% or more and less than 10%, an area ratio of the bainite is 5% to 20%, a total amount of the other phase is less than 8%, an equivalent circle diameter of a carbonitride including one or both of Nb and Ti is 1 nm or more and 10 nm or less, and a tensile strength is 590 MPa or more,

said method comprising:

heating a slab having a composition consisting of: C: 0.020% or more and 0.080% or less; Si: 0.20% or more and 1.00% or less; Mn: 0.80% or more and 2.30% or less; P: 0.0050% or more and 0.1500% or less; S: 0.0020% or more and 0.0150% or less; Al: 0.010% or more and 0.100% or less; N: 0.0010% or more and 0.0100% or less; one or more of Nb and Ti which satisfy a requirement of 0.005%≤Nb+Ti<0.030%; and a remainder including Fe and unavoidable impurities, to 1150° C. or more and 1280° C. or less;

finishing a finish rolling under a temperature of Ar 3 ° C. or more and 1050° C. or less;

pickling and then cold-rolling a hot-rolled steel sheet, which is coiled under a temperature range of 450° C. or more and 650° C. or less, under a reduction of 40% or more and 70% or less; thereafter

heating into a temperature range of Ac 1 ° C. or more and (Ac 1 +40) ° C. or less under a rate of 2° C./sec or more and 4° C./sec or less;

annealing the cold-rolled steel sheet under the temperature range of Ac 1 ° C. or more and (Ac 1 +40)° C. or less and under a holding time of 10 sec or more and 200 sec or less; and

primary cooling immediately after the annealing into a steel sheet temperature range of 600° C. or more and 720° C. or less under a cooling rate of 10° C./sec or less in a course from the annealing to arriving at a normal temperature,

wherein the Ar 3 ° C. and the Ac 1 ° C. is a Ar 3 transformation temperature and a Ac 1 transformation temperature, respectively, calculated from Expressions 1 and 2,

Ar 3 =910−325×[C]+33×[Si]+287×[P]+40×[Al]−92×([Mn]+[Mo]+[Cu])−46×([Cr]+[Ni])  (Expression 1),

Ac 1 =723+212×[C]−10.7×[Mn]+29.1×[Si]  (Expression 2), and

elements noted in brackets represents an amount of the elements by mass %, respectively.

2. A method for manufacturing a plated steel sheet, comprising:

plating the cold-rolled steel sheet manufactured by the method according to claim 1 after the annealing and the cooling.

3. The method for manufacturing a plated steel sheet according to claim 2 , comprising:

heat treating the plated steel sheet under a temperature range of 450° C. or more and 600° C. or less with 10 seconds or longer.

4. A method for manufacturing a cold-rolled steel sheet, the cold-rolled steel sheet consisting of, by mass %:

C: 0.020% or more and 0.080% or less;

Si: 0.20% or more and 1.00% or less;

Mn: 0.80% or more and 2.30% or less;

P: 0.0050% or more and 0.1500% or less;

S: 0.0020% or more and 0.0150% or less;

Al: 0.010% or more and 0.100% or less;

N: 0.0010% or more and 0.0100% or less;

one or more of Nb and Ti which satisfy a requirement of 0.005%≤Nb+Ti<0.030%; and

a remainder including Fe and unavoidable impurities,

wherein a structure comprises a ferrite and a bainite and optionally an other phase,

wherein the optional other phase, when present, includes one or more of a pearlite, a residual austenite, and a martensite, said optional other phase, when present, is present in a total amount of up to less than 8%,

an area ratio of the ferrite is 80% or more and less than 95%,

an area ratio of a non-recrystallization ferrite in the ferrite is 1% or more and less than 10%,

an area ratio of the bainite is 5% to 20%,

an equivalent circle diameter of a carbonitride including one or both of Nb and Ti is 1 nm or more and 10 nm or less, and

a tensile strength is 590 MPa or more, said method comprising:

heating a slab having a composition consisting of: C: 0.020% or more and 0.080% or less; Si: 0.20% or more and 1.00% or less; Mn: 0.80% or more and 2.30% or less; P: 0.0050% or more and 0.1500% or less; S: 0.0020% or more and 0.0150% or less; Al: 0.010% or more and 0.100% or less; N: 0.0010% or more and 0.0100% or less; one or more of Nb and Ti which satisfy a requirement of 0.005%≤Nb+Ti<0.030%; and a remainder including Fe and unavoidable impurities to 1150° C. or more and 1280° C. or less;

finishing a finish rolling under a temperature of Ar 3 ° C. or more and 1050° C. or less;

pickling and then cold-rolling a hot-rolled steel sheet, which is coiled under a temperature range of 450° C. or more and 650° C. or less, under a reduction of 40% or more and 70% or less; thereafter

heating into a temperature range of Ac 1 ° C. or more and (Ac 1 +40)° C. or less under a rate of 2° C./sec or more and 4° C./sec or less;

annealing the cold-rolled steel sheet under the temperature range of Ac 1 ° C. or more and (Ac 1 +40)° C. or less and under a holding time of 10 sec or more and 200 sec or less; and

primary cooling immediately after the annealing into a steel sheet temperature range of 600° C. or more and 720° C. or less under a cooling rate of 10° C./sec or less in a course from the annealing to arriving at a normal temperature,

wherein the Ar 3 ° C. and the Ac 1 ° C. is a Ar 3 transformation temperature and a Ac 1 transformation temperature, respectively, calculated from Expressions 1 and 2,

Ar 3 =910−325×[C]+33×[Si]+287×[P]+40×[Al]−92×([Mn]+[Mo]+[Cu])−46×([Cr]+[Ni])  (Expression 1),

Ac 1 =723+212×[C]−10.7×[Mn]+29.1×[Si]  (Expression 2), and

elements noted in brackets represents an amount of the elements by mass %, respectively.

5. A method for manufacturing a plated steel sheet,

comprising:

plating the cold-rolled steel sheet manufactured by the method according to claim 4 after the annealing and the cooling.

6. The method for manufacturing a plated steel sheet

according to claim 5 , comprising:

heat treating the plated steel sheet under a temperature range of 450° C. or more and 600° C. or less with 10 seconds or longer.