Cold-rolled steel sheet and method of manufacturing same

1. A cold-rolled steel sheet, comprising, as a chemical composition, in % by mass:

C: 0.100% or more and less than 0.500%;

Si: 0.8% or more and less than 4.0%;

Mn: 1.0% or more and less than 4.0%;

P: less than 0.015%;

S: less than 0.0500%;

N: less than 0.0100%;

Al: less than 2.000%;

Ti: 0.020% or more and less than 0.150%;

Nb: 0% or more and less than 0.200%;

V: 0% or more and less than 0.500%;

B: 0% or more and less than 0.0030%;

Mo: 0% or more and less than 0.500%;

Cr: 0% or more and less than 2.000%;

Mg: 0% or more and less than 0.0400%;

Rem: 0% or more and less than 0.0400%;

Ca: 0% or more and less than 0.0400%; and

a remainder of Fe and impurities,

wherein the total amount of Si and Al is 1.000% or more,

wherein a metallographic structure contains 40.0% or more and less than 60.0% of a polygonal ferrite, 30.0% or more of a bainitic ferrite, 10.0% to 25.0% of a residual austenite, and 15.0% or less of a martensite, by an area ratio,

wherein, in the residual austenite, a proportion of the residual austenite in which an aspect ratio is 2.0 or less, a length of a long axis is 1.0 μm or less, and a length of a short axis is 1.0 μm or less, is 80.0% or more,

wherein, in the bainitic ferrite, a proportion of the bainitic ferrite in which an aspect ratio is 1.7 or less and an average value of a crystal orientation difference in a region surrounded by a boundary in which a crystal orientation difference is 15° or more is 0.5° or more and less than 3.0°, is 80.0% or more,

wherein a connection index D value of the martensite, the bainitic ferrite, and the residual austenite is 0.70 or less, and

wherein a tensile strength is 980 MPa or more, a 0.2% proof stress is 600 MPa or more, a total elongation is 21.0% or more, and a hole expansion ratio is 30.0% or more.

2. The cold-rolled steel sheet according to claim 1 ,

wherein the connection index D value is 0.50 or less and the hole expansion ratio is 50.0% or more.

3. The cold-rolled steel sheet according to claim 1 or 2 , comprising, as the chemical composition, in % by mass:

one or two or more of

Nb: 0.005% or more and less than 0.200%;

V: 0.010% or more and less than 0.500%;

B: 0.0001% or more and less than 0.0030%;

Mo: 0.010% or more and less than 0.500%;

Cr: 0.010% or more and less than 2.000%;

Mg: 0.0005% or more and less than 0.0400%;

Rem: 0.0005% or more and less than 0.0400%; and

Ca: 0.0005% or more and less than 0.0400%.

4. A hot-rolled steel sheet which is used for manufacturing the cold-rolled steel sheet according to claim 1 or 2 , comprising, as a chemical composition, in % by mass:

C: 0.100% or more and less than 0.500%;

Si: 0.8% or more and less than 4.0%;

Mn: 1.0% or more and less than 4.0%;

P: less than 0.015%;

S: less than 0.0500%;

N: less than 0.0100%;

Al: less than 2.000%;

Ti: 0.020% or more and less than 0.150%;

Nb: 0% or more and less than 0.200%;

V: 0% or more and less than 0.500%;

B: 0% or more and less than 0.0030%;

Mo: 0% or more and less than 0.500%;

Cr: 0% or more and less than 2.000%;

Mg: 0% or more and less than 0.0400%;

Rem: 0% or more and less than 0.0400%;

Ca: 0% or more and less than 0.0400%; and

a remainder of Fe and impurities,

wherein the total amount of Si and Al is 1.000% or more,

wherein a metallographic structure contains a bainitic ferrite,

wherein, in the bainitic ferrite, an area ratio of the bainitic ferrite in which an average value of a crystal orientation difference in a region surrounded by a boundary in which a crystal orientation difference is 15° or more is 0.5° or more and less than 3.0°, is 80.0% or more, and

wherein a connection index E value of pearlite is 0.40 or less.

5. A method of manufacturing a cold-rolled steel sheet according to claim 1 , the method comprising:

casting a steel ingot or a slab including, as a chemical composition, C: 0.100% or more and less than 0.500%, Si: 0.8% or more and less than 4.0%, Mn: 1.0% or more and less than 4.0%, P: less than 0.015%, S: less than 0.0500%, N: less than 0.0100%, Al: less than 2.000%, Ti: 0.020% or more and less than 0.150%, Nb: 0% or more and less than 0.200%, V: 0% or more and less than 0.500%, B: 0% or more and less than 0.0030%, Mo: 0% or more and less than 0.500%, Cr: 0% or more and less than 2.000%, Mg: 0% or more and less than 0.0400%, Rem: 0% or more and less than 0.0400%, Ca: 0% or more and less than 0.0400%, and a remainder of Fe and impurities, in which the total amount of Si and Al is 1.000% or more;

hot rolling including a rough rolling in which the steel ingot or the slab is reduced at 40% or more in total in a first temperature range of 1000° C. to 1150° C., and a finish rolling in which the steel ingot or the slab is reduced at 50% or more in total in a second temperature range of T1° C. to T1+150° C., and the hot rolling being finished at T1−40° C. or more to obtain a hot-rolled steel sheet when a temperature determined by compositions specified in the following Equation (1) is set to be T1;

first cooling of cooling the hot-rolled steel sheet after the hot rolling at a cooling rate of 20° C./s to 80° C./s to a third temperature range of 600° C. to 650° C.;

holding the hot-rolled steel sheet after the first cooling for time t seconds to 10.0 seconds determined by the following Equation (2) in the third temperature range of 600° C. to 650° C.;

second cooling of cooling the hot-rolled steel sheet after the holding, to 600° C. or less;

coiling the hot-rolled steel sheet at 600° C. or less so that in a microstructure of the hot-rolled steel sheet after coiling, the connection index E value of the pearlite is 0.40 or less, and in the bainitic ferrite, an area ratio of the bainitic ferrite in which an average value of a crystal orientation difference in a region surrounded by a boundary in which a crystal orientation difference is 15° or more is 0.5° or more and less than 3.0°, is 80.0% or more to obtain the hot-rolled steel sheet;

pickling the hot-rolled steel sheet;

cold rolling the hot-rolled steel sheet after the pickling so that a cumulative rolling reduction is 40.0% to 80.0% to obtain a cold-rolled steel sheet;

annealing of holding the cold-rolled steel sheet after the cold rolling for 30 to 600 seconds in a fourth temperature range after raising the temperature to the fourth temperature range of T1−50° C. to 960° C.;

third cooling of cooling the cold-rolled steel sheet after the annealing at a cooling rate of 1.0° C./s to 10.0° C./s to a fifth temperature range of 600° C. to 720° C.; and

heat treating of holding the cold-rolled steel sheet for 30 seconds to 600 seconds after cooling the temperature to a sixth temperature range of 150° C. to 500° C. at the cooling rate of 10.0° C./s to 60.0° C./s,

T1(° C.)=920+40×C 2 −80×C+Si 2 +0.5×Si+0.4×Mn 2 −9×Mn+10×Al+200×N 2 −30×N−15×Ti  Equation (1)

t (seconds)=1.6+(10×C+Mn−20×Ti)/8  Equation (2)

here, element symbols in the equations indicate the amount of elements in % by mass.

6. The method of manufacturing a cold-rolled steel sheet according to claim 5 ,

wherein the steel sheet is coiled at 100° C. or less in the coiling.

7. The method of manufacturing a cold-rolled steel sheet according to claim 6 , comprising:

holding the hot-rolled steel sheet for 10 seconds to 10 hours after raising the temperature to a seventh temperature range of 400° C. to an Al transformation point between the coiling and the pickling.

8. The method of manufacturing a cold-rolled steel sheet according to any one of claims 5 to 7 , comprising:

reheating the cold-rolled steel sheet to a temperature range of 150° C. to 500° C. before holding the cold-rolled steel sheet for 1 second or more after cooling the cold-rolled steel sheet to the sixth temperature range in the heat treating.

9. The method of manufacturing a cold-rolled steel sheet according to any one of claims 5 to 7 , further comprising:

hot-dip galvanizing the cold-rolled steel sheet after the heat treating.

10. The method of manufacturing a cold-rolled steel sheet according to claim 9 , further comprising:

alloying of performing the heat treatment within an eighth temperature range of 450° C. to 600° C. after the hot-dip galvanizing.

11. The method of manufacturing a cold-rolled steel sheet according to claim 8 , further comprising:

hot-dip galvanizing the cold-rolled steel sheet after the heat treating.