Cold-rolled steel sheet and method of manufacturing same

A method of manufacturing a cold-rolled steel sheet, 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×C2−80×C+Si2+0.5×Si+0.4×Mn2−9×Mn+10×Al+200×N2−30×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.