Hot-rolled steel sheet and method of producing the same

A method of producing a hot-rolled steel sheet, the method comprising:

• heating a steel ingot or a slab including, by mass %,
• C:
• a content [C] of 0.02% to 0.07%,
• Si:
• a content [Si] of 0.001% to 2.5%,
• Mn:
• a content [Mn] of 0.01% to 4%,
• Al:
• a content [Al] of 0.001% to 2%,
• Ti:
• a content [Ti] of 0.015% to 0.2%,
• P:
• a limited content [P] of 0.15% or less,
• S:
• a limited content [S] of 0.03% or less,
• N:
• a limited content [N] of 0.01% or less, and
• the balance consisting of Fe and unavoidable impurities, in which the contents [Ti], [N], [S], and [C] satisfy the following expressions (a) and (b), at SRTmin° C., which is a temperature determined according to the following expression (d) and 1150° C., to 1260° C.;
• performing a first hot rolling in which reduction is performed once or more at a rolling reduction of 40% or higher per pass in a temperature range of 1000° C. to 1200° C.;
• starting a second hot rolling in a temperature range of 1000° C. or higher within 150 seconds after a finish of the first hot rolling;
• performing a reduction in the second hot rolling in a temperature range of (T1+30)° C. to (T1+200)° C., wherein a temperature determined by components of the steel sheet according to the following expression (e) is represented by T1° C. so as to obtain a total reduction ratio of 50% or higher, with at least one of a rolling reduction ratio of 30%;
• performing a third hot rolling in which a total rolling reduction is lower than or equal to 30% in a temperature range of less than (T1+30)° C.;
• finishing the hot rollings at the Ar3 transformation temperature or higher;
• performing a primary cooling under conditions of a cooling rate of 50° C./sec or higher, a temperature change of 40° C. or more and 140° C. or less, and a cooling end temperature of (T1+100)° C. or lower such that a waiting time t (second) from a finish of a final pass of a large reduction pass to a start of cooling satisfies the following expression (f), wherein said large reduction pass is defined as a pass of a rolling reduction of 30% or higher in the temperature range of (T1+30)° C. to (T1+200)° C.;
• performing a secondary cooling at a cooling rate of 15° C./sec or higher within 3 seconds from the finish of the primary cooling;
• and
• performing a coiling in a temperature range of 550° C. to lower than 700° C.,
• 0%≦([Ti]−[N]×48/14−[S]×48/32) (a)
• 0%≦[C]−12/48×([Ti]−[N]×48/14−[S]×48/32) (b)
• SRTmin=7000/{2.75−log([Ti]×[C])}−273 (d)
• T1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[0]+100×[Mo]+100×[V] (e)
• t≦2.5×t1 (f)
• where t1 is represented by the following expression (g),
• t1=0.001×((Tf−T1)×P1/100)2−0.109×((Tf−T1)×P1/100)+3.1 (g)
• where Tf represents a temperature (° C.) after a final reduction at a rolling reduction of 30% or higher, and P1 represents the rolling reduction (%) during the final reduction at a rolling reduction of 30% or higher.