Hot-rolled steel sheet and method for producing same

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

• first-hot-rolling a steel in a temperature range of 1000° C. to 1200° C. under conditions such that at least one pass whose reduction is 40% or more is included so as to control an average grain size of an austenite in the steel to 200 μm or less, wherein the steel includes, as a chemical composition, by mass %,
• C:
• 0.01% to 0.4%,
• Si:
• 0.001% to 2.5%,
• Mn:
• 0.001% to 4.0%,
• Al:
• 0.001% to 2.0%,
• P:
• limited to 0.15% or less,
• S:
• limited to 0.03% or less,
• N:
• limited to 0.01% or less,
• O:
• limited to 0.01% or less, and
• a balance consisting of Fe and unavoidable impurities;
• second-hot-rolling the steel under conditions such that, when a temperature calculated by a following Expression 5 is defined as T1 in unit of ° C. and a ferritic transformation temperature calculated by a following Expression 6 is defined as Ar3 in unit of ° C., a large reduction pass whose reduction is 30% or more in a temperature range of T1+30° C. to T1+200° C. is included, a cumulative reduction in the temperature range of T1+30° C. to T1+200° C. is 50% or more, a cumulative reduction in a temperature range of Ar3 to lower than T1+30° C. is limited to 30% or less, and a rolling finish temperature is Ar3 or higher;
• first-cooling the steel under conditions such that, when a waiting time from a finish of a final pass in the large reduction pass to a cooling start is defined as t in unit of second, the waiting time t satisfies a following Expression 7, an average cooling rate is 50° C./second or faster, a cooling temperature change which is a difference between a steel temperature at the cooling start and a steel temperature at a cooling finish is 40° C. to 140° C., and the steel temperature at the cooling finish is T1+100° C. or lower;
• second-cooling the steel to a temperature range of 600° C. to 800° C. under an average cooling rate of 15° C./second to 300° C./second after finishing the second-hot-rolling;
• holding the steel in the temperature range of 600° C. to 800° C. for 1 second to 15 seconds;
• third-cooling the steel to a temperature range of a room temperature to 350° C. under an average cooling rate of 50° C./second to 300° C./second after finishing the holding;
• coiling the steel in the temperature range of the room temperature to 350° C.,
• T1=850+10×([C]+[N])×[Mn] (Expression 5),
• here, [C], [N], and [Mn] represent mass percentages of C, N, and Mn respectively,
• Ar3=879.4−516.1×[C]−65.7×[Mn]+38.0×[Si]+274.7×[P] (Expression 6),
• here, in Expression 6, [C], [Mn], [Si] and [P] represent mass percentages of C, Mn, Si, and P respectively,
• t≦2.5×t1 (Expression 7),
• here, t1 is represented by a following Expression 8,
• t1=0.001×((Tf−T1)×P1/100)2−0.109×((Tf−T1)×P1/100)+3.1 (Expression 8),
• here, Tf represents a celsius temperature of the steel at the finish of the final pass, and P1 represents a percentage of a reduction at the final pass.