Patent ID: 9631265
Date: 2017-04-25
CPC Classifications: C21D,C22C,Y10T

Claim:
1. 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 Ar 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., here, [C], [N], and [Mn] represent mass percentages of C, N, and Mn respectively, here, in Expression 6, [C], [Mn], [Si] and [P] represent mass percentages of C, Mn, Si, and P respectively, here, t1 is represented by a following 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.