Patent ID: 9587287
Date: 2017-03-07
CPC Classifications: C21D,C22C

Claim:
1. A manufacturing method of a bainite-containing-type hot-rolled steel sheet comprising: on a slab containing: in mass%, C: greater than 0.07 to 0.2%; Si: 0.001 to 2.5%; Mn: 0.01 to 4%; P: 0.15% or less (not including 0%); S: 0.03% or less (not including 0%); N: 0.01% or less (not including 0%); Al: 0.001 to 2%; and a balance being composed of Fe and inevitable impurities, performing first hot rolling on the slab in which rolling at a reduction ratio of 40% or more is performed one time or more in a temperature range of not lower than 1000° C. nor higher than 1200° C.; performing second hot rolling in which rolling at 30% or more is performed in one pass at least one time in a temperature region of not lower than T1+30° C. nor higher than T1 +200° C. determined by Expression (1) below; and setting the total of reduction ratios in the second hot rolling to 50% or more; performing final reduction at a reduction ratio of 30% or more in the second hot rolling and then starting primary cooling in a manner that a waiting time period t second satisfies Expression (2) below; setting an average cooling rate in the primary cooling to 50° C./second or more and performing the primary cooling in a manner that a temperature change is in a range of not lower than 40° C. nor higher than 140° C.; within three seconds after completion of the primary cooling, performing secondary cooling in which cooling is performed at an average cooling rate of 15° C./second or more; and after completion of the secondary cooling, performing air cooling for 1 to 20 seconds in a temperature region of lower than an Ar3 transformation point temperature and an Ar1 transformation point temperature or higher and next performing coiling at 450° C. or higher and lower than 550° C., wherein, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the element (mass%). wherein, t1 is obtained by Expression (3) below, wherein, in Expression (3) above, Tf represents the temperature of the slab obtained after the final reduction at a reduction ratio of 30% or more, and P1 represents the reduction ratio of the final reduction at 30% or more.