| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet having high strength and high toughness satisfying the yield strength YS:≥960 MPa, and suitable as the one for the structural member of a large-sized industrial construction machine, and to provide a method for producing the hot-rolled steel sheet. SOLUTION: A steel stock comprising C, Si, Mn, P, S, Al, Nb, Ti, Mo, Cr and B in proper ranges is heated at 1,100 to 1,250°C, and is subjected to finish rolling where the inlet side temperature in the finish rolling lies in the range of 900 to 1,100°C, the outlet side temperature in the finish rolling lies in the range of 800 to 900°C, and a cumulative draft in a recrystallizedγregion is 60 to 90%. Thereafter, immediately, cooling is started, and is performed within 30 s from the start of the cooling to a cooling stop temperature of (an Ms point+50°C) or below at a cooling velocity more than a martensite formation critical cooling velocity, and next, the steel is held for 10 to 60s in the temperature range of (a cooling stop temperature±100°C) and is thereafter coiled so as to obtain a steel sheet having a structure consisting of an M phase or a tempered M phase as the main phase, and in which the aspect ratio of oldγgrains in the cross-section in the rolling direction is 3 to 18 and combining high strength and high toughness. COPYRIGHT: (C)2011,JPO&INPIT |
