High tensile steel products excellent in the resistance to delayed fracture and process for production of the same

摘要

The invention provides tensile strength 600MPa above, be suitable as high tensile steel and manufacture method thereof that building industry machinery such as uses at excellent in delayed fracture resistance.Particularly, a kind of steel, contain C, Si, Mn, Al, N, P, S, contain one or more elements among Mo, Nb, V, Ti, Cu, Ni, Cr, W, B, Ca, REM, the Mg as required, surplus is made of Fe and unavoidable impurities, the mean value of the length-to-diameter ratio of original austenite grain is more than 3 in whole thickness of slab direction, and perhaps the cementite fraction of coverage at lath interface further is below 50%, and perhaps delayed fracture resistance degree of safety index further is more than 75%.In addition, a kind of method of making the high tensile steel of excellent in delayed fracture resistance, wherein, after casting above-mentioned steel, be not cooled to below the Ar3 transformation temperature, perhaps, reheat is to more than the Ac3 transformation temperature, the rolling rate in the non-recrystallization district of carrying out then is the hot rolling more than 30%, then from being cooled to temperature below 350 ℃ with the speed of cooling more than 1 ℃/second more than the Ar3 transformation temperature, carries out tempering then below the Ac1 transformation temperature.