| 摘要 |
<p>The invention provides high tensile steel products which have tensile strength of 600MPa or above and are excellent in the resistance to delayed fracture and thus suitable for construction and industrial machinery use and a process for the production of the same. A steel which contains C, Si, Mn, Al, N, P, S and, if necessary, one or more elements selected from among Mo, Nb, V, Ti, Cu, Ni, Cr, W, B, Ca, REM and Mg with the balance consisting of Fe and unavoidable impurities and in which the average of aspect ratios of prior austenite grains is 3 or above over the whole plate thickness; a steel as described above in which the cementite covering ratio in the lath boundary is 50% or below; a steel as described above in which the safety index of delayed fracture resistance is 75% or above; and a process for the production of high tensile steel products excellent in the resistance to delayed fracture which comprises casting a steel as described above, either inhibiting the cast steel from being cooled to a temperature of the Ar3 transformation temperature or below or reheating the cast steel to a temperature of the Ac3 transformation temperature or above, hot-rolling the resulting cast steel at a reduction of 30% or above in the non-recrystallization region, cooling the hot-rolled steel from a temperature of the Ar3 transformation temperature or above to a temperature of 350°C or below at a cooling rate of 1°C/s or above, and then tempering the resulting rolled steel at a temperature of the Ac1 transformation temperature or below.</p> |
