The high tension steel sheet and the production manner which are superior in resistance retarded breaking property

摘要

High tensile strength steels that have both favorable delayed fracture resistance and a tensile strength of 600 MPa or higher and are suitably used in construction machinery, tanks, penstocks, and pipelines, as well as methods for manufacturing such steels are provided. More specifically, what is provided is a steel preferably containing elements C, Si, Mn, Al, N, P, and S; one or more of Mo, Nb, V, Ti, Cu, Ni, Cr, W, B, Ca, REM, and Mg if necessary; and Fe and unavoidable impurities as the balance, wherein the average aspect ratio of prior austenite grains calculated over the entire thickness is at least three and, if necessary, hydrogen is charged into the steel whose cementite covering ratio of laths is 50% or lower and the hydrogen contained in the steel is sealed by zinc galvanizing, the safety index of delayed fracture resistance calculated using the formula described below being at least 75% when a slow strain rate test is performed with the strain rate set to 1 10 -3 /s or lower. A steel having the composition described above is cast, protected from cooling to the Ar 3 transformation temperature or lower or heated to a temperature equal to or higher than the Ac 3 transformation temperature once again, hot rolled with the rolling reduction for non-recrystallization regions set to 30% or higher, directly quenched from a temperature equal to or higher than the Ar 3 transformation temperature, and then tempered so that the maximum temperature at the middle of the steel thickness is equal to or lower than the Ac 1 transformation temperature. The safety index of delayed fracture resistance (%) = 100 (X 1 /X 0 ), where X 0 : reduction of area of a specimen substantially free from diffusible hydrogen, and X 1 : reduction of area of a specimen containing diffusible hydrogen.