Delayed cracking in hot stamping with hot trimming for ultra-high strength steel components

Abstract

Hydrogen-induced delayed cracking in hot stamping with hot trimming for ultra-high strength steel components at various trimming temperatures was investigated. The trimming temperature of a heated quenchable sheet was adjusted by rapid cooling with the upper punch and die, and then, the sheet was trimmed and die-quenched. A cathode hydrogen charging test was performed to examine the occurrence of delayed cracking at trimmed edges, and then, tensile strength and total elongation of the hydrogen-charged specimen were measured from the tensile test. Below, a martensite transformation start temperature of 420 °C, the fracture surface and the tensile residual stress became large, and delayed cracking was caused on the fracture surfaces of the sheared edges. Although no delayed cracking of the hydrogen-charged specimen occurred above 420 °C, the tensile strength and total elongation were reduced by hydrogen embrittlement. The critical temperature of delayed cracking for a thin sheet having 1.0 mm in thickness rose to 600 °C.