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

摘要

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 degrees 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 degrees 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 degrees C.