HYDROGEN INDUCED DELAYED FRACTURE BEHAVIOR OF A LOW-CARBON Mn-B TYPE ULTRA-HIGH STRENGTH STEEL SHEET AFTER HOT STAMPING

Abstract

In consideration of the light weight and the impact safety of cars, in recent years, hot stamping has been increasingly applied to the manufacture of car parts, which has driven forward the increasing use of ultra-high strength steel sheet in the automobile industry. However, when the tensile strength of steel is more than about 1200 MPa, the steel sheet is very susceptible to hydrogen induced delayed fracture. Up to now, some research has been carried out on delayed fracture behavior of both high strength and ultra-high strength steel sheet. However, the delayed fracture behavior of a low-carbon Mn-B type steel sheet widely used for hot stamping has not been paid enough attention and fully studied. There are also a scarcity of reports on hydrogen induced delayed fracture behavior of this steel sheet after hot stamping. With the development of the hot stamping, the evaluation of delayed fracture behavior of ultra-high strength steel sheet for hot stamping, especially the delayed fracture behavior of steel sheet after hot stamping has become very urgent. For these reasons, hydrogen induced delayed fracture behavior of a low-carbon Mn-B type ultra-high strength hot stamping steel sheet at both the hot stamped status and the common quenched and tempered status was studied by means of constant load delayed fracture test and hydrogen thermal desorption spectrometry (TDS) analysis. The results show that both the critical delayed fracture stressc and delayed fracture life of