Influence of Retained Austenite and Strain on Delayed Fracture of Ultra-high Strength Steel Sheet

Abstract

Influence of retained austenite and strain on delayed fracture of ultra-high strength steel sheets with tensile strength of 1200MPa class was investigated. Volume of retained austenite in the steel was changed by annealing at various temperature. Plastic strain was induced by bending. The tendency of delayed fracture was observed by measuring the time to fracture when the steel was cathodically charged with hydrogen. Occurrence of delayed fracture is affected by the volume of retained austenite; the steel that has a large amount of retained austenite has a susceptibility to delayed fracture. When the steel had no plastic strain, delayed fracture was not observed in it, but as the steel was plastically deformed, delayed fracture occurred. Increasing the strain, a strong susceptibility for delayed fracture was shown. Delayed fracture was affected by applied stress after plastic deformation; increasing applied stress, a strong susceptibility for delayed fracture was shown. Existence of retained austenite, plastic deformation and applied stress a re necessary to occurrence of delayed fracture of ultra-high strength steel sheet.