Effect of initial microstructures before annealing on ductility of TRIP-aided steel sheets

Abstract

The TRIP-aided steel sheets have been known as the excellent formable high strength steel sheet with high elongation property. However, its stretch-flangeability,which is another necessary property of products, is inferior. In this research,the stretch-flangeability of the retained austenite steel sheets with the annealed martensitic or bainitic matrix of 0.15C-1.5Si-2.0Mn and 0.20C-1.5Si-1.5Mn steels were investigated. Initial microstructures as bainite,martensite,and ferrite-martensite were prepared by hot rolling, and following annealing heat treatment was applied. These steel sheets applied with such process increase the stretch-flangeability also elongation than the conventional retained austenite steel with polygonal-ferrite matrix and block-like retained austenite. Furthermore, the higher stretch-flangeability shows in the case that initial microstructure of the hot rolled sheet is full-bainite or full-martensite rather than ferrite-martensite microstructure. In these steel sheets, lath-type retained austenite morphology and annealed martensitic or bainitic matrix without polygonal-ferrite causing high HV ratio are considered as effective factors for increasing stretch-flangeability.