Damage development in low alloy TRIP-aided steels during air-bending

► Cracking mainly by decohesion of martensite/ferrite interface. ► Damage nucleates at hard bands for an average strain close to 0.10. ► Sub-surface cracks may appear when hard bands are located below the surface. ► Relationship between damage, band thickness and “mesoscopic” strain was proposed. In order to optimize the metallurgical quality of advanced high strength steels for automotive applications, the present study aims at understanding damage mechanisms involved in air-bending of two low alloy TRansformation Induced Plasticity (TRIP)-aided steels. Air-bending tests were performed together with metallographic investigations of damage development in bent specimens. In order to assess the role of hard bands induced by chemical segregations, air-bending tests on specimens with various locations of the main hard band (with respect to the neutral fibre) were performed. Cracking initiates from the outer surface or from just below, mainly by decohesion between ferrite and secondary (martensite) phases, at hard bands located close to the outer surface. From these examinations, together with a simple kinematics analysis of bending, a quantitative relationship between cracking, local thickness of hard band and local “mesoscopic” strain is proposed.