Development of TRlP-aided cold rolled steel sheets with the excellent stretch-flange formability

To develop the TRIP-aided cold rolled steel sheet with excellent stretch-flange formability, cold rolling of 0-50% was employed on the hot bands having martensite or ferrite-martensite microstrure, followed by intercritical annealing and austemper treatment. When hot band of martensite microstructure was subjected to cold rolling up to 20% and subsequent heat-treatment, the cold rolled steel sheet has relatively high stretch-flange formability. This may be caused by the uniform fine microstructure composing of annealed martensite lath matrix and interlath retained austenite. If hot band of ferrite-martensite microstructure with 60 vol% ferrite was subjected to cold rolling up to 20% and subsequent heat-treatment, the TRIP-aided cold rolled steel sheet achieved higher stretch-flange formability than that of martensite microstructure. In this case, the ferrite phase is considered to suppress the plastic deformation of martensite and prevent martensite lath substructure vanishing during cold rolling. Then finally the microstructure containing the fine uniform matrix and filmy lath-like retained austenite obtains after annealing.