2004 Volume 90 Issue 8 Pages 581-587
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 sub-structure vanishing during cold rolling. Then finally the microstructure containing the fine uniform matrix and filmy lath-like retained austenite obtains after annealing.