Subcritical heat treatment of cold rolled multi-phase TRIP steel.

摘要

The high strength of TRIP (TRansformation Induced Plasticity) steel enables high fuel efficiency by reducing the weight of a car without any loss of safety. In addition, the good formability shown by TRIP steel extends the application of this type of steel to complicated component shapes. TRIP steels can be used in the cold rolled and 'TRIP annealed' condition. After cold rolling, TRIP steels are subjected to a complicated anneal/heat-treatment encompassing intercritical annealing followed by bainitic transformation. Because the 'TRIP annealing' is designed to produce the complex microstructures, comprised of ferrite, bainite, retained austenite, which achieve the best combination of strength and formability; the processing of these steels requires careful selection and delicate control of temperatures and holding times. In this context, this research work has been carried out to design an alternative process for TRIP steel, using a relatively simple subcritical heat-treating process. Basically, the concept is to cold roll the TRIP steel to produce strain induced martensite, as well as work hardened ferrite and bainite, and then to 'conventionally' anneal the steel to produce tempered martensite and bainite, as well as recrystallized ferrite. For this purpose, 'subcritical heat treatment' of multiphase cold rolled TRIP steel at 200, 300, 400, 500, 550, 600, 650°C for a hour were done then the microstructure, mechanical properties were investigated with x-ray diffraction, OM and SEM, tensile testing and hardness measuring. The mechanical properties after the subcritical heat treatment at 600°C and over are approaching those of traditional TRIP processed steel hence this process can be a promising alternative and worth further study.