Development of Coated TRIP Steels at Dofasco

摘要

The TRIP (TRansformation Induced Plasticity) steels possess unique mechanical properties (high tensile strength with extraordinary ductility and high strain hardening capacity), excellent crash worthiness, and are considered to be one of the advanced high strength steels (AHSS) to be used in the new generation of vehicles. TRIP steels have been studied extensively in last decade. In published research work, the IBT (Isothermal Bainitic Transformation) process, which required a rapid quenching to bainitic region (～400°C) from intercritical temperature and holding for a few minutes followed by air cooling to room temperature, was a common approach to obtain the desirable microstracture and mechanical properties. High silicon (1-2%Si) alloying was widely used to suppress the formation of cementite in the IBT process so that sufficient carbon enrichment in austenite could take place. The carbon enrichment makes it possible to keep the austenite in metastable condition, an essential condition for TRIP effect, at room temperature. However, the IBT process is not applicable and the high silicon content is detrimental to the coating surface quality on hot-dip galvanizing lines To improve the hot dip galvanizability while retaining the TRIP properties, Si must be fully or partially replaced by other elements with similar metallurgical behavior. Aluminum seems to be effective in this respect. This paper presents the results of lab-scale development of low silicon, high aluminum TRIP steel substrates at Dofasco using the thermal cycles of hot-dip galvanizing lines.