Effect of Cold Rolling and Isothermal Holding before Austenitization on Mechanical Properties in Steels

Abstract

Effect of pre-treatments before austenitization on mechanical properties in steels has been investigated. Pre-treatments are cold rolling and isothermal holding below A1 before austenitization. Pre-treatments were performed in order to introduce dislocation network in austenite (γ), which acts as nucleation site for “bainite laths within γ grain (BWING)”. During the pre-treatment of isothermal holding below A1, carbon segregation and/or carbide formation might occur at grain boundary of ferrite (α). Thin γ layer might be formed around the region where carbon segregation and/or carbide formation occurred in the beginning of austenitization. Transformation strain due to α → γ would be effectively relaxed by the deformation of thin γ film, because α had been already hardened by the cold rolling. Dislocations might be introduced dominantly into the thin γ film, and stable dislocation network even in the high temperature region of γ might be formed. BWING nucleates around the dislocation network to relax the strain field around dislocation network. BWING would nucleate also at ( BWING / γ ) interface cooperatively to relax the strain around the interface, resulting in the formation of “aggregate of bainite laths with nearly parallel slip systems between neighboring bainite laths (ALPS)”. Larger ALPS containing a lot of BWINGs might be formed in the pre-treated steel, and improves tensile strength and toughness.