Abstract
Effects of process parameters of the ausforming such as temperature, strain and strain rate on the martensitic start temperature, kinetics of isothermal bainitic transformation and microstructure refinement of a low carbon carbide-free bainitic steel were investigated. It was found that applying plastic deformation to untransformed austenite during intermediate temperatures decreased the martensite start temperature of steel and enabled isothermal bainitic transformation at low temperatures. Hereby, ausforming significantly generated heterogeneous nucleation sites, which accelerated the overall kinetics of bainitic transformation and thus increased bainitic phase fraction in steel. In addition, the ausforming enhanced the stability of austenite that led to reduced amount of martensite after cooling down to room temperature. Finally, the ausforming parameters and observed microstructure features were correlated and discussed along with the hardness of steel.
Highlights
The requirement of high-performance steels with lower production cost and better machinability has led to development of carbide free bainitic (CFB) steel
It was evidenced that carbide precipitation and/or growth of existing carbides could lead to an inhomogeneous distribution of alloying element in austenite, which resulted in a discontinuous characteristic of austenite to martensite transformation between the martensite start (Ms) and martensite finish (Mf) temperature
Such discontinuous transformation was likely caused by ausforming accelerated/induced carbide precipitation. It seemed that larger difference between ausforming and Ms temperature led to more noticeable slope change
Summary
The requirement of high-performance steels with lower production cost and better machinability has led to development of carbide free bainitic (CFB) steel. This CFB steel mainly consisted of fine lath matrix of bainitic ferrite (BF) embedded with retained austenite (RA). This RA was certainly a residual product from the process of carbon partitioning between supersaturated bainitic ferrite and surrounding austenite during a bainitic transformation. Such partitioning resulted in a formation of film-like or blocky RA that exhibited different stabilities [1]. It was obviously shown that controlling the stability of RA played an important role in balancing mechanical properties (ductility, strength, and toughness) regarding the TRIP effect, which strongly depended on its chemical composition and morphological feature
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