Shaping mechanism of ultrafine metastable austenite in HSLA steels through a cumulative process of hot rolling, partitioning and tempering

Abstract

We have uniquely combined hot rolling, partitioning and tempering to formulate a new pathway for tailoring shape, size and stability of ultrafine metastable austenite (γ) in a low-alloy steel and understand the associated mechanism. The formation of 1-D ultrafine γ in partitioned-tempered steels was governed by 1-D micro-segregation of Mn/Ni in partitioned steels because nucleation of 1-D ultrafine was promoted in 1-D micro-segregation region of Mn/Ni during the initial stage of tempering. Migration of carbon from adjacent martensite to γ in micro-segregation region restricted nucleation of 2-D/3-D ultrafine γ. Tuning of alloying design and partitioning process effectively reduced 1-D micro-segregation of Mn/Ni, restrained the nucleation of 1-D ultrafine γ, and promoted the formation of 2-D/3-D ultrafine γ grains between martensite. Thus, low level of 1-D micro-segregation of Mn/Ni in partitioned steels contributed to the refinement of γ. But high degree of micro-segregation of Mn/Ni at the phase boundary of γ in partitioned and tempered steels significantly improved its stability, which was induced by austenitization (between martensite and austenite) and ferritic transformation (between annealed martensite and austenite). The micro-segregation of Mn/Ni design approach presented here for low alloy steels can be extended to other multiphase microstructure alloys.