Grain Refinement of As Cast Austenite by Dynamic Recrystallization in HSLA Steels

Abstract

Recent progress of steel manufacturing process necessitates to refine an extremely coarse austenitic microstructure evolved in a strand cast steel. Grain refinement of as cast austenite by dynamic recrystallization in HSLA steels was studied by using a hot working simulator. The specimens were prepared from a strand cast slab and the hot rolled steel plate of 0.09%C-1.14%Mn-2.26Ni-0.54Mo-0.045%V steel supplied from a steel plant, and also laboratory heat ingots of 0.14%C-1.45%Mn and 0.14%C-1.45%Mn-0.018%Ti steels. Variations of the true stress-true strain curve and dynamically recrystallized grain size with the deformation temperature, strain rate and the initial γ grain size were investigated by hot compression test. It was confirmed that dynamically recrystallized grain size in as cast steels was determined simply by steady state flow stress or Zener-Hollomon parameter, but was not influenced by the initial grain size. Austenitic grain size variation with the reheating temperature in the as cast 0.09%C-2.26Ni-Mo-V steel was very small, and flow stress in the as cast Ti-bearing steel was markedly higher compared with those of the hot rolled plate of this steel or the C-Mn steel. These appeared to be caused by grain growth suppression due to the interdendritic phase enriched with carbon or alloying elements, and dispersion of the micro segregation region with high hardness, respectively. Finally, the direct hot deformation experiment after levitation melting and solidification was conducted, where the Ti-bearing steel was reheated at the temperature from 1 743 to1 773 K and hot deformed by tensile strain at 1 523 K. It was confirmed that very coarse γ grain size in an order of mm was much refined down to 130 to 170 μm by dynamic recrystallization.