Effects of dynamic recrystallization on γ grain refinement and improvement of micro segregation of as cast austenite in 9% Ni steel

Effects of dynamic recrystallization on γ grain refinement and improvement in micro segregation of elements such as Ni and Mn of as cast austenite in 9% Ni steel were investigated by two kinds of experimental methods. The first one was a hot compression test using the specimens prepared from the strand cast slab and the hot rolled plate of 9% Ni steel, and the other was a hot tensile straining test in the austenitic region after levitation melting and solidification. In the hot compression test, variations in onset strain and flow stress of steady state flow of dynamic recrystallization with hot deformation conditions were investigated. The onset strain was found to decrease below 0.25 at the temperatures above 1523 K and the strain rate below 1×10-2s-1. The activation energy value obtained from steady state flow stress was 421 kJ/mol. Dynamically recrystallized γ grain size in as cast austenite of this steel was controlled simply by Z value with no dependence on the initial γ grain size. This beneficial feature of dynamic recrystallization was confirmed by the experiment of tensile straining in austenite formed after levitation melting and solidification, where extremely coarse initial γ grain size of around 1.9 mm was markedly refined down to 140 μm by straining such a small strain as 0.40. Micro segregation ratio of Ni and Mn in the strand cast 9% Ni steel examined by EPMA analysis was 1.20 and 1.36, respectively. These values were found to decrease continuously with reduction in strain rate in hot deformation of austenite. That is, dynamic recrystallization in austenite taken place at lower strain rate deformation is much more effective for homogenization of segregated elements compared with high strain rate deformation.