EFFECT OF LOW TEMPERATURE HOT DEFORMATION ON THE MICROSTRUCTURE OF NB/TI BEARING LOW CARBON STEELS

摘要

As a research for developing fine-grained high strength low carbon steels, the effects of low temperature hot deformation processes on the microstructure were investigated by applying hot deformation processes with two reheating temperatures and various reduction ratios to the 0.15wt.% carbon steels with microalloying elements such as Nb and Ti. When the steels were reheated at 1200°C, Nb containing steel showed coarse austenite grain size of 90 μm whereas Ti-Nb steel did fine one of 58 μm, because Ti-Nb carbonitrides in Ti-Nb steel could suppress the austenite grain growth. In the case of 60% reduction at 800°C, the austenite grains in Nb steel partially recrystallized while those in Ti-Nb steel fully recrystallized probably due to finer prior austenite grains. For one pass heavy deformation of Ti-Nb steel, the ferrite grain size was not sensitively changed by austenitizing temperature and reduction ratio and, severe deformation of 80% reduction was not essentially necessary to refine ferrite grains to about 3 μm which could be obtained through lighter deformation of 40% reduction. For deformation of 80% in total reduction for Nb steel, ferrite grain size obtained from three pass deformation (40% reduction per pass) were about 2 μm which was smaller than about 4 μm obtained from one pass severe deformation, presumably due to non-recrystallized austenite condition in the multi-pass deformation.