Formation Mechanism of Coarse Austenite Grain during Hot Forging and Cooling in Case Hardening Steel

Abstract

Abnormally coarse microstructure consisting of coarse pearlite and bainite has been sometimes observed in case hardening steels when they are slowly cooled after hot forging. In order to avoid this abnormal microstructure, it is of industrial importance to clarify its formation mechanism. In the present study, effects of hot deformation condition and cooling rate on the formation of austenite grain coarsening during cooling were investigated by a thermomechanical simulator for JIS SCM420 (0.20C-0.26Si-0.82Mn-1.03Cr-0.15Mo steel, in mass%). Coarse microstructures were observed when the specimen was deformed slightly at higher temperature after large deformation and subsequently cooled at slow cooling rate. In order to clarify the formation mechanism of coarse austenite grain, strain distribution (GOS: Grain Orientation Spread) in reconstructed austenite orientation map were analyzed for specimens quenched just after deformation. In the condition where coarse austenite formed, the GOS map made it clear that strain was introduced inhomogeneously grain by grain. This result suggested that abnormal austenite grain growth during slow cooling was induced by inhomogeneous strain distribution because growth of recrystallized grains (relatively low dislocation density) into work hardened grains (relatively high dislocation density) was driven by strain energy difference in addition to reduction of g rai n bou nd a r y area. Keywords: case hardening steel; h o t d e f o r m a t i o n ; continuous cooling; microstructure; grain growth; recrystallization; electron backscattering diffraction.