DEFORMATION BEHAVIOURS OF MILD STEEL IN HOT WORKING.

Equipment was constructed which enabled metallic materials to be quenched by hydrogen gas immediately after dynamic tension ( epsilon equals 18 sec** minus **1) at high tempertures. With this equipment the deformation behaviors of mild steel (C equals 0. 036 wt%) were studied over the temperature range of 25 degree C to 1200 degree C. Measurements of mechanical properties and slip line observations revealed the presence of five temperature regions where mechanisms seemed to be different; (1) the low temperature deformation region between 25 and 300 degree C, (2) the blue brittleness region between 300 and 600 degree C, (3) the high temperature deformation region of the alpha -phase between 600 degree C and A//1 point (723 degree C), (4) the alpha - gamma mixed phase region between A//1 and A//3 point ( congruent 895 degree C), and (5) the gamma -phase region above A//3 point. True stress vs strain curves in region (3) showed steady state flow in the range of high strain above 40 similar 50%. The size of subgrains which were formed in the original grains by deformation was nearly constant in the range of high strains and also the values of microhardness and coercive force on specimens which were hot deformed and then immediately quenched were nearly constant in this range. The recrystallized-grains did not appear during hot deformation. They indicate that deformation of alpha -iron in region (3) is controlled by dynamic recovery. The structural change due to dynamic recovery is considered to retard or prevent crack initiation, thereby leading to increase in hot ductility. The remarkable decrease of ductility with increasing temperature in region (4) may be explained by the fact that most of deformation was concentrated in the region of the alpha -phase in which flow stress was lower and ductility is larger than that of the gamma -phase and consequently nonuniform deformation occurred considerably.