INFLUENCE OF MICRO-ALLOYING ADDITIONS ON THE HOT DUCTILITY OF STEELS HEATED DIRECTLY TO THE TEST TEMPERATURE.

Abstract

Six microalloyed steels have been tested at temperatures in the range 600 to 1100 degree C using strain rates between 3 multiplied by 10** minus **2 and 3 multiplied by 10** minus **4 s** minus **1. No relationship could be found between the value for Q, the activation energy for deformation in the austenite, and hot ductility performance. At a strain rate of 3 multiplied by 10** minus **3 s** minus **1, the C-Mn-Al, C-Mn-Al-Ca, C-Mn-V-Al, and C-Mn-Nb-Al steels showed a loss of ductility in the temperature range 750 to 1000 degree C due to grain boundary precipitation which retarded the onset of dynamic recrystallization and encouraged grain boundary sliding. Decreasing the strain rate from 3 multiplied by 10** minus **2 to 3 multiplied by 10** minus **4 s** minus **1 deepened and broadened the ductility troughs in all the steels except the Ti-containing steel and introduced a ductility trough in the behavior of the C-Mn steel in the temperature range 800 to 900 degree C. The Ca and Ti containing steels, both of which contained precipitates, gave superior hot ductility over the plain C-Mn steel, particularly at low strain rates.