Reverse austenite transformation behavior of equal channel angular pressed low carbon ferrite/pearlite steel

Abstract

Reverse austenite transformation characteristics of ultrafine grained ferrite/pearlite low carbon steel were examined by using a dilatometry with a heating rate of 10 degrees C/s, and compared with those of coarse grained counterpart. Ultrafine grained steel was prepared by equal channel angular pressing at 500 degrees C, resulting in a microstructure consisting of ultrafine ferrite grains and pearlite with partially dissolved pearlitic cementite. Reverse austenite transformation start and finish temperatures of ultrafine grained steel were lower than those of coarse grained steel. Reverse austenite transformation of coarse grained steel occurred with the two serial stages of pearlite -> austenite followed by proeutectoid ferrite -> austenite. In the case of ultrafine grained steel, reverse austenite transformation was manifested by the three serial stages of carbon-supersaturated ferrite -> austenite, pearlite -> austenite, and proeutectoid ferrite -> austenite in the order. The formation of carbon-supersaturated ferrite in ultrafine grained steel is associated with carbon dissolution from pearlitic cementite during equal channel angular pressing. The effect of equal channel angular pressing on reverse austenite transformation temperatures can be explained in terms of the difference in shear stress driving transformation between two steels under assumption that the nature of carbon-supersaturated ferrite in ultrafine grained steel is similar to that of conventional martensite.