MICROSTRUCTURE REFINEMENT OF EUTECTOID STEEL BASED ON DIVORCED EUTECTOID TRANSFORMATION

For high carbon steels, the (alpha+theta) microduplex structure consisting of ultrafine ferrite matrix and dispersed cementite particles demonstrates a good balance between strength and ductility as compared with a, normal microstructure, lamellar pearlite in eutectoid steel or lamellar pearlite plus pro-eutectoid cementite in hypereutectoid steel. The divorced eutectoid transformation (DET) has been confirmed to be very effective in ultrahigh carbon steels for the production of the (alpha+theta) microduplex structure. hi ultrahigh carbon steels, DET takes place during slow cooling of a mixing microstructure of austenite plus dispersed undissolved cementite particles formed by the intercritical annealing within the (gamma+theta) two phase range. Due to the absence of the (gamma+theta) two phase range, DET is difficult to take place in eutectoid steel. In the present work, DET was realized in eutectoid steel by a special thermal-mechanical treatment, which involved two-stage hot deformation in the temperature ranges of A(1) to A(r1) and A(1) to A(c1) and subsequent Slow Cooling. The microstructure evolution during such treatment was studied by hot uniaxial compression tests using a Gleeble 1500 hot simulation test machine in combination with SEM and EBSD. The results indicate that during hot deformation in the temperature range of A(1) to A(c1) after hot deformation of undercooled austenite in the temperature range of A(1) to A(r1), the re-austenization could be controlled by the applied strain, leading to the formation of the mixing microstructure of austenite plus undissolved cementite particles at certain conditions. During subsequent slow cooling, DET took place, resulting in the formation of an ultrafine (alpha+theta) microduplex structure with alpha-Fe grains less than 3 mu m and theta-Fe3C particles less than 0.5 mu m.