Thermal stability of high temperature deformation induced ferrite in a low carbon steel

A series of unidirectional compression tests for a low carbon steel were performed on a thermal simulating machine Gleeble 3 500, and the thermal stability of high temperature deformation induced ferrite (DIF) was studied systematically through the post-treatments such as isothermally hot holding time above the austenite-ferrite equilibrium transformation temperature Ae(3), cooling rates and annealing temperatures after deformation. The results show that the DIF volume fraction decreases during isothermally hot holding and finally DIF disappears. The DIF grains showed rapid growth at low annealing temperatures (< 200 degrees C) and relatively stable at middle annealing temperatures (200-500 degrees C), lastly rapid growing at high annealing temperatures (500-700 degrees C). Compared with the microstructure of DIF with martensite in quenched specimens, the proeutectoid ferrite and pearlite appear in controlled cooled specimens at low cooling rates (i.e. 10 degrees C/s and 1 degrees C/s). The present experiments revealed that DIF is not stable at the high temperature and hardly conserved in hot mills.