Interaction between ferrite recrystallization and austenite formation in dual-phase steel manufacture

In this publication, the effect of heating rate on microstructural evolution of manganese segregated cold reduced dual phase steels is systematically studied for different inter-critical temperatures and holding times. At slow heating rate, completion of ferrite recrystallization before austenite formation led to the preferential formation of austenite on the ferrite grain boundaries leading to a necklace austenite (now martensite) morphology. The slower austenite formation kinetics has been attributed to longer diffusion paths dictated by larger ferrite grain sizes. In medium heating rate condition, microstructure before austenite formation had partially recrystallized ferrite and partially spheroidized cementite. Rapid austenite growth occurred along the rolling direction in carbon-rich cementite regions and dislocation-rich recovered ferrite regions. The presence of partially recrystallized ferrite grains restricted the austenite growth in the normal direction and therefore enabled the formation of thin martensite bands parallel to the rolling direction. At fast heating rate, the microstructure before austenite formation predominately contained un-recrystallized ferrite and un-spheroidized cementite and therefore enabled faster austenite formation kinetics. Thicker martensite bands are formed at fast heating rates due to the absence of recrystallized grains, thereby, enabling the growth of austenite in all directions with a higher preference to the rolling direction