Improving Toughness of Heavy Steel Plate by Deformation Distribution Under Low Finish Cooling Temperature

The significant role of deformation distribution in toughness improvement of heavy steel plate under low finish cooling temperature was investigated. Deformation distribution was conducted by changing temperature-holding thickness in two-stage control rolling. The results show that the low finish cooling temperature always inhibits the ferrite transformation. However, when heavy deformation was applied at noncrystallization region, extensive ferrite was formed and ferrite was also effectively refined. Hence, homogeneous ferrite microstructure through the thickness with the ferrite volume fraction of 82.4% and grain size refined to 6.7 mu m at quarter thickness of 40-mm heavy steel plate was obtained when the deformation at nonrecrystallization region reaches 70%. Thus, high toughness can be achieved, showing that the fully ductile fracture can be maintained at -60 A degrees C and the ductile-to-brittle transition temperature is lowered to -91 A degrees C. The improved toughness is ascribed to the high ferrite volume fraction, refinement of ferrite and hard phase colony and the increase in the percent of high-angle grain boundaries and average grain boundary misorientation.