Abstract
Effects of controlled rolling conditions on strength and toughness of Mn-Mo-Nb steels were compared between their structure of ferrite · pearlite and of bainite. The change in strength and toughness with the rolling conditions between slab reheating and finish rolling was nearly the same with each other. The bainite steel showed superior strength and toughness to the ferrite · pearlite steel. Toughness of the bainite steel decreased with increase in bainite area fraction resulted from the variation of controlled rolling conditions.
The investigation was also extended to the effects of cooling rate and coiling temperature, as cooling parameters after controlled rolling, on strength and toughness of Mn-Mo-Nb bainite steels different in carbon and molybdenum contents. Both strength and toughness of these steels were improved by increasing cooling rate and lowering coiling temperature, although these conditions resulted in the increase in bainite area fraction. These mutually contradictory effects of bainite area fraction on toughnss of bainite steel were attributed to the difference in appearance of bainite and size of the coexisting polygonal ferrite.
The remarkable increase in toughness caused by the accelerated cooling rate after finish rolling corresponded well to the increase in area fraction of the extra fine (1-2μ) polygonal ferrite which nucleated on the deformation band of unrecrystallized austenite and remained small during cooling.
