Influence of coiling temperature on toughness of hot rolled microalloyed steels

The effect of coiling temperature on the toughness of hot rolled microalloyed steels was investigated. Heats with different Nb and/or Ti contents were laboratory hot rolled in austenitic conditions. Coiling temperatures were varied between 400 degrees C and 700 degrees C. A reduction of more than 50% was given below the recrystallization-stop temperature of the steels. Finishing temperature was around 860 degrees C and thickness of the finished strips was about 6 mm. The transformation behaviour of the steels was studied by deformation dilatometry in order to select an appropriate thermomechanical treatment and to correlate the influence of coiling temperature with transformation kinetics. The influences of grain size, grain boundary cementite, pearlite morphology, coherency of precipitates and solute carbon on toughness of the steels were examined. It was found that the most important parameter in achieving a good toughness in the hot rolled condition is the carbide morphology, By selecting coiling temperatures below the pearlite start temperature of the particular steels, a fine carbide (pearlite) dispersion was obtained with excellent toughness properties. Although a possible harmful influence of the presence of coherent precipitates on the toughness of the microalloyed steels should not be completely excluded, it is thought to have a minor effect at these temperatures.