Effect of solution temperature and rolling schedule on niob,ium steel controlled-rolled into ferrite temperature range

An alloy containing 0.03%C, 1.0%Mn, 0.05%Nb, and 0.006%N was solution treated at either 1300 or 1020°C prior to hot rolling. The solution treatments were designed such that all the calculated volume fraction of NbCN was taken into solution at 1300°C but only two-thirds was in solution at 1020°C, with the aim of grain refining the austenite. The material was controlled rolled over a range of finishing temperatures between 870 and 600°C. Mechanical and impact properties were determined and the microstructure observed by thin foil electron microscopy. Decreasing the finishing rolling temperature was found to increase the yield stress for both solution-treatments but good impact properties were only found for the 1020°C treatment. The variation in properties is explained in terms of the observed microstructure.