Hot ductility in Nb-bearing high-strength low-alloy steels.

The effect of Nb content in HSLA steels on hot ductility was investigated under various thermal cycles. The increase of Nb content was found to deteriorate hot ductility in all the thermal cycles investigated. The change in hot ductility with temperature gave rise to only one trough in the temperature region from 700 to 900°C, which covered the lower temperature region of the austenite (I), and the γ-α region (II). The strain concentration in the ferrite primarily reduced hot ductility in the region II, and the increase of Nb content reduced hot ductility remarkably in region I, where grain boundary precipitates such as Nb(CN) primarily controlled hot ductility. The latter fact was confirmed through the studies of fractography and reversibility of hot ductility behavior. The improvement of hot ductility in the Nb-bearing steels was shown to be attained by adoption of the thermal cycle corresponding to hot charging process of the ingot or the slab, and also by the decrease of N content or the addition of the small amount of Ti. Relation between hot ductility and hot deformation mechanism, such as dynamic recrystallization or grain boundary sliding, is discussed.