Effect of thermal cycle and nitrogen content on the gor ductility of boron-bearing steel

Abstract

Hot ductility of Boron (B)-bearing steel has been examined in view of slab corner cracking problem. Addition of B to the low carbon steel reduced its hot ductility under a thermal cycle in which samples were cooled directly to the test temperature before straining. The change in hot ductility of B-bearing steel with deformation temperature showed one trough in the temperature range of 800-1 000 degrees C, which covered the lower temperature region of austenite single phase (region (I)), and near the austenite/ferrite transformation temperature (Ae(3)) (region (II)). An abrupt temperature decrease and reheating before straining heavily deteriorated the hot ductility of B-bearing steel in the region (I). In all steels, the strain concentration in the film-like ferrite primarily reduced hot ductility in region (II) regardless of the addition of B and the thermal cycles before straining. The ductility reduction of B-bearing steel is caused by the distribution and amount of BN precipitation, which is determined by the thermal cycles and the N content. Increase in the N content remarkably reduced hot ductility of B-bearing steel in region (I), where the behavior of BN precipitates controlled hot ductility. The results shows that the improvement of hot ductility in B-bearing steel can be attained by decreasing the N content and by avoiding an abrupt temperature decrease in the secondary cooling stage of the slab after solidification.