Effect of Nb on microstructure and mechanical properties between base metal and high heat input coarse-grain HAZ in a Ti-deoxidized low carbon high strength steel

In this paper, a Ti-deoxidized low carbon high strength steel plate with different Nb content was obtained by thermal mechanical control processing (TMCP) methods. The coarse-grain heat-affected zone (CGHAZ) under high heat input of 300 kJ/cm conditions was simulated. The effects of Nb on microstructure and mechanical properties in base metal (BM) and CGHAZ were analyzed. The results showed that high content of Nb addition is not conducive to the improvement of toughness, especially in high heat input CGHAZ. A certain amount of Nb inhibites the formation of ferrite, while promotes the bainite transformation with coarse carbides during high heat input welding process. Although high-angle grain boundary has a certain hindrance to the crack propagation, the coarse carbides caused by the enhanced hardenability in Nb-bearing steels stimulates the crack initiation sensitively. Besides, the uneven prior austenite grains may also take responsibility to the deterioration of toughness in Nb-bearing CGHAZ. In general, the content of Nb should be carefully added when impact toughness of CGHAZ is considered during high heat input welding in Ti-deoxidized low carbon high strength steel.