Effect of compound addition of Nb-B on hot ductility of Cr-Mo alloy steel

The hot tensile tests were conducted in this study to investigate the combined effect of Nb and B on hot ductility of 25CrMo alloy steel in temperature range of 700-1100 degrees C with a strain rate of 0.5 s(-1). Besides, the influences of DRX grains growth behavior, ferrite transformation, precipitates and B non-equilibrium segregation on hot ductility were also investigated by use of OM, FE-SEM, TEM, AES and SIMS. The results indicated that the amount of Nb(C,N) particles decreased with increasing deformation temperatures. Moreover, the precipitation of BN on Nb(C,N) particles, which precipitated in matrix abundantly rather than at grain boundaries, reduced stress acting on grain boundaries, and the coarsening BN particles precipitating on Nb(C,N) reduced the pining effect of fine Nb(C,N) particles on dislocations. Besides Nb(C,N) and BN particles, there were M2B, M23C6 and M7C3 precipitates in gamma + alpha two-phase region. The mechanism for hot ductility improvement by compound addition of Nb-B in single phase austenite region was that although the DRX was retarded, the DRX grains were refined obviously by adding Nb and B. Meanwhile, the B atoms segregated rapidly to grain boundaries occupying the voids and enhancing grain boundary cohesion. Besides, the BN particles, precipitating on Nb(C,N) particles in matrix rather than at grain boundaries, reduced the adverse effects on hot ductility. The improvement of hot ductility in gamma + alpha two-phase region was attributed to the restraint of ferrite transformation by the addition of B and the deformation induced precipitation of carbides, which consumed strengthening alloy elements in austenite.