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 °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 γ+α 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 γ+α 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.

在这项研究中进行了热拉伸试验，以研究Nb和B对25CrMo合金钢在700–1100°C温度范围内，应变速率为0.5 s ^-1时的热延性的综合影响。。此外，还通过OM，FE-SEM，TEM，AES和SIMS研究了DRX晶粒生长行为，铁素体相变，析出物和B非平衡偏析对热延展性的影响。结果表明，随着变形温度的升高，Nb（C，N）颗粒的数量减少。此外，BN在Nb（C，N）颗粒上的沉淀大量沉淀在基体中而不是在晶界处沉淀，减小了作用在晶界上的应力，而粗大的BN颗粒沉淀在Nb（C，N）上则降低了钉扎效应Nb（C，N）微粒在位错上的分布。除了Nb（C，N）和BN粒子外，还有M ₂ B，M ₂₃ C ₆和M ₇ C ₃在γ+α两相区析出。通过在单相奥氏体区域复合添加Nb-B来改善热延展性的机理是，尽管DRX受到阻碍，但通过添加Nb和B可以使DRX晶粒明显地细化。同时，B原子迅速偏析到占据晶界的B原子上。空隙并增强晶界凝聚力。此外，BN粒子沉淀在基体中而不是晶界处的Nb（C，N）粒子上，减少了对热延展性的不利影响。γ+α两相区热延展性的提高归因于通过添加B来抑制铁素体相变，以及因形变引起的碳化物析出，从而消耗了奥氏体中的强化合金元素。