Adverse effect of niobium and boron on hot deformation behavior of sulfur-containing steel

The hot deformation behaviors of sulfur-containing gear steel 20MnCr5 containing three different contents of Nb and B (0, 0.021%Nb, and 0.024%Nb–0.0022%B) were investigated. Hot compression and tenssion tests were carried out by Gleeble3800 at the austenite region from 850 to 1150 °C and the adverse effects of Nb and B were analyzed by the fracture, microstructure and precipitate observations. Hot compression tests showed that the proportions of instable area in hot processing maps of 0.021%Nb and Nb–B steels were higher and the deformability of Nb free steel was better. The tensile deformation experiments showed that the reduction areas of Nb free, 0.021%Nb and Nb–B steels were 92%–99%, 84%–98% and 67%–97%, respectively. The addition of Nb or Nb and B inhibited the dynamic recrystallization during hot deformation, and consequently, more deformed grains were then formed in 0.021%Nb and Nb–B steels thus to obtain the microstructure with worse uniformity and then deteriorate the deformability. In addition, the interaction between inclusions and microalloyed elements was also significant. NbC particles of 0.021%Nb and Nb–B steels dynamically precipitated during deformation and precipitated together with MnS thus to worsen the deformability, resulting in the decrease of reduction area.