Mechanism of hot ductility loss and recovery in C-Mn/Nb-Microalloyed Steels

Abstract

Hot ductility loss and recovery mechanisms of C-Mn and C-Mn-Nb steels were investigated with various thermal histories before hot deformation. The austenite grain boundaries are embrittled by the segregated sulfur. During holding at a temperature, a convex segregation profile of sulfur, which is due to the precipitation of MnS particles, is formed in a holding time versus segregation concentration plot. The precipitation start curve of MnS particles is expressed by a C curve in a time versus temperature plot. The recovery of hot ductility is attributed to the increase in intergranular fracture strength which results from the decrease in grain boundary segregation concentration of sulfur. The recovery of hot ductility is encouraged by the fine MnS particles uniformly distributed in the matrix which act as void nucleation site before the grain boundary cracking. The poor hot ductility of low-alloy steels containing Nb is primarily due to segregation of S to austenite grain boundaries, which decreases their cohesion strength. When grain boundaries are embrittled by the segregated S, formation of Nb(C,N) particles accelerates loss of hot ductility due to the additional matrix strengthening effect. The recovery of hot ductility is due to both decrease in segregation concentration of S at austenite grain boundaries which is accompanied by the MnS reaction, and to coarsening of Nb(C,N) precipitates. Hot ductility of reheating and as-cast tensile tests were compared by newly-designed as-cast tensile tests. Reheating tests caused no ductility loss, but as-cast tests caused loss of hot ductility due to microstructural differences between reheated and as-casted specimens and enhanced S segregation during solidification. S and P segregate competitively to the grain boundaries or inter-dendritic regions and S segregates preferentially at them. Accordingly, S segregation is the main reason for the low ductility of the steels. Ductility increased as S content decreased and Mn content increased. High Mn content increased MnS precipitation temperature; this increase is beneficial to reduce S segregation at grain boundaries. In addition, hot ductility increased with holding time. A change of MnS morphology from plate or rod shapes to globular shapes and stress concentration around detached MnS contributed to recovery of hot ductility.