Correlation Between MnS Precipitation, Sulfur Segregation Kinetics, and Hot Ductility in C-Mn Steel

In a C-Mn steel without Nb, the mechanism of hot ductility loss and recovery has been understood. The specimens were solution treated at 1673 K (1400 A degrees C), subsequently cooled to 1273 K (1000 A degrees C) at a rate of 1 or 20 K/s and finally held at the temperature. At a rate of 1 K/s, the segregation concentration of sulfur at prior austenite grain boundaries decreases gradually with holding time. At the rate of 20 K/s, the segregation concentration of sulfur shows a convex profile in a time versus segregation concentration plot. Such segregation behaviors of sulfur are deeply related to the MnS reaction during cooling to 1273 K (1000 A degrees C) or holding at the temperature. The high-temperature intergranular fracture observed in this steel is due to the sulfur segregated at the grain boundaries. The recovery of hot ductility results from the combination between the decrease in sulfur segregation concentration governed by the MnS reaction and the overall decohesion at the interface of the MnS particles which act as a strong sink of the free sulfur tending to segregate to the grain boundaries.