For the sake of understanding the mechanism of the cracking in continuous casting and hot direct rolling, the mechanical behavior at elevated temperatures and in a wide range of strain rates must be investigated. Many previous reports described the hot ductility in low speed tensile tests, but only a few researchers have been concerned with high speed tensile tests.
Hot ductility of austenite in low and plain carbon steels has been examined at strain rates up to 200/s by using a hot working simulator. When Mn: S ratio is less than 20 in carbon steels, or sulfur content is greater than 30ppm in the carbon steel without addition of manganese, the embrittlement with intergranular fracture is observed. The embrittlement is restrained if the sulfur content is less than the order of 10ppm or Mn: S ratio is greater than 50. Thin-layered precipitates of MnS are observed on the fracture surfaces in a 0.19 % carbon steel with Mn: S ratio of 35. The effect of carbon content on the embrittlement is poorly understood. For a low carbon steel containing 0.26Mn and 0.0155, the ductility increases with decrease of the cooling rate from the solution-treating temperature or decrease of the strain rate when the solution-treating temperature is 1573K. If the temperature is up to 1673K, however, the recovery of ductility is not achieved despite of slow cooling and low strain rate. On the other hand, the ductility in ferrite and austenite-ferrite two phase region is good at the strain rate of 10^-2/s.