HOT-DUCTILITY RECOVERY BY MANGANESE SULFIDE PRECIPITATION IN LOW MANGANESE MILD-STEEL

The hot-ductility of low Mn mild steel has been investigated as a function of temperature, annealing time, and cooling rate after soaking treatment at varied levels of Mn and S contents. A drastic reduction of hot-ductility was brought about by soaking treatment at 1 250-degrees-C, followed by a slow cooling at a rate less than approximately 30-degrees-C/s, although its ductility was excellent upon heating to the test temperature directly from room temperature. A further deterioration was made by an increased S content. This hot-ductility loss was attributed to S segregation to austenite grain boundaries. An extremely fast cooling rate after soaking, greater than 100-degrees-C/s for example, resulted in an improved hot-ductility. The binding energy between S atom and austenite grain boundary has been determined to be 198.4 kJ/mol from the S content dependence of hot-ductility. The hot-ductility of the embrittled steel improved by annealing at a test temperature. This was accompanied by MnS precipitation. A pre-deformation and a reduced S content accelerated the hot-ductility recovery. The apparent activation energy for the hot-ductility recovery has been determined to be 461.2 - 491.3 kJ/mol. This high activation energy has been discussed in terms of the diffusion of Mn and the binding energy between S and austenite grain boundary. The presence of free O retarded the recovery of the hot-ductility, rendering the reduction of S content useless.