2009 Volume 49 Issue 9 Pages 1424-1431
The hot ductility of low manganese mild steels was investigated at a high strain rate of 22 s−1 to simulate a roughing mill in the hot rolling process for various contents of manganese and sulfur. With all steels, ductility loss occurred in a low temperature range in the austenite region after reheating at 1573 K. A decrease in the manganese content and an increase in the sulfur content deteriorated hot ductility. A low reheating temperature and a long holding time before tensile deformation improved hot ductility. Tensile strength decreased at reduction-in-area below 40% at a constant temperature, and had no relationship with chemical composition. The brittle fracture surfaces were intergranular fractured ones and were observed at reduction-in-area below 40%. (Fe,Mn)S precipitates existed in small dimples on the brittle fracture surfaces. The quantities of the precipitated sulfides were calculated by Thermo-Calc, and increased with increasing manganese and sulfur contents. An increase in the manganese content promoted coarse (Fe,Mn)S at the same temperature. The sizes of the sulfides were also correlated with thermal diffusion at various temperatures and holding times. Hot ductility was understood by the quantity and size of the precipitated sulfides at high reheating temperatures. Embrittlement was accelerated by an increase in the quantity of precipitated sulfides and a decrease in the precipitate size. At low reheating temperatures, improvement of hot ductility was recognized as a result of a decrease in solved sulfur in the reheating process.