Tin-induced hot ductility degradation and its suppression by phosphorus for a Cr-Mo low-alloy steel

Using a Gleeble 1500D thermomechanical simulator, the samples of 1Cr-0.5Mo low-alloy steel, undoped, doped with Sn and doped with P+Sn, were heated at 1300 °C for 3 min and then cooled at a rate of 5 °C s−1 down to different temperatures between 700 and 1050 °C, followed by tensile deformation until fracture. The results show that the hot ductility of the steel, evaluated by the reduction in area, can be substantially deteriorated by Sn, but it is considerably enhanced by P+Sn, especially in the range 850-1000 °C. Therefore, the detrimental effect of Sn on the steel hot ductility is completely suppressed by the beneficial effect of P. Grain boundary chemistry measurements indicate that the hot ductility deterioration by Sn may mainly arise from its grain boundary segregation. Nanohardness measurements and microscopy demonstrate that, in the austenite-ferrite region, the hot ductility enhancement by P+Sn is mainly caused by the P-induced solid solution hardening of ferrite layers formed along austenite grain boundaries, and in the austenite region, by the P-promoted dynamic recrystallization of austenite.