Effects of 0.4 % Si and 0.02 % P Additions on Surface Hot Shortness in 0.1%C-0.5%Mn Steels Containing 0.5 % Cu

Abstract

The objective of this paper is to examine the effects of Si and P in low carbon steels on surface hot shortness due to Cu. Susceptibility of the steels to surface hot shortness was evaluated by a new method using tensile tests which is proposed by the present authors. Tensile tests were carried out after heating specimens at 1 000, 1 100 and 1 200°C in air and in Ar gas. Tensile tests using specimens implanting a Cu rod, observation of oxidation rate by thermogravimetry, optical microscopy and EPMA of steel/scale interface region etc. were also performed. At 1 100°C, single additions of 0.4 % Si and 0.02 % P were effective to decrease susceptibility to surface hot shortness, although these increased the oxidation rate. Duplex addition of 0.4 % Si and 0.02 % P decreased the oxidation rate and exhibited a substantial effect on a decrease in the susceptibility. Addition of Si decreased the amount of Cu-enriched phase at steel/scale interface. This is contributable to the reduction of the susceptibility to surface hot shortness. Internal oxidation of Si is thought to decrease the amount of the Cu-enriched phase. Single addition of 0.02 % P seems to increase slightly the amount of the Cu-enriched phase. A critical stress exists to fracture the specimens by Cu-enriched liquid phase. The additions of Si and P increase this critical stress. Silicon also contributes to a decrease in the growth rate of the crack created by the penetration. At 1 200°C, the susceptibility to surface hot shortness in all steels decreased compared with that at 1 100°C, but trends of effects of single and duplex additions of 0.4 % Si and 0.02 % P on the susceptibility were similar to those at 1 100°C. The oxidation rate for all steels was much higher than at 1 100°C, but the amount of Cu-enriched phase at steel/scale interface was reduced compared with at 1 100°C. The amount of the Cu-enriched phase in the steels containing 0.4 % Si is smaller than that in other steels. Liquid phase which appears in scale at temperatures higher than 1 177°C (eutectic temperature of FeO-2FeO·SiO₂) is a contributing factor in both increases of the oxidation rate and occlusion of Cu into the scale at 1 200°C.