EFFECTS OF 0.4-PERCENT SI AND 0.02-PERCENT P ADDITIONS ON SURFACE HOTSHORTNESS IN O.1-PERCENT-C-0.5-PERCENT-MN STEELS CONTAINING 0.5-PERCENT CU

The objective of this paper is to examine the effects of Si and P in l ow carbon steels on surface hot shortness due to Cu. Susceptibility of the steels to surface hot shortness was evaluated by a new method usi ng tensile tests which is proposed by the present authors. Tensile tes ts were carried out after heating specimens at 1 000, 1 100 and 1 200 degrees C in air and in Ar gas. Tensile tests using specimens implanti ng a Cu rod, observation of oxidation rate by thermogravimetry, optica l microscopy and EPMA of steel/scale interface region etc. were also p erformed. At 1 100 degrees C, single additions of 0.4% Si and 0.02% P were effective to decrease susceptibility to surface hot shortness, al though these increased the oxidation rate. Duplex addition of 0.4% Si and 0.02% P decreased the oxidation rate and exhibited a substantial e ffect on a decrease in the susceptibility. Addition of Si decreased th e amount of Cu-enriched phase at steel/scale interface. This is contri butable to the reduction of the susceptibility to surface hot shortnes s. Internal oxidation of Si is thought to decrease the amount of the C u-enriched phase. Single addition of 0.02% P seems to increase slightl y the amount of the Cu-enriched phase. A critical stress exists to fra cture the specimens by Cu-enriched liquid phase. The additions of Si a nd P increase this critical stress. Silicon also contributes to a decr ease in the growth rate of the crack created by the penetration. At 1 200 degrees C, the susceptibility to surface hat shortness in all stee ls decreased compared with that at 1 100 degrees C, but trends of effe cts of single and duplex additions of 0.4% Si and 0.02% P on the susce ptibility were similar to those at 1 100 degrees C. The oxidation rate for all steels was much higher than at 1 100 degrees C, but the amoun t of Cu-enriched phase at steel/scale interface was reduced compared w ith at 1 100 degrees C. The amount of the Cu-enriched phase in the ste els containing 0.4% Si is smaller than that in other steels. Liquid ph ase which appears in scale at temperatures higher than 1 177 degrees C (eutectic temperature of FeO-2FeO . SiO2) is a contributing factor in both increases of the oxidation rate and occlusion of Cu into the sca le at 1 200 degrees C.