Abstract
In the lst report, (Tetsu-to-Hagane Vol. 41, No. 1, 1955 p. 17-22) it was shown that the black, spots acted as defect points for the hot wbrking. In this report, the following studies were carried out. The chemical compositions of the black spots were analysed and their microstructures were examined. About O•15% of sulphur and O.4% manganese were contained and sulphide particles were concentrated remarkably in these spots. The hot-working properties of steel containing the same amount of sulphur as that of the. black spots were investigated by the impact-bending tests and tensile tests at high temperatures. By this investigation, It was shown that the hot-working properties were injured when the sulphur content surpassed O•15%. Therefore, sulphur was found most influential factor for defect points. But increase of manganese content was effective to the improvement in the hot-working Properties of the high-sulphur steel.
Beside the numerous MnS inclusions, FeS inclusions were detected in the black spots of some billets. When the billets or ingots containing FeS inclusions were heated at the temperatuee from 800°C to 1200°C for several hours, it was found that the FeS converted into the MnS.
