| 摘要 |
PURPOSE:To carry out the stable operation always freely from generation of surface cracks, by rolling just cast hot billet, without reheating, at temperatures lower than the embrittlement release temperature corresponding to various surface cooling speeds, previously obtained for every different grade of steel. CONSTITUTION:The cooling curves 8, 9, 10, starting at the solidification point 11, and the embrittlement release curve 12 obtained by connecting the border points at which the embrittlement is released, that is, the embrittlement release points 11a- 11c, are previously prepared by directly rolling the hot cast billet of the specified grade of steel, like as the illustrated diagram. Next, the hot cast billet, without being reheated, is directly rolled at temperatures lower than above curve 12. That is, if the hot cast billet is cooled to above embrittlement release zone, the restriction on the chemical composition is greatly relaxed. For example, in case of the carbon steel (C<=0.1%) of oxygen content 200ppm or less and Mn/S ratio 7 or more, the surface crack owing to the low melting point non-metallic inclusion, and the austenitic intercrystalline cracks owing to the solid-solution type sulfide to be caused at 1200 deg.C or lower temperatures, can be prevented. |
