Influence of hot top and mould design on the formation of central porosities and loose structure in heavy forging ingot was analysed by using finite element method. The results of the analysis were compared with those of sectioning investigation of 100 and 135 t ingots and the influence of mould and hot top design on the internal defects has been made clear quantitatively. The result shows that the geometry of hot top and mould design plays most important role in the manufacture of sound heavy ingots. The central porosities and loose structure are liable to increase when the rate of vertical solidification at the centerline of ingot exceeds the value of about 10 mm/ min, and the defects are strengthened at the area where the rate of solidification is accelerated. For 0.25%C-3.5%Ni-Cr-Mo-V steel ingot, "A" segregation begins to form when the rate of transverse (horizontal) solidification decreases to the value of about 0.8 mm/min.

Synopsis Spalling phenomena of the backup rolls used for hot strip mills and their prevention are discussed. The spalling on the surfaces of backup rolls is classified into two categories: excess rolling contact fatigue resulting from the local wear in roll barrel and propagation of the surface cracks generated by thermal shock in the mill accidents such as squeezing and roll slip, and also by incomplete removal of the cracks in roll dressing. Factors for preventing the spalling phenomena are studied from the experiments in both production mills and laboratory. The spalling tendencies are correlated with the carbon and chromium contents, areal fractions of carbides, and fracture toughness values of roll materials. To prevent the spalling, therefore, it is necessary to control and improve both properties of roll materials and procedures of roll maintenance.

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