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It is commonly observed that severe V-channel segregates are associated with the equi-axed crystal zone in the central region of a continuously cast strand. This type of segregation may deteriorate the qualities of products especially for sour gas service. The modification of V-segregates is found to be attained by electromagnetic stirring at the final stage of solidification of casts. The laboratory scale stirring experiments have been carried out in order to eliminate V-segregation and the f ollowing results have been obtained. (1) V-segregation streaks can be eliminated by stirring at the final stage of solidification with either linear or rotary type stirrer. (2) The upward flow of molten steel at the solidification front induced by the linear type stirrer may prevent the interdendritic liquid flow toward the bottom which leads to the formation of V-segregates. (3) The presence of adequate amount of fine equi-axed crystals at the final stage of solidification is the essential condition for the elimination of V-segregates by the rotary type stirrer. Alternating stirring direction around the cast axis is preferred for the prevention of coagulation of equiaxed crystals and results in the modification of packing of them. (4) In addition to the above results, the timing and intensity of stirring with which the best result can be obtained have been discussed.

SynopsisThe unidirectionally solidified large scale (USL) ingots, which have low height and large expanse, were developed based on the results of the simulation experiments with the ammonium chloride solution. The USL ingots were found to be f ree from V segregates, the loose structure and the accumulation of non-metallic inclusions. The intensity of top segregation of USL ingot closely related to that of the negative segregation in the region which solidified from the mold wall. Both segregations, however, can be minimized by optimizing the mold wall design and they can be completely removed by conditioning to obtain the highly homogeneous and sound ingot. It was found that the wider the ingot, the milder the top segregation. It was realized that the USL ingot was suitable for production of high grade heavy gauge plates of the large unit weight. The manufacturing process of high grade heavy gauge plates was completed by the combination of USL ingots and the low-speed heavy-reduction plate rolling technique. The qualities of the plates produced by the process were found to show superior mechanical properties, homogeneity, cleanliness and workabilities for manufacturing pressure vessels. The maximum thickness and the size of the plate were both substantially expanded.The formation mechanisms of the sedimental crystal zone and the top segregation in the ingot were also suggested in the course of development of the USL ingots.

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