Application of “flat’ closed-bottom submerged nuzzles for decreasing rejections of slabs because of longitudinal cracks

In the process of steel continuous casting it was discovered, that due to relatively small distance between a submerged nuzzle and a mold walls, formation of “scull crust” takes place in the area of small and big radius of the billet. It resulted in deterioration of heat-away in mold, conditions are formed for origination of longitudinal cracks following its further opening in the CCM secondary cooling zone. To decrease the number of rejections continuously casted slab billets due to existence of longitudinal cracks, it was proposed to use “flat” closed-bottom submerged nuzzles. It was shown, that in contrast to cylindrical form of a series closed-bottom submerged nuzzle, the proposed one has rectangular section with chamfered butt facet in the are of nuzzle submerging into the mold melt, which enables to ensure better fluidity of slag-forming mixture between the nuzzle and the mold walls. This effect results in onsiderable improving evenness of heat-away. To confirm the effectiveness of the pilot submerged nuzzles application, in 2019 their pilot-industrial tests were accomplished in the campaign of casting of carbon and peritectic steels to produce 200 mm thick slabs at CCM No. 4. In the process of the tests when casting various steels, the same slag-forming mixtures were used. As a result of the tests the decrease of rejections of continuously casted slabs due to longitudinal cracks formation was confirmed.