Research On The Quality Defects Of Continuous Casting Slab For F550 Ship Plates

With the implementation of the strategy of China’s marine power, the state pays high attention to the development of the steel used in shipbuilding and marine engineering.The marine engineering equipment and high technology ships was ranked developed as a key development industry in Chinese manufacturing 2025. The level of steel development and research would be improved by the projects which strengthened the base of China’s industry.The development of China’s shipbuilding industry promoted the shipbuilding steel which was inclined to use high strength and toughness and had good welding and corrosion resistance.F550 ship plate steel has high strength, high toughness and other characteristics which is the important structural steels for building high-level ship.Due to that the development and research of China’s high-performance F550 ship plate steel was starting too late, the production technology was unstable and the quality of continuous casting slab was poor, the quality of ship plates steel produced by our country could not meet the F550 level requirements, which influenced the development of China’s high-level shipbuilding equipment manufacturing. In this paper,the influence rules of various process parameters to the quality of continuous casting slab for F550 ship plate steel were studied through analyzing non-metallic inclusions existent form,the segregation law of some chemical elements and the formation mechanism of the corner and cross cracks in the continuous casting slab for F550 ship plate steel, which provided technical guidance for the production of high quality F550 continuous casting slab. The concrete conclusions were as follows:(1)The non-metallic inclusions in the F550 grade cast slab were mainly consisted of alumina inclusions and oxide-sulfide complex inclusions. The inclusions mainly came from the secondary oxidation, casting powder or the undish covering agent fluxs. The quality of cast slab would be improved by the means of preventing the formation of inclusions, such as voiding secondary oxidation,improving the cast starting protection measures.(2)The elements such as C, P, S and Mn had different degrees of segregation in the cast slab. The P, S and C were all strong segregation elements, the average segregation degree of Mn element is low. Impurity element content and controlled rolling process could improve the element uniformity and decreased the influence of element segregation to quality of slab.(3)The corner and cross cracks were related to slab composition. The incidence of corner and cross cracks were increased with the C, S, N and Al content of casting slab.Therefore, the C content of casting slab should be far away from peritectic reaction point(Wt% = 0.1). And the S content and the N content should be both less than 40 ppm. Inaddition, the Al content shoud be reduced to a proper degree. The increase of Mn / S ratio could reduce the incidence of corner and cross cracks. It was suggested that Mn content should be controlled on upper limit. While the influence of process parameters on corner and cross cracks was weak.