Current status of research on the formation and control of longitudinal cracks on the surface of continuous casting slabs

Abstract: Surface longitudinal cracks are serious quality defects of cast slabs, which are prone to significant expansion during the rolling process and seriously affect production efficiency and product quality. A large amount of research at home and abroad focuses on how to control surface cracks in continuous casting slabs. This article reviews and analyzes the formation and control methods of typical longitudinal cracks on the surface of continuous cast slabs. Factors affecting the surface defects of continuous casting billets mainly include steel grade composition, continuous casting parameters (mold cooling intensity, pulling speed, superheat, mold taper, etc.) and the physical and chemical properties of the selected mold powder. Through the coordinated regulation of various factors, uniform heat transfer on the surface of the slab and stable growth of the shell are promoted, and the occurrence of longitudinal cracks on the surface of the slab is suppressed.

Keywords: continuous casting; surface longitudinal cracks; steel grade composition; continuous casting process parameters; mold powder

With the development of the steel industry, continuous casting technology has become one of the core technologies in the steel industry. As the “heart” of the continuous casting machine, the continuous casting mold undergoes complex, high-temperature, multi-phase, and instantaneous physical and chemical changes. Continuous casting billet defects mainly include internal defects and surface defects. Among them, surface defects can be divided into vertical cracks, transverse cracks, depressions, reticular cracks, subcutaneous pinholes and macro inclusions. According to statistics, surface crack defects in continuous casting billets account for more than 50% of various defects in continuous casting. Cracks in continuous casting billets are the main defects that affect the output of continuous casting machines and the quality of cast billets. In particular, surface longitudinal cracks are usually regarded as the most serious defects among cracks. At the same time, longitudinal cracks in continuous casting billets are particularly prone to severe expansion during the rolling process, seriously affecting production efficiency. Surface defects (dents, cracks, etc.) of the continuous casting billet originate from the initial solidification stage of the molten steel and are affected by the heat transfer between the molten steel and the cooling copper mold of the mold, the initial solidification of the molten steel, and the physical and chemical properties of the selected mold slag. In particular, it is greatly affected by the uniformity of heat transfer and the uniform growth of the billet shell during the initial solidification process of molten steel. First of all, changes in the composition of steel types are characteristics of the steel type itself. Changes in composition affect its heat transfer efficiency or solidification efficiency, and affect its high-temperature phase change process. Secondly, continuous casting parameters are also an important link that affects the uniform heat transfer in the mold, such as mold cooling intensity, casting speed, etc. Finally, the physical and chemical properties of the mold flux will affect the heat transfer between the molten steel and the mold, which is mainly determined by the structural distribution of the slag film formed by the penetration of the mold flux into the gap between the mold and the shell. Therefore, this article will review the influence rules and mechanisms of longitudinal cracks on the surface of continuous casting billet from steel grade composition, continuous casting parameters and mold slag. Pay special attention to the steel grade composition (C, S, etc.), continuous casting parameters (mold cooling intensity, pulling speed, superheat, mold taper, etc.) and the physical and chemical properties of the mold slag. It is recommended to promote uniform heat transfer on the surface of the slab and stable growth of the shell through coordinated regulation of various factors, and inhibit the occurrence of longitudinal cracks on the surface of the slab.

Featured charts

Figure: Relationship between carbon content and crack index

Figure: Crack diagram of medium carbon steel

Figure: Distribution of mold powder in the copper mould tube

Conclusion

1) Longitudinal defects on the surface of the continuous casting billet originate from the initial solidification stage of the molten steel and are affected by the heat transfer between the molten steel and the cooling copper mold of the mold, the initial solidification behavior of the molten steel, and the type of protective slag selected. In particular, it is greatly affected by the uniformity of heat transfer and the uniform growth of the billet shell during the initial solidification process of molten steel. It is recommended to select a specific type of mold slag according to the solidification characteristics of the steel type to match the appropriate heat transfer and lubrication properties.

2) During the continuous casting process, special attention should be paid to the steel composition, continuous casting parameters (mold cooling intensity, pulling speed, superheat, mold taper, etc.), as well as the physical and chemical properties of the selected mold powder. Through coordinated regulation between various processes, uniform heat transfer on the surface of the slab and stable growth of the shell are promoted, and the occurrence of longitudinal cracks on the surface of the slab is suppressed.

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