Selection of internal design of crystallizer sleeves for continuous casting of 200 mm billets

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The crystallizer is the most critical and most important functional unit of a continuous casting machine (hereinafter referred to as CCM). It is the main technological unit of the CCM, an assembly for removing heat during the solidification of the solidifying metal and the formation of the ingot. The main requirement for the crystallizer is to provide maximum heat removal from the solidifying steel to the cooling water, and to obtain a strong ingot shell with a good surface at the outlet of the crystallizer, which would not be destroyed by the heat of the liquid phase and the ferrostatic pressure. During the reconstruction of the CCM by the contractor company, drawings of crystallizers and crystallizer sleeves with a double‑cone internal geometry for casting 200 mm continuous cast billets were developed. The 200 mm continuous cast billet is a billet for the production of hot‑rolled seamless steel pipes. During the operation of the crystallizer sleeves with a double‑cone internal geometry, an accelerated wear of the protective coating in the lower part of the sleeve was noted, as well as an increased rejection of hot‑rolled pipes due to defects on the outer surface of the pipes. In order to identify the cause of the defect formation, complex metallographic studies of the outer surface of the pipes were carried out. As a result of the metallographic study, surface defects classified as steel mill scale on the outer surface of the pipes were identified. To minimize the “steel mill scale” defect and reduce the wear of the protective coating of the crystallizer sleeves, work was carried out to select the optimal conditions for the crystallization of the continuous cast billet: the use of crystallizer sleeves with a three‑cone internal geometry compared to a double‑cone was investigated; maintaining the temperature difference of the water at the inlet and outlet of the crystallizer within specified limits (ΔT) was tested. It was found that the use of a crystallizer sleeve with a three‑cone internal geometry for casting 200 mm billets and the stable behavior of the ΔT parameter made it possible to ensure the required quality of the surface of seamless pipes, reduce the wear of the protective coating of the crystallizer sleeves and increase the productivity of the CCM.