Practically the whole steel, including the free-cutting one, is casting by continuous casting machines (CCM). The technological parameters of the free-cutting steel continuous casting have their features, since the high content of Sulphur and phosphor in the free-cutting steel, necessary for its high machinability, increases the probability of fractures and emergency breach of metal. The purpose of the work was study of influence of temperature-speed mode of A12 free-cutting steel casting on cooling agent consumption in the secondary cooling zone of radial type billet CCM. The CCM mold was 900 long with supporting system on the mold frame of two rows of rollers. The secondary cooling (SC) zone comprised four zones of 340, 2360, 3460 and 3610 mm length. The cooling in the first zone is accomplished by water, in the others – by water-air mist. The study was carried out by application of mathematical simulation method to the process of billet secondary cooling, elaborated by V.N. Selivanov. The temperature of the free-cutting steel billet surface was calculated by zones of secondary cooling at various steel temperature in the tundish. The flow density and water consumption in various SC zones determined, necessary to reach the required variation of billet surface temperature. The dependence of total and specific water consumption for the billet cooling on the metal temperature in the CCM tundish determined.
Free-cutting steel gains specific working properties thanks to the high content of sulfur and phosphorus. These elements, especially sulfur, have a rather high tendency to segregation. Therefore, segregation defects in free-cutting steel continuously cast billets can be significantly developed. The aim of the work was to study the influence of the chemical composition of freecutting steel and casting technological parameters on the quality of the macrostructure of continuously cast billets. A metallographic assessment of the internal structure of cast metal made of free-cutting steel and data processing by application of correlation and regression analysis were the research methods. The array of production data of 43 heats of free-cutting steel of grade A12 was studied. Steel casting on a five-strand radial type continuous casting machine was carried out by various methods of metal pouring from tundish into the molds. Metal of 19 heats was poured with an open stream, and 24 heats – by a closed stream through submerged nozzles with a vertical hole. High-quality billets had a cross-sectional size of 150×150 mm. The macrostructure of high-quality square billets made of free-cutting steel of A12 grade is characterized by the presence of central porosity, axial segregation and peripheral point contamination, the degree of development of which was in the range from 1.5 to 2.0 points, segregation cracks and strips – about 1.0 points. In the course of casting with an open stream, almost all of these defects are more developed comparing with the casting by a closed stream. As a result of correlation and regression analysis, linear dependences of the development degree of segregation cracks and strips both axial and angular on the sulfur content in steel and on the ratio of manganese content to sulfur content were established. The degree of these defects development increases with growing of sulfur content in steel of A12 grade. These defects had especially strong development when sulfur content in steel was of more than 0.10%. To improve the quality of cast metal, it is necessary to have the ratio of the manganese content to the sulfur content in the metal more than eight.
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