Металознавство та обробка металів (Mar 2021)
The cooling rate effect during a continuously cast billet solidification on the dendritic structure features of carbon steel
Abstract
The analysis of the formation process of the cast structure of carbon steel grade ОС (ДСТУ ГОСТ 4728:2014) after the completion of its crystallization with a change in a wide range of metal cooling rate during solidification of a continuously cast billet (ССB) with a diameter of 450 mm has been carried out. The effect of the cooling rate during the solidification of ССB Ø 450 mm on the parameters of the chemical heterogeneity of the distribution of silicon and manganese in the microstructure of carbon steel has been shown. It has been determined that the effect of the metal cooling rate during the solidification of the investigated CCB on the size of dendritic crystals is described by the inversely proportional relationship: у = 423.75 х-0,161. With a change in the cooling rate of the metal during solidification from 106 до 1 °C/min, the size of the dendrites in the direction from the surface to the central layers of the CCB Ø 450 mm increased by ~ 8 times, and the density of the dendritic structure of carbon steel ОС decreases by 65 times. In this case, the nature of its dependence on the intensity of heat removal is the opposite nature of the change in the size of dendrites. It has been established that by varying the cooling rate in the range 1 – 106 °C/min, one can achieve a significant change in the average size and density of dendritic crystals while maintaining the constancy of the volume fraction of segregation areas of silicon and manganese ~ 24% in carbon steel (0.42 – 0.50 % wt. C). It has been determined that in the entire investigated range of cooling rates 1 – 106 °C/min, the coefficients of dendritic segregation КдI and КдII of silicon and manganese change insignificantly and amount to 1.8-1.9 and 1.5 for КдI and КдII, respectively. In this case, the values of the coefficients КдI and КдII for both elements are practically constant in both pearlite and ferrite. It has been proven that both silicon and manganese have high diffusion mobility only at sufficiently high temperatures, when steel is in a solid-liquid state. Based on the results of X-ray microanalysis, it has been established that the heterogeneity of the distribution of chemical elements, which is formed as a result of dendritic segregation of silicon and manganese, is the primary and constant component of the microstructure of carbon steel.
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