Abstract
The statistical correlation was applied to analyze the specific and quantitative correlation relationship between the solidification structure and central segregation along the casting direction in carbon steel billet. On this basis, the segregation formation mechanism of the solute element and related control strategy were investigated. It is found that the equiaxed crystal zone fluctuation along the casting direction determines the fluctuation degree of central segregation. At the same time, the central segregation at a certain position is mostly affected by the equiaxed crystal zone width at the hysteretic position. Moreover, the casting speed can influence the columnar to equiaxed transition (CET) fluctuation along the casting direction by affecting the flow of molten steel in the billet. Overall, the segregation mechanism of solute elements along the casting direction can be summarized into two aspects: First, with the growth of columnar crystals in the initial stage, the segregated solutes are continuously enriched and distributed in the equiaxed crystal zone after CET. The fluctuation of the equiaxed crystal zone will affect the distribution of the enriched solute in the billet and cause the fluctuation of the central segregation. Second, due to the solidification shrinkage at the end of solidification, the solute-enriched liquid phase at the hysteretic position is pumped to the solidification endpoint and forms the central V-shaped segregation. Meanwhile, the stable solidification structure (columnar crystal length or equiaxed crystal zone width) along the casting direction and control measures preceded equiaxed crystal zone formation are beneficial to reduce the central V-shaped segregation.
Highlights
In the era of rapid industrial development, steel products, especially high-end steel products, play an essential role in national economic development and state construction.Nowadays, continuous casting (CC) is a major production process of high-end steel products in the world due to higher efficiency and lower energy consumption
To investigate the discrete relationship between the equiaxed crystal zone width and the central segregation area ratio, the variation coefficient was introduced to quantitatively describe the discrete degree of these two variables, which can avoid the to quantitatively describe the discrete degree of these two variables, which can avoid the comparison error caused by the different scales of the two sets of data
The variation coefficient of the equiaxed crystal zone width and the central segregation area ratio can be obtained by Equation (5)
Summary
In the era of rapid industrial development, steel products, especially high-end steel products, play an essential role in national economic development and state construction. Central carbon segregation along the casting direction occurs due to the carbon solubility difference between the liquid phase and solid phase, which can induce tremendous quality control issues in high-end steel products [1,2]. This kind of defect is difficult to eliminate in reheating and rolling processes and will be left into the subsequent products, which will affect the tensile yield strength and low-temperature impact toughness [3,4]. This research can provide theoretical guidance for controlling the internal defects and improving the quality uniformity along casting direction in CC billets
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