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Longitudinal surface cracks are prevalent in the continuous casting of hypo-peritectic beam blanks, primarily related to multiphysics non-uniformity (multi-phase field, flow field, temperature field) within the mold. Therefore, multi-phase and coupled flow-thermal models were constructed to investigate the non-uniform multiphysics characteristics in the beam blank mold with a single nozzle and its effect on the formation of longitudinal cracks. First, the three-phase flow model reveals significant unevenness slag distribution along the web width, positively correlated to surface velocity of the molten steel. Subsequently, considering the impact of the solidification shell, surface velocity decreases nonlinearly with increasing immersion depth, influenced by web vortex position in the X-direction, which is also influenced by mold curvature, and interaction between web and side vortexes. Furthermore, the temperature field distribution inside the mold is non-uniform. Under the effect of the single-web recirculation flow, the temperature near the meniscus at the NN (position near the nozzle) side of the web is lower than that at the FN (position far away from the nozzle) side, resulting in more intensive initial cooling at this position. Meanwhile, a reheating zone exists at the mold exit of the NN side. Statistical analysis revealed that the absolute values of Spearman coefficient between the non-uniformity of shell thickness and the temperature and flow velocity of the molten steel exceed 0.6, which is related to the depth of the web vortex. Finally, from the perspective of the influence of SEN's immersion depth on multiphysics, the high surface velocity of the steel, strong cooling near the meniscus, and non-uniform shell thickness at the mold exit can be relieved with an immersion depth of 75 mm. The plant experiment confirmed that increasing the immersion depth from 65 to 75 mm leads to a relative reduction of 39 and 43 pct in the longitudinal cracks ratio for strands and rolled products, respectively. [ABSTRACT FROM AUTHOR]

To obtain a reliable hereditary law of carbon segregation, the carbon element content in the entire area of the longitudinal sections of billet, wire rod and bolt samples is quantitatively calculated through the method of metal macrostructure grayscale analysis (MGA). Based on the calculation results, the hereditary behavior of carbon distribution characteristics at the "geometric similar" positions of the selected samples is quantitatively analyzed from three aspects: carbon element content, longitudinal carbon fluctuation and transverse carbon uniformity. The results show that the distribution characteristics of the carbon element content and carbon variation coefficient on the longitudinal sections of the selected samples are approximately the same, which can quantitatively reflect the hereditary behavior of the carbon segregation, and the corresponding hereditary equations during the rolling process are fitted. Then, the central segregation index, which can represent the transverse carbon uniformity, on the longitudinal sections of selected samples is calculated, and the corresponding hereditary equation is also obtained. According to the hereditary equation of central segregation index during the rolling process, to produce rolled products with more uniform quality and mechanical properties, the central segregation index of the corresponding billet should be controlled below 1.05. [ABSTRACT FROM AUTHOR]

Surface depressions are common defects in the continuous casting process of hypo-peritectic steel, categorised into transverse and longitudinal depressions. While the formation processes of these depressions are well understood, their influence on the propagation of longitudinal surface cracks remains unclear. This study developed a new method for large-area surface depth measurement of the strand based on optical focusing. Utilising this method, we investigated the impact of surface depressions on the three-dimensional propagation of longitudinal surface crack in terms of width, depth and casting direction through macrostructure characteristics and finite element static analysis. The results indicated that transverse and longitudinal depressions affect crack propagation differently. Specifically, the depth of longitudinal depressions increases crack width, while transverse depressions can inhibit it to some extent. Macrostructure analysis of the cross-section revealed a continuous segregation line at the bottom of the depression, closely related to heat transfer, thermal stress, and the flow of solute-enriched liquid steel. Statistical analysis indicated that the depth of longitudinal depressions significantly affects the length of the segregation line, which in turn influences crack depth. However, no surface cracks were found at the bottom of transverse depressions adjacent to longitudinal depressions. This may be due to the banded grains at the bottom of transverse depressions resisting tensile thermal stress parallel to the strand's width direction. Along the casting direction, crack propagation primarily occurs along the bottom of longitudinal depressions due to continuous thermal disturbances at the meniscus. Meanwhile, the transverse depression can restrict the propagation of the existing crack while promoting the formation of the new crack at the other site. Finally, the study proposed mechanisms by which depressions influence the propagation of the longitudinal surface crack. This research further elucidates the impact patterns of surface depressions on macrostructure and stress distribution and how these factors subsequently influence crack propagation in three dimensions. It underscores the importance of controlling surface depressions to mitigate the longitudinal surface crack. [ABSTRACT FROM AUTHOR]

Under the current background of high-speed continuous casting, it is essential to investigate the effect of casting speed on the stability of the solidification structure and quality of billets. In this paper, a three-dimensional transient numerical model was established to simulate the continuous casting process, and the model was verified through the temperature measurement data and the actual solidification structure characteristics of billets. On this basis, the effect of casting speed on columnar to equiaxed transition (CET) position fluctuation along the casting direction was investigated, and the corresponding control measures of CET position fluctuation were presented. The internal quality stability of billets will not always deteriorate with the increase of the casting speed, and there is an optimal casting speed. Meanwhile, the casting speed can affect the fluctuation of the heat transfer by changing the molten steel convection fluctuation in billets, and finally, resulting in the fluctuation of CET position and central quality along the casting direction. Under a certain specific water amount, the fluctuation of liquid convection and heat transfer at the CET region is relatively decreased, and the solidification structure along the casting direction is more uniform when the casting speed is controlled in the intermediate range. During the high-speed continuous casting process, the cooling intensity of the secondary cooling can be appropriately increased, when the optimal casting speed is undeterminable, to reduce the CET fluctuation and improve the uniformity of the billets central quality. This paper can provide methods and theoretical references for determining the optimal casting speed and improving the internal quality of billets under the background of high-speed continuous casting. [ABSTRACT FROM AUTHOR]

What is nucleation core origin during alloy solidification, especially for equiaxed grains? Different dependence degrees of the magnitude or occurrence of element content variation could shed light on this long-standing issue in actual large ingots. Here, based on etched surface height and grayscale, element content distributions within the solid fraction in continuous casting billets and additive manufacturing samples are first obtained by only a two-dimensional surface. Then, combined with the phylogenetic trees, the rank correlation is applied to measure the dependence of content differences during initial solidification. Assessments of external dependence degrees are helpful to determine nucleation core origin and low internal dependence degree facilitates grain refinement. Moreover, in continuous casting, some nucleation cores in the central equiaxed grain zone are confirmed to originate from the edge-chilled zone and high equiaxed grain area ratio under a low superheat, which is attributed to the low ratio of temperature gradient to growth rate rather than remelting fewer cores originating from the chilled zone. In addition, the floating behavior of separated grains originating from the chilled zone can be affected by gravity force, but these grains should be more active when increasing the casting superheat that may weaken the influence of gravity to a certain extent. [ABSTRACT FROM AUTHOR]

Macrosegregation remains one of main defects affecting metal materials properties, which is mainly caused by interdendritic fluid flow during solidifying. However, as for controlling actual specific segregation, it is still difficult to effectively measure or simulate this kind flow instead of pure fluid flow, especially in complex casting processes of high-grade materials. Herein, a new method for obtaining velocity magnitude and direction of interdendritic fluid flow during metal solidifying is proposed from boundary layer and standard deviation obtained by measuring etched surface heights of the actual ingot and using statistical principles. Taking continuous casting bloom of GCr15 bearing steel as an example, it is indicated that the calculated velocity magnitudes under different sides and superheats can be explained by process features and, hence, solidification mechanism. The velocity magnitude and fluctuation are higher on the inner curve side and under low superheat. Meanwhile, it is found that the fluctuation extent of secondary arm spacing is more relevant with interdendritic fluid flow, although its magnitude is mainly determined by the cooling rate. Moreover, on the basis of the calculated velocity directions and magnitudes, there is a positive correlation between segregation area ratio and the effective ratio between interdendritic flow velocity and growth velocity especially in the equiaxed grain zone, which corresponds with classic macrosegregation formation theory. The above findings and comparison with other results demonstrate the validity of the new approach, which can obtain the magnitude and the direction of interdendritic fluid velocity for two or three-dimensional multiscale velocity distribution by tailoring measuring length and numbers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]