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Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice. Water cooling is a usual way to enhance production efficiency. However, the combination of the two factors will generate internal defects, such as shrinkage porosity and hot crack. The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test. A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted. The billets were examined by X-ray high energy industrial CT, and the compactness was reconstructed in three dimensions. Microstructure near the crack was observed using scanning electron microscopy, and the solidification process and grain size were studied by high temperature confocal microscopy. Moreover, thermomechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack. A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot. The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio, and watercooling further generates excessive tensile stress tearing the liquid films around the porosities. Then, hot cracks begin to propagate along grain boundaries. The grain size in the upper and center of the ingot is large, which leads to an inverted cone shape defects zone in the ingot center.

In this paper, the temperature field and heat transfer stress mathematical model of the unidirectional solidification ingot of the traditional water-cooled chassis and the improved water-cooled chassis are established, and the numerical simulation calculation of 45 tons unidirectional solidification ingot is carried out by using the finite element simulation analysis method. The results show that adding proper forced cooling conditions to the bottom of ingot is beneficial to the balanced advance of the solidification front of molten steel. The improvement of the bottom forced cooling scheme obviously shortens the solidification time of the ingot. The water cooling at the bottom of the ingot is adapted to the air gap distribution to achieve uniform air gap distribution. The ingot solidifies evenly in the longitudinal direction to avoid bottom warpage.

Shrinkage cavity has significant influence on macrosegregation in steel ingots. An arbitrary Lagrangian-Eulerian (ALE) model based on volume averaging method is developed to predict the coupled formation progress of macrosegregation and shrinkage cavity during solidification of steel ingots. The combined effect of thermal-solutal convection and solidification shrinkage on macrosegregation is considered in the model. A specially designed mesh update algorithm is proposed to consider the formation of shrinkage cavity. The streamline-upwind/Petrov–Galerkin (SUPG) stabilized finite element algorithm is adopted to solve the conservation equations. Two solution methods for the energy conservation equation are proposed, i.e. the temperature-based solver and enthalpy-based solver. A Pb-48wt.%Sn solidification benchmark is used for validation. Then, the ALE model is applied to a Fe-3.6wt.%C industrial steel ingot. The formation progress of macrosegregation coupled with shrinkage cavity is predicted. By comparison with the predictions of the finite element model and finite volume model, the effect of shrinkage cavity formation on macrosegregation is investigated. Results show that the formation of shrinkage cavity can significantly change the segregation region and segregation degree at the hot top. It is demonstrated that the ALE model can predict the coupled formation of macrosegregation and shrinkage cavity in steel ingots.

The paper deals with the inoculant–freigrator effect on some parameters of the structure: grain size and contamination index. The “beads” extracted from steelmaking slags were used as the inoculant. The effect of the fractional composition and the number of introduced “beads” were investigated. The “beads” were preliminarily crushed to the fraction of 100 – 1 500 μm and were introduced into the melt in the amount of 0,5 – 1,5 % by mass. It was established that the introduction of the “beads” of the fraction of 500 – 600 microns in the amount of 1 – 1,5 % as the inoculant contributed to the grinding of grain, reduced the tendency to segregation and dendre formation.

A new type of optimized riser structure was proposed, in which a low-emissivity board was inserted into the hollow interlayer of the material to increase the yield of the steel ingot. The insulation effect of different riser heights on the solidification process of steel ingot is numerical simulated. The results show that when the riser height can be reduced from 350 mm to 300 mm, the ingot yield can be increased by 1,92 %. As the number of low-emissivity boards is increases, the riser insulation performance is also increased.

Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice. Water cooling is a usual way to enhance production efficiency. However, the combination of the two factors will generate internal defects, such as shrinkage porosity and hot crack. The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test. A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted. The billets were examined by X-ray high energy industrial CT, and the compactness was reconstructed in three dimensions. Microstructure near the crack was observed using scanning electron microscopy, and the solidification process and grain size were studied by high temperature confocal microscopy. Moreover, thermomechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack. A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot. The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio, and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities. Then, hot cracks begin to propagate along grain boundaries. The grain size in the upper and center of the ingot is large, which leads to an inverted cone shape defects zone in the ingot center. [ABSTRACT FROM AUTHOR]

The effect of a hollow riser on the upper heat dissipation of 76 tons and 8 corners ingots is studied by numerical simulation in this paper. Through the simulation of the heat transfer of the traditional riser, the 1 gap riser and the 2 gaps riser ingot, this paper draws a conclusion that the solidification time of the riser part of the riser is the longest in the 2 gaps scheme, and the solidification time of the central molten steel is 31417s, which is far higher than the other two schemes. The heat flux in the riser of the 2 gaps scheme is obviously larger than the other two schemes, which slows down the advance of the solidification front. The 2 gaps scheme is stronger than the other two schemes for reducing the cooling speed of the riser line center and the shoulder joint. It is particularly significant to reduce the cooling rate of the shoulder joints, which is more conducive to expanding the angle of the solidification front and fully filling.

In this paper, the temperature field and heat transfer stress mathematical model of the unidirectional solidification ingot of the traditional water-cooled chassis and the improved water-cooled chassis are established, and the numerical simulation calculation of 45 tons unidirectional solidification ingot is carried out by using the finite element simulation analysis method. The results show that adding proper forced cooling conditions to the bottom of ingot is beneficial to the balanced advance of the solidification front of molten steel. The improvement of the bottom forced cooling scheme obviously shortens the solidification time of the ingot. The water cooling at the bottom of the ingot is adapted to the air gap distribution to achieve uniform air gap distribution. The ingot solidifies evenly in the longitudinal direction to avoid bottom warpage. [ABSTRACT FROM AUTHOR]

Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the internal quality, and a central shrinkage crack band of 1,400 mm in height and 120 mm in width, was found at a distance of 450 mm under the riser bottom line. Then, thermo-mechanical simulation using an elasto-viscoplastic finite-element model was conducted to analyze the stress-strain evolution during ingot solidification. A new criterion considering mush mechanical property in the brittle temperature range as well as shrinkage porosity was used to identify the shrinkage crack potential, where the degree of shrinkage porosity is regarded as a probability factor using a modified sigmoid function. Different casting processes, such as pouring speed, mould preheating and riser insulation, were optimized with the simulation model. The results show that fast pouring, proper mould preheating and good riser insulation can alleviate shrinkage crack potential in the ingot center.

The paper deals with the inoculant-freigrator effect on some parameters of the structure: grain size and contamination index. The "beads" extracted from steelmaking slags were used as the inoculant. The effect of the fractional composition and the number of introduced "beads" were investigated. The "beads" were preliminarily crushed to the fraction of 100 - 1 500 µm and were introduced into the melt in the amount of 0,5 - 1,5% by mass. It was established that the introduction of the "beads" of the fraction of 500 - 600 microns in the amount of 1 - 1,5% as the inoculant contributed to the grinding of grain, reduced the tendency to segregation and dendre formation. [ABSTRACT FROM AUTHOR]

A new type of optimized riser structure was proposed, in which a low-emissivity board was inserted into the hollow interlayer of the material to increase the yield of the steel ingot. The insulation effect of different riser heights on the solidification process of steel ingot is numerical simulated. The results show that when the riser height can be reduced from 350 mm to 300 mm, the ingot yield can be increased by 1,92%. As the number of low-emissivity boards is increases, the riser insulation performance is also increased. [ABSTRACT FROM AUTHOR]

A model for fluid and heat transfer of early stage of filling of a 19 ton ingot mould was built, and the flow and tempera-ture distribution characteristics during early stage of filling were investigated. Numerical simulations of mould filling with a series of en-trance nozzles with the different top diameters and bottom diameters were carried out, and the effects of entrance nozzle structure on mould powder entrapment during early stage of filling were investigated. The results show that level fluctuation in molten steel is strong and solidified shell grows fast so that mould powder can be entrapped easily. When top-end interior diameter is larger than bottom-end, the velocity of molten steel stream depends on the bottom-end interior diameter, and decreases with the increasing of op-end interior di-ameter. As for 19 ton steel ingot, the risk of mould powder entrapment can be greatly decreased when the bottom-end interior diameter is larger than 90 mm.

The electromagnetic feeding method was used to improve solidification quality of steel ingot, and the feeding principle of electromagnetic riser was introduced. The distribution characteristics of magnetic field in electromagnetic riser were investigated before and after pouring steel by the measures of numerical and physical simulation. The results showed that there’s a good symmetry of magnetic field distribution in electromagnetic feeding riser before pouring steel, but both symmetry and uniformity of magnetic field distribution are broken after pouring steel into riser, and magnetic induction intensity of the surface is higher than that of the inner of feeding riser.

P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack of feeding in the ingot center often generates many defects, such as porosity and crack. A six-ton P91 ingot was cast and sliced, and a representative part of the longitudinal section was inspected in more detail. The morphology of crack-like defects was examined by X-ray high energy industrial CT and reconstructed by 3D software. There are five main portions of defects larger than 200 mm3, four of which are interconnected. These initiated from continuous liquid film, and then were torn apart by excessive tensile stress within the brittle temperature range (BTR). The 3D FEM analysis of thermo-mechanical simulation was carried out to analyze the formation of porosity and internal crack defects. The results of shrinkage porosity and Niyama values revealed that the center of the ingot suffers from inadequate feeding. Several criteria based on thermal and mechanical models were used to evaluate the susceptibility of hot crack formation. The Clyne and Davies’ criterion and Katgerman’s criterion successfully predicted the high hot crack susceptibility in the ingot center. Six typical locations in the longitudinal section had been chosen for analysis of the stresses and strains evolution during the BTR. Locations in the defects region showed the highest tensile stresses and relative high strain values, while other locations showed either low tensile stresses or low strain values. In conclusion, hot crack develops only when stress and strain exceed a threshold value at the same time during the BTR.

The generic mathematical model and computational algorithm considering hydrodynamics, heat and mass transfer processes during casting and forming steel ingots and castings are offered. Usage domains for turbulent, convective and non-convective models are determined depending on ingot geometry and thermal overheating of the poured melt. The expert system is developed, enabling to choose a mathematical model depending on the physical statement of a problem.

Based on the mechanisms of macrosegregation evolution in large steel ingots, an approach named as gradual-cooling solidification (GCS) was proposed to alleviate the macrosegregation of large steel ingots. A three-phase mixed columnar dendritic-equiaxed solidification model was employed to investigate the approach. The solidification model considered the dendritic structure of equiaxed grains, nucleation and growth of equiaxed crystals, growth of columnar trunks, thermal-solutal buoyancy, sedimentation of equiaxed crystals, and columnar-to-equiaxed transition (CET). After the verification of model accuracy by a reported experiment results of a 55-ton steel ingot, this model has been used to study the GCS approach. The simulation results showed that the GCS approach has significant potential to alleviate the macrosegregation of the large steel ingot; e.g., for a 55-ton steel ingot, the variation range of segregation value decreased from 0.854 in the conventional casting case to 0.077 in the GCS case. [ABSTRACT FROM AUTHOR]

The present communication presents results of an investigation aimed at developing a ratio between the top mass and the body mass of the forging octagonal dead-melted steel ingot. Technological solutions for the application of various exothermic and thermal insulating materials in the exterior part of the ingot are advanced on the ground of an analysis of the shape and length variation of the pipe zone and shrinkage porosity in conformity with the numerical computer modeling of crystallization. [ABSTRACT FROM AUTHOR]

A mixed columnar dendritic-equiaxed three-phase solidification model that considers the secondary dendrite arm spacing (SDAS), nucleation and growth of equiaxed crystals, sedimentation of equiaxed crystals, growth of columnar dendritic trunks, and columnar-to-equiaxed transition (CET) was employed to study the formation of macrosegregation in steel ingots of different sizes. A reported experimental case (55,000 kg ingot) was used to verify the rationality of the current three-phase model. The predicted segregation patterns were in good agreement with the experimental results. The formation of macrosegregation in a series of steel ingots with different sizes (50 kg, 400 kg, 3300 kg, 6000 kg, 25,000 kg, 55,000 kg, and 97,000 kg) was studied using this solidification model. The results showed that the severity of macrosegregation increased with the increasing ingot weight. The thermal-solutal buoyancy and sedimentation of the dendritic equiaxed grains played dominated roles in determining the final segregation patterns in the different ingots. Additionally, the maximum negative segregation increased until the ingot weight exceeded 25,000 kg, which was caused by limited packing of the dendritic equiaxed grains. [ABSTRACT FROM AUTHOR]

The effect of a hollow riser on the upper heat dissipation of 76 tons and 8 corners ingots is studied by numerical simulation in this paper. Through the simulation of the heat transfer of the traditional riser, the 1 gap riser and the 2 gaps riser ingot, this paper draws a conclusion that the solidification time of the riser part of the riser is the longest in the 2 gaps scheme, and the solidification time of the central molten steel is 31417s, which is far higher than the other two schemes. The heat flux in the riser of the 2 gaps scheme is obviously larger than the other two schemes, which slows down the advance of the solidification front. The 2 gaps scheme is stronger than the other two schemes for reducing the cooling speed of the riser line center and the shoulder joint. It is particularly significant to reduce the cooling rate of the shoulder joints, which is more conducive to expanding the angle of the solidification front and fully filling. [ABSTRACT FROM AUTHOR]

The electromagnetic feeding method was used to improve solidification quality of steel ingot, and the feeding principle of electromagnetic riser was introduced. The distribution characteristics of magnetic field in electromagnetic riser were investigated before and after pouring steel by the measures of numerical and physical simulation. The results showed that there's a good symmetry of magnetic field distribution in electromagnetic feeding riser before pouring steel, but both symmetry and uniformity of magnetic field distribution are broken after pouring steel into riser, and magnetic induction intensity of the surface is higher than that of the inner of feeding riser. [ABSTRACT FROM AUTHOR]

In this paper, a complete three-dimensional numerical model of mold filling and solidification of steel ingots is presented. The risk of powder entrapment and defects formation during filling is analyzed in detail, demonstrating the importance of using a comprehensive geometry, with trumpet and runner, compared to conventional simplified models. By using a case study, it was shown that the simplified model significantly underestimates the defects sources, reducing the utility of simulations in supporting mold and process design. An experimental test was also performed on an instrumented mold and the measurements were compared to the calculation results. The good agreement between calculation and trial allowed validating the simulation. [ABSTRACT FROM AUTHOR]

Three-dimensional mathematical model of the solidification of low-carbon steel ingot in a chilled mould has been formulated and invest-igated in the paper. The model is based on theoretical knowledge and it is supplemented with experience data. In the mathematical model the thermo physical data are incorporated which are temperature dependent, what gives nonlinearity to the model. The model is solved by means of the explicit finite difference method - the method of enthalpy. The solidification algorithm has been solved with computer SPERRY 1106 in program language ASCII FORTRAN. The time of solidification, the optimum stripping time, the thickness of the solid skin in any direction, and the potentional prints of possible defects in ingot may be obtained on the basis of the model.

Part 1 of this two-part investigation presented a multiphase solidification model incorporating the finite diffusion kinetics and ternary phase diagram with the macroscopic transport phenomena (Wu et al., 2013). In Part 2, the importance of proper treatment of the finite diffusion kinetics in the calculation of macrosegregation is addressed. Calculations for a two-dimensional (2D) square casting (50 x 50 mm²) of Fe-0.45 wt.%C-1.06 wt.%Mn considering thermo-solutal convection and crystal sedimentation are performed. The modeling result indicates that the infinite liquid mixing kinetics as assumed by classical models (e.g., the Gulliver-Scheil or lever rule), which cannot properly consider the solute enrichment of the interdendritic or inter-granular melt at the early stage of solidification, might lead to an erroneous estimation of the macrosegregation. To confirm this statement, further theoretical and experimental evaluations are desired. The pattern and intensity of the flow and crystal sedimentation are dependent on the crystal morphologies (columnar or equiaxed); hence, the potential error of the calculated macrosegregation caused by the assumed growth kinetics depends on the crystal morphology. Finally, an illustrative simulation of an engineering 2.45-ton steel ingot is performed, and the results are compared with experimental results. This example demonstrates the model applicability for engineering castings regarding both the calculation efficiency and functionality. [ABSTRACT FROM AUTHOR]

We analyze theoretically and experimentally solute redistribution in the dendritic solidification process and positive segregation during solidification of steel ingots. Positive segregation is mainly caused by liquid flow in the mushy zone. Changes in the liquid steel velocity are caused by the temperature gradient and by the increase in the solid fraction during solidification. The effects of buoyancy and of the change in the solid fraction on segregation intensity are analyzed. The relationships between the density change, liquid fraction and the steel composition are considered. Such elements as W, Ni, Mo and Cr decrease the effect of the density variations, i.e. they show smaller tendency to segregate. Based on the modeling and experimental results, coefficients are provided controlling the effects of chemical composition, secondary dendrite arm spacing and the solid fraction. [ABSTRACT FROM AUTHOR]

We analyze theoretically and experimentally solute redistribution in the dendritic solidification process and positive segregation during solidification of steel ingots. Positive segregation is mainly caused by liquid flow in the mushy zone. Changes in the liquid steel velocity are caused by the temperature gradient and by the increase in the solid fraction during solidification. The effects of buoyancy and of the change in the solid fraction on segregation intensity are analyzed. The relationships between the density change, liquid fraction and the steel composition are considered. Such elements as W, Ni, Mo and Cr decrease the effect of the density variations, i.e. they show smaller tendency to segregate. Based on the modeling and experimental results, coefficients are provided controlling the effects of chemical composition, secondary dendrite arm spacing and the solid fraction.