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The effect of the pouring rate on the carbon macrosegregation in 30Cr15Mo1N ingots has been clarified by investigating changes in the content of carbon‐containing precipitated phases, cooling rate, secondary dendrite arm spacing (SDAS) and the flow characteristics of molten steel. During the solidification process, the solute‐enriched liquid steel migrates toward the center of the ingot under the action of convection, resulting in an increasing segregation ratio from the edge to the center. However, the growth of equiaxed grains partially restricts the flow of solute‐enriched liquid steel, which leads to a maximum segregation ratio not at the center but at approximately a quarter radius from the center. Furthermore, due to the upward flow of molten steel in the center, positive segregation occurs at the top. An increment in the pouring rate reduces the SDAS by enhancing the cooling rate, and thus the permeability decreases. It aggravates the carbon macrosegregation horizontally. However, increasing the pouring rate decreases the carbon macrosegregation vertically. This is due to the more intense flow of molten steel and the larger liquid phase region at higher pouring rate, which reduces the solute enrichment degree and promotes compositional homogeneity above and below the ingot. [ABSTRACT FROM AUTHOR]

In this study, to clarify the evolution mechanism of dendrite structure and shrinkage porosity in 30Cr15Mo1N ingots under varying pouring rates, the changes in cooling rate, temperature gradient, secondary dendrite arm spacing (SDAS), columnar to equiaxed transition (CET), and area of shrinkage porosity with pouring rate are investigated. The cooling rate and temperature gradient calculated by using the ProCAST software increase with the pouring rate, leading to a great solidification rate. As a result, the SDAS decreases and the position of CET moves toward the center of 30Cr15Mo1N ingot. Additionally, the area proportion of shrinkage porosity increases with the increase in pouring rate. This phenomenon can be mainly due to the less overlap between columnar dendrite and higher permeability with the larger SDAS, which are conducive to the feeding of the molten steel. In addition, the delayed formation of semi‐solidified shell at the top of the ingot at low pouring rate minimizes the impact of solid contraction on the surface sink, which results in a flatter top surface of the ingot at low pouring rate. [ABSTRACT FROM AUTHOR]

The removal mechanism of vanadium addition on nitrogen pores in 30Cr15Mo1N ingot is systematically investigated through statistical analysis and theoretical calculation. The nitrogen solubility is obtained by the thermodynamic model, and the nitrogen content is obtained by combining the ProCAST software and the C–K model. Then, according to the formula of critical nucleation radius and growth rate of nitrogen bubbles, the variation of critical nucleation radius and growth rate of nitrogen bubbles with different vanadium contents is calculated. As the vanadium content increases, the number and mean area of nitrogen pores decrease. In comparison to the 0 V ingot, the number of nitrogen pores decreases in the 0.2 V ingot, but there is an increase in the number of large‐sized nitrogen pores (>3000 μm in diameter). When the vanadium content increases to 0.4, the nitrogen pores are completely removed. The results demonstrate that increasing vanadium content can remove nitrogen pores. Since the solidification mode remains unchanged with increasing vanadium content, increasing the vanadium content cannot remove nitrogen pores by changing the solidification mode of the 30Cr15Mo1N ingots. Therefore, increasing the vanadium content removes the nitrogen pores mainly by inhibiting the nucleation of nitrogen bubbles and promoting nitrogen bubble overflow in 30Cr15Mo1N ingots. [ABSTRACT FROM AUTHOR]

This paper presents a method based on Darcy's law to determine the pressure drop between the dendrite tip and root during the formation of porosity. After verification by the measurement result of the porosity region in 30Cr15Mo1N ingot, this method is employed to investigate the change in the pressure drop with solidification pressure, considering various factors such as the second dendrite arm spacing, solid fraction, cooling rate, temperature gradient, density, liquidus temperature, and solidus temperature. The relationship between the pressure drop and solidification pressure determines the formation of porosity. The results demonstrate that as the solidification pressure increases from 0.3 to 0.7 MPa, the pressure drop decreases from 1.572 to 0.676 MPa. When the solidification pressure is below 0.6 MPa, the pressure drop consistently exceeds the solidification pressure, leading to inadequate interdendritic feeding of the molten steel and subsequent porosity formation. Conversely, when the solidification pressure ranges between 0.6 and 0.7 MPa, the pressure drop undergoes minimal change and is approximately equal to the solidification pressure, which leads to sufficient interdendritic feeding. Consequently, increasing the solidification pressure effectively suppresses the formation of porosity by reducing the hindrance to molten steel feeding and enhancing its feeding capacity. Furthermore, a more accurate determination of the constant kC was obtained to calculate the pressure drop in 30Cr15Mo1N ingot, within the range of 2.77 to 3.11. [ABSTRACT FROM AUTHOR]

In the process of electroslag remelting (ESR) for large-sized slab ingots, controlling the surface quality of the slab ingot is challenging due to its relatively high width-to-thickness ratio. In this study, a three-dimensional dynamic mathematical model for single-electrode ESR slab ingots was developed using dynamic mesh technology, with the aid of the commercial software FLUENT. The model is based on the electromagnetic field equation, flow field equation, and energy equation. A detailed analysis of various physical fields and the distribution law of the metal pool shape was conducted. According to the calculation results, the maximum flow velocity of the molten slag was found below the consumable electrode, with the range of maximum velocity at different time points varying between 4.35 × 10−2 and 4.88 × 10−2 m/s. The range of the maximum temperature for the slag bath at different time points was between 2118 and 2122 K. As the remelting continued, the impact of the forced cooling of the bottom plate on the temperature of the metal pool weakened. Consequently, the temperature gradient of the electroslag ingot gradually decreased, the depth of the metal pool increased, and the height of the metal liquid head in the metal pool rose. The effects of different voltages, filling ratios, and mold chamfers on the shape of the metal pool were investigated using the established mathematical model. Based on the research findings from the mathematical model, the technical processes for ESR J80 large-sized slab ingots were improved, providing solutions to improve the surface quality of the ESR large-sized slab ingots. [ABSTRACT FROM AUTHOR]

Elimination mechanism of shrinkage porosity during pressurized solidification process of 19Cr14Mn4Mo1N high-nitrogen steel ingot was systematically investigated by experimental and calculational methods. According to the formation mechanism and morphological characterization, there are three types of shrinkage porosity in the 19Cr14Mn4Mo1N ingots, namely conventional shrinkage porosity, gas-shrinkage porosity, and gas-induced shrinkage porosity. It is shown that with increasing solidification pressure from 0.3 to 0.4 MPa, the dominant type of shrinkage porosity transfers from gas-shrinkage porosity to conventional shrinkage porosity, and the size of gas-shrinkage porosity under 0.3 MPa is larger than that of under 0.4 MPa. Under 0.6 MPa, the shrinkage porosity virtually disappears completely. The results demonstrate that increasing solidification pressure can eliminate shrinkage porosity. Since there are only a few variations in the cooling rate at the center of ingots with increasing solidification pressure from 0.3 to 0.6 MPa, increasing the solidification pressure cannot eliminate shrinkage porosity defects by changing the cooling rate of 19Cr14Mn4Mo1N ingot. Therefore, increasing the solidification pressure eliminates shrinkage porosity mainly by inhibiting the nucleation and growth of nitrogen pore and promoting the feeding flow. [ABSTRACT FROM AUTHOR]

Aiming at the challenge of reproducing the continuous casting (CC) steel in the laboratory, we developed a novel physical simulator to simulate the solidification process of CC slab. One characteristic physical simulation (PS) sample was obtained from the physical simulator, whose cooling intensity was controlled by cooling water to simulate the heat transfer of the CC slab. The simulation accuracy and reliability were verified by comparing the similarities of surface heat flux and solidification structure between the PS sample and the CC slab. The results demonstrate that the PS sample could accurately reproduce the heat transfer and solidification process of the CC slab. In the mold cooling stage, a similar air gap rapidly formed on both the PS sample and CC slab surface, leading to a similarly sharp reduction in the heat flux. In the stage corresponding to secondary cooling zone, the heat transfer on the PS sample surface via a thickening air gap was also basically similar to that on the CC slab surface through a vapor film, resulting in a similarly slow decrease in the heat flux. Additionally, the PS sample has similar solidification structure characteristics with the CC slab due to heat transfer similarity. Particularly, the crystal zone proportion, columnar to equiaxed transition position, and variation trends of dendrite arm spacing and Mo segregation in both were almost identical. [ABSTRACT FROM AUTHOR]

In this paper, the influence mechanism of pressure on nitrogen bubble formation during solidification process in 30Cr15Mo1N ingot was investigated by experimental and calculation methods. The effects of pressure on the critical nucleation radius and the growth rate of nitrogen bubbles were investigated. The growth of nitrogen bubbles was closely associated with the direction of solidification. In addition, nitrogen bubbles were generally egg-shaped and spherical in the columnar and equiaxed regions, respectively. With increasing pressure from 0.3 to 0.5 MPa, an increment existed in the critical nucleation radius and a decrement in the growth rate of nitrogen bubbles, mainly caused by the increment of the Gibbs free energy change of the system for nucleation and the internal pressure in the nitrogen bubbles, respectively. It led to decrements in number and mean area of nitrogen bubbles in 30Cr15Mo1N ingots with increasing pressure from 0.3 to 0.5 MPa. Based on the effect of pressure on formation of nitrogen bubbles, a special fitting formula was proposed to obtain the critical pressure for the 30Cr15Mo1N cylindrical ingot without nitrogen bubbles: P c = max 0.8867 [ pct\;N ] 0 + 0.098 , 0.053 e [ pct\; N ] 0 / 0.212 + 0.0415 , which is applicable under low pressure (≤ 2 MPa). [ABSTRACT FROM AUTHOR]

In this research, the effect of solidification pressure on phase transformation temperature and sequence, amount of proeutectoid ferrite, characteristics of pearlite and carbonitrides have been investigated by microstructure observation, crystal structure and component analysis, and thermodynamic calculation. The microstructure of 30Cr15Mo1N ingot is mainly composed of martensite, austenite, pearlite, ferrite, M23(C,N)6 and M2(C,N). With increasing solidification pressure, M23(C,N)6 precipitation temperature, austenite (γ) and ferrite (δ) formation temperature increase, and M2(C,N) precipitation temperature and ferrite (α) formation temperature decrease. Under 0.5 MPa, the precipitation sequence during solidification process of 30Cr15Mo1N ingot is "L → δ → γ → M2(C,N) → M23(C,N)6 → α + martensite/retained austenite → pearlite". There is a negligible change in phase transformation sequence with increasing solidification pressure from 0.5 to 2 MPa. In addition, the increment in solidification pressure is beneficial to significantly increase the amount of proeutectoid ferrite and reduce pearlite lamellar spacing by enhancing cooling of 30Cr15Mo1N ingot. Meanwhile, with increasing solidification pressure from 0.5 to 2 MPa, the size of M2(C,N) and the amount of precipitated phases (pearlite and carbonitrides) have a decreasing trend, and the change in the size of M23(C,N)6 can be neglected. [ABSTRACT FROM AUTHOR]

In order to clarify the effects of pressure on the second dendrite arm spacing (SDAS) and columnar to equiaxed transition (CET) of 30Cr15Mo1N ingot, changes in solidification parameters, cooling rate and columnar dendrite tip growth rate with pressure have been investigated. A formula is proposed to obtain the quantitative correlation between pressure (≤ 2 MPa) and interfacial heat transfer coefficient: hi=(28.01P2+299.28P+1129.10)t−0.24. And then, A formula is proposed to calculate SDAS with cooling rate: λ2=21.48×R−0.77, which is applicable for low pressure (≤2 MPa) and low cooling rate (0.4–1.4 K · s−1). There are tiny changes in solidification mode and equilibrium partition coefficient with increasing pressure from 0.5 to 2 MPa. Therefore, increasing pressure refines SDAS mainly by enlarging cooling rate. Pressurization can increase columnar dendrite tip growth rate by enhancing undercooling, and inhibit nucleation and growth of equiaxed dendrite by decreasing constitutional undercooling. It results that the distance between the center of ingot and CET position decreased with increasing pressure from 0.5 to 2 MPa. And a formula is proposed to calculate columnar dendrite tip growth rate with undercooling: V=1.33×10‐5ΔT2+2.71×10‐7ΔT3, which is suitable for low pressure (≤2 MPa). [ABSTRACT FROM AUTHOR]

In order to clarify the influence mechanism of high pressure on inclusion number distribution, the changes in phase transition sequences, key properties, and heat transfer boundary conditions with pressure have been investigated. Furthermore, the inclusion number distribution of H13 die steel ingot under different pressures (0.1, 1, and 2 MPa) has been numerically simulated by the mathematical model verified using the experimental results of cooling rate and the parameter ηn. The changes in key properties and phase transition sequences with pressure were investigated by Thermo-Calc software. With increasing pressure from 0.1 to 1000 MPa, the suppression of ferritic phase (δ) formation and improvement of liquidus/solidus temperature are obvious, but the density, thermal expansion coefficient, specific heat, and latent heat of solidification barely change. Meanwhile, the effect of pressure on heat transfer boundary conditions has been quantitatively revealed with formulas proposed by experimental measurement and numerical calculation: hf,0.1 = 1137.4t−0.23 (for 0.1 MPa), hf,1.0 = 1294.3t−0.23 (for 1 MPa), and hf,2.0 = 1501.6t−0.23 (for 2 MPa). During solidification process, gravity, buoyancy, and drag forces play the key roles in affecting the movement behavior of inclusions. Moreover, their net force drives inclusions to sink downward near the tip of columnar dendrite, and move counterclockwise. With increasing pressure from 0.1 to 2 MPa, the enrichment degrees of inclusions in ingot decrease, and the distribution of inclusion number becomes more uniform due to the stronger escaping ability of inclusions and the weaker inclusion trapping ability of the mushy zone. [ABSTRACT FROM AUTHOR]

The influence of solidification pressure on the thermodynamic and kinetic parameters of 19Cr14Mn4Mo0.9N high nitrogen steel is clarified by Thermo‐Calc/DICTRA software, including phase diagram, solidification mode, liquidus/solidus temperature, nitrogen solubility, partition coefficient, diffusion coefficient, driving force for nucleation, and diffusion coefficient. On this basis, the effect of pressure on critical nucleation radius and nucleation rate is also revealed. In the calculated vertical sections of 19Cr14Mn4Mo‐xN alloy phase diagram, δ single‐phase region decreases, both γ single‐phase region and three‐phase region L + δ + γ increase with increasing pressure. Additionally, the reaction mode of solidification transition gradually turns from peritectic to eutectic reaction, and the solidification mode changes from FA to A mode. Increasing the partial pressure of nitrogen can suppress the formation of ferrite trap and increase nitrogen solubility. Meanwhile, the partition coefficients of C, N, Cr, and Si increase, that of Mn and Mo decreases, resulting in decrease of C, N, and Cr microsegregation and improvement of Si, Mn, and Mo microsegregation. Pressurization has a small impact on diffusion coefficients, but it obviously promote the driving force for produced phase (δ and γ) nucleation, this lead to the decrease of critical nucleation radius, improvement of nucleation rate, and grain refinement of 19Cr14Mn4Mo0.9N high nitrogen steel. [ABSTRACT FROM AUTHOR]

The nitrogen gas pore-formation mechanism was discussed with regard to the solidification of 21.5Cr-1.5Ni duplex stainless steels (DSSs) by considering nitrogen segregation and solubility. The segregation behavior of nitrogen was investigated with phase transformation using experimental detection methods and Thermo-Calc software calculations. The process associated with the formation of gas pores was illustrated clearly. The factors that influenced the formation of gas pores, including shrinkage, nitrogen content, solidification pressure, and alloying elements (Mn and Cr), were discussed in detail. The formation of nitrogen-rich phases [austenite phase (FCC), AlN, and hexagonal close packed] is beneficial to eliminate nitrogen segregation and suppressing gas pore formation. The nitrogen-depleted phase (ferrite phase (BCC)) exhibits an opposite effect. Regular gas pores are initially formed in locations consisting of the austenite phase. As the gas pores lengthen, ferrite and austenite phases alternately form around the gas pores. Solidification shrinkage can promote the formation of irregular gas pores at the centerline of the ingots. Increasing the nitrogen content is favorable to the formation of gas pores. Increasing solidification pressure is effective with regard to suppressing the formation of gas pore defects in DSSs. Increasing the Mn content can reduce the likelihood of gas pore formation; this can be attributed to the increased nitrogen solubility in the residual liquid surrounding the dendrites and the formation tendency of the nitrogen-rich phase. Increasing the Cr content exhibits a dual effect on gas pore formation, which is caused by the increased nitrogen solubility and segregation in the residual liquid. [ABSTRACT FROM AUTHOR]

The cellular automaton-finite element (CAFE) model is used to simulate solidification structures of 19Cr14Mn0.9N high nitrogen steel under different solidification pressures. The effect of solidification pressure on model parameters is firstly investigated. After validation, this model is used to clarify the effect of solidification pressure on compactness degree with number of grains and primary dendrite arm spacing ( [ABSTRACT FROM AUTHOR]

The effect of nitrogen content on as-cast microstructure, particularly the precipitates (M 7 C 3 , M 2 (C, N), Fe 3 C) of 55Cr17Mo1VN plastic die steel was investigated. It's found that higher nitrogen content would enlarge the solidification region, and increase the area ratio of precipitates. The eutectic M 7 C 3 carbides and M 2 (C, N) carbonitride are mainly distributed in the interdendritic region. The eutectic M 7 C 3 carbides mainly possess coarse (M 7 C 3 -C) and lamellar (M 7 C 3 -L) morphologies. With increasing nitrogen content from 0.16% to 0.31%, the M 7 C 3 carbide transforms from M 7 C 3 -C to M 7 C 3 -L and promotes M 2 (C, N) carbonitride precipitates. Based on Bramfitt's two-dimensional lattice misfit theory, Al 2 O 3 inclusion is the very potent heterogeneous nucleation site for the formation of M 2 (C, N). In contrast, M 2 (C, N) could be a poor heterogeneous nucleation site for M 7 C 3. Eutectoid transformation γ-Fe → α-Fe + Fe 3 C occurred in 0.23% and 0.31% nitrogen content samples when solidifying, which caused the outer layer carbides in lamellar morphology. The excessive precipitates weaken the austenite stability and eventually promote martensite transformation. [Display omitted] • The formation behavior of eutectic carbides (M 7 C 3) and carbonitride (M 2 (C, N)) was investigated. • The M 7 C 3 carbides mainly possess coarse and lamellar morphologies. • With the increase of nitrogen content coarse M 7 C 3 transformed to lamellar M 7 C 3. • Al 2 O 3 inclusion is the very potent nucleation catalyst for the formation of M 2 (C, N), while M 2 (C, N) is a poor potent nucleation catalyst for M 7 C 3. • High nitrogen content promotes the Fe 3 C precipitates. [ABSTRACT FROM AUTHOR]

Shrinkage porosity in high-nitrogen stainless steel ingots can be eliminated through pressurized solidification. However, in practical industrial settings, achieving the necessary solidification pressure to inhibit shrinkage porosity is often time-consuming, labor-intensive, and challenging. In this study, the area ratio, formation height, type, and morphology of shrinkage porosity were investigated in the ingots with the different vanadium and nitrogen contents by experimental observations and statistical analysis. Then, the inhibition mechanism of the vanadium-nitrogen synergy on shrinkage porosity is revealed via an analysis that amalgamates the feeding resistance and critical feeding driving force. This indicated that the difficulty in generating shrinkage porosity intensified in the 0.2 V ingot compared with that in the 0 V ingot, and the primary type of shrinkage porosity changed from net shrinkage porosity to island shrinkage porosity and gas-shrinkage porosity. Gas-shrinkage porosity were more readily formed and relied on the presence of nitrogen pores. In the 0.4 V ingot, the shrinkage porosity virtually disappeared completely. This could be attributed to the suppression of the nucleation and growth of nitrogen pores with increasing vanadium content. Meanwhile, the dendritic growth restriction factor increased through the vanadium-nitrogen synergistic effect, inhibiting dendrite growth and resulting in a decrease in feeding resistance. This reduced the critical feeding driving force and promoted the feeding flow between the dendrites, ultimately inhibiting the formation of shrinkage porosity. This study revealed the mechanism by which the synergistic effect of vanadium and nitrogen inhibits the formation of shrinkage porosity during the solidification of 30Cr15Mo1N ingots, providing an efficient method for improving the solidification quality of high-nitrogen stainless steel, which is extremely helpful for its manufacture. [Display omitted] • Vanadium-nitrogen synergy changes the type of shrinkage porosity. • Quantification of feeding resistance and critical feeding driving force. • Game relationship between feeding resistance and driving force was elucidated. [ABSTRACT FROM AUTHOR]