Metallurgical and Materials Transactions B最新文献

As a typical metallurgical defect, macrosegregation seriously affects the internal quality of the continuous casting billet, and it cannot be solved by processes such as high-temperature diffusion and rolling. For continuous casting billet, the solidification shrinkage and thermal shrinkage of the microstructure directly affect the macrosegregation defect. In order to reveal the effects of solidification shrinkage and thermal shrinkage on the melt flow, microstructure distribution, and solute segregation, a multiphase solidification model based on the Eulerian–Eulerian approach was established in this work. The growth behaviors of the columnar dendrite trunk and the columnar dendrite tip were fully considered, as well as the nucleation, growth, free migration of equiaxed grains, and the columnar-to-equiaxed transition (CET). Besides, the corresponding relationship between the secondary dendrite arm spacing (SDAS) and the cooling rate has also been taken into account in the model, which makes the net mass transport source term of the mass conservation equations more accurate. The calculation results show that when no any shrinkage behavior is considered in the model, the melt flow velocity in front of the solidification end will gradually decrease until it is the same as the casting speed, and the segregation index at the billet center will gradually increase until it reaches the maximum value at the solidification end. Both thermal shrinkage and solidification shrinkage can generate a negative pressure zone in the billet center, sucking the poor-solute melt located the upstream of continuous casting strand flows towards the solidification end, and mixing with the enriched-solute melt before the solidification end, thereby inhibiting macrosegregation. However, compared with the solidification shrinkage, the effect of thermal shrinkage on reducing the positive segregation index in the billet center is limited.

Abstract

In this work, the random pore model (RPM) is utilized for the kinetic study of hematite reduction to Iron with CO. This can significantly contribute to the more effective design of reduction reactors in Iron production plants. Indeed, the developed RPM in this work employs a real pore size distribution (PSD) of the solid reactant, resulting in more realistic and accurate kinetic parameters. Accordingly, the kinetic parameters were calculated via RPM using the data from the reduction experiments of a highly porous pure hematite pellet. Validation of such kinetic parameters by different pure hematite and industrial pellets with various porous structures demonstrated RPM as the most comprehensive non-catalytic gas–solid reactions model. The activation energy obtained for the mentioned reaction was calculated at 25.5 kJ/mol. In addition, oxygen ions showed a mean diffusion coefficient of 1.18 × 10⁻¹⁶ m²/s for the industrial pellets through the Iron product layer. Furthermore, the importance of adjusting the CO–CO₂ ratio on the conversion in the reduction reactor was discussed. The results of this work could help reduce the amount of required CO and CO₂ product during the reduction of hematite to Iron.

Graphical Abstract

For decades, the fixed-grid method (FGM) has undergone extensive development and widespread application in addressing phase change problems. Nonetheless, comparative studies on various FGMs in convective regime are considerably scarce. Moreover, it has been proven that two-dimensional (2D) numerical simulations can cause large deviations from experimental observations. Therefore, this study, based on a reference experiment involving gallium melting, seeks to comprehensively and quantitatively compare three prevalent FGMs: enthalpy method (EM), total enthalpy method (TEM), and heat source method (HSM). The TEM validates overestimation of temperature at low Péclet numbers, as the heat dissipation induced by non-uniform thermal properties in solid and liquid phases is not accounted for. To address this issue, a revised TEM has been introduced. The three FGMs were implemented within the OpenFOAM software, with over 150 simulations conducted on 3D meshes. The comparison focused on evaluating the numerical robustness, accuracy and stability of these FGMs, along with exploring their similarities and differences in flow patterns and velocities. Results obtained reveal that EM offers accuracy but lacks robustness, TEM manifests relatively large errors and instability due to oscillation with variations in grid size and time step, while HSM excels in robustness, accuracy, and stability. Under an identical discretization scheme, all FGMs predict similar melt front shapes, vortex structures, and velocity magnitudes. However, with the upwind scheme, the velocity magnitude of the secondary flow is approximately 50 pct of that with high-order schemes, yet it tends to overestimate the melting rate. The reason lies in the limited capacity of the slow secondary flow to effectively disrupt the stable and persistent vortex in the primary flow direction, consequently enhancing heat transfer efficiency in this direction.

The titanium distribution ratio (({L}_{Ti})) between ferrosilicon (FeSi) melt and the CaO-SiO₂-Al₂O₃ slag was measured at 1773 K (1500 °C). FeSi-slag equilibration was carried out using various slag compositions. The ({L}_{Ti}) was presented against the Vee ratio (pct CaO/pct SiO₂ = C/S) between 0.3 and 1.2, and Al₂O₃ content from 5 to 20 mass pct. The ({L}_{Ti}) exhibited minima at about C/S=0.7(±0.1) at a fixed Al₂O₃ content. In a C/S < 0.7 regime, i.e., relatively acidic melts, Ti⁴⁺ ions were considered as network forming [TiO₄]-tetrahedron unit in the aluminosilicate framework. However, in a C/S > 0.7 regime, i.e., relatively basic melts, Ti⁴⁺ ions were considered to form the [TiO₅]-pyramid structure unit compensated by Ca²⁺. When adding Al₂O₃ into the basic melts over 10 mass pct, [AlO₄]-tetrahedrons take Ca²⁺ ions for charge compensation, resulting in a decrease of stability of [TiO₅] unit because of Ca²⁺ depletion. At a greater than 10 mass pct Al₂O₃ content in the basic melts, Ti⁴⁺ replaced Al³⁺ to form the [TiO₄]-tetrahedron unit, decreasing the activity coefficient of TiO₂ in the slag.

In this paper, the hydrogen direct reduction of iron ore fines is numerically studied by using the Dense Discrete Phase Model (DDPM) in the fluidized bed. The fluidization behavior at different inlet gas velocities (U_g) as well as the influence of U_g and hydrogen concentration on reduction degree (RD) are comprehensively investigated. The result indicates the increase of time-averaged solids volume fraction for the same cross-sectional heights with increasing U_g when the bed height (H) exceeds 0.06 m. Furthermore, the reduction rate of mineral powder increases with higher U_g value, and the RD reaches almost 100 pct after 4000 seconds of reduction time with U_g ranging from 0.35 to 0.65 m/s. The reduction rate increases noticeably with the increase of hydrogen concentration in the range of 10 to 100 pct, and Fe₂O₃ can be completely converted to Fe under condition of 65 pct H₂ concentration after 4000 seconds. Moreover, higher H₂ concentration leads to faster rate of Fe₂O₃ consumption and Fe production. The mass fraction peak values of Fe₃O₄ and FeO are in the range of 0.29 to 0.34 and 0.21 to 0.24 under different H₂ concentrations, respectively.

Feeding strip significantly enhances continuous cast slab quality. To clarify its impact on inclusion distribution, a mathematical model coupling flow, solidification and inclusion motion have been developed. The upper recirculation, lower recirculation, and unformed recirculation flow occur during continuous casting. Under the resultant forces of drag, virtual mass, pressure gradient, Saffman, gravity, buoyancy, etc., the inclusion motion can be divided into two stages: Injection and Split flow. Feeding strip mainly affects inclusion motion by altering the drag, virtual mass, pressure gradient, and Saffman forces, which are closely related to the molten steel flow. After feeding strip, the lower recirculation on the strip feeding side is compressed, while it on the no-feeding side is expanded. The unformed recirculation flow on strip feeding side squeezes the flow below lower recirculation on no-feeding side. A higher strip feeding speed promotes downward inclusion motion, increasing the chance of being captured between the slab edge and strip. Unformed recirculation flow guides inclusions on the no-feeding side toward the slab edge, while expanded flow directs them toward the center. Consequently, inclusions on strip feeding side gradually gather between slab edge and quarter, while inclusions on no-feeding side first gather toward center and then toward edge of slab with increased strip feeding speed.

Five turbulence inhibitor (TI) designs are evaluated to define the highest performance to float non-metallic inclusions through the turbulence length scale analysis. The flow structures in the flow mushrooms, formed by the entry jet and its impact with a TI, generate coherent structures in the boundary layers’ walls of this device. The second invariant of the velocity gradient, Q, analyzes these structures. In the mushroom region, the inhibitor yielding the largest magnitudes of this second invariant has the most significant efficiency to float inclusions. Other criteria like the wall shear stress, the turbulent viscosity ratio, and the kinetic energy/friction velocity ratio are proved to be as valuable as the Q criterion to assess the performance of a given TI to float inclusions. This theory was tested numerically through the dynamics of amine particles in a tundish water model to simulate the dynamics of the non-metallic inclusions in steel and with amine powder injection experiments. The mass of powder escaping through the strand decreased as the absolute magnitudes of these criteria rose.