Carlos Antônio da Silva

Numerical simulation of solute trapping phenomena using phase-field solidification model for dilute binary alloys

Numerical simulation of solute trapping during solidification, using two phase-field model for dilute binary alloys developed by Kim et al. [Phys. Rev. E, 60, 7186 (1999)] and Ramirez et al. [Phys. Rev. E, 69, 05167 (2004)] is presented here. The simulations on dilute Cu-Ni alloy are in good agreement with one dimensional analytic solution of sharp interface model. Simulation conducted under small solidification velocity using solid-liquid interface thickness (2λ) of 8 nanometers reproduced the solute (Cu) equilibrium partition coefficient. The spurious numerical solute trapping in solid phase, due to the interface thickness was negligible. A parameter used in analytical solute trapping model was determined by isothermal phase-field simulation of Ni-Cu alloy. Its application to Si-As and Si-Bi alloys reproduced results that agree reasonably well with experimental data. A comparison between the three models of solute trapping (Aziz, Sobolev and Galenko [Phys. Rev. E, 76, 031606 (2007)]) was performed. It resulted in large differences in predicting the solidification velocity for partition-less solidification, indicating the necessity for new and more acute experimental data.

The effect of adding boron in solidification microstructure of dilute iron-carbon alloy as assessed by phase-field modeling

Alloying element like boron, even in small addition, is well known to improve hardenability of steels. Its application can improve mechanical properties of steels and reduce alloying costs. Despite these benefits is not easy to cast boron steels, mainly in dynamical solidification process like continuous casting, due to their crack susceptibility1,2. The strategy of using Phase-Field simulation of the solidification process is based on its proved capacity of predicting realistic microstructure that emerge during solidification under conditions even far from equilibrium3-5. Base on this, some comparative simulations were performed using a three component dilute alloy in a two dimensional domain under unconstrained (isothermal) and constrained (directional) solidification. Simulation results suggested two fragile mechanisms: one related to a deep dendritic primary arms space and other due to the remelting of this region at low temperature. Both resulted mainly from the high boron segregation in interdendritic regions.

Análise numérica bidimensional da morfologia dendrítica do níquel e do ferro utilizando o método de campo de fase.

Nesse trabalho, sao apresentados os resultados de simulacoes bidimensionais da formacao de dendritas durante a solidificacao do niquel e do ferro puros, utilizando um modelo de campo de fase. A morfologia obtida mostrou-se qualitativamente semelhante a forma dendritica, com bracos primarios e secundarios bem desenvolvidos. Alem disso, as velocidades de crescimento em regime permanente, calculadas no presente trabalho, apresentaram boa conformidade com os dados experimentais em uma grande faixa de super-resfriamentos. Foi observado que a espessura da interface apresenta um forte efeito sobre a morfologia e a velocidade de solidificacao. Interfaces espessas reduzem o tempo computacional; entretanto podem gerar resultados inconsistentes. A espessura que apresentou os melhores resultados sem aumento acentuado do tempo de processamento foi 4x10-8m.

Kinetics of Ferrous Iron Oxidation by Moderately Thermophilic Microorganisms

The biological oxidation of ferrous iron is an important sub-process in the bioleaching of metal sulphides and other bioprocesses such as the removal of H2S from gases, the desulphurization of coal and the treatment of acid mine drainage (AMD). Owing to the numerous potential applications of ferrous iron biooxidation, several works were carried out with mesophilic microorganisms, especially with Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans, and only few studies were performed with moderately thermophilic and thermophilic microorganisms. The latter two have potential to be applied to the leaching of refractory sulphides such as chalcopyrite and requires a smaller degree of reactor cooling during bioleaching. The ferrous iron oxidation kinetics in the presence of Sulfobacillus thermosulfidooxidans (DSMZ 9293) was studied. The experiments were carried out in batch mode (2L CSTR) and the effect of ferrous iron concentration (2-20g/L) on the ferrous iron consumption and bacterial growth rate was studied. The Monod equation was applied to describe the growth kinetics of this microorganism and values of μ max and KS around 0.33 h -1 and 1.18g/L, respectively, were achieved, similarly to those reported for mesophilic microorganisms.

LIME-BASED FLUX INJECTION FOR PIG IRON REFINING IN INDUSTRIAL LADLES

A software for lime-based flux injection for desulfurization of pig iron in ladle has been developed with the objective of predicting end sulfur content and temperature in one of the steel plants situated in the Minas Gerais State, Brazil, namely Belgo Mineira S.A. The software is also capable of assessing the influence of various process parameters such as flow rate, flux composition, depth of injection, amount of carry-over slag, initial temperatures of the charging materials, etc., on the process of refining. The mathematical model on which the computer code and the software are based, takes into consideration the contribution of the top slag, the injected particles, and the particles adhering to the bubbles, in the process of desulfurization. Transport phenomena parameters used in the model were chosen after an extensive literature survey. The thermodynamic parameters involved in the various exothermic and endothermic reactions have been considered in the thermal balance. A comparison between the results obtained from injection trials with 30-ton ladle showed that the predicted values using the software are reasonably in agreement with the actually observed values with respect to end sulfur content.

STUDY OF FLOW MODIFICATION AND INCLUSION REMOVAL IN SLAB TUNDISH DUE TO BOTTOM GAS INJECTION

Com o intuito de quantificar o papel da remocao de inclusoes, avalia-se o padrao de fluxo no interior do distribuidor. Modelos matematicos de separacao de inclusao, incluindo a influencia da coalescencia de inclusoes tambem sao discutidos. A presente analise sugere que no tocante a inclusoes de pequenas dimensoes, parâmetros operacionais como fracao de fluxo pistonado deixam de ser criticos, uma vez que a probabilidade de separacao e desprezivel. Entretanto, tecnicas como o uso de cortina de gas sao relevantes para melhor controle do conteudo de inclusao. Resultados de modelamento fisico e matematico sao apresentados considerando a associacao de uma barreira porosa e um plugue anelar de injecao posicionado ao redor do veio. Os resultados confirmam as expectativas ja que a modificacao de fluxo, exposta pela simulacao matematica, permite que inclusoes fiquem retidas no reator, registrando valores de saida abaixo de 3% no caso do veio com aspersao de gas.

Hybrid model associating thermodynamic calculations and artificial neural network in order to predict molten steel temperature evolution from blowing end of a BOF for secondary metallurgy

The developed model is an association of thermodynamic calculations for dissolution of alloys, slag formers and the deoxidation reaction in the molten steel with two artificial neural network (ANN) models trained with industrial data, to predict the molten steel temperature drop from the blowing end of the BOF until the first measurement at secondary metallurgy. To calculate the associated energy for deoxidation, an experiment was designed to set up the parameters for oxygen partitioning among deoxidants, with timed aluminum addition during teeming being the main parameter. The temperature control in the teeming stage presented a standard deviation for the error of prediction of 5.46 oC, for transportation from the rinsing station to the secondary metallurgy of 2.79 oC. The association of all calculations presented an error standard deviation of 7.49 oC. The operational validation presented superior accuracy compared with the current method for controlling the temperature, resulting in a reduction in the aluminum consumption for heating at secondary metallurgy with a potential economy of U

Replacement of fluorspar in the desulfurization of hot metal

4.07 million per year for a steel shop producing 5 million tons of steel yearly. The artificial neural network model confirmed its capacity for modeling a complex multivariable process and the separation of thermodynamic calculation provides a better adaptability to different steel grades with different teeming strategies.

Numerical Simulation of Recirculating Flow and Physical Model of Slag–Metal Behavior in an RH Reactor: Application to Desulfurization

In the steel industry, lime is used as a desulfurizing agent and fluorspar as flux in the hot metal desulfurization process. However there are environmental concerns regarding the use of fluorspar, even when its harmful effects are not fully known. In order to reduce such risks and also to anticipate possible future restrictions, which could undermine the sustainability of steel production, it is proposed to replace the fluorspar for alternative materials in the composition of the desulfurizing lime based mixture. For this purpose new mixtures using aluminum dross, ilmenite, sodalite and ulexite were tested, comparing them with a reference mixture containing fluorspar. The impact of these mixtures on the refractories commonly used in hot metal pretreatment was also checked, since this is an important cost parameter for the steel industries. The laboratory tests showed ulexite as a feasible substitute since the lime (84.03%) – ulexite (10.97%) – graphite (5%) mixture presented the highest desulfurization ratio and low refractory wear (less than the standard fluorspar mixture). The sodalite mixture was the worst by comparison. The ilmenite and aluminum dross mixtures showed similar behavior when compared to the fluorspar mixture.

A Study of Intermix in a Six-Strand Billet Caster

Computational fluid dynamics (CFD) techniques and a 1:7.5 physical model of an RH degasser have been used to evaluate the flow of gas and metal inside an RH reactor for vacuum degassing of liquid steel. The effect of gas injection on the gas spatial distribution, steel circulation rate and flow field inside the ladle, snorkels and vacuum chamber have been assessed. N-pentane oil was employed to evaluate the average residence time as well as the slag droplet size distribution. The predicted radial gas distribution and liquid circulation rate have been validated against experimental data from a physical model. The results with incorporation of the virtual mass force coefficient of 0.25 and the turbulence dispersion force showed better predictions of gas distribution in the up-snorkel as well as circulation rate. Full-scale simulations were performed, and the predicted circulation rate was significantly affected by the argon bubble expansion. Data from these simulations were used to analyze the degree of desulfurization performed by the addition of desulfurizing agents inside the vacuum chamber. A model of the kinetics of desulfurization based on the results from the physical model and CFD simulation and on slag dispersion inside liquid steel yields degrees of desulfurization similar to the industrial trials reported in the literature.