Thermodynamic study of non-metallic inclusions in al-killed calcium treated steel

O objetivo geral do trabalho foi o estudo da formação de inclusões não-metálicas no lingotamento contínuo (LC) para um aço acalmado ao alumínio e tratado com cálcio (SAE 8620). Os objetivos específicos foram: 1) A obtenção das fases e compostos formados nas inclusões em função da composição e temperatura de lingotamento adotadas para o aço SAE 8620. 2) Estabelecer condições de composição química do aço para a formação de inclusões menos deletérias à lingotabilidade do SAE 8620. Com base em trabalhos da literatura e em trabalhos anteriores dos autores, simulações foram realizadas via FactSage e seus bancos de dados. As simulações foram realizadas utilizando a composição química global do aço SAE 8620 no distribuidor de LC. O resultado é a composição do aço e inclusões não-metálicas (óxidos e sulfetos) na temperatura de interesse. Os resultados mostraram a formação de diferentes óxidos sólidos e a formação de fase líquida nas inclusões pela variação do teor de cálcio no aço. Foi possível determinar a composição das inclusões como função dos teores de alumínio e cálcio no aço SAE 8620. Para uma determinada composição e temperatura do aço SAE 8620, também foi possível estabelecer uma faixa de teor de cálcio onde as inclusões são formadas por fase líquida predominante. Foi ainda possível calcular o percentual de fase líquida e sólida nas inclusões, bem como sua composição em termos de óxidos.The main goal was the study of non-metallic inclusion formation in the continuous casting (CC) for Al-killed, calcium treated steel (SAE 8620). The specific goals were: 1) To obtain the phases and compounds formed in the inclusions as a function of composition and casting temperature adopted for the SAE 8620 steel. 2) To establish conditions of steel chemical composition for the formation of less harmful inclusions to the SAE 8620 castability. Based in the literature and previous work of the authors, were carried out simulations using the software FactSage and databases. The simulations were realized using the global chemical composition of SAE 8620 steel in the CC tundish. The results were steel and non-metallic inclusions (oxides and sulphites) composition in the aimed temperature. The results showed the formation of different solid oxides and the formation of liquid phase in the inclusions by the variation of calcium content in the steel. It was possible to determine the inclusion composition as a function of aluminum and calcium content of SAE 8620 steel. For the specific composition and temperature of SAE 8620 steel, it was possible to establish a range of calcium content were the inclusions are formed by predominant liquid phase. It was possible to calculate the percent of liquid and solid phase in the inclusions, as well the composition in terms of oxides

Fluorine-free mould slags for slab casting : utilisation of the hot thermocouple technique to build TTT diagrams

The elimination of fluorine from slags is necessary because F-bearing materials pollute the environment and cause machinery corrosion at steelworks. Regarding the development of F-free mould slags for slab casting, the main problem is how to control the heat transfer between steel shell and mould. In commercial products the heat transfer control is obtained through the crystallisation of cuspidine (3CaO.2SiO2 .CaF2 ) from mould slag. Thus, the fluorine-free slag which would be used as the substitute must show similar crystallisation behaviour. This paper shows results of a cooperation work between Technische Universität Bergakademie Freiberg and Universidade Federal do Rio Grande do Sul. Ti-bearing slags were studied through the Single Hot Thermocouple Technique, building time-temperature-transformation (TTT) diagrams. The results indicate that the system CaO-SiO2 -TiO2 -Na2 O can be used as substitute for F-bearing slags, regarding crystallisation behaviour, since the incubation times at TTT diagram can be similar to commercial products. This is possible because addition of Na2 O shorten intensely the incubation times from the CaO-SiO2 -TiO2 slag system

Evaluation of zinc removal and compressive strength of self-reducing pellets composed of Electric Arc Furnace Dust

The amount of Electric Arc Furnace Dust (EAFD) is continuously increasing in mini-mill steel plants. This dust is considered a hazardous waste because of the presence of elements like lead, cadmium and chromium. Among many different treatment processes for this issue, there is the possibility of returning the EAFD back to the Electric Arc Furnace. This article presents a study of the compressive strength of selfreducing cold bonded pellets as well as their zinc removal, in an agglomerate containing EAFD, petroleum coke (PET) and Portland cement. The effects of the reductant and binder employed were discussed. Moreover, an apparatus was built to prevent zinc gas reoxidation inside an electric vertical laboratory furnace. Thus, the fraction of weight loss complemented the result of a previous study indicating the optimal content of PET source usage between 10 to 15%, in mass. Zinc removal and additional X Ray Diffraction outcomes are shown and discussed, concluding that 80% of zinc removal for this system could indicate the possibility of the EAFD reuse

Study of morphology, size, distribution and precipitation on oxides of inclusions of manganese in resulphurised steels

O enxofre tem influência direta nas propriedades mecânicas nas diversas classes de aços, principalmente se formar FeS que reduz a resistência no trabalho a quente, devido ao seu baixo ponto fusão. A fim de que o teor de enxofre seja controlado na produção do aço, tem sido aplicada a adição de elementos de liga como o manganês. O manganês reduz consideravelmente a solubilidade do enxofre pela formação de sulfetos de manganês com a redução da temperatura. Estas inclusões controlam o crescimento do grão e facilitam a usinagem, aumentando a vida útil das ferramentas de corte. No entanto, prejudicam a qualidade dos aços e provocam problemas como redução na dutilidade e qualidade superficial de chapas laminadas a quente. Portanto, este trabalho fará uma abordagem sobre as influencias de processos, objetivando estudar sua morfologia, distribuição e tamanho médio, assim como, a precipitação dos sulfetos de manganês sobre óxidos; usando análise de imagens produzidas a partir de amostras retiradas de aço ressulfurado.Sulfur has a direct influence on the mechanical properties of the various grades of steel, especially if it forms FeS, which reduces the resistance to hot rolling due to its low melting point. In order that the sulfur content is controlled in the production of steel, the addition of alloying elements such as manganese has been applied. Manganese considerably reduces the solubility of sulfur by the formation of manganese sulphides with the reduction of temperature. These inclusions control grain growth and facilitate machining, increasing the life of cutting tools. However, they impair the quality of the steels and cause problems such as reduction in the ductility and surface quality of hot-rolled sheets. Therefore, this work will focus on the influences of processes, aiming to study their morphology, distribution and average size, as well as the precipitation of manganese sulphides on oxides; using image analysis produced from samples taken from resulphurised steel

Analysis and verification of process variables as causes for macroinclusions and scrapping in a special steel melt shop

The technology used in manufacturing steel for mechanical constructions has made great progress in recent years, resulting in a remarkable reduction of the impurities in special steel. One of these impurities is known as macroinclusion, the presence of which can cause serious defects in the steel structure. In a melt shop, where the semi-finished product is formed, there are several variables that can cause the formation of impurities in the steel and must be controlled. Therefore, by analyzing the influence of the primary melt shop variables, including: the iron and manganese oxides of the ladle slag (FeO+MnO), the stopper rod level variation and the argon pressure in the shroud between ladle and tundish in continuous casting; it was possible to establish a relationship between scrap generation and these process variables, through the creation of an investigation method. As result it was possible to identify the scrapping relationship with the presence of high FeO and MnO contents in the slag, during ladle furnace starting operations, along with stopper rod level variation in the tundish

Slag foaming fundamentals : a critical assessment

Slag foaming is part of steelmaking process and could bring several benefits: it helps to save energy, improves productivity, enhances the refractory service life, decreases noise pollution and protects the bath from nitrogen incorporation. Unfortunately, slag foaming is a highly dynamic process that is difficult to control. There are factors that limit the quality of the foam generated on the slag, such as: basicity, FeO content, surface tension, viscosity, carbon and oxygen injection. This paper aims to discuss the main factor that induces foaming, mathematical models proposed by different authors and the use of isothermal solubility diagram (ISD) to predict the foam quality

Estudo do comportamento de redução e fusão de misturas de carepa/coque de petróleo

Self-reducing tests were carried out under isothermal and non-isothermal condition in a muffle furnace, aiming to assess the reduction and melting of a self-reducing blend of mill scale and petroleum coke (85-15% in weight). The products obtained were analyzed by mass loss and wet analysis. Further investigations for the products from the non-isothermal condition were done by X-ray diffraction, nude eye inspection and carbon analyzer. It was observed that mass loss fraction and metallization degree are directly related and both increase with time and temperature. In the non-isothermal maximum mass loss was achieved in 8 minutes, reaching metallization degrees above 90%. It was observed that the reduction of iron oxide occurs mainly in solid state and the smelting of the samples is directly related to the iron carburization process. Thus, the use of self-reducing mixtures shows a possible way to recycle mill scale.Testes de autorredução em condições isotérmicas e não isotérmicas em forno mufla, foram propostos para avaliar a redução e fusão de uma mistura autorredutora de carepa e coque de petróleo (85-15% em massa). Os produtos obtidos foram analisados através de análises de perda de massa e via úmida. Para os produtos obtidos nas condições não isotérmicas também foram realizadas análises de difração de raios-X, inspeção a olho nu e análise de carbono. Observou-se que a fração reagida e o grau de metalização obtidos estão diretamente relacionados e ambos aumentam com o tempo e temperatura. Os ensaios não isotérmicos levaram 8 minutos para atingir a perda de massa máxima da mistura, alcançando graus de metalização superiores a 90%. Verificou-se que a redução dos óxidos de ferro ocorre principalmente no estado sólido e que a fusão das amostras está diretamente relacionada com o processo de carburação do ferro. Desta forma, o uso de misturas autorredutoras indica uma possibilidade de reciclagem de carepa

Self-reducing pellets composed of wastes from steelmaking production for their recycling

This paper presents a study about the recycling of a mixture of EAF filter dust and mill scale; and slag, in the form of self-reducing pellets. At first, chemical and morphological characteristics of the material were investigated. Pelletizing was carried out manually, with the use of graphite 100% fixed carbon and molasses as binder with moisture. The results of reduction tests under the temperature of 1200 ºC for the wastes could be analyzed according to the use of different contents of reducing agent, time and mass loss. It was found that 20% of the carbonaceous material was the best configuration for a maximum mass loss, for both wastes. Furthermore, the pellets composed of stainless steel slag, added to the metallic bath presented a chromium recovery of around 95%, indicating its potential reuse

Analysis of secondary refining slag parameters with focus on inclusion cleanliness

Secondary refining slag samples with different chemical compositions (45-54 % CaO, 7-13 % Al2O3, 20-29 % SiO2, 9-16 % MgO, 0-5 % CaF2) were investigated to verify the influence of their effective viscosity on inclusion cleanliness of DIN 38MnS6 in a steelmaking plant. The steel samples were collected during the production process for analysis of inclusions. Using the commercial software FactSage 6.4, thermodynamic calculations were performed to determine the effective viscosity, solid fraction, liquid fraction and MgO saturation point of these slags at 1560ºC. The results showed that all the slags were saturated in MgO, revealing a better protection of the ladle refractory. The addition of 2 to 5 % of CaF2 reduced the effective viscosity values for the analyzed slags from 0.45 Pa∙s to 0.10 Pa∙s , in comparison to the slags without the addition of CaF2, with an effective viscosity of 0.40 Pa∙s, 0.27 Pa∙s and 0.22 Pa∙s, decreasing the level of non-metallic inclusions for some of the analysed heats. However, it was detected during the manufacturing process that high slag fluidity and re-oxidation events continue to be a challenge associated with reducing the level of non-metallic inclusions