Advances in Science and Technology Vol. 70

Abstract: Slide gates are used to control the flow rate of molten steel in continuous casting. The properties that are important for materials for slide gate application are corrosion resistance, thermal shock resistance, abrasion resistance, and strength. However, operating conditions and in particular steel chemistries vary by shop and require development of different refractory material qualities. Alumina-carbon refractories are the prevalent family for slide gate application due to their superior thermo-mechanical properties. However, they suffer from extensive corrosion in aggressive applications. On the other hand, magnesia based refractories show promising corrosion resistance, but they are poor in terms of thermal shock and abrasion resistance. A novel approach using material based solution was employed to utilize a combination of properties of both refractory material families. This approach resulted in a material composite that jointly exhibits the thermomechanical properties and abrasion resistance of alumina based refractory and the corrosion resistance of magnesia based refractory. This paper will present the properties, key limiting factors, and results of usage at three steel plants.

Abstract: In this report, the trends in refractories technologies for iron and steel production at Nippon Steel Corporation in the last few decades are outlined. Amid dramatic changes in crude steel production accompanying increased ratios in higher quality of steel and sophisticated refining methods, in in our refractory ceramics R&D Div., actual machines have been realized by practicing developed outcomes with technologies for every production process relating to: refractory material quality, furnace building or execution, reparation, diagnosis, demolition, and recycling. Thus, continuing reduction in the unit consumption of refractories has been steadily performed. Additionally, technical issues are reduced to further enhance and maintain our international competitiveness in refractories technologies.

Abstract: Significant advances in the mechanical design of ceramic parts were realized since the pioneer works of Kingery and Hasselman to define thermal shock resistance. But for high heterogeneous refractories and contrasted local phase properties the use of these criteria is not always convincing because the assumptions made are too simplicist. First, we underline how thermal shock resistance parameters helped to improve the global performance of ceramics and make some comments on their limitations for refractory materials. Then we show how numerical tools are useful for the design of refractory structures at high temperatures through several approaches we have developed for refractory structures: prediction of macroscopic thermal shock resistance of heterogeneous refractories using multi-scale analysis, finite element methods applied to a specific structure considering either a two-scale approach to describe the thermo-elastic quasi brittle behavior of heterogeneous materials, at the macroscopic scale approach considering homogeneous microstructure.

Abstract: Simulations presented here aim to show reasons for undesirable product heterogeneity and possible defects. Potential negative influences are for example insufficient deaeration, friction between the bulk material and the mould, deformation of the mould and an inappropriate mould filling. In a first step data were collected from a production process. This includes the load and displacement during moulding and the mass of the mix filled into the mould. For the Finite Element modelling with ABAQUS the brick material was simulated with a Drucker-Prager model including a cap. The results show the influence of the friction between mix and mould on the densities and stresses in the pressed blank.

Abstract: Damage is a crucial characteristic of refractory castables and has to be considered to simulate correctly the behaviour of refractory structures. But, damage modelling by finite element simulations remains difficult. Indeed, the use of a continuum damage model with softening leads to strain localization phenomena. Numerical results depend on the mesh. Several numerical methods allow solving this meshing dependence by introducing an internal length in the material constitutive laws. In this paper, a regularization method has been applied with the damage plasticity model, considering a scalar value for damage. This model enables to take into account permanent strains due to plasticity and damage before and after the peak stress in tension and compression. Thermomechanical simulations are performed with this model to predict damage in a gas-burner. The damage level is evaluated after a thermal simulation generating high temperature gradients. Interests to take into account damage in the refractory structures are discussed. Sensitivity of results to material properties is studied. This work gives an example of using thermomechanical simulations to improve the design of refractory castable structures and to help in the material choice.

Abstract: Gasification is used by industry to convert low cost carbon into materials for producing electricity and chemical products such as fertilizer, plastics, and Fischer-Tropsch liquids. It is considered a critical technology in the success of the DOE’s Near Zero Emissions Advanced Fossil Fuel Power Plants, could play a key role in defining long-term energy security in both power and liquid fuels, and is considered a leading candidate for H2 production in a hydrogen based economy. Molten slag originating from mineral impurities in the carbon feedstock is of concern to slagging gasifier operation, with some gasifiers generating over 100 tons per day. Molten slag attacks and wears away the internal lining of the gasifier vessel by two major mechanisms, chemical dissolution and spalling. The main component in current refractory linings is chrome oxide. NETL is researching new types of refractory materials as an alternative to the high chrome oxide refractories currently used, with the goal of improving performance and providing a predictable service life. It has previously developed and patented the use of phosphate additives to improve the wear and spalling resistance of high chrome oxide refractories, and is exploring chrome and non-chrome oxide materials through laboratory testing and the use of thermodynamic modeling. In this paper, the results of a different additive (carbon) was evaluated by laboratory testing and found to improve slag penetration resistance. Data on this evaluation will be presented, and is proposed as a means of improving the molten slag wear resistance of the refractory. The use of it as a means of improving refractory service life in the field is patent pending.

Abstract: Criteria and tools are presented for the selection of appropriate refractory materials to be used in pulverised coal combustion reactors for the production of electrical energy. One of the main causes of failure of refractory materials is the high temperature corrosion due to chemical attack of the slag formed from the coal ash and possible additives. Selection criteria are based on the phenomena involved in the corrosion mechanism. The developed tools are based on thermodynamics of chemical reactions between oxides systems and permitted to evaluate the classes of materials in terms of chemical corrosion resistance as a function of slag composition and temperature. The effects of the chemical interaction between slag and refractory on the physical and chemical properties of the slag, such as viscosity, melting point, change of composition, solid phase formation, were also investigated. Experimental tests resulted in agreement with the indications obtained from the thermodynamic evaluation. Preliminary evaluation is an important tool for reducing the number of laboratory trials often expensive and time consuming. These chemical aspects are complementary to the engineering plant aspects and must be taken into consideration for a successful realization of the process.

Abstract: The present work aims the calculation of the mass transfer coefficient in the slag bath area of a gas-stirred steel ladle. The mass transfer in dependence on influencing factors like flow rate and velocity of purging gas was investigated. For this purpose computational fluid dynamics calculations were performed. Due to the fact that during refractory corrosion diffusion-controlled mechanisms take place simulations considering diffusion were performed. From the resultant concentration of the diffusive component in the slag in dependence on time the mass flux is known. With the help of Fick´s 1st law that describes the diffusive mass flux the mass transfer coefficient may be calculated. From a dimensional analysis followed by a parameter study conducted with the help of computational fluid dynamics calculations a relation for the mass transfer in the slag bath area of the gas-stirred steel ladle was deduced.

Abstract: Fluoride emissions during continuous casting operations could cause health problems and equipment corrosion. For these reasons, nowadays free fluor mould fluxes are developed, using other oxides in order to substitute fluoride compounds. Although physical properties of the flux must be garantizied to prevent operation problems and product defects, it is relevant to analyse the effect of the new fluxes on the nozzle wear mechanisms. In this paper, the influence of B2O3 and Li2O on the wear of the nozzle materials is determined. A post mortem study on a nozzle used for steel billet casting that it was in contact with fluorides fluxes were carried out to identify the real wear mechanisms acting during operation. To determine and compare the effect of B2O3 and Li2O on the wear, static corrosion tests were developed on the same type of nozzle material. A similar corrosion mechanisms were identified in both cases: refractory melt infiltration and grain dissolution, but less penetration depth was observed for the flux containing B2O3 and Li2O. The results were correlated with some physical properties of the fluxes such as viscosity and surface tension.