Water modelling of level fluctuation in thin slab continuous casting mould

Abstract

A water modelling experiment was conducted to study the meniscus instability in a continuous thin slab casting mould using particle image visualisation. The results show that the level fluctuation, circulation centre position and jet impinging depth are unsteady and periodic with a similar period. The probability distributions of the fluctuating meniscus and wave height have been obtained with the highest frequency near the average position. The flow pattern and meniscus profile may be momentarily asymmetrical, and the phase difference of level fluctuation in the two sides of mould centreline is a half period. The average meniscus profile, the highest and lowest meniscus positions are generally symmetrical about the mould centreline. The wave height mainly depends on the jet impinging depth and circulation centre position. The wave height increases as the jet impinging position rises and the circulation centre approaches to the submerged entry nozzle.     

Flow pattern and alloy dissolution during tapping of steel furnaces

Abstract

Alloying of steel during tapping from BOFs and EAFs has been studied by computational fluid dynamics in two-dimensional axisymmetric models of two ladles. The flow patterns and particle trajectories have been computed for six different levels of steel in the tapping ladle, five different alloy sizes, two alloy injection points, and three types of bulk alloy (FeMn, SiMn, and FeSi75). Based on the fluid dynamics in the ladle and a definition of good alloying practice, conclusions with regard to alloy sizing and timing of alloy addition have been established. The computational results support findings in plant tests, which show the benefit of using small sized alloys. Furthermore, a method that allows us to estimate the optimum feeding rate for alloy additions during steelplant operation has been developed. Results from full scale tests in steelplants are shown.     

Effect of exciting current of high frequency electromagnetic field on initial solidification of steel during electromagnetic continuous casting

Abstract

A three-dimensional finite element model of the electromagnetic field and temperature field of electromagnetic continuous casting (EMCC) process was developed. The aim was to investigate the effects of induction heat of high frequency electromagnetic field on the early solidification process of molten steel in mould under various conditions of exciting current parameters. The results show that the induction heat has significant effects on the early solidification process, which appear as increasing the billet surface temperature, thinning the initial solidified shell and lowering the starting point of the initial solidification. The increases in exciting current frequency and density make the effects of induction heat on solidification process increase remarkably. Especially, with the exciting current frequency increase, the early solidification process and shell growth become non-uniform in billet circumferential direction. Furthermore, if the exciting current density exceeds a certain value, there occurs a high temperature region in the top of molten steel column, and then the initial solidification rate is greatly decreased. As a conclusion, the effects of induction heat on initial solidification process must be considered when the exciting current frequency and density are adjusted during the electromagnetic continuous casting process.     

Simulation and application of dynamic heat transfer model for improvement of continuous casting process

Abstract

Numerical simulation is being increasingly used to improve the existing cooling systems. In order to attain highest quality strand, a two-dimensional dynamic mathematical heat transfer model of billet continuous casting of low carbon steel has been presented. This model can be used to compute the billet temperature distribution and shell thickness, especially it can be used to simulate the solidification process which is caused by frequently variational casting conditions. The fluctuation of measured temperature has been reduced to <10°C with thermal imaging system. The online model can monitor surface temperature and shell thickness in the casting process. So it provides the possibility for the online process control. For the validation of the dynamic model, a lot of billet surface temperature and shell thickness measurement were carried out on an actual casting machine. Finally, the dynamic model has been used for adjusting the operating parameters to improve the casting speed.     

Development of de-alloyed zones during oxidation: effects on microstructure and spallation behaviour

Abstract

The development of de-alloyed zones during oxidation of martensitic and austenitic steels and Ni based superalloys has been reviewed and the influence of de-alloying on local creep strength has been assessed. The de-alloyed zones in martensitic steels have similar, possibly higher, strength than the bulk material, whereas in Ni based superalloys the de-alloyed zone is significantly weaker than the bulk alloy. The effect in austenitic steels varies according to the strengthening phases present in the alloys: the de-alloyed zone is weaker in alloys strengthened by chromium carbides and/or γ′ but has similar strength in alloys strengthened by niobium carbide.     

Use of titanium and zirconium in centrifugally cast heat resistant steel

Abstract

Low carbon 25Cr–35Ni steel (HP type steel) modified with titanium and zirconium has been produced by centrifugal casting. The different phases present in the as cast and aged conditions were described by light optical and scanning electron microscopy with secondary electron imaging and energy dispersive spectroscopy. Results suggest that the use of titanium as a microalloying element reduces secondary precipitation during aging. Moreover, secondary precipitates in the microalloyed steel are much finer and more evenly distributed. On the other hand, zirconium oxides was found to be potential nucleation sites for primary titanium rich carbides contributing to an optimum distribution of these carbides in the tubes. These differences together with the higher stability of the titanium containing primary carbides are responsible for the improvement on ductility and creep resistance found in the present work.     

Effects of pH on formation and morphology of barium titanate powders prepared by coprecipitation process

Abstract

The effects of pH on the formation and microstructure of barium titanate (BaTiO₃) prepared by a coprecipitation process have been investigated. After calcination of the precipitates at 800°C, pure BaTiO₃ was formed in samples prepared from oxalic solutions with pH in the range 5-7, and the powders obtained exhibited spherical shape with a size range of 5-10 μm. In contrast, small particles (around 0·1 μm) with Ti enriched phases (BaTi₃O₇, BaTi₄O₉) were produced at pH 1, and BaCO₃ with irregular shape appeared at pH 10. The phases and morphologies of the calcined powders markedly influenced the sintering behaviour and dielectric properties of BaTiO₃. After sintering at 1350°C, densified ceramics with high dielectric constants were derived from the calcined powders containing pure BaTiO₃ phase. These results show that close control of the pH of the oxalic solutions is required to produce BaTiO₃ powders with high sinterability and high dielectric constant.     

Structural analysis considering shrinkage defect of cast part

Abstract

Flow and solidification of fusion metal at high temperature may introduce defects in cast components. As a results, many components have unsound internal areas. However, most engineers do not consider the effect of shrinkage defects when designing components; it is generally assumed that the material is completely sound. The material property reduction method is one approach to taking into account the effect of unsound areas, but it cannot consider stress concentration effects around the shrinkage. To compensate for this limitation, a shape simplification method is proposed. The method reconstructs shrinkage defects as hollow spheroid primitives based on shrinkage shape data obtained from industrial computerised tomography. The shape simplification method offers a smaller number of elements than other methods for modelling of shrinkages, and is also able to calculate the stress concentration. The present study examines the effect of shrinkage on a component structure subject to practical loads. It is possible to improve the productivity and reliability of cast products by such considerations.     

Truchas – a multi-physics tool for casting simulation

Abstract

The Truchas code was developed at Los Alamos National Laboratory under the Advanced Simulation and Computing Program. This open source multi-physics simulation software is designed to run in a scalable parallel computing environment. The capabilities of the code and numerical implementation are briefly described. The advantages and limitations of large three dimensional simulations will be discussed, and two example simulations are shown that demonstrate the utility of the fluid flow, heat transfer, phase change and solid mechanics capabilities. Validation of a code such as Truchas is a difficult task because of the complexity of the coupling between different physical phenomena being modelled and the poor understanding of phenomena such as heat transfer across interfaces. The challenges associated with verification and validation of complex simulation tools and integration into the design process are also discussed.     

Physical and mathematical modelling of argon shroud for protection of steel stream during casting

Abstract

Physical modelling of argon shroud was carried out using a two-dimensional 1 : 1 scaled model in order to gain a basic understanding of air entrainment during casting. The flow pattern of protective gas was determined with particle image velocimetry technique. Computational fluid dynamics calculations were also conducted. The results of the particle image velocimetry measurements and computational fluid dynamics modelling showed mutual agreement. The main findings of the present study are the following. First, the use of vertical gas inlet outside the collar on the top of the vertical runner should be avoided. Second, the distance between the ladle and the collar should be minimised to reduce the oxygen entrainment by convection and diffusion. Third, a modest flowrate of argon gas should be used. Further increase in the gas flow does not seem to improve the protection.