Materials Science Forum Vol. 762

Abstract: For the production and development of Advanced High-Strength Steels adequate understanding of the formation mechanisms of the metallic microstructure is crucial. The superior properties of these steels are based on a sometimes delicate balance between thermodynamic (in) stability and dynamic processes, in which thermodynamic driving force and interface kinetics determine the development of the microstructure of the steel. In order to achieve further development and optimisation of such steels, experimental and modelling studies should go beyond microstructural characterisation in terms of average properties only. In this paper some examples will be given in which full (3D-) microstructures are simulated on the basis of the evolution of diffusional transformations. Although nucleation is not understood to sufficient extent to be predicted quantitatively, growth can adequately be described as governed by short-range diffusion at the interface (the basis for the interface mobility) and, in case of a partitioning phase transformation, the long-range diffusion behaviour (most notably of carbon). Whereas in the literature often one of the two processes is assumed to be rate-determining (interface control or diffusion control), physical modelling taking both into account ("mixed-mode growth") has also been effectuated. The widely used technique of Phase Field modelling and an alternative mixed-mode approach based on Cellular Automata will be presented and compared in this paper. Whereas Phase Field modelling is applicable to a wider range of processes, the Cellular-Automata method is highly efficient and allows 3D-simulations of entire process cycles within very limited computation times. Examples of these modelling techniques applied to the development of microstructures in Dual-Phase and Quenching-&-Partitioning steels are given.

Abstract: Most of the commercial metallic materials undergo at least one hot deformation stage during fabrication. Hot deformation processing leads to the production of plates, strips, rods, pipes and other shapes at lower overall cost when compared to the cold deformation/annealing route. Comprehensive study of the metallurgical phenomena during hot deformation has enormous potential application in the control of industrial rolling processes. Understanding of the microstructural and mean flow stress evolution lead to sound steel developments and innovative rolling schedules. The models predict parameters such as grain size, fractional softening (static and dynamic) and strain induced precipitation which are useful to improve rolling schedules. Effects such as incomplete softening and strain accumulation can be easily detected as well as their consequences on the final grain size and mechanical properties. In this regard, special attention must be given to steels, the most important metallic material in terms of history, present and future. In this paper, three hot rolling routes will be analyzed in order to produce high strength linepipe steels. Examples were selected on how the use of modelling during development stage can help to meet mechanical properties, mainly toughness and drop weight tear test. Firstly, it is presented a brief overview on mathematical models applied to hot rolling. Thin slab casting/direct rolling, hot strip mill and plate mill are exemplified in the present work. The development of new steel grades can greatly accelerated with the aid of modelling, which is an useful, low-cost technique.

Abstract: A software code for simulating thermal stress in castings was developed using finite difference method (FDM). The one way coupled map model was adopted to treat the relationship between the solidification and the thermal stress in castings. A method of killing the mold element whose temperature begins to decrease was employed to deal with the mold/casting interaction. The thermal stress and deformation in a stress frame casting were experimentally studied and analyzed by this software code. The results simulated and the residual stress measured are in good agreement. This indicates that the software code is a useful tool for predicting thermal stress and deformation in castings. Therefore, it provides a scientific way to improve the casting dimensional accuracy and reduce the defects caused by thermal stress.

Abstract: Thermal stress simulation is an important part in the numerical simulation of casting process. It provides engineers with insights into the evolution of displacement, strain and stress of castings in the solidification process. With thermal stress simulation, some defects of casting, i.e. hot tearing, cold cracking and large deformation can be predicted and the engineers are instructed to optimize and improve the casting process. Based on the finite difference method (FDM), this paper presents an integrated numerical method to simulate the thermal stress and deformation of casting in the solidification process. Practical examples show that the method is capable to predict stress distribution and deformation as well as the defects in the experiment.

Abstract: Nowadays, most 3D casting process CAD systems are developed based on second development tools of various 3D CAD systems. However the type and version escalation of various 3D CAD systems in foundry enterprises greatly limit the application of 3D casting process CAD systems. By the data exchange between the neutral file format of 3D geometric model, these problems can be well solved. In this study, by taking advantage of universal CAD data-exchange standard iges for data exchange, a practical 3D CAD system was developed based on opencascade geometry kernel. The system has the function of design and modeling of practical casting process such as part information extraction, parting surface, gating system, riser system and so on. Finally taking a steel casting for example, the casting process is exported in the file of stl and the simulation analysis is carried by intecast software which demonstrate the system based on iges neutral file can well support the integration of casting CAD and CAE.

Abstract: This paper summarizes and discusses our recent work on modelling of several steelmaking processes. The work started by developing a detailed sub-model for a single gas bubble reacting in liquid steel. The key feature in this model was an approach based on LOMA, Law of Mass Action, which was employed for defining the chemical rate of a reaction in a robust way. The bubble reaction model was then coupled with a new simulator concept for the AOD process, Argon-Oxygen Decarburization. After a successful validation, the same approach was used to model chemical reactions and chemical heating of liquid steel in the CAS-OB process, Composition Adjustment by Sealed Argon Bubbling Oxygen Blowing, using a supersonic lance. Finally, a new model was developed and implemented into the existing AOD process model for slag reduction with slag droplets. The purpose of this paper is to present a generalised framework for applying and validating the LOMA approach into modelling of metallurgical unit operations. In addition, the use of Computational Fluid Dynamics (CFD) in the validation and verification work is discussed.

Abstract: Top slag emulsification is a significant phenomenon in refining metallurgy. During bottom-or side-blowing, the flowing steel detaches small droplets from the top slag. The interfacial energy between liquid slag and steel is one of the most important factors affecting to emulsification. Surface energy, which can be described by interfacial tension, is the dominant property when determining slag emulsification. During chemical reactions, mass transfer between the phases decreases the interfacial tension at the slag-steel interface. The change in the interfacial tension affects the droplet formation.In this paper, the effect of interfacial tension on the emulsification was studied with Computational Fluid Dynamics (CFD) modelling. Three cases were simulated by considering a 3-phase system consisting of slag, steel and gas. A small area, where a 15 mm slag layer lies on top of the liquid steel, was simulated applying three different interfacial tensions, while keeping other properties unaltered. Gas was included to enable a free slag top-surface. The droplet diameter, size distribution and amount of droplets are in the scope of interest. It was found that the Sauter mean diameter of the slag droplets increased as the interfacial tension increased. The emulsification fraction varied between 1.621.95%.

Abstract: In the production of steel, the CAS-OB process is used for composition adjustment, temperature control and removal of various dissolved impurities. In this work we have studied the CAS-OB process with CFD and focused on the behavior of the slag layer, which is produced on the top of molten steel. Dynamic mesh adaption has been applied to resolve the slag layer boundary in a detailed way. As a result the time dependent evolution of the slag layer is presented. The chosen approach to describe the process offers an effective and promising way to study this complex system. In the future this model will be validated against experimental data.

Abstract: The steelmaking field has been seeing an increased demand of reducing hydrogen and nitrogen in liquid steel before casting. This is often accomplished by vacuum treatment. This paper focuses on developing a numerical model to investigate the removal of hydrogen and nitrogen from the melt of medium carbon steel in a commercial vacuum tank degasser. An activity coefficient model and the eddy-cell expression are implemented in the ANSYS FLUENT code to compute the activities of related elements and mass transfer coefficients of hydrogen and nitrogen in liquid steel. Several cases are simulated to assess the effect of gas flow rate and initial nitrogen content in liquid steel on degassing process and the calculated results are compared with industrial measured data.

Abstract: Numerical and physical simulation on model samples can provide data for various aspects of metal forming, without resorting to time-consuming and costly full-scale tests. This paper presents examples of modeling of the deformation of a slab with a liquid core. The use of soft reduction can enhance the homogeneity of the structure, which improves the quality of cast billets. Mathematical modeling is described here where the fluid layer is taken into account by the influence of boundary conditions in the crust in the form of ferrostatic pressure, which allows calculation of the intensity of deformation, total deformation and strain. It also provides a novel method for studying the process of soft reduction. It is based on a physical model of the slab consisting of a closed solid shell made of a calibrated lead shot and the Wood's alloy. To simulate the liquid molten metal, the interior of the shell is filled with gelatin. This approach can be applied to further studies on deformation processes and the penetration of deformation into complex metallic systems.