Seznam abstraktů 2017: SEKCE A - POKROKOVÁ VÝROBA ŽELEZA, OCELI A LITINY
Datum: 24.5.2017
Vacuum metallurgy processes are now the primary processes of manufacturing new metallic materials. Requirements concerning product purity and accuracy in determining chemical composition constitute a huge challenge arising from the needs of new technologies. Examples include the required parameters of metal powders applied in 3D printing.
The subject of this paper is a kinetic model of the alloy steelmaking process, based upon the tank theory. Assumptions to this model enable the process to be simulated in real time. The method of determination of the equilibrium state at the metal-gas interface with the use of the Equilibr module from the FactSage® program is an essential component of the model. The computations were verified on the basis of laboratory heats made in a VIM furnace.
The open eye and tundish slag entrainment are originated with various factors such as ladle shroud misalignment, inappropriate impact pad design, and so on. Therefore, it is essential to control flow characteristics in order to secure quality of liquid steel in tundish. In this work, numerical analysis and water model test were conducted to investigate detrimental effect of the ladle shroud misalignment. Then evaluation on performance and optimization of impact pad design were carried out. For the optimization, flow characteristics of tundish were analyzed in case of baseline and new impact pad design with biased off-vertical angle of ladle shroud. In addition, the cleanness of final products was also investigated.
The reaction between Al in the liquid steel and SiO2 in lime-silica-based mould fluxes within the continuous casting process of high Al steel causes chemical compositional changes in the mould slag, subsequently affecting the surface quality of slabs. In order to solve the aforementioned problem, lime-alumina-based mould powder has been developed. In this new approach molten steel/mould slag interaction is inhibited and the surface quality of slabs is increased by the development of lime-alumina based mould powder. However, the mould slag tends to crystalline easily, which results in mould lubrication deterioration. In this study, the crystallinity of lime-alumina-based mould powder changes with the effects of increasing CaO/Al2O3 ratio have been observed through the XRF (X-ray fluorescence), thermal analysis, XRD (X-ray Diffraction), optical microscopy and SEM (Scanning Electron Microscopy). The results show that crystallinity of lime-alumina-based mould fluxes suppressed lower CaO/Al2O3 ratio.
Microsegregation in general is the consequence of the different solubility of alloying and residual elements in liquid and solid steel. The result is an enrichment of segregating elements between the dendrites due to incomplete diffusion equalization. This enrichment favours the precipitation of inclusions (MnS) during solidification as well as the formation of hot tear segregations (HTS) along primary grain boundaries. Hence, the modelling of microsegregation phenomena is an important task in order to predict the final product quality and to optimize the process parameters in continuous casting of steel.
A microsegregation model based on the analytical solution proposed by Ohnaka was implemented in an in-house 1D-FV solidification simulation software. Considering equilibrium partition coefficients from FactSage (FSStel2015) solidus temperatures of various steel grades were calculated. These results show very good correspondence with values determined from an in-situ hot tensile experiment.
Finally, the results were applied to calculate solidification in a continuous slab casting machine and to analyse the strain in the mushy zone subsequently. For selected steel grades the probability of HTS formation was predicted and the influence of operating parameters was quantified.
In the secondary cooling zone of the continuous casting process water/air nozzles are used to cool down the strand. The cooling has to be sufficient enough to ensure a certain shell thickness and finally the complete solidification of the material. Further a controlled and uniform cooling strategy is important to minimize the amount of defects in the solidified steel. For the determination of the water distribution and the cooling characteristic of water/air sprays at defined operation parameters the Nozzle Measuring Stand (NMS) at the Montanuniverstaet Leoben is used.
In this work the measuring principle of the NMS is explained and the influence of several nozzle operation parameters on water/air sprays, in particular on their cooling characteristic is shown. In a first step the water distribution at different nozzle distances, water flows and air pressures were determined. Afterwards the heat transfer coefficient (HTC) for every parameter modification was measured. The surface start temperature for all experiments was set at 950 °C. In order to show the change in cooling intensity over the width of the spray, HTC measurements were performed at several positions. Finally the results of the experiments and possible links between the operation parameters, water distribution and HTC are discussed.
Optimal regulation algorithms and complex numerical simulations are utilized in contemporary production processes. High quality of products, minimum of scrap, and reduction of carbon footprint are current trends in industry processes including the continuous casting process of steel. The paper shows a utilization of the 3D transient solidification model and an original fuzzy regulator for the improvement of the surface slab quality in the continuous casting of high-strength low-carbon steel. An optimal casting scenario was proposed based on the real casting measurement and statistical evaluation. The relationship between the temperature history of the slab surface and the number of surface defects was statistically evaluated. Based on statistical data, the fuzzy regulator and the solidification model were used to find an optimal cooling strategy as a function of the casting speed and of the casting temperature. Though results are demonstrated for a specific slab caster and for a specific grade of steel, the presented concept is general from its nature and it can be used for any caster or any grade of steel.
The paper deals with the study of key thermophysical properties - phase transition temperatures (liquidus, solidus and peritectic transformation) with use of DTA (Differential Thermal Analysis) and “direct” thermal analysis (TA). The study presents the basic principles of these methods, their characteristics, advantages, disadvantages and results obtained with these two very often used methods at heating and cooling process. There are presented results from the high temperature region (above) 1000 °C with focus on the melting and solidifying region of Fe-C-Cr based metallic alloys. The paper discusses obtained results with two mentioned methods at heating/cooling process, with different loads of analyzed samples and other factors that can influence the obtained results. The evaluation of heating/cooling curves and DTA–curves at heating and cooling process is demonstrated. The obtained experimental phase transition temperatures are mutually compared and discussed. Temperatures of liquidus, solidus and peritectic transformation are discussed with values theoretically calculated by selected polynomial equations, by SWs, such as Thermo-Calc, Compu-Therm and IDS (solidification analysis package). Differences between liquidus, solidus and peritectic transformation temperatures were encountered (in some cases substantial) between theoretical and experimental values.
Three alloys based on Fe-C-Cr were studied. These contained carbon in a range of 0,31 – 0,38 wt.% and chrome 1,06 – 4,99 wt.%. Temperatures of solidus (TS), liquidus (TL) and peritectic transformation (TP) were studied in high temperature region. These temperatures were obtained using two thermal analysis methods: "Direct" Thermal Analysis (TA) and Differential Thermal Analysis (DTA). The Setaram Setsys 18TM was used for experiments with use of DTA method. Measurements were done in inert atmosphere of pure argon by heating rate of 10°C/min. The Netzsch STA 449 F3 Jupiter was used for experiments with use of "direct" TA method. Measurements were done in inert atmosphere of pure argon by heating and cooling rate of 5°C/min. Phase transformation temperatures were obtained by heating and cooling process and were also approximated to “equilibrium conditions”. Experimental data were compared and discussed with calculation results using IDS (solidification analysis package) and SW Thermo Calc with use of the TCFE8 (Thermo-Calc Fe-based alloys) database. Temperatures of solidus (TS), liquidus (TL) and peritectic transformation (TP) were obtained and it is possible to make following conclusions in the case of liquidus temperature.
In this work we propose a novel way to model solidification during continuous processes using a Generalized Finite Difference Method (GFDM). The meshfree nature of this approach gives the advantage of naturally capture the motion and form of the phase boundaries without the need of adaptive remeshing algorithms. This approach is based on the works of S. Tiwari, J. Kuhnert, E.O. Resendiz-Flores and F.R. Saucedo-Zendejo, which provide a background to solve general elliptic equations in a meshfree framework and their application to solve multiphase and heat transfer problems. The features behind this approach, details of its implementation and numerical results of the simulation of some two-dimensional benchmarks problems are presented.
Authors developed new theoretical conceptions about impurities segregation mechanism at the time of alloys solidification on the basis of present microvolumes with different chemical composition at alloys solidification times. This conception permit to establish and optimize technologic parameters that influence to produce main kinds of macroheterogeneity (soundness). Derived model conceptions were confirm final condition of real continuous casting billets by using advanced analyzing methods.
The spray cooling is a common cooling method in the steel industry (continuous casting, hot rolling, product cooling or heat treatment). The water is sprayed on the hot surface by the nozzle which transforms the water stream into droplets. Sprays are able to provide very high cooling rates with the benefit of the regulation of the cooling process. The spray cooling is influenced by several parameters. They can be divided into two groups. The first group deals with spray parameters like water impact density, impact pressure, water temperature, water additives and droplets size. The second group is connected with cooled surface: temperature, velocity, roughness and presence of scales. This paper describes non-stationary spray cooling experimental measurement and deals with factors influencing the spray cooling. The influence of each factor is experimentally investigated and the influence on the heat transfer coefficient or Leidenfrost temperature is described based on experimental results and or existing publications.
Datum: 25.5.2017
The research purpose is to upgrade the mathematical modelling and computer simulation of casting of steel.
Based on theoretical analyse of physical processes which exist in casting systems the proper mathematical model is established and computer software is developed.
On the basis of control volume method, the algorithm for prediction of mechanical properties and microstructure distribution in steel casting has been developed. The computer simulation of casting of steel is consisted of two parts: numerical calculation of transient temperature field in process of solidification and cooling of casting to the final temperature, and of numerical calculation of mechanical properties. The hardness and microstructures of casting has been predicted based on CCT-diagrams.
Physical properties that were included in the model, such as heat conductivity coefficient, heat capacity and surface heat transfer coefficient were obtained by the inversion method.
The algorithm is completed to solve 3-D situation problems such as the casting of complex cylinders, cones, spheres, etc. The established model of steel casting can be successfully applied in the practice of casting.
From the perspective of increasingly stringent requirements for quality of ingots and cast steel, the knowledge of solidus temperature (TS) and especially liquidus temperature (TL) of produced steel grade is necessary. There are options to determine these temperatures: using empirical relationships or calculations from specialized programs that use thermodynamic databases. These calculations use the data from steel chemical composition and final temperature of phase transformation is then its reflection. TS and TL values then may not completely correspond to reality as well. For confrontation of calculated and experimental TL and TS, the use of methods of thermal analysis is very appropriate. At the Faculty of Metallurgy and Materials Engineering are 3 modern devices for high temperature thermal analysis. To determination of TL and TS of real steel grades, the parallel method of thermal analysis to currently mass-enhanced differential thermal analysis (DTA) was used. Analogous steel samples by direct thermal analysis method (PTA) were analysed. The simultaneous use of both methods allows disadvantages of each method reduce significantly and recommend TL and TS for practical use in casting process in conditions of industrial partners. The paper is focused on the discussion of TL and TS of steels cast into the mould (9 melts; 7 steel grades) analysed in the project TA0410035. The submitted evaluation shows an importance of the parallel utilization of methods, accuracy and reproducibility of the results and also the deviations of the experimentally determined TL and TS and empirically calculated values of the industrial partner and values obtained by thermodynamic calculations.
Experimental and theoretical investigation of temperatures of phase transformations (temperature of liquidus TL, temperature of peritectic transformation TP, temperature of solidus TS, temperature of the end of α->γ transformation Ac3, and temperature of the start of eutectoid transformation Ac1) of these alloys were measured. All investigated temperatures were also calculated by Thermo-Calc.
Comparison of measured and calculated temperatures of phase transformations was performed and good agreement was found. Discussion of identified differences was also done.
In MATERIAL AND METALURGICAL RESEARCH LtD. we made a proposal, producted and verificated of oxygen-nitrogen nozzle. This nozzle is part of a vacuum and pressurized induction melting furnace (VPIM) and it work at a pressure 10-20 kPa(a). Experimental melt of increasing the nitrogen content in molten steel by nitrogen gas will be carried out at low pressure. First, we will use the oxygen-nitrogen nozzles and than we will use a porous block located at the bottom of the casting ladle.