The aim of this thesis work is to increase the knowledge of inclusion behavior at the steel-slag interface by mathematical modeling and in-situ Confocal Scanning Laser Microscope experiments. Mathematical models based on the equation of motion predicting liquid and solid inclusion behavior was first investigated. Four main forces, the buoyancy force, the added mass force, the rebound force and the drag force, act on the inclusion as it crosses the interface. There are three types of behavior an inclusion at the steel-slag interface can adopt. These are a) pass, which means that the inclusion is separated to the slag, b) remain, where the inclusion stays at the interface without being fully transferred to the slag or c) oscillate, and the inclusion rises and descends at the interface until the motion is dampened out by the interfacial forces. The studies showed the importance of accurate experimental physical property data. Application of the models to industrial conditions illustrated that useful plots could be made showing the industry how to optimize their interfacial properties in the ladle and tundish to obtain maximum inclusion separation. In-situ Confocal Scanning Laser Microscope (CSLM) experiments were carried out in order to study agglomeration of liquid and semi liquid inclusions at the steel-gas and steel-slag interfaces and in the slag. Liquid-liquid inclusion agglomeration at steel-gas and steel-slag interfaces was seen to not occur without using force. However, when already transferred to the slag the inclusions agglomerated freely due to a higher free energy force. Comparison of experimental and theoretical agglomeration force showed good agreement between experiments and theory. The main conclusion of this work is that inclusion separation is a complex field of study and there exist no model that takes everything into account. Here the tendency for inclusion transfer and how to manipulate the physical properties for inclusion separation together with agglomeration experiments have been studied. For the future maybe coupling of models for computational fluid dynamics, agglomeration, inclusion separation, dissolution and slag entrainment in addition with experimental physical property data can provide a better overview and understanding. / QC 20100823

The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties. The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys. Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness. Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting. The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting. The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties / QC 20100901

The present work was initiated to investigate the stability, structural and thermodynamic properties of transition metal carbides, nitrides and carbo-nitrides by atomistic simulations and experimentations. The interatomic pair potentials of Cr-Cr, Mn-Mn, Fe-Fe, C-C, Cr-C, Mn-C, Fe-C, Cr-Fe, Cr-N and Mn-N were inverted by the lattice inversion method and ab initio cohesive energies, and then employed to investigate the properties of Cr-, Mn- and Fe-carbides by atomistic simulations in this work. For the binary M7C3 carbide, the structural properties of M7C3 (M = Cr, Mn, Fe) were investigated by atomistic simulations. The results show that the stable structure for these compounds is hexagonal structure with P63mc space group. The cohesive energy of M7C3 calculated in this work indicates that the stability of carbides decreases with the increasing in metal atomic number. Further, the vibrational entropy of Cr7C3 was calculated at different temperatures and compared with the entropy obtained by experimentations. The comparison demonstrates that the main contribution to the entropy is made by the vibrational entropy. For the binary τ-carbides, the structural properties of Cr23C6 and Mn23C6, as well as the vibrational entropy of Cr23C6 were computed. Further, the site preference of ternary element Fe among 4a, 8c, 32f and 48h symmetry sites in Cr23-xFexC6 was studied. It has been seen that Fe atoms would firstly occupy 4a sites and then 8c sites. The lattice constant and stability of Cr23-xFexC6 were also computed with different Fe content. In order to understand the relative stability of the transition metal carbides and nitrides, the standard formation Gibbs energies of carbides and nitrides for Cr, Mn and Fe were compared. The order of carbon and nitrogen affinities for Cr, Mn and Fe was further clarified by the comparison of the interatomic pair potentials among Cr-C, Mn-C, Fe-C, Cr-N and Mn-N. It was found that Cr-N interaction was very strong in comparison with other binary interactions above and consequently, nitrogen addition would lead to a strong decrease in the thermodynamic activity of chromium in Cr-containing alloys. This was confirmed by the investigations of thermodynamic activities of Cr in the Fe-Cr-N and Fe-Cr-C-N alloys. The activities were measured in the temperature range 973-1173 K by solid-state galvanic cell method involving CaF2 solid electrolyte under the purified N2 gas. In addition, the analysis of nitrogen content and phase relationships in the Fe-Cr-N and Fe-Cr-C-N alloys equilibrated at 1173 K were carried out by inert-gas fusion thermal conductivity method, X-ray diffraction and scanning electron microscopy technique. The experimental results show that the solubility of nitrogen in the alloys decreases with the decreasing chromium content, as well as the increasing temperature. The addition of nitrogen to the alloys was found to have a strong negative impact on the Cr activity in Fe-Cr-N and Fe-Cr-C-N systems. / QC 20100929

QC 20100927

The key to the understanding of the mechanical behavior of dual phase (DP) steels is to a large extent to be found in the microstructure. The microstructure is in its turn a result of the chemical composition and the process parameters during its production. In this thesis the connection between microstructure and mechanical properties is studied, with focus on the microstructure development during annealing in a continuous annealing line. In-line trials as well as the lab simulations have been carried out in order to investigate the impact of alloying elements and process parameters on the microstructure. Further, a dislocation model has been developed in order to analyze the work hardening behavior of DP steels during plastic deformation. From the in-line trials it was concluded that there is an inheritance from the hot rolling process both on the microstructure and properties of the cold rolled and annealed product. Despite large cold rolling reductions, recrystallization and phase transformations, the final dual phase steel is still effected by process parameters far back in the production chain, such as the coiling temperature following the hot rolling. Lab simulations showed that the microstructure and consequently the mechanical properties are impacted not only by the chemical composition of the steel but also by a large number of process parameters such as soaking temperature, cooling rate prior to quenching, quench and temper annealing temperature. / QC 20101004

The present study was carried out to investigate the formation and evolution of non-metallic inclusions during ingot casting. Emphasize have been on understanding the types of inclusions formed and developed through the casting process and on the development of already existing inclusions carried over from the ladle during casting. Industrial experiments carried on at Uddeholm Tooling together with laboratory work and Computational Fluid Dynamics (CFD) simulations. Ingots of 5.8 tons have been sampled and the types of inclusions together with their distribution within the ingot have been characterized. Two new types of inclusions have been found. Type C1 is found originated from casting powder and in the size from a few μm to 30 μm. Type C2 is of macro inclusion type sizing up to 70 μm. The presence of C2 inclusions are few but very detrimental for the quality of the steel. Both types, C1 and C2 consist of alumina, indicating that reoxidation is the main reason for their existence. The protecting argon shroud has been studied by the use of a 1:1 scaled 2D model. Both flow pattern and oxygen measurement have been carried out. CFD has also been used as an auxiliary tool. It has been found that the oxygen pickup through argon gas shroud depends mostly on the distance between the ladle and the collar placed on top of runner. Further increase of gas flow rate above 2.5 m3.h-1 had very little effect on the oxygen distribution since both the flushing effect and the entraining effect with respect to oxygen are enhanced by further increase of inert gas flow rate. In the case of dual gas inlet, the flow in the shroud was found much less diffused compared with either vertical or horizontal injection system. The oxygen content in this arrangement was also greatly reduced. Studies of the runner after casting revealed a sparse non-metallic network structure around the periphery of the steel rod remained in the runner. The surface of the refractory had been severely attacked by the mechanical force from the streaming steel. The erosions of the centre stone and the end stone were on the other hand negligible. CFD calculations showed that the flow at those locations is almost stagnant. The surface of the refractory in contact with the steel was found to have an increased content of alumina. The source for the alumina could come from either exchange reaction of dissolved aluminium replaces the silica or reoxidation products origin from oxygen pick up during the transfer from the ladle to the vertical runner. Inclusions were also found entrapped in the steel refractory interface. It was also found that a formation of a liquid slag film as early as possible during casting would increase the possibility to remove inclusions and especially inclusions generated by the casting powder. / QC 20100617

<p>The thesis is based on two supplements with two major objectives. In the first supplement equilibrium top slag-steel bulk and inclusions-steel bulk were investigated by comparison between calculated and measured oxygen activity values. This was done by applying different oxide activity models for slags combined with thermodynamic calculations. In the second supplement the inclusion composition is studied during the ladle refining process. The inclusion composition is related to top slag composition and other parameters during ladle</p><p>treatment.</p><p>The work was carried out by collecting data during well controlled sampling procedures at two different steel plants. Extensive inclusion analyses in Scanning Electron Microscope, SEM, were done. The data was used together with thermodynamics for a description of the interaction between slag-steel-inclusion interaction during ladle treatment.</p><p>Evaluation of inclusion composition during the ladle refining have revealed that the majority of the inclusions belonged to the system Al₂O₃-CaO-MgO-SiO₂and showed a continuous composition change throughout the ladle refining process, from high Al₂O_3, via MgO-spinel to finally complex types rich in CaO and Al₂O_3. The final composition after vacuum treatment was found to be close to the top slag composition. Small process parameter changes and practical variations during ladle refining were proven to give large differences of the inclusion composition.</p><p>Finally, it was concluded that equilibrium does not exist between top slag and steel bulk, with respect to oxygen, for the studied conditions. However, the equilibrium does exist between the steel bulk and inclusion.</p>

<p>Continuous caster moulds are the last and most important stage in the steelmaking process, where inclusions can either be generated or removed. With increasing casting speed using conventional immersion nozzles critical problems, such as unstable bulk mould flow have been noticed. Mould flux entrapment due to vortex and shearing action from the oscillating surface waves have become of particular concern. It is therefore necessary to have a calm inlet flow at the entrance of the mould. Recently, it has been acknowledged that a swirl blade placed at the upstream of the immersion nozzle effectively resolves the problems arising from unstable bulk mould flow. Therefore, to increase the knowledge of effect of swirling flow on the flow pattern in the mould, fundamental mathematical models of a billet mould equipped with a swirl blade in the nozzle have been developed. The model was used to study the effect of divergent angle of the immersion nozzle and mould aspect ratio on the flow field and temperature distribution inside billets moulds. Data from water model experiments were used to verify the mathematical model predictions. A fairly good agreement was found between physical modeling data and predictions, which ensured that the numerical model is reliable. Thereafter, the differences between square and round billet moulds were studied. Next, the effect of changing aspect ratio of the rectangular mould on the fluid flow and heat transfer, while keeping mould surface area constant, was studied. Two types of immersion nozzles, bottomless and conventional, were also analyzed during the research. The model moulds were changed gradually from a square billet with an aspect ratio of 1x1 to a rectangular billet with an aspect ratio of 3x1. First, the temperature and velocity distributions were calculated. Later, unsteady calculations were done to determine velocity fluctuations on the meniscus level for two types of nozzles and several moulds geometries.</p>

A good slag practice is essential for production of a high-quality stainless steel. In addition, the electrical and material efficiency of the electric arc furnace (EAF) can considerably be improved by a good slag practice. The metallurgical properties of the slag are strongly influenced by its high-temperature microstructure. Thus, characterization of the phases within the EAF slag as well as the determination of the amount of these phases is of high importance.In addition, the knowledge about the chemical composition of the liquid slag and solid phases at the process temperatures is instrumental in developing a good slag practice.In order to study the slag in EAF high-chromium stainless steelmaking, slag samples were collected from 14 heats of AISI 304L steel (two samples per heat) and 7 heats of duplex steel (three samples per heat).The selected slag samples were petrographically studied both using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) and light optical microscopy (LOM). In some cases, X-ray diffraction (XRD) analyses were also performed. Moreover, computational thermodynamics was used to determine the equilibrium phases in the EAF steelmaking slags at the process temperatures. In addition, parameter studies were performed on the factors influencing the equilibria.More specifically, a petrographical and thermodynamic characterization was performed on the EAF austenitic steelmaking slags. Thereafter, the microstructural evolution of the slag during the EAF duplex steelmaking process was investigated. Moreover, an investigation with focus on the total amount of precipitates within the high-chromium stainless steelmaking slags was done. Finally, the foamability of these slags was quantified and evaluated.The petrographic investigations showed that, during the refining stage, in both austenitic and duplex cases, the main constituent of the EAF slag is a melt consisting of liquid oxides. In addition, the slag samples contain solid spinel particles. However, before ferrosilicon-addition (FeSi), the slag may also contain solid stoichiometric calcium chromite. Moreover, depending on the slag basicity, the slag may contain solid dicalcium silicate at the process temperatures.The evolution of the slag during the refining stage of the EAF was graphically illustrated in the calculated isothermal phase diagrams for the slag system Al2O3-Cr2O3-CaO-MgO-SiO2-TiO2.It was found that the only critical parameter affecting the amount of solid spinel particles in the slag is the chromium-oxide content. More specifically, it was shown that the amount of the spinel particles in the slag increases with an increased chromium-oxide content of the slag. It wasvialso shown that a higher basicity and a lower temperature of the slag contribute to the dicalcium silicate precipitation.In order to evaluate and quantify the foamability of the slags, the slag’s physical properties influencing its foaming index were determined. Computational thermodynamics was used as a tool to calculate the weight fractions of the solid phases within the slag at different EAF process stages. The computational thermophysics was used to estimate the viscosity of the liquid part of the slag samples at the process temperatures. The apparent viscosity of the samples was calculated by combining the above results. By estimating the density, surface tension and the foaming-gas bubble size, the foaming index of the slag samples were quantified. It could be shown that the foaming index of the EAF high-chromium stainless steelmaking slag may be on its minimum as the slag’s basicity takes a value in the range of 1.2 – 1.5. A basicity value of around 1.50 – 1.60 can be suitable for enhancing the foaming index of the slag, during the refining period in EAF high-chromium stainless steelmaking.

Compared to more common used austenitic stainless steels, ferritic stainless steels contain very low amounts of the expensive alloying element Ni. In addition, they have good corrosion properties, but are sometimes suffering from poor weldability and bad mechanical properties. This is mainly due to the presence of large grains after casting and large grain growth during heat treatment or welding. Processes for reducing the grain size (grain refining) of metal alloys are widely known and proven before to be suitable for many alloys. A successful grain refining process can increase the strength of an alloy without decreasing the ductility. This can be achieved by different methods, such as rolling or cooling. In this work, the focus has been on studying the aspect from a metallurgist point of view, to analyse the possibilities to create small particles in the liquid stage to enhance the solidification. The focus has been on oxide and nitride formation for nucleation of smaller grains during solidification. This study was made by forming particles, develop the analysis methods and thereafter to study the effect of particles on the structure of a ferritic stainless steel. The particles were formed by additions of Ti, Ce and Zr in to a liquid Fe-20mass% Cr alloy containing different amounts of oxygen and nitrogen. The electrolytic extraction technique was used to achieve a good understanding of the reaction processes and the particles size, number, composition and morphology. The grain sizes and the particles were then studied in as-cast samples as well as in specimens heat treated for 60 minutes at 1200, 1300 and 1400°C in a Confocal Scanning Laser Microscope (CSLM). Also, based on Scanning Electron Microscope (SEM) determinations and Thermo-Calc calculations, the precipitated particles were divided as primary and secondary particles. Thereafter, the grain refining potential was studied for each of these types. In this work, particles engineering by using small particles have been proven suitable for the Fe-20mass% Cr ferritic stainless steel alloys. Although the work has been based on small-scale samples and experiments, the results show good tendencies with respect to grain refining as well as a clear relationship between the particles in the steels and the microstructures. It was found that Ti-Ce additions in Fe-20mass% Cr alloys resulted in complex oxides including Ti, Ce and Cr. These oxides were observed to have high agglomeration tendencies and therefore to form larger particles or clusters. The grain refining potential on the solidification structure was insignificant, despite a relatively large amount of particles. However, Ti-Zr additions in Fe- 20mass% Cr alloys on the other hand resulted in primary precipitated particles such as ZrO2 and ZrO2+ZrN. Furthermore, ZrN nucleated ferrite during solidification, which lead to a clearly observed grain refining effect. This effect was also increased with an increased number of particles. The amount of particles (primary and secondary) was also found to increase with an increased amount of nitrogen. Secondary particles (mostly TiN) were precipitated near grain boundaries, which lead to a location based pinning effect of the grain growth during heat treatment at 1200 °C. This pinning effect was increased by an increased nitrogen content and thereby an increased number of particles. Finally, the pinning effect was clearly reduced during heat treatment at 1400 °C. / QC20100524