The cleanliness of steel impacts the success of steel production and the physical and chemical properties of the final product. Improving the cleanliness of steel, therefore, becomes a necessity in the present time, with an ever-increasing demand for high-quality steel products. The cleanliness can be improved by removing the harmful inclusions through flotation or by modifying their composition and morphology to less detrimental forms. The present study focuses on better understanding the second approach, a specific modification method commonly known as the calcium (Ca) treatment in advanced high strength steel (AHSS) production. The chemical and morphological evolutions of Al2O3 inclusions under experimental and industrial conditions, as well as the formation of CaS and MnS inclusions, were studied in this work. Six laboratory experiments with different combinations of calcium and sulfur contents of liquid steel were conducted. Samples were taken at different time durations after calcium addition. The inclusions on the sample cross-sections were analyzed using an automated SEM-EDS system to obtain their chemical, size distribution, population, and morphological information. Similar steps were taken in the analysis of industrial samples. The findings obtained based on the automated SEM-EDS analyses were further supported and validated against other analysis results such as manual SEM analysis, thermodynamics, and kinetics calculations. The modification mechanism for Al2O3 inclusions was established in the first part of the study. After adding 10 ppm, 20 ppm, and 35 ppm Ca, small-sized calcium aluminates CAx (C and A denote CaO and Al2O3, respectively) inclusions become the primary oxygen bearer instead of Al2O3 inclusions. The modification extent of the CAx inclusions depends on the Ca content. CaS inclusions also form at the early stage of calcium treatment. In the later stage, CaS inclusions act as the Ca source to modify the remaining Al2O3 inclusions to CAx inclusions and simultaneously modify the existing CAx inclusions until equilibrium is reached. CaO inclusions only form in steel containing 20 ppm S and 35 ppm Ca; the primary oxygen bearer will change from Al2O3 to CaO, followed by a transformation from CaO to CaS. In other S and Ca contents, CaO inclusions do not form. This finding clarified that modification of Al2O3 inclusions is mainly driven by dissolved Ca and CaS inclusions, with CaO showing a minor direct impact. Moreover, the experimental studies showed that total area fractions of liquid and semi-liquid inclusions, which are inclusions that are partially liquid and partially solid, are correlated with the thermodynamic stability of CaS inclusions. The fraction of liquid inclusions decreases after the area fraction of CaS inclusions drastically increases when steel chemistries allow stable CaS inclusions to precipitate. The correlation between the steel chemistries and inclusions was improved by iv incorporating more data from industrial heats. Three modification indexes were proposed to estimate the control of CAx, CaS, and MnS inclusions. The fraction of CAx inclusions with more than 50 pct liquid, and the area fraction of CaS and MnS inclusions in tundish samples were correlated with the Ca, Al, Mn, and S contents of liquid steel. Later, these modification indexes were incorporated to evaluate the effectiveness of calcium treatment quantitatively. This makes the present study the first to discuss the correlations between Ca, Al, Mn, and S contents and the number of inclusions in the open literature. The correlations were validated against industrial data, they may be used in industry to determine the optimum Ca content for inclusion control and modification. Based on the experimental and industrial data, the coarsening of CaS inclusions was initially governed by mass transport, then shifted to collision-related mechanisms. When agitation is absent, Brownian motion shows the most significant impact on the growth of CaS inclusions, while turbulent flow is the critical cause of collision and coagulation when the melt is stirred, such as in industrial conditions. It has been found that CAx inclusion growth mainly occurs in the early stage after Ca addition. The potential reason is that the lack of attraction prevents coagulation after CAx inclusions are modified to liquid and semi-liquid. / Thesis / Doctor of Philosophy (PhD)

Abstract Non-metallic inclusions are unavoidable particles in steel and are often detrimental to the steelmaking process and the mechanical properties of the steel. While it is not feasible to remove all inclusions, in the well-established calcium treatment of aluminium-killed steel, solid aluminium oxides are transformed into less harmful liquid calcium aluminates. The main objective of this work was to develop a new inclusion identification and classification method. The presented method offers valuable information on phases of the inclusions. This data is often buried within simple inclusion classification criteria. The method offers the best approximation of the phases in each inclusion detected with minimal time, if no time-consuming elemental map analyses are available. In this work, applications for the inclusion phase identification method are presented. Utilising the method, the dispersion and evolution of inclusions during the sampling of liquid steel in lollipop samples is investigated, as well as the evolution of inclusions during continuous casting. New information is obtained on the elongation of inclusions and formation of oxide–sulphide stringers during hot rolling. The results for the investigated steels show that with sulphur contents higher than 10 ppm, calcium aluminates were back-modified to alumina and spinel inclusions during casting. However, with decreasing sulphur contents, and adequate simultaneous calcium treatment of oxides, Al₂O₃ and spinel formation is hindered or even avoided. The most elongated inclusions are often also found in stringers. According to the results of this study, low melting calcium aluminate inclusions are not the most elongated oxide inclusions in the hot rolled product. With moderate calcium treatment, an optimal oxide composition can be obtained, found within the C12A7–CA–MgO composition. These liquid inclusions ensure good castability, while they do not easily elongate or fragment into detrimental stringers after continuous casting. / Tiivistelmä Epämetalliset sulkeumat ovat väistämättä osa terästä, ja ne aiheuttavat ongelmia prosessille ja teräksen mekaanisille ominaisuuksille. Sulkeumien poiston sijaan niiden koostumusta muokataan kalsiumkäsittelyllä. Tällöin kiinteät alumiinioksidit muuttuvat vähemmän ongelmallisiksi suliksi kalsiumaluminaateiksi. Tämän työn päätavoitteena oli kehittää uusi sulkeumien tunnistus- ja luokittelumenetelmä. Sulkeuma-analyysien data voi jäädä hyödyntämättä, jos luokitteluun käytetään yksinkertaisia kriteerejä. Työssä esitetty menetelmä antaa parhaan arvion sulkeumissa olevista faaseista, mikäli sulkeumista ei ole alkuainekarttoja, joiden muodostaminen vie huomattavasti aikaa. Väitöskirjassa esitetään sulkeumien tunnistus- ja luokittelumenetelmän soveltamiskohteita. Menetelmän avulla tutkitaan sulkeumien muutosta ja jakaantumista sulan teräksen näytteenotossa ja sulkeumakuvan muutosta jatkuvavalun aikana. Uutta tietoa esitetään oksidi-sulfidiketjujen muodostumisesta kuumavalssauksen aikana. Tutkituilla teräksillä rikkipitoisuuden noustessa yli 10 ppm:n sulat kalsiumaluminaatit takaisinmuokkautuvat kiinteiksi alumiinioksideiksi tai spinelleiksi jatkuvavalun aikana. Riittävän matalalla rikkipitoisuudella ja kohtuullisella kalsiumkäsittelyllä alumiinioksidien ja spinellien muodostumista voidaan hillitä tai jopa estää se. Tutkimuksessa esitetään, että kuumavalssauksessa venyvimmät sulkeumat muodostavat myös lukuisten sulkeumien oksidi-sulfidiketjuja, jotka ovat tuotteen ominaisuuksien kannalta haitallisia. Tulosten mukaan kalsiumaluminaatit, jotka sulavat matalimmissa lämpötiloissa, eivät kuitenkaan ole taipuvaisimpia venymään valssauksessa. Maltillisella kalsiumkäsittelyllä saavutetaan optimaalinen oksidikoostumus C12A7–CA–MgO-faasiseurueesta. Nämä sulkeumat ovat sulia jatkuvavalussa varmistaen teräksen valettavuuden. Toisaalta ne eivät helposti veny tai rikkoonnu ketjuiksi valssauksessa jatkuvavalun jälkeen.

To stay competitive in the steel industry, steelmakers are utilizing different production methods to reduce production cost without compromising on the quality of their products. In steelmaking, the production process plays a significant role on the steel cleanliness. Recent increasing demands on the cleanliness level therefore require optimization of production process to meet the requirement. Often, the types and distribution of non-metallic inclusions in steel determines the steel cleanliness. In order to optimize the production process, complete assessment of the non-metallic inclusions in the steel is necessary, leading to implementation of measures to control and/or remove non-metallic inclusions in the steel. The present study was performed to investigate the cleanliness level of steel bars produced from a high frequency induction furnace (HF) route at Uddeholms AB. Experimental studies were carried out and characteristics such as number, composition, size distribution and morphology of non-metallic inclusions were investigated. Total oxygen and total nitrogen content were also measured for indirect assessment. Further, the production operations at the HF were observed and evaluated to determine their influence on the inclusion characteristics. The characteristics obtained were compared with characteristics of inclusions in steel bars produced from an electric arc furnace production (EAF) route at Uddeholms AB and a competitor producer sample. The results showed that the level of cleanliness varies from different production routes and is hence dependent on the process at each production route. The number, maximum and mean size of inclusions were found to be higher in the HF route compared to the other routes. More so, there were differences in the types of oxide inclusions observed from each process route. However, sulphide inclusions exhibit similar characteristics from the different process routes. Further, the compositions of oxide inclusions observed from the HF route were found to be closely related to the steel chemistry. More importantly, the types of inclusions formed in the HF route were found to be sensitively affected by the extent of aluminium and calcium contents in the steel. Thus, the oxide inclusion types in the HF samples could be traced to the extent of different additions and operations such as deoxidation and calcium treatment that were carried out during the steelmaking process.

This study aims on characterization of non-metallic inclusions (oxides and sulfides) in steel with and without Calcium treatment. In this paper, the three-dimensional (3-D) investigation method of inclusions on film filter after Electrolytic Extraction (EE) of steel samples is applied on two different steel grades (280, 316L). Image analysis of non-metallic inclusions is carried out on Scanning Electron Microscope (SEM) images following with inclusion characteristic analysis. Steel samples with calcium treatment in the production process and reference samples was investigated and compared in density, size and composition. It shows that higher magnification and larger inclusion size correspond to more accurate result. With restriction of equipment and measurement, 2μm is the smallest size could be measured in ×500 magnification within 5% error. The comparison of distribution of inclusions varies with different zones: center has the largest size with least number of NMI; surface has the smallest size with largest number of NM; middle part has the median properties. In 316L steel, Ca treatment increases the number of oxides inclusions significantly; the content of Ca and Si in oxides inclusions is also increased. Two shapes of oxides inclusions, oval and elongated ones, are found with Ca treatment. In 280 steel, the total amount of NMI decreases slightly with Ca treatment; CaS is found in spherical inclusions; Because of earlier formation of larger size spherical inclusions, higher Ca content is found than that in smaller size spherical inclusions.

Inclusion removal is critical for the production of clean steel. A better understanding of removal processes require knowledge of the effect of process parameters on dissolution kinetics. The present research focuses on the kinetics of calcium aluminate inclusion dissolution in relevant steelmaking slags that contain CaO, Al2O3 and SiO2. In-situ observation of inclusion dissolution in slag is conducted using a high temperature, confocal scanning laser microscope (HT-CSLM). The particles used in this experimental work are produced in the laboratory and the production technique is explained in detail. The change in particle size is recorded with time and the effects of temperature, slag composition and inclusion morphology are investigated. The images are extracted from video and they are analysed to record the change in equivalent radius of a single particle during the dissolution process. The original and normalized dissolution data is used to determine the dissolution mechanism and to improve existing dissolution models. It has been found that an increase in temperature increases the dissolution rate. At 1550°C and 1600°C, there is no product layer formation at the slag-inclusion interface and so, the dissolution process is faster. Slag composition shows a significant influence on the dissolution kinetics due to differences in the dissolution driving force and viscosity. Additionally, the dissolution rate depends on the morphology of inclusion as available reaction sites vary significantly. Rate limiting steps are discussed based on the shrinking core model and diffusion in stagnant fluid model. It is shown that the rate limiting step for dissolution is the diffusion v through a product layer at 1500°C whereas it is mass transfer in slag at 1550°C and 1600°C. The diffusion coefficient of alumina is obtained by applying a one-dimension diffusion model. The calculated results varied between 5.5×10-11 and 2.6×10-10 m2/s depending on experimental conditions. Slag viscosity was found to be an important parameter for the modelling of the dissolution process. A modification to the correlation between the correction coefficient and slag viscosity was proposed. This modification improved the prediction of the dissolution path for calcium aluminate and alumina inclusions in steelmaking slags. This novel study provides an understanding of dissolution mechanisms and it offers data on the dissolution rate of CA2 inclusions in the slags related to the process of steelmaking. The results from this work can be used by steelmakers to aid in process design. / Thesis / Master of Applied Science (MASc) / The present work is a pioneer study on the dissolution of calcium aluminate particles in liquid oxide mixtures using the unique real-time observation approach. Experiments were conducted to provide a better understanding of the effects of various steelmaking conditions on inclusion removal during the refinement of liquid steel. An existing dissolution model is further refined by introducing an additional parameter that is correlated to the properties of oxide mixtures. It has been found that the dissolution model can be applied not only to calcium aluminate inclusions but also to alumina inclusions. Hence, the approach proves the potential university nature of the dissolution model. A clear understanding of the dissolution kinetics of inclusions helps to optimize the current steelmaking routes and enhance the removability of inclusions. Steel with a minimum amount of inclusions has better properties from all aspects, which improves its applicability in all fields.

Afin de réduire les effets nocifs des inclusions d'alumine et d'améliorer la coulabilité de l'acier en fusion, le traitement au calcium est largement utilisé pour transformer les inclusions solides en inclusions liquides. Cependant, le traitement au calcium donne des résultats très irréguliers, difficilement explicables. Même si de nombreux efforts ont été déployés pour comprendre le comportement du calcium dans l’acier liquide, il n’a pas encore été prédit avec précision. Par conséquent, le mécanisme par lequel le calcium se dissout dans la masse fondue et transforme les inclusions solides doit être compris pour optimiser les conditions de traitement tels que la vitesse d'injection, la profondeur d'injection, le diamètre du fil d'injection, le temps d'agitation, etc. Afin de comprendre le mécanisme par lequel le calcium se dissout dans l'acier liquide, des expériences en laboratoire ont été effectuées dans un petit four à induction d'une capacité de 2,5 kg de métal. L'objectif est de confronter les résultats expérimentaux avec les résultats du modèle numérique développé. La remontée des gouttelettes de calcium ou des bulles dans l’acier liquide est un problème à trois phases (gouttelettes ou bulles de calcium/acier liquide/air au sommet). Par conséquent, une plateforme de calcul scientifique interne (ICI-tech) basée sur des méthodes par éléments finis est adaptée pour permettre la modélisation de telles solutions. Les écoulements triphasiques sont validés à l’aide de références classiques issues de la littérature. Le modèle de dissolution a été mis en oeuvre dans notre logiciel et la validation du modèle de dissolution a été réalisée. Les gouttelettes et les bulles de calcium sont étudiées dans l’acier liquide et leur coefficient de transfert de masse moyen est indiqué. Afin d'étudier le changement de phase calcium liquide/gaz, un modèle de nucléation a été implémenté dans le code ICI-tech. Un test typique est effectué où la croissance d'une bulle (vapeur d'eau) dans une eau uniformément surchauffée et la croissance d'une bulle de calcium dans du calcium liquide uniformément surchauffé sont calculées. / In order to diminish the harmful effects of aluminate inclusions and improve the castability of molten steel, calcium treatment is widely used in Aluminum killed steels. However, calcium treatment gives irregular results. Even with many efforts done to understand the behavior of calcium in liquid steel, it is not yet accurately predicted. Therefore, the mechanism by which the calcium dissolves into the liquid steel and transforms the solid inclusions must be understood to optimize the process conditions such as injection speed, injection depth, injection wire diameter, stirring time, etc. In order to understand the mechanism by which the calcium dissolves into the liquid steel, laboratory scale experiments have been performed in a small induction furnace of 2.5 kg metal capacity. The calcium injections are performed at the temperature below and above the boiling point of calcium. Then, the corresponding yields (calcium recovery) are compared. Rise of calcium droplet or bubble in liquid steel is a three-phase problem (calcium droplet or calcium bubble/liquid steel/air at the top). Therefore, an in-house scientific computational platform (ICI-tech) based on finite element methods is adapted to allow the modeling of such three-phase flows, which is validated using the classical benchmark issued from the literature. The dissolution model has been implemented in our software, and their validation has been performed. Thereafter, the rise of Calcium droplets and bubbles are studied in the liquid steel, and their respective average mass transfer coefficient in the liquid steel is reported. In order to study the calcium liquid/gas phase change, the nucleation model has been implemented in the code (ICI-tech). A typical test is performed where the growth of a bubble (water vapor) in uniformly superheated water and growth of a calcium bubble in uniformly superheated Ca liquid is computed.

The non-metallic inclusions (NMI) have large influence on steel properties. Therefore, evaluation of inclusion size, number, composition and morphology by using modern analytical techniques are very important for control of steel production and steel quality. Three dimensional analysis method of electrolytic extraction is applied in this work. Metals are dissolved and undissolved inclusions are collected on the film filter. Scanning Electron Microscope (SEM) with Energy Dispersive Spectrometer (EDS) is applied to observe different non-metallic inclusions. Apart from electrolytic extraction, several other different methods can be used to analyse the inclusions. Each of them has their advantages and drawbacks. A part of this work compares different methods for investigation of inclusions. To modify the inclusion size, number and morphology, calcium or zirconium can be added in to the steel with certain amount according to the steel grade. This work discussed how inclusion size, number and morphology changed after the modification. In samples of Heat A, the addition of Zr leads to the appearance of ZrO2 clusters. Large size particles disappeared while the peak value for the number of inclusions per unit volume didn’t decrease. The electrolytic extraction and fractional gas analysis (FGA) results show some differences on the inclusion compositions. In samples of Heats B and C, the effect of calcium treatment is investigated. Calcium treatment modified the spinels with large content of Al2O3 into spherical CaS+CaO-MgO-Al2O3 inclusions. Large size inclusions were removed after calcium treatment.Key