Clean Steel Production Research Articles

Mathematical Modelling of Magnesia Carbon Refractories Reactions with Steel

The production of super clean steel has put in clear evidence the importance of the refractory in influencing the degree on internal cleanliness of steel. In fact chemical species transfer from the refractory to the steel bath. To reduce the steel contamination caused by the refractory it is necessary to know the interaction between the refractory and the steel. A mathematical model of the modifications of the refractory surface in contact with the steel has been developed. The work is focused on magnesia‐carbon refractory (taking into account the presence of reducing agents too, as Al, Si and Mg) and carbon steel. The model calculates the new phases formed when liquid steel and refractory are in contact. The model is based on the following mechanism of reactions: (i) Magnesia reacts with graphite forming gaseous CO and Mg. (ii) The other oxides present in the refractory (mainly CaO and SiO2 which constitutes a glassy phase) pass into the steel, giving dissolved elements and dispersed oxides. (iii) Gaseous Mg reacts at the interface refractory/steel with oxygen dissolved in the steel forming solid MgO. (iv) The MgO generated and the oxides originating from the glassy phase form the new oxide system. The newly formed phase can potentially be transferred to the bath through the steel flow against the refractory walls. The results of the model simulations have been validated by experimental tests.

Effect of Melt Depth and Nozzle Type on the Mixing Behavior in Bottom-Blown Steelmaking Ladle – A Water Model Approach

It is generally well known that a steelmaking ladle operation plays an important role in the production of clean steel. A turbulent mixing of melt with Ar gas bubbling from the ladle bottom can homogenize a melt temperature and can control precisely chemical compositions of steel. In order to figure out these phenomena, a quantitative analysis of fluid flow behavior of gas and melt during a ladle operation is required and special concerns should be focused on effects of operating parameters on the perfect mixing time of melt. In this study, as a basic approach, effects of operating parameters such as a melt depth (aspect ratio) and a nozzle type (one-hole or porous plug) on the mixing behavior in ladle operation are investigated. Water model experiments are carried out to simulate these melt behaviors in steelmaking ladle. As a result, it was found that there exist an optimized melt depth and a nozzle type at a given gas flow rate, which affect significantly on the mixing behavior of melt.

IISI study on clean steel

Clean steel encompasses a multitude of concepts that are based on fulfilling customer requests. Clean steel can be produced in many ways depending on the existing equipment and detailed customer demands. A common feature of all clean steel production is tight process control along with continuous monitoring. Quality assurance is essential and it still requires many improvements. There are no specific obstacles to clean steelmaking, as the industrial processes are available, sufficiently understood and controlled. The major issues to be addressed remain disturbances that occur in industrial reality. This asks for continuous improvements in processes and equipment and for further development of quality assurance systems to achieve full control of any possible detrimental effect.

A Mathematical Model to Study Liquid Inclusion Behavior at the Steel-Slag Interface

The separation of non-metallic inclusions at the interface between the steel and the slag in the ladle, tundish and mold is an essential part of the production of clean steel. It is therefore, of great importance to have a deep understanding of the phenomena controlling the transfer of inclusions from the steel to the slag layer. In this work a mathematical model, derived from the equation of particle motion, have been used to study the transfer of liquid inclusions to slags. The effects of the drag, added mass, buoyancy and rebound force on the inclusion transfer are considered. The model relies, to a great extent, on the availability of accurate information of the magnitude of a number of physical properties of the involved phases. Among those properties, the interfacial tension between the phases and the slag viscosity were found to be the most critical. Due to the fact that the availability of experimentally obtained high-temperature physical property data, relevant to the industrial conditions, is scarce in the literature several different model descriptions have been used in this work to estimate these properties. The mathematical model has been used to investigate the separation of liquid non-metallic inclusions, of different size and composition, to a number of different industrial ladle slag compositions.

Determination of inclusion characteristics in ‘low‐carbon’ steel during up‐hill teeming

Clean steel production requires good control of the size distribution and the composition of the inclusions at all stages of the steel making process. So far, there exists very little information on the behavior of the inclusion population during ingot casting of steel. Therefore, this study focus on providing information on the inclusion population before and during up‐hill teeming of ‘low‐carbon’ steel, based on plant trails carried out at Ovako Steel's facilities in Hofors, Sweden. Liquid steel samples were acquired from the casting ladle just before the start of the filling of the ingot molds and from the rising steel in the molds during the filling of the molds. Thereafter, the inclusion compositions for the different size classes were determined using a scanning electron microscope equipped with energy dispersive X‐ray. It was found that the inclusion compositions were more complex in the samples taken in the mold in comparison to the ones taken in the ladle. In some cases large exogenous inclusions most likely formed due to a reaction with the mold flux were found. When a mold was filled without mold flux reoxidation of the steel due to reaction with the atmosphere took place. The inclusion size distributions were also determined using light optical microscopy and classified according to the Swedish standard SS111116 (JK chart II). The number of inclusions found in the sample taken in the mold were higher than in the sample taken in the ladle. The general conclusions are that more in‐depth research is necessary to understand the reactions taking place during filling of an ingot.

Determination of macroinclusions during clean steel production

There are very few macroinclusions in clean steel, but the few that exist are very harmful for material properties of the final steel product. To date, very little information on large, so called macroinclusions in clean steel production has been presented. Therefore, the present study has focused on providing information on these inclusions during different stages of the steelmaking process, based on plant trials carried out at Uddeholm Tooling, Hagfors, Sweden. Macroinclusion size distributions have been determined using optical microscopy and classified according to a modified version of the Swedish standard SS 111116, as well as using immersed ultrasonic scanning. For the sake of completeness, the size distribution of microinclusions has also been determined using optical microscopy. Inclusion compositions were determined using a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Two types of liquid steel sampler were used in the investigation: a rapid solidifying (RS) sampler and the LSHR (liquid sampling hot rolling) sampler, suitable for immersed ultrasonic scanning. The results are critically discussed with respect to process conditions, such as alloy additions that took place during the plant trials.

Process metallurgical evaluation and application of very fine bubbling technology

The potential of VFB(very fine bubbling)-technology in steelmaking, developed for the production of super clean steels, was investigated. Recent RD 2] and to a level of 10 ppm under argon protective atmosphere [2]. The perspective of industrial application of the VFB technology to a 56-t ladle furnace of Helliniki Halyvourgia S.A., Greece, in order to improve steel cleanliness, requires additional R&D efforts. It is important to define the limits of VFB technology in respect of alloys dissolution, mixing time and homogenisation of steel and slag/metal reactions. In this work, a gas driven bubble aqueous reactor model simulating the bottom gas stirred ladle by means of gas injection through a SPP and a conventional porous plug was studied. Various operating conditions as well as different positions for the porous plug with and without a top oil layer were simulated. Tests concerning mixing time, solid-liquid mass transfer and critical gas flow rate, liquid/liquid mass transfer, using the SPP and a conventional porous plug have been performed. The evaluation of experimental results delivered important information for the design and operation of steel ladles, applying VFB-technology. Experimental results with SPP bubbles' agitated steel (1600 °C) in laboratory and technical scale experiments in IF and VIF are presented and discussed.

Modelling of ladle glaze interactions

A potential source of inclusions in liquid steel is ladle glaze, the slag/inclusion coating on the refractory lining of the ladle which is always present after casting and before reuse of the ladle. The composition and quantity of this glaze can vary considerably, depending on the range of steel qualities being made and, in order to model the effect of glaze on inclusion/top slag formation, a combined fluid flow-thermodynamic model has been developed. The behaviour of ladle glaze for various different steels has been studied under pilot plant conditions and the results incorporated into the mathematical model. The implications of the pilot plant and model results are discussed in the context of the production of clean steel.

Avaliação do tamanho, da morfologia e da composição química de inclusões não metálicas em aço baixo carbono desoxidado ao alumínio

A presença de inclusões não metálicas endógenas e exógenas prejudica a lingotabilidade e as propriedades mecânicas dos aços. A busca contínua pelo controle da quantidade, do tamanho, da morfologia e da composição química de inclusões não metálicas tem sido tema de estudos nas últimas décadas, em especial durante o processo de refino secundário dos aços. Através do controle, principalmente, do tamanho e da composição química das inclusões, é possível obter um produto final de boa qualidade. A identificação da natureza e o controle da formação de inclusões são de suma importância para a obtenção de aços limpos. Este trabalho tem como objetivo fazer uma investigação de inclusões não metálicas em aço de baixo teor de carbono(SAE 1015), desoxidado ao alumínio, através de um método de dissolução parcial e total da matriz metálica e posterior análise ao Microscópio Eletrônico de Varredura (MEV), acoplado a um sistema de análise por energia dispersiva (EDS), permitindo uma visualização do tamanho, da forma e da composição química das inclusões com maior precisão, sem interferência da matriz ferrítica.

Special porous plugs for the production of super clean steels

Fine bubblers to be used as special porous plugs (SPP), have been devised for fine bubbling within steel melts for improving the cleanliness efficiently by floating the micro-inclusions and removing the dissolved gases. The development principles of the magnesia-spinel porous plugs are presented. Laboratory water simulator tests have shown that small (< 1 mm), non-coagulating bubbles, with uniform distribution are formed. The characterisation and testing of the SPP has shown that under steelmaking conditions clogging from metal infiltration is limited. Erosion, cracking or changes in microstructure were not detected in high temperature long-term endurance tests. With combined treatment in a vacuum induction furnace under a 3 kPa pressure and Ar purging of 18 min through the new SPP, the total oxygen content of the steel melt was decreased from 65 to 7.7 ppm. An application has been filed for a patent for the method for the production of fine bubblers to be used as porous plugs.

Thermodynamic Stabilities of Lanthanum Tungstates Using a Calcium Fluoride Solid Electrolyte Galvanic Cell

A knowledge of the thermodynamic stabilities of the lanthanum tungstates measured as the Gibbs energies of formation from the free oxides is important as lanthanum is used as a powerful desulphurizing agent and tungsten as a special alloying element in the production of clean steels and special steels. These data will also be useful as the system La2O3 WO3 can be a model since it exhibits many compounds between the end members. Excepting one EMF work, no information is available in the literature on the thermodynamics of this system. This system exhibits the compounds 5 La2O3·22 WO3, La2 O3 · 3 WO3, La2O · 2 WO3, 7 La2O3 · 8 WO3,La2O · WO3, 3 La2O3 · 2 WO3 and 5 La2O3 · 2 WO3. The compounds were prepared from reagent grade free oxides by solid state sintering at high temperature followed by X-ray diffraction. The standard Gibbs energies of formation of the compounds from the free oxides were determined by operating the following galvanic cells with calcium fluoride as solid electrolyte. O2(g), Pt/ La2O3 · LaOF// CaF2/ / 5 La2O3 · 22 WO3, WO3, LaOF/Pt, O2(g) O2(g), Pt/La2O3 · LaOF//CaF2//5 La2O3 · 22 WO3, La2O3 · 3 WO3, LaOF/Pt, O2(g) O2(g), Pt/La2O3 · LaOF//CaF2// La2O3 · 3 WO3, La2O3 · 2 WO3 LaOF/Pt, O2(g) O2(g), Pt/La2O3 · LaOF//CaF2// La2O3 · 2 WO3, 7 La2O3 · 8 WO3, LaOF/Pt, O2(g) O2(g), Pt/La2O3 · LaOF//CaF2//La2O3 · WO3, 3La2O3 · 2 WO3 LaOF/Pt, O2(g) O2(g), Pt/La2O3 · LaOF//CaF2//3 La2O3 · 2 WO3, 5La2O3 · 2 WO3 LaOF/Pt, O2(g) O2(g), Pt/ La2O3 LaOF// CaF2/ / 3 La2O32 WO3, La2O3, 2 WO3 LaOF/Pt, O2(g) Stable, reproducible EMFs were obtained in the temperature range 1123 to 1273 K. The net cell reactions are respectively as follows: 5 La2O3 + 22 WO3 = 5 La2O3 · 22 WO3 7 La2O3 + 3 (5 La2O3 · 22 WO3) = 22 (La2O3 · 3 WO3) La2O3 + 2 (La2O3 · 3 WO3) = 3 (La2O3 · 2 WO3) 3 La2O3 + 4 (La2O3) = 7 La2O3 · 8 WO3 La2O3 + 7 La2O3 · 8 WO3 = 8 (La2O3 · WO3) La2O3 + 2 (La2O3 · WO3) = 3 La2O3 · 2 WO3 2 La2O3 + 3 La2O3 · 2 WO3 = 5 La2O3 · 2 WO3 The standard Gibbs energies of formation of the different lanthanum tungstates are then calculated by appropriately combining from the above cell reactions and their Gibbs energy changes obtained from the experimental EMF data. The present data are discussed in the light of EMF data reported in the literature for WO3-rich compounds in the system.

Cold Model Study on Inclusion Removal from Liquid Steel Using Fine Gas Bubbles.

A water model study was undertaken to investigate inclusion removal from liquid steel using fine gas bubbles, which were created by injecting air into a shroud nozzle just underneath the slide gate. The water flow rate was varied in the range of 2.0-3.2 m 3 hr -1 which is equivalent to 1.77-2.83 m s -1 of the linear velocity at the exit of the shroud nozzle. The ratio of gas to liquid flow rates was varied in the range of 0-12 % by volume. Polyethylene, PVC and ABS particles were used to imitate the inclusion. The characteristics of the bubble formation varied with the gas inlet position along the shroud: the closer the slide gate, the better the mixing between the gas and liquid phases. This was dependent on the slide gate opening: the smaller the opening, the more turbulence and hence the better mixing of the gas and the liquid. The relative removal efficiency was fairly independent of such variables as liquid flow rate, i.e., linear velocity, the slide gate opening (up to 58 % by area), and inclusion concentration. However, this was greatly affected by the wettability, i.e., contact angle of the inclusion with the liquid: the larger the contact angle, the higher the efficiency. It is concluded that the governing factor which determines the removal efficiency of the inclusion from liquid steel is the wettability of the inclusion with the steel, and the idea proposed in the present study should be an effective means for production of clean steels.

Thermodynamic assessment of Mg deoxidation reaction of liquid iron and equilibria of [Mg]-[Al]-[O] and [Mg]-[S]-[O

The deoxidation reaction of magnesium was investigated thermodynamically employing the equilibrium system between magnesium vapour and liquid iron in the molybdenum chamber sealed with an iron cover at 1873 K as a fundamental study to address the clean steel production technology in the steelmaking process. The previously reported thermodynamic data for magnesium deoxidation reaction are limitedly in good agreement with only their respective specific Mg concentration range, but fail to explain the thermodynamic equilibria generally over the wider range of magnesium concentration beyond the limited range. Therefore, the equilibrium constant, K Mg for the magnesium deoxidation reaction as well as the first and second-order interaction parameters between magnesium and oxygen were determined over the extensive magnesium mass content range covering up to 0.04 %. Furthermore, the phase stability diagram based on the equilibria of [Mg]-[Al]-[O] in liquid iron for the purpose of controlling the oxide inclusions in the steelmaking process was accomplished using the determined thermodynamic parameters. The equilibria of [Mg]-[S]-[O] were also discussed in order to evaluate the utilisation of Mg as a desulphurizing agent as well as deoxidizer in the production process of low carbon steels.

清浄鋼製造のための溶鋼流動制御

清浄鋼製造のための溶鋼流動制御

Inclusion Control in High-Performance Steels

Progress of clean steel production, fundamentals of oxide and sulphide inclusions as well as inclusion morphology in normal and calcium treated steels are described. Effects of cleanliness and inclusion control on steel properties are discussed. In many demanding constructional and engineering applications the nonmetallic inclusions have a quite decisive role in steel performance. An example of combination of good mechanical properties and superior machinability by applying inclusion control is presented.

Secondary metallurgy and continuous casting practice for clean steel production

The industrial procedure for the production of clean steel by tank degassing will be explained. During this kind of secondary metallurgy, a simultaneous degassing as well as metal-slag reaction occurs. During continuous casting process, a reoxidation and resulphurisation must be prevented. Optimum results for steel cleanness can be achieved with a vertical bending caster type.