Ladle Treatment Research Articles

Industrial uses of slag (the use and re-use of iron and steelmaking slags)

Traditionally, iron ore has been reduced with coke in a blast furnace and the hot metal product of the blast furnace containing carbon, manganese, silicon, sulphur and phosphorous, was subsequently refined in a steelmaking furnace. During ironmaking as well as during steelmaking, significant amounts of slag are produced. Two decades ago, more than 13 million tonne (mt) of blast furnace slag and 4 mt of steelmaking slag per annum were produced in the USA alone. It is therefore not surprising that many attempts have been made to re-use iron and steelmaking slags. However, these slags are not merely metallurgical waste products but, on the contrary, their compositions and physical properties are judiciously designed to optimise the operations in the respective metallurgical reactors. The variation in composition and properties of blast furnace slags are largely determined by the gangue in the ore and ash in the coke and much progress has been made in the re-use of such slag. With respect to steelmaking slag, emphasis has in recent years been placed on hot metal treatment before refining and ladle treatment after decarburisation. The bulk removal of silicon, phosphorous and sulphur from the hot metal product of the blast furnace by specially designed slags prior to refining in a basic oxygen furnace, has reduced tap-to-tap time, lowered cost and produced steel of much higher quality. The use of synthetic slags in ladle refining techniques subsequent to decarburisation in the basic oxygen furnace has resulted in reduced hydrogen and nitrogen contents and reduced solute impurities. Moreover, inclusion shape and composition control, again by the use of specially designed slags has led to the achievement of much improved mechanical properties of the steel. In these revised process routes, slags of different compositions are produced in the variety of unit processes and hence the re-use of steelmaking slags has become much more complex. Clearly, a thorough understanding of the design and use of these different slags is a prerequisite to the development of re-use strategies. Japanese steelmakers have spearheaded attempts to reduce slag volumes in the individual unit processes and to strive towards 'slagless steelmaking' by recycling slags internally. Steelmaking slag volumes have been reduced on commercial scale from about 140 kg t−1 steel to 60 kg t−1 steel. Moreover, model predictions have shown that in principle, all slag can be recycled and the phosphorous extracted to produce fertiliser. This development is of particular significance since the supply of low-phosphorous iron ores is diminishing at an ever increasing rate.

Impact of slag–refractory lining reactions on the formation of inclusions in steel

The present study was carried out to investigate the impact of slag– refractory lining reactions on the formation of inclusions during ladle treatment of steel. The experiments were conducted on an industrial scale in the ladle at Uddeholm Tooling AB in Hagfors, Sweden. The inclusion chemistry and population during ladle treatment were studied along with the composition of the ladle glaze, taken from the ladle lining. The inclusions in the steel were classified into four groups according to the Swedish standard SS 111116. SEM/EDS analyses were carried out to identify the phases present in both the inclusions and the ladle glaze. The number of inclusions in the steel before deoxidation was found to increase with the ladle age, i.e. the number of times the ladle had been in use. A similar increase was also found after vacuum degassing and before casting. A great portion of inclusions before casting was found to be supplied by ladle glaze. This observation was further confirmed and explained by thermodynamic analysis. The present results show that ladle glaze is a major source of inclusions in the ladle at Uddeholm Tooling.

Ladle glaze: major source of oxide inclusions during ladle treatment of steel

Samples of ladle lining covered by glaze were taken from industrial ladles of different ages at Uddeholm Tooling AB, Hagfors, Sweden. It was found in the samples taken at and below the slag line that a slag infiltrated layer was covered by an outer layer containing many MgO 'islands' of various sizes. The microstructure of the infiltrating slag was the same as the matrix of the outer layer. The slag was found to decompose into the compound 3CaO.Al2O3 and a liquid phase during the cooling process. The former phase along with tiny MgO particles from the ladle glaze was found to be one of the major sources of inclusions during the degassing and flotation periods of ladle treatment. Thermodynamic analysis indicated that the reaction between the ladle glaze and the slag from the electric arc furnace resulted in the formation of MgO–Al2O3 spinel and an oxide solution, which were also the main inclusions found at the initial stages of ladle treatment. Evidence of this reaction was found in the lining samples taken above the slag line.

Effect of ladle age on formation of non-metallic inclusions in ladle treatment

An experimental study was carried out at Uddeholm Tooling, Hagfors, Sweden to examine the impact of ladle glaze on the formation of non-metallic inclusions during the ladle treatment process. Steel samples were taken at various stages of the process and from ladles of different ages. Inclusion numbers were counted under an optical microscope. It was found that the total number of inclusions increased with the ladle age before deoxidation and at the end of the ladle treatment. The increase was substantial after the ladle had been used more than 18 times. Inclusions having the smallest sizes were found to make a large contribution to this increase. This observation was further confirmed by the difference between total oxygen content and dissolved oxygen content in the steel samples, which also showed an increase with ladle age. Analysis by SEM–EDX revealed two types of inclusions before casting, namely, inclusions consisting only of an oxide solution having a composition very close to 3CaO. Al2O3, and inclusions consisting of the same oxide solution as well as MgO phase. This finding was in accordance with the reported result that both 3CaO. Al2O3 and MgO were present in the slag infiltrated layer of the ladle glaze. It was concluded that ladle glaze is the foremost source of non-metallic inclusions in tool steel during ladle treatment.

Evaluation of low carbon Al-killed/Cr-stabilized steel to be used in the automobile industry

An Al-killed/Cr-stabilized low carbon steel was produced in the steel industry using electric arc furnace, vacuum degassing, ladle treatment and continuous casting route. The resulting slabs were then hot rolled at 1100 °C, coiled at 600 °C, cold rolled and annealed at 700 °C. After evaluation of the microstructure, texture and mechanical properties, the fully recrystallized coils fulfilled the target properties established by the automobile industry.

Hydrogen control during steel making for medium carbon wheels

The control of hydrogen is essential to avoid flakes, internal microcracks and other types of steel defects. Vacuum degassing, introduced in late 1950s for bringing down the hydrogen of liquid steel, continues to be an important refining operation today to meet stringent requirement of hydrogen, which is sometimes as low as 1 ppm or lower. The net hydrogen level in steel depends on the extent of its pick up and removal during various stages of steel making. The measures to control pick up mainly include the use of inputs with low moisture or hydrogen content and those for removal are efficient degassing in VAD, control of ladle slag basicity etc. The hydrogen level in medium carbon wheel steel at Durgapur Steel Plant is controlled by utilising a number of measures in BOF and ladle treatment practices. These include avoidance of late addition of lime in converter, minimisation of carry over slag, delayed and just adequate deoxidation in ladle, recarburisation with low hydrogen pet coke, efficient degassing in VAD at deep vacuum and intense purging, restricting ladle slag basicity to less than 1.2 etc. The measures developed and implemented during the investigation resulted in hydrogen level of 1.2–1.8 ppm consistently.

Model of Desulphurization of Steel Melts in Ladles - Deoxidation and Slag Formers Calculation

The authors present their own model applicable for effective desulphurization of steel by ladle treatment. The model is based on a thermodynamic approach (equilibrium data) and technological data (correction factors). It consists of two parts. In the first part the authors present the formulae derived for estimation of the additions of deoxidation agents which ensure sufficiently low oxygen level prior to the desulphurization process, while the other part gives a qualitative and quantitative selection of slag formers to achieve low final levels of sulphur in steel. For the deoxidation process two variants were considered: (i) with Al only (for low silicon steels) or (ii) Al-Si (for silicon steels) as deoxidizers. For the desulphurization process three variants were assumed as to ladle slag composition: (i) slag consisting of a fraction of furnace slag, lime addition and deoxidation products, (ii) slag made of synthetic CaO-Al 2 O 3 and (iii) slag based on lime and fluorspar. The model formulae for desulphurization were derived using the sulphide capacity concept which relies on the optical basicity. In addition, rough estimates of the slag liquidus temperatures are given. A numeric example of the model application and the model algorithm (appendix) are enclosed.

La investigación siderúrgica en el CENIM

In 2002 the National Centre for Metallurgical Research (CENIM) celebrated its 39^^^ anniversary. During this time, steel has been the most studied material of all the metals and alloys researched in the different projects funded by national science and technology plans, European metallurgical research programmes, and by the iron and steelmaking companies who over the years have placed their trust in the research work undertaken at CENIM. Two generations of researchers have dedicated their careers to studying the different aspects of steel, from primary metallurgy considering iron ores and enrichment processes, the thermodynamics of reactions in the blast furnace and in the ladle, thermal and thermomechanical treatments, the physical metallurgy of steels, their mechanical properties, and finally their deterioration in a wide range of media of different aggressivities. CENIM maintains its committment to iron and steel research, drawing attention to the vast amount of research work undertaken and financed within the framework of the ECSC programme since Spain joined the European Community in January 1986.

Reoxidation during ladle treatment

Cleanliness of steel is mainly determined by the presence of oxide inclusions. The reoxidation of liquid steel before, during and after ladle treatment at the Sidmar steel plant was quantified for three steel grades, as it is a potential source of inclusions. A mass balance calculation was performed to compare the expected and measured steel composition. The total reoxidation was calculated from the reactions of the dissolved elements. A dissimilarity in behaviour of the steel grades was found. The resultant compositions of the slags were calculated and compared with measured compositions. Some differences between computed slag compositions and average measured values are explained. Finally, the relation between the reoxidation and internal cleanliness at the end of ladle treatment was examined.

Thermodynamic examination of inclusion modification and precipitation from calcium treatment to solidified steel

Calcium treatment is a well established method to transform oxide and sulphide inclusions in steel to less harmful inclusions which can have beneficial effects on properties. Transformation of solid alumina clusters to liquid calcium aluminates by calcium is a much used technique to avoid nozzle blocking in continuous casting of aluminium deoxidised steels. Calcium treatment is also a key method to improve machinability, which can be further improved by increased sulphur content. Resulphurised steels, however, tend to have casting problems due to deficient modification of oxides or formation of solid CaS inclusions which also have a tendency to clog the nozzle. In this study, the formation and transformation of liquid and solid inclusions containing oxidic and sulphidic components were examined by thermodynamic calculations. A quasichemical slag model was applied to calculate equilibrium oxide and sulphide inclusions in steel. Calculations were carried out over a wide temperature range to study the formation of species in the melt corresponding to ladle treatment conditions as well as casting. To examine inclusion behaviour during casting and solidification, the interdendritic solidification model, IDS, was coupled with thermodynamic equilibrium calculations. The results show the conditions in which liquid inclusions can be formed with calcium treatment. The calculations were performed at different temperatures and varying calcium, aluminium and sulphur contents. The effect of total oxygen content was also studied. The main components in liquid calcium aluminates were CaO and Al2O3 plus some SiO2 and CaS. The stability relations of different compounds alter as a function of temperature. An abrupt change occurs during solidification when the partition of the inclusion forming elements between the liquid and solid iron phases are taken into account.

Influence of ferromanganese additions on microalloyed engineering steel

Plant trials with the addition of four different low carbon and medium carbon ferromanganese alloys to microalloyed engineering steels have been carried out at Ovako Steel, Hofors, Sweden. The process route for this type of steel at Ovako is electric arc furnace (EAF), ladle arc furnace (LAF), ASEA-SKF vacuum degasser, LAF and finally uphill teeming. For each ferromanganese alloy type, five experiments were accomplished by adding alloy at the ladle furnace station, i.e. 20 charges in total. During the experiments, liquid steel rapid solidification samples were taken to study the behaviour of inclusion formation and removal during ladle treatment. Also, samples for LSHR testing (liquid sampling hot rolling) were acquired for one heat of each of the ferromanganese containing alloys. Microinclusion assessments were carried out according to the method described in Swedish standard SS 111116. Macroinclusion assessments were carried out using immersed ultrasonic testing. Element analysis and mappings were done using a scanning electron microscope to establish the effect of ferromanganese addition on the composition of inclusions present in the steel. In samples taken 3 min after the ferromanganese addition to the LAF, about 30% of the analysed inclusions contain Al2O3, MnO and MnS. The Al2O3 content is concluded to originate from the deoxidation practice, while MnO and MnS contents are likely to originate from the addition of ferromanganese. The results also show that the addition of ferromanganese alloys leads to an increased amount of macroinclusions in the LAF up to 9 min after alloy addition. Steel samples taken after degassing but just before casting indicate that all the tested ferromanganese alloys give the same result with respect to the studied microinclusion characteristics at the end of ladle treatment. Last, the inclusion composition in the final steel samples is unrelated to the type of ferromanganese alloy used. In this case, the main compositions are Al2O3-MgO and MnS.

State of the Art in Evaluation and Control of Steel Cleanliness.

This paper first reviews the current “state-of-the-art” in the evaluation of steel cleanliness, discussing over 20 different methods. The demand for cleaner steels requires lowering non-metallic oxide inclusions and also controlling their morphology, composition and size distribution. Because no single method can measure all of these aspects accurately, it is best to combine several methods together to quantify steel cleanliness in a given operation. Owing to the cost, time requirements, and sampling difficulties, steel cleanliness is widely inferred using total oxygen, nitrogen pick-up, and other indirect methods. Recent cleanliness values using these indicators are summarized for LCAK at many steel plants around the world. Secondly, this paper reviews operating practices to improve steel cleanliness at the ladle, tundish and continuous caster, emphasizing findings quantified with plant measurements. Inclusions come from many sources, including deoxidation, reoxidation, slag entrapment, refractory wear, and chemical reactions. They generate many defects such as cracks and slivers in the steel product. Steel cleanliness is controlled by attention to a wide range of important operating conditions throughout the steelmaking and casting processes. For ladle operations, FeO and MnO in the slag, ladle treatments, and inclusion modification are discussed. For tundish operations, tundish depth and capacity, casting transitions, refractory lining, tundish flux; gas stirring, and flow controls are discussed. Important transfer operations from ladle to tundish and from tundish to mold, such as argon protection, sealing issues, and SEN clogging are summarized. Caster operations reviewed include the effect of casting speed, fluid flow pattern control, surface level control, and caster curvature.

Modelling of Temperature Distribution in Refractory Ladle Lining for Steelmaking

As a first step towards optimisation of ladle treatment work, mathematical modelling of heat transfer in the ladle has been undertaken. A numerical model considering heat transfer is developed which can be used for prediction of the ladle lining temperature fields during steel casting sequences depending on wear rates and used lining materials. The model is based on Fourier differential equations. In the cases of dolomite brick, the calculational results are compared with temperature measurements from earlier publications for the periods of charged ladle, teeming, empty state and preheating whereby a good agreement is found.

Chemical characteristics of inclusions formed at various stages during the ladle treatment of steel

Industrial data were analysed to shed some light on the formation and growth of non-metallic inclusions during the ladle treatment of a particular grade of tool steel, Orvar Supreme (Fe-0·39C-1·0Si-0·4Mn- 5·2Cr-1·0Mo-0·9V). Seven types of inclusions were detected in samples taken along the processing evolution of the steel. The types of inclusions present were found to vary with the various stages of that evolution. While additions of aluminium to the steel bath were found to affect the composition of the inclusions, only a small number of pure alumina inclusions, agglomerated as clusters, were observed during the initial stages of deoxidation. Ladle glaze was found to be the major source of the inclusions. Most of those left in the steel before tapping were found to be of very small size and to contain high concentrations of Al2O3 and CaO and relatively minor ones of MgO and FeO.

Slag-metal reactions during ladle treatment with focus on desulphurisation

Within several cooperative projects, KTH (Royal Institute of Technology), Ovako Steel AB, and MEFOS have investigated the desulphurisation of bearing steel during vacuum degassing. The work includes thermodynamic calculations of the slag-metal equilibrium, CFD modelling of slag-metal reactions, and plant trials. Results from the various studies are presented and discussed in this paper. Models for predicting slag properties (sulphide capacity, viscosity, and oxide activities) in liquid slags as functions of slag composition and temperature have been used for the calculation of data which have been employed in static and dynamic modelling of sulphur refining. The results from static modelling show that the method allows fast and easy evaluation of the theoretical desulphurisation conditions during degassing at Ovako Steel AB, as well as theoretical determination of the parameters that have the greatest influence on the equilibrium sulphur distribution. The conclusion from dynamic modelling is that the vacuum degassing operation can be described dynamically with the present knowledge of sulphide capacity, sulphur distribution, viscosity, and oxide activities of ladle slags if this knowledge is combined with fluid flow modelling to derive the overall kinetics. The presented model approaches have been found useful in understanding the sulphur refining process at Ovako Steel AB. The dynamic modelling concept is also believed to have potential for dynamic descriptions of other slag-metal reactions in steelmaking.

The influence of stirring method on hydrogen removal during ladle treatment

A mathematical model based on fundamental transport equations for vacuum degassing in an Asea-SKF ladle furnace was developed earlier and predicted hydrogen values from the model were found to agree well with experimental measurements from plant trials. In the present study, the mathematical model was used to predict the hydrogen removal for four different stirring methods: 1–2) combined gas and induction stirring with the inductive stirrer working upwards or downwards and 3–4) gas stirring with two different locations for two porous plugs. Plant data was used as input to the calculations. The hydrogen calculation was validated by simulation of an actual heat and comparison with measured data. After 44 minutes of stirring the predicted and measured hydrogen contents were 1.6 and 1.8 ppm, respectively. This agreement was considered satisfactory and the model was then used to study the effect of different stirring methods on hydrogen refining. A change in the stirring method was found to affect the hydrogen refining. Combined gas and induction stirring using downward stirring was found to result in the lowest final hydrogen content in the steel as it rendered the most favourable kinetic conditions for hydrogen refining. Einfluß der Rührmethode auf die Wasserstoffentfernung bei der Pfannenbehandlung. Ein mathematisches Modell, das auf den grundlegenden Transportgleichungen für die Vakuumentgasung in einem Asea-SKF-Pfannenofen basiert, wurde zu einem früheren Zeitpunkt entwickelt. Die mit diesem Modell vorausberechneten Wasserstoffwerte stimmten mit den Ergebnissen experimenteller Messungen aus Anlagenversuchen überein. In der vorliegenden Studie wurde das mathematische Modell nun eingesetzt, um die Reduzierung des Wasserstoffgehalts für vier verschiedene Rührmethoden vorherzusagen: 1–2: Kombiniertes Gas- und Induktionsrühren mit einem nach oben und unten arbeitenden induktiven Rührer sowie 3–4: Gasspülen mit zwei an zwei verschiedenen Orten plazierten porösen Spülsteinen. Als Input für die Berechnungen dienten Anlagendaten genutzt. Die Wasserstoffberechnung wurde durch Simulation einer Schmelze und Vergleich mit gemessenen Daten validiert. Nach 44 Minuten Rühren stellten sich die vorausberechneten und gemessenen Wasserstoffgehalte von 1,6 und 1,8 ppm ein. Diese Übereinstimmung wurde als ausreichend betrachtet und das Modell wurde sodann genutzt, um den Effekt verschiedener Rührmethoden auf die Reduzierung des Wasserstoffgehalts zu untersuchen. Es hat sich gezeigt, daß eine Veränderung der Rührmethode einen Einfluß auf den Wasserstoffgehalt ausübt. Das kombinierte Gas- und Induktionsrühren von oben nach unten führte zu den geringsten Wasserstoffgehalten im Stahl, da sich so die günstigsten kinetischen Bedingungen für die Wasserstoffreduzierung einstellten.

Control of inclusions in a resulphurised steel

The formation of non‐metallic endogenous inclusions in ladle‐furnace treatment of a resulphurised C‐Mn Al‐killed steel was investigated. A model based on equilibrium reactions and using as input data compositions and temperatures measured at various stages of secondary steelmaking was developed and applied to different routine and experimental heats.Microstructural analyses were carried out on samples systematically extracted from ladle and from tundish. Detailed characterisation of inclusion morphology and composition was carried out to investigate their properties in the present resulphurised steel and to evaluate reactions taking place during different phases of ladle treatment. The experimental results were also used to validate the theoretical model. A good agreement between predicted concentrations of inclusions and experimentally measured values was obtained.

Improvement of Castability and Quality of Continuously Cast Steel.

Thermodynamic fundamentals on the systems Fe-Al-O and Fe-Al-Ca-S-O being important to metallurgy of Ca-treated Al-killed steels are analysed and discussed. The formation of liquid Ca aluminates for the improvement of castability is pursued in view of the production of Ca-treated Al-killed steels. The "Liquid window" is theoretically analysed and experimentally investigated. The top slag is formed after tapping and its composition is fairly constant during ladle treatment for both with and without RH treatment, respectively. However, the top slag of the degassed heats shows a lower oxygen level compared to the non-degassed heats. It is found that there exists an "inclusion path" during the ladle treatment. The inclusion path for the heats with RH treatment considerably differs from that of the heats without RH treatment and it greatly influences the efficiency of Ca treatment.

A Thermodynamic and Kinetic Model of Reoxidation and Desulphurisation in the Ladle Furnace.

A thermodynamic and kinetic model of reoxidation and desulphurisation in the ladle furnace has been developed by using a two-dimensional fluid-flow model combined with equations expressing the thermodynamics of reoxidation and desulphurisation. More specifically, ladle vacuum treatment of a gas-stirred ladle has been simulated. In order to describe the activities of the oxide components of the slag, expressions by Ohta and Suito 1) were adopted. The thermodynamic model describes the slag/metal equilibria between Al, Si, O, Mn and S in the molten steel and Al 2 O 3 , SiO 2 , FeO, MnO and S in the slag. A fluid dynamic model that considers the slag, steel and argon phases derives the kinetics. Results from an isothermal calculation have been heuristically compared with industrial plant data and they show good agreement. The model results have also revealed that the reduction of silica plays a part in aluminium loss during ladle treatment.

Development of an API 5L X-70 grade steel for sour gas resistance pipeline application

An API 5L X-70 grade steel for large diameter pipeline application with sour gas resistance was developed by electric arc furnace processing, furnace ladle treatment, vacuum degassing, ladle treatment, and continuous casting, followed by three different controlled rolling schedules and air cooling or accelerated cooling. Mechanical properties equivalent to those of an API 5L X-70 grade steel were achieved in slabs with content ranges from 0.25 to 0.37 wt% C, 1.31 to 1.53 wt% Mn, 0.082 to 0.095 wt% Nb, and 0.008 to 0.015 wt% Ti. The slabs were processed by using a controlled rolled and accelerated cooling schedule.