Modeling of the BOF Tapping Process: the Reactions in the Ladle
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ORIGINAL RESEARCH ARTICLE Modeling of the BOF Tapping Process: The Reactions in the Ladle DALI YOU, CHRISTIAN BERNHARD, PETER MAYER, JOSEF FASCHING, GERALD KLOESCH, ROMAN RO¨ SSLER, and RAINER AMMER A tapping process model of the steel from the basic oxygen furnace (BOF) addressing the reactions in the ladle is proposed. In the model, the effective equilibrium reaction zone (EERZ) method is applied to describe the steel/slag interfacial reaction. The equilibrium reactions in the bulk steel (steel/inclusion/lining wear) and slag (liquid slag/slag additions/lining wear) are considered. The thermodynamic library—ChemApp is used to perform thermodynamic calculation. The process model includes most of the actions during the tapping process, such as the additions of ferroalloys and slag formers, carryover slag entrapment and air pick-up. After the calibration by the industrial measurements of two plants, the model is applied to study the influence of the amount of carryover slag. https://doi.org/10.1007/s11663-021-02153-2 Ó The Author(s) 2021 I. INTRODUCTION Modeling simulation is an effective method to study the reactions in the steel tapping and refining process. STEEL tapping from the basic oxygen furnace (BOF) Galindo et al.[1] proposed a thermodynamic model on into the ladle is the critical first step in ladle treatment. steel deoxidation processes during tapping and refining. Deoxidation and alloying start during the tapping In the model, the thermodynamic equilibrium of inclu- process, resulting in the generation of the primary sion and metal, inclusion and entrapped slag by steel inclusions and affecting the subsequent inclusion con- and mass balance were considered, while the interfacial trol. The refining slag is formed from the addition of reaction of steel and slag was not accounted for. The slag formers, BOF carryover slag and separated inclu- model predictions suggested that the addition sequence sions during steel tapping. Steelmakers intend to min- of alloys played an important role regarding the amount imize the amount of BOF carryover slag due to its and types of inclusions. A similar model of the tapping negative effects. The chemical composition and qualities process and its applications in deoxidation practice was of the refining slag further influence the steel composi- presented by Cicutti and Capurro.[2] Besides the ther- tion and cleanliness due to the interfacial reaction. The modynamic reactions, the fluid flow of steel and alloy tapping process is a kind of ‘black box’, as information additions and alloy dissolution in the tapping ladle were from samples before the start of ladle refining is rarely studied separately.[3,4] However, comprehensive model- available. To effectively control the steel refining process ing work on the reactions of the tapping process is and achieve high quality steel, it is important and missing. A variety of models of ladle refining processes necessary to track the reactions and composition have been reported, as summarized in the former changes in the steel, slag and inclusions during the publication, which offered valuable references for the tapping process. simulation of the tapping process.[5] The present project aims at modeling the entire steel refining process from liquid steel tapping and refining to the solidification to track the changes in the steel, slag and inclusions. It is believed that the comprehensive model is gaining more importance because each of the DALI YOU and CHRISTIAN BERNHARD are with the metallurgical factors in the process can affect the steel Montanuniversita¨ t Leoben, Franz Josef Straße 18, A-8700 Leoben, quality and production. The basic concept of the project Austria. Contact e-mail: [email protected] PETER MAYER, is to link metallurgical models to a thermodynamic JOSEF FASCHING, and GERALD KLOESCH are with the database. In the former study, microsegregation and voestalpine Stahl Donawitz GmbH, Kerpelystraße 199, A-8700 ¨ inclusion formation during the cooling and solidification Leoben, Austria. ROMAN ROSSLER and RAINER AMMER are [6–9] with the voestalpine Stahl GmbH, voestalpine Straße 3, A-4020 Linz, process of steel were simulated; the ladle refining and Austria. Manuscript submitted January 27, 2021; accepted March 11, 2021. METALLURGICAL AND MATERIALS TRANSACTIONS B Ruhrstahl–Heraeus (RH) models were constructed and The schematic of the present model is displayed in validated through comparing the reported industrial Figure 1. In the tapping process of steel, liquid steel with [5,10] practice. a mass of DmBOFÀst (Eq. [1]) is tapped from the BOF to For the present study, a comprehensive model of the the ladle at each time step (Dt). The tapping flow rate steel tapping process addressing the reactions in the (Vst) is calculated in Eq. [2]. The steel tapping results in ladle was proposed. In the model, thermodynamic turbulent mixing of steel, carryover slag, added slag equilibrium calculations were performed using thermo- formers and alloying additions. The EERZ method is dynamic library—ChemApp.[11] The effective equilib- applied to describe the steel/slag interfacial reaction. rium reaction zone (EERZ) method was applied to The validation of this method for treating the interfacial describe the steel/slag interfacial reaction.[12,13] The reaction has been illustrated by several applications equilibrium reactions in the bulk steel (steel/inclusion/ from different authors.[14–17] In the EERZ method, an lining wear) and slag (liquid slag/slag additions/lining effective equilibrium reaction zone containing both steel wear) are considered. Most of the actions, such as the and slag phases (the region between the dashed lines in additions of ferroalloys and slag formers, carryover Figure 1) is defined. The mass of steel (Dmst) and slag slag entrapment and air pick-up, were included. The (Dmsl) present in the interfacial reaction per time step is model was calibrated by comparing the predicted calculated based on the mass transfer coefficient compositions of the steel and slag with the measure- (kst and ksl), as given in Eqs. [3] and [4]. The mass ments from a total of 10 heats from two plants. The transfer coefficient of steel is an adjustable parameter composition developments of the steel, slag and inclu- and calibrated based on plant measurements. The mass sions of an example case were presented. The influence transfer coefficient of slag is assumed to be one tenth of of the amount of carryover slag was studied using the that of steel.[18] The steel and slag phases return to the calibrated model. bulk phase after the interfacial reaction and approach thermodynamic equilibrium, respectively. As mentioned above, thermodynamic equilibrium calculation is carried II. MODELING out using ChemApp. The inclusions formed are assumed to be in equilibrium state with bulk steel and floated at a Following the concept of the project, the proposed time-independent, constant rate (rfl) to the interface. tapping process modeling work applied ChemApp to The inclusion floating rate is an adjustable parameter link the metallurgical model to the thermodynamic and parametrized by the total oxygen and Al content database of FactSage. FactSage and ChemApp are measured. The mass of the inclusions present in the products of GTT Technologies, Herzogenrath, Ger- interfacial reaction at each time step is calculated by many. The program language of the model is Eq. [5]. The gas phase generated is assumed to be FORTRAN. completely removed from the system. Fig. 1—Schematic of the tapping process model. METALLURGICAL AND MATERIALS TRANSACTIONS B As shown in Figure 1, BOF carryover slag is added to process and measurements. Note that lining wear is the bulk slag during the steel tapping. The total amount added as the main oxide components: Al2O3 is added to of the carryover slag is estimated based on the overall the steel and MgO is added to the slag; other compo- mass balance. The total slag carryover is divided into nents are also considered. The gas phase generated is pre-slag, vortex slag and post slag; the relevant propor- assumed to be out of the system. The temperature in the tions of total carryover slag depend on the tapping system is considered as prescribed according to the practice of a steel plant.[19,20] It is assumed that the process data; the heat is not balanced. pre-slag (mpreÀsl) is added to the ladle before tapping; the DmBOFÀst ¼ VstDt ½1 vortex slag is added in a mass of DmBOFÀsl (Eqs. [6]and [7]) at each calculation step; the post slag (mpostÀsl)is added after the end of tapping. The chemical compo- sition of the BOF slag and the proportion of the partial mBOFÀst Vst ¼ ½2 amount of carryover slag are defined according to the ttap data and experience of the particular plant. Besides the carryover slag, slag formers such as lime and magnesium oxide are presumed to be added to and dissolved in the Dmst ¼ kstAq Dt ½3 bulk slag at a constant dissolution rate. The dissolution st rate for all slag formers (Vsl) is assumed as 1 kg/s. Note that the dissolution rates of the slag formers and alloys are only the first assumptions and will have to be Dmsl ¼ kslAqslDt ½4 parameterized in the future. The mass of the slag former that enters into the system at each time step is calculated by means of Eq. [8]. Dmincl ¼ rflminclDt ½5 For alloy additions, it is assumed that graphite and Al as buoyant alloys float to the surface of the steel after addition and thus, they are added to the steel/slag Dm ¼ V Dt ½6 interfacial zone.[21] The alloys with comparably higher BOFÀsl BOFÀsl density such as FeSi, FeMn and FeCr are added to the bulk steel. During the addition, the alloys can be mvorÀsl oxidized by atmosphere. The recovery rate defines the VBOFÀsl ¼ ½7 ratio of the alloying elements dissolving in the steel ttap (rreÀX) and oxidizing during addition (1 À rreÀX).