An optimal and consistent production process in a BOF

Transcription

1 Steelmaking Computer supported calculation and evaluation of BOF slag composition The addition of the correct quantity of slag formers for optimum metallurgical operation, lining wear and yield is an essential aspect of the BOF process. COAT (control optimisation and analysis tool) is software used to calculate the quantities required and facilitate process improvement through data display and management. Authors: Manfred Bock, Alfred K Louis, Roman Müller and Christian Oehler Saar-Metallwerke GmbH and ISCA Prof. Louis An optimal and consistent production process in a BOF shop is only possible by controlling the metallurgical process within close limits. The complex relation between the addition of slag formers and the steel quality is governed by well-established metallurgical equations, necessitating use of computers to achieve the optimal result in real time. This optimisation is especially important with regard to the constantly increasing demands on the final steel quality. Depending on the quality of the input materials and the lance blowing practice this is very often a difficult task to achieve. The importance of the correct slag composition often receives insufficient consideration, especially in cases when low phosphorous and sulphur contents are required. The slag composition is not only of importance for the metallurgical results, it has also a great influence on converter lining life and steel yield, and hence the total cost of the BOF steel production. METALLURGICAL BACKGROUND The quality of the lime and dolomite used is highly important for the optimisation of the blowing process. The degree of burning, variations in the quantity of fine particles and variations in CaO, MgO, SiO 2 and H 2 O contents, both in the delivery condition and right before charging into the converter, heavily influences process stability and therefore the metallurgical results. The lime and dolomite fines, especially, have a great influence, as particles below 5-6mm are extracted by the off-gas system before reaching the bath surface and so are unavailable for use. Often as much as 30% of lime and dolomite are lost as fines, but unfortunately this loss is not consistent, otherwise it could be easily compensated for by adding correspondingly greater amounts. A separation between fine and coarse particles already takes place when lime and dolomite is charged into the storage bunker by forming a dumping cone. Depending on the ratio of fine and coarse particles extracted from the bunker and conveyed into the converter, varying metallurgical results will be experienced. Hence, in order to achieve a more stable process many steel plants screen the lime just before charging it into the converter storage bunker. COAT: CONTROL OPTIMISATION AND ANALYSIS TOOL This software package is a joint product of Saar Metallwerke GmbH and ISCA Prof. Louis, both located in Saarbrücken, Germany. Saar-Metall is a leading manufacturer of BOF lance tips and all kinds of water cooled copper fittings for metallurgical industries, supplied the metallurgical knowledge fundamental for the software package. ISCA Prof. Louis, a company specialising in mathematical modelling of technological processes and the development of the corresponding software, contributed its know-how in the production of the software package. The software is easy to handle and runs on any PC with Windows XP, although other platforms and their integration into productions systems are possible. Optimisation with COAT The computer model calculates the optimum slag composition, and hence the correct amounts of lime and dolomite to be added. The model considers the chemical composition of the input materials, degree of saturation of the slags and desired metallurgical aim figures. The bases of the calculation are the slag basicity, the corresponding Fe contents in slag in the equilibrium condition and the aim phosphorous content in steel at blow end. The result of this calculation can be easily adapted to existing heat balance calculation software. Furthermore, based on the slag analysis for each heat prior to tapping, the lime loss (lost as fines) is calculated. This figure can then be used for the calculation of the following heat, so that a continuous adaption of the lime loss rate is ensured, providing better computation of required lime and hence stability of the slag composition. a 55

2 r Fig 1 Input screen r Fig 2 Output screen The correct calculation of the amounts of necessary slag formers to be added is a great help for the steel plant operator to minimise the consumption figures and hence the cost of BOF steel making. Because the CaO saturation level in a given blow is unknown to the plant operators, the BOF slag is often either below or above the saturation lines in the ternary CaO-FeO-SiO 2 phase diagram[1,2]. Slags are, therefore, often super- or under-saturated. Undersaturated slag is especially detrimental for the converter lining in cases where the slag formers are also added too late during blowing [3,4]. Super-saturated slag contains an undissolved share of the added slag formers which protect the lining and often leads to converter build-ups. Such slags also have a higher viscosity and lower reactivity, providing no metallurgical advantages or, under curtain circumstances, even disadvantages. Even a saving of just 1kg lime/tls offers a considerable saving potential per year, not only from less added lime, but also through lower iron losses due to smaller slag volumes. COAT also provides a proposal for a well-adapted lance pattern. The lance distance to the steel bath surface is calculated in relation to the Si content of the hot metal (slag volume) and the blowing hardness of the lance (resulting from the nozzle design or the oxygen exit velocity and the number of nozzles). It is clear that lance tips from different suppliers can have different designs for the same application, thus demanding a different lance pattern. The proposal of the software for the lance pattern is optimised for the Saar-Metall lances but can be adapted to the pattern for different lance suppliers when their design is known. The program starts with an introduction screen as shown in Figure 1. Here, for example, the Si contents of the different slag formers are introduced. The final result of the optimisation step is then given as shown in Figure 2 as easy to read results directly usable in the production unit. Evaluation with COAT In order to optimise the slag composition it is essential to continuously survey the slag analysis after each heat. Varying quality of the input materials can result in undesired changes in the process and in the metallurgical results. These figures are provided by an evaluation package within the COAT software. All relevant heat data are stored automatically in a data bank, allowing evaluations to be performed. One possible outcome (Figure 3) shows a relationship between slag basicity and Fe. The evaluation program can be started either to check individual heats or for a larger number of heats in certain time intervals. Furthermore, heats from different shifts can be easily compared with each other. In the case of trial series or test heats a fast and easy evaluation of the results is also possible. The results can be shown either as frequency distributions, including the average figures and deviations, or as specific relationships which are of special 56

3 Steelmaking q Fig 3 Evaluation example, % Fe in slag vs basicity (super-saturated slags) interest to the BOF steel maker. The slag phase equilibrium lines for technical slags are also determined (indicated as a red line in the corresponding figures, see later). These diagrams give immediate information of what has to be done in order to improve the results. Also, slow progressing changes can be detected early, so that countermeasures can be taken in time. Overall, the work of the BOF steel maker is very much facilitated by using the new software. RESULTS ACHIEVED In the following, a few examples from various BOF shops are demonstrated. For each evaluation step two diagrams are shown: one case with a bad result to be improved and a second which comes quite close to the ideal case. Figures 4 and 5 show frequency distributions of the Si content in hot metal to be processed by two BOF shops. Immediately recognisable is shop No.1 where problems are to be expected during blowing, especially if the lance pattern is not properly adapted to the wide range Si content. Figures 6 and 7 show the frequency distribution of the slag Fe contents. Figure 6 indicates a normal or typical distribution, whereas Figure 7 indicates a high average figure and large variation. The lance tip is not responsible for the poor results shown in Figure 7, as is often assumed. The reason is either that the lance pattern is not properly adapted to the hot metal chemistry or a wrong calculation of the addition of the necessary slag formers is made, including basicity, which is set too high. Another reason for high slag Fe is often the slag basicity, as clearly shown in Figure 3 (BOF operating with predominately lime supersaturated slag), and less so in Figure 8 (BOF operating with predominately lime under-saturated slag). In both figures the basicity (CaO/SiO 2 ) is plotted against slag Fe content. In Figure 9, where the plant operates close to the target saturation line (shown in red) the relationship is a r Fig 4 Si distribution in hot metal BOF shop No.1 r Fig 5 Si distribution in hot metal BOF shop No.2 57

4 r Fig 6 Slag Fe distribution, normal r Fig 7 Slag Fe distribution, poor r Fig 8 Slag Fe vs basicity (under-saturated slags) r Fig 9 Slag Fe vs basicity (close to target) more clearly seen. Thus opportunities exist to improve yield by optimising basicity and slag volume. It is sometimes found that the amounts of added lime and dolomite are calculated without regard to important input variables, even the hot metal Si content. Figure 10 indicates an example where no clear relationship is discernible. Even worse, Figure 11 shows an inverse relationship. In contrast, Figure 12 indicates a better working practice for the lime and dolomite addition rates. Despite small exceptions a clear relationship between the addition rates and the Si content is identifiable. In some BOF shops hot metal with high P content has to be processed. In order to avoid problems with dephosphorisation and to obtain the turn down P content required for the steel grade, an exact adjustment of the optimal slag composition is especially important. Figure 13 indicates the relationship between slag P 2 O 5 content and the steel turn down P content. In addition it indicates whether something has to be done with regard to the total slag quantity. A precondition is, of course, that the slag has a suitable composition for dephosphorisation. It is known that many factors determine the P content in steel at the end of blowing. These include the slag composition (Fe content), the amount of slag and its influence on the P 2 O 5 content, the timing of the addition of the slag formers, the time of the formation of a liquid slag, bath movement (bottom stirring) and the tapping temperature. The variations in Figure 13 are the result of all these parameters. The influence of these and other parameters on the P content can be demonstrated with the help of COAT, as far as they are registered in the data bank. Figure 13 just indicates that the P 2 O 5 activity is almost constant from a P 2 O 5 content of 2% and above. In order to obtain the lowest steel P contents, independently of 58

5 Steelmaking r Fig 11 Lime rate; inverse relationship with Si content r Fig 10 Lime addition independent of Si content the slag Fe content and the tapping temperature, P2O5 contents of less than 1.2 % should be obtained. Furthermore a statement about the lime losses can be made, as shown in Figure 14. The given figures are calculated based on the theoretically achievable basicity from the additions made, versus the achieved basicity, analysed from the slag composition. The data shows a mean loss of about 25% CaO. r Fig 12 Relationship between lime and dolomite additions and Si content SUMMARY The COAT software package for the adjustment of an optimal BOF slag composition by calculating the necessary amounts of slag formers to be added, is an ideal aid for the BOF steel plant operator. A simple qualified result evaluation program allows for a continuous survey of the BOF process by the operators and the supervision personal. MS REFERENCES r Fig 13 Relationship between P and P2O5 Manfred Bock is Metallurgical Consultant and Christian Oehler is Director, Sales International, both at Saar-Metallwerke GmbH, Saarbrücken, Germany. Alfred K Louis is responsible for Applied Mathematics and Scientific Computing and Roman Müller is Senior Scientist, both at ISCA-Prof. Louis, Saarbrücken, Germany. r Fig 14 Calculated fines losses of lime and dolomet CONTACT: dr.c.oehler@saarmetall.de [1] KH Obst, E Schürmann, W Münchberg, G Mahn, D Nolle, Arch. Eisenhüttenwesen 51(10) pp (1980). [2] N Bannenberg, Stahl und Eisen 111(6) pp71-76 (1991). [3] N Bannenberg, B Bergmann, H Gaye, Steel Research 63(10) pp (1992). [4] N Bannenberg, H Haege, P Umland, Stahl und Eisen 117(9) pp (1997). 59