The final two decades of the 20th century saw the birth

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1 BOF process control in the 21st century: latest developments and retrofit/upgrade options The latest developments in sublance-based BOF process control, including slag sampling and phosphorous measurement fully automatic at the push of a single button all help to reduce energy use, converter additions and tap-to-tap times. Retrofitting or upgrading systems in existing plants enable the same benefits to be achieved as in greenfield plants. Authors: Bart van den Berg, Edo Engel, Peter Gootjes and Guido van Hattum Danieli Corus The final two decades of the 20th century saw the birth of sublance-based process control for BOF steelmaking plants. As this method gained acceptance and became standard practice in most of the world, BOF process control systems were also developed to offer new measurement possibilities, process monitoring tools and increased levels of process automation. r Fig 1 Process control concept AUTOMATIC STEELMAKING Since basic oxygen steelmaking is a rapid and complex process, good process control is essential to achieving high plant output and quality, and minimising costs. Many improvements to measurement techniques and process monitoring tools have been made over recent decades. The amount of process-related information currently available to the operator, both online and at intervals, enables the highest hitting rates and substantially reduced tap-totap times for maximum plant output to be achieved, as well as cost reduction through optimised use of converter additions, scrap, etc. The Automatic Steelmaking System developed by Danieli Corus consists of a set of hardware and software components that can be implemented individually or in combination. After the initial installation, a system can be upgraded with additional modules. The system integrates all available tools and combines them within the BOF Static Dynamic Process Model, which essentially forms the heart of the system. All information from the plant level 1 equipment, as well as the individual process monitoring and control tools, is fed to the model, which is a decision support system as well as automation system for the steelmaking process. The system integrates operation and exchange of information with plant systems, from raw material ordering all the way up to, for example, the plant s enterprise resource planning and manufacturing execution systems (see Figure 1). 50

2 STEELMAKING AND CASTING r Fig 2 Probability to safely start tapping the finished heat and remaining under the maximum allowed P level The model was first developed at the IJmuiden steel plant and further improved during implementation in other plants worldwide. It has been improved for over 30 years now and can be fine-tuned to any plant. The model consistently achieves heats within the predetermined target window for temperature and carbon content using a high level of plant automation. With this level of automation, energy consumption, tap-to-tap times and additives consumption are further reduced. The system has reached a level of sophistication that allows for fully computerised operation, such that after the hot metal and scrap have been charged (in automatic mode), the operator can start the heat with one click of a mouse. Oxygen lance control, converter material additions systems and all process control equipment can work together in computer mode for the entire heat, until it is ready for tapping with no further human intervention required. After tapping, ladle stirring additions and wire feeders can also be operated automatically. More than 30 Static Dynamic Process Control systems, capable of running multiple oxygen steelmaking converters in full automatic mode, have been installed so far. THE SAFE TAPPING DECISION SUPPORT SYSTEM Although BOF process control and automation have reached such levels of sophistication that it has become possible to fully automate the BOF process from charge to tap, some aspects may be more attractive to control semiautomatically with the information provided by a decision support system. This option is becoming increasingly attractive now that critical parameters with respect to liquid steel quality can be measured online which in the past could only be measured by taking a sample and waiting for laboratory analysis. A great benefit for the operator in the converter control room in deciding whether to start tapping or not is the addition of a decision support system. Rather than showing the measured data on the operator screen and letting the operator interpret the data, process conditions are presented through a newly developed concept called Safe Tapping, and developed with a solid knowledge base that was accumulated over more than 40 years of process control in BOF steelmaking. This is a graphical information tool that informs the operator through a multi-colour graph whether or not it is safe to start tapping. After each measurement, a marker will be shown on the Safe Tapping operator screen. An example for phosphorus is shown in Figure 2. When a marker is in the green area, the probability is that the phosphorous content is within limits sufficient to start tapping; when the marker is in the yellow area, the a 51

3 r Fig 3 Phosphorous content determined by sublance TSO versus laboratory analysis of steel sample r Fig 4 Probe with provision for slag sampling system recommends waiting for laboratory results; when the marker is in the red area, the heat needs additional processing. The Safe Tapping decision support system was built initially for phosphorous measurement, but can be customised to include any set of process parameters (such as carbon content and bath temperature windows) based on operator requirements regarded as essential for tapping the liquid steel. This gives the operator full control of the BOF process at high levels of confidence. For the first implementation of this decision support system, model results and actual laboratory results were compared, demonstrating that out of a total of 1,094 heats, only 20 received green tapping advice with respect to P, whereas laboratory results showed that the system should have recommended waiting for laboratory analysis. This represents a 1.8% false-positive result only. LATEST DEVELOPMENTS IN BOF PROCESS CONTROL Online phosphorous measurement Controlling P levels during the steelmaking process has always been difficult. With the increased use of high P iron ores, the measurement of P at end point in BOF steelmaking continues to be a subject of interest because of its impact on steel quality. BOF slag is complex in nature and contains several oxides like CaO, SiO 2, P 2 O 5, MgO, MnO, and FeO. Towards the end of blow, the kinetics of different reactions slows down such that slag composition approaches a kind of pseudo-equilibrium with the metal. Thus, application of thermodynamic models alone for direct estimation of P distribution does not yield sufficiently reliable information for a start tapping decision. Over a decade ago, the first attempts were made to use signals from the end-of-blow sublance measurement for P determination; three years ago Danieli Corus took up the challenge to go beyond existing technologies and develop real-time P measurement. Key aspects of the measurement, such as the oxygen sensor, the measurement cycle, data recording and data processing, were subsequently optimised for P determination. This was done under two strict conditions: that existing sublance performance for carbon and end temperature control should not deteriorate, and that during plant trials, the P measurement should not influence normal steelmaking operation in any way. Because the normal measurement cycle only focuses on the measurement in the steel bath, a new cycle was needed that also takes into account the measurement from the slag, since slag composition also needs to be known. Existing hardware was improved to allow for measurements in both the steel bath and slag layer. The standard oxygen sensor, as applied for carbon and temperature measurement of the steel, had to be 52

4 STEELMAKING AND CASTING improved to give a stable oxygen activity signal in the slag. Through a series of hardware modifications to the sublance data processing computer, it was possible to measure simultaneously the oxygen level of two oxygen sensors: one for steel and one for slag. Because of this configuration, no disturbance of normal sublance measurements occurs and production is not affected by the development work. The most complex step was to convert the process data into information and to find a suitable way of presenting the measured P content. The first step was to extract key components of a full-size level 2 process control model. These components were combined into a miniature process model, focused on calculating phosphorous. This P model was ported to run on a process computer, allowing it to calculate the phosphorous from sublance probe measurement data in real time. First results of the development work were an optimised sublance measurement cycle and an improved oxygen sensor. By adding oxygen activity from the sublance end of blow measurement and real-time level 2 process data (such as hot metal composition and material additions), P can be determined in real time. The results from field trials are shown in Figure 3. It should be mentioned that no alterations were made to normal converter operation to achieve more favourable conditions for P measurements. Also, no precautions were taken with respect to converter addition material quality, quantities or other process control parameters. It was shown that it was possible to limit the error to 20ppm up to a P content in the steel bath of 300ppm. P-lab-TSO sample is the result of the lab analysis of the sample in the TSO probe; P-measured-DIRC is the measurement indicated by the DIRC measurement computer. DIRC VI Recently, a new version of the DIRC measurement computer was developed. This integrates a flat, front-panel PC with touch screen in a single housing, and is substantially more compact than that of its predecessor, DIRC V. It allows for web browser-based access from, eg, desktop computers elsewhere in the plant or from tablets carried by operating personnel. The more modern equipment used for DIRC VI offers enhanced flexibility with respect to future additions to the system. Currently, radio frequency identification (RFID) reader signals can be processed for remote monitoring of the correct use and lifetime of equipment, such as the sublance body, as well as probe identification. SLAG SAMPLING To improve working conditions for the converter bay operator, two types of slag sampling probes have been developed. To r Fig 5 Revised, counterweighted arrangement avoid the risks of manual slag sampling, Danieli Corus now offers a TSO-type probe containing a small slag box as well as a dedicated slag probe that will take a slag sample at any given time during the blowing process. This probe is a great improvement for operational personnel in terms of health and safety aspects, as well as a more consistent way of taking slag samples (see Figure 4). UPGRADES AND RETROFITS Highly advanced BOF process control systems have now become commonplace. Most of the constituent equipment and systems are relatively easy to retrofit, replace or upgrade, but the sublance and probe attaching equipment requires closer investigation. These need space, but in some cases space may be scarce and, in addition, the units have to operate in exceptionally harsh environments. Worldwide, it is noted that most available systems are vulnerable in the harsh environment above the converter hood. The Danieli Corus system has proven its mechanical robustness with trouble-free operations for more than 30 years. When reviewing both greenfield and existing BOF plant designs, conditions may be found that are sub-optimal or even prohibitive towards implementing sublancebased process control. Plant layouts may be designed for different systems or may not have design provisions for such systems at all. Danieli Corus has successfully worked towards designs for sublances and probe attaching equipment in a number of such cases, three of which will be highlighted here. Replacement of existing system Danieli Corus was contracted to replace three existing sublance systems at the a 53

5 PANEL "C " 18 0 r Fig 6 Sublance arrangement and movement at converter No t converters at the No. 1 steelmaking plant of Shagang Iron & Steel (Group) Co., Ltd. These replaced three existing systems of European origin, the performance of which had been unsatisfactory for the client. During the project, it was found that the existing probe-attaching robot had lost its ability to attach probes and that the existing sublance had lost most of its working speed. Shagang already used Danieli Corus systems on other converters which were implemented earlier, and with excellent results. eliminated, so are the transverse movements of hoses and cables. The straightforward movements and reduced weight allow for a drastic reduction of the measurement cycle duration by around 25 seconds. Whether or not this design can implemented in other steel plants depends on space restrictions in each specific case. The latest generation of measurement computer, DIRC VI, and phosphorous determination, were also included in the upgrade, which allows for future additions to the measurement system. New mechanical arrangement In India, seven sublance systems for JSW Steel s two basic oxygen steel plants in Toranagallu, were successfully commissioned. The first three were implemented at BOF plant No. 1; the remaining four at BOF plant No. 2. The client s desire to have easy access to the oxygen lance introduced additional space constraints for the installation of the sublances. To accommodate this, the design was revised and made more compact. The winch platform found with traditional slewing type sublances could be eliminated by basing the design on a vertical lifting movement, leading to a substantially lower weight for the entire system (see Figure 5). The counterweighted arrangement, similar to that of oil well pumps or classic drawbridges, gives the system greatly improved mechanical stability. Since slewing is Space restrictions At an existing BOF plant in North America, Danieli Corus was asked to replace the Bethlehem-type sublance systems with modern design units. Converters 1 and 2 were positioned in-line and were interconnected. Relatively conventional arrangements based on side-shift cars for moving the sublance into position could be used, albeit with modifications to the plant. The third converter, however, was positioned as a stand-alone unit and space restrictions were exceptionally stringent. During the feasibility study phase, a three-dimensional laser scan of the entire BOF plant was made. Given the nature of this equipment, scans took a long time to make and, in order to eliminate the risk of interference between the plant equipment and the scanning equipment, these 54

6 STEELMAKING AND CASTING activities were carefully planned during time windows offered by plant maintenance. The results of the scans were modelled and reviewed, leading to a list of design alterations for the plant: ` In a number of locations, steel structures had to be cut and modified ` Existing piping had to be re-routed to allow for the implementation of an automatic probe charger ` Some platforms had to be modified ` One of the platforms had to be replaced by a flip-up platform in order to allow for the sublance movements ` Oxygen lances were found to be located slightly offcentre, imposing further space restrictions that had to be overcome during the design phase This review process and the plant surveys that had to be executed before the sublance systems could be designed, were time-consuming. The resulting boundaries for the design required substantial creativity of the system designers, but eventually systems could be designed for all three converters whereas previously, there was serious doubt among the plant operators whether this could be achieved. A parallelogram arrangement was designed for vessel 3 in order to allow for the implementation of a new sublance (see Figure 6). CONCLUSIONS The competitive environment in which a BOF plant operates, demands optimised performance that can only be achieved with high levels of process control and process automation. A wide variety of process control hardware can be integrated into a single process control system that allows for automatic steelmaking: operating the converter in full computer mode. Also, new breakthrough developments such as online phosphorous measurement now allow for further performance enhancement of the BOF shop. The Safe Tapping Decision Support System adds greatly to the usability of modern process control systems, providing the operator with clear and concise advice with respect to process parameters for more effective decision making. In cases where existing equipment needs to be replaced or where no control systems are in place, serious restrictions may apply. However, based on comprehensive site surveys and innovative mechanical design, solutions can be found to overcome these restrictions and state-of-the-art process control can be implemented. MS Bart van den Berg, Edo Engel, Peter Gootjes and Guido van Hattum are with Danieli Corus, Ijmuiden, The Netherlands. CONTACT: comms.office@danieli-corus.com