Design and Implementation of Ferroalloy Charging Control System for No. 2 Slab LF Furnace of Panzhihua Iron and Steel Group

[Abstract] This paper introduces the composition, configuration, and control functions of the ferroalloy charging control system for the No. 2 slab LF furnace of Panzhihua Iron and Steel Group. It plays a positive role in improving weighing accuracy, shortening charging time, and ensuring the quality of molten steel.
[Keywords] Automatic control of alloy charging in LF furnace
1 Introduction
The LF furnace, also known as a ladle refining furnace, is used to improve the quality of molten steel by heating, fine-tuning the alloy composition, desulfurizing, deoxidizing, slagging, controlling inclusions, and homogenizing the temperature. It is a crucial production link between steelmaking and continuous casting processes, coordinating the production rhythms of converters and continuous casting machines, and is increasingly widely used in steel mills. The precise addition of controllable amounts of ferroalloys and slagging materials to the molten steel, allowing for precise control of its chemical composition, plays a vital role in regulating the quality of the steel. This article uses the No. 2 slab LF furnace of Panzhihua Iron and Steel Group as an example to provide a brief introduction to the control of the automatic ferroalloy feeding system in the LF furnace.
Brief Description of 2LF Furnace Iron Alloy Process
The LF furnace ferroalloy process includes ferroalloy charging, batching, discharging, and feeding. Its process flow is shown in Figure 1.

2.1 Loading: Ferroalloys or slag materials are loaded into 2m³ bottom-opening silos according to their type, transported by truck to the furnace span, and then lifted by the crane in the furnace span and hoisted onto the 10m³ ferroalloy high-level silo.
2.2 Batching: Each ladle of molten steel from the steelmaking converter requires the addition of different quantities and types of alloy materials due to variations in composition and the different steel grades produced. The alloy materials and slag are weighed according to the batching list confirmed by the operators and unloaded from the high-level silo to the weighing hopper via a vibrating feeder. Weighing hopper No. 1 weighs silos #1 to #6 according to the batching list; weighing hopper No. 2 weighs silos #7 to #10 according to the batching list; and weighing hopper No. 3 weighs silos #11 to #15 according to the batching list. The material and weighing hopper parameters are shown in the table below.

Material and Weighing Hopper Parameter Table

2.3 Discharge: Discharge includes normal discharge and emergency discharge. Normal discharge is the process of transporting the pre-mixed material from each weighing hopper to the intermediate hopper of the LF furnace. Under normal batching and production conditions, normal discharge involves starting the equipment sequentially in the reverse direction of material flow. Emergency discharge occurs when batching or production is abnormal, emptying the material from each weighing hopper into the emergency hopper.
2.4 Feeding: Feeding is the process of transporting the material from the intermediate hopper into the LF furnace by controlling the ball valve on the alloy charging gate of the furnace cover and the vibrating feeder of the intermediate hopper.
3 Control System Design
The control system consists of a basic automation system (L1) and a secondary process computer system (L2). The basic automation system includes an automatic control system and an HMI monitoring system, which performs basic control functions such as field data acquisition, process loop control, parameter setting, equipment sequence control, equipment operation monitoring, and alarms. The secondary computer system performs process control functions such as casting planning, casting model calculation, batching model calculation, and steel composition logistics tracking. The two systems work together to control the automatic feeding of LF furnace ferroalloys, achieving accurate batching and automatic feeding functions, and timely monitoring of the type and quantity of materials added. The control system configuration diagram is shown in Figure 2.

Figure 2 Control System Configuration Diagram

The basic automation system control station uses the Siemens S7-400 from Germany, with a CPU414-2DP CPU board, communicating with L2 and higher-level systems via Ethernet. The control station has five ET200 remote I/O stations, communicating via PROFIBUS-DP interfaces. The system programming software uses STEP7, a platform based on Microsoft Windows 2000/XP operating systems, which is easy to install and has a user-friendly interface. The monitoring software uses Wonderware INTOUCH, which is extremely convenient for screen graphic creation, data reading and writing, communication configuration, trend recording, and alarm recording. The system layout is reasonable, and the network operates stably with smooth communication.
The process control computer system consists of one server, one development and maintenance terminal, one operator station, and corresponding Ethernet. The server uses disk array technology to ensure the reliability of system data. The server, development and maintenance terminal, and operator station are installed in the LF furnace operating room and are connected to each other via a star Ethernet network. The secondary computer communicates with the basic automation system through the server. The server communicates with the host computer system via Ethernet.
4 Control Functions
The control of the feeding system includes the control of the feeding system, the control of the batching system, the control of the discharge system, and the control of the feeding system.
4.1 Material Feeding Control
1) Manual feeding: Feed periodically based on the low-level alarm signals of the ferroalloy and slag high-level silos on the operation screen or based on the operator's experience. An ultrasonic level gauge is used to detect the alloy level in the silo via ultrasonic echo positioning. When the level is below 15%, an alarm is displayed on the operation station to remind the operator to replenish the alloy in time.
2) The feeding process should be interlocked with the dust removal valve of the high-level silo.
3) The L2 operation screen displays the names of each silo. If the material type corresponding to the silo changes, the actual material is entered through the L1 operation screen and uploaded to the L2 system. At the same time, the correspondence table between the silo number and the stored material name on the L1 operation screen will be automatically adjusted accordingly.
4.2 Ingredient Control
1) Batching control includes local manual, remote manual, and automatic batching control modes. The local and remote control modes are switched on the local control panel, while the automatic and remote manual control modes are switched on the control room screen.
2) Manual operation at the machine side. Not used for production, but for equipment debugging and maintenance. Debugging and maintenance personnel can manually start and stop each vibrating feeder at the machine side.
3) Manual ingredient preparation
a) On the operation screen, switch the "Operation Mode" to "Manual".
b) The batching system is in manual control mode in the control room. The operator can manually start and stop each vibrating feeder. When approaching the weighing end point, the vibrating feeder can be manually switched to a weak vibration mode to achieve slow feeding.
c) Manual batching is used for minor adjustments to the alloy type after batching or for replenishing alloy in the weighing hopper when production requirements change. Under normal production conditions, batching is done automatically.
4) Automatic batching
a) On the operation screen, switch the "Operation Mode" to "Automatic".
b) Molten steel is sent to the testing and analysis system after temperature measurement and sampling. The system analyzes the steel composition and transmits the results to the L2 level of the process control system via computer. The L2 system considers factors such as the refined steel grade, molten steel temperature, chemical composition, and equipment performance, and uses a mathematical model to calculate the required amount of alloy to be added and the heating time. Pressing the "Request Level 2" button on the screen downloads the alloy and slag to be added to the LF furnace to the basic automation system, which displays the suggested settings in the HMI screen. Operators confirm the suggested settings based on practical experience, or modify the computer's calculation results. In case of process calculation anomalies, operators must manually input the settings on the operation screen.
c) After checking and confirming that the suggested area setpoint data is correct, press the "Confirm" button on the operation screen. The control system will then verify the setpoint, including the following:
 The material level signal of the high-level silo that needs to be unloaded is not "empty".
 The set weight value of each material is greater than the "drop" after the corresponding high-level silo vibrating feeder stops feeding.
The total weight and volume of all materials weighed by a certain weighing hopper must not exceed the weighing range and effective volume of the weighing hopper.
If the setting value verification fails, an alarm should alert the operator. Once the setting value passes verification, the setting value from the suggestion area is sent to the receiving area; settings in the receiving area cannot be modified, only accepted or cleared. After the setting value is transferred from the suggestion area to the receiving area, the setting value in the suggestion area is automatically reset to zero.
d) After the set value verification is successful, the operator triggers the "Automatic Batching Start" button, and the control system checks whether the "batching" prerequisites are met. At the start of computer-controlled batching, it checks whether the electrical system is "ready," the corresponding weighing hopper is empty, and the vibrating feeder under the corresponding weighing hopper is in a stopped state. If the check fails, the batching process stops and an alarm is triggered to the operator; if normal, the "Automatic Batching Start" button is disabled during automatic batching, and the material set value cannot be modified; the control system immediately and automatically starts the vibrating feeder of the corresponding hopper to begin automatic weighing. During automatic batching, the "Automatic Batching Stop" button takes priority; if the "Automatic Batching Stop" button is pressed during automatic batching, the system stops immediately.
e) Materials in different weighing hoppers can be weighed simultaneously. For the same weighing hopper, the weighing is carried out by cumulative weighing. That is, if there are two materials to be weighed in a weighing hopper, they must be weighed in sequence. After one material is weighed, the weighing hopper does not need to be emptied before the weighing of the second material begins immediately. The weights are accumulated.
f) For each weighing hopper, the weighing process is the process of the weighing hopper weight value (the weight signal fed back by the weighing hopper pressure head) approaching the target value. When the weight value differs from the target value by a certain amount (switching to low speed), the "speed change point" from high speed to low speed is reached, and the vibrating feeder operates in a weak vibration state. When the weight value differs from the target weight by a certain amount (drop difference), a stop vibration signal is issued, stopping the vibrating feeder. The material that continues to fall due to inertia (drop difference) reaches the set value, and the weighing of one type of material is completed.
g) After all the set materials have been weighed in sequence, the computer automatic batching process will automatically end.
4.3 Material Discharge Control
1) The material feeding system is equipped with three control modes: "automatic", "remote manual", and "local manual". The switching between "local manual" and "remote" is set on the local control panel, while the switching between "automatic" and "remote manual" is set on the HMI control screen.
2) Remote manual material feeding. When the operation location is remote and the manual mode is selected on the HMI, the material feeding system is in remote manual control mode, and the operator can manually start and stop the material feeding system.
3) Manual operation at the machine. Not used for production, but for equipment debugging and maintenance. Debugging and maintenance personnel can manually start and stop each material feeding system at the machine.
4) The operator judges the actual weighing situation of the weighing hopper and the production situation, and decides to select the "normal discharge" or "emergency discharge" mode on the HMI.
5) Automatic Normal Discharge. When the "Automatic Discharge" mode is selected on the HMI, first confirm that the weighing operation of the weighing hopper has been completed and the vibrating feeder of the intermediate hopper is in a stopped state; the operator presses the "Normal Discharge" button on the HMI, and the "Normal Discharge" and "Accident Discharge" buttons are disabled, realizing automatic normal discharge.
a) Automatic normal material discharge, with belts starting sequentially in the reverse direction of the material flow, following the order of vertical and horizontal belt conveyors, with a delay.
b) Upon receiving the signal that the belt conveyor is running, the vibrating feeder of the weighing hopper with material starts sequentially according to the numbering order of the hoppers, but the weighing hoppers cannot discharge material at the same time.
c) When the weight of the material in the weighing hopper decreases to the "bias value", the vibrating feeder stops after a 5-second delay.
d) After the last weighing bucket vibrating feeder stops vibrating, the horizontal belt conveyor will stop after a 10-second delay.
e) Stop the vertical belt conveyor 20 seconds after the horizontal belt conveyor stops, and the normal material discharge process ends.
6) Automatic Emergency Discharge. When a production anomaly occurs, the material needs to be automatically discharged into the emergency hopper. First, confirm that the weighing operation of the weighing hopper has been completed and that the vibrating feeder of the intermediate hopper is in a stopped state; the operator can select the "Automatic Discharge" mode on the HMI and press the "Emergency Discharge" button. The "Normal Discharge" and "Emergency Discharge" buttons will be disabled, realizing automatic emergency discharge.
a) For automatic emergency discharge, start the equipment in the reverse direction of the material flow by opening the emergency discharge three-way valve and the horizontal belt conveyor.
b) Upon receiving the signal that the belt conveyor is running, the vibrating feeder of the weighing hopper with material starts sequentially according to the numbering order of the hoppers, but the weighing hoppers cannot discharge material at the same time.
c) When the weight of the material in the weighing hopper decreases to the "bias value", the vibrating feeder stops after a 5-second delay.
d) After the last weighing bucket vibrating feeder stops vibrating, the horizontal belt conveyor will stop after a 10-second delay.
e) Close the emergency discharge three-way valve 3 seconds after the horizontal belt conveyor stops, and the emergency discharge process ends.
4.4 Feeding Control
1) Alloy cutting is only permitted when the electrode is in a high position.
2) The feeding system is equipped with three control modes: automatic, remote manual, and on-site manual. The switching point between on-site and remote control is located at the machine, while the switching point between automatic and remote manual control modes is on the HMI operation screen.
3) Remote manual feeding. When the operation location is remote and the manual mode is selected on the HMI, the feeding system is in remote manual control mode, and the operator can manually start and stop the intermediate hopper vibrator and the feeding ball valve.
4) Manual operation at the machine. Not used for production, but for equipment debugging and maintenance. Debugging and maintenance personnel can manually start and stop each feeding system equipment at the machine location.
5) Automatic feeding. After normal material discharge is completed, the operator can press the "Automatic Feeding" button on the HMI to automatically start the feeding control as needed.
a) First, open the nitrogen seal on the charging chute, and after a 2-second delay, open the alloy charging ball valve above the furnace cover.
b) After the alloy feeding is turned on, start the vibrating feeder under the intermediate hopper after a 2-second delay.
c) After the vibrating feeder under the intermediate hopper is started, it will be stopped after a 10-second delay.
d) After the vibrating feeder under the intermediate hopper stops, the alloy feeding gate ball valve on the furnace cover is closed after a 2-second delay, and then the nitrogen seal of the feeding chute is closed after a 2-second delay, thus ending the automatic feeding process.
4.5 Interlocking Processing
1) When a piece of equipment malfunctions, all equipment in front of it (in the opposite direction of material conveying) should stop immediately, while the equipment behind it should continue to operate until the material is empty, in order to avoid material accumulation.
2) The belt conveyor is equipped with a belt misalignment switch, a slippage detector, and a pull rope switch, and their signals must be interlocked with the relevant control equipment.
3) The operation screen and emergency control panel are equipped with an "emergency stop" button, which enables the equipment to be reliably stopped in case of emergency during operation, so as to ensure equipment safety.
5. Conclusion
The ferroalloy charging system for the No. 2 slab LF furnace of Panzhihua Iron & Steel Group Co., Ltd. was put into operation in February 2010. Currently, the system is operating well with a 100% automatic operation rate. The typical batching cycle is 1 minute and 30 seconds, with an accuracy of 1.5%.