Design and analysis of PLC control system in metallurgical continuous casting electrical system

　　Abstract: the production efficiency of continuous caster and slab quality are directly affected by the automation level of metallurgical continuous casting production. The application of PLC control system greatly improves the automation level of metallurgical continuous casting electrical control system, and ensures the precision of metallurgical industry and the quality of castings. Therefore, it is necessary to strengthen the application of PLC control system in metallurgical continuous casting electrical system, constantly optimize its design, give full play to the function and role of PLC control system, and improve the overall level of metallurgical continuous casting electrical control.

　　Continuous casting process is the intermediate link between steelmaking and steel rolling, which can convert liquid steel into billets, so the continuous casting process directly affects the metallurgical steel rolling quality and yield. With the emergence of PLC control system, this system has been used in metallurgical continuous casting electrical at present, which has improved the continuous casting technology and greatly improved the automatic control level of continuous casting machine.

　　1. Main composition and functions of PLC control system

　　1.1 basic composition of PLC control system

　　The s7-300/400 system of Siemens uses PLC control system. Its mature hardware technology and software system make the data more convenient. At present, PLC control system has been used in the field of metallurgical automation. Its intelligent system is a simulation of human thinking activities. As a technology with a wide range of design, PLC control system integrates many disciplines, and the system is complex, so that the intelligent control process of metallurgical automation system can be replaced by it.

　　1.2 configuration of PLC control system

　　The PLC control system consists of 2 sets of PLC cabinets, 2 control stations, 1 engineer station and 1 network printer. The hardware configuration of the system is to set 2 control stations to prevent emergency control failure and ensure the reliability of the system. Two sets of control stations are used on the host and auxiliary machines of metallurgical automation production. The fault-tolerant Ethernet interface module, S7-400 control processor, power supply module, redundancy module and i/o module constitute its main components. Intel CPU 2.8GhZ is the main configuration of two control stations and one engineer station. It uses Windows XP SP3 operating system and 19lcd display, with 4GB RAM storage capacity. The software configuration includes the programming software step 7 v5.5 SP2 of the control station, the monitoring software fix 6.15 running version of two control stations, and the monitoring software fix 6.15 developed version of one engineer station.

　　1.3 functions of PLC control system

　　The application of PLC control system needs to initialize the data processing system first. Usually, the system needs to be restarted for initialization after the control system is put into operation, after the precision spot check or after the system fails. This process first needs to read the status information and condition information of process equipment, and display the process flow chart and equipment operation status. After all timers and timers are cleared, set the equipment operation mode to "manual" mode, and then the operator "manual" operation to specify the mode and status of each equipment. Under the condition that the current state of the equipment is normal and the operation conditions are met, the operator sets the operation mode and switches the program operation from "manual" mode to "automatic" mode. In addition, the operation site shall also be selected. The "remote control / near control" permission selection switch shall be set on the local operation box, and the operation site of the equipment to be controlled on the PLC shall be set to the "near control" position. The operation indicator light and operation button on the local operation box shall be used for the start (start) and stop (close) operations of the field equipment. During the "remote control" operation, it is necessary to set a selection switch for remote operation at the PLC control station in the central control room. For the fault occurred during the operation of the PLC control system, another control station will send an alarm signal in time and print the fault report. Generally, PLC fault consists of communication fault, CPU fault and redundant board fault. At this time, the system will automatically switch to redundant equipment without interference, and the operation station will send an alarm signal. When non redundant board fault occurs, professionals will confirm and eliminate it.
 

　　2. Application of PLC automatic control system in metallurgical continuous casting

　　2.1 control of ladle turret

　　The ladle turret is driven to rotate by the motor and rotates from the rising position to the falling position after positioning. The ladle turret of the metallurgical continuous caster needs to carry two ladles. During the casting process, it is necessary to switch between the casting and to be cast states. In this process, the pressure on the turret is large, and the turret will fluctuate under the long-term rotation state. Therefore, in order to improve the efficiency of the rotation process, it is necessary to accurately control the rotation speed and time of the turntable to prevent the rotation time from being too long. The rotation angle shall be accurately controlled during the rotation of the ladle turret, and the rotation angle shall be detected by the pulse encoder. For the detection of pulse encoder, two synchronous switches must be installed next to the ladle turret to clear the pulse and do a good job of detection. If the ladle turret rotates to the casting position, it can be stopped by PLC control coil.

　　2.2 equipment cooling water control

　　During the operation of metallurgical continuous caster, the water flow shall be strictly controlled, including the water flow temperature, pressure and flow rate of equipment cooling water. The water pump pressurization cooling water system can supply water to the continuous caster equipment, cool the water, and pressurize it again after the cooling tower is cooled. The water pump shall pressurize the water flow from the cooling tower, the PLC control system shall detect the pressure and temperature of the pressurized water flow, and the cooling water supplied to the caster equipment must meet the requirements. The temperature and flow of water flow can be controlled through the PLC control system before the water from the continuous caster enters the next circulating process.

　　2.3 sliding nozzle control

　　The liquid metal in the ladle flows into the tundish through the sliding nozzle, so the opening and closing state of the ladle must be controlled to ensure the accuracy of the nozzle flow and effectively prevent safety accidents. Moreover, the signal for detecting the liquid level of the tundish needs to use the weighing sensor, which should be used to set the program and process in combination with the signal detected by the PLC control system. This calculation result provides a basis for controlling the movement of the sliding nozzle of the ladle.
 

　　3. Design strategy of PLC control system in metallurgical continuous casting

　　3.1 tension straightening hydraulic station

　　The automatic operation of the tension straightening hydraulic station requires the selection of the working oil pump. When the system pressure is lower than 5.5mpa, the working oil pump will be started. After 1 minute delay, the solenoid valve will be powered on and the system will start to supply oil. If the system pressure is greater than 7MPa, the solenoid valve loses power and the oil pump will stop supplying oil. Therefore, the low level signal of the oil tank can be cascaded into the control circuit of the oil pump and solenoid valve, so that in case of low level due to serious leakage of the pipeline, the system will automatically close the oil pump and solenoid valve after a time delay, so as to ensure the safety of the system equipment under abnormal conditions. The tension straightening hydraulic station can give an audible and visual alarm, and display the alarm information in the monitoring screen of the upper computer.

　　3.2 automatic ingot feeding and drawing process

　　The functions of the automatic ingot feeding and drawing process are to run / stop the tension leveler, the dummy bar storage motor, and automatically lift / press the pull-down roll and the stripper roll. The automatic ingot feeding and drawing process can ensure the safe operation of the continuous casting production process and effectively avoid huge economic losses and personal safety. Therefore, the dual control of limit switch and time interlock shall be designed in the program of automatic ingot feeding to ensure that the equipment control is completed by the limit switch under normal conditions. In case of limit switch failure, the control is carried out by the timer, so as to provide guarantee for the safety and reliability of control operation.

　　3.3 automatic slab cutting during slab cutting

　　There are two working modes: full-automatic and semi-automatic. The length signal sent by the length measuring instrument is the cutting command of full-automatic mode. When the slab reaches a fixed length, the semi-automatic cutting mode will send the cutting command artificially. Once the cutting instruction is received, the cutting machine will immediately start cutting the billet, and the clamping cylinder solenoid valve and the preheating gas, preheating oxygen and cutting oxygen solenoid valves will be powered on respectively. After the slab is cut off, the return cylinder solenoid valve is powered on, all other solenoid valves will be powered off, and the cutter will also return to the initial position for automatic cutting of the slab.

　　3.4 pusher master order

　　The push and return contacts of the controller control the working stroke of the pusher. The control design of this system is for its safety interlock. When the step-by-step turnover cooling bed is working, the pusher cannot push the steel, and the pusher cannot turn the steel at the same time, so that the pusher will not conflict with the steel tipper in the process of turning the steel. Generally, after the pusher completes a steel pushing cycle, the action blocking of the steel tipper shall be unlocked, and then the steel tipper shall continue the next steel tipping process.

　　3.5 step type turnover cooling bed

　　In order to shorten the storage interval of the slab on the cooling bed, the reverse step-by-step turnover cooling bed can be used, and the slab conveying uses the forward rotation of the cooling bed. The forward and reverse transfer points of the master controller installed on the reducer shaft control the forward, reverse and stop actions of the cooling bed. This control task accurately controls the contacts of the master controller so that the cooling bed can stop at the lower gear position when it stops, because if the cooling bed stops at the upper gear position, it will hinder the pushing operation of the pusher. During the automatic operation of the cooling bed, the cooling bed can be continuously rotated forward only after the return action of the pusher in the design program for a period of time. Because the action of the pusher immediately after the return of the cooling bed is easy to collide with the pusher and damage the equipment. Therefore, in order to ensure continuous forward rotation of the cooling bed for 8 weeks and complete the transportation of up to 8 billets at a time, the calculation of the forward rotation action time of the cooling bed should be carried out in combination with the technical parameters such as the motor speed of the cooling bed, the reduction ratio of the reducer and the reduction ratio of the master controller. Through this program design, the closing action of the master contact can immediately stop the cooling bed. When controlling the cooling bed to stop, the inertia of the motor will not immediately open the master contact, so as to ensure the normal operation of the cooling bed without failure to stop.
 

　　4. Conclusion

　　Industry affects the level of economic construction in our country. The application of PLC control system in continuous casting machine in metallurgical enterprises is conducive to the unification of social, economic and ecological benefits. Therefore, it is necessary to realize the importance of using PLC control system in metallurgical continuous casting electrical system, constantly optimize the design of PLC system, and promote the automation of metallurgical continuous casting electrical system.