Abstract: This paper introduces the hardware composition, configuration, control strategy, and control functions of the cooling water control system of Xinggang No. 1 billet continuous casting machine. It plays a crucial role in producing qualified continuously cast billets.
Keywords: Automatic control of continuous casting cooling water
1. Overview
Continuous casting is a complex production process characterized by multiple variables, nonlinearity, large time lag, and strong coupling. During continuous casting, the cooling and solidification of the billet is controlled by heat transfer changes related to the casting speed. The billet cooling process consists of two parts: primary cooling control and secondary cooling control. Primary cooling is the cooling of the crystallizer, while secondary cooling is divided into multiple cooling zones, each containing multiple cooling loops. The water spray control of each loop is determined based on process parameters such as casting speed, molten steel superheat, steel grade, and cooling water temperature. This paper focuses on the design and practice of the cooling water control system for the No. 1 square billet continuous casting machine at Xinggang Steel in Hebei Province.
2. Hardware design of the control system
The No. 1 billet continuous casting machine at Hebei Xinggang Steel is a 3-machine, 3-strand continuous casting machine. The hardware configuration of the continuous casting machine's automation system is shown in Figure 1. The cooling water control system is equipped with three sets of SIEMENS S7-400 PLC systems to independently control the cooling water for each strand. The PLC system includes a CPU module, a power supply module, and corresponding input/output modules. The CPU uses a Siemens S7-400 series CPU. Each strand's cooling water PLC system includes a master station, three slave stations, a fieldbus, and a high-speed industrial Ethernet. The fieldbus network connects to three slave stations: a remote I/O slave station on the 4.5-meter platform (No. 1), a remote I/O slave station on the 4.5-meter platform (No. 2), and a remote I/O slave station on the 10-meter platform (No. 3). The master station's high-speed industrial Ethernet is connected to the high-speed industrial Ethernet of the HMI (Human-Machine Interface) system for data communication. Each slave station control cabinet is installed near the cooling water distribution room to save on cabling and achieve low-cost automation.
Figure 1: Hardware configuration diagram of the continuous casting machine automation system
3. Software configuration and control software algorithm
3.1 System Software Configuration
The computer operating system uses Windows XP Chinese operating system. The PLC control system development software uses Siemens Step7 V5.4 programming software. The basic automation HMI monitoring system development uses Siemens WinCC 6.2 development version and runtime software, WinCC 6.2 server and redundant options.
3.2 Typical Algorithms for Cooling Water Control Software
Firstly, a modular program design is proposed, in which different functional modules are designed as low-coupling functional blocks (FBs) that are called multiple times by the corresponding program. This reduces the error rate of program execution and improves program reliability. At the same time, it improves the maintainability of the program and extends its life cycle.
3.2.1 Interpolation Algorithm
An interpolation algorithm is used for processing the secondary cooling water meters: Assuming the real-valued function f(x) on the interval [a, b] has values f[x0], ..., f(xn) at n+1 distinct points x0, x1...xn in that interval, the value of f(x) at a certain point in [a, b] is estimated. Multiple water meters are retrieved using a single function block.
3.2.2 Smoothing PID Algorithm
PID control is an effective method for dynamic quality correction of continuous systems. PID controllers, which control based on the proportional (P), integral (I), and derivative (D) of the deviation, have independent control parameters, and parameter selection is simple and effective. In the switching of various control modes of continuous casting cooling water, the PID setpoint is used to automatically track the measured value for a smooth transition, thereby achieving disturbance-free switching and stable control transition of the system.
4. Control Functions
The function of the cooling water PLC system is to set and adjust the parameters of each process flow in the continuous casting machine, monitor the parameters and equipment status, and display alarms. The cooling water system for each flow flow includes: crystallizer cooling water, secondary cooling water, closed-loop equipment cooling water, open-loop equipment water, and cutting and granulation water.
4.1 Crystallizer Cooling Water Control
The cooling water volume in the crystallizer is primarily determined by preventing steel leakage and reducing surface defects in the cast billet. Excessive water volume can cause cracks in the billet; insufficient water volume will result in a thin billet shell, leading to leakage. The water flow rate within the crystallizer must be sufficient to remove heat from the copper walls, preventing heat accumulation and subsequent temperature rise. If the copper wall temperature exceeds 100°C, the water will boil, causing water bubbles to cover the copper plate surface and scale buildup, thus worsening heat transfer within the crystallizer.
Water entering through the lower inlet pipe of the crystallizer flows at high speed around the crystallizer, carrying away heat, and then flows out through the upper pipe. The temperature difference between the inlet and outlet water is generally 4–8℃. Each crystallizer has a wide-face return water flow regulating valve and a narrow-face return water flow regulating valve. The flow regulation of each loop is controlled by a cooling PLC. Crystallizer cooling water detection includes measuring the temperature (temperature difference) and pressure of the inlet and outlet crystallizers, as well as the flow rate and valve position detection. The field loop control diagram is shown in Figure 2.
Figure 2. Crystallizer cooling water control circuit diagram
The loop control method is as follows:
1) The crystallizer cooling water control mode is divided into two modes: manual and automatic. You can select the mode on the HMI screen.
2) In manual mode, the PLC directly controls the valve opening value set on the HMI.
3) In automatic mode, the PLC adjusts the circuit control according to the set flow value of the control system.
4) Manual shutdown. In maintenance mode, the inlet water shut-off valve and return water shut-off valve can be manually opened or closed via the HMI screen.
5) Minimum flow rate setting control. Regardless of whether it is manual or automatic, the flow rate setpoint of the regulating valve must not be lower than the minimum flow rate setting value.
4.2 Secondary Cooling Water Control
The control of the secondary cooling water plays a crucial role in ensuring uniform cooling of the billet, improving its surface quality, reducing temperature drop, and achieving hot delivery. The quality of the secondary cooling water commissioning directly affects the system control accuracy, and consequently, the quality of the billet.
The secondary cooling system of the No. 1 billet continuous casting machine is divided into 5 zones, each with two flow control valves (one wide and one narrow), totaling 10 flow control valves for secondary cooling water. Flow regulation in each loop is controlled by a cooling PLC. Secondary cooling water monitoring includes measuring secondary cooling water temperature, pressure, and flow rate, as well as measuring secondary cooling air pressure and valve position. Secondary cooling water control modes include L2, L1, and operator-controlled automatic modes. The secondary cooling control loop diagram is shown in Figure 3.
Figure 3 Secondary cooling control loop diagram
The control method for the secondary cooling circuit is as follows:
1) The secondary cooling water control mode is divided into two modes: manual and automatic. You can select the mode on the HMI screen.
2) Manual opening control. After selecting the HMI screen mode, the opening degree of the regulating valve can be set on the HMI screen. The PLC directly controls the opening degree of the regulating valve according to the set value to realize open-loop control.
3) Automatic Flow Control. On the HMI screen, selecting the automatic mode completes the closed-loop flow control function. The closed-loop circuit adjusts the opening of the regulating valve to ensure the actual flow value matches the set flow value. Automatic flow control has three modes: Level 2 cooling dynamic model (L2), Level 1 water meter (L1), and Operator Setting (OP).
In L2 mode, the flow setpoints for the control valves in each zone are provided by a Level 2 computer.
In L1 mode, the flow setting value of each zone regulating valve is calculated by the Level 1 PLC based on the Level 1 water meter and the actual pulling speed. Up to 10 water meters can be set on the HMI screen.
In OP mode, the flow setting values of the control valves in each zone are manually entered by the operator on the HMI screen.
The loop control logic diagram is shown in Figure 4.
Figure 4. Control logic diagram of secondary cooling water
4.3 Air conditioning control for aerosol cooling
The air-water mixed cooling system has advantages such as high water flow density, good water droplet atomization, large cooling area, small temperature fluctuation range of the billet surface, and low cooling water consumption. Therefore, the control of the pressure kinetic energy of the air-water mixed cooling system is very necessary.
The aerosol cooling air acts on zones 2-5 of the secondary cooling system. Each zone has two pressure regulating valves (one wide and one narrow), for a total of eight aerosol cooling air pressure regulating valves. Each air circuit is equipped with air pressure detection. The secondary cooling air control methods include L1 level gas meter, operator, and manual modes. The control logic diagram for aerosol cooling air regulation is shown in Figure 5.
Figure 5 Control logic diagram of aerosol cooling air conditioning
The second method for controlling cold air is as follows:
1) The air conditioning control mode is divided into two modes: manual and automatic. You can select the mode on the HMI screen.
2) Manual opening control. After selecting the HMI screen mode, the opening degree of the regulating valve can be set on the HMI screen. The PLC directly controls the opening degree of the regulating valve according to the set value to realize open-loop control.
3) Automatic Pressure Control. Selecting the automatic mode on the HMI screen enables closed-loop pressure control. The closed-loop control circuit adjusts the opening of the regulating valve to ensure the actual pressure matches the set pressure value. Automatic pressure control has two modes: Level 1 PLC (L1) setting and Operator Setting (OP).
In L1 setting mode, the pressure setting value is calculated based on the gas gauge and the actual amount of cooling water.
In OP mode, the pressure setting is entered by the operator on the HMI screen.
4.4 Closed-loop cooling water control for equipment
The closed-loop water cooling system includes: cooling for the roller conveyor and frame of the sector section, cooling for the roller conveyor and frame of the leveling unit, and cooling water for the cutting machine. The return flow rate of each cooling water circuit is monitored by a flow switch and regulated by a manual valve. The return water temperature of each circuit is monitored, and low flow alarms and high temperature alarms are displayed on the HMI.
4.5 Equipment Open-Circuit Cooling Water Control
The equipment's open-circuit cooling water, also known as external cooling water, is regulated by a manual valve on the external cooling water of the billet discharge area roller conveyor. The water volume is monitored using a flow switch.
5. Conclusion
The cooling water control system for Xinggang No. 1 billet continuous casting machine was put into production in May 2011. To date, the system has been operating well with a 100% automatic operation rate. All performance indicators of the cast billets meet production standards and satisfy user requirements.
About the author: Zhang Yan (1974-), engineer, mainly engaged in the design and integration of automatic control systems.
Tan Zhouhua (1965-) is a senior engineer and chief technology officer, engaged in the design review of electrical and automation engineering projects.
