1 Introduction
The rapid development of steel enterprises in the 21st century has intensified competition, rendering low-end products uncompetitive. This necessitates the development of competitive high-end products. Germany was one of the first countries in the world to adopt RH refining technology, which emerged as early as the late 1950s. RH stands for the first letters of the two German manufacturers using this technology at the time. Currently, there are over 100 RH refining furnaces worldwide. RH refining technology has been widely adopted in the United States, Japan, and Western Europe. Steelmaking production lines face increasingly stringent requirements for cost control and production pace, while rolling mills demand higher quality steel, including stricter temperature control and higher carbon content. The RH (Ruhrstahl Heraeus) system is a secondary refining process equipment used to produce high-quality steel, widely applied in decarburization, deoxidation, degassing, temperature increase, composition adjustment, and ultra-low carbon steel smelting. Process control technology has evolved through two stages: manual control and automatic control. During the automatic control period, the process control system has gone through three development stages: the distributed control stage, the centralized control stage and the distributed control stage [13]. The RH vacuum treatment automatic control system has evolved from the relay logic control system and electric unit combination instrument system in the 1960s and 1970s to the current PLC network control system. The RH molten steel vacuum treatment adopts a two-level computer control system. Basic automation includes electrical control and instrumentation control, which is a PLC control system that combines electrical and instrumentation; secondary automation uses a Pentium PC to compile mathematical models to realize process control. With the continuous development of industrial automation technology, the emergence of enterprise internal information networks, client/server mode and fieldbus technology has also affected the structure of RH vacuum treatment automation and computers. Now the RH vacuum treatment automatic control system adopts a PLC system, and fieldbus technology is used at the field control level. Computer network technology is used at the basic automation level and process control management level. In terms of transmission system, fully digital thyristor rectifier devices and fully digital AC inverter devices have completely replaced the original analog control AC/DC power supply devices. In terms of field detection instruments, intelligent instruments with fieldbus communication capabilities have completely replaced the original analog detection instruments.
2. Basic process of RH vacuum treatment
The RH vacuum treatment process flow is shown in the figure below:
Figure 1 RH vacuum treatment production process
The RH system is a secondary refining process equipment used to produce high-quality steel. The entire metallurgical reaction takes place in a vacuum tank lined with refractory bricks. The interior of the vacuum tank is lined with refractory bricks to ensure that it will not be burned through by the high-temperature molten steel. The lower part of the vacuum tank has two immersion tubes for insertion into the molten steel; the inner walls of the immersion tubes are lined with refractory bricks. A hot-bent pipe is installed at the upper part of the vacuum tank. The extracted gases are discharged into the atmosphere through the hot-bent pipe at the top of the vacuum tank and a gas cooler.
Before the molten steel is processed, the immersion tubes are first immersed in the ladle of steel to be processed. As the vacuum pump continuously removes gas from the vacuum tank, the atmospheric pressure on the surface of the molten steel decreases, and the pressure difference between the inside and outside of the vacuum tank forces the molten steel to flow from the immersion tubes into the vacuum tank. The two immersion tubes at the bottom of the vacuum tank are the riser and the downcomer, respectively. During the steel processing, the vacuum circulation system continuously blows argon gas into the riser, creating a higher static pressure difference relative to the downcomer. This difference forces the molten steel to enter the vacuum tank from the riser and then flow out from the downcomer under its own gravity, thus continuously circulating the molten steel. Under the action of the vacuum pump, gases such as argon, hydrogen, and carbon monoxide in the molten steel in the vacuum tank are continuously extracted, keeping the vacuum tank in a state of constant vacuum. At the same time, the molten steel in the vacuum tank undergoes a series of chemical reactions during circulation, such as dehydrogenation, deoxidation, and heating reactions. Usually, in order to meet the requirements of a certain steel grade and to achieve precise control of the steel composition, RH also incorporates an alloying process during the circulating degassing process.
3. Theoretical Analysis of Control Systems
In RH vacuum processing, metallurgical reactions are primarily based on thermodynamic and kinetic theories. During vacuum metallurgy, the vacuum has a significant impact on reactions such as degassing, carbon deoxidation, and decarburization.
3.1 Principle of molten steel circulation
The main equipment of the RH (Refrigerant Refractory) system is a cylindrical vacuum chamber, 7-10 meters high. At the bottom are two refractory tubes, one called the riser and the other the downcomer, both referred to as insertion tubes, which are inserted into the molten steel during processing. When the vacuum chamber is evacuated, the molten steel rises approximately 1.4 meters under atmospheric pressure and enters the vacuum chamber through the insertion tubes. Argon gas is introduced into the riser; upon entering the molten steel, the argon gas expands upon heating, pushing the liquid level up. This causes the molten steel level on the riser side of the vacuum chamber to be significantly higher than on the downcomer side. The molten steel flows towards the downcomer side, then flows back to the ladle, thus creating a circulating flow of molten steel. The number of cycles (cycle factor C) is determined by the following formula:
———Circulation Flow
— Processing capacity
t — Degassing time
The circulation flow rate is determined by the following formula:
---------Diameter of the riser pipe
----------Diameter of the downcomer
G-----------Flow rate of driving gas
H---------- Length of the insertion tube
3.2 Generation Control Method
A programmable logic controller (PLC) is a computer specifically designed and manufactured for industrial applications. It features abundant input/output interfaces and strong driving capabilities. However, PLC products are not tailored to any particular industrial application. In practical applications, the hardware must be selected and configured according to actual needs, and the software must be designed and programmed according to control requirements. Essentially, it is a computer specifically designed for industrial control, and its hardware structure is basically the same as that of a microcomputer, as shown in the figure.
Figure 2 Hardware System Diagram
4. Architecture Design
The RH control system consists of a two-level control system: a primary basic automation system and a secondary process computer control system. The two levels of systems perform different functions:
I. Basic Automation System (Level 1): Manages the entire RH production process, consisting of PLCs (including remote I/O distributed across various control panels), OMS (Human Machine Interface), engineering workstations, programming stations, and an industrial Ethernet network. It collects field signals and performs basic functions such as logical sequence control of electrical equipment, process loop control, equipment operation, equipment monitoring, and alarms.
II. Process Computer Control System (Level 2): This is a relatively independent system based on the Level 1 system. It manages and optimizes the entire RH system's production process and provides interfaces for the establishment of more advanced management information systems for the steel plant and the Panzhihua Iron and Steel ERP system. See the diagram below for the control system structure:
Figure 3 Control System Structure
The RH system's control is divided into two parts: basic automation control (L1) and process computer control (L2). Level 1 control is primarily responsible for basic automation control, including control of field actuators, parameter feedback, and AC drive control of motors, mainly implemented using Siemens industrial control computer systems and Siemens PLC systems. Level 2 control is responsible for data model building, dynamic model control, production planning and process tracking, and reporting systems, mainly implemented using industrial Ethernet.
All electrical and instrumentation equipment is housed in a separate control cabinet, with signals connected to the I/O modules of the process control unit. All solenoid valves and motor controls are controlled via relay outputs on the PLC or auxiliary relays. The required number of I/O units is determined by the number of drives, instruments, and other electrical components. Therefore, the number of I/O units is determined by the program structure, taking into account all required drive and control functions, including a 20% redundancy capacity. A 30% redundancy capacity is provided for memory. At least one empty slot is reserved per rack. Except for sequential control and interlocking, all critical tasks such as monitoring, management, and process value control will be implemented in the basic automation system (Level 1) CPU.
The RH basic control system is primarily used for real-time monitoring and control of the steelmaking process. It implements functions such as acquiring field signals, data processing, logical judgment, and controlling field equipment. The control software in the PLC is implemented using Siemens' Step7 V5.4 programming tool. The screen monitoring software uses WinCC 6.2, providing a robust HMI for monitoring and controlling the production process, as well as archiving and further processing production data.
4.1 PID Control for Circulating Gas Flow Regulation
The challenge of PID control of circulating gas flow lies in ensuring that the total circulating gas flow rate set by the operator is distributed to the regulating valves on the four branch pipes through PID program control, while simultaneously feeding back the output to the regulating valves for further control. After careful study, we adopted a cascade PID control method in actual production. PID control mainly includes three parameters: setpoint SV, feedback value PV, and output value MV. SV is compared with PV to obtain the value of MV. The total circulating gas flow rate set by the operator is divided by 4, and the SV value is then evenly distributed to the PID controllers on the four branch pipes. The process values of the flow meters collected on the four branch pipes are used as the PV value, and the output of each PID controller is the MV value, controlling their respective regulating valves. However, in actual production, we found that when occasional blockages occur in individual pipelines, the total circulating gas volume in the vacuum tank differs significantly from the setpoint, resulting in very poor circulation. Some improvements were made to the control system. The main controller is a virtual controller that does not directly control the field equipment. The total circulating gas flow rate set by the operator is the SV value of the main controller. The sum of the gas flow rates collected from the flow transmitters on the four branches is the PV value of the main controller. These two values are compared to obtain the MV value on the main pipeline. This MV value is divided by 4 to obtain the SV value of the PID controller on the four branches. This SV value is then compared with the feedback value of the flow transmitter on that branch to perform PID regulation control of the flow regulating valve on that branch.
Figure 4 RH circulating current regulation control loop
5 System Testing and Application
The L1 subsystem consists of two Siemens PLC control stations, two HMI servers, and two client machines that can share power, electronic, and electrical components. The servers and client machines are equipped with WinCC monitoring software for controlling and monitoring the status of field devices. One engineering station is used for L1 software development and system maintenance, and it is equipped with Step7 programming software. Two L1 machines are used for alarm reporting and programming/maintenance printers. Communication between the master and substations is via PROFIBUS, and communication between the control station and the server, and between the server and client machines, is via Ethernet.
(1) System software testing
Since the server, client, and engineering station all use the Windows XP operating system, and the server and client have WinCC monitoring software installed, while the engineering station has Step7 programming software installed, after installing the above software, we will conduct various operation experiments in Windows XP, WinCC, and Step7 respectively to check whether the system software is running normally.
(2) Communication function test: By downloading the PLC control program and observing the changes in the device's actions and status on the monitoring screen, we can determine whether the communication between the host computer and the PLC is normal. By operating the screen, we can determine whether the communication between the engineer station and the host computer is normal. By checking the indicator lights on the CPU, power supply, communication module and I/O module, we can determine whether the communication between the master station and the substation is normal.
(3) Digital output signal test
Using the forced function provided by Step7 software, force each output point and observe whether the indicator lights of the corresponding points on the template are lit. If the field equipment is controlled by a relay, observe whether the relay is engaged. Also, contact the field personnel to confirm whether the equipment is operating normally. This way, you can test whether the output of each output point on the digital output template is normal, and also check whether the wiring of the field equipment is normal.
6. Conclusion
This paper provides an in-depth analysis and study of the control requirements, control theory, control level, control system structure, and control functions of the newly built automatic control system for RH vacuum treatment. The selection of the automatic control system for RH vacuum treatment followed the principles of advanced technology, reliability, practicality, and cost-effectiveness; the equipment selection and level of equipment reached the current advanced level. This work has certain reference value for the selection of automatic control systems in future new and renovation projects of Panzhihua Iron and Steel Group.
