Abstract: Based on the research and assimilation of the process and control technology of the all-hydrogen bell-type annealing furnace, the control system of the Panzhihua Iron and Steel Group's all-hydrogen bell-type annealing furnace was upgraded and developed. A two-level architecture of basic automation system and process computer was adopted and connected to the production management system, realizing the automation and informatization of the all-hydrogen bell-type furnace annealing process. Keywords: all-hydrogen bell-type furnace; annealing; automatic control 1 Introduction The cold rolling bell-type furnace of Panzhihua Iron and Steel Group is an all-hydrogen bell-type annealing furnace designed and manufactured by LOI Company of Germany in the early 1990s. Its control system adopts the early S5-115U control system, and spare parts cannot be purchased; the original system cannot support the current production management system, so the control system must be upgraded to solve the problem of control system replacement and realize the informatization of bell-type furnace production. The all-hydrogen bell-type furnace is a bell-type annealing equipment with a high level of annealing technology. It has the advantages of high production efficiency, excellent annealed product quality, and low medium and energy consumption. It is an important piece of equipment for improving and enhancing the surface quality of cold-rolled products, especially cold-rolled strip steel, and has been widely used in the production of cold-rolled products. Based on the assimilation of process and control technologies, Panzhihua Iron and Steel Information Company successfully upgraded the control system of the original all-hydrogen bell-type annealing furnace. This automatic control system targets the annealing temperature curve to achieve fuzzy temperature control during the annealing process. Operational results show that the upgrade was successful, not only reducing the failure purging rate but also improving the productivity of the all-hydrogen bell-type furnace. It also integrates the bell-type furnace production system into the Panzhihua Iron and Steel Group's production management system, realizing the automation and informatization of the all-hydrogen bell-type furnace annealing process. 2. Brief Description of the Bell-Type Furnace Annealing Process The all-hydrogen bell-type furnace equipment includes a furnace platform, inner bell, heating bell, and cooling bell. The process is as follows: The furnace platform is loaded with material; the inner bell is placed and tightened using a hydraulic device; the sealing of the H2 valve and the inner bell is checked to ensure system safety; the air inside the inner bell is purged with nitrogen to reduce the oxygen content to below 1%, preparing for the introduction of hydrogen; a heating bell is placed outside the inner bell, with the combustion zone between the inner bell and the heating bell being purged with air to remove residual gas from the combustion zone. When the oxygen content inside the inner shroud is below 1%, heating and ignition begin; hydrogen is used to replace the nitrogen in the inner shroud to achieve a full hydrogen atmosphere annealing process; hydrogen flow is controlled and purged during the heating and soaking stages of the annealing process; a heat seal test is performed; the heating shroud is cooled; the heating shroud/heat dissipation/cooling shroud is removed; the cooling shroud is cooled, and water spray is used for cooling; the hydrogen inside the furnace is purged with nitrogen; and the steel coil is hoisted out of the furnace. 3. Control System Design Based on the characteristics of the full hydrogen bell-type furnace process and the need for automatic control, the automation system architecture adopts a two-level control system and a three-level network system. The two-level control system consists of a process control computer (L2) system and a basic automation (L1) system; the three-level network consists of a process control level Ethernet, a basic automation (L1) level Ethernet, and a device level fieldbus. Each distributed I/O station is connected to the controller via a PROFIBUS bus. The automation system configuration diagram is shown in Figure 1. Based on the optimal cost-performance ratio, a Siemens S7 series PLC system was selected as the basic automation system. The control system is designed as a system where one PLC controls two all-hydrogen bell-type annealing furnaces. The system includes three distributed I/O stations (furnace valve station ET200M, heating hood ET200M, and cooling hood ET200M), a fieldbus, an operator panel, and a monitoring and display system. Two servers, two front-end data acquisition units, one development station, and two operator stations are set up on an L1 high-speed Ethernet network. The two servers are equipped with two 3COM general-purpose Ethernet cards. One card communicates with the front-end computers, and the other exchanges data with the monitoring operator station, completing the exchange and transmission of monitoring data in a server-client manner. One of the two front-end data acquisition computers operates normally, while the other is on hot standby with redundancy and is equipped with dual network cards. One network card communicates with 21 PLCs, and the other exchanges data with the monitoring operator station and the database computer. The historical database server primarily provides historical data storage and access, while the information server mainly provides access to real-time data and facilitates data communication between the operation screen and the PLC. The monitoring software uses Wonderware's IAS version, operating in a client/server mode, fully meeting the requirements for process monitoring, alarm messages, historical data archiving, real-time data, and annealing curve display for the 39-seat bell-type cold rolling furnace. The L2 system includes a data server, one operator station, and one maintenance station, primarily responsible for historical data storage, annealing curve management, and connection with the cold rolling L3 system. The automatic control system boasts good openness, employing a distributed I/O control scheme, which improves the reliability and anti-interference capability of the control system while reducing its cost, achieving a rational configuration. The control system has the following characteristics: (1) High reliability: It adopts distributed I/O, and the PLC, communication network and distributed I/O are electrically isolated from each other. It can completely prevent the failure of any device from affecting the normal operation of other devices in the network. It can be disconnected from or connected to the network at any time without affecting the operation of the network system. The signal connection line is short, the signal is processed locally, and the interference introduced is small. The distributed power supply of the modules can also effectively reduce the interference introduced by the power supply or the system failure caused by the power supply failure. (2) Good flexibility: When the signal needs to be added due to the change of process, the new signal can be connected to the nearby distributed I/O cabinet, which is convenient for construction. (3) Significant economic benefits: Compared with the centralized control scheme, the distributed I/O control scheme can save a lot of cable investment. All signals that need to enter the PLC cabinet can be connected to the distributed I/O cabinet on site. Only one power line and one communication network cable are needed from the distributed I/O cabinet to the PLC cabinet. The installation materials and manpower required for laying cables can also be greatly reduced. The PLC configuration of this control system effectively utilizes the functions and advantages of the PLC itself, reduces the system cost, and improves the scalability, reliability and practicality of the system. (4) The system has good development potential. The selection of hardware equipment and software and network configuration of the automated system follow the principles of strong universality and good openness, which facilitates future software development, porting, system upgrades and hardware expansion. 4 Control functions The annealing process of the all-hydrogen hood annealing furnace is based on the steel grade and specifications of the steel coil. It receives the annealing temperature curve from the process computer as the control target and completes the interlocking control and loop control of the entire annealing process, including automatic ignition control, combustion process control, operation of cooling fan, frequency conversion control of circulating fan, furnace temperature and pressure monitoring and control, nitrogen and hydrogen flow control, cooling water system control, operation of various valves, and emergency purging control. The control functions complete the following according to the process: (1) Control of inner hood setting and inner hood clamping and loosening device. (2) Conduct cold sealing test of H2 valve and furnace inner hood system. (3) Control of H2 and N2 purging process and detection of H2 and N2 flow. (4) Setting up and igniting the heating hood, heating and homogenizing during the annealing process, and hot sealing test of the furnace platform. (5) Temperature regulation during the cooling process. (6) Monitoring of various process variables, including furnace platform temperature measurement, cooling water flow and temperature monitoring, inner hood pressure monitoring, inner hood pressure safety device monitoring, control temperature measurement, O2 content measurement, nitrogen and hydrogen flow monitoring, and monitoring and control of related processes. (7) Detection and alarm of various faults during the annealing process, and annealing treatment under special circumstances according to the corresponding situation. (8) Complete communication with the process computer, receive various data such as annealing temperature curves from the process computer (including various set values, such as annealing time setting, annealing temperature setting, purging time setting, purging amount setting, etc.), and transmit process monitoring status information and fault information from PLC to the process computer. (9) Receive production data sent by the production management system and upload actual production data. (10) Manual operation function for testing and maintenance operations. 5. Implementation of Key Control Functions 5.1 Furnace Temperature Fuzzy Adaptive Control The heating program allows for temperature control in up to 16 time periods. The start and end temperatures for each time period can be set via the process computer, the OP control panel, or the monitoring screen. There are 12 heating burners arranged in two layers, with the bottom burner used for waste hydrogen combustion. Each burner is equipped with its own ignition device and flame monitoring system. Before ignition, the combustion space is purged with maximum combustion air for 5 minutes. Then, ignition occurs, and all 12 burners are controlled. During the heating phase, the top row of burners is controlled cyclically, while the bottom row is controlled continuously. After heating, during the heat preservation phase, the bottom burners are controlled cyclically, and the top burners are turned off. If the bottom burners cannot maintain the set temperature, the top burners will re-enter the control. The entire temperature control automatically implements fuzzy switching control of the 12 nozzles based on the temperature curve. 5.2 Automatic Speed ​​Control of the Circulating Fan in the Furnace: The circulating fan in the all-hydrogen bell-type furnace adopts stepless frequency conversion speed regulation control. Based on the heating conditions and the steel grade and specifications of the steel coil, the speed is controlled according to a pre-selected curve of circulating fan speed versus start time and start temperature. This ensures that the fan speed matches the specific gravity and temperature of the atmosphere. Due to high thermal efficiency, the annealing cycle is significantly shortened, by 30%–40% compared to traditional bell-type furnaces, resulting in a substantial reduction in energy consumption and electricity consumption. 5.3 Automatic H2 Flow Control: Hydrogen flow control is achieved by opening and closing a servo motor, controlled according to a pre-selected curve of hydrogen flow rate versus start time and start temperature. Similar to the heating program, hydrogen flow control allows for a maximum of 16 time periods for flow control. The flow rate, time, and start temperature for each time period can be set via the process computer, the OP operation panel, or the monitoring screen. 5.4 Emergency Purging Control For safety reasons, the all-hydrogen bell-type furnace will generate nitrogen emergency purging when the furnace pressure is lower than 2.5 mbar, the oxygen content in the furnace is higher than 1%, or the high temperature sealing test fails. After emergency purging, annealing or cooling can only continue in N2 atmosphere, which prolongs the annealing cycle and seriously affects the surface quality of the strip steel. 5.5 Handling of Abnormal Situations Emergency purging caused by emergency power outage, pressure switch false alarm, or failure of high temperature sealing test (confirmed not to be caused by leakage) can be replaced with H2 after purging. The specific operation method is as follows: (1) For emergency purging of the furnace platform during the heating stage caused by the above reasons, after confirming that there is no leakage, the following operations are performed: ① Turn off the heating hood; ② Put the furnace platform in the basic state; ③ Start N2 pre-purging, and re-ignite after the end; ④ Reset the annealing time, hydrogen flow rate and time after ignition. The annealing time is the total annealing time minus the actual annealing time plus 1-2 hours. The purging time is adjusted accordingly based on the annealing time. (2) If the high-temperature sealing test fails and it is confirmed that the leak is not the cause, the following operations can be performed: ① Place the furnace platform in the basic state; ② Start the N2 pre-purging and restart the ignition after it is completed; ③ Reset the heating and H2 purging parameters. The H2 purging flow rate is 30 m3/h and the time is 30 min. (3) For emergency purging that occurs during the cooling stage, after it is completed, the H2 inlet is forcibly opened, the N2 inlet is closed, the N2 outlet is opened, and the outlet is closed after 20 min. 6 Conclusion The upgrade and transformation of the Panzhihua Iron and Steel Group's cold rolling all-hydrogen bell furnace control system was put into production in October 2008. It has been running for 2 months and the system is running well so far. The automatic operation rate has reached 100%. The various performance indicators of the coils have reached the production standards and meet the user requirements.