1. Introduction The temperature control system of the continuous hot-dip galvanizing annealing furnace uses two solenoid valves, one large and one small, to open and close, allowing gas to enter the radiant tubes for combustion. This only provides two control modes: high and low fire. Gas flow cannot be adjusted; it can only be regulated through manual adjustment valves on each gas branch pipe and each radiant branch pipe (a total of 294 valves). This leads to the following problems during production: ① Manual valve opening during production results in slow furnace temperature adjustment, failing to meet production demands; ② The annealing furnace area has a harsh environment, making on-site manual adjustment highly dangerous and labor-intensive; ③ The mixed gas supplied by Baosteel currently has a calorific value of 3000-4000 kcal, which is not only far higher than the original annealing furnace design requirement of 2000 kcal + 10%, but also has a large fluctuation range, making temperature control of the annealing furnace extremely difficult. The lagging furnace temperature control leads to unstable galvanized sheet quality; ④ High gas consumption and low thermal energy utilization increase production costs, and a large amount of excess gas burns in the flue gas pipeline, causing equipment damage. This increases equipment maintenance and product costs. These problems can be completely solved through modification. 2. Modification Plan Through analysis and discussion, a modification plan was proposed: The original design of the continuous hot-dip galvanizing annealing furnace included 10 DN100 gas branch pipe one-way manual valves, which were replaced with DN100 electric regulating valves; an electrical control system was established to realize electric regulation and remote control functions; the cooling fan at the end of the annealing furnace was changed from a conventional motor to a frequency converter motor. 2.1 System Introduction The continuous hot-dip galvanizing annealing furnace electrical control system consists of one host computer, one programmer, one Siemens S7-300 system, and five S7200 PLCs. The host computer is an Advantech industrial computer, using a Siemens CP5611 communication card, which is powerful. The programmer is a laptop computer, and the PLC programming software is STEP7V5.3. The human-machine interface software is WINCC V6.0, and the system's Chinese platform is Windows 2000. The PLC uses MPI (Multi-Point Interface) to form a master-slave control mode, with the S7-300 as the master station. Data transmission between the central rack and the expansion rack is realized through the interface template IM365. Five sets of S7-200 PLCs form five slave stations. The connection between the PLC and the PROFIBUS-DP network is established by the CP343 communication template, forming a complete fieldbus control system. As shown in Figure 1, 2.2, the hardware configuration of the newly added PLC slave station is as follows: power supply module PS307, 5A; interface template IM153 1; analog input module SM331AI 8x1281T; analog output module SM332AO 4x1281T; PROFIBUS bus connector 6ES7972 0BA12---OXA0. The newly added PLC station was configured as the 6th slave station of the original system through hardware configuration. An electric regulating valve control function block was written in the PLC program. Through analog input and output modules, the electric regulating valve's opening degree from 0 to 100% was adjusted and monitored. The program is shown in Figure 2. Using Siemens WinCC software, a complete HMI software system, and a high-performance information integration and exchange platform, an electric regulating operation control screen was created. Operators could operate the system via mouse on the host computer to adjust and monitor the real-time opening degree of the electric regulating valve. 2.3 Cooling Fan Selection: A Siemens MM440 series frequency converter (37kW) was selected for control. It integrates a built-in Class A EMC filter and a basic operation panel (BOP), with an additional PROFIBUS DP communication module to achieve communication with the PLC (CPU313C 2DP). The electrical control schematic diagram of the variable frequency motor is shown in Figure 3. [ALIGN=CENTER] [/ALIGN] 3. Retrofit Effect After the annealing furnace system was retrofitted and put into operation, tracking and checking various data during use showed that it had good overall benefits. It can monitor the opening status of the gas regulating valve in real time, and adjust the gas flow rate accordingly based on changes in the calorific value of the gas and the required furnace temperature, thereby improving the gas utilization rate and effectively reducing production costs. The stable furnace temperature control improves the product yield (from the original 76% to 92%). It also improves the working environment for operators, ensures equipment safety and civilized production, reduces the workload of maintenance personnel, and provides a guarantee for production.