[Abstract] This paper mainly introduces the application of the S7-400 series PLC system in the walking-beam furnace of hot strip mill at Taiyuan Iron & Steel Co., Ltd. It highlights the advantages of the system in practical applications and the convenience of its operating system. A brief description of the system's network structure is also provided. [Keywords] Walking-beam furnace, PROFIBUS, ET200, MIP network I. Introduction to the Furnace Process The main function of the walking-beam furnace in the hot strip mill is to feed cold steel billets into the furnace for heating, bringing the steel to the temperature required for rolling. The entire heating furnace consists of the furnace body, feeding roller conveyors (A1-A8) (where A3 is the slab weighing roller conveyor and A1 is the slab length measuring roller conveyor), in-furnace walking beams, discharge roller conveyors (C1, C3, C5, D4, D5), and intermediate roller conveyors (C2, C4, C5, C6, D1, D2). These components work together to complete the entire process of billet loading, walking, and unloading. The proper operation of the heating furnace directly affects the normal operation of the entire rolling line. To align with technological advancements and maximize automation, we adopted the Siemens S7-400 system to control the heating furnace equipment during the upgrade. Below are two photos of the heating furnace. Figure A shows the loading of cold billets into the furnace, and Figure B shows the unloading of heated billets from the furnace: [align=center] Figure A[/align] [align=center] Figure B[/align] II. Production Process Overview Slabs stored in the slab bay are placed on the loading roller conveyor in the order arranged according to the production plan using a semi-gantry crane. They are then sent to the weighing roller conveyor for weight and grade verification. Qualified slabs are sent to the slab length measuring roller conveyor for length measurement. After length measurement, they are sent to the four heating furnaces respectively via the furnace front loading roller conveyor according to the production plan. The slabs are accurately positioned at the designated furnace front position on the furnace front loading roller conveyor, and then advanced into the furnace by a pusher or loading machine via a slide or direct lifting. They are pushed onto the fixed beams inside the heating furnace, and then the walking beams lift the slabs, allowing them to be heated in a step-by-step manner within the furnace. To ensure the slabs are accurately positioned on the fixed beams and that the slab gaps are correct, a laser position detector is installed at the furnace inlet to monitor the slab's position. Under normal circumstances, the walking beam transports the slabs within the furnace in a "positive cycle" operation, meaning the slabs move towards the discharge end. The slabs are heated by stepping along the walking beams of the heating furnace, passing through each section. When the slab, heated to the specified temperature, is detected by the laser position detector at the furnace discharge end, the walking beam stops after completing the current positive cycle. Upon receiving a steel demand signal from the rolling mill, the slab ejector places the slab on the center line of the discharge roller conveyor, and then transports it to the front roller conveyor of the rolling mill for rolling. III. Control System Composition We use an S7400 416 PLC, which possesses powerful logic and floating-point calculation capabilities, rich addressing modes, a complete communication protocol with the host computer, and comprehensive mathematical function functions. It can quickly respond to signals from sensors, smoothly transporting the steel billets within the furnace and avoiding the possibility of impacts from the mechanical mechanisms. It reliably realizes manual, semi-automatic, and automatic operation of the heating furnace equipment. Each heating furnace is configured with two PLCs, one for drive control and the other for combustion control, based on the controlled objects and functions. Each PLC consists of a main frame and a remote ET200 substation. The main frame is equipped with an Ethernet card for communication with the secondary computer and HMI server. Due to the large number of signal exchanges between the drive PLCs and the common PLC, and to reduce the communication load on the Ethernet while ensuring real-time signal transmission, the drive PLCs communicate with the common PLC via an MPI network. Given the large number of remote devices, some of which are geographically concentrated, the structure of the CPU communicating with the remote ET200 via PROFIBUS ensures both rapid overall system response and significantly reduces wiring costs, making it a highly cost-effective solution. [align=center]The common PLC hardware configuration diagram is shown in Figure 1[/align] IV. Connection between PLC and Drive An MPI network connection is used between the PLC and the drive. Global variables are used for communication between the drive and the PLC. The communication method of global variables relies more on the hardware configuration and does not require separate programming. The communication variables can be directly referenced, and the hardware configuration of global variables can be downloaded online without restarting the PLC. Furthermore, the network connection eliminates the original point-to-point hardwired connection, reducing cable usage and the number of potential failure points. V. Interlocking Conditions for Walking Beam Furnace Operation 1. Pusher: The pusher is in its original position; the B roller conveyor stops and the slab is positioned; the walking beam is at its lower limit; the charging furnace door is open; at least 3/4 of the slab's width is pushed onto the fixed beam; CMD10, CMD12, CMD14, CMD16, CMD01, and CMD02 must detect the presence of steel before the pusher can operate automatically. The necessary condition for manual operation is that the walking beam is at its origin. This operation is mainly used for charging empty furnaces. 2. Charging Furnace Door: When the charging furnace door is working normally, both furnace doors open and close simultaneously; when the pusher pushes steel, the pusher can only push steel when the furnace door is opened to its upper limit; the furnace door closes after the pusher returns to the standby position; the charging furnace door can only open automatically when the walking beam returns to its lower limit, i.e., the origin; the charging furnace door can be manually operated after the above interlocks are released. 3. Walking Beam: The walking beam (WB) is interlocked with the laser detector. When the slab advances to the blocking LD1, LD2, or LD3, the WB stops and waits, or steps in place, or rises and waits after completing the current positive cycle. The WB is interlocked with the charging furnace door; the WB is only allowed to operate when the charging furnace door is closed. The WB is interlocked with the pusher; the WB cannot start if the pusher is not in the standby position. When continuously charging the furnace, if after the WB completes one cycle, the distance between the slab and the previous slab after the pusher's maximum stroke has delivered the slab into the furnace is still greater than 50mm, then the WB cannot start in the next cycle and will return to the origin or step. If it is only discharging and not loading, the interlock must be released before it can operate. The WB is interlocked with the discharge furnace door; the discharge furnace door is only allowed to operate when the WB is in the origin position. The WB is interlocked with the ejector; the WB cannot start if the ejector is not in the standby position. Stroke control of the furnace bottom lifting mechanism: When the furnace bottom lifting mechanism approaches its upper or lower limit, a signal is sent by the lifting position detector, causing the drive motor to be de-energized and the brake to stop when it reaches the upper or lower limit. The acceleration, deceleration, and stroke control of the WB furnace bottom horizontal mechanism: The horizontal movement of the furnace bottom is accomplished by a horizontal hydraulic cylinder. The acceleration, deceleration, and stroke of the horizontal movement are achieved by the acceleration/deceleration signals sent by the horizontal position detector through a hydraulic proportional system. The acceleration/deceleration time can be controlled by data input from a computer. Each step forward of the walking beam is transmitted to the computer by a pulse generator to achieve automatic tracking of the slab within the furnace. 4. Discharge furnace door: The discharge furnace door is only allowed to open automatically after the walking beam returns to its origin. The two furnace doors operate synchronously during normal operation; therefore, both furnace door position control switches must be open (or closed) to confirm the door is in position. When the tapping machine is tapping steel, the tapping machine support rod is only allowed to move forward when the furnace door is half-open or fully open. After the tapping machine completes its tapping task and returns to the standby position, the furnace door closes. The tapping furnace door can be manually operated after the above interlock is released. 5. Tapping Machine: There are heated slabs in the heating furnace, and the slabs block the laser detector; the rolling mill needs steel. The tapping machine and the walking beam cannot operate simultaneously. The discharge rollers stop, and there are no slabs on the rollers; before the discharge rod moves into the furnace, it confirms that the furnace door is half-open or fully open and sends a signal. WB is at the origin, and double discharge first exits from the rolling mill side, then from the non-rolling mill side. The tapping machine is operated by operator room No. 3. VI. Conclusion Since its implementation in the hot rolling mill's heating furnace, the S7-400 PLC control system has demonstrated unprecedented automation. From slab feeding, roller conveyor operation, data tracking, steel loading, to steel unloading, the entire process is automated, eliminating human error, improving equipment accuracy, and reducing operator workload. In terms of maintenance, it significantly reduces maintenance workload, improves equipment performance, and facilitates convenient online parameter modification, allowing for timely adjustments to interlock protection and action sequences based on actual site conditions. Practical experience has proven that the S7-400 PLC control system provides a reliable guarantee for the stable operation of the walking beam heating furnace.