Modification design of hydraulic system of special-shaped billet continuous casting and straightening machine

Abstract: An in-depth analysis was conducted on the working principle, design features and existing defects of the original hydraulic system of Maanshan Iron and Steel’s special-shaped billet continuous casting and straightening machine. On this basis, a transformation plan was proposed to use a proportional relief valve to replace the DC motor relief valve to adjust the pressure of the hydraulic system of the tension and straightening machine. It improves the dynamic response speed and equipment reliability of the hydraulic system, while reducing the spare parts procurement cost and shortening the spare parts supply cycle.

Keywords: special-shaped blank; tension and straightening machine; hydraulic system; modification design

In the face of fierce market competition, it has become the consensus of many steel companies to reduce production costs, improve continuous casting ratio and continuous casting billet quality. Among the many factors that affect the quality of the slab, the continuous casting and straightening machine plays the role of guiding and straightening the slab, and is one of the key factors affecting the quality of the slab. The first domestic near-net-shape profile billet continuous casting machine introduced by Maanshan Iron and Steel Company from the United States in the 1990s had the advantages of high automation level, low energy consumption, high yield and small subsequent rolling processing volume. However, during use, it was found that the hydraulic system pressure dynamic response was poor. The key valve DBG DC motor relief valve had a small use range, high spare parts cost, and long procurement cycle. With the advancement of enterprise technology, the original hydraulic system of the tension and leveling machine can no longer meet production requirements and needs to be upgraded.

In recent years, with the continuous advancement of electro-hydraulic proportional technology, the application range of proportional control technology has become wider and wider. After repeated comparisons, it was finally decided to use the DBE proportional relief valve instead of the DBG DC motor relief valve to adjust the pressure of the hydraulic system of the tension and leveling machine. This can improve the dynamic response speed of the hydraulic system and equipment reliability, and reduce the cost of spare parts procurement.

Working principle of the hydraulic system of the front tension leveling machine after modification

Maanshan Iron and Steel No. 3 Steel Rolling Plant has a three-stream near-net-shape beam continuous caster, each stream is equipped with four tension levelers. Each tension and leveling machine is controlled by a hydraulic valve group and a hydraulic cylinder. The hydraulic cylinder drives the tensioning and leveling roller to provide power for feeding the dummy bar, drawing the billet and straightening the billet from arc to level. There are a total of twelve sets of hydraulic valves. The power of the hydraulic system of the tension and leveling machine is provided by the pump source under normal circumstances and by the accumulator group in the event of an accident. Figure 1 is the schematic diagram of the hydraulic system of the tension leveling machine before modification. This loop has two operating modes: normal PLC control and emergency state control.

1. Electromagnetic reversing valve 2. One-way throttle valve 3. DC motor relief valve 4. Hydraulic reversing valve 5. Pilot pressure reducing valve 6. Pilot relief valve 7. Pressure sensor

Figure 1 Principle diagram of the hydraulic system of the front tension leveling machine after modification

Normal PLC control: The tension leveling machine has different pressure values in different modes, and various pressure values are preset on the MMI manually or automatically. Through the PID controller in the PLC, the 24 V pulse signal is output and enters the DC motor relief valve 3. The rotation of the DC motor in the relief valve 3 drives the reducer and cam to rotate to change the tension of the spring. Thereby changing the regulating pressure of the relief valve 3. The relief valve 3 is directly connected to the remote control port of the pilot pressure reducing valve 5 and the pilot relief valve 6, so the pulse signal sent by the PLC can remotely adjust the output pressure value of the pilot pressure reducing valve 5. The pressure value is returned to the PID controller in the PLC through the pressure sensor 7 and compared with the set value. If it does not match the set value, a pulse signal is sent again to adjust the pressure value of the relief valve 3 until the output value of the pilot pressure reducing valve 5 is equal to the set value, forming a simple closed-loop dynamic pressure regulation circuit. Emergency state control: In the power outage state, external compressed air drives a set of pneumatic valve groups. The accident accumulator group provides power to the hydraulic system of the tension and leveling machine through the P pipe, and at the same time, the pressure oil in the accident accumulator group enters the P1 pipe. Under the action of the control oil, the hydraulic reversing valve 4 changes direction, and the remote control ports of the pilot pressure reducing valve 5 and the pilot relief valve 6 are sealed. The pressure oil in the accident accumulator is no longer decompressed by the pilot pressure reducing valve 5 and overflowed by the pilot relief valve 6, but directly acts on the slab with a system pressure of 17.5 MPa until the hot slab is completely cooled. The leakage of the valve is reduced and the pressure holding time is long.

Analysis of the hydraulic system of the tension and leveling machine before modification

The DC motor relief valve changes the traditional manual pressure adjustment to a DC motor pressure adjustment, combining the speed and flexibility of electric control with the advantages of hydraulic transmission. It realizes continuous and remote control, and can prevent the occurrence of pressure shock phenomenon during pressure conversion. It has a simple structure, has low requirements on system cleanliness, and can realize closed-loop pressure control. In the event of a sudden power outage, the original pressure can be maintained unchanged, which is particularly important for continuous casting machines, so it was relatively advanced at the time.

The DC motor relief valve drives the reducer and cam to rotate through the rotation of the motor to change the tension of the spring and adjust the pressure of the relief valve. In this way, it takes 12 s to reduce the pressure of the tension and leveling machine from the system pressure of 17.5 MPa to the working pressure of 2.1 MPa corresponding to a certain specification of continuous casting billet. The adjustment time is long and the dynamic response is poor. When large dimensional changes occur on the end face of the tension and leveling machine, the pressure changes cannot keep up. When the pressure of the upper roller of the tension and leveling machine loses pressure, the pressure exerted by the upper roller on the slab is too small instantly, which may easily cause the slab to move and cause an accident. At the same time, due to the small scope of use of this valve, the production cost is high, the spare parts are expensive, and the delivery cycle is long.

In the event of a power outage, the system pressure of 17.5 MPa is directly applied to the billet until the hot billet is completely cooled. Although the pressure holding time was long, because the pressure was 3 to 8 times higher than the normal working pressure, the slab was severely deformed and could not be pulled out normally from the secondary cooling chamber. The control oil port of the hydraulic reversing valve 4 has been directly connected back to the fuel tank, so that the hydraulic reversing valve 4 does not reverse direction during an accident. The pilot pressure reducing valve 5 and the pilot relief valve 6 work normally under the action of the DC motor relief valve 3, keeping the original working pressure unchanged. Due to the leakage of the pressure reducing valve, although the pressure holding time is reduced, it can still be maintained for a certain period of time, which can meet the working requirements in the event of an accident.

Working principle of hydraulic system of tension and leveling machine after modification

Through the above analysis, the modification of the hydraulic system of the tension and leveling machine must ensure the following three points: ① The pressure dynamic response must be fast; ② When an accident occurs, a certain pressure must be maintained, which can not only suppress the cast slab but not overpress it (that is, the deformation is too large) ;③Reduce costs. To achieve the above three points, use a proportional relief valve to replace the DC motor relief valve, and slightly modify the hydraulic system principle. The principle of the modified hydraulic system of the tension and leveling machine is shown in Figure 2.

1. 4. Electromagnetic reversing valve 2. One-way throttle valve 3. Proportional relief valve 5. Pilot pressure reducing valve 6. Pilot relief valve 7. Pressure sensor 8. Cartridge relief valve 9. Cartridge check valve

Figure 2 Schematic diagram of the hydraulic system of the tension and leveling machine after modification

Proportional valve control is a new control method between the on-off control of ordinary hydraulic valves and servo valve control. Its control accuracy is close to that of servo systems, but its anti-pollution, reliability and economy are closer to ordinary valve control. hydraulic system. In recent years, with the continuous advancement of hydraulic technology, it has become easier for hydraulic oil to meet the NAS7 cleanliness level required by proportional valves. Therefore, using DBE proportional relief valve instead of DBG DC motor relief valve can greatly improve the dynamic response characteristics of pressure. It can also reduce investment costs and maintenance costs. DBE proportional relief valves are relatively low-priced, commonly used valves, and have short supply cycles, making them a better choice. Under normal conditions, the electromagnetic reversing valve 4 is energized, and the remote control ports of the pilot pressure reducing valve 5 and the pilot relief valve 6 are connected to the proportional relief valve 3. The proportional relief valve remotely controls the working pressure of the hydraulic system of the tension and straightening machine. In the event of a sudden power outage, the electromagnetic reversing valve 4 is powered off and reversed. The pilot pressure reducing valve 5 and the pilot relief valve 6 work normally under the action of the cartridge relief valve 8. The accident pressure is controlled by the cartridge relief valve 8 and is set to 6 MPa. It can not only hold the billet down and prevent it from sliding down, but also prevent it from being deformed too much. After an accident, the continuous casting billet can be smoothly pulled out from the secondary cooling chamber, reducing the waste billet and the maintenance time of the continuous casting machine. The back pressure of the cartridge check valve 9 is 0.05 MPa, so that the set pressure of the pilot relief valve 6 is always 0.05 MPa higher than the pressure of the pilot pressure reducing valve 5. In this way, when the upper roller of the tension and leveling machine encounters an increase in pressure caused by external loads such as impurities and oxide scale in the slab, overflow will occur from the pilot relief valve 6 before the pilot pressure reducing valve 5 has reacted. Maintain system pressure stability.

Conclusion

As the level of continuous casting technology continues to improve, the requirements for equipment are also getting higher and higher. Only by continuously improving the level of equipment can we meet the needs of production. After the transformation of the hydraulic system of the tension and leveling machine, the dynamic response performance has been greatly improved, the equipment reliability has been further enhanced, and the maintenance cost will be significantly reduced.

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