Failure Analysis and Treatment of Mold Vibration System of Slab Continuous Casting Machine

Abstract: This article introduces the mold vibration system of the 4″ slab continuous caster of Tianjin Iron and Steel Co., Ltd., and conducts a logical analysis of the fault phenomenon based on the principle of the hydraulic system.Use troubleshooting methods to find faults, find out where the fault occurred, and repair it to ensure the smooth progress of production.

Keywords: continuous casting machine; copper mould tube; vibration system; electro-hydraulic servo valve; hydraulic cylinder; position sensor; fault; treatment

1 Overview

The 4″ 180/200/250×1320-2100 mm slab caster of Tianjin Iron and Steel Co., Ltd. is designed by Voestalpine. It is a straight mold, continuous bending, continuous straightening, and curved slab caster.

The crystallizer vibration of the continuous casting machine adopts a hydraulic control system. The entire machine is controlled by a PLC system and remotely operated and controlled by a computer, with a high degree of automation.

2 System introduction

The 4* slab continuous casting machine mold vibration hydraulic control system uses hydraulic servo valves as input signal conversion and amplification components. The system can control high-power hydraulic flow output with low-power electrical signal input to obtain high control accuracy and fast response speed and position control. The movement of its hydraulic actuator can track random control signal changes with high precision. It is an electro-hydraulic servo valve, servo amplifier, and sensor system that integrates mechanics, hydraulics, and electricity.

There are two hydraulic cylinders in the crystallizer vibration device of the 4″ slab continuous casting machine. When working, the two hydraulic cylinders are required to vibrate synchronously. The principle of the vibration system is shown in Figure 1.

There are two hydraulic servo valves 2, and each hydraulic servo valve controls the movement of a hydraulic cylinder 3. When the hydraulic servo valve receives an electrical signal input, the hydraulic flow is output in proportion, and the position sensor 4 feeds back the position signal of the hydraulic cylinder rod to the PLC. The system controls the telescopic movement of the hydraulic cylinder rod according to changes in the control signal, thereby controlling the up and down vibration of the crystallizer. Remote control of the vibration of the copper mould tube is achieved through PLC and computer. Accumulator 1 and oil return accumulator 5 play a role in reducing system impact. Reasonable adjustment of the oil return pressure of oil return valve block group 6 can make the system vibration relatively stable.

The crystallizer vibration control system parameters such as amplitude, vibration frequency and non-sinusoidal coefficients are set on the PLC as follows:

Minimum vibration frequency: 40cpm

Maximum vibration frequency: 400cpm

Maximum stroke at minimum frequency: 190 mm

Maximum stroke at maximum frequency: 190mmTAU

Minimum value of non-sinusoidal coefficient: 0.3 mm

Maximum value of TAU non-sinusoidal coefficient: 0.7 mm

3 Fault conditions and fault finding and management

During the steel drawing process, the following situation occurred: when the drawing speed was set to 1.2 m/min, the continuous casting mold vibration 2*hydraulic cylinder position exceeded the tolerance alarm, causing the vibration to stop and the drawing and straightening to stop. The system is normal after restarting. The pulling speed was set to 1.2 m/min again, and the vibration pattern on the computer screen and the actual vibration of the continuous casting mold were both normal. When the pulling speed is set to 1.5m/min, the vibration graphic display on the computer screen is slightly deviated. When the pulling speed is set to 1.8 m/min, the vibration graphic display on the computer screen deviates greatly. When the pulling speed is set to 2.0 m/min, the continuous casting mold vibration and the tension leveler stop. Select the tail blank mode to automatically calibrate the position, re-install the spindle for test run, and the vibration will stop in a short time. After restarting the relevant systems repeatedly, the problem still exists. Carry out logical analysis based on the principles of the hydraulic system, eliminate them one by one, and finally find out the fault location, solve the problem and ensure normal production.

First, the set parameters were checked to eliminate unreasonable parameter settings. The working conditions of PLC components and the connection conditions of each line were checked, but the fault point was not found. After investigation and analysis, it is believed that there is a greater possibility of a problem with the hydraulic servo valve control system. Then further investigation will be conducted from the following aspects.

(1) Check the oil source pressure, no abnormality.

(2) Check the cleanliness of the oil. The actual measurement is NAS1, which is normal.

(3) Further check the internal feedback of the servo valve. By setting a certain current signal of the hydraulic servo valve on the computer (between ±10 mA), check the opening degree of the 1″ hydraulic servo valve and the 2″ hydraulic servo valve. By comparison, the openings of the two valves are almost the same, ruling out servo valve failure.

(4) Check whether the output signal of the sensor is normal, and repeatedly measure the lifting position of the continuous casting mold, that is, the lifting position of the hydraulic cylinder rod in manual and positioning modes. It was found that the lifting position of the 2″ hydraulic cylinder rod displayed on the computer was very different from the actual position. In fact, the lifting position of the 2″ hydraulic cylinder rod was basically the same. It was initially judged that there was a problem with the position signal of the 2″ hydraulic cylinder rod.

Afterwards, the cables were rewired and powered on for a test. There was still a problem with the position signal. Then the position sensor was tested for static resistance. It was found that the resistance value was much different from the resistance value of the 1″ hydraulic cylinder. Finally, it was determined that the position sensor failed.

1. Accumulator 2. Hydraulic servo valve 3. Vibration hydraulic cylinder 4. Position sensor 5. Loop accumulator 6. Oil return valve block

Figure 1 Principle diagram of the mold vibration system of the slab continuous casting machine

After replacing the 2″ mold vibration hydraulic cylinder assembly (including servo valve, position and pressure sensor, etc.), restart the continuous casting mold vibration system.

It was found that the opening degree of the new servo valve when the current output was 0mA was 8.7%, and it still could not work normally. Afterwards, the parameters were modified again, the zero position was adjusted, etc., but the fault was still not found. The hydraulic cylinder assembly is provided by VAI Company as a whole. After consultation with foreign experts, it was confirmed that the new hydraulic cylinder assembly had a neutral line that was not removed when it left the factory. This neutral connection causes the 24 V power supply and the 4 mA~24 mA signal to share the same ground, causing signal drift. After removing the neutral line, the fault point is eliminated.

Shortly after the casting started, the hydraulic vibration of mold 1* became abnormal intermittently, making the operation unstable and unable to guarantee normal production. Then conduct troubleshooting:

(1) The system oil pressure is normal, and the hydraulic pump and relief valve work normally;

(2) There is no jamming phenomenon in the actuator;

(3) After checking, the input and output electrical signals of the hydraulic servo amplifier are normal;

(4) Check the electrical signal of the hydraulic servo valve. After setting a certain opening degree of the electro-hydraulic servo valve on the computer, actually measure the continuous casting mold position. It was found that the 1* hydraulic cylinder did not change with the change of the servo valve opening, but the computer screen displayed normal, that is, after the control signal was input, the actuator did not move, and when the input electrical signal changed, the hydraulic output did not change accordingly. Therefore, it is determined that the hydraulic servo valve 1 is not working properly. After replacing the hydraulic servo valve, the vibration is normal. The maximum and minimum values of vibration amplitude (as shown in Figure 2), non-sinusoidal coefficients, oil return pressure, etc. were recalibrated and production resumed.

Figure 2 Schematic diagram of vibration range

4 Conclusion

Through the search and treatment of vibration faults of the 4* slab continuous caster, we have deepened our understanding and mastery of the hydraulic servo valve system, and accumulated practical experience for future judgment and search of faults in the system. When using a hydraulic servo valve system, pay attention to the following points:

(1) Read the instruction manual carefully and pay attention to whether the control coil connection method of the electro-hydraulic servo valve is correct;

(2) Whether the connection method of feedback sensors (such as displacement, force, speed, etc. sensors) meets the design needs and whether the work is reliable;

(3) Whether the oil source pressure and physical and chemical indicators are stable and meet the design requirements, and whether the system accumulator inflation pressure is normal;

(4) The servo valve and servo amplifier need to be zeroed (in order to adjust the system zero position, a bias voltage is sometimes added);

(5) After the hydraulic control system is put into operation, the following recorded data should be checked regularly: oil temperature, oil pressure, oil contamination level, operating stability, zero deviation of the actuator, and signal tracking by the actuator.

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