Asymmetrical 3-roller plate rolling machine is the key equipment of pressure vessel manufacturing enterprises, mainly used for bending and rounding steel plates of petrochemical pressure vessels such as plate welding.
- Its advantages are a relatively uniform force on plates and precise control of forming dimensions;
- The disadvantage is that the entire length of the sheet cannot be bent, and there must be a certain length of unbent straight section at both ends of the metal plate, that is, in China, we call it the head allowance or unbent flat end.
Some large-scale pressure vessel manufacturers in China have used the process of pre-bending first, then cutting off the unbent straight end before rolling the plate, so they have to consider the above-mentioned process requirements when ordering and leaving a length margin for cutting the unbent straight end. During the process, it is necessary to transfer to other stations to cutting the unbent straight-end portion of the steel plate, which not only prolongs the manufacturing cycle, but also increases the manufacturing cost. Therefore, it is necessary to adopt a new bending process to carry out roll forming of pressure vessels without unbent straight end parts.
Why does the steel plate rolling process produce unbent flat ends?
Taking a 3500mm PRT 3-roll steel plate rolling machine as an example, the relevant parameters are shown in the left Figure 1. The upper roll of the steel plate rolling machine is a movable roll that can move up and down, left and right, and the two lower rolls are fixed rolls. The plate rolling process is a double fulcrum beam Bending forms subject to concentrated loads.
When the steel plate is curved, the lower surface of the steel plate is in contact with the highest point B of the lower roll. When the upper roll presses down and exceeds the yield limit of the material, the steel plate produces plastic deformation. A certain area of the AB section in the left picture cannot be deformed and becomes a barrel joint, the head allowance/unbent flat end of the steel plate.
Reducing the margin of the pressure head/unbent flat end
The size of the pressure head allowance/unbent flat end is proportional to the minimum center distance between the upper and lower rollers of the plate roller in the horizontal direction, and decreases as the radius of curvature R of the barrel section becomes smaller. Although in the actual operation process, factors such as the strength of the plate, the thickness of plate, and the width of the plate can be considered comprehensively, and the upper roller can be moved horizontally along the horizontal direction, thereby reducing the margin of the pressure head/unbent flat end.
However, the greater the traverse distance, the farther the top roller deviates from the center of the hydraulic cylinder, the smaller the deformation ability applied to the steel plate, and the worse the bending ability. Even so, there is always a section at the end of the plate that does not have the condition of a double fulcrum beam, and cannot pass through the maximum plastic deformation zone of the lower part of the upper roller.
Therefore, the current process plan cannot eliminate the pressure head allowance/unbent flat end.
The improved plate rolling process
The improved rolling process is as follows:
Firstly, roll bending the middle position of the steel plate length of the cylindrical section, and then roll bending the two ends of the steel plate of the cylindrical section, so that the steel plate can close the longitudinal seam and the straight section is at a flat angle. After the longitudinal seam is welded school circle, the detailed steps of the plate rolling process are as follows:
4 steps of plate rolling
- The incoming steel plate is net size, and the grooves of the longitudinal welds at both ends of the barrel joint have been processed.
- Curve from the middle part of the steel plate length of the cylindrical section to both ends. The radius of curvature after the curve is the final radius R of the product, and a straight section with a length of 400~600 mm is left at each end.
- The straight section at both ends of the steel plate is bent to a radius of R1 (R1=0.7~0.8R) arc until the steel plate can close the longitudinal seam without indentation, so that the barrel joint is in a “D” shape, as shown in the left Figure 2: Assemble and weld the longitudinal seam of the barrel joint, pay attention control welding. The resulting angular deformation makes the weld reinforcement as small as possible.
- Correction circle: Use the upper roller to press down the straight section and bend it, gradually increasing the curvature. When it is close to the theoretical size R, correct the circle circle by circle until it reaches the theoretical radius R, and the circle calibration is over. It should be noted that when there is a pressure head allowance, the circle calibration of the barrel joint is only to correct the angular deformation caused by the longitudinal seam welding of the barrel joint, which is a local small-scale correction.
4 notes of correction circle
When there is no pressure head allowance for the circular adjustment of the cylindrical joint, the large-scale cylindrical steel plate needs to be greatly deformed. If there are linear defects such as cracks in the longitudinal seam, it is easy to expand during the circular adjustment.
Therefore, attention should be paid during the operation:
- The curved shape should satisfy that the steel plate can close the longitudinal seam and not be concave;
- When welding the longitudinal seam of the cylindrical joint, strictly control the angular deformation caused by welding, and the weld reinforcement should be ground evenly;
- Carry out UT flaw detection on the longitudinal weld of the barrel joint before rounding;
- When correction circle, determine the pressing amount of the upper roller each time according to the plate thickness and material, and gradually press down, and the pressing amount should not be too large.
Using the above-mentioned process plan, a company bent a total of 10 steel plates for a hydrogenation reactor. Satisfy the requirements of the design drawings Figure 3 Application of curved technology without pressure head allowance
Compared with the original plate rolling process, the plate rolling process without indenter margin reduces the steel plate cutting unbent flat end process, saves the purchase cost of the steel plate, and avoids the station turnover the repeated loading and unloading of the 3-roll station of plate roller greatly shortens the production cycle and improves the economic benefits, which has a guiding role in the manufacturing of plate welding pressure vessel cylinders.
Works Cited: Research on Curving Technology of Symmetrical Three-Roll Plate Bending Machine