Application of high strength steel in bending machine

Application of high strength steel in bending machine
When the high-strength steel is bent by the free bending method on the CNC bending machine, various defects are produced due to the sheet properties, equipment status, bending process parameters and other factors, which reduce the production efficiency and cause economic losses. In this paper, the main defect types of high strength steel under free bending are analyzed, and the corresponding improvement measures and countermeasures are put forward.
Bending cracking
When the R angle inside the bending is small to a certain value, the longitudinal material outside the plate may have cracks or microcracks at the R angle of the bending part due to excessive deformation, as shown in Figure 1. At the early stage of trial production of high-strength steel bending parts, bending cracking defects occurred frequently, which not only caused waste of plates, but also affected the normal progress of the project. The main factors causing cracking of high-strength steel bending parts are as follows.
Bending radius
The ultimate bending radius to prevent the outer fiber from cracking is called the minimum bending radius. The minimum bending radius is related to the mechanical properties of the material, the fiber direction of the sheet, the surface and side quality of the sheet, and the thickness of the sheet. Generally, with the increase of the material strength, the plasticity will decrease, and the minimum bending radius that the material can adopt will be larger.
Bend line positioning method
During the bending process, the bending line needs to be positioned to ensure the accuracy of the bending parts. The traditional manual line positioning methods mainly include manual or laser marking positioning method, process gap positioning method and machine tool stop positioning method.

In the process of bending trial production of bp500 plate, when the process notch positioning method is adopted, the bending part cracks at the positioning notch. After notching the sheet metal, the original smooth edge shape changes sharply, and the stress at the notch is concentrated. When bending the sheet metal, the stress value of the material at the notch will first reach the strength limit, resulting in cracking. This also shows that bp500 plate has high requirements for material surface quality and side quality when bending process is adopted, and the surface shall be free of cracks, scratches, burrs, notches and other defects on the side. Therefore, the bp500 plate cannot be bent and positioned by the process notch positioning method, and the machine block positioning method without damage to the plate shall be preferred for bending.

Bending springback
When the bending part is taken out of the die, the bending angle and radius are inconsistent with the die, which is called bending springback. The main factors that affect the springback of bending parts are: mechanical properties of materials, relative bending radius r/t, bending mode, bending center angle and shape of bending parts. The springback angle is directly proportional to the yield strength and hardening index of the material, and inversely proportional to the elastic modulus. The yield strength of protective steel bp500 is not less than 1250MPa, so its rebound trend is much larger than that of ordinary plate.
The bending angles at the left and right ends of the bending part are inconsistent
During the bending process of bp500 material bending parts, it is found that some bending parts with long bending lines have inconsistent bending angles at the left and right ends. The causes of inconsistent bending angles at the left and right ends of the bending part are as follows: ⑴ the parallelism between the bending machine slider and the workbench plane exceeds the tolerance when it reaches the bending end position; (2) the parallelism between the installation surface of the upper formwork and the working bottom surface exceeds the tolerance; (3) the parallelism between the V-shaped groove of the bending lower die and the installation bottom surface exceeds the tolerance; (4) the thickness of the folded plate is inconsistent at the left and right ends.
When the lower bending die is rotated by 180 °, that is, when the left and right ends are turned around for bending, the angle difference between the left and right ends of the bending part still exists, and the size values are exchanged left and right. It is speculated that the contact positions of the left and right ends are different when the upper bending die and the lower bending die are closed. After further investigation, it is found that the arc size of the working bottom surface of the bending upper die is inconsistent, which leads to the out of tolerance parallelism between the installation bottom surface of the bending upper die and the working bottom surface, resulting in the inconsistency between the left and right bending angles. By re machining the working bottom arc of the bending upper die, the flatness reached 0.05mm/m, and the problem of inconsistent bending angles at the left and right ends of the bending parts was solved.
Concluding remarks
To sum up, in the free bending of high-strength plates, the minimum bending radius and springback trend need to be tested first to ensure the consistency of bending machine accuracy, mold accuracy and sheet thickness, and then optimize and adjust the free bending process of the bending machine, such as the optimization of positioning mode, so as to effectively reduce the defects of high-strength plate bending parts and improve the product qualification rate.