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Roller design as the core content of the cold-formed steel pass design occupies an important position in the cold roll forming technology. Under a certain rolling mill load strength, how to make the deformation of each pass of the sheet material even for the same basic roll diameter is an important objective of the roll design. Whether it succeeds or not has a direct impact on the dimensional accuracy and deformation defects of the final profile, as well as the subsequent roll design, production cost and processing cycle.
Using dynamic explicit finite element method to simulate the cold roll forming process to judge whether the roll flower design is reasonable is a better choice. ANSYS is a versatile, powerful finite element analysis software. The secondary development function is used to realize the input of the preset roller parameters through the interface, the cold bending forming finite element model is established, and the LSDYAN solver is used to carry out the simulation calculation and analysis of the whole process, and the equivalent plastic stress, strain and the like of the sheet material can be obtained. According to this, it is judged whether the design of roller parameters is reasonable, and the roller flowers are optimized.
1 Cold Roll Forming FEM Simulation Parameters and Processes
In the cold-formed forming process, the plates are successively passed through the forming rolls arranged in the forming direction, and are continuously bent from the plane to various cross sections required. The ANSYS/LSDYNA was used to simulate the molding process, ie, the input parameters of the cold bending roller were entered and the finite element model was established for analysis. Taking the channel steel as an example, the steel type adopted during the simulation is Q235, the sheet size is 8mm×682mm, the finished product has a bending radius of 34mm, and the bending angle is 77.1°. Taking into account the rebound, design the initial flower pattern (Figure 1) and the cold roll forming parameters (Table 1). 
Fig.1 Cold Roll Formed Roller Table No.1 Channel Initial Cold Roller Pattern Neutral Surface Parameters 
The ANSYS parametric APDL language parametric modeling and analysis of the cold bending process and the output sheet plastic stress and strain analysis process are shown in Figure 2. 
Figure 2 program analysis flow chart
2 The establishment of cold-formed finite element model
Under the ANSYS interface, click on the CHANNEL button on the toolbar and run the CHANNEL.MAC file in the background to initialize the parameters. By entering the blank width, thickness, and mechanical properties such as yield strength, Young's modulus, etc., a sheet metal model is created. The model of the roll is established by inputting the length of the bending edge, bending radius and angle of each pass.
In the simulation, the material selection model of the sheet material is a bilinear follow-up hardening model (BKIN). This material model contains the Bauschinger effect, which is applicable to the Von Misses yield criterion.
Due to the symmetry of the sheet material and the roll, in order to save the calculation cost and storage space, take half of it for simulation analysis. Taking into account the sheet material deformation between passes, the rack roll spacing is 800mm, and the length of a given sheet material is 1000mm. . The roller is defined as a rigid contact body. The unit uses SHELL163 to replace the entire roll by the outer surface of the roll. This reduces the number of finite element units and shortens the calculation time.
When defining the contact, it is a very complicated boundary nonlinear problem because of the constant contact, separation, and re-contact changes between the sheet material and each roll. Therefore, the contact between the sheet and the roll is automatic surface contact (ASTS), and the contact function is controlled by the penalty function method. The penalty function coefficient is set to 0.6. At this time, once the contact penetration phenomenon is calculated, a large amount will be applied. The value of the penalty function returns it to the contact surface for accurate calculations. The friction between the sheet and the roll is a Coulomb friction model with a static friction coefficient of 0.2 and 0.1 respectively.
When the load is applied, for the sheet material, only the symmetrical section needs to be symmetrically constrained. For the roll, all five degrees of freedom constraints are zero except for the rotation around the axis. When defining the initial speed of the sheet, the initial speed should be the same as the line speed at the basic roll diameter of the roll. The roll only needs to give it active axial rotation speed. For example, if the roller has an angular velocity of 2 rad/s, multiplying the angular velocity by the roller diameter is the initial speed of the sheet.
When setting up the solution, if the sheet material is applied at the true speed, then the computational runtime will be hundreds of hours or thousands of hours, which is obviously unrealistic. We can reduce the running time by increasing the speed of the sheet or by mass scaling, but these will reduce the accuracy of the solution, so choosing the right speed is critical. After the setting is completed, the final generated solution model for the program is shown in Figure 3. 
Figure 3 The cold-formed finite element model
3 Cold bending simulation initial stress strain diagram
After the analysis and solution are completed, the average plastic strain equivalent curve (see Fig. 5) can be automatically output at the 5 deformation points (see Fig. 4) in the bending deformation zone, which can be judged from the increase of the strain in each pass. Whether the distribution of bending angle and bending radius of each pass is reasonable. From Figure 5, it can be seen that the equivalent plastic strain value at the location on the deformation zone is gradually increasing as the sheet passes through each pass. However, this increase is not uniform. In the fourth and fifth passes, the equivalent stress in the deformed zone has the largest increase, reaching 0.13 and 0.11, respectively accounting for 26% and 22% of the total deformation. The time is relatively small. 
Figure 4 Schematic diagram of relative position of sheet material and roll 1 and location of deformation area 
Figure 5 The average equivalent strain curve of the initial equivalent curve In a certain rolling mill load strength, for the same basic roll diameter, in order to make the sheet material in all passes evenly deformed, you can adjust the initial set of roller parameter values, to the roll flower Correct the optimization.
4 Cold-strain simulation optimization after stress and strain diagram
The results of the above analysis were used to optimize the reduction of the amount of change in the angle and radius of the four-fifth pass, properly assign to other passes, correct the parameters, and obtain the parameters of the cold-rolled roll after optimizing the channel steel, as shown in Table 2. Table 2 Optimization of Cold Rolled Roller Chart Neutral Surface Parameters 
After the re-simulation, the average equivalent plastic strain curve of the five positions of the optimized bending deformation area with time is shown in Fig.6. It can be seen that the equivalent strain in the four-fifth order has been greatly reduced, with an increase of 0.06 and 0.09, which is 14% and 18% of the total strain. Corresponding strains of other passes have been increased to achieve the goal of uniform distribution of the sheets. 
Figure 6 The average curve of equivalent strain after optimization
5 Conclusion
Under the ANSYS interface, the cold roll forming process simulation was initially implemented using the ADPL language. Based on this, it can be determined whether the roll angle and bending radius distribution of the pre-roller rolls are reasonable. The program enters the preset roller parameters, automatically modeling and calculation, and then obtains the deformed stress-strain diagram or numerical value, and then manually judges and corrects the roller flowers so that the deformation of the sheet material in each pass is even, and the production is qualified for cold bending. Products and shortening the product cycle play an important role.
Although the use of ANSYS secondary development to simulate the cold bending forming method is achievable, there are still many problems that need to be solved, such as expanding product range, proposing better material models, flexible selection of routes, and more reasonable Roller descriptions, etc., are worth studying in future work.
Cold forming is an important area for sheet metal deep processing. It is a plastic forming method in which a forming unit composed of a plurality of rollers having a certain surface shape bends and deforms the billet gradually to obtain a uniform section product. So far, the cold-formed forming technology has achieved considerable development. However, due to the complexities of geometry, kinematics, dynamics, and material science in the molding process, cold-formed molding is still an engineering technique that is highly empirical.