Introduction to thickened plates
Tailo redrolled blanks (TRB) are often referred to as variable-thickness rolled blanks (VRB) in China. It was first researched in the early nineties by the Institute of Metal Research of RWTH Aachen University in Germany, and its core is "flexible rolling technology", that is, in the process of steel plate rolling, the roll gap is controlled in real time by the computer, and the pre-customized variable cross-section thickness plate is obtained along the rolling direction. Due to the characteristics of controllable thickness combination, the thickener plate can flexibly select the thickness combination of the sheet according to the bearing mode of the automobile structure and the body assembly conditions, which greatly improves the design space of the automobile parts and realizes the lightweight of the automobile through the merging of parts.
Thickened plate provides a variety of flexible design solutions for body stiffness and strength, and has become an important technical choice for automobile lightweight, which can achieve weight reduction while ensuring vehicle safety, and is also favored by the world's major automobile manufacturers. The application technology of foreign thickener plate has developed from the research stage of the laboratory to industrial production, and many foreign high-end models, especially German models, such as BMW, Audi, Mercedes-Benz, Volkswagen and Opel, have begun to use a large number of thickener materials. In China, the research on thickened plates is still in the exploratory stage, and Baosteel Research Institute has successfully carried out the rolling of cold-rolled thickened plates using the pilot rolling mill, and the large rolling mill of Baosteel's VRB industry has the ability to supply in batches. However, the thickness change and deformation heterogeneity of the thickened plate form its uniqueness, especially in the field of mold design and manufacturing, which is still in the blank stage. Therefore, it is worth in-depth study of key issues such as the positioning of the blank of the punching die of the thickened plate, the clearance of the mold parts in the thickness transition zone, the processing of the profile of the mold parts, and the rebound treatment.
The following is an analysis of the forming process of a certain model of roof beam using thicker plate material and its process performance, completing the design and manufacture of thickening plate mold, and successfully testing the parts of the thickened plate top cover beam.
Analysis of the forming process of the top cover beam of the thickened plate
Fig.1. VRB roof beam
The roof beam part is an important safety bearing part of the automobile body in the body frame, which has a great impact on improving the side impact safety and the stiffness of the layout structure joint, so its structural characteristics should have a large thickness at the position of the joint between the two ends and the side wall B-pillar to increase the stiffness. VRB top cover beam is used as shown in Figure 1, and the thickness of both ends is 12mm, middle thickness 08mm, the material is HC340LA.
Fig.2. Transition mode of the thickness transition zone of the VRB roof beam
The thickness transition of the VRB top cover beam adopts a uniform transition, and the two ends are composed of 12mm stepwise linear transition to 08mm, and the transition mode of its thickness transition zone is shown in Figure 2.
Thickening plate transition zone simulation
Fig.3. Schematic diagram of the discretization of the transition zone of the thickener
The difference between the thickened plate and the general steel plate of equal thickness is that there is a transition zone of continuous thickness change in the sheet metal during the rolling process, and this continuous change in thickness also leads to the heterogeneity of the sheet metal in material properties, resulting in the fact that the parameters of a single material model that are conventionally used for forming simulation cannot be directly used for the forming of the thickening plate**. On the one hand, the partition discretization method is used to discretize the transition zone into sheets connected by different stepped thickness zones (see Fig. 3), and the same material properties are used in the equal thickness zone, so that the material characteristics of the transition region of the thickened plate can be effectively simulated, and the material parameters of the discrete regions of different thicknesses can be reached through two thickness differences.
Fig.4. Variable thickness sheet setup in Dynaform software
At the same time, the initial thickness of the blank can be set by keyword control in Dynaform software, so that the blank can be changed from equal thickness to unequal thickness, and the distribution of material thickness change in Dynaform software is shown in Figure 4.
Rough calculation analysis of the autoform forming process
At present, the commonly used stamping CAE** analysis software is AutoForm and Dynaform, both of which can simulate and analyze the stamping process of the whole process. AutoForm is much more computationally efficient than DynaForm, and it is easy to set up. The analysis process is first analyzed by Autoform, and after the analysis results are stabilized by CAD modeling, the actuarial calculation is carried out by Dynaform to analyze the cracking, wrinkling and rebound of the sheet.
Fig.5. Determining the stamping direction
By adjusting the stamping center coordinates and carrying out the stamping direction rotation, determine the stamping direction of the part (see Fig. 5), ensure that there can be no negative angle, and the demoulding angle is guaranteed to be more than 3 °, and ensure the rationality of drawing and the convenience of subsequent trimming and punching, and it can be seen from Fig. 5 that the overall stamping angle of the part profile is in a safe state. 
Fig. 6 **Analyze part surface design
The design of the part profile part can be divided into two parts: pressing surface and process supplement. The design of the pressing surface is controlled by means such as cross-section lines, and the principle of adding cross-sectional lines is that the less the better, because the more cross-sectional lines, the more difficult it is to control the curvature change of the pressing surfaceThe process supplement first needs to confirm the shape of the main section, which is concave, punch fillet, side wall inclination angle and parting line width, etc., to fully ensure the overall smoothness of the process supplement surface, as shown in Figure 6. 
Fig.7. Setup of variable thickness sheet metal in AutoForm
Since AutoForm cannot directly set the variation of unequal material thickness, the blank of unequal material thickness can be set by adding tailored blanks by adding tailored blanks, as shown in Figure 7. The crimping force is calculated according to the following formula:
where:- Tensile strength of the material, mpa;h——Sheet thickness, mm;l- Parting line length, mm. Since the forming of the top cover beam does not require much material inflow, the material inflow on the edge pressing ring can be controlled by drawing ribs. 
Fig.8. FLD diagram of the formation of the roof beam and the beam
Fig.9. Schematic diagram of the formation and wrinkling of the roof beam
Through the above characterization method and forming process of thickened plate sheet metal characteristics of forming parameters set and analyzed, the analysis is shown in Figure 8 and Figure 9, the results show that the material property characterization method is used for VRB parts stamping and forming numerical simulation process is stable, high precision, reliable results, the part adopts the thickening plate scheme when the drawing forming performance is good, and there is no cracking phenomenon on the deep-drawn part. The wrinkling of the part can be judged by the creping criterion, and the surface undulation height is greater than 0At 03mm, it can be considered that there is a risk of wrinkling in this area. From the analysis results, it can be seen that the wrinkling of this piece in the trimming area of the part meets the requirements.
Actuarial analysis of the forming process Dynaform
Since the profile constructed in AutoForm is a relatively coarse mesh data, it cannot be directly applied to process actuarial and mold structure design, so it is necessary to combine the rough calculation data provided by AutoForm to build a CAD digital model. According to the relevant requirements of the mold design of the top cover beam, the pressing surface and the process supplementary design of the part are completed, as shown in Figure 10.
Fig.10. Forming profile of the top cover beam
Import the finished process surfaces described above in Dynaform and mesh them. Due to the high requirements of actuarial science for the quality of the grid, the quality inspection of the grid must be carried out, and the internal gaps, local sharp corners, overlapping grids must not appear, and the normal direction must be unified.
Fig. 11: Implementation of variable thickness sheet metal in dynaform
The initial thickness of the sheet can be defined in the dynaform calculation file, which can be set as shown in Figure 11, so that a uniform transition between the thin and thick areas of the sheet can be achieved. According to the boundary conditions determined in the rough calculation, and combined with the above variable thickness sheet setting method, the actuarial analysis of the thickening plate scheme of the roof beam parts is carried out. In the Dynaform analysis, it is usually necessary to analyze whether the sheet is cracked in combination with the FLD plot and the downgauging rate, and for the unidirectional and bidirectional tensile regions, the thinning of more than 25 is considered to be crackedAt the same time, in the biaxially stretched area, the thinning is between the limit material thickness and 25, which can be judged by the FLD diagram.
Fig.12 Schematic diagram of the thinning rate of the top cover beam after forming
As can be seen from Figure 12, the thinning of the formed part at the corner is too large, which may lead to the actual stamping cracking on the site, and the process profile of the cracking position needs to be modified until the cracking is eliminated in the actuarial calculation of Dynaform. When the part is formed and stabilized, the part rebound calculation is added to the later simulation process, and the boundary condition is supported rebound, and the rebound result is shown in Figure 13.
Fig.13. Actuarial rebound results of the roof beam
As can be seen from Figure 13, the springback of the part is negative at both ends, with a maximum value of 1About 5mm, there is basically no deviation or local deviation in the middle of the part, which is not more than 07mm, according to the simulation results, it can be judged that the state of the part after leaving the mold is: in the stamping direction, both sides are bent downward, and the middle is upturned.
Fig.14. Compensation scheme for local springback
In order to carry out the springback treatment in advance and save the time of later debugging, the springback compensation of the parts was optimized according to the above simulation results. There are two common methods of springback compensation: global compensation and local compensation. Due to the particularity of the unequal thickness plate, and the rebound area is concentrated at both ends, the rebound treatment is carried out by local compensation, as shown in Figure 14. In the stamping direction, the two ends of the part collapse downward, so the two ends need to be lifted a little along the opposite direction of the rebound to achieve the compensation effect, and at the same time, considering that the thickness of the transition area of the unequal material thickness is inconsistent, the rebound treatment of the transition area may cause the transition area of the sheet metal to be difficult to match with the transition area of the mold part, and affect the coloring effect of the mold part, so only the end is selected to the profile close to the transition area for rebound compensation, and the maximum change of the profile is 20mm。
Fig.15 Mold surface design of the drawing process
After the above analysis and mold profile design and improvement, the mold profile of the top cover beam drawing process obtained is shown in Figure 15.
VRB thickening plate top cover beam mold development
Mold profile treatment
In the process of processing, the traditional mold surface processing method cannot be used for the thickening plate mold parts, and the processing data of the upper and lower model surfaces need to be processed according to the characteristics of the thickening plate.
Fig.16. Thickening plate mold surface treatment
Under normal circumstances, the processing number model surface of the convex, concave die and the edge pressing ring are all on the same side, and the profile is overlapping, and the processing data is generally offset to the upper profile through the CNC machining program, and the lower profile surface used to process the convex, concave die and the edge pressing ring is obtained. There is a material thickness transition area in the thickened plate, and the material thickness cannot be offset by machining methods, so it is necessary to reflect the material thickness offset in the processing data, and make the profile of the convex, concave die and edge pressing ring. In the design process stage, the upper surface of the part is taken from the profile, then the upper and lower surfaces of the digital model are directly used in the convex and concave die areas in the processing data, and the concave model surface is not moved, and the process supplementary area of the punch and the edge pressing ring profile face are offset by the concave model to offset the material thickness downward, and the material thickness transition area is connected by a bridge, as shown in Figure 16.
Mold structure design
If the sheet displacement occurs during the forming process of the thickened plate, it is easy to have a large thickness deviation, so it is necessary to control the sheet movement state of the thickened plate in the whole deformation process.
Fig.17. Side view of the rimming ring
Fig.18. Counterweight positioning plate
It can be seen from Fig. 17 that the pressing surface of the top cover beam is high at both ends, low in the middle, when the sheet is just put in, the middle will sag under the action of gravity, and the positioning of the plate is unstable, especially the thickening plate is characterized by the thickness of the middle and both sides, and the middle strength becomes poor, and the middle sagging of the sheet can lead to positioning failure, which leads to the inconsistency of the transition area of the sheet thickness and the transition area of the mold thickness, and affects the drawing forming effect. Therefore, the counterweight positioning plate as shown in Fig. 18 can be used in the middle of the hemming ring to control the positioning accuracy of the sheet.
Fig.19. Top cover beam mold lower die
Considering the convenience of on-site debugging in the later stage, it is necessary to arrange an adjustment cushion block on the lower die near the transition area of the sheet thickness, so as to facilitate the adjustment of the gap and control the flow of the sheet, as shown in Figure 19.
Fig.20. Thickened plate top cover beam
Based on the above research on the processing and manufacturing of thicker plate mold parts, the trial production of the parts of the top cover beam was completed, the surface of the parts was smooth and flat, no cracks and wrinkles were generated, and the size of the parts met the requirements, as shown in Figure 20. Original Author:ZHANG Wen1,2, GAO Yongsheng1,2
Author's Affiliation:1Baoshan Iron & Steel Co., Ltd. *** Research Institute;2.State Key Laboratory of Automotive Steel Development and Application Technology (Baosteel).