With the aggravation of energy crisis and environmental problems, energy conservation and safety have become the most important starting point of automobile manufacturing industry. In order to achieve the above objectives, reducing vehicle weight is a very effective method, which leads to the rapid development and application of advanced high-strength steel.
The use of hot forming technology can greatly improve the stiffness and strength of the overall body structure, and greatly improve the vehicle crash safety and NVH performance; A large number of applications of this technology can effectively reduce the weight of BIW, reduce energy consumption, reduce environmental pollution, and improve the economic performance of the vehicle. At present, according to the requirements of body structure strength, hot forming technology is mainly applied to the production of high-strength components such as front and rear bumpers, A pillars, B pillars, C pillars, roof reinforcement beams, underbody channel frames, dashboard brackets, door inner panels, door crash beams, etc. (see Figure 1). The proportion of hot formed parts in BIW of the whole vehicle can reach more than 45%.
At present, hot forming technology has been widely used in automobile manufacturing companies at home and abroad. Major domestic automobile manufacturers have widely used hot formed parts for the production and design of vehicle models. The number of hot formed parts used in a single vehicle has generally reached 6 to 10, and the highest number of vehicle models has reached 24.
However, these hot stamping technology and die technology have been monopolized by foreign companies for a long time. The hot forming dies used by most manufacturers are imported and expensive. This time, we cooperated with Wuhan Iron and Steel Research Institute to develop the B pillar hot forming mold and parts for a certain passenger car of Dongfeng Company using Wuhan Iron and Steel WHT1500HF hot forming steel. This paper introduces the development of hot forming die and parts with B pillar as an example.
Development of B pillar thermoforming parts
The hot forming evaluation material of B column parts is WHT1500 steel with a thickness of 1.80mm. The material mechanical properties are shown in Table 1, the hot forming process test parameters are shown in Table 2, and the mechanical property curve is shown in Figure 1.
Analysis of thermoforming process
1. Boundary conditions
The single acting die structure is adopted, that is, the female die is on the top, the male die is on the bottom, and the stamping direction is as shown in Figure 3
Boundary condition: the initial temperature of the sheet is 800 ° C, and the sheet transfer time is not more than 3.5s; The waiting time of the sheet on the die before clamping is 3.5s; The clearance between die and blank holder is 1.5 x material thickness; The blank holder pressure is 1T; The forming and clamping speed is 150 mm/s; The holding pressure during quenching is 400t; The initial temperature of the mold is 100 ℃ on the surface and 20 ℃ inside.
1. Boundary conditions
Molding analysis
The blank of the part is unfolded by Pamstamp to get the sheet, and the forming module is analyzed in Pamstamp. In the forming simulation, the maximum thinned area is located on the side of the lower half area of the neutral column (MAX-23% as shown in the figure), and the thickened area is located on the front of the lower middle area of the central column as shown in the figure (+23.2% as shown in Figure 4). According to this situation, it was decided to pay attention to this area during mold processing and part debugging.
3. Simulation analysis of forming process
In the forming module of Pamstamp, observe the forming condition of the sheet metal during the movement of the mold. Figure 5 shows the upper mold is 40mm, 20mm, 5mm away from the lower dead center and the last formed part. At 5mm from the upper mold to the lower dead center, it can be found that there is wrinkling under the middle column. In the debugging phase of the mold, strong pressing will be done here.
4. Temperature field analysis
The temperature distribution of the part at the moment of forming when the upper die is at the lower dead point is obtained through the analysis module in the Pamstamp module. In order to obtain the martensite structure at last, the part temperature can be observed to be higher than 665 ℃ through simulation, which conforms to the standard and meets the quenching transformation conditions.
5. Distribution of martensite in quenched parts
Carry out analysis, as shown in Fig. 6a, the parts are quenched after 10s in the die, and more than 90% of the parts have been transformed into martensite except for the influence of blank holder position; Because the blank holder will be cut off by laser cutting. Fig. 6b shows the pressure holding (10s) temperature. When the part is completed, the surface temperature is less than 200 ° C, and the temperature at the blank holder is higher, which is consistent with the distribution of martensite.
6. Summary of formability analysis
(1) The analysis results show that the B pillar part is produced by WHT1500 hot forming, and the process is feasible.
(2) The area at the bottom of the part is the risk area of material wrinkling. It is recommended to increase the pressing force and reduce the die clearance to control the material flow.
(3) The fillet at the top of the part is at high risk of cracking. It is recommended to reduce the size of the sheet metal.
mould design
1. Design process of hot stamping mold
The design process of hot stamping mold is shown in Figure 7.
2. Mold structure design
(1) The hot stamping die material shall be H13 hot working die steel.
(2) Calculation and layout of cooling water channel
① Calculation formula of cooling channel
Where, mw is the mass of water flowing through the mold in unit time (kg/h); N is the number of pipes; Qw is the cooling water flow of a single pipe (m3/h); ρ W is the density of cooling water at a certain temperature (kg/m3), 1000 kg/m3; D is the diameter of cooling water hole (m); V is the flow velocity of cooling water (m/s); Tu is the unit time, 3600s.
Where, Re is Reynolds number; V is kinematic viscosity (m2/s), V=1.3077 at 10 ℃ × 10-6m2/s。
② Cooling channel layout
The diameter of cooling pipe is 10-14mm; The center distance between adjacent pipes is 17~20mm; The minimum distance from the pipe center to the profile shall be more than 15mm. The cooling systems of each insert are independent of each other, and the cooling channels between adjacent inserts are not connected with each other.
epilogue
Through the practice of using domestic hot forming steel to develop hot forming dies and parts for the main engine plant, we have learned and mastered the change rules of hot stamping die technology and heating and cooling organization, and gained some practical experience. During the research and development of the hot forming die for the B pillar of a passenger car, the die structure design of the part was completed through the CAE forming analysis of the part, the simulation of the change of the strength of the part in the temperature field and the check of the die strength. After debugging the hot stamping forming of the die, metallographic analysis and tensile test of the parts, the structure and strength of the parts are ensured to meet the expected requirements. The results show that the application of the designed and manufactured hot forming die for the B pillar of passenger car meets the technical requirements of the product.
