In order to reduce fuel consumption and reduce emissions, high-strength steel stamping parts are used in car body manufacturing. The use of high-strength steel can reduce the weight of parts by reducing the wall thickness, and realize the lightweight of the car body. At the same time, its ultra-high strength can improve the safety of vehicle collisions and meet the requirements of lightweight and improved safety. Therefore, it is used in the automotive field. increasingly widespread. However, as the strength of high-strength steel plate increases, its stamping formability decreases. The higher the strength, the more difficult it is to form. Especially when the strength exceeds 1000MPa, some parts with complex shapes cannot be formed by conventional cold stamping processes. Hot forming technology is a new technology specially used for forming high-strength steel plate stamping parts. It can form stamping parts with a strength of up to 1500MPa, and there is almost no springback when forming at high temperature. It has the advantages of high forming accuracy and good forming performance. The industry’s general attention and quickly become a hot technology in the field of automobile manufacturing. At present, this technology has been developed and applied rapidly abroad, and automobile companies such as American General Motors, Ford and German Volkswagen are all using this technology to manufacture high-strength stamping parts. However, the research and application of this technology in China is still in its infancy. At present, only a few domestic companies have spent huge sums of money to introduce production lines from abroad for the production of hot stamping parts, but they do not have the ability to independently develop hot stamping related technologies and equipment. . Now, the typical stamping parts of automobiles are selected, the experimental research on the hot forming process is carried out, the main influencing factors of the hot forming process are discussed, and the experimental trial production of the typical parts is completed.
Thermoforming process principle
The hot forming technology is different from the traditional cold stamping forming process. The cold stamping forming is mostly carried out at room temperature. In the hot forming technology, the sheet is stamped and formed in a red hot state. The process is shown in Figure 1. The boron alloy steel plate after feeding is sent into the heating device and heated to 880-950 ℃ to make it austenitized, and then sent to the stamping die with cooling pipe inside for stamping forming. Quenching makes it undergo phase transformation and transforms austenite into martensite, so that the formed parts are strengthened and the strength is greatly improved. For example, the strength of boron alloy steel plate at room temperature before forming is 500-600MPa. After cooling and quenching after forming, the strength of stamping parts can reach 1500MPa, and the strength is increased by more than 250%. Therefore, this technology is also called stamping hardening technology and can be widely used in Production of high-strength car bumpers, door bumpers, A-pillars, B-pillars, C-pillars, roof frames, middle tunnels and other security and structural parts.
Taking a certain type of automobile reinforcement plate as an example, the hot forming experiment of high-strength steel stamping parts is carried out. The steel plate material used in the experiment is low carbon boron alloy steel 22MnB5, and the main chemical composition is shown in Table 1. Figure 2 is a forming die with cooling channels. In the experiment, the steel plate was first heated to 900 ℃ in a heating furnace, and kept for 5 minutes, so that the steel plate was uniformly austenitized, and a protective layer was applied on the surface of the steel plate before heating to prevent the surface of the steel plate from being oxidized. After the steel plate is heated, it is quickly put into the stamping die with cooling pipes, and the hydraulic press quickly descends to realize stamping forming. After the die is completely closed, it is kept for 10 s, so that the steel plate is fully cooled and quenched in the die to realize the transformation from austenite to martensite. full transformation. Figure 3 is the obtained test piece. The microstructure analysis of the formed piece is carried out on the diced samples. The results show that the formed piece is lath-like martensite after quenching, as shown in Figure 4. The tensile mechanical properties of the formed parts were tested, and the yield strength reached 1 036 MPa and the tensile strength reached 1 547 MPa, which fully met the product performance requirements of high-strength steel hot stamping parts.
Analysis and discussion
In the hot forming process of high-strength steel stamping parts, heating, forming and cooling are three very important links, which directly affect the transformation of austenite to martensite and the performance of the product. Therefore, the selection of process parameters in the three stages is very important. .
(1) In the heating stage, the uniform austenitization of the sheet should be ensured without the grain growth. Therefore, a reasonable heating temperature and holding time must be adopted. Excessive heating temperature will cause the surface of the sheet to be overburned and crystallized. Grain growth, and the length of holding time affects the uniformity of austenitization. Excessive holding time will lead to grain growth, which in turn affects the mechanical properties of the formed parts.
(2) In the forming stage, the sheet metal must be stamped and formed in the austenite state, and a high forming speed needs to be adopted, so that the workpiece can be rapidly formed in an instant, so as to avoid excessive heat loss caused by too slow forming speed and Excessive temperature drop. Therefore, a hydraulic press capable of high-speed forming should be used in the hot stamping process.
(3) In the cooling stage, the formed part is cooled and quenched by the surface of the mold, and a phase transformation occurs, so that the austenite is transformed into martensite to achieve strengthening. However, this phase transformation is related to the cooling rate. Only when the cooling rate exceeds a certain critical value can austenite be transformed into martensite. If the cooling rate is too low, other structures such as bainite will appear in the formed parts. Affect the strength of the formed parts. The research shows that in the hot stamping process, the minimum cooling rate (critical cooling rate) to realize the transformation from austenite to martensite is 27 ℃/s . Therefore, in order to ensure the transformation of austenite to martensite in the hot stamping process, the cooling rate of the mold to the formed part must be greater than this value, but the higher the cooling rate is not the better, the excessive cooling rate will cause the formed part to crack.
In addition, because the hot forming process is quite different from the traditional stamping process in many other aspects, such as the design method of the formed part, the mold design method and even the process debugging method, the hot forming process has its own unique side. All of these will have a great impact on the hot forming process and the quality of the formed parts. In practice, directly using the methods and means in the cold stamping process may not be able to deal with the related problems of hot forming, but requires careful analysis and summary, and continuous experiments. Means to explore more suitable technical methods and measures for hot forming.
The influence of the main process parameters on the forming is discussed and analyzed through the hot forming experiment of typical automobile stamping parts. The test results show that the yield strength of the formed typical parts exceeds 1000MPa, the tensile strength reaches 1547MPa, and its microstructure is ideal strip martensite. Hot forming technology is the latest technology in the field of automotive high-strength steel stamping parts production. It can be used to produce ultra-high-strength stamping parts with a strength of up to 1500MPa, which can meet the requirements of automotive lightweight and high safety at the same time. It has almost no springback, With the advantages of high forming accuracy, it can be widely used in the manufacture of body safety parts and other structural parts, and has a very broad application prospect.
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