Introduction to Precision Stamping Technology (Hot Stamping)

The steel plate used in hot stamping is a special boron alloy steel plate, which is different from the traditional cold-formed ultra-high-strength steel. Cold-formed high-strength steel plates such as dual-phase steels and complex-phase steels, which are now widely used, are generally formed by cold stamping at room temperature, and the microstructure and mechanical strength of the parts before and after forming basically do not change. The steel plate used in hot stamping is not very strong at room temperature, and the tensile strength is only 400~600MPa, which has good plasticity and formability; it is formed and quenched by the hot stamping process. The microstructure is transformed from the original ferrite and pearlite into uniform martensite, the tensile strength can reach more than 1500MPa, the hardness can reach 50HRC, and there is basically no rebound, with high dimensional accuracy. Boron is added to the steel plate, the purpose of which is to improve the quenching performance of the steel plate and make the structure transformation of the plate material proceed smoothly. In addition, in order to improve the strength of the material and other mechanical properties, a variety of alloy trace elements such as Ti, Cr, Mo, Cu, and Ni are also added.　　Hot stamping forming consists of the following processes: 　　1. Blanking: It is the first process in hot stamping forming, which punches the sheet out of the required outer contour blank.　　 2. Austenitization: Including heating and holding two stages. The purpose of this process is to heat the steel plate to a suitable temperature, so that the steel plate is completely austenitized and has good plasticity. The equipment used for heating is a dedicated continuous heating furnace. After the steel plate is heated to a temperature above the recrystallization temperature, the surface is easily oxidized to form oxide scale, which will adversely affect the subsequent processing. In order to avoid or reduce the oxidation of the steel plate in the heating furnace, an inert gas protection mechanism is generally installed in the heating furnace, or the surface of the plate is treated with anti-oxidation.　　3. Transfer: refers to taking the heated steel plate out of the heating furnace and putting it into the hot forming mold. In this process, it is necessary to ensure that the steel plate is transferred to the mold as quickly as possible. On the one hand, it is to prevent oxidation of the steel plate at high temperature, and on the other hand, to ensure that the steel plate is still at a higher temperature during forming. Has good plasticity.　　 4. Stamping and quenching. After the steel plate is placed in the mold, the steel plate should be stamped and formed immediately to avoid excessive temperature drop that affects the formability of the steel plate. After forming, the mold must be closed and held for a period of time. On the one hand, it is to control the shape of the part, and on the other hand, the steel plate is quenched by the cooling device set in the mold to make the part form a uniform martensite structure and obtain a good size. Precision and mechanical properties. Studies have shown that for the currently commonly used hot stamping steels, the minimum cooling rate to transform austenite to martensite is 27-30°C/s. Therefore, it is necessary to ensure that the cooling rate of the die to the sheet metal is greater than this critical value.　　 5. Follow-up processing. After the molded part is taken out of the mold, some follow-up treatments are required, such as removing the oxide scale on the surface of the part by pickling or shot peening, as well as trimming and drilling the part. Due to the high strength of hot stamping parts, it cannot be trimmed and drilled with traditional methods, but must be completed with laser technology.