High strength steel plate processing hot forming technology

The actual hot forming process first heats a high-strength boron alloy steel plate with a strength of 500-600 MPa at room temperature to 880-950 ℃ to uniformly austenitize it, and then sends it into a mold with an internal cooling system for stamping and forming. After that, it is rapidly cooled and quenched under pressure to transform austenite into martensite, and the formed part is strengthened and hardened, greatly improving its strength. For example, after cooling and quenching inside the mold, the strength of the stamped part can reach 1500 MPa, an increase of more than 250% in strength. Therefore, this technology is also known as "stamping hardening" technology. In actual production, hot stamping process is divided into two types, namely direct process and indirect process. After cutting, the steel plate is directly heated and stamped into shape, mainly used for workpieces with relatively simple shapes and little deformation. For some workpieces with complex shapes or deep drawing depths, indirect processes are required, which first pre deform the cut steel plate and then heat it up for hot stamping. For the hot forming technology of high-strength steel plates, we need to focus on the selection of steel, surface coating of hot forming steel, mold design, and inspection of hot forming parts.

The selection of steel for hot forming is an important factor in ensuring the performance of hot formed parts. The hot formability of high-strength steel plates mainly includes the following forms: deep drawing formability, bulging formability, and extended flange formability. It is generally believed that the formability of deep drawing depends on the Lankford value of the plastic strain of the steel plate; The formability of bulging depends on the ductility of the steel plate; The formability of the extended flange depends on the local deformation energy and microstructure uniformity of the steel plate. B plays an important role in the homogenization of the microstructure that governs the formability and bending formability of the extended flange, so F+B and B single structures have been consistently used; And in order to achieve the goal of high strength, low-carbon M. The principle of 22MnB5 steel in martensitic steel is consistent with this, which is a typical hot stamping material steel. It uses titanium and boron microalloying methods to obtain high formability and extremely high strength through hot forming and rapid cooling. At present, hot formed MnB steel plates have been used in major automobile manufacturing companies in Europe, America, and Japan. For example, the new Golf V6 car has 5 components made of MnB steel, and the latest sixth generation PASSAT model has 9 such components. Figure 5 shows the CCT (Continuous Cooling Transformation) curve of boron containing steel developed by Baosteel for hot stamping forming. After heating and insulation in the single-phase austenite zone at around 950 ℃, when the cooling rate is greater than 15 ℃/s, the structure of the steel plate transforms into a fully martensitic structure with a hardness of HV450-500 and a strength of 1300-1500MPa.

The crushing performance of hot formed parts (low intrusion after collision) determines that they are very suitable for use as safety components. The processing of hot formed parts usually requires processes such as laser cutting, punching holes, spot welding, cold forming, assembly, and painting, so there are many aspects that need to be tested for hot formed parts. Firstly, it is necessary to conduct mechanical performance testing, shape testing, thickness distribution testing, and introduced internal stress testing on the hot formed parts. Different methods should also be used for physical performance testing according to the different requirements of different parts. For a qualified hot formed part, it should meet the requirements of high strength, lightweight, and safety, while also having good strength and toughness combination, dimensional stability, machinability (geometric dimension stability), weldability, and fatigue resistance.