Generally, hot stamping is to heat the steel plate to about 900 ℃ austenite before stamping, and then cool the parts by rapidly injecting water into the stamping die. Through this thermal cycle and cooling process, the strength of the steel plate can be greatly improved, and the tensile strength can be increased from about 500MPA under delivery to about 1500MPa after hot stamping. According to different processes, hot stamping can be divided into direct hot stamping and indirect hot stamping.
01 Direct Hot Stamping Technology
After blanking, the hot formed steel plate is directly heated to the austenitizing temperature without pre forming, and then put into the die for rapid forming. Once the stamping shape reaches the predetermined value, the parts are immediately quenched and hardened (see figure). This process is mainly used for workpieces with simple shape and little deformation. Due to the low cost of direct forming process, it is also widely used.
02 Indirect hot stamping technology
The hot formed steel plate is first formed to 90%~95% of the final shape in the conventional cold forming die, and then the preformed parts are heated and austenitized for hot stamping forming and quenching hardening (see Figure 2). For some parts with complex shape or large drawing depth, indirect hot stamping can avoid forming cracks, and the preforming of parts can reduce the relative displacement between materials and dies, thus reducing the wear of die surface under high temperature. Generally, indirect hot stamping process must be used for parts made of galvanized coated hot formed steel.
At present, hot formed parts are widely used in automobile body manufacturing at home and abroad, and the usage ratio is increasing rapidly year by year. In the future, the hot forming production line in China will gradually spread to the automobile industrial areas in central China, southwest and southeast China. The development direction of hot stamping process will be to use high conductivity die steel to accelerate the cooling process, or use more effective cooling system to improve the cooling speed of parts, thereby improving production efficiency and shortening the forming cycle. In the next 10 years, hot forming technology will have more applications in China’s automobile industry, and the competition in the hot forming market will be more intense.