Form hardening
Press hardening is a process of hot forming of sheet metal that is used specifically in automobile construction. In the literature, this production process is also referred to as press hardening . During form hardening, a sheet metal is heated to a temperature of approx. 950 ° C and cooled during the shaping process. The term is derived from the procedure in which the component to be manufactured is hardened in a hot forming tool (the mold).
Objective of the procedure
Due to the objective of producing as resource-saving, inexpensive and environmentally friendly as possible, press hardening has gained considerably in importance in recent years. The advantage of this method is to achieve the same or even higher strength of molded sheet metal parts with a reduced use of material and thus reduced mass than would be achieved with conventional forming techniques.
Procedure
The process on which press hardening is based is very similar to that of cold forming of sheet metal: A sheet (rectangular shape) or a blank is cut from an endless strip ( coil ) made of steel (in this case a boron-manganese-steel alloy) . Basically, a circuit board can assume any geometric shape and also already have the two-dimensional basic shape of the finished component. A three-dimensional component is formed from the blank in a forming process.
The subsequent production steps depend on the process to be used. In the case of large drawing depths, it is imperative to form indirectly. The procedure is called indirect, as the final component shape is not produced directly in just one forming process, but in at least two. Analogous to the indirect process, there is the direct process in which the shaping can be produced in just one step.
The first forming process of the indirect process is called pulling forward. When drawing, the sheet metal is preformed in the cold state with a conventional tool, which reduces the drawing depth in the heated state and eliminates the risk of the material cracking due to excessive stretching.
Both manufacturing processes are identical below. The blanks or basic parts to be formed are austenitized in a continuous furnace up to 60 m long at approx. 950 ° C, which changes the lattice structure of the material. The then red-hot metal sheets are transferred (e.g. a robot ) to a press in which a special, water-cooled tool is installed. Hydraulic presses are usually used for this task ; Currently there are considerations to use mechanical presses that are driven by servo motors. During the closing process of the press, the material is reshaped by the tool. After reaching the bottom dead center, the holding time begins, in which the sheet is quenched and the heat is extracted within a few seconds. The holding time is usually less than 20 seconds and depends on factors such as the component surface, the amount of material, the thermal conductivity of the tool steel and the hardness of the component to be achieved.
After the holding time has elapsed, the component is removed at a temperature between 150 and 250 ° C. The rapid cooling creates a martensitic structure in the steel, which is responsible for the high hardness values. (See also hardening of steel , first paragraph)
During press hardening, the component removed from the tool does not correspond to the final shape in which it is installed. A final trimming is necessary due to the material supplies required for production.
Tools
The construction of a hot forming tool can be considerably more complex than that of a conventional one. On the one hand, this is due to the fact that cooling channels have to be incorporated in the punch and die, which transport the cooling medium as close as possible to the printing surfaces in order to ensure optimal heat dissipation from the board. On the other hand, the tool has to perform the work of several conventional forming operations. On a traditional press line (this is the combination of several presses in one system, which are connected by a transfer that transports the component from press to press), a component is formed in several steps, so-called operations. This avoids excessive stress on the material in critical areas and the formation of cracks (compare deep drawing ). In the individual operations, material is fetched at certain points and given its final shape in the next tool stage. The requirement of the hot forming tool is to cover these steps, which are divided into up to 6 operations in a press line, in just one step. To make this possible, additional operations can be built into the tools that can be controlled via hydraulic cylinders and other media and, during the closing process of the press, for example, fetch material that assumes its final shape when the press reaches the bottom dead center.
Due to the structure described above, cutting elements or knives can only be integrated to a limited extent in hot forming tools, which is why the final component shape usually has to be produced afterwards. The extraordinary hardness of the steel, along with a tensile strength of more than 1,000 N / mm², generally no longer allows the components to be cut from the hot-stamped component with conventional cutting tools. 3D laser processing , primarily with CO 2 lasers , but recently also increasingly with solid-state lasers , has proven to be a tried and tested, but also very expensive and energy-consuming process . The final component shape is extracted from the hot-formed sheet metal using a laser beam. An alternative that can be used for simple geometries is hard cutting, in which the component is given its final shape using cutting tools on a press line.