Hot stamping is a hot forming method used in sheet metal forming, specifically in production of high-strength steel components for crash-relevant parts in the automotive industry.
The first developments in hot stamping in Sweden by Volvo in Olofström and Norrbottens Järnverks AB in Luleå (Jergeus 2012).
The first announcement for a European patent on hot stamping (SE7315058-3) in Sweden by Carl-Erik Ridderstrale of Plannja HardTech, a division of Plannja AB, which in turn is a subsidiary of Swedish Steel AB (SSAB), for manufacturing farm implements. The motivation is the need for lightweight and strong steel components (Plannja 1973; Keremedjiev 2011).
A marketing campaign toward the European automotive industry started, and the first contracts for side impact beams are signed for Saab 9000 (Klebert 2017).
With the Saab 9000, Saab Automobile AB is the first vehicle manufacturer who opts for a hardened boron steel component (Karbasian and Tekkaya 2010).
The first publication of a patent on hot stamping in Germany (DE2452486C2) by Plannja AB (Plannja 1985).
The first automotive series production starts by Plannja AB (Jergeus 2012).
The number of produced parts amounts to 3 million parts in this year (Schuler 2013).
The great breakthrough on the European market for Plannja AB is achieved by the contract for side impact beams for Ford (AP&T 2017).
Decisions are made on establishing a manufacturing plant in Mason, Michigan, USA, with planned start of manufacture in the middle of 1998. As this US investment is taking place, Luleå is developing into a technical center for research and development in process and tool technologies. The focus on the automotive industry is followed by the removal of saws and wear parts from the manufacturing (Reynolds 2000).
The number of produced parts amounts to 8 million parts/year (Schuler 2013).
The Spanish company Gestamp Automoción buys Plannja HardTech from SSAB primarily because of their know-how. The first aim is to complete the ongoing investment with a hot stamping line at Haynrode in Germany (SSAB 2004).
For the first time, the number of produced parts amounts to more than 100 million parts/year (Schuler 2013).
The patent EP1645345A2 expires, so new competitors to Gestamp appear: Benteler, TKS, and the original equipment manufacturers (OEM) VW/Audi, BMW, Fiat, and Volvo, among others (Jergeus 2012).
According to forecasts by Schuler Group, the number of parts increases to 500–600 million parts/year (Schuler 2013).
The microstructure transformation into austenite starts when the blank is heated up to the material-specific temperature Ac1 and is completed when the blank passes the Ac3 temperature. The heated and austenitized material is then formed in the tool with a cooled punch and is quenched until it reaches a target temperature of 100 °C ≤ θ ≤200 °C. The contact with the tools automatically quenches the blank. The composition of the resulting structure primarily depends on the cooling rate. If the continuous cooling occurs quickly enough, it results in martensite, which leads to very high-strength products.
Diffusion-Controlled and Diffusionless Phase Transformations
Cooling rates above the critical cooling rate lead to an undercooling of the γ-Fe crystal and to a lattice sharing with α-Fe crystals. At such high cooling rates, the time is not sufficient for carbon diffusion, as it is present in diffusion-controlled transformations. Because the α-Fe crystals are only able to resolve a little CO2 into the steady state, a tetragonal lattice structure occurs. This is associated with a sudden increase in volume by up to 3%, which manifests itself in a change in length. This type of structure is called martensite and is characterized by low formability but offering very high strength. These properties make the martensitic structure desirable in many parts of the steel, and this is the reason why press hardening is carried out.
Properties of Manganese-Boron Steel
Manganese-boron steels are used in hot stamping due to the properties of the alloying elements, manganese and boron. The alloying element manganese in particular has a strength-increasing effect. It also delays the transformation of undercooled austenite in the secondary structure of ferrite, pearlite, and bainite. But the latter is of secondary importance since boron reduces the critical cooling rate to a much greater extent. The delay of the austenite transformation increases the time slot for carrying out the forming process.
The following facts cause the significant reduction of the critical cooling rate and slow formation of a secondary structure in alloys with a higher boron percentage:
The alloying element boron delays the heterogeneous nucleation on the grain boundaries through a reduction of the surface energy since boron is withdrawn at the grain boundaries (Babu et al. 1998). Thus, grain boundaries in alloys with a higher boron percentage are less effective nucleation sites than steel with an equal alloy composition except for the boron percentage. Usually, in commercial boron-alloyed steels, boron concentrations from 0.001 to 0.003 mass.−% are used. Higher levels may lead to the formation of borides at the austenite grain boundaries, which accelerates the formation of ferrite and thus contradicts the mentioned aim of a reduction of the critical cooling rate through the alloying with boron. Manganese-boron steels are characterized, in contrast to other quenched and tempered steels, by a low critical cooling rate of approximately 30 K/s. A deformation of the material has to be done before reaching the martensite start temperature, since martensite displays very high strength and hardness but has only low deformability. For example, the Vickers hardness can reach 470 HV in MnB steels. Plastic deformation leads to an increase of the critical cooling rate and also to a reduction of the martensite start temperature. The first is due to an increase of the internal energy, which is why less energy is required for a diffusion-controlled transformation. The variation of the martensite start temperature is due to the dislocation during the forming process (Naderi et al. 2008). This hinders a diffusionless phase transformation, which is why a higher driving force for the transformation of austenite into martensite is required than for non-transformed austenite. This driving force is achieved by lowering the martensite start temperature.
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