## 4 Applications
### Applications in the Automotive Industry  
#### 4.3.1 Car Body Parts  
It is well accepted that the use of hot-stamped steel or Press-Hardened Steel (PHS) in reinforcement elements of the car body structure offers the possibility for significant reduction of vehicles' weight at moderate costs [66]. However, the production of PHS parts is also very challenging. Today, OEMs and suppliers offer a broad range of PHS products that are based on tailored steel properties and adapted coating systems, including monolithic and patched blanks, tailor tempered blanks (TTB), tailor welded blanks (TWB), and tailor rolled blanks (TRB) [67]. For that reason, there is a high demand for methods that allow for the precise monitoring of the PHS production process in order to maintain it to fixed targets and to ensure that PHS products in all variants meet the high quality requirements. This is crucial for the automotive industry in order to maintain and improve its competitiveness in a global market [68]. The mechanical properties of the PHS are adjusted not in the steel plant during the process, but only afterwards during hot forming. In order to meet the material specifications, all process parameters during heating, forming, and quenching must be kept within strict tolerance bands. Nevertheless, process disturbances, such as furnace malfunctions, wear on the pressing tool, or breakdown of the cooling circuit in the press could affect the material quality. Deteriorated material properties could also result if the heated blank cools down too fast during transfer between the furnace and press. In the case of AlSi-coated blanks, a complicated coating layer structure is developed in the furnace. In order to achieve the optimum press-hardening results and paint adhesion, this layer structure has to meet strict specifications. The application possibilities of 3 MA for quality assurance and process monitoring in PHS production have been researched, since the first use of PHS in large-scale automotive production in 2004 [56]. Based on the calibrated 3 MA technique, quantitative values of hardness, tensile strength (Rm), elastic limit (Rp0.2), fracture elongation (A50) and uniform elongation (Ag) of the PHS material can be determined. The 3 MA calibrations are generally steel-grade specific. Thus, different calibration functions for the hardenable and the nonhardenable parts in a TWB have to be used. Additionally, each coating type requires an individual calibration. The diagrams in Figures 18 and 19 show the result of a calibration for an aluminum-silicon (AlSi)-coated steel of grade 22 MnB5. Usually, for hardness and Rm, the 3 MA parameters provide the best correlations, whereas the correlations for A50 and Ag (not shown here) are the worst. Figure 17: Measuring procedure for determining the cold ends at the plate head and at the plate root. To a lesser extent, the steel sheet thickness also influences the 3 MA signals. Therefore, two separate calibrations for sheet thicknesses that are smaller and larger than 1.0 mm have proved to be beneficial. On the other hand, it was shown that batch or supplier variations have no detectable effect on the calibration functions. The widespread AlSi coating often develops a layer structure with a diffusion layer that consists of ferromagnetic Al-Fe phases. Besides the steel substrate, this diffusion layer will produce a second contribution to the 3 MA signals, which is distinguishable from the steel's signal contribution. This is shown in Figure 20.  
Figure 19: Results of the calibration for mechanical properties in AlSi-coated PHS: (**a**) hardness and (**b**) fracture. Figure 20: Barkhausen Noise (BN) signal (left) and IP signal of an AlSi-coated PHS part; the characteristic double-peak structure is clearly visible. Figure 18: Results of the calibration for the mechanical properties in AlSi-coated Press-Hardened Steel (PHS): (**a**) tensile strength and (**b**) yield strength.