## 4 Applications
### Applications in the Steel Industry  
#### 4.2.1 Inline Strip Steel Testing  
Cold rolled and recrystallizing annealed sheet steel is a high-value product that is used, among other uses, for automotive body shell parts. Material parameters, such as yield strength Rp0.2, tensile strength Rm, and anisotropy parameters \(\mathrm{r}_{\mathrm{m}}\) and \(\Delta\mathrm{r}\) must be met with sufficient accuracy. In production, the strips are welded one after another to a coil of several kilometers in length. Typically, the material properties from above are tested by destructive tensile tests. It is clear that these tests cannot be done over the entire strip length. Therefore, only samples from the beginning and the end of the coil are taken for these tests. These samples undergo extensive and time-consuming tensile tests before the coil can be released for shipping. The time delay between sample collection and testing can reach up to several hours. A quick detection of, and reaction to, deviations in the production process is not possible with this time delay between sampling and testing. In order to overcome these limitations, nondestructive testing (NDT) systems for the continuous inline determination of mechanical parameters in the entire steel strip are required. More than 20 years ago, Fraunhofer IZFP started research activities that were related to the development of such NDT equipment for strip steel testing. A first prototype system is described in a PhD thesis from 1997 [63]. This system, as shown in Figure 4, was installed in a production line of ThyssenKrupp Steel in Duisburg, Germany. The prototype system was integrated after annealing and skin-path-rolling processes and just before coiling. At this point, the strip feed is in the range of 300 m/min. In this equipment, measuring quantities, as derived from the Incremental Permeability (IP) method, were used as the only micromagnetic parameters. Here, the narrow-band-filtering suppressing most of the wide-band environmental noise, which is typically present in strip steel production lines, has turned out to be particularly advantageous. Measuring quantities, as derived from the IP signal, were \(\mathrm{\mu}_{\mathrm{MAX}}\), \(\mathrm{H}\mathrm{c}\mathrm{\mu}\), as well as \(\mathrm{DH25\mu}\), \(\mathrm{DH50\mu}\), and \(\mathrm{DH75\mu}\) (see Section 3.4). These parameters have shown to be sensitive to the yield strength Rp0.2 of the strip steel. A further disturbance to be considered is the lift-off variation due to vibrations or ripple of the running strip. The electromagnetic signal detection depends on the distance between the sensor coil and the material's surface, i.e., the lift-off. In order to reduce its disturbing influence to an acceptable level, controlling the lift-off in the range to 2 mm \(\pm\) 0.5 mm was necessary, which was realized by the pretension of the strip with two rollers that were integrated into a roller-table. Further attempts were made to determine correlations between the nondestructive measuring parameters and anisotropy parameters of the steel, like the mean vertical anisotropy \(\mathrm{r}_{\mathrm{m}}\) and the planar anisotropy \(\Delta\mathrm{r}\). For this purpose, it turned out to be advantageous to also implement the so-called EMAT technique (electromagnetic acoustic transducer), in addition to the IP technique. The EMAT principle is described elsewhere [64]. The EMAT sensors were used to measure the phase velocity of the first symmetrical shear-horizontal plate mode of an ultrasound (US) wave that was travelling in the rolling direction (EMAT_RD) and 45\({}^{\circ}\) to this (EMAT_45\({}^{\circ}\)). Based on these measurements, \(\mathbf{r}_{\text{m}}\) and \(\Delta\mathbf{r}\) could be determined [51]. Investigations were conducted on a soft deep-drawing material of interstitial-free steel (IF-steel). Three different grades of IF steel with differences in nitrogen, niobium, and titanium contents were used. The inline calibration was carried out on 353 strips of these steel grades.