## 4 Applications
### Applications in the Steel Industry
#### 4.2.1 Inline Strip Steel Testing

Table 7: Steel grade classes and results of verification measurements for Rm and Rp0.2. Figure 10: Results of the verification of the calibration; comparison of the values from 3 MA measurements with destructively measured values: (**a**) for tensile strength, Rm and (**b**) for yield strength, Rp0.2. where \(u_{nd}\) and \(u_{d}\) are the uncertainties of the nondestructive measurement and of the destructive measurement, respectively. The values of the RMSE, as shown in Table 7, are close to the known measuring uncertainties of the destructive tensile test, indicating that the measuring uncertainty of 3 MA is in the same range. Inline measurements of Rm and Rp0.2 with 3 MA can be used in order to precisely determine the acceptance range of a coil, as is shown in Figure 11. The threshold level (acceptance line) of 260 MPa is not exceeded in the strip position range x \(>\) 200 m and x \(<\) 2400 m.  
In later developments, the 3 MA systems for inline strip steel testing were also calibrated to surface hardness and residual stress at different depths. These calibration results are shown in Figure 12.  
Calibration to hardness and residual stress was made with different steel grades from tool and spring steels (75 Cr1, CK75, 80 CrV2), low and unalloyed steels (C55, C75), and heat-treatable steel (50 CrMo4), providing hardness levels that were between 40 and 54 HRC (Rockwell hardness type C). Using X-ray diffraction determined the reference values for residual stress. The different depths for the x-ray measurements were uncovered by electropolishing. Again, the measuring results from the calibrated 3 MA system were compared with the results from destructive measurements. For hardness, the RMSE was in the range of 1 HRC. For residual stress, RMSE values that were between 1 nd 47 MPa were determined, dependent on the steel grade and the measuring depth (see Table 8). Figure 11: Inline measurement of yield strength Rp0.2 in a coil of 2500 m in length; destructive measurements of Rp0.2 at the beginning, at the end and in the middle of the strip are also shown as red dots; the acceptance line indicating the threshold level of Rp0.2 = 260 MPa is shown. Figure 12: Results of the calibration to: (**a**) hardness and (**b**) residual stress at the surface and at depths of 50, 100 and 200 \(\upmu\)m (all RMSE values in MPa). Based on these calibrations, the hardness and residual stress can be determined as a function of strip position. Figure 13 shows a typical example. At the beginning of the strip, strong variations of residual stress can be observed. The hardness is increased. After x = 60 m, the strip has calmed down, i.e., hardness and residual stress have reached their \"normal\" values. Meanwhile, a wide range of 3 MA installations for inline strip steel inspection exists. Figure 14 shows two of them. In the image of Figure 14a, the 3 MA probe is applied above a large deflection roller. On the other side, the probe head may also be applied with a robot, as shown in Figure 14b. Meanwhile 3 MA applications on hot strip, up to 300 \({}^{\circ}\)C can be realized, if the probe head is actively cooled with pressurized air. Other optimizations of the probe head design allow for a lift-off of up to 5 mm. Figure 14: Further variants of 3 MA probe head application on the running strip: (**a**) on a large deflection roller and (**b**) guided by a robot. Figure 13: Inline measurement of hardness and residual stress at the surface and at depths of 50, 100, and 200 \(\upmu\)m. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Steel Grade**} & \multicolumn{4}{c}{**Residual Stress, RS**} \\ \cline{2-5}  & **At the Surface** & **At 50 \(\upmu\) Depth** & **At 100 \(\upmu\)m Depth** & **At 200 \(\upmu\)m Depth** \\ \cline{2-5}  & **RMSE [MPa]** & **RMSE [MPa]** & **RMSE [MPa]** & **RMSE [MPa]** \\ \hline
75 Cr1 & 47 & \(1\) & 3 & 4 \\
80 CrV2 & 15 & \(1\) & 9 & 31 \\
50 CrMo4 & 4 & 4 & 4 & 11 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Steel grades and results of verification measurements for residual stress.