## 3 Results and Discussion
### Influence of Thermomechanical Treatment and Matrix Chemistry  
Surface heterogeneity in low-alloyed high-strength steels with respect to MBN and the corresponding decarburization is a function of two different aspects. The first one is linked with the duration of the high-temperature cycle during hot rolling. The second one is associated with the chemical composition, especially the C content in the matrix. Figure 16 clearly depicts that the degree of surface heterogeneity for 1100 QL (expressed in MBN and PP values) is much less pronounced than that for 1100 MC (see Figure 5).  
The MC concept represents direct quenching of the sheet from the hot rolling temperatures when the final sheet thickness is obtained. The QL concept represents hot rolling followed by moderate self-cooling. The final martensite microstructure is obtained during the next heat treatment step when heating on the austenitizing temperature without rolling process assistance is followed by rapid cooling. The exact conditions with respect to hot rolling (especially the hot rolling temperature and duration of the high-temperature cycle) as well as quenching are not known (kept secret on the side of a producer). It is considered that the prolongation of high-temperature regimes results in a higher degree of surface decarburization. For this reason, the QL concept should suffer more from decarburization. However, the MBN and _PP_ fluctuation range is much lower for the QL concept as contrasted against the MC concept. Also, the metallographic images depict that the near-surface microstructure alterations are less pronounced; see Figure 17. Furthermore, the _HV0.1_ depth profiles demonstrate that the valuable differences in microhardness can be found in the very near surface only (about 20 \(\upmu\)m), and _HV0.1_ in the subsurface layers are quite similar; see Figure 18. Being so, it is considered that the differences in the C content play a major role. Due to the much lower C content in 1100 QL (0.08%), the diffusion rate of C is much slower than that for 1100 MC with a C content of 0.15%. Expressed in other words, the C gradient becomes steeper at elevated temperatures for the steels with higher C content, which in turn accelerates the decarburizing process.  
Monitoring HSLA steels undergoing thermo-mechanical treatment using MBN is a challenging task. On the one hand, the contribution of remnant magnetization can be fully removed by sufficiently demagnetizing. This process can be quite easily checked by visual observations of consecutive MBN bursts. However, variable decarburization mixed with the components' real loading makes the application of the MBN technique for monitoring the components in use a debatable issue since the synergistic contribution of an altering microstructure and the superimposing influence of stress might be difficult to unwrap. This aspect needs further research together with deeper insight into decarburization quantification and its influence on microstructure features (the presence of fine carbides). A clear understanding of the complex behavior is vital of importance with respect to the reliable application of the MBN technique especially in the automotive and civil industries.  
Figure 16: MBN and _PP_ along the sheet width for \(1100\) QL: (**a**) MBN QL and (**b**) _PP_.