Non-destructive magnetic evaluation of microstructure and mechanical properties of advanced high-strength steels

Because of more demanding mechanical properties in sheet steel products in the automotive industry, the use of multi-phase steels, capable of complying with those mechanical requirements, has been growing along the last decades. The non-destructive characterization of these steel grades, therefore, has become an active research line worldwide. In the present work, the magnetic characterization by magnetic hysteresis BH loops of various strip samples of industrially cold rolled and annealed DP800 (dual phase) and CP800 (complex phase) steels at several positions with varying processing temperatures in the coil length has been performed. In addition, a detailed EBSD characterization of the samples has allowed determining the fraction of the various microconstituents, namely, ferrite, bainite, martensite and retained austenite. Finally, the mechanical characterization has been done through hardness and tensile tests on all the samples. All the results regarding these three aspects have been analysed carefully to extract likely relationships among them. Clear one-to-one linear correlations have been found between all these features, even though large variations in mechanical/magnetic properties and microstructures have been attained. Specifically, for volume fractions of bainite approximately between 15% to 75%, the yield and tensile strengths show a linear correlation with the coercive field, Hc. Thus, the coercive field may be a candidate to become a parameter for the prediction of the mechanical properties if a previous calibration is performed on these steel grades with multi-phase microstructures.