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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.

On-line non-destructive magnetic monitoring (NDM) techniques are relevant to evaluate the quality of steels and their fluctuations during forming process. The on-line monitoring faces different issues. Due to magneto-mechanical coupling, stress that may be applied during NDM may have a strong effect on the magnetic behavior that may lead to erroneous estimations of the microstructure quality of the material. The multiaxial character of the stress state is another strong issue. In this communication, some new results showing the effect of a uniaxial and biaxial stress on the magnetic behavior of 3 grades of hot-rolled steels are presented (from low carbon steel to AHSS).

Weight savings in mobility and transport are mandatory in order to reduce CO 2 emissions and energy consumption. The steel industry offers weight saving solutions by a growing portfolio of Advanced High Strength Steel (AHSS) products. AHSS owe their strength to their largely refined and complex microstructures, containing multiple metallurgical phases. Optimal control of the thermo-mechanical processing of AHSS requires inline sensors for real-time monitoring of evolution and consistency of microstructure and material properties. To coordinate and accelerate European development activities in this domain, the project ”Online Microstructure Analytics (OMA)” was established in 2019, constituting of a consortium of 14 specialised research organisations. The EU-funded OMA project, with a total budget exceeding 6 MEU, focuses on inline sensing techniques to monitor the consistency and homogeneity of microstructure, texture and mechanical properties for automotive steels and in particular for AHSS.