Micromagnetic testing for rolled steel

In forming of steel by hot and cold rolling a broad range of semi-finished and final products can be produced with very specific, custom-tailored technological properties. Here, the characteristics of mechanical-technological tests (tensile, hardness, creep, fatigue test) are often of central importance. Obviously the traditionally process orientated steel industry has a strong interest to replace timeconsuming and expensive destructive mechanical tests by non-destructive (nd) methods, which can be applied in closed vicinity or even integrated within the production process. Micro-magnetic testing, i.e. nd material characterization with electromagnetic methods is generally suited for this purpose, because the mechanical properties of a ferromagnetic material and its intrinsic magnetic properties are influenced by the same microstructure parameters. Furthermore the micro-magnetic measuring parameters are sensitive to load-induced and residual stresses. The so called 3MA technique (Multiparameter Micromagnetic Microstructure and stress Analyzer) is a combination of four micro-magnetic methods. After calibration it allows to predict mechanical properties like tensile, yield strength and hardness but also residual stresses in steels. Recently, applications of this technique for different testing tasks in steel industry have been investigated in three research projects, funded by the former European Coal and Steel Community (ECSC). A first project was concerned in the improved non-destructive determination of mechanical properties of cold rolled and recrystallising annealed sheet steel. In this context a fully automated 3MA testing system for in-line determination of tensile and yield strength in moving steel strips was implemented in a hot dip galvanizing line. Task of a second project was the determination of mechanical properties in heavy plates. For this purpose a 3MA system was integrated in a remotely controllable testing trolley, which can be applied in a rolling conveyor. Finally in a third ECSC project it was analyzed, if the extension of 3MA by a Dynamic Magnetostriction module allows to precisely monitoring internal stresses in strip and plate.