Mechanical Property Determination of Heavy Steel Plates and Cold Rolled Steel Sheets by Micro-magnetic Ndt

The presentation is to results recently obtained in collaborative research in the European steel programme performed by Fraunhofer-IZFP and co-operative steel industry producing rolling products. The objective was and is to characterize i.e. to predict mechanical properties like hardness, yield strength, tensile strength, deep drawing parameters rm and ∆r, etc., mainly online by use of contact-less NDT. The approach performed developed a decade before is by micro-magnetic NDT was optimized and especially adopted to sensitively classify a wider range of steel grades. The contribution reports to the technology development, the transducer optimization and the technology qualification. Introduction: Mechanical properties are quality parameters characterizing the fitness for use of rolled products. State of the art to determine yield strength and tensile strength is the selection of standardized specimens at the end of the process and destructive testing according to the definition in the inspection laboratory using standard tensile testing machines. Hardness measurements are performed by using standardized indentation techniques according to Brinell or Vickers. Not satisfying for the producer is the fact that the material – per definition – is destroyed or locally damaged and that this kind of material examination can be performed only statistically and not continuously online. There is therefore the tendency to develop non-destructive (ndt) techniques which – after a calibration by using the destructive techniques can replace the destructive test. In the institute of the authors the topic is an objective and for years ndt-technology was developed and introduced into online process monitoring and control of production lines in the steel industry [1]. Besides ultrasonic techniques based on phase velocity measurements to characterize texture in cold rolled steel sheets and the vertical and planar anisotropy parameter (rm and ∆r) by using EMAT [2] mainly electromagnetic, respectively micro-magnetic techniques were applied [3] and introduced into practice under the acronym 3MA (Micro-magnetic, Multiparameter, Microstructure and Stress Analysis). Micro-magnetic techniques are especially suitable to determine mechanical properties because there are many similarities between the interaction with microstructure parameters and lattice defects and movement of dislocations under mechanical loads and the movement of Bloch-walls under magnetic loads. Figure 1 documents this fact. microstructure parameter: vacancies dissolved atoms dislocations precipitates grain boundaries phase boundaries inclusions pores mechanical properties hardness, yield strength tensile strength, upper shelf FATT Micro-magnetic parameters 3MA Strengthening by lattice defects is impeding of dislocation movement lattice defects impede Bloch wall movements Figure 1: Micro-magnetic microstructure characterization, 3MA Figure 2: 3MA inspection System 3MA is the combination of different micro-magnetic testing procedures: The measurement of the magnetic Barkhausen noise, the incremental permeability, the higher harmonic analysis of the tangential magnetic field and an eddy current impedance measurement. Over all, 22 different parameters are derived by the measurement and can be combined in a multiple regression model in order to determine the mechanical properties [3]. The special intelligence is designed into the transducer head, based on a u-shaped yoke magnet, which by time-multiplexing allows the allocation of all parameters. The 3MA-systen is shown in Figure 2. The magnetic field strength applied was up to 40A/cm. Application on heavy steel plates: In co-operation with the companies Dillinger Hütte AG, IRSID, Fabrique de Fer, Salzgitter AG and ThyssenKrupp-Stahl AG a research project was performed under the umbrella of the European Community of Coal and Steel. Table 1 documents the material selected by the companies of the different partners. Coupons were taken from the running production and destructively tested in the inspection laboratory of one partner. Each of the 88 coupons was measured at 8 different positions in order to have a better statistical approach. The values from position 1-4 were used for calibration and the values from position 5-8 for verification. The regression model applied here was based on 17 unknown coefficients and polynomials in the micro-magnetic parameters up to the 2 order but also non-linear terms like mixed products and quotients. As target functions hardness, yield strength and tensile strength were selected. As function of the hardness a regression coefficient R with R= 0.97 can be found and a residual standard deviation ∆Hardness=11.9 HB-units. Table 1. Inspected material Steel grade Heat treatment Thickness range in Specimens