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Online testing yield strength and deep-drawing parameters of strip steel

Industrial Requirements

Cold rolled and recrystallizing annealed sheet steel is a high-value product that is used, among other uses, for automotive body shell parts. Material parameters, such as yield strength Rp0.2, tensile strength Rm, and anisotropy parameters rm and ?r must be met with sufficient accuracy. In production, the strips are welded one after another to a coil of several kilometers in length. Typically, the material properties from above are tested by destructive tensile tests. It is clear that these tests cannot be done over the entire strip length. Therefore, only samples from the beginning and the end of the coil are taken for these tests. These samples undergo extensive and time-consuming tensile tests before the coil can be released for shipping. The time delay between sample collection and testing can reach up to several hours. A quick detection of, and reaction to, deviations in the production process is not possible with this time delay between sampling and testing.

Technology Application

In order to overcome these limitations, nondestructive testing (NDT) systems for the continuous inline determination of mechanical parameters in the entire steel strip are required. More than 20 years ago, Fraunhofer IZFP started research activities that were related to the development of such NDT equipment for strip steel testing. A first prototype system is described in a PhD thesis from 1997. This system, as shown in Figure below, was installed in a production line of ThyssenKrupp Steel in Duisburg, Germany.

Figure: First prototype system for continuous testing of strength (Rp0.2) and deep‐drawing (rm and ?r) parameters in strip steel based on a combination of the Incremental Permeability (IP) and the EMAT measuring technique: (a) Top view of the sensor, showing the IP sensor (IP) as well as the two EMAT sensors (EMAT_RD and EMAT_45°); and, (b) Complete table carrier with two stabilization rollers.

The prototype system was integrated after annealing and skin-path-rolling processes and just before coiling. At this point, the strip feed is in the range of 300 m/min. In this equipment, measuring quantities, as derived from the Incremental Permeability (IP) method, were used as the only micromagnetic parameters. Here, the narrow-band-filtering suppressing most of the wide-band environmental noise, which is typically present in strip steel production lines, has turned out to be particularly advantageous. Measuring quantities, as derived from the IP signal, were uMAX, Hcu, as well as DH25u, DH50u, and DH75u. These parameters have shown to be sensitive to the yield strength Rp0.2 of the strip steel.

Results of the calibration to yield strength Rp0.2 based on measuring parameters from IP measurements, combined with sheet thickness; nondestructively predicted values of yield strength (from IP) are shown as a function of destructively measured values (from ref.): (a) for a steel grade IF_1 and (b) for a steel grade

For that reason, it makes sense to either use an individual calibration model for each sheet thickness or to integrate the sheet thickness as a parameter in the calibration functions. The latter variant was applied in the work that is described here. As shown in Figure above, the correlation coefficients R2 were quite high (0.7–0.8) and the root mean square errors RMSE were quite low (4–6 MPa). The calibrations for rm and ?r provided values for R2 that were in the range between 0.6 and 0.8 and RMSEs between 0.04 and 0.06. Nevertheless, it has to be considered that the value ranges are quite low for these calibrations, which promotes small RMSE values.

Online measurement of yield strength Rp0.2 in a coil of 2300 m in length. Destructive measurements of Rp0.2 at the beginning and at the end of the strip are also shown as white squares with error bars.

Online measurement of vertical anisotropy rm and planar anisotropy ?r in a coil of 2300 m in length.

Based on the above calibrations, continuous inline measurements of Rp0.2, rm, and ?r were made. Figures above show some of the results. In the measuring plot of upper Figure, it can be observed that the strip leaves the acceptance range (130–180 MPa) at position x = 2100 m. This is later confirmed by the destructive measurements. The results of the inline measurements of rm and ?r in the same coil are shown in lower Figure.

For this reason, historical facts prove that the integration of EMAT (Electromagnetic Acoustic Transducer) technology and incremental permeability IP technology is conducive to the continuous measurement of strip yield strength and deep drawing properties such as anisotropic parameters. For more information, please contact LAB GAGES, or go on to visit our 3MA series Nondestructive Testing Systems Product Center.