EMG IMPOC is used worldwide in the steel industry to monitor the mechanical properties of ferromagnetic steel strip directly in the production process. The measuring principle is based on measuring the electromagnetic properties of the steel to be examined and using correlation relationships with the results of mechanical testing to generate statistical models for mechanical parameters such as tensile strength and yield strength of the steel strip.
Based on these models and the magnetic field values measured inline, the tensile strength and yield strength of the steel can then be displayed online without delay and used, for example, for process control or optimisation.
But how exactly are these electromagnetic signals related to the properties of the steel? A scientific paper from the European research project Product Uniformity Control (PUC) [1] investigated precisely these relationships and developed models to better understand and evaluate the IMPOC signal physically.
How does EMG IMPOC work? The physical measurement principle behind the inline determination of mechanical properties
EMG IMPOC is based on an electromagnetic measurement principle that is only suitable for ferromagnetic steel strips. As the strip runs through the production line, electromagnetic coils generate a short, strong magnetisation pulse. This pulse magnetises the steel locally to near magnetic saturation. After the magnetisation pulse has ended, a remanent magnetisation remains in the material.
Highly sensitive magnetic field sensors, positioned a few centimetres behind the magnetisation unit, measure the gradient of the residual magnetic field strength of the remanent magnetic field in A/m2. This measured value is stored as a characteristic IMPOC value – also known as the IMPOC raw value.
Since the magnetic properties of steel are closely linked to its microstructure, the measured signal also changes when the strength or microstructure of the material changes. On this basis, statistical correlation models can be used to determine mechanical characteristics such as yield strength or tensile strength along the entire length of the strip. This provides the user with continuous online information about the material properties without having to take samples or interrupt the production process.
EMG IMPOC and microstructure: Why magnetic signals allow conclusions to be drawn about strength
EMG IMPOC exploits a fundamental relationship in materials physics: the magnetic properties of ferromagnetic steel are closely related to its microstructure. Changes in structure, phase proportions, dislocation density or grain size influence not only the mechanical properties of the steel, but also its magnetic behaviour.
It is precisely this relationship that was scientifically investigated in the European research project Product Uniformity Control. The researchers were able to show that differences in the microstructure lead to measurable changes in the magnetic response of the material. Particularly relevant here are the hysteresis properties of the steel, which describe how magnetisation and magnetic field influence each other.
Since the mechanical strength of modern steels is strongly determined by their microstructure, indirect correlations arise between magnetic measurement signals and mechanical characteristics. The EMG IMPOC system uses these correlations to draw conclusions about the yield strength and tensile strength of the strip steel from the measured magnetic field gradient. This allows the mechanical properties to be measured indirectly during the ongoing production process.
EMG IMPOC in the focus of research: findings from the RFCS project Product Uniformity Control
EMG IMPOC was intensively investigated in the European research project Product Uniformity Control (PUC). A key result of this work is described in the paper "Modelling the IMPOC Response for Different Steel Strips". The aim of this study was to better understand the physical mechanisms behind the IMPOC measurement signal and to develop a model for calculating the sensor response.
The work focused on how the electromagnetic signal is formed after the magnetisation pulse was generated. To this end, the researchers developed a numerical model that solves Maxwell's equations for a ferromagnetic material with non-linear and hysteretic magnetisation. The magnetic B-H curves of the respective steel serve as important input variables.
The simulations show how different steel grades, microstructures and material parameters influence the IMPOC signal. The image (according to [1]) shows the good agreement between the IMPOC values and the simulated values for the measurement signal for a DP steel. At the same time, it became clear that process variables such as strip speed or strip thickness also change the measurement result.
Limitations and influencing factors of inline measurement in steel production
Like any inline measurement technology, EMG IMPOC is influenced by various process and material parameters. Research from the RFCS Product Uniformity Control project and practical use of the system in production show that, in addition to the microstructure of the steel, factors such as strip speed, strip thickness, strip tension and the distance between the sensor and the strip surface can also alter the measured signal. These effects sometimes overlap and can vary in intensity depending on the steel grade.
In order to obtain reliable measurement results, a good physical understanding of the measurement principle is therefore necessary. This is precisely where scientific modelling came into play in the project: by combining experimental measurements and numerical simulations, the most important influencing factors could be identified and better understood. The findings were directly incorporated into the further development of the EMG IMPOC system. They contributed to a more robust interpretation of measurement signals, made the inline determination of mechanical properties in industrial practice even more reliable, and enabled the system to be further developed for new steel grades, such as AHSS grades.
Conclusion
Our deep dive shows that EMG IMPOC combines industrial measurement technology, materials’ physics and data-based modelling to create a powerful tool for modern steel production. Scientific work such as that carried out in the context of the RFCS project has laid important foundations for understanding how electromagnetic signals relate to the microstructure and mechanical properties of steel. At the same time, the broad industrial application – around 90 systems are now in use worldwide – shows that the technology has proven itself in practice. Current developments such as the IMPOC Booster, which incorporates additional process and material data into the modelling, further improve the accuracy and robustness of inline prediction.
What's next?
If you would like to learn more about EMG IMPOC, please visit our website, download the latest product brochure or contact our sales or product management team directly (impoc@emg-automation.com). We would be happy to help you leverage the possibilities of inline determination of mechanical properties in your production.
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Source:
[1] Skarlatos, A.; Reboud, C.; Svaton, T.; Martinez-de-Guerenu, A.; Kebe, T.; Van den Berg, F. (2016): Modelling the IMPOC Response for Different Steel Strips. 19th World Conference on Non-Destructive Testing (WCNDT 2016).