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Inline Inspection System Revolutionizes Rolled Steel Sheet Quality Assurance

The flawless material density and surface finish of cold-rolled steel is essential for subsequent processing into high-quality products. The smallest material defects of all kinds can lead to material weaknesses in the form of inhomogeneities only becoming visible at the very end of a processing operation such as coating, lead to material breaks in forming processes or even damage tools in the production process. This not only entails high follow-up costs, but ultimately also the risk of losing customer confidence.

So far, manufacturers have used optical inspection systems with camera and sensor technology as well as industrial image processing to check the surface quality of strip steel. However, if the material has internal inclusions, shell defects or surface defects that cannot be detected optically, these have so far remained undetected with conventional testing and measuring methods.

Representation of a rolling and coating defect, material thickness 660 µm, and its signal curve

IMS Messsysteme. a pioneer for innovative testing and measuring systems for the steel, aluminum and non-ferrous metals industry, has developed the first inclusion detection system (IDS) for ultra-thin sheets up to 1.0 mm thick for the early detection of internal inclusions and shell defects using magnetic leakage flux. The system detects previously undetectable defects in coated and uncoated ultra-fine and thin sheets from 0.1 to 1 mm thickness precisely, without contact and at high speed in the ongoing process. By using the inclusion detection system, steel cold rolling mills can ensure the homogeneous material structure and thus the perfect quality of strip steel before it is further processed with high degrees of deformation.

“Unlike an optical surface inspection system, the Inclusion Detection System (IDS) is able to detect defects that are often completely internal to the material and for which there is no discernible sign on the material surface. With the further developed IDS, we enable finest and now also fine sheet metal producers and their customers to check the quality of the material for internal inclusions, shell and minimal surface defects before further processing. Internal inclusions, for example, lead to material fractures in the deep-drawing process, and shell defects with a small difference in height that are covered by a coating often only become visible in the form of optical irregularities on finished painted car parts” explains Dipl. Ing. Christian Knackstedt, Division Manager Sales & Marketing.

If the system recognizes such faulty areas that would lead to rejects and customer complaints in the subsequent processing steps, early on and during the ongoing process, these areas can be specifically extracted from the strip. In this way, manufacturers can ensure and document the consistently high quality of their steel products. Customers, who then process the strip steel into high-quality products, benefit in particular from this.

Electromagnets are used transversely to the rolling direction to magnetize the material. The performance of these can be adapted to the nature, structure and geometry of the material to be inspected, and they can be switched off for maintenance and cleaning purposes. One magnet each and the sensor line it contains are combined into a compact sensor module. The modules are designed with protection class IP 64, which enables them to be used directly in harsh environments. To ensure mechanical stability, the rows of sensor modules and their carriers are cooled with water.

The digitized sensor signals from up to 8 sensor modules are fed to a common Gig-E hub and converted to the Gig-E camera standard. The Gig-E hubs are connected to a camera computer, which takes care of signal pre-processing, error detection, feature calculation, classification, and control and adjustment of the sensor modules. The visualization of the errors and the connection to the customer database takes place via the higher-level database server. This is where the error images are saved and this is also where the production and training database is located.

The IDS is particularly useful for manufacturers of fine and ultra-fine sheet metal who have to guarantee the highest quality standards for the automotive and food industries. The image processing used with feature calculation and classification distinguishes existing material defects based on their size and type of defect. The measuring system and the defect classification is adapted depending on the customer specification and material.

By using the system, users can not only guarantee the high quality of their products, but also document the material structure thanks to the complete evaluation. The costs of purchasing the system can be amortized within a short period of time, as companies reduce the effort required for rework, their scrap rate and possible costs for complaints or even claims for recourse due to material defects.

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