Inline surface inspection of very fast hot wire rods
voest Wire Rod Austria recently opened the world’s most advanced wire rod mill in Styria. In order to maintain a leadership position in the harsh competition of the international steel industry, high product quality is of essential importance. Within the scope of Vision+ JOANNEUM RESEARCH developed an innovative optical high-speed inspection system for gapless wire surface testing. The system thus makes a valuable contribution to ensuring product quality and builds an important complement to the existing quality management system of the wire rod mill.
During the hot-rolling process in industrial wire production semi-finished steel billets are heated to red heat and subjected to a multi-stage transformation. Starting from a square profile the processed product successively ends up in circular wire rod. At the end of the rolling train the wire rod attains a velocity of up to 400 km/h and a total length of several kilometres. The different wire products have diameters from 5 up to 60 mm and serve as semi-finished goods e.g. for screws, bolts and chassis components.
The steadily increasing customer requirements for the quality of the produced wire rod demand increasingly better control of the process parameters on the one hand, and also a best possible control of the finished product on the other hand.
The produced red hot wire rod is wound in coils for further transport and processing. In this condition, a surface inspection of the wire is not feasible, therefore the need arises to inspect the wire in red-hot state at maximum feed speed after the last rolling step, see Fig 1.
Fig. 1: Inline test image acquisition system in action at the rolling train (© JR)
A very nifty surface inspection concept is based on the use of line scan cameras and image based defect detection algorithms. In order to ensure a high image quality at resolutions of about 0.5 mm per scan line the high wire feed rate requires high sampling frequencies and, consequently, high light input.
Steel processing plants in general and rod mills in particular represent a very rough environment for sensitive sensors, optics and electronic components. Large temperature fluctuations, vibrations, moisture, dust and dirt, as well as 24/7 operation greatly afflict a process-integrated inspection System.
The camera based inspection system developed by JOANNEUM RESEARCH meets the described requirements. It consists of three line scan cameras in order to comprehensively capture the whole wire surface. Each camera is controlled by a dedicated computing unit which receives the captured image data and performs defect detection as well. The necessary light input in mega lux range is provided by circularly arranged, water cooled high performance LED light bars.
Fig. 2: Detected defects during production (© JR)
The defect detection algorithms rely on fast decision tree based classifiers using statistic image features. Beside single defects it is also possible to identify periodic errors which allow conclusions to be drawn about a defective roll. Fig. 2 shows examples of detected surface defects during production process.
The images necessary for training the defect detection algorithms were taken from 1813 km of wire rod with different diameters. To be able to handle the enormous training data amount of several terabytes “WireInspector” was developed. The software allows lot-based visualisation of very long wire images as well as analysis and annotation of surface defects. Fig. 3 shows the graphical user interface (GUI) of WireInspector.
Fig. 3: WireInspector. Software for visualisation and
annotation of wire surface defects (© JR)
Impact and effects
The steel industry has been subject to the strong trend of globalization for decades. Competition on the steel market is steadily becoming tougher. In order to prevail against competitors from countries with low wage levels and rudimentary or no environmental regulations, it is essential to focus on high-tech products, on automation and last but not least on the highest quality.
The implemented image based inline inspection system by JOANNEUM RESEARCH enables a robust 100% in-process testing of high speed, red hot wire surface and comprehensive documentation. The system makes an important contribution to the sustainable assurance of product quality at voestalpine Wire Rod Austria and, not least, to hundreds of qualified local workplaces. The achieved solution will be transferred to a productive system in a follow-up project by voestalpine Wire Rod Austria and JOANNEUM RESEARCH in the near future.