A Novel Method to Measure Cleanliness in Steel Using Ultrasonic C-Scan Image Analysis
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INTRODUCTION
CLEAN steel in terms of its content of macroinclusions has always been a big problem for steel producers, because its severity greatly affects the quality and reliability of the finished product. It has been reported that total oxygen (TO) in steel strongly influences the surface quality of auto grade steel, particularly by generating sliver defects and macroinclusion in steel.[6] To study and control steel cleanliness, it is critical to have accurate methods for its evaluation. Measurement techniques range from direct methods, which are accurate but costly, to indirect methods, which are fast and inexpensive, but only reliable as relative indicators.[1] The present article illustrates a novel method to measure cleanliness levels in samples taken by a specific sampling device[2] from liquid steel using ultrasonic C-scan image analysis and high-frequency focused beam probes. It is a direct method to measure cleanliness in steel samples and is different from other methods such as Mannesmann inclusion detection by analysis of surfboards (MIDAS)[3] or liquid sampling hot rolling (LSHR).[4] In this method, ultrasonic testing and analysis is done on the cast samples (samples taken from liquid steel from tundish or ladle using a special sampling device) only, whereas in MIDAS and LSHR methods, hot rolling of the cast samples is required due to which the results are delayed. The present method can be used by the steel plants, which do not have the experimental facilities for hot rolling of the small steel samples. The advantages over the microscopic and spectroscopic methods[1] are that these methods provide the
planar (x-y plane only) information about the inclusions, whereas the present method provides the volumetric information due to through thickness transmission of ultrasonic beam and this volumetric information is provided on the x-y plane of the C-scan image. Although the slime extraction (electrolysis) method[1] appears to be more accurate and provides information from the comparatively large billet/slab samples, it takes too much time and decisions based on the results are delayed. The methods mentioned such as use of a liquid metal cleanliness analyzer (LIMCA)[1] require a specific ultrasonic sensor system to withstand liquid steel temperature, and the experiment cost can be high due to consumption of the sensor in hot steel during each experiment. The authors[5] developed a method using a conventional ultrasonic flaw detector with 10 MHz frequency probe by which different casts of continuously cast billets and slabs could be ranked on the basis of an index formulated from the low, medium, and high amplitude ultrasonic reflections from the inclusions. The testing was manually performed on hot-forged billet/slab slices, and scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) analysis revealed typical inclusion sizes of 210 to 325 lm. The present method discussed in this article using ultrasonic C-scan imaging is fairly accurate and provides realistic and quick information about the inclus
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