Why do the Fixed Volume Gauge standard and GB/T150 require certain steel plates to undergo ultrasonic testing?

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What are the considerations behind such regulations? This is because raw steel plates are prone to internal defects such as folding, delamination, inclusions, and cracks during the production process (such as rolling). For steels used in special applications like pressure vessels, manufacturers can use ultrasonic testing to carry out quality grading; steel plates that do not meet the required standards are handled in accordance with relevant material management regulations. Folding (double skin) involves the formation of a folded metal layer on the surface or near the surface of the steel plate during rolling. In layered slab billets, defects such as shrinkage cavities, non-metallic inclusions, or metal oxides and sulfides fail to weld together due to flattening during rolling; these defects usually appear in the middle or near the middle of the steel plate, and are generally parallel to the surface of the plate. Inclusions are caused by the failure to weld together during rolling due to a low compression ratio, and their locations are distributed randomly. Among them, internal inclusions refer to central carbon-manganese segregation that results in a high number of low-melting-point inclusions in the molten steel, whereas external non-metallic inclusions are formed by impurities introduced from sources such as the degradation of the furnace wall refractory materials. Cracks are mainly white spots or heat treatment cracks. White spots are small cracks formed when hydrogen does not have enough time to disperse during rolling cooling; they pose a significant risk (refining furnaces can help prevent such defects effectively). Slab heat treatment cracks are caused by improper heat treatment, or by thermal stresses in the heat-affected zone during cutting, resulting in cracks that are perpendicular to the rolling direction of the steel plate; they generally occur on the surface of the steel plate, but occasionally also inside it.