Steel heavy plates, grade S355, micro‐alloyed with Vanadium‐V and/or Niobium‐Nb plus Titanium‐Ti in thicknesses from 5 to 60 mm, 200.000‐350.000 t/y, are produced according to EN 10025 at STOMANA S.A., a company of the SIDENOR Group in Pernik Bulgaria, and are exported to the European Market. These plates fulfil high quality standards as they are used for constructions and engineering applications (e.g. high‐building constructions, bridges, shipping applications, cranes, etc.). Often intermediate and/or final products (slabs and plates, respectively) suffer from surface and/or internal defects, which deteriorate the final product's quality. The purpose of this paper is to look at the challenging task of eliminating the external and especially the internal defects.
ELKEME performs root‐cause analysis and proposes improvement actions. For these purposes light optical metallography (LOM) and scanning electron microscopy (SEM) with EDS were applied. For the analysis a NIKON SMZ 1500 stereoscope (up to 100x), a NIKON epiphot 300 inverted metallographic microscope (up to 1000x) and a Philips XL‐40 SEM were used.
Most surface defects are attributed to copper (having its origin mainly from scrap or from mould's wear due to bad lubrication), or casting powder entrapping, cracks at deep oscillation mark points or transverse cracking, with the majority occurring during continuous casting. High‐copper amounts in the steel cause hot shortness issues. Hot tears in the surface of “as‐cast” material lead to flakes and tears in the plates after hot rolling. The torn surfaces are heavily oxidized and decarburized if oxidizing‐conditions exist in the reheating‐furnace. Internal defects are related with large‐concentrated MnS stringers and entrapped in the steel desoxidation products. Additionally, based on carbon amount of the cast steel, macro‐segregation can lead to crack initiation and propagation along the centreline.
This work refers to industrial research widely applied and focused. Sampling and root cause analysis is never easy in an industrial environment. The most difficult part is to identify the critical process conditions that reflect to negative quality issues in the final product.
Internal defects, especially centreline segregation and inclusion clustering, are important imperfections that deteriorate material properties and jeopardize the products’ structural integrity. The paper discusses possible root‐causes in relation to the overall production processes, concluding in improvement actions for in‐plant operation given the equipment limitations of the very specific production site.
Papaefthymiou, S. (2013), "Root cause analysis of surface and internal defects of micro‐alloyed S355 heavy plates", International Journal of Structural Integrity, Vol. 4 No. 1, pp. 91-107. https://doi.org/10.1108/17579861311303654Download as .RIS
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