Digital measurements of non-metallic inclusions in steel

The experience of many-year research has shown that optimizing the steel chemical composition and microstructural characteristics, as well as reducing its contamination with non-metallic inclusions (NMI), it is possible to significantly increase the corrosion resistance of oilfield pipeline steels and increase the time of their trouble-free operation. The influence of complex NMIs on the steel corrosion resistance is determined by both the chemical composition of NMIs and their quantitative ratios. Therefore, obtaining metal products of the required quality is possible only when using the “control by structure” principle. In the work, based on the analysis of brightness fields of images (on a sample scale) in 256 shades of gray, the authors proposed digital, metrologically supported procedures for measuring the NMI heterogeneity of low-carbon oilfield steels: eliminating the heterogeneity of field illumination, justifying the criteria for binarization and noise filtering. For low-carbon steels of various types of melting, the authors identified the key role of dispersed non-metallic inclusions ranging in size from 5–10 μm2 to 2 nm2 in the formation of the corrosion resistance of steels. This may explain why, in some cases, there is no interrelation between the corrosion rate and the fracture resistance of steels, the formation of which is determined by larger particles. When representing the NMI as a set of random points on the plane, the distribution of distances between the nearest ones is estimated based on Voronoi polyhedra statistics. The study shows that an increase in the kurtosis coefficient of distributions of polyhedra areas is accompanied by an increase in the corrosion rate of the steels under study. This indicates the negative impact of heterogeneity in the arrangement of dispersed NMIs on the corrosion resistance of steels.