In this diploma thesis, the influence of nitrogen alloying technology on the cleanliness
of austenitic stainless steel XM‑19 was investigated. Three industrial heats were
compared in which nitrogen was introduced by different routes: by top blowing through
a lance (heat 331352), by a combination of lance blowing and bottom stirring via an
argon purging plug (heat 331821), and by the addition of a nitrogen‑bearing
ferrochromium alloy (heat 334288). The aim of the work was to determine how the
selected nitrogen alloying methods affect the number, size and chemical composition
of oxide inclusions.
In the experimental part, final VOD samples from all three heats were analysed,
together with an additional sample taken from the intermediate tundish of the
continuous casting machine for heat 331821. Steel cleanliness was evaluated by total
oxygen analysis and automated inclusion analysis using a field emission scanning
electron microscope. The chemical compositions of slags in different process stages
were also compared.
For all three technological routes, the target nitrogen content was achieved in the final
steel. Automated inclusion analysis showed that oxide inclusions within the CaO
Al2O3–MgO system dominate in all samples, with Ca-aluminate inclusions being more
pronounced in heat 334288, while heats 331352 and 331821 contain a higher fraction
of solid aluminates and spinel phases. Total oxygen analysis revealed that, among the
final VOD samples, heat 334288 was the cleanest, whereas heat 331352 exhibited the
highest oxygen content; the additional tundish sample from heat 331821 showed the
lowest total oxygen level, confirming further inclusion flotation at this stage of the
process.
Nitrogen alloying via the argon purging plug achieved a higher nitrogen yield and a
more stable process than nitrogen blowing with a top lance. Alloying with the nitrided
ferroalloy FeCrLC-N likewise enabled the target nitrogen content to be reached, with
automated analysis indicating a shift in inclusion chemistry towards Ca-aluminate
compositions, while total oxygen measurements pointed to an even cleaner steel
compared to the other two heats. It was found that a reliable assessment of the
cleanliness of XM-19 steel is best obtained by combining total oxygen measurements
with automated inclusion analysis, the latter primarily providing insight into the
chemical composition and size of non-metallic inclusions.
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