The paper deals with the formation of coarse nitride non-metallic inclusions in steel. The thermodynamic calculations for the determination of the equilibrium solubility of nitrogen in the steel melt at a given temperature and chemical composition are presented. Some of the negative effects of excessive nitrogen contents, like blow holes, strain aging and, most importantly, nitride non-metallic inclusion formation are discussed. Five types of the coarse nitride non-metallic inclusions that occur in industrial practice are presented: aluminium, boron, niobium, titanium, and zirconium nitrides. Examples of the negative effect of coarse nitrides from case studies are also shown. The basic thermodynamic tools and values of the parameters for calculating the nitride precipitation in the steel melt are given in the paper.

The effects of titanium and zirconium additions on the non-metallic inclusions and microstructure of PK942 (X11CrNiMo12) steel were investigated. Laboratory steel charges with additions of Ti, Zr and a combination of both elements were melted. The inclusions' size, quantity, distribution and composition were analysed using optical and scanning electron microscopy (SEM). Other than the difference in d-ferrite quantity, there are no significant differences in the microstructure. The Ti and Zr additions had a significant effect on the non-metallic inclusions, and while the addition of Ti did have an effect on the composition of inclusions, the addition of Zr also had a great effect on the size distribution and the number of non-metallic inclusions. The result of the Zr addition was a large reduction in the total non-metallic inclusion surface area. The additions of both titanium and zirconium had similar effects to the addition of Zr. Thermodynamic reactions were considered to explain the modification of the non-metallic inclusions. Keywords: non-metallic inclusions, clean steel, de-oxidation with Ti, de-oxidation with Zr, creep resistant steel S pomo~jo opti~ne ter elektronske mikroskopije se je preiskal vpliv dezoksidacije jekla PK942 (X11CrNiMo12) s cirkonijem, titanom ter kombinacijo obeh. Poudarek raziskave je bil na spremembi velikosti, razporeditve ter sestave nekovinskih vklju~kov v jeklu PK942. Razen v majhni razliki dele`a d-ferita, ni bilo bistvenega vpliva na osnovno mikrostrukturo jekla. Ve~ji vpliv pa se je pokazal na velikost, {tevilo ter kemi~no sestavo nekovinskih vklju~kov. Analiza nekovinskih vklju~kov je pokazala, da Ti vpliva predvsem na kemi~no sestavo, med tem ko ima Zr zelo velik vpliv tudi na velikostno porazdelitev ter {tevilo nekovinskih vklju~kov, rezultat~esar je manj{i skupni povr{inski dele`nekovinskih vklju~kov. Dodatek kombinacije Zr in Ti ima podoben vpliv kot dodatek Zr, vendar v manj{em obsegu. Pri razlagi modifikacije vklju~kov so se upo{tevale osnovne termodinami~ne reakcije oksidacije.

The production of clean steel is associated with high-quality steel grades for demanding applications. The formation of oxide inclusions mainly depends on the deoxidation practice; it is usually carried out through Al additions, but alumina inclusions can have detrimental effects. An alternative zirconium inclusion modification was used in a creep-resistant steel to improve the cleanliness of laboratory-made steel. The thermodynamics behind the inclusion modification are presented, the reaction products are identified and the steel cleanliness improvement is quantified. The resulting influence of zirconium addition on non-metallic inclusions and mechanical properties is discussed. While the Zr additions drastically reduce the non-metallic inclusion size and area, additions above a certain amount result in the formation of zirconium nitrides that ultimately soften the martensitic steel due to the depletion of nitrogen in solid solution.