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Non-metallic inclusions in high manganese austenitic alloys

Grajcar A., Galisz U., Bulkowski L.

Archives of Materials Science and Engineering

2011

Vol. 50, nr 1

21-30

Purpose: The aim of the paper is to identify the type, fraction and chemical composition of non-metallic inclusions modified by rare-earth elements in an advanced group of high-manganese austenitic C-Mn-Si-Al-type steels with Nb and Ti microadditions. Design/methodology/approach: The heats of 3 high-Mn steels of a various content of Si, Al and Ti were melted in a vacuum induction furnace and a modification of non-metallic inclusions was carried out by the mischmetal in the amount of 0.87 g or 1.74 g per 1 kg of steel. Evaluation of the metallurgical purity of steels with non-metallic inclusions was done basing on determination their fraction, type, size and morphology. Stereological parameters of the inclusions were assessed by the use of automatic image analyzer cooperating with light microscope. EDS method was used to assess the chemical composition of non-metallic inclusions. Findings: It was found that the steels are characterized by high metallurgical purity connected to low concentrations of phosphorus and gases at a slightly higher sulphur content, introduced to a melt together with electrolytic manganese. The steels contain fine sulfide inclusions with a mean size from 21 to 25 ěm2 in a majority and their fraction equals from 0.047 to 0.09%, depending on sulphur content. MnS, carbonitrides of the (Ti,Nb)(C,N) type and complex carbosulfides containing Mn, Ti and Nb were identified in steels. The beneficial influence in decreasing a fraction of non-metallic inclusions and their susceptibility to elongate in a rolling direction has a higher addition of mischmetal and titanium microaddition. A modification of the chemical composition of non-metallic inclusions by Ce, La and Nd proceeds in an external zone of inclusions. Research limitations/implications: Further investigations relating the type and morphology of non-metallic inclusions to mechanical properties of sheets at various sections according to the rolling direction are needed. Practical implications: The knowledge of the type and morphology of non-metallic inclusions forming in high-Mn alloys and the effectiveness of their modification has a significant meaning for metallurgical and steel making technologies. Originality/value: A problem of the identification of non-metallic inclusions modified by REE and titanium is a new topic in studies on the metallurgical purity of advanced high-strength high-manganese steels.