MnO x SiO2 x Al2O3 inclusion in high carbon wire rod steel


Figure 1: X-ray map of constituent elements of liquid MnO x SiO2 x Al2O3 inclusions with several crystallized SiO2 phase in liquid high carbon steel from the ladle. SEM, scale bar: 10 µm.

Inclusion name: MnO x SiO2 x Al2O3
Record No.: 851
Inclusion formula: MnO x SiO2 x Al2O3
Inclusion type (Macro/Micro/Nano): Macro
Inclusion type (Exogenous/Indigenous): No data
Inclusion classification: Oxide
Inclusion composition in weight %: 35–40% MnO, 55–65% SiO2, 1–2% Al2O3 and traces of CaO and TiO2.
Sample: High carbon wire rod steel
Steel composition in weight %: 0.70-0.80% C, 0.65-0.70% Mn, 0.16-0.30% Si, max. 0.020% P, max. 0.020% S, max. 60 ppm N.
Note: Fig. 1 presents the SEM image of a typical liquid rhodonite inclusion of liquid steel sample collected from the LF. The rhodonite inclusions found in the liquid steel samples were actually round duplex inclusions, which contained several crystallized SiO2 phases in the silicate matrix. The phase separation seen in the image occurred on cooling although initially at liquid steel temperatures these inclusions were liquid and of similar chemistry.
Thermodynamically, rhodonite is formed at relatively higher oxygen potential arising from the reoxidation of Mn–Si–Al killed liquid steel. Unless proper care is taken in handling and during all operations subsequent to the ladle treatment, reoxidation of liquid steel is practically unavoidable. The reoxidation product, rhodonite, is mainly responsible for the slag patch or scum formation during casting of high carbon semi-killed steel billets. In many instances mould breakouts have occurred mainly due to entrapment of scum by the solidifying shell in the mold. Owing to its lower melting point rhodonite inclusions deform easily and discontinuously at the hot rolling temperature, giving rise to very large oxide stringers in the wire rods. Presence of such bulky stringers adversely affects the ductility of steel. Fortunately, occurrence of such inclusions has been effectively controlled in the modified practice.
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