Metamorphic effect of rare earth La on inclusion of Y40Mn free cutting steel

Abstract

The quantity, size, morphology and distribution of MnS inclusion in sulfur series free-cutting steels significantly influence their properties. The addition of rare earth elements, particularly lanthanum (La), markedly impacts the morphology, size, distribution, and quantity of the inclusion in steel. This study examined the effects of varying La content (x = 0, 11, 18, 32, 39 ppm) on the morphology, size, and distribution of MnS inclusion in Y40Mn free-cutting steel using FactSage thermodynamic calculations, SEM-EDS, optical microscopy, and electron probe analysis. The sample of 32 ppm La taken after adding La 2 min and 5 min was non-aqueous electrolysised, and their three-dimensional was analyzed. Thermodynamic calculation shows that (Mn, Fe) S-liq and FeS-liq begin to precipitate in Y40Mn steel without rare earth La added at 1420°C. At 1175°C, the liquid phase content decreases rapidly, and the solid phase inclusions of FeS and MnS commence precipitation concurrently. When the temperature is cooled to 975°C, the liquid phase completely disappears and transforms into solid inclusions. Upon the addition of rare earth La, La₂SO₂ precipitates within the steel, and transitions into La₂S₃ between 1250°C and 1300°C, eventually existing in the steel as La₂S₃. The MnS inclusion in Y40Mn steel typically forms elongated strips at the grain boundary. However, the introduction of rare earth La modifies these inclusions into ellipsoid or spherical shapes, and induces notable grain refinement. When the content of La is 32 ppm, MnS and the composite inclusions containing rare earth sulfur oxides and MnS after metamorphism were best refined, in which the percentage of less than 2 μm is 73.37% and 54.65%, respectively. Moreover, the inclusion morphology changes when MnS inclusions transform into complex rare earth inclusions with rare earth sulfur oxides serving as nucleation cores for MnS.