Precipitation of MnS inclusions in steel affects the mechanical properties of the material significantly. The evolution of MnS inclusions along the continuous casting slab thickness and its influencing factors has not been clearly established and comprehensively studied. In this paper, solidification macrostructure, sulfur segregation and MnS inclusions in the continuous casting slab of medium carbon structural steel 45# were studied by various methods, including the metallographic observations, elemental analysis, scanning electron microscope (SEM) with Energy Dispersive Spectrometer (EDS) observation, automatic particle analysis, and thermodynamic calculations. The 2D/3D morphologies of MnS inclusions suggest that the sulfides turn from globular to rodlike, and further to dendritic shape along the slab thickness progressively. Furthermore, it was found that MnS inclusions are remarkably aggregated in the columnar crystals and the equiaxed crystals mixed zone, where the sulfides have the largest average diameter of 6.35 μm and the second maximum area fraction of 0.025% along the slab thickness. In order to reveal the mechanism of this phenomenon, the precipitation temperature of MnS inclusion in the 45# steel was clarified by thermodynamic calculation and experimental observation, and the quantitative relationships among the distribution of sulfur content, secondary dendrite arm spacing (SDAS), and precipitation area fraction of MnS inclusions were discussed. Moreover, the inclusion size was numerically predicted to compare with the measured value. The results indicate that the large SDAS, high sulfur content and low cooling rate accounting for the large-size aggregated MnS inclusions in the mixed zone. Unfortunately, the dendritic MnS inclusions, even if the average diameter exceeds 52 μm, can act as the nucleation sites for ferrites, and the distribution of the sulfides promotes uneven microstructure in the steel.

In the current article, the influence of titanium on the hot ductility of peritectic-microalloyed steels, in the temperature range 600°C to 950°C, was investigated by tensile testing. The morphology of the fracture and the behaviour of precipitates, inclusions and microstructures near the fracture surface were observed and discussed. It was found that within the test temperature range, the reduction in area values of titanium-bearing steels were almost greater than 40%, which were much larger than those of titanium-free steels. This improvement was due to the formation of (Ti_xNb_1-x)(C_yN_1-y) precipitates and titanium-aluminium oxides with high titanium content that could act as the nucleation sites for dimples to promote the occurrence of ductile fracture. In addition, the austenite grain was refined and more uniform in size. The thickness together with the maintain temperature range of the ferrite films which distribute only along austenite grain boundaries were reduced.

The effects of yttrium (Y) addition on the formation of inclusions in 0.5C-1Cr-0.2Mo-0.1V spring steel were studied via thermodynamic calculations and scanning electron microscope (SEM) coupled with energy dispersive spectrum (EDS). According to thermodynamic calculations of Gibbs free energy, [Y] reacts with [O] and [S] but not with [C] in molten steel, with Y ₂ O ₂ S and Y ₂ O ₃ being more easily formed inclusions in steel at 1873 K than YS and Y ₂ S ₃ when the Y content is less than 0.06%. The experimental results show that the inclusions in spring steel without Y addition are mainly large irregular aluminum silicate, Al ₂ O ₃ -SiO ₂ -TiO ₂ and MnS inclusions. The addition of 0.011% Y can eliminate large inclusions and form square Y ₂ O ₃ and complex Y ₂ O ₃ -Y ₂ O ₂ S inclusions with Y ₂ O ₂ S in the core and surrounding Y ₂ O ₃ . When the Y content increased to 0.019%, the size of Y ₂ O ₃ decreased, and isolated spherical Y ₂ O ₂ S formed. The addition of 0.031% Y can further decrease the size of Y ₂ O ₂ S, and Y ₂ O ₃ is replaced by YS. A good consistency was observed between the calculated result of the Gibbs free energy and the experimental result. Moreover, a three-dimensional stability diagram of precipitation under equilibrium conditions at 1873 K is established to predict the formation sequence of Y-containing inclusions with different Y contents. The formation sequences are Y ₂ O ₂ S → Y ₂ O ₃ with Y contents in the range of 0.01−0.02%, YS → Y ₂ O ₂ S → Y ₂ O ₃ with Y contents in the range of 0.02−0.06% and Y ₂ S ₃ → YS → Y ₂ O ₂ S → Y ₂ O ₃ with Y contents in the range of 0.06−0.08%.

The morphology and types of inclusion, as well as the microstructure, fundamentally affect the properties of high-strength peritectic steel. Rare earth elements not only modify inclusions but also act on the transformation of the microstructure. In this paper, the evolution mechanism of yttrium for the inclusions and microstructure in high-strength peritectic steel was investigated through experimental testing and thermodynamic analysis. The results show that yttrium treatment can modify the main large-sized irregular inclusions into spherical or near-spherical rare earth inclusions, accompanied by a reduction in the number density, area fraction, average diameter, and aspect ratio of inclusions. The evolution route for the inclusions follows Al₂O₃ + MnS + Al₂O₃-MnS→Y₂O₃ + Y-O-S + Y-S + Y-O-S-MnS with yttrium addition. The microstructural characteristics of yttrium-free steel show significant differences from those of yttrium-containing steel. Compared to yttrium-free steel, the yttrium-0.015 wt.% steel shows a refined austenite structure with more uniform size distribution and the absence of grain boundary ferrite films. The Y₂O₃ and Y₂O₂S inclusions mainly formed in liquid steel were found along the austenite grain boundary to prevent the grain growth and the formation of ferrite films. Additionally, after adding rare earth yttrium, the fraction of high-angle grain boundaries (HAGBs) increases, together with a decrease in the fraction of low-angle grain boundaries (LAGBs) in steel. The research results can provide a theoretical basis for the application of adding rare earth yttrium to high-strength peritectic steel.