Abstract
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.
Highlights
Non-metallic inclusions in steels usually have a detrimental effect on the material performance
In order to analyze the relationship between MnS inclusions behavior and cast features, the macrostructure of continuous casting slab of medium carbon structural steel 45# was obtained by macroetching methods
The main findings are as follows: (1) The optical microscope and scanning electron microscope (SEM)-Energy Dispersive Spectrometer (EDS) observation of MnS inclusions morphology suggest that the sulfides are in globular, rodlike and dendritic shape, and distribution varies along the slab thickness of the 45# steel, the former is mainly presented near the slab edge, while the latter two are generally found in the columnar zone, mixed zone and equiaxed zone
Summary
Non-metallic inclusions in steels usually have a detrimental effect on the material performance. With the improvement of the quality requirements of steel products, the control of inclusions has been a long-term problem during the steelmaking process. As one of the typical inclusions in steels, is beneficial in improving machinability of free-cutting steels [1,2], as well as slowing grain growth [3] and promoting the precipitation of intragranular ferrite [4]. The material properties in the transversal direction are weakened due to the anisotropy caused by the elongation of MnS inclusions during hot rolling [5]. Different steels have different requirements for the morphology and distribution of MnS inclusions. Many studies on the formation and control of MnS inclusions have been carried out through the years [6,7,8]
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