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In order to investigate the relationship between microstructure and MnS inclusion in non-quenched and tempered steel, and cooling rate during horizontal directional solidification, 49MnVS steel was used to conduct the experiments with a self-designed device. The mathematical effect of cooling rate on dendritic arm spacing and mean diameter of MnS particles (dMnS) were determined by using linear regression method. The results show that the length of dendrite from solid–liquid interface to end-solidification decreased with increasing the withdrawal velocity (ϑ). dMnS has a similar value in the area of the steady directional solidification; the value of dMnS was 4.1, 3.6, 3.3, 2.8 and 2.3 μm at withdrawal velocity of 50, 75, 100, 150 and 200 μm/s, respectively. dMnS increased with reducing ϑ or RC (interface cooling rate). MnS precipitated in the gaps between dendrites and was influenced by secondary dendritic arm spacing. Besides, a new concept of the ‘Precipitation Unit Space’ (PUS) was proposed and the relationships between dMnS, VPUS (volume of PUS) and RC were obtained.

To analyse the effects of solute elements on formation of MnS in the micro-alloyed non-tempered steel, a novel coupling model of micro-segregation and MnS precipitation during solidification was bu...

Controlling the formation of MnS inclusions during solidification influences the mechanical properties and machinability of the resulfurized steel. A coupled segregation&ndash, nucleation&ndash, growth model was developed by the finite-difference method involving solute redistribution, heterogeneous nucleation and growth kinetics. Laboratory solidification experiments were performed under various cooling rates in resulphurised 49MnVS steel. In this work, the influence of cooling rate on solute redistribution and growth size of MnS inclusions were simulated using the current coupled model, and the calculated results can provide a valuable reference for MnS formation. Increasing of the cooling rate led to early precipitation and refinement of formed MnS inclusions. Based on the simulation results and experimental data, mathematical relationships between the growing size of MnS with the cooling rate in the low ductility temperature region and in the whole solidification were obtained.

In this study, the precipitation behavior of MnS was calculated during the solidification process of the medium sulfur, non-quenched and tempered 49 MnVS steel. And the precipitation process was directly observed by a high-temperature confocal laser scanning microscope (HT-CLSM) equipped with a gold-image furnace. Besides, the precipitated particles were proved to be MnS by using the Scanning Electron Microscope-Energy Dispersive spectroscopy (SEM-EDS) analysis. The modified thermodynamic calculation indicated that the MnS precipitated from the residual liquid steel when the product of manganese and sulfur concentrations exceeded the equilibrium value at 1417.0 °C. Meanwhile, the results calculated by FactSage software shown that the beginning precipitation temperature of MnS is about 1411.0 °C. The observation of HT-CLSM demonstrated that the primary solid on the free surface of the liquid steel was visualized at 1485.0 °C during the solidification process (cooled from 1520.0 °C to room temperature with a rate of −30 °C/min). Then, the MnS particles started to form at 1437.0 °C and rapidly grew up into shape on the solid-liquid boundary. And the precipitation of MnS continued to form in a large amount (90% of the total) in the next 10 degrees. The enrichment of Mn and S near the local melt surface may be conducive the MnS formation, which results a higher actual precipitation temperature than the calculated value.