Critical precipitation behavior of MnTe inclusions in resulfurized steels during solidification

Te treatment is an effective method for modifying sulfide inclusions,and MnTe precipitation has an important effect on thermal brittleness and steel corrosion resistance.In most actual industrial applications of Te treatment,MnTe precipitation is unexpected.The critical precipitation behavior of MnTe inclusions was investigated through scanning electron microscopy,transmission electron microscopy,machine learning,and first-principles calculation.MnTe preferentially precipitated at the container mouth for sphere-like sulfides and at the interface between MnS grain boundaries and steel matrix for rod-like sulfides.The MnS/MnTe interface was semicoherent.A composition transition zone with a rock-salt structure exhibiting periodic changes existed to maintain the semicoherent interface.The critical precipitation behavior of MnTe inclusions in resulfurized steels involved three stages at varying temperatures.First,Mn(S,Te)precipitated during solidification.Second,MnTe with a rock-salt structure precipitated from Mn(S,Te).Third,MnTe with a hexagonal NiAs structure transformed from the rock-salt structure.The solubility of Te in MnS decreased with decreasing temperature.The critical precipitation behavior of MnTe inclusions in resulfurized steels was related to the MnS precipitation temperature.With the increase in MnS precipitation temperature,the critical Te/S weight ratio decreased.In consideration of the cost-effectiveness of Te addition for industrial production,the Te content in resulfurized steels should be controlled in accordance with MnS precipitation temperature and S content.

Formation and evolution behavior of inclusions in Al-killed resulfurized free-cutting steel with magnesium treatment

To reveal the formation and evolution behavior of inclusions during magnesium treatment and resulfurization in Al-killed free-cutting steel, both laboratory experiments and thermodynamic calculations were carried out. The extracted samples were polished and analyzed by scanning electron microscopy and energy-dispersive spectroscopy. The results indicated that MgO formed immediately and then transferred to MgO·Al2O3 with the homogenization of [Mg] in steel in the case of low magnesium addition (w[Mg] < 0.0020%). However, MgO would keep stable in the case of high magnesium addition (w[Mg] ≥ 0.0020%). In the process of resulfurization, the addition of sulfur forced the transformation of MgO to MgO·Al2O3 accompanied with the formation of MgS if the mass fraction of [S] in steel was in the range 0.15%-0.70%. MgS precipi-tated as a solid solution containing a quantity of MnS, and the composition of the sulfide solution was calculated to be (Mg0.85Mn0.15)S when the mass fraction of [Mn] was 1.2%. The formation mechanisms of inclusions after magnesium and sulfur additions were discussed comprehensively, and proper models for the evolution of inclusions were set up.

Influence of desulfurization slag containing oxides of vanadium and titanium on semi-steel pre-desulfurization

The desulfurization and resulfurization of slag containing oxides of vanadium and titanium(CaO–SiO_(2)–MgO–Al_(2)O_(3)–V_(2)O_(3)–TiO_(2)–FeO)in the pretreatment of semi-steel desulfurization at 1623 K were investigated.Based on the ionic structure theory of slag,it could be concluded that V_(2)O_(3) and TiO_(2) combined with(O_(2)–)to form complex anions including VO_(3)^(3–),TiO_(4)^(4–),etc.,which reduced the activity of(O_(2)–)and decreased the sulfur partition ratio(LS).And FeO enhanced the activity of[O]at the slag–metal interface,which decreased the desulfurization capacity of the slag.Compared to the binary basicity,the presented basicity expression containing V_(2)O_(3) and TiO_(2) described the relationship between LS and basicity more accurately.Considering the problems of dilute slag,a large amount of residual slag and much resulfurization in the slagging-off process after pretreatment,the effect of CaO+C-based slag modifier on the resulfurization,melting point and viscosity of the desulfurization slag was investigated.It was proposed that adding 5%–10%of the slag modifier to the desulfurization slag after desulfurization decreased the resulfurization effectively.And the added modifier adjusted the slag viscosity well,which helped to reduce iron loss and residual slag.

Application of fractal theory to study morphology of manganese sulfide inclusion in resulfurized free-cutting steels

The morphology and distribution of manganese sulfide(MnS)inclusions have a significant influence on the comprehensive performance,which is an important research field for resulfurized steels.Based on the experiments of non-aqueous electrolyte and scanning electron microscope observation,the fractal theory was employed to study the three-dimensional morphologies of MnS inclusions.The results showed that the edge fractal dimension of MnS inclusions was between 1.59 and 1.88.In addition,similar morphology of MnS inclusions had a close fractal dimension.The MnS edge fractal dimension is highly positively correlated with the morphological parameters.The multifractal spectrums of MnS inclusions on two-dimensional plane of as-cast and as-rolled resulfurized free-cutting steels were calculated.The large-size MnS inclusions belong to large probability subset,while the small-size MnS inclusions belong to small probability subset.The multifractal spectrum can truly and effectively reflect the difference and non-uniformity of distribution of MnS inclusions on 2D plane.On the premise of similar content of MnS,with the refinement of MnS inclusions,the multifractal spectrum width and the multifractal spectrum symmetry parameter were decreased.The multifractal spectrum provides a new method for studying the second phase in materials.