In the present paper, the reduction reaction of high-purity MoO₃ with CO–15 vol % CO₂ mixed gases in the temperature range of 901–948 K is investigated via the thermogravimetric analysis technology. The results show that reduction of MoO₃ to MoO₂ follows a three-step reaction process, viz., MoO₃ is first reduced into Mo₉O₂₆, followed by Mo₄O₁₁, and finally to MoO₂. The reaction sequences of MoO₃ → Mo₉O₂₆ → Mo₄O₁₁ → MoO₂ are proposed, which are quite different from those observed on reduction of MoO₃ by pure H₂ or CO gases. Pure Mo₉O₂₆ and Mo₄O₁₁ could be synthesized once suitable time was controlled. Rate-controlling steps for the reduction from MoO₃ to Mo₉O₂₆ and Mo₉O₂₆ to MoO₂ (include both Mo₉O₂₆ to Mo₄O₁₁ and Mo₄O₁₁ to MoO₂) are interfacial chemical reactions, with the activation energies of 318.326 and 112.047 kJ/mol, respectively. This study also discovers that the as-synthesized MoO₂ keeps the same platelet-shaped and smooth morphology as the MoO₃ raw material; however, its particles size gradually increased as the reaction proceeds due to the formation of low-melting-point eutectic and the sticking of different particles.

Thermodynamic analysis of the precipitation behavior, growth kinetic, and control mechanism of MnS inclusion in U75V heavy rail steel was conducted in this study. The results showed that solute element S had a much higher segregation ratio than that of Mn, and MnS would only precipitate in the solid–liquid (two-phase) regions at the late stage during the solidification process at the solid fraction of 0.9518. Increasing the cooling rate had no obvious influence on the precipitation time of MnS inclusion; however, its particle size would be decreased greatly. The results also suggested that increasing the concentration of Mn would lead to an earlier precipitation time of MnS, while it had little effect on the final particle size; as to S, it was found that increasing its concentration could not only make the precipitation time earlier but also make the particle size larger. Adding a certain amount of Ti additive could improve the mechanical properties of U75V heavy rail steel due to the formation of TiO _x –MnS or MnS–TiS complex inclusions. The precipitation sequences of Ti ₃ O ₅ → Ti ₂ O ₃ → TiO ₂ → TiO → MnS → TiS for Ti treatment were determined based on the thermodynamic calculation.

Tire cord steel is widely used in the tire production process of the vehicle manufacturing industry due to its excellent strength and toughness. Titanium nitride (TiN) inclusion, existing in tire rod, has a seriously detrimental effect on the fatigue and drawing performances of the tire steel. In order to control its amount and morphology, the precipitation behavior of TiN during solidification in SWRH 92A tire cord steel was analyzed by selected thermodynamic models. The calculated results showed that TiN cannot precipitate in the liquid phase region regardless of the selected models. However, the precipitation of TiN in the mushy zone would occur at the final stage during the solidification process (at solid fractions greater than 0.98) if the LRSM (Lever-rule model was applied for the N and Scheil model for Ti) or Ohnaka models (without considering the effect of carbon on secondary dendrite arm spacing (SDAS)) were adopted. For the Ohnaka model, in the case when the effect of carbon on SDAS was considered, TiN would probably precipitate in the solid phase zone rather than precipitate in the liquid phase region or mushy zone.