Reasonable regulation of nonmetallic inclusions in steel can significantly improve its strength, toughness, and corrosion resistance. In this paper, EH420 marine steel was treated with Mg, Ce, and Mg-Ce to modify the inclusions. The effects of different treatments on the morphology, composition, size distribution, induced intragranular ferrite (IGF) nucleation, and pitting resistance of inclusions were systematically analyzed using various methods. The results show that the Mg-Ce composite treatment can modify irregular MgAl₂O₄ inclusions into spherical Mg-Ce-O composite inclusions and MgO-dominated inclusions. The density of inclusions is increased from 74.8/mm² to 186.0/mm², and the average size of inclusions is decreased from 2.60 μm to 1.07 μm. The Mg-Ce-O composite inclusions are effective inclusions for inducing IGF. Furthermore, the pitting potential is increased from −503 mV to −487 mV, and the corrosion rate is decreased. The order of average electronic work function is ΦMgO < ΦCe₂O₃ < Φα-Fe < ΦAl₂O₃. Ce₂O₃ is hard to induce pitting corrosion due to its similar electronic work function to the steel matrix. Thus, the Mg-Ce composite treatment is better than that for Mg and Ce treatment alone, and has better application prospects.

To understand and clarify the formation mechanisms and evolution of complex inclusions in Ti-Ca deoxidized offshore structural steel, inclusions in industrial steel were systematically investigated. The number density of total inclusions generally decreased from Ladle Furnace (LF), Vacuum Degassing (VD), Tundish to the final product except for Ti and Ca addition. The major inclusions during the refining process were CaO-Al₂O₃-SiO₂-(MgO)-TiO_x and CaO-Al₂O₃-SiO₂. CaO-Al₂O₃-SiO₂-(MgO)-TiO_x inclusion initially originated from the combination of CaO-SiO₂-(MgO) in refining slag or refractory and deoxidization product Al₂O₃ and TiO₂. With the refining process proceeding and Ca addition, the Al₂O₃ concentration in the CaO-Al₂O₃-SiO₂-(MgO)-TiO_x inclusions gradually dropped while the CaO and TiO₂ concentrations gradually increased. The CaO-Al₂O₃-SiO₂ inclusions originally came from refining slag, existing as 2CaO∙ Al₂O₃∙ SiO₂, and maintained a liquid state during the early stage of LF. After Ca treatment, it was gradually transferred to 2CaO∙ SiO₂ due to Al₂O₃ continuously being reduced by Ca. The liquidus of 2CaO∙ SiO₂ inclusion was higher than that of molten steel, so they presented as a solid-state during the refining process. After welding thermal simulation, CaO-Al₂O₃-SiO₂-(MgO)-TiO_x inclusions were proven effective for inducing intragranular acicular ferrite (IAF) while CaO-Al₂O₃-SiO₂ was inert for IAF promotion. Additionally, Al₂O₃-MgO spinel in multiphase CaO-Al₂O₃-SiO₂-(MgO)-TiO_x inclusion has different formation mechanisms: (1) initial formation as individual Al₂O₃-MgO spinel as a solid-state in molten steel; (2) and it presented as a part of liquid inclusion CaO-Al₂O₃-SiO₂-(MgO)-TiO_x and firstly precipitated due to its low solubility.

The liquidus temperature and temperature drop coefficients of medium manganese steel were systematically studied using Factsage and differential scanning calorimetry (DSC) experiments. The results indicated that the temperature drop coefficients of C, Mn, Cr, Si, and Al were complex, while the coefficients of Mo, V, and Nb were of a constant value. Based on the temperature drop coefficients, the empirical formula for calculating the liquidus temperature of medium manganese steel was established. The liquidus temperature calculated using the empirical formula was 1422.7°C, while that obtained by the DSC experiment was 1422.9°C. By comparison with different calculation formulas, the liquidus temperature obtained from the formula that was constructed in this study was much closer to the experiment one, indicating the high accuracy of the empirical formula in predicting the liquidus temperature of medium manganese steel.

The number density, size and composition of inclusions in U75V heavy rail steel during ladle furnace (LF)–RH refining process were studied by an Aztec-Feature inclusion automatic software which equipped with a field emission microscope. The results showed that the MnO–SiO ₂ –Al ₂ O ₃ -type inclusions were the main inclusions at LF start. The MnO–SiO ₂ –Al ₂ O ₃ -type inclusions were transformed into the CaO–SiO ₂ –Al ₂ O ₃ -type inclusions during the LF refining process because of the Ca and Al elements which were brought by ferroalloy. At the RH start stage, the main inclusions were CaO–SiO ₂ –Al ₂ O ₃ -type inclusions with low melting temperature and CaO–MgO–Al ₂ O ₃ -type inclusions with high melting temperature in the sample. And, the CaO–MgO–Al ₂ O ₃ -type inclusions were obviously removed during RH vacuum treatment. The average compositions of CaO–SiO ₂ –Al ₂ O ₃ -type inclusions had no obvious change from RH vacuum holding for 15 min, RH vacuum break to RH soft blowing.