To reduce slag entrapment in 150 × 1270 mm slab continuous casting molds at the Tang Steel Company, the effect of submerged entrance nozzle (SEN) depth and casting speed on the phenomenon was studied by computational fluid dynamics simulations. Then, the slag entrapment behavior in continuous casting molds, utilizing Large Eddy Simulation (LES) by coupling the volume of fluid (VOF) method, was also used. Finally, the effect of several common oils usually used to simulate slag in water modelling on slag entrapment was discussed and the water modelling results were used to validate the numerical simulation findings. The results showed that the optimum scheme is a submerged depth of SEN 90 mm and a casting speed of 1.6 m/min. Under optimal conditions, the maximum surface velocity is smallest (0.335 m/s) and the maximum slag entrapment ratio (0.44%) appears in the position of 0.1 m below the meniscus after 15 s. The water modelling results were in good agreement with the numerical simulation results.

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.

As the most typical high-strength stainless steel, the heat-treated AISI 440C steel has low machinability. It is also not weldable due to the high carbon content. Additive manufacturing, specifically laser powder bed fusion (LPBF), offers a promising solution for fabricating near-net shape components with this alloy. To address the limited research on the LPBF of AISI 440C steel, this work investigates the LPBF processability, microstructure and phase evolution of the AISI 440C steel, focussing on the role of post-processing treatments on improving mechanical properties, wear and corrosion resistance compared to its wrought counterpart. Our results indicate that highly dense AISI 440C components with > 99.98% relative density can be produced using optimised LPBF parameters. However, the low martensite start (Ms) temperature of the steel and the fast-cooling feature of the LPBF processing result in the formation of an austenite-dominant microstructure in the as-LPBF-fabricated parts with low strength and hardness. Post-processing treatment is required to enable the martensite transformation. Deep cryogenic treatment (DCT) at -196°C in liquid nitrogen had no effect, so conventional quenching and tempering were used. This facilitated M ₂₃ C ₆ carbide precipitation, increased the Ms temperature and promoted martensitic transformation. After the heat treatment, the LPBF-fabricated AISI 440C steel exhibits significant improvements, including an ultimate tensile strength over 2.2 GPa compared with the as-LPBF-fabricated steel, enhanced toughness, doubled elongation (7.6%), and comparable hardness to the wrought counterpart. These improvements are attributed to the higher fraction of retained austenite with refined grains and precipitation of fine carbides. In addition, the heat-treated LPBF steel also exhibits the lowest specific wear rate, and superior corrosion resistance compared with the wrought 440C steel, due to improved passivation from the higher amount of retained austenite and finer carbides. The present results provide a foundation for broadening the practical applications of the AISI 440C steel via additive manufacturing.