Optimization of metallurgical process for 38CrMoAl steel used in marine vessels

38CrMoAl steel is widely applied in marine ship components including plate frame structures and marine engine transmission shafts. The marine service environment involves salt spray corrosion, alternating humid and hot conditions, and wave impact erosion, which puts forward high comprehensive quality requirements for 38CrMoAl steel to extend the service life of marine ships. As a high-aluminum steel, 38CrMoAl steel encounters certain technical difficulties in the smelting process, including the adjustment of aluminum content, the control of refining slag composition, the design of continuous casting mold flux composition, the improvement of molten steel cleanliness, and the guarantee of castability during continuous casting. In this study, 38CrMoAl high-aluminum steel continuous casting billets with a cross-sectional dimension of ϕ600 mm were trial-produced via the process of converter smelting, ladle furnace(LF) refining, vacuum degasser(VD) vacuum treatment and continuous casting. The obtained billets were used to produce rolled or forged materials with a diameter below ϕ300 mm. The formation mechanism of typical defects of 38CrMoAl high-aluminum steel was analyzed, and the key quality control points of the converter, LF refining, VD vacuum treatment and continuous casting processes were systematically summarized. The controlled composition ranges of LF refining slag were 50% to 55% for w ((CaO)), 3% to 5% for w ((SiO₂)), 30% to 35% for w((Al₂O₃)), 10 to 13 for binary basicity R(w((CaO))/w((SiO₂))), 5% to 9% for w ((CaF₂)), and 0.20% to 0.40% for w ((MnO))+w ((TFe)). A segmented high vacuum pressure holding process of "10 min+5 min" was adopted in the VD vacuum procedure. After the first 10 min high vacuum pressure holding, the vacuum was broken, and aluminum ingots were added in a single operation to adjust the aluminum content to the target value. The second 5 min high vacuum pressure holding was immediately carried out after aluminum addition. No calcium wire feeding or calcium treatment was conducted after the completion of VD vacuum treatment. The designed composition ranges of continuous casting mold flux were 17% to 20% for w((CaO)), 40% to 45% for w((SiO₂)), no more than 2.0% for w((Al₂O₃)), 0.37 to 0.50 for basicity, and 1.5% to 2.5% for w((Li₂O)). The optimized process supports the continuous casting of 38CrMoAl steel for 9 heats with a single casting duration of no less than 500 min. The continuous casting billets present excellent surface quality and an extremely low content of non-metallic inclusions, which meet the requirements of subsequent hot forming processing and can be massively applied to the manufacturing of key components for marine ships.