Development Technology for Prevention of Macro-segregation in Casting of Steel Ingot by Insert Casting in Vacuum Atmosphere

摘要

Important large components, such as rotors for power generation steam turbines, pressure vessels and reaction vessels, are manufactured through ingot casting. However, it is quite tasking to manufacture the materials with sufficient properties because of the macro-segregation of ingots. To develop effective and versatile macro-segregation countermeasures in the casting of large steel ingots for manufacturing large parts for power plants, the insert casting in vacuum atmosphere, in which a core material similar in composition as the base steel, is placed at the center of the mold, was studied. The effectiveness of the proposed insert casting as a macro-segregation countermeasure was evaluated in the casting experiments with 0.5 mass% carbon steel using cast iron mold with a 150 mm square inner cross section, insulated to reduce solidification rate. In addition, it is now confirmed that good bonding between the core material and base material can be achieved even under conditions where bonding by normal insert casting in air atmosphere is hard to achieve. The behavior of the core material melting and the solidification of the molten steel in the experiments of macro-segregation reproduction casting and insert casting were investigated using the direct finite difference method. The mechanism by which this method suppresses the macro-segregation formation and solidification conditions for the suppression, the reasons, and conditions for good bonding in this insert casting are clarified by the analyses. Furthermore, the cause of internal crack formation in the insert casting was investigated and guidelines for preventing the crack formation were presented.