Generation And Control Of Globular Inclusion During The Refining Process

Globular inclusions are the brittle inclusions in steel, which are isolated and cannot deform. There is a big difference between globular inclusions and steel substrateofthermal expansion and deformability.The existence of globular inclusions will severely damage the continuity of the steel substrate, and is a major cause of fatigue failure of steel shafts.It is important to control the generation and conversion of globular inclusions in the refining process in order to product clean shafts steel. The key point is to reduce the generation of globular inclusions in the steel as much as possible.42Cr Mo A crankshaft steel is taken as an example in this paper, the generation conditions of classical globular inclusions are carefully studied through thermodynamic calculations with the help of metallurgical thermodynamics and Factsage thermodynamic software. The precipitation of calcium and magnesium in the steel are controlled in the process of refining desulfurization by controlling refining slag composition, refining temperature and Als content. Hence, these researches have supported the theoretical basis on generation and control of globular inclusions. The industrial test has invalidated the control efficiency of globular inclusions.The thermodynamic studies shown that, C and Al in the steel can reducethe Mg O and Ca O in the slag in varying degrees, and the maximum precipitation amount of calcium is 12 ppm and magnesium 11.9ppm in molten steel. When it comes to 42 Cr Mo A crankshaft steel, the calcium and magnesium content of the molten steel increases with the increase of refining slag basicity, Als content and refining temperature, while decreases with the increase of Fe O contents. Liquid calcium aluminate inclusions and Mg O·A12O3spinel inclusions will be generated when calcium content in the molten steel is in the range of 0.8~8.2ppm and magnesium content is in the range of o.45~13.3ppm, and Al2O3 in the slag 10~45%, basicity bigger than 2, Mg O smaller than 15%, Fe O smaller than 15%, acid-soluble aluminum smaller than 0.06% and refining temperature at 1773~1883K.The industrial test results have shown that, high basicity refining slag has a strong desulfurization ability. The contents of Ca, Mg and Al in steel will gradually increase with the refining process going on, which has illustrated that, high basicity refining slag is unbeneficial to the control of globular inclusions.