The inclusion of the MgO·Al2O3 (MA) spinel and CaO–Al2O3 are occasionally observed during the refining of Al–killed steel, even without the intentional additions of Ca and Mg. Many studies have focused on the source of Mg and Ca; however, especially for the formation of CaO–Al2O3–type inclusions, some recent results showed that Ca was difficult to dissolve from refining slag, even when the Al content in molten steel was high. To confirm these differences, industrial experiments were designed in this study, and the effects of the FeO and MnO contents, as well as the impurities of the alloying materials, were discussed. The results showed that, when the FeO and MnO contents in slag were high (about 10 mass%) at the start of LF (ladle furnace), all inclusions remained as Al2O3, despite alloying. Using the slag with low FeO + MnO content (<1~2 mass%), the Al2O3 inclusions changed to the MA spinel, but CaO–Al2O3 inclusions were not observed, indicating that CaO–Al2O3 inclusions were difficult to form by the steel/slag reactions under the current conditions. Only for the molten steel that contained a low level of dissolved oxygen and a large amount of Fe–Si, which contained Ca as the impurity was added, CaO–Al2O3 inclusions were generated.
Inclusions containing Mg existed in low carbon aluminium killed steel even though Mg is not added during LF treatment. To investigate the mass transfer mechanism of Mg in low carbon aluminium killed steel, both industrial practice and kinetic calculations were carried out in the present work. The results from industrial practice showed that Mg concentration in molten steel and inclusions increased with refining time during ladle furnace treatment. The inclusion size tended to become smaller with the increase of Mg concentration in the inclusions. The erosion rate of refractory with different composition was tallied. A refractory-slag-metal-inclusion multiphase reaction model was developed to investigate mass transfer mechanism underlying the variation of Mg among the steel, the slag, inclusions and the refractory. The calculated results exhibited a good predictability of the content of Mg in the molten steel, slag and inclusions. The results showed that Mg dissolved into molten steel in two ways: the first is in the way of slag/steel reaction, the second is in the way of refractory erosion which is the main way.
The paper presents a failure analysis of tower axle in the water tank wire drawing machine. Material of the tower axle is 40Cr steel. In order to analyze the failure reasons of tower axle in the water tank wire drawing machine, we take experimental means such as morphological analysis, mechanical property testing, micro-metallography and scanning electron microscopy (SEM) of fracture observation, etc. The results show that the fracture of tower axle belongs to brittle fracture. The content of inclusions is more, and the composition is complex in the material. The heat treatment technology is improper. Improper quenched and tempered processing technologies cause more reticular and blocky ferrite in the steel. These are the main reasons for fracture of the tower axle.
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