Influence of Inclusions on Impact Properties of J55 Steel

J55 steels are mainly used in the field of petroleum casing due to their excellent mechanical properties. Microstructure of J55 can improve its impact properties without losing strength. The characteristics of MnS inclusions including composition, morphology, size, mumber and area with different S content and Ca/S ratio in plates were studied utilizing SEM-EDS (scanning electron microscopy-energy dispersive spectrometry) and ASPEX. The effects of inclusions characteristics and molten steel compositions on impact properties were also investigated. The results demonstrate that inclusions affecting impact properties are mainly massive string typed MnS, especially those with size larger than 90 μm. Moreover, the number density and area density of MnS inclusions also have effect on the impact properties. The impact properties of steel are obviously improved by controlling S content to less than 0.004 0 mass% or Ca/S ratio more than 0.5 in plates, which ensures that the generation of larger size MnS inclusions is effectively inhibited and the number of MnS inclusions is reduced.