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FAQ: Why is the Ca:S ratio specified in steel specifications?


Frequently Asked Questions

Calcium is added to steel during manufacture for the reduction of sulphur and oxygen contents and also to control the shape of remaining sulphide inclusions. Following injection into the ladle, calcium combines with sulphur and oxygen, and reaction products are carried into the slag.

Rolling of steel generally results in the flattening of non-metallic inclusions, the extent of this flattening depending on the relative inclusion:matrix strengths at rolling temperature. The presence of elongated inclusions results in anisotropy of toughness and ductility properties. Ca additions strengthen sulphide inclusions and reduce the extent of deformation, thus giving more isotropic properties to the steel. Excessive additions of Ca can lead to the formation of CaOS clusters. The optimum Ca:S ratio is approximately 2.

Rolled inclusions in steel provide planes of weakness which may result in cracking problems due to weld shrinkage strains (lamellar tearing), or hydrogen accumulation at the interfaces ( hydrogen pressure induced cracking). There are also some additional benefits of Ca additions to ERW/HFI welded pipe.

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