Morphology Study on Inclusion Modifications Using Mg⁻Ca Treatment in Resulfurized Special Steel

In resulfurized special steel, MnS and Al₂O₃ are two main inclusions that deteriorate fatigue life and machinability. It is important that these two inclusions should be well controlled to increase steel quality and usage performance. In the present study, a Mg⁻Ca treatment was employed to modify the MnS and Al₂O₃ inclusions in resulfurized steels to reduce detrimental effects on fatigue life and machinability. In the laboratory study, Ni⁻Mg alloy was added to 16MnCrS5 and 49MnVS3 steels. Both Al₂O₃ and CaO⁻Al₂O₃ were gradually modified to MgO·Al₂O₃ and MgO, being surrounded by MnS, that is, a complex inclusion with an oxide core and sulfide outer layer was formed. The amount of the complex inclusion increased with Mg content. In the hot forging experiment, non-Mg treated inclusions were in the morphology of long strip, while those with Mg treatment were seen to be less deformed with spherical morphology of low aspect ratio in which case inclusions had less effect on steel mechanical properties. The Mg⁻Ca treatment was also applied to the manufacture of resulfurized special steel in steel plants. The scanning electron microscope⁻energy dispersive spectrometer and statistical results agreed well with those in the laboratory studies.