Characteristics and Formation Mechanism of Ca–Mg–Al–Si–O + (Ca,Mn)S Duplex Inclusions in Ca–S Free-Cutting Steel

Abstract

Large numbers of long strip-shaped pure MnS inclusions in steel will result in anisotropy of mechanical properties. To obtain more duplex (Ca,Mn)S inclusions in steel through adding Ca can decrease the proportion of long strip-shaped pure MnS inclusions, and anisotropy of mechanical properties can be reduced. In this paper, based on three heats of Ca–S free cutting steel with Al content under 0.01%, the characteristics and formation of Ca–Mg–Al–Si–O + (Ca,Mn)S duplex inclusions in bars were analyzed. The results indicate these duplex inclusions can be classified as four types, named as “Type-C”, “TypeMC-H”, “TypeMC-L”, and “Type-M”, respectively. For Type-C, although they behave spherical, CaS is enriched in (Ca,Mn)S and CaO is enriched in core oxides. For TypeMC-H, CaS content in (Ca,Mn)S is appropriate, they behave spindle-shaped, but CaO content in core oxides closes to Type-C. The formation of Type-C or TypeMC-H consumes lots of Ca element, which makes the overall number of duplex inclusions decrease. For TypeMC-L, their compositions and shapes are both appropriate. For Type-M, although they have higher aspect ratios, their formation can reduce the formation of pure MnS inclusions and improve the distribution of sulfides. Under the condition with specific Ca/S ratio in steel, to obtain more duplex (Ca,Mn)S inclusions for reducing anisotropy of mechanical properties, numbers of Type-C and TypeMC-H should be decreased, and numbers of TypeMC-L and Type-M should be increased. The key is to make SiO₂ content in RH-end oxides as lower as possible, and Al content in steel should be controlled not less than 0.007%.

Graphical abstract