Herein, the nucleation, growth, aggregation, sintering, and densification of sulfide in 1215MS free-cutting steel billet with a total oxygen concentration of 70 ppm are investigated. A portion of oxygen in the steel dissolves in sulfide to form oxysulfide, Mn(S,O), which can be single-particle spherelike (SPS), doubleparticle rodlike (DPR), and multi-particle rodlike (MPR), and SiO 2 can precipitate at the outer edge of the SPS and connection in the DPR and MPR inclusion particles to form Mn(S,O)-SiO 2 inclusion, which is formed by the change of liquid inclusion, that undergoes monotectic reaction to form oxides and oxysulfide. The possible phase-transition process can be expressed as (Mn,Si) x (S, O) y (l) ! Mn(S,O)(l ! s) þ (Mn,Si) x O y (s) ! Mn(S,O)(s) þ SiO 2 (s). Oxysulfide inclusions can aggregate under the influence of van der Waals and viscous drag forces. The larger the particle, the greater the critical spacing and aggregation force. MPR oxysulfide can be divided into three parts: droplet, flat surface, and neck, which have decreasing chemical potentials in turn. Sintering and densification occur after the aggregation of particles, where surface diffusion of elements dominates, while grain boundary diffusion and bulk diffusion dominate in the densification of particles. Meanwhile, the formation and evolution of five different morphologies of sulfides in 1215MS free-cutting steels are discussed.
