Characterization of Microstructure and Non-metallic Inclusions in Zr–Al Deoxidized Low Carbon Steel

Laboratory experiments were carried out to investigate non-metallic inclusions and microstructures in Zr–Al deoxidized low carbon steel. Contents of [Zr] in the steel samples were of different levels while [Al] varied in the range of 0.0012–0.0032%. It was found that inclusions and the developed microstructures varied greatly with the change of [Zr] in steel. In the high [Zr] (0.072%) steel sample with [S] about 0.012%, the produced ZrO2 inclusions in deoxidization favored the uniform precipitation of MnS and all inclusions were composed of dual phases of ZrO2+MnS. Microstructures consisted of pearlites and ferrites after water quenching directly from liquid to solid. Ferrites developed by ZrO2+MnS inclusions were not observed. With [Zr] at a medium level about of 0.0085% and [S] at about 0.012%, complex (ZrO2–TiOx)–(Al2O3–SiO2–MnO–(MnS)) inclusions were formed. The microstructures of steel were characterized by fine and interlocked intra-granular ferrites (IAF). SEM observation indicated that those complex inclusions were good nucleation sites for IAF. In the [Zr] steel sample with low [Zr] of about 0.0008% and [S] of about 0.0017%, very typical IAF was also induced by the complex (MgO–Al2O3–SiO2–MnO) inclusions of very high number density.