Effect of rare earth Ce on MnS inclusions in high strength IF steel containing phosphorus during a continuous casting and rolling process

The control of MnS inclusions in high strength IF steel containing phosphorus was analyzed. The inclusion statistics and two-dimensional morphologies of the samples at the slab thicknesses of 1/8, 1/2, and 7/8 and in hot rolling, cold rolling, and continuous annealing processes were observed and compared via an ASPEX scanning electron microscope (SEM). In addition, the three-dimensional morphologies of the inclusions extracted from the electrolysis of billet samples and inclusions extracted from the original rolling process samples were observed and compared. The results show that the amount distribution of MnS inclusions in the center of the slab is obviously larger than that near the surface of the slab. When a rare earth element is added, it is preferentially combined with the S in the steel and precipitates earlier than MnS in the solidification process, forming small spherical inclusions, which can significantly reduce the size and quantity of MnS inclusions at various positions of the slab. The size of MnS inclusions of the steel strip without a rare earth element addition is approximately 10 μm in each rolling process, which is inherited. During the rolling process, MnS inclusions become longer, but there is no fragmentation and dispersion. S–O–Ce inclusions are formed after adding a rare earth element. These inclusions are spherical, 2–5 μm in size, and distributed independently, which do not affect the structure continuity of the strip steel and benefit the relevant properties of the products.