Effect of Rare Earth Ce on the Morphology and Distribution of Al2O3 Inclusions in High Strength IF Steel Containing Phosphorus during Continuous Casting and Rolling Process

• Published in: ISIJ International Vol. 61; no. 3; pp. 657 - 666
• Main Authors: Wang, Hao, Bao, Yan-ping, Zhi, Jian-guo, Duan, Chen-yi, Gao, Shuai, Wang, Min
• Format: Journal Article
• Language: English
• Published: The Iron and Steel Institute of Japan 15.03.2021
• Subjects: Al2O3 inclusion; high strength IF steel containing P; rare earth Ce
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.ISIJINT-2020-053

This paper analyzes the control of Al2O3 inclusions in high strength IF steel containing phosphorus. The inclusion statistics and two-dimensional morphology of samples with slab thickness of 1/8, 1/2 and 7/8, and the samples with hot rolling, cold rolling and continuous annealing processes were observed and compared by ASPEX SEM. In addition, the three-dimensional morphology of the inclusions extracted from the electrolysis of slab samples and the original morphology of inclusions from the rolling process samples were observed and compared. The results show that: When rare earth Ce is added to the steel, the combination of Ce with activity O and S in the steel has lower Gibbs free energy, and it is easy to generate CeAlO3, Ce2O2S, Ce2O3, composite rare earth inclusions combined with other inclusions. The concentration and supersaturation of aluminum and oxygen are reduced, and the ability of single particle Al2O3 to aggregate into large-scale cluster inclusions is reduced. The average size of Al2O3 inclusions decreased from 5–7 µm to 2–5 µm in each thickness direction. The morphology of inclusions changed from long strip, sharp angle and cluster to spherical, spindle and round surface. Meanwhile, the number density of Al2O3 inclusions increase, but the area density decreases during continuous casting and rolling process. The size of Al2O3 inclusions in the steel without rare earth addition is mostly large-scale strip shape, and it is crushed during rolling,resulting quality defects on the exposed surface and causing the stamping crack problem. When rare earth is added to the steel, the inclusions in the steel change into small-sized circular inclusions with dispersion distribution. The modulus of elasticity is close to that of the steel matrix, which will not affect the continuity of the strip structure, and is beneficial to the relevant properties of the product.