A New Grain Refiner for Ferritic Steels

• Published in: Metallurgical and materials transactions. B, Process metallurgy and materials processing science Vol. 48; no. 6; pp. 2902 - 2912
• Main Authors: Li, Ming, Li, Jian-Min, Zheng, Qing, Qiu, Dong, Wang, Geoff, Zhang, Ming-Xing
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.12.2017; Springer Nature B.V
• Subjects: BACKSCATTERING; Characterization and Evaluation of Materials; Chemistry and Materials Science; Columnar structure; CRYSTALLOGRAPHY; Delta ferrite; ELECTRON DIFFRACTION; FERRITES; Ferritic stainless steels; FERRITIC STEELS; Ferrous alloys; GRAIN BOUNDARIES; GRAIN ORIENTATION; GRAIN REFINEMENT; GRAIN SIZE; IRON BORIDES; LANTHANUM BORIDES; LANTHANUM OXIDES; LANTHANUM SULFATES; MATERIALS SCIENCE; Metallic Materials; Microstructure; Model matching; Nanotechnology; NUCLEATION; Structural Materials; Surfaces and Interfaces; TEMPERATURE RANGE 0273-0400 K; THERMODYNAMICS; Thin Films; Electron Backscatter Diffraction (EBSD); La2O3 Particles; Average Equiaxed Grain Size (AEGS); Model EEM; LaB6 Particles
• ISSN: 1073-5615; 1543-1916
• DOI: 10.1007/s11663-017-1101-y

A new grain refiner, LaB 6 , was identified for ferritic steels based on the crystallographic calculation using the edge-to-edge matching model. Addition of 0.5 wt pct LaB 6 led to a reduction of the average grain size from 765 to 92  μ m and the proportion of the columnar structure from 35 to 8 pct in an as-cast Fe-4Si ferritic alloy. Although LaB 6 was supposed to act as an active inoculant for δ-ferrite, thermodynamic calculation indicated that LaB 6 is not thermodynamically stable in the melt of the Fe-4Si alloy. It was subject to decompose into La and B solutes. Consequently, both La and B reacted with Fe, O and S, forming different compounds. Microstructural examination at room temperature observed La 2 SO 2 and La 2 O 3 particles within the ferrite grains and Fe 2 B along the grain boundaries in the samples. Through EBSD analysis, a reproducible orientation relationship between ferrite and La 2 SO 2 was identified. In addition, the edge-to-edge matching calculation also predicted the high potency for La 2 SO 2 to be an effective nucleant for δ-ferrite. It was considered that the grain refinement of LaB 6 was attributed to the enhanced heterogeneous nucleation of δ-ferrite by La 2 SO 2 , and the solute effect of B due to the high Q -value in ferrite.