A New Grain Refiner for Ferritic Steels

作者：Li Ming; Li Jian Min; Zheng Qing; Qiu Dong; Wang Geoff; Zhang Ming Xing^*

来源：Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science, 2017, 48(6): 2902-2912.

DOI：10.1007/s11663-017-1101-y

A new grain refiner, LaB6, was identified for ferritic steels based on the crystallographic calculation using the edge-to-edge matching model. Addition of 0.5 wt pct LaB6 led to a reduction of the average grain size from 765 to 92 mu m and the proportion of the columnar structure from 35 to 8 pct in an as-cast Fe-4Si ferritic alloy. Although LaB6 was supposed to act as an active inoculant for delta-ferrite, thermodynamic calculation indicated that LaB6 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 La2SO2 and La2O3 particles within the ferrite grains and Fe2B along the grain boundaries in the samples. Through EBSD analysis, a reproducible orientation relationship between ferrite and La2SO2 was identified. In addition, the edge-to-edge matching calculation also predicted the high potency for La2SO2 to be an effective nucleant for delta-ferrite. It was considered that the grain refinement of LaB6 was attributed to the enhanced heterogeneous nucleation of delta-ferrite by La2SO2, and the solute effect of B due to the high Q-value in ferrite.

• 出版日期2017-12