Effects and Mechanism of Lanthanum/Cerium on Refinement of the Primary δ-Ferrite in Solidification of Steels

Author:Ji Yun Ping

Supervisor:ren hui ping


Degree Year:2019





The effect of lanthanum/cerium on the macrostructure and microstructure of steels in solidification have attracted increasing interest and attention of steel researchers and engineers for decades.It is generally considered that the grain refinement of steels in solidification by addition of lanthanum/cerium is attributed to the heterogeneous nucleation of RE inclusions or/and the solute effect of lanthanum/cerium.However,the previous reports about the related action mechanism of lanthanum/cerium are inconsistent and the hard data are lacking to support the existing viewpoints at present.For the steels with δ-ferrite as the primary phase,study on the refining effect of lanthanum/cerium on the 8-ferrite is particularly challenging because the δ-ferrite is subject to phase transformations in the subsequent cooling process,including the direct transformation from δ-ferrite to y-austenite or the peritectic reaction(δ+L→y)and then the solid-state phase transformation of γ-austenite.In order to retain the primary δ-ferrite to room temperature after solidification and avoid the interference to microstructure analysis caused by the solid-state phase transformations,the model alloy of Fe-4%Si will be selected as the study object.The experimental materials were prepared by adding different amounts of’ La/Ce into the Fe-4%Si alloy.The refining effect of La/Ce on the primary δ-ferrite and the mechanism of La/Ce were studied.The researches aim to clarify the previously controversial and confused reports and provide the fresh insight into the understanding of the effect of La/Ce on the refinement of δ-ferrite in solidification of steels.The refining effect of La/Ce on the primary 8-ferrite was investigated by comparing the solidified structure of Fe-4%Si and Fe-4%Si-La/Ce alloys.The microscopic characterization techniques of materials,the thermodynamic calculations using the FactSage Software System and the crystallography calculation using the edge-to-edge matching(E2EM)model were employed to study the possibility and the potency of the La/Ce inclusions with high melting points acting as the heterogeneous nucleation sites of the primary δ-ferrite.The directional solidification experiments were designed and conducted.The Electron Probe X-ray Micro-Analyser(EPMA)was used to perform the qualitative and quantitative microchemical-analysis of the directional solidified samples.The growth restriction parameter(β)of the Fe-4%Si alloys with different La/Ce contents was calculated Then the solute effect of La/Ce existed in the form of atoms on the grain size ofδ-ferrite in solidification of steels was revealed.The results show that the addition of La/Ce can refine δ-ferrite in steels.The majority of Ce-inclusions formed in the alloys with different contents of Ce from 0.0080%to 0.0325%are Ce2O2S,and no other types of Ce inclusions are observed probably because of the small fractions of them.Similarly,the majority of La-inclusions formed in the alloys with different contents of La from 0.0060%to 0.0283%are La2O2S.The La2O2S/Ce2O2S in the Fe-4%Si alloys with different La/Ce contents is spherical or ellipsoidal in shape.The distribution of La2O2S/Ce2O2S particles in the alloys is dispersed and the size distribution of them concentrates in the range of 1.0-2.0μm.For the first time,two reproducible ORs between Ce2O2S and δ-ferrite and one reproducible OR between La2O2S andδ-ferrite are experimentally determined using the EBSD technology,which agree well with the predictions using the E2EM model.This confirms that La2O2S and Ce2O2S can act as effective heterogeneous nucleation sites for the primary δ-ferrite during solidification of steels from the crystallographic point view.The ORs are as follows(δ-ferrite is denoted by δ-Fe for convenience): OR(1):[2110]ce2o2s//[110]δ-Fe,(0002)ce2o2s 1.0°from(110)δ-Fe,(0110)ce2o1s1.0°from(002)δ-Fe OR(2):[1101]Ce2o2s//[100]δ-Fe,(1120)ce2o2s3.0°from(011)δ-Fe,(1102)ce2o2s 3.0°from(011)δ-Fe 0R(3):(0002)La2o2S//(110)δ-Fe,(0110)La2o2s1.5°from(002)δ-Fe,[2110],a2o2s1.5°from[110]δ-Fe.It is confirmed that the La/Ce atoms dissolved in alloy melts can enrich at the solid/liquid interfaces during solidification of the alloys through the long-running EPMA point quantitative analysis.Because the equilibrium distribution coefficients of La and Ce are both much less than 1.the controlling factor for the crystal growth in solidification of the Fe-4%Si alloys with different La/Ce contents is the supply of the solvent atoms on the basis of the new concept for growth restriction of β.The growth restriction effect of the solute of La/Ce is the blockage of the supply of the solvent atoms of Fe during the solidification of the Fe-4%Si alloys,which can cause the solute enrichment of La/Ce and Si ahead of the growing solid/liquid interface.Calculation of the β value of the Fe-4%Si-La/Ce system,which is comprehensively considered the nature of solutes,solute concentrations and solidification conditions,indicates that the solute effect of La/Ce on the grain refinement of δ-ferrite in solidification of steels is weak.Moreover,in the actual solidification process of steels,the cooling velocity can probably conceal the solute effect of La/Ce on the grain refinement of δ-ferrite.It is the heterogeneous nucleation effect of La2O2S/Ce2O2S in-situ formed by adding La/Ce in Fe-4%Si alloy that governs the grain refinement efficiency ofδ-ferrite during the solidification of the alloys.This confirms the experimental observations.The heterogeneous nucleation effect of high melting-point La/Ce containing inclusions is also the governing factor for the grain refinement of the 8-ferrite during solidification of steels.