稀土镧和铈对钢液凝固过程中细化δ相的作用及机理研究

The effects of lanthanum and cerium on the solidified structure of steels have been attracted interest and attention of researchers over the years. But the previous reports are inconsistent. For the related action mechanism, the hard data are lacking to support the existing viewpoints at present. For the steels which the primary phases in solidification are δ-ferrite, study on the effects of lanthanum and cerium on the solidified structure is particularly challenging because of the solidification process followed by solid-state phase transformations. The model alloy of Fe-4 %Si will be selected as the study object in order to avoid the solid-state phase transformations which can interfere in the microstructure analysis. Based on the thermodynamic calculation, the experimental materials will be prepared by adding different amounts of the single element of La/Ce to Fe-4 %Si alloy and simultaneously controlling the contents of oxygen and sulfur. The comparative study on the solidified structure of Fe-4 %Si and Fe-4 %Si-RE alloys will be carried out to clarify whether the rare earth elements of lanthanum and cerium can refine the δ-ferrite. The microscopic characterization techniques of materials, the crystallography calculation and the first principles calculation will be used to study the possibility and the potency of the various rare earth inclusions with high melting point acting as the heterogeneous nucleation sites of the primary phase δ-ferrite. The directional solidification experiments will be designed and the growth restriction factor theory will be applied to understand the effects of the solute lanthanum and cerium on the constitutional supercooling and then reveal the effects of the constitutional supercooling on the grain size of solidified structure. The researches aim to clarify the previously controversial and confused reports and provide the fresh insight into the understanding of the effects of lanthanum and cerium on the refinement of δ-ferrite in solidification of steels. The solid and reliable related acting mechanism of lanthanum and cerium will be theorized.

稀土镧、铈对钢的凝固组织影响的研究一直备受关注，但以往的研究没有形成统一的认识，相关机理缺乏确凿的研究数据来支撑已有观点。对于初生相为δ铁素体的钢，因凝固后发生固态相变，难以将初生相保留到室温，造成实验研究的困难。为了避开凝固后发生固态相变对微观组织分析的干扰，本项目选择Fe-4 %Si模型合金作为研究载体，结合热力学计算，设计不同的单一稀土（镧、铈）加入量及可控制的氧、硫含量冶炼制备试验材料。通过对比研究Fe-4 %Si与Fe-4 %Si-RE合金的凝固组织，搞清镧、铈是否能够细化δ相。采用材料微观表征技术，并通过晶体学计算、第一性原理计算，研究各类高熔点稀土夹杂物作为δ相异质形核核心的可能性；通过设计定向凝固实验，结合生长抑制因子理论的应用，研究稀土以原子形式存在对成分过冷的影响进而对凝固组织的影响。澄清以往研究的混乱报道，提出稀土作用的新见解，建立扎实可靠的相关稀土作用机理的理论。

以往研究认为镧/铈能够细化钢的铸态组织，其作用机理是高熔点稀土夹杂物对初生相的异质形核作用或/和镧/铈的溶质作用。对于凝固初生相δ铁素体，因凝固后发生固态相变，难以将初生相保留到室温，造成实验研究的困难。因此以往的研究没有形成统一认识，缺乏确凿的研究数据支撑已有观点。本项目选择能够将初生δ铁素体保留到室温的Fe-4 %Si合金作为研究载体，通过设计不同的镧/铈加入量熔炼制备实验材料，研究镧/铈对初生δ铁素体的细化作用；采用晶体学预测、热力学计算、第一性原理计算和微观表征技术，研究不同铈/镧添加形成的高熔点稀土夹杂物作为初生δ铁素体异质形核核心的可能性及效用；设计定向凝固实验，进行微区成分分析，对生长抑制参数β的理论计算，研究以原子形式存在的Ce/La对初生δ铁素体的细化作用。研究表明，选择Fe-4 %Si合金作为研究载体，方便研究镧/铈对钢液凝固初生相δ铁素体的细化作用，添加单一稀土能够细化初生δ铁素体。添加RE（Ce/La）的Fe-4 %Si合金中稀土夹杂物的形成顺序为RE2O3 > RE2O2S > RE的硫化物，RE硫化物可能为RE2S3、RE3S4或RES，其类型与RE含量和S含量的比值有关。通过EBSD菊池线对位向关系的表征，首次测得Ce2O2S/La2O2S与δ铁素体之间存在可重复的晶体学位向关系，与晶体学预测一致。从晶体学的角度证实了Ce2O2S和La2O2S可以能够钢液凝固过程δ铁素体异质形核的有效核心。第一性原理计算为晶体学预测提供了能量学的依据。考虑到实际钢中以原子形式存在的稀土极少，相比高熔点Ce/La夹杂物的异质形核作用，Ce/La对于细化δ铁素体的溶质作用很小。初生δ铁素体的细化主要受高熔点La/Ce夹杂物的异质形核作用控制。研究工作澄清了以往的混乱报道，提出镧/铈相关作用机理的新见解，丰富了稀土在钢中作用机理的理论体系。