稀土氧硫化物对汽车钢韧塑性的影响机制与控制研究

Continuous improvement of toughness and plasticity is an important direction in the field of automobile steel. It was in part known that the additions of rare-earth (RE) elements can improve the ductility of the automobile steel via modifying the large size inclusions into fine and uniformly dispersed RE-oxysulfides in the matrix. Our early studies have already attacked two long-time bottlenecks of technologies related with the industrialization of RE-contained steel. The one was the mechanical properties of the RE-contained steels were always fluctuated dramatically in an unstable way, and the other one was that nozzle clogging comes across seriously. Within this background, the present project aims to clarify the crucial influences on the purity and total oxygen content in RE-contained steels via undergoing joint multi-scale modeling, simulations and experimental studies. The main purposes of the project render a systematical study of (i) the interaction bonding between RE elements (La, Ce) and the interstitial or impurities (O, S, P) in steel, (ii) the characterization the RE elements and their reaction products in steel matrix, (iii) the formation and control mechanism of sub-micron-size (RE)2O2S, and (iv) the elucidation of the effect of size, morphology, amount and distribution of (RE)2O2S on the microstructures and properties of Re-contained steels during the deformation and phase transformation. In terms of fundamental mechanism studied here, the generic technologies for the manufacturing of sub-micron RE-oxysulfide-contained steels will be developed, with a key technical index as requested that the size of more than 50% (RE)2O2S is less than 1μm and with the toughness and plasticity enhanced over 20%. The project can provide a theoretical guidance and the technical support for the high quality third generation automobile steels and will promote the utilization of rare earth resources in equilibrium, as well.

高韧塑性是汽车钢发展的重要方向。而稀土金属可以变质和细化钢中的大尺寸夹杂物成为均匀、弥散、细小的稀土氧硫化物，从而显著提高汽车钢的韧性和成形塑性。在突破稀土加入钢中导致的工艺不顺行和性能不稳定两个难题基础上，本项目将采用多尺度模拟计算与实验研究相结合的方法，澄清纯净度及氧含量在稀土钢应用中的关键作用，研究镧、铈轻稀土与钢中元素间的交互作用规律，表征稀土及其反应产物在钢基体中的存在形式与分布状态，揭示亚微米稀土氧硫化物的形成与控制机理；阐明形变与相变过程中稀土氧硫化物的尺寸、形貌、数量与分布对组织与韧塑性的影响机制；开发亚微米级稀土氧硫化物钢制备原型技术，实现钢中50%以上稀土氧硫化物尺寸小于1μm的目标；在典型汽车钢品种中示范，使其韧性与成形塑性提高20%以上。为第三代汽车用钢的高质量制备提供指导，并促进稀土资源平衡利用。

持续提高韧塑性是汽车钢轻量化发展的主流方向，而精细的工艺控制和夹杂/组织调控是获得最佳强韧性匹配的首选途径。稀土添加有望实现该目标：化合态稀土变质夹杂，将钢中粗大团簇状Al2O3类夹杂改性为细小球状RE2O2S，钢渣反应吸附部分夹杂从而净化钢液；固溶态稀土作用于再结晶与相变热/动力学进而细化基体组织，可共同提高汽车钢的韧性与成形塑性。本项目通过多尺度模拟与多手段表征，澄清了纯净度即氧含量在稀土钢应用中的关键作用，明确不同纯净度下稀土元素与炼钢元素间的交互作用序列以及稀土合金化适用条件；表征了亚微米RE2O2S形成与演变过程，阐明稀土对Al2O3类夹杂的变质改性机理及减少MnS、TiN类夹杂形核基底的正面效应；基于稀土及其反应产物的存在形式与分布状态，研究了化合/固溶态稀土元素分配与夹杂物生成、析出行为的关联及其对形变、再结晶与相变过程中元素配分、组织演变和韧塑性改善的作用机制，搭建起“化合/固溶稀土-夹杂/组织-性能”基础架构及亚微米稀土氧硫化物钢制备理论基础。同时，基于先研超纯/超高纯稀土金属制备和双低氧洁净化技术，稀土添加后全流程保护浇铸，突破了稀土钢工业化应用时水口絮瘤、性能波动两大瓶颈，开发出亚微米稀土氧硫化物钢原型制备技术（50%以上RE2O2S尺寸小于1μm），并优选3类典型汽车钢在本钢、鞍钢、首钢等企业示范化应用，生产过程稳定顺行。为第三代高品质汽车钢研制提供理论/技术指导，并为稀土资源平衡利用开拓应用出口。