北京科技大学罗海文教授团队与上海交通大学金学军教授团队联合申请了国家自然科学基金重点项目“复相组织超高强高韧钢中多类型析出强化及相变强化耦合机制研究”。

申请团队在多相组织高强韧中锰钢前期研究中，既发现经典析出强化位错理论模型显著低估了实际强化贡献，也发现中锰钢中两种纳米粒子同时析出比一种粒子的析出强化更大。据此，提出了与现有马氏体时效超高强高韧钢不同的组织设计思路，即在中锰成分体系的多相组织钢中引入不同类型的纳米粒子，旨在通过不同类型纳米析出相（绕过、剪切机制）的叠加强化，并与相变强化耦合来获得超高强度和高塑性。本项目主要研究如下内容：①关于单一类型粒子在多相组织中析出强化的机制与模型；②多类型纳米粒子（绕过、剪切机制）在多相组织中的沉淀强化的叠加机制与模型；③纳米析出相对多相组织超高强度钢屈服与加工硬化行为的影响；包括纳米粒子对亚稳相机械稳定性和相变的影响，沉淀强化与相变强化的耦合机制等。与含有贵合金元素的马氏体时效钢不同，研究可望通过合金素华设计将超高强钢的强度-塑性边界上限提高到一个新水平。前期工作初步证实了上述思路的可行性。

Major Project of National Natural Science Foundation of China (NSFC), “Fundamental research on multi-phased ultrahigh-strength and ductile steel via multi-type precipitation and transformation hardening”

During the previous extensive researches on the multi-phased medium-Mn steels, we have found that the classical physical modeling on the precipitation strengthening may underestimate the actual measured contribution; moreover, the precipitation hardening resulting from two different types of nanoparticles is more than that merely from the single type. Based on these findings, we propose a new strategy on designing the ultrahigh strength steel, which is different from the existing well-known maraging steel. We plan to introduce two very different types of nanoparticles, which can interact with dislocations by both cutting-through and looping mechanisms, into the multi-phased medium-Mn steel containing meta-stable phase. In this case, two types of precipitation strengthening including both cutting and by-passing together with transformation hardening can be all expected during deformation, which should lead to significant increases in both yield and ultimate tensile strength. The following aspects are suggested for the future research. ① Why did the calculated precipitation hardening on VC precipitation in medium-Mn steel in the previous research significantly underestimate the actually measured contribution? A new physical model which is consistent with the experimental measurement shall be studied; ② when two very different types of nanoparticles, one for cutting-through and the other for by-passing, are both present in the multi-phased microstructures, does the stronger strengthening effect resulting from one type shield the weaker one from the other type? If not, how can the strengthening from each type of nanoparticles be superimposed? What is the quantitative model on this overlapped strengthening contribution? ③ How does the precipitation hardening affect the yielding and work hardening during deformation; in particular, how does it affect the mechanical stability of retained austenite and strain-induced transformation, and how can the precipitation strengthening and transformation hardening coupled for the best combination of ultrahigh strength and high ductility? By answering these questions, we may expect that the trade-off boundary of strength-ductility in such lean alloys can be further improved to a new record by the proposed research. Moreover, the medium-Mn steel, as proposed for research in this project, is a relatively lean alloying design, since it contains just economic alloying element of Mn rather than those expensive alloying elements of Ni, Mo, Ti and Co, the latter are commonly used in the conventional ultrahigh-strength maraging steels. The preliminary researches performed by the applicants confirm the feasibility of proposal.