核乏燃料储运用难加工高硼不锈钢薄板的组织调控及高效制备

Coarse, hard and brittle eutectic borides with a low melting point and a network-like distribution existing in the initial solidification structure of high boron containing stainless steels lead to some problems such as bad hot workability, very narrow processing widow and so on, due to which the development of compact and high-performance fabrication technology for high boron containing stainless steels is hobbled. In order to solve this bottle-neck problem, we propound to design a new thermodynamic phase diagram and new chemical composition based on the introduction of new ‘Liquid→Liquid+δ’ transformation at the beginning of solidification to change the solidification behavior, by means of which the borides can be greatly refined and dispersed fundamentally. Also, much higher solidus temperature can be achieved so as to broaden processing widow. In the present project, this work will elucidate the principle of designing a new thermodynamic phase diagram and new chemical composition, clarify the new phase transformation behavior during sub-rapid solidification and the corresponding controlling mechanism. Moreover, the microstructure evolution, deformation behavior and failure mechanism during hot-working will also be revealed. The synergistic effects of a new thermodynamic phase diagram and new chemical composition, sub-rapid solidification as well as thermal-mechanical treatment on microstructure evolution and materials performance will be systematically explained. Thus, the key theoretical points of microstructure controlling and high-efficient fabrication for hard-to-work and high boron containing stainless steels will be established. Two new types of compact and high-performance fabrication process, ‘Die casting of thin slab + Side welding protecting material onto rolled slab + Hot continuous rolling’ and ‘Twin roll strip casting + Single-pass hot rolling’, will both be invented and developed to produce thin gauge hot rolled prototype steel with a combination of excellent thermal neutron absorption performance, good mechanical properties and favorable corrosion resistance.

凝固组织中粗大、网状分布的低熔点硬脆共晶硼化物导致高硼不锈钢热加工性非常差、热加工温度窗口极为狭窄等问题，严重制约了其短流程、高性能制备技术的发展。针对这一瓶颈，申请人提出通过基于引入L→L+δ的新型相图及成分设计来改变凝固过程的相变行为，从根本上实现硼化物的微细化与离散化控制，并大幅提高固相线温度以扩大热加工温度窗口。将阐明新型相图及化学成分的优化设计原理，弄清（亚快速）凝固过程基于新相图及成分设计的相变行为及控制机理，揭示热加工过程的组织演变规律、变形机制及破坏机理，系统阐明新相图及成分设计、（亚快速）凝固过程、形变热处理对组织演变的协同调控作用以及对材料性能的综合影响。据此建立难加工高硼不锈钢组织调控及高效制备的关键理论要点，发明“模铸薄板坯＋侧边焊接组坯＋热连轧”和“双辊薄带连铸＋一道次热轧”两种新型短流程、高性能制备技术，并获得热中子吸收、力学、耐腐蚀性能俱佳的薄规格热轧原型钢。

高硼不锈钢凝固组织中粗大、网状分布的低熔点硬脆共晶硼化物导致热加工性非常差、热加工温度窗口极为狭窄等问题，严重制约了短流程、高性能制备技术的发展。针对这一瓶颈问题，首先，研究了模铸条件下的凝固行为及热变形行为。具有正交结构的层片状(Cr,Fe)2B在奥氏体晶界上呈连续网状分布，且具有堆垛层错亚结构。硼化物破碎产生的孔洞在宽展导致的拉应力作用下继续增殖、长大及聚合，进而诱发热轧边裂。为了改善热塑性，提出了引入铝合金化的新型成分设计。加入1.5%Al能够明显提升液相线和固相线进而扩大热加工温度区间。并且，使L+γ两相区温度区间缩小，使初生奥氏体的生长受到抑制。从而消除了网状不均匀分布特征，改善了硼化物在基体中的分布均匀性，进而在一定程度上改善了热塑性。但是，加入过量的铝会导致(Cr,Fe)2B异常粗大，使热塑性恶化。然后，研究了无硼钢/高硼钢异种材料焊接过程的金属学行为及焊接接头的力学行为，为焊接组坯提供了支撑。提出通过焊接组坯+复合热轧来抑制热轧边裂并提升力学性能。无硼钢包覆层作用于高硼钢芯层边部的附加压应力能够有效抑制热轧边部裂纹的扩展。“三明治”层间界面通过奥氏体再结晶和元素扩散实现了良好的冶金结合。另外，无硼钢包覆层具有较好的加工硬化能力，并在“几何约束”作用下对芯层产生附加压应力，这两个方面共同延缓了芯层颈缩的发生，使复合板塑性显著优于非复合板。最后，将近终形双辊连铸技术引入高硼不锈钢板的制备流程，揭示了亚快速凝固过程中硼化物的演变规律及细化机制。奥氏体和硼化物之间存在特定的取向关系，且两者之间的界面错配度较低，这有利于硼化物的形核和细化。所以，薄带连铸板材的强度和塑性均优于模铸+热轧板材。通过上述工作，基本阐明了高硼不锈钢的组织演变及性能优化控制机理，发展了“模铸薄板坯＋侧边焊接组坯＋热轧＋固溶”和“双辊薄带连铸＋热轧＋固溶”两种原型制备工艺。该工作为发展我国核乏燃料安全储运用高硼不锈钢制造技术提供了重要的理论和技术支撑。基于本项目，发表（含录用）SCI收录论文9篇，获授权中国发明专利4项。入选2021年中组部“万人计划”青年拔尖人才1人，培养博士研究生2名。

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