金属粗大晶粒热变形的微区协调机制及其对组织演变和塑性损伤的影响机理研究

The project aims at the refinement of very large grains that often exists in the 316LN and SA508-3 steel which are often used respectively as the steels to manufacture the main pipes and equipment in the nuclear power plants. The project will take the incompatible deformation mechanism of the large grains as the main line, through the experiment and crystal plasticity finite element analysis, to investigate the influence of grain sizes, orientation and degree of mixed grains on the micro-regional deformation distribution around large grains, to establish the characterization methods of trigger condition of intragranular recrystallization nucleation in the large grains, and to reveal the mechanisms of large grains fragmentation and refinement through recrystallization induced by the uneven micro-regional strain distribution. The project will also investigate the influence of micro-regional stress and strain gradient on the intragranular and intergranular micro-crack initiation, to clarify the relationship among the degree of mixed grains, stress state, micro-regional strain gradient and corresponding micro-crack nucleation and propagation, to reveal the mechanism of micro-regional ductile damage induced by the incompatible deformation of large grains. By coupling the crystal plasticity finite element and cellular automaton method, the simulation method of large grains recrystallization will be established, and aiming at the refinement of coarse grains, the method to determine the forming limit of the heavy forgings will be proposed. The research of this project will yield a set of effective computational methods for the deformation and refinement of large grains, and the achievement will provide analysis method and technology basis for the elimination of coarse grains in heavy forgings.

本项目针对核电主管道用钢316LN和核岛主设备用钢SA508-3中经常出现的粗大晶粒问题开展研究。以粗大晶粒的微区变形机制不协调为主线，通过实验与晶体塑性有限元分析，探索晶粒尺寸、位向及混晶程度对粗大晶粒微区变形分布的影响规律，建立晶内再结晶形核触发条件的表征方法，揭示微区应变诱发粗大晶粒破碎与多机制再结晶的细化机理。研究微区应力与应变梯度对晶内及晶间微裂纹萌生的影响规律，厘清混晶程度—应力状态—微区应变梯度—微裂纹形核与扩展的对应关系，揭示粗大晶粒变形不协调诱发微区塑性损伤的机理。基于晶体塑性有限元与元胞自动机方法的耦合，建立粗大晶粒再结晶的模拟方法，并以细化粗大晶粒为目标，提出大锻件成形极限参数范围的确定方法。通过项目研究，形成一套有效的粗大晶粒变形与细化的计算方法，为消除大锻件的粗大晶粒提供分析方法和工艺制定依据。

重大工程中经常涉及到超大型结构制造，例如核电蒸发器的一体化顶盖、主管道、发电机组低压转子等，这些结构件的锻件通常由重达几百吨的钢锭锻造而成。大锻件的热制造通常会遇到粗大晶粒的问题，这些粗大晶粒如不能加以解决，将严重影响锻件的性能，甚至导致锻件质量不合格。课题以毫米级粗大晶粒及混晶组织的细化与变形损伤为研究内容，以大晶粒引起介观尺度变形不均匀为线索，通过理论研究与实验测试，完成了以下主要工作：（1）开展了含有毫米级粗大晶粒与混晶组织的材料热变形组织演变研究，发现伴随着变形引起晶内滑移带的产生，再结晶形核常发生在粗大晶粒内部，并由小角度晶界逐渐演变成大角度晶界。（2）构建了含有大晶粒和混晶的代表体元模型及其晶体塑性有限元解法。模拟结果表明，软取向粗大晶粒优先发生变形，而硬取向粗大晶粒则承担了比平均应力高得多的应力，而高应力又可以启动其它滑移系，从而在硬取向晶粒内部形成滑移带。模拟结果与实验观察得到的现象相吻合，揭示了粗大晶粒的细化机理。将晶体塑性有限元与元胞自动机方法相集成，可视化地模拟了晶粒的演变过程。（3）在GTN-Thomason空洞损伤模型基础上，提出了经动态再结晶百分比修正的空洞形核与空洞聚合的临界应变条件，建立了热变形细观损伤模型。提出了包含应力状态与动态再结晶影响的唯像断裂模型，建立了晶粒尺寸与空洞扩展的概率型关系，比较准确地再现了晶粒尺寸对材料延伸率的影响。（4）提出了采用小变形和静态再结晶机理细化奥氏体不锈钢粗大晶粒的方法。.以该项目成果为依据，在制定大锻件的锻造工艺时，可以充分考虑消除粗大晶粒的锻造条件，在保证材料不发生明显损伤的条件下通过大变形达到控制大锻件晶粒度的目的，从而为提升重大工程中的大构件服役性能提供理论与技术支持。