异步热轧高锰奥氏体无磁钢的退火孪晶形成机理

For high managanese austenitic steel, the behaviour of strengthening and roughness is closely related to annealing twins. But how to obtain huge amounts of small size annealing twins in hot rolling process is still a difficult problem need to solve. This project presents a new way of thinking, huge amounts of small size annealing twins will be obtained by cross shear hot rolling process for high manganese austenitic steel. The technology is supposed to utilize cross shear hot rolling to drive the metal in cross shear region to produce a severe plasticity deformation so that the shear stress and accumulated strain during deformation can promote huge amounts of small annealing twins. Based on the design of chemical composition and hot rolling technology, influence of parameters of cross shear hot rolling process, including speed ratio, schedule of reduction rolling, initial and finish rolling temperature, coiling temperature and cooling method on microstructure and mechanical property will be investigated for non-magnetic high managanese austenitic steel in the laboratory. During serve plasticity deformation process the influence of shear stress and accumulated strain on stacking fault energy, nucleation, grain refinement and texture of annealing twins will be explored so as to reveal the formation mechanism of annealing twins in non-magnetic high manganese austenitic steel during cross shear hot rolling. A new revised stacking fault energy model with factors of shear stress and accumulated strain for non-magnetic high managanese austenitic steel will be established. The relation of inheritance and evolution between annealing twins and deformation twins will be intensive studied. Finally, the control method of annealing twins during cross shear hot rolling will be proposed. Undoubtedly, the research results possess significance for development and application of high strengthening and roughness high managanese austenitic steel.

高锰奥氏体钢的强韧化行为与其退火孪晶组织密切相关，但如何在热轧过程中获得大量细小的退火孪晶组织是目前亟待解决的难题。本项目提出通过异步热轧技术实现高锰奥氏体钢产生大量细小孪晶组织的新思路，即在异步热轧过程搓轧区内的金属发生深度塑性变形，由此产生的剪应力和累积应变将有利于获得大量细小的退火孪晶组织。在实验室进行高锰奥氏体无磁钢的成分和异步热轧工艺设计的基础上，考察不同速比、变形分配、开轧温度、终轧温度、卷取温度和冷却方式等异步热轧工艺参数对退火孪晶组织和力学性能的影响；探索深度塑性变形的剪应力和累计应变对层错能、孪晶形核、孪晶细化和孪晶织构的影响规律；揭示异步热轧过程高锰奥氏体无磁钢的退火孪晶形成机理；建立新的考虑剪应力和累积应变的层错能计算模型；深入研究退火孪晶与形变孪晶的遗传和演变关系，最终提出异步热轧过程退火孪晶组织的控制方法。研究结果对于高强韧高锰奥氏体钢的开发与应用具有重要意义。

高锰奥氏体钢的强韧化行为与其退火孪晶组织密切相关，但如何在热轧过程中获得大量细小的退火孪晶组织是目前亟待解决的难题。本项目提出通过异步热轧技术实现高锰奥氏体无磁钢产生大量细小孪晶组织的新思路，即在异步热轧搓轧区内金属发生深度塑性变形，由此产生的剪应力和累积应变将有利于获得大量细小的退火孪晶组织。.以30Mn20Al4V高锰无磁钢为研究对象，建立了高锰奥氏体钢的异步热轧工艺理论基础，提出了异步热轧高锰奥氏体钢退火孪晶组织的演变规律和控制方法。分析了化学成分对于高锰奥氏体钢层错能和退火孪晶形成的影响。研究了冷轧变形分配、退火温度和退火时间在再结晶晶粒形核和长大阶段对退火孪晶界占大角度晶界比例的影响规律，并以此为基础，分析了常规热轧变形分配和轧制道次数等工艺参数与退火孪晶演变的作用关系。利用有限元模拟计算了异步热轧不同异速比、变形量条件下剪应力应变场分布，并根据数值模拟结果对不同轧制速度、轧制温度、异速比、变形分配和异步热轧过程中高锰无磁钢晶粒细化、织构转动、退火孪晶形核、孪晶细化、力学性能之间的关系进行了分析，得到了优化的异步热轧工艺参数。并对冷变形过程中退火孪晶和形变孪晶的遗传和演变关系进行了考察研究。.结果表明，热轧过程退火孪晶比例与晶粒尺寸存在显著依赖关系，随着晶粒尺寸增加，呈现先增加后减小趋势，当晶粒尺寸过大，孪晶比例基本保持不变。而且退火孪晶比例对退火温度、退火时间等工艺参数以及再结晶织构变化表现不敏感。异步热轧产生的附加剪应力和深度塑性变形提高了奥氏体在变形和再结晶过程中的晶界迁移速率，促使晶粒长大过程发生堆垛错排的几率增高，同时诱发大量Σ3特殊晶界反应，从而促进了高锰奥氏体钢中大量细小的退火孪晶组织的形成。由于异步热轧显著的晶粒细化和孪晶细化作用，高锰奥氏体钢热轧板强度提高约100 MPa，延伸率从47%提高到60%。因此，给出能够获得细小孪晶组织的异步热轧工艺参数为：轧辊速度接近1m/s或更高，异速比为1.2，终轧温度在880℃~920℃，最后两三道次的总变形量超过60%。