梯度超细晶钢的动态相变机理与制备技术

It is one of the most difficult challenges to simultaneously improve strength, ductility, and toughness of steel. The interplay relationship between strength and ductility or toughness cannot be broken through by the strengthening and toughening mechanism at microscopic scale alone. On the other hand, ultrafine grained steel has been appeared the great potential for improving properties. Its apparent drawbacks of low work-hardening capacity and difficult fabrication of large-size pieces, however, limit the industrial application at the moment. The recent research has shown that the optimized properties can be achieved by architecture design on multiple length scales in either the structure or constituent at both the microscopic and macroscopic levels. The architectured materials provides an important paradigm shift for developing new high-strength steels with high toughness, as well as high ductility by processing innovation..To optimize the strength, ductility, and toughness, this study therefore proposes a new process to architecture a gradient structure with ultrafine grains in the thickness direction. During interpass of hot rolling, instantaneously ultrafast cooling is applied to produce lath-type low-temperature constituents as starting microstructure, which can effectively isolate and refine the prior austenite. Reverse transformation will rapidly occur in the surface layer by high thermal conductivity from higher-temperature core. Subsequent rolling deformation promotes strain-induced transformation and dynamic recrystallization or recovery through cooperative mechanism, forming ultrafine grained multi-phase structure. The attention will be draw on dynamic transformations for forming ultrafine grains, characterization of gradient structure and cooperation behavior of multi-length scale constituents. The effort will also made to build the corresponding relationship between architectured microstructure and overall improvement of properties. Thus, the long-standing curse on lowering ductility and toughness with increasing strength will possibly be broken in gradient ultrafine grained steel. Finally, the research will provide fundamental supports for industrial processing to produce low-cost and high-performance steels for highly diverse applications.

同时提高强度、塑性和韧性是结构用钢发展面临的重要挑战之一，但仅靠单一微观尺度上的强韧化机制设计已难以解决这一固有矛盾。超细晶钢虽极具性能提升潜力，但由于其加工硬化能力低、塑性差和大尺寸制备困难而无法满足结构用钢应用需求。通过在钢材表层选择性超细晶化，实现微观结构尺寸在宏观尺度上的梯度变化，可有效克服单一超细晶结构的性能缺点，同时发挥梯度层次结构的相互协调作用，使钢材整体性能得到优化和提高。.本项目提出在厚度方向上构筑梯度超细晶结构的研究思路，以实现高强度、高塑性及高韧性为研究目标，基于轧制中间坯即时超快冷工艺形成的板条初始组织，通过逆相变与应变耦合作用，重点研究动态应变诱导铁素体相变、动态再结晶或回复的超细晶形成机理，阐明厚度方向超细晶到粗晶结构梯度变化特征及其塑性变形协调机制，验证综合性能优势，并掌握梯度超细晶钢制备技术，为发展满足多样化工业应用需求的低成本、高性能结构用钢奠定研究基础。

金属材料研究领域长期以来寻求突破“强度-塑性-韧性”难以兼顾的瓶颈，而晶粒尺寸细化被认为是最有前景的方法。控轧控冷技术大规模应用成功将热轧钢板平均晶粒尺寸细化到5 μm左右，验证了晶粒细化对综合力学性能提高的显著效果。但是，尽管过去30年里超细晶实验室研究获得重要进展，但工业化结构钢材晶粒尺寸细化却一直无法突破。本项目基于工业化控轧控冷工艺，提出在粗轧和精轧阶段实施即时超快冷工艺，获得了5-12mm厚度的梯度超细晶大尺寸低碳低合金钢板，得以评价了工程级别尺寸的力学性能。(1)12mm厚度梯度超细晶钢板：表层到心部晶粒尺寸从0.82μm到2.09 μm，屈服强度达到550MPa，断后延伸率23%，具有更高的加工硬化能力，-60°C冲击功达到400 ±11 J，尤其在-196 °C液氮条件下，冲击功稳定在40±5 J，比均匀细晶钢高出近6倍。(2)5mm厚度微观织构和晶粒尺寸双梯度超细晶钢板：表层到心部铁素体平均晶粒尺寸从1.04 μm到3 μm，微观织构从表层的强烈(110) <001>织构转为1/4厚度的(100) <110>织构，最终在心部演变成(112) <110>织构。表层超细晶层屈服强度高达788 MPa，而钢板整体屈服强度为487 MPa，抗拉强度高达728MPa，展现了优异的加工硬化能力，总延伸率达到23%。同时双梯度结构展示了优异的上平台和液氮-196 °C冲击韧性(78 J)，是单一晶粒尺寸梯度超细晶钢的3.25倍，验证了双梯度结构优异性能。.本项目在科学上的意义是突破了以往梯度金属材料研究单一晶粒尺寸梯度的局限，获得了具有微观织构和晶粒尺寸两个维度双梯度超细晶钢板。同时简单工艺制备出最厚12mm的大尺寸超细晶钢板，得以评价标准尺寸试样冲击断裂韧性和全厚度拉伸性能，这种大尺寸力学性能评价突破了以往超细晶材料试样小无法全面评价强度、塑性和冲击韧性的局限性，具有重要的工程意义。本项目实施验证了双梯度结构可以有效突破单一超细晶和单一尺寸梯度材料无法同时提高强度、塑性、韧性的瓶颈，而这三个性能对结构材料工程化应用十分关键。本项目设计钢种化学成分简单、基于工业化制备工艺、强度-塑性-韧性优异，这三个特点为超细晶研究从实验室走向工程化应用迈出了更近一大步，有望在开发优异加工硬化能力和优异超低温韧性的结构用钢方面获得应用。