晶粒尺寸、相组分、析出物分布可控的超高强韧钢的先进制备及细观力学行为

The main target and research focus of steel development is how to achieve both the higher strength and better toughness simultaneously. Based on the scientific issue how simultaneously to keep a good ductility and high strength of metallic materials, ultrahigh-strength steels will be prepared by tailoring grain size, content and morphology of austenite, size and distribution of precipitates. The preparation methods mainly consist of a continuous rolling accompanied with the new generation TMCP(Thermo-Mechanical-Controlled Processing) technique, with subsequently intercritical annealing and austempering holding. It is expected to promote the work-hardening ability of ultrahigh-strength steel by the comprehensively strengthening and toughening effects, resulting from grain refinement (Hall-Petch relationship), transformation-induced-plasticity (TRIP), precipitation hardening and twinning-induced-plasticity (TWIP). By investigating the deformation behavior of ultrahigh-strength steels under complex conditions/environment, the main objective of this project is intended to explore relationship between mechanical properties and the volume fraction along with the morphology of nanoscale austenite in TWIP/TRIP steels with various grain sizes. It is predicted that some breakthroughs will be achieved through a successful execution of this program in the following several aspects:. (1) proposing a new method to improve a better trade-off between the strength and ductility of TWIP/TRIP steels; (2) providing a significantly experimental and theoretical basis for the development of a new generation of steels, which might be potentially used as the frame of the new generation of light-weight vehicles. The investigated steels are intended to possess excellent properties, including ultrahigh strength, good ductility, high security, and good formability, enhanced energy-absorption-capacity, and low specific gravity; (3)shedding a light on enriching the theories related to twinning and transformation processes of metallic materials, which has significantly important academic value; (4) providing a reference for the deformation of metallic materials with twin microstructures and metastable austenite. A new microstructure-based damage theory will be developed to predict accurately the service performance and deformation damage of polycrystalline materials under the complex service conditions. This is extremely important for both theory and practice to the engineering application and requirement of key materials with ultrahigh strength.

高强度高韧性是钢铁材料主要发展方向和研究热点。针对“如何使金属材料在获得超高强度的同时保持良好塑性变形能力”这一科学问题，采用以超快冷为核心的新一代TMCP轧制技术，通过两相区退火及等温热处理，制备晶粒尺寸、奥氏体体积分数与形态、析出物尺寸与分布可控的超高强TWIP/TRIP钢。通过纳米尺度片状奥氏体可以促进稳态马氏体相变，结合孪生的动态Hall-Petch效应、析出强化效应等复合强韧化手段，改善超高强TWIP/TRIP钢的加工硬化性能。系统研究纳米尺度奥氏体的体积分数和形态对具有不同晶粒尺寸的TWIP/TRIP钢在复杂应力下的微观结构及力学性能变化的影响规律，提出实现TWIP/TRIP钢强度与塑性最佳匹配的技术措施。.该研究取得的成果将为高强、超塑、成型性能更优的新一代轻量化汽车用钢的研发提供参考，为工程材料的使役性能预估提供理论参考，对丰富孪生及相变领域材料学理论具有重要的学术价值。

高强度高韧性是钢铁材料主要发展方向和研究热点。针对“如何使金属材料在获得超高强度的同时保持良好塑性变形能力”这一科学问题，采用不同的轧制技术，通过两相区退火及等温热处理，制备晶粒尺寸、奥氏体体积分数与形态、析出物尺寸与分布可控的超高强TWIP/TRIP钢。通过纳米尺度片状奥氏体可以促进稳态马氏体相变，结合孪生形成纳米尺度孪晶的动态Hall-Petch效应、析出强化效应等复合强韧化手段，改善超高强TWIP/TRIP钢的加工硬化性能。系统研究纳米尺度奥氏体的体积分数和形态对具有不同晶粒尺寸的TWIP/TRIP钢在复杂应力下的微观结构及力学性能变化的影响规律，提出实现TWIP/TRIP钢强度与塑性最佳匹配的技术措施。. 研究结果表明：晶粒尺寸和应变速率对Fe-20Mn-0.6C TWIP钢的变形行为产生显著影响。在恒定应变速率下，屈服强度和极限抗拉强度均随晶粒尺寸的减小而显著提高，延伸率略有降低。晶粒尺寸为3.5μm的钢抗拉强度为1930MPa，延伸率为66%。当真应变大于0.35±0.02，即临界应变时，应变局部化和软化均有发生，且随着应变速率和应变的增加，应变局部化和软化更为明显。对于Cu-TRIP钢采用820oC中间临界退火180 s，淬火至180oC，保持300 s，然后在空气中冷却至室温，调整低锰中碳钢析出物的成分和微观结构组成，产生多相组织：67%的马氏体，10%的残余奥氏体和23%的铁素体(体积分数)，以及纳米尺寸的析出物。获得超高抗拉强度2800 MPa和良好的延性(真应变为0.18)。变形过程中，残余奥氏体向马氏体的变形诱导转变产生局部加工硬化效应，延迟了局部颈缩并导致均匀的延展性。. 该研究取得的成果将为高强、超塑、成型性能更优的新一代轻量化汽车用钢的研发提供参考，为工程材料的使役性能预估提供理论参考，对丰富孪生及相变领域材料学理论具有重要的学术价值。