低温和晶粒尺寸对奥氏体不锈钢力学行为的影响机理

Austenitic stainless steel (ASS) is widely used in the manufacture of cryogenic vessels due to its good low-temperature toughness, resistance to corrosion and formability, and it is significant to reduce the weight of equipment for energy saving and environmental protection though improving the yield strength of ASS. Although the yield strength of ASS can be improved by conventional strain-hardening technology, the formation of the crystal defects and the martensite result in a decrease in the low-temperature toughness and an increase in the sensitivity of environmental cracking for ASS, i.e., the increment in the yield strength limited. Therefore, the idea of fine-grained strengthening is proposed for ASS in this project, and the corresponding macro- and micro-mechanical behavior at the low-temperature will be studied, including the following three main parts: first, statistical relationships among temperature, grain size and mechanical properties for ASS; second, the role of temperature and grain size in the austenite stability, yielding and work-hardening behavior of ASS, then putting forward a mechanism of temperature and grain size to effect on the mechanical behavior for ASS; third, establishing quantitative physical metallurgy models of mechanical properties and a physical metallurgy stress-strain model considering the role of grain size and temperature. The expected results of this project have two-fold effect. First, it provides data and theoretical basis for fine-grained strengthening ASS to manufacture lightweight cryogenic container. Second, it is helpful to understand the mechanical behavior of other steels with austenite under cryogenic conditions.

奥氏体不锈钢由于其良好的低温韧性、耐腐蚀性和成型性被广泛应用于深冷容器的制造，提高其屈服强度以减轻设备重量对节能和环保都有重要意义。传统应变强化技术虽然可以提高奥氏体不锈钢的屈服强度，但引入的晶体缺陷和马氏体会使其低温韧性降低、环境开裂敏感性提高，限制了屈服强度的提高程度。为此本项目拟采用细晶强化奥氏体不锈钢思路，并对其低温宏-微观力学行为开展研究，主要包括：一、明确温度、晶粒尺寸与奥氏体不锈钢力学性能间的统计规律；二、明确温度、晶粒尺寸在奥氏体稳定性、屈服和加工硬化行为中的作用，进而提出温度、晶粒尺寸影响奥氏体不锈钢力学行为的微观机理；三、建立力学性能与温度、晶粒尺寸间的定量物理冶金模型及具有温度和晶粒尺寸关联的应力-应变物理冶金模型。本项目的预期研究成果不仅可以为细晶强化奥氏体不锈钢制深冷容器的轻量化提供数据和理论基础，而且对其他含有奥氏体相钢铁材料的低温力学行为的理解也有一定帮助。

本项目以奥氏体不锈钢（简称“不锈钢”）制深冷容器轻量化的工程需求、低温和晶粒尺寸双重作用下的奥氏体稳定性、组织演变与宏微观力学行为关联性的科学问题为背景，在组织制备、20–298 K力学性能测试、低温和晶粒尺寸相关的力学行为以及构建物理冶金模型等方面取得主要研究进展，相关结果也有助于理解其它亚稳金属的低温力学行为。.发现室温下超细晶不锈钢不再像固溶态不锈钢那样出现三阶段加工硬化，说明了室温应变诱导马氏体（SIM，包含bcc结构SIMα′和hcp结构SIMε）相变的硬化作用弱于位错动态回复的软化作用。通过降低环境温度来抑制位错动态回复并加速SIM相变以改善超细晶不锈钢加工硬化能力的方案，最终使超细晶不锈钢呈现三阶段加工硬化和良好的强塑性配合。.通过预应变和热处理调控奥氏体的晶体缺陷密度及晶粒尺寸使奥氏体呈现所需的稳定性，进而调控不锈钢的屈服和加工硬化等宏微观力学行为。证实了在细晶至粗晶尺度范围内，初始晶体缺陷密度是影响奥氏体稳定性的关键因素，进而也是影响宏微观力学行为的重要因素。.室温/低温原位高能X射线试验结果表明，超细晶不锈钢中的SIMα′相变是温度和取向相关的多变体相变，且SIMα′相变以奥氏体→SIMα′机制为主、奥氏体→SIMε→SIMα′机制为辅。超低温（173 K）下SIM相变在塑性变形初期被抑制，晶体缺陷在塑性变形中期集中转变为SIM，导致出现SIM含量突增的现象。在合适的温度（253 K）下，奥氏体各晶面较其他温度对应晶面都发生了明显且连续的SIMα′相变，这是导致超细晶不锈钢在该温度下呈现较好强塑性配合的主因。.物理冶金模型分析结果表明，超细晶不锈钢中各相在相变速率突增时伴随着明显的应力配分。超细晶不锈钢的加工硬化行为是各相体积分数演变、相变速率、各相应力和相应力变化速率的综合作用体现。奥氏体和SIMα′的晶体缺陷减小速度都随着降温而减小，适当降温可使奥氏体和SIMα′的晶体缺陷增大速度高于其他温度对应的增大速度，进而最大化的储存晶体缺陷以改善超细晶不锈钢的加工硬化能力。