Heat treatments, microstructures and properties of TRIP steels without hold time during the Bainite transformation

这个项目是我们与捷克共和国的同事合作提出的，它考虑了TRIP钢铁公司。其显微组织由铁素体、贝氏体和残余奥氏体的混合物组成，这是特殊的热机械处理和适当合金化的结果。TRIP钢的强度组合可达到1000 Mpa以上，延伸率约为30%。然而，为了获得这些令人印象深刻的性能，需要获得大约10%-15%的适当稳定的残余奥氏体。在实践中，TRIP钢加工中的一个关键问题是在300-450℃的贝氏体相变温度范围内保持，贝氏体在这里形成，残余奥氏体变得稳定：这一保持阶段是使用TRIP钢的一个重要技术限制。该项目的目标是更深入地了解采用连续冷却的新加工路线，以取代现有的TRIP钢加工程序，并将组织特征信息与由此产生的机械性能直接联系起来。三种化学成分适合TRIP效应的低合金钢将被选为实验方案：(I)用硅和锰合金化；(Ii)用另一种元素取代硅；(Iii)用Nb微合金化以细化组织。系统地比较了常规热处理和无贝氏体恒温步的新型连续冷却方法。为了充分利用这些新的加工方法，有必要了解连续冷却条件对残余奥氏体的稳定和对足够数量的贝氏体转变的影响。尤其是残余奥氏体区的形态需要详细描述。虽然合金化概念和加工路线的发展是我们在皮尔森的合作伙伴的主要关注点，但这里描述的项目的主要贡献将是在微观结构分析和机械表征领域。这包括金相、电子显微镜和X射线衍射，用于定量分析相组成和残余奥氏体的稳定性和形态，以及力学测试(还包括：循环行为的首次实验)，以全面表征宏观力学和相变行为。特别强调了残余奥氏体的特征及其稳定性；将使用纳米压痕来进一步分析单个微观结构成分的机械行为。该联合项目将提供有关新的TRIP钢的加工、组织和性能之间复杂关系的详细信息，从而消除等温保持步骤，这最终可能导致这些具有成本效益的钢的使用大幅扩大。

This project, which we propose in collaboration with colleagues from Czech republic, considers TRIP steels. Their microstructure consists of a mixture of ferrite, bainite and retained austenite as a result of special thermo-mechanical treatments and suitable alloying. TRIP steels can reach combinations of strengths over 1000 MPa with ductility around 30%. However, to achieve these impressive properties, it is necessary to obtain about 10-15% of properly stabilized retained austenite. In practice one crucial problem in processing of TRIP steels is the holding at the bainitic transformation temperature in the 300 - 450 °C range, where bainite forms and retained austenite becomes stabilized: This holding stage represents an important technological constraint in utilizing TRIP steels. The objective of this project is to obtain a deeper insight into novel processing routes with continuous cooling to replace the existing procedures of TRIP steel processing, and to directly relate information on microstructural features with the resulting mechanical properties. Three low-alloyed steels with chemical compositions suitable for the TRIP effect will be selected for the experimental program: (i) alloyed with silicon and manganese; (ii) substitution of silicon by another element; (iii) micro-alloying with niobium for microstructural refinement. Conventional heat treatments will be systematically compared to novel continuous cooling approaches without the bainitic hold step. To fully utilize these new processing approaches, it is necessary to understand the influence of continuous cooling conditions on the stabilization of retained austenite and on the transformation of sufficient amounts of bainite. The morphologies of regions of retained austenite in particular need to be characterized in detail. While the development of alloying concepts and processing routes are the main focus of our partners in Pilsen, the main contributions of the project described here will be in the areas of microstructural analysis and mechanical characterization. This includes metallography, electron microscopy and X-ray diffraction for quantitative analysis of phase fractions and stability and morphology of retained austenite, and mechanical testing (also: first experiments on cyclic behavior) to fully characterize the macroscopic mechanical and transformation behavior. Special emphasis is placed on a characterization of retained austenite and its stability; nanoindentation will be used to further analyze the mechanical behavior of the individual microstructural constituents. This joint project will provide detailed information on the complex relationships between processing, microstructures and properties of the new TRIP steels and thus allow for an elimination of the isothermal holding step, which may eventually lead to substantial expansion in the use of these cost-effective steels.