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

我们与捷克共和国的同事合作提出的这个项目考虑了TRIP钢。它们的显微组织由铁素体、贝氏体和残余奥氏体的混合物组成，这是特殊热机械处理和适当合金化的结果。TRIP钢可以达到超过1000 MPa的强度与约30%的延展性的组合。然而，为了获得这些令人印象深刻的性能，必须获得约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 °C范围内的贝氏体转变温度下保持，其中贝氏体形成并且残留奥氏体变得稳定：该保持阶段代表了使用TRIP钢的重要技术约束。该项目的目标是更深入地了解采用连续冷却的新工艺路线，以取代现有的TRIP钢加工工艺，并将显微组织特征信息与所得机械性能直接联系起来。将选择三种具有适合TRIP效应的化学成分的低合金钢用于实验方案：（i）与硅和锰合金化;（ii）用另一种元素替代硅;（iii）与铌微合金化以细化显微组织。传统的热处理将系统地比较，新的连续冷却方法没有贝氏体保持步骤。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稳定化和足够量的贝氏体转变的影响。残余奥氏体区域的形态特别需要详细表征。虽然合金化概念和工艺路线的开发是我们在比尔森的合作伙伴的主要重点，但本文所述项目的主要贡献将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，用于定量分析相分数和残余奥氏体的稳定性和形态，以及机械测试（也包括：首次循环行为实验），以充分表征宏观机械和相变行为。特别强调的是残余奥氏体及其稳定性的表征，纳米压痕将被用来进一步分析个别微观结构成分的机械行为。该联合项目将提供有关新型TRIP钢的加工、微观结构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成本效益高的钢的使用大幅扩展。

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.