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

点击翻译按钮获取中文摘要

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.