The microstructural evolution and properties for varied austempering routes are investigated in a cold-rolled bainitic steel. Special attention is given to the effect of retained austenite (RA) in terms of its fraction, carbon concentration, and morphology resulting from different austempering routes on mechanical properties and stretch flangeability. Bimodal sized bainitic laths are provided, and the carbon concentration of RA maintains the highest value through the two-step austempering. Total elongation (TEL) is remarkably enhanced for the two-step austempering, deviating from the exponential relationship between tensile strength (TS) and TEL as maintained by the one-step austempering. Considering the two plateaus of the strain-hardening exponent, it is considered that the hierarchical stability of RA is provided by the two-step austempering, leading to the postponed necking point so as to improve the uniform elongation. Two-step austempering could provide more complete bainitic transformation as well as more stable film-like RA, supplying a promising way to improve the combination of strength, ductility, and stretch flangeability.

中文翻译：

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通过在1000 MPa级冷轧贝氏体钢中进行两步回火来增强强度，延展性和拉伸凸缘性的平衡

在冷轧贝氏体钢中研究了各种奥氏体回火路线的组织演变和性能。就残余奥氏体（RA）的分数，碳浓度和不同的奥氏体回火路线对机械性能和拉伸凸缘性产生的形态而言，应给予特别注意。提供了双峰大小的贝氏体板条，通过两步回火，RA的碳浓度保持最高值。两步回火的总伸长率（TEL）显着提高，这与拉伸强度（TS）和TEL之间的指数关系有所不同一步一步的保持。考虑到应变硬化指数的两个平台，认为通过两步回火提供了RA的层级稳定性，导致了缩颈点的延迟，从而提高了均匀伸长率。两步回火可提供更完整的贝氏体转变以及更稳定的薄膜状RA，为改善强度，延展性和拉伸凸缘性的组合提供了一种有希望的方法。