Design of novel high-strength bainitic steels

In medium carbon steels containing sufficient quantities of silicon, it is possible to obtain a microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite and some martensite. The mechanism by which bainite grows, places strict limits on the temperature range for transformation and on the maximum fraction of transformation that can ever be achieved. These criteria are embedded in the concept of T-o curve. Thus, thermodynamic theory relying on the T-o concept can be used to design an optimum microstructure, avoiding large regions of austenite known to be detrimental for toughness. At the same time, detailed kinetic theory must be used to ensure that the hardenability of the steel is sufficient for industrial application and that the microstructure can be generated by continuous cooling transformation. The aim of the present work was to see how far these concepts can be extended to achieve the highest ever combination of strength and toughness in bulk-samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements.