ReviewOverview of processing, microstructure and mechanical properties of ultrafine grained bcc steels
Introduction
Among the different strengthening mechanisms, grain refinement is the only method to improve both strength and toughness simultaneously. Therefore, ultrafine grained steels with relatively simple chemical compositions, strengthened primarily by grain refinement, have great potential for replacing some conventional low alloyed high strength steels. The main benefits behind such an approach are to avoid additional alloying elements; to avoid additional heat treatments like soft annealing, quenching and tempering; and to improve weldability owing to lower required carbon contents and other alloying elements when compared with other high strength steels. A further high potential domain for such ultrafine grained steel is the possibility for high strain rate superplasticity at medium and elevated temperatures [1]. In general, the term ultrafine grain is used here in the context of average grain sizes between 1 and 2 μm in diameter; submicron refers to grain sizes between 100 and 1000 nm; while nanostructured refers to grain sizes below about 100 nm.
The purpose of this overview is to provide a detailed introduction to the processing technologies, to the resulting microstructures, and to the mechanical properties associated with ultrafine grained body centered cubic (bcc) steels.
Section snippets
Microstructure characterization of ultrafine grained steels
Ultrafine grained ferrite microstructures can be quite different due to the various methods and heat treatments applied as well as the differences in the chemical compositions and the initial microstructures. In this section, characterization of ultrafine grained bcc steel microstructures will be discussed in detail.
Effect of grain size on strength
The yield stress for bcc steels processed by different methods is plotted in Fig. 2 as a function of the inverse square root of the grain size for grain sizes ranging from 45 to 0.2 μm. The ultrafine microstructures (grain size less than 2 μm) were produced by various techniques: the open symbols display the results from the SPD methods; the full symbols in gray represent the results from the advanced thermomechanical process routes (ATP); the full symbols in black show the results from the
Toughness improvement in ultrafine grained steels
While several studies examined tensile properties of ultrafine grained steels, Charpy impact properties were less commonly investigated due to limitations in the sample size typically available from laboratory-scale process set-ups.
The impact properties of ultrafine grained IF, low/medium carbon and Nb–V–Ti microalloyed steels have been reported by Tsuji et al. [116], Hanamura et al. [105], Song et al. [44] and Sjong et al. [117]. Fig. 6 shows the impact transition curves of the medium carbon
Conclusions
Processing, microstructure and mechanical properties of ultrafine grained bcc steels were discussed and compared with several of their coarse grained counterparts. The following conclusions can be drawn based on the interpretations presented in this paper:
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Ultrafine grained bcc steels can be produced by severe plastic deformation techniques or advanced thermomechanical processing routes. For the severe plastic deformation methods, a well-designed strain path is more important and also more
Acknowledgements
The authors would like to express their gratitude for the financial support of the European Coal and Steel Community (ECSC). The support of the Max-Planck-Institut für Eisenforschung in Germany and the sponsors of the Advanced Steel Processing and Products Research Center at the Colorado School of Mines in the USA are gratefully acknowledged.
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