Review
Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels

https://doi.org/10.1016/j.msea.2006.08.095Get rights and content

Abstract

Ultrafine grained steels with grain sizes below about 1 μm offer the prospect of high strength and high toughness with traditional steel compositions. These materials are currently the subject of extensive research efforts worldwide. Ultrafine grained steels can be produced either by advanced thermomechanical processes or by severe plastic deformation strategies. Both approaches are suited to produce submicron grain structures with attractive mechanical properties. This overview describes the various techniques to fabricate ultrafine grained bcc steels, the corresponding microstructures, and the resulting spectrum of mechanical properties.

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:

  • (1)

    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.

References (131)

  • R.Z. Valiev et al.

    Scripta Mater.

    (2003)
  • I.V. Alexandrov et al.

    Scripta Mater.

    (2001)
  • V.V. Stolyarov et al.

    Nanostruct. Mater.

    (1999)
  • R.Z. Valiev

    Mater. Sci. Eng. A

    (1997)
  • V.M. Segal

    Mater. Sci. Eng. A

    (1999)
  • Y. Fukuda et al.

    Acta Mater.

    (2002)
  • J. Kim et al.

    Scripta Mater.

    (2001)
  • D.H. Shin et al.

    Acta Mater.

    (2001)
  • Y.T. Zhu et al.

    Mater. Sci. Eng. A

    (2000)
  • Y. Saito et al.

    Acta Mater.

    (1999)
  • N. Tsuji et al.

    Scripta Mater.

    (2002)
  • N. Tsuji et al.

    Scripta Mater.

    (1999)
  • Y. Ivanisenko et al.

    Scripta Mater.

    (2003)
  • Y. Ivanisenko et al.

    Acta Mater.

    (2003)
  • R.Z. Valiev et al.

    Ann. Chim. Sci. Matér.

    (2002)
  • R.Z. Valiev et al.

    Mater. Sci. Eng. A

    (1991)
  • D.H. Shin et al.

    Acta Mater.

    (2000)
  • N. Kamikawa et al.

    Sci. Technol. Adv. Mater.

    (2004)
  • N. Tsuji et al.

    Scripta Mater.

    (2002)
  • Y. Saito et al.

    Scripta Mater.

    (1998)
  • P.D. Hodgson et al.

    Scripta Mater.

    (1999)
  • N. Tsuji et al.

    Scripta Mater.

    (1997)
  • S.V.S. Murty et al.

    Scripta Mater.

    (2005)
  • R. Song et al.

    Acta Mater.

    (2005)
  • R. Song et al.

    Scripta Mater.

    (2005)
  • R. Song et al.

    Acta Mater.

    (2005)
  • Y.I. Son et al.

    Acta Mater.

    (2005)
  • G. Azevedo et al.

    Mater. Sci. Eng. A

    (2005)
  • K.-T. Park et al.

    Scripta Mater.

    (2004)
  • R. Ueji et al.

    Acta Mater.

    (2002)
  • N. Tsuji et al.

    Scripta Mater.

    (2002)
  • R. Ueji et al.

    Sci. Technol. Adv. Mater.

    (2004)
  • P.J. Hurley et al.

    Scripta Mater.

    (1999)
  • P.J. Hurley et al.

    Mater. Sci. Eng. A

    (2001)
  • W.J. Kim et al.

    Mater. Lett.

    (2001)
  • D.H. Shin et al.

    Acta Mater.

    (2000)
  • X. Huang et al.

    Mater. Sci. Eng. A

    (2003)
  • M. Hölscher et al.

    Acta Metall.

    (1994)
  • S.C. Hong et al.

    Mater. Sci. Eng. A

    (2002)
  • A. Bodin et al.

    Mater. Charact.

    (2001)
  • G.H. Akbari et al.

    Acta Mater.

    (1997)
  • D.H. Shin et al.

    Scripta Mater.

    (2000)
  • M.Y. Liu et al.

    Mater. Lett.

    (2003)
  • A.A.W. Thompson

    Acta Metall.

    (1975)
  • A.A.W. Thompson

    Acta Metall.

    (1977)
  • W.M. Baldwin

    Acta Metall.

    (1958)
  • H. Conrad

    Acta Metall.

    (1963)
  • H. Conrad et al.

    Mater. Sci. Eng.

    (1967)
  • C.S. Pande et al.

    Nanostruct. Mater.

    (1993)
  • H.W. Song et al.

    Nanostruct. Mater.

    (1999)
  • Cited by (557)

    View all citing articles on Scopus
    View full text