Materials Science and Engineering A 441 (2006) 1–17
Review
Overview of processing, microstructure and mechanical properties
of ultrafine grained bcc steels
R. Song
a,∗
, D. Ponge
b
, D. Raabe
b,∗∗
, J.G. Speer
a
, D.K. Matlock
a
a
Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA
b
Max-Planck-Institut f¨ ur Eisenforschung, Max-Planck-Str. 1, 40237 D¨ usseldorf, Germany
Received 18 July 2006; received in revised form 3 August 2006; accepted 25 August 2006
Abstract
Ultrafine grained steels with grain sizes below about 1 m offer the prospect of high strength and high toughness with traditional steel com-
positions. 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.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Ultrafine grained steels; Microstructure; Thermomechanical processing; Severe plastic deformation; Mechanical properties
Contents
1. Introduction ............................................................................................................... 2
2. Methods of producing ultrafine grained steels ................................................................................ 2
2.1. Introduction ........................................................................................................ 2
2.2. Severe plastic deformation ........................................................................................... 2
2.2.1. Severe plastic deformation techniques for steels ................................................................ 2
2.2.2. Equal-channel angular pressing ............................................................................... 2
2.2.3. Accumulative roll bonding ................................................................................... 3
2.2.4. High pressure torsion ........................................................................................ 3
2.2.5. Bi-directional large strain deformation ......................................................................... 3
2.3. Advanced thermomechanical processes ................................................................................ 3
2.3.1. Introduction ................................................................................................ 3
2.3.2. Recrystallization of austenite during hot deformation ........................................................... 3
2.3.3. Strain-induced ferrite transformation .......................................................................... 3
2.3.4. Intercritical hot rolling ....................................................................................... 4
2.3.5. Dynamic recrystallization of ferrite during warm deformation ................................................... 4
2.3.6. Pronounced recovery of ferrite during warm deformation and annealing .......................................... 4
2.3.7. Cold rolling and annealing of martensitic steel ................................................................. 4
2.4. Summary of the two strategies of producing ultrafine grained steels ...................................................... 4
2.4.1. Differences ................................................................................................. 4
2.4.2. Similarities ................................................................................................. 5
3. Microstructure characterization of ultrafine grained steels ...................................................................... 7
3.1. Microstructure of ultrafine grained steels produced by SPD techniques .................................................... 7
∗
Corresponding author. Tel.: +1 303 273 3624; fax: +1 303 273 3016.
∗∗
Corresponding author. Tel.: +49 211 6792 340.
E-mail addresses: rsong@mines.edu (R. Song), d.raabe@mpie.de (D. Raabe).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2006.08.095