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Ferrite morphology and residual phases in continuously cooled low carbon steels

Journal Article


Abstract


  • Although much research has been conducted on the isothermal transformation products of medium to high carbon hardenable steels, relatively little has been reported for transformation of low carbon structural steels under continuous cooling conditions. The trend towards reduced carbon levels (less than about 0.1 wt% C) has been driven by demands for formability and weldability, challenging steel designers to maintain strength by microalloying and/or thermomechanical controlled processing. Although control of the ferritic products formed in low carbon steels after hot rolling, normalizing and welding is essential in order to ensure adequate strength and toughness, understanding of the microstructures formed on continuous cooling is still limited. In addition, transformation mechanisms remain controversial because of polarization of researchers into groups championing diffusional and displacive theories for the transformation of austenite over a wide range of cooling rates. The present review compares and draws together the main ferrite classification schemes, and discusses some critical issues on kinetics and mechanisms, in an attempt to rationalize the effects of cooling rate, prior austenite structure and composition on the resulting ferrite structure and its mechanical properties. It is concluded that with increasing cooling rate the ferritic product becomes finer, more plate-like, more dislocated, more carbon supersaturated, more likely to be formed by a displacive mechanism, harder and stronger. Other conclusions are that: (i) `bainitic ferrite', which is a pervasive form of ferrite in continuously cooled low carbon steels, is different from the conventional upper and lower bainites observed in higher carbon steels, insofar as the co-product `phase' is typically martensite-austenite islands rather than cementite; and (ii) low carbon bainite rather than martensite is the dominant product at typical fast cooling rates (<500 K/s) associated with commercial processing.

Publication Date


  • 1999

Citation


  • Dunne, D. P. (1999). Ferrite morphology and residual phases in continuously cooled low carbon steels. Materials Forum, 23, 63-76.

Scopus Eid


  • 2-s2.0-0033342394

Web Of Science Accession Number


Start Page


  • 63

End Page


  • 76

Volume


  • 23

Abstract


  • Although much research has been conducted on the isothermal transformation products of medium to high carbon hardenable steels, relatively little has been reported for transformation of low carbon structural steels under continuous cooling conditions. The trend towards reduced carbon levels (less than about 0.1 wt% C) has been driven by demands for formability and weldability, challenging steel designers to maintain strength by microalloying and/or thermomechanical controlled processing. Although control of the ferritic products formed in low carbon steels after hot rolling, normalizing and welding is essential in order to ensure adequate strength and toughness, understanding of the microstructures formed on continuous cooling is still limited. In addition, transformation mechanisms remain controversial because of polarization of researchers into groups championing diffusional and displacive theories for the transformation of austenite over a wide range of cooling rates. The present review compares and draws together the main ferrite classification schemes, and discusses some critical issues on kinetics and mechanisms, in an attempt to rationalize the effects of cooling rate, prior austenite structure and composition on the resulting ferrite structure and its mechanical properties. It is concluded that with increasing cooling rate the ferritic product becomes finer, more plate-like, more dislocated, more carbon supersaturated, more likely to be formed by a displacive mechanism, harder and stronger. Other conclusions are that: (i) `bainitic ferrite', which is a pervasive form of ferrite in continuously cooled low carbon steels, is different from the conventional upper and lower bainites observed in higher carbon steels, insofar as the co-product `phase' is typically martensite-austenite islands rather than cementite; and (ii) low carbon bainite rather than martensite is the dominant product at typical fast cooling rates (<500 K/s) associated with commercial processing.

Publication Date


  • 1999

Citation


  • Dunne, D. P. (1999). Ferrite morphology and residual phases in continuously cooled low carbon steels. Materials Forum, 23, 63-76.

Scopus Eid


  • 2-s2.0-0033342394

Web Of Science Accession Number


Start Page


  • 63

End Page


  • 76

Volume


  • 23