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Global Journal of Engineering Research

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Transformation kinetics of microalloyed steels after hot controlled rolling

AR Morgridge

Abstract


Transformation kinetics of austenite into ferrite after controlled hot rolling has been investigated in three microalloyed steels (Nb, Nb-Ti and C-Mn-V) using hot interrupted compression tests on the Gleeble 1500 within the testing temperature range of 875°C-1100°C. Holding times were varied between 0.5 and 30s, strain rates of between 9.5 and 20/s and strains up to 0.9 were employed. Peak strain varied from 0.4 to 0.45 at 20/s for both C-Mn and Nb steels at 1000°C. As a result of strain accumulation, dynamic recrystallization occurred at the beginning of the 3rd pass in both C-Mn and, in Nb, steels. Measured grain sizes after reheating were 87.2, 57.4 and 50.2microns for C-Mn-V, Nb and Nb-Ti steels respectively. After undergoing full static, dynamic and metadynamic recrystallisation, austenite grains were greatly refined down to measured 19.1, 18.0 and 17.5microns for C-Mn-V, Nb, and Nb-Ti steels. These were the final austenite grain sizes before transformation to ferrite at the transformation temperature. Predicted ferrite grain sizes after transformation were 12.0 and 9.41microns for C-Mn and Nb steels respectively. With retained strain (0.38), better grain refinement and hence predicted lower ferrite grain values of 9.42 and 7.91microns were for C-Mn-V and Nb steels respectively. Measured ferrite grain sizes after transformation were 9.2, 6.0 and 5.1 microns respectively for C-Mn-V, Nb and Nb-Ti steels. The finer the ferrite grain size, the higher the predicted lower yield stress (L.Y.S) for C-Mn steel. 347 Mpa (transformed from dynamically recrystallised austenite grains) and 317Mpa (transformed from statically recystallised austenite grains). Measured L.Y.S. from tensile tests was 361Mpa.

Keywords: transformation kinetics, microalloyed steels, hot rolling, recrystallisation

Global Journal of Engineering Research Vol.4 (1&2) 2005: 5-11



http://dx.doi.org/10.4314/gjer.v4i1.18930
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