Transformation kinetics of microalloyed steels after hot controlled rolling

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