Thermal effect of bainitic transformation in tube steel by accelerated cooling

M.L. Lobanov, G.M. Rusakov, V.N. Urtsev, M.L. Krasnov, E.D. Mokshin, A.V. Shmakov, S.I. Platov

Abstract

On the original laboratory bench, which allows to simulate the technological cooling rates of hot-rolled sheets in the process of controlled thermomechanical processing, the thermal effect of bainitic transformation in low-carbon low-alloy tube steel 06G2MB type is determined.We have designed original laboratory bench, which allows to model technological rates of cooling of hot-rolled sheets in the process of controlled thermomechanical processing (TMCP). We have used samples cut from the industrial sheet of pipe steel type 06G2MB, which was intended for production of large diameter pipes with strength X80. We have obtained dependencies of temperatures from the time at cooling rates 100-500 оС/s. All processing modes resulted in almost identical structures formed mainly due to bainitic transformation. Dispersion of structures increased with increasing cooling rate. Thermotechnical calculations were carried out assuming that the temperature equalization occurs instantly along with the sample thickness. Times of transformation are 1-9 s. Most of the transformation at all cooling modes in conditions close to isothermal occurred. Athermic nature of bainitic transformation in TMCP was recorded. In the studied interval of cooling rates, the temperature of bainitic transformation start was 660-730 ºC. The heat effect was approximately equal to 120 kJ/kg, which is twice the thermal effect of martensite transformation for the low-carbon steels. This fact allows suggesting that the kinetics of bainitic transformation is largely determined by the energy of slowest process, namely, the re-arrangement of carbon atoms of carbon in the austenite occurring in parallel to the shear adjustment of FCC lattices into BCC. It is presumed that energy contribution of the redistribution process of carbon atoms (if their amount is small) to the thermal effect of bainitic transformation at least compare with energy effect of shear lattice rearrangement.

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