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An original laboratory bench, which allows for modeling technological rates of cooling of hot-rolled sheets in the process of controlled thermomechanical processing (TMCP) has been designed. Samples cut from an industrial sheet of 06G2MB type pipe steel used in production of large diameter pipes with strength grade X80 have been used. Time dependencies of the actual temperature at initial cooling rates of 100-500 K / s have been obtained. All processing modes resulted in almost identical structures mainly formed due to the bainitic transformation. The dispersion of structures decreased with the increase of the cooling rate. Thermotechnical calculations were carried out assuming that the temperature equalization over the sample thickness was instant. The times of transformation amounted to 1-9 s. Most of the transformation at all cooling modes occurred in conditions close to the isothermal one. The athermal nature of the bainitic transformation in TMCP was recorded. Within the observed interval of cooling rates, the temperature at the starting point of the bainitic transformation was 660-730°C. The heat effect approximately was equal to 120 kJ / kg, which is twice larger than the thermal effect of the martensite transformation for low-carbon steels. This fact suggests that the kinetics of the bainitic transformation is largely determined by the energy of the slowest process, namely, the rearrangement of carbon atoms in the austenite occurring parallelly to the shear transformation of FCC lattice into BCC one. It is assumed that energy contribution of the redistribution process of carbon atoms (if their amount is small) to the thermal effect of the bainitic transformation is comparable, at least, to the energy effect of the shear lattice rearrangement.

The article is devoted the experimental studies of deformation and temperature parameters of rolling the pipe steel on the rolling mill 5000 PJSC “MMK”. Knowledge of the regularities of the change in the surface temperature of work piece during hot rolling under the conditions of thermo mechanical deformation processes is of great theoretical and practical importance. The temperature conditions determine the energy and force parameters of rolling, as well as the mechanical properties and microstructure of the finished products. The experimental rolling was carried out in two stages: roughing at 6 passes at a temperature of 1050-900 ° C and finishing at 18-20 passes at a temperature below the start of the phase transformation of 900 ° C. The finishing rolling has been carried out in a two-phase region at the stage of polymorphic transformation in the steel. Significant differences in the nature of temperature changes at different stages of rolling are revealed.