Thermal effect of bainitic transformation in tube steels during accelerated cooling

Abstract

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.

Development of scientific ideas on phase and structural transformations in steels for different application

“…Особое внимание уделено физическим процессам, протекающим при контролируемом охлаждении листового проката из ННВТС [27,28]. Для исследования процесса распада аустенита при непрерывном охлаждении и определения удельных тепловых эффектов фазовых превращений (Qпр) разработан оригинальный лабораторный стенд, позволяющий моделировать технологические скорости охлаждения листов и производить расчет тепловых эффектов [29].…”

Forecasting the Microstructure of a Microalloyed Steel Plate Strip

“…Thus, it is not possible to use the current model to obtain reliable results. Therefore, it was decided to create an author's phenomenological model based on the experimental data obtained in the study, described in detail in [21,22] The main objectives of the study were:…”

Determination of Thermal Effects of Diffusionless Phase Transformations in Low-Carbon Low-Alloy Steels at High Cooling Rates

“…где первое слагаемое дает основной вклад, а второе -поправку на отведенную тепловую энергию. Экспериментальные зависимости T i (t) как для высокотемпературной (i ≡ γ), так и низкотемпературной (i ≡ α) областей на термограммах хорошо аппроксимируются выражениями: Алгоритм численного расчета тепловых эффектов с использованием метода конечных разностей приведен в [12]. При его проведении время протекания фазового превращения (t ft s ) определяли из анализа зависимости dT(t)/dt от t (рис.…”