Institute of Ferrous Metallurgy named after Z. I. Nekrasov (Dnepr, Ukraine):

E. V. Parusov, Cand. Eng., Senior Researcher, Head of the Dept., e-mail: xxdneprxx@gmail.com
V. A. Lutsenko, Dr. Eng., Leading Researcher
I. N. Chuiko, Cand. Eng., Senior Researcher
O. V. Parusov, Cand. Eng., Senior Researcher

The kinetics of supercooled austenite decomposition depends on numerous factors, the most important of which are the chemical composition of the steel and the temperature-rate conditions of cooling. When carrying out the main technological operations of heat treatment of steel, the austenite decomposition occurs in most cases with a constant decrease in temperature, therefore, studies of the features of the behavior of austenite during continuous cooling are of great practical importance. The kinetics of austenite decomposition during continuous cooling in high-carbon steels have been studied. The features of the structure formation during continuous cooling in C80D2 and C82D steels alloyed with carbide-forming elements are shown. When carrying out a comparative analysis, C86D carbon steel was used as the reference one. The heating temperature of the samples was (1010–1030) ± 10 °C, which is higher than the temperatures traditionally used in practice and is due to the need to increase the stability of austenite by increasing the average austenite grain size before the start of cooling. For the steels under study, structural diagrams were constructed and a comparative analysis of the features of the austenite decomposition kinetics in relation to the reference steel C86D was carried out. For steels alloyed with carbide-forming elements (C80D2^V, C82D^V, C82D^Cr, C82D^CrV), a significant decrease in the lower critical cooling rate is characteristic, which, at high degrees of overcooling, can lead to the formation of structures formed by intermediate and shear mechanisms. For cooling rates lower than the lower critical one, the amount of pearlite is maximum, however, there are 6 point pearlite areas in the steel structure, which is an unacceptable indicator. Cooling of С80D2^V, С82D^V, С82D^Cr, С82D^CrV and С86D steels is recommended to be carried out in the intercritical area with rates V_min = (0.7÷0.8) · Vmax to the temperature interval of pearlite transformation (t_i – 15÷20 °C) followed by quasi-isothermal holding and further cooling. The research results made it possible to improve the industrial cooling modes of coiled steel to achieve at least 90% pearlite with 1 point in the steel structure.

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