INVESTIGATION OF THE AUSTENITE DECAY KINETICS DURING CONTINUOUS COOLING OF STEEL WITH 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са FOR RAILWAY RAILS OF THE NEW GENERATION

Purpose. Investigation of the supercooled austenite decay kinetics during continuous cooling of steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca for railway rails of a new generation with increased operational properties to develop parameters of differential cooling. Method. The investigation of the austenite decay kinetics was carried out using differential thermal analysis. Samples were cooled at different cooling rates, temperature control was performed using chromel-aluminum thermocouples installed in the center of the sample. Microstructural investigations are performed using optical (AxioVert 200M mat) and electronic (SEM-106) microscopes. The interplate distance in pearlite is determined by the linear method, the diagonals are located perpendicular to the plate packages. Vickers hardness at a load of 10 kg. The number of structural components is recognized on microstructural images using the imageJ software complex. Results. The TKD analysis showed that at cooling rates of 0,06...5,96 °C/s, the structure of the investigated steel consists of pearlite. As the cooling rate increases, the morphology changes and the dispersion of pearlite increases: from medium pearlite to sorbitol-like. It should be noted that in the cooling rate range of 0,06...0,07°С/s, tertiary cementite is formed from ferrite. The diagram shows that at a cooling rate of 5.96 °С/s the hardness is 468 НВ (432 НВ), and the structure does not contain bainite. At a cooling rate of 1.47 °С/s, the hardness is 356 НV (345 НВ). This is slightly below the minimum allowable value (370 HV), but by interpolation it can be determined that the cooling rate must be at least 2.5 °C/s to achieve a hardness of at least 393 HV (370 HV). That is, when the rolling surface of the rail is cooled at a rate of 5.96 °C/s, the central volumes of the rail head at a rate of at least 2.5 °C/s, high-strength rails of the following categories can be manufactured: higher (DSTU (National Standard of Ukraine) 4344:2004) and R400HT (EN 13674-1:2011) from steel with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca. Scientific novelty. Constructed thermokinetic diagram of steel austenite decay with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са. Practical value. It was established that with the use of experimental steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca it is possible to achieve a hardness above 400NV without the formation of needle-like structures, therefore this is recommended chemical composition for the manufacture of high-strength rails of the R400NT category according to EN 13674-1-2011.