KINETICS OF BAINITE TRANSFORMATION IN HOT-WORKED AND ULTRAFINE AUSTENITE OF ALLOY-STEELS

The isothermal bainite transformation kinetics of low- and medium-carb on alloy steels after hot deformation and thermocycling treatment (TC) has been studied by the magnetometric method. Deformation was produce d by rolling at 800 degrees C-1000 degrees C. TC included three cycles of rapid (30 degrees C/s) electro-heating up to 900 degrees C-1000 de grees C followed by quenching. Hot deformation retards bainite transfo rmation in the lower part of bainite range (< 400 degrees C) and accel erates it in the upper part (> 400 degrees C). The effect is greater f or higher strain and lower deformation temperature. Deformation also d iminishes the overall bainite fraction below 400 degrees C and increas es the completeness of reaction above 400 degrees C. Thermocycling pro vides: more uniform ultrafine austenite grains with average size of 3 mu m - 4 mu m compared to 12 mu m - 14 mu m after conventional heating . The transformation rare after TC was almost the same as after conven tional heating in low-carbon steel bill was faster in medium-carbon wi th no change in the completeness of reaction. The degree of accelerati on increased with rise of transformation temperature. There are a numb er of factors which are responsible for a change in bainite transforma tion kinetics in austenite deformed al high temperatures or subjected to TC. One of the most important factors is the refinement of structur e, another one is structural and chemical inhomogenity of austenite du e to deformation or to rapid heating. To estimate the contribution of these factors, gamma-transformation was studied after different austen itizing conditions (low-temperature hearing at 850 degrees C-950 degre es C; high-temperature heating at 1100 degrees C; step treatment - aus tenitizing at 1100 degrees C, then cooling to 950 degrees C-850 degree s C). The results obtained make it possible to conclude that the struc tural and chemical inhomogeneity of austenite activate the bainite rea ction while the grain refinement retards it. Intensity of the effect p roduced by each factor depends strongly on the steel composition.