STUDY OF THE FINE STRUCTURE OF THE PRODUCTS OF THE INTERMEDIATE AND SHEAR MECHANISMS OF AUSTENITE TRANSFORMATION OF LOW-CARBON LOW-ALLOY STEEL

The post-war recovery of Ukraine will require an increase in the scale of production of rolled metal for the construction industry. The use of high-strength steels in the construction industry is the main reason for the emergence of increased requirements for these steels. At the same time, it should be noted that since the 80s of the last century, foreign manufacturers have been producing rolled metal for building metal structures according to technological schemes that include the use of accelerated cooling. The use of this particular technology makes it possible to achieve a higher level of operational properties (compared to classical controlled rolling) by replacing the pearlite phase in the structure of the rolled metal with a dispersed bainite component. Therefore, the task of researching the morphological features of the structural state, which is formed in low-carbon, low-alloy steels when using accelerated cooling in the technological scheme of production, is relevant. Purpose of the article. study of the morphology of structural components that are formed in low-carbon, low-alloy steels as a result of the action of intermediate and/or shear mechanisms of transformation of supercooled austenite. Conclusion. The morphology of the structural components that are formed in low-carbon, low-alloy steels as a result of the action of the intermediate and shear mechanisms of transformation of supercooled austenite has been studied. It was established that heat treatment in the temperature range of 550...450 °С with a holding time of 20 minutes leads to the formation of bainite of different morphological forms (upper bainite and acicular ferrite) and rack dislocation martensite (observed at a holding temperature of 450 °С) in the structure of low-carbon, low-alloy steels. With the use of microdiffraction analysis, it is shown that the boundary between two colonies of upper bainite has a structure that corresponds to the boundaries of a special type. Combined analysis of the microdiffraction and dark-field data showed that only residual austenite is found between the ferrite rails in the acicular ferrite packets (holding temperature 500 °C). Along with this, the carbonitride phase is also present in the colonies of rail dislocation martensite between the rails simultaneously with residual austenite. The conducted set of studies showed that the differences in the morphology of the structural components, which are formed during exposure in the temperature range of 550...450 °C, are caused, first of all, by the difference in the speeds and, as a result, the limitations of the crystal lattice shift processes. The higher the transformation temperature, the greater the diffusion activity of atoms, the lower the rate of shear transformation of γ-Fe into α-Fe.