CONSTRUCTION OF A MATHEMATICAL MODEL OF THE DEPENDENCE OF THE COEFFICIENT OF RESISTANCE TO CRACK PROPAGATION ON THE PARAMETERS OF THE STRUCTURAL STATE OF LOW-CARBON LOW-ALLOY STEEL

Obtaining a material with an optimal combination of strength properties and fracture toughness is impossible without finding a compromise both at the production stage and at the operation stage. In this case, one should take into account both the technical capabilities of production facilities and the economic feasibility of using rolled metal with an increased level of operational properties. From this point of view, problems first of all arise in combining a number of desired properties in one material. Thus, it is known that technological schemes of production, which lead to an increase in the strength of rolled metal, reduce its fracture toughness. On the other hand, high-strength steels with high toughness are characterized by complex microstructures. That is why, elucidating the fundamental dependencies between the structural state of low-carbon low-alloy steels and the kinetics of fracture propagation is an urgent task of modern materials science. Purpose of the article. Construction of a mathematical model of the relationship between the crack propagation resistance coefficient and the parameters of the structural state of low-carbon, low-alloy steels. Conclusion. Mathematical models of the relationship between the crack propagation resistance coefficient (KIC) and the parameters of the structural state of low-carbon low-alloy steels were obtained. The morphological features of the structural state of low-carbon low-alloy steel after various heat treatment regimes were investigated. The conducted set of studies showed that at a treatment temperature of 650 °C, the steel structure contains ferritic and pearlitic components. Reducing the holding temperature to 600 °C leads to the appearance of a bainite component in the steel structure simultaneously with the ferritic and pearlitic components. At holding temperatures of 550 and 500 °C, only morphological types of bainite are present in the steel structure. Reducing the holding temperature to 450 and 400 °C leads to the formation of bainite and martensitic components in different percentage ratios in the steel structure. The influence of heat treatment regimes on the complex of mechanical properties and on the value of the coefficient of resistance to crack propagation of low-carbon low-alloy steel was investigated. Using the mathematical apparatus of one-dimensional regression analysis, models of the dependence of the KIC parameter were obtained: on the percentage of ferritic and pearlitic components in the form of a second-degree polynomial; on the percentage of bainite and martensitic components in an exponential form. Using the mathematical apparatus of multiple regression analysis, a 3-D model dependence of the KIC parameter on the parameters of the structural state, which are formed by implementing the diffusion (percentage of ferritic and pearlitic structural components) and shear (percentage of bainite and martensitic structural components) mechanisms of transformation of supercooled austenite of low-carbon low-alloy steels was obtained.