Formation of mechanical properties of low alloy steel surface under thermomechanical treatment
Detection of physical mechanisms of formation and evolution of structure-phase states and dislocation substructures in steels is one of the important problems of condensed-state physics and modern material science because it forms the basis of development and formation of effective methods for increasing the service characteristics of articles. Experimental investigations of structures and phase states being formed in a cross-section of articles as a result of thermomechanical treatment are very significant for understanding the physical nature of transformations as they make it possible to change structure and mechanical characteristics purposefully. Thermomechanical treatment of low carbon steel 09G2S (0.09 wt.% C, 2 wt.% Mn, 1 wt.% Si) is done by rolling of H-beam DP 155 and forced water cooling in the process of rolling on rolling mill 450 open joint-stock company “Evraz - Western Siberian metallurgical combine” By methods of physical material science the investigations of structure-phase states, defect substructure mechanical and tribological properties of H-beam surface from steel 09G2S being formed as a result of thermomechanical hardening in the process of rolling on rolling mill. The qualitative correspondence of change in microhardness and scalar dislocation density along the cross-section of H-beam has been established. The processes were analyzed and the analysis of mechanisms contributing to the formation of nano-dimensional cementite phase in the conditions of thermomechanical treatment of low-alloy steel was done. It was shown that the cementite particles were formed: - in dispersion of cementite plates of pearlite colonies by its cutting with moving dislocations; - in dissolution of cementite plates of pearlite colonies and its repeated precipitation on dislocations, boundaries of subgrains and grains; - the decomposition of solid solution of carbon in α-iron occurring in the conditions of “self-tempering” of martensite loads to formation of particles precipitated in the volume of martensite crystals on dislocations and at boundaries of martensite crystals. - in diffusion γ → α transformation in the conditions of high degree of deformation and high temperatures of treatment a dispersion of lamellar pearlite structure is observed. It is established that the phenomenon of increase in hardness of steel surface layer is a multi-factor, morphologically multi-component one and is determined by the nature of γ → α transformation.