OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology Thermo-mechanical treatment The importance of obtaining a fine grain size in terms of increasing both strength and toughness is evident from the original work of Hall [81] and Petch [82], who experimentally showed for a number of polycrystalline metals that the yield stress σ0.2 at constant strain is associated with grain diameter d by a certain ratio [83]. Thermomechanical treatment of steel can be divided into three large groups depending on whether the deformation process occurs before, during or after the phase transformation. Processes used include high temperature thermomechanical treatment (HTMT), controlled rolling and low temperature thermomechanical treatment (LTMT). When rolling plain carbon-manganese (C-Mn) steel sheet (Figure 11), the grain size can be reduced from 10 µm to 5 µm when the sheet is controlled by the degree of deformation and accelerated cooling. This reduction in grain size increases the yield strength of the steel by approximately 80 MPa according to the well-known Hall-Petch relationship [81–83]. Fig. 11. Schematic diagram of grain refinement in steels during deformation Controlled rolling is a means by which the properties of steels can be improved to levels equivalent to those of higher alloy or heat treated steels. Controlled rolling consists of three stages: (a) deformation in the recrystallization region at high temperatures, (b) deformation in the non-recrystallization region, and (c) deformation in the austenite-ferritic region. The significance of deformation in the nonrecrystallization region lies in the division of austenite grains into several blocks as a result of the introduction of deformation bands into the grains. Deformation in the austenitic-ferritic region gives a mixed structure consisting of equiaxed grains and subgrains. The fundamental difference between conventionally hot-rolled and controlled-rolled steels is that ferrite nucleation occurs exclusively at the austenite grain boundaries in the former, while in the latter it occurs within the grains as well as at the grain boundaries, resulting in finer structures. Conventional controlled rolling aims to produce flattened austenite grains through plastic deformation, resulting in an increase in nucleation sites for the transformation of austenite to ferrite. This process then results in the formation of fine ferrite grains measuring approximately 5 to 8 µm in size. Traditional controlled rolling usually involves high heating temperatures to achieve complete transition of microalloying elements, i.e. Nb and V, into solid solution. During the rolling process [14, 51–56, 84–98], which occurs below zero recrystallization temperature, deformation precipitation of Nb(C,N) occurs, causing complete suppression of recrystallization between each pass. The small size of the austenite grain leads to the refinement of ferrite grains. When deforming in the non-recrystallized temperature range of the austenite phase (below the non-recrystallization tempera
RkJQdWJsaXNoZXIy MTk0ODM1