The structure and mechanical properties of the 09Mn2Si high-strength low-alloyed steel after the five-stage helical rolling (HR) were studied. It was revealed that the fine-grained structure had been formed in the surface layer ≈ 1 mm deep as a result of severe plastic strains. In the lower layers, the “lamellar” structure had been formed, which consisted of thin elongated ferrite grains oriented in the HR direction. It was shown that the five-stage HR resulted in the increase in the steel fatigue life by more than 3.5 times under cyclic tension. The highest values of the number of cycles before failure were obtained for the samples cut from the bar core. It was demonstrated that the degree of the elastic energy dissipation in the steel samples under loading directly depended on the area of the grain boundaries as well as on the grain shapes. The fine-grained structure possessed the maximum value of the average torsional energy among all the studied samples, which caused the local material structure transformation and the decrease in the elastic energy level. This improved the crack resistance under the cyclic mechanical loading. The effect of the accumulation of the rotational strain modes at the grain boundaries was discovered, which caused the local structure transformation at the boundary zones. In the fine-grained structure, the formation of grain conglomerates was observed, which increased the values of the specific modulus of the moment of force. This could be mutually compensated due to the small sizes of grains. At the same time, the coarse-grained structures were characterized by the presence of the small number of grains with a high level of the moments of forces at their boundaries. They could result in trans-crystalline cracking.

The aim of the paper was to investigate the helical rolling parameters (a number of passes) for the microstructural modification and the low-temperature impact toughness improvement of the 09Mn2Si High Strength Low-Alloyed (HSLA) steel. In order to achieve this purpose, work spent to crack initiation and propagation was analyzed and compared with patterns of fracture surfaces. The microstructure and impact toughness values were presented in the temperature range from +20 to –70°C. Also, the fracture mechanisms in individual regions on the fracture surfaces were discussed. In addition, a methodology for computer simulation of the process was developed and implemented within the framework of the excitable cellular automata method and its integration with the kinetic theory of fracture. Finally, a theoretical analysis of the effect of grain shapes and orientations on the strain response patterns of a certain meso-volume simulating the material after the helical rolling was carried out.

A new hybrid discrete-continuum cellular automata approach is proposed to simulate the process of new phase/grain nucleation and growth. The method couples classical thermomechanics and the logics of cellular automata switching. Within the framework of the hybrid discrete-continuum cellular automata method, the space occupied by the simulated specimen is represented as a cellular automaton—a set of ordered active elements. Every element imitates an immovable region of space related to a part of material being characterized by the certain numerical parameters. The proposed approach enables calculating the magnitude of the local force moments and simulating dissipation of torsion energy leading to the formation of new defect structures. To illustrate the capacity of the proposed hybrid discrete-continuum cellular automata approach, the numerical simulations of thermally activated recrystallization of pure titanium near crack faces were conducted. The 3D cellular automaton simulated the microstructure evolution of the V-notched specimen region that imitated the crack tip vicinity at high homologous temperatures. Calculation of heat expansion with simultaneous thermal stresses accumulation and microrotation initiation was incorporated in the simulations permitting thereby to evaluate the local entropy and to monitor the evolution of crystal defects from initiation to storage. Perspectives of the proposed algorithms for simulations of the mechanical behavior of materials experiencing thermally induced twining or phase transformations are discussed.