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.

The effect of high temperature helical rolling (HR) on structure and fatigue life of 09Mn2Si pipe steel has been studied. With the use of transmission electron microscopy there was revealed that rolling gives rise to refinement of ferrite grains and cracking (fracturing) of cementite plates within the pearlite phase. The effect manifests itself to the greatest extent in the surface layer where due to the rolling the level of plastic deformation was the highest. Data of microhardness measurements confirms the gradient pattern of strain hardening over the cross section during the HR occurs while the most intensive microhardness increasing take place at the depth of up to 3 mm. According to the mechanical testing results the helical rolling of 09Mn2Si steel gives rise to increasing the level of deforming stress at the yield plateau as well as the proportionality limit with a general decrease in the relative elongation. At the same time, despite the strain hardening resulting from the helical rolling the mechanisms of plastic deformation which manifest themselves in the form of parabolic hardening with a smooth decrease in the flow stress level after neck formation are preserved in the steel. During the cyclic tension the number of cycles prior to failure increases from 2.5 to 3.8 times that depends on the location of specimens’ cutting from the rolled rod. The highest improvement in fatigue fracture resistance is registered for specimens cut out from the core of the rolled rods.

The structure and mechanical properties of ductile ferrite-pearlite 09Mn2Si steel after helical rolling (HR) were studied. It has been shown that five passes of the helical rolling (with a gradual temperature decrease at each pass) resulted in a finely dispersed structure formation in the rod surface layer (up to ∼1 mm thick). This indicated the development of intensive plastic deformation there. At the same time, a lamellar structure consisting of thin elongated ferrite grains has formed in the underlying layers. The largest strain hardening was evident in the surface layer up to 3 mm thick. A yield tooth and plateau were evident on plastic flow curve. The length of the yield plateau was increased up to 2 times compared to as-received steel. This was accompanied with tensile strength increasing by 30%, while the elongation at break decreased. Data of cyclic testing have shown that fatigue life of the HR-treated samples increased by 3.8 times. The highest fatigue durability was characteristic for specimens cut out from the core of HR processed rod.