Influence of deformation thermocyclic treatment on microstructure, phase composition, tensile strength and impact toughness of low-alloy steel

Increased requirements for the strength properties of low-alloy steels stimulate the search for their new processing methods. Such improvements are accompanied by lowering impact toughness, especially at subzero temperatures. In this study, the possibilities of enhancing both tensile strength and low-temperature impact toughness of the Х70 grade steel by deformation thermocyclic treatment were investigated. Six cycles of heating the blanks, their helical rolling and cooling in water were carried out. In each cycle, the heating temperatures were gradually reduced from the initial level of 920°C down to the final value of 830°C. For comparison, two cooling media (water and air) were used after the final cycle. Then, the blanks were tempered at 550°C. After final cooling in air, the strength properties of the steel increased by about 15 % compared to that in the hot-rolled condition, while it enhanced by 2.5 times (σUTS up to 1200 MPa) after final cooling in water. For both media, it was not possible to improve the impact toughness. After final cooling in air, the reason was the formation of lath bainite areas, characterized by high microhardness values, in the soft ferrite matrix. In the second case, the martensitic microstructure with microhardness levels of up to 320 HV50 was observed. Additional tempering of the steel at 550°C significantly increased impact toughness at subzero temperatures (KCV−40°C = 220 – 260 J / cm2). Its combination with preliminary final cooling in water ensured both high strength properties (σUTS up to 900 MPa) and the absence of the ductile-to-brittle transition down to −70°C. The reason was the formed fine-grained microstructure, in which uniformly distributed phases possessed similar properties. The obtained results enabled to recommend the applied mode of deformation thermocyclic treatment for the Х70 grade steel that ensures the achievement of improved both strength properties and impact toughness.