Effect of accelerated cooling after cross-helical rolling on formation of structure and low-temperature fracture toughness of low-carbon steel

Abstract

The effect of accelerated cooling after cross-helical rolling of X70 low-carbon steel on the formation of structures and mechanical properties under static tension and impact bending was investigated. The use of interrupted accelerated cooling of steel after cross-helical rolling with exposure at 530 °C (mode I) and continuous accelerated cooling (mode II) leads to the formation of different types and ratios of structures in steel. After rolling according to mode I, the structure is characterized by the presence of ferrite, troostite, granular bainite, and fine Fe3C carbides. After rolling according to mode II, the structure is characterized by the formation of lath bainite and large sections of the martensitic-austenitic (MA) component up to 1 – 2 µm in size. It is shown that a decrease in the fineness of ferrite grains in steel after cross-helical rolling in modes I and II from 12 to 4.6 – 4.3 μm, the formation of a bainitic phase, and hardening of the matrix with carbides led to an increase in the yield strength of steel up to 440 and 490 MPa and tensile strength up to 760 and 880 MPa. Carrying out helical rolling according to mode I makes it possible to significantly increase the low-temperature fracture toughness of steel (KCV–70 °С = 160 J/cm2) compared to the hot-rolled state (KCV–70 °С = 11 J/cm2) and reduce the cold brittleness of steel to the temperatures below –50 °C. The use of continuous accelerated cooling (mode II) does not allow increasing the cold resistance of steel due to the formation of the lath bainite structure and large areas of the MA component.