Effect of helical rolling on the bainitic microstructure and impact toughness of the low-carbon microalloyed steel
Ferrite-bainite microstructures and impact toughness of the X65 low-carbon microalloyed steel were investigated after helical rolling at 1000, 920, 850, and 810 °C followed by continuous cooling in air. After helical rolling at 1000 °C, granular bainite with large areas of the massive-shape martensite-austenite constituent (d = 1.5 μm) and a high fraction of twinned martensite (d > 2.0 μm) were observed in the steel. This caused a decrease in impact energy at low test temperatures (for example, 70 J at –70°С). Lowering the helical rolling temperature contributed to a reduction of dimensions of both ferrite-bainite and martensite-austenite constituent areas, as well as the replacement of the latter by a slender type one and an improvement in fracture toughness at the low temperatures. The highest impact energy level (210 J at –70 °C) was achieved after helical rolling at 850 °C due to the formation of a homogeneous microstructure, which included dispersed ferrite grains, granular bainite and small fractions of the slender type martensite-austenite constituent (d = 0.1–0.7 μm). In this case, areas of twinned martensite were absent.