Ultrafine Carbide-Free Bainite in High-Carbon Steel After Continuous Annealing with Different Cooling Rates

In addition to conventional isothermal heat treatment, ultrafine carbide-free bainite can be obtained by continuous annealing of steels, which is of great interest to scientists and research communities. The resulting microstructure consists of bainitic sheaves composed of bainitic ferrites interwoven with austenite films in nanoscale, being separated by austenite microblocks resulting in GPa-strength level. This article aims to investigate the microstructural characteristics and impact toughness of high-carbon steel after continuous cooling heat treatment at cooling rates of 0.1, 0.15, 0.2 and 0.3 °C/min. Results indicated that bainitic ferrites and austenite films of less than 270 nm and different volume fractions of bainitic sheaves and high carbon retained austenite could be attained. Wider size distribution of bainitic ferrites was achieved by a slower cooling rate due to the progressive transformation of the primary austenite to bainite. Finally, it has been shown that impact toughness values were increased by decreasing the cooling rate of the steel due to a more effective TRIP effect and obtaining more bainitic sheaves with different orientations, while premature bainite transformation and the presence of martensite deteriorated the impact toughness when the highest cooling rate was applied.