Abstract
Tensile/impact behaviour of lower bainite obtained in high-Si steel 55Si3Mn2CrMoVNb was studied using SEM, TEM, and XRD. Specimens were austenitized at 900 °C and isothermally treated at 250, 270, and 300 °C with holding up to 600 min. The heat treatment results in the formation of cementite-free lower bainite/retained austenite structure, where retained austenite was found as blocky “islands” and interlaths “films”. The width of bainitic ferrite laths decreases from 170–240 μm to 45–80 μm with holding temperature decreasing. This results in increasing UTS (to 1700 MPa) and hardness (to 52 HRC). The optimal combination of mechanical properties (UTS 1397–1522 MPa, hardness 45–47 HRC, total elongation 18–21 %, U-notched impact toughness 105–139 J cm^–2) refers to holding at 300 °C to be associated with higher amount of retained austenite (30–33 %). With prolonging the bainitizing duration the hardness and ductility decreases while impact toughness increases. Prolonged holding at 300 °C leads to a continuation of bainite transformation and precipitation of transitional carbides within ferrite laths.

Key words
carbide-free lower bainite, retained austenite, phase transformations, microstructure, mechanical properties

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