Mechanical properties of carbide-free lower bainite in complex-alloyed constructional steel: Effect of bainitizing treatment parameters

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 m to 45-80 m 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.