Effects of thermo-mechanical treatments on the microstructure and mechanical properties of a 460 MPa grade low carbon bainitic ferrite steel

Abstract

The effects of deformation at 900 °C and 1050 °C along with various cooling rates on the microstructure and mechanical properties of a 460 MPa grade low carbon bainitic ferrite steel are investigated. Without prior high temperature deformation, the microstructures can be polygonal ferrite, pearlite, granular ferrite, acicular ferrite or bainite ferrite, depending on the cooling rate. High temperature deformation moves the transformation curves toward the left side in the continuous cooling transformation (CCT) diagram and promotes the formation of polygonal ferrite, granular ferrite, and acicular ferrite, which leads to a lower hardness. Electron backscattering diffraction (EBSD) results indicated that with a cooling rate of 40 °C/s, high proportion of low angle grain boundary (LAGB), large orientation gradient, and high dislocation density and residual stress can be obtained by deformation at high temperature before cooling. Low cooling rate reduces the proportion of LAGB, orientation gradient, dislocation density, and residual stress due to the release of residual stress and the consumption of dislocations. Deformation before cooling slightly lowers the transformation start temperature (Ar3) while accelerating the transformation speed of the supercooled austenite. The yield and tensile strength increase with the increase in cooling rate. Furthermore, the impact energy value increases with the increase of cooling rate, and it slightly decreases when the microstructure fully changes to bainite ferrite owing to the significant increase in strength. Impact toughness is more sensitive to strength at low temperatures.