Effect of Twinning on the Mechanical Properties of Bainitic Steels

Abstract

Bainitic steels are able to provide ultra-high strengths but show limited ductility. Authors have worked to develop bainitic steels with different chemical compositions and various heat treatment processing routes. In most of the studies, the toughness values have been achieved because of the very high strength values obtained. However, limited ductility has been a concern in these developed steels resulting in poor formability. Very limited literature is available where authors have developed bainitic grades with good combination of high strength-high ductility. There is a necessity to develop high toughness bainitic steels with improved ductility and hence improved formability. The main objective of the present dissertation work was to develop bainitic steel with high strength and high ductility by obtaining maximum volume fraction of bainite phase mixture in the microstructure, reducing the cementite/carbide content in bainite, and obtaining bainitic-ferrite as thin-plates. To achieve this objective, the steel chemistry-heat treatment combination was very carefully designed. The thermo-mechanical processing comprised of hot forging and hot rolling (mechanical portion of processing) followed by austempering (heat treatment or thermo-portion of thermo-mechanical processing). Bainite was obtained by isothermal holding at a low transformation temperature. The soaking time for isothermal transformation at the bainitic transformation temperature (350℃) was varied over a wide range in three distinct periods of 1h, 1 day, and 3 days to study the behavior and mechanical properties of bainitic-ferrite and RA obtained in the microstructure under different conditions. The results of tensile testing showed that microstructure comprising of high volume fraction of bainite (in the form of thin bainitic-ferrite) along with some amount of stable retained austenite results in good strength-ductility combination in bainitic steel. The bainitic steel (1 day sample) comprising of bainite (76.45%) and retained austenite (23.55%) showed the best properties with high strength (1507.29 MPa) and good ductility (30.0%). Characterization results showed thin plates of bainite-ferrite uniformly distributed throughout the micrograph along with RA which was present in two forms viz. fine austenite and blocky austenite. Thin films of RA in-between the sheaves of bainite-ferrite helped to improve the mechanical properties. For the given thermo-mechanical processing conditions applied to the steel chemistry, the microstructure of the bainitic steel with best properties showed presence of stacking faults (SFs)/twins in the microstructure.