In-situ synchrotron diffraction studies on hot deformation of austenite in a high strength quenched and tempered structural steel

摘要

The effect of plastic deformation of austenite at elevated temperatures on the kinetics of phase transformations during continuous cooling was studied in a high strength quenched and tempered structural steel S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt.%)) by means of in-situ synchrotron diffraction. The steel was heated to 900°C (above Ac3) in the austenite region and elongated by 6% followed by quenching to room temperature. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the local d-spacings between lattice planes. The plane specific diffraction elastic constants of austenite at 900°C in the steel were determined from the local d-spacings. The effect of the deformation of austenite on the phase transformation kinetics was studied. The evolution of lattice parameters and the phase fraction of the bcc phases during the quenching process were calculated. The calculated plane specific elastic constants of austenite at 900°C varied between 32 GPa to 140 GPa for the different {hkl} reflections of austenite. The deformation of austenite at 900°Cresulted in the formation of a mixture of 38% bainite, 59% martensite and 3% retained austenite after quenching to room temperature. Without hot deformation, austenite transformed to 9% bainite and 88% martensite with 3% retained austenite. The presence of the bainitic and the martensitic phases was observed from the change in the slopes of the lattice parameters of the bcc phase during quenching and confirmed by microscopy.