Effect of Austenitizing Temperature on Microstructure and Properties of 2 200 MPa Grade Ultra-High Strength Steel

The microstructure and mechanical properties of a new 2 200 MPa grade ultra-high strength steel after different austenitizing temperature treatments were investigated through mechanical tests， optical microscopy （OM）， scanning electron microscopy （SEM）， and other methods.The results showed that the strength and toughness and plasticity first increased and then decreased with the increase of austenitizing temperature. At 940°C austenitizing temperature， the optimal strength-toughness balance was achieved， with a tensile strength of 2 222.5 MPa， a yield strength of 1 725.5 MPa， an elongation after fracture of 11%， a reduction of area of 42.5%， and an impact energy of 51.05 J.At lower austenitizing temperatures， the impact fracture morphology was primarily characterized by hole-aggregating toughness dimples with fine grain size.Fine secondary phases （with a diameter of only 0.75 µm） were observed on the fracture surface. Based on phase diagram calculations and EDS（energy spectrum analysis）， the secondary phase was identified as M23C6.With increasing austenitizing temperature ， the morphology of the impact fracture surface transitioned to a combination of dimples and quasi-cleavage， accompanied by the disappearance of secondary phases and gradual coarsening of grains.