Microstructure and mechanical properties of low carbon bainite steel treated by quenching and tempering

The effects of quenching at 915℃ and tempering at 490~640℃ on the microstructure and mechanical properties of C-Mn-Mo-Cu-Nb-Ti-B based low-carbon microalloyed steel were investigated.The microstructure,the shape and distribution of precipitates,and the fractograph of the steel were observed by scanning electron microscopy(SEM) and transmission electron microscopy(TEM).The volume fraction of retained austenite was measured by X-ray diffractometry(XRD).The results show that a multiphase microstructure with bainite,martensite and retained austenite is achieved after quenching and tempering.The bainite lath is 250 nm in width.The volume fraction of retained austenite decreases with the tempering temperature rising,and it is 2.1% after tempering at 520℃.The quantity of precipitates increases sharply after heat treatment,and the proportion of precipitates in a size range of 6 to 15 nm is above 70%.After quenching at 915℃ and tempering at 520℃,the test steel has a yield strength of 915 MPa,a tensile strength of 990 MPa and a-40℃ impact energy of 95 J.Fine precipitates and narrow lath bainite can enhance the strength,but the retained austenite existing between bainite laths can significantly improve the toughness.