Effect of tempering temperature on microstructure and properties of boron-nickel added Nb-containing H-beam steel

To lower ductile-brittle transition temperature(50%,FATT) of high strength low alloy(HSLA) H-beam steel,a boron-nickel added Nb-containing HSLA steel were tempered at 550,600 and 650 ℃,respectively,after quenched at 910 ℃.Microstructure and mechanical properties,especially cryogenic toughness of the experimental steel were investigated by means of SEM,XRD,HRTEM,EDS analysis,uniaxial tensile and Charpy impact tests.The results indicate that the quenched and tempered steel exhibits good combination of strength and toughness.Their FATTs are all below-70 ℃ and the impact energy tested at-90 ℃ is about 100 J.The improvement of fracture toughness is the result of recovery and recrystallization of the quenched steel during tempering.Complex carbides precipitated in the tempered steel promote grain refinement and have little influence on cryogenic toughness.