Study On Microstructures And Properties Of The Yield Strength 500MPa And 1100MPa Grade High Strength Steels

With the rapid development of social economy,the performance of high strength steel is required to improve.And low cost and high performance of high strength steel has become one of the most important researches focus in the field of iron and steel.In this paper,the yield strength of 500MPa and 1100MPa grade high strength engineering machinery steels were selected,and the influence of controlled rolling,controlled cooling and heat treatment processes on microstructures and properties of steels were investigated,and some reliable data can be used for practical production.The main research contents and results are showed as following:(1)The phase transition curves of dynamic and static continuous cooling of the steels were determined by thermal expansion method combined with metallographic observation.The results showed that the microstructure of the steels changed from ferrite and pearlite to granular bainite and lath bainite with the increase of cooling rate.The start and finish phase transformation temperatures can be increased by the deformation.(2)The effects of controlled rolling,controlled cooling and tempering on microstructures and properties of 500MPa grade high strength steels were studied.The results showed that the on-line quenching process,and the yield strength and tensile strength were 667MPa and 1216MPa for the steel,respectively,but the corresponding elongation and impact energy at-40? were reduced.The optimum tempering process was tempering at 600? for 1h,and its corresponding yield strength was 686MPa and 978MPa,respectively,and the elongation is high,and impact energy at-40? was more than 100J.(3)The effect of quenching and tempering process on microstructures and properties of two kinds of the yield strength 1100MPa grade high strength steels were studied.The results showed that the yield strength,tensile strength and impact energy at-20? were increased with of quenching temperature.The optimum quenching temperature for the two steels were 890? and 910?,respectively.When the tempering temperature was 180?300? the yield and tensile strength of the steels changed inconspicuously and the elongation first increased and then decreased with the increase of tempering temperature.The yield and tensile strength of the steels decreased and the elongation and the impact energy at low temperature increased when the tempering temperature was higher than 300?.The optimum tempering process for the two steels were at 200? and 180? for 0.5h,respectively.The yield strength was more than 1170MPa,the elongation was about 10%and impact energy at-20? was about 50J,and good mechanical properties were obtained for the steel.