| In order to achieve ultra-high strength and high toughness of steel,the more valuable element Co is generally added,which greatly increases the cost of steel.At the same time,because the material is vulnerable to strong dynamic load in the process of use,how to achieve low cost,ultra-high strength,high toughness and good dynamic properties has become a key issue in the development of new steel grades.In this paper,low alloy ultra-high strength DT506 steel designed is taken as the research object,which is mainly characterized by no Co and low Ni.Continuous cooling transformation test,heat treatment test and split Hopkinson pressure bar test were carried out on DT506 steel.The law of continuous cooling transformation of undercooled austenite,the effect of heat treatment process on its microstructure and properties and the effect of yield ratio on dynamic mechanical properties are analyzed systematically.The main research results are as follows.(1)The DT506 steel was heated to 950?for 10 minutes,and then cooled to room temperature at a cooling rate of 0.028?/s?19?/s.The phase transformation points Ac1 and Ac3 were measured to be 665?and 775?,respectively,and MS and Mf were 265?and 90?,respectively.The complete martensite structure is obtained when the cooling rate is higher than 0.138?/s,and the bainite and martensite two-phase structure are obtained when the cooling rate decreases to 0.055?/s.(2)Normalizing temperature and tempering time have little effect on the quasi-static mechanical properties of DT506.While quenching temperature and tempering temperature have significant effects on its quasi-static mechanical properties.After quenching at 810??1000?,the tensile strength and yield strength of DT506 steel increased at first and then decreased gradually.And reached the peak value after quenching at 860?,and then decreased gradually.The impact work decreases gradually between 810??860?,and decreases continuously when the temperature rises to 900?and reaches the maximum value.On the other hand,the fracture toughness showed an increasing trend.(3)After tempering at 150??680?,the tensile strength of DT506 steel decreases gradually with the increase of tempering temperature,while the yield strength increases first and reaches the peak value at 350?,then decreases gradually.The impact work gradually increases and reaches the peak value after tempering at260?,and then decreases rapidly.The tempering brittleness appears at 450?,and the impact work reaches the lowest value at this time.The optimal mechanical properties of DT506 steel were obtained after air cooling at 920?×1h+oil quenching at 880?×1h+260?×3h air cooling.The tensile strength of DT506 steel was 1915.5MPa,the yield strength was 1531.5MPa,the impact work was 64J,and the fracture toughness was 108MPam1/2.(4)The stress limit and yield strength of DT506 steel increase with the increase of strain rate at 0.001s-1?3100s-1 strain rate,and the material shows a strong strain rate strengthening effect.At the same strain rate,the lower the yield ratio of DT506steel,the higher the stress limit and yield strength of the material,and the stronger the strain rate sensitivity.The dynamic constitutive models of C-S and J-C are established,which provides a theoretical basis for the engineering application of DT506 steel. |
PDF Full Text Request