| With the country's increasing investment in environmental protection,hydropower has received more and more attention,which has also set higher requirements for the property of steel for hydropower steel.Hydropower steel need to achieve sufficient strength,toughness and welding properties,and the development of high heat welding hydropower steels can not only reduce production costs,but also improve productivity.In this paper,600MPa grade hydropower steel is designed and prepared,and systematic heat treatment experiments and mechanical properties tests are carried out.Based on this,the differences in welding properties between conventional metallurgical steel and oxide metallurgical steel are compared and studied.The main research contents of this article are as follows:(1)With reference to the composition range of Q490S and 12MnNiVR,the effects of various alloy elements on the mechanical properties of steel were studied.Through the optimization of the composition,the principle of reducing the carbon content to improve the welding properties and increasing the manganese content to improve the strength and toughness was designed and prepared 600MPa grade hydropower steel.(2)In order to study the dynamic recrystallization behavior of the experimental steel,a single pass compression experiment was performed to obtain the true stress-true strain curve,and the relationship between the true stress and the deformation temperature,degree of deformation,and strain rate was studied.The results show that the smaller the strain rate and the higher the deformation temperature,the easier it is for dynamic recrystallization to occur.The lowest temperature at which dynamic recrystallization occurs is 900?,and the highest strain rate is 5s-1.Based on this,the hyperbolic sine model was used to fit the flow stress.(3)Under the undeformed and deformed conditions,the continuous cooling transformation behavior of the experimental steel was studied.Aci=760? and Ac3=838? were measured.The static CCT curve and the dynamic CCT curve were drawn by the thermal expansion method.The distortion energy accumulated in the deformation can promote the phase transition,which leads to an increase in the phase transition point of the dynamic CCT curve.With the increase of the cooling rate from 0.5?/s to 50?/s,the microstructure transformation under undeformed conditions is:ferrite+bainite,bainite,martensite;microstructure transformation under deformation conditions is:pearlite+ferrite,ferrite+bainite,bainite.(4)The experimental steel was subjected to two-stage controlled rolling,and the rolled steel plates were subjected to two different heat treatment methods:Reheat Quenching +Tempering(RQ-T),Direct Quenching+Tempering(DQ-T),and the mechanical properties were tested.The results show that direct quenching retains the deformed structure,increases the distortion energy of the grains,and can effectively improve the strength of the steel,but reduces the plasticity and toughness of the steel.The optimal heat treatment process for RQ-T steel is 920?+50min(quenching)? 630?+40min(tempering),and the optimal heat treatment process for DQ-T steel is 630?+60min(tempering).(5)A high heat welding thermal simulation experiment was performed on the experimental steel.When the heat input is increased from 100kJ/cm to 200kJ/cm,the charpy impact energy(-20?)of the conventional metallurgical steel HAZ is reduced from 94J to 23 J,and the charpy impact energy(-20?)of oxide metallurgical steel(2 furnaces)decreased from 155J,145J to 135J,91J.This shows that the resistance to high heat energy welding of oxide metallurgical steel is significantly better than that of conventional metallurgical steel.Analysis of inclusions in steel shows that the composition of inclusions in conventional metallurgical steels is Ti2O3CaS composite inclusions,the inclusions in oxide metallurgical steels are ZrO2-Al2O3-MgOMnS composite inclusions,and precipitation in conventional metallurgical steel The amount of material is significantly less than that of oxide metallurgical steel. |
PDF Full Text Request