Processing,Microstructures And Mechanical Properities Of An Ultra-high Strength Automotive Plate Spring Steel

As China's industry transitions to high-end industries,the requirements for equipment parts and related devices are becoming higher and higher.As a basic component,spring steel has a large demand in various fields of industry,and its main role is to store energy and reduce vibration.The aerospace and automotive industries place great demands on the strength of spring steel.Therefore,the research and development of ultra-high strength automobile leaf spring steel with ultra-long fatigue life is particularly important,which is of great significance for the lightweight design and application of automobiles.The aim of this thesis is to develop an ultra-high strength automobile leaf spring steel with a strength of about 2000 MPa.The chemical composition was designed first,and then the continuous cooling transformation behavior,dynamic recrystallization,decarburization behavior,hardenability,and microstructure and mechanical properties of heat treated steel were carried out.Finally,automobile leaf spring steel has been developed with good comprehensive performance.The main research contents and results of this thesis are summarized as follows.(1)The chemical composition of the experimental steel was designed and simulation calculated by using JMatPro thermodynamic software,and the fundamental mechanical properties with respects to the contents of various main elements were initially calculated.The optimally designed chemical composition of the experimental steel is 0.54%C,1%Si,0.9%Mn,1%Cr,1%Ni,0.03%Nb,0.1%V,0.0022%B.(2)The continuous cooling transformation behavior of the experimental steel was studied and the factors affecting the transformation process were analyzed.The Aci and Ac3 of this steel were determined to be 701? and 751?,respectively.The thermal expansion method combined with the metallographic method was used to measure and draw the static CCT curve of the experimental steel in a phase change dilatometer.When the cooling rate is 0.1?/s,the obtained microstructure is ferrite and pearlite.When the cooling rate is 0.2 ?/s,a bainite structure appears,while the cooling rate is 5 ?/s or more,the microstructures are all of martensite.(3)Using a single-pass thermal simulation compression experiment,the true stress-true strain curve of the experimental steel was obtained and the dynamic recrystallization behavior under different conditions was studied.The results show that higher deformation temperature,slower strain rate and larger amount of deformation are all conducive to the occurrence of dynamic recrystallization of the experimental steel.(4)The effect of heating temperature on the decarburization behavior of the experimental steel surface was studied,and it was found that 750?800? was fully decarburized,and 850? and above were partially decarburized,and the decarburized layer increased with increasing heating temperature The depth increases first and then decreases.At the same time,the effect of holding time on the decarburization of the experimental steel surface was studied.The rule is that the depth of the decarburized layer increases with the holding time.(5)The hardenability of the experimental steel was studied by measuring the hardness of the as-quenched steel.After observation of the metallographic microstructure and measuring of Rockwell hardness of the experimental steel from the surface to the centeral layer,it was found that the hardness value was stable to be?58 HRC and the microstructures from the surface to the central layer are of martensitic.The full thickness of the experimental steel is 21 mm,which indicates that the experimental steel has good hardenability.(6)A study was made on the rolling and heat treatment process for this newly developed ultra-high strength spring steel,while the optimal heat treatment process was obtained.The optimal heat treatment process of the rolled and air-cooled steel is 920?×30 min+375?×60 min.In this case,the tensile and yield strengths are 2001 MPa and 1636 MPa,respectively,while the elongation is 20%and the percentage reduction of area is 49%.The yield ratio is 0.82 and the impact energy is 29.6 J.