Research On Processing And Properties Of A New Type Of Si-Cr Plate Spring Steel With Variable Thickness

Leaf spring is one of the most important elastic elements in the automobile suspension system,its main function is to ensure that the bearing system(frame or load-bearing body)of the automobile has a certain elastic connection with the wheel or axle.Because of its relatively simple production process,high reliability,low cost,widely used in transport tools.With the high-speed and high-quality development of automobile manufacturing industry,the demand of high-strength automobile leaf spring is increasing year by year.With the advantages of light weight and reasonable structure,the few pieces of variable cross-section leaf spring is gradually eliminating the traditional steel plate spring in recent years.The subject of this research is a new type of Si-Cr leaf spring that is designed to give guidance on industrial production through its processing technology and the study of the variable cross-section area properties.The main contents of this paper are as follows:(1)The continuous cooling phase transition of the expermental steel was studied.The temperature of AC1 and AC3 of the expermental steel were measured by Formastor-FII automatic phase change expander instrument and the decomposition products of supercooled austenite at different cooling rates were studied.Thr-ough the microstructure observation and hardness testing,the CCT diagram and microstructure transformation of the test steel were obtained,which provided guidance for the subsequent heat treatment process setting.(2)The effect of deformation on the continuous cooling transformation of supercooled austenite was studied.The decomposition products of supercooled austenite at different cooling rates were measured by DIL805 deformation dilatometer with 10%,30%and 50%single pass deformation.The dynamic CCT diagram and microstructure transformation were obtained.The results show that when the cooling rate is lower than 0.5?/s the microstructure are pearlite and ferrite.When the cooling rate is 1?/s,the martensite begins to appear and as the amount of deformation increases the amount of martensite increases.With the decrease of single pass deformation,the temperature of the pearlite begins to transformation is rise and the temperature of the pearlite finish to transformation is decrease at the same cooling rate.Meanwhile both the temperature range of the pearlite transformation and the pearlite termination transition cooling rate will increase,in other words,martensite critical transition cooling rate increases.(3)The effect of heating system on decarburization performance was studied.With the increase of heating temperature,the thickness of decarburization layer increases continuously,especially at 900?,the total decarburization layer with ferrite mainly appears.At 900?,950?,1000? for different times,the results show that the decarburization layer is mainly composed of ferrite decarburization at 900? and the decarburization layer is mainly dominated by partial decarburization at 950? and 1000? as well the thickness of decarburization layer increases with the increasing of holding time.(4)The microstructure and properties of variable cross-section area were studied.When the reduction rate is 10%the strength is the highest,the yield strength is 1485MPa and the tensile strength is 1645MPa.On the whole,the strength increases first and then decreases with the increase of the reduction rate.The elongation and impact energy are contrary to the strength,but all the elongation are higher than 8%.The hardness is maximum when the reduction rate is 10%.The thickness of the decarburized layer decreases with the increase of the reduction rate.Properties meet the needs of industrial production.