Divorced Eutectoid Transformation For Spheroidization Of κ-Pearlite In The High Specific Elastic Modulus Steel

Automobile lightweight is an effective method to save energy,reduce emission and improve the actual safety performance of the automobile,which is the primary development trend of the automobile industry.At present,one of the main methods to realize lightweight is to apply high-strength steel,which can effectively reduce the thickness of components on the premise of ensuring the safety performance of the vehicle.Secondly,when using high strength,reducing the material density can further reduce the weight of the car body,and avoid the stiffness problem caused by the thickness reduction when using high strength steel.The low density of steel can be achieved by adding light-alloy elemental,Aluminum.However,when more than 5wt.%Al is added into steel,the elastic modulus of the material will be reduced to below 150 GPa.The decrease of the elastic modulus will lead to the deterioration of the stiffness and the impact of safety.It is an effective way to improve the overall elastic modulus of the material by introducing the ceramic phase into the metal matrix.The κ-carbide is a kind of hard ceramic phase with high elastic modulus.Owning to its high elastic modulus,the consumption of Al element in ferrite matrix can improve the elastic modulus of material,and then achieve the improvement of the overall elastic modulus.In this paper,aiming at solve a high-Al low-density with low modulus,a kind of lowdensity steel and a high specific modulus is developed based on κ-pearlite(1)The design of high specific elasticity modulus steel.The problem of low elastic modulus in high Al steel is solved by introducing high elastic modulus κ-carbide into ferroalloy.According to the calculation of thermodynamics,three kinds of composition steels are designed.The composition of alloy A is Fe-1.4C-5Al wt.%,the composition of alloy B is Fe-1.0C-7Al wt.%,and the composition of alloy C is Fe-1.4C-7Al wt.%.The theoretical elastic modulus of alloy A,B and C is 212.4 GPa,199.5 GPa,203.6 GPa,respectively;The theoretical density is 7.25 g/cm3,7.09 g/cm3,7.06 g/cm3,respectively.(2)The divorced eutectoid spheroidization improves toughness.Based on the existence of high brittle κ-pearlite in the microstructure of experimental steel,the spheroidization treatment is carried out by using the principle of divorced eutectoid to improve the toughness.The elongation of hot rolled alloy a is less than 1%,and the elongation after spheroidization is increased to 5%,which is nearly five times higher than that of hot rolled.However,the microstructure show that there are some block carbides in alloy A,which can deteriorate the ductility.(3)The influence mechanism of composition optimization and heat treatment process parameters.The composition design of alloy B and alloy C can effectively inhibit the formation of block net-like carbides.After the divorced eutectoid spheroidization,the elongation increased from 5.49%to 15.63%and the actual specific elastic modulus is 25.4 ×1010 cm2/s2;The elongation of alloy C increased from 1.93%to 13.23%after spheroidization,and the actual specific elastic modulus is 27.1×1010 cm2/s2,respectively.The initial austenitizing parameters,subcritical isothermal parameters and cooling rate are the main factors affecting the spheroidization.When austenitizing temperature increase,the amount of κ-carbides dissolved in the austenite increasing,meanwhile,the less the amount of carbides after spheroidization;When the greater undercooling degree is,the easier to obtain lamellar mode carbides,at the same time the smaller the undercooling degree is,the easier to obtain the divorced eutectoid mode carbides;The cooling speed affects the precipitation of carbides in the supersaturated austenite,the faster the cooling speed is,the carbides in the supersaturated austenite are not thoroughly precipited..