Effect Of Niobium On Microstructure, Mechanical Properties And Oxidation Characteristics Of Ultra-high Strength Spring Stee

With the development of lightweight and high-performance development goals in the automotive and railway industries,more and more attention has been paid to the research on high-strength spring steel used in spring parts.Niobium microalloying is a typical microalloying technology used in ultra-high-strength spring steel,which affects various properties and production systems of ultra-high-strength spring steel.In order to study the effect of niobium element in 2100 MPa ultra-high-strength spring steel,the65 SiCrV ultra-high-strength spring steel is taken as the research object.As well as material analysis methods and means such as OM,SEM,TEM,EDS,XRD and Fact Sage thermochemical calculation software,the heating phase transformation characteristics,continuous cooling phase transformation characteristics and heat treatment microstructure of the steel with a mass fraction of 0.017 % niobium were studied.The main research results are as follows:（1）The initial forging microstructures of the two 65 SiCrV steels are pearlite + a small amount of ferrite + a small amount of undissolved carbides;after adding niobium,the pearlite lamellar spacing and ferrite size of the 65 SiCrV steel are both finer.change.After isothermal holding at 850-1200 ℃ for 1 h,the average austenite grain size of both steels grows parabolically and exponentially with increasing temperature;The austenite grains of the steel are refined,and the grain growth tendency of the steel is also suppressed to a certain extent.（2）During the continuous heating process from room temperature to 1200 °C at different heating rates of 0.1-50 °C/s,both 65 SiCrV steels underwent transformation from pearlite + ferrite to austenite and carbide dissolution.In the process of heating,the austenite transformation temperature range is also increasing with the increase of the heating rate;the addition of niobium also refines the austenite grains of 65 SiCrV steel during continuous heating.The transformation start temperature has little effect,but the austenite transformation end temperature is delayed by about 5-20 °C.（3）The continuous cooling curve（CCT）results show that the addition of niobium increases the pearlite transformation start temperature of 65 SiCrV steel by about 12-20 °C,but does not change its pearlitic transformation end temperature;the addition of niobium does not change the 65 SiCrV steel.The martensite transformation temperature of the steel is around 238-117 ℃.When the cooling rate is 3 ℃/s,martensite begins to appear in the steel,and when the cooling rate is 5 ℃/s,it is all martensite.（4）The heat treatment results show that different quenching temperatures of 860-920 ℃（tempering temperature of 390 ℃）have no significant effect on the tensile properties of the two 65 SiCrV steels,and the tensile strength and area shrinkage are2100-2200 MPa,respectively.and 35-40 % fluctuation;but at 350-430 ℃ with the increase of tempering temperature（quenching temperature of 880 ℃）,the strength of the two steels decreased significantly and the plastic toughness increased significantly,tensile strength,area reduction rate and impact The change of toughness is about 2250-1750 MPa,34-45 % and 20-50 J respectively;the addition of niobium improves the strength and plasticity of 65 SiCrV steel,and the tensile strength,area reduction and impact toughness are increased by about 20-40 MPa,2-3 % and 6-10 J.Through microstructure observation,it is found that the addition of niobium refines the tempered troostite structure of 65 SiCrV steel after heat treatment,and its strengthening increment mainly comes from grain refinement strengthening and precipitation strengthening.（5）Under different orthogonal experimental conditions of holding temperature（1050-1150 °C）,holding time（60-90 min）,heating rate（8-20 °C/min）and oxygen concentration（2-7 vol.%）,There is no fully decarburized layer but only a half decarburized layer in the two 65 SiCrV steels.The thickness of the decarburized layer is about 500-850 μm;the oxidation kinetic curves of the two steels are similar to the linear law,and the oxidation rate The constant increases significantly with the increase of the holding temperature,and the oxide layer is divided into an obvious three-layer structure（Fe₂SiO₄ + FeO,Fe₃O₄,Fe₂O₃）from the inside to the outside.However,after the addition of niobium,the thickness of the decarburized layer of 65 SiCrV steel decreases,while the total oxidation weight gain and the thickness of the oxide layer increase,and the density of the oxide layer decreases.