Effects of Trace Rare Earth Elements on Microstructure and Properties of Cold Formed Ultra High Strength Automobile Steel
With the rapid rise of production and sales volume for new energy automobile,the quantity demanded of ul-tra high strength automobile steel is increasing.The problems such as spring back,deformation and cracking in cold form-ing process of ultra high strength automobile steel become the main factors restricting its applications.Therefore,it is of practical application values and huge market potential to develop a kind of automobile steel with high strength and excellent formability.It is found in many studies that the comprehensive properties of automobile steel could be improved by adding rare earth elements.In this paper,two groups of composition schemes are designed and the same process parameters of an-nealing of reverse transformation(ART)are used.The influencing mechanisms of rare earth elements on the microstructure and properties of cold formed ultra high strength automobile steel are studied by means of tensile test,scanning electron mi-croscopy and X-ray diffraction(XRD).The results showed that the microstructure of experimental steel was martensite,retained austenite and trace ferrite under the ART process,but content of retained austenite of experimental steel with rare earth in the two composition schemes was slightly higher than that of experimental steel without rare earth,which provided the microstructure basis for transformation-induced plasticity(TRIP)effect in subsequent forming process.Under the op-timum heat treatment process parameters of quenching at 800℃and holding for 5 min.,then annealing at 645℃and holding for 15 min.,the content of retained austenite in experimental steel with rare earth in the first scheme is increased by 8.1%compared with that of experimental steel without rare earth,percentage elongation after fracture is increased by 21.8%as well as product of strength and elongation(PSE)is increased by 17.2%and reaches 28.47 GPa·%;in the sec-ond scheme,the content of retained austenite in experimental steel with rare earth is increased by 4%compared with that of experimental steel without rare earth,percentage elongation after fracture is increased by 2.8%as well as PSE is increased by 5.1%and reaches 34.8 GPa·%,which illustrate that the comprehensive properties of ultra high strength automobile steel could be improved to different extent by adding rare earth elements.
cold formedultra high strength automobile steelrare earth microalloyingART annealing
万方数据
维普
