Study On The Structure And Mechanical Properties Of High Strength Steel For Automobile

With the continuous development of automobile industry,the problem of automobile lightweight has attracted much attention,and high strength steel has become a widely used automobile steel.In this paper,two kinds of high strength automobile steels are studied:Low alloy ferritic steel containing Cu and High strength spring steel with 55Cr Si.Firstly,the low alloy ferritic steel containing Cu was studied,and combined with Q&P+isothermal aging process,the steel with certain retained austenite and Cu rich precipitate phase was prepared.The high toughness（ferrite+certain retained austenite）and high strength（precipitation strengthening of Cu rich phase）of the low alloy ferritic steel containing Cu were realized.Secondly,the effects of non-metallic inclusions and hydrogen embrittlement on the tensile fracture behavior of 55Cr Si high strength spring steel were studied.In this paper,LECO-836 oxygen nitrogen hydrogen analyzer is used to measure the content of O,N and H in 55Cr Si spring steel,combined with OM,SEM XRD,universal testing machine,HXD-1000 TMC Vickers,and the microstructure,fracture morphology and mechanical properties of two kinds of high strength steels were observed and tested by these equipments.The experimental results are as follows:After Q&P and isothermal aging treatment on the same composition of Cu containing low alloy ferritic steel,the microstructure produced high strength lath like martensite and retained austenite,but after different isothermal aging time,it was found that the size of bainite ferrite was different,and the size of bainite ferrite was the smallest when the isothermal time was 20 minutes,and the final structure is composed of bainite ferrite and austenite.The results of tensile test show that the microhardness is the highest at 20 min isothermal time,which is 421 HV_0.2,2.1%higher than that at 10 min.At this time,the maximum tensile strength is 1193.4 MPa,and the maximum elongation is 20%.By increasing the content of Mn and Ni,the strength and toughness of low alloy ferritic steel are improved.The increase of Mn content increased the tensile strength from 691.6 MPa to 1193.4 MPa,the yield strength from 570 Mpa to 760 MPa,the elongation from 15%to 20%,and the value of strong plastic product from 10.3 GPa%to 23.9 GPa%.The results showed that the increase of Mn content not only improved the plasticity of the material,but also the strength.The tensile strength value is 1151.8MPa,the yield strength value is 740 MPa,the elongation value is 17.5%,and the strong plastic product value is 20.1%when increases the content of Ni element.It shows that the increase of Ni content on the basis of 1.01 wt.%has no significant effect on the improvement of the strength and toughness of the material.The precipitation strengthening effect of Ni on Cu rich precipitates is not obvious.The tensile strength and elongation of 55Cr Si spring steel are reduced due to the combined effect of nonmetallic inclusion and hydrogen.And inclusion with Mg O-Al₂O₃spinel nonmetallic inclusion which has a sharp shape and are tightly bound to the matrix.For the inclusion with a mixture of Ca O,Si O₂and part of Al₂O₃,the fracture surface of the specimen showed an evident gap at the inclusion/matrix interfaces.For the specimen containing Mg O-Al₂O₃spinel inclusion,the fracture surface in the“fisheye”area is mainly composed of three regions including typical quasi-cleavage mixed intergranular fracture,dimple mixed transgranular fractures and ductile fracture from the interior to the edge,whereas there is no obvious transition zone from brittle fracture to dimple fracture in the“fisheye”area of the specimen containing inclusions with a mixture of Ca O,Si O₂and part of Al₂O₃.