| To the high-strength plasticity and lightweight potential,austenitic low-density steels have attracted great attention in automotive manufacturing and the armor protection field.To the characteristics of high manganese,high aluminum and high carbon,this type of steel generally suffers from several problems,such as large as-cast microstructure,non-metallic inclusions are difficult to control,easy grain growth in solution treatment,and narrow hot working process interval,etc.Therefore,it is very difficult to control the microstructure and properties as well as machinability of this type of steel.In view of the above problems,the Fe-30Mn-9Al-1Si-0.5Mo(Mn30A19SiMo)steel for armor in a certain country as the research object.The effect of electroslag remelting on the microstructure and properties of non-metallic inclusion in the as-cast state,as well as the microstructure and properties in the rolled state,solution+aging state,and the toughening mechanism of this steel were studied.Finally,taking the Mn30A19.5 steel as the research object,the relationship between machinability and processing technology of this steel,the solution temperature of this steel,and the aging structure and properties of this steel as well as the toughening mechanism were studied via hot processing map.The main research conclusions are as follows:(1)The as-cast microstructure of Mn30A19SiMo low density steel is composed of austenite,ferrite,and a small amount of κ-carbides,the nonmetallic inclusion are mainly compound inclusions rich in O,S,etc.It is mainly dominated by MnS non-metallic inclusions,with about 78.3%content.The size of non-metallic inclusions mainly concentrates in the range of 1-10 μm.After electroslag remelting,the non-metallic inclusions are mainly Al-Oxides inclusions,and the MnS non-metallic inclusions decrease by about 86.6%and non-metallic inclusions of size above 5 μm decreased by 95.1%compared with vacuum induction melting.(2)The rolled microstructure of the electroslag remelted Mn30A19SiMo low density steel is mainly composed of 71.8%austenite and 28.2%strip ferrite,and the average grain size of austenite is around 19.5 μm.During the solution at 1020℃ for 3 h and aging at 530℃ for 12 h,the steel has the best combination advantages of strength and plasticity,the tensile strength is 1042 MPa,and the elongation after fracture is nearly 30%.Microband induced plasticity(MBIP)effect is the main strengthening and toughening mechanism of austenitic Mn30A19SiMo steel.(3)The vacuum induction melting Mn30A19.5 low density steel constructed based on the prasad instability criterion,the hot working diagram of a single pass compression of 60%at 850-1050℃ and 0.1-10s-1 shows that the steel has a relatively high process window narrow,there is a thermal processing safety window only under the process conditions of 980-1040℃ and strain rate of 0.1-1s-1.(4)The forging microstructure of wrought Mn30A19.5 low density steel is composed of austenite,a large amount of networked carbides,the austenite grain size is around 82.2μm.As solution treatment at 1000-1050℃ for 0.5-3 h,the austenite microstructure gradually homogenized.After solution treatment at 1020℃ for 2 h,the austenite showed a uniform equiaxed shape,the grain size was 67.7 μm.After aging at 530℃ for 9 h,the tensile strength of this steel is the highest(1180MPa),and the elongation after fracture is only 3.2%.The presence of brittle phases such as β-Mn and the presence of MnS and silicate non-metallic inclusions in the steel are the main reasons for the low strength and toughness of the steel.MB IP effect is the main strengthening and toughening mechanism of austenitic Mn30Al9.5 steel. |
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