Study On Microstructure And Properties Of Cr And Mo Alloyed Low Nickel Steel

At present,the development of Ni-containing cryogenic steel is toward a direction of reduction and low-cost.As Ni is a rare and precious metal element,reducing the content of Ni in steel can effectively save resources and reduce costs.Therefore,the production and development of low nickel LNG steel is particularly important.9%Ni steel is widely used in LNG tank production,so a lot of research has been done on low Ni steel plate at home and abroad.It was found that the low Ni steel plate produced by traditional QT(Quenching and tempering)and QLT(Quenching,lamellarizing and tempering)process had some problems such as poor toughness or insufficient strength.In order to obtain good mechanical properties,a new process is proposed to produce low Ni steel.However,due to the decrease of Ni content,the strength of low Ni steel plate produced by this process is also reduced.Therefore,it is proposed to produce low Ni steel by alloying with Cr and Mo in order to improve the strength of steel plate.In this paper,the microstructures and mechanical properties of low Ni steel under TMCP-UFC-LT process and the transformation regulation of low Ni steel during continuous cooling were systematically studied.The effect of Cr and Mo on phase transformation behavior and microstructure and properties of low Ni steel was discussed.The effects of two-phase temperature,tempering temperature and tempering time on the reversed austenite content,morphology and distribution in low Ni steel were analyzed.On this basis,the strengthening and toughening mechanism of Cr and Mo alloyed low Ni steel under TMCP-UFC-LT process is clarified.The main works of the present paper are as follows:(1)Microstructure transformation of undercooled austenite in low Ni steel during static and dynamic continuous cooling was studied by thermal simulation experiments.The results show that in the process of static and dynamic continuous cooling of low Ni steel,there are three transformation zones:proeutectoid ferrite zone,bainite zone and martensite zone.At high cooling rate,the structure of low Ni steel is all martensite.(2)The phase transformation rule of low Ni steel with different contents of Cr and Mo was studied,and the effect of Cr and Mo on the phase transformation behavior of low Ni steel was analyzed.The results show that Cr and Mo can increase the diffusion activation energy of C in austenite,hinder the diffusion of C,delay the transformation of undercooled austenite and reduce the ferrite transformation point.With the increase of Cr and Mo content,the critical cooling rate of martensite decreases and the Vickers hardness increases.The hardenability and hardness of low Ni steel are improved by Cr and Mo.(3)The effect of Cr and Mo content on the mechanical properties of low Ni steel was studied.The results show that the yield strength and tensile strength of experimental steel increase with the increase of Cr and Mo content.After adding 0.5%Cr and 0.1%Mo,the yield and tensile strength of low Ni steel reach 630MPa and 730MPa respectively due to the effect of solution strengthening,which meet the strength requirement of 9%Ni steel.(4)The microstructure transformation process of low Ni steel based on TMCP-UFC-LT process was studied.The results show that the tempered structure of low Ni steel produced by TMCP-UFC-LT process includes critical ferrite,tempered martensite and reversed austenite.Alloy element distribution occurs during the holding process in the two-phase zone,and austenite stabilized elements eventually enrich in reverse austenite.(5)The effects of two-phase temperature,tempering temperature and tempering time on the structure and properties of low Ni steel were studied.The strengthening and toughening mechanism of Cr and Mo alloyed low Ni steel under TMCP-UFC-LT process was analyzed in combination with the change regulation of reversed austenite content and morphology.The results show that acicular reversed austenite can passivate the crack tip and hinder the crack propagation,and can improve cryogenic toughness more effectively.Proper dispersion and stable acicular reversed austenite,fine grains and solid solution strengthening of Cr and Mo are the main reasons for good mechanical properties of Cr and Mo alloyed low Ni steel treated by TMCP-UFC-LT process.