| Low density high strength steels have good comprehensive properties such as low density,corrosion resistance,high strength and toughness,etc.This type of steels has great potential in automobile,military industry,construction and other fields,and also has a wide application prospect in automobile lightweight and safety.Among different types of low density steels,Fe-Mn-Al-C low density steels are favored by researchers because of their higher specific strength and good plasticity.When Ni is added to Fe-Mn-Al-C low density steel,the intermetallic compound B2 phase is formed,the steel has excellent work hardening behavior,which is due to the non-shearing property of non-coherent B2 phase.However,at present,there is little research on the microalloying and strengthening mechanism of austenitic low density steel containing Ni.At the same time,considering that the austenitic low density steel containing Ni is newly developed,there is a lack of basic understanding of the effects of various hot deformation parameters on the deformation behavior and microstructure.Therefore,a kind of low density and high strength steel with yield strength higher than 1350MPa grade has been developed in this paper.And evaluate its thermal deformation behavior and effective to optimize the process parameters,provide theoretical basis for the follow-up of hot rolling process parameters;Then,the microstructure and mechanical properties of low density steel in hot rolling state were systematically studied.It is expected that these studies can provide theoretical basis and data support for the industrialization of new low density and high strength steel materials.This thesis mainly got the following results:austenitic base on three kinds of low density experiment of steel in the deformation rate of 0.01?10 s-1,deformation temperature 900??1050?under the condition of the hot compression experiment,shows that with the increase of deformation temperature,increase the degree of recrystallization,when fully recrystallization,with the increase of temperature,recrystallization grain size has grown up.With the increase of strain rate at a certain deformation temperature,the degree of dynamic recrystallization becomes more obvious and the size of recrystallized grain decreases.In combination with the microstructure of the experimental steel and the thermal working chart,it can be seen that in the region of high temperature and low strain rate,better thermal working performance can be obtained,and with the ch ange of Ni content,the instability region of the thermal working chart also changes.As can be seen from the figure,the optimal range of hot working process for the test steel is:0Ni steel:deformation temperature is 1000?1050?and deformation rate is 0.01?0.3s-1;For 3Ni steel,the deformation temperature is 950?1050?and the deformation rate is 0.05?0.6s-1.5Ni:the deformation temperature is 950?1000?,the deformation rate is 0.02?0.05s-1,the deformation temperature is 1030-1050?,the deformation rate is 0.01?0.6s-1.The strength of nano-sized precipitates of B2 structure and?-Carbide and MC second phases depended on the content of precipitates.With the increase of Ni content,the tensile strength,yield strength and hardness of hot rolled steel all show an upward trend.The yield strength of 5Ni steel exceeds 1350 MPa,which is 116 MPa higher than that of 0Ni steel,mainly due to the higher content of B2 phase in 5Ni steel(16.9%VS5.7%).The precipitation strengthening mechanism of MC phase in 0 Ni,3Ni and 5Ni steels is mainly mechanism.The theoretical calculation shows that the contribution of MC to the yield strength of experimental steels is 380 MPa,322 MPa and 364MPa and contribution of B2 to the yield strength of experimental steels is110MPa?242MPa and 358MPa.It can be seen that the strength of the experimental steel is significantly improved by the formation of nial B2 phase after the addition of Ni in the low density steel. |
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