Study On Corrosion Resistance And Cryogenic Properties Of Fe-20/27Mn-4Al-0.3C Non-magnetic Steel

Non-magnetic steel is a kind of functional material and it has broad application prospects.Fe-Mn-Al-C non-magnetic steel has the advantages of low density,high strength,non-magnetic and low cost.It has been widely used in the manufacture of special ships and cryogenic engineering.However,the researchs on the corrosion laws of Fe-Mn-Al-C non-magnetic steel in sea water and the deformation mechanism under cryogenic conditions are limited.This study is supported by the national natural science foundation of China"Formation mechanism of annealing twins in high manganese non-magnetic steels during asymmetrical hot rolling process ".Based on the study of the microstructure and properties of the hot rolled plate,in order to further improve the corrosion resistance and mechanical properties of the non-magnetic steel in the actual environment,the corrosion laws,the mechanical properties and microstructure at cryogenic environment of Fe-20/27Mn-4Al-0.3C(designated as 20Mn and 27Mn)non-magnetic steels were investigated.The main conclusions of this study are listed as follows:(1)Through the full immersion corrosion experiment,the average corrosion rate of 20/27Mn in the marine environment was calculated.The profile and phase composition of the corrosion product were studied,and the corrosion law of high-Mn non-magnetic in the marine environment was studied.The results showed that the corrosion type of high manganese non-magnetic steel was local corrosion and the average corrosion rate gradually decreased with the prolongation of the experiment time.The corrosion products could be divided into the outer rust layer and the inner rust layer which had different structures.The corrosion products mainly included lepidocrocite,goethite,magnetite,iron oxide and manganese oxide.(2)Trough the electrochemical corrosion experiment,the polarization curve of 20/27Mn high manganese non-magnetic steels in NaCl solution was tested and the effect of solid solution treatment on corrosion resistance of non-magnetic steel was studied.The results showed that 20Mn steel had a higher corrosion potential,and the corrosion rate was lower when compare with 27Mn steel.The temperature and time of solution treatment had significant influence on the grain size of the non-magnetic steel and the ratio of CSL grain boundary.The fine grain and the higher ratio of CSL grain boundary were beneficial to the improvement of corrosion resistance.(3)The stacking fault energy of 20/27Mn high manganese non-magnetic steels at different temperatures was estimated.The effects of deformation temperature(room temperature and-196℃),solid solution temperature(800℃～1000℃)and tensile rate on the mechanical properties and deformation microstructure of non-magnetic steel were studied through the tensile tests at room temperature and low temperature.The result showed that the stacking fault energy of the non-magnetic steel decreased with the decrease of temperature.The main deformation mechanism of 20/27Mn non-magnetic steels at-196℃ cryogenic environment was twin,and the strain-induced martensite was found during the deformation process of 20Mn steel.Under the cryogenic environment,the yield strength,tensile strength and elongation of two kinds of non-magnetic steels were all obviously improved.With the increase of the tensile rate,the volume fraction of strain-induced martensite and the elongation of 20Mn steel decreased.When the solution temperature was 800℃,the critical stress for the formation of deformation twins increased due to the small grain size,which was not conducive to the TWIP effect and lead to the decrease of elongation.When the solid solution temperature was 1000℃,the deformation twin was easier to form.(4)The characteristics of deformed microstructure of 20/27Mn steels under different true strain during the tensile process at-196℃ were studied.The results showed that the number of deformation twins was very small in the deformation microstructure of the non-magnetic steels when the true strain was 0.04.Then the number of deformation twins increased significantly in the process of deformation.For 20Mn steel,the strain-induced martensite transformation was found and the volume fraction of martensite gradually increased during the cryogenic tensile process.The austenite microstructure of 27Mn steel was more stable,and there was no the formation of strain-induced martensite during the process of cryogenic tensile.(5)The deformation mechanism of 20/27Mn high manganese non-magnetic steels in cryogenic environment was studied.The results showed that the plastic deformation process of 20Mn steel in cryogenic environment could be divided into three different stages.In the first stage,dislocation gliding was the main deformation mechanism and the strain hardening rate decreased significantly.In the second stage,twin became the main deformation mechanism and the formation of strain-induced martensite was found.The strain hardening rate increased obviously.In the third stage,the speed of twin’s formation slowed down.Although the strain-induced martensite continued to generate,the strain hardening rate decreased.The TWIP effect and TRIP effect were found in the cryogenic tensile process of 20Mn steel.The plastic deformation process of 27Mn steel at low temperature could be divided into the same three stages.However,the twin became the main deformation mechanism of 27Mn steel and there was no the formation of strain-induced martensite due to the higher stacking fault energy.Therefore,the strain hardening rate was lower than that of 20Mn steel.