Mechanical Properties And Corrosion Resistance Of Fe-Mn-Al-C-Cr-N System High Manganese Steels

Twinning induced plasticity(TWIP) steels present good mechanical properties caused by the formation of mechanical twins during the deformation process.Based on the previous work and under the frame of replacing Ni and Cr with Mn and Al,respectively,a Fe-Mn-Al-C-Cr-N-system steel was designed in this study.The new steels possess good mechanical properties,corrosion resistance and lower cost and have the potential of partly replacing the traditional stainless steel in the field of daily necessities and architectural decoration.Surface modification is a good choice for further enhancing the corrosion resistance,wear resistance,decoration of these steels.In view of low cost and viable operation in practical industrial application,gas nitriding processes are widely used to modify the surface properties.But the surface of steel sheet will become hard and brittle after nitriding,and this is unfavorable to the subsequent plastic processing.Thus,it should be carried out as the last step in a manufacturing procedure.That is,plastic deformation should be exerted on the cold-rolled and annealed steel sheet prior to nitriding to avoid surface defects.The hot deformation behaviors,mechanical properties,mechanism of twinning during the deformation,high temperature oxidation behavior and the corrosion behaviors in 3.5% NaCl solution and the effect of surface modification by gas nitriding of the Fe-Mn-Al-C-Cr-N system high manganese steel were systematically studied.It is expected that the above involved researches can provide theoretical and technological basis for the development and application of low cost stainless steel The main researches and conclusions are as follow:(1)The hot deformation and recrystallization behaviors of the studied high manganese steel were investigated by uniaxial compression tests.During the initial stage of plastic deformation,the stress was improved with the increasing of strain and reached a peak value rapidly.After that,due to the demineralization mechanism of recovery and recrystallization,the stress kept steady or decreased to a curtain value and then kept steady.In addition,the hot deformation behavior of this steel is sensitive to deformation rate.Moreover,the constitutive equations for this high manganese steel deformed at 950~1150℃.(2)The effect of Cr on microstructure and mechanical properties of the Fe-Mn-Al-C-Cr-N system high manganese steels was investigated by uniaxial tensile test conducted at room temperature.When the content of Cr is lower than 2.35%,no Cr23C6 can be observed.While Cr content is higher than 2.35%,Cr23C6 can be seen obviously.Furthermore,both the amount of Cr23C6 and the yield strength increased with the improvement of Cr content in matrix.When the content of Cr is increased to 5.47%,a large number of precipitates distributed on the grain boundary,that weaken the adhesion between grain boundaries and lead to the reduction of tensile strength.(3)The effect of annealing processes on the microstructure and mechanical properties of the Fe-Mn-Al-C-Cr-N system high manganese steel was studied.With the increasing of annealing temperature,the average grain size and the frequency of ∑3 grain boundaries were improved,while the yield and tensile strengths were reduced and the elongation was increased.VWith the improvement of deformation level,the microstructure changed from high density dislocation to slip bands during tensile deformation,and deformation twins produced finally.(4)The effects of Cr and N,oxidation temperature and time on the high-temperature oxidation behaviors of the Fe-Mn-Al-C-Cr-N system high manganese steels have been researched.With the increasing of Cr and N,the oxidation weight gain was decreased.In addition,the weight gain was also increased with the increasing of temperature and the extending of time.Moreover,the increasing of N in the matrix can improve the compactness of the oxide.Furthermore,there are three distinct stages in the mass gain curves at temperature higher than 800 ℃,while the oxidation process can be divided into two stages at temperature lower than 800 ℃.(5)The anti-corrosion abilities of the Fe-Mn-Al-C-Cr-N system high manganese steels with different content of Cr and annealed by various process were characterized through the typical polarization curves measured by CorrTest 4 electrochemical workstation in 3.5%NaCl solution.It shows that the increase of Cr in the matrix improved the corrosion resistance.The effect of annealing processes on the anti-corrosion property mainly depends on the degree of recrystallization and the distribution of coincidence site lattice(CSL)grain boundaries.When the recrystallization process just finished and grains were inhomogeneous and not start to grow,the average grain size had a great influence on anti-corrosion ability.With the increasing of grain size,the anti-corrosion ability of this high manganese steel is weakened.When the recrystallized grain growth took place,the occurrence frequency of CSL grain boundary made the dominant effect on the anti-corrosion ability.The anti-corrosion ability was optimized with increasing the frequency of CSL,especially ∑3n grain boundary.(6)The effect of gas nitriding processes on the surface properties of the cold rolled and pre-strained high manganese steel has been studied.It was indicated that nitriding temperature has a tremendous modification effect on surface properties.When the nitriding temperature was lower than 500 ℃,the main composition of the modified layers was S-phase and the nitrogen concentration in modified layers decreases nearly linearly.Fe4N formed with S-phase and step-like distribution of nitrogen can be observed in the nitriding layer,when the treatment temperature was 600 ℃.At the same time,the improvement of nitriding temperature lead to the thickness of nitriding layers increased.When the steel sheets were nitrided at 400 ℃,the thickness of nitriding layers increased unremarkable and the structure of the nitriding layers didn’t change with nitriding time.