Study On The Properties Of A New Corrosion-resistant Fe-Mn-Al-Cr-C TWIP Steel

In order to improve the corrosion resistance of Fe-Mn-Al-C austenitic TWIP steel,the design idea of composite addition of corrosion resistant elements Al and Cr was adopted.According to the calculation formula of stacking fault energy and empirical analytical equation of austenitic stability zone and composition of Fe-Mn-Al steel,Two new Fe-27Mn-3Al-7Cr-0.3C and Fe-27Mn-3Al-7Cr-0.5C austenitic TWIP steels with stacking fault energy of 36.6m J/m² and 42.3 m J/m² were designed.X-ray diffraction（XRD）,Scanning Electron Microscope（SEM）,Electron Backscattered Diffraction（EBSD）and universal electronic testing machine（10T）were used to study the mechanical properties and microstructure changes of two TWIP steels during tensile process.The corrosion resistance of Na₂SO₄ in 1 mol/L simulated atmospheric environment was studied by anodic polarization curve and electrochemical impedance spectroscopy.The composition,structure and compactness of passivated film of Fe-27Mn-3Al-7Cr-0.5C TWIP steel were studied by Mott-Schottky curve,Auger electron spectroscopy and X-ray photoelectron spectroscopy,and the conventional high-aluminum Fe-28Mn-9Al-0.8C austenitic steel was introduced for comparison.The results show that:After solution treatment,both Fe-27Mn-3Al-7Cr-0.3C and Fe-27Mn-3Al-7Cr-0.5C austenitic TWIP steels produced high density deformation twins and high work hardening rate during the tensile process at room temperature,and the work hardening rate showed a large plateau with the true strain curve.The tensile strength is 900 MPa and 950 MPa,and the elongation after fracture is 100%and 115%,respectively.The strong plastic volume of Fe-25Mn-3Al7Cr-0.5C austenitic TWIP steel is up to 115 MPa%,which can meet the requirements of the third generation of automotive steel.Compared with Fe-28Mn-9Al-0.8C steel,the anodic polarization curves of Fe-27Mn-3Al-7Cr-0.3C and Fe-27Mn-3Al-7Cr-0.5C austenitic TWIP steel show self-passivation in 1 mol/L Na₂SO₄ solution.The self-corrosion potential Ecorr increased from-560 m V to-340 m V and-350 m V,respectively,and the blunt current density Iss decreased from 11.8μA/cm² to 3.8μA/cm² and 3.9μA/cm²,respectively.The compound addition of Al and Cr can significantly improve the passivity of steel.The passivation film of Fe-27Mn-3Al-7Cr-0.5C austenitic TWIP steel is divided into two layers,the outer layer is mainly composed of Fe₂O₃,Mn₂O₃ and Al₂O₃ with N-type semiconductor characteristics,and the inner layer is mainly composed of Cr₂O₃ with P-type semiconductor characteristics.The double-layer structure can effectively block the outward migration of metal ions in the matrix.The passivation film resistance of Fe-28Mn-9Al-0.8C steel increases from 3.8 k?·cm² to 14.1 k?·cm²,and the densification of passivation film is significantly improvedCorrosion resistance of TWIP steel is adversely affected by grain boundary stress accumulation caused by pre-strain,but coincidence site lattice（CSL）grain boundary and small angle grain boundary with low interfacial energy generated during tensile process can reduce interfacial sensitivity and thus improve corrosion resistance.Fe-27Mn-3Al-7Cr-0.5C steel with large CSL grain boundaries and small angle grain boundaries generated by 0.6 true strain has better passivation performance in 1 mol/L Na₂SO₄ solution,the dimensional bluff current density I_ss decreases to 2.8μA/cm²,and the passivation film resistance increases to 15.5 k?·cm².The results show that CSL grain boundary and small Angle grain boundary can eliminate the adverse effect of stress and improve the corrosion resistance of steel.