Composition Design, Properties And Phase Transformation Mechanism Of 19Cr Economical Duplex Stainless Steels With TRIP Effect

In combination with superior mechanical and corrosion properties of ferritic and austenitic stainless steels, duplex stainless steels（DSSs）, containing both ferrite（a） and austenite（7）g) phase, exhibit better mechanical strength and resistance to both general and localized corrosion. For these properties, DSSs are suitable for ma ny applications, such as chemical, petrochemical, pulp and paper, power and gas industries, particularly in chloride containing media. In recent years, as the result of high cost of raw material such as N i & Mo and cap-and-trade system in association with CO₂ emission, lean alloying DSSs, which are nearly N i & Mo free and could exhibit better mechanical properties, are becoming important subjects on developing advanced structural steels. For these reasons, Fe-Cr-Mn-N series economical 19 Cr DSSs with transformation- induced plasticity（TRIP） effect, which could exhibit higher strength and ductility comparing to conventional ones, are actively being developed by replacing expensive Cr and N i with low cost Mn and N in this dissertation.First of all, effect of alloying elements on microstructure and properties were investigated. Transmission electron microscopy（TEM）, scanning electron microscopy（SEM）, X-ray diffraction（XRD）, Raman spectroscopy（RS） and Thermo-Calc thermal-calculation were employed to study the effect of Mn, N i and Si on microstructure, mechanical properties at room temperature and high-temperature oxidation properties at 1100 ℃ in air. Chemical compositions have been optimized, and effects of hot/cold working, aging treatment and tempering treatment on microstructure and properties have also been investigated. The conclusions are shown as follows:1) Three new series of nearly N i-free 19Cr-x Mn-0.3Ni-0.2N（x = 5.5 15）, 19Cr-10Mn-0.3Ni-y N（y = 0.2 0.3） and 19Cr-5.5Mn-0.3Ni-z Si-0.2N（z = 0.5 2.5） economical duplex stainless steels have been prepared and characterized. All these DSSs were balanced with ferrite-austenite relation after solution-treated at 1100 ℃. Ferrite volume fraction and tensile strength decreased with the increase of Mn content. Elongation firstly increased, reaching the summit when Mn content is 10 wt. %, and then decreased with the continuous increase of Mn content. DSS with 10 wt. % Mn content exhibited comprehensive mechanical properties as transformation induced plasticity: tensile strength was about 800 MPa and elongation exceeded 65 %. Induced ferrite diffusion layer between matrix and oxidation film became thicker with the increase of oxidation time and Mn content during oxidation at 1100 ℃ in air. DSS with higher Mn content showed more compact and continuous Cr₂O₃ film, which is benefit for high-temperature oxidation resistance. Self-repairing behavior of ferrite diffusion layer occurred in 10 wt. % Mn content DSS as the increased diffused Mn content induced more ferrite phase transformation. Both tensile strength and elongation increased with the increase of N content at room temperature. Weight gains per unit area were in parabolic relation to time during high-temperature oxidation at 1100 ℃ in air. DSS with higher N content showed worse high-temperature oxidation resistance with the increase of oxidation time. In DSS with higher N content, high-temperature internal oxidation behavior preferred to occur at and penetrate into the local breakaway regions in ferrite diffusion layer upon the austenite substrate regions, due to the increased outward diffusion of nitrogen ions. While the oxide film on the continuous ferrite diffusion layer parts remained protective as the formation of continuous and protective Cr₂O₃ layer in the inner layer of oxide film. Ferrite volume fractions increased and tensile strength decreased with the increase of Si content. Elongation firstly increased, reaching the summit when Si content was 1.5 wt. %, and then decreased with the continuous increase of Si content. DSS with 1.5 wt. % Si content showed the best ductility at room temperature, the elongation could almost reach 52 %. Continuous and compact Cr₂O₃ and Si O₂ film formed in the inner layer of oxide film during oxidation at 1100 ℃ in air, which was beneficial to high-temperature oxidation resistance. During tensile deformation of DSS with comprehensive mechanical properties at room temperature, ferrite phase deforms by slip, and austenite mainly deforms by deformation- induced martensite transformations according to g â†' e-martensite â†'a’-martensite. Orientation relationship between e-martensite and g was close to the S-N relationship, while a’-martensite and g showed K-S and N-W relationships. According to the result of mechanical properties at room temperature and high-temperature oxidation properties at 1100 ℃, the optimized representative economical DSS chemical composition was 00Cr19Mn10 N i0.3N0.25.2) During cold-rolling of solution-treated 00Cr19Mn10 N i0.3N0.25 DSS at room temperature, dislocation slip dominated the deformation mode of ferrite phase. Deformation mechanism of austenite phase was different with the increase of rolling reductions. Dislocation slip and strengthening effect of twin boundaries caused pile-up phenomenon at the initial deformation stage. When cold-rolling reductions exceeded about 50 %, induced a’-martensite generated in the deformed austenite phase. Work-hardening effect of deformed austenite phase was higher than ferrite phase. Cold-rolling deteriorated pitting corrosion resistance in Na C l aqueous solution. Pitting corrosion always initiated in ferrite and phase boundary in the solution-treated DSS. While additional pitting holes appeared in deformed austenite phase. 00Cr19Mn10 N i0.3N0.25 DSS showed obvious strain-rate dependence effect during tensile deformation at room temperature. Induced martensite content decreased as the insufficient transformation induced plasticity effect at higher strain rate condition. Therefore, both tensile strength and elongation decreased with the increase of strain rate. Micro-crack preferred to initiate at the a/g phase boundary.3) σ phase precipitated in a/a grain boundary after aging treatment at 750 o C, which could be obstacle to dislocation slip and increase tensile strength at room temperature. Pitting corrosion resistance in Na C l aqueous solution decreased with the increase of aging time. Nubbly Cr-rich a’Cr phase generated in ferrite grain as spinodal decomposition and amount of Cr23C6 precipitations appeared in a/a and a/g grain boundaries during aging at 475 ℃, which could bring decrease in ductility and pitting corrosion resistance properties. Mechanical properties and corrosion resistance of 350 ℃ aged samples were similar to that of solution-treated DSS at room temperature.4) During tempering treatment at 500 850 ℃ of the 80 % cold-rolling reductions DSS, the reverse transformation of induced martensite to austenite could finish in a short time. After tempering treatment at 850 ℃ for 30 min, equiaxial recrystallization grain size was about 2 4 mm. Therefore, DSS could show the best combination of strength and ductility during tensile test at room temperature, tensile strength almost reaching 1000 MPa with an elongation about 60 %.The above results indicates that the suggested optimum chemical compositions of 19 Cr economical duplex stainless steels with TRIP effect are: C ≤ 0.03 wt. %, Cr 18.0 20.0 wt. %, Mn 9.0 10.0 wt. %, Ni 0.2 0.4 wt. %, N 0.15 0.3 wt. %, Si ≤ 1.0 wt. %, B 80 ppm, Ce or Y 0.005 0.20 wt. %, balanced with Fe.