| 022Cr25Ni7Mo4N SDSS (super duplex stainless steel alloy)isrepresentative of steel, mainly used in harsh corrosive media, especiallychlorine environment, such as water, oil and gas exploitation. The alloy ofhigh chromium, molybdenum and nitrogen balance of high compositiondesign, has a high resistance to stress corrosion cracking, pitting andcrevice corrosion resistance performance, but also in a range of organicand inorganic acids also have a very low corrosion rate. The alloy iswidely used in the oil and gas, petrochemical and chemical industries,pulp, marine engineering and other industries, etc.Currently, the super duplex stainless steel in China is still in thedevelopment stage. Since the alloy in the thermal process, the austeniteand ferrite uneven distribution of stress and strain in the hot-rolled, hotforging process is easy to crack. Meanwhile, the dual-phase steel inthermal processing prone to precipitate. Therefore, the alloy tubes, platesand other products are difficultly made. This super duplex stainless steelfor the production difficulties encountered in the process, at the conclusion of Baosteel Special Steel duplex stainless steel, especially022Cr25Ni7Mo4N SDSS production.Based on the thermal processingaccording to the needs of key parameters,022Cr25Ni7Mo4N SDSS isstudied under different heating processes and properties, and thermaldeformation behavior of high-temperature thermoplastic law. The resultsfor the rational development and optimization022Cr25Ni7Mo4N SDSSthermal processing parameters has important theoretical significance andpractical value. The main contents and conclusionsare as follows:1.The Thermo-Calc thermodynamic phase diagram calculationsoftware was used in this article, combined with the heat treatment of022Cr25Ni7Mo4N SDSS at different temperatures, the microstructure andproperties of this alloy were studied. The results indicated that superduplex stainless steel cast ferritic phase volume fraction is about45%. Asthe changing of heat treatment in different temperature,022Cr25Ni7Mo4N SDSS of ferrite and austenite phase ratio will change.The ferrite and austenite phase ratio changed little below1150℃, and theferrite phase ratio will increase with the increasing of temperature above1150℃. As the temperature continues to increase, the degree of austeniteis significantly improved ball.When the temperature is lower than980℃, the brittle phase σ phase could be precipitated, it may soften the material.Therefore, the steel should be heat treated above1000℃during thethermal processing.2.022Cr25Ni7Mo4N SDSS was hot tensile tested on thermalsimulation machine Themecmater-3800for a super duplex stainless steeldeformation temperature of900~1350℃, deformation rate of1s-1. Andthe microstructure of the alloy was studied at different temperaturesthrough GSM, SEM, EPMA method The results indicated that thedeformation resistance of the material decrease with the increasing oftemperature, the plasticity increase with increasing of temperature. As-castthermoplastic shows a little weak, such as RA about80%. Thefundamental reason is that less developed and thick dendrites in the phaseboundary itself is flat and easy to crack, and the segregation of harmfulimpurities will further exacerbate the phase boundary weakening. Thealloy thermoplastic curve has two inflection points, appeared in about1000℃and1300℃.The ductility decreased due to the grain boundarymelting above1300℃plastic points. Another plastic limit point could befound below1000℃because of the σ phase. Thus，the suitable thermalprocessing temperature should in the range of1050~1250℃. 3. The isothermal compression experiment of022Cr25Ni7Mo4N wasconducted in the deformation rate of0.1~10s-1with the maximumreduction of80%in the constant strain rate. The hot deformation behaviorwas studied from the true stress-true strain curve which were obtainedfrom the compression test. On the basis of the dynamic material model,the thermal processing maps could be obtained. From the stress-straincurve, it can be seen that temperature and strain rate are sensitive to theflow stress during the hot deformation process.The flow stress decreaseswith the decreasing of temperature and the strain rate. During the hotdeformation process, the dynamic softening (recovery or recrystallization)was likely to occur when reducing strain rate or increasing thedeformation temperature. The thermal processing map indicated that theprocessing window is narrow in the hot deforamtion. A dynamicrecrystallization behaviour could happen in this region, but the efficientenergy consumption is low in this region (less than0.45), and a strain ratesensitivity is also relatively low. The deformation region is variable to benarrow when increasing the deformation degree. Therefore, it is necessaryto conduct the process in the dynamic recrystallization regions, namelyenergy consumption is much higher and avoiding instability zone, whichis due to the higher power dissipation efficiency in the dynamic recrystallization region. On the other hand, the deformation rate should beconsidered in the hot deformation haviour. In the high deformationtemperature and high strain rate the material should be product in lowerdefrmation dgreee., large deformation degree should be conducted afterentering into the high energy consumption and high efficiency strain rateindes region.At present, Baosteel has prototyped022Cr25Ni7Mo4N SDSShot-extruded tubes and hot rolled plate, and has achieved commercialproduction. |
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