توليد و مشخصه يابي فولاد زنگ نزن 201L فوق ريزدانه/نانو ساختار حاوي تيتانيوم با استفاده از فرايند ترموديناميكي مارتنزيت

چكيده انگليسي :

Production and Characterization of Ti Modified Nano Ultrafine Grained 201L Stainless Steel through Martensite Thermomechanical Treatment Saeed Sadeghpour s sadeghpour@ma iut ac ir Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisors Ahmad Kermanpur ahmad k@cc iut ac ir Abbas Najafizadeh a najafi@cc iut ac irAbstractBecause of their extremely small grain sizes nanograined NG materials exhibit very high strength However truly NG materials grain sizes below 100 nm also have drawbacks they are often produced insmall quantities and exhibit limited tensile elongation to failure Several methods have been suggested interms of microstructural design to improve the limited ductility of NG materials In recent yearsthermomechanical processes using strain induced martensitic transformation and its reverse transformation Martensite Treatment were named as a method that produces NG metastable austenitic steels This methodis a combination of severe cold deformation and phase reversion annealing approach NG and ultrafinegrained UFG steels produced through this method have high tensile strength and excellent ductility Nonetheless there is a significant interest to enhance the ductility in the NG UFG materials produced bymartensite treatment It is the aim of this research to study the effect of accompanying martensite process bynanosize precipitates in order to produce a NG UFG stainless steel with an improvement in its ductility Inthis regard a Ti containing microalloyed 201L stainless steel was prepared in a laboratory vacuum inductionfurnace The cast ingot was homogenized at 1200 C for 12h hot rolled to 8 mm thick and water quenched The strip was then solution treated at 1200 C for various times After preparing the initial microstructure specimens were cold rolled from 5 to 0 5 mm and subsequently annealed in the temperature range of750 900 C for 30 3600 s X ray diffraction and a Ferritescope instrument were used to determine the phasefraction of austenite and martensite after cold deformation and after annealing The structural features wereexamined in optical microscope scanning electron microscope SEM field emission SEM and atomic forcemicroscope Precipitation behavior was evaluated by electrical resistivity and microhardness tests Distribution and chemical composition of the precipitates were examined using energy dispersivespectroscopy in the SEM The saturation thickness reduction of martensite formation during the cold rollingat room temperature in Ti modified 201L stainless steel was about 30 The results showed that themartensite transformation was suppressed with increasing strain rate due to adiabatic heating The mainfindings were that through the martensite treatment a structure with a grain size of about 45 12 nm achievedafter 90 cold rolling followed by reversion annealing at 900 C for 60 s This NG steel possessed a highyield strength 1000 MPa six times higher than that of initial coarse grained steel very high tensilestrength 1330 Mpa and exceptional elongation of 42 due to both precipitates and strain inducedmartensite that form during tensile test The results show that the deformation mechanisms of NG UFGaustenitic stainless steels with high strength high ductility combination can be distinctly different from theCG counterpart as documented via nanoindentation The differences in the deformation mechanisms ofNG UFG and CG steels are distinctly reflected in the force displacement plots and attributed to differencesin austenitic stability associated with the grain size effect These results are discussed and their possibleeffects on the final microstructure and mechanical properties are inferred Key Words Ti modified stainless steel Thermomechanical traetment Strain induced martensite metastable austenite Nano Ultrafine grained Reversion annealing Cold rolling

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توليد و مشخصه يابي فولاد زنگ نزن 201L فوق ريزدانه/نانو ساختار حاوي تيتانيوم با استفاده از فرايند ترموديناميكي مارتنزيت

فولاد زنگ نزن حاوي Ti , آستنسيت نيمه پايدار , ريز دانه , آنيل بازگشتي , نورد سرد