توسعه فرايند مارتنزيت جهت توليد فولاد فوق ريزدانه / نانو ساختار AISI 201 حاوي نيوبيوم

Development of Martensite Process to Produce Ultrafine/Nanostructured AISI 201 Stainless Steel Containing Niobium

تمايل به گسترش نسل سوم فولادهاي با استحكام بالا سبب توسعه نسل سوم فولادهاي با استحكام بالا شده است. هدف تحقيق حاضر ريز كردن دانه هاي فولاد AISI 201 حاوي نيوبيوم تا محدوده نانو متري و يا فوق ريزدانه به كمك فرايند مارتنزيت و در نهايت مقايسه خواص مكانيكي آن ( به ويژه انعطاف پذيري) با فولاد فاقد نيوبيوم بود. بدين منظور نمونه ها پس از عمليات ريخته گري، همگن سازي، فورج داغ و آنيل انحلالي تحت فرايند مارتنزيت قرار گرفنتد. كرنش نمونه مورد شده در دماي 25°C در درصد كاهش ضخامت 50% رخ داد. بازگشت كامل آستنيت در دماهاي 850 و 900°C به ترتيب در زمان هاي 300 و 60 ثانيه انجام شد. اندازه دانه ها در نمونه آنيل شده در دماي 900°C به مدت 60 ثانيه برابر و 10±90 نانومتر بدست آمد. نمونه نانو ساختار شده، استحكام تسليم، استحكام كششي و درصد ازدياد طولي به ترتيب برابر 1140MPa و 1407MPa و 37% (بالاتر از انعطاف پذيري فولاد نانو ساختار فاقد نيوبيوم) از خود نشان داد.

In recent years there is a great interest to production of third generation of high advanced strenght steels including austenitic stainless steels. The aim of this study was to produce ultrafine/nanostructure AISI 201 austenitic stainless steel containing niobium through martensite process and comparing the mechanical properties with steel without niobium. The ingots casted in metallic mould, homogenized as 1200 ºC. In order to decrease the amount of delta ferrite and dissolution of niobium and choromium carbides, solution annealing treatment was carried out at 1200 ºC for 9hr after hot forging. The specimens quenched in water. The strips with the initial thickness of 10 mm were cold rolled to different reduction of 10 to 90%. Multipass cold rolling was performed at the room and zero temperatures with reduction in thickness of 0.1 mm per each pass. The reversion of martensite to austenite was carried out by an isothermal annealing for different times from 15 to 1800s.The identification of phases was carried out using X-ray diffraction (XRD) and Feritscope. The specimen microstructures were observed by optical and scanning electron microscopy (SEM). The grain size was measured using image analyzer software. XRD results showed that the intensity of austenite peaks decrease and in return martensite peaks increase during cold rolling. Saturation strain of cold rolled specimen at room temperature was about 0.7. Reversion annealing results showed that complete austenite reversion at 700, 750 and 800 ºC needs to longer times.The reversion rate is much faster at higher annealing temperatures. Austenite reversion at 850 and 900 ºC completed for 300 and 60s respectively. These are longer than completed reversion annealing time in AISI 201 wihout niobium steel. The difference was due to pin effect of niobium. The grain size of specimen annealed at 900ºC for 60s was 90±10nm .. The ultimate tensile strenght, yield srenght, and elongation of specimen annealed at 900 ºC for 60s were 1407 MPa, 1170 MPa, and 37%. The higher elongation compared to nanostructured AISI 201 without niobium steel was related to promotion of deformation mechanisms such as strain induced martensite due to carbide precipitation in ultrafine/nanostructured samples.