تاثیر افزودن نایوبیم و دمای آستنیته بر ریزساختار و خواص مکانیکی حاصل از فرآیند مارتنزیتی یک فولاد کم‌کربن

نوع مقاله: مقاله پژوهشی

نویسندگان

1 کارشناسی ارشد مهندسی مواد، گروه مهندسی مواد، دانشکده فنی و مهندسی، واحد علوم و تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران.

2 استادیار، گروه مهندسی مواد، دانشکده فنی و مهندسی، واحد علوم و تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران.

چکیده

در این پژوهش تاثیر افزودن نایوبیم و دمای آستنیته بر خواص مکانیکی و ریزساختار فولادی کم‌کربن پس از اعمال فرآیند مارتنزیتی مورد بررسی قرار گرفته است. نمونه‌ها پس از ریخته‌گری و نورد داغ؛ در دماهای 900، 1000، 1100 و C 1200 به‌مدت یک ساعت آستنیته و سپس در آب کوئنچ شدند که حاصل آن ریزساختار مارتنزیتی (همراه مقدار کمی فریت ویدمن‌اشتاتن) بود. بیشینه سختی در نمونه حاوی نایوبیم آستنیته‌شده در دمای C 1200 حاصل شد. سختی این نمونه 238 ویکرز و استحکام کششی آن نیز MPa 859 بود. با این حال، این نمونه افزایش طول نسبی ناچیزی در حد 1 درصد از خود نشان داد. در مرحله بعد، کاهش 50 درصدی ضخامت نمونه‌های بدون نایوبیم و حاوی نایوبیم به‌وسیله نورد سرد و آنیل آنها به مدت min 90 در دمای C 600 انجام گرفت. بررسی‌های ریزساختاری با میکروسکوپ‌های نوری و الکترونی روبشی (SEM) نشان داد که ریزساختار نمونه بدون نایوبیم کاملاً آنیل و دانه‌ها هم‌محور شده‌اند و تاثیر کل فرآیند مارتنزیتی در آن خنثی شده است. آزمون کشش نیز نشان‌دهنده مشابهت استحکام نمونه آنیل‌شده و نورد داغ شده بود. از طرف دیگر، در نمونه‌ حاوی نایوبیم، ساختاری ریزدانه حاصل شد که سختی آن (در مقایسه با حالت کوئنچ‌شده) تا حد زیادی حفظ شده بود. بررسی نتایج آزمون کشش نیز نشان داد که فرآیند مارتنزیتی در نمونه‌ نایوبیم‌دار بسیار موثر بوده به گونه‌ای که پس از آنیل، استحکام کششی همچنان بیش از MPa 700 بود. علیرغم استحکام بالا، افزایش طول نسبی این نمونه در سطحی بیش از 15 درصد حفظ شد.

کلیدواژه‌ها


عنوان مقاله [English]

Effects of Nb addition and austenitizing temperature on the mechanical properties and microstructure of a low-carbon steel after martensite process

نویسندگان [English]

  • Nasim Narimani-Rad 1
  • Hamidreza Najafi 2
  • M.Reza Afshar 2
1 B.Sc., Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
2 Assistant Professor, Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
چکیده [English]

The effects of niobium addition (about 0.15 wt.%) and austenitizing temperature on the mechanical properties and microstructure of a low carbon steel (containing less than 0.2 wt.% carbon) after martensite process were investigated in this research. After casting and hot rolling at 1050 C, the samples were austenitized at 900, 1000, 1100 and 1200 C for one hour followed by quenching in water. The as-quenched microstructures consisted of martensite along with a small amount of widmanstatten ferrite. The maximum hardness and strength were obtained from the Nb-containing sample austenitized at 1200 °C. The hardness and tensile strength were 238 Vickers and 859 MPa, respectively. However, this sample showed an elongation of less than 1%. After 50% reduction in the thickness of the as-quenched samples by cold rolling, annealing was carried out at 600 °C for 90 min. Optical and scanning electron microscopy (SEM) showed that the microstructure of the niobium-free sample was completely annealed and the martensite process effects were eliminated. The tensile strength of this sample was approximately the same as that of the hot rolled specimen. On the other hand, the Nb-containing sample exhibited a fine-grained microstructure. In comparison with the as-quenched condition, the hardness of the microalloyed specimen was essentially preserved after martensite process. The results of the tensile test also revealed that the martensite process led to a good combination of strength and elongation in the microalloyed specimen because its tensile strength and elongation were higher than 700 MPa and 15%, respectively.

کلیدواژه‌ها [English]

  • Martensite process
  • Microalloyed steel
  • Grain refinement
  • Cold rolling
  • Annealing
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