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Investigation on Effects of Cold Roll Prior to Intercritical Annealing on Microstructure and Mechanical Properties of Ferrite - Martensite Dual Phase Steel

Hosseinifar, Farinaz | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52968 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ekrami, Ali Akbar
  7. Abstract:
  8. This study investigates effect of cold roll prior to inter-critical annealing on microstructure and mechanical properties of ferrite-martensite dual phase steel. Samples were heated to 850oC for 1h followed by oil quench, then the steel sheet were cold rolled by 0%,10%,15% and 20% reduction in thickness, Then an inter-critical annealing(750oC,120min) was performed to generate a ferrite-martensite microstructure. Microstructural study showed that increase in applied Cold roll, leading to increase volume fraction of martensite and decrease in ferrite grain size. Mechanical properties of dual phase steel were tested by Tensile test, impact test and micro-hardness Test. Results shows that increase in applied cold roll cause increase in ultimate tensile strength, yield strength, micro-hardness and toughness (e.g. Total elongation, uniform elongation and Impact energy). Improvement in mechanical properties can be related to increase in martensite volume fraction and grain refinement in investigated dual phase steel. Analysis of strain hardening behavior of DP steels by Hollomon analysis showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (nI) increased with increasing volume fraction of martensite. The energy absorption capacity (UE × UTS) increased with increase cold roll deformation. The fracture surface of tensile specimens also showed an increase in toughness due to grain refinement caused by increase applied cold roll
  9. Keywords:
  10. Martensite ; Mechanical Properties ; Microstructure ; Ferrite-Martensite Dual Phase Steels ; Cold Working ; Work Hardening ; Fractography

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