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Effect of Cold Rolling and Intercritical Annealing Temperature of TRIP Steel Sheet on Martensitic Transtormation during Retained Austenite Deformation

Shayanfar, Navid | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 43763 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Akbarzadeh, Abbas
  7. Abstract:
  8. TRIP steel with good plasticity properties, has ferrite, bainite and retained austenite phases at ambient temperature. According to austenite to martensite phase transformation during plastic deformation, this grade of steel has high elongation and ultimate tensile strength, simultaneously. There are some parameters which affect the stability of retained austenite, such as: imposed strain, strain rate, temperature of deformation, grain size, weight percentage of different alloy elements and crystallographic orientation of existing phases. Mechanical properties of TRIP steels are improved by increasing stability of retained austenite. TRIP steels have been considered a Jot, in automotive industry due to their enhanced mechanical properties. The aim of this research is studying the effect of cold rolling and temperature of intercritical annealing on mechanical and physical properties of this steel, with a specified chemical composition. Thus, hot and then cold rolling, annealing and finally tensile test has been applied. Also, micro structural studies have been done with optic and electronic microscopes. For a better investigation on stability of retained austenite, XRD test has been applied. The results indicate that middle annealing temperature causes to the most stable retained austenite. This temperature is the average of AC 1 and AC3 • At this temperature,
    volume percentage of high temperature austenite, in intercritical region, is more than AC 1 , on

    the other hand , stability of this austenite is more than the one which forms at higher temperature near AC3. Results of tensile tests, which had been performed in 3 directions : RD, ND and 45° with rolling direction, has been analyzed. Yield strength, ultimate tensile strength, elongation, stretcher strain and work hardening rate have been calculated. As intercritical annealing temperature increases, yield strength and elongation decrease, stretcher strain increases and UTS increases followed by a decrease . According to retained austenite volume fraction this behavior can be justified
  9. Keywords:
  10. Retained Austenite ; Cold Rolled Sheet ; Transformation Induced Plasticity (TRIP)Steel ; X Ray Diffraction ; Interciritical Annealing

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