Loading...

Texture and Microstructure of Ultra Fine Grained Structure Produced by Accumulative Roll Bonding (ARB) and Warm Rolling Processes in an IF Steel

Kolahi, Alireza | 2009

850 Viewed
  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 39124 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Akbarzadeh, Abbas
  7. Abstract:
  8. Accumulative Roll Bonding (ARB) process has been developed during last decade to produce the Ultra Fine Grained (UFG) structures with high strength. In the present study, a Ti-IF steel was deformed up to equivalent strain of 4.8 at 500 ˚C using ARB process. For comparing the ARB with conventional forming processes, some samples were deformed with similar equivalent strain to ARB conditions at 500 ˚C and 300 ˚C as warm rolling. Moreover, the thermo-mechanical warm rolling was simulated by warm torsion. The ARB and warm rolled samples with largest strains were annealed at 750 ˚C. The microstructure and texture were studied using Electron Back Scattered Diffraction (EBSD) method. The micro hardness and tensile testing were conducted to ARB and warm rolled samples. The results indicated that with increasing the strain, in all processes, the grains refine gradually and strength and hardness increase due to increasing the flow stress by zener-hollomon (Z) parameter. Flow stress and Z parameter enhancement due to increasing strain result in pancaking of the grains and concurrently developing the sub-boundaries (dislocation boundaries). These sub-grains transform to high angle boundaries by lattice rotation and the ultra fine grains appear. There are power law relationships between the grain size and strain, flow stress and Z parameter during deformation. The shear texture (ND║<110>) appears at the surface of ARB samples due to shear strain resulted from rolling without lubrication. On the other hand, rolling fibre texture is observed in the mid-thickness of ARB samples by plane strain rolling. Also in the warm rolled sample, the rolling fibre textures are present due to plane strain rolling condition. The same strain, strain rate and deformation temperature results in similar range of the grain size and misorientations obtained by warm rolling and ARB methods. However, there are some differences between the microstructures in two methods. In spite of severe deformation by ARB, strong RD and ND fibre textures in the warm rolled samples are accompanied by very elongated grains and shear bands in the microstructure, respectively. Stability of shear and rolling fibre orientations during annealing resulted in similar texture gradient before and after annealing in both ARB and warm rolling samples. The warm torsion samples at the same deformation temperature and strain, as opposed to the warm rolling and ARB processes, have higher grain size and more sub-grains (lower misorientation angles). This is attributed to very low strain rate leading to lower flow stress and zener-hollomon parameter
  9. Keywords:
  10. Accumulative Roll Bonding (ARB) ; Titanium Interstitial Free Steel (Ti-IF Steel) ; Electron Back-Scattered Diffraction (EBSD) ; Microstructural Changes ; Rolling Texture

 Digital Object List

  • محتواي پايان نامه
  •   view

 Bookmark

No TOC