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Hot Workability of a Free-cutting Steel with Severe Sulfur Segregation During Continuous Casting

Naghdy, Soroosh | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42754 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Akbarzadeh, Abbas
  7. Abstract:
  8. Splitting in ingot cast structures and transverse cracks in continuous cast structures are the main problems of hot rolling of low carbon resulfurized free-cutting (LCRF) steels. Presence of high volume fraction of manganese sulfide inclusions in cast structure increases the risk of alligatoring in hot rolling. Because of high sulfur content of these steels and probability of formation of low melting point phases, minimum level of manganese and maximum level of copper and tin is necessary. Morphology of manganese sulfide is another important factor in hot forming of these steels, which can be controlled by level of deoxidation in steel making. In fact, MnS2 is present in fully killed steels and MnS1 in semi killed steels. In this research, hot workability of LCRF steels in cast and worked structures is investigated using hot compression and hot tensile tests. Results show that MnS2 inclusions have more activation energy (330 kJ/mol) than MnS1 (315 kJ/mol) in hot deformation. Due to more plasticity of MnS2 inclusions, anisotropy in rolling and transverse directions increases and probability of occurrence of alligatoring in last passes of rolling is high. Deformation of sulfide inclusions show that MnS1 at 6.4 effective strain will resist against deformation, but MnS2 will disperse in the matrix. According to the benefits of bigger sizes of sulfide inclusions in machinability, presence of MnS1 is more preferabale than MnS2. Then, with cleaner steel making process and using minimum level of manganese, less anisotropy and more workability of the MnS1 inclusions, can be achieved
  9. Keywords:
  10. Hot Workability ; Dynamic Recrystallization ; Low Carbon Resulfurized Free Cutting Steel (LCRF) ; Manganese Sulfide (II) ; Manganese Sulfide (IV)

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