Designing Ferroalloys for Niobium and Titanium Additions to Steel

  • Author / Creator
    Shah, Syed Jawad Ali
  • The use of microalloyed steels have accelerated exponentially in the past few decades due to their better mechanical properties as compared to traditional carbon steel. One of the main factor that contribute to their high strength and toughness values is the presence of uniformly dispersed fine carbide, nitrides and/or carbonitride precipitates. Titanium, niobium and/or vanadium are usually added in form of their respective ferroalloys to form such precipitates. However, if the size of these precipitates is coarser then they adversely effects the mechanical properties. Presence of such coarse niobium- and/or titanium-rich particles have been reported extensively in literature. Very limited understanding is available on the source of these coarse particles in microalloyed steels. Two different school of thoughts exists: one contributing the existence of such particles to undissolved phases from their respective ferroalloys while other to precipitation at high temperatures as a result of segregation.

    In order to understand and compare both ideologies, a need of extensive study of ferroalloys was required. In this thesis both ferroniobium and ferrotitanium are characterized in details in order to identify their phases, especially high melting temperature phases as compared to steelmaking temperature. Differential scanning calorimeter (DSC) is used to study different phase transformations during the course of solidification of ferroniobium alloy in order to better understand its phase evolution mechanism. For first school of thoughts: once the nature of high melting temperature phase(s) of ferroniobium is determined, different steel samples are studied and characterized to relate coarse niobium-rich particles to high melting temperature phase(s). For second school of thought: thermodynamic study of steel system is done both under equilibrium and using Scheil solidification models, in order to check for precipitation at high temperature.
    Although current study supports second school of thought still ternary alloys are made to eliminate the high melting temperature phase(s) of ferroniobium alloy. This is done by addition of aluminum to as-received ferroniobium alloy followed by characterization and phase study. The whole study incorporates the use of different characterization tools including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC).

  • Subjects / Keywords
  • Graduation date
    Fall 2015
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.