Time-Dependent Evolution of Ti-Bearing Oxide Inclusions During Isothermal Holding at 1573 K (1300 °C)

Ti-bearing oxide inclusions are utilized to enhance the properties of steel used in shipbuilding, multi-purpose gas carriers, and pipelines in accordance with the principles of oxide metallurgy. Conventionally, Ti-bearing oxide inclusions have been assumed to be stable during isothermal holding. The authors propose the possibility of modifying Ti-bearing oxide inclusions by appropriate heat treatment. To provide fundamental information on the evolution of Ti-bearing inclusions in solid steel during isothermal holding, the effect of the isothermal holding time on their evolution at 1573 K (1300 °C) was studied systematically. For heterogeneous phase Al+Ti oxide in the as-cast alloys, the composition and size distribution were maintained during isothermal holding. In contrast, homogeneous phase Al-Ti oxide in the as-cast alloys changed to a heterogeneous oxide consisting of an Al-rich part and a Ti-rich part and its shape changed from spherical to irregular. The composition of the Al-Ti oxide changed upon isothermal holding for 0.5 hour, and it occurred earlier in the Al-Ti oxide with a higher Al content. The evolution mechanism is considered to involve the precipitation of Al₂O₃ from the Al-Ti oxide owing to crystallization of glassy oxide during isothermal holding. The results challenge the long-held opinion that Ti-bearing oxide inclusions are maintained in solid steel during isothermal holding, which is the first step toward modifying oxide inclusions by heat treatment as a new concept in oxide metallurgy.