The evolution of inclusions with Ce addition and Ca treatment in Al-killed steel during RH refining process was investigated through experimental observations and thermodynamic calculations. The results indicated that the typical inclusions before Ce addition are CaO-Al 2 O 3 inclusions, which were a liquid state during RH refining. After Ce addition, the typical inclusions was transformed from calcium aluminate inclusion to (Ca-Ce-SO) + (Ce-Al-Ca-O) complex inclusion. After Ca treatment, the types and morphologies of typical inclusions in steel did not change. Experimental observation and thermodynamic calculations shown that a certain amount of Ca addition can't affect the formation of Ce-containing inclusion, which may indicate that Ca treatment should not be carried out for rare earth treated steel.
The mechanism of inclusion evolution after rare earth addition based on oxide metallurgy was investigated experimentally and using thermodynamic calculations, where Ce-La was added to Al-killed high strength steel during Ruhrstahl-Heraeus refining to modify the oxide inclusions within the steel. The typical inclusions observed before Ce-La addition were spherical magnesium aluminate spinel inclusions. And fewer individual Al2O3 inclusions and Al2O3–TiOx inclusions were also observed. The addition of Ce-La led to transformation of MgO · Al2O3 spinel inclusions to (Ce,La)2O3, (Ce,La)2O2S and (Ce,La)2O2S + MgO · Al2O3 inclusions. Thermodynamic calculations indicated that Ce-La combined with dissolved oxygen and sulfur in molten steel to form rare earth inclusions, while the remainder of the Ce and La modified MgO · Al2O3 to (Ce,La)2O3 and (Ce,La)2O2S.
Oxide metallurgy technology plays an important role in inclusion control and is also applied to improve the weldability of high strength steel. Based on the requirements of the weldability in high strength steel, the influencing factors of weld heat affected zone (HAZ) as well as the development and application status of oxide metallurgy technology are summarized in this review. Moreover, the advantages and difficulties in the application of rare earth (RE) oxide metallurgy technology are analyzed, combined with the performance mechanism of RE and its formation characteristics of fine and high melting point RE inclusions with distribution dispersed in liquid steel. With the weldability diversities of different high strength steels, the research status of weldability of high strength steel with high carbon equivalent and the effects of RE on the microstructure and properties of HAZ are discussed, and some suggestions about further research in the future are proposed.
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