Abstract
The transient evolution of inclusions during Al and Ti additions in a Fe-20 mass pct Cr alloy was investigated using polished cross sections and electrolytic extraction. After Al addition, the evolution of Al2O3-based inclusions based upon the area and particle size passed through the following three main stages with time: Particle agglomeration, inclusion floating, and a slow decrease of the remaining Al2O3-based inclusions. Titanium wire was fed into the steel at the end of the floating stage after Al addition when the Ostwald ripening process was finished. Immediately after Ti addition, the transient phase of Ti oxide was readily generated on the existing Al2O3-based inclusion and disappeared due to Al reduction as time progressed. The formation of the transient TiOx phase was affected by the low disregistry between Al2O3 and TiOx and the local Ti supersaturation, which cannot be predicted by the equilibrium relations of Ti–O–N or Ti–Al–O in the high-Cr-containing melt. Because of the local supersaturation of dissolved [%Ti] and [%N] shortly after Ti addition, TiN associated with existing inclusions and three types of individual TiN including single cubes, twinned inclusions, and clusters were identified. In order to minimize the Ti loss caused by the formation of Ti-rich zones during the transient stages, the removal of large Al2O3-based particles including aggregates, clusters, and flower-shaped inclusions should be promoted by stirring before Ti addition. After Ti addition, Brownian and turbulent were the major factors affecting the collision of particles smaller than the threshold of 2.7 μm. The agglomeration of inclusions larger than this threshold was mainly dominated by turbulent and Stokes’ collisions.
Highlights
Aluminum and titanium are both important alloying elements in steels
The main conclusions obtained are as follows: (1) the characteristics of Al2 O3- based inclusions are not new for low-carbon Al-killed steels, the current laboratory experiments identified that the deoxidation products in a liquid Fe-20 mass pct Cr alloy passed through three main stages with time: Particle agglomeration, inclusion floating and a slow decrease of remaining Al2 O3 -based inclusions
(2) After Ti addition, TiOx was immediately generated onto the existing Al2 O3 -based inclusion as a transitional product and thermodynamically reduced into Al2 O3 by Al as time progressed
Summary
Aluminum and titanium are both important alloying elements in steels. Aluminum provides one of the most economical and efficient approaches for deoxidation in steels. Various factors including the sequence of alloy addition [18,20,32,33], titanium source [34], titanium/aluminum ratio [35], and element contents [19,36,37,38,39] have been examined their impacts on the inclusion behaviors during Al and Ti additions, several fundamental questions remain about the evolution mechanism of the inclusion behaviors. Electrolytic extraction was employed to extract intact inclusions from the steel matrix, allowing of inclusion behaviors by taking samples at various times during Al and Ti additions. Electrolytic three-dimensional investigations of inclusion characteristics including composition, morphology, extraction was employed to extract intact inclusions from the steel matrix, allowing three-dimensional and structure during the transient stage after. Agglomeration mechanism of inclusions after Ti addition was discussed in this paper
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