Transient Evolution of Inclusions during Al and Ti Additions in Fe-20 Mass pct Cr Alloy

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.

Evolution of Inclusions and Cleanliness in Ti-Bearing IF Steel Produced via the BOF–LF–RH–CC Process

“…Figure 8 shows morphologies of Al 2 O 3 •TiO x inclusions during RH treatment and tundish processes, indicating that there are mainly three types of Al 2 O 3 •TiO x inclusions. The first type is similar to the shape of Al 2 O 3 inclusions, including spherical and polygonal shapes, and the size is usually smaller than 5 µm, as shown in Figure 8a-d Similar morphologies of the first two types of Al 2 O 3 •TiO x inclusions were observed by Dorrer [19], Yang [23], Wang [24], Bai [25] and Yang [26]. The third type of Al 2 O 3 •TiO x inclusions, in which the Al-Ti-O is distributed in the outer part and the Al 2 O 3 is distributed in the inner part, was also reported by Basu [14], Park [20], Wang [27], Doo [28], Qin [29] and Sun [30].…”

“…In addition, the formation of TiOx inclusions is possible at the steel/slag interface based on Reaction (R3). Many scholars [23][24][25][26][27][28][29][30] have studied the formation mechanism of Al2O3•TiOx inclusions, and their studies are good for analyzing the formation mechanisms of various Al2O3•TiOx inclusions. The schematic of the process is depicted in Figure 14.…”

Effect of Yttrium Addition on Nonmetallic Inclusions in FeCrAl Alloys

“…It can be inferred that the high Cr content in FeCrAl alloys greatly suppresses the formation of Al 2 O 3 inclusions. Bai et al [ 29 ] considered that an increase in Cr content reduces the deoxidization ability of Al in stainless steel. Similarly, Ohta et al [ 30 ] investigated the Al‐O equilibrium in Fe‐10, 20, and 40 mass% Cr alloys and found that the O content increased with increasing Cr content at a given Al content.…”

“….41 [29] 0.03 [29] Al 0.025 0.045 0.03 0.033 [3] -6.6 r Al O =0.012, [25] r Al;O O =327 300/T þ 127 [26] r Al Al =-0.001, [3] r O Al =-107 þ 275 000/T, [28] r O;Al Al =47.4 [28] Figure…”

“…Here, a Al 2 O 3 , a Al , and a O represent the activities of Al 2 O 3 , Al, and O, respectively, taking the mass fraction of 1% as the standard state, and Al 2 O 3 is a pure substance with a value of 1. f i represents the activity coefficient of i, which is calculated by Wagner's formalism; [wt%j] is the content of j in the molten alloy; and e j i , r j i , and r ðj,kÞ i represent the firstand second-order interaction coefficients of elements in the molten alloy. [29] considered that an increase in Cr content reduces the deoxidization ability of Al in stainless steel. Similarly, Ohta et al [30] investigated the Al-O equilibrium in Fe-10, 20, and 40 mass% Cr alloys and found that the O content increased with increasing Cr content at a given Al content.…”