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Inclusion flotation-driven channel segregation in solidifying steels.

Li D, Chen XQ, Fu P, Ma X, Liu H, Chen Y, Cao Y, Luan Y, Li Y - Nat Commun (2014)

Bottom Line: An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels.Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation.This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

View Article: PubMed Central - PubMed

Affiliation: Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.

ABSTRACT
Channel segregation, which is featured by the strip-like shape with compositional variation in cast materials due to density contrast-induced flow during solidification, frequently causes the severe destruction of homogeneity and some fatal damage. An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation. Here we discover a new force of inclusion flotation that drives the occurrence of channel segregation. It originates from oxide-based inclusions (Al2O3/MnS) and their sufficient volume fraction-driven flotation becomes stronger than the traditionally recognized inter-dendritic thermosolutal buoyancy, inducing the destabilization of the mushy zone and dominating the formation of channels. This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

No MeSH data available.


Related in: MedlinePlus

Existence and elimination of CS.The sectioned surfaces of four 0.5-ton ingots have been treated by the VCD technique and cut along the axle plane. (a) Experiment II: CS disappears with T.O=1.0 × 10−3 wt.%. (b) Experiment III: CS has been significantly reduced with T.O=1.5 × 10−3 wt.%. (c) Experiment IV: CS occurred with T.O=2.0 × 10−3 wt.% in the presence of a small amount of OI. (d) Experiment V: CS has disappeared in this ingot poured in the air by argon protection as long as the oxygen concentration has been limited to a low level T.O=0.7 × 10−3 wt.%. The arrows denote the presence of CS.
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f2: Existence and elimination of CS.The sectioned surfaces of four 0.5-ton ingots have been treated by the VCD technique and cut along the axle plane. (a) Experiment II: CS disappears with T.O=1.0 × 10−3 wt.%. (b) Experiment III: CS has been significantly reduced with T.O=1.5 × 10−3 wt.%. (c) Experiment IV: CS occurred with T.O=2.0 × 10−3 wt.% in the presence of a small amount of OI. (d) Experiment V: CS has disappeared in this ingot poured in the air by argon protection as long as the oxygen concentration has been limited to a low level T.O=0.7 × 10−3 wt.%. The arrows denote the presence of CS.

Mentions: To address this mechanism, we have designed five Experiments (I–V), as compiled in Table 1. Substantial differences are observed in the fully dissected, etched longitudinal sections of ingots I and II (Figs 1a and 2a). Experiment I exhibits typical CS with some narrow, vertical and centre-inclined axial-symmetrical strip-like chains (Fig. 1a), whereas the CS disappears in Experiment II (Fig. 2a). Their differences are observed in the employed deoxidation techniques and the pouring methods, which result in a highly distinct oxygen concentration. In the former I, the T.O is 5.6 × 10−3 wt.% with the AD and air pouring treatments, whereas the VCD and vacuum pouring treatments yield T.O=1.0 × 10−3 wt.% in the latter II (Table 1). The analysis reveals that the inclusions enriched in the CS region of the Experiment I (Fig. 1b) are primarily composed of Al2O3, MnS and minor amounts of bubble-like cavities (see Supplementary Figs 10–16 and Supplementary Note 4). A special feature has been regularly observed in the CS region, in which the majority of MnS (Fig. 1d,e) tends to combine with Al2O3 (Fig. 1b) to form the Al2O3/MnS oxide-based inclusions (OIs); they almost have a diameter in the range of ~5~50 μm. The typical morphologies include the MnS-like impurity precipitates surrounding the centred Al2O3 (Fig. 1c). Some main elements are also promoted to segregate, which corresponds to the occurrence of OIs (see Supplementary Table 2). Compared with Experiment I, the amount and size of the OIs, which are dependent on the oxygen concentration, have been significantly reduced in Experiment II (Fig. 2a). Thus, the contrasting experiments between I and II imply that the oxygen concentration seems to be the crucial factor for CS formation.


Inclusion flotation-driven channel segregation in solidifying steels.

Li D, Chen XQ, Fu P, Ma X, Liu H, Chen Y, Cao Y, Luan Y, Li Y - Nat Commun (2014)

Existence and elimination of CS.The sectioned surfaces of four 0.5-ton ingots have been treated by the VCD technique and cut along the axle plane. (a) Experiment II: CS disappears with T.O=1.0 × 10−3 wt.%. (b) Experiment III: CS has been significantly reduced with T.O=1.5 × 10−3 wt.%. (c) Experiment IV: CS occurred with T.O=2.0 × 10−3 wt.% in the presence of a small amount of OI. (d) Experiment V: CS has disappeared in this ingot poured in the air by argon protection as long as the oxygen concentration has been limited to a low level T.O=0.7 × 10−3 wt.%. The arrows denote the presence of CS.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4263320&req=5

f2: Existence and elimination of CS.The sectioned surfaces of four 0.5-ton ingots have been treated by the VCD technique and cut along the axle plane. (a) Experiment II: CS disappears with T.O=1.0 × 10−3 wt.%. (b) Experiment III: CS has been significantly reduced with T.O=1.5 × 10−3 wt.%. (c) Experiment IV: CS occurred with T.O=2.0 × 10−3 wt.% in the presence of a small amount of OI. (d) Experiment V: CS has disappeared in this ingot poured in the air by argon protection as long as the oxygen concentration has been limited to a low level T.O=0.7 × 10−3 wt.%. The arrows denote the presence of CS.
Mentions: To address this mechanism, we have designed five Experiments (I–V), as compiled in Table 1. Substantial differences are observed in the fully dissected, etched longitudinal sections of ingots I and II (Figs 1a and 2a). Experiment I exhibits typical CS with some narrow, vertical and centre-inclined axial-symmetrical strip-like chains (Fig. 1a), whereas the CS disappears in Experiment II (Fig. 2a). Their differences are observed in the employed deoxidation techniques and the pouring methods, which result in a highly distinct oxygen concentration. In the former I, the T.O is 5.6 × 10−3 wt.% with the AD and air pouring treatments, whereas the VCD and vacuum pouring treatments yield T.O=1.0 × 10−3 wt.% in the latter II (Table 1). The analysis reveals that the inclusions enriched in the CS region of the Experiment I (Fig. 1b) are primarily composed of Al2O3, MnS and minor amounts of bubble-like cavities (see Supplementary Figs 10–16 and Supplementary Note 4). A special feature has been regularly observed in the CS region, in which the majority of MnS (Fig. 1d,e) tends to combine with Al2O3 (Fig. 1b) to form the Al2O3/MnS oxide-based inclusions (OIs); they almost have a diameter in the range of ~5~50 μm. The typical morphologies include the MnS-like impurity precipitates surrounding the centred Al2O3 (Fig. 1c). Some main elements are also promoted to segregate, which corresponds to the occurrence of OIs (see Supplementary Table 2). Compared with Experiment I, the amount and size of the OIs, which are dependent on the oxygen concentration, have been significantly reduced in Experiment II (Fig. 2a). Thus, the contrasting experiments between I and II imply that the oxygen concentration seems to be the crucial factor for CS formation.

Bottom Line: An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels.Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation.This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

View Article: PubMed Central - PubMed

Affiliation: Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.

ABSTRACT
Channel segregation, which is featured by the strip-like shape with compositional variation in cast materials due to density contrast-induced flow during solidification, frequently causes the severe destruction of homogeneity and some fatal damage. An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation. Here we discover a new force of inclusion flotation that drives the occurrence of channel segregation. It originates from oxide-based inclusions (Al2O3/MnS) and their sufficient volume fraction-driven flotation becomes stronger than the traditionally recognized inter-dendritic thermosolutal buoyancy, inducing the destabilization of the mushy zone and dominating the formation of channels. This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

No MeSH data available.


Related in: MedlinePlus