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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

Nucleation of OIs and their inclusion-driven flotation.(a) The first principles calculations simulated the nucleating process Mn+nS clusters by trapping S and Mn ions on the Al2O3 (0001) surface. The upper panels denote the local geometric structural details for the Mn or Mn+nS adsorptions. Blue and yellow balls denote the Mn ions and S ions, respectively. Depending on the S introduction to the Mn+nS atom complex, the bonding length between Mn and the nearest neighbouring O1 atoms has been slightly reduced from n=0 to n=2 and is subsequently increased for the cases of n=3 and 4. This feature corresponds to the stable adsorption for n=0, 1 and 2 due to the enhanced attraction between the trapped Mn atoms and the trapped O1 atoms on the surface. The increasing distance for the n=3 and n=4 cases reveals the weakening of the adsorption. (b) The adsorption energies of the Mn+nS atom complex on the surface as a function of the trapped S ions. (c) The binding energies have been obtained from the Mn+nS complex with respect to the extra-S atom and the already formed Mn+(n−1)S complex on the Al2O3 (0001) surface. (d) The simulated CS of Fe-0.36 wt.% C steel coupled with the flotation of the initial 500 alumina particles (OIs) with a diameter of 15 μm is unidirectionally solidified in a cavity (100 × 60 mm2). (e) Detailed information of the liquid flow patterns in the mushy zone and the distribution of solid particles, which demonstrates that the floating particles perturb the flow field by accelerating the local flow velocity and altering its direction. The isolines of the solid fraction are also superimposed in the figure. The legend on the right shows the number of particles n0.
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f3: Nucleation of OIs and their inclusion-driven flotation.(a) The first principles calculations simulated the nucleating process Mn+nS clusters by trapping S and Mn ions on the Al2O3 (0001) surface. The upper panels denote the local geometric structural details for the Mn or Mn+nS adsorptions. Blue and yellow balls denote the Mn ions and S ions, respectively. Depending on the S introduction to the Mn+nS atom complex, the bonding length between Mn and the nearest neighbouring O1 atoms has been slightly reduced from n=0 to n=2 and is subsequently increased for the cases of n=3 and 4. This feature corresponds to the stable adsorption for n=0, 1 and 2 due to the enhanced attraction between the trapped Mn atoms and the trapped O1 atoms on the surface. The increasing distance for the n=3 and n=4 cases reveals the weakening of the adsorption. (b) The adsorption energies of the Mn+nS atom complex on the surface as a function of the trapped S ions. (c) The binding energies have been obtained from the Mn+nS complex with respect to the extra-S atom and the already formed Mn+(n−1)S complex on the Al2O3 (0001) surface. (d) The simulated CS of Fe-0.36 wt.% C steel coupled with the flotation of the initial 500 alumina particles (OIs) with a diameter of 15 μm is unidirectionally solidified in a cavity (100 × 60 mm2). (e) Detailed information of the liquid flow patterns in the mushy zone and the distribution of solid particles, which demonstrates that the floating particles perturb the flow field by accelerating the local flow velocity and altering its direction. The isolines of the solid fraction are also superimposed in the figure. The legend on the right shows the number of particles n0.

Mentions: As the dissolved oxygen concentration is ≪1.0 × 10−3 wt.% in steel melt treated by the AD technique, the Al2O3 in the CS would most likely not form via the chemical reaction between aluminium and oxygen during the solidification. In the refining process, the high melting point oxide of Al2O3 (>2,000 °C) is unavoidably formed in traditionally metallurgical practices. If the AD technique is adopted in the final stage of the refining process, a certain amount of Al2O3 particles with diameters ≪10 μm should not float rapidly, according to the Stokes law, which dictates that the floating velocity is proportional to the square of the particle’s diameters. Thus, these small particles will be buried into melts. Owing to the weak wettability of Al2O3 particles during the solidification process, they would not only be inclined to agglomerate together but would also adsorb the surrounding S, Mn or other ions to form a larger OI (Fig. 1b) and become more buoyant. While the inter-dendritic microsegregation certainly enhances these adsorptions, their behaviours would also be chemically evidenced by our first-principles calculations. Owing to the strong electronic hybridizations among S and its nearest neighbours O and Al on the surface, α-Al2O3 favourably traps the free S ion and binds with Mn ion to initially nucleate the MnS-like cluster (Fig. 3a). As shown in Fig. 3b, the calculations also indicate that, as the number of S ions trapped by one absorbed Mn ion on the surface increases, the adsorption energies (Fig. 3b) and the binding energies (Fig. 3c) of the Mn-nS-like clusters weaken. Similar behaviours have been observed for the adsorption of additional Mn ions. The calculations highlight the following trend: as nucleated Mn+nS-like clusters coarsen to the crystalline phase on the surface, their interface binding energies with the substrate of Al2O3 weaken, which eventually causes their separation (see Supplementary Note 5 and Supplementary Figs 17–20). This situation is experimentally identified in the CS strips (see Supplementary Fig. 10).


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)

Nucleation of OIs and their inclusion-driven flotation.(a) The first principles calculations simulated the nucleating process Mn+nS clusters by trapping S and Mn ions on the Al2O3 (0001) surface. The upper panels denote the local geometric structural details for the Mn or Mn+nS adsorptions. Blue and yellow balls denote the Mn ions and S ions, respectively. Depending on the S introduction to the Mn+nS atom complex, the bonding length between Mn and the nearest neighbouring O1 atoms has been slightly reduced from n=0 to n=2 and is subsequently increased for the cases of n=3 and 4. This feature corresponds to the stable adsorption for n=0, 1 and 2 due to the enhanced attraction between the trapped Mn atoms and the trapped O1 atoms on the surface. The increasing distance for the n=3 and n=4 cases reveals the weakening of the adsorption. (b) The adsorption energies of the Mn+nS atom complex on the surface as a function of the trapped S ions. (c) The binding energies have been obtained from the Mn+nS complex with respect to the extra-S atom and the already formed Mn+(n−1)S complex on the Al2O3 (0001) surface. (d) The simulated CS of Fe-0.36 wt.% C steel coupled with the flotation of the initial 500 alumina particles (OIs) with a diameter of 15 μm is unidirectionally solidified in a cavity (100 × 60 mm2). (e) Detailed information of the liquid flow patterns in the mushy zone and the distribution of solid particles, which demonstrates that the floating particles perturb the flow field by accelerating the local flow velocity and altering its direction. The isolines of the solid fraction are also superimposed in the figure. The legend on the right shows the number of particles n0.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Nucleation of OIs and their inclusion-driven flotation.(a) The first principles calculations simulated the nucleating process Mn+nS clusters by trapping S and Mn ions on the Al2O3 (0001) surface. The upper panels denote the local geometric structural details for the Mn or Mn+nS adsorptions. Blue and yellow balls denote the Mn ions and S ions, respectively. Depending on the S introduction to the Mn+nS atom complex, the bonding length between Mn and the nearest neighbouring O1 atoms has been slightly reduced from n=0 to n=2 and is subsequently increased for the cases of n=3 and 4. This feature corresponds to the stable adsorption for n=0, 1 and 2 due to the enhanced attraction between the trapped Mn atoms and the trapped O1 atoms on the surface. The increasing distance for the n=3 and n=4 cases reveals the weakening of the adsorption. (b) The adsorption energies of the Mn+nS atom complex on the surface as a function of the trapped S ions. (c) The binding energies have been obtained from the Mn+nS complex with respect to the extra-S atom and the already formed Mn+(n−1)S complex on the Al2O3 (0001) surface. (d) The simulated CS of Fe-0.36 wt.% C steel coupled with the flotation of the initial 500 alumina particles (OIs) with a diameter of 15 μm is unidirectionally solidified in a cavity (100 × 60 mm2). (e) Detailed information of the liquid flow patterns in the mushy zone and the distribution of solid particles, which demonstrates that the floating particles perturb the flow field by accelerating the local flow velocity and altering its direction. The isolines of the solid fraction are also superimposed in the figure. The legend on the right shows the number of particles n0.
Mentions: As the dissolved oxygen concentration is ≪1.0 × 10−3 wt.% in steel melt treated by the AD technique, the Al2O3 in the CS would most likely not form via the chemical reaction between aluminium and oxygen during the solidification. In the refining process, the high melting point oxide of Al2O3 (>2,000 °C) is unavoidably formed in traditionally metallurgical practices. If the AD technique is adopted in the final stage of the refining process, a certain amount of Al2O3 particles with diameters ≪10 μm should not float rapidly, according to the Stokes law, which dictates that the floating velocity is proportional to the square of the particle’s diameters. Thus, these small particles will be buried into melts. Owing to the weak wettability of Al2O3 particles during the solidification process, they would not only be inclined to agglomerate together but would also adsorb the surrounding S, Mn or other ions to form a larger OI (Fig. 1b) and become more buoyant. While the inter-dendritic microsegregation certainly enhances these adsorptions, their behaviours would also be chemically evidenced by our first-principles calculations. Owing to the strong electronic hybridizations among S and its nearest neighbours O and Al on the surface, α-Al2O3 favourably traps the free S ion and binds with Mn ion to initially nucleate the MnS-like cluster (Fig. 3a). As shown in Fig. 3b, the calculations also indicate that, as the number of S ions trapped by one absorbed Mn ion on the surface increases, the adsorption energies (Fig. 3b) and the binding energies (Fig. 3c) of the Mn-nS-like clusters weaken. Similar behaviours have been observed for the adsorption of additional Mn ions. The calculations highlight the following trend: as nucleated Mn+nS-like clusters coarsen to the crystalline phase on the surface, their interface binding energies with the substrate of Al2O3 weaken, which eventually causes their separation (see Supplementary Note 5 and Supplementary Figs 17–20). This situation is experimentally identified in the CS strips (see Supplementary Fig. 10).

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