The Experimental Characterization and Numerical Simulation of A-Segregates in 27SiMn Steel
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I.
INTRODUCTION
AS a typical casting defect of steel ingot and casting, macrosegregation cannot be eliminated via the downstream processes such as forging and heat treatment, and hence will cause a low quality, low material utilization and poor quality consistency. Of all types of macrosegregation in steel ingots, A-segregates (or channel segregations) are the most prevalent and notorious ones and often distributed linearly in two sides of the ingot body. To clarify and understand its formation conditions and mechanism, massive efforts, ranging from experimental characterization, theoretical analysis, to numerical simulations have been made over the past decades.[1–5] In terms of the dissection experiments of steel ingots, the previous researches stressed mainly macro-scale observations such as the occurrence of channel or not, and their dimensions, shape, and positions,[3,6–9] but the detailed micro-scale characterization of structure, composition, and defects are rarely involved. On the other hand, due to the rather low level of smelting technology, the impurities of O, S, and P were very high in the earlier steel ingots, and thus the severe A-segregates can be always observed in the dissected surface. They appear to penetrate through the whole ingot body. However, the steelmaking techniques have been improved significantly Y.F. CAO, Y. CHEN, P.X. FU, H.W. LIU, and D.Z. LI are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016 Liaoning, P.R. China. e-mail: [email protected] Manuscript submiited September 28, 2016. Article published online February 15, 2017 2260—VOLUME 48A, MAY 2017
in recent years, and the contents of O, S, and P can be controlled at an extremely low level via the purifying technology. For instance, based on a large amount of steel ingot dissections and characterization, Li et al.[10] found that when the oxygen content is lower than 0.001 wt pct, the A-segregates in most steels such as the 1045 steel can be reduced obviously and even eliminated regardless of the ingot weight and size. Further, they observed that, besides the enrichment of the main solute elements, there exists obvious aggregation of inclusions (such as Al2O3 and MnS) and even cavities in A-segregates. Hence, a new driving force of channel segregation formation in steels, inclusion flotation, has been proposed based on the extensive experimental analysis and multi-scale simulations.[10] Recently, via the comprehensive characterization of macrosegregation and microstructure evolutions in steels, Pickering et al.[11,12] provided a thorough study on the structure and composition of A-segregates in SA508 Grade 3 pressure-vessel steel, and high C and Si steel. Obviously, the structure and the composition vary with the different steels, and hence the advanced experimental analysis technologies, such as 3D X-ray microtomography technique,[13] are necessary to characterize the A-segregates in-depth. Besides the experimental methods, the prediction of channel
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