Delta-Ferrite Distribution in a Continuous Casting Slab of Fe-Cr-Mn Austenitic Stainless Steel
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THE solidification mode of austenitic stainless steels varies among the F, FA, AF, and A modes.[1] In most austenitic stainless steel grades, the solidification mode is the FA mode, which means ferrite as the primary phase solidified from the liquid metal, and the ferrite is subsequently transformed to austenite. In this case, the austenite is the only stable phase at room temperature. However, due to the non-equilibrium nature of the solidification process, some ferrite remained in the casting products at room temperature, i.e., as residual ferrite, which is crucial for hot rolling and welding. With regard to the welding, the crack sensitivity increases when the residual ferrite fraction is less than 2 pct.[2] However, the hot working performance declines significantly when the residual ferrite fraction is more than 6 pct.[3 5] Thus, to avoid hot cracks in welds, it is necessary to control the ferrite fraction within 3-8 pct in the welds.[6] The ferrite fraction could be controlled by chemical composition design under optimum cooling
CHAO CHEN is with the Department of Metallurgy, Taiyuan University of Technology, Yingze West Street No. 79, Taiyuan 030024, Shanxi Province, P.R. China. Contact e-mail: [email protected] GUOGUANG CHENG is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 100083 Beijing, P.R. China. Manuscript submitted January 15, 2017.
conditions. Since the late 1970s, the delta-ferrite-related issues in castings and continuous casting products have been widely studied and reported.[7–25] The delta-ferrite distributions in continuous casting slabs, billets, horizontal casting, strip casting, etc. are summarized in Table I. The delta-ferrite fractions vary from 0 to 10 pct, and most are in the range of 0 to 5 pct. The distribution of delta-ferrite in the continuous casting slab has been investigated, and the ‘‘M’’ type* distribu*The ‘M’ type refers to the delta-ferrite fractions being lower both at the edge and at the center part in the width (thickness) direction, while the fractions are higher at the 1/4 and 3/4 parts (or near those locations) in the width (thickness) direction. In contrast, the ‘A’ type refers to the delta-ferrite fractions being lower at the edge but highest at the center part in the width direction.
tion has been widely reported. Some of the noteworthy results were obtained from individual figures in technical articles. The articles were focused on stainless steel casting performance, casting machine improvement, etc. rather than on the delta-ferrite distribution in stainless steel castings. A comprehensive study was carried out by Kim et al.[21] They discussed the mechanism of the ‘‘M’’ type distribution and assumed that the segregation at the center could modify the solidification mode. However, Pereira and Beech[10] summarized the factors influencing the delta-ferrite content of cast austenitic stainless steels, and they found that the chemical composition and cooling rate are key factors. In addition, as summarized in Table
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