Reduction of Anisotropy in Cold-Rolled Duplex Stainless Steel Sheets by Using Sigma Phase Transformation
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TRODUCTION
DSSS are characterized by having a microstructure consisting of two phases, austenite and ferrite, present in approximately the same volume fractions. The DSS grade EN 1.4462, also called 2205, is nowadays the most widely used DSS grade for numerous applications in different industrial sectors such as marine transportation, petrochemical, pulp and paper, desalination, etc., due to its outstanding combination of mechanical properties and corrosion resistance.[1–6] DSSs are mainly used as cast or flat products, i.e., plates, sheets, and strips.[6] The general industrial sequence for manufacturing rolled DSSs is illustrated in Figure 1. The continuously cast thick (approximately 200 mm) slab is first hot rolled in several passes in order to achieve a large thickness reduction as long as there is not too much austenite present in the DSS. Note that the austenite volume fraction increases with decreasing temperature, inducing an intensification of the forces needed for rolling operations. Afterward, the steel coil is annealed at temperatures in the range of 1283 K to 1353 K (1010 C to 1080 C) to dissolve all precipitates and to recover the ductility for the subsequent cold G. FARGAS, Assistant Professor, and M. ANGLADA and A. MATEO, Professors, are with the Department of Materials Science and Metallurgical Engineering-CIEFMA, Universitat Polite`cnica de Catalunya, 08028 Barcelona, Spain. Contact e-mail: gemma.fargas@ upc.edu N. AKDUT, Professor, is with the International Research & Education Center, Department of Materials Science and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan. Manuscript submitted December 1, 2010. Article published online June 8, 2011 3472—VOLUME 42A, NOVEMBER 2011
rolling down to the required final sheet thickness. The process ends with a last annealing at around 1273 K (1000 C), intended to leave the DSS with the suitable properties for common applications. Through this industrial process, the steel undergoes microstructural and crystallographic changes, which affect also the mechanical properties of the final product. Since during rolling both phases align in the rolling direction, the microstructure becomes morphologically anisotropic and also marked crystallographic textures appear. As a result, the mechanical properties of rolled DSS are strongly direction dependent and their anisotropy is stronger than that observed in single-phase stainless steels.[7–10] This effect tends to rise with increasing rolling reduction, and quite distinct from findings on other commonly used structural materials, DSS flat products have a significantly higher strength in the transverse direction than in the rolling one. Other publications confirm this fact: Hutchinson et al.[8] reported 15 pct higher yield stress for transversal specimens than for longitudinal ones working with 4-mm-thick plates of a DSS 2377, quite similar to DSS 2205; and Mateo et al.[10] found that both yield stress and ultimate tensile strength were 9 pct higher for transversal oriented samples in 5-mm-thick plates of DSS
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