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THE demerit of annealed austenitic stainless steels is that their yield strength (Rp0.2) is quite low, e.g., 230 to 260 MPa for AISI 304 type, and 350 to 380 MPa for AISI 301LN. Tensile strength (Rm) is in the range of 600 to 800 MPa and total elongation from 45 to 60 pct. An interest in improving the mechanical properties of common austenitic stainless steels so that they could be better used, e.g., in structural applications and automotive safety components, has led to the development of new processing routes.[1,2] As is well known, nitrogen alloying is quite beneficial in increasing the strength by the solid-solution strengthening.[3] However, there is a M.C. SOMANI, Senior Researcher, P. JUNTUNEN, Researcher, and L.P. KARJALAINEN, Professor, are with the Materials Engineering Laboratory, Department of Mechanical Engineering, University of Oulu, 90014 Oulu, Finland. R.D.K. MISRA, Professor of Materials Science and Engineering and Director, is with Center for Structural and Functional Materials, University of Louisiana at Lafayette, LA 70504-4130. Contact e-mail: [email protected] A. KYRO¨LA¨INEN, General Manager, is with the Raahe Region Technology Center Ltd., 92100 Raahe, Finland. Manuscript submitted April 24, 2008. Article published online January 13, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

low limit for nitrogen solubility in conventional grades and problems with hot ductility may also appear with increased nitrogen content. Cold rolling is a technique applied industrially to increase the strength. The yield strength of austenitic stainless steels, and particularly that of metastable alloys, can be drastically improved by cold deformation that generates strain-induced martensite in addition to ordinary strain hardening of highalloyed austenite. However, ductility is consequently decreased. The strength levels of commercial temperrolled grades are classified in the design manual for structural stainless steel.[4] Typical reported values for C850 (1/4 Hard) grade are Rp0.2
650 MPa, Rm
970 MPa, and the total elongation A80
30 pct.[5] Partial annealing of cold-rolled structure has been found to enhance combination of strength and ductility for AISI 304, Rp0.2
450 MPa and 40 pct elongation.[6] A special process of thermomechanical treatment consisting of a double cold-deformation annealing cycle for a 10Cr-5Ni-8Mn steel has resulted in a combination of Rp0.2
780 MPa, Rm
1100 MPa, and 30 pct elongation, based on nanosized grain structure.[7] In this context, a novel processing route of controlled reversion annealing of the heavily cold-deformed

VOLUME 40A, MARCH 2009—729

martensite in metastable Cr-Ni austenitic stainless steels has been employed resulting in highly refined austenite grain size.[8,9] In the first stage, the deformation of austenite in the vicinity of room temperature (RT) leads to straininduced shear transformation of austenite to martensite, and then upon annealing this heavily deformed martensite transforms back to fine-grained austenite either through a martensitic shea