Enhanced Mechanical Properties of a Novel High-Nitrogen Cr-Mn-Ni-Si Austenitic Stainless Steel via TWIP/TRIP Effects
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INTRODUCTION
AUSTENITIC stainless steels have been increasingly used in the last 10 years.[1] These high-performance steels are indispensable in many diverse fields of chemical, marine, and nuclear industries since they conjugate good mechanical properties and excellent corrosion and oxidation resistance.[2] The fcc austenite structure imparts to the steel greater toughness and formability compared to most common bcc ferritic steels at room temperature. Austenitic steels do not show the ductile-to-brittle transition that ferritic steels show. There has always been a considerable interest in developing low-cost austenitic stainless steels with similar or improved properties, for instance, replacing nickel with other cheaper alloying elements.[3–7] In this search for new high-performance austenitic stainless steels with reduced amounts of nickel, manganese is generally considered as the obvious replacement element. However, it is not simply possible to replace nickel by equal amounts of manganese since this element is not as strong an austenite former. Interstitial elements such as carbon or nitrogen must be added to assist in M. POZUELO, Postdoctoral Student, formerly with the Department of Materials Technology, Max Planck Institut fu¨r Eisenforschung, Max Planck Strasse 1, is with the Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095-1595. G. FROMMEYER, Professor, is with the Department of Materials Technology, Max Planck Institut fu¨r Eisenforschung, Max Planck Strasse 1, D-04237 Du¨sseldorf, Germany. Contact e-mail: [email protected] J.E. WITTIG, Professor, is with the Department of Materials Science and Engineering, Vanderbilt University, Nashville, TN 37232. J.A. JIME´NEZ, Postdoctoral Student, is with the Departamento de Metalurgia Fı´ sica, CENIM, CSIC, 28040 Madrid, Spain. Manuscript submitted November 12, 2008. Article published online June 20, 2009 1826—VOLUME 40A, AUGUST 2009
stabilizing the austenitic structure. It should be noted that a decrease in the Ni content will diminish the corrosion resistance in chloride and flouride containing aqueous solutions.[8,9] In the past 30 years, several austentic stainless steel grades with low or even without nickel content based on the system Fe-Cr-Mn-N have been developed. These grades include the so-called 200 series.[10] The austenitic grades completely free of nickel have compositions of the type 17 to 18 pct Cr, 12 to 15 pct Mn, 0.4 to 0.5 pct N, and 0.1 pct C. In general, these steels exhibit at room temperature superior strength, work hardening, and similar ductility compared to Cr-Ni stainless steels.[11–13] However, most of nickel free stainless steels show a strong increase in yield strength and a severe loss in ductility associated with precipitations of nitrides or carbonitrides during an annealing treatment at temperatures between 600 °C and 900 °C.[14,15] Thus, the nitrogen content should be reduced to a lower level of
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