The strain dependence of postdynamic recrystallization in 304 H stainless steel
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lity production of 304 stainless steel requires a detailed understanding of the softening kinetics during interpass times; this can help to ensure accurate thickness and microstructure control. Both dynamic and static recrystallization can occur during and after deformation, respectively. Once the strain exceeds a certain critical value (ec), dynamic recrystallization (DRX) is initiated, which is subsequently followed by both static recrystallization (SRX) and metadynamic recrystallization (MDRX).[1–7] The observations by Djaic and Jonas on high carbon steel[8] as well as by Cho et al. on Nb microalloyed steel[9] indicate that abrupt changes in recrystallization time can take place at strains of about ep (the peak strain) that involve a transition from strain dependence to strain independence. These abrupt changes are associated with MDRX becoming the dominant softening process. Uranga et al.[10] have studied this transition strain and reported that the strainindependent range begins at about e* 5 1.7ep in a Nb microalloyed steel. Zahiri et al.[11] have noted that the critical strain e* in IF steels is just greater than ep and that the value depends on that of the Zener-Hollomon parameter Z. They showed that the critical strain e* fell in the range 0.6 to 1.5 when 9 3 1012 # Z # 1014 s"1. There is no currently available information regarding the value of e* in 304 stainless steels. The objective of this study was therefore to analyze the interpass softening behavior of 304 H stainless steel in the DRX 1 MDRX range and then to identify the critical strains at which the rate of MDRX becomes strain-independent. In this way, characteristics associated with e* could also be clarified. A. NAJAFIZADEH, Professor, is with the Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran. Contact e-mail: abbas.najafi[email protected] J.J. JONAS, Professor, and G.R. STEWART, Postdoctoral Fellow, are with the Department of Metallurgical Engineering, McGill University, 3610 University Street, Montre´al, Canada H3A 2B2. E.I. POLIAK, Senior Researcher, is with Research Laboratories, Mittal Steel Co., East Chicago, IN, U.S.A. Manuscript submitted August 30, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
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MATERIALS AND EXPERIMENTAL PROCEDURE
The chemical composition of the 304 H stainless steel used in this work is given in Table I. This material was supplied in the form of a hot-rolled bar with a diameter of 7.94 mm. Cylindrical samples 7.9 mm in diameter and 11.5 mm in height were prepared with their axes aligned along the rolling direction. One- and two-hit hot compression tests were carried out on a computer-controlled servohydraulic MTS machine equipped with a radiant furnace. The MTS hot compression machine was programmed to operate at constant true strain rate by incremental calculation of the current true strain. The samples were deformed in an argon atmosphere after the specimens were preheated at 1200 °C for 15 minutes. They were then cooled to the test temperature at 1 °C/second and held for 5 m
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