Nitrogen Effects in Dual-Phase Sheet Steel
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INTRODUCTION
ADVANCED high strength steels (AHSS), such as dualphase (DP) steel, are increasingly prevalent in automobile manufacturing, due to their favorable combination of high strength and formability, which allows for weight reduction. In order to meet design requirements, manufacturers of DP steel must be able to produce sheet of consistent properties to fit a wide range of applications. Nitrogen was identified as an influential element in AHSS, which can significantly impact mechanical properties.[1–3] The behavior of nitrogen may be influenced by processing details. For example, AlN precipitation is very sensitive to coiling temperature conditions.[4] The sensitivity of nitrogen to processing conditions therefore might be a source of mechanical property variability in sheet production. The scope of this work is to develop an understanding of the behavior of nitrogen in DP sheet steels under different processing conditions with a focus on providing information to optimize nitrogen composition and processing to achieve consistent and beneficial mechanical properties.
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METHODS
A series of alloy variations was created based on a commercial DP800 steel composition (wt pct) 0.095C, 1.98Mn, 0.2Cr, and 0.2Mo, with systematic additions of nitrogen and aluminum to create a range of free nitrogen TYSON W. BROWN, Graduate Student, JOHN G. SPEER and DAVID K. MATLOCK, Professors, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] DENNIS M. HAEZEBROUCK, Senior Research Consultant, is with the United States Steel Research and Technology Center, Munhall, PA 15120. Manuscript submitted February 23, 2011. Article published online August 30, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
and nitrogen-related behaviors, along with residual levels of 0.013 to 0.029Si, 0.0030 to 0.0039S, and 0.0044 to 0.0088P. Five alloys were considered, containing nitrogen levels of 0.003, 0.008, and 0.015 wt pct, with aluminum levels of 0.02 and 0.08 wt pct. Aluminum was selected as a nitrogen stabilizer because of its common application, but also because AlN provides limited contributions to strength when precipitated at high temperature.[4] The measured concentrations of N, Al, and elements that strongly affect nitrogen solubility are shown in Table I, along with descriptive abbreviations for the alloys. As a perspective on the experimental alloy design, Figure 1 shows the solubility product of AlN in austenite (Leslie et al.[5]) plotted at a common hot-strip mill slab reheating soak temperature, 1573 K (1300 °C), and at the planned intercritical annealing temperature, 1033 K (760 °C). Also shown are the Al and N levels in the experimental steels. This figure does not account for the small reduction in soluble Al due to the presence of aluminum oxide, but provides an estimate of the levels of free nitrogen expected if AlN precipitation reached thermodynamic equilibrium. Two coiling temperatures were selected in order to influence the amount of AlN pr
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