Failure during Sheared Edge Stretching of Dual-Phase Steels
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NTRODUCTION
DUAL-PHASE
sheet steels are being extensively used in automotive applications in order to reduce vehicle mass, which can lead to better fuel economy and lower carbon emissions. One of the limitations in forming these types of advanced high-strength steels (AHSSs) is the fracture that occurs when sheared edges are stretched. A number of investigators have studied aspects of this process. The results of these investigations have identified conflicting results on the factors that contribute to shear edge failures when sheared edges of dual-phase steels are stretched. The intent of the present study is to provide insight on the important steps of the failure process in stretching sheared edges by evaluating a steel with a constant bulk carbon content but processed to different dual-phase microstructures. Analyzing the failure process during sheared edge stretching must consider the literature on strength of phases, fracture in the hard constituent, interface conditions, decohesion between phases, microstructural morphology, and the effect of shearing. The information from the literature review provides the basis for the proposed failure sequence. The data from experimental investigation in this study evaluate the factors that contribute to the failure as identified from the review of the literature. B.S. LEVY, President, is with the B.S. Levy Consultants Ltd, 1700 E. 56th St., Suite 3705, Chicago, IL 60637. M. GIBBS, formerly Graduate Student with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, is now Engineer with the ArcelorMittal, Burns Harbor, IN. C.J. VAN TYNE, FIERF Professor, is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines. Contact e-mail: [email protected] Manuscript submitted December 11, 2012. Article published online April 10, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
A. Hard Constituent and Ferrite Strength Krauss[1] has shown that the strength of martensite is determined by its carbon content and that Vickers hardness is a linear function of carbon content, up to somewhere between 0.3 and 0.5 pct carbon. Thereafter, the slope of the curve gradually decreases. McFarland[2,3] evaluated the stress–strain properties of 100 pct martensite steels with carbon contents ranging from 0.06 to 0.25 pct. For carbon contents ranging between 0.09 and 0.15 pct, McFarland found rapid strain hardening between the proportional limit and the 0.2 pct offset yield strength, followed by strainhardening exponents (n-values) of 0.21/0.22, up to a strain of 0.03. Total elongation failure was ranging from 3 to 4 pct, which indicates fairly low tensile ductility. In relating tensile strength to percent carbon, McFarland[3] combined data from two separate experiments with data from Leslie et al., and a data set that used results from Kurdjumov et al., and Aborn. The equations from each regression were similar, and showed that TS (in ksi) = 120 + 546 9 (wt pct C). These results exhibit excellent agreement with Hasegawa et al
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