Microstructures of hot-rolled high-strength steels with significant differences in edge formability
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I. INTRODUCTION
A MAJOR goal within the automotive industry continues to be the reduction of vehicle weight. Significant research and development efforts have concentrated on the weight reduction associated with cold-rolled sheet-steel products. However, substantial weight reduction can also be realized by the development of more-formable, high-strength, hotrolled sheet steels for applications including suspension systems and wheels.[1–13] Additional benefits associated with some of the newest hot-rolled steels are a comparatively low cost of alloying and improved product consistency.[13] Research of steels for wheel-disk applications has emphasized different aspects of formability. Hilsen et al.[1] described bulk formability* as the “ability of a steel to be *In a recent handbook,[14] the terms “bulk formability” and “sheet formability” were used to describe two different types of formability. In the remainder of this article, the latter term will be used to describe a material’s response to stamping and similar forming processes.
stamped into a part without failure occurring within the body of the part from exhaustion of ductility.” They noted that determination of this property is achieved with cup-drawing experiments, forming-limit diagrams, and measures of ductility from conventional tensile tests. Edge formability was described as the “ability of the steel to be stamped into a R.D.K. MISRA, Stuller Endowed Chair in Metallurgy and Professor, is with the Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70504. S.W. THOMPSON, Associate Professor, is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. T.A. HYLTON, Director-Quality Control and Technical Services, Pipe and Conduit, is with LTV-Copperweld Tubular Products Company, Independence, OH 44131. A.J. BOUCEK, Manager-High Strength Steels, is with LTV Steel, Technology Center, OH 44131. At the time the work was carried out R.D.K. Misra was at LTV Steel, Technology Center. Manuscript submitted October 19, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
part without failure by fracture or excessive thinning at a sheared edge or hole.”[1] In this case, property evaluation is achieved with various bend tests and hole-expansion tests. Various approaches to hole-expansion testing have been utilized;[1,3–13, 15–19] however, the essence of the test is to determine the capability of a material to avoid failure during the extension of a sheared edge. Two factors that have a major influence on hole expansion in steel sheets are nonmetallic inclusions and the condition of the hole edge prior to hole-expansion testing.[1–4,15–17] Specifically, hole-expansion performance is reduced by the presence of elongated manganese sulfide inclusions and burrs associated with severely cold-worked material at the edge of a hole. These negative contributions can be minimized by sulfide-shape control and an intermediate hole-milling process prior to testing, respectively. Several other factors
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