A New Paradigm for Designing High-Fracture-Energy Steels
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major goal in the development of steels for structural and other applications is increased strength without sacrificing ductility and toughness. Most highstrength steels are martensitic. The strength of martensitic steels increases with carbon content. A high carbon content, however, leads to poor weldability as a result of the formation of a brittle heat-affected zone adjacent to the weld. One can overcome this problem by using steels with a low carbon content and by enhancing the strength with precipitates. This was the basis for the development of HSLA-80 and HSLA-100 Cu-precipitationstrengthened steels,[1–6] which now are used in Naval applications, mining and dredging equipment, heavy-duty truck frames, and are beginning to be used in bridge applications. More recently, Cu-alloyed steels also are being studied as potential high-strength, highformability steels.[7] Depending on the strength and toughness requirements for the specific application, Cu-precipitation-strengthened steels may be supplied M.E. FINE and Y.-W. CHUNG, Professors, and S. VAYNMAN and D. ISHEIM, Research Professors, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. Contact e-mail: m-fi[email protected] S.P. BHAT, Principal Research Engineer, is with ArcelorMittal Global Research and Development, East Chicago, IN 46132. C.H. HAHIN, Bridge Engineer, is with the Illinois Department of Transportation, Springfield, IL 62764. Manuscript submitted June 17, 2010. Article published online October 5, 2010 3318—VOLUME 41A, DECEMBER 2010
in various conditions, including as-rolled, as-rolled and aged, normalized and aged, or quenched and aged. All conditions take advantage of copper precipitation to achieve the combination of improved strength and toughness. The primary purpose of this study is to provide a new insight into the underlying mechanisms at play in achieving a high-impact fracture energy at low temperatures in Cu-bearing low-carbon steels. It has been known since the 1930s that the addition of Cu to steels leads to precipitation strengthening.[8] Over the years, the topic has been studied extensively.[9–14] Figure 1, extracted from the studies of Lahiri et al.,[11] shows the results for a binary Fe-1.67 at. pct Cu. After solution treatment at 1273 K (1000 °C) and aging at 773 K (500 °C), the flow stress at 0.2 pct strain increases rapidly with little or no incubation time, but the change in Young’s modulus seems to require an incubation period of approximately 30 minutes. The body-centered cubic (bcc) Cu-Fe precipitation thus seems to occur in two stages. The initial stage does not lead to a change in Young’s modulus. The aging of Cu-containing steels occurs in stages, beginning first as Cu clustering in the matrix, followed by bcc Cu alloy precipitates, transitioning into the 9R structure, and finally face-centeredcubic Cu precipitates. Using field ion microscopy and atom probe, Goodman et al.[15] observed the formation of coherent, coplanar bcc Cu precipitates that contain a substantial a
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