LowDensity Steels

Authors: Radhakanta Rana

Publish Date: 2014/08/29

Volume: 66, Issue: 9, Pages: 1730-1733

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Abstract

Furthermore even though automobiles account for a major portion of the economy in the advanced world eg 3–35 of gross domestic product in the USA3 they are also one of the largest sources of greenhouse gas emissions in the world attracting stringent environmental regulations4 These circumstances compel the automotive manufacturers to continuously look for potential avenues that could reduce emissions Lowering the density of steels is therefore considered an effective approach in this endeavor that would allow use of a thicker gauge in automobiles for stiffness while leading to greater fuel efficiency and reducing emissionsThe first information on “lowdensity steels” dates back to as early as the 1930s with Korter and Ton5 reporting on the use of the FeMnAlC system for the first time Subsequently in the 1950s Ham and Carin6 dwelt on the possibility of replacing costly Ni and Cr in stainless steels by cheaper Mn and Al respectively and in the process bringing about advantage in the density too During the 1980s and 1990s Febased aluminides FeAl and Fe3Al were researched thoroughly7 8 9 10 as less expensive replacements for stainless steels and Nibased superalloys while work in earlier mentioned substitution of Ni and Cr in stainless steels continued1112 However serious efforts on understanding and developing lowdensity steels in the context of automotive applications have been seen only recently Academic as well as industrial interest in this area is reflected by a recent publication of a “viewpoint set” in Scripta Materialia 13 and by a number of patents taken out by steel companies14It is in the above backdrop that work on the present topic was undertaken Although several approaches could be considered for lightweighting from a materials perspective such as steelmatrix composites steel foams steelbased laminates etc the present compilation of articles focuses only on the bulk ferrous alloys that are less expensive and are amenable to mass production These bulk iron alloys rely primarily on alloying with a high amount of aluminum considering its high solubility in iron and effectiveness in reducing the density of iron14 However since the FeAl intermetallic alloys have been well documented in the past7 8 9 10 and have not responded favorably to bulk ductilization after extensive work by several groups they are also excluded from the scope of this special topic which primarily focuses on the solid solutioniron alloys As will be observed later a good balance has been maintained in the compilation between overview and review articles and original contributions by academic as well as industrial colleagues from around the globe The articles deal with a host of areas such as fundamentals of processing phase transformations and microstructure development deformation characteristics and mechanisms of strengthening effects of alloying elements thermodynamics of high Alcontaining steels and alloy development A list of the articles published under the current special topic can be found at the end of this commentary highlights from each of the articles are presented belowThe article by Rana et al sets the tone of the article series by providing a comprehensive overview of the different strategies for lightweighting of ferrous materials and presenting briefly the physical metallurgy of bulk iron alloys These lowdensity steels FeAl and FeMnAlC alloys can be ferritic austenitic duplex or triplex alloys depending on the phase constitution and the same alloys can exhibit precipitation strengthening strain aging or various plasticity effects such as transformationinduced plasticity TRIP twinninginduced plasticity TWIP shearbandinduced plasticity SIP and microbandinduced plasticity MBIP It is shown that even with a reduced Young’s modulus due to high Al addition in a stiffnessdominated situation lowdensity steels can still result in weight saving in comparison with highstrength lowalloy HSLA steelsThe fact that Al is actually an “old” alloying element in steels but was not looked into in the past as a densityreducing element has been underlined with some historical perspective in the review prepared by Zuazo et al Among others prediction of phase diagrams on Al addition in the FeC system using CALPHAD led to new thermodynamic database that would provide a better insight into the evolution of complex microstructures in lowdensity steelsA review of the past work on FeMnAlC alloys carried out at Pohang Steel Company POSCO and Pohang Institute of Science and Technology POSTECH both in South Korea can be found in the article authored by Sohn et al They have reported on a thermodynamicsbased alloy design throughprocess microstructural evolution and tensile properties of Alrich and Mnlean duplex lowdensity steels Austenite nucleated from fine and broken κcarbides possessed high mechanical stability causing a very high yield ratio The enhanced ductility of these duplex lowdensity steels at similar strength levels of conventional automotive steels is indeed advantageous

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