Effect of recycling on residuals, processing, and properties of carbon and low-alloy steels
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I.
INTRODUCTION
THE long-standing
concern of metallurgists and metal users with residual elements in steel ] stems from the effects of residuals on processing and product as well as from a desire for consistency in such properties. The present interest2'3 is intensified by: (a) the exponential increase in the use of scrap-intensive electric furnace steelmaking (Figure 1), (b) the potential for recycling substantial amounts of the ferrous fraction of municipal solid wastes,4'5 and (c) the increasing importance of so-called nonstandard properties, such as temper embrittlement, that may be influenced by residuals. 3 The purpose of this paper is to examine the relationships between: (1) recycling and residuals as well as the trends in both, (2) residuals and processing phenomena, i.e., hot shortness and adherent scale, and (3) residuals and product properties of steel. The paper focuses on carbon and lowalloy steels, mainly in the as-rolled, normalized, or annealed conditions. These types of steels are produced in very large tonnages and consume large quantities of scrap.
II.
to relatively high levels with continued recycling, and (b) significantly affect certain critical properties of steel at residual concentrations of a few hundredths to a few tenths of a percent. An initial screening of the periodic table and literature yielded nine elements that are difficult to remove in steelmaking and that many metallurgists consider to be residuals, namely, Cu, Ni, Cr, Mo, Sn, As, Co, Sb, and W. 2'6-1~Although A1, Pb, Nb, Ti, V, Zn, Zr, and the rare earths may cause problems in steelmaking, they are not considered as residuals for purposes of this paper since they partition to the dust, slag, or insoluble molten Pb pool rather than to the steel. 2'9
B. Recycling Residual elements are introduced into steel as impurities in the ore, flux, and coke, and as contaminants and alloying elements in the scrap. It is generally agreed that scrap is the major source of residuals, s'H Duckett 12considered a simplified iterative model of recycling. Assuming that steel products are repeatedly recovered
RESIDUALS AND RECYCLING
Before considering processing and properties, we will more precisely define and identify residual elements, consider the relationship between the recycling of scrap and the levels of residual elements, and lastly the levels, sources, and trends in residuals.
A. Definition and Identification of Residual Elements Although residual elements have been variously defined, 3'6-8 in this paper they will include those metallic and metalloid elements that: (a) are not efficiently removed from the liquid metal in steelmaking and may therefore build up
E.T. STEPHENSON is Senior Scientist, Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016. This paper is based on a presentation made at a symposium on "Metallurgical Implications of Recycling" held at the Louisville meeting of The Metallurgical Society of AIME, October 14, 1981, under the sponsorship of the TMS Non-Ferrous Metals Committee.
METALLURGICALT
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