cast steel product, different alloying combinations were required in order to meet the mechanical properties. This signaled the age of microalloying and high strength low alloy steel production. Today, EAF melting accounts for nearly 70% of all steel production in the U.S. The microalloyed steels are known
by a number of names, including AHSS, UHSS, TRIP and Dual Ten, and other specialty alloyed steels, such as Cor Ten, also are available. The alloy levels can vary by alloying element(s) and concentration (Table 1). In addition to residual levels, higher
This steel scrap fl ow chart indicates how the material progresses from various scrap sources to your metalcasting facility.
acceptable criteria for steel scrap. For example, the specifi cation may dictate the exclusion of heavily painted steel scrap. Once the metalcasting facility and the supplier are in agreement, the metalcaster should employ a procedure ensuring compliance to the specifi ca- tion by the supplier. It is also suggested the metalcaster
keep the supplier informed of future needs for material. The supplier/pro- cessor of the scrap steel (and other consumables and supplies) possesses a fi nite supply of product. In the case of steel scrap, suffi cient material must be secured and processed in order to meet specifi cations.
Residual Element Levels and Coatings Residual elements in charge materi-
als can include intentional additions of alloying elements (manganese, chromium, molybdenum, nickel), as well as tramp elements. Many of these elements do not pose any metallurgi- cal risk, as long as allowable analytical levels are maintained. Historically, residual elements con-
tained in returns are well known, as the metalcasting facility typically analyzes the metal before pouring. If alloying or re- sidual elements that pose a risk for future heats are present in returns, these returns should be segregated for use in heats that require or can tolerate those levels. The steel scrap portion of the metal-
lic charge is more diffi cult to quantify elementally. Steel scrap typically is a blend of a number of different steel
38
grades processed and shipped under a number of different designations. The designations include cut plate and structural (P & S), No. 1 heavy melt, No. 1 bundles, and others. The selling price of each of these grades is a function of purity level. The grades that are lowest in total residual level are considered prime and priced higher. Metalcasting facilities must balance the price of each grade versus the cost of higher residual levels. For castings that can tolerate higher alloy concentrations, a grade of steel scrap with higher overall residuals may be acceptable. Residual levels in steel scrap have
undergone a signifi cant change as the melting and casting method for steel in the U.S. evolved from blast furnace/ convertor/ingot cast steel to electric arc furnace (EAF) continuously cast steel. Steel produced in integrated mills (blast furnace/convertor/ingot cast) were typically alloyed with manganese, chro- mium and molybdenum. As production shifted to EAF melted and continuously
Table 1. Residual Levels in Steel Scrap Element Mo Ti
Level 0.1-0.5%
Cb/Nb Mn P
Cr B
Cu V
0.03-0.05% 0.02-0.04% 0.10-2.0% 0.02-0.08% 0.02-0.50%
0.001-0.003% Up to 2.0% 0.01-0.08%
percentages of steel now are being coated. At least 40% of all steel sheet material is coated for rust prevention. These coatings may contain zinc, alu- minum, cadmium, tin or combinations of alloys. Except for tin, most of the coatings are volatilized during melting. The effl uent from the vaporization of the coatings may contain concentra- tions suffi cient to label the dust or sludge collected from dust collector systems hazardous waste. Knowledge and care must be taken to avoid either the unintentional discharge of harmful waste or contamination of the melt by coatings that are not volatilized.
Effects and Solutions
The production of iron castings, particularly ductile iron, requires closer control of many residual elements. The al- lowable levels of manganese, chromium, vanadium, phosphorous and many other elements are much lower than the toler- able levels in gray iron castings and/or steel castings. However, this does not lessen the need for a full understanding of the residual elements that may be present in today’s steel scrap stream. Increased residual levels may have ac-
ceptable metallurgical benefi ts, depend- ing on the type and/or grade of ferrous casting being produced. In cases where the level of a particular element may be excessive, dilution of that element is typi- cally the only solution. In the production of iron castings, pig iron is the most reli- able charge material for dilution. In steel castings, it is less desirable due to its high (4%) carbon level.
MC About the Author
Eugene Muratore is senior foundry metallurgist for Rio Tinto Iron and Titanium America, Chicago. For More Information
Visit
www.metalprices.com for pricing on a variety of primary and scrap metals.
MODERN CASTING / February 2011
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