Some residual elements will remain in the final steel product due to the scrap melting process.
Elements such as Zn, Pb, and Sb, which have high vapor pressures, can be easily removed by evaporation at steelmaking temperatures. However, elements like Cu and Sn are difficult to remove using conventional steelmaking methods because their oxidation potentials are lower than that of Fe. Cu, Sn, Sb, and As tend to concentrate at the steel surface, where their content can reach 20 to 140 times the average level. At rolling temperatures between 1000–1300 °C, these elements can enter a liquid phase and penetrate along grain boundaries at the billet surface, forming a network of cracks during rolling.
Harmful residual elements in scrap with the scrap recycling and coating steel products for many times the number of growing, these residual elements in the solidification of steel when the enrichment behavior of steel products to reduce the mechanical properties, smelting may also cause leakage, furnace cover life reduction and other hazards. All kinds of furnace materials such as scrap, iron, scrap substitutes (such as DRI) and ferroalloys may bring in a certain amount of residual elements, DRI residual element content is the lowest, pig iron residual element content is also low, scrap is the main source of residual elements.
Harmful impurity elements such as Sb, Sn, and As tend to segregate at grain boundaries or sub-grain boundaries, significantly reducing the fatigue strength, stress corrosion resistance, and thermal creep resistance of high- and medium-temperature heat-resistant alloy steels, and increasing their brittleness.
With repeated recycling of scrap and coated steel products, the concentration of residual elements tends to accumulate. During solidification, this enrichment of residual elements can degrade the mechanical properties of steel products. Additionally, during smelting, they may cause problems such as molten steel leakage and reduced furnace cover life.
Various furnace charge materials—such as scrap, pig iron, scrap substitutes (e.g., DRI), and ferroalloys—can introduce residual elements. Among them, direct reduced iron (DRI) contains the lowest level of residuals, followed by pig iron. Scrap remains the primary source of residual elements.
Currently, in addition to strictly controlling the quality of scrap, the main method for reducing residual element content is to partially replace scrap with low-residual furnace materials. Common substitutes include direct reduced iron (DRI), blast furnace iron, cold pig iron, decarburized granulated iron, and iron carbide.
