Classification of Residual Elements
Residual elements in steel are categorized into three groups based on their oxidation potential: fully retained elements, partially retained elements, and minimally retained elements.
Fully retained elements—Cu, Ni, Co, As, W, Mo, Sn, Sb—have a lower oxidation potential than iron. These elements do not participate in oxidation reactions during steelmaking and are almost entirely retained in the final steel product.
Partially retained elements—S, P, Mn, Cr, C, H, N—have oxidation potentials similar to that of iron. During melting, only a portion of these elements is oxidized and removed. The degree of removal depends on the specific characteristics of each element.
Minimally retained elements—Pb, Zn, V, Ti, Si, Al, Zr, Mg, Ca, Nb—have higher oxidation potentials than iron. These elements are preferentially oxidized during melting and mostly enter the slag phase, with only small amounts remaining in the steel.
Sources of Residual Elements
Residual elements in steel primarily originate from iron ore and scrap. Iron ores often contain coexisting elements such as V, Ti, P, As, Sn, Sb, and rare earth elements (Re), which are introduced into the steel during smelting. In short-process steelmaking, the main sources of residual elements are alloyed steel scrap, coated or plated steel (e.g., with tin, nickel, copper, chromium, or zinc), and various non-ferrous metals.
Among all residual elements, copper is present in the largest quantity, mostly entering the steelmaking furnace through automotive scrap. Antimony (Sb) and arsenic (As) mainly come from primary iron ore. While they can be diluted by the addition of clean scrap, they tend to accumulate gradually in recycled steel. Hydrogen (H) and nitrogen (N) are primarily absorbed from the furnace atmosphere during steelmaking. Their content is influenced by both the steel composition and the specific steelmaking process used.
Segregation of Residual Elements in Steel
Most residual elements exhibit strong segregation tendencies. Elemental segregation can occur both during solidification and through subsequent solid-state phase transformations. These transformations require extended diffusion time. Segregated elements can lead to the formation of inclusions, resulting in localized regions with higher hardness compared to the rest of the ingot. In the solid state or during thermal processing, residual elements may segregate at grain boundaries. This phenomenon contributes to grain boundary embrittlement, such as Type II temper embrittlement in alloy steels, commonly caused by the segregation of P, Sn, As, and Sb.
Effects of Residual Elements
Beneficial fully retained elements such as Ni, Co, W, and Mo enhance the hardenability of steel. Copper (Cu) plays a dual role: it can cause hot shortness (embrittlement during high-temperature processing), but it also improves resistance to atmospheric corrosion. Harmful residual elements such as Sn, As, and Sb not only exacerbate copper-induced embrittlement but also contribute to Type II temper embrittlement. Among them, tin (Sn) is particularly detrimental, as it significantly reduces the high-temperature mechanical properties of steels and alloys.
Among the partially retained elements, chromium (Cr) improves oxidation resistance, corrosion resistance, and hardenability, but also increases the tendency for temper embrittlement; nitrogen (N) helps refine austenite grain size but may lead to strain aging in steel; hydrogen (H) is a harmful element that causes internal defects such as white spots and cracking, especially in low-alloy high-strength steels.
