Classification of the residual elements
Residual elements in steel are divided into three categories according to their oxidation potential, fully reserved elements, partial reserved elements and minimal reserved elements.
The oxidation potential of the fully reserved elements Cu,Ni,Co,As,W,Mo,Sn,Sb is lower than iron, does not participate in the oxidation reaction during steelmaking, and eventually accumulates almost entirely in the steel product.
The oxidation potential of partial reserved elements S,P,Mn,Cr,C,H,N is close to iron, and at melting course, only a part of the element is oxidized and removed, and the degree of removal is related to the characteristics of the element itself.
The oxidation potential of the minimal reserved elements Pb,Zn,V,Ti,Si,Al,Zr,Mg,Ca,Nb is higher than iron. During the melting process, they are first oxidized into the slag, and only a small part of the remaining into the product.
Source of the residual elements
The residual elements in steel mainly come from ore or scrap. Co-grown iron ore includes V, Ti, P, As, Sn, Sb, Re (rare earth elements), etc., which will be carried into the steel during smelting. For short process steel mills, the largest sources of residual elements in steel are alloyed steel in scrap, coated or plated steel (such as tin, nickel, copper, chromium, zinc) and non-ferrous metals.
The largest amount of residual elements is copper, which enters the steelmaking furnace mainly from automotive scrap. Residual Sb and As in steel mainly originate from primary iron ore, and they can be diluted when scrap containing these impurities enters recycling, but the residual amount will gradually accumulate in the steel. H and N in steel mainly originate from the shop atmosphere during steelmaking, and their content depends mainly on the different steel compositions and steelmaking processes.
Segregation of residual elements in steel
Most of the residual elements in the steel have a strong bias; the bias process of an element can occur either in the solidification process of the steel, but also in the subsequent solid state phase change, but requires a long diffusion time. After the formation of inclusions, the hardness of this part of the material is also higher than the rest of the ingot. As opposed to solidification segregation, residual elements in the solid state phase change, or heating, will produce grain boundary segregation, such as the second type of temper brittleness of steel is mainly P, Sn, As, Sb grain boundary segregation caused by.
Effect of residual elements
Fully reserved elements of Ni, Co, W, Mo can improve the hardenability of steel, is the beneficial element; On the one hand Cu can cause copper embrittlement at steel high-temperature hot work, on the other hand can improve the ability of steel to resist atmospheric corrosion; residual elements Sn, As, Sb belong to the harmful elements, it is not only in the steel to strengthen the copper embrittlement, more importantly, it will lead to the second type of tempering brittleness of alloy steel; Sn is One of the extremely harmful residual elements in steel, Sn will greatly reduce the high temperature mechanical properties of steel and alloys.
In addition to C, Mn, S, P conventional elements, partial reserved elements Cr can improve the oxidation resistance of steel, increase the corrosion resistance and hardenability of steel, but also increases the tempering brittleness of steel; N is good for controlling the grain size of austenite, but also causes strain aging of steel; H in steel is a harmful and unhelpful element, which can lead to white spots, low-alloy high-strength steel internal cracking, etc..
