On the market, copper has the largest content of residual elements, and copper is mainly fed into steelmaking furnaces from automobile scrap. It is estimated that the average copper content in the mixed steel scrap of the current steelmaking plant is about 0.3%, and the specific content depends on the source and proportion of the alloy steel.
The residual Sb and As in steel are mainly derived from primary iron ore. When scrap steel containing these impurities is recycled, they can be diluted, but the residual amount will gradually accumulate in the steel.
The H and N in steel mainly come from the atmosphere of the workshop during steelmaking, and their content mainly depends on the composition of different steel grades and the steelmaking process.
2. Segregation of residual elements in steel
Many residual elements exist and play a role in the form of segregation in steel. Most residual elements have strong segregation ability in steel; the segregation process of this element can occur either in the solidification process of molten steel or the subsequent solid phase transformation, but it requires a long diffusion time.
The main segregation elements in the riser part of the ingot are S, P, and C, followed by Sb, N, As, H, and Sn. After segregation and formation of inclusions, the hardness of this part of the material is also higher than the other parts of the ingot.
Compared with solidification segregation, residual elements will produce grain boundary segregation during solid phase transformation or heating. For example, the second type of temper brittleness of steel is mainly caused by P, Sn, As, and Sb grain boundary segregation.
3. Brief description of the role of residual elements
8 kinds of fully retained elements
Ni, Co, W, Mo can improve the hardenability of steel and are beneficial elements;
On the one hand, Cu can cause copper brittleness during high-temperature thermal processing of steel, but on the other hand, it can improve the ability of steel to resist atmospheric corrosion;
The residual elements Sn, As, Sb are harmful elements, which not only strengthen the copper brittleness in steel, but more importantly, it will cause the second type of temper 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.
7 partially retained elements
C, Mn, S, and P are conventional control elements;
Cr can improve the oxidation resistance of steel, increase the corrosion resistance and hardenability of steel, but also increase the temper brittleness of steel;
N is conducive to controlling the grain size of austenite, but it also causes strain aging of steel;
H in steel is a harmful and unhelpful element, which can cause white spots, cracks in low-alloy high-strength steel, etc.