The Effect And Control Study Of Tramp Elements In Steel

The tramp elements such as copper, arsenic, tin and so on whose oxidation potential are less than iron would be completely into the liquid steel in the steelmaking process. And they will bring about many effect and hazard on the quality and performance of steel by three different kinds of mechanisms such as solidification segregation, grain boundary segregation, and precipitation enrichment between steel matrix and oxide layer.A domestic steel has not self-sufficient mine, and the sources of iron ores used are complicated and miscellaneous, which leads to the relative high level of tramp elements in hot metal such as copper, arsenic, tin, antimony etc. Moreover, it results in some seriously adverse impact on the surface quality and mechanical properties of hot/cold rolled coils produced by CSP process. For example some black spots appear on the surface of acid cleaned cold rolled coil where the tramp elements enrich; and some edge defects including edge cracks, edge deletions arise on hot rolled coils etc. Based on the actual production of the steel, the issues related to the tramp elements and how to control and reduce them has been studied systematically in this paper.The problems caused by tramp elements on continuous casting thin slab of steel 08Al have been studied. The research results indicate that copper, arsenic, tin does not have obvious macrosegregation. However, the segregation degree is relatively larger near the edge of slab. Moreover, in the center line arsenic and tin central segregation is more significant than copper, which is consistent with the theoretical calculations. Meanwhile, there are obvious tramp elements enriched phases in the interface between steel matrix and oxide layer of the slab.The dearsenization and detinning studies on low arsenic hot metal from the steel with CaC2-CaF2 slag have been carried out. The results indicate that it could be accomplished desulfurization and dearsenization simultaneously, but it is impossible to carry out dephosphorization and detinning in the experimental condition. Meanwhile, the dearsenization is a solid-liquid reaction, of which the restrictive step is the arsenic diffusion through the boundary layer of hot metal to CaC2 particle surface. And the higher hot metal temperature and better bath stirring are more beneficial to the dearsenization, but the sulfur would have a great adverse impact on it. Moreover, the experimental results of induction furnace show that the arsenic content of hot metal decreases with the increasing of the reaction time, and the lowest arsenic content could be 0.019%, which means the largest dearsenization rate is 34.5% when the reaction time is between 15 and 20 minutes with 60% CaC2 and 40% CaF2.The purification of lanthanum and yttrium on the tramp elements in steel has been researched. Thermodynamic analysis shows that lanthanum, cerium and yttrium is likely to react with arsenic, tin and antimony at general melting temperature. However, the reactions do not occur in the experimental conditions. Therefore a small amount of the rare earth compound inclusions with tramp elements in steel would be the product of elements segregation during solidification.The effects of rare earth content on the quality and performance of steel have also been studied. The results indicate that compared with adding 0.02% lanthanum of steel, 0.0078% yttrium is beneficial to suppress the austenite grain growth, increase the yield strength and improve the plasticity in the experimental conditions. But the latter easily leads to increasing the yield ratio of steel. Moreover, they have the equivalent benefit on improving the transverse impact toughness of steel, and have no obvious effect on increasing the tensile strength. In addition, after adding rare earth, the tramp elements enrichment layer between steel matrix and oxide layer becomes less apparent, and the arsenic content of enrichment phases decreases, but the copper content increases. Moreover, the enrichment of silicon, manganese, phosphorus, chromium, nickel and oxygen reduces in the oxide layer near the interface. But these elements enriched phases increase significantly between interface and steel matrix especially for the experimental steel with adding 0.02% lanthanum.