There will remain some residual elements in final steel product through scrap melting process.
Zn, Pb, Sb and other residual elements have high vapor pressure and be easily removed by evaporation at steelmaking temperature. Cu, Sn and other elements are difficult to remove by normal steelmaking methods because their oxidation potential energy is lower than that of Fe. Cu, Sn, Sb, As elements in the steel surface enrichment can reach the average content of 20 ~ 140 times. In 1000 ~ 1300 ℃ rolling temperature is a liquid phase, and to the billet surface grain boundaries within the penetration of grain boundaries, so that the billet in the rolling process to form a network of cracks. The Sb, Sn, As and other harmful impurity elements increase the steel in the grain boundary or sub-grain boundary bias, can significantly reduce the fatigue strength, stress corrosion resistance and thermal creep properties of high and medium temperature hot alloy steel, increasing its brittleness tendency.
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.
At present,except strict controlling the quality of scrap, mainly through the use of low residual elements content of the furnace material to replace part of the scrap smelting, in order to reduce the content of diluted residual elements. Commonly used alternative furnace materials are direct reduced iron (DRI), blast furnace iron, cold pig iron, decarburized granulated iron, iron carbide, etc.
