The problem of residual elements in steel is one of the important problems faced by the metallurgical industry. In the steelmaking process, steelmaking raw materials (including molten iron, scrap steel and ferroalloys, etc.) will bring a large amount of impurity elements into the steelmaking furnace. Some of the impurity elements can be removed, but some of the impurity elements will remain in the steel. This part of the impurities (unintentionally added alloying elements) is collectively referred to as residual elements.
These residual elements are one of the main factors that cause instability in the quality of steel. Some residual elements are easy to segregate, even if their content is very low, they will have a strong negative effect on the properties of steel.
For example, residual titanium in bearing steel is a typical case. Ti easily reacts with N to produce high-hardness inclusions, which greatly affect the service life of bearing steel.
1. The source of residual elements in steel
my country is a country with a lot of symbiotic iron ore. The symbiotic iron ore includes V, Ti, P, As, Sn, Sb, Re (rare earth elements), etc., which are brought into steel during smelting.
In addition to the residual elements brought into the molten iron by primary iron ore, the largest source of residual elements in molten steel is scrap steel, which is mainly divided into:
(1) Alloy steel in scrap steel. At present, steel mills do not have economical and effective technology to separate alloy steel and normal carbon steel, and some medium and high alloy steels contain a wide variety of alloying elements. In the recycling of steel, these alloying elements will enter the steel as residual elements;
(2) Surface coating or plating in scrap steel. Among them, the most problematic is the tin plate, which enters the scrap steel cycle as a can. Other coatings include copper, nickel and chromium. Galvanized plates are also widely used, but zinc can be basically removed in steelmaking without consideration. ;
(3) Non-ferrous metals contained in scrap steel raw materials. The most important is automobile scrap steel, which contains some micro motors, and the main impurity is copper.