problem of residual elements in steel
The problem of residual elements in steel is one of the important problems faced by the metallurgical industry. In the steel-making process, steel-making raw materials (including molten iron, scrap steel and ferro-alloys etc) will bring a large amount of impurity elements into the steel-making 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 alloy elements) are collectively called residual elements,these residual elements are one of the main factors causing instability of steel quality. 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.
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 as following:
(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 plating layers include copper, nickel, and chromium.
(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.
The residual elements in steel have a certain promotion effect on the production of steel:
(1)8 Fully retained elements
Ni, Co, W, Mo can improve the hardenability of steel and are beneficial elements; Cu can cause copper brittleness during high-temperature hot working of steel on the one hand, but on the other hand can improve the ability of steel to resist atmospheric corrosion; residual element Sn , As, Sb are harmful elements, it not only strengthens 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, and Sn will greatly reduce high temperature mechanical properties of steel and alloys.
(2)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 beneficial to control the austenite the grain size, but it will also cause the strain aging of steel; H in steel is a harmful and unhelpful element, which can cause white spots and cracks in low-alloy high-strength steel.
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