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Chemical compositions of steel

Chemical Composition :- 

The chemical composition of steel is of great importance since it determines the potential mechanical properties of the finished steel product and controls the degree of corrosion resistance and weld- ability of the material For this reason structural steel specifications always provide a table of chemical composition limits within which the steel producer must develop his own particular recipe. The purpose of the specified chemical composition is not to provide the detailed chemical formula
necessary to produce a certain type of steel but to provide safeguards. The steel producer is informed that he must keep within the limits which are deemed to be acceptable for the type of steel considered. Within these limits, which may be broad or narrow, the steel producer has complete freedom to use his skill and knowledge to make steel with the required mechanical properties. Each producer selects a combination of quantities of elements, which fall within the requirements of the applicable specification, provide the required mechanical and other properties, and are most suitable
from the point of view of his particular material supply and steel- making facilities. For this reason actual heats of steel seldom, if ever, will contain the elements in the exact combination of quantities called for in the applicable specification.
PERIODIC TABLE

Obviously the steel producer has more latitude, and is in a better position to keep production costs down if the requirements as to chemical composition are kept to a minimum. Thus the aim of the specification is to impose only the chemistry which is deemed essential and to provide the broadest possible limits. The limits placed on the various elements are therefore usually specified as either a maximum or minimum percentage and, only where considered to be essential, as a range between
minimum and maximum. In some cases, the permissible range of a particular element is specified because it tells the steel producer that he must produce the steel in a certain way. For instance a specification which states that the percentage of silicon is to be within the range 0.15/0.30 tells the steel producer that killed steel is required, whereas if the percentage of silicon is specified to be 0.30
maximum, the steel producer is informed that the steel may be semi-killed or killed at his option.

Generally, a maximum limit is placed on elements which the steel producer has to reduce in the refining process (i.e. carbon, sulphur, phosphorus) and a minimum limit is placed on elements which the steel producer has to add (i.e. metallic alloying elements).

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