Accidental impurities in steels are considered chemical elements in it, which were not specially added to steel, including, indirectly, eg, when deoxidizing steel, and which cannot be removed by simple metallurgical processes. In English they are called "residual elements", which could be translated as “residual elements”.
These impurities do not include the so-called "permanent" impurities - carbon, manganese, silicon, sulfur and phosphorus. The content of permanent impurities is usually present in any quality certificate for steel. The content of accidental impurities is controlled only for special steels.
Accidental impurities in steels
Accidental impurities in steels primarily include:
The presence of these impurity elements in steel can significantly affect their mechanical properties.. Therefore it is understandable, what is necessary to identify and assess the impact - adverse or beneficial - of these impurities, to keep their content in steel within acceptable limits.
Accidental impurities in steel - or, at least, some of them - have an impact on technological processes and modes - from steel casting to final annealing, as well as, possibly, for all their mechanical properties.
It is necessary to clearly separate those impurities, which affect steel because they are in solid solution - this is:
and those of them, which influence through their segregation along different boundaries, Firstly, along grain boundaries is:
- arsenic and
Influence of accidental impurities on technology
Possible metallurgical effects of various impurities in steel on the technological processes of its production and the final properties of steel products are presented below..
- Molybdenum, chrome and lead affect recrystallization and rolling forces in the production of hot rolled steel strips.
- All incidental impurities affect the transformation of austenite to ferrite and hardenability of steel.
- Zinc and lead reduce plasticity during hot deformation.
- Molybdenum, chrome and lead affect the recrystallization process during steel annealing.
- Copper, nickel, arsenic and tin adversely affect the surface quality of hot rolled and pickled steel strip. The reason is the aggravation of the phenomenon of red brittleness of steel.
- Antimony, tin and arsenic promote embrittlement of steels along grain boundaries. This is especially true for coiling steel strips, as well as annealing low carbon steels.
All incidental impurities in steel adversely affect the mechanical properties of sheet steel products - strips, sheets, slabs.
Accidental impurities in steel can adversely affect metal coating processes - hot and electrolytic.
- Molybdenum, chromium, copper and nickel negatively affect the weldability of high speed steels.
Steel scrap - a source of accidental impurities in steel
Accidental impurities fall into steel with impurities of ore, coke, flux and steel scrap. Steel scrap is considered the most important source of such impurities.. The following impurities are most commonly encountered: copper, nickel, chromium, molybdenum and lead. Acceptable limits for the content of these impurities in steel depend mainly on the requirements for the products., for the manufacture of which this steel is used.
The main problem in the remelting of steel scrap is precisely the control of the level of unwanted elements - impurity elements, to ensure the required steel cleanliness, required by steel manufacturers. Currently, most steels are low carbon and low alloy steels and ultra deep drawing steels.. The properties of these steels are very sensitive to impurity content.. Therefore, in sheet products made of such steels, the content of copper and nickel is limited. 0,2 %; chromium - 0,1 %; molybdenum and tin - 0,03 %.
Influence of accidental impurities on the properties of steel
Although the effect of accidental impurities on properties can be very small, sometimes even a small change in some properties can significantly affect the scrap rate of steel products with special requirements. Generally accepted, that an increase in the content of impurities in steel affects the various properties of steels as follows.
- The strength and hardness of steel increases with increasing copper content, nickel, molybdenum tin and antimony. Chromium has different effects on steels: in some cases increases it, while in others it reduces.
- The ductility of steel decreases unambiguously with increasing copper content., molybdenum and tin. Nickel and chromium can affect the plastic properties of steel in different ways..
- The degree of work hardening of steel decreases with an increase in the content of all impurities.
- The toughness of steel increases with increasing copper and nickel content, decreases with an increase in the content of molybdenum and tin.
- The hardenability of steel increases with increasing copper content, nickel and chromium. Other impurities or have no effect on hardenability, or have little influence on her.
- The weldability of steels decreases with an increase in the content of all impurities.
- Corrosion resistance of steels increases with increasing copper content, nickel, chrome and tin.
- An increase in the content of tin and antimony in the steel leads to an aggravation of the temper brittleness of the steel..