In any steel making process small amounts of trace elements are left behind in the molten steel. These are normally called residual elements. Residual elements can have direct negative consequences on the weldability and toughness of steel. They may have positive and negative effects or they may have positive effects. Skilled steelmaking keeps a balance between the different effects to optimise the steel for the purpose that it is going to be used for.
For some carbon steels such as EN10025-2 S355J2+N there are very few alloying elements and consequently few residual elements. For more complex offshore and high yield steels however the alloying process is a critical part of how the required properties are achieved.
Residual Elements in Steel
Increasing the quantity of carbon and manganese in steel gives higher tensile and yield properties. Ductility is lowered and the steel also suffers from increased embrittlement and low weldability
Increase sulphur and phosphorous increase the strength of the steel but also increases the brittleness. The consequence of this is low weldability, hot cracking, reduced ductility and reduce impact toughness
Increasing the Silicon content of the steel lowers the ductility transition temperature, which reduces the temperature at which the steel will tend to shatter. It also reduces the weldability of the steel
Hydrogen and oxygen cause brittleness and reduce ductility and the toughness of steel. Hydrogen Induced Cracking is one failure mechanism for steel plates that have too much hydrogen in them (though as a result of the environment rather than the steel making process)
Nitrogen increases the embrittlement of steel and thus decreases its impact toughness
Adding Tin to steel can reduce toughness. Because it tends to form inclusions it also reduces plasticity.
Increasing the Copper content of steel can increase the low temperature notch toughness if the steel is not precipitation hardened (age hardening). However when precipitation hardening occurs hardness and tensile strength increase which reduces toughness.
Residual Elements in Heavy Plate
Generally for the use of heavy plate the key residual elements are the levels of sulphur and phosphorous. The lower these can be reduced the higher the quality of the steel. The main mechanisms for achieving this are vacuum degassing and steel desulphurisation.