Sulfur is generally regarded as a harmful element in steel, which mainly exists in the form of MnS and FeS, which can cause the thermal brittleness of steel to be reduced, and the mechanical properties of steel, especially the yield limit strength, plasticity and wear resistance to be reduced. The presence of sulfur also has adverse effects on corrosion resistance and weldability of steel. General requirements of ordinary steel (S) of 0.050% or less, omega omega (S) of 0.030% or less in the high-quality steel, advanced omega (S) of 0.020% or less of high quality steel.
Sulfur segregates seriously in steel and deteriorates the quality of steel. At high temperatures, the plasticity of the steel is reduced. It takes the form of FeS with a lower melting point; FeS alone has a melting point of only 1190 ° C, while the eutectic temperature of forming a eutectic crystal with iron in steel is even lower, only 988 ° C, and when the steel solidifies, the iron sulfide will settle at the primary grain boundary. When the steel is rolled at 1100-1200℃, the FeS on the grain boundary will melt, which greatly weakens the binding force between the grains and leads to the hot brittleness of the steel. Therefore, the sulfur should be strictly controlled, generally controlled at 0.020%-0.050%.
In order to prevent brittleness due to sulfur, enough manganese should be added so that it forms MnS with a higher melting point. If the sulfur content in the steel is high, due to the production of SO2 during welding, pores and porosity will be formed in the welding metal.
In some steel (such as cutting steel, magnetic steel), adding an appropriate amount of sulfur can improve the machinability, workability and magnetism of steel.
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