The types of sulfides are roughly divided into three categories:

Type I: spherical,

Type II: film or chain of grain boundary,

Type III: Block



The type of sulfide is closely related to the degree of deoxidation, which is related to the residual oxygen content of the molten steel;

For medium carbon steel, it is roughly as follows:

1) When the residual aluminum content is less than or equal to 0.004%, it is type I; the oxygen-rich droplet-shaped sulfide is formed before the molten steel solidifies, and it will remain in the matrix during the solidification process until the solidification is completed.

2) When the residual aluminum content is between 0.005% and 0015%, it is type II; deoxidation is not complete, and it will solidify at the grain boundary at the end of solidification to form a strip or chain.

3) When the residual aluminum content is more than 0.015%, it is type III; deoxidation is good, and the bulk aluminum oxysulfide (high melting point, solidified before the matrix is solidified) is formed before the solidification of the molten steel. ;-Less impact on mechanical properties.

Among them, Type I sulfide has a slight influence on the mechanical properties, but at this time the oxygen content in the molten steel is too high, there will be a lot of pores; so it is not desirable. Type II sulfide exists in the grain boundary in the form of film or chain, which has a great impact on mechanical properties, especially ductility and toughness; this type of sulfide should be avoided during steelmaking. Type III sulfide is massive and has less impact on mechanical properties.

However, the above critical aluminum content will be affected by the carbon, silicon and manganese of molten steel, as shown in the following figure:

It can be seen from this figure that when the residual aluminum content of WCB (carbon content is about 0.2%) is above 0.03%, there will be no hidden worries of type II sulfide.

Article source: Cast Steel Sharing Alliance

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