Sulfide shape control

W. G. Wilson and A. McLean, "Desulphurization of Iron and Steel and Sulfide Shape Control," The Iron and Steel Society of AIME, Warrendale, Pa.,... 

Even today, however, the alloy is favored over mischmetal for multiple ladle practices and for a combined deoxidation-desulfur-ization-sulfide shape control effect using large ladle additions in the production of critical line pipe qualities. This latter application is threatened by substitution with calcium injection as we will see later. Because it was based on bastnasite, an exclusively American ore at the time, and because of its metallurgical limitations, RES never took hold extensively in Europe and in Japan, The recent emergence of a very large Chinese bastnasite deposit may prompt renewed interest in some sort of direct reduced REM alloy during the eighties. 

With ladle plunging or additions through a degassing unit, the costs per ton can go as high as 12.00 per ton for guaranteed sulfide shape control and maximum transverse properties which generally require a RE/S equal to U instead of 3. 

Figure 18. Economic optimization of desulfurization plus sulfide shape control, using mischmetal at 50% recovery, by ladle addition for critical applications...

As the Kawasaki research team mentioned earlier nave concluded, there is also, with calcium, a Ca/S requirement as with the REM S for total MnS substitution 32), They have proposed the most reliable estimate so far of retained calcium requirement for "homogeneous sulfide shape control" which is summarized in Table IV and Figure 17. 

Figure 23 Qualitative summary of the evolution of steel properties discussed in this chapter as a function of the retained REM content showing an early maximum for all properties associated with sulfide shape control and tramp element control but quite a different story for hydrogen and oxidation resistance...

The shape of manganese sulfide inclusions can be partially altered by cross rolling. This method is being used on plate mills for producing high quality steel. Even when this technique is used, the sulfide shape control is incomplete because the inclusions still retain a biaxial ellipsoidal shape. Furthermore, cross rolling on modern continuous hot strip mills is currently impractical. 

Fig. 21. Effect of Ce and Ca additions on the variation in total crack length. The symbol SSC represents sulfide shape control elements such as rare earths and Ca.

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