(2) Processing . 



(a) Deoxidation and Classification . Steels are classified on 

 the basis of the amount of gases evolved during solidification. In the 

 manufacture of steel from pig iron, excess carbon is removed by the exposure 

 of the molten metal to oxygen or air until the desired carbon content is 

 reached. Oxygen dissolves in the molten metal and reacts with carbon to 

 form bubbles of carbon monoxide, which rise to the surface. Other sources 

 of oxygen include that contained in materials added to the slag or molten 

 metal and that present as a product of chemical reactions that occur during 

 the steelmaking process. Because carbon and oxygen may continue to react 

 during solidification, or the steel may have an unacceptably high oxygen 

 content, deoxidation in the ladle of the molten steel may be required. The 

 deoxidation practice, which may or may not be specified by the steel specifi- 

 cation, is often an important consideration in choosing a steel for a 

 particular application. Steels having decreasing degrees of gas evolution 

 or deoxidation are termed "rimmed," "capped," "semikilled," and "killed." 

 Figure 47 shows sketches of typical ingot cross sections corresponding to 

 these degrees of deoxidation. 



K ILLED 



SEMIKILLED 



CAPPED 



RIM MED 



Figure 47. Typical ingot structures 



Rimmed steels do not receive any silicon or aluminum deoxidation before 

 being poured into ingots. Carbon and oxygen dissolved in the molten steel 

 continue to combine forming small hubbies which effervesce during solidifica- 

 tion. Some effervescence is also due to the much lower solubility of 

 oxygen in solid steel compared to that in molten steel. Chemical composition 



205 



