varies widely throughout rimmed steel ingots. The area near the surface is 

 much lower in carhon, sulfur and phosphorus than the remainder of the ingot. 

 This low carbon skin persists to the finished mill product which contributes 

 to the superior deep drawing properties of rimmed steel. 



Capped steel is similar to rimmed steel except that the effervescence of 

 gas from the molten steel in the ingot mold is only allowed to occur for a 

 minute or so before a cast-iron cap is placed on the mold. Capped steel has 

 a thinner low carbon rim than rimmed steel but is of more uniform composi- 

 tion. Capped steel is used for plate, strip, pipe, trim plates, wire, and 

 bars. 



Semikilled steel is produced by additions of silicon and other deoxida- 

 tion elements during the manufacture. These additions are carefully made in 

 order to balance evolution of gases with solidification shrinkage. Semi- 

 killed steel is used for structural shapes, plate, pipe, forging billet and 

 bars. 



Killed steel is produced by the addition of excess deoxidizers over the 

 amount required to fully remove all oxygen from the ladle. As the molten 

 killed steel solidifies, a large shrinkage cavity forms, called the pipe, as 

 shown in Figure 47. It is necessary to cut off the top of killed steel 

 ingots at the bottom of the pipe to avoid producing a defect, known as a 

 seam, in the rolled product. Killed steel is more uniform in both composi- 

 tion and mechanical properties than semikilled steel. However, because of 

 the low yield of product per ingot, killed steel is more costly than semi- 

 killed steel. 



The choice of deoxidizer used is often dictated by the specification 

 chosen by the user after considering the end use. If high temperature use 

 is contemplated (for instance for use in a steam boiler) silicon coarse- 

 grain killed steel will be specified because of its improved resistance to 

 deformation at high temperatures. When improved resistance to brittle 

 failure is desired, particularly for service at temperatures below 20 

 Celsius (68 Fahrenheit), a silicon-aluminum dioxidation practice will be 

 specified so as to produce a steel having a fine-grain structure. Vacuum 

 degassing is also used when premium quality is required. 



(b) Heat Treatment . Mechanical properties of steel can be 

 altered by heat treatment. When steel is heated above a critical tempera- 

 ture (the specific temperature depends on the composition), transformation 

 of the microstructure into a single phase solid solution occurs. This solid 

 solution is called austenite. The temperature at which transformation takes 

 place is called the austenitizing temperature. Steel heated to the austen- 

 itizing temperature and allowed to cool in the furnace to a temperature low 

 enough for the steel to be handled is said to be annealed. Annealing is 

 performed to reduce hardness, improve machinability, and facilitate cold 

 working. 



Normalizing is the process of heating the steel above the austenitizing 

 temperature, allowing sufficient time for transformation to occur, and then 

 removing the steel from the furnace and cooling in air. Normalizing is 

 performed to refine grain size and homogenize microstructure, improve 

 machinability, and or provide the desired mechanical properties. Normalizing 



206 



