Semikilled steels

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Low-carbon plate and sheet are made in three qualities fully killed with silicon and aluminum, semikilled (or balanced), and rimmed steel. Fully killed steels are used for pressure vessels. Most general-purpose structural mild steels are semikilled steels. Rimming steels have minimum amounts of deoxidation and are used mainly as thin sheet for consumer applications. 

The strength of mild steel can be improved by adding small amounts (not exceeding 0.1%) of niobium, which permits the manufacture of semikilled steels with yield points up to 280 N/mm. By increasing the manganese... 

These steels account for over 90% of all steel production, There are numerous varieties, depending upon carbon content and method of production. In one classification, there are killed steels, semikilled steels, rimmed steels, and capped steels, These are described in considerable detail under Iron Metals, Alloys, and Steels. 

As a grain-refining addition, niobium has a number of advantages. Unlike aluminum, it does not seriously affect the high-temperature yield stress or creep strength. Also, whereas aluminum-treated steel is necessarily fully killed, niobium may be added to semikilled steel. The latter is particularly advantageous in Britain, where industry is geared to the production and use of semikilled carbon steel. 

Deoxidation Practice. As far as deoxidation is concerned, carbon steel may be divided into three categories rimming steel, killed and semikilled steel. 

The terms semikilled, rimmed, and capped refer to many carbon steels that have been partially deoxidized or not deoxidized at all. Examples include ASTM A53 and API 5L [10] for pipe and ASTM A36, a structural steel specification. (While these specifications do no require a killing practice, some, such as API 5L, are normally supplied in the killed condition). Although plain carbon steels are often permitted in benign services such as chemically treated utility water or air lines, killed carbon steels are generally used for at least three reasons (1) There is virtually no cost difference between the two steels. The killed carbon steel is usually preferred because of its lower defect density and higher maximum code-allowable stress at higher service temperatures. 

National standards for carbon steel pipe used at higher temperatures and pressures commonly require killed steel. For normal duties the deoxidation practice is not normally specified. Nevertheless, most seamless pipe is either killed or semikilled, except where specially ordered. An exception to this generalization is Germany, where... 

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