Deoxidized steel is steel that has a some or all of the oxygen removed from the melt during the steelmaking process. Liquid steels contain dissolved oxygen after their conversion from molten iron, but the solubility of oxygen in steel decreases with temperature. As steel cools, excess oxygen can cause blowholes or precipitate FeO. Therefore, several strategies have been developed for deoxidation. This may be accomplished by adding metallic deoxidizing agents to the melt either before or after it is tapped, or by vacuum treatment, in which carbon dissolved in the steel is the deoxidizer.
There are four types, ranging from fully deoxidized to slightly deoxidized: killed, semi-killed, rimmed, and capped. Note that none of the various types are better than the other, but that each is useful in its own regard.
Killed steel is steel that has been completely deoxidized by the addition of an agent before casting, so that there is practically no evolution of gas during solidification. They are characterized by a high degree of chemical homogeneity and freedom from gas porosity. The steel is said to be "killed" because it will quietly solidify in the mould, with no gas bubbling out. It is marked with a "K" for identification purposes.
Common deoxidizing agents include aluminium, ferrosilicon and manganese. Aluminium reacts with the dissolved gas to form aluminium oxide. Aluminium also has the added benefit of forming pin grain boundaries, which prevent grain growth during heat treatments. For steels of the same grade a killed steel will be harder than rimmed steel.
The main disadvantage killed steels is that it suffers from deep pipe shrinkage defects. To minimize the amount of metal that must be discarded because of the shrinkage, a large vertical mold is used with a hot top. Typical killed-steel ingots have a yield of 80% by weight.
Commonly killed steels include alloy steels, stainless steels, heat resisting steels, steels with a carbon content greater than 0.25%, steels used for forgings, structural steels with a carbon content between 0.15 and 0.25%, and some special steels in the lower carbon ranges. It is also used for any steel castings. Note that as the carbon content decreases the greater the problems with non-metallic inclusions.
Semi-killed steel is mostly deoxidized steel, but the carbon monoxide leaves blowhole type porosity distributed throughout the ingot. The porosity eliminates the pipe found in killed steel and increases the yield to approximately 90% by weight. Semi-killed steel is commonly used for structural steel with a carbon content between 0.15 to 0.25% carbon, because it is rolled, which closes the porosity. It is also used for drawing applications.
Characteristics of rimmed steels.
⇒ Structural steels containing 0.15 to 0.25% carbon are generally semi-killed. ⇒ In semi-killed steel, the aim is to produce metal free from surface blowhole and pipe. ⇒ The surface should have a sound kin of considerable thickness. ⇒ They are used for general structural applications. ⇒ During solidification of semi-killed steel, gas is evolved in the body of the ingot, tending to compensate in part or entirely for the shrinkage accompanying solidification. ⇒ Since pipe cavities are minimized, semi-killed steels are usually cast in big-end -down molds without hot-tops. ⇒ This type of steel is suitable for drawing operation (except severe drawing).
Rimmed steel, also known as drawing quality steel, has little to no deoxidizing agent added to it during casting which causes carbon monoxide to evolve rapidly from the ingot. This causes small blow holes in the surface that are later closed up in the hot rolling process. Another result is the segregation of elements; almost all of the carbon, phosphorus, and sulfur move to the center of the ingot, leaving an almost perfect "rim" of pure iron on the outside of the ingot. This gives the ingot an excellent surface finish because of this iron rim, but also form the most segregated composition. Most rimmed steel has a carbon content below 0.25% carbon, a manganese content below 0.6%, and is not alloyed with aluminum, silicon, and titanium. This type of steel is commonly used for cold-bending, cold-forming, cold-heading and, as the name implies, drawing. Due to the non-uniformity of alloying elements it is not recommended for hot-working applications.
Capped steel starts as rimmed steel but part way through the solidification the ingot is capped. This can be done by literally covering the ingot mold or by adding a deoxidizing agent. The top of the ingot then forms into a solid layer of steel, but the rim of the rest of the ingot is thinner than in a rimmed steel. Also there is less segregation of impurities.
The yield of rimmed and capped steel is slightly better than that of semi-killed steel. These types of steels are commonly used for sheet and strip metal because of their excellent surface condition. It is also used in most cold-working applications.
Due to production processes, as the carbon content of rimmed and capped steel increases above 0.08%, the cleanliness decreases.
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