Killed carbon steel
Encyclopedia
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 metalic 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.
, 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 treatment
s. 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 steel
s, heat resisting steels, steels with a carbon content greater than 0.25%, steels used for forging
s, structural steel
s 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 casting
s. Note that as the carbon content decreases the greater the problems with non-metallic inclusion
s.
, which closes the porosity. It is also used for Drawing (manufacturing)
applications.
, 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.
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.
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
that has a some or all of the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
removed from the melt during the steelmaking
Steelmaking
Steelmaking is the second step in producing steel from iron ore. In this stage, impurities such as sulfur, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce the exact steel required.-Older...
process. Liquid steels contain dissolved oxygen after their conversion from molten iron, but the solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
of oxygen in steel decreases with temperature. As steel cools, excess oxygen can cause blowholes or precipitate FeO
Iron(II) oxide
Iron oxide, also known as ferrous oxide, is one of the iron oxides. It is a black-colored powder with the chemical formula . It consists of the chemical element iron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. Iron oxide should not be confused with rust,...
. Therefore, several strategies have been developed for deoxidation. This may be accomplished by adding metalic deoxidizing agents
Deoxidizer
A deoxidizer is a chemical used in a reaction or process to remove oxygen. In comparison with antioxidants, deoxidizers are not used for stabilization during storage but for oxygen removal during manufacture...
to the melt either before or after it is tapped, or by vacuum treatment, in which carbon dissolved in the steel is the deoxidizer.
Types
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
Killed steel is steel that has been completely deoxidized by the addition of an agent before castingCasting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
, so that there is practically no evolution of gas during solidification. They are characterized by a high degree of chemical homogeneity
Homogeneous (chemistry)
A substance that is uniform in composition is a definition of homogeneous. This is in contrast to a substance that is heterogeneous.The definition of homogeneous strongly depends on the context used. In Chemistry, a homogeneous suspension of material means that when dividing the volume in half, the...
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
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, ferrosilicon
Ferrosilicon
Ferrosilicon, or ferrosilicium, is a ferroalloy, an alloy of iron and silicon with between 15% and 90% silicon. It contains a high proportion of iron silicides. Its melting point is about 1200 °C to 1250 °C with a boiling point of 2355 °C...
and manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
. Aluminium reacts with the dissolved gas to form aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
. Aluminium also has the added benefit of forming pin grain boundaries
Grain boundary
A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material...
, which prevent grain growth
Grain growth
Grain growth is the increase in size of grains in a material at high temperature. This occurs when recovery and recrystallisation are complete and further reduction in the internal energy can only be achieved by reducing the total area of grain boundary...
during heat treatment
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
s. 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 steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
s, heat resisting steels, steels with a carbon content greater than 0.25%, steels used for forging
Forging
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...
s, structural steel
Structural steel
Structural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and mechanical properties...
s 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 casting
Steel casting
Steel casting is a specialized form of casting involving various types of steel. Steel castings are used when cast irons cannot deliver enough strength or shock resistance....
s. Note that as the carbon content decreases the greater the problems with non-metallic inclusion
Inclusion
Inclusion may refer to:- Metallurgy :*Inclusion , a type of metal casting defect*Inclusions in Aluminium Alloys, solid particles in liquid aluminium alloy- Social inclusion of persons :...
s.
Semi-killed
Semi-killed steel is mostly deoxidized steel, but the carbon monoxide left 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 rolledRolling (metalworking)
In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling...
, which closes the porosity. It is also used for Drawing (manufacturing)
Drawing (manufacturing)
Drawing is a metalworking process which uses tensile forces to stretch metal. It is broken up into two types: sheet metal drawing and wire, bar, and tube drawing. The specific definition for sheet metal drawing is that it involves plastic deformation over a curved axis...
applications.
Rimmed
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, phosphorusPhosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
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
Surface finish
Surface finish, also known as surface texture, is the characteristics of a surface. It has three components: lay, surface roughness, and waviness.-Lay:...
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
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
. This type of steel is commonly used for cold-bending
Bending (metalworking)
Bending is a manufacturing process that produces a V-shape, U-shape, or channel shape along a straight axis in ductile materials, most commonly sheet metal. Commonly used equipment include box and pan brakes, brake presses, and other specialized machine presses...
, cold-forming, cold-heading
Heading (metalworking)
Heading is a metalworking process which incorporates the forging, extruding and upsetting process. It is often performed in the cold state, resulting in cold working...
and, as the name implies, drawing. Due to the non-uniformity of alloying elements it is not recommended for hot-working applications.
Capped
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
Sheet metal
Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material...
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.