The General Definition of Steel as Piping Material will explain by Piping & Fabrication more detail and this is the explanation of Steel.
Steel is defined as an alloy of iron with not more than 2.0 weight percent carbon. The most common method of producing steel is to refine pig iron by oxidation of impurities and excess carbon, which have a greater affinity for oxygen than iron.
The principal reduction methods used are the basic oxygen process (BOP) and the electric furnace process, each representing a type of furnace in which the refining takes place. The BOP primarily uses molten pig iron as the initial furnace charge; the electric furnace can use a charge of selected steel scrap. Another process, called the basic open-hearth process, is no longer in use in the United States. Although it constituted the major steel producing process for decades, it has succumbed to the more advanced and economical BOP and electric furnaces.
The pig iron is reduced to the desired steel composition through use of acid and/or basic reactions with fluxing agents, heat, oxygen, and time.
Excess carbon is oxidized and lost as gas; impurities float to the surface. Often desired alloying elements are added to the molten pool. The steel can be further refined by using one of various methods of vacuum degassing. As the name suggests, the molten steel is passed through a vacuum chamber with the purpose of removing entrained gases such as oxygen, hydrogen, and carbon dioxide. This operation is performed when extra steel purity is desired, and it results in improved and more uniform properties in the final product form.
The molten steel is then cast into molded ingots, which are then further reduced by hot working in rolling and drawing operations. Alternately, the molten steel may be directly cast into continuous smaller billet or hollow products. The latter process is called continuous casting and has become the preferred method of making steel since it avoids the costly ingot reduction operations.
Alloying additions are made, if required, to the molten steel either while in the reducing furnace as already noted, in the ladle into which the steel is put, or in the ingot into which steel is poured from the ladle.
While the steel is molten in the furnace, oxygen is forcibly injected into it to refine the charge. The oxygen combines with excess carbon and is released as a gas. Excess oxygen is, however, unavoidably left in the molten steel. This results in the formation of oxide inclusions in the steel, or porosity, which appear upon solidification. The process of removing the oxygen is known as deoxidizing practice. Deoxidation is achieved by adding silicon, aluminum, or other deoxidizing agents to the molten steel, the amount of which determines the degree of deoxidation and the type of steel seated. The common names given to these various steel types are killed steel, semikilled steel, and rimmed steel.
Steel of the killed type is deoxidized almost completely; that is, sufficient deoxidizing agent is added to the molten pool to combine with all the excess entrained oxygen. The result is a large number of tiny oxides in the melt. The lack of gas in the molten pool gives the effect of ‘‘killing’’ any visible bubbling activity of the steel, thus the name. Killed steel has a more uniform composition than any of the other types, and usually possesses the best formability at room temperature. A finegrained structure results from this practice because the many oxides formed act as initiation sites of new grains upon solidification and subsequent recrystallization.
This fine-grained character offers toughness superior to the other types of steel. Rimmed steel employs no purposeful addition of deoxidizing agents, and is characterized by relatively violent bubbling and stirring action in the ingot mold. This type exhibits a marked variation in composition across and from top to bottom of the ingot. The outer rim or outer edge of the solidified ingot is relatively pure and ductile material. The amounts of carbon, phosphorous, sulfur, and nonmetallic inclusions in this rim are lower than the average composition of the whole ingot.
The amount of these constituents in the inner portion or core is higher than the ingot average. This type of steel costs less to make than the other types and is widely used for structural applications, where good surface appearance of the final product is desired.
Semikilled steel is only partially deoxidized with silicon, aluminum, or both, taking advantage of the positive attributes of killed and rimmed steel. After casting, or teaming into the ingot molds, the steel is normally further reduced in size and modified in shape by mechanical working. The majority of the reduction process is done hot. During hot working, sufficient heat is maintained to ameliorate the working effects and maintain a structure that is ductile throughout the reduction process.
The steel in the form of ingot, slab, bar, or billet is first brought to the proper temperature throughout and is then passed through rolls or dies. The flow of metal is continuous and preferentially in one (longitudinal) direction. The cross-sectional area is reduced, and the metal is shaped the desired form. The internal structure of the steel is also favorably affected. The working reduces the grain size of the material, and tends to homogenize the overall structure, compared with cast or unworked steel.
Steel is defined as an alloy of iron with not more than 2.0 weight percent carbon. The most common method of producing steel is to refine pig iron by oxidation of impurities and excess carbon, which have a greater affinity for oxygen than iron.
The principal reduction methods used are the basic oxygen process (BOP) and the electric furnace process, each representing a type of furnace in which the refining takes place. The BOP primarily uses molten pig iron as the initial furnace charge; the electric furnace can use a charge of selected steel scrap. Another process, called the basic open-hearth process, is no longer in use in the United States. Although it constituted the major steel producing process for decades, it has succumbed to the more advanced and economical BOP and electric furnaces.
The pig iron is reduced to the desired steel composition through use of acid and/or basic reactions with fluxing agents, heat, oxygen, and time.
Excess carbon is oxidized and lost as gas; impurities float to the surface. Often desired alloying elements are added to the molten pool. The steel can be further refined by using one of various methods of vacuum degassing. As the name suggests, the molten steel is passed through a vacuum chamber with the purpose of removing entrained gases such as oxygen, hydrogen, and carbon dioxide. This operation is performed when extra steel purity is desired, and it results in improved and more uniform properties in the final product form.
The molten steel is then cast into molded ingots, which are then further reduced by hot working in rolling and drawing operations. Alternately, the molten steel may be directly cast into continuous smaller billet or hollow products. The latter process is called continuous casting and has become the preferred method of making steel since it avoids the costly ingot reduction operations.
Alloying additions are made, if required, to the molten steel either while in the reducing furnace as already noted, in the ladle into which the steel is put, or in the ingot into which steel is poured from the ladle.
While the steel is molten in the furnace, oxygen is forcibly injected into it to refine the charge. The oxygen combines with excess carbon and is released as a gas. Excess oxygen is, however, unavoidably left in the molten steel. This results in the formation of oxide inclusions in the steel, or porosity, which appear upon solidification. The process of removing the oxygen is known as deoxidizing practice. Deoxidation is achieved by adding silicon, aluminum, or other deoxidizing agents to the molten steel, the amount of which determines the degree of deoxidation and the type of steel seated. The common names given to these various steel types are killed steel, semikilled steel, and rimmed steel.
Steel of the killed type is deoxidized almost completely; that is, sufficient deoxidizing agent is added to the molten pool to combine with all the excess entrained oxygen. The result is a large number of tiny oxides in the melt. The lack of gas in the molten pool gives the effect of ‘‘killing’’ any visible bubbling activity of the steel, thus the name. Killed steel has a more uniform composition than any of the other types, and usually possesses the best formability at room temperature. A finegrained structure results from this practice because the many oxides formed act as initiation sites of new grains upon solidification and subsequent recrystallization.
This fine-grained character offers toughness superior to the other types of steel. Rimmed steel employs no purposeful addition of deoxidizing agents, and is characterized by relatively violent bubbling and stirring action in the ingot mold. This type exhibits a marked variation in composition across and from top to bottom of the ingot. The outer rim or outer edge of the solidified ingot is relatively pure and ductile material. The amounts of carbon, phosphorous, sulfur, and nonmetallic inclusions in this rim are lower than the average composition of the whole ingot.
The amount of these constituents in the inner portion or core is higher than the ingot average. This type of steel costs less to make than the other types and is widely used for structural applications, where good surface appearance of the final product is desired.
Semikilled steel is only partially deoxidized with silicon, aluminum, or both, taking advantage of the positive attributes of killed and rimmed steel. After casting, or teaming into the ingot molds, the steel is normally further reduced in size and modified in shape by mechanical working. The majority of the reduction process is done hot. During hot working, sufficient heat is maintained to ameliorate the working effects and maintain a structure that is ductile throughout the reduction process.
The steel in the form of ingot, slab, bar, or billet is first brought to the proper temperature throughout and is then passed through rolls or dies. The flow of metal is continuous and preferentially in one (longitudinal) direction. The cross-sectional area is reduced, and the metal is shaped the desired form. The internal structure of the steel is also favorably affected. The working reduces the grain size of the material, and tends to homogenize the overall structure, compared with cast or unworked steel.
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