Classification of Steel
Steel is an alloy mainly composed of iron and carbon, and its carbon content is generally less than 2.11%. Steel is a very important metal material in economic construction. According to chemical composition, steel can be divided into carbon steel and alloy steel. Carbon steel is an alloy obtained by pig iron smelting. In addition to iron and carbon, it also contains a small amount of impurities such as manganese, silicon, sulfur and phosphorus. Carbon steel has certain mechanical properties, good technological properties and low price. Therefore, carbon steel has been widely used. However, with the rapid development of modern industry and science and technology, the properties of carbon steel can not fully meet the needs, so people have developed various alloy steels. Alloy steel is a multi-element alloy obtained by purposefully adding some elements (called alloy elements) on the basis of carbon steel. Compared with carbon steel, the properties of alloy steel have been improved remarkably, so it has been used more and more widely.
Because of the variety of steel products, in order to facilitate production, storage, selection and research, it is necessary to classify steel products. According to the different uses, chemical composition and quality of steels, steels can be classified into many categories:
Classification by use
According to the use of steel, it can be divided into three categories: structural steel, tool steel and special performance steel.
Structural steel: 1. Steel used for various machine parts. It includes carburized steel, quenched and tempered steel, spring steel and rolling bearing steel.
2. Steel used as engineering structure. It includes A, B, special steel and common low alloy steel in carbon steel.
Tool steel: Steel used to make various tools. According to the different uses of tools, they can be divided into cutting tool steel, die steel and measuring tool steel.
Special performance steel: steel with special physical and chemical properties. It can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel and so on.
Classification by chemical constituents
According to the chemical composition of steel, it can be divided into carbon steel and alloy steel.
Carbon steel: according to carbon content, it can be divided into low carbon steel (carbon content < 0.25%), medium carbon steel (0.25% < carbon content < 0.6%) and high carbon steel (carbon content < 0.6%).
Alloy steel: According to the content of alloy elements, it can be divided into low alloy steel (total content of alloy elements < 5%), medium alloy steel (total content of alloy elements = 5% - 10%) and high alloy steel (total content of alloy elements > 10%).
In addition, according to the different kinds of main alloying elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium nickel steel, chromium manganese titanium steel and so on.
Classification by quality
According to the content of harmful impurities, phosphorus and sulphur in steel can be divided into ordinary steel (phosphorus content < 0.045%, sulphur content < 0.055%; or both phosphorus and sulphur content < 0.050%); high quality steel (phosphorus and sulphur content < 0.040%); high quality steel (phosphorus content < 0.035%, sulphur content < 0.030%).
In addition, according to the types of smelting furnaces, steel is divided into open-hearth steel (acid open-hearth furnace, alkaline open-hearth furnace), air converter steel (acid converter, alkaline converter, oxygen top-blown converter steel) and electric furnace steel. According to the degree of deoxidation during smelting, the steel is divided into boiling steel (incomplete deoxidation), killed steel (complete deoxidation) and semi-killed steel.
In naming steel products, steel mills often combine the three classification methods of use, composition and quality. For example, steel is called ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, high-quality carbon tool steel, alloy structural steel, alloy tool steel, etc.
IV. Mechanical Properties of Metal Materials
The properties of metal materials are generally divided into two categories: technological properties and service properties. The so-called process performance refers to the performance of metal materials in the cold and hot working conditions during the manufacturing process of mechanical parts. The technological properties of metal materials determine their adaptability to processing and forming in the manufacturing process. Due to different processing conditions, the required technological properties are also different, such as casting performance, weldability, malleability, heat treatment performance, cutting processing and so on. The so-called service performance refers to the performance of metal materials under the service conditions of mechanical parts, which includes mechanical properties, physical properties, chemical properties and so on. The use range and service life of metal materials are determined by their performance. In the mechanical manufacturing industry, the general mechanical parts are used in normal temperature, atmospheric pressure and non-strong corrosive medium, and in the process of use, each mechanical part will bear different loads. The resistance of metal materials to damage under load is called mechanical properties (or mechanical properties).
The mechanical properties of metal materials are the main basis for the design and material selection of parts. The mechanical properties required for metal materials will vary with the nature of external loading (such as tension, compression, torsion, impact, cyclic loading, etc.). Common mechanical properties include strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit. Various mechanical properties are discussed below.
1. strength
Strength refers to the resistance of metal materials to failure (excessive plastic deformation or fracture) under static load. Because the load acts in the form of tension, compression, bending and shearing, the strength can also be divided into tensile strength, compressive strength, flexural strength, shear strength and so on. There is a certain relationship between various strengths. Tensile strength is usually used as the most basic strength index.
2. plasticity
Plasticity refers to the ability of metal materials to produce plastic deformation (permanent deformation) without destroying under load.
3. hardness
Hardness is an index to measure the degree of hardness and softness of metal materials. Pressure hardness method is the most commonly used method to measure hardness in production. It uses a certain geometry.
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