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What are the main properties of steel? What are the corresponding indicators for each performance?

The main properties of steel include mechanical properties and process properties. Mechanical properties are the most important properties of steel, including tensile properties, plasticity, toughness and hardness. Process performance is the performance of the steel during various processes, including cold bendability and weldability.

(1) Tensile properties. The indexes indicating the tensile properties of the steel include yield strength, tensile strength, yield ratio, elongation, and reduction in area.

Yielding refers to the phenomenon that the load of the steel sample does not increase during the stretching process, and the sample continues to deform. The minimum stress at the time of yielding is called the yield point or the yield limit. In structural design, the yield strength is generally used as the design basis.

Tensile strength refers to the ratio of the maximum load to the original cross-sectional area of the specimen before the specimen is stretched.

The ratio of the yield point (yield strength) of the steel to the tensile strength is called the yield ratio. The greater the yield ratio, the higher the reliability of the structural parts. The general carbon steel yield ratio is 0.6-0.65, the low-alloy structural steel is 0.65-0.75, and the alloy structural steel is 0.84-0.86.

Elongation refers to the percentage of the length of the gauge length of the metal material after stretching, and the percentage of the original gauge length; the reduction ratio of the section refers to the cross section of the neck after the metal specimen is broken. The maximum reduction in area and the percentage of the original cross-sectional area. The greater the elongation and the reduction of the section, the better the plasticity of the steel.

(2) Cold bending performance. Cold bending performance refers to the ability of steel to resist bending deformation at normal temperature, indicating the plasticity of steel under severe conditions. After the steel is bent at a predetermined bending angle a and a core diameter d, it is evaluated by examining the outer and side surfaces of the bending portion for cracks, delamination or fracture.

Through cold bending, defects such as stress and impurities inside the steel can be revealed, and it can also be used for inspection of welding quality of steel, which can reveal defects such as cracks and impurities of the welded part on the curved surface.

(3) Impact toughness. Impact toughness refers to the ability of steel to resist impact loads without breaking.

A pendulum impact bending test is commonly used in engineering to determine the ability of a material to resist impact loads, that is, to measure the impact energy Ak consumed by the impact load sample being broken, in joules (J). The impact toughness of steel is an indicator for measuring the quality of steel. Especially for components that are often subjected to load impact, such as heavyweight crane beams, it is subject to the identification of impact toughness. The greater the impact toughness, the better the impact toughness of the steel.

(4) Hardness. Hardness refers to the ability of a metal to resist the pressing of a hard object against a surface. Hardness is not a pure physical quantity, but a comprehensive performance index reflecting elasticity, strength, and plasticity.

The hardness is expressed by Brinell hardness, Rockwell hardness, Vickers hardness, and Shore hardness. The most commonly used method is Brinell hardness. It is a ball of a certain diameter (steel ball or cemented carbide ball), which is pressed against the surface of the sample with the corresponding test force. After the specified holding time, the test force is removed and the surface is measured. The indentation diameter is calculated for its hardness value.

(5) Fatigue damage. Under the action of alternating stress, the steel suddenly breaks down under the static tensile strength of the static load, and even breaks below the static load yield strength. This damage is called fatigue failure. The stress index of steel fatigue failure is expressed by fatigue strength (or fatigue limit), which refers to the maximum stress value of the specimen under the action of alternating stress without fatigue failure. Generally, the steel is subjected to an alternating load of 1×107 weeks, and the maximum stress that can be withstood without damage is taken as the fatigue strength. When designing a structure that is subjected to alternating loads and requires fatigue checking, the fatigue strength of the steel used should be known.