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Take a look at the following details: The four steel grades, 20, 20G, 20g, and 20R, are all carbon structural steels with a carbon content of 0.20%. But there are significant differences between them, and the price differences are quite substantial. If the correct selection is not made during design and use, it can lead to catastrophic accidents in severe cases, or it can affect the product’s service life in less serious cases. It may also result in the misuse of high-quality materials. The differences among these four steel grades, as well as the points that should be taken into consideration in their design and use, are outlined below: The applicable standards and uses vary; 1. Grade 20 steel is used for ordinary steel pipes, and there are three standards for it. Standard GB8162 applies to seamless steel tubes for structural use ； Standard GB8163 applies to general seamless steel pipes for fluid transportation ； Standard GB3087 applies to seamless steel tubes for low and medium pressure boilers ； 2. 20G is a high-quality carbon structural steel used for manufacturing steam boilers, pipes, and other components that require high pressures or higher. It is widely used to manufacture economizers, superheaters, water wall, feedwater pipes, main steam pipes, etc., for media temperatures ≤ 430°C. Standard GB5310 applies to seamless steel tubes for high-pressure boilers. 3. 20g is used to manufacture carbon steel plates with a thickness of 60–120 ram for various pressure-bearing components in boilers. Standard GB713 applies to carbon steel and low-alloy steel used in boilers; it also applies to such steel. 4. 20R is suitable for carbon steel with a thickness of 6–10 mm for pressure-bearing components in medium and low-temperature pressure vessels. Standard GB6654 applies to steel plates for pressure vessels. The difference between 16Mn and Q345: Q345 steel serves as a replacement for older grades such as 12MnV, 14MnNb, 18Nb, 16MnRE, 16Mn, and others, rather than merely replacing the 16Mn steel alone. In terms of chemical composition, 16Mn and Q345 also differ. More importantly, there are significant differences in the thickness grouping sizes for the two steel grades based on their yield strengths, and this will inevitably lead to changes in the allowable stress for materials of certain thicknesses. Therefore, it is inappropriate to simply apply the allowable stress of 16Mn steel to Q345 steel; instead, the allowable stress should be determined anew based on the new thickness classification for steel grades. The proportion of the main constituent elements in Q345 steel is basically the same as that in 16Mn steel; the difference lies in the addition of trace alloying elements such as V, Ti, and Nb. A small amount of V, Ti, and Nb alloying elements can refine the grain structure, enhance the toughness of the steel, and significantly improve its overall mechanical properties. It is for this reason that the thickness of the steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be superior to those of 16Mn steel, especially its low-temperature performance, which is not available in 16Mn steel. The allowable stress of Q345 steel is slightly higher than that of 16Mn steel.