Question 19 in August: For carbon steel and low-alloy steel, what thicknesses should be used in the normalized state?

Plate materials used for normalizing: 1. 20R and 16MnR with a shell thickness greater than 30 mm should be used in the normalized state. 2. Other pressure-bearing components such as flanges, tube sheets, and flat covers, made of 20R and 16MnR with a thickness greater than 50 mm, should also be used in the normalized state. 3. Additionally, 15MnVR with a thickness greater than 16 mm should be used in the normalized state. Reason: 1. Normalizing can improve the mechanical properties of the plate materials; at the crystal structure level, it helps to refine the grains and enhance the structural organization, thereby giving the plates better impact toughness, plasticity, and overall mechanical performance. 2. In terms of the rolling equipment used for producing steel plates domestically, the rolling ratio for thicker plates is smaller; once a plate reaches a certain thickness, its internal density and central microstructure become slightly poorer. Normalizing treatment can effectively improve the internal density and central microstructure of the plate.

20R and 16MnR used for shell thicknesses greater than 30 mm, 20R and 16MnR used for other pressure components (flanges, tube sheets, flat covers, etc.) with thicknesses greater than 50 mm, and 15MnVR with thicknesses greater than 16 mm should be used in the normalized state. This is mainly due to the limitations imposed by domestic rolling equipment; the rolling ratio for thicker plates is lower, resulting in slightly poorer density within the steel plates as well as lower quality of the central structure ； Furthermore, normalizing the steel plate can refine the grain structure and improve its microstructure, thereby endowing the plate with better toughness, plasticity, and overall mechanical properties.

Carbon steel and low-alloy steel for pressure vessels, in accordance with clause 4.2.5 on page 9 of standard GB150: 1. For 20R and 16MnR materials with a shell thickness greater than 30 mm, they must be used in the normalized state. 2. Other major pressure-bearing components such as flanges, tube sheets, and flat covers with a thickness of more than 50 mm made of 20R and 16MnR must be used in the normalized state. 3. 15MnVR with a thickness greater than 16 mm must be used in the normalized state. Normalizing treatment can improve the mechanical properties of the sheet metal, as well as its microstructure, thereby endowing it with better impact toughness, plasticity, and overall mechanical performance.

The following carbon steel and low-alloy steel plates should be used in the normalized state: 1. 20R and 16MnR for shells with a thickness greater than 30 mm. 2. 20R and 16MnR for other pressure-bearing components (flanges, tube sheets, flat covers, etc.) with a thickness greater than 50 mm. 3. 15MnVR with a thickness greater than 16 mm. (—) Purposes of normalization: 1. To refine the microstructure, eliminate overheating defects caused by hot working, and normalize the structure. 2. For low-carbon steels (C < 0.3%), to increase hardness and improve machinability. 3. For medium-carbon steels, as a substitute for quenching and tempering treatment.

Clause 4.2.5 of GB150 stipulates that the following materials shall be used in the normalized state: 1) 20R and 16MnR with a shell thickness greater than 30 mm; 2) 20R and 16MnR for other pressure-bearing components (flanges, tube sheets, flat covers, etc.) with a thickness greater than 50 mm; 3) 15MnVR with a thickness greater than 16 mm. The reason for this is that, due to the limitations of domestic rolling equipment, the rolling ratio for thicker steel plates is lower, resulting in slightly poorer internal density and microstructural quality of the steel. Additionally, normalizing the steel helps to refine the grain structure, improve the microstructure, and enhance the steel’s toughness, plasticity, and overall mechanical properties

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Several floors said that “thicker steel plates are rolled to be smaller” – what does that mean?

Clause 4.2.5 of GB150-1998 stipulates that the following carbon steel and low-alloy steel plates shall be used in the normalized state: a) 20R and 16MnR used for shells with a thickness greater than 30 mm； b) 20R and 16MnR with a thickness greater than 50 mm for other pressure components (flanges, tube sheets, flat covers, etc.) ； c) 15MnVR with a thickness greater than 16 mm. Of course, many steel grades have been changed nowadays; for example, 20R has been replaced by Q245R, and 16MnR by Q345R, among others.

GB150, Page P9: 4.2.5 Plates used for normalizing: 1. 20R and 16MnR with a shell thickness greater than 30 mm should be used in the normalized state. 2. Other pressure-bearing components such as flanges, tube sheets, and flat covers made of 20R and 16MnR with a thickness greater than 50 mm should also be used in the normalized state. 3. Additionally, 15MnVR with a thickness greater than 16 mm should be used in the normalized state

The rolling ratio can be understood as the ratio of the thickness of the original steel billet to the thickness of the steel plate after rolling. For billets with a consistent thickness, the rolling ratio is naturally lower for thick-walled plates than for thin-walled plates; the grains in the central part of thick-walled plates are not refined due to rolling, resulting in poor microstructural uniformity, which in turn affects their mechanical properties.