Q420 steel plate is a low-alloy high-strength structural steel with excellent mechanical properties and low-temperature impact toughness. The low-temperature impact grade of this steel is determined based on the results of impact tests and is mainly divided into the following three grades:

1. Grade Q420A: It has good low-temperature impact toughness and does not require special low-temperature impact tests. At temperatures below -20°C, no significant embrittlement occurs, ensuring structural reliability.
2. Grade Q420B: It has a certain level of low-temperature impact toughness and requires low-temperature impact tests. At temperatures below -20°C, some embrittlement may occur, but it does not affect the overall performance of the structure.
3. Grade Q420C: It has relatively poor low-temperature impact toughness and requires strict low-temperature impact tests. At temperatures below -20°C, significant embrittlement may occur, which can impact structural reliability.

Different grades of Q420 steel plates have varying application requirements. For example, in critical projects such as bridges, high-rise buildings, and hydropower stations, Grade Q420A steel plates are typically selected to ensure structural safety and stability. In contrast, for some general projects, Grade Q420B or Q420C steel plates may be chosen to control costs.

Additionally, the low-temperature impact grade of Q420 steel plates is influenced by other factors. For instance, the chemical composition of the steel, smelting process, heat treatment process, and forming process can all affect the low-temperature impact grade. Therefore, when selecting Q420 steel plates, it is necessary to consider various factors comprehensively based on specific usage requirements and design specifications to choose the appropriate low-temperature impact grade.

In summary, the low-temperature impact grade of Q420 steel plates is determined based on impact test results and is divided into three grades: A, B, and C. Different grades of steel plates have varying application requirements, and the appropriate low-temperature impact grade must be selected based on specific usage needs and design requirements. At the same time, it is essential to control influencing factors such as the chemical composition of the steel, smelting process, heat treatment process, and forming process to ensure the low-temperature impact performance of the steel.