The reasons for the failure of low-temperature impact toughness in Q390E steel may include the following aspects:

1. Unreasonable chemical composition. For example, excessively high carbon content or excessively low manganese content may lead to low-temperature impact failure.
2. Inappropriate heat treatment process. During heat treatment, temperature and cooling rate directly affect the low-temperature toughness of the steel plate. Excessively high or low temperatures, as well as excessively fast or slow cooling rates, may reduce low-temperature impact performance.
3. Unreasonable plate forming process. During the forming process, the steel plate undergoes significant stretching and compression, which may cause cracks and fractures at low temperatures, thereby reducing low-temperature impact performance.
4. Unreasonable content of alloying elements. The content of elements such as chromium and nickel is related to low-temperature strength. Chromium can improve the corrosion resistance of steel but has little effect on low-temperature toughness, while nickel can improve the toughness of steel but has little effect on corrosion resistance. Therefore, appropriate additions of elements such as nickel and chromium can simultaneously enhance the toughness and corrosion resistance of the steel plate, thereby improving its low-temperature impact performance.

To address this issue, the following measures can be taken:

1. Optimize the material formulation and reasonably control the chemical composition, such as carbon and manganese content.
2. Optimize the heat treatment process by establishing appropriate heating and cooling systems to achieve good low-temperature toughness.
3. Optimize the plate forming process to reduce potential cracks and fractures during forming.
4. Reasonable combination of alloying elements, with appropriate additions of elements such as nickel and chromium to improve the toughness and corrosion resistance of the steel plate.

In summary, the reasons for the failure of low-temperature impact toughness in Q390E steel are multifaceted and require comprehensive consideration of material formulation, heat treatment process, plate forming process, and alloying element content. By implementing corresponding measures, the low-temperature impact performance of the steel plate can be effectively improved.