Q420E steel plate is a type of high-strength low-alloy structural steel defined in the Chinese national standard GB/T 1591-2018 "High Strength Low Alloy Structural Steels." The "E" indicates the quality grade of the material, typically associated with impact testing at -40°C. The hot rolling process is a critical step in the production of Q420E steel plate, where temperature control is essential to ensure the microstructure and macroscopic properties of the steel plate.

1. Heating Temperature:

   • The hot rolling process begins by heating the steel billet to an appropriate temperature, usually between 1200°C and 1300°C. This temperature range helps reduce oxidation and decarburization inside the billet while ensuring the plasticity of the steel for subsequent rolling.

2. Soaking Time:

   • The billet needs to be held in the heating furnace for a certain period to achieve uniform temperature, known as soaking time. Soaking not only ensures temperature consistency but also helps reduce internal stresses.

3. Rolling Temperature Control:

   • During hot rolling, the temperature of the billet gradually decreases. To maintain an appropriate rolling temperature, temperature monitoring and adjustments of the rolling mill are necessary. The rolling temperature is typically controlled between 950°C and 1100°C to ensure good plasticity of the steel plate.

4. Cooling Rate:

   • After hot rolling, the steel plate requires rapid cooling to form a fine grain structure, enhancing the material's strength and toughness. Cooling rate control can be achieved through water cooling or air cooling, but overly rapid cooling should be avoided to prevent internal stresses.

5. Finishing Temperature:

   • The finishing temperature is the temperature at which the steel plate exits the last rolling mill during hot rolling, typically not lower than 850°C, to ensure good plasticity during the rolling process.

6. Coiling Temperature:

   • The coiling temperature is the temperature at which the steel plate is coiled, usually controlled between 650°C and 750°C. This temperature range helps prevent cracking during the coiling process.

7. Temperature Uniformity:

   • Maintaining uniform temperature across the steel plate throughout the hot rolling process is crucial. Temperature non-uniformity can lead to inconsistent material properties, affecting the quality of the final product.

8. Automated Control System:

   • Modern hot rolling production lines are typically equipped with advanced automated control systems that can monitor and adjust the temperature of heating furnaces and rolling mills in real-time, ensuring process stability and product quality.

By precisely controlling the various temperature parameters in the hot rolling process, high-quality products that meet the performance requirements of Q420E steel plate can be produced. Temperature control not only affects the microstructure of the steel plate, such as grain size and phase transformation, but also directly impacts its mechanical properties, such as yield strength, tensile strength, and impact toughness.