Q420D is a steel grade defined in the Chinese national standard GB/T 1591-2018 "High Strength Low Alloy Structural Steels," where the "D" indicates the quality grade of the material, typically associated with impact testing at -20°C. Q420D steel can be produced through different rolling processes, including hot rolling and normalized rolling. The following is a comparative analysis of hot rolling and normalized rolling processes:

1. Hot Rolling Process:

   • The hot rolling process involves rolling the steel at high temperatures, followed by natural or controlled cooling.
   • Hot-rolled Q420D steel does not undergo subsequent heat treatment during production; it cools directly from the high-temperature rolled state to room temperature.
   • This process helps achieve a fine grain structure, improving the material's strength and toughness but may sacrifice some plasticity.

2. Normalized Rolling Process:

   • Normalized rolling involves normalizing the steel during the rolling process, which means heating it to a certain temperature, holding it, and then cooling it at an appropriate rate to refine the grain structure.
   • Normalizing treatment can reduce internal stresses generated during rolling, improve the material's plasticity and toughness, and enhance weldability.
   • Normalized-rolled Q420D steel typically exhibits better impact toughness, especially in low-temperature applications.

Comparative Analysis:

• Microstructure: Hot rolling may result in coarser grains, while normalizing helps refine the grains and improve material uniformity.
• Mechanical Properties: Hot-rolled Q420D may have higher yield strength and tensile strength, but its plasticity may be slightly lower than that of normalized-rolled material. Normalized rolling can enhance the material's plasticity and toughness.
• Production Cost: Normalizing treatment may increase production costs due to the additional heat treatment step.
• Application Areas: Hot-rolled Q420D is suitable for structural components requiring high strength and hardness, while normalized-rolled Q420D is more appropriate for applications requiring good toughness and weldability.

In practical applications, the choice between hot-rolled and normalized-rolled Q420D depends on the specific requirements of the final product. If higher strength and hardness are needed, hot-rolled material may be selected; if better toughness and weldability are required, normalized-rolled material may be chosen. Additionally, cost and production efficiency are factors to consider when selecting the process.