Q420C and Q420NC are steel grades defined in the Chinese national standard GB/T 1591-2018 "High Strength Low Alloy Structural Steels." Their main differences lie in the quality grade and possible through-thickness performance requirements.

1. Q420C:

   • "C" indicates the quality grade of the material, typically related to impact testing at 0°C.
   • Q420C steel plates require a yield strength of at least 420 MPa and a tensile strength between 520 and 680 MPa.
   • Q420C generally does not specify through-thickness performance and is suitable for general structural and engineering applications.

2. Q420NC:

   • "NC" indicates that the material has specific through-thickness performance. "N" stands for normalized or normalized-rolled condition, and "C" represents impact testing at 0°C.
   • In addition to meeting the mechanical property requirements of Q420C, Q420NC also possesses specially treated performance in the thickness direction, such as Z-direction performance grades Z15, Z25, Z35, etc. These grades indicate higher impact energy absorption requirements in the thickness direction.

Yield Variation:

• For both Q420C and Q420NC, yield strength is the stress value at which the material begins to undergo permanent deformation under external force. Variations in yield strength are mainly influenced by the material's chemical composition, microstructure, and production process.
• Within the same thickness range, the yield strength requirements for Q420C and Q420NC are the same, both being no less than 420 MPa. However, since Q420NC may undergo normalization treatment or possess specific through-thickness performance, its microstructure may be more uniform, which could affect the distribution and consistency of yield strength.
• Through-thickness performance requirements (such as Z-direction performance) may enable Q420NC to exhibit better yield strength uniformity in the thickness direction, especially in thick plate materials, which helps improve structural reliability and safety.

In practical applications, the choice between Q420C and Q420NC depends on the specific requirements of the structural design. If the structure demands higher performance in the thickness direction or requires better welding performance and impact toughness, Q420NC may be selected. If through-thickness performance is not a critical requirement, Q420C may be a more cost-effective option. Engineers should choose the appropriate material based on specific engineering needs and safety standards.