The relationship between steel plate thickness and hardness isn't a simple linear correlation; it's directly affected by the production process (especially the cold rolling and annealing processes). This can be analyzed from the following perspectives:
1. During the cold rolling process: Thickness and hardness are indirectly negatively correlated (due to deformation).
During the cold rolling stage (before annealing), the change in steel plate thickness is primarily determined by the rolling deformation:
After multiple cold rolling passes, the thickness of the original steel plate (such as hot-rolled plate) gradually decreases, while the deformation (i.e., "reduction") increases.
The greater the deformation, the more the grains within the metal are elongated and flattened, increasing dislocation density and causing work hardening, leading to higher hardness.
Therefore, in the cold-rolled (unannealed) state, thinner steel plates from the same batch generally have higher hardness. For example:
A steel plate with an original thickness of 3mm, when cold-rolled to 2mm, has a slightly higher hardness than its original state.
Continuing cold rolling to 1mm, the deformation increases, further increasing the hardness. 2. After Annealing: The Correlation Between Thickness and Hardness Weakens (Depending on the Annealing Process).
Cold-rolled steel sheets typically require annealing (e.g., continuous annealing) to eliminate work hardening and restore plasticity. At this point, the relationship between thickness and hardness is dominated by the annealing process:
Annealing temperature and time determine the degree of recrystallization: When the temperature is high enough and the time is sufficient, work hardening is completely eliminated, grains regenerate uniformly, and the hardness decreases significantly compared to the cold-rolled state.
At this point, if steel sheets from the same batch but different thicknesses undergo the same annealing process, the hardness differences decrease, and the effect of thickness on hardness is reduced.
For example, cold-rolled steel sheets of 1mm and 2mm may have similar hardnesses (e.g., 110-130 HV) after the same annealing process. The effect of thickness differences on hardness is much smaller than in the cold-rolled state. 3. Exceptions may exist for different steel grades or processes.
If the thickness of steel plates varies significantly (e.g., ultra-thin plates <0.3mm and thick plates >5mm), the difference in heat transfer efficiency during annealing may result in thinner plates being more prone to overheating, coarsening grains, and slightly lower hardness than thicker plates (this depends on the specific process).
For certain specially treated steel plates (such as surface quenching), thickness may affect hardenability, and thus the difference in hardness between the surface and core. However, this is a local hardness variation and not a direct correlation between overall thickness and hardness.
Summary
When cold rolled without annealing: thickness and hardness are indirectly related-the thinner the thickness (greater deformation), the higher the hardness.
After annealing: The effect of thickness on hardness decreases, and hardness is primarily determined by the annealing process (degree of recrystallization). Hardness can be consistent across the same batch of steel plates of varying thickness.
Thus, steel plate thickness itself does not directly determine hardness. Instead, it indirectly affects hardness through its influence on rolling deformation and annealing uniformity. The core influencing factors remain work hardening and the degree of recrystallization.