Heat treatment process and performance improvement of ABS Grade B shipbuilding steel plate

I. Normalising Process and Its Optimisation

Normalising is the most commonly used heat treatment process for ABS Grade B steel plates. It aims to refine the grain structure, relieve internal stresses, and ensure more uniform and stable mechanical properties.

Traditional normalising process parameters are as follows: heating temperature of 880–920 °C, holding time of 1–2 hours (adjusted according to plate thickness), followed by natural cooling. This process suffers from issues such as insufficient grain refinement and incomplete elimination of internal stresses, which can lead to significant fluctuations in impact toughness.

Optimisation measures include:

Adjusting the heating temperature to 900–910 °C and extending the holding time to 1.5–2.5 hours, based on the plate thickness and chemical composition, to ensure sufficient grain refinement;

Employing staged cooling: first air cooling to 600 °C, followed by forced air cooling to room temperature, to avoid increased brittleness caused by excessive cooling rates, whilst further eliminating internal stresses.

Following optimisation, the impact toughness of the steel plate increased by 10%–15%, the yield strength by 5%–8%, and performance stability improved significantly.

II. Temperature-Modified Control Process (TMCP)

The Temperature-Modified Control Process (TMCP) is suitable for medium- and heavy-gauge plates with a thickness greater than 50 mm. It achieves grain refinement and performance enhancement by controlling rolling temperature and cooling rate. The core parameters are optimised as follows:

The final rolling temperature is controlled at 800–850 °C to prevent excessive grain coarsening caused by high temperatures;

The cooling rate is controlled at 5–10 °C/s; accelerated cooling inhibits grain growth, thereby improving strength and toughness.

For thick ABS Grade B steel plates, replacing conventional normalising with the TMCP process can significantly enhance impact toughness and fatigue resistance, whilst reducing production costs.

III. Quenching and Tempering (Q&T)

For ABS Grade B steel plates with specific requirements, quenching and tempering (Q&T) may be employed to further enhance strength and hardness. However, as this process is relatively costly, it is only suitable for ship structural components with extremely high performance requirements.