Normalizing process: Heat the steel plate to 850–900°C, hold it for a sufficient time (1.5–2 minutes per millimeter of thickness) to ensure uniform austenitization, then cool it in still air. This process refines the ferrite-pearlite microstructure of Q235NH, eliminates the banded structure formed during rolling, and moderately increases the yield strength by 10–30 MPa (the specific value depends on the original grain size and cooling rate).
Tempering auxiliary treatment: If the hardness increases slightly after normalizing, low-temperature tempering at 200–300°C can be performed to reduce residual stress without significantly reducing the increased strength. This step helps maintain the steel's ductility (elongation at break remains above 20%) while avoiding embrittlement.
Quenching and tempering (Q&T): Quenching Q235NH (heating to 850–900°C, then water quenching) will form hard and brittle martensite, which can significantly improve yield strength (up to 400 MPa or more), but this method is not recommended. The reasons are:
Q235NH has low hardenability due to its low carbon content (≤0.18%) and limited alloying elements; quenching can only achieve surface hardening, and the core remains ferrite-pearlite, resulting in uneven strength distribution.
The martensitic structure will sharply reduce the steel's ductility and toughness, making it prone to cracking during subsequent processing or use.
Excessive quenching temperature or rapid cooling will damage the distribution of corrosion-resistant alloy elements (Cu, Cr), leading to uneven patina formation and reduced atmospheric corrosion resistance.
Annealing: Annealing (heating to 700–750°C, slow cooling) will soften the steel plate, reduce internal stress, but lower the yield strength, so it is not suitable for strength improvement.
Control the heating temperature strictly: Exceeding 900°C will cause grain coarsening, which not only fails to improve strength but also reduces toughness.
Avoid rapid cooling methods: Air cooling is the only acceptable cooling method for Q235NH during strength-improving heat treatment.
Post-heat treatment surface treatment: After heat treatment, oxide scales on the surface should be removed by shot blasting to ensure that the corrosion-resistant alloy elements can react with air and moisture normally, and the patina formation ability is not affected.
