Strictly control the hot-forming temperature range
The optimal hot-forming temperature is 900–1100°C. Below 900°C, the steel's ductility is insufficient, leading to a high risk of cracking, tearing, or deformation defects during forming. Above 1100°C, the steel grains will coarsen significantly, which reduces the post-forming toughness and may affect the uniformity of patina formation.
For thick plates (≥20 mm), ensure uniform heating throughout the material-use a furnace with uniform temperature distribution and extend the holding time appropriately (1.5–2 minutes per millimeter of thickness) to avoid internal and external temperature gradients that cause uneven deformation.
Adopt the correct heating and cooling methods
Heating precaution: Avoid direct flame heating (e.g., oxy-acetylene torch) on local areas, as this can cause overheating, grain burning, or surface oxidation pits. Use a batch furnace or continuous heating furnace for overall heating.
Cooling precaution: Air cooling is mandatory after forming. Never use water quenching or rapid cooling-this will introduce excessive residual stress, cause surface microcracks, and even lead to brittle fracture. For large or thick formed parts, place them in a dry, ventilated area to cool slowly and evenly, which helps refine the microstructure and maintain mechanical stability.
Protect the steel surface during forming
Prevent scale and oxidation: Apply a high-temperature anti-oxidation coating on the surface before heating, which can reduce the formation of thick oxide scales. If scale does form during hot-forming, remove it promptly after cooling using shot blasting or mechanical grinding-thick, loose scales will delay patina formation and cause localized corrosion.
Avoid surface scratches and indentations: Use clean, smooth forming dies and tools; avoid using hard metal tools to strike the steel surface during forming. Minor scratches can be self-repaired by the patina, but deep indentations may trap moisture and cause long-term localized corrosion.
Post-forming treatment precautions
If overheating causes grain coarsening (identified by reduced toughness during testing), perform a normalizing treatment (heat to 850–900°C, hold for a proper time, then air-cool) to refine the grains and restore the material's mechanical properties.
Do not apply non-breathable coatings (e.g., epoxy paint) immediately after forming-this will block the contact between the steel surface and air/moisture, disabling the patina formation mechanism. If temporary protection is needed, use a removable, breathable rust inhibitor.
Store the formed parts in a dry, well-ventilated environment, and avoid stacking them directly on the ground (use wooden pallets to prevent moisture accumulation and crevice corrosion).
