1.What type of protective gas is typically used in the annealing process of cold-rolled coils?
During annealing, cold-rolled coils primarily use a mixture of hydrogen (H₂) and nitrogen (N₂) gases, with the hydrogen proportion typically ranging from 5% to 100% (up to 100% in a full-hydrogen furnace). Hydrogen has strong reducing properties, effectively removing the oxide layer from the strip surface and preventing oxidation; nitrogen serves as a carrier gas to regulate the atmosphere and reduce the risk of explosion.
2.Why must the protective gas contain hydrogen?
Reduced iron oxide: Reduces iron oxide that may form during annealing to metallic iron, avoiding surface defects.
Improved surface quality: The reduction reaction generates water vapor, which is then expelled with the gas flow, ensuring a bright, oxide-free strip surface.
Enhanced thermal conductivity: Hydrogen has a much higher thermal conductivity than nitrogen, shortening the annealing cycle and improving production efficiency.
3.How should the ratio of protective gas be adjusted according to the process?
All-hydrogen furnace (100% H₂): Used for applications requiring high surface quality (e.g., automotive exterior panels), achieving efficient reduction and rapid cooling.
Nitrogen-hydrogen mixture (H₂ 5%-40%): Used for ordinary deep-drawing steel or strip steel with lower surface requirements, balancing safety and cost.
Pure nitrogen (100% N₂): Used only for low-temperature processes or special steel grades (e.g., intermediate annealing of silicon steel), avoiding excessive hydrogen permeation.
4.How can the safety of protective gases be ensured during use?
Strict airtightness testing: The furnace body adopts a fully sealed structure to prevent air infiltration.
Atmosphere control: Oxygen concentration is monitored in real time using an oxygen analyzer to ensure it remains below the lower explosive limit (typically <0.5%).
Purge procedure: Before annealing, the air inside the furnace is fully replaced with nitrogen, and after annealing, hydrogen is purged with nitrogen to avoid dangerous mixing.
5.What effect does the protective gas have on the final properties of cold-rolled coils?
Surface Quality: Prevents defects such as oxidation and pitting, ensuring adhesion of subsequent coatings (e.g., zinc plating).
Mechanical Properties: A pure hydrogen atmosphere prevents surface decarburization or carburization, ensuring stable strip strength and deep-drawing performance.
Production Efficiency: A high-hydrogen atmosphere significantly shortens the annealing cycle (e.g., a full-hydrogen furnace saves over 30% more energy than a conventional furnace), reducing production costs.