The Q345B rectangular tube is a nickel-iron-chromium heat-resistant alloy, which is enhanced with a certain amount of silicon to achieve good high-temperature strength. Due to its high high-temperature strength, excellent resistance to high-temperature oxidation, and resistance to medium corrosion, it is increasingly applied in the aerospace, petrochemical, and chemical metallurgy industries. Since the coal drying process uses high-temperature gas to carry out low-temperature pyrolysis of low-rank coal, and the gas is heated in a gas heating furnace before entering the pyrolysis section of the furnace, the gas contains steam and a small amount of corrosive gases like H2S, thus requiring the material used to make the heating furnace to be resistant to corrosion.