| 摘要 |
PROBLEM TO BE SOLVED: To prevent development of tempering color and uneven plating by lowering inner pressure of an inner cover before igniting a burner, making flow of atmospheric gas in the inner cover after holding this state for a fixed time, returning back the inner pressure to the ordinary pressure, igniting the burner in an outer cover and heating. SOLUTION: In raising of a dew point developed in a heat-up process, the dew point develops at a first peak at 100-400 deg.C and a second peak at 400-600 deg.C in an atmospheric temp. range and the raising of the dew point at each peak is restrained. For restraining this raising, evacuation is executed before ignition and the inner pressure is reduced to >=5 Torr and the holding time T (sec) in the inner pressure is made to not lower than the max. value of 35×[W.Ra/(ρt)] to prevent the raising of the dew point in the first peak. Wherein, W: coil wt. (ton), Ra: surface roughness (μm),ρ: sp. gr. (g/cm3) of a steel and (t): sheet thickness (mm) of the coil. Then, atmospheric gas flow rate at a room temp. and ordinary pressure per one hour, flowing into the inner cover, is regulated to >=2 times of gap vol. between the cover and the charging coil to prevent the raising of the dew point in the second peak.
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