| 摘要 |
PROBLEM TO BE SOLVED: To obtain particularly good surface gloss of a steel by controlling a concn. of Fe ions in an aq. soln. of nitric acid in a nitric acid pickling tank to the value equal to or below the specified one. SOLUTION: Since, in a nitric acid pickling tank in which passivating treatment is executed in the final stage of a pickling line, the solubility of Fe ions is small, the generation of smuts and their adhesion to a steel strip occur. Therefore, the concn. of Fe ions in an aq. soln. of nitric acid in the nitric acid pickling tank as the most important factor for the suppression of the generation of smuts is controlled to <=2.0 wt.%. For controlling this, e.g., the concn. of Fe ions in the aq. soln. of nitric acid in the tank is analyzed by instrumental analysis or the like continuously or by the batch, and, based on the result, the replacement of the aq. soln. of nitric acid or the dilution by new nitric acid is executed. The concn. of nitric acid in the aq. soln. of nitric acid in the tank is preferably controlled to 80 to 120 g/l. Moreover, the soln. temp. of the tank is suitably controlled to 40 to 60 deg.C. As the particularly effective object, a stainless steel cold rolled steel strip having <=15 wt.% Cr content or the like is exemplified.
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