The optimum conditions of pickling are the following: (a) the concentration of hydrochloric acid solution is 0.01 to 0.06wt.%, (b) the temperature of pickling solution is 30 to 80deg.C, and (c) the transporting velocity of cold rolled steel sheet is 50 to 200m/min. The pickling process consists of (a) calculating of boundary time of unpickling and overpickling with concentration of HCl under condition of holding at fixed temperature of pickling solution, (b) calculating of fitting constant with boundary time, (c) graphicalizing of calculated boundary time by put fitting constant into equation (5a) tIC = 1 / (AIC + BIC[HCl ) and (5b) tIIC = 1 / (AIIC + BIIC[HCl ) (Here, tIC : boundary time of unpickling(sec), tIIC : boundary time of overpickling(sec) [HCl : concentration of hydrochloric acid(wt.%), AIC, AIIC, BIC, BIIC : fitting constant) and (d) pickling of cold rolled sheet during the range of time between tIC line and tIIC line.
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