| 摘要 |
PROBLEM TO BE SOLVED: To provide a new method for producing a hot dip galvanized steel sheet free from nonplating, excellent in adhesion, moreover low in a yield ratio and having good workability. SOLUTION: A plating base sheet contg., by weight, 0.005 to 0.15% C, 0.3 to 3.0% Mn and 0.05 to 1.0% Mo is subjected to annealing at the Ac1 transformation point to the Ac3 transformation point for at least one time, is cooled and is thereafter pickled to remove a surface concentrated layer, next, it is heated to the temp. range of the Ac1 transformation point to the Ac3 transformation point, is cooled in the temp. region from the heating temp. at least to the plating bath temp. at the rate equal to or above the critical cooling rate CR ( deg.C/sec) expressed by log CR=-3.50 (Mowt.%)-1.20 (Mnwt.%)-0.16 (Siwt.%)-2.0 (Crwt.%)-0.08 (Niwt.%+Cuwt.%)-0.32 (Pwt.%)+3.50 in the case of B<=0.0006 wt.% and expressed by log CR=-3.50 (Mowt.%)-1.20 (Mnwt.%)-0.16 (Siwt.%)-2.0 (Crwt.%)-0.08 (Niwt.%+Cuwt.%)-0.32 (Pwt.%)+3.20 in the case of B>0.0006 wt.%, is applied with hot dip galvanizing and is successively cooled in the temp. region to 300 deg.C similarly at the rates in the above relations.
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