Nkk Corporation

Michihiro Hiasa

Takeshi Fujita

Tadashi Inoue

Fusato Kitano

Takeo Ishii

The present invention provides a method of manufacturing a cold rolled steel sheet and a hot-dip zinc-coated steel sheet having excellent surface properties, an excellent resistance to natural aging and an excellent dent resistance of panel and, thus, adapted to the manufacture of steel sheet for outer panels of vehicles. The method comprises the steps of preparing steel consisting of 0.005 to 0.012% by weight of C, 0.01 to 0.4% by weight of Si, 0.15 to 1.0% by weight of Mn, 0.01 to 0.08% by weight of P, at most 0.02% by weight of S, 0.01 to 0.1% by weight of sol. Al, at most 0.004% by weight of N, and at least one element selected from the group consisting of 0.01 to 0.2% by weight of Nb and 0.04 to 0.1% by weight of Ti, and satisfying the condition 1.2&lE;(12/93) (Nb%/C%)&plus;(12/48) (Ti*%/C%)&lE;2.5, applying a hot rolling and a cold rolling to the steel, soaking the cold rolled steel sheet at T(&deg; C.) meeting a formula Ac3&gE;T(&deg; C.)&gE;157log(X)&plus;737, and cooling the steel sheet after the soaking step at a cooling rate meeting a formula R(&deg; C./sec)&lE;&minus;35&plus;162/X. Ti*% (noted above)&equals;Ti%&minus;(48/14)N%&minus;(48/32)S%. When Ti* in the above equation is not larger than 0, Ti* is regarded as 0.

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Tokyo, Japan

Michihiro Hiasa