Method of production and production facility of high strength cold rolled steel sheet excellent in chemical convertibility

1. A method of production of a cold rolled steel sheet excellent in chemical convertibility comprising using;

a continuous annealing furnace where a cooling method of a cooling zone including part of all of a temperature range of the steel sheet of 600 to 250° C. following heating for recrystallization is one or more of gas cooling, effusion cooling, and cooling pipe cooling, or

a joint cold rolled steel sheet/hot dip galvanized steel sheet facility having such a continuous annealing furnace, to continuously anneal cold rolled steel sheet and produce the cold rolled steel sheet,

said method characterized by;

exposing the surface of the steel sheet in said steel sheet temperature range to an atmospheric gas of a ratio of hydrogen partial pressure and water vapor partial pressure (H 2 /H 2 O) of 0.008 to 2.4 so as to oxidize said surface,

pickling the sheet by an acid fluid containing 1 to 20 weight % of hydrochloric acid or sulfuric acid in a temperature range between 60° C. to 95° C. at the annealing furnace outlet side so the sheet has no bald spots and has a P ratio of 0.80 or more, then

iron- or Ni-plating the sheet to 1 to 50 mg/m 2 ,

wherein the P ratio is the X-ray diffraction intensity ratio P/(P+H) of the phosphophyllite (100) plane P and hopeite (020) plane H.

2. The method of production of the cold rolled steel sheet excellent in chemical convertibility as set forth in claim 1 ,

said method characterized by;

supplying the inside of the furnace in said steel sheet temperature range with atmospheric gas containing oxygen or water vapor,

measuring the oxygen concentration or dew point in the furnace,

controlling the amount of supply of the atmospheric gas containing the oxygen or water vapor from the measurement results,

pickling the sheet at the annealing furnace outlet side, then

iron- or Ni-plating the sheet to 1 to 50 mg/m 2 .

3. The method of production of the cold rolled steel sheet excellent in chemical convertibility as set forth in claim 1 , characterized by making the steel sheet run outside of the furnace so as to expose the surface of the steel sheet to an iron oxidizing atmosphere at part or all of the said 600 to 250° C. steel sheet temperature range following heating for recrystallization.

4. The method of production of the cold rolled steel sheet excellent in chemical convertibility as set forth in any one of claims 1 to 3 , characterized in that said cold rolled steel sheet contains, by mass %, Si: 1.0 to 2.0% and/or Mn: 2.0 to 3.0%.