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Process for producing deep-drawing, non-ageing, cold rolled steel strips having excellent paint bake-hardenability by continuous annealing
| Details |
Inventors: Takahashi, Nobuyuki; Shibata, Masaaki; Furuno, Yoshikuni;
Assignee: Nippon Steel Corporation (Tokyo, JP)
Primary Examiner: Skiff; Peter K.
Assistant Examiner:
Attorney, Agent or Firm: Cushman, Darby & Cushman
Al-killed steels containing 0.001-0.01% C., not larger than 1.5% Mn, 0.005-0.20% Al, not larger than 0.007% N and B in amounts determined by the ratio of B/N ranging from 0.5 to 2.5, and optionally containing not less than 1% Si and 0.04 to 0.12% P are subjected to ordinary hot and cold rolling operations, then soaked in a temperature range of from 730.degree. C. to A.sub.3 point by a continuous annealing process, and rapidly cooled from a temperature between the soaking temperature to 450.degree. C. down to a temperature not higher than 250.degree. C. at an average cooling rate not less than 60.degree. C./second, without subsequent overageing treatments, to obtain deep-drawing, non-ageing cold rolled steel sheets and strips having excellent press formability and paint bake-hardenability. |
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DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in more details hereinbelow.
The essential features of the present invention and various limitations made in the present invention will be explained.
Regarding the chemical composition of the steel strips according to the present invention, carbon is one of the most important elements and must be limited to the range of from 0.
001 to 0.
01% in order to restrict the occurrence of yield point elongation in the as-annealed condition when the steel is rapidly cooled from a temperature between the soaking temperature and 450.
degree.
C.
and to provide less ageing and excellent paint bake-hardening property.
When the carbon content is less than 0.
001%, no enough paint bake-hardening can be obtained, but when it exceeds 0.
010% a significant yield point elongation develops under the as-annealed condition, and the ageing property increases and the elongation greatly deteriorates.
A preferable carbon range is from 0.
002 to 0.
006%.
Manganese is essential for preventing the hot embrittlement of the steel, but excessive manganese contents will produce excessive hardness of the steel.
Therefore, in the present invention, the upper limit of the manganese content is 1.
5%, and the manganese may be contained in various amounts within the defined range depending on the desired strength of the products.
For example, when low strength deep-drawing cold rolled steel strips are desired the manganese content is maintained at about 0.
6% or less, and for special applications it may be maintained less than about 0.
3%.
Naturally larger manganese contents are maintained for obtaining high strength steel sheets.
Aluminum must be contained in amounts not less than 0.
005% as soluble aluminum for desired deoxidation of the steel, but aluminum contents of 0.
2% or larger will very often cause surface defects.
Therefore, the aluminum content should be desirably maintained not more than 0.
06%.
Nitrogen, when contained in excessive amounts, is harmful to the object of the present invention to restrict the occurrence of yield point elongation in the as-annealed condition and assure less ageing
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