Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-02-28
2001-04-17
Sheehan, John P (Department: 1742)
Metal treatment
Stock
Ferrous
C148S330000, C420S121000, C420S124000, C420S126000, C420S127000
Reexamination Certificate
active
06217675
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a steel sheet, more particularly to a cold rolled steel sheet having improved bake hardenability.
BARCKGROUND ART
For example, Japanese Patent Laid-Open Nos. 141526/1980 and 141555/1980 disclose a method for improving the bake hardenability of cold rolled steel sheets. Specifically, regarding niobium-containing steels, a method is known wherein niobium is added in an amount depending upon the contents of carbon, nitrogen, and aluminum in the steel to limit, in terms of at. %, niobium/(carbon in solid solution+nitrogen in solid solution) to a certain range, thereby regulating the content of carbon in solid solution and the content of nitrogen in solid solution in steel sheets and, in addition, regulating the cooling rate after annealing. Another method known in the art is such that titanium and niobium are added in combination to prepare a steel sheet having excellent bake hardenability (Japanese Patent Laid-Open No. 45689/1986). Mere regulation of the content of carbon in solid solution to the certain range, however, leads to only an expectation of an improvement in bake hardenability of about 30 MPa at the highest. Increasing the amount of carbon in solid solution in order to further improve the bake hardenability results in deteriorated age hardenability which poses a problem that pressing after storage for a long period of time causes a stripe pattern called “stretcher strain.” For this reason, satisfying both excellent bake hardenability and excellent age hardenability has been regarded as difficult and thus has been a problem to be solved for many years.
Against this, Japanese Patent Laid-Open Publication Nos. 109927/1987 and 120217/1992 disclose that both bake hardenability and age hardenability are provided by utilizing molybdenum. According to finding by the present inventors, these methods specify only the content range of molybdenum as the additive element. In fact, however, the proposed methods are technically very unstable because the contemplated effect can be attained in some cases and cannot be attained in other cases depending upon the carbon content and the titanium and niobium contents. For example, in the prior art, regarding the addition of molybdenum, a mere description is found such that the amount of molybdenum added is in the range of 0.001 to 3.0% or in the range of 0.02 to 0.16%. That is, in the above methods, only sole use of molybdenum is accepted. Mere regulation of the amount of molybdenum added cannot provide a constant effect, and the level of the baking effect is 50 MPa in some cases and is as low as 10 MPa in other cases.
On the other hand, on the market, lightening of automobiles has led to an ever-increasing demand for an improvement in bake hardenability, and further improved bake hardenability and delay aging have become required in the art.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a cold rolled steel sheet which is simultaneously improved in both bake hardenability and delay aging, can ensure a stable bake hardening level, and, in addition, has larger bake hardenability than the prior art product.
According to one aspect of the present invention, there is provided a cold rolled steel sheet having improved bake hardenability, comprising by weight
carbon: 0.0013 to 0.007%,
silicon: 0.001 to 0.08%,
manganese: 0.01 to 0.9%,
phosphorus: 0.001 to 0.10%,
sulfur: not more than 0.030%,
aluminum: 0.001 to 0.1%, and
nitrogen: not more than 0.01%, said steel sheet further comprising
titanium: 0.001 to 0.025% and
niobium: 0.001 to 0.040%, the titanium and niobium contents satisfying k value defined by the following formula:
k
=%C−12/93×%Nb−12/48
33
(%Ti−48/14×%N)≧0.0008
wherein k=0 when %Ti−48/14×%N≦0,
said steel sheet containing molybdenum as an additive on a level satisfying the following formulae:
0.005≦%Mo≦0.25
and
0.1
×{square root over ( )}k≦
%Mo≦5
×{square root over ( )}k
wherein k is as defined above.
According to a preferred embodiment of the present invention, boron is further added on a level satisfying the following formulae:
0.005
×{square root over ( )}k≦
%B≦0.08
×{square root over ( )}k,
and
%Mo/300≦%B≦%Mo/4.
Further, according to a preferred embodiment of the present invention, the dislocation density is 50 to 3,000 dislocation lines per &mgr;m
2
of plane field.
REFERENCES:
patent: 5356494 (1994-10-01), Okada et al.
patent: 5558726 (1996-09-01), Yatoh et al.
patent: 5954896 (1999-09-01), Koyama et al.
patent: 61-250113 (1986-11-01), None
patent: 63-241122 (1988-10-01), None
patent: 1-191739 (1989-08-01), None
patent: 4-323346 (1992-11-01), None
patent: 5-125484 (1993-05-01), None
patent: 5-331553 (1993-12-01), None
Goto Koichi
Taniguchi Hirokazu
Yamazaki Kazumasa
Kenyon & Kenyon
Nippon Steel Corporation
Sheehan John P
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