Metal treatment – Stock – Ferrous
Patent
1997-03-19
1999-01-05
Yee, Deborah
Metal treatment
Stock
Ferrous
148603, 148652, 148651, C21D 804, C22C 3804, C22C 3806
Patent
active
058556966
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to ultra low carbon, cold rolled steel sheet and galvanized steel sheet, for deep drawing, improved in fatigue properties of the base metal and spot weld zone, and processes for producing the same. The cold rolled steel sheets according to the present invention are those which, after press forming, are used for applications such as automobiles, domestic electric appliances, and buildings, and include both surface untreated cold rolled steel sheets in the narrow sense and cold rolled steel sheets, in the broad sense, which have been subjected to surface treatment for rust preventive purposes, such as Zn plating or alloyed Zn plating, and further provided with an organic film on the plating.
The galvanized steel sheets according to the present invention are similarly those which, after press forming, are used for applications such as automobiles, domestic electric appliances, and buildings and have been subjected to surface treatment for rust preventive purposes, such as galvanizing or alloyed galvanizing.
BACKGROUND ART
Technical advance of vacuum degassing process for a molten steel in recent years has facilitated the production of ultra low carbon steels, leading to an ever-increasing demand for ultra low carbon steel sheets having good workability.
It is well known that such ultra low carbon steel sheets generally contain at least one element selected from the group consisting of Ti and Nb. Ti and Nb exhibit a strong, attractive interaction with interstitial solid solution elements (C, N) in the steel to easily form carbonitrides, enabling a steel free from interstitial solid solution elements (IF steel: interstitial free steel) to be easily produced. IF steels are free from interstitial solid solution elements causative of strain aging and deteriorated workability and, hence, feature a non-aging property and very good workability. Further, the addition of Ti and Nb plays an important role, that is, it refines the diameter of grains, of a hot rolled steel sheet of an ultra low carbon steel, which are likely to be coarsened, and improves the deep drawability of a cold rolled, annealed steel sheet. However, ultra low carbon steels with Ti and Nb added thereto have the following problems. First of all, the production cost is high because the cost associated with the addition of expensive elements such as Ti and Nb is added to the cost of vacuum treatment for achieving ultra low carbon. Secondly, the absence of C and N in solid solution in product sheets results in drawing-induced embrittlement or disappearance of paint bake hardening property (BH property). Thirdly, the base metal and the spot weld zone have poor fatigue properties. The reason for this is that the strength of the material is low due to the nature of the ultra low carbon steel and, in addition, the microstructure of heat-affected zone in the spot welded area is coarsened to form a brittle area. Fourthly, Ti and Nb are strong oxide formers, and the formed oxides deteriorate the surface quality.
A large amount of research and development have been done with a view to solving the above problems of IF steels. One conceivable means for solving the above problems is to use as a base material an ultra low carbon steel with Ti and Nb not added thereto. this is because the use of a steel not containing Ti and Nb, as the base material naturally leads to the solution of the above first, second, and fourth problems. The adoption of such means is found, for example, in Japanese Unexamined Patent Publications (Kokai) No. 63-83230, No. 63-72830, No. 59-80724, No. 60-103129, No. 1-184251, No. 58-141355, and No. 6-93376. In all of the above publications, attention is drawn to the properties influencing the press moldability of an ultra low carbon steel sheet not containing Ti and Nb, such as r value and elongation, and the BH property and the fabrication embrittlement resistance.
Regarding the fatigue properties as the third problem, however, only a few studies have been made. Japanese Unexamined Patent Publi
REFERENCES:
patent: 5470403 (1995-11-01), Yoshinaga et al.
patent: 5486241 (1996-01-01), Ushioda et al.
Fujii Shiro
Itami Atsushi
Sakuma Yasuharu
Tezuka Makoto
Ushioda Kohsaku
Nippon Steel Corporation
Yee Deborah
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