Metal treatment – Stock – Ferrous
Patent
1994-02-18
1995-11-28
Yee, Deborah
Metal treatment
Stock
Ferrous
148603, 148533, 428659, C21D 804, C22C 3806, C22C 3804, B32B 1500
Patent
active
054704030
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a cold rolled steel sheet and a hot dip zinc-coated cold rolled steel sheet having excellent paint bake hardenability, non-aging properties and formability, and a process for producing the same.
The cold rolled steel sheet according to the present invention is used for automobiles, electrical appliances and buildings after press forming. Moreover, the cold rolled steel sheet of the invention includes both a cold rolled steel sheet in a strict sense, namely, a cold rolled steel sheet without surface treatment, and a cold rolled steel sheet subjected to surface treatment such as Zn coating and alloyed Zn coating. Since the steel sheet of the invention has both strength and formability, the thickness of the sheet can be reduced, that is, the sheet can be lightened compared with conventional steel sheets, when used. Accordingly, the use of the sheet is considered to contribute to the maintenance of the global environment.
BACKGROUND OF THE INVENTION
Since the recent progress of vacuum degassing molten steel has made the ingot-making of extremely low carbon steel easy, demand for extremely low carbon steel sheets having good formability has further been increasing. Among the steel sheets, those extremely low carbon steel sheets in which Ti and Nb are compositely added and which are disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 59-31827 and 59-38337 have extremely good formability, bake hardening (BH) properties and excellent hot dip zinc coating characteristics. Accordingly, the extremely low carbon steel sheets occupy an important position. However, the amount of BH of the steel sheets does not exceed the level of conventional BH steel sheets, and has a drawback that they cannot secure their cold non-aging properties when a further amount of BH is tried to be imparted thereto.
Furthermore, there are many disclosures with regard to extremely low carbon steel sheets excellent in formability and containing neither Ti nor Nb, for example, in Japanese Examined Patent Publication (Kokoku) No. 53-22052, and Japanese Unexamined Patent Publication (Kokai) Nos. 58-136721 and 58 -141335.
On the other hand, many attempts have heretofore been made to increase the strength of steels while the formability thereof is being secured. Particularly, in the case of steels which have a tensile strength from 30 to 50 kgf/mm.sup.2 and to which the present invention relates, the strength thereof has been increased by adding P, Si, etc., to the steels and utilizing these solid solution strengthening mechanisms. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 59-31827 and 59-33837 disclose processes for producing high strength cold rolled steel sheets having a tensile strength of up to a class of 45 kgf/mm.sup.2 by adding mainly Si and P to extremely low carbon steel sheets to which Ti and Nb have been added. Japanese Examined Patent Publication (Kokai) No. 57-57945 disclose a typical prior technique relating to a process for producing a high strength cold rolled steel sheet by adding P to an extremely low carbon steel to which Ti has been added.
Moreover, in extremely low carbon steels containing neither Ti nor Nb, Japanese Examined Patent Publication (Kokoku) No. 58-57492 and Japanese Unexamined Patent Publication (Kokai) No. 58-48636 disclose techniques for highly strengthening steels by adding P, and Japanese Unexamined Patent Publication (Kokai) No. 57-43932 discloses a technique for utilizing Si.
As described above, heretofore P firstly and Si secondly have often been used as strengthening elements for reasons as described below. It has been considered that the addition of small amounts of P and Si increases the strength of a steel because of their very high solid solution strengthening capability, that the addition does not lower the ductility and deep drawability of the steel much, and that the addition cost does not increase much. However, in reality, when strengthening the steel with only these elements is tried, not o
REFERENCES:
patent: 4368084 (1983-01-01), Irie et al.
Abstract of JP 4-214820 published Aug. 5, 1992.
Abstract of JP 5-78783 published Mar. 30, 1993.
Abstract of JP 5-78784 published Mar. 30, 1993.
Supplementary European Search Report EP 93 91 3564 Nov. 18, 1994.
Akisue Osamu
Nishimura Kunio
Ushioda Kohsaku
Yoshinaga Naoki
Nippon Steel Corporation
Yee Deborah
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