Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Patent
1997-09-11
2000-12-12
Speer, Timothy
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
428939, 427433, 148533, B32B 1518, C21D 109
Patent
active
061596221
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a galvannealed steel sheet suitable for use in automobiles, plated with a metallic coating having excellent powdering resistance and chipping resistance, as well as to a manufacturing method thereof.
BACKGROUND OF THE INVENTION
A galvannealed steel sheet has recently been in widespread use in various sectors of industry such as automobiles, electric home appliances, construction materials, and the like because of their excellent weldability, paintability, corrosion resistance, economic merits, and the like. A high strength galvannealed steel sheet having good press formability is also demanded from the viewpoint of promoting safety and weight reduction of automobiles. Therefore, the galvannealed steel sheet is required which can meet all of the aforesaid requirement.
Normally, the galvannealed steel sheet is manufactured by heating up a hot--dip galvanized steel sheet to a temperature in the range of 500 to 600.degree. C. for a retention time of 3 to 60 seconds in a heating furnace for Fe--Zn alloying. By applying a Fe--Zn alloying treatment as above, a Zn layer composing an original metallic coating is turned into a Fe--Zn alloy layer containing normally 8 to 12 wt % of Fe. A coating weight of the metallic coating after the treatment, that is, a Fe--Zn alloy layer, is normally 20 to 70 g /m.sup.2 of the surface on one side of the steel sheet.
In application of the galvannealed steel sheet for manufacturing automobile body parts, such properties as powdering resistance and chipping resistance are important. Powdering is a phenomenon in which the metallic coating is broken into fine pieces and exfoliated at sites where the steel sheet is subjected to compressive deformation during press forming, and the like. Not only corrosion resistance is degraded at sites of the steel sheet where powdering occurs, but also fine pieces of the exfoliated coating, adhered to press dies, give rise to a cause for surface defects of a formed product. Various measures have been adopted for preventing powdering, including reduction in a Zn coating weight, restriction on Al concentration in a plating bath, restriction on Fe--Zn alloying conditions and Fe content of a galvannealed coating.
Chipping is a phenomenon in which the galvannealed coating exfoliates from the surface of a base metal, occurring, for example, when pebbles, and the like, collide with a running automobile, and the impact force of the pebbles is applied to the painted surface of the automobile body. Automobiles in service in cold environments are susceptible to the chipping phenomena.
Since both powdering and chipping are phenomena whereby the galvannealed coating exfoliates, it has been considered that enhancement in powdering resistance would be accompanied by improvement in chipping resistance. However, it has since been found that enhanced powdering resistance does not necessarily result in improved chipping resistance, and adhesion property at the interface between the base metal and the galvannealed coating needs to be enhanced in order to improve chipping resistance.
For example, a method of manufacturing a galvannealed steel sheet focusing on improvement in the adhesive property at the interface between the base metal and the galvannealed coating is disclosed in Japanese Patent Publication Laid-open (Kokai) No. Hei 2-97653. The steel sheet according to the aforesaid invention has a micro-structure formed by diffusion of Zn into the grain boundaries on the surface of a base metal. The steel sheet described above is manufactured by plating a base metal in a hot-dip galvanizing bath containing Al in a concentration set much higher than for normal cases, and by applying the Fe--Zn alloying treatment at higher temperature than for normal cases. However, use of a plating bath containing Al in higher concentration requires application of the Fe--Zn alloying treatment at higher temperature and for longer period of time than for normal cases. Powdering resistance tends to be impaired when the Fe--Zn al
REFERENCES:
patent: 5897967 (1999-04-01), Hori et al.
"Coating Microstructure Assessment and Control For Advanced Product Properties of Galvannealed IF Steels," by W. van Koesveld et al., Galvatech '95 Conference Proceedings(Sep. 17-21, 1995), pp. 343-355.
"Laboratory Investigations on the morphology of the Coating and the Forming Behavior of Galvannealed Steel Sheet," by R. Brisberger et al., Galvatech '95 Conference Proceedings(Sep. 17-21, 1995), pp. 753-759.
Hori Masahiko
Miki Keiji
Nakamori Toshio
Resnick Jason
Speer Timothy
Sumitomo Metal Industries Ltd.
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