Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Patent
1997-09-12
2000-02-29
Zimmerman, John J.
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
427436, 427431, 427406, 148276, 148533, 148601, 148602, B32B 1518, B05D 118, C23C 2800, C21D 802
Patent
active
060307145
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a zinc and zinc-alloy hot-dip-coated steel sheet having a decreased number of bare spots and excellent coating adhesion, and a method for manufacturing the same.
BACKGROUND ART
Zinc and zinc-alloy hot-dip-coated steel sheets are mainly used for automobile bodies because of low cost and excellent corrosion resistance, and in addition to the corrosion resistance due to coating, coating adhesion during press working is required for applying the steel sheets to automobile bodies. When coating adhesion deteriorates, coated layers peel as a powder or blocks, which phenomenon sometimes causes galling in press forming or deteriorates corrosion resistance of the portions from which the coated layer peels; and also, peeled fragments disadvantageously inflict the steel sheet.
As a conventional technique for improving coating adhesion, Japanese Patent Laid-Open No. 61-276961 discloses a technique in which alloying Fe with Zn at a high temperature ranging from 700 to 850.degree. C. is performed after zinc hot-dip-coating. However, alloying at a high temperature lead to not only higher costs but also increased expenses for equipment such as rolls.
Additionally, in Japanese Patent Laid-Open No. 3-232926, steel contains at least one of Zr, La, Ce, Y, and Ca, and the cooling rate from recrystallization annealing to coating is set to not less than 50.degree. C./sec. The cost is raised due to the addition of Zr or the like to steel and productivity deteriorates because the sheet-feeding rate has to be lowered due to the cooling capacity.
Furthermore, in Japanese Patent Laid-Open No. 2-163356, the O, Al, and N contents in steel are set to not more than 0.0045 wt %, (25.times.N wt %) to 0.15 wt %, and not more than 0.0030%, respectively. Moreover, restrictions on the Ti, Si, and P contents, and Si (wt %)+P (wt %).gtoreq.Ti (wt %) must be satisfied according to Japanese Patent Laid-Open No. 6-81101. Anyway, the desired steel-sheet properties such as strength and drawing cannot be always achieved by such content restrictions, and there is a possibility that coating adhesion will deteriorate because of deviations from a predetermined composition range.
In Japanese Patent Laid-Open No. 4-333552, coating adhesion is improved by carrying out Ni pre-plating before galvanizing. However, in general, a continuous galvanizing line (hereinafter referred to as "CGL") does not have such equipment, and a large investment is required for improving equipment or the like.
Meanwhile, automobile bodies are required to be lighter because of recent regulations for exhaust gas. Thinning the steel sheets is a method for lightening the automobile bodies. According to this method, it is necessary for ensuring safety to increase steel-sheet strength corresponding to the decreased thickness. Thus, high tensile-strength steel sheets have been developed for strengthening the steel sheets by increasing the steel contents of elements such as Si, Mn, and P. Since steel sheets for automobiles are subjected to press forming, excellent material characteristics with a high r-value (high Lankford value) are required, and in particular, the addition of these elements is essential for high-tensile strength steel sheets.
In the case of zinc hot-dip-coating such steel sheets, recrystallization annealing at a high temperature ranging from approximately 700 to 900.degree. C. is necessary to attain excellent material characteristics. In the CGL, recrystallization annealing is generally carried out under a nitrogen atmosphere in the presence of hydrogen (hereinafter referred to as reduction annealing), and although this atmosphere is a reducing atmosphere for Fe, it is an oxidizing atmosphere for some elements such as Si, Mn, and P. Thus, elements such as Si, Mn, and P (referred to as readily oxidizable elements) which are more oxidizable than Fe externally diffuse during reduction annealing and bond to oxygen on the surface of steel sheets to form oxides (called as "surface segregated layer"). Since these oxides signif
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Fujibayashi Nobue
Kyono Kazuaki
Miyaoka Motonori
Totsuka Nobuo
Kawasaki Steel Corporation
Resnick Jason
Zimmerman John J.
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