Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-02-08
2003-10-21
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S648000, C428S659000, C428S677000, C428S678000, C428S939000
Reexamination Certificate
active
06635359
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a highly corrosion resistant Al—Zn—Mg—Si alloy-plated steel material and to a process for its production.
BACKGROUND ART
Zn plating of steel surfaces for improved corrosion resistance has been widely known in the prior art, and materials with Zn platings are currently produced in mass. Zn—Al alloy platings have even been proposed as a means of further improving corrosion resistance. Such an Zn—Al alloy plating is proposed in Japanese Patent No. 617,971. Specifically, there is disclosed an alloy plating comprising Al at 25-75%, Si at 0.5% or more of the Al content and with the remainder consisting of substantially Zn, wherein the Zn—Al alloy obtained exhibits excellent corrosion resistance as well as satisfactory adhesion to steel sheets and an attractive outer appearance. Such Zn—Al alloy platings provide especially excellent corrosion resistance compared to conventional Zn platings.
It is currently the situation, however, that when Zn—Al plated steel sheets fabricated in this manner are subjected to cutting, the exhibited corrosion resistance at the cut edges is insufficient. This occurs because, although corrosion of the steel sheet sections exposed at the cut edges is prevented by the sacrificial rusting effect of the Zn, the Zn component is lost from the Zn-segregated sections of the Zn—Al alloy plating layer, thus lowering the corrosion resistance. Also, when the plating layer is further coated with paint or laminated with a plastic film, the corrosion product resulting from selective corrosion of Zn accumulates, creating film blisters or so-called edge creep, and thus notably reducing the product value.
As a means of improving the corrosion resistance of cut edges of painted Zn—Al alloy platings, Japanese Patent No. 1,330,504 discloses an alloy plating containing Mg at 0.01-1.0% in a Zn—Al alloy layer, and although a slight effect is exhibited, the technique does not provide a thorough solution to the problem of edge corrosion. A similar technique is disclosed in Japanese Examined Patent Publication HEI No. 3-21627, as a plating which comprises 3-20% Mg, 3-15% Si and the remainder Al and Zn with an Al/Zn ratio of 1-1.5, and which is characterized by having a structure with Al-rich dendritic crystals as well as Zn-rich dendritic crystals and an intermetallic compound phase comprising Mg
2
Si, MgZn
2
, SiO
2
and Mg
32
(Al,Zn)
49
.
The results of experimentation by the present inventors have revealed that although plated steel sheets employing the platings disclosed in the prior art sometimes exhibit vastly improved corrosion resistance compared to Zn—Al plated steel sheets containing no Mg or Si, the workability of the platings differs depending on the Mg and Si content, and on the proportion and the form and size of the deposited Mg
2
Si phase, such that considerable variation is exhibited in terms of the corrosion resistance. Particularly as concerns the size of the Mg
2
Si phase, the observed size also differs depending on the method of observing the structure, and especially depending on the sample embedding angle when observing the cross-sectional composition, and it was found to be important to carry out measurement of the size by a more precise method while controlling the size.
It was also found that if the content of the deposited Mg
2
si phase is kept at above a certain value, even outside of the range of the composition disclosed in the aforementioned prior art, there exists a range in which the corrosion resistance is vastly improved compared to conventional Zn—Al plated steel sheets.
Another prior art example of controlling the amount of the Mg
2
Si phase in the plating phase is found in U.S. Pat. No. 3,026,606, which discloses a technique whereby the Mg
2
Si phase in the Al plating phase is controlled in a range of 4-25% and the thickness of the alloy phase produced at the interface between the plating phase and the base iron is minimized; however, the Mg
2
Si phase is not utilized as the means for improving corrosion resistance.
The present invention provides a highly corrosion resistant Zn—Al—Mg—Si alloy-plated steel sheet having a controlled content of Mg and Si added to a Zn—Al based plating and a controlled deposition amount and deposition form of the Mg
2
Si phase which exhibits an effect of improving corrosion resistance, as well as a process for its production.
DISCLOSURE OF THE INVENTION
As a result of diligent research aimed at solving the problems described above, the present inventors have completed the present invention upon finding that by adding Mg and Si in an appropriate range to Zn—Al alloy and controlling the structure thereof, it is possible to provide an alloy plating with not only unpainted corrosion resistance but also exceptional edge creep resistance at cut edge sections after painting, which has not been achievable by the prior art.
In other words, the gist of the present invention is as follows. (
1) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance, characterized by comprising, in terms of wt %,
Al: at least 45% and no greater than 70%
Mg: at least 3% and less than 10%
Si: at least 3% and less than 10%,
with the remainder Zn and unavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and the plating layer contains a bulky Mg
2
Si phase.
(2) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance, characterized by comprising, in terms of wt %,
Al: at least 45% and no greater than 70%
Mg: at least 1% and less than 5%
Si: at least 0.5% and less than 3%,
with the remainder Zn and unavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and the plating layer contains a scaly Mg
2
Si phase.
(3) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance according to (1) or (2) above, characterized by further comprising, as the Zn—Al—Mg—Si alloy plating composition, one or more from among In: 0.01-1.0%, Sn: 0.1-10.0%, Ca: 0.01-0.5%, Be: 0.01-0.2%, Ti: 0.01-0.2%, Cu: 0.1-1.0%, Ni: 0.01-0.2%, Co: 0.01-0.3%, Cr: 0.01-0.2%, Mn: 0.01-0.5%, Fe: 0.01-3.0% and Sr: 0.01-0.5%.
(4) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance, characterized in that the bulky Mg
2
Si phase of (1) above has a long diameter mean size of 3-50 &mgr;m, the area ratio of particles with a long diameter exceeding 100 &mgr;m is no more than 10% of the bulky Mg
2
Si phase, and the ratio of the short diameter to the long diameter is at least 0.4, as observed with a 5° inclination polished cross-section.
(5) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance, characterized in that the scaly Mg
2
Si phase of (2) above has a long diameter mean size of 3-50 &mgr;m, and the ratio of the short diameter to the long diameter is less than 0.4, as observed with a 5° inclination polished cross-section.
(6) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance according to (1), (3) or (4) above, characterized in that the total content of the bulky and scaly Mg
2
Si phases in the plating layer is 10-30% as the area ratio when observed with a 5° inclination polished cross-section, and the area ratio of bulky Mg
2
Si to the total Mg
2
Si phase is at least 1%.
(7) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance according to (2), (3) or (5) above, characterized in that the content of the scaly Mg
2
Si phase in the plating layer is at least 3% as the area ratio when observed with a 5° inclination polished cross-section.
(8) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance according to any one of (1) to (7) above, characterized by having a preplating layer containing one or more from among Ni, Co, Zn, Sn, Fe and Cu and/or an intermetallic compound phase comprising two or more from among Ni, Co, Zn, Sn, Fe and Cu, at the interface between the plating layer and the steel material.
(9) A Zn—Al—Mg—Si alloy-plated steel material with excellent corrosion resistance according to any one of (1) to (8) above, characterized in that the plat
Goto Osamu
Kurosaki Masao
Maki Jun
Morimoto Yasuhide
Nishimura Kazumi
Kenyon & Kenyon
Koehler Robert R.
Nippon Steel Corporation
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