This invention relates to a high Al hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet whose plating layer has an Al content on a level of more than 10 to 22 mass %.
The good corrosion resistance of hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheets produced using a plating bath containing suitable amounts of Al and Mg in Zn has long made them a focus of various development and research. In the production of hot-dip plated steel sheet of this type, however, spot-like crystal phase appears on the plated steel sheet surface. After standing for a while, the spot portions turn grayish black and give the sheet surface an ugly appearance. Despite being excellent in corrosion resistance, therefore, hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet has been slow to gain acceptance as an industrial product.
Through extensive studies the inventors ascertained that the spot-like crystal phase is Zn11Mg2 phase. Based on this finding, they defined a metallic structure for a Znxe2x80x94Alxe2x80x94Mg plating layer containing Al: 4-10% and Mg: 1-4% that inhibits crystallization of the Zn11Mg2 phase and presents a good appearance. They also developed a production method for obtaining the metallic structure. The metallic structure and production method are described in JPA. 10-226865 and JPA. 10-306357.
Thanks to the metallic structure and production method proposed by the inventors, it has become possible to produce industrial-quality hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet with a plating layer Al content on the 4-10% level that does not have an ugly spotted appearance. However, no study has been reported regarding whether production of such a high-quality hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet is possible when the Al content is high, e.g., when the plating layer contains Al in excess of 10 mass %. The literature also offers little data regarding the corrosion resistance of hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet having an Al content in such a high region.
On the other hand, it is known that increasing the Al content of a Zn-base plating offers such advantages as improved heat resistance. This suggest that it could well be worth while to look into the feasibility of developing commercial Znxe2x80x94Alxe2x80x94Mg plated steel sheet products in the high Al region of an Al content exceeding 10 mass %. In fact, however, little research has been done in this direction.
The reason for this can be traced at least in part to the reported corrosion resistance of Znxe2x80x94Al plated steel sheet found in outdoor exposure tests. These show that corrosion resistance improves with increasing Al content up to a plating layer Al content of around 10 mass % but then begins to degenerate when the content exceeds about 10 mass %. It was held that the tendency to degenerate in corrosion resistance would continue up to an Al content of approximately 20 mass % (See Iron and Steel, 1980, No. 7, p.821-834, FIG. 2). As nothing contrary to this was reported, it came to be considered an established theory. In including Al in a Zn-base plating layer, therefore, the ordinary practice is, from the viewpoint of corrosion resistance (particularly outdoor exposure performance), to avoid the Al content range of approximately 10-20 mass %.
Moreover, when the Al content of the plating layer exceeds 10 mass %, an alloy layer composed mainly of an intermetallic compound between the steel sheet base metal and the plating layer very readily forms. This, too, has hindered development of hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet in the high Al content region. Formation of this alloy layer markedly degrades plating adhesion, making use in applications where forming property is important difficult.
An object of the present invention is therefore to determine the upper limit of Al content and Mg content in an industrially producible hot-dip Zn-base plating layer and to provide a high corrosion resistance hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet that, in the high Al content region exceeding 10 mass %, has excellent quality thoroughly capable of standing up to practical use as an industrial product.
An in-depth study carried out by the inventors clarified that, differently from the known corrosion resistance behavior of an Al-containing Zn-base plated steel sheet, the corrosion resistance (particularly the outdoor exposure performance) of a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet does not degenerate whatsoever when the Al content of the plating layer exceeds 10 mass %. This corrosion resistance behavior, which is not predictable from conventional knowledge, was concluded to be an effect produced by combined addition of Al and Mg.
In the hot-dip plating layer Al content region of greater than around 5 mass %, the melting point of the plating metal rises with increasing Al content, and the plating bath temperature must be raised proportionally during the plating operation. However, increasing the plating bath temperature shortens the service life of the equipment in the plating bath and tends to increase the amount of dross in the bath. The higher the Al concentration, therefore, the more desirable it is to keep the bath temperature as low as possible, i.e., keep the bath temperature as close to the melting point as possible. From the viewpoint of obtaining a plated steel sheet of good appearance when using a Znxe2x80x94Alxe2x80x94Mg system, it is important to maintain the metallic structure of the plating layer in the specified form explained in the following. An effective way to achieve this is, it was found, to set the plating bath temperature high, for example, to set a plating bath temperature that is 40xc2x0 C. or more higher than the melting point. Production of a plated steel sheet with good surface appearance at low cost and high productivity is therefore not easy in the high plating layer Al content region above 10 mass %.
Further study showed that inclusion of suitable amounts of Ti and B in the plating layer markedly inhibited generation of the Zn11Mg2 crystal phase that degrades surface appearance. This led to the discovery that the range of plating bath temperature conditions within which Znxe2x80x94Alxe2x80x94Mg plated steel sheet with good surface appearance is obtainable can be expanded. Moreover, this effect was found also to be well expressed in the high plating layer Al content region above 10 mass %. In other words, combined addition of Ti and B was found to enable production of hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet having a plating layer Al content exceeding 10 mass % at a low plating bath temperature closer to the melting point of the plating metal.
Moreover, it was ascertained that inclusion of a suitable amount of Si in the plating layer of a such a high Al hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet markedly reduces the amount of alloy layer generated and, as such, is highly effective for improving plating adherence. The present invention was accomplished based on the foregoing newly acquired knowledge.
Specifically, the present invention achieves the foregoing object by providing a high Al hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet obtained by forming on a steel sheet surface a hot-dip plating layer comprising, in mass %, Al: more than 10 to 22%, Mg: 1-5%, Ti: 0.002-0.1% and B: 0.001-0.045%, and, optionally, Si: 0.005-0.5% and the balance of Zn and unavoidable impurities.
As a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet enabling a good surface appearance to be obtained with high reliability, the present invention further provides a high Al hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet obtained by forming on a steel sheet surface a hot-dip Zn-base plating layer of a composition containing, in mass %, Al: more than 10 to 22% and Mg: 1-5%, which plating layer exhibits a metallic structure of [primary crystal Al phase] mixed in a matrix of [Al/Zn/Zn2Mg ternary eutectic crystal structure]. In a preferred aspect, the present invention provides a plated steel sheet wherein substantially no Zn11Mg2 phase is present in these metallic structures. By xe2x80x9csubstantially no Zn11Mg2 phase is presentxe2x80x9d is meant that the Zn11Mg2 phase is not detected by X-ray diffraction.
The invention further provides plated steel sheets having preferable compositions of the hot-dip Zn-base plating layer exhibiting the aforesaid metallic structure. Specifically, the invention provides as four embodiments plated steel sheets whose hot-dip Zn-base plating layer composition comprises:
i) in mass %, Al: more than 10 to 22%, Mg: 1-5% and the balance of Zn and unavoidable impurities,
ii) in mass %, Al: more than 10 to 22%, Mg: 1-5%, Ti: 0.002-0.1%, B: 0.001-0.045% and the balance of Zn and unavoidable impurities,
iii) in mass %, Al: more than 10 to 22%, Mg: 1-5%, Si: 0.005-0.5% and the balance of Zn and unavoidable impurities, and
iv) in mass %, Al: more than 10 to 22%, Mg: 1-5%, Ti: 0.002-0.1%, B: 0.001-0.045%, Si: 0.005-0.5% and the balance of Zn and unavoidable impurities.