JPH09111433A - Al-Zn alloy plated steel sheet excellent in workability and method for producing the same - Google Patents

Abstract

(57) [PROBLEMS] To provide a hot-dip Al-Zn alloy-plated steel sheet having a plating film that can withstand severe processing and a method for producing the same. SOLUTION: Al in weight% is 25 to 75%, and the amount of Al is 0.5.
~ 5.0% of Si, one or two of Zr and Hf in a total amount of 0.01 to 0.50%, and Ti: 0 to 0.50%, and the balance having a surface plating film consisting of zinc and unavoidable impurities. A hot dip Al-Zn alloy plated steel sheet. The workability of the coating is further improved by performing a heat treatment in which the steel sheet is heated to 150 to 350 ° C.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip Al-Zn alloy-plated steel sheet excellent in workability of a coating and a method for producing the same.

2. Description of the Related Art Al is 25 to 75%, and the amount of Al is 0.5 to 5.0.
% Hot-rolled Al-Zn alloy-plated steel sheet having a coating coating consisting of zinc and unavoidable impurities has a corrosion resistance several times higher than that of hot-dip galvanized steel sheet, It has heat resistance and heat reflectivity of
Moreover, it also has the corrosion resistance of the end face, which is not possible with hot-dip aluminum plating. A typical example of such a steel sheet is Z
There are n-55% Al-1.6% Si alloy plated steel sheets, but due to their excellent corrosion resistance, building material products such as roofing materials and wall materials, guard rails, civil engineering products such as wiring pipes and soundproof walls, automobile exhaust systems, and electrical equipment. It is rapidly spreading to applications such as home appliances such as washing machines and microwave ovens. However, the workability of the coating film of the hot dip Al-Zn alloy-plated steel sheet is poor, and for example, when the radius of curvature of bending becomes small, cracks occur in the plating layer and the corrosion resistance of that portion is poor. There are several reasons why the coating processability is poor. First, since the alloy of the coating film is a hypereutectic alloy of Al with respect to Zn, if it is cooled slowly from the molten state, the Zn-rich phase fills the space between the dendrites with the Al-rich primary crystals. It becomes a solidified tissue. In this case, the rapid cooling after plating makes the structure of the coating finer, but the residual strain increases and the workability thereof deteriorates. Also, Al in the plating is
Al-Fe reacts rapidly with Fe at a temperature around the plating bath
To make an alloy, Si is added to the bath to suppress it. However, this reaction could not be completely suppressed, and A
A layer mainly containing an intermetallic compound such as 1-Fe or Al-Fe-Si is formed near the interface between the steel and the plating. These compounds are extremely brittle and reduce the processability of the coating. Further Al
The content of Si for suppressing the reaction between Fe and Fe comes to exceed the solid solubility limit near the solidification temperature of the plating alloy,
Needle-like hard metal crystals of Si precipitate between the dendrites, which also act as the origin of cracks during deformation and deteriorate the workability of the coating. In order to improve the inferior workability of such an Al-Zn alloy plated layer, there is a method of performing a heat treatment after plating, but it is not effective if it is not heated at a low temperature for a long time (for example, 200 ° C, 24 hours). Can't get Since such a long-time process is difficult to perform on a continuous line,
An invention for reducing the heating time by performing shot blasting treatment on a plated steel sheet is disclosed in JP-A-4-41657. Since the blasting process shortens the time, it is said that the heat treatment of the plating layer is possible even with a short heating time for baking the paint when coating on the plated steel sheet.
However, both the low temperature and long time treatment and the shot blasting treatment require separate steps, which lengthens the steps and results in a considerable cost increase. On the other hand, a method of adding an element such as Sr, V or Cr to the plating bath to improve the workability of the plating film has been reported (AL
ZINC PLUS: A NEW CONTINUOUS HOT-DIP 55% Al-Zn PROTE
CTIVE COATING: M.Lamberigts, et al; GALVATEC'92, Am
sterdam (1992), p.412). By adding a small amount of these elements, the structure of the plating layer is refined, the structure of the interface between the steel and the plating layer is changed, and the acicular precipitated particles of Si are spheroidized to improve the workability of the plating film. However, the workability improving effect is not always sufficient. As described above, regarding the Al-Zn alloy plated steel sheet, various studies have been made for improving the workability of the plating film, but provision of a steel sheet having stable and sufficient film workability,
It is hard to say that a low-cost manufacturing method that can be easily adopted industrially has been established.

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot-dip Al-Zn alloy plated steel sheet having a plating film that can withstand severe working and a method for producing the same.

DISCLOSURE OF THE INVENTION The inventors of the present invention have performed hot-dip Al-Zn alloy plating on a steel sheet, that is, 25 to 75% Al,
In order to improve the workability of a plating film containing 1% of O.5 to 5.0% of Si and the balance of Zn and unavoidable impurities, various studies were conducted with the main problem of improving the structure of the film. It was The thermal history from melting to solidification of the Al-Zn alloy after adhesion on the surface of the steel sheet is basically the same as that of a normal hot dip galvanizing line except that the bath temperature is as high as around 600 ° C, and cannot be largely changed. Therefore, in order to change the solidification structure under almost constant cooling conditions, it was considered that various additive elements should be studied. Therefore, as a result of examining the influence of the additional element on the solidification state of the Al-Zn alloy, Z
It has been found that the addition of an appropriate amount of r, Hf or Ti has a remarkable effect of particularly lowering the hardness of the coating and improving the workability. Further, it has been clarified that when these elements are added, heating after plating further enhances the effect of improving the workability of the coating. From these findings, the limit of the effect of addition and the effective range of heat treatment were clarified, and the present invention was completed. The gist of the present invention is as follows. (1) Al: 25-75% by weight%, 0.5-5.0% of Al amount
The total amount of one or two of Si, Zr and Hf of 0.01 to
A hot-dip Al-Zn alloy plated steel sheet containing 0.50% and Ti: 0 to 0.50%, and the balance having a surface plating film consisting of zinc and unavoidable impurities. (2) The molten Al-Zn according to (1) above, which is characterized by performing a heat treatment of heating at 150 to 350 ° C after plating.
Manufacturing method of alloy plated steel sheet. The reason why the hardness of the coating is lowered by the addition of Zr, Hf, or Ti is not clear, but it is considered that one of the reasons is that it has the effect of refining the metal structure of the coating.
When the molten Al-Zn alloy adheres to the steel sheet and is rapidly cooled and solidifies from the supercooled state, Zr, Hf or Ti is
It is presumed that an intermetallic compound with Al or Zn was rapidly formed, and these were finely dispersed to form many solidification nuclei, which changed the solidification structure. Further, strain and internal stress remain in the rapidly solidified coating, which tends to deteriorate the workability of the coating. However, the heat treatment releases the residual stress of the structure, further improving the workability. Since the structure is made fine in the present invention, it is considered that the strain can be released easily and a sufficient effect can be obtained in a short time.

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the Al of the plating layer is
The reason why the content is limited to 25 to 75% is that the corrosion resistance of the steel sheet becomes inferior even if the content is more or less than this range. With respect to this Al content, Si is 0.5 to 5.0 of the Al content.
%. The addition of Si is for suppressing the alloying reaction of Fe and Al that rapidly progresses at the plating temperature to prevent the development of the alloy layer, maintain the corrosion resistance and improve the adhesion of the plating film. If the amount of Al is less than 0.5%, the suppressing effect is insufficient. If the amount is too large, not only the effect is saturated, but also the generation of bath dross increases and the product surface quality is deteriorated, and the metal Si particles in the plating layer increase and the workability of the coating deteriorates. Up to 5.0% of the Al amount. The typical composition is Al: 55%, Si: 1.6%
(2.9% of Al amount). The alloy of the plating layer contains 0.01 to 0.50% of the total amount of Zr and Hf. By adding these elements to the bath for the purpose of inclusion in the plating layer, the workability of the coating is greatly improved. In that case,
If the content in the coating is less than 0.01%, the effect of improving the workability is small, and if it exceeds 0.50%, the effect is saturated and the dross of the bath increases. Therefore, the upper limit of the content is 0.50%. Ti also has the effect of refining the structure of the coating and improving workability. If Zr and Hf are contained to obtain a sufficient effect, Ti may not be added.
It is possible to further improve the homogenization of the coating and to reduce the amounts of Zr and Hf to obtain the same effect. The desirable content for obtaining the effect of addition is 0.01% or more, but the effect is saturated even if a large amount is added, so the upper limit of the content in the plating film alloy is 0.50%. In the case of the steel sheet of the present invention, the workability of the coating film is as-is, even after being plated on the hot dip coating line.
It is superior to that of the conventional hot-dip Al-Zn alloy plated steel sheet. However, the workability of the coating can be further improved by the heat treatment of heating in the temperature range of 150 to 350 ° C. The holding time of heating is not particularly specified, but longer is better when the temperature is low and shorter if the temperature is high. If the temperature is lower than 150 ° C, the workability improvement effect is small even if it is kept for a long time, and if the temperature exceeds 350 ° C, not only the surface changes color from gray to gray black, but also the corrosion resistance decreases and the alloy layer develops. Adhesion of the plating will decrease. Therefore, the heating temperature range is set to 150-350 ℃,
Desirable is heating at 170 to 340 ℃ for about 2 to 10 minutes. Such heat treatment can be realized by baking the paint when the coated steel sheet is further coated, and when processing is performed after coating the surface, cracks that occur in the plated portion of the coating base are suppressed and Corrosion resistance during use can be improved.

[Example] As a material steel plate, a low carbon Al-killed steel plate having a plate thickness of 0.8 mm (compositional analysis value: weight% ... C: 0.04, Si: 0.0
1, Mn: 0.26, P: 0.015, S: 0.012, sol.Al:
0.023, N: 0.001), width 80mm, length 200m
Preheating, reduction annealing, and hot dip coating were performed using a vertical type hot dip galvanizing tester capable of directly charging a hot dip plating bath from a reducing atmosphere using a test piece of m 2. The plating conditions are shown in Table 1. To prepare the components of the plating bath, 9
9.99% Zn, 99.9% Al pure metal and 13% Si
-Al, 5% Zr-Al, 5% Hf-Al, 5% Ti-
A mother alloy such as Al was used as appropriate. Table 2 shows the composition of the coating film of the trial plating. When performing heat treatment after plating, put the test piece in a constant temperature electric furnace heated to a predetermined temperature,
A method of holding for a predetermined time was adopted. The atmosphere is in the atmosphere.

[Table 1]

[Table 2] As a performance evaluation of the steel sheet after plating, surface appearance investigation, 2
A t-bend or close-bend (t = 0) test and a corrosion resistance test were performed. The surface appearance was evaluated comprehensively by visual inspection, using a color difference meter and a gloss meter together. To evaluate the workability by bending, a test piece with a width of 15 mm and a length of 100 mm is cut out, bent for 2 t at right angles to the longitudinal direction or closely bent, and then embedded in resin to polish the cross section of the bent part. The plating film was observed with an electron microscope. In this case, among the cracks generated in the coating on the outside of the bending, the number of cracks reaching from the surface to the underlying iron was counted and used as the evaluation value. The smaller the number, the better the workability of the coating. Evaluation of corrosion resistance is
JIS-Z-2371 with test pieces that seal the end and back surfaces
The salt spray test based on the above was performed, and the time until the occurrence of red rust was measured. Table 2 also shows these results. As is clear from these results, the steel sheet having the coating composition defined by the present invention has excellent workability and corrosion resistance. Although it has sufficient workability even when it is plated, further heat treatment further reduces the occurrence of cracks due to bending and improves workability.

EFFECTS OF THE INVENTION The hot-dip Al-Zn alloy-plated steel sheet of the present invention retains its advantages of excellent corrosion resistance, heat resistance and heat reflectivity as it is, and the workability of the coating is greatly improved as compared with the prior art. . In addition, the workability of the plating film is further improved by heating during baking coating, and stable corrosion resistance can be maintained even in the processed portion.

Claims (2)

[Claims] 1. Al: 25-75% by weight%, 0.5 of Al amount
~ 5.0% of Si, one or two of Zr and Hf in a total amount of 0.01 to 0.50%, and Ti: 0 to 0.50%, and the balance having a surface plating film consisting of zinc and unavoidable impurities. A hot dip Al-Zn alloy plated steel sheet. 2. The method for producing a hot-dip Al—Zn alloy-plated steel sheet according to claim 1, wherein after the plating, a heat treatment of heating in a temperature range of 150 to 350 ° C. is performed.