JPH09256132A - Hot dip aluminum-zinc alloy plated steel sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot dip Al-Zn alloy plated steel sheet having a plating film withstanding severe working and to provide a method for producing the same. SOLUTION: This hot dip aluminum-zinc alloy plated steel sheet has a plating layer of an Al-Zn ally contg., by weight, 25 to 75% Al, one or >= two kinds among Mo, W, Nb and Ta by 0.01 to 0.50% in total, one or two kinds of V and Cr by 0 to 0.50% in total and Si by 0.5 to 5.0% to the content of Al. By subjecting the steel sheet to heat treatment of executing heating at 150 to 350 deg.C, the workability of the film can furthermore be improved.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a hot-dip aluminum-zinc alloy-plated steel sheet having a plated layer having excellent workability,
It relates to the manufacturing method.

[0002]

2. Description of the Related Art Molten aluminum-zinc (hereinafter referred to as "Al-") containing 25 to 75% of Al and 0.5 to 5.0% of the amount of Al, and the balance Zn having Zn and unavoidable impurities.
Zn abbreviated) alloy-coated steel sheet has corrosion resistance several times higher than that of hot-dip galvanized steel sheet, has heat resistance and heat reflectivity comparable to hot-dip aluminum-plated steel sheet, and has an end surface that is not found in hot-dip aluminum-plated steel sheet. It also has corrosion resistance. Zn-55 is a typical example of such a steel sheet.
% Al-1.6% Si alloy plated steel sheet is available, but due to its excellent corrosion resistance, building material products such as roofing materials and wall materials, civil engineering products such as guardrails, wiring pipes and soundproof walls, automobile exhaust systems, electric washing machines, It is rapidly spreading to applications such as home appliances such as microwave ovens.

However, the hot-dip Al-Zn alloy plated steel sheet is
When the workability of the plating layer is poor, for example, when the radius of curvature of bending becomes small, cracks occur in the plating layer, and the corrosion resistance of that portion deteriorates. The surface plating is an alloy containing Al, Zn and Si as described above, but an alloy containing Fe is formed between the base steel plate and the plating during the plating process. The evaluation of the plating layer, especially the workability thereof, is often dependent on the properties and amount of the alloy or intermetallic compound formed at this interface. Therefore, hereinafter, the term "plated layer" includes the alloy layer containing Fe or the like formed at this interface unless otherwise specified.

There are several reasons why the workability of the plated layer is poor. First, since the alloy composition of the plated layer is a hypereutectic alloy of Al with respect to Zn, when the molten state is cooled, the Zn-rich phase fills the space between the dendrites of the Al-rich primary crystals. It solidifies and becomes a heterogeneous tissue. In the case of plating, the structure of the plating layer tends to become fine because it is rapidly cooled, but the residual strain increases and the workability deteriorates. On the contact surface with steel, Al in the plating layer is Fe
Rapidly reacts with Al to produce an Al-Fe alloy, and Si is added to the bath for the purpose of suppressing this. However, this reaction cannot be completely suppressed, and Al--Fe and Al
A layer mainly containing an intermetallic compound of —Fe—Si is formed in the vicinity of the interface between the steel and the plating, and these compounds are extremely brittle, which causes the workability of the plated layer to be deteriorated. Furthermore, Si added to suppress the reaction between Al and Fe is
The content exceeds the solid solubility limit near the solidification temperature of the plating alloy, and needle-like hard metal crystals of Si precipitate between the dendrites, which also becomes the starting point of cracks during deformation and It deteriorates workability.

In order to improve the inferior workability of the Al-Zn alloy plated layer, there is a method of performing heat treatment for removing strain after plating. However, if the temperature is raised too much, alloying of Al and Fe will proceed. Low temperature for a long time (eg 200 ° C, 24 hours)
If it is not heated, a sufficient effect cannot be obtained. However, in a continuous line, it is difficult to perform heat treatment for a long time, and a separate process is required. In order to improve this, Japanese Patent Laid-Open No. 41657/1992 discloses an invention in which a plated steel sheet is shot-blasted to shorten the heating time. Since the heating time can be shortened by the blast treatment, it is said that the heat treatment of the plating layer can be performed even for a short time for the purpose of baking the paint during coating on the plated steel sheet. However,
The shot blasting process requires a separate process, which requires a longer process, resulting in a considerable cost increase.

On the other hand, it has been reported that an element such as Sr, V or Cr is added to the plating bath to improve the workability of the plating layer (ALZINC PLUS: A NEW CONTIN.
UOUSHOT-DIP 55% Al-Zn PROTECTIVE COATING: M.Lambe
rigts, et al; GALVATEC'92, Amsterdam (1992), p.412
reference). This is to improve the workability of the plating layer by refining the structure of the plating layer, changing the structure of the interface between the steel and the plating layer, and spheroidizing the acicular precipitate particles of Si by adding a small amount of the above elements. However, the workability improving effect is not always sufficient.

As described above, various measures have been studied for improving the workability of the plated layer on the surface of the Al—Zn alloy plated steel sheet. However, at present, satisfactory steel sheets have not necessarily been obtained, and it is hard to say that a low-cost manufacturing method that can be industrially easily adopted is well established.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a molten Al-Zn having a surface plating layer that withstands severe processing.
An object is to provide an alloy-plated steel sheet and a method for manufacturing the same.

[0009]

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 layer containing 1% of O.5-5.0% of Si and the balance Zn and unavoidable impurities, various studies were conducted with the main problem of improving the structure. . The thermal history from melting to solidification of the Al-Zn alloy after adhering to 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, Can not change. Therefore, it was considered that various additive elements should be studied in order to change the solidification structure under substantially constant cooling conditions and further improve the workability.

Therefore, Al-Zn containing the above-mentioned small amount of Si
As a result of various studies on the influence of additional elements on the solidification structure of the alloy and the workability of the plated steel sheet, Mo, W, Nb, Ta,
It has been found that the addition of a small amount of V or Cr has the effect of improving the structure of the plating layer and lowering the hardness, or improving the workability. Further, it was found that the effect of improving the workability becomes more remarkable by including these elements in appropriate amounts in combination. However, when they were added in combination, there were cases where the effect appeared simply depending on the total amount added, and cases where the effect more than the expected effect appeared and there was a synergistic effect.

It seems that there are mainly two effects for improving the workability of the plated layer by adding these elements. One is the effect of refining the solidification structure or the metal structure of the plating layer. This is because when the molten Al-Zn alloy adheres to the steel sheet and is rapidly cooled and solidifies from a supercooled state, it rapidly forms an intermetallic compound with Al or Zn and a second phase, and these are finely divided. It is presumed that a large number of solidification nuclei were dispersed to change the solidification structure. The workability of the micronized structure is improved, and the hardness is lowered depending on the subsequent heat history, and the workability is further improved. Elements having such an effect are Mo, W, Nb and Ta among the above. These elements are the elements of group A.

The other is the effect of changing the morphology of a brittle alloy layer mainly composed of Al-Fe or Al-Fe-Si intermetallic compounds formed at the interface with the plating in contact with steel.
It is presumed that this suppresses the occurrence of cracks in the brittle alloy layer, which becomes the starting point of cracking of the plating layer during processing, and consequently improves the workability of the plating layer. It is V and Cr that are considered to bring about such an effect. These elements are defined as B group elements. Unlike the elements of the A group, these elements of the B group have almost no effect on the structure of the alloy that occupies most of the plating layer, so that the effect of improving the workability seems to be small. However, even if the elements in the same group are contained in combination, only the effect corresponding to the total amount can be obtained, whereas if the elements of group A and elements of group B are added in combination, they are synergistic. The improvement effect appears.

Strain and internal stress remain in the rapidly solidified alloy, which tends to deteriorate the workability of the plating layer, but the heat treatment releases the residual stress of the structure, further improving the workability. It was also revealed that it has an effect. In particular, in the present invention, since the structure was made finer by the addition of the above-mentioned elements, it was thought that the strain could be released easily and a sufficient effect could be obtained in a short time.

The present invention has been completed by clarifying the effective content range and the effective range of heat treatment based on these findings. The gist of the present invention is as follows.

(1) 25% to 75% Al, Mo,
One or more of W, Nb, and Ta in a total amount of 0.01 to
0.50%, 0 or 0.50 in total of one or two of V and Cr
%, And Al containing 0.5 to 5.0% of the amount of Al
-A hot-dip aluminum-zinc alloy-plated steel sheet having a Zn alloy plating layer.

(2) The method for producing a hot-dip aluminum-zinc alloy-plated steel sheet according to claim 1, characterized in that after plating, heat treatment is performed by heating in a temperature range of 150 to 350 ° C.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the Al of the plating layer is
The content is limited to 25 to 75%. The reason is that the corrosion resistance of the steel sheet becomes inferior even if it is more or less than this range. That is, when the Al content is less than 25%, heat resistance and excellent corrosion resistance due to Al are lost, and when it exceeds 75%, the anticorrosion effect of the cut end surface is lost.

This plating layer contains Si with an Al content.
0.5 to 5.0% is included. The addition of Si suppresses the alloying reaction of Fe and Al that rapidly progresses at the plating temperature, and prevents the development of the brittle Al-Fe intermetallic compound phase that develops at the boundary between the plating layer and the base steel sheet, It has the effect of maintaining the adhesion of the layer and further improving the corrosion resistance. When the Si content is less than 0.5% with respect to the Al amount, the effect of suppressing alloying is insufficient. Further, 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 moreover, the metal Si particles in the plating layer increase and the workability of the coating deteriorates. Therefore, the upper limit is 5.0% of the Al amount.
Up to. The typical composition is Al: 55%, Si:
1.6% (2.9% of Al content).

In the alloy, the elements of group A, namely M
o, W, Nb or Ta, one or more of them in a total amount of 0.01-0.
Include in the range of 50%. For the purpose of inclusion in the plating layer,
By adding these elements to the bath, the processability of the coating is greatly improved. In that case, if the total content in the coating is less than 0.01%, the effect of improving the workability is small. On the other hand, if it exceeds 0.50%, the workability deteriorates. Therefore, the upper limit of the total content is 0.50%. If the amount is too large, the workability of the plating layer will deteriorate.
This is probably because the amount of intermetallic compounds and second phase increases.

B group elements, ie V or Cr
The inclusion of one or two of these also has the effect of improving the workability of the plating layer. When the effect of containing the elements of the A group is sufficient, it is not necessary to add the element in particular, but by adding the element of the B group to this, the workability can be further improved. In the case of the elements of group A, the effect of addition is remarkable, but if the content is too large, the workability deteriorates. On the other hand, the elements of the B group tend to be slightly inferior in the effect of addition, but the adverse effect is small even if added in excess. Therefore, the effect can be greatly enhanced by adding the element of group B after adding the element of group A. Further, it is possible to reduce the content of the element of the A group to obtain the same effect.

The desirable content for obtaining the effect of the addition of the element of the B group, that is, V or Cr is 0.01% or more in total. However, even if added in a large amount, the effect is saturated, so the upper limit of the content of one or two of V or Cr in the plating layer is 0.50% in total.

In the case of hot dipping Al-Zn alloy, Z
The temperature is higher than in the case of n plating and Al is F
e tends to easily form an intermetallic compound, and an alloy layer containing Fe is formed at the contact interface between the plating and the steel sheet. Since this alloy layer is extremely brittle and causes peeling of the plating layer and generation of cracks during processing, Si is added to suppress its formation, but the effect is not sufficient. The addition of the elements of the B group has no effect of suppressing the formation of the alloy layer, but exhibits the effect of reducing cracks in the plating layer which are presumed to be caused by the alloy layer formed. The reason for this is not clear, but it is considered that one of the reasons for becoming brittle is to make acicular Si particles spherical.

In the case of the steel sheet of the present invention, the workability of the plating layer is better than that of the conventional hot-dip Al—Zn alloy-plated steel sheet even when it is plated on the hot-dip galvanizing line.
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 limited, 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 to 350 ° C, but 1 is preferred.
Heating at 70 to 340 ℃ for 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 the steel sheet after coating on the surface, cracks that occur in the plated portion of the coating base are suppressed. However, the corrosion resistance when using the steel sheet can be improved.

[0025]

[Example] As a material steel plate, a low carbon Al killed steel plate having a plate thickness of 0.8 mm (compositional analysis value: wt%, C: 0.05, Si:
0.01, Mn: 0.24, P: 0.015, S: 0.011, sol.A
L: 0.028, N: 0.0023) width 80mm, length 20
Using a 0 mm test piece, a vertical hot dip galvanizing tester capable of being directly charged into the hot dip plating bath from a reducing atmosphere was used to perform preheating, reduction annealing, and then hot dip plating. The plating conditions are shown in Table 1. For adjusting the components of the plating bath, 99.99% Zn, 99.9% Al, 13% Si-Al master alloy, and Mo, W, and N containing about 5% of each element.
An Al master alloy of b, Ta, V and Cr was used as appropriate. Table 2 shows the composition of the coating film of the trial plating. When heat treatment was performed after plating, the test piece was placed in a constant temperature electric furnace heated to a predetermined temperature and kept for a predetermined time. The atmosphere is in the atmosphere.

[0026]

[Table 1]

[0027]

[Table 2]

As a performance evaluation of the plated steel sheet, a surface appearance investigation, a 2t bending or a contact bending (t = 0) test, and a corrosion resistance test were conducted. 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 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 processed plating layer 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. The corrosion resistance was evaluated by performing a salt spray test according to JIS-Z-2371 on a test piece whose end face and back face were sealed, and measuring the time until the occurrence of red rust. 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, the heat treatment further reduces the occurrence of cracks due to bending and improves the workability.

[0030]

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 significantly improved as compared with the conventional case. 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. As described above, the present invention can provide a hot-dip Al-Zn alloy plated steel sheet having better workability than ever before.

Claims (2)

[Claims] 1. A weight percentage of 25-75% Al, Mo, W,
0.01 to 0.50 in total of one or more of Nb and Ta
%, One or two of V and Cr in a total amount of 0 to 0.50%,
And Al-Z containing Si of 0.5 to 5.0% of Al amount
A hot-dip aluminum-zinc alloy plated steel sheet having an n-alloy plated layer. 2. The method for producing a hot-dip aluminum-zinc alloy plated steel sheet according to claim 1, wherein after the plating, heat treatment is performed by heating in a temperature range of 150 to 350 ° C.