JP2002371345A - Manufacturing method of hot-dip Zn-Al-Mg alloy plated steel sheet - Google Patents

Abstract

(57) [Problem] To provide a method for producing a hot-dip Zn-Al-Mg alloy-plated steel sheet having excellent corrosion resistance, workability and surface appearance. SOLUTION: Mg: 0.50 to 4.5%, Al: 4.3 / [5.3−
Mg (%)] to 5.5%, with the balance being immersed in a hot-dip plating bath consisting of Zn and unavoidable impurities, pulled up to adjust the amount of plating applied, and cooled until the solidification of the plating film is completed. Speed is [5 / Mg (%)-0.9]-[5 / Mg (%) + 10]
(The unit is ° C./second).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip Zn-Al-Mg alloy-plated steel sheet having excellent corrosion resistance and workability and excellent surface appearance.

[0002]

2. Description of the Related Art In recent years, galvanized steel sheets that are inexpensive and have excellent corrosion resistance have been widely used in the industrial fields such as automobiles, home appliances, civil engineering, construction materials, and the like. Various methods have been studied for improving the corrosion resistance of hot-dip galvanized steel sheets. For example, US Pat. ~
A hot-dip Zn-Al-Mg alloy-plated steel sheet having excellent corrosion resistance using a hot-dip coating bath comprising 17%, Mg: 1 to 5% and the balance of Zn has been proposed.

[0003] These steel sheets may be used after being worked after painting, or may be used without painting (hereinafter simply referred to as "unpainted use"). At the time of non-painting use, it is also important that the plating film has an excellent appearance (design) in addition to corrosion resistance and workability.

[0004] Spangles usually occur in the molten Zn-Al-Mg alloy plating film. If the spangle is large, light is scattered and reflected when unpainted, and the appearance is impaired. In addition, if the sponge pattern becomes larger than a certain limit, a turtle-shaped luster pattern (turtle pattern) is likely to be generated, and the surface appearance is remarkably impaired even when used without painting or after painting. Further, in the molten Zn-Al-Mg alloy plating film, surface defects such as adhesion of black dross are liable to occur, and therefore, the molten Zn-Al-Mg has excellent surface appearance free from these defects as well as corrosion resistance and workability. There is a need for Mg alloy plated steel sheets.

In general, it is known that a hot-dip galvanized steel sheet can be finely spangled by increasing the cooling rate when the coating layer solidifies. In addition,
No. 6-158257 discloses that Al: 3 to 10%, Ti: 0.01 to 1
%, Mg: 0.01 to 1%, the balance being Zn and unavoidable impurities, and a hot-dip Zn-Al plated steel sheet having a spangle size of 0.5 mm or less has been proposed. This proposal has the effect of improving corrosion resistance by including Ti in the plating film, and has the effect of spangle coarsening due to the spangle refining effect of fine intermetallic compounds of Ti and Al precipitated in the plating layer. Thus, the effect of improving the appearance of the plating surface is also reduced by reducing the crevices generated by the formation.

[0006]

The rapid cooling method at the time of solidification of a coating layer as a method for refining spangles of a hot-dip galvanized steel sheet is not necessarily a good method when the plating film contains Mg. Absent. This is because if the cooling rate at the time of solidification of the Mg-containing plating film is increased, the plating film is hardened, and problems such as damage to the plating film during processing and deterioration of the corrosion resistance of the portion are likely to occur.

Further, the steel plate proposed in Japanese Patent Application Laid-Open No. 6-158257 requires a complicated plating bath component management because a quaternary plating bath is used in its production, and is an easy spangle miniaturization method. In addition, there is a problem that the workability of the plating film obtained is not always good. Further, since the above method is intended for a plating film having a Mg content of 1% or less, there is a limit in the corrosion resistance of the plating film obtained. As described above, it has been difficult to obtain a hot-dip Zn-Al-Mg alloy-plated steel sheet having excellent corrosion resistance and workability and stably having fine spangles by the conventional method.

The present invention has solved these problems, and has excellent corrosion resistance and workability, and also has excellent surface appearance in molten Zn-A.
An object of the present invention is to provide a method for producing an l-Mg alloy plated steel sheet.

[0009]

SUMMARY OF THE INVENTION We have found that molten Z
As a result of repeated studies on methods for improving the corrosion resistance and surface appearance of n-Al-Mg alloy plated steel sheets, the following findings were obtained.

A. The surface appearance of hot-dip Zn-Al-Mg alloy plated steel sheets with spangle patterns of various sizes was examined from the viewpoint of designability. As a result, when the average diameter of the spangles of the plating film exceeded 2.0 mm, The scattering of light is large and the glaring feeling is remarkable, and the design property when using unpainted is not good. Also, the average diameter of spangles in the turtle pattern is
In many cases, the spangle is visually recognized when it exceeds 2.0 mm. In this sense, setting the average diameter of the spangle to 2.0 mm or less was effective for improving the design.

B. Mg of the hot-dip coating has an effect of making spangles inconspicuous. This is because, as the Mg concentration in the plating bath increases, Al, Zn, Zn-
This is because the ternary eutectic structure of Zn / Al / Zn-Mg intermetallic compound having a eutectic structure of Mg or the like becomes randomly oriented, and the grain boundary depth between spangles called dentz becomes shallow. .

Therefore, if the action of Mg is used to improve the appearance of spangles of a hot dip coating film containing Mg, the spangle pattern can be made finer at a slower cooling rate than in hot dip galvanizing. That is, by adjusting the cooling rate at the time of solidification of the plating film in accordance with the Mg content of the plating film, spangles can be made fine without impairing workability.

As a result of various studies from this viewpoint, Mg
In order to reduce the average diameter of spangles to 2 mm or less without impairing the workability of the plating film containing, the cooling rate at the time of solidification of the plating film is calculated as [5 / Mg (%)-0.9] (unit: ℃ / sec) or more and the value calculated by [5 / Mg (%) + 10] (unit is ℃ / sec).

The present invention has been completed based on these findings, and its gist lies in the following method for producing a hot-dip Zn-Al-Mg alloy plated steel sheet. Base material, Mg in mass%
Hot-dip plating containing 0.50% or more and 4.5% or less, containing Al in a value calculated by 4.3 / [5.3-Mg (%)] (unit is mass%) and 5.5% or less, with the balance being Zn and unavoidable impurities After immersing in a bath and pulling up to adjust the coating weight, the cooling rate up to 350 ° C should be higher than the value calculated by [5 / Mg (%)-0.9], [5 / Mg (%) + [10] A method for producing a hot-dip Zn-Al-Mg alloy-plated steel sheet, wherein the steel sheet is cooled so as to have a value equal to or less than the value calculated in (10).

[0015]

Embodiments of the present invention will be described below in detail. The type of base material is not particularly limited,
Either a hot rolled steel plate or a cold rolled steel plate may be used.
The steel type can be any steel such as Al-killed steel, ultra-low carbon steel containing Ti, Nb, etc., low carbon steel, alloy steel containing Si, Mn, P and other alloys, and high-tensile steel having various characteristics. Can be used.

The base material is preferably subjected to a pretreatment by a known method such as washing with an aqueous alkaline solution or grinding the surface with a nylon brush or the like to make the surface suitable for hot-dip plating. Next, it is desirable to heat to 600 ° C. or more or a recrystallization temperature or more in a reducing atmosphere, hold for a required time, and cool to near the plating bath temperature. The reducing atmosphere is preferably an atmosphere composed of 5 to 30% by volume of hydrogen, the balance being nitrogen, and having a dew point of -60 to -0 ° C.

The base material is then cooled to a temperature close to the plating bath temperature, immersed in a hot-dip plating bath whose chemical composition has been adjusted so as to obtain a predetermined plating film composition, pulled up to adjust the amount of plating applied, and then cooled.

The chemical composition of the hot-dip plating bath is as follows. The Al and Mg contents of the plating film are almost the same as the contents of the respective elements in the plating bath. Mg: Mg improves the corrosion resistance of the plating film,
It has the effect of making the spangle of the plating film fine. Mg
Of the plating bath to ensure the effect of improving corrosion resistance
The concentration should be 0.50% or more. From the viewpoint of spangle miniaturization in addition to the improvement in corrosion resistance, the content is preferably 1.0% or more, more preferably 1.5% or more.

In the present invention, as described below, the plating bath contains Al in order to stably increase the amount of Mg dissolved in the plating bath. However, in order to ensure the workability of the plating film, the Al content is increased. To 5.5% or less. In that case, the upper limit of Mg that can be contained in the plating bath without generating Mg-based dross is 4.5%, so the Mg concentration in the plating bath is 4.5% or less.

Al: When Al is contained in the hot-dip Zn plating bath, Al forms an oxide film on the surface of the Zn bath and has an effect of preventing Mg from being oxidized. Originally, the amount of Mg dissolved in molten Zn is only about 0.1%, but by adding an appropriate amount of Al corresponding to the desired Mg concentration, Mg in the plating bath can be dissolved.
The amount of dissolution can be stably increased. Further, by the oxide film forming action, the generation of Mg-based dross on the surface of the plating bath can be suppressed.

In order to obtain the above effect, the plating bath should be 4.3 /
A of more than the value (unit is%) calculated by [5.3-Mg (%)]
l. Desirably, it is not less than a value calculated by 5.5 / [5.3-Mg (%)] (unit is%).

On the other hand, if the Al concentration in the plating bath exceeds 5.5%, primary crystals of Al are formed during the solidification of the plating film, and the workability of the plating film is impaired. To avoid this, the Al concentration in the plating bath should be 5.5% or less. 5.0% or less, more preferably 4.0%, from the viewpoint of improving processability
% Or less.

The balance is Zn and unavoidable impurities.
The temperature of the plating bath is not particularly limited. However, if the temperature is too low, the viscosity of the plating bath increases and it becomes difficult to control the amount of plating.
If the temperature of the plating bath is excessively high, the evaporation of Zn from the plating bath becomes severe.

The method of adjusting the coating weight after dipping in the plating bath and pulling up is arbitrary, and may be a commonly used method such as a gas squeezing method. The coating weight is preferably 10 g / m ² or more to ensure corrosion resistance, and is preferably 300 g / m ² or less from the viewpoint of workability and cost reduction.

The average diameter of the spangle of the plating film is 2.0
mm or less, after adjusting the coating weight, at least until the solidification of the plating film is completed, the cooling rate is more than the value calculated by [5 / Mg (%)-0.9], [5 / Mg (%) + 1
0] (the unit is ° C / sec). The cooling rate is determined after the solidification of the plating film is completed after the base material is pulled out of the plating bath.
It means the cooling rate until reaching 350 ° C.

When the cooling rate is less than the value calculated in the above [5 / Mg (%)-0.9] (unit: ° C./sec), the average diameter of spangle size exceeds 2.0 mm, and It is likely to occur. The average diameter is determined from the number of spangle boundaries that cross a line segment having a length of 100 mm and extending in an arbitrary direction at arbitrary 10 locations of the plating film.

If the upper limit of the cooling rate exceeds the value calculated by [5 / Mg (%) + 10] (unit: ° C./sec), the workability of the plating film is reduced, and the cost of cooling equipment and manufacturing cost are reduced. Increase. Desirably, the value is not more than the value calculated by [5 / Mg (%) + 5] (unit: ° C / sec).

The cooling method is not particularly limited, and may be a known method such as cooling, air cooling by blowing gas, or steam spraying by spraying a steam mixture. Conditions other than those described above, such as chromate treatment and other post-treatments,
Processing such as temper rolling may be arbitrarily performed. For example, as the post-treatment, a chromate treatment (a coating type, a reaction type, an electrolytic type, etc.) using a known chromate solution or a resin chromate solution, an inorganic coating containing no chromium and mainly containing a phosphate compound or the like. Treatment, or coating treatment with polyester paint such as epoxy polyester, polyester, melamine polyester, urethane polyester or acrylic paint to improve lubricity, or as a coating film, polyester, amino, epoxy, acrylic, urethane, fluorine, etc. Coating treatment or the like containing a resin as a main component may be performed alone or in combination.

According to the production method of the present invention, the plating film is excellent in corrosion resistance and workability, and has little black hard Mg dross on the surface of the plating film, small spangles, no turtle pattern, and extremely excellent design. Things can be obtained stably. Therefore, in the industrial fields such as automobiles, home appliances, civil engineering, and building materials, it can greatly contribute to the expansion of unpainted use.

[0030]

EXAMPLE A low-carbon cold-rolled steel sheet containing 0.04% of C and 0.25% of Mn and having a thickness of 0.80 mm was used as a plating base material. This was degreased and washed with a 75 ° C NaOH aqueous solution, annealed at 800 ° C in a reducing atmosphere using a continuous hot-dip plating simulator, cooled to near the plating bath temperature, and the chemical composition and temperature of the plating bath were varied. Immerse in the modified molten Zn-Al-Mg alloy plating bath, then lift it up from the plating bath and adjust the amount of adhesion per side to 100 g / m ² by gas squeezing method.
After lifting from the plating bath, the cooling rate until the solidification was completed was changed in various ways within the range of 0.2 to 20 ° C./sec, and then cooled to room temperature to obtain various plated steel sheets. After plating, skin pass rolling at an elongation of 0.5% was performed. These steel sheets were evaluated by the following methods.

Spangle size: The number of spangles cut along a line having a length of 100 mm in any direction on the surface of the plating film was measured, and the average diameter of the spangle was determined from this. This was similarly measured at ten locations, and the average value was defined as the average spangle diameter.

Corrosion resistance: The corrosion resistance was evaluated using a combined cycle tester. Cyclic corrosion test conditions were as follows: 5% NaCl salt spray at 35 ° C: 1 hour, dry atmosphere at 60 ° C and 20-30% relative humidity: 4 hours, 50 ° C and 95% relative humidity Hold in a humid atmosphere: 3 hours, 8 hours / cycle total.

Particularly good when the number of cycles in which white rust does not occur exceeds 20 cycles (marked with ◎), good when not more than 20 cycles and 15 cycles or more (marked with ○), and when not more than 15 cycles Was evaluated as poor (x mark), and the corrosion resistance was evaluated.

Workability: A plated steel sheet is cut into a size of 15 mm × 100 mm, subjected to bending deformation with an inner bending radius of 0.80 mm, and the section of the plating film is observed with a scanning electron microscope, and the cracking rate (bending curved surface total length) The ratio of the length of the film cracked portion to the cracking rate) was measured, and the case where the crack occurrence rate was 20% or less was particularly good (◎), and the case where the crack occurrence rate was more than 20% and 25% or less was good (○) ), Defective if over 25% (x mark)
Was evaluated.

Table 1 shows the plating conditions and the results of the performance inspection of the plating film.

[0036]

[Table 1]

As shown in Table 1, Test Nos. 1 to 12 satisfying the conditions specified by the present invention were performed on the surface of the plating film.
The system dross was small, the spangle average diameter was 2.0 mm or less, and the design was excellent and the workability was good.

On the other hand, in Test Nos. 13, 15, 17 and 20, in which the cooling rate at the time of solidification of the plating film was too slow, the average diameter of the spangle was large and the design was not good. In addition, these plating films had a turtle pattern. In Test Nos. 14, 16, and 18, in which the cooling rate during solidification of the plating film was too fast, the workability was not good. M of plating bath
Test No. 19, where the g concentration was too low, did not have good corrosion resistance. In Test No. 20 in which the Al concentration was too low and in Test No. 21 in which the Mg concentration was too high, a large amount of Mg-based dross was generated and the surface appearance was poor. Test No. 22, in which the Al concentration was too high, showed poor processability.

[0039]

According to the production method of the present invention, a hot-dip Zn-Al-Mg alloy-plated steel sheet having excellent corrosion resistance and workability, small spangles, and excellent design can be easily obtained. This steel sheet is particularly suitable for use in unpainted automobiles, home appliances, civil engineering construction, building materials, and the like, and has extremely high industrial utility value.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Takebayashi F-term in Sumitomo Metal Industries Co., Ltd. 4-5-33 Kitahama, Chuo-ku, Osaka City, Osaka 4K027 AA05 AA22 AB02 AB05 AB44 AC12 AC15 AC18 AC72 AE02 AE03

Claims (1)

[Claims] (1) a base material having a Mg content of at least 0.50% by mass;
It is immersed in a hot-dip plating bath containing not more than 4.5%, not less than 5.5% of Al and a value calculated by 4.3 / [5.3-Mg (%)] (unit is mass%) and not more than 5.5%, with the balance being Zn and unavoidable impurities. After pulling up and adjusting the coating weight, the cooling rate up to 350 ℃ is more than the value calculated by [5 / Mg (%)-0.9], calculated by [5 / Mg (%) + 10] A method for producing a hot-dip Zn-Al-Mg alloy-plated steel sheet, wherein the steel sheet is cooled so as to be equal to or less than a predetermined value (all units are in degrees Celsius / second).