JP2627788C - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention reduces the spangle pattern unique to hot-dip zinc-aluminum alloy-plated steel sheets (hereinafter simply referred to as Zn-Al-plated steel sheets), and shows the appearance after plating and painting. The present invention relates to a steel sheet which is improved and has improved corrosion resistance and plating adhesion. (Prior art) Hot-dip galvanized steel sheet is a product having the widest demand field among surface-treated steel sheets. The quality requirements of consumers have become more advanced year by year, and recently, product development of multi-component Zn-Al plated steel sheets for improving workability and paintability, including corrosion resistance, has been actively performed. Representative examples of Zn-Al plated steel sheets include commercially available Al: 55 wt% and Si: 1.5 wt% under the name of Galvalume.
And a Zn-Al-Si-based Zn-Al-plated steel sheet composed of the remaining Zn. The plating layer is composed of α phase (Al rich) and β phase (Zn rich) + eutectic phase of α phase, so it shows high rust prevention to spread rust, but does not have sufficient sacrificial anticorrosion action, so it prevents edge protection. Not enough rust. Also, Japanese Patent Application Laid-Open No. 50-104731 discloses Al: 5-2.
Zn-Al-Si-based Zn-Al composed of 0 wt%, Si: <5 wt% and residual Zn
Japanese Patent Application Laid-Open No. 54-23033 discloses that a plated steel sheet contains Al: 2 to 20 wt%,
: 0.001 to 0.5 wt%, Sn: 0.01 to 0.1 wt%, and a Zn-Al-Si-Sn-based Zn-Al-plated steel sheet comprising Zn. However, in any case where the Al content exceeds 5 wt%, there is a problem that the α-phase in which Al-rich is developed precipitates as a primary crystal in the plating film, and the sacrificial corrosion protection effect of the plating layer is reduced. Alternatively, while exhibiting excellent corrosion resistance in a severely corrosive environment, intergranular corrosion is extremely likely to occur due to the effect of added Sn, which poses a practical problem. Unlike a normal hot-dip galvanized steel sheet, the appearance of the Zn-Al-plated steel sheet has a unique spangle-like spangle pattern. However, this has a disadvantage that it has an adverse effect of intergranular corrosion on the corrosion resistance, and a spangle pattern appears on the appearance of the coating. As a method of improving this, there is a quenching treatment after plating, but this alone is not sufficient for the above-mentioned effect of eliminating the turtle shell spangle pattern. (Problems to be Solved by the Invention) The present invention is to suppress the generation of a unique tortoiseshell spangle pattern which hardly disappears even in a quenching treatment generated in a Zn-Al plated steel sheet. (Means for Solving the Problems) The present invention relates to the following: Al: 3.0 to 10 wt%; Si: Al: 0.2 to 0.45%;
g: 0.01 to 1.0 wt%, the total of unavoidable impurities is less than 0.02 wt%, and the balance is plated with a Zn plating bath. It is an aluminum alloy plated steel sheet. (Action) The present invention focuses on the fact that the turtle shell spangle pattern is caused by the wettability of the plating bath at the interface with the iron base material, and as a result of intensive studies, it is found that the addition of a small amount of Si to the Zn-Al plating bath is effective. It was made based on. The technical improvement points in the present invention are as follows. A small amount of Si is added to the Zn-Al plating bath to increase the wettability between the steel plate and the plating, thereby reducing the spangle pattern unique to the Zn-Al plated steel plate. Also,
At the same time, Al enriched at or near the grain boundaries of the plating layer or the Fe-based alloy layer is eutectic-solidified by Si, so that the active Al is passivated, thereby improving the plating adhesion and the corrosion resistance. Prevent intergranular corrosion due to unavoidable impurities such as Pb in the Zn-Al plating layer,
Further, Mg is used in combination to suppress the anode corrosion in the Zn-rich β phase, expand the passivation region, and further increase the corrosion resistance. Hereinafter, the reasons for limiting the components of the plating bath in the present invention will be described.・ Aluminum In the Zn—Al plated steel sheet of the present invention, the effect of Al is Al in the plating bath.
Depends on quantity. As the amount of Al in the plating bath increases, the plating layer becomes out of phase and η
The phase (Zn), the β phase, and the α phase are intricately mixed, and when the Al content is further increased, the crystallization of the α phase and the β phase becomes dominant, resulting in high corrosion resistance of the Zn-Al plated steel sheet. Al is 3.
If it is less than 0 wt%, the formation of the eutectic Al-Zn is small, so that the corrosion resistance of the plating layer is not sufficient. On the other hand, when the amount of Al in the bath exceeds 10.0 wt%, the interdiffusion reaction between Al and Fe at the plating-steel interface excessively progresses, causing abnormal growth of the Fe-Al-based alloy layer which is brittle for processing. This causes adverse effects on corrosion resistance such as generation of spot-like red rust in a humid atmosphere as well as plating adhesion. Furthermore, corrosion of the steel plate or the plating pot is large in the plating bath, and the eluted Fe is mixed in the plating layer as an Fe-Al-based compound (floating dross) to impair the plating appearance, which is not practical. From the above, the amount of Al is set to 3.0 to 10.0 wt%, preferably 4.
0 to 7.0 wt%.・ Silicon Silicon is the most important element in eliminating tortoiseshell spangles and obtaining a smoother appearance. When a small amount of Si is added to the Zn-Al plating bath, the wettability between the steel sheet and the plating is improved, and in the solidification process of the plating layer, Si is taken into each Zn-Al eutectic, causing a further decrease in melting point and a decrease in grain size. This is thought to reduce the difference in solidification point between the boundary and the eutectic, and to solidify relatively uniformly. However, the spangle pattern peculiar to the Zn-Al plated steel sheet is eliminated and the plating appearance is smoothed. Simultaneously, the enriched Al at or near the grain boundaries of the plating layer or the Fe-based alloy layer is eutectic-solidified by Si, so that active Al
To improve the corrosion resistance by preventing corrosion from the grain boundaries. Si is Al
If it is less than 0.2%, the contribution to the improvement of wettability with the steel sheet is weak, and eutectic with Al enriched in the grain boundary etc. in the plating layer, and the absolute amount of Si in the plating layer is reduced, so that the corrosion resistance is reduced. The effect of improvement cannot be expected very much. Further, the effect of suppressing the growth of the Fe-based alloy layer by Si is governed by the action of Al, and the effect is hardly recognized. On the other hand, when Si exceeds 0.45% of Al, the effect of reducing the tortoiseshell spangle pattern is saturated. From the above viewpoints, the range of Si is set to 0.2 to 0.45% of Al, preferably 0.3 to 0.45% of Al. -Magnesium Mg has the effect of further improving the corrosion resistance by crystallizing into the Zn-rich β phase of the Zn-Al-Si plating layer, but it is necessary to use Mg within a range that does not cause brittle fracture and local corrosion of the plating layer. is there. If the content of Mg is less than 0.01 wt%, there is almost no effect of preventing the sensitivity of impurities such as Pb or the like to prevent intergranular corrosion of the plating layer, and even if the eutectic body is formed with Zn, Al, and Si as plating bath components. There is almost no effect of expanding the passivation region of the layer, and it is difficult to achieve high corrosion resistance. If Mg exceeds 1% by weight, Mg segregating at the crystal grain boundaries of the plating layer becomes a cathode, causing selective corrosion of the β phase, causing brittle destruction due to intergranular corrosion of the plating layer, and thus causing a practical problem. From the above, the range of Mg is 0.01 to 1.0 wt%, preferably 0.0 to 1.0 wt%.
3 to 0.5 wt%. Inevitable impurities Inevitable impurities are Pb and Sn which have the effect of forming themselves as a cathode and promoting the anodization of Zn (Zn → Zn ²⁺ + 2e) when a local battery is formed by contact with Zn.
And elements such as Cd. Therefore, these elements induce intergranular corrosion of the plating layer and its brittle fracture, leading to plating exfoliation, and are not always desirable for increasing the corrosion resistance of the plating layer. In the present invention, it is necessary to eliminate this as much as possible. There is. For the above reasons, in the present invention, the total amount of impurity elements which are inevitably mixed in the smelting process of the plating metal is set to less than 0.02 wt%, but preferably 0%.
. 01 wt% or less. (Examples) Table 1 shows the results of hot-dip coating of an annealed aluminum killed steel sheet having a thickness of 0.3 mm and a width of 914 mm, and summarizes the plating appearance, the appearance after painting, the bare corrosion resistance, and the plating adhesion. is there. Hot-dip plating was performed using a Sendzimer hot-dip line under the following conditions. Line speed 150m / min Pretreatment Non-oxidizing furnace outlet plate temperature 600-650 ° C Reduction furnace outlet plate temperature 790-830 ° C Reduction furnace gas composition 25% H ₂ , 75% N ₂ hot-dip bath temperature 450 ° C Plating weight (gas Wiping control) 100-120 g / m ² plating after cooling on one side Cooling was adjusted by adjusting the pressure and distance to the plating surface in a molten state using a phosphate-based aqueous solution using a special nozzle. In addition, the performance evaluation of the Zn—Al alloy plated steel sheet thus obtained was performed by the following method. Plating appearance Visually evaluated according to the following criteria ◎: Almost no grain boundaries are observed ○: Very few grain boundaries are observed △: Grain boundaries are considerably observed ×: Grain boundaries are clearly observed (can also be felt by touch) Rear appearance Lacquer spray coating (red: Acrylic lacquer paint made by Dai Nippon Paint) 15μ
The appearance after m was visually judged according to the following criteria. ：: Spangled pattern was hardly recognized. ○: Spangled pattern was slightly observed partially. △: Spangled pattern was slightly observed. ×: Spangled pattern was observed. Bare corrosion resistance clearly observed on the entire surface Evaluated by rusting after 72 hours of salt spray test according to JIS Z-2371. ◎: White rust generation small ○: White rust generation large △: White rust generation large ×: Severe white rust generation Plating adhesion Adhesive steel ingot with a load of 5 kg having a hemisphere with a radius of 3/4 inch was naturally dropped from a height of 500 mm. The convex-plated surface was forcibly peeled off with Cellotape (registered trademark) and evaluated according to the following criteria. ：: No plating peeling ○: Fine point-like (several points) peeling ：: Peeling with a certain area ×: Peeling over the whole The following is a brief description based on Table 1. Plating Appearance The most significant feature of the present invention, the effect of improving the smoothness of the plating surface layer by Si, is shown in Example N.
o. 1 to 17 clearly show. The spikelet pattern unique to the Zn-Al-plated steel sheet is comparative example No. As shown in Nos. 28 to 31, this phenomenon occurs regardless of the amount of Al. To prevent this, it is effective to improve the wettability between the steel sheet and the plating by a trace amount of Si and to solidify the eutectic Al at the grain boundaries. You can see clearly. The appearance after plating is basically the same as the plating appearance, and the appearance after painting is governed by the degree of spangle pattern of the underlying plating layer. Example No. In Nos. 1 to 17, the effect of adding Si is clearly exhibited. Bare Corrosion Resistance In the present invention, the effectiveness of Al, Si, and Mg as alloying components for increasing the corrosion resistance of the plating layer was discussed. No. 18-22 (low Al), No. 23 to 27 (rich in Al), No. 28 to 31 (Si small), 32-35 (M
g small) and comparison with all examples. It is thought that the function of each alloy component is different for the high corrosion resistance of the plating layer, but it can be said in common that the eutectic in some form with the base metal Zn has an important point of high corrosion resistance, It is considered that the eutectic electrochemically widened the passivation region of Zn and the corrosion current was reduced. Plating Adhesion Uniform formation of the Fe-Al-Zn-based ternary alloy layer by Al is the most important factor for ensuring adhesion. Comparative Example No. 23 to 27 are Fe-Al because Al is high.
It is probable that the adhesion could not be ensured due to insufficient control of the alloy layer. Further, the effect of improving the plating adhesion by Si is shown in Comparative Example No. It is shown by comparison between Examples 28 to 31 and the Examples, and it can be seen that Si, like Al, has an effect of controlling the Fe-based alloy layer. * Wt% to Al (Effect of the Invention) As described above, the present invention smoothes the plating appearance and the appearance after painting by eliminating the peculiar turtle shell spangle pattern, which is a basic defect of the Zn-Al plated steel sheet, This is an epoch-making hot-dip zinc-aluminum alloy-plated steel sheet that has succeeded in improving the corrosion resistance and plating adhesion while making it more beautiful.

Claims (1)

Claims (1) Al: 3.0 to 10 wt%, Si: 0.2 to 0.45% of Al, Mg: 0.01 to 1.0 wt%, and the sum of unavoidable impurities is A highly corrosion-resistant hot-dip zinc-aluminum alloy-plated steel sheet having excellent surface smoothness, characterized by being plated with a plating bath of Zn at less than 0.02 wt%.