JPH042796A - Hot dip galvanized steel sheet having zn-cr surface layer - Google Patents

Abstract

PURPOSE:To render lustrous appearance in a naked state and to maintain rust resistance over a long period of time by forming a Zn-Cr plating layer on a hot dip galvanized layer formed as an underlayer. CONSTITUTION:A hot dip galvanized layer is formed as an underlayer on a steel sheet and a plating layer consisting of 5-40% Cr and the balance Zn is formed as a surface layer on the underlayer by >=1g/m<2>. The Zn-Cr plating layer has satisfactory corrosion resistance in the naked state in spite of the small thickness and inhibits the formation of red rust and the surface discolors hardly even after exposure to the open air over a long period of time. When the surface layer is chromated, the adhesion to a coating film and the corrosion resistance after coating are improved.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is applicable to civil engineering architectural structures, industrial machinery, vehicles, electrical equipment,
This relates to rust-preventing steel materials used outdoors, such as in agriculture.

(Prior art) Steel materials used outdoors, such as building materials, are either bare or coated, but in either case, long-term rust prevention is required. Or, is Z
, hot-dip plated steel sheet mainly composed of n, e.g. Fe5-20
Zn-plated steel sheet containing 5% or more of Ag, or molten Zn- coated steel sheet containing 5% or more of Ag, or further containing rare earth metals.
Al-plated steel plates and the like are exclusively used.

, for Zn-Cr plating, see JP-A-88-2432.
No. 95 discloses the electroplating method in Japanese Patent Application Laid-open No. 1-111864.
The publication discloses a vapor deposition plating method.

(To be solved by the invention: 1IIWJ) Corrosion resistance of hot-dip Zn-plated steel sheets or hot-dip-plated steel sheets mainly composed of Zn can be improved by making them thicker, but when plastic working is applied, they adhere to the mold, causing the plating to peel off or the plating to peel off. layer becomes more susceptible to damage.

In addition, if it is exposed outdoors for a long period of time for collecting stool, it has the disadvantage that even if no red rust appears, the surface turns mottled and gray or turns black, deteriorating its appearance. When used as a paint base, the adhesion to the paint film is not sufficient, often causing paint film peeling.

Although it is known that Zn--Cr plated steel sheets have excellent corrosion resistance, since they are produced by electroplating or vapor deposition plating, it is impossible to economically produce thick coated products.

(Means for Solving the Problems) The present invention has been made to advantageously solve the above-mentioned problems, and its characteristics are as follows: A plating layer consisting of 5 to 40% Cr and the remainder Zn is applied to the surface at a rate of 1 g/rW. ? or more, and the lower layer is Z
These are a two-layer plated steel sheet consisting of a hot-dip plating layer mainly composed of n or Zn, and a two-layer plated steel sheet with a chromate treatment film applied to the outermost layer.

(Function) Zn-Cr plating has good corrosion resistance even when thinned, and not only does it suppress the occurrence of red rust, but even after long-term outdoor exposure, the surface hardly changes color and maintains its original luster for a long time. can. Furthermore, when used as a paint base, it has good adhesion to the paint film and extremely excellent paint corrosion resistance.

Since such characteristics are surface properties, by applying a Zn-Cr plating layer on top of a thick hot-dip Zn plating or a hot-dip plating layer mainly composed of Zn, the hot-dip Zn
It was discovered that the disadvantages conventionally associated with n-based plated steel sheets could be overcome.

The Cr content of the Zn-Cr plating layer is preferably 5 to 40%; if it is less than 5%, the corrosion resistance is insufficient and there is a tendency to whiten upon exposure like Zn, so that the characteristics of the present invention cannot be exhibited. C: If it exceeds 40%, the plating layer becomes excessively hard and brittle, cracks occur when subjected to plastic working, and whitening due to the underlying Zn progresses over long-term exposure, or in severe cases, Zn- This is not preferable because the Cr layer is missing.

If the thickness of the Zn-Cr plating layer is Ig/rrr or more, it is sufficient to improve the surface quality, and if it is made even thicker, the corrosion resistance will be improved, but considering the economic cost,
It is practical to carry out at 0g1rd or less.

In addition to Zn and Cr, the Zn-Cr plating layer includes Fe,
Nl, Co, Mn, Sn, Mg, All.

Even if small amounts of Si, Ti, P, C, B, N, O, etc. are contained, the effects of the present invention are not lost.

The lower layer of molten Zn or a hot-dip plating layer mainly composed of Zn has a basis weight of 30 g:/d for applications where corrosion resistance is important.
Applied at ~600 g/rd. As the hot-dip coating layer mainly containing Zn, an alloyed hot-dip Zn-plated steel sheet containing 5 to 20% of Fe, a hot-dip Zn-/J-plated steel sheet containing 5% or more of AI, or further containing a rare earth metal can be used.

These hot-dip Zn-based plating layers contain Fe.

In addition to AI, Pb, Sb, Cd, Sn, Mg, Ca.

A small amount of Mn, N1, Cr, P, La, Ce, Nd, etc. may be contained.

A hard and brittle alloy layer is formed between the lower hot-dip coating layer and the base steel sheet during hot-dip coating, but as the weight increases, the alloy layer at the interface also becomes thicker, so plastic working is necessary. This makes it easier for the plating to peel off from here. This is because shearing force is generated due to friction between the surface of the plating layer and the mold.

When a Zn-Cr plating layer containing 5 to 40% Cr is present on the surface layer, the sliding property within the mold is improved, so even if the lower layer has a thick coating weight, peeling of the plating can be reduced, and the surface mold caking can be prevented. Also, it becomes difficult to accept the damage caused by the plating layer.

When a chromate treatment is applied to a Zn-Cr surface layer containing 5 to 40% Cr, the effect of improving corrosion resistance is extremely high.

If the Zn plating surface is subjected to chromate treatment and a salt spray test is performed, the white rust prevention effect will last for about 24 to 100 hours, but when the Zn-Cr plating layer is chromate treated, the salt water spray test will last more than 1000 hours. However, there is no white rust or even discoloration.

It is believed that the corrosion resistance of Zn-Cr plating is due to the surface being covered with insoluble Cr corrosion products in a corrosive environment and becoming passivated, but chromate treatment intentionally strengthens this Cr corrosion product layer. It is presumed that corrosion resistance will be greatly improved since this corresponds to adding a

As the chromate treatment, any of coating type chromate treatment, reactive type chromate treatment, and electrolytic type chromate treatment is highly effective in improving corrosion resistance.

For coating type and reactive chromate treatment, Cr+6C
In addition to r+3, inorganic colloids, acids such as phosphoric acid, fluorides, or water-soluble or emulsion type organic resins may be added.

For example, as a treatment liquid containing phosphoric acid and fluoride, chromic acid 30g/II, phosphoric acid 10g/II-potassium titanium fluoride 4g/litre, and sodium fluoride 0.5g/N.

The treatment liquid containing silica is chromic acid 50g/II.

Of which, 40% trivalent chromium, S io 2100g/j
! There is.

Examples of inorganic colloids include SiO2゜AfIO
Nocolloids such as TiO2, Zr0, 23' or oxygen acids such as molybdic acid, tungstic acid, vanadate acid and their salts, or phosphoric acids such as phosphoric acid and polyphosphoric acid that react with zinc in plating to form poorly soluble salts;
Alternatively, it contains one or more types of fluorosilicides, titanium fluorides, phosphates, etc. that produce poorly soluble salts through reactions such as hydrolysis.

These colloids are effective in fixing small amounts of hexavalent chromium in the chromate film.

In addition, phosphoric acids such as phosphoric acid and fluorides are particularly effective in promoting the reaction between the plating layer and chromate. The amount of these inorganic colloids added varies depending on the type of additive, but for example, for phosphoric acid, it is 1 to 200 g/RSS
For iO2, it is 1 to 800 g/l.

In some cases, an organic resin that can be stably mixed with chromate such as acrylic resin may be added.

For electrolytic chromate treatment, in addition to chromic acid, sulfuric acid,
Those to which phosphoric acid, halogen ions, etc. are added, or and 5102. Those to which inorganic colloids such as A11203 are added, and those to which cations such as Co and Mg are added are also applicable. Usually, cathodic electrolysis is performed, but anodic electrolysis and alternating electrolysis can also be added.

The film amount of the chromate layer is 5 to 100 mg/r as Cr.
rf is suitable. If it is less than 5 mg/rd, the effect will be insufficient, and if it exceeds 100i+g/rrf, the adhesion of the chromate layer will deteriorate, which is not preferred in practice.

By applying chromate treatment, in addition to the above-mentioned bare corrosion resistance and durability of appearance, adhesion with a paint film as a base for painting and paint corrosion resistance are also improved.

Next, the manufacturing method of the present invention will be described.

After hot dipping a steel plate using the usual method, degrease and pickle if necessary, and apply Zn-C by electroplating or vapor deposition plating.
Perform r plating. To perform electroplating, add a Cr electrodeposition accelerator to a sulfuric acid bath containing Zn2-one and Cr3-one with a pH of 3 or less, or a chloride bath, and set the bath temperature to 40 to 70°C.
Current density 50-250A/dm2, flow rate 0.5-3 m
Cathodic electrolysis is carried out under the conditions of /see.

The Cr electrodeposition accelerator is a polyoxyalkylene derivative such as polyethylene glycol, a cationic surfactant such as polyamide sulfonic acid, etc. at 0.01 to 20 g/fl, or an oxide colloid such as S 102 at 2 to 200 g/l.
Added.

To perform vapor deposition plating, preheat the steel plate if necessary and heat it to 1G-
The crucible is introduced into a vacuum chamber at 2 to 10 Torr, and Zn and C are heated and melted using an electron gun or the like, and metal vapor is generated from the crucible and deposited on a steel plate.

(Example) Various hot-dip Zn-based plated steel sheets with a thickness of 0.7 mm were heated at 60°C.
5A in a 1N-NaOH aqueous solution with the sample as a cathode.
/d■ Electrolytic degreasing was performed for 2.4 seconds, and after washing with water, Zn-Cr electroplating was performed.

The plating conditions were: Zn2"4 ion 56g/l, Cr3+ ion 44g/j!, Na ion 15g/I as sulfate, pH-1, 3, 50°C plating bath with addition of polyethylene glycol Ig/N. using a plating solution flow rate of 60 m/min and a current density of 120 A/d+a2.
A Zn-Cr plating layer with r15% was obtained.

The alloy composition was adjusted by changing the Zn2~on and Cr3~on concentrations.

Chromate treatment was performed using an electrolytic method and a coating method.

Electrolytic method is CrO50g/N 5H3PO410tr/I
.

Electrolytic treatment was carried out at 5 A/dm2 in a 40° C. bath consisting of H2SO40.5 g/II using the plated sample as a cathode.

The amount of film was adjusted by the amount of coulombs.

The coating method is partially reduced chromic acid (Cr'/Cr”m6/4)
3.8g/II as Cr amount, acrylic resin lo
The sample was dip-coated in a treatment solution consisting of g/i' 5H3P 04 log/1 and squeezed with a roll to dry. The amount of film was adjusted by adjusting the bath concentration and roll pressure.

The produced samples and comparative examples were evaluated in the following tests.

(1) Weather resistance test The specimens were exposed outdoors for one year and changes in appearance were evaluated.

0: No change, glossy △-: Surface white with white rust 23711.: Compliant> The test was conducted for 1000 hours and evaluated based on the area where red rust occurred.

O: No red rust △: Within 10% of the area where red rust occurs ×: Area where red rust occurs 10~100% (3) Paint corrosion resistance test Pre-treatment degreasing with iron phosphate treatment liquid, apply powder polyester paint for 40μs, and apply knife A crosscut was drawn to reach the steel plate, and the spread width of the blister from the crosscut portion was evaluated after 500 hours in a salt spray test (according to JIS Z 2371).

0: within 2.5 u △ : 2.5 dragon to 5 dragon The test piece was
The sample was passed through a mold with a square bead of 1 cm, and while applying a load during pressing, the sample was pulled out using a tensile tester and slid for 200 m+s. As the pressing load increases, the plating layer adheres to the mold, and peeled plating pieces clog the mold, causing a sudden increase in the pull-out load.

This was taken as the breaking load, and the workability was evaluated based on its magnitude.

Table 1 shows the test results of Examples of the present invention and Comparative Examples.

Table 1 clearly shows the effects of the present invention.

(Effects of the invention) According to the present invention, it is possible to maintain a glossy and beautiful appearance and rust resistance for a long period of time when used outdoors, and even when used as a paint, it has excellent under-coating corrosion resistance, and even when processed, there is no damage to the plating layer. This produces rust-proof steel sheets that are difficult to resist, and can be used as steel materials for civil engineering and building structures, industrial machinery, vehicles, electrical equipment, agriculture, etc., resulting in remarkable industrial effects.

Claims (2)

[Claims] 1. A hot-dip galvanized steel sheet having a Zn-Cr surface layer having a plating layer consisting of 5 to 40% Cr and residual Zn of 1 g/m^2 or more on the surface layer, and a lower layer consisting of Zn or a hot-dip coating layer mainly composed of Zn. 2. The hot dip galvanized steel sheet having a Zn-Cr surface layer according to claim 1, wherein the outermost layer is coated with a chromate treatment film.