JPH03138391A - Production of surface treated steel sheet excellent in coating suitability and corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a surface treated steel sheet excellent in coating suitability and corrosion resistance by subjecting an alloying hot dip galvanized steel sheet to degreasing treatment by means of an aqueous solution containing NaOH and/or Na3PO4 and a surfactant, such as cationic, anionic, or nonionic surfactant, and then applying electroplating with metal or alloy to the above steel sheet. CONSTITUTION:For example, a Zn-Fe alloying hot dip galvanized steel sheet is cleaned by degreasing the surface of this steel sheet by treating the surface by means of an aqueous solution containing NaOH and/or Na3PO4 by 10-150g/l and a surfactant, such as cationic surfactant, anionic surfactant, or nonionic surfactant, by 0.1-1g/l and then carrying out water washing. Subsequently, after the above steel sheet is subjected, if necessary, to pickling treatment by means of a sulfuric acid solution of 1-20g/l concentration, the steel sheet is subjected to electroplating with metal, such as Fe and Ni, or Fe-P, Zn-Fe, or Zn-Ni alloy into monolayer or laminated state.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a surface-treated steel sheet that has excellent coating properties such as chemical conversion treatment properties, electrodeposition coating properties, and paint @adhesion properties, and corrosion resistance.

(Problems to be solved by conventional technology and inventions) In recent years, the problem of car body corrosion has arisen in regions where rock salt is sprayed for the purpose of winter snow melting, such as northern Europe, Canada, and the northern United States. Corrosion-resistant surface-treated steel sheets are being used to replace cold-rolled steel sheets, and their usage is increasing year by year.

Among them, alloyed hot-dip galvanized steel sheet (abbreviated as GA) is one of them; ■It is possible to form relatively thick sheets.

■Excellent weldability.

■Cost is relatively low.

Due to these advantages, it is used more often than other surface-treated steel sheets (corrosion-proof steel sheets for car bodies). At the same time, quality requirements for GA are also becoming stricter.

That is, when GA is used as is as a rust-preventing steel sheet for automobiles and C is used, automobile manufacturers have pointed out that chemical conversion treatment (phosphate treatment), electrophoretic painting finish, and post-painting corrosion resistance are poor.

As a countermeasure, for example, as in Japanese Patent Publication No. 58-15554, Fe-Zn with Fe% of 40% or more is added to the surface of GA.
There is a method of applying alloy coating.

In addition, thin Fe or Ni, Fe-Pl on the surface of GA
Alternatively, there is also a method of applying alloy plating such as Zn-Ni.

However, in this way, the GA coat surface Fe, Ni.

When polishing Fe-Zn, Fe-P, Zn-Ni, etc., the following problems occur on the actual production line. For example, when a two-layer plated steel sheet is manufactured using the following process, when the two-layer plated steel sheet is usually subjected to phosphate treatment by automobile manufacturers before electrodeposition painting, there is often a problem that a pale white streak pattern appears on the surface. Ta.

When this white striped pattern was observed with a scanning electron microscope, it was found that the phosphate crystals were incomplete and the matrix appeared sparse in places.

As a result of intensive investigation into countermeasures to solve this problem, we found that by clarifying the cause of the problem, it could be solved by devising a pretreatment method before the upper layer plating.

[Means to solve the problem]

That is, the gist of the present invention is to contain GA in the amount of 10 to 150 g/l of one or more of caustic soda and sodium phosphate, and further contain 0.1 to 1 g of cationic, anionic, or nonionic surfactant. /It is degreased in an aqueous degreasing solution containing It. Alternatively, as a subsequent treatment, the steel sheet is pickled in an aqueous sulfuric acid solution having a concentration of 1 to 20 g/j2, and subjected to preliminary metal plating, alloy plating, or multilayer plating of these, thereby producing a surface-treated steel sheet with excellent paintability and corrosion resistance. It is.

The present invention will be explained in detail below.

As a result of investigating the cause of the pale white color after phosphate treatment, which is the first problem, using an EPMA analyzer, we found that the pale white part contains more St elements (possibly due to the formation of SiO□) than the normal part. I discovered many things. As a result of further investigation into the cause of this 5i(h formation), it was found that it was caused by sodium orthosilicate (Na, Sin, ) in the degreasing liquid agent.

In other words, when GA is immersed in a degreasing solution containing conventionally used orthosilica soda, the GA is removed by a hydrolysis reaction.
A coating surface 5i02 is deposited. The amount of precipitation for 5in2 is G
Although there are variations due to differences in the thickness and activity of the oxide film on the surface of A coating, the pale white abnormal area after phosphate treatment does not disappear completely no matter the degreasing conditions (sodium orthosilicate concentration, temperature, immersion time) are selected. .

As a result of investigating the selection of degreasing agents, GA was first degreased. For the degreasing treatment, a degreasing liquid consisting of one or both of caustic soda and sodium phosphate and a surfactant is used. Caustic soda is used for alkaline degreasing (vegetable oils and fats) and has a strong saponification effect. Also, sodium phosphate is weakly alkaline and has little saponification, emulsification, or dispersion effect, but when used for degreasing zinc, etc.
It has the characteristic of not damaging the material.

Also, since it has the effect of softening hard water, it improves the degreasing effect and the washing effect. These degreasing liquids can be used alone or in combination with a surfactant. However, its concentration is 10g/j! If the degreasing aqueous solution is less than 100 ml, even if it penetrates into the inside of the fat, it will not be able to swell and weaken the adhesion between the fat and the plating surface. On the other hand, 150g/
If the concentration exceeds jQ, the plating surface will be easily attacked by alkali and should be avoided.

Additionally, surfactants significantly improve the cleaning effect. This is to enhance the penetration, wetting, and cleaning power of the alkali into the metal surface. Furthermore, the removed oil is emulsified and floats in the liquid, preventing it from adhering to the plating surface again. For the above reasons, active agents commonly used in degreasing treatments, such as anionic surfactants such as petroleum-based detergents, cationic surfactants pyridine-based, and nonionic surfactants ester-based, are used. However, the concentration of surfactant in the degreased aqueous solution is 0.1 g/f.
If the molecular weight is less than 1, even those having lipophilic and woodophilic groups, which are characteristic of activators, will not exhibit excellent activity unless their molecular weight is considerably large. Its limit is 0.1 g/j2. Another 1g/! Concentrations exceeding 100% are not effective and often cause problems such as foaming. Further, after degreasing, the activator remains on the surface of the mesh and damages the surface of the upper layer.

The GA that has been degreased with such a degreasing aqueous solution is free from oil and fat contamination and has a surface with good activity. However, when degreasing in a sodium phosphate degreasing agent solution, light color unevenness (difference in gray and black color) may occur in the phosphate coating depending on various conditions such as the solution temperature and treatment time during the degreasing treatment. This is due to the formation of a small amount of zinc phosphate on the surface, so in order to remove this uneven appearance, it is recommended to perform pickling treatment in a sulfuric acid solution after degreasing. If the concentration of pickling treatment is less than 1%, the GA coating surface cleaning effect will be low, and if it exceeds 20g/IL, the GA coating surface G
The cracks in the A plating layer are etched, leading to deterioration in workability.

GA that has undergone such a pretreatment process is plated with metal such as Fe, Ni, or plated with metal such as Fe-P, Zn-Fa,
Alloy plating such as Zn-Ni or electroplating of these layers is applied. The thus obtained surface-treated steel sheet is free from the problematic white streak pattern and has excellent paintability and corrosion resistance.

(Examples and Effects of the Invention) Example-1 A sample of an alloyed hot-dip galvanized steel sheet (GA) was coated with a steel sheet (thickness: 00 .8mm plating amount is 4 on the front side.
0g/lr? , the back side is 40g/m', the plating layer composition is Z
n: 89%, Fe: 11%) with NaOH or Na2P
After immersing the sample in a degreasing aqueous solution containing O4 and a cationic surfactant concentration of 1 g/It at a temperature of 50 DEG C. to 60 DEG C. for 10 seconds, the sample was washed with water and brushed.

For rough pickling treatment) l, SO4 concentration: 1% (1
After being immersed in a solution of 0g/u) at 20°C to 25°C for 5 seconds, it was washed with water.

Further, in the upper layer plating, Zn-Fe (80%) Zn -
Although three types of tests were conducted, P (1 to 2%) and Zn-N1 (11%), Zn-Fe (80%) alloy plating is shown as a representative example. The plating bath composition, plating conditions, and resulting fleck properties are as follows.

Plating bath composition Plating conditions ZnSO4”7)120 16g/fL Current density 80A/cm”FeSO4・7H2043θg/fL
Plating time 2.8 seconds H2SO411g/j!
Temperature 50℃p)l 1 Plating properties Coating weight 3.1-3.7g/rrl' Alloy composition
The appearance after peeling of Zn-Fe (77N85 turtle) was observed with the naked eye, and quality was judged as follows.

◎ Surface uniform with no abnormalities) O Slight running pattern △ Slight linear pattern (black and white unevenness) X Linear pattern (clear black and white unevenness) Full-deep drug PB308D treatment) was applied as follows.

Degreasing 40℃ x 2'' Surface conditioning 1.4g/ft Room temperature x 2''
Chemical formation 43℃ x 2' TA23. F to O, 8 Coating weight: 2.5 g/rn” Judgment of the chemical conversion coating was made using the following standards (observation with the naked eye, but SEM observation was also performed if necessary).

◎ Good (dense plate-like crystals) ○ Good texture (dense, but with some needle-like crystals) △ Linear FA-like white part with scratches (no crystals) X There is a lot of scratches in the white part of the linear pattern .

Next, ED painting was performed and the surface appearance after ED was observed.

The ED conditions are as follows.

Electrodeposition paint Power Top U-80 Gray made by Nippon Paint Electrodeposition conditions 220VX 3 minutes 28℃ 150mm between poles
Polar ratio: contrast 1:3 (sample) Paint film thickness 20-25μ Baking conditions 180℃ x 3 minutes ED coating surface judgment criteria ◎ Surface flat ○ A few small dents can be seen / 70 x 150mm
△ 5 to 10 dents/70x 150+Il@X 50 dents/70x150mm Table 1 shows a summary of the test results when Zn-Fe (~80%) alloy plating was applied.

The effects of the present invention are clearly evident.

Claims (1)

[Claims] 1. An alloyed hot-dip galvanized steel sheet containing 10 to 150 g/l of one or both of caustic soda and sodium phosphate, and further containing 0 to 150 g/l of a cationic, anionic or nonionic surfactant. A surface-treated steel sheet with excellent paintability and corrosion resistance, characterized in that after degreasing in an aqueous degreasing solution containing 1 to 1 g/l, the surface is coated with a corrosion-resistant metal plating, a corrosion-resistant alloy plating, or a laminated plating thereof. manufacturing method. 2 Alloyed hot-dip galvanized steel sheet containing 10 to 150 g/l of one or both of caustic soda and sodium phosphate, and further containing 0.1 to 1 g/l of cationic, anionic, or nonionic surfactant. After degreasing in a degreasing aqueous solution, followed by pickling in a sulfuric acid solution with a concentration of 1 to 20 g/l, the surface is plated with a corrosion-resistant metal, a corrosion-resistant alloy, or a laminated plating thereof. A method for manufacturing surface-treated steel sheets with excellent paintability and corrosion resistance.