JPH0323939A - Double layer plated steel sheet - Google Patents

Abstract

PURPOSE:To exercise coating properties equivalent to that of phosphate treatment by providing a Zn plated or Zn alloy plated lower layer, providing an Fe plated, an Fe-B alloy plated or an Fe-Zn-B alloy plated upper layer and having the upper layer contain specified water-soluble polymer. CONSTITUTION:A lower layer is Zn plated or Zn based alloy plated, while an upper layer is Fe plated, Fe-Zn alloy plated with Fe 80wt.% or more, or Fe-B alloy plated with B 0.001 - 3.0wt.%, or else Fe-Zn-B alloy plated with Fe 80wt.% and B 0.001 - 3.0wt.%, and 0.01 - 20wt.% water-soluble polymer of a derivative formed with a polymer composed one or both of hydroxystyrene of average molecular weight of 2000 - 1,000,000 and its derivative is contained in said upper layer. The water-soluble high molecular contains average 2 - 15 hydroxyl radicals and average 1 - 10 sulfone radicals or sulfonate radicals per 1000 molecular weight, and in order to contain 0.01 - 20wt.% in the upper layer, 0.1 - 100g/l water-soluble polymers are added into sulfuric acid bath and chloride bath of Fe plating of Fe-Zn alloy plating, and plating is carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a plated steel sheet that exhibits excellent coating film adhesion even when coated without pre-painting treatment such as phosphate treatment.

(Advanced technology) In recent years, Zn-Ni has been used as galvanized steel sheets for automobiles.
Zn-based alloy electroplated steel sheets such as Zn-based alloys and Zn--Fe based alloys are increasingly being used in large quantities.

However, since these plated steel sheets have poor phosphate treatment properties, craters occur in the coating film during electrodeposition coating, resulting in poor paint quality.
There was a problem that they were inferior in their reciprocity.

Therefore, the present inventors applied Zn-based alloy plating to the lower layer, and applied Fe plating to the upper layer to create a plated steel sheet that solved the warping problem.
Less than 50-100u+t%, 80,001+lIt% or more, preferably 0.001-3. Owt% Fe −
Plated steel sheet for painting with Zn alloy plating
-228498) or Zn plating or Zn-based alloy plating on the lower layer, and B (boron) or S (sulfur) on the upper layer, preferably 0.001 to 3, Out% of the former, and 0.00 1-0 of the latter. A plated steel sheet (Japanese Unexamined Patent Application Publication No. 12-253796) which was subjected to Fe plating containing .4u+t% and had excellent electrodeposition coating properties was previously provided.

These plated steel sheets have a lower layer coated with Zn or a Zn-based alloy that has excellent corrosion resistance, and an upper layer coated with a high Fe-Zn alloy or Fe plated that has excellent paintability, and by containing B, it is easy to coat by electrodeposition. The first layer is electroplated to prevent the formation of craters.

(Problem to be solved by the invention) However, when painting this plated steel sheet, it is necessary to perform phosphate treatment in the same way as conventional plated steel sheets, so care must be taken to manage the treatment liquid and dispose of the waste liquid. I was in a hurry.

The present invention provides a multilayer plated steel sheet that exhibits the same paintability as a phosphate-treated steel sheet without phosphate treatment.

(Means for Solving the Problems) The present inventors have discovered that even without phosphate treatment on the multi-scrap plated steel sheet as described above, it is possible to obtain the same paintability as when phosphate treatment is applied. As a result of various studies in order to improve the
Zn joint venture plating, BO. OO1-3. Oart% Fe
-B alloy plating or Fe80wt% or more, BO. OO
1-3.0+ut% Fe-Zn-B alloy plating, and the upper layer is a water-soluble derivative of hydroxystyrene with an average molecular weight of 2,000 to 1,000,000, a sulfonated product of one or both of its derivatives. This was made possible by containing 0.01 to 20...L% of the polymer.

In the present invention, the upper layer is plated with Fe-Zn alloy or Fe-Zn
In the case of B alloy plating, the Fe amount was set to 80u+t% or more because if the Fe amount is too low, the coating film blisters that occur in the corrosion resistance test will become large even if a water-soluble polymer is added. It is from.

Further, the reason why B is added to Fe plating or Fe-Zn alloy plating is to prevent the occurrence of coating film craters during electrodeposition coating, as in the conventional method. For this reason, the addition amount was also set to 0.001 to 3.0% by weight, similar to the case of Jubei's plated steel sheets.

Sulfonated polymers of hydroxystyrene, one or both of its derivatives are added to enable coating properties equivalent to those obtained with phosphate treatment without phosphate treatment. Sulfonated products of hydroquine styrene polymers are those in which a hydroxyl group and a sulfone group or a sulfonic acid group are directly bonded to an aromatic ring (e.g., poly-1), monohydroxystyrene sulfonic acid, poly-
1) sulfonated monohydroxystyrene (alkali metal salts, etc.), and its derivatives include those in which an alkyl group is introduced into ethylene (hydroxy-α-methylstyrene, hydroxy-α-ethylstyrene, hydroxy-β-ethylstyrene, etc.).
methylstyrene, etc.), those with a hydroxyalkyl group or sulfoalkyl group dedicated to the aromatic ring (3-hydroxymethyl-4-hydroxystyrene, 3-hydroxyethyl-
4-hydroxystyrene, 3-sulfomethyl-4-hydroxystyrene, 3-sulfoethyl-4-hydroxystyrene, etc.).

Since this water-soluble polymer has a sulfonic group, it dissolves in an aqueous plating bath, and when electrolyzed, it is eutectoided with the plating metal on the cathode, although the reason is unclear. Also,
This plating layer containing a water-soluble polymer has good adhesion to the coating film when electrodeposited on the surface, probably because the polymer has a chemical affinity with the coating film. For this reason,
Shows excellent coating adhesion even without phosphate treatment.

If the water-soluble polymer has a low weight average molecular weight of less than 2,000, sufficient coating film adhesion cannot be obtained.
When the molecular weight exceeds 1 million, the solubility in the plating bath decreases, and the amount added to the bath is limited, so eutectoid to the extent that paint film adhesion can be obtained without phosphate treatment. things become difficult.

The solubility of a water-soluble polymer in a plating bath is affected not only by its molecular weight but also by the density of sulfonic groups and sulfonic acid groups. Inferior, requires one or more on average. However, if the number exceeds 10 on average, the adhesion of the coating film decreases, which is not preferable.

In addition, the ability to improve paint film adhesion by water-soluble polymers is also affected by the density of hydroxyl groups, and if the number of hydroxyl groups is less than 562 per molecule fl = 1000, sufficient paint film adhesion cannot be obtained. 15
If it exceeds 1, the secondary adhesion will decrease.

The amount of water-soluble polymer contained in the plating layer is 0,Olw
If it is less than t%, a sufficient coating film adhesion improvement effect cannot be obtained, and even if it is contained in an amount exceeding 20u+t%, the improvement effect is saturated, and even if it is contained more than that, no improvement effect is observed.
Moreover, the workability of the plating layer is slightly reduced.

In order to contain B in the upper layer, plating can be carried out by adding a boron compound such as boric acid or metaboric acid to an electric plating bath as in the past.
To contain +t%, Fe plating or high Fe-Zn
Add 0.1 to 0.1% of water-soluble polymer to the sulfuric acid bath and chloride bath for alloy plating.
Plating may be performed by adding 1009/g.

If the amount of plating on the upper layer is less than 0.59/m2 (one side), the lower layer cannot be completely covered, and if it is more than 109/+o2, the cost will increase, so it should be 0.5 to 109/m2.
It is preferable to use m2 (one side). There is no particular limit to the amount of plating deposited on the lower layer. This lower layer may be formed by directly plating Zn or a Zn alloy by electroplating, vapor deposition plating, hot-dip plating, or the like, or may be formed by plating Zn by the method described above and then alloying it by thermal diffusion.

Next, the present invention will be explained with reference to examples.

('J!.Example) Example J After a cold-rolled steel sheet was subjected to pretreatment such as degreasing and pickling in the usual manner, lower layer plating and upper layer plating were applied under the conditions shown in Table 1.

The water-soluble polymers added to the plating bath during upper layer plating were as follows.

Water-soluble polymer A weight average molecular weight: 10000, number of hydroxyl groups per molecular weight 1000: 3, number of sulfone groups = 5, -7 = 8- water-soluble polymer B weight average molecular weight: 2 0 0 0 0, Molecular weight 1000
Number of hydroxyl groups per unit: 5, number of sulfone groups = 5, water-soluble polymer C weight average molecular weight: 30000, molecular weight 1000
Number of hydroxyl groups per unit: 5, number of sulfone groups: 2.Next, the plated steel sheet obtained as above was coated with cationic electrodeposition coating (Power Top U manufactured by Nippon Paint) without being subjected to phosphate treatment. -30, coating thickness 20μ+n) and a similar cationic electrodeposition coating using a phosphate treatment (Bonderite $3030 manufactured by Nippon Parriki Lining Co., Ltd.) on a galvanized steel sheet using the following properties. compared with.

(1> Paint film adhesion After immersing the coated steel plate in 40℃ hot water for 100 days, 1+
An n+n cross-cut test was conducted to evaluate the state of paint film peeling with cellophane tape according to the following criteria.

Paint film peeling condition Symbol 0/100~ 5/100 06/100~
25/100 △26/100~100/100
X (2) Paint film blistering A cross cut is made on the painted steel plate to reach the steel cable ground, and one cycle (24 hours) is a salt spray test according to JIS Z 2371! ! 12 hours → 60℃ drying 6 hours → wet test (50
50 cycles of the corrosion test were conducted for 6 hours (°C, R I {95% or more), the maximum bulging width from the cross-cut portion was measured, and it was evaluated based on the following criteria.

Less than 3+nm 0 3~10+n+n　△ 9 =10 1 0mIIl, 1i1 × The above results are shown in Table 2.

13 Example 2 A cold-rolled steel plate was heated to 50 vol% 112-N2M- at 800°C.
After preheating in an ambient atmosphere for 30 seconds, the 0. (2) A hot-dip Zn-plated steel sheet was prepared by immersing it in a 4wt% 8l-Zn plating bath for 2 seconds. In addition, the plated steel sheet produced in this manner was heated to alloy the plating layer, thereby producing an alloyed Zn-plated steel sheet having a plating layer composition of Zn-9u+t%Fe.

Next, upper layer plating was applied to the hot-dip Zn-plated steel sheet and the alloyed hot-dip Zn-plated steel sheet produced as described above under the following conditions.

(1) Plating bath composition or sulfate iron 2509/e, zinc sulfate
O~1009/g, sulfuric acid sorg
709/e, metaboric acid sorg 10-509/e
．． , water-soluble polymer 0-209/e, (water-soluble polymer is the same as that used in the example) (2) Plating conditions lul+ 1.5-2.5 Bath temperature
50°C, current density 408/dm2 (adjusted by Plating W111 or Bath Assemblage II) Next, the upper layer plating was applied under the above conditions, and then cation was applied without phosphate treatment. The coating properties were compared in the same manner as in Example 1 when electrodeposition coating was applied and when cationic electrodeposition coating was applied to a conventionally plated steel plate subjected to phosphate treatment.

The above results are shown in Table 3.

]4 15 16 (Effects of the invention) As described above, the steel sheet of the present invention can produce a coating film equivalent to that obtained when a conventional plated steel sheet is subjected to phosphate treatment, even without phosphate treatment before painting. Indicates adhesion. Therefore, the phosphate treatment that was carried out before electrocoating when manufacturing automobiles in the United States is no longer necessary, and the process Fl! Problems with liquid management and waste liquid treatment will be resolved, and productivity improvements and cost reductions can be expected.

Claims (2)

[Claims] (1) The lower layer is Zn plating or Zn alloy plating,
The upper layer is Fe plating, Fe-Zn alloy plating with Fe80wt% or more, Fe-B alloy plating with B0.001-3.0wt%, or Fe80wt% or more, B0.001-3.
Along with 0 wt% Fe-Zn-B alloy plating,
In these upper layers, 0.01 to 20 wt% of a water-soluble polymer consisting of a sulfonated polymer of hydroxystyrene with an average molecular weight of 2,000 to 1 million, or one or both of its derivatives is added.
A multi-layer plated steel sheet characterized by containing: (2) The water-soluble polymer has an average of 2 to 15 hydroxyl groups and an average of 1 to 10 sulfonic groups or sulfonic acid groups per 1000 molecular weight, Multi-layer plated steel plate.