JPS59182958A - Alloyed and galvanized steel sheet and its production - Google Patents

Abstract

PURPOSE:To obtain an alloyed and galvanized steel sheet having improved and stabilized corrosion resistance after painting by subjecting a steel strip which is a base material to galvanizing contg. Pb, etc. and adjusting the coating weight of zinc then subjecting the strip to a heating and alloying treatment. CONSTITUTION:A steel strip which is a base material subjected to an ordinary pretreatment is plated with a molten zinc bath contg. <=0.05% Pb or a molten zinc bath contg. 0.1-0.15% Al, 0.03-0.5% Sn, and the coating weight of zinc is adjusted to <=45g/m<2>. The strip is then subjected to a heating and alloying treatment at a furnace temp. of 350-700 deg.C in a suitable treating furnace. The alloyed and galvanized steel sheet having <=0.5 Al/Zn in the atom concn. ratio between the Al and Zn existing in the compsn. down to about 20Angstrom from the surface of the plated steel sheet, 10-20mum surface roughness and <=40mg/m<2> content of Pb existing in the plating film per unit area on one surface is obtd. by such treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an alloyed hot-dip galvanized steel sheet with excellent corrosion resistance after painting and a method for manufacturing the same.

Alloyed hot-dip galvanized steel sheets (hereinafter referred to as "alloyed steel sheets" for convenience of explanation) are particularly excellent in weldability, paintability, and corrosion resistance among galvanized steel sheets, and their demand has been increasing in recent years. It is used as a steel plate for painting home appliances, automobiles, steel furniture, vending machines, etc.

1.Although the alloyed steel sheet has the above-mentioned excellent properties, it also has several drawbacks. One of these is corrosion resistance after painting, which has relatively large lot-to-lot and intra-lot variations.While some lots are extremely good, there are also lots that are quite poor, and there is demand for this. It may cause trouble at home. The appearance of defective products with poor corrosion resistance after painting or the occurrence of unevenness is noticeable when special IC's extremely simple coating base treatment and coating threads are employed. Although it is empirically clear that the cause of such a phenomenon lies in the plating material factor, there has been no report on the investigation of this cause.

This invention was made with the aim of improving and stabilizing the quality of alloyed steel sheets, especially in terms of corrosion resistance after painting.

The inventors are particularly interested in alloyed steel sheets that are suitable for applications such as one-coat painting after ordinary zinc phosphate treatment. There are many applications for coating plated copper plates, such as multiple coats using electrodeposition coating, or multiple coats after phosphate treatment and chromate sealing, but in such cases, the quality of the plated material The contribution rate of slight variations to the performance of the final product, that is, the ease with which it can be eaten after painting, is extremely low. However, in the case of one-coat painting without a chromate seal, which is used by some consumers of home appliances, steel furniture, etc., the quality of the plating material is mostly reflected in the performance of the final product. Therefore, it is an object of this invention to propose an alloyed steel sheet that has sufficient deterioration resistance after surface painting, and a method for manufacturing the same, in a relatively simple form of surface treatment for painting as described above. .

The gist of this invention is that the atomic concentration ratio of M and Zn present in the elemental composition up to a depth of 2OA on the surface of the alloyed steel sheet is Ae/Zn < 0.5, and the surface roughness is 10 μm or more.
The alloyed steel sheet is characterized in that the plating film has a pb thickness of 20 μm or less, and the amount of PB present in the plating film is 40 m Q /m 2 or less per unit area on one side of the steel sheet.

One of the causes of poor corrosion resistance after coating of alloyed steel sheets is
The zinc phosphate film is insufficiently formed. As for the evaluation method of resistance after painting, for example, after painting a test steel plate, cross-shaped scratches are added to the coated steel plate and a salt water spray test is carried out for about 240 hours, or a 3% salt water test is carried out for about 100 hours. After conducting the immersion test, the peeling width of the peeled coating film is evaluated by a taping test on the scratched area.

Incidentally, it is empirically known that the state of zinc phosphate crystals in alloyed steel sheets is inferior to that of other plated steel sheets, such as electrogalvanized or hot-dip galvanized steel sheets. However, in actual comparison with other plated steel sheets, such as in the accelerated test mentioned above, the fact that the alloyed steel sheet is still superior even when the loft variation of the alloyed steel sheet is taken into account is simply due to the zinc phosphate treatment. This indicates that sex alone is not involved in the ability to eat food after painting.

Therefore, in order to overcome the drawbacks of conventional alloyed steel sheets, the inventors conducted intensive studies on material factors for corrosion resistance after painting, and finally completed the present invention.

That is, one of them is, for example, X-ray photoelectron spectroscopy (ES
CA) The atomic concentration ratio of M and Zn in the composition up to a depth of 2OA from the surface of the alloyed steel sheet is I'd!
/Zn < 0.5.

According to the inventors' study, chemical conversion treatment properties are improved by combining the factors involved in corrosion resistance after painting.
//It is a chart showing the relationship between Zn and chemical conversion treatment properties. Here, the chemical conversion property is defined as - 900 mmV vs. until the corrosion potential of the specimen becomes noble and passivated when the steel sheet is immersed in the required zinc phosphate treatment solution.
The evaluation is based on the time required to return from an initial potential of about SCE to -700 mv vs SCE. The result shown in FIG. 1 is the above-mentioned N! /Zn<0.5 indicates good chemical conversion treatment properties.

In other words, if A//Zn exceeds 0.5, the time required for passivation becomes longer and the chemical conversion treatment property deteriorates, so in this invention, A//Zn is specified to be 0.5 or less.

In this invention, the surface roughness of the alloyed copper plate was limited to 10 to 20 μm because, as is clear from Figure 2, the surface roughness Rmax value has a large effect on the corrosion resistance of the alloyed steel plate after painting. However, it is preferable to have a larger Rmax value; however, when Rmax ) is 20 μm, the sharpness of the painted finish is deteriorated, and when Rmax (10 μm), the corrosion resistance after painting is deteriorated.

Furthermore, in this invention, the pb content in the plating film of the alloyed steel sheet is set to 40 mf/T' per unit area on one side.
The reason why it was restricted to r12 or less was because, as shown in FIG. 3, it was found that as the amount of PB in the plating film increases, the corrosion resistance after coating deteriorates by 1IIIt, and the distance between the corners is 4 Q m 97 m2.

In addition, the post-painting mI corrosion resistance in Figures 2 and 3 refers to plating test pieces (60 pieces x 150 mm) that were degreased with a commercially available weak alkaline cleaner, washed with water, immediately treated with zinc phosphate, and then treated with acrylic. Apply 25 μm of resin baking paint, make cross-shaped scratches, and apply 3% NacI! ,3
The evaluation was made during peeling of the scratched area (on both sides) using commercially available cellophane tape after immersion in a solution at 5° C. for 1 oohr.

Next, a method for producing the above-mentioned alloyed steel sheet of the present invention will be explained.

In this invention method, a steel plate is pretreated with PB (0.0
, 05% (of course, the general 0.1~
0.15% M and Fe etc. are not allowed! After plating (contains I impurities), the basis weight is adjusted to 45 g/m2 or less per unit surface of the steel plate using an air knife, etc.
It is characterized by heat-alloying treatment at a furnace temperature of 350 to 700°C (including batch-type heat-alloying treatment)
, size, instead of pb in plating Rakuchu (10'3~0.5
It is characterized in that after plating in a molten zinc bath to which Sn is added, a conventional heat alloying treatment is performed.

In this invention method, the furnace temperature is limited to 350 to 700°C because alloying treatment is extremely difficult at a furnace temperature of less than 350°C, and when it exceeds 700°C, Al/Z on the plating surface layer
n increases, especially when the pb content in the plating bath is 0.05%
This is because if the Zn basis weight exceeds 45 g/m2, both chemical conversion treatment properties and post-painting corrosion resistance become poor. According to the inventors' study, the main factors that affect the corrosion resistance after coating are the interaction between the PA1 degree during plating and the alloying temperature.
Plated in a plating bath with an im degree of over 0.5% and heated to 700°C.
When alloying is performed at a furnace temperature above the above, extreme variations occur in the chemical conversion properties of the product, increasing the probability of producing inferior products. Therefore, if plating is done with as little PB as possible,
Even if the alloying treatment temperature exceeds 700°C, there are almost no defective products.

However, reducing PB in the plating bath has disadvantages such as a decrease in the fluidity of the plating bath and a decrease in the number of products that can be manufactured. In order to deal with this problem, in this invention, in place of PB in the plating bath, 0.03 to 0.5
This paper presents a method of plating with a plating bath to which % of Sn is added. In this case, since there is no influence of PB, there is no problem even if the alloying treatment temperature is considerably high up to 1200°C.

However, rapid alloying treatment in a high-temperature furnace is not necessarily preferable because it reduces the Rmax value of the plated surface, and it is usually most preferable to perform alloying treatment at a furnace temperature of 750° C. or lower. In this invention method, the Sn content is reduced to 0.
．． The reason why it is limited to 0.03 to 0.5% is that less than 0.03% will lead to a decrease in the fluidity of the plating bath, and if it exceeds 0.5%, it will not only lead to an increase in cost, but will also effectively become saturated. It is. The reason for limiting the M content to 0.1 to 0.15% is that at 0.15%, alloying is delayed and operability deteriorates, and when Al < 0.1%, ordinary zinc It becomes difficult to adjust the plating bath when switching between plated products.

Methods for reducing the atomic concentration ratio A//Zn of M and Zn on the surface of the alloyed steel sheet in this invention include, for example, mechanically polishing the surface, pickling or cathodic protection, and electrolytic acid treatment. A washing method and an alloying method in a non-oxidizing atmosphere can also be adopted.

Examples of the present invention will be described below.

Example: After plating with a hot-dip galvanizing device (Sendzima one-way type) that can pass a copper strip with a width of 300 rrrm and a thickness of 0.8,
Alloyed steel sheets were produced in an alloying furnace (furnace length: 40 CIl) in which the furnace temperature could be adjusted (furnace length: 40 CIl), and the resulting alloyed steel sheets were evaluated by a salt water immersion test after coating (coating thickness: 25μ). The resulting hinutogram is shown in FIG. 4 in comparison with the case of the conventional method. The manufacturing conditions for the alloyed steel sheets were as shown in Table 1.

As is clear from the results shown in FIG. 4, it can be seen that according to the method of the present invention, there is little variation in the corrosion resistance of alloyed steel sheets after coating, and that stable quality can be obtained.

Chapter 1 Table Just 17, Bath temperature: 460℃±3℃ Fe content in plating film Rate: 9-13%

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between Al/Zn (atomic concentration ratio) and chemical conversion properties in the composition up to 2 o^ depth from the surface of the alloyed steel sheet, and Figure 2 shows the surface roughness Rma of the alloyed steel sheet.
Figure 3 is a diagram showing the relationship between the x value and the corrosion resistance of the coating, Figure 3 is a diagram showing the relationship between the PB content in the plating film and the corrosion resistance after painting, and Figure 4 is the graph showing the relationship between the alloyed steel plate in the example of this invention after painting. This is a hinutogram showing the evaluation results by a salt water immersion wiper. Applicant: Sumitomo Metal Industries, Ltd.
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Claims (1)

[Claims] 1. AI present in the elemental composition up to a depth of 2OA on the surface of an alloyed hot-dip galvanized steel sheet! and Zn atomic concentration ratio is At /Zn (Q, 5, surface roughness is 10prn
An alloyed hot-dip galvanized steel sheet, characterized in that the amount of PB present in the plating film is 40 mf/m2 or less per unit area on one side of the steel sheet. 2 A base steel strip that had undergone normal pretreatment was plated in a molten zinc bath containing 0.05% or less of PB, and the zinc weight was adjusted to 44 M'/m2 or less per unit area on one side of the steel plate. A method for producing an alloyed hot-dip galvanized steel sheet, which is then heat-alloyed at a furnace temperature of 35°C. 3. The base metal copper strip that has been subjected to normal pretreatment is subjected to N! 0.1~0
．． 15%, Sn o, o 3 to 0.5%, residual ratio 1 Law.