JPS58157990A - Surface treatment of steel plate - Google Patents

Abstract

PURPOSE:To enhance the weldability of a steel plate and the adhsive strength in coating without deteriorating the mechanical characteristics, by electroplating the surface of the steel plate with a zinc-iron mixtur and by irradiating the surface with laser light to alloy the surface. CONSTITUTION:The suface of a steel plate is plated with a zinc-iron mixture by an electroplating method using a zinc-iron mixture plating bath. The surface of the plated layer is irradiated with oaser light to heat only the plated layer for a short time. By the heating at least the surface of the plated layer is aoolyed crystallographically to form a zinc-iron alloy layer (delta1 phase) having high adhesive strength in coating.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention mainly relates to a method for surface #& treatment of steel plates for automobiles, and includes 4! This is a method for treating zinc-based chemicals.

Conventionally, hot-dip galvanized steel sheets or electrogalvanized steel sheets have been used as surface-treated steel sheets for automobiles.
However, they all have advantages and disadvantages, and the reality is that a surface-treated steel sheet that fully satisfies all the mechanical properties required for automotive and other uses, such as corrosion resistance, weldability, and paintability, is still not available. be.

For example, in hot-dip galvanizing, the amount of plating deposited is usually 45 t/wt or more per WL, which forces the plating layer to be extremely thick. However, it has the disadvantage of poor weldability, and when hot-dip galvanized steel sheets are used as a base material for painting, the adhesion of the paint is poor, that is, the surface of the 6th layer in the middle and the top layer. There are problems such as poor adhesion with paints, which can cause blistering and peeling of the paint film, and a decrease in corrosion resistance. Conventionally, iron is diffused and the plating layer is crystallographically transformed into a zinc-iron alloy layer (phase A). In this case, the paint adhesion of the surface is greatly improved. Improved. However, even when the middle layer of molten galvanized metal is alloyed, the plating layer itself is thick, so the drawback of poor weldability cannot be overcome.Furthermore, as mentioned above, the ninth step of alloying the plating layer was conventionally done using a heating furnace. Since the entire plated steel sheet was heated, the heat treatment had a negative effect on the mechanical properties of the base steel sheet, which could lead to fatal defects, especially in the case of tensile steel sheets for automobiles).
In the case of a single-sided plated steel plate, there are problems such as the cold abrasive surface being oxidized during heating, resulting in tensile force 2-.

On the other hand, in the case of electrolytic galvanizing, the amount of plating deposited is 2
At 0 to 30 f/l, the plating layer is relatively thin and its rounding weldability is good, but the paint adhesion is poor, causing peeling and blistering of the paint film, resulting in poor coating corrosion resistance. There are drawbacks. To solve this problem, it is possible to heat the plated layer after electroplating to alloy the plated layer as in the case of alloyed hot-dip galvanized steel. There are problems in that it adversely affects mechanical properties and in the case of single-sided plating, it may cause temper color.

This invention was made in view of the above circumstances, and
The object of the present invention is to provide a method for obtaining a galvanized surface-treated steel sheet that has good paint adhesion and excellent weldability without adversely affecting the mechanical properties of the base steel sheet.4
It is.

In other words, the present inventors have found that the electroplating method is advantageous in that it can reduce the thickness of the plating layer from the problem of Sm property, and that the ninth method for improving paint adhesion is to use the plating layer on a zinc-iron base or gold plate. If it is effective to form a phase (-1 phase) and it is possible to alloy it by heating only the surface of the middle layer, it will improve paint adhesion without impairing the mechanical properties of the base steel sheet. After conducting various experiments and studies, we found that if a zinc-iron mixed plating layer was formed in advance by electroplating, the process could be improved without heating the entire plated steel sheet.
By irradiating the plated layer surface box with laser light and heating only the surface area, it is possible to alloy the plated layer.
As a result, it was discovered that a surface-treated steel sheet with excellent weldability and paint adhesion could be obtained without impairing the mechanical properties of the base steel sheet, and Maruno was able to develop this invention.

Therefore, in the surface treatment method of the present invention, iron-zinc mixed plating is applied to the surface of a steel sheet (by electroplating), and then the surface of the plating layer is irradiated with laser light to crystallize the plating layer. It is characterized by being alloyed with.

The surface treatment method of the present invention will be explained in more detail below.

In carrying out the method of this invention, first, a zinc-iron mixed plating is applied to the surface of a steel plate by electroplating. This S combination electroplating can be carried out using a zinc-iron mixed plating bath according to a conventional method. It is desirable to form a zinc-iron alloy (Jt phase), and from that point of view, the iron content is 8 to 15w.
t%, and the balance is preferably set to be substantially zinc. If the iron content is less than 8 vt-, a sufficient amount of δ1 phase will not be generated, and a sufficient effect of improving paint adhesion will not be obtained.
If it exceeds 15vtd, the processing adhesion of the plating layer itself will be poor. On the other hand, in order to improve weldability, it is desirable that the coating amount of zinc-iron mixed electroplating be less than 4 (Lr/v) on one side.If it exceeds 40V, weldability will deteriorate and welding costs rises.

As mentioned above, when zinc-iron mixed electroplating is applied, the plating layer is crystallographically not yet an alloy phase (J1 phase) with excellent paint adhesion, but contains a mixture of iron and zinc phases. The situation is such that Therefore, after this, laser light was irradiated onto the surface of the L-plated layer to heat only the plating layer for a short time.
From at least the surface portion of the plating layer, preferably from the surface, I 1111m or more is crystallographically alloyed, and zinc-
Iron alloy phase (a.

phase) is generated. The thickness of alloyed here is 1jIm
If the thickness is less than that, the effect of alloying, that is, the effect of improving paint adhesion, will be small. Therefore, as mentioned above, it is desirable to alloy the thickness of IIC 11 km or more. The heating temperature for short-time heating is
The temperature is 300 to 600℃ to efficiently generate the J1 phase.
It is desirable to do so.

At temperatures below 300°C, it is difficult to efficiently generate a one-day phase in a short period of time, and at temperatures above 600°C, diffusion of iron from the iron matrix occurs, and the iron content in the alloy layer exceeds 15 wt'l. There is a risk.

The laser used for laser light irradiation as mentioned above is
Gas lasers such as CO2 lasers or solid-state lasers such as YAG lasers are used.The laser light irradiation conditions are such that the surface temperature of the plating layer is within the range of 300 to 600"C as described above. The specific conditions are ~100W1! degree depending on the thermal conductivity and light reflectance of the object to be treated, etc., and 1a1 per fi0.
The J1 phase obtained by short-time heating with such laser light irradiation has extremely excellent coating adhesion, and in this laser light irradiation, only the plating layer can be coated. Since it is heated for a short time, there is no risk of adversely affecting the mechanical properties of the base steel sheet.

Examples of this invention are described below.

Example 1 After electrolytically degreasing and pickling the surface of a high-tensile steel plate for automobiles with a thickness of 0.6 mm and a width of 100 mm and a length of 20 mm, a zinc-iron mixed plating bath having the bath composition shown in Table 1 was applied. Using current density 6
Zinc-iron mixed plating was performed at 0 Jy'dm2 and a bath temperature of 50°C. Here, the amount of plating deposited was 20 Vwl per side, and the composition of the resulting plating layer was 90 ftlG of zinc.
, iron IQ wtchi.

The nine zinc-iron mixed plated steel sheets thus obtained contain CO
□ Laser light irradiation was performed using a gas laser (output 2 kgI) to alloy the 11-mixed plated sheet to a thickness of about 2 μm from the surface. However, the laser beam irradiation time is 1 d/0.
The temperature reached by moths on the surface of the middle layer is 4.
It is estimated to be around 50℃ and is round.

After carrying out a chemical conversion treatment on the surface of the treated steel sheet (as in Example IK above), a cationic electrodeposition coating was applied to a thickness of 1771 m. Then, a salt spray test was conducted in accordance with JIS Z2371, and the bulge width of the coating film from the cross-cut area after 30 days was examined.The results shown in the upper row of Table 2 were obtained. A steel sheet that was subjected to zinc-iron mixed electroplating under the same conditions as in 1 without being irradiated with laser light, a steel sheet that was subjected to normal electrogalvanization, and a cold-rolled steel sheet that was not plated were used as comparison materials. Comparative materials were subjected to the same chemical conversion treatment and cationic electrodeposition coating as described above, and then subjected to a salt spray test. The results are also shown in Table 2.

Table 1 (Zinc-iron hot plating bath composition) From the results shown in Table 2, No. 29, it can be seen that the method of the present invention, therefore, in plated steel sheets subjected to laser beam irradiation after zinc-iron mixed electroplating, other It is clear that the surface-treated steel sheet exhibits superior coating adhesion and corrosion resistance compared to medium-cold abrasive steel sheets.

Example 2 After electrolytically degreasing and pickling the surface of a general-purpose cold-rolled steel plate (5 PCC material) with a thickness of 0.7, width of 100 cm, and length of 200 cm, the same composition as that of Example 1 was prepared. Zinc-iron mixed plating was carried out using medium temperature.However, the amount of plating deposited and the composition of the plating layer were almost the same as in Example 1. The thus obtained zinc-iron mixed plated steel sheet was heated for a short period of time by single-beam irradiation using a K YAG laser (output 40 W) to reduce almost the entire thickness of the plated layer (approximately 3 μm). was alloyed. However, the laser light irradiation time is 1
a11 (#) 0.0 seconds, and the maximum temperature reached on the surface of the plating layer was estimated to be about 500°C.

After chemical conversion treatment was applied to the surface of the mourning steel plate treated in accordance with Example 2 above, three layers of cationic electrodeposition, intermediate coating, and upper me were applied to a total coating thickness of about 100 mm. And J.I.
8Z2341 was tested with salt water spray Vh, 9
In addition to examining the swelling width of the coating film from the cross-cut area that will appear after 0 days, 1) After immersing the above-mentioned 3-layer coated steel plate in pure water at 50°C for 240 hours, make a square cross-cut on the surface EliK2. Create 100 squares, perform a peeling test (secondary adhesion test) using adhesive tape, and check the number of @O's until the coating film does not peel off.These test results are shown in Table 3. For comparison purposes, the same comparative materials as those described above were subjected to chemical conversion treatment and three-layer coating, and then
The results of a test similar to the above are also described in the 3rd ti.

Table 3 From the results shown in Table 3, the coating adhesion and corrosion resistance after the three-layer coating were found to be superior to the plated steel sheets that were subjected to radar light irradiation after zinc-iron mixed electroplating according to the method of the present invention. It is clear that it is much superior to other surface-treated steel sheets and cold-rolled steel sheets9, and that the secondary adhesion after the three-layer coating is also much better than other surface-treated steel sheets. It is.

As is clear from the above explanation, since the surface treatment method of the present invention basically adopts the electroplating method, excellent weldability can be obtained by making the plating layer thinner. Furthermore, since the middle layer is a zinc-iron alloy layer, excellent paint adhesion and corrosion resistance can be obtained. Since a zinc-iron mixed metal intermediate layer is formed in advance without diffusing iron, and only that mixed plating layer is alloyed by short-term heating with laser light irradiation, it does not adversely affect the mechanical properties of the base steel sheet. It is different from conventional zinc plating maim process, and even in the case of single-sided galvanizing, it is possible to effectively prevent the occurrence of oxidation of the minobu surface due to the rounding process of alloying, resulting in temper color. In comparison, it is much better and you can get the same effect as before.

Applicant: Kawasaki Steel Corporation

Claims (4)

[Claims] (1) Apply zinc-iron mixed plating (by electroplating) to the surface of the steel plate, and then apply the zinc-iron mixed plating layer 01
! A method for treating a steel sheet, characterized by irradiating the plate with a laser beam to crystallographically alloy at least the surface of the mixed plating layer. (2) The surface treatment method according to claim 1, wherein the zinc-iron mixed plating is applied so that the composition of the mixed plating layer is 8 to 15 vtl of iron and the balance is zinc. (3) Is the coating amount of the zinc-iron mixed plating K multiplied by D 4 G on one side of the steel plate? /v or less, the method of OII box plating according to claim - $11, in which zinc-iron mixed plating is applied so that the zinc-iron ratio is less than or equal to /v. (4) Patent claim 0111 where the alloying of the middle layer of the heated layer by the laser beam irradiation is performed by rolling it out from the plated box to a thickness of 1 μm or more! OVa image processing method described in item 1.