JPS6114235B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention mainly relates to a method for surface treatment of steel sheets for automobiles, and in particular to a method for zinc-based plating treatment. Hot-dip galvanized steel sheets or electrogalvanized steel sheets have traditionally been used as surface-treated steel sheets for automobiles, but each has its advantages and disadvantages.
The reality is that a surface-treated steel sheet that satisfactorily satisfies all of the various properties required for applications such as automobiles, such as corrosion resistance, weldability, and paintability, has not yet been obtained. For example, in hot-dip galvanizing, the amount of plating deposited is usually 45 g/m ² or more on one side, and the plating layer must be considerably thick. Therefore, although it has excellent corrosion resistance, it has the disadvantage of poor weldability. In addition, when hot-dip galvanized steel sheets are used as a base material for coating, the adhesion of the coating between the surface of the galvanized layer and the paint on it is poor, resulting in poor coating film. There are problems such as blistering and peeling of the paint, and a decrease in paint corrosion resistance. Therefore, after hot-dip galvanizing, the iron is diffused from the base steel sheet into the plating layer by heating, and as a result, the plating layer becomes a zinc-iron alloy (δ ₁ phase) crystallographically.
Conventionally, this has been done, and in this case, the coating adhesion of the surface is greatly improved. However, even when the hot-dip galvanized layer is alloyed, the disadvantage of poor weldability cannot be overcome because the galvanized layer itself is thick, as described above. Furthermore, as mentioned above, in order to alloy the plating layer, the entire plated steel sheet was conventionally heated in a heating furnace, and the heat treatment adversely affected the mechanical properties of the base steel sheet.
Particularly in the case of high-strength steel sheets for automobiles, fatal defects may occur, and in the case of single-sided plated steel sheets, there are problems such as oxidation of the cold-rolled surface during heating, resulting in temper color. On the other hand, in the case of electrogalvanizing, the plating layer is relatively thin with a plating thickness of 20 to 30% g/ ^m2 .
Therefore, although the weldability is good, there are drawbacks such as poor paint adhesion, peeling and blistering of the paint film, and low paint corrosion resistance. In order to overcome this drawback, it may be possible to alloy the plating layer by heating after electroplating, as in the case of alloyed hot-dip galvanizing, but in this case as well, the base steel sheet is heated as described above. There are problems in that it adversely affects mechanical properties and in the case of single-sided plating, it causes temper color. This invention was made in view of the above circumstances, and provides 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. The purpose is to provide a method for obtaining In other words, the present inventors found that the electroplating method is advantageous in terms of weldability because it allows the thickness of the plating layer to be reduced, and that in order to improve paint adhesion, the plating layer should be formed of a zinc-iron alloy phase (δ If it is effective to form _{a single} phase) and in that case it is possible to alloy it by heating only the surface of the plating layer, it is possible to improve the 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 surface of the plating layer could be irradiated with laser light without heating the entire plated steel sheet. We discovered that by heating only the surface portion, it is possible to alloy the plating layer, and as a result, a surface-treated steel sheet with excellent weldability and paint adhesion can be obtained without impairing the mechanical properties of the base steel sheet. This led to this invention. Therefore, in the surface treatment method of the present invention, iron-zinc mixed plating is applied to the surface of a steel plate by electroplating, and then the surface of the plating layer is irradiated with laser light to crystallographically alloy the plating layer. It is characterized by this. 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. In this case, electroplating can be carried out using a zinc-iron mixed plating bath according to a conventional method. Here, it is desirable that the composition of the zinc-iron mixed plating layer be determined so that a zinc-iron alloy (δ ₁ phase) can be efficiently produced by short-time heating by subsequent laser light irradiation, and from that point of view, Iron content 8~15wt
%, and the balance is desirably set to be substantially zinc. If iron is less than 8wt%, sufficient amount of δ ₁
Since a phase is not generated, a sufficient effect of improving paint adhesion cannot be obtained, and if it exceeds 15 wt%, the processing adhesion of the plating layer itself will be poor. On the other hand, the amount of plating deposited in zinc-iron mixed electroplating is preferably 40 g/m ² or less on one side in order to improve weldability.
If it exceeds 40 g/m ² , weldability deteriorates and welding costs increase. As mentioned above, when zinc-iron mixed electroplating is applied, the plating layer has not crystallized into an alloy phase (δ ₁ phase) that has excellent paint adhesion.
The iron phase and zinc phase are mixed. Therefore, after this, the surface of the plating layer is irradiated with laser light to heat only the plating layer for a short time, and at least the surface part of the plating layer, preferably 1 μm from the surface, is heated.
The above is alloyed crystallographically to generate a zinc-iron alloy phase (δ ₁ phase). If the thickness to be alloyed is less than 1 μm, the effect of alloying, that is, the effect of improving paint adhesion will be small, and therefore, as mentioned above, it is desirable to alloy the thickness to be 1 μm or more. Further, the heating amount temperature during short-time heating of the surface of the plating layer by laser beam irradiation is preferably 300 to 600°C in order to efficiently generate the _δ1 phase. At temperatures below 300℃, it is difficult to efficiently generate the δ ₁ phase in a short period of time, and at temperatures above 600℃, diffusion of iron from the iron matrix may occur, and the iron content in the alloy layer may exceed 15wt%. There is. As described above, the laser used for laser light irradiation may be any gas laser such as a CO ₂ laser or solid laser such as a YAG laser. As mentioned above, the laser beam irradiation conditions should be set so that the surface temperature of the plating layer is within the range of about 300 to 600℃, and the specific conditions will depend on the thermal conductivity and light reflectance of the object to be treated. It varies depending on the case, but usually, for example, in the case of a CO ₂ laser, the output
About 400W~2KW, output 20~ for YAG laser
It is sufficient to irradiate at about 100W for about 0.01 to 1 second per ¹ cm2. The δ ₁ phase obtained by short-time heating with laser light irradiation has extremely excellent paint adhesion, and since only the plating layer is heated for a short time during laser light irradiation, the mechanical strength of the base steel sheet is reduced. There is no risk of adverse effects on characteristics, etc. 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, width of 100 mm, and length of 200 mm, current density was applied using a zinc-iron mixed plating bath with the bath composition shown in Table 1. Zinc-iron mixed plating was applied at 60 A/dm ² and bath temperature of 50°C. Here, the plating adhesion amount is per one side.
20 g/m ² , and the composition of the resulting plating layer was 90 wt% zinc and 10 wt% iron. The thus obtained zinc-iron mixed plating steel sheet was irradiated with laser light using a CO ₂ gas laser (output 2 KW) to alloy the mixed plating layer to a thickness of about 2 μm from the surface. However, the laser beam irradiation time is 1cm ²
It was estimated that the maximum temperature reached on the surface of the plating layer was about 450℃. After chemical conversion treatment was applied to the surface of the steel plate treated in Example 1 above, cationic electrodeposition coating was applied to a thickness of 17 μm. Then, a salt spray test was conducted in accordance with JIS Z 2371, and the width of the bulge of the coating film from the cross-cut portion after 30 days was examined, and the results shown in the upper row of Table 2 were obtained. For comparison, a steel sheet that was subjected to zinc-iron mixed electroplating under the same conditions as in Example 1 and then not irradiated with laser light, a steel sheet that was subjected to ordinary electrogalvanization, and a cold-rolled steel sheet that was not plated. Table 2 also shows the results of a salt spray test performed on these comparative materials using a steel plate as a comparative material and subjected to the same chemical conversion treatment and cationic electrodeposition coating as described above. Table 1 (zinc-iron mixed plating bath composition)

【table】

[Table] From the results shown in Table 2, it can be seen that the galvanized steel sheets that were irradiated with laser light after zinc-iron mixed electroplating using the method of the present invention were compared with other surface-treated steel sheets and cold-rolled steel sheets. Excellent coating adhesion,
It is clear that it exhibits corrosion resistance. Example 2 After electrolytically degreasing and pickling the surface of a general-purpose cold-rolled steel plate (SPCC material) with a thickness of 0.7 mm, width of 100 mm, and length of 200 mm, a plating bath with the same composition as that used in Example 1 was used. Then mixed zinc-iron plating was performed. However, the amount of plating deposited and the composition of the plating layer are almost the same as in Example 1. The zinc-iron mixed plated steel sheet obtained in this way was subjected to short-time heat treatment for the plated layer by laser light irradiation using a YAG laser (output 40W), and the plated layer was heated to almost the entire thickness (approximately 3 μm).
was alloyed. However, the laser beam irradiation time is 1cm ²
It was estimated that the maximum temperature reached on the surface of the plating layer was about 500℃. After chemical conversion treatment was applied to the surface of the steel plate treated in Example 2 above, a three-layer coating consisting of cationic electrodeposition, intermediate coating, and top coating was applied to a total thickness of approximately 100 μm.
I went there. A salt spray test was then conducted in accordance with JIS Z 2341 to examine the bulge width of the coating film from the cross-cut portion after 90 days, and the above three-layer coated steel sheet was immersed in pure water at 50°C for 240 hours. , put a 2mm square cross cut on the surface.
100 squares were created and a peel test (secondary adhesion test) using adhesive tape was conducted to determine the number of squares where the coating film did not peel off. The results of these tests are shown in Table 3. For comparison, Table 3 also shows the results of a test similar to that described above performed on a comparative material similar to that described above after chemical conversion treatment and three-layer coating.

[Table] From the results shown in Table 3, it can be seen that the plated steel sheets that were subjected to laser beam irradiation after zinc-iron mixed electroplating according to the method of the present invention had better coating adhesion and corrosion resistance after three-layer coating. It is clear that the secondary adhesion after the three-layer coating is also much better than that of other surface-treated steel sheets. 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. Moreover, since the plating layer is a zinc-iron alloy layer, it is possible to obtain excellent paint adhesion and paint corrosion resistance, and when alloying the plating layer, iron is not diffused from the base steel sheet. Since a zinc-iron mixed plating layer is formed in advance and only the mixed plating layer is alloyed by short-term heating with laser light irradiation, there is no risk of adversely affecting the mechanical properties of the base steel sheet. In addition, even in the case of single-sided plating, it is possible to effectively prevent the occurrence of temper color caused by oxidation of the cold-rolled surface due to alloying treatment, which is much more effective than conventional galvanized surface treatments. Various excellent effects can be obtained.

Claims (1)

[Claims] 1. Applying zinc-iron mixed plating to the surface of a steel plate by electroplating, and then irradiating the surface of the zinc-iron mixed plating layer with laser light to crystallize at least the surface of the mixed plating layer. A method for surface treatment of steel sheets characterized by chemical alloying. 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 wt% iron and the balance is zinc. 3. The surface treatment method according to claim 1, wherein zinc-iron mixed plating is applied so that the amount of plating deposited in the zinc-iron mixed plating is 40 g/m ² or less per side of the steel sheet. 4. The surface treatment method according to claim 1, wherein the mixed plating layer is alloyed by the laser beam irradiation over a thickness of 1 μm or more from the plating surface.