JP2000309897A - Production of electrogalvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel sheet excellent in plating adhesion by executing dip pickling or, preferably, executing electrolytic pickling by the electric quantity equal to or above a specified one with a pickling soln. added with a specified amt. of thiourea and then executing electrogalvannealing of a Zn alloy. SOLUTION: Dip pickling or, preferably, electrolytic pickling is executed by the electric quantity of >=30 c/dm2 with a pickling soln. added with 15 to 500 ppm thiourea. The Zn alloy is the one contg. one or more kinds among Co, Fe, Ni, Cr and Mn, and preferably is a composite Zn alloy contg. these by 0.01 to 10 wt.% in total and a high molecular organic matter composed of C, H and O of 0.01 to 10 wt.% by the C content. The high-molecular organic matter composed of C, H and O is dextrin, dextran or at least one kind of organic compd. of high molecular weight having the effect equal to that of these and/or an organic compd. of high molecular weight produced therefrom by the reaction of decomposition, polymerization, or the like, at the time of electroplating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a Zn-based alloy electroplated steel sheet having improved adhesion of a plating film, and more particularly to a method for producing a composite Zn-based alloy electroplated steel sheet containing a polymer organic substance.

[0002]

2. Description of the Related Art In recent years, Zn-based alloy electroplated steel sheets have been used to improve the rust prevention performance of automobiles. Currently, this type of plated steel sheet, Zn-Ni system, Zn-Fe system, etc.
Various types of Zn-based alloy electroplating have been adopted.Zn-based alloy electroplating has better corrosion resistance than Zn plating, but has higher internal stress of the plating film and poor adhesion to the base steel sheet. There are drawbacks.

[0003] Even in the past, several methods have been proposed as means for mainly improving the condition of the surface of the base material and solving the plating adhesion. For example, in Japanese Patent Application Laid-Open No. 63-140098, a method of polishing the surface of a steel sheet with an abrasive containing abrasive grains before plating, or further polishing the surface of the steel sheet with an abrasive containing abrasive grains, and then in a Zn-based alloy plating solution. A method is disclosed in which a Zn-based alloy plating of 0.1 to ² g / m ² is applied on a steel sheet at a current density of 1 to 10 A / dm ² and then a normal Zn-based alloy is electroplated with a plating solution of the same type. I have.

Japanese Patent Application Laid-Open No. 5-230689 discloses a method in which the surface is polished as a pretreatment for plating, followed by pickling (including electrolytic pickling) for 10 seconds or less, and then electroplating.

[0005] In either method, the pickling treatment involves:
Uses acidic solutions such as hydrochloric acid and sulfuric acid. In addition, there is also known a method in which a small amount of plating is performed in advance, such as by performing pre-treatment plating such as Ni plating, so as to improve the adhesion to a subsequently provided plating film.

[0006]

However, when producing a composite Zn-based alloy electroplated steel sheet containing a high molecular organic substance consisting of C, H, and O, a special element is concentrated on the surface of the base material. Compared to conventional Zn-based alloys other than base metal,
It was difficult to obtain the same adhesion.

[0007] Pretreatment plating also has an insufficient practical effect, and in particular, in the case of the composite Zn-based alloy electroplating described above, the effect of improving the adhesion is not sufficient. A first object of the present invention is to provide a method for producing a Zn-based alloy electroplated steel sheet which solves such problems of the prior art and has improved plating adhesion.

A more specific object of the present invention is to provide a composite comprising at least one of Co, Fe, Ni, Cr and Mn and a high molecular organic substance comprising C, H and O, which has stabilized adhesion.
An object of the present invention is to provide a method for producing a Zn-based alloy electroplated steel sheet.

[0009]

Before electroplating a metal plate, a pretreatment for activating the metal surface by degrease and pickling the base material is performed. The pickling method at the time of the pretreatment is a soaking pickling method in which a base material is immersed in an acid solution to dissolve and remove oxides, and a method in which the base material is immersed in an acid solution to give an electric potential difference between the base material and the solution to perform electrolysis. There is an electrolytic pickling method in which the mechanical action of the generated gas and the reducing power of the nascent hydrogen are added to promote the dissolution and removal of the oxide.

In the immersion pickling method, a plated steel sheet having good adhesion can be obtained by strictly controlling working conditions such as the concentration, temperature, and purity of the pickling solution.
It is greatly affected by properties of the pickling solution such as impurity ions in the pickling solution such as e ³⁺ , Ni ²⁺ and Pb ²⁺ . In addition, depending on the surface properties of the base material, the oxide removing power may be insufficient to cause poor adhesion. On the other hand, the electrolytic pickling method is widely used as a pretreatment means because the pickling rate is high and the pickling rate can be adjusted by controlling the amount of current during electrolysis.

The present inventors have intensively studied the factors that deteriorate the adhesion by conducting a surface analysis of the base material and a test in which the pickling conditions were changed in order to obtain stable adhesion of the composite Zn-based alloy electroplating. .

As a result, a composite Zn containing a high molecular organic substance
It has been found that the adhesiveness of the electroplated steel sheet is greatly affected by the surface condition of the base metal and the pretreatment method. It has been found that the plating adhesion greatly depends on the micro-order physical shape of the base material surface. It is considered that the cause of the deterioration of plating adhesion in the base material having the physical shape of the micro order is that the plating cracking property of the micro-shaped portion is poor. Is estimated to deteriorate.

That is, in the case where the adhesion of the plating film is unfavorable, many very small depressions (micro pits) having a diameter of several tens to several hundreds of nm (nanometers) were observed on the surface of the base material before plating. Deterioration of adhesion of plating film
It was inferred that the plating material of the plating material on the micro pits was not good, and as a result, the bond between the plating film and the base material surface was weakened. Micropits often occur in association with fluctuations in electrolytic pickling conditions applied as a pretreatment for plating, and this tendency was particularly pronounced in the anodic electrolytic pickling method in which electrolysis is performed using a base material as an anode. .

In order to understand the cause of the poor adhesion in more detail, the present inventors used an extremely low-carbon cold-rolled steel sheet as a base material, set the same pickling time, and determined the degree of pickling in units of electrolytic pickling. After various pretreatments were performed by changing the amount of electricity per area, plating was performed under the same conditions to produce a zinc alloy-based plated steel sheet, and the adhesion of the plating film was examined. As a result, it was found that the adhesion of the plating film was poor when a large number of microscopic unevenness micropits were observed on the surface of the base material.

The change in the shape of the surface of the base material causing the deterioration of the adhesion is caused by, for example, the influence of coexisting elements in the pickling solution causing the dissolution of the base material anode due to the deterioration of the pickling solution.
It is caused by the conditions of electrolytic degreasing and electrolytic pickling.

From these results, the inventors of the present invention carried out immersion pickling or pickling with a pickling solution containing 15 ppm or more of thiourea as a treatment for preparing a micro-order shape of the base material surface before plating.
It has been found that by performing electrolytic pickling with an amount of electricity of 30 c / dm ² or more and then performing electroplating, the plating adhesion of the composite Zn-based alloy electroplate is stabilized.

Of course, the present invention intends to improve the adhesion of the composite zinc alloy plating film described above, but the effect of improving the adhesion of the plating film according to the present invention is not limited to such composite plating film and alloy plating. In general, it is effective in improving the adhesion of the electroplated film to steel.

Therefore, in a broad sense, the present invention provides a pickling solution containing thiourea in an amount of 15 ppm or more and 500 ppm or less, by immersion pickling or preferably by electrolytic pickling at an electric quantity of 30 c / dm ² or more, Next, for example, a method for producing an electroplated steel sheet having excellent plating adhesion, which is performed by electroplating a Zn-based alloy.

In the present invention, more specifically, the Zn
The alloy is a Zn-based alloy containing one or more selected from the group consisting of Co, Fe, Ni, Cr, and Mn. Further, as a preferred embodiment, the Zn-based alloy is Co, Fe, Ni,
This is a composite Zn-based alloy containing one or more selected from the group consisting of Cr and Mn, a total of 0.01 to 10 wt%, and a high molecular weight organic material composed of C, H, and O.

[0020]

Next, an embodiment of the present invention will be described in detail. In the present invention, when the plating film is C
one or more members selected from the group consisting of o, Fe, Ni, Cr, and Mn, 0.01 to 10 wt% in total, and C, H, O
When a composite Zn-based alloy plating film containing a high-molecular organic material consisting of 0.01 to 10 wt% in terms of C content exhibits particularly remarkable plating adhesion, the present invention will be described below with reference to that example. explain.

As the plating base material, it is preferable to use a known cold-rolled steel plate such as a drawing steel plate or a high-tensile steel plate. However, the present invention is not limited to this, and a high-tensile steel plate used for reducing the weight of an automobile body may be used. For example, an arbitrary metal plate can be used as a base material according to a purpose of use and a part to be used.

In the pretreatment of the base material, after degreased by a known method, immersion pickling or electrolytic pickling is performed using a known acid solution, for example, a sulfuric acid aqueous solution. The present invention eliminates the influence of coexisting elements that promote the dissolution of the base metal anode in the pickling solution by adding a very small amount of thiourea to the pickling solution to a concentration of 15 to 500 ppm, and makes the surface of the steel sheet uniform. It utilizes an effect that has not been expected in the past, such as etching to reduce minute irregularities on the surface due to intragranular corrosion.

According to the present invention, the surface of a steel sheet is etched by using a pickling treatment solution containing a very small amount of thiourea and setting the pickling conditions to a non-energized state or an electric quantity of 30 c / dm ² or more. In order to make the surface shape of the base material in micro order uniform, the plating adhesion is stable even when, for example, a plating treatment is performed on a composite Zn-based alloy electroplated steel sheet containing a high-molecular organic substance composed of C, H, and O. It becomes.

Conventionally, thiourea is known as an inhibitor in pickling, but in this case, it is added at a considerably high concentration of 0.1 to 0.5%. This is for surface finish and corrosion control.

However, according to the present invention, minute irregularities are eliminated by adding a very small amount, and this is peculiar to thiourea, which is not found in other conventional inhibitors (starch, aniline, etc.). A phenomenon, and C, H,
When such pickling is performed as a preliminary treatment when performing electroplating of a composite Zn-based alloy containing a high-molecular organic substance composed of O, a particularly remarkable effect can be obtained. that is,
It is thought to prevent micro-order etch pits.

Here, the action of thiourea in the present invention is summarized as follows. Prior to electroplating a metal plate, a pretreatment for activating the metal surface by degrease and pickling the base material is performed. The pickling method at the time of pretreatment includes a soaking pickling method in which a base material is dipped in a pickling solution to dissolve and remove oxides, and a gas generated by dipping in a pickling solution to give an electric potential difference and electrolysis. There is an electrolytic pickling system which promotes dissolution and removal of oxides by adding a mechanical action by hydrogen and a reducing power by hydrogen at the nascent stage. The surface after pickling has the properties of the pickling solution (Fe
^3+, Ni ^2+, surface properties (purity impurity ion) and the base metal, such as Pb ^2+, composition, process, is greatly influenced by the extent and the like) of the heat treatment, sometimes the plating adhesion failure occurs.

Even if the plating conditions are the same, the adhesion level of the plating film may fluctuate. When the adhesion of the plating film was not favorable, many very small dents having a diameter of several tens to several hundreds of nm were observed on the surface of the base material before plating. The reason why the plating adhesion is deteriorated in the base material having the micro-order physical shape is considered to be poor plating resistance of the micro-shaped portion.
In order to improve the adhesion deterioration, it is necessary to reduce micro-order irregularities.

The effect of the addition of thiourea is to suppress the influence of impurities that promote dissolution of the base metal anode in the pickling solution.
(Inhibitor effect) and minute irregularities on the anode metal surface cause a difference in the concentration gradient of thiourea in the liquid phase,
As a result, the diffusion current is affected, and the current is more concentrated on the convex portion, and as a result, it is conceivable that the fine irregularities on the surface disappear (fine irregularities disappearance effect).

The preferred pickling conditions as a pretreatment for plating in the present invention are as follows, for example. Immersion pickling : H ₂ SO ₄ 5%, temperature 55 ° C., thiourea 100 ppm Electrolytic pickling : H ₂ SO ₄ 5%, temperature 55 ° C., thiourea 100 ppm, charge density 30 c / dm ² Preferred embodiment of the present invention According to this, as a Zn-based alloy to be plated, a Zn-based alloy containing one or more selected from the group consisting of Co, Fe, Ni, Cr and Mn as an alloy element is exemplified.

More specifically, the total content of the above alloy elements in the coating is limited to 0.01 to 10 wt%, for the following reasons. Add 0.01% or more to maintain good corrosion resistance (perforation resistance) without painting.

The balance other than the alloying elements is substantially Zn. If the addition amount of alloying elements is low, Zn in the plating film
Exists in the form of an η phase having excellent sacrificial corrosion resistance, but when the addition amount of the alloying element exceeds 10%, the η phase disappears, the sacrificial corrosion protection function of the plating film is reduced, and the end face corrosion resistance after painting is impaired. It is. The η phase has ductility and maintains crystallographic consistency with a steel plate or an aluminum alloy plate, and thus has excellent adhesion to a base metal plate. Therefore, the upper limit of the content of the alloying element is 10%. Preferably, it is 1 to 5%.

Further, in a preferred embodiment of the present invention,
The high-molecular organic substance consisting of C, H, and O has a C content of 0.01 to
Only 10wt% is contained in the plating film to form a composite plating film.

Here, the high-molecular organic substance comprising C, H, and O means dextrin, dextran or at least one high-molecular-weight organic compound having the same effect as these, and / or decomposed during electroplating. Alternatively, it means a high-molecular-weight organic compound produced from these by a reaction such as polymerization. The high-molecular organic substance composed of C, H, and O is contained in an amount of 0.01 to 10% in terms of C content based on the weight of the entire film.

When the content of such a high molecular organic substance is less than 0.01%, the corrosion resistance without painting becomes insufficient.
If the content exceeds 10%, plating work becomes difficult, and corrosion resistance is impaired. Preferably, it is 1 to 5%.

In the present invention, examples of thiourea to be added to the pickling solution include diethyl thiourea and dibutyl thiourea. If the added amount of thiourea is less than 15 ppm, the adhesion to the steel sheet may be reduced, and it is necessary to add 15 ppm or more in order to obtain stable adhesion.
On the other hand, if it exceeds 500 ppm, it is economically disadvantageous, so the upper limit is made 500 ppm. Preferably, it is 100 to 200 ppm.

Here, the difference between the action of thiourea as an inhibitor generally added to the pickling solution and that of the present invention will be described as follows. In the past, inhibitors have been added at a fairly high concentration of 0.1-0.5%, and the base iron is partially exposed and dissolved by pickling, thus preventing roughening of the steel sheet, hydrogen embrittlement, iron loss, etc. In this case, the function of smoothing only the portion having the large unevenness is used. Therefore, such an inhibitor has been added only in the case of descaling such as before and after heat treatment. In order to improve the adhesion of the plating film, it has not been used for the purpose of smoothing fine irregularities.

In the present invention, the coating amount of the plating film is optional, but is preferably 5 to 100 g / m ² . Next, the present invention will be described more specifically with reference to working examples and effects thereof.

[0038]

[Example] After degreasing a 0.8 mm-thick SPCC cold-rolled steel sheet,
The pickling treatment was performed by a combination of the addition of thiourea to the pickling solution to which the coexisting element was added and the electrolysis conditions. The pickling conditions are summarized in Table 2.

The cold-rolled steel sheet thus degreased and pickled is used as a composite Zn-Co containing each alloying element and a high molecular organic substance composed of C, H and O by using a plating bath having the following composition.
An electroplating of 30 g / m ^{2 was} performed.

[0040] <Plating _{conditions> ZnSO 4 · 7H 2 O:} 1~40 _{wt% CoSO 4 · 7H 2 O:} 0~40 _{wt% NiSO 4 · 6H 2 O:} 0~40 wt% FeSO ₄ · 7H ₂ O: 0-40 wt% Na ₂ sO _4: 5 to 15% by weight dextrin: 0-40 wt% pH: 1.8 temperature: 50 ± 2 ° C. solution flow rate: 1.0 m / sec current density: 20 to 150 A / dm ² such Test materials were sampled from the composite Zn-based alloy electroplated steel sheet obtained in the above, and the plating adhesion of these test materials was evaluated by the DuPont impact peel test (1.0 kg × 50 cm, 6 mmφ, bare backing).
Was evaluated.

That is, after the DuPont impact peeling test, the surface opposite to the impact was peeled off with a peeling test tape, and the lightness (L value) of the plating particles adhered to the tape was measured using a color difference meter CR manufactured by Minolta.
The measured L value was converted to a contact code, and this contact code was used as a guideline for the evaluation of the adhesion. The adhesion code is shown in Table 3. In addition, the test materials were also evaluated for perforation resistance, end face corrosion resistance, workability, and weldability in the following manner.

Perforation resistance : salt spray (5% NaCl, 35 ° C.,
7 hours) → Dry (50 ° C, 2 hours) → Wet (RH 85%, 50%)
(15 ° C, 15 hours) as one cycle.
After the 0 cycles, evaluation was made based on the maximum corrosion depth.
◎: 0 mm, ○: less than 0.1 mm, Δ: 0.1 to 0.3 mm, ×: more than 0.3 mm.

End face corrosion resistance : The same corrosion cycle test as in the case of the above-described perforation resistance was performed for 60 cycles, and evaluated by the area ratio of red rust occurrence on the end face. ◎: No red rust, ○: 5%
Below, △: 10% or less, ×: 30% or less.

Workability : A cylindrical deep drawing test was conducted, and the workability was evaluated based on the peeling rate of the plating film on the side wall surface. ◎: no peeling, ○: less than 10%, Δ: less than 30%, ×: 30% or more.

Weldability : Resistance spot welding was performed and evaluated by the number of continuous hits until the nugget diameter became 3.6 mm or more. ◎: 2000 times or more, ○: 2000 to 1500 times, △: 1500 to
1000, ×: Less than 1000 times.

The results of these tests are summarized in FIGS. 1, 2 and Table 4. In Comparative Examples in Table 4, thiourea was not added during pickling. The content of the organic substance was measured by dissolving the plating film with 5% sulfuric acid, decomposing the high molecular organic substance in the solution into glucose, and quantifying the amount of glucose by the phenol sulfuric acid method. As the molecular weight, a number average molecular weight measured by gel permeation chromatography was used.

[0047]

[Table 1]

[0048]

[Table 3]

[0049]

[Table 4]

FIG. 1 is a graph showing how the adhesion of the plating film changes when the addition amount of thiourea is variously changed in the case of Example No. 1; The data are shown for both the case of only the case and the case of carrying out electrolytic pickling with a current flow of 30 c / dm ² .

From the results of FIG. 1, it can be seen that if the added amount of thiourea is 15 ppm or more, a region having good adhesion is obtained. However, when continuous treatment is performed, thiourea is decomposed or pickled solution is taken out. Since the urea concentration decreases, it is more desirably about 100 ppm.

FIG. 2 shows the relationship between the amount of electrolytic pickling electricity and the adhesion code in the case of electrolytic pickling in which thiourea was added to the pickling solution at 100 ppm. From the results of FIG. It can be seen that stabilization of the contact cord was achieved by washing and immersion or at an electric quantity of 30 c / dm ² or more.

From the above results, 0.01 to 10 wt% of Co, Fe,
In order to ensure the plating adhesion of a composite Zn-based alloy electroplated steel sheet containing at least one or more of Ni, Cr, and Mn and a high molecular organic material consisting of 0.01 to 10 wt% of C, H, and O,
It can be seen that it is necessary to add 15 ppm or more of thiourea to the pickling solution, and it is more preferable that the amount of electricity for pickling be 30 c / dm ² or more as an electrolytic pickling treatment method.

[0054]

According to the present invention, the adhesion of C, H, O to the micro-order physical shape of the base material surface is greatly affected by the combination of the addition of thiourea and the conditions of electrolytic pickling.
The plating adhesion of the composite Zn-based alloy electroplated steel sheet containing high-molecular-weight organic material was secured, which enabled efficient production and stable quality.

[Brief description of the drawings]

FIG. 1 is a graph showing the amount of thiourea added and the adhesion code.

FIG. 2 is a graph showing the quantity of pickling electricity and the adhesion code.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kenya Tadada 3 Oaza Hikari, Kashima City, Ibaraki Prefecture Sumitomo Metal Industries Kajima Works F-term (reference) 4K024 AA15 AA17 AA18 AA19 AA20 AB01 AB19 BA03 BC01 CA02 DA02 DA03 GA01 4K053 PA02 PA12 QA01 QA07 RA15 RA51 RA54 TA09 TA16

Claims (4)

[Claims] 1. An immersion pickling or electrolytic pickling with a pickling solution containing thiourea in an amount of 15 ppm or more and 500 ppm or less.
A method for producing a Zn-based alloy electroplated steel sheet having excellent plating adhesion, which comprises electroplating a Zn-based alloy. 2. The method for producing an electroplated steel sheet according to claim 1, wherein said electrolytic pickling is performed at an electric quantity of 30 c / dm ² or more. 3. The electroplating according to claim 1, wherein the Zn-based alloy is a Zn-based alloy containing one or more selected from the group consisting of Co, Fe, Ni, Cr, and Mn. Steel plate manufacturing method. 4. The method according to claim 1, wherein the Zn-based alloy is at least one selected from the group consisting of Co, Fe, Ni, Cr, and Mn.
The method for producing an electroplated steel sheet according to any one of claims 1 to 3, which is a composite Zn-based alloy containing 0.01 to 10 wt% and a high-molecular organic substance composed of C, H, and O in a C content of 0.01 to 10 wt%. .