JP5858175B1 - Method for producing electrolytic chromated steel sheet - Google Patents

Abstract

The method for producing an electrolytic chromate-treated steel sheet according to the present invention includes a step of sequentially degreasing and pickling a raw steel sheet, a surface temperature of the raw steel sheet of 47 ° C. or lower, 150 g / l or more of chromic acid, and the chromium A step of cathodic electrolysis treatment of the material steel plate at a current density of 120 A / dm ² or less in a high concentration chromic acid bath containing sulfate ions having a concentration of 0.003 to 0.006 times the concentration of the acid and a fluorine compound; , A step of immersing the raw steel plate in a low-concentration chromic acid bath containing less than 60 ° C. containing chromic acid of 30 g / l to 60 g / l and a fluorine compound, and a step of washing the raw steel plate with water It is characterized by having.

Description

  The present invention is suitable as a material for beverage cans, food cans, 18L cans, pail cans, art cans, etc., and can be used for electrolytic chromate treatment steel plates that can be polished and welded with high efficiency and stability. The present invention relates to a method of manufacturing a steel plate.

  Electrochromated steel sheets that have a chromate film consisting of a chromium metal hydrate layer and a chromium chromium oxide layer on the surface of the material steel sheet are cheaper and more paint-coating than tin-plated steel sheets (so-called “blink”). In recent years, it has been widely used as a material for beverage cans, food cans, art cans, 18L cans, pail cans, and the like.

  On the other hand, electrolytic chromate-treated steel sheets are inferior in weldability compared to tin-plated steel sheets, and usually it is necessary to perform welding after mechanically polishing and removing the surface chromate film that impairs weldability immediately before welding. However, when carrying out the so-called polishing welding method, in which welding is performed after the chromate film is mechanically removed in this way, the number of welding lines is increased and it is necessary to introduce processing equipment for polishing. This is disadvantageous in terms of production cost.

  Moreover, when implemented in industrial mass production, a large amount of polishing scraps (metal powder) is generated. As a result, for example, in the can manufacturing line, problems such as polishing scraps being mixed into the can and a burden required for maintenance such as cleaning of the welding machine are increased. Further, such a polishing welding method has a problem that the exposed metal surface is oxidized by heat input during welding and changes to dark blue or black, and the appearance after welding deteriorates.

  For these reasons, there is a strong demand for the development of an electrolytic chromate-treated steel sheet that exhibits sufficient weldability even when the so-called non-abrasive welding method is used for welding without polishing, and various techniques have been proposed in the past. Has been.

  However, in the electrolytic chromate-treated steel sheet obtained by the prior art, the weldability when the non-abrasive welding method is applied (hereinafter referred to as “abrasive weldability”) is improved to some extent, but the non-abrasive welding in the steel sheet width direction is observed. The non-abrasive weldability at the edge in the width direction deteriorates. When the non-abrasive weldability deteriorates at the width direction edge portion as described above, it is necessary to cut off the edge portion, which causes problems such as poor yield. In addition, the electrolytic chromate-treated steel sheet obtained by the conventional technique has a problem that the weldable current range (ACR) of non-abrasive welding is also small.

  Furthermore, in the prior art, the non-abrasive weldability of the electrolytic chromate-treated steel sheet is improved to some extent, but it is difficult to balance with other characteristics such as corrosion resistance against an alkaline solution and surface appearance deterioration. In particular, when the corrosion resistance with respect to the alkaline solution is deteriorated, when an alkaline solution containing a surfactant is filled as the contents of the can, such as an 18L can or a pail can, the deterioration of the corrosion resistance particularly in the processed part becomes a problem. Also, in the can-making line, the electrolytic chromate-treated steel sheet is usually paint-baked or film-laminated and then welded to make a can. However, in the conventional technology, after the electrolytic chromate-treated steel sheet is paint-baked or film-laminated, various properties such as corrosion resistance are produced. Deteriorates.

In order to solve such a problem, Patent Document 1 discloses that when producing an electrolytic chromate-treated steel sheet, the material steel sheet is subjected to cathodic electrolytic treatment in a high-concentration chromic acid bath and then immersed in a low-concentration chromic acid bath. Techniques to apply have been proposed. By this method, on at least one side of the material steel plate, the amount of adhesion per side is 90 mg / m ^{2 to} 190 mg / m ² or less, and the amount of adhesion per side is 3 to 8 mg / m in terms of metal chromium. a chromate film composed of a uniform chromium hydrated oxide layer of m ^2, which is dehydrated during paint baking and / or film lamination among all the chromium hydrated oxides constituting the chromium hydrated oxide layer. The insoluble chromium hydrated oxide, which has physical properties that do not dissolve in hot alkali, is 1 to 5 mg / m ² in terms of metal chromium, and the proportion of the insoluble chromium hydrated oxide in the total chromium hydrated oxide is mass. A chromate film with a ratio of less than 68% is formed.

  According to the technique proposed in Patent Document 1, by forming a desired chromate film as described above, it is excellent in naked corrosion resistance against an alkaline solution containing a surfactant, as well as corrosion resistance under a coating film and a film. An electrolytic chromate-treated steel sheet with excellent abrasive weldability and a wide weldable current range (ACR) for non-abrasive welding can be obtained. Moreover, according to the technique proposed in Patent Document 1, it is possible to efficiently and stably provide an electrolytic chromate-treated steel sheet having desired characteristics while suppressing deterioration of the non-abrasive weldability at the edge portion in the width direction.

JP 2005-298864 A

  However, in the technique proposed in Patent Document 1, it is difficult to increase productivity when manufacturing an electrolytic chromated steel sheet having a desired chromate film. When producing an electrolytic chromate-treated steel sheet by subjecting the material steel sheet to cathodic electrolysis treatment and immersion treatment, an electrolytic bath (high-concentration chromic acid bath) for performing cathode electrolysis treatment and an immersion bath for performing immersion treatment are usually used. The material steel plate is passed through a chromate treatment line in which (low concentration chromic acid bath) is arranged in series. And in the technique proposed by patent document 1, in order to manufacture the electrolytic chromate processing steel plate which provided the desired chromate film | membrane as mentioned above, the steel plate after cathodic electrolysis processing should be immersed in an immersion tank for 2 seconds or more. Necessary. As specified in Patent Document 1, when the immersion treatment time is less than 2 seconds, a uniform chromium hydrated oxide layer cannot be obtained, and various characteristics such as non-abrasive weldability vary greatly in the width direction of the steel sheet. .

  On the other hand, in order to increase the productivity of the electrolytic chromate-treated steel sheet, it is essential to increase the line speed of the chromate treatment line. However, if the line speed is increased, the immersion treatment time is inevitably shortened. Therefore, in the technique proposed in Patent Document 1, when the line speed of the chromate treatment line is increased, the immersion treatment time becomes insufficient, and as a result, an electrolytic chromate treated steel plate having a desired chromate film is stably produced. It becomes difficult.

  The present invention has been made in view of the above circumstances, and even when the line speed of the chromate treatment line is increased, an electrolytic chromate treatment capable of forming a desired chromate film over the entire width direction of the raw steel plate. It aims at proposing the manufacturing method of a steel plate.

Specifically, when manufacturing the electrolytic chromate-treated steel plate by passing the material steel plate through the chromate treatment line in which the electrolytic bath and the immersion bath are arranged in series as described above, the immersion treatment time in the immersion bath is 2 seconds. Even when shortened to less than, metal chromium layer with an adhesion amount per side of more than 90 mg / m ^{2 and} 190 mg / m ² or less, and an adhesion amount per side of 3 to 8 mg / m in terms of metal chromium ^{2. A} chromate film comprising a uniform chromium hydrated oxide layer, wherein all chromium hydrated oxides constituting the chromium hydrated oxide layer are dehydrated and heated during paint baking and / or film lamination. The insoluble chromium hydrated oxide (hereinafter also referred to as “alkali insoluble chromium hydrated oxide”), which has physical properties that do not dissolve in alkali, is 1 to 5 mg / m ² in terms of metallic chromium. And the proportion of the total chromium hydrated oxide is It aims at providing the manufacturing method of the electrolytic chromate processing steel plate which can form the chromate film | membrane whose mass ratio is less than 68% over the whole width direction of a raw material steel plate.

  As described above, an electrolytic chromate-treated steel sheet having a chromate film composed of a metal chromium layer and a chromium hydrated oxide layer on a steel sheet material is subjected to cathodic electrolytic treatment in a high-concentration chromic acid bath and then to a low concentration. It is obtained by subjecting it to immersion treatment in a chromic acid bath. When the raw steel plate is subjected to cathodic electrolysis in a high-concentration chromic acid bath, metallic chromium and chromium hydrated oxide are deposited on the surface of the raw steel plate. In this cathodic electrolysis treatment, excessive chromium hydrated oxide is deposited on the surface of the material steel plate. However, by applying the subsequent immersion treatment, a part of the chromium hydrated oxide is dissolved, and the chromium hydrated oxide is deposited as desired. It can be an amount.

  Here, in the case of producing an electrolytic chromated steel plate by passing a material steel plate through a chromate treatment line in which an electrolytic bath (high concentration chromic acid bath) and an immersion bath (low concentration chromic acid bath) are arranged in series, When the line speed is increased, the immersion treatment time in the immersion tank (low concentration chromic acid bath) is inevitably shortened. When the immersion treatment time in the immersion bath (low-concentration chromic acid bath) is thus shortened, there is a concern that the amount of chromium hydrated oxide dissolved during the immersion treatment becomes non-uniform.

  Therefore, the present inventors performed a cathodic electrolytic treatment on the raw steel sheet in a high concentration chromic acid bath, and then performed an immersion treatment in a low concentration chromic acid bath for an immersion treatment time of less than 2 seconds. The form of the chromate film formed (the amount of metal chromium layer and chromium hydrated oxide layer deposited, the amount of alkali insoluble chromium hydrated oxide deposited, the alkali insoluble chromium in the total chromium hydrated oxide) Various factors affecting the ratio of hydrated oxide) were studied.

  As a result, as the current density during cathodic electrolysis increases, the mass ratio of alkali-insoluble chromium hydrated oxide to the total chromium hydrated oxide constituting the chromium hydrated oxide layer (hereinafter referred to as “alkaline-insoluble chromium ratio”). ”), And it became very difficult to keep it below 68%.

  FIG. 1 is a diagram showing the results of measuring the alkali-insoluble chromium ratio in the plate width direction for an electrolytic chromate-treated steel plate subjected to cathodic electrolysis at a high current density.

The electrolytic chromate-treated steel sheet includes a step of sequentially degreasing and pickling a raw steel sheet (plate width: 910 mm), a surface temperature of the raw steel sheet of 45 ° C., 170 g / l of chromic acid, A step of subjecting the material steel plate to cathodic electrolysis at a current density of 130 A / dm ² in a high concentration chromic acid bath containing sulfate ions having a concentration 0.0055 times the concentration and a fluorine compound; and 50 g / l chromic acid. And a step of immersing the raw steel plate in a low-concentration chromic acid bath at 45 ° C. containing a fluorine compound for 1.8 seconds, and subsequently washing the raw steel plate with water in this order. This is an electrochromated steel sheet.

  In addition, the alkali-insoluble chromium ratio shown in FIG. 1 is obtained by cutting a steel plate having a length of 100 mm in the rolling direction from the obtained electrolytic chromate-treated steel plate, further dividing the steel plate into 5 mm widths, and a test piece having a length of 100 mm × width of 5 mm. (Test specimen whose length direction is parallel to the rolling direction) was prepared, and the alkali-insoluble chromium ratio was measured for each 5 mm width using these test specimens.

  In addition, the adhesion amount of the total chromium hydrated oxide which comprises a chromium hydrated oxide layer, and the adhesion amount of alkali-insoluble chromium hydrated oxide were measured as follows. Moreover, the alkali-insoluble chromium ratio was calculated | required in (adhesion amount of alkali-insoluble chromium hydrated oxide) / (adhesion amount of all chromium hydrated oxide) x100 (%).

<Adhesion amount of all chromium hydrated oxide>
Divided test pieces were prepared by dividing each test piece into four equal parts, and the chromium adhesion amount (a) was measured by fluorescent X-ray for each. Next, the chromic hydrated oxide on the surface layer was removed by immersing two of the four divided test pieces in 7.5 normal NaOH solution heated to 110 ° C for 10 minutes, and then the amount of chromium deposited by fluorescent X-rays. (B) was measured. By subtracting the average of the two measured values of (b) from the average of the four measured values of (a), the adhesion amount of the total chromium hydrated oxide constituting the chromium hydrated oxide layer was determined in terms of metallic chromium.

<Adhesion amount of alkali-insoluble chromium hydrated oxide>
The remaining two divided test pieces among the four divided in the above were placed in a furnace at 210 ° C., and after the temperature of each divided test piece reached 210 ° C., air baking was performed for 10 minutes. It was immersed for 10 minutes in a 7.5N NaOH solution heated to 0 ° C., and the chromium content (c) was measured by fluorescent X-rays. By subtracting the average of the two measurement values of (b) from the average of the two measurement values of (c), the amount of the alkali-insoluble chromium hydrated oxide was determined in terms of metal chromium.

As shown in FIG. 1, when the current density of the cathodic electrolysis treatment is 130 A / dm ² and the treatment time of the immersion treatment is 1.8 seconds, the alkali insoluble chromium ratio is less than 68% over almost the entire region in the plate width direction. The alkali insoluble chromium ratio is 68% or more at the edge in the plate width direction (the portion from the edge to 5 mm). At the edge in the width direction of the steel sheet, the current tends to change due to the concentration of current during the cathodic electrolysis, so it is assumed that the above-mentioned characteristic failure at the outermost edge in the plate width is due to the current density of the cathodic electrolysis. Is done.

Therefore, the inventors measured the alkali-insoluble chromium ratio for electrolytic chromate-treated steel sheets subjected to cathodic electrolytic treatment at various current densities of 85 to 130 A / dm ² . The electrolytic chromated steel sheet used for the measurement was manufactured under the same conditions as described above except that the current density was changed to 85 to 130 A / dm ² . Further, the alkali-insoluble chromium ratio was measured in the same manner as described above.

  FIG. 2 is a diagram showing the relationship between the alkali-insoluble chromium ratio and the current density at the edge in the plate width direction (portion from the edge to 5 mm). Note that the vertical axis in FIG. 2 indicates the alkali-insoluble chromium ratio in the test piece at one edge width direction (OP) of the steel plate and the alkali in the test piece at the other edge direction (DR) of the steel plate. It is an average value with the insoluble chromium ratio.

As shown in FIG. 2, it can be seen that when the current density of the cathodic electrolysis exceeds 120 A / dm ² , the alkali-insoluble chromium ratio at the outermost edge in the plate width direction is 68% or more. From these results, the current density of the cathodic electrolysis treatment was applied when forming a chromate film on the steel plate material by subjecting the material steel plate to a cathodic electrolysis treatment in a high concentration chromic acid bath and then to an immersion treatment in a low concentration chromic acid bath. It has been clarified that the alkali-insoluble chromium ratio can be reduced to less than 68% by appropriately adjusting the A in the range below 120 A / dm ² .

As described above, by setting the current density of the cathodic electrolysis treatment to 120 A / dm ² or less, it is possible to solve the problem of characteristic failure at the outermost edge in the plate width direction. However, when the line speed was increased, depending on the production conditions of the electrolytic chromate-treated steel sheet, it was frequently confirmed that the alkali-insoluble chromium ratio was 68% or more, not limited to the outermost edge in the sheet width direction.

  Therefore, the present inventors investigated the correlation between the above phenomenon and manufacturing conditions, and tried to investigate the cause. As a result, it became clear that the temperature of the low concentration chromic acid bath used for the immersion treatment is strongly related to the above phenomenon.

FIG. 3 is a diagram showing the results of measuring the alkali-insoluble chromium ratio for an electrolytic chromate-treated steel sheet subjected to immersion treatment using low-concentration chromic acid baths at various temperatures of 40 to 70 ° C. The electrolytic chromated steel sheet used for the measurement was manufactured under the same conditions as above except that the current density was changed to 90 to 120 A / dm ² and the temperature of the low concentration chromic acid bath was changed to 40 to 70 ° C. did. Further, the alkali-insoluble chromium ratio was measured in the same manner as described above.

  As shown in FIG. 3, when the bath temperature of the low-concentration chromic acid bath is 60 ° C. or higher, the alkali-insoluble chromium ratio may be 68% or higher. From these results, the line speed is increased when the steel plate is subjected to cathodic electrolytic treatment in a high concentration chromic acid bath and then subjected to immersion treatment in a low concentration chromic acid bath to form a chromate film on the steel plate material. In this case, it was revealed that the alkali-insoluble chromium ratio can be suppressed to less than 68% over the entire plate width direction by setting the bath temperature of the low-concentration chromic acid bath used for the immersion treatment to less than 60 ° C.

The present invention has been made based on the above-described findings, and the gist of the present invention is as follows.
[1] A step of sequentially performing degreasing and pickling on the steel sheet,
The surface temperature of the raw steel plate is 47 ° C. or lower, and high concentration chromium containing 150 g / l or more chromic acid, sulfate ions having a concentration of 0.003 to 0.006 times the concentration of the chromic acid, and a fluorine compound. A step of subjecting the material steel plate to cathodic electrolysis at a current density of 120 A / dm ² or less in an acid bath;
A step of immersing the steel sheet in a low-concentration chromic acid bath of less than 60 ° C. containing chromic acid of 30 g / l or more and 60 g / l or less and a fluorine compound;
Washing the material steel plate with water;
In this order, a method for producing an electrolytic chromate-treated steel sheet.

[2] In the above [1], the concentration of the fluorine compound in the low concentration chromic acid bath is 0.01 to 0.04 times the chromic acid concentration in terms of fluorine ions, and the immersion treatment in the low concentration chromic acid bath A method for producing an electrolytic chromate-treated steel sheet having a time of less than 2 seconds.

  According to the present invention, even when the line speed of the chromate treatment line is increased, a desired chromate film can be formed over the entire width direction of the material steel plate, and the electrolytic chromate excellent in non-abrasive weldability. High-efficiency and stable production of treated steel sheets becomes possible.

It is a figure which shows the plate width direction distribution of an alkali-insoluble chromium rate about an electrolytic chromated steel plate. (Cathode electrolysis current density: 130 A / dm ² , immersion treatment time: 1.8 seconds) It is a figure which shows the relationship between the current density of a cathode electrolytic treatment, and the alkali-insoluble chromium rate of an electrolytic chromate-treated steel plate (plate width direction most edge part). It is a figure which shows the relationship between the bath temperature of the low concentration chromic acid bath used for an immersion process, and the alkali-insoluble chromium rate of an electrochromated steel plate (plate width direction center part). It is a schematic diagram which shows the welding state at the time of carrying out resistance welding by superposing | stacking an electrolytic chromate processing steel plate.

The present invention is a method for producing an electrolytic chromate-treated steel sheet having excellent abrasive weldability. As an electrolytic chromate-treated steel sheet with excellent non-abrasive weldability, at least one side of the material steel sheet has a metal chromium layer with an adhesion amount per side of more than 90 mg / m ^{2 and} 190 mg / m ² or less, and an adhesion amount per side. Is a chromate film consisting of a uniform chromium hydrated oxide layer of 3 to 8 mg / m ² in terms of metal chromium, and the amount of adhesion of alkali-insoluble chromium hydrated oxide per side in terms of metal chromium An electrolytic chromate-treated steel sheet provided with a chromate film that is 1 to 5 mg / m ² and has a mass ratio of less than 68% in the total chromium hydrated oxide is exemplified. By using such a chromate film, an electrolytic chromate-treated steel sheet having excellent corrosion resistance and excellent non-abrasive weldability can be obtained.

  In addition, by adjusting the adhesion amount and mass ratio of the alkali-insoluble chromium hydrated oxide to a predetermined range as described above, the overlapped portions of the electrolytic chromated steel plates are resistance-welded with a pair of welding electrodes. In this case, the electrolytic chromate treatment has a physical property that the contact electrical resistance (R1) at the contact of the overlapped portion is 0.15 times or more than the contact electrical resistance (R2) at the contact of the welding electrode and the electrolytic chromated steel plate. It can be a steel plate. And by setting it as the electrolytic chromate processing steel plate which has such a physical property, the suitable heat_generation | fever form is obtained at the time of welding, and defects, such as a dust and a splash, in a welded part outer surface can be suppressed effectively.

  In the present invention, the material steel plate is first degreased and pickled in order, and then subjected to cathodic electrolysis. The surface temperature of the material steel plate during the cathodic electrolysis treatment not only affects the deposition efficiency of metallic chromium but also the hydration degree of the chromium hydrated oxide. When the surface temperature exceeds 47 ° C., the alkali-insoluble chromium ratio increases, which may adversely affect the non-abrasive weldability. Therefore, the surface temperature is set to 47 ° C. or lower.

  The surface temperature of the material steel plate during the cathodic electrolytic treatment is determined by the bath temperature and the steel plate temperature just before entering the electrolytic cell. In addition, in the case of a vertical electrolytic cell known as a general electrolytic chromate treatment tank, the surface temperature of the material steel plate is slightly higher than the bath temperature due to the effect of Joule heat. The bath temperature of the acid bath is preferably at least less than 47 ° C.

  A high-concentration chromic acid bath used for cathodic electrolysis is a high-concentration chromic acid containing 150 g / l or more of chromic acid, sulfate ions having a concentration of 0.003 to 0.006 times the concentration of the chromic acid, and a fluorine compound. Take a bath.

  Sulfate ions contained in the high-concentration chromic acid bath are added as an auxiliary agent, and are added in order to obtain a desired metal chromium content and chromium hydrated oxide content. When paint baking or film lamination is performed on the electrolytic chromate-treated steel sheet, a part of the total chromium hydrated oxide constituting the chromium hydrated oxide layer is dehydrated and converted to alkali-insoluble chromium hydrated oxide. Here, the degree of dehydration and change to alkali-insoluble chromium hydrated oxide depends particularly on the sulfate ion concentration contained in the high concentration chromic acid bath.

  When the sulfate ion concentration is less than 0.003 times the chromic acid concentration, the alkali-insoluble chromium ratio increases to 68% or more, and good non-abrasive weldability cannot be obtained. On the other hand, if the sulfate ion concentration exceeds 0.006 times as compared with the concentration of chromic acid, the proper adhesion amount of the target chromium hydrated oxide layer may not be ensured. Therefore, the sulfate ion concentration contained in the high-concentration chromic acid bath is 0.003 to 0.006 times the chromic acid concentration.

As the high concentration chromic acid bath, for example, a CrO ₃ —Na ₂ SiF ₆ —SO ₄ ²⁻ based high concentration chromic acid bath is suitable. This high-concentration chromic acid bath can deposit metallic chromium stably for a long period of time and with high efficiency, and at the same time, it can simultaneously deposit a film serving as the basis of a chromium hydrated oxide suitable for non-abrasive weldability. Further, as the high concentration chromic acid bath, for example, a high concentration chromic acid bath such as CrO ₃ —NaF—SO ₄ ²⁻ system can be applied.

  The concentration of chromic acid in the high concentration chromic acid bath is preferably 200 g / l or less. The concentration of the fluorine compound in the high concentration chromic acid bath is preferably 0.01 times or more and 0.04 times or less compared with the chromic acid concentration in terms of fluorine ions.

The current density of cathodic electrolysis is extremely important for reducing the alkali-insoluble chromium ratio. When manufacturing the electrolytic chromate-treated steel sheet by passing the material steel plate through the chromate treatment line in which the electrolytic bath (high concentration chromic acid bath) and the immersion bath (low concentration chromic acid bath) are arranged in series, the line speed is increased. When it is necessary to increase the current density, it is necessary to form a desired chromate film in a short time. However, when the current density of the cathodic electrolysis is excessively high, the alkali-insoluble chromium ratio is increased. In particular, when the current density of the cathodic electrolysis exceeds 120 A / dm ² , the alkali-insoluble chromium ratio becomes high at the edge portion in the width direction of the steel sheet, and it becomes difficult to make it less than 68%.

Therefore, the current density of the cathodic electrolysis treatment is 120 A / dm ² or less. However, extremely low current density in cathodic electrolysis is not preferable from the viewpoint of productivity. In addition, as the current density of the cathodic electrolysis decreases, the amount of chromium hydrated oxide that precipitates together with the metal chromium during the cathodic electrolysis tends to increase. Therefore, when the current density of the cathodic electrolysis is significantly reduced, a large amount of chromium hydrated oxide is precipitated in the cathodic electrolysis, and a part of the chromium hydrated oxide is dissolved in the subsequent immersion treatment to obtain a desired adhesion amount. It becomes difficult to do. In particular, when the immersion treatment time is shortened to less than 2 seconds as the line speed increases, the above problem becomes remarkable. Therefore, it is preferable that the current density of the cathode electrolytic treatment to 40A / dm ² or more, more preferably, to 50A / dm ² or more, and even more preferably to 55 A / dm ² or more.

  Subsequently, an immersion treatment is performed on the material steel plate subjected to the cathodic electrolytic treatment in a low concentration chromic acid bath. In this immersion treatment, the chromium hydrated oxide excessively deposited by the cathodic electrolysis treatment is dissolved to form a chromium hydrated oxide layer having a desired adhesion amount, and an extra auxiliary agent in the chromium hydrated oxide is formed. Remove. Moreover, the ratio of the alkali-insoluble chromium hydrated oxide contained in the chromium hydrated oxide layer is reduced through this immersion treatment.

  The low-concentration chromic acid bath used in the immersion treatment is a low-concentration chromic acid bath of less than 60 ° C. containing chromic acid of 30 g / l or more and 60 g / l or less and a fluorine compound. If the concentration of chromic acid contained in the low-concentration chromic acid bath is less than 30 g / l, the effect of removing excess auxiliary agent in the chromium hydrated oxide may not be sufficiently exhibited. On the other hand, when the concentration of chromic acid contained in the low-concentration chromic acid bath exceeds 60 g / l, the hydrated chromium oxide is dissolved inhomogeneously and a uniform chromium hydrated oxide layer cannot be obtained.

The concentration of the fluorine compound is preferably 0.01 to 0.04 times the chromic acid concentration in terms of fluorine ions. If the concentration of the fluorine compound is less than 0.01 times the chromic acid concentration in terms of fluorine ions, the effect of removing excess auxiliary agent in the chromium hydrated oxide may not be sufficiently exhibited. On the other hand, if the concentration of the fluorine compound is more than 0.04 times the chromic acid concentration in terms of fluorine ions, the chromium hydrated oxide is dissolved inhomogeneously and a uniform chromium hydrated oxide layer may not be obtained. Examples of the fluorine compound contained in the low concentration chromic acid bath include NH ₄ F and NaF.

Here, the bath temperature of the low-concentration chromic acid bath is extremely important in reducing the alkali-insoluble chromium ratio. When the line speed is increased, if the bath temperature of the low-concentration chromic acid bath is 60 ° C. or higher, the alkali-insoluble chromium ratio may be 68% or higher, not limited to the edge portion in the plate width direction. Therefore, the bath temperature of the low concentration chromic acid bath is set to less than 60 ° C. However, if the bath temperature of the low-concentration chromic acid bath is extremely low, there is a concern that the reactivity of the dipping treatment will be poor. As the low concentration chromic acid bath, for example, a CrO ₃ —NH ₄ F-based low concentration chromic acid bath is suitable. Further, a low concentration chromic acid bath such as a CrO ₃ —NaF system can also be applied.

  With the above manufacturing conditions, the desired chromate film can be formed even if the immersion treatment time is less than 2 seconds, and the line speed can be increased.

  Subsequently, the material steel plate subjected to the immersion treatment is washed with water. After immersion treatment in a low-concentration chromic acid bath, washing with water, particularly warm water (rinsing) improves paint adhesion and laminate film adhesion after paint baking when coating baking or film laminating on electrolytic chromate-treated steel sheets. To do. In order to obtain such an effect, washing with warm water of 60 ° C. or higher is preferable, and 80 ° C. or higher is more preferable.

  The reason why the coating adhesion and the laminating film adhesion described above are improved by the above immersion treatment and water washing is not clear, but the film is formed by the interaction of the removal of the auxiliary agent and the hot water rinse (polymer with OH group). It is speculated that this has contributed. An indicator of this change appears in reducing the alkali-insoluble chromium ratio.

  The above description is merely an example of the embodiment of the present invention, and various modifications can be made within the scope of the claims. Next, examples of the present invention will be described in detail below.

  After performing normal degreasing and pickling on both sides of a cold rolled steel sheet with a thickness of 0.32mm and a width of 900mm, which is a raw steel sheet, it is subjected to cathodic electrolysis treatment and immersion treatment according to the conditions shown in Table 1, followed by washing with water and drying. Then, oil was applied to produce an electrolytic chromate-treated steel sheet. Note that the surface temperature of the material steel plate during the cathodic electrolysis treatment was 40 ° C. or lower in any of the test examples.

  For each electrolytic chromate-treated steel sheet produced, a test piece was prepared according to the following method, and the amount of metal chromium layer deposited on the center and edge of the steel sheet, and the total chromium hydrated oxide constituting the chromium hydrated oxide layer. The adhesion amount and the adhesion amount of alkali-insoluble chromium hydrated oxide were measured. Moreover, the alkali-insoluble chromium ratio was calculated | required from the obtained measured value by (adhesion amount of alkali-insoluble chromium hydrated oxide) / (adhesion amount of all chromium hydrated oxide) x100 (%).

<Preparation of test piece (steel plate width direction edge and center)>
From each of the obtained electrolytic chromate-treated steel sheets, a steel sheet having a length of 100 mm was cut out in the rolling direction, and the width from one of the sheet width direction outermost edge part (OP) and the other sheet width direction outermost edge part (DR), respectively. A 5 mm test piece was collected to prepare a test piece having a length of 100 mm and a width of 5 mm (a test piece having a length direction parallel to the rolling direction). Further, from each of the obtained electrolytic chromate-treated steel plates, a steel plate having a length of 100 mm was cut in the rolling direction, a test piece having a width of 5 mm was taken from the central portion in the width direction of the steel plate, and a test piece having a length of 100 mm × width of 5 mm was obtained. (Test specimen whose length direction was parallel to the rolling direction) was prepared.

  Divided test pieces were prepared by dividing each test piece into four equal parts, and the chromium adhesion amount (a) was measured by fluorescent X-ray for each. Next, two of the four divided test pieces were immersed in a 7.5 normal NaOH solution heated to 110 ° C. for 10 minutes to remove the surface chromium hydrated oxide, and then attached with chromium X-rays. The value obtained by measuring the amount was defined as the adhesion amount (b) of the metal chromium layer.

  Further, the average of the two measured values of (b) was subtracted from the average of the four measured values of (a) to determine the total amount of hydrated chromium oxide (converted to chromium metal).

  Further, the remaining two divided test pieces out of the four divided in the above were placed in a furnace at 210 ° C., and after each divided test piece temperature reached 210 ° C., air-baking was performed for 10 minutes. Was immersed in a 7.5 N NaOH solution heated to 110 ° C. for 10 minutes, and the chromium content (c) was measured by fluorescent X-ray. Then, the average of the two measured values of (b) was subtracted from the average of the two measured values of (c) to obtain the amount of alkali-insoluble chromium hydrated oxide deposited (in terms of chromium metal).

  The adhesion amount of the metal chromate layer at the edge portion is the adhesion amount of the metal chromate layer of the test piece at one edge portion (OP) and the adhesion amount of the metal chromate layer of the test piece at the other edge portion (DR). The average value. Similarly, the amount of all chromium hydrated oxide deposited on the edge portion is the same as the amount of all chromium hydrated oxide deposited on the test piece on one edge (OP) and the amount of test piece on the other edge (DR). It was set as the average value with the adhesion amount of the total chromium hydrated oxide. The adhesion amount of the alkali-insoluble chromium hydrated oxide at the edge is determined by the amount of alkali-insoluble chromium hydrated oxide at the test piece at one edge (OP) and the alkali of the test piece at the other edge (DR). It was set as the average value with the adhesion amount of insoluble chromium hydrated oxide.

  Furthermore, according to the following method, when resistance welding is performed on the overlapped portions of the electrolytic chromated steel sheets with a pair of welding electrodes, the contact electrical resistance (R1) at the contact of the overlapped portion and the welding electrode And the ratio (R1 / R2) of the contact electrical resistance (R2) at the contact point of the electrolytic chromated steel sheet.

<Measurement of contact electrical resistance ratio (R1 / R2)>
Each manufactured electrolytic chromate-treated steel sheet was placed in a furnace at 210 ° C., and after the steel sheet temperature reached 210 ° C., it was baked for 10 minutes. As shown in FIG. 4, the overlapped end portions 1 a and 1 b of the electrolytic chromated steel sheet are squeezed with a pair of copper electrodes 2 a and 2 b having an area of 2 mmφ as shown in FIG. A current of 1 A was applied at a pressure of 980 N, R1 and R2 were obtained from the potential difference after 3 seconds, and the contact electrical resistance ratio of R1 / R2 was calculated from these.

  These results are shown in Table 2. In addition, all the adhesion amounts of Table 2 are adhesion amounts per one side of the steel sheet.

  As shown in Table 2, any of the electrochromated steel sheets (steel plates Nos. 1 to 7, 10, and 11) manufactured according to the conditions defined in the present invention have a desired chromate film. On the other hand, the electrolytic chromate-treated steel sheet (steel sheets No. 8, 9) produced by a method that does not satisfy the conditions of the present invention has an alkali-insoluble chromium ratio of 68% or more.

1a, 1b Overlapping edges of electrolytic chromated steel sheet
2a, 2b Welding electrode

Claims (1)

A process of sequentially performing degreasing and pickling on the steel sheet,
The surface temperature of the raw steel plate is 47 ° C. or lower, and high concentration chromium containing 150 g / l or more chromic acid, sulfate ions having a concentration of 0.003 to 0.006 times the concentration of the chromic acid, and a fluorine compound. In the acid bath, the step of cathodic electrolytic treatment of the material steel plate at a current density of 55 A / dm ² or more and 120 A / dm ² or less,
The material steel plate in a low concentration chromic acid bath of less than 60 ° C. containing chromic acid of 30 g / l or more and 60 g / l or less and a fluorine compound having a concentration of 0.01 to 0.04 times the concentration of chromic acid in terms of fluorine ions And a step of performing an immersion treatment for 1.8 seconds or less ,
Washing the material steel plate with water;
Have a in this order,
On at least one surface of the steel sheet, and metallic chromium layer adhesion amount per one side is not more than 90 mg / m ² Ultra 190 mg / m ^2, the adhesion amount per one side is 3 to 8 mg / m ² reckoned as metal chromium chromium A chromate film comprising a hydrated oxide layer, the amount of adhesion of one side of the alkali-insoluble chromium hydrated oxide being 1 to 5 mg / m ² in terms of metallic chromium , and the total chromium hydrated oxide A method for producing an electrolytic chromate-treated steel sheet, comprising producing an electrolytic chromate-treated steel sheet having a chromate film whose proportion by weight is less than 68% .

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