KR930010342B1 - Weldable tin free steel sheet having a chromium bilayer and method for producing the same - Google Patents

Abstract

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Description

Electrolytic chromic acid treatment excellent in weldability and manufacturing method thereof

1 is a diagram showing the relationship between the amount of metal chromium after dissolution of chromium hydroxide formed by immersion in hot sodium hydrogen peroxide solution and the floating current of the steel base exposed through the metal chromium layer. It shows what is obtained by using a fluoride bath (CrO ₃ : 100 g / l, NH ₄ F: 5 g / l) at a cathode current density of 50 A / dm 2 at ° C., and B and C are each 50 A / dm 2 at 60 ° C. Mixture bath (CrO ₃ : 150g / l, H ₂ SO ₄ : 0.8g / l, Na ₂ SiF ₆ : 5g / l) and sulfuric acid bath (CrO ₃ : 160g / l, H ₂ SO ₄ : 2.5 under cathode current density What is obtained by using g / l) is shown.

2 is an example showing the effect of the immersion time in the chromic acid electrolyte on the remaining amount of insoluble chromium hydroxide, in which A and B are the same as described above for formation and dissolution of chromium hydroxide. It shows what was obtained by using a fluoride bath and a mixed bath.

3 is a diagram showing the effect of the amount of metal chromium on the electrical contact resistance of an electrolytic chromic acid-treated steel sheet produced by using the fluoride bath in A after heating at 210 ° C. for 20 minutes and B before heating.

4 shows the effect of the amount of metal chromium on the second possible current range in welding an electrolytic chromic acid treated steel sheet produced by using the fluoride bath after heating at 210 ° C. for 20 minutes.

The black spots in FIGS. 3 and 4 are each capable of electrolytic chromic acid treated steel sheets with granular deposits of metal chromium produced by anodization after heating for 20 minutes at 210 ° C. for 20 minutes in the mixing bath described above. The second current range and the electrical contact resistance are shown.

The present invention relates to a method for producing an electrolytic chromic acid treated steel sheet excellent in weldability.

Specifically, the present invention relates to a method for producing a double layer electrolytic chromic acid treated steel sheet, which is composed of a lower layer of flatly deposited metal chromium and an upper layer of uniformly formed insoluble chromium hydroxide.

This method uses a chromic acid electrolyte as a small amount of fluorine compound to form a triple layer consisting of a bottom layer of metal chromium, an intermediate layer of insoluble chromium hydroxide, and an upper layer of insoluble chromium hydroxide on a steel base. Is characterized by dissolving an insoluble chromium hydroxide.

By using such an electrolytic chromic acid treated steel sheet, it is possible to manufacture a can body welded at high speed without removing the plating layer from the welded portion.

The method for producing an electrolytic chromic acid treated steel sheet having excellent weldability according to the present invention has been developed through careful research.

An object of the present invention is to use an electrochromic acid treatment having a bilayer consisting of a lower layer of limited amount of metal chromium and an upper layer of limited amount of insoluble chromium hydroxide on a steel base by using a chromic acid electrolyte as a small amount of fluorine compound (fluoride bath). By providing a steel sheet.

The method according to the invention is characterized by the following elements.

(1) Use of a fluoride bath to form the triple layer on a steel base.

(2) Dissolving insoluble chromium hydroxide which becomes an upper layer in the triple layer after forming the triple layer on a steel base by immersing in the fluoride bath.

(3) Limitation in the range of the amount of attached metal chromium.

(4) Limitation in the range of the amount of insoluble chromium hydroxide formed.

Electrolytic chromic acid treated steel sheet according to the present invention is easy to weld at high speed without removing the plating layer, and also lacquered except for the welded parts such as food canned bodies, aerosol can bodies and other can bodies. Applications that require good weldability can be used. Furthermore, according to the present invention, the electrolytic chromic acid treated steel sheet has better corrosion resistance after lacquer than conventional electrolytic chromic acid treated steel sheets, so that it can be used for the end of the can and the drawing can.

In the description, soluble chromium hydroxide means chromium hydroxide which is easily dissolved in an additive-free fluoride bath or chromic acid solution before drying, and insoluble chromium hydroxide is chromium that is difficult to dissolve in an additive-free fluoride bath or chromic acid solution before drying. It means a hydroxide oxide.

Recently, the use of cheap electrochromic acid treated steel plates consisting of a double layer of lower layers of chromium and an upper layer of chromium hydride has rapidly changed in the canning of food and beverages, and of expensive electroplating tins in 5 gallon and other cans. Is happening.

This is because tin used in the production of tin is very expensive, and electrolytic chromic acid treated steel sheet has excellent lacquer adhesion compared with tin. A typical metal can made of an electrolytic chromic acid treated steel sheet is composed of two can ends and one can body, except for the drawing can. In the case of an electrolytic chromic acid treated steel sheet, the joining of the can body is performed with nylon adhesive using the Toyo Seam (trade name) and the Mira Seam (trade name) method.

Also known is a method of joining the can body of an electrolytic chromic acid treated steel sheet by electric welding.

However, in the case of joining the can body of the electrolytic chromic acid treated steel sheet by electric welding such as the Sooudronic method, in order to easily obtain a can body which is well bonded at high speed, it is also possible to mechanically and from the surface of the electrochromic acid treated steel sheet. Chemically, the metal chromium layer and the chromium hydroxide must be removed.

Therefore, the corrosion resistance of the welded portion of the electrochromic acid treated steel can body is significantly reduced even if this welded portion is covered with a lacquer after welding.

In view of the above background, the development of can materials which are cheaper than tin and can be easily welded at high speed without removing the plating layer has been required, especially in the food canning portion.

Recently, various methods of manufacturing electrolytic chromic acid treated steel sheets which are easily welded at high speed without removing the plating layer have been proposed.

For example, the methods shown in Japanese Unexamined Patent Publications No. 57-19752, 57-36986, Japanese Unexamined Patent Publication Nos. 61-213398, and 63-186894 are already known.

JP 57-19752 is an electrolytic chromic acid-treated steel sheet having excellent weldability between a double layer composed of a lower layer of metal chromium of 3 to 40 mg / m 2 and an upper layer of nonmetallic chromium of chromium oxide of 2 to 15 mg / m 2 as chromium. It is about.

Japanese Patent Application No. 57-19752 seeks to improve the weldability of an electrolytic chromic acid treated steel sheet by forming a porous metal chromium layer with a small amount of metal chromium.

However, this is because the iron oxide film having a high electrical resistance is formed by oxidation of the steel base which is optionally exposed through the pores of the metal chromium during the curing of the lacquer, and it should be considered that the corrosion resistance as well as the weldability is reduced.

Japanese Patent Application No. 57-36989 discloses an electrolytic chromic acid-treated steel sheet having excellent weldability and formability, which are made of metal chromium of 0.5 to 30 mg / m 2 and chromium hydroxide of 2 to 50 mg / m 2 as chromium. To produce, it is characterized by using chromic acid electrolyte as a small amount of ions such as sulfate ion, nitrate ion and chloride ion.

This method for producing electrolytic chromic acid treated steel sheet is intended to improve the corrosion resistance which is reduced by the reduction of the amount of metal chromium by improving the quality of the chromium hydroxide layer.

However, the weldability of the electrolytic chromic acid treated steel sheet obtained by this method cannot be improved because the exposed steel plate surface is oxidized by heating the lacquered coated electrolytic chromic acid treated steel sheet similarly to the above-mentioned special publication 57-19752.

Japanese Patent Application Laid-Open No. 61-213398 is a welded product having excellent corrosion resistance after lacquering, which includes a flat metal layer of 10 to 40 mg / m 2 and 3 to 30 mg / m 2 of chromium hydroxide uniformly formed as chromium. It relates to an electrolytic chromic acid treated steel sheet for cans.

The method for producing an electrolytic chromic acid-treated steel sheet is characterized by dissolving the partially attached metal chromium as anodizing after formation of a double layer composed of metal chromium and chromium hydroxide.

However, it is recognized that the weldability of the electrolytic chromic acid treated steel sheet obtained by this method cannot be improved because the exposure of the steel base increases due to an increase in porosity in the metal chromium layer.

Japanese Patent Application Laid-Open No. 63-186894 relates to an electrolytic chromic acid treated steel sheet for welded cans containing 50 to 150 mg / m 2 of metal chromium and 5 to 20 mg / m 2 of chromium hydroxide. The electrolytic chromic acid treated steel sheet obtained by Japanese Patent Application Laid-Open No. 63-186894 is characterized by granular adhesion of metal chromium.

This electrolytic chromic acid treated steel sheet was developed based on the fact that the electrical contact resistance used as one of the weldability evaluation indexes is reduced by the granular adhesion of metal chromium.

The weldability is usually evaluated by the second possible current range in welding, that is, the wider the second current range in welding, the better the weldability is.

Regarding the determination of the second possible current range in welding, the upper limit corresponds to the welding condition in which defects such as spraying are found, and the lower limit corresponds to the welding condition in which breakage occurs in the welded portion by a tearing test. do.

However, since many samples are required to determine the second possible current range in welding, the normal weldability is evaluated by a simple electrical contact resistance measurement method that has a clear correlation with the second possible current range in welding.

Although the electrical contact resistance has a clear correlation with the second possible current range for welding electrolytic chromic acid-plated steel plates of almost the same form as metal chromium, this correlation is true for electrolytic chromic acid-treated steel plates with other forms of metal chromium. It is not recognized.

For example, in the case of an electrolytic chromic acid treated steel sheet containing a granular metal chromium deposit obtained in Japanese Patent Application Laid-Open No. 63-186894, the second current range that can be welded is narrow despite low electrical contact resistance.

Therefore, even if the electrical contact resistance is used as the weldability evaluation index of the electrolytic chromic acid treated steel sheet containing almost the same type of metal chromium, the weldability of the electrolytic chromic acid treated steel sheet cannot be evaluated only by the electrical contact resistance.

In general, two methods are well known for producing conventional electrochromic acid treated steel sheets.

The first method is a one-step method in which metal chromium and chromium hydroxide are formed in a single action using a single electrolyte mixture. In the second method, metal chromium is first formed using a single electrolyte mixture as a chromium plating solution, and then chromium hydroxide is formed. It is a two-step method that is used as another electrolyte mixture and formed on the metal chromium layer.

In these two types of methods, chromic acid electrolytes of fluorine compounds (fluoride baths), sulfuric acid mixtures (sulfuric acid baths), or both additives (mixing baths) are usually used.

The film in the electrolytic chromic acid treated steel sheet produced by the above method is composed of a triple layer consisting of a bottom layer of metal chromium, an intermediate layer of insoluble chromium hydroxide mainly equivalent to chromium oxide, and an upper layer of soluble chromium hydroxide. And Yoshidakei et al. (Kyozo Hyomen Kijuchu, Vol. 30, No. 7, 1979, p. 338). That is, the chromium hydroxide is formed of a double layer of another structure.

The steel base used in the production of the electrolytic chromic acid treated steel sheet according to the present invention may be any cold rolled steel sheet generally used in the production of electroplating tin and ordinary electrolytic chromic acid treated steel sheet. The thickness of the steel base is preferably about 0.1 to about 0.35 mm.

The electrolytic chromic acid treated steel sheet according to the present invention is produced by the following method: The triple layer comprising metal chromium, an insoluble chromium hydroxide and a soluble chromium hydroxide, degreased with alkali, pickled with acid and washed with water. The soluble chromium hydroxide is dissolved by immersion in an electrolyte and then washed with water and dried.

In the above process, if dissolution of the chromic acid solution and the soluble chromium hydroxide used for the formation of the triple layer includes a different mixture for controlling the concentration of each mixture, water washing may be added after the formation of the triple layer. have.

In the present invention, it is preferable to add a fluoride bath for the following reasons:

(1) The portion of the steel base exposed through the pores of the formed metal chromium layer is smaller in the sulfuric acid bath and the mixing bath in the same amount of metal chromium, as shown in FIG.

That is, the steel base is constantly covered with a small amount of metal chromium compared to that in the sulfuric acid bath.

In the case of mixed baths, the addition of large amounts of sulfate ions is not preferred because the steel base portion exposed from the metal chromium layer is increased.

(2) Flat adhesion of the metal chromium is obtained using a fluoride bath, and granular adhesion of the metal chromium is obtained using a sulfuric acid bath.

Therefore, the steel surface is sufficiently covered with a small amount of metal chromium compared to that of the sulfuric acid bath.

(3) Since the structure of the water-soluble chromium hydroxide is not disturbed by the fluoride bonded to the hydroxyl radical in the chromium hydroxide or to the chromium hydroxide having about the same capacity as the combined water, the thin and uniform soluble chromium hydroxide is the fluoride bath. It is formed using.

On the other hand, the structure of the soluble chromium hydroxide is thick, non-uniform solubility because it is interrupted by bound sulphate ions, such as trivalent chromium, coordinated by hydroxyl radicals or by water bound with coordination number 6. Chromium hydride oxide is formed by using a sulfuric acid bath.

(4) A film containing a thinner and more uniform insoluble chromium hydroxide is formed by using a fluoride bath compared with that in a sulfuric acid bath.

In the present invention, in order to continuously manufacture the electrolytic chromic acid treated steel sheet having excellent weldability at high speed, it is essential that each layer of the formed triple layer be thin and uniform.

Therefore, since the fluoride bath has various advantages described above, the use of the fluoride bath in the present invention is preferable.

In the case of fluoride baths, the following electrolyte conditions should be selected for the formation of the triple layer on a steel base:

Concentration of chromic acid: 50 to 300 g / l, more preferably 80 to 200 g / l.

Concentration of fluorine compound: 1.0 to 10.0% by weight, more preferably 1.0 to 8.0% by weight of chromic acid.

Temperature of electrolyte: 40-60 degreeC, More preferably, 50-60 degreeC

Cathode current density: 20 to 100 A / dm 2, more preferably 40 to 100 A / dm 2.

The amount of soluble chromium hydroxide in the triple layer formed decreases with increasing concentration of chromic acid at a higher temperature of the electrolyte, under higher cathodic current density, by weight ratio of the appropriate fluorine compound.

Therefore, in order to form a thin soluble chromium hydroxide, it is not preferable to use a fluoride bath containing 30 g / l or less of chromic acid at a cathode current density of 20 A / dm 2 or less at a temperature of a fluoride bath of 40 ° C. or less.

Moreover, since the current efficiency for the deposition of metal chromium is significantly reduced, it is not preferable to use chromic acid having a temperature of 30 g / l or less, a fluoride bath having a temperature of 60 ° C. or more and a fluoride bath having a current density of 20 A / dm 2 or less. .

However, the concentration of chromic acid of 300 g / l or more and the cathode current density of 100 A / dm 2 or more are not suitable for the deposition of a small amount of metal chromium from an economical point of view. The presence of the fluorine compound in the electrolyte used for producing the electrolytic chromic acid treated steel sheet according to the present invention is essential for the uniform triple layer. If the weight percentage of the fluorine compound to chromic acid is 1.0 or less or 10.0 or more, the current efficiency for the deposition of the metal chromium is significantly reduced, and the uniformity in the adhesion of the metal chromium layer is also reduced.

In particular, when the weight percentage of the fluorine compound to chromic acid is 1.0 or less, weldability is remarkably reduced because insoluble chromium hydroxide is formed thickly and unevenly. The fluorine compound is selected from the group consisting of hydrofluoric acid, boric acid fluoride, silicic acid fluoride, ammonium difluoride, alkali metal difluoride, ammonium fluoride, alkali metal fluoride, ammonium fluoride salt, alkali metal fluoride salt, ammonium fluoride salt, alkali metal fluoride metal salt and aluminum fluoride. It is preferred that it is at least one compound selected. In the chromic acid electrolyte, these fluorine compounds dissociate to fluorine ions and have all functions as so-called adjuvants, so these fluorine compounds can be used alone or in combination. The soluble chromium hydroxide formed as an upper layer of the triple layer should be removed before washing with water and drying in the method for producing an electrolytic chromic acid treated steel sheet having excellent weldability according to the present invention.

Although chromic acid solution may be used without or with other additives, in order to dissolve the soluble chromium hydroxide in the triple layer, from an economic point of view, a fluoride bath used for forming the triple layer on a steel base. It is appropriate to add.

The concentration of chromic acid and the temperature of the solution for dissolving the formed flexible chromium hydroxide should be adjusted to 30 g / l or more and 40 ° C. or more, respectively, in order to dissolve the soluble chromium hydroxide efficiently in a short time.

The concentration of chromic acid of 300 g / l or more and the temperature of 60 ° C. or more are not suitable for dissolution of soluble chromium hydroxide from an economic point of view.

In the method for producing an electrolytic chromium acid treated steel sheet according to the present invention, the immersion time is a very important factor.

As shown in FIG. 2, the amount of insoluble chromium hydroxide becomes substantially constant at immersion time of 2.5 seconds or more.

In the present invention, the immersion time should be controlled in the range of 2.5 to 10 seconds, because in the case of continuously manufacturing the electrolytic chromic acid treated steel sheet at high speed by the present invention, immersion of 10 seconds or more requires a large space to be. Anodization in the fluoride bath after formation of the triple layer is also contemplated for dissolution of the soluble chromium hydroxide formed.

However, this method is not suitable for the present invention because the portion of the steel base exposed through the metal chromium layer is increased by the anode dissolution of the metal chromium.

In the electrolytic chromic acid treated steel sheet according to the present invention, the amount of metal chromium should be controlled within the range of 45 to 90 mg / m 2. If the amount of metal chromium is 45 mg / m 2 or less, excellent weldability cannot be obtained after heating for lacquer cure, in which the exposed portion of the steel base through the formed metal chromium layer is markedly as shown in FIG. Because it increases.

The exposed portion of the steel base through the formed metal chromium increases the electrical contact resistance, as shown in FIG. 3, and leads to a narrow second current range in welding, as shown in FIG. This is because iron oxide films with high electrical resistance are formed due to oxidation of the exposed steel base.

If the amount of metal chromium is greater than 90 mg / m 2, even if the portion of the steel base exposed from the metal chromium layer is reduced by an increase in the amount of metal chromium, that of the steel base, as shown in FIGS. 3 and 4 The weldability is also reduced due to the increase in the amount of metal chromium with poor forging ability compared with

3 and 4 show that in the case of the electrolytic chromic acid treated steel sheet having a shape substantially the same as that of the metal chromium according to the present invention, the electrical contact resistance has a clear correlation with the second current range possible for welding. Can be.

In contrast, other electrolytic chromic acid treated steel sheets containing deposits of granular metal chromium have a narrow second current range possible in welding despite low electrical contact resistance as shown in FIGS. 3 and 4.

It is well known that the weldability of conventional electrolytic chromic acid treated steel sheets depends on the amount of chromium hydroxide having a high electrical resistance after heating for curing the coated lacquer.

That is, the smaller the amount of chromium hydroxide, the better the weldability. However, the reduction of the amount of chromium hydroxide is not suitable from the standpoint of corrosion resistance after lacquering, even if it improves weldability.

In particular, the presence of soluble chromium hydride is not suitable for the production of electrolytic chromic acid treated steel sheet having excellent weldability, because the uniformity of the soluble chromium hydroxide formed is reduced even if a small amount of the soluble chromium hydroxide is reduced.

Substantially, it is impossible to uniformly weld the can bodies of a conventional electrochromic acid treated steel sheet having a small amount of soluble chromium hydroxide without removing the plated layer at high speed because the electrolytic contact resistance in the welded part is partially Because it is different.

On the other hand, insoluble chromium hydroxide in the triple layer is formed uniformly compared with soluble chromium hydroxide. Moreover, the amount of insoluble chromium hydroxide formed is almost constant without the effect of electrolyte conditions for the formation of the triple layer.

In the present invention, the amount of chromium of the insoluble chromium hydroxide formed is preferably controlled within the range of 3 to 7 mg / m 2 as chromium. Insoluble chromium hydroxides containing 2 mg / m 2 or less and 10 mg / m 2 or more as chromium together with metal chromium, and soluble chromium hydroxides except for any special electrolyte conditions, such as using chromic acid electrolytes with small amounts of additives, It's hard. If the amount of insoluble chromium hydroxide is 3 mg / m 2 or less, the corrosion resistance after lacquering is deteriorated even though the weldability is improved. Weldability is gradually reduced by an increase in the amount of insoluble chromium hydroxide by an increase in the amount of soluble chromium hydroxide.

In the present invention, the allowable upper limit of the amount of insoluble chromium hydroxide is 7 mg / m 2 as chromium in order to perform uniform welding at high speed without removing the plating layer.

The invention is illustrated through the following examples. These examples are intended to illustrate the invention but are not limited thereto. Others are obvious to those skilled in the art.

The cold rolled steel sheets having a thickness of 0.22 mm according to Examples 1 to 5 were electrolytically degreased in a 70 g / l solution of sodium hydroxide, washed with water and then washed with 100 g / l solution of sulfuric acid and then washed with water. In accordance with the following methods.

The triple layer consisting of the bottom layer of the metal chromium, the intermediate layer of the insoluble chromium hydroxide and the upper layer of the soluble chromium hydroxide is formed by the fluoride bath, washed with water, immersed in the fluoride bath (chromic acid solutions of Examples 4 and 5), dry.

In each embodiment, the conditions are shown in detail. Condition A is an electrolyte condition for the formation of a triple layer consisting of a bottom layer of metal chromium, an intermediate layer of insoluble chromium hydroxide and an upper layer of soluble chromium hydroxide. Condition B is the dissolution condition of the soluble chromium hydroxide formed.

Example 1

Condition A

Composition of electrolyte

Condition B

Example 2

Condition A

Composition of electrolyte

Condition B

Example 3

Condition A

Electrolyte composition

Condition B

Example 4

Condition A

Electrolyte composition

Condition B

Example 5

Condition A

Electrolyte composition

Condition B

Comparative Example 1

Steel plates of the same kind pretreated as in Example 1 were plated with metal chromium, worked up under the following conditions, washed with water and dried.

Conditions for Metal Chrome Attachment

Composition of electrolyte

Electrolyte composition

Comparative Example 2

Steel plates of the same kind pretreated as in Example 1 were treated under the following conditions, washed with water and then dried.

Treatment condition

Composition of electrolyte

Comparative Example 3

The same kind of steel plate pretreated as in Example 1 was plated with metal chromium and then treated with an anode under the following conditions.

Conditions for Metal Chrome Attachment

Electrolyte composition

The steel thus treated is then treated with a cathode, cleaned and dried under the following conditions.

Cathodic Treatment Condition

Electrolyte composition

[Comparative Example 4]

The same kind of steel sheet pretreated as in Example 1 was plated with metal chromium and then treated with an anode under the following conditions.

Metal Chrome Attachment Conditions

Electrolyte composition

The steel sheet thus treated was then treated with a cathode under the following conditions, washed with water and dried.

Cathodic Treatment Condition

Composition of electrolyte

[Comparative Example 5]

The same kind of steel plate pretreated as in Example 1 was treated with a cathode under the same conditions as in Example 2, after which the steel sheet thus treated was washed with water and dried without dissolving the soluble chromium hydroxide formed.

Comparative Examples 1, 2, 3 and 4 show Japanese Patent Application Nos. 57-19752, 57-36986, Japanese Patent Application Nos. 61-213398 and 63-186984, respectively.

The steel sheet portion exposed through the formed metal chromium layer, the electrical contact resistance, the second current range possible for welding, and the corrosion resistance after lacquering of the steel sheets treated in the examples and the comparative examples are fluorescent X in insoluble chromium hydroxide. After the measurement of the amount of metal chromium and chromium by the ray method, it is measured by the following measuring method.

The results are shown in the accompanying table.

(1) The steel base portion exposed from the formed metal chromium layer (uniform metal chromium layer).

A sample of chromium hydroxide cut into a size of 50 mm x 50 mm is immersed in 300 g / l of a sodium chloride solution at 95 ° C for 5 minutes to dissolve. Thereafter, the specimen is sealed with vinyl tape except for the measured portion having a diameter of 30 mm. The floating current of the steel base in the sealed specimen is measured by using the anodic polarization method at a polarization rate of 125 mv / min. The value of this current corresponds to the portion of the steel base exposed through the metal chromium layer, ie, indicating the uniformity of the metal chromium layer.

(2) electrical contact resistance

First, the specimen is heated at 210 ° C. for 20 minutes and then cut into a size of 20 mm × 100 mm, and then a pair of specimens is inserted between a pair of copper plate electrodes (diameter: 65 mm, thickness: 2 mm) rotating at 5 m / min. The electrical contact resistance is calculated from the voltage between a pair of copper plate electrodes subjected to a 5 ampere direct current and a 50 kg load.

(3) weldable second current range

The specimens are heated at 210 ° C. for 20 minutes and then cut to a size of 100 mm × 50 mm. A pair of specimens overlapped with 0.4 mm width were welded at a frequency of 60 Hz with a load of 50 kg at 7.2 m / min. The upper limit of the second current is determined by the condition in which defects such as spraying are found, and the lower limit is determined by the condition in which cracks are generated in the welded part by performing tearing measurements using a plurality of samples. The second current range available for welding is determined from the above difference of the second currents.

(4) Corrosion Resistance after Recreational (Measurement 1)

The sample is coated with a 60 mg / dm 2 epoxy-phenolic lacquer and then baked at 210 ° C. for 10 minutes. After the crosshairs are drawn by a laser 10 m wide and 15 μm deep on the surface of the coated specimen, the specimen is immersed for 2 weeks at 38 ° C. in a solution containing 1.5% citric acid and 1.5% sodium chloride. Corrosion of the scratched portion of the coated specimen is divided into five grades: 5 is number, 4 is right, 3 is beautiful, 2 is positive, and 1 is false.

(5) Fibrous corrosion resistance after lacquering (Test 2) (4) Lacquered and cross-crossed specimens are formed by an Erichsen measuring machine. The degree of corrosion in the scratches of the formed specimens immersed in a solution containing 3% sodium chloride was divided into 5 grades after being stored for 10 days at 45 ° C and 85% RH, where 5 is number, 4 is right, and 3 is Mi, 2 is positive, 1 is false.

TABLE 1

Legend

* 1 Cr ^o represents metal chromium and Cr ^ox represents Cr in chromium hydroxide.

Claims (5)

In the manufacturing method of the electrolytic chromic acid treated steel plate for high speed welding which has the lower layer of the flatly adhered metal chromium, and the double layer which consists of an insoluble chromium hydroxide and the insoluble upper layer in chromic acid solution, (A) 45-58 Fluorine compounds in an amount comprising 3.3 to 7.5% by weight of chromic acid at a temperature of < RTI ID = 0.0 > [deg.] C. < / RTI > which are catalyzed by cathodic treatment of essentially clean steel sheets in an 80-200 g / l chromic acid electrolyte at a cathode current density of 30-70 A / dm 2. Forming a triple layer consisting of a bottom layer of chromium hydroxide insoluble in the chromic acid electrolyte, and a bottom layer of metal chromium hydroxide insoluble in the chromic acid electrolyte, and a top layer of chromium hydroxide soluble in the chromic acid solution; (B) dissolving the soluble chromium hydroxide by immersing the steel sheet covered with the triple layer formed by step (a) in a chromic acid solution for a dipping time of 2.5 to 10 seconds; (C) washing with water and drying. The soluble chromium hydroxide is dissolved at a temperature of 45 to 58 ° C. in a chromic acid solution containing 80 to 200 g / l of chromic acid and at least one fluorine compound at 3.3 to 7.5 wt% of chromic acid. A method performed by soaking for 10 seconds of soaking time. The method of claim 1, wherein the dissolution of the soluble chromium hydroxide is carried out by immersing in a chromic acid solution containing 80 to 200 g / l of chromic acid at a temperature of 45 to 58 ℃ for 2.5 to 10 seconds immersion time. The fluorine compound according to claim 1 or 2, wherein the fluorine compound is hydrofluoric acid, boric acid fluoride, silicic acid fluoride, ammonium difluoride, alkali difluoride metal, ammonium fluoride, alkali fluoride metal, ammonium fluoride salt, alkali metal fluoroborate salt, ammonium fluoride silicate, At least one compound selected from the group consisting of alkali metal fluoride alkali salts and ammonium fluoride. 3 to 7 mg / m 2 of chromium hydroxide oxide high speed weldable electrochromic acid treated steel sheet on a 0.1 to 0.35 mm steel base as 45 to 90 mg / m 2 chromium.

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