阅读说明：本技术 罐用钢板及其制造方法 (Steel sheet for can and method for producing same ) 是由 中川祐介 铃木威 须藤干人 小岛克己 马场雄也 曾凡洋 山中洋一郎 德井俊介 于 2018-06-05 设计创作，主要内容包括：本发明提供焊接性优异的罐用钢板及其制造方法。上述罐用钢板在钢板的表面从上述钢板侧起依次具有金属铬层及水合氧化铬层,上述金属铬层的附着量为50～200mg/m<Sup>2</Sup>,上述水合氧化铬层的铬换算附着量为3～30mg/m<Sup>2</Sup>,上述金属铬层包含厚度为7.0nm以上的基部和在上述基部上设置的粒状突起,该粒状突起的最大粒径为200nm以下,且单位面积个数密度为30个/μm<Sup>2</Sup>以上。(The invention provides a steel sheet for a can with excellent weldability and a manufacturing method thereof. The steel sheet for can has a metallic chromium layer and a hydrated chromium oxide layer on the surface of the steel sheet in this order from the steel sheet side, and the amount of the metallic chromium layer deposited is 50 to 200mg/m 2 To aboveThe chromium content of the hydrated chromium oxide layer is 3 to 30mg/m in terms of chromium deposition 2 The metallic chromium layer comprises a base having a thickness of 7.0nm or more and granular protrusions provided on the base, the granular protrusions having a maximum particle diameter of 200nm or less and a number density per unit area of 30 particles/. mu.m 2 The above.)

1. A steel sheet for cans, which has a metallic chromium layer and a hydrated chromium oxide layer on the surface of the steel sheet in this order from the steel sheet side,

the adhesion amount of the metal chromium layer is 50-200 mg/m²，

The chromium-converted adhesion amount of the hydrated chromium oxide layer is 3-30 mg/m²，

The metallic chromium layer comprises a base having a thickness of 7.0nm or more and granular protrusions provided on the base, the granular protrusions having a maximum particle diameter of 200nm or less and a number density per unit area of 30 particles/μm²The above.

2. Steel sheet for cans according to claim 1,

the chromium content of the hydrated chromium oxide layer is more than 15mg/m²And is 30mg/m²The following.

3. Steel sheet for cans according to claim 1 or 2,

the number density per unit area of the granular protrusions is 200/μm²The above.

4. A method for producing a steel sheet for cans according to any one of claims 1 to 3, using an aqueous solution containing a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid, the method comprising the steps of:

treatment 1 comprising cathodic electrolysis treatment C1 was carried out on a steel sheet using the aqueous solution; and

and (3) performing treatment 2 on the steel sheet subjected to the cathodic electrolysis treatment C1 by using the aqueous solution for more than 2 times, wherein the treatment 2 comprises anodic electrolysis treatment A1 and cathodic electrolysis treatment C2 after the anodic electrolysis treatment A1.

5. The method for manufacturing a steel sheet for a can according to claim 4,

the current density of the anodic electrolytic treatment A1 was 0.1A/dm²Above and below 5.0A/dm²，

The electric quantity density of the anode electrolysis treatment A1 is more than 0.3C/dm²And less than 5.0C/dm²，

The current density of the cathodic electrolysis treatment C2 is lower than 60.0A/dm²，

The electricity density of the cathodic electrolysis treatment C2 is lower than 30.0C/dm²。

6. The method of manufacturing steel sheet for can use according to claim 4 or 5, wherein one of the aqueous solutions is used in the cathodic electrolysis treatment C1, the anodic electrolysis treatment A1 and the cathodic electrolysis treatment C2.

Technical Field

The present invention relates to a steel sheet for can and a method for manufacturing the same.

Background

Cans, which are containers used for beverages and foods, are used worldwide because they can store contents for a long period of time. Cans are broadly classified into two-piece cans in which a bottom portion and a body portion are integrally formed by drawing, ironing, drawing, and bending a metal plate, and then the can bottom portion and the body portion are seamed with an upper lid, and three-piece cans in which a body portion is obtained by processing a metal plate into a cylindrical shape and welding the metal plate with a seam welding (wire seam), and the body portion and both ends thereof are seamed with lids.

Conventionally, Sn-plated steel sheets (so-called tinplate) have been widely used as can steel sheets.

In recent years, an electrolytic chromate-treated steel sheet (hereinafter also referred to as "tin-free steel (TFS)") having a metallic chromium layer and a hydrated chromium oxide layer has been widely used because it is cheaper and has excellent paint adhesion compared to tin-plated steel sheet.

From reduction of cleaning effluent and CO₂From the viewpoint of such environmental measures, a can using a steel sheet laminated with an organic resin film such as PET (polyethylene terephthalate) has attracted attention as an alternative technique capable of omitting painting and a subsequent baking treatment. From this point of view, it is expected that the range of application of TFS having excellent adhesion to an organic resin film will still expand in the future.

On the other hand, TFS may have inferior solderability compared to tin plate. The reason for this is that the hydrated chromium oxide layer on the surface layer undergoes a dehydration condensation reaction by baking treatment after coating or heat treatment after lamination of the organic resin film, and the contact resistance increases. In particular, since the baking treatment after coating is a high temperature as compared with the heat treatment after laminating the organic resin films, the weldability tends to be inferior.

Thus, for the current TFS, mechanical grinding is performed to remove the hydrated chromium oxide layer before it is to be welded, thereby enabling it to be welded.

However, in industrial production, there are many problems such as a risk of the metal powder after grinding being mixed into the contents, an increase in maintenance load such as cleaning of the can making apparatus, and a risk of fire occurrence due to the metal powder.

For this reason, for example, patent documents 1 and 2 propose techniques for soldering TFS without polishing.

Disclosure of Invention

The techniques disclosed in patent documents 1 and 2 are techniques in which a large number of defective portions are formed in a metallic chromium layer by performing an anodic electrolytic treatment between a former stage and a latter stage of cathodic electrolytic treatment, and the metallic chromium is formed into a granular protruded shape by the latter stage of cathodic electrolytic treatment.

According to this technique, the granular protrusions formed of metallic chromium destroy a hydrated chromium oxide layer as a welding inhibitor of the surface layer at the time of welding, thereby making it possible to expect a reduction in contact resistance and an improvement in weldability.

However, the inventors of the present application have studied the steel sheets for cans specifically described in patent documents 1 and 2, and as a result, have found that weldability may be insufficient.

Accordingly, an object of the present invention is to provide a steel sheet for a can having excellent weldability and a method for producing the same.

Means for solving the problems

The inventors of the present invention have made intensive studies to achieve the above object and as a result, have found that weldability of a steel sheet for can is improved by densifying granular protrusions of a metallic chromium layer, and have completed the present invention.

Namely, the present invention provides the following [1] to [6 ].

[1]A steel sheet for can, which has a metallic chromium layer and a hydrated chromium oxide layer on the surface of the steel sheet in this order from the steel sheet side, wherein the amount of the metallic chromium layer deposited is 50 to 200mg/m²The chromium-equivalent adhesion amount of the hydrated chromium oxide layer is 3 to 30mg/m²The metallic chromium layer comprises a base having a thickness of 7.0nm or more and granular protrusions provided on the base, the granular protrusions having a maximum particle diameter of 200nm or less and a number density per unit area of 30 particles/μm²The above.

[2]Above-mentioned [1]The steel sheet for can, wherein the chromium-converted adhesion amount of the hydrated chromium oxide layer is more than 15mg/m²And is 30mg/m²The following.

[3]Above-mentioned [1]Or [ 2]]The steel sheet for can, wherein the number density per unit area of the granular protrusions is 200 pieces/. mu.m²The above.

[4] A method for producing a steel sheet for cans, which comprises obtaining the steel sheet for cans according to any one of the above [1] to [3] using an aqueous solution containing a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid, the method comprising the steps of:

treatment 1 comprising cathodic electrolysis treatment C1 was carried out on a steel sheet using the above aqueous solution; and

and (3) performing treatment 2 on the steel sheet subjected to the cathodic electrolysis treatment C1 by using the aqueous solution for more than 2 times, wherein the treatment 2 comprises anodic electrolysis treatment A1 and cathodic electrolysis treatment C2 after the anodic electrolysis treatment A1.

[5]Above-mentioned [4]The method for producing a steel sheet for cans, wherein the anodic electrolytic treatment A1 has a current density of 0.1A/dm²Above and below 5.0A/dm²The electrical density of the anodic electrolytic treatment A1 was more than 0.3C/dm²And less than 5.0C/dm²The current density of the cathodic electrolysis treatment C2 is less than 60.0A/dm²The electrical density of the cathodic electrolysis treatment C2 is lower than 30.0C/dm²。

[6] The method for producing a steel sheet for cans according to [4] or [5], wherein one of the aqueous solutions is used for the cathodic electrolysis C1, the anodic electrolysis A1 and the cathodic electrolysis C2.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a steel sheet for a can having excellent weldability and a method for producing the same can be provided.

Drawings

Fig. 1 is a cross-sectional view schematically showing an example of a steel sheet for a can according to the present invention.

Detailed Description

[ Steel sheet for Can ]

Fig. 1 is a cross-sectional view schematically showing an example of a steel sheet for a can according to the present invention.

As shown in fig. 1, a steel sheet 1 for a can has a steel sheet 2. The steel sheet 1 for can further has a metallic chromium layer 3 and a hydrated chromium oxide layer 4 on the surface of the steel sheet 2 in this order from the steel sheet 2 side.

The metallic chromium layer 3 includes a base 3a covering the steel sheet 2 and granular protrusions 3b provided on the base 3 a. The thickness of the base 3a is 7.0nm or more. The granular protrusions 3b have a maximum particle diameter of 200nm or less and a number density per unit area of 30 particles/. mu.m²The above. The amount of the metallic chromium layer 3 adhering to the substrate including the base 3a and the granular protrusions 3b is 50 to 200mg/m²。

The hydrated chromium oxide layer 4 is disposed on the metal so as to conform to the shape of the granular protrusions 3bOn the chromium layer 3. The chromium-converted adhesion amount of the hydrated chromium oxide layer 4 is 3 to 30mg/m²。

The adhesion amount is the amount of adhesion on one side of the steel sheet.

The respective configurations of the present invention will be described in more detail below.

Steel plate

The type of the steel sheet is not particularly limited. In general, steel sheets (e.g., low carbon steel sheets and ultra-low carbon steel sheets) that can be used as container materials can be used. The method of producing the steel sheet, the material and the like are not particularly limited. The steel sheet can be produced through the steps of hot rolling, pickling, cold rolling, annealing, temper rolling, and the like from a general steel sheet production step.

Metal chromium layer

The steel sheet for a can of the present invention has a metallic chromium layer on the surface of the steel sheet.

The metal chromium in the general TFS functions to suppress exposure of the steel sheet as a base material to the surface to improve corrosion resistance. If the amount of metallic chromium is too small, the steel sheet may be inevitably exposed and the corrosion resistance may deteriorate.

The amount of chromium metal layer deposited is 50mg/m for the reason that the corrosion resistance of the steel sheet for can is excellent²As described above, 60mg/m is preferable for further improving the corrosion resistance²Above, more preferably 65mg/m²The above, more preferably 70mg/m²The above.

On the other hand, if the amount of metallic chromium is too large, metallic chromium having a high melting point covers the entire surface of the steel sheet, and the welding strength may be reduced during welding, and generation of dust may become significant, thereby deteriorating weldability.

The reason why the weldability of the steel sheet for can is excellent is that the amount of adhesion of the metallic chromium layer is 200mg/m²Hereinafter, 180mg/m is preferable for the reason that weldability is more excellent²Below, 160mg/m is more preferable²The following.

Method for measuring adhesion amount

The amount of the metal chromium layer deposited and the amount of the hydrated chromium oxide layer deposited in terms of chromium, which will be described later, were measured in the following manner.

First, the amount of chromium (total amount of chromium) was measured using a fluorescent X-ray apparatus for a steel sheet for cans on which a metallic chromium layer and a hydrated chromium oxide layer were formed. Next, the steel sheet for can was immersed in 6.5N-NaOH at 90 ℃ for 10 minutes in an alkali treatment, and then the amount of chromium (amount of chromium after alkali treatment) was measured again using a fluorescent X-ray apparatus. The amount of chromium after the alkali treatment was used as the amount of adhesion of the metallic chromium layer.

Next, the (alkali-soluble chromium amount) — (total chromium amount) - (chromium amount after alkali treatment) was calculated, and the alkali-soluble chromium amount was set as the chromium-equivalent adhesion amount of the hydrated chromium oxide layer.

Such a metallic chromium layer comprises a base and granular protrusions provided on the base.

Next, the above portions included in the metallic chromium layer will be described in detail.

Base of metallic chromium layer

The base of the metallic chromium layer mainly plays a role of covering the surface of the steel sheet to improve corrosion resistance.

In addition to the corrosion resistance required by the general TFS, the base of the metallic chromium layer of the present invention is required to have a sufficiently uniform thickness in order to prevent the steel sheet from being exposed by breaking the base when the steel sheets for cans inevitably come into contact with each other by the granular protrusions provided on the surface layer during processing.

From the above-described viewpoints, the inventors of the present application conducted a friction test between steel sheets for cans and examined rust resistance. As a result, it was found that when the thickness of the base portion of the metallic chromium layer was 7.0nm or more, the rust resistance was excellent. That is, the thickness of the base portion of the metallic chromium layer is 7.0nm or more for the reason of excellent corrosion resistance of the steel sheet for can, and is preferably 9.0nm or more, more preferably 10.0nm or more for the reason of further excellent corrosion resistance.

On the other hand, the upper limit of the thickness of the base portion of the metallic chromium layer is not particularly limited, and is, for example, 20.0nm or less, preferably 15.0nm or less.

(method of measuring thickness)

The thickness of the base of the metallic chromium layer was measured in the following manner.

First, a cross-sectional sample of a steel sheet for a can having a metallic chromium layer and a hydrated chromium oxide layer formed thereon was prepared by a Focused Ion Beam (FIB) method, and observed at 20000 times using a scanning Transmission Electron Microscope (TEM). Next, by observing the cross-sectional shape in the bright field image, focusing on the portion where only the base portion is present without the granular protrusions, the thickness of the base portion was calculated from the intensity curves (horizontal axis: distance, vertical axis: intensity) of chromium and iron by line analysis by energy dispersive X-ray spectroscopy (EDX). In this case, more specifically, in the intensity curve of chromium, a point having an intensity of 20% of the maximum value is defined as the outermost layer, an intersection with the intensity curve of iron is defined as a boundary point with iron, and the distance between the two points is defined as the thickness of the base.

The amount of the base of the metallic chromium layer is preferably 10mg/m for the reason that the steel sheet for cans is excellent in rust resistance²Above, more preferably 30mg/m²Above, more preferably 40mg/m²The above.

Granular protrusions of metallic chromium layer

The granular protrusions of the metallic chromium layer are formed on the surface of the base part, and mainly play a role in reducing the contact resistance between the steel sheets for the can and improving weldability. The mechanism of the presumption of the decrease in the contact resistance is explained below.

Since the hydrated chromium oxide layer coated on the metallic chromium layer is a non-conductive coating film, the resistance is higher than that of metallic chromium, and this becomes a factor of inhibition of welding. When the granular protrusions are formed on the surface of the base portion of the metallic chromium layer, the granular protrusions break the hydrated chromium oxide layer by the surface pressure when the steel sheets for cans are brought into contact with each other during welding, and become conduction points for the welding current, and the contact resistance is greatly reduced.

If the number of granular protrusions of the metallic chromium layer is too small, the number of current-carrying points during welding may decrease, and the contact resistance may not be reduced, resulting in poor weldability. By forming the granular protrusions at a high density, contact resistance can be reduced even in the case where the hydrated chromium oxide layer as the insulating layer is thick. This makes it possible to achieve paint adhesion, corrosion resistance under a coating film, weldability, and the like with an excellent balance.

The number density per unit area of the granular protrusions was 30/μm for the reason that the weldability of the steel sheet for cans was excellent²As described above, the number density per unit area of the granular protrusions is preferably 200/μm for the reason that the weldability is more excellent²More preferably 1000 pieces/. mu.m²Above, more preferably more than 1000 pieces/μm²。

The upper limit of the number density per unit area of the granular projections may be too high, which may affect the color tone, and the number density per unit area is preferably 10000/μm for the reason that the surface appearance of the steel sheet for can is more excellent²The following. More preferably 5000 particles/. mu.m²The following.

However, the inventors of the present application have found that if the maximum particle size of the granular protrusions of the metallic chromium layer is too large, the color tone of the steel sheet for can is affected, and a brown pattern may be formed to deteriorate the surface appearance. The reason is considered as follows: the granular protrusions absorb light on the short wavelength side (blue system) to attenuate the reflected light thereof, thereby exhibiting a reddish-brown system color; the granular protrusions scatter the reflected light to reduce the reflectance of the whole to be dark; and so on.

Therefore, the maximum particle diameter of the granular protrusions of the metallic chromium layer is 200nm or less. The steel sheet for cans has excellent surface appearance. This is considered to be because the absorption of light on the short wavelength side or the scattering of reflected light can be suppressed by reducing the diameter of the granular protrusions.

The maximum particle diameter of the granular protrusions of the metallic chromium layer is preferably 150nm or less, more preferably 100nm or less, and still more preferably 80nm or less, for the reason that the surface appearance of the steel sheet for cans is more excellent.

The lower limit of the maximum particle diameter is not particularly limited, and is preferably 10nm or more, for example.

(method of measuring particle diameter and number density per unit area of granular protrusions)

The particle diameter and the number density per unit area of the granular protrusions of the metallic chromium layer were measured in the following manner.

First, a sample for observation was prepared by an extraction replica method by carbon vapor deposition on the surface of a steel sheet for cans on which a metallic chromium layer and a hydrated chromium oxide layer were formed, a photograph was taken at 20000 times using a scanning Transmission Electron Microscope (TEM), the taken photograph was binarized using software (trade name: ImageJ) to analyze the image, the area occupied by the granular protrusions was subjected to inverse calculation, and the particle size and the number density per unit area were calculated by perfect circle conversion. The maximum particle size is the maximum particle size in an observation field photographed 20000 times and 5 fields, and the number density per unit area is the average of 5 fields.

Hydrated chromium oxide layer

Hydrated chromium oxide is precipitated simultaneously with metallic chromium in the surface of the steel sheet, and mainly plays a role in improving corrosion resistance. In addition, hydrated chromium oxide improves both the post-coating corrosion resistance such as the under-coating corrosion resistance and the coating adhesion. The chromium content of the hydrated chromium oxide layer was 3mg/m in terms of chromium deposition for the reason of ensuring corrosion resistance and paint adhesion of the steel sheet for cans²As described above, 10mg/m is preferable for further improving the corrosion resistance and the paint adhesion²Above, more preferably more than 15mg/m²。

On the other hand, hydrated chromium oxide has a lower conductivity than metallic chromium, and if the hydrated chromium oxide is excessive, an excessive electric resistance is generated during welding, and various welding defects such as generation of dust and spatter and blowholes accompanying the overwelding may be caused, thereby deteriorating weldability of the steel sheet for can.

Therefore, the chromium content of the hydrated chromium oxide layer is 30mg/m in terms of chromium deposition because of the excellent weldability of the steel sheet for cans²Hereinafter, the chromium-equivalent adhesion amount of the hydrated chromium oxide layer is preferably 25mg/m for the reason of further excellent weldability²Below, more preferably 20mg/m²The following.

The measurement method of the chromium-converted adhesion amount of the hydrated chromium oxide layer is as described above.

[ method for producing Steel sheet for Can ]

Next, a method for producing a steel sheet for cans of the present invention will be described.

The method for producing a steel sheet for cans of the present invention (hereinafter also simply referred to as "the method for producing the present invention") uses an aqueous solution containing a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid to obtain the steel sheet for cans of the present invention, and the method for producing a steel sheet for cans includes the steps of: treatment 1 comprising cathodic electrolysis treatment C1 was carried out on a steel sheet using the above aqueous solution; and performing treatment 2 on the steel sheet subjected to the cathodic electrolysis treatment C1 by using the aqueous solution for more than 2 times, wherein the treatment 2 comprises anodic electrolysis treatment A1 and cathodic electrolysis treatment C2 after the anodic electrolysis treatment A1.

Generally, in the cathodic electrolysis treatment in an aqueous solution containing a hexavalent chromium compound, a reduction reaction occurs on the surface of the steel sheet to precipitate metallic chromium, and hydrated chromium oxide (which is an intermediate product leading to metallic chromium) is precipitated on the surface thereof. The hydrated chromium oxide is unevenly dissolved by being intermittently subjected to electrolytic treatment or by being immersed in an aqueous solution of a hexavalent chromium compound for a long period of time, and granular protrusions formed of metallic chromium are formed by the subsequent cathodic electrolytic treatment.

By performing the anodic electrolytic treatment at intervals of the cathodic electrolytic treatment, the metallic chromium is dissolved at a plurality of sites over the entire surface of the steel sheet, and becomes a starting point of the granular protrusions formed by the metallic chromium formed in the subsequent cathodic electrolytic treatment. In the cathodic electrolysis treatment C1 before the anodic electrolysis treatment A1, the base of the metallic chromium layer was precipitated, and in the cathodic electrolysis treatment C2 after the anodic electrolysis treatment A1, the granular protrusions of the metallic chromium layer were precipitated.

The respective deposition amounts can be controlled by electrolysis conditions in the respective electrolysis treatments.

The aqueous solution and each electrolytic treatment used in the production method of the present invention will be described in detail below.

Aqueous solution

The aqueous solution used in the production method of the present invention contains a hexavalent chromium compound, a fluorine-containing compound, and sulfuric acid.

The fluorine-containing compound and sulfuric acid in the aqueous solution exist in a state of being dissociated into fluoride ions, sulfate ions, and hydrogen sulfate ions. The above-mentioned substances generally act as a catalyst for the reduction reaction and oxidation reaction of hexavalent chromium ions present in an aqueous solution, which are carried out in the cathodic electrolysis treatment and the anodic electrolysis treatment, and therefore are added to a chromium plating bath as an auxiliary.

The aqueous solution used for the electrolytic treatment contains a fluorine-containing compound and sulfuric acid, and thereby the amount of chromium deposited in terms of chromium oxide hydrate layer of the steel sheet for can obtained can be reduced. The reason for this is not clear, but it is considered that the amount of the generated oxide is decreased due to the increase in the amount of the anion in the electrolytic treatment.

It is preferable to use only one aqueous solution among the cathodic electrolysis treatment C1, the anodic electrolysis treatment A1 and the cathodic electrolysis treatment C2.

Hexavalent chromium compounds

The hexavalent chromium compound contained in the aqueous solution is not particularly limited, and examples thereof include chromium trioxide (CrO)₃) Potassium dichromate (K)₂Cr₂O₇) Isochromate, potassium chromate (K)₂CrO₄) And chromate salts, and the like.

The content of the hexavalent chromium compound in the aqueous solution is preferably 0.14 to 3.00mol/L, and more preferably 0.30 to 2.50mol/L, in terms of Cr.

Fluorine-containing Compound

The fluorine-containing compound contained in the aqueous solution is not particularly limited, and examples thereof include hydrofluoric acid (HF), potassium fluoride (KF), sodium fluoride (NaF), and fluorosilicic acid (H)₂SiF₆) And/or salts thereof, and the like. Examples of the salt of fluorosilicic acid include sodium fluorosilicate (Na)₂SiF₆) Potassium fluosilicate (K)₂SiF₆) Ammonium fluorosilicate ((NH)₄)₂SiF₆) And so on.

The content of the fluorine-containing compound in the aqueous solution is preferably 0.02 to 0.48mol/L, more preferably 0.08 to 0.40mol/L in terms of F.

Sulfuric acid

Sulfuric acid (H) in aqueous solution₂SO₄) In the amount of SO₄ ^2-The amount is preferably 0.0001 to 0.1000mol/L, more preferably 0.0003 to 0.0500mol/L, and further preferably 0.0010 to 0.0500 mol/L.

Sulfuric acid is used in combination with a fluorine-containing compound, thereby improving the electrolytic efficiency in the adhesion of the metallic chromium layer. When the content of sulfuric acid in the aqueous solution is within the above range, it becomes easy to control the size of the granular protrusions of the metallic chromium layer precipitated in the cathodic electrolysis treatment C2 to an appropriate range.

In addition, sulfuric acid also has an effect on the formation of generation sites of granular protrusions of the metallic chromium layer in the anodic electrolytic treatment. When the content of sulfuric acid in the aqueous solution is within the above range, the granular protrusions of the metallic chromium layer are less likely to become too fine or too coarse, and an appropriate number density can be obtained more easily.

The liquid temperature of the aqueous solution in each electrolysis treatment is preferably 20 to 80 ℃, and more preferably 40 to 60 ℃.

Cathodic electrolytic treatment C1 (treatment 1)

Metallic chromium and hydrated chromium oxide were precipitated in the cathodic electrolysis treatment C1.

In this case, the cathode electrolysis treatment C1 preferably has an electric energy density (product of current density and energization time) of 20 to 50C/dm from the viewpoint of ensuring an appropriate deposition amount and an appropriate thickness of the base of the metallic chromium layer²More preferably 25 to 45C/dm²。

Current Density (Unit: A/dm)²) And energization time (unit: sec. (sec)) is appropriately set in accordance with the above-described charge density.

The cathodic electrolysis treatment C1 may not be a continuous electrolysis treatment. That is, the cathodic electrolysis treatment C1 may be an intermittent electrolysis treatment in which electrolysis is performed at a plurality of electrodes in an industrial production process, and thus, an electroless immersion time inevitably occurs. In the case of the intermittent electrolytic treatment, the total charge density is preferably within the above range.

Anodic electrolytic treatment A1

The anodic electrolytic treatment A1 served to dissolve the metallic chromium precipitated in the cathodic electrolytic treatment C1 and formed sites for the generation of granular projections of the metallic chromium layer in the cathodic electrolytic treatment C2.

At this time, if the dissolution in the anodic electrolytic treatment a1 is too strong, the number density per unit area of the granular protrusions decreases due to a decrease in the number of generation sites, the dissolution proceeds unevenly, the distribution of the granular protrusions varies, or the thickness of the base portion of the metallic chromium layer decreases to less than 7.0nm in some cases.

If the current density of the anodic electrolytic treatment a1 is too high, corrosion resistance and the like may be adversely affected. The reason is assumed to be that a part of the metallic chromium layer is excessively dissolved at a level of a desired limit or more, and a generation site where the thickness of the base portion of the metallic chromium layer is less than 7.0nm is locally formed.

The metallic chromium layer formed by cathodic electrolysis C1 and initial anodic electrolysis A1 was primarily the base. In order to make the thickness of the base of the metallic chromium layer 7.0nm or more, it is necessary to ensure that the amount of metallic chromium after the cathodic electrolysis treatment C1 and the initial anodic electrolysis treatment A1 is 50mg/m²The above.

From the above viewpoints, in order to make it easy to form a metallic chromium layer having granular protrusions in the subsequent cathodic electrolysis treatment C2, it is preferable that the current density of the anodic electrolysis treatment A1 (the current density of each time since the anodic electrolysis treatment A1 is carried out 2 times or more) be appropriately adjusted, preferably to 0.1A/dm²Above and below 5.0A/dm²。

By setting the current density at 0.1A/dm²In this way, the generation sites of the granular protrusions are sufficiently formed, and the granular protrusions are sufficiently generated and uniformly distributed easily in the subsequent cathodic electrolysis treatment C2, which is preferable.

Further, the current density is controlled to be lower than 5.0A/dm²Therefore, corrosion resistance and corrosion resistance under the coating film are improved, and therefore, this is preferable. This is presumably because the metal chromium dissolved in the primary anodic electrolytic treatment does not increase unexpectedly and the generation sites of the granular projections do not become excessively large, and therefore, the local thinning of the thickness of the base portion of the metal chromium layer can be suppressed.

The electric density of the anodic electrolytic treatment A1 (the electric density of each of the anodic electrolytic treatments A1 was carried out 2 or more times) was preferably more than 0.3C/dm²And is less than 5.0C/dm². More preferably more than 0.3C/dm²And is 3.0C/dm²Below, more preferably more than 0.3C/dm²And is 2.0C/dm²The following. The charge density is the product of the current density and the energization time.

The energization time (unit: sec.) is in accordance with the aboveCurrent Density (Unit: A/dm)²) And the electricity density (unit: c/dm²) And (4) setting the parameters appropriately.

The anodic electrolytic treatment A1 may not be a continuous electrolytic treatment. That is, the anodic electrolytic treatment a1 may be an intermittent electrolytic treatment in which electrolysis is performed in a plurality of electrodes in an industrial production, and an electroless immersion time inevitably occurs. In the case of the intermittent electrolytic treatment, the total charge density is preferably within the above range.

Cathodic electrolysis treatment C2

As described above, metallic chromium and hydrated chromium oxide are precipitated in the cathodic electrolysis treatment. In particular, in the cathodic electrolysis treatment C2, granular protrusions of the metallic chromium layer were generated starting from the above-described generation sites. At this time, if the current density and the electric density are too high, granular protrusions of the metallic chromium layer may grow rapidly and the grain size may become coarse.

From the above viewpoint, the current density of the cathodic electrolysis C2 (the current density of each cathodic electrolysis C2 is preferably less than 60.0A/dm)²More preferably less than 50.0A/dm²More preferably, it is less than 40.0A/dm². The lower limit is not particularly limited, but is preferably 10.0A/dm²Above, more preferably more than 15.0A/dm²。

For the same reason, the electric density of the cathodic electrolysis C2 (the electric density of each of the cathodic electrolysis C2 cycles of 2 or more) is preferably less than 30.0C/dm²More preferably 25.0C/dm²Hereinafter, more preferably 7.0C/dm²The following. The lower limit is not particularly limited, but is preferably 1.0C/dm²Above, more preferably 2.0C/dm²The above.

The energization time (unit: sec.) is appropriately set in accordance with the above-described electric energy density and electric energy density.

The cathodic electrolysis treatment C2 may not be a continuous electrolysis treatment. That is, the cathodic electrolysis treatment C2 may be an intermittent electrolysis treatment in which electrolysis is performed in a plurality of electrodes in industrial production and an electroless immersion time inevitably occurs. In the case of the intermittent electrolytic treatment, the total charge density is preferably within the above range.

Number of treatments 2 including A1 and C2

In the manufacturing method of the present invention, 2 treatments including anodic electrolysis treatment a1 and cathodic electrolysis treatment C2 were performed 2 or more times on a steel sheet subjected to cathodic electrolysis treatment C1.

The number of the above-mentioned treatments 2 is preferably 3 or more, more preferably 5 or more, and further preferably 7 or more. By repeatedly performing the above-described process 2, the generation site of the granular protrusions forming the metallic chromium layer (the anodic electrolytic process a1) and the granular protrusions forming the metallic chromium layer (the cathodic electrolytic process C2) are repeatedly performed, and therefore the granular protrusions of the metallic chromium layer can be formed more uniformly and at high density. Therefore, even when the amount of the hydrated chromium oxide layer deposited is increased in order to improve corrosion resistance or the like, the uniform and high-density granular protrusions act to increase the number of contact points during welding, and the contact resistance is reduced, thereby improving weldability.

The upper limit of the number of times of the above-mentioned treatment 2 is not particularly limited, and from the viewpoint of controlling the thickness of the base portion of the metallic chromium layer formed in the cathodic electrolysis treatment C1 to an appropriate range, excessive repetition is preferably avoided, and for example, 30 times or less, and preferably 20 times or less are preferable.

Post-treatment

Post-treatment may also be performed after treatment 2 including anodic electrolytic treatment A1 and cathodic electrolytic treatment C2.

For example, from the viewpoint of ensuring paint adhesion and corrosion resistance under a coating film, an aqueous solution containing a hexavalent chromium compound may be used to perform immersion treatment or cathodic electrolysis treatment on a steel sheet in order to control the amount of a hydrated chromium oxide layer and modify the amount of the hydrated chromium oxide layer.

Even if such post-treatment is performed, the thickness of the base portion of the metallic chromium layer and the particle diameter and number density of the granular protrusions are not affected.

The hexavalent chromium compound contained in the aqueous solution used for the post-treatment is not particularly limited, and examples thereof include chromium trioxide (CrO)₃) Potassium dichromate (K)₂Cr₂O₇) Equal weight chromiumAcid salt, potassium chromate (K)₂CrO₄) Isochromates; and so on.