TWI845179B - Surface treated steel plate and manufacturing method thereof - Google Patents

Abstract

The present invention provides a surface-treated steel plate that can be manufactured without using hexavalent chromium and has excellent film corrosion resistance, coating corrosion resistance, film wettability, coating secondary adhesion, and excellent weldability. The surface-treated steel plate of the present invention has a Ni-containing layer arranged on at least one side of the steel plate, a metal Cr layer arranged on the aforementioned Ni-containing layer, and an oxidized Cr layer arranged on the aforementioned metal Cr layer, and the water contact angle is less than 50°, and the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface relative to Cr is less than 5.0%.

Description

Surface treated steel plate and manufacturing method thereof

The present invention relates to a surface treated steel plate, and in particular to a surface treated steel plate having excellent film corrosion resistance, coating corrosion resistance, film wettability, coating secondary adhesion, and weldability. The surface treated steel plate of the present invention can be applied to containers such as cans. In addition, the present invention relates to a method for manufacturing the surface treated steel plate.

Sn-plated steel (blik) has excellent corrosion resistance, weldability, processability, and is easy to manufacture. It has been used as a material for various metal cans such as beverage cans, food cans, barrels (pail), and 18-liter cans for more than 200 years.

However, since Sn is an expensive material, Wuxi steel (TFS) which is a surface treated steel sheet that does not use Sn has been developed. Wuxi steel is a surface treated steel sheet that forms a metal Cr layer and an oxide Cr layer on the surface of the steel sheet. It is usually manufactured by electrolytically treating the steel sheet in an electrolyte containing hexavalent Cr (patent documents 1~3). Wuxi steel sheets are now commonly used as container steel sheets to replace tinned steel sheets due to their excellent corrosion resistance and coating adhesion. However, this Wuxi steel sheet lacks weldability because it has an insulating film of chromium oxide on the surface.

In addition, Ni-plated steel sheets are known to have excellent weldability and do not use Sn as surface-treated steel sheets (Patent Documents 4 and 5). However, when Ni-plated steel sheets are used as materials for welding cans, in order to ensure corrosion resistance or coating adhesion, an aqueous solution containing hexavalent Cr must be used on the Ni-plated steel sheets to provide a chromate treatment film.

In recent years, due to the increasing awareness of the environment, the world is moving towards limiting the use of hexavalent chromium. Therefore, even in the field of surface-treated steel sheets used in containers, etc., it is required to establish a manufacturing method that does not use hexavalent chromium.

Methods for forming surface-treated steel sheets without using hexavalent chromium include methods proposed in known patent documents 6 and 7. In this method, a surface-treated layer is formed by electrolytic treatment in an electrolyte containing a trivalent chromium compound such as alkaline chromium sulfate. Prior art documents Patent documents

Patent document 1: Japanese Patent Publication No. 58-110695 Patent document 2: Japanese Patent Publication No. 55-134197 Patent document 3: Japanese Patent Publication No. 57-035699 Patent document 4: Japanese Patent Publication No. 11-117085 Patent document 5: Japanese Patent Publication No. 2007-231394 Patent document 6: Japanese Patent Publication No. 2016-505708 Patent document 7: Japanese Patent Publication No. 2015-520794

Invent the problem you want to solve

According to the method proposed in Patent Documents 6 and 7, a surface treatment layer can be formed without using hexavalent chromium. In addition, according to Patent Documents 6 and 7, by the above method, a surface treated steel sheet having excellent adhesion to a resin film in a wet environment (hereinafter referred to as "film wet adhesion") and excellent adhesion to a paint in a wet environment (hereinafter referred to as "paint secondary adhesion") can be obtained.

However, the surface treated steel sheet obtained by the conventional method proposed in Patent Documents 6 and 7 has excellent film wetting adhesion and coating secondary adhesion, but poor weldability. When used as a substitute for the surface treated steel sheet produced by the method using hexavalent chromium, its performance is sufficient.

Therefore, there is a demand for surface treated steel sheets that can be manufactured without using hexavalent chromium and that have excellent film corrosion resistance, coating corrosion resistance, film wetting adhesion, coating secondary adhesion, and weldability.

This invention was completed in view of the above actual situation. The purpose of this invention is to provide a surface-treated steel plate that can be manufactured without using hexavalent chromium and has excellent film corrosion resistance, coating corrosion resistance, film wetting adhesion, coating secondary adhesion, and weldability. Means for solving the problem

The inventors of the present invention have carefully studied to achieve the above-mentioned purpose and have obtained the following findings (1) and (2).

(1) In a surface-treated steel sheet having a metallic Cr layer and an oxidized Cr layer on a Ni layer, by controlling the water contact angle and the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface relative to Cr within specific ranges, a surface-treated steel sheet having excellent film corrosion resistance, coating corrosion resistance, film wetting adhesion, coating secondary adhesion, and weldability can be obtained.

(2) The surface treated steel sheet can be manufactured by subjecting it to cathodic electrolytic treatment in an electrolyte prepared by a specific method containing trivalent chromium ions, and then performing a final water wash using water having a conductivity below a predetermined value.

The present invention is completed based on the above findings. The main contents of the present invention are as follows.

1. A surface treated steel plate, comprising a steel plate, a Ni-containing layer disposed on at least one surface of the steel plate, a metal Cr layer disposed on the Ni-containing layer, and an oxidized Cr layer disposed on the metal Cr layer, wherein the water contact angle is less than 50°, and the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface relative to Cr is less than 5.0%.

2. For the surface treated steel plate as described in 1 above, the Ni adhesion amount of the Ni-containing layer is not less than 200 mg/ ^m2 and not more than 2000 mg/ ^m2 per single side of the steel plate.

3. The surface treated steel sheet as described in 1 or 2 above, wherein the Cr adhesion amount of the metal Cr layer is 2 mg/m ² or more and less than 40 mg/m ² per single side of the steel sheet.

4. The surface treated steel plate according to any one of 1 to 3 above, wherein the Cr adhesion amount of the oxidized Cr layer is not less than 0.1 mg/ ^m2 and not more than 15.0 mg/ ^m2 per single side of the steel plate.

5. A surface-treated steel plate as described in any one of 1 to 4 above, wherein the atomic ratio of Ni to Cr in the surface of the surface-treated steel plate is less than 100%.

A method for manufacturing a surface-treated steel plate, which comprises a steel plate, a Ni-containing layer disposed on at least one surface of the steel plate, a metal Cr layer disposed on the Ni-containing layer, and an oxidized Cr layer disposed on the metal Cr layer, and comprises the following steps: an electrolyte preparation step of preparing an electrolyte containing trivalent chromium ions, a cathodic electrolysis step of cathodically electrolyzing the steel plate having at least one Ni-containing layer on one side in the electrolyte, and a water washing step of washing the cathodically electrolyzed steel plate at least once, In the electrolyte preparation step, a trivalent chromium ion source, a carboxylic acid compound, and water are mixed, and the pH is adjusted to 4.0-7.0. At the same time, the temperature is adjusted to 40-70°C to prepare the electrolyte. In the water washing step, at least in the last water washing, water with a conductivity of less than 100μS/m is used. Effect of the invention

According to the present invention, a surface treated steel sheet is provided which does not use hexavalent chromium and has excellent film corrosion resistance, coating corrosion resistance, film wetting adhesion, coating secondary adhesion, and weldability. The surface treated steel sheet of the present invention can be used as a material for containers, etc.

Form of implementation of the invention

Hereinafter, the method for implementing the present invention will be specifically described. In addition, the following description is an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

The surface treated steel sheet in one embodiment of the present invention comprises a steel sheet, a Ni-containing layer disposed on at least one surface of the steel sheet, a metal Cr layer disposed on the Ni-containing layer, and an oxidized Cr layer disposed on the metal Cr layer. The present invention is important that the water contact angle of the surface treated steel sheet is 50° or less, and the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface to Cr is 5.0% or less. The constituent elements of the surface treated steel sheet are described below.

[Steel Plate] The aforementioned steel plate is not particularly limited, and any steel plate can be used. The aforementioned steel plate is preferably a steel plate for cans. The aforementioned steel plate can be, for example, an ultra-low carbon steel plate or a low carbon steel plate. The manufacturing method of the aforementioned steel plate is not particularly limited, and a steel plate manufactured by any method can be used. Usually, the aforementioned steel plate can be a cold-rolled steel plate. The aforementioned cold-rolled steel plate can be manufactured, for example, by a general manufacturing step of hot rolling, pickling, cold rolling, and tempering rolling.

The composition of the steel plate is not particularly limited, and the Cr content is preferably 0.10 mass % or less, and more preferably 0.08 mass % or less. When the Cr content of the steel plate is set to the above range, the surface of the steel plate will not be excessively Cr-concentrated, and as a result, the atomic ratio of Ni to Cr in the surface of the surface-treated steel plate finally obtained can be 100% or less. In addition, the steel plate may contain C, Mn, P, S, Si, Cu, Ni, Mo, Al, and inevitable impurities within a range that does not impair the effects of the present invention. In this case, the steel plate may be a steel plate having a composition specified in, for example, ASTM A623M-09.

In one embodiment of the present invention, it is preferred that the weight percent of the The steel plate is composed of the remaining Fe and inevitable impurities. Among the above composition, the lower the content of Si, P, S, Al, and N, the better the composition is. Cu, Ni, Cr, Mo, Ti, Nb, B, Ca, Sn, and Sb can be added arbitrarily.

The thickness of the steel plate is not particularly limited, but is preferably 0.60 mm or less. Here, "steel plate" is defined as including "steel strip". In addition, the lower limit of the thickness is not particularly limited, but is preferably 0.10 mm or more.

[Ni-containing layer] When surface-treated steel plates are used as steel plates for cans, they are generally welded by resistance welding such as wire seam welding. Ni is an element with excellent forge welding properties, so by configuring a Ni-containing layer, weldability can be improved. In other words, when a Ni-containing layer is present, lower resistance heating can be achieved, and excellent welding strength can be obtained, so the lower limit of welding current can be expanded.

The Ni-containing layer may be provided on at least one side of the steel plate or on both sides. The Ni-containing layer may cover at least a portion of the steel plate or the entire surface on which the Ni-containing layer is provided. Furthermore, the Ni-containing layer may be a continuous layer or a discontinuous layer. The discontinuous layer may be, for example, a layer having an island structure.

The Ni-containing layer may be any layer containing nickel, for example, one or both of a Ni layer and a Ni alloy layer. For example, when a Ni alloy layer is formed by diffusion annealing after Ni plating, it is also included in the Ni alloy layer. In addition, the Ni alloy layer may be, for example, a Ni-Fe alloy layer.

The Ni-containing layer is preferably a Ni-based plating layer. Here, "Ni-based plating layer" is defined as a plating layer having a Ni content of 50 mass % or more. In other words, the Ni-based plating layer is a plating layer composed of a Ni plating layer or a Ni-based alloy.

The aforementioned Ni-based coating may be a dispersed coating (composite coating) in which solid microparticles are dispersed in Ni or a Ni-based alloy as a matrix. The aforementioned solid microparticles are not particularly limited, and microparticles of any material may be used. The aforementioned microparticles may be either inorganic microparticles or organic microparticles. The aforementioned organic microparticles may include, for example, microparticles composed of resins. The aforementioned resin may be any resin, but it is preferably a fluororesin, and more preferably polytetrafluoroethylene (PTFE). The aforementioned inorganic microparticles are not particularly limited, and microparticles composed of any inorganic material may be used. The aforementioned inorganic material may be, for example, a metal (including an alloy), a compound, or another monomer. Among them, it is preferred to use microparticles composed of at least one selected from the group consisting of oxides, nitrides, and carbides, and it is preferred to use microparticles of metal oxides. Examples of the metal oxides include aluminum oxide, chromium oxide, titanium oxide, and zinc oxide.

The particle size of the microparticles used in the above-mentioned dispersion coating is not particularly limited, and particles of any size can be used. However, it is preferred that the diameter of the microparticles does not exceed the thickness of the dispersion coating layer containing the Ni layer. Typically, the diameter of the above-mentioned microparticles is preferably 1nm~50μm, and more preferably 10nm~1000nm.

The amount of Ni deposited in the Ni-containing layer is not particularly limited and may be any amount. However, from the viewpoint of further improving the weldability and corrosion resistance of the surface-treated steel plate, the amount of Ni deposited is 200 mg/m ² or more per single side of the steel plate, and more preferably 250 mg/m ² or more. In addition, when the amount of Ni deposited exceeds 2000 mg/m ² , the effect of improving weldability is saturated. Therefore, from the viewpoint of reducing excessive costs, it is preferred that the amount of Ni deposited is set to 2000 mg/m ² or less, and more preferably 1800 mg/m ² or less.

The Ni adhesion amount of the aforementioned Ni-containing layer is measured by the calibration curve method of fluorescent X-rays. A plurality of steel plates with known Ni adhesion amounts are prepared, and the intensity of fluorescent X-rays from Ni is measured in advance. The relationship between the measured intensity of fluorescent X-rays and the Ni adhesion amount is linearly approximated as a calibration curve. The intensity of fluorescent X-rays from Ni of the surface-treated steel plate is measured, and the Ni adhesion amount of the aforementioned Ni-containing layer can be measured using the above calibration curve.

The formation of the aforementioned Ni-containing layer is not particularly limited, and can be performed by any method such as electroplating. When the Ni-containing layer is formed by electroplating, any plating bath can be used. Examples of plating baths that can be used include a Watts bath, an aminosulfonic acid bath, or a nickel bath. When forming a Ni-Fe alloy layer as a Ni-containing layer, after forming a Ni layer on the surface of a steel plate by electroplating or the like, the Ni-Fe alloy layer can be formed by annealing.

The surface side of the Ni-containing layer may contain Ni oxide or may not contain Ni oxide at all, but from the viewpoint of further improving the secondary adhesion of the coating and the resistance to sulfidation blackening, it is preferred that the surface side of the Ni-containing layer does not contain Ni oxide. Ni oxide is formed by dissolved oxygen contained in the washing water after Ni plating, but it is preferred to remove the Ni oxide contained in the Ni-containing layer by the pretreatment described below.

[Metallic Cr layer] A metallic Cr layer exists on the aforementioned Ni-containing layer.

The amount of the metal Cr layer is not particularly limited and may be any value. However, from the viewpoint of further improving the corrosion resistance, the amount of the metal Cr layer is preferably 2 mg/ ^m2 or more, more preferably 4 mg/ ^m2 or more, based on the amount of Cr on each side of the steel plate. In addition, the upper limit of the amount of the metal Cr layer is not particularly limited. If the amount of the metal Cr layer is too much, the contact resistance increases and the weldability may be impaired. Therefore, from the viewpoint of more stably ensuring the weldability, the amount of the metal Cr layer is preferably less than 40 mg/ ^m2 , more preferably less than 35 mg/ ^m2 , based on the amount of Cr on each side of the steel plate.

In addition, the amount of Cr adhesion in the metal Cr layer can be measured by a fluorescent X-ray method. Specifically, first, a fluorescent X-ray device is used to measure the amount of Cr (total Cr amount) in the surface-treated steel plate. Then, the surface-treated steel plate is subjected to an alkaline treatment by being immersed in 7.5N-NaOH at 90°C for 10 minutes, and then is thoroughly washed with water. Then, a fluorescent X-ray device is used to measure the amount of Cr (the amount of Cr after alkaline treatment). Furthermore, for the steel plate after the metal Cr layer and the oxidized Cr layer are peeled off, a fluorescent X-ray device is used to measure the amount of Cr (the amount of Cr in the original plate). The metal Cr layer and the oxidized Cr layer can be peeled off using, for example, a commercially available hydrochloric acid-based chromium plating stripping agent. The value obtained by subtracting the original plate Cr amount from the alkali-treated Cr amount is used as the Cr adhesion amount of the metal Cr layer per single side of the steel plate. In addition, the total Cr amount is used to calculate the Cr adhesion amount as the oxidized Cr layer described later.

The metal Cr constituting the metal Cr layer may be amorphous Cr or crystalline Cr. That is, the metal Cr layer may contain one or both of amorphous Cr and crystalline Cr. The metal Cr layer produced by the method described below generally contains amorphous Cr, and further, may contain crystalline Cr. Although the formation mechanism of the metal Cr layer is unknown, it is considered that when the amorphous Cr is formed, it is partially crystallized to form a metal Cr layer containing both amorphous and crystalline phases.

The ratio of the total crystalline Cr of the amorphous Cr and crystalline Cr contained in the metal Cr layer is preferably 0% to 80%, and more preferably 0% to 50%. Here, the ratio of the crystalline Cr can be measured by observing the metal Cr layer with a scanning electron microscope (STEM). Specifically, first, a STEM image is obtained at a magnification of about 2 million to 10 million times with a beam diameter that can obtain a resolution of less than 1 nm. In the obtained STEM image, the area where the lattice interference fringes can be confirmed is the crystalline phase, and the area where the maze pattern can be confirmed is the amorphous phase, and the areas of the two are calculated. From this result, the ratio of the area of crystalline Cr to the total area of amorphous Cr and crystalline Cr was calculated.

[Oxidized Cr layer] An oxidized Cr layer exists on the aforementioned metal Cr layer. The amount of the oxidized Cr layer is not particularly limited and may be any value. However, from the viewpoint of further improving corrosion resistance, the amount of the oxidized Cr layer is preferably 0.1 mg/ ^m2 or more based on the amount of Cr attached to each side of the steel plate. In addition, there is no particular upper limit on the amount of the oxidized Cr layer. When the amount of the oxidized Cr layer is too much, the contact resistance increases and weldability may be impaired. Therefore, from the viewpoint of ensuring more stable weldability, the amount of the oxidized Cr layer is preferably 15.0 mg/ ^m2 or less based on the amount of Cr attached to each side of the steel plate. In addition, the amount of Cr attached in the oxidized Cr layer can be measured by fluorescent X-ray method. Specifically, the amount of Cr attached to the oxidized Cr layer can be obtained by subtracting the amount of Cr after alkali treatment from the total amount of Cr measured using the aforementioned fluorescent X-ray device.

One or both of the metal Cr layer and the oxidized Cr layer may contain C. However, when the metal Cr layer and the oxidized Cr layer contain too much C, the weld heat-affected portion hardens during welding, resulting in cracking. Therefore, the atomic ratio of the C content in the metal Cr layer to Cr is preferably 40% or less, and more preferably 35% or less. Similarly, the atomic ratio of the C content in the oxidized Cr layer to Cr is preferably 40% or less, and more preferably 35% or less. The metal Cr layer and the oxidized Cr layer may not contain C, and therefore, the lower limit of the C content contained in the metal Cr layer and the oxidized Cr layer, respectively, to the atomic ratio of Cr, may be 0%.

The C content in the metallic Cr layer and the C content in the oxidized Cr layer can be measured by X-ray photoelectron spectroscopy (XPS). Specifically, the determination of the C content obtained by XPS is carried out by calculating the C atomic ratio/Cr atomic ratio by the relative sensitivity coefficient method using the integrated intensity of the narrow spectrum of Cr2p and C1s measured by XPS.

In addition, since the C from contamination is detected on the top layer of the surface treated steel plate, in order to accurately measure the C content in the oxidized Cr layer, the top layer can be sputtered to a depth of, for example, 0.2nm or more in terms of _SiO2 . In addition, the C content in the metal Cr layer can be measured by sputtering the top layer after the above-mentioned alkali treatment to a depth of 1/2 of the thickness of the metal Cr layer.

The thickness of the metal Cr layer used for the above measurement can be obtained using the following steps. First, the Cr atomic ratio and Ni atomic ratio are measured by XPS every 1nm in the depth direction from the top layer after alkali treatment. Then, the cubic formula that approximates the relationship between the Ni atomic ratio/Cr atomic ratio of the top layer after alkali treatment and the depth is obtained by the least square method. Using the obtained cubic formula, the depth of the top layer where the Ni atomic ratio/Cr atomic ratio becomes 1 is calculated, which is taken as the thickness of the metal Cr layer.

The above measurement can be performed using, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI. The X-ray source is monochromatic AlKα ray, the voltage is 15 kV, the beam diameter is 100 μmφ, the take-off angle is 45°, and the sputtering conditions are to accelerate Ar ions at a voltage of 1 kV and a sputtering rate of 1.50 nm/min in terms of _SiO2 .

Although the mechanism by which the metallic Cr layer and the oxidized Cr layer contain C is unknown, it is considered that in the step of forming the metallic Cr layer and the oxidized Cr layer on the steel plate, the carboxylic acid compound contained in the electrolyte is decomposed and incorporated into the film.

The presence form of C in the metallic Cr layer and the oxidized Cr layer is not particularly limited. When it exists as a precipitate, the corrosion resistance may be reduced due to the formation of local cells. Therefore, the sum of the volume fractions of carbides or clusters with a clear crystal structure is preferably 10% or less, and more preferably completely absent (0%). The presence or absence of carbides can be confirmed by, for example, composition analysis obtained by energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) attached to a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The presence or absence of clusters can be confirmed by, for example, cluster analysis of data obtained after three-dimensional composition analysis by a three-dimensional atom probe (3DAP).

The metal Cr layer may contain O. The upper limit of the O content in the metal Cr layer is not particularly limited. When the O content is high, oxidized Cr will precipitate, and the corrosion resistance may be reduced due to the formation of local cells. Therefore, the atomic ratio of O content to Cr is preferably 30% or less, and more preferably 25% or less. The metal Cr layer may not contain O, and therefore, the lower limit of Cr contained in the metal Cr layer is not particularly limited, and may be 0%.

The content of O in the metal Cr layer can be measured by composition analysis such as EDS and WDS attached to SEM or TEM, or 3DAP.

One or both of the metal Cr layer and the oxidized Cr layer may contain Ni. The upper limit of the Ni content in the metal Cr layer is not particularly limited, and the atomic ratio of Ni to Cr is preferably less than 100%. Similarly, the upper limit of the Ni content in the oxidized Cr layer is not particularly limited, and the atomic ratio of Ni to Cr is preferably less than 100%. The metal Cr layer and the oxidized Cr layer may not contain Ni, and therefore, the lower limit of the atomic ratio of Ni to Cr is not particularly limited and may be 0%.

The Ni content in the surface of the surface treated steel sheet, i.e., the surface of the oxidized Cr layer, is not particularly limited. The lower the Ni content, the better the film wettability and secondary adhesion of the coating. Therefore, the atomic ratio of Ni to Cr in the surface of the surface treated steel sheet is preferably 100% or less, more preferably 80% or less.

The content of Ni in the metallic Cr layer and the oxidized Cr layer is the same as the content of C and can be measured by XPS. The atomic ratio of Ni to Cr in the surface of the surface treated steel sheet, that is, the surface of the oxidized Cr layer, can be measured by XPS. The atomic ratio can be calculated using the narrow spectrum of Cr2p and Ni2p.

Although the mechanism by which the metallic Cr layer and the oxidized Cr layer contain Ni is unknown, it is considered that in the step of forming the metallic Cr layer and the oxidized Cr layer on the steel plate, a small amount of Ni contained in the Ni-containing layer dissolves in the electrolyte, and Ni is incorporated into the film.

In addition to Cr, O, Ni, C and K, Na, Mg and Ca described later, the above-mentioned metal Cr layer and oxidized Cr layer may also contain metal impurities such as Cu, Zn, Sn, Fe, etc. contained in the aqueous solution or S, N, Cl, Br, etc. However, when these elements exist, the film wettability and the secondary adhesion of the coating may be reduced. Therefore, the content of Fe in the metal Cr layer and the oxidized Cr layer is preferably less than 10% in atomic ratio to Cr, and more preferably, it is completely free of (0%). The total atomic ratio of elements other than Cr, O, Ni, C, K, Na, Mg, Ca, and Fe to Cr is preferably less than 3%, and more preferably, it is completely free of (0%). The content of the above-mentioned elements is not particularly limited, for example, it can be measured by XPS in the same way as the content of C. In particular, when XPS is used to determine the Fe content, the narrow spectrum of Fe2p is used, but there is a case where the quantitative value of the Fe content is calculated to be higher than the actual value due to overlap with the NiLLM peak. As mentioned above, the Fe content is different from other elements, and the atomic ratio of Fe to Cr is preferably controlled below 10%.

The metal Cr layer and the oxidized Cr layer are preferably free of cracks. The presence or absence of cracks can be confirmed by, for example, cutting the film cross section with a focused ion beam (FIB) or the like and directly observing with a transmission electron microscope (TEM).

In addition, the surface roughness of the surface treated steel plate of the present invention does not change greatly due to the formation of the metal Cr layer and the oxidized Cr layer, and is generally roughly the same as the surface roughness of the base steel plate used. The surface roughness of the surface treated steel plate is not particularly limited, and the arithmetic average roughness Ra is preferably 0.1 μm or more and 4 μm or less. In addition, the ten-point average roughness Rz is preferably 0.2 μm or more and 6 μm or less.

[Water contact angle] In the present invention, it is important that the water contact angle of the surface treated steel plate is 50° or less. By making the water contact angle 50° or less, the surface of the surface treated steel plate is highly hydrophilic, and a strong hydrogen bond is formed between the resin contained in the coating and the surface treated steel plate, resulting in high adhesion even in a wet environment. From the perspective of further improving the secondary adhesion of the coating, the water contact angle is preferably 48° or less, and more preferably 45° or less. From the perspective of improving adhesion, the lower the water contact angle, the better, so its lower limit is not particularly limited and can be 0°. However, from the perspective of ease of manufacturing, it is preferably 3° or more, and more preferably 6° or more. Furthermore, the aforementioned water contact angle can be measured by the method described in the embodiment.

Although the mechanism by which the surface of the surface-treated steel sheet is hydrophilized is unknown, when a metal Cr layer and an oxidized Cr layer are formed by cathodic electrolysis in an electrolyte, carboxylic acids or carboxylates contained in the electrolyte are decomposed and incorporated into the film, and hydrophilic functional groups such as carboxyl groups are given to the surface. However, as described later, when the electrolyte is not prepared under specific conditions, the surface of the surface-treated steel sheet will not be hydrophilized even if carboxylic acids or carboxylates are contained in the electrolyte. Although the mechanism by which the preparation conditions of the electrolyte affect the hydrophilization of the surface of the surface-treated steel sheet is unknown, it is speculated that when the electrolyte is appropriately prepared under the conditions described later, hydrophilic functional groups such as carboxyl groups form complexes that are easily given to the surface.

In addition, Patent Documents 1 to 5 propose that the composition of the hydrated chromium oxide layer on the surface of the surface treated steel sheet produced using the conventional hexavalent chromium bath has a significant impact on the adhesion of the coating or film in a wet environment. In a wet environment, water that penetrates the coating or film will hinder the adhesion of the interface between the coating or film and the hydrated chromium oxide layer. Therefore, when a large number of hydrophilic OH groups exist in the hydrated chromium oxide layer, the expansion and wetting of water in the interface is promoted, and the adhesion is reduced. Therefore, in the conventional surface treatment of steel sheets, the OH groups are reduced due to the progress of carbonylation (oxo) of hydrated chromium oxide, that is, the surface is hydrophobicized, thereby improving the adhesion with the coating or film in a wet environment.

In contrast, the present invention forms a strong hydrogen bond at the interface between the coating and the surface-treated steel plate by making the surface hydrophilic to a level close to super-hydrophilicity. Therefore, high adhesion can be maintained even in a humid environment. This is based on a technical idea that is completely opposite to the above-mentioned conventional technology.

[Atomic ratio of adsorbed elements] As mentioned above, the surface treated steel plate of the present invention has a high hydrophilicity with a water contact angle of 50° or less, and its surface is chemically active. Therefore, the surface of the surface treated steel plate is easy to adsorb cations of elements such as K, Na, Mg, and Ca. When the inventors simply set the water contact angle to less than 50°, it was found that the original adhesion could not be exerted due to the influence of the adsorption of the aforementioned cations. In the present invention, by reducing the amount of the aforementioned cations adsorbed on the surface of the surface treated steel plate, the adhesion to the resin is improved, and excellent film wetting adhesion and coating secondary adhesion can be achieved.

Specifically, the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface of the surface treated steel plate to Cr is 5.0% or less, preferably 3.0% or less, and more preferably 1.0% or less. The lower the total atomic ratio, the better, so the lower limit is not particularly limited and may be 0%. The total atomic ratio can be measured by the method described in the embodiment.

[Manufacturing method] The manufacturing method of the surface-treated steel plate in one embodiment of the present invention uses the method described below to manufacture the surface-treated steel plate having the above-mentioned characteristics.

A method for manufacturing a surface-treated steel plate in one embodiment of the present invention is a method for manufacturing a surface-treated steel plate having a Ni-containing layer on at least one surface of the steel plate, a metal Cr layer disposed on the aforementioned Ni-containing layer, and an oxidized Cr layer disposed on the aforementioned metal Cr layer, and comprises the following steps (1) to (3). Each step is described below. (1) An electrolyte preparation step of preparing an electrolyte containing trivalent chromium ions (2) A cathodic electrolytic treatment step of cathodically electrolytically treating the steel plate having the Ni-containing layer in the aforementioned electrolyte (3) A water washing step of washing the steel plate after the aforementioned cathodic electrolytic treatment at least once

[Electrolyte preparation step] (i) Mixing In the above-mentioned electrolyte preparation step, first, a trivalent chromium ion source, a carboxylic acid compound, and water are mixed to form an aqueous solution.

The trivalent chromium ion source may be any compound as long as it can provide trivalent chromium ions. The trivalent chromium ion source may be, for example, at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate.

The content of the trivalent chromium ion source in the aqueous solution is not particularly limited, but is preferably 3 g/L to 50 g/L, more preferably 5 g/L to 40 g/L, in terms of trivalent chromium ions. The trivalent chromium ion source may be BluCr (registered trademark) TFSA from Atotech.

The aforementioned carboxylic acid compound is not particularly limited, and any carboxylic acid compound can be used. The aforementioned carboxylic acid compound can be at least one of a carboxylic acid and a carboxylate, preferably at least one of an aliphatic carboxylic acid and an aliphatic carboxylate. The carbon number of the aforementioned aliphatic carboxylic acid is preferably 1 to 10, more preferably 1 to 5. In addition, the carbon number of the aforementioned aliphatic carboxylate is preferably 1 to 10, more preferably 1 to 5. The content of the aforementioned carboxylic acid compound is not particularly limited, and is preferably 0.1 mol/L to 5.5 mol/L, more preferably 0.15 mol/L to 5.3 mol/L. The aforementioned carboxylic acid compound can use BluCr (registered trademark) TFS B from Atotech.

In the present invention, water is used as a solvent for preparing the electrolyte. The water is an ion exchange resin, etc. It is preferred to use ion exchange water from which cations have been removed in advance, or water of high purity such as distilled water. As described later, from the perspective of reducing the amount of K, Na, Mg, and Ca contained in the electrolyte, it is preferred to use water with a conductivity of 30μS/m or less.

In order to reduce the amount of K, Na, Mg, and Ca adsorbed on the surface of the surface-treated steel plate, the aqueous solution preferably does not contain K, Na, Mg, and Ca intentionally. Therefore, the components added to the aqueous solution of the trivalent chromium ion source, carboxylic acid compound, and pH adjuster described in detail below preferably do not contain K, Na, Mg, and Ca. As a pH adjuster, when the pH is lowered, hydrochloric acid, sulfuric acid, nitric acid, etc. are used, and when the pH is raised, ammonia water, etc. are preferably used. K, Na, Mg, and Ca that are inevitably mixed into the aqueous solution or electrolyte are allowed, but the total concentration of K, Na, Mg, and Ca is preferably 2.0 mol/L or less, more preferably 1.5 mol/L or less, and even more preferably 1.0 mol/L or less.

In order to effectively inhibit the generation of hexavalent chromium at the anode in the cathodic electrolytic treatment step and improve the stability of the above-mentioned electrolyte, it is preferred that at least one type of halogenide ion is contained in the above-mentioned aqueous solution. The content of the halogenide ion is not particularly limited, but is preferably 0.05 mol/L to 3.0 mol/L, and more preferably 0.10 mol/L to 2.5 mol/L. In order to contain the above-mentioned halogenide ions, BluCr (registered trademark) TFS C1 and BluCr (registered trademark) TFS C2 of Atotech can be used.

It is preferred that hexavalent chromium is not added to the aqueous solution. In the cathodic electrolytic treatment step, the extremely small amount of hexavalent chromium formed at the anode is removed, and the electrolyte does not contain hexavalent chromium. In the cathodic electrolytic treatment step, the extremely small amount of hexavalent chromium formed at the anode is reduced to trivalent chromium, so the hexavalent chromium concentration in the electrolyte does not increase.

The aqueous solution preferably does not intentionally add metal ions other than trivalent chromium ions. The metal ions are not limited, and may include Cu ions, Zn ions, Ni ions, Fe ions, Sn ions, etc., each preferably being 0 mg/L to 40 mg/L, more preferably 0 mg/L to 20 mg/L, and most preferably 0 mg/L to 10 mg/L. Among the metal ions, Ni ions dissolve in the electrolyte during the cathodic electrolysis step when the steel plate is immersed in the electrolyte, and form eutectoid in the film, but will not affect the film wettability, adhesion, secondary adhesion, and weldability of the coating. The Ni ion concentration is preferably 0 mg/L to 40 mg/L, more preferably 0 mg/L to 20 mg/L, and most preferably 0 mg/L to 10 mg/L. In addition, the Ni ion concentration is preferably within the above range when the bath is built, but even in the cathodic electrolytic treatment step, the Ni ion concentration in the electrolyte is preferably maintained within the above range. When the Ni ion is controlled within the above range, the formation of the metal Cr layer and the oxidized Cr layer will not be hindered, and the metal Cr layer and the oxidized Cr layer of the necessary thickness can be formed.

(ii) Adjustment of pH and temperature Secondly, the aforementioned electrolyte is prepared by adjusting the pH of the aforementioned aqueous solution to 4.0~7.0 and the temperature of the aforementioned aqueous solution to 40~70°C. In order to manufacture the aforementioned surface-treated steel sheet, it is not enough to simply dissolve the trivalent chromium ion source and the carboxylic acid compound in water. It is important to properly control the pH and temperature as mentioned above.

pH: 4.0~7.0 The aforementioned electrolyte preparation step is to adjust the pH of the mixed aqueous solution to 4.0~7.0. When the pH is less than 4.0 or exceeds 7.0, the water contact angle of the surface-treated steel plate manufactured using the obtained electrolyte is higher than 50°. The pH is preferably 4.5~6.5.

Temperature: 40~70℃ In the aforementioned electrolyte preparation step, the temperature of the mixed aqueous solution is adjusted to 40~70℃. When the temperature is less than 40℃ or exceeds 70℃, the water contact angle of the surface treated steel plate manufactured using the obtained electrolyte is higher than 50°. In addition, the holding time in the temperature range of 40~70℃ is not particularly limited.

Through the above steps, the electrolyte used in the following cathodic electrolysis treatment step can be obtained. In addition, the electrolyte prepared in the above steps can be stored at room temperature.

[Cathode electrolysis step] Next, the steel plate having a Ni-containing layer on at least one side is subjected to cathodic electrolysis in the electrolyte obtained in the electrolyte preparation step. By the cathodic electrolysis, a metal Cr layer and an oxidized Cr layer can be formed on the Ni-containing layer.

The temperature of the electrolyte during the cathodic electrolytic treatment is not particularly limited, but is preferably in the temperature range of 40°C to 70°C in order to efficiently form the metal Cr layer and the oxidized Cr layer. From the viewpoint of stably manufacturing the surface-treated steel sheet, the temperature of the electrolyte is monitored during the cathodic electrolytic treatment step, and is preferably maintained in the above temperature range.

The pH of the electrolyte during the cathodic electrolytic treatment is not particularly limited, but is preferably 4.0 or higher, more preferably 4.5 or higher. Furthermore, the aforementioned pH is preferably 7.0 or lower, more preferably 6.5 or lower. From the viewpoint of stable production of the aforementioned surface-treated steel sheet, the pH of the electrolyte is monitored during the cathodic electrolytic treatment step, and it is preferred to maintain the aforementioned pH range.

The current density in the above-mentioned cathodic electrolytic treatment is not particularly limited, and can be appropriately adjusted to form the desired surface treatment layer. However, when the current density is too high, the C content in the metal Cr layer increases, and the weldability may be deteriorated. Therefore, the current density is preferably less than 5.0A/ ^dm2 , and more preferably less than 3.0A/ ^dm2 . The lower limit of the current density is not particularly limited. When the current density is too low, hexavalent Cr is generated in the electrolyte, and the stability of the bath may collapse. Therefore, the current density is preferably 0.01A/ ^dm2 or more, and more preferably 0.03A/ ^dm2 or more.

The number of times the steel sheet is subjected to cathodic electrolysis is not particularly limited and can be any number. In other words, the cathodic electrolysis can be performed using an electrolytic treatment device having any number of passes (1 or more than 2). For example, it is preferable to continuously perform cathodic electrolysis while conveying the steel sheet (steel strip) through a plurality of passes. In addition, when the number of cathodic electrolysis (i.e., the number of passes) is increased, the number of electrolytic cells corresponding thereto must be increased, so the number of cathodic electrolysis (the number of passes) is preferably 20 or less.

There is no particular limit to the electrolysis time per channel. However, if the electrolysis time per channel is too long, the conveying speed (line speed) of the steel plate will decrease, which will reduce productivity. Therefore, the electrolysis time per channel is preferably less than 5 seconds, and more preferably less than 3 seconds. There is no particular limit to the lower limit of the electrolysis time per channel, but if the electrolysis time is shortened too much, the line speed will have to be increased accordingly, making control difficult. Therefore, the electrolysis time per channel is preferably more than 0.005 seconds, and more preferably more than 0.01 seconds.

The amount of metal Cr formed by cathodic electrolysis can be controlled by the total electric density represented by the product of current density, electrolysis time and number of channels. As mentioned above, if the amount of metal Cr is too much, the contact resistance becomes large, which sometimes affects the weldability. If the amount of metal Cr is too little, the corrosion resistance is sometimes damaged. Therefore, the amount of Cr attached to each side of the steel plate of the metal Cr layer is controlled to be more than ² mg/m2 and less than 40 mg/ ^m2 . However, the relationship between the amount of metal Cr layer and the total electric density varies due to the structure of the device used in the cathodic electrolysis step, so the actual electrolysis treatment conditions only need to be adjusted in accordance with the device.

The type of anode used in the cathodic electrolytic treatment is not particularly limited, and any anode can be used. As the aforementioned anode, an insoluble anode is preferably used. As the aforementioned insoluble anode, at least one selected from the group consisting of an anode in which one or both of a platinum group metal and an oxide of a platinum group metal are coated on Ti and a graphite anode is preferably used. More specifically, as the aforementioned insoluble anode, an anode in which platinum, iridium oxide or ruthenium oxide is coated on the surface of Ti as a substrate is exemplified.

In the above-mentioned cathodic electrolytic treatment step, the concentration of the electrolyte changes frequently due to the influence of liquid carry-out or carry-in, water evaporation, etc., for the formation of the metal Cr layer and the oxidized Cr layer on the steel plate. Since the concentration of the electrolyte changes in the cathodic electrolytic treatment step due to the structure of the device and the manufacturing conditions, from the perspective of more stably manufacturing the surface-treated steel plate, it is better to monitor the concentration of the components contained in the electrolyte in the cathodic electrolytic treatment step and maintain it within the above-mentioned concentration range.

Furthermore, before the aforementioned cathodic electrolytic treatment, the steel plate having the Ni-containing layer may be subjected to any pretreatment. By performing the pretreatment, the natural oxide film existing on the surface of the Ni-containing layer is removed, and the surface can be activated. The aforementioned pretreatment method is not particularly limited, and any method may be used, for example, pickling by immersion in dilute sulfuric acid may be performed.

After the above-mentioned pre-treatment, it is preferred to perform water washing from the viewpoint of removing the previous treatment liquid attached to the surface.

When the Ni-containing layer is formed on the surface of the base steel plate, it is preferred to perform a pretreatment on the base steel plate. As the pretreatment, any treatment may be performed, but at least one of degreasing, pickling and water washing is preferably performed.

By performing degreasing, rolling oil and anti-rust oil attached to the steel plate can be removed. The above-mentioned degreasing is not particularly limited and can be performed by any method. After degreasing, in order to remove the degreasing treatment liquid attached to the surface of the steel plate, it is preferably washed with water.

In addition, by performing pickling, the natural oxide film existing on the surface of the steel plate is removed, and the surface can be activated. The pickling mentioned above is not particularly limited and can be performed by any method. After pickling, in order to remove the pickling treatment liquid attached to the surface of the steel plate, it is preferably washed with water.

[Water washing step] Then, the steel plate after the cathodic electrolysis treatment is washed with water at least once. By washing with water, the electrolyte remaining on the surface of the steel plate can be removed. The above-mentioned water washing is not particularly limited and can be performed by any method. For example, a water washing tank is set downstream of the electrolytic tank for cathodic electrolysis treatment, and the steel plate after the cathodic electrolysis treatment can be continuously immersed in water. In addition, the steel plate after the cathodic electrolysis treatment can also be washed with water by spraying water.

The number of times of water washing is not particularly limited, and may be 1 time or 2 times or more. However, in order to avoid too many water washing tanks, the number of water washing is preferably 5 times or less. Furthermore, when water washing treatment is performed 2 times or more, each water washing can be performed by the same method or by different methods.

In the present invention, it is important to use water with a conductivity of 100μS/m or less in at least the last water washing in the aforementioned water washing treatment step. This can reduce the amount of K, Na, Mg and Ca adsorbed on the surface of the surface-treated steel plate, and as a result, the adhesion can be improved. Water with a conductivity of 100μS/m or less can be produced by any method. The aforementioned water with a conductivity of 100μS/m or less can be, for example, ion exchange water or distilled water. In addition, the lower limit of the aforementioned conductivity is not particularly limited, but excessive reduction will lead to an increase in manufacturing costs. Therefore, from the perspective of manufacturing costs, the aforementioned conductivity is preferably 1μS/m or more, more preferably 5μS/m or more, and more preferably 10μS/m or more.

In the above-mentioned water washing treatment step, when water washing is performed two or more times, the above-mentioned effect can be obtained if water with a conductivity of 100 μS/m or less is used for the final water washing, so any water can be used for the water washing other than the final water washing. Although water with a conductivity of 100 μS/m or less can be used for the water washing other than the final water washing, from the viewpoint of cost reduction, only water with a conductivity of 100 μS/m or less is used for the final water washing, and general water such as tap water and industrial water is preferably used for the water washing other than the final water washing.

From the viewpoint of further reducing the amount of K, Na, Mg and Ca adsorbed on the surface of the surface-treated steel plate, the conductivity of the water used for the final water washing is preferably 50 μS/m or less, and more preferably 30 μS/m or less.

The temperature of the water used for the washing treatment is not particularly limited and can be any temperature. However, if the temperature is too high, it will cause excessive burden on the washing equipment, so the temperature of the water used for washing is preferably below 95°C. In addition, the lower limit of the temperature of the water used for washing is not particularly limited, but it is preferably above 0°C. The temperature of the water used for the washing treatment can also be room temperature.

The washing time of each washing treatment is not particularly limited, but is preferably 0.1 seconds or more, more preferably 0.2 seconds or more, from the viewpoint of improving the washing effect. The upper limit of the washing time of each washing treatment is not particularly limited, but is preferably 10 seconds or less, more preferably 8 seconds or less, because the productivity decreases when the line speed decreases when manufacturing on a continuous production line.

After the above-mentioned water washing treatment step, optional drying can also be performed. The drying method is not particularly limited, and for example, a conventional drying machine or electric furnace drying method can be used. The temperature during the drying treatment is preferably below 100°C. If it is within the above range, the deterioration of the surface treatment film can be suppressed. In addition, the lower limit is not particularly limited, and is usually around room temperature.

The use of the surface-treated steel plate of the present invention is not particularly limited, and is particularly suitable for use as a surface-treated steel plate for containers used in the manufacture of various containers such as food cans, beverage cans, barrels, and 18-liter cans. Example

In order to confirm the effect of the present invention, a surface treated steel plate was produced in the following procedure and its characteristics were evaluated.

(Electrolyte preparation step) First, an electrolyte having the composition A to G shown in Table 1 is prepared under the conditions shown in Table 1. That is, each component shown in Table 1 is mixed with water to prepare an aqueous solution, and then the aqueous solution is adjusted to the pH and temperature shown in Table 1. In addition, electrolyte G is equivalent to the electrolyte used in the embodiment of patent document 6. When the pH is increased, ammonia water is used. When the pH is decreased, sulfuric acid is used in electrolytes A, B, and G, hydrochloric acid is used in electrolytes C and D, and nitric acid is used in electrolytes E and F.

(Formation of Ni-containing layer) In addition, Ni is electroplated on both sides of the steel plate to obtain a Ni-plated steel plate having a Ni-containing layer on both sides of the steel plate. The aforementioned Ni electroplating uses a Watts bath. Furthermore, before the aforementioned Ni electroplating, the aforementioned steel plate is subjected to electrolytic degreasing, water washing, pickling by immersion in dilute sulfuric acid, and water washing in sequence. The aforementioned Ni electroplating is performed by changing the charge density to make the Ni adhesion amount of the Ni coating layer as shown in Tables 2 and 3. The Ni adhesion amount of the aforementioned Ni-containing layer is measured by the aforementioned fluorescent X-ray calibration curve method. After the Ni plating layer is formed, it is washed with water and kept in a wet state for the next cathode electrolysis treatment step. In addition, some embodiments form a Ni-Fe alloy layer as a Ni-containing layer. That is, after the Ni plating layer is formed by the above method, the Ni-Fe alloy layer is formed by annealing.

As the steel plate, a can steel plate (T4 original plate) having a Cr content as shown in Tables 2 and 3 and a plate thickness of 0.17 mm was used.

(Cathode electrolysis step) Next, the Ni-plated steel plate is subjected to cathodic electrolysis under the conditions shown in Tables 2 and 3. The electrolyte during the cathodic electrolysis is maintained at the pH and temperature shown in Table 1. The charge density during the cathodic electrolysis is the value shown in Tables 2 and 3, and the electrolysis time and the number of channels are appropriately changed. As the anode during the cathodic electrolysis, an insoluble anode coated with iridium oxide is used for Ti as a substrate. After the cathodic electrolysis, it is washed with water and dried at room temperature using a blower.

(Water washing step) Next, the steel plate after the above cathodic electrolysis treatment is subjected to water washing. The above water washing treatment is performed 1 to 5 times under the conditions shown in Tables 2 and 3. The method of each water washing and the conductivity of the water used are shown in Tables 2 and 3.

For each of the surface-treated steel plates obtained, the amount of Cr attached to the metal Cr layer per single side of the steel plate and the amount of Cr attached to the oxidized Cr layer per single side of the steel plate were measured by the aforementioned method. Similarly, the C atomic ratio of the metal Cr layer was measured by the aforementioned method. In addition, the "C atomic ratio" of the metal Cr layer shown in Tables 4 and 5 is a value expressed by the atomic ratio of the C content in the metal Cr layer relative to the Cr. In addition, for each of the surface-treated steel plates obtained, the water contact angle, the amount of adsorbed elements, and the atomic ratio of Ni in the outermost surface were measured by the following method. The measurement results are shown in Tables 4 and 5.

(Water contact angle) The water contact angle was measured using an automatic contact angle meter CA-VP manufactured by Kyowa Interface Science Co., Ltd. The surface temperature of the surface treated steel plate was set to 20℃±1℃, and distilled water at 20±1℃ was used. Distilled water was dripped onto the surface of the surface treated steel plate in a droplet volume of 2μl. After 1 second, the contact angle was measured using the θ/2 method, and the average of the contact angles of 5 drops was taken as the water contact angle.

(Amount of adsorbed elements) The total atomic ratio of K, Na, Mg and Ca adsorbed on the surface of the surface treated steel plate relative to Cr was measured by XPS. No sputtering was performed during the measurement. The atomic ratio was quantified by the relative sensitivity coefficient method from the integrated intensity of the narrow spectrum of K2p, Na1s, Ca2p, Mg1s and Cr2p on the outermost surface of the sample, and (K atomic ratio + Na atomic ratio + Ca atomic ratio + Mg atomic ratio) / Cr atomic ratio was calculated. XPS measurement was performed using a scanning X-ray photospectrometer PHI X-tool manufactured by Ulvac-Phi. The X-ray source was monochromatic AlKα radiation, the voltage was 15kV, the beam diameter was 100μmφ, and the take-off angle was 45°.

(Atomic ratio of Ni on the outermost surface) The atomic ratio of Ni content on the outermost surface of the surface treated steel plate relative to Cr was measured by XPS. No sputtering was performed during the measurement. The atomic ratio was quantitatively determined by the relative sensitivity coefficient method from the integrated intensity of the narrow spectrum of Ni2p and Cr2p on the outermost surface of the sample, and the Ni atomic ratio/Cr atomic ratio was calculated. XPS measurement was performed using a scanning X-ray photospectrometer PHI X-tool manufactured by Ulvac-Phi. The X-ray source was monochromatic AlKα radiation, the voltage was 15kV, the beam diameter was 100μmφ, and the take-off angle was 45°.

Furthermore, the film wetting and adhesion, coating secondary adhesion, and weldability of the obtained surface treated steel plates were evaluated by the following methods. The evaluation results are recorded in Tables 4 and 5.

(Sample preparation) Laminated steel plates were prepared in the following order as samples for evaluation of film corrosion resistance and film wettability.

On both sides of the obtained surface treated steel plate, a polyethylene terephthalate film with an isophthalic acid copolymer having a stretching ratio of 3.1×3.1, a thickness of 25μm, a copolymerization ratio of 12 mol%, and a melting point of 224°C was laminated to produce a laminated steel plate. The aforementioned lamination is carried out under the condition that the crystallization degree of the resin film is less than 10%. Specifically, the steel plate feeding speed is 40m/min, the nip length of the rubber roller is 17mm, and the time from pressing to water cooling is 1sec. In addition, the crystallization degree of the resin film is obtained by the density gradient tube method according to JIS K7112. In addition, the nip length refers to the length of the portion where the rubber roller and the steel plate meet in the conveying direction.

In addition, a coated steel plate was prepared by the following procedure as a sample for evaluation of the corrosion resistance of the coating and the secondary adhesion of the coating.

The surface of the surface treated steel plate was coated with epoxyphenol coating and dried at 210°C for 10 minutes to produce a coated steel plate. The coating adhesion was 50 mg/dm ² .

(Film Corrosion Resistance, Coating Corrosion Resistance) A cross cut was made on the film surface of the laminated steel plate and the coating surface of the coated steel plate using a cutter to reach the bottom metal (steel plate). The laminated steel plate and the coated steel plate cut into the cross cut were immersed in a 55°C test solution containing a mixed aqueous solution of 1.5% by mass of citric acid and 1.5% by mass of salt for 96 hours. After immersion, washing and drying, cellophane adhesive tape was attached to the film surface of the laminated steel plate and the coating surface of the coated steel plate, and the tape was peeled off by pulling it off. For film corrosion resistance, the film peeling width (the total width of the left and right sides extending from the cut part) of the laminated steel plate is measured at any 4 locations of the cross cut part, and the average value of the 4 locations is calculated as the corrosion width. For coating corrosion resistance, the film peeling width (the total width of the left and right sides extending from the cut part) of the coated steel plate is measured at any 4 locations of the cross cut part, and the average value of the 4 locations is calculated as the corrosion width. Film corrosion resistance and coating corrosion resistance are evaluated based on the following 4 criteria. In practice, an evaluation of 1 to 3 indicates excellent corrosion resistance. 1: Corrosion width less than 0.3mm 2: Corrosion width greater than 0.3mm but less than 0.5mm 3: Corrosion width greater than 0.5mm but less than 1.0mm 4: Corrosion width greater than 1.0mm

(Film Wet Adhesion) Film wet adhesion is evaluated by using the above laminated steel plate through a 180° peeling test in a steaming environment at a temperature of 130°C and a relative humidity of 100%. The specific procedure is as follows.

First, cut out 3 test pieces with the surface as the object surface and 3 test pieces with the back as the object surface from each of the above-mentioned laminated steel plates, a total of 6 test pieces. The size of each test piece is 30 mm in width and 100 mm in length. Then, at a position 15 mm above the length direction of each test piece, leave the film on the object surface, and cut the film and the steel plate on the side opposite to the object surface. Fix the cut test piece from the bottom to 15 mm in the length direction of the test piece in a manner that the steel plate is perpendicular to the ground, and hang down the film on the object surface in a connected state at a portion with a width of 30 mm and a length of 15 mm above the cutting position. Next, install a 100g hammer at the 30mm wide and 15mm long part of the hanging part.

The test piece in this state was placed in a steaming environment at a temperature of 130°C and a relative humidity of 100% for 30 minutes, and then opened to the atmosphere. The length of the film peeling off from the surface-treated steel plate on the target surface was taken as the film peeling length. For each laminated steel plate, the average value of the film peeling length in the 6 test pieces was obtained. The average value of the film peeling length obtained was used to evaluate the film wettability and adhesion according to the following 4 criteria. In practice, when the evaluation is 1 to 3, it means that the film wettability and adhesion are excellent. 1: Peeling length less than 20mm 2: Peeling length more than 20mm and less than 40mm 3: Peeling length more than 40mm and less than 60mm 4: Peeling length more than 60mm

(Secondary adhesion of coating) Two coated steel plates made under the same conditions were sandwiched with nylon adhesive films, laminated with the coated surfaces facing each other, and bonded under the conditions of pressure of 2.94×10 ⁵ Pa, temperature of 190°C, and pressing time of 30 seconds. Then, the test pieces were divided into 5 mm wide test pieces. The divided test pieces were immersed in a 55°C test solution composed of a mixed aqueous solution containing 1.5 mass% citric acid and 1.5 mass% salt for 168 hours. After immersion, the two steel plates of the divided test pieces were peeled off with a tensile testing machine, and the tensile strength when pulled off was measured. The average value of the three test pieces is evaluated according to the following criteria. In practice, when the evaluation is 1 to 3, it means that the secondary adhesion of the coating is excellent. 1: 2.5 kgf or more 2: 2.0 kgf or more and less than 2.5 kgf 3: 1.5 kgf or more and less than 2.0 kgf ×: less than 1.5 kgf

(Weldability) For the surface treated steel sheet obtained, after heat treatment at 210℃×10 minutes, assuming a coating baking step, two samples were clamped by a DR type 1 mass% Cr-Cu electrode (electrode processed with a tip diameter of 2.3mm and a curvature R40mm) and energized under the following conditions. ･Transistor power supply manufactured by Amada Miyachi: MDA-8000A ･Welding head: AH-200 ･Pressure: 40kgf ･Electrification time: 1.6msec. (slope 0.2msec.) ･Waveform: rectangular wave

The lower limit current obtained from sufficient strength and the upper limit current without scattering are used to find the appropriate current range (= upper limit current - lower limit current), and the evaluation is performed according to the following 4 criteria. In practice, when the evaluation is 1~3, it means that the weldability is excellent. 1: 2.5kA or more 2: 2.0kA or more, but less than 2.5kA 3: 1.5kA or more, but less than 2.0kA 4: less than 1.5kA

From the results shown in Table 4 and Table 5, it can be seen that the surface treated steel sheets meeting the conditions of the present invention, even though they are not manufactured using hexavalent chromium, still have excellent film corrosion resistance, coating corrosion resistance, film wetting and adhesion, coating secondary adhesion, and weldability.

Claims (10)

A surface treated steel plate comprises a steel plate, a Ni-containing layer disposed on at least one surface of the steel plate, a metal Cr layer disposed on the Ni-containing layer, and an oxidized Cr layer disposed on the metal Cr layer, wherein the water contact angle is less than 50°, and the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface relative to Cr is less than 5.0%. In the surface treated steel plate of claim 1, the Ni adhesion amount of the aforementioned Ni-containing layer is not less than 200 mg/ ^m2 and not more than 2000 mg/ ^m2 per single side of the aforementioned steel plate. In the surface treated steel sheet of claim 1 or 2, the Cr adhesion amount of the metal Cr layer is not less than 2 mg/m ² and not more than 40 mg/m ² per single side of the steel sheet. The surface treated steel plate of claim 1 or 2, wherein the Cr adhesion amount of the aforementioned oxidized Cr layer is not less than 0.1 mg/ ^m2 and not more than 15.0 mg/ ^m2 per single side of the aforementioned steel plate. The surface treated steel plate of claim 3, wherein the Cr adhesion amount of the aforementioned oxidized Cr layer is not less than 0.1 mg/ ^m2 and not more than 15.0 mg/ ^m2 per single side of the aforementioned steel plate. The surface treated steel plate of claim 1 or 2, wherein the atomic ratio of Ni to Cr in the surface of the surface treated steel plate is less than 100%. The surface treated steel plate of claim 3, wherein the atomic ratio of Ni to Cr in the surface of the surface treated steel plate is less than 100%. The surface treated steel plate of claim 4, wherein the atomic ratio of Ni to Cr in the surface of the surface treated steel plate is less than 100%. The surface treated steel plate of claim 5, wherein the atomic ratio of Ni to Cr in the surface of the surface treated steel plate is less than 100%. A method for manufacturing a surface-treated steel plate comprises a steel plate, a Ni-containing layer disposed on at least one surface of the steel plate, a metal Cr layer disposed on the Ni-containing layer, and an oxidized Cr layer disposed on the metal Cr layer. The method comprises the following steps: preparing an electrolyte containing trivalent chromium ions, subjecting the steel plate having the Ni-containing layer on at least one side to cathodic electrolysis in the electrolyte. The invention further comprises a step of cathodic electrolysis treatment, and a step of washing the steel plate after the cathodic electrolysis treatment at least once, wherein in the electrolyte preparation step, a trivalent chromium ion source, a carboxylic acid compound, and water are mixed, and the electrolyte is prepared by adjusting the pH to 4.0-7.0 and the temperature to 40-70°C. In the washing step, at least in the last washing, water with a conductivity of less than 100μS/m is used.