BRPI0819870B1 - STEEL PLATE FOR FUEL TANKS AND PRODUCTION METHOD OF THE SAME - Google Patents

Abstract

Fuel tank steel sheet and method of manufacture thereof The present invention relates to a fuel tank steel sheet which includes a zn-ni alloy electrodeposition layer disposed on at least one of its surfaces and a coating chromate arranged on the alloy electroplating layer. The change in the amount of chromium in the chromate coating immersed in boiling water for 30 minutes is 2% or less of the amount of chromium in the chromate coating not immersed in boiling water. A value l indicating the color tone of the steel plate is 55 or more. The difference between the maximum and minimum values of the l value is 3 or less. The chroming solution is applied to the zn-ni alloy electroplating layer and then heated. the chroming solution contains chromic acid having a mass ratio (trivalent chromium) / (total chromium) of more than 0.5, phosphoric acid having a mass ratio (phosphoric acid) / (total chromium) of 0.1 to 5 , 0, and an organic reducing agent. The steel sheet looks good and has excellent long-term corrosion resistance for fuels such as gasoline, alcohols and gasoline-alcohol blend.

Description

[001] The present invention relates to steel plates for fuel tanks. The present invention relates in particular to a steel sheet for fuel tanks used in environments containing gasoline that contains a small amount of water or environments such as environments where formic acid is produced due to the degradation of gasoline, containing metallic organic acids highly corrosive and also refers to the steel sheet production method.

Background of the Technique [002] Conventional gas tanks for automobiles or motorcycles were generally produced from steel plates coated with a tin (Sn)-lead (Pb) alloy containing 20% by weight or less of lead (Pb) as described, for example, in Examined Japanese Patent Application Publication No. 5761833 or multilayer coated steel sheets including a nickel (Ni) electrogalvanization layer and a SnPb layer formed by hot dipping.

[003] Steel sheets coated with Sn-Pb alloy have excellent working capacity and excellent resistance to chemicals such as gasoline. However, the coating layers are soft and vulnerable and have no galvanic action on iron (Fe) because the coating layers are electrochemically more noble than Fe. Therefore, in the case of gas tanks made of alloy-coated steel sheets Sn-Pb are used in environments containing water, the presence of defects such as punctures or fractures causes problems due to the fact that gasoline drips through the

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2/26 holes drilled in the plates and / or the combustion filters are clogged with red rust caused by corrosion.

[004] In consideration of the deteriorating oil situation, the following substances are used as automobile fuels in some cases: an alcohol such as methyl alcohol, ethyl alcohol, or methyl-t-butyl ether; their derivatives (hereinafter also referred to as alcohols); and mixtures of alcohols and gasoline. Sn-Pb alloy coated steel sheets are readily corroded with the water contained in the alcohols; alcohol oxides, such as formaldehydes, acetaldehydes, formic acid, and acetic acid; and impurities, and therefore are unsuitable for tanks for such fuels.

[005] After the Sn-Pb alloy coated steel sheets are pressed, the Sn-Pb alloy coated steel sheets can be reduced locally in corrosion resistance due to peeling or skinning of the coating even if the coated steel sheets are Sn-Pb alloy are painted, because the coating layers have no galvanic action on Fe. In steel sheets coated with Sn-Pb alloy, the secondary adhesion between the coating layers and the paint layers is small. Therefore, the layers of paint can be peeled off, for example, by the impact caused by stones that reach the fuel tanks of motorcycles in motion. This causes red rust or paint bubbles.

[006] On the other hand, in view of environmental problems, the use of harmful substances such as Pb and hexavalent chromium (Cr) tends to be avoided. Unexamined Japanese Patent Application Publication No. 2005-290556 describes a chrome steel sheet which is a Pb-free steel sheet for fuel tanks in which the dissolution of hexavalent Cr is inhibited.

[007] This steel plate includes an electroplating layer

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3/26 of Zn-Ni alloy on at least one of its surfaces and a chromate coating disposed on the electrodeposition layer. The electrodeposition layer contains 5 to 30% by mass of Ni and the amount of electrodeposition layer for a single surface is 1 to 40 g / m ² . The change in the amount of chromium in the chromate coating immersed in boiling water for 30 minutes is 2% or less of the amount of chromium in the chromate coating not immersed in boiling water. The chromate coating is obtained in such a way that a chromatic solution is applied to the electrodeposition layer and then heated. The chromatic solution contains chromic acid having a mass ratio (trivalent chromium / total chromium) greater than 0.5, phosphoric acid having a mass ratio (phosphoric acid) / (total chromium) of 0.1 to 5.0 and a organic reducing people.

[008] In this steel plate, the chromate coating has a color interference. This steel sheet has portions of surface having different shades of color and is inferior in appearance.

[009] It is an objective of the present invention to provide a steel sheet for fuel tanks and to provide a method for producing the steel sheet. The steel plate is lead-free and has a good appearance and excellent long-term corrosion resistance for fuels such as gasoline, alcohols, and gasoline mixed with alcohol.

Description of the Invention [1] A steel sheet for fuel tanks includes an electroplating layer of Zn-Ni alloy disposed on at least one surface of the steel sheet and a chrome coating disposed on the electroplating layer of the alloy. The change in the amount of chromium in the chromate coating immersed in boiling water for 30 minutes is 2% or less of the amount of chromium in the chromate coating not immersed in boiling water. An L value indicating the

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4/26 color of the steel sheet is 55 or more. The difference between the maximum and the minimum of the L value is four or less.

[2] In the steel plate for fuel tanks specified in item [1], the electrodeposition layer of the alloy includes a Zn oxide sublayer that is located at the top and has a thickness of 20 nm or less and a content of P of an atomic percent or less.

[3] In the steel plate for fuel tanks specified in item [1] or [2], the electrodeposition layer of the alloy has a surface with an average grain size of 0.8 mm or more.

[4] The method of producing a steel sheet for fuel tanks includes the formation of an electroplating layer of Zn-Ni alloy on at least one surface of the steel sheet, applying a chromatic solution to the electroplating layer of the turns on, and then heating the chromatic solution. The electrodeposition layer of the alloy includes a Zn oxide sublayer that is located at the top and has a thickness of 20 nm or less and a P content of one atomic percent or less. The chromatic solution contains chromic acid having a mass ratio (trivalent chromium) / (total chromium) of more than 0.5, phosphoric acid having a mass ratio (phosphoric acid) / (total chromium) from 0.1 to 5, 0, and an organic reducing agent.

[5] The method of producing a steel sheet for fuel tanks includes the formation of an electroplating layer of Zn-Ni alloy on at least one surface of the steel sheet, applying a chromate coating solution to the electroplating layer. alloy, and then heat the chromatic solution. The alloy electrodeposition layer has a surface with an average grain size of 0.8 mm or more. The chromatic solution

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5/26 contains chromic acid having a mass ratio (trivalent chromium) / (total chromium) of more than 0.5, phosphoric acid having a mass ratio (phosphoric acid) / (total chromium) from 0.1 to 5, 0, and an organic reducing agent.

Brief Description of the Drawings [0010] Figure 1 is a photograph of a surface of an electroplating layer of Zn-Ni alloy having a coating grain size of 1.0 mm (Example 1).

[0011] Figure 2 is a photograph of a surface of an electroplating layer of Zn-Ni alloy having a coating grain size of 0.3 mm (Example 1).

Best Mode for Carrying Out the Invention [0012] To prevent Zn coated steel sheets from rusting, a technique for forming a chromate coating on a chrome plating layer has been widely used. The inventors studied that steel sheets coated with Zn including chromate coatings formed as described above are used for fuel tanks. As a result, the inventors obtained the findings below.

(A) Among Zn-coated steel sheets, a steel sheet with Zn-Ni alloy electrodeposition has excellent corrosion resistance for fuels such as gasoline, alcohols, and gasoline mixed with alcohol.

(B) A chromate coating is able to safely prevent the dissolution of Cr even if the chromate coating is used in environments containing gasoline that contains a small amount of water or in environments, such as environments where formic acid is produced due to degradation of the gasoline, containing highly corrosive metallic organic acids. The change in the amount of chromium in the chromate coating immersed in water

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6/26 boiling for 30 minutes is 2% or less of the chromium in the chromate coating not immersed in the boiling water.

(C) A steel sheet can be prevented from deteriorating its appearance by setting an L value indicating a shade of color and the difference between the maximum and minimum of the L value, the L value being an index used to assess the degree irregularity of the steel sheet surface.

(D) To adjust the L value of a steel sheet to a predetermined value or less and to adjust the difference between the maximum and minimum values of the L value to a predetermined value or less, it is preferred that the alloy coating layer of Zn-Ni is located at the top and has a thickness of 20 nm or less and a P content of one atomic percent or less.

(E) To adjust the L value of a steel sheet to a predetermined value or less and to adjust the difference between the maximum and minimum of the L value to a predetermined value or less, it is preferred that after an electrodeposition layer is formed of Zn-Ni alloy so that the average grain size of the Zn-Ni alloy is 0.8 pm or more, a predetermined chromate coating is formed.

[0013] The present invention was made on the basis of the above findings. The present invention will now be explained in detail.

1) Zn-Ni alloy electrodeposition layer [0014] A Zn-Ni alloy electrodeposition layer is effectively prevented from being corroded due to water, alcohol oxides such as formaldehyde, acetaldehyde, formic acid, and / or acetic acid, and containing impurities in fuels, such as alcohols and gasoline mixed with alcohol, different from conventional gasoline. Therefore, at least one surface of a steel sheet that contacts a fuel needs to include the electroplating layer of Zn-Ni alloy.

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7/26 [0015] The electroplating layer of the Zn-Ni alloy is not particularly limited and preferably contains 5 to 30% by mass of Ni and the amount of electroplating layer of the ZnNi alloy per face is 1 to 40 g / m ² .

[0016] When the Ni content in the coating layer is less than five percent by mass, corrosion of coating defects cannot be avoided and, therefore, sufficient corrosion resistance cannot be achieved in some cases. At the same time, when the Ni content is greater than 30 percent by weight, the coating layer has high hardness and therefore cracks are formed in the coating layer during pressing to cause corrosion. Therefore, the Ni content of the coating layer is preferably 5 to 30% by weight. When the amount of surface coating layer is less than 1 g / m ² , sufficient corrosion resistance cannot be achieved in some cases. At the same time, when its quantity is greater than 40 g / m ² , the pressing work capacity can be reduced. Therefore, the amount of the surface coating layer is preferably 1 to 40 g / m ² .

[0017] The electroplating layer of the Zn-Ni alloy preferably includes a zinc oxide sublayer that is located at the top and has a thickness of 20 nm or less and a P content of one atomic percent or less. This allows the appearance of the electroplating layer of Zn-Ni alloy subjected to chrome plating to be kept stable.

[0018] In the present invention, the L value that indicates the color tone of the steel sheet is 55 or more and the difference between the maximum and minimum values of the L value is 4 or less. To achieve these requirements, the Zn-Ni alloy electrodeposition layer preferably has surface conditions such that the thickness of the oxide sublayer

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8/26 Zn is 20 nm or less and the P content of the zinc oxide sublayer is 1% atomic or less. When the thickness of the Zn oxide sublayer is greater than 20 nm and the P content of the sublayer is greater than 1% atomic, light interference is likely to occur because the coating layer has a blackish tone and extremely fine irregularities are formed in the coating crystals, so that light dispersion is avoided. The oxide layer is preferably less than 20 nm thick.

[0019] To adjust the thickness of the Zn oxide sublayer of the Zn-Ni alloy electrodeposition layer to 20 nm or less, the electrodeposition time for chrome plating is preferably 120 hours or less. This is because when this time is greater than 120 hours, the thickness of the oxide layer exceeds 20 nm and therefore the coating layer has a blackish tone, resulting in an increase in the difference between color tones.

[0020] The thickness of an oxide or hydroxide layer can be determined by a combination of Auger electronic spectroscopy (AES) and spraying of Air ions. After the layer is sprayed to a predetermined depth, the composition of a region located at the depth it can be determined in such a way that the peak intensity of each target element is corrected with a relative sensitivity factor. The O content originating from the oxide or hydroxide peaks reaches a certain depth at the maximum (its content can reach the maximum at the top of the layer) and then decreases to a constant value. The thickness of the oxide layer can be determined in such a way that the next time is converted to the base of the spray rate of a SiO2 layer having a known thickness: the time it takes to spray the oxide layer to a portion that has a of O equal to half the sum of the maximum O content and such constant value and that

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9/26 is located under a position in which the O content reaches its maximum value.

[0021] The P content can be determined by a combination of X-ray photoelectronic spectroscopy (XPS) and air ion spraying. The measurement is performed in the same way as above, while the P concentration profile is determined at depth direction. The value of the P concentration that reaches its maximum value at a depth corresponding to the thickness of the oxide layer is determined to be the P content of the oxide layer.

[0022] The electroplating layer of Zn-Ni alloy preferably has a surface with an average grain size of 0.8 mm or more. This allows the appearance of the electroplating layer of Zn-Ni alloy subjected to chrome plating to be kept stable.

[0023] In the present invention, the value L, which indicates the color tone of the steel sheet, is 55 or more and the difference between the maximum and minimum values of the value L is 4 or less. To achieve these requirements, the Zn-Ni alloy electrodeposition layer preferably has surface conditions such that the thickness of the Zn oxide sublayer is 20 nm or less and the P content of the Zn oxide sublayer is 1% atomic or less or the surface of the Zn-Ni alloy electrodeposition layer is coated with a chromate coating that preferably has a surface with a grain size of 0.8 mm or more. When the grain size is less than 0.8 mm, light interference is likely to occur because the coating layer has a blackish tint and light scattering is avoided. At the same time, when its grain size is 0.8 mm or more, light interference is avoided because the coating layer is whitish in color and light scattering is likely to occur.

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10/26 [0024] Although the upper limit of its grain size is not limited, it is probably difficult to form Zn-Ni alloy grains having a size of 2 mm or more by an electroplating process. In the electrodeposition process, to reach a large grain size, the number of sites generating precipitate cores needs to be reduced so that the grains are greatly developed. To ensure this, the current density used for electrodeposition needs to be low. However, a reduction in current density leads to a reduction in the speed of a coating line or a reduction in production capacity and is therefore not preferred.

[0025] The average grain size can be determined in such a way that an electronic scanning microphotograph with a magnification of 3000-20000x is observed, the number of grains per unit area is counted, and the equivalent circle diameter of the grains is then determined.

2) Chromate coating [0026] The change in the amount of Cr in the chromate coating immersed in boiling water for 30 minutes is 2% or less of the amount of Cr in the chromate coating not immersed in boiling water as described above. The chromate coating is able to safely prevent the dissolution of Cr even if the chromate coating is used in environments containing highly corrosive metallic organic acids; therefore, the chromate coating has excellent resistance to fuels such as gasoline.

[0027] The change in the amount of Cr in the chromate coating immersed in boiling water for 30 minutes can be determined by a boiling water resistance test specified in JIS K 5400-1990 8.20 in such a way that the amount of Cr in the chromate coating does not immersed in boiling water and the amount of Cr in the coating

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11/26 chromate immersed in boiling water for 30 minutes is measured by X-ray fluorescence spectrometry. In X-ray fluorescence spectrometry, the amount of Cr is determined from a calibration curve between the amount of Cr and the number of Cr count determined using standard samples in which the amount of Cr is known.

[0028] The dissolution of hexavalent Cr is evaluated by a method in which the type of liquid for dissolution, the temperature of dissolution, and the time of dissolution are defined and the concentration of Cr dissolved in the liquid is used for evaluation as specified in a Volvo Leach test (Volvo Standard News 1990.10) or a method in which the amount of Cr dissolved by alkaline degreasing is used for evaluation as specified in Unexamined Japanese Patent Application Publication No. 10-46353. In the present invention, the dissolution of hexavalent Cr is evaluated from the change in the amount of Cr in the chromate coating immersed in boiling water for 30 minutes. This is because the amount of dissolved Cr reaches a constant value after the chromate coating is immersed in boiling water for 30 minutes and there is a good correlation between its quantity and the amount of remaining chromate coating.

[0029] In the chromate coating according to the present invention, the L value, which indicates the color tone of the steel sheet, is 55 or more and the difference between the maximum and minimum values of the L value is 4 or less and preferably 3 or any less. Chromed steel sheets, particularly chromed steel sheets with trivalent chromium, commonly exhibit color interference. Color interference ideally depends on the thickness of an oxide coating and the equation reflected light + transmitted light = influence of white (complementary) light. The variation of an oxide coating on a plate

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12/26 steel causes irregularity in color interference. This leads to deterioration in appearance. In the steel sheet according to the present invention, the amount of chromate coating is preferably within a range of 10 to 50 mg / m ² on the base of metallic Cr and the oxide coating is thick enough to cause irregularity in the color interference. Therefore, in the present invention, the L value, which indicates the color tone of the steel sheet, is 55 or more and the difference between the maximum and minimum values of the L value is 4 or less and preferably 3 or less. An increase in the L value increases whiteness and a reduction in the L value increases blackness. Since the difference between the maximum and minimum values of the L value is 4 or less, the difference between color tones can be minimized and deterioration of appearance can be avoided. The reason that the lower limit of the L value is limited to 55 or more is that an increase in blackness increases the difference between color tones. When the L value is less than 55, the difference between the color tones is conspicuous even if the difference between the maximum and minimum values of the L value is 4 or less. This leads to deterioration in appearance. The L value can be measured by the method (for example, a multi-light source spectrocolorimeter, MSC -1S-2B, produced by Suga Test Instruments Co., or similar) specified in JIS Z 8722.

[0030] The chromate coating is not particularly limited except that the treated chromate coating has a predetermined color tone and the amount of chromium dissolved in the treated chromate coating is within a predetermined range. The chromate coating can be formed in such a way that, for example, the chrome plating solution described below is applied to the electrodeposition layer of the Zn-Ni alloy and then heated. The amount of chromate coating is preferably 10 to 50 mg / m ² on the base of metallic Cr. This is because a corrosion resistance

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Sufficient 13/26 cannot be achieved when its quantity is less than 10 mg / m ² and a high cost is incurred when its quantity is greater than 50 mg / m ² .

3) Production method [0031] A steel plate production method according to the present invention includes a step of forming the electroplating layer of the Zn-Ni alloy on at least one surface of the steel sheet and a step of forming the coating chromate in the electrodeposition layer of the alloy.

[0032] The production method is not particularly limited except that the change in the amount of chromium in the chromate coating immersed in boiling water for 30 minutes is 2% or less of the amount of chromium in the chromate coating not immersed in boiling water, the value L , which indicates the color tone of the steel plate, is 55 or more and the difference between the maximum and minimum values of the L value is 4 or less.

[0033] For example, chromatization to allow the amount of dissolved chromium to be within if a predetermined range can be performed in such a way that the chromatization solution is applied to the layer and then heated. The chromatization solution contains chromic acid having a mass ratio ((trivalent chromium) / (total chromium)) of trivalent chromium to total chromium of more than 0.5, phosphoric acid having a mass ratio ((phosphoric acid) / (total chromium)) of phosphoric acid for total chromium from 0.1 to 5.0, and an organic reducing agent.

[0034] A chromate coating produced as described above has a problem by the fact that unlike hexavalent chromate coating, this chromate coating is unlikely to look good. In particular, a Zn-Ni coating and a chromate coating treated in different lines

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14/26 generally have defective appearances. The inventors carried out investigations and found that a surface without a chromate coating is maintained in appropriate conditions. One way is as follows: an electroplating layer of Zn-Ni alloy is formed to include a Zn oxide sublayer that is located at the top and has a thickness of 20 nm or less and a P content of 1 Atomic% or less before forming the chromate coating. It has been confirmed that such a surface condition is effective in achieving a good appearance.

[0035] Furthermore, it has been found that a good appearance can be achieved in such a way that an electroplating layer of Zn-Ni alloy is formed so as to have a surface having an average grain size of 0.8 mm and a coating chromate is then formed. [0036] The amount of Zn oxide sublayer, which is located on top of the Zn-Ni alloy electrodeposition layer, and the P content in the Zn oxide sublayer can be adjusted and / or the average grain size of the Electroplating layer of the Zn-Ni alloy can be adjusted.

[0037] The conditions for forming the electroplating layer of the Zn-Ni alloy are not particularly limited. The coating layer preferably contains 5 to 30% by weight and the amount of the coating layer is preferably 1 to 40 g / m ² . [0038] It is an effective way to implement the present invention that a coating is placed on the final section of an electroplating line and the chrome plating is performed immediately after the chromate coating is formed.

[0039] The presence of a large amount of P in the oxide layer is disadvantageous in appearance. Therefore, if degreasing and / or surface conditioning is carried out prior to chrome plating, fine degreasing or

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15/26 surface conditioning solution or powerful cleaning is performed so that no P remains.

[0040] The technique of suppressing the P content is not particularly limited and a common technique can be used. For example, a powerful cleaning is performed after degreasing or conditioning the surface or the concentration of a used treatment solution is reduced.

[0041] After the Zn-Ni alloy electrodeposition layer is formed, the chrome plating solution is applied to the Zn-Ni alloy electrodeposition layer. In the present invention, the chromate coating needs to be formed in the electrodeposition layer of Zn-Ni alloy so that the change in the amount of Cr in the chromate coating immersed in boiling water for 30 minutes is 2% or less. Therefore, the chrome plating solution, which contains chromic acid having a mass ratio ((trivalent chromium) / (total chromium)) of trivalent chromium to total chromium of more than 0.5, phosphoric acid having a mass ratio (( phosphoric acid) / (total chromium)) of phosphoric acid for total chromium from 0.1 to 5.0, and the organic reducing agent, can be applied to the electroplating layer of Zn-Ni alloy and then heated.

[0042] The hexavalent Cr contained in the chrome plating solution reacts with the organic reducing agent during heating and, therefore, reduced in trivalent Cr. When the mass ratio of trivalent Cr to total Cr is 0.5 or less, the amount of hexavalent Cr is excessive and therefore hexavalent Cr remains in the chromate coating after heating. Therefore, if the chromate coating is immersed in boiling water, the hexavalent Cr is dissolved; therefore, the change in the amount of Cr in the chromate coating immersed in boiling water for 30 minutes exceeds 2% and a high corrosion resistance for fuels such as gasoline cannot be achieved.

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16/26

When the mass ratio of phosphoric acid to total chromium is less than 0.1, the trivalent Cr is polymerized in a precipitated gel and the chromate coating cannot maintain its properties. At the same time, when this ratio is greater than 5.0, the phosphoric acid remains in the chromate coating; then, the phosphoric acid is dissolved to cause corrosion and / or blackening.

[0043] The organic reducing agent, which is contained in the chrome plating solution, is preferably at least one selected from the group consisting of diols and sugars. Among the diols, particularly preferred are ethylene glycol, propylene glycol, trimethylene glycol, or 1,4butane diol. Among sugars, advantageously suitable are glycerin, polyethylene glycol, sucrose, lactose, sucrose, glucose, or fructose.

[0044] The organic reducing agent is preferably contained in the chrome plating solution so that the mass ratio of the organic reducing agent to the total Cr is 0.1 to 0.4. This is because the chrome plating solution did not have a sufficient reduction effect when its mass ratio is less than 0.1 and the stability of the chrome plating solution cannot be maintained when its mass ratio is greater than 0.4. To increase the stability of the chrome plating solution, the organic reducing agent is preferably added to the chrome plating solution immediately before using the chrome plating solution.

[0045] In order to increase corrosion resistance, the chrome plating solution may contain an inorganic inhibitor as required. Examples of inorganic inhibitors include inorganic colloids such as silica, ZrO2, TiO2, zirconium sulfate, and aluminum bisphosphate and heteropoly acids such as phosphomolybdic acid, silicotungstenic acid, and phosphovanadomolybdic acid. The presence of the inorganic inhibitor in the chrome plating solution inhibits the reaction of the organic reducing agent with hexavalent Cr and therefore promotes

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17/26 dissolution of hexavalent Cr when the chromate coating is immersed in boiling water; then the content of inorganic inhibitor is preferably adjusted so that the mass ratio of the inorganic inhibitor to hexavalent Cr is less than 0.05. The reason why the inorganic inhibitor slows down the reaction rate of the organic reducing agent with hexavalent Cr is unclear but it is probably because the inorganic inhibitor is ionized in the chrome plating solution or interacts with hexavalent Cr after the inorganic inhibitor is dispersed in the chrome plating.

[0046] In order to increase the reactivity of the chrome plating solution with the electroplating layer of Zn-Ni alloy, the chrome plating solution may contain an acid such as hydrofluoric acid, sulfuric acid or hydrochloric acid.

[0047] To inhibit the dissolution of Cr from the chromate coating, the chrome plating solution may also contain a water-soluble or water-dispersible polymeric compound. Examples of water-soluble or water-dispersible polymeric compounds include polyvinyl alcohol, polyacrylic acids, polyacrylic amides, epoxy ester polymers, melamine alkyl resins, natural polymeric compounds such as starch and casein, partial alkyl silicate hydrolysates, partial alkyl phosphate hydrolysates, and silanes such as silane and epoxy silane emulsifying agents. The water-soluble or water-dispersible polymeric compound has a function as a protective layer that inhibits the dissolution of Cr from the chromate coating and protects the chromate coating from external mechanical shock. The water-soluble or water-dispersible polymeric compound has terminal function groups acting as reducers for hexavalent Cr ions. Therefore, to ensure the stability of the chrome plating solution, the content of the compound is preferably adjusted so that the mass ratio of the compound to the hexavalent Cr is lower

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18/26 than 0.05.

[0048] The chrome plating solution is applied to the layer and then heated. The chrome plating solution is preferably heated so that the temperature of the steel sheet is 120 ° C or higher. When its temperature is less than 120 ° C, the reduction of Cr does not happen sufficiently and therefore an increased amount of Cr can be dissolved from the chromate coating when the chromate coating is immersed in boiling water.

[0049] Before the chrome plating solution is applied to the layer, an aqueous solution containing Ti colloid is applied to the layer then dried, with which the dissolution of Cr from the chromate coating can be inhibited. This is probably because the colloid of Ti absorbed in the electrodeposition layer of Zn-Ni alloy acts as a reactive site with the chrome plating solution, which is acidic, and therefore the reduction of hexavalent Cr in trivalent Cr is promoted during heating.

[0050] The aqueous solution containing Ti colloid preferably has a concentration of Ti colloid of 1 to 10 ppm by volume and a pH 7.5 to 10 and is preferably applied in the layer at a temperature of 40 ° C to 60 ° C for 1 to 30 seconds.

[0051] The steel sheet for fuel tanks, according to the present invention, preferably contains, for example, in% by mass, 0.0007% to 0.0050% of C, 0.5% or less of Si, 2 , 0% or less of Mn, 0.1% or less of P, 0.015% or less of S, 0.01% to 0.20% of Al, 0.01% or less of N, 0.005% to 0, 08% Ti, 0.001% to 0.01% B, the rest being Fe and the inevitable impurities. The steel sheet has excellent deep drawing capacity.

[0052] The reason for limiting each component is described below.

C: 0.0007% to 0.0050%

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19/26 [0053] C adversely affects the capacity of deep drawing and, therefore, its content is preferably 0.0050% or less. When its content is less than 0.0007%, the improvement in the deep drawing capacity is not achieved and an increase in the decarburization cost is caused. Therefore, the C content is preferably 0.0007% to 0.0050%.

Si: 0.5% or less [0054] Si has the function of increasing the strength of steel; then, the steel sheet can contain Si depending on the desired strength. However, a Si content exceeding 0.5% causes a reduction in the deep embossing capacity; then, the Si content is preferably 0.5% or less.

Mn: 2.0% or less [0055] Mn, like Si, has the function of increasing the strength of steel; then, the steel sheet may contain Mn depending on the desired strength. However, when its content is greater than 2.0%, a reduction in the capacity for deep drawing is caused; then the Mn content is preferably 2.0% or less.

P: 0.1% or less [0056] P precipitates at the grain edges to strengthen the grain edges, prevents welded portions from fracturing, and has the function of increasing the strength of the steel. However, when its content is greater than 0.1%, a reduction in the capacity for deep printing is caused; then the P content is preferably 0.1% or less. To ensure the prevention against fracture of welded portions, the P content is more preferably 0.01% to 0.05%.

S: 0.015% or less [0057] S adversely affects the deep drawing capacity and therefore its content is preferably 0.015% or less.

Al: 0.01% to 0.20%

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20/26 [0058] Al is used to deoxidize steel or to increase the yield of a carbonitride-forming element such as Ti. When its content is less than 0.01%, its effect is insufficient. At the same time, when its content is greater than 0.20%, its effect is saturated. Therefore, the Al content is preferably 0.01% to 0.20%.

N: 0.01% or less [0059] N adversely affects the deep drawing capacity and, therefore, its content is preferably 0.01% or less. Ti: 0.005% to 0.08% [0060] Ti forms a precipitate with C or N in the steel to reduce the amount of C or N solute and therefore has an effect of increasing the deep drawing capacity. When its content is less than 0.005%, its effect is small. At the same time, when its content is greater than 0.08%, its effect is saturated. Therefore, the Ti content is preferably 0.005% to 0.08%.

B: 0.001% to 0.01% [0061] B, like P, has the function of preventing welded portions from fracturing. When its content is less than 0.001%, its effect is small. At the same time, when its content is greater than 0.01%, a reduction in deep drawing capacity is caused. Therefore, the B content is preferably 0.001% to 0.01% and more preferably 0.001% to 0.004%.

[0062] The reason why B and P prevent welded fracture portions is probably as described below. The fracture of the welded portions is probably due to the weakening of the liquid metal which is caused in such a way that Cu, which is the main component of an electrode and / or Zn, which is a coating component, is liquefied during welding and enters the grain edges on steel to weaken grain edges. B and P precipitate at the grain edges and therefore prevent the welded portions from

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21/26 fracture.

[0063] The rest is Fe and the inevitable impurities. The content of each unavoidable impurity may be within the usual range. For example, the O content is 0.010% or less.

[0064] The steel plate preferably also contains 0.0005% to 0.0050% Nb in addition to the above components because of the increased deep drawing capacity.

Example 1 [0065] Electroplating steel sheets with Zn-Ni alloy (a Ni content of 12% by mass and an amount per surface of 20 g / m ² ) were prepared by a usual method using a steel sheet laminated to hot containing, in mass%, 0.0015% C, 0.01% Si, 0.08% Mn, 0.011% P, 0.008% S, 0.05% Al, 0.0019% of N, 0.035% of Ti, 0.003% of Nb, and 0.004% of B, the rest being Fe and the inevitable impurities.

[0066] In this operation, a Zn-Ni electrodeposition layer was formed on each plate in such a way that the current density used was varied and the speed of an electrodeposition line was varied on two levels: 90 mpm and 160 mpm. The surface of the Zn-Ni alloy electrodeposition layers had an average grain size of 1.0 or 0.3 mm. Figures 1 and 2 show one of the surfaces of the electrodeposition layer having an average grain size of 1.0 mm and one of the surfaces of the electrodeposition layer having an average grain size of 0.3 mm respectively. The average grain size was determined in such a way that an electronically scanned micrograph with a range of 3000-20000x of grain was observed, the number of grains per unit area was counted, and the diameter of the equivalent grain circle was then determined.

[0067] Some of the plates had their surface conditioned

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22/26 such that these plates were immersed in a solution of disodium hydrogen phosphate with a pH 10 to 50 ° C, with which an acid electrodeposition solution was neutralized. The resulting plates were washed with water, while a surface portion of each layer was formed in a Zn oxide sublayer containing P. The other plates were not conditioned.

[0068] The thickness of the oxide or hydroxide layer can be determined by a combination of Auger electronic spectroscopy (AES) and spraying of Air ions. Then, the content of each element in a portion of the surface was measured by AES and the layer was then sprayed to a predetermined depth, the content of the element in a surface portion of the resulting layer was measured by AES. This operation was repeated, while the distribution of the element was measured in the direction of the depth of the layer. The O content originating from the oxide or hydroxide peaks at a certain depth and then decreases to a constant value. The thickness of the oxide layer was determined to be the depth of the portion that had an O content equal to half the sum of the maximum O content with the constant value and which was located under a position where the O content reaches a value maximum. For pre-treatment, air spraying was carried out for 30 seconds, while a contamination layer was removed from a specimen surface.

[0069] Measurement by electronic x-ray spectroscopy (XPS) was performed in the same manner as above, while the profile of P concentration was determined in the direction of depth. The value of the P concentration that reached a maximum value at a depth corresponding to the thickness of the oxide layer was determined to be the P content of the oxide layer.

[0070] The plates were chromed immediately after

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23/26 coating (a lapse of 10 seconds), after a lapse of 100 hours from the coating in such a way that or after a lapse of 200 hours from the coating in such a way that the chrome plating solutions shown in Table 2 were applied to the electroplating layers of Zn-Ni alloy with coating cylinders. The resulting sheets were heated to the temperature shown in Table 2, were prepared as Samples ^Nos 1 to 9 having chromate coatings with a Cr amount shown in Table 2. The term heating temperature herein means the maximum temperature that each steel sheet reaches. The chrome steel sheets obtained as described above were investigated for the L value, resistance to corrosion by gasoline, resistance to the dissolution of Cr, and appearance. Measurement methods and assessment standards are as described below. The Table summarizes the results obtained.

L-value [0071] The L-value of each plate was measured by a method (for example, a multi-luminous source spectrocolorimeter, MSC -1S-2B, produced by Suga Test Instruments Co. or similar) specified in JIS Z 8722.

[0072] The surface of the steel plate was measured for the maximum value of the L value and the minimum value of the L value and the difference between them was calculated.

Corrosion resistance for gasoline [0073] An unworked sample with a size of 20 mm x 100 mm and a disk sample with a 60 mm diameter hole were immersed in a fuel, prepared by mixing unleaded gasoline and a solution aqueous with a concentration of 500 ppm by volume of formic acid together with a mass ratio of 1: 1, for one month at room temperature and then measured for the percentage of red rust area. The

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24/26 average of the measurements obtained. Each plate was evaluated for resistance to corrosion by gasoline based on the following standards:

A: a percentage of red rust area of less than 50% (target of the present invention) and

B: a percentage of red rust area of 50% or more.

Resistance to Cr dissolution [0074] The change in the amount of Cr was determined by a boiling water resistance test specified in JIS K 5400-1990 8.20 in such a way that the amount of Cr in each chromate coating not immersed in boiling water by 30 minutes were measured by X-ray fluorescence spectrometry. In X-ray fluorescence spectrometry, the amount of Cr is determined from the calibration curve between the amount of Cr and the number of Cr count determined using standard samples in which the amount of Cr was known.

A: a reduction in the amount of Cr of 2% or less

B: a reduction in the amount of Cr of more than 2% Appearance [0075] Chromed steel sheets were evaluated visually for appearance.

A: good

B: slightly irregular

C: irregular

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Table 1

Coating n ° Average size of Zn-Ni alloy coating grain (Mm) Condition of the oxide layer located at the top of the rev. newly formed ZnNi alloy Coating time to chrome plating Alloys ZnNi directly before cromage m Crom act Resistance to Cr dissolution Resistance to gasolin a L value Difference between maximum and minimum values of Value L Appearance Grades Esp. of the oxide layer (nm) P content in the oxide layer (atomic%) Coating oxide layer thickness. (nm) 1 1 18 0.2 10 sec 18 B B B 60 1.5 THE Ex. Comp. 2 1 18 0.2 10 sec 18 The THE THE 59 2.0 THE Example 3 1 16 0.2 100 h 24 The THE THE 57 2.7 THE Example 4 1 17 0.2 200 h 30 The THE THE 56 3.5 B Example 5 0.3 17 0.2 10 sec 18 The THE THE 57 2.5 THE Example 6 0.3 18 0.2 100 h 25 The THE THE 58 3.6 B Example 7 0.3 19 0.2 200 h 32 The THE THE 49 5.5 Ç Ex. Comp. 8 0.3 12 1.2 100 h 18 The THE THE 57 3.9 B Example 9 0.3 12 1.2 200 h 24 The THE THE 50 5.3 Ç Ex. Comp.

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Table 2

No. Chrome plating solution Heating temperature Target amount of Cr (mg / m ² ) Resistance to Cr dissolution Mass ratio of Trivalent Cr to total Cr Mass ratio of phosphoric acid to total Cr Reduto Agent Organic Mass ratio of organic reducing agent to Total Cr The 0.55 1.2 Glucose 0.4 150 ° C 35 THE B 0.4 0 Glucose 0.4 150 ° C 35 B

[0076] As is clear from the above, the examples of the present invention have excellent resistance to gasoline and good appearances. In contrast, the comparative examples have resistance to corrosion by gasoline or an L value outside the scope of the present invention and, therefore, are inferior in resistance to gasoline or appearance.

That is, examples of the present invention include chromate coatings having good appearances. Chromate coatings have excellent long-term corrosion resistance for fuels such as gasoline, alcohols, and gasoline-alcohol mixtures and inhibit the dissolution of Cr due to immersion in boiling water. Industrial Applicability [0078] A steel sheet according to the present invention has excellent corrosion resistance for fuels such as gasoline, alcohols and gasoline-alcohol mixture and is good looking and therefore suitable for, for example, fuel tanks such as gas tanks for cars or motorcycles.

Claims (2)

1. Steel plate for fuel tanks, characterized by the fact that said steel plate comprises an electroplating layer of Zn-Ni alloy disposed on at least one surface of a steel plate and a chromate coating disposed on the electroplating layer alloy, in which said steel plate is resistant to chromium dissolution, whereas the change in the amount of chromium in the chromate coating immersed in boiling water for 30 minutes is 2% or less of the amount of chromium in the non-immersed chromate coating in boiling water, an L value that indicates the color tones of the steel plate is 55 or more, and the difference between the maximum and minimum values of the L value is 4 or less, in which the change in the amount of chromium in the chromate coating was measured by X-ray fluorescence spectrometry, in which the amount of chromium in the X-ray fluorescence spectrometry is determined from the calibration curve between the amount of chromium and the number of chromium counts determined using standard samples in which the amount of Cr is known, where the alloy electrodeposition layer includes a Zn oxide sublayer that is located at the top and has a thickness of 20 nm or less and a P content of 1% atomic or less, and where the alloy electrodeposition layer has a surface with an average grain size of 0.8 pm or more. 2. Method of producing a steel sheet for fuel tanks as defined in claim 1, characterized by the fact that said method comprises the formation of an electroplating layer of Zn-Ni alloy on at least one surface of a Petition 870190038883, of 04/25/2019, p. 4/9 2/2 steel plate, applying a chrome plating solution to the alloy electroplating layer, and then heating the chrome plating solution, where the alloy electroplating layer includes a Zn oxide sublayer that is located in its top and which has a thickness of 20 nm or less and a P content of 1% atomic or less and the chrome plating solution contains chromic acid having a mass ratio (trivalent chromium) / (total chromium) of more than 0.5 , phosphoric acid having a mass (phosphoric acid) / (total chromium) ratio of 0.1 to 5.0, and an organic reducing agent.