WO2009069830A1

WO2009069830A1 - Steel sheet for fuel tanks and process for manufaturing the sheet

Info

Publication number: WO2009069830A1
Application number: PCT/JP2008/072107
Authority: WO
Inventors: Michitaka Sakurai; Nobuo Baba; Chiyoko Tada; Masayasu Nagoshi; Wataru Tanimoto
Original assignee: Jfe Steel Corporation
Priority date: 2007-11-28
Filing date: 2008-11-28
Publication date: 2009-06-04
Also published as: EP2233610B1; MX2010005154A; JP5315677B2; JP2009127126A; CN101878325A; BRPI0819870B1; BRPI0819870A2; CN101878325B; EP2233610A4; EP2233610A1

Abstract

A steel sheet for fuel tanks which has a Zn-Ni alloy coating electroplated on at least one side of the sheet and a chromate coating formed on the alloy coating. The chromate coating exhibits a change in the build-up of chromium after immersion in boiling water for 30 minutes of within 2% based on the build-up of chromium before the immersion, an L value (indicating the surface color tone of steel sheet) of 55 or above, and a difference between the maximum and minimum L values of within 3. The chromate coating can be formed by applying a chromate treatment fluid which contains chromic acid at a trivalent chromium/total chromium mass ratio exceeding 0.5, phosphoric acid at a phosphoric acid/total chromium mass ratio of 0.1 to 5.0 and an organic reducing agent onto the above alloy coating and heating the resulting sheet. The steel sheet exerts excellent corrosion resistance against fuels such as gasoline, alcohols, and alcohol blended gasolines over a long period and is excellent in surface appearance.

Description

Steel plate for fuel tank and manufacturing method thereof

Technical field

The present invention is mainly used for steel plates for fuel tanks, especially when gasoline contains a small amount of water, or when formic acid is generated due to deterioration of gasoline, under high environmental conditions including highly corrosive metal and organic acids. The present invention relates to a fuel tank copper plate and a manufacturing method thereof.

Background art

book

Conventionally, materials for gasoline tanks such as automobiles and motorcycles include tin (Sn) -lead (Pb) containing 20 mass% or less of lead (Pb) as disclosed in Japanese Patent Publication No. 57-61833, for example. Mainly used are steel plates plated with alloys, or multi-layered steel plates in which a Sn—Pb alloy is melted onto a nickel (Ni) electroplated layer.

However, Sn_Pb alloy-plated steel sheet has excellent workability and chemical resistance to gasoline, etc., but the plating layer is soft and easily damaged, and electrochemically more iron (Fe). Because it is noble, it has no sacrificial anticorrosive action on Fe. Therefore, when a gasoline tank using Sn—Pb alloy steel plate is used in an environment containing moisture, if there are defects such as pinholes and cracks in the plating layer, pitting corrosion will occur on the steel plate and gasoline will leak. Problems such as clogging of the combustion filter occur due to red soot caused by corrosion.

In recent years, considering the worsening of the petroleum situation, as automobile fuels, alcohols such as methyl alcohol, ethyl alcohol or methyl-t-butyl ether and derivatives thereof (hereinafter also referred to as alcohols) themselves, or these Mixed gasoline may be used. However, Sn-Pb alloy steel plates are easily corroded by alcohol oxides and impurities such as water, formaldehyde, acetoaldehyde, formic acid and Z or acetic acid contained in alcohols. It is unsuitable as a material for use. In addition, when a copper plate with Sn-Pb alloy is pressed, the plating layer does not have a sacrificial anti-corrosion effect on Fe, so even if the outer surface is applied, local corrosion resistance decreases due to plating peeling or mold galling. Sometimes. In addition, Sn_Pb alloy-plated steel sheets have a low secondary adhesion at the interface between the coating and the plating. Or it may cause swelling of the paint film.

On the other hand, due to recent environmental problems, there is a movement to refrain from the use of harmful substances such as Pb and hexavalent chromium (Cr). JP 2005-290556 discloses a steel plate for fuel tanks that does not use Pb. A chromate-treated steel sheet that suppresses the elution of hexavalent Cr has been proposed.

This steel sheet has an electric Zn-Ni alloy plating layer containing 5 to 30 mass% of Ni and having an adhesion amount per surface of 1 to 40 g / m ² on at least one side of the steel sheet. A chrome film is provided on the surface layer, and the chrome film has a change in the amount of chromium deposited after being immersed in boiling water for 30 minutes within 2% of the amount of chromium deposited before immersion. The chromate film has a chromic acid with a mass ratio (trivalent chromium) / (total chromium) of more than 0.5 on the alloy plating layer, and a mass ratio (phosphoric acid) / (total chromium). It was obtained by applying a chromate treatment solution containing phosphoric acid of 0.1 to 5.0 and an organic reducing agent, followed by heating. However, in this steel sheet for fuel tanks, it was found that the chromate film surface exhibits an interference color, and in some cases, the color tone is partially different within the surface of the steel sheet and the surface appearance may be inferior.

The object of the present invention is to provide fuel tanks with excellent corrosion resistance over a long period of time, without using Pb, to fuels such as gasoline, alcohols and alcohol-mixed gasoline, and with a good surface. The object is to provide a steel plate and a method for producing the same. Disclosure of the invention

The gist of the present invention is as follows.

[1] At least one surface of the steel sheet has an electro-zinc-nickel alloy plating layer, and further has a chromate coating on the alloy plating layer, and the chromat coating is in boiling water for 30 minutes. The change in the amount of chromium after immersion is within 2% of the amount of deposit before the immersion. A steel plate for fuel tanks, characterized by an L value of 55 or more, and a difference between the maximum and minimum L values of 4 or less.

[2] In the above [1], the thickness of the Zn oxide layer on the surface of the alloy is 20 nm or less, and the P content of the Zn oxide layer is 1% or less in terms of atomic% (denoted as at%). A steel plate for a fuel tank, characterized in that

[3] The steel plate for a fuel tank according to [1] or [2], wherein an average crystal grain size of the surface of the alloy plating layer is 0.8 μm or more.

[4] An electric Zn—Ni alloy plating layer is formed on at least one surface of the steel plate, the thickness of the Zn oxide layer of the alloy plating surface layer is 20 nm or less, and the P content of the Zn oxide layer is 31%. In addition, the chromic acid having a mass ratio (trivalent chromium) / (total chromium) of more than 0.5 is formed on the upper layer of the alloy plating layer, and the mass ratio (phosphoric acid) / (total chromium). A method for producing a steel plate for a fuel tank, comprising applying a chromate treatment solution containing 0.1 to 5.0 phosphoric acid and an organic reducing agent and heating.

[5] An electric Zn—Ni alloy plating layer is formed on at least one surface of the steel sheet, and the average crystal grain size of the surface of the alloy plating phase is set to 0.8 / zm or more. Chromate treatment containing chromic acid with a mass ratio (trivalent chromium) / (total chromium) of more than 0.5, phosphoric acid with a mass ratio (phosphoric acid) / (total chromium) of 0.1 to 5.0 and an organic reducing agent in the upper layer A method for producing a steel plate for a fuel tank, characterized by applying a liquid and heating. Brief Description of Drawings

Fig. 1 is a photograph showing the surface of an electroplated Zn-Ni alloy plating layer when the plating crystal grain size is 1.0 // m (Example 1).

Fig. 2 is a photograph showing the surface of the electroplated Zn-Ni alloy plating layer when the crystal grain size is 0.3 / im (Example 1). BEST MODE FOR CARRYING OUT THE INVENTION

Conventionally, in order to protect Zn-plated steel sheets, it has been widely practiced to form a chromate film on the plating layer by chromate treatment. The inventors have When the applicability of a zinc-plated steel sheet with a ROME film to a fuel tank was examined, the following findings were obtained.

(A) As a Zn-plated steel sheet, an electric Zn_Ni alloy-plated steel sheet has better corrosion resistance against fuels such as gasoline, alcohols and alcohol-mixed gasoline.

(B) When the chromate film is a film in which the change in the amount of Cr deposited after immersion in boiling water for 30 minutes is within 2% of the amount of Cr deposited before immersion, a small amount of water is contained in the gasoline. Even when used in an environment containing organic acids with high metal corrosivity, such as when formic acid is produced due to gasoline deterioration, elution of Cr can be reliably prevented.

(C) As an index for evaluating the degree of unevenness on the surface of the steel sheet, by defining the difference between the L value representing the color tone and the maximum and minimum values of the L value, it is possible to prevent a decrease in surface ^ Mi.

(D) In order to make the difference between the maximum value and minimum value of the L value and the L value on the steel sheet surface less than a predetermined amount, the Zn oxide layer thickness of the surface layer of the Zn-Ni alloy adhesive layer should be 20 nm or less. And the P content in the oxide layer is preferably 31% and 1% or less.

(E) In order to make the difference between the L value and the maximum value and minimum value of the L value on the surface of the steel sheet not more than a predetermined amount, the Zn-Ni alloy average crystal grain size should be 0.8 / zm or more. -It is preferable to form a predetermined chromate film after forming the Ni alloy plating layer.

The present invention has been developed based on the above findings, and details thereof will be described below.

1) Electric Zn_Ni alloy plating layer

The electric Ζη-Ni alloy plating layer is different from conventional gasoline, that is, alcohol such as alcohols and alcohols containing alcohol, alcohol such as formaldehyde, acetoaldehyde, formic acid Z or acetic acid. Effectively suppresses corrosion caused by oxides and impurities. Therefore, it is necessary to form this electric Zn_Ni alloy plating layer at least on the surface of the steel sheet in contact with the fuel.

At this time, the electroplated Zn—Ni alloy plating layer is not particularly specified, but it is desirable that Ni is contained in an amount of 5 to 30 mass% and the adhesion amount per side is 1 to 40 g / m ² .

If the Ni content in the plating layer is less than 5 ma _SS %, corrosion from the film defects cannot be suppressed and sufficient corrosion resistance may not be obtained. On the other hand, when the Ni content exceeds 30 mass%, It becomes harder and cracks occur during press working, and corrosion tends to proceed from this crack. Therefore, the amount of Ni in the plating layer should be 5-30 mass%. Also, if the coating amount per side of the plating layer is less than 1 _g / m ² , sufficient corrosion resistance cannot be obtained, while if it exceeds 40 g / m ² , the press workability may deteriorate. Therefore, the adhesion amount per one side of the plating layer is desirably 1 to 40 g / m ² . ,

In addition, the thickness of the Zn oxide layer on the surface of the Zn—Ni alloy plating layer is preferably 20 nm or less, and the P content contained in the Zn oxide layer is preferably 1% or less in at%. With such a configuration, ^ after the chromate treatment can be kept stable.

As will be described later, in the present invention, the L value representing the color tone of the steel sheet surface is 55 or more, and the difference between the maximum value and the minimum value is 4 or less. In order to satisfy this, the surface state of the electroplated Zn-Ni alloy plating layer before the chromate treatment is as follows: Zn oxide layer is 20 nm or less, and the P content of the Zn oxide layer is 31%, 1% It's preferable to be below. If the thickness of the oxide layer exceeds 20 nm and the P content in the oxide layer exceeds 1% at%, the color tone of the plating film itself becomes slightly dark and the surface of the plated crystal is very Fine irregularities are formed on the surface, and light interference is suppressed by suppressing irregular reflection of light. The oxide layer is preferably lower than 2 Onm.

In order to keep the thickness of the Zn oxide layer on the surface layer of electro-Zn-Ni plating to Onm or less, the time from applying electro-Zn-Ni to applying chromate treatment must be within 120 hours. Is preferred. This is because when the time exceeds 120 hours, the thickness of the oxide film exceeds 20 nm, so that the color tone of the plating film itself becomes dark and the difference in color tone is promoted.

The thickness of the oxide or hydroxide layer can be determined by Auger electron spectroscopy (AES) combined with Ar ion sputtering. After sputtering to a predetermined depth, the composition at that depth can be determined by correcting the relative sensitivity factor from the peak intensity of each element to be measured. The O content due to oxides or hydroxides decreases and becomes constant after it reaches its maximum value at a certain depth (this may be the outermost layer). The thickness of the oxide layer is the sum of the maximum value and the constant value at a position where the O content is deeper than the maximum value. The sputtering time for 1Z2 can be determined by conversion based on the sputtering rate of an SiO 2 film with a known film thickness.

The P content can be determined by X-ray photoelectron spectroscopy (XPS) combined with Ar ion sputtering. By performing the same measurement, the P concentration profile in the depth direction was obtained, and the value at which the P concentration was the maximum with respect to the depth corresponding to the thickness of the oxide layer was determined as the P content of the oxide layer. did.

In addition, the average crystal grain size of the surface of the Zn—Ni alloy plating layer is preferably 0.8 / zm or more. With such a configuration, ^ after chromate treatment can be kept stable.

In the present invention, the L value representing the color tone of the steel sheet surface is 55 or more, and the difference between the maximum value and the minimum value is 4 or less. In order to satisfy this, as described above, the Zn oxide layer is 20 nm or less and the P content of the Zn oxide layer is aty. Or less, or the crystal grain size of the surface of the Zn—Ni alloy plating layer on which the chromate film is applied is preferably 0.8 μm or more. When the crystal grain size is less than 0.8 / zm, the color tone of the plating film itself becomes slightly blackish, and the light reflection is suppressed by suppressing the irregular reflection of light. On the other hand, when the crystal grain size is 0.8 zm or more, the color tone of the plating film itself becomes slightly whitish, and light interference is likely to occur, and light interference is less likely to occur.

Although there is no upper limit to the crystal grain size, it seems that it is difficult in the electroplating process to make the size of the Zn_Ni alloy crystal in electroplating 2 / zm or more. In the electroplating process, in order to increase the crystal grain size, it is necessary to increase the grain growth by reducing the number of nucleation sites for precipitation. In order to do this, it is necessary to lower the current density in the electroplating, which is not preferable because the line speed of the plating line is lowered and the productivity is deteriorated.

The average crystal grain size can be obtained as a circle-equivalent grain size by observing 3000 to 20000 times scanning electron micrographs and counting the number of crystals per unit area.

2) Chromate film As described above, the chromate film of the present invention is a film in which the change in the amount of Cr deposited after immersion for 30 minutes in boiling water is within 2% of the amount of deposit deposited before immersion. With such a coating, even if used in an environment containing highly corrosive organic acids, Cr elution can be reliably prevented, and excellent corrosion resistance against fuels such as gasoline can be obtained.

The change in Cr adhesion after immersion in boiling water for 30 minutes is based on the boiling water resistance test described in 8.20 of JIS K 5400-1990. It can be determined by measuring by X-ray fluorescence. In the X-ray fluorescence method, the amount of Cr deposited is determined from a Cr count and a calibration curve for the amount of Cr deposited in advance using a standard sample with a known amount of Cr deposited.

Regarding the elution of hexavalent Cr, the type of elution liquid, elution temperature and elution time, such as Volvo Leach Test (Volvo Standard News 199 1.10), are determined and evaluated by the concentration of Cr eluted in the liquid. In addition, there is a method of evaluating by the amount of Cr elution by Al force degreasing described in JP-A-10-46353, but in the present invention, it is evaluated by the change in the amount of Cr adhered by immersion in boiling water for 30 minutes. The reason is that the Cr elution amount becomes constant by being immersed in boiling water for 30 minutes, and this amount has a good correlation with the remaining amount of chromate film when used as a fuel tank.

In the chromate film of the present invention, the L value representing the color tone of the steel sheet surface is 55 or more, and the difference between the maximum value and the minimum value of the L value is 4 or less, preferably 3 or less. Chromated steel sheets, especially trivalent chromium chromated steel sheets, often exhibit interference colors. The interference color ideally depends on the film thickness of the oxide film, and the relationship “reflected light + transmitted light = white (complementary color relationship)” holds. Therefore, when the oxide film thickness of the steel plate fluctuates, interference color unevenness may occur, and the surface will be damaged. Steel sheet of the present invention, the amount of deposition of the chromate film is preferably in the range of 10~50 _m g / _m ² of metal Cr terms, unevenness in interference color is in the oxide film thickness tends to occur. Therefore, in the present invention, the L value representing the color tone of the copper plate surface is 55 or more, and the difference between the maximum value and the minimum value of the L value is within 4, preferably within 3. Larger L values increase whiteness, and smaller values increase blackness. When the difference between the maximum and minimum L values is within 4, the difference in color tone can be minimized and the surface appearance can be prevented from deteriorating. The lower limit of the L value is 55 or more because the difference in color tone seems to increase as the blackness increases. If the L value is less than 55, even if the difference between the maximum and minimum L values is within 4, the difference in color will be noticeable and the surface appearance will be impaired. The L value can be measured by the method specified in JIS Z8722 (for example, Suga Test Instruments multi-light source spectrocolorimeter MSC-1S-2B).

The above-described chromate film of the present invention is not particularly limited as long as the color tone after treatment and the amount of elution from the mouth are within a predetermined range. For example, it can be formed by applying a chromate treatment liquid, which will be described later, onto the electroplated Zn—Ni alloy plating layer and then heating. The adhesion amount of the chromate film is preferably 10 to 50 mg / m ² in terms of metal Cr. This is because if it is less than 10 mg / m ² , sufficient corrosion resistance cannot be obtained, while if it exceeds 50 mg / m ² , the cost increases.

3) Manufacturing method

The steel sheet for a fuel tank of the present invention includes a step of forming an electric Zn_Ni alloy plating layer on at least one steel plate surface, and a step of forming a chromate film on the upper layer of the alloy plating layer.

In the above process, the change in the amount of deposit after the chromate film is immersed in boiling water for 30 minutes is within 2% of the amount of chromium before immersion, and the L value representing the color tone of the copper plate surface is 55 or more. The manufacturing method is not limited as long as the difference between the maximum and minimum L values is 4 or less.

For example, for chromate treatment to keep the chromium elution amount within the specified range, the mass ratio of trivalent Cr to chromic acid and total Cr with a mass ratio ((trivalent chromium) / (total chromium)) exceeding 0.5 It can be produced by applying a chromate treatment solution containing phosphoric acid and an organic reducing agent having a ratio ((phosphoric acid) / (total chromium)) of 0.1 to 5.0 and heating.

The chromate film produced in this way has a problem that it is difficult to obtain a good appearance like a hexavalent chromate film. In particular, when Zn-Ni plating and chromate film were processed on separate lines, they were often defective. As a result of intensive studies by the inventors, it was found that a good surface can be obtained by making the surface condition before applying the chromate film appropriate. One method is to apply an electroplated Zn-Ni alloy plating layer and reduce the thickness of the zinc oxide layer on the alloy plating surface to 20 nm or less before forming a chromate film on the upper layer. Below, the P content of the oxide layer may be 1% or less in at%. It was confirmed that a good appearance could be secured by using such a surface state. It is also possible to ensure a good appearance by forming a gold-plated layer so that the average crystal grain size of the surface of the electro-plated Zn-Ni alloy is 0.8 / xm and then forming a chromate film. I found.

Either or both of the control of the amount of the Zn oxide layer on the surface of the Zn-Ni plating layer and the P ratio in the oxide and the control of the average crystal grain size of the Zn-Ni plating surface may be used. . There are no particular restrictions on the plating conditions for forming the electroplated Zn—Ni alloy plating layer, but the plating layer contains 5-30 mass% Ni and the plating layer adhesion is 1-40 g / m ^2. It is preferable.

In addition, it is an effective method for carrying out the present invention to dispose the coater in the final section of the electroplating line and form a plating film, and immediately apply chromate treatment.

When the P content in the oxide layer is high, it is disadvantageous to ^, so when degreasing or surface conditioning is performed before chromate treatment, the cleaning is strengthened so that P does not remain, or degreasing and surface adjustment are performed. It is effective to make the liquid thinner.

Further, the method for suppressing the P content is not particularly limited, but a normal method may be used such as strengthening washing after degreasing or surface conditioning treatment or reducing the concentration of the treatment liquid. After forming the electroplated Zn-Ni alloy plating layer, apply the chromate treatment solution onto the electroplated Zn-Ni alloy plating layer. Here, in the present invention, it is necessary to form a chromate film on the electroplated Zn—Ni alloy plating layer, in which the change in Cr adhesion after immersion for 30 minutes in boiling water is within 2%. Chromic acid with a mass ratio of trivalent Cr to total Cr ((trivalent chromium) / (total chromium)) exceeding 0.5 and mass ratio ((phosphoric acid) / (total chromium)) to total Cr is 0. A chromate treatment solution containing 1 to 5.0 phosphoric acid and an organic reducing agent is applied onto the electroplated Zn-Ni alloy plating layer and heated in the next layer.

Hexavalent Cr in the chromate treatment liquid reacts with the organic reducing agent during heating and is reduced to trivalent Cr. If the mass ratio of trivalent Cr to total Cr is 0.5 or less, the amount of hexavalent Cr Excessive amount of hexavalent Cr remains in the chromate film after heating. Therefore, ku This hexavalent Cr elutes when the mouth coating is immersed in boiling water, so the change in the amount of Cr deposited after immersion in boiling water for 30 minutes exceeds 2%, and it has excellent corrosion resistance against fuels such as gasoline. It can no longer be obtained. If the mass of phosphoric acid with respect to the total Cr is less than 0.1, trivalent Cr will be polymerized to form a gel-like precipitate, and the properties as a chromate treatment solution cannot be maintained. On the other hand, if this ratio exceeds 5.0, phosphoric acid remains in the chromate film, and this phosphoric acid elutes in a wet environment, causing pitting corrosion and darkening of the skin.

As the organic reducing agent to be contained in the chromate treatment solution, it is preferable to use at least one selected from diols and saccharides. Among the diols, particularly preferred are ethylene glycol, propylene glycol, trimethylene glycol, and 1,4-butanediol. On the other hand, among saccharides, glycerin, polyethylene glycol, saccharose, ratatose, sucrose, glucose, or fructose are advantageously suitable.

This organic reducing agent is preferably contained in the chromate treatment liquid so that the mass ratio with respect to total Cr is 0.1 to 0.4. This is because if it is less than 0.1, a sufficient reduction effect cannot be obtained, while if it exceeds 0.1, the stability of the chromate treatment solution may not be maintained. The organic reducing agent is preferably added to the chromate treatment solution immediately before the chromate treatment solution is applied in order to increase the stability of the chromate treatment solution. In order to improve the corrosion resistance, the chromate treatment solution may contain an inorganic inhibitor as necessary. Examples of such inorganic inhibitors include inorganic colloids such as silica, Zr 0 ₂ , Ti0 ₂ , zirconium sulfate, and aluminum biphosphate, and heteropoly acids such as phosphomolybdic acid, key tungstic acid, and phosphovanadmolybdic acid. Illustrated. However, when these inorganic inhibitors are present in the chromate treatment solution, the reaction between hexavalent Cr and the organic reducing agent is delayed, and when the chromate film is immersed in boiling water, the elution of hexavalent Cr is promoted. The content is preferably less than 0.05 by mass ratio with respect to hexavalent Cr. The reason why inorganic inhibitors delay the reaction rate between hexavalent Cr and organic reducing agents is not clear, but the reason is that they are ionized in solution or interact with hexavalent Cr ions when dispersed. Conceivable. In addition, the chromate treatment solution may contain acids such as hydrofluoric acid, sulfuric acid, and hydrochloric acid for the purpose of promoting the reactivity with the electroplated Zn—Ni alloy plating layer.

In addition, the chromate treatment liquid may contain a water-soluble or water-dispersible polymer compound in order to further suppress Cr elution from the chromate film. Examples of water-soluble or water-dispersible polymer compounds include polyvinyl alcohol, polyacrylic acid, polyacrylamide, epoxy ester polymer, melamine alkyd resin polymer, natural polymer compounds such as starch and gazein, Examples include partial hydrolysates of alkinosilicates, partial hydrolysates of alkyl phosphates, and silane compounds such as silane coupling agents and epoxy silanes. These water-soluble polymers and water-dispersible polymer compounds have an effect of suppressing Cr elution from the chromate film and a protective film against external mechanical shock, but the terminal functional group is hexavalent. Since it acts as a reducing agent for Cr-ion, to ensure the stability of the treatment liquid, the content is preferably less than 0.05 as a mass ratio with respect to hexavalent Cr.

After applying chromate treatment solution, heat. At this time, it is preferable to heat the steel plate so that the temperature of the steel plate is 120 ° C or higher. Below 120 ° C, the reduction of Cr does not proceed sufficiently, and the amount of Cr eluted from the chromate film may increase when immersed in boiling water.

Furthermore, it is possible to further suppress Cr elution from the chromate film by applying and drying an aqueous solution containing Ti colloid before applying the chromate treatment solution. This is because the Ti colloid acts as an active site for reaction with the acidic chromate treatment solution by adsorbing the Ti colloid on the electro-ηη-Ni alloy plating layer. This is presumably because the reaction reduced to Cr is promoted.

The aqueous solution containing Ti colloid is preferably applied with an aqueous solution containing pH: 7.5 to 10 and temperature: 40 to 60 ° C. containing Ti colloid at a concentration of 1 to 10 volppm for 1 to 30 seconds. The steel plate used for the fuel tank steel plate according to the present invention is, for example, mass%, C: 0.0007 to 0.0050%, Si: 0.5% or less, Mn: 2.0% or less, P: 0.1. % Or less, S: 0.015% or less, A 1: 0.01 to 0.20%, N: 0.01% or less, Ti: 0.005 to 0.08%, and ：: 0.001 to 0.00. 01% A cold-rolled copper plate that is excellent in deep drawability and contains Fe and inevitable impurities is preferable.

Hereinafter, the reasons for limitation of each component will be described.

C: 0.0007 to 0.0050%

Since C adversely affects deep drawability, the content is preferably set to 0.0050% or less. Further, even if the content is less than 0.0007%, the deep drawability is not improved, but rather the cost of the decarburization treatment is increased. Therefore, the C content is preferably set to 0.0007% or more and 0.0050% or less.

Si: 0.5% or less

Since Si has an action of increasing the strength of steel, it can be added according to a desired strength. However, if the amount exceeds 0.5%, the deep drawability deteriorates, so the Si amount is preferably 0.5% or less.

Mn: 2.0% or less

Mn, like Si, has the effect of increasing the strength of steel, so it can be added according to the desired strength. However, if the amount exceeds 2.0%, the deep drawability deteriorates, so the Mn amount is preferably 2.0% or less.

P: 0.1% or less

P segregates at the grain boundaries and strengthens the grain boundaries, thereby suppressing cracks in the weld and strengthening the steel. However, if the amount exceeds 0.1%, the deep drawability deteriorates. Therefore, the P amount is preferably 0.1% or less. In order to more reliably suppress cracks in the welded portion, it is more preferable that the P content be 0.01% or more and 0.05% or less.

S: 0.015% or less

Since S adversely affects deep drawability, the amount is preferably set to 0.015% or less.

A1: 0.01-0.20%

A1 is added to deoxidize steel and improve the yield of carbonitride-forming elements such as Ti. However, if the amount is less than 0.01%, the effect of addition is poor, whereas if it exceeds 0.20% The effect is saturated. Therefore, the A1 amount is preferably 0.01% or more and 0.20% or less.

N: 0.01% or less

Since N adversely affects deep drawability, the amount is preferably 0.01% or less.

Ti: 0.005-0.08%

Ti has the effect of improving the deep drawability by forming precipitates with C and N in the steel and reducing solid solution C and N. However, if the amount is less than 0.005%, the effect is small, while if it exceeds 0.08%, the effect is saturated. Therefore, the Ti content is preferably set to 0.005% or more and 0.08% or less.

B: 0.001 to 0.01%

B, like P, has the effect of suppressing cracks in the weld. However, if the amount is less than 0.001%, the effect is small, while if it exceeds 0.01%, the deep drawability deteriorates. Accordingly, the B content is preferably 0.001% or more and 0.01% or less, and more preferably 0.001% or more and 0.004% or less.

The reason why B and P suppress cracks in the weld is considered as follows.

In other words, weld cracking is presumed to be due to the liquid metal embrittlement, which is the main ingredient of the electrode, Cu, which is a squeezed zinc, which becomes liquid during welding and penetrates into the steel grain boundaries to embrittle the grain boundaries. In this respect, B and P tend to pray to the grain boundaries, so the grain boundaries are strengthened to suppress these weld cracks.

The balance is Fe and inevitable impurities. Here, the amount of inevitable impurities may be within a normal range, for example, O is 0.000% or less.

In addition to the above components, addition of Nb in an amount of 0.0005 to 0.0005% or more is suitable for improving the deep drawability. Example 1

In mass%, C: 0.0015%, Si: 0.01%, Mn: 0.08%, P: 0.011%, S: 0.008%, A1: 0.05%, N: 0.00. 0019%, Ti: 0.035%, Nb: 0.003% and B: 0.004%, The remainder was made of cold-rolled steel sheet with a composition of Fe and inevitable impurities, and an electric Zn-Ni alloy-plated steel sheet (Ni: 12 mass%, adhesion amount per side: 20 g / m ² ) was prepared by a conventional method. At this time, by changing the electric plating line speed to two levels of 90mpm and 16Ompm and changing the current density, the average grain size of the plating layer surface is 1.0 / zni and 0.3 μπι A Zn—Ni alloy plating layer was formed. Figures 1 and 2 show photographs of the surface with average grain sizes of 1. Ομπι and 0.3 / zm, respectively. The average crystal grain size was determined as a circle equivalent grain size by observing 3000 to 20000 times scanning electron micrographs, counting the number of crystals per unit area.

Subsequently, as a surface conditioning treatment, the surface of the Zn oxide containing P is contained by neutralizing the acidic electroplating solution by immersing it in disodium hydrogen phosphate at 50 ° C and pH 10 and washing it with water. A layer was formed. On the other hand, the condition that the surface adjustment treatment is not performed was also performed.

The thickness of the oxide or hydroxide layer can be determined by Auger electron spectroscopy (AES) combined with Ar ion sputtering. The content of each element on the surface is measured by AES, and after performing sputtering with Ar to a predetermined depth, the content of each element on the surface is measured by AES. The composition distribution of each element in the vertical direction was measured. The content of 0 due to oxides and hydroxides reaches a maximum at a certain depth and then decreases and becomes constant. The oxide thickness was defined as the depth at which the 0 content was deeper than the maximum value and half the sum of the maximum value and the constant value. As a pretreatment, Ar contamination was performed for 30 seconds to remove the contamination layer on the surface of the specimen.

In addition, by performing the same measurement using X-ray photoelectron spectroscopy (XPS), the P concentration profile in the depth direction is obtained, and the P concentration is compared to the depth corresponding to the thickness of the oxide layer. The maximum value was defined as the P content of the oxide layer.

Then, after plating, conditions for chromate treatment immediately (elapsed time 10 seconds), conditions for chromate treatment after 100 hours after plating, and conditions for chromate treatment after 200 hours after plating. Then, after applying the chromate treatment solution shown in Table 2 using a roll coater, the mixture was heated to the heating temperature shown in Table 2, Sample Nos.! ~ 9 'with Cr coating amount of Cr adhesion shown in Table 2 were prepared. Here, the heating temperature is the maximum temperature reached by the steel sheet. The chromate-treated steel sheets obtained as described above were examined for L value, gasoline corrosion resistance, Cr elution resistance, and surface appearance. The measurement method and evaluation criteria are as follows. The results obtained are also shown in Table 1.

"Direct

The L value was measured by a method defined in JIS Z8722 (for example, a multi-light source spectrocolorimeter MSC-1S-2B manufactured by Suga Test Instruments Co., Ltd.).

In addition, the maximum L value and the minimum L value in the steel plate surface were measured and the difference was calculated.

Corrosion resistance against gasoline

The sample size 20 negation X 100 mm prior to processing, the sample was processed in a blank diameter of 60瞧, unleaded gasoline and Concentration: 500v _O LPPM mass ratio and formic acid aqueous solution of 1: mixed fuel in 1, After immersion at room temperature for 1 month, the area ratio of red cocoon occurrence was measured and the average of these was obtained. The corrosion resistance against gasoline was evaluated according to the following criteria.

◯: Red cocoon occurrence area ratio is less than 50% (target of the present invention)

X: Red cocoon occurrence area ratio is 50% or more

Cr elution resistance

The change in the amount of Cr deposited was determined by measuring the amount of Cr deposited before and after immersion in boiling water for 30 minutes using the fluorescent X-ray method based on the test for boiling water resistance described in 8.20 of JIS K 5400-1990. . In the fluorescent X-ray method, the Cr adhesion amount was determined from a Cr count number and a calibration curve for the Cr adhesion amount prepared in advance using a standard sample with a known Cr adhesion amount.

○: Cr adhesion reduction rate is 2% or less

X: Cr adhesion reduction rate exceeds 2%

Surface appearance

The visual appearance after chromate treatment was evaluated.

Y: Good

Δ: Slightly uneven

X: Uneven

Table 2

As described above, in the present invention example, the gasoline resistance is excellent and the surface appearance is good.

On the other hand, in the comparative examples, the Cr elution resistance or the L value is outside the range of the present invention, so either the gasoline resistance or the surface appearance is inferior.

That is, according to the present invention, excellent corrosion resistance is exhibited over a long period of time with respect to fuel such as gasoline, alcohols alone or alcohols mixed gasoline, and chrome elution by boiling water immersion is suppressed and the surface appearance is good It has a rometo film. Industrial applicability

The steel sheet of the present invention has excellent corrosion resistance to fuels such as gasoline, alcohol fuel and alcohol-mixed gasoline, and also has a good surface. For example, fuel such as gasoline tanks for automobiles and motorcycles. Ideal as a steel plate for tanks.

Claims

The scope of the claims

1. At least one side of the steel sheet has an electro-zinc-nickel alloy plating layer, and further has a chromate coating on the alloy plating layer, which is immersed in boiling water for 30 minutes. The change in the amount of chrome deposition after that is within 2% of the amount of deposits before dipping, the L value representing the color of the steel sheet surface is 55 or more, and the difference between the maximum and minimum L values is within 4 A steel plate for fuel tanks characterized by

2. The fuel according to claim 1, wherein the thickness of the Zn oxide layer of the alloy plating surface layer is 20 nm or less, and the P content of the Zn oxide layer is 1% or less at%. Steel plate for tanks.

3. The steel plate for a fuel tank according to claim 1 or 2, wherein an average crystal grain size of the surface of the alloy plating layer is 0.8 / Zm or more.

4. At least one side of the steel sheet is formed with an adhesive layer of an electrical Zn-Ni alloy, the thickness of the Zn oxide layer of the surface of the alloy is 20 nm or less, and the P content of the Zn oxide layer is at% In addition, the chromic acid with a mass ratio (trivalent chromium) / (total chromium) of more than 0.5, and the mass ratio (phosphoric acid) / ( A method for producing a steel plate for a fuel tank, characterized by applying a chromate treatment solution containing phosphoric acid having a total chromium content of 0.1 to 5.0 and an organic reducing agent, followed by heating.

5. After forming an electric Zn_Ni alloy plating layer on at least one side of the steel sheet and setting the average crystal grain size of the surface of the alloy plating phase to 0.8 / m or more, In the upper layer, chromic acid with a mass ratio (trivalent chromium) / (total chromium) of more than 0.5, phosphoric acid and organic with a mass ratio (phosphoric acid) / (total amount) of 0.1 to 5.0 A method for producing a steel plate for a fuel tank, wherein a chromate treatment solution containing a reducing agent is applied and heated.