JP2014031549A - Steel sheet for fuel tank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high corrosion-resistant steel sheet for fuel tank excellent in press workability, outer surface corrosion resistance and inner surface corrosion resistance as well as excellent in resistance weldability.SOLUTION: A steel sheet for fuel tank of the present invention has zinc-based plating layers and trivalent chromate layers formed successively on both surfaces of a steel sheet, a first composite coating comprising metal powders of Al and Ni and an amine-modified epoxy resin formed on the chromate layer formed on one surface of the steel sheet, and a second composite coating comprising an acrylic emulsion resin, silica, a lubricant and one or more electrically conductive particles selected from a group consisting of a metal particle, a metal compound particle and a graphite particle, which have a particle size of 0.01 to 3.0 μm by median size (50%D) formed on the trivalent chromate layer formed on the other surface of the steel sheet.

Description

  The present invention relates to a steel plate for a fuel tank that has good press workability, outer surface corrosion resistance, and inner surface corrosion resistance, and is excellent in resistance weldability.

  Steel plates for fuel tanks are required to have many characteristics in addition to corrosion resistance. The required characteristics are roughly classified into two, one is a characteristic necessary for manufacturing as a tank, and the other is a characteristic required when used as a fuel tank.

  The characteristics required for production include press workability for processing into a complicated shape, spot weldability for attaching parts, seam weldability for joining tank bodies, and the like.

On the other hand, since the fuel tank is an important safety part, its required characteristics require corrosion resistance from the tank outer surface environment, that is, corrosion resistance against salt damage, wetting, mud, etc. (hereinafter referred to as “outer surface corrosion resistance”). is there.
Also, on the inner surface of the tank, it is corroded by organic acids (formic acid, acetic acid, etc.) generated by long-term storage of condensed water and gasoline due to humidity changes, and as a mixture of alcohol fuel and biofuel that has been increasing in use in recent years. Corrosion due to organic acids and impurities is regarded as a problem, and corrosion resistance against deteriorated fuel (hereinafter referred to as “internal corrosion resistance”) is also an important required characteristic. When the inner surface corrosion resistance is low, the surface treatment layer inside the tank and iron rust, especially floating corrosion products, will cause clogging of the filter in the fuel circulation system and hinder fuel supply.

  Examples of steel plates that have been put to practical use as fuel tank steel plates so far include, for example, Pb—Sn alloy hot-dip plated steel plates as disclosed in Patent Document 1 and Zn as disclosed in Patent Document 2. Examples thereof include materials obtained by subjecting a system-plated steel sheet to a thick chromate treatment. In particular, the Pb—Sn alloy hot-dip steel sheet has long been used as a product having characteristics as a fuel tank.

However, the Pb-Sn alloy hot-dip steel sheet of Patent Document 1 was prohibited from using Pb after July 2007 by the EU ELV Directive (Automobile Disposal Directive), so it was necessary to move to other materials. It was.
Further, regarding the inner surface corrosion resistance of the material of Patent Document 2, although it has a rust prevention function due to the sacrificial anticorrosive action of zinc, the elution rate of zinc in water is large, a large amount of floating white precipitate is generated, and the fuel There was a problem that the filter was clogged in the circulation system. In addition, after zinc elution, there is a disadvantage that red rust (iron rust) of the base steel is generated, which is insufficient as a performance for use as a steel plate for fuel tanks.

  From the above situation, development of new steel plates for fuel tanks has progressed, and several materials have been proposed. As typical examples, Patent Documents 3 to 5 disclose materials having characteristics necessary for a fuel tank.

Patent Document 3 has an intermetallic compound coating layer containing Al, Fe, Si on the surface of a steel plate and having a thickness of 5 μm or less, and the surface of the intermetallic compound coating layer has a weight%. In addition, there is disclosed a hot-dip galvanized steel sheet having a coating layer composed of Si: 2 to 13%, Mg: more than 3% to 15%, and the balance being substantially Al.
Patent Document 4 discloses a tin-zinc alloy-plated steel sheet comprising 70-99% tin, the balance zinc, and unavoidable impurities.
Patent Document 5 discloses a technique in which an organic resin film is provided on a material obtained by subjecting a galvanized steel sheet to chromate treatment to further improve inner surface corrosion resistance and outer surface corrosion resistance. In this technique, by adding conductive metal powder to the coating on the inner surface side of the tank, the organic resin coating improves corrosion resistance and prevents resistance weldability from being reduced due to the organic resin coating.

Japanese Patent Publication No.57-61833 Japanese Patent Publication No.53-19981 Japanese Patent No. 4264157 Japanese Patent No. 3126623 Japanese Patent No. 3328578

However, the hot-dip aluminum-plated steel sheet of Patent Document 3 has a problem that cracks are likely to occur in the plating during processing, and corrosion progresses from the end portion. Further, there is a problem in that massive Mg ₂ Si or Al ₃ Mg ₂ phase is precipitated in the plating layer, and the local dissolution of the plating layer starting from them occurs.

  Furthermore, the tin-zinc alloy-plated steel sheet of Cited Document 4 has a problem that the sacrificial anticorrosive ability is low, and the corrosion of the base iron progresses due to scratches or the like.

  Moreover, in the technique of patent document 5, in the spot welding process in an actual line, there is a problem in the number of hitting points that can be welded without electrode maintenance (hereinafter referred to as spot welding continuous spotting property), and further improvement has been desired. . In addition, when the steel plate is welded at a high current exceeding the appropriate range, cracks in the welded portion may occur, and it is necessary to prevent the occurrence.

  The present invention advantageously solves the above-described problems of the prior art, and provides a steel sheet for a highly corrosion-resistant fuel tank that is excellent in press workability, inner surface corrosion resistance, and outer surface corrosion resistance, and in particular, has further improved resistance weldability. For the purpose.

  Accordingly, the present inventors have focused on galvanized steel sheets that are superior in sacrificial anticorrosive ability and are advantageous in versatility and procurement, and have made extensive studies to improve the resistance weldability of steel sheets for fuel tanks using galvanized steel sheets. As a result, it was found that a steel plate for a fuel tank having target characteristics can be obtained by adding specific components to the base material in a specific range and including particles having conductivity in the coating on the outer surface of the tank. It was.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
In mass%, C: 0.0007 to 0.0050%, P: 0.005 to 0.10%, Al: 0.01 to 0.20%, Ti: 0.005 to 0.08%, B: 0.0012 to 0.01%, S: 0.015% or less, N: 0.01% or less , Si: 0.5% or less and Mn: 2.0% or less on both surfaces of the steel sheet,
A zinc-based plating layer and a trivalent chromate layer are sequentially formed,
On the chromate layer formed on one surface side of the steel sheet, a first composite film containing Al and Ni metal powder and an amine-modified epoxy resin is formed,
On the trivalent chromate layer formed on the other surface side of the steel sheet, one or two selected from the group consisting of acrylic emulsion resin, silica, lubricant, and metal particles, metal compound particles and graphite particles A steel plate for a fuel tank, comprising the second composite film containing the above conductive particles having a particle diameter in the range of 0.01 to 3.0 μm in median diameter (50% D).

  ADVANTAGE OF THE INVENTION According to this invention, while having favorable press workability, outer surface corrosion resistance, and inner surface corrosion resistance, it becomes possible to provide the steel plate for fuel tanks excellent in resistance weldability.

It is sectional drawing typically shown about an example of the steel plate for fuel tanks obtained in the Example. It is the figure which showed the state of seam welding typically.

Hereinafter, the present invention will be specifically described.
(steel sheet)
The steel plate for a fuel tank of the present invention is in mass%, C: 0.0007 to 0.0050%, P: 0.005 to 0.10%, Al: 0.01 to 0.20%, Ti: 0.005 to 0.08%, B: 0.0012 to 0.01%, S: A steel plate containing 0.015% or less, N: 0.01% or less, Si: 0.5% or less, and Mn: 2.0% or less is used.

・ C: 0.0007 to 0.0050 mass%
C is a component that adversely affects deep drawability and is preferably reduced as much as possible. For this reason, the upper limit of the C content is set to 0.0050 mass% or less. However, even if the C content is less than 0.0007% by mass, not only the deep drawability cannot be improved any more, but also a more advanced decarburization treatment is required, resulting in an increase in cost. For this reason, the lower limit is set to 0.0007 mass%.

P: 0.005-0.10 mass%
P, like B, is one of the most important components among the components of the steel sheet in the present invention. In particular, the segregation of P at the grain boundaries strengthens the grain boundaries, and has the effect of suppressing weld cracking and the effect of strengthening steel. For this reason, it adds according to desired intensity | strength. However, if the P content exceeds 0.10% by mass, the deep drawability deteriorates. For this reason, P content shall be 0.10 mass% or less. Note that the lower limit of the P content is set to 0.005% by mass from the viewpoint of obtaining an effect of suppressing cracks in the weld.
In particular, when it is necessary to further suppress cracks in the weld zone, the P content is preferably in the range of 0.01 to 0.05 mass%. When the P content is 0.01% by mass or more, the effect of suppressing cracks in the welded portion is sufficient. On the other hand, if it is 0.05% by mass or less, the deep drawability does not deteriorate.

Al: 0.01-0.20 mass%
Al is added to improve the yield of deoxidation and carbonitride-forming elements. However, when the Al content is less than 0.01% by mass, the effect of addition is small. On the other hand, even if the content exceeds 0.20% by mass, an effect commensurate with the content cannot be obtained. Therefore, Al content shall be the range of 0.01-0.20 mass%.

Ti: 0.005-0.08 mass%
Ti combines with C in steel and precipitates as a carbide, and has an effect of preventing deep drawability deterioration due to solute C. If the Ti content is less than 0.005% by mass, the above effects are small. Moreover, even if it contains exceeding 0.08 mass%, the effect corresponding to content will not be acquired. Therefore, Ti content shall be the range of 0.005-0.08 mass%.

B: 0.0012 to 0.01% by mass
B is one of the most important components among the composition components of the steel sheet in the present invention, and particularly has an action of effectively preventing cracks in the welded portion. In order to prevent weld cracking, the B content needs to be 0.0012% by mass or more. On the other hand, if it exceeds 0.01% by mass, the deep drawability deteriorates. For this reason, B content shall be the range of 0.0012-0.01 mass%.
The reason why weld cracking occurs is that, during welding, Cu, which is the main component of the electrode, and Zn, which is the plating component, form a liquid metal, which causes brittle cracks by entering the grain boundaries. Presumed. As described above, when B is positively contained in the steel sheet, it is considered that B is segregated at the grain boundary, and as a result of strengthening the grain boundary, cracking of the weld zone can be suppressed. Moreover, more preferable content is 0.0012-0.004 mass%.

S: 0.015 mass% or less
S is a component that adversely affects deep drawability and is preferably reduced as much as possible. For this reason, the upper limit of the S content is 0.015% by mass.

N: 0.01% by mass or less
N is a component that adversely affects deep drawability and is preferably reduced as much as possible. For this reason, the upper limit of the content is set to 0.01% by mass.

Si: 0.5% by mass or less, Mn: 2.0% by mass or less
Since both Si and Mn have the effect of increasing the strength of the steel, they are added according to the desired strength. However, when the addition amounts of Si and Mn exceed 0.5% by mass and 2.0% by mass, respectively, the deep drawability deteriorates. For this reason, content of Si and Mn shall be 0.5 mass% or less and 2.0 mass% or less, respectively.

The basic components have been described above.
The steel sheet for a fuel tank of the present invention can be a hot-rolled steel sheet or a cold-rolled steel sheet that has the above-described basic components and the balance of which is composed of Fe and inevitable impurities.
Nb: 0.0005 to 0.0050 mass%
In the present invention, it is preferable to contain Nb in the range of 0.0005 to 0.0050 mass% when the crystal grain of the hot-rolled sheet is refined and the deep drawability after cold rolling and annealing is improved.

Moreover, in this invention, although it does not prescribe | regulate especially about the impurity component contained unavoidable in a steel plate, what is necessary is just in the range which contains an unavoidable impurity component normally.
For example, O as an unavoidable impurity component may be within a range of 0.010 mass% or less.

(Zinc-based plating layer)
In the steel sheet for fuel tank of the present invention, zinc-based plating layers are formed on both surfaces of the steel sheet.
Since the zinc-based plating layer shows a lower potential than the iron base (plated steel sheet), the occurrence of red rust (iron rust) is suppressed by the sacrificial anticorrosive action of zinc even in the pressed parts where the plating layer is damaged. In particular, the outer surface corrosion resistance of the fuel tank is improved.

  There is no particular limitation on the zinc-based plating layer. For example, electro galvanizing, electro zinc-nickel alloy plating, electro zinc-cobalt alloy plating, electro zinc-iron alloy plating, hot dip galvanizing, alloying hot dip galvanizing, hot dip zinc-aluminum plating, hot dip zinc-magnesium plating, melting In addition to zinc-aluminum-magnesium plating, etc., zinc-based dispersion plating in which silica, alumina, organic resin, etc. are dispersed in the plating layer, and multilayer plating in which these are laminated are listed.

Also, zinc-based plating layer, it is preferable to set the adhesion amount per one side and 10 to 200 g / m ^2. When the adhesion amount is 10 g / m ² or more, the sacrificial anticorrosive action of zinc is sufficient and the corrosion resistance is not insufficient. On the other hand, if the adhesion amount is 200 g / m ² or less, an effect of improving corrosion resistance can be expected, which is not only economical but also does not deteriorate weldability. The adhesion amount is more preferably in the range of 15 to 100 g / m ² .

(Trivalent chromate layer)
In the fuel tank steel sheet of the present invention, a trivalent chromate layer is formed on the zinc-based plating layer.
The trivalent chromate layer, in addition to the effect of improving the corrosion resistance, is to ensure sufficient adhesion between the organic resin contained in the first and second composite films formed on the upper layer and the zinc-based plating layer. It is a necessary intermediate layer (the first composite film and the second composite film will be described later).

Moreover, it is preferable that the adhesion amount of the said trivalent chromate layer shall be 5-200 mg / m < ² > per single side in conversion of metal chromium. When the adhesion amount is 5 mg / m ² or more, the corrosion resistance is sufficient. In addition, the adhesion between the organic resin layer and the zinc plating layer in the composite film is sufficient. For this reason, the said composite film of a sliding part does not peel at the time of press work. Furthermore, since the composite film does not peel off, the plating layer does not peel off, and the processed portion corrosion resistance of the inner and outer surfaces of the tank is sufficient. On the other hand, if it is 200 mg / m ² or less, the trivalent chromate film itself does not become brittle, so that the trivalent chromate layer does not peel off at the processed sliding portion. Accordingly, peeling of the upper composite film does not occur, and the processed portion corrosion resistance of the inner and outer surfaces becomes sufficient. The more preferable range of the amount of the trivalent chromate layer deposited is 10 to 100 mg / m ² .

The formation of the trivalent chromate layer can be performed according to a usual trivalent chromate treatment method, and is not particularly limited.
The treatment liquid used for the trivalent chromate treatment is obtained by a method in which chromic acid (CrO ₃ ) is used as a starting material and is changed to Cr ³⁺ using a reducing agent. Furthermore, as reducing agents, polysaccharides such as starch, fructose and sucrose, organic acids such as succinic acid and formic acid, or inorganic compounds such as phenols, hydrogen peroxide, phosphorous acid and hypophosphorous acid are used. Can be used.
In addition, after carrying out trivalent chromate treatment of the zinc-based plated steel sheet, a trivalent chromate layer is formed through a drying process such as hot air drying.

(First composite film)
The steel sheet for fuel tanks of the present invention forms a first composite film containing Al and Ni metal powder and an amine-modified epoxy resin on the chromate layer formed on one surface side of the steel sheet.
The first composite film has excellent corrosion resistance against deteriorated fuel mixed with organic acids (formic acid, acetic acid, etc.) generated by oxidation of gasoline, organic acids and impurities as contaminants of alcohol fuel and biofuel. It has durability and serves as a barrier layer that prevents direct contact between the lower zinc-based plating layer and the chromate layer and the deteriorated fuel.

-Metal powder The metal powder in the first composite film is contained mainly for the purpose of ensuring resistance weldability. That is, since a film made of an organic resin generally has high electrical insulation, when the film thickness is 1 μm or more, exposure of the lower chromate layer cannot be expected at all, and a current point cannot be secured, so resistance welding is difficult. . Therefore, in the present invention, it is necessary to increase the conductivity of the coating by dispersing the required amount of metal powder in the first composite coating located on the inner surface side of the fuel tank.

  In addition, in order to ensure resistance weldability in this invention, it may be insufficient only to contain the said metal powder in a 1st composite film. When a sufficient cross-linking reaction is caused by containing an organic resin curing agent in the first composite film, it is difficult to expect the effect that the organic resin melts due to heat generated during welding and the film is eliminated. The periphery of the remaining part causes poor welding, resulting in insufficient lap between the nuggets. In addition, when the crosslinking is insufficient and the curing agent remains as an unreacted material, corrosion factors (acid, chlorine ions, etc.) intrude due to the fact that the cohesive strength of the part is low or the hydrophilicity is high. And the inner surface corrosion resistance of the tank becomes insufficient. Considering these cases, the first composite film preferably contains no organic resin curing agent.

About the metal powder contained in a 1st composite film, it is preferable to have a property with high specific resistance and a large calorific value. Specifically, Ni, Al, Fe, Cu, etc. are mentioned.
Among the above metals, Ni is most useful because of its excellent corrosion resistance to methanol and high specific resistance. In addition, Al has a lower specific resistance and melting point than Ni, and is not necessarily optimal for welding. However, as described later, by adding Al in a scaly (flaked) shape to the first composite film, It is useful for suppressing the permeation of organic acids and impurities having corrosive properties, and improving the internal corrosion resistance.

Therefore, in the first composite coating of the present invention, by combining Al and Ni metal powders and containing them in an appropriate ratio, the conductivity of the coating is improved to improve resistance weldability and the permeation of corrosive ions is suppressed. Thus, the inner surface corrosion resistance can also be improved. Furthermore, the first composite film may contain metal powders such as Fe and Cu in addition to the essential component Al and Ni metal powders.
Moreover, as a shape of the said metal powder, any of powder shape and scale shape (flakes shape) may be sufficient.

  The Ni metal powder is preferably granular with an average particle size of 1 to 9 μm. When the average particle size is 1 μm or more, there is no shortage of energization points. On the other hand, if the average particle size is 9 μm or less, the energization point can be effectively secured, so that resistance weldability can be improved with a small amount. Further, the coating does not become porous and the inner surface corrosion resistance does not deteriorate. Furthermore, powdering of the coating film during press working is not a problem. More preferably, it is 2-7 micrometers.

The metal powder of Al preferably has a scale shape with an average major axis of 8 to 18 μm, an average minor axis of 1 to 10 μm, and a thickness of 1 to 5 μm. When the average major axis is 8 μm or more and the average minor axis is 1 μm or more, the area of the scale is not too small, so the permeation suppression ability of organic acids and impurities with corrosiveness is not lowered, and the internal corrosion resistance is reduced. do not do. On the other hand, when the average major axis is 18 μm or less and the average minor axis is 10 μm or less, the film does not become too porous, so that the film has sufficient strength and does not become brittle, and powdering may occur. In addition, the inner surface corrosion resistance of the press-processed portion does not deteriorate. Furthermore, when the average thickness is 1 μm or more, the inner surface corrosion resistance life is not shortened. On the other hand, when the average thickness is 5 μm or less, the ratio of the Al powder exposed on the surface of the first composite film does not increase excessively, so that resistance weldability does not deteriorate, which is preferable.
The Al powder preferably has an average major axis of 10 to 15 μm, an average minor axis of 5 to 8 μm, and an average thickness of 2 to 4 μm.

  The total amount of Ni and Al metal powders in the first composite film is preferably 30 to 110 parts by mass with respect to 100 parts by mass of the organic resin component. When the total blending amount is 30 parts by mass or more, there is no shortage of energization points, and the resistance weldability does not deteriorate because the conductivity is not inferior. Moreover, when the said total compounding quantity is 110 mass parts or less, 1st composite film itself does not become weak, the powdering resistance at the time of press work does not fall, and inner surface corrosion resistance does not fall. The total blending amount is more preferably 45 to 100 parts by mass with respect to 100 parts by mass of the organic resin component.

  Furthermore, when the total compounding amount of the Ni and Al metal powders in the first composite film is within the above-described preferred range, the resistance is controlled by setting the Ni / Al ratio (mass ratio) to 80/20 to 30/70. Weldability and inner surface corrosion resistance can be improved in a well-balanced manner. When the Ni / Al ratio is 30/70 or more, the amount of Ni having a high specific resistance is not insufficient, and resistance weldability does not deteriorate. Further, when the Ni / Al ratio is 80/20 or less, the amount of Al having the function of suppressing the penetration of fuel does not decrease, so the inner surface corrosion resistance does not decrease. The Ni / Al ratio is more preferably 70/30 to 40/60.

Amine-modified epoxy resin An amine-modified epoxy resin is used as the organic resin contained in the first composite film. The first composite film has excellent corrosion resistance and durability against deteriorated fuel containing gasoline, alcohol, formic acid, etc., and adhesion to the base plate (steel plate + zinc-based plating layer + chromate layer) and It is necessary to have excellent press workability, and by using an amine-modified epoxy resin, excellent press workability, corrosion resistance against deteriorated fuel, and adhesion to a base plate can be ensured.

  Here, the amine-modified epoxy resin is one in which the oxirane ring of the epoxy resin forming the main skeleton is opened by an amine.

  Examples of the epoxy resin forming the main skeleton of the amine-modified epoxy resin include bisphenol A type epoxy resin, bisphenol F type epoxy resin, cycloaliphatic epoxy resin, hydantoin type epoxy resin, novolac type epoxy resin, and glycidyl ester type epoxy. Examples thereof include resins. Among these, bisphenol A type epoxy resin and bisphenol F type epoxy resin are excellent in stability as a coating material in forming the first composite film, and can stably obtain a film excellent in press workability and internal corrosion resistance. It is more preferable in terms of a wide range of production conditions. In addition, the epoxy resin may be used alone or as an epoxy ester resin obtained by reacting a dicarboxylic acid such as adipic acid, azelaic acid, sebacic acid, phthalic acid, dimer acid or the like, and polyalkylene glycol diglycidin ether is used in combination. May be.

  In the amine-modified epoxy resin, examples of the amine added to the oxirane ring of the epoxy resin include monoalkanolamines such as ethylethanolamine and ethanolamine, and dialkanolamines such as diethanolamine, dipropanolamine, and dibutanolamine. Primary or secondary amines can be mentioned. Among these, diethanolamine is preferable in that it has excellent stability after addition and high adhesion to the chromate layer and the metal powder.

  In the amine-modified epoxy resin, the number of moles of alkanolamine added to 1 equivalent of the oxirane ring of the epoxy resin as the main skeleton is preferably 0.2 to 1.0 mole. When the epoxy equivalent is 500 to 1000, the number of moles of alkanolamine is preferably 0.2 to 0.6 mole, and when the epoxy equivalent is 1000 to 5000, the number of moles of alkanolamine is more preferably 0.6 to 1.0 mole. When the number of moles of alkanolamine added to 1 equivalent of the oxirane ring of the epoxy resin is 0.2 or more, the degree of amine modification is not insufficient, so the affinity between the metal powder and the amine-modified epoxy resin does not decrease, and the metal during press working Since the powder does not detach from the coating and the plating layer does not peel off, press workability does not deteriorate. In addition, for the same reason, corrosive ions do not easily stay between the resin / metal powder in the film, and sufficient hydrophobicity is obtained, which makes it difficult to attract corrosive ions such as formate ions into the film. Thus, the corrosion resistance (internal corrosion resistance) against the deteriorated fuel having corrosiveness is also sufficient. On the other hand, when the number of moles of the alkanolamine to be added is 1.0 mole or less, there is no excess alkanolamine content, which is economical. Further, the excess amine does not increase the water absorption and the inner surface corrosion resistance does not decrease.

  As described above, the amine-modified epoxy resin reinforces the interface between the metal powder in the first composite film and the main skeleton epoxy resin. Furthermore, it has the effect of improving the interfacial adhesion between the first composite film and the chromate layer as a feature when using an amine-modified epoxy resin. The effect of strengthening the interface improves the flat corrosion resistance, suppresses film peeling during press working, and further improves the inner surface corrosion resistance of the pressed portion.

  The amine-modified epoxy resin preferably has a weight average molecular weight in the range of 5000 to 50000. When the weight average molecular weight is 5000 or more, the molecular weight of the main skeleton epoxy resin is not too low, the intermolecular force is not insufficient, and the toughness of the film is not insufficient. The desired press workability can be obtained. On the other hand, when the weight average molecular weight is 50000 or less, the amount of alkanolamine added to the oxirane ring at the end of the molecule does not decrease, so the affinity between the resin and the metal powder is not insufficient, Therefore, the desired internal corrosion resistance can be obtained.

Other In addition, the first composite film may further contain one or more of resins other than amine-modified epoxy resins, for example, urethane-modified epoxy resins, urethane resins, epoxy resins, acrylic resins, or olefin resins. . When these resins are contained, the mass of these resins is also taken into account as the organic resin component that is the basis of the total blending amount of the above metal powder.

  Furthermore, additives such as a lubricant, a coupling agent, a pigment, a thixotropic agent, and a dispersant can be added to the first composite film as necessary.

  The thickness of the first composite film is preferably 1 to 10 μm. If it is 1 μm or more, sufficient inner surface corrosion resistance required as an inner surface layer can be obtained. Moreover, if it is 10 micrometers or less, the improvement effect of internal surface corrosion resistance and press workability can be anticipated, and resistance weldability will not fall.

The first composite film is formed by preparing a paint containing the amine-modified epoxy resin, Al and Ni metal powders, and various additives that are appropriately added as necessary, and using this as a chromate on the inner surface side. It can carry out by the method of apply | coating as an upper layer of a layer.
For the preparation of the coating, a sand mill, an attritor or the like is used for an amine-modified epoxy resin obtained by adding alkanolamine to an epoxy resin having an epoxy equivalent of 500 to 5000 and reacting at room temperature to 100 ° C. for 4 to 5 hours. Thus, the metal powder and various additives added as necessary can be blended at a predetermined blending ratio.

(Second composite film)
The steel sheet for a fuel tank of the present invention comprises an acrylic emulsion resin, silica, a lubricant, metal particles, metal compound particles, and graphite particles on a trivalent chromate layer formed on the other surface side of the steel plate. A second composite film containing one or more selected from the group and conductive particles having a particle diameter in the range of 0.01 to 3.0 μm in median diameter (50% D) is formed.

-Acrylic emulsion resin The second composite film contains an acrylic emulsion resin as a base resin.
Steel tanks for fuel tanks require excellent press workability, so it is necessary to form a tough film. To form a tough film, a film is formed by a cross-linking reaction between the base resin and the curing agent. There is a technique to make it. However, in that case, there is a risk of poor welding as described above. In addition, even when conductive particles described later are used, the improvement effect due to the addition of conductive particles cannot be expected.
Therefore, in the present invention, as a result of examining a resin system that obtains a tough film without using a curing agent, it is possible to use an epoxy resin, an alkyd resin, an acrylic resin, a urethane resin, a polyvinyl butyral resin, a phenol resin, a melamine resin, and the like. As a result of further investigation and further investigation, it was found that an acrylic emulsion resin is optimal.

  When using a water-soluble resin, it is necessary to add a large amount of hydrophilic functional groups for water solubilization, which increases the hydrophilicity of the film and facilitates the entry of corrosion factors (acid, chloride ions, etc.) There is a tendency that the corrosion resistance of the outer surface of the tank is insufficient. On the other hand, an emulsion resin is optimal because it does not cause such a problem.

  The acrylic emulsion resin is an emulsion resin obtained by emulsion polymerization in water in the presence of an emulsifier or a dispersant, a vinyl monomer mixture containing at least one selected from an acrylic ester and a methacrylic ester, It refers to a resin that has been neutralized or modified as necessary.

  Examples of the monomer component of the vinyl monomer mixture used for preparing the acrylic emulsion resin include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, (meth) Isopropyl acrylate, butyl (meth) acrylate (n-, i-, t-), hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl (meth) acrylate, (meth) acrylic C1-C18 (1-24) alkyl ester or cycloalkyl ester of acrylic acid or methacrylic acid such as decyl acid, lauryl (meth) acrylate, stearyl (meth) acrylate, cyclohexyl (meth) acrylate, Acrylic acid such as methoxyethyl (meth) acrylate, ethoxybutyl (meth) acrylate, or C2-C18 alkoxyalkyl of methacrylic acid, carboxyl group-containing unsaturated monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, 2-hydroxyethyl (meth) acrylate, 2 -Or 3-hydroxypropyl (meth) acrylate or other acrylic resin or methacrylic acid hydroxyalkyl ester having 2 to 8 carbon atoms, N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) Nitrogen-containing alkyl (meth) acrylates such as acrylate, acrylamide, methacrylamide, N-butoxymethyl (meth) acrylamide, N, N-dimethylaminopropyl (meth) acrylamide, N, N-dimethylaminoethyl (meth) acrylamide, etc. Polymerizable amides , It can be cited glycidyl (meth) acrylate, or an epoxy group-containing unsaturated monomers such as allyl glycidyl ether, vinyl acetate, styrene, alpha-chlorostyrene, acrylonitrile and the like.

  The emulsion polymerization reaction for obtaining the emulsion resin is performed by supplying the monomers and the polymerization initiator all at once at a predetermined temperature while stirring in an aqueous medium in the presence of an emulsifier.

  The emulsifier is usually used in an amount of 0.05 to 10% by mass, preferably 0.1 to 5% by mass, based on the total amount of monomers. Specific examples of emulsifiers include cationic emulsifiers such as stearylamine hydrochloride, lauryltrimethylammonium chloride, trimethyloctadecylammonium chloride, potassium oleate, sodium laurate, sodium dodecylbenzenesulfonate, sodium alkanesulfonate, sodium alkylnaphthalenesulfonate Anionic emulsifiers such as sodium dialkylsulfosuccinate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkyl allyl ether sulfate, polyoxyethylene alkyl phosphate ester, polyoxyethylene alkyl allyl phosphate ester, polyoxyethylene alkyl ether, poly Oxyethylene alkyl allyl ether, polyoxyethylenepropyl block Rimmer, polyethylene glycol fatty acid esters, nonionic emulsifiers such as polyoxyethylene sorbitan fatty acid esters, lauryl betaine, amphoteric emulsifiers such as such as lauryl dimethyl amine oxide.

  In addition, a water-soluble polymer such as polyvinyl alcohol, hydroxyethyl cellulose, a water-soluble acrylic copolymer, a copolymer of sodium styrenesulfonate, and the like can be used alone or in combination with the above-described emulsifier.

The monomer concentration during the polymerization is usually 30 to 70% by mass, preferably 35 to 65% by mass. As for the polymerization initiator, generally used radical polymerization initiators such as persulfates such as ammonium persulfate, potassium persulfate, and sodium persulfate, 2,2′-azobis (2,4-dimethylpareronitrile) An azo polymerization initiator such as benzoyl peroxide or peroxide polymerization initiator such as lauryl peroxide can be used.
In addition, the usage-amount of a radical polymerization initiator should just be about 0.1-10 mass% with respect to the monomer whole quantity, Preferably it may be about 0.3-5 mass%.
The polymerization reaction time is usually about 2 to 16 hours, and the polymerization temperature is usually about 60 to 100 ° C.

In order to reduce the contact area between the mold and the steel plate during pressing as much as possible, it is important to form a coating with high hardness. For this purpose, an acrylic emulsion resin with a high glass transition point (Tg) is used. It is effective to use.
Specifically, the Tg of the acrylic emulsion resin of the second composite film is preferably 0 to 90 ° C. When Tg is 0 ° C. or higher, the hardness of the film is not low and is not too soft at the die or steel plate surface temperature during pressing. As a result, the mold / steel plate contact ratio does not increase, and the press workability does not become poor. On the other hand, when Tg is 90 ° C. or lower, the film is not too brittle and press workability does not become poor. A more preferable Tg is 50 to 80 ° C.
Furthermore, it is preferable to add general additives such as reaction accelerators, stabilizers, and dispersants to the acrylic emulsion resin as long as they do not impair the gist of the present invention.
In the present invention, the acrylic emulsion resin is used in order to further improve the adhesion between the second composite film and the trivalent chromate layer and improve the corrosion resistance even in a severe environment such as a wet environment. It is preferable to add an epoxy resin. Examples of the epoxy resin include bisphenol A type epoxy resin, bisphenol F type epoxy resin, cycloaliphatic epoxy resin, hydantoin type epoxy resin, novolac type epoxy resin, and glycidyl ester type epoxy resin. When these resins are added to the acrylic emulsion resin, a combination of the acrylic emulsion resin and the epoxy resin becomes the base resin of the second composite film. In order to obtain the above effect, the epoxy resin is preferably added in the range of 20 to 60% by mass with respect to the entire base resin.

Silica The silica contained in the second composite film is blended for the purpose of imparting corrosion resistance on the outer surface of the tank.
Examples of the type of silica include colloidal silica, organosilica sol, silica powder, or organic silicate that becomes silica by dehydration condensation (for example, ethyl silicate or the like is used in combination with an acid catalyst).

  The average particle size of the silica is preferably in the range of 5 to 70 nm in order to uniformly disperse the silica in the second composite film.

  Furthermore, it is preferable that the compounding quantity of the silica contained in a said 2nd composite film shall be 5-80 mass parts of silica with respect to 100 mass parts of said base resins. If it is 5 parts by mass or more, the corrosion resistance does not decrease. If it is 80 parts by mass or less, the film does not become brittle, mold galling does not occur during press working, and press workability does not deteriorate. More preferably, it is 20-60 mass parts. If it is 60 parts by mass or less, resistance weldability is not lowered.

  In the present invention, a silane coupling agent can also be used as a reaction accelerator for the acrylic emulsion resin and silica. Examples of the silane coupling agent used include γ- (2-aminoethyl) aminopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, and the like.

・ Lubricant The lubricant contained in the second composite film is intended to maintain good press workability of the film by forming a lubricating layer between the film and the mold during press processing. Is formulated as
As the lubricant, a wax made of a polymer of an olefinic hydrocarbon such as polyethylene, polypropylene, polybutylene or the like is preferable, and these may be used in combination.
Further, a lubricant containing fluorine may be used.

  The blending amount of the lubricant in the second composite film is preferably 1 to 40 parts by mass of the lubricant with respect to 100 parts by mass of the base resin. When the blending amount of the lubricant is 40 parts by mass or less, the film strength of the second composite film to be formed does not decrease and the lubricity does not decrease. On the other hand, if it is 1 part by mass or more, the lubricity will not be insufficient. More preferably, the content is 5 to 30 parts by mass.

  The average particle size of the lubricant is preferably 1 to 7 μm. When the average particle size is 1 μm or more, the amount of lubricant protruding from the second composite film does not decrease, and press workability does not deteriorate. Moreover, if it is 7 micrometers or less, a 2nd composite film will not become too weak, the powdering resistance of a film will not fall, and press workability will not be inferior.

  The softening point of the lubricant may be any as long as it is in the range of 70 to 150 ° C., and two or more kinds of lubricants having different softening points for the purpose of further improving the press workability. May be used in combination. When the softening point of the lubricant is 70 ° C. or higher, the elastic modulus of the lubricating layer does not decrease significantly even under severe pressing conditions with heat generation, the lubricity does not decrease, and the press workability is excellent. On the other hand, if it is 150 ° C. or lower, the softening of the lubricant does not become insufficient and the lubricating layer does not become too tough, so the lubricity is not inferior and the press workability is excellent.

-Conductive particles And, in the present invention, among the performances of the fuel tank steel plate, particularly when resistance welding is performed such that the electrode contacts the outer surface of the tank, more specifically, the second composite coating. In order to further improve the weldability and enable continuous welding, conductive particles are further contained in the second composite coating.

  Various types of conductive particles are known as the conductive particles contained in the second composite film. In the present invention, one or two or more selected from the group consisting of metal particles, metal compound particles, and graphite particles are used. Use particles.

  As the metal particles, it is preferable to use particles such as nickel, tin, and copper, and alloy particles typified by stainless steel such as SUS304L, SUS316, and SUS430. Among these, it is more preferable to use particles of nickel, tin, and stainless steel.

As the metal compound particles, it is preferable to use conductive metal oxide particles. The conductive metal oxide particles are typified by tin oxide powder. This metal oxide particle is not only a single composition but also a composite oxide, a core using inexpensive particles and a metal oxide with excellent conductivity on the surface, a composite treatment, etc. preferable.
For example, as the powder of tin oxide, “Nano Tek Tinoxide” (manufactured by CI Kasei Co., Ltd.), and as the colloidal dispersion of tin oxide, “Cerames S-8” (manufactured by Taki Chemical Co., Ltd.), As an ATO (antimony-doped tin oxide) powder, “SN-100P” (manufactured by Ishihara Sangyo Co., Ltd.), and as a colloid dispersion of ATO, “SN-100D” (manufactured by Ishihara Sangyo Co., Ltd.),
Examples of the colloidal dispersion of the antimony zinc composite oxide include “CELNAX CX-Z300H (registered trademark)” (manufactured by Nissan Chemical Industries, Ltd.).

Examples of the graphite particles include graphite powder, a choroid sol dispersed in an organic solvent or water, and the like. Examples of commercially available powder types include “AUP” (manufactured by Nippon Graphite Industry Co., Ltd.), “TGP-05” (manufactured by Tokai Carbon Co., Ltd.), “GP-60S”, “GP-82”, “ GP-78 ”,“ GP-63 ”(manufactured by Hitachi Powdered Metals Co., Ltd.), etc. Colloidal sols dispersed in organic solvents and water include“ Hitasol GA-66 ”,“ AB-1 ”,“ “GA-315” (manufactured by Hitachi Powdered Metals Co., Ltd.), “Bunny Lay C-9A”, “BP-4” (manufactured by Nippon Graphite Industries Co., Ltd.), and the like.

  Moreover, the particle diameter of the electroconductive particle mentioned above shall be 0.01-3.0 micrometers by a median diameter (50% D). If the thickness exceeds 3.0 μm, the conductive particles protruding from the film during press working will come into contact with the mold and press workability will deteriorate, and the corrosion resistance of the press processed part will deteriorate. On the other hand, if the thickness is less than 0.01 μm, it is difficult to form an energization path in the film, and resistance weldability, particularly spot weldability, tends to decrease. A more preferable median diameter range is 0.02 to 1.0 μm, and further preferably 0.02 to 0.1 μm. In this range, the dispersibility in the paint is better, and the spot weldability is even better.

Moreover, it is preferable to make the compounding quantity of the electroconductive particle contained in a said 2nd composite film into the range of 5-30 mass parts of electroconductive particles with respect to 100 mass parts of base resins. If the blended amount of the conductive particles is 30 parts by mass or less, the toughness of the film is not impaired, the powder does not powder during press processing, and the press workability is not deteriorated. Corrosion resistance does not deteriorate. On the other hand, when the blending amount of the conductive particles is 5 parts by mass or more, an effect of adding conductive particles is obtained, and resistance weldability, particularly spot weldability, is not deteriorated.
Moreover, the more preferable range of the compounding quantity of the said electroconductive particle is 11-30 mass parts.

  Further, the second composite film only needs to contain the silica, lubricant, and conductive particles in the above amounts in the acrylic emulsion resin (base resin), and further appropriately contain other additives and the like. May be.

  The thickness of the second composite film is preferably 0.5 to 3.0 μm. If it is 0.5 μm or more, the press workability required for the outer surface layer can be sufficiently obtained. Moreover, if it is 3.0 micrometers or less, sufficient resistance weldability will be acquired.

In addition, the formation of the second composite film is performed by using the acrylic emulsion resin, silica, lubricant, and one or more kinds of conductive particles selected from the group consisting of metal particles, metal compound particles, and graphite particles. If necessary, a coating material containing various resins or additives that are appropriately added can be prepared and applied as an upper layer of the outer chromate layer.
The preparation of the coating is carried out by using a sand mill, an attritor or the like on an acrylic emulsion resin adjusted by emulsion polymerization or a base resin further containing an epoxy resin, silica, lubricant, conductive particles and, if necessary. Various additives to be added can be blended at a predetermined blending ratio.

  Furthermore, there is no problem even if a lubricating oil is applied depending on the difficulty of pressing, which is preferable from the viewpoint of preventing damage to the coating film.

A steel plate for a fuel tank as a sample was produced according to the following steps. A schematic cross-sectional configuration of the obtained fuel tank steel plate is shown as an example in FIG. Reference numeral 1 denotes a steel plate, 2 denotes a zinc-based plating layer, 3 denotes a trivalent chromate layer, 4 denotes a first composite coating, and 5 denotes a second composite coating.
(1) After heating the steel material (slab) having the chemical composition shown in Table 1 to 1200 ° C and hot rolling it under the conditions of finishing temperature: 880 ° C and winding temperature: 600 ° C to make the plate thickness 3.5mm Then, cold rolling was performed at a reduction ratio of 77% to obtain a cold rolled steel strip.
(2) Next, after recrystallization annealing was performed at 830 ° C. using a continuous annealing line, temper rolling was further performed under the condition of a reduction ratio of 0.8%, thereby producing a steel sheet to be plated having a thickness of 1.0 mm.
(3) Then, the zinc-type plating layer of the kind shown in Table 2, and the adhesion amount per single side | surface was given to both surfaces of the produced steel plate to be plated.
(4) Subsequently, using a roll coater on the upper layer of the zinc-based plating layer, after applying so as to have a predetermined Cr adhesion amount after drying, baking is performed so that the ultimate plate temperature becomes 100 ° C. A trivalent chromate layer having an adhesion amount per side shown in 2 was formed.
(5) A first composite film and a second composite film having the conditions shown in Table 2 were formed on each surface of the formed trivalent chromate layer.

-Formation method of 1st composite film In addition, the said 1st composite film was formed with the following methods.
First, in a reactor equipped with a reflux condenser, a stirrer, a thermometer, and a nitrogen gas blowing device, Epicoat 1007 (manufactured by Yuka Shell Epoxy Co., Ltd., epoxy resin: epoxy equivalent = 2000) 2000 g (one equivalent of oxirane ring) And 1000 g of toluene were added, and the temperature was raised to 80 ° C. after substitution with nitrogen to obtain a uniform solution. Next, 52.5 g of diethanolamine (0.8 mol with respect to 1 equivalent of the oxirane ring) was added dropwise over 30 minutes, and then the amine was modified by reacting for 1 hour to prepare an amine-modified epoxy resin. The resulting amine-modified epoxy resin had a weight average molecular weight of 35,000.
Subsequently, Al and Ni metal powder, an organic solvent, and other additives were added and kneaded to prepare a suspension. The Al metal powder was scaly with an average major axis of 13 μm, an average minor axis of 5 μm and an average thickness of 2 μm, and the Ni metal powder was granular with an average particle diameter of 5 μm, and the amounts shown in Table 2 were added. The amount of the organic solvent was 60 to 85% by mass of the whole suspension. The resin mixture (suspension) is applied by roll application so as to have a predetermined thickness, and is baked under conditions such that the ultimate plate temperature after 10 to 30 seconds is 100 to 200 ° C. A composite film was formed.

-Formation method of 2nd composite film Moreover, the 2nd composite film was formed with the following methods.
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, etc., 29.18 parts by weight of deionized water, 0.13 parts by weight of emulsifier A (polyoxyethylene nonylphenyl ether ammonium sulfate), 0.4 parts by weight of emulsifier B (polyoxyethylene alkyl ether) And heated to 80 ° C., 0.1 parts by weight of ammonium persulfate is added and stirred for 5 minutes. Next, a monomer premix consisting of 21.92 parts by weight of deionized water, 0.52 parts by weight of emulsifier A, 12.6 parts by weight of styrene, and 1.2 parts by weight of methyl methacrylate is dropped into the reaction vessel over 3 hours. After completion of dropping, the mixture was kept at 80 ° C. for 2 hours, then cooled to 40 ° C., adjusted to pH 8.5 with dimethylaminoethanol, and an aqueous dispersion of a stable acrylic emulsion resin (Tg: 80 ° C.) was obtained. In some examples, an aqueous epoxy resin, which will be described later, was blended with the aqueous dispersion of the acrylic emulsion resin to form a base resin.
Next, colloidal silica having an average particle diameter of 5 to 70 nm, a lubricant (polyethylene wax, an average particle diameter of 2 μm, a softening point of 120 ° C.) and conductive particles are added as shown in Table 2, and deionized water is added. And kneaded to prepare a suspension. The amount of solvent (such as deionized water) was 75 to 90% by mass of the entire suspension. This resin mixture is applied by roll application so as to have a predetermined thickness, and is baked under conditions such that the ultimate plate temperature after 10 to 30 seconds is 100 to 200 ° C., thereby forming a second composite film. did.

(Evaluation)
The following evaluation was performed about each sample of the obtained steel plate for fuel tanks.

(A) Evaluation of press workability Cylindrical processing was performed under the following conditions, and press workability was evaluated by investigating the limit drawing ratio and powdering resistance. The evaluation results are shown in Table 2.
<Pressing conditions>
・ Oil coating ・ ・ ・ ・ ・ ・ ・ ・ Rust preventive oil Z5 (made by Idemitsu Oil Co., Ltd.) 1g / m ² oil coating ・ Punch diameter and shape ・ ・ ・ 33mmφ flat bottom cylinder ・ Clear ・ ・ ・ ・ 1mm
・ Blank diameter ・ ・ ・ Various changes ・ Wrinkle presser load ・ ・ ・ ・ 29420N (3tf)
・ Aperture speed ... 60mm / sec.

-Limit drawing ratio For the evaluation of the limit drawing ratio, under the above pressing conditions, each steel plate was cylindrically processed with the second composite coating (outer surface) side as the die side and the first composite coating (inner side) surface as the punch side. The limit drawing ratio of the sample (the maximum value of the blank diameter / punch diameter of the drawn-out sample) was determined and evaluated according to the following evaluation criteria.
○: 2.1 ≦ limit aperture ratio Δ: 2.0 ≦ limit aperture ratio <2.1
×: Limit drawing ratio <2.0

-Powdering resistance Evaluation of powdering resistance was performed by investigating the degree of powdering of the resin film on the side wall of the cup when each sample was subjected to cylindrical processing under the following conditions.
<Pressing conditions>
・ Oil coating ・ ・ ・ ・ ・ ・ ・ ・ Rust preventive oil Z5 (made by Idemitsu Oil Co., Ltd.) 1g / m ² oil coating ・ Punch diameter and shape ・ ・ ・ 33mmφ flat bottom cylinder ・ Clear ・ ・ ・ ・ 1mm
・ Blank diameter ... 60mm
・ Wrinkle presser load ・ ・ ・ ・ 29420N (3tf)
・ Aperture speed ... 60mm / sec.
For the evaluation of powdering, the ratio (= spot count of C after processing / C spot count of C before processing) calculated from the measured value of C count before and after processing by EPMA was determined according to the following criteria.
○: 0.8 ≦ C count ratio Δ: 0.2 ≦ C count ratio <0.8
×: C count ratio <0.2

(B) Resistance weldability evaluation method Resistance weldability was evaluated in terms of seam weldability and spot weldability. The evaluation results are shown in Table 2.

-Seam weldability As shown in FIG. 2, seam welding was performed under the following conditions.
(I) Continuous weldability As a preliminary experiment, welding was performed while changing various welding currents to obtain an appropriate current range. The appropriate current range is according to the peel tensile test method (JIS Z 3141-1996: test method for seam welded joints). Load is applied until the test piece breaks, the test piece breaks the base metal, and the weld cross section Observed with an optical microscope, the current range was such that the nugget wrap was good and no blowholes were generated. The cross-section observation sample is obtained by embedding a welded specimen in a resin, polishing it, and then etching it.
As a result of the evaluation described above, the appropriate current range was 11.5 to 16 kA. Accordingly, the welding current for the continuous welding test was set to 12 kA which is the lower limit current value of the appropriate current range +0.5 kA. The welding conditions are shown below.
<Seam welding conditions>
・ Electrode ・ ・ ・ ・ ・ ・ Chromium-copper alloy ・ Electrode shape ・ ・ ・ ・ Electrode diameter 230mmφ, electrode thickness 8.0mm, end 4mmR, center tip 15mmR (width 4.5mm)
・ Welding method ・ ・ ・ ・ Double hook, lap seam welding ・ Pressure force ・ ・ ・ 3923N (400kgf)
-Energizing time ...-2/50 sec energization on, 1/50 sec energization off
・ Cooling ・ ・ ・ ・ ・ ・ Internal water cooling ・ Welding speed ・ 2.5m / min.
・ Welding current ... 12kA
Under the above conditions, the first composite coatings of test pieces with a size of 500 x 300 mm are overlapped, welded in the long side direction, and approximately 2 m of welding was performed with one set of test pieces, for a total of 300 test pieces A continuous welding test of 600 m in total was carried out. In the middle, a sample for a tensile test was taken out every 10 m of welding distance, and a load was applied by a peel tension test method until the test piece broke, and the form of breakage was observed. From the form of fracture, the continuous weldability of seam welding was evaluated according to the following criteria.
◎: When the continuous welding distance exceeds 500 m, the fracture mode is the base metal fracture ○: Continuous welding distance exceeds 300 m, the fracture mode is the base material fracture within 500 m, Δ: The continuous welding distance exceeds 300 m, and the fracture mode is within 500 m, Or, within 300 m of continuous welding, the fracture type is the base metal fracture ×: Within 300 m of continuous welding distance, the fracture type is fracture in the nugget (ii) Crack at the weld zone 18 kA, +2 kA higher than the upper limit 16 kA above, and other conditions Was welded under the same welding conditions as above, and the presence or absence of cracks in the weld was evaluated by observing the cross section of the weld. As the weld cross-section observation sample, a sample taken in parallel with the welding direction and 50 mm from the center of the weld was embedded in resin, polished, and then etched. This was observed with an optical microscope over the entire embedded 50 mm range, and the number of weld cracks was counted and evaluated according to the following evaluation criteria.
○: No occurrence △: 1 to 2 ×: 3 or more

-Spot weldability (i) Continuous weldability In this test, a welding experiment was conducted by a two-stage energization method. As a preliminary experiment, the first stage welding current was fixed at 6.5 kA, and welding was performed while changing the second stage welding current in various ways to obtain an appropriate current range. The appropriate current range is such that the lower limit is 4√t (t = plate thickness) or more and the upper limit is a current range in which welding does not occur. As a result, the appropriate current range was 5.5 kA to 7.5 kA.
From the above, the welding current for the continuous welding test was set to 7.0 kA which is the upper limit current value of the appropriate current range -0.5 kA. The welding conditions other than the current value are shown below.
<Spot welding conditions>
Plate assembly ... 2 layers, DR type (1st composite film contacts),
CF type (2nd composite film contacts)
Energization condition: Two-stage energization method with the following conditions: ... 1961N (200kgf)
Welding current: 1st stage / 6.5kA, 2nd stage / 7.0kA
Welding time: 1st / 5th cycle, 2nd / 12th cycle cooling: Internal water cooling Under the above conditions, the first composite film is applied to the DR-type electrode using a plurality of test pieces with a size of 100 x 200mm. Then, the CF composite electrode was overlapped so that the second composite film was in contact with it, and continuous welding was performed. A test piece of 20 mm × 80 mm was used for every 20 striking points on the way to check the welding state on the way.
That is, the welded part of a 20 mm × 80 mm welded test piece was peeled off, the major axis and minor axis of the button were measured, and the hit point with the minor axis satisfying 4√t or more was regarded as acceptable. The continuous dotability was evaluated according to the following evaluation criteria based on the number of hit points.
◎: 800 points or more ○: 600 points or more and less than 800 points △: 300 points or more and less than 600 points ×: Less than 300 points (ii) Upper limit value of the appropriate current range for weld cracking + 2 kA, current value of 9.5 kA, other conditions Was welded under the same welding conditions as above, and the presence or absence of cracks in the weld was evaluated by observing the cross section of the weld. As the cross-sectional observation sample, a sample cut at the center of the nugget was embedded in a resin, polished, and then etched. All of the nugget cross sections were observed with an optical microscope, the number of occurrences of cracks in the welds was counted, and evaluated according to the following evaluation criteria.
○: No occurrence △: 1 to 2 ×: 3 or more

(C) External surface corrosion resistance evaluation method The external surface corrosion resistance of the second composite film was evaluated as follows. Under the conditions of the JASO-M610 method (1 cycle: salt spray 2 hours → 60 ° C, drying at 20-30% relative humidity 4 hours → 50 ° C, relative humidity 98% for 2 hours) And the side wall part outside the cylindrical cup squeeze sample was evaluated. The cylindrical cup squeezed sample was assumed to be an actual tank, and was pressed so that the surface in contact with the die was the second composite coating surface, that is, the second composite coating surface was outside the cup. The press working conditions are shown below.
<Pressing conditions>
・ Oil coating ・ ・ ・ ・ ・ ・ ・ ・ Rust preventive oil Z5 (made by Idemitsu Oil Co., Ltd.) 1g / m ² oil coating ・ Punch diameter and shape ・ ・ ・ 33mmφ flat bottom cylinder ・ Clear ・ ・ ・ ・ 1mm
・ Blank diameter ... 60mm
・ Wrinkle presser load ・ ・ ・ ・ 29420N (3tf)
・ Aperture speed ... 60mm / sec.
The test time was 300 cycles for the flat portion, and 100 cycles for the cross cut portion and the side wall portion outside the cylindrical cup drawn sample. After completion of the test, the respective maximum thickness reduction values were measured, and the outer surface corrosion resistance was evaluated according to the following criteria. The evaluation results are shown in Table 2.
In actual tank production, it is common to apply a top coat to the upper layer of the film. However, in order to evaluate the performance of the second composite film, the performance is measured without the top coat. evaluated.
A: Thickness reduction amount <0.3 mm
○: 0.3 mm ≦ thickness reduction value <0.5 mm
Δ: 0.5 mm ≦ thickness reduction value <1.0 mm
×: 1.0 mm ≦ thickness reduction value (perforated)

(D) Inner surface corrosion resistance evaluation method About the inner surface corrosion resistance of the 1st composite membrane | film | coat, it evaluated using the plane part and the cylindrical cup squeezed sample. The cylindrical cup squeezed sample was assumed to be an actual tank, and was pressed so that the surface in contact with the punch was the first composite coating surface, that is, the first composite coating surface was inside the cup. The press working conditions are shown below.
<Pressing conditions>
・ Oil coating ・ ・ ・ ・ ・ ・ ・ ・ Rust preventive oil Z5 (made by Idemitsu Oil Co., Ltd.) 1g / m ² oil coating ・ Punch diameter and shape ・ ・ ・ 33mmφ flat bottom cylinder ・ Clear ・ ・ ・ ・ 1mm
・ Blank diameter ... 60mm
・ Wrinkle presser load ・ ・ ・ ・ 29420N (3tf)
・ Aperture speed ... 60mm / sec.
When investigating the flat part, after thoroughly stirring the test fuel consisting of unleaded gasoline / 500 mass ppm formic acid aqueous solution = 1/1 (mass ratio), immerse a test piece of 20 mm x 100 mm at room temperature. The area ratio (%) of red rust after immersion for 1 month was measured.
When evaluating the inside of the cup, about 80% of the volume of the fuel was put into the cup, stirred sufficiently, and left at room temperature for 1 month. The red rust area ratio (%) on the inner surface of the subsequent cup was measured.
Since the fuel is separated from the formic acid aqueous solution in the lower layer and the unleaded gasoline in the upper layer from the specific gravity permutation, the red rust generation area ratio in each part (gasoline phase, water phase) is measured, and (i) And each of (ii) evaluated internal corrosion resistance. The evaluation results are shown in Table 2.
○: Area ratio of red rust occurrence <50%
Δ: 50% ≦ red rust area ratio <80%
×: 80% ≦ Red rust occurrence area rate

* 1 Plating types EG: Electrogalvanized EZN: Zinc-12 mass% Nickel alloy plated GA: Alloyed hot dip galvanized
* 2 Type of conductive particles A: Nickel metal powder B: Graphite: “TGP-05” (manufactured by Tokai Carbon Co., Ltd.)
C: Antimony zinc complex oxide: “CELNAX CX-Z300H (registered trademark)” (manufactured by Nissan Chemical Industries, Ltd.)
D: ATO (antimony-doped tin oxide) powder: “SN-100P” (Ishihara Sangyo Co., Ltd.)
E: Colloidal dispersion of ATO: “SN-100D” (Ishihara Sangyo Co., Ltd.)
* 3 Base resin a of the second composite film a: Aqueous epoxy resin: Adeka Resin EM (manufactured by Adeka Co., Ltd.) in an acrylic emulsion resin aqueous dispersion Acrylic emulsion resin: Epoxy resin = 50:50 (mass ratio) Formulated to be
b: Aqueous epoxy resin: W2821R70 (Mitsubishi Chemical Co., Ltd.) blended in an aqueous dispersion of acrylic emulsion resin so that acrylic emulsion resin: epoxy resin = 70: 30 (mass ratio).
* The underlined part in the table indicates that it is outside the scope of the present invention.

  From the results of Table 2, it was found that the fuel tank steel plates of the examples showed good results in all of press workability, seam weldability, spot weldability, outer surface corrosion resistance, and inner surface corrosion resistance. On the other hand, the steel sheet for fuel tanks of the comparative example resulted in failure for any of the above evaluation items.

  According to the present invention, it is possible to provide a steel sheet for a high corrosion resistance fuel tank that is excellent in press workability, outer surface corrosion resistance, and inner surface corrosion resistance, and in particular has improved resistance weldability as compared with a conventional fuel tank steel sheet. . Moreover, since it does not contain lead which is a harmful substance at all like a turn-plated steel sheet, it has extremely high industrial value as a steel sheet for fuel tanks.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Zinc-type plating layer 3 Trivalent chromate layer 4 1st composite film 5 2nd composite film

Claims (1)

In mass%, C: 0.0007 to 0.0050%, P: 0.005 to 0.10%, Al: 0.01 to 0.20%, Ti: 0.005 to 0.08%, B: 0.0012 to 0.01%, S: 0.015% or less, N: 0.01% or less , Si: 0.5% or less and Mn: 2.0% or less on both surfaces of the steel sheet,
A zinc-based plating layer and a trivalent chromate layer are sequentially formed,
On the chromate layer formed on one surface side of the steel sheet, a first composite film containing Al and Ni metal powder and an amine-modified epoxy resin is formed,
On the trivalent chromate layer formed on the other surface side of the steel sheet, one or two selected from the group consisting of acrylic emulsion resin, silica, lubricant, and metal particles, metal compound particles and graphite particles A steel plate for a fuel tank, comprising the second composite film containing the above conductive particles having a particle diameter in the range of 0.01 to 3.0 μm in median diameter (50% D).

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