JP5953901B2 - Steel plate for fuel tank and manufacturing method thereof - Google Patents

Description

  The present invention is particularly excellent in press formability in the tank manufacturing process, can be welded, has excellent corrosion resistance on the inner surface of the tank, and has no hexavalent chromium remaining in the chromate film, and a method for manufacturing the same, The present invention relates to a fuel tank, and more particularly to a fuel tank steel sheet suitable for a motorcycle fuel tank.

  Conventionally, steel sheets for motorcycle fuel tanks used in Japan have been cold-rolled steel sheets. However, for the purpose of improving corrosion resistance on the inner surface of the tank, Zn-Ni plating is applied only to the inner surface, or a chromate treatment is applied to the upper layer. Applied steel plates have been developed and are now in general use.

In Patent Document 1, an electrical Zn—Ni alloy plating layer containing 5 to 30% by mass of Ni is formed on at least one side of a steel plate with an adhesion amount of 1 to 40 g / m ² per side, and then on the alloy plating layer. After applying an aqueous solution containing Ti colloid and drying, chromic acid having a mass ratio (trivalent chromium) / (total chromium) of more than 0.5 and a mass ratio (phosphoric acid) / (total chromium) of 0. A method for producing a steel plate for a fuel tank is disclosed in which a chromate treatment solution containing 1 to 5.0 phosphoric acid and an organic reducing agent is applied and heated.

  Patent Document 2 discloses a chromate-treated steel sheet that suppresses elution of hexavalent chromium as a fuel tank steel sheet that does not use Pb. In this steel sheet, the mass ratio of trivalent chromium to total chromium is 0.4 to 0.8, and the mass ratio to total chromium is 0.2 to 0.2 in a chromium aqueous solution in which 5 to 50 g / liter of chromium is dissolved. Mainly mixed with 1.5 phosphoric acid, 0.1 to 1.0 hydrofluoric acid, 0.5 to 20.0, liquid phase silica having a pH of 2 to 5, and 0.05 to 0.3 sulfuric acid A chromate treatment solution containing a solution and an aqueous curing agent solution to which 2 to 10% by mass of an epoxy-based silane is added, the pH is 2 to 3, and the mass ratio to the main solution is 0.05 to 0.5 is zinc. Or it is a steel plate coated on a steel plate plated with a zinc-based alloy.

Japanese Patent No. 4654714 Japanese Patent No. 3418177

  However, since it is necessary to press the steel plate for fuel tanks under particularly high pressing conditions, if a steel plate having a Zn—Ni alloy plating layer described in Patent Document 1 is used, the Zn—Ni plating layer of the bead portion is used. Powdering occurs and accumulates. For this reason, a friction coefficient rises and press troubles, such as a crack and a galling, generate | occur | produce at the time of press molding. Moreover, when the steel plate described in Patent Document 2 is used as a steel plate for a fuel tank, it becomes a hard film because silica is added and a hardener is added. Will occur. As a result, the friction coefficient increases and the press formability is inferior. Furthermore, such film damage reduces the protective function of the film and inferior internal corrosion resistance. Furthermore, when the curing reaction of the film component proceeds by the curing agent, it becomes difficult to eliminate the film by the exothermic reaction at the time of welding, which causes a problem of poor welding.

  The present invention has been made in view of the above problems, and has excellent press formability even in severe press conditions in a tank press working process when manufacturing a steel plate for a fuel tank, and has weldability and a tank. An object of the present invention is to provide a steel plate for a fuel tank that is excellent in corrosion resistance on the inner surface side and in which hexavalent chromium does not remain in the chromate film, a method for producing the same, and a fuel tank.

That is, the gist configuration of the present invention is as follows.
(1) The surface which becomes the inner surface side of the tank has a pure zinc plating layer containing 5 to 1000 mass ppm of Ni and having an adhesion amount of 1 to 150 g / m ² , and on the pure zinc plating layer, Chromates containing chromic acid with a ratio (trivalent chromium) / (total chromium) of more than 0.5, phosphoric acid with a mass ratio (phosphoric acid) / (total chromium) of 0.1-5.0 and an organic reducing agent It has a chromate film formed by heating after applying the treatment liquid, and the chromate film has a change rate of the chromium adhesion amount before and after the immersion when immersed in boiling water for 30 minutes within 2.0%. A steel plate for a fuel tank, characterized in that there is.
(2) The steel plate for a fuel tank according to (1), wherein the chromate treatment liquid contains an inorganic inhibitor having a mass ratio with respect to hexavalent chromium of less than 0.05.
(3) The fuel according to (1) or (2), wherein the chromate film is formed by applying an aqueous solution containing a Ti colloid, applying the chromate treatment liquid, and then heating. Steel plate for tanks.
(4) A pure galvanized layer containing 5 to 1000 mass ppm of Ni and having an adhesion amount of 1 to 150 g / m ² is formed on the inner surface of the tank, and the mass ratio is then formed on the pure galvanized layer. Chromate treatment containing (chromium) with (trivalent chromium) / (total chromium) exceeding 0.5, phosphoric acid with a mass ratio (phosphoric acid) / (total chromium) of 0.1 to 5.0 and an organic reducing agent A method for producing a steel plate for a fuel tank, wherein a chromate film is formed by heating after applying the liquid.
(5) The method for producing a steel plate for a fuel tank according to (4), wherein an inorganic inhibitor having a mass ratio of less than 0.05 to hexavalent chromium is added to the chromate treatment solution.
(6) The method for producing a steel sheet for a fuel tank according to (4) or (5), wherein an aqueous solution containing a Ti colloid is applied on the pure galvanized layer, and then the chromate treatment solution is applied. .
In the present invention, the fact that hexavalent chromium does not remain in the chromate film means a chromate film in which the rate of change in the amount of chromium deposited before and after immersion in boiling water for 30 minutes is within 2.0%. .

  According to the present invention, in the tank press working process when manufacturing a steel plate for a fuel tank, it has excellent press formability even under severe press conditions, and has excellent weldability and corrosion resistance on the inner surface of the tank, and chromate. It is possible to provide a steel plate for a fuel tank in which no hexavalent chromium remains in the film, a method for producing the same, and a fuel tank.

It is the seam welding apparatus schematic used for the resistance weldability test, and sectional drawing of an electrode (Example 1).

(1) Pure galvanized layer The present inventors are actual presses that are high-speed and high surface pressure in a tank press working process when manufacturing steel plates for fuel tanks for motorcycles, for example, for steel plates having various plating compositions. Were simulated, a sliding test was performed, and a friction coefficient was measured. The inventors of the present invention have studied a steel plate that has less powdering and maintains a constant coefficient of friction under the following sliding conditions.
-Contact area between mold and sample: 3mm x 10mm
・ Sliding distance: 1mm
・ Pressure: 130.4 MPa
・ Sliding speed: 1.0 m / min
・ Lubricant: Antirust oil ・ Friction coefficient μ = Pulling load / Pressure force As a result, when a pure galvanized layer containing 5 to 1000 mass ppm of Ni is used as the plating composition, the initial friction coefficient is low and the powder It was found that there were few rings. Therefore, in the case of severe press conditions with high surface pressure in the production of fuel tank steel plates, it has been found that a fuel tank steel plate having a pure galvanized layer is optimal and press trouble can be eliminated.

In the present invention, Ni is added to the pure galvanized layer in an amount of 5 to 1000 mass ppm. The pure galvanized layer may be more easily dissolved by moisture or organic acids such as formic acid and acetic acid mixed as impurities in the biofuel, compared to the Zn-Ni plated layer. Dissolved zinc corrosion products can clog the filters in the tank and adversely affect the fuel supply. Therefore, in order to achieve both the internal corrosion resistance against water and organic acids and the press formability described above, further investigations have resulted in the need to add 5 to 1000 ppm by mass of Ni in the pure galvanized layer. It came to the conclusion that it is.
If the amount of Ni is less than 5 ppm by mass, the effect of improving corrosion resistance cannot be obtained. If it exceeds 1000 mass ppm, the initial coefficient of friction increases, and adhesion occurs and grows easily. Preferably, it is 5-500 mass ppm. In order to add Ni to the pure zinc plating layer, plating may be performed in a zinc plating bath containing 10 to 2000 ppm by mass of Ni.

The coating weight of pure zinc plated layer of the present invention is required to be ^{1g / m 2 ~150g / m 2} . If the amount of plating is less than 1 g / m ² , sufficient corrosion resistance cannot be obtained, and if it exceeds 150 g / m ² , welding becomes difficult. At the manufacturing site, there is a case where a method of removing the pure galvanized layer before welding, for example, by grinding, is taken in order to promote bonding and prevent electrode deterioration. If the amount is in the above range, it is within the scope of the present invention. The pure galvanized layer may be formed by any commonly used galvanizing method, and methods such as electrogalvanizing and hot dip galvanizing are applicable. In addition, the pure zinc plating layer in this invention means the plating layer whose zinc other than the component inevitably mixed is zinc. The present invention is characterized in that Ni is intentionally added to such a pure galvanized layer.

(2) Chromate film In the present invention, on the pure galvanized layer, a chromate film having a change rate of chromium adhesion within 2.0% before and after immersion when immersed in boiling water for 30 minutes is provided. It is characterized by. Here, the change of the chromium adhesion amount before and after being immersed in boiling water for 30 minutes is based on the boiling water resistance test described in JIS K 5400-1990, 8.20, and the chromium adhesion amount before and after immersion in boiling water for 30 minutes. Is obtained by measuring by the fluorescent X-ray method. In the fluorescent X-ray method, the chromium adhesion amount is determined from a chromium count number prepared in advance using a standard sample with a known chromium adhesion amount and a calibration curve of the chromium adhesion amount. When hexavalent chromium is present in the chromate film, hexavalent chromium may be eluted in an aqueous solution such as condensed water or rainwater that may enter the tank. Therefore, it is important to measure the amount of hexavalent chromium eluted from the film. Regarding the elution of hexavalent chromium, a method for determining the type of elution liquid, elution temperature, elution time, such as the Volvo Leach Test (Volvo Standard News 1991.10), and evaluating the concentration of chromium eluted in the liquid, As described in JP-A-10-46353, there is a method in which the chromium adhesion amount of a steel sheet sample before and after alkali elution treatment is measured by fluorescent X-rays, and the elution amount is evaluated from the difference. The present invention is a method for measuring the amount of chromium deposited on a steel sheet sample by fluorescent X-ray, and can be evaluated by the difference in the amount of chromium deposited before and after the treatment, that is, the rate of change in the amount of chromium deposited. . In the present invention, in view of the measurement error at the time of measuring the chromium adhesion amount, the change rate of the chromium adhesion amount is set to 2.0% or less so that hexavalent chromium does not remain in the chromate film.

  In order to form the chromate film as described above, chromic acid having a mass ratio of trivalent chromium to total chromium of more than 0.5, phosphoric acid having a mass ratio of 0.1 to 5.0 with respect to total chromium, and organic reduction It is necessary to heat the chromate treatment solution containing the agent after coating.

  The chromate treatment liquid is in a state where hexavalent chromium and trivalent chromium are mixed. Among these, hexavalent chromium is reduced to trivalent chromium by reaction with an organic reducing agent during heating after coating. When the amount of hexavalent chromium is excessive among the total chromium amount of these hexavalent chromium and trivalent chromium combined, hexavalent chromium remains in the chromate film after heating. When hexavalent chromium remains in the chromate film, there is a possibility that chromium will elute in an aqueous solution such as condensed water or rain water that may enter the tank. As a method for controlling the residual hexavalent chromium amount, it is effective to limit the mass ratio between the total chromium amount and the trivalent chromium amount in the chromate treatment solution. As a result of various studies, it has been found that when the mass ratio of trivalent chromium to total chromium exceeds 0.5, no hexavalent chromium remains. Therefore, the mass ratio of trivalent chromium to total chromium is set to more than 0.5.

  The amount of phosphoric acid is 0.1 to 5.0 with respect to the total chromium. By adding phosphoric acid, a trivalent chromium-phosphoric acid-based film is formed, the amount of trivalent chromium is increased, and a dense film is formed, improving the barrier effect and improving the corrosion resistance. When the mass ratio of phosphoric acid is less than 0.1, trivalent chromium is polymerized to form a gel-like precipitate, so that the properties as the chromate treatment liquid cannot be maintained and cannot be applied to the steel sheet. On the other hand, when this ratio exceeds 5.0, phosphoric acid remains excessively in the chromate film, and this phosphoric acid elutes in a wet environment, causing pitting corrosion and blackening of plating. In addition, hexavalent chromium may remain in the chromate film.

  In the present invention, the chromate treatment liquid contains an organic reducing agent in addition to the chromic acid and phosphoric acid. As the organic reducing agent to be contained in the chromate treatment liquid, it is preferable to use at least one selected from diols and saccharides. Among the diols, ethylene glycol, propylene glycol, trimethylene glycol, 1,4-butanediol, and the like are particularly preferable. On the other hand, among saccharides, glycerin, polyethylene glycol, saccharose, lactose, sucrose, glucose or fructose are advantageously suitable.

  This organic reducing agent is preferably contained in the chromate treatment solution so that the mass ratio with respect to the total chromium is 0.1 to 0.4. If it is less than 0.1, a sufficient reduction effect cannot be obtained, and hexavalent chromium may remain in the chromate film. On the other hand, if it exceeds 0.4, the stability of the chromate treatment solution cannot be maintained. The organic reducing agent is preferably added to the chromate treatment solution immediately before the chromate treatment solution is applied to the pure galvanized layer from the viewpoint of improving the stability of the chromate treatment solution.

The chromate treatment solution may contain an inorganic inhibitor as necessary for the purpose of improving the corrosion resistance. Examples of such inorganic inhibitors include inorganic colloids such as silica, ZrO ₂ , TiO ₂ , zirconium sulfate, and aluminum diphosphate, and heteropolyacids such as phosphomolybdic acid, silicotungstic acid, and phosphovanadomolybdic acid. However, if an inorganic inhibitor is present in the chromate treatment solution, the reaction between hexavalent chromium and the organic reducing agent may be delayed, and elution of hexavalent chromium may be promoted when the chromate film is immersed in boiling water. For this reason, content of an inorganic inhibitor shall be less than 0.05 by mass ratio with respect to hexavalent chromium. The reason why the iron-free inhibitor delays the reaction rate between hexavalent chromium and the organic reducing agent is not clear, but the factor is that it interacts with hexavalent chromium ions when ionized or dispersed in solution. it is conceivable that.

  In addition, the chromate treatment solution may contain an acid such as hydrofluoric acid, sulfuric acid and hydrochloric acid for the purpose of promoting the reactivity with the pure zinc plating layer.

  Further, the chromate treatment liquid may contain a water-soluble or water-dispersible polymer compound in order to promote the reduction reaction from hexavalent chromium to trivalent chromium in the chromate film. Examples of water-soluble or water-dispersible polymer compounds include polyvinyl alcohol, polyacrylic acid, polyacrylamide, epoxy ester polymers, melamine alkyd resin polymers, natural polymer compounds such as starch and casein, and partial hydrolysis of alkinosilicate esters. Products, partial hydrolysates of alkyl phosphates, silane compounds such as silane coupling agents and epoxy silanes. These water-soluble or water-dispersible polymer compounds have an effect of promoting the hexavalent chromium reduction reaction in the chromate film and a protective film against external mechanical impact. However, since the terminal functional group acts as a reducing agent for hexavalent chromium ions, in order to ensure the stability of the treatment liquid, the content thereof should be less than 0.05 by mass ratio with respect to hexavalent chromium. preferable.

The chromate film of the present invention is formed by applying a special chromate treatment liquid as described above onto a pure galvanized layer and then heating. The adhesion amount of the chromate film is preferably about 10 to 50 mg / m ^{2 in} terms of metal chromium. Deposition amount of chromate film is not sufficient corrosion resistance can not be obtained with less than 10 mg / m ² reckoned as metal chromium, whereas weldability exceeds 50 mg / m ² is reduced, the cost becomes high. In heating after application of the chromate treatment solution, it is preferable to heat the steel plate so that the temperature of the steel plate is 120 ° C. or higher. When the heating temperature is less than 120 ° C., the reduction reaction of hexavalent chromium does not proceed sufficiently, and there is a risk that hexavalent chromium remains in the chromate film when immersed in boiling water.

  In the present invention, by providing the above-mentioned chromate film on the above pure zinc plating layer, the corrosion resistance is improved and the occurrence of white rust which is a corrosion product of the pure zinc plating layer is prevented.

  Furthermore, it is preferable to apply and dry an aqueous solution containing Ti colloid prior to applying the chromate treatment solution. By performing such a Ti colloid treatment, hexavalent chromium does not remain in the chromate film. This is because when Ti colloid is adsorbed onto the pure galvanized layer, Ti colloid acts as a reaction active site with the acidic chromate treatment solution, and thus hexavalent chromium is reduced to insoluble trivalent chromium during heating. This is probably because the reaction is accelerated. When applying such an aqueous solution containing Ti colloid, it is preferable to apply an aqueous solution containing pH: 7.5 to 10 and temperature: 40 to 60 ° C. containing Ti colloid having a concentration of 1 to 10 volppm for 1 to 30 seconds. In addition, the drying conditions should just dry a water | moisture content, For example, the method of drying with 60-80 degreeC hot air, the method of putting in a 100-120 degreeC thermostat for about 20 seconds, etc. are mentioned.

  The above pure galvanized layer and chromate film must be formed on the inner surface side of the tank, but may be processed on the outer surface side of the tank as necessary. Furthermore, in order to improve corrosion resistance, an organic film can be further provided on the upper layer of chromate.

  The steel plate used for the fuel tank steel plate of the present invention may be a cold-rolled steel plate, 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, Al: 0.01-0.20%, N: 0.01% or less, Ti: 0.005-0.08% and B: A cold-rolled steel sheet containing 0.001 to 0.01% and having a balance of Fe and inevitable impurities and excellent in deep drawability is suitable.

  Hereinafter, the reason for limitation of each component is demonstrated.

C: 0.0007 to 0.0050%
Since C adversely affects deep drawability, the content needs to be 0.0050% or less. Moreover, 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 0.0007 to 0.0050%.

Si: 0.5% or less Si has an action of increasing the strength of steel, and 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 boundary to strengthen the grain boundary, suppresses cracking of the welded portion, and has an effect of strengthening the steel. However, if the amount exceeds 0.1%, deep drawability deteriorates, so the P amount is preferably 0.1% or less. In addition, in order to suppress the crack of a weld part more reliably, it is more preferable to make P amount 0.01-0.05%.

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

Al: 0.01-0.20%
Al is added for deoxidizing steel and improving the yield of carbonitride-forming elements such as Ti. However, if the amount is less than 0.01%, the effect of addition is poor. On the other hand, if the amount exceeds 0.20%, the effect is saturated. Therefore, the Al amount is preferably 0.01 to 0.20%.

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 the solid solution C and N. However, if the amount is less than 0.005%, the effect is small. On the other hand, if the amount exceeds 0.08%, the effect is saturated, so the Ti amount is preferably 0.005 to 0.08%.

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. Therefore, the B content is preferably 0.001 to 0.01%, and more preferably 0.001 to 0.004%.

  The reason why B or P suppresses cracks in the weld is considered as follows. That is, the weld crack is presumed to be due to liquid metal embrittlement in which Cu as the main component of the electrode and Zn as the plating component become liquid during welding and enter the grain boundaries of the steel to embrittle the grain boundaries. In this respect, since B and P are easily segregated at the grain boundary, the grain boundary is strengthened to suppress such weld cracking.

  The balance is Fe and inevitable impurities. Here, the amount of inevitable impurities should just be in a normal range, for example, O is 0.010% or less.

  In addition to the above components, adding Nb in an amount of 0.0005 to 0.0050% or more is suitable for improving deep drawability.

  The steel plate for a fuel tank of the present invention can be used as a material for a fuel tank, particularly a material for a fuel tank for a motorcycle.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited to these Examples.

About the steel plate used for the steel plate for fuel tanks, a cold-rolled steel plate having a thickness of 0.8 mm (mass%, C: 0.0015%, Si: 0.01%, Mn: 0.08%, P: 0.011) %, S: 0.008%, Al: 0.05%, N: 0.0019%, Ti: 0.035%, Nb: 0.003% and B: 0.004%, the balance being Fe And the cold-rolled steel sheet having an inevitable impurity composition) was subjected to electrogalvanizing with the plating adhesion amount shown in Table 1 to form a pure galvanized layer.
Next, chromate treatment was performed on one surface of the pure galvanized layer. Specifically, the chromate treatment liquid shown in Table 1 was applied by a roll coater and then heated to form a chromate film. The heating temperature and chromium adhesion amount are shown in Table 1. In addition, heating temperature is the highest temperature which a steel plate reaches | attains.
No. Regarding No. 22, before the chromate treatment, a Ti colloid treatment was performed in which an aqueous solution containing Ti colloid was applied to a pure galvanized layer. As treatment conditions, an aqueous solution of pH = 8, 60 ° C. containing 8 vol ppm of Ti colloid was spray-coated for 5 seconds, and then dried with hot air at 60 ° C.
Moreover, about the obtained steel plate for fuel tanks, chromium elution resistance, slidability, internal corrosion resistance, and resistance weldability were evaluated. Each evaluation method is as follows. For all the evaluation items, the examples of the present invention that satisfy the target level of the present invention are those that are not judged as Δ or ×.
(1) Chromium adhesion rate change rate measurement Based on the boiling water immersion method (JIS K 5400-1990), it was measured by fluorescent X-rays, and the change rate of the chromium adhesion rate before and after immersion was determined by the following formula. In view of the measurement error when measuring the chromium adhesion amount, it was determined that no hexavalent chromium remained in the chromate film if the change rate of the chromium adhesion amount was within 2.0%.
Rate of change (%) = (Amount of chromium before immersion−Amount of chromium after immersion) / Amount of chromium attached before immersion × 100
About the rate of change (%) obtained by the said measurement, it determined as follows.
○: Change rate within 2.0% ×: Change rate over 2.0% (2) Slidability test A slidability test was conducted as a press formability evaluation, and a friction coefficient was measured. The friction coefficient was calculated by the following formula (1). Each measurement condition is as follows.
Friction coefficient μ = Pulling load / Pressure force (1)
-Contact area between mold and sample: 3mm x 10mm
・ Sliding distance: 1mm
・ Pressure: 130.4 MPa
・ Sliding speed: 1.0 m / min
-Lubricating oil: Antirust oil About the friction coefficient (mu) obtained by the said test, it determined as follows.
◎: Less than 0.10 ○: 0.10 or more and less than 0.13 Δ: 0.13 or more and less than 0.15 ×: 0.15 or more (3) Blank using a punch with inner surface corrosion resistance of φ50 mm and shoulder R of 2.75 mm A sample with a diameter of φ100 mm was processed, and a sample processed into a cup shape with a height of 30 mm was used as a test material. The following two types of fuels were added and allowed to stand at room temperature for one month, and then the inner surface corrosion resistance was evaluated from the amount of rust generated inside.
(Liquid A) Unleaded gasoline: 30 ml, ion-exchanged water: 3 ml
(Liquid B) Unleaded gasoline: 15 ml, concentration: 500 volppm formic acid aqueous solution: 15 ml
Since the fuel is separated from the aqueous solution layer in the lower layer and unleaded gasoline in the upper layer due to the difference in specific gravity, the rust generation area ratio was measured for each layer. And the following standards evaluated the corrosion resistance with respect to gasoline with the following standards.
◎: White rust occurrence area ratio less than 30% ○: 30% or more and less than 50% △: 50% or more and less than 80% ×: 80% or more (4) Chromate film surface of test piece of resistance weldability 500 × 300mm They were joined together (the electrode contact surface was plated and no chromate film), and welding was performed by lap seam welding (see FIG. 1). The welding conditions are as follows.
-Electrode: Chrome-copper alloy electrode shape Electrode diameter 230mmφ, electrode thickness 8.0mm, end 4mmR
Center tip 15mmR (4.5mm width)
・ Welding method: Two-piece, lap seam welding ・ Pressure force: 400kgf (3923N)
Energizing time: 2/50 sec energization on, 1/50 sec energization off
・ Cooling: Internal water cooling ・ Welding speed: 2.5 m / min
・ Welding current: Various changes Continued to this sample by the peel tensile test method (JIS Z 3141: Seam welded joint test method), a load was applied until the sample broke. The welding current (kA) that is sufficient was determined as an appropriate current, and the range was determined. Seam weldability was evaluated according to the following criteria.
○: 4 kA or more Δ: 3 kA or more and less than 4 kA ×: less than 3 kA The results obtained are shown in Table 1.

From the results shown in Table 1, all of the examples of the present invention are excellent in press formability even under severe press conditions required when manufacturing a steel plate for a fuel tank. Furthermore, it has excellent weldability and corrosion resistance. Furthermore, no hexavalent chromium remains.
In addition, No. No. 29 could not be applied because of the gelation of the chromate treatment solution and could not be evaluated.

  The fuel tank steel plate used in the present invention can be suitably used as a material for fuel tanks of motorbikes, lawn mowers, tillers, outboard motors, etc., in addition to motorcycles.

Claims (6)

The surface which becomes the inner surface side of the tank has a pure zinc plating layer containing 5 to 1000 mass ppm of Ni and having an adhesion amount of 1 to 150 g / m ² , and on the pure zinc plating layer, a mass ratio (3 in valence chromium) / (total chromium) of a chromate treatment liquid greater than 0.5 of the chromic acid, the mass ratio (phosphoric acid) / (total chromium) contains phosphoric acid and an organic reducing agent 0.1 to 5.0 A steel plate for a fuel tank having a chromate film formed, wherein the chromate film has a change rate of chromium adhesion before and after immersion in boiling water for 30 minutes within 2.0%. .   The steel plate for a fuel tank according to claim 1, wherein the chromate treatment liquid contains an inorganic inhibitor having a mass ratio of less than 0.05 to hexavalent chromium. Wherein on the pure zinc plated layer, adsorbed has a Ti was, steel sheet for fuel tank according to claim 1 or 2, characterized in Rukoto further having a said chromate film. A pure zinc plating layer containing 5 to 1000 ppm by mass of Ni and having an adhesion amount of 1 to 150 g / m ² is formed on the inner surface of the tank, and then a mass ratio (trivalent) is formed on the pure zinc plating layer. Apply chromate treatment solution containing chromic acid with (chromium) / (total chromium) exceeding 0.5, phosphoric acid with mass ratio (phosphoric acid) / (total chromium) of 0.1-5.0 and organic reducing agent. A method for producing a steel plate for a fuel tank, wherein a chromate film is formed by heating.   The method for producing a steel plate for a fuel tank according to claim 4, wherein an inorganic inhibitor having a mass ratio with respect to hexavalent chromium of less than 0.05 is added to the chromate treatment liquid.   6. The method for producing a steel sheet for a fuel tank according to claim 4, wherein the chromate treatment liquid is applied after applying an aqueous solution containing Ti colloid on the pure galvanized layer.

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