JP2018021227A - Cleaning liquid, method for cleaning steel welded structure, and method for manufacturing steel welded structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cleaning liquid that well removes oxide film and fume on steel welded structure.SOLUTION: Provided are: a cleaning liquid for cleaning steel welded structure and which is an acidic solution containing EDTA or a salt thereof; and a method for cleaning steel welded structure in which a steel welded structure is immersed in an acidic solution containing EDTA or a salt thereof.SELECTED DRAWING: None

Description

  The present invention relates to a cleaning liquid for cleaning a steel welded structure, a method for cleaning a steel welded structure, and a method for manufacturing a steel welded structure.

  In a structure in which painting is performed after welding of automobile parts and the like, the welded portion is significantly poorer in paintability than the base material portion, and it is difficult to obtain desired corrosion resistance. This is caused by an oxide film or fumes adhering to the surface of the steel plate near the weld. These are attached so as to be in close contact with the steel sheet surface, and are difficult to remove by a general chemical conversion treatment or an electrodeposition coating line. Therefore, a technique for removing an oxide film, fumes and the like attached to the surface of the steel plate near the weld is desired.

  As such a technique, for example, Patent Document 1 discloses a fume removal method in which a galvanized steel sheet to which fume adheres is immersed in a solvent having a pH of 8 to 11 to which a chelating agent such as EDTA or DTPA is added.

JP 2014-188528 A

  According to the fume removal method described in Patent Document 1, it is said that by using an alkaline solution containing EDTA, a chelate action is generated in zinc and EDTA on the surface of the galvanized steel sheet, and the fume can be removed. However, with respect to the fume generated when welding to a steel plate not subjected to galvanization, the alkaline solution used in the fume removing method described in Patent Document 1 does not cause chelation between the iron surface and EDTA. There was a problem that could not be removed. In Patent Document 1, no attention is paid to the removal of the oxide film.

  Then, an object of this invention is to provide the cleaning liquid useful for the removal of the oxide film and fume on a steel welded structure. Further, as a pretreatment step of chemical conversion treatment, a method for cleaning a steel welded structure that can satisfactorily remove oxide films and fumes on the steel welded structure, and further, a steel welded structure provided with a cleaning step by the cleaning method It aims at providing the manufacturing method of a thing.

As a result of intensive studies to achieve the above object, the present inventors have found that the object can be achieved with the following configuration, and have completed the present invention.
That is, the present invention relates to a cleaning solution for cleaning a steel welded structure, which is an acidic solution containing EDTA or a salt thereof.

The concentration of EDTA or a salt thereof in the cleaning liquid described above may be 10 to 70 mmol / L.
The pH of the cleaning liquid described above may be 3-5.
The above-described cleaning liquid may contain polyphosphoric acid as a pH adjuster.
The temperature of the cleaning liquid described above may be 20 to 80 ° C.

  The present invention also relates to a method for cleaning a steel welded structure in which the steel welded structure is immersed in an acidic solution containing EDTA or a salt thereof.

In the above-described method for cleaning a steel welded structure, the concentration of EDTA or a salt thereof in the acidic solution may be 10 to 70 mmol / L.
In the above-described method for cleaning a steel welded structure, the acidic solution may have a pH of 3 to 5.
In the above-described method for cleaning a steel welded structure, the acidic solution may contain polyphosphoric acid as a pH adjuster.
In the above-described method for cleaning a steel welded structure, the temperature of the acidic solution may be 20 to 80 ° C.
In the above-described method for cleaning a steel welded structure, the immersion time of the steel welded structure in an acidic solution may be 1.5 to 5 minutes.

In the above-described method for cleaning a steel welded structure, the concentration of EDTA or a salt thereof in the acidic solution is X (mmol / L), the immersion time is Y (minutes), and X and Y are the following formula (1): You may be satisfied with the relationship.
Y ≧ −0.03X + 2.7 (1)

  In the above-described method for cleaning a steel welded structure, when the steel welded structure is immersed in an acidic solution, at least one of stirring of the acidic solution and peristalsis of the steel welded structure may be performed. .

  Further, the present invention provides a steel welded structure comprising the steps of obtaining a steel welded structure by welding to a steel material and the step of cleaning the steel welded structure by the above-described method for cleaning a steel welded structure. It also relates to the manufacturing method.

  In the above-described method for manufacturing a steel welded structure, the steel material may be welded under welding conditions where the welding heat input is 0.3 to 15 kJ / cm.

  In the above-described method for manufacturing a steel welded structure, the steel material may be welded using a shield gas having an inert gas concentration of 70% by volume or more.

  In the method for manufacturing a steel welded structure described above, the steel material may be welded at a welding speed of 1.5 m / min or less.

  The cleaning liquid of the present invention is useful for removing oxide films and fumes on a steel welded structure. In addition, according to the method for cleaning a steel welded structure of the present invention, an oxide film and fumes on the steel welded structure can be favorably removed as a pretreatment step for chemical conversion treatment. Furthermore, according to the method for manufacturing a steel welded structure of the present invention, it is possible to manufacture a steel welded structure from which an oxide film and fume are well removed.

1 is a photomicrograph of the surface of a test piece according to Example 48. FIG. FIG. 2 is a photomicrograph of the surface of the test piece according to Example 2. FIG. 3 is a graph in which EDTA concentration: X (mmol / L) is plotted on the horizontal axis and immersion time: Y (min) is plotted on the vertical axis for Examples 1 to 33.

  Hereinafter, embodiments for carrying out the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below.

<Cleaning liquid>
First, the cleaning liquid according to the present invention will be described. The cleaning liquid of the present invention is a cleaning liquid for cleaning a steel welded structure, and is an acidic solution containing EDTA or a salt thereof. An oxide film is formed on the surface of a steel welded structure obtained by welding to a steel material. Further, fumes generated during welding adhere to the steel material surface or the oxide film surface. Here, when the steel welded structure is cleaned with the cleaning liquid of the present invention, the cleaning liquid enters the interface between the oxide film or fume and the steel material base, and peels off the oxide film or fume. Further, the cleaning solution dissolves part of the oxide film and fumes and collects iron, zinc ions, and the like by the chelating action of EDTA. According to the cleaning liquid of the present invention, oxide films and fumes on the steel welded structure can be removed by these actions.

The cleaning liquid of the present invention contains EDTA (Ethylene Diamine Tetraacetic Acid, ethylenediaminetetraacetic acid) or a salt thereof. In the present specification, including EDTA or a salt thereof includes a case where both EDTA and a salt thereof are included.
Examples of the EDTA salt include Na salt (2Na salt, 3Na salt, 4Na salt), K salt (2K salt, 3K salt, 4K salt), and Ca · Na salt (Ca · 2Na salt). It is done. These may be hydrates. Among them, EDTA Na salt such as EDTA2Na (edetate disodium) is more preferable because of its high chelating action.
EDTA or a salt thereof can be used alone or in combination of two or more.

The cleaning liquid of the present invention is an acidic solution. When the cleaning liquid is acidic (that is, less than pH 7), the above-described action is effectively exhibited, and the oxide film and fume on the steel welded structure can be removed well. The pH of the cleaning liquid is preferably 5 or less, more preferably 4 or less.
On the other hand, the lower limit of the pH of the cleaning liquid is not particularly limited. However, when the pH is low, for example, when applied to a welded structure using a galvanized steel sheet, galvanization on the galvanized steel sheet may be eroded. From this viewpoint, the pH of the cleaning liquid is preferably 3 or more.

  The concentration of EDTA or a salt thereof in the cleaning liquid of the present invention is not particularly limited, but in order to exhibit the above cleaning effect more effectively, it is preferably 10 mmol / L or more, and 30 mmol / L or more. More preferably, it is more preferably 60 mmol / L or more. On the other hand, if the concentration of EDTA or a salt thereof is too high, precipitation may occur and it may be difficult to perform uniform cleaning. From this viewpoint, it is preferably 70 mmol / L or less.

Examples of the solvent used in the cleaning liquid of the present invention include tap water, distilled water, and ion exchange water.
Moreover, you may contain polyphosphoric acid, hydrochloric acid, nitric acid, a sulfuric acid, etc. suitably as a pH adjuster for adjusting pH to the range mentioned above. Here, it is preferable to use polyphosphoric acid from the viewpoint of the stability of the cleaning liquid, the ease of pH adjustment, the waste liquid treatment cost, and the like.
Further, the cleaning liquid of the present invention further contains a chelating agent other than EDTA or a salt thereof, such as citric acid or DTPA (Diethylene Triamine Pentaacetic Acid), as long as the effect of the present invention is not inhibited. May be. Furthermore, as long as the effects of the present invention are not inhibited, known additives such as surfactants and pH buffering agents that can be added to the cleaning liquid may be appropriately contained.

  The temperature of the cleaning liquid when using the cleaning liquid of the present invention is not particularly limited, but is preferably higher in order to exhibit the above-described cleaning effect more effectively. From this viewpoint, it is preferably 20 ° C. or higher. More preferably, it is 30 ° C. or higher. On the other hand, if the temperature is too high, the cleaning liquid may evaporate, and the concentration control of the cleaning liquid may be difficult. Therefore, the temperature is preferably 80 ° C. or less, and more preferably 60 ° C. or less.

<Washing method of steel welded structure>
Next, a method for cleaning a steel welded structure according to the present invention will be described. The method for cleaning a steel welded structure of the present invention (hereinafter also referred to as the cleaning method of the present invention) includes immersing the steel welded structure in an acidic solution containing EDTA or a salt thereof having a pH of 5 or lower, that is, the above-described cleaning solution. Including. According to the method for cleaning a steel welded structure of the present invention, an oxide film and fume on the steel welded structure can be removed satisfactorily.

  The acidic solution used in the cleaning method of the present invention is the above-described cleaning solution, and preferred embodiments thereof can be similarly applied.

In the cleaning method of the present invention, the immersion time of the steel welded structure in the acidic solution can be appropriately adjusted in consideration of the concentration of EDTA or its salt in the cleaning solution, and is not particularly limited. In order to exhibit the cleaning effect more effectively, it is preferably 1.5 minutes or longer.
On the other hand, the upper limit of the immersion time is not particularly limited, but when the immersion time is long, for example, when applied to a welded structure using a galvanized steel sheet, the galvanization on the galvanized steel sheet may be eroded. From this viewpoint, the immersion time is preferably 5 minutes or less, and more preferably 4 minutes or less.

Furthermore, in the cleaning method of the present invention, the concentration of EDTA or its salt in the acidic solution is X (mmol / L), the immersion time is Y (minutes), and X and Y have the relationship of the following formula (1): It is preferable to satisfy.
Y ≧ −0.03X + 2.7 (1)
Here, as demonstrated in the examples described later, a better cleaning effect can be obtained by performing cleaning under conditions that satisfy the relational expression (1).

  In the cleaning method of the cleaning method of the present invention, when the steel welded structure is immersed in the acidic solution, it is preferable to perform at least one of stirring of the acidic solution and peristalsis of the steel welded structure. If it does in this way, since EDTA or its salt can be continuously supplied to the steel welded structure surface used as a candidate for processing, the above-mentioned cleaning effect can be exhibited more effectively.

Although the stirring method in the case of stirring an acidic solution is not specifically limited, stirring using a pump, stirring using a stirring bar, etc. are illustrated. Here, as conditions for stirring the acidic solution using a pump, for example, a solution having a volume of 0.01 to 100 times the tank volume may be sent per minute. Moreover, what is necessary is just to stir with the rotation speed of 0.01-1000 rpm as conditions in the case of stirring an acidic solution using a stirring bar, for example.
Moreover, what is necessary is just to rock a steel welded structure, for example in the period of 6 to 120 times per minute as conditions in the case of rocking a steel welded structure.

In addition, in the washing | cleaning method of this invention, you may perform alkali degreasing | defatting and water washing as needed before the immersion to the acidic solution mentioned above. For alkali degreasing, for example, an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, or the like can be used. Moreover, as conditions of alkali degreasing, the process by pH 12-14, temperature 20-80 degreeC, and immersion time 1-3 minutes is illustrated, for example.
Moreover, in the washing | cleaning method of this invention, you may wash with water and dry as needed after the immersion to the acidic solution mentioned above.

<Method for producing steel welded structure>
Next, a method for manufacturing a steel welded structure according to the present invention will be described. The method for manufacturing a steel welded structure of the present invention (hereinafter also referred to as the manufacturing method of the present invention) includes a step of obtaining a steel welded structure by welding to a steel material (hereinafter also referred to as a welding step), and the steel described above. A step of cleaning the steel welded structure by a method of cleaning the welded structure (hereinafter also referred to as a cleaning step). According to the manufacturing method of the present invention, a steel welded structure from which an oxide film and fume are well removed can be manufactured by cleaning the steel welded structure by the above-described cleaning method.

(Welding process)
In the welding process in the manufacturing method of the present invention, known welding methods such as arc welding and laser welding can be applied without particular limitation.

  Here, the state of the oxide film generated in the vicinity of the weld bead is greatly influenced by a change in cooling rate due to a difference in welding heat input, a protective effect by an inert gas during welding, and the like. Therefore, in order to more effectively clean the vicinity of the weld bead in the cleaning process following the welding process, the thickness of the oxide film can be controlled by appropriately controlling various conditions such as welding heat input, shielding gas, and welding speed in the welding process. It is preferable to control the sheath adhesion.

  The welding heat input affects the cooling rate and the amount of thermal strain of the steel sheet. When the cooling rate is slow and the holding time at high temperature is long, the film thickness of the oxide film increases, which becomes a factor that hinders the reaction between the acidic solution and the steel material base. In addition, in the appropriate heat input range, thermal distortion occurs in the steel during the cooling process, and the oxide film layer cracks due to the difference in the coefficient of linear expansion between the base steel and the oxide film, and the reaction between the steel substrate and the solution Becomes easy. From these viewpoints, the welding heat input in the welding process is preferably 0.3 to 15 kJ / cm. When the welding heat input exceeds 15 kJ / cm, the cooling rate is excessively slowed, so that the thickness of the oxide film increases and the cleanability may be deteriorated. Further, if the welding heat input is less than 0.3 kJ / cm, the thermal distortion of the steel material due to welding is suppressed, so that the adhesion of the oxide film is high, and the cleanability may be reduced. The welding heat input is more preferably 1.0 kJ / cm or more, and more preferably 10 kJ / cm or less.

In addition, the welding heat input in this specification is defined by the following formula in the case of arc welding.
{Welding current I (A) × welding voltage V (V) × 60} / {welding speed (cm / min) × 1000}

Moreover, the welding heat input in this specification is defined by the following formula in the case of laser welding.
{Laser output (kW) x 60} / Welding speed (cm / min)

Shielding gas is used for both laser welding and arc welding to protect the weld from the surrounding atmosphere. In general, a mixed gas mainly composed of CO ₂ is often used as the shielding gas in order to suppress pore defects in the weld due to nitrogen in the atmosphere. However, an oxidizing gas such as CO ₂ contributes to an increase in the thickness of the oxide film because it promotes oxidation of the steel sheet surface near the weld. Therefore, in the present invention, it is preferable to employ a shield gas in which the concentration of an inert gas such as Ar, He, N ₂ is 70% by volume or more. The concentration of the inert gas in the shield gas is more preferably 80% by volume or more.

  The welding speed affects the welding heat input and the time during which the weld is protected by the shielding gas. When the welding speed is excessively fast, the vicinity of the welded portion is exposed to the atmosphere at a high temperature exceeding 800 ° C., which may promote surface oxidation. Therefore, the welding speed is preferably 1.5 m / min or less, and more preferably 1.2 m / min or less.

  Although the steel material welded in this welding process is not specifically limited, Typically, it is a steel plate. The steel plate may be a non-plated steel plate or a steel plate subjected to various plating treatments such as a galvanized steel plate or an aluminum plated steel plate. By welding to these steel materials, an oxide film or fume is formed on the surface.

(Washing process)
In the cleaning process, the steel welded structure obtained in the welding process is cleaned by the above-described cleaning method. Thereby, the oxide film and fume on a steel welded structure can be removed satisfactorily.

(Other processing or process)
In the production method of the present invention, a chemical conversion treatment, a coating step, and the like can be appropriately applied after the cleaning step. Here, according to the manufacturing method of the present invention, since the oxide film and the fumes on the steel welded structure are satisfactorily removed by the cleaning step, the subsequent chemical conversion treatment can be performed satisfactorily. Can be applied with good paintability, and a welded structure excellent in corrosion resistance can be obtained.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples, and may be implemented with modifications within a range that can be adapted to the gist of the present invention. All of which are within the scope of the present invention. Further, the welding conditions described here are merely examples, and the present embodiment is not limited to the following welding conditions.

(Test conditions)
In each example, a bead-on-plate welding was performed on a JIS G 3141 SPCC steel plate having a thickness of 2.3 mm, a width of 70 mm, and a length of 200 mm under the predetermined welding conditions shown in Table 1 to prepare a test piece. Subsequently, after performing alkaline degreasing by immersing in a sodium hydroxide aqueous solution adjusted to pH 13 and a temperature of 40 ° C. for 2 minutes, it was successively immersed in two tanks in which tap water was stored and washed twice.

“MAG” in the row of shielding gas in Table 1 is a mixed gas of 20% by volume of CO ₂ and 80% by volume of Ar. Further, “50% CO ₂ ” is a mixed gas of 50 volume% CO ₂ and 50 volume% Ar.

  Next, a cleaning liquid was prepared in a 500 ml beaker under the predetermined conditions shown in Table 2, and the test piece was immersed in the cleaning liquid under the predetermined conditions shown in Table 2 to perform a cleaning process. Thereafter, the welded structure was taken out from the cleaning liquid, immersed in two tanks in which tap water was stored in order, washed twice with water, and then dried at 60 ° C. to complete the treatment.

  The “stirring only” in the row of the stirring method in Table 2 means that a stirrer that is rotated by a magnetic force is added to the bottom of the beaker while stirring the test piece in the cleaning solution, and stirring is performed at a rotation speed of 150 rpm. Represents that. In addition, “stirring and shaking” means that the test piece was rocked at a cycle of 30 times per minute using an automatic test piece rocking device together with stirring under the above-described conditions.

(Evaluation of cleaning effect)
Regarding the cleaning effect, the surface of the steel plate in each example after cleaning was observed using a stereomicroscope, and the surface area of the portion covered with the oxide film in the field of view was less than 10%. Those with less than 30% were evaluated as ◯, those with 30% or more and less than 50% were evaluated as Δ, and those with 50% or more were evaluated as ×, and those other than evaluation were evaluated as ×. These results are shown in Table 3.

Moreover, the microscope picture of the test piece surface which concerns on the example 48 whose evaluation of a cleaning effect is x is shown in FIG. In FIG. 1, a blackish portion is a portion covered with an oxide film, and a whiteish portion is a portion where a steel plate substrate is exposed. That is, in the test piece shown in FIG. 1, it can be seen that many portions of the surface are covered with the oxide film.
On the other hand, a micrograph of the surface of the test piece according to Example 2 in which the evaluation of the cleaning effect is A is shown in FIG. In the test piece shown in FIG. 2, it can be seen that the whole is a whitish portion, that is, the oxide film is well removed.

(Evaluation of erosion to zinc plating)
In each example, a galvanized steel sheet was used instead of a non-plated steel sheet, and a test piece was prepared by welding in the same manner as described above, followed by cleaning. Here, the basis weight of the galvanized steel sheet was 45 g / m ² .
And about the erosion to galvanization, the weight of the test piece before and after the cleaning treatment is measured, and the galvanization residual amount is 40 g / m ² or more, and the one having less than 40 g / m ^{2 and} 30 g / m ² or more. ○, less than 30 g / m ^{2 and} 20 g / m ² or more were evaluated as Δ, and less than 20 g / m ² was evaluated as ×. These results are also shown in Table 3.

As shown in Table 3, in Example 47, the pH of the cleaning solution was 7, which was outside the range specified in the present invention, and a large amount of oxide film remained even after cleaning.
In Example 48, the pH of the cleaning solution was 9, which was outside the range specified in the present invention, and a large amount of oxide film remained after cleaning.
In Example 49 using a cleaning solution containing citric acid instead of EDTA, a large amount of oxide film remained after cleaning.
On the other hand, in Examples 1-46 which are an Example, the evaluation result of the cleaning effect was pass, and the oxide film was able to be removed favorably.

Moreover, about Examples 1-33, the graph which plotted EDTA density | concentration: X (mmol / L) on the horizontal axis and immersion time: Y (min) on the vertical axis | shaft is shown in FIG. Here, in the graph of FIG. 3, an example in which the evaluation of the cleaning effect is “◎” or “プ ロ ッ ト” is plotted as “◯”, and an example in which the evaluation of the cleaning effect is “Δ” is plotted as “Δ”.
Then, as shown in FIG. 3, in the region satisfying the following formula (1), the evaluation result of the cleaning effect is ◎ to ○, and it can be seen that a particularly good cleaning effect can be obtained.
Y ≧ −0.03X + 2.7 (1)

Claims (17)

A cleaning liquid for cleaning a steel welded structure,
A cleaning solution which is an acidic solution containing EDTA or a salt thereof.   The cleaning solution according to claim 1, wherein the concentration of the EDTA or a salt thereof in the cleaning solution is 10 to 70 mmol / L.   The cleaning liquid according to claim 1 or 2, wherein the cleaning liquid has a pH of 3 to 5.   The washing | cleaning liquid of any one of Claims 1-3 containing polyphosphoric acid as a pH adjuster.   The temperature of the said cleaning liquid is 20-80 degreeC, The cleaning liquid of any one of Claims 1-4.   A method for cleaning a steel welded structure, wherein the steel welded structure is immersed in an acidic solution containing EDTA or a salt thereof.   The method for cleaning a steel welded structure according to claim 6, wherein the concentration of the EDTA or a salt thereof in the acidic solution is 10 to 70 mmol / L.   The method for cleaning a steel welded structure according to claim 6 or 7, wherein the acidic solution has a pH of 3 to 5.   The method for cleaning a steel welded structure according to any one of claims 6 to 8, wherein the acidic solution contains polyphosphoric acid as a pH adjuster.   The temperature of the said acidic solution is 20-80 degreeC, The cleaning method of the steel welded structure of any one of Claims 6-9.   The method for cleaning a steel welded structure according to any one of claims 6 to 10, wherein the immersion time of the steel welded structure in the acidic solution is 1.5 to 5 minutes. The concentration of the EDTA or its salt in the acidic solution is X (mmol / L), the immersion time is Y (minutes), and X and Y satisfy the relationship of the following formula (1): A method for cleaning a steel welded structure according to any one of the preceding claims.
Y ≧ −0.03X + 2.7 (1)   13. When dipping the steel welded structure in the acid solution, at least one of stirring of the acid solution and peristalsis of the steel welded structure is performed. A method for cleaning a steel welded structure according to claim 1. A process of obtaining a steel welded structure by welding to a steel material;
A method for manufacturing a steel welded structure, comprising the step of cleaning the steel welded structure by the method for cleaning a steel welded structure according to any one of claims 6 to 13.   The method for manufacturing a steel welded structure according to claim 14, wherein the steel material is welded under a welding condition in which a welding heat input is 0.3 to 15 kJ / cm.   The method for manufacturing a steel welded structure according to claim 14 or 15, wherein the steel material is welded using a shielding gas having an inert gas concentration of 70% by volume or more.   The method for manufacturing a steel welded structure according to any one of claims 14 to 16, wherein the steel material is welded at a welding speed of 1.5 m / min or less.