JP2013221177A - Method for producing surface treated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treated steel sheet being a surface treated galvanized steel sheet which has a resin film essentially composed of a thermosetting resin and including a wax on the plating face of a galvanized steel sheet, and which can be press-formed without performing lubricating treatment, and which satisfies all of galling resistance, coating adhesion and solvent resistance on high levels.SOLUTION: A coating liquid in which a wax with the average particle diameter of 0.2 to 3 μm is incorporated into a coating liquid essentially composed of a thermosetting resin by 5 to 15 mass% based on the total solid content of the wax is prepared. The thermosetting resin has a composition regulated in such a manner that its curing completion temperature is made higher than the melting temperature of the wax. The coating liquid is applied to the surface of a galvanized steel sheet, thereafter, primary baking for ≥5 s is performed in a temperature region which is higher than the melting temperature of the wax and in which fluidity remains in the coating liquid, and next, secondary baking is performed at the setting completion temperature of the thermosetting resin or higher to form a surface treated film with a film thickness of ≥0.5 μm.

Description

  The present invention relates to a method for producing a zinc-based plated surface-treated steel sheet having a resinous surface-treated film containing a wax on a plated surface of a zinc-based plated steel sheet. The present invention relates to a method for producing a zinc-plated surface-treated steel sheet that is particularly excellent in mold galling resistance, paint adhesion, and solvent resistance, and that is suitable for applications in which coating is performed after washing with an organic solvent after press molding.

  Zinc-based plated surface-treated steel sheets coated with a wax-containing resinous coating on zinc-plated steel sheets are excellent in rust prevention and can be press-molded without lubrication such as applying press oil. It is used in a wide range of applications such as home appliances, automobiles, and building materials. Among these uses, after performing advanced molding, some of the dirt adhered during molding is washed and removed with a solvent, followed by painting to produce a product. For such applications, in particular, mold galling resistance, solvent resistance, and adhesion to the top coat are required. Patent Documents 1 and 2 below propose surface-treated steel sheets that satisfy this requirement.

  Patent Document 1 (Japanese Patent No. 2617835) discloses a method for producing a lubricated plated steel sheet in which a resin containing wax is applied after chromate treatment, and the film is immediately baked to reach a final plate temperature of 90 to 200 ° C. ing. By baking immediately after application, the surface concentration of the wax in the film is suppressed, and the adhesion of the top coat is ensured. However, since it is necessary to use heat treatment equipment that can cope with rapid heating, the equipment cost becomes high. Furthermore, even if the film is cured first, if baked at a temperature higher than the melting point of the wax, part of the liquefied wax may ooze out and concentrate on the surface of the film. Sex is reduced. Considering the adhesion with the top coat, it is necessary to lower the baking temperature as much as possible and to shorten the baking time. However, the resin cannot be cured sufficiently and the solvent resistance is lowered.

  Patent Document 2 (Japanese Patent No. 2622927) discloses a coated steel sheet containing spherical polyethylene wax particles in the main component urethane-based resin and having a resin film on which the spherical shape of the wax particles is retained on the steel sheet surface. ing. In order to keep the wax particles spherical, it is necessary to heat dry the film at a temperature lower than the melting point of the wax. Therefore, the curing of the resin does not proceed sufficiently, and the solvent resistance is also unsatisfactory.

  In this way, the conventional technology aims to achieve both mold caulking and adhesion with top coating by suppressing the surface concentration of the wax by curing the resin without melting the wax as much as possible. It is. However, since the baking is performed at a relatively low temperature, the resin cannot be sufficiently cured, and the solvent resistance cannot satisfy the high solvent resistance required by some users.

Japanese Patent No. 2617835 Japanese Patent No. 2622927

  The present invention relates to a method for producing a zinc-based plated surface-treated steel sheet having a wax-containing resin film on a plated surface of a zinc-based plated steel sheet, each having a high level of mold galling resistance, paint adhesion, and solvent resistance. The challenge is to establish it.

According to the present invention, on a plated surface of a zinc-based plated steel sheet, a method for producing a surface-treated steel sheet having a surface treatment film containing a thermosetting resin as a main component and containing a wax,
A coating solution containing 5 to 15% by mass of a wax having an average particle size of 0.2 to 3 μm based on the total solid content in a coating solution containing a thermosetting resin as a main component is prepared. The resin has a composition adjusted so that its curing completion temperature is higher than the melting temperature of the wax,
After this coating solution is applied to the surface of the galvanized steel sheet, primary baking is performed for 5 seconds or more in a temperature range where the melting temperature of the wax is higher and the fluidity remains in the coating solution, and then the resin system in the coating is cured. Secondary baking is performed at a temperature equal to or higher than the completion temperature to form a surface treatment film having a thickness of 0.5 μm or more.
A method for producing a surface-treated steel sheet excellent in mold galling resistance, paint adhesion and solvent resistance is provided.

In the present invention, the temperature range where the fluidity remains in the coating solution is a temperature range where thickening is observed when heated at a rate of 2 ° C./min with a viscoelasticity meter.
In a preferred embodiment, the thermosetting resin is an epoxy resin / curing agent / curing temperature adjusting agent mixture, and the wax is polyethylene wax.

  In the present invention, the curing completion temperature of the thermosetting resin is a temperature at which curing is completed when heated at a rate of 2 ° C./min using a viscoelasticity meter (a temperature at which viscoelasticity rapidly decreases in a viscoelastic curve). Means. The melting temperature of the wax is a temperature at which the wax is completely melted after being softened.

  According to the present invention, when forming a resin film containing a wax on the plated surface of a zinc-based plated steel sheet, the thermosetting resin is adjusted so that its curing completion temperature is higher than the melting temperature of the wax, Baking of the coating after coating is performed in two stages: primary baking at a temperature range that is higher than the melting temperature of the wax but remains fluid in the coating solution, and secondary baking at a temperature equal to or higher than the resin curing completion temperature. .

  In this way, the wax is melted before the complete curing of the resin proceeds, the molten wax is taken into the resin film, and then the resin is cured, so that the wax is uniformly concentrated in the film without concentrating on the surface layer. The resin can be completely cured while present in a dispersed state. As a result, coating adhesion can be ensured, and during severe molding, the wax is melted from the film by the frictional heat to impart mold galling resistance to the film. Furthermore, the resin can be sufficiently baked, and the resin can be in a fully cured state in which the resin is sufficiently cross-linked, so that the solvent resistance can be secured.

  Therefore, the surface-treated zinc-based plated steel sheet produced by the method according to the present invention is excellent in mold galling and can be used for hardness press molding without lubrication treatment, and thus is useful for press molding applications. . In addition, because it has excellent solvent resistance and coating adhesion, it can be painted with the surface treatment film left after molding by simply removing the dirt by solvent washing. Can be formed. Therefore, the surface-treated zinc-based plated steel sheet produced by the method of the present invention is suitable for applications in which a coating is applied after press molding and commercialized, and is particularly useful for the production of automobile parts.

Hereinafter, the manufacturing method of the surface-treated zinc-based plated steel sheet of the present invention will be described more specifically. However, the following description is for illustrative purposes and does not limit the present invention.
Zinc-based plating of a zinc-based plated steel sheet as a base material includes pure zinc plating and various types of zinc alloy plating. An example of zinc alloy plating is zinc alloy plating with Ni, Fe, Al or the like. Zinc-based plating is processed by electroplating or hot dipping. Alloyed hot dip galvanizing can be employed in which a heat treatment is performed after hot dip galvanizing to form a Zn—Fe alloy plating film. The amount of plating adhesion is not particularly limited and varies depending on the plating technique, but generally the range of 10 to 100 g / m ² is preferable from the balance between sacrificial corrosion resistance and production cost.

  Most zinc-based plated steel sheets are double-sided plated, but may be single-sided plated steel sheets. In that case, a surface treatment film is formed on the plated surface according to the present invention. In the case of a double-sided plated steel sheet, a surface treatment film can be formed on the double-sided or single-sided plating surface according to the present invention.

According to the present invention, first, a coating solution containing a thermosetting resin as a main component and containing 5 to 15% by mass of a wax having an average particle diameter of 0.2 to 3 μm based on the total solid content is prepared.
As the wax, it is preferable to use a synthetic wax because it can impart excellent mold galling resistance. Synthetic waxes suitable for use include polyolefin waxes such as polyethylene, polypropylene and polybutene, and polytetrafluoroethylene wax. Of these, polyethylene wax is particularly preferred.

  A wax having an average particle diameter of 0.2 to 3 μm is used. When the average particle size of the wax is smaller than 0.2 μm, the mold galling resistance is unsatisfactory, and when it is larger than 3 μm, the coating adhesion is unsatisfactory. For the same reason, the amount of wax in the film (and therefore the amount of wax based on the total solid content of the coating solution) is 5 to 15% by mass. The preferred average particle size of the wax is 0.5-2 μm.

  The thermosetting resin is preferably at least one resin having at least one functional group selected from a hydroxyl group, an isocyanate group, a carboxyl group, a glycidyl group, and an amino group. Specific examples include epoxy resins, alkyd resins, acrylic resins, urethane resins, polyvinyl butyral resins, phenol resins, melamine resins, and the like. If necessary, an appropriate curing agent is combined with the resin. For example, in the case of an epoxy resin, it is preferable to use a phenol resin (novolak type) as a curing agent, but other epoxy curing agents can also be used. The resin system can be selected from those commonly used in paints, particularly steel sheet paints.

  The thermosetting resin / curing agent combination to be used is selected so that the curing completion temperature is higher than the melting point (melting temperature) of the wax in accordance with the melting point of the wax to be blended. The curing completion temperature can also be adjusted by blending a small amount of an additional component other than the curing agent. In the present invention, such an additive is referred to as a curing temperature adjusting agent.

  Melamine (2,4,6-triamino-1,3,5-triazine, raw material for melamine resin) as a curing temperature regulator that can increase the curing completion temperature when the thermosetting resin is an epoxy resin Proved to be effective. By blending a small amount of melamine, the curing completion temperature of the epoxy resin can be made higher than the melting point of the wax. It is unclear why melamine exerts such an effect, but it is thought to be because it has an action that hinders the reaction between the epoxy resin and the curing agent.

  The blending amount of the curing temperature adjusting agent blended in a small amount to adjust the curing temperature may be 3% by mass or less, preferably 1% by mass or less, based on the main component resin (eg, epoxy resin). Even in such a small amount, the curing temperature of the resin system can be adjusted to a desired temperature.

  A coating solution is prepared by adding and dispersing the above wax in a resin solution in which a resin system (main resin and, if necessary, a curing agent and / or a curing temperature adjusting agent) is dissolved in an appropriate solvent as necessary. can do. The coating solution may be aqueous or organic solvent. The solid content concentration of the coating solution is adjusted to have a viscosity suitable for coating. The coating liquid may further contain components other than those described above. Examples of such components include coloring pigments, rust preventive pigments (eg, silica), surfactants and antifoaming agents. The coating liquid used in the present invention can also be prepared by adding a wax and, if necessary, a curing temperature adjusting agent to a commercially available primer coating for a steel sheet.

  The prepared coating solution is applied onto the plated surface of the galvanized steel sheet as the base material. The coating is typically performed using various roll coating apparatuses commonly used for coating steel sheets, but other coating methods (eg, spray coating) should be used if the coating thickness can be adjusted. You can also. Before coating, it is preferable to clean the surface of the galvanized steel sheet by a conventional method such as degreasing using an alkaline degreasing solution.

  The coating thickness can be adjusted according to the required corrosion resistance, but in order to ensure mold galling resistance, the film thickness after baking the coating film should be 0.5 μm or more. Although the upper limit of the film thickness is not particularly defined, it is preferably set to 3.0 μm or less from the viewpoint of press debris. The film thickness may be smaller or larger than the average particle diameter of the wax used. A preferable film thickness is in the range of 0.5 to 1.0 μm.

  After coating the coating solution containing wax on the surface of the zinc-based plated steel sheet, primary baking is performed for 5 seconds or more in the temperature range where the melting temperature of the wax is higher but the fluidity remains in the coating solution. Secondary baking is performed at a temperature equal to or higher than the curing completion temperature of the resin system (main resin + curing agent and curing temperature adjusting agent if present).

  The primary baking is preferably performed at a temperature 10 ° C. or more, preferably 20 ° C. or more higher than the melting temperature of the wax so that the wax melts quickly during heating for a short time. On the other hand, it is preferable that the temperature be 20 ° C. or more lower than the temperature at which the fluidity of the coating solution disappears so that the resin does not harden significantly during the primary baking. If the resin is cured too much at this stage, it is not possible to sufficiently prevent the wax from thickening.

  In the case of a resin system in which the curing temperature adjusting agent (eg, melamine) is blended with the main resin (eg, epoxy resin) and the curing completion temperature of the resin system is increased, the primary baking is performed at a temperature higher than the curing completion temperature of the main resin. It is preferable to do. For example, as shown in Examples described later, when the curing completion temperature of the main epoxy resin is 150 ° C. and the curing completion temperature of the resin system containing melamine is 230 ° C., the primary baking temperature is an intermediate value between the two. It can be within the range of ± 20 ° C. In addition, primary baking is performed by two or more stages of heating, such as first a low temperature (eg, a temperature near the curing completion temperature of the main resin), then a higher temperature (eg, the above-mentioned intermediate value ± 20 ° C.). It can also be done.

  This primary baking is performed for a time of 5 seconds or more. If this baking time is shorter than 5 seconds, the melted wax cannot be sufficiently dispersed in the film. The upper limit of the primary baking time is not particularly specified, but considering productivity, it is preferably within 15 seconds, more preferably within 10 seconds.

  Thereafter, secondary baking is performed at a temperature equal to or higher than the curing completion temperature of the resin system used in the coating solution to completely cure the resin. The secondary baking temperature may be the same as the curing completion temperature of the resin system, and the upper limit is not specified. However, considering the energy cost, it is advantageous that the temperature is 10 ° C. higher than the curing completion temperature. The heating time for the secondary baking is set so that the resin system of the film is completely cured. A preferred baking time is within 10 seconds, more preferably within 5 seconds.

  The primary baking and the secondary baking are advantageously performed continuously in the same heating furnace. Thus, the wax dispersed in the film is fixed in the film, and a surface-treated galvanized steel sheet having good coating adhesion, mold galling resistance, and solvent resistance is obtained. This surface-treated zinc-based plated steel sheet can be press-formed without lubrication treatment, can withstand severe press-forming, and can be coated after being washed with a solvent after press-forming.

(1) Substrate zinc-based plated steel sheet Electrogalvanized steel sheet (double-sided plating, plating coverage: 20 g / m ² per side) based on a cold rolled steel sheet of ultra-low carbon steel with a thickness of 0.8 mm is 200 mm × What was cut into 300 mm and subjected to alkali degreasing was used as a test substrate.

(2) Preparation of coating solution Urethane-modified epoxy resin as thermosetting resin, novolak type phenol resin as curing agent, melamine as curing temperature adjusting agent, polyethylene wax having the average particle size shown in Table 1 as wax, and epoxy resin as solvent Nine types of coating liquids having the solid content compositions shown in Table 1 were prepared using the above-mentioned thinner.

  Among these, since the coating liquid No. 8 has an average particle size of wax exceeding 3 μm and the coating liquid No. 9 does not contain wax, both are coating liquids for comparison. Further, as can be seen from the coating liquid No. 1, the curing completion temperature of the urethane-modified epoxy resin / phenol resin curing agent system is 150 ° C. However, by adding a very small amount of melamine, the curing completion temperature of the resin system is 230. It became ℃.

(3) Formation of surface treatment film The coating solution was applied to one side of the test base material of the above-described galvanized steel sheet with a bar coater so as to have a predetermined dry film thickness, and under any of the heating conditions shown in Table 2 Baking was performed in an oven to form a cured film on the plated surface, and a surface-treated zinc-based plated steel sheet provided with a resin film containing wax was obtained. Of the heating conditions shown in Table 2, conditions 1 and 5 are performed by one-stage heating, condition 3 is that the heating time for primary baking is shorter than 5 seconds, and condition 6 is that the primary baking temperature is melting of the wax. Since the temperature is not higher than the temperature, both are out of the scope of the present invention.

The obtained surface-treated galvanized steel sheet was used as a test piece, and the mold galling resistance, paint adhesion and solvent resistance were investigated by the following methods. In both cases, ○ or higher is acceptable.
(1) Mold galling resistance The test piece was molded into a U-shape using a crank press with the surface-treated surface (resin-coated surface) facing outward (die and punch shoulder R = 5 mm, clearance: plate The galling resistance was evaluated according to the following criteria, with the appearance of the molded product (the portion subjected to sliding by the mold) as the evaluation object. It shows in Table 3 with the coating liquid number (number of Table 1) and heating condition number (number of Table 2) which used the test result.

<Evaluation criteria>
A: No mold galling; O: No mold galling but metallic luster; X: Mold galling.
(2) Coating adhesion Commercially available melamine alkyd paint (Glymine # 100, manufactured by Shinto Paint Co., Ltd.) was sprayed on the surface-treated surface of the test piece with a spray coating device so as to form a 25 μm coating film after drying, 180 Baked at 20 ° C. for 20 minutes. After that, in order to simulate the paint adhesion over time, it was immersed in warm water at 80 ° C. for 2 hours. The coating adhesion was evaluated according to the following criteria according to the state of peeling of the coating film by peeling.

<Evaluation criteria>
○: The number of peeling masses is less than 10; ×: The number of peeling masses is 10 or more.
(3) Solvent resistance The surface treated surface of the test piece is rubbed 10 times with a load soaked with ethanol and MEK (methyl ethyl ketone) at a load of 500 g, and the presence or absence of dissolution of the film is visually determined and evaluated according to the following criteria. did.

<Evaluation criteria>
A: No discoloration; B: No discoloration at first glance, but slight discoloration is observed depending on the viewing angle; x: Clear discoloration.

  As can be seen from Table 3, the coating adhesion is higher than the melting temperature of the wax, and after the primary baking for 5 seconds or more in the temperature range where the fluidity remains in the coating solution, the coating adhesion is secondary above the curing temperature of the resin system. The case where baking occurred was good. The mold galling resistance was good when the film thickness was 0.5 μm or more. The solvent resistance was good when baked at a temperature equal to or higher than the curing completion temperature of the resin. When a film was formed under the coating liquid composition and baking (heating) conditions within the scope of the present invention, the coating adhesion, mold galling resistance and solvent resistance were all satisfactory.

Claims (3)

On the plated surface of the zinc-based plated steel sheet, a method for producing a surface-treated steel sheet having a surface-treated film containing a thermosetting resin as a main component and containing a wax,
A coating solution containing 5 to 15% by mass of a wax having an average particle size of 0.2 to 3 μm based on the total solid content is prepared in a coating solution containing a thermosetting resin as a main component. The thermosetting resin has a composition adjusted so that its curing completion temperature is higher than the melting temperature of the wax,
After this coating solution is applied to the surface of the galvanized steel sheet, primary baking is performed for 5 seconds or more in the temperature range where the melting temperature of the wax is higher and the fluidity remains in the coating solution, and then the curing of the thermosetting resin is completed. Secondary baking is performed at a temperature higher than the temperature to form a surface treatment film with a film thickness of 0.5 μm or more.
A method for producing a surface-treated steel sheet excellent in mold galling resistance, paint adhesion and solvent resistance.   The method for producing a surface-treated steel sheet according to claim 1, wherein the thermosetting resin is a mixture of epoxy resin / curing agent / curing temperature adjusting agent, and the wax is polyethylene wax.   The method for producing a surface-treated steel sheet according to claim 2, wherein the curing temperature adjusting agent is melamine.