JP3223813B2 - Pre-coated steel sheet with excellent formability and corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precoated steel sheet suitable for applications requiring excellent formability and corrosion resistance. The precoated steel sheet of the present invention is used after being subjected to final coating after forming, and is suitable for, for example, home appliances and building materials, and can also be used for automobiles.

[0002]

2. Description of the Related Art Conventionally, in order to obtain a coated steel sheet, a method has been adopted in which a surface-treated steel sheet such as a plated steel sheet is formed and then coated. This method is called post-coating because it is painted after molding. However, this post-coat method required large-scale facilities including environmental measures for the painting process. On the other hand, a so-called pre-coated steel sheet obtained by applying a coating to a coil-shaped or sheet-shaped steel sheet or a plated steel sheet can be made into a product simply by forming and processing such a complicated coating such as a post-coating method.
Since it does not require a baking process, it is now widely used for home appliances and interior materials.

[0003] The properties required for a coating film of a precoated steel sheet include many items such as high quality appearance, high hardness, high workability, stain resistance, chemical resistance, water resistance, and corrosion resistance. Above all, formability and corrosion resistance after working are important factors that determine the quality level of a precoated steel sheet, and various studies have hitherto been made to satisfy these at a high level.
The formability of a coating film here means that the coating film itself is less damaged in the process of bending, cutting, drawing, etc. when processing a flat metal plate into various shapes. The greater the degree of elongation and flexibility, the better the moldability. The term "corrosion resistance" refers to little or no peeling of the coating film and no rust from the base steel sheet.

Conventionally, in order to improve such properties, a technique of modifying the polyester resin with an epoxy resin to improve the adhesion of the coating film (JP-A-60-141768), using a coupling agent as a pretreatment. In addition, measures such as a technique of combining this with a specific primer (JP-A-63-97266) are being studied. However, it is difficult for the coating films obtained by these methods to satisfy the corrosion resistance after severe processing.

[0005]

As described above, the pre-coated steel sheet currently used is not satisfactory in formability and coating film performance after forming, and has a problem in particular in corrosion resistance in a formed portion and an end face. . For this reason, conventionally, measures have been taken in order to secure corrosion resistance, such as further applying a coating or relaxing the forming conditions. However, these methods have impaired productivity and efficiency, which are the advantages of precoated steel sheets. Therefore, an object of the present invention is to provide a pre-coated steel sheet which is excellent in formability and shows excellent corrosion resistance even after being subjected to severe forming processing by being used after being subjected to final coating after forming.

[0006]

In order to achieve the above object, a precoated steel sheet according to the present invention has the following configuration. (1) A zinc-based plated steel sheet having a basis weight of plating of 15 g / m ² or more on one side has a chemical conversion coating on both sides,
A primer coating containing strontium chromate in an amount of 1.0 g / m ² or more in the upper layer of the chemical conversion coating on one side of the steel sheet, and further having an overcoat on the upper layer;
A precoated steel sheet having a back coating containing 1.0 g / m ² or more of strontium chromate as an upper layer of the chemical conversion coating on the other side of the steel sheet, wherein the top coating has an elongation of 20% or more and a coating of The coefficient of dynamic friction of the surface is 0.
15 or less, and the total strontium chromate content of the primer coating and the back coating is 4.0 g / m2.
Pre-coated steel sheet with excellent formability and corrosion resistance, characterized by being ² or more. (2) The precoated steel sheet according to the above (1), characterized in that the main component of the resin constituting the overcoat film is a polyester resin, and is excellent in formability and corrosion resistance.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The base steel sheet used for the precoated steel sheet of the present invention is a hot-dip galvanized steel sheet, hot-dip galvanized alloy coated steel sheet, galvannealed steel sheet, electro-galvanized steel sheet, composite galvanized steel sheet, electric alloy Various types of galvanized steel sheets such as galvanized steel sheets and the like, and the basis weight of plating on one side thereof must be 15 g / m ² or more. If the basis weight of plating is less than 15 g / m ² , corrosion resistance becomes insufficient irrespective of the top coat, the primer coat and the back coat. There is no particular upper limit on the basis weight of plating, but it is preferable to set the upper limit to about 90 g / m ² from the viewpoint of workability and the like.

A chemical conversion coating is formed on the surface (both surfaces) of the plated steel sheet by a phosphate treatment, a chromate treatment or the like. This chemical conversion coating is formed for the purpose of improving the adhesion and corrosion resistance between the plated steel sheet and the upper coating film. The phosphate treatment includes zinc phosphate treatment, iron phosphate treatment, and lead phosphate. Treatment and the like, and examples of the chromate treatment include a reaction type chromate treatment, an electrolytic type chromate treatment, and a coating type chromate treatment. In the chromate treatment, silica, phosphoric acid, a resin component, and the like can be added to the chromate treatment solution for the purpose of improving corrosion resistance and adhesion.

[0009] A primer coating is formed on the chemical conversion coating on one side of the steel sheet, and an overcoat is further formed on the primer coating. The type of resin used for the primer coating is not particularly limited, but from the viewpoint of processability and adhesion between the topcoat coating and the chemical conversion coating, a polyester coating, an epoxy coating, a urethane coating or a resin coating mixture of these. It is preferable that A rust preventive pigment is added to the primer coating to improve corrosion resistance. As this rust-preventive pigment, strontium chromate, zinc chromate, barium chromate or the like, which is a fine powder of a so-called poorly soluble chromate, can be used. Used as an essential rust preventive pigment. The amount of strontium chromate added is 1.0 g / m ² or more from the viewpoint of ensuring corrosion resistance, and 4.0 g in total with strontium chromate contained in the backside coating film.
/ M ² or more. The thickness of the primer coating is preferably in the range of 3 to 15 μm from the viewpoint of corrosion resistance and economy. Further, the amount of strontium chromate which can be contained in the coating film is limited to about 50% by weight, and if it exceeds this, the performance such as adhesion and processability is reduced. Therefore, considering the film thickness and the strontium chromate content in the coating film, the upper limit of the amount of strontium chromate added is 20.0 g / m 2.
It is preferably ² .

For the top coat, it is preferable to use a polyester resin as a main component of the resin constituting the overcoat film. Among them, a resin obtained by the reaction of a polyester polyol with an amino resin and / or an isocyanate compound is particularly preferable. Most preferred. The reason why the polyester resin is preferable as the resin for forming a coating film is as follows. That is, as a resin for forming a coating film,
Polyester-based, epoxy-based, acrylic-based, vinylidene fluoride-based, silicone polyester-based, and vinyl chloride-based resins can be considered. Among them, epoxy-based, acrylic-based, and silicone polyester-based resins have poor flexibility, and Cracking and peeling of the coating film are likely to occur during molding. Further, the vinyl chloride resin has a poor workability at the time of molding because the coating film is soft, and is not preferable in view of environmental pollution and the like. In this respect, the polyester resin does not cause the above-mentioned problem at all.

The overcoat film has a kinetic coefficient of friction of 0.
Must be 15 or less. If the coefficient of kinetic friction exceeds 0.15, the friction resistance of the coating film at the time of molding increases and the moldability decreases, and as a result, the corrosion resistance of the processed portion also decreases.
In order to make the kinetic friction coefficient of the surface of the overcoat film 0.15 or less, it is preferable to add a wax component to the overcoat film. Examples of the wax component include hydrocarbon waxes such as paraffin and low molecular weight polyethylene, silicone waxes, amide waxes, and ester waxes. The amount of these wax components is not particularly limited, but is preferably a solid content of the resin component 1.
The amount is 0.03 to 8.0 parts by weight, particularly preferably 0.05 to 5.0 parts by weight, per 100 parts by weight. If the amount of the wax component is less than 0.03 parts by weight, the above-mentioned coefficient of kinetic friction is hardly obtained, and the lubricity of the coating film surface becomes insufficient, resulting in poor moldability. On the other hand, if the amount exceeds 8.0 parts by weight, the lubricating property becomes excessive, and the press at the time of molding becomes insufficient, so that an unnecessary slip phenomenon or the like occurs.

Further, the elongation of the top coat must be 20% or more. Here, the elongation percentage of the top coat is to make the top coat a free film having a thickness of 20 μ,
This is equivalent to the elongation percentage when the elongation is measured. However, a hard coating film having an elongation percentage of less than 20% does not have sufficient elongation to withstand molding processing, so that the coating film cracks during molding processing. And the like, and as a result, the coating effect of the coating film becomes insufficient and causes a reduction in corrosion resistance.

On the other side of the steel sheet, a back coating film is formed on the upper side of the chemical conversion coating. The type of resin used for the back surface coating is not particularly limited, but is preferably a polyester-based, epoxy-based, urethane-based or a resin-based coating obtained by mixing these from the viewpoint of processability and adhesion to the base. . It is also possible to provide a further upper layer coating on the upper layer of the backside coating. A rust-preventive pigment is added to the back coating to improve corrosion resistance. Also for this rust preventive pigment, strontium chromate is used for the same reason as the above-mentioned primer coating film. The amount of strontium chromate added is 1.0 from the viewpoint of ensuring corrosion resistance.
g / m ² or more, and the total amount with strontium chromate contained in the primer coating film must be 4.0 g / m ² or more. The thickness of the back coating is corrosion resistance,
It is preferable to set it in the range of 3 to 15 μm from the viewpoint of economy. Further, the amount of strontium chromate which can be contained in the coating film is limited to about 50% by weight, and if it exceeds this, the performance such as adhesion and processability is reduced. Therefore, considering the film thickness and the strontium chromate content in the coating film, the upper limit of the amount of strontium chromate added to the back coating film is 20.0 g / m ^2.
It is preferable that

In preparing the coating composition for forming each coating film, an ordinary disperser or kneader such as a sand grind mill, a ball mill, a blender or the like can be selected and used. The coating method is not particularly limited, but it is generally preferable to apply by a method such as roll coater coating or curtain flow coating. The coating film is formed by applying the coating composition, baking the coating composition with a heating means such as hot air drying, infrared heating, or induction heating to crosslink the resin to obtain a cured coating film. The baking conditions at the time of heat curing are as follows: primer coating, top coating, back coating at 200 to 250 ° C. for about 40 seconds to
Baking may be performed for about 3 minutes.

[0015]

EXAMPLES Precoated steel sheets were produced using the coating compositions prepared as described below, and their properties were examined. The conditions for preparing the coating composition, the manufacturing conditions for the precoated steel sheet, and the method for evaluating the coating film performance are shown below. (1) Preparation of coating composition for primer coating film Table 1 shows coating compositions P-1 to P-4 for primer coating film. These coating compositions are listed in Table 1 in 125 parts of a urethane-modified epoxy resin (“Epokey 834” manufactured by Mitsui Toatsu Chemicals, Inc.).
In which the pigment is added in a blending amount of

(2) Preparation of the paint composition for the top coat and the paint composition for the back coat Table 2 shows the paint compositions F-1 to F-3 for the top coat and the paint composition for the back coat Indicates B-1. In a reaction vessel equipped with a heating device, a stirrer, a rectification column, a decompression device, and a thermometer, dimethyl terephthalate, dimethyl isophthalate, ethylene glycol, neopentyl glycol, and manganese acetate catalyst were added in the amounts shown in Table 2. The transesterification reaction was carried out at 180 to 210 ° C. in a charged nitrogen atmosphere to distill off methanol. Further, trimethylolpropane was added, and while heating to 250 ° C., the pressure was gradually reduced to 10 mmHg to carry out a polymerization reaction to obtain a polyester polyol. The obtained polyester polyol was adjusted to a nonvolatile content of 40 to 60% with a cyclohexanone solvent.
Further, the molecular weight of the polyester polyol was adjusted by the polymerization reaction time. The molecular weight was measured by gel permeation chromatography using a standard polystyrene calibration curve to determine the number average molecular weight. Next, the above-mentioned polyester polyol was mixed with a curing agent, a pigment, a curing catalyst and additives in the amounts shown in Table 2 and then dispersed for 1 hour using a sand mill containing glass beads having a diameter of about 1 mm.
Further, by adding cyclohexanone, the non-volatile content becomes 40-6.
0%, and a coating composition F-1 for a top coat.
To F-3 and a coating composition B-1 for a back coating film were obtained.

(3) Manufacturing conditions of the pre-coated steel sheet According to the conditions shown in Tables 3 and 4, the base steel sheet is subjected to a chemical conversion treatment, and then a primer is applied as necessary.
An overcoat was applied to the uppermost layer to produce precoated steel sheets of the present invention and comparative examples. The dynamic friction coefficient of the overcoated film of the precoated steel sheet thus obtained was measured using an ASTM 189 tester using a HEIDON-14 type tester manufactured by Haydon.
According to No. 4, it was determined from the load caused by sliding at a load of 500 g.

(4) Evaluation of coating film performance The coating film performance of the precoated steel sheet was evaluated as follows. (A) T-bend workability A 2T bending test was performed in a room at 20 ° C., and the coating film in the processed portion was observed with a 30-fold loupe to check for cracks in the coating film, and evaluated according to the following criteria. A: No crack was observed in the coating film. ：: Fine cracks were observed in the coating film. ×: Clear cracks were observed in the coating film. (B) Corrosion resistance of flat part A salt water spray test (SST) was performed for 240 hours using test pieces prepared by applying tape seals to the top and bottom of a flat pre-coated steel sheet and making the right and left sides become upper and lower burrs, respectively. After that, a tape peeling test was performed on the coating film at the cut end of the test piece, and the corrosion resistance was evaluated as follows. ：: Surface coating peeling of 3 mm or less ○: Surface coating peeling of more than 3 mm, 5 mm or less Δ: Surface coating peeling of more than 5 mm, 10 mm or less ×: Surface coating peeling of more than 10 mm

(C) Corrosion Resistance of Processed Part A flat sliding test was performed on a flat precoated steel sheet with a high load of 500 kgf / cm ² , the center of the sliding part was cut, and all other cut end faces were tape-sealed. Thereafter, a salt spray test (SST) was conducted for 240 hours, and then a tape peeling test was performed on the coating film at the cut end of the test piece, and the corrosion resistance was evaluated as follows. ：: Surface coating peeling of 3 mm or less ○: Surface coating peeling of more than 3 mm, 5 mm or less Δ: Surface coating peeling of more than 5 mm, 10 mm or less ×: Surface coating peeling of more than 10 mm

Tables 3 and 4 show the results of the evaluation of the coating film performance together with the coating composition of the precoated steel sheet. According to Tables 3 and 4, any one of the basis weight of the base-coated steel sheet, the elongation percentage of the coating film of the top coat, the coefficient of kinetic friction of the coating film surface, and the total amount of strontium chromate added to the primer coating film and the back surface coating film is shown. Are out of the range of the present invention Comparative Examples 1 to
In No. 6, either the moldability or the corrosion resistance is inferior.
On the other hand, in Examples 1 to 3 of the present invention, extremely excellent moldability and corrosion resistance were obtained.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

[0025]

As described above, the pre-coated steel sheet of the present invention is excellent in both formability and corrosion resistance, and therefore has high formability and corrosion resistance in applications such as home appliances, building materials and automobiles. It is extremely useful as a pre-coated steel sheet to be used in required parts.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masaru Sagiyama 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-7-126826 (JP, A) JP-A-5 JP-A-146750 (JP, A) JP-A-7-195613 (JP, A) JP-A-5-220449 (JP, A) JP-A-7-284728 (JP, A) JP-A-8-267662 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) B05D 7 / 14,7 / 24,1 / 36 B32B 15/08 C23C 28/00 C23C 22/24-22/28

Claims (2)

(57) [Claims] 1. A coating weight per one side of 15 g / m ^2.
The zinc-coated steel sheet has a chemical conversion coating on both surfaces, and a primer coating containing strontium chromate at 1.0 g / m ² or more on an upper layer of the chemical conversion coating on one side of the steel sheet. A precoated steel sheet having a top coat and having a back coat containing 1.0 g / m ² or more of strontium chromate as an upper layer of the chemical conversion coating on the other side of the steel sheet, wherein the top coat has an elongation percentage. A molding having a kinetic friction coefficient of not more than 20% and a coating film surface of not more than 0.15, and a total strontium chromate content of the primer coating film and the back coating film of not less than 4.0 g / m ^2. Pre-coated steel sheet with excellent workability and corrosion resistance. 2. The precoated steel sheet having excellent moldability and corrosion resistance according to claim 1, wherein the main component of the resin constituting the overcoat film is a polyester resin.