JPH09272175A - Guard film-free type precoated steel plate with substrate of cold-rolled steel plate excellent in coating-film adhesion, punching resistance, and pressure mark resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a precoated steel plate which is excellent in coating film adhesion, punching resistance, and pressure mark (scratch) resistance. SOLUTION: (1) The total Cr adhesion of a chromate composition is specified to be 10-40mg/m<2> . (2) As a non-Cr type rust preventive pigment in a primer film, basic phosphite in which zinc-potassium or zinc-potassium phosphate is specified as the base is used in a main pigment system, and organic phosphonic acid is used as an auxiliary pigment system. (3) A top paint film is provided with PM resistance (4) The particle size of resin particle aggregate is controlled to be 2-50μm, and its content as a solid weight percentage is controlled to be 1-50 %. (5) The melting point of polyethylene lubricant is made 80-130 deg.C, and its content as solid weight percentage is controlled to be 1-5%. (6) The surface glossiness of a back paint film is controlled to be at least 50-100% of that of the surface top paint film.

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 excellent in coating film adhesion in punching machining which is particularly required for automobile inner layer members such as oil filters and dashboards or home electric appliance members such as audio products, and punching machining. The present invention relates to a guard film-free type precoated steel sheet that has both the perforation resistance (punching resistance) and the pressure mark resistance represented by a packaging material.

[0002]

2. Description of the Related Art In recent years, there is a strong need for cost reduction of coated steel sheets in the fields of home appliances and automobiles, and post-coat precoating (hereinafter simply referred to as PC
Abbreviation for pre-coated metal called M. ) While progress is being made,
Market demand for guard film-free precoated steel sheet that achieves both coating adhesion after processing, perforation resistance (hereinafter simply referred to as punching resistance) and pressure mark resistance (hereinafter simply referred to as PM property) due to continuous punching processing Is increasing. In particular, PCM represented by an inner layer member such as an oil filter for automobiles is premised to have a low price including a substrate, and even if it is deep drawn without sacrificing other physical properties. It is essential that the membrane is alive.

Further, in the development of PCM which is frequently used as an outer panel of audio and cooking products and which is required to have high functionality, it is first necessary to prevent the handling flaw of the appearance of the top coating on the front side, which occurs in the process of molding products by customers. It is necessary, and as a countermeasure for this on the production side, a guard film having a film thickness of several tens to several hundreds of μm is separately attached to the coated surface and provided to the market, so both the manufacturing cost and the product price must be high. I am in a situation where I cannot get it.

In addition, as a post-treatment of the guard film, there is a tendency that the cost burden on the consumer caused by the guard film such as manual work for peeling the film from the product which has been finally molded and incineration work increases year by year, and it is wide. From a global environmental standpoint in the sense, there is a growing demand for precoat film non-guarding. However, this corresponding technology on the manufacturing side has a problem to be further considered. In the production of this guard film-free type precoat under a highly productive line, it is often stored as a coil product on the floor until it becomes a cut plate product. As a problem, it is also necessary to solve the indentation transfer flaw (PM resistance) on the coated surface due to the weight of the coil during this period, and as a problem to be solved, it is imposed on the precoat coating film including the floor mounting method of the coil product.

[0005] With regard to imparting the PM resistance to the PCM coating film, some customers are demanding it, and it is becoming essential to maintain the PM resistance by a packaging material such as expanded polystyrene when the final product is transported. With the guard film free, depending on the unit weight of the coil, not a few linear or strip-shaped indentation marks are generated in the coating film in the plate width direction depending on the unit weight of the coil, and this occurs at a constant pitch in the coil longitudinal direction, greatly reducing productivity. To invite
In this respect, the greatest problem to be solved is countermeasures against pressure-induced scratches on the coating film (hereinafter simply referred to as PM resistance).

[0006] Further, in the precoat having PM resistance, it is required that the continuous perforation (punching resistance) by machining is basically cleared without impairing the performance of other coating films. It is essential to give both punching resistance and PM resistance to the film. This punching resistance is the abrasion resistance of the punching die due to rubbing with the coating film, and the consequent occurrence of burrs around the holes in the steel sheet, or the powdered release coating is pressure-bonded to the die. As a result, this causes a flaw on the coated surface, and it is an important issue to satisfy both of these, including the coating film adhesion. In this sense, at present, there is almost no technology disclosed in the pre-coated steel sheet in which the adhesion of coating film, the punching resistance and the PM resistance are imparted at the same time using the cold rolled steel sheet as the substrate in the prior art or the manufacturing method thereof.

As an issue related to the regulation of industrial waste, an era in which commercialization using raw materials that conflict with this must be avoided in the future, including recycling, is about to come. For various surface treated steel sheets,
Particularly in coated steel sheets, Cr-based rust preventive pigments and the like are often used for imparting high rust preventive properties, and there is a risk of violating the waste regulations. As a conventional technique in response to this situation, the present inventors have previously filed Japanese Patent Application No. 6-209339 as a product patent.
Japanese Patent Application No. 6-209340, Japanese Patent Application No. 7-62666,
Japanese Patent Application No. 7-74899, Japanese Patent Application No. 7-184276 and Japanese Patent Application No. 7-186056 have been proposed. However, since the substrate is a plated steel plate for PCM, cost and global environment protection It is not always satisfying the market demand.

[0008]

In the prior art as described above, the present invention is to provide the PCM on the market at a low cost and with a high function. First, the substrate is limited to the cold rolled steel sheet, and the adhesion of the coating film. In order to obtain a stable and high-quality coating, the coating surface treatment technology was specified, the primer coating was made of a non-Cr system, and the top coating was modified to provide both punching resistance and PM resistance. The improved technology aims to provide a guard film-free precoat on the market at a low cost. In particular, from the producer to the final commercialization by the consumer, it must be a pre-coat that does not cause scratches during processing, including punching, and for that purpose the following coating pretreatment and coating modification There are technical issues involved.

As the base chromate treatment, it is necessary to improve the adhesion of the film itself to the base material of the cold-rolled steel sheet as the original plate and the adhesion of the upper coating film. It is necessary. Further, it is necessary for the primer coating film to have high rust prevention and high adhesion as a coated steel sheet by specifying a non-Cr pigment. Further, as the top coating film, the aggregate in the coating film may have elasticity and pressure resistance (providing PM resistance) to the coating film, which is suitable for the storage and transportation environment of processed products, and may shorten the mold life. It is necessary to modify the coating film satisfying all the coating film performances such as softening (providing PM resistance) and sacrificing other coating film performances. That is, it is basically required to improve the adhesion of the underlying chromate film to the base material, and to design a coating film that is appropriately hard and has a high elasticity.

In order to solve the above problems, first, it is necessary to suppress the insolubility of the underlying chromate film formed on the surface of the base cold-rolled steel sheet and to suppress the brittle fracture of the film. High-corrosion resistance is provided by the primer coating film, and further, the coating film forming specifications sufficiently follow the deformation processing of the substrate in consideration of the change of the coating film aggregate and the glass transition point (hereinafter simply referred to as Tg) of the coating film. The improvement of the adhesion of coating film in deep drawing press processing and continuous punching processing, etc. by suppressing the abrasion of the punching die and the adhesion of the release coating to the punching die, and the PM resistance of the coating film. There is a point to give.

That is, the glass transition point (hereinafter, simply referred to as Tg) of the coating film is controlled within a specific range, and the coating film design of a shape having high elasticity by enhancing impact resistance is basically used for improving punching resistance. However, it is necessary to control the hardness of the aggregate in the coating film, and in the present invention, it is essential to apply a resin type organic aggregate that hardly dissolves or reacts with the main resin of the binder. Since inorganic aggregates (silica grains) having a high degree of crystallinity, which are frequently used in the prior art, are hard, they must be excluded from the coating film outside the system. Also, the stable P coating resistance of the top coating
In order to obtain M property, the present invention is proposed based on the knowledge that the surface state of the back coating film, for example, the appearance gloss of the back coating film needs to be controlled to be equal to or lower than the glossiness of the top coating film. It came to.

[0012]

[Means for Solving the Problems] The present invention has the following technical concept as a product which solves the above-mentioned drawbacks of the conventional technique and responds to the recent market needs for cost reduction of prepainted steel sheets. Based on a highly designed PCM, an inexpensive PCM using a cold-rolled steel sheet having a base chromate film, a primer coating and a top coating, which has both adhesion, punching resistance and PM resistance. It came to propose. Making the underlying chromate film insoluble by specifying the chromate film composition and amount of adhesion and improving the adhesion to the base material and the upper coating film by it. Corrosion resistance imparting by specifying the properties of the non-Cr pigment as an environment-friendly primer coating resin Tg , Imparting elasticity to the top coating film by optimizing the molecular weight and compounding amount of the binder resin, and imparting PM resistance and punching resistance by optimizing the particle size and compounding amount of the organic resin aggregate Stabilization of PM resistance of top coating film by control

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The coating composition of the steel sheet of the present invention will be described below. An extremely low carbon steel plate made of a low carbon steel plate or a high purity steel having a lower carbon than that as a substrate, and an ultra low carbon plated surface with at least one element of at least one iron group element such as Ni as an adhesion amount of 20 mg / m ² or less. A carbon steel plate is used. A chromate composition in which Cr ³⁺ is 40 to 60% by weight ratio to the total Cr and vapor phase silica having a primary average particle diameter of 3 to 50 mμ is 0.5 to 2.0 in the total Cr amount ratio on the upper layer of the substrate. It is characterized in that the total amount of deposited Cr is 10 to 40 mg / m ² .

In the upper layer, a basic phosphite-based rust preventive pigment whose base is zinc potassium or zinc potassium phosphate is used as a main pigment, and the main pigment is 10 to 100 parts by weight based on 100 parts by weight of the resin. Part, and further, after forming a polymer polyester resin-based primer coating film in which an organic phosphonate having an alkyl group as an auxiliary pigment is specifically blended with the main pigment, a glass transition point of the coating film on the uppermost layer ( T
g) 5 to 70 ° C., average molecular weight 15000 to 5000
The melamine curable molecular polyester resin of 0 has a solid content weight ratio of 30 to 90%, and the organic resin particles having a primary average particle diameter of 2 to 50 μm which are hardly fused with this resin as an aggregate have a solid content weight ratio of 1 to 50%. Melting point as lubricant is 80-130 ℃
Of a guard film free type excellent in coating adhesion, punching resistance and PM resistance, characterized by forming a top coating film containing 1 to 5% by weight of polyethylene wax in solid content ratio. It is a precoated steel sheet.

Further, the surface glossiness of the back surface coating film is controlled to at least 50 to 100% of the front side top coating film, which is a guard film free type precoat excellent in punching resistance and PM resistance. It is a steel plate. The essence is as follows, in order to improve the adhesion of the coating film around the punch hole edge portion by deep drawing processing or continuous punching processing, it is necessary to modify the morphology of the underlying chromate film to be insoluble.
Therefore, the Cr ³⁺ ratio and the amount of adhesion in the chromate composition are set in a specific range. The point of applying an environment-friendly primer coating film by specifying the form of the non-Cr pigment and its blending ratio. In imparting PM resistance to the coating film, the Tg and the molecular weight of the resin are set in a specific range, whereby appropriate hardness and elasticity are imparted to the coating film.

In order to suppress die wear and improve punching resistance, organic resin particles having a specified average particle diameter are applied as coating film aggregates. By controlling the surface glossiness of the back side coating film to be at least equal to or lower than that of the front side top coating film, PM resistance of the top coating film is improved.
The point to get the sex stable. Incidentally, in designing the top coating film of the present invention, coloring of the coating film may be freely carried out by a known technique, and addition of design property as the top coating film such as metallicization and appearance gloss is also particularly limited. Not something.

Hereinafter, the constitutions of these coatings and their operation limits will be described in detail. (1) Regarding PCM original plate The cold rolled steel plate as a PCM original plate applied to the steel plate of the present invention is, for example, a cold rolled steel plate manufactured by a known technique referred to in JIS G3101 or a high-purity steel in which the amount of carbon in the steel is further restricted. The ultra-low carbon steel sheet may be used. Further, in the application of the ultra-low carbon steel plate which is flash-plated with an iron group element such as Ni, which is applied to the surface of the cold-rolled steel plate for imparting chemical conversion treatment property, at least the amount of plating is controlled within a specific range. Needs to be done.

That is, in the present invention, the cold-rolled steel plate of the substrate, in particular, the flash plating deposition amount of the iron group element such as Ni formed on the surface of the ultra-low carbon steel plate is 30 mg /
It is controlled to m ² or less. Beyond this, it is almost impossible to expect an increase in the Cr fixing ratio of the upper chromate film, and it becomes difficult to follow the deformation of the coating film in deep drawing, and it becomes difficult to stably obtain excellent coating film adhesion. Preferably 10
It is better to control to less than mg / m ² .

The reason for this is that, unlike low-carbon steel sheets, the surface of ultra-low-carbon steel sheets of high-purity steel is enriched with components in the steel such as C (Fe ₃ C, etc.), Mn, and S that become cathode sites for the iron matrix. Since it is relatively small, there is a characteristic that the dissolution reaction of the base iron is unlikely to occur. This means that the reduction reaction of soluble Cr ⁶⁺ to Cr ³⁺ cannot be expected so much from the interfacial reaction with the chromate film located in the upper layer. That is, unlike low-carbon steel sheets, it is difficult to make the chromate film insoluble in ultra-low-carbon steel sheets, and if the surface of this ultra-low carbon steel sheet is subjected to iron group flash plating, etc. The surface concentrated layer of the steel component that becomes is forcibly dissolved and removed by pickling during plating, and there is almost no potential difference between the plating metal of the iron group element and the base Fe in the chromic acid atmosphere. It is presumed that this is because the dissolution reaction of the base iron was further reduced, which in turn suppressed the insolubility of the chromate film.

(2) Underlying chromate film: Cr ³⁺ is 40 to 60% in the total Cr amount ratio in the upper layer of the steel sheet, and vapor phase silica having a primary average particle size of 3 to 50 mμ is in the total Cr amount ratio of 0.5 to 50%. It is characterized in that the chromate composition of 2.0 forms 10 to 40 mg / m ² as the total Cr adhesion amount. First, in the chromate film mainly composed of Cr ⁶⁺ and Cr ³⁺ , in order to stably obtain high adhesion of the coating film, in particular, the ratio of Cr ³⁺ occupying in the film is increased as much as possible so that It is necessary to change the form to a chromate film. If the Cr ³⁺ concentration ratio of the composition is less than 40%, the water resistance of the chromate film is not sufficient, so that the chromate film undergoes cohesive failure due to deep drawing, making it difficult for the film to follow deformation processing of the substrate. However, it becomes difficult to secure high stability of coating film adhesion, which is not preferable. On the other hand, if it exceeds 60%, the viscosity of the liquid as the chromate composition increases, which hinders the control of the liquid and the uniform film forming property as the chromate film in the continuous coating operation, which is not preferable. Therefore, in the present invention, the Cr ³⁺ ratio of the chromate composition is 40 to 60%, preferably 45 to 60%.
55% is good.

The role of the vapor phase silica is to prevent scratches on the coating film and to improve the adhesion of the coating film. For that purpose, the particle diameter as the primary average particle diameter and the compounding ratio of silica / total Cr are appropriate. Need to be designed. The silica particle size is 3 mμ
If the amount is less than 50%, the anchoring effect on the coating film is halved, and it is difficult to stabilize the coating film adhesion to a high level including prevention of scratches on the coating film. On the other hand, if it exceeds 50 mμ, secondary agglomeration in the treatment liquid causes coarsening and precipitation or the like occurs, which makes it difficult to form a uniform film and is not preferable. Therefore, the preferable particle size range of the silica is 3 to 50 mμ, and 10 to 30 mμ is preferable.

Next, as the compounding ratio of the silica, silica / total C
If the ratio of r is less than 0.5, the amount of silica contained in the chromate film is insufficient, and the intended effect of preventing scratches on the coating film and improving the adhesion of the coating film declines, which is not very good. On the other hand, if the silica compounding ratio exceeds 2.0, on the contrary, the silica content in the coating film becomes excessive, which has an adverse effect on the prevention of scratches on the coating film and the improvement of the adhesiveness of the coating film. .
The compounding ratio of the silica to the total Cr is 0.7-2.
0, and preferably 0.7 to 1.5.

Further, the chromate thus formed
The appropriate amount range of the composition is as follows:
5-40 mg / m^Two May be formed, preferably 10 to
30 mg / m^Two Is good. The adhesion amount is 5 mg / m^Two Less than
Has insufficient coating corrosion resistance and is 40 mg / m ^Two In super
The chromate film form is difficult to insolubilize, which is
Induced cohesive failure of chromate film due to concentration of processing stress
Eventually, the adhesion of the coating film on the processed part is reduced and punching resistance
Is also a good idea in terms of cost and cost.
Absent.

(3) Anticorrosion Pigment in Primer Coating In order to maintain the corrosion resistance of the steel sheet of the present invention, as a rustproofing pigment in the primer coating, the basic base is specified as zinc potassium or zinc potassium phosphate. Mainly phosphate
An organic phosphonate having an alkyl group is used together therewith in an appropriate ratio. As the alkyl group of the organic phosphonic acid,
Any of trimethyl type, triethyl type, triisopropyl type and triphenyl type may be used.

The basic phosphite of the main pigment is resin 100
If the weight ratio to the parts is less than 10 parts, sufficient corrosion resistance cannot be obtained even in an appropriate ratio with the organic phosphonate of the auxiliary pigment,
On the other hand, if the amount exceeds 100 parts, the rust preventive effect is saturated and it is difficult to obtain sufficient coating film adhesion. From the above, the proper mixing ratio of the basic phosphite of the main pigment is preferably 30 parts to 70 parts by weight with respect to 100 parts of the resin.

If the weight ratio of the main pigment consisting of the basic phosphite to the organic phosphonate used together with the main phosphite is less than 0.02, the rust preventive property against undercoat corrosion is insufficient. However, sufficient corrosion resistance cannot be obtained. On the other hand, if the ratio exceeds 0.5, the rust preventive property is saturated, but the paint life is short, which is a bad idea including the paintability. Therefore, the proper mixing ratio of the basic phosphite of the main pigment and the organic phosphonate of the auxiliary pigment used in combination is 0.02 to 0.5, preferably 0.03 to 0.3. Is good.

The particle size of the rust-preventive pigment is not particularly limited in the present invention, but it is preferable to control the primary average particle size to about 30 to 1 μm for corrosion resistance and processing as a coated product. It can be said that the productivity is higher in terms of maintainability and smoothness of the coating appearance. The binder resin system of the anticorrosive pigment for primer coating is not particularly limited in the present invention, but is preferably polyester, polyolefin, nylon, polyacryl, polyurethane, polypropylene, epoxy, polyamide, phenol, Polyethylene may be used.

(4) Main resin of top coating film Suitable molecular weight of high molecular polyester resin as main resin for binder In the present invention, in order to impart PM resistance to the top coating film, first, the coating film is appropriately selected. It is premised that the resin is made hard to withstand the applied pressure, and for that purpose, it is necessary to specify the molecular weight range of the high molecular weight polyester resin of the binder resin. When the molecular weight of the resin is less than 15,000, the crosslink density becomes too high, and the coating film is prone to work cracks, and the coating film peeling is likely to rust upon mechanical impact such as punching. On the other hand, if it exceeds 50,000, defects in the appearance of the coating, such as generation of cracked skin and cracks, are likely to occur, which is not a good measure including cost. Therefore, in the present invention, the molecular weight range of the resin is 15,000 to 50,000, and preferably 20,000 to 30,000.

Appropriate Blending Ratio of Polymer Polyester Resin In the present invention, the blending amount of the binder resin is preferably 30 to 90% as a solid content weight ratio. If it is less than 30%, the resin effect as a binder for the pigment is lowered, so that the coating film tends to be brittle, the coating film adhesion is deteriorated, and the coating film has poor scratch resistance and processability. It becomes difficult to obtain a painted appearance. On the other hand, if it exceeds 90%, the coating film becomes soft and the core PM resistance and abrasion resistance are deteriorated, and the coating performance including appearance is greatly deteriorated. Not. Therefore, the appropriate compounding amount of the resin is 30 to 90%, preferably 40 to 7%.
0% is good.

Coating Film Tg (Glass Transition Point) The coating film Tg of the present invention is a basic factor to be controlled especially in order to obtain the PM resistance of the top coating film more stably. If the Tg is less than 5 ° C, the PM resistance is not sufficient, and if the Tg exceeds 70 ° C, the PM resistance of the coating film is saturated, but the coating film tends to harden due to adhesion of the coating film and work cracking tends to occur. It is difficult to obtain excellent punching resistance, and it is difficult to obtain a scratched surface or cracks, which makes it difficult to obtain at least a uniform coating appearance. Therefore, the coating film Tg in the present invention is preferably 5 to 70 ° C, and more preferably 20 to 60 ° C.

Proper particle size and proper compounding amount of organic resin particle aggregate In the present invention, the organic resin aggregate imparts appropriate hardness and elasticity to the coating film, and the punching resistance and scratch resistance of the coating film are improved. It is for imparting scratches. Resin particle properties of the resin aggregate, which are insoluble or sparingly soluble in the binder resin from the viewpoint of imparting elasticity to the coating film and have a function of sufficiently controlling the appearance gloss of the coating film. Must. For that purpose, it is necessary to design the particle size and composition sufficiently.

First, when the average particle size of the resin particle aggregate is less than 2 μm, the elasticity of the coating film is excessively imparted, so that the punching resistance is sufficiently imparted, but the coating film is softened. On the contrary, PM resistance and scratch resistance are rather lowered and become unstable, which is not preferable. In addition, the appearance of the coating film is poor in the function of adjusting gloss and gloss, which is not a good idea including cost. On the other hand, if the average particle size exceeds 50 μm, the appearance of the coating film will be large, and it will be difficult to control the glossiness of the coating film in particular, and the scratch resistance will decrease, which is not a good idea including productivity. . Therefore, the particle size range of the organic resin particle aggregate is preferably 2 to 50 μm, and more preferably 3 to 30 μm.

Next, regarding the compounding amount of the resin particle aggregate,
If the solid content weight ratio is less than 1%, the elasticity of the coating film due to the aggregate is insufficient, and although good PM resistance is maintained, it is difficult to achieve punching resistance at the same time, and deterioration of scratch resistance and appearance gloss are observed. It is not preferable because the control of (1) becomes unstable. Further, if it exceeds 50%, sufficient PM resistance and punching resistance can be achieved at the same time, but it becomes difficult to control the gloss of the coating film appearance, which is not preferable. Therefore, the content of the resin particle aggregate is preferably 1 to 50%, more preferably 3 to 20%. Incidentally, in the present invention, polymethylmethacrylate, polypropylene, polyamide, polyacrylonitrile, polyester, alkyl silicone, melamine-formaldehyde, polyurethane and polyvinylidene fluoride may be applied as the resin particle aggregate, and the effect is obtained. There is no difference in.

Proper amount of polyethylene lubricant The polyethylene lubricant is intended to improve the scratch resistance of the coating film and the workability in press working. If the lubricant is less than 1% by weight of the solid content, the deterioration of the coating film performance cannot be avoided.
In addition, punching resistance is not sufficient due to the occurrence of flaws due to pressure-bonding of the release coating film. On the other hand, if it exceeds 5%, a water-cooled pattern at the time of baking the coating film becomes conspicuous, and it is difficult to obtain a uniform coating appearance. Therefore, the proper blending amount of the lubricant is 1 to 5%, preferably 1.5 to 3%.

Next, when the melting point of the lubricant is less than 80 ° C., the strength of the lubricating film formed on the surface of the coating film is not sufficient, so that the scratch resistance is deteriorated and the essential PM resistance, and further, the coating film after baking. It is not so preferable because water cooling pattern is generated on the surface and the commercial value is reduced. On the other hand, when the melting point of the lubricant exceeds 130 ° C., the uniform dispersibility in the paint is slightly lacking, so that it becomes difficult to form a uniform lubricating film on the paint surface, and uneven appearance and instability of scratch resistance are caused. Not invited. Therefore, in the present invention, the melting point of the lubricant is preferably 80 to 130 ° C, preferably 1
The temperature is preferably 0 to 120 ° C. Further, in the present invention, a lubricant of silicone type and fluorine type other than polyethylene may be used as the lubricant, and there is no difference in the effect.

(4) Surface Glossiness of Back Coating Film In the present invention, the surface glossiness of the back coating film is a control factor for obtaining more stable PM resistance of the top coating film. When the gloss of the back coating is less than 50% of the gloss of the top coating, the PM resistance of the top coating decreases, and when it exceeds 100%, the PM resistance of the top coating also decreases. . That is, in order to stably obtain the PM resistance of the top coating film, it is essential to maintain the range of the present invention as the back surface glossiness, preferably 60 to 90%.

[0037]

EXAMPLES The effects of the present invention will be described in more detail below with reference to examples and comparative examples. High-productivity line, degreasing with alkali by known technology, thickness 0.7mm, width 9
First, the undercoat chromate treatment specified in Tables 1 to 4 is roll-coated on both sides of a 14 mm low-carbon steel sheet at a line speed of 70 m / min, then dried at a sheet temperature of 80 ° C. and cooled to room temperature to start the coating process. The surface side of the steel sheet subjected to the base treatment is 2C
2B coating, 1C1B coating on the back side, the coating system used for these is any organic solvent-based melamine-curable polymer polyester resin-based coating described in Table 1 to Table 4 specified by the present invention, Paint-baked under specified conditions.

At this time, for the primer coating of the 2C2B coated surface, the present invention uses a high molecular polyester resin-based primer made of the specific pigments shown in Tables 1 to 4 after roll coating to a solid film thickness of 5 μm. On top of that, as a top coating film, the coating composition of the present invention whose components are specified in Tables 1 to 4 is curtain-coated as a solid film so as to have a thickness of 18 μm, and each is baked under standard conditions and water-cooled and dried. The surface gloss of the top coating at this time is 80%.
Was controlled.

On the other hand, for coating the back surface of the steel sheet, the coating composition of the present invention whose glossiness of the coating film is adjusted is specified in Tables 1 to 4 by a well-known roll coating method so that the coating film thickness becomes 1 μm as a solid film. The coating is controlled so that it will be baked at a predetermined plate temperature, washed with water and dried. Regarding the coating film performance of the precoated steel sheet of the present invention produced in this manner, Table 5 corresponds to Tables 1 to 4.
~ Summarized in Table 8. Further, as to the effect thereof, as described below, as a precoated steel sheet having both a high level of stable coating adhesion, pressure mark resistance (PM resistance) and punching resistance, it is possible to stably produce the same. It has come to provide an epoch-making pre-coated steel sheet that is not available in the market at low cost.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

(1) Applicable substrate As the cold rolled steel sheet applied to the present invention, No. 1 to No. 12 and N for the comparative example.
o. 13-No. As shown in 15, it may be either a low carbon steel plate or an ultra low carbon steel plate. In addition, in an ultra-low carbon steel sheet, the amount of iron group element such as Ni that is flash-plated is preferably 30 mg / m ² or less, and preferably 10 m, in order to impart chemical conversion treatment property to the surface.
It is more effective to control it to g / m ² or less. This is because the steel component concentrated layer is forcibly removed from the surface of the steel sheet by pickling during the plating or the plating metal has a small potential difference with the base iron in the chromic acid film, so the local battery between the base Fe and Soluble Cr
^It is considered that the reduction reaction of ⁶⁺ to Cr ³⁺ became difficult to occur.

(2) Effect of chromate film In the present invention, the role of the chromate film is to change the form of the film to be sparingly soluble. The greatest feature is that the film is sufficient even at low temperature drying at 100 ° C or lower. The point is that the Cr adhesion amount is specified so that it becomes insoluble. This makes it difficult for the chromate film to break even if the processing stress is concentrated, and the adhesion of the film to the substrate is clearly improved, which brings about high stability of the film adhesion, and deep drawing as a prepainted steel sheet. And punching resistance is stably imparted.

Regarding this effect, as an example of the present invention, the effect of the Cr ³⁺ ratio in the chromate composition was examined. 5
And No. 16-No. No. 19 shows the comparative example. Two
0-No. 21. Regarding the proper particle size range of the vapor phase silica, No. 5 and No. 22-N
o. No. 26 shows the comparative example. 27-No. 28. Further, regarding the proper mixing ratio of the vapor phase silica, No. 5 and No. 29-No. No. 33 shows the comparative example. 34-No. 35. Further, regarding the proper range of the amount of chromate adhesion specified by the present invention, No. 5 and No. 36-No. No. 40 shows the comparative example. 41-No. 42. As is clear from these examples, the chromate film specified by the present invention makes it possible to stably ensure excellent coating film adhesion without impairing other coating film performances. Punching resistance and PM resistance (pressure mark resistance) can be consistently achieved at a high level.

(3) Primer coating film As the effect of applying the non-Cr type anticorrosive pigment according to the present invention to the primer coating film, the pigment type and its compounding ratio in Example No. 5 and No. 43-No. No. 51 shows the comparative example. 52-No. 53. In addition, regarding the combined effect of the organic phosphonate which is the rust preventive auxiliary pigment of the present invention, Example No. 5, No. 54-No. 55. As is clear from these examples, by suitable combination of the anticorrosion pigment and the anticorrosion auxiliary pigment specified by the present invention, the coating corrosion resistance and the coating corrosion resistance can be stabilized at a high level without impairing the coating performance such as coating adhesion. It is possible to achieve both.

(4) Action and effect of main resin Regarding action and effect of the main resin for imparting PM resistance to the coating film, the suitable molecular weight range of the present invention is shown in Example No. 5
And No. 58-No. No. 61 shows the comparative example. 6
2-No. 63. Further, regarding the proper Tg of the main resin, No. 5 and No. 64-No. 67
No. 68-No. 69. Further, regarding the proper mixing ratio of the main resin,
o. 5 and No. 70-No. No. 73 shows a comparative example.
74-No. 75. As is clear from these examples, the coating film is resistant to P without impairing other properties of the coating film.
In order to impart M property, it is important to design the Tg of the coating film within the range of the present invention, and in addition, in order to stably obtain uniform appearance and PM resistance of the coating film. Indicates that proper designing with the resin molecular weight and blending amount referred to in the present invention is necessary.

(5) Appropriate Mixing of Resin Particle Aggregate in Coating Film The resin aggregate is present in the coating film to impart scratch resistance, but in the present invention, a machine such as punching is used. In terms of punching property during processing, the main purpose is to suppress die wear, and for that purpose, it is important to control the particle size of the aggregate and the compounding amount thereof within an appropriate range. In the present invention, No. 5 and N
o. 76-No. 82, the comparative example No. 83-No.
84. In addition, regarding the proper blending amount,
o. 5 and No. 85-No. 91, the comparative example No.
92-No. Shown at 93. As is clear from these examples, in order to achieve both punching resistance and other coating performance without deteriorating the performance of other coatings, the present invention requires that the particle size and blending amount of the resin particle aggregate be the same. It turns out that it is necessary to control within the appropriate range.

(6) Appropriate Mixing of Polyethylene Lubricant This lubricant is in the form of powder, and its purpose is to appropriately increase the scratch resistance by imparting sliding property to the coating film, and it is necessary to optimize the mixing. Is. Examples of the lubricant according to the present invention are No. 5 and No. 94-No. No. 98, a comparative example. 99-No. 100 is shown. As is clear from this example, by imparting lubricity to the coating film, steel plate handling flaws in stable press working and the like are eliminated, and in particular, by imparting PM resistance to the coating film, it is characterized by guard film free. In the steel sheet of the present invention, it is essential to properly mix the lubricant.

In addition, it is understood that when the content of the lubricant exceeds the upper limit, a water-cooled pattern after baking is likely to occur on the appearance of the coating film, which is not preferable. Further, regarding the melting point of the lubricant, the examples according to the present invention are 5 and No. 10
1 to No. No. 103 of the comparative example. 104-No. ~
Shown at 105. As is clear from this example, when the melting point range of the lubricant of the present invention is out of the proper range, if it is less than the lower limit, not only good scratch resistance but also stable PM resistance which is the object of the present invention. It can be seen that it is difficult to obtain, and if the upper limit is exceeded, a water-cooled pattern is generated on the coating film, which greatly impairs the commercial value in appearance.

(7) Glossiness of Backside Coating In the present invention, in order to obtain stable PM resistance of the topside coating, it is preferable to control the glossiness of the backside coating within an appropriate range. . In this case, the back surface glossiness is controlled by comparing the glossiness of the top coating film. With respect to this point, the embodiment No. 5 and No. 106-No. No. 109 of the comparative example. 110-No. 111. As is clear from this, if the back surface glossiness control range of the present invention is deviated, it becomes difficult to stably maintain the PM resistance to the top coating surface having a high commercial value under the high productivity line, and the cost is reduced. It is not a good idea to include it.

[0053]

[Table 5]

[0054]

[Table 6]

[0055]

[Table 7]

[0056]

[Table 8]

(Note) * 1. Cold rolled steel sheet CR: JIS G3101 standard low carbon steel sheet SR: Ultra low carbon steel sheet (JIS G3101 standard low carbon steel sheet with carbon content controlled to about 1/10) Plating adhesion amount: fluorescent X-ray analysis, unit Is mg / m ² * 2. Chromate film Trivalent Cr ratio: Weight ratio of Cr ³⁺ to the total Cr amount in the chromate composition Particle size of vapor phase silica: Indicates the primary average particle size, and the display unit is m
μ Silica blending ratio: Display unit is expressed as wt% as a ratio to total chromium amount Adhesion amount: Displayed as total Cr amount (fluorescent X-ray analysis)

* 3. Primer coating using high molecular polyester resin Anticorrosion pigment ZK: Basic potassium potassium phosphite, primary average particle size applied in the range of 10 mμ to 2 μm Antirust pigment PK: Basic zinc potassium phosphite, primary average particle Applicable in the range of diameter 10 mμ to 2 μm P1: Triphenyl type organic phosphonic acid P2: Triisopropyl type organic phosphonic acid P3: Trimethyl type organic phosphonic acid P4: Triethyl type organic phosphonic acid

* 4. Top coating film properties High molecular polyester resin by curing melamine resin Tg refers to the glass transition point of the cured coating film, measured by TMA. All blending amounts are weight ratios to coating film solids Organic aggregates are resins Refers to particle aggregates. Particle size refers to the primary average particle size * 5. Glossiness of back side coating 60 degree The glossiness of the top side coating due to specular reflection * 6. Cr fixing rate It is the ratio of the amount of residual Cr deposited on the steel sheet before and after immersion in boiling pure water for 60 minutes and immersion in boiling water. X-ray fluorescence analysis

* 7. Adhesion of coating film Cylindrical drawing process (50w x 50L x 50H [m
m]), squeezing ratio 2.1 to 2.3, uncoated oil 1 mm square grid pattern of skirt part × 100 pieces → pure water boiling 30
Minute → Taping peeling within 30 minutes ◎: No coating peeling, ◯: Very slight peeling, △: Peeling
Small, ×: Large peeling * 8. PM resistance evaluation After leaving 168 hrs in a wet atmosphere of load 80 kg / cm ² , RH 95%, 40 ° C. on a test piece in which a back surface coating was overlaid on the top coating surface, remove the test piece and inspect the surface for indentation. Visual evaluation. ◎: Initial appearance and no change 〇: Very slight indentation transfer when transparent △: Slight indentation transfer when viewed from the front ×: Clear indentation transfer

* 9. Punching resistance Continuous punching punching device: Continuous punching by electric crank press, 4 pieces per second Die: Uses a die with hard surface finish. 4 sets of blades with a wall thickness of 1 mm x width of 4.5 mm are set. Drilling resistance evaluation: Visually evaluate the state of burr formation around the 300,000th punching hole ◎: No burr, ◯: Slightly There are blemishes, but there are no burrs, △: several burrs occur, ×: frequent burrs occurrence of coating powder: ◎: none, ◯: no blemishes that occur slightly, △: adhered to the mold, blemishes Is sporadic, ×: It adheres to the mold considerably and there are frequent defects.

* 10. Coating corrosion resistance Oil free cylindrical drawing (50w x 50L x 50H [m
m]), squeeze ratio 2.2, oil-free, visual evaluation of the occurrence of coating film swelling on the processed side surface / cross-cut area CCT60 cycle (24 hrs / cycle) SST 6 hrs → left 1 hrs → dry 70 ° C, RH60
%, 4 hrs → leave 2 hrs → wet 49 ° C, RH 98%, 4 hrs → leave 2
hrs → cooling −20 ° C., 4 hrs → leaved for 1 hrs ⊚: no swelling, ◯: slightly swelling, Δ: fine partial swelling,
×: Swelling on the entire surface * 11. Appearance of coating (discoloration, visual evaluation of coating defects) ◎: Uniform appearance, ◯: Very slightly uneven pattern through the watermark,
△: Partially uneven, ×: Uneven over all surface

[0063]

As described above, in order to meet the market demand for an inexpensive precoated steel sheet, the present invention provides the adhesion of the coating film in the processed portion by deep drawing and the pressure resistant mark of the coating film when the substrate is a cold rolled steel sheet. As a result of a detailed study on the compatibility of the properties (PM resistance) and punching property, the pre-coated steel sheet based on the following technical idea was successfully developed and brought to the market. (1) By forming a chromate composition having a specified Cr ³⁺ ratio and a primary average particle size of vapor phase silica and a ratio of the silica / total Cr amount in a specified amount, the film becomes a highly sparingly soluble film. We succeeded in changing the form, and it became possible to provide it to the market as a precoated steel sheet that can maintain stable coating film adhesion at a high level even in severe forming processes. (2) By specifying the anti-corrosion pigment in the primer coating film to be a non-Cr type, it is possible to eliminate Cr elution from the coated product and provide it to the market as a pre-coated steel sheet that is kind to the global environment.

(3) The elasticity of the coating film is imparted by specifying the aggregate in the top coating film as the resin particle type aggregate, and by specifying the particle size and the blending amount thereof. Since die wear during processing is suppressed, continuous punching properties for steel sheets can be stably obtained. (4) Tg of top coating film, molecular weight of resin and proper mixture thereof, melting point of lubricant and proper mixture thereof, and further specification of appearance glossiness of back coating film, etc. Due to the synergistic effect of, good pressure mark resistance can be stably obtained regardless of whether the coil product or the cut plate product is loaded (achievement of non-guard film of precoated steel plate).

Claims (5)

[Claims] 1. A low-carbon steel sheet having a Cr ³⁺ / total Cr weight ratio of 40 to 60% and a gas phase silica having a primary average particle diameter of 3 to 50 mμ in a total Cr amount ratio of 0.5 to 2.0. A basic phosphite-based rust preventive pigment formed by forming a chromate composition consisting of 10 to 40 mg / m ² as a total Cr deposition amount and further specifying the base as zinc potassium or zinc potassium phosphate in the upper layer. Polymer polyester resin-based primer coating film in which 10 to 100 parts by weight to 100 parts by weight of resin and the ratio of the organic phosphonate of the auxiliary pigment to the anticorrosion pigment are 0.02 to 0.5 by weight ratio. After forming, the top layer has a glass transition point (Tg) of 5 as a top coating.
Melamine-curable high molecular weight polyester resin having an average molecular weight of 15,000 to 50,000 at a temperature of 70 ° C to 3% by weight of solid content.
0-90%, primary average particle size 2 which is hardly fused with this resin
Form a top coating film in which organic resin particles having a particle size of 50 μm are used as an aggregate in an amount of 1 to 50% by weight of solid content, and a polyethylene wax having a melting point of 80 to 130 ° C. as a lubricant is included in an amount of 1 to 5% by weight of solid content. A guard precoated steel sheet having excellent coating film adhesion, punching resistance and pressure mark resistance. 2. The substrate has a carbon content of about 1 of that of a low carbon steel plate.
A precoated steel sheet excellent in coating film adhesion, punching resistance and pressure mark resistance according to claim 1, characterized in that it is made of an extremely low carbon steel sheet of / 10 or less. 3. Ni, Co, Fe on the surface of an ultra low carbon steel sheet
The precoated steel sheet excellent in coating film adhesion, punching resistance and pressure mark resistance according to claim 1 or 2, characterized in that at least one of them forms an adhesion amount of 30 mg / m ² or less. 4. The coating according to claim 1, wherein the organic phosphonate of the rust preventive pigment used in the primer coating is any one of trimethyl, triethyl, triisopropyl and triphenyl. Precoated steel sheet with excellent film adhesion, punching resistance and pressure mark resistance. 5. The coating adhesion, punching resistance and resistance according to claim 1, wherein the surface gloss of the back coating is controlled to be at least 50 to 100% of the front top coating. Pre-coated steel sheet with excellent pressure mark properties.