JP3068998B2 - Pre-coated steel sheet with excellent backside grounding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvanized steel sheet subjected to a chemical conversion treatment on a substrate, and more particularly to a precoated steel sheet having a conductive coating on one side of the steel sheet. The present invention relates to the applied precoated steel sheet.

[0002]

2. Description of the Related Art In recent years, there has been a high demand for cost reduction of surface-treated steel sheets in the field of home appliances and automobiles, and as the precoating of post-coats has progressed further, there is also a high demand for high-performance steel sheets having different functions on both sides. In particular, in the case of pre-coated steel sheets for home appliances frequently used for outer panels of audio and video products, there is an increasing demand for painted steel sheets which oblige a grounding property on a back service coat surface. As an example of the prior art in which a thin film is applied to impart electrical conductivity to a steel sheet, an example is given to a weldable coated steel sheet for an automobile as follows, but there is almost no disclosure in the field of a pre-coated steel sheet for home appliances.

[0003] For example, Japanese Patent Application Laid-Open No. 61-23766 and Japanese Patent Publication No.
There is JP-A No. 20004, and zinclometal is a typical marketed product. However, these coating film performances are disadvantageous in that, for example, crack-free coating films for processing required in the field of home electric appliances cannot be basically achieved.

[0004] Next, a chromium compound is mixed with a water-dispersible resin to improve the corrosion resistance of a steel sheet.
And Japanese Patent Application Laid-Open No. 61-584 are proposed, but none of them has a conductive function in a coating film and is not practical as a back surface grounding property. Further, Japanese Patent Application Laid-Open No. Sho-60-50 discloses a thin film coating of an organic composite silicate resin film obtained by blending a silane coupling agent and silica sol with a water dispersible resin.
181 and JP-A-60-149786. However, these may be used as spot weldability of steel sheets where the applied voltage is high and the coating film is destroyed by resistance heating under high pressure, but it is not suitable for the grounding of home appliances for which the applied voltage and pressure are small. On the other hand, a contact resistance by soft touch without destruction of the coating film is required, and in this case, the disclosed technology is not practical at all.

As described above, the prior art disclosed as a weldable painted steel sheet in the automobile field is a pre-coated steel sheet in the field of home appliances, and a steel sheet such as front non-guard / backside conductive required for audio and video products. At present, there are few disclosure cases of precoated steel sheets having different coating functions on the front and back. In addition, in the prior art described above, disclosure of the type of conductive metal powder is only listed in a unified manner, the difference in properties of each metal powder and behavior and performance in the coating film No feature differences are disclosed.

[0006] In the case of a pre-coated steel sheet in which the appearance of a product is most important, it is necessary to firstly work to prevent handling defects on the appearance of the coating film. For example, in coil coating in a high-productivity line, the coil is in a coil state until primary storage of the product or secondary processing in a slitter line or the like. In this state, the top coating with high designability and the back coating inevitably come into contact,
At this time, the appearance of the coating film on the back side is transferred as a linear mark in the width direction of the sheet from the coil's own weight to the top painted surface, which appears as indentation marks at a constant pitch in the longitudinal direction of the coil, greatly reducing productivity. However, there has been no disclosure of this pressure mark prevention measure (hereinafter simply referred to as PM resistance) based on the properties of the coating film on the back surface.

[0007]

In contrast to the above prior art, the present invention relates to a high productivity line coil coating.
It is an object of the present invention to impart stable grounding properties to a back coating film of a precoated steel sheet having a high Tg type top coating film having a glass transition point (Tg) of 50 ° C. or higher. In this case, it should be noted that it is necessary to stably impart grounding property to the back surface coating while considering the PM resistance to the high Tg type top coating surface having high designability in the coil coating. is there. For this purpose, first, the Tg range of the high molecular polyester resin, which is the binder resin for the back surface coating, is specified, and various conductive pigments are preliminarily selected for the conductivity of the coating.
Based on the knowledge that the filler is optimal, the appropriate range including the film thickness is examined for the shape, particle size and compounding ratio of this metal Ni filler, and furthermore, the surface roughness of the coated back side of the applicable original plate is reduced. It is intended to stably impart the grounding property of the back coating while maintaining the PM resistance of the top coating by specificity.

[0008]

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention is based on the following technical concept and, in order to impart a grounding property to the back surface of a pre-coated steel plate, a design of a back surface coating including the surface roughness of the original plate. It is intended to be suitable. The front and back of the coated steel sheet is sandwiched by thin copper sheet pieces, and the contact resistance with the backside coating when applying a slight voltage and load from above is 1Ω or less, and it can be obtained stably without variation depending on the place (Ensure backside grounding). The weight of the painted coil is about 5 tons, and the back film appearance is not transferred to the top film with high designability by the weight of the coil (ensure PM resistance). In the formation of the backside coating film to achieve both, the appropriateness of the form and the mixing ratio of the conductive metal Ni filler and the appropriateness of the film thickness are obtained in conformity with the polyester resin of the binder, and the formed backside coating film Is specified from the viewpoint of the PM resistance of the top coating film.

[0009] Extraction of a roughness element to be controlled as the surface roughness of the base original plate to be the target surface, and the like. That is, in imparting the grounding property to the back surface of the pre-coated steel sheet, it is necessary to design the back surface in consideration of PM resistance to the top painted surface. To achieve this, first, the Tg of the back coating and its appropriate film thickness in consideration of the PM resistance of the top coating, the shape and particle size of the metal Ni filler serving as the conductor of the back coating, and the compounding ratio thereof It is necessary to improve the suitability of In addition, in order to stably secure the grounding property on the back surface, it was necessary to control the surface roughness of the base plate, particularly the appropriate control of PPI and Ra. As a result, the present invention was proposed. Things.

Hereinafter, the constitution of the backside coating film of the present invention will be described first. In order to stably impart the grounding property to the back surface coating of the steel plate of the present invention, first, the back surface of the steel plate is 70 to 15 PPI.
0, Ra controlled the roughness to 0.5 to 2.0 μm, and then subjected to appropriate chemical conversion treatment on both sides of the steel sheet.
When a high-molecular polyester resin is used as a binder and Ni fillers having different particle diameters of 0.1 to 50 μm are blended into the binder, the ratio of the flake-like Ni / chain-like Ni as the filler is expressed by the weight ratio of the resin. 1 to 10, the N
i is characterized in that the total amount of the filler contains 10 to 200% by weight of the resin component, and the Tg of this coating film is 0 to 40.
° C, and is formed as a solid film of 0.5 to 5 µm.

The gist of the invention is that the grounding property of the back coating film can be stably obtained in consideration of the PM resistance of the top painted surface as a coil product. In order to obtain the former back coating having no adverse effect on the PM resistance to the top coating, it is basically necessary to lower the Tg of the back coating from the top coating, and Ni which is a conductor of the back coating is required. The point is that the form and the mixing ratio of the filler are specified, the film thickness of the back surface coating film is specified, and further, the surface roughness of the base plate is specified. Here, it is to be noted that if the back surface coating film of the present invention needs to be colored, there is no problem if a coating with a coloring pigment is performed in the category of the prior art.

The steel sheet applied to the present invention may be any of a bare steel sheet and a galvanized steel sheet, and the zinc-based steel sheet may be any one obtained by a known plating method. For example, in an electroplating system, Zn
Plating, a Zn-based alloy-plated steel sheet in which the alloy element is at least one of Ni, Cr and Fe may be used. Further, in the electro-dispersion plating system, Zn-Ni, Zn-
SiO ₂ , TiO ₂ , ZrO ₂ , BaC based on Fe
A Zn-based dispersion alloy-plated steel sheet obtained by uniformly dispersing and depositing a metal oxide such as rO ₄ may be used. Further, a hot-dip galvanized steel sheet, a Zn—Al-based alloy-coated steel sheet, and a galvanized steel sheet obtained by alloying them may be used in a hot-dip coating system.

[0013]

The operation limits of the control factors constituting the back surface of the steel sheet of the present invention will be described below. (1) Surface roughness of original plate In order to stably obtain the grounding property of the back coated surface of the present invention without affecting the PM resistance of the top coating film surface, first, the surface roughness of the target original plate back surface should be as follows: In particular, control of PPI and Ra is required. On the other hand, if the PPI is less than 70, the surface of the original plate becomes too smooth, and the thickness of the coating film on the back surface tends to be excessively smooth, and the conductive Ni in the coating film is originally unevenly distributed, so that a stable grounding property is obtained. It is not a good idea under the productivity line because it becomes difficult. On the other hand, if the PPI is more than 150, it is difficult to stably manage the roughness of the original plate from the viewpoint of the life of the roll roughness and the cost of the original plate. Therefore, the PPI of the original plate roughness is preferably 70 to 150, particularly 90 to 120.
Is preferred.

Next, when Ra is less than 0.5 μm, it is necessary to excessively increase the mixing ratio of the metal Ni filler necessary for imparting the grounding property of the back surface coating, and the uniform dispersibility of the Ni filler in the paint pan is required. Or the lowering of the processing adhesion, or the surface roughness of the back coating film due to the Ni filler is easily transferred to the top coating film surface, and it is difficult to impart a back grounding property that is also compatible with the so-called PM resistance of the top coating film, Not very good, including cost. On the other hand, if Ra exceeds 2.0 μm, it is difficult to stably manage the roughness of the original plate from the viewpoint of the life of the roll roughness and the cost of the original plate. Accordingly, Ra as a raw plate roughness is 0.5 to
2.0 μm is preferred, and particularly preferably 0.8 to 1.5 μm.

(2) Form of Ni filler for imparting conductivity to the backside coating film and its compounding ratio First, as the high molecular polyester resin of the binder resin forming the backside coating film of the present invention, there is a conventional technology. What is necessary is just to be obtained. Also, the coloring of the back surface coating film can be easily performed by coating the film with a commercially available coloring pigment according to the conventional technique. It is important to control the shape, particle size, and mixing ratio of the metal Ni filler used for imparting conductivity to the back surface coating film within the following specific ranges. Among the forms of the Ni filler, the shape is most preferably flaky in order to transfer the appearance of the back surface coating to the top coating having high designability in the coil coating, that is, to obtain stable PM resistance. It is preferable to use a linear and a chain Ni filler in combination.

First, regarding the total amount ratio of the Ni filler,
A resin content weight ratio of 10 to 200% is allowed. This Ni
If the compounding ratio is less than 10%, lowering of the grounding property on the back side is caused. In this state, in order to secure the grounding property, the coating thickness should be 0.5 μm.
However, it is difficult to achieve the PM resistance of the top coating film as a coil product, including the appearance of the back coating film, with the film thickness being easily reduced. On the other hand, if it exceeds 200%, the uniform dispersibility of the Ni filler in the paint is impaired, and the essential grounding property of the back surface is reduced. Not. Therefore, the total amount of the Ni filler is preferably from 10 to 200% by weight, and more preferably from 20 to 100% by weight.

Next, regarding the suitable form of the Ni filler and the compounding ratio thereof, if the shape ratio of flake Ni / chain Ni is less than 1.0 in terms of the weight ratio of the resin, the ratio of chain Ni becomes excessive, and the specific gravity is increased. The difference causes a problem in uniform dispersibility in the paint, and a decrease in the Ni transfer rate into the paint film causes a decrease in the function of forming an electric circuit by Ni on the back paint film, leading to an increase in paint film resistance and grounding property. It is not so preferable because it exhibits a decrease.
On the other hand, if the ratio of Ni exceeds 10, the flake-like Ni tends to segregate in the surface layer of the coating film, and the conductive circuit with the chain-like Ni is easily short-circuited. Is difficult to obtain, which is not so preferable. Flaky N
The shape ratio of i / chain Ni is 1 to 10 in terms of resin content weight ratio.
In particular, 3 to 7 are preferable.

In the Ni filler having such a different form, it is necessary to control the particle size within an appropriate range. The Ni
If the particle size is less than 0.1 μm, Ni is too fine and tends to segregate on the surface of the coating film, which hinders the formation of an electric circuit of the coating film by the essential Ni, which in turn makes the grounding property of the back surface unstable. Therefore, it is not very preferable. In addition, the particle size is 50μ.
If it exceeds m, the back surface coating film is apt to be roughened, which causes a decrease in PM resistance to the top coated surface as a coil product, which is not preferable in terms of commercial value. Therefore, the N
The preferred particle size of the i-filler is preferably from 0.1 to 50 μm, more preferably from 5 to 20 μm.

(3) Tg of Back Coating In the formation of the back coating of the present invention as described above, the Ni filler is contained in order to basically guarantee the PM resistance of the top coating. It is necessary to control the Tg of the backside coating film within an appropriate range. When the Tg of the back surface coating film is less than 0 ° C., the coating film is basically soft, and thus a handling flaw is likely to occur in a secondary processing step such as press working. Conversely, the PM resistance of the back coating decreases, or the unevenness of the back coating is promoted by the Ni filler of the skeleton, which is difficult to remedy by adjusting the gloss of the coating. In any case, the softness of the back surface coating often causes a decrease in commercial value, such as a decrease in PM property, and is not preferred. Further, when the Tg of the back coating exceeds 40 ° C., the Tg difference from the top coating is reduced, so that there is a decrease in PM resistance to the top coating and a decrease in adhesion as the back coating, which is not preferable. . Therefore, the Tg of the backside coating film of the present invention using a high-molecular polyester resin as a binder is 0 to 40 ° C.
And preferably 15 to 25 ° C.

(4) Appropriate Thickness of Backside Coating In order to impart stable grounding properties to the backside coating, it is necessary to mix Ni filler having the specific properties as described above, but the thickness of the film naturally limits. There is. When the thickness of the coating film is less than 0.5 μm, for example, in roll coating or the like, the coating film lacks uniform paintability due to the breakage of the coating film and the productivity is poor.
In addition, it is difficult to achieve both PM resistance and the top painted surface as a coil product, so this is not very advantageous. On the other hand, if it exceeds 5 μm, the grounding property and adhesion as the back surface coating film become unstable, and this is not very advantageous including cost. Therefore, a suitable backside coating thickness for achieving both PM resistance to the top painted surface and backside grounding property is preferably 0.5 to 5.0 μm, and more preferably 1 to 3 μm.

[0021]

The effects of the present invention will be described below in detail with reference to examples. 0.7m thick steel plate with roughness controlled in specific range
m, line speed 70 on both sides of steel plate 914mm wide
After the silica sol-containing chromate treatment with a total chromium deposition amount of 50 mg / m ² as a pre-coating treatment according to the prior art at m / min, the roll coating process of 2C2B (2 coats 2 bake) is immediately started. Top design (2C2
B) Both primer coating and top coating use an organic solvent-based high-molecular polyester resin paint, and a flexible coat 600 EU primer (manufactured by Nippon Paint) is applied as a primer coating to a thickness of 5 μm as a solid coating, and the top coating is applied to the upper layer. After a flexible coat 500 paint (manufactured by Nippon Paint Co., Ltd.) having a Tg of 60 ° C. as a film, the thickness of which is controlled to 17 μm as a solid film, the film is baked under standard conditions.
Water cooled and dried. On the other hand, for the back surface coating on the steel plate back surface controlled to a specific roughness, by a known roll coating method, the Ni filler-containing high-molecular polyester resin paint specified in Examples of the present invention described in Tables 1 to 8 is applied. Painted with a specific film thickness. The back painted surface manufactured by the method of the present invention manufactured in this way has a PM resistance to the top painted surface having high designability.
It can be seen that this is an epoch-making technology which does not sacrifice the characteristics and does not exist in the prior art, which also achieves stable backside grounding.

(1) Effect of Surface Roughness of Backside Surface of Steel Sheet In order to stably obtain grounding properties of the backside coating film of the present invention, it is important to specify the surface roughness of the steel sheet. PPI and Ra specified by the present invention as the surface roughness of the back surface of the steel sheet
No. was applied to the examples regarding the appropriate range of No. 1 to No. 24, and the comparative example is No. 24. 25-No. 32. Further, as an example in which the present invention is applied to various types of plated steel sheets manufactured by a conventional technique as an underlying base sheet, the examples are described as Nos. 73 and No. 97-N
o. Shown at 105. As is clear from this, if the surface roughness of the back surface of the steel sheet is out of the specific range of the present invention for both PPI and Ra, it becomes difficult to achieve both PM resistance to the top coated surface in coil coating and essential back surface grounding. I understand. In other words, in order to stably obtain the grounding property of the back coating film in consideration of PM resistance to the top coating surface with high designability according to the present invention, regardless of the plating system, the appropriateness of the back surface roughness of the base steel sheet It is necessary to control the roughness, especially P
The effectiveness of controlling PI and Ra within the range specified by the present invention is clearly seen.

(2) Appropriate Compounding of Conductive Metal Ni Filler for Backside Coating The present invention is most concerned with the addition of the Ni filler in imparting the backside grounding property. The reason is that the distribution form of the metal Ni filler contained in the base material is appropriately distributed, and in addition, a stable grounding property is obtained in consideration of the PM resistance of the top painted surface.
For that purpose, it is necessary to mix the metal Ni filler having a different shape in a specific range including its form. First, flake-like Ni (NoF) and chain-like Ni (No
As the particle size combination effect of (C), the embodiment of the present invention is No.
33-No. No. 65, and the comparative example is No. 66-No.
70.

Thus, when the particle size falls below the lower limit of the range specified by the present invention, the Ni filler is too fine and tends to segregate on the surface layer of the cross section of the coating film. It is difficult to expect, and when the applied voltage and load are small, the contact resistance of the coating film increases, and it becomes difficult to provide the expected back surface grounding property. On the other hand, when the particle size exceeds the upper limit of the present invention, the pick-up property of the Ni filler to the coating roll varies, and the distribution of the Ni filler in the coating film is destabilized. It becomes difficult to obtain, and in some cases, this causes protrusion of the Ni filler to the surface of the back coating film, and this leads to deterioration of the PM resistance of the top coated surface. In any case, it is important to control the appropriate particle size of the Ni filler for imparting a stable grounding property to the back surface coating while maintaining the PM resistance of the top coated surface within a specific range of the present invention. You can see that there is.

Next, in the present invention, it is necessary to appropriately control the shape ratio of the Ni filler. When the Ni filler having a different shape from the flake shape (F) and the chain shape (C) is used in combination, the shape ratio (F / C) is specified as a resin component weight ratio. With respect to the shape ratio of the invention of the present application, the example is No. 71-No. No. 75, and the comparative example was No. 75. 76 ~
No. 77. If the Ni shape ratio of F / C of the present invention is out of the specified range, the back coating appearance and the P coating resistance of the top coating surface are reduced.
It can be seen that it is difficult to achieve compatibility of the coating film properties such as the M property, the adhesion of the backside coating film, and the backside grounding property, and there is a decrease in productivity.

Further, the total amount of the Ni filler with respect to the high-molecular polyester resin is the same as that of the embodiment of the present invention. 7
8 to No. No. 88, and the comparative example was No. 88. 89-No. 9
0 is shown. In imparting a stable grounding property to the backside coating film of the present invention, it is necessary to raise the distribution density of the Ni filler in the coating film cross section and the surface as a premise, and in this sense, specific control of the total Ni amount is indispensable. It is. However, also in this case, needless to say, the PM resistance on the top coating film surface is taken into consideration. As can be seen from the examples and comparative examples, it can be seen that, even if the total Ni content is out of the specific range of the present invention, the back coating properties do not satisfy the above-mentioned purpose, and the commercial value of the pre-coated steel sheet to which the present invention is intended is impaired. . As mentioned above,
In order to properly form the electric circuit on the back coating and ensure stable grounding while considering the PM resistance of the top coating, the present invention uses the appropriate ratio of the shape and particle size of the metal Ni filler. It turns out that control within the scope of the invention is essential.

(3) Appropriate thickness of the back surface coating In the present invention, it is necessary to control the thickness of the back surface coating to an appropriate range. Regarding this point, the embodiment of the present invention is referred to as No. 1. 91-No. 94 and N for Comparative Example
o. 95-No. 96. As is clear from this, when the specific thickness of the present invention is deviated, not only the maintenance of PM resistance to the top coating film having high commercial value, but also the securing of the grounding property of the originally intended rear coating film becomes difficult, Not very good, including cost. It should be noted that the coloring of the back surface coating film of the present invention can be dealt with in the category of a conventional coating material using a commercially available pigment.

(4) Appropriate Tg of Back Coating Film The Tg of the back coating film satisfying the grounding property of the present invention is defined as the PM resistance to the front and back coating surfaces of the precoated steel sheet.
It is to maintain sex basically. With respect to the Tg range of the back surface coating film specified by the present invention, the examples are described as Nos. 106 ~
No. No. 111, and the comparative example is No. 112-No. 1
FIG. As is clear from this, when the Tg of the backside coating film specified by the present invention deviates, on the low Tg side, scratches due to insufficient hardness of the coating film and frequent handling scratches on the backside coating film in a product processing step occur. Invite.

On the other hand, since the coating film is soft, the PM resistance of the top coating appearance to the back coating film is reduced, and at the same time, the PM resistance of the back coating film to the top coating surface by the conductive Ni filler is reduced. Even if the grounding property on the back surface is satisfied, the coating film appearance flaw and the adhesion are greatly reduced, and the commercial value is lost. On the other hand, when the Tg exceeds the upper limit of the present invention, the coating film is hardened, so that there is a decrease in adhesion as a back coating film and a decrease in PM resistance to a top coating film, and as a commercial value, Decline overall. In any case, it is understood that the Tg of the back coating film must be controlled within the range of the present invention in terms of commercial value.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

[0034]

[Table 5]

[0035]

[Table 6]

[0036]

[Table 7]

[0037]

[Table 8]

(Note) * 1. Plating EZ: Electro zinc plating ZN: Electric Zn-Ni alloy plating (Ni; 11.5%) EC: Electric Zn-Cr-Ni alloy plating (Cr; 10)
%, Ni; 2%) EF : Electrical Zn-Fe alloy plating (Fe; 15%) ZS: Electrical _{Zn-Ni-SiO 2 (Ni} ; 12%, Si
O ₂ ; 3%) ZT: Electric Zn—Ni—TiO ₂ (Ni; 12%, Ti
O _2; 3%) ZB: Electrical _{Zn-Ni-BaSO 4 (Ni} ; 12%, B
aSO _4; 3%) ZS: Electrical _{Zn-Fe-SiO 2 (Fe} ; 10%, Si
O _2; 3%) GZ: molten Zn plating GA: molten Zn-Al plating (Al; 5%) GF: alloyed hot-dip Zn plating (Fe; 8~11%)

* 2. Surface roughness The roughness of a steel sheet in the width direction. PPI (number of irregularities per 1 inch length), Ra (average surface roughness) * 3. Metal Ni Filler Properties in Back Coating Film F: Flake Ni, C: Chain Ni The compounding ratio of Ni fillers having different shapes of F / C is represented by resin weight ratio. The primary average particle size of the filler is expressed as [(major axis + minor axis) / 2]. Coating Tg is TMA (probe method)
By measurement.

* 4. Uniformity of coating film appearance (visual appearance) ：: uniform, 〇: slightly roping, △: partial roping, ×: entire roping or stripe unevenness * 5. Grounding property of the back coating film A copper piece is sandwiched between the front and back of the steel sheet, and the contact resistance between the upper and lower copper pieces is measured five times at different locations, and the average value is set (applied voltage 1 V, load 50 g / cm ² ). ◎: 0.5Ω or less, Δ: 1Ω or less, Δ: 2Ω or less, ×:
Over 5Ω

* 6. Backside coating adhesion Primary adhesion; Repeated processing of the same processed part with OT contact bending 3
Time cellophane tape peeling. Secondary adhesion: After being immersed in pure water and boiled for 2 hours and then left for 24 hours, the same evaluation as the primary was performed. ：: No peeling of coating film, Δ: 5% or less of peeling, Δ: 10% or less of peeling, ×: Over 10% of peeling * 7. Pressure mark resistance to top coatings The top painted surface / back painted surface of the steel sheet is faced and the load is 6
0kg / cm ² , 40 ° C. × 48 hrs. ◎: No change, Δ: Very slight indentation through the mark, Δ: Thin indentation, ×: Clear indentation

[0042]

As described above, according to the present invention, as a result of various studies including the need for imparting the grounding property to the back side of the coated steel sheet, including securing the stability thereof, the present invention includes the following surface roughness characteristics of the original steel sheet. We came up with a design for the backside coating. That is, the suitable range of PPI and Ra was particularly specified as the back surface roughness of the original plate. Metal Ni filler was used as the conductive circuit forming ability of the backside coating film, but its shape and particle size were specified, and their appropriate blending ratio was specified. There is an appropriate range of Tg in the backside coating and it is specified. The thickness range has been specified as the thickness of the backside coating. Due to these synergistic effects, the present invention stably imparts the grounding property of the back surface coating without impairing the pressure mark resistance to the top design surface with high designability in coil coating under a high productivity line. This is a revolutionary pre-coated steel sheet that has never been successful before and has been offered to the market. It should be noted that the requirement for coloring of the back surface coating film of the present invention as described above can be met by appropriately blending a coloring pigment in the category of the related art.

────────────────────────────────────────────────── ⁷ Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI B32B 15/08 104 B32B 15/08 104Z (56) References JP-A-63-778 (JP, A) JP-A-5-185030 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B05D 1/00-7/26 B32B 15/08 B32B 15/08 104

Claims (2)

(57) [Claims] 1. The steel sheet according to claim 1, wherein the primary average particle size is equal to 0.
The ratio of flake-like Ni / chain-like Ni is 1 to 1 in terms of a resin content weight ratio, with a shape ratio of an irregular shaped metal Ni filler of 1 to 50 μm.
0, the total amount of Ni filler is 10 to 200 in terms of resin weight ratio
% Of a high-molecular polyester resin coating film as a solid film having a thickness of 0.5 to 5 μm and a glass transition point (Tg) of 50 ° C. or higher for a top coating film and a back coating film Tg of 10 to 40 μm. Pre-coated steel sheet excellent in grounding property on the back side, which is characterized by a temperature of ℃. 2. The back surface roughness of a coating original plate is 7 in PPI.
2. The precoated steel sheet according to claim 1, wherein the precoated steel sheet is controlled to have a Ra of 0 to 150 and a Ra of 0.5 to 2.0 [mu] m.