JP2007105770A - Method for bending precoated steel sheet with film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To bend a precoated steel sheet with a film without generating a surface defect by suppressing the breakage of the film when applying a bending work to the steel sheet by using a surface protecting film. <P>SOLUTION: When applying the bending work to the precoated steel sheet on which the surface protecting film is stuck, the film having characteristics that is ≥12.5 N/cm in tensile strength and ≤150% in tensile breaking elongation is used as the surface protecting film and also the bending work is performed under die conditions of a clearance of ≥33% and a punch radius of ≥4.6t (where, t is the thickness of a steel sheet to be worked). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of bending a coated steel plate or the like without bending wrinkles or the like on the surface of the steel plate to be processed.

In recent years, stainless steel plates have been used for the outer plates of household electrical appliances aimed at high design, and in some cases, stainless steel plates with a clear coating have been used. In the case where a stainless steel plate is used as an outer plate constituting a home electric appliance, a desired shape product is produced by subjecting a steel plate that has been subjected to shearing and appropriately cut into a shape to bending such as press forming.
From the viewpoint of designability, wrinkles and dirt on the product due to processing are considered as one of processing defects that impair the appearance. For this reason, in order to suppress the appearance defect due to the processing flaw, a vinyl chloride-based, polyolefin-based, or polyester-based surface protective film is attached and then bent.

  These surface protective films are required to have high tensile strength and high tensile elongation at break in order to suppress the occurrence of film breakage. Generally, the tensile strength is 10 to 40 N / cm and the elongation is high. About 150 to 600% is used. As seen in Patent Document 1, a protective film that defines only the breaking elongation in the vertical direction and the horizontal direction has also been proposed. Further, as can be seen in Patent Document 2, a protective film that defines only tensile strength has also been proposed.

By the way, when sticking a film on the steel plate surface, various types of adhesives such as rubber and acrylic are used. The adhesive strength between the film and the steel plate strongly affects the film peeling, and the stronger the adhesive strength, the less the film peeling occurs. However, the adhesive strength is greatly affected by the surface condition of the steel sheet, and when a surface treatment such as an acrylic silicone-based clear coating is applied, the adhesive strength to the coated steel sheet is greatly reduced.
JP 9-316411 A JP-A-8-60112

  One of the processing defects that occur when bending a steel sheet using a surface protective film is the occurrence of dents on the product due to film peeling residue caused by film breakage that occurs during bending. This defect is greatly influenced by the strength and ductility of the film and the adhesive strength between the film and the steel plate. Particularly recently, an acrylic silicone-based clear coating film is often formed on the surface of the steel sheet in consideration of design and fingerprint resistance, but when such a clear coating film is formed, the film as described above. The adhesive strength to the coated steel sheet becomes very weak, film breakage occurs, and film peeling tends to occur.

  In general, when bending a steel sheet using a die having a die and a punch, the smaller the clearance and the smaller the punch R, the better the shape accuracy of the bent product. However, when the same bending process is applied to the steel sheet to which the surface protective film is adhered, the smaller the clearance and the smaller the punch R, the easier the film breaks, and the surface defects due to the film breakage tend to appear. . In particular, when bending is performed using a film having excellent ductility for surface protection, as shown in FIG. 1, the film is stretched and wrinkles are generated in the punch R portion, and the film is crushed by the punch stroke. Acts as a brake between the punch and the steel sheet, causing film breakage.

  The present invention has been devised to solve such a problem, and by selecting the characteristics of the surface protective film to be used and the mold conditions for bending, it suppresses film breakage and generates surface defects. An object of the present invention is to provide a method for bending a coated steel plate with no film.

The bending method of the coated steel sheet with a film of the present invention achieves the object, and when bending the coated steel sheet to which the surface protective film is attached, the tensile strength as the protective film is 12.5 N / cm or more, A film having a tensile elongation at break of 150% or less is used, and bending is performed under a mold condition of a punch R of 33% or more and a punch R of 4.6 t (where t is the thickness of the steel plate to be processed). It is characterized by that.
In addition, the tensile strength and tensile elongation at break of the film specified in the present invention are measured according to JIS Z0237.

  According to the method of the present invention, since the film breakage of the surface protective film affixed to the coated steel sheet is suppressed, it is possible to eliminate the occurrence of dents due to film peeling residue that is likely to occur during bending. In addition, since film breakage is suppressed, film removal after processing can be easily performed, resulting in a reduction in work costs.

The inventors of the present invention have repeatedly studied the conditions that do not cause film breakage that causes surface defects when bending a coated steel sheet with a surface protective film attached thereto using a die and a die having a punch. It was.
As a result, in the case of a steel plate with resin coating, especially a steel plate with acrylic silicone coating, because the film is easily peeled regardless of the strength of the adhesive applied to the film in advance, It has been found that it is not necessary to consider the adhesive strength as a factor causing film breakage. If the film is easily peeled off, use a film having a predetermined tensile strength even if the tensile elongation at break is relatively small, and mold it under mold conditions with a relatively large clearance and punch R. It was found that film breakage can be suppressed.
Details will be described below.

Usually, as shown in FIG. 1, the bending of the steel sheet is performed by pressing the end of the steel plate 1 from the end of the die 2 from above while holding the steel plate 1 placed on the die 2 with the plate retainer 4. The punch 3 is pressed.
When the punch 3 presses downward against the die 2 and the steel plate 1, the film 5 adhered to the steel plate surface slides strongly with the punch 3, and is stretched by a shearing force.
At this time, if the balance between the tensile strength of the film and the tensile elongation at break is poor, film breakage is likely to occur.

By the way, a film having a tensile strength of 10 to 40 N / cm and a tensile elongation at break of 6 to 550% is often used as a surface protective film for a coated stainless steel sheet. Various films are each coated with an adhesive having a unique adhesive force.
Therefore, the present inventors stuck various films having different characteristics to the coated stainless steel sheet and repeated experiments on the bending workability.
The details of the results are left in the description of the examples, but as described above, the adhesive strength of the film has an effect on the cause of film breakage when the surface protective film is applied to the coated stainless steel sheet and subjected to bending. It was found that the film was affected only by the properties of the film itself and the mold conditions.

The reason why the adhesive force of the adhesive applied to the film does not affect The following two reasons can be considered as the cause of the film breakage easily occurring in the bending process.
One is a case where the film breakage tends to occur because the breaking strength against the frictional resistance becomes low when the strength (tensile strength) of the film is low. The other is that when a film with extremely large elongation is used, the film is stretched by the frictional force between the punch and the film, and the film stays at the tip of the punch. This is the case. For this reason, it is considered effective to use a material having a lower ductility and a higher tensile strength than conventional steel plate processing films.

On the other hand, it is considered effective to increase the adhesive strength of the film as one of the measures for suppressing film breakage. However, when a large amount of lubricating oil is used, or when an acrylic silicone coating is applied to the surface of the steel sheet, the adhesion of the film becomes very low and the film easily peels off. When the film is peeled off, the film is easily stretched and deformed by sliding with the punch, and the film is easily cut. In this case, increasing the adhesive strength of the film is not necessarily an effective measure for film breakage.
As described above, as a measure for suppressing film breakage that can be applied under a wide range of processing conditions, the present invention has reached the point that it is effective to use a steel sheet processing film having a low ductility and a high tensile strength.

Reasons for Enlarging Die Clearance and Punch R As described above, when trying to bend a steel plate with a surface protective film attached using a die and a die, the die used The smaller the clearance and the smaller the punch R, the easier the film breaks and the surface defects due to the film breakage are more likely to appear.
This phenomenon can be explained as follows. That is, when the mold clearance is small, as shown in FIG. 2, the distance between the die tip and the punch tip is shortened, so that the bending moment during molding is reduced and the bending force is increased. For this reason, the sliding resistance of the film surface stuck on the steel plate is increased, and the film is easily cut.

Further, when the punch R is small, the distance between the die tip and the punch tip is similarly shortened, so that the bending moment at the time of molding is reduced and the bending force is increased. For this reason, the sliding resistance of the film surface stuck on the steel plate is increased, and the film is easily cut.
In the present invention, as described in detail in Examples below, when bending a steel sheet with a protective film having specific characteristics using a mold, the mold conditions are defined in the claims. By doing so, the occurrence of film breakage could be prevented.

Example 1:
No. 4 (JIS R6001) surface-finished SUS430 with a thickness of 0.65 mm is chemically coated with E253NI manufactured by Nihon Parkerizing Co., Ltd. and SPDIF P98 Clear HT manufactured by Nihon Fine Coatings Co., Ltd. as a primer. A steel plate coated with Nippon Fine Coatings ZD06 with a total thickness of 11 μm as an acrylic clear was used as an original plate.
A steel plate obtained by attaching a 70 μm thick film (Nitto Denko Corporation; V-5320) having a tensile strength of 12.5 N / cm and a tensile elongation at break of 150% to this original plate was used as a test material.

  After punching this coated steel sheet with a film to □ 30 mm × 50 mm, the clearance is variously changed in the range of 9 to 66% under the condition of a bending flange length of 5 mm shown in FIG. When a molding test was performed in which the punch R variously changed in the range of 5t to 4.6t was pushed into the die of the die R4.6t at a pushing speed of 1 mm / s, the results shown in Table 1 were obtained. In Table 1, those that could be molded without problems were evaluated as “Good”, and those that caused film breakage were evaluated as “Poor”.

From this result, it is understood that a clearance of 33% or more and a punch R of 4.6 t or more are appropriate.
The above results were obtained when a film having a tensile strength of 12.5 N / cm, a tensile elongation at breakage of 150%, and a thickness of 70 μm was used. The film has a tensile strength of 30 N / cm, It should be noted that the same results were obtained when a universal craft tape with a tensile elongation at break of 6% (Teraoka Seisakusho) was used.

Example 2:
Various films having different properties in a tensile strength of 10 to 40 N / cm and tensile elongation at break of 6 to 550% under the mold conditions having a clearance of 33% and a punch R of 4.6 t, A steel plate adhered to the same clear-coated stainless steel plate as used in Example 1 was subjected to a forming test. The results are shown in Table 2 and FIG. The indentation speed and the evaluation criteria are the same as in the examples.
From this result, it can be seen that a film having a tensile strength of 12.5 N / cm or more and a tensile elongation at break of 150% or less is optimal.

Diagram explaining how film breakage occurs during bending The figure explaining the relationship between the die clearance, the size of the punch R, and the distance between the die tip and the punch tip The figure explaining the molding conditions of the indentation test conducted as an example Diagram showing the relationship between film properties and moldable range

Claims (1)

  When bending the coated steel sheet to which the surface protective film is adhered, a film having the characteristics that the tensile strength is 12.5 N / cm or more and the tensile elongation at break is 150% or less is used as the protective film. A bending method for a coated steel sheet with a film, characterized in that bending is performed under a die condition of a clearance R and a punch R of 4.6 t (where t is the thickness of the steel sheet to be processed).

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