JP5140373B2 - Multilayer paperboard - Google Patents

Description

  The present invention relates to a multilayer paperboard which is excellent in printability and cosmetics and has excellent surface strength.

  In recent years, a cardboard case has been required to have a display function, and thus a corrugated cardboard multilayer paperboard that is compatible with cosmetic printing has been demanded. In particular, flexographic printing is excellent in economic efficiency and environmentally friendly water-based inks are mainly used. Therefore, flexographic printing is often applied as cosmetic printing, and it is considered that it will become more popular in the future.

  Until now, as a multilayer paperboard for corrugated board corresponding to such cosmetic printing, for example, as shown in Patent Document 1, a method of providing a coating layer containing a pigment and a binder on the surface to be printed has been used. Yes. However, although the coating layer can be used to enhance the cosmetics of the paper and improve the printing effect, there is a problem that the manufacturing cost increases. In addition, by providing a coating layer, the surface of the coating layer has high whiteness, so it is excellent in appearance and appearance, but when such a paperboard is used for a cardboard case, for example, during transportation or in a warehouse There is a problem that dirt such as dust adhering to is very noticeable and gives consumers an impression that the product is dirty or old.

  Also, as a multi-layer paperboard for corrugated board corresponding to cosmetic printing, white ink is printed on the entire surface of unbleached multilayer paperboard, and other colors such as red and indigo are printed on it to give cosmetics. There is also a way to do it. However, since white ink is expensive, there is a problem that the manufacturing cost is high, and in multi-color printing for obtaining cosmetics, one color needs to be printed with white ink, and the number of colors is also limited. There is also a problem.

A multilayer paperboard (hereinafter referred to as a white liner) formed by using bleached pulp as a raw material pulp for the surface layer is excellent in printability and print quality, and can increase the commercial value. However, the white liner, like the multi-layer paperboard with the above coating layer, has a high whiteness and is excellent in appearance and appearance, but it is very dirty due to dust adhering to it during transportation and warehouse. There is a problem that the product is noticeable and gives the impression that the product is dirty or old.
JP 2007-182656 A

  The present invention has been made in view of the above-described circumstances, and the object of the present invention is to maintain a predetermined surface strength while reducing expensive bleached pulp and to provide a white cosmetic appearance. An object of the present invention is to provide a multi-layer paperboard that does not stand out from dirt such as dust adhering to it during processing or use.

The above object of the present invention, at least a base sheet by a surface layer portion and the lower portion is configured, the superficial layer is a multilayer paper making paperboard having a white layer of at least two layers of the surface white layer and underlying white layer, wherein The raw material pulp constituting the surface white layer is made of softwood bleached kraft pulp as a main raw material, the whiteness of the raw material pulp constituting the white layer is set to 65% to 85%, and the basis weight of the whole white layer is 10 to 45 g / m ² (dry), the basis weight of the surface white layer is 29 to 1 g / m ² (dry), and the whiteness of the surface layer portion surface is 40% to 60%. This is accomplished by providing paperboard.

The above-mentioned object of the present invention is characterized in that the raw material pulp constituting the lower white layer is mainly hardwood bleached kraft pulp, and the basis weight of the lower white layer is 1 to 29 g / m ² (dry). This is achieved effectively by providing a multilayer paperboard.

  Furthermore, the object of the present invention is to provide a multilayer paperboard characterized in that the surface strength of the surface layer portion surface is 10 A or more and the Parker print surf smoothness is 6.0 to 8.0 μm. Is achieved more effectively.

According to the multilayer paperboard according to the present invention, a base paper is constituted by at least a surface layer portion and a lower layer portion, and the surface layer portion has at least two white layers of a surface white layer and a lower white layer. And the whiteness of the raw material pulp which comprises a white layer shall be 65% or more and 85% or less, the basic weight of the whole white layer shall be 10-45 g / m < ² > (dry), and the whiteness of the surface layer part surface shall be 40% Since it is 60% or less, the printability can be improved and, as before, a large amount of white ink is printed on the entire surface of the unbleached multilayer paperboard, and other colors are printed. Even if printing is not performed, it is possible to exhibit a cosmetic property with a whiteness (product color) that is not conventional. In addition, dirt during transportation is less noticeable and the appearance is better than that of the conventional white liner.

  In the following, the details of the multilayer paperboard (hereinafter referred to as “main paperboard”) according to the present invention will be described in detail. The base paper is composed of a surface layer portion and a lower layer portion, and the surface layer portion is arranged on the surface layer side. Consists of two white layers, a white layer (hereinafter referred to as “surface white layer”) and a white layer (hereinafter referred to as “lower white layer”) disposed on the lower layer side, and the lower layer portion is composed of one layer. An example will be described. The multilayer paperboard according to the present invention is not limited to the following embodiments, and it is needless to say that the configuration can be changed as appropriate without departing from the scope of the claims.

  In this way, the paperboard is formed by multilayer papermaking because the basis weight of the paperboard is generally heavier than that of western paper. This is because the paper is made better and the quality of the paperboard is better.

Moreover, this paperboard makes the whiteness of the raw material pulp which comprises a white layer 65% or more and 85% or less, and sets the basic weight of the whole white layer to 10-45 g / m < ² > (dry), but the whiteness of the surface layer part surface Is 40% or more and 60% or less. The basis weight (dry) is a value measured according to JIS-P8124 (hereinafter the same).

As a result, the present paperboard exhibits an unprecedented product color and beauty, and can save forest resources. In addition, when the paperboard is processed into a cardboard case, dirt such as paper dust adhered during transportation of the cardboard or in a warehouse becomes inconspicuous, and an effect of relieving the impression that the product is dirty or old can be obtained. That is, the whiteness of the raw material pulp constituting the white layer of the present paperboard is set to 65% or more and 85% or less, and the total basis weight of the white layer is set to 10 to 45 g / m ² (dry). The structure can be seen through the surface layer. As a result, it is possible to obtain an effect such as having cosmetics with a feeling of whiteness that has not been heretofore present, and that dirt during transportation is less noticeable than conventional white liners.

  In addition, when the whiteness of the raw material pulp which comprises a white layer is less than 65%, it will become difficult to make the whiteness of the surface layer part surface of this paperboard into 40% or more. On the other hand, when the whiteness exceeds 85%, whiteness can be obtained, but the production cost of the raw material pulp increases.

Further, when the total basis weight of the white layer is less than 10 g / m ² (dry), there is no cushioning property and printability is poor. On the other hand, when the total basis weight of the white layer exceeds 45 g / m ² (dry), a multilayer paperboard having the same appearance as the conventional white liner is obtained.

  Furthermore, if the whiteness of the surface layer part surface of the present paperboard is less than 40%, it will not be a multilayer paperboard having the desired cosmetic properties of the present application, and if the whiteness of the surface layer part surface exceeds 60%, It approaches the texture of the white liner, and dirt such as dust adhering during transportation or warehouse becomes conspicuous.

  Examples of raw material pulp for the surface white layer and the lower white layer include chemicals such as hardwood bleached kraft pulp (LBKP), softwood bleached kraft pulp (NBKP), hardwood semibleached kraft pulp (LSBKP), and softwood semibleached kraft pulp (NSBKP). Pulp, Stone Grand Pulp (SGP), Pressurized Stone Grand Pulp (PGW), Refiner Grand Pulp (RGP), Thermo Grand Pulp (TGP), Chemi Grand Pulp (CGP), Ground Pulp (GP), Thermo Mechanical Pulp (TMP) ) Etc. are manufactured from pulp bleached from mechanical pulp, tea waste paper, craft envelope waste paper, magazine waste paper, newspaper waste paper, flyer waste paper, office waste paper, corrugated waste paper, upper white waste paper, Kent waste paper, imitation waste paper, old paper waste paper, etc. Disaggregated / deinked waste paper pulp or disaggregated / deinked / bleached waste paper pulp Flop, or may be used kenaf, hemp, various known bleaching pulp such as chemically or mechanically produced bleached pulp from non-wood fibers such as reed.

  Among these raw material pulps, manufactured from LBKP, NBKP, upper white waste paper, and Kent waste paper in order to achieve a balanced and efficient achievement of the role as the surface layer of this paperboard and various quality characteristics as the bookboard. It is preferable to use the used waste paper pulp.

  Furthermore, it is more preferable that the raw material pulp of the surface white layer is made of NBKP as a main raw material, and the raw material pulp of the lower white layer is made of LBKP as a main raw material. Thereby, it can be made excellent in printability, without reducing the surface strength of this paperboard.

That is, the raw material pulp constituting the surface white layer is made of NBKP having a strong inter-fiber strength, so that the lower white layer has the cushioning property of the surface white layer while maintaining the surface strength of the surface layer. Better printability can be obtained. Therefore, the raw material pulp which comprises a lower layer white layer uses LBKP as a main raw material, and acts on the cushioning property of a surface layer white layer, and the compactness of a surface layer white layer effectively. Thereby, even if the basis weight of the surface white layer is reduced to 29 to 1 g / m ² as described later, good surface strength and printability can be obtained.

  Specifically, NBKP is blended in an amount of 55 to 100 mass%, preferably 65 to 90 mass%, in the raw material pulp used for the surface white layer. As a result, it is possible to effectively prevent the surface strength of the paperboard from being lowered and to improve the bursting strength required for the base paper for corrugated cardboard. If the NBKP content of the raw material pulp of the surface white layer is less than 55% by mass, the fiber length is long and the content of NBKP with thick fibers decreases, so the surface strength decreases, Ruled line breakage is likely to occur during box processing. On the other hand, although the content of NBKP can be 100% by mass, the content of NBKP is 90% by mass or less because there is a risk of unevenness in strength due to unevenness of texture, deterioration of appearance, and high cost. It is preferable to do.

  Moreover, 55-100 mass% of LBKP is mix | blended with the raw material pulp used for a lower layer white layer, Preferably 65-90 mass% is mix | blended. Thereby, the fiber formation is good, the cushioning property of the paper layer is improved, and the printability is excellent. When the content of LBKP in the raw material pulp of the lower white layer is less than 55% by mass, the fiber length is short, the content of thin LBKP is small, and the cushioning property of the paper layer is lowered. On the other hand, the content of LBKP can be set to 100% by mass, but since the dehydrating property during paper making is poor and the productivity may be deteriorated, the content of LBKP may be 90% by mass or less. preferable.

  Moreover, the raw material pulp used for layers other than the white layer of this paperboard (lower layer part in this embodiment) is not specifically limited, It can be set as a well-known structure. However, in order to make the surface whiteness of the surface layer portion of the paperboard 40% or more and 60% or less, and to make the paperboard have a white cosmetics, a corrugated paperboard is used for layers other than the white layer. Waste paper pulp produced as the main raw material is preferably used. Furthermore, it is preferable to mix as much waste paper pulp as possible from the viewpoint of reducing energy consumption and the load on the environment. In addition, for example, in order to give the strength required when the present paperboard is used in a corrugated cardboard case, it is more preferable to use a raw material pulp having a high bursting strength and compressive strength and to differentiate the functions of the surface layer portion and the lower layer portion. .

  In addition, as an internal sizing agent used for the raw material pulp constituting the present paperboard, for example, rosin size, alkyl ketene dimer (AKD), alkenyl anhydrous succinic acid conventionally used for producing multilayer paperboard A well-known thing, such as an acid (ASA), can be used without a restriction | limiting. The internal sizing agent may be used alone or in combination of two or more. However, it is preferable to use a rosin-based size as an internal sizing agent for the present paperboard. When a rosin-based sizing agent is used, an excellent effect is obtained with respect to the anti-slip property required when processed into a corrugated cardboard case, compared to the case where other sizing agents are used. In addition, as this rosin-type sizing agent, "R-50" by Modern Chemical Co., Ltd. can be used, for example.

  In addition, when making this paperboard, various conventionally used nonionic, cationic and anionic retention agents, freeness improvers, paper strength improvers, and other internal additives for papermaking are required. It can be selected and used as appropriate.

  Further, for example, sulfuric acid band, aluminum chloride, sodium aluminate, basic aluminum compounds such as basic aluminum chloride and basic polyaluminum hydroxide, water-soluble aluminum compounds such as water-degradable alumina sol, ferrous sulfate In addition, a polyvalent metal compound such as ferric sulfate or silica sol may be internally added. Other papermaking aids such as various starches, polyacrylamide, urea resin, melamine resin, epoxy resin, polyamide resin, polyamide, polyamine resin, polyamine, polyethyleneimine, vegetable gum, polyvinyl alcohol, latex, polyethylene oxide or hydrophilic cross-linked polymer Various compounds such as a particle dispersion and derivatives or modified products thereof may be added, applied, applied, or impregnated.

  Furthermore, internal additives for papermaking such as dyes, fluorescent brighteners, pH adjusters, antifoaming agents, pitch control agents or slime control agents can be added as appropriate according to the intended use.

This paperboard uses the raw material pulp mentioned above, It is preferable that the basic weight of a surface white layer is 29-1 g / m < ² > (dry), and the basic weight of a lower layer white layer is 1-29 g / m < ² > (dry). . When the basis weight of the surface white layer exceeds 29 g / m ² (dry), the surface strength of the raw material pulp having interfiber bond strength increases, but the whiteness of the surface of the surface layer portion increases. The paperboard does not have the desired texture. On the other hand, when the basis weight of the surface white layer is less than 1 g / m ² (dry), the surface strength of the surface layer of the multilayer paperboard is lowered. Moreover, if the basis weight of the lower white layer is less than 1 g / m ² (dry), the cushioning property of the surface layer portion is lost and the printability is poor. On the other hand, when the basis weight of the lower white layer exceeds 29 g / m ² (dry), the strength of the paper layer is increased, but the whiteness of the surface layer portion surface is increased, so that the paperboard does not have a desired texture. .

Furthermore, the total basis weight of the paperboard is not particularly limited, and is preferably 100 to 500 g / ^{m 2,} and more preferably a 140~300g / ^{m 2.} If the basis weight of the entire paperboard is less than 100 g / m ² , there is a concern that poor coating suitability during the production of the paperboard and associated printing suitability may occur. On the other hand, if the basis weight exceeds 500 g / m ² , the thickness of the multi-layer paperboard is too thick, which makes it difficult to wind up.

  The papermaking method for the base paper of the present paperboard is not particularly limited, and any of an acidic papermaking method, a neutral papermaking method, and an alkaline papermaking method may be used. Also, the paper machine is not particularly limited, and various known paper machines such as a long net paper machine, a twin wire paper machine, a circular net paper machine, and a short net combination machine can be used. .

  This paperboard is adjusted to the specified raw material composition as described above. However, in order to improve the interfiber bonding and the surface strength, the wet paper strength agent is added to the pulp solids to the raw material pulp constituting the white layer. More preferably, 0.1 to 1.0% by mass is added. As a specific example of the wet paper strength agent, it is preferable to use an amphoteric polyacrylamide type paper strength agent that is less contaminated in the paper making process and is excellent in printability, but it is of course not limited thereto. This amphoteric polyacrylamide is a polymer in which a cationic group is introduced into polyacrylamide, for example, a copolymer of a cationic monomer such as acrylamide-vinyl monomer copolymer, acrylamide-dimethylaminoethyl methacrylate copolymer and acrylamide. And Mannich-modified polyacrylamide, Hoffmann-decomposed polyacrylamide, and the like.

In the present paperboard, it is desirable to coat or impregnate the surface white layer with a coating agent. Examples of the coating agent include various known ones such as styrene / butadiene copolymer latex (SBR), vinyl acetate, acrylic, polyvinyl alcohol (PVA), polyacrylamide (PAM), water-proofing agent and water-repellent agent. And may be used alone or in combination of two or more as required. The coating amount of the coating agent is not particularly limited, but it is preferably 0.2 to 2.5 g / m ² in terms of solid content. That is, even if the coating amount is more than 2.5 g / m ² , the surface strength cannot be improved enough to meet the coating amount, while the coating amount is less than 0.2 g / m ² . If there is, there is a tendency that fluffing occurs on the surface of the surface white layer due to friction or the like and white spots are generated during printing.

As a method of applying the coating liquid, it can be applied by a known coating means such as a bar coater, a rod coater, an air knife or the like. Moreover, it can also print by well-known printing means, such as a gravure printing machine and a flexographic printing machine. Among these, this paperboard is preferable because it uses a bar coater coating machine to improve surface properties and excellent printability, and is also suitable for calendering in the previous step of coating the coating liquid. Is preferred. Thereby, the surface of the base paper (surface of the surface layer portion) becomes dense, the coating liquid can be efficiently kept on the surface of the base paper, and the board can be made more excellent in printability.

  Furthermore, by adjusting the addition amount of the internal sizing agent to the surface white layer and the lower white layer, the surface white layer can be efficiently coated or impregnated with the coating agent. In particular, if the amount of the internal sizing agent added is 0.1% by mass for the solid white layer and 0.3% by mass for the lower white layer, the coating liquid absorbs or impregnates more than necessary. It is more preferable because it is not performed.

  The paperboard formed as described above is a numerical value measured using the wax pick measuring method specified in JIS-P8129, has a surface strength of 10 A or more, and has a measured value of Parker Print Surf Smoothness Meter of 6. It is preferably 0 to 8.0 μm, preferably 6.5 to 7.6 μm.

  When the surface strength is less than 10 A, the surface strength of the multilayer paperboard may be insufficient in practice. In addition, surface layer swelling occurs during printing, and the printability is poor due to paper dust or the like.

  Moreover, since the measured value of the Parker Print Surf smoothness meter approximates the printing pressure at the time of flexographic printing, the measured value is considered to represent the smoothness at the time of printing fairly accurately. That is, when the measured value by the Parker Print Surf smoothness meter is low, the measurement surface is excellent in smoothness and the ink penetration is reduced, but the ink is poor in drying property and the ink is rubbed and smeared. On the other hand, when the value measured by the Parker Print Surf smoothness meter is high, the measurement surface is rough, the ink fixing property to the surface of the surface of the multilayer paperboard is poor, and problems such as printing blur are likely to occur.

  Therefore, in this multilayer paperboard, when the value measured by the Parker Print Surf smoothness meter exceeds 8.0 μm, the ink fixing property is inferior, and problems such as printing blur are likely to occur. On the other hand, if it is less than 6.0 μm, the ink Although it is excellent in fixability and reproducibility, it is inferior in the drying property of the ink and easily causes ink smearing.

The present paperboard formed as described above has a whiteness of 65% to 85% of the raw material pulp constituting the white layer, and a basic layer weight of 10 to 45 g / m ² (dry) of the entire white layer. Since the whiteness of the surface is set to 40% or more and 60% or less, it is necessary to perform cosmetic printing by printing a large amount of white ink on unbleached multilayer paperboard and printing other colors as before. Disappears.

  As described above, in the multilayer paperboard according to the present invention, the case where the base paper is made in three layers, that is, a surface layer portion composed of two white layers of a surface white layer and a lower white layer, and a lower layer portion of one layer is described in detail. As described above, the multilayer paperboard is not particularly limited as long as it has three or more layers, and may have any number of layers. That is, the white layer may be composed of three or more layers, or the lower layer portion is composed of two or more layers, for example, composed of an intermediate layer and a lower layer, or composed of two or more lower layers, and the configuration is appropriately determined. It may be changed.

  In order to confirm the effect of the multilayer paperboard according to the present invention, the following various samples were prepared, and a test for evaluating the quality of each sample was performed. In addition, in a present Example, the numerical value which shows a mixing | blending, a density | concentration, etc. is a numerical value based on the weight of a solid content or an active ingredient. Moreover, since the pulp, chemical | medical agent, etc. which are shown in a present Example are only examples, it cannot be overemphasized that this invention is not restrict | limited by these Examples, and can be selected suitably.

  16 types of paperboards according to the present invention (referred to as “Example 1” through “Example 16”) and five types of paperboards (referred to as “Example 1” through “Example 16”) “Comparative Example 1” to “Comparative Example 5”) were prepared as shown in Table 1.

Example 1
The raw material slurry for the surface white layer is a rosin-based sizing agent (trade name: R-50) as an internal sizing agent in a raw pulp in which softwood kraft pulp (NBKP) and upper white waste paper are blended at a mass ratio of 55:45. (Manufactured by Modern Chemical Co., Ltd.) 0.1% by mass as a solid content. In addition, a raw material slurry for the lower white layer was mixed with a raw material pulp in which hardwood kraft pulp (LBKP) and upper white waste paper were blended at a mass ratio of 55:45. R-50 Modern Chemical Co., Ltd.) was added to prepare 0.3% by mass. Furthermore, the raw material slurry for the lower layer part (in this embodiment, the middle layer and the back layer) is 100% by mass of the corrugated waste paper (WP), and the wet paper strength agent (amphoteric polyacrylamide) is 0.5% by mass in solid content. It was prepared by adding.

Using these raw material slurries, the paper layer of the surface layer portion consisting of the surface white layer and the lower layer white layer and the lower layer portion consisting of the middle layer and the back layer on the paper machine, the basis weight of the surface white layer is 10 g / m ² , basis weight of 15 g / ^{m 2} of the lower white layer, and the total basis weight of the lower part to fit the paper making in a 195 g / ^{m 2} to prepare a base paper. The basis weight was measured according to JIS-P8124.

On the surface of this base paper, that is, on the surface of the surface white layer, polyacrylamide (PAM) (trade name: “Haricoat G” / manufactured by Harima Kasei Co., Ltd.) as a coating solution is solid in a bar coater. A multilayer paperboard having a four-layer structure by coating on the surface of the surface white layer at a coating amount of 5 g / m ² and then calendering with a machine calender under a linear pressure of 30 kgf / cm (Example 1) Got.

(Examples 2 to 16)
Mixing of raw material pulp of surface white layer, basis weight of surface white layer, mixing of raw material pulp of lower layer white layer, basis weight of lower layer white layer, whiteness of raw material pulp constituting white layer, basis weight of lower layer part, coating A multilayer paperboard obtained in the same manner as in Example 1 except that the coating amount of the working liquid was set as shown in Table 1.

(Comparative Examples 1-5)
Mixing of raw material pulp of surface white layer, basis weight of surface white layer, mixing of raw material pulp of lower layer white layer, basis weight of lower layer white layer, whiteness of raw material pulp constituting white layer, basis weight of lower layer part, coating A multilayer paperboard obtained in the same manner as in Example 1 except that the construction of the working fluid is as shown in Table 1.

  Table 2 shows the results of quality evaluation for all of these Examples and Comparative Examples, that is, evaluation tests on product whiteness, surface strength, smoothness, printability, and production cost.

  The “product whiteness (%)” in Table 2 is the surface whiteness of the surface layer, and is a value measured in accordance with the hunter whiteness test method for paper and pulp of JIS-P8123.

  The “surface strength (A)” is a value measured based on “2.1 Method Using Wax” described in 1994 “Method for testing surface strength of paper and paperboard” of JIS-P8129.

  “Smoothness (μm)” is a value measured in accordance with ISO 8791-4 under a pressure of 1.0 MPa using a Parker Print Surf smoothness meter (SE-115 manufactured by Lorentzen & Wettre).

“Printability” means a KRK universal printing aptitude test made by Kumagaya Riki Kogyo Co., Ltd. using a standard tack grade ink (product number 404.380.000) used in an IGT printability tester defined in JIS-P8129. It was printed using a machine and evaluated. The evaluation criteria are as follows.
A: There is no peeling of ink, and there is no effect on the printed surface.
◯: There is slight peeling of the ink, but there is no effect on the printed surface.
Δ: Ink is peeled off and there is a place where a base appears on the printed surface.
X: The ink peeled off and the appearance of the ground on the printed surface was markedly inappropriate.

  Furthermore, “manufacturing cost” is a relative evaluation of the cost of manufacturing each example and comparative example, and was evaluated in four stages: “◎”, “○”, “△”, and “×”. .

As shown in Table 2, the multilayer paperboard according to the present invention, that is, the multilayer paperboard according to Examples 1 to 16, is excellent in quality evaluation, that is, while reducing the amount of expensive bleached pulp used. It can be seen that, while maintaining a predetermined surface strength and having a cosmetic white appearance, dirt such as dust adhering during processing and use does not stand out.

Claims (3)

A base paper is composed of at least a surface layer portion and a lower layer portion, and the surface layer portion is a multilayer paperboard having at least two white layers of a surface white layer and a lower white layer,
The raw material pulp that constitutes the surface white layer is mainly made of softwood bleached kraft pulp,
The whiteness of the raw material pulp constituting the white layer is 65% or more and 85% or less,
The basis weight of the entire white layer is 10 to 45 g / m ² (dry) ,
The basis weight of the surface white layer is 29-1 g / m ² (dry),
A multilayer paperboard having a whiteness of 40% to 60% on the surface of the surface layer portion. Raw material pulp constituting the lower white layer is mainly hardwood bleached kraft pulp,
^2. The multilayer paperboard according to claim 1, wherein a basis weight of the lower white layer is 1 to 29 g / m ² (dry).   3. The multilayer paperboard according to claim 1, wherein the surface strength of the surface layer portion surface is 10 A or more, and the Parker print surf smoothness is 6.0 to 8.0 μm.