Method for manufacturing shallow-drawn molding

Abstract

To provide a shallow-drawn molding made of austenite ferrite two-phase stainless steel and capable of suppressing the occurrence of twisting.SOLUTION: A load applied on a punch when a blank made of two-phase stainless steel is pushed in increases along with the increase of a molding height and, at at least molding bottom dead center, the blank is held between a second support provided on one side of the punch and the punch. When the moving distance of the punch from the blank pressing start time to the molding dead center of the punch is a maximum distance, a load P0.8 applied on the punch when the moving distance of the punch from the pressing start time reaches a maximum distance 80% and a load P0.9 applied on the punch when the moving distance of the punch from the pressing start time reaches a maximum distance 90%, and a maximum load Pmax applied on the punch from the pressing start time to the molding dead center satisfy "Pmax-P0.9) / (P0.9-P0.8)≥15.0".SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a shallow-drawn molded article and a method for manufacturing the same.

Background Art

[0002] Conventionally, in food manufacturers, aluminum, which is lightweight and has excellent press formability, has been used as a material for food trays used in the cooking and manufacturing of various foods. In addition, the use of stainless steel, which is excellent in corrosion resistance and strength, as a material for food trays is also increasing.

[0003] For example, Patent Document 1 discloses a food stainless steel container. Patent Document 1 describes that as stainless steel, martensitic, ferritic, austenitic, or duplex stainless steel can be used.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Food trays used in ovens are required to have heat distortion resistance in order to prevent snagging, food from falling off, and uneven baking of food inside the oven. In recent years, due to the need to comply with HACCP (Hazard Analysis and Critical Control Point), food trays with excellent abrasion resistance and magnetism have been demanded. Specifically, abrasion resistance is required so that when cooked food is scraped off the food tray, the material of the food tray is not scraped to produce fragments, scraps, etc., and magnetism is required so that even if such fragments are generated, metal foreign objects in the food can be detected by a magnetic separator. On the other hand, from the perspective of improving the working environment, weight reduction is also required for food trays. Therefore, the present inventors have considered using austenite-ferrite dual-phase stainless steel (hereinafter, also simply referred to as dual-phase stainless steel), which is excellent in heat distortion resistance, abrasion resistance, and magnetism, and can be lightweight by thinning the plate, as a material for food trays. Dual-phase stainless steel has sufficient strength compared to aluminum. Therefore, when using dual-phase stainless steel as the blank material, sufficient thinning can be achieved compared to using aluminum. As a result, when using dual-phase stainless steel as the blank material, weight reduction similar to that when using aluminum as the blank material can be achieved.

[0006] However, as a result of the study by the present inventors, it has been found that when using dual-phase stainless steel as the material, it may not be possible to manufacture a container with a desired shape. Specifically, it has been found that when manufacturing a container by performing a shallow drawing process on a blank made of a dual-phase stainless steel plate, the formed container (formed product) may be twisted.

[0007] Therefore, an object of the present invention is to provide a method for manufacturing a shallow drawing formed product made of austenite-ferrite dual-phase stainless steel that can suppress the occurrence of twisting during press forming.

Means for Solving the Problems

[0008] The inventor has conducted studies to suppress the deformation that occurs in a formed product when manufacturing a shallow drawing formed product from a blank made of duplex stainless steel. Specifically, an attempt was made to manufacture the food tray shown in FIG. 1 by performing shallow drawing on a duplex stainless steel sheet. In FIG. 1, (a) is a plan view of the food tray, and (b) is a front view of the food tray. The food tray 100 shown in FIG. 1 has a cylindrical vertical wall portion 102, a bottom portion 104 that closes one end of the vertical wall portion 102, and a flange portion 106 that extends outward from the other end of the vertical wall portion 102.

[0009] Since duplex stainless steel has a higher specific gravity than aluminum, when using a duplex stainless steel sheet as a blank, it is necessary to reduce the thickness to achieve weight reduction. However, as a result of the studies by the inventors, it was found that duplex stainless steel has high yield strength (strength) and poor shape freezing property. Therefore, when the thickness is reduced, the rigidity decreases, and the amount of deformation generated by the release of residual stress after forming increases. Specifically, as shown in FIG. 2, it was found that large torsions may occur in the food tray 100. Note that FIG. 2 shows a front view of the food tray 100.

[0010] Therefore, the inventor conducted studies to suppress the occurrence of such torsions. As a result, it was found that the forming load during shallow drawing has a great influence on the torsion generated in the food tray 100. Specifically, it was found that the torsion generated in the food tray 100 can be sufficiently suppressed by greatly increasing the forming load at or near the bottom dead center of forming.

[0011] The present invention has been made based on the above findings, and the gist thereof is a method for manufacturing the following shallow drawing formed product.

[0012] (1) A method for manufacturing a shallow drawing molded product, comprising sandwiching a blank made of a duplex stainless steel between a hollow first support portion disposed on one side of the blank in the thickness direction of the blank and an annular blank holder disposed on the other side of the blank in the thickness direction, and pushing the inside of the portion of the blank sandwiched between the first support portion and the blank holder toward the one side by a punch. When pushing the blank, the load acting on the punch increases as the forming height increases. At least at the forming bottom dead center, sandwich the blank between a second support portion provided on the one side of the punch and the punch. When the moving distance of the punch from the start point of pressing the blank by the punch to the forming bottom dead center is defined as the maximum distance, When the moving distance of the punch from the start point of pressing reaches 80% of the maximum distance, the load P acting on the punch 0.8 , when the moving distance of the punch from the start point of pressing reaches 90% of the maximum distance, the load P acting on the punch 0.9 , and the maximum load P acting on the punch between the start point of pressing and the forming bottom dead center max satisfy the following formula (i), which is a method for manufacturing a shallow drawing molded product. (P max - P 0.9 ) / (P 0.9 - P 0.8 ) ≥ 15.0 ··· (i)

[0013] (2) At the forming bottom dead center, the maximum load P max acts on the punch, which is the method for manufacturing a shallow drawing molded product according to (1) above.

[0014] (3) The method for manufacturing a shallow drawing molded product according to (1) or (2) above, wherein the first support portion and the second support portion are separate members.

[0015] (4) The method for manufacturing a shallow drawing molded product according to (1) or (2) above, wherein the first support portion and the second support portion are integrally formed.

Advantages of the Invention

[0016] According to the present invention, a shallow drawing molded product made of austenite-ferrite dual-phase stainless steel with suppressed generation of torsion during press forming can be obtained.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0018] Hereinafter, a manufacturing method of a shallow drawing molded product according to an embodiment of the present invention will be described. In the following, the case of manufacturing the food tray 100 shown in FIG. 1 will be described.

[0019] (Outline of the manufacturing method) FIG. 3 is a schematic perspective view showing a mold for manufacturing the food tray 100, and FIG. 4 is a view showing the manufacturing method of the food tray 100.

[0020] As shown in FIG. 3, the mold 10 used in the manufacturing method according to one embodiment of the present invention includes a die 12, a punch 14, a blank holder 16, and a support plate 18. The die 12 has a hollow shape. In the present embodiment, a through hole penetrating the die 12 in the vertical direction is formed in the central portion of the die 12. In the present embodiment, the die 12 has an annular support surface 12a and a wall surface 12b extending downward from the inner edge of the support surface 12a.

[0021] The blank holder 16 is disposed above the die 12. A through hole penetrating in the vertical direction is formed in the central portion of the blank holder 16. In the present embodiment, the blank holder 16 has a pressing surface 16a formed in an annular shape so as to face the support surface 12a in the vertical direction.

[0022] The punch 14 is formed to have a size that can be inserted into the through holes of the die 12 and the blank holder 16 from above. In the present embodiment, the punch 14 has a substantially rectangular pressing surface 14a. Although illustration is omitted, a load for performing shallow drawing forming is applied to the punch 14 by a biasing device such as a gas spring. Since a known configuration of a press machine can be used as the biasing device for applying a load to the punch 14, detailed description thereof is omitted.

[0023] The support plate 18 is formed to have a size that can be inserted into the through hole of the die 12 from below. The support plate 18 has a support surface 18a formed in a substantially rectangular shape so as to face the pressing surface 14a in the vertical direction. In the present embodiment, the pressing surface 14a and the support surface 18a have the same area, but it is preferable that the area of the support surface 18a is set to be equal to or larger than the area of the pressing surface 14a. More specifically, it is preferable that the pressing surface 14a and the support surface 18a are provided such that the outer edge of the pressing surface 14a is positioned inside the outer edge of the support surface 18a. In the present embodiment, the die 12 corresponds to the first support portion, and the support plate 18 corresponds to the second support portion.

[0024] As shown in Fig. 4(a), when manufacturing the food tray 100, the die 12 and the blank holder 16 clamp the outer peripheral portion (the portion that becomes the flange portion 106) of the blank 20. In this embodiment, the blank 20 is clamped by the die 12 and the blank holder 16 such that the thickness direction of the blank 20 is the vertical direction. Specifically, the blank 20 placed on the support surface 12a of the die 12 is pressed from above by the pressing surface 16a of the blank holder 16. In this embodiment, the outer peripheral portion of the blank 20 is pressed by the blank holder 16 so that a pressure of, for example, 3 to 15 MPa is applied to the blank 20 from the blank holder 16. As the blank 20, an austenite-ferrite two-phase stainless steel sheet (hereinafter simply referred to as a two-phase stainless steel sheet) is used.

[0025] Next, as shown in Fig. 4(b), the punch 14 is lowered, and the inside of the portion of the blank 20 sandwiched between the die 12 and the blank holder 16 is pushed downward by the punch 14. Specifically, the punch 14 is moved downward until the blank 20 is sandwiched between the pressing surface 14a of the punch 14 and the support surface 18a of the support plate 18. Thereby, the food tray 100 shown in Fig. 1 is formed. In this embodiment, during the period from the start of pressing the blank 20 by the punch 14 until the punch 14 reaches the forming bottom dead center, the vertical position of the support plate 18 with respect to the die 12 is fixed by the support member 50.

[0026] Thereafter, the punch 14 and the blank holder 16 are raised, and the food tray 100 is taken out. In this way, the food tray 100 is manufactured. Part or all of the flange portion 106 may be cut and removed after demolding. Note that a biasing device such as a gas spring may be provided on the support member 50 so that the support member 50 can move up and down. In this case, after the punch 14 and the blank holder 16 are raised, the food tray 100 can be easily taken out by raising the support plate 18.

[0027] (Regarding the shallow drawing molded product and the blank) In this specification, a shallow drawing molded product means a drawing molded product in which the drawing depth (the height of the vertical wall portion in the thickness direction of the bottom: height h in the food tray 100 in FIG. 1(b)) is smaller than the diameter of the circle (the circle indicated by the dashed-dotted line in the food tray 100 in FIG. 1(a)) circumscribing the inner surface of the open end of the vertical wall portion (the minor axis in the case of an ellipse). In the food tray 100 according to the present embodiment, the drawing depth (height h) is smaller than the length of the short side of the vertical wall portion 102 in plan view.

[0028] In the present embodiment, a duplex stainless steel sheet is used as the material of the blank 20. As the material of the blank, for example, SUS821L1 can be used. More specifically, in mass%, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 2.00 to 4.00%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.50 to 3.50%, Cr: 19.60 to 24.00%, Mo: 0.01 to 0.60%, Cu: 0.01 to 1.50%, N: 0.0100 to 0.2000%, Al: 0 to 0.050% or less, Nb: 0 to 0.500%, Ti: 0 to 0.500%, Co: 0 to 0.30%, V: 0 to 0.50%, Sn: 0 to 0.500%, Ga: 0 to 0.050%, Zr: 0 to 0.50%, Ta: 0 to 0.050%, B: 0 to 0.0050%, W: 0 to 0.50%, Mg: 0 to 0.0100%, REM: 0 to 0.0100% are included, the balance being Fe and impurities, and a duplex stainless steel sheet having a PREN value (pitting index) represented by the following formula (a) of 20 or more can be used as the blank. PREN = Cr + 3.3Mo + 16N ···(a) However, each element symbol in the formula represents the content (mass%) of each element contained in the steel sheet.

[0029] The thickness of the blank 20 is, for example, 0.10 to 1.50 mm. From the viewpoint of weight reduction, the thickness of the blank 20 is preferably 0.60 mm or less.

[0030] (Regarding the load acting on the punch during shallow drawing forming) FIG. 5 is a diagram showing an example of the relationship between the moving distance of the punch 14 and the load acting on the punch 14 during shallow drawing forming. In FIG. 5, the horizontal axis represents the moving distance of the punch 14 (i.e., the forming height) after the pressing of the blank 20 by the punch 14 is started, and the vertical axis represents the load acting on the punch 14. The load acting on the punch 14 can be measured, for example, by attaching a load cell to the punch 14. In the following description, the moving distance of the punch 14 means the moving distance of the punch 14 from the start point of pressing the blank 20 by the punch 14.

[0031] In FIG. 5, D max represents the moving distance of the punch 14 to the bottom dead center of forming, and D 0.9 is D max shows a value 0.9 times that of D, and D 0.8 is D max shows a value 0.8 times that of D. Also, in FIG. 5, P max represents the maximum value of the load acting on the punch 14 from the start point of pressing the blank 20 by the punch 14 to the bottom dead center of forming. In the example of FIG. 5, at the bottom dead center of forming, the maximum load (P max ) acts on the punch 14. P 0.9 represents the load acting on the punch 14 when the moving distance of the punch 14 reaches D 0.9 , and P 0.8 represents the load acting on the punch 14 when the moving distance of the punch 14 reaches D 0.8 . In the present embodiment, the moving distance D max corresponds to the maximum distance.

[0032] As shown in FIG. 5, in the present embodiment, the load acting on the punch 14 when the punch 14 presses the blank 20 increases as the moving distance (forming height) of the punch 14 increases. Also, in the present embodiment, the loads P max , P 0.9 and P 0.8 are adjusted such that the following formula (i) is satisfied. (P max -P 0.9 ) / (P 0.9 -P 0.8)≥15.0 ···(i)

[0033] By adjusting the load acting on the punch 14 so as to satisfy the above formula (i), the forming load on the blank 20 can be greatly increased at or near the forming bottom dead center. Thereby, it is possible to suppress the occurrence of torsion in the food tray 100. Note that the value of the left side of the above formula (i) is preferably 30.0 or more, more preferably 35.0 or more, and even more preferably 40.0 or more.

[0034] (Modification example) In the above-described embodiment, the case where the shallow drawing forming is performed on the blank 20 with the position of the support plate 18 with respect to the die 12 fixed has been described. However, as long as the blank 20 is sandwiched between the punch 14 and the support plate 18 at least at the forming bottom dead center, the support plate 18 may move with respect to the die 12 during the shallow drawing forming. For example, after the punch 14 starts pressing on the blank 20 and before the punch 14 reaches the forming bottom dead center, the blank 20 may be sandwiched between the punch 14 and the support plate 18. In this case, for example, with the blank 20 sandwiched between the punch 14 and the support plate 18, the punch 14 and the support plate 18 may be lowered to the forming bottom dead center. However, even when performing the shallow drawing forming in this way, the load acting on the punch 14 increases as the moving distance (forming height) of the punch 14 increases, and the load P max , P 0.9 and P 0.8 satisfy the above formula (i).

[0035] In the above-described embodiment, the case where the first support portion (die 12) and the second support portion (support plate 18) are constituted by separate members has been described. However, the first support portion and the second support portion may be integrally formed. FIG. 6 shows a mold 10a according to a modified example. The mold 10a is different from the above-described mold 10 in that it includes a die 30 instead of the die 12 and the support plate 18. The die 30 has a hollow first support portion 32 and a plate-like second support portion 34 integrally formed at the lower end portion of the first support portion 32. The first support portion 32 has an annular support surface 32a and a cylindrical wall surface 32b extending downward from the inner edge of the support surface 32a to the upper surface 34a of the second support portion 34.

[0036] When manufacturing the food tray 100 using the mold 10a shown in FIG. 5, first, the blank 20 placed on the support surface 32a of the first support portion 32 is pressed from above by the pressing surface 16a of the blank holder 16. Next, the punch 14 is moved downward until the blank 20 is sandwiched between the pressing surface 14a of the punch 14 and the upper surface 34a of the second support portion 34. Thereby, the food tray 100 shown in FIG. 1 is formed. Also in this embodiment, the load acting on the punch 14 increases as the moving distance (forming height) of the punch 14 increases, and the loads P max 、P 0.9 and P 0.8 satisfy the above-described formula (i). Thereby, it is possible to suppress the occurrence of torsion in the food tray 100.

[0037] In the above-described embodiment, the case of manufacturing the food tray 100 having the vertical wall portion 102 with a rectangular cross section (rectangular shape) as a shallow drawing molded product has been described. However, the shape of the shallow drawing molded product according to the present invention is not limited to the above example. For example, as shown in FIG. 7, the present invention may be applied when manufacturing a shallow drawing molded product 100a having a cylindrical vertical wall portion 102a. Also, although not shown, the vertical wall portion may have a triangular cross section or a polygonal cross section with five or more sides.

[0038] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

Example

[0039] Using a stainless steel plate obtained from steel having the chemical composition shown in Table 1 as a blank, the food tray 100 shown in FIG. 1 was manufactured under the manufacturing conditions of Test Nos. 1 to 7 shown in Table 2. In Tests Nos. 1 to 3, 5, and 6, the food tray 100 was manufactured using a mold similar to the mold 10 shown in FIGS. 3 and 4. On the other hand, in Tests Nos. 4 and 7, the food tray 100 was manufactured using a mold having a configuration in which the support plate 18 was removed from the mold 10 shown in FIGS. 3 and 4. The dimensions of the blank were width: 520 mm × length: 720 mm × thickness: 0.5 mm, and the dimensions (design values) of the food tray 100 were width: 450 mm × length: 650 × drawing depth: 28 mm.

[0040]

Table 1

Table 2

[0041] (Regarding Test Nos. 1 to 4) To confirm the effects of the present invention, the twist height of each obtained food tray was measured. The twist height means the difference between the actual height and the design value of the flange portion 106 when the food tray 100 is placed on a flat surface. In the present embodiment, as shown in FIG. 2, the value obtained by subtracting the design value (28 mm) of the drawing depth from the average value of the height h1 of the flange portion 106 on one side in the long side direction of the food tray 100 and the height h2 of the flange portion 106 on the other side was defined as the twist height. As a result, in the food tray of Test No. 4 without using the support plate 18, the left side value of the above formula (i) was 4.0, and the forming load could not be increased significantly at the bottom dead center of forming or in the vicinity thereof. Further, the twist height was 43 mm, and a large twist occurred in the food tray 100.

[0042] The reduction rates of the torsional heights of Tests No. 1 to 3 with respect to the torsional height of Test No. 4 were evaluated. Specifically, the reduction rates (%) of the torsional heights of Tests No. 1 to 3 with respect to the torsional height of Test No. 4 were calculated by the following formula (b). Reduction rate = 100 - ((torsional height of Tests No. 1 to 3 / torsional height of Test No. 4) × 100) ···(b)

[0043] In the column of torsional suppression evaluation in Table 2, "〇" means that the reduction rate calculated by the above formula was 30% or more, "◎" means that the reduction rate was 40% or more, and "×" means that the reduction rate was less than 30%.

[0044] As shown in Table 2, in Tests No. 1 and 2 where the left side value of the above formula (i) was 15.0 or more, the torsion in the food tray 100 could be sufficiently suppressed. On the other hand, in Test No. 3 where the left side value of the above formula (i) was less than 15.0, the torsion in the food tray 100 could not be sufficiently suppressed.

[0045] (Regarding Tests No. 5 to 7) Similar to Tests No. 1 to 4 using steel type a, for Tests No. 5 to 7 using steel type b, the reduction rates of the torsional heights of Tests No. 5 and 6 with respect to the torsional height of Test No. 7 without using the support plate 18 were calculated. The reduction rates were calculated in the same manner as Tests No. 1 to 4. In the column of torsional suppression evaluation in Table 2, "〇" means that the reduction rate was 30% or more, and "◎" means that the reduction rate was 40% or more. As shown in Table 2, even when using steel type b, in Tests No. 5 and 6 where the left side value of the above formula (i) was 15.0 or more, the torsion in the food tray 100 could be sufficiently suppressed.

Industrial Applicability

[0046] According to the present invention, a shallow drawing molded product with suppressed torsion can be obtained.

Explanation of Reference Numerals

[0047] Molds 10, 10a Dies 12, 30 Punches 14 Blank holders 16 Support plates 18 Blanks 20 Food trays 100, 100a Vertical wall portions 102, 102a Bottoms 104 Flange portions 106

Claims

1. A method for manufacturing a shallow drawing molded product, comprising sandwiching a blank made of a duplex stainless steel between a hollow first support portion disposed on one side of the blank in the thickness direction of the blank and an annular blank holder disposed on the other side of the blank in the thickness direction, and pushing the inside of the portion of the blank sandwiched between the first support portion and the blank holder toward the one side by a punch, wherein: the load acting on the punch when pushing the blank increases as the forming height increases; at least at the forming bottom dead center, sandwiching the blank between a second support portion provided on the one side of the punch and the punch; when the moving distance of the punch from the start point of pressing the blank by the punch to the forming bottom dead center is defined as the maximum distance; The load P acting on the punch when the moving distance of the punch from the start point of pressing reaches 80% of the maximum distance ０．８ , the load P acting on the punch when the moving distance of the punch from the start point of pressing reaches 90% of the maximum distance ０．９ , and the maximum load P acting on the punch between the start point of pressing and the forming bottom dead center ｍａｘ satisfy the following formula (i), a method for manufacturing a shallow drawing molded product (P ｍａｘ - P ０．９ ) / (P ０．９ - P ０．８ ) ≥ 15.0 ··· (i)

2. At the forming bottom dead center, the maximum load P acts on the punch ｍａｘ The method for manufacturing a shallow drawing formed product according to claim 1, wherein this occurs

3. The method for manufacturing a shallow drawing molded product according to claim 1 or 2, wherein the first support portion and the second support portion are separate members.

4. The method for manufacturing a shallow drawing molded product according to claim 1 or 2, wherein the first support portion and the second support portion are integrally formed.

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