Shallow-drawn product and method for manufacturing same

Abstract

Provided is a shallow-drawn product made of austenitic-ferritic duplex stainless steel in which the occurrence of twisting during press forming is suppressed. A food tray 100 is a shallow-drawn product and comprises a rectangular cylindrical vertical wall portion 102 and a bottom portion 104 closing one end portion of the vertical wall portion 102. When viewed from the thickness direction of the bottom portion 104, the open end of the vertical wall portion 102 has a rectangular shape. The drawn depth of the food tray 100 is less than the length of each side of the open end of the vertical wall portion 102. The food tray 100 is made of austenitic-ferritic duplex stainless steel. The metallographic structure in a central portion of the bottom portion 104 has an austenite phase with an area ratio of 40% or less.

Description

Shallow-drawn product and its manufacturing method

[0001] The present invention relates to a shallow-drawn product and a method for producing the same.

[0002] Traditionally, food manufacturers have used aluminum, which is lightweight and has excellent press formability, as the material for food trays used in the preparation and manufacturing of a variety of foods. However, stainless steel, which has excellent corrosion resistance and strength, is also increasingly being used as a food tray material.

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

[0004] Japanese Patent Application Laid-Open No. 2002-97579

[0005] Food trays used in ovens require heat distortion resistance to prevent snagging, food dropping, and uneven cooking. In recent years, food trays with excellent wear resistance and magnetic properties are required to comply with HACCP (Hazard Analysis and Critical Control Point) standards. Specifically, wear resistance is required to prevent the material of the food tray from being scraped off when cooked food is scraped off the food tray, thereby preventing debris and chips from forming. Even if such debris does occur, magnetic properties are required to enable magnetic separators to detect metallic contaminants in the food. Meanwhile, lightweight food trays are also required to improve the working environment. Therefore, the inventors investigated the use of austenitic-ferritic duplex stainless steel (hereinafter simply referred to as duplex stainless steel) as a material for food trays. Duplex stainless steel has excellent heat distortion resistance, wear resistance, and magnetic properties, and can be made thinner for lighter weight. Duplex stainless steel has sufficient strength compared to aluminum. Therefore, when duplex stainless steel is used as the blank material, it is possible to make the blank thinner than when aluminum is used, which makes it possible to achieve the same weight reduction as when aluminum is used as the blank material.

[0006] However, as a result of studies by the present inventors, it has been found that when conventional duplex stainless steel, which generally has high strength and high yield strength, is used as a material, it may not be possible to manufacture a container of a desired shape. Specifically, it has been found that when a container is manufactured by shallow drawing a blank made of duplex stainless steel sheet, twisting may occur in the container (formed product) after forming. The metal structure of duplex stainless steel is composed of an austenite phase and a ferrite phase, and the ratio thereof is generally about 1:1. Thus, duplex stainless steel has high yield strength due to the combination of the hard austenite phase and the soft ferrite phase. It is believed that this high yield strength causes significant shape deformation after press forming, resulting in twisting.

[0007] Therefore, an object of the present invention is to provide a shallow-drawn product made of austenitic-ferritic duplex stainless steel in which the occurrence of twisting during press forming is suppressed, and a method for manufacturing the same.

[0008] The present inventors have conducted research into suppressing deformation that occurs in shallow-drawn products manufactured from blanks made of duplex stainless steel. Specifically, they attempted to manufacture a food tray as shown in Figures 1A and 1B by shallow-drawing a duplex stainless steel sheet. Note that Figure 1A is a plan view of the food tray, and Figure 1B is a front view of the food tray. Food tray 100 shown in Figures 1A and 1B has a cylindrical vertical wall portion 102, a bottom portion 104 that closes one end of vertical wall portion 102, and a flange portion 106 that extends outward from the other end of 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 reduce weight. However, as a result of the inventors' studies, it was found that conventional duplex stainless steel, which has high strength and high yield strength, does not have excellent press formability, and that reducing the thickness reduces rigidity, resulting in a large amount of deformation after forming. Specifically, as shown in Figure 2, it was found that a large twist may occur in a food tray 100. Note that Figure 2 shows a front view of a food tray 100.

[0010] Therefore, the present inventors conducted research to prevent the occurrence of twisting as described above. As a result, they found that the proportion of the austenite phase in the duplex stainless steel sheet has a significant effect on the twisting that occurs in food tray 100. Specifically, they found that by setting the proportion of the austenite phase in the duplex stainless steel sheet to 40% or less in terms of area ratio, and thereby lowering the yield strength compared to conventional duplex stainless steel sheets, it is possible to prevent shape deformation after press forming. This makes it possible to sufficiently prevent twisting from occurring in food tray 100.

[0011] The present invention was made based on the above findings.

[0012] (1) A shallow-drawn product according to one aspect of the present invention comprises a rectangular cylindrical vertical wall portion and a bottom portion closing one end of the vertical wall portion, the opening end of the vertical wall portion having a rectangular shape when viewed in the thickness direction of the bottom portion, and the drawing depth is smaller than the length of each side of the opening end, and is made of austenitic-ferritic duplex stainless steel, and the metal structure in the center of the bottom portion has an austenite phase of 40% or less in area ratio.

[0013] (2) In the shallow-drawn product, the metal structure in the center of the bottom portion may have an austenite phase with an area ratio of 25% or more.

[0014] (3) The shallow-drawn product may further include a flange portion provided at the other end of the vertical wall portion.

[0015] (4) The shallow-drawn product may be a food tray.

[0016] (5) A method for producing a drawn product in one aspect of the present invention is a method for producing a shallow drawn product by shallow drawing a blank made of austenitic-ferritic duplex stainless steel, the shallow drawn product having a rectangular tubular vertical wall portion and a bottom portion closing one end of the vertical wall portion, the opening end of the vertical wall portion having a rectangular shape when viewed in the thickness direction of the bottom portion, and the drawing depth being smaller than the length of each side of the opening end, wherein the metal structure of the blank has an austenite phase of 40% or less in area ratio.

[0017] (6) In the manufacturing method, the metal structure of the blank may have an austenite phase of 25% or more in area ratio.

[0018] According to the present invention, a shallow-drawn product made of austenitic-ferritic duplex stainless steel can be obtained in which the occurrence of twisting during press forming is suppressed. Furthermore, since the present invention allows the use of austenitic-ferritic duplex stainless steel, which has high hardness, excellent wear resistance, and magnetic properties, as the material for the shallow-drawn product, a shallow-drawn product suitable in particular for food trays can be obtained.

[0019] Fig. 1A is a plan view showing a food tray. Fig. 1B is a front view showing a food tray. Fig. 2 is a view showing a twisted food tray. Fig. 3 is a view showing a mold used in a method for manufacturing a food tray. Fig. 4A is a view showing a method for manufacturing a food tray. Fig. 4B is a view showing a method for manufacturing a food tray. Fig. 5 is a view showing a reference example of a shallow-draw molded product.

[0020] A shallow-drawn product and a method for manufacturing the same according to an embodiment of the present invention will be described below. Note that the following describes the case where a food tray 100 shown in Figures 1A and 1B is manufactured as the shallow-drawn product according to the present invention.

[0021] (Method of Manufacturing Food Tray) First, we will explain the method of manufacturing food tray 100. Fig. 3 is a schematic perspective view showing a mold for manufacturing food tray 100, and Figs. 4A and 4B are diagrams showing the method of manufacturing food tray 100.

[0022] As shown in Figure 3, the mold 10 used in the manufacturing method according to this embodiment includes a die 12, a punch 14, and a blank holder 16. The blank holder 16 is disposed above the die 12. A through hole penetrating in the vertical direction is formed in the center of the die 12 and the blank holder 16. The punch 14 is sized so that it can be inserted into the through hole of the die 12 and the blank holder 16.

[0023] 4A , when manufacturing food tray 100, first, blank 20 is placed on die 12, and the outer periphery of blank 20 (the portion that will become flange portion 106) is clamped between die 12 and blank holder 16. An austenitic-ferritic duplex stainless steel plate (hereinafter simply referred to as duplex stainless steel plate) is used as blank 20. The duplex stainless steel plate used as blank 20 will be described later.

[0024] Next, as shown in Figure 4B, punch 14 is lowered to shallow-draw blank 20. This forms food tray 100 shown in Figures 1A and 1B. Thereafter, punch 14 and blank holder 16 are raised, and food tray 100 is removed. In this manner, food tray 100 is manufactured. Note that part or all of flange portion 106 may be cut and removed after release from the mold.

[0025] In this embodiment, the vertical wall portion 102 of the food tray 100 has a rectangular cylindrical shape. The opening end of the vertical wall portion 102 has a rectangular shape with a pair of long sides and a pair of short sides in a plan view (viewed from the thickness direction of the bottom portion 104). In one example of a shallow-drawn product according to the present invention, the drawing depth (the height of the vertical wall portion in the thickness direction of the bottom portion: height h in the food tray 100 of FIG. 1B ) is smaller than the length of each side of the opening end (edge ​​of the hole) of the rectangular cylindrical vertical wall portion. In the food tray 100 according to this embodiment, the drawing depth (height h) is smaller than the length L of the short side of the opening end (edge ​​of the hole) of the vertical wall portion 102 in a plan view.

[0026] (Regarding the Blank and Shallow-Drawn Product) In this embodiment, a duplex stainless steel sheet is used as the blank material. For example, SUS821L1 can be used as the blank material. More specifically, in mass %, the blank material contains: 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, and Nb: 0 to 0.50%. A duplex stainless steel sheet containing 0%, Ti: 0-0.500%, Co: 0-0.30%, V: 0-0.50%, Sn: 0-0.500%, Ga: 0-0.050%, Zr: 0-0.50%, Ta: 0-0.050%, B: 0-0.0050%, W: 0-0.50%, Mg: 0-0.0100%, REM: 0-0.0100%, with the balance being Fe and impurities, can be used as the blank. Note that the shallow-drawn product according to the present invention is obtained by shallow-drawing a single duplex stainless steel sheet and differs from shallow-drawn products obtained by shallow-drawing a clad material composed of a duplex stainless steel sheet and another metal sheet. In other words, in the method for manufacturing a shallow-drawn product according to the present invention, clad materials are excluded from the blank material.

[0027] (Press formability) The duplex stainless steel sheet used as the blank in this embodiment has a metallographic structure in which the area fraction of the austenite phase is 40% or less, with the remainder being ferrite phase and precipitates. By setting the area fraction of the austenite phase in the metallographic structure to 40% or less, it is possible to reduce the yield strength compared to conventional duplex stainless steel sheets. This improves the press formability of the blank and suppresses shape deformation after press forming. As a result, it is possible to suppress the occurrence of twist in the shallow-drawn product (in this embodiment, the food tray 100). From the viewpoint of further suppressing the occurrence of twist in the shallow-drawn product, the area fraction of the austenite phase is preferably 35% or less, and more preferably 30% or less.

[0028] The area fraction of the austenite phase can be measured using an electron backscatter diffraction (EBSD) analyzer. Specifically, measurements are performed in a 100 μm square region at the center of the blank's thickness at measurement intervals (steps) of 1 μm. The FCC phase is then identified from the analysis results, and its area fraction is calculated as the area fraction of the austenite phase. In this embodiment, the above measurement is performed five times at different measurement locations, and the average value is used as the area fraction.

[0029] (Ductility) In order to sufficiently improve press formability, it is preferable that the duplex stainless steel sheet used as the blank has excellent ductility. Specifically, the ductility (total elongation) of the duplex stainless steel sheet used as the blank is preferably 25% or more, and more preferably 30% or more. The ductility (total elongation) of the duplex stainless steel sheet can be measured in accordance with JIS Z 2241:2022 by taking a JIS No. 13B test piece with the longitudinal direction being the rolling direction.

[0030] (Magnetic Properties) In order to enable a magnetic separator to detect broken pieces of food tray 100 if they become mixed in with food, the duplex stainless steel plate from which food tray 100 is made preferably has sufficient magnetic properties. In this embodiment, the area ratio of the austenite phase in the metal structure of the duplex stainless steel plate from which food tray 100 is made is 40% or less. In other words, the area ratio of the ferrite phase in the metal structure is 60% or more. This provides sufficient magnetic properties.

[0031] (Corrosion Resistance) In order to accommodate a variety of foods (foods containing salt) and frequent cleaning, it is preferable that food tray 100 have excellent corrosion resistance. From this perspective, it is preferable to use a duplex stainless steel plate having a PREN value (pitting corrosion index) of 20 or more, as shown in the following formula (a): PREN = Cr + 3.3Mo + 16N (a) where each element symbol in the formula represents the content (mass %) of each element contained in the steel plate.

[0032] (Robustness) In order to reduce life cycle costs, it is preferable that food tray 100 have excellent robustness (strength). Specifically, the 0.2% yield strength of the duplex stainless steel plate that serves as the material (blank) of food tray 100 is preferably 300 MPa or more, more preferably 350 MPa or more, and even more preferably 400 MPa or more. The 0.2% yield strength of the duplex stainless steel plate can be measured in accordance with JIS Z 2241:2022 by taking a JIS No. 13B test piece with the longitudinal direction as the rolling direction.

[0033] (Wear Resistance) In order to prevent the surface of food tray 100 from being scraped when scraping off food adhering to the surface of food tray 100, it is preferable that food tray 100 have excellent wear resistance. Specifically, the surface hardness of the duplex stainless steel plate from which food tray 100 is made is preferably 200 HV or more, more preferably 210 HV or more, in Vickers hardness. Vickers hardness can be measured in accordance with JIS Z 2244-1:2020, using a test force of 1.0 kgf. Note that the area ratio of the austenite phase in the metal structure of the blank is preferably 25% or more, so that the strength (0.2% proof stress) and wear resistance (Vickers hardness) of the duplex stainless steel plate from which food tray 100 is made satisfy the above requirements.

[0034] (Heat Distortion Resistance) Food tray 100 used in an oven preferably has excellent heat distortion resistance. Specifically, the thermal expansion coefficient of the duplex stainless steel plate that is the material for food tray 100 is 15 (×10 －６ / °C), and 14 (×10 －６ / °C) or less. The thermal expansion coefficient of the duplex stainless steel sheet can be measured in accordance with JIS Z 2285:2003.

[0035] A duplex stainless steel sheet having the above-described metal structure and properties can be manufactured, for example, as follows. Steel melted by known means (e.g., an electric furnace) is cast into a slab using a continuous casting machine. The resulting slab is heated and hot-rolled. The heating temperature of the slab is not particularly limited and may be set appropriately. After hot rolling is complete, the resulting hot-rolled sheet is annealed (hot-rolled annealing) to produce a hot-rolled annealed sheet. In hot-rolled annealing, the hot-rolled sheet is held at 1050 to 1150°C for 10 seconds or more, and then cooled to 400°C or less at an average cooling rate of 20°C / s or more. After hot-rolled annealing, pickling may be performed as needed.

[0036] The obtained hot-rolled annealed sheet is cold-rolled to obtain a cold-rolled sheet. The rolling reduction ratio of the cold rolling is 50 to 90%. After the cold rolling, finish annealing is performed. In the finish annealing, the cold-rolled sheet is held at 1040 to 1120°C for 5 seconds or more, then cooled to 850°C at an average cooling rate of 30°C / s or more, and then cooled to 400°C or less at an average cooling rate of 20°C / s or more. The hot-rolled annealed sheet may be cold-rolled multiple times (for example, twice). In this case, intermediate annealing may be performed between cold rolling. The conditions for the intermediate annealing may be the same as those for the above-mentioned hot-rolled annealing. When cold rolling is performed multiple times, the total rolling reduction ratio for the multiple cold rollings is 50 to 90%.

[0037] A shallow-drawn product manufactured from the above-described duplex stainless steel sheet (blank) has a metal structure similar to that of the above-described duplex stainless steel sheet (blank). In this embodiment, the metal structure at the center of the plate thickness of the center of the bottom of the shallow-drawn product (bottom 104 in the above-described food tray 100) satisfies the above requirements. In addition, it is preferable that the shallow-drawn product has the above-described properties (ductility, corrosion resistance, robustness (0.2% proof stress), wear resistance (Vickers hardness), and heat distortion resistance (coefficient of thermal expansion)) described for the duplex stainless steel sheet (blank). In this embodiment, it is preferable that the center of the bottom of the shallow-drawn product (bottom 104 in the above-described food tray 100) satisfies the requirements for each property described for the duplex stainless steel sheet (blank).

[0038] The thickness of the blank (shallow-drawn product) is, for example, 0.10 to 1.50 mm. From the viewpoints of weight reduction and strength, the thickness of the blank (shallow-drawn product) is preferably 0.40 to 0.60 mm. In this embodiment, duplex stainless steel, which has superior strength and yield strength to aluminum, is used as the blank material, so the blank thickness can be made smaller than when aluminum is used as the blank material. As a result, the shallow-drawn product according to this embodiment can be made as lightweight as a shallow-drawn product using aluminum as the blank material.

[0039] In the above-described food tray 100, the opening edge (edge ​​of the hole) of the vertical wall portion 102 has a rectangular shape in a plan view, but the vertical wall portion may be formed so that the opening edge (edge ​​of the hole) is square or approximately square. Furthermore, the shallow drawing method and the mold used therefor described above are merely examples, and the present invention can be practiced using various known shallow drawing methods and molds used therefor.

[0040] (Reference Example) Figure 5 is a diagram showing a shallow-drawn product according to a reference example. As shown in Figure 5, the shallow-drawn product 100a according to the reference example differs from the food tray 100 described above, which has a vertical wall portion 102 with a rectangular cross section, in that the shallow-drawn product 100a according to this reference example has a cylindrical vertical wall portion 102a. The shallow-drawn product 100a according to this reference example can also be manufactured from the same duplex stainless steel sheet as the blank for the food tray 100 described above. Furthermore, although not shown, a shallow-drawn product whose vertical wall portion has a triangular cross section or a polygonal cross section with pentagons or more can also be manufactured from the same duplex stainless steel sheet as the blank described above. Note that in these shallow-drawn products according to the reference examples, the drawing depth is set smaller than the diameter of the largest circle that fits inside the opening edge (hole edge) of the vertical wall portion in a plan view.

[0041] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0042] Stainless steel sheets Nos. 1 to 7 shown in Table 2 were obtained from steels having the chemical compositions shown in Table 1. The austenite area ratio of the duplex stainless steel sheets Nos. 1 to 6 was adjusted by appropriately changing the annealing temperature during steel sheet production between 850 and 1050°C. The 0.2% yield strength, surface hardness, ductility, thermal expansion coefficient, austenite area ratio, and magnetic properties of the manufactured stainless steel sheets were measured. The 0.2% yield strength and ductility were measured in accordance with JIS Z 2241:2022. The surface hardness (Vickers hardness) was measured in accordance with JIS Z 2244-1:2020, using a test force of 1.0 kgf. The thermal expansion coefficient was measured in accordance with JIS Z 2285:2003. The austenite area ratio was measured using the method described above. Magnetic properties were investigated by determining whether the sheets adhered to a magnet. The measurement results are shown in Table 2. In the column for magnetism in Table 2, ◯ indicates that the sample stuck to the magnet, and × indicates that the sample did not stick to the magnet.

[0043]

[0044] 1A and 1B was manufactured by the above-described method using the obtained stainless steel plate as a blank. The dimensions of the blank were width: 520 mm x length: 720 mm, and the dimensions (design values) of food tray 100 were width: 453 mm x length: 653 mm x drawing depth: 28 mm.

[0045] To confirm the effects of the present invention, the twist height of each food tray was measured. The twist height refers to the difference between the actual height of flange portion 106 when food tray 100 is placed on a flat surface and the design value. In this example, as shown in FIG. 2 , the twist height was determined by subtracting the design value (28 mm) of the drawing depth from the average value of height h1 of flange portion 106 on one side of food tray 100 in the long side direction and height h2 of flange portion 106 on the other side. As a result, the twist height of test No. 5, in which the area fraction of the austenite phase in the blank was 53.7%, was 43 mm, indicating significant twisting.

[0046] The reduction rates of the twist heights of Test Nos. 1 to 4, 6, and 7 relative to the twist height of Test No. 5 were evaluated. Specifically, the reduction rates (%) of the twist heights of Test Nos. 1 to 4, 6, and 7 relative to the twist height of Test No. 5 were calculated using the following formula (b): Reduction rate = 100 - ((twist height of Test Nos. 1 to 4, 6, or 7 / twist height of Test No. 5) x 100) (b)

[0047] In the column for twist suppression evaluation in Table 2, "◯" 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%.

[0048] As shown in Table 2, in Tests Nos. 1 to 3, in which a duplex stainless steel plate with an austenite phase area ratio of 40% or less was used as the blank, twisting in food tray 100 was sufficiently suppressed. Furthermore, the blanks in Tests Nos. 1 to 3 had sufficient strength (0.2% yield strength) compared to the blank in Test No. 7. These results demonstrate that, according to the present invention, sufficient strength can be ensured in food tray 100 even when the blank thickness is reduced.

[0049] Furthermore, the surface hardness of the blanks of Test Nos. 1 to 3 was sufficiently higher than the surface hardness of the blank of Test No. 7, and the thermal expansion coefficients of the blanks of Test Nos. 1 to 3 were sufficiently lower than the thermal expansion coefficient of the blank of Test No. 7. These results demonstrate that the present invention can ensure excellent wear resistance and thermal distortion resistance in food tray 100.

[0050] According to the present invention, a shallow-drawn product in which the occurrence of twisting is suppressed can be obtained.

[0051] REFERENCE SIGNS LIST 10 mold 12 die 14 punch 16 blank holder 20 blank 100 food tray 100a shallow-drawn product 102, 102a vertical wall portion 104 bottom portion 106 flange portion

Claims

1. A shallow-drawn product having a rectangular tubular vertical wall and a bottom that closes one end of the vertical wall, the opening end of the vertical wall having a rectangular shape when viewed in the thickness direction of the bottom, and a drawing depth that is smaller than the length of each side of the opening end, said shallow-drawn product being made of austenitic-ferritic duplex stainless steel, and the metal structure in the center of the bottom has an austenite phase of 40% or less by area.

2. The shallow drawn product according to claim 1, wherein the metal structure in the center of the bottom portion has an austenite phase of 25% or more by area.

3. A shallow drawn product according to claim 1 or 2, further comprising a flange portion provided at the other end of the vertical wall portion.

4. The shallow-drawn product according to claim 1 or 2, which is a food tray.

5. A method for producing a shallow-drawn product by shallow-drawing a blank made of austenitic-ferritic duplex stainless steel, the shallow-drawn product having a rectangular vertical wall portion and a bottom portion closing one end of the vertical wall portion, the opening end of the vertical wall portion having a rectangular shape when viewed in the thickness direction of the bottom portion, and the drawing depth being smaller than the length of each side of the opening end, wherein the metal structure of the blank has an austenite phase area ratio of 40% or less.

6. The method for producing a shallow drawn product according to claim 5, wherein the metal structure of the blank has an austenite phase of 25% or more by area.

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