Patent Publication Number: US-2021169273-A1

Title: Method of manufacturing cooking utensil and cooking utensil

Description:
DOMESTIC PRIORITY 
     This application is a divisional of U.S. patent application Ser. No. 16/222,201 entitled “Method of Manufacturing Cooking Utensil and Cooking Utensil,” filed Dec. 17, 2018 incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to a method of manufacturing a cooking utensil and a cooking utensil. Specifically, the present invention relates to surface treatment for a cooking surface of a cooking utensil such as a Peking pan made of cast iron. 
     Conventionally, to prevent burning and facilitate cooking, surface treatment is important for a cooking utensil having a cooking surface, such as a Peking pan or a frying pan. 
     To allow for comfortable cooking with an iron frying pan, the pan is seasoned before use by heating it empty and frying vegetable trimmings in it to form an oil film on the surface of the pan. However, the oil film tends to disappear easily and thus causes the pan to burn. Such burning necessitates repeated care such as empty heating, which is troublesome. 
     A known fluorine resin coated cooking utensil prevents burning and allows for comfortable cooking even without the above-described seasoning. However, although care such as empty heating is unnecessary for this type of cooking utensil, the surface of the cooking utensil has low oil wettability and hampers uniform oil spreading. The fluorine resin coated cooking utensil thus causes an oil pool below the lower surface of a cooking target, hampering desired uniform browning. Eventually, the coating on the cooking utensil may peel off or deteriorate. 
     To solve this problem, a pan disclosed in Japanese Laid-Open Patent Publication No. 2008-154649 has a pan body made of soft iron, a nitride layer, and an oxide layer. The nitride layer is formed on the surface of the pan body by gas soft nitriding. The oxide layer is formed on the surface of the nitride layer by oxidation. 
     Also, a frying pan made of iron disclosed in Japanese Laid-Open Patent Publication No. 2013-70751 has a frying pan body made of iron, a nitride layer, and an oxidation surface layer. The nitride layer is formed on the frying pan body by nitriding. The oxidation surface layer is formed on the nitride layer. Specifically, a slightly uneven surface is formed on the nitride layer through blasting. The uneven surface is then subjected to oxidation to form an oxidation coating. In this manner, the oxidation surface layer having the slightly uneven surface is formed on the nitride layer. 
     Through such surface nitriding, the cooking utensil is provided with a scratch resistant and high-strength surface that allows for easy formation of an oil film. 
     Also, although the surface formed by nitriding tends to rust quickly, the oxidation coating formed on this surface provides a rust-resistant frying pan. 
     However, on the other hand, complicated manufacturing procedures are required. 
     Accordingly, it is an objective of the present invention to provide a method of manufacturing a cooking utensil in which a surface treatment is performed on a cooking surface to provide high durability and rust resistance by a simple procedure, as well as a cooking utensil that is manufactured by this manufacturing method. 
     SUMMARY 
     In accordance with one aspect of the present disclosure, a method of manufacturing a cooking utensil is provided that includes: forming a cooking utensil made of a cast iron by casting; and forming a nitride layer by performing a nitriding on a cooking surface of the cooking utensil made of the cast iron. 
     In accordance with another aspect of the present disclosure, a cooking utensil is provided that includes a cooking utensil made of a cast iron that is formed by a casting and a nitride layer that is formed on a cooking surface of the cooking utensil made of the cast iron. 
     Other aspects and advantages of the present disclosure will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be understood by reference to the following description together with the accompanying drawings: 
         FIG. 1  is a perspective view showing casting molds used in a method of manufacturing a Peking pan according to an embodiment and a 
       Peking pan as a thin cast-iron product cast using the molds; 
         FIG. 2  is a schematic view showing the configuration of the surface of the cooking surface of the Peking pan of the embodiment. 
         FIG. 3  is a front view showing the closed molds and heating portions; 
         FIG. 4  is a flowchart representing the casting step using the molds of the embodiment; 
         FIG. 5  is a flowchart representing the nitriding step of the embodiment; 
         FIG. 6  is a plan view showing a manufacturing apparatus of the embodiment; 
         FIG. 7  is a cross-sectional view showing a melting furnace of the embodiment; 
         FIG. 8  is a perspective view illustrating a molten-metal pouring step of the embodiment; and 
         FIG. 9  is a side view showing the molds and a mold clamping device of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The inventor of the present invention has proposed a super-thin frying pan made of spheroidal graphite cast iron formed by atmosphere casting using molds (Japanese Laid-Open Patent Publication No. 2010-274327). The thickness of the frying pan is approximately 1.0 mm Specifically, the term “thin” herein refers to the thickness of a product that is approximately 5 mm or less. Particularly, a product having a thickness of 2 mm or less is referred to as a “super-thin” product. The optimal thickness of a frying pan or a Chinese pan is 0.3 to 1.5 mm, or, particularly, 1.0 to 1.4 mm. The molds enable mass production of these products with the optimal thickness, which has not been achieved in the industry. Specifically, as long as the product has such a thin section even partially in the cooking surface, the product may include a section having a greater thickness in other sections. For example, if a certain frying pan has a pan body with a thickness of 2 mm or less and a handle with a thickness of 10 to 50 mm, the frying pan is referred to as a “super-thin” frying pan, as described herein. 
     Compared to conventional casting using sand molds, casting using molds is extremely efficient due to the reusability of the molds. 
     Specifically, if a super-thin frying pan is manufactured using conventional porous molds such as sand molds, the cooking surface having the unevenness transferred from the molds must be smoothed using a sander after the product is removed from the molds. In other words, the oxide film (the “mill scale”) must be temporarily peeled off. 
     Therefore, conventionally, the product after polishing is subjected to rust-resistant treatment before shipment, as in the case of an iron frying pan. The user thus has to heat the product to a high temperature and fry vegetable trimmings in the product. This produces black rust afterward and the black rust forms an oxide film. 
     However, in the case of a product made of cast iron formed by mold atmosphere casting, the cavity surface of the mold is transferred to the surface of the product, thus providing extremely improved smoothness for the product. Therefore, in this case, the surface polishing after opening the molds is unnecessary. As a result, the product is completed with a mill scale (black rust: Fe 3 O 4 ) maintained on the cooking surface. In this state, provided is a preferable cooking utensil that requires no seasoning. 
     As has been described, the “mill scale” is maintained on the cooking surface of the cooking utensil made of cast iron, which is provided by casting. In this state, nitriding is performed on the cooking surface. As has been found by the inventor of the present invention, the cooking utensil with this cooking surface exerts optimal performance as a cooking utensil and, besides, simplifies the manufacturing procedure. 
     On the other hand, depending on the purpose of the product, the surface of the cooking surface may be machined through polishing, grinding, or cutting, thus exposing a cementite layer and, additionally, the base of the cast iron. Nitriding is then performed on the surface of the cementite layer or the cast-iron base. This optimizes the composition of the nitride that should be produced and the depth of the nitride layer. As a result, a preferable cooking utensil is obtained. 
     The nitriding herein refers to nitriding in a broad sense, including soft nitriding. The nitride layer includes a compound layer and a diffusion hardening layer. 
     It is preferable that the casting be performed with molds and be atmosphere casting. However, the casting may include casting using sand molds or porous molds, casting in vacuum or in an inert gas atmosphere, and casting involving air suction and pressing. 
     Based on the above-described conventional techniques that have been provided by the inventor of the present invention, the present disclosure is accomplished by the inventor of the present invention in order to improve the durability of the mill scale, thus obtaining a maintenance-free cooking utensil. 
     A method of manufacturing a cooking utensil and a cooking utensil according to an embodiment will now be described, citing an example of a super-thin Chinese pan with a handle (Peking pan) that is made of spheroidal graphite cast iron and has a thickness of 1.4 mm. 
     With reference to  FIG. 1 , a Peking pan P will hereafter be described. The Peking pan P is manufactured by the manufacturing method according to the present embodiment. 
     Material and Composition 
     The Peking pan P of the present embodiment is made of spheroidal graphite cast iron. The spheroidal graphite cast iron will now be described. Cast irons include white cast iron, gray cast iron, CV cast iron, and spheroidal graphite cast iron. Spheroidal graphite cast iron is specified basically according to “JIS G 5502”. Spheroidal graphite cast iron is a cast iron that contains spheroidal graphite in the structure. Compared to gray cast iron, spheroidal graphite cast iron has a continuous base and thus exhibits improved toughness. Spheroidal graphite cast iron is thus also referred to as ductile cast iron. To obtain spheroidal graphite cast iron, Fe containing high contents of molten C and Si is inoculated with Mg or Ce. Molten-metal material is thus formed and then inoculated with ferrosilicon (Fe—Si). The molten-metal material is then cooled slowly such that graphite deposits and is spheroidized during solidification. To obtain a certain type of spheroidal graphite cast iron of FCD800 to FCD400 separately from other types, the proportions of perlite and ferrite in the base structure are controlled. Typically, to provide a great amount of perlite, Cu, Sn, Mg, Cr, or Mn, each as a perlite stabilizing element, is added by an appropriate amount. In contrast, to obtain a ferrite base, the contents of the perlite stabilizing elements are minimized while a relatively high content of a graphitizing element, such as Si, is employed. Particularly, by restricting the content of Mn to 0.45% to 0.35% or less, elongation is varied as in FCD400-10 and FCD400-15. 
     In the present embodiment, toughness, which is resistance to break, is needed more than hardness as a characteristic of a super-thin Peking pan. The Peking pan P thus employs a base mainly containing ferrite. 
     Casting Molds 
     When casting spheroidal graphite cast iron, a rapid temperature drop hampers deposition of spheroidal graphite and forms cementite (Fe 3 C). The cast iron is thus whitened and becomes hard but fragile. Such solidification as cementite without deposition of spheroidal graphite is referred to as “chilling”. If the material is maintained in a molten state at a high temperature for a long time, the amount of molten Mg decreases due to oxidation or evaporation and thus graphite spheroidization does not occur (referred to as “fading”). 
     Since graphite deposits in spheroidal shapes during solidification, spheroidal graphite cast iron has a relatively great volume, which is a specific characteristic. Therefore, molds must be tolerant to complicated volume variation, as well as original thermal expansion. 
     Due to problems involved in temperature control and volume variation, as has been described, cooking utensils made of spheroidal graphite cast iron have conventionally been considered to be non-castable unless porous molds are employed. However, the inventor of the present invention has achieved the manufacturing of the cooking utensils made of spheroidal graphite cast iron using metal-mold casting. 
     As a result, a product immediately after casting is provided as a final product with a mill scale maintained, only with burr removal performed, basically without polishing. 
     Shape of Peking Pan P 
     The shape of the Peking pan P will now be described. As illustrated in  FIG. 1 , the Peking pan P is a typical Peking pan including a body  11  and a handle  12 . The handle  12  is formed integrally with the body  11 . The body  11  includes a substantially flat, disk-like inner bottom section  14  and a round-surface-like peripheral section  13 . The diameter of the inner bottom section  14  is 13 cm. The peripheral section  13  extends upright from the inner bottom section  14 . The handle  12  extends diagonally upward from the body  11 . The diameter of the body  11  is approximately 29 cm. The body  11  has uniform thickness. Specifically, the thickness of the inner bottom section  14  and the thickness of the peripheral section  13  are both 1.4 mm and exceptionally small for a Peking pan made of cast iron. 
     The handle  12  has a base portion and a distal end section on the opposite side to the base portion. The base portion is continuous with the body  11 . The handle  12  has a substantially circular cross-sectional shape in the range between the base portion and the distal end section. That is, the handle  12  is shaped like a circular tube in the range between the base portion and the distal end section. The distal end section of the handle  12  is open. The distal end section of the handle  12  is shaped to have an upper section that is slightly projected with respect to a lower section. A stopper hole  16  for storing is formed in the upper section of the distal end section of the handle  12 . 
     Surface Structure of Peking Pan P 
       FIG. 2  is a schematic view representing the surface structure of the Peking pan P and does not represent the accurate thickness of each of the layers. Referring to the drawing, an oxide film layer L 2  (an oxide film) is arranged on a base portion L 1  made of spheroidal graphite cast iron. The oxide film layer L 2  is a mill scale (black rust, triiron tetroxide, Fe 3 O 4 ). The mill scale is formed by the atmospheric oxygen bonding with the base portion L 1  at a high temperature during casting. In conventional cases using sand molds, the mill scale must be grinded with a sander or the like to be smoothed. In cases of mold casting, the mill scale is used as a cooking surface. 
     In the present embodiment, the surface of the oxide film layer L 2  is subjected also to nitriding. This forms a nitride layer NL on the oxide film layer L 2 . In the embodiment, the nitride layer NL has a diffusion hardening layer L 3  and a compound layer L 4 . The diffusion hardening layer L 3  is located on the oxide film layer L 2 . The compound layer L 4  is located on the diffusion hardening layer L 3 . The compound layer L 4  configures the outermost surface S of the Peking pan P. The compound layer L 4  is formed of ε-Fe 2-3 N, γ′-Fe 4 N, and Si 3 N 4 . In the diffusion hardening layer L 3 , Si 3 N 4  deposits by a slight amount. While the content of Si is small in iron or iron steel, the content of Si is great in spheroidal graphite cast iron. Therefore, Si 3 N 4  is generated through nitriding. Particularly, Si 3 N 4  is also generated in the diffusion hardening layer L 3 , which is spaced from the surface. This further improves the durability of the Peking pan P. 
     These nitride compounds all exhibit a high Vickers hardness, thus improving the durability of the Peking pan P. 
     Also, since the surface of the Peking pan P has a porous structure, the surface readily allows for formation of an oil film as a cooking surface. 
     In addition, the base portion L 1 , the oxide film layer L 2 , the diffusion hardening layer L 3 , and the compound layer L 4  are integrated with one another and do not peel off from one another. 
     The thickness of the oxide film layer L 2  (the oxide film) is several tens of micrometers. The oxide film layer L 2  thus blocks oxygen and effectively suppresses generation of red rust (ferric oxide: Fe 2 O 3 ) in the base portion L 1 . Specifically, the thickness of the oxide film may be several micrometers to several millimeters. 
     Manufacturing Apparatus 
     Next, a manufacturing apparatus  1  for the Peking pan P of the present embodiment will be described. 
     Configuration of Mold 
     As shown in  FIGS. 1 and 6 , the manufacturing apparatus  1  includes multiple mold sets  2 . Each of the mold sets  2  is configured for atmosphere casting (gravity casting). Each mold set  2  thus basically does not need equipment for pressing or drawing molten metal. The mold sets  2  each include a fixed mold  3  and a movable mold  4 . The fixed mold  3  and the movable mold  4  are separate from each other by means of vertical separating surfaces  33 ,  43 . 
     With reference to  FIG. 3 , when the separating surface  33  of the fixed mold  3  and the separating surface  43  of the movable mold  4  are held in contact with each other, the inner surfaces of cavity surfaces  32 ,  42  form a cavity  25  shaped in correspondence with the Peking pan P (see  FIG. 1 ). 
     As shown in  FIGS. 3 and 8 , at the time of casting, funnel-shaped molten-metal pouring ports  38 ,  48  are attached to upper surfaces at positions corresponding to molten-metal ports  34 ,  44 . This configuration facilitates pouring molten metal M into the molten-metal ports  34 ,  44 . 
     Referring to  FIG. 6 , the movable mold  4  and the fixed mold  3  are in an open state. In this state, the fixed mold  3  is moved to a predetermined position, as illustrated in  FIG. 8 , and then fixed. Subsequently, for the pouring of molten metal, the movable mold  4  is brought into contact with the fixed mold  3 . The molds  3 ,  4  are then clamped together and the molten metal is poured into the molds  3 ,  4 . With reference to  FIGS. 6 and 9 , the manufacturing apparatus  1  includes multiple metal-mold support devices  60 . Each of the metal-mold support devices  60  supports the corresponding one of the mold sets  2  such that, when removing a product from the molds, the movable mold  4  is separated from the fixed mold  3  to open the mold set  2  and the separating surface  43  of the movable mold  4  is reversed downward to remove the complete Peking pan P downward. This will be later described in detail. 
     Heating Device  20  for Molds 
     As shown in  FIG. 3 , two heating devices  20  are arranged close to and on opposite sides of each of the mold sets  2 . 
     Each of the heating devices  20  includes a gas burner set. Each mold set  2  has body cavity surfaces  32   a ,  42   a . A fixed-metal-mold heating portion  32   e  and a movable-metal-mold heating portion  42   c  are formed on the outer surface of the body cavity surface  32   a  and the outer surface of the body cavity surface  42   a , respectively. Hereinafter, the fixed-metal-mold heating portion  32   e  and the movable-metal-mold heating portion  42   c  will be referred to collectively as a “heating portion  24 ”. The gas burner sets of the heating devices  20  heat, with direct flames, the corresponding surfaces of the heating portion  24  in the directions represented by the arrows in  FIG. 3 . 
     Each of the gas burner sets includes main burners  20   a  and a pilot burner  20   b . Each of the main burners  20   a  includes multiple gas nozzles. A heating control device  23  serves as a control section to selectively open and close a non-illustrated valve of each of the gas nozzles. Each gas nozzle thus ejects combustible gas. The combustible gas is supplied by a gas supply device  22 . Each of the pilot burners  20   b  constantly provides a flame for igniting the corresponding ones of the main burners  20   a  toward the heating portion  24  of the mold set  2 . When the main burners  20   a  are deactivated, the pilot burners  20   b  heat the mold set  2  to such an extent that the temperature of the mold set  2  gradually decreases. In this manner, a rapid temperature drop of the mold set  2  is made unlikely to occur. By maintaining a certain level of high temperature as has been described, graphite spheroidization is promoted and chilling is limited. Also, a mill scale having a greater thickness is generated. 
     Temperature Control on Molds 
     Radiation thermometers  21  are arranged, each as a temperature measuring section, in the vicinity of the mold set  2  to measure the surface temperatures of the corresponding outer surfaces of the mold set  2 . The radiation thermometers  21  remotely measure the surface temperatures of the heating portion  24  of the mold set  2  in correspondence with infrared rays emitted by the mold set  2 . 
     The manufacturing apparatus  1  includes the heating control device  23 . The heating control device  23  controls the heating devices  20  depending on the surface temperatures of the mold set  2  measured by the radiation thermometers  21 . The heating control device  23  includes a publicly known computer and, according to a program memorized in advance, preheats the mold set  2  to a predetermined temperature at a predetermined timing and then retains the heat. 
     As has been described, each mold set  2  of the present embodiment includes the heating portion  24 . The fixed mold  3  and the movable mold  4  each has a uniform thickness D 1 , as measured in the direction of the thickness of the cavity surface  32 ,  42  of the mold  3 ,  4 . Specifically, the heating portion  24  having the thickness D 1  is formed in the sections of the cavity surfaces  32 ,  42  that correspond to the thin plate-like body  11  of the Peking pan P, for example. The thickness D 1  is uniformly 20 mm Other sections of frames  31 ,  41  have thicknesses of approximately 30 mm Therefore, if the fixed-metal-mold heating portion  32   e  and the movable-metal-mold heating portion  42   c  are uniformly heated, the heat is transmitted to the interior of the mold set  2  uniformly and rapidly from the surfaces of the mold set  2 . The heat simultaneously reaches the inner walls of the cavity surfaces  32 ,  42 , thus raising the temperatures of the cavity surfaces  32 ,  42  uniformly. This makes it unnecessary to perform actual temperature measurement in the interior of the cavity  25 . That is, the temperatures of the cavity surfaces  32 ,  42  are controlled simply by measuring the surface temperatures of the fixed-metal-mold heating portion  32   e  and the movable-metal-mold heating portion  42   c , which correspond to the heating portion  24 . 
     Material of Molds 
     Each mold set  2  is made of spheroidal graphite cast iron and thus provides effective heat conduction. Therefore, the temperatures in the interiors of the cavity surfaces  32 ,  42  are quickly raised by heating the fixed-metal-mold heating portion  32   e  and the movable-metal-mold heating portion  42   c . Also, spheroidal graphite cast iron has a great heat capacity, thus providing a stable heat environment for the molten metal M in the cavity  25 . Further, with respect to heating or cooling, spheroidal graphite cast iron thermally expands or contracts to an equal extent with the casting. As a result, the stress acting on the casting at the time of cooling is extremely small. In addition, spheroidal graphite cast iron has a greater tensile strength than not only sand molds but also other metal castings (see JIS G5502). Therefore, even if the thickness of the mold set  2  is small, the mold set  2  is unlikely to be damaged by a great thermal change. Also, since the casting target is a super-thin product, the fact that the absolute value of the dimension change caused by strain is small is advantageous in enabling the mold casting. 
     Mold Support Device 
     As illustrated in  FIG. 6 , each of the mold support devices  60  supports the associated one of the mold sets  2 . Each mold support device  60  is configured to be movable on two rails  61  such that the molten-metal pouring port  48  of the corresponding mold set  2 , which the mold support device  60  supports, can be moved to a molten-metal pouring point  56  in the vicinity of a melting furnace  50 . 
     With reference to  FIG. 9 , the mold support device  60  includes a base  62 . The base  62  has an axle  64 . The axle  64  includes wheels  63  on opposite ends of the axle  64  to be movable on the rails  61 . The axle  64  is driven by a motor  65 . 
     A fixed-metal-mold support portion  66  supports the fixed mold  3  and is arranged on an end of the base  62 . The corresponding one of the heating devices  20  is arranged on the backside of the fixed mold  3 . 
     A movable-metal-mold support portion  67  supports the movable mold  4  and is arranged on the opposite end of the base  62 . The corresponding one of the heating devices  20  is arranged on the backside of the movable mold  4 . 
     The movable-metal-mold support portion  67  supports the movable mold  4  through a movement mechanism  68 . The movable mold  4  is moved to be selectively clamped to and separated from the fixed mold  3 . The movable-metal-mold support portion  67  includes a pivot mechanism  69  having a pivot shaft  69   a . The pivot shaft  69   a  is arranged horizontally. The movable-metal-mold support portion  67  is capable of pivoting the movable mold  4  by 90 degrees such that the separating surface  43  faces vertically downward. When the separating surface  43  faces vertically downward, a belt conveyor  70  is located vertically downward from the movable mold  4 . After the Peking pan P is removed from the molds and released downward, the belt conveyor  70  receives the Peking pan P and transports the Peking pan P to the site at which the subsequent step takes place. 
     Melting Furnace 
     In the present embodiment, the material is melted using a super-small-sized melting furnace  50 , as shown in  FIG. 7 . The melting furnace  50  is a high-frequency induction furnace and includes high-frequency-induction heating coils  51  and levitation coils  52 . The melting furnace  50  lifts the material by means of the levitation coils  52  and heats and melts the material using eddy currents from the high-frequency-induction heating coils  51 . Cooling pipes  58  are arranged in the interiors of corresponding side walls  50   a  of the melting furnace  50 . Coolant circulates in the cooling pipes  58 , thus cooling the side walls  50   a.    
     Referring to  FIG. 8 , the melting furnace  50  is securely attached to a furnace base  59 . The furnace base  59  tilts about a pivot shaft  59   a  to pour the molten metal M floating in the melting furnace  50  into the molten-metal pouring port  48  of the corresponding one of the mold sets  2  that is located at the molten-metal pouring point  56  ( FIG. 6 ). 
     As shown in  FIG. 6 , a power source device  53 , a cooling device  54 , and a melting-furnace control device  55  are arranged in the vicinity of the melting furnace  50 . The power source device  53  supplies electric power to the melting furnace. 
     The cooling device  54  circulates refrigerant in the cooling pipes  58  ( FIG. 7 ). The melting-furnace control device  55  heats the melting furnace  50  and controls the tilting of the furnace base  59  for pouring the molten metal. 
     Casting Method Using Molds 
     With reference to the flowchart shown in  FIG. 4 , a method of casting the Peking pan P as a super-thin cast-iron product using the mold set  2 , which has the above-described configuration, will now be described. 
     Outline of Manufacturing Method 
     A method of manufacturing the Peking pan P is performed as illustrated in  FIG. 4 , using the manufacturing apparatus  1 , as will be described. First, using the fixed mold  3  and the movable mold  4  of the mold set  2  for the Peking pan P, a mold clamping step (S 1 ) is carried out to clamp the molds  3 ,  4  with the separating surfaces  33 ,  43  located between the molds  3 ,  4 . This step is followed by a metal-mold preheating step (S 2 ) in which the heating control device  23  measures the surface temperatures of the heating portion  24  of the mold set  2  by means of the radiation thermometers  21  and the heating devices  20  heat the mold set  2  to a target temperature that is set in advance. Subsequently, in a molten-metal-material producing step (S 3 ), the molten-metal material is melted in advance by the melting furnace  50  (see  FIGS. 6 and 7 ). The molten-metal material, which has been melted, is then poured into the heated mold set  2  (see  FIG. 8 ) in a metal-mold molten-metal pouring step (S 4 ). Next, a metal-mold heat-retaining step (S 5 ) is performed to hamper the rapid cooling of the mold set  2 , into which the molten metal has been poured. This step is followed by a casting solidifying step (S 6 ) to harden the molten-metal material, which has been poured, while hampering a rapid temperature change in the mold set  2 . After the casting is hardened, a metal-mold opening step (S 7 ) is carried out to separate the fixed mold  3  and the movable mold  4  from each other at the separating surfaces  33 ,  43 . Then, a mold removal step (S 8 ) is performed to remove the Peking pan P, which has been cast (see  FIG. 9 ). This step is followed by a shaping step (S 9 ) in which the casting-burr removal from the obtained Peking pan P is mainly performed. After such shaping, the Peking pan P is directly subjected to a nitriding step (S 10 ), and then to an after-treatment step (S 11 ) in which the Peking pan P is provided as a complete product. 
     Hereinafter, the respective steps will be described in detail. 
     Mold Clamping Step (S 1 ) 
     As shown in  FIG. 9 , the movable mold  4  is set in the mold support device  60  in advance and supported by the movement mechanism  68  such that the movable mold  4  selectively contacts and separates from the fixed mold  3 . In  FIG. 9 , the movable mold  4  is in an open state. In this state, if necessary, a mold wash is re-applied onto the walls of the cavity in the mold set  2  and dried at 120° c. to 200° c. 
     Then, in the state shown in  FIG. 9 , the movable mold  4  is moved by the movement mechanism  68  in the upper left direction as viewed in the drawing such that the separating surfaces  33 ,  43  come into contact with each other. Subsequently, the separating surface  43  is pressed against the separating surface  33  by means of a non-illustrated hydraulic mechanism arranged in the movement mechanism  68 . In this manner, the mold clamping step (S 1 ) is performed to clamp the mold set  2 , as illustrated in  FIG. 3 . 
     Metal-Mold Preheating Step (S 2 ) 
     Metal-Mold Preheating 
     Next, the metal-mold preheating step (S 2 ) is carried out to heat the mold set  2 , which has been clamped, to a target temperature that has been set in advance. As shown in  FIG. 3 , the two heating devices  20  are arranged in the vicinity of the clamped mold set  2  on the opposite sides of the mold set  2 . In the metal-mold preheating step (S 2 ), the heating devices  20  heat the mold set  2  as a whole in the directions represented by the arrows in  FIG. 3 . It may be desirable, at this stage, to intensely heat a downstream side on which a temperature drop tends to occur in the molten metal M or to focally heat a narrow section in the cavity  25  in which flowability tends to lower. In the present embodiment, however, a uniform target temperature is employed. 
     Setting Preheating Temperature 
     The first goal of the preheating of the mold set  2  is to hamper cementite formation caused by rapid cooling, which hampers graphite spheroidization. 
     The second goal is to provide improved molten-metal run in the molds. The molds are made of spheroidal graphite cast iron and are super-thin, with the width of the cavity varying from 0.5 to 1.5 mm. Therefore, even without the occurrence of chilling, a lowered flowability directly causes chipping, sink marks, or pinholes. Therefore, to maintain high flowability during molten-metal run, it is necessary to stop the heat absorption from the molten metal by the molds. In this regard, the higher the preheating temperature of the molds, the more effective such preheating. 
     The third goal is to ensure that the preheating temperature is set such that, even though the preheating of the mold set  2  deforms the molds, the dimension accuracy of the casting is maintained within the acceptable range. 
     The fourth goal is to maintain the strength and durability of the mold set  2  without being lowered by a change in the structure configurations of the mold set  2  caused by the preheating of the mold set  2 . 
     The fifth goal is to ensure that the preheating temperature is set such that a sufficient mill scale, which is important in the present embodiment, is formed. 
     The preheating temperature is set to achieve the above-described goals. 
     To achieve the goals, it is effective to maintain the temperature in the interior of the mold set  2  at a temperature higher than or equal to the A1 transformation point. The A1 transformation point is a temperature at which, for example, when the molten metal M is poured into and thus contacts the mold set  2 , cementite formation does not occur and a perlite structure or a ferrite structure is formed. Although the A1 transformation point varies depending on the elements, the A1 transformation point is approximately 727° C. typically in spheroidal graphite cast iron. 
     However, it is difficult to accurately measure the temperature in the interior of the mold set  2 . The temperature in the interior of the cavity is thus estimated from the surface temperature of the mold set  2 . In the present embodiment, the heating portion  24  employed by the mold set  2  has a uniform small thickness. This enables temperature control from the exterior. That is, the method of the present embodiment employs a temperature that falls in an annealing temperature range, thus providing graphite spheroidization. This limits the deterioration of the molds caused by heating. 
     Molten-Metal-Material Producing Step (S 3 ) 
     While the mold clamping step S 1  and the metal-mold preheating step S 2  are performed, the molten-metal material (the molten metal M) of spheroidal graphite cast iron is prepared for the metal-mold molten-metal pouring step (S 4 ). 
     The molten-metal material may be produced in any suitable manner. In the present embodiment, the molten metal M is produced using the melting furnace  50 , as shown in  FIG. 7 . 
     According to conventional methods, from the view point of molten-metal run, the molten metal M is heated preferably to a temperature higher than or equal to 1400° C., or, more preferably, to a temperature of 1500° C. to 1600° C., to improve the flowability of the molten metal M. In the present embodiment, from the view point of preventing fading and chilling, the mold set  2  is heated to such a temperature that the temperature of the molten metal M reaches 1280° C. In this manner, the super-thin Peking pan P is manufactured. 
     Metal-Mold Molten-Metal Pouring Step (S 4 ) 
     After producing the molten-metal material in the above-described manner (S 3 ), the mold set  2 , which has been clamped (S 1 ) and preheated (S 2 ), is moved to the molten-metal pouring point  56 , as shown in  FIG. 1 . The melting furnace  50  is then tilted about the pivot shaft  59   a  by means of the furnace base  59 . Subsequently, the molten-metal material (the molten metal M) is poured from the melting furnace  50  directly into the molten-metal port cup that is formed by the molten-metal pouring ports  38 ,  48 . 
     Metal-Mold Heat Retaining Step (S 5 ) and Casting Solidifying Step (S 6 ) 
     Subsequently, the mold set  2 , which has been preheated, lowers the temperature of the molten metal M slowly, thus causing graphite deposition. As has been described, the thickness of each mold, into which the molten metal has been poured, is small. The molds thus release heat effectively compared to not only conventional sand molds or shell molds but also thick block-like molds. Therefore, even by natural cooling, solidification is completed substantially within one minute. At the time of such solidification, the preheated mold set  2  and the Peking pan P exhibit equal expansion rates and thus thermally contract in similar manners as the temperature lowers. The mold set  2  is thus unlikely to strain. 
     After pouring the molten metal, the casting solidifying step (S 6 ) is performed through natural cooling. That is, as the temperature of the molten metal M lowers in the mold set  2 , the molten metal M solidifies. In the present embodiment, in addition to this, the mold set  2  is heated as needed by the main burners  20   a  and the pilot burners  20   b  of the heating devices  20  following the molten metal pouring, regardless of before or after the casting solidifying step (S 6 ). In this manner, the metal-mold heat-retaining step (S 5 ) is carried out to retain the heat of the mold set  2  to a target temperature that is set in advance. 
     In the metal-mold heat retaining step, the lowering of the temperature of the molten metal M is controlled to limit the chilling caused by a rapid temperature drop. Therefore, the target temperature is set with this point of view taken into consideration. 
     The target temperature is set to a temperature unequal to, for example, lower than, the preheating temperature. Specifically, the target temperature is set to provide such a condition that the molten metal M is cooled in a manner providing effective graphite spheroidization. 
     Also, the target temperature may be lowered gradually as the time elapses following the molten metal pouring to provide such a temperature condition that provides effective graphite spheroidization. 
     This condition does not necessarily have to be provided through precise temperature control but may be provided by, for example, adjusting the heating time. 
     In the present embodiment, the preheating temperature is 900° C. and the heat is retained at 500° C. after the molten metal is poured. The heat retaining in the present embodiment is carried out such that the temperature reaches a value lower than or equal to the A1 transformation point, at which the casting completely solidifies, in one minute or longer and two minutes or shorter. The heat retaining may be ended at this time or continued until a subsequent cycle of molten metal pouring. Continuing the heat retaining in this manner facilitates formation of the mill scale on the surface of the casting. 
     Metal-Mold Opening Step (S 7 ) 
     When the temperature of the casting reaches the A1 transformation point, the structure is fixed and hardened. Afterward, the metal-mold opening step (S 7 ) is carried out by a non-illustrated mold clamping device to open the mold set  2 . 
     Mold Removal Step (S 8 ) 
     Subsequently, the mold removal step (S 8 ) is performed. That is, the movable mold  4  is separated from the fixed mold  3  and pivoted with the separating surface  43  facing downward such that the Peking pan P is removed from the molds. If necessary, a knock pin may be arranged in the molds when the Peking pan P is removed from the molds. 
     Shaping Step (S 9 ) 
     Next, the shaping step (S 9 ) is performed. In the present embodiment, the casting-burr removal step is performed after the casting. That is, in a direction parallel to the movement direction of the movable mold  4 , the thin cast-iron product is subjected to fusion cutting using a laser beam or the casting burrs formed on the peripheral edge of the thin cast-iron product are struck or grinded using a hammer or a sander. Since the casting time for the Peking pan P in the present embodiment is short, the obtained mill scale (oxide film) is thin. Therefore, the interior of the body  11 , as the cooking surface of the Peking pan P, is basically not subjected to any treatment or machining such that the mill scale is maintained in a non-treated or non-machined state. 
     Nitriding Step (S 10 ) 
     After completing the burr removal in the shaping step, the nitriding step (S 10 ) is carried out. The nitriding step (S 10 ) will be later described in detail. 
     After-Treatment Step (S 11 ) 
     In the after-treatment step, the product may be washed and then subjected to decorative finishing by painting or using fat or oil or wax, as needed. 
     Nitriding S 101  to S 107   
     The nitriding step (S 10 ) will hereafter be described in detail with reference to  FIG. 5 . 
     In the present disclosure, the term “nitriding” refers to nitriding in a broad sense, unless otherwise specified. That is, the “nitriding” herein represents a generic concept of nitriding and includes not only the nitriding mainly employing nitrogen but also soft nitriding mainly employing nitrogen and carbon, oxynitriding mainly employing nitrogen and oxygen, and sulfonitriding mainly employing nitrogen and sulfur. 
     The methods of nitriding herein include heating in molten salt, heating in a gas atmosphere, and heating in gas plasma. 
     In the present embodiment, gas soft nitriding is performed in which heating is carried out in a gas atmosphere mainly containing nitrogen and carbon. 
     In the gas soft nitriding, a nitriding target is set in a furnace (not shown) and a gas mixture of NH 3 , N 2 , and CO 2  is introduced into the furnace (S 101 ). 
     Subsequently, heating is started on the gas mixture in this atmosphere to 530° C. to 600° C. (in the present embodiment, 550° C.) (S 102 ), thus nitriding the target using the N element obtained through decomposition of NH 3  and carburizing the target using the C element obtained through decomposition of CO 2 . This forms, on the surface of the product, the nitride layer NL (the compound layer L 4  and the diffusion hardening layer L 3 ) formed by a nitride with a thickness of approximately 5 to 100 μm. To accomplish such formation of the nitride layer NL, gas-flow-amount adjustment is performed (S 103 ). Specifically, it is desirable that the thickness of the nitride layer be 5 μm or greater, or, particularly, 40 μm or greater. In the present embodiment, the thickness of the nitride layer NL is 40 μm or greater. It is also desirable that the nitride layer contain Si 3 N 4 . 
     The heating is stopped after a predetermined time (in the present embodiment, two hours) elapses and the nitriding is completed (S 104 ). 
     When the temperature drops to a predetermined temperature (in the present embodiment, 150° C.) or lower, the furnace is opened to remove the Peking pan P (S 105 ). 
     All or some of the completed Peking pans P are used as samples and subjected to a hardness test (a HV hardness test) (S 106 ). The hardness test is performed according to “JIS Z 2244—Vickers Hardness Test Method”. The present embodiment employs a micro Vickers test in which the Vickers hardness is measured using a test force of approximately 1 kgf (approximately 9.8N) or less. That is, the test is conducted as a non-destructive inspection, without substantially damaging the products. In the present embodiment, a micro Vickers test is carried out using the automatic indentation measurement and automatic XY stage system ARS900 from FUTURE-TECH Corporation, under a load of 0.01 kgf (approximately 0.098N). 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Depth (μm) 
                 Hardness (HV) 
                 Result 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 0 
                 1081 
                 Passed 
               
               
                   
                 10 
                 701 
                 Passed 
               
               
                   
                 20 
                 561 
                 Passed 
               
               
                   
                 40 
                 479 
                 Passed 
               
               
                   
                 60 
                 639 
                 Passed 
               
               
                   
                 80 
                 611 
                 Passed 
               
               
                   
                 100 
                 345 
                 Failed 
               
               
                   
                 120 
                 345 
                 Failed 
               
               
                   
                   
               
            
           
         
       
     
     According to the test, whether the nitriding step (S 101  to S 105 ) has been successful is determined. In the present embodiment, any product is considered to have passed the test if the product exhibits, for example, 400 HV0.01 or greater at the depth of 40 μm. However, depending on the target product, the criteria for passing the test can be varied. Specifically, it is desirable that the Vickers hardness of the surface of the nitride layer be 350 HV0.01 or greater, or, particularly, 500 HV0.01 or greater. 
     Those of the products that have passed the test (S 107 : YES) are subjected to a polishing step (S 108 ). The others, which have not shown sufficient hardness, are determined to have failed the test (S 107 : NO) and subjected to additional nitriding (S 102  to S 105 ). 
     Specifically, correlation is found between the effect of nitriding and the time of nitriding. Therefore, by prolonging the nitriding time of two hours in the present embodiment to approximately eight to ten hours, the nitride layer can be formed to a further depth. In conventional iron frying pans, the nitride layer cannot be formed to a sufficient depth even by prolonging the treatment time. However, in the present embodiment, cast iron is employed. As a result, the nitride layer NL, or, particularly, the diffusion hardening layer L 3  formed from Si 3 N 4 , is formed to a sufficient depth. 
     Also, soft nitriding is suitable for spheroidal graphite cast iron. This provides nitriding to a greater depth in a shorter treatment time than nitriding on conventional iron products. 
     In the polishing step (S 108 ), the Peking pan P may be polished using a sander or the outermost surface layer may be removed by lapping such that the Peking pan P is provided as a complete product. 
     Operation and Advantages of Embodiment 
     The Peking pan P of the above-described embodiment operates in the following manner and has the following advantages. 
     (1) The present embodiment provides a high Vickers hardness compared to a case of cast iron without nitriding. As shown in Table 1, for the depths from 100 to 120 μm, the Vickers hardness is approximately 345 HV0.01. The value corresponds to the Vickers hardness of the spheroidal graphite cast iron of the present embodiment. The hardness of the outermost surface (the depth of 0 μm) obtained through the treatment of the present embodiment is 1081 HV0.01, which is significantly high compared to the spheroidal graphite cast iron. The hardness indicates an extremely high scratch resistance. As a result, a turner or knife that is used for cooking is unlikely to form a scratch on the Peking pan P. 
     (2) The Vickers hardness becomes gradually lower toward a greater depth until it reaches 479 HV0.01 at the depth of 40 μm. However, at the depths of 60 to 80 μm, the Vickers hardness recovers to as high as 639 to 611 HV0.01. The super-thin Peking pan P made of spheroidal graphite cast iron of the present embodiment exhibits toughness not only on the surface but also in the interior. The Peking pan P is thus hardly deformable despite its super-thinness. The Peking pan P also shows improved anti-fatigue performance. 
     (3) The present embodiment improves rust resistance. That is, a rust-resistant mill scale (oxide film) is formed below the nitride layer NL, thus hampering rust formation, compared to the conventional techniques having nitride layers only (Japanese Laid-Open Patent Publications No. 2008-154649 and No. 2013-70751). As a result, it is unnecessary to arrange an oxidation layer additionally on the nitride layer, unlike the conventional techniques. This simplifies the manufacturing procedure. 
     (4) The present embodiment allows for formation of an oil film. Even when spheroidal graphite cast iron is molded in metal mods, thermal contraction forms a slightly uneven skin in the cast iron. Also, the compound layer L 4  of the present embodiment is porous and thus readily allows for formation of an oil film when used for cooking. This hampers, in cooking, not only burning but also oil repelling or oil gathering, unlike fluorine resin coated cooking utensils. As a result, uniform food browning is brought about. 
     (5) The present embodiment maintains a desirable outer appearance. The scratch resistance not only maintains the desirable outer appearance but also hampers surface contamination. 
     (6) The present embodiment improves heat resistance. Since the nitride layer is highly heat-resistant, the quality of the surface is unlikely to change even after being heated empty. Also, in case burning occurs, the burnt object adhering to the surface can be burned off, by heating the surface empty. 
     (7) The present embodiment improves anti-corrosion performance. The compound layer L 4  in the outermost surface is dense and thus improves the anti-corrosion performance. 
     (8) The present embodiment, compared to the conventional techniques (Japanese Laid-Open Patent Publications Nos. 2008-154649 and 2013-70751), provides a preferable cooking utensil by a simple manufacturing procedure. In the embodiment, the mill scale, as the oxide film formed on the cast iron obtained by atmosphere casting, is maintained in a non-treated or non-machined state and, in this state, nitriding is performed on the mill scale. This simplifies the manufacturing procedure, compared to the conventional techniques. 
     (9) Since cast iron originally contains Si by 2% or more, Si 3 N 4  is produced in the compound layer L 4 . Therefore, compared to the conventional techniques (Japanese Laid-Open Patent Publications Nos. 2008-154649 and 2013-70751), the present embodiment provides preferable surface hardening with enhanced hardness. 
     (10) Particularly, since the present embodiment employs spheroidal graphite cast iron, Si 3 N 4  is also produced in the diffusion hardening layer L 3 , which is located deeper than the compound layer L 4 . This improves toughness compared to the conventional techniques (Japanese Laid-Open Patent Publications No. 2008-154649 and 2013-70751). 
     (11) Since cast iron typically contains a high content of C, soft nitriding (N+C) can be provided, simply by carrying out nitriding (N). 
     The above-described embodiment may be modified as follows. 
     In the above-described embodiment, after the Peking pan P is removed from the opened molds, the interior of the body  11  as the cooking surface is subjected to nitriding, with the mill scale maintained in a non-treated or non-machined state. 
     However, in order to use the elements of the cast iron, a section or the whole of the mill scale may be removed. In this case, nitriding is carried out on the cementite layer or the base of the cast iron. That is, before nitriding, at least a section of the mill scale may be removed by air blasting, wiping using a brush or cloth, washing with water, washing agent, solvent, or acid, shot blasting, polishing with a sander or grindstone, or lapping. 
     The above-described removal of the mill scale is performed for purposes including removal of foreign objects from the Peking pan P, removal of a section of the outermost layer and a section of the lower layer below the outermost layer, surface smoothing, and size adjustment. 
     Through such surface treatment, the thin compound layer of Fe 3 C (cementite) with high hardness below the mill scale may be exposed. Nitriding is performed on the surface of the compound layer. 
     Also, the timing for opening the molds at the time of casting may be delayed to retain the heat such that the mill scale (black rust, Fe 3 O 4 ) is formed with a greater thickness. In this case, nitriding is carried out on the thicker mill scale. 
     The “cast iron” described in the present disclosure is not restricted to the spheroidal graphite cast iron illustrated in the embodiment. The disclosure may be applied broadly to cast irons formed by atmosphere casting, including white cast iron, gray cast iron (FC: normal cast iron), spheroidal graphite cast iron (FCD), CV cast iron (FCV), white heart malleable cast iron (FCMW), black heart malleable cast iron (FCMB), and perlite malleable cast iron (FCMP). 
     The cooking utensil of the present disclosure may be, other than the Peking pan P of the embodiment, a frying pan, a cocotte, a skillet, a Japanese omelet pan, a cooking iron plate, a pot, a hot plate, an octopus ball pan, a Japanese fish-shaped cake maker, a pan for baking Japanese round-shaped cakes containing read-bean paste, a hot sandwich maker, a sukiyaki pan, an iron kettle, or a rice cooker. That is, the present disclosure mainly targets alternatives for the cooking utensils that cause burning on the cooking surface and thus require fluorite resin coating. The disclosure is also applicable to a grill grid or grid. 
     The present disclosure is applicable to not only a super-thin cast-iron product with a thickness of approximately 1.4 mm, as in the embodiment, but also a Dutch oven or cooking iron plate having a greater thickness. 
     The molds are not restricted to the molds of the embodiment. That is, the casting may be performed using sand molds or shell molds. Such casing with sand molds or shell molds may be employed as long as the cooking surface does not need sander polishing and the casting can be provided as a complete product without polishing, even if the cooking surface is slightly uneven depending on the purpose.