How Japanese swords are made

Abstract

To provide a method of manufacturing a Japanese sword that can forge a sword with minimum reduction of steel.SOLUTION: A method of manufacturing a Japanese sword includes: a first step S1 of heating and beating out steel; a second step S2 of crushing the beaten steel; a third step S3 of selecting steel of good quality from the crushed steel; a fourth step S4 of stacking the selected steel; a fifth step S5 of heating and integrating the stacked steel; and a sixth step S6 of heating and beating out the integrated steel, and then folding it in two layers. In the first step S1, the steel is heated to a temperature of 800°C or lower and rolled to a thickness of 0.3 mm or less, and the sixth step S6 is repeated three times or four times.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a Japanese sword that can be forged while minimizing the loss of steel. [Background technology]

[0002] A conventional method for manufacturing Japanese swords is known, for example, from the method described in Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7050373 Summary of the Invention [Problem to be solved by the invention]

[0004] In the traditional Japanese sword manufacturing process, the steel is flattened, then folded back and stacked into two pieces, a process that is repeated about 15 times. 15 =32,768 layers of steel are formed, which is said to result in a strong Japanese sword.

[0005] However, it is said that after the reforging process, only about 10 to 20 percent of the steel's weight remains. The reason for this is thought to be that an oxide film is likely to form on the surface of the high-temperature steel during the forging process, and this oxide film peels off during the forging process. Incidentally, even if the loss per reforging process is 10%, the steel remaining after 15 forging processes is only 0.9% of the original weight. 15 =0.21, which means that only about 20% remains.

[0006] Therefore, an object of the present invention is to provide a method for manufacturing a Japanese sword that can be forged while minimizing the loss of steel. [Means for solving the problem]

[0007] The method for manufacturing a Japanese sword of the present invention comprises a first step of heating and beating steel, a second step of crushing the beated steel, a third step of selecting high-quality steel from the crushed steel, a fourth step of stacking the selected steel, a fifth step of heating the stacked steel to unite it, and a sixth step of heating and beating the united steel and then folding it back to make two layers.In the first step, the steel is heated to a temperature of 800°C or less and beated to a thickness of 0.3 mm or less, and the sixth step is repeated three or four times. [Effects of the Invention]

[0008] According to the method for manufacturing Japanese swords of the present invention, the heating temperature of the steel in the first step is set to 800°C or less, which is lower than conventional temperatures, thereby minimizing the oxidation of the steel. Furthermore, by hammering the steel to a thickness of 0.3 mm or less, which is thinner than conventional methods, in the first step, impurities can be effectively removed. Furthermore, by reducing the number of times the sixth step is performed to three or four times, the amount of steel that oxidizes can be reduced. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is an explanatory diagram showing the steps of the method for manufacturing a Japanese sword of the present invention. [Figure 2] FIG. 10 is an explanatory diagram showing the folding forging process. DETAILED DESCRIPTION OF THE INVENTION

[0010] The method for manufacturing a Japanese sword of the present invention will be described in detail based on the following examples. Note that the present invention is not limited to the following examples, and various modifications are possible within the scope of the concept of the present invention.

[0011] Referring to Figure 1, first, steel that will be used as the raw material for the Japanese sword is prepared (S0). For this steel, a type called tamahagane, which has an appropriate carbon content and is of excellent quality, is preferably used.

[0012] In the first step, the steel is heated and hammered (S1), a process called Mizuheshi.

[0013] In conventional methods, steel is heated to 1000°C or higher in this process. However, heating to 1000°C or higher makes steel susceptible to oxidation, resulting in the loss of some of the valuable raw materials. Therefore, in the method of the present invention, the steel is heated to a temperature of 800°C or lower to minimize oxidation of the steel. The heating temperature of the steel is preferably 700 to 800°C, which is the quenching temperature of steel, and more preferably 750 to 800°C. This is because a low heating temperature reduces the ductility of the steel, making it difficult to process, while a high heating temperature causes rapid oxidation of the steel.

[0014] In addition, in conventional methods, steel is beaten to a thickness of about 5 to 10 mm in this step. In contrast, in the method of the present invention, steel is beaten to a thickness of 0.3 mm or less, and lumps of hard and brittle impurities, such as silicon oxide, contained in the steel are crushed and removed from the steel. Note that the thickness of the steel after beating is preferably as thin as possible to remove impurities, but in consideration of workability, a thickness of 0.1 to 0.3 mm is preferable, and a thickness of 0.1 to 0.2 mm is even more preferable.

[0015] Finally, the burnt black skin on the surface is removed by hitting it with a hot steel foil on a damp anvil.

[0016] Next, in the second step, the steel that was hammered out in the first step is crushed (S2). This step is called "shredding." As a result, thin strips of steel are obtained.

[0017] In the third process, high-quality steel is selected from the steel crushed in the second process (S3), and in the fourth process, the steel selected in the third process is stacked (S4). Here, the steel is stacked on a platform made of the same quality steel. This platform also has a handle.

[0018] Next, in the fifth process, the steel piled up in the fourth process is heated and united (S5). This process is called "tsumi-bokashi." By heating it up to about 1300°C over a long period of time, the steel becomes completely united.

[0019] Furthermore, in the sixth process, the steel that was united in the fourth process is heated and hammered out, and then folded back to form two sheets (S6). This process is called folding forging.

[0020] This process will be explained with reference to Figure 2. This process is carried out while heating to over 1000°C in order to adhere the two overlapping steel sheets together. First, the steel is hammered out (S61), chiseled horizontally to fold it back (S62), and then stacked into two sheets (S63). Next, if this process is to be repeated, the two overlapping steel sheets are hammered out again into one sheet (S64), chiseled in a different vertical direction than in the previous process, and folded back (S65), and then stacked into two sheets (S66). If this process is to be repeated subsequently, the two overlapping steel sheets are hammered out again into one sheet (S61), and the same process is repeated thereafter.

[0021] In conventional methods, this process of folding the steel back and creating two layers after forging is repeated approximately 15 times. However, prolonged heating at temperatures above 1000°C and forging causes a large amount of oxidation in the steel, resulting in the loss of 80 to 90 percent of the valuable raw materials before and after the folding and forging process. Therefore, in the method of the present invention, this process of folding the steel back and creating two layers after forging is repeated only three or four times to prevent oxidation of the steel. Note that performing this process twice is undesirable because the surface of the resulting steel is too rough, while performing this process five or more times is undesirable because it increases the loss of steel due to oxidation.

[0022] The rest of the process is the same as before: the shin-gane and kawa-gane are produced using the methods described above, the shin-gane is wrapped in kawa-gane, the shin-gane is hammered out to the length of the sword, the hi-tsukuri is forged, and the Japanese sword is then completed through the processes of doki-oki, quenching, blacksmithing, inscription-kiri and polishing.

[0023] As described above, the method for manufacturing a Japanese sword of the present invention comprises the following steps: a first step (S1) of heating and forging steel; a second step (S2) of crushing the forged steel; a third step (S3) of selecting high-quality steel from the crushed steel; a fourth step (S4) of stacking the selected steel; a fifth step (S5) of heating and combining the stacked steel; and a sixth step (S6) of heating and forging the combined steel, then folding it back to form two sheets. In the first step (S1), the steel is heated to a temperature of 800°C or less and forged to a thickness of 0.3 mm or less. The sixth step (S6) is repeated three or four times. Therefore, according to the method for manufacturing a Japanese sword of the present invention, by heating the steel to a temperature of 800°C or less, which is lower than conventional temperatures, oxidation of the steel can be prevented. Furthermore, by forging the steel to a thickness of 0.3 mm or less, which is thinner than conventional methods, impurities can be effectively removed in the first step (S1). Furthermore, by reducing the number of times of the sixth step S6 to three or four times, which is less than the conventional number of times, the amount of steel that oxidizes can be reduced.

[0024] Silicon oxide, the main impurity, is brittle and has almost no viscosity at room temperature. Steel, on the other hand, retains sufficient viscosity even when heated. This method takes advantage of the large difference in ductility between steel and silicon oxide to remove the silicon oxide present in the steel. Specifically, a small piece less than half the size of a quail's egg or a chicken's egg is cut from a block of steel, and this small piece is heated and beaten to a thickness of less than 0.3 mm, during which the silicon oxide falls off and is removed. In this process, the heating temperature is kept below 800°C to minimize oxidation of the steel due to heating. At this temperature, the silicon oxide in the steel remains almost completely ductile.

[0025] The lower the temperature of the steel, the less efficient the hammering process becomes, so to thin the steel to 0.3 mm or less, the steel must be heated and hammered several times. This method ensures that at least half of the original weight remains. [Explanation of symbols]

[0026] S1 First process S2 Second process S3 Third process S4 Fourth process S5 Fifth step S6 Sixth process

Claims

[Claim 1] A method for manufacturing a Japanese sword comprising a first step of heating and beating steel, a second step of crushing the beated steel, a third step of selecting high-quality steel from the crushed steel, a fourth step of stacking the selected steel, a fifth step of heating and uniting the stacked steel, and a sixth step of heating and beating the united steel and then folding it back to make two layers, wherein in the first step the steel is heated to a temperature of 800°C or less and beated to a thickness of 0.3 mm or less, and the sixth step is repeated three or four times.

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