日本刀の強度-靱性バランスと刃金用玉鋼の折返し鍛錬による炭素量,焼入れ硬さの変化 Strength-toughnes s Balance of the Japanese Sword and Changes in Carbon Content and as Quenched Hardness of HAGANE Steel with Cyclic Folding Forging
Micro-strucure, micro-hardness and micro-absorbed impact energy in the Japanese sword have been investigated to clarify excellent mechanical properties of the Japanese sword. The Japanese sword specimen used in the present research has been made by using TSUKURIKOMI process which combines four kinds of steels ; HAGANE (edge), SHINGANE (core), MUNEGANE (back) and KAWAGANE (side) steels, with different carbon contents. By this process, HASAKI (edge) side becomes high carbon steel and MUNE (back) side possesses low carbon steel. The cooling velocity in quenching of the Japanese sword is controlled by TSUCHIOKI treatment which coats the clay thinner in the HASAKI side and thicker in the MUNE side. The HASAKI side is quickly cooled and the MUNE side is slowly cooled. The micro-structure in the HASAKI side shows martensite while the MUNE side shows the coexist structure of ferrite and pearlite. The HASAKI side has a lower value while the MUNE side shows a higher value in the micro absorbed impact energies obtained with the 1.0 and 0.7mm square miniaturized specimens. It has been shown clearly that the TSUKURIKOMI and the TSUCHIOKI processes give the excellent gradated balance of strength-toughness to the Japanese sword. The ORIKAESHI (folding) forging has an effect both on the carbon content and as quenched hardness in HAGANE steel. The most suitable times of ORIKAESHI cycles which adjust to the carbon content of 0.55-0.60mass% and hardness of 800HV1 have been determined to be thirteen times. These times of ORIKAESHI cycles correspond to the optimum traditional cycles lying between twelve and fifteen times. The present research from the viewpoint of the metallurgy sheds light on the empirical rule in the traditional Japanese sword processing.