2003 年 44 巻 8 号 p. 1577-1582
We have developed a new manufacturing process for nickel-free austenitic stainless steel. In combination with machining and a nitrogen absorption treatment, this process makes it possible to form small precise devices. The new manufacturing process can be used to manufacture small devices with a great deal of precision and parts with a maximum thickness or diameter of 4 mm. However, the temperature for the nitrogen absorption, 1473 K, was sufficiently high for grain growth, and coarsening was observed after nitrogen absorption. Therefore, a nitrogen absorption treatment that allows the retention of strength and ductility is performed with a grain refinement process before nitrogen absorption. In this study, we attempted the refinement of grains by thermo-mechanical treatment before nitrogen absorption treatment in order to increase the mechanical properties after nitrogen absorption treatment. The mechanical properties and microstructures of Fe–24Cr–2Mo with fine grains before and after nitrogen absorption treatment were evaluated to understand the effects of grain refinement on nitrogen absorption. The austenitic phase was obtained only from the surface to a 0.5-mm depth in the alloy with nitrogen absorption at 1473 K for 7.2 ks. The balance between strength and elongation in the alloy with nitrogen absorption at 1473 K for over 10.8 ks was the same as that in conventional austenitic stainless steel. The value of ultimate tensile strength in the alloy with nitrogen absorption increased with the grain refinement process attempted in this study. The elongation in the alloy with nitrogen absorption over 18.0 ks decreased because of grain growth. Therefore, grain refinement before nitrogen absorption treatment can increase the mechanical properties of nickel-free austenitic stainless steel.
