HIGH-STRENGTH MOLDED PRODUCT COMPRISING ULTRAFINE GRAIN STRUCTURE STEEL AND PROCESS FOR PRODUCING THE SAME

摘要

Steel is hot worked or warm worked and is further cold worked to prepare a wire rod or a steel wire having a bamboo nanostructure with an average grain diameter of not more than 200 nm in the cross-section in a direction C. Next, in a desired limited site, a cross-section other than a cross-section parallel to the direction L is forged, e.g., by cold pressing using a tool or a mold to form an equiaxed nanostructure having an average grain diameter of not more than 200 nm at this site. Screws are mentioned as a typical example of the molded product. For example, a high-strength microscrew may be mentioned in which the axial outer diameter is not more than 1.6 mm, the surface layer part in the head recess has an equiaxed nanostructure and has a hardness of HV ≥ 300, and the screw body part has a bamboo nanostructure and has a hardness of HV ≥ 250. The chemical composition of the steel is a carbon steel having a carbon content of C 0.02%, and the addition of a strengthening element is not always required. The molded product produced from a wire rod/steel wire strengthened by ultrarefinement has a higher strength than the conventional level, and the strength of a desired limited site is higher than that of the body. There is also provided a production process of the molded product.