When a mechanical component having a complex shape is manufactured conventionally, in many cases, each of a plurality of components is first manufactured individually, and then, they are combined to obtain the shape of the product. In this case, parts having a complex shape such as a gear are often cut before being combined. On the other hand, in recent years, in order to reduce manufacturing costs, forming components having a shape similar to that of product by punching, hole expanding, forging, or the like is promoted. As a result, a number of the components can be reduced and manufacturing can be performed with fewer processes. When a large deformation is applied, a hot working in which deformation resistance is low is employed, and when it is necessary to work with good accuracy of shape, a cold working is employed. If the steel sheet is worked to be a complex shape similar to that of the product, the steel sheet needs a workability better than in the conventional case in which each of a plurality of parts is manufactured, and then, they are combined. That is, in a conventional steel sheet, if the steel sheet is punched, expanded, or forged so as to be a complex shape, the steel sheet may become cracked or the dimensional accuracy of the product may be deteriorated. In addition, of course, the product after working may require properties such as toughness, strength, wear resistance equal to or more than the conventional art. In order to solve the problems, Patent Documents 1 to 3 propose techniques as follows.
Patent Document 1 proposes a steel reclining seat gear of which a raw material is a steel sheet excellent in notched tensile elongation ratio, in which C: 0.15% to 0.50% and S: 0.01% or less in terms of mass %, and a relationship [% P]≤6×[% B]+0.005 is satisfied. Patent Document 1 focuses on a strong correlation between punchability and the notched tensile elongation ratio, and proposes that the notched tensile elongation ratio and the punchability can be enhanced by increasing a grain size of a carbide dispersed in the steel sheet.
Patent Document 2 proposes a high carbon steel which includes C: 0.70% to 1.20% in terms of mass %, and in which a grain size of carbide dispersed in ferrite matrix is controlled. Since the notched tensile elongation ratio of the steel, which has a close relationship with the punchability, is enhanced, the steel is excellent in punchability. In addition, since a configuration of MnS is controlled by further including Ca in the steel, the punchability of the steel is further enhanced.
Patent Document 3 proposes a steel for gear excellent in cold forgeability, which includes C: 0.10% to 0.40% and S: 0.010% or less in terms of mass %, in which shape of the inclusion is categorized in accordance with ASTM-D method, and in which the shape and the number of the inclusions are set within a range.
In addition, in order to control an amount and/or a configuration of inclusions in the steel, Ca and/or REM (Rare Earth Metal) has been added. The inventors have proposed a technique in which Ca and REM were added to a thick steel plate for structure including 0.08% to 0.22% of C in terms of mass % to control oxide (inclusion) formed in the steel as a mixture phase state of high-melting phase and low-melting phase for preventing the oxide (inclusion) from elongation during rolling and for preventing erosion of a continuous-casting nozzle and an internal inclusion defect from occurring.