Rod material for non-tempered machine component, steel rod for non-tempered machine component, and non-tempered machine component

Abstract

This steel rod for a non-tempered machine component has a chemical composition comprising, in mass%, prescribed amounts of C, Si, Mn, Cr, Mo, Ti, Al, B, Nb and V, and has limited P, S, N and O, with the remainder comprising Fe and impurities; if the content of C, in mass%, is denoted as [C%], the structure includes at least 75*[C%]+25 vol% of bainite, with the remainder comprising at least one of ferrite and pearlite; if the average aspect ratio of a bainite block in a second surface layer section of the steel rod is denoted as R1, R1 is at least 1.2; if the average particle size of the bainite block in a third surface layer section of the steel rod is denoted as PS3 [mu]m, and the average particle size of the bainite block in a third center section of the the steel rod is denoted as PC3 [mu]m, PS3 satisfies formula (c), and PS3 and PC3 satisfy formula (d); the standard deviation of the particle size of the bainite block in the structure is not more than 8.0 [mu]m; and the tensile strength is 800 MPa to 1600 MPa. (c) PS3<=20 / R1 (d) PS3 / PC3<=0.95.

Description

technical field [0001] Non-tempered machine parts with a tensile strength of 800 MPa to 1600 MPa are used in shaft-shaped automotive parts such as bolts, torsion bars, and stabilizers, and various industrial machines. [0002] The present invention relates to the non-quenched and tempered machine part, the steel wire used to manufacture it, and the wire rod used to manufacture the steel wire. [0003] In addition, the non-tempered machine parts targeted by the present invention also include bolts for automobiles or construction. [0004] Hereinafter, the wire rod for non-heat-treated mechanical parts may be simply referred to as wire rod, the steel wire for non-heat-treated machine parts may be simply referred to as steel wire, and the non-heat-treated machine part may be simply referred to as machine part. [0005] This application claims priority based on Japanese Patent Application No. 2015-013385 filed in Japan on January 27, 2015 and Japanese Patent Application No. 2015-...

AI Technical Summary

Problems solved by technology

[0028] Furthermore, the steel wires and wire rods used to manufacture them cannot be obtained by the prior art

[0029] In addition, in these conventional technologies, since pearlite or pseudo-pearlite does not contain bainite as the main structure, the tensile strength of the steel wire increases, and the deformation resistance becomes high during cold working, so the mold If the load increases, or even if it is a structure containing bainite, the grain size or standard deviation of the bainite block is large, so the ductility decreases, processing cracks tend to occur, and the cold workability decreases significantly.

[0030] Therefore, it is difficult to obtain good hydrogen embrittlement resistance in non-tempered high-strength mechanical parts with a tensile strength of 800 MPa or more, especially 1200 MPa or more.

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