Bolt, and steel material for bolts

Abstract

Provided are: a bolt exhibiting excellent delayed fracture resistant properties at a high strength level not less than 1200 MPa but less than 1600 MPa in terms of tensile strength where the possibility of delayed fracture occurring is extremely high; and a bolt steel material serving as a material for said bolt. The bolt has a compositional makeup satisfying formulae (1) and (2), and has a tensile strength not less than 1200 MPa but less than 1600 MPa. Herein, (1): 0.48<=Mo / 1.4+V<1.10. (2): 0.80<Mo / V<3.00, wherein the contents (mass%) of Mo and V contained in the bolt are respectively substituted into Mo and V.

Description

technical field [0001] The invention relates to a bolt and a steel material for the bolt. Background technique [0002] With the increase in performance and weight of automobiles and industrial machinery, and the increase in the size of civil engineering structures, bolts are required to have higher strength. [0003] For bolts, alloy steel materials for machine structures such as SCM435 and SCM440 specified in JIS G 4053:2016 are used. Bolts are made of alloy steel used for mechanical structures and then quenched and tempered to adjust their strength. [0004] In order to increase the strength of the bolt, it is sufficient to increase the carbon content of the steel or lower the tempering temperature. [0005] However, when the tensile strength of the bolt exceeds 1200 MPa, delayed fracture which is a type of hydrogen embrittlement becomes a problem. Delayed fracture is a phenomenon in which parts placed under static stress suddenly undergo brittle fracture after a certa...

AI Technical Summary

Problems solved by technology

[0007] If the bolt is used outdoors, especially in an environment where seawater, snowmelt salt, etc. are scattered, the amount of hydrogen intrusion increases due to the adhesion of salt, and the possibility of delayed fracture increases.

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