High-strength bolt
View Patent ↗A high-strength bolt is provided that has high strength and excellent hydrogen embrittlement resistance characteristics. A bolt according to this invention has a chemical composition consisting of, in mass %, C: 0.22 to 0.40%, Si: 0.10 to 1.50%, Mn: 0.20 to less than 0.40%, P: 0.020% or less, S: 0.020% or less, Cr: 0.70 to 1.45%, Al: 0.005 to 0.060%, Ti: 0.010 to 0.045%, B: 0.0003 to 0.0040%, N: 0.0015 to 0.0080% and O: 0.0020% or less, with a balance being Fe and impurities, and satisfying Formula (1) and Formula (2), and with the high-strength bolt having a tensile strength of 1000 to 1300 MPa. 0.50≤C+Si/10+Mn/5+5Cr/22≤0.85 (1) Si/Mn>1.0 (2) Where, a content (mass %) of a corresponding element is substituted for each symbol of an element in Formula (1) and Formula (2).
1. A high-strength bolt comprising a chemical composition consisting of, in mass %:
C: 0.22 to 0.40%,
Si: 0.10 to 1.50%,
P: 0.020% or less,
S: 0.020% or less,
Cr: 0.70 to 1.45%,
Al: 0.005 to 0.060%,
Ti: 0.010 to 0.045%,
B: 0.0003 to 0.0040%,
N: 0.0015 to 0.0080%,
O: 0.0020% or less,
Cu: 0 to 0.50%,
Ni: 0 to 0.30%,
Mo: 0 to 0.04%,
V: 0 to 0.05%, and
Nb: 0 to 0.050%, with a balance being Fe and impurities, and that satisfies Formula (1) and Formula (2),
wherein the high-strength bolt has a tensile strength of 1000 to 1300 MPa,
wherein a critical diffusible hydrogen content ratio HR, defined by Formula (A), is 1.20 or more:
0.50≤C+Si/10+Mn/5+5Cr/22≤0.85 (1)
Si/Mn>1.0 (2)
HR=Hc/Href, (A)
where, a content (mass %) of a corresponding element is substituted for each symbol of an element in Formula (1) and Formula (2), and
where, in Formula (A), a critical diffusible hydrogen content Hc is defined by a maximum hydrogen amount at which a hydrogen introduced test specimen does not rupture in a constant load test at tensile stress of nominal stress of 1080 MPa, and a critical diffusible hydrogen content Href is defined by a critical diffusible hydrogen content of a steel having a chemical composition corresponding to SCM435 of JIS G4053 (2008) as a reference.
2. The high-strength bolt according to claim 1 , wherein:
at a root of a thread of the high-strength bolt, an absolute value of a compressive residual stress of an outer layer of the root of the thread is 10 to 90% of the tensile strength of the high-strength bolt.
3. The high-strength bolt according to claim 1 , containing one or more types selected from a group consisting of, in mass %:
Cu: 0.02 to 0.50%,
Ni: 0.03 to 0.30%,
Mo: 0.01 to 0.04%, and
V: 0.005 to 0.05%.
4. The high-strength bolt according to claim 3 , containing, in mass %: Nb: 0.0015 to 0.050%.
5. The high-strength bolt according to claim 4 , wherein:
at a root of a thread of the high-strength bolt, an absolute value of a compressive residual stress of an outer layer of the root of the thread is 10 to 90% of the tensile strength of the high-strength bolt.
6. The high-strength bolt according to claim 3 , wherein:
at a root of a thread of the high-strength bolt, an absolute value of a compressive residual stress of an outer layer of the root of the thread is 10 to 90% of the tensile strength of the high-strength bolt.
7. The high-strength bolt according to claim 1 , containing, in mass %: Nb: 0.0015 to 0.050%.
8. The high-strength bolt according to claim 7 , wherein:
at a root of a thread of the high-strength bolt, an absolute value of a compressive residual stress of an outer layer of the root of the thread is 10 to 90% of the tensile strength of the high-strength bolt.