HIGH STRENGTH STEEL MATERIAL AND HIGH STRENGTH BOLT EXCELLENT IN DELAYED FRACTURE RESISTANCE AND METHODS OF PRODUCTION OF SAME

1 . A steel material which is excellent in delayed fracture resistance containing, by mass %,

C: 0.10 to 0.55%,

Si: 0.01 to 3%, and

Mn: 0.1 to 2%,

further containing one or both of

V: 1.5% or less and Mo: 3.0% or less, the contents of V and Mo satisfying

V+1/2Mo>0.4%,

further containing one or more of

Cr: 0.05 to 1.5%,

Nb: 0.001 to 0.05%,

Cu: 0.01 to 4%,

Ni: 0.01 to 4%, and

B: 0.0001 to 0.005%, and

having a balance of Fe and unavoidable impurities, the structure being a mainly tempered martensite structure,

the surface of the steel material being formed with

(a) a nitrided layer having a thickness from the surface of said steel material of 200 μm or more and a nitrogen concentration of 12.0 mass % or less and higher than the nitrogen concentration of said steel material by 0.02 mass % or more and

(b) a low carbon region having a depth from the surface of said steel material of 100 μm or more to 1000 μm or less and having a carbon concentration of 0.05 mass % or more and 0.9 time or less the carbon concentration of said steel material.

2 . A steel material which is excellent in delayed fracture resistance as set forth in claim 1 characterized in that due to the presence of said nitrided layer and low carbon region, the amount of absorption of hydrogen in the steel material is 0.5 ppm or less and the critical diffusible hydrogen content of the steel material is 2.00 ppm or more.

3 . A steel material which is excellent in delayed fracture resistance as set forth in claim 1 further characterized in that said steel material contains, by mass %, one or more of

Al: 0.003 to 0.1%,

Ti: 0.003 to 0.05%,

Mg: 0.0003 to 0.01%,

Ca: 0.0003 to 0.01%, and

Zr: 0.0003 to 0.01%.

4 . A steel material which is excellent in delayed fracture resistance as set forth in claim 1 characterized in that said nitrided layer has a thickness of 1000 μm or less.

5 . A steel material which is excellent in delayed fracture resistance as set forth in claim 1 characterized in that said tempered martensite has an area ratio of 85% or more.

6 . A steel material which is excellent in delayed fracture resistance as set forth in claim 1 characterized in that said steel material has a compressive residual stress at the surface of 200 MPa or more.

7 . A steel material which is excellent in delayed fracture resistance as set forth in claim 1 characterized in that said steel material has a tensile strength of 1300 MPa or more.

8 . A bolt which is excellent in delayed fracture resistance obtained by working a steel material containing, by mass %,

C: 0.10 to 0.55%,

Si: 0.01 to 3%, and

Mn: 0.1 to 2%,

further containing one or both of

V: 1.5% or less and

Mo: 3.0% or less, the contents of V and Mo satisfying

V+1/2Mo>0.4%,

further containing one or more of

Cr: 0.05 to 1.5%,

Nb: 0.001 to 0.05%,

Cu: 0.01 to 4%,

Ni: 0.01 to 4%, and

B: 0.0001 to 0.005%, and

having a balance of Fe and unavoidable impurities, the structure being a mainly tempered martensite structure,

the surface of said bolt being formed with

(a) a nitrided layer having a thickness from the surface of said bolt of 200 μm or more and a nitrogen concentration of 12.0 mass % or less and higher than the nitrogen concentration of said steel material by 0.02 mass % or more and

(b) a low carbon region having a depth from the surface of said bolt of 100 μm or more to 1000 μm or less and having a carbon concentration of 0.05 mass % or more and 0.9 time or less the carbon concentration of said steel material.

9 . A high strength bolt which is excellent in delayed fracture resistance as set forth in claim 8 characterized in that due to the presence of said nitrided layer and low carbon region, the amount of absorption of hydrogen in the bolt is 0.5 ppm or less and the critical diffusible hydrogen content of the bolt is 2.00 ppm or more.

10 . A high strength bolt which is excellent in delayed fracture resistance as set forth in claim 8 characterized in that said steel material further contains, by mass %, one or more of

Al: 0.003 to 0.1%,

Ti: 0.003 to 0.05%,

Mg: 0.0003 to 0.01%,

Ca: 0.0003 to 0.01%, and

Zr: 0.0003 to 0.01%.

11 . A high strength bolt which is excellent in delayed fracture resistance as set forth in claim 8 characterized in that said nitrided layer has a thickness of 1000 μm or less.

12 . A high strength bolt which is excellent in delayed fracture resistance as set forth in claim 8 characterized in that said tempered martensite has an area ratio of 85% or more.

13 . A high strength bolt which is excellent in delayed fracture resistance as set forth in claim 8 characterized in that said bolt has a compressive residual stress at the surface of 200 MPa or more.

14 . A high strength bolt which is excellent in delayed fracture resistance as set forth in claim 8 characterized in that said bolt has a tensile strength of 1300 MPa or more.

15 . A method of production of a high strength steel material which is excellent in delayed fracture resistance as set forth in claim 1 ,

said method of production of a high strength steel material which is excellent in delayed fracture resistance characterized by

(1) heating a steel material having a composition as set forth in claim 1 to form a low carbon region having a depth from the surface of said steel material of 100 μm or more to 1000 μm or less and having a carbon concentration of 0.05 mass % or more and 0.9 time or less the carbon concentration of said steel material, then cooling as it is to make the steel material structure a mainly martensite structure, then

(2) nitriding said steel material at over 500° C. to 650° C. or less to form on the surface of the steel material a nitrided layer having a nitrogen concentration of 12.0 mass % or less and higher than the nitrogen concentration of said steel material by 0.02 mass % and having a thickness from the surface of said steel material of 200 μm or more and to make the steel material structure a mainly tempered martensite structure.

16 . A method of production of a high strength steel material which is excellent in delayed fracture resistance as set forth in claim 15 characterized in that said nitrided layer has a thickness of 1000 μm or less.

17 . A method of production of a bolt which is excellent in delayed fracture resistance as set forth in claim 8 ,

said method of production of a bolt which is excellent in delayed fracture resistance characterized by

(1) heating a bolt obtained by working a steel material having a composition as set forth in claim 8 to form a low carbon region having a depth from the surface of said bolt of 100 μm or more to 1000 μm or less and having a carbon concentration of 0.05 mass % or more and 0.9 time or less the carbon concentration of said steel material, then cooling as it is to make the steel material structure a mainly martensite structure, then

(2) nitriding said bolt at over 500° C. to 650° C. or less to form on the surface of the bolt a nitrided layer having a nitrogen concentration of 12.0 mass % or less and higher than the nitrogen concentration of said steel material by 0.02 mass % and having a thickness from the surface of said bolt of 200 μm or more and to make the steel material structure a mainly tempered martensite structure.

18 . A method of production of a bolt which is excellent in delayed fracture resistance as set forth in claim 17 , characterized in that said nitrided layer has a thickness of 1000 μm or less.

19 . A steel material which is excellent in delayed fracture resistance as set forth in claim 2 further characterized in that said steel material contains, by mass %, one or more of

Al: 0.003 to 0.1%,

Ti: 0.003 to 0.05%,

Mg: 0.0003 to 0.01%,

Ca: 0.0003 to 0.01%, and

Zr: 0.0003 to 0.01%.

20 . A steel material which is excellent in delayed fracture resistance as set forth in claim 2 characterized in that said nitrided layer has a thickness of 1000 μm or less.