Drive shaft and method for producing drive shaft

1. A drive shaft comprising:

a hollow tubular body formed of a medium carbon steel; and

solid stub shafts formed of a medium carbon steel and joined to both ends of the hollow tubular body,

wherein, when grain size is expressed as grain size number, grain size of the hollow tubular body at joint parts where the hollow tubular body and the solid stub shafts are joined together ranges from #5 to #9, and grain size of the solid stub shafts at the joint parts ranges from #10 to #12.

2. The drive shaft according to claim 1 , wherein the medium carbon steel serving as a material of the solid stub shafts contains, by weight, 0.45-0.51% C, 0.25% or less Si, 0.30-0.50% Mn, 0.015% or less P, 0.008-0.020% S, 0.2% or less Cu, 0.1% or less Ni, 0.1-0.2% Cr, at least one of 0.05-0.25% Mo, 0.03-0.08% Nb, and 0.01-0.05% Ti, 0.02-0.04% Al, and 10-30 ppm B, and a balance of Fe and unavoidable impurities, and the grain size number of the medium carbon steel ranges from #9 to #11.

3. The drive shaft according to claim 2 , wherein at least one of NbC, Mo 2 C, and TiC precipitates in a grain boundary in metallic structure of the solid stub shafts.

4. The drive shaft according to claim 1 , wherein the medium carbon steel serving as a material of the hollow tubular body contains, by weight, 0.43-0.47% C, 0.20% or less Si, 0.60-0.90% Mn, 0.010% or less P, 0.008-0.020% S, 0.1% or less Cu, 0.1% or less Ni, 0.05% or less Cr, at least one of 0.05% or less Mo, 0.01-0.03% Nb, and 0.01-0.05% Ti, and 0.02-0.04% Al, and a balance of Fe and unavoidable impurities, and the grain size number of the medium carbon steel ranges from #4 to #9.

5. The drive shaft according to claim 1 , wherein part of the hollow tubular body penetrates into the solid stub shafts by a thickness of 1-30 μm at the joint parts.

6. A production method for obtaining a drive shaft by joining solid stub shafts formed of a medium carbon steel to both ends of a hollow tubular body formed of a medium carbon steel, the production method comprising:

obtaining the solid stub shafts by cold forging; and

joining the hollow tubular body and the solid stub shafts together by friction welding, and setting, when grain size is expressed as grain size number, grain size of the hollow tubular body at joint parts where the hollow tubular body and the solid stub shafts are joined together to range from #5 to #9 and setting grain size of the solid stub shafts at the joint parts to range from #10 to #12.

7. The production method according to claim 6 , wherein, during the friction welding, the solid stub shafts and the hollow tubular body rotating relative to each other are brought into contact, a thrust force is applied by a thrust force applying device to the hollow tubular body or the solid stub shafts such that the solid stub shafts are relatively pressed against the hollow tubular body to generate frictional heat to soften the hollow tubular body and the solid stub shafts, and then additional thrust force is applied to join the hollow tubular body and the solid stub shafts together by solid phase joining.

8. The production method according to claim 6 , wherein the medium carbon steel used as a material for the solid stub shafts contains, by weight, 0.45-0.51% C, 0.25% or less Si, 0.30-0.50% Mn, 0.015% or less P, 0.008-0.020% S, 0.2% or less Cu, 0.1% or less Ni, 0.1-0.2% Cr, at least one of 0.05-0.25% Mo, 0.03-0.08% Nb, and 0.01-0.05% Ti, 0.02-0.04% Al, and 10-30 ppm B, and a balance of Fe and unavoidable impurities, and the grain size number of the medium carbon steel ranges from #9 to #11.

9. The production method according to claim 8 , further comprising:

annealing the drive shaft at a temperature of 650-720° C. for 30-90 minutes after the joining by friction welding; and

hardening the drive shaft after the annealing,

wherein at least one of NbC, Mo 2 C, and TiC is precipitated in a grain boundary in metallic structure of the solid stub shafts.

10. The production method according to claim 6 , wherein the medium carbon steel used as a material for the hollow tubular body contains, by weight, 0.43-0.47% C, 0.20% or less Si, 0.60-0.90% Mn, 0.010% or less P, 0.008-0.020% S, 0.1% or less Cu, 0.1% or less Ni, 0.05% or less Cr, at least one of 0.05% or less Mo, 0.01-0.03% Nb, and 0.01-0.05% Ti, and 0.02-0.04% Al, and a balance of Fe and unavoidable impurities, and the grain size number of the medium carbon steel ranges from #4 to #9.