Electric resistance welded steel pipe excellent in deformability and fatigue properties after quenching

1. An electric resistance welded steel pipe comprising, by mass %,

C: 0.30 to 0.55%,

Si: 0.01 to 0.30%,

Mn: 0.5 to 1.5%,

Ca: 0.0010 to 0.0030%,

S: 0.0005 to 0.0050%, and

O: 0.0005 to 0.0050%,

restricting

P: 0.020% or less,

N: 0.0050% or less, and

Al: 0.050% or less,

having a balance of Fe and impurities,

having contents of Ca, O, and S satisfying

0.10≦[Ca](1-124[O])/1.25[S]≦2.50,

having Ca-based inclusions present at a base material and electric resistance weld zone with an average particle size of 1.0 to 10 μm and density of 3 to 300/mm 2 , and

having a difference ΔHv of a maximum hardness of the electric resistance weld zone and average hardness of a base material part of 100≦ΔHv≦500,

wherein:

heat treatment of the welded steel pipe is not performed after pipe-making, and

the welded steel pipe is in a state of the pipe as-produced, without performing heat treatment.

2. The electric resistance welded steel pipe as set forth in claim 1 , further comprising, by mass, Mo: 0.25% or less.

3. The electric resistance welded steel pipe as set forth in claim 1 or 2 , further comprising, by mass %, Ti: 0.030% or less and B: 0.0050% or less.

4. The electric resistance welded steel pipe as set forth in claim 1 or 2 , further comprising, by mass %, one or more of Ni: 1.00% or less, Cu: 1.00% or less, Cr: 1.50% or less, Mo: less than 0.05%, Nb: 0.050% or less, and V: 0.040% or less.

5. The electric resistance welded steel pipe as set forth in claim 1 or 2 , wherein the base material has a thickness of 4 to 12 mm.

6. The electric resistance welded steel pipe as set forth in claim 1 , wherein the content of C is 0.35 to 0.55%.