High strength electric resistance welded steel pipe and method for producing high strength electric resistance welded steel pipe

1. High strength electric resistance welded steel pipe characterized by having a chemical composition consisting of, by mass %,

C: 0.040 to 0.070%,

Si: 0.10 to 0.50%,

Mn: 1.60 to 2.00%,

Nb: 0.020 to 0.080%,

V: 0.060% or less,

Ti: 0.010 to 0.025%,

Mo: 0.20 to 0.40%,

Ni: 0.10 to 0.50%,

Al: 0.050% or less,

3 Mo %+Ni %: more than 1.00%, and

a balance of Fe and unavoidable impurities,

optionally P: 0.030% or less, S: 0.004% or less, N: 0.006% or less, O: 0.004% or less, Cu: 0.10 to 0.50%, Cr: 0.05to 0.50%, Ca: 0.0005 to 0.0040% and REM: 0.0005 to 0.0050%, and wherein

when using a point defined as a point ¼ of the thickness in the thickness direction from the surface in the base material part of the steel pipe as a reference point, the metal structure in a region having a width of 0.5 mm in both the thickness directions from the reference point as a center consists of polygonal ferrite: 10 area % or less and a balance of bainitic ferrite, and

a thickness is 15.0 to 19.8 mm, and wherein the mechanical properties of the metal structure are such that the tensile strength in the axial direction exceeds 725 MPa, the yield strength in the axial direction is from 655 to 758 MPa, and the Charpy absorbed energy in the circumferential direction at 0° C. is 22 J or more.

2. The high strength electric resistance welded steel pipe according to claim 1 , wherein the polygonal ferrite has an average particle diameter of 20 μm or less.

3. The high strength electric resistance welded steel pipe according to claim 1 , wherein (3Mo %+Ni %+Cu %) is more than 1.20%.

4. The high strength electric resistance welded steel pipe according claim 3 , wherein the polygonal ferrite has an average particle diameter of 20 μm or less.

5. A method of producing the high strength electric resistance welded steel pipe according to claim 1 , comprising:

hot rolling at 790° C. or more a hot-rolled steel plate having a chemical composition consisting of, by mass %,

C: 0.040 to 0.070%,

Si: 0.10 to 0.50%,

Mn: 1.60 to 2.00%,

Nb: 0.020 to 0.080%,

V: 0.060% or less,

Ti: 0.010 to 0.025%,

Mo: 0.20 to 0.40%,

Ni: 0.10 to 0.50%,

Al: 0.050% or less,

3 Mo %+Ni %: more than 1.00%, and

a balance of Fe and unavoidable impurities,

optionally P: 0.030% or less, S: 0.004% or less, N: 0.006% or less, O: 0.004% or less, Cu: 0.10 to 0.50%, Cr: 0.05 to 0.50%, Ca: 0.0005 to 0.0040% and REM: 0.0005 to 0.0050%

cooling the steel plate at an average cooling rate of 8 to 15° C./sec,

coiling the steel plate at 500 to 630° C.,

cooling the steel plate at a cooling rate exceeding 5° C./sec at the beginning of transformation,

forming the steel plate into a tube,

welding end portions of the steel plate by electric resistance welding,

heating the outer surface of the weld zone to 950 to 1050° C., and cooling the inner surface of the weld zone at a cooling rate of 8° C./sec or more to a cooling stop temperature of the outer surface of 600 to 450° C. to obtain the high strength electric resistance welded steel pipe.

6. The method for producing the high strength electric resistance welded steel pipe according to claim 5 , wherein the temperature at the start of the transformation is less than 665° C.

7. The method for producing the high strength electric resistance welded steel pipe according to claim 5 , wherein the hot-rolled steel plate is hot-rolled steel plate obtained by heating a slab and hot rolling it with a cumulative rolling reduction rate at 950° C. or less of 50% or more.