High-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and method of manufacturing the same

A high-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and excellent HIC resistance comprising:

• a chemical composition consisting of, on a mass percent basis,
• C:
• 0.025% to 0.084%, Si:
• 0.10% to 0.30%,
• Mn:
• 0.70% to 1.80%, P:
• 0.001% to 0.018%,
• S:
• 0.0001% to 0.0029%, Al:
• 0.01% to 0.10%,
• Nb:
• 0.001% to 0.065%, V:
• 0.001% to 0.065%,
• Ti:
• 0.001% to 0.033%, Ca:
• 0.0001% to 0.0035%,
• N:
• 0.0050% or less, O:
• 0.0030% or less
• and the remainder being Fe and incidental impurities,
• wherein Pcm defined by formula (1) is 0.20 or less,
• Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (1)
• wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, and B denote amounts (mass %) of corresponding elements,
• a microstructure which includes 90% by area or more of quasi-polygonal ferrite having a grain size of 10 μm or less in each of a base steel portion and an electric resistance welded portion of the steel pipe,
• a yield strength YS of 400 MPa or more,
• and
• an absorbed energy vE−50 of 150 J or more at −50° C. in a Charpy impact test.

A method of manufacturing a high-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and excellent HIC resistance, comprising:

• a hot-rolling step of producing a hot-rolled steel strip from steel by heating, hot-rolling, cooling, and coiling,
• a pipe-forming step of continuously roll-forming the hot-rolled steel strip after the hot-rolling step to form a tubular product having a substantially circular cross section and butt-welding circumferential ends of the tubular product by electric resistance welding to produce an electric resistance welded steel pipe, wherein
• the steel has a chemical composition consisting of C:
• 0.025% to 0.084%, Si:
• 0.10% to 0.30%,
• Mn:
• 0.70% to 1.80%, P:
• 0.001% to 0.018%,
• S:
• 0.0001% to 0.0029%, Al:
• 0.01% to 0.10%,
• Nb:
• 0.001% to 0.065%, V:
• 0.001% to 0.065%,
• Ti:
• 0.001% to 0.033%, Ca:
• 0.0001% to 0.0035%,
• N:
• 0.0050% or less, O:
• 0.0030% or less on a mass percent basis,
• and the remainder being Fe and incidental impurities,
• wherein Pcm defined by formula (1) is 0.20 or less,
• Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (1)
• wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, and B denote the amounts (mass %) of the corresponding elements,
• the hot-rolling step is performed by heating the steel to a temperature of 1200° C. to 1280° C., maintaining the temperature for 90 min or more, hot-rolling the steel at a hot-rolling reduction of 20% or more in an unrecrystallized austenite region, after completion of the hot-rolling, cooling the steel to a finish cooling temperature of 630° C. or less at a cooling rate of 7° C./s to 49° C./s, the cooling rate being an average cooling rate at a temperature from 780° C. to 630° C. at a central portion in the thickness direction, and coiling the steel at a coiling temperature of 400° C. or more and less than 600° C.,
• the pipe-forming step is followed by a heat treatment that includes heating the electric resistance welded portion of the electric resistance welded steel pipe on a production line such that the electric resistance welded portion has a temperature of 800° C. to 1150° C. over the total wall thickness, then cooling the electric resistance welded portion to a finish cooling temperature of 630° C. or less at a cooling rate of 7° C./s to 49° C./s, the cooling rate being the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction, and then allowing the electric resistance welded portion to air-cool,
• and
• the base steel portion and the electric resistance welded portion of the electric resistance welded steel pipe have a yield strength YS of 400 MPa or more and an absorbed energy vE−50 of 150 J or more at −50° C. in a Charpy impact test.