High chromium martensitic steel pipe having excellent pitting resistance and method of manufacturing

A high-Cr martensitic steel pipe for line pipes having absorbed energy for heat-affected zone at 0.degree. C. of more than 170 J, and corrosion rate of mother material including welded portions, under the condition of immersing in a corrosion-test liquid of 20% NaCl solution in which a carbonic acid gas of 3.0 MPa was saturated at 80.degree. C., of lower than 0.1 mm/Year, manufactured from a steel consisting essentially of:

• C:
• about 0.03 wt % or less, Si:
• about 0.5 wt % or less, Mn:
• about 0.5 to 3.0 wt %, Cr:
• about 10.0 to 14.0 wt %, Ni:
• about 0.2 to 2.0 wt %, Cu:
• about 0.2 to 0.7 wt %, N:
• about 0.03 wt % or less, V:
• about 0.01-0.1 wt % and the balance being Fe and incidental impurities, and having an X value of about 12.2 or more, wherein said X value is represented by the following equation (1):
• X value=(Cr %)+3(Cu %)-3(C %) (1),
• said steel being subjected to formation of a pipe with a pipe-making machine, austenitizing treatment, quenching and tempering.

A high-Cr martensitic steel pipe for line pipes having absorbed energy for heat-affected zone at 0.degree. C. of more than 170 J, and corrosion rate of mother material including welded portions, under the condition of immersing in a corrosion-test liquid of 20% NaCl solution in which a carbonic acid gas of 3.0 MPa was saturated at 80.degree. C., of lower than 0.1 mm/Year, manufactured from a steel consisting essentially of:

• C:
• about 0.03 wt % or less, Si:
• about 0.5 wt % or less, Mn:
• about 0.5 to 3.0 wt %, Cr:
• about 10.0 to 14.0 wt %, Ni:
• about 0.2 to 2.0 wt %, Cu:
• about 0.2 to 0.7 wt %, N:
• about 0.03 wt % or less, V:
• about 0.01-0.1 wt % and the balance being Fe and incidental impurities, and having an X value of about 12.2 or more, said X value being represented by the following equation (1):
• X value=(Cr %)+3(Cu %)-3(C %) (1),
• said steel being subjected to formation of a pipe with a pipe-making machine, austenitizing treatment, quenching and heat treatment in a dual-phase region.

A high-Cr martensitic steel pipe for line pipes having absorbed energy for heat-affected zone at 0.degree. C. of more than 170 J, and corrosion rate of mother material including welded portions, under the condition of immersing in a corrosion-test liquid of 20% NaCl solution in which a carbonic acid gas of 3.0 MPa was saturated at 80.degree. C., of lower than 0.1 mm/Year, manufactured from a steel consisting essentially of:

• C:
• about 0.03 wt % or less, Si:
• about 0.5 wt % or less, Mn:
• about 0.5 to 3.0 wt %, Cr:
• about 10.0 to 14.0 wt %, Ni:
• about 0.2 to 2.0 wt %, Cu:
• about 0.2 to 0.7 wt %, N:
• about 0.03 wt % or less, V:
• about 0.01-0.1 wt % and the balance being Fe and incidental impurities, and having an X value of about 12.2 or more, said X value being represented by the following equation (1):
• X value=(Cr %)+3(Cu %)-3(C %) (1),
• said steel being subjected to formation of a pipe with a pipe-making machine, austenitizing treatment, quenching, heat treatment in a dual-phase region, and tempering.