High strength line pipe steel having low yield ratio and excellent in low temperature toughness

A high strength line pipe steel having a low yield ratio and excellent in low temperature toughness, containing, in terms of percent by weight:

• C:
• 0.05 to 0.10%,
• Si:
• not greater than 0.6%,
• Mn:
• 1,7 to 2.2%,
• P:
• not greater than 0.015%,
• S:
• not greater than 0.003%,
• Ni:
• 0.1 to 1.0%,
• Mo:
• 0.15 to 0.50%,
• Nb:
• 0.01 to 0.10%,
• Ti:
• 0.005 to 0.030%,
• Al:
• not greater than 0.06%,
• B:
• 0.0003 to 0.0020%,
• N:
• 0.001 to 0.006%, and
• the balance of Fe and unavoidable impurities:
• having a P value, defined by the following general formula, within the range of 2.5 to 4.0;
• and
• having a micro-structure comprising martensite, bainite and ferrite, wherein a ferrite fraction is 20 to 90%, said ferrite contains 50 to 100% of worked ferrite, and a ferrite mean grain size is not greater than 5 .mu.m;
• P value=2.7C+0.4Si+Mn+0.45Ni+2Mo.

A high strength line pipe steel having a low yield ratio and excellent in low temperature toughness, containing, in terms of percent by weight:

• C:
• 0.05 to 0.10%,
• Si:
• not greater than 0.6%,
• Mn:
• 1.7 to 2.5%,
• P:
• not greater than 0.015%,
• S:
• not greater than 0.003%,
• Ni:
• 0.1 to 1.0%,
• Mo:
• 0.35 to 0.50%,
• Nb:
• 0.01 to 0.10%,
• Ti:
• 0.005 to 0.030%,
• Al:
• not greater than 0.06%,
• Cu:
• 0.8 to 1.2%,
• Cr:
• up to 0.6%,
• V:
• up to 0.10%,
• N:
• 0.001 to 0.006%, and
• the balance of Fe and unavoidable impurities;
• having a P value, defined by the following general formula, within the range of 2.5 to 3.5;
• and
• having a micro-structure comprising martensite, bainite and ferrite , wherein a ferrite fraction is from 20 to 90%, said ferrite contains 50 to 100 of worked ferrite, and a ferrite me an grain size is not greater than 5 .mu.m;
• P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+.