Steel pipe having high formability and method for producing the same

A steel pipe, excellent in formability, having a chemical composition comprising, in mass,

• 0.0001 to 0.50% of C,
• 0.001 to 2.5% of Si,
• 0.01 to 3.0% of Mn,
• 0.001 to 0.2% of P,
• 0.05% or less of S,
• 0.01% or less of N,
• 0.01 to 2.5% of Al and
• 0.01% or less of O
• in a manner to satisfy the expressions (1) and (2) below, with the balance consisting of Fe and unavoidable impurities, characterized in that:
• the relationship between the tensile strength (TS) and the n-value of the steel pipe satisfies the expression (3) below;
• the volume percentage of its ferrite phase is 75% or more;
• the average grain size of the ferrite is 10 μm or more;
• and the crystal grains of the ferrite having an aspect ratio of 0.5 to 3.0 account for, in area percentage, 90% or more of all the crystal grains composing the ferrite,
• (203{square root over ( )}C+15.2Ni−44.7Si−104V−31.5Mo+30Mn+11Cr+20Cu−700P−200Al)<−20 (1)
• (44.7Si+700P+200Al)>80 (2)
• n≧−0.126×ln(TS)+0.94 (3).

A steel pipe, excellent in formability, having a chemical composition comprising, in mass,

• 0.0001 to 0.50% of C,
• 0.001 to 2.5% of Si,
• 0.01 to 3.0% of Mn,
• 0.001 to 0.2% of P,
• 0.05% or less of S,
• 0.01% or less of N,
• 0.2% or less of Ti and
• 0.15% or less of Nb
• in a manner to satisfy the expression 0.5≦(Mn+13Ti+29Nb)≦5, with the balance consisting of Fe and unavoidable impurities, characterized by having the property that the ratio of the X-ray intensity in the orientation components of {111}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 5.0 or larger and the ratio of the X-ray intensity in the orientation component of {111}<112> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is below 2.0.

A method to produce a steel pipe, excellent in formability, having a chemical composition comprising, in mass,

• 0.0001 to 0.50% of C,
• 0.001 to 2.5% of Si,
• 0.01 to 3.0% of Mn,
• 0.001 to 0.2% of P,
• 0.05% or less of S and
• 0.01% or less of N,
• with the balance consisting of Fe and unavoidable impurities, characterized by heating the steel pipe, having the property that the ratio of the X-ray intensity in every one of the orientation components of {001}<110>, {116}<110>, {114}<110> and {112}<110> on the plane at the center of the wall thickness of the mother pipe before diameter reduction to the random X-ray intensity is 3 or smaller, to a temperature in the range from 650° C. or higher to 1,200° C. or lower and by applying working under a condition of a diameter reduction ratio of 30% or more and a wall thickness reduction ratio of 5% or more to 30% or less, so that the steel pipe has an r-value of 1.4 or larger in the axial direction of the steel pipe and the property that the average of the ratios of the X-ray intensity in the orientation component group of {110}<110> to {332}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 3.5 or larger, and/or the ratio of the X-ray intensity in the orientation component of {110}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 5.0 or larger.

A method to produce a steel pipe, excellent in formability, having a chemical composition comprising, in mass,

• 0.0001 to 0.50% of C,
• 0.001 to 2.5% of Si,
• 0.01 to 3.0% of Mn,
• 0.001 to 0.2% of P,
• 0.05% or less of S and
• 0.01% or less of N,
• with the balance consisting of Fe and unavoidable impurities, characterized by heating the steel pipe, having the property that the ratio of the X-ray intensity in one or more of the orientation components of {001}<110>, {116}<110>, {114}<110> and {112}<110> on the plane at the center of the wall thickness of the mother pipe before diameter reduction to the random X-ray intensity exceeds 3 to a temperature in the range from (Ac3−50)° C. or higher, to 1,200° C. or lower and by applying working under a condition of a diameter reduction ratio of 30% or more and a wall thickness reduction ratio of 5% or more to 30% or less, so that the steel pipe has an r-value of 1.4 or larger in the axial direction of the steel pipe and the property that the average of the ratios of the X-ray intensity in the orientation component group of {110}<110> to {332}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 3.5 or larger, and/or the ratios of the X-ray intensity in the orientation component of {110}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 5.0 or larger.

A method to produce a steel pipe, excellent in formability, having a chemical composition comprising, in mass,

• 0.0001 to 0.50% of C,
• 0.001 to 2.5% of Si,
• 0.01 to 3.0% of Mn,
• 0.001 to 0.2% of P,
• 0.05% or less of S,
• 0.01% or less of N,
• 0.01 to 2.5% of Al and
• 0.01% or less of O
• in a manner to satisfy the expressions (1) and (2) below, with the balance consisting of Fe and unavoidable impurities, characterized by heating the mother pipe to 850° C. or higher at diameter reduction, applying the diameter reduction under a diameter reduction ratio of 20% or more in the temperature range from below the Ar3 transformation temperature to 750° C. or higher and completing the diameter reduction at 750° C. or higher;
• so that the relationship between the tensile strength (TS) and the n-value of the steel pipe satisfies the expression (3) below, the volume percentage of its ferrite phase is 75% or more, the average grain size of the ferrite is 10 μm or more, and the crystal grains of the ferrite having an aspect ratio of 0.5 to 3.0 account for, in area percentage, 90% or more of all the crystal grains composing the ferrite,
• (203{square root over ( )}C+15.2Ni−44.7Si−104V−31.5Mo+30Mn+11Cr+20Cu−700P−200Al)<−20 (1)
• (44.7Si+700P+200Al)>80 (2)
• n≧−0.126×ln(TS)+0.94 (3).

A method to produce a steel pipe, excellent in formability, having a chemical composition comprising, in mass,

• 0.0001 to 0.50% of C,
• 0.001 to 2.5% of Si,
• 0.01 to 3.0% of Mn,
• 0.001 to 0.2% of P,
• 0.05% or less of S,
• 0.01% or less of N,
• 0.2% or less of Ti and
• 0.15% or less of Nb
• in a manner to satisfy the expression 0.5≦(Mn+13Ti+29Nb)≦5, with the balance consisting of Fe and unavoidable impurities, characterized by heating the mother pipe to a temperature of the Ac3 transformation temperature or higher at diameter reduction, applying the diameter reduction under a diameter reduction ratio of 40% or more in the temperature range of the Ar3 transformation temperature or higher, completing the diameter reduction at a temperature equal to or higher than the Ar3 transformation temperature, commencing cooling within 5 sec. after completing the diameter reduction, and cooling the diameter-reduced steel pipe to a temperature of (Ar3−100)° C. or lower at a cooling rate of 5° C./sec. or more, so that the steel pipe has the property that the ratio of the X-ray intensity in the orientation component of {111}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 5.0 or larger and the ratio of the X-ray intensity in the orientation component of {111}<112> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is below 2.0.

A method to produce a steel pipe, excellent in formability, having a chemical composition comprising, in mass,

• 0.0001 to 0.50% of C,
• 0.001 to 2.5% of Si,
• 0.01 to 3.0% of Mn,
• 0.001 to 0.2% of P,
• 0.05% or less of S,
• 0.01% or less of N,
• 0.2% or less of Ti and
• 0.15% or less of Nb
• in a manner to satisfy the expression 0.5≦(Mn+13Ti+29Nb)≦5, with the balance consisting of Fe and unavoidable impurities, characterized by heating the mother pipe to a temperature of the Ac3 transformation temperature or higher at diameter reduction, applying the diameter reduction under a diameter reduction ratio of 40% or more in the temperature range of the Ar3 transformation temperature or higher, subsequently applying another step of the diameter reduction under a diameter reduction ratio of 10% or more in the temperature range from Ar3 to (Ar3−100)° C., and completing the diameter reduction at a temperature in the range from Ar3 to (Ar3−100)° C., so that the steel pipe has the property that the ratio of the X-ray intensity in the orientation component of {111}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 5.0 or larger and the ratio of the X-ray intensity in the orientation component of {111}<112> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is below 2.0.