Steel pipe with good magnetic properties and method of producing the same
A steel pipe blank having a composition containing 0.5% or less C and 85% or more Fe in terms of mass percent is heated, and stretch-reducing is then performed so that the diameter decrease ratio is 15% or more and the rolling finishing temperature is (the Ar 3 transformation point−10)° C. or lower. Consequently, a structure in which the ratio of X-ray diffraction intensity obtained from the plane in which the <100> direction of crystal grains is preferentially oriented parallel to the circumference direction and the <011> direction of crystal grains is preferentially oriented parallel to the rolling direction of the steel pipe to that obtained for a three-dimensionally randomly oriented sample is 3.0 or more is formed, and the r-value is increased, thereby improving the magnetic properties of the steel pipe.
1. A steel pipe with good magnetic properties comprising a composition containing less than 0.01% C and 95% or more Fe in terms of mass percent, has a structure in which a ratio of X-ray diffraction intensity from 1) a plane in which the <100> direction of crystal grains is mostly oriented parallel to a circumference direction and the <011> direction of crystal grains is mostly oriented parallel to a rolling direction of the steel pipe to 2) a three-dimensionally randomly oriented sample is 3.0 or more, and has a maximum relative permeability of 2500 or more and a magnetic flux density under a magnetic field condition with a magnetizing force of 200 A/m of 0.8 T or more.
2. The steel pipe according to claim 1 , having an r-value in the rolling direction of 2.0 or more.
3. The steel pipe according to claim 1 , wherein the structure has an average crystal grain size of 20 μm or more.
4. The steel pipe according to claim 1 , wherein the composition comprises less than 0.01% C, 0.45% or less Si, 0.1% to 1.4% Mn, about 0.01% or less S, 0.025% or less P, 0.01% to 0.06% Al, and 0.005% or less N in terms of mass percent, the balance being Fe, and inevitable impurities.
5. The steel pipe according to claim 1 , wherein the composition comprises less that 0.01% C, more than 0.45% and 3.5% or less Si, 0.1% to 1.4% Mn, 0.01% or less S, 0.025% or less P, more than 0.06% and 0.5% or less Al, and 0.005% or less N in terms of mass percent, the balance being Fe, and inevitable impurities.
6. The steel pipe according to claim 4 , wherein the composition further comprises at least one group selected from the following Groups D to F in terms of mass percent:
Group D: at least one selected from 0.05% or less Ti, 0.05% or less Nb, and 0.005% or less B;
Group E: at least one selected from 5% or less Cr, 5% or less Ni, and 0.05% or less Mo; and
Group F: at least one of 0.005% or less Ca and 0.05% or less REM.