STEEL PIPE HAVING EXCELLENT ELECTROMAGNETIC PROPERTIES AND PROCESS FOR PRODUCING THE SAME

摘要

A steel pipe with good magnetic properties and a method of producing the same are proposed. Specific solutions are as follows. 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. Furthermore, when annealing is performed at a temperature in the range of 550°C to the Ac 1 transformation point after the stretch-reducing, the crystal grain size is coarsened to further improve the magnetic properties. Cold drawing may be performed prior to the annealing. When a steel pipe having a high-purity composition containing less than 0.01% C and 95% or more Fe is used as the steel pipe blank, the magnetic properties are further improved. In order to further improve the magnetic properties, appropriate amounts of Si and Al are preferably contained. In addition, an appropriate content of Cr improves the magnetic properties in the high-frequency range.