Tubular metal body, method of producing same, liner for pressure vessel and method of producing same

b a a A tubular metal body comprises a tube extruded through a porthole die and composed of a plurality of components joined to one another with a plurality of joint portions extending over the entire length of the tube. The base material metal of the extruded tube in each of the joint portions is subjected to a modifying treatment to produce finely divided crystal grains. The modifying treatment for the extruded tube is conducted preferably by frictionally agitating each joint portion using a probe of a friction agitation joining tool . The tubular metal body is available with an increased length in a larger size and has high pressure resistance.