PROCESS FOR PRODUCING STEEL PIPE HAVING HIGH DUCTILITY AND STRENGTH

摘要

A steel product having a structure composed mainly of ferrite or ferrite plus pearlite or ferrite plus cementite. A steel pipe produced from this steel product by rolling at a ferrite recrystallization temperature such that the reduction of area is greater than 20%. This steel pipe is characterized by grain size not greater than 3 mu m, preferably not greater than 1 mu m, elongation greater than 20%, tensile strength (TS : MPa) and elongation (El : %) whose product is greater than 10000, and percent ductile fracture greater than 95%, preferably 100%, measured by Charpy impact test on an actual pipe at -100 DEG C. The structure is characterized by C : 0.005-0.03%, Si : 0.01-3.0%, Mn : 0.01-2.0%, and Al : 0.001-0.10% on a weight basis, and is composed of ferrite or ferrite and a secondary phase, with ferrite grains being not greater than 3 mu m and the secondary phase having an areal ratio not more than 30%. The steel pipe is produced from a steel pipe stock having the above-mentioned composition by heating at a temperature of (Ac1 + 50 DEG C) to 400 DEG C and subsequently performing reducing on it at a rolling temperature of (Ac1 + 50 DEG C) to 400 DEG C such that the cumulative reduction of diameter is greater than 20%. The reducing is preferably performed such that at least one of rolling passes reduces the diameter by more than 6% per pass. The steel pipe will have high ductility and high strength and will be superior in toughness and stress corrosion cracking resistance, if the content of C, Si, Mn, and other alloying elements is limited low and reducing is performed at the temperature specified above. The resulting steel pipe has good fatigue resistance and is suitable for use as line pipe. <IMAGE>