Method for making steel tubes or pipes of increased acidic gas resistance

Method of making welded steel pipes which can be used for conveying acidic gas and/or oils, with compressive stresses present on the inner side facing the acidic gas and/or oil, characterized by the combination of the following method steps:

• a. provided and thermomechanically rolling a steel having a composition of
• 0.02 to 0.20% C.
• 0.10 to 0.60% Si
• 0.60 to 1.50% Mn
• max. 0.02% P
• max. 0.005% S
• 0.01 to 0.16% Al
• 0.001 to 0.01% Ca,
• The ratio Ca:
• S being greater than 2.25 and the product Ca.times.S equal to or smaller than 0.001, and depending on the required strength characteristics of the finished steel pipe, at least one alloy elements of the following group:
• max. 0.35% CR
• max. 1.0% Mo
• max. 0.03% B
• max. 0.70% Ni or Cu and Ni
• max. 0.15% V and/or
• max. 0.15% Nb,
• the balance iron and unavoidable impurities, forming a strip with pearlitic-ferritic and/or bainitic structure:
• b. forming a pipe out of this strip with a ratio of wall thickness to diameter of 1 to 25 to 1 to 160 and welding the strip edges together;
• c. and heating the outer side of the welded pipe remote from the acidic oil and/or gas or the weld seam region in max. 400 mm width to a temperature of 300.degree. to 680 degrees C., higher by at least 100 degrees C. than the temperature of the inner side and thereafter cooling from the inside with water or air in an amount of 1 to 2000 liters per meter tube length, with a tube feed in the seam direction during the heating and subsequent cooling of 0.4 to 30 m/min.