FIELD: metallurgy.
SUBSTANCE: in process of steel production it is preliminary deoxidised with Si and Mn, further there is introduced Ti for obtaining composition of steel containing wt %: C from 0.010 to 0.050, Si from 0.01 to 0.50, Mn from 0.50 to 2.00, S from 0.0001 to 0.0050, Ti from 0.003 to 0.030, O from 0.0001 to 0.0080, B from 0.0003 to 0.0030, P 0.050 or less, Al 0.020 or less, Mo to less, than 0.10, when required, one or more from Cu from 0.05 to 1.50, Ni from 0.05 to 5.00, Cr from 0.02 to 1.50, V from 0.010 to 0.100, Nb from 0.001 to 0.200, Zr from 0.0001 to 0.0500, Ta from 0.0001 to 0.0500, Mg from 0.0001 to 0.0100, Ca from 0.0001 to 0.0050, REM 0.0001 to 0.0050, Y from 0.0001 to 0.0050, Hf from 0.0001 to 0.0050, Re from 0.0001 to 0.0050, W from 0.01 to 0.50, iron and unavoidable impurities - the rest. Produced steel is cast into a slab which is heated to temperature 1000°C or higher. The slab is hot rolled to sheet at relative reduction 2.5 or more in the region of temperatures preceding crystallisation corresponding to 900°C or lower. Further, there is performed water cooling stopped at temperature 600°C or lower. The produces sheet is formed into a pipe and butt sections are seam welded on internal and external surfaces. structure of the steel sheet consists of polygonal ferrite occupying 20 % of area or less and bainite occupying 80 % of area or more. Also, actual dimension of a grain in basic material of steel amounts to 20 mcm or less, while actual dimension of a grain in a zone of thermal influence at welding is 150 mcm or less.
EFFECT: high strength and excellent low temperature ductility of steel.
13 cl, 1 dwg, 4 tbl, 1 ex
