STEEL SHEETS FOR ULTRAHIGH-STRENGTH HEADER PIPES AND ULTRAHIGH-STRENGTH HEADER PIPES POSSESSING EXCELLENT LOW TEMPERATURE IMPACT RESISTANCE AND METHODS OF THEIR FABRICATION

摘要

FIELD: metallurgy.;SUBSTANCE: invention refers to metallurgy, particularly to production of steel sheet for fabricating of ultrahigh-strength header pipes possessing excellent low temperature impact resistance. The technical result of the invention consists in provision of tangential strength of not lower than 900 MPa, avoiding at the same time increasing of axial tension strength of pipes fabricated by means of seam welding of sheet edges. To achieve this technical result sheets are fabricated out of steel, containing in mass.%: C from 0.03 to 0.07; Si not more, than 0.6; Mn from 1.5 to 2.5; P not more than 0.015; S not more than 0.003; Ni from 0.1 to 1.5; Mo from 0.15 to 0.60; Nb from 0.01 to 0.10; Ti from 0.005 to 0.030; Al not more than 0.06, one or more elements out of the group: B, N, V, Cu, Cr, Ca, REM (rare earth metals) and Mg at required quantities, the remaining percentage is iron and unavoidable impurities; the sheet has a ratio (Hv-ave_p)/(Hv-M*): medium hardness Hv-ave_p according to Vickers in the direction of thickness to martensite hardness- Hv-M*, determined by carbon contents constituing from 0.8 to 0.9, tensile strength from 880 to 1080 MPa, a hardenability index P within the range from 2.5 to 4.0; P and Hv-M* are calculated from the expression, correspondingly: P=2,7C+0,4Si+Mn+0,8Cr+0,45(Ni+Cu)+(1+)Mo-1+, where (=1, when B3 h/mln and (=0, when B3 h/mln), Hv-M* =270+1300C, where C-carbon, mass.%. The method of sheet fabricating includes production of slab, heating of slab to 1000-1250C, roughing within the temperature of recrystallisation, rolling within the non-recrystallising austenite range at 900C or lower at a summary degree of rolling crimping of not less than 75%, and then it is cooled from the austenite stage at a ratio of 1 to 10C/sec up to obtaining the temperature of 500C or lower in the centre of sheet thickness. A header pipe is produced by means of molding of a steel sheet into a tubular form at coinciding of steel sheet rolling direction with longitudinal direction of a pipe, and then welding of pipe edges is performed.;EFFECT: provision of upgraded tangential strength, avoiding increasing of axial tensile strength, performed by means of seam welding of sheet edges.;35 cl, 3 tbl, 3 dwg, 11 ex