HIGH STRENGTH SHEET STEEL HAVING LOW RATIO OF YIELD STRENGTH TO TENSILE STRENGTH, EXCELLENT AS OF RESISTANCE TO POST-DEFORMATION AGING, METHOD OF ITS PRODUCTION AND HIGH STRENGTH WELDED STEEL PIPE MADE THEREOF Russian patent published in 2017 - IPC C22C38/14 C21D8/02 B23K9/23 

FIELD: metallurgy.

SUBSTANCE: steel has a chemical composition containing in wt %: C: 0.03-0.08, Si: 0.01-1.0, Mn: 1.2-3.0, P: 0.015 or less, S: 0.005 or less, Al: 0.08 or less, Nb: 0.005-0.07, Ti: 0.005-0.025, N: 0.010 or less, O: 0.005 or less, Fe and unavoidable impurities are the rest. The steel has a metallographic structure including the bainite phase, island martensite and polygonal ferrite phase in the surface areas within 5 mm from the top and the bottom surfaces. The share of the island martensite area is 3-15%, and the equivalent diameter of the island martensite circle is 3.0 mcm or less. The share of the polygonal ferrite phase area in the surface areas is 10-80%. Steel has a change in Vickers hardness in the thickness direction of ΔHV30 or less, a change in Vickers hardness in the width direction of ΔHV30 or less, maximum in Vickers hardness in the steel sheet surface areas of HV230 or less, yield strength to strength limit ratio 85% or less, and elongation 22% or more with a full thickness tensile test.

EFFECT: steel has high strength and resistance to post-deformation aging.

4 cl, 1 dwg, 4 tbl, 1 ex