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 chemical composition comprising, 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 - the rest. The steel has a metallographic two-phase structure consisting of a phase of bainite and island martensite. The portion of the island martensite phase area is 3-15%, and the size of the island martensite grains in the form of an equivalent circle diameter is 3.0 mcm or less. 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