HIGH-STRENGTH HOT-DIP GALVANISED STEEL SHEET HAVING EXCELLENT RESISTANCE TO DELAYED FRACTURE, AND METHOD FOR PRODUCTION THEREOF Russian patent published in 2016 - IPC C21D8/02 C21D9/46 C22C38/06 C22C38/58 C23C2/06 C23C2/28 B21B3/00 

FIELD: metallurgy.

SUBSTANCE: invention relates to a hot-rolled galvanised sheets. High-strength galvanised steel sheet comprises a sheet of steel containing, in wt%: C from 0.075 to 0.400, Si from 0.01 to 2.00, Mn from 0.80-3.50, P from 0.0001 to 0.100, S from 0.0001 to 0.0100, Al from 0.001 to 2.00, O from 0.0001 to 0.0100, N from 0.0001 to 0.0100, iron and inevitable admixtures - balance, and hot layer formed on steel sheet surface. In range of 1/8 to 3/8 thickness of steel sheet, centre of which is 1/4 thickness of steel sheet from surface of steel sheet, steel sheet structure comprises from 40 vol% to 90 vol% ferrite phase, residual austenite phase is 5 vol% or less, and ratio of non-recrystallised ferrite to entire ferrite phase is 50 vol% or less. Ratio of diameters of crystal grains in ferrite phase in said steel sheet is from 0.75 to 1.33. Said ratio of diameters of crystal grains is defined as ratio of average grain diameter in direction of said rolling phase to average diameter of said grain phase in sheet width direction, and ratio of lengths of solid structures in form of islands dispersed in ferrite phase is between 0.75 to 1.33. Said ratio of lengths of solid structures defined as average length of said structures in rolling direction to length of said secondary structures in sheet width direction, and further average aspect ratio of inclusions contained in steel sheet is from 1.0 to 5.0. Surface layer of steel sheet is in form of a decarburised layer having a thickness of 0.01 to 10.0 mcm, with an average grain diameter in decarburised oxide layer is 30 to 500 nm, and average density in decarburised oxide layer is in range of 10×10¹² oxides/m² to 10×10¹⁶ oxides/m².

EFFECT: technical result is to increase resistance to delayed fracture sheet.

11 cl, 21 tbl