FIELD: metallurgy.
SUBSTANCE: invention relates to metallurgy. Cold-rolled and heat-treated steel sheet has chemical composition containing in wt.%: 0.17 ≤ carbon ≤ 0.25, 2 ≤ manganese ≤ 3, 0.9 ≤ silicon ≤ 2, aluminum ≤ 0.09, 0.01 ≤ molybdenum ≤ 0.2, phosphorus ≤ 0.02, sulfur ≤ 0.03, nitrogen ≤ 0.09, and optionally one or more of the elements: 0 ≤ chrome ≤ 0.3, 0 ≤ niobium ≤ 0.06, 0 ≤ titanium ≤ 0.06, 0 ≤ vanadium ≤ 0.1, 0 ≤ calcium ≤ 0.005, 0 ≤ boron ≤ 0.010, 0 ≤ magnesium ≤ 0.05, 0 ≤ zirconium ≤ 0.05, 0 ≤ cerium ≤ 0.1, and the rest includes iron and unavoidable impurities. Sheet has microstructure containing 50-75% of bainite, 10–30% of residual austenite, 15–40% of distributed martensite, 0–10% of ferrite and 0–5% of fresh martensite in surface fractions, as well as a ferrite-rich layer extending at a depth of up to 50 mcm from both surfaces of said steel sheet, wherein said ferrite-rich layer has average ferrite content of 55–80% in surface fractions.
EFFECT: sheet has tensile strength greater than or equal to 1170 MPa, expansion factor of the hole greater than or greater than 30%, as well as high resistance to liquid metal embrittlement.
17 cl, 2 dwg, 4 tbl, 7 ex
