Cold rolled, annealed and tempered steel sheet and method of manufacturing the same

Abstract

A cold rolled, annealed and partitioned steel sheet, made of a steel having a composition including, by weight percent: C: 0.05 - 0.18%, Mn: 6.0 - 11.0%, Mo: 0.05 - 0.5%, B: 0.0005 - 0.005%, S ≤ 0.010%, P ≤ 0.020%, N ≤ 0.008%, and including optionally one or more of the following elements, in weight percentage: Al < 3%, Si ≤ 1.20%, Ti ≤ 0.050%, Nb ≤ 0.050%, Cr ≤ 0.5%, V ≤ 0.2%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure including, in surface fraction, from 0% to 30% of ferrite, such ferrite, when present, having a grain size below 1.0 µm, from 8% to 40% of retained austenite, the fraction of austenite islands with a size above 0.5 µm being below or equal to 5% from 30 to 92% of partitioned martensite less than 3% of fresh martensite, a carbon [C]A and manganese [Mn]A content in austenite, expressed in weight percent, such that the ratio ([C]A2 x [Mn]A) / (C%2 x Mn%) is below 18.0, C% and Mn% being the nominal values in carbon and manganese in weight %.

Description

Claims

1-13. (canceled)

14. A cold rolled, annealed and partitioned steel sheet, made of a steel having a composition comprising, by weight percent: C: 0.05 - 0.18%Mn:6.0 - 11.0%Mo: 0.05 - 0.5%B: 0.0005 - 0.005%S ≤ 0.010%P ≤ 0.020%N ≤ 0.008%

15. The steel sheet as recited in claim 14 wherein the carbon content is from 0.08% to 0.15%.

16. The steel sheet as recited in claim 14 wherein the manganese content is from 6.0% to 9%.

17. The steel sheet as recited in claim 14 wherein the aluminium content is from 0.2% to 2.2%.

18. The steel sheet as recited in claim 14 wherein the microstructure comprises from 5% to 25% of ferrite, from 15% to 30% of retained austenite and from 45% to 80% of partitioned martensite.

19. The steel sheet as recited in claim 14 wherein the microstructure comprises no ferrite, from 20% to 30% of retained austenite and from 70% to 80% of partitioned martensite.

20. The steel sheet as recited in claim 14 wherein the tensile strength is above or equal to 1270 MPa, the uniform elongation UE is above or equal to 10.0% the total elongation TE is above or equal to 14.0% and wherein TS, TE and the carbon and silicon contents satisfy the following equation: (TSxTE) /(C%+Si%/4) > 50 000 MPa.% wherein C% and Si% refer to the nominal wt% in C and Si of the steel.

21. The steel sheet as recited in claim 14 wherein the hole expansion ratio is above or equal to 15%.

22. The steel sheet as recited in claim 14 wherein the yield strength YS is above or equal to 1000 MPa.

23. The steel sheet as recited in claim 14 wherein the LME index is below 0.36.

24. The steel sheet as recited in claim 14 wherein the steel has a carbon equivalent Ceq lower than 0.4%, the carbon equivalent being defined as Ceq = C% + Si%/55 + Cr%/20 + Mn%/19 -Al%/18 + 2.2P% - 3.24B% -0.133×Mn%×Mo% with elements being expressed by weight percent.

25. A resistance spot weld of two steel parts made of the cold rolled, annealed and partitioned steel sheet as recited in claim 14, the resistance spot weld having an α value of at least 30 daN/mm2.

26. A press hardened and partitioned steel part having a composition comprising, by weight percent: C: 0.05 - 0.18%Mn: 6.0 - 11.0%Mo: 0.05 - 0.5%B: 0.0005 - 0.005%S ≤ 0.010%P ≤ 0.020%N ≤ 0.008%and optionally one or more of the following elements: Al: < 3%Si ≤ 1.20%Ti ≤ 0.050%Nb ≤ 0.050%Cr ≤ 0.5%V < 0.2%a remainder of the composition being iron and unavoidable impurities resulting from processing, and havinga microstructure comprising, in surface fraction, from 0% to 30% of ferrite, such ferrite, when present, having a grain size below 1.0 µm, from 8% to 40% of retained austenite, the fraction of austenite islands with a size above 0.5 µm being below or equal to 5%, from 30 to 92% of partitioned martensite, less than 3% of fresh martensite, and a carbon [C]A and manganese [Mn]A content in austenite, expressed in weight percent, such that the ratio ([C]A2 x [Mn]A) /(C%2 x Mn%) is below 18.0, C% and Mn% being the nominal values in carbon and manganese in weight %.