Cold rolled and heat-treated steel sheet and method of manufacturing the same

1 . A cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent:

C: 0.3-0.4%

Mn: 2.0-2.6%

0.8≤Si<1.5%

Al: 0.01-0.6%

Mo: 0.15-0.5%

Cr: 0.3-1.0%

0.0010%≤Nb≤0.06%

0.0010%≤Ti≤0.06%

Ni≤0.8%

S≤0.010%

P≤0.020%

N≤0.008%

Cu≤0.03%

and optionally one or more of the following elements, in weight percentage:

B: 0.0003-0.005%

V≤0.2%

a remainder of the composition being iron and unavoidable impurities resulting from processing,

the steel sheet having a microstructure consisting of, in surface fraction:

between 15% and 30% of retained austenite, said retained austenite having a carbon content of at least 0.7%;

between 70% and 85% of tempered martensite;

at most 5% of fresh martensite; and

at most 5% of bainite;

wherein the cold-rolled and heat-treated steel sheet has a yield strength YS of at least 1100 MPa, a tensile strength TS of at least 1470 MPa, a total elongation TE of at least 13%, a hole expansion ratio HER of at least 15% and an LME index of less than 0.70.

2 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the chromium content is between 0.6% and 0.8%.

3 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the silicon content is below 1.4%.

4 . The cold-rolled and heat-treated steel sheet as recited in claim 3 wherein the silicon content is below 1.3%.

5 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the cumulated amount of silicon and aluminum is equal to or above 1.6%.

6 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the aluminum content is between 0.2% and 0.5%.

7 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the molybdenum content is between 0.20% and 0.40%.

8 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes at most 2% of fresh martensite.

9 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes at most 2% of bainite.

10 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes no bainite and no fresh martensite.

11 . The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the cold-rolled and heat-treated steel sheet is coated with Zn or a Zn alloy or with Al or an Al alloy.

12 . A method for manufacturing a cold-rolled and heat-treated steel sheet, comprising the following successive steps:

casting a steel to obtain a slab, said steel having a composition comprising, by weight percent:

C: 0.3-0.4%

Mn: 2.0-2.6%

0.8≤Si≤1.5%

Al: 0.01-0.6%

Mo: 0.15-0.5%

Cr: 0.3-1.0%

0.0010%≤Nb≤0.06%

0.0010%≤Ti≤0.06%

Ni≤0.8%

S≤0.010%

P≤0.020%

N≤0.008%

Cu≤0.03%

and optionally one or more of the following elements, in weight percentage:

B: 0.0003-0.005%

V≤0.2%

a remainder of the composition being iron and unavoidable impurities resulting from processing;

reheating the slab at a temperature T reheat comprised between 1150° C. and 1300° C.,

hot rolling the reheated slab at a temperature higher than Ar3 to obtain a hot rolled steel sheet;

coiling the hot rolled steel sheet at a coiling temperature T coil between 200° C. and 700° C.;

optionally pickling said hot rolled steel sheet;

optionally annealing the hot rolled steel sheet, to obtain a hot-rolled and annealed steel sheet;

optionally pickling said hot-rolled and annealed steel sheet;

cold rolling the hot-rolled and annealed steel sheet to obtain a cold rolled steel sheet;

reheating the cold-rolled steel sheet to an annealing temperature between Ac3 and Ac3+100° C. and maintaining the cold-rolled steel sheet at the annealing temperature for a holding time between 30 s and 600 s, to obtain, upon annealing, a fully austenitic structure;

quenching the cold-rolled steel sheet at a cooling rate comprised between 0.1° C./s and 200° C./s, to a quenching temperature Tq between (Ms-140° C.) and (Ms-75° C.) and optionally maintaining it at Tq for a holding time between 1 and 200 s;

reheating the cold-rolled steel sheet to a partitioning temperature between 350° C. and 500° C., and maintaining the cold-rolled steel sheet at the partitioning temperature for a partitioning time between 30 s and 2000 s; and

cooling the cold-rolled steel and heat-treated sheet to room temperature to obtain the cold-rolled steel and heat-treated sheet as recited in claim 1 .

13 . The method as recited in claim 12 wherein the coiling temperature T coil is between 450° C. and 650° C.

14 . The method as recited in claim 12 wherein the hot rolled steel sheet after coiling comprises a grain boundary oxidation layer having a maximum thickness of 5 μm.

15 . The method as recited in claim 12 wherein the hot rolled steel sheet is annealed at a temperature between 500 and 800° C., during 1000 s to 108000 s.