Cold rolled and annealed 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: 0.03-0.25%

Mn: 3.5-8%

Si: 0.1-2.0%

Al: 0.03-2.0%

Ti≤0.080%

Nb≤0.080%

V≤0.2%

V+Ti+Nb>0.01%

S≤0.010%

P≤0.020%

N≤0.008%

and optionally including at least one of the following elements, in weight percentage:

Mo: 0.1-0.5%

Cr: 0.01-1%

B: 0.0005-0.004%

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

the cold-rolled and heat-treated steel sheet having a microstructure consisting of, in surface fraction:

between 10% and 30% of retained austenite, the retained austenite being present as austenite films having an aspect ratio of at least 3 and as Martensite Austenite islands, less than 8% of such Martensite Austenite islands having a size above 0.5 μm,

at most 10% of fresh martensite, and

balance with recovered martensite containing precipitates of at least one element chosen from the group consisting of: niobium, titanium and vanadium.

2. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein aluminium content is 0.03%-1.2% by weight percent.

3. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein niobium content is 0.010%≤Nb≤0.080% by weight percent.

4. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein carbon content is at least 0.10%-0.25% by weight percent.

5. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein a fraction ratio between the Martensite Austenite islands having the size above 0.5 μm and the austenite films is below 1.0.

6. The cold-rolled and heat-treated steel sheet as recited in claim 5 wherein the fraction ratio is below 0.5.

7. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the surface fraction of the fresh martensite is below 5%.

8. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein less than 5% of the Martensite Austenite islands have the size above 0.5 μm.

9. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein said aspect ratio is at least 8%.

10. 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.

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 Al or an Al alloy.

12. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the cold-rolled and heat-treated steel sheet has a yield strength YS of at least 700 MPa, a tensile strength TS of at least 900 MPa, a uniform elongation UE of at least 12%, and a hole expansion ratio HER of at least 20%.

13. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein from 2% to less than 8% of the Martensite Austenite islands have the size above 0.5 μm.

14. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the surface fraction of the fresh martensite is from 1% to below 10%.

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

casting a steel so as to obtain a slab, the steel having a composition comprising by weight percent:

C: 0.03-0.25%

Mn: 3.5-8%

Si: 0.1-2.0%

Al: 0.03-2.0%

Ti≤0.080%

Nb≤0.080%

V≤0.2%

V+Ti+Nb>0.01%

S≤0.010%

P≤0.020%

N≤0.008%

and optionally including at least one of the following elements, in weight percentage:

Mo: 0.1-0.5%

Cr: 0.01-1%

B: 0.0005-0.004%

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

reheating the slab at a temperature T reheat 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 20° C. and 600° C.;

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

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

reheating the cold-rolled steel sheet to a first annealing temperature above 860° C. and maintaining the cold-rolled steel sheet at the first annealing temperature for a holding time between 30 s and 600 s, so as to obtain, upon annealing, a fully austenitic structure;

quenching the cold-rolled steel sheet at a cooling rate between 0.5° C./s and 200° C./s, to a quenching temperature between 20° C. and Ms-50° C. and maintaining the cold-rolled steel sheet at the quenching temperature for a holding time between 1 and 200 s;

reheating the cold-rolled steel sheet to a second annealing temperature between TO and 720° C., TΘ designating the temperature above which cementite is dissolved upon heating, and maintaining the cold-rolled steel sheet at the second annealing temperature for a time between 100 s and 2000 s;

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

16. The method as recited in claim 15 wherein the annealing of the hot-rolled steel sheet is a batch annealing, performed at a temperature 500° C. and 680° C. for 1000 s to 50000 s.

17. The method as recited in claim 15 wherein the reheating rate of the cold rolled steel sheet to the first annealing temperature is between 1 and 200° C./s.

18. A method for producing a spot welded joint of at least two steel sheets, comprising the steps of:

providing the cold-rolled and heat-treated steel sheet manufactured according to the method as recited in claim 15 ;

providing a second steel sheet;

spot welding the cold-rolled and heat-treated steel sheet to the second steel sheet.

19. The method as recited in claim 18 wherein the second steel sheet is a cold-rolled and heat-treated steel sheet manufactured according to the method as recited in claim 15 .

20. A method for producing a spot welded joint of at least two steel sheets, comprising the steps of:

providing the cold-rolled and heat-treated steel sheet as recited in claim 1 ;

providing a second steel sheet;

spot welding the cold-rolled and heat-treated steel sheet to the second steel sheet.

21. The method as recited in claim 20 wherein the second steel sheet is a cold-rolled and heat-treated steel sheet as recited in claim 1 .