Cold rolled and heat-treated steel sheet and method of manufacturing thereof

1 . A cold-rolled and heat-treated steel sheet, the steel of the steel sheet having a composition comprising, in weight percentage:

0.17%≤carbon≤0.25%,

2%≤manganese≤3%,

0.9%≤silicon≤2%,

0%≤aluminum≤0.09%,

0.01%≤molybdenum≤0.2%,

0%≤phosphorus≤0.02%,

0%≤sulfur≤0.03%,

0%≤nitrogen≤0.09%,

and optionally one or more of the following elements:

0%≤chromium≤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%,

a balance including iron and unavoidable impurities,

the steel sheet having a microstructure comprising 50% to 80% of bainite, 10% to 30% of residual austenite, 15% to 50% of partitioned martensite, 0% to 10% of ferrite and 0% to 5% fresh martensite in area fractions, and a ferrite-enriched layer extending up to 50 microns from both surfaces of the steel sheet, the ferrite-enriched layer having a mean ferrite content from 55% to 80% in area fraction.

2 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the composition includes 2.2% to 2.9% of manganese.

3 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the composition includes 0.18% to 0.23% of carbon.

4 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the composition includes 1% to 1.9% of silicon.

5 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the composition includes 0.05% to 0.15% of molybdenum.

6 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the microstructure contains 55% to 75% of bainite.

7 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the microstructure contains residual 12% to 25% of residual austenite.

8 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the microstructure contains 15% to 45% of partitioned martensite.

9 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the steel sheet has a tensile strength greater than or equal to 1170 MPa, and a hole expansion ratio of 30% or more.

10 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the steels sheet has a yield strength greater than or equal to 780 MPa, and a total elongation of 12.0% or more.

11 . The cold-rolled and heat-treated steel sheet as recited in claim 1 , wherein the ferrite-enriched layer up to 50 microns from both surfaces contains 60% to 78% of ferrite in area fraction.

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

providing a semi-finished product with a steel composition comprising, in weight percentage:

0.1%≤carbon≤0.25%,

2%≤manganese≤3%,

0.9%≤silicon≤2%,

0%≤aluminum≤0.09%,

0.01%≤molybdenum≤0.2%,

0%≤phosphorus≤0.02%,

0%≤sulfur≤0.03%,

0%≤nitrogen≤0.09%,

and optionally one or more of the following elements:

0%≤chromium≤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%,

a balance including iron and unavoidable impurities,

reheating the semi-finished product to a temperature from 1000° C. to 1280° C.;

rolling the semi-finished product completely in the austenitic range wherein the hot rolling finishing temperature is greater than or equal to 850° C. to obtain a hot rolled steel sheet;

cooling the hot rolled steel sheet at a cooling rate above 30° C./s to a temperature below or equal to 550° C.; and coiling the hot rolled steel sheet and keeping the temperature of coiled sheet below 500° C.;

cooling the hot rolled steel sheet;

cold rolling the hot rolled steel sheet with a reduction rate from 35 to 70% to obtain a cold rolled steel sheet;

annealing the cold rolled steel sheet in two steps heating during which the dew point is controlled from −15° C. to +15° C. and wherein:

the first step starts from heating the steel sheet from room temperature to a temperature HT1 from 600° C. to 800° C., with a heating rate HR1 from 2° C./s to 70° C./s,

the second step starts from heating further the steel sheet from HT1 to a soaking temperature TA from Ac3-10° C. and Ac3+100° C., with a heating rate HR2 from 0.1° C./s and 10° C./s or less, HR2 being lower than HR1, then

annealing at TA during 10 to 500 seconds, time being selected to obtain a minimum percentage of 90% austenite, the dew point being controlled from −10° C. to +10° C. during the annealing; then

cooling the cold rolled steel sheet from TA to cooling stop temperature CS1 from Ms−5° C. to Ms−100° C. with a cooling rate CR1 greater than 30° C./s; then

heating the cold rolled steel sheet from CS1 temperature to an overaging temperature TOA from 250° C. to 580° C. at an average heating rate HR3 from 1° C./s to 100° C./s; then

overaging the cold rolled steel sheet at TOA during 5 to 500 seconds to obtain the cold-rolled and heat-treated steel sheet as recited in claim 1 .

13 . The method as recited in claim 12 , wherein the HT1 temperature is from 625° C. to 775° C.

14 . The method as recited in claim 12 , further comprising coating the cold rolled steel sheet with zinc or a zinc-based alloy.

15 . The method as recited in claim 12 , further comprising performing a scale removal process on the hot rolled steel sheet.

16 . The method as recited in claim 12 , further comprising subjecting the hot rolled steel sheet to an annealing at a temperature from 350° C. to 750° C. for a duration of 1 h to 96 h.

17 . The method as recited in claim 16 , further comprising performing a scale removal process on the hot rolled annealed steel sheet.

18 . A method comprising employing the cold rolled steel sheet produced according to the method of claim 12 , for manufacture of a structural or safety part of a vehicle.

19 . A method comprising employing the cold-rolled and heat-treated steel sheet as recited in claim 1 , for manufacture of a structural or safety part of a vehicle.