Cold rolled steel sheet and a method of manufacturing thereof

1. A cold rolled heat treated steel sheet having a composition comprising the following elements, expressed in percentage by weight:

0.1%≤Carbon≤0.5%

1%≤Manganese≤3.4%

0.5%≤Silicon≤2.5%

0.03%≤Aluminum≤1.5%

0%≤Sulfur≤0.003%,

0.002%≤Phosphorus≤0.02%

0%≤Nitrogen≤0.01%

and optionally containing one or more of the following elements

0.05%≤Chromium≤1%

0.001%≤Molybdenum≤0.5%

0.001%≤Niobium≤0.1%

0.001%≤Titanium≤0.1%

0.01%≤Copper≤2%

0.01%≤Nickel≤3%

0.0001%≤Calcium≤0.005%

0%≤Vanadium≤0.1%

0%≤Boron≤0.003%

0%≤Cerium≤0.1%

0%≤Magnesium≤0.010%

0%≤Zirconium≤0.010%

a remainder being iron and unavoidable impurities caused by processing;

a microstructure of the cold rolled heat treated steel sheet comprising in area fraction, 10 to 30% Residual Austenite, 50 to 85% Bainite, 1 to 20% Quenched Martensite, and less than 30% Tempered Martensite, the Bainite consisting of Lath Bainite and Granular Bainite.

2. The cold rolled heat treated steel sheet as recited in claim 1 wherein the composition includes 0.7% to 2.4% of Silicon.

3. The cold rolled heat treated steel sheet as recited in claim 1 wherein the composition includes 0.03% to 0.9% of Aluminum.

4. The cold rolled heat treated steel sheet as recited in claim 3 wherein the composition includes 0.03% to 0.6% of Aluminum.

5. The cold rolled heat treated steel sheet as recited in claim 1 wherein the composition includes 1.2% to 2.3% of Manganese.

6. The cold rolled heat treated steel sheet as recited in claim 1 wherein the composition includes 0.03% to 0.5% of Chromium.

7. The cold rolled heat treated steel sheet as recited in claim 1 wherein a sum of the Bainite and the Residual Austenite is equal to 70% or more.

8. The cold rolled heat treated steel sheet as recited in claim 1 wherein a sum of the Tempered Martensite, and the Quenched Martensite is more than or equal to 20% and the Quenched Martensite is higher than 10%.

9. The cold rolled heat treated steel sheet as recited in claim 1 wherein the cold rolled heat treated steel sheet has an ultimate tensile strength 1100 MPa, and a total elongation 14.0% or more.

10. The cold rolled heat treated steel sheet as recited in claim 9 wherein a yield strength to ultimate tensile strength ratio is greater than or equal to 0.65.

11. The cold rolled heat treated steel according to claim 1 , wherein said composition includes, in percentage by weight: 1.2%≤Manganese≤2.2%.

12. The cold rolled heat treated steel according to claim 1 , wherein said microstructure includes, in area fraction, less than 3% ferrite.

13. A method of production of the cold rolled heat treated steel sheet as recited in claim 1 comprising the following successive steps:

providing semi-finished product having a steel composition comprising the following elements, expressed in percentage by weight:

0.1%<Carbon<0.5%

1%<Manganese<3.4%

0.5%<Silicon<2.5%

0.03%<Aluminum<1.5%

0%<Sulfur<0.003%

0.002%<Phosphorus<0.02%

0%<Nitrogen<0.01%

and optionally containing one or more of the following elements

0.05%<Chromium<1%

0.001%<Molybdenum<0.5%

0.001%<Niobium<0.1%

0.001%<Titanium<0.1%

0.01%<Copper<2%

0.01%<Nickel<3%

0.001%<Calcium<0.005%

0%<Vanadium<0.1%

0%<Boron<0.003%

0%<Cerium<0.1%

0%<Magnesium<0.010%

0%<Zirconium<0.010%

a remainder being iron and unavoidable impurities caused by processing;

reheating the semi-finished product to a temperature between 1200° C. and 1280° C.;

rolling the semi-finished product in an austenitic range wherein the hot rolling finishing temperature is above Ac3 to obtain a hot rolled steel sheet;

cooling the hot rolled steel sheet at a cooling rate above 30° C./s to a coiling temperature below 600° C. and coiling the hot rolled steel sheet;

cooling the hot rolled steel sheet to room temperature;

optionally performing a scale removal process on the hot rolled steel sheet;

optionally annealing the hot rolled steel sheet at temperature between 400° C. and 750° C.;

optionally performing a scale removal process on the hot rolled steel sheet;

cold rolling the hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet;

heating the cold rolled steel sheet at a rate greater than 3° C./s to a soaking temperature between Ac3 and Ac3+100° C. and holding the cold rolled steel sheet for a time of 10 to 500 seconds;

cooling the cold rolled steel sheet at a rate greater than 20° C./s to a temperature below 500° C. to define an annealed cold rolled steel sheet;

cooling the annealed cold rolled steel sheet to room temperature;

optionally tempering the annealed cold rolled steel sheet between 120° C. and 250° C.;

heating the annealed cold rolled steel sheet at a rate greater than 3° C./s to a soaking temperature between Ac3 and Ac3+100° C. and holding the annealed cold rolled steel sheet of 10 to 500 seconds;

cooling the annealed cold rolled steel sheet at a rate greater than 20° C./s to a temperature range between T Cmax and T Cmin wherein:

T Cmax =565−601*(1−Exp(−0.868*C))−34*Mn−13*Si−10*Cr+13*Al−361*Nb

T Cmin =535−601*(1−Exp(−0.868*C))−34*Mn−13*Si−10*Cr+13*AI−361*Nb; and

bringing the annealed cold rolled steel sheet to a holding temperature range between 350° C. and 550° C. for a time period between 5 and 500 seconds and then cooling the annealed cold rolled steel sheet to room temperature with a cooling rate of at least 1° C./s to obtain the cold rolled heat treated steel sheet; and

optionally coating the cold rolled heat treated steel sheet;

thereby producing the cold rolled heat treated steel sheet of claim 1 .

14. The method as recited in claim 13 wherein the coiling temperature is below 570° C.

15. The method as recited in claim 13 wherein the soaking temperature for first or second annealing is between Ac3 and Ac3+50° C.

16. The method as recited in claim 13 wherein the cooling of the cold rolled steel sheet or the cooling of the annealed cold rolled steel sheet is greater than to a temperature below 500° C.

17. A structural or safety part of a vehicle made according to the method as recited in claim 13 .

18. The part as recited in claim 17 wherein the part is obtained by flexible rolling of the cold rolled heat treated steel sheet.

19. A vehicle comprising the part as recited in claim 17 .

20. A structural or safety part of a vehicle comprising the cold rolled and heat treated steel sheet as recited in claim 1 .

21. The part as recited in claim 20 wherein the part is obtained by flexible rolling of the cold rolled and heat treated steel sheet.

22. A vehicle comprising the part as recited in claim 20 .