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

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

0.10%≤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 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 and heat treated steel sheet comprising by area fraction, 10 to 30% Residual Austenite, 10 to 40% Bainite, 5% to 50% Annealed Martensite, 1% to 20% Quenched Martensite and less than 30% Tempered Martensite, wherein the Bainite and the Residual Austenite is more than or equal to 25%, wherein the microstructure is free of pearlite, ferrite and cementite.

2. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the composition includes, in percentage by weight, 1% to 2% of Silicon.

3. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the composition includes, in percentage by weight, 0.03% to 1.0% of Aluminum.

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

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

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

7. The cold rolled and heat treated steel sheet as recited in claim 1 wherein a sum of the Tempered Martensite, the Quenched Martensite and the Annealed Martensite is more than or equal to 20% by area fraction, and the Annealed Martensite is greater than 10% to 50% by area fraction.

8. The cold rolled and heat treated steel sheet as recited in claim 1 wherein a Carbon content of the Residual Austenite is between 0.9 to 1.1%.

9. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the cold rolled and heat treated steel sheet has an ultimate tensile strength of 950 MPa or more and a total elongation of 15% or more.

10. The cold rolled and heat treated steel sheet as recited in claim 9 wherein the cold rolled and heat treated steel sheet has an ultimate tensile strength of 1000 MPa or more and a yield strength to ultimate tensile strength ratio greater than or equal to 0.5.

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

12. A vehicle comprising the part as recited in claim 11 .

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

providing a semi-finished product with the composition as recited in claim 1 ;

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

rolling the semi-finished product in in a temperature range, in ° C., of Ac3 to Ac3+100° C. wherein a 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 sheet to room temperature;

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

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

optionally performing a further 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;

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

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

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

performing a second annealing by heating the annealed steel sheet at a rate greater than 30C/s to a soaking temperature, in ° C., between T soaking and Ac3 and holding the annealed steel sheet for a time of 10 to 500 seconds;

cooling the annealed steel sheet at a rate greater than 20° C./s to a temperature range, in ° C., between Tc max and Tc min ;

bringing the annealed steel sheet to a temperature range between 350° C. and 550° C. for a time of 5 to 500 seconds and cooling the annealed steel sheet down to room temperature with a cooling rate of at least 10C/s to obtain the cold rolled heat treated steel sheet having the microstructure as recited in claim 1 ;

wherein

Ac3=901−262*C−29*Mn+31*Si−12*Cr−155*Nb+86*Al,

T soaking =830−260*C−25*Mn+22*Si+40*Al,

Tc max =565−601*(1-Exp(−0.868*C))−34*Mn−13*Si−10*Cr+13*Al−361*Nb, and

Tc min =565−601*(1-Exp(−1.736*C))−34*Mn−13*Si−10*Cr+13*Al−361*Nb,

wherein C, Mn, Si, Cr, Al and Nb in the above equations represent the respective content in percentage by weight of elements in the steel composition.

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 hot rolling finishing temperature is between Ac3 and Ac3+100° C.

16. The method as recited in claim 13 wherein the coiling temperature is between 350° C. and 570° C.