Cold rolled and 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 0.4%

0.5%≤Silicon≤2.5%

0.03%≤Aluminum≤1.5%

Sulfur≤0.003%.

0.002%≤Phosphorus≤0.02%

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%

Vanadium≤0.1%

Boron≤0.003%

Cerium≤0.1%

Magnesium≤0.010%

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, 5 to 50% Annealed Bainite, 10 to 40% of Bainite, 1% to 20% Quenched Martensite, and less than 30% Tempered Martensite wherein the Bainite and the Residual Austenite are 30% or more.

2. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the composition includes 0.7%≤Silicon≤2.2%.

3. The cold rolled and heat treated steel sheet as recited in claim 2 wherein the composition includes 1%≤Silicon≤2.2%.

4. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the composition includes 0.03%≤Aluminum≤1.0%.

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

6. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the composition includes 0.05%≤Chromium≤0.5%.

7. The cold rolled and heat treated steel sheet as recited in claim 1 wherein a sum of the Residual Austenite and the Bainite is greater than 35%.

8. The cold rolled and heat treated steel sheet as recited in claim 1 wherein a sum of the Annealed Bainite and the Bainite is greater than 45%.

9. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the Residual Austenite is between 15 and 30%.

10. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the Bainite is between 15% and 40%.

11. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the cold rolled and heat treated steel sheet has a tensile strength greater than 950 MPa and total elongation of 20% or more.

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 above 600 MPa and a ratio of yield strength to tensile strength of 0.6 or more.

13. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the cold rolled and heat treated steel sheet has a tensile strength between 1000 MPa and 1100 MPa and a total elongation of 23% or more.

14. The cold rolled and heat treated steel sheet as recited in claim 1 wherein the microstructure does not contain Ferrite.

15. A method of production of a cold rolled and heat treated steel sheet as recited in claim 1 , the method comprising the following steps:

providing a steel with the composition to define a semi-finished product;

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

rolling the semi-finished product in the 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 sheet to room temperature;

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

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

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

then performing a first annealing by 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 sheet for a time of 10 seconds to 500 seconds;

then cooling the cold rolled sheet at a rate greater than 25° C./s to a temperature between 380° C. and 480° C. and holding the cold rolled steel sheet for a time of 10 and 500 seconds;

cooling the cold rolled steel sheet to room temperature to obtain a cold-rolled and annealed steel sheet;

then performing a second annealing by heating the cold rolled and annealed steel sheet at a rate greater than 3° C./s to a soaking temperature between T soaking and Ac3 and holding the cold rolled and annealed steel sheet for a time of 10 seconds to 500 seconds;

then cooling the cold rolled and annealed steel sheet at a rate greater than 20° C./s to a temperature range between Tc max and Tc min ; wherein Tc max and Tc min are defined as follows:

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

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 are in wt. % of the elements in the steel composition; and

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

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

17. The method as recited in claim 13 wherein the first annealing soaking temperature is between Ac3 and Ac3+75° C. for the 10 to 500 seconds.

18. The method as recited in claim 13 wherein the second annealing is a continuous annealing between T soaking and Ac3 for the 10 to 500 seconds to have an Austenite to Annealed Bainite ratio between 50:50 to 90:10.

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

20. The part as recited in claim 19 wherein the part is obtained by flexible rolling of the cold rolled and heat treated steel sheet.

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