Tempered and coated steel sheet having excellent formability and a method of manufacturing the same

1. A tempered and coated steel sheet having a composition comprising the following elements, expressed in percentage by weight:

0.17%≤carbon≤0.25%,

1.8%≤manganese≤2.3%,

0.5%≤silicon≤2.0%,

0.03≤aluminum≤1.2%,

sulphur≤0.03%,

phosphorus≤0.03%,

the remainder of the composition composed of iron and unavoidable impurities caused by processing,

the microstructure of said steel sheet comprising in area fraction, 4 to 20% residual austenite, 0 to 15% of ferrite, 40 to 85% tempered bainite and a minimum of 5% of tempered martensite, wherein the cumulated amounts of tempered martensite and residual austenite is between 10 and 30%, and

a yield strength above 700 MPa, an ultimate tensile strength above 950 MPa, a hole elongation ratio above 18%, a total elongation above 12%,

and wherein the tempered martensite is composed of laths elongated in one direction inside each grain issued from a primary austenite grain, in which iron carbide sticks which are 50 to 200 nm long are precipitated between the laths.

2. A tempered and coated steel sheet according to claim 1 , wherein the composition includes 0.6% to 1.8% of silicon.

3. A tempered and coated steel according to claim 1 , wherein the composition includes 0.03% to 0.6% of aluminium.

4. A tempered and coated steel sheet according to claim 1 , wherein the cumulated amounts of tempered martensite and residual austenite is between 10% and 25%.

5. A tempered and coated steel sheet according to claim 1 , wherein, the cumulated amounts of tempered martensite and residual austenite is more than or equal to 15% and the percentage of tempered martensite is higher than 10%.

6. A tempered and coated steel sheet according to claim 1 , wherein the carbon content of residual austenite is between 0.9 to 1.1%.

7. A tempered and coated steel sheet according to claim 1 , wherein said tempered and coated steel sheet has an ultimate tensile strength of 1000 MPa to 1100 MPa and a hole expansion ratio above 20%.

8. A structural or safety part of a vehicle comprising a steel sheet according to claim 1 .

9. A vehicle comprising a structural or safety part according to claim 8 .

10. A tempered and coated steel sheet according to claim 1 , wherein the composition further comprises one or more of the following elements:

chromium≤0.4%,

molybdenum≤0.3%,

niobium≤0.04%,

titanium≤0.1%.

11. A tempered and coated steel sheet according to claim 1 , wherein the tempered steel sheet is coated with a zinc or zinc alloy coating.

12. A tempered and coated steel sheet according to claim 1 , wherein

0.750%≤silicon≤2.0%, and

0.750%≤aluminum≤1.2%.

13. A tempered and coated steel sheet according to claim 1 , wherein the ferrite comprises ferrite formed during a cooling after annealing of the steel sheet.

14. A tempered and coated steel sheet according to claim 1 , wherein the microstructure consists of the ferrite, the tempered bainite, the tempered martensite and the residual austenite.

15. A method of production of the tempered and coated steel sheet according to claim 1 , the method comprising the following successive steps:

providing a semi-finished product having the composition;

reheating said semi-finished product to a temperature above Ac3;

hot rolling the semi-finished product in an austenitic range, wherein the hot rolling finishing temperature is between 750° C. and 1050° C. to obtain a hot rolled steel sheet;

cooling the hot rolled steel sheet at a cooling rate 20 to 150° C./s to a coiling temperature of less than or equal to 600° C.; and coiling the hot rolled steel sheet;

cooling the hot rolled steel sheet to room temperature;

annealing the hot rolled steel sheet at temperature between 400° C. and 750 to obtain an annealed hot rolled steel sheet;

cold rolling the annealed hot rolled steel sheet with a reduction rate between 30 and 80% to obtain a cold rolled steel sheet;

then heating the cold rolled steel sheet at a rate between 1 to 20° C./s to a soaking temperature above Ae3 where it is held during less than 600 seconds;

then cooling the cold rolled steel sheet at a rate greater than 5° C./s to a temperature above Ms and less than 475° C. and holding the cold rolled steel sheet at such temperature during 20 to 400 seconds;

then cooling the cold rolled steel sheet at cooling rate not greater than 200° C./s down to room temperature;

then reheating the annealed steel sheet at a rate between 1° C./s to 20° C./s to a soaking temperature between 440° C. and 600° C. where it is held during less than 100s and then hot dipping the annealed steel sheet in a bath zinc or zinc alloy coating for tempering and coating it,

cooling the tempered and coated steel sheet to room temperature at a cooling rate between 1° C./s and 20° C./s.

16. A method according to claim 15 , wherein the composition further comprises one or more of the following elements:

chromium≤0.4%,

molybdenum≤0.3%,

niobium≤0.04%,

titanium≤0.1%.

17. A method according to claim 15 , wherein the coiling temperature is above 400° C.

18. A method according to claim 15 , further comprising performing scale removal process on the hot rolled steel sheet.

19. A method according to claim 15 , further comprising performing scale removal process on the annealed hot rolled steel sheet.