COLD-ROLLED STEEL PLATE COATED WITH ZINC OR A ZINC ALLOY, METHOD FOR MANUFACTURING SAME, AND USE OF SUCH A STEEL PLATE
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Abstract
The present invention provides a “TRIP effect” cold-rolled and annealed steel sheet which has improved formability and weldability, strength between 780 and 900 MPa and elongation at fracture greater than 19%, the composition of which includes the following elements in percentages expressed by weight: 0.17%≦C≦0.25%, 1.5%≦Mn≦2%, 0.50%≦Si≦1%, 0.50%≦Al≦1.2%, whereby Si+Al≧1.30%, the remainder of the composition including iron and the inevitable impurities resulting from processing. The microstructure of the sheet includes 65 to 85% ferrite and 15 to 35% islands of martensite and residual austenite. The average size of these islands of martensite and residual austenite is less than 1.3 micrometers and their shape factor is less than 3.
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Citations
38 Claims
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1-17. -17. (canceled)
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18. A cold-rolled steel sheet coated with zinc or zinc alloy, the composition of which includes the following, expressed by weight:
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0.17%≦
C≦
0.25%
1.5%≦
Mn≦
2.0%
0.50%≦
Si≦
1%
0.50%≦
Al≦
1.2%
B≦
0.001%
P≦
0.030%
S≦
0.01%
Nb≦
0.030%
Ti≦
0.020%
V≦
0.015%
Cu≦
0.1%
Cr≦
0.150%
Ni≦
0.1%
0%≦
Mo≦
0.150%
whereby Si+Al≧
1.30%,the remainder of the composition consists of iron and the inevitable impurities resulting from processing, a microstructure of the composition includes, with the contents expressed in area percentage; of 65 to 85% ferrite, and of 15 to 35% islands of martensite and residual austenite, the ferrite containing less than 5% non-recrystallized ferrite, a total content of residual austenite being between 10 and 25% and a total martensite content being less than or equal to 10%, an average size of the martensite and residual austenite islands being less than 1.3 micrometers, and an average shape factor of the islands being less than 3, a mechanical strength Rm of the sheet being between 780 and 900 MPa, and an elongation at fracture A % being greater than or equal to 19%. - View Dependent Claims (19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38)
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Specification