MANUFACTURING METHOD FOR VERY HIGH-STRENGTH COLD-ROLLED DUAL-PHASE STEEL SHEETS AND SHEETS SO PRODUCED

1 . A cold-rolled and annealed Dual-Phase steel sheet having a strength between 980 and 1100 MPa, and a breaking elongation greater than 9%, wherein the cold-rolled and annealed Dual-Phase steel sheet comprises iron, the following elements expressed by weight:

055%≦C≦0.095%

2%≦Mn≦2.6%

0.005%≦Si≦0.35%

S≦0.005%

P≦0.050%

0.1≦Al≦0.3%

0.05%≦Mo≦0.25%

0.2%≦Cr≦0.5%

it being understood that Cr+2Mo≦0.6%

Ni≦0.1%

0.010≦Nb≦0.040%

0.010≦Ti≦0.050%

0.0005≦B≦0.0025%

and 0.002%≦N≦0.007%

and inevitable impurities resulting from smelting.

2 . The steel sheet as claimed in claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:

0.12%≦Al≦0.25%

3 . The steel sheet as claimed in claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:

0.10%≦Si≦0.30%

4 . The steel sheet as claimed in claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:

0 . 15 %≦Si≦0.28%

5 . The steel sheet as claimed in claim 1 , wherein the composition of said steel comprises, the content being expressed by weight:

P≦0.015%

6 . The steel sheet as claimed in claim 1 , wherein said steel sheet has a microstructure and the microstructure comprises a surface area fraction of 35 to 50% martensite.

7 . The steel sheet as claimed in claim 6 , wherein the microstructure further comprises of a surface area fraction of 50 to 65% ferrite.

8 . The steel sheet as claimed in claim 6 , wherein the microstructure further comprises of surface area fractions of 1 to 10% bainite and 40 to 64% ferrite.

9 . The steel sheet as claimed in claim 1 , wherein its non-recrystallized ferrite surface area fraction, compared to the whole of the ferritic phase, is less than or equal to 15%.

10 . The steel sheet as claimed in claim 1 , wherein the ratio of its yield strength R e to its strength R m is such that: 0.6 5 Re/R m ≦0.8.

11 . The steel sheet as claimed in claim 1 , wherein it is continuously galvanized.

12 . The steel sheet as claimed in claim 1 , wherein it includes a galvannealed coating.

13 . A manufacturing method for producing a cold-rolled and annealed Dual-Phase steel sheet, comprising: casting a steel having a composition as claimed in claim 1 ,

as a semi-finished product,

bringing said semi-finished product to a temperature 1150° C.≦T R 1250° C.,

hot-rolling said semi-finished product with an end-of-roiling temperature T FL ≧Ar3 to obtain a hot-rolled product,

coiling said hot-rolled product at a temperature T bob such as: 500° C. T bob ≦570° C.,

descaling said hot-rolled product to obtain a descaled hot-rolled product, and

cold-rolling said descaled hot-rolled product with a reduction of between 30 and 80% to obtain a cold-rolled product,

heating said cold-rolled product at a rate 1° C./s≦V C ≦5° C./s to an annealing temperature TM such as: Ac1+40° C.≦T M ≦Ac3−30° C. at which the product is held for a time: 30 s≦t M ≦300 s so as to obtain a heated and annealed product with a structure comprising austenite, then;

cooling said heated and annealed product to a temperature less than the temperature M 5 at a rate V high enough for ail of said austenite to transform to martensite.

14 . A manufacturing method for producing a cold-rolled, annealed and galvanized Dual-Phase steel sheet comprising cooling a heated and annealed product with a structure comprising austenite as claimed in claim 13

at a rate V R high enough to prevent the transformation of said austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T Zn is reached, then;

continuously galvanized galvanizing said product by immersing said product in a bath of zinc or Zn alloy at a temperature 450° C.≦T Zn ≦480° C. to obtain a galvanized product, then;

cooling said galvanized product to the ambient temperature at a rate V′ R greater than 4° C./s to obtain a cold-rolled, annealed and galvanized steel sheet.

15 . A manufacturing method for producing a cold-rolled and galvannealed Dual-Phase steel sheet, comprising cooling a heated and annealed product with a structure comprising austenite as claimed in claim 13

at a rate V R high enough to prevent the transformation of said austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T Zn is reached,

continuously galvanizing said product by immersing said product in a bath of zinc or Zn alloy at a temperature 450° C.≦T Zn ≦480° C. to obtain a galvanized product,

heating said galvanized product at a temperature TG between 490 and 550° C. for a time t G between 10 and 40 s to obtain a galvannealed product, then;

cooling said galvannealed product to the ambient temperature at a rate V″ R greater than 4° C./s, to obtain a cold-rolled and galvannealed steel sheet.

16 . The manufacturing method as claimed in claim 13 , wherein said temperature T M is between 760 and 830° C.

17 . The manufacturing method as claimed in claim 14 , wherein said rate of cooling V R is greater than or equal to 15° C./s.

18 . (canceled)

19 . A motor vehicle safety part comprising a steel sheet manufactured according to the method of claim 13 .

20 . A cold-rolled and annealed Dual-Phase steel sheet having a strength between 980 and 1100 MPa, and a breaking elongation greater than 9%, wherein said cold-rolled and annealed Dual-Phase steel sheet consists essentially of iron, the following elements expressed by weight:

0.055%≦C≦0.095%

2%≦Mn≦2.6%

0.005%≦Si≦0.35%

S≦0.005%

P≦0.050%

0.1≦Al≦0.3%

0.05%≦Mo≦0.25%

0.2%≦Cr≦0.5%

it being understood that Cr+2Mo≦0.6%

Ni≦0.1%

0.010≦Nb≦0.040%

0.010≦Ti≦0.050%

0.0005≦B≦0.0025%

and 0.002%≦N≦0.007%

and inevitable impurities resulting from smelting.