Process for manufacturing a galvanized or a galvannealed steel sheet by DFF regulation

1. A process for manufacturing a hot-dip galvanized or galvannealed steel sheet having a TRIP microstructure comprising ferrite, residual austenite and optionally martensite, bainite or a mixture thereof, wherein a composition of the steel sheet comprises Fe and: by weight

0.01≦C≦0.22%

0.50≦Mn≦2.0%

0.2≦Si≦2.0%

0.005≦Al≦2.0%

Mo<1.0%

Cr≦1.0%

P<0.02%

Ti≦0.20%

V≦0.40%

Ni≦1.0%

Nb≦0.20%

and unavoidable impurities resulting from smelting, wherein the process comprises:

oxidizing said steel sheet in a direct flame furnace where an atmosphere comprises air and fuel with an air-to-fuel ratio between 0.80 and 0.95;

forming a layer of iron oxide having a thickness from 0.05 to 0.2 μm on the surface of the steel sheet;

forming at least one internal oxide of Si oxide, Mn oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide of Si and Al, complex oxide comprising Mn and Al, and complex oxide comprising Si, Mn and Al;

reducing said layer of iron oxide, at a reduction speed from 0.001 to 0.01 μm/s;

growing the internal oxide in depth of steel sheet;

completing the reduction of the layer of iron oxide;

hot-dip galvanizing said reduced steel sheet to form a zinc-coated steel sheet; and

optionally, subjecting said hot-dip coated steel sheet to an alloying treatment to form a galvannealed steel sheet.

2. The process according to claim 1 , wherein said steel sheet comprises, by weight, P<0.015%.

3. The process according to claim 1 , wherein said steel sheet comprises, by weight, Mo≦0.01%.

4. The process according to claim 1 , comprising oxidizing the steel sheet by heating the steel sheet from ambient temperature to a heating temperature T1.

5. The process according to claim 4 , wherein said temperature T1 is between 680 to 800° C.

6. The process according to claim 1 , wherein the reduction of said layer of iron oxide comprises a heat treatment performed in a furnace having an atmosphere comprising from 2 to less than 15% by volume of hydrogen, the balance of the composition being nitrogen and unavoidable impurities.

7. The process according to claim 6 , wherein the atmosphere comprises from 2 to less than 5% by volume of hydrogen.

8. The process according to claim 6 , wherein said heat treatment comprises heating from the heating temperature T1 to a soaking temperature T2, soaking at said soaking temperature T2 for a soaking time t2, and cooling from said soaking temperature T2 to a cooling temperature T3.

9. The process according to claim 8 , wherein said soaking temperature T2 is between 770 and 850° C.

10. The process according to claim 8 , wherein said soaking time t2 is between 20 and 180 s.

11. The process according to claim 8 , wherein said cooling temperature T3 is between 460 to 510° C.

12. The process according to claim 8 , wherein said reduction is performed in a radiant tube furnace or in a resistance furnace.

13. The process according to claim 1 , wherein the process manufactures a hot-dip galvanized steel sheet and the hot-dip galvanizing is performed by hot-dipping said reduced steel sheet in a molten bath comprising from 0.14 to 0.3% by weight of aluminium, the balance being zinc and unavoidable impurities.

14. The process according to claim 13 , wherein the temperature of said molten bath is between 450 and 500° C.

15. The process according to claim 1 , wherein, the process manufactures a hot-dip galvannealed steel sheet and the hot-dip galvanizing is performed by hot-dipping said reduced steel sheet in a molten bath comprising from 0.08 to 0.135% by weight of aluminium, the balance being zinc and unavoidable impurities.

16. The process according to claim 15 , wherein molybdenum of said steel sheet is less than 0.01% by weight.

17. The process according to claim 15 , wherein said alloying treatment is performed by heating said zinc coated steel sheet at a temperature T4between 460 and 510° C. for a soaking time t4 between 10 and 30 s.