Process for manufacturing steel, for hot forming or quenching in a tool, having improved ductility

1. A process for manufacturing a coated steel part, the process comprising:

hot rolling or cold rolling a steel sheet, comprising, Fe and, by weight:

0.040%≦C≦0.100%;

0.80%≦Mn≦2.00%;

Si≦0.30%;

S≦0.005%;

P≦0.030%;

0.010%≦Al≦0.070%;

0.015%≦Nb≦0.100%;

0.030%≦Ti≦0.080%;

N≦0.009%;

Cu≦0.100%;

Ni≦0.100%;

Cr≦0.100%;

Mo≦0.100%;

Ca≦0.006%,

to obtain a rolled sheet;

precoating the rolled sheet with a precoating, which is an alloy comprising zinc or aluminum, to obtain a coated sheet;

cutting the coated sheet to obtain a blank;

heating the blank to a temperature Tc in a furnace to form, by alloying the steel and the precoating, an alloyed layer on a surface of the blank, wherein the alloying is carried out over an entire thickness of the alloyed layer, and the steel comprises an entirely austenitic structure after heating;

removing the blank from the furnace, and, optionally, hot deforming the blank, to obtain a part precursor; and

cooling the part precursor within a tool under conditions to obtain intended mechanical properties in a coated steel part,

wherein the steel of the steel part has a microstructure comprising at least 75% equiaxed ferrite, martensite in an amount not less than 5% but not exceeding 20%, and bainite in an amount not exceeding 10%, expressed as surface fractions.

2. A process for manufacturing a part, comprising:

hot rolling or cold rolling a steel sheet comprising, Fe and, by weight:

0.040%≦C≦0.100%;

0.80%≦Mn≦2.00%;

Si≦0.30%;

S≦0.005%;

P≦0.030%;

0.010%≦Al≦0.070%;

0.015%≦Nb≦0.100%;

0.030%≦Ti≦0.080%;

N≦0.009%;

Cu≦0.100%;

Ni≦0.100%;

Cr≦0.100%;

Mo≦0.100%;

Ca≦0.006%;

to obtain a rolled sheet;

cutting the sheet to obtain a blank;

heating the blank to a temperature Tc in a furnace to give the steel a completely austenitic structure;

removing the blank from the furnace, and, optionally, hot deforming the blank, to obtain a part precursor;

cooling the part precursor in a tool under conditions suitable to obtain intended mechanical properties in a cooled part;

wherein the steel of the steel part has a microstructure comprising at least 75% equiaxed ferrite, martensite in an amount not less than 5% but not exceeding 20%, and bainite in an amount not exceeding 10%, expressed as surface fractions.

3. The process of claim 1 , wherein the temperature T C is between 880 and 950° C. for between 3 and 10 minutes.

4. The process of claim 1 , wherein an average cooling rate V C between the temperature T C and 400° C. is between 30 and 80° C./s.

5. The process of claim 4 , wherein the average cooling rate V C between the temperature T C and 400° C. is between 35 and 60° C./s.

6. The process of claim 1 further comprising the step(s) of welding the blank and/or cold deforming the blank.

7. The process of claim 2 , wherein the temperature T C is between 880 and 950° C. for between 3 and 10 minutes.

8. The process of claim 2 , wherein an average cooling rate V C between the temperature T C and 400° C. is between 30 and 80° C./s.

9. The process of claim 8 , wherein the average cooling rate V C between the temperature T C and 400° C. is between 35 and 60° C./s.

10. The process of claim 2 , further comprising the step(s) of welding the blank and/or cold deforming the blank and/or coating the cooled part.

11. The process of claim 2 , wherein the steel part has an average ferrite grain size of 6 microns or less.

12. The process of claim 2 , wherein the martensite is in the form of martensite islands and the martensite islands are 6 microns or less.

13. The process of claim 2 , wherein the martensite islands are smaller than the ferrite grains.

14. The process of claim 2 , wherein the steel part has a thickness of from 0.5 to 4 mm.