Low-density steel having good drawability
View Patent ↗The invention relates to a hot-rolled ferritic steel sheet, the composition of the steel of which comprises, the contents being expressed by weight: 0.001≦C≦0.15%, Mn≦1%, Si≦1.5%, 6%≦Al≦10%, 0.020%≦Ti≦0.5%, S≦0.050%, P≦0.1%, and, optionally, one or more elements chosen from: Cr≦1%, Mo≦1%, Ni≦1%, Nb≦0.1%, V≦0.2%, B≦0.010%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the average ferrite grain size d IV measured on a surface perpendicular to the transverse direction with respect to the rolling being less than 100 microns.
1. A hot-rolled ferritic steel sheet, the composition of the steel of which comprises, the contents being expressed by weight:
0.001≦C≦0.15%
Mn≦1%
Si≦1.5%
7.5%≦Al≦10%
0.020%≦Ti≦0.5%
S≦0.050%, and
P≦0.1%
the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, wherein said hot-rolled ferritic steel sheet, resulting from hot-rolling of said steel composition, comprises kappa (κ) precipitates and non-equiaxed ferrite grains wherein an average grain size div of the non-equiaxed ferrite grains, measured on a surface perpendicular to the transverse direction with respect to the hot-rolling is less than 100 microns and wherein the non-equiaxed ferrite grains have an elongation in a direction of the hot rolling.
2. The steel sheet according to claim 1 , wherein the composition comprises, the contents being expressed by weight:
0.001%≦C≦0.010%
Mn≦0.2%.
3. The steel sheet according to claim 1 , wherein the composition comprises, the contents being expressed by weight:
0.010%<C≦0.15%
0.2%<Mn≦1%
4. The steel sheet according to claim 1 , wherein the composition comprises, the contents being expressed by weight:
7.5%≦Al≦8.5%.
5. The steel sheet according to claim 1 , wherein the content of carbon in solid solution is less than 0.005% by weight.
6. The steel sheet according to claim 1 , wherein a strength R m is equal to or greater than 400 MPa.
7. The steel sheet according to claim 3 , wherein a strength R m is equal to or greater than 600 MPa.
8. A process for manufacturing a hot-rolled ferritic steel sheet in which:
a steel composition according to claim 1 is supplied;
said steel is cast in the form of a semi-finished product; then
said semi-finished product is heated to a temperature of 1150° C. or higher; then
said semi-finished product is hot-rolled so as to obtain a sheet using at least two rolling operations carried out at temperatures above 1050° C., the reduction ratio of each of said at least two operations being equal to or greater than 30%, the time elapsing between each of said at least two rolling operations and the next rolling operation being equal to or greater than 10 s; then
the rolling is completed at a temperature T ER of 900° C. or higher; then
said sheet is cooled so that the time interval t p elapsing between 850 and 700° C. is greater than 3 s in order to cause the precipitation of κ precipitates; and then
said sheet is coiled at a temperature T coil between 500 and 700° C. to form the hot rolled ferritic steel sheet according to claim 1 .
9. The process for manufacturing a hot-rolled sheet according to claim 8 , wherein said casting is carried out directly in the form of casting a thin slab or thin strip between counter-rotating rolls.
10. A process for manufacturing a cold-rolled and annealed steel sheet, in which:
a hot-rolled steel sheet manufactured according to claim 8 is supplied; then
said sheet is cold-rolled with a reduction ratio between 30 and 90% in order to obtain a cold-rolled sheet; then
said cold-rolled sheet is heated to a temperature T′ at a rate V h greater than 3° C./s; and then
said sheet is cooled at a rate V c less than 100° C./s,
said temperature T′ and said rate V h being chosen in order to obtain complete recrystallization, a linear fraction f of intergranular κ precipitates of less than 30% and a content of carbon in solid solution of less than 0.005% by weight.
11. The manufacturing process according to claim 10 , wherein said cold-rolled sheet is heated to a temperature T′ between 750 and 950° C.
12. The manufacturing process according to claim 10 , wherein a sheet of the composition which comprises, the contents being expressed by weight:
0.010%<C≦0.15%
0.2%<Mn≦1%
is supplied and in that said cold-rolled sheet is heated to a temperature T′ chosen in order to prevent the dissolution of K precipitates.
13. The manufacturing process according to claim 10 , wherein a sheet of the composition which comprises, the contents being expressed by weight:
0.010%<C≦0.15%
0.2%<Mn≦1%
is supplied and in that said cold rolled sheet is heated to a temperature T′ between 750 and 800° C.
14. A skin part or structural part in the automotive field comprising a steel sheet according to claim 1 .
15. The steel sheet according to claim 1 , comprising 0.007%≦Cr<1%.
16. The steel sheet according to claim 1 , wherein the average ferrite grain size d IV , measured on a surface perpendicular to the transverse direction with respect to the hot-rolling, is 40 microns or larger and less than 100 microns.
17. The steel sheet according to claim 1 , wherein the steel has a reduced density of less than 7.3.
18. The steel sheet according to claim 1 , wherein the steel is obtained by a process comprising at least two rolling operations carried out at temperatures above 1050° C. wherein a reduction ratio of each of said at least two operations is equal to or greater than 30%.
19. The steel sheet according to claim 1 , wherein the steel has a ferritic structure at ambient temperature.
20. The steel sheet according to claim 1 , wherein the steel has a ferritic matrix at all temperatures during manufacturing, from solidification after casting.
21. The steel sheet according to claim 1 , further comprising one or more elements selected from the group consisting of:
Cr ≦1%
Mo ≦1%
Ni ≦1%
Nb ≦0.1%
V ≦0.2%, and
B ≦0.010%.
22. The steel sheet according to claim 1 , wherein the steel structure composition is homogenous.