Galvanized or galvannealed silicon steel
View Patent ↗The invention deals with a hot-dip galvanized or galvannealed steel sheet wherein, the composition of the steel sheet comprises, by weight: 0.01≦C≦0.22%, 0.50≦Mn≦2.0%, 0.2≦Si≦3.0%, 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%, the balance of the composition being iron and unavoidable impurities resulting from the smelting, and the steel sheet comprises a layer of an internal nitride of at least one type of nitride selected from a Si nitride, Mn nitride, Al nitride, complex nitride comprising Si and Mn, or Al and Si, or Al and Mn, or complex nitride comprising Si, Mn and Al, said steel sheet comprising no further outer layer of iron nitride.
1. A process for manufacturing a hot-dip galvanized or galvannealed steel sheet, comprising:
a) subjecting a steel sheet having a composition comprising by weight:
0.01≦C≦0.22%
0.50≦Mn≦2.0%
0.2≦Si≦3.0%
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%,
wherein the balance of the composition is iron and unavoidable impurities resulting from smelting,
to an annealing in a furnace to form an annealed steel sheet, wherein the furnace comprises:
a first heating zone wherein the steel sheet is pre-heated from ambient temperature to a heating temperature T1, in a non nitriding atmosphere having a Dew Point less than −30° C.,
a second heating zone wherein the pre-heated steel sheet is heated from the heating temperature T1 to a heating temperature T2, in a nitriding atmosphere having a Dew Point between −30 and −10° C.,
a third heating zone wherein the pre-heated steel sheet is further heated from the heating temperature T2 to a soaking temperature T3, in a non nitriding atmosphere having a Dew Point less than −30° C.,
a soaking zone wherein the heated steel sheet is soaked at the soaking temperature T3 for a time t3, in a non nitriding atmosphere having a Dew Point less than −30° C. and,
a cooling zone wherein the steel sheet is cooled from the soaking temperature T3 to a temperature T4, in a non nitriding atmosphere having a Dew Point less than −30° C., and
(b) hot-dip galvanizing the annealed steel sheet to form a zinc-based coated steel sheet.
2. The process of claim 1 , wherein the nitriding atmosphere in the second heating zone comprises, by volume, 3 to 10% of ammonia, 3 to 10% of hydrogen, the balance of the composition being nitrogen and unavoidable impurities.
3. The process of claim 1 or 2 , wherein the heating temperature T1 is between 450 and 550° C.
4. The process of claim 1 or 2 , wherein the heating temperature T2 is between 480 and 750° C.
5. The process of claim 1 or 2 , wherein the soaking temperature T3 is between 720 and 850° C.
6. The process of claim 1 or 2 , wherein the time t3 is between 20 and 180 s.
7. The process of claim 1 or 2 , wherein the non nitriding atmosphere in the first heating, third heating, soaking and cooling zones comprises 3 to 10% by volume of hydrogen, the balance of the composition being nitrogen, and unavoidable impurities.
8. The process of claim 1 or 2 , wherein the temperature T4 is between 460 and 510° C.
9. The process of claim 1 or 2 , wherein a hot-dip galvanized steel sheet is produced, and wherein the hot-dip galvanizing is performed by hot-dipping the reduced steel sheet in a molten bath comprising from 0.14 to 0.3% by weight of aluminum, the balance being zinc and unavoidable impurities.
10. The process of claim 1 or 2 , wherein a hot-dip galvannealed steel sheet is produced, and wherein the hot-dip galvanizing is performed by hot-dipping the reduced steel sheet in a molten bath comprising from 0.08 to 0.135% by weight of aluminum, the balance being zinc and unavoidable impurities.
11. The process of claim 10 , wherein the content of molybdenum of the steel sheet is less than 0.01% by weight.
12. The process of claim 10 , wherein the alloying treatment is performed by heating the zinc-based coated steel sheet at a temperature T5 between 460 and 510° C. for a soaking time t5 between 10 and 30 s.
13. The process of claim 11 , wherein the alloying treatment is performed by heating the zinc-based coated steel sheet at a temperature T5 between 460 and 510° C. for a soaking time t5 between 10 and 30 s.
14. The process of claim 9 , wherein the temperature of the molten bath is between 450 and 500° C.
15. The process of claim 10 , wherein the temperature of the molten bath is between 450 and 500° C.
16. The process of claim 1 , wherein the steel sheet has a TRIP microstructure comprising ferrite and residual austenite.
17. The process of claim 1 , wherein the steel sheet comprises at least one layer of an internal nitride of at least one type of nitride selected from the group consisting of Si nitride, Mn nitride, Al nitride, complex nitride comprising Si and Mn and complex nitride comprising Si, Mn and Al.
18. The process of claim 17 , wherein the layer of internal nitride is formed at a depth between 2.0 and 12.0 μm from the surface of the steel sheet.
19. The process of claim 17 , wherein the steel sheet comprises no further layer of iron nitride.
20. The process of claim 1 further comprising:
(c) subjecting the zinc-based coated steel sheet to an alloying treatment to form a galvannealed steel sheet.
21. The process of claim 16 , wherein the TRIP microstructure includes martensite or bainite.
22. The process of claim 16 , wherein the TRIP microstructure includes martensite and bainite.
23. The process of claim 1 wherein ammonia is introduced to form the nitriding atmosphere.