Method for the manufacture of a hardened part which does not have LME issues
The present invention provides a method for the manufacture of a hardened part. The method includes the provision of a steel sheet pre-coated with a metallic coating including from 2.0 to 24.0% by weight of zinc, from 1.1 to 7.0% by weight of silicon, optionally from 1.1 to 8.0% by weight of magnesium when the amount of silicon is between 1.1 and 4.0%, and optionally additional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each additional element being less than 0.3% by weight, the balance being aluminum and unavoidable impurities and residuals elements, wherein the ratio Al/Zn is above 2.9. The method further includes the cutting of the coated steel sheet to obtain a blank, the thermal treatment of the blank at a temperature between 840 and 950° C. to obtain a fully austenitic microstructure in the steel, the transfer of the blank into a press tool, the hot-forming of the blank to obtain a part, the cooling of the part in order to obtain a microstructure in steel being martensitic or martensitic-bainitic or made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.
1. A method for the manufacture of a hardened part comprising the following steps:
A) providing a steel sheet pre-coated with a metallic coating comprising:
2.0 to 24.0% by weight of zinc;
1.1 to 7.0% by weight of silicon;
optionally from 0.5 to 3.0% by weight of magnesium when the amount of silicon is between 1.1 and 4.0%,
and optionally additional elements chosen from Pb, Ni, Zr or Hf, a content by weight of each additional element being less than 0.3% by weight,
a balance being aluminum, unavoidable impurities and residual elements resulting from feeding ingots or from a passage of the steel sheet in a molten bath; and
a ratio Al/Zn by weight greater than 2.9;
B) cutting the coated steel sheet to obtain a blank;
C) performing a thermal treatment of the blank at a temperature between 840 and 950° C. to obtain a fully austenitic microstructure in the steel;
D) transferring the blank into a press tool;
E) hot-forming the blank to obtain a part; and
F) cooling the part in order to obtain a microstructure in the steel being martensitic or martensitic-bainitic or made of at least 75% of equiaxed ferrite, 5 to 20% of martensite and bainite in an amount less than or equal to 10%.
2. The method according to claim 1 , further comprising the magnesium at 0.5 to 3.0% by weight and the silicon between 1.1 and 4.0% by weight.
3. The method according to claim 1 , further comprising at least one of the additional elements chosen from Pb, Ni, Zr, or Hf.
4. The method according to claim 1 , wherein the ratio Al/Zn is between 5 and 9.
5. The method according to claim 1 , wherein a ratio Zn/Si is between 2.9 and 8.
6. The method according to claim 1 , wherein the coating comprises from 2.0 to 5.0% by weight of silicon.
7. The method according to claim 6 , wherein the coating comprises from 2.1 to 4.9% by weight of silicon.
8. The method according to claim 1 , wherein the coating comprises from 1.5 to 3.5% by weight of silicon.
9. The method according to claim 1 , wherein the coating comprises from 4.5 to 5.5% by weight of silicon.
10. The method according to claim 1 , wherein the coating comprises from 5.0 to 19.0% by weight of zinc.
11. The method according to claim 10 , wherein the coating comprises from 5.0 to 15.0% by weight of zinc.
12. The method according to claim 11 , wherein the coating comprises from 10.0 to 15.0% by weight of zinc.
13. The method according to claim 1 , wherein the coating comprises from 0.5 to 3.0% by weight of magnesium.
14. The method according to claim 13 , wherein the coating comprises from 1.0 to 2.9% by weight of magnesium.
15. The method according to claim 1 , wherein the coating comprises from 3.1 to 8.0% by weight of magnesium.
16. The method according to claim 15 , wherein the coating comprises from 4.0 to 8.0% by weight of magnesium.
17. The method according to claim 1 , wherein the coating comprises greater than 71% by weight of aluminum.
18. The method according to claim 1 , wherein the coating comprises greater than 76% by weight of aluminum.
19. The method according to claim 1 , wherein a thickness of the coating is between 5 and 50 μm.
20. The method according to claim 19 , wherein the thickness is between 10 and 35 μm.
21. The method according to claim 20 , wherein the thickness is between 12 and 18 μm.
22. The method according to claim 20 , wherein the thickness is between 26 and 31 μm.
23. A method for the manufacture of a hardened part comprising the following steps:
A) providing a steel sheet pre-coated with a metallic coating comprising:
2.0 to 24.0% by weight of zinc;
1.1 to 7.0% by weight of silicon;
optionally from 0.5 to 3.0% by weight of magnesium when the amount of silicon is between 1.1 and 4.0%,
and optionally additional elements chosen from Pb, Ni, Zr or Hf, a content by weight of each additional element being less than 0.3% by weight,
wherein the coating does not comprise elements selected among Cr, Mn, Ti, Ce, La, Nd, Pr, Ca, Bi, In, Sn and Sb or combinations thereof,
a balance being aluminum, unavoidable impurities and residual elements resulting from feeding ingots or from a passage of the steel sheet in a molten bath; and
a ratio Al/Zn by weight greater than 2.9;
B) cutting the coated steel sheet to obtain a blank;
c) performing a thermal treatment of the blank at a temperature between 840 and 950° C. to obtain a fully austenitic microstructure in the steel;
D) transferring the blank into a press tool;
E) hot-forming the blank to obtain a part; and
F) cooling the part in order to obtain a microstructure in the steel being martensitic or martensitic-bainitic or made of at least 75% of equiaxed ferrite, 5 to 20% of martensite and bainite in an amount less than or equal to 10%.
24. The method according to claim 1 , wherein step C) is performed during a dwell time between 1 to 12 minutes in an inert atmosphere or an atmosphere comprising air.
25. The method according to claim 1 , wherein during step E) the hot-forming of the blank is at a temperature between 600 and 830° C.
26. The method as recited in claim 1 wherein step D) occurs after step C).
27. The method as recited in claim 1 wherein the steel sheet has a base made of 22MnB5 steel.
28. The method as recited in claim 1 wherein the steel sheet has a base with a carbon content by weight of between 0.24% and 0.38%.
29. The method as recited in claim 1 wherein the steel sheet has a base with a composition by weight percentage including: 0.24%≤C≤0.38%; 0.40%≤Mn≤3%; 0.10%≤Si≤0.70%; 0.015%≤Al≤0.070%; 0%≤Cr≤2%; 0.25%≤Ni≤2%; 0.020%≤Ti≤0.10%; 0%≤Nb≤0.060%; 0.0005%≤B≤0.0040%; 0.003%≤N≤0.010%; 0.0001%≤S≤0.005%; 0.0001%≤P≤0.025%; it being understood that the contents of titanium and nitrogen satisfy Ti/N>3.42.
30. The method as recited in claim 1 wherein the steel sheet has a base with a composition by weight percentage including: 0.040%≤C≤0.100%; 0.80%≤Mn≤2.00%; 0%≤Si≤0.30%; 0%≤S≤0.005%; 0%≤P≤0.030%; 0.010%≤Al≤0.070%; 0.015%≤Nb≤0.100%; 0.030%≤Ti≤0.080%; 0%≤N≤0.009%; 0%≤Cu≤0.100%; 0%≤Ni≤0.100%; 0%≤Cr≤0.100%; 0%≤Mo≤0.100%; 0%≤Ca≤0.006%.
31. The method as recited in claim 1 wherein the press tool after the hot forming has no coating smearing.
32. The method as recited in claim 1 , wherein the coating does not include In and the coating does not include Sn.