Steel for press hardening and press hardened part manufactured from such steel

Patent metadata

Field	Value
Publication number	US-2022017987-A1
Application number	US-202117491785-A
Country	US
Kind code	A1
Filing date	Oct 1, 2021
Priority date	Jul 9, 2015
Publication date	Jan 20, 2022
Grant date	—

Abstract

A press hardened steel part is provided. The steel of the part has a chemical composition including, in weight: 0.062%≤C≤0.095%, 1.4%≤Mn≤1.9%, 0.2%≤Si≤0.5%, 0.020%≤Al≤0.070%, 0.02%≤Cr≤0.1%, wherein: 1.5%≤(C+Mn+Si+Cr)≤2.7%, 0.040%≤Nb≤0.060%, 3.4×N≤Ti≤8×N wherein: 0.044%≤(Nb+Ti)≤0.090%, 0.0005≤B≤0.004%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020%, optionally: 0.0001%≤Ca≤0.003%, and the remainder being Fe and unavoidable impurities. The microstructure of the part includes, in a majority of the part, in surface fractions: less than 40% of bainite, less than 5% of austenite, less than 5% of ferrite, and a remainder being martensite. The martensite is fresh martensite and self-tempered martensite.

First claim

What is claimed is:

1 . A press hardened steel part comprising:
a chemical composition including, by weight:
0.062≤C≤0.095%;
1.4%≤Mn≤1.9%;
0.2%≤Si≤0.5%;
0.020%≤Al≤0.070%;
0.02%≤Cr≤0.1%;
 wherein: 1.5%≤(C+Mn+Si+Cr)≤2.7%;
0.040%≤Nb≤0.060%;
3.4×N≤Ti≤8×N;
 wherein: 0.044%≤(Nb+Ti)≤0.090%;
0.0005≤B≤0.004%;
0.001%≤N≤0.009%;
0.0005%≤S≤0.003%;
0.001%≤P≤0.020%;
a remainder being Fe and unavoidable impurities; and a microstructure including, in a majority of the part, in surface fractions: less than 40% of bainite, less than 5% of austenite, less than 5% of ferrite, a remainder being martensite, the martensite consisting of fresh martensite and self-tempered martensite.

2 . The press hardened steel part according to claim 1 , wherein 1.7%≤(C+Mn+Si+Cr)≤2.3%.

3 . The press hardened steel part according to claim 1 , wherein 0.065%≤C≤0.095%.

4 . The press hardened steel part according to claim 1 , wherein the microstructure comprises at least 5% in surface fraction of the self-tempered martensite.

5 . The press hardened steel part according to claim 1 , wherein a sum of the fresh martensite and the self-tempered martensite surface fractions is between 65 and 100%.

6 . The press hardened steel part according to claim 1 , wherein an average size of titanium nitrides is less than 2 micrometers in outer zones between one quarter thickness of the press hardened steel part and a closest surface of the press hardened steel part.

7 . The press hardened steel part according to claim 1 , wherein an average length of sulfides is less than 120 micrometers in outer zones between one quarter thickness of the press hardened steel part and a closest surface of the press hardened steel part.

8 . The press hardened steel part according to claim 1 , comprising at least one hot deformed zone (A) with a deformation quantity ε c higher than 0.15, and at least one zone (B) with the deformation quantity ε c of less than 0.05, the at least one hot deformed zone (A) and the at least one zone (B) experienced the same cooling cycle in press hardening.

9 . The press hardened steel part according to claim 8 , wherein a difference in hardness between the at least one zone (B) and the at least one hot deformed zone (A) is more than 20 HV.

10 . The press hardened steel part according to claim 8 , wherein an average lath width of a martensitic-bainitic structure in the at least one hot deformed zone (A) is reduced by more than 50% as compared to a lath width of a martensitic-bainitic structure in the at least one zone (B).

11 . The press hardened steel part according to claim 8 , wherein an average lath width of a martensitic-bainitic structure in the at least one hot deformed zone (A) is less than 1 μm.

12 . The press hardened steel part according to claim 8 , wherein an average lath width of a martensitic-bainitic structure in the at least one zone (B) is between 1 and 2.5 μm.

13 . The press hardened steel part according to claim 1 , further comprising a metallic coating coating the press hardened steel part.

14 . The press hardened steel part according to claim 13 , wherein the metallic coating is zinc-based alloy, or zinc alloy.

15 . The press hardened steel part according to claim 13 , wherein the metallic coating is aluminum-based alloy, or aluminum alloy.

16 . The press hardened steel part according to claim 1 , wherein the press hardened steel part has a yield stress between 700 and 950 MPa, a tensile stress TS between 950 and 1200 MPa, and a bending angle higher than 75°.

17 . The press hardened steel part according to claim 1 , wherein the press hardened steel part has a variable thickness.

18 . The press hardened steel part according to claim 17 , wherein the variable thickness is produced by a continuous flexible rolling process.

19 . A press hardened laser welded steel part, comprising at least one first steel part, the at least one first steel part being a part according to claim 15 , welded with at least one second steel part, a composition of which includes from 0.065 to 0.38% carbon by weight, and wherein a weld metal between the at least one first steel part and the at least one second steel part has an aluminum content less than 0.3% in weight, and wherein the at least one first steel part, the at least one second steel part, and the weld metal, are press hardened in a same operation.

20 . A process for manufacturing a press hardened laser welded steel part, comprising the successive steps of:
providing at least one first steel sheet with a composition containing, by weight:
0.062≤C≤0.095%;
1.4%≤Mn≤1.9%;
0.2%≤Si≤0.5%;
0.020%≤Al≤0.070%;
0.02%≤Cr≤0.1%;
 wherein: 1.5%≤(C+Mn+Si+Cr)≤2.7%;
0.040%≤Nb≤0.060%;
3.4×N≤Ti≤8×N;
 wherein: 0.044%≤(Nb+Ti)≤0.090%;
0.0005≤B≤0.004%;
0.001%≤N≤0.009%;
0.0005%≤S≤0.003%;
0.001%≤P≤0.020%; and
 a remainder being Fe and unavoidable impurities,
the at least one first steel sheet precoated with a metallic precoating of aluminum, or aluminum-based alloy, or aluminum alloy;
providing at least one second steel sheet with a composition containing, by weight, from 0.065 to 0.38% of carbon, the at least one second steel sheet precoated with a metallic precoating of aluminum, or aluminum-based alloy, or aluminum alloy;
removing a portion of a thickness of the aluminum precoating at upper and lower sides along one side of a periphery of the at least one first steel sheet and the at least one second steel sheet;
creating a welded blank by laser welding the at least one first steel sheet and the at least one second steel sheet, such that an aluminum content in a weld metal is lower than 0.3% by weight, the laser welding being performed along the periphery wherein the portion of the thickness of the aluminum precoating has been removed;
heating the welded blank and holding the welded blank at a temperature Tm between 890 and 950° C., a holding duration Dm at the temperature being between 1 and 10 minutes, so as to obtain a heated welded blank;
transferring the heated welded blank within a forming press, the transfer duration Dt being less than 10 s;
hot forming the heated welded blank in the forming press so as to obtain a welded formed part; and
cooling the welded formed part at a first cooling rate CR1 between 40 and 360° C./s in a temperature range between 750 and 450° C., and at a second cooling rate CR2 between 15 to 150° C./s in a temperature range between 450° C. and 250° C., wherein CR2<CR1.

21 . The process for manufacturing a press hardened laser welded steel part according to claim 20 , wherein the holding duration Dm is between 1 and 6 minutes.

22 . The process for manufacturing a press hardened laser welded steel part according to claim 20 , further comprising extracting the heated welding blank from a heating device prior to the transferring.

23 . A press hardened steel part manufactured according to claim 22 .

24 . A structural or safety part for a vehicle comprising:
the press hardened steel part according to claim 1 .

25 . The press hardened steel part according to claim 1 , further comprising 0.0001%≤Ca≤0.003%.

26 . A structural or safety part for a vehicle comprising:
the press hardened laser welded steel part according to claim 19 .

Assignees

• Arcelormittal

Inventors

• Drillet Pascal
• Poirier Maria
• Sarkar Sujay

Classifications

• C21D8/02 Primary

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• C21D1/673

  for die quenching · CPC title

• C22C38/44

  with molybdenum or tungsten · CPC title

• C22C38/48

  with niobium or tantalum · CPC title