Steel for Press Hardening and Press Hardened Part Manufactured from such Steel

Patent metadata

Field	Value
Publication number	US-2018202017-A1
Application number	US-201615741290-A
Country	US
Kind code	A1
Filing date	Jun 10, 2016
Priority date	Jul 9, 2015
Publication date	Jul 19, 2018
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 - 33 . (canceled)

34 : A press hardened steel part comprising:
a chemical composition of the steel 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%;

a remainder being Fe and unavoidable impurities; and a microstructure of the steel 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, a remainder being martensite, the martensite consisting of fresh martensite and self-tempered martensite.

35 : The press hardened steel part according to claim 34 , wherein 1.7%≤(C+Mn+Si+Cr)≤2.3%.

36 : The press hardened steel part according to claim 34 , wherein 0.065%≤C≤0.095%.

37 : The press hardened steel part according to claim 34 , wherein the microstructure comprises at least 5% in surface fraction of self-tempered martensite.

38 : The press hardened steel part according to claim 34 , wherein a sum of fresh martensite and of self-tempered martensite surface fractions is between 65 and 100%.

39 : The press hardened steel part according to claim 34 , wherein an average size of titanium nitrides is less than 2 micrometers in outer zones between one quarter thickness of the part and a closest surface of the part.

40 : The press hardened steel part according to claim 34 , wherein an average length of sulfides is less than 120 micrometers in outer zones between one quarter thickness of the part and a closest surface of the part.

41 : The press hardened steel part according to claim 34 , comprising at least one hot deformed zone (A) with a deformation quantity ε c higher than 0.15, and at least one zone (B) having experienced the same cooling cycle in press hardening as zone (A), wherein the deformation quantity ε c is less than 0.05.

42 : The press hardened part according to claim 41 , wherein the difference in hardness between the at least one zone (B) and the at least one hot deformed zone (A) is more than 20 HV.

43 : The press hardened part according to claim 41 , wherein an average lath width of the 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 the martensitic-bainitic structure in the at least one zone (B).

44 : The press hardened part according to claim 41 , wherein an average lath width of the martensitic-bainitic structure in the at least one hot deformed zone (A) is less than 1 μm.

45 : The press hardened part according to claim 41 , wherein an average lath width of the martensitic-bainitic structure in said at least one zone (B), is comprised between 1 and 2.5 μm.

46 : The press hardened steel part according to claim 33 , further comprising a metallic coating the part.

47 : The press hardened steel part according to claim 46 , wherein the metallic coating is zinc-based alloy, or zinc alloy.

48 : The press hardened steel part according to claim 46 , wherein the metallic coating is aluminum-based alloy, or aluminum alloy.

49 : The press hardened part according to claim 34 , wherein a yield stress is between 700 and 950 MPa, a tensile stress TS is between 950 and 1200 MPa, and a bending angle is higher than 75°.

50 : The press hardened steel part according to claim 34 , wherein the press hardened steel part has a variable thickness.

51 : The press hardened steel part according to claim 50 , wherein the variable thickness is produced by a continuous flexible rolling process.

52 : A press hardened laser welded steel part, comprising at least one first steel part of the weld being a part according to claim 48 , welded with at least one second steel part, a composition of which includes from 0.065 to 0.38% of carbon in weight, and wherein the 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.

53 : A process for manufacturing a press hardened steel part comprising the successive steps of:
providing a steel semi-product with a composition including:
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;
hot rolling such semi-product to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a coiling temperature Tc comprised between 550° C. and Ms, Ms being a martensitic transformation start temperature of the steel sheet, so as to obtain a coiled steel sheet; optionally cold rolling the coiled steel sheet; annealing the steel sheet at an annealing temperature Ta in order to obtain less than 10% of unrecrystallized area fraction, so as to obtain an annealed steel sheet; cutting the annealed steel sheet to a predetermined shape, so as to obtain a blank; heating the blank and holding the 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 blank; transferring said heated blank within a forming press, the transfer duration Dt being less than 10 s; hot forming the heated blank in the forming press so as to obtain a formed part; cooling the formed part at a cooling rate CR1 between 40 and 360° C./s in a temperature range between 750 and 450° C., and at an cooling rate CR2 between 15 to 150° C./s in a temperature range between 450° C. and 250° C., wherein CR2≤CR1.

54 : The process for manufacturing a press hardened steel part according to claim 53 , wherein the cold rolled sheet is cold rolled with a rolling ratio between 50 and 80%.

55 : The process for manufacturing a press hardened steel part according to claim 53 , wherein the annealing temperature Ta is between 800 and 850° C.

56 : The process for manufacturing a press hardened steel part according to claim 53 , wherein the annealing temperature Ta is between 800 and 835° C.

57 : The process for manufacturing a press hardened steel part according to claim 53 , wherein the blank is cold formed before heating the blank at the temperature Tm.

58 : The process for manufacturing a press hardened steel part according to claim 53 , wherein the hot forming is performed with a deformation quantity ε c higher than 0.15 in at least one hot deformed zone of the part,

59 : The process for manufacturing a press hardened steel part according to claim 53 , wherein the annealed steel sheet is precoated with metallic precoating before cutting the annealed steel blank to a predetermined shape.

60 : The process for manufacturing a press hardened steel part according to claim 59 , wherein the metallic precoating is zinc, or zinc-based alloy, or zinc alloy.

61 : The process for manufacturing a press hardened steel part according to claim 59 , wherein

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

• C22C21/02

  with silicon as the next major constituent · CPC title

• C21D8/0278

  involving a particular surface treatment  (C21D8/0294 takes precedence) · CPC title

• C22C38/44

  with molybdenum or tungsten · CPC title

• C22C38/46

  with vanadium · CPC title