Press hardened part with high resistance to delayed fracture and a manufacturing process thereof

Patent metadata

Field	Value
Publication number	US-11725255-B2
Application number	US-201817311677-A
Country	US
Kind code	B2
Filing date	Dec 18, 2018
Priority date	Dec 18, 2018
Publication date	Aug 15, 2023
Grant date	Aug 15, 2023

Abstract

A press hardened coated steel part with high resistance to delayed fracture, the coating containing (Fe x —Al y ) intermetallic compounds resulting from the diffusion of iron into an aluminum or an aluminum-based alloy, or an aluminum alloy of a precoating, wherein the chemical composition of the steel includes, in weight: 0.16%≤C≤0.42%, 0.1%≤Mn≤3%, 0.07%≤Si≤1.60%, 0.002%≤Al≤0.070%, 0.02%≤Cr≤1.0%, 0.0005≤B≤0.005%, 0.002%≤Mg≤0.007%, 0.002%≤Ti≤0.11%, 0.0008%≤O≤0.005%, wherein (Ti)×(O) 2 ×10 7 ≤2, 0.001%≤N≤0.007%, 0.001%≤S≤0.005%, 0.001%≤P≤0.025% and optionally one or more elements selected from the list of: 0.005%≤Ni≤0.23%, 0.005%≤Nb≤0.060%, the remainder being Fe and unavoidable impurities, and wherein the microstructure includes at least 95% martensite.

First claim

What is claimed is:

1. A press hardened coated steel part, comprising a base of steel and a coating, the coating containing (Fex-Aly) intermetallic compounds resulting from the diffusion of iron into an aluminum or an aluminum-based alloy, or an aluminum alloy of a precoating, wherein the chemical composition of the steel comprises, in weight:
0.16%≤C≤0.42%
0.1%≤Mn≤3%
0.07%≤Si≤1.60%
0.002%≤Al≤0.070%
0.02%≤Cr≤1.0%,
0.0005≤B≤0.005%
0.002%≤Mg≤0.007%
0.002%≤Ti≤0.11%
0.0008%≤0≤0.005%
wherein (Ti)×(0) 2 ×107≤2
0.001%≤N≤0.007%
0.001%≤S≤0.005%
0.001%≤P≤0.025%
and optionally one or more elements selected from:
0.005%≤Ni≤0.23%, and
0.005%≤Nb≤0.060%,
a remainder being Fe and unavoidable impurities,

and wherein a microstructure of the steel includes at least 95% martensite, by area or volume fraction;
wherein a tensile strength of the press hardened steel part is between 1400 and 2000 MPa;
and wherein, the press hardened steel part has a resistance to delayed fracture greater than or equal to 3×10 16 ×TS −4.345 +100, wherein TS is said tensile strength.

2. The press hardened coated steel part as recited in claim 1 wherein, 0.18%≤C≤0.35%.

3. A press hardened coated steel part, comprising a base of steel and a coating, the coating containing (Fex-Aly) intermetallic compounds resulting from the diffusion of iron into an aluminum or an aluminum-based alloy, or an aluminum alloy of a precoating, wherein the chemical composition of the steel comprises, in weight:
0.16%≤C≤0.42%
0.55%≤Mn≤1.40%
0.07%≤Si≤1.60%
0.002%≤Al≤0.070%
0.02%≤Cr≤1.0%,
0.0005≤B≤0.005%
0.002%≤Mg≤0.007%
0.002%≤Ti≤0.11%
0.0008%≤0≤0.005%
wherein (Ti)×(0) 2 ×107≤2
0.001%≤N≤0.007%
0.001%≤S≤0.005%
0.001%≤P≤0.025%
and optionally one or more elements selected from:
0.005%≤Ni≤0.23%, and
0.005%≤Nb≤0.060%,
a remainder being Fe and unavoidable impurities,

and wherein a microstructure of the steel includes at least 95% martensite, by area or volume fraction.

4. The press hardened coated steel part as recited in claim 1 wherein, 0.07%≤Si≤0.30%.

5. A press hardened coated steel part, comprising a base of steel and a coating, the coating containing (Fex-Aly) intermetallic compounds resulting from the diffusion of iron into an aluminum or an aluminum-based alloy, or an aluminum alloy of a precoating, wherein the chemical composition of the steel comprises, in weight:
0.16%≤C≤0.42%
0.1%≤Mn≤3%
0.07%≤Si≤1.60%
0.002%≤Al≤0.070%
0.02%≤Cr≤1.0%,
0.0005≤B≤0.005%
0.002%≤Mg≤0.007%
0.002%≤Ti≤0.11%
0.0008%≤0≤0.005%
wherein (Ti)×(0) 2 ×107≤2
0.001%≤N≤0.007%
0.001%≤S≤0.005%
0.001%≤P≤0.025%
and optionally one or more elements selected from:
0.005%≤Ni≤0.23%, and
0.005%≤Nb≤0.060%,
a remainder being Fe and unavoidable impurities,

wherein a microstructure of the steel includes at least 95% martensite, by area or volume fraction; and
wherein an average size d av of oxides, carbonitrides, sulfides and oxisulfides is less than 1.7 μm and wherein at least one of conditions (C1) or (C2) is fulfilled:
(C1): the sum N(MgO+MgO—Al 2 O 3 ) of the numbers of MgO and MgO—Al 2 O 3 particles per area unit is higher than 90 per mm 2 ,
(C2): the number N(MgO-TixOy) of MgO-TixOy particles per area unit is higher than 100 per mm 2 with the average size lower than 1 μm.

6. The press hardened coated steel part as recited in claim 1 , wherein the microstructure contains bainite and/or ferrite.

7. The press hardened coated steel part as recited in claim 1 , wherein a thickness of the coated steel part is comprised between 0.8 and 4 mm.

8. The press hardened coated steel part as recited in claim 1 , wherein a yield stress is higher than 1000 MPa.

9. A process for manufacturing a press hardened coated steel part according to claim 1 , comprising the following and successive steps:
providing liquid steel comprising by weight: 0.16%≤C≤0.42%, 0.1%≤Mn≤3%, 0.07%≤Si≤1.60%, 0.002%≤Al≤0.070%, 0.02%≤Cr≤1.0%, 0.0005≤B≤0.005%, 0.002%≤Ti≤0.11%, 0.001%≤0≤0.008% wherein (Ti)×(0) 2 ×107≤2, 0.001%≤N≤0.007%, and optionally: 0.005%≤Ni≤0.23%, 0.005%≤Nb≤0.060%, 0.001%≤S≤0.005%, and 0.001%≤P≤0.025%, a remainder being Fe and unavoidable impurities, then;
adding Mg or Mg-alloy at a temperature T addition so as to obtain a further liquid steel with a chemical composition by weight: 0.16%≤C≤0.42%, 0.1%≤Mn≤3%, 0.07%≤Si≤1.60%, 0.002%≤Al≤0.070%, 0.02%≤Cr≤1.0%, 0.0005≤B≤0.005%, 0.002%≤Mg≤0.007%, 0.002%≤Ti≤0.11%, 0.0008%≤0≤0.005%, wherein (Ti)×(0) 2 ×107≤2, 0.001%≤N≤0.007%, 0.001%≤S≤0.005%, 0.001%≤P≤0.025%, and optionally one or more elements selected from: 0.005%≤Ni≤0.23%, and 0.005%≤Nb≤0.060%, a remainder being Fe and unavoidable impurities, the temperature T addition being between T liquids and (T liquids +70° C.); then
casting the liquid steel in the form of a semi-product, a duration t D elapsing between the addition of Mg or Mg alloy and the solidification start of the liquid steel being less than 30 minutes; then
heating the semi-product at a temperature between 1250 and 1300° C. so to obtain a heated semi-product; then
rolling the semi-product so to obtain a rolled steel sheet; then
precoating the rolled steel sheet with aluminum or aluminum-based alloy, or aluminum alloy so to obtain a precoated steel sheet; then
cutting the precoated steel sheet so to obtain a precoated steel blank; then
heating the precoated steel blank so to obtain a heated blank with a full austenitic structure; then
hot press forming the heated blank so to obtain a hot press formed part, then
cooling the hot press formed part while maintaining the hot press formed part in a press tooling so as to obtain a press hardened coated steel part with a microstructure comprising at least 95% martensite, by area or volume fraction;

wherein a tensile strength of the press hardened steel part is between 1400 and 2000 MPa;
and wherein, the press hardened steel part has a resistance to delayed fracture greater than or equal to 3×10 16 ×TS −4.345 +100, wherein TS is said tensile strength.

10. The process as recited in claim 9 , wherein the duration t D is less than 1 minute.

11. The process as recited in claim 9 , wherein the duration t D is less than 10 s.

12. The process as recited in claim 9 , wherein a cooling rate Vs at a surface of the semi-product is higher than 30° C./s.

13. The process as recited in claim 9 , wherein the heating of the precoated steel blank is performed up to a temperature Om between 890 and 950° C. and a total dwell time t m between 1 and 10 minutes.

14. The process as recited in claim 9 , wherein the heating of the precoated steel blank is performed in a furnace with an atmosphere having a dew point comprised between +10 and +25° C.

15. The process as recited in claim 9 , wherein a thickness of the precoated steel sheet is comprised between 0.8 and 4 mm.

16. The process as recited in claim 9 , wherein a yield stress of the press hardened coated steel part is higher than 1000 MPa.

17. A method for fabrication of structural or safety parts of motor vehicles comprising performing the process as recited in claim 9 .

18. A method for fabrication of structural or safety parts of motor vehicles comprising using the press hardened coated steel as recited in claim 1 .

Assignees

• Arcelormittal

Inventors

• Remy Blandine
• Sturel Thierry
• Lucas Emmanuel
• Boi Gianni

Classifications

• C21D8/02

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

• C23C2/26

  After-treatment (C23C2/14 takes precedence) · CPC title

• C22C38/12

  containing tungsten, tantalum, molybdenum, vanadium, or niobium · CPC title

• C23C2/50

  Controlling or regulating the coating processes (C23C2/14 takes precedence) · CPC title

• C21D2211/002

  Bainite · CPC title