Hot-formed previously welded steel part with very high mechanical resistance and production method

Patent metadata

Field	Value
Publication number	US-10828729-B2
Application number	US-201715637913-A
Country	US
Kind code	B2
Filing date	Jun 29, 2017
Priority date	Jul 26, 2011
Publication date	Nov 10, 2020
Grant date	Nov 10, 2020

Abstract

A welded steel part with a very high mechanical strength is provided. The welded steel part is obtained by heating followed by hot forming, then cooling of at least one welded blank obtained by butt welding of at least one first and one second sheet. The at least one first and second sheets including, at least in part, a steel substrate and a pre-coating which includes an intermetallic alloy layer in contact with the steel substrate, topped by a metal alloy layer of aluminum or aluminum-based alloy. A method for the fabrication of a welded steel part and the fabrication of structural or safety parts for automotive vehicles are also provided.

First claim

What is claimed is:

1. A method of fabricating a welded steel part comprising:
providing at least a first and a second steel sheet each having a steel substrate and a pre-coating which includes an AlSiFe intermetallic alloy layer in contact with the steel substrate,
the first and second steel sheets having different compositions or thicknesses,
a composition of the steel substrate of the first and the second steel sheets comprising the following elements, expressed in percent by weight:
0.10%≤C≤0.5%;
0.5%≤Mn≤3%;
0.1%≤Si≤1%;
0.01%≤Cr≤1%;
Ti≤0.2%;
Al≤0.1%;
S≤0.05%;
P≤0.1%; and
0.0002%≤B≤0.010%,

the balance being iron and unavoidable impurities from processing;
laser butt welding the first and the second steel sheets at respective peripheral edges of the first and second steel sheets to form a weld metal zone by using a filler wire over at least a portion of the length of the weld metal zone; then
heating the welded blank to give the weld metal zone a microstructure that contains no ferrite;
wherein the weld metal zone has an increased carbon content with respect to a carbon content of the substrate of the first or second steel sheet having the higher carbon content, such that a mechanical strength of the weld metal zone is greater than that of the first and second steel sheet;
wherein the welded steel part that has a mechanical strength greater than 1230 MPa.

2. The method of claim 1 , wherein a fracture occurs in the base metal and not in the weld metal zone when the weld joint is subjected to a uniaxial tensile stress perpendicular to the joint.

3. The method of claim 1 , wherein the increased carbon content of the weld metal zone resulting from the welding operation and constituting the bond between the first and second sheets is at least 1.27 times a carbon content of the substrate of the first or second steel sheet having the higher carbon content.

4. The method of claim 1 , wherein an Mn content of the filler wire is 4% by weight, and an Mn content of the steel substrate of the first or second sheet is 2.3% by weight.

5. The method of claim 1 , wherein a carbon content of the filler wire is 0.6%≤C≤1.5% by weight, and wherein the substrate of the first and second steel sheets have a carbon content between 0.1%≤C≤0.5% by weight.

6. The method of claim 1 , wherein the filler wire comprises the following elements, expressed in percent by weight:
0.6%≤C≤1.5%;
1%≤Mn≤4%;
0.1%≤Si≤0.6%;
Cr≤2%; and
Ti≤0.2%;
the balance being iron and unavoidable impurities from processing.

7. The method of claim 1 , further comprising, prior to the providing step, removing the metal alloy layer from a portion of the first and second steel sheets, leaving in place the intermetallic alloy layer.

8. The method of claim 7 , further comprising, after the heating step, hot forming and heating the welded blank to obtain a steel part, then cooling the steel part at a controlled rate.

9. The method of claim 1 , wherein, on respective cut edges of the peripheral edges of the first and second sheets destined to be subjected to the welding operation, aluminum or aluminum alloy is removed.

10. The method of claim 9 , wherein the aluminum or aluminum alloy on the respective cut edges results from a previous cutting operation of each of the first and second sheets.

11. The method as in claim 1 , wherein an Mn content of the filler wire is 1%≤Mn≤4% by weight and an Mn content of the substrate of the first and second steel sheet is 0.8%≤Mn≤2.3% by weight.

12. The method of claim 1 , wherein an Mn content of the filler wire is 1%≤Mn≤4% by weight and an Mn content of the substrate of the first or second steel sheet is 0.8%≤Mn≤1.8% by weight.

13. The method of claim 1 , wherein said heating comprises heating to a temperature of 900 degrees C.

14. A method for the fabrication of a welded steel part comprising
providing at least a first steel sheet with a pre-coating including an intermetallic alloy layer;
providing at least a second steel sheet with a pre-coating including an intermetallic alloy layer, the first and second steel sheets having different compositions or thicknesses;
forming a welded blank by butt laser welding the at least first and the at least second steel sheets to form a weld metal zone and form a bond between the at least first and the at least second sheets, wherein a filler metal wire is used over at least a portion of the weld metal zone;
heating the welded blank in an austenitic range followed by hot forming and then cooling to form the welded steel part;
wherein the weld metal zone has an increased carbon content with respect to a carbon content of the substrate of the first or second steel sheet having the higher carbon content, such that a mechanical strength of the weld metal zone is greater than that of the first and second steel sheet;
wherein a ratio between a hardness of the weld metal zone and a hardness of a substrate of the first or second steel sheet that has the higher carbon content Cmax is greater than 1.029+(0.36 Cmax), where Cmax is expressed in percent by weight.

15. The method of claim 14 , wherein the intermetallic alloy layer is an AlSiFe intermetallic alloy layer.

16. The method of claim 14 , wherein the C carbon content of the first or second steel sheet having the higher carbon content is 0.15%≤C≤0.25% by weight.

17. The method of claim 16 , wherein the C carbon content of the steel sheet having the higher carbon content is 0.2%≤C≤0.25% by weight.

18. The method of claim 17 , wherein the C carbon content of the filler wire is 0.65%≤C≤0.75% by weight.

19. The method of claim 14 , wherein an Mn content of the filler wire is 1%≤Mn≤4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.8% %≤Mn≤2.3% by weight.

20. The method of claim 14 , wherein an Mn content of the filler wire is 1%≤Mn≤4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.8% %≤Mn≤1.8% by weight.

21. The method of claim 14 , wherein an Mn content of the filler wire is 1%≤Mn≤4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.8% %≤Mn≤2.3% by weight.

22. The method of claim 14 , wherein an Mn content of the filler wire is 1.95%≤Mn≤2.05% by weight.

23. The method of claim 14 , wherein an Mn content of the filler wire is 1%≤Mn≤4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.5%≤Mn≤2.3% by weight.

24. The method of claim 14 , wherein the carbon content of the weld metal zone is less than or equal to 0.35% by weight.

25. The method of claim 14 , wherein a microstructure of the weld metal zone contains no ferrite.

26. The method of claim 14 , wherein a microstructure of the weld metal zone is martensitic.

27. The method of claim 14 , wherein the heating comprises heating to a temperature of 900 C.

Assignees

• Arcelormittal

Inventors

• Cretteur Laurent
• Yin Qingdong
• Schmit Francis
• Ehling Wolfram

Classifications

• B23K35/3053 Primary

  Fe as the principal constituent · CPC title

• C22C38/04

  containing manganese · CPC title

• C21D9/48

  deep-drawing sheets · CPC title

• C21D2251/02

  Clad material · CPC title

• B23K26/60

  Preliminary treatment · CPC title