US 11,911,847 B2
Method for producing a press-hardened laser welded steel part and press-hardened laser welded steel part
Francis Schmit, Clermont (FR); Maria Poirier, Villers-Saint-Paul (FR); and Sadok Gaied, Saint Maximin (FR)
Assigned to ARCELORMITTAL, Luxembourg (LU)
Appl. No. 16/976,392
Filed by ArcelorMittal, Luxembourg (LU)
PCT Filed Feb. 26, 2019, PCT No. PCT/IB2019/051528
§ 371(c)(1), (2) Date Aug. 27, 2020,
PCT Pub. No. WO2019/166941, PCT Pub. Date Sep. 6, 2019.
Prior Publication US 2021/0053154 A1, Feb. 25, 2021
Int. Cl. B23K 26/322 (2014.01); B23K 26/323 (2014.01); B23K 26/12 (2014.01); B23K 26/32 (2014.01); C21D 9/50 (2006.01); C21D 9/46 (2006.01); C21D 1/673 (2006.01); C21D 1/18 (2006.01); B23K 35/30 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/06 (2006.01); C22C 38/42 (2006.01); C22C 38/44 (2006.01); C22C 38/50 (2006.01); C22C 38/54 (2006.01); C22C 38/58 (2006.01); B62B 3/02 (2006.01); B62B 3/10 (2006.01); B62B 3/12 (2006.01); B23K 101/18 (2006.01); B23K 103/04 (2006.01); B23K 101/00 (2006.01); B23K 26/24 (2014.01); C22C 38/48 (2006.01)
CPC B23K 26/322 (2013.01) [B23K 26/123 (2013.01); B23K 26/32 (2013.01); B23K 26/323 (2015.10); B23K 35/3073 (2013.01); B62B 3/02 (2013.01); B62B 3/106 (2013.01); B62B 3/12 (2013.01); C21D 1/185 (2013.01); C21D 1/673 (2013.01); C21D 9/46 (2013.01); C21D 9/50 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/06 (2013.01); C22C 38/42 (2013.01); C22C 38/44 (2013.01); C22C 38/50 (2013.01); C22C 38/54 (2013.01); C22C 38/58 (2013.01); B23K 26/24 (2013.01); B23K 2101/006 (2018.08); B23K 2101/185 (2018.08); B23K 2103/04 (2018.08); B62B 2202/22 (2013.01); B62B 2205/30 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01); C22C 38/48 (2013.01)] 19 Claims
OG exemplary drawing
 
1. A method for producing a press-hardened laser welded steel part comprising the following successive steps:
providing a first precoated steel sheet and a second precoated steel sheet, each of the first and second precoated steel sheets comprising a steel substrate, at least one of the first and second precoated steel sheets having, on at least one of its main faces, an aluminum-containing precoating comprising at least 50% by weight of aluminum,
the first precoated steel sheet having a first thickness and the second precoated steel sheet having a second thickness,
the steel substrate of the first precoated steel sheet having, after press-hardening, an ultimate tensile strength greater than the ultimate tensile strength, after press-hardening, of the steel substrate of the second precoated steel sheet, and
the product of the first thickness by the ultimate tensile strength, after press-hardening, of the first precoated steel sheet being greater than the product of the second thickness by the ultimate tensile strength, after press-hardening, of the second precoated steel sheet; then
butt welding the first precoated steel sheet and the second precoated steel sheet using laser welding so as to obtain a weld joint between the first and second precoated steel sheets thereby obtaining a welded blank, the butt welding step optionally including using a filler material;
heating the welded blank to a heat treatment temperature, the heat treatment temperature being at least 10° C. lower than the full austenitization temperature of the weld joint and at least 15° C. higher than a minimum temperature Tmin, where

OG Complex Work Unit Math
where
Ac3(WJ) is the full austenitization temperature of the weld joint, in ° C. and Al is the content of aluminum in the weld joint, in wt. %,
and αICmax is the maximum intercritical ferrite content of the weld joint, calculated using the following formula:

OG Complex Work Unit Math
where
TS1 is the ultimate tensile strength of a stronger one of the two steel substrates after press-hardening, in MPa,
TS2 is the ultimate tensile strength of a weaker one of the two steel substrates after press-hardening, in MPa,
CFW is the carbon content of the filler material, in wt. %,
β is the proportion of the filler material added to a weld pool, comprised between 0 and 1,
ρ is the ratio between the thickness of the precoated steel sheet comprising the weaker steel substrate and the thickness of the precoated steel sheet comprising the stronger steel substrate (ρ=t2/t1);
holding the welded blank at the heat treatment temperature for a time comprised between 2 and 10 minutes;
press-forming the welded blank into a steel part; and
cooling the thus formed steel part with a cooling speed greater than or equal to the critical martensitic or bainitic cooling speed of the most hardenable steel substrate among the steel substrates of the first and second precoated steel sheets so as to obtain a press-hardened welded steel part.