	US 11,850,821 B2
	Hot-pressed member, cold-rolled steel sheet for hot-pressed member, and method for producing the same
Katsutoshi Takashima, Tokyo (JP); Minoru Tanaka, Tokyo (JP); Kenji Tsuzumi, Tokyo (JP); Shingo Arakawa, Tokyo (JP); and Michitaka Sakurai, Tokyo (JP)
Assigned to JFE STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/425,847
Filed by JFE Steel Corporation, Tokyo (JP)
PCT Filed Dec. 26, 2019, PCT No. PCT/JP2019/051231 § 371(c)(1), (2) Date Jul. 26, 2021, PCT Pub. No. WO2020/158285, PCT Pub. Date Aug. 6, 2020.
Claims priority of application No. 2019-015062 (JP), filed on Jan. 31, 2019.
Prior Publication US 2022/0118741 A1, Apr. 21, 2022
Int. Cl. C21D 6/00 (2006.01); B32B 15/01 (2006.01); B21B 3/02 (2006.01); C21D 8/02 (2006.01); C21D 9/46 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/04 (2006.01); C22C 38/06 (2006.01); C22C 38/08 (2006.01); C22C 38/16 (2006.01); C22C 38/26 (2006.01); C22C 38/28 (2006.01); C22C 38/32 (2006.01); C23C 2/02 (2006.01); C23C 2/26 (2006.01); C23C 2/28 (2006.01); C25D 3/22 (2006.01)

CPC B32B 15/015 (2013.01) [B21B 3/02 (2013.01); C21D 6/001 (2013.01); C21D 6/002 (2013.01); C21D 6/005 (2013.01); C21D 6/008 (2013.01); C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 8/0236 (2013.01); C21D 8/0263 (2013.01); C21D 9/46 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/08 (2013.01); C22C 38/16 (2013.01); C22C 38/26 (2013.01); C22C 38/28 (2013.01); C22C 38/32 (2013.01); C23C 2/02 (2013.01); C23C 2/024 (2022.08); C23C 2/0224 (2022.08); C23C 2/26 (2013.01); C23C 2/28 (2013.01); C21D 2211/008 (2013.01); C25D 3/22 (2013.01)]

4 Claims

1. A hot-pressed member comprising:

a steel sheet; and

a surface layer disposed on a surface of the steel sheet,

the steel sheet having a chemical composition containing, in mass%,

C: 0.28% or greater and less than 0.50%,

Si: 0.01% or greater and 1.5% or less,

Mn: 1.0% or greater and 2.2% or less,

P: 0.05% or less,

S: 0.005% or less,

Al: 0.01% or greater and 0.50% or less,

N: 0.01% or less,

Sb: 0.001% or greater and 0.020% or less, and

one or more selected from

Mo: 0.005% or greater and 0.35% or less,

Cr: 0.005% or greater and 0.35% or less,

Nb: 0.001% or greater and 0.05% or less,

Ti: 0.001% or greater and 0.05% or less,

B: 0.0002% or greater and 0.0050% or less,

Ca: 0.005% or less,

V: 0.05% or less,

Cu: 0.50% or less,

Ni: 0.50% or less, and

Sn: 0.50% or less,

with a balance being Fe and incidental impurities,

wherein, in the hot-pressed member,

the steel sheet has a microstructure in which a prior austenite average grain diameter is 8 μm or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 μm of the surface of the steel sheet of the hot-pressed member,

a Ni diffusion region having a thickness of 0.5 μm or greater exists in the surface layer,

a standard deviation of Vickers hardness values of the hot-pressed member is 35 or less,

Mndif, a degree of Mn segregation in a sheet thickness direction, obtained by the following equation, is 0.20 mass% or less:

Mndif (mass%) =Mnmax (mass%)−Mnmin (mass%) ≤0.20, where

Mnmax (mass%) is a maximum value of amounts of Mn measured by EPMA line analysis in a sheet thickness direction, and

Mnmin (mass%) is a minimum value of the amounts of Mn measured by EPMA line analysis in a sheet thickness direction, and

the hot-pressed member has a tensile strength of 1780 MPa or greater, and

the hot-pressed member has satisfactory delayed fracture resistance if a bolt-tightened test piece, having a bolt secured to the hole of the nut of a welded body, upon immersion in a solution of hydrochloric acid (pH=2.0) at room temperature for 100 hours or more, displayed no fracture under either of the following loads:

(i) a load of Fs at which the nut was peeled off from the bolt-tightened test piece when an indentation-peel test was conducted in accordance with JIS B 1196: 2001, or

iii) a load of 0.7xFs,

wherein the bolt-tightened test piece was prepared by welding an M8 bolt in a hole of 9 mm diameter in the center of a test piece measuring 50 mm×150 mm cut from the hot-pressed member, and

wherein welding was performed at welding conditions of an electrode force of 3200 N, a welding time of 5 cycles (50 Hz), a welding current of 12 kA, and a holding time of 10 cycles (50 Hz).