	US 12,480,190 B2
	Steel
Yutaka Neishi, Tokyo (JP); Keisuke Chiba, Tokyo (JP); Kohei Kanetani, Kashiba (JP); and Takashi Sada, Kashiwara (JP)
Assigned to NIPPON STEEL CORPORATION, Tokyo (JP); and JTEKT CORPORATION, Aichi (JP)
Appl. No. 17/905,883
Filed by NIPPON STEEL CORPORATION, Tokyo (JP); and JTEKT CORPORATION, Kariya (JP)
PCT Filed Mar. 31, 2021, PCT No. PCT/JP2021/014004 § 371(c)(1), (2) Date Sep. 8, 2022, PCT Pub. No. WO2021/201159, PCT Pub. Date Oct. 7, 2021.
Claims priority of application No. 2020-063054 (JP), filed on Mar. 31, 2020.
Prior Publication US 2023/0118113 A1, Apr. 20, 2023
This patent is subject to a terminal disclaimer.
Int. Cl. C22C 38/46 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/04 (2006.01); C22C 38/06 (2006.01); C22C 38/20 (2006.01); C22C 38/22 (2006.01); C22C 38/24 (2006.01); C22C 38/26 (2006.01); C22C 38/28 (2006.01); C22C 38/32 (2006.01); C22C 38/44 (2006.01); C21D 6/00 (2006.01); C21D 8/02 (2006.01); C21D 9/00 (2006.01)

CPC C22C 38/46 (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 38/20 (2013.01); C22C 38/22 (2013.01); C22C 38/24 (2013.01); C22C 38/26 (2013.01); C22C 38/28 (2013.01); C22C 38/32 (2013.01); C22C 38/44 (2013.01); C21D 6/002 (2013.01); C21D 6/004 (2013.01); C21D 6/005 (2013.01); C21D 6/008 (2013.01); C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 8/0263 (2013.01); C21D 9/0081 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01); C21D 2211/009 (2013.01)]

2 Claims

1. A steel comprising, in mass %:

C: 0.25 to 0.45%,

Si: 0.10 to 0.50%,

Mn: 0.40 to 0.70%,

P: 0.015% or less,

S: 0.005% or less,

Cr: 0.80 to 1.50%,

Mo: 0.17 to 0.30%,

V: 0.24 to 0.40%,

Al: 0.005 to 0.100%,

N: 0.0300% or less,

O: 0.0015% or less, and

the balance being Fe and impurities, wherein

on a precondition that a content of each element in the steel falls within a range described above, Formula (1) to Formula (4) are satisfied, and

wherein:

a microstructure of the steel is composed of:

ferrite and pearlite having a total area fraction of 5.0 to 100.0%, and

a hard phase composed of bainite or bainite and martensite having a total area fraction of 0 to 95.0%;

the steel contains CaO—CaS—MgO—Al₂O₃composite oxides containing:

at least one selected from the group consisting of CaO and Cas,

MgO, and

Al₂O₃,

a proportion of a total area of the CaO—CaS—MgO-Al₂O₃composite oxides with respect to a total area of oxides in the steel is 30.0% or more; and

among oxides in the steel, a number density of oxides having an equivalent circle diameter of 20.0 μm or more is 15.0 pieces/mm²or less:

1.50<0.4Cr+0.4Mo+4.5V<2.45 (1)

2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<3.50 (2)

Mo/V≥0.58 (3)

(Mo+V+Cr)/(Mn+20P)≥2.00 (4)

where each symbol of an element in Formula (1) to Formula (4) is to be substituted by a content of a corresponding element in mass %, and is to be substituted by “0” if the corresponding element is not contained.