	US 12,454,734 B2
	High strength steel sheet having excellent workability and method for manufacturing same
Jae-Hoon Lee, Gwangyang-si (KR); and Min-Seo Koo, Gwangyang-si (KR)
Assigned to POSCO, Pohang-si (KR)
Appl. No. 17/785,868
Filed by POSCO, Pohang-si (KR)
PCT Filed Nov. 24, 2020, PCT No. PCT/KR2020/016636 § 371(c)(1), (2) Date Jun. 15, 2022, PCT Pub. No. WO2021/125594, PCT Pub. Date Jun. 24, 2021.
Claims priority of application No. 10-2019-0169608 (KR), filed on Dec. 18, 2019.
Prior Publication US 2023/0023692 A1, Jan. 26, 2023
Int. Cl. C21D 9/46 (2006.01); C21D 6/00 (2006.01); C21D 8/02 (2006.01); C22C 38/00 (2006.01); C22C 38/06 (2006.01); C22C 38/20 (2006.01); C22C 38/24 (2006.01); C22C 38/26 (2006.01); C22C 38/28 (2006.01); C22C 38/30 (2006.01); C22C 38/32 (2006.01); C22C 38/34 (2006.01); C22C 38/44 (2006.01); C22C 38/58 (2006.01)

CPC C21D 9/46 (2013.01) [C21D 6/004 (2013.01); C21D 6/005 (2013.01); C21D 6/007 (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); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/005 (2013.01); C22C 38/008 (2013.01); C22C 38/06 (2013.01); C22C 38/20 (2013.01); C22C 38/24 (2013.01); C22C 38/26 (2013.01); C22C 38/28 (2013.01); C22C 38/30 (2013.01); C22C 38/32 (2013.01); C22C 38/34 (2013.01); C22C 38/44 (2013.01); C22C 38/58 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)]

8 Claims

1. A steel sheet comprising:

by wt %, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and unavoidable impurities, wherein a total content (Si+Al) of Si and Al is 2.51 to 6.0 wt %; and

as microstructures, 30 to 70 vol % of tempered martensite, 10 to 45 vol % of bainite, 10 to 40 vol % of retained austenite, 3 to 20 vol % of ferrite, and unavoidable structures,

wherein the steel sheet satisfies the following [Relational Expression 1] and [Relational Expression 2],

1.02≤[Si+Al]_F/[Si+Al]_av≤1.45 [Relational Expression 1]

in the above Relational Expression 1, [Si+Al]_Fis an average total content (wt %) of Si and Al included in the ferrite, and [Si+Al]_avis an average total content (wt %) of Si and Al included in the retained austenite,

V(1.2 μm,γ)/V(γ)≥0.1 [Relational Expression 2]

in the above Relational Expression 2, V(1.2 μm, γ) is a fraction (vol %) of the retained austenite having an average grain size of 1.2 μm or more, and V(γ) is a fraction (vol %) of the retained austenite of the steel sheet.