	US 12,252,759 B2
	High strength steel sheet having excellent workability and method for manufacturing same
Jae-Hoon Lee, Gwangyang-si (KR); Min-Seo Koo, Gwangyang-si (KR); and Tae-Oh Lee, Gwangyang-si (KR)
Assigned to POSCO CO., LTD, Pohang-si (KR)
Appl. No. 17/785,168
Filed by POSCO, Pohang-si (KR)
PCT Filed Nov. 23, 2020, PCT No. PCT/KR2020/016559 § 371(c)(1), (2) Date Jun. 14, 2022, PCT Pub. No. WO2021/125589, PCT Pub. Date Jun. 24, 2021.
Claims priority of application No. 10-2019-0169613 (KR), filed on Dec. 18, 2019.
Prior Publication US 2023/0025863 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 38/02 (2006.01); C22C 38/04 (2006.01); C22C 38/06 (2006.01); C22C 38/08 (2006.01); C22C 38/12 (2006.01); C22C 38/14 (2006.01); C22C 38/16 (2006.01); C22C 38/22 (2006.01); C22C 38/34 (2006.01); C22C 38/38 (2006.01)

CPC C21D 9/46 (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); 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 38/12 (2013.01); C22C 38/14 (2013.01); C22C 38/16 (2013.01); C22C 38/22 (2013.01); C22C 38/34 (2013.01); C22C 38/38 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)]

9 Claims

1. A high strength steel sheet having excellent workability, 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, a balance of Fe, and unavoidable impurities; and

as microstructures, ferrite which is a soft structure, and tempered martensite, bainite, and retained austenite which are hard structures,

wherein the high strength steel sheet satisfies the following [Relational Expression 1], [Relational Expression 2], and [Relational Expression 3].

0.4≤[H]_F/[H]_TM+B+γ≤0.9 [Relational Expression 1]

where [H]_Fand [H]_TM+B+γ are nanohardness values measured using a nanoindenter, [H]_Fis an average nanohardness value Hv of the ferrite which is the soft structure, and [H]_TM+B+γ is the average nanohardness value Hv of the tempered martensite, the bainite, and the retained austenite which are the hard structures,

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

where 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 the fraction (vol %) of the retained austenite of the steel sheet,

V(lath, γ)/V(γ)≥0.5 [Relational Expression 3]

where V(lath, γ) is the fraction (vol %) of the retained austenite in a lath form, and V(γ) is the fraction (vol %) of the retained austenite of the steel sheet.