US 12,338,515 B2
Ultra-thick steel excellent in brittle crack arrestability and manufacturing method therefor
Hak-Cheol Lee, Pohang-si (KR)
Assigned to POSCO CO., LTD, Pohang-si (KR)
Appl. No. 17/297,940
Filed by POSCO, Pohang-si (KR)
PCT Filed Nov. 29, 2019, PCT No. PCT/KR2019/016702
§ 371(c)(1), (2) Date May 27, 2021,
PCT Pub. No. WO2020/111860, PCT Pub. Date Jun. 4, 2020.
Claims priority of application No. 10-2018-0151871 (KR), filed on Nov. 30, 2018.
Prior Publication US 2022/0025491 A1, Jan. 27, 2022
This patent is subject to a terminal disclaimer.
Int. Cl. C22C 38/04 (2006.01); C21D 1/18 (2006.01); C21D 8/00 (2006.01); C21D 8/02 (2006.01); C22C 38/00 (2006.01); C22C 38/08 (2006.01); C22C 38/12 (2006.01); C22C 38/14 (2006.01); C22C 38/16 (2006.01)
CPC C22C 38/04 (2013.01) [C21D 1/18 (2013.01); C21D 8/005 (2013.01); C21D 8/021 (2013.01); C22C 38/002 (2013.01); C22C 38/08 (2013.01); C22C 38/12 (2013.01); C22C 38/14 (2013.01); C22C 38/16 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] 5 Claims
 
1. A steel comprising:
by wt %, 0.02 to 0.07% of C, 1.8 to 2.2% of Mn, 0.7 to 1.2% of Ni, 0.005 to 0.02% of Nb, 0.005 to 0.02% of Ti, 0.1 to 0.4% of Cu, 0.01% or less of P, 0.004% or less of S, and a balance of Fe and inevitable impurities, wherein grains that have a high angle grain boundary of 15° or more, as measured by EBSD, have an average grain size of 15 μm or less in the t/4−(3*t)/8 region, where t represents a thickness of the steel, and the same below,
wherein a microstructure of a test piece collected in the t/4 region of the steel includes a composite structure of acicular ferrite and granular bainite, and further includes island martensite as a second phase,
wherein the acicular ferrite is included in a ratio of 60 to 80 area %, the granular bainite is included in a ratio of 20 to 40 area %, the island martensite is included in a ratio of greater than 0% 10 area % or less, and
wherein a naval research laboratory-drop weight test (NRL-DWT) specified in ASTM E208-06 is performed on a test piece collected in the t/4 region of the steel, a Nil-ductility transition (NDT) temperature is −45° C. or lower.