US 12,435,400 B2
Thick composite-phase steel having excellent durability and manufacturing method therefor
Hyun-Taek Na, Gwangyang-si (KR); and Sung-Il Kim, Gwangyang-si (KR)
Assigned to POSCO, Pohang-si (KR)
Appl. No. 17/779,096
Filed by POSCO, Pohang-si (KR)
PCT Filed Nov. 26, 2020, PCT No. PCT/KR2020/016925
§ 371(c)(1), (2) Date May 23, 2022,
PCT Pub. No. WO2021/112488, PCT Pub. Date Jun. 10, 2021.
Claims priority of application No. 10-2019-0158289 (KR), filed on Dec. 2, 2019.
Prior Publication US 2022/0403489 A1, Dec. 22, 2022
Int. Cl. C22C 38/26 (2006.01); C21D 8/02 (2006.01); C22C 38/28 (2006.01); C22C 38/38 (2006.01); C23C 2/06 (2006.01)
CPC C22C 38/26 (2013.01) [C21D 8/0226 (2013.01); C22C 38/28 (2013.01); C22C 38/38 (2013.01); C23C 2/06 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01)] 4 Claims
OG exemplary drawing
 
1. A composite phase steel having excellent material and durability uniformity and a thickness of 5 mm or more, the composite-phase steel comprising:
by wt %, C: 0.05 to 0.15%, Si: 0.01 to 1.0%, Mn: 1.0 to 2.3%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Nb: 0.005 to 0.07%, Ti: 0.005 to 0.11%, and a balance of Fe and inevitable impurities; and
a microstructure comprising: a mixed phase of ferrite and bainite as a base structure; a pearlite phase; a martensite and austenite (MA) phase; and a martensite phase, wherein an area fraction of each of the pearlite phase and the martensite and austenite (MA) phase is less than 5%, and an area fraction of the martensite phase is less than 10%,
wherein, when a coil in a wound state is divided, in a lengthwise direction, into three parts: head, mid, and tail parts, a product of tensile strength, elongation, and fatigue strength of an outer wound portion of the coil, which is a region of the head part and the tail part, is 25×105 MPa2·% or greater, and a product of tensile strength, elongation, and fatigue strength of an inner wound portion of the coil, which is a region of the mid part, is 24×105 MPa2·% or greater.