US 12,134,811 B2
High-strength hot-rolled steel sheet having excellent bendability and low-temperature and method for manufacturing same
Sung-Il Kim, Gwangyang-si (KR); Hee-Sung Kang, Gwangyang-si (KR); and Hyun-Seok Tak, Gwangyang-si (KR)
Assigned to POSCO CO., LTD, Pohang-si (KR)
Filed by POSCO CO., LTD, Pohang-si (KR)
Filed on Jun. 23, 2023, as Appl. No. 18/213,539.
Application 18/213,539 is a division of application No. 16/772,931, granted, now 11,732,339, previously published as PCT/KR2018/014462, filed on Nov. 22, 2018.
Claims priority of application No. 10-2017-0178826 (KR), filed on Dec. 22, 2017.
Prior Publication US 2023/0332281 A1, Oct. 19, 2023
Int. Cl. C22C 38/22 (2006.01); C21D 8/02 (2006.01); C21D 9/46 (2006.01); C22C 38/04 (2006.01)
CPC C22C 38/22 (2013.01) [C21D 8/0226 (2013.01); C21D 8/0257 (2013.01); C21D 9/46 (2013.01); C22C 38/04 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] 3 Claims
OG exemplary drawing
 
1. A method for manufacturing a hot-rolled steel sheet, the method comprising:
reheating a steel slab to a temperature within a range of 1200° C. to 1350° C., the steel slab comprising, by weight percentage (wt %), C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.5%, Al: 0.01 to 0.1%, Cr: 0.3 to 1.2%, Mo: 0.001 to 0.5%, P: 0.001 to 0.01%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Nb: 0.001 to 0.06%, Ti: 0.005 to 0.03%, V: 0.001 to 0.2%, B: 0.0003 to 0.003%, a remainder of iron (Fe), and other unavoidable impurities, wherein a content relationship (T) of C, Mn, Cr, and Mo, expressed by Relational Expression 1 below, satisfies 1.0 to 2.5;
finish hot rolling the reheated steel slab to a temperature within a range of 850° C. to 1150° C. to manufacture a hot-rolled steel sheet;
cooling the hot-rolled steel sheet to a temperature within a range of 500° C. to 700° C. at a cooling rate of 10° C./s to 70° C./s after the finish hot rolling;
coiling the hot-rolled steel sheet within a temperature range of 500° C. to 700° C. after the cooling;
performing a first heat treatment to preserve heat of the hot-rolled steel sheet or to heat the hot-rolled steel sheet within a temperature range of 350° C. to 500° C. after the coiling;
performing first cooling to cool the hot-rolled steel sheet to a room temperature at a cooling rate of 0.001° C./s to 10° C./s after the first heat treatment;
performing a second heat treatment to reheat the hot-rolled steel sheet to a temperature within a range of 850° C. to 1000° C. and to maintain the reheated hot-rolled steel sheet for 10 minutes to 60 minutes after the first cooling;
performing second cooling to cool the hot-rolled steel sheet to a temperature within a range of 0° C. to 100° C. at a cooling rate of 10° C./s to 100° C./s after the second heat treatment;
performing a third heat treatment to reheat the hot-rolled steel sheet to a temperature within a range of 100° C. to 500° C. and to thermally treat the reheated hot-rolled steel sheet for 10 minutes to 60 minutes after the second cooling; and
performing third cooling to cool the hot-rolled steel sheet to a temperature within a range of 0° C. to 100° C. at a cooling rate of 0.001° C./s to 100° C./s after the third heat treatment,
T=[C]+{[Mn]/(0.85[Cr]+1.3[Mo])}  Relational Expression 1:
where C, Mn, Cr, and Mo refer to weight contents of elements, respectively.