US 11,851,727 B2
Hot-rolled steel sheet having excellent expandability and method for manufacturing same
Hyun-Taek Na, Gwangyang-si (KR); and Seok-Jong Seo, Gwangyang-si (KR)
Assigned to POSCO Co., Ltd., Pohang-si (KR)
Filed by POSCO Co., Ltd, Pohang-si (KR)
Filed on Dec. 20, 2022, as Appl. No. 18/085,061.
Application 18/085,061 is a division of application No. 16/956,417, granted, now 11,560,607, previously published as PCT/KR2018/013943, filed on Nov. 15, 2018.
Claims priority of application No. 10-2017-0177510 (KR), filed on Dec. 21, 2017.
Prior Publication US 2023/0129303 A1, Apr. 27, 2023
Int. Cl. C21D 9/46 (2006.01); B23K 11/00 (2006.01); B23K 31/02 (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 38/24 (2006.01); C22C 38/26 (2006.01); C22C 38/28 (2006.01); B23K 103/04 (2006.01); B23K 101/06 (2006.01)
CPC C21D 9/46 (2013.01) [B23K 11/002 (2013.01); B23K 31/027 (2013.01); C21D 8/0226 (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 38/24 (2013.01); C22C 38/26 (2013.01); C22C 38/28 (2013.01); B23K 2101/06 (2018.08); B23K 2103/04 (2018.08); C21D 2211/002 (2013.01); C21D 2211/004 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] 2 Claims
OG exemplary drawing
 
1. A method for manufacturing a hot-rolled steel sheet for an electric resistance welded steel pipe, comprising steps of:
reheating a steel slab including 0.15 wt % to 0.22 wt % of carbon (C), 0.1 wt % to 1.0 wt % of silicon (Si), 0.8 wt % to 1.8 wt % of manganese (Mn), 0.001 wt % to 0.03 wt % of phosphorus (P), 0.001 wt % to 0.01 wt % of sulfur (S), 0.001 wt % to 0.19 wt % of soluble aluminum (Sol.Al), 0.3 wt % to 1.0 wt % of chromium (Cr), 0.01 wt % to 0.05 wt % of titanium (Ti), 0.025 wt % or less of niobium (Nb), 0.035 wt % or less of vanadium (V), 0.001 wt % to 0.01 wt % of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities, Mn and Si satisfying the following Relational Expression 1, to a temperature range of 1180 to 1300° C.;
finish hot rolling the reheated steel slab at a temperature of Ar3 or higher to prepare a hot-rolled steel sheet;
first cooling the hot-rolled steel sheet to a temperature range of 550 to 750° C. at a cooling rate of 20° C./s or higher;
second cooling the hot-rolled steel sheet at a cooling rate of 0.05 to 2.0° C./s within a range satisfying the following Relational Expression 3 after the first cooling;
third cooling the hot-rolled steel sheet at a cooling rate of 20° C./s or higher to a temperature range of room temperature to 400° C., after the second cooling; and
coiling the hot-rolled steel sheet after the third cooling,
4<Mn/Si<12  [Relational Expression 1]
where, Mn and Si mean a weight content of each element
0≤t−ta≤3  [Relational Expression 3]
where, the ta is [250+(65.1[C])+(9.2[Mn])+(20.5[Cr])−(4.7[Si])−(4.8[Sol.Al])−(0.87Temp)+(0.00068Temp 2)], where t refers to a second cooling maintaining time (seconds, sec.), ta is a second cooling maintaining time to secure an optimal phase fraction (seconds, sec.), and Temp refers to a second cooling intermediate temperature, which means a temperature of an intermediate point between the second cooling start point and the second cooling end point, and each alloy component means a weight content.