US 9,809,869 B2
Thick-walled high-strength hot rolled steel sheet having excellent hydrogen induced cracking resistance and manufacturing method thereof
Kinya Nakagawa, Fukuyama (JP); and Chikara Kami, Kurashiki (JP)
Assigned to JFE Steel Corporation, Tokyo (JP)
Filed by JFE STEEL CORPORATION, Tokyo (JP)
Filed on Dec. 12, 2016, as Appl. No. 15/375,410.
Application 15/375,410 is a division of application No. 13/146,751, previously published as PCT/JP2010/051647, filed on Jan. 29, 2010.
Claims priority of application No. 2009-019339 (JP), filed on Jan. 30, 2009; and application No. 2009-019342 (JP), filed on Jan. 30, 2009.
Prior Publication US 2017/0088916 A1, Mar. 30, 2017
Int. Cl. C21D 8/02 (2006.01); C21D 9/46 (2006.01); C22C 38/54 (2006.01); C22C 38/46 (2006.01); C22C 38/44 (2006.01); C22C 38/50 (2006.01); C22C 38/48 (2006.01); C22C 38/06 (2006.01); C22C 38/04 (2006.01); C22C 38/02 (2006.01); C22C 38/00 (2006.01); C22C 38/42 (2006.01)
CPC C21D 9/46 (2013.01) [C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C22C 38/002 (2013.01); C22C 38/005 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/42 (2013.01); C22C 38/44 (2013.01); C22C 38/46 (2013.01); C22C 38/48 (2013.01); C22C 38/50 (2013.01); C22C 38/54 (2013.01)] 10 Claims
 
1. A method of manufacturing a thick-walled high-strength hot rolled steel sheet having surface layer hardness of 230HV or less in terms of Vickers hardness, wherein in manufacturing a hot rolled steel sheet by applying hot rolling consisting of rough rolling and finish rolling to a raw steel material having a composition which contains by mass % 0.02 to 0.08% C, 1.0% or less Si, 0.50 to 1.85% Mn, 0.03% or less P, 0.005% or less S, 0.1% or less Al, 0.02 to 0.10% Nb, 0.001 to 0.05% Ti, 0.0005% or less B, and Fe and unavoidable impurities as a balance, such that the composition satisfies the following formula: (Ti+Nb/2)/C<4, where Ti, Nb, and C are contents of the respective elements by mass %, and after the finish rolling is finished, a first cooling step in which the hot rolled steel sheet is cooled by accelerated cooling at an average surface cooling rate of 30° C./s or more until a surface temperature becomes 500° C. or below, a second cooling step in which the hot rolled steel sheet is cooled by air cooling for 10 s or less after the first cooling step is finished, and a third cooling step in which the hot rolled steel sheet is cooled by accelerated cooling to a temperature which falls within a temperature range from 350° C. or above to a temperature below 600° C. at the center of a sheet-thickness at an average cooling rate of 10° C./s or more at the center of the sheet-thickness are applied to the hot rolled steel sheet, and the hot rolled steel sheet is coiled in a coil shape after the third cooling step is finished.