US 9,809,874 B2
Steel sheet suitable for impact absorbing member and method for its manufacture
Yasuaki Tanaka, Tokyo (JP); Kaori Kawano, Tokyo (JP); Masahito Tasaka, Tokyo (JP); Yoshiaki Nakazawa, Tokyo (JP); Takuya Nishio, Tokyo (JP); Masayuki Wakita, Tokyo (JP); Jun Haga, Tokyo (JP); and Toshiro Tomida, Tokyo (JP)
Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION, Tokyo (JP)
Appl. No. 14/391,215
Filed by NIPPON STEEL & SUMITOMO METAL CORPORATION, Tokyo (JP)
PCT Filed Apr. 8, 2013, PCT No. PCT/JP2013/060625
§ 371(c)(1), (2) Date Oct. 8, 2014,
PCT Pub. No. WO2013/154071, PCT Pub. Date Oct. 17, 2013.
Claims priority of application No. 2012-088944 (JP), filed on Apr. 10, 2012; and application No. 2012-088945 (JP), filed on Apr. 10, 2012.
Prior Publication US 2015/0075680 A1, Mar. 19, 2015
Int. Cl. C22C 38/38 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/04 (2006.01); C22C 38/06 (2006.01); C22C 38/12 (2006.01); C22C 38/14 (2006.01); C22C 38/22 (2006.01); C22C 38/24 (2006.01); C22C 38/26 (2006.01); C22C 38/28 (2006.01); C21D 8/04 (2006.01); C21D 9/48 (2006.01); C21D 1/20 (2006.01); C21D 1/26 (2006.01); C21D 8/02 (2006.01); C21D 9/46 (2006.01); C22C 38/32 (2006.01)
CPC C22C 38/38 (2013.01) [C21D 1/20 (2013.01); C21D 1/26 (2013.01); C21D 8/0236 (2013.01); C21D 8/0263 (2013.01); C21D 8/0436 (2013.01); C21D 8/0463 (2013.01); C21D 8/0473 (2013.01); C21D 9/46 (2013.01); C21D 9/48 (2013.01); C22C 38/001 (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 38/12 (2013.01); C22C 38/14 (2013.01); C22C 38/22 (2013.01); C22C 38/24 (2013.01); C22C 38/26 (2013.01); C22C 38/28 (2013.01); C22C 38/32 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] 5 Claims
 
1. A method for manufacturing a steel sheet having a microstructure containing, by area %, bainite: more than 50%, martensite: at least 3% and at most 30%, and retained austenite: at least 3% and at most 15%, the remainder consisting of ferrite having an average grain diameter of less than 5 μm; and mechanical properties in which the product of uniform elongation and hole expansion ratio is at least 300%2, and an effective flow stress when 5% true strain is applied is at least 900 MPa, comprising following steps (A) to (C):
(A) a hot rolling step in which a slab having a chemical composition containing, by mass %, C: at least 0.08% and at most 0.30%, Mn: at least 1.5% and at most 3.5%, Si+Al: at least 0.50% and at most 3.0%, P: at most 0.10%, S: at most 0.010%, N: at most 0.010%, Cr: 0 to at most 0.5%, Mo: 0 to at most 0.5%, B: 0 to at most 0.01%, Ti: 0 to less than 0.04%, Nb: 0 to less than 0.030%, V: 0 to less than 0.5%, Ca: 0 to at most 0.010%, Mg: 0 to at most 0.010%, REM: 0 to at most 0.050%, and Bi: 0 to at most 0.050%, the remainder being Fe and impurities is subjected to multi-pass hot rolling in which rolling is completed at a temperature of at least Ar3 point, the obtained steel sheet is cooled to a temperature range of at least 620° C. and at most 720° C. under a cooling condition in which cooling is started within 0.4 seconds after completion of rolling, and an average cooling rate is at least 600° C./sec, as well as a time required for cooling from completion of rolling in a rolling pass which is two passes before the last rolling pass to 720° C. is at most 4 seconds, and the steel sheet is held in the temperature range for at least 1 second and at most 10 seconds, thereafter being cooled to a temperature range of at least 300° C. and at most 610° C. at an average cooling rate of at least 10° C./sec and at most 100° C./sec, and being wound up to obtain a hot-rolled steel sheet;
(B) a cold rolling step in which the hot-rolled steel sheet obtained by the hot rolling step is subjected to cold rolling of a rolling reduction of at least 40% and at most 70% to be formed into a cold-rolled steel sheet; and
(C) an annealing step in which the cold-rolled steel sheet obtained by the cold rolling step is subjected to a heat treatment in which the steel sheet is held in a temperature range of at least (Ac3 point−30° C.) and at most (Ac3 point +100° C.) for at least 10 seconds and at most 300 seconds, and then is cooled at an average cooling rate of at least 15° C./sec in a temperature range of at least 500° C. and at most 650° C., thereafter being held in a temperature range of at least 300° C. and at most 500° C. for at least 30 seconds and at most 3000 seconds.