US 11,814,695 B2
Method for manufacturing high-strength galvanized steel sheet and high-strength galvanized steel sheet
Mai Aoyama, Tokyo (JP); Minoru Tanaka, Tokyo (JP); Yoshitsugu Suzuki, Tokyo (JP); Yoshiyasu Kawasaki, Tokyo (JP); and Hideyuki Takahashi, Tokyo (JP)
Assigned to JFE STEEL CORPORATION, Tokyo (JP)
Appl. No. 15/779,157
Filed by JFE STEEL CORPORATION, Tokyo (JP)
PCT Filed Nov. 8, 2016, PCT No. PCT/JP2016/004829
§ 371(c)(1), (2) Date May 25, 2018,
PCT Pub. No. WO2017/090236, PCT Pub. Date Jun. 1, 2017.
Claims priority of application No. 2015-230246 (JP), filed on Nov. 26, 2015.
Prior Publication US 2018/0327884 A1, Nov. 15, 2018
Int. Cl. C21D 9/46 (2006.01); C23C 2/02 (2006.01); C23C 2/06 (2006.01); C22C 38/14 (2006.01); C22C 38/60 (2006.01); C21D 6/00 (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 38/08 (2006.01); C22C 38/12 (2006.01); C22C 38/16 (2006.01); C22C 38/28 (2006.01); C21D 8/04 (2006.01); C21D 9/48 (2006.01); C21D 1/76 (2006.01); C21D 1/18 (2006.01); B32B 15/01 (2006.01); C22C 38/38 (2006.01); C23C 2/40 (2006.01)
CPC C21D 9/46 (2013.01) [B32B 15/013 (2013.01); C21D 1/185 (2013.01); C21D 1/76 (2013.01); C21D 6/004 (2013.01); C21D 6/005 (2013.01); C21D 6/008 (2013.01); C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 8/0236 (2013.01); C21D 8/0263 (2013.01); C21D 8/0273 (2013.01); C21D 8/0278 (2013.01); C21D 8/0463 (2013.01); C21D 8/0473 (2013.01); C21D 9/48 (2013.01); C22C 38/00 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/005 (2013.01); C22C 38/008 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/08 (2013.01); C22C 38/12 (2013.01); C22C 38/14 (2013.01); C22C 38/16 (2013.01); C22C 38/28 (2013.01); C22C 38/38 (2013.01); C22C 38/60 (2013.01); C23C 2/02 (2013.01); C23C 2/06 (2013.01); C23C 2/40 (2013.01); C21D 2211/001 (2013.01); C21D 2211/005 (2013.01)] 18 Claims
 
1. A method for manufacturing a galvanized steel sheet with a tensile strength (TS) of 590 MPa or more, the method comprising:
hot-rolling a steel slab to form a steel sheet, the steel slab having a composition comprising:
C: 0.030% to 0.250%, by mass %,
Si: 0.01% to 3.00%, by mass %,
Mn: 2.00% to 10.00%, by mass %,
P: 0.001% to 0.100%, by mass %,
S: 0.0001% to 0.0200%, by mass %,
N: 0.0005% to 0.0100%, by mass %,
Ti: 0.005% to 0.200%, by mass %, and
Fe and inevitable impurities;
performing pickling on the steel sheet;
performing a first heat treatment on the steel sheet in such a manner that the steel sheet is held in a temperature range of 650° C. to 850° C. for 600 s to 21,600 s in an atmosphere having an H2 concentration of 0.1% by volume to 25.0% by volume and a dew point of −45° C. to 0° C. in such a state that a surface of the steel sheet is exposed;
performing first cooling of the steel sheet;
performing pickling on the steel sheet under such conditions that a pickling weight loss is 0.03 g/m2 to 5.00 g/m2 in terms of Fe;
performing a second heat treatment on the steel sheet in such a manner that the steel sheet is held in a temperature range of 600° C. to 830° C. for 20 s to 900 s in an atmosphere having an H2 concentration of 0.05% by volume to 25.0% by volume and a dew point of −10° C. or lower;
performing second cooling of the steel sheet; and
then performing galvanizing on the steel sheet,
wherein a concentration of solute Mn at a depth of 5 μm or less from a surface of the steel sheet is 1.50% by mass or less,
a value obtained by dividing an average mass percentage of Mn in retained austenite by an average mass percentage of Mn in ferrite is 2.0 or more, and
at least one of (i) an x-ray fluorescence count of a number of Zn particles in a coating layer peeled from a portion of the galvanized steel sheet to which alloying is performed after performing the galvanizing is less than 5,000, and (ii) a coating layer disposed on a portion of the galvanized steel sheet to which no alloying is performed after performing the galvanizing does not peel off with a cellophane tape under a ball impact test carried out under conditions including a ball mass of 1.8 kg and a drop height of 100 cm.