CPC C21D 9/46 (2013.01) [B32B 15/013 (2013.01); C21D 6/001 (2013.01); C21D 6/002 (2013.01); C21D 6/005 (2013.01); C21D 6/007 (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); 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 38/08 (2013.01); C22C 38/10 (2013.01); C22C 38/12 (2013.01); C22C 38/14 (2013.01); C22C 38/16 (2013.01); C22C 38/18 (2013.01); C22C 38/60 (2013.01); C23C 2/06 (2013.01); C23C 2/40 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/003 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01); C21D 2211/009 (2013.01)] | 10 Claims |
1. A steel sheet comprising, as a chemical composition, by mass %:
C: 0.050% to 0.500%;
Si: 0.01% to 3.00%;
Mn: 0.50% to 5.00%;
P: 0.0001% to 0.1000%;
S: 0.0001% to 0.0100%;
Al: 0.001% to 2.500%;
N: 0.0001% to 0.0100%;
O: 0.0001% to 0.0100%;
Ti: 0% to 0,300%;
V: 0% to 1.00%;
Nb: 0% to 0.100%;
Cr: 0% to 2.00%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
Co: 0% to 2.00%;
Mo: 0% to 1.00%;
W: 0% to 1.00%;
B: 0% to 0.0100%;
Sn: 0% to 1.00%;
Sb: 0% to 1.00%;
Ca: 0% to 0.0100%;
Mg: 0% to 0.0100%;
Ce: 0% to 0.0100%;
Zr: 0% to 0.0100%;
La: 0% to 0.0100%;
Hf: 0% to 0.0100%;
Bi: 0% to 0.0100%;
REM: 0% to 0.0100%; and
a remainder including Fe and impurities;
wherein a steel structure in a ⅛ to ⅜ thickness range centered on a ¼ thickness position from a surface contains, by volume fraction,
a soft ferrite: 0% to 30%,
a retained austenite: 3% to 40%,
a fresh martensite: 0% to 30%,
a sum of pearlite and cementite: 0% to 10%, and
a remainder including hard ferrite,
wherein soft ferrite is defined as ferrite that does not contain retained austenite in the grains, and wherein hard ferrite is defined as ferrite containing retained austenite in the grains,
in the ⅛ to ⅜ thickness range, a number proportion of retained austenite having an aspect ratio of 2.0 or more in the total retained austenite is 50% or more,
when a region having a hardness of 80% or less of a hardness of the ⅛ to ⅜ thickness range is defined as a soft layer, the soft layer having a thickness of 1 to 100 μm from the surface in a sheet thickness direction is present,
in ferrite contained in the soft layer, a volume fraction of grains having an aspect ratio of 3.0 or more is 50% or more,
a volume fraction of retained austenite in the soft layer is 80% or less of the volume fraction of the retained austenite in the ⅛ to ⅜ thickness range, and
when an emission intensity at a wavelength indicating Si is analyzed in the sheet thickness direction from the surface by a radio-frequency glow discharge analysis method, a peak of the emission intensity at the wavelength indicating Si appears in a range of more than 0.2 μm and 10.0 μm or less from the surface.
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