Patent ID: 11946111
Assignee: JFE STEEL CORPORATION
Field: Materials, metallurgy (Chemistry)
Classification: CPC C | IPC C

Claim 0:
1. A steel sheet comprising:
a composition that contains, in terms of mass %
C: 0.048% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 1.50% or more and 3.00% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Al: 0.001% or more and 1.000% or less,
N: 0.0005% or more and 0.0100% or less, and the balance being Fe and unavoidable impurities;

a microstructure containing, in terms of area fraction, 20% or more and 86% or less of ferrite and 5% or more of martensite, wherein the ferrite has an average crystal grain size of 5.0 μm or more and 20 μm or less, the martensite has an average size of 15 μm or less, a ratio of the average crystal grain size of the ferrite to the average size of the martensite (ferrite average crystal grain size/martensite average size) is 0.5 to 10.0, a ratio of a hardness of the martensite to a hardness of the ferrite (martensite hardness/ferrite hardness) is 1.0 or more and 5.0 or less, and, in a texture of the ferrite, an inverse intensity ratio of γ-fiber to α-fiber is 0.8 or more and 7.0 or less; and
a tensile strength of 780 MPa or more, a yield ratio of 65% or less, and an index of the YP planar anisotropy |ΔYP| of 50 MPa or less,
wherein the index of the YP planar anisotropy |ΔYP| is determined by formula (1) below:

ΔYP=(YPL−2×YPD+YPC)/2  (I)

where YPL, YPD, and YPC respectively represent values of YP measured from JIS No. 5 test pieces taken in three directions, namely, the rolling direction (L direction) of the steel sheet, a direction (D direction) 45° with respect to the rolling direction of the steel sheet, and a direction (C direction) 90° with respect to the rolling direction of the steel sheet, by a tensile test in accordance with the description of JIS Z 2241 (2011) at a crosshead speed of 10 mm/min.