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

Claim 20:
21. A method for producing the coated steel sheet according to claim 6, the method comprising:
producing a hot-rolled steel sheet by heating a steel slab having 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,
at least one element selected from:
Cr: 0.01% or more and 1.00% or less,
Nb: 0.001% or more and 0.100% or less,
V: 0.001% or more and 0.100% or less,
Ti: 0.001% or more and 0.100% or less,
B: 0.0001% or more and 0.0100% or less,
Mo: 0.01% or more and 0.50% or less,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 1.00% or less,
As: 0.001% or more and 0.500% or less,
Sb: 0.001% or more and 0.200% or less,
Sn: 0.001% or more and 0.200% or less,
Ta: 0.001% or more and 0.100% or less,
Ca: 0.0001% or more and 0.0200% or less,
Mg: 0.0001% or more and 0.0200% or less,
Zn: 0.001% or more and 0.020% or less,
Co: 0.001% or more and 0.020% or less,
Zr: 0.001% or more and 0.020% or less, and
REM: 0.0001% or more and 0.0200% or less, and

the balance being Fe and unavoidable impurities;

rough-rolling the heated steel slab; in subsequent finish-rolling, hot-rolling the rough-rolled steel slab under conditions of a finish-rolling inlet temperature of 1020° C. or higher and 1180° C. or lower, a rolling reduction in a final pass of the finish rolling of 5% or more and 15% or less, a rolling reduction in a pass before the final pass of 15% or more and 25% or less, and a finish-rolling delivery temperature of 800° C. or higher and 1000° C. or lower; after the hot-rolling, cooling the hot-rolled steel sheet under a condition of an average cooling rate of 5° C./s or more and 90° C./s or less; coiling the cooled steel sheet under a condition of a coiling temperature of 300° C. or higher and 700° C. or lower;
producing a heat-treated steel sheet by heating the hot-rolled steel sheet obtained in the method under conditions of a maximum attained temperature of a T1 temperature or higher and a T2 temperature or lower and an average heating rate of 50° C./s or less in a temperature range of 450° C. to [T1 temperature—10° C.]; and, after the heating, performing cooling and pickling;
producing a steel sheet by re-heating the heat-treated steel sheet to a temperature equal to or higher than the T1 temperature; and cooling the re-heated steel sheet under a condition of an average cooling rate of 3° C./s or more in a temperature range of [T1 temperature—10° C.] to 550° C., wherein a dew point in a temperature range of 600° C. or higher is −40° C. or lower; and
coating the steel sheet comprising 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.