Thin high-strength cold-rolled steel sheet and method of producing the same
A steel having a composition containing C: more than 0.20% and 0.45% or less, Si: 0.50% to 2.50%, Mn: 2.00% or more and less than 3.50%, and one or two selected from Ti: 0.005% to 0.100% and Nb: 0.005% to 0.100% is hot-rolled and cold-rolled. The steel sheet is heated to 800° C. to 950° C. and cooled to a cooling-end temperature of 350° C. to 500° C. at a cooling rate of 5° C./s or more to form a steel sheet having a microstructure including martensite and bainite phases such that the total proportion of the martensite and bainite phases is 80% or more by volume. The steel sheet is heated to 700° C. to 840° C. and maintained at 700° C. to 840° C., cooled to a cooling-end temperature of 350° C. to 500° C. at a cooling rate of 5 to 50° C./s, and maintained within the above temperature range for 10 to 1800 s.
1. A high-strength cold-rolled steel sheet comprising:
a composition containing, by mass,
C: more than 0.25% and 0.45% or less,
Si: 0.50% to 2.50%,
Mn: 2.00% or more and less than 3.50%,
P: 0.001% to 0.100%,
S: 0.0200% or less,
N: 0.0100% or less,
Al: 0.01% to 0.100%,
and one or two elements selected from
Ti: 0.005% to 0.100% and
Nb: 0.005% to 0.100%,
the balance being Fe and inevitable impurities, and
a microstructure including, by volume, 15% or more and 70% or less ferrite phase and more than 15% and 40% or less retained austenite phase, the balance being 30% or less (not including 0%) martensite phase or including 30% or less (not including 0%) martensite phase and 10% or less (including 0%) pearlite phase and/or carbide, wherein
crystal grains of the retained austenite phase have an average diameter of 2.0 μm or less and an aspect ratio of 2.0 or more,
a tensile strength of the high-strength cold-rolled steel sheet is 980 MPa or more,
an in-plane anisotropy δTS of the high-strength cold-rolled steel sheet in terms of tensile strength defined by Formula (1) below is 25 MPa or less, and
an in-plane anisotropy δEl of the high-strength cold-rolled steel sheet in terms of total elongation defined by Formula (2) below is 10% or less:
δTS=(TS L +TS C −2×TS D )/2 (1)
where δTS: in-plane anisotropy (MPa) in terms of tensile strength TS, TS L : tensile strength (MPa) in a direction parallel to a rolling direction (L direction), TS C : tensile strength (MPa) in a direction (C direction) perpendicular to the rolling direction, and TS D : tensile strength (MPa) in a direction (D direction) inclined at an angle of 45° with respect to the rolling direction,
δEl=(El L +El C −2×El D )/2 (2)
where δEl: in-plane anisotropy (%) in terms of total elongation El, El L : total elongation (%) in a direction parallel to the rolling direction (L direction), El C : total elongation (%) in a direction (C direction) perpendicular to the rolling direction, and El D : total elongation (%) in a direction (D direction) inclined at an angle of 45° with respect to the rolling direction.
2. The high-strength cold-rolled steel sheet according to claim 1 , further comprising a plating layer selected from a hot-dip galvanizing layer, a hot-dip galvannealing layer, and an electrogalvanizing layer, the plating layer deposited on a surface of the high-strength cold-rolled steel sheet.
3. The thin high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains, by mass, one or more groups selected from Groups A to D below:
Group A: one or more elements selected from
B: 0.0001% to 0.0050%,
Cr: 0.05% to 1.00%, and
Cu: 0.05% to 1.00%
Group B: one or two elements selected from
Sb: 0.002% to 0.200% and
Sn: 0.002% to 0.200%
Group C: Ta: 0.001% to 0.100%
Group D: one or more elements selected from
Ca: 0.0005% to 0.0050%,
Mg: 0.0005% to 0.0050%, and
REM: 0.0005% to 0.0050%.
4. The high-strength cold-rolled steel sheet according to claim 3 , further comprising a plating layer selected from a hot-dip galvanizing layer, a hot-dip galvannealing layer, and an electrogalvanizing layer, the plating layer deposited on a surface of the high-strength cold-rolled steel sheet.
5. A method of producing the high-strength cold-rolled steel sheet according to claim 1 in which a steel is subjected to a hot-rolling step, a pickling step, a cold-rolling step, and annealing step in this order to form a cold-rolled steel sheet,
wherein the hot-rolling step includes heating the steel and forming the steel into a hot-rolled steel sheet having a predetermined thickness,
the cold-rolling step includes cold-rolling the hot-rolled steel sheet at a rolling reduction of 30% or more to form the hot-rolled steel sheet into a cold-rolled steel sheet having a predetermined thickness,
the annealing step includes first and second annealing treatments,
the first annealing treatment including heating the cold-rolled steel sheet to an annealing temperature of 800° C. to 950° C. and subsequently cooling the cold-rolled steel sheet to a cooling-end temperature of 350° C. to 500° C. at a cooling rate such that an average cooling rate between an annealing temperature and a cooling-end temperature is 5° C./s or more to form the cold-rolled steel sheet into a cold-rolled and annealed steel sheet having a microstructure including a martensite phase and a bainite phase such that a total volume fraction of the martensite phase and the bainite phase is 80% or more, and
the second annealing treatment including heating the cold-rolled and annealed steel sheet to an annealing temperature of 700° C. to 840° C., holding the cold-rolled and annealed steel sheet at 700° C. to 840° C. for 10 to 900 s, subsequently cooling the cold-rolled and annealed steel sheet to a cooling-end temperature range of 350° C. to 500° C. at a cooling rate such that the average cooling rate between the annealing temperature and the cooling-end temperature is 5 to 50° C./s, and holding the thin cold-rolled and annealed steel sheet within the cooling-end temperature range for 10 to 1800 s.
6. The method according to claim 5 , wherein, subsequent to the second annealing treatment included in the annealing step, any one of a hot-dip galvanizing treatment, a set of a hot-dip galvanizing treatment and an alloying treatment, and an electrogalvanizing treatment is performed.
7. The method according to claim 5 , wherein the composition further contains, by mass, one or more groups selected from Groups A to D below:
Group A: one or more elements selected from
B: 0.0001% to 0.0050%,
Cr: 0.05% to 1.00%, and
Cu: 0.05% to 1.00%
Group B: one or two elements selected from
Sb: 0.002% to 0.200% and
Sn: 0.002% to 0.200%
Group C: Ta: 0.001% to 0.100%
Group D: one or more elements selected from
Ca: 0.0005% to 0.0050%,
Mg: 0.0005% to 0.0050%, and
REM: 0.0005% to 0.0050%.
8. The method according to claim 7 , wherein, subsequent to the second annealing treatment included in the annealing step, any one of a hot-dip galvanizing treatment, a set of a hot-dip galvanizing treatment and an alloying treatment, and an electrogalvanizing treatment is performed.