High-strength cold-rolled steel sheet, high-strength coated or plated steel sheet, method of producing high-strength cold-rolled steel sheet, and method of producing high-strength coated or plated steel sheet
A high-strength cold-rolled steel sheet comprises: a chemical composition that contains C, Si, Mn, P, S, N, Al, Ti, Nb, and B with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1; and a steel microstructure in which: an area fraction of ferrite is 12% or more and less than 30%; a total area fraction of tempered martensite and bainite is 55% or more and 85% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 20% or less; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.
1. A high-strength cold-rolled steel sheet comprising:
a chemical composition that contains, in mass %,
C: 0.06% or more and 0.15% or less,
Si: 0.10% or more and 1.8% or less,
Mn: 2.00% or more and 3.50% or less,
P: 0.050% or less,
S: 0.0050% or less,
N: 0.0060% or less,
Al: 0.010% or more and 1.0% or less,
Ti: 0.005% or more and 0.075% or less,
Nb: 0.005% or more and 0.075% or less, and
B: 0.0002% or more and 0.0040% or less
with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1, where [mol % N] and [mol % Ti] are respectively a content of N and a content of Ti in steel in mol %; and
a steel microstructure in which:
an area fraction of ferrite is 12% or more and less than 30%;
a total area fraction of tempered martensite and bainite is 55% or more and 85% or less;
an area fraction of quenched martensite is 15% or less;
an area fraction of retained austenite is 1% or more and 10% or less;
an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 20% or less, where [% Mn] is a content of Mn in the steel in mass %;
a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more;
an area fraction of a residual microstructure is less than 3%; and
an average grain size of the low-Mn ferrite is 10 μm or less.
2. The high-strength cold-rolled steel sheet according to claim 1 , wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of
V: 0.200% or less,
Cr: 0.20% or less,
Mo: 0.20% or less,
Cu: 0.30% or less,
Ni: 0.30% or less,
Sb: 0.100% or less,
Sn: 0.100% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
REM: 0.0050% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Zr: 0.0200% or less, and
Co: 0.100% or less.
3. A high-strength coated or plated steel sheet comprising:
the high-strength cold-rolled steel sheet according to claim 1 ; and
a coated or plated layer on at least one side of the high-strength cold-rolled steel sheet.
4. A method of producing a high-strength cold-rolled steel sheet, the method comprising:
subjecting a steel slab having the chemical composition that contains, in mass %
C: 0.06% or more and 0.15% or less,
Si: 0.10% or more and 1.8% or less,
Mn: 2.00% or more and 3.50% or less,
P: 0.050% or less,
S: 0.0050% or less,
N: 0.0060% or less,
Al: 0.010% or more and 1.0% or less,
Ti: 0.005% or more and 0.075% or less,
Nb: 0.005% or more and 0.075% or less, and
B: 0.0002% or more and 0.0040% or less
with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1, where [mol % N] and [mol % Ti] are respectively a content of N and a content of Ti in steel in mol % to hot rolling to obtain a hot-rolled sheet;
subjecting the hot-rolled sheet to pickling;
subjecting the hot-rolled sheet after the pickling to cold rolling to obtain a cold-rolled sheet;
thereafter performing a first heating process in which the cold-rolled sheet is heated to a first heating temperature of Ac 1 point or higher and (Ac 3 point−50° C.) or lower and held in a first heating temperature range of Ac 1 point or higher and (Ac 3 point−50° C.) or lower for 10 s or more, wherein the A C1 point and the A C3 point are calculated by the following formulae (1) and (2)
Ac 1 (° C.)=751−16[% C]+35[% Si]−28[% Mn]−5.5[% Cu]−16[% Ni]+13[% Cr]+3.4[% Mo] (1);
Ac 3 (° C.)=881−206[% C]+53[% Si]−15[% Mn]−27[% Cu]−20[% Ni]−1[% Cr]+41[% Mo] (2)
where [% M] denotes the content (mass %) of M in steel;
thereafter performing a second heating process in which the cold-rolled sheet is heated to a second heating temperature of Ac 3 point or higher and (Ac 3 point+40° C.) or lower at a heating rate of 10° C./s or more and held in a second heating temperature range of Ac 3 point or higher and (Ac 3 point+40° C.) or lower for 5 s or more and 60 s or less;
thereafter performing (a) a first cooling process in which the cold-rolled sheet is cooled to a first cooling stop temperature of 500° C. or lower and higher than Ms point at a first cooling rate of 10° C./s or more, and then performing one selected from (b1) holding at the first cooling stop temperature for 10 s or more and 60 s or less or (b2) cooling from the first cooling stop temperature to higher than Ms point at a third cooling rate of less than 10° C./s for 10 s or more and 60 s or less, wherein the Ms point is calculated by the following formula (3)
Ms(° C.)=561−474[% C]−7.5[% Si]−33[% Mn]−17[% Ni]−17[% Cr]−21[% Mo] (3)
where [% M] denotes the content (mass %) of M in steel;
thereafter performing a second cooling process in which the cold-rolled sheet is cooled to a second cooling stop temperature of (Ms point−100° C.) or lower and 100° C. or higher at a second cooling rate of 10° C./s or more; and
thereafter performing a reheating process in which the cold-rolled sheet is reheated to a reheating temperature range of the second cooling stop temperature or higher and 450° C. or lower and held in the reheating temperature range of the second cooling stop temperature or higher and 450° C. or lower for 10 s or more and 1800 s or less, to obtain the high-strength cold-rolled steel sheet.
5. A method of producing a high-strength coated or plated steel sheet, the method comprising
subjecting, after the reheating process as recited in claim 4 , the high-strength cold-rolled steel sheet to coating or plating treatment to obtain the high-strength coated or plated steel sheet.
6. An automotive part made of, in at least part thereof, the high-strength cold-rolled steel sheet according to claim 1 .
7. An automotive part made of, in at least part thereof, the high-strength coated or plated steel sheet according to claim 3 .