High-strength galvanized steel sheet and method for manufacturing the same
A high-strength galvanized steel sheet having a chemical composition containing, by mass %, C: 0.07% to 0.25%, Si: 0.01% to 3.00%, Mn: 1.5% to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 1.50%, N: 0.001% to 0.008%, Ti: 0.003% to 0.200%, B: 0.0003% to 0.0050%, and the balance being Fe and inevitable impurities, in which the relationship Ti>4N is satisfied, and a microstructure including, in terms of area ratio in a cross section located at ¼ of the thickness from the surface of a base steel sheet, a ferrite phase in an amount of 70% or less (including 0%), a bainite phase and a tempered bainite phase in an amount of 20% or less (including 0%) in total, a tempered martensite phase in an amount of 25% or more, and a retained austenite phase in an amount of less than 3% (including 0%), in which the average crystal grain diameter of the tempered martensite phase is 20 μm or less, in which a variation in the Vickers hardness of the tempered martensite phase is 20 or less in terms of standard deviation, and in which the number density of carbides having a minor axis length of 0.05 μm or more in the tempered martensite phase is 3×10 6 particles/mm 2 or less, as well as a method for manufacturing the steel sheet, is disclosed.
1. A high-strength, galvanized steel sheet having a chemical composition containing, by mass %:
C: 0.07% to 0.25%,
Si: 0.01% to 3.00%,
Mn: 1.5% to 4.0%,
P: 0.100% or less,
S: 0.02% or less,
Al: 0.01% to 1.50%,
N: 0.001% to 0.008%,
Ti: 0.003% to 0.200%,
B: 0.0003% to 0.0050%, and
the balance being Fe and inevitable impurities,
wherein
the relationship Ti>4N is satisfied, and
a microstructure including, in terms of area ratio in a cross section located at ¼ of the thickness from the surface of a base steel sheet:
a ferrite phase in an amount of 70% or less (including 0%),
a bainite phase and a tempered bainite phase in an amount of 20% or less (including 0%) in total,
a tempered martensite phase in an amount of 25% or more, and
a retained austenite phase in an amount of less than 3% (including 0%),
wherein the average crystal grain diameter of the tempered martensite phase is 20 μm or less, wherein a variation in the Vickers hardness of the tempered martensite phase is 20 or less in terms of standard deviation, and wherein the number density of carbides having a minor axis length of 0.05 μm or more in the tempered martensite phase is 3×10 6 particles/mm 2 or less.
2. The-strength, galvanized steel sheet according to claim 1 , the chemical composition further containing, by mass %, at least one group selected from the group consisting of Group A to C:
Group A, which contains at least one selected from:
Cr: 0.01% to 2.00%,
Mo: 0.01% to 2.00%,
V: 0.01% to 2.00%,
Ni: 0.01% to 2.00%, and
Cu: 0.01% to 2.00%
Group B, which contains:
Nb: 0.003% to 0.200%, and
Group C, which contains at least one selected from:
Ca: 0.001% to 0.005%, and
REM: 0.001% to 0.005%.
3. A method for manufacturing a high-strength, galvanized steel sheet, the method comprising performing the following processes in the following order:
a hot rolling process in which, after having performed finish rolling on a slab having the chemical composition according claim 1 , cooling is performed such that a total time during which the hot-rolled steel sheet is retained in a temperature range of 600° C. to 700° C. is 10 seconds or less and in which coiling is performed at a coiling temperature of lower than 600° C.,
a cold rolling process in which cold rolling is performed with a rolling reduction of more than 20%,
an annealing process in which heating is performed to an annealing temperature of 750° C. to 950° C. at an average heating rate of 15° C./s or less and in which the heated steel sheet is held at the annealing temperature for 30 seconds or more,
a first cooling process in which cooling is performed at an average cooling rate of 3° C./s or more,
a galvanizing process in which galvanizing is performed,
a second cooling process in which, after having performed cooling to a temperature equal to or higher than the Ms temperature at an average cooling rate of 1° C./s or more, cooling is performed to a temperature of 100° C. or lower at an average cooling rate of 100° C./s or more, and
a tempering process in which reheating is performed to a temperature of 350° C. or lower and in which the reheated steel sheet is held at the temperature for 1 second or more.
4. The method for manufacturing a high-strength, galvanized steel sheet according to claim 3 , wherein, after having performed galvanizing in the galvanizing process, an alloying treatment is further performed on the galvanizing layer by heating the galvanized steel sheet to a temperature of 460° C. to 600° C.
5. A method for manufacturing a high-strength, galvanized steel sheet, the method comprising performing the following processes in the following order:
a hot rolling process in which, after having performed finish rolling on a slab having the chemical composition according claim 2 , cooling is performed such that a total time during which the hot-rolled steel sheet is retained in a temperature range of 600° C. to 700° C. is 10 seconds or less and in which coiling is performed at a coiling temperature of lower than 600° C.,
a cold rolling process in which cold rolling is performed with a rolling reduction of more than 20%,
an annealing process in which heating is performed to an annealing temperature of 750° C. to 950° C. at an average heating rate of 15° C./s or less and in which the heated steel sheet is held at the annealing temperature for 30 seconds or more,
a first cooling process in which cooling is performed at an average cooling rate of 3° C./s or more,
a galvanizing process in which galvanizing is performed,
a second cooling process in which, after having performed cooling to a temperature equal to or higher than the Ms temperature at an average cooling rate of 1° C./s or more, cooling is performed to a temperature of 100° C. or lower at an average cooling rate of 100° C./s or more, and
a tempering process in which reheating is performed to a temperature of 350° C. or lower and in which the reheated steel sheet is held at the temperature for 1 second or more.
6. The method for manufacturing a high-strength, galvanized steel sheet according to claim 5 , wherein, after having performed galvanizing in the galvanizing process, an alloying treatment is further performed on the galvanizing layer by heating the galvanized steel sheet to a temperature of 460° C. to 600° C.