High-strength hot-dip galvanized steel sheet having excellent yield strength and formability
A hot-dip galvanized steel sheet contains specific amounts of C, Si, Mn, P, S, Ti, Al, and N with the remainder being iron and unavoidable impurities. Bainitic ferrite, martensite, retained γ, and ferrite (α) are present each in a specific area ratio. The remainder γ has a specific C concentration. Sub-grains in the recrystallized α and un-recrystallized α have a specific grain diameter. The surface area ratio of α and worked α having a grain diameter of 5 μm or more is 5% or less. The average particle diameter of TiC particles inside α grains is 10 nm or less.
1. A high strength hot-dip galvanized steel sheet, comprising:
a steel sheet having a galvanized layer formed on a surface thereof, the steel sheet including following components by mass %,
C: 0.05 to 0.3%,
Si: 1 to 3%,
Mn: 1 to 3%,
P: 0.1% or less (inclusive 0%),
S: 0.01% or less (inclusive 0%),
Ti: 0.02 to 0.2%,
Al: 0.001 to 0.1%,
N: 0.002 to 0.03%, and
iron,
wherein the steel sheet satisfies Formula (1) and Formula (2):
10[C]+0.4[Si]+[Mn]≦4.5 Formula (1), and
0.8×(723−10.7[Mn]−16.9[Ni]+29.1[Si]+16.9[Cr])+0.2×(910−203√[C]+44.7[Si]+31.5[Mo]−30[Mn]−11[Cr]−20[Cu]+700[P]+400[Al]+400[Ti])≦780 Formula (2)
(where the components in [ ] indicate contents thereof in mass % in the steel sheet,
the steel sheet includes, by an area ratio based on an entire microstructure,
bainitic ferrite: 40 to 65%,
martensite+retained austenite: 15% or more,
the martensite: 15% or less,
the retained austenite: 5% or more, and
ferrite: 20 to 40%,
the retained austenite has a C concentration CγR of 0.9 mass % or more,
the steel sheet includes a plurality of ferrite particles including recrystallized ferrite particles and un-recrystallized ferrite particles such that an average grain diameter of the recrystallized ferrite and an average grain diameter of sub-grains in the un-recrystallized ferrite particles are each 3 μm or less,
the steel sheet includes ferrite particles having a grain diameter of 5 μm or more such that an area ratio of the ferrite particles having a grain diameter of 5 μm or more is 5% or less,
the steel sheet includes worked ferrite such that an area ratio of the worked ferrite is 5% or less, and
the ferrite particles include TiC particles having an average grain diameter of 10 nm or less.
2. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes at least one of:
Cr: 0.01 to 3%,
Mo: 0.01 to 1%,
Cu: 0.01 to 2%,
Ni: 0.01 to 2%,
B: 0.00001 to 0.001%,
Ca: 0.0005 to 0.01%/,
Mg: 0.0005 to 0.01%, and
REM: 0.0001 to 0.01%.
3. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.01 to 3% of Cr.
4. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.01 to 1% of Mo.
5. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.01 to 2% of Cu.
6. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.01 to 2% of Ni.
7. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.00001 to 0.001% of B.
8. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.0005 to 0.01% of Ca.
9. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.0005 to 0.01% of Mg.
10. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet further includes 0.0001 to 0.01% of REM.
11. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet includes 0.10 to 0.3% of C.
12. The high strength hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet consists of C, Si, Mn, P, S, Ti, Al, N, and the balance being iron and unavoidable impurities.
13. The high strength hot-dip galvanized steel sheet according to claim 12 , wherein the steel sheet further includes at least one of:
Cr: 0.01 to 3%,
Mo: 0.01 to 1%,
Cu: 0.01 to 2%,
Ni: 0.01 to 2%,
B: 0.00001 to 0.001%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%, and
REM: 0.0001 to 0.01%.