High-strength steel sheet excellent in impact resistance and manufacturing method thereof, and high-strength galvanized steel sheet and manufacturing method thereof
The present invention provides a high-strength steel sheet excellent in impact resistance. The high-strength steel sheet contains predetermined contents of C, Si, Mn, P, S, Al, Ti, N, and O, with the balance being iron and inevitable impurities, and has a steel sheet structure in which, in a ⅛ thickness to ⅜ thickness region across ¼ of a sheet thickness, 1 to 8% retained austenite is contained in volume fraction, an average aspect ratio of the retained austenite is 2.0 or less, an amount of solid-solution Mn in the retained austenite is 1.1 times an average amount of Mn or more, and TiN grains having a 0.5 μm average grain diameter or less are contained, and a density of AlN grains with a 1 μm grain diameter or more is 1.0 pieces/mm 2 or less, wherein a maximum tensile strength is 900 MPa or more.
1. A high-strength steel sheet containing, in mass %,
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.0001 to 0.0050%,
Al: 0.001 to 0.050%,
Ti: 0.0010 to 0.0150%,
N: 0.0001 to 0.0050%, and
O: 0.0001 to 0.0030%,
with the balance being iron and inevitable impurities, and
having a steel sheet structure wherein a volume fraction of retained austenite is 1 to 8%, an average aspect ratio of the retained austenite is 2.0 or less, and an amount of solid-solution Mn in the retained austenite is at least 1.1 times an average amount of Mn in the steel sheet, wherein the steel sheet structure comprises TiN grains having an average grain diameter of 0.5 μm or less, and wherein a density of AlN grains with a grain diameter of 1 μm or more in the steel sheet structure is 1.0 grain/mm 2 or less, and
wherein a tensile strength of the steel sheet is 900 MPa or more.
2. The high-strength steel sheet according to claim 1 ,
wherein: the steel sheet structure further contains, in volume fraction, 10 to 75% ferrite, a total of 10 to 50% of one or both of bainitic ferrite and bainite, combined, and 10 to 50% tempered martensite, and
wherein pearlite is limited to 5% or less in volume fraction, and fresh martensite is limited to 15% or less in volume fraction.
3. The high-strength steel sheet according to claim 1 , further containing, in mass %, one or two or more of
Nb: 0.0010 to 0.0150%,
V: 0.010 to 0.150%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Mo: 0.01 to 1.00%, and
W: 0.01 to 1.00%, and/or
in total 0.0001 to 0.5000 mass % of one or two or more of Ca, Ce, Mg, Zr, Hf, and REM.
4. The high-strength galvanized steel according to claim 1 , wherein a galvanized layer is formed on a surface.
5. The high-strength galvanized steel sheet according to claim 4 , wherein a coating film made of a phosphorus oxide and/or a composite oxide containing phosphorus is formed on the surface of the galvanized layer.
6. A manufacturing method of the high-strength steel sheet of claim 1 , the method comprising:
a hot-rolling step in which a slab containing, in mass %,
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.0001 to 0.0050%,
Al: 0.001 to 0.050%,
Ti: 0.0010 to 0.0150%,
N: 0.0001 to 0.0050%, and
O: 0.0001 to 0.0030%,
with the balance being iron and inevitable impurities
is heated to 1210° C. or higher, reduction is performed under a condition satisfying the following (Expression 1) at least in a temperature range of 1100 to 1000° C., the reduction is finished at a finish hot-rolling temperature that is not lower than a higher temperature of 800° C. and an Ar 3 transformation point nor higher than 970° C., coiling is performed in a temperature region of 750° C. or lower, and cooling is performed at an average cooling rate of 15° C./hour or less;
a cold-rolling step in which cold-rolling is performed at a reduction ratio of 30 to 75% after the hot-rolling step; and
a continuous annealing step of performing, after the cold-rolling step, annealing where heating is performed in a temperature range of 550 to 700° C. at an average heating rate of 10° C./second or less, a maximum heating temperature is set to a temperature between (an Act transformation point+40) and 1000° C., cooling is performed in a temperature range of the maximum heating temperature to 700° C. at an average cooling rate of 1.0 to 10.0° C./second, cooling is performed in a temperature range of 700 to 500° C. at an average cooling rate of 5.0 to 200.0° C./second, and a retention process is performed in a temperature range of 350 to 450° C. for 30 to 1000 seconds,
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7. A method of manufacturing a high-strength galvanized steel sheet, wherein, in the continuous annealing step of the manufacturing method according to claim 6 , a galvanized layer is formed on a surface of the steel sheet by applying electrogalvanization after the retention process.
8. A manufacturing method of a high-strength galvanized steel sheet, wherein, in the continuous annealing step of the manufacturing method according to claim 6 , after the cooling in the temperature range of 700 to 500° C., the steel sheet is immersed in a galvanizing bath to form a galvanized layer on a surface of the steel sheet before the retention process in the temperature range of 350 to 450° C. or after the retention process.
9. The manufacturing method of the high-strength galvanized steel sheet according to claim 8 , wherein, after being immersed in the galvanizing bath, the steel sheet is re-heated to 460 to 600° C. and is retained for two seconds or longer to alloy the galvanized layer.
10. The manufacturing method of the high-strength galvanized steel sheet according to claim 8 , wherein, after the galvanized layer is formed, a coating film made of a phosphorus oxide and/or a composite oxide containing phosphorus is applied on a surface of the galvanized layer.
11. The manufacturing method of the high-strength galvanized steel sheet according to claim 9 , wherein, after the galvanized layer is alloyed, a coating film made of a phosphorus oxide and/or a composite oxide containing phosphorus is applied on a surface of the alloyed galvanized layer.
12. A method of manufacturing a high-strength galvanized steel sheet according to claim 6 , wherein the slab further comprises, in mass %,
one or two or more of
Nb: 0.0010 to 0.0150%,
V: 0.010 to 0.150%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Mo: 0.01 to 1.00%, and
W: 0.01 to 1.00%, and/or
in total 0.0001 to 0.5000 mass % of one or two or more of Ca, Ce, Mg, Zr, Hf, and REM.