High-strength steel sheet and high-strength zinc-coated steel sheet which have excellent ductility and stretch-flangeability and manufacturing method thereof
View Patent ↗This high-strength steel sheet includes by mass percentage: 0.05 to 0.4% of C; 0.1 to 2.5% of Si; 1.0 to 3.5% of Mn; 0.001 to 0.03% of P; 0.0001 to 0.01% of S; 0.001 to 2.5% of Al; 0.0001 to 0.01% of N; 0.0001 to 0.008% of O; and a remainder composed of iron and inevitable impurities, wherein a steel sheet structure contains by volume fraction 10 to 50% of a ferrite phase, 10 to 50% of a tempered martensite phase, and a remaining hard phase, wherein a 98% hardness is 1.5 or more times as high as a 2% hardness in a range from ⅛ to ⅜ of a thickness of the steel sheet, wherein a kurtosis K* of the hardness distribution between the 2% hardness and the 98% hardness is −1.2 to −0.4, and wherein an average crystal grain size in the steel sheet structure is 10 μm or less.
1. A high-strength steel sheet comprising by mass percentage:
0.05 to 0.4% of C;
0.1 to 2.5% of Si;
1.0 to 3.5% of Mn;
0.001 to 0.03% of P;
0.0001 to 0.01% of S;
0.001 to 2.5% of Al;
0.0001 to 0.01% of N;
0.0001 to 0.008% of 0; and
a remainder composed of iron and inevitable impurities,
wherein a steel sheet structure contains by volume fraction 10 to 45% of a ferrite phase, 10 to 50% of a tempered martensite phase, and a remaining hard phase which is selected from the group consisting of a bainitic ferrite phase, a bainite phase, a fresh martensite phase, and a mixture thereof,
wherein when a plurality of measurement regions with diameters of 1 μm or less are set in a range from ⅛ to ⅜ of a thickness of the steel sheet, hardness measurement values in the plurality of measurement regions are arranged in ascending order to obtain a hardness distribution, an integer N0.02 which is a number obtained by multiplying a total number of the hardness measurement values by 0.02 and, if present, by rounding up a decimal number, is obtained, a hardness of a measurement value which is an N0.02-th largest value from a smallest hardness measurement value is regarded as a 2% hardness, an integer N0.98 which is a number obtained by multiplying the total number of the hardness measurement values by 0.98 and, if present, by rounding down the decimal number is obtained, and a hardness of a measurement value which is an N0.98-th largest value from the smallest hardness measurement value is regarded as a 98% hardness, the 98% hardness is 1.5 or more times as high as the 2% hardness,
wherein a kurtosis K* of the hardness distribution between the 2% hardness and the 98% hardness is equal to or more than −1.2 and equal to or less than −0.4,
wherein an average crystal grain size in the steel sheet structure is 10 μm or less,
wherein a tensile strength is 900 MPa or more,
wherein a difference between a maximum value and a minimum value of Mn concentration in a base iron in a thickness range from ⅛ to ⅜ of the steel sheet is equal to or more than 0.4% and equal to or less than 3.5% when converted into the mass percentage, and
wherein the hard phase includes a fresh martensite phase of 10% or less.
2. The high-strength steel sheet according to claim 1 ,
wherein when a section from the 2% hardness to the 98% hardness is equally divided into 10 parts, and 10 1/10-sections are set, a number of the hardness measurement values in each 1/10-section is 2 to 30% of a number of all measurement values.
3. The high-strength steel sheet according to claim 1 ,
wherein the hard phase includes any one of or both a bainitic ferrite phase and a bainite phase of 10 to 45% by a volume fraction.
4. The high-strength steel sheet according to claim 1 ,
wherein the steel sheet structure further includes 2 to 25% of a retained austenite.
5. The high-strength steel sheet according to claim 1 , further comprising by mass percentage one or more of:
0.005 to 0.09% of Ti; and
0.005 to 0.09% of Nb.
6. The high-strength steel sheet according to claim 1 , further comprising by mass percentage one or more of:
0.0001 to 0.01% of B;
0.01 to 2.0% of Cr;
0.01 to 2.0% of Ni;
0.01 to 2.0% of Cu; and
0.01 to 0.8% of Mo.
7. The high-strength steel sheet according to claim 1 , further comprising by mass percentage:
0.005 to 0.09% of V.
8. The high-strength steel sheet according to claim 1 , further comprising one or more of Ca, Ce, Mg, and REM excluding the Ce at 0.0001 to 0.5% by mass percentage in total.
9. A high-strength zinc-coated steel sheet,
wherein the high-strength zinc-coated steel sheet is produced by forming a zinc-plated layer on a surface of the high-strength steel sheet according to claim 1 .
10. A manufacturing method of a high-strength steel sheet according to claim 1 , the method comprising:
a hot rolling process in which a slab containing chemical constitutents according to claim 1 is heated up to 1050° C. or higher directly or after cooling once, a hot rolling is performed thereon at a higher temperature of one of 800° C. and an Ar a transformation point, and a winding is performed in a temperature range of 750° C. or lower such that an austenite phase in a structure of a rolled material after rolling occupies 50% by volume or more;
a cooling process in which the steel sheet after the hot rolling is cooled from a winding temperature to (the winding temperature—100° C. at a rate of 20° C./hour or lower while a following Equation (1) is satisfied; and
a process in which continuous annealing is performed on the steel sheet after the cooling,
wherein in the process in which continuous annealing is performed,
the steel sheet is annealed at a maximum heating temperature of 750 to 1000° C.,
a first cooling in which the steel sheet is cooled from the maximum heating temperature to a ferrite transformation temperature range or lower and maintained in the ferrite transformation temperature range for 20 to 1000 seconds is subsequently performed,
a second cooling in which the steel sheet is cooled at a cooling rate of 10° C./second or higher on average in a bainite transformation temperature range and cooling is stopped within a range from a martensite transformation start temperature—98° C. to the martensite transformation start temperature is subsequently performed,
the steel sheet after the second cooling is maintained in a range from a second cooling stop temperature to the martensite transformation start temperature for 2 to 1000 seconds,
the steel sheet is subsequently reheated up to a reheating stop temperature, which is equal to or more than a bainite transformation start temperature—100° C., at a rate of temperature increase of 10° C./second or higher on average in the bainite transformation temperature range, and
a third cooling in which the steel sheet after the reheating is cooled from the reheating stop temperature to a temperature which is lower than the bainite transformation temperature range and maintained in the bainite transformation temperature range for 30 seconds or more is performed:
[
Equation
1
]
[
∫
T
c
-
100
T
c
9.47
×
10
5
·
exp
(
-
18480
T
+
273
)
·
t
(
T
)
·
ⅆ
T
]
0.5
≥
1.0
(
1
)
[where, t(T) in Equation (1) represents maintaining time (seconds) of the steel sheet at a temperature T° C. in the cooling process after the winding], so that a steel sheet according to claim 1 is formed.
11. The manufacturing method of the high-strength steel sheet according to claim 10 ,
wherein the winding temperature after the hot rolling is equal to or more than a Bs point and equal to or less than 750° C.
12. The manufacturing method of the high-strength steel sheet according to claim 10 , further comprising between the cooling process and the continuous annealing process:
a cold rolling process in which the steel sheet is subjected to acid pickling and a cold rolling at rolling reduction from 35 to 80%.
13. The manufacturing method of the high-strength steel sheet according to claim 10 ,
wherein a sum of a time during which the steel sheet is maintained in the bainite transformation temperature range in the second cooling and a time during which the steel sheet is maintained in the bainite transformation temperature range in the reheating is 25 seconds or less.
14. The manufacturing method of the high-strength steel sheet according to claim 10 , wherein the steel sheet is dipped into a zinc plating bath in the reheating in manufacturing the high-strength steel sheet.
15. A manufacturing method of a high-strength zinc-coated steel sheet according to claim 10 , wherein the steel sheet is dipped into a zinc plating bath in the bainite transformation temperature range in the third cooling in manufacturing the high-strength steel sheet.
16. A manufacturing method of a high-strength zinc-coated steel sheet according to claim 10 , wherein a zinc electroplating is performed after manufacturing the high-strength steel sheet.
17. A manufacturing method of a high-strength zinc-coated steel sheet according to claim 10 , wherein a hot-dip zinc-plating is performed after manufacturing the high-strength steel sheet.