Steel sheet and method of manufacturing steel sheet
View Patent ↗A steel sheet, including: as chemical components, by mass %, 0.05% to 0.35% of C; 0.05% to 2.0% of Si; 0.8% to 3.0% of Mn; 0.01% to 2.0% of Al; equal to or less than 0.1% of P; equal to or less than 0.05% of S; equal to or less than 0.01% of N; and the balance including iron and inevitable impurities, wherein the steel sheet comprises 50% or more of a ferrite phase, a bainite phase, and a tempered martensite phase, 3% or more of a retained austenite phase, and 50% or more of the crystal grains of the retained austenite phase satisfy Expression 1, wherein a carbon concentration at a position of center of gravity is Cgc and a carbon concentration at a grain boundary is Cgb.
1. A steel sheet comprising:
as chemical components, by mass %,
0.05% to 0.35% of C;
0.05% to 2.0% of Si;
0.8% to 3.0% of Mn;
0.01% to 2.0% of Al;
less than or equal to 0.1% of P;
less than or equal to 0.05% of S;
less than or equal to 0.01% of N; and
the balance including iron and inevitable impurities,
wherein:
the steel sheet has a 25° C. elongation of not less than 20%, and a 150° C. elongation of not less than 21%;
the steel sheet comprises in area ratio, 3% or more of a retained austenite phase, 50% or more of a total of a ferrite phase, a bainite phase and a tempered martensite phase;
a number ratio of 50% or more of crystal grains of the retained austenite phase satisfy Expression 1,
a number ratio of 40% or more of the crystal grains of the retained austenite phase are small-diameter crystal grains having an average grain size greater than or equal to 1 μm and less than 2 μm,
a number ratio of 20% or more of the crystal grains of the retained austenite phase are large diameter crystal grains having an average grain size greater than or equal to 2 μm,
a number ratio of 51% or more of the small diameter crystal grains satisfy Expression 2,
a number ratio of 51% or more of the large diameter crystal grains satisfy Expression 3,
Cgb/Cgc≧ 1.2; Expression 1:
CgbS/CgcS> 1.3; Expression 2:
1.3 >CgbL/CgcL> 1.1; Expression 3:
wherein:
in Expression 1, Cgc represents a carbon concentration at a center of gravity, and Cgb represents a carbon concentration at a grain boundary;
in Expression 2, CgcS represents a carbon concentration of small particles at a center of gravity, and CgbS represents a carbon concentration of small particles at a grain boundary;
in Expression 3, CgcL represents a carbon concentration of large particles at a center of gravity, and CgbL represents a carbon concentration of large particles at a grain boundary.
2. The steel sheet according to claim 1 , further comprising, in the chemical components, by mass %, at least one of:
0.01% to 0.5% of Mo;
0.005% to 0.1% of Nb;
0.005% to 0.2% of Ti;
0.005% to 0.5% of V;
0.05% to 5.0% of Cr;
0.05% to 5.0% of W;
0.0005% to 0.05% of Ca;
0.0005% to 0.05% of Mg;
0.0005% to 0.05% of Zr;
0.0005% to 0.05% of rare earth metals;
0.02% to 2.0% of Cu;
0.02% to 1.0% of Ni; and
0.0003% to 0.007% of B.
3. The steel sheet according to claim 1 ,
wherein an average carbon concentration in the retained austenite phase is equal to or higher than 0.7% and equal to or less than 1.5%.
4. The steel sheet according to claim 1 ,
wherein the steel sheet has a galvanized film provided to at least one surface.
5. The steel sheet according to claim 1 ,
wherein the steel sheet has a galvannealed film provided to at least one surface.
6. The steel sheet according to claim 2 ,
wherein the steel sheet has a galvanized film provided to at least one surface.
7. The steel sheet according to claim 2 ,
wherein the steel sheet has a galvannealed film provided to at least one surface.
8. The steel sheet according to claim 3 ,
wherein the steel sheet has a galvanized film provided to at least one surface.
9. The steel sheet according to claim 3 ,
wherein the steel sheet has a galvannealed film provided to at least one surface.
10. A method of manufacturing a steel sheet of claim 1 , the method comprising:
a hot-rolling process of manufacturing a hot-rolled steel sheet by performing hot rolling on a slab having a composition as recited in claim 1 at a finishing temperature of equal to or higher than 850° C. and equal to or less than 970° C.;
an air-cooling process of performing air cooling on the hot-rolled steel sheet for a time of equal to or longer than 1 second and equal to or shorter than 10 seconds;
a coiling process of cooling the air-cooled hot-rolled steel sheet to a temperature range of equal to or less than 650° C. at an average cooling rate of equal to or higher than 10° C./sec and equal to or less than 200° C./sec and thereafter coiling the steel sheet in a temperature range of equal to or less than 650° C.;
a cold-rolling process of performing pickling on the coiled hot-rolled steel sheet at a rolling reduction ratio of equal to or higher than 40% and thereafter performing cold rolling on the steel sheet, thereby manufacturing a cold-rolled steel sheet;
an annealing process of performing annealing on the cold-rolled steel sheet at a maximum temperature of equal to or higher than 700° C. and equal to or less than 900° C.;
a holding process of cooling the annealed cold-rolled steel sheet in a temperature range of equal to or higher than 350° C. and equal to or less than 480° C. at an average cooling rate of equal to or higher than 0.1° C./sec and equal to or less than 200° C./sec, and holding the steel sheet in this temperature range for a time of equal to or longer than 1 second and equal to or shorter than 1000 seconds; and
a final cooling process of primarily cooling the cold-rolled steel sheet in a temperature range from 350° C. to 220° C. at an average cooling rage of equal to or higher than 5° C./sec and equal to or less than 25° C./sec, and secondarily cooling the steel sheet in a temperature range from 120° C. to near room temperature at an average cooling rate of equal to or higher than 100° C./sec or equal to or less than 5° C./sec,
wherein the hot-rolling process comprises multiple passes of rolling, wherein the rolling is performed with a strain amount of equal to or less than 20% in a sixth pass and with a strain amount of equal to or less than 15% in a seventh pass, wherein the sixth and seventh passes are the final two passes in the hot rolling process.
11. The method according to claim 10 ,
wherein a slab which is re-heated to 1100° C. or higher after being cooled to 1100° C. or less is used in the hot-rolling process.
12. The method according to claim 10 , further comprising an immersion process of immersing the steel sheet in a hot-dip galvanizing bath after the holding process.
13. The method according to claim 12 further comprising an alloying treatment process of performing an alloying treatment in a range of equal to or higher than 500° C. and equal to or less than 580° C. after the immersion process.
14. A method of manufacturing a steel sheet of claim 2 , the method comprising:
a hot-rolling process of manufacturing a hot-rolled steel sheet by performing hot rolling on a slab having a composition as recited in claim 2 at a finishing temperature of equal to or higher than 850° C. and equal to or less than 970° C.;
an air-cooling process of performing air cooling on the hot-rolled steel sheet for a time of equal to or longer than 1 second and equal to or shorter than 10 seconds;
a coiling process of cooling the air-cooled hot-rolled steel sheet to a temperature range of equal to or less than 650° C. at an average cooling rate of equal to or higher than 10° C./sec and equal to or less than 200° C./sec and thereafter coiling the steel sheet in a temperature range of equal to or less than 650° C.;
a cold-rolling process of performing pickling on the coiled hot-rolled steel sheet at a rolling reduction ratio of equal to or higher than 40% and thereafter performing cold rolling on the steel sheet, thereby manufacturing a cold-rolled steel sheet;
an annealing process of performing annealing on the cold-rolled steel sheet at a maximum temperature of equal to or higher than 700° C. and equal to or less than 900° C.;
a holding process of cooling the annealed cold-rolled steel sheet in a temperature range of equal to or higher than 350° C. and equal to or less than 480° C. at an average cooling rate of equal to or higher than 0.1° C./sec and equal to or less than 200° C./sec, and holding the steel sheet in this temperature range for a time of equal to or longer than 1 second and equal to or shorter than 1000 seconds; and
a final cooling process of primarily cooling the cold-rolled steel sheet in a temperature range from 350° C. to 220° C. at an average cooling rage of equal to or higher than 5° C./sec and equal to or less than 25° C./sec, and secondarily cooling the steel sheet in a temperature range from 120° C. to near room temperature at an average cooling rate of equal to or higher than 100° C./sec or equal to or less than 5° C./sec,
wherein the hot-rolling process comprises multiple passes of rolling, wherein the rolling is performed with a strain amount of equal to or less than 20% in a sixth pass and with a strain amount of equal to or less than 15% in a seventh pass, wherein the sixth and seventh passes are the final two passes in the hot rolling process.
15. The method according to claim 14 ,
wherein a slab which is re-heated to 1100° C. or higher after being cooled to 1100° C. or less is used in the hot-rolling process.
16. The method according to claim 14 ,
further comprising an immersion process of immersing the steel sheet in a hot-dip galvanizing bath after the holding process.
17. The method according to claim 16 further comprising an alloying treatment process of performing an alloying treatment in a range of equal to or higher than 500° C. and equal to or less than 580° C. after the immersion process.