High-strength cold-rolled steel sheet and method of manufacturing thereof
View Patent ↗A high-strength cold-rolled steel sheet includes, by mass %, C: 0.10% to 0.40%, Mn: 0.5% to 4.0%, Si: 0.005% to 2.5%, Al: 0.005% to 2.5%, Cr: 0% to 1.0%, and a balance of iron and inevitable impurities, in which an amount of P is limited to 0.05% or less, an amount of S is limited to 0.02% or less, an amount of N is limited to 0.006% or less, the microstructure includes 2% to 30% of retained austenite by area percentage, martensite is limited to 20% or less by area percentage in the microstructure, an average particle size of cementite is 0.01 μm to 1 μm, and 30% to 100% of the cementite has an aspect ratio of 1 to 3.
1. A high-strength cold-rolled steel sheet comprising: by mass%,
C: 0.10% to 0.40%;
Mn: 0.5% to 4.0%;
Si: 0.005% to 2.5%;
Al: 0.005% to 2.5%;
Cr: 0% to 1.0%; and
a balance of iron and inevitable impurities, wherein
an amount of P is limited to 0.05% or less,
an amount of S is limited to 0.02% or less,
an amount of N is limited to 0.006% or less,
a microstructure thereof includes 2% to 30% of retained austenite by area percentage,
martensite is limited to 20% or less by area percentage in the microstructure,
an average particle size of cementite in the microstructure is 0.01 μm to 1 μm, and
30% to 100% of the cementite has an aspect ratio of 1 to 3.
2. The high-strength cold-rolled steel sheet according to claim 1 , further comprising, by mass %, one or more kinds of:
Mo: 0.01% to 0.3%;
Ni: 0.01% to 5%;
Cu: 0.01% to 5%;
B: 0.0003% to 0.003%;
Nb: 0.01% to 0.1%;
Ti: 0.01% to 0.2%;
V: 0.01% to 1.0%;
W: 0.01% to 1.0%;
Ca: 0.0001% to 0.05%;
Mg: 0.0001% to 0.05%;
Zr: 0.0001% to 0.05%; and
REM: 0.0001% to 0.05%.
3. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein a total amount of Si and Al is 0.5% to 2.5%.
4. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein an average grain size of the retained austenite is 5 μm or less.
5. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein the microstructure includes, by area percentage, 10% to 70% of ferrite.
6. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein the microstructure includes, by area percentage, 10% to 70% of ferrite and bainite in total.
7. The high-strength cold-rolled steel sheet according to claim 1
wherein the microstructure includes, by area percentage, 10% to 75% of bainite and tempered martensite in total.
8. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein an average grain diameter of the ferrite is 10 μm or less.
9. The high-strength cold-rolled steel sheet according to claim 5 or 6
wherein 0.003 to 0.12 particles of the cementite having the aspect ratio of 1 to 3 is included in an area of 1 μm 2 .
10. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein, in a central portion of a sheet thickness thereof, a random intensity ratio X of a {100} <001>orientation of the retained austenite and the average value Y of a random intensity ratio of a {110} <111>to {110} <001>orientation group of the retained austenite satisfies a following equation (14),
4<2X+Y<10 (14).
11. The high-strength cold-rolled steel sheet according to claim 1
wherein, in a central portion of a sheet thickness thereof, the ratio of the random intensity ratio of a {110} <111>orientation of the retained austenite to a random intensity ratio of a {110} <001>orientation of the retained austenite is 3.0 or less.
12. The high-strength cold-rolled steel sheet according to claim 1 , further comprising a zinc coating on at least one surface thereof.
13. The high-strength cold-rolled steel sheet according to claim 1 , further comprising a galvannealed coating on at least one surface thereof.
14. A method of manufacturing a high-strength cold-rolled steel sheet, the method comprising:
a first process in which a slab having the chemical composition according to claim 1 is hot-rolled at a finishing temperature of 820° C. or higher so as to produce a hot-rolled steel sheet;
a second process in which, after the first process, the hot-rolled steel sheet is cooled and coiled in a coiling temperature CT° C. of 350° C. to 600° C.;
a third process in which the hot-rolled steel sheet that has undergone the second process is cold-rolled in a reduction in thickness of 30% to 85% so as to produce a cold-rolled steel sheet;
a fourth process in which, after the third process, the cold-rolled steel sheet is heated and annealed at an average heating temperature of 750° C. to 900° C.;
a fifth process in which the cold-rolled steel sheet that has undergone the fourth process is cooled at an average cooling rate of 3 ° C/s to 200 ° C/s and held in a temperature range of 300° C. to 500° C. for 15 seconds to 1200 seconds; and
a sixth process in which the cold-rolled steel sheet that has undergone the fifth process is cooled, wherein
in the second process, a first average cooling rate CR1° C/s from 750° C. to 650° C. is 15 ° C/s to 100 ° C/s, a second average cooling rate CR2° C/s from 650° C. to the coiling temperature CT° C. is 50 ° C/s or less, a third average cooling rate CR3° C/s from after coiling to 150° C. is 1 ° C/s or less, and the coiling temperature CT° C. and the first average cooling rate CR1° C/s satisfy a following equation (15), and
in the fourth process, in a case in which the amounts of Si, Al, and Cr are represented by [Si], [Al], and [Cr] in terms of mass%, respectively, an average area S μm 2 of pearlite included in the hot-rolled steel sheet that has undergone the second process, the average heating temperature T° C., and a heating time t satisfy a relationship of a following equation (16),
1500 <CR1×(650 -CT) <15000 (15)
2200 >T ×log (t)/(1 +0.3[Si] +0.5[Al] +[Cr] +0.5S) >110 (16).
15. The method of manufacturing the high-strength cold-rolled steel sheet according to claim 14 ,
wherein a total of the reduction in thickness of last two steps in the first process is 15% or more.
16. The method of manufacturing the high-strength cold-rolled steel sheet according to claim 14 ,
wherein the cold-rolled steel sheet that has undergone the fifth process and is to undergo the sixth process is coated with zinc.
17. The method of manufacturing the high-strength cold-rolled steel sheet according to claim 14 ,
wherein the cold-rolled steel sheet that has undergone the fifth process and is to undergo the sixth process is galvanized and annealed in 400° C. to 600° C. for alloying.
18. The method of manufacturing the high-strength cold-rolled steel sheet according to claim 14 ,
wherein an average heating rate from 600° C. to 680° C. in the fourth process is 0.1 ° C/s to 7° C/s.
19. The method of manufacturing the high-strength cold-rolled steel sheet according to claim 14 ,
wherein, before the first process, the slab is cooled to 1000° C. or lower and reheated to 1000° C. or higher.