Hot-rolled steel sheet, cold-rolled steel sheet, galvanized steel sheet, and methods of manufacturing the same

1. A method of manufacturing a hot-rolled steel sheet, the method comprising,

first hot rolling carried out at least once at a rolling reduction ratio of 20% or more in a temperature range of 1000° C. to 1200° C., and an austenite grain diameter is set to 200 μm or less, wherein

an ingot or a slab containing, by mass %:

C: 0.0001% to 0.40%,

Si: 0.001% to 2.5%,

Mn: 0.001% to 4.0%,

P: 0.001% to 0.15%,

S: 0.0005% to 0.03%,

Al: 0.001% to 2.0%,

N: 0.0005% to 0.01%,

O: 0.0005% to 0.01%,

and further comprising one or two or more of:

Ti: 0.001% to 0.20%,

Nb: 0.001% to 0.20%,

V: 0.001% to 1.0%,

W: 0.001% to 1.0%,

B: 0.0001% to 0.0050%,

Mo: 0.001% to 1.0%,

Cr: 0.001% to 2.0%,

Cu: 0.001% to 2.0%,

Ni: 0.001% to 2.0%,

Co: 0.0001% to 1.0%,

Sn: 0.0001% to 0.2%,

Zr: 0.0001% to 0.2%,

As: 0.0001% to 0.50%,

Mg: 0.0001% to 0.010%,

Ca: 0.0001% to 0.010%, and

REM: 0.0001% to 0.1%

and balance composed of iron and inevitable impurities;

second hot rolling in which a total of rolling reduction ratios is 50% or more is carried out in a temperature range of T1+30° C. to T1+200° C.;

third hot rolling in which a total of rolling reduction ratios is less than 30% is carried out in a temperature range of T1° C. to lower than T1+30° C.; and

hot rolling ends at an Ar3 transformation temperature or higher,

where, T1 is a temperature determined by steel sheet components, and expressed by the following formula 1,

T1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  (Formula 1),

wherein an average value of an X-ray random intensity ratio of a {100} <011> to {223} <110> orientation group at least in a sheet thickness central portion that is in a sheet thickness range of ⅝ to ⅜ from a steel sheet surface is 1.0 to 6.0 in the hot-rolled steel sheet, an X-ray random intensity ratio of a {332} <113> crystal orientation is 1.0 to 5.0 in the hot-rolled steel sheet; and

rC which is an r value in a direction perpendicular to a rolling direction is 0.70 to 1.10, and r30 which is an r value in a direction that forms an angle of 30° with respect to the rolling direction is 0.70 to 1.10 in the hot-rolled steel sheet.

2. The method of manufacturing a hot-rolled steel sheet according to claim 1 ,

wherein, in the second hot rolling in the temperature range of T1+30° C. to T1+200° C., the ingot or slab is rolled at least once at a rolling reduction ratio of 30% or more in a pass.

3. The method of manufacturing a hot-rolled steel sheet according to claim 1 ,

wherein, in the first hot rolling in a temperature range of 1000° C. to 1200° C., the ingot or slab is rolled at least twice at a rolling reduction ratio of 20% or more, and the austenite grain diameter is set to 100 μm or less.

4. The method of manufacturing a hot-rolled steel sheet according to claim 1 ,

wherein, in a case in which a pass in which a rolling reduction ratio is 30% or more in the temperature range of T1+30° C. to T1+200° C. is defined as a large reduction pass, a waiting time t from completion of a final pass of the large reduction pass to initiation of cooling employs a configuration that satisfies the following formula 2,

t 1≦ t≦t 1×2.5  (Formula 2)

where t1 is expressed by the following formula 3;

t 1=0.001×(( Tf−T 1)× P 1) 2 −0.109×(( Tf−T 1)× P 1)+3.1  (Formula 3)

where Tf represents a temperature after the final pass, and P1 represents a rolling reduction ratio in the final pass.

5. The method of manufacturing a hot-rolled steel sheet according to claim 4 ,

wherein a temperature of the steel sheet increases by 18° C. or less between respective passes of the second hot rolling in the temperature range of T1+30° C. to T1+200° C.

6. A method of manufacturing a cold-rolled steel sheet, the method comprising,

pickling the hot-rolled steel sheet according to claim 1 ;

cold-rolling the hot-rolled steel sheet at 20% to 90%, and obtaining the cold-rolled steel sheet;

annealing the cold-rolled steel sheet at a temperature range of 720° C. to 900° C. for a holding time of 1 second to 300 seconds;

acceleration-cooling the cold-rolled steel sheet at a cooling rate of 10° C./s to 200° C./s from 650° C. to 500° C.; and

holding the cold-rolled steel sheet at a temperature of 200° C. to 500° C.

7. The method of manufacturing a cold-rolled steel sheet according to claim 6 ,

wherein, in the second hot rolling in the temperature range of T1+30° C. to T1+200° C., rolling at a rolling reduction ratio of 30% or more in a pass is carried out at least once.

8. The method of manufacturing a cold-rolled steel sheet according to claim 6 ,

wherein, in the first hot rolling in the temperature range of 1000° C. to 1200° C., rolling at a rolling reduction ratio of 20% or more is carried out at least twice, and the austenite grain diameter is set to 100 μm or less.

9. The method of manufacturing a cold-rolled steel sheet according to claim 6 ,

wherein, in a case in which a pass in which a rolling reduction ratio is 30% or more in the temperature range of T1+30° C. to T1+200° C. is defined as a large reduction pass, a waiting time t from completion of a final pass of the large reduction pass to initiation of cooling employs a configuration that satisfies the following formula 4,

t 1≦ t≦t 1×2.5  (Formula 4)

where t1 is expressed by the following formula 5;

t 1=0.001×(( Tf−T 1)× P 1) 2 −0.109×(( Tf−T 1)× P 1)+3.1  (Formula 5)

where Tf represents a temperature after the final pass, and P1 represents a rolling reduction ratio in the final pass.

10. The method of manufacturing a cold-rolled steel sheet according to claim 9 ,

wherein a temperature of the steel sheet increases by 18° C. or less between respective passes of the second hot rolling in the temperature range of T1+30° C. to T1+200° C.

11. A method of manufacturing a galvanized steel sheet, the method comprising,

winding the hot-rolled steel sheet according to claim 1 in a temperature range of 680° C. to room temperature;

pickling the hot-rolled steel sheet;

cold-rolling the hot-rolled steel sheet at 20% to 90%, and obtaining a cold-rolled steel sheet;

heating the cold-rolled steel sheet to a temperature range of 650° C. to 900° C.;

annealing the cold-rolled steel sheet for a holding time of 1 second to 300 seconds;

cooling the cold-rolled steel sheet at a cooling rate of 0.1° C./s to 100° C./s from 720° C. to 580° C.; and

galvanizing treating the cold-rolled steel sheet.

12. The method of manufacturing a galvanized steel sheet according to claim 11 ,

wherein, in the second hot rolling in the temperature range of T1+30° C. to T1+200° C., rolling at a rolling reduction ratio of 30% or more in a pass is carried out at least once.

13. The method of manufacturing a galvanized steel sheet according to claim 11 ,

wherein, in the first hot rolling in the temperature range of 1000° C. to 1200° C., rolling at a rolling reduction ratio of 20% or more is carried out at least twice, and the austenite grain diameter is set to 100 μm or less.

14. The method of manufacturing a galvanized steel sheet according to claim 11 ,

wherein, in a case in which a pass in which a rolling reduction ratio is 30% or more in the temperature range of T1+30° C. to T1+200° C. is defined as a large reduction pass, a waiting time t from completion of a final pass of the large reduction pass to initiation of cooling employs a configuration that satisfies the following formula 6,

t 1≦ t≦t 1×2.5  (Formula 6)

where t1 is expressed by the following formula 7;

t 1=0.001×(( Tf−T 1)× P 1) 2 −0.109×(( Tf−T 1)× P 1)+3.1  (Formula 7)

where Tf represents a temperature after the final pass, and P1 represents a rolling reduction ratio in the final pass.

15. The method of manufacturing a galvanized steel sheet according to claim 14 ,

wherein a temperature of the steel sheet increases by 18° C. or less between respective passes of the second hot rolling in the temperature range of T1+30° C. to T1+200° C.