Hot-rolled steel sheet and production method thereof

1. A hot-rolled steel sheet being a steel sheet having a chemical composition, by mass %, of

C: 0.02% to 0.5%,

Si: 0.001% to 4.0%,

Mn: 0.001% to 4.0%,

Al: 0.001% to 4.0%,

P: 0.15% or less,

S: 0.03% or less,

N: 0.01% or less,

O: 0.01% or less,

and the balance comprising Fe and unavoidable impurities,

wherein a sum of a content of the Si and a content of the Al is 1.0% to 4.0% in the chemical composition of the steel sheet,

an average pole density of an orientation group from {100}<011> to {223}<110>, which is a pole density expressed by an arithmetic average of pole densities of respective crystal orientations {100}<011>, {116}<110>, {114}<110>, {112}<110>, and {223}<110>, is 1.0 to 6.5, and a pole density of a crystal orientation {332}<113> is 1.0 to 5.0 in a sheet thickness central portion within a range of ⅝ to ⅜ of a sheet thickness,

a microstructure of the steel sheet includes a plurality of grains,

the microstructure of the steel sheet includes, by an area ratio, 2% to 30% of retained austenite, 20% to 50% of ferrite, 10% to 60% of bainite, 20% or less of pearlite, and 20% or less of martensite, and

rC that is a Lankford value in a direction orthogonal to a rolling direction is 0.70 to 1.10, and r30 that is a Lankford value in a direction forming an angle of 30° with the rolling direction is 0.70 to 1.10.

2. The hot-rolled steel sheet according to claim 1 ,

wherein the chemical composition of the steel sheet further includes, by mass %, one or more selected from the group consisting of

Ti: 0.001% to 0.2%,

Nb: 0.001% to 0.2%,

V: 0.001% to 1.0%,

W: 0.001% to 1.0%,

Cu: 0.001% to 2.0%,

B: 0.0001% to 0.005%,

Mo: 0.001% to 1.0%,

Cr: 0.001% to 2.0%,

As: 0.0001% to 0.50%,

Mg: 0.0001% to 0.010%,

REM: 0.0001% to 0.1%,

Ca: 0.0001% to 0.010%,

Ni: 0.001% to 2.0%,

Co: 0.0001% to 1.0%,

Sn: 0.0001% to 0.2%, and

Zr: 0.0001% to 0.2%.

3. The hot-rolled steel sheet according to claim 1 ,

wherein a volume average diameter of the grains is 1 μm to 4 μm.

4. The hot-rolled steel sheet according to claim 1 ,

wherein the average pole density of the orientation group from {100}<011> to {223}<110> is 1.0 to 5.0, and the pole density of the crystal orientation {332}<113> is 1.0 to 4.0.

5. The hot-rolled steel sheet according to claim 1 ,

wherein among the plurality of grains, an area ratio of grains which exceed 20 μm in grain size is limited to 10% or less.

6. The hot-rolled steel sheet according to claim 1 ,

wherein with regard to at least 100 grains of the retained austenite and the martensite, a standard deviation of a distance L MA between the grains closest to each other is 5 μm or less.

7. The hot-rolled steel sheet according to claim 2 ,

wherein a volume average diameter of the grains is 1 μm to 4 μm.

8. The hot-rolled steel sheet according to claim 2 ,

wherein the average pole density of the orientation group from {100}<011> to {223}<110> is 1.0 to 5.0, and the pole density of the crystal orientation {332}<113> is 1.0 to 4.0.

9. The hot-rolled steel sheet according to claim 2 ,

wherein among the plurality of grains, an area ratio of grains which exceed 20 μm in grain size is limited to 10% or less.

10. The hot-rolled steel sheet according to claim 2 ,

wherein with regard to at least 100 grains of the retained austenite and the martensite, a standard deviation of a distance L MA between the grains closest to each other is 5 μm or less.

11. A production method of a hot-rolled steel sheet, the production method comprising:

(a) a first hot-rolling process, wherein the first hot-rolling process comprises rolling the steel, at least one or more times, at a rolling reduction ratio of 40% or more and a temperature range of 1,000° C. to 1,200° C.

wherein the steel comprises, by mass %,

C: 0.02% to 0.5%,

Si: 0.001% to 4.0%,

Mn: 0.001% to 4.0%,

Al: 0.001% to 4.0%,

P: 0.15% or less,

S: 0.03% or less,

N: 0.01% or less,

O: 0.01% or less, and

the balance comprising Fe and unavoidable impurities, and

wherein a sum of a content of the Si and a content of the Al is 1.0% to 4.0%;

(b) a second hot-rolling process of performing hot rolling,

wherein the second hot-rolling process is controlled to include large-rolling-reduction passes with a rolling reduction ratio of 30% or more in a temperature range of T1+30° C. to T1+200° C., wherein T1 is calculated by a following Expression 1,

wherein an accumulative rolling reduction ratio in the temperature range of T1+30° C. to T1+200° C. is 50% or more,

wherein an accumulative rolling reduction ratio in a temperature range, that is higher than or equal to Ar 3 ° C. and lower than T1+30° C., is limited to 30% or less, and

wherein a rolling terminal temperature is Ar 3 ° C. or higher, Ar 3 being calculated by the following formula:

Ar 3 =879.4−516.1×C−65.7×Mn+38.0×Si+274.7×[P]

wherein [C], [Mn], [Si] and [P] respectively represent a content of C, a content of Mn, a content of Si and a content of P, in mass percent;

(c) a primary cooling process of performing cooling,

wherein a standby time t (second) from completion of a final large-rolling-reduction pass among the large-rolling-reduction passes in step (b) to a start of the primary cooling process satisfies Expression 2;

(d) a separate cooling process of cooling the steel to a temperature T3, in ° C., within a range of 630° C. to 800° C. at an average cooling rate of 10° C./s to 100° C./s;

(e) a retention process of performing a retaining, wherein the steel is retained within the temperature range of 630° C. to 800° C. for 1 second to 20 seconds, or a slow cooling process of a slow cooling, wherein the steel is slowly cooled from the temperature T3, in ° C., to a temperature range within lower than T3, in ° C., and higher than or equal to 550° C. at an average cooling rate of 20° C./s or less;

(f) a winding process of performing a winding of the steel in a temperature range of 350° C. to 500° C.; and

(g) an air cooling process of performing a cooling of the steel with air, wherein the steel is subjected to a temperature range control step of retaining the steel at the temperature range of 350° C. to 500° C. for 30 minutes to 300 minutes, and the steel is subsequently cooled by air;

wherein:

T 1=850+10×([C]+[N])×[Mn]  (Expression 1)

wherein [N] represents a content of N, in mass percent;

t≦ 2.5× t 1  (Expression 2)

wherein t1 is expressed by the following Expression 3,

t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+3.1  (Expression 3)

wherein Tf represents a celsius temperature of the steel at the time of completion of the final large-rolling-reduction pass, and P1 represents a percentage of the rolling reduction ratio during the final large-rolling-reduction pass.

12. The production method of the hot-rolled steel sheet according to claim 11 ,

wherein the steel further comprises, by mass %, one or more selected from the group consisting of

Ti: 0.001% to 0.2%,

Nb: 0.001% to 0.2%,

V: 0.001% to 1.0%,

W: 0.001% to 1.0%,

Cu: 0.001% to 2.0%,

B: 0.0001% to 0.005%,

Mo: 0.001% to 1.0%,

Cr: 0.001% to 2.0%,

As: 0.0001% to 0.50%,

Mg: 0.0001% to 0.010%,

REM: 0.0001% to 0.1%,

Ca: 0.0001% to 0.010%,

Ni: 0.001% to 2.0%,

Co: 0.0001% to 1.0%,

Sn: 0.0001% to 0.2%,

Zr: 0.0001% to 0.2% of Zr, and

wherein a temperature calculated by the following Expression 4 in place of the temperature calculated by the Expression 1 is set as the T1,

T 1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]+100×[V]  (Expression 4)

here, [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] represent mass percentages of the content of C, the content of N, the content of Mn, the content of Nb, the content of Ti, the content of B, the content of Cr, the content of Mo, and the content of V in the steel, respectively.

13. The production method of the hot-rolled steel sheet according to claim 11 ,

wherein the standby time t (second) further satisfies the following Expression 5 using t1,

t<t 1  (Expression 5).

14. The production method of the hot-rolled steel sheet according to claim 11 ,

wherein the standby time t (second) further satisfies the following Expression 6 using t1,

t 1≦ t≦t 1×2.5  (Expression 6).

15. The production method of the hot-rolled steel sheet according to claim 11 ,

wherein in the primary cooling process, the average cooling rate is 50° C./s or more, a cooling temperature variation that is a difference between a steel temperature at the start time of a cooling and a steel temperature at the finish time of the cooling is 40° C. to 140° C., and the steel temperature at the finish time of the cooling is T1+100° C. or lower.

16. The production method of the hot-rolled steel sheet according to claim 11 ,

wherein the final large-rolling-reduction pass is a final pass in the temperature range of T1+30° C. to T1+200° C.

17. The production method of the hot-rolled steel sheet according to claim 11 ,

wherein, in the temperature range control step, a temperature variation rate is −40° C./h to 40° C./h.

18. The production method of the hot-rolled steel sheet according to claim 11 ,

wherein the primary cooling process is performed between rolling stands.

19. The production method of the hot-rolled steel sheet according to claim 12 ,

wherein the standby time t (second) further satisfies the following Expression 5 using t1,

t<t 1  (Expression 5).

20. The production method of the hot-rolled steel sheet according to claim 12 ,

wherein the standby time t (second) further satisfies the following Expression 6 using t1,

t 1≦ t≦t 1×2.5  (Expression 6).