Hot-rolled steel sheet and method of producing the same

1. A hot-rolled steel sheet comprising, by mass %,

C: a content [C] of 0.02% to 0.07%,

Si: a content [Si] of 0.001% to 2.5%,

Mn: a content [Mn] of 0.01% to 4%,

Al: a content [Al] of 0.001% to 2%,

Ti: a content [Ti] of 0.015% to 0.2%,

P: a limited content [P] of 0.15% or less,

S: a limited content [S] of 0.03% or less,

N: a limited content [N] of 0.01% or less, and

the balance consisting of Fe and unavoidable impurities,

wherein the contents [Ti], [N], [S], and [C] satisfy the following expressions (a) and (b);

an average pole density of an orientation group {100}<011> to {223}<110>, which is represented by an arithmetic mean of pole densities of orientations {100}<011>, {116}<110>, {114}<110>, {112}<110, and {223}<110> is 1.0 to 4.0 and a pole density of a crystal orientation {332}<113> is 1.0 to 4.8, in a thickness center portion which is a thickness range of ⅝ to ⅜ from the surface of the steel sheet;

an average grain size in the thickness center portion is less than or equal to 10 μm and a grain size of a cementite precipitating in a grain boundary in the steel sheet is less than or equal to 2 μm; and

an average grain size of precipitates containing TiC in grains is less than or equal to 3 nm and a number density per unit volume is greater than or equal to 1×10 16 grains/cm 3 ,

0%≦([Ti]−[N]×48/14−[S]×48/32)  (a)

0%≦[C]−12/48×([Ti]−[N]×48/14−[S]×48/32)  (b).

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

wherein the average pole density of the orientation group {100}<011> to {223}<110> is less than or equal to 2.0 and the pole density of the crystal orientation {332}<113> is less than or equal to 3.0.

3. The hot-rolled steel sheet according to claim 2 , further comprising, by mass %,

Nb: a content [Nb] of 0.005% to 0.06%,

wherein the contents [Nb], [Ti], [N], [S], and [C] satisfy the following expression (c),

0%≦[C]−12/48×([Ti]+[Nb]×48/93−[N]×48/14−[S]×48/32)  (c).

4. The hot-rolled steel sheet according to claim 2 , further comprising

one or two or more selected from the group consisting of, by mass %,

Cu: a content [Cu] of 0.02% to 1.2%,

Ni: a content [Ni] of 0.01% to 0.6%,

Mo: a content [Mo] of 0.01% to 1%,

V: a content [V] of 0.01% to 0.2%,

Cr: a content [Cr] of 0.01% to 2%,

Mg: a content [Mg] of 0.0005% to 0.01%,

Ca: a content [Ca] of 0.0005% to 0.01%,

REM: a content [REM] of 0.0005% to 0.1%, and

B: a content [B] of 0.0002% to 0.002%.

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

wherein the average grain size is less than or equal to 7 μm.

6. The hot-rolled steel sheet according to claim 5 , further comprising, by mass %,

Nb: a content [Nb] of 0.005% to 0.06%,

wherein the contents [Nb], [Ti], [N], [S], and [C] satisfy the following expression (c),

0%≦[C]−12/48×([Ti]+[Nb]×48/93−[N]×48/14−[S]×48/32)  (c).

7. The hot-rolled steel sheet according to claim 5 , further comprising

one or two or more selected from the group consisting of, by mass %,

Cu: a content [Cu] of 0.02% to 1.2%,

Ni: a content [Ni] of 0.01% to 0.6%,

Mo: a content [Mo] of 0.01% to 1%,

V: a content [V] of 0.01% to 0.2%,

Cr: a content [Cr] of 0.01% to 2%,

Mg: a content [Mg] of 0.0005% to 0.01%,

Ca: a content [Ca] of 0.0005% to 0.01%,

REM: a content [REM] of 0.0005% to 0.1%, and

B: a content [B] of 0.0002% to 0.002%.

8. The hot-rolled steel sheet according to claim 1 , further comprising, by mass %,

Nb: a content [Nb] of 0.005% to 0.06%,

wherein the contents [Nb], [Ti], [N], [S], and [C] satisfy the following expression (c),

0%≦[C]−12/48×([Ti]+[Nb]×48/93−[N]×48/14−[S]×48/32)  (c).

9. The hot-rolled steel sheet according to claim 8 , further comprising

one or two or more selected from the group consisting of, by mass %,

Cu: a content [Cu] of 0.02% to 1.2%,

Ni: a content [Ni] of 0.01% to 0.6%,

Mo: a content [Mo] of 0.01% to 1%,

V: a content [V] of 0.01% to 0.2%,

Cr: a content [Cr] of 0.01% to 2%,

Mg: a content [Mg] of 0.0005% to 0.01%,

Ca: a content [Ca] of 0.0005% to 0.01%,

REM: a content [REM] of 0.0005% to 0.1%, and

B: a content [B] of 0.0002% to 0.002%.

10. The hot-rolled steel sheet according to claim 1 , further comprising

one or two or more selected from the group consisting of, by mass %,

Cu: a content [Cu] of 0.02% to 1.2%,

Ni: a content [Ni] of 0.01% to 0.6%,

Mo: a content [Mo] of 0.01% to 1%,

V: a content [V] of 0.01% to 0.2%,

Cr: a content [Cr] of 0.01% to 2%,

Mg: a content [Mg] of 0.0005% to 0.01%,

Ca: a content [Ca] of 0.0005% to 0.01%,

REM: a content [REM] of 0.0005% to 0.1%, and

B: a content [B] of 0.0002% to 0.002%.

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

heating a steel ingot or a slab including, by mass %,

C: a content [C] of 0.02% to 0.07%,

Si: a content [Si] of 0.001% to 2.5%,

Mn: a content [Mn] of 0.01% to 4%,

Al: a content [Al] of 0.001% to 2%,

Ti: a content [Ti] of 0.015% to 0.2%,

P: a limited content [P] of 0.15% or less,

S: a limited content [S] of 0.03% or less,

N: a limited content [N] of 0.01% or less, and

the balance consisting of Fe and unavoidable impurities, in which the contents [Ti], [N], [S], and [C] satisfy the following expressions (a) and (b), at SRTmin° C., which is a temperature determined according to the following expression (d) and 1150° C., to 1260° C.;

performing a first hot rolling in which reduction is performed once or more at a rolling reduction of 40% or higher per pass in a temperature range of 1000° C. to 1200° C.;

starting a second hot rolling in a temperature range of 1000° C. or higher within 150 seconds after a finish of the first hot rolling;

performing a reduction in the second hot rolling in a temperature range of (T1+30)° C. to (T1+200)° C., wherein a temperature determined by components of the steel sheet according to the following expression (e) is represented by T1° C. so as to obtain a total reduction ratio of 50% or higher, with at least one of a rolling reduction ratio of 30%;

performing a third hot rolling in which a total rolling reduction is lower than or equal to 30% in a temperature range of less than (T1+30)° C.;

finishing the hot rollings at the Ar3 transformation temperature or higher;

performing a primary cooling under conditions of a cooling rate of 50° C./sec or higher, a temperature change of 40° C. or more and 140° C. or less, and a cooling end temperature of (T1+100)° C. or lower such that a waiting time t (second) from a finish of a final pass of a large reduction pass to a start of cooling satisfies the following expression (f), wherein said large reduction pass is defined as a pass of a rolling reduction of 30% or higher in the temperature range of (T1+30)° C. to (T1+200)° C.;

performing a secondary cooling at a cooling rate of 15° C./sec or higher within 3 seconds from the finish of the primary cooling; and

performing a coiling in a temperature range of 550° C. to lower than 700° C.,

0%≦([Ti]−[N]×48/14−[S]×48/32)  (a)

0%≦[C]−12/48×([Ti]−[N]×48/14−[S]×48/32)  (b)

SRTmin=7000/{2.75−log([Ti]×[C])}−273  (d)

T 1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[0]+100×[Mo]+100×[V]  (e)

t≦ 2.5× t 1  (f)

where t1 is represented by the following expression (g),

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

where Tf represents a temperature (° C.) after a final reduction at a rolling reduction of 30% or higher, and P1 represents the rolling reduction (%) during the final reduction at a rolling reduction of 30% or higher.

12. The method of producing a hot-rolled steel sheet according to claim 11 ,

wherein the primary cooling is performed between rolling stands and the secondary cooling is performed after passage through a final rolling stand.

13. The method of producing a hot-rolled steel sheet according to claim 12 ,

wherein the waiting time t (second) further satisfies the following expression (h),

t 1≦ t≦ 2.5× t 1  (h).

14. The method of producing a hot-rolled steel sheet according to claim 12 ,

wherein the waiting time t (second) further satisfies the following expression (i),

t<t 1  (i).

15. The method of producing a hot-rolled steel sheet according to claim 11 ,

wherein the waiting time t (second) further satisfies the following expression (h),

t 1≦ t≦ 2.5× t 1  (h).

16. The method of producing a hot-rolled steel sheet according to claim 11 ,

wherein the waiting time t (second) further satisfies the following expression (i),

t<t 1  (i).

17. The method of producing a hot-rolled steel sheet according to claim 11 ,

wherein a temperature increase between passes in the second hot rolling is lower than or equal to 18° C.

18. The method of producing a hot-rolled steel sheet according to claim 11 ,

wherein the steel ingot or the slab further includes, by mass %,

Nb: a content [Nb] of 0.005% to 0.06%, and

the contents [Nb], [Ti], [N], [S], and [C] satisfies the following expression (c),

0%≦[C]−12/48×([Ti]+[Nb]×48/93−[N]×48/14−[S]×48/32)  (c).

19. The method of producing a hot-rolled steel sheet according to claim 18 ,

wherein the steel ingot or the slab further includes one or two or more selected from the group consisting of, by mass %,

Cu: a content [Cu] of 0.02% to 1.2%,

Ni: a content [Ni] of 0.01% to 0.6%,

Mo: a content [Mo] of 0.01% to 1%,

V: a content [V] of 0.01% to 0.2%,

Cr: a content [Cr] of 0.01% to 2%,

Mg: a content [Mg] of 0.0005% to 0.01%,

Ca: a content [Ca] of 0.0005% to 0.01%,

REM: a content [REM] of 0.0005% to 0.1%, and

B: a content [B] of 0.0002% to 0.002%.

20. The method of producing a hot-rolled steel sheet according to claim 11 ,

wherein the steel ingot or the slab further includes one or two or more selected from the group consisting of, by mass %,

Cu: a content [Cu] of 0.02% to 1.2%,

Ni: a content [Ni] of 0.01% to 0.6%,

Mo: a content [Mo] of 0.01% to 1%,

V: a content [V] of 0.01% to 0.2%,

Cr: a content [Cr] of 0.01% to 2%,

Mg: a content [Mg] of 0.0005% to 0.01%,

Ca: a content [Ca] of 0.0005% to 0.01%,

REM: a content [REM] of 0.0005% to 0.1%, and

B: a content [B] of 0.0002% to 0.002%.