Hot-rolled steel sheet and method for producing same

1. A method for producing a hot-rolled steel sheet, comprising:

first-hot-rolling a steel in a temperature range of 1000° C. to 1200° C. under conditions such that at least one pass whose reduction is 40% or more is included so as to control an average grain size of an austenite in the steel to 200 μM or less, wherein the steel includes, as a chemical composition, by mass %,

C: 0.01% to 0.4%,

Si: 0.001% to 2.5%,

Mn: 0.001% to 4.0%,

Al: 0.001% to 2.0%,

P: limited to 0.15% or less,

S: limited to 0.03% or less,

N: limited to 0.01% or less,

O: limited to 0.01% or less, and

a balance consisting of Fe and unavoidable impurities;

second-hot-rolling the steel under conditions such that, when a temperature calculated by a following Expression 5 is defined as T1 in unit of ° C. and a ferritic transformation temperature calculated by a following Expression 6 is defined as Ar 3 in unit of ° C., a large reduction pass whose reduction is 30% or more in a temperature range of T1+30° C. to T1+200° C. is included, a cumulative reduction in the temperature range of T1+30° C. to T1+200° C. is 50% or more, a cumulative reduction in a temperature range of Ar 3 to lower than T1+30° C. is limited to 30% or less, and a rolling finish temperature is Ar 3 or higher;

first-cooling the steel under conditions such that, when a waiting time from a finish of a final pass in the large reduction pass to a cooling start is defined as t in unit of second, the waiting time t satisfies a following Expression 7, an average cooling rate is 50° C./second or faster, a cooling temperature change which is a difference between a steel temperature at the cooling start and a steel temperature at a cooling finish is 40° C. to 140° C., and the steel temperature at the cooling finish is T1+100° C. or lower;

second-cooling the steel to a temperature range of 600° C. to 800° C. under an average cooling rate of 15° C./second to 300° C./second after finishing the second-hot-rolling;

holding the steel in the temperature range of 600° C. to 800° C. for 1 second to 15 seconds;

third-cooling the steel to a temperature range of a room temperature to 350° C. under an average cooling rate of 50° C./second to 300° C./second after finishing the holding;

coiling the steel in the temperature range of the room temperature to 350° C.,

T 1=850+10×([C]+[N])×[Mn]  (Expression 5),

here, [C], [N], and [Mn] represent mass percentages of C, N, and Mn respectively,

Ar 3 =879.4−516.1×[C]−65.7×[Mn]+38.0×[Si]+2743×[P]  (Expression 6),

here, in Expression 6, [C], [Mn], [Si] and [P] represent mass percentages of C, Mn, Si, and P respectively,

t≦ 2.5× t 1  (Expression 7),

here, t1 is represented by a following Expression 8,

t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+3.1  (Expression 8),

here, Tf represents a celsius temperature of the steel at the finish of the final pass, and P1 represents a percentage of a reduction at the final pass.

2. The method for producing the hot-rolled steel sheet according to claim 1 ,

wherein the waiting time t further satisfies a following Expression 10,

0≦ t<t 1  (Expression 10).

3. The method for producing the hot-rolled steel sheet according to claim 1 ,

wherein the waiting time t further satisfies a following Expression 11,

t 1≦ t≦t 1×2.5  (Expression 11).

4. The method for producing the hot-rolled steel sheet according to claim 1 ,

wherein, in the first-hot-rolling, at least two times of rollings whose reduction is 40% or more are conducted, and the average grain size of the austenite is controlled to 100 μm or less.

5. The method for producing the hot-rolled steel sheet according to claim 1 ,

wherein the second-cooling starts within 3 seconds after finishing the second-hot-rolling.

6. The method for producing the hot-rolled steel sheet according to claim 1 ,

wherein, in the second-hot-rolling, a temperature rise of the steel between passes is 18° C. or lower.

7. The method for producing the hot-rolled steel sheet according to claim 1 ,

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

8. The method for producing the hot-rolled steel sheet according to claim 1 ,

wherein, in the holding, the steel is held in a temperature range of 600° C. to 680° C. for 3 seconds to 15 seconds.

9. The method for producing the hot-rolled steel sheet according to claim 1 ,

wherein the first-cooling is conducted at an interval between rolling stands.

10. A method for producing a hot-rolled steel sheet, comprising:

first-hot-rolling a steel in a temperature range of 1000° C. to 1200° C. under conditions such that at least one pass whose reduction is 40% or more is included so as to control an average grain size of an austenite in the steel to 200 μm or less, wherein the steel includes, as a chemical composition, by mass %,

C: 0.01% to 0.4%,

Si: 0.001% to 2.5%,

Mn: 0.001% to 4.0%,

Al: 0.001% to 2.0%,

P: limited to 0.15% or less,

S: limited to 0.03% or less,

N: limited to 0.01% or less,

O: limited to 0.01% or less, and

at least one selected from the group consisting of:

Mo: 0.001% to 1.0%,

Cr: 0.001% to 2.0%,

Ni: 0.001% to 2.0%,

Cu: 0.001% to 2.0%,

B: 0.0001% to 0.005%,

Nb: 0.001% to 0.2%,

Ti: 0.001% to 0.2%,

V: 0.001% to 1.0%,

W: 0.001% to 1.0%,

Ca: 0.0001% to 0.01%,

Mg: 0.0001% to 0.01%,

Zr: 0.0001% to 0.2%,

Rare Earth Metal: 0.0001% to 0.1%,

As: 0.0001% to 0.5%,

Co: 0.0001% to 1.0%,

Sn: 0.0001% to 0.2%,

Pb: 0.0001% to 0.2%,

Y: 0.0001% to 0.2%, and

Hf: 0.0001% to 0.2%, and

a balance consisting of Fe and unavoidable impurities;

second-hot-rolling the steel under conditions such that, when a temperature calculated by a following Expression 9 is defined as T1 in unit of ° C. and a ferritic transformation temperature calculated by a following Expression 6 is defined as Ar 3 in unit of ° C., a large reduction pass whose reduction is 30% or more in a temperature range of T1+30° C. to T1+200° C. is included, a cumulative reduction in the temperature range of T1+30° C. to T1+200° C. is 50% or more, a cumulative reduction in a temperature range of Ar 3 to lower than T1+30° C. is limited to 30% or less, and a rolling finish temperature is Ar 3 or higher;

first-cooling the steel under conditions such that, when a waiting time from a finish of a final pass in the large reduction pass to a cooling start is defined as t in unit of second, the waiting time t satisfies a following Expression 7, an average cooling rate is 50° C./second or faster, a cooling temperature change which is a difference between a steel temperature at the cooling start and a steel temperature at a cooling finish is 40° C. to 140° C., and the steel temperature at the cooling finish is T1+100° C. or lower;

second-cooling the steel to a temperature range of 600° C. to 800° C. under an average cooling rate of 15° C./second to 300° C./second after finishing the second-hot-rolling;

holding the steel in the temperature range of 600° C. to 800° C. for 1 second to 15 seconds;

third-cooling the steel to a temperature range of a room temperature to 350° C. under an average cooling rate of 50° C./second to 300° C./second after finishing the holding;

coiling the steel in the temperature range of the room temperature to 350° C.,

T 1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]+100×[V]  (Expression 9),

here, [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] represent mass percentages of C, N, Mn, Nb, Ti, B, Cr, Mo, and V respectively;

Ar 3 =879.4−516.1×[C]−65.7×[Mn]+38.0×[Si]+274.7×[P]  (Expression 6),

here, in Expression 6, [C], [Mn], [Si] and [P] represent mass percentages of C, Mn, Si, and P respectively,

t≦ 2.5× t 1  (Expression 7),

here, t1 is represented by a following Expression 8,

t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+3.1  (Expression 8),

here, Tf represents a celsius temperature of the steel at the finish of the final pass, and P1 represents a percentage of a reduction at the final pass.