Method of producing a hot rolled steel sheet
View Patent ↗In a hot-rolled sheet, an average value of pole densities 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> in a thickness center portion of a thickness range of ⅝ to ⅜ from a surface of the steel sheet, is 1.0 to 6.5 and a pole density of a crystal orientation {332}<113> is 1.0 to 5.0; and a Lankford value rC in a direction perpendicular to a rolling direction is 0.70 to 1.10 and a Lankford value r30 in a direction that forms 30° with respect to the rolling direction is 0.70 to 1.10.
1. A method of producing a hot-rolled steel sheet, comprising:
performing a first hot rolling which reduces a steel ingot or a slab including, by mass %,
C: a content [C] of 0.0001% to 0.40%,
Si: a content [Si] of 0.001% to 2.5%,
Mn: a content [Mn] of 0.001% to 4.0%,
P: a content [P] of 0.001% to 0.15%,
S: a content [S] of 0.0005% to 0.10%,
Al: a content [Al] of 0.001% to 2.0%,
N: a content [N] of 0.0005% to 0.01%,
O: a content [O] of 0.0005% to 0.01%, and
a balance consisting of iron and unavoidable impurities,
and which includes at least one pass at a rolling reduction of 40% or higher in a temperature range of 1000° C. to 1200° C. so as to control an austenite grain size to be less than or equal to 200 μm;
performing a second hot rolling in which, when a temperature determined by components of the steel sheet according to a following expression 2 is represented by T 1 ° C., a total rolling reduction is larger than or equal to 50% in a temperature range of (T 1 +30°) C. to (T 1 +200°) C.;
performing a third hot rolling in which a total rolling reduction is lower than or equal to 30% in a temperature range of T 1 ° C. to less than (T 1 +30°) C.;
finishing the hot rollings at T 1 ° C. or higher; and
performing a primary cooling between rolling stands such that, when a pass of a rolling reduction of 30% or higher in the temperature range of (T 1 +30°) C. to (T 1 +200°) C. is defined as a large reduction pass, a waiting time t (second) from a finish of a final pass of a large reduction pass to the start of cooling satisfies a following expression 3,
T1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]+100×[V] (Expression 2)
t≦t 1×2.5 (Expression 3)
(wherein t 1 is represented by a following expression 4)
t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+3.1 (Expression 4)
(wherein Tf represents the temperature (° C.) of the steel sheet at the time of the finish of the final pass, and P 1 represents the rolling reduction (%) during the final pass).
2. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein the waiting time t (second) further satisfies a following expression 5,
t<t 1 (Expression 5).
3. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein the waiting time t (second) further satisfies a following expression 6,
t 1 ≦t≦t 1×2.5 (Expression 6).
4. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein a cooling temperature change, which is a difference between a steel sheet temperature at a time of a start of the cooling and a steel sheet temperature at the time of the finish of the cooling in the primary cooling, is 40° C. to 140° C., and
the steel sheet temperature at the time of the finish of cooling in the primary cooling is lower than or equal to (T 1 +100°) C.
5. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein in the second hot rolling of the temperature range of (T 1 +30°) C. to (T 1 +200°) C., the reduction is performed at least once in one pass at a rolling reduction of 30% or higher.
6. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein in the first hot rolling, the reduction is performed at least twice at a rolling reduction of 40% or higher to control an austenite grain size to be less than or equal to 100 μm.
7. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein a secondary cooling starts after passing through a final rolling stand and within 10 seconds from the finish of the primary cooling.
8. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein in the second hot rolling, an increase in the temperature of the steel sheet between passes is lower than or equal to 18° C.
9. The method of producing a hot-rolled steel sheet according to claim 1 ,
wherein the steel ingot or the slab further includes one or more selected from, by mass %,
Ti: a content [Ti] of 0.001% to 0.20%,
Nb: a content [Nb] of 0.001% to 0.20%,
V: a content [V] of 0.001% to 1.0%,
W: a content [W] of 0.001% to 1.0%,
B: a content [B] of 0.0001% to 0.0050%,
Mo: a content [Mo] of 0.001% to 2.0%,
Cr: a content [Cr] of 0.001% to 2.0%,
Cu: a content [Cu] of 0.001% to 2.0%,
Ni: a content [Ni] of 0.001% to 2.0%,
Co: a content [Co] of 0.0001% to 1.0%,
Sn: a content [Sn] of 0.0001% to 0.2%,
Zr: a content [Zr] of 0.0001% to 0.2%,
As: a content [As] of 0.0001% to 0.50%,
Mg: a content [Mg] of 0.0001% to 0.010%,
Ca: a content [Ca] of 0.0001% to 0.010%, and
REM: a content [REM] of 0.0001% to 0.1%.