High strength hot rolled steel products for line-pipes excellent in low temperature toughness and production method of the same
View Patent ↗The present invention provides high strength hot rolled steel plate for line-pipes superior in low temperature toughness, and a method of production of the same, containing, by mass %, C: 0.01 to 0.1%, Si: 0.05 to 0.5%, Mn: 1 to 2%, P: ≦0.03%, S: ≦0.005%, O: ≦0.003%, Al: 0.005 to 0.05%, N: 0.0015 to 0.006%, Nb: 0.005 to 0.08%, and Ti: 0.005 to 0.02%, where N−14/48×Ti>0% and Nb−93/14×(N−14/48×Ti)>0.005%, and a balance of Fe and unavoidable impurities, said steel plate characterized in that its microstructure is a continuously cooled transformed structure, a reflected X-ray intensity ratio {211}/{111} of the {211} plane and {111} plane parallel to the plate surface in the texture at the center of plate thickness is 1.1 or more, and an in-grain precipitate density of the precipitates of Nb and/or Ti carbonitrides is 10 17 to 10 18 /cm 3 .
1. High strength hot rolled steel product for line-pipes, comprising, by mass %,
C: 0.01 to 0.1%,
Si: 0.05 to 0.5%,
Mn: 1 to 2%,
P: ≦0.03%,
S: ≦0.005%,
O: ≦0.003%,
Al: 0.005 to 0.05%,
N: 0.0015 to 0.006%,
Nb: 0.005 to 0.08%, and
Ti: 0.005 to 0.02%, where
N−14/48×Ti>0% and
Nb−93/14×(N−14/48×Ti)>0.005%, and
a balance of Fe and unavoidable impurities, wherein
the steel product is in the form of a steel plate having a continuously cooled transformed structure microstructure and a thickness of 14 mm or more,
a reflected X-ray intensity ratio, {211}/{111}, of the {211} plane and {111} plane parallel to the plate surface in the texture at the center of plate thickness of 1.1 or more, and
an in-grain precipitate density of the precipitates of Nb and/or Ti carbonitrides of 10 17 to 10 18 /cm 3 .
2. The high strength hot rolled steel product for line-pipes as set forth in claim 1 , further comprising one or more of:
V: 0.01 to 0.3%,
Mo: 0.01 to 0.3%,
Cr: 0.01 to 0.3%,
Cu: 0.01 to 0.3%,
Ni: 0.01 to 0.3%,
B: 0.0002 to 0.003%,
Ca: 0.0005 to 0.005%, and
REM: 0.0005 to 0.02%.
3. A method of production of high strength hot rolled steel products for line-pipes, comprising heating a steel slab comprising, by mass %,
C: 0.01 to 0.1%,
Si: 0.05 to 0.5%,
Mn: 1 to 2%,
P: ≦0.03%,
S: ≦0.005%,
O: ≦0.003%,
Al: 0.005 to 0.05%,
N: 0.0015 to 0.006%,
Nb: 0.005 to 0.08%, and
Ti: 0.005 to 0.02%, where
N−14/48×Ti>0% and
Nb−93/14×(N−14/48×Ti)>0.005%, and
a balance of Fe and unavoidable impurities to a temperature in the range of a temperature satisfying the formula:
SRT(° C.)=6670/(2.26−log [% Nb][% C])−273
to a temperature of 1230° C.,
holding the heated steel slab in that temperature region for 20 minutes or more,
then hot rolling the heated steel slab in a pre-recrystallization temperature region at a total reduction rate of 65% or more,
ending that rolling at an Ar 3 transformation point temperature or more,
then starting cooling within 5 seconds,
cooling in the temperature region ranging from the temperature at the start of cooling to 700° C. at a cooling rate of 15° to 50° C./second, and
coiling at 450° C. to 650° C.,
wherein an in-grain precipitate density of the precipitates of Nb and/or Ti carbonitrides of the hot rolled steel products is 10 17 to 10 18 /cm 3 .
4. The method as set forth in claim 3 , further comprising cooling before rolling in the pre-recrystallization temperature region.
5. The method as set forth in claim 3 , wherein the steel slab further comprises one or more of:
V: 0.01 to 0.3%,
Mo: 0.01 to 0.3%,
Cr: 0.01 to 0.3%,
Cu: 0.01 to 0.3%,
Ni: 0.01 to 0.3%,
B: 0.0002 to 0.003%,
Ca: 0.0005 to 0.005%, and
REM: 0.0005 to 0.02%.
6. The method as set forth in claim 5 , further comprising cooling before rolling in the pre-recrystallization temperature region.
7. The method as set forth in claim 3 , wherein the cooling rate for cooling in the temperature region ranging from the temperature at the start of cooling to 700° C. is 16° C./second to 25° C./second.