Steel plate for line pipe excellent in strength and ductility and method of production of same
View Patent ↗The present invention provides steel plate for line pipe excellent in strength and ductility and a method of production of the same. The steel plate has a steel composition containing, by mass %, C: 0.04 to 0.15%, Si: 0.05 to 0.60%, Mn: 0.80 to 1.80%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.08%, and Al: 0.003 to 0.08%, having a balance of iron and unavoidable impurities, and having a value of Ceq shown by the following formula <1> of 0.48 or less, comprised of a mixed structure of ferrite and pearlite or ferrite and pearlite partially containing bainite in which a ferrite percentage is 60 to 95%, having a yield strength of 450 MPa or more, and having an amount of hydrogen contained in the steel of 0.1 ppm or less: Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+Nb+V+Ti)/5+5B <1>.
1. Steel plate for line pipe having a steel composition containing, by mass %,
C: 0.07 to 0.15%,
Si: 0.05 to 0.60%,
Mn: 0.80 to 1.80%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.01 to 0.08%, and
Al: 0.003 to 0.08%,
having a balance of iron and unavoidable impurities, and having a value of Ceq shown by the following formula <1> of 0.48 or less, comprised of a mixed structure of ferrite and pearlite or ferrite and pearlite partially containing bainite in which a ferrite percentage is 60 to 95%, having a yield strength of 450 MPa or more and a total elongation of 20% or more, and having an amount of hydrogen contained in the steel of 0.1 ppm or less:
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+Nb+V+Ti)/5+5B <1>.
2. Steel plate for line pipe as set forth in claim 1 , characterized in that said steel further contains, by mass %, one or more of
Cu: 0.05 to 0.70%,
Ni: 0.05 to 0.70%,
Cr: 0.80% or less,
Mo: 0.30% or less,
B: 0.0003 to 0.0030%,
V: 0.01 to 0.12%,
Ti: 0.003 to 0.030%,
N: 0.0010 to 0.0100%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0003 to 0.0030%, and
REM: 0.0005 to 0.0050%.
3. A method for production of steel plate for line pipe characterized by continuously casting molten steel having a composition of either of claim 1 or 2 to obtain a cast slab, reheating said cast slab to 950 to 1250° C. in temperature region, then hot rolling at a temperature region of 850° C. or less by a cumulative reduction rate of 40% or more, ending the hot rolling in a 700 to 750° C. temperature region, then air cooling down to 350° C. or less, then cooling in a 300 to 100° C. temperature range for 10 hours or more or a 200 to 80° C. temperature range for 100 hours or more.
4. A method for production of steel plate for line pipe characterized by continuously casting molten steel having a composition of either of claim 1 or 2 to obtain a cast slab, reheating said cast slab to 950 to 1250° C. in temperature region, then hot rolling at a temperature region of 850° C. or less by a cumulative reduction rate of 40% or more, ending the hot rolling in a 700 to 750° C. temperature region, then cooling down to 100° C. or less, then reheating the steel plate to 250 to 300° C. temperature range, holding it at that temperature region for 1 minute or more, then cooling.