Heavy wall seamless steel pipe for line pipe and a manufacturing method thereof

Abstract

A heavy wall seamless steel pipe for line pipe with a high strength and increased toughness, which has a chemical composition, by mass %, that consists of C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, and 0.0002 to 0.005%, in total, of at least one selected from Ca, Mg and REM, and the balance Fe and impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%, and that P and S among impurities are not more than 0.05% and not more than 0.005% respectively. It may contain 0.0003 to 0.01% of boron. A manufacturing method thereof is characterized by cooling rate, heating condition for piercing, and heat treating after pipe making.

1 . A heavy wall seamless steel pipe for line pipe with a high strength and increased toughness, which has a chemical composition, by mass %, that consists of C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, fi: 0.004 to 0.010%, N: 0.002 to 0.008%, and 0.0002 to 0.005%, in total, of at least one selected from Ca, Mg and REM, and the balance Fe and impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%, and that P and S among impurities are not more than 0.05% and not more than 0.005% respectively.  
   
   
       2 . A heavy wall seamless steel pipe for line pipe with a high strength and increased toughness, which has a chemical composition, by mass %, that consists of C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, B: 0.0003 to 0.01%, and 0.0002 to 0.005%, in total, of at least one selected from Ca, Mg and REM, and the balance Fe and impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%, and that P and S among impurities are not more than 0.05% and not more than 0.005% respectively.  
   
   
       3 . A method of manufacturing a heavy wall seamless steel pipe for line pipe with a high strength and increased toughness characterized by comprising the following steps (a) to (e): 
 (a) Forming a billet with a round cross section by continuous casting of molten steel that has the chemical composition according to  claim 1  or  2 .    (b) Cooling the billet to the room temperature at not less than 6° C./min of an average cooling rate between 1400 and 1000° C.    (c) Heating the billet to the temperature between 1150 and 1280° C. at not more than 15° C./min of an average heating rate between 550 and 900° C., then piercing and rolling, to make a seamless pipe.    (d) Cooling forcedly the seamless pipe to a temperature of not higher than 100° C. at not less than 8° C./min of an average cooling rate between 800 and 500° C., immediately after pipe making, or after isothermal treating at the temperature between 850 and 1000° C. immediately in succession with pipe making, or after heating to the temperature between 850 and 1000° C. after once cooling in succession with pipe making.    (e) Tempering the seamless pipe at the temperature between 500 and 690° C.    
   
   
       4 . A method of manufacturing a heavy wall seamless steel pipe for line pipe with a high strength and increased toughness characterized by comprising the following steps (a) to (0): 
 (a) Forming a bloom or slab with a square cross section by continuous casting of molten steel that has the chemical composition according to  claim 1  or  2 .    (b) Cooling the bloom or slab to the room temperature at not less than 8° C./min of an average cooling rate between 1400 and 1000° C.    (c) Heating the bloom or slab to the temperature between 1150 and 1280° C. at not more than 15° C./min of an average heating rate between 550 and 900° C., and then cooling to the room temperature, to form a billet with a round cross section by forging and/or rolling.    (d) Heating the billet to the temperature between 1150 and 1280° C., then piercing and rolling, to make a seamless pipe.    (e) Cooling forcedly the seamless pipe to a temperature of not higher than 100° C. at not less than 8° C./min of an average cooling rate between 800 and 500° C., immediately after pipe making, or after isothermal treating at the temperature between 850 and 1000° C. immediately in succession with pipe making, or after heating to the temperature between 850 and 1000° C. after once cooling in succession with pipe making.    (f) Tempering the seamless pipe at the temperature between 500 and 690° C.    
   
   
       5 . A method of manufacturing a heavy wall seamless steel pipe for line pipe with a high strength and increased toughness characterized by comprising the following steps (a) to (e): 
 (a) Forming a billet with a round cross section by continuous casting of a molten steel that has the chemical composition according to  claim 1  or  2 .    (b) Cooling the billet to the room temperature at not less than 6° C./min of an average cooling rate between 1400 and 1000° C.    (c) Isothermal treating the billet during not less than 15 minutes at the temperature between 550 and 1000° C., and heating to the temperature between 1150 and 1280° C., then piercing and rolling, to make a seamless pipe.    (d) Cooling forcedly the seamless pipe to a temperature of not higher than 100° C. at not less than 8° C./min of an average heating rate between 800 and 500° C., immediately after pipe making, or after isothermal treating at the temperature between 850 and 1000° C. immediately in succession with pipe making, or after heating to the temperature between 850 and 1000° C. after once cooling in succession with pipe making.    (e) Tempering the seamless pipe at the temperature between 500 and 690° C.    
   
   
       6 . A method of manufacturing a heavy wall seamless steel pipe for line pipe with a high strength and increased toughness characterized by comprising the following steps (a) to (0): 
 (a) Forming a bloom or slab with a square cross section by continuous casting of a molten steel that has the chemical composition according to  claim 1  or  2 .    (b) Cooling the bloom or slab to the room temperature at not less than 8° C./min of an average cooling rate between 1400 and 1000° C.    (c) Isothermal treating the bloom or slab during not less than 15 minutes at the temperature between 550 and 1000° C., and heating to the temperature between 1150 and 1280° C., then forging and/or rolling, to form a billet with a round cross section, and then cooling to the room temperature.    (d) Heating the billet to the temperature of 1150 to 1280° C., then piercing and rolling, to make a seamless pipe.    (e) Cooling forcedly the seamless pipe to a temperature of not higher than 100° C. at not less than 8° C./min of an average cooling rate between 800 and 500° C., immediately after pipe making, or after isothermal treating at the temperature between 850 and 1000° C. immediately in succession with pipe making, or after heating to the temperature between 850 and 1000° C. after once cooling in succession with pipe making.    (f) Tempering the seamless pipe at the temperature between 500 and 690° C.