Seamless steel pipe for line pipe and a process for its manufacture

1. A seamless steel pipe for line pipe characterized by having a chemical composition consisting essentially of, in mass percent, C: 0.02-0.08%, Si: at most 0.5%, Mn: 1.8-3.0%, Al: 0.001-0.10%, Mo: greater than 0.4% to 1.2%, N: 0.002-0.015%, Ca: 0.0002-0.007%, Cr: 0-1.0%, Ti: 0-0.03%, Ni: 0-2.0%, Nb: 0-0.03%, V: 0-0.2%, Cu: 0-1.5%, and a remainder of Fe and impurities, wherein the contents of the impurities are at most 0.03% for P, at most 0.005% for S , at most 0.005% for O, and less than 0.0005% for B, and wherein the value of Pcm calculated by the following Equation (1) is at least 0.185 and at most 0.250, the pipe having a microstructure primarily comprising bainite and having a length of cementite of at most 20 micrometers:

Pcm =[C]+[Si]/30+([Mn]+[Cr]+[Cu])/20+[Mo]/15+[V]/10+ 5 [B]  (1)

wherein [C], [Si], [Mn], [Cr], [Cu], [Mo], [V], and [B] are numbers respectively indicating the content in mass percent of C, Si, Mn, Cr, Cu, Mo, V, and B and further wherein the pipe has a toughness of 100 J or more which is evaluated as the minimum value of the absorbed energy impact measured in a Charpy impact test at −40 ° Cm using a test piece measuring 10 mm wide and 10 mm thick and having a V-notch with a depth of 2 mm.

2. A seamless steel pipe for line pipe as set forth in claim 1 wherein the chemical composition contains, in mass percent, one or more elements selected from the group consisting of Cr: 0.02-1.0%, Ti: 0.003-0.03%, Ni: 0.02-2.0%, Nb: 0.003-0.03%, V: 0.003-0.2%, and Cu: 0.02-1.5%.

3. A process of manufacturing a seamless steel pipe for line pipe characterized by heating a steel billet having a chemical composition consisting essentially of, in mass percent, C: 0.02-0.08%, Si: at most 0.5%, Mn: 1.8-3.0%, Al: 0.001-0.10%, Mo: greater than 0.4% to 1.2%, N: 0.002-0.015%, Ca: 0.0002-0.007%, Cr: 0-1.0%, Ti: 0-0.03%, Ni: 0-2.0%, Nb: 0-0.03%, V: 0-0.2%, Cu: 0-1.5%, and a remainder of Fe and impurities, wherein the contents of the impurities are at most 0.03% for P, at most 0.005% for S, at most 0.005% for 0, and less than 0.0005% for B, and wherein the value of Pcm calculated by the following Equation (1) is at least 0.185 and at most 0.250, the pipe having a microstructure primarily comprising bainite and having a length of cementite of at most 20 micrometers

Pcm =[C]+[Si]/ 30+([Mn]+[Cr]+[Cu])/20+[Mo]/15+[V]/10+ 5 [B]  (1)

wherein [C], [Si], [Mn], [Cr], [Cu], [Mo], [V], and [B] are numbers respectively indicating the content in mass percent of C, Si, Mn, Cr, Cu, Mo, V, and B, forming the billet into a seamless steel pipe by hot tube rolling with a starting temperature of 1250-1100° C. and a finishing temperature of at least 900° C., reheating for soaking the resulting steel pipe at a temperature of at least 900° C. and at most 1000° C., quenching the pipe under conditions such that the average cooling rate from 800° C. to 500° C. at the center of the wall thickness is at least 1° C. per second, and then tempering the quenched pipe at a temperature of from 500° C. to less than the Ac 1 transformation temperature,

wherein the pipe has a toughness of 100 J or more which is evaluated as the minimum value of the absorbed energy impact measured in a Charpy impact test at −40 ° Cm using a test piece measuring 10 mm wide and 10 mm thick and having a V-notch with a depth of 2 mm.

4. A process as set forth in claim 3 wherein the seamless steel pipe which is formed by hot tube rolling is initially cooled before quenching.

5. A process as set forth in claim 3 wherein the seamless steel pipe which is formed by hot tube rolling is immediately quenched.

6. A process as set forth in claim 3 , wherein the chemical composition contains, in mass percent, one or more elements selected from the group consisting of Cr: 0.02-1.0%, Ti' 0.003-0.03%, Ni: 0.02-2.0%, Nb: 0.003-0.03%, V: 0.003-0.2%, and Cu″ 0.02-1.5%.