Abstract: The seamless steel pipe according to the present disclosure includes a base material and a decarburized layer formed on the surface of the base material. The chemical composition of the base material consists of, in mass %, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.0300% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.20%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, V: 0.01 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, and with the balance being Fe and impurities. The base material has a yield strength of 655 MPa or more, and a yield ratio of 85.0% or more. The decarburized layer has a depth of 150 ?m or less.

Abstract: A piercer plug with increased recyclability is provided. A piercer plug (1) has a chemical composition of, in mass %: 0.15 to 0.30% C; 0.4 to 1.2% Si; 0.2 to 1.5% Mn; 0.1 to 2.0% Ni; 0 to 4.0% Mo and 0 to 4.0% W, where the total content of Mo and W is 1.0 to 6.0%; higher than 1.0% and not higher than 4.0% Cr; 0 to 0.2% B; 0 to 1.0% Nb; 0 to 1.0% V; 0 to 1.0% Ti; and balance Fe and impurities, the plug including a tip portion (2) and a trunk portion (3) made of the same material as the tip portion (2) and contiguous to the tip portion (2). The trunk portion (3) includes a cylindrical portion (5) having a hole used to mount a bar. The tip portion (2) is harder than the cylindrical portion (5).

Abstract: A piercer plug with increased wear resistance is provided. A piercer plug includes: a plug body; and a sprayed coating formed on the surface of the plug body. The sprayed coating contains an iron-based alloy and oxides thereof. The chromium concentration determined by analyzing the sprayed coating with X-ray fluorescence analysis is 3 to 20 mass %.

Abstract: A piercer plug having good base-material deformation resistance is provided. A method of manufacturing a piercer plug (10) includes the steps of; preparing a plug body (1) including a tip portion (11) and a cylindrical portion (12) having a hole (121) usable to attach a bar and located rearward of the tip portion (11); forming a build-up layer (2) on the surface of the tip portion (11); and heating the plug body (1) such that the temperature of the tip portion (11) with the build-up layer (2) formed thereon is not lower than the austenite transformation temperature and the temperature of the cylindrical portion (12) is lower than the austenite transformation temperature.

Abstract: A piercer plug with increased recyclability is provided. A piercer plug (1) has a chemical composition of, in mass %: 0.15 to 0.30% C; 0.4 to 1.2% Si; 0.2 to 1.5% Mn; 0.1 to 2.0% Ni; 0 to 4.0% Mo and 0 to 4.0% W, where the total content of Mo and W is 1.0 to 6.0%; higher than 1.0% and not higher than 4.0% Cr; 0 to 0.2% B; 0 to 1.0% Nb; 0 to 1.0% V; 0 to 1.0% Ti; and balance Fe and impurities, the plug including a tip portion (2) and a trunk portion (3) made of the same material as the tip portion (2) and contiguous to the tip portion (2). The trunk portion (3) includes a cylindrical portion (5) having a hole used to mount a bar. The tip portion (2) is harder than the cylindrical portion (5).

Abstract: A piercer plug having good base-material deformation resistance is provided. A method of manufacturing a piercer plug (10) includes the steps of; preparing a plug body (1) including a tip portion (11) and a cylindrical portion (12) having a hole (121) usable to attach a bar and located rearward of the tip portion (11); forming a build-up layer (2) on the surface of the tip portion (11); and heating the plug body (1) such that the temperature of the tip portion (11) with the build-up layer (2) formed thereon is not lower than the austenite transformation temperature and the temperature of the cylindrical portion (12) is lower than the austenite transformation temperature.