Abstract: A threaded joint with excellent tensile strength, comprising: a pin having an externally threaded part comprising a first and second externally threaded sections across a step on an outer periphery on a distal-end side of a first steel pipe or tube; and a box having an internally threaded part comprising a first and second internally threaded sections across the step on an inner periphery on a distal-end side of a second steel pipe or tube, the externally threaded part being screwed to the internally threaded part to join the pin and the box. The threaded joint has a first and second thread sequence with lengths L1 and L2, respectively, and cross-sectional areas P1, P2, B1, and B2 at end positions 4, 5, 6, and 7, respectively, satisfying Formulas (1) and (2): (P2, B2)min?P1/{L1/(L1+L2)}(1) and (P2, B2)min?B1/{L2/(L1+L2)}(2), where (P2, B2)min represents P2 or B2, whichever is smaller.

Abstract: A threaded joint with excellent tensile strength, comprising: a pin having an externally threaded part comprising a first and second externally threaded sections across a step on an outer periphery on a distal-end side of a first steel pipe or tube; and a box having an internally threaded part comprising a first and second internally threaded sections across the step on an inner periphery on a distal-end side of a second steel pipe or tube, the externally threaded part being screwed to the internally threaded part to join the pin and the box. The threaded joint has a first and second thread sequence with lengths L1 and L2, respectively, and cross-sectional areas P1, P2, B1, and B2 at end positions 4, 5, 6, and 7, respectively, satisfying Formulas (1) and (2): (P2, B2)min?P1/{L1/(L1+L2)}(1) and (P2, B2)min?B1/{L2/(L1+L2)}(2), where (P2, B2)min represents P2 or B2, whichever is smaller.

Abstract: A threaded joint for oil well steel pipe is provided. The threaded joint includes a pin provided with a male thread, and a box provided with a female thread threadedly engaged with the male thread, and at an outer periphery side sealing portion, a cross-sectional area S1 of the pin at a seal point, and a cross-sectional area S0 of a pipe body portion, which is an unprocessed pin portion, satisfy an expression (S1/S0)×100?15(%), at an inner periphery side sealing portion, a cross-sectional area S2 of the box at a seal point, and a cross-sectional area S0? of a pipe body portion, which is an unprocessed box portion satisfy an expression (S2/S0?)×100?20(%), and a surface area S3 of a contact portion of an intermediate shoulder between thread rows and the cross-sectional area S0 satisfy the expression (S3/S0)×100?10(%).

Abstract: Provided is a threaded joint for an oil well tubing which possesses sufficient sealability when the external pressure is applied to the threaded joint by suppressing the plastic deformation of a nose portion and sufficient galling resistance. The sufficient galling resistance is provided by preventing the occurrence of galling at the time of making up the threaded joint during verification test performed in accordance with ISO13679:2002 even when the threaded joint for an oil well tubing has a small thickness. Parameters including sizes of the threaded joint at positions of a seal point and shoulder portions satisfy formula 1 ((Ds12?Ds02)/(D12?D02)?0.30), formula 2 (tan ?>(?D+?)/{2(Lt?Ls)}), and Ls/Ln is set to 0.2 to 0.6.

Abstract: A threaded pipe joint, when acted upon by a tensile load, does not break on the female thread-side of a first thread section of a second thread series that extends from an intermediate shoulder. The threaded pipe joint reliably undergoes tensile rupture on the female thread-side of a first thread section of a first thread series, which is the original critical cross-section site. The threaded pipe joint is configured so that when the male and female threads of the intermediate shoulder are screwed together, the load flank-side thread gaps L1G and L2G of a first thread series, which is the radially inward thread series, and a second thread series, which is the radially outward thread series, on the two sides of the intermediate shoulder always satisfy the relationship L1G<L2G.

Abstract: A threaded joint for oil well steel pipe is provided. The threaded joint includes a pin provided with a male thread, and a box provided with a female thread threadedly engaged with the male thread, and at an outer periphery side sealing portion, a cross-sectional area S1 of the pin at a seal point, and a cross-sectional area S0 of a pipe body portion, which is an unprocessed pin portion, satisfy an expression (S1/S0)×100?15(%), at an inner periphery side sealing portion, a cross-sectional area S2 of the box at a seal point, and a cross-sectional area S0? of a pipe body portion, which is an unprocessed box portion satisfy an expression (S2/S0?)×100?20(%), and a surface area S3 of a contact portion of an intermediate shoulder between thread rows and the cross-sectional area S0 satisfy the expression (S3/S0)×100?10(%).

Abstract: Provided is a threaded joint for an oil well tubing which possesses sufficient sealability when the external pressure is applied to the threaded joint by suppressing the plastic deformation of a nose portion and sufficient galling resistance. The sufficient galling resistance is provided by preventing the occurrence of galling at the time of making up the threaded joint during verification test performed in accordance with ISO13679:2002 even when the threaded joint for an oil well tubing has a small thickness. Parameters including sizes of the threaded joint at positions of a seal point and shoulder portions satisfy formula 1 ((Ds12?Ds02)/(D12?D02)?0.30), formula 2 (tan ?>(?D+?)/{2(Lt?Ls)}), and Ls/Ln is set to 0.2 to 0.6.

Abstract: A threaded pipe joint, when acted upon by a tensile load, does not break on the female thread-side of a first thread section of a second thread series that extends from an intermediate shoulder. The threaded pipe joint reliably undergoes tensile rupture on the female thread-side of a first thread section of a first thread series, which is the original critical cross-section site. The threaded pipe joint is configured so that when the male and female threads of the intermediate shoulder are screwed together, the load flank-side thread gaps L1G and L2G of a first thread series, which is the radially inward thread series, and a second thread series, which is the radially outward thread series, on the two sides of the intermediate shoulder always satisfy the relationship L1G<L2G.

Abstract: A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (&mgr;m), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac3 transformation point, and applying reducing at an average rolling temperature of &thgr;m (° C.) and a total reduction ratio Tred (%) within s temperature range of from 400 to Ac3 transformation point, with di, &thgr;m and Tred being in a relation satisfying a prescribed equation.

Abstract: The steel pipe has a structure composed mainly of ferrite or ferrite plus pearlite or ferrite plus cementite. The steel pipe is characterized by grain size not greater than 3 &mgr;m, preferably not greater than 1 &mgr;m, elongation greater than 20%, tensile strength (TS:MPa) and elongation (E1:%) whose product is greater than 10000, and percent ductile fracture greater than 95%, preferably 100%, measured by Charpy impact test on an actual pipe at −100° C. The structure is characterized by C: 0.005-0.03%, Si: 0.01-3.0%, Mn: 0.01-2.0%, and Al: 0.001-0.10% on a weight basis, and is composed of ferrite or ferrite and a secondary phase, with ferrite grains being not greater than 3 &mgr;m and the secondary phase having an areal ratio not more than 30%. A steel pipe stock having the above-mentioned composition is heated at a temperature of (Ac1+50° C.) to 400° C. and subsequently reduced at a rolling temperature of (Ac1+50° C.) to 400° C.

Abstract: A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (&mgr;m), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac3 transformation point, and applying reducing at an average rolling temperature of &thgr;m (° C.) and a total reduction ratio Tred (%) within s temperature range of from 400 to Ac3 transformation point, with di, &thgr;m and Tred being in a relation satisfying a prescribed equation.

Abstract: A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (&mgr;m), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac3 transformation point, and applying reducing at an average rolling temperature of &thgr;m (°C.) and a total reduction ration Tred (%) within s temperature range of from 400 to Ac3 transformation point, with di, &thgr;m and Tred being in a relation satisfying a prescribed equation.

Abstract: This invention provide a plug and a mandrel bar for use in rolling a seamless steel pipe of which life can be substantially improved more than that of the prior art under a severe condition of use always exposed to a high temperature and high load. The entire plug or the mandrel bar or at least the surface layer of the extremity end and the work section of it is made of ceramics. In addition, this invention provides a method for manufacturing a seamless steel pipe using such a plug or mandrel bar as described above.

Abstract: A lubricant is provided for use in hot working such as cross-roll rolling of a seamless steel pipe. It effectively reduces the friction coefficient between the working tools and the material under the hot work, thus extending the lives of the tools. The lubricant contains: (A) from about 10 to about 60 wt % of alkali silicate; (B) from about 1 to about 20 wt % of silane coupling agent; (C) from about 0.1 to about 5.0 wt % of at least one kind of alkali compound selected from a group consisting of an alkali hydroxide, an alkali carbonate, an alkali borate and an alkali salt of mineral acid; and (D) from about 30 to about 70 wt % of water. Alternatively, the lubricant contains: (A) from about 10 to about 60 wt % of alkali silicate; (B) from about 1 to about 20 wt % of silane coupling agent; (C) from about 0.1 to about 5.

Abstract: A forged roll for rolling a seamless steel pipe, having excellent biting properties and wear resistance, and manufacturing method. The forged roll comprises a high carbon alloy cast steel comprising about 1.10-1.85 wt % carbon, about 0.3-1.2 wt % silicon, about 0.4-1.5 wt % manganese, about 0.5-2.0 wt % nickel, about 0.5-2.0 wt % chromium and iron. Heat treatment for spheroidal carbide formation is performed so that the spheroidal carbide covers about 35-55% of the area of the roll metallurgical structure.

Abstract: A lubricant is provided for use in hot working such as cross-roll rolling of a seamless steel pipe. It effectively reduces the friction coefficient between the working tools and the material under the hot work, thus extending the lives of the tools. The lubricant contains: (A) from about 10 to about 60 wt % of alkali silicate; (B) from about 1 to about 20 wt % of silane coupling agent; (C) from about 0.1 to about 5.0 wt % of at least one kind of alkali compound selected from a group consisting of an alkali hydroxide, an alkali carbonate, an alkali borate and an alkali salt of mineral acid; and (D) from about 30 to about 70 wt % of water. Alternatively, the lubricant contains: (A) from about 10 to about 60 wt % of alkali silicate; (B) from about 1 to about 20 wt % of silane coupling agent; (C) from about 0.1 to about 5.

Abstract: A forged roll for rolling a seamless steel pipe, having excellent biting properties and wear resistance. The forged roll includes a high carbon alloy cast steel comprising about 1.10-1.85 wt % carbon, about 0.3-1.2 wt % silicon, about 0.4-1.5 wt % manganese, about 0.5-2.0 wt % nickel, about 0.5-2.0 wt % chromium and iron. Heat treatment for spheroidal carbide formation is performed so that the spheroidal carbide covers about 35-55% of the area of the roll metallurgical structure.