An as-rolled type K55 electric resistance welded oil well pipe includes, in terms of % by mass, 0.30 to 0.50% of C, 0.05 to 0.40% of Si, 0.50 to 1.20% of Mn, 0 to 0.030% of P, 0 to 0.020% of S, 0.002 to 0.080% of Al, 0 to 0.0080% of N, 0 to 0.30% of Cu, 0 to 0.30% of Ni, 0 to 0.30% of Cr, 0 to 0.10% of Mo, 0 to 0.10% of V, 0 to 0.050% of Nb, 0 to 0.030% of Ti, 0 to 0.0100% of Ca, and the balance being Fe and impurities. In the pipe, a metallographic structure at a position of of a pipe thickness in an L cross-section at a base metal 90 position is a ferrite-pearlite structure in which prior -grains are flattened, includes grain boundary ferrite and intragranular ferrite, and has a rate of a total area of the grain boundary ferrite and the intragranular ferrite, of 10 to 30%.

An as-rolled type K55 electric resistance welded oil well pipe includes, in terms of % by mass, 0.30 to 0.50% of C, 0.05 to 0.40% of Si, 0.50 to 1.20% of Mn, 0 to 0.030% of P, 0 to 0.020% of S, 0.002 to 0.080% of Al, 0 to 0.0080% of N, 0 to 0.30% of Cu, 0 to 0.30% of Ni, 0 to 0.30% of Cr, 0 to 0.10% of Mo, 0 to 0.10% of V, 0 to 0.050% of Nb, 0 to 0.030% of Ti, 0 to 0.0100% of Ca, and the balance being Fe and impurities. In the pipe, a metallographic structure at a position of of a pipe thickness in an L cross-section at a base metal 90 position is a ferrite-pearlite structure in which prior -grains are flattened, includes grain boundary ferrite and intragranular ferrite, and has a rate of a total area of the grain boundary ferrite and the intragranular ferrite, of 10 to 30%.

In a tandem type cold rolling mill of a circulating oil-feeding system for continuously rolling a steel sheet by feeding a coolant serving as a rolling oil and a cooling water to each stand, an edge heater for heating both edge portions of the steel sheet to not lower than 60 C. as a steel sheet temperature at an entry side of a roll bite is arranged at an upstream side of the first stand in the cold rolling mill and a device for jetting a coolant having a concentration higher than that of the coolant fed to the firsts stand onto surfaces of both edge portions of the steel sheet is arranged between the edge heater and the first stand. The cold rolling mill can be used to roll a hard-to-roll material such as silicon or stainless steel sheet without causing an edge crack or sheet breakage in low-speed rolling.

A method for producing a semifinished product with locally different material thicknesses may involve preparing a multilayer, metal material composite, which has a plurality of layers with different ductilities, and rolling the material composite in a method for flexible rolling through a rolling gap formed between two rollers. The rolling gap may be configured such that regions with different material thicknesses are formed. In some cases, the multilayer, metal material composite is rolled at room temperature. Further, the plurality of layers of the multilayer, metal material composite may include a first outer layer disposed on a first side of a middle layer and a second outer layer disposed on a second side of the middle layer, with the second side of the middle layer being opposite the first side.

A method for producing a semifinished product with locally different material thicknesses may involve preparing a multilayer, metal material composite, which has a plurality of layers with different ductilities, and rolling the material composite in a method for flexible rolling through a rolling gap formed between two rollers. The rolling gap may be configured such that regions with different material thicknesses are formed. In some cases, the multilayer, metal material composite is rolled at room temperature. Further, the plurality of layers of the multilayer, metal material composite may include a first outer layer disposed on a first side of a middle layer and a second outer layer disposed on a second side of the middle layer, with the second side of the middle layer being opposite the first side.

A process of producing a duplex stainless steel tube comprises the steps of: a) producing an ingot or a continuous casted billet of said duplex stainless steel; b) hot extruding the ingot or the billet obtained from step a) into a tube; and c) cold rolling the tube obtained from step b) to a final dimension thereof.
The outer diameter D and the wall thickness t of the cold rolled tube is 50-250 mm respectively is 5-25 mm, and, for the cold rolling step, R and Q are set such that the following formula is satisfied:
Rp0.2target=416.53+113.26.Math.log Q+4.0479.Math.R+2694.9.Math.C %82.750.Math.(log Q).sup.20.04279.Math.R.sup.22.2601.Math.log Q.Math.R+16.9.Math.Cr %+26.1.Math.Mo %+83.6.Math.N %+Z(1)
wherein Rp0.2target is targeted yield strength and is 800-1100 MPa and 0<Q<3.6.

Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 25% or less of a ferrite phase, 75% or more of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 m from the surface in the thickness direction, the area ratio of a ferrite phase is 5% to 20%, and a tensile strength is 1180 MPa or more.

Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 25% or less of a ferrite phase, 75% or more of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 m from the surface in the thickness direction, the area ratio of a ferrite phase is 5% to 20%, and a tensile strength is 1180 MPa or more.

Provided are a ferrite-based stainless steel having improved heat radiation property and processability and method for preparing same. A ferritic stainless steel according to an embodiment of the disclosure, includes, in % by weight, carbon (C): 0.0005 to 0.02%, nitrogen (N): 0.005 to 0.02%, chromium (Cr): 10.0 to 17.0%, titanium (Ti): 0.02 to 0.30%, niobium (Nb): 0.10 to 0.60%, and the remainder of iron (Fe) and other inevitable impurities, and the ferritic stainless steel is plated with aluminum (Al) having a thickness of 5 to 50 m. Therefore, heat dissipation and workability of ferritic stainless steel may be improved by controlling ferritic stainless steel alloy composition, aluminum (Al) thickness and manufacturing method.

Provided are a ferrite-based stainless steel having improved heat radiation property and processability and method for preparing same. A ferritic stainless steel according to an embodiment of the disclosure, includes, in % by weight, carbon (C): 0.0005 to 0.02%, nitrogen (N): 0.005 to 0.02%, chromium (Cr): 10.0 to 17.0%, titanium (Ti): 0.02 to 0.30%, niobium (Nb): 0.10 to 0.60%, and the remainder of iron (Fe) and other inevitable impurities, and the ferritic stainless steel is plated with aluminum (Al) having a thickness of 5 to 50 m. Therefore, heat dissipation and workability of ferritic stainless steel may be improved by controlling ferritic stainless steel alloy composition, aluminum (Al) thickness and manufacturing method.