A preferable aspect of the present invention provides: an ultra-high-strength hot-rolled steel sheet containing, by weight, one or two of 0.40-0.60% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, 0.9-1.5% of Ni, and 0.9-1.5% of Cu, 1.1% or more of Cu+Ni, 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B, 0.006% or less (excluding 0%) of N, and the balance Fe and other impurities, the alloy elements satisfying relational formulas 1 and 2 below, wherein a microstructure of the hot-rolled steel sheet comprises, by volume, 7% or more of ferrite and 93% or less of perlite; a steel pipe and a member each using the same; and manufacturing methods therefor. [Relational formula 1] (Mn/Si) #3 (weight ratio) [Relational formula 2] (Ni/Si) #1 (weight ratio)

Provided is a welding management system that appropriately manages welding. The welding management system (100) includes a plurality of local magnetic sensors (10a to 10f) provided around a target joining position (K) and a correction magnetic sensor (20) curved to surround the target joining position (K). A data processing unit (30) generates information of a joining state of the target joining position (K) based on a difference between a detection value of each of the plurality of the local magnetic sensors (10a to 10f) and a detection value of the correction magnetic sensor (20).

The present invention relates to an air conditioner. The air conditioner according to the present embodiment has a refrigeration capacity of 2 kW to 7 kW, inclusive, and uses R410a as a refrigerant circulating therein, and since a refrigerant pipe therein includes a ductile stainless steel pipe made of a material containing, at least, chrome (Cr), nickel (Ni), manganese (Mn) and copper (Cu), the refrigerant pipe can maintain strength and hardness as good as or better than those of a copper pipe, while also maintaining good processability.

The present invention relates to an air conditioner. The air conditioner according to the present embodiment has a refrigeration capacity of 7 kW to 11 kW, inclusive, and uses a refrigerant R134a as a refrigerant, and since a refrigerant pipe therein is made of a ductile stainless steel material having 1% or less of a delta-ferrite matrix structure with respect to the grain size area thereof, and includes a suction pipe guiding the suction of the refrigerant into a compressor and having an outer diameter of 15.88 mm, the refrigerant pipe can maintain strength and hardness as good as or better than those of a copper pipe, while also maintaining good processability.

An electrode unit of a joining apparatus includes an abutting section provided with a tapered outer circumferential surface that contacts a tapered pressure-receiving surface by being inserted into an inside of a first metal member. The tapered outer circumferential surface inclines in a direction of getting closer to an axial center of the abutting section from one end side to another end side. At least part of the abutting section has a thickness elastically deformable in such a manner that the tapered outer circumferential surface aligns with the tapered pressure-receiving surface during application of a pressurizing force to the first metal member and a second metal member.

A preferable aspect of the present invention provides: an ultra-high-strength hot-rolled steel sheet containing, by weight, one or two of 0.40-0.60% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, 0.9-1.5% of Ni, and 0.9-1.5% of Cu, 1.1% or more of Cu+Ni, 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B, 0.006% or less (excluding 0%) of N, and the balance Fe and other impurities, the alloy elements satisfying relational formulas 1 and 2 below, wherein a microstructure of the hot-rolled steel sheet comprises, by volume, 7% or more of ferrite and 93% or less of perlite; a steel pipe and a member each using the same; and manufacturing methods therefor. [Relational formula 1] (Mn/Si) #3 (weight ratio) [Relational formula 2] (Ni/Si) #1 (weight ratio)

In a main metal piece 60 and an inner cylinder 70 serving as cylindrical bodies before welding, a second end surface 75 is an inclined surface that inclines toward an end portion opposite from the second end surface 75 as the second end surface 75 extends toward the center axis, and an angle 2a is 5 to 15. Thus, the distance between a first end surface 65 and the second end surface 75 positioned in resistance welding increases toward the center axis, and an angle c (=angle 2a) between the first end surface 65 and the second end surface 75 is 5 to 15.

Provided is an electric resistance welded clad steel pipe or tube in which a region where solidification microstructure is formed, i.e. a region in a weld particularly having significant influence on properties, is reduced without impairing its function as a clad pipe or tube. An electric resistance welded clad steel pipe or tube comprises: a first layer made of carbon steel or low-alloy steel as base metal; and a second layer placed on one surface of the first layer, and made of stainless steel or a nickel-containing alloy as cladding metal, wherein the base metal is not exposed at a cladding metal-side surface of the electric resistance welded clad steel pipe or tube in a weld, and no solidification microstructure is contained in each of circular sections of 0.1 mm in radius respectively centered at specific three positions in a plane perpendicular to a pipe or tube longitudinal direction.

Provided is a welding management system that appropriately manages welding. The welding management system (100) includes a plurality of local magnetic sensors (10a to 10f) provided around a target joining position (K) and a correction magnetic sensor (20) curved to surround the target joining position (K). A data processing unit (30) generates information of a joining state of the target joining position (K) based on a difference between a detection value of each of the plurality of the local magnetic sensors (10a to 10f) and a detection value of the correction magnetic sensor (20).

A steel pipe, consisting of, in terms of mass %: from 0.06% to 0.25% of C, 0.50% or less of Si, 1.00% to 1.80% of Mn, 0.030% or less of P, 0.020% or less of S, 0.08% or less of Al, 0.008% or less of N, 0.080% or less of Nb, and a remainder consisting of Fe and unavoidable impurities, wherein a compressive residual stress at an outer surface measured by an X-ray method is 250 MPa or more, and a compressive residual stress at a position at a depth of 1 mm from the outer surface measured by the X-ray method is 70% or more of the compressive residual stress at the outer surface measured by the X-ray method.