Provides is a non-oriented electrical steel sheet suitable for use in a rotor of an IPM motor that has excellent magnetic flux density B.sub.50 and high-frequency iron loss properties, high tensile strength and fatigue strength, and little variation in tensile strength. The non-oriented electrical steel sheet has a predetermined steel sheet chemical composition and a microstructure in which a ratio of non-recrystallized microstructure is 5% or more and 70% or less and the number of inclusion having a diameter of 5 μm or more is not more than 5 counts/mm.sup.2.

The steel sheet of the present invention has a steel microstructure containing, in area fraction, martensite: from 20% to 100%, ferrite: from 0% to 80%, and another metal phase: 5% or less, and in which a ratio of a dislocation density in metal phases on a surface of the steel sheet to a dislocation density in the metal phases in a thicknesswise central portion of the steel sheet is from 30% to 80%. The maximum amount of warpage of the steel sheet when the steel sheet is sheared to a length of 1 m in a rolling direction is 15 mm or less.

An electric resistance welded steel pipe includes a base metal zone and an electric resistance welded zone. The base metal zone has a predetermined chemical composition and a microstructure including, by volume, ferrite: more than 30%, and bainite: 10% or more. The total volume fraction of the ferrite and the bainite is 70% or more and 95% or less. The balance being one or two or more phases selected from pearlite, martensite, and austenite. Further, when regions surrounded by boundaries between adjacent crystals having a misorientation of 15° or more are defined as crystal grains, the average size of the crystal grains is less than 7.0 μm, and the volume fraction of crystal grains having a size of 40.0 μm or more is 30% or less. A compressive residual stress generated in the inner and outer surfaces of the steel pipe in the axial direction is 250 MPa or less.

A non-oriented electrical steel sheet produced by hot-rolling a steel slab containing Si: 2.8 to 6.5 mass % and Zn: 0.0005 to 0.0050 mass % followed by cold rolling and finish annealing, a coating agent containing at least one element from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi is applied to the surface after annealing forming an insulation coating with nitriding-suppressing ability. Alternatively, an intermediate layer containing at least one element from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi and having a nitriding-suppressing ability forms on the steel sheet iron matrix after the annealing and forms an insulation coating, without above elements, on the intermediate layer thus obtaining a non-oriented electrical steel sheet wherein a high strength rotor core with and stator core with excellent magnetic is simultaneously obtained, and a motor core including a stator core and rotor core from the steel sheet.

In a production of a non-oriented electrical steel sheet by hot-rolling a steel slab containing Si: 2.8 to 6.5 mass % and Zn: 0.0005 to 0.0050 mass % followed by cold rolling and finish annealing, a coating agent containing at least one element selected from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi is applied to the steel sheet surface after the finish annealing to form an insulation coating with a nitriding-suppressing ability. Alternatively, an intermediate layer containing at least one element selected from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi and having a nitriding-suppressing ability is formed on the steel sheet iron matrix after the finish annealing and form an insulation coating not containing above elements is formed on the intermediate layer thus to obtain a non-oriented electrical steel sheet from which a rotor core with high strength and stator core with excellent magnetic properties after the stress-relief annealing can be obtained at the same time, and a motor core comprising a stator core and rotor core is produced from the steel sheet.

A non-oriented electrical steel sheet produced by hot-rolling a steel slab containing Si: 2.8 to 6.5 mass % and Zn: 0.0005 to 0.0050 mass % followed by cold rolling and finish annealing, a coating agent containing at least one element from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi is applied to the surface after annealing forming an insulation coating with nitriding-suppressing ability. Alternatively, an intermediate layer containing at least one element from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi and having a nitriding-suppressing ability forms on the steel sheet iron matrix after the annealing and forms an insulation coating, without above elements, on the intermediate layer thus obtaining a non-oriented electrical steel sheet wherein a high strength rotor core with and stator core with excellent magnetic is simultaneously obtained, and a motor core including a stator core and rotor core from the steel sheet.

In a production of a non-oriented electrical steel sheet by hot-rolling a steel slab containing Si: 2.8 to 6.5 mass % and Zn: 0.0005 to 0.0050 mass % followed by cold rolling and finish annealing, a coating agent containing at least one element selected from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi is applied to the steel sheet surface after the finish annealing to form an insulation coating with a nitriding-suppressing ability. Alternatively, an intermediate layer containing at least one element selected from Sn, Sb, P, S, Se, As, Te, B, Pb, and Bi and having a nitriding-suppressing ability is formed on the steel sheet iron matrix after the finish annealing and form an insulation coating not containing above elements is formed on the intermediate layer thus to obtain a non-oriented electrical steel sheet from which a rotor core with high strength and stator core with excellent magnetic properties after the stress-relief annealing can be obtained at the same time, and a motor core comprising a stator core and rotor core is produced from the steel sheet.

A spot welded joint of at least two steel sheets is provided. At least one of the steel sheets presents yield strength above or equal to 600 MPa, an ultimate tensile strength above or equal to 1000 MPa, uniform elongation above or equal to 15%. The base metal chemical composition includes 0.05≤C≤0.21%, 4.0≤Mn≤7.0%, 0.5≤Al≤3.5%, Si≤2.0%, Ti≤0.2%, V≤0.2%, Nb≤0.2%, P≤0.025%, B≤0.0035%, and the spot welded joint contains a molten zone microstructure containing more than 0.5% of Al and containing a surface fraction of segregated areas lower than 1%, said segregated areas being zones larger than 20 μm.sup.2 and containing more than the steel nominal phosphorus content.

A spot welded joint of at least two steel sheets is provided. At least one of the steel sheets presents yield strength above or equal to 600 MPa, an ultimate tensile strength above or equal to 1000 MPa, uniform elongation above or equal to 15%. The base metal chemical composition includes 0.05≤C≤0.21%, 4.0≤Mn≤7.0%, 0.5≤Al≤3.5%, Si≤2.0%, Ti≤0.2%, V≤0.2%, Nb≤0.2%, P≤0.025%, B≤0.0035%, and the spot welded joint contains a molten zone microstructure containing more than 0.5% of Al and containing a surface fraction of segregated areas lower than 1%, said segregated areas being zones larger than 20 μm.sup.2 and containing more than the steel nominal phosphorus content.

A cold-rolled and heat treated steel sheet, has a composition comprising 0.1%≤C≤0.4%, 3.5%≤Mn≤8.0%, 0.1%≤Si≤1.5%, Al≤3%, Mo≤0.5%, Cr≤1%, Nb≤0.1%, Ti≤0.1%, V≤0.2%, B≤0.004%, 0.002%≤N≤0.013%, S≤0.003%, P≤0.015%. The structure consists of, in surface fraction: between 8 and 50% of retained austenite, at most 80% of intercritical ferrite, the ferrite grains, if any, having an average size of at most 1.5 μm, and at most 1% of cementite, the cementite particles having an average size lower than 50 nm, martensite and/or bainite.

A cold-rolled and heat treated steel sheet, has a composition comprising 0.1%≤C≤0.4%, 3.5%≤Mn≤8.0%, 0.1%≤Si≤1.5%, Al≤3%, Mo≤0.5%, Cr≤1%, Nb≤0.1%, Ti≤0.1%, V≤0.2%, B≤0.004%, 0.002%≤N≤0.013%, S≤0.003%, P≤0.015%. The structure consists of, in surface fraction: between 8 and 50% of retained austenite, at most 80% of intercritical ferrite, the ferrite grains, if any, having an average size of at most 1.5 μm, and at most 1% of cementite, the cementite particles having an average size lower than 50 nm, martensite and/or bainite.

A cold-rolled steel plate (1) and a manufacturing method therefor. The chemical composition of the steel plate (1) in percentage by weight is: C 0.15-0.25%, Si 1.50-2.50%, Mn 2.00-3.00%, P≤0.02%, S≤0.01%, Al 0.03-0.06%, N≤0.01%, with the balance being Fe and impurities. The surface layer has an inner oxide layer (2) with a thickness of 1-5 μm, and there is no enrichment of Si or Mn on the surface. The steel plate (1) has good phosphating performance and formability, with a tensile strength of ≥1180 MPa and an elongation of ≥14%, and has a complex-phase structure of ferrite, martensite, and retained austenite, the content of the retained austenite being not lower than 5%. A dew point is at −25° C. to 10° C. in continuous annealing, such that external oxidation transitions to internal oxidation.