A steel sheet with a tensile strength (TS) of 1180 MPa or more, a member, and a method for producing them. In a region of the steel sheet within 4.9 μm in the thickness direction, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more. The lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy the following formula (1):

L.sub.Si+L.sub.Mn≤(T.sub.Si+T.sub.Mn)/4  (1).

A low thermal expansion cast steel having a sufficient strength even at a high temperature and having a low coefficient of thermal expansion, that is, a low thermal expansion cast steel comprising, by mass %, C: 0 to 0.100%, Si: 0 to 1.00%, Mn: 0 to 1.00%, Co: 8.0 to 13.0%, and Ni satisfying −2.5×% Ni+85.5≤% Co≤−2.5×% Ni+90.5 (% Ni and % Co respectively being contents of Ni and Co (mass %)) and having a balance of Fe and unavoidable impurities and having, upon being subjected to suitable heat treatment, a 0.2% proof stress of a tensile test at 300° C. of 125 MPa or more, having an average coefficient of thermal expansion at 25 to 300° C. of 4.0 ppm/° C. or less, and having a Curie temperature of 250° C. or more.

A non-oriented electrical steel sheet including a base metal that has a chemical composition including, in mass%, C: 0.0010 to 0.0040%, Si: 4.0 to 5.0%, Mn: 0.20% or less Al: 0.010% or more and less than 0.050%, P: 0.030% or less, S: 0.0030% or less, N: 0.0005 to 0.0030%, O: 0.0100 to 0.0400%, Ca: less than 0.0010%, Ti: less than 0.0050%, Nb: less than 0.0050%, Zr: less than 0.0050%, V: less than 0.0050%, Cu: less than 0.20%, Ni: less than 0.50%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, and a balance: Fe and impurities, in which the base metal has an O content of less than 0.0050% in a region excluding a portion from a surface of the base metal to a position of 10 .Math.m in a depth direction of the base metal.

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. The steel pipe having a composition including C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%. The steel pipe having a structure composed of 90% or more by volume of a bainitic ferrite phase and 10% or less (including 0%) by volume of a second phase. The bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion. The steel pipe having high strength and toughness that is maintained through post-weld heat treatment, including heating to a temperature of 600° C. or more.

A Si/Mn ratio of steel material components of a base material is not less than 0.27 nor more than 0.90 in mass ratio, an internal oxide layer having a thickness of not less than 1 μm nor more than 30 μm is provided right below an oxide scale of a steel sheet surface layer portion, and regarding the internal oxide layer, an internal oxide in a crystal grain of the internal oxide layer is an oxide containing Si and having a thickness of not less than 10 nm nor more than 200 nm in a crystal grain in a range of greater than 0% and 30% or less of a thickness of the internal oxide layer from an interface between the internal oxide layer and base iron toward a direction of the surface layer oxide scale, one or more branches of the internal oxide exist in a cross section of 1 μm×1 μm square, and in any crystal grain boundary having a length of 1 μm, one or more of the internal oxides in the crystal grain are connected to an internal oxide of the crystal grain boundary to form a net-like structure.

A steel sheet has a specified chemical composition and a specified steel microstructure. An average grain size of ferrite and/or bainitic ferrite is 7.0 μm or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the grain size of the ferrite and/or the bainitic ferrite to the average grain size of the ferrite and/or the bainitic ferrite is 10% or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the area fraction of as-quenched martensite to the area fraction of the as-quenched martensite is 10% or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the area fraction of retained austenite to the area fraction of the retained austenite is 10% or less.

A galvannealed steel sheet includes: a scale-removed rolled steel sheet; and a galvannealed layer arranged on the scale-removed rolled steel sheet. When ten measurement points of the galvannealed steel sheet are set in a transverse direction by equally dividing a line-segment having a reference length of 50 mm by 10, a minimum P content of the galvannealed layer in the ten measurement points is 50% or more as compared with a maximum P content therein.

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. The steel pipe having a composition including C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%. The steel pipe having a structure composed of a 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less (including 0%) by volume of a second phase. The bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion. The steel pipe having high strength, toughness, and high resistance that is maintained through post-weld heat treatment.

The present invention manufactures a steel sheet for a rotor core for an IPM motor, wherein the steel sheet has a magnetic flux density B.sub.8000 of 1.65 T or more as measured when magnetic field strength is 8000 A/m, and a residual magnetic flux density Br of 0.5 T or more as measured at that time, and optionally, a coercivity Hc of 100 A/m or more as measured after magnetization reaches 8000 A/m. By using the steel sheet manufactured according to the present invention for a rotor core of an IPM motor, it is possible to increase further an output torque in a high-speed rotational range and raise further the maximum rotational speed.

The present invention relates to a grain oriented electrical steel sheet having excellent core loss and to a method for manufacturing same. The electrical steel sheet according to one aspect of the present invention may have a composition comprising, by weight %, Si: 1.0 to 4.0%, Al: 0.1 to 4.0%, and a rare earth element: 0.0001 to 0.5% by the total content of the whole rare earth element.