A high-strength steel sheet with excellent delayed fracture resistance, having a tensile strength of 1700 MPa or larger, including a predetermined component composition, having a martensite structure whose ratio accounts for 95 area % or more of the entire metallographic structure, and having a transition metal carbide whose ratio accounts for 0.8 volume % or more of the entire metallographic structure.

Provided is a grain-oriented electrical steel sheet having both low iron loss and good magnetostrictive properties, with which a transformer having excellent properties can be manufactured. The grain-oriented electrical steel sheet of the present disclosure has a linear strain region extending linearly in a direction intersecting the rolling direction, where the linear strain region has a region having compressive stress in the rolling direction, and a region adjacent in a rolling direction to the region having compressive stress has a region having tensile stress in the rolling direction.

Provided is a grain-oriented electrical steel sheet having both low iron loss and good magnetostrictive properties, with which a transformer having excellent properties can be manufactured. The grain-oriented electrical steel sheet of the present disclosure has a linear strain region extending linearly in a direction intersecting the rolling direction, where the linear strain region has a region having compressive stress in the rolling direction, and a region adjacent in a rolling direction to the region having compressive stress has a region having tensile stress in the rolling direction.

A hot-rolled steel sheet has, as a chemical composition, by mass %: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; and Ni: 0.02% to 0.50%, in which among measurement points at which elemental analysis is performed at a measurement pitch of 1 μm using an EPMA in a region of 250 μm×250 μm on a surface, the percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%.

A hot-rolled steel sheet has, as a chemical composition, by mass %: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; and Ni: 0.02% to 0.50%, in which among measurement points at which elemental analysis is performed at a measurement pitch of 1 μm using an EPMA in a region of 250 μm×250 μm on a surface, the percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%.

A steel sheet includes a predetermined composition, in which a microstructure at a ¼ thickness position from a surface in a sheet thickness direction includes, by vol %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the steel sheet stretched by 10% at the ¼ thickness position from the surface in the sheet thickness direction, a number density of voids having a maximum diameter of 1.0 μm or more is 1.0×10.sup.9 pieces/m.sup.2 or less.

A steel sheet includes a predetermined composition, in which a microstructure at a ¼ thickness position from a surface in a sheet thickness direction includes, by vol %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the steel sheet stretched by 10% at the ¼ thickness position from the surface in the sheet thickness direction, a number density of voids having a maximum diameter of 1.0 μm or more is 1.0×10.sup.9 pieces/m.sup.2 or less.

The invention provides an endless rolling method based on temperature uniformity control, and belongs to the field of iron and steel metallurgy. By optimizing the process path, a new layout mode is adopted, a double heat storage soaking furnace and a descaling box are additionally arranged, transverse and longitudinal bonding magnetic induction heating device is adopted, transverse and longitudinal temperature uniform of the slab in the rolling process is realized, the cross section temperature difference is reduced, and the product quality is improved. On the basis of five-stand arrangement of a traditional finish rolling mill, a rolling mill is additionally arranged to serve as a standby rolling mill, such that on-line non-shutdown change roller of the finish rolling mill is realized. The method of the invention realizes a full-continuous production of production and meets the high-quality development requirements of iron and steel metallurgy, such that traditional cool rolling can be replaced with hot rolling, traditional thick-specification strip can be replaced with high-added-value thin specification strip. There is important significance in the aspects of productivity optimization layout, green manufacturing, intelligent manufacturing and the like.

Disclosed is, as a high-strength steel plate of API X80 grade or higher with a thickness of 38 mm or more, a thick steel plate for structural pipes or tubes that exhibits high strength in the rolling direction and excellent Charpy properties at its mid-thickness part without addition of large amounts of alloying elements. The thick steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure at its mid-thickness part that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50%, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80% or more with respect to the whole area of the ferrite; a tensile strength of 620 MPa or more; and a Charpy absorption energy vE.sub.−20+ C. at −20° C. at the mid-thickness part of 100 J or more.

A transverse field induction heating apparatus for the inductive heating of sheet metal in a rolling mill includes an upper inductor and a lower inductor. The upper inductor includes two adjacently positioned upper partial induction loops which are series-connected and fed an electrical current in opposite directions. The lower inductor includes two adjacently positioned lower partial induction loops which are series-connected and fed an electrical current in opposite directions. The electrical current in both partial induction loops is oriented in an opposing direction. Each of the upper and lower partial induction loop is structured to be moved individually perpendicular to a sheet axis and includes a rounded head positioned adjacent to each other such that the rounded head is shaped as a hammer head.