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.

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 metal powder having a composition including the following elements, expressed in content by weight: 6.5%≤Si≤10%, 4.5%≤Nb≤10%, 0.2%≤B≤2.0%, 0.2%≤Cu≤2.0%, C≤2% and optionally containing Ni≤10 wt % and/or Co≤10 wt % and/or Cr≤7 wt % and/or Zr as a substitute for any part of Nb on a one-to-one basis and/or Mo as a substitute for any part of Nb on a one-to-one basis and/or P as a substitute for any part of Si on a one-to-one basis, the balance being Fe and unavoidable impurities resulting from the elaboration, the metal powder having a microstructure including at least 5% in area fraction of an amorphous phase, the balance being made of crystalline ferritic phases with a grain size below 20 μm and possible precipitates, the metal powder having a mean sphericity SPHT of at least 0.80.

A method of manufacturing steel strip including the steps of: casting molten steel into slabs; reheating the slabs at 1150° C. or more for 1 hour or more; hot rolling the steel into a strip, preferably with an average F1 slab entry temperature above 1000° C.; coiling the hot rolled steel strip; batch annealing the steel strip: at an intercritical temperature (i.e. between Ac1 and Ac3), preferably below 700° C.; in non-oxidising and non-nitrogenated atmosphere; total annealing time at least 5 hours, preferably at least 10 hours to get Mn enrichment in austenite such that Mn content is at least 1.25 times bulk Mn content of the steel and C enrichment such that C content is at least 1.2 times bulk C content of the steel; cooling the steel after batch annealing in air, forced air or water quench.

A high-strength structural steel material having excellent fatigue crack propagation inhibitory characteristics according to an aspect of the present invention contains, by weight, 0.02-0.12% of C, 1.7-2.5% of Mn, 0.01-0.8% of Si, 0.005-0.5% of Al, and the balance Fe and unavoidable impurities, wherein a microstructure of the structural steel sheet material is divided into a surface layer portion outside and a central portion inside along a thickness direction; the surface layer portion comprises tempered bainite as a matrix structure, fresh martensite as a second structure, and austenite as a residual structure; and the central portion comprises lath bainite.

A high-strength structural steel material having excellent fatigue crack propagation inhibitory characteristics according to an aspect of the present invention contains, by weight, 0.02-0.12% of C, 1.7-2.5% of Mn, 0.01-0.8% of Si, 0.005-0.5% of Al, and the balance Fe and unavoidable impurities, wherein a microstructure of the structural steel sheet material is divided into a surface layer portion outside and a central portion inside along a thickness direction; the surface layer portion comprises tempered bainite as a matrix structure, fresh martensite as a second structure, and austenite as a residual structure; and the central portion comprises lath bainite.

A method for manufacturing a grain-oriented electrical steel sheet, according to an embodiment of the present invention includes: heating a slab, based on 100 wt % of a total composition thereof, including N at 0.0005 wt % to 0.015 wt %, Ti at 0.0001 wt % to 0.020 wt %, V at 0.0001 wt % to 0.020 wt %, Nb at 0.0001 wt % to 0.020 wt %, B at 0.0001 wt % to 0.020 wt %, and the remaining portion including Fe and other impurities, and then hot rolling it to prepare a hot-rolled steel sheet; annealing the hot-rolled steel sheet; after the hot-rolled steel sheet is annealed, cooling the hot-rolled steel sheet, and then cold rolling it to prepare a cold-rolled steel sheet; decarburization-annealing the cold-rolled steel sheet and then nitriding-annealing it, or simultaneously performing the decarburization-annealing and the nitriding-annealing; and final-annealing the decarburization-annealed and nitriding-annealed steel sheet.

A method for manufacturing a grain-oriented electrical steel sheet, according to an embodiment of the present invention includes: heating a slab, based on 100 wt % of a total composition thereof, including N at 0.0005 wt % to 0.015 wt %, Ti at 0.0001 wt % to 0.020 wt %, V at 0.0001 wt % to 0.020 wt %, Nb at 0.0001 wt % to 0.020 wt %, B at 0.0001 wt % to 0.020 wt %, and the remaining portion including Fe and other impurities, and then hot rolling it to prepare a hot-rolled steel sheet; annealing the hot-rolled steel sheet; after the hot-rolled steel sheet is annealed, cooling the hot-rolled steel sheet, and then cold rolling it to prepare a cold-rolled steel sheet; decarburization-annealing the cold-rolled steel sheet and then nitriding-annealing it, or simultaneously performing the decarburization-annealing and the nitriding-annealing; and final-annealing the decarburization-annealed and nitriding-annealed steel sheet.

A grain-oriented electrical steel sheet that includes a base coating with a high TiN ratio advantageous for the application of tension to the steel sheet and has excellent magnetic property is provided. The grain-oriented electrical steel sheet includes: a base coating having a peak value PTiN of TiN in the form of osbornite, observed in a range of 42°<2θ<43° and a peak value PMg.sub.2SiO.sub.4 of Mg.sub.2SiO.sub.4 in the form of forsterite, observed in a range of 35°<2θ<36° of both more than 0 and satisfying a relationship PTiN≧PMg.sub.2SiO.sub.4, in thin-film X-ray diffraction analysis; and an iron loss W.sub.17/50 of 1.0 W/kg or less.

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, 30% or more of a ferrite phase, 40% to 65% 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 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 40% to 55% and the total area ratio of a bainite phase having a grain diameter of more than 5 μm and/or a martensite phase having a grain diameter of more than 5 μm is 20% or less, and a tensile strength is 980 MPa or more.