A hot-rolled steel sheet for a non-oriented electrical steel sheet containing, in mass %, components of C: 0.0010% to 0.0050%, Si: 1.90% to 3.50%, Al: 0.10% to 3.00%, Mn: 0.05% to 2.00%, P: 0.10% or less, S: 0.005% or less, N: 0.0040% or less and B: 0.0060% or less with a remainder consisting of Fe and impurities, in which, in a sheet width-direction end portion of the hot-rolled steel sheet for a non-oriented electrical steel sheet, a C concentration [atom %] in a crystal grain boundary is 3.0 or more times a P concentration [atom %], and the C concentration [atom %] in the crystal grain boundary of the hot-rolled steel sheet for a non-oriented electrical steel sheet is 3.5 or more times a C concentration in a crystal grain.

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.

Provided are high-strength steel plate having excellent low-temperature impact toughness and method of manufacturing the same. The present disclosure relates to a high-strength steel plate comprising, by weight %, carbon (C): 0.04-0.12%, silicon (Si): 0.1-0.5%, manganese (Mn): 1.2-2.5%, phosphorus (P): 0.01% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01-0.08%, niobium (Nb): 0.01-0.08%, chromium (Cr): 0.01-0.5%, nickel (Ni): 0.4-1.0%, copper (Cu): 0.5% or less, molybdenum (Mo): 0.01-0.5%, vanadium (V): 0.05% or less, titanium (Ti): 0.005-0.02%, boron (B): 0.001-0.0025%, nitrogen (N): 0.002-0.01%, the balance Fe and inevitable impurities, a Ceq value being less than 0.55.

A preparation method of glassless grain-oriented silicon steel includes the following operations. During a decarburization annealing, a thickness of an oxide film on a surface of strip is 1.5-2.5 μm; an atomic weight ratio of Si element and Fe element in the oxide film satisfies: Si/(Si+Fe)≥0.76; during a high-temperature annealing, a cooling stage includes sequentially: cooling with an inner cover when a temperature drops from 1200° C. to 500° C.; wherein a protective gas is a mixed gas containing nitrogen and hydrogen, and a volume percentage of the hydrogen in the mixed gas is >3%; cooling with the inner cover when the temperature drops from 500° C. to 200° C.; wherein the protective gas is nitrogen; and cooling in air by removing the inner cover when the temperature is <200° C.

This hot-rolled steel sheet has a predetermined chemical composition, in which in a case where the thickness is denoted by t, a metallographic structure at a t/4 position from the surface includes, by area fraction, 77.0% to 97.0% of bainite or tempered martensite, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite, in the metallographic structure, the average grain size excluding the residual austenite is 7.0 μm or less, the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10.sup.6 carbides/mm.sup.2 or more, a tensile strength is 980 MPa or more, and an average Ni concentration on the surface is 7.0% or more.

Disclosed is a hot-rolled steel strip having a tensile strength greater than 875 MPa and containing in mass-%: C 0.06-0.12, Si 0-0.5, Mn 0.70-2.20, Nb 0.005-0.100, Ti 0.01-0.10, V 0.11-0.40, whereby the total amount of V+Nb+Ti is 0.20-0.40 Al 0.005-0.150, B 0-0.0008, Cr 0-1.0, whereby the total amount of Mn+Cr is 0.9-2.5, Mo 0-0.5, Cu 0-0.5, Ni 0-1.0, P 0-0.05, S 0-0.01, Zr 0-0.1 Co 0-0.1 W 0-0.1 Ca 0-0.005, N 0-0.01, balance Fe and unavoidable impurities, and having a microstructure at ¼ thickness that is: at least 90% martensite and bainite with island-shaped martensite-austenite (MA) constituents, the remainder being: less than 5% polygonal ferrite and quasi-polygonal ferrite, less than 5% pearlite, less than 5% austenite, so that the total area percentage is 100%.

Disclosed are a wire rod for a steel fiber having a strength of 1,500 MPa or more without performing LP heat treatment during a wire drawing process, a steel fiber and, a method for manufacturing the same. The wire rod for a high-strength steel fiber according to the present disclosure includes, in percent by weight (wt %), 0.01 to 0.03% of C, 0.05 to 0.15% of Si, 1.0 to 2.0% of Mn, 0.05 to 0.15% of P, 0.005% or less (excluding 0) of Al, 0.01% or less (excluding 0) of N, 0.03% or less (excluding 0) of S, 0.02 to 0.08% of Sn, and the remainder of Fe and inevitable impurities, wherein a microstructure is single-phase ferrite.

A hot-rolled steel sheet has a predetermined chemical composition in which a microstructure includes 99% or more of martensite by volume fraction and a remainder in microstructure including residual austenite and ferrite, in a cross section parallel to a rolling direction, an average aspect ratio of prior austenite grains is less than 3.0, a proportion of sulfides having an aspect ratio of more than 3.0 among sulfides having an area of 1.0 μm.sup.2 or more is 1.0% or, less, in a thickness middle portion, and a pole density of {211} <011> orientation is 3.0 or less, and a tensile strength TS is 980 MPa or higher.

A steel sheet with excellent surface quality, and a manufacturing method therefor are provided. The present invention provides a pickled steel sheet with excellent surface quality, comprising, by wt %, carbon (C) in an amount greater than or equal to 0.05% and less than 0.4%, 0.5% or less of silicon (Si) (excluding 0%), 0.05% or less of phosphorus (P), 0.03% or less of sulfur (S), 0.01% or less of boron (B), 0.1-2.5% of manganese (Mn) and/or chromium (Cr), and the balance of iron (Fe) and inevitable impurities, wherein the average thickness of an internal oxide layer and/or a tantalum layer, which are formed on the surface layer of the steel sheet, is 1-10 μm, and the standard deviation of the thickness of the internal oxide layer and/or the tantalum layer in the length direction of the steel sheet is 2 μm or less.

Provided are a high-carbon steel sheet having good surface quality and a manufacturing method therefor. The present invention provides a high-carbon pickled steel sheet having good surface quality, the steel sheet containing, in weight %, 0.4% or more and less than 1.2% of carbon (C), 0.5% or less (excluding 0%) of silicon (Si), 0.05% or less of phosphorus (P), 0.03% or less of sulfur (S), 0.1 to 2.5% of at least one of manganese (Mn) and chromium (Cr), and the balance of iron (Fe) and inevitable impurities, wherein the average thickness of an internal oxide layer and/or a decarburized layer formed in a surface layer portion of the steel sheet is 1 to 10 μm and the standard deviation of the thickness of the internal oxide layer and/or the decarburized layer in the length direction of the steel sheet is 2 μm or less.