Provided is an ultra-high-strength reinforcing bar and a method for manufacturing the same are disclosed. In an exemplary embodiment, the ultra-high-strength reinforcing bar includes an amount of 0.10 to 0.45 wt % carbon (C), an amount of 0.5 to 1.0 wt % silicon (Si), an amount of 0.40 to 1.80 wt % manganese (Mn), an amount of 0.10 to 1.0 wt % chromium (Cr), an amount greater than 0 and less than or equal to 0.2 wt % vanadium (V), an amount greater than 0 and less than or equal to 0.4 wt % copper (Cu), an amount greater than 0 and less than or equal to 0.5 wt % molybdenum (Mo), an amount of 0.015 to 0.070 wt % aluminum (Al), an amount greater than 0 and less than or equal to 0.25 wt % nickel (Ni), an amount greater than 0 and less than or equal to 0.1 wt % tin (Sn), an amount greater than 0 and less than or equal to 0.05 wt % phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % sulfur (S), an amount of 0.005 to 0.02 wt % nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities.

A flat product of steel with yield strength Rp 0.2 of 660 to 820 MPa, BH2 value greater than 30 MPa, a hole expansion ratio greater than 30%, and a microstructure having a first main component at a proportion of at least 50%, including one or more individual components of ferrite, tempered bainite, and tempered martensite, each with less than 5% carbides, and a second main component at a proportion of 5% to 50%, including one or more individual components of martensite, residual austenite, bainite or perlite, with the steel having a following chemical composition (in weight %): C: 0.04 to 0.12; Si: 0.03 to 0.8; Mn: 1 to 2.5: P: max. 0.08; S: max. 0.01; N: max. 0.01; Al: up to 0.1; Ni+Mo; up to 0.5; Nb: up to 0.08; Ti: up to 0.2; Nb+Ti: min, 0.03; Cr: up to 0.6; the remainder being iron including unavoidable steel-associated elements.

The rail having a chemical composition containing C: 0.70-1.00 mass %, Si: 0.50-1.60 mass %, Mn: 0.20-1.00 mass %, P: ≤0.035 mass %, S: ≤0.012 mass %, Cr: 0.40-1.30 mass %, where Ceq defined by the formula (1) is 1.04-1.25,
Ceq=[% C]+([% Si]/11)+([% Mn]/7)+([% Cr]/5.8)  (1) where [% M] is the content in mass % of the element M, the balance being Fe and inevitable impurities, where Ceq(max) is ≤1.40, where the Ceq(max) is determined by the formula (2) using maximum contents of C, Si, Mn, and Cr obtained by subjecting a region between specified positions to EPMA line analysis; and a pearlite area ratio in the region is 95% or more,
Ceq(max)=[% C(max)]+([% Si(max)]/11)+([% Mn(max)]/7)+([% Cr(max)]/5.8)  (2) where [% M(max)] is the maximum content of the element M.

A steel sheet includes a predetermined chemical composition and a metal structure represented by, in area fraction, ferrite: 50% to 95%, granular bainite: 5% to 48%, martensite: 2% to 30%, and upper bainite, lower bainite, tempered martensite, retained austenite, and pearlite: 5% or less in total.

Disclosed is a steel for an alloy structure, the chemical elements of the steel being, in percentage by mass: 0.35-0.45% of C, 0.27-0.35% of Si, 0.6-0.8% of Mn, 0.015-0.05% of Al, 0.06-0.1% of V, 0.2-1.0% of Zr, 0.001-0.005% of Mg, 0.025% or less of P, 0.015% or less of S, 0.005% or less of N, 0.001% or less of 0, the balance being Fe and other inevitable impurities. In addition, also disclosed is a manufacturing method for the steel for an alloy structure, the method comprising steps of: (1) smelting, refining, and casting; (2) blooming and cogging; (3) secondary hot rolling to form a product; and (4) heat treatment including quenching and tempering. The steel for an alloy structure is designed by adding trace alloy elements, the steel for an alloy structure is further strengthened and toughened, and the manufacturing cost is low.

The present invention provides steel sheet having both bendability and hydrogen embrittlement resistance. The steel sheet of the present invention includes a central part of sheet thickness and a surface sort part formed at one side or both sides of the central part of sheet thickness. The microstructure of the central part of sheet thickness comprises, by volume ratio, 60% or more of tempered martensite, respectively less than 30% of ferrite, bainite, pearlite, and retained austenite, and less than 5% of as-quenched martensite. A thickness of the surface soft part is more than 10 μm per side and 15% or less of a thickness of the central part of sheet thickness, an average hardness of the surface soft part is 0.90 time or less of an average hardness of the central part of sheet thickness, the surface soft part includes carbides in a number density of 1×10.sup.4/mm.sup.2 or more, an average particle size of the carbides is 0.250 μm or less, and a standard deviation of a log of a particle size is 0.05 or less.

This hot-stamping formed body has a predetermined chemical composition and has a metallographic structure consisting of, by area ratio, a total of 10% to 30% of ferrite and granular bainite and a remainder in microstructure consisting of one or more of martensite, bainite, and tempered martensite, and, in textures of a surface layer region and an inside region, ratios between a pole density of an orientation group consisting of {001}<1-10> to {001}<−1-10> and a pole density of an orientation group consisting of {111}<1-10> to {111}<−1-12> are controlled.

A preferable aspect of the present invention provides: a hot-rolled steel sheet with excellent impact resistance containing, by weight, 0.35-0.55% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, one or two of 0.1-1.0% of Ni and 0.1-1.0% of Cu, 0.4% or more of Cu+Ni, 0.006% or less (excluding 0%) of N, and the balance Fe and other impurities, the alloy elements satisfying relational formulas 1 to 3 below, wherein a microstructure of the hot-rolled steel sheet comprises, by volume, 10% or more of ferrite and 90% or less of pearlite; a steel pipe and a member each using the same; and manufacturing methods therefore.

(Mn/Si)≥3 (weight ratio)  [Relational formula 1]

(Ni+Cu)/(C+Mn)≥0.2 (weight ratio)  [Relational formula 2]

(Ni/Si)≥(weight ratio)  [Relational formula 3].

A hot-rolled steel plate/strip for sulfuric acid dew point corrosion resistance and manufacturing method therefor. In said method, elements such as Sn and Cu remaining in steel scrap are fully utilized to smelt molten steel, and micro-alloy elements such as Cr, Ti, and Sb are selectively added; in a smelting process, basicity of slag, types and melting points of inclusions in steel, and a free oxygen content and an acid-soluble aluminum (Als) content in molten steel are controlled, a cast strip (11) is casted by means of twin-roll strip continuous casting, the cast strip (11) exits from crystallization rolls (8a, 8b) and directly enters a lower closed chamer (10) having a non-oxidizing atmosphere, then enters, in a closed condition, an on-line rolling mill (13) for hot rolling, after rolling, strip steel is cooled by means of gas atomization cooling, and finally the strip steel is wound up. The steel can be widely applied to the fields of products, such as tobacco baking apparatuses, air preheater heat exchange elements in industries such as petroleum, chemical industry, electric power, and metallurgy, delivery pipe, flue, and stack manufacturing structural parts, and boiler preheater and economizer equipment, of which the use environments have requirements for sulfuric acid dew point corrosion resistance performance.

A high-strength thin-gauge checkered steel plate/strip and a manufacturing method therefor, wherein residual elements such as Sn and Cu in steel scrap are fully utilized as alloy elements in the smelting of molten steel, and the steel has selectively added micro-alloy elements such as B; during the smelting process, the alkalinity of the slag, the types of inclusion in the steel and the melting point thereof, the content of free oxygen and the content of soluble aluminum (Als) in the molten steel are controlled; and twin-roll thin-strip continuous casting is performed to cast a cast strip (11); after exiting crystallization rollers (8a, 8b), the cast strip (11) directly enters a lower sealed chamber (10) containing a non-oxidizing atmosphere, and enters an online rolling machine (13) in a sealed manner so as to undergo hot rolling, then after rolling, the strip steel is cooled by means of air atomization. The resultant steel roll can be used directly as hot-rolled checkered plate/strip, or as a finished checkered plate/strip after being cut and finished, and is widely applicable to the fields of architecture, mechanical production, automobile, bridges, transportation, ship building, etc.