The present invention relates to a steel material including: 0.25 mass %?C?0.37 mass %; 0.08 mass %?V?0.28 mass %; 6.60 mass %?Mn+Cr?7.40 mass %; Mn/Cr?0.150; Mn?0.60 mass %; Cr?6.60 mass %; Cu+Ni?0.84 mass %; 0.40 mass %?Si?0.90 mass %; 0.60 mass %?Mo?2.00 mass %; 0.001 mass %?Al?0.080 mass %; and 0.003 mass %?N?0.040 mass %, with the balance being Fe and unavoidable impurities.

Provided are a QT heat treated high carbon hot rolled steel sheet, a high carbon cold rolled steel sheet, a QT heat treated high carbon cold rolled steel sheet, and manufacturing methods thereof, wherein the QT heat treated high carbon hot rolled steel sheet comprises, in weight %, C: 1.0-1.4%, Si: 0.1-0.4%, Mn: 0.1-0.8%, Cr: 0.3-11%, W: 0.05-2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, and the balance being Fe and other inevitable impurities, the average size of carbides being 0.1-20 m.

Provided are a QT heat treated high carbon hot rolled steel sheet, a high carbon cold rolled steel sheet, a QT heat treated high carbon cold rolled steel sheet, and manufacturing methods thereof, wherein the QT heat treated high carbon hot rolled steel sheet comprises, in weight %, C: 1.0-1.4%, Si: 0.1-0.4%, Mn: 0.1-0.8%, Cr: 0.3-11%, W: 0.05-2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, and the balance being Fe and other inevitable impurities, the average size of carbides being 0.1-20 m.

A die steel with a high thermal diffusion coefficient includes 0.30-0.40 wt. % of C, 0.05-0.10 wt. % of Si, 2.50-3.40 wt. % of Mo, 0.01-0.05 wt. % of Nb, 0.30-0.50 wt. % of Co, 0.01-0.05 wt. % of RE, the rest is Fe and unavoidable impurities, wherein in the die steel, P?0.15 wt. %, S?0.025 wt. %. A preparation method of the die steel includes steps of melting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering.

A die steel with a high thermal diffusion coefficient includes 0.30-0.40 wt. % of C, 0.05-0.10 wt. % of Si, 2.50-3.40 wt. % of Mo, 0.01-0.05 wt. % of Nb, 0.30-0.50 wt. % of Co, 0.01-0.05 wt. % of RE, the rest is Fe and unavoidable impurities, wherein in the die steel, P?0.15 wt. %, S?0.025 wt. %. A preparation method of the die steel includes steps of melting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering.

Disclosed are wire rods and parts with improved delayed fracture resistance, and methods for manufacturing the same. The wire rod with improved delayed fracture resistance according to the present disclosure contains, by wt %, 0.15-0.30% of C, 0.15-0.25% of Si, 0.95-1.35% of Mn, 0.030% or less of P, 0.030% or less of S, 0.015-0.030% of Ti, 0.0010-0.0040% of B, 0.0010-0.0080% of N, and Fe and inevitable impurities as the balance, and satisfies formula 1 of 2.05.5[Si]+[Mn]2.4, where [Si] and [Mn] represent the contents (wt %) of the corresponding elements.

Disclosed in the present specification are: a high-strength wire rod for cold heading, having superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, the rod being applicable to a bolt, etc.; a heat-treated component; and a method for manufacturing the same. According to an exemplary embodiment, the high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N, and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 210.sup.19/m.sup.3 or more of aluminum nitride having a diameter of 5-50 nm.

The present disclosure provides a method for controlling an amount of silicon added to ductile cast iron, a method for casting ductile cast iron, and a cast product, which relate to the technical fields of metallurgical and cast iron alloys. The method for controlling an amount of silicon added to ductile cast iron includes smelting ductile cast iron using scrap steel as a raw material. After the scrap steel is melted into molten iron, a copper alloy is added so that the molten iron has a copper equivalent of 0.8% to 1.0%, wherein the copper equivalent is controlled by formula (II). Then, ferrosilicon is added so that the content of silicon added to the molten iron satisfies formula (I).

The present disclosure provides a method for controlling an amount of silicon added to ductile cast iron, a method for casting ductile cast iron, and a cast product, which relate to the technical fields of metallurgical and cast iron alloys. The method for controlling an amount of silicon added to ductile cast iron includes smelting ductile cast iron using scrap steel as a raw material. After the scrap steel is melted into molten iron, a copper alloy is added so that the molten iron has a copper equivalent of 0.8% to 1.0%, wherein the copper equivalent is controlled by formula (II). Then, ferrosilicon is added so that the content of silicon added to the molten iron satisfies formula (I).

A high strength, high toughness steel alloy is disclosed. The alloy has the following broad weight percent composition. TABLE-US-00001 Element Broad C 0.35-0.55 Mn 0.6-1.2 Si 0.9-2.5 P 0.01 max. S 0.001 max. Cr 0.75-2.0 Ni 3.5-7.0 Mo + W 0.4-1.3 Cu 0.5-0.6 Co 0.01 max. V + (5/9) Nb 0.2-1.0 Fe Balance
Included in the balance are the usual impurities found in commercial grades of steel alloys produced for similar use and properties. Also disclosed is a hardened and tempered article that has very high strength and fracture toughness. The article is formed from the alloy having the broad weight percent composition set forth above. The alloy article according to this aspect of the invention is further characterized by being tempered at a temperature of about 500 F. to 600 F.