A method for producing a machine component excellent in pitting resistance characteristics and toughness includes a carburizing step, performed on a steel material containing 0.13-0.30% C and 0.90-2.00% Cr in mass % and at least one of Si, Mn, Ni, Mo, Nb, V, Ti, B, Al, and N, balance Fe and unavoidable impurities; heating the material to 850-1030° C. to attain carbon concentration in a surface of 0.8-1.5%; cooling the material at an average rate of 5° C./sec or lower from a temperature higher than the A.sub.cm point of a surface layer to a cooling end temperature that is at least 50° C. lower than the A.sub.1 point to cause the surface layer to have a pearlite or bainite structure with dispersion; spheroidizing annealing at a temperature not higher than the A.sub.cm point at the surface layer; heating the material to not higher than the A.sub.cm point at the surface layer; and performing tempering.

A method for producing a machine component excellent in pitting resistance characteristics and toughness includes a carburizing step, performed on a steel material containing 0.13-0.30% C and 0.90-2.00% Cr in mass % and at least one of Si, Mn, Ni, Mo, Nb, V, Ti, B, Al, and N, balance Fe and unavoidable impurities; heating the material to 850-1030° C. to attain carbon concentration in a surface of 0.8-1.5%; cooling the material at an average rate of 5° C./sec or lower from a temperature higher than the A.sub.cm point of a surface layer to a cooling end temperature that is at least 50° C. lower than the A.sub.1 point to cause the surface layer to have a pearlite or bainite structure with dispersion; spheroidizing annealing at a temperature not higher than the A.sub.cm point at the surface layer; heating the material to not higher than the A.sub.cm point at the surface layer; and performing tempering.

The present invention relates to a steel for a mold, which has a composition containing, on % by mass basis: 0.35%≤C≤0.40%, 0.003%≤Si≤0.20%, 0.72%≤Mn≤0.94%, 5.65%≤Cr≤6.00%, 1.65%≤Mo≤2.00%, 0.71%≤V≤0.90%, and 0.001%≤N≤0.080%, with the balance being Fe and inevitable impurities.

The present invention relates to a steel for a mold, which has a composition containing, on % by mass basis: 0.35%≤C≤0.40%, 0.003%≤Si≤0.20%, 0.72%≤Mn≤0.94%, 5.65%≤Cr≤6.00%, 1.65%≤Mo≤2.00%, 0.71%≤V≤0.90%, and 0.001%≤N≤0.080%, with the balance being Fe and inevitable impurities.

Provided is a steel material which can achieve excellent fatigue strength even when a carburized steel component is produced by welding before carburizing treatment. The steel material has a chemical composition containing: in mass %, C: 0.09 to 0.16%, Si: 0.01 to 0.50%, Mn: 0.40 to 0.60%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90 to 2.00%, Mo: 0.10 to 0.40%, Al: 0.005 to 0.030%, Ti: 0.010 to less than 0.050%, Nb: 0.010 to 0.030%, N: 0.0080% or less, O: 0.0030% or less, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.0050%, and the balance: Fe and impurities, and satisfying Formula (1) to Formula (3) according to the description. In a cross section parallel to an axial direction of the steel material, an amount of Mn sulfide is 70.0 pieces/mm.sup.2 or less, and an amount of oxide is 25.0 pieces/mm.sup.2 or less.

The present invention provides a novel heat exchange medium to replace lead. A carbon-steel wire 1A heated in a heating furnace 11 is passed through a bath 12A filled with a liquid-phase Mg—Al—Ca alloy 20 obtained by melting a Mg—Al—Ca alloy in which the main constituent elements are Mg (magnesium), Al (aluminum) and Ca (calcium). When it passes through the bath 12A, the carbon-steel wire 1A, which has been heated for example to about 950° C. in the heating furnace 11, is cooled to about 550° C. The Mg—Al—Ca alloy is non-toxic and has no environmental impact as well.

The steel material for a carburized bearing part according to the present invention contains, by mass %, C: 0.25 to 0.45%, Si: 0.15 to 0.45%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.60 to 2.00%, Mo: 0.10 to 0.35%, V: 0.20 to 0.40%, Al: 0.005 to 0.100%, Ca: 0.0002 to 0.0010%, N: 0.0300% or less and O: 0.0015% or less, with the balance being Fe and impurities, and satisfies Formulae (1) to (3).
1.20<0.4Cr+0.4Mo+4.5V<2.75  (1)
A1/A2>0.50  (2)
2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V>2.55  (3)
Formula (2) shows an area fraction of sulfides containing Ca in an amount of 1 mol % or more among sulfides having an equivalent circular diameter of 1 μm or more.

The steel material for a carburized bearing part according to the present invention contains, by mass %, C: 0.25 to 0.45%, Si: 0.15 to 0.45%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.60 to 2.00%, Mo: 0.10 to 0.35%, V: 0.20 to 0.40%, Al: 0.005 to 0.100%, Ca: 0.0002 to 0.0010%, N: 0.0300% or less and O: 0.0015% or less, with the balance being Fe and impurities, and satisfies Formulae (1) to (3).
1.20<0.4Cr+0.4Mo+4.5V<2.75  (1)
A1/A2>0.50  (2)
2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V>2.55  (3)
Formula (2) shows an area fraction of sulfides containing Ca in an amount of 1 mol % or more among sulfides having an equivalent circular diameter of 1 μm or more.

A damper spring having an excellent fatigue limit is provided. A damper spring according to the present embodiment includes a nitrided layer formed in an outer layer, and a core portion that is further inward than the nitrided layer. The chemical composition of the core portion consists of, in mass%, C: 0.53 to 0.59%, Si: 2.51 to 2.90%, Mn: 0.70 to 0.85%, P: 0.020% or less, 5: 0.020% or less, Cr: 1.40 to 1.70%, Mo: 0.17 to 0.53%, V: 0.23 to 0.33%, Cu: 0.050% or less, Ni: 0.050% or less, Al: 0.0050% or less, Ti: 0.050% or less, N: 0.0070% or less, and Nb: 0 to 0.020%, with the balance being Fe and impurities. In the core portion, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 500 to 8000 pieces/μm.sup.2.

The present disclosure discloses a method for producing high strength hot rolled steel. The method includes casting a steel slab of a composition, comprising in weight %: carbon (C) of about 0.45 wt. %-1.2 wt. %, manganese (Mn) of about 0.0-1.0 wt. %, silicon (Si) of about 0.0-0.5 wt. %, niobium (Nb) up-to 0.03 wt. %, sulphur (S) up-to 0.05 wt. % of S, phosphorous (P) up-to 0.05 wt. %, nitrogen (N) 0.002 wt. %-0.012 wt. % and balance being Iron (Fe) optionally along with incidental elements. The method also involves, heating, hot rolling, cooling, coiling the steel and retaining the steel at an ambient temperature to produce high strength hot rolled steel with 75-95% spheroid microstructure and 5-25% pearlite microstructure.