Abstract: A steel wire which has an excellent fatigue limit when made into a spring is provided. A chemical composition of the steel wire according to the present embodiment 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, S: 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, Ca: 0 to 0.0050%, and Nb: 0 to 0.020%, with the balance being Fe and impurities. In the steel wire, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 500 to 8000 pieces/?m2.

Abstract: A steel wire which has an excellent fatigue limit when made into a spring is provided. A chemical composition of the steel wire according to the present embodiment 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, S: 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, Ca: 0 to 0.0050%, and Nb: 0 to 0.020%, with the balance being Fe and impurities. In the steel wire, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 500 to 8000 pieces/?m2.

Abstract: A valve spring which has an excellent fatigue limit is provided. A chemical composition of the valve spring according to the present embodiment contains, in mass %, C: 0.50 to 0.80%, Si: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less. S: 0.020% or less: Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, Ca: 0.0001 to 0.0050%, and N: 0.0100% or less, with the balance being Fe and impurities. In the valve spring, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces/?m3, and a numerical proportion of Ca sulfides with respect to a total number of oxide-based inclusions and sulfide-based inclusions is 0.20% or less.

Abstract: A steel wire which has excellent cold coiling workability, and which has an excellent fatigue limit when made into a spring is provided. A chemical composition of the steel wire according to the present, embodiment containing, in mass %, C: 0.50 to 0.80%, Si: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, and N: 0.0100% or less, with the balance being Fe and impurities. In the steel wire, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces/?m3.

Abstract: A low-carbon steel slab producing method includes: adding Ti to a molten steel decarbonized to have a carbon concentration of 0.05 mass % or less, and subsequently adding at least one of La and Ce to adjust a constitution, and producing a smelted molten steel; and pouring the smelted molten steel into a casting mold via a tundish; wherein at least one of La and Ce in a total amount of 0.2 to 1.2 times an increased amount of oxygen in the smelted molten steel during contained in the tundish is added to the smelted molten steel in the tundish, so as to obtain a steel slab having inclusions which contain oxides of Ti and at least one of La and Ce as chief components, and so as to make a composition of each of the inclusions have a mass ratio of 0.1 to 0.7, in terms of (La2O3+Ce2O3)÷TiOn (n=1˜2).

Abstract: A low-carbon steel slab producing method includes: adding Ti to a molten steel decarbonized to have a carbon concentration of 0.05 mass % or less, and subsequently adding at least one of La and Ce to adjust a constitution, and producing a smelted molten steel; and pouring the smelted molten steel into a casting mold via a tundish; wherein at least one of La and Ce in a total amount of 0.2 to 1.2 times an increased amount of oxygen in the smelted molten steel during contained in the tundish is added to the smelted molten steel in the tundish, so as to obtain a steel slab having inclusions which contain oxides of Ti and at least one of La and Ce as chief components, and so as to make a composition of each of the inclusions have a mass ratio of 0.1 to 0.7, in terms of (La2O3+Ce2O3)÷TiOn (n=1˜2).

Abstract: A low-carbon steel slab producing method includes: adding Ti to a molten steel decarbonized to have a carbon concentration of 0.05 mass % or less, and subsequently adding at least one of La and Ce to adjust a constitution, and producing a smelted molten steel; and pouring the smelted molten steel into a casting mold via a tundish; wherein at least one of La and Ce in a total amount of 0.2 to 1.2 times an increased amount of oxygen in the smelted molten steel during contained in the tundish is added to the smelted molten steel in the tundish, so as to obtain a steel slab having inclusions which contain oxides of Ti and at least one of La and Ce as chief components, and so as to make a composition of each of the inclusions have a mass ratio of 0.1 to 0.7, in terms of (La2O3+Ce2O3)÷TiOn (n=1˜2).