Abstract: A workpiece for a fuel injection component is made of a steel having compositions, by mass %, of C: 0.08 to 0.16%, Si: 0.10 to 0.30%, Mn: 1.00 to 2.00%, S: 0.005 to 0.030%, Cu: 0.01 to 0.30%, Ni: 0.40 to 1.50%, Cr: 0.50 to 1.50%, Mo: 0.30 to 0.70%, V: 0.10 to 0.40%, s-Al: 0.001 to 0.100%, and Fe and unavoidable impurities as remaining components. After heating the workpiece to a temperature of 950° C. or more and 1350° C. or less, the workpiece is subjected to a hot forging, and thereafter cooled at an average cooling rate of 0.1° C./sec. or more in a temperature range from 800° C. to 500° C., and at the average cooling rate of 0.02° C./sec. or more and 10° C./sec. or less in the subsequent temperature range from 500° C. to 300° C. to set an area ratio of a bainite structure after hot forging to 85% or more.

Abstract: The present invention provides a carburized part which is formed by processing a steel into a shape of a part and performing a carburizing treatment on the steel, the steel having a composition consisting essentially of, in terms of % by mass: 0.10% to 0.40% of C; 0.05% to 2.00% of Si; 0.30% to 2.00% of Mn; 0.30% to 3.00% of Cr; 0.025% or less of N; and as a pinning particle forming element which forms a pinning particle by nitrification, one or two or more elements selected from: 0.020% to 0.100% of Al; 0.01% to 0.20% of Nb; and 0.005% to 0.20% of Ti, and optionally: 0.80% or less of Mo, with the remainder being Fe and inevitable impurities, in which a crystal grain size number of a surface layer of the part at a depth of 50 ?m or less from a surface is greater than 5, and the crystal grain size number of an inner portion of the part at a depth of 3 mm or more from the surface is 5 or less.

Abstract: A workpiece for a fuel injection component is made of a steel having compositions, by mass %, of C: 0.08 to 0.16%, Si: 0.10 to 0.30%, Mn: 1.00 to 2.00%, S: 0.005 to 0.030%, Cu: 0.01 to 0.30%, Ni: 0.40 to 1.50%, Cr: 0.50 to 1.50%, Mo: 0.30 to 0.70%, V: 0.10 to 0.40%, s-Al: 0.001 to 0.100%, and Fe and unavoidable impurities as remaining components. After heating the workpiece to a temperature of 950° C. or more and 1350° C. or less, the workpiece is subjected to a hot forging, and thereafter cooled at an average cooling rate of 0.1° C./sec. or more in a temperature range from 800° C. to 500° C., and at the average cooling rate of 0.02° C./sec. or more and 10° C./sec. or less in the subsequent temperature range from 500° C. to 300° C. to set an area ratio of a bainite structure after hot forging to 85% or more.

Abstract: The present invention provides a carburized part which is formed by processing a steel into a shape of a part and performing a carburizing treatment on the steel, the steel having a composition consisting essentially of, in terms of % by mass: 0.10% to 0.40% of C; 0.05% to 2.00% of Si; 0.30% to 2.00% of Mn; 0.30% to 3.00% of Cr; 0.025% or less of N; and as a pinning particle forming element which forms a pinning particle by nitrification, one or two or more elements selected from: 0.020% to 0.100% of Al; 0.01% to 0.20% of Nb; and 0.005% to 0.20% of Ti, and optionally: 0.80% or less of Mo, with the remainder being Fe and inevitable impurities, in which a crystal grain size number of a surface layer of the part at a depth of 50 ?m or less from a surface is greater than 5, and the crystal grain size number of an inner portion of the part at a depth of 3 mm or more from the surface is 5 or less.