High-Strength Steel Machined Product and Method for Manufacturing the Same, and Method for Manufacturing Diesel Engine Fuel Injection Pipe and Common Rail

A high-strength steel machined product having excellent hardenability, comprising:

• 0.1 to 0.7% of C;
• 2.5% or less (excluding 0%) of Si;
• 0.5 to 3% of Mn;
• 1.5% or less of Al;
• 0.01 to 0.3% as the sum of one or more of Nb, Ti, and V;
• 2.0% or less (excluding 0%) of Cr;
• 0.5% or less (excluding 0%) of Mo;
• 2.0% or less of Ni;
• 0.7 to 3.0% as the sum of two or more of Cr, Mo, and Ni;
• 0.75 to 0.90% of carbon equivalent (Ceq) defined by the following formula;
• and the balance of Fe and inevitable impurities,
• wherein the metal structure is composed of a mother-phase structure containing 50% or more (volume percentage to the entire structure, same is applied to the following structures) of lathy bainitic ferrite and 20% or less as the sum of polygonal ferrite and granular bainitic ferrite, and a secondary-phase structure has 5 to 30% of retained austenite and 5% or less of martensite.
• Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 Formula