| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot-forged steel which has sufficiently enhanced strength though not being heat-treated and also keeps its machinability. SOLUTION: A method for manufacturing a high-strength non-heat-treated hot-forged steel having a ferrite-pearlite structure in which fine V carbides are precipitated includes: preparing a steel including, by mass%, 0.2 to 0.8% C, 0.5% or less Si, 0.4 to 1.0% Mn, 0.2 to 0.8% V, 0.05% or less S, 0.05% or less P, 0.01% or less N and the balance Fe with unavoidable impurities; heating the steel to a temperature of (Tvc+50)°C or higher (Tvc is such a temperature that V carbides in the steel having the component are completely dissolved); hot-forging the steel at a temperature between 850°C and the above temperature; cooling the steel from the finishing temperature in the hot forging step to a temperature between 550°C and 720°C at a rapid cooling rate of 1.5°C/s or higher; and cooling the steel from a temperature between 550°C and 720°C to 400°C at a cooling rate of 0.1°C/s or higher but lower than 1.5°C/s. COPYRIGHT: (C)2010,JPO&INPIT
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