| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel to be carburized which is superior in cold-forgeability while securing strength and toughness in the inner part of the steel material except its carburized layer, and has a superior effect of inhibiting crystal grains from coarsening in a carburization step. SOLUTION: A slab of the steel to be carburized comprises, by mass%, 0.1 to 0.4% C, 0.5% or less Si, 2.0% or less Mn, 0.1% or less Al, 0.1 to 0.5% Ti, 0.05 to 1.0% Mo, 0.007% or less S and 0.006% or less N in a range of satisfying the following expression (1): 0.5≤([%C]/12)/[ä[%Ti]-(48[%S]/32)-(48[%N]/14)}/48+([%Mo]/96)]≤2.0, wherein [%M] represents the content (mass%) of an element M. The manufacturing method includes: heating the slab to 1,200°C or higher; then rolling the slab; subsequently heating the steel bar to 900 to 1,050°C; then rolling the steel bar; finishing the rolling at a finishing temperature of 800 to 950°C; and then cooling the bar at a temperature range of 500 to 800°C with a rate of 0.1 to 2.0°C/s. COPYRIGHT: (C)2010,JPO&INPIT
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