Carburizing steel having excellent cold forgeability and method of manufacturing the same

Abstract

A carburizing steel has a composition containing, in mass %, C: 0.1-0.35%; Si: 0.01-0.22%; Mn: 0.3-1.5%; Cr: 1.35-3.0%; P: 0.018% or less; S: 0.02% or less; Al: 0.015-0.05%; N: 0.008-0.015%; and O: 0.0015% or less, each being contained in an amount within a range satisfying formulas (1), (2) and (3) below, and the balance of the composition being Fe and incidental impurities, and the carburizing steel having microstructures before spheroidizing annealing such that a total microstructure proportion of ferrite and pearlite is 85% or more and an average ferrite grain size is 25 μm or less.3.1≧{([% Si] / 2)+[% Mn]+[% Cr]}≧2.2   (1)[% C]−([% Si] / 2)+([% Mn] / 5)+2[% Cr]≧3.0   (2)2.5≧[% Al] / [% N]≧1.7   (3)[% M] represents content (in mass %) of element M.

Description

TECHNICAL FIELD[0001]The present invention relates to a carburizing steel having excellent cold forgeability, which can be suitably used in, for example, automobiles and other various industrial equipment, and a method for manufacturing the same.BACKGROUND ART[0002]As vehicle weight is reduced more and more for better energy saving, there is an increasing demand for gears of smaller sizes for use in automobiles or the like, and the gears have to bear increasingly higher load exerted thereon. Furthermore, such gears have to bear increasingly higher load exerted thereon as the engine output is made higher and higher. Durability of a gear is primarily determined by degrees of gear tooth root fracture caused by bending fatigue and pitting fatigue fracture of gear tooth surface.[0003]Conventionally, gears have been manufactured by preparing gear materials with case hardening steel, which is specified in JIS G 4053 (2003) as SCM420H, SCM822H and so on, and subjecting the gear materials to...

AI Technical Summary

Benefits of technology

The present invention concerns a type of steel that can be used to make parts like gears. By subjecting it to a specific process involving cold forging, the steel becomes better at resisting fatigue when bent or contacted. This makes it easier and more efficient to create parts that can withstand fatigue stresses and have good strength.

Problems solved by technology

However, since such gears are not endurable in use under high stress conditions, attempts have been made to improve strength against bending fatigue of a gear tooth root and pitching resistance by changing steel materials, changing heat treatment methods, and furthermore, using work hardening processes on surfaces, and so on.

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