The steel for hot carburization and the hot forging component null mass for hot carburization which are superior in hot

摘要

PROBLEM TO BE SOLVED: To provide a high temperature carburizing steel excellent in high temperature carburizability, capable of preventing the reduction of hardness in an outermost layer as well as suppressing the occurrence of coarse grains in a high temperature carburizing/quenching process and also capable of attaining desired hardening layer depth, and also provide a hot forged member for high temperature carburizing. ;SOLUTION: The steel and the member have a composition in which 0.1-0.5% C, 0.015-0.05% Al, 0.005-0.05% Nb and 0.006-0.02% N are contained and respective contents of P, Ti, V, B, Mg and O are limited. Further, the amount of precipitation of Nb(CN) and AlN after hot rolling is specified, the structure fraction of bainite is made to ≤30%, and the score of ferrite banding in a structure in the cross section parallel to a hot rolling direction is made to 1-5. Moreover, hardness Hv is regulated to a value ranging from (H-20) to (H+40) (where H is a hardness index defined by equation H=273.5C%+39.1Si%+54.7Mn%+30.4Cr%+136.7Mo%+18.2Ni%+128.7Nb%).;COPYRIGHT: (C)2001,JPO