Soft-nitrided parts made of non-heat treated steel

Abstract

Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ′ nitride of more than 20 μm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1%, N: 0.010 to 0.030%, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%. The non-heat treated steel may contain one or more elements of 0.001 to 0.1% of Nb, 0.01 to 1.0% of Mo:, 0.01 to 1.0% of Cu, 0.01 to 1.0% of Ni and 0.001 to 0.005% of B, 0.01 to 0.1% of S and 0.0001 to 0.005% of Ca. This invention provides machinery parts, based on this non-heat treated steel for soft-nitriding, which has further improved high fatigue strength and excellent bending straightening property after soft-nitrided even omitting thermal refining.

1 . Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ′ nitride of more than 20 μm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1%, N: 0.010 to 0.030%, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%.  
     
     
         2 . Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ′ nitride of more than 20 μm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1%, N: 0.010 to 0.030%, and either or both of one or more elements selected from the first group mentioned below and one or more elements selected from the second group mentioned below, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%. 
 The first group: 0.001 to 0.1% of Nb, 0.01 to 1.0% of Mo, 0.01 to 1.0% of Cu, 0.01 to 1.0% of Ni and 0.001 to 0.005% of B.    The second group: 0.01 to 0.1% of S and 0.0001 to 0.005% of Ca.