Kobe Steel, Ltd.

Nariaki Okamoto

Yosuke Shindo

Mutsuhisa Nagahama

A case hardening steel, which has excellent cold forgeability and excellent crystal grain coarsening prevention characteristics after carburization, contains, in mass %, 0.05-0.20% of C, 0.01-0.1% of Si, 0.3-0.6% of Mn, 0.03% or less of P (excluding 0%), 0.001-0.02% of S, 1.2-2.0% of Cr, 0.01-0.1% of Al, 0.010-0.10% of Ti, 0.010% or less of N (excluding 0%), and 0.0005-0.005% of B, with the balance consisting of iron and unavoidable impurities. The density of Ti-based precipitates having circle-equivalent diameters less than 20 nm in the case hardening steel is 10-100 pieces/&#x3bc;m; the density of Ti-based precipitates having diameters of 20 nm or more in the case hardening steel is 1.5-10 pieces/&#x3bc;m; and the case hardening steel has a Vickers hardness of 130 HV or less.

Case hardening steel, method for producing same, and mechanical structural part using case hardening steel

A case hardened steel with excellent cold forgeability and excellent impact properties after case hardening processing contains C, Si, Mn, S, Cr, Al, Ti, Nb, B, and N, with the balance being iron and unavoidable impurities. Of precipitates containing Ti and/or Nb, precipitates having a size of not less than 20 &#x3bc;mare at a number density of not more than 1.0/mm. Of precipitates containing Ti and/or Nb, precipitates having a size of more than 5 &#x3bc;mand less than 20 &#x3bc;mand containing Mn and S are at a number density of more than 0.7/mmand not more than 3.0/mm. The ferrite fraction is more than 77% by area.

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Kobe, Japan

Nariaki Okamoto