CASE-HARDENING STEEL AND CASE-HARDENED STEEL MEMBER

摘要

There is provided a case-hardening steel in which in inclusion evaluation using an extreme value statistical method, when an estimated area S is 30,000 mm2, an estimated value of the maximum size (√area)S of sulfide-based inclusions present in the estimated area S is 49 μm or less and an estimated value of the maximum size (√area)Ox of oxide-based inclusions present in the estimated area S is 80 μm or less, and the number of sulfide-based inclusions having a length of more than 20 μm and a thickness of more than 2 μm per 1 mm2 is limited to 200 or less,

主权项

1. A case-hardening steel comprising, as a chemical composition, by mass %:
C: 0.05% to 0.30%; Si: 0.40% to 1.5%; Mn: 0.2% to 1.0%; S: 0.001% to 0.050%; Cr: 1.0% to 2.0%; Mo: 0.02% to 0.8%; Al: 0.001% to 0.20%; N: 0.003% to 0.03%; Nb: 0% to 0.10%; Cu: 0% to 0.2%; Ni: 0% to 1.5%; V: 0% to 0.20%; Ca: 0% to 0.0050%; Mg: 0% to 0.0050%; Sb: 0% to 0.050%; P: limited to 0.030% or less; O: limited to 0.0020% or less; Ti: limited to 0.005% or less; and a balance consisting of Fe and impurities, wherein the following Equations (1) and (2) are satisfied, in inclusion evaluation using an extreme value statistical method, when an estimated area S is 30,000 mm2, an estimated value of the maximum size (√area)S of sulfide-based inclusions present in the estimated area S is 49 μm or less and an estimated value of the maximum size (√area)Ox of oxide-based inclusions present in the estimated area S is 80 μm or less, and the number of sulfide-based inclusions having a length of more than 20 μm and a thickness of more than 2 p.m per 1 mm2 is limited to 200 or less,
12×Si(%)+25×Mn(%)+Cr(%)+2×Mo(%)≦25   (1),31×Si(%)+15×Mn(%)+23×Cr(%)≧50   (2), here, Si(%), Mn(%), Cr(%) and Mo(%) represent the amount of each element by mass % in Equations (1) and (2).