TECHNICAL FIELD

The disclosure relates to a case hardening steel applied for machine structure components used in the field of construction machinery and automobiles, in particular, to a case hardening steel having excellent cold forgeability and excellent fatigue strength after carburizing treatment.

BACKGROUND

Since automobile components or the like are often produced by cold forming a steel bar, the material therefor is required to have good cold forgeability. Therefore, the material is normally subjected to softening annealing to spheroidize carbide and improve cold forgeability. Further, in terms of the chemical composition of steel, proposals have been made to reduce the content of Si which greatly affects deformation resistance.

JP3623313B discloses that, by reducing Si content and, further by reducing the amount of other alloying elements to such an extent as to compensate for the quench hardenability improving effect provided by dissolved B, hardness is decreased and cold forgeability is improved.

Further, JP3764586B proposes a case hardening steel ensuring cold workability obtained by combining a chemical composition where Si and Mn which are solid-solution-strengthening elements are reduced and quench hardenability is ensured by dissolved B, with certain production conditions.

The techniques disclosed in JP '313 and JP '586 utilize the quench hardenability improving effect provided by B. However, the quench hardenability improving effect of B is greatly influenced by the cooling rate. On the other hand, since most cold-forged products have complicated shapes, the cooling rate inside components at the time of carburizing and quenching tends to become non-uniform and, as a result, dimensional accuracy after carburizing treatment decreases or component strength becomes insufficient.

Further, although Ti is added to prevent a reduction in the quench hardenability improving effect of B, since nitrides of Ti are generated in the solidification stage of casting, they tend to become coarse, and become the origin of fatigue fracture to shorten the lifetime of components.

It could thus be helpful to provide a case hardening steel exhibiting good cold forgeability and having excellent fatigue strength after carburizing treatment.

SUMMARY

We discovered that by applying an appropriate chemical composition and appropriately managing the addition amount of Si, Cr, and Mn, a case hardening steel with excellent cold forgeability and fatigue strength can be obtained.

We thus provide:(1) A case hardening steel having a chemical composition containing

C: 0.10 mass % to 0.35 mass %,

Si: 0.01 mass % to 0.13 mass %,

Mn: 0.30 mass % to 0.80 mass %,

P: 0.02 mass % or less,

S: 0.03 mass % or less,

Al: 0.01 mass % to 0.045 mass %,

Cr: 0.5 mass % to 3.0 mass %,

B: 0.0005 mass % to 0.0040 mass %,

Nb: 0.003 mass % to 0.080 mass %, and

N: 0.0080 mass % or less

in a range satisfying following formulas (1) and (2),

Ti as an impurity: 0.005 mass % or less, and

the balance being Fe and incidental impurities:
3.0[% Si]+9.2[% Cr]+10.3[% Mn]≥10.0  (1)
3.0[% Si]+1.0[% Mn]<1.0  (2)
where [% M] represents the content of element M (mass %).(2) The case hardening steel according to aspect (1) wherein the chemical composition further contains one or more of

Cu: 0.5 mass % or less,

Ni: 0.5 mass % or less, and

V: 0.1 mass % or less.

A case hardening steel with both excellent cold forgeability and high fatigue strength can be provided.

