Steel for machine structural use and induction-hardened steel component

Abstract

There is provided a steel for machine structural use including, as a chemical composition, by mass %, C: 0.40% to 0.70%, Si: 0.15% to 3.00%, Mn: 0.30% to 2.00%, Cr: 0.01% or more and less than 0.50%, S: 0.003% to 0.070%, Bi: more than 0.0001% and 0.0050% or less, N: 0.0030% to 0.0075%, Al: 0.003% to 0.100%, and P: 0.050% or less; and as necessary, B, Mo, Ni, Cu, Ca, Mg, Zr, Rem, Ti, Nb, V, Sb, Te, and Pb, a remainder including Fe and impurities, in which Expressions 290×C+50×Si+430≥620 and d+3σ<20 are satisfied, and a presence density of MnS having an equivalent circle diameter of smaller than 2.0 μm is 300 pieces / mm2 or more in a cross section parallel to a longitudinal direction.

Description

TECHNICAL FIELD OF THE INVENTION[0001]The present invention relates to a steel for machine structural use and an induction-hardened steel component, and particularly relates to an induction-hardened steel component adopted in power transmission components (for example, gears, bearings, CVT sheaves, and shafts) used for automobiles, construction machines, farm machines, power generating windmills, and other industrial machinery, and a steel for machine structural use as a base forming material thereof.[0002]Priority is claimed on Japanese Patent Application No. 2015-205631, filed on Oct. 19, 2015, the content of which is incorporated herein by reference.RELATED ART[0003]In the related art, power transmission components such as gears have often been used after being subjected to surface-hardening treatment. As a method for surface-hardening treatment, carburizing, nitriding, or induction hardening has been employed. Among these, “carburizing” is aimed at surface-hardening performed by...

AI Technical Summary

Benefits of technology

The present invention provides a steel that can be easily machined and has strong surface fatigue strength after being hardened. This allows for the creation of excellent induction-hardened steel components that have exceptional surface fatigue strength. The steel also reduces the cost of manufacturing and improves the quality of components used in automobiles and industrial machinery.

Problems solved by technology

However, batch processing performed in a gas atmosphere is the mainstream of carburizing treatment, requiring considerable energy and cost, for example, heating to a temperature near 930° C. and retention for several hours or longer.

In addition, in an actual operation, there are also a problem of environmental deterioration accompanying treatment of carburizing materials and a problem of difficulties in realizing in-line processing.

However, in this case, since hardness of the steel increases before cutting, machinability deteriorates.

That is, in a case where components which have been manufactured through carburizing are manufactured through induction hardening, the biggest problem is the machinability of a steel, and there are demands for a steel of which machinability does not deteriorate even though the steel is hardened due to the increased C content.

However, in the technology of Patent Document 1, although the C content contained in a steel is needed to be at least 0.35% or more in order to acquire appropriate surface hardness of a component to be subjected to induction hardening, no examination has been performed in regard to the improvement of machinability.

Therefore, in the steel for induction hardening of Patent Document 1, workability, particularly machinability is low.

In such steels having high hardness, there is a high possibility that breakage such as chippage of a tool will occur during cutting, and applicable cutting conditions are limited, and there are cases where productivity is degraded.

Although Patent Document 3 describes that machinability of a steel for induction hardening is improved, since no comparative examination is performed between machinability of a steel of the invention and machinability of a case hardening steel, it is uncertain whether favorable machinability can be achieved to the same extent as that of a case hardening steel.

In addition, in Patent Document 3, since the Al content needs to be more than 0.100%, austenitic transformation is unlikely to be completed during induction heating, and there is concern that hardenability is degraded.

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