Case hardening steel, carburized component, and manufacturing method of case hardening steel

Abstract

A case hardening steel includes, as a chemical composition, by mass %, C: 0.10% to 0.30%, Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S: 0.003% to 0.020%, Cr: 0.40% to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to 0.20%, Bi: 0.0001% to 0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0% to 0.50%, B: 0% to 0.0050%, Nb: 0% or more and less than 0.040%, P: limited to 0.050% or less, N: limited to 0.0060% or less, O: limited to 0.0025% or less, and a remainder including an iron and impurities, and satisfies Ti / S≥6.0, in which, in a longitudinal section, a maximum diameter of Ti-based precipitates predicted by extreme value statistics under a condition that an inspection standard area is 100 mm2, a number of inspections is 16 visual fields, and an area where prediction is performed is 30,000 mm2, is 40 μm or less.

Description

TECHNICAL FIELD OF THE INVENTION[0001]The present invention relates to a case hardening steel, a carburized component, and a manufacturing method of a case hardening steel, and particularly to a case hardening steel which is excellent in coarse grain prevention properties and fatigue properties during carburizing, a manufacturing method thereof, and a carburized component which is obtained from the case hardening steel.[0002]Priority is claimed on Japanese Patent Application No. 2015-256254, filed on Dec. 28, 2015, the content of which is incorporated herein by reference.RELATED ART[0003]Carburized components such as gears, bearing components, rolling components, shafts, and constant-velocity joint components are typically manufactured by a method of forging medium carbon alloy steel for machine structural use specified in, for example, JIS G 4052, JIS G 4104, JIS G 4105, and JIS G 4106, machining the forged steel into a predetermined shape, and performing carburizing and quenching ...

AI Technical Summary

Benefits of technology

The present invention provides a case hardening steel with excellent coarse grain prevention properties during carburizing, which can suppress heat treatment strain and achieve excellent fatigue properties after carburizing and quenching. The case hardening steel has a predetermined chemical composition and the maximum diameter of Ti-based precipitates is controlled to be in a predetermined range. The manufacturing method of the case hardening steel involves heating and hot rolling processes, followed by a cooling process. The carburized component made from the case hardening steel has less heat treatment strain and excellent fatigue properties.

Problems solved by technology

For example, when a shaft is bent due to heat treatment strain, the function of the shaft is impaired.

In addition, high heat treatment strain in gears and constant-velocity joint component causes noise and vibration.

The greatest cause of heat treatment strain in carburized components is coarse grains generated during carburizing.

However, since this high depth carburizing typically takes a long period of time of a dozen or so hours to several tens of hours, from the viewpoint of energy saving, a reduction in the carburizing time is required.

That is, the carburizing time can be reduced by setting the carburizing temperature, which is about 930° C. in typical carburizing, to be in a temperature range of 990° C. to 1090° C. However, when high temperature carburizing is performed in a temperature range of 990° C. to 1090° C. in order to reduce the carburizing time, coarse grains are generated, and there may be cases where fatigue properties such as rolling fatigue properties needed for carburized components are not sufficiently obtained.

In addition, gears, bearing components, and rolling components which receive high contact pressure are generally large components.

However, in the case hardening steels disclosed in Patent Documents 1 and 2, a reduction in pinning force due to coarsening of precipitates used for pinning of grains and the balance between Ti and S contents are not considered, and there is a possibility that Ti-based precipitates used for the pinning effect may be insufficient.

As a result, in the case hardening steels disclosed in Patent Documents 1 and 2, in a case where switching from hot forging to cold forging, annealing after the cold forging is omitted, or high temperature carburizing and the like are performed, there is a possibility that the coarse grain prevention properties may be insufficient.