For example, for mechanical parts such as gears, bearing parts and shafts to be used in automobiles, JIS steel types such as SCr420 are generally used after having been processed into the shapes of parts and then subjected to a surface-hardening treatment by carburization hardening to improve abrasion resistance, fatigue strength and the like.
The carburization hardening is a high-temperature, long-duration heat treatment that is likely to cause coarsening of crystal grains.
For this reason, various studies and proposals to prevent crystal grains from becoming coarse have been conventionally made.
A technique of pinning grain boundaries by precipitating particles such as AlN in a dispersed state at a manufacturing step before a carburizing treatment has been widely known as a useful technique for preventing crystal grains from becoming coarse.
For example, techniques of this kind are disclosed in, for example, Patent Document 1 and Patent Document 2 below.
However, such techniques which allow pinning of grain boundaries by utilizing precipitate particles are incapable of sufficiently preventing an abnormal grain growth in which abnormal coarsening of crystal grains occurs locally.
The term “abnormal grain growth” used herein refers to a phenomenon occurring due to a cause that, though a pinning force of precipitate particles is greater than a driving force for crystal grain growth in the initial carburizing stage, the magnitude relation between these forces comes to reverse and the driving force for crystal grain growth becomes greater than the pinning force of precipitate particles in the middle of the carburizing. Such a reversal of these forces takes place through a cause that the pinning force is reduced by solid solution formation of precipitate particles during the carburizing, by coarsening of precipitates through Ostwald growth, and the like.
In addition, as to the parts which are subjected to cold forging, a distribution of plastic distortions is introduced into the inside of the parts at the time of the forging, and a reversal of magnitude takes place between the pinning force and driving force of crystal grain growth in regions where the distortion is great, thereby causing abnormal grain growth of crystal grains.
FIG. 1(B) shows the occurrence of abnormally grown grains model-wise.
(a) of FIG. 1(B) shows a state at the initial stage of carburization, and p represents a precipitate particle (a pinning particle). In the state at the initial stage of carburization, a large number of precipitate particles p are interposed between grain boundaries, and the grain boundaries between crystal grains q are pinned and restrained, thereby inhibiting the crystal grains q from growing to a larger size.
However, some of the precipitate particles p pinning grain boundaries disappear by forming a solid solution during carburization, and the pinning (restraint) by such precipitate particles p is broken (comes undone), and some adjacent pairs of crystal grains thus made free from the pinning at the grain boundaries coalesce and grow into one crystal grain.
Crystal grains which have increased in size in such a manner can gain power for grain growth, and under a relative reduction in the pinning force of precipitate particles p, each crystal grain breaks the pinning of grain boundaries by the precipitate particles p and swallows one neighboring crystal grain after another, thereby continuing the grain growth.
That is, once the grain boundary pinning by precipitate particles p has been broken, the pinning-broken crystal grain boundaries function as the center of grain growth, and from such grain boundaries, the grain growth of the crystal grain occurs chain-reactionally to develop into abnormal grain growth and finally abnormally form giant crystal grains Q as shown in (b) of FIG. 1(B).
(c) of FIG. 1(B) shows an example of abnormally-grown grains (a photograph of crystal grains after carburization).
Incidentally, the photograph of this example is a photograph of the central portion of a steel material listed as Comparative Example 1 in Table 1 in the case where the steel material has been subjected to a carburizing treatment at 1,100° C.
When such abnormal grain growth occurs, heat treatment distortion develops due to local improvement of hardenability and thus causes problems of making noises and vibrations or reducing the fatigue strength.
Conventionally, in such a case, measures have been taken so that greater precipitate particles are precipitated in a dispersed state to further improve the power of grain boundary pinning by the precipitate particles. However, occurrence of the abnormal grain growth cannot be sufficiently prevented by such measures.
Particularly in recent years, the use of a technique of raising carburization temperatures to reduce the carburizing time, a technique of performing cold forging for reduction of manufacturing costs of parts and techniques adaptable to environmental protection such as vacuum carburization performed to reduce emissions of CO2 in the middle of manufacturing and to improve the strength have been widespread. However, the abnormal grain growth has been more likely to occur under these techniques. Accordingly, there have been demands for measures allowing for effective inhibition of such abnormal grain growth.
In addition, as another background art relating to the present invention, an invention of “a case hardening steel excellent in cold workability and crystal grain coarsening properties” has been disclosed in Patent Document 3 below, and this document discloses the point that, since AlN particles currently in use for pinning crystal grain boundaries are solid-solved or increased in size thereof in a region at a temperature of 900° C. or higher and thus are unable to have much effect on prevention of grain coarsening at the time of the carburizing treatment, the prevention of grain coarsening is attempted by adding Nb and Al to steel and causing these elements to be combined with C and N, thereby forming fine composite precipitates.
However, the invention disclosed in Patent Document 3 is basically different from the present invention in a point that an excessive amount of Nb is added in contrast to the present invention in which the addition of Nb is avoided as an impurity.
As still another background art relating to the present invention, an invention of “a case hardening steel excellent in crystal grain-coarsening resisting properties, fatigue properties and machinability, and a manufacturing method thereof” has been disclosed in Patent Document 4 below, and this document has discloses the point that, without impairing the crystal grain-coarsening resisting properties, fatigue properties and machinability are improved by properly adjusting the grain size distribution of Ti precipitates in the steel.
However, the substance of the disclosure made in Patent Document 4 consists of precipitating 10 pieces/mm2 or more of Ti precipitates having a size of 1.0 μm to 5.0 μm and all the steels 1 to 26 according to the invention disclosed in Patent Document 4 include an excessive amount of Ti as compared to an amount of N and do not fall within the scope of the expression (1) in the present invention. The invention disclosed in Patent Document 4 is therefore different from the present invention.
As still another background art relating to the present invention, an invention of “a steel for carburized parts which is excellent in cold workability, allows prevention of crystal grains from coarsening at the time of carburization and has excellent impact-resisting properties and impact fatigue-resisting properties” has been disclosed in Patent Document 5 below, and this document discloses the point that Ti or both Ti and Nb are added to steel in such amounts as not to impair cold workability and machinability and allow to be precipitated in the form of carbides or nitrides thereof, thereby allowing prevention of crystal grain coarsening at the time of the carburization.
Claim 1 of Patent Document 5 discloses that the Ti content is limited to 0.1% to 0.2%, the N content is limited to 0.01% or less, and the Al content is limited to 0.005% to 0.05%. However, in Examples 1 to 11 actually disclosed therein, an excessive amount of Ti is added as compared to an amount of N, in terms of molar ratio, for precipitating TiC. The concept of this disclosure is therefore opposite to that of the present invention and outside the scope of expression (1) in the present invention.
In addition, in Claim 2 of Patent Document 5, the Ti content is limited to 0.025% to 0.05%, the Nb content is limited to 0.03% to 0.2%, the N content is limited to 0.01% or less, and the Al content is limited to 0.005% to 0.05%. Therefore, it is different from the present invention in that an excessive amount of Nb is added.