1. Field of the Invention
The present invention relates to a throw-away tip made of cermet and a cutting tool having high cutting performance and, more particularly, to a throw-away tip made of cermet and a cutting tool having a fine micro structure with less variations in characteristics among individual pieces.
2. Description of Related Art
Such cermet constituted from a carbonitride phase comprising a composite metal carbonitride of Ti and at least one kind of metals of groups 4a, 5a and 6a of the Periodic Table other than Ti, and a binder phase of Co and/or Ni, and throw-away tips made of such cermet of which surface is coated with the carbonitride phase coating layer such as TiC, TiN or TiCN formed by chemical deposition or physical deposition process have been used for continuous cutting or intermittent cutting of workpiece made of steel or the like (see, for example, Japanese Unexamined Patent Publication No. 5-222551 and Japanese Unexamined Patent Publication No. 4-289003).
It has been in practice to control the grain size of the carbonitride phase of cermet in order to increase the hardness and strength of the cermet thereby to improve the wear resistance and chipping resistance of the throw-away tip. For example, it is described to control the mean grain size to 2 μm or less inside the cermet in Japanese Unexamined Patent Publication No. 5-192804 and Japanese Unexamined Patent Publication No. 6-17229.
However, although it is indispensable to prepare a fine powder as the stock material in order to control the carbonitride phase so as to consist of fine grains as described in Japanese Unexamined Patent Publication No. 5-192804 and Japanese Unexamined Patent Publication No. 6-17229, it leads to such a problem as the material powder such as carbides, nitrides and carbonitrides that constitute the carbonitride phase agglomerate, and requires it to raise the firing temperature since the material becomes difficult to sinter. As a result, melding and/or decomposition of the binding phase are accelerated, thus leading to inhomogeneous structure due to segregation of the binding phase, generation of voids on the surface and/or inside of the sintered material, resulting in considerable variations in the mechanical characteristics and cutting performance among throw-away tips thus manufactured.
Therefore, it has been necessary to set the parameters for using a replacement tool insert to the parameters of using a throw-away tip of low cutting performance when the throw-away tip that has been used in machining a predetermined number of workpieces is automatically changed regardless of the wear condition and, even when a throw-away tip having high performance is formed, the high performance cannot be utilized and the tool cost is increased.
Throw-away tips used in metal cutting operations are made mainly of cemented carbide consisting of a carbonitride phase made of tungsten carbide that is held together by a binder phase of Co (for example, Japanese Unexamined Patent Publication No. 8-57703 and Japanese Unexamined Patent Publication No. 2001-329331) and TiCN-based cermet consisting of a carbonitride phase made of composite metal carbonitride of Ti and at least one kind of metals of groups 4a, 5a and 6a of the Periodic Table other than Ti that is held together by a binder phase of Co and/or Ni (for example, Japanese Unexamined Patent Publication No. 2001-277008 and Japanese Unexamined Patent Publication No. 9-239605). Cutting tools made of cemented carbide are used in wide applications ranging from rough cutting to finishing, and throw-away tips made of TiCN-based cermet are used in finishing operations due to high wear resistance and better characteristic with regard to resistance to the reaction with steel.
In recent years, however, depletion of the resources for tungsten carbide is feared. Accordingly, demands are increasing for throw-away tips made of TiCN based cermet that show high cutting performance in wide applications, particularly in rough cutting operations.
However, the cutting tool receives greater impact during rough cutting than finishing. As a result, the throw-away tips made of TiCN-based cermet disclosed in Japanese Unexamined Patent Publication No. 2001-277008 and Japanese Unexamined Patent Publication No. 9-239605, when used in rough cutting operations, are likely to be chipped prematurely due to the impact of cutting operation, thus showing performance poorer than that of cutting tools made of cemented carbide.
Moreover, as new hard-to-cut metals such as Pb-free free cutting steel are put into use, the TiCN-based cermet of the prior art does not show sufficient cutting performance in finishing operation. Accordingly, there is a demand for cermet having better cutting performance in finishing operations to which cermet has been preferably used.