TECHNICAL FIELD

The present invention relates to a coated cutting tool.

BACKGROUND ART

It is well known to use a coated cutting tool in which a coating layer having a total thickness of 3 μm to 20 μm is formed by a chemical vapor deposition method on the surface of a substrate composed of a cemented carbide, for cutting steel, cast iron, and the like. Known examples of the above-mentioned coating layer include a coating layer consisting of a single layer of one kind selected from the group consisting of a Ti carbide, nitride, carbonitride, oxycarbide and oxycarbonitride, and aluminum oxide, or consisting of multiple layers of two or more kinds selected from the foregoing.

Patent Publication JP-A-06-158325 describes a surface-coated cermet cutting tool with improved wear resistance of a hard coating layer, in which a hard coating layer comprising multiple layers of two or more kinds selected from a Ti carbide, nitride, carbonitride and oxycarbonitride, and aluminum oxide is formed by chemical vapor deposition on the surface of a cermet substrate made of a tungsten carbide based cemented carbide or a titanium carbonitride based cermet, wherein at least one of the constituent layers of the hard coating layer excluding the first layer in contact with the substrate surface is composed of titanium carbonitride, and at least one of the titanium carbonitride layers is composed of titanium carbonitride showing the highest peak intensity on a (422) plane in X-ray diffraction.

Japanese Patent No. 5768308 describes a surface-coated cutting tool comprising a substrate and a coating film formed on the substrate, wherein the coating film comprises at least one TiCN layer, the TiCN layer includes a columnar crystal region, the columnar crystal region has a composition of TiCxNy, where 0.65≤x/(x+y)≤0.90, a plane spacing of (422) planes is 0.8765 Å to 0.8780 Å, and TC(422) is the largest among the orientation indexes TC(hkl).

Japanese Patent No. 5884138 describes a surface-coated cutting tool comprising a substrate and a coating film formed on the substrate, wherein the coating film comprises at least one TiCN layer, the TiCN layer includes a columnar crystal region, the columnar crystal region has a composition of TiCxNy, where 0.65≤x/(x+y)≤0.90, a plane spacing of (422) planes is 0.8765 Å to 0.8790 Å, and TC(220) is the largest among the orientation indexes TC(hkl).

SUMMARY

Technical Problem

The speed, feed and cutting depth of cutting processes have significantly increased in recent times, which, in combination with increased strength of work materials, has created a demand for further improvement in wear resistance and fracture resistance of tools compared to those involved in the prior art. In particular, in recent years, strength of cast iron has been increased to enable thickness reduction, and the number of cutting processes in which a load is exerted on a coating cutting tool, such as high-speed cutting of cast iron, has increased. When the conventional coated cutting tools such as described in Patent Publication JP-A-06-158325, Japanese Patent No. 5768308 and Japanese Patent No. 5884138 are used under such severe cutting conditions, the tool life cannot be extended due to rapid wear, or chipping and fracture may occur due to insufficient toughness.

Under such a background, the surface-coated cermet cutting tool described in Patent Publication JP-A-06-158325 may have low hardness and insufficient wear resistance due to a low carbon content in the coating layer. Further, the surface-coated cutting tool described in Japanese Patent No. 5768308 may have insufficient adhesion or insufficient wear resistance because the temperature at which the Ti carbonitride layer in the coating layer is formed is low. In the surface-coated cutting tool described in Japanese Patent No. 5884138, a certain effect can be considered to be achieved in terms of the wear resistance of the Ti carbonitride layer in the coating layer, but the tool may have insufficient toughness, which can sometimes result in insufficient chipping resistance and fracture resistance.

The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a coated cutting tool which has excellent wear resistance and fracture resistance and which accordingly allows for an extended tool life.

Solution to Problem

The present inventors have conducted extensive research on extending the tool life of a coated cutting tool from the above perspective, and have found that with the below-described configuration in which the coating layer comprises at least one Ti carbonitride layer and a texture coefficient of a (331) plane in the Ti carbonitride layer falls within a specified range, excellent balance between the wear resistance and the fracture resistance is achieved, and as a result, the tool life can be extended, even when the atomic ratio of carbon in the Ti carbonitride layer is above 0.65 and not more than 0.90.

That is, the present invention is as follows.[1]

A coated cutting tool comprising a substrate and a coating layer formed on a surface of the substrate, wherein

the coating layer comprises a lower layer and an upper layer formed on a surface of the lower layer,

the lower layer comprises a Ti compound layer comprising one or more layers selected from the group consisting of a Ti carbide layer, a Ti nitride layer, a Ti carbonitride layer, a Ti carboxide layer, and a Ti oxycarbonitride layer,

an average thickness of the lower layer is 3.0 μm or more and 15.0 μm or less,

the upper layer comprises an α-aluminum oxide layer,

an average thickness of the upper layer is 3.0 μm or more and 15.0 μm or less,

the Ti compound layer comprises at least one Ti carbonitride layer,

the Ti carbonitride layer has a composition represented by a following formula (i):
Ti(CxN1-x)  (i),wherein in the formula, x represents an atomic ratio of a C element to a total of the C element and an N element in the Ti carbonitride layer, and satisfies 0.65<x≤0.90), and

In the Ti carbonitride layer, a texture coefficient TC (331) of a (331) plane represented by a following formula (1) is 1.5 or more and 4.0 or less:

T⁢C⁡(3⁢3⁢1)=I⁡(33⁢1)I0⁡(3⁢3⁢1)⁢{18⁢∑I⁡(hkl)I0⁡(hkl)}-1(1)
wherein in the formula (1), I(hkl) represents a peak intensity of an (hkl) plane in X-ray diffraction of the Ti carbonitride layer, I0(hkl) represents an average value of a standard diffraction intensity of the (hkl) plane of Ti carbide in a JCPDS card number 32-1383 and a standard diffraction intensity of the (hkl) plane of Ti nitride in a JCPDS card number 32-1420, and (hkl) indicates eight crystal planes (111), (200), (220), (311), (331), (420), (422), and (511).[2]

The coated cutting tool according to [1], wherein the texture coefficient TC(331) in the Ti carbonitride layer is 2.0 or more and 4.0 or less.[3]

The coated cutting tool according to [1] or [2], wherein grains constituting the Ti carbonitride layer have an average grain size of 0.3 μm or more and 2.0 μm or less.[4]

The coated cutting tool according to any one of [1] to [3], wherein the coating layer comprises an outer layer formed on a surface of the upper layer,

the outer layer comprises a Ti nitride layer and/or a Ti carbonitride layer,

an average thickness of the outer layer is 0.1 μm or more and 5.0 μm or less.[5]

The coated cutting tool according to any one of [1] to [4], wherein an average thickness of the entire coating layer is 7.5 μm or more and 25.0 μm or less.[6]

The coated cutting tool according to any one of [1] to [5], wherein the substrate is composed of any of a cemented carbide, a cermet, a ceramic and a cubic boron nitride sintered body.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a coated cutting tool that has excellent wear resistance and fracture resistance, thereby making it possible to extend the tool life.

