JP2013078837A - Surface-coated cutting tool with hard coating layer exhibiting superior chipping resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface-coated cutting tool with a hard coating layer exhibiting superior chipping and fracture resistance in high-speed intermittent cutting.SOLUTION: A hard coating layer comprises a lower layer and an upper layer formed through chemical vapor deposition, wherein (a) the lower layer is at least one Ti carbonitride layer and one or more Ti compound layers having a total average layer thickness of 3-20 μm, (b) the upper layer is an alumimum oxide layer having an average layer thickness of 1-25 μm, and the at least one Ti carbonitride layer constituting the lower layer has a columnar longitudinal growth TiCN crystal structure in which fine-grain TiCN is dispersed and distributed.

Description

  In the present invention, high-speed intermittent cutting of various steels and cast irons with high heat generation and intermittent / impact loads acting on the cutting edge, the hard coating layer has excellent chipping resistance, which makes it possible to The present invention relates to a surface-coated cutting tool (hereinafter referred to as a coated tool) that exhibits excellent cutting performance over use.

Conventionally, the surface of a tool substrate (hereinafter collectively referred to as a tool substrate) generally composed of tungsten carbide (hereinafter referred to as WC) -based cemented carbide or titanium carbonitride (hereinafter referred to as TiCN) -based cermet. In addition,
(A) Ti carbide (hereinafter referred to as TiC) layer, nitride (hereinafter also referred to as TiN) layer, carbonitride (hereinafter referred to as TiCN) layer formed by chemical vapor deposition of the lower layers. A Ti compound layer consisting of one or more of a carbon oxide (hereinafter referred to as TiCO) layer and a carbonitride oxide (hereinafter referred to as TiCNO) layer,
(B) an aluminum oxide (hereinafter referred to as Al ₂ O ₃ ) layer in which the upper layer is formed by chemical vapor deposition;
A coated tool formed by forming a hard coating layer composed of (a) and (b) above is known, and this coated tool is known to be used for cutting various steels and cast irons. It has been.
However, the above-described coated tool has a problem that chipping loss or the like is likely to occur under a cutting condition in which a heavy load is applied to the cutting edge, and the tool life is short-lived. Proposals have been made.

For example, in Patent Document 1, a hard coating layer is composed of a single layer of TiCN or a laminate of two or more layers, and one or two or more of these constituent layers are (a) vertically grown from a granular crystal structure. A crystal structure that changes to a crystal structure, (b) a crystal structure that changes from a granular crystal structure to a vertically grown crystal structure, and a crystal structure that changes from this vertically grown crystal structure to a granular crystal structure, and (c) a crystal structure that changes from a vertically elongated crystal structure to a granular crystal structure. It has been proposed to improve the chipping resistance of the coated tool by constituting one or two or more crystal structures.
In Patent Document 2, the hard coating layer is formed of a single layer or a multilayer including a columnar TiCN layer, and TiCN is located at a distance of 1/5 of the thickness of the TiCN layer from the upper end of the TiCN layer. The ratio of the horizontal average grain size d1 of the columnar crystal grains to the horizontal average grain size d2 of the TiCN columnar crystal grains at a position 2/5 of the thickness of the TiCN layer from the lower end of the TiCN layer is 1. It has been proposed to provide a coated tool that can withstand long-time cutting including intermittent cutting by having a configuration of ≦ d1 / d2 ≦ 1.3.

Japanese Patent Laid-Open No. 6-8009 JP-A-10-109206

  In recent years, there is a strong demand for energy saving and energy saving in cutting, and with this, coated tools are increasingly used under harsh conditions. Even when a coated tool is used for high-speed interrupted cutting that is accompanied by high heat generation and more intermittent and impact loads are applied to the cutting edge, the mechanical impact resistance and thermal shock resistance of the lower layer Therefore, chipping and chipping are likely to occur on the cutting edge due to a high load during cutting, and as a result, the service life is reached in a relatively short time.

  In view of the above, the inventors of the present invention have an excellent hard coating layer even when used in high-speed intermittent cutting with high heat generation and intermittent and impact loads acting on the cutting edge. As a result of earnest research on coated tools that have excellent shock absorption and, as a result, excellent chipping resistance and fracture resistance over long-term use, the following findings were obtained.

That is, in the case where the conventional lower layer including the Ti carbonitride layer is formed as the hard coating layer, the Ti carbonitride layer is formed in a columnar shape in a direction perpendicular to the substrate. Therefore, hardness and wear resistance are improved, but on the other hand, as the anisotropy of the Ti carbonitride layer increases, the toughness of the Ti carbonitride layer decreases, resulting in sufficient chipping resistance and fracture resistance. In addition, the tool life was not satisfactory.
Therefore, the present inventors have intensively researched the Ti carbonitride layer in the Ti compound layer constituting the lower layer of the hard coating layer, and as a result, relaxed the anisotropy of the Ti carbonitride layer and improved toughness. The inventors have found a novel finding that the chipping resistance and the chipping resistance of the hard coating layer can be improved by increasing the thickness.

  Specifically, at least one Ti carbonitride layer constituting the lower layer has a columnar vertically grown TiCN crystal structure, and fine TiCN is dispersed and distributed in the structure. The anisotropy of the carbonitride layer is relaxed and the toughness is increased (hereinafter referred to as a modified TiCN layer).

The modified TiCN layer having the above-described configuration can be formed by, for example, the following chemical vapor deposition method.
On the surface of the tool base, the reaction gas composition (volume%) is TiCl ₄ : 1.7 to 1.9%, TDMAT (tetrakisdimethylaminotitanium): 0.06 to 0.10%, CH ₃ CN: 0.7 _{~0.9%, N 2: 20%} , H 2: remainder, as the reaction atmosphere pressure, as 5～12KPa, the reaction atmosphere temperature, as 820-970 ° C., by performing chemical vapor deposition, fine TiCN A columnar vertically grown TiCN crystal structure in which is dispersed in the film can be obtained. Here, in the present invention, fine TiCN means a granular TiCN crystal phase or an amorphous TiCN phase or a mixed phase of a granular TiCN crystal phase and an amorphous TiCN phase. That is, when a Ti carbonitride layer is formed by the above-described chemical vapor deposition method, the fine TiCN dispersed and formed in the film is (1) a granular TiCN crystal phase due to a subtle difference in film formation conditions. (2) Amorphous TiCN phase, (3) A mixed phase of granular TiCN crystal phase and amorphous TiCN phase were confirmed. Moreover, it was confirmed that in any of the cases (1) to (3), there was no particular difference in the effect that the anisotropy of the Ti carbonitride layer described above was relaxed and the toughness was increased. Therefore, in the present invention, the above (1) to (3) are collectively referred to as fine grain TiCN.

  When the surface density of the cross section of the fine TiCN in the Ti carbonitride layer has a surface density distribution form that periodically changes at a period of 0.5 μm to 5 μm along the layer thickness direction, the surface of the fine TiCN Due to the presence of the low density region, the columnar vertically grown TiCN crystal exhibits excellent properties such as excellent hardness and wear resistance, and the presence of the high surface density region of the fine TiCN makes it excellent by the fine TiCN. In addition, it exhibits a high characteristic of shock absorption and can have the above characteristics at a high level. Therefore, the hard coating layer has excellent chipping resistance and fracture resistance, especially when it is used for high-speed intermittent cutting of steel or cast iron that generates high heat and has intermittent and impact loads on the cutting edge. The present inventors have found that excellent wear resistance can be exhibited over a long period of use.

The present invention has been made based on the above findings,
“(1) In a surface-coated cutting tool in which a hard coating layer is provided on the surface of a tool base made of tungsten carbide-based cemented carbide or titanium carbonitride-based cermet,
The hard coating layer is composed of a lower layer and an upper layer chemically vapor-deposited,
(A) The lower layer includes at least one Ti carbonitride layer, and has a total average layer thickness of 3 to 20 μm, a Ti compound layer composed of one layer or two or more layers,
(B) the upper layer is an aluminum oxide layer having an average layer thickness of 1 to 25 μm;
Consists of
The at least one Ti carbonitride layer constituting the lower layer of (a) has a columnar vertically grown TiCN crystal structure, in which fine TiCN is dispersed and distributed, and the fine TiCN Is a granular TiCN crystal phase or an amorphous TiCN phase or a mixed phase of a granular TiCN crystal phase and an amorphous TiCN phase. A surface-coated cutting tool, wherein an average particle diameter R of the fine TiCN is more than 50 nm and 300 nm or less.
(2) The surface-coated cutting according to (1), wherein the surface density of the cross section of the fine TiCN present in at least one Ti carbonitride layer constituting the lower layer is 5 to 30%. tool.
(3) (1) or (2) characterized in that the surface density of the cross section of the fine TiCN has a surface density distribution form that periodically changes along the layer thickness direction at a period of 0.5 to 5 μm. ) Surface-coated cutting tool. "
It has the characteristics.

  The present invention will be described in detail below.

Lower Ti compound layer:
It includes at least a Ti carbonitride layer, and is composed of one or two or more Ti compound layers of Ti carbide layer, nitride layer, carbonitride layer, carbonate layer and carbonitride oxide layer. The lower layer can be formed under normal chemical vapor deposition conditions, but at least one Ti carbonitride layer is formed by another method as described later. The Ti compound layer constituting the lower layer itself has a high temperature strength, and due to the presence of the Ti compound layer, the hard coating layer has a high temperature strength, and any of the upper layer composed of the tool base and Al ₂ O ₃ In addition, it has an action that contributes to improving the adhesion of the hard coating layer to the tool substrate, but if the total average layer thickness is less than 3 μm, the above action cannot be sufficiently exerted, If the total average layer thickness exceeds 20 μm, chipping is likely to occur. Therefore, the total average layer thickness was set to 3 to 20 μm.

At least one Ti carbonitride layer in the lower layer:
At least one Ti carbonitride layer in the lower layer has a columnar vertically grown TiCN crystal structure, and fine TiCN is dispersed and distributed in the structure. By adopting such a configuration, impact resistance is improved, and excellent chipping resistance is exhibited. However, for each crystal grain of the columnar vertically grown TiCN crystal, when the particle width in the direction parallel to the substrate surface is w and the average value is the average particle width W, the maximum particle width of the average particle width W is more than 50 nm. If it is small, the wear resistance over long-term use cannot be ensured. On the other hand, if it exceeds 2000 nm, the chipping resistance and chipping resistance deteriorate due to the coarsening of the particles. Therefore, the average particle width W of the columnar vertically grown TiCN crystal is preferably 50 to 2000 nm. In addition, for each crystal grain of the columnar vertically grown TiCN crystal, the grain length in the direction perpendicular to the substrate surface is l, the ratio of the grain width w and l is the aspect ratio a of each crystal grain, When the average aspect ratio A determined for the crystal grains is the average aspect ratio A, if the average aspect ratio A is smaller than 5, the high wear resistance characteristic of the columnar vertically grown TiCN cannot be ensured. On the other hand, the toughness is lowered, and the chipping resistance and fracture resistance are lowered. Therefore, the average aspect ratio A of the columnar vertically grown TiCN crystal is desirably 5-50. Here, in the present invention, when one particle of the columnar vertically grown TiCN crystal is measured, the constant direction maximum diameter in the direction parallel to the substrate surface is called the particle width w, while the constant direction tangent in the direction perpendicular to the substrate surface. The diameter is called the particle length l.
Further, for fine TiCN, when the particle size of each fine TiCN is r and the average value is the average particle size R, if the average particle size R is 50 nm or less, the impact resistance by dispersing and distributing fine TiCN On the other hand, if it exceeds 300 nm, the toughness is lowered. Therefore, the average particle diameter R of the fine TiCN is preferably more than 50 nm and 300 nm or less. Here, in the present invention, the long axis diameter, which is the longest diameter of the precipitated phase of each fine TiCN, is called the particle diameter r of the fine TiCN.
In addition, if the surface density of the fine TiCN is less than 5%, the effect of dispersing and distributing the fine TiCN is not exhibited, while if it exceeds 30%, the growth of the columnar vertically grown TiCN crystal is inhibited, On the contrary, the wear resistance decreases. Therefore, the surface density of the cross section of the fine TiCN is desirably 5 to 30%. Further, the fine TiCN is not uniformly distributed, but is further improved in impact resistance by adopting a surface density distribution form in which the surface density periodically changes along the layer thickness direction with a period of 0.5 to 5 μm. Improves.
Hereinafter, the Ti carbonitride layer modified as described above is referred to as a “modified TiCN layer”.

Upper layer Al ₂ O ₃ layer:
Al ₂ O ₃ layer constituting the upper layer is already well known to have high temperature hardness and heat resistance, but if the average layer thickness is less than 1 μm, it will ensure wear resistance over a long period of use. On the other hand, if the average layer thickness exceeds 25 μm, the Al ₂ O ₃ crystal grains are liable to be coarsened. Since the chipping property and chipping resistance are lowered, the average layer thickness is set to 1 to 25 μm.

Formation of dispersed finely divided TiCN:
The fine TiCN of the present invention can be formed by performing chemical vapor deposition under the following conditions during the formation process of the lower layer formed under normal chemical vapor deposition conditions.
By adding TDMAT, which is the core of fine TiCN, to the reaction gas, finely divided TiCN having a distributed distribution is formed.
Reaction gas composition (volume%):
TiCl _4: 1.7~1.9%,
TDMAT: 0.06-0.10%
CH ₃ CN: 0.7~0.9%
N ₂ : 20%
H ₂ : residual reaction atmosphere temperature: 820 to 970 ° C.
Reaction atmosphere pressure: 5-12 kPa,
FIG. 1 is a schematic diagram showing a fine TiCN distribution form of a modified TiCN layer included in the lower layer of the present invention formed under the above chemical vapor deposition conditions.
Further, the fine TiCN is formed to have a surface density distribution form in which the surface density periodically changes along the layer thickness direction with a period of 0.5 to 5 μm by periodically changing the amount of TDMAT added. The FIG. 2 shows a schematic diagram thereof.
This will be described in more detail with reference to FIG.
FIG. 3 is a surface density distribution pattern diagram showing an example of a layer thickness direction position-surface density correlation in a lower layer having a surface density distribution in which the fine TiCN of the present invention formed under the chemical vapor deposition conditions changes periodically. Indicates.
This surface density distribution pattern can be obtained by the following method.
First, the lower layer is divided into 0.1 μm thickness width regions parallel to the tool base surface (in FIG. 4, the sections partitioned by a plurality of parallel lines drawn in parallel to the tool base surface are 0. It corresponds to a thickness width region of 1 μm), and the area occupied by fine TiCN existing in each divided thickness width region is measured over a total length of 10 μm and measured using a scanning electron microscope (magnification 50000 times). Then, the surface density (%) of the thickness width region of 0.1 μm is obtained, and the surface density obtained in each thickness width region is graphed along the layer thickness direction, whereby a layer as shown in FIG. Create a surface density distribution pattern in the thickness direction.
FIG. 5 is a diagram schematically showing the growth state of columnar vertically grown TiCN crystal grains in the columnar vertically grown TiCN crystal structure layer.

  In the present invention, the structure in which fine TiCN is dispersed and distributed in the columnar vertically grown TiCN crystal structure has a structure in which each columnar vertically elongated growth occurs when force is applied to the columnar vertically grown TiCN crystal structure due to the presence of the fine TiCN. Since a shift occurs in the TiCN crystal, a large toughness is generated. As a result, the impact absorption is enhanced, and the effect of improving chipping resistance and chipping resistance is exhibited.

  The coated tool of the present invention comprises a chemically vapor-deposited lower layer and an upper layer as a hard coating layer, (a) the lower layer includes at least one Ti carbonitride layer, and 3 to 3 Ti compound layer composed of one or more layers having a total average layer thickness of 20 μm, (b) the upper layer is an aluminum oxide layer having an average layer thickness of 1 to 25 μm, and constitutes the lower layer At least one Ti carbonitride layer has a columnar vertically grown TiCN crystal structure, and fine TiCN is dispersed and distributed in the structure, which is accompanied by high heat generation of steel, cast iron, etc. Even when used for high-speed intermittent cutting where intermittent and impact high loads are applied to the cutting edge, it has excellent chipping resistance and chipping resistance, resulting in excellent wear resistance over a long period of use. Longer life of coated tools achieved It is what is done.

The schematic diagram of the fine grain TiCN distribution form of the modified TiCN layer contained in the lower layer of the present invention is shown. The schematic diagram of the modified TiCN layer contained in the lower layer which takes the form of surface density distribution which changes periodically is shown. The area density distribution form figure showing the correlation of the layer thickness direction position-area density in a lower layer is shown. The schematic model for demonstrating the method of calculating | requiring the surface density distribution form figure of FIG. 3 is shown. The figure which represented typically the growth state of the columnar vertically grown TiCN crystal grain in the columnar vertically grown TiCN crystal structure layer is shown.

  Next, the coated tool of the present invention will be specifically described with reference to examples.

As raw material powders, WC powder, TiC powder, ZrC powder, VC powder, TaC powder, NbC powder, Cr ₃ C ₂ powder, TiN powder, and Co powder each having an average particle diameter of 1 to 3 μm are prepared. The raw material powder is blended in the blending composition shown in Table 1, added with wax, ball mill mixed in acetone for 24 hours, dried under reduced pressure, and press-molded into a green compact of a predetermined shape at a pressure of 98 MPa. The green compact is vacuum-sintered in a vacuum of 5 Pa at a predetermined temperature within a range of 1370 to 1470 ° C. for 1 hour. After sintering, the cutting edge is subjected to a honing process of R: 0.07 mm. Thus, tool bases A to E made of a WC-base cemented carbide having an insert shape specified in ISO · CNMG120212 were manufactured.

In addition, as raw material powders, TiCN (mass ratio TiC / TiN = 50/50) powder, Mo ₂ C powder, ZrC powder, NbC powder, TaC powder, WC powder, all having an average particle diameter of 0.5 to 2 μm. Co powder and Ni powder are prepared, and these raw material powders are blended in the blending composition shown in Table 2, wet mixed by a ball mill for 24 hours, dried, and pressed into a compact at a pressure of 98 MPa. The green compact was sintered in a nitrogen atmosphere of 1.3 kPa at a temperature of 1540 ° C. for 1 hour. After sintering, the cutting edge portion was subjected to a honing process of R: 0.09 mm. Tool bases a to e made of TiCN-based cermet having an insert shape of standard / CNMG12041 were formed.

Next, a normal chemical vapor deposition apparatus is used on the surfaces of these tool bases A to E and tool bases a to e,
(A) As a lower layer of the hard coating layer, a Ti compound layer is formed by vapor deposition under the conditions shown in Tables 3 and 4 and the target layer thickness shown in Table 6.
(B) At this time, when the Ti carbonitride layer constituting the Ti compound layer is formed under the k to o conditions shown in Table 4, the maximum value and the minimum value of the TDMAT capacity% shown in Table 4 The Ti compound layer is formed by vapor deposition while periodically changing the addition amount.
(C) Next, the coated tool of the present invention is formed by vapor-depositing a hard coating layer comprising the upper layer (Al ₂ O ₃ layer) having the target layer thickness shown in Table 6 under the conditions shown in Table 3. 15 was produced.

When at least one modified TiCN layer in the lower layer of the inventive coated tools 1 to 10 is observed over a plurality of fields using a scanning electron microscope (magnification 50000 times), the film shown in FIG. The film structure in which fine grain TiCN exists in the grain boundaries and grains of the columnar crystals shown in the structural schematic diagram was confirmed.
Further, when at least one modified TiCN layer in the lower layer of the coated tools 11 to 15 of the present invention was observed over a plurality of fields using a scanning electron microscope (magnification 50000 times), it is shown in FIG. The film structure in which fine grain TiCN exists in the grain boundaries and in the grains of the columnar crystals shown in the schematic diagram of the film configuration was confirmed.
Further, at least one modified TiCN layer in the lower layer of the coated tools 1 to 15 of the present invention is observed over a plurality of fields using a transmission electron microscope (magnification 200000 times), and the fine TiCN is an electron. As a result of line diffraction, the fine TiCN was confirmed to be a granular TiCN crystal phase or an amorphous TiCN phase or a mixed phase of a granular TiCN crystal phase and an amorphous TiCN phase.

For comparison purposes, the surfaces of the tool bases A to E and the tool bases a to e are the same as the coated tools 1 to 15 of the present invention under the conditions shown in Tables 3 and 5 and the target layer thicknesses shown in Table 7. Then, a Ti compound layer as a lower layer of the hard coating layer was formed by vapor deposition. At this time, in order to form the Ti carbonitride layer constituting the Ti compound layer, TDMAT was not added, and a columnar vertically grown TiCN crystal structure was formed.
Next, as an upper layer of the hard coating layer, the upper layer composed of Al ₂ O ₃ layers was formed by vapor deposition under the conditions shown in Table 3 and the target layer thicknesses shown in Table 7, thereby comparing the comparative coating tools 1 to 1 in Table 7. 15 was produced.

Moreover, the layer thickness of each component layer of this invention coated tool 1-15 and comparative coated tool 1-15 is measured using a scanning electron microscope (5000 times magnification), and the layer thickness of five points in an observation visual field is obtained. When the average layer thickness was determined by measurement and averaged, the average layer thickness was substantially the same as the target layer thickness shown in Tables 6 and 7.
Moreover, about this invention coated tool 1-15 and comparative coated tool 1-15, the columnar vertically long growth which comprises the Ti carbonitride layer contained in a lower layer similarly using a scanning electron microscope (magnification 5000 times). The grain width w and grain length l of the TiCN crystal were measured for columnar vertically grown TiCN crystals existing in the range of a total length of 10 μm in the horizontal direction with respect to the tool substrate, and the average value of the grain width w obtained for each crystal grain. The average aspect ratio A, which is the average value of the aspect ratio a defined as the ratio of the average grain width W and the grain width w and the grain length 1 obtained for each crystal grain, was obtained.
Moreover, about this invention coated tools 1-10, the area which fine TiCN which exists in the carbonitride layer of Ti contained in a lower layer occupies the tool base | substrate similarly using a scanning electron microscope (50000 times magnification). The vertical direction was measured over the thickness of the TiCN film thickness, and the tool base and the horizontal direction were measured over a total length of 10 μm to determine the surface density (%) of the cross section.
For the coated tools 11 to 15 of the present invention, using a scanning electron microscope (50000 times magnification), the Ti carbonitride layer contained in the lower layer is 0.1 μm thick in parallel with the tool base surface. The area of fine TiCN existing in each divided width area is measured over a total length of 10 μm, and the cross-sectional surface of the fine TiCN existing in the thickness area of 0.1 μm Density (%) was determined.

Next, with respect to the inventive coated tools 1-15 and comparative coated tools 1-15, a cutting test was performed under the conditions shown in Table 8, and the flank wear width of the cutting edge was measured in any cutting test.
Table 9 shows the measurement results.

  From the results shown in Table 6 and Table 9, in the coated tool of the present invention, the Ti carbonitride layer constituting the lower layer of the hard coating layer has a columnar vertically grown TiCN crystal structure, Even if it is used for high-speed intermittent cutting with high heat generation of steel, cast iron, etc., and intermittent / impact high loads acting on the cutting edge due to the distribution of fine TiCN in It is apparent that it has excellent chipping resistance, and as a result, exhibits excellent cutting performance over a long period of use.

  On the other hand, the comparative coated tools 1 to 15 in which the fine TiCN is not dispersed and distributed in the Ti carbonitride layer constituting the lower layer of the hard coating layer is accompanied by high heat generation and intermittently on the cutting edge.・ When used for high-speed intermittent cutting with high impact loads, it is clear that chipping, chipping, etc. will lead to short life.

  As described above, the coated tool of the present invention has excellent chipping resistance in high-speed intermittent cutting with high heat generation such as steel and cast iron and intermittent and impact high load acting on the cutting edge. It exhibits defect resistance and can extend the service life.

Claims (3)

In a surface-coated cutting tool in which a hard coating layer is provided on the surface of a tool base composed of tungsten carbide-based cemented carbide or titanium carbonitride-based cermet,
The hard coating layer is composed of a lower layer and an upper layer chemically vapor-deposited,
(A) The lower layer includes at least one Ti carbonitride layer, and has a total average layer thickness of 3 to 20 μm, a Ti compound layer composed of one layer or two or more layers,
(B) the upper layer is an aluminum oxide layer having an average layer thickness of 1 to 25 μm;
Consists of
The at least one Ti carbonitride layer constituting the lower layer of (a) has a columnar vertically grown TiCN crystal structure, in which fine TiCN is dispersed and distributed, and the fine TiCN Is a granular TiCN crystal phase or an amorphous TiCN phase or a mixed phase of a granular TiCN crystal phase and an amorphous TiCN phase. A surface-coated cutting tool, wherein an average particle diameter R of the fine TiCN is more than 50 nm and 300 nm or less.   2. The surface-coated cutting tool according to claim 1, wherein the surface density of the cross-section of the fine TiCN existing in at least one Ti carbonitride layer constituting the lower layer is 5 to 30%. 3. The surface density distribution form in which the surface density of the cross section of the fine TiCN periodically changes along the layer thickness direction at a period of 0.5 to 5 [mu] m. Surface coated cutting tool.