Surface-coated cutting tools

Abstract

A surface-coated cutting tool comprising a tool body and a coating layer on the tool body, wherein: the average thickness of the coating layer is 0.5-5.0 µm; the coating layer has a Ti-boride composite layer; and the Ti-boride composite layer has a crystalline phase and an amorphous phase, and has an average composition such that when expressed by the compositional formula TiBx, the atomic ratio x satisfies 1.5≤x≤3.0.

Description

[Technical Field]

[0001] The present invention relates to a surface-coated cutting tool (hereinafter sometimes referred to as a coated tool). This application claims priority to Japanese Patent Application No. 2020-55010, filed on March 25, 2020. The entire contents of said Japanese patent application are incorporated herein by reference. [Background technology]

[0002] Conventionally, coated tools have been known in which a tool substrate is made of cemented carbide or the like and a coating layer is formed on the surface of the tool substrate by a vapor deposition method. These coated tools have wear resistance, but various proposals have been made to further improve this wear resistance, including a proposal for a coating layer containing boron.

[0003] For example, Patent Document 1 proposes a coated tool having a Ti-boride layer with an average thickness of 0.5 to 5 μm on the surface of a tool substrate, the layer being configured as a composite structure of crystal grains with multiple average grain sizes, the composite structure being composed of secondary crystal grains with an average grain size of 20 to 70 nm, which are composed of aggregates of primary crystal grains with an average grain size of 10 to 15 nm, and tertiary crystal grains with an average grain size of 300 to 600 nm, which are aggregates of the secondary crystal grains. This coated tool is said to be able to suppress peeling of the soft coating layer due to welding during high-speed cutting of soft, difficult-to-cut materials.

[0004] Furthermore, for example, Patent Document 2 proposes a coated tool having a boride phase, which is a region containing a chemical bond between element M (M is one or more elements selected from Ti, W, Zr, Hf, V, Nb, Ta, Mo, and Cr) and B, and a carbide phase, which is a region containing a chemical bond between element M and C. This coated tool is said to have better wear resistance than a coated tool having a coating layer consisting only of TiB2. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-139795 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-166055 Summary of the Invention [Problem to be solved by the invention]

[0006] While cutting equipment has become increasingly sophisticated and automated, there is a growing demand for cutting difficult-to-cut materials, including titanium-based alloys and austenitic stainless steel, which are prone to welding during cutting.

[0007] The present invention has been made in view of the above circumstances and proposals, and an object of the present invention is to provide a coated tool that exhibits excellent crack resistance and wear resistance over long periods of use, particularly when used in high-speed intermittent cutting of Ti-based alloys and austenitic stainless steels.

[0008] Here, high-speed intermittent cutting of Ti-based alloys refers to cutting at a cutting speed of 70 m / min or faster, in which the cutting edge of a cutting tool alternates between cutting and idling, and high-speed intermittent cutting of austenitic stainless steel refers to cutting at a cutting speed of 100 m / min or faster, in which the cutting edge of a cutting tool alternates between cutting and idling. [Means for solving the problem]

[0009] A surface-coated cutting tool according to an embodiment of the present invention includes a tool substrate and a coating layer on the tool substrate, the coating layer is directly over the tool substrate; The coating layer has an average thickness of 0.5 to 5.0 μm, The coating layer is a Ti boride layer. and , The Ti boride layer has a composition represented by the formula: TiB x When expressed as above, the average composition satisfies the atomic ratio x of 1.5≦x≦3.0, and further has a crystalline phase composed of crystal grains with a hexagonal crystal structure and an amorphous phase. death , the crystalline phase has a hexagonal crystal structure and an average grain size of 2 to 30 nm, and the crystalline phase occupies an area ratio of 50 to 95% of the Ti boride layer; Regarding the hexagonal crystal grains constituting the crystalline phase of the Ti boride layer, when the peak intensities of the 001 diffraction line, the 100 diffraction line, and the 101 diffraction line in X-ray diffraction are Ih(001), Ih(100), and Ih(101), respectively, the following relationship is satisfied: 0.01≦Ih(001) / {Ih(001)+Ih(100)+Ih(101)}≦0.80. and the coating layer is directly overlying a lower layer directly overlying the tool substrate; the lower layer is one or more of a Ti carbide layer, a nitride layer, a carbonitride layer, a carbonate layer, and a carbonitride oxide layer, each having a total average thickness of 0.1 to 2.0 μm; The coating layer has an average thickness of 0.5 to 5.0 μm, The coating layer is a Ti boride layer. and , The Ti boride layer has a composition represented by the formula: TiB x When expressed as above, the average composition satisfies the atomic ratio x of 1.5≦x≦3.0, and further has a crystalline phase composed of crystal grains with a hexagonal crystal structure and an amorphous phase. death , the crystalline phase has a hexagonal crystal structure and an average grain size of 2 to 30 nm, and the crystalline phase occupies an area ratio of 50 to 95% of the Ti boride layer; Regarding the hexagonal crystal grains constituting the crystalline phase of the Ti boride layer, when the peak intensities of the 001 diffraction line, 100 diffraction line, and 101 diffraction line in X-ray diffraction are Ih(001), Ih(100), and Ih(101), respectively, the relationship 0.01≦Ih(001) / {Ih(001)+Ih(100)+Ih(101)}≦0.80 is satisfied.

[0010] Furthermore, the surface-coated cutting tool according to the embodiment has the following features: (1) may be satisfied.

[0012] ( 1 ) The nanoindentation hardness of the Ti boride layer is 30 to 50 GPa. [Effects of the Invention]

[0014] According to the above, even when used in high-speed intermittent cutting of Ti-based alloys and austenitic stainless steels, excellent crack resistance and wear resistance are exhibited over a long period of use. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have recognized the following points regarding the coated tools described in Patent Documents 1 and 2.

[0016] (1) The coated tool described in Patent Document 1 enables high-speed cutting of soft, difficult-to-cut materials such as Al-based alloys. However, sufficient consideration is not given to high-speed, interrupted cutting of materials that are prone to welding during cutting, such as Ti-based alloys and austenitic stainless steels.

[0017] (2) The coated tool described in Patent Document 2 exhibits excellent peeling resistance and wear resistance by suppressing welding during cutting of hard, difficult-to-cut materials such as Ti-based alloys and Al-Si alloys with a high Si content. However, further improvement in wear resistance is required.

[0018] Based on the above findings, the present inventors have conducted extensive research into Ti-boride layers as coating layers, and have made the novel discovery that wear resistance is improved when the Ti-boride coating layer has a crystalline phase and an amorphous phase.

[0019] A surface-coated cutting tool according to one embodiment of the present invention will be described below. In this specification and claims, when a numerical range is expressed as "A to B" (where A and B are both numerical values), the range includes an upper limit (B) and a lower limit (A), and the upper limit (B) and the lower limit (A) have the same units. The numerical values ​​also include tolerances.

[0020] Average coating thickness: The coating layer has a Ti boride layer, and its average layer thickness is preferably 0.5 to 5.0 μm. This is because if the average layer thickness is less than 0.5 μm, it is difficult to maintain wear resistance for a long period of time, while if it exceeds 5.0 μm, chipping is likely to occur. A more preferable range of the average layer thickness is 1.0 to 2.5 μm.

[0021] The average thickness of the coating layer is measured as follows. For example, a focused ion beam (FIB) system, a cross-section polisher (CP), or the like is used to cut the coating layer at a longitudinal cross section (a cross section perpendicular to the tool substrate surface when the tool substrate surface is treated as a flat surface, ignoring minute irregularities on the tool substrate surface) at an arbitrary position to prepare a specimen for observation. The longitudinal cross section is then observed at multiple locations (e.g., five locations) using a scanning electron microscope (SEM), and the average thickness is calculated by arithmetically averaging the layer thicknesses.

[0022] Average composition of Ti boride layer: The Ti boride layer has the composition formula: TiB x When expressed as x, it is preferable that the average composition satisfies the atomic ratio x of 1.5≦x≦3.0. The reason is that if x is less than 1.5, adhesive wear tends to progress at the contact area between the Ti boride layer and the workpiece material such as a Ti-based alloy, while if x exceeds 3.0, the crystal structure of the Ti boride becomes disordered, resulting in a decrease in hardness. A more preferable range of x is 1.8 to 2.2.

[0023] The boron content is measured as follows. Using an electron probe micro analyzer (EPMA), an electron beam is irradiated onto the surface of the coating layer or onto five locations on a longitudinal cross section of the coating layer at any desired position. The characteristic X-rays obtained from each location corresponding to the elements that make up the coating layer are analyzed to quantify the content of each element, and the results are arithmetically averaged.

[0024] Crystalline and amorphous phases of Ti boride layer: It is preferable that the Ti boride layer has a crystalline phase and an amorphous phase. Although the reason for this is unclear, the presence of the amorphous phase produces boron oxide on the abrasion surface between the Ti boride layer and the workpiece Ti-based alloy during cutting. As a result, it is assumed that solid lubrication is imparted to the Ti boride layer, improving its wear resistance.

[0025] Furthermore, the hexagonal crystal grains (hexagonal crystals) that make up the crystalline phase are preferably fine crystal grains, and more preferably have an average grain size in the range of 2 to 30 nm. The reason for this is believed to be as follows: Since the destruction units of the coating layer due to welding are crystal units, if the crystal grains are fine crystals, that is, if the crystal grains are small, the destruction units will be small. As a result, wear of the Ti-boride layer that involves destruction is suppressed, and the wear resistance of the layer is improved.

[0026] The average grain size of the crystal grains that make up the crystalline phase is determined as follows: Analysis is performed using an automated crystal orientation mapping (ACOM)-TEM with a transmission electron microscope (TEM) to define the grain boundaries. The area enclosed by the grain boundaries is then defined as a crystal grain, and the maximum length of that crystal grain is defined as the grain size. The grain size of each of five arbitrary crystal grains is determined, and the arithmetic average is taken as the average grain size.

[0027] Here, the differentiation between crystalline and amorphous phases is performed as follows. Specifically, a longitudinal section is observed using a TEM, and an image is obtained at a magnification sufficient to distinguish, for example, a few nanometers on the observation surface. Then, this image is subjected to FFT image transformation processing to select bright spots corresponding to the lattice constant (e.g., bright spots (including circular) corresponding to the (001) plane of a hexagonal crystal structure). Further, an inverse FFT transformation processing is performed, followed by binarization processing. This processing enables the enhancement of the crystalline structure of the lattice fringes and angles with the lattice constant selected as each bright spot. Similar processing is performed for each lattice constant to create an enhanced image corresponding to each lattice constant, such as a (001) plane enhanced image or a (100) plane enhanced image. Finally, each of the enhanced images is combined by ORing the enhanced images.

[0028] Then, the binarized image is expanded so that the lattice spacing with the largest lattice constant is filled, resulting in an image in which the lattice spacing of at least the largest lattice width is densely filled. In this case, the filled-in portions represent the crystalline phase, and the unfilled portions represent the amorphous phase. The magnification is not particularly limited as long as it allows the above-mentioned lattice constant to be observed.

[0029] Unlike ORing, this is a process in which the logical sum of pixels in the same position in two or more images is calculated for each pixel in the image to obtain the resulting image. Specifically, if a specific pixel is a bright point in one of the images, it is considered a bright point, and if it is a dark point in all the images, it is considered a dark point.

[0030] Using the above-mentioned method for distinguishing between crystalline and amorphous phases, the crystalline phase and amorphous phase are distinguished for each of five arbitrary visual fields. The area percentage of the crystalline phase in each visual field is calculated, and the arithmetic average of the calculated area percentages is taken as the area percentage of the crystalline phase. The area percentage of the crystalline phase is more preferably 50 to 95% by area. The reason for this is as follows.

[0031] If the area ratio of the crystalline phase is less than 50% by area, the hardness will decrease due to the small amount of crystalline phase in the Ti boride layer, and the coating layer may not perform as well as it should. On the other hand, if the area ratio of the crystalline phase exceeds 95% by area, the wear resistance will decrease and grain boundary fracture will become dominant, causing entire crystal grains to fall off at the grain boundaries, resulting in poor cutting performance. Here, the reason for the decrease in wear resistance is thought to be that the Ti boride layer is less likely to form boron oxide during cutting.

[0032] Nanoindentation hardness of Ti-boride layer: The Ti boride layer preferably has a nanoindentation hardness of 30 to 50 GPa. When the nanoindentation hardness is in this range, chipping resistance and wear resistance are further improved. This is presumably because, when the nanoindentation hardness is in this range, the Ti boride layer has a crystalline phase and an amorphous phase, which reliably improves wear resistance.

[0033] Here, the nanoindentation hardness was measured based on the nanoindentation test method (ISO14577), by polishing the surface of the Ti boride layer and indenting it with a Berkovich indenter made of diamond at a load of 1.96 × 10 -3The test is carried out at a pressure of 200 mgf. At least 10 random points are measured, and the arithmetic average of the measurements is taken as the hardness measurement value. In this measurement, the distance between each measurement point is at least 20 times the indentation depth during the test.

[0034] Crystal orientation of the crystalline phase in the Ti boride layer: The hexagonal 011 diffraction line, 100 diffraction line, and 101 diffraction line measured by X-ray diffraction are diffraction peaks due to the (001), (100), and (101) planes, respectively. When these peak intensities are designated as Ih(001), Ih(100), and Ih(101), respectively, it is more preferable to satisfy the relationship 0.01≦Ih(001) / {Ih(001)+Ih(100)+Ih(101)}≦0.80. Satisfying this relationship further improves wear resistance.

[0035] Here, the diffraction peak intensities of the (001), (100), and (101) planes of the hexagonal crystal can be measured using X-ray diffraction with a 2θ / θ focusing optical system using Cu-Kα radiation (wavelength λ: 0.15405 nm).

[0036] The (001) plane of a hexagonal crystal is sometimes expressed as the (0001) plane. Similarly, the (100) plane is sometimes expressed as the (10-10) plane, (1-100) plane, (01-10) plane, (-1100) plane, (-1010) plane, or (0-110) plane. Similarly, the (101) plane is sometimes expressed as the (10-11) plane, (1-101) plane, (01-11) plane, (-1101) plane, (-1011) plane, or (0-111) plane. These are equivalent plane indices.

[0037] Other layers (bottom layer): In this embodiment, when a lower layer containing a Ti compound (not limited to a stoichiometric compound) layer having a total average layer thickness of 0.1 to 2.0 μm and consisting of one or more layers selected from the group consisting of a Ti carbide layer, a nitride layer, a carbonitride layer, a carbonate layer, and a carbonitride oxide layer is provided adjacent to the tool substrate in addition to the coating layer, the effect of this layer, combined with the effect of this lower layer, can provide even better chipping resistance and thermal crack resistance.

[0038] If the total average thickness of the lower layers is less than 0.1 μm, the effect of the lower layers is not fully exhibited, whereas if it exceeds 2.0 μm, the crystal grains in the lower layers tend to become coarse, making chipping more likely to occur.

[0039] Tool base: (1)Material Any conventionally known substrate for this type of tool substrate can be used as the tool substrate, as long as it does not impede the achievement of the above-mentioned object. Examples include cemented carbide (WC-based cemented carbide, those containing Co in addition to WC, or those containing carbonitrides of Ti, Ta, Nb, etc.), cermets (those containing TiC, TiN, TiCN, etc. as their main components), ceramics (titanium carbide, silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, etc.), and cBN sintered compacts, and any of these is preferred.

[0040] (2) Shape There are no particular restrictions on the shape of the tool base, as long as it is a shape that can be used as a cutting tool, and examples thereof include the shape of an insert and the shape of an end mill. [Example]

[0041] Next, examples will be described, but the present invention is not limited to these examples.

[0042] The raw material powders were WC powder, VC powder, TaC powder, NbC powder, Cr3C2 powder, and Co powder, all with average particle sizes of 1 to 3 μm. These raw material powders were blended according to the composition shown in Table 1, wet mixed in a ball mill for 72 hours, and dried. They were then pressed into a green compact at a pressure of 100 MPa. This green compact was sintered in a vacuum of 6 Pa at a temperature of 1400°C for 1 hour. After sintering, the cutting edge was honed to an R of 0.03 to produce tool substrates 1 and 2 made of WC-based cemented carbide with insert shapes conforming to ISO standard CNMG120408.

[0043] Furthermore, the same raw material powders as above were compounded according to the composition shown in Table 1. The mixture was wet mixed in a ball mill for 72 hours, dried, and then pressed into a green compact at a pressure of 100 MPa. This green compact was sintered in a vacuum of 6 Pa at a temperature of 1400°C for 1 hour to produce a 4 mm diameter sintered round bar for forming a cemented carbide substrate. Furthermore, the sintered round bar was ground to produce tool substrates (end mill shape) 3 and 4 made of WC-based cemented carbide, each with a cutting edge diameter and length of 2 mm x 4 mm and a four-flute square shape with a 40 degree helix angle.

[0044] Subsequently, a lower layer (provided only on some of the tool substrates) and a coating layer were formed on these tool substrates 1 to 4 according to the following procedures (a) to (d).

[0045] (a) Each of the tool substrates 1 to 4 was ultrasonically cleaned in acetone and, in a dried state, mounted along its outer periphery at a predetermined radial distance from the central axis on a rotating table in a high-power pulse sputtering apparatus. Meanwhile, a Ti target and a sintered Ti and boron target were placed in four positions facing each other across the rotating table in the high-power pulse sputtering apparatus.

[0046] (b) The inside of the apparatus was evacuated and heated to 500°C using a heater while maintaining a vacuum of 0.1 Pa or less. Then, a DC bias voltage of -200 V was applied to the tool substrate, which was rotating and revolving on the rotary table. Then, argon (hereinafter referred to as Ar) gas was introduced into the apparatus as a reactive gas, creating an atmosphere of 2.0 Pa. Furthermore, a current of 40 A was passed through a tungsten filament provided in the apparatus to excite Ar ions, and the tool substrate was subjected to Ar bombardment for 1 hour.

[0047] (c) Ar gas and nitrogen gas were introduced into the apparatus as reactive gases to create a reactive atmosphere of 0.6 Pa, and high-power pulse sputtering was performed on the Ti target under the specified pulse sputtering conditions shown in Table 2. As a result, a TiN layer having an average thickness shown in Table 3 was formed on the surface of the tool substrate as the lower layer of the coating layer. However, the lower layer was not formed on all of the tool substrates.

[0048] (d) Subsequently, the nitrogen gas among the gases introduced into the apparatus was turned off and switched to Ar gas, and the atmosphere inside the apparatus was set to 0.5 Pa. The nitrogen gas was sufficiently discharged, and the atmosphere inside the apparatus was filled with only Ar gas. Thereafter, high-power pulse sputtering was performed on a sintered compact target made of Ti and boron under the predetermined pulse sputtering conditions shown in Table 2 for a time corresponding to the layer thickness, and the coated inserts 1 to 13 and coated end mills 14 to 26 shown in Table 3 (hereinafter collectively referred to as Examples 1 to 26) were obtained. However, Examples 4 to 6, 10 to 13, 17 to 19, and 23 to 26 are reference examples. ) were produced, respectively.

[0049] For comparison purposes, lower layers and coating layers were formed on these tool substrates 1 to 4 under the conditions shown in Table 4 according to the procedures (a) to (d) above, and comparative coated inserts 1 to 7 and comparative coated end mills 11 to 17 (8 to 10 are missing numbers, and hereinafter collectively referred to as Comparative Examples 1 to 7 and 11 to 17) were manufactured as comparative coated tools shown in Table 5. However, lower layers were not formed on all tool substrates.

[0050] [Table 1]

[0051] In Table 1, "-" indicates that it is not contained.

[0052] [Table 2]

[0053] In Table 2, the examples are a and b and the tool base symbol is * When a and b are α and β, Example a indicates that tool base α was used, and Example b indicates that tool base β was used (a, b, α, and β are numbers), and "-" indicates that the corresponding processing was not performed.

[0054] [Table 3]

[0055] In Table 3, the examples are a and b and the tool base symbol is * When α and β are used, Example a uses tool substrate α, and Example b uses tool substrate β (a, b, α, and β are numbers). Also, the strength ratio ** is the value of Ih{001} / {Ih{001}+Ih{100}+Ih{101}}, and "-" indicates that it does not exist.

[0056] [Table 4]

[0057] In Table 4, the comparative examples are a and b, and the tool base symbol is * When a and b are α and β, comparative example a indicates that tool base α was used, and comparative example b indicates that tool base β was used (a, b, α, and β are numbers), and "-" indicates that the corresponding treatment was not performed.

[0058] [Table 5]

[0059] In Table 5, the comparative examples are a and b, and the tool base symbol is * When α and β are used, Comparative Example a uses tool base α, and Comparative Example b uses tool base β (a, b, α, and β are numbers). Also, the strength ratio ** is the value of Ih{001} / {Ih{001}+Ih{100}+Ih{101}}, and "-" indicates that it does not exist.

[0060] Next, the following cutting tests 1 and 2 were carried out on Examples 1 to 26 and Comparative Examples 1 to 17, and the results are shown in Tables 6 and 7.

[0061] For Examples 1 to 13 and Comparative Examples 1 to 7, a dry high-speed intermittent cutting test (cutting test 1) was carried out under the following conditions, with each being screwed to a fixing jig at the tip of a tool steel bit.

[0062] Cutting test 1 Workpiece: JIS SUS316L round bar with four longitudinal grooves evenly spaced along the length Cutting speed: 180m / min Cut: 2mm Feed: 0.2mm / rev Cutting time: 10 minutes

[0063] After the cutting test, the flank wear width was measured and the presence or absence of chipping was observed. However, if chipping occurred before the end of the cutting time, cutting was stopped and the time from the start of cutting was measured. Table 6 shows the test results.

[0064] [Table 6]

[0065] In Table 6, the cutting time (minutes) until the end of life in the comparative example refers to the cutting time (minutes) until the end of life is reached due to chipping.

[0066] Next, for Examples 14 to 26 and Comparative Examples 11 to 17, a wet high-speed interrupted cutting test (cutting test 2) was carried out in side machining using an end mill under the following conditions.

[0067] Cutting test 2 Workpiece: Block of Ti-based alloy (Ti-6%Al-4%V alloy by mass%) (width 100mm x length 250mm) Cutting speed: 130m / min Rotation speed: 20690 min -1 Cutting depth: 2.0 mm Feed: 0.05mm / rev End mill blade outer diameter: 2mm

[0068] The cutting length was 150m (cutting time was about 144 minutes), the flank wear width was measured, and the occurrence of chipping was observed. However, if chipping occurred before the cutting length reached 150m, cutting was stopped and the time from the start of cutting was measured. Table 7 shows the test results.

[0069] [Table 7]

[0070] In Table 7, the cutting time (minutes) until the end of life in the comparative example indicates the cutting time (minutes) until the end of life is reached due to chipping.

[0071] From the results shown in Tables 6 and 7, it can be seen that the examples in which the coating layer using the Ti boride layer has a crystalline phase and an amorphous phase exhibit excellent adhesion resistance and wear resistance in high-speed intermittent cutting of materials that have high adhesion to coated tools, such as various Ti-based alloys and austenitic stainless steels. In contrast, in the comparative example, the cutting edge wear progresses quickly during high-speed intermittent cutting of the material with high weldability, and it is clear that the cutting edge reaches the end of its useful life in a relatively short period of time.

[0072] The above-disclosed embodiments are merely illustrative in all respects and are not restrictive. The scope of the present invention is defined by the claims, not by the above-disclosed embodiments, and is intended to include meanings equivalent to the claims and all modifications within the scope of the claims.

Claims

1. 1. A surface-coated cutting tool having a tool substrate and a coating layer on the tool substrate, the coating layer is directly over the tool substrate; the coating layer has an average thickness of 0.5 to 5.0 μm; the coating layer is a Ti-boride layer, The Ti boride layer has a composition represented by the formula: TiB ｘ When expressed as above, the average composition satisfies the atomic ratio x of 1.5≦x≦3.0, and further has a crystalline phase constituted by crystal grains of a hexagonal crystal structure and an amorphous phase, the crystalline phase has a hexagonal crystal structure and has an average grain size of 2 to 30 nm, and the crystalline phase occupies an area ratio of 50 to 95 area % of the Ti-boride layer; Regarding the crystal grains of the hexagonal crystal structure constituting the crystalline phase of the Ti-boride layer, when the peak intensities of the 001 diffraction line, the 100 diffraction line and the 101 diffraction line in X-ray diffraction are Ih(001), Ih(100) and Ih(101), respectively, the following relationship is satisfied: 0.01≦Ih(001) / {Ih(001)+Ih(100)+Ih(101)}≦0.

80. A surface-coated cutting tool characterized by:

2. 1. A surface-coated cutting tool having a tool substrate and a coating layer on the tool substrate, the coating layer is directly overlying a lower layer directly overlying the tool substrate; the lower layer is one or more of a Ti carbide layer, a nitride layer, a carbonitride layer, a carbonate layer, and a carbonitride oxide layer, having a total average thickness of 0.1 to 2.0 μm; the coating layer has an average thickness of 0.5 to 5.0 μm; the coating layer is a Ti-boride layer, The Ti boride layer has a composition represented by the formula: TiB ｘ When expressed as above, the average composition satisfies the atomic ratio x of 1.5≦x≦3.0, and further has a crystalline phase constituted by crystal grains of a hexagonal crystal structure and an amorphous phase, the crystalline phase has a hexagonal crystal structure and has an average grain size of 2 to 30 nm, and the crystalline phase occupies an area ratio of 50 to 95 area % of the Ti-boride layer; Regarding the crystal grains of the hexagonal crystal structure constituting the crystalline phase of the Ti-boride layer, when the peak intensities of the 001 diffraction line, the 100 diffraction line and the 101 diffraction line in X-ray diffraction are Ih(001), Ih(100) and Ih(101), respectively, the following relationship is satisfied: 0.01≦Ih(001) / {Ih(001)+Ih(100)+Ih(101)}≦0.

80. A surface-coated cutting tool characterized by:

3. 3. The surface-coated cutting tool according to claim 1, wherein the Ti-boride layer has a nanoindentation hardness of 30 to 50 GPa.

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