JP6416624B2 - Method for cutting cold tool steel and method for producing cold mold material - Google Patents

Description

  The present invention relates to a cutting method for cold tool steel suitable for tool materials, particularly cold mold materials for molding home appliances, mobile phones and automobile-related parts, and a method for producing cold mold materials using the cutting methods. It is about.

  In cold tools used for press forming such as bending, drawing, and punching of plate materials at room temperature, a steel material adjusted to a hardness of 60 HRC or higher by quenching and tempering has been proposed in order to improve its wear resistance. (For example, see Patent Documents 1 to 3). Such a hard steel material is difficult to cut into a tool shape after quenching and tempering. Therefore, normally, after roughing in an annealed state with low hardness, the working hardness is adjusted to 60 HRC or higher. In this case, since the shape after the rough machining is heat-treated and deformed by quenching and tempering, after the quenching and tempering, the final tool shape is adjusted by performing another finish cutting for correcting the deformation. The main cause of heat treatment deformation of the tool by quenching and tempering is that the volume of the steel material expands due to the transformation of the steel material that was a ferrite structure into a martensite structure in the annealed state.

In addition to the steel materials described above, many pre-hardened steels that have been adjusted to the working hardness in advance have been proposed. In pre-hardened steel, after cutting all the way to the final tool shape, there is no need for quenching and tempering, so the heat treatment deformation of the tool due to quenching and tempering can be eliminated, and the above-mentioned finishing cutting can also be omitted. Technology. Regarding this technique, there is known a method of cutting a high-hardness steel material using a cutting tool whose durability is improved by improving a base material or a film of the cutting tool (for example, Patent Document 4, 5).
In addition, as an approach from steel materials, while optimizing the amount of undissolved primary carbides in the hardened steel materials that reduce machinability, while ensuring a quenching and tempering hardness exceeding 55 HRC Cold tool steel having excellent machinability has been proposed (see, for example, Patent Document 6). On the other hand, an oxide ((FeO) ₂ · SiO ₂ , Fe ₂ SiO ₄ or (FeSi) having a melting point of 1200 ° C. or lower is used to suppress tool wear caused by friction between the cutting tool and the steel material during cutting. A cold tool steel has been proposed in which a self-lubricating property is imparted by adding an element that forms a) Cr ₂ O ₂ ) and forming the oxide on the mold surface by heat generated during cutting (for example, And Patent Document 7).

JP 2008-189982 A JP 2009-132990 A JP 2006-193790 A Japanese Patent Laid-Open No. 2003-1504 JP 2010-1115764 A JP 2001-316769 A JP 2005-272899 A

  Patent Documents 4 and 5 disclose cutting of SKD11, which is a typical cold tool steel having a hardness of 60 HRC. However, in the cutting of SKD11 in which a large amount of undissolved primary carbide is formed, if the base hardness is increased to a high hardness of 60 HRC, the tool life is short and the machinability is not improved sufficiently.

  On the other hand, cold tool steel in which the formation amount of undissolved primary carbide as disclosed in Patent Document 6 is controlled to be low, and low melting point oxide as disclosed in Patent Document 7 are used as a self-lubricating film. Even in the case of cold tool steel, when the hardness becomes higher than 60 HRC, the damage and wear of the tool increases, and the machinability may not be improved sufficiently.

  The present invention has been made in view of the above circumstances. Under such circumstances, a cold tool steel cutting method capable of dramatically improving machinability for cold tool steel having a high hardness of 60 HRC or more, and a cold die using the cutting method. There is a need for a method of manufacturing the material.

  The present inventors have intensively studied methods for improving the machinability of cold tool steel. And the knowledge that there existed the combination of the optimal component range of cold tool steel and cutting conditions which can improve machinability in the cutting of the cold tool steel whose quenching and tempering hardness is 60 HRC or more was obtained. The present invention has been achieved by specifying such an optimal combination.

  That is, the first invention includes cold tool steel containing 0.6% to 1.2% C (carbon) by mass ratio and having a hardness adjusted to 60 HRC or more, including metal (including semimetal). This is a cold tool steel cutting method in which cutting is performed using a coated cutting tool coated with an AlTi nitride film in which Al is more than 50% by atomic ratio of the portion.

It is preferable that the cold tool steel further contains Cr (chromium) in a mass ratio of 3.0% or more and less than 8.0%.
Further, the cold tool steel further includes Mo (molybdenum) and W (tungsten), which may be included as a single element or as a composite compound. These Mo and W are in a mass ratio (as a single element or a composite compound, respectively), and the value of {Mo amount + (1/2 × W amount)} is in the range of 0.5% to 2.0%. It is preferable that it is contained. Here, the amount of Mo represents the mass ratio of Mo in steel, and the amount of W represents the mass ratio of W in steel.

Further, the cold tool steel further has a mass ratio of Al (aluminum) of 0.01% or more and less than 0.3%, Mn (manganese) of 0.3% to 2.0%, and S (sulfur). It is preferable to contain 0.02%-0.1%.
In the cold tool steel cutting method of the present invention, the cold tool steel is preferably cut at a cutting speed of 120 m / min or more.

In addition, cold tool steel is a mass ratio,
C: 0.6% to 1.2%
Si: 0.7% to 2.5%
Mn: 0.3% to 2.0%
S: 0.02% to 0.1%,
Cr: 3.0% or more and less than 5.0%,
Mo and W: 0.5% ≦ {Mo amount + (1/2 × W amount)} ≦ 2.0% (Mo amount: mass ratio of Mo in steel (mass%), W amount: in W steel) Mass ratio (mass%) in
Al: 0.04% or more and less than 0.3%,
The balance is preferably composed of the remaining Fe (iron) and inevitable impurities.
In this case, it is more preferable that the above S, Cr, and Al are cold tool steels having a machinability index MP value of more than 0 (that is, MP> 0) obtained by the following relational expression.
[Relational expression]
MP = 21.9 × S amount + 124.2 × (Al amount / Cr amount) −2.1
In the above relational expression, the S amount is the mass ratio (mass%) of the S in the steel, the Cr amount is the mass ratio (mass%) of the Cr in the steel, and Al is the Al steel in the steel. The mass ratio (mass%) is expressed respectively.
These cold tool steels are preferably cut at a cutting speed of 160 m / min or more.

  Moreover, 2nd invention is a manufacturing method of the cold mold material which manufactures cold mold material by cutting cold tool steel using the cutting method of said cold tool steel.

According to the present invention, a cold tool steel cutting method and a cold mold material that are excellent in machinability, have a long tool life, and perform high-efficiency machining even when machining cold tool steel having a high hardness of 60 HRC or more. A manufacturing method is provided.
Therefore, this is an indispensable technique for the practical application of pre-hardened cold tool steel.

In Example 1, it is the digital microscope photograph which showed the external appearance of the flank and rake face of the cutting tool used for the cutting method of the example of this invention. In Example 1, it is the digital microscope photograph which showed the external appearance of the flank and rake face of the cutting tool used for the cutting method of the comparative example. It is an element map figure when the deposit formed on the surface of the cutting tool of Drawing 1A analyzes EPMA (electron beam microanalyzer). It is an element map figure when the deposit formed on the surface of the cutting tool of Drawing 1B is analyzed by EPMA (electron beam microanalyzer). In Example 2, it is the electron micrograph which showed the external appearance of the flank and rake face of the cutting tool used for the cutting method of this invention example and a comparative example. In Example 3, it is the electron micrograph which showed the external appearance of the flank and rake face of the cutting tool used for the cutting method of the example of this invention and the comparative example.

  The feature of the present invention is that the formation of primary carbide is suppressed by adjusting the carbon content contained in the cold tool steel, and further, the cutting tool used for the processing is made of AlTi nitride mainly composed of Al. It has been found that the machinability is greatly improved even in a cold tool steel having a high hardness of 60 HRC or higher by applying a coated cutting tool.

“HRC” represents Rockwell Hardness (HR), which is a measure of indentation hardness. The following is in accordance with JIS B7726 under the conditions of indenter: 120 ° conical diamond, test load: 150 kgf. This is a value obtained by an expression.
HR = 100-500h
(H: Actual indentation depth [mm] with reference load (10 kgf) as zero point)
In the following, “%” (mass%) representing the mass ratio may be simply expressed as “%”.

First, it will be described from the viewpoint of cold work tool steel.
(1) C: 0.6% to 1.2%
The cold tool steel in the present invention contains C in a range of 0.6% to 1.2% by mass ratio.
In the present invention, adjusting the carbon (C) amount of the cold tool steel is extremely important for improving the machinability. For example, since SKD11, which is a typical cold tool steel, contains 1.4% to 1.6% by mass of carbon, a large amount of undissolved primary carbide is formed. According to the study by the present inventors, in the case of SKD11, it was confirmed that the area ratio of the amount of primary carbide was about 8%. If a large amount of primary carbide having a high hardness such as SKD11 is present, when the base hardness is increased to a high hardness of 60 HRC or higher, the progress of tool wear becomes significant even if the cutting tool is improved.

  The “area ratio of the amount of primary carbide” is the area ratio of primary carbide in the cut surface when the cold tool steel is cut. This is obtained by observing with an optical microscope, taking a plurality of tissue photographs of one visual field composed of a predetermined region, obtaining an area ratio of primary carbides having an equivalent circle diameter of 5 μm or more in each visual field, and calculating an average value thereof. This is the value obtained.

The inventors of the present invention have studied to reduce the amount of primary carbide so as to improve the machinability in a cold work tool steel having a hardness of 60 HRC or higher. And it confirmed that C content had a strong influence on the amount of primary carbide contained in cold tool steel.
C is an important element that imparts hardness to the cold tool steel by forming carbides in the steel and forming a solid solution in the matrix. If the C content is less than 0.6% and becomes too small, the amount of C that forms carbides and dissolves in the matrix decreases, so it becomes difficult to impart a hardness of 60 HRC or higher. On the other hand, if the C content exceeds 1.2%, the toughness tends to decrease due to an increase in the amount of undissolved primary carbide when quenched, but more primary carbide is formed than that. Therefore, the machinability is greatly reduced.
In the present invention, by setting the C content to 0.6% to 1.2%, the cold tool steel is given a hardness of 60 HRC or more, and the area ratio of undissolved primary carbide is 5% or less. Can be easily reduced. This tends to improve machinability.

  In order to increase the hardness of the cold tool steel, the C content is preferably 0.65% or more, and more preferably 0.70% or more. Moreover, in order to improve machinability, it is preferable to set it as 1.0% or less, More preferably, it is 0.9% or less.

  The area ratio of the amount of primary carbide is preferably 4% or less, more preferably 3% or less, still more preferably 2% or less, and particularly preferably 1% or less.

  In the cold tool steel according to the present invention, components other than C can be appropriately designed based on common technical knowledge. For example, Si, Cr, Mn, W, Mo, Al, P, S, Ni, V, Cu , Nb or the like (remainder, Fe), and an appropriate element can be selected. Further, by mass ratio, Si: 0.1% to 3.0%, Cr: 3.0% to 9.0%, Mn: 0.3% to 2.0%, {Mo amount + (1 / 2 × W amount)}: 0.5% to 2.0%, Al: 0.5% or less, P: 0.05% or less, S: 0.1% or less, Ni: 1.0% or less, V: 1.0% or less, Cu: 1.0% or less, Nb: 0.5% or less, and the like can be appropriately selected from elements, and a desired content can be designed and configured.

  Furthermore, in order to reduce the area ratio of the primary carbide of the cold tool steel while maintaining a high hardness of 60 HRC or more, the Cr, Mo, and W contents that easily form the primary carbide should be controlled within the following ranges. Is preferred. Specifically, it is as follows.

(2) Cr: 3.0% or more and less than 8.0% It is preferable that Cr in the cold tool steel is contained in a range of 3.0% or more and less than 8.0%.
Cr imparts hardness to the cold tool steel by forming M ₇ C ₃ carbide in the structure after quenching and tempering. In addition, a part of the material is present as insoluble carbide during quenching heating, and has an effect of suppressing the growth of crystal grains. And when Cr is 3.0% or more, a desired amount of carbide is obtained, and it is easy to achieve a hardness of 60 HRC or more. On the other hand, by making Cr less than 8.0%, the amount of undissolved primary carbide is reduced, and toughness is improved. And by suppressing the excessive formation of the low melting point oxide containing Cr, it is possible to improve the function of an Al ₂ O ₃ protective film made of Al, which will be described later, and to significantly improve the machinability.

In addition, when V or Nb that forms hard MC carbide is added for the purpose of suppressing crystal grain growth or imparting hardness, the formation of coarse MC carbide is suppressed by coexisting M ₇ C ₃ carbide. There is an effect, and when Cr is 3.0% or more, the effect is sufficiently obtained and machinability is improved. For this reason, it is preferable that Cr shall be 3.0 mass% or more and less than 8.0 mass%.
Note that “M” represents V, Nb, Cr, W, Mo, and the like (the same applies hereinafter).
The Cr content is more preferably 3.1% or more, and even more preferably 3.5% or more. Further, the upper limit of the Cr content is preferably 7.0% or less, more preferably less than 5.0%, and particularly preferably 4.8% or less.

(3) Mo and W: 0.5 mass% ≦ {Mo amount + (1/2 × W amount)} ≦ 2.0%
The cold tool steel according to the present invention satisfies 0.5% by mass ≦ {Mo amount + (1/2 × W amount)} ≦ 2.0% by using Mo and W as individual elements or composite compounds, respectively. It is preferable to contain in the range.

  Mo and W are elements that improve the hardness by precipitation strengthening (secondary hardening) of fine carbides during tempering. However, at the same time, since the decomposition of residual austenite that occurs during tempering is delayed, if it is excessively contained, residual austenite tends to remain in the structure after quenching and tempering. Further, since Mo and W are expensive elements, the amount of addition should be reduced as much as possible for practical use. Therefore, the addition amount of these elements is preferably in a range satisfying 0.5% to 2.0% in the relational expression of {Mo amount + (1/2 × W amount)}.

Among the above, a preferred embodiment for improving the machinability of the cold tool steel will be described.
The cold tool steel in the present invention is, by mass ratio, C: 0.6% to 1.2%, Si: 0.7% to 2.5%, Mn: 0.3% to 2.0%, S : 0.02% to 0.1%, Cr: 3.0% or more and less than 5.0%, Mo and W (single or combined): 0.5% ≦ {Mo amount + (1/2 × W amount) )} ≦ 2.0%, Al: 0.04% to less than 0.3%, the balance Fe, and unavoidable impurities.

Since Si is an element that tends to form a corundum oxide with Al ₂ O ₃ in addition to having a stronger oxidation tendency than Fe and Cr, Fe-based oxides and Cr-based oxides that lower the melting point of oxides to suppress the formation of oxides, it has the effect of promoting the formation of the Al ₂ O ₃ protective coating. Moreover, it is an element which solidifies in steel and gives hardness to cold tool steel. In order to acquire these effects, it is preferable that Si is 0.7% or more by mass ratio with respect to the whole steel. However, if too much, the hardenability and toughness are remarkably lowered, so the mass ratio with respect to the whole steel is preferably 2.5% or less. Furthermore, the mass ratio of Si is preferably 0.8% or more. A more preferable mass ratio of Si is 2.0% or less.

In addition to controlling the Si content to the above-described range, by applying cold tool steel in which the Cr and Al contents are more preferably controlled, Al ₂ O ₃ that is a high melting point oxide, A composite lubricating protective film comprising MnS, which is a ductile inclusion, is preferable because it is stably formed on the tool edge. Specifically, by mass ratio,
C: 0.6% to 1.2%, Si: 0.7% to 2.5%, Mn: 0.3% to 2.0%, S: 0.02% to 0.1%, Cr: 3.0% or more and less than 5.0%, Mo and W (single or combined): 0.5% ≦ {Mo amount + (1/2 × W amount)} ≦ 2.0%, Al: 0.04 It is preferable to use a cold tool steel comprising less than ˜0.3%, the balance Fe, and unavoidable impurities, and having a machinability index MP value of more than 0 determined by the following relational expression.
MP = 21.9 × S amount + 124.2 × (Al amount / Cr amount) −2.1
[S amount: mass ratio of S in steel, Cr amount: mass ratio of Cr in steel, Al: mass ratio of Al in steel]
In the above, it is more preferable that the S amount is 0.03% to 0.8% by mass ratio and the ratio of Al amount / Cr amount is 0.02 to 0.06.

The adjustment of the machinability index MP is a preferable requirement for forming a sufficient amount of a composite lubricating protective film composed of Al ₂ O ₃ and MnS on the tool surface during cutting. The cold tool steel applied to the preferable cutting method of the present invention forms Al ₂ O ₃ which is a high melting point oxide on the surface of the cutting tool by heat generated during the cutting process. Since the melting point of Al ₂ O ₃ is about 2050 ° C., which is much higher than the cutting temperature, Al ₂ O ₃ functions as a protective film for the cutting tool. Furthermore, a sufficient amount of S contained in the preferred cold tool steel in the present invention forms MnS. In addition to being rich in ductility, MnS is well-familiar with Al ₂ O ₃ , so it is deposited on the above Al ₂ O ₃ protective film, and these serve as a good composite lubricating protective film.

On the other hand, Cr, which is a main component of cold tool steel, tends to form a low melting point oxide. That is, Cr contained excessively with respect to the amount of Al in the steel tends to be a factor that hinders the function of the Al ₂ O ₃ protective coating. Then, as a result, it can become a factor that inhibits the function of the composite lubricating protective coating consisting of Al ₂ O ₃ and MnS. Therefore, the preferable cold tool steel according to the present invention preferably contains a sufficient amount of Al of 0.04% or more and adjusts the balance between the amount of Al in the steel and the amount of Cr (Al / Cr). . And the function of said composite lubrication protective film is exhibited by adjusting S amount corresponding to this.

  Based on the above effects, the interrelationship of the degree of influence on the self-lubricating properties of S, Cr, and Al was studied in detail. As a result, in the case of cold tool steel satisfying the component composition of the present invention, the above-mentioned influence degree by these three elements is “21.9 × S amount + 124.2 × (Al amount / Cr amount) −2.1. It was confirmed that the machinability of the present invention can be evaluated with high accuracy by using the machinability index MP as a value based on this relational expression. And when this MP value becomes large, the machinability improvement effect by the composite lubricating protective film using the high melting point oxide of the present invention is exhibited. Specifically, if the component composition is adjusted to exceed 0, this The effect is fully demonstrated.

  Cold tool steel that satisfies the above-mentioned relational expression is designed so that a sufficient amount of a composite lubricating protective film is formed on the surface of the cutting tool. Is effectively demonstrated. The cutting speed is preferably 120 m / min or more, more preferably 160 m / min or more, further preferably 180 m / min or more, and particularly preferably 200 m / min or more. Also, increasing the cutting speed leads to shortening of the time required for cutting.

The cutting method of the present invention is preferably applied to milling, which is intermittent cutting in which oxygen is provided during processing and an oxidation protective film is easily formed.
Moreover, since the supply source of oxygen required for oxidation is the atmospheric atmosphere during cutting, there is no restriction on the use of lubricating oil (dry or wet). However, in order to form an oxidation protective film, it is preferable to use a dry method in which the cutting temperature is likely to rise, and this is a method suitable for making the cutting oil free as required recently.

Next, elements other than those described above that can be included in the cold tool steel according to the present invention will be described.
-V: It is preferable to contain at 1.0% or less.
V (vanadium) has the effect of forming various carbides and increasing the hardness of the cold tool steel. In addition, the formed insoluble MC carbide has an effect of suppressing the growth of crystal grains. In particular, by adding it in combination with Nb, which will be described later, the MC carbide that has not been dissolved yet during quenching heating becomes fine and uniform, and has the function of effectively suppressing crystal grain growth. On the other hand, MC carbide is hard and causes a decrease in machinability. Excessive V addition excessively forms coarse MC carbides and reduces the toughness and machinability of the cold tool steel. Therefore, even when V is added, V is preferably 1.0% or less, more preferably 0.7% or less.

-Nb: It is preferable to contain at 0.5% or less.
Nb (niobium) functions to suppress the coarsening of crystal grains by forming MC carbides. However, if added excessively, coarse MC carbides are excessively formed, and the toughness and machinability of the steel are lowered. Therefore, when Nb is added, Nb is preferably 0.5% or less. More preferably, it is 0.3% or less.

-Ni: It is preferable to contain at 1.0% or less.
Ni (nickel) is an element that improves the toughness and weldability of steel. Moreover, since tempering after quenching has the effect of precipitating as Ni ₃ Al and increasing the hardness of the steel, it is effective to add it according to the amount of Al. On the other hand, Ni is an expensive metal and is an element whose addition amount should be reduced as much as possible for practical use. In the present invention, similarly, the amount of Cr, which is an expensive metal, can be significantly reduced as compared to JIS-SKD11, which is a typical cold tool steel, so that the amount of Ni added can be increased accordingly. it can. Therefore, even when Ni is added, it may be added up to 1.0% or less.

Cu is preferably contained at 1.0% or less.
Cu (copper) precipitates as ε-Cu during tempering after quenching, and has the effect of increasing the hardness of the steel. However, Cu is an element that causes hot brittleness of a steel material. Therefore, even when adding Cu, it is preferable to set it as 1.0% or less. In order to suppress hot brittleness due to Cu, it is also preferable to add Ni at the same time. And it is further more preferable that Cu and Ni at this time shall be substantially the same amount.

-P: It is preferable to contain 0.05% or less.
P (phosphorus) is an element inevitably contained in steel. And when too much, it is an element which reduces hot workability and toughness. Therefore, in the present invention, the content (mass ratio) of P in the steel is preferably 0.05% or less, and more preferably 0.03% or less.

Next, I will describe from the standpoint of cutting tools.
The cutting tool for cutting cold tool steel according to the present invention is a coated cutting tool coated with an AlTi nitride film in which the atomic ratio of the metal (including metalloid) portion is greater than 50%.
In the present invention, the semimetal means boron or silicon.

By limiting the primary carbide contained in the cold tool steel with a high hardness to a certain amount or less, the damage to the cutting tool is reduced and the life of the cutting tool tends to be improved. However, in order to improve the machinability of cold tool steel having a base hardness of 60 HRC or more, the hard coating of the coated cutting tool used for the machining is important. That is, when cutting a hard material, the cutting edge of the tool becomes high temperature, so that a hard film covering the cutting tool needs to have excellent heat resistance. Furthermore, in order to improve the machinability of the high-hardness material, it has been found that it is particularly important to reduce the coefficient of friction with the work material during cutting.
As a result of examining various hard films, the AlTi nitride film having a high Al content is excellent in the heat resistance of the film itself, and further, the friction coefficient with the work material during cutting is reduced. The knowledge that it is in a tendency was acquired. Specifically, it is an AlTi nitride film having an Al content of more than 50 atomic% in the metal (including metalloid) portion of the nitride film. A more preferable Al content is 60 atomic% or more, and further 65 atomic% or more.
Further, since the composite lubricating protective film used in the preferred cutting method of the present invention is formed on the surface of the cutting tool by heat during cutting, it is effective to increase the cutting speed. And if it is a nitride film of AlTi with a large content of Al, it has excellent heat resistance and can reduce the coefficient of friction with the workpiece during cutting. Therefore, it is preferable because the cutting speed can be further increased.

The AlTi nitride film according to the present invention is a coated cutting tool having excellent durability by making the crystal structure specified by XRD (X-ray diffraction) a cubic structure. In order to change the crystal structure of the hard coating to a cubic structure, the Al content in the metal (including metalloid) portion is preferably 75 atomic% or less. If the crystal structure specified by XRD is a cubic structure, the AlTi nitride film according to the present invention may contain other metal (including semi-metal) elements. The film thickness of the hard coating is preferably 1 μm to 6 μm.
The AlTi nitride film having a large Al content according to the present invention tends to have a hexagonal crystal structure when the absolute value of the negative bias voltage applied to the substrate during film formation is small. Therefore, it is preferable to make the absolute value of the negative bias voltage applied to the base material larger than −90 V during coating.

The base material used for the cutting tool is preferably a WC-based cemented carbide having an excellent balance between hardness and toughness. Since the present invention is a method for cutting a cold tool steel having a high hardness of 60 HRC or higher, it is preferable that the base material of the cutting tool used for the cutting has a hardness of 93.0 HRA or higher. Further preferable hardness is 93.5 HRA or more.
It is preferable to metal bombard the surface of the base material before coating with the hard coating. By providing an intermediate film having a thickness of 10 nm or less immediately above the base material by metal bombardment, adhesion between the base material and the AlTi nitride film having a high Al content is enhanced. Furthermore, excellent durability can be exhibited even in high-speed machining.

The above-mentioned “HRA” represents Rockwell Hardness (HR), which is a measure of indentation hardness. The following is in accordance with JIS B7726 under the conditions of indenter: 120 ° conical diamond and test load: 60 kgf. This is a value obtained by an expression.
HR = 100-500h
(H: Actual indentation depth [mm] with reference load (10 kgf) as zero point)
In the following, “%” (mass%) representing the mass ratio may be simply expressed as “%”.

Next, a preferred embodiment for improving the machinability of the cold tool steel will be described.
-Cold tool steel contains Al: 0.01% or more and less than 0.3%, Mn: 0.3% to 2.0%, S: 0.02% to 0.1% by mass ratio. It is preferable.
Even cold tool steel with a reduced amount of primary carbide, when the hardness becomes 60HRC or higher, when the cutting speed is increased, the lubrication characteristics during cutting are insufficient and sudden cracks occur in the honing area There is. Therefore, the present inventors have examined means for improving machinability that can widely correspond to the component composition of cold tool steel. As a result, we focused on the effectiveness of self-lubrication. And when self-lubricating action effect using the low melting point oxide like patent document 7 was examined, it discovered that this had the subject depending on cutting temperature. In other words, the low melting point oxide having self-lubricating properties is a composite oxide containing Fe and Cr that are generally contained in a large amount in a steel material. It fluctuates greatly and a stable lubricating effect cannot be obtained.

Then, when earnestly researching a technique for improving the machinability of cold tool steel without using a low melting point oxide, Al ₂ O ₃ which is a high melting point oxide and MnS which is a high ductility inclusion. The method of forming the composite lubricating protective film to be formed on the surface of the cutting tool by heat during cutting was found. The effect of this composite lubricating protective film does not vary in response to a wide range of cutting temperatures, and even when an element that forms a hard MC carbide such as Nb or V is added, good machinability can be secured. And there exists the component range of the steel raw material which can form this composite lubrication protective film, and since this was able to be specified, machinability can be improved further.

Al forms a high melting point oxide, Al ₂ O ₃ , on the cutting tool surface during cutting and functions as a protective film. And since Al contains 0.01% or more, the protective film of sufficient thickness is formed and a tool life improves. Al is more preferably 0.04% or more, and still more preferably 0.05% or more. However, when a large amount of Al is added, since much Al ₂ O ₃ is formed as inclusions in the steel material, the machinability of the steel material is lowered. For this reason, the upper limit of the Al addition amount is preferably less than 0.3%, more preferably 0.15% or less.

Mn acts as a good lubricating film on the Al ₂ O ₃ protective film formed on the cutting tool surface. And it is an austenite formation element, and it dissolves in steel and improves hardenability. However, if the added amount is too large, a large amount of retained austenite remains after quenching and tempering, which causes aging during tool use. Further, since the easily form a Fe and Cr and a low melting oxide, is a factor that inhibits the function of the Al ₂ O ₃ protective coating. Therefore, the Mn content is preferably 0.3% to 2.0%, more preferably 0.4% or more. Further, Mn is more preferably 1.5% or less.

S acts as a good lubricating film on the Al ₂ O ₃ protective film formed on the cutting tool surface. In order to sufficiently exhibit such a lubricating action, addition of 0.02% or more is preferable. The upper limit of the S content is preferably 0.1% because S deteriorates the toughness of the steel. The S content is more preferably 0.03% or more. More preferably, it is 0.08% or less.

By controlling Al, Mn, and S contained in the cold tool steel within the above range, the above-described composite lubricating protective film is formed even in high-speed cutting with a cutting speed of 120 m / min or more. It is preferable because sudden chipping is easily suppressed.
The cutting speed in the present invention is the speed of the work surface (blade edge). That is, in the case of a blade-tip replaceable tool, the speed is the speed of the outermost edge of the insert when the insert tip is set, and in the case of a turning tool such as a drill or end mill, the speed of the outer peripheral edge.

  According to the cutting method of the present invention, high-hardness pre-hardened steel can be cut, so that heat treatment deformation caused by quenching and tempering is excluded, and finishing cutting can be omitted. Therefore, a cold mold material for molding home appliances, mobile phones and automobile-related parts can be manufactured more efficiently than before.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist thereof.

Example 1
Necessary materials were melted using a high-frequency induction melting furnace to produce ingots having chemical components shown in Table 1. Next, hot forging was performed on the ingot so that the forging ratio was about 10, and after cooling, annealing was performed at 860 ° C. And after performing the quenching process by air cooling from 1030 degreeC to this annealing material, it adjusts hardness by tempering twice at 500 to 540 degreeC, and the test piece for evaluating machinability (No. 7) was produced.

The machinability test was performed by plane cutting using an insert PICOmini manufactured by Hitachi Tool Co., Ltd. as a cutting edge exchangeable tool corresponding to cutting of a hard material. The insert was based on a WC-based cemented carbide with a hardness of 93.5 HRC. As described below, the present invention example includes an AlTi nitride film in which Al is more than 50% in the atomic ratio of the metal (including metalloid) portion, and the comparative example is a TiAl system in which Al is less than 50%. Inserts (coated cutting tools) each coated with a nitride film were used.
Cutting conditions were dry, cutting speed 210 m / min, rotation speed 5570 rpm, feed speed 2228 mm / min, feed amount per blade 0.4 mm / blade, depth of cut 0.15 mm, width of cut 6 mm, number of teeth 1 .

The nitride film in the insert used in the example of the present invention is a cathode arc ion plating method in which an alloy containing 70% Al and 30% Ti is used as a cathode electrode in an atomic ratio of a metal (including a semimetal) portion. Used to form a film. While maintaining the substrate temperature at 450 ° C. with a heater, a bias of −100 V was applied to the substrate, N ₂ gas was introduced, and arc discharge was started to form a film. The film thickness of the formed nitride film was 4.5 μm on the tool clearance surface side. The film composition of the metal part obtained from the quantitative evaluation by the electron probe microanalyzer was 69% Al and 31% Ti by atomic ratio. The crystal structure specified by XRD (X-ray diffraction) was a cubic structure.

The nitride film in the insert used in the comparative example was formed by a cathode arc ion plating method using an alloy containing 50% Al and 50% Ti as the cathode electrode in atomic ratios. While maintaining the substrate temperature at 450 ° C. with a heater, a bias of −50 V was applied to the substrate, N ₂ gas was introduced, and arc discharge was started to form a film. The film thickness of the formed nitride film was 4.3 μm on the tool clearance surface side. The film composition of the metal part obtained from the quantitative evaluation by the electron probe microanalyzer was 52% Ti and 48% Al by atomic ratio. The crystal structure specified by XRD (X-ray diffraction) was a cubic structure.

  The evaluation of machinability is based on test piece no. 7 was cut under the above cutting conditions with a cutting distance of 25 m, and the amount of wear after the end of cutting in the insert (tool wear amount) was measured using an optical microscope. These evaluation results are shown in Table 2.

  FIG. 1A is a digital microscope photograph showing the appearance of the cutting edge after cutting of the cutting tool used in the cutting method of the present invention, and FIG. 1B shows the end of cutting of the cutting tool used in the cutting method of the comparative example. It is the digital microscope photograph which showed the external appearance of the back blade edge | tip. 1A and 1B, the upper photograph shows the rake face side, and the lower photograph shows the flank face side.

FIG. 2A is an analysis result of distribution of Al, O, Mn, and S by an electronic probe microanalyzer on the rake face side of the cutting tool used in the cutting method of the present invention. FIG. 2B is a cutting of the comparative example. It is the analysis result of distribution of Al, O, Mn, and S by the electronic probe microanalyzer in the rake face side of the cutting tool used for the method. The high concentration part of each element is shown in light color. As shown in the upper left photographs of FIGS. 2A and 2B, since Al is originally included in the nitride film, the area where Al ₂ O ₃ is formed cannot be determined from the Al distribution alone. It is judged that Al ₂ O _{3 is} formed in a region where both Al and O are present at a high concentration.

As is apparent from Table 2, FIG. 1A and FIG. 1B, the amount of tool wear in the inserts used in the examples of the present invention was extremely small compared to the inserts used in the comparative examples. The inserts used in the examples of the present invention showed excellent wear resistance without the defects seen in the inserts used in the comparative examples.
Moreover, according to FIG. 2A and FIG. 2B, in any of the insert used by the example of this invention, and the insert used by the comparative example, O, Mn, and S are the same area | regions of the blade edge | tip on the surface of the blade edge | tip after completion | finish of cutting. It is confirmed that they are distributed. From this fact, in any insert, it is presumed that a composite lubricating protective film composed of Al ₂ O ₃ and MnS is formed on the surface of the cutting edge in the cutting process.
Further, when the concentration of Al in the nitride film is increased, the heat resistance of the film itself is improved, and the friction coefficient with the work material during cutting tends to decrease. Thus, according to the cutting method of the present invention example, it was confirmed that the cold tool steel can be cut at a high speed while maintaining the excellent wear resistance of the cutting tool by the synergistic effect of the composite lubricating protective film and the coating film. .

(Example 2)
Necessary materials were melted using a high-frequency induction melting furnace to produce ingots having chemical components shown in Table 3. Next, hot forging was performed on these ingots so that the forging ratio was about 10, and after cooling, annealing was performed at 860 ° C. And after performing the quenching process by air cooling from 1030 degreeC to these annealing materials, it adjusts to the hardness of 60HRC by two tempering processes at 500 to 540 degreeC, and the test piece for evaluating machinability (Nos. 1 to 6) were produced.

  And test piece No. Primary carbides distributed in 1-6 structures were evaluated. First, a 15 mm × 15 mm cross section parallel to the length direction (stretching direction) of the test piece was specified, and this cross section was polished into a mirror surface using diamond slurry. Next, the cross section was corroded using 10% nital so that the boundary between the primary carbide and the base when the cross sectional structure was observed became clear. Then, the cross section after corrosion was observed with an optical microscope having a magnification of 200 times, and 20 views of a tissue photograph of one field of view of an area of 877 μm × 661 μm were taken. Then, by processing the tissue photograph, primary carbides having an equivalent circle diameter of 5 μm or more observed in the cross-sectional structure are extracted, and the area ratio of the primary carbides in the cross-sectional structure is set as an average value for 20 fields of view. Asked.

  And the machinability was evaluated with the coated cutting tool with respect to the test piece after measuring the distribution state of the primary carbide. The machinability test was performed by plane cutting using an insert PICOmini manufactured by Hitachi Tool Co., Ltd. as a cutting edge exchangeable tool corresponding to cutting of a hard material. The insert was based on a WC-based cemented carbide with a hardness of 93.5 HRC. In the example of the present invention, an AlTi nitride film having an Al ratio of more than 50% in the atomic ratio of the metal (including metalloid) portion is provided, and in the comparative example, a TiAl series nitride film having an Al content of less than 50% is provided on the surface. The coated insert was used.

The nitride film in the insert used in the examples of the present invention was formed using a cathode arc ion plating method in which an alloy containing 70% Al and 30% Ti in atomic ratios was used as a cathode electrode. While maintaining the substrate temperature at 450 ° C. with a heater, a bias of −100 V was applied to the substrate, N ₂ gas was introduced, and arc discharge was started to form a film. The film thickness of the formed nitride film was 4.5 μm on the tool clearance surface side. The film composition of the metal part obtained from the quantitative evaluation by the electron probe microanalyzer was 69% Al and 31% Ti by atomic ratio. The crystal structure specified by XRD (X-ray diffraction) was a cubic structure.
Before the hard coating is applied, the pressure in the furnace is evacuated to 8 × 10 ⁻³ Pa or less, the negative bias voltage applied to the substrate is set to −1000 V, and a coil magnet is provided on the outer periphery of the target. Using the prepared cathode, Ti bombardment treatment was performed to form an intermediate film of 10 nm or less.

The nitride film in the insert used in the comparative example was formed by a cathode arc ion plating method using an alloy containing 50% Al and 50% Ti as the cathode electrode in atomic ratios. While maintaining the substrate temperature at 450 ° C. with a heater, a bias of −50 V was applied to the substrate, N ₂ gas was introduced, and arc discharge was started to form a film. The film thickness of the formed nitride film was 4.3 μm on the tool clearance surface side. The film composition of the metal part obtained from the quantitative evaluation by the electron probe microanalyzer was 52% Ti and 48% Al by atomic ratio. The crystal structure specified by XRD (X-ray diffraction) was a cubic structure.
Before the hard coating is applied, the pressure in the furnace is evacuated to 8 × 10 ⁻³ Pa or less, the negative bias voltage applied to the substrate is set to −1000 V, and a coil magnet is provided on the outer periphery of the target. Using the prepared cathode, Ti bombardment treatment was performed to form an intermediate film of 10 nm or less.

  Cutting conditions were dry, cutting speed 120 m / min, rotation speed 5570 rpm, feed speed 2228 mm / min, feed amount per blade 0.4 mm / blade, depth of cut 0.15 mm, width of cut 6 mm, number of teeth 1 . The cutting distance (L) was set to 50 m, and the amount of tool wear on the flank after cutting was measured using an electron microscope. These evaluation results are shown in Table 4.

FIG. 3 is an observation photograph of the insert after the cutting test. Sample piece No. with high carbon content. No. 1 had a higher amount of primary carbide than the cold tool steel according to the example of the present invention, and reached its life due to early chipping.
On the other hand, the cold tool steel according to the example of the present invention has a specimen No. The carbon content was less than 1, and the amount of primary carbide was reduced. Then, the cold tool steel according to the example of the present invention is cut using a coated cutting tool coated with an AlTi nitride film having an Al ratio of more than 50% in the atomic ratio of the metal (including semimetal) portion. The tool wear was suppressed. It was confirmed that even if the amount of primary carbide in the cold tool steel is small, tool wear is not suppressed unless the hard coating of the coated cutting tool used for cutting is appropriate.
Among the examples of the present invention, the sample piece No. in which the cold tool steel has a preferable composition range. In the cutting process of No. 5, a sufficient amount of the composite lubricating protective film was formed, so that tool wear did not occur and the tool life was extremely good.
Sample piece No. In the cutting process of No. 6, the wear width of the tool tended to be small, but the composite lubrication protective film was not formed, and the lubricity was inferior compared to other examples of the present invention, so that the honing part was chipped.

(Example 3)
Sample piece No. For No. 5, machinability was evaluated by changing the cutting speed. The cutting tool used is the same as in Example 2.
Cutting conditions are dry, cutting speed 160, 200 m / min, rotation speed 5570 rpm, feed speed 2228 mm / min, feed amount per blade 0.4 mm / blade, depth of cut 0.15 mm, depth of cut 6 mm, number of teeth 1 It was. The cutting distance (L) was 50 m, and the amount of tool wear on the flank after cutting was measured with an electron microscope. These evaluation results are shown in Table 5.

  FIG. 4 is an observation photograph of the insert after the cutting test. In the cutting method of the present invention example, it was confirmed that almost no tool wear occurred even when the cutting speed was increased. On the other hand, in the cutting method of the comparative example, the tool wear tends to increase when the cutting speed is improved, and the tool life is shortened at a cutting distance shorter than that of the present invention.

The disclosure of Japanese application 2012-181486 is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards mentioned in this specification are to the same extent as if each individual document, patent application, and technical standard were specifically and individually described to be incorporated by reference, Incorporated herein by reference.

Claims (7)

It contains 0.6% to 1.2% C by mass ratio, and further Cr is 3.0 % to less than 5.0% , Al is 0.04 % to less than 0.3%, and Mn is mass ratio. A cold tool steel containing 0.3% to 2.0%, 0.02% to 0.1% S and having a hardness adjusted to 60 HRC or more, the base material is a WC-based cemented carbide, And the cutting method of the cold tool steel cut | disconnected using the coating cutting tool by which AlTi nitride film with more than 50% of Al is contained by the atomic ratio of the metal part containing a semimetal.   The cold tool steel further includes Mo and W in a mass ratio of 0.5% ≦ {Mo amount + (1/2 × W amount)} ≦ 2.0% (Mo amount: mass of Mo in steel). The cutting method for cold tool steel according to claim 1, which is contained in a range that satisfies a ratio, a W amount: a mass ratio of W in steel).   The method for cutting cold tool steel according to claim 1 or 2, wherein cutting is performed at a cutting speed of 120 m / min or more. By mass ratio,
C: 0.6% to 1.2%
Si: 0.7% to 2.5%
Mn: 0.3% to 2.0%
S: 0.02% to 0.1%,
Cr: 3.0% or more and less than 5.0%,
Mo and W: 0.5% ≦ {Mo amount + (1/2 × W amount)} ≦ 2.0% (Mo amount: mass ratio of Mo in steel, W amount: mass ratio of W in steel) ,
Al: 0.04% or more and less than 0.3%,
A cold tool steel consisting of the remaining Fe and inevitable impurities and having a hardness adjusted to 60 HRC or more is covered with an AlTi-based nitride film in which Al is more than 50% in the atomic ratio of the metal part including the metalloid. Of cold tool steel, which is cut using a coated coated cutting tool. By mass ratio,
C: 0.6% to 1.2%
Si: 0.7% to 2.5%
Mn: 0.3% to 2.0%
S: 0.02% to 0.1%,
Cr: 3.0% or more and less than 5.0%,
Mo and W: 0.5% ≦ {Mo amount + (1/2 × W amount)} ≦ 2.0% (Mo amount: mass ratio of Mo in steel, W amount: mass ratio of W in steel) ,
Al: 0.04% or more and less than 0.3%,
The balance Fe, and inevitable impurities,
A cold tool steel whose machinability index MP obtained by the following relational expression is greater than 0 and whose hardness is adjusted to 60 HRC or more is less than 50% in the atomic ratio of the metal part including the semimetal. A method for cutting cold tool steel, which uses a coated cutting tool coated with a large amount of an AlTi nitride film.
MP = 21.9 × S amount + 124.2 × (Al amount / Cr amount) −2.1
[S amount: mass ratio of S in steel, Cr amount: mass ratio of Cr in steel, Al: mass ratio of Al in steel]   The method for cutting cold tool steel according to claim 4 or 5, wherein cutting is performed at a cutting speed of 160 m / min or more.   The manufacturing method of the cold mold material which manufactures cold mold material by cutting cold tool steel with the cutting method of the cold tool steel of any one of Claims 1-6.