JP2004360062A - Tool member having excellent high temperature strength property, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tool member which has excellent high temperature strength, and in which crystal grains are fined, and to provide its production method. <P>SOLUTION: The tool member is the one subjected to quenching and tempering, and consists of a tool steel comprising, by mass, 0.1 to 3.0% C and 1.0 to 18.0% Cr. Oxides with a grain size of ≤25 nm are dispersed into the structure by ≥750 pieces per μm<SP>3</SP>, and also, the maximum crystal grain size by old austenitic grain boundaries is ≤10 μm. Preferably, the maximum grain size of the oxides dispersed into the structure is ≤25 nm. In the method of producing the tool member, a powdery mixture of tool steel powder and oxide powder mixed so that the content of C is controlled, by mass, to 0.1 to 3.0%, Cr to 1.0 to 18.0% and oxides to 0.3 to 5.0% is subjected to mechanical milling, is thereafter subjected to compacting and forming, and is quenched and tempered. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a tool member having improved high-temperature strength, which is optimal for various types of tool members such as a press die, a die casting die, an extrusion tool, a cutting tool, a punch, and a die, and a method of manufacturing the same.

  Conventionally, JIS steel grade SKD61-based alloy tool steel and SKH2-based high-speed steel have been used in the field of warm tools and cutting tools. Usually, such a tool member is machined into a product shape in an annealed (low-hardness) state of the tool steel material, and then quenched and tempered to adjust the hardness, and is finished into a product tool after finishing. . (In the case of pre-hardened steel, the product is machined and finished to the product shape in a quenched and tempered state and turned into a product tool.)

In such members, for example, a technique for improving the mechanical properties of high-speed steel has been proposed (see Patent Document 1). This proposal is excellent in that the carbide diameter and the crystal grain diameter are reduced to increase the room temperature strength.
JP-A-2002-105513

  Although the technique disclosed in Patent Document 1 described above is advantageous in that the strength at room temperature is increased, in terms of high-temperature strength, there is a concern that carbides that carry the strength grow at high temperatures and cannot maintain the strength. In order to reduce the cost of products manufactured using tools, it is necessary to develop tool members that can extend the life of the tools used and withstand high loads, and then improve the high-temperature strength described above. Is a big challenge.

  An object of the present invention is to solve the problem that the carbides responsible for strength grow at a high temperature and cannot maintain the strength, and further improve the strength (including the high-temperature strength) by refining the crystal grains as compared with the conventional technique. An object of the present invention is to provide a tool member and a manufacturing method thereof.

  The present inventor studied the problem that carbides, which were mainly introduced as particles bearing strength, grow at high temperatures and cannot maintain strength, and disperse oxides that are stable and do not grow much at high temperatures finely. Adopted. The present inventors have intensively studied the optimum composition and manufacturing method, and found that strength (including high-temperature strength) and toughness can be greatly improved even when the amount of carbide is reduced, and the present invention has been achieved.

That is, the present invention is a tool member that has been quenched and tempered, and is made of tool steel containing 0.1 to 3.0% by mass of C and 1.0 to 18.0% of Cr by mass%. Is a tool member excellent in high-temperature strength characteristics in which oxides having a particle size of 25 nm or less are dispersed at 750 or more per 1 μm ³ and the crystal grain size due to the prior austenite grain boundary is 10 μm or less at maximum. Preferably, the particle size of the oxide dispersed in the tissue is at most 25 nm or less.

  And the manufacturing method of the tool member excellent in the high temperature strength characteristic of the present invention is as follows: C: 0.1 to 3.0%, Cr: 1.0 to 18.0%, and 0.3% of oxide by mass%. The mixed powder of the tool steel powder and the oxide powder mixed so as to have a volume of up to 5.0% by volume is mechanically milled, then solidified, and then quenched and tempered. Further, if necessary, the method of manufacturing a tool member according to the present invention is capable of performing machining before quenching and tempering, and processing into a tool member having a predetermined shape.

  ADVANTAGE OF THE INVENTION According to this invention, the crystal grain of a tool member can be made very fine and the high temperature strength characteristic can be improved remarkably. Therefore, in order to reduce the product cost, the technology is indispensable for practical use of a tool member that can withstand a longer life and a higher load of a tool to be used.

  As described above, an important feature of the present invention is that a technique for finely dispersing an oxide in its structure is employed as a means for improving the strength of a tool member, particularly, high-temperature strength.

  First, the reason why the oxide forming the basis of the present invention is finely dispersed will be described. By finely dispersing the oxide, crystal grain growth of the base material can be effectively suppressed. By keeping the crystal grains fine, it is possible to utilize the strengthening of the crystal grain refinement, and it is possible to replace the precipitation strengthening by carbide which has been responsible for the strength of the conventional material. When the strength is increased by using precipitation strengthening, the toughness tends to deteriorate.On the other hand, the grain refinement strengthening of the present invention does not significantly impair or improve the toughness. It is effective for improvement.

Further, oxides such as yttrium-based (Y ₂ O ₃ ), titanium-based (TiO ₂ ), and aluminum-based (Al ₂ O ₃ ) are usually subjected to heat treatment at a higher temperature than carbides formed in a tool member. Because it does not grow much during or during use, it is possible to use crystal grain refinement strengthening even in the high temperature range where carbide growth occurs and the amount of precipitation strengthening decreases significantly in conventional materials, and it is possible to dramatically increase high temperature strength. it can.

Therefore, in order to effectively utilize the above effects, it is important to simultaneously adjust the size and the number density of the oxide dispersed in the tissue. In the case of the tool member of the present invention, the dispersed state of the oxide is such that 750 or more oxides having a particle size of 25 nm or less are dispersed per 1 μm ³ , and preferably the maximum diameter of the oxide itself is 25 nm or less. I do. It is particularly preferable that the maximum diameter of the oxide be 15 nm or less and 20,000 or more per 1 μm ³ .

  Note that the dispersion state of the oxide may be evaluated from the result of thin film observation using a transmission electron microscope. By this means, it can be confirmed that the oxide is dispersed in the tissue. For example, a 400,000-fold dark field image (FIG. 1) and an element image obtained by performing element distribution mapping and forming an image only with a transmission electron beam of Fe (FIG. 1) By using one field of view in 2), the size (maximum diameter) and number density of the oxide can be obtained. By using the element image of Fe, the number of oxides can be accurately observed.

  That is, the length of the longest direction of the oxide in the dark-field image is measured, the nominal particle size may be obtained from the ASTM cutting method, and the nominal particle size may be determined as the particle size. What is necessary is just to let a nominal particle diameter be the maximum diameter. Further, the number of oxides having a diameter of 2 mm or more in the Fe element image may be counted, and the number may be divided by an observation volume (observation area × thin film sample thickness) to obtain an oxide number density. Note that the electronic images in FIGS. 1 and 2 are obtained when the test material A evaluated in later (Example 1) was tempered at 500 ° C.

  Hereinafter, the reasons for limiting the components and the crystal grain size of the tool member, which are preferable for maximizing the effects of the present invention, will be described in detail.

  C and Cr are elements that enhance quenchability and are very important in producing a quenched and tempered tool member that forms the basis of the present invention. Such an element that enhances the hardenability must be adjusted so that a sufficient hardenability can be ensured in order to realize the tool member of the present invention.

C: 0.1 to 3.0% by mass
C is an important element that enhances abrasion resistance and seizure resistance by partially dissolving in the matrix and imparting strength and partially forming carbides. In the case of using a tool member such as an intermediate tool or a cutting tool, the utility of the present invention is particularly improved. In addition, when C, which is a solid solution interstitial atom, is co-added with a substitutional atom having a high affinity for C such as Cr, an I (interstitial atom) -S (substitutional atom) effect; drag of a solute atom It is also expected to act as resistance and increase strength. However, if the content is less than 0.1% by mass, sufficient hardness and wear resistance as a tool member cannot be secured. On the other hand, an excessive addition causes a decrease in toughness and hot strength, so the upper limit is made 3.0% by mass.

Cr: 1.0 to 18.0% by mass
Since Cr has the effect of enhancing hardenability and forming carbides to strengthen the matrix and improve wear resistance, when the object of the present invention is a tool member such as a hot tool or a cutting tool, , Particularly an element that improves the usefulness of the present invention, and must be added at least 1.0% by mass. However, excessive addition causes a decrease in hardenability and hot strength, so the upper limit is 18.0% by mass.

The crystal grain size due to the prior austenite grain boundary is 10 μm or less at most. For a tool member which is used after quenching and tempering, the effect of introducing the oxide of the present invention makes the grain refinement effect by the “old austenite grain boundary after quenching and tempering”. Crystal grain size "is reflected. In the case of a normal tool member, the crystal grain size due to the former austenite grain boundary is at least about 20 μm, but the strengthening amount by the refinement of the crystal grain becomes large in a region having an average crystal grain size of 10 μm or less. In order to strengthen by utilizing crystal grain refinement strengthening, the crystal grain size of the tool member according to the present invention is set to 10 μm or less at the maximum. Preferably it is 5 μm or less, more preferably 1 μm or less.

  The evaluation of the crystal grain size based on the prior austenite grain boundary in the present invention may be performed based on the results of thin film observation using a transmission electron microscope (for example, a 40,000-fold dark field image with four visual fields). That is, the length of the largest crystal grain in the dark field image in the longest direction is measured, the nominal grain size is obtained from the ASTM cutting method, and the nominal grain size is determined as the maximum grain size. The dark-field image shown in FIG. 3 is obtained when the test material A evaluated in later (Example 1) is tempered at 500 ° C.

  In addition, the component composition of the tool member of the present invention may contain, for example, Mo, W if necessary, except that it contains C: 0.1 to 3.0% and Cr: 1.0 to 18.0% by mass%. , V, Ni, Co, and the like can be added, and a tool steel composition as described in JIS can be applied.

Next, a method for manufacturing a tool member according to the present invention will be described.
In order to achieve a tool member having a fine grain structure of the present invention, for example, a method of solidifying and forming a powder produced by a mechanical milling method can be applied, and this is finally achieved by quenching and tempering the crystal grains. This is a preferable method for manufacturing a tool member in which growth can occur. That is, C: 0.1 to 3.0% by mass%, Cr: 1.0 to 18.0%, and tool steel powder mixed so that the oxide becomes 0.3 to 5.0% by volume. This is a method for manufacturing a tool member that is subjected to mechanical milling of a mixed powder of a metal powder and an oxide powder, then solidified and formed, and then quenched and tempered. It can be.

  Conventionally, a mechanical milling method using an apparatus such as an attritor or a ball mill has been used as a means for reducing the crystal grain size of a raw material powder used for the milling, and has also been proposed in the field of tool steel (Patent Document 1). reference). In the present invention, the powder after the treatment by the mechanical milling method is also solidified and formed.Here, in the case of the present invention, a mixed powder further mixed with an oxide powder is used as a raw material powder before the mechanical milling, and the powder is machined to an atomic level. By performing the mixing, crystal grain refinement enhancement and dispersion enhancement by oxide particles that are stable even at high temperatures can be achieved.

Oxide: 0.3 to 5.0% by volume
Oxides are thermally stable even at high temperatures, so they are an important substance in effectively suppressing grain growth during heat treatment of tool members and use at high temperatures. 0.3% by volume is required. However, if the amount of the oxide is too large, the moldability at the time of solidification molding is deteriorated, and the toughness of the tool member is deteriorated. Therefore, the upper limit is set to 5.0% by volume.

  Next, the powder processed by the mechanical milling method is solidified and formed. The solidified powder is formed, and in a later step, the crystal grains grow by a heat treatment at the time of quenching and tempering. That is, in order to make the crystal grain size of the tool member produced according to the present invention fine, it is desirable that the powder produced by the mechanical milling method as the raw material has a fine crystal grain size. Therefore, the ultrafine refining of the crystal grains of the powder by the mechanical milling method is preferably 100 nm or less on average, more preferably 50 nm or less.

  In addition, high-temperature solidification such as sintering, HIP, hot rolling, and hot extrusion can be applied to the solidification molding means, and HIP, hot rolling, and hot extrusion are preferable because a completely dense material is easily obtained. The solidified material is then subjected to a normal forging / rolling process and machining in an annealed state, if necessary, and then quenched and tempered to complete a tool member.

  Further, it is very important that the tool member of the present invention is a material that can be quenched and tempered in order to manufacture a tool member having excellent high-temperature strength characteristics. That is, in order to efficiently manufacture a tool member having a large volume dimension that can sufficiently cope with a high alloy type tool member or a large tool, it is preferable that the temperature at the time of solidification molding is higher. However, in a high-temperature region exceeding 900 ° C., phase transformation occurs in almost all component systems of iron-based materials, and crystal grains grow. In addition, an extremely large crystal grain size results from the two phase transformations at the time of temperature increase and at the time of temperature decrease. In this respect, in the case of a material that can be quenched, the phase transformation in which crystal grain growth occurs occurs only once when the temperature is raised, and the crystal grain growth does not occur when the temperature is lowered, so that the fine grain size can be maintained and the grain refinement strengthened Can be used.

Specimen A shown in Table 1 was prepared by subjecting a mixed powder of an alloy powder produced by a gas atomizing method and a commercially available Y ₂ O ₃ oxide powder to mechanical milling at 100 rpm for 2 hours at 2300 rpm using a planetary ball mill. It is a prepared powder. The composition corresponds to SKD61, to which Y ₂ O ₃ oxide is added to be 3% of the total volume.

  The conditions of the mechanical milling were adjusted by adjusting the device factors so that the average crystal grain size of the powder after the treatment was 100 nm or less, observation using a transmission electron microscope (100,000-fold dark field image in one field) and The average crystal grain size of the powder after the treatment of the test material A, calculated using the half width by X-ray diffraction, was about 30 nm.

  Next, after the above-mentioned powder of the test material A was placed in a metal container and solidified by rolling at 1000 ° C., a standard material of SKD61 was prepared together with a melted SKD61 material prepared separately in Table 1 as a comparative material. A quenching treatment at 1020 ° C., which is a quenching and tempering temperature, and a tempering at 500 ° C. and 550 ° C. were performed. Then, the maximum crystal grain size of the prior austenite grain size after quenching and tempering, the oxide having a grain size of 25 nm or less, the maximum diameter and the number per unit volume, the quenched state and the hardness after tempering were measured. The evaluation of the crystal grain size was performed based on the results of thin film observation using a transmission electron microscope (40,000 magnifications, 4 visual fields). The evaluation of the oxide is based on the results of thin film observation using a transmission electron microscope (maximum diameter: 400,000-fold dark field image per field, number per unit volume: 400,000 element image of Fe, 1 field). went. The hardness was measured using a micro Vickers hardness tester. Table 2 shows the results.

The tempered structure of the test material A was composed of old austenite crystal grains having a size of about 0.1 to 0.5 μm, regardless of the tempering temperature, and the maximum crystal grain size measured was 0.5 μm. . On the other hand, regardless of the tempering temperature, the tempered structure of SKD61 produced by the melting method has an average austenite grain size of 22 μm on average, and the crystal grain size of the present invention is smaller than the general grain size of about 20 μm. Extremely fine. The maximum diameter of the oxide of the tempered material at 500 ° C. of the test material A was 9.4 nm, and there were 123,577 particles per 1 μm ^3. The maximum diameter of the oxide of the tempered material at 550 ° C. of the test material A was 9.8 nm. There were 118,904 per 1 μm ³ . Note that no oxides having a particle size exceeding 25 nm were found in the structures of both test materials.

  On the other hand, in the structure of the tempered material of SKD61 produced by the melting method, very large oxides (such as alumina-based) exceeding 1 μm were present, but oxides having a particle size of 25 nm or less were not observed by this method. Was.

  The material of the present invention has a very high quenching hardness due to the refinement of crystal grains, and the hardness is substantially maintained even when tempered at 500 ° C. and 550 ° C., which are the temperature ranges where warm tools are often used. It has excellent hardness and high-temperature strength. On the other hand, it can be seen that the hardness of the comparative material is considerably reduced after tempering. In the case of the material of the present invention, when the structure at the time of quenching was also examined, it was found to be crystal grains (large-angle grains) having a size of about 0.1 to 0.5 μm. Due to the existence of very fine and numerous oxides, the growth of crystal grains during the tempering process (use process) as well as the growth of crystal grains during the solidification molding and quenching processes at high temperatures could be suppressed.

Specimen B shown in Table 3 was obtained by subjecting a mixed powder of an alloy powder produced by a gas atomization method and a commercially available Y ₂ O ₃ oxide powder to mechanical milling at a rotation speed of 2300 rpm for 100 hours using a planetary ball mill. It is a prepared powder. The composition corresponds to a high-speed steel containing almost no primary carbide (hereinafter referred to as a matrix high-speed steel), to which Y ₂ O ₃ oxide is added so as to be 3% of the total volume. The condition setting of the mechanical milling is in accordance with (Example 1), and is calculated using observation using a transmission electron microscope (one dark field image of 100,000 magnification) and the half width by X-ray diffraction. The average crystal grain size of the powder after the treatment of the test material B was about 20 nm.

  Next, after the above-mentioned powder of the test material B was placed in a metal container and solidified by rolling at 1000 ° C., the powder was mixed with a matrix high speed steel prepared separately as shown in Table 3 as a comparative material. The matrix high speed steel was quenched at 1140 ° C., which is a standard quenching and tempering temperature, and tempered at 500 ° C. and 550 ° C. In the same manner as in (Example 1), the oxide having the largest grain size of the prior austenite grain size after the quenching and tempering and the grain size of 25 nm or less, the maximum size and the number per unit volume, the quenched state and The hardness after tempering was measured. Table 4 shows the results.

The tempered structure of the test material B was composed of old austenitic crystal grains having a size of about 0.1 to 0.4 μm, regardless of the tempering temperature, and the maximum crystal grain size measured was 0.4 μm. . On the other hand, regardless of the tempering temperature, the tempered structure of the matrix high-speed steel produced by the smelting method has an average austenite grain size of 22 μm on average, which is smaller than the general grain size of about 20 μm. The particle size is very fine. The maximum diameter of the oxide of the sample B tempered material at 500 ° C. is 10.2 nm, and there are 109971 per 1 μm ³ of the oxide. The maximum diameter of the oxide of the sample B 550 ° C. tempered material is 9.5 nm. There were 114357 per 1 μm ³ . Note that no oxides having a particle size exceeding 25 nm were found in the structures of both test materials.

  On the other hand, in the structure of the tempered material of the matrix high-speed steel manufactured by the smelting method, very large oxides exceeding 1 μm (such as alumina-based) were present, but oxides having a particle size of 25 nm or less were not used in this method. Not observed.

  The material of the present invention has a very high quenching hardness due to the refinement of crystal grains, and further rises slightly after tempering at 500 ° C. and 550 ° C., which is also a temperature range where warm tools are often used. I have. On the other hand, it can be seen that the hardness of the comparative material is considerably reduced after tempering. In addition, also in the case of the test material B, the structure at the time of quenching was composed of crystal grains (large-angle grains) having a size of about 0.1 to 0.4 μm.

  According to the present invention, by extremely refining the crystal grains of the tool member and dramatically improving the high-temperature strength characteristics, not only the life becomes longer than the conventional material, but also the conventional material cannot withstand. Applicable to high load environments.

It is a transmission electron micrograph (dark-field image) which shows the structure of the tool member of the present invention. It is a transmission electron micrograph (Fe element image) which shows the structure of the tool member of this invention. It is a transmission electron micrograph (dark-field image) which shows the structure of the tool member of the present invention.

Claims (4)

A quenched and tempered tool member comprising tool steel containing 0.1 to 3.0% by mass of C and 1.0 to 18.0% by mass of Cr, and having a grain size of 25 nm or less in the structure. A tool member excellent in high-temperature strength characteristics, characterized in that 750 or more oxides are dispersed per 1 μm ³ and the crystal grain size due to prior austenite grain boundaries is 10 μm or less at maximum. The tool member having excellent high-temperature strength characteristics according to claim 1, wherein the particle size of the oxide dispersed in the structure is 25 nm or less at the maximum. C: 0.1 to 3.0% by mass, Cr: 1.0 to 18.0%, and tool steel powder mixed so that the oxide is 0.3 to 5.0% by volume and oxidation. A method for producing a tool member having excellent high-temperature strength characteristics, comprising mechanically milling a mixed powder of material powders, solidifying and then quenching and tempering. The method according to claim 3, wherein the mixed powder of the tool steel powder and the oxide powder is mechanically milled, then solidified, machined, and quenched and tempered.