JP2006328521A - Tool for precision working and tool steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tool steel used for the material of a tool for cold working, which has both high hardness and high toughness, and to provide a method of producing a tool for precision working using the same. <P>SOLUTION: The tool steel has an alloy composition comprising, by weight, 0.45 to 0.90% C, 0.1 to 2.0% Si, 0.1 to 1.5% Mn, 3.0 to 8.0% Cr and 0.02 to 0.5% V, and further comprising Mo and W so as to satisfy Mo+0.5W: 0.1 to 2.5%, and the balance Fe with impurities, and in which, provided that L=15.5C(%)+Cr(%), 14≤L≤20 is satisfied. In the tool steel, the number of carbides with a size of ≥10 μm expressed in terms of diameter is ≤10 pieces in a field of 0.5 mm<SP>2</SP>, and, when A-based, B-based and C-based inclusions are measured in accordance with the method of JIS G0555, (dA+dB+dC)60×400≤0.01% is satisfied. The tool steel is held at ≥1,150°C for ≥5 hr, thereafter, the shape of a tool is imparted thereto, then, quenching is performed from 1,000 to 1,060°C, and subsequently, tempering is performed at 500 to 65°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a tool, particularly a tool such as forging or pressing, which is suitable for precise machining, and a tool steel used as the material thereof.

Various molds correspond to the fact that products manufactured using them are miniaturized, combined or enhanced in performance, the shape becomes more complicated and high dimensional accuracy is required. The demand for increasingly complex shapes and higher dimensional accuracy must be met. For example, the dimensional accuracy is often required to be 10 μm or less.

In the case where the mold is required to have a high hardness of HRC 55 or higher, the wear resistance determines the mold life, so that it is important to achieve a high hardness and to disperse the carbide appropriately. Taking an example of cold tool steel represented by JIS SKJD11 and SKH51, a large number of large carbides having a high hardness and exceeding 10 μm are distributed.

As a result of studying the intention of providing a cold work tool steel having both high toughness and wear resistance, the applicant has studied the balance between the V content and the Cr content, and Cr: 6.0 In the tool steel containing ~ 9.0% and V1.0 ~ 2.3%, by satisfying the relationship of 0.35> V / Cr> 0.15, the morphology of the primary crystallized carbide is controlled and fine It was discovered that a large number of crystallized VCs had good results (Patent Document 1).

Whether the above-mentioned known tool steel or the tool steel disclosed by the applicant, it has been experienced that when a mold is produced using the tool steel as a material, an unsatisfactory product is produced with respect to dimensional accuracy. For example, when machining with a small-diameter end mill, troubles such as irregularities of about 10 μm and breakage of the cutting tool occur even though the machining accuracy itself is 1 μm or less. When the reason was investigated, it turned out that it is because a large sized carbide | carbonized_material exists.

Since the carbide has a very high hardness and the carbide cannot be processed with a cutting tool, the tool in contact with the large carbide breaks or deviates from a specified position. The same problem occurs when electric discharge machining or wire electric discharge machining is performed regardless of cutting. Regarding surface modification treatment and mirror finishing, troubles such as failure to obtain desired dimensional accuracy, large surface roughness, and surface treatment film peeling off due to carbides occur. . In order to suppress the formation of giant carbides that are harmful to the toughness of the tool, there is a proposal of a cold tool steel that attempts to optimize the balance between the C content and the Cr content (Patent Document 2). I have not stepped into the existence form.
JP-A-10-025554 JP-A-10-060596

The object of the present invention is to provide a tool steel in which large carbides and inclusions are reduced as much as possible without lowering the hardness, by taking the current state of the above technology one step forward, and using it as a material, with high dimensional accuracy. It is to realize a tool and its manufacturing method.

The tool steel of the present invention has a basic alloy composition in weight%, C: 0.45-0.90%, Si: 0.1-2.0%, Mn: 0.1-1.5. %, Cr: 3.0 to 8.0%, and V: 0.02 to 0.5%, Mo and W are contained so that Mo + 0.5W: 0.1 to 2.5% And the balance consists of Fe and inevitable impurities,
When L = 15.5C (%) + Cr (%), 14 ≦ L ≦ 20
The number of carbides having a diameter of 10 μm or more in terms of diameter is 10 or less in a 0.5 mm ² field of view, and inclusions of A, B, and C are defined in JIS G0555. When measured according to the method, (dA + dB + dC) 60 × 400 ≦ 0.01%.

The tool steel of the present invention has few crystallized carbides, especially large ones, and by applying an appropriate heat treatment, the amount of residual austenite and residual stress is low, so that it can be easily processed into a precise mold. . The manufactured mold has both high toughness and high wear resistance, has high dimensional stability, has high performance as a mold for precision machining, and has a long life.

In addition to the above basic alloy components, this tool steel can additionally contain one or more of the following optional components.
1) P: 0.03% or less, S: 0.03% or less, Al: 0.03% or less, O: 0.005% or less, and N: 0.025% or less 2) Cu: 1.0% or less, Ni: 1.0% or less, Co: 1.0% or less and B: 0.01% or less 1 type or 2 types or more 3) Se: 0.05% or less, Te: 0.05% or less, Pb: 0.05% or less, Bi: 0.05% or less, and Ca: 0.05% or less 4) Nb: 0.1% or less, Ta: 0. 1% or less, Ti: 0.1% or less, Zr: 0.1% or less, Mg: 0.1% or less, and REM: 0.1% or less

The tool of the present invention using such a tool steel is made of a tool steel having any of the alloy compositions described above, held at a temperature of 1150 ° C. or higher for at least 5 hours, and then subjected to plastic working to change the shape of the tool. A tool having the carbides and inclusions described above. This tool is subjected to heat treatment consisting of quenching from 1000 to 1060 ° C. and tempering from 500 to 650 ° C. following the above steps, the amount of retained austenite after heat treatment is 5% or less, and the surface residual stress is within ± 200 MPa. The hardness can be in a state of 55 to 65 HRC. Thereby, a tool capable of performing desired precision machining is realized.

Below, the development process which reached | attained the tool steel of this invention and the knowledge acquired there are demonstrated. The inventor first attempted to keep the hardness at the same level as that of the conventional tool steel and to reduce as much as possible large carbides (crystallized carbides having a diameter equivalent to 10 μm or more). For this, it is necessary to adjust the amounts of C and Cr, Mo, W and V as carbide forming elements, but these are also elements that determine the hardness obtained by quenching and tempering, In particular, since the amounts of C and Cr are important, the “L” value defining the amount of carbide is limited to an appropriate range. Moreover, not only carbides but also inclusions in steel cause troubles during processing, as with carbides. Therefore, inclusions in the A system, B system, and C system should be reduced as much as possible. These are the first findings.

Next, it is necessary to reduce the amount of carbide crystallized as much as possible and to reduce the size. As a result of the experiment, the limit of “10 or less carbide per 0.5 mm ² in diameter equivalent to 10 μm” was found. As a means for realizing this, it has been found that it is effective to dissolve carbide as much as possible in the tool manufacturing process. If the amount of carbides is reduced by optimizing the alloy composition, the number of large carbides can be reduced. However, in actual production, there are large variations depending on the casting size of the ingot and the parts in the ingot. By following the forging / rolling start temperature of about 1100 to 1140 ° C. that has been adopted for conventional tool steel, the carbide is maintained as it is, and finally, depending on the part, a large amount of large carbide remains. Become. In view of this, a measure was taken to maintain the temperature at 1150 ° C. or higher exceeding the conventional heating temperature for 5 hours or more. This reduces the amount of carbide and reduces the size, thereby achieving the above goals. A preferable heating temperature is 1180 to 1250 ° C. This is the second finding.

Finally, the achievement of high dimensional accuracy is achieved when the mold dimensions are determined from the design value in the state of use as a mold, that is, after quenching and tempering, and processing into a mold shape and surface treatment are completed. It is necessary not to fluctuate. The reason why the mold dimensions fluctuate at this stage is that the austenite structure remaining in the steel decomposes due to temperature conditions during use, the passage of time, processing load, etc., and leads to dimensional changes. It is conceivable that the stress remaining therein is released by heat treatment or processing to cause a dimensional change.

In order to reduce residual austenite and residual stress, it is necessary to perform proper quenching and tempering. Residual austenite decomposes when the tempering temperature is 500 ° C. or higher, and the residual stress is released at a similar temperature. Therefore, a tempering temperature of 500 ° C. or higher may be adopted, but the hardness decreases as the tempering temperature increases. Therefore, the upper limit of 650 ° C. was selected as a condition for obtaining HRC 55 or higher. When the quenching temperature is less than 1000 ° C., the hardness of the carbide is small and the hardness is low, and when it exceeds 1060 ° C., the crystal grain size becomes very large and the toughness is lowered. Therefore, quenching is performed from 1000 to 1060 ° C, and tempering is performed at 500 to 650 ° C. This is the third finding.

In order to decompose the retained austenite, it is also effective to perform sub-zero treatment after quenching. This treatment needs to be performed at 0 ° C. or less, but can be easily realized by, for example, (dry ice + alcohol). If even a little retained austenite exists after tempering, it is recommended to perform tempering again at a temperature of about 300 to 480 ° C. Thereby, a little residual austenite is also stabilized, and the dimensional change can be suppressed to a very small level. By combining the above three findings, extremely high dimensional accuracy and shape stabilization can be achieved for the mold.

In the tool steel of the present invention, by satisfying the above basic alloy composition, crystallized carbide mainly composed of M7C3 and secondary carbide mainly composed of M23C6 are generated. The former production amount is in the range of 0.01 to 5% by weight. In order to obtain a hardness of HRC 55 or higher by quenching / tempering, the above alloy composition is required. The reason why the alloy composition, carbide distribution, and amount of inclusions are determined as described above will be described below.

C: 0.45-0.90%
C is indispensable for increasing the hardness of martensite during quenching, and combines with carbide-forming elements such as Cr, Mo, V, etc. to form carbides. It works to improve wear. In order to ensure the hardness HRC 55 or more after quenching / tempering described above, the presence of 0.45% or more is necessary. If it exceeds 0.90%, the amount of crystallized carbide increases, and the toughness decreases.

Si: 0.1 to 2.0%
Si is added as a deoxidizer and contained in the steel. It is a component that contributes to improving the high-temperature tempering hardness, and for that purpose, 0.1% or more is added. Addition of a large amount impairs hot workability and reduces toughness as a characteristic after quenching and tempering, so 2.0% is made the upper limit.

Mn: 0.1 to 1.5%
Mn improves hardenability. In order to obtain a hardness of HRC 55 or higher, addition of 0.1% or more is required. If it is present in a large amount, the hot workability is lowered, so the addition is limited to 1.5%.

Cr: 3.0-8.0%
Cr dissolves in the matrix to improve hardenability and contribute to improvement in hardness, and also forms carbides and increases wear resistance. In order to acquire this effect, it is necessary to add 3.0% or more. Addition exceeding 8.0% results in the formation of an excessive amount of crystallized carbides, resulting in deterioration of toughness and machinability as characteristics after quenching and tempering, so this is the upper limit.

V: 0.02-0.5%
The action of V is to form stable carbides to prevent coarsening of crystal grains, and to form fine carbides and contribute to improvement of wear resistance and hardness. In order to obtain such an effect, 0.02% or more of V is required. If the addition is too large, the amount of crystallized carbide will increase, the machinability will deteriorate, and the hot workability will deteriorate, so an addition amount within 0.5% is selected.

Mo equivalent (Mo + 0.5W): 0.1-2.5%
Both Mo and W are dissolved in the matrix to enhance the hardenability and improve the hardness of the steel. At the same time, carbide is formed to increase wear resistance and to improve temper softening resistance. These effects are obtained when the Mo equivalent is 0.1% or more. An excessive Mo equivalent reduces hot workability, toughness, and machinability, so the value is set to 2.5%.

When L = 15.5C (%) + Cr (%), 14 ≦ L ≦ 20
In the present invention, as described above, it is an important premise to keep the crystallized carbide in a small amount. Therefore, the amount of C and Cr, which are the main elements forming the crystallized carbide, is controlled by the value of L. In order for the amount of crystallized carbide to be in the range of 0.01 to 5% by weight, L must be in the range of 14 to 20.

Carbide with a diameter of 10 μm or more in terms of diameter: 10 or less in a field of 0.5 mm ² Here, “diameter conversion” means the diameter of a circle when the carbide is converted into a circle having the same projected area. . Since the crystallized carbide generally has a large diameter of 10 μm or more in terms of diameter, it is within 10 and preferably within 5 in a visual field of 0.5 mm ² .

A system, B system and C system inclusions: (dA + dB + dC) 60 × 400 ≦ 0.01%
Any of these inclusions reduces the machinability, so the content is limited to 0.01% or less. Thereby, troubles during the electric discharge machining and wire electric discharge machining and grinding are reduced, and the surface roughness is improved.

P: 0.03% or less, S: 0.03% or less, Al: 0.03% or less, O: 0.005% or less, and N: 0.025% or less, both in the steel Inevitably contained. Among them, Al and O are constituents of B-type and C-type inclusions, and S is a constituent of A-type inclusions, which affect the respective generation amounts. When these components become large, toughness and machinability are impaired, so that the content is within the above allowable limit. By actively reducing these, stable toughness can be maintained, but a balance with manufacturing costs must be considered.

One or more of Cu: 1.0% or less, Ni: 1.0% or less, Co: 1.0% or less, and B: 0.01% or less These elements are dissolved in the matrix, Improve hardenability. It also helps to improve toughness and thereby maintain weldability through a mechanism that lowers the impact transition temperature. Co also has the effect of increasing the high temperature strength. In any case, the effect is saturated even when added in a large amount, so the above-mentioned addition limits were determined from that viewpoint.

Se: 0.05% or less, Te: 0.05% or less, Pb: 0.05% or less, Bi: 0.05% or less, and Ca: 0.05% or less Although it is an element that improves the properties and is added for that purpose, Se and Te combine with Mn to increase A-based inclusions, and Ca forms an oxide. Pb and Bi alone precipitate at the grain boundaries. In the present invention, which is intended to provide a precise tool, an amount within a range not exceeding the above-mentioned limit is selected even when it is added so as not to roughen the surface roughness during precision machining. A preferable range is 0.01% or less.

Nb: 0.1% or less, Ta: 0.1% or less, Ti: 0.1% or less, Zr: 0.1% or less, Mg: 0.1% or less, and REM: 0.1% or less Alternatively, two or more kinds of these elements can be added in anticipation of an effect of improving toughness due to refinement of carbides and refinement of crystal grains. If the addition amount is too large, the toughness is lowered and the weldability becomes unfavorable, so 0.1% is made the upper limit.

Quenching: 1000-1060 ° C / Tempering: 500-650 ° C
By performing heat treatment under these conditions, a tool having a residual austenite amount of 5% or less, a surface residual stress within ± 200 MPa, and a hardness of HRC 55 to 65 can be obtained. In order to obtain the hardness HRC55, a quenching temperature of 1000 to 1060 ° C. is necessary, and a tempering temperature of 500 ° C. or more is necessary for decomposition of residual austenite and reduction of surface residual stress. Since the hardness decreases as the tempering temperature increases, in order to secure HRC55, the limit was set to 650 ° C.

Each 50 kg of steel having an alloy composition (% by weight, balance Fe) shown in Table 1 was melted in a high-frequency vacuum melting furnace and cast into an ingot. Each ingot was held at the plastic working temperature shown in Table 2 for 5 hours or more and then forged hot to form a square bar with a side of 35 mm. After forging, it was gradually cooled and subjected to spheroidizing annealing treatment (maintained at 800 ° C. and gradually cooled at a cooling rate of 18 ° C./hour). The following tests were conducted on each steel of the examples and comparative examples. The test results are shown together in Table 2.

[Measurement of carbide content and inclusion content]
The surface of the polished sample is corroded (picric acid-ethanol or Virella solution), and a field of view of 0.5 mm ² or more is observed with an optical microscope (magnification 200), image analysis is performed, and the average grain system of carbides and the circle equivalent The number of carbides having a diameter of 10 μm or more was measured, and the result was converted to the number per 0.5 mm ² field of view. Furthermore, (dA + dB + dC) 60 × 400 was measured by the method defined in JISG0550.

[Precision processing test] End mill cutting test Tool: Solid carbide end mill (diameter 0.2mm), 2-flute Cutting speed: 350m / min Feed: 0.01mm / rev.
Cutting depth: 0.05mm width, 2mm height
Tool cooling method: Air blow Tool life: Cutting distance to breakage

[Hardness]
What was heat-treated under the quenching and tempering conditions shown in Table 2 was measured on the Rockwell C scale.
[Amount of retained austenite]
A test piece having a side of 10 mm and a thickness of 2 mm was cut out from the test piece whose hardness was measured, and the intensity of each phase was measured using an X-ray diffractometer. The ratio of the austenite phase to the martensite phase intensity was used as the residual ratio of the austenite phase.
[Residual stress]
A test piece having a side of 30 mm and a length of 40 mm was subjected to heat treatment under the quenching and tempering conditions shown in Table 2, and the residual stress on the surface was measured by X-ray diffraction.
[Impact resistance] A Charpy test piece having a Charpy impact value of 10R notch was cut out and heat-treated so that the rolling direction of the material was the length direction of the test piece. The test was conducted at room temperature according to the method defined in JISZ2242.

[Mirror finish]
A test piece of 50 mm × 10 mm × 30 mm was ground after grinding (# 180 → # 240 → # 400 → # 800 → # 1500 → # 3000 → # 14000) by mechanical polishing (after # 3000) Using diamond abrasive grains), the maximum surface roughness Ry was measured.

The following can be understood from the data of the examples. In the steel of Comparative Example 1, both C and Cr are excessive amounts outside the scope of the present invention, and the value of L exceeds the upper limit of the present invention, so there are 10 large carbides. . The presence of such coarse carbides hindered precise processing, and reduced Charpy impact value and specularity. In Comparative Example 2, contrary to Comparative Example 1, the Cr amount is too low and the value of L is also below the lower limit. Therefore, even when quenching and tempering under appropriate conditions, the lower limit HRC55 of hardness is not obtained. Therefore, it cannot be used as a mold material. In Comparative Examples 3 and 6, the amounts of P, S, and O are higher than the regulation values, so that the amount of inclusions is increased, and the Charpy impact value and the specularity are low.

In Comparative Example 4, since the amount of V is excessive, the amount of C and Cr is within the range of the present invention, and the balance between them is appropriate, but the amount of crystallized carbides increases and precision workability is increased. Both Charpy impact value and specularity are low. Since the comparative example 5 had too much Mo amount and the temperature of plastic working (hot forging) was low, a large amount of crystallized carbide remained. Therefore, this steel has poor precision workability, Charpy impact value and specularity. In Comparative Example 7, since the amount of Si exceeded the limit of the present invention, the Charpy impact value remained at a low value. From the heat treatment conditions of Comparative Examples 1 and 6, it can be seen that the tempering temperature is too low, the amount of retained austenite and the residual stress are both high, and as a result, the precision workability is low.

The tool steel of the present invention exhibits its performance best when used as a die material for cold working. Specific examples include bending dies, punching dies, cutting blades, rolling dies, punch members such as gears, drawing dies, forging dies, dies, swaging dies, and the like. It is also useful as a material for structural members and has applications such as plastic working tools, screw members, cam members, seal plates, and gauges. In addition, the tool steel of the present invention is also suitable for use as a mold for molding various plastics and rubbers, for example, an injection mold or an extrusion mold.

The above molds and structural members include those subjected to various surface treatments, those subjected to mirror finishing and chemical etching, of course, CVD treatment, PVD treatment, TD treatment, nitriding, chromium plating, Ni- There are P-plated, leather wrinkles, and satin finish. Products subjected to surface modification such as electron beam irradiation, thermal spraying, and shot peening are also included in the tool of the present invention.

Claims (7)

By weight, C: 0.45-0.90%, Si: 0.1-2.0%, Mn: 0.1-1.5%, Cr: 3.0-8.0%, and V : In addition to 0.02 to 0.5%, Mo and W are contained so that Mo + 0.5W: 0.1 to 2.5%, with the balance being Fe and inevitable impurities,
When L = 15.5C (%) + Cr (%), 14 ≦ L ≦ 20
The number of carbides having a diameter of 10 μm or more in terms of diameter is 10 or less in a 0.5 mm ² field of view, and inclusions of A, B, and C are defined in JIS G0555. Tool steel, wherein (dA + dB + dC) 60 × 400 ≦ 0.01% when measured according to the method. In addition to the alloy components described in claim 1, P: 0.03% or less, S: 0.03% or less, Al: 0.03% or less, O: 0.005% or less, and N: 0.025% or less The tool steel of Claim 1 containing 1 type, or 2 or more types of these. In addition to the alloy components described in claim 1 or 2, one or two of Cu: 1.0% or less, Ni: 1.0% or less, Co: 1.0% or less, and B: 0.01% or less The tool steel of Claim 1 or 2 containing the above. In addition to the alloy components according to any one of claims 1 to 3, Se: 0.05% or less, Te: 0.05% or less, Pb: 0.05% or less, Bi: 0.05% or less, and Ca: The tool steel according to any one of claims 1 to 3, containing 0.05% or less of one kind or two or more kinds. In addition to the alloy components according to any one of claims 1 to 4, Nb: 0.1% or less, Ta: 0.1% or less, Ti: 0.1% or less, Zr: 0.1% or less, Mg: The tool steel according to any one of claims 1 to 4, comprising one or more of 0.1% or less and REM: 0.1% or less. The tool steel having the alloy composition according to any one of claims 1 to 5 is used as a material, and the shape of the tool is given by performing plastic working after being held at a temperature of 1150 ° C or higher for at least 5 hours. Tool for precision machining with carbides and inclusions. The tool according to claim 6, which is subjected to a heat treatment comprising quenching from 1000 to 1060 ° C. and tempering from 500 to 65 ° C., a residual austenite amount after the heat treatment is 5% or less, and a surface residual stress is ± 200 MPa. A tool for precision machining with a hardness of HRC55-65.