JP2005344202A - Method for nitriding metal mold and method for evaluating nitrided metal mold - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for nitriding an alloy tool steel for a metal mold and to provide a method for evaluating the nitrided metal mold. <P>SOLUTION: The method for nitriding a metal mold comprises selecting the ratio between gases used in nitriding, a salt bath compositional ratio, a treatment temperature, or a treatment time so as to satisfy the condition that, in a metal mold formed by nitriding an alloy tool steel, the difference δ2θ between the angles of diffraction and/or between the difference δW of the half-width values from the (200) faces and/or the (211) faces of α-phases in the area surface-nitrided and the area not surface-nitrided is 0.3° or larger and/or 0.3° or larger. In one embodiment of the above method, the difference δ2θ between the angles of diffraction from the α phases is 0.6° or larger and/or the difference δW between the half-widths is 0.5° or larger. The method for evaluating the nitrided metal mold is also provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a nitriding method of an alloy tool steel for a mold and an evaluation method thereof.

  In general, this mold is often nitrided and used for the purpose of increasing the hardness of the mold surface and imparting compressive residual stress and lubricity using a metal alloy tool steel for the mold as a base material. On the other hand, nitriding also increases the risk of large cracks in the mold, so it is necessary to perform nitriding appropriately, but at present it is often done empirically at present. is there. In order to extend the service life by increasing the hardness of the mold, for example, Japanese Patent Laid-Open No. 10-287965 (Patent Document 1) aims to provide a hot working mold having excellent heat crack resistance. In addition, the amount of C in the diffusion layer of the nitride layer is 80% or more and less than 100% compared with the amount of C in the base, and the depth of the hardened layer higher than the base hardness is 100 μm or less. A type has been proposed.

  Moreover, for the purpose of ensuring high heat check resistance, for example, Japanese Patent Laid-Open No. 2000-334544 (Patent Document 2) includes a base material hardness of 30 to 44 HRC, a nitride layer hardness of 500 to 900 HV, It is disclosed that the nitrided layer has a hardening depth of 80 μm or less and does not form a compound layer. In addition, for the purpose of imparting high wear resistance, Japanese Patent Laid-Open No. 2001-73087 (Patent Document 3) is characterized in that the hardness within 25 μm from the surface of the nitride layer is 1100 HV or more. Are disclosed. Furthermore, Japanese Patent Laid-Open No. 2001-316795 (Patent Document 4) discloses that the composition ratio on the gas oxysulfurizing surface can be optimized using X-rays.

Japanese Patent Laid-Open No. 10-287965 JP 2000-334544 A JP 2001-73087 A JP 2001-316895 A

  However, Patent Document 1 described above discloses that there is an optimum value for the amount of C and the hardening depth in the diffusion layer, but the compressive residual stress which is one of the purposes of nitriding is appropriately applied. There is no mention of indicators for judging whether or not. In Patent Document 2, as in Patent Document 1, the compressive residual stress of the surface layer plays an important role in the nitrided mold, but the judgment index is not mentioned at all. Further, Patent Document 3 does not mention any index for measuring crack resistance including heat check. Furthermore, Patent Document 4 does not mention any optimization of the base material base targeted in the present invention by using the X-ray method.

  On the other hand, nitriding is an inexpensive and simple surface hardening method, and is widely applied to molds. However, hot molds have improved heat crack resistance and cold molds have improved crack resistance. It is essential. So far, the microstructure control of the nitrided layer and the optimization of the hardness distribution have been proposed, but the optimal range varies depending on the type of mold material, application, and usage conditions. The current situation is.

  In order to solve the above-mentioned problems, the inventors have conducted extensive research. As a result, the X-ray diffraction angle and the half-value width of the base of the mold material are changed by nitriding using a general X-ray method. The present invention provides a nitriding method and an evaluation method thereof that can perform optimum nitriding in any method such as gas nitriding, ion nitriding, and salt bath nitriding. In other words, by selecting the type of gas or salt bath components used in the nitriding process, the processing temperature, and the processing time so that the X-ray diffraction angle and the half width at the base of the mold material are equal to or greater than a predetermined value, the mold surface It is possible to generate a compressive residual stress in an optimum range. Due to this compressive residual stress, the occurrence of heat cracks and cracks on the mold surface is remarkably suppressed.

The gist of the invention is that
(1) In a die obtained by nitriding alloy tool steel, the diffraction angle from the (200) plane and / or (211) plane of the α phase of the portion where the surface is nitrided and the portion where nitriding is not performed The gas ratio or salt bath composition ratio, processing temperature, and processing time are selected so that the difference δ2θ is 0.3 ° or more and / or the half-value width difference δW is 0.3 ° or more. A method for nitriding a mold.

(2) In the method described in the above (1), the diffraction angle difference δ2θ from the (200) plane and / or the (211) plane of the α phase is 0.6 ° or more and / or the half width difference δW. Is a mold nitriding method, characterized by being 0.5 ° or more.
(3) From the value of diffraction angle difference δ2θ and / or half-value width difference δW from (200) plane and / or (211) plane of α phase as described in (1) or (2) above, mold There is a mold evaluation method characterized in that the deterioration of the surface of the mold is evaluated and the mold is replaced.

  As described above, according to the present invention, it is possible to perform an optimum and stable nitriding treatment on the mold surface, and the excellent effect of prolonging the life of the mold can be achieved.

  Hereinafter, the present invention will be described in detail with reference to the drawings. Alloy tool steels for molds according to the present invention include cold tool steels such as JIS-SKD11, hot tool steels such as JIS-SKD61, and tools obtained by adding various elements to these JIS steel types and improving them. It can be used for steel. Using the X-ray diffraction method, the initial state of the tool tool steel for molds before use of the mold is obtained from the surface of the mold that has been subjected to nitriding treatment and the α phase of the portion that has not been subjected to nitriding treatment. The diffraction angle and half width were measured. FIG. 1 is a diagram showing an example of X-ray diffraction patterns of α-Fe (ferrite) and γ-Fe (austenite) by X-ray diffraction. When the horizontal axis represents the diffraction angle (2θ degrees) and the vertical axis represents the intensity, as shown in this figure, the peak values are α (200), γ (220), α (211), and γ (311). It can be seen that they appear in order. In the present invention, the α (200) plane and (211) plane, which are the measured peak values, were used.

  FIG. 2 is a diagram showing a diffraction angle and a half width at a peak value by X-ray diffraction. Here, the diffraction angle means that when X-rays are applied to the material, the X-rays are diffracted by a lattice plane specific to the material, and an X-ray diffraction pattern is obtained as shown in the figure. The angle at which the maximum peak height is obtained by this X-ray diffraction pattern is called the diffraction angle. Further, the peak width at the half of the maximum peak height according to the X-ray diffraction pattern is referred to as a half width (W). FIG. 3 is a diagram showing a change state of a diffraction angle and a half width at a peak value by X-ray diffraction. FIG. 3A shows a state where the diffraction angle is shifted depending on the internal state of the material. FIG. 3B shows a state in which the half width is expanded depending on the internal state of the material.

  As described above, the diffraction angle is deviated or the half width is widened depending on the internal state of the material. Therefore, in the initial state, the diffraction angle from the α phase on the mold surface shifts by 1 ° or more to a lower angle side than the portion not subjected to nitriding treatment, and the half width is expanded by 0.8 ° or more. It was confirmed. In addition, as the number of use cycles increases, the difference between the diffraction angle and the half-value width obtained from the α phase of the nitridized mold surface and the non-nitrided part is small. Become. Specifically, the nitriding is performed so that the diffraction angle from the α phase on the mold surface shifts 0.3 ° or more to the lower angle side than the diffraction angle obtained internally, and the half width is expanded 0.3 ° or more. By selecting the gas ratio, salt bath composition ratio, treatment temperature, or treatment time used in the treatment, an appropriate range of compressive residual stress is imparted to the mold surface.

  When the difference in diffraction angle δ2θ obtained from the (200) plane of the α phase and / or the (211) plane is less than 0.3 °, the compressive residual stress (MPa) value is lower than the target compressive residual stress value, and Since the value of hardness (HV) at a position of 30 μm from the surface cannot be obtained, the number of cycles until the occurrence of heat cracks is reduced, so the angle is set to 0.3 ° or more. When the half width difference δW is less than 0.3 °, the compression residual stress (MPa) value is lower than the target compression residual stress value as in the case of the diffraction angle difference δ2θ. The number of cycles is reduced. Accordingly, the angle is set to 0.3 ° or more. More preferably, the diffraction angle difference δ2θ is 0.6 ° or more and / or the half-value width δW is 0.5 ° or more.

  As described above, nitriding is performed so that the difference in diffraction angle δ2θ and / or the half width δW obtained from the (200) plane and / or (211) plane of the α phase is 0.3 °, respectively. The gas ratio, salt bath composition ratio, treatment temperature, or treatment time used for the treatment is selected. As a specific method thereof, for example, the gas used for the gas nitriding treatment is a nitriding atmosphere containing ammonia gas. Gas is used, and the ammonia gas content is 20 to 60%. As the salt bath composition ratio, the content of cyanate is 30 to 60%. Further, the processing temperature is 450 to 600 ° C., the processing time is 5 to 30 hours, and the secondary nitriding is a predetermined temperature for selecting a nitriding atmosphere gas in the range of 200 to 450 ° C. The treatment time is 0.5 to 5 hours.

  On the other hand, the deterioration of the mold can be evaluated using these, and it can be used for the judgment evaluation of whether or not it is necessary for the mold replacement. For example, the diffraction angle difference δ2θ and / or the half-value width δW obtained from the (200) plane and / or (211) plane of the α phase measured during use was measured before or at the beginning of use, respectively. The mold is exchanged when the diffraction angle difference δ2θ and / or the half-value width δW of the mold becomes 1/2 to ３.

(Example 1)
Hereinafter, the present invention will be specifically described with reference to examples.
A mold steel (equivalent to SKD61) was melted in a 1-t vacuum induction melting furnace, and the obtained steel ingot was forged to φ100 mm and then annealed to obtain a test material. The specimen was quenched from 1030 ° C. and then tempered twice at various temperatures of 550 to 650 ° C. Nitriding was performed by plasma nitriding at a processing temperature of 500 to 600 ° C. for 1 to 10 hours. In the heat check test, a test piece having a diameter of 60 × 60 mm was used. After heating to 600 ° C., rapid cooling to room temperature with 20 ° C. water was performed repeatedly. The presence or absence of heat cracks was confirmed every 200 cycles. The results are shown in Table 1.

  As shown in Table 1, no. 1-4 are examples of the present invention. 5-7 are comparative examples. Comparative Example No. No. 5 has a small diffraction angle difference δ2θ of 0.2, so that the compressive residual stress is low, the hardness at the 30 μm position from the surface is low, and the number of cycles until the occurrence of heat cracks is small. Comparative Example No. No. 6 has a small half-value width difference δW as small as 0.2, so that the compressive residual stress is low and the number of cycles until the occurrence of heat cracks is small. Comparative Example No. 7 has a diffraction angle difference δ2θ as low as 0.1 and a half-value width difference δW as small as 0.2, so that the compressive residual stress is low and the hardness at 30 μm position from the surface is It is low and it turns out that the number of cycles until heat crack generation is small. On the other hand, the present invention example No. It turns out that 1-4 is excellent also in any performance.

(Example 2)
Next, a mold steel (equivalent to SKD11) was melted in a 1-t vacuum induction melting furnace, and the obtained steel ingot was forged to φ100 mm and then annealed to obtain a test material. The specimen was quenched from 1030 ° C. and then tempered repeatedly at each temperature of 450 to 550 ° C. twice. Nitriding was performed by plasma nitriding at a processing temperature of 400 to 520 ° C. for 1 to 10 hours. In the fatigue test, a rotating bending test piece having a parallel part φ6 × 10 mm was used, and the evaluation was performed with a stress that does not cause cracking 10 ⁶ times. The results are shown in Table 2.

As shown in Table 2, no. Nos. 8 to 11 are examples of the present invention. 12 to 14 are comparative examples. Comparative Example No. No. 12 has a diffraction angle difference δ 2θ as small as 0.2, so the compressive residual stress is low and the stress value at which cracking does not occur after 10 ⁶ times is small. Comparative Example No. No. 13 has a small half-value width difference δW of 0.2, so the compressive residual stress is low and the stress value at which cracking does not occur after 10 ⁶ times is small. Comparative Example No. No. 14 has a diffraction angle difference δ2θ as low as 0.1 and a half-value width difference δW as small as 0.2, so that the compressive residual stress is low and cracking does not occur after 10 ⁶ times. Stress value is small. On the other hand, the present invention example No. It can be seen that 8 to 11 are all excellent in performance.

It is a figure which shows an example of the X-ray-diffraction pattern of (alpha) -Fe (ferrite) and (gamma) -Fe (austenite) by X-ray diffraction. It is a figure which shows the diffraction angle and half value width in the peak value by X-ray diffraction. It is the figure which showed the change state of the diffraction angle and half value width in the peak value by X-ray diffraction.

Claims (3)

In a die obtained by nitriding alloy tool steel, the difference δ2θ in diffraction angle from the (200) plane and / or (211) plane of the α phase between the nitridated portion and the non-nitrided portion on the surface is The gas ratio, salt bath composition ratio, treatment temperature, or treatment time used for the nitriding treatment is selected such that the difference δW of 0.3 ° or more and / or the half-value width difference is 0.3 ° or more. The nitriding method of the mold. 2. The method according to claim 1, wherein a diffraction angle difference δ2θ of the α phase from the (200) plane and / or the (211) plane is 0.6 ° or more and / or a half width difference δW is 0.5. A mold nitriding method characterized by being at least ° C. Deterioration of the mold surface is evaluated from the value of the diffraction angle difference δ2θ and / or the half-value width difference δW of the α phase according to claim 1 or 2 from the (200) plane and / or the (211) plane. A method for evaluating a mold, wherein the mold is replaced.

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