JP5225596B2 - Method for strengthening alloy steel for hot mold and alloy steel for hot mold formed by suppressing generation of thermal fatigue crack by the method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for strengthening a hot-die alloy steel and a hot-die alloy steel that suppresses the occurrence of thermal fatigue cracks by the method, and more particularly, die casting represented by SKD61 steel. Method for strengthening hot mold alloy steel capable of suppressing the occurrence of so-called cracks called heat cracks or heat checks due to thermal fatigue of hot mold alloy steel such as molds, and thermal fatigue caused by the method The present invention relates to an alloy steel for hot metal molds which suppresses the generation of cracks.

(1) Mold strengthening method Conventionally, in the die-cast casting, the above-mentioned heating / cooling is required for a mold that requires severe operating conditions by repeated rapid heating and cooling of molding and applying a release agent. As a low cycle fatigue phenomenon due to thermal cycle due to expansion and compression due to thermal cracks, cracks due to thermal fatigue such as fine turtle-shaped surface cracks (heat check) and independent long surface cracks (heat cracks) on the mold surface Has occurred, reducing the life of the mold.

  Therefore, various surface treatments are used for the purpose of stabilizing the product surface. The purpose of this treatment is to prevent the occurrence of cracks in the mold, and to improve the thermal shock resistance and seizure properties.

  These processes include, for example, performing nitriding a plurality of times and performing shot peening for the purpose of applying compressive residual stress in combination with the nitriding process (Patent Document 1). In the manufacturing process of the above-mentioned SKD61 steel die-casting mold, the surface of the mold is subjected to nitriding treatment such as gas nitriding to form a nitrided layer of about 70 μm, and then a compressive residual stress is applied by shot peening to obtain a material hardness In contrast to HV450, the hardness of the nitrided layer was increased to HV700-1200, and the maximum value of compressive residual stress was -1000-1400 MPa.

(2) Nanocrystallization by shot peening Although the structure change and the formation mechanism of the nanocrystal structure by the shot peening have not yet been fully elucidated, the surface of the material to be treated is nanocrystallized by shot peening. Thus, it has been proposed to perform various modifications to the material to be treated.

  For example, aluminum alloy is processed with zircon shot and nano-crystallized to improve hardness and corrosion resistance (Patent Document 2), ultrasonic peening to HV320 or lower steel grade (Patent Document 3), shot peening metal material In addition to making the surface layer amorphous, part of the particles and the metal material surface layer are plastically deformed in the metal material surface layer to cause mechanical alloying between them. By adopting a nanocrystal structure, a nanocrystal structure having a desired alloy composition can be obtained, or a desired composition can be provided around the nanocrystal. For example, by using magnesium alloy as shot peening particles for a metal material containing aluminum as a main component, the surface layer of the metal material containing aluminum as a main component can have an aluminum alloy composition containing magnesium. Is disclosed (Patent Document 4).

Prior art document information of the present invention includes the following.
JP 2003-253422 A JP 2005-231252 A JP 2004-169100 A JP 2003-201549 A

(1) Problems with the conventional mold strengthening method In the above conventional method, the nitrided layer is decomposed by heating by practical casting, and the high hardness and compressive residual stress obtained by nitriding and shot peening are reduced. Since the increase in hardness due to nitriding is excessively high, micro-cracks propagated all at once due to thermal shock, leading to failure.

  In die casting, in order to produce high-precision castings, molten metal such as aluminum alloy is filled into the mold at high pressure, but the molten aluminum alloy reacts with the mold to form an intermetallic compound. This causes a erosion phenomenon that erodes and falls off. There is a case where an iron nitride layer is provided on the surface by the nitriding treatment, and this has the purpose of suppressing the reaction between the mold and the molten aluminum by the iron nitride. However, since such a nitride layer decomposes and disappears during use, it is not possible to maintain the long-term resistance to melting damage (performance for suppressing melting damage).

(2) Efficacy and problems of nanocrystallization In the conventional nanocrystallization method described above, nanocrystallization near the surface can be performed relatively easily by shot peening. It is a nanocrystallization method, and does not provide a nanocrystal structuring technique for hard metals such as hot die alloy steel to be treated in the present invention. In addition, the effect of conventional shot peening has been understood for many years to suppress the occurrence of cracks by applying compressive residual stress, and compressive residual in materials exposed to high temperatures, such as alloy steel for hot molds. It was thought that the stress was relaxed and had no effect. For this reason, there have been few cases where the effect of shot peening has been studied in detail for alloy steels for hot molds with relatively high hardness. Naturally, the suppression of thermal fatigue cracks in the treated material and the nanostructure of the surface texture The relationship with crystallization was not considered at all.

However, as a result of research by the inventors of the present invention, it is effective in suppressing thermal fatigue cracks in alloy steel for hot molds by forming a nanocrystal layer without a layered work structure described later on the surface. It was confirmed that there was.

(3) Object of the present invention The object of the present invention is to achieve nanocrystallization by collision of particles on the surface of die casting mold steel represented by SKD61 steel, and if necessary, the nanocrystal structure and plasma nitriding. An object of the present invention is to strengthen the die-casting mold capable of suppressing the occurrence of thermal fatigue cracks by forming a nitrogen diffusion layer at least, and to provide such a strengthened mold steel.

  In addition to nanocrystallization on the mold surface, or if necessary, nanocrystallization and formation of a nitrogen diffusion layer, the present invention provides a chromium nitride layer having a great effect of suppressing reaction with aluminum on the outermost surface. This ceramic layer is not decomposed at the operating temperature of a normal die-casting mold, and therefore the object is to maintain the resistance to melting for a long time.

Therefore, the method for suppressing thermal fatigue cracking of the alloy steel for hot molds of the present invention is as follows: C: 0.32 to 0.42%, Si: 0.80 to 1.20, Mn: 0.50 or less, P: 0.030 or less, S: 0.020 or less, Cr: 4.50 to 5.50, Mo: 1.00 to 1.50, V: 0.30 to 1.15, and other materials to be treated as alloy steels for hot molds made of HV450 to 520 steel containing Fe and inevitable impurities In contrast, a solid-gas two-phase mixed fluid of particles with a hardness of HV550 to 1100 and an average particle size of 50 μm (particle size distribution 10 to 120 μm) and high-pressure gas is applied to the surface of the material to be treated from the nozzle at an arc height of 0.10 mmN to 0.19 mmN. It is sprayed so that a plastic strain is uniformly applied along the surface of the material to be processed, and a nanocrystalline structure without a layered processed structure is generated in the vicinity of the surface.

  When particles collide with the material to be treated with a relatively high collision energy, a processed structure folded in layers is formed near the surface of the material to be treated as shown in FIG. The “layered texture” in the present invention refers to such a layered structure formed by machining.

  The nanocrystalline structure without the layered texture is generated by causing dynamic continuous recrystallization in the vicinity of the surface of the material to be processed by jetting a solid-gas two-phase mixed fluid of particles and high-pressure gas. Can do.

  Here, “dynamic continuous recrystallization” refers to recrystallization caused by exceeding a certain limit of dislocation density during so-called ultra-strong processing, which is imparted with extremely high plastic strain.

In this way, a radical nitriding treatment can be further applied to the material to be processed in which a nanocrystal structure without a layered structure is generated.

Wherein said particles are carbon steel, high speed tool steel, preferably steel particles and the like of stainless steel having a particle size distribution of 10 to 120 [mu] m, the particles of hardness HV550~1100 as described above Inject with gas so that the arc height is 0.10mmN ~ 0.19mmN.

  As an example, when the hardness of the particles is, for example, about HV800, the hardness is, for example, HV1000 by an injection pressure of 0.4 MPa to 0.6 MPa using a gravitational injection device, which will be described later, and 0.2 MPa to 0.3 MPa for a direct pressure injection device. In the above case, the above-described arc height can be obtained by injecting at an injection pressure of 0.2 MPa to 0.4 MPa using a gravity type injection device which will be described later.

Further, in the radical nitridation treatment by said plasma, said without forming a film-like nitride layer on the surface of the material to be treated, it is possible to diffuse nitrogen on the surface layer of the nanocrystalline tissue near the surface.

  Furthermore, in addition to the application of the nanocrystal layer and the subsequent application of the nitrogen diffusion layer by radical nitriding, a hard ceramic film such as a chromium nitride film may be applied to the outermost surface for the purpose of improving the resistance to melting damage.

  According to the present invention, by improving the hardness and toughness by imparting a nanocrystalline structure without a layered processed structure obtained by adjusting the degree of processing by particle injection to a predetermined arc height, It can suppress the generation of cracks due to wear loss and thermal fatigue, and can significantly improve the service life. This nanocrystalline structure is thermally stable in a high temperature range of about 650 ° C., and its excellent function can be maintained for a long time without grain growth when heated by a molten aluminum alloy. In addition, it is possible to improve the erosion resistance by diffusing nitrogen into the nanocrystal structure by plasma nitriding as necessary, or by providing a film-like nitride layer on the surface by physical vapor deposition. As a result, it has become possible to provide a hot mold made of a long-life alloy steel as a practical mold.

  Next, an embodiment of the present invention will be described below.

1. Overall structure The method of strengthening hot mold alloy steel according to the present invention is such that particles of HV550 to 1100 are made to have an arc height of 0.10 mmN to 0.19 mmN with respect to the alloy steel for hot mold as the material to be processed. Injecting with compressed air to give a nanocrystal structure without the layered structure described above in the vicinity of the surface of the material to be treated;
If necessary, it includes a step of diffusing nitrogen into the nanocrystalline structure by plasma nitriding (radical nitriding),
Furthermore, it is comprised by the process of forming a hard ceramic layer on the surface by a physical vapor deposition method as needed.

2. Processing object Alloy steel for hot molds to be processed by the method of the present invention is a process of rapid heating and cooling such as molding and mold release agent, such as molds used for die casting. Used in molds that require use in severe temperature changes, for example, die cast mold steel represented by SKD61 steel. The object of the present invention is the composition of C: 0.32 to 0.42%, Si: 0.80 to 1.20, Mn: 0.50 or less, P: 0.030 or less, S: 0.020 or less, Cr: 4.50 to 5.50, Mo: 1.00 to 1.50, V: 0.30 to 1.15, others are Fe and inevitable impurities, HV450 to 520 Steel.

3. Particle injection (1) Particles The particles to be injected together with the compressed air for the above objects to be processed are HV550 to 1100 whose hardness is higher than that of the material to be processed.

  The material used is not particularly limited as long as it has the above-mentioned hardness and can impart a nanocrystalline structure without a layered processed structure near the surface by colliding with the object to be treated. Various things can be used. In this embodiment, carbon steel particles having an average particle size of about 50 μm and a particle size distribution of 10 to 120 μm were used.

(2) Injection conditions The injection conditions such as the injection pressure, injection speed, and injection time for injecting the particles correspond to the particle size, material, hardness, specific gravity, etc. In addition, the conditions under which nanocrystallization can be performed without producing a layered texture are selected.

  When the impact energy of particles increases relative to the deformation resistance (hardness) of the workpiece, as will be described later, a non-uniform plastic deformation region with cracks and notches is formed in the workpiece, The nanocrystal structure generated in the vicinity of the surface of the material to be processed also becomes a discontinuous processed structure (layered processed structure) folded in layers by enclosing such cracks and notches (see FIG. 3B). Such a nanocrystal structure with notches and cracks and a layered structure cannot sufficiently contribute to the improvement of thermal fatigue characteristics. Therefore, depending on the hardness of the material to be treated and the particles to be ejected, the collision energy of the particles is not a discontinuous processed structure (layered processed structure) folded in layers as described above. It is necessary to adjust so as to produce a continuous nanocrystalline structure without accompanying.

  In order to obtain a nanocrystalline structure without such a layered structure, for example, particles having a hardness of HV550 to HV1100 and an average particle size of 50 μm (particle size distribution of 10 to 120 μm) and an arc height of 0.10 mmN By injecting at an injection speed, injection pressure, injection time, etc. of ˜0.19 mmN, it is possible to obtain a nanocrystal structure without a layered processed structure as described above.

  For example, when the particle hardness is about HV800, the injection pressure is set to 0.2 MPa to 0.6 MPa using a gravity type injection device described later, and when using a direct pressure type injection device, the injection pressure is set to 0.2 MPa to 0.3 MPa. For example, when the hardness is HV1000 or higher, the gravity pressure can be set to 0.2MPa to 0.4MPa by using a gravitational injection device, and processing can be performed at an arc height in the above numerical range by injecting the material. Can give a continuous nanocrystal structure without a layered structure.

(3) Injector, etc. As an injector for injecting the above-mentioned particles onto the object to be treated, the particles are sucked or pumped from the particle supply source communicating with the nozzle by the air flow ejected from the nozzle communicating with the compressed air supply source. Any type of blasting can be used as long as it can eject a solid-gas two-phase mixed fluid of particles and compressed air from the nozzle, regardless of the type such as gravity type or direct pressure type. The device can be used as the aforementioned spray device for spraying particles.

  As an example, when the above-described particle injection device is the gravity blast device 10, one end of the blast hose 28 is connected to the lower end of the recovery tank 17 via the abrasive adjuster 26 and communicated with the other end of the blast hose 28. By supplying clean compressed air from a compressor and an air cleaner (not shown) to the injection nozzle 31 through the compressed air supply pipe 32, the inside of the blast hose 28 becomes negative due to this compressed air, and the inside of the recovery tank 17 The particles are sucked and fed, mixed with the compressed air, and sprayed from the spray nozzle 31 toward the material to be treated (see FIG. 1).

  In the case of the direct pressure blasting apparatus 10, a recovery tank 17 is provided at the upper part, and an abrasive tank 24 is communicated with a lower part of the recovery tank 17 via a dump valve (not shown) that can be opened and closed. One end of the blast hose 28 is connected via the adjuster 26. Clean compressed air from a compressor and an air cleaner (not shown) is supplied from one end of the abrasive tank 24 and the blast hose 28, and the polishing material in the abrasive tank 24 is introduced into the injection nozzle 31 at the tip of the blast hose 28. , Spraying from the spray nozzle 31 toward the material to be treated, causing the particles to collide.

  When particles are injected under the same conditions, the particle injection using the direct pressure injection device described above is more effective than the particle injection using the gravity injection device. Since energy tends to be relatively increased and a layered processed structure is easily formed, in the present invention aiming at formation of a nanocrystal structure without a layered processed structure, it is preferable to use a gravity type jetting apparatus.

(4) Action by particle injection / collision As mentioned above, continuous nanocrystals that do not have a layered texture near the surface unless the conditions for particle collision are selected appropriately according to the hardness of the material to be treated. An organization cannot be granted.

  In the following, the influence of the conditions for colliding particles on the morphology of the nanocrystal structure will be described.

  If the collision energy of the particles is too large for the alloy steel for hot molds whose hardness is in the range of HV450 to 520, the nanocrystal structure is due to deformation from multiple directions as shown in Fig. 3 (b). As a presumed curved layered work structure, it is generated discontinuously along the surface with a clear boundary from the surrounding work hardening region. Also, cracks are often observed in this layered structure.

  In this way, if the impact energy of the particles is too large relative to the hardness of the material to be treated, significant irregularities are formed on the surface in the initial stage of the particle impact, and the convex portions are folded by the subsequent impact and move toward the inside of the material. Intrusion occurs, and the formation of irregularities and folding of the convex parts are repeated by the subsequent collision. In this way, plastic strain concentrates in the region subjected to repeated colliding with folding, and when the dislocation density exceeds a certain critical value, it finally nanocrystallizes by dynamic continuous recrystallization. The formed nanocrystalline structure appears to be a curved layered processed structure, which is a trace of repeated folding due to particle collisions, and the formation of a clear interface with the surrounding work hardening region is caused by plastic strain. This is because when the concentrated layered structure is folded and penetrates into the interior, it joins with the work hardening region with less plastic strain to form a new interface. Cracks often observed inside the layered structure are: It is considered that the joining at the time of folding occurs at a place where the joining is not sufficient. In this case, since the concentration of local plastic strain becomes the cause of the nanocrystal structure, the nanocrystal structure is distributed discontinuously along the surface, and it is difficult to uniformly generate the entire surface. The nanocrystalline structure formed through such a process exists discontinuously along the surface, and is accompanied by notches and cracks caused by folding, and cannot exert a sufficient effect for improving thermal fatigue characteristics.

  On the other hand, when the particles collide under an appropriate condition according to the hardness of the material to be processed, it is possible to impart a continuous nanocrystal structure without the layered structure described above. When the collision energy of the particles is adjusted appropriately, plastic strain is uniformly applied along the surface without forming severe irregularities when the particles collide. In this case, the plastic strain applied is the largest on the surface of the workpiece and continuously decreases toward the inside, and the region where the plastic strain exceeds a critical value near the surface is the nanocrystalline structure by dynamic continuous recrystallization. It is thought that it leads to. As a result, it is possible to form a nanocrystalline structure uniformly on the entire surface at a certain depth along the surface.

  When steel particles with HV550 to HV1100 and average particle size of 50 μm (particle size distribution of 10 to 120 μm) are used as the particles, the particles collide so that the arc height is in the range of 0.10 mmN to 0.19 mmN. It is possible to generate a nanocrystalline structure without the layered processed structure as described above.

  As an example, when the particle hardness is about HV800, the injection pressure is set to 0.2 MPa to 0.6 MPa using a gravity type injection device described later, or the injection pressure is set to 0.2 MPa to 0.3 MPa using a direct pressure type injection device. For HV1000 and above, the arc height can be within the above numerical range by spraying the material to be processed with the spray pressure of 0.2MPa to 0.4MPa using the same gravitational spray device. The nanocrystal structure without it was able to be given.

4). Plasma nitriding By injecting particles as described above, the material to be treated with the nanocrystallized surface texture is then nitrided by plasma nitriding as necessary to form a nitride below the surface. In addition to suppressing the occurrence and growth of heat cracks, improvement in resistance to melting is achieved.

  Ion nitriding or radical nitriding can be considered as such a plasma nitriding method. Ion nitriding is because the surface layer of the mold is removed by sputtering, and a film-like 5-15 μm nitride layer is formed on the surface. As shown in FIG. 4B, the nanocrystallized surface layer is removed, and the effect of suppressing the generation and growth of heat cracks disappears. On the other hand, in radical nitriding, as shown in FIG. 4 (c), the surface layer of the mold is not removed by sputtering and a nitride layer is not formed on the surface. A nitriding treatment can be performed by forming a nitrogen diffusion layer of about 80 to 100 μm while maintaining it.

5. Formation of a hard ceramic film When a nitriding treatment is performed without a film-like nitride layer on the surface as described above, a hard ceramic film such as a chromium nitride (CrN) film can be formed by physical vapor deposition if necessary. Done.

  For example, since a chromium nitride (CrN) film does not decompose at the use temperature of the die-cast mold as described above, it is possible to take measures against melting damage after nanocrystallization by forming the chromium nitride film.

  The formation of a chromium nitride (CrN) film or the like by this physical vapor deposition method is not particularly limited as long as it is based on the physical vapor deposition method. Various known methods such as vacuum vapor deposition, sputtering, ion plating, vacuum arc method, dynamic mixing, etc. It can be done by the method.

  When a hard ceramic film is formed on the surface of the material after nitriding in this way, radical nitridation that allows direct PVD treatment on the surface after nitridation is used as the nitriding method described above. It is convenient to adopt it.

  Hereinafter, test examples conducted in connection with the present invention will be described.

1. Particle collision condition confirmation test In this test, SKD61 steel, which is an alloy for hot molds, was subjected to electric discharge machining, the abnormal layer was removed, and a test specimen with a hardness of HV490 (HRC48) was formed by gravity, Particle injection was performed using two types of direct pressure injection devices.

  Prepare two types of particles to be injected: carbon steel particles (HV approx. 800) and high-speed steel particles (HV1000 or higher) (both have an average particle size of 50 μm and a particle size distribution of 10 μm to 120 μm), and the nozzle is fixed. Then, the jet was performed for 30 seconds without changing the collision position of the particles. The test conditions here are shown in Table 1.

  The results of the above confirmation test are shown in Table 2.

  In the example in which carbon steel particles were injected at a pressure of 0.4 MPa using a gravity-type injection device, the formation of a layered structure (folding) could not be confirmed [Fig. 5 (a)], but carbon steel particles were directly pressed. In the example in which injection was performed at an injection pressure of 0.5 MPa with this injection device, generation of a layered texture was confirmed [FIG. 5 (b)].

From the above results, when other machining conditions are the same,
(1) As the injection pressure increases, a lamellar processed structure tends to occur.
(2) As the hardness of the particles increases, a layered texture is likely to occur.
(3) The direct pressure type is more likely to produce a layered structure than the gravity type.
The result was confirmed.

  In addition, when the arc height is in the range of 0.10 mmN to 0.19 mmN, a nanocrystalline structure without a layered structure is generated regardless of differences in the injection method, injection pressure, particle hardness, and other factors. Was confirmed. As an example of nanocrystallization, FIG. 6 shows a microstructure by a transmission electron microscope in the vicinity of the surface when carbon constituent particles are jetted at a pressure of 0.4 MPa by a gravity jet device.

  Therefore, when a layered structure is generated, the particle collision energy is reduced by adjusting it to adapt to each of the above conditions, for example, by adjusting the arc height to be in the range of 0.10 mmN to 0.19 mmN. , The occurrence can be prevented.

2. Confirmation test of heat crack occurrence state (1) Test piece
Using a test piece made of SKD61 steel [Hardness HV490 (HRC48)], a heat crack occurrence state confirmation test was conducted. The test piece used in this test is shown in FIG.

  As shown in FIGS. 7 (a) and 7 (b), the test piece used in this example is a cylinder having a diameter of 58 mm and a thickness of 20 mm obtained by grinding the flat surface by grinding and turning the outer peripheral surface by turning. Is cut into four equal parts in the diameter direction as shown in FIG.

(2) Processing conditions Samples prepared by performing the treatment shown in the following table (Table 3) on the test pieces were prepared.

  In Table 3 above, carbon steel particles with an average particle diameter of 50 μm (HV of about 800; see Table 1) were used for the particle injection, and the injection time was 30 seconds. Both test pieces were jetted in a fixed position.

(3) Test method Each of the above samples (samples 1 to 9) was contacted with a block heated to 570 ° C for 160 seconds, and then immersed in water at 100 ° C for 15 seconds.

  The above treatment was set to 1 cycle, and after repeating this treatment for 2000 cycles, occurrence of heat cracks was confirmed.

(4) Test Results FIGS. 8 to 10 show the results of confirming the occurrence of heat cracks as described above, and FIGS.

(4-1) Relationship between particle injection and thermal fatigue crack As shown in Fig. 8, in comparison with the untreated sample (Sample 1), the sample injected with particles (Sample 2 and 3) In both cases, it was confirmed that both the number of heat cracks per mm and the depth of heat cracks decreased.

  In addition, even in the case of the sample in which the particles are jetted, the sample 3 without the layered processed structure has both the number of heat cracks per 1 mm and the heat crack depth compared to the sample 2 in which the layered processed structure is formed. It was confirmed that the resistance to thermal fatigue cracks was low.

(4-2) Relationship between nitriding and thermal fatigue cracks As shown in Fig. 9, the samples subjected to ion nitriding (samples 4, 6, and 7) are more resistant to thermal fatigue cracks than untreated materials. Was confirmed to decrease.

  On the other hand, as shown in FIG. 10, in the samples subjected to radical nitriding (samples 5, 8, and 9), the sample 8 in which the layered processed structure is formed is changed to the untreated sample (sample 1 in FIG. 8). In comparison, the number of heat cracks per mm was increased, and the sample subjected only to radical nitriding (sample 5) was compared with the untreated sample (sample 1 in FIG. 8) per mm of heat crack. In the other sample (sample 9) in which the arc height was within the scope of the present invention and the nanocrystal structure without the layered structure was formed. Can perform nitriding while maintaining improved resistance to thermal fatigue cracks by performing particle injection even after radical nitriding treatment, and improved resistance to thermal fatigue cracks. , Better resistance to erosion It was confirmed that the above can be obtained together.

3. Formation of chromium nitride (CrN) film A hard ceramic film such as a chromium nitride (CrN) film is formed by physical vapor deposition on the surface of the material to be processed after plasma nitriding such as radical nitriding.

  As described above, it is possible to take measures against melting damage after nanocrystallization by forming a chromium nitride (CrN) film that does not decompose at the use temperature of the die-cast mold.

  By forming this chromium nitride (CrN) film, a ceramic layer that suppresses the reaction with aluminum can be formed, and the resistance to melting can be maintained for a long time.

Schematic explanatory drawing of a gravity type blast processing apparatus. Schematic explanatory drawing of a direct pressure blast processing apparatus. A cross-sectional electron micrograph of a sample in which a layered processed structure is generated by particle injection. It is explanatory drawing of the process state in each process by this invention, (a) after particle injection, (b) after ion nitriding, (c) shows the state after radical nitriding, respectively. It is an electron micrograph of SKD61 steel after particle injection. (A) shows the case where the layered structure is not generated, and (b) shows the case where the layered structure is generated. An electron microscope that captured the nanocrystalline structure formed on the surface of SKD61 steel (same sample as Fig. 5 (a)) in which particles were injected under conditions that do not generate a layered structure (gravity 0.4MPa, arc height 0.14mmN) Photo. Explanatory drawing of the test piece used for the occurrence check test of a heat crack. Graph showing the distribution of heat cracks (no nitriding). The graph which shows the distribution condition of a heat crack (ion nitriding). The graph (radical nitriding) which shows the distribution condition of a heat crack. The cross-sectional electron micrograph of the sample 1 which shows the generation | occurrence | production state of a heat crack. The cross-sectional electron micrograph of the sample 2 which shows the generation | occurence | production state of a heat crack. The cross-sectional electron micrograph of the sample 3 which shows the generation | occurrence | production state of a heat crack. The cross-sectional electron micrograph of the sample 4 which shows the generation | occurrence | production state of a heat crack. The cross-sectional electron micrograph of the sample 5 which shows the generation | occurrence | production state of a heat crack. The cross-sectional electron micrograph of the sample 6 which shows the generation | occurrence | production state of a heat crack. The cross-sectional electron micrograph of the sample 7 which shows the generation | occurrence | production state of a heat crack. The cross-sectional electron micrograph of the sample 8 which shows the generation | occurrence | production state of a heat crack. The cross-sectional electron micrograph of the sample 9 which shows the generation | occurrence | production state of a heat crack.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Blasting device 17 Collection tank 24 Abrasive material tank 26 Abrasive material adjuster 28 Blast hose 31 Injection nozzle 32 Compressed air supply pipe

Claims (9)

  C: 0.32 to 0.42%, Si: 0.80 to 1.20, Mn: 0.50 or less, P: 0.030 or less, S: 0.020 or less, Cr: 4.50 to 5.50, Mo: 1.00 to 1.50, V: 0.30 to 1.15, others are Fe Compared with the material to be processed, which is alloy steel for hot mold made of HV450 ~ 520 steel containing inevitable impurities, the hardness is HV550 ~ 1100 and the average particle size is 50μm (particle size distribution 10 ~ 120μm) And a high-pressure gas solid-gas two-phase mixed fluid are sprayed from the nozzle onto the surface of the material to be treated so that the arc height is 0.10 mmN to 0.19 mmN, and plastic strain is uniformly applied along the surface of the material to be treated. A method for inhibiting thermal fatigue cracks in alloy steel for hot molds, characterized in that a nanocrystalline structure is produced.   The nanocrystalline structure is generated by injecting a solid-gas two-phase mixed fluid of the particles and high-pressure gas onto the surface of the material to be treated, and causing dynamic continuous recrystallization in the vicinity of the surface of the material to be treated. Item 2. A method for suppressing thermal fatigue cracks in alloy steel for hot molds according to Item 1.   A material for which the nanocrystalline structure is formed by the method according to claim 1 or 2, wherein radical nitriding treatment is performed by plasma on a material to be subjected to thermal fatigue cracking in a hot die alloy steel. Suppression method.   The method for suppressing thermal fatigue cracks in alloy steel for hot die according to any one of claims 1 to 3, wherein the particles are carbon steel particles.   4. Nitrogen is diffused into the nanocrystal structure of the surface layer in the vicinity of the surface without forming a film-like nitride layer on the surface of the material to be processed by nitriding with the plasma. A method for inhibiting thermal fatigue cracks in the alloy steel for hot die described.   6. The film made of hard ceramics is applied to the surface of the material to be treated by physical vapor deposition after the diffusion treatment of nitrogen into the surface nanocrystal structure near the surface by the nitriding. A method for suppressing thermal fatigue cracks in alloy steel for hot molds. C: 0.32 to 0.42%, Si: 0.80 to 1.20, Mn: 0.50 or less, P: 0.030 or less, S: 0.020 or less, Cr: 4.50 to 5.50, Mo: 1.00 to 1.50, V: 0.30 to 1.15, others are Fe Generation of thermal fatigue cracks characterized by having a nanocrystalline structure without a layered work structure near the surface of alloy steel for hot mold made of HV450-520 steel containing inevitable impurities and inevitable impurities Alloy steel for hot molds that suppresses   8. The alloy steel for hot mold according to claim 7, wherein nitrogen is diffused into the nanocrystal structure of the surface layer in the vicinity of the surface by radical nitriding treatment with plasma. 9. The hot die alloy steel according to claim 8, further comprising a hard ceramic film formed by physical vapor deposition on a surface of the material to be treated.