Heat treatment method of hot work die steel

Abstract

The invention belongs to the field of steel smelting, and particularly relates to a heat treatment method of hot work die steel, which comprises the following steps: 1, preheating steel to a quenching temperature higher than AC3 + 30 DEG C; secondly, the bulk materials are cooled and placed in static air, a cooling medium is graphite or quartz sand or carborundum or silicon iron, the temperature ranges from-70 DEG C to 470 DEG C, the bulk materials are in a vibration fluidization state or a static state, and the critical temperature point of the steel materials and the stability of austenite are considered in the large-range temperature; 3, when the hot die steel is cooled to the temperature of the cooling medium; fourthly, the workpiece in the third step is subjected to follow-up tempering treatment; and vibration fluidization cooling is controlled in a bulk cooling medium, so that the specified structure morphology is obtained, and the metal performance is improved.

Description

Technical Field

[0001] The invention belongs to the metallurgy and machinery manufacturing industries, is used for heat treatment of metal products, and specifically relates to a heat treatment method for hot working die steel. Background Art

[0002] When fluidized bed quenching is performed on steel workpieces, the workpieces are generally fixed on a self-vibrating component and then cooled. This method is feasible for some longer workpieces, but it is not applicable when quenching batches of small parts with complex geometries, as the hardening effect is relatively poor. Summary of the invention

[0003] In order to solve the above problems, the present invention provides a heat treatment method for hot working die steel, and the specific technical scheme is as follows:

[0004] Step 1: Preheat the steel to a temperature higher than A C3 +30℃ quenching temperature;

[0005] Step 2: Cooling in bulk materials, placed in still air, the cooling medium is graphite, quartz sand, corundum or ferrosilicon, the temperature is -70℃~470℃, vibrating fluidized state or static state. This wide range of temperature takes into account the critical temperature point of steel materials and the stability of austenite;

[0006] Step 3: When the hot mold steel cools to the cooling medium temperature;

[0007] Step 4: The workpiece in step 3 is subjected to subsequent tempering treatment to obtain a steel material that meets the requirements.

[0008] As a further improvement: the particle size of the bulk material is 5-1000 microns.

[0009] As a further improvement: For hot working die steel with low austenite stability, the cooling process in step 2 uses a bulk material cooling medium with a temperature of 0 to 30°C to cool the workpiece under a vibrating fluidized state. When the austenite begins to decompose into bainite, the cooling of the cooling medium is stopped and the workpiece continues to be cooled.

[0010] As a further improvement: For hot working die steels with high austenite stability, the austenitizing temperature is н +10℃~Μ н +15℃, in the cooling process of step 2, a bulk material cooling medium with a temperature of 0~30℃ is used to cool the workpiece under a vibrating fluidized state, and the workpiece temperature reaches Μ н +10℃~Μ н +15℃, stop vibrating fluidization and cooling of the cooling medium, and continue cooling the workpiece.

[0011] As a further improvement: For hot working die steel with low austenite stability, the cooling process in step 2 uses a bulk material cooling medium at -10 to 30°C to cool the workpiece under a vibrating fluidized state until the workpiece temperature reaches M н +10℃~Μ н +15℃, stop cooling with multiple cooling media and continue cooling the workpiece.

[0012] As a further improvement: For high carbon alloy hot working die steel, in the cooling process in step 2, a bulk material cooling medium at -70°C to 0°C is used for cooling under a vibrating fluidized state. When the cooling temperature reaches 0 to -30°C, the vibrating fluidized state is stopped and cooling is continued.

[0013] As a further improvement: In step 2, when the workpiece temperature reaches M н -15℃~Μ н When the temperature reaches +15℃, the vibration fluidization of the bulk material is stopped, and isothermal aging cooling is carried out in the stationary bulk material.

[0014] In the heat treatment process of steel, granular materials such as graphite, quartz sand, corundum, ferrosilicon or their mixtures are used as cooling media. The particle size of the cooling medium is 5-1000 microns, the temperature ranges from -70 to 470°C, and it is in a vibrating fluidized state or a static state, allowing the cooling system to be selected within a larger speed range. The cooling of the workpiece should take into account the critical point temperature and austenite stability of the steel type used, in order to achieve the required organizational morphology and mechanical property level, and have the lowest residual stress level.

[0015] In order to develop and design the vibration cooling device, the experimental parameters selected are: vibration frequency of 20-70Hz (perpendicular to the longitudinal axis of the bed), the particle size of the bulk material is 5-1000μm, otherwise it will cause uneven cooling of the parts, particles with a size less than 5 microns are prone to form clumps, and particles with a size greater than 1000 microns will deteriorate the heat transfer conditions because the porosity of the bulk material is larger at this time.

[0016] Beneficial effects of the present invention: The present invention provides a heat treatment method for hot working die steel, which can be used to process various types of steel workpieces, and also has a hardening heat treatment method for workpieces with complex geometric shapes. By controlling the vibration fluidization cooling in the bulk cooling medium, it is ensured that the specified organizational morphology is obtained and the metal properties are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is the austenite isothermal decomposition curve of 9XФ steel;

[0018] Figure 2 This is the thermodynamic curve of austenite decomposition of 5XNM steel;

[0019] Figure 3 is the austenite isothermal decomposition curve of y8 steel;

[0020] Figure 4 This is the cooling (quenching) curve of ШX15 steel;

[0021] Figure 5 Schematic diagram of isothermal decomposition of supercooled austenite of 6XС steel. DETAILED DESCRIPTION

[0022] In order to deepen the understanding of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The embodiments are only used to explain the present invention and do not limit the scope of protection.

[0023] Example 1

[0024] This embodiment provides a heat treatment hardening process for 9ХФ steel cold cutting metal tools (strip thickness is 20 mm), in which a workpiece with low austenite stability is cooled in a supercooled state below the critical point under a bulk material vibrating fluidized state, and the bulk material temperature is 30°C to 0°C. When the workpiece is cooled to a temperature below the nose of the austenite decomposition curve of bainite, the cooling of the bulk material is stopped, and then the workpiece is continued to be cooled to the medium temperature under the bulk material vibrating fluidized state. In order to achieve martensitic transformation, the critical cooling rate should not be less than 23°C / second. Critical point temperature of 9ХФ steel: АС1=740°C; M н = 185°C. The isothermal decomposition curve of supercooled austenite in steel with low austenite stability is shown in Figure 1 According to this figure, the workpiece is quenched in the fluidized state of bulk material.

[0025] The cold cutting tool metal heat treatment (rectangular bar 90×200×20mm, after finishing) is carried out by preheating the parts to 500°C at a rate of ≤150°C / hour in a box-type electric furnace, isothermally aging at this temperature for 20 minutes, and then further heating to the quenching temperature of 880°C in a salt bath furnace (BaCl2-80%, KCl-20%), keeping warm for 15 minutes, and cooling (quenching) under the vibration fluidization state of the bulk material. The bulk material consists of 85% silicon carbide and 15% graphite with a particle size of 0.1 mm.

[0026] The bed of the cooling device vibrates stably at a frequency of 40Hz, and water at a temperature of 10-20℃ is passed through the empty bed of the cooling device at a pressure of 2.5atm. When the temperature of the metal of the part reaches 400-390℃, that is, the decomposition temperature of the unstable austenite to bainite region, the vibrator and water are turned off. When the temperature drops below the nose of the minimum stable temperature curve of the transformation of austenite to bainite, the cooling of the workpiece is carried out under the conditions of stationary bulk material and closing the energy-carrying medium (cold water). The workpiece is cooled to the medium temperature (25-300С) through the empty bed, and then the workpiece is taken out and sent for tempering. The metal of the cold cutting tool reaches a hardness of 61-62HRC after work hardening (quenching), which meets the requirements of standard documents.

[0027] The presence of graphite in the bulk material (cooling medium) protects the surface of the machined product from decarburization and scale formation. After quenching, the parts are tempered at 240-250°C to a hardness of 57-58HRC.

[0028] Example 2

[0029] This embodiment takes the heat treatment hardening process of 5XNM hot working die steel as an example. Figure 2 The graph of thermodynamic curve of austenite decomposition of 5XNM steel is shown. The peculiarity of this steel is that austenite has a high stability when supercooled below the critical point (in the pearlite and bainite transformation temperature zone). According to the manual of production standard classification catalog, the limiting rate of cooling of austenitic steels for martensite transformation cannot be lower than 1.5 ° C / sec. Based on this, according to the classification catalog, in order to calculate the limiting cooling rate, it is necessary to draw a tangent to the austenitization temperature relative to the nose of the minimum stability curve of austenite in the bainite transformation zone. The critical temperature point of 5XNM steel is: Α С1 =710℃; A C3 =790℃; M н =230℃.

[0030] From austenitization to M н In the temperature range of +10~15℃, for hot working die steel with high austenite stability, the bulk material is cooled in the vibrating fluidized state. The bulk material temperature is 30℃~0℃. When the workpiece is cooled to Μ н+10~15℃, then stop the vibration fluidization and cooling of the bulk material, and then the workpiece continues to cool to the medium temperature in the stationary bulk material. Taking the heat treatment of a small hot-deformed 5XHM steel stamping (after fine processing) with a diameter of 150mm and a height of 140mm as an example, the workpiece is placed in a box-type electric furnace and preheated to 600℃ at a rate of ≤350℃ / hour, isothermally aged at this temperature for 30 minutes, and then further heated to a quenching temperature of 850℃ in a box-type electric furnace (on a tray with a waste carburizer) and kept warm for 150 minutes. The stamping is cooled (quenched) in the vibration fluidization of bulk materials, which are composed of 85% silicon carbide and 15% graphite with a particle size of 0.1mm, until the workpiece metal temperature reaches 50-80℃. The cooling device bed body vibrates at a frequency of 40Hz until the workpiece metal temperature reaches 50-80℃. Then take out the workpiece for tempering. After heat treatment hardening (quenching), the hardness of stamping metal is 59-60HRC, which meets the requirements of standard documents. The presence of graphite in bulk materials (cooling medium) can protect the surface of the working part of the machined product from decarburization and scale formation. After quenching, tempering is carried out immediately at a temperature of 420℃, and the hardness reaches 47-48HRC.

[0031] Example 3

[0032] This embodiment takes the heat treatment hardening process of У8 hot working die steel as an example. Figure 3 , after austenitizing to Μ н +10…15℃ temperature range, for hot working die steel with low austenite stability, the bulk material is cooled in the state of vibration fluidization, and the bulk material temperature is -10~+30℃. When the workpiece is cooled to Μ н +10~15℃, then stop cooling the bulk material, and then continue to cool the workpiece to the medium temperature under the vibration fluidization state of the bulk material. In the supercooled state below the critical point, austenite of У8 steel tends to decompose (pearlite transformation temperature range). According to the decomposition isothermal thermodynamics, the critical cooling rate of martensite transformation is not less than ~120℃ / second. From the phase diagram, it can be seen that in order to calculate the critical cooling rate, the tangent of the austenitizing temperature must be drawn relative to the nose of the lowest stability of austenite in the pearlite transformation region. Take the heat treatment hardening process of У8 steel door processing tools (chisel, blade thickness 4mm) as an example. The critical temperature point of У8 steel is: А С1 =820℃; A C3 =870℃; M н =220℃.

[0033] The chisel (after fine machining) is heat treated by using bulk material vibration fluidization technology. First, the part is preheated to 600℃ in a box-type electric furnace at a speed of ≤350℃ / hour, isothermally aged at this temperature for 5 minutes, then heated to quenching temperature 790+5℃ in a salt bath furnace (BaCl2-80%, KCl-20%), aged for 3 minutes, and cooled in the state of bulk material vibration fluidization. The bulk material consists of 85% silicon carbide and 15% graphite, with a particle size of 0.1mm. The bed of the cooling device vibrates steadily at a frequency of 70Hz. When the austenitizing temperature reaches 230-235℃ (Μ н =220℃), for У8 steel with low austenite stability, bulk material vibration fluidization cooling is adopted. 10 minutes before quenching cooling and 1-2 minutes after the hot workpiece is placed in the cold bulk material, the liquid coolant (mixture, such as industrial alcohol + dry ice-carbon dioxide) flows through the empty bed shell to cool the bulk material.

[0034] When the metal of the part reaches 230-235℃, the heat carrier of the vibrating fluidized bed body is turned off, and the workpiece is continued to be cooled to the medium temperature under the vibrating fluidization state of the bulk material. After quenching, the workpiece is taken out of the equipment and transferred to the tempering furnace to be heated to 220℃ and kept warm for 60 minutes. The hardness of the workpiece after tempering reaches 56-57HRC.

[0035] Example 4

[0036] This embodiment takes the heat treatment process of ШX15 steel as an example. Figure 4 This is the cooling (quenching) curve of ШX15 steel. The workpiece is quenched and cooled to 0℃~-30℃ in a bulk material vibrating fluidized bed, and the bulk material temperature is -70℃~0℃. Then the vibrating fluidization is stopped, and isothermal aging is carried out at -25℃. Then the cooling of the bulk material is stopped, the bulk material is heated to above 0℃, and the workpiece is taken out and placed in the air.

[0037] The heat treatment of the workpiece is preheated to 600°C at a rate of ≤350°C / h in a box-type electric furnace, isothermally aged at this temperature for 5 minutes, and then heated to the quenching temperature of 850+5°C in a salt bath furnace (BaCl2-80%, KCl-20%), kept warm for 3 minutes, and the workpiece is cooled in a vibrating fluidized state of bulk material, which is composed of 85% silicon carbide and 15% graphite with a particle size of 0.1 mm. The bed body of the cooling device vibrates at a frequency of 70Hz. 10 minutes before quenching cooling and 1-2 minutes after the hot workpiece is placed in the cold bulk material, the liquid coolant (mixture, such as industrial alcohol + dry ice-carbon dioxide) flows through the empty bed shell to cool the bulk material.

[0038] The metal temperature of the inner surface of the bed (in contact with the bulk material of the vibrating fluidization) can reach -45-47°C. When the temperature of the workpiece metal reaches 200-210°C (when the austenitizing temperature is 850+5°C, the martensite transformation starting temperature M н The temperature of the martensitic transformation is 180-185°C, the martensitic transformation end temperature Mк is -30°C, and then the connection between the energy carrier and the empty vibrating fluidized bed body is closed, and the workpiece continues to cool to the medium temperature in the vibrating fluidized state of the bulk material.

[0039] In order to cold work and reach the critical point temperature Mк, the workpiece is further cooled to -25℃ in the vibrating fluidized bulk material, and a liquid coolant (a mixture of industrial alcohol + "dry ice"-carbon dioxide) is used to flow through the empty body of the vibrating fluidized bed, and the cooling is continued for 10 minutes (5 minutes before the workpiece begins to be supercooled, and 5 minutes after the workpiece at 30-40℃ is placed in the supercooled bulk material). After the workpiece is isothermally insulated, it is taken out of the vibrating fluidized bed cooling device and transferred to the tempering furnace, where the workpiece is heated to 170+10℃ and then kept warm for 90 minutes. The martensitic transformation end temperature (Mк) of ШX15 steel depends on the austenitizing temperature and is below 0℃. When the austenitizing temperature is 850+5℃, Mк is -30℃. After cold working and tempering at 170℃ (90 minutes), the hardness of the workpiece reaches 56-57HRC.

[0040] Example 5

[0041] In this embodiment, 6XC steel (A C1 =770℃; A C3 = 835℃) as an example, refer to Figure 5 The workpiece is cooled from the austenitizing temperature to M н +10~15℃ to M н After -10 to 15°C, the vibratory fluidized cooling of the bulk material is stopped, and isothermal aging is performed in the stationary bulk material until the transformation from austenite to bainite is completed, and then the workpiece is cooled in air.

[0042] The heat treatment system of tools made of 6XС steel is as follows: the product is preheated to 550℃ at a rate of ≤150℃ / h in a box-type electric furnace, isothermally kept for 15 minutes, and then further heated to the quenching temperature of 860+5℃ in a salt bath furnace (BaCl2-80%, KCl-20%), kept for 12 minutes, and cooled in a vibrating fluidized state of bulk material, which consists of 85% silicon carbide and 15% graphite with a particle size of 0.1 mm. The presence of graphite in the bulk material (cooling medium) prevents decarburization and scale formation on the surface of the machined parts during isothermal quenching. The main body of the cooling device bed vibrates at a frequency of 70Hz.

[0043] The tool is continuously cooled in the cooling device until the metal temperature reaches 250-240°C (temperature Mн =230°C). After the workpiece is removed from the vibrating fluidized bed cooling device, it is transferred to a similar quenching device. In this device, before the workpiece is placed in the bulk material, the bulk material and the fluidized bed body are preheated to 230-240°C by superheated steam at 360-380°C through the water jacket of the fluidized bed body. Subsequently, the steam supply to the cooling device water jacket is stopped, the vibrator is turned off, and the tools are slowly cooled (they are actually isothermally aged in the hot bulk material) for 15 minutes. After the tools have been isothermally aged in the bulk material at 240-250°C for 15 minutes, the product is removed and hung vertically in the air to cool to room temperature.

[0044] After the above heat treatment, the hardness and impact toughness of the 6XC steel tool surface are measured to be 51-52HRC and KCU 20 =35-36J / cm 2 .

[0045] In summary of the above embodiments, the method of heat treatment of hot working die steel by vibration fluidized cooling of bulk materials adopted in this embodiment is efficient, has good process performance, and has more universal application. Compared with the prototype, it can be directly used in heat treatment workshops and mechanical manufacturing, tool and other workshops in factories. Due to the reasonable selection of temperature-time parameters in the cooling process of hot working die steel, energy consumption and production costs are reduced.

Claims

1. A heat treatment method for hot working die steel, the specific steps of which are: Step 1: Preheat the steel to a temperature higher than A C3 +30℃ quenching temperature; Step 2: Cooling in bulk materials, placing in still air, the cooling medium is graphite, quartz sand, corundum or ferrosilicon, the temperature is -70℃~470℃, vibrating fluidized state or static state; Step 3: When the hot mold steel cools to the cooling medium temperature; Step 4: The workpiece in step 3 is subjected to subsequent tempering treatment to obtain a steel material that meets the requirements.

2. The heat treatment method for hot working die steel according to claim 1, characterized in that: The particle size of the bulk material is 5-1000 microns.

3. The heat treatment method for hot working die steel according to claim 1, characterized in that: For hot working die steel with low austenite stability, in the cooling process in step 2, a bulk material cooling medium with a temperature of 0 to 30°C is used to cool the workpiece under a vibrating fluidized state. When the austenite begins to decompose into bainite, the cooling of the cooling medium is stopped and the workpiece is continued to be cooled.

4. The heat treatment method for hot working die steel according to claim 1, characterized in that: For hot working die steel with high austenite stability, when the austenitizing temperature is between Mн+10℃ and Mн+15℃, in the cooling process in step 2, a bulk material cooling medium with a temperature of 0 to 30℃ is used to cool the workpiece under a vibrating fluidized state. When the workpiece temperature reaches Mн+10℃ to Mн+15℃, the vibrating fluidization and cooling of the cooling medium are stopped, and the workpiece continues to be cooled.

5. The heat treatment method for hot working die steel according to claim 1, characterized in that: For hot working die steel with low austenite stability, in the cooling process in step 2, a bulk material cooling medium with a temperature of -10 to 30°C is used to cool the workpiece under a vibrating fluidized state. When the workpiece temperature reaches Μн+10°C to Μн+15°C, the cooling of the cooling medium is stopped and the workpiece is continued to be cooled.

6. A heat treatment method for hot working die steel according to claim 1, characterized in that: For high carbon alloy hot working die steel, in the cooling process in step 2, a bulk material cooling medium at -70°C to 0°C is used for cooling under a vibrating fluidized state. When the cooling temperature reaches 0 to -30°C, the vibrating fluidized state is stopped and cooling is continued.

7. The heat treatment method for hot working die steel according to claim 1, characterized in that: In step 2, when the temperature of the workpiece reaches Mн-15°C~Mн+15°C, the vibration fluidization of the bulk material is stopped, and isothermal aging cooling is performed in the stationary bulk material.

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