A heat treatment method for mold steel

Abstract

The invention belongs to the technical field of steel material processing, and specifically relates to a heat treatment method for mold steel, comprising the following steps: step S1: preheating, placing the mold steel in a preset vacuum heat treatment furnace for preheating treatment, and keeping the temperature after the preheating treatment is completed to obtain the preheated mold steel; step S2: vacuum quenching, placing the preheated mold steel in a vacuum heat treatment furnace for quenching treatment, raising the temperature to 600°C-700°C at a heating rate of 20-30°C / min, and keeping it for 30-60min; and heating the mold steel at a heating rate of 15-25°C / min. The temperature is raised to 800°C-900°C at a heating rate of 10-15°C / min and maintained for 30-60min; the temperature is raised to 1000°C-1050°C at a heating rate of 10-15°C / min and maintained for 30-60min; the temperature is cooled for 150min at a cooling pressure of 1.3MPa; step S3: measuring the deformation of the steel after vacuum quenching; step S4: performing three high-temperature temperings at a tempering temperature of 600°C-650°C and keeping the temperature for 1.5-2h; step S5: cutting the cooled mold steel to complete the heat treatment.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material processing, and in particular relates to a heat treatment method for die steel. Background Art

[0002] Mold steel is a type of steel specially used to manufacture various types of molds, such as cold stamping dies, hot forging dies, die-casting molds, etc. It is widely used in the fields of automobiles, machinery, electronics, home appliances, medical equipment, etc. It is an indispensable material for manufacturing high-precision and high-performance molds.

[0003] As the requirements for mold precision and performance gradually increase, mold steel has higher requirements for its hardness, wear resistance, thermal stability and other properties. Heat treatment process is the most important process step in the mold steel processing process. It can improve the performance of mold steel, such as improving hardness, improving toughness, and enhancing thermal stability. It can also expand the application range of mold steel materials. For example, different types of molds have different requirements for mold steel performance. Through heat treatment, a mold steel material can be used for a variety of molds. Therefore, it is necessary to further improve the heat treatment process of mold steel. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a heat treatment method capable of improving the performance of die steel.

[0005] The present invention provides a heat treatment method for mold steel, which comprises the following steps:

[0006] Step S1: preheating, placing the mold steel in a preset vacuum heat treatment furnace for preheating treatment, and keeping the temperature after the preheating treatment is completed to obtain the preheated mold steel;

[0007] Step S2: vacuum quenching, placing the preheated mold steel in a vacuum heat treatment furnace for quenching treatment, raising the temperature to 600°C-700°C at a heating rate of 20-30°C / min, and maintaining it for 30-60min; raising the temperature to 800°C-900°C at a heating rate of 15-25°C / min, and maintaining it for 30-60min; raising the temperature to 1000°C-1050°C at a heating rate of 10-15°C / min, and maintaining it for 30-60min; cooling for 150min at a cooling pressure of 1.3MPa;

[0008] Step S3: measuring the deformation of the steel material after vacuum quenching;

[0009] Step S4: perform high temperature tempering three times, the tempering temperature is 600°C-650°C, and the temperature is kept for 1.5-2h;

[0010] Step S5: Cut the cooled mold steel to complete the heat treatment.

[0011] Preferably, between steps S3 and S4, a cryogenic treatment step is further included, comprising the following steps:

[0012] Pretreatment: Clean and dry the mold steel surface;

[0013] Furnace loading: Place the mold steel in a cryogenic treatment furnace;

[0014] Cooling: lower the temperature of the cryogenic treatment furnace to -130℃~-150℃;

[0015] Insulation: Maintain deep freezing temperature for 0.5-1.5h;

[0016] Heating: Raise the temperature of the cryogenic treatment furnace to room temperature.

[0017] Preferably, in the vacuum quenching in step S2, cooling the mold steel after heating and heat preservation comprises the following steps:

[0018] When the temperature of the mold steel is 1050℃-500℃, the cooling temperature rate T1 is controlled to: T1>20℃ / min;

[0019] When the temperature of the mold steel is 500℃-250℃, the cooling temperature rate T2 is controlled as: 5℃ / min≤T2<15℃ / min.

[0020] Further preferably, when the temperature of the mold steel is 500°C-400°C, the cooling temperature rate T2 is controlled to be: 5°C / min≤T2<15°C / min;

[0021] When the temperature of the mold steel is 400℃-250℃, the cooling temperature rate T3 is controlled as: 3.5℃ / min≤T3<8℃ / min.

[0022] Preferably, after the three temperings are completed in step S4, the mold steel is subjected to gas soft nitriding treatment, comprising the following steps:

[0023] Pretreatment: Clean and dry the mold steel surface;

[0024] Furnace loading and preheating: Place the mold steel in a nitriding furnace and heat to 550℃-570℃;

[0025] Ventilation and reaction: Add nitriding source into the nitriding furnace and maintain the temperature for 2-3h;

[0026] Cooling and unloading: After the reaction is completed, cool to room temperature to complete the gas soft nitriding treatment.

[0027] Preferably, the nitriding source is ammonia and one or more of urea, formamide and triethanolamine.

[0028] Preferably, after the three temperings are completed in step S4, the die steel is subjected to hard nitriding treatment, comprising the following steps:

[0029] Pretreatment: Clean and dry the mold steel surface;

[0030] Furnace loading and preheating: Place the mold steel in a nitriding furnace and heat to 480℃-540℃;

[0031] Ventilation and reaction: A mixture of nitrogen and ammonia is introduced into the nitriding furnace and the temperature is maintained for 15-70 hours;

[0032] Cooling and unloading: After the reaction is completed, cool to room temperature to complete the hard nitriding treatment.

[0033] Preferably, after the three temperings are completed in step S4, the mold steel is subjected to surface coating treatment, comprising the following steps:

[0034] Pretreatment: Clean, dry and polish the surface of the mold steel;

[0035] Preheating: preheating the mold steel and cladding material;

[0036] Laser cladding: Cover the cladding material on the mold steel surface, scan the mold steel surface with a laser beam, or perform laser beam scanning simultaneously when delivering the cladding material to the mold steel surface.

[0037] Preferably, the laser beam current is 250A-300A, and the laser scanning speed is 2-5mm / s.

[0038] Preferably, the cladding material is a composite metal powder of Ni-Cr-Mo.

[0039] The technical solution provided by the present invention has at least the following advantages compared with the prior art:

[0040] 1. By adopting a specific vacuum quenching process, the speed of heating and cooling during the quenching process is limited. On the premise of ensuring the hardness performance of the mold steel, the toughness of the mold steel is also guaranteed, so that the mold steel after heat treatment has higher hardness, more stable material structure, and enhanced toughness.

[0041] 2. By surface treating the mold steel after quenching and tempering, the surface hardness and wear resistance of the mold steel can be effectively improved, and the wear caused by friction between the mold and the workpiece after the mold is applied to the mold can be effectively reduced, so that the mold can still maintain a high precision after long-term use and a long service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The above and other purposes, features and advantages of the present invention will become more clear by more specific description of the preferred embodiments of the present invention shown in the accompanying drawings. The same reference numerals indicate the same parts in all the accompanying drawings, and the accompanying drawings are not deliberately scaled to the actual size, and the focus is on illustrating the main purpose of the present invention.

[0043] Figure 1 This is a cooling rate curve diagram of vacuum quenching in Example 1 of the present invention;

[0044] Figure 2 This is a schematic diagram of sampling mold steel after heat treatment according to Example 1 of the present invention;

[0045] Figure 3 This is a physical picture of sampling the die steel after heat treatment in Example 1 of the present invention;

[0046] Figure 4 This is the metallographic structure diagram of the die steel sample after heat treatment in Example 1 of the present invention. DETAILED DESCRIPTION

[0047] The technical solution of the present invention is further described in detail below in conjunction with specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0048] In order to better illustrate the scheme of the present invention, the following are specific embodiments.

[0049] An embodiment of the present invention provides a heat treatment method for mold steel, which comprises the following steps:

[0050] Step S1: preheating, placing the mold steel in a preset vacuum heat treatment furnace for preheating treatment, and keeping the temperature after the preheating treatment is completed to obtain the preheated mold steel;

[0051] Step S2: vacuum quenching, placing the preheated mold steel in a vacuum heat treatment furnace for quenching treatment, raising the temperature to 600°C-700°C at a heating rate of 20-30°C / min, and maintaining it for 30-60min; raising the temperature to 800°C-900°C at a heating rate of 15-25°C / min, and maintaining it for 30-60min; raising the temperature to 1000°C-1050°C at a heating rate of 10-15°C / min, and maintaining it for 30-60min; cooling for 150min at a cooling pressure of 1.3MPa;

[0052] Step S3: measuring the deformation of the steel material after vacuum quenching;

[0053] Step S4: perform high temperature tempering three times, the tempering temperature is 600°C-650°C, and the temperature is kept for 1.5-2h;

[0054] Step S5: Cut the cooled mold steel to complete the heat treatment.

[0055] In this embodiment, the mold steel is heat treated by vacuum quenching process, which isolates the mold steel from contacting with oxygen in the air in a vacuum environment and producing oxidation reaction, causing the surface quality of the mold steel to decrease; it also avoids the problem of decarburization during the quenching process, thereby avoiding affecting the hardness of the mold steel. In this embodiment, by accurately controlling the three uniform heating and heat preservation, the thermal stress generated by the mold steel during the quenching and heating process can be reduced, and the staged heating can give the mold steel enough time to be evenly heated, reducing the temperature difference between the inner and outer layers; in addition, the staged heating can ensure the uniform transformation of the structure in the mold steel. The different alloy elements and organizational structures in the steel structure have a specific transformation process in different temperature ranges. The staged heating can provide suitable conditions for the organizational transformation. For example, after the first stage is heated to the pearlite to austenite transformation temperature range, it is maintained for a period of time to allow the pearlite to fully transform into austenite. In this way, in the subsequent quenching process, due to the uniform organizational transformation, a more ideal quenching effect can be obtained, such as a finer and more uniform martensite structure, thereby improving the performance of the mold steel.

[0056] The heat treatment method of this embodiment adopts a three-time high-temperature tempering process after vacuum quenching. On the one hand, it is to eliminate the quenching stress. Due to the complex distribution of quenching stress inside the mold steel, one tempering is not enough to eliminate the quenching stress inside the mold steel. There may be residual tensile stress and compressive stress areas. Three high-temperature tempering can eliminate stress as much as possible. Secondly, multiple tempering can stabilize the structure of the mold steel. The structure of the mold steel after quenching is in a metastable state. The structure transformation after the first tempering is not complete. Through multiple high-temperature tempering, the structure can be made more stable.

[0057] In a preferred embodiment, between steps S3 and S4, a cryogenic treatment step is further included, comprising the following steps:

[0058] Pretreatment: Clean and dry the mold steel surface;

[0059] Furnace loading: Place the mold steel in a cryogenic treatment furnace;

[0060] Cooling: lower the temperature of the cryogenic treatment furnace to -130℃~-150℃;

[0061] Insulation: Maintain deep freezing temperature for 0.5-1.5h;

[0062] Heating: Raise the temperature of the cryogenic treatment furnace to room temperature.

[0063] In a preferred embodiment, the mold steel that has completed vacuum quenching is subjected to deep cryogenic treatment, which can, on the one hand, reduce the stress generated in the mold steel during quenching, thereby enhancing the dimensional stability of the mold steel, and on the other hand, refine the grains in the mold steel, thereby improving the toughness of the mold steel; improving the uniformity of the mold steel structure. The uniform structure can make the stress borne by the mold during operation more evenly distributed, reduce stress concentration, effectively improve the fatigue resistance of the mold steel, and extend the service life of the mold.

[0064] In this embodiment, in step S2 vacuum quenching, cooling the mold steel after heating and heat preservation includes the following steps:

[0065] When the temperature of the mold steel is 1050℃-500℃, the cooling temperature rate T1 is controlled to: T1>20℃ / min;

[0066] When the temperature of the mold steel is 500℃-250℃, the cooling temperature rate T2 is controlled as: 5℃ / min≤T2<15℃ / min.

[0067] Furthermore, when the temperature of the mold steel is 500°C-400°C, the cooling temperature rate T2 is controlled to be: 5°C / min≤T2<15°C / min;

[0068] When the temperature of the mold steel is 400℃-250℃, the cooling temperature rate T3 is controlled as: 3.5℃ / min≤T3<8℃ / min.

[0069] Different cooling rates will affect the final microstructure of the mold steel. The cooling rate of vacuum quenching has a great influence on the toughness of the mold steel. In this embodiment, by accurately controlling the cooling rate of quenching and avoiding too fast a cooling rate, the martensitic structure can be made finer and more uniform. The fine and uniform martensitic structure has better comprehensive properties, including higher strength and toughness.

[0070] In other preferred embodiments, after the three temperings are completed in step S4, the mold steel is subjected to gas soft nitriding treatment, comprising the following steps:

[0071] Pretreatment: Clean and dry the mold steel surface;

[0072] Furnace loading and preheating: Place the mold steel in a nitriding furnace and heat to 550℃-570℃;

[0073] Ventilation and reaction: Add nitriding source into the nitriding furnace and maintain the temperature for 2-3h;

[0074] Cooling and unloading: After the reaction is completed, cool to room temperature to complete the gas soft nitriding treatment.

[0075] Furthermore, the nitriding source is ammonia and one or more of urea, formamide, and triethanolamine, which undergo thermal decomposition reaction at the soft nitriding temperature to produce active nitrogen atoms and carbon atoms, thereby causing a nitriding reaction on the surface of the mold steel.

[0076] In this embodiment, the mold steel after tempering is subjected to soft nitriding treatment, so that a nitrided layer with high hardness can be formed on the surface of the mold steel, which can improve the surface hardness and wear resistance of the mold steel.

[0077] In other preferred embodiments, after the three temperings are completed in step S4, the die steel is subjected to hard nitriding treatment, including the following steps:

[0078] Pretreatment: Clean and dry the mold steel surface;

[0079] Furnace loading and preheating: Place the mold steel in a nitriding furnace and heat to 480℃-540℃;

[0080] Ventilation and reaction: A mixture of nitrogen and ammonia is introduced into the nitriding furnace and the temperature is maintained for 15-70 hours;

[0081] Cooling and unloading: After the reaction is completed, cool to room temperature to complete the hard nitriding treatment.

[0082] In this embodiment, the mold steel after tempering is subjected to hard nitriding treatment, which can form a high-hardness nitrided layer on the surface of the mold steel, greatly improving the surface hardness of the mold steel, enabling it to withstand higher pressure and friction; and can significantly improve the wear resistance of the mold steel.

[0083] In other preferred embodiments, after the three temperings are completed in step S4, the mold steel is subjected to surface coating treatment, including the following steps:

[0084] Pretreatment: Clean, dry and polish the surface of the mold steel;

[0085] Preheating: preheating the mold steel and cladding material;

[0086] Laser cladding: Cover the cladding material on the mold steel surface, scan the mold steel surface with a laser beam, or perform laser beam scanning simultaneously when delivering the cladding material to the mold steel surface.

[0087] Furthermore, the laser beam current is 250A-300A, and the laser scanning speed is 2-5mm / s.

[0088] Furthermore, the cladding material adopts Ni-Cr-Mo composite metal powder.

[0089] In this embodiment, laser cladding is applied to the tempered die steel to form a fine structure coating on the die steel surface, which has a strong resistance to high temperature friction loss.

[0090] To further illustrate this solution, specific embodiments are described below.

[0091] Example 1

[0092] (1) Raw materials: XF328R steel is used as the raw material and the steel is cut into a shape of 400 mm × 400 mm × 400 mm.

[0093] (2) Heat treatment method:

[0094] Before heat treatment, 7 thermocouples (1#-7#) are inserted into the vacuum heat treatment furnace to detect the heat treatment temperature on the steel surface.

[0095] Step S1: preheating, placing the steel in a preset vacuum heat treatment furnace for preheating treatment at a temperature of 300°C, and keeping the temperature after the preheating treatment is completed to obtain a preheated mold steel;

[0096] Step S2: vacuum quenching, placing the preheated mold steel in a vacuum heat treatment furnace for quenching treatment, raising the temperature to 650°C at a heating rate of 20°C / min, and maintaining it for 60 minutes; raising the temperature to 850°C at a heating rate of 15°C / min, and maintaining it for 60 minutes; raising the temperature to 1020°C at a heating rate of 1°C / min, and maintaining it for 60 minutes; cooling for 150 minutes at a cooling pressure of 1.3MPa; when the temperature of the mold steel is 1050°C-500°C, the cooling temperature rate T1 is controlled to be: T1>20°C / min; when the temperature of the mold steel is 500°C-400°C, the cooling temperature rate T2 is controlled to be: 5°C / min≤T2<15°C / min; when the temperature of the mold steel is 400°C-250°C, the cooling temperature rate T3 is controlled to be: 3.5°C / min≤T3<8°C / min.

[0097] Step S3: measuring the deformation of the steel material after vacuum quenching;

[0098] Step S4: perform high temperature tempering three times, the tempering temperature is 630°C, and the temperature is kept for 1.5 hours.

[0099] (3) Performance testing

[0100] A. The actual measured temperature of vacuum quenching is shown in the following table:

[0101] Table 1 Vacuum quenching temperature data

[0102]

[0103]

[0104] like Figure 1 As shown, the cooling rate curve is plotted for all the data of the cooling process measured by 1#-7# thermocouples. It can be seen that the trends of the cooling temperature curves measured by 1#-7# thermocouples are basically consistent, indicating that the cooling rate control of the steel meets the requirements of this plan.

[0105] B. Conduct performance tests on the cooled steel.

[0106] a) Sampling

[0107] like Figure 2 As shown, samples are taken from two places of the steel. Position ① is at 1 / 4 of the diagonal of the steel. Two samples are taken from the surface and core respectively, with L1-1 and L1-3 in the longitudinal direction and H1-2 and H1-4 in the transverse direction. Position ② is at the center of the diagonal. Two samples are taken from the surface, with L2-2 in the longitudinal direction and H2-1 in the transverse direction. The samples are as follows: Figure 3 As shown; sampling size and label are as follows:

[0108] Table 2 Sample size and number table

[0109]

[0110] b) The sample was subjected to hardness test (Rockwell hardness tester) and V-notch impact test (using GB / T229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials" for testing). The test results are as follows:

[0111] Table 3 Hardness test and V-notch impact test results

[0112]

[0113]

[0114] c) Scan the sample with SEM to observe the metallographic structure of the sample, as follows: Figure 4 shown.

[0115] It can be seen that the metallographic morphology of each sample is slightly different, which is caused by the different cooling rates at different positions, but the metallographic structure of each sample is uniform and has no defects, which is within the standard range recognized by NADCA (North American Die Casting Association).

[0116] The mold steel treated by the heat treatment method of this embodiment has excellent hardness and impact resistance. It can also be seen from the metallographic structure that the mold steel structure after the heat treatment has no defects and is within the recognized range of international standards.

[0117] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A heat treatment method for mold steel, characterized in that: The following steps are involved: Step S1: preheating, placing the mold steel in a preset vacuum heat treatment furnace for preheating treatment, and keeping the temperature after the preheating treatment is completed to obtain the preheated mold steel; Step S2: vacuum quenching, placing the preheated mold steel in a vacuum heat treatment furnace for quenching treatment, raising the temperature to 600°C-700°C at a heating rate of 20-30°C / min, and maintaining it for 30-60min; Raise the temperature to 800-900°C at a heating rate of 15-25°C / min and keep it for 30-60min; raise the temperature to 1000-1050°C at a heating rate of 10-15°C / min and keep it for 30-60min; cool for 150min at a cooling pressure of 1.3MPa; Step S3: measuring the deformation of the steel material after vacuum quenching; Step S4: perform high temperature tempering three times, the tempering temperature is 600°C-650°C, and the temperature is kept for 1.5-2h; Step S5: Cut the cooled mold steel to complete the heat treatment.

2. The heat treatment method for mold steel according to claim 1, characterized in that: Between step S3 and step S4, a cryogenic treatment step is also included, including the following steps: Pretreatment: Clean and dry the mold steel surface; Furnace loading: Place the mold steel in a cryogenic treatment furnace; Cooling: lower the temperature of the cryogenic treatment furnace to -130℃~-150℃; Insulation: Maintain deep freezing temperature for 0.5-1.5h; Heating: Raise the temperature of the cryogenic treatment furnace to room temperature.

3. The heat treatment method for mold steel according to claim 1, characterized in that: In step S2 vacuum quenching, the mold steel that has been heated and kept warm is cooled, including the following steps: When the temperature of the mold steel is 1050℃-500℃, the cooling temperature rate T1 is controlled to: T1>20℃ / min; When the temperature of the mold steel is 500℃-250℃, the cooling temperature rate T2 is controlled as: 5℃ / min≤T2<15℃ / min.

4. The heat treatment method for mold steel according to claim 3, characterized in that: When the temperature of the mold steel is 500℃-400℃, the cooling temperature rate T2 is controlled as follows: 5℃ / min≤T2<15℃ / min; When the temperature of the mold steel is 400℃-250℃, the cooling temperature rate T3 is controlled as: 3.5℃ / min≤T3<8℃ / min.

5. The heat treatment method for mold steel according to claim 1, characterized in that: After the three temperings are completed in step S4, the mold steel is subjected to gas soft nitriding treatment, including the following steps: Pretreatment: Clean and dry the mold steel surface; Furnace loading and preheating: Place the mold steel in a nitriding furnace and heat to 550℃-570℃; Ventilation and reaction: Add nitriding source into the nitriding furnace and maintain the temperature for 2-3h; Cooling and unloading: After the reaction is completed, cool to room temperature to complete the gas soft nitriding treatment.

6. The heat treatment method for mold steel according to claim 5, characterized in that: The nitriding source is one or more of ammonia, urea, formamide and triethanolamine.

7. The heat treatment method for mold steel according to claim 1, characterized in that: After the three temperings are completed in step S4, the die steel is subjected to hard nitriding treatment, including the following steps: Pretreatment: Clean and dry the mold steel surface; Furnace loading and preheating: Place the mold steel in a nitriding furnace and heat to 480℃-540℃; Ventilation and reaction: A mixture of nitrogen and ammonia is introduced into the nitriding furnace and the temperature is maintained for 15-70 hours; Cooling and unloading: After the reaction is completed, cool to room temperature to complete the hard nitriding treatment.

8. The heat treatment method for mold steel according to claim 1, characterized in that: After the three temperings are completed in step S4, the mold steel is subjected to surface coating treatment, including the following steps: Pretreatment: Clean, dry and polish the surface of the mold steel; Preheating: preheating the mold steel and cladding material; Laser cladding: Cover the cladding material on the mold steel surface, scan the mold steel surface with a laser beam, or perform laser beam scanning simultaneously when delivering the cladding material to the mold steel surface.

9. The heat treatment method for mold steel according to claim 8, characterized in that: The laser beam current is 250A-300A, and the laser scanning speed is 2-5mm / s.

10. The heat treatment method for mold steel according to claim 8, characterized in that: The cladding material is Ni-Cr-Mo composite metal powder.

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