A high-hardness, micro-deformation, clean heat treatment control method for cold working dies

Abstract

The invention discloses a high-hardness micro-deformation clean heat treatment control method for a cold working die, including two-stage isothermal high-pressure gas quenching, filling a vacuum furnace with high-pressure gas quenching gas, stopping filling the high-pressure gas quenching gas after the surface temperature of the cold working die is reduced to 50°C above the metastable austenite region through gas circulation, and continuing to fill the high-pressure gas quenching gas until the surface temperature of the cold working die is reduced to below the martensite point and then stopping; then continue to heat up to the lower bainite region for insulation, so that the core undergoes a lower bainite transformation, and then cools to room temperature; and then high-temperature tempering 2-3 times. The present invention adopts two-stage vacuum high-pressure gas quenching isothermal, that is, vacuum high-pressure gas quenching isothermal quenching in the metastable austenite region and the lower bainite region, and the two-stage isothermal controls the thermal stress and tissue stress to control the mold deformation, thereby achieving precise control of the organization and stress, thereby controlling the mold deformation within 0.05%.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment, and in particular to a high-hardness micro-deformation clean heat treatment control method for a cold working die. Background Art

[0002] For the Cr12MoV cold working die made of high carbon and high chromium ledeburite steel, the original annealed state structure is eutectic carbide and granular pearlite, as shown in the attached figure. The black structure in the backscattering diagram is eutectic carbide, which is a carbide rich in Cr, usually distributed in a band or mesh form. The carbide is very stable and difficult to eliminate during quenching and tempering. In addition, the Ms (martensite transformation start temperature) and Mf point (martensite transformation end temperature) of the material are low, and a large amount of residual austenite exists at room temperature after quenching. Due to the organizational characteristics of the eutectic carbide and the transformation of martensite and the existence of residual austenite during the quenching process, the deformation of the die processed by the material is difficult to control, which has become a technical difficulty of the current cold working die.

[0003] The current heat treatment methods either have poor control effects on mold deformation, resulting in poor mold dimensional stability, or have difficulty in achieving precise control of mold deformation and organization.

[0004] For example:

[0005] Related technology 1, application number CN201911086269.5, patent application named "A method for heat treatment of hot working die steel", this process adopts multi-stage cooling for hot working die steel, including high temperature zone cooling, medium temperature zone cooling, low temperature zone cooling and furnace cooling and correction in sequence, and finally the hot working die steel after quenching and cooling is tempered to control the deformation to less than 0.1% of the diagonal length of the bottom surface of the die. The invention adopts 600℃, 400℃, and 250℃ three sections of isothermal, among which 400℃ isothermal is easy to form upper bainite, which seriously affects the performance of the die. And the invention controls the deformation by 0.1%, and the deformation control effect is poor.

[0006] Related technology 2, application number CN201410349478.5, patent application named "Micro-deformation quenching heat treatment process for cold-working die steel thin parts", this process controls the oil quenching process of the workpiece, the surface temperature is lower than the Ms point, the core temperature is Ms+30℃~Ms+50℃, and low-temperature tempering is performed after quenching. However, the holding time in the bainite zone of the oil quenching process used in this method is extremely short, and the transformation of the lower bainite cannot be achieved well. Moreover, although the low-temperature tempering process can control the instantaneous deformation after the heat treatment is completed, the subsequent aging decomposition of the residual austenite makes the dimensional stability of the mold poor. In the intermediate transition time from the completion of the heat treatment of the workpiece to the assembly, large dimensional fluctuations will occur, thus affecting the subsequent assembly.

[0007] Related technology 3, application number CN201110119192.4, patent application titled "A quenching heat treatment process for low heat resistance hot working die steel", the process uses air cooling, water cooling, and then air cooling and oil cooling to quench the low heat resistance hot working die steel. This method is difficult to achieve precise control of deformation and structure for molds with high precision and performance requirements.

[0008] In summary, the heat treatment methods in the prior art cannot solve the problem that the Cr12MoV cold working die is prone to large deformation during quenching, and cannot achieve precise control of the structure and stress, thereby affecting subsequent processing and assembly. Summary of the invention

[0009] The purpose of the present invention is to provide a high-hardness micro-deformation clean heat treatment control method for cold working dies to solve the technical problems in the prior art that cold working dies are prone to large deformation during quenching and cannot achieve precise control of structure and stress.

[0010] In order to solve the above technical problems, the present invention specifically provides the following technical solutions:

[0011] A high-hardness micro-deformation clean heat treatment control method for a cold working die comprises the following steps:

[0012] Solution treatment: Place the cold working die in a vacuum furnace, heat it to the quenching temperature and keep it warm;

[0013] Two-stage isothermal high-pressure gas quenching: Fill the vacuum furnace with high-pressure gas quenching gas, and stop filling the high-pressure gas quenching gas after the surface temperature of the cold-working mold drops to the metastable austenite region, so that the gas circulates in the furnace;

[0014] After the core temperature of the cold working die drops to 50°C above the metastable austenite region, continue to fill with high-pressure quenching gas until the surface temperature of the cold working die drops below the martensite point;

[0015] When the core temperature of the cold-working die drops to the transition temperature between upper bainite and lower bainite, the furnace is heated to the lower bainite zone and kept isothermal for 3-5 hours, and then high-pressure quenching gas is continuously charged until the cold-working die is cooled to room temperature;

[0016] Tempering: Tempering treatment is performed on the cold working mold after double-stage isothermal high-pressure gas quenching cooling.

[0017] Furthermore, the cold working die is a Cr12MoV cold working die made of high-carbon and high-chromium ledeburite steel, and its original annealed structure is eutectic carbide and granular pearlite.

[0018] Further, the pressure of the high-pressure quenching gas when it is charged into the vacuum furnace is 0.6-1.0 MPa;

[0019] The high-pressure quenching gas is an inert gas.

[0020] Furthermore, the high-pressure quenching gas is nitrogen or argon.

[0021] Furthermore, the tempering temperature is 490-510°C.

[0022] Furthermore, the tempering is performed 2-3 times.

[0023] Furthermore, the solution treatment is carried out at a heating rate of ≤300°C / h, the temperature is raised to 1020°C-1025°C and the temperature is maintained.

[0024] Furthermore, the holding time of the solution treatment is not less than 0.5 h, and the holding time is calculated from the time when the center of the mold reaches temperature.

[0025] Furthermore, before the mold is subjected to solution treatment, a process of preheating the mold is also included, and the preheating treatment includes two heating and heat preservation steps.

[0026] Furthermore, the two heating and heat preservation steps in the preheating treatment are specifically as follows:

[0027] Heat the mold to 600-650℃ at a heating rate of ≤300℃ / h and keep it warm for 0.5-1h;

[0028] Then increase the temperature to 800-850℃ at a heating rate of ≤300℃ / h and keep it at that temperature for 0.5-1h.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The present invention adopts two-stage vacuum isothermal high-pressure gas quenching, i.e., vacuum high-pressure gas quenching isothermal quenching in the metastable austenite zone and the lower bainite zone, and reduces the surface temperature to a relatively low temperature below the Ms point in the intermediate stage of isothermal quenching in the metastable austenite zone and the lower bainite zone, and then heats up to the lower bainite zone when the core is cooled to the transition zone between the upper bainite and the lower bainite, so that the martensitic phase transformation occurs on the mold surface, and is beneficial to accelerate the core of the mold, thereby avoiding the formation of upper bainite in the core. The two-stage isothermal gas quenching controls thermal stress and structural stress, thereby controlling mold deformation, and controlling the mold deformation within 0.05%, while maintaining the mechanical properties of the mold, and having high hardness. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0032] Figure 1 The original annealed structure of the Cr12MoV cold working die in the present invention: (a) backscattering image; (b) secondary electron image;

[0033] Figure 2 It is a schematic diagram of the heat treatment process in the present invention;

[0034] Figure 3 It is a schematic diagram of the cold stamping die in the present invention;

[0035] Figure 4 Schematic diagram of the mold microstructure in Example 1 of the present invention: (a) surface; (b) core. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] like Figure 2 As shown, the present invention provides a high-hardness micro-deformation clean heat treatment control method for a cold working die, which mainly realizes die deformation control by adopting two-stage vacuum high-pressure isothermal gas quenching.

[0038] The current heat treatment methods either have poor control effects on mold deformation, resulting in poor mold dimensional stability, or have difficulty in achieving precise control of mold deformation and organization.

[0039] A high-hardness micro-deformation clean heat treatment control method for a cold working die comprises the following steps:

[0040] Solution treatment: Place the cold working die in a vacuum furnace, heat it to the quenching temperature and keep it warm;

[0041] Two-stage isothermal high-pressure gas quenching: Fill the vacuum furnace with high-pressure gas quenching gas, and stop filling the high-pressure gas quenching gas after the surface temperature of the cold-working mold drops to the metastable austenite region, so that the gas circulates in the furnace;

[0042] After the core temperature of the cold working die drops to 50°C above the metastable austenite region, continue to fill with high-pressure quenching gas until the surface temperature of the cold working die drops below the martensite point;

[0043] When the core temperature of the cold-working die drops to the transition temperature between upper bainite and lower bainite, the furnace is heated to the lower bainite zone and kept isothermal for 3-5 hours, and then high-pressure quenching gas is continuously charged until the cold-working die is cooled to room temperature;

[0044] Tempering: Tempering treatment is performed on the cold working mold after double-stage isothermal high-pressure gas quenching cooling.

[0045] The cold working die is a Cr12MoV cold working die made of high-carbon and high-chromium ledeburite steel, and its original annealed state structure is eutectic carbide and granular pearlite.

[0046] The metastable austenite region is the intermediate temperature range between the ferrite pearlite transformation curve and the bainite transformation curve in the CCT transformation curve. When the temperature is cooled to this temperature range at an appropriate cooling rate, the austenite structure can be maintained without other phase changes.

[0047] This process uses vacuum gas quenching to control the quenching process, which can achieve precise control of cooling speed and temperature, ensuring high hardness while effectively controlling mold deformation and dimensional accuracy stability, and achieving precise control of two-stage quenching.

[0048] The pressure of the high-pressure quenching gas when it is charged into the vacuum furnace is 0.6-1.0 MPa.

[0049] When performing high-pressure gas quenching, the selection of the gas quenching gas pressure is very important. It must be able to ensure the cooling speed under this gas pressure, so as to accurately control the mold structure and stress. In this embodiment, the gas quenching gas pressure is preferably 0.6-1.0MPa. If the gas pressure is too high, greater than 1.0MPa, the cooling speed is too fast, and the surface temperature is difficult to control, so that the mold thermal stress and structural stress cannot be accurately controlled, and the mold deformation cannot be controlled. If the gas pressure is too low, less than 0.6MPa, the cooling speed is too slow, and pearlite structure is easily formed in the mold, which cannot meet the basic structure and performance requirements.

[0050] As a preferred embodiment, the high-pressure quenching gas is an inert gas.

[0051] The high pressure gas quenching gas is nitrogen or argon. In the gas quenching process, selecting an inert gas as the cooling medium can effectively prevent the metal from being oxidized during the cooling process, while avoiding decarburization and maintaining the purity and surface quality of the material.

[0052] This process is different from the vacuum high-pressure gas quenching method in the prior art. This process uses two stages of vacuum high-pressure gas quenching isothermal quenching: that is, vacuum high-pressure gas quenching isothermal quenching in the metastable austenite zone and the lower bainite zone. At the same time, when the lower bainite zone isothermal, the surface is first lowered to a lower temperature below the Ms point. When the temperature of the mold core drops to the transition zone between the upper bainite and the lower bainite, the furnace is heated to the lower bainite zone isothermal. On the one hand, it causes martensitic phase transformation on the surface, and on the other hand, it is conducive to accelerating the cooling of the mold core to avoid the formation of upper bainite in the core, thereby ensuring the performance of the mold.

[0053] The characteristics and difficulties of this process are that, on the one hand, the process obtains a high cooling rate through high-pressure gas quenching, reaches the isotherm of the metastable austenite zone in a short time, and avoids the formation of pearlite; on the other hand, the surface is first lowered to below the Ms point before the second isothermal stage to reduce the temperature, forming a gradient microstructure transformation with martensite transformation on the surface and bainite transformation below the core. The combined effect of the two can effectively avoid the formation of pearlite for cold-working die steel with a large critical cooling rate, and at the same time obtain the lower bainite transition structure of the surface martensite core. The formation of lower bainite can reduce the microstructure stress, and further control the microstructure stress on the basis of controlling the thermal stress in the two-stage isothermal stage, thereby effectively controlling the deformation. This process can control the deformation below 0.05%.

[0054] In order to further adjust the mechanical properties and internal structure of the cold working die, a tempering process is further adopted for the die after quenching.

[0055] Specifically, the tempering temperature is 490-510°C. The number of tempering is 2-3 times. The tempering process of the present application is different from the tempering method in the prior art. The present application adopts multiple high-temperature tempering processes, and the tempering temperature reaches 490-510°C. On the one hand, the residual austenite can be gradually decomposed, so that the residual austenite can be fully decomposed to stabilize the size. The mold after double-stage isothermal high-pressure gas quenching and high-temperature tempering can achieve small deformation and good dimensional stability; on the other hand, the hardness of the core of the mold is gradually increased to meet the use requirements.

[0056] In order to fully dissolve the solute atoms in the alloy into the lattice of the solvent metal and form a uniform solid solution structure, the cold working die needs to be heated for solid solution treatment before gas quenching. Preferably, the solid solution treatment in this application is heated to 1020℃-1025℃ at a heating rate of ≤300℃ / h and kept warm. The holding time of the solid solution treatment is not less than 0.5h, and the holding time is calculated from the time when the core of the die reaches the temperature.

[0057] Heat the mold workpiece to 1020℃-1025℃ to ensure that various elements in the alloy can be dissolved in the base metal as much as possible. Then keep it at 1020℃-1025℃ to ensure that the temperature of each part inside the alloy is uniform and some small-sized eutectic carbides are dissolved back into austenite as much as possible.

[0058] The solution treatment heats the alloy to the single-phase austenite region and rapidly cools it in this state, so that the alloy elements are fully dissolved in the austenite, improving its stability.

[0059] Before the mold is subjected to solution treatment, a process of preheating the mold is also included, and the preheating treatment includes two heating and insulation steps.

[0060] The two heating and insulation steps in the preheating treatment are as follows:

[0061] Heat the mold to 600-650℃ at a rate of ≤300℃ / h and keep it warm for 0.5-1h. The holding time can be increased or decreased according to the size of the mold.

[0062] Then increase the temperature to 800-850℃ at a heating rate of ≤300℃ / h and keep it warm for 0.5-1h. The holding time can be increased or decreased according to the mold size.

[0063] This process can enhance the control of mold deformation by preheating, reducing the heating rate and performing two-stage preheating.

[0064] The heating rate during the solution treatment is ≤300℃ / h. If the heating process is too fast, it will cause a temperature difference between the core and the surface of the mold, thereby causing thermal stress and structural stress. In the subsequent quenching process, the stress gradually accumulates, which is not conducive to deformation control.

[0065] The following is further described in conjunction with specific embodiments:

[0066] The mold used in the experiment adopts a cold stamping mold insert, as shown in the following figure. Figure 3 As shown. Thermocouples are inserted into the mold surface and core to collect their temperatures.

[0067] Embodiment 1:

[0068] Step 1: Use a three-coordinate measuring machine to detect the original size of the mold.

[0069] Step 2: In a vacuum furnace, heat the mold to 650°C at a heating rate of 300°C / h and keep it warm for 0.5h, then heat it to 850°C at a heating rate of 300°C / h and keep it warm for 0.5h, and then heat it to 1020°C at a heating rate of 300°C / h and keep it warm for 1.5h.

[0070] Step 3: Use nitrogen as the quenching medium and fill it into the vacuum furnace at a pressure of 0.6MPa. When the surface temperature drops to 560℃ (metastable austenite region), keep it warm; when the core temperature drops to 610℃, continue to fill nitrogen to reduce the surface temperature to 100℃ (below Ms point) and maintain it for a short time; when the core temperature drops to 330℃, start heating and keep the furnace temperature within the range of 280-300℃ (lower bainite region) for 5h; then continue to fill nitrogen and cool it to room temperature. The core and surface structures after quenching are as follows: Figure 4 shown.

[0071] Step 4: Transfer the mold to the tempering furnace, keep it at 495℃ for 5h, and temper it 3 times.

[0072] Step 5: Use three-coordinate measurement to detect the size of the mold after heat treatment and compare it with that before heat treatment.

[0073] The maximum change rate of the mold size is 0.05%. Through hardness testing, the surface and core hardness are detected, the surface hardness is greater than 59HRC, and the core hardness is greater than 58HRC.

[0074] Comparative Example 1:

[0075] Step 1: Use a three-coordinate measuring machine to detect the original size of the mold.

[0076] Step 2: In a vacuum furnace, heat the mold to 650°C at a heating rate of 300°C / h and keep it warm for 0.5h, then heat it to 850°C at a heating rate of 300°C / h and keep it warm for 0.5h, and then heat it to 1020°C at a heating rate of 300°C / h and keep it warm for 1.5h.

[0077] Step 3: Use nitrogen as the quenching medium, fill it into the vacuum furnace at a pressure of 0.6MPa, and cool the mold to room temperature.

[0078] Step 4: Transfer the mold to the tempering furnace, keep it at 495℃ for 5h, and temper it 3 times.

[0079] Step 5: Use three-coordinate measurement to detect the size of the mold after heat treatment and compare it with that before heat treatment.

[0080] The maximum change rate of the mold size is 0.1%. Through hardness testing, the surface and core hardness are detected, the surface hardness is greater than 59HRC, and the core hardness is greater than 58HRC.

[0081] The test results are shown in the following table:

[0082]

[0083] The following conclusions can be drawn from the analysis of the above table:

[0084] The process provided by the present application adopts two stages of vacuum high-pressure gas quenching isothermal quenching: namely, vacuum high-pressure gas quenching isothermal quenching in the metastable austenite zone and the lower bainite zone, and at the same time, when the lower bainite zone isothermal quenching, the surface is first lowered to a lower temperature below the Ms point, and when the temperature of the core of the mold drops to the transition zone between the upper bainite and the lower bainite, the furnace is heated to the lower bainite zone isothermal. On the one hand, it causes martensitic phase transformation on the surface, and on the other hand, it is conducive to accelerating the cooling of the core of the mold to avoid the formation of upper bainite in the core, thereby ensuring the performance of the mold.

[0085] Compared with the vacuum high-pressure gas quenching method in the prior art, the method provided by the present application can control the mold deformation to less than 0.05%.

[0086] The above embodiments are only exemplary embodiments of the present application and are not intended to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and protection scope of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the protection scope of the present application.

Claims

1. A high hardness micro-deformation clean heat treatment control method for cold working dies, characterized in that: The following steps are involved: Solution treatment: Place the cold working die in a vacuum furnace, heat it to the quenching temperature and keep it warm; Two-stage isothermal high-pressure gas quenching: Fill the vacuum furnace with high-pressure gas quenching gas, and stop filling the high-pressure gas quenching gas after the surface temperature of the cold-working mold drops to the metastable austenite region, so that the gas circulates in the furnace; After the core temperature of the cold working die drops to 50°C above the metastable austenite region, continue to fill with high-pressure quenching gas until the surface temperature of the cold working die drops below the martensite point; When the core temperature of the cold-working die drops to the transition temperature between upper bainite and lower bainite, the furnace is heated to the lower bainite zone and kept isothermal for 3-5 hours, and then high-pressure quenching gas is continuously charged until the cold-working die is cooled to room temperature; Tempering: Tempering treatment is performed on the cold working mold after double-stage isothermal high-pressure gas quenching cooling.

2. The high hardness micro-deformation clean heat treatment control method for cold working dies according to claim 1 is characterized in that: The cold working die is a Cr12MoV cold working die made of high-carbon and high-chromium ledeburite steel, and its original annealed state structure is eutectic carbide and granular pearlite.

3. A high hardness micro-deformation cleaning heat treatment control method for a cold working die according to claim 1 or 2, characterized in that: The pressure of the high-pressure quenching gas when it is charged into the vacuum furnace is 0.6-1.0MPa; The high-pressure quenching gas is an inert gas.

4. The high hardness micro-deformation cleaning heat treatment control method for cold working dies according to claim 3 is characterized in that: The high pressure quenching gas is nitrogen or argon.

5. The high hardness micro-deformation cleaning heat treatment control method for cold working dies according to claim 4 is characterized in that: The tempering temperature is 490-510°C.

6. A high-hardness micro-deformation clean heat treatment control method for cold working dies according to claim 5, characterized in that: The number of tempering is 2-3 times.

7. A high-hardness micro-deformation clean heat treatment control method for cold working dies according to claim 6, characterized in that: The solution treatment is carried out at a heating rate of ≤300°C / h, rising to 1020°C-1025°C and maintaining the temperature.

8. The high-hardness micro-deformation clean heat treatment control method for cold working dies according to claim 7, characterized in that: The holding time of solution treatment shall not be less than 0.5h, and the holding time shall be calculated from the time when the center of the mold reaches temperature.

9. A high-hardness micro-deformation clean heat treatment control method for cold working dies according to claim 8, characterized in that: Before the mold is subjected to solution treatment, a process of preheating the mold is also included, and the preheating treatment includes two heating and insulation steps.

10. A high hardness micro-deformation clean heat treatment control method for cold working dies according to claim 9, characterized in that: The two heating and insulation steps in the preheating treatment are as follows: Heat the mold to 600-650℃ at a heating rate of ≤300℃ / h and keep it warm for 0.5-1h; Then increase the temperature to 800-850℃ at a heating rate of ≤300℃ / h and keep it at that temperature for 0.5-1h.

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