Clean heat treatment control method for achieving high hardness and minimal deformation of cold-work die

Abstract

Disclosed in the present invention is a clean heat treatment control method for achieving high hardness and minimal deformation of a cold-work die, comprising: double-stage isothermal high-pressure gas quenching: introducing a high-pressure gas quenching gas into a vacuum furnace and stopping introduction of the high-pressure gas quenching gas once the surface temperature of a cold-work die has been cooled to a metastable austenite zone; enabling the temperature of a core portion of the cold-work die to be reduced to 50ºC above the metastable austenite zone by means of gas circulation, and then continuing to introduce the high-pressure gas quenching gas until the surface temperature of the cold-work die is reduced to a martensite point or below; then continuing to heat the cold-work die to a lower bainite zone, maintaining the temperature to enable the core portion to undergo lower bainite transformation, and then cooling the cold-work die to room temperature; and then carrying out high-temperature tempering two or three times. According to the present invention, two-stage vacuum high-pressure gas quenching isothermal treatment, i.e., vacuum high-pressure gas quenching and isothermal quenching in the metastable austenite zone and the lower bainite zone, is performed, and thermal stress and structural stress are controlled by means of the two-stage isothermal treatment so as to control the deformation of the die, thereby achieving accurate control of the structure and stress, controlling the deformation of the die to be within 0.05%.

Description

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

[0001] The present invention relates to the field of heat treatment technology, 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 from high-carbon, high-chromium ledeburite steel, the original annealed microstructure is eutectic carbides and granular pearlite, as shown in the attached image (Error! Reference source not found). The black structure in the backscattered image is eutectic carbides, Cr-rich carbides typically distributed in bands or networks. These carbides are very stable and difficult to eliminate during quenching and tempering. Furthermore, the material has low Ms (martensitic transformation start temperature) and Mf point (martensitic transformation end temperature), resulting in a large amount of retained austenite at room temperature after quenching. Due to the microstructure of these eutectic carbides, the transformation of martensite during quenching, and the presence of retained austenite, mold deformation in this material is difficult to control, a current technical difficulty in cold-working die processing.

[0003] 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 structure.

[0004] For example:

[0005] Related technology 1, application number CN201911086269.5, patent application titled “A method for heat treatment of hot working die steel”, this process uses multi-stage cooling for hot working die steel, including sequential high temperature zone cooling, medium temperature zone cooling, low temperature zone cooling and furnace cooling and correction. 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 three isothermal sections of 600°C, 400°C, and 250°C, among which the 400°C isothermal is prone to forming upper bainite, which seriously affects the performance of the die. Moreover, the invention controls the deformation by 0.1%, and the deformation control effect is poor.

[0006] Related technology 2, application number CN201410349478.5, is a patent application titled “Micro-deformation quenching heat treatment process for thin cold-working die steel parts.” This process controls the oil quenching process of the workpiece, so that the surface temperature is lower than the Ms point and the core temperature is reduced to Ms+30℃~Ms+50℃, followed by low-temperature tempering after quenching. However, the oil quenching process used in this method has an extremely short holding time in the bainite zone, making it impossible to effectively achieve the transformation of lower bainite. Moreover, although the low-temperature tempering process can control the instantaneous deformation after the heat treatment is completed, the subsequent aging decomposition of the retained austenite makes the mold dimensional stability poor. Large dimensional fluctuations will occur during the transition period between the completion of the workpiece heat treatment and assembly, thus affecting subsequent assembly.

[0007] Related Technology 3, application number CN201110119192.4, entitled "A Quenching Heat Treatment Process for Low-Heat-Resistance Hot-Work Die Steel," uses air cooling, water cooling, and then air cooling and oil cooling to quench low-heat-resistance hot-work die steel. This method makes it difficult to achieve precise control of deformation and microstructure for molds requiring high precision and performance.

[0008] In summary, the existing heat treatment methods cannot solve the problem that Cr12MoV cold working dies are prone to large deformation during quenching, and cannot achieve precise control of 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, so as to solve the technical problem 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 and 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 quenching gas, and stop filling the high-pressure quenching gas after the surface temperature of the cold working die 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 inject 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 injected 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] Furthermore, 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 kept constant.

[0024] Furthermore, the holding time of the solution treatment is not less than 0.5h, and the holding time is calculated from the time when the core 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 holding steps.

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

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

[0028] Then increase the temperature to 800-850℃ at a 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 and isothermal quenching in the metastable austenite region and the lower bainite region, and reduces the surface temperature to a lower temperature below the Ms point in the intermediate stage of isothermal quenching in the metastable austenite region and isothermal quenching in the lower bainite region. When the core is cooled to the transition zone between the upper bainite and the lower bainite, the temperature is raised to the lower bainite region, so that a martensitic phase transformation occurs on the mold surface, and it 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] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other implementation drawings based on the provided drawings without inventive effort.

[0032] FIG1 is the original annealed structure of the Cr12MoV cold working die of the present invention: (a) backscattered image; (b) secondary electron image;

[0033] FIG2 is a schematic diagram of the heat treatment process of the present invention;

[0034] FIG3 is a schematic diagram of a cold stamping die in the present invention;

[0035] FIG4 is a schematic diagram of the mold microstructure in Example 1 of the present invention: (a) surface; (b) core. DETAILED DESCRIPTION

[0036] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0037] As shown in FIG2 , the present invention provides a method for controlling high-hardness, micro-deformation, and clean heat treatment of a cold-working die, which mainly realizes die deformation control by adopting two-stage vacuum high-pressure isothermal gas quenching.

[0038] 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 structure.

[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 and 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 quenching gas, and stop filling the high-pressure quenching gas after the surface temperature of the cold working die 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 inject 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 injected 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 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 rate and temperature, ensuring high hardness while effectively controlling mold deformation and dimensional accuracy stability, and realizing precise control of two-stage quenching.

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

[0049] When performing high-pressure gas quenching, the selection of the quenching gas pressure is crucial. It must ensure a consistent cooling rate at this pressure, thereby enabling precise control of the mold's microstructure and stress. In this embodiment, the gas quenching pressure is preferably between 0.6 and 1.0 MPa. If the pressure is too high, exceeding 1.0 MPa, the cooling rate is too rapid, making it difficult to control the surface temperature, resulting in an inability to precisely control mold thermal and microstructural stresses, and consequently, mold deformation. If the pressure is too low, below 0.6 MPa, the cooling rate is too slow, and pearlite tends to form within the mold, failing to meet basic microstructure 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. During 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 differs from existing vacuum and high-pressure gas quenching methods in that it employs a two-stage vacuum and high-pressure gas quenching isotherm: austempering the metastable austenite and lower bainite regions. During the lower bainite isotherm, the surface is first cooled to a lower temperature below the Ms point. Once the mold core temperature has dropped to the transition zone between upper and lower bainite, the furnace temperature is raised to the lower bainite region. This not only allows for martensitic transformation on the surface, but also accelerates cooling of the mold core, preventing the formation of upper bainite there, thereby ensuring mold performance.

[0053] The unique features and challenges of this process lie in its ability to achieve a high cooling rate through high-pressure gas quenching, quickly reaching the metastable austenite isotherm and preventing pearlite formation. Furthermore, the surface temperature is lowered to below the Ms point before the second isothermal stage, resulting in a gradient microstructure transformation, with martensite predominant on the surface and bainite predominant below the core. This combined effect effectively prevents pearlite formation for cold-working die steels with high critical cooling rates, while simultaneously achieving a transitional microstructure with martensite on the surface and lower bainite in the core. The formation of lower bainite reduces microstructure stress, further controlling microstructure stress based on the thermal stress control achieved during the two isothermal stages, thereby effectively controlling deformation. This process can control deformation to 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 times 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 retained austenite can be gradually decomposed, so that the retained austenite is fully decomposed to stabilize the size. After the two-stage isothermal high-pressure gas quenching and high-temperature tempering treatment, the mold 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] To fully dissolve the solute atoms in the alloy into the solvent metal lattice and form a uniform solid solution structure, the cold working die needs to be heated for solution treatment before gas quenching. Preferably, the solution treatment in this application is performed at a heating rate of ≤300°C / h to 1020°C-1025°C and then maintained at this temperature. The solution treatment holding time is not less than 0.5h, and this holding time is calculated from the time the core of the die reaches temperature.

[0057] The mold workpiece is heated to 1020-1025°C to ensure that the various elements in the alloy are dissolved as much as possible in the base metal. Then, the temperature is kept at 1020-1025°C to ensure that the temperature of each part of the alloy is uniform and some small eutectic carbides are dissolved back into the austenite as much as possible.

[0058] Solution treatment heats the alloy to the single-phase austenite region and rapidly cools it in this state, allowing the alloy elements to fully dissolve in the austenite and improve 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 holding 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 mold size.

[0062] Then increase the temperature to 800-850℃ 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 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 solution treatment is ≤300°C / h. If the heating rate is too fast, it will cause a temperature difference between the core and surface of the mold, resulting in thermal stress and structural stress. During the subsequent quenching process, the stress gradually accumulates, which is not conducive to deformation control.

[0065] The following is further described with reference to specific embodiments:

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

[0067] Example 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 warm for 0.5h, then heat it to 850°C at a heating rate of 300°C / h and keep warm for 0.5h, and finally heat it to 1020°C at a heating rate of 300°C / h and keep warm for 1.5h.

[0070] Step 3: Nitrogen is used as the quenching medium and is introduced into the vacuum furnace at a pressure of 0.6 MPa. When the surface temperature drops to 560°C (metastable austenite region), the temperature is maintained. When the core temperature drops to 610°C, nitrogen is continued to be introduced, bringing the surface temperature down to 100°C (below the Ms point) and maintaining this temperature for a short period. When the core temperature drops to 330°C, the temperature is raised, maintaining the furnace temperature between 280-300°C (lower bainite region) for another 5 hours. Nitrogen is then continued to be introduced, and the temperature is brought down to room temperature. The core and surface microstructures after quenching are shown in Figure 4.

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

[0072] Step 5: Use three coordinates 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 warm for 0.5h, then heat it to 850°C at a heating rate of 300°C / h and keep warm for 0.5h, and finally heat it to 1020°C at a heating rate of 300°C / h and keep warm for 1.5h.

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

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

[0079] Step 5: Use three coordinates 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] The following conclusions can be drawn from the analysis of the above table:

[0083] The process disclosed in this application utilizes a two-stage vacuum high-pressure gas quenching isothermal process: vacuum high-pressure gas quenching isothermal quenching in the metastable austenite region and the lower bainite region. During the lower bainite isothermal process, the surface is first cooled to a lower temperature below the Ms point. Once the core temperature of the mold has fallen to the transition zone between upper and lower bainite, the furnace temperature is raised to the lower bainite isothermal temperature. This not only allows for a martensitic transformation on the surface, but also accelerates cooling of the mold core, preventing the formation of upper bainite there, thereby ensuring mold performance.

[0084] 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 below 0.05%.

[0085] The above embodiments are merely exemplary embodiments of the present application and are not intended to limit the scope of the present application. The scope of protection 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 scope of protection of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the scope of protection 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 and heat it to the quenching temperature and keep it warm; Two-stage isothermal high-pressure gas quenching: Fill the vacuum furnace with high-pressure quenching gas, and stop filling the high-pressure quenching gas after the surface temperature of the cold working die 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 inject 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 injected 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 a cold working die according to claim 1, 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 structure is eutectic carbide and granular pearlite.

3. A high-hardness micro-deformation clean 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 filled into the vacuum furnace is 0.6-1.0 MPa; The high-pressure quenching gas is an inert gas.

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

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

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

7. The high-hardness, micro-deformation, clean heat treatment control method for a cold working die according to claim 6, characterized in that: The solution treatment is carried out at a heating rate of ≤300℃ / h to 1020℃-1025℃ and then kept warm.

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

9. The high-hardness, micro-deformation, clean heat treatment control method for a cold working die 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 holding steps.

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

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