A cryogenic treatment process for reducing the retained austenite content of steel materials

Abstract

The application belongs to the technical field of material processing, and particularly relates to a cryogenic treatment process for reducing residual austenite content of steel materials. The process comprises the following steps: providing quenched steel materials; lowering the quenched steel materials to-170 to-196 DEG C for first holding; raising the first holding steel materials to-70 to-90 DEG C for second holding; and raising the second holding steel materials to room temperature. The cryogenic treatment process takes into account the transformation characteristics of continuous transformation stage and isothermal transformation stage, can effectively reduce the residual austenite content of quenched steel materials while reducing the treatment time and cost, and further plays the role of cryogenic treatment in improving microstructure and optimizing macroscopic performance.

Description

Technical Field

[0001] This invention belongs to the field of material processing technology, specifically relating to a cryogenic treatment process for reducing the residual austenite content in steel materials. Background Technology

[0002] Quenching is a fundamental heat treatment process to ensure that steel materials achieve good properties. Martensitic transformation is the main characteristic of steel materials during quenching. Because the pre-formed martensite inhibits subsequent martensitic transformations, a portion of retained austenite (10-30%) is always present in the microstructure after quenching. Retained austenite is a metastable phase with low strength, and it is easily induced to transform into martensite under external stress, causing uncontrollable deformation of mechanical parts during use and affecting dimensional stability. Therefore, reducing the retained austenite content in quenched steel is of great significance for improving the comprehensive mechanical properties of steel materials, the quality of mechanical parts, and their service life.

[0003] Cryogenic treatment increases the driving force for the transformation of austenite to martensite by lowering the temperature, thus promoting the martensitic phase transformation. Therefore, it is widely used to reduce the residual austenite content in steel materials after quenching. During cryogenic treatment, the martensitic phase transformation occurs in the continuous cooling stage and the isothermal stage. Therefore, the amount of residual austenite transformation is closely related to process parameters such as the cooling rate, cryogenic treatment temperature, and cryogenic holding time.

[0004] Conventional cryogenic treatment processes typically involve cooling the material to a low temperature at a slow rate followed by a holding period. For example, CN201810834120.X discloses a cryogenic treatment process for AerMet100 steel, which involves cooling to (-115) to (-125)℃ at a rate of 3–5℃ / min, holding for 1 hour, and then allowing it to naturally return to room temperature. However, as the temperature decreases, the martensite nuclei consumed by the residual austenite transformation during the continuous cooling phase inhibit subsequent isothermal transformations. Therefore, the practice of directly holding the material after it has been cooled to the lowest temperature in this process does not organically combine the martensitic phase transformations at different stages of cryogenic treatment, and thus does not offer a significant advantage in effectively reducing the residual austenite content. Furthermore, when the cryogenic treatment temperature is low, using a fixed slow cooling rate will significantly reduce the efficiency of cryogenic treatment. In general, most cryogenic treatment processes are currently developed based on practical experience. However, the variation of residual austenite with cooling rate, cryogenic treatment temperature, and cryogenic holding time during cryogenic treatment is still unknown, which will have a significant impact on the accurate formulation of cryogenic treatment process parameters.

[0005] In summary, the lack of clear theoretical guidance for the formulation of cryogenic treatment process parameters currently results in limited effectiveness of existing cryogenic treatment processes in eliminating retained austenite. Therefore, it is necessary to propose a novel cryogenic treatment process based on the transformation law of retained austenite at low temperatures to effectively reduce the retained austenite content of quenched steel materials and further leverage the role of cryogenic treatment in improving microstructure and optimizing macroscopic properties. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the inventors of this invention, based on the principle that the transformation of paramagnetic austenite into ferromagnetic martensite during martensitic phase transformation causes changes in the saturation magnetic moment of the material, employed a magnetic measurement system (MPMS) to reflect the characteristics and laws of martensitic phase transformation in situ during cryogenic treatment. This information guides the invention in providing a cryogenic treatment process that can significantly reduce the residual austenite content in steel materials, thereby further enhancing the role of cryogenic treatment in improving microstructure and optimizing macroscopic properties.

[0007] Specifically, the present invention provides the following technical solution:

[0008] A cryogenic treatment process for reducing the residual austenite content in steel materials includes the following steps:

[0009] S1. Provide quenched steel materials;

[0010] S2. Cool the quenched steel material to -170 to -196°C for the first heat preservation.

[0011] S3. After the first insulation, heat the steel material to -70 to -90℃ (more preferably -80℃) for a second insulation.

[0012] S4. Heat the steel material after the second insulation to room temperature.

[0013] Conventional cryogenic treatment processes typically involve holding the temperature at a minimum after it has been lowered to the minimum, followed by a warming to room temperature. This invention reveals that because martensite nuclei are largely consumed during the initial continuous cooling transformation, holding at the minimum temperature inhibits the isothermal martensitic transformation. The novel cryogenic treatment process proposed in this invention uses a warming phase after the minimum temperature to thermally activate and induce martensite nuclei formation, followed by prolonged holding at the measured optimal residual austenite isothermal transformation temperature of -70 to -90°C, thereby promoting isothermal martensite formation.

[0014] Preferably, in step S1, the quenched steel material is steel material that has been quenched and cooled to room temperature. More preferably, it is steel material that has been cooled to room temperature for no more than 2 hours. Too long a time interval will increase the difficulty of the transformation of the retained austenite into martensite after quenching.

[0015] Preferably, in step S2, the quenched steel material is first cooled to -40 to -60°C at a cooling rate of 10 to 20°C / min.

[0016] Then, the temperature is lowered to -150 to -165°C (more preferably -160°C) at a cooling rate of 0.5 to 2°C / min.

[0017] Finally, the temperature is lowered to -170 to -196°C at a cooling rate of 10 to 20°C / min.

[0018] During the continuous cooling phase, conventional cryogenic treatment processes typically employ a fixed, slow cooling rate. However, the novel cryogenic treatment process proposed in this invention employs a method of first rapidly passing through a transformation-insensitive temperature zone (room temperature to -60°C) and then slowly passing through a transformation-sensitive temperature zone (-60°C to -160°C). This effectively improves cryogenic treatment efficiency and saves energy consumption without affecting the continuous cooling transformation of residual austenite.

[0019] Preferably, in step S2, the first heat preservation time is 0.5 to 1 hour. The purpose of selecting the above-mentioned first heat preservation time is to use the shortest possible heat preservation time to ensure that the material temperature is uniform and that the workpiece is thoroughly cooled. If the heat preservation time is too long, in addition to increasing the processing cost, it may also inhibit further martensitic phase transformation during the heating and second heat preservation.

[0020] Preferably, in step S3, the steel material after the first heat treatment is heated to -70 to -90°C at a heating rate of 0.5 to 2°C / min. The purpose of using this relatively slow heating rate is to induce sufficient martensite nucleation through thermal activation, providing conditions for the subsequent isothermal transformation.

[0021] Preferably, in step S3, the second heat treatment time is 6–12 hours. The purpose of using this second heat treatment time is to allow sufficient time for the isothermal transformation of retained austenite to increase.

[0022] Preferably, in step S4, the steel material after the second heat treatment is heated to room temperature at a heating rate of 10–20 °C / min. This relatively fast heating rate is used to improve the efficiency of the cryogenic treatment.

[0023] By employing the above technical solution, the present invention has at least the following advantages and beneficial effects:

[0024] 1) Based on the characteristics and laws of martensitic phase transformation at low temperature, this invention precisely formulates the process parameters for cryogenic treatment, providing a simple and effective new cryogenic treatment process to further enhance the role of cryogenic treatment in improving microstructure and optimizing macroscopic properties.

[0025] 2) The novel cryogenic treatment process provided by this invention takes into account the transformation characteristics of both the continuous transformation stage and the isothermal transformation stage, and can effectively reduce the residual austenite content of steel materials after quenching while reducing processing time and processing costs.

[0026] 3) The novel cryogenic treatment process provided by this invention is easy to operate, pollution-free, and highly applicable, and has a wide range of application value. Attached Figure Description

[0027] Figure 1 The graph shows the temperature-time variation of the cryogenic treatment process in Example 1; where RT is room temperature, the cooling / heating rate k1 is 15℃ / min, the cryogenic treatment temperature T1 is -43℃, the cooling / heating rate k2 is 1℃ / min, the cryogenic treatment temperature T2 is -153℃, the minimum cryogenic treatment temperature T3 is -180℃, and the cryogenic treatment isothermal temperature T4 is -80℃.

[0028] Figure 2 The variation of residual austenite content with temperature is shown.

[0029] Figure 3 The relationship between the residual austenite transformation rate and the cooling rate during the continuous cooling process from -43℃ to -153℃ is given.

[0030] Figure 4 The effect of different cryogenic temperatures on the transformation of residual austenite during isothermal processes.

[0031] Figure 5 The microstructure changes before and after continuous cooling are shown in (a) to (c), which are the microstructure characteristics before continuous cooling, after continuous cooling to -80℃ and after continuous cooling to -180℃, respectively. (d) is the diffraction calibration of the twin structure. In the figure, M represents martensite, MT represents martensitic twin, and Diff represents the selected area electron diffraction region. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1

[0034] This embodiment provides a cryogenic treatment process for reducing the residual austenite content in steel materials. The cryogenic treatment is carried out in a programmable cryogenic treatment device (SLX-80 type) developed by the Institute of Physics and Chemistry, Chinese Academy of Sciences. The steel material used is high-carbon chromium GCr15 steel, with the following chemical composition (wt.%): carbon (C) = 0.95–1.05, silicon (Si) = 0.15–0.35, manganese (Mn) = 0.25–0.45, chromium (Cr) = 1.40–1.65, nickel (Ni) ≤ 0.30, copper (Cu) ≤ 0.25, molybdenum (Mo) ≤ 0.10, sulfur (S) ≤ 0.020, phosphorus (P) ≤ 0.027, and the remainder is iron (Fe). The residual austenite content of the steel material after austenitizing and oil quenching at 850℃ is approximately 13%.

[0035] like Figure 1 As shown, the processing technology specifically includes the following steps:

[0036] The quenched steel material was cooled from room temperature to -43℃ (T1) at a rate of 15℃ / min;

[0037] Next, the temperature was lowered from -43°C to -153°C (T2) at a rate of 1°C / min;

[0038] Next, the temperature was lowered from -153℃ to the minimum cryogenic treatment temperature of -180℃ (T3) at a rate of 15℃ / min, and held at that temperature for 0.5h.

[0039] Next, the temperature was raised from the lowest cryogenic treatment temperature to -80℃ (T4, cryogenic treatment isothermal temperature) at a rate of 1℃ / min and held for 10 hours.

[0040] Finally, the temperature was raised from -80°C to room temperature at a rate of 15°C / min.

[0041] After the above-mentioned cryogenic treatment, the austenite content of the steel material decreased to 2.1%.

[0042] Figure 2 The curves show the variation of residual austenite content with temperature during continuous cooling and heating. The curves indicate that during continuous cooling, the martensitic transformation undergoes three phases: an incubation stage from room temperature to T1, a transformation stage from T1 to T2, and a slowdown stage from T2 to T3.

[0043] During the incubation stage, the transformation rate is not affected by the cooling rate because the driving force required for the martensitic phase transformation is insufficient. Therefore, rapid cooling should be carried out in this stage to improve the efficiency of cryogenic treatment.

[0044] When the temperature drops from T2 to T3, the residual austenite content decreases very little. However, cooling to the lowest temperature T3 promotes the transformation during the subsequent warming and holding process. Therefore, adopting a rapid cooling method in the T2 to T3 temperature range can not only not affect the transformation effect, but also improve the processing efficiency.

[0045] During continuous heating, as the temperature rises from T3 to T4, the content of retained austenite further decreases, indicating that the number of martensite nuclei increases during continuous heating, and the retained austenite continues to transform into martensite. However, when the temperature exceeds T4, the change in the content of retained austenite is very small. It can be seen that the transformation during heating mainly occurs in the temperature range of T3 to T4.

[0046] Figure 4 The effect of different cryogenic isothermal temperatures on the transformation of retained austenite during isothermal processes. For example... Figure 4 As shown, the lower the holding temperature, the smaller the change in the residual austenite content with increasing holding time. Figure 4 An increase in the ordinate indicates a decrease in the content of retained austenite. Figure 1 In the process route, holding at the lowest temperature T3 is not related to eliminating residual austenite; its purpose is to ensure the sample cools thoroughly and the temperature distribution is uniform. Holding at -80℃ (T4) shows the best reduction effect in residual austenite with increasing holding time. Figure 1 The process route selects T4 temperature as the holding temperature after heating, which is both the highest temperature in the residual austenite transformation temperature zone during the heating process and the holding temperature with the fastest transformation during the isothermal process.

[0047] Figure 3 This represents the relationship between the residual austenite transformation rate and the cooling rate during a continuous cooling process from -43℃ to -153℃. For example... Figure 3 As shown, during the continuous transformation stage, the rate of non-uniform martensite nucleation per unit time increases with decreasing cooling rate, leading to a higher maximum transformation rate. Therefore, a slow cooling rate of 1℃ / min should be implemented during this stage to increase the amount of retained austenite transformed.

[0048] Figure 5 The images show transmission electron microscopy (TEM) images of the material's microstructure before and after continuous cooling treatment. In the images, M represents martensite, and MT represents martensitic twins. Figure 5 (a) The microstructure before continuous cooling shows coarse martensite laths, which is due to the large initial austenite grain size during quenching. Figure 5(b) It can be seen that after continuous cooling to -80℃ cryogenic treatment, the number of fine martensite twins in the microstructure increases. This is because the retained austenite is small after quenching and is subjected to greater compressive stress under the action of the surrounding quenched martensite laths. Therefore, after cryogenic treatment, it transforms into a smaller nascent martensite twin structure. The increase in nascent fine martensite twins in the microstructure is an important manifestation of cryogenic treatment reducing the content of retained austenite, and the number of nascent twins further increases as the cryogenic treatment temperature decreases to -180℃ (see...). Figure 5 (c)).

[0049] Comparative Example 1

[0050] The only difference from Example 1 is that the cryogenic treatment isothermal temperature is -40°C, while other conditions remain unchanged. After cryogenic treatment, the austenite content of the steel material decreased to 10.2%.

[0051] The only difference from Example 1 is that the cryogenic treatment isothermal temperature was -138°C, while other conditions remained unchanged. After cryogenic treatment, the austenite content of the steel material decreased to 7.2%.

[0052] The only difference from Example 1 is that the cryogenic treatment isothermal temperature is -196°C, while other conditions remain unchanged. After cryogenic treatment, the austenite content of the steel material decreased to 6.5%.

[0053] The above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A cryogenic treatment process for reducing the residual austenite content in steel materials, characterized in that, Includes the following steps: S1. Provide quenched steel materials; S2. The quenched steel material is first cooled to -40~-60℃ at a cooling rate of 10~20℃ / min. Then, cool to -150 to -165℃ at a cooling rate of 0.5~2℃ / min. Finally, the temperature is lowered to -170~-196℃ at a cooling rate of 10~20℃ / min for the first heat preservation. S3. After the first insulation, heat the steel material to -70~-90℃ for a second insulation. S4. Heat the steel material after the second insulation to room temperature.

2. The cryogenic treatment process according to claim 1, characterized in that, In step S1, the quenched steel material is steel material that has been quenched and cooled to room temperature.

3. The cryogenic treatment process according to claim 1, characterized in that, In step S2, the first heat preservation time is 0.5~1h.

4. The cryogenic treatment process according to claim 1, characterized in that, In step S3, the steel material after the first heat preservation is heated to -70~-90℃ at a heating rate of 0.5~2℃ / min.

5. The cryogenic treatment process according to claim 1 or 4, characterized in that, In step S3, the second heat preservation time is 6~12 hours.

6. The cryogenic treatment process according to claim 1, characterized in that, In step S4, the steel material after the second heat preservation is heated to room temperature at a heating rate of 10~20℃ / min.

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