Online heat treatment process obtaining method for optimizing medium-temperature tempered structure and temper brittleness resistance of 30MnSi deformed steel bar based on orthogonal test

Abstract

The invention discloses a 30MnSi deformed steel bar medium-temperature tempering process optimization method based on an orthogonal test, and belongs to the technical field of metal material heat treatment process optimization. The whole quenching-tempering process is simulated through an MMM-200 thermal simulation experiment machine, grain boundary impurity segregation is inhibited in combination with a forced water cooling process, and on the premise that the tensile strength is kept to be larger than or equal to 870 MPa, the impact energy is improved by 30%, and the grain boundary brittleness risk is remarkably reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of optimizing heat treatment processes of metal materials, and particularly relates to a method for obtaining an on-line heat treatment process for optimizing the microstructure and temper embrittlement resistance of medium-temperature tempered 30MnSi ribbed steel based on orthogonal experiments. Background Art

[0002] The conventional process for 30MnSi ribbed steel uses on-line residual heat quenching (870 - 900°C) and medium-temperature tempering at 450°C. The micro-structure is tempered troostite, and the tensile strength is about 870 MPa. However, this process has the following problems:

[0003] 1. Risk of temper embrittlement: The tempering temperature of 450°C is in the sensitive region of the second type of temper embrittlement (450 - 650°C). If the cooling rate after tempering is insufficient, it is easy to cause segregation of grain boundary impurity elements (such as P, Sn, Sb), resulting in reversible temper embrittlement and a significant decrease in impact toughness.

[0004] 2. Insufficient tissue uniformity: During medium-temperature tempering, the precipitation morphology and distribution of carbides are affected by the cooling rate, and it is easy to form locally coarsened carbides, reducing the toughness of the material.

[0005] The existing technology lacks quantitative correlation analysis of tempering temperature, holding time, and cooling rate, and it is difficult to avoid the risk of brittleness while maintaining the target performance. Summary of the Invention

[0006] Aiming at the potential temper embrittlement and tissue uniformity problems in the medium-temperature tempering process of 30MnSi ribbed steel, the purpose of the present invention is to provide a method for obtaining an on-line heat treatment process for optimizing the microstructure and temper embrittlement resistance of medium-temperature tempered 30MnSi ribbed steel based on orthogonal experiments. By means of thermo-simulation experiments, the influence of process parameters on the troostite structure, tensile strength, and brittleness sensitivity is quantified, and a process window with low brittleness and high stability is obtained.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A method for obtaining an on-line heat treatment process for optimizing the microstructure and temper embrittlement resistance of medium-temperature tempered 30MnSi ribbed steel based on orthogonal experiments, comprising:

[0009] 1). Orthogonal experiment design:

[0010] Factors and levels: Three key process parameters are selected, and each parameter is set at three levels:

[0011] A. Quenching temperature: 860°C, 880°C, 900°C;

[0012] B. Tempering temperature: 420°C, 450°C, 480°C;

[0013] C. Cooling rate after tempering: 10 °C / s - air cooling, 30 °C / s - water spray cooling, 50 °C / s - forced water cooling;

[0014] 2). Simulation experiment process:

[0015] Use the MMS-200 thermomechanical simulation testing machine to simulate the on-line heat treatment process after rolling:

[0016] 2.1). Quenching stage: Heat the specimen to the set quenching temperature - Factor A, hold for 5 minutes and then cool to room temperature at a fixed rate by water cooling.

[0017] 2.2). Tempering stage: Heat up to the tempering temperature - Factor B, hold for 30 - 50 minutes and then cool at the set cooling rate - Factor C.

[0018] 3). Performance detection and optimization objectives:

[0019] Microstructure: Analyze the proportion of tempered troostite by SEM and EBSD, target ≥ 95%; carbide size ≤ 0.5 μm and the degree of segregation of grain boundary impurities;

[0020] Mechanical properties: Tensile strength ≥ 870 MPa, elongation ≥ 12%, impact energy ≥ 40 J;

[0021] Brittleness sensitivity: Detect the contents of P and Sn at grain boundaries by Auger electron spectroscopy AES, target value ≤ 0.01 wt%.

[0022] Furthermore, the orthogonal experimental design adopts the following experimental matrix: Use the L9 orthogonal array to design 9 groups of experiments.

[0023] Furthermore, the range of optimal process parameters is: quenching temperature 870 - 890 °C, tempering temperature 440 - 460 °C, cooling rate ≥ 40 °C / s.

[0024] Furthermore, the optimal process parameters are: quenching temperature 880 °C, tempering temperature 450 °C, cooling rate 50 °C / s.

[0025] Furthermore, the cooling medium for the forced water cooling is an aqueous solution containing 0.1% polyvinyl alcohol, which inhibits the formation of the steam film to improve the cooling uniformity.

[0026] Furthermore, the best process obtained by this method reduces the segregation amount of grain boundary impurities by 50% and enhances the performance stability: the uniformity of the troostite structure is improved to more than 95%, and the fluctuation range of the tensile strength ≤ ±10 MPa.

[0027] Furthermore, on the premise of maintaining the tensile strength ≥ 870 MPa, the best process obtained by this method increases the impact energy by 30% and significantly reduces the risk of grain boundary brittleness.

[0028] Furthermore, the following steps are also included:

[0029] 4). Data analysis and process optimization:

[0030] Determine the influence weights of various factors on brittleness sensitivity through range analysis and variance analysis;

[0031] With the multi-objective optimization conditions of tensile strength ≥ 870 MPa, maximum impact energy, and minimum grain boundary impurities, screen the optimal parameter combination.

[0032] Temper embrittlement inhibition mechanism:

[0033] Rapid cooling to block segregation: Forced water cooling (50 °C / s) significantly shortens the cooling time after tempering and inhibits the diffusion of impurity elements such as P and Sn to the grain boundaries.

[0034] Temperature-time window optimization: Tempering at 450 °C with short-time holding (40 min) not only ensures the full precipitation of carbides but also avoids grain boundary weakening caused by long-term high temperature.

[0035] Compared with the prior art, the beneficial technical effects of the present invention:

[0036] Precisely avoid the brittle sensitive area: Through the coordinated control of tempering temperature and cooling rate, avoid the second type of temper embrittlement caused by slow cooling in the 450 - 650 °C range.

[0037] Suppress impurity segregation: Adopt forced water cooling (50 °C / s) to shorten the cooling time after tempering and inhibit the diffusion of impurity elements such as P and Sn to the grain boundaries.

[0038] Coupled optimization of microstructure and properties: Improve the uniformity of troostite by controlling the carbide size (≤ 0.5 μm), taking into account both strength and toughness.

[0039] 1. Controllable brittleness risk: The forced water cooling process increases the cooling rate after tempering to 50 °C / s, reducing the grain boundary impurity segregation by 50%.

[0040] 2. Enhanced performance stability: The uniformity of the troostite microstructure is increased to over 95%, and the fluctuation range of the tensile strength is ≤ ±10 MPa.

[0041] 3. High process compatibility: The optimized parameters can be directly mapped to the water cooling system of the production line without additional equipment investment. Specific implementation method

[0042] A method for obtaining an on-line heat treatment process for optimizing the medium-temperature tempering microstructure and anti-temper embrittlement of 30MnSi threaded steel based on orthogonal experiments, including the following steps:

[0043] 1. Orthogonal experiment design:

[0044] Factors and levels: Three key process parameters are selected, and each parameter is set at three levels:

[0045] A. Quenching temperature (°C): 860, 880, 900

[0046] B. Tempering temperature (°C): 420, 450, 480 (avoid the first type of brittleness zone of 200 - 350 °C, and focus on optimizing the second type of brittleness sensitive zone)

[0047] C. Cooling rate after tempering (°C / s): 10 (air cooling), 30 (water mist cooling), 50 (forced water cooling)

[0048] Test matrix: Use the L9(3 3 ) orthogonal array to design 9 groups of experiments (see Table 1).

[0049] 2. Simulation experiment process:

[0050] Use the MMS-200 thermomechanical simulation tester to simulate the on-line heat treatment process after rolling:

[0051] 2.1. Quenching stage: Heat the specimen to the set quenching temperature (Factor A), hold for 5 min, and then water-cool to room temperature at a fixed rate (50 °C / s).

[0052] 2.2. Tempering stage: Heat up to the tempering temperature (Factor B), hold for 40 min, and then cool at the set cooling rate (Factor C).

[0053] 3. Performance detection and optimization objectives:

[0054] Microstructure: Analyze the proportion of tempered troostite (target ≥ 95%), carbide size (≤ 0.5 μm), and the degree of grain boundary impurity segregation through SEM and EBSD.

[0055] Mechanical properties: Tensile strength ≥ 870 MPa, elongation ≥ 12%, impact energy ≥ 40 J (V-notch).

[0056] Brittleness sensitivity: Detect the P and Sn contents at grain boundaries using Auger electron spectroscopy (AES), and the target value is ≤ 0.01 wt%.

[0057] 4. Data analysis and process optimization:

[0058] Determine the influence weights of each factor on brittleness sensitivity through range analysis and variance analysis.

[0059] With the multi-objective optimization conditions of tensile strength ≥ 870 MPa, maximum impact energy, and minimum grain boundary impurities, screen the optimal parameter combination.

[0060] Example 1

[0061] 1. Implementation of orthogonal experiment:

[0062] Conduct 9 groups of experiments according to the parameters in Table 1, and take the average value after repeating each group 3 times.

[0063] Table 1: Orthogonal experiment design table (L9(3 3 ))

[0064]

[0065]

[0066] 2. Determination of the optimal process:

[0067] The experimental results show that the parameters of the 5th group (A2B2C3) (quenching at 880 °C, tempering at 450 °C, forced water cooling at 50 °C / s) are the optimal combination:

[0068] Microstructure: 96% fine tempered troostite, average carbide size 0.45 μm, P content at grain boundaries 0.008 wt%.

[0069] Mechanical properties: Tensile strength 885 MPa, elongation 13.5%, impact energy 45 J.

[0070] Brittleness risk: The impact energy is increased by 30% compared with the traditional process (air cooling), and the brittleness tendency at grain boundaries is significantly reduced.

[0071] The embodiments described above are only for describing the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. An online heat treatment process acquisition method for optimizing the microstructure and temper brittleness resistance of medium-temperature tempered 30MnSi threaded steel based on orthogonal experiments, characterized in that, Including: 1). Orthogonal experiment design: Factors and levels: Three key process parameters are selected, and each parameter is set at three levels: A. Quenching temperature: 860°C, 880°C, 900°C; B. Tempering temperature: 420°C, 450°C, 480°C; C. Cooling rate after tempering: 10°C / s - air cooling, 30°C / s - water mist cooling, 50°C / s - forced water cooling; 2). Simulation experiment process: Use the MMS-200 thermal simulation experiment machine to simulate the on-line heat treatment process after rolling: 2.1). Quenching stage: Heat the specimen to the set quenching temperature - Factor A, hold for 5 minutes, and then water-cool to room temperature at a fixed rate. 2.2). Tempering stage: Heat up to the tempering temperature - Factor B, hold for 30 - 50 minutes, and then cool at the set cooling rate - Factor C. 3). Performance detection and optimization objectives: Microstructure: Analyze the proportion of tempered troostite by SEM and EBSD, with the target ≥ 95%; Carbide size ≤ 0.5μm and the degree of grain boundary impurity segregation; Mechanical properties: Tensile strength ≥ 870 MPa, elongation ≥ 12%, impact energy ≥ 40 J; Brittleness sensitivity: Use Auger electron spectroscopy (AES) to detect the P and Sn contents at the grain boundaries, and the target value ≤ 0.01 wt%.

2. The method for obtaining an on-line heat treatment process for optimizing the medium-temperature tempering structure and anti-temper embrittlement of 30MnSi threaded steel based on orthogonal experiments according to claim 1, wherein The orthogonal experiment design adopts the following experimental matrix: Use the L9 orthogonal table to design 9 groups of experiments.

3. The method for obtaining an on-line heat treatment process for optimizing the intermediate temperature tempering structure and anti-tempering brittleness of 30MnSi threaded steel based on orthogonal experiments according to claim 1, wherein the optimal process parameter range is: quenching temperature 870 - 890°C, tempering temperature 440 - 460°C, cooling rate ≥ 40°C / s.

4. The method for obtaining an on-line heat treatment process for optimizing the intermediate temperature tempering structure and anti-tempering brittleness of 30MnSi threaded steel based on orthogonal experiments according to claim 3, wherein the optimal process parameters are: quenching temperature 880°C, tempering temperature 450°C, cooling rate 50°C / s.

5. The method for obtaining an on-line heat treatment process for optimizing the microstructure and temper embrittlement resistance of medium-temperature tempered 30MnSi threaded steel based on orthogonal experiments according to claim 1 or 4, characterized in that, The cooling medium for the forced water cooling is an aqueous solution containing 0.1% polyvinyl alcohol, which inhibits the formation of the steam film to improve the cooling uniformity.

6. The method for obtaining an on-line heat treatment process for optimizing the medium-temperature tempering structure and anti-temper embrittlement of 30MnSi threaded steel based on orthogonal experiments according to claim 5, characterized in that, The best process obtained by this method reduces the grain boundary impurity segregation amount by 50%, and enhances the performance stability: the uniformity of the troostite structure is improved to more than 95%, and the fluctuation range of the tensile strength ≤ ±10 MPa.

7. The method for obtaining an on-line heat treatment process for optimizing the microstructure and temper brittleness resistance of medium-temperature tempered 30MnSi threaded steel based on orthogonal experiments according to claim 5, characterized in that, The best process obtained by this method increases the impact energy by 30% on the premise of maintaining the tensile strength ≥ 870 MPa, and significantly reduces the grain boundary brittleness risk.

8. The method for obtaining an on-line heat treatment process for optimizing the microstructure and temper brittleness resistance of medium-temperature tempered 30MnSi threaded steel based on orthogonal experiments according to claim 1, characterized in that, It also includes the following steps: 4). Data analysis and process optimization: Determine the influence weight of each factor on the brittleness sensitivity through range analysis and variance analysis; With the tensile strength ≥ 870 MPa, maximizing the impact energy, and minimizing the grain boundary impurities as the multi-objective optimization conditions, screen the optimal parameter combination.

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