Heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel

Abstract

The present invention discloses a heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel, the process steps of which are as follows: (1) first quenching: heating the martensitic steel material to 820-950°C and holding the temperature for 100-375 seconds, and then water quenching; (2) tempering: heating the material after the first quenching to 500-700°C and holding the temperature; (3) second quenching: rapidly heating the tempered material to 820-950°C and holding the temperature, the total time of the rapid heating and holding process being 100-375 seconds; and water quenching after holding the temperature. This method uses a secondary quenching process + rapid heating + shortened holding time process to ensure that the material matrix is ​​more than 98% martensite structure, while ensuring that a large number of copper-rich nanoparticles with a size of 13±7nm are present in the material, and plays an effective hydrogen capture role; the resulting martensitic steel matrix structure is more than 99% martensite structure, the width of the martensite lath is 200-400nm, the size of the martensite block is 2-4μm, and the average grain size is 6-8μm.

Description

Technical Field

[0001] The invention relates to a heat treatment method, in particular to a heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel. Background Art

[0002] Steel is currently the most widely used material in the world. Due to its excellent mechanical properties, it is often used in the military, automotive, and other fields. Currently, lightweighting vehicles is a major trend in automotive development. Relevant data shows that for every 10% reduction in vehicle weight, fuel consumption can be reduced by 6% to 8%. This means that lightweighting has both energy-saving and environmental benefits. However, lightweighting is inconsistent with vehicle safety and compliance with head-on and side impact regulations. A highly effective approach is to use high-strength and ultra-high-strength lightweight materials. This approach can reduce fuel consumption and emissions while ensuring compliance with safety regulations.

[0003] However, a pressing challenge in developing ultra-high-strength martensitic steels is reducing their hydrogen embrittlement susceptibility. This is because a material's strength is directly proportional to its hydrogen embrittlement susceptibility: greater strength corresponds to a higher hydrogen embrittlement index. To address this issue, alloying element precipitates within the material are often used as "hydrogen traps" to improve the steel's resistance to hydrogen embrittlement. Precipitates such as titanium carbide, niobium carbide, and vanadium carbide are typically incorporated into the material to act as hydrogen traps, enhancing the material's resistance to hydrogen embrittlement. However, cost reduction is a critical issue for commercial low-alloy steels, and the amount of alloying elements added must be strictly controlled. Generally, the amount of alloying elements added is below 0.1 wt%, with Nb being even lower. Recent research suggests that Cu may enhance the steel's hydrogen embrittlement resistance. However, Cu precipitation is typically achieved through a solution-assisted aging treatment, and the precipitation temperature range is between 500°C and 700°C. For quenched martensitic steels, the quench-and-temper process cannot achieve dispersed Cu precipitation while maintaining high strength. At present, no method has been found to achieve the dispersed distribution of Cu-rich nanoparticles in low-alloy steel (total alloying element content <1wt%) cold-formed martensitic steel with a tensile strength exceeding 1500MPa. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel, so as to achieve effective hydrogen capture.

[0005] In order to solve the above technical problems, the process steps adopted by the present invention are as follows: (1) first quenching: heating the martensitic steel material to 820-950°C and keeping the temperature for 100-375s, and then water quenching;

[0006] (2) Tempering: Raise the temperature of the material after the first quenching to 500℃~700℃ and keep it warm;

[0007] (3) Second quenching: Rapidly heat the tempered material to 820-950℃ and keep it warm. The total time of the rapid heating and holding process is 100-375s; water quench after holding.

[0008] In step (3) of the present invention, the cooling rate of water quenching is 50°C / s or above. Furthermore, the cooling rate of water quenching is 150°C / s or above.

[0009] In step (2) of the present invention, the holding time is 30 to 60 minutes.

[0010] The martensitic steel material of the present invention contains 0.2-0.35 wt% of C and 0.01-0.5 wt% of Cu.

[0011] The beneficial effects of the above technical solution are as follows: through a combination of a secondary quenching process, rapid heating, and a shortened holding time, along with appropriate process parameters, the present invention achieves the presence of a large number of copper-rich nanoparticles with a size of 13±7 nm, while ensuring a matrix structure of at least 98% martensite, thereby effectively capturing hydrogen. The resulting martensitic steel matrix is ​​at least 99% martensite, with martensite lath widths ranging from 200 to 400 nm, martensite block sizes ranging from 2 to 4 μm, and an average grain size of 6 to 8 μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0013] Figure 1 Schematic diagram of the process of the heat treatment experiment of the present invention;

[0014] Figure 2 is a scanning electron microscope (SEM) image of the heat treatment experiment described in the present invention;

[0015] Figure 3 This is a high-resolution transmission electron microscopy (HRTEM) image of the QT-100s sample from the heat treatment experiment;

[0016] Figure 4 is a high-resolution transmission electron microscopy (HRTEM) image of copper nanoparticles in the QT-100s sample from the heat treatment experiment;

[0017] Figure 5 It is a graph of the thermal desorption test and simulated deconvolution results in the heat treatment experiment;

[0018] Figure 6This is a graph showing the hydrogen trap activation energy obtained by simulation calculation based on the Kissinger formula in the heat treatment experiment;

[0019] Figure 7 It is a diagram of the U-bending test results in the heat treatment experiment. DETAILED DESCRIPTION

[0020] Example 1-5: The heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel adopts the following process.

[0021] (1) The martensitic steel suitable for this heat treatment method contains C 0.20-0.35wt% and Cu 0.01-0.5wt%, and further Cu 0.05-0.5wt%; preferably, the following steel type, whose composition, calculated by element weight content, is: C 0.20-0.35%, Si 0.16-0.36%, Mn 0.30-0.60%, P≤0.01%, S≤0.006%, Al≤0.04%, B0.001-0.003%, Cr 0.02-0.05%, Mo 0.003-0.008%, V 0.01-0.05%, Ti 0.01-0.04%, Nb0.02%-0.05%, Cu 0.01-0.5%, and the balance is Fe and unavoidable impurities.

[0022] (2) This heat treatment method uses a first quenching, tempering, and second quenching process. In the first quenching process, the martensitic steel material is heated to 820-950°C for a holding time of 100-375 seconds, and then water quenched to completely dissolve the Cu element in the matrix; the cooling rate of the water quenching is 50°C / s or above, preferably 150°C / s or above.

[0023] The tempering process is as follows: the material after the first quenching is heated to 500° C. to 700° C. and kept warm for 30 to 60 minutes.

[0024] The second quenching process involves rapidly heating the tempered material to 820-950°C and holding the temperature. The total time for this rapid heating and holding process is 100-375 seconds. To achieve this rapid heating, the sample is quickly transferred to a furnace pre-set to the target temperature (820-950°C). To achieve optimal results, the rapid heating rate is 2°C / s or higher, and the holding time is 60-150 seconds. After holding, the material is water quenched, with a cooling rate of 50°C / s or higher, preferably 150°C / s or higher. Specific process parameters for each embodiment are shown in Table 1.

[0025] Table 1: Process parameters of various examples

[0026]

[0027] (3) The matrix structure of the martensitic steel obtained by this heat treatment method is more than 99% martensitic, with a martensitic lath width of 200-400 nm, a martensitic block size of 2-4 μm, and an average grain size of 6-8 μm. The C and Cu contents and internal structure of the martensitic steel obtained in each example are shown in Table 2.

[0028] Table 2: Internal structure of martensitic steel obtained in various examples

[0029]

[0030] Heat treatment experiments and testing: Using an ultra-high-strength low-alloy martensitic steel as an example, we added 0.2-0.35% C and 0.1-0.5% Cu. We subjected it to various heat treatments and analyzed its microstructural evolution and copper precipitation. We also compared it with a group of copper-free low-alloy martensitic steel plates of the same strength level to demonstrate the role of copper-rich nanoparticles in improving the steel's hydrogen embrittlement resistance.

[0031] (1) Heat treatment experiment:

[0032] The above heat treatment experiments were carried out on steel plates using multiple muffle furnaces. Figure 1 First, the cold-rolled steel sheet was heated to 900°C and held at this temperature for 375 seconds before being water quenched. After quenching, the four samples were heated to the tempering temperatures of 550°C, 600°C, 650°C, and 700°C, respectively, and held at these temperatures for 30 minutes. The temperature was then rapidly raised to 900°C and held at this temperature for a total of 100 seconds. After the 900°C holding period, the steel was water quenched. The control example was identical to the experimental example above, except for the tempering temperature. The effects of Cu-rich nanoparticle precipitation on the four samples and the control example are shown in Table 3.

[0033] Table 3: Effect of tempering temperature selection on the precipitation of Cu-rich nanoparticles

[0034]

[0035] First, the cold-rolled steel sheets were heated to 900°C and held at this temperature for 375 seconds before being water-quenched. The quenched samples were heated to the tempering temperature of 550°C and held at this temperature for 30 minutes before being rapidly heated to 900°C and held there. The total heating and holding times for the four samples were 50 seconds, 75 seconds, 100 seconds, and 125 seconds, respectively. After the 900°C holding period, the samples were water-quenched. Six groups of samples were obtained: QT-50, QT-75, QT-100, QT-125, QT-175, and QT-375. The control example was identical to the experimental example above, except for the total heating and holding times. The effects of Cu-rich nanoparticle precipitation on the six samples and the control example are shown in Table 4.

[0036] Table 4: Effect of total heating and holding time on the precipitation of Cu-rich nanoparticles

[0037]

[0038] (2) Microstructure analysis:

[0039] The microstructures of QT-50, QT-75, QT-100 and QT-125 samples were observed using a scanning electron microscope. Figure 2 The material structure is primarily composed of ferrite and martensite, with significant changes in the structure occurring with different holding times. This change is primarily manifested in the varying proportions of ferrite and martensite. It can be seen that as the holding time increases for the four samples, the proportions of ferrite and martensite decrease, with the proportions of ferrite decreasing to 50%, 30%, <1%, and <0.5%, respectively.

[0040] The QT-100 sample was analyzed by transmission electron microscopy. Figure 3 and Figure 4 . Figure 3 middle, Figure 3 (a) and (b) are martensitic structures observed under a transmission electron microscope. The results show that the martensitic effect of the material is good, and the martensitic laths and martensitic blocks have fine structures. The thickness of the martensitic laths is 300-400nm, and the block size is 1-2μm. Figure 4 middle, Figure 4 (a) and Figure 4 (b) is a transmission electron microscope (TEM) image of copper-rich nanoparticles in the material. It can be seen that the nanoparticles are finely dispersed in the material matrix. Figure 4 (c) shows a high-resolution transmission electron microscopy (HRTEM) image of copper-rich nanoparticles. (A) in (c) shows a magnified view of the nanoparticles. (B) in (c) shows the inverse Fourier transform (IFFT) calculation of the nanoparticles, with a 2.11 nm interplanar spacing. (D) shows the energy dispersive spectroscopic (EDS) analysis of the nanoparticles. The copper-rich nanoparticles are 13 ± 2 nm in size, demonstrating that this heat treatment method successfully produces a large number of copper-rich nanoparticles in the quenched martensitic steel.

[0041] (3) Hydrogen embrittlement resistance test:

[0042] The hydrogen embrittlement resistance of 1500MPa martensitic steel plates containing 0.2% Cu and 0% Cu was analyzed by U-bending test and thermal desorption analysis (TDA) to demonstrate the effect of Cu addition on the hydrogen embrittlement resistance of the material. Figure 5, it can be seen that the sample containing 0.2% Cu has a significant peak shift to the right and a decrease in peak value compared to the 0% Cu sample, which indicates that the hydrogen traps inside the material have changed, mainly manifested in an increase in hydrogen trap activation energy and a dispersed distribution of diffused hydrogen. Through the deconvolution simulation results of the low-temperature peak of the test curve, it can be seen that the sample containing 0.2% Cu has an additional Peak 4 compared to the 0% Cu sample, which is believed to be the result of the dispersed copper-rich nanoparticles in the material playing a role in hydrogen capture. In order to further confirm, the Kissinger formula was used to simulate and calculate the activation energies of different hydrogen traps, and the results are shown in Figure 6 It can be seen that the activation energy of Peak 4 is 35.5KJ / mol, which is close to the research results. The results of the U-bending test on the two steel plates are shown in Figure 7 , it can be seen that the Cu-containing sample exhibits better resistance to hydrogen embrittlement. This proves that the 13±7nm copper-rich nanoparticles present in martensitic steel can act as hydrogen capture, effectively dispersing the diffusible hydrogen in the material, and its activation energy is slightly higher than that of the grain boundary.

Claims

1. A heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel, characterized in that: The process steps are as follows: (1) first quenching: heating the martensitic steel material to 820-950℃ and keeping it at that temperature for 100-375s, and then water quenching; (2) Tempering: Raise the temperature of the material after the first quenching to 500℃~700℃ and keep it warm; (3) Second quenching: Rapidly heat the tempered material to 820-950°C and keep it warm. The total time of the rapid heating and holding process is 100-375s; water quench after holding; The martensitic steel comprises, by weight of elements, the following components: C 0.20-0.35%, Si 0.16-0.36%, Mn 0.30-0.60%, P ≤ 0.01%, S ≤ 0.006%, Al ≤ 0.04%, B 0.001-0.003%, Cr 0.02-0.05%, Mo 0.003-0.008%, V 0.01-0.05%, Ti 0.01-0.04%, Nb 0.02%-0.05%, Cu 0.01-0.5%, with the remainder being Fe and unavoidable impurities; and copper-rich nanoparticles are dispersed in the martensitic steel.

2. The heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel according to claim 1, characterized in that: The copper-rich nanoparticles have a size of 13±7 nm.

3. The heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel according to claim 1, characterized in that: In the step (3), the cooling rate of water quenching is 50°C / s or above.

4. The heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel according to claim 3, characterized in that: The cooling rate of the water quenching is 150°C / s or above.

5. The heat treatment method for achieving dispersed distribution of copper-rich nanoparticles in martensitic steel according to any one of claims 1 to 4, characterized in that: In the step (2), the holding time is 30 to 60 minutes.

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