A process for treating bainitic steel with ultrafine retained austenite

Abstract

This invention discloses a processing technology for bainitic steel with ultrafine retained austenite, belonging to the field of bainitic steel heat treatment technology. The invention includes: (1) heating and holding a cast steel billet to completely transform its internal structure into austenite, obtaining a steel billet; (2) water quenching the steel billet obtained in step (1) to 3-5°C below the Ms temperature; (3) performing isothermal treatment on the water-quenched steel billet in step (2), followed by air cooling to room temperature; and (4) tempering the cooled steel billet in step (3) to obtain bainitic steel with ultrafine retained austenite. This invention obtains bainitic steel with good comprehensive performance through a simple and efficient processing technology, which is beneficial for bainitic steel to meet more application needs.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for bainitic steel, and relates to a treatment process for bainitic steel with ultrafine retained austenite. Background Technology

[0002] Steel plays a vital role in modern industry, and its performance improvements and innovations continuously drive the development of engineering technology. Bainitic steel, due to its excellent strength and toughness, is widely used in machinery manufacturing, the automotive industry, and structural materials. With the continuous development of engineering technology, the performance requirements for steel materials are also increasing. Therefore, improving the comprehensive performance of bainitic steel to meet higher application requirements is of great significance.

[0003] Retained austenite can significantly improve the toughness and impact resistance of materials, thereby further optimizing the overall performance of bainitic steel and enabling it to meet higher application requirements.

[0004] Therefore, it is necessary to provide a bainitic steel processing technology with ultrafine retained austenite in order to obtain bainitic steel with superior comprehensive properties. Summary of the Invention

[0005] To overcome the problems in the background art, the purpose of this invention is to provide a method for rapidly obtaining bainitic steel with ultrafine retained austenite through a quenching and partitioning process. By controlling the quenching and partitioning process, bainitic steel with ultrafine retained austenite can be obtained rapidly. While maintaining high toughness, it does not require long-term isothermal treatment, which greatly improves the preparation efficiency, reduces production costs, and has significant benefits.

[0006] This invention processes cast steel billets to obtain bainitic steel with ultrafine retained austenite, thereby further enhancing the comprehensive properties of bainitic steel and broadening its application range.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0008] The processing technology includes the following steps:

[0009] (1) Melting and casting: The proportioned materials are placed in a boiler, heated and kept warm until the internal temperature of the entire steel reaches the required temperature, and then cast to obtain a cast steel billet.

[0010] (2) Full austenite homogenization: The cast steel billet is placed in a box furnace and heated to 850℃±20℃ at a rate of 65-80℃ / h to fully homogenize austenite. The holding time is 1h to ensure that the internal structure of the steel billet is completely transformed into austenite.

[0011] (3) Rapid quenching steps to form bainite transformation precursor: After the steel billet is fully homogenized with austenite, it is placed in the quenching tank for water quenching. The surface temperature of the steel billet is monitored and it is quenched to 3-5℃ below the martensite transformation temperature Ms.

[0012] (4) Quenching and partitioning of bainite rapid isothermal transformation process: After the rapid quenching behavior forms the bainite transformation precursor, the billet is quickly placed in a constant temperature salt bath, and the billet temperature is kept constant for isothermal treatment to obtain bainite. The billet is kept at a temperature of 1 hour for every 50 mm of effective thickness, and then air-cooled to room temperature.

[0013] (5) Tempering to refine the residual austenite: The billet is tempered again to refine the austenite, and the remaining large austenite structure is refined to obtain bainitic steel with ultrafine austenite, thus completing the entire process.

[0014] Preferably, the water quenching time in step (3) is 5-10 seconds.

[0015] Preferably, in step (3), the isothermal treatment temperature is 320-380℃, and the isothermal treatment time is 1 hour for every 50mm effective thickness. The effective thickness = nominal thickness - corrosion allowance - thickness negative deviation - forming thinning layer thickness. The nominal thickness refers to the thickness of the steel standard specification after adding the design thickness to the steel thickness negative deviation; the corrosion allowance refers to the pre-added thickness margin considering corrosion from the contact medium (including the atmosphere) during the material's service life, and its value is determined by the corrosion rate of the medium on the material and the design life of the component; the thickness negative deviation is determined according to standard GB 150.1-2011, according to the material standard; and the forming thinning layer thickness is the minimum thickness required to meet the requirements after forming.

[0016] Preferably, in step (5), the tempering temperature is 260-300℃ and the tempering time is 0.5-2h.

[0017] This invention employs a rapid quenching process. When cooled to 3-5°C below Ms, the formation of a large number of bainite transformation precursors within this specific temperature range is due to the decreasing stability of austenite and the limited diffusion ability of atoms, although short-range diffusion and rearrangement are still possible to a certain extent. The presence of these precursors provides nucleation sites for bainite transformation, resulting in a more uniform and finer bainite microstructure in terms of morphology and distribution. Subsequent bainite transformation can proceed more efficiently, thereby shortening the time required for the entire bainite formation and thus reducing the overall heat treatment process time. Simultaneously, during rapid quenching, the localized stress and strain induced by the rapid temperature drop and the formation of bainite transformation precursors act on the retained austenite. This stress and strain promotes the transformation of the retained austenite. Plastic deformation and segmentation gradually refine the originally large blocky structure into an ultrafine retained austenite structure. The ultrafine retained austenite structure has a high interfacial energy and volume fraction, which can effectively hinder dislocation movement during subsequent deformation, thereby enhancing the strength of the material. At the same time, due to the synergistic effect of the uniform and fine bainite structure, which is a precursor to bainite transformation, and the ultrafine retained austenite, the bainite structure provides a certain strength support through its own phase structure characteristics, while the retained austenite can undergo martensitic transformation under stress and deformation, producing a phase transformation strengthening and plasticizing effect. The combined effect of the two significantly enhances the strength and toughness of the material, enabling the material to have high strength while also possessing good toughness and plastic deformation capacity, meeting the requirements of comprehensive material performance in engineering applications.

[0018] The isothermal stage involves holding the material at a temperature of 1 hour for every 50 mm of effective thickness to ensure the bainite transformation proceeds fully. According to the isothermal kinetics of bainite transformation, this transformation is a time-consuming process. During this process, the diffusion and rearrangement of atoms require a certain amount of time to reach equilibrium. This method of holding ensures that bainite forms uniformly throughout the steel and makes the microstructure more stable, thereby further optimizing the material's properties.

[0019] The beneficial effects of this invention are:

[0020] 1. This invention significantly shortens the overall heat treatment time by optimizing quenching, isothermal transformation, and tempering processes. This means that more material can be produced within the same production cycle to meet market demand. Simultaneously, it effectively reduces energy consumption and equipment usage time, thereby significantly lowering production costs. This makes the production of bainitic steel more economical and enhances its overall market competitiveness.

[0021] 2. This invention employs a rapid quenching process, which generates a large number of bainite transformation precursors within the steel. On one hand, this provides a driving force for accelerating the subsequent isothermal transformation of bainite, which is beneficial for improving the bainite transformation efficiency. Ultimately, this results in a highly efficient overall processing procedure. On the other hand, the local stress and strain induced by the bainite transformation precursors act on the retained austenite, causing it to undergo plastic deformation and segmentation, gradually refining the originally large blocky structure into an ultrafine retained austenite microstructure.

[0022] 3. In the process of controlling the quenching composition, the present invention obtains an ultrafine austenitic structure while rapidly forming bainite, which can effectively maintain the strength and toughness of the material and improve its impact resistance.

[0023] 4. The processing technology of this invention is simple and convenient, with relatively low cost, and is suitable for industrial application. The comprehensive properties of the final bainitic steel can be adjusted by simply controlling the water quenching temperature and time, making it easy to adapt to the diverse application needs of bainitic steel. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of the present invention.

[0025] Figure 2 This is an electron microscope image of the microstructure of bainitic steel in Example 1 of the present invention.

[0026] Figure 3 This is an electron microscope image of the microstructure of bainitic steel in Example 2 of the present invention.

[0027] Figure 4 This is an electron microscope image of the microstructure of bainitic steel in Comparative Example 1 of the present invention.

[0028] Figure 5 The images show the XRD patterns of the bainitic steels in Examples 1-2 and Comparative Example 1. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to specific embodiments; in the embodiments and comparative examples of the present invention, unless otherwise specified, all chemical reagents used in the experiments were commercially available analytical grade reagents.

[0030] In the embodiments and comparative examples of this invention, the as-cast slab used is NM300 low-alloy wear-resistant steel material. The slab composition by mass fraction includes: C: 0.15%, Si: 1.2%, Mn: 1.7%, Al: 0.5%, Cu: 0.05%, Cr: 0.024%, with the balance being Fe and unavoidable impurities. The as-cast slab is produced by conventional smelting and casting methods.

[0031] Example 1

[0032] This embodiment uses the following method to process the as-cast steel billet:

[0033] (1) Place the cast slab into the box furnace, set the heating rate of the box furnace to 65℃ / h, set the heating temperature to 850±20℃, and set the holding time to 1h. Then the box furnace heats and holds the cast steel billet according to the set parameters, so that the microstructure in the cast slab is completely transformed into austenite.

[0034] (2) The slab processed in step (1) is placed in a quenching tank for water quenching. The surface temperature of the slab is monitored. After water quenching for 10 seconds, when the temperature reaches 280℃ (5℃ below Ms), the slab is taken out and quickly transferred to a constant temperature salt bath at 350℃ for isothermal treatment. In this embodiment, the effective thickness of the slab is 50mm. Therefore, after isothermal treatment for 1 hour, the slab is taken out and air-cooled to room temperature.

[0035] (3) The steel billet that was air-cooled to room temperature in step (2) was tempered at 280℃ for 0.5h to obtain bainitic steel.

[0036] The microstructure of the bainitic steel in this embodiment was observed using an electron microscope, and the results are as follows: Figure 2 As shown, X-ray diffraction experiments were performed on the bainitic steel of this embodiment, and the results are as follows. Figure 5 As shown in Table 1, the hardness and impact toughness of the bainitic steel in this embodiment were measured.

[0037] pass Figure 2 It can be seen that the matrix structure of the bainitic steel obtained in this embodiment is uniformly distributed with ultrafine retained austenite. This is because when cooled to 280°C, a large number of bainite transformation precursors are generated in this specific temperature range, and the resulting local stress and strain act on the retained austenite. This stress and strain causes the retained austenite to undergo plastic deformation and segmentation, so that its originally large blocky structure is gradually refined into an ultrafine retained austenite structure.

[0038] Example 2

[0039] This embodiment uses the following method to process the as-cast steel billet:

[0040] (1) Place the cast steel billet into the box furnace, set the heating rate of the box furnace to 80℃ / h, set the heating temperature to 850±20℃, and set the holding time to 1h. Then the box furnace heats and holds the cast steel billet according to the set parameters, so that the microstructure in the cast steel billet is completely transformed into austenite.

[0041] (2) The steel billet processed in step (1) is placed in a quenching pool for water quenching. The surface temperature of the steel billet is monitored. After water quenching for 5 seconds, when the temperature reaches 282℃ (3℃ below Ms), the steel billet is taken out and quickly transferred to a constant temperature salt bath at 320℃ for isothermal treatment. In this embodiment, the effective thickness of the steel billet is 50mm. Therefore, after isothermal treatment for 1 hour, the steel billet is taken out and air-cooled to room temperature.

[0042] (3) The steel billet that was air-cooled to room temperature in step (2) was tempered at 260℃ for 2 hours to obtain bainitic steel.

[0043] The microstructure of the bainitic steel in this embodiment was observed using an electron microscope, and the results are as follows: Figure 3 As shown, X-ray diffraction experiments were performed on the bainitic steel of this embodiment, and the results are as follows. Figure 5 As shown in Table 1, the hardness and impact toughness of the bainitic steel in this embodiment were measured.

[0044] pass Figure 3 It can be seen that the microstructure of the bainitic steel prepared in this embodiment is similar to that of the bainitic steel in Example 1.

[0045] Example 3

[0046] This embodiment uses the following method to process the as-cast steel billet:

[0047] (1) Place the cast steel billet into the box furnace, set the heating rate of the box furnace to 70℃ / h, set the heating temperature to 850±20℃, and set the holding time to 1h. Then the box furnace heats and holds the cast steel billet according to the set parameters, so that the microstructure in the cast steel billet is completely transformed into austenite.

[0048] (2) The steel billet processed in step (1) is placed in a quenching pool for water quenching. The surface temperature of the steel billet is monitored. After water quenching for 5 seconds, when the temperature reaches 281℃ (4℃ below Ms), the steel billet is taken out and quickly transferred to a constant temperature salt bath at 380℃ for isothermal treatment. In this embodiment, the effective thickness of the steel billet is 50mm. Therefore, after isothermal treatment for 1 hour, the steel billet is taken out and air-cooled to room temperature.

[0049] (3) The steel billet that was air-cooled to room temperature in step (2) was tempered at 300℃ for 1 hour to obtain bainitic steel.

[0050] The bainitic steel prepared in this embodiment has similar properties to the bainitic steel in Example 1.

[0051] Comparative Example 1

[0052] This comparative example uses the same parameters as Example 1 for the experiment. The difference is that in this comparative example, the steel billet is water quenched to room temperature, then tempered, and finally isothermal treated for 6 hours.

[0053] The microstructure of the bainitic steel in this embodiment was observed using an electron microscope, and the results are as follows: Figure 4 As shown, X-ray diffraction experiments were performed on the bainitic steel of this embodiment, and the results are as follows. Figure 5 As shown in Table 1, the hardness and impact toughness of the bainitic steel in this embodiment were measured.

[0054] pass Figures 2-4 It can be seen that the bainitic steel microstructure obtained in the examples and comparative examples is similar.

[0055] Table 1

[0056] Matrix hardness / HBW <![CDATA[Impact toughness / J·cm -2 > Example 1 294 132 Example 2 300 128 Comparative Example 1 292 136

[0057] As can be seen from Table 1, the processing technology of the present invention can complete the processing of bainitic steel in a short time, so that the bainitic steel maintains a high level of hardness and impact toughness, and thus has excellent comprehensive properties.

[0058] In summary, the processing technology of this invention can easily and efficiently produce bainitic steel with excellent overall performance, thus broadening the application range of bainitic steel.

Claims

1. A processing method for bainitic steel with ultrafine retained austenite, characterized in that: The processing technology includes the following steps: (1) Heat and hold the as-cast slab to completely transform the internal structure of the as-cast slab into austenite, and obtain the slab; (2) The slab obtained in step (1) is water quenched to 3-5°C below the martensitic transformation initiation temperature; (3) The slab after water quenching in step (2) is subjected to isothermal treatment. After isothermal treatment, the slab is air-cooled to room temperature. The isothermal treatment temperature is 320-380℃, and the isothermal treatment time is 1 hour for every 50mm effective thickness. (4) The slab cooled in step (3) is tempered to obtain bainitic steel with ultrafine retained austenite.

2. The processing technology according to claim 1, characterized in that: In step (1), the heating rate is 65-80℃ / h, the heating temperature is 850±20℃, and the holding time is 1h.

3. The processing technology according to claim 1, characterized in that: The water quenching time in step (2) is 5-10 seconds.

4. The processing technology according to claim 1, characterized in that: In step (4), the tempering temperature is 260-300℃ and the tempering time is 0.5-2h.

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