Treatment process for bainite steel with superfine retained austenite

Abstract

The invention discloses a bainite steel treatment process with superfine retained austenite, and belongs to the technical field of bainite steel heat treatment. The method comprises the following steps: (1) heating an as-cast billet, preserving heat, and completely converting the internal structure of the as-cast billet into austenite to obtain a billet; (2) carrying out water quenching on the steel billet obtained in the step (1) until the temperature is 3-5 DEG C below the Ms temperature; (3) carrying out isothermal treatment on the steel billet subjected to water quenching in the step (2), and after isothermal treatment, carrying out air cooling on the steel billet to room temperature; and (4) the steel billet cooled in the step (3) is subjected to tempering treatment, and the bainite steel with the superfine retained austenite is obtained. The bainite steel with good comprehensive performance is obtained through the simple and efficient treatment process, and the bainite steel can meet more application requirements.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of bainite steel and relates to a treatment process of bainite steel with ultrafine residual austenite. Background Art

[0002] Steel materials play a vital role in modern industry, and their performance improvement and innovation continue to promote the development of engineering technology. Bainitic steel is widely used in machinery manufacturing, automobile industry, structural materials and other fields due to its excellent strength and toughness. With the continuous development of engineering technology, the performance requirements for steel materials are also increasing. Therefore, it is of great significance to improve the comprehensive performance of bainitic steel so that it can meet higher application requirements.

[0003] Retained austenite can significantly improve the toughness and impact resistance of the material, thereby further optimizing the comprehensive performance of bainitic steel and allowing the performance of bainitic steel to meet higher application requirements.

[0004] Therefore, it is necessary to provide a bainitic steel processing process with ultrafine retained austenite to obtain bainitic steel with relatively excellent comprehensive properties. Summary of the invention

[0005] In order to overcome the problems in the background technology, the purpose of the present invention is to provide a method for quickly obtaining bainitic steel with ultrafine retained austenite through a quenching and partitioning process. By regulating the quenching and partitioning process, bainitic steel with ultrafine retained austenite structure can be quickly obtained. While maintaining strong toughness, long-term isothermal treatment is not required, which greatly improves the preparation efficiency, reduces production costs, and has obvious benefits.

[0006] The present invention processes the cast steel billet to obtain bainitic steel with ultrafine residual austenite, so that the comprehensive performance of the bainitic steel is further enhanced and improved, making its application range wider.

[0007] In order to achieve the above object, the present invention is implemented by the following technical solutions:

[0008] The treatment process comprises the following steps:

[0009] (1) Melting and pouring: The prepared materials are placed in a boiler, heated and kept warm until the temperature inside the entire steel reaches the required temperature, and then cast to obtain a cast steel billet;

[0010] (2) Full austenite homogenization: Place the as-cast steel billet in a box furnace and heat it to 850°C ± 20°C at a rate of 65-80°C / h for full austenite homogenization. The holding time is 1h to ensure that the structure in the steel billet is completely transformed into austenite.

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

[0012] (4) Rapid isothermal transformation of bainite by quenching and partitioning: 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 temperature is kept constant for 1 hour for every 50 mm effective thickness, and then air-cooled to room temperature;

[0013] (5) Tempering and refining of residual austenite: The steel billet is subjected to austenite refining and tempering treatment again to refine the remaining large austenite structure, thereby obtaining bainitic steel with ultrafine austenite, thus completing the entire process.

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

[0015] Preferably, in step (3), the isothermal treatment temperature is 320-380°C, the isothermal treatment time is 1 hour for every 50mm effective thickness, and the effective thickness = nominal thickness - corrosion allowance - negative thickness deviation - thickness of the formed thinning layer. The nominal thickness refers to the thickness of the steel material rounded to the standard specification after adding the design thickness to the negative thickness deviation of the steel material; the corrosion allowance refers to the thickness allowance added in advance to take into account the corrosion of the material by the contact medium (including the atmosphere) during the service life, and its value is determined by the corrosion rate of the medium on the material and the design life of the parts; the negative thickness deviation is determined according to the standard GB 150.1-2011 and the provisions of the material standard, and the thickness of the formed thinning layer is the minimum thickness to ensure the requirements after forming.

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

[0017] The present invention adopts a rapid quenching process. When cooled to 3-5°C below Ms, a large number of bainite transformation precursors are generated in this specific temperature range because the stability of austenite begins to decrease, and the diffusion capacity of atoms is limited but still can undergo a certain degree of short-range diffusion and rearrangement; the existence of these precursors constructs nucleation sites for bainite transformation, making the formed bainite structure more uniform and fine in morphology and distribution; the subsequent bainite transformation can be carried out more efficiently, thereby shortening the time required for the entire bainite formation, thereby shortening the overall duration of the heat treatment process; at the same time, during the rapid quenching process, the local stress and strain caused by the rapid drop in temperature and the formation of bainite transformation precursors act on the residual austenite; this stress and strain promote the residual austenite to undergo Plastic deformation and segmentation cause its originally large block structure to be gradually refined into ultrafine retained austenite structure; the ultrafine retained austenite structure has higher interface energy and volume fraction, which can effectively hinder dislocation movement in the subsequent deformation process, thereby enhancing the strength of the material; at the same time, due to the synergistic effect between the uniform and fine bainite structure of the bainite transformation precursor and the ultrafine retained austenite, the bainite structure provides a certain strength support through its own phase structure characteristics, and the retained austenite can undergo martensitic phase transformation when subjected to stress deformation, resulting in phase transformation strengthening and plasticization effects. The combined effect of the two significantly enhances the strength and toughness of the material, so that the material has high strength while also having good toughness and plastic deformation ability, meeting the requirements for comprehensive material performance in engineering applications.

[0018] In the isothermal stage, the insulation time is 1 hour for every 50mm effective thickness in order to allow the transformation of bainite to proceed fully; according to the isothermal kinetics of bainite transformation, the transformation of bainite is a process that takes time to complete; in this process, the diffusion and rearrangement of atoms require a certain amount of time to reach a state of equilibrium; through this insulation method, it can be ensured that bainite is formed evenly throughout the steel, and the structure is made more stable, thereby further optimizing the performance of the material.

[0019] Beneficial effects of the present invention:

[0020] 1. The present invention significantly shortens the overall heat treatment time by optimizing the process steps of quenching, isothermal transformation, tempering, etc. This means that more materials can be produced within the same production cycle to meet market demand; at the same time, it can effectively reduce energy consumption and equipment use time, thereby significantly reducing production costs. This makes the production of bainitic steel more economical and improves overall market competitiveness.

[0021] 2. The present invention adopts a rapid quenching process to produce a large amount of bainite transformation precursors in the steel. On the one hand, it provides a driving force for accelerating the subsequent isothermal transformation of bainite, which is beneficial to improving the bainite transformation efficiency, and ultimately makes the entire processing process more efficient; on the other hand, the local stress and strain caused by the bainite transformation precursor act on the residual austenite, causing the residual austenite to undergo plastic deformation and segmentation, so that the originally larger block structure is gradually refined into an ultrafine residual austenite structure.

[0022] 3. In the process of regulating the quenching distribution, the present invention obtains ultrafine austenite structure while rapidly forming bainite, which can effectively maintain the toughness of the material and improve the impact resistance.

[0023] 4. The treatment process of the present invention is simple and convenient, with relatively low cost, and is suitable for industrial promotion and application. The comprehensive properties of the bainitic steel finally obtained can be adjusted by simply adjusting the water quenching temperature and time, which is convenient for adapting to the diverse application needs of bainitic steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a process flow chart of the treatment process.

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

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

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

[0028] Figure 5 1 is the XRD diagram of the bainitic steel in Examples 1-2 and Comparative Example 1. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with specific examples. In the examples and comparative examples of the present invention, chemical reagents not otherwise specified are all commercially available analytically pure for experiments.

[0030] In the embodiments of the present invention and the comparative examples, the cast slab used is NM300 low alloy wear-resistant steel material, and the slab components include, by mass fraction, C: 0.15%, Si: 1.2%, Mn: 1.7%, Al: 0.5%, Cu: 0.05%, Cr: 0.024%, and the remainder is Fe and unavoidable impurities; the cast slab is made by conventional smelting and casting.

[0031] Example 1

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

[0033] (1) The as-cast slab is placed in a box furnace, and the heating rate of the box furnace is set to 65°C / h, the heating temperature is set to 850±20°C, and the holding time is 1h. The box furnace heats and holds the as-cast steel billet according to the set parameters, so that the structure of the as-cast slab is completely transformed into austenite.

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

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

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

[0037] pass Figure 2 It can be seen that ultrafine retained austenite is evenly distributed in the matrix structure of the bainitic steel obtained in this embodiment. This is because when cooling to 280°C is adopted, a large amount of bainite transformation precursors are generated in this specific temperature range, and the induced local stress and strain act on the retained austenite; this stress and strain promotes the plastic deformation and segmentation of the retained austenite, so that its originally large block 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) The as-cast steel billet is placed in a box-type furnace, and the heating rate of the box-type furnace is set to 80°C / h, the heating temperature is set to 850±20°C, and the holding time is 1h. The box-type furnace heats and holds the as-cast steel billet according to the set parameters, so that the structure in the as-cast steel billet is completely transformed into austenite.

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

[0042] (3) The steel billet air-cooled to room temperature in step (2) is tempered at 260° C. for 2 h to obtain bainitic steel.

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

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

[0045] Example 3

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

[0047] (1) The as-cast steel billet is placed in a box-type furnace, and the heating rate of the box-type furnace is set to 70°C / h, the heating temperature is set to 850±20°C, and the holding time is 1h. The box-type furnace heats and holds the as-cast steel billet according to the set parameters, so that the structure in the as-cast steel billet is completely transformed into austenite.

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

[0049] (3) The steel billet air-cooled to room temperature in step (2) is tempered at 300° C. for 1 h to obtain bainitic steel.

[0050] The bainitic steel prepared in this example 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 experimentation, except that in this comparative example, the steel billet is water quenched to room temperature, then tempered, and finally isothermally treated for 6 hours.

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

[0054] pass Figure 2-Figure 4 It can be seen that the bainite steel structures obtained in the embodiment and the comparative example are 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] It can be seen from Table 1 that the treatment process of the present invention can complete the treatment of bainitic steel in a relatively short time, so that the bainitic steel maintains a relatively excellent hardness level and impact toughness, and the bainitic steel has relatively excellent comprehensive properties.

[0058] In summary, through the processing technology of the present invention, bainitic steel with excellent comprehensive performance can be obtained more simply and efficiently, making the application range of bainitic steel wider.

Claims

1. A process for treating bainitic steel with ultrafine retained austenite, characterized in that: The treatment process comprises the following steps: (1) heating and keeping the as-cast slab warm so that the structure of the as-cast slab is completely transformed into austenite to obtain a slab; (2) water quenching the slab obtained in step (1) to a temperature 3-5° C. below the martensitic transformation start temperature; (3) performing isothermal treatment on the slab after water quenching in step (2), and after the isothermal treatment, air cooling the slab to room temperature; (4) Tempering the slab after cooling in step (3) to obtain bainitic steel with ultrafine retained austenite.

2. The treatment process according to claim 1, characterized in that: In the step (1), the heating rate is 65-80°C / h, the heating temperature is 850±20°C, and the insulation time is 1h.

3. The treatment process according to claim 1, characterized in that: The water quenching time in step (2) is 5-10s.

4. The treatment process according to claim 1, characterized in that: In the step (3), the isothermal treatment temperature is 320-380° C., and the isothermal treatment time is 1 hour for every 50 mm of effective thickness.

5. The treatment process according to claim 1, characterized in that: In the step (4), the tempering temperature is 260-300° C. and the tempering time is 0.5-2 h.

Images