A high-strength high-plasticity quenching-partitioning steel based on dynamic partitioning and a preparation method thereof

Abstract

The application discloses a kind of high-strength high-plastic quenching-partitioning steel based on dynamic partitioning and a preparation method thereof, and relates to the technical field of high-strength steel processing.The method comprises preparing a steel ingot, homogenizing the ingot, and then forging it into a billet;the billet is hot-rolled to a thickness of 10 mm;the hot-rolled steel plate is heated to 800-900°C, held for 30-180 s, then cooled to 180-250°C, held for 10-60 s, reheated to 300-500°C, and then cooled to room temperature to obtain high-strength high-plastic quenching-partitioning steel based on dynamic partitioning.The application utilizes dynamic partitioning to obtain a large amount of high-dislocation-density martensite and low-C film-shaped austenite, which improves dislocation strengthening in martensite while ensuring good stability of the overall austenite, and is of great significance for expanding the microstructure design and mechanical property control window of quenching-partitioning steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-strength steel processing, and particularly relates to a high-strength and high-plasticity quenching and partitioning steel based on dynamic partitioning and a preparation method thereof. BACKGROUND

[0002] As a key to realize and promote the double carbon goal, iron and steel materials, especially automobile iron and steel materials, need to continuously improve their comprehensive mechanical properties, and therefore advanced high-strength steels with optimized mechanical properties gradually become the main research object of the automobile industry. As a representative of advanced high-strength steels, quenching and partitioning steels have attracted widespread attention due to their excellent mechanical properties and low manufacturing cost. The good ductility of quenching and partitioning steels is derived from the transformation induced plasticity (TRIP) effect of residual austenite during deformation, and the martensite provides a hard matrix for the material.

[0003] At present, the performance improvement of quenching and partitioning steels includes the following aspects:

[0004] (1) Increase the dislocation density of martensite and improve the yield strength of the material. Mishnev et al. [Mishnev R, Borisova Y, Gaidar S, et al. Q&P Response of a Medium Carbon Low Alloy Steel. Metals, 2023, 13(4): 689.] explored the relationship between microstructure and mechanical properties of quenched and partitioned steel under different partitioning temperature conditions. The results showed that the lower the partitioning temperature, the higher the dislocation density in the martensite lath, and the higher the yield strength of the material under the effect of dislocation strengthening. (2) Increase the volume fraction and stability of austenite, and produce high-efficiency TRIP effect to improve the elongation of the material. According to the research of traditional quenched and partitioned steel, the volume fraction of residual austenite is greatly affected by the quenching temperature. Some reports [Bansal G K, Tripathy S, Chandan A K, et al. Influence of quenching strategy on phase transformation and mechanical properties of low alloy steel. Materials Science and Engineering: A, 2021, 826: 141937.] said that with the increase of quenching temperature, the volume fraction of residual austenite increases, and the plasticity of the material improves. In addition, the stability of residual austenite in quenched and partitioned low alloy steel is also sensitive to the quenching temperature. For example, Liu et al. [Liu L, He B B, Cheng G J, et al. Optimum properties of quenching and partitioning steels achieved by balancing fraction and stability of retained austenite. Scripta Materialia, 2018, 150: 1-6.] studied the microstructure formed under different quenching temperatures and found that when the quenching temperature decreased from 240℃ to 150℃, a large number of blocky residual austenite was replaced by film-like residual austenite, and the C content increased continuously, which made the stability of residual austenite continuously enhanced.

[0005] It is worth noting that although a lot of research has been carried out on the design of quenching-partitioning process, the following shortcomings still exist: (1) In the traditional isothermal partitioning process, the partitioning temperature is high, the recovery of dislocations is intensified, resulting in a decrease in dislocation density, which in turn reduces the yield strength of the material; (2) In the traditional low alloy quenching-partitioning steel, not only a large amount of film austenite is retained at low quenching temperature, but also the C content is high, resulting in excessive stability of the residual austenite and a decrease in the uniform elongation of the material during deformation; (3) In the actual industrial production process, it is difficult to achieve long-term isothermal partitioning, the furnace temperature is prone to fluctuation, and the partitioning temperature will also determine the volume fraction and C content of the residual austenite, which restricts the large-scale application of quenching-partitioning process in industrial production. Summary of the Invention

[0006] To address the shortcomings of the aforementioned background technologies, this invention provides a high-strength, high-ductility quenched-partitioned steel based on dynamic partitioning and its preparation method. This invention utilizes dynamic partitioning to obtain a large amount of martensite with high dislocation density and low-carbon film austenite. While ensuring the good stability of the overall austenite, it enhances dislocation strengthening within the martensite, which is of great significance for expanding the microstructure design and mechanical property control window of quenched-partitioned steel. Simultaneously, this invention not only overcomes the shortcomings of isothermal carbon partitioning but also improves production efficiency and reduces production costs. This invention successfully obtains low-carbon quenched-partitioned steel with superior mechanical properties using a dynamic partitioning process, possessing significant theoretical research value and broad market prospects.

[0007] The first objective of this invention is to provide a method for preparing high-strength, high-ductility quenched and partitioned steel based on dynamic partitioning, comprising the following steps:

[0008] The steel ingot is prepared by holding it at 1100~1250℃ for 2~4 hours for homogenization treatment, and then forging it into a steel billet with a thickness of 40-60mm at 1050~1150℃.

[0009] The steel billet is held at 1100~1200℃ for 2~4 hours, and then hot rolled to a thickness of 10mm to obtain a hot-rolled steel plate;

[0010] Hot-rolled steel plates are heated to 800-900℃ at a heating rate of 2-10℃ / s and held for 30-180s. Then, they are cooled to 180-250℃ at a cooling rate of 5-15℃ / s and held for 10-60s. The plates are then heated to 300-500℃ and cooled to room temperature to obtain high-strength, high-plasticity quenched steel based on dynamic partitioning.

[0011] Preferably, the heating rate when heating to 300-500℃ is 0.5~1.5℃ / s; the cooling rate to room temperature is 5-15℃ / s.

[0012] Preferably, the hot-rolled steel plate is heated to 800-900℃ at a heating rate of 2-10℃ / s and held for 30-180s to obtain austenite, and then cooled to 180-250℃ at a cooling rate of 5-15℃ / s and held for 10-60s to obtain martensite.

[0013] Preferably, during hot rolling, the initial rolling temperature is 1050~1100℃, and the rolling termination temperature is not lower than 850-900℃.

[0014] Preferably, the steel ingot is prepared using the following composition by mass percentage: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe.

[0015] Preferably, the steel ingot is made by melting raw materials according to the mass percentage of the chemical composition.

[0016] The second objective of this invention is to provide a high-strength, high-plasticity quenched steel based on dynamic partitioning.

[0017] The third objective of this invention is to provide an application of dynamically partitioned high-strength, high-plasticity quenched steel in automobile manufacturing.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] This invention provides a dynamic partitioning-based high-strength, high-ductility quenched-partitioned steel and its preparation method. The dynamic partitioning-based high-strength, high-ductility quenched-partitioned steel provided by this invention has a low carbon content in the film-like austenite within the retained austenite, exhibiting good stability. This invention first involves heating to A... C3 The above process involves brief heat treatment to obtain austenite, followed by quenching to obtain a certain amount of martensite. Dynamic partitioning then occurs during slow heating. Because the partitioning temperature is lower than traditional isothermal partitioning, the carbon content of the austenite is slightly lower after partitioning. During the secondary quenching and cooling process, with the formation of fresh martensite, a large amount of low-carbon film austenite is retained at room temperature, exhibiting a uniform elongation of 12%–16.7% during stretching.

[0020] This invention achieves dynamic recovery of dislocations during a slow heating process, as dynamic partitioning occurs. Compared to isothermal recovery, the recovery temperature is lower in the dynamic process, resulting in slower dislocation recovery in martensite and an increase in dislocation density. This leads to a yield strength of 1069-1178 MPa and a tensile strength of 1649-1722 MPa at room temperature. Attached Figure Description

[0021] Figure 1This is a heat treatment process diagram of a high-strength, high-plasticity quenched steel based on dynamic partitioning according to the present invention.

[0022] Figure 2 The images show SEM images of the microstructure of Example 1, and the KAM values ​​of martensite at room temperature.

[0023] Figure 3 The images show the SEM images of the microstructure of conventionally quenched and partitioned steel 1, as well as the KAM values ​​of martensite at room temperature.

[0024] Figure 4 It is the full width at half maximum (FWHM) of the austenite XRD peaks of Example 1 and conventional quenched-partitioned steel 1.

[0025] Figure 5 These are the room temperature tensile stress-strain curves of Example 1 and conventional quenched-divided steel 1.

[0026] Figure 6 These are the room temperature tensile stress-strain curves of Example 2 and conventional quenched-divided steel 2. Detailed Implementation

[0027] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention.

[0028] The first aspect of this invention provides a method for preparing high-strength, high-ductility quenched-partitioned steel based on dynamic partitioning, comprising the following steps:

[0029] The steel ingot is prepared by holding it at 1100~1250℃ for 2~4 hours for homogenization treatment, and then forging it into a steel billet with a thickness of 40-60mm at 1050~1150℃.

[0030] The steel billet is held at 1100~1200℃ for 2~4 hours, and then hot rolled to a thickness of 10mm to obtain a hot-rolled steel plate;

[0031] Hot-rolled steel plates are heated to 800-900℃ at a heating rate of 2-10℃ / s and held for 30-180s. Then, they are cooled to 180-250℃ at a cooling rate of 5-15℃ / s and held for 10-60s. The plates are then heated to 300-500℃ and cooled to room temperature to obtain high-strength, high-plasticity quenched steel based on dynamic partitioning.

[0032] The heating rate when heating to 300-500℃ is 0.5~1.5℃ / s; the cooling rate to room temperature is 5-15℃ / s.

[0033] Hot-rolled steel plates are heated to 800-900℃ at a heating rate of 2-10℃ / s and held for 30-180s to obtain austenite. Then, they are cooled to 180-250℃ at a cooling rate of 5-15℃ / s and held for 10-60s to obtain martensite.

[0034] During hot rolling, the initial rolling temperature is 1050~1100℃, and the rolling termination temperature is not lower than 850-900℃.

[0035] When preparing steel ingots, the following composition by mass percentage is selected: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe.

[0036] Steel ingots are made by melting raw materials according to the mass percentage of their chemical composition.

[0037] In one embodiment, a high-strength, high-plasticity quenched steel based on dynamic partitioning and its preparation method are disclosed, wherein the composition by mass percentage is: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe and unavoidable impurities.

[0038] See Figure 1 As shown, a dynamically partitioned high-strength, high-ductility quenched-partitioned steel and its preparation method are disclosed. This method utilizes slow heating to both slow dislocation recovery in martensite and reduce the carbon content in austenite. Following rapid cooling, a low-carbon film-like austenite is formed at room temperature, providing a new approach for preparing next-generation high-strength, high-ductility quenched-partitioned steels. Specifically, the preparation method for dynamically partitioned high-strength, high-ductility quenched-partitioned steel includes the following steps:

[0039] (1) The ingot is obtained by smelting according to the mass percentage of the chemical composition, and is homogenized by holding at 1200℃ for 2h. Then it is forged into a steel billet with a thickness of 40-60mm at 1100℃. The steel billet is held at 1150℃ for 2h, and then hot rolled to 10mm, wherein the rolling termination temperature is not lower than 850-900℃. Finally, it is cooled to room temperature to form a hot-rolled martensitic steel plate.

[0040] (2) Heat the hot-rolled steel plate to the austenitizing termination temperature (A). C3 The temperature is above 800-900℃ and held for a short time to completely austenitize the martensite through reverse austenite transformation. The heating rate is 2-10℃ / s, the heating temperature is 800-900℃, and the holding time is 30-180s.

[0041] (3) The austenite is rapidly cooled to between the start and end temperatures of the martensitic transformation (Ms-Mf) and held for a short time, so that part of the austenite is transformed into martensite. The cooling rate is 5-15℃ / s, the end temperature is 180-250℃, and the holding time is 10-60s.

[0042] (4) Then slowly heat to a certain temperature, and then quickly cool to room temperature to obtain high strength and high plasticity quenched steel based on dynamic distribution. The heating rate is 0.5-1.5℃ / s, the heating temperature is 300-500℃, and the cooling rate is 5-15℃ / s.

[0043] This invention optimizes the traditional partitioning process parameters by designing the isothermal partitioning process as dynamic partitioning. This utilizes the slower dislocation recovery during heating to generate high-dislocation-density martensite, while appropriately reducing the C content of the film-like retained austenite. Specifically, during the slow heating process in step (4), process parameters such as partitioning temperature and partitioning time are optimized. By controlling the dislocation recovery of martensite and the C content of austenite during dynamic partitioning, a two-phase microstructure of high-dislocation-density martensite and C-depleted film-like austenite is obtained. During the subsequent quenching process, a small amount of C-depleted austenite transforms into secondary martensite, while more C-depleted film-like austenite is retained at room temperature. Simultaneously, designing the isothermal partitioning as a dynamic temperature-variable process reduces the instability of microstructure and mechanical properties caused by temperature fluctuations, making the process more suitable for large-scale industrial production.

[0044] The second aspect of the present invention provides a high-strength, high-plasticity quenched steel based on dynamic partitioning.

[0045] The third aspect of this invention provides an application of dynamically partitioned high-strength, high-plasticity quenched steel in automobile manufacturing.

[0046] It should be noted that, unless otherwise specified, the experimental methods used in this invention are all conventional methods; and the reagents and materials used, unless otherwise specified, are all commercially available.

[0047] Example 1

[0048] Alloy composition (mass percentage): 0.38% C, 2.98% Mn, 1.49% Si, with the remainder being Fe.

[0049] Preparation process:

[0050] (1) The selected chemical composition of the material is put into a vacuum furnace for smelting to obtain an ingot, then it is kept at 1200℃ for 2 hours for homogenization treatment, and then forged into a 50mm thick steel billet at 1100℃. The steel billet is kept at 1150℃ for 2 hours, and then hot rolled to 10mm. The initial rolling temperature is 1050℃, the rolling termination temperature is not lower than 870℃, and finally cooled to room temperature to form a hot-rolled martensitic steel plate.

[0051] (2) Use a thermal expansion apparatus to heat the hot-rolled martensitic steel plate to 850°C at a rate of 5°C / s and hold it at that temperature for 60s;

[0052] (3) The sample was cooled from 850℃ to 195℃ at 10℃ / s and held for 30s. Then it was heated to 450℃ at 0.85℃ / s and finally cooled to room temperature at 10℃ / s to obtain quenched-partitioned steel.

[0053] Tissue: The room temperature tissue obtained using the process of Example 1 is as follows Figure 2 As shown, it contains 80% martensite (gray area) and 20% retained austenite (white area), with the retained austenite exhibiting a high nuclear average orientation (KAM) value. Figure 4 As shown, the FWHM of the austenite XRD peak in Example 1 is high, and the C content is low. The room temperature microstructure of conventionally quenched-partitioned steel 1 is as follows. Figure 3 As shown, it contains martensite (gray area) and retained austenite (white area), with a martensite volume fraction of 70% and a retained austenite volume fraction of 30%. The martensite has a high KAM value. Figure 4 As shown, the FWHM of the austenite XRD peak in conventionally quenched and fractionated steel 1 is low, indicating that its C content is higher.

[0054] Mechanical performance test results, such as Figure 5 As shown: Conventional quenched-partitioned steel 1, which has the same nominal composition as Example 1 and isothermally partitioned at 450°C for 300s, has a yield strength of 1028 MPa, a tensile strength of 1634 MPa, and a uniform elongation of 15.7%; while in Example 1, the yield strength of dynamically partitioned quenched-partitioned steel is increased to 1178 MPa, the tensile strength is increased to 1722 MPa, and the uniform elongation is 14.7%.

[0055] Example 2

[0056] Alloy composition (mass percentage): 0.38% C, 2.98% Mn, 1.49% Si, with the remainder being Fe.

[0057] Preparation process:

[0058] (1) The slab is obtained by smelting according to the selected chemical composition and homogenized by holding at 1200℃ for 2h. Then it is forged into a 50mm thick slab at 1100℃. The slab is held at 1150℃ for 2h and then hot rolled to 10mm. The initial rolling temperature is 1100℃, the rolling termination temperature is not lower than 870℃, and finally it is cooled to room temperature to form a hot-rolled martensitic steel plate.

[0059] (2) Use a thermal expansion apparatus to heat the hot-rolled martensitic steel plate to 850°C at a rate of 5°C / s and hold it at that temperature for 60s;

[0060] (3) The sample was cooled from 850℃ to 195℃ at 10℃ / s and held for 30s. Then it was heated to 450℃ at 1.25℃ / s and finally cooled to room temperature at 10℃ / s to obtain quenched-partitioned steel.

[0061] Mechanical performance test results (such as) Figure 6 As shown): Conventional quenched-partitioned steel 2, which has the same nominal composition as Example 2 and isothermally partitioned at 450°C for 200s, has a yield strength of 1015MPa, a tensile strength of 1632MPa, and a uniform elongation of 15.3%; while in Example 2, the yield strength of dynamically partitioned quenched-partitioned steel is increased to 1095MPa, the tensile strength is increased to 1685MPa, and the uniform elongation is 14.5%.

[0062] This invention describes preferred embodiments and their effects. However, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to include both the preferred embodiments and all changes and modifications falling within the scope of this invention.

[0063] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-strength, high-ductility quenched-partitioned steel based on dynamic partitioning, characterized in that, Includes the following steps: The steel ingot is prepared by holding it at 1100~1250℃ for 2~4 hours for homogenization treatment, and then forging it into a steel billet with a thickness of 40-60mm at 1050~1150℃. The steel billet is held at 1100~1200℃ for 2~4 hours, and then hot rolled to a thickness of 10mm to obtain a hot-rolled steel plate; Hot-rolled steel plates are heated to 800-900℃ at a heating rate of 2-10℃ / s and held for 30-180s. Then, they are cooled to 180-250℃ at a cooling rate of 5-15℃ / s and held for 10-60s. The plates are then heated to 300-500℃ and cooled to room temperature to obtain high-strength and high-plasticity quenched steel based on dynamic distribution. The heating rate when heating to 300-500℃ is 0.5~1.5℃ / s; the cooling rate to room temperature is 5-15℃ / s; When preparing steel ingots, the following composition by mass percentage is selected: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe.

2. The method for preparing high-strength, high-ductility quenched-partitioned steel based on dynamic partitioning according to claim 1, characterized in that, Hot-rolled steel plates are heated to 800-900℃ at a heating rate of 2-10℃ / s and held for 30-180s to obtain austenite. Then, they are cooled to 180-250℃ at a cooling rate of 5-15℃ / s and held for 10-60s to obtain martensite.

3. The method for preparing high-strength, high-ductility quenched-partitioned steel based on dynamic partitioning according to claim 1, characterized in that, During hot rolling, the initial rolling temperature is 1050~1100℃, and the rolling termination temperature is not lower than 850-900℃.

4. The method for preparing high-strength, high-ductility quenched-partitioned steel based on dynamic partitioning according to claim 1, characterized in that, Steel ingots are made by melting raw materials according to the mass percentage of their chemical composition.

5. A high-strength, high-ductility quenched-partition steel based on dynamic partitioning obtained by the method of any one of claims 1 to 4.

6. The application of the dynamic partitioning high-strength and high-plasticity quenched-partitioned steel as described in claim 5 in automobile manufacturing.

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