A nanostructured bainitic steel and its preparation method

Abstract

The application discloses a nano-structured bainite steel and a preparation method thereof, and belongs to the technical field of steel materials. The preparation method comprises the following steps: subjecting raw material steel to austenitizing treatment; cooling to a bainite transformation temperature, and performing first-stage isothermal treatment; cooling to 20-200 DEG C below a martensite transformation start temperature; heating to the bainite transformation temperature, and performing second-stage isothermal treatment; after the second-stage isothermal treatment, cooling to room temperature, and obtaining the nano-structured bainite steel. Martensite is introduced between the first-stage and second-stage isothermal treatment, the austenite block is divided, the transformation rate of the bainite in the second-stage isothermal treatment is accelerated, the heat treatment time is shortened, and the bainite structure is refined; the bainite structure formed in the early stage serves as a flexible phase, can stabilize a part of thin film-shaped austenite, absorbs phase change distortion caused by the martensite formed in the intermediate stage, and avoids the cracking phenomenon of high-carbon steel in ordinary quenching or quenching and partitioning.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, specifically to a nanostructured bainitic steel and its preparation method. Background Technology

[0002] Nanostructured bainitic steels have attracted widespread attention due to their excellent strength-toughness balance, showing great application potential in automotive steel, rail steel, bearing steel, and wear-resistant steel. However, traditional nanostructured bainitic steels are generally designed with high carbon content, resulting in low bainitic transformation temperatures. These lower transformation temperatures prolong the bainitic transformation time, sometimes to more than ten days. During the bainitic transformation, alloying elements (mainly carbon) diffuse from bainitic ferrite to the surrounding untransformed austenite, gradually accumulating at the interface between bainitic ferrite and austenite. This leads to a slowdown or even stagnation of the transformation rate in the later stages of the bainitic transformation, thus correspondingly extending the required transformation time.

[0003] In recent years, many scholars have proposed optimizing alloying elements (such as adding Al and Co) or using hot or warm deformation, multi-stage isothermal treatment, etc., in order to accelerate the rate of bainitic transformation. However, the heat treatment process for nanostructured bainitic steel still takes several hours or even days. These technical drawbacks limit the engineering application of nanostructured bainitic steel.

[0004] In the field of multiphase steel preparation, CN107227433A discloses "a high-performance martensitic-austenitic dual-phase steel and its preparation method," which requires rapid quenching to Ms-10℃ in the first step to form a martensitic structure. Due to the rapid quenching speed and the formation of martensite structure through shear transformation, the steel is prone to quenching cracks. CN10590809A discloses "a vanadium-titanium composite-added steel plate with high fatigue strength and its manufacturing method," which also requires complete austenite to form martensite structure in the first step to obtain a quenched plate, and also suffers from the problem of easy quenching cracks. Therefore, how to form a more flexible bainitic structure and solve the problem of easy quenching cracks has become an urgent problem to be solved in the field of high-strength / ultra-high-strength steel. Summary of the Invention

[0005] To address the aforementioned technical problems in the existing technology, this invention provides a nanostructured bainitic steel and its preparation method. The method involves a first-stage isothermal treatment after austenitization, cooling to the bainitic transformation temperature range to form a certain amount of bainitic ferrite. Then, cooling to below the martensitic transformation initiation temperature forms a certain amount of martensite. This process divides the untransformed austenite blocks, refines the untransformed austenite, and reduces quenching cracks. It also accelerates the second-stage bainitic transformation and refines the bainitic ferrite formed in the second stage.

[0006] This invention discloses a method for preparing nanostructured bainitic steel, comprising the following steps: obtaining raw steel; subjecting the raw steel to austenitization treatment; after austenitization treatment, cooling to the bainite transformation temperature and performing a first-stage isothermal treatment; after the first-stage isothermal treatment, cooling to 20-200°C below the martensite transformation initiation temperature; reheating to the bainite transformation temperature and performing a second-stage isothermal treatment; after the second-stage isothermal treatment, cooling to room temperature to obtain nanostructured bainitic steel.

[0007] In the first stage of isothermal treatment, when the transformation amount of bainitic ferrite is 30-70%, the temperature is cooled to 20-200°C below the martensitic transformation initiation temperature. The transformation amount of bainitic ferrite in the first stage isothermal treatment is determined by the isothermal treatment duration, which is 10-240 minutes. The second stage isothermal treatment time is 30-60 minutes. The treatment temperatures for the first and second stages are the same.

[0008] The raw material steel has the following composition: C: 0.40-0.65 wt%; Si: 1.2-1.8 wt%; Mn: 1.5-2.5 wt%; Cr: 0-0.8 wt%; V: 0.04-0.1 wt%; with the balance being Fe.

[0009] The temperature-time transformation curve of the raw steel is tested to obtain the bainitic transformation kinetic curve. Based on the bainitic transformation kinetic curve, nanostructured bainitic steel is prepared. The bainitic transformation kinetic curve includes the following parameters: upper transformation temperature, bainitic transformation temperature range, and martensitic transformation onset temperature; wherein, the bainitic transformation temperature range includes: bainitic transformation onset temperature and bainitic transformation temperature.

[0010] The specific preparation method includes the following steps: raw steel is produced by conventional smelting, forging, or rolling; the bainite transformation kinetic curve of the raw steel is obtained; the raw steel is austenitized at 880℃; after austenitization, it is cooled to 340-360℃ for a first-stage isothermal treatment for 20-240 minutes, so that the austenite in the steel is transformed into 30-70% bainitic ferrite, with the remainder being untransformed austenite; after the first-stage isothermal treatment, it is cooled to 20-200℃ below the martensite transformation initiation temperature; it is heated to 340-360℃ for a second-stage isothermal treatment for 30-60 minutes; and cooled to room temperature to obtain nanostructured bainitic steel.

[0011] The present invention also provides a nanostructured bainitic steel prepared by the above-described preparation method. Tests show that the nanostructured bainitic steel has a tensile strength of 1400-1700 MPa, an elongation after fracture of 25-35%, a strength-to-plasticity product of 40-49 GPa%, and a yield strength of 800-1200 MPa.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: between the first stage and the second stage of isothermal treatment, martensite is introduced to divide the austenite blocks, accelerate the transformation rate of bainite in the second stage of isothermal treatment, shorten the heat treatment time, and refine the bainite structure; the bainite structure formed in the early stage, as a flexible phase, can stabilize a portion of the thin-film austenite, absorb the phase transformation distortion caused by the martensite formed in the intermediate stage, and avoid the cracking phenomenon that occurs in high carbon steel during ordinary quenching or quenching-partitioning process. Attached Figure Description

[0013] Figure 1 This is a flowchart of the preparation method of the nanostructured bainitic steel of the present invention;

[0014] Figure 2 This is a schematic diagram of the temperature curves for the preparation of nanostructured bainitic steel;

[0015] Figure 3 This is the microstructure of the nanostructured bainitic steel in Example 3;

[0016] Figure 4 This is the microstructure of the nanostructured bainitic steel in Example 4;

[0017] Figure 5 This is the temperature-time-transition curve of Example 5;

[0018] Figure 6 This is the microstructure of the nanostructured bainitic steel of Example 6. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] The present invention will now be described in further detail with reference to the accompanying drawings:

[0021] A method for preparing nanostructured bainitic steel, such as Figure 1 and 2 As shown, it includes the following steps:

[0022] Step 101: Obtain raw steel.

[0023] Specifically, the composition of the raw material steel is as follows: C: 0.40-0.65 wt%; Si: 1.2-1.8 wt%; Mn: 1.5-2.5 wt%; Cr: 0-0.8 wt%; V: 0.04-0.1 wt%; with the balance being Fe.

[0024] Step 102: Austenitize the raw steel. Specifically, heat the raw steel to above the upper transformation temperature Ac3 to perform austenitization.

[0025] Step 103: After austenitization, cool to the bainite transformation temperature BT and perform the first stage of isothermal treatment. The bainite transformation temperature BT is lower than the bainite transformation initiation temperature Bs.

[0026] Step 104: After the first stage of isothermal treatment, cool to 20-200℃ below the martensitic transformation start temperature Ms to introduce martensitic structure, referred to as martensite.

[0027] Specifically, in the first stage of isothermal treatment, when the transformation amount of bainitic ferrite is 30-70%, the temperature is cooled to 20-200°C below the martensitic transformation initiation temperature. More specifically, the transformation amount of bainitic ferrite in the first stage of isothermal treatment is determined by the isothermal treatment duration, which is 10-240 minutes. The correlation between the transformation amount and the isothermal treatment duration can be determined through specific tests.

[0028] Step 105: Reheat to the bainite transformation temperature for the second stage of isothermal treatment.

[0029] The second stage of isothermal treatment takes 30-60 minutes. The treatment temperatures for the first and second stages of isothermal treatment can be the same, allowing the same heat treatment furnace to be used in industrial applications, thus shortening the traditional multi-step heat treatment process.

[0030] Step 106: After the second stage of isothermal treatment, cool to room temperature to obtain nanostructured bainitic steel.

[0031] Martensite is introduced between the first and second isothermal treatment stages. This introduction of martensite into the untransformed austenite blocks disrupts the bainite transformation, accelerating the bainite transformation rate in the second isothermal treatment. This significantly shortens the heat treatment time and refines the bainite microstructure. The initially formed bainite microstructure acts as a flexible phase, stabilizing a portion of the thin-film austenite and absorbing the phase transformation distortion caused by the martensite formed in the intermediate stage. This avoids cracking in high-carbon steel during ordinary quenching or quench-partitioning processes, thus preventing quenching cracks. The second-stage martensite increases the dislocation density of the first-stage bainite, providing confinement and strengthening.

[0032] The obtained nanostructured bainitic steel has a tensile strength of 1400-1700 MPa, an elongation after fracture of 25-35%, a strength-ductility product (strength-ductility product) of 40-49 GPa%, and a yield strength of 800-1200 MPa. It has good application prospects in the fields of automotive steel, wear-resistant steel, and rail steel.

[0033] The specific preparation method includes the following steps:

[0034] Step 201: Produced into raw steel through conventional smelting, forging, or rolling.

[0035] Step 202: Obtain the bainite transformation kinetic curve of the raw steel.

[0036] The bainitic transformation kinetic curve can be obtained by testing the temperature-time transformation curve of the raw steel. The bainitic transformation kinetic curve includes the following parameters: upper transformation temperature, bainitic transformation temperature range, and martensitic transformation onset temperature; wherein, the bainitic transformation temperature range includes: bainitic transformation onset temperature and bainitic transformation temperature.

[0037] Step 203: Austenitize the raw steel at 880℃.

[0038] Step 204: After austenitization, cool to 340-360℃ for the first stage of isothermal treatment, with an isothermal time of 20-240 minutes, so that the austenite in the steel is transformed into 30-70% bainitic ferrite, and the remainder is untransformed austenite.

[0039] Step 205: After the first stage of isothermal treatment, cool to 20-200℃ below the martensitic transformation start temperature;

[0040] Step 206: Heat to 340-360℃ for the second stage of isothermal treatment for 30-60 minutes;

[0041] Step 207: Cool to room temperature to obtain nanostructured bainitic steel.

[0042] Example 1

[0043] The preparation method of nanostructured bainitic steel includes the following steps:

[0044] 1) The raw steel is produced by conventional smelting, forging or rolling, and the raw steel contains the following components and mass percentages: C: 0.42 wt%; Si: 1.3 wt%; Mn: 2.0 wt%; Cr: 0.6 wt%; V: 0.7 wt%.

[0045] 2) Test the temperature-time-transformation curve of the above raw steel to determine the bainite transformation temperature range and the bainite transformation kinetic curve at different temperatures.

[0046] 3) Austenitize the raw steel at 880℃ for 45 minutes.

[0047] 4) Cool the steel obtained in step 3) to 360°C for the first stage of isothermal treatment for 10 minutes, so that the austenite in the steel is transformed into 30% bainitic ferrite, and the remainder is untransformed austenite.

[0048] 5) Using a quenching expansion tester, the martensitic transformation start temperature of the untransformed austenite was tested to be 240℃. The steel obtained in step 4) was cooled to 160℃.

[0049] 6) The steel obtained in step 5) is reheated to 360°C for a second stage of isothermal treatment for 60 minutes, and then cooled to room temperature to obtain nanostructured bainitic steel, denoted as product 1.

[0050] Example 2

[0051] The preparation method of nanostructured bainitic steel includes the following steps:

[0052] 1) The raw steel is produced by conventional smelting, forging or rolling, and the raw steel contains the following components and mass percentages: C: 0.45wt%; Si: 1.2wt%; Mn: 1.8wt%; Cr: 0.6wt%; V: 0.7wt%.

[0053] 2) Test the temperature-time-transformation curve of the above raw steel to determine the bainite transformation temperature range and the bainite transformation kinetic curve at different temperatures.

[0054] 3) Austenitize the raw steel at 880℃ for 45 minutes.

[0055] 4) Cool the steel obtained in step 3) to 360°C for the first stage of isothermal treatment for 20 minutes, so that the austenite in the steel is transformed into 50% bainitic ferrite, and the remainder is untransformed austenite.

[0056] 5) Using a quenching expansion tester, the martensitic transformation start temperature of the untransformed austenite was tested to be 230℃. The steel obtained in step 4) was cooled to 140℃.

[0057] 6) The steel obtained in step 5) is reheated to 360°C for a second stage of isothermal treatment for 45 minutes, and then cooled to room temperature to obtain nanostructured bainitic steel, denoted as product 2.

[0058] Example 3

[0059] The preparation method of nanostructured bainitic steel includes the following steps:

[0060] 1) The raw steel is produced by conventional smelting, forging or rolling, and the raw steel contains the following components and mass percentages: C: 0.40%; Si: 1.4%; Mn: 1.5%; Cr: 0.55%; V: 0.05 wt%.

[0061] 2) Test the temperature-time-transformation curve of the above raw steel to determine the bainite transformation temperature range and the bainite transformation kinetic curve at different temperatures;

[0062] 3) The raw steel is austenitized at 880℃ for 45 minutes;

[0063] 4) Cool the steel obtained in step 3) to 360°C for the first stage of isothermal treatment for 45 minutes, so that the austenite in the steel is transformed into 65% bainitic ferrite, and the remainder is untransformed austenite.

[0064] 5) The martensitic transformation initiation temperature of the untransformed austenite was tested using a quenching expansion tester and found to be 220°C. The steel obtained in step 4) was then cooled to 120°C.

[0065] 6) The steel obtained in step 5) is reheated to 360℃ for a second stage of isothermal treatment for 45 minutes, then cooled to room temperature to obtain nanostructured bainitic steel, denoted as Product 3. The microstructure of Product 3 is as follows: Figure 3 As shown, B represents bainite.

[0066] Example 4

[0067] The preparation method of nanostructured bainitic steel includes the following steps:

[0068] 1) The raw steel is produced by conventional smelting, forging or rolling, and the raw steel contains the following components and mass percentages: C: 0.45 wt%; Si: 1.4 wt%; Mn: 1.8 wt%; Cr: 0.6 wt%; V: 0.1 wt%.

[0069] 2) Test the temperature-time-transformation curve of the above raw steel to determine the bainite transformation temperature range and the bainite transformation kinetic curve at different temperatures.

[0070] 3) Austenitize the raw steel at 880℃ for 45 minutes.

[0071] 4) Cool the steel obtained in step 3) to 360°C for the first stage of isothermal treatment for 75 minutes, so that the austenite in the steel is transformed into 70% bainitic ferrite, and the remainder is untransformed austenite.

[0072] 5) Using a quenching expansion tester, the martensitic transformation start temperature of the untransformed austenite was tested to be 210℃. The steel obtained in step 4) was then cooled to 100℃.

[0073] 6) The steel obtained in step 5) is reheated to 360℃ for a second stage of isothermal treatment for 30 minutes, then cooled to room temperature to obtain nanostructured bainitic steel, denoted as Product 4. The microstructure of Product 4 is as follows: Figure 4 As shown, B represents bainite.

[0074] Example 5

[0075] The preparation method of nanostructured bainitic steel includes the following steps:

[0076] 1) The raw steel is produced by conventional smelting, forging or rolling, and the raw steel contains the following components and mass percentages: C: 0.65wt%; Si: 1.5wt%; Mn: 2.4wt%; Cr: 0.5wt%; V: 0.6wt%.

[0077] 2) Test the temperature-time-transformation curve of the above-mentioned raw steel, such as... Figure 5 As shown, the bainite transformation temperature range and the bainite transformation kinetic curves at different temperatures are determined. Figure 5 The transformation curves are shown for transformation variables ranging from 10% to 50%.

[0078] 3) Austenitize the raw steel at 880℃ for 45 minutes.

[0079] 4) Cool the steel obtained in step 3) to 340°C for the first stage of isothermal treatment for 120 minutes, so that the austenite in the steel is transformed into 50% bainitic ferrite, and the remainder is untransformed austenite.

[0080] 5) Using a quenching expansion tester, the martensitic transformation start temperature of the untransformed austenite was tested to be 125℃. The steel obtained in step 4) was cooled to 20℃.

[0081] 6) The steel obtained in step 5) is reheated to 340°C for a second stage of isothermal treatment for 60 minutes, and then cooled to room temperature to obtain nanostructured bainitic steel, which is denoted as product 5.

[0082] Example 6

[0083] The preparation method of nanostructured bainitic steel includes the following steps:

[0084] 1) The raw steel is produced by conventional smelting, forging or rolling, and the raw steel contains the following components and mass percentages: C: 0.65wt%; Si: 1.8wt%; Mn: 2.5wt%; Cr: 0.4wt%; V: 0.1wt%.

[0085] 2) Test the temperature-time-transformation curve of the above raw steel to determine the bainite transformation temperature range and the bainite transformation kinetic curve at different temperatures.

[0086] 3) Austenitize the raw steel at 880℃ for 45 minutes.

[0087] 4) Cool the steel obtained in step 3) to 340°C for the first stage of isothermal treatment for 240 minutes, so that the austenite in the steel is transformed into 60% bainitic ferrite, and the remainder is untransformed austenite.

[0088] 5) Using a quenching expansion tester, the martensitic transformation start temperature of the untransformed austenite was tested to be 110℃. The steel obtained in step 4) was cooled to 20℃.

[0089] 6) The steel obtained in step 5) is reheated to 340℃ for a second stage of isothermal treatment for 60 minutes, then cooled to room temperature to obtain nanostructured bainitic steel, denoted as product 6. The microstructure of product 3 is as follows: Figure 6 As shown, B represents bainite, M represents martensite, and RA represents retained austenite.

[0090] Comparative Example 1

[0091] Unlike Example 1, in step 4), after the steel underwent the first stage of isothermal treatment at 360°C, it was cooled to room temperature for 120 minutes, and this was designated as Comparative Product 1.

[0092] Comparative Example 2

[0093] Unlike Example 5, in step 4), after the steel underwent the first stage of isothermal treatment at 340°C, it was cooled to room temperature for 120 minutes, and this was designated as Comparative Product 2.

[0094] Comparative Example 3

[0095] Unlike Example 6, in step 4), after the steel was subjected to the first stage of isothermal treatment at 340°C, it was cooled to room temperature for 240 minutes, and this was designated as Comparative Product 3.

[0096] Comparative Example 4

[0097] Unlike Example 6, only the steel was subjected to a first stage of isothermal treatment at 340°C and then cooled to room temperature for 480 minutes, which was designated as Comparative Product 4.

[0098] Using a universal tensile testing machine and standard tensile specimens, the mechanical properties of the specimens in each embodiment and comparative example were determined according to the provisions of the national standard GB / T228.1-2021. The performance range is shown in Table 1.

[0099] Table 1

[0100]

[0101]

[0102] This invention, based on bainitic transformation kinetics, involves isothermally cooling austenitized steel to the bainitic transformation temperature BT to form a certain amount of bainitic ferrite. The steel is then cooled to a temperature below the martensitic transformation initiation temperature of the untransformed austenite to form a certain amount of martensite, thus segmenting and refining the untransformed austenite blocks. Simultaneously, the formed martensitic structure accelerates the second-stage bainitic transformation and refines the bainitic ferrite formed in this stage. The isothermal temperatures of the first and second stages are the same, and a "temperature interference" exists between them. This temperature interference forms a certain amount of martensite, ultimately accelerating the bainitic transformation and refining the bainitic structure.

[0103] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

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