Multi-graded high specific strength and ductility medium manganese steel based on friction stir processing and preparation method thereof

Abstract

The application relates to the field of high-strength steel plate production and provides a multi-gradient high-strength plastic product medium manganese steel based on friction stir processing and a preparation method, the method comprising the following steps: S1, smelting, forging and rolling according to predetermined medium manganese steel alloy components to obtain an initial slab; S2, annealing the initial slab in a two-phase zone to obtain an annealed slab; and S3, performing friction stir processing on the annealed slab, wherein the rotating speed of a stirring head is 50-200 rpm, the processing speed is 50-150 mm / min, the reduction is 0.05-0.3 mm, and the pin inclination angle is 2-4 DEG. Through rolling, two-phase zone annealing and then friction stir processing, the austenite, grain size and KAM in the medium manganese steel product are distributed in a gradient from top to bottom along the thickness direction, compared with traditional homogeneous medium manganese steel with the same alloy components, the tensile strength is greatly improved without significantly reducing the plasticity, and the strength-plasticity product is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel plate production, and in particular to a multi-gradient high-strength, high-ductility manganese steel based on friction stir processing and its preparation method. Background Technology

[0002] With the development of the automotive manufacturing industry, resource and energy shortages and environmental problems pose significant challenges. Therefore, improving fuel efficiency and energy conservation and emission reduction have attracted widespread attention in the automotive industry, while lightweighting of vehicles while ensuring passenger safety has become a future trend. Based on this trend, researchers have developed third-generation high-strength automotive steel. Medium-manganese steel, as a representative of this generation, has gained widespread attention for its low alloy composition and excellent mechanical properties. Medium-manganese steel is mainly composed of approximately 3–12% Mn, less than 0.4% C, and low concentrations of additional alloying elements (such as Al, Si, and V). After hot rolling, it typically exhibits a fully martensitic structure, while after critical annealing, it develops a dual-phase microstructure of ultrafine α-ferrite and retained austenite. During deformation, a transformation-induced plasticity (TRIP) effect occurs, resulting in high strength and high plasticity. When a vehicle is subjected to impact, the high strength-ductility product of the automotive steel can absorb more energy, ensuring the safety of the occupants. Currently, further increasing strength mainly involves adding alloying elements. Adding alloying elements increases the weight of the material and raises costs. To ensure passenger safety and meet the requirements of lightweight vehicles, it is crucial to address the key issue of increasing the strength-ductility product of medium-manganese steel.

[0003] In recent years, gradient structure materials have gradually gained attention due to their ability to improve mechanical properties to a certain extent due to their special structure. The invention patent "A High-Strength, High-Duty Medium-Manganese Steel with Gradient Structure and Its Preparation Method" (authorization announcement number CN114480811A) discloses a process of torsion treatment followed by two-phase annealing of hot-rolled medium-manganese steel in the two-phase region to create an austenite content gradient and improve the overall material performance. However, this invention requires post-processing two-phase annealing, making the process relatively more complex, and it only considers the austenite content gradient structure, resulting in a single gradient structure. The invention patent "A Composite Gradient Structure Medium-Manganese Steel and Its Preparation Method" (CN114480808A) discloses a torsion treatment of hot-rolled medium-manganese steel in the two-phase region to prepare a novel medium-manganese steel with a composite gradient structure where phase transformation martensite, austenite, and ferrite, grain size, and dislocation density all exhibit a gradient distribution from the surface to the interior, thus improving the overall material performance. However, this invention uses a torsion process, which is only applicable to rod-shaped materials and has limited application to thickness gradients in plate-shaped materials. The invention patent "Preparation Method of High-Strength Steel with Austenite Content Gradient Based on Friction Stir Processing" (Authorization Announcement No. CN115058571A) discloses a method for preparing high-strength steel with austenite content using friction stir processing. This method creates a temperature gradient along the thickness direction of the steel plate, thereby producing an austenite content gradient. However, this invention only considers the austenite content gradient, resulting in a single gradient structure, and the changes in performance after processing are not clearly discussed.

[0004] Based on the above problems, this invention prepares a novel medium-manganese steel with a multi-gradient structure in terms of microstructure, grain size, and dislocation density in the thickness direction, which is not limited to the austenite content gradient, but also improves the comprehensive performance of the material. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a multi-gradient high strength-ductility-product medium-manganese steel based on friction stir processing and its preparation method. By utilizing two-phase annealing and friction stir processing, a novel microstructure feature is achieved in the matrix, where the microstructure, grain size, dislocation density, etc., obtained in traditional medium-manganese steel exhibit a gradient distribution, thereby improving the strength-ductility-product of medium-manganese steel.

[0006] The present invention adopts the following technical solution:

[0007] On one hand, the present invention provides a method for preparing high-strength, high-ductility manganese steel with a multi-gradient structure based on friction stir processing, comprising:

[0008] S1. The initial slab is obtained by smelting, forging and rolling according to the predetermined medium manganese steel alloy composition.

[0009] S2. Anneal the two-phase region of the initial slab to obtain the annealed slab;

[0010] S3. Set the friction stir processing parameters and perform friction stir processing on the annealed slab to obtain the multi-gradient high-strength and high-ductility manganese steel.

[0011] In addition to any of the possible implementations described above, another implementation is provided in which, in step S1, the chemical composition of the medium manganese steel alloy, by mass percentage, is: C: 0.1-0.4%, Mn: 3-7%, with the balance being Fe and unavoidable impurities.

[0012] It should be noted that there are no special requirements for the composition of medium manganese steel in this invention. As long as the billet matrix contains austenite that has undergone reverse transformation and the austenite has high stability, the strength-ductility product of the steel can be improved by rolling, two-phase annealing, and friction stirring.

[0013] In addition to any of the possible implementations described above, another implementation is provided in which the chemical composition of the medium manganese steel alloy, by mass percentage, is: C: 0.12%, Mn: 4.5%, with the balance being Fe and unavoidable impurities.

[0014] In addition to any of the possible implementations described above, another implementation is provided in which, in step S1, a steel ingot is obtained by smelting, the steel ingot is heated to 1200°C and held for 2 hours, then forged into a 30mm thick steel ingot, then air-cooled to room temperature, heated to 1200°C and held for 2 hours, then hot-rolled into a 6mm thick steel plate, and then water-cooled to room temperature to obtain an initial slab; in step S2, the initial slab is reheated to 650°C and held for 6 hours, and then air-cooled to room temperature.

[0015] In addition to any of the possible implementations described above, another implementation is provided in which the friction stir processing parameters in step S3 are: stirring head rotation speed of 50-200 rpm, processing speed of 50-150 mm / min, pressing amount of 0.05-0.3 mm, and pin tilt angle of 2-4°.

[0016] In addition to any of the possible implementations described above, another implementation is provided in which the friction stir processing parameters in step S3 are: stirring head rotation speed of 150 rpm, processing speed of 150 mm / min, pressing amount of 0.2 mm, and pin tilt angle of 2.5°.

[0017] For medium manganese steel, excessively low stirring head speed and processing speed, coupled with low processing temperature, prevent proper plastic flow of the metal, resulting in numerous defects in the matrix. Conversely, excessively high stirring head speed and processing speed, combined with higher temperature, lead to a gradual increase in grain size. Heat transfer along the thickness direction causes austenite transformation, preventing the formation of an effective grain size gradient and ultimately reducing performance. The aforementioned process parameters enable friction stir processing of medium manganese steel without shrinkage cavities or cracks. Temperature control is crucial during friction stir processing, maintaining a low temperature (<1000℃). Excessively high temperatures result in larger grain sizes, hindering the formation of a grain size gradient and reducing the synergistic effect of strength and plasticity in the material.

[0018] In addition to any of the possible implementations described above, another implementation is provided in which, in step S2, after the two-phase region annealing, the surface of the initial slab is polished with sandpaper, rinsed with alcohol, and dried.

[0019] In addition to any of the possible implementations described above, a further implementation is provided in which the processing temperature of the friction stir process in step S3 does not exceed a certain degree.

[0020] On the other hand, the present invention also provides a multi-gradient high-strength plasticity manganese steel, which is obtained by the above-mentioned preparation method of multi-gradient high-strength plasticity manganese steel based on friction stirring.

[0021] In addition to any of the possible implementations described above, another implementation is provided in which the manganese steel in the multi-gradient high-strength plastic product has a phase gradient, a grain size gradient, and a KAM gradient.

[0022] In this invention, friction stirring after annealing in the two-phase region of medium manganese steel can achieve a synergistic effect of multiple structural gradients, including the austenite content gradient. Compared with direct friction stirring after rolling, which does not produce an austenite content gradient due to the high processing temperature, the gradient between the hard and soft regions in the thickness direction is reduced, and the synergistic effect of multiple gradients is weakened. This results in a reduction in the additional HDI strengthening and hardening, thereby reducing the overall strength and plasticity of the material.

[0023] This invention involves hot-rolling and annealing a steel billet to produce an austenite + ferrite two-phase microstructure. Then, friction stir processing is used to induce different martensitic phase transformations in the austenite at different positions along the thickness direction, resulting in a gradient structure of martensite and austenite phases. Furthermore, due to friction stir, the dislocation density and grain size of the sample change from top to bottom, creating a composite gradient structure. During deformation, in addition to the synergistic effects of TRIP effect, grain refinement, and dislocation strengthening, the formation of distinct soft and hard regions along the thickness direction due to the multiple gradients, with a significant gradient degree, results in substantial additional HDI strengthening and hardening. This allows the material to maintain high plasticity while significantly increasing strength, thereby improving the strength-ductility product of medium manganese steel.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. The medium-manganese steel with a multi-gradient structure prepared by this invention mainly includes phase gradient (austenite, transformed martensite), grain size gradient, and KAM gradient. During deformation, this novel medium-manganese steel exhibits a synergistic effect of TRIP effect, strain gradient effect (HDI strengthening), dislocation strengthening effect, and grain refinement strengthening effect, thus significantly improving the strength-ductility product of the medium-manganese steel simultaneously.

[0026] 2. The preparation method of the present invention is simple and easy to implement, does not require changes to the composition or addition of components, the process is relatively simple, the cycle is short, and it is highly operable. Attached Figure Description

[0027] Figure 1 The diagram shows a schematic of the preparation of multi-gradient high-strength, high-ductility hot-rolled manganese steel by friction stir processing.

[0028] Figure 2 The image shown is an EBSD image of different regions in the thickness direction of the manganese steel in the multi-gradient structure prepared in the example.

[0029] Figure 3 The figure shows the hardness distribution curves from the upper surface to the lower surface of the medium manganese steel with multi-gradient structure and the homogeneous medium manganese steel prepared in the examples.

[0030] Figure 4 The figure shown is a comparison of the engineering stress-strain curves of medium-manganese steel with a multi-gradient structure prepared in the example and medium-manganese steel with the same homogeneous structure.

[0031] Figure 5 The figure shown is a comparison of the engineering stress-strain curves of the medium manganese steel with a multi-gradient structure prepared in the example and the medium manganese steel that was directly subjected to friction stir processing without annealing.

[0032] Figure 6The diagram shown is a flowchart illustrating a method for preparing high-strength, high-ductility manganese steel based on friction stir processing according to an embodiment of the present invention. Detailed Implementation

[0033] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that the technical features or combinations of technical features described in the following embodiments should not be considered in isolation, but can be combined with each other to achieve better technical effects.

[0034] like Figure 6 As shown in the figure, an embodiment of the present invention provides a method for preparing multi-gradient high-strength-ductility manganese steel based on friction stir processing, comprising:

[0035] S1. The initial slab is obtained by smelting, forging and rolling according to the predetermined medium manganese steel alloy composition.

[0036] S2. Anneal the two-phase region of the initial slab to obtain the annealed slab;

[0037] S3. Set the friction stir processing parameters and perform friction stir processing on the annealed slab to obtain the multi-gradient high-strength and high-ductility manganese steel.

[0038] In one specific embodiment, in step S1, the chemical composition of the medium manganese steel alloy, by mass percentage, is: C: 0.1-0.4%, Mn: 3-7%, with the balance being Fe and unavoidable impurities.

[0039] In one specific embodiment, the chemical composition of the medium manganese steel, by mass percentage, is: C: 0.12%, Mn: 4.5%, with the balance being Fe and unavoidable impurities.

[0040] In one specific embodiment, in step S1, a steel ingot is obtained by smelting, the steel ingot is heated to 1200°C and held for 2 hours, then forged into a 30mm thick steel ingot, then air-cooled to room temperature, heated to 1200°C and held for 2 hours, then hot-rolled into a 6mm thick steel plate, and then water-cooled to room temperature to obtain an initial slab; in step S2, the initial slab is reheated to 650°C and held for 6 hours, and then air-cooled to room temperature.

[0041] In one specific embodiment, in step S3, the stirring friction processing parameters are: stirring head rotation speed of 50-200 rpm, processing speed of 50-150 mm / min, pressing amount of 0.05-0.3 mm, and pin tilt angle of 2-4°.

[0042] In one specific embodiment, in step S3, the parameters for friction stir processing are: stirring head rotation speed of 150 rpm, processing speed of 150 mm / min, pressing amount of 0.2 mm, and pin tilt angle of 2.5°.

[0043] In one specific embodiment, in step S2, after the two-phase region is annealed, the surface of the initial slab is polished with sandpaper, rinsed with alcohol, and dried.

[0044] In one specific embodiment, in step S3, the processing temperature of friction stir processing is not higher than 1000 degrees.

[0045] This invention provides a multi-gradient high-strength manganese steel, obtained using the above-described method for preparing multi-gradient high-strength manganese steel based on friction stir processing.

[0046] In one specific embodiment, the manganese steel in the multi-gradient high-strength plastic product has a phase gradient, a grain size gradient, and a KAM gradient.

[0047] Example 1

[0048] Medium manganese steel is smelted, forged, rolled, and annealed in the two-phase region according to a predetermined alloy composition to obtain an annealed plate, which is then subjected to friction stir processing. The processing procedure is as follows: Figure 1 As shown, the parameters for the preparation and processing of medium manganese steel annealed plates are all preferred parameters:

[0049] The chemical composition of medium manganese steel alloy, by mass percentage, is: C: 0.12%, Mn: 4.5%, with the balance being Fe and unavoidable impurities.

[0050] The steel ingot is heated to 1200℃ and held for 2 hours, then forged into a 30mm thick steel ingot, then air-cooled to room temperature, heated to 1200℃ and held for 2 hours, then hot-rolled into a 6mm thick steel plate, then water-cooled to room temperature, then heated to 650℃ and held for 6 hours, and then air-cooled to room temperature.

[0051] The parameters for friction stir machining are: stirring head rotation speed of 150 rpm, machining speed of 150 mm / min, pressing amount of 0.2 mm, and pin tilt angle of 2.5°.

[0052] The microstructure of the manganese steel in the gradient structure prepared in this embodiment, from the upper surface to the lower surface, is shown in the diagram. Figure 2 As shown. The left side shows the EBSD phase diagram for each region, with black representing ferrite and white representing austenite. Along the thickness direction, the austenite content from top to bottom is 0.4% and 2.3%, respectively. The grain sizes from top to bottom are 2.66 μm and 12.25 μm, respectively. The right side shows the corresponding KAM diagram for each region. Along the thickness direction, the KAM values ​​from top to bottom are 0.62° and 1°, respectively. The hardness distribution of manganese steel along the thickness direction in the gradient structure is shown below. Figure 3 As shown, the hardness gradually decreases from top to bottom. The engineering stress-strain curve of manganese steel in the gradient structure is as follows. Figure 4 As shown, the tensile strength is 1035 MPa, the elongation is 35.06%, and the strength-ductility product is 36.29 GPa·s.

[0053] Comparative Example 1

[0054] The alloy composition of the medium-manganese steel in this comparative example is the same as that in Example 1, but the preparation method is different. The main difference is that the stirring friction processing was not performed on the basis of the example. The remaining steps S1 and S2 are the same as those in the example, and a homogeneous medium-manganese steel is finally obtained.

[0055] The engineering stress-strain curve of manganese steel in the homogeneous structure of this comparative example is shown below. Figure 4 As shown, the tensile strength is 650 MPa, the elongation is 38.27%, and the strength-ductility product is 24.88 GPa·%. Its ductility is slightly lower than that of manganese steel in the gradient structure of this invention, but its tensile strength and strength-ductility product are significantly lower than those of manganese steel in the gradient structure of this invention.

[0056] The comparison shows that, compared with the homogeneous manganese steel in Comparative Example 1, the manganese steel in the gradient structure of Example 1 has a 59.2% increase in tensile strength, a slightly lower elongation, and a 45.9% increase in strength-ductility product.

[0057] Comparative Example 2

[0058] The alloy composition of the medium manganese steel in this comparative example is the same as that in Example 1, but the preparation method is different. The main difference is that the steel is subjected to friction stirring without annealing. The remaining steps S1 and S3 are the same as in the example, and the medium manganese steel that has been subjected to friction stirring without annealing is finally obtained.

[0059] The engineering stress-strain curve of manganese steel in the homogeneous structure of this comparative example is shown below. Figure 5 As shown, the tensile strength is 1118 MPa, the elongation is 25.65%, and the strength-ductility product is 18.68 GPa·%. Its tensile strength is slightly higher than that of manganese steel in the gradient structure of this invention, but its plasticity and strength-ductility product are significantly reduced.

[0060] The comparison shows that the medium manganese steel with gradient structure in Example 1 has slightly lower tensile strength, higher elongation by 36.7%, and higher strength-ductility product by 26.5% compared with the medium manganese steel in Comparative Example 2 that was not annealed and subjected to friction stir processing.

[0061] While embodiments of the present invention have been provided herein, those skilled in the art should understand that modifications can be made to the embodiments without departing from the spirit of the invention. The above embodiments are merely exemplary and should not be construed as limiting the scope of the invention.

Claims

1. A method for preparing multi-gradient high-strength, high-ductility manganese steel based on friction stir processing, characterized in that, The method includes: S1. The initial slab is obtained by smelting, forging, and rolling according to the predetermined medium manganese steel alloy composition. S2. Anneal the initial slab in the two-phase region to produce an austenite + ferrite two-phase structure, and obtain the annealed slab. S3. Using friction stir processing, the annealed slab is subjected to friction stir processing to form a multi-gradient structure from top to bottom in the thickness direction. The process parameters of friction stir processing are: stirring head speed of 50-200 rpm, processing speed of 50-150 mm / min, reduction of 0.05-0.3 mm, tilt angle of 2-4°, and the processing temperature of friction stir processing is not higher than 1000℃, thus obtaining the multi-gradient high-strength and high-ductility manganese steel.

2. The method for preparing multi-gradient high-strength, high-ductility manganese steel based on friction stir processing as described in claim 1, characterized in that, In step S1, the chemical composition of the medium manganese steel alloy, by mass percentage, is: C: 0.1~0.4%, Mn: 3~7%, with the balance being Fe and unavoidable impurities.

3. The method for preparing multi-gradient high-strength, high-ductility manganese steel based on friction stir processing as described in claim 2, characterized in that, The chemical composition of the medium manganese steel alloy, by mass percentage, is: C: 0.12%, Mn: 4.5%, with the balance being Fe and unavoidable impurities.

4. The method for preparing multi-gradient high-strength, high-ductility manganese steel based on friction stir processing as described in claim 1, characterized in that, In step S1, steel ingots are obtained by smelting. The steel ingots are heated to 1200℃ and held for 2 hours, then forged into 30mm thick steel ingots, and then air-cooled to room temperature. After being heated to 1200℃ and held for 2 hours, they are hot-rolled into 6mm thick steel plates, and then water-cooled to room temperature to obtain the initial slab. In step S2, the initial slab is reheated to 650℃ and held for 6 hours, and then air-cooled to room temperature.

5. The method for preparing multi-gradient high-strength, high-ductility manganese steel based on friction stir processing as described in claim 1, characterized in that, In step S3, the parameters for friction stir processing are: stirring head rotation speed of 150 rpm, processing speed of 150 mm / min, pressing amount of 0.2 mm, and tilt angle of 2.5°.

6. The method for preparing multi-gradient high-strength, high-ductility manganese steel based on friction stir processing as described in claim 1, characterized in that, In step S2, after the two-phase region is annealed, the surface of the annealed slab is polished with sandpaper, rinsed with alcohol, and dried.

7. A multi-gradient high-strength, high-ductility medium-manganese steel, characterized in that, The multi-gradient high-strength plastic volume manganese steel is obtained using the preparation method of multi-gradient high-strength plastic volume manganese steel based on friction stir processing as described in any one of claims 1-6.

8. The multi-gradient high-strength, high-ductility manganese steel as described in claim 7, characterized in that, The multi-gradient high-strength ductile steel has phase gradient, grain size gradient and KAM gradient.

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