High-strength high-plasticity light-weight hot-rolled medium-manganese steel and preparation method thereof

Abstract

The application relates to the technical field of metal materials, in particular to a high-strength high-plasticity light hot-rolled medium-manganese steel and a preparation method thereof. The chemical components of the steel are as follows in percentage by weight: C: 0.2%-0.3%, Si: 0.1%-0.2%, Mn: 8%-10%, Al: 4%-5%, Cu: 0.2%-0.4%, P<=0.01%, S<=0.01%, total amount of other inevitable impurity elements <=0.05%, and the rest is Fe. The steel blank is heated to 1050-1150 DEG C and kept; then hot rolling is carried out, and the final rolling temperature is controlled to be 680-700 DEG C; after rolling, the steel plate is air-cooled to 540-580 DEG C and coiled, and no quenching, tempering or annealing treatment is needed after coiling. Excellent comprehensive performance of tensile strength >=1500 MPa, elongation >=25%, density 7.1-7.3 g / cm 3 is achieved. The process is simple, energy-saving, cost-reducing, and particularly suitable for automobile light-weight structural parts.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, specifically to a high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel and its preparation method. Background Technology

[0002] Since the beginning of the 21st century, with the rapid development of the global automotive industry, especially the new energy vehicle industry, the performance requirements for automotive steel have expanded from traditional strength and plasticity to comprehensive indicators such as low density (lightweight) and high vibration damping. Medium manganese steel, due to its excellent synergistic potential in strength and plasticity, has become one of the important research and development directions for automotive lightweight steel.

[0003] Currently, extensive research has been conducted both domestically and internationally on lightweight medium-manganese steel, primarily focusing on achieving target properties through adjusting alloy composition and optimizing heat treatment processes. For example: Chinese patent application CN111996465A discloses "A hot-rolled ultra-high strength lightweight medium-manganese steel plate for automobiles and its preparation method." This method employs a composition design with high carbon (0.75%~0.80%), high manganese (7.6%~8.0%), and high aluminum (7.4%~8.0%), and obtains a multiphase microstructure through a quenching and tempering heat treatment process. While this method can achieve high strength (tensile strength 1410~1680MPa) and certain plasticity (elongation 15%~30%), its excessively high carbon content may deteriorate the cold working and welding performance of the steel; excessively high aluminum content easily leads to the formation of coarse δ-ferrite, impairing overall performance; and it must rely on complex heat treatment processes (quenching + tempering), increasing production costs and energy consumption.

[0004] Chinese patent application CN119120860A discloses a "high-strength, multi-gradient lightweight cold-rolled medium-manganese steel and its preparation method." This technology employs a cold rolling process and requires multiple two-phase annealing processes and a final friction stir process to construct a gradient structure in the thickness direction. This method has an extremely lengthy process route, involving cold rolling, annealing, and special processing, resulting in a long production cycle and high costs, making it difficult to meet the efficiency and economic requirements of large-scale industrial production.

[0005] Chinese patent application CN117248159A discloses "A vanadium microalloyed lightweight high-strength steel and its preparation method." This technology improves performance by adding the microalloying element V and employing warm rolling and subsequent heat treatment processes. However, its problems are: the addition of the expensive microalloying element (V) increases raw material costs; the obtained strength level (tensile strength 790~870MPa) is significantly lower than the target of this invention (≥1500MPa); and its process still includes warm rolling and specialized heat treatment steps, making it insufficiently simplified.

[0006] In summary, existing lightweight medium-manganese steels may have problems with machinability, weldability, and microstructure control due to their alloy composition design (such as excessively high C and Al content), or their manufacturing process must rely on subsequent quenching, tempering, fractional annealing, and even complex cold rolling and special processing, resulting in complex production processes, high energy consumption, and high costs. Summary of the Invention

[0007] To overcome the shortcomings of the prior art, the present invention provides a high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel and its preparation method. This medium-manganese steel has a reasonable composition design, can effectively suppress harmful phases, and can be directly obtained in the hot-rolled state without complex subsequent heat treatment, exhibiting high strength, high plasticity, and low density.

[0008] To achieve the above objectives, the present invention employs the following technical solution: A high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel is composed of the following chemical composition by weight percentage: C: 0.2%~0.3%, Si: 0.1%~0.2%, Mn: 8%~10%, Al: 4%~5%, Cu: 0.2%~0.4%, P≤0.01%, S≤0.01%, other unavoidable impurity elements total ≤0.05%, the remainder is Fe.

[0009] The effect of selecting the above alloying elements and their contents: Carbon (C) is the most important solid solution strengthening element in steel, expanding the austenite phase region and inhibiting the formation of δ-ferrite. However, excessively high C content can deteriorate the cold working and weldability of steel. Considering the overall properties of steel, this invention adopts a medium carbon content of 0.2% to 0.3%.

[0010] Mn is not only a major solid solution strengthening element, but it can also expand the austenite phase region, inhibit the formation of δ-ferrite, and increase the content of room-temperature austenite in steel, thereby improving the steel's plasticity. However, excessively high Mn content increases production difficulty and costs. Therefore, this invention precisely controls the Mn content to 8%~10%.

[0011] Al significantly reduces the density of steel and also has a solid solution strengthening effect. However, excessive aluminum will form coarse δ-ferrite, which will worsen the overall properties of the steel. Generally speaking, the formation of δ-ferrite is inevitable when the aluminum content exceeds 5%. Therefore, this invention precisely controls the Al content to 4%~5%.

[0012] Cu is a chemically stable element that can improve the thermal conductivity of steel. Like carbon (C), it also expands the austenite phase region, inhibits the formation of δ-ferrite, and improves the weldability of steel. Therefore, this invention precisely controls the Cu content to 0.2%~0.4%.

[0013] Si is a common element in steel, possessing solid solution strengthening properties and inhibiting cementite formation (indirectly inhibiting δ-ferrite formation). However, increasing the Si content increases the brittleness of the steel and deteriorates its surface quality. Therefore, this invention precisely controls the Si content to 0.1%~0.2%.

[0014] Impurity elements such as sulfur (S) and phosphorus (P) can impair the performance of steel in various ways, and their content should be controlled below 0.01%.

[0015] The above-mentioned steel has a tensile strength ≥1500MPa, an elongation after fracture (A50) ≥25%, and a density of 7.1~7.3g / cm³. 3 .

[0016] The microstructure of the above-mentioned steel is a multiphase structure containing ferrite, austenite and martensite, wherein the volume fraction of retained austenite is 18% to 28%.

[0017] The preparation method of the above-mentioned high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel specifically includes the following steps: 1. Provide steel billets: smelt and cast steel billets with the above chemical composition.

[0018] 2. Heating: Heat the qualified steel billet in a heating furnace to 1100±50℃ and hold for 1.5~2.5h. This allows the alloying elements to be fully dissolved and the composition to be uniform.

[0019] Preferably, the heating temperature is 1080~1120℃ and the holding time is 1.8~2.2h.

[0020] 3. Hot rolling: The heated steel billet undergoes multiple hot rolling passes, with a final rolling temperature of 680~700℃. The reduction rate of the final rolling pass is ≥5%. The total deformation of hot rolling is ≥70%.

[0021] 4. Cooling and coiling: After rolling, the steel plate is air-cooled to 540~580℃ and then coiled to obtain hot-rolled steel plate. The total deformation of hot rolling is ≥70%, and the cooling rate of air cooling is 5~15℃ / s.

[0022] Preferably, the winding temperature is 550~570℃.

[0023] In the preparation method, after winding in step 4), no quenching, tempering or fractional annealing treatment is performed.

[0024] Compared with the prior art, the beneficial effects of the present invention are: 1. Hot-rolled steel plates with ultra-high strength, high plasticity and low density were obtained, with excellent comprehensive mechanical properties.

[0025] Traditionally, to achieve a combination of high strength and high plasticity, medium manganese steel typically relies on high carbon content or complex subsequent heat treatments to control its microstructure. This invention combines a "C-Mn-Al-Cu" composition system design with a simplified process of "low-temperature final rolling + air cooling," directly obtaining an ideal multiphase microstructure in the hot-rolled state without the need for subsequent heat treatment. Specifically: (1) Source of high strength: The medium carbon content (0.2%~0.3%) and high manganese content (8%~10%) provide a significant solid solution strengthening effect. More importantly, by controlling the lower final rolling temperature (680~700℃) and the specific air cooling process, an appropriate amount of hard martensite phase is formed in the microstructure, which provides a very high strength foundation for the steel plate, making the tensile strength stably reach more than 1500MPa (see examples, up to 1880MPa).

[0026] (2) Source of high plasticity: The addition of Mn and Cu effectively expands the austenite phase region and inhibits the formation of δ-ferrite. The key is that the final rolling temperature and cooling regime designed in this invention promote "dynamic, long-range distribution" of alloying elements (especially Mn) during phase transformation, allowing a large amount of austenite to be stably retained at room temperature (residual austenite content ≥18%, up to 28% in the examples). These stable austenites, capable of undergoing the TRIP effect during deformation, greatly improve the uniform elongation and total elongation of the material, ensuring that the elongation after fracture (A50) is not less than 25% (up to 30% in the examples).

[0027] (3) Lightweight effect: The addition of aluminum (4%~5%) significantly reduces the density of the steel. The density of the steel in this invention is 7.1~7.3 g / cm³. 3 Compared to ordinary high-strength steel (density approximately 7.85 g / cm³), 3 This reduces costs by approximately 5% to 7%, directly contributing to the lightweighting of vehicles such as automobiles.

[0028] 2. It completely eliminates the complex and energy-intensive subsequent heat treatment process, greatly simplifying the process and significantly reducing production costs and energy consumption.

[0029] Existing technologies, in order to achieve high performance, cannot avoid complex and energy-intensive heat treatment or processing steps such as quenching and tempering, multiple annealing, and even friction stir processing. This invention, by precisely controlling the hot rolling process parameters (especially the final rolling temperature and coiling temperature), can simultaneously control and optimize the microstructure during rolling and cooling, thus achieving "rolling instead of heat treatment".

[0030] (1) Simplified process: The preparation process of the present invention is only: heating → hot rolling → air cooling and coiling. All additional heat treatment furnaces and corresponding operations are eliminated, and the process flow is shortened by more than 60%.

[0031] (2) Energy saving and consumption reduction: It directly avoids the large amount of electricity or gas consumption required for the heat treatment process. It is estimated that compared with the traditional "hot rolling + quenching and tempering" process, the present invention can save about 70% to 80% of the energy consumption in the heat treatment process.

[0032] (3) Improved efficiency: The production cycle is significantly shortened, the rolling line operation rate is increased, and it is more suitable for large-scale continuous industrial production.

[0033] 3. It adopts an economical alloy design, does not rely on expensive microalloying elements, and the raw material cost is controllable.

[0034] Unlike existing technologies that add microalloying elements such as V, Ti, and Nb to refine grains or strengthen precipitation, this invention relies entirely on the rational proportions of conventional elements such as C, Mn, Al, Cu, and Si to achieve its performance targets.

[0035] (1) Reduced cost: The expensive microalloys such as V and Nb were abandoned, and the main alloying elements are all bulk raw materials, which significantly reduced the alloy cost per ton of steel.

[0036] (2) Improve rolling efficiency: It avoids the problem of increased deformation resistance that may be caused by microalloying elements, making the hot rolling process smoother and helping to improve rolling speed and production efficiency.

[0037] 4. It effectively inhibits the formation of harmful δ-ferrite, ensuring the uniformity of the structure and the stability of the performance.

[0038] In high-Al lightweight steels, the formation of coarse δ-ferrite is a key issue impairing strength and ductility. This invention employs a multi-dimensional approach to synergistically suppress its formation: (1) Composition design: The upper limit of Al content is controlled at 5%, which reduces the thermodynamic driving force for the formation of δ-ferrite from the source; at the same time, the added Cu element and appropriate amounts of C and Mn have the effect of expanding the austenite phase region and inhibiting the formation of ferrite.

[0039] (2) Process coordination: The lower heating temperature (1050~1150℃) avoids the formation of too much δ phase at high temperature, while the specific rolling cooling process promotes the stabilization of austenite and further squeezes the space for the formation of δ ferrite.

[0040] (3) Effect: The final microstructure is uniform, mainly composed of fine ferrite, austenite and martensite, avoiding performance fluctuations and weak links caused by coarse δ ferrite.

[0041] In summary, this invention, through the organic combination of the composition system and a highly simplified hot rolling process, successfully resolves the contradiction between "performance and process complexity and cost" that is prevalent in the production of existing lightweight medium-manganese steel. It provides a high-performance lightweight steel plate solution that can be directly hot-rolled, has excellent comprehensive performance, low production cost, and is suitable for large-scale manufacturing, and has significant industrial application value. Attached Figure Description

[0042] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention. Detailed Implementation

[0043] This invention discloses a high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel and its preparation method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0044] Examples 1-6: Preparation and Properties of the Steel of the Present Invention Designed according to the six chemical compositions shown in Table 1, the process involves smelting and continuous casting to obtain continuous cast billets with the corresponding compositions. After cutting and cleaning, the billets are heated, hot-rolled, and cooled and coiled according to the process parameters shown in Table 2. The specific steps are as follows: 1. Smelting and Continuous Casting: The alloy was smelted in a converter, refined in an LF furnace, and degassed under RH vacuum to precisely control the alloy composition to the range in Table 1. Subsequently, a continuous slab with a thickness of 230 mm was obtained through full-process protective casting.

[0045] 2. Heating: The slab is fed into a walking beam furnace and heated according to the homogenization temperature and holding time in Table 2 to ensure that the alloying elements are fully dissolved and the composition is homogenized.

[0046] 3. Hot rolling: After the heated slab is descaled by high-pressure water, it undergoes multiple passes of rough rolling and finish rolling. During the finish rolling stage, the final rolling temperature is controlled to the value specified in Table 2, and the reduction rate of the last pass is greater than 5%, with a total deformation of approximately 75%.

[0047] 4. Cooling and Coiling: The rolled steel plate immediately enters the laminar flow cooling system. By controlling the air cooling (natural thermal radiation and convection) rate at about 10℃ / s, the steel plate is cooled to the coiling temperature specified in Table 2, and then coiled to obtain a hot-rolled steel plate coil with a thickness of 3.0mm.

[0048] 5. Subsequent processing: The coiled steel is directly air-cooled to room temperature without any form of quenching, tempering, annealing or other subsequent heat treatment.

[0049] The obtained hot-rolled steel plates were subjected to mechanical property tests, density measurements, and microstructure analysis. The results are shown in Table 3. The metallographic structure was observed using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques, and the content of retained austenite was determined using X-ray diffraction (XRD).

[0050] The chemical composition of the steel in this embodiment is shown in Table 1, the hot rolling process parameters are shown in Table 2, and the mechanical property parameters are shown in Table 3.

[0051] Table 1 Chemical composition (wt%) of embodiments of the present invention Table 2 Hot rolling process parameters of the embodiments of the present invention Table 3 Mechanical performance parameters of embodiments of the present invention As shown in Table 3, all hot-rolled steel sheets prepared according to the composition and process of this invention have tensile strengths exceeding 1500 MPa, with the highest reaching 1880 MPa; elongation after fracture is above 25%, with the highest reaching 30%; and density ranges from 7.12 to 7.25 g / cm³. 3 This design achieves a perfect balance between high strength, high plasticity, and lightweight. The retained austenite content remains above 18%, providing a fundamental guarantee for excellent plasticity.

[0052] Comparative Example 1 (Comparison with Traditional Heat Treatment Processes) The same chemical composition as in Example B of this invention was used for the cast billet. The billet was heated to 1200℃ and held for 2 hours, then hot-rolled to a final rolling temperature of 900℃, and immediately water-quenched to room temperature. The steel plate was then tempered at 250℃ for 1 hour. Its properties were tested as follows: tensile strength 1580 MPa, elongation 19%, and density 7.20 g / cm³. 3 .

[0053] Analysis: While the traditional "hot rolling + quenching + tempering" process can achieve high strength, its plasticity (19%) is significantly lower than that of Example B (30%) using the process of this invention. Furthermore, this process adds two energy-intensive steps—quenching and tempering—resulting in a longer production cycle and higher costs. This demonstrates that the "heat treatment-free" process of this invention achieves better overall performance while simultaneously saving energy and reducing consumption.

[0054] Comparative Example 2 (Comparison with insufficient Cu content) A chemical composition was prepared with C, Mn, and Al contents similar to those in Example E, but without the addition of Cu. Specifically, the composition was: C: 0.22%, Si: 0.17%, Mn: 9.5%, Al: 4.7%, and P and S were the same as before. The hot-rolling process was identical to that in Example E (see Table 2). The resulting steel plate had the following properties: tensile strength 1690 MPa, elongation 21%, and density 7.16 g / cm³. 3 Metallographic observation revealed the presence of a small amount of banded δ-ferrite in the microstructure.

[0055] Analysis: The lack of Cu significantly reduced the elongation of the steel, and harmful δ-ferrite appeared in the microstructure. This proves that the Cu element added in this invention not only helps with solid solution strengthening, but more importantly, it synergistically expands the austenite region with C and Mn, effectively inhibiting the formation of δ-ferrite, thereby ensuring high plasticity and microstructure uniformity.

[0056] Comparative Example 3 (Comparison of excessively high final rolling temperature) The same chemical composition and heating regime as in Example A were used. However, during hot rolling, the final rolling temperature was increased to 750°C (exceeding the 680-700°C range of this invention), while the coiling temperature was still controlled at 540°C. The resulting steel sheet had the following properties: tensile strength of 1480 MPa, elongation of 22%, and retained austenite content of only 12%.

[0057] Analysis: When the final rolling temperature is too high, the austenite grains after rolling are coarse, and the elemental distribution is insufficient during subsequent cooling, resulting in a significant reduction in the amount of stable retained austenite in the final microstructure (only 12%). Both its strength and plasticity decline significantly, failing to meet the objectives of this invention. This confirms that controlling the final rolling temperature within the range defined in this invention is the key process for achieving "dynamic, long-range manganese distribution," obtaining a high content of stable retained austenite, and high plasticity.

[0058] Explanation of metallographic views The metallographic structure of Example 1 of the present invention was observed (see attached). Figure 1 The following features can be clearly seen: (1) Fine and uniform structure: The microstructure is composed of fine lath / block structure with small grain size and no obvious coarse grains, which is the basis of high strength.

[0059] (2) Obvious multiphase structure: The microstructure is a typical mixed structure of ferrite (F), austenite (A) and martensite (M). Among them, white residual austenite is distributed in islands or films between laths or at grain boundaries.

[0060] (3) No harmful δ-ferrite: No continuous or coarse blocky δ-ferrite was observed throughout the field of view, which proves the effectiveness of the composition and process of the present invention in suppressing this harmful phase.

[0061] (4) Relationship with performance: The large amount of retained austenite in the figure (corresponding to the high volume fraction in Table 3) directly explains the source of the material’s high elongation; the fine martensite plates and solid solution strengthening contribute to the high strength; the uniform and dense microstructure ensures the stability of performance.

[0062] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel, characterized in that, It is composed of the following chemical components in weight percentage: C: 0.2%~0.3%, Si: 0.1%~0.2%, Mn: 8%~10%, Al: 4%~5%, Cu: 0.2%~0.4%, P≤0.01%, S≤0.01%, other unavoidable impurity elements total ≤0.05%, the remainder is Fe.

2. The high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 1, characterized in that, The steel has a tensile strength ≥1500MPa, an elongation after fracture (A50) ≥25%, and a density of 7.1~7.3g / cm³. 3 .

3. A high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 1 or 2, characterized in that, The microstructure of the steel is a multiphase structure containing ferrite, austenite and martensite, wherein the volume fraction of retained austenite is 18% to 28%.

4. A method for preparing high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel as described in any one of claims 1 to 3, characterized in that, The preparation method specifically includes the following steps: 1) Providing a steel billet: smelting and casting to obtain a steel billet having the chemical composition of claim 1; 2) Heating: Heat the steel billet to 1050~1150℃ and hold for 1.5~2.5 hours; 3) Hot rolling: The heated steel billet is hot rolled in multiple passes, with a final rolling temperature of 680~700℃; 4) Cooling and coiling: After rolling, the steel plate is air-cooled to 540~580℃ and then coiled to obtain hot-rolled steel plate; In the preparation method, after winding in step 4), no quenching, tempering or fractional annealing treatment is performed.

5. The method for preparing high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 4, characterized in that, In step 1), the smelting includes smelting in a converter or electric furnace, followed by vacuum refining in an LF furnace or RH furnace; the casting is continuous casting of steel to obtain continuous slabs.

6. The method for preparing high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 4, characterized in that, In step 2), the heating temperature is 1080~1120℃ and the holding time is 1.8~2.2h.

7. The method for preparing high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 4, characterized in that, In step 3), the reduction rate of the last rolling pass is ≥5%.

8. The method for preparing high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 4, characterized in that, In step 3), the total deformation of the hot rolling is ≥70%.

9. The method for preparing high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 4, characterized in that, In step 4), the winding temperature is 550~570℃.

10. The method for preparing high-strength, high-plasticity, lightweight hot-rolled medium-manganese steel according to claim 4, characterized in that, In step 4), the air cooling rate is 5~15℃ / s.

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