980MPa-grade light steel suitable for industrial production and production method of 980MPa-grade light steel

Abstract

The invention discloses 980MPa-grade light steel suitable for industrial production and a production method of the 980MPa-grade light steel, and relates to the technical field of steel and iron. The 980MPa-grade light steel comprises the following chemical components: C, Si, Mn, Al, V, P, S and the balance of Fe and inevitable residual elements and impurities. An optimized Mn and Al alloy content ratio is adopted, the Al content is reasonably designed, and a trace amount of V is added. The finally prepared 980MPa-grade light steel has high yield strength, tensile strength, ductility and cold bending performance, the industrial production difficulty and production cost of high-manganese and high-aluminum light steel are reduced, the product is suitable for manufacturing of ultra-high-strength light parts with complex deformation, and research result transformation and practical application of the light steel are promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of steel, and in particular to a 980 MPa-grade lightweight steel suitable for industrial production and a production method thereof. Background Art

[0002] Lightweighting is the future development path of the automotive industry. Steel materials with high strength and good plasticity are ideal materials for the manufacture of automobile body structures. However, there is a "trade-off" relationship between strength and plasticity. It is far from enough to achieve the goal of lightweighting by simply increasing the strength of the material.

[0003] Traditional automotive steel faces a technical bottleneck due to the intertwined constraints between strength and ductility. Simply increasing material strength is insufficient to meet lightweighting requirements. This led to the emergence of low-density steel as a research hotspot in the early 21st century. Fe-Al-based low-density steels, in particular, have attracted considerable attention due to their ability to achieve a 10% reduction in specific gravity by adding 5%-8.5% aluminum. However, existing research is largely limited to the laboratory stage, and comprehensive systematic research on Fe-Mn-Al-C-based low-density steels, including alloy composition design, strengthening mechanism analysis, and production process optimization, remains significantly insufficient.

[0004] Chinese patent CN201810362095.X discloses a high-strength and toughness steel for automotive use with a strength-ductility product greater than 45 GPa·% and its preparation method; Chinese patent CN201810507557.2 discloses a 1200 MPa-grade high-strength, high-ductility, low-density steel plate and its manufacturing method; the preparation methods for the low-density steels provided above all require hot rolling, cold rolling, and heat treatment to achieve the desired performance, and the published indicators are only mechanical properties, with little mention of process performance, and the products are all simple deformation parts. Although high-manganese aluminum alloy lightweight steels have significant lightweight advantages, the high production cost and difficulty reflected in their high alloy composition system have seriously restricted their industrial production process.

[0005] Therefore, there is an urgent need to seek a lightweight steel that is suitable for continuous casting production, has excellent comprehensive performance, and is suitable for large-scale industrial production. It can meet the requirements of directly manufacturing ultra-high-strength and lightweight components, and is conducive to promoting the transformation and practical application of lightweight steel research results.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The object of the present invention is to provide a 980 MPa grade lightweight steel suitable for industrial production and a production method thereof, so as to solve the above technical problems.

[0008] The present invention is achieved in that: In a first aspect, an embodiment of the present invention provides a 980 MPa grade lightweight steel suitable for industrial production. The chemical composition of the 980 MPa grade lightweight steel is as follows, calculated by weight percentage: C: 0.20%-0.30%, Si: 0.30%-0.50%, Mn: 2.50%-4.0%, Al: 1.40%-2.5%, V: 0.010%-0.050%, P: ≤0.020%, S: ≤0.005%, the rest are Fe and unavoidable residual elements and impurities.

[0009] In a second aspect, an embodiment of the present invention provides a method for producing the aforementioned 980 MPa grade lightweight steel, comprising the following steps: Smelting and continuous casting are carried out according to the chemical composition of 980MPa grade lightweight steel to produce slabs; Heating the slab; The slab after the heating treatment is rolled to obtain strip steel; The strip steel is cooled and coiled to obtain a hot rolled coil; The hot rolled coil is heat treated to produce 980MPa grade lightweight steel.

[0010] The present invention has the following beneficial effects: The 980 MPa-grade lightweight steel suitable for industrial production provided by the embodiments of the present invention adopts an optimized ratio of Mn and Al alloy contents and rationally designs the Al content, thereby avoiding the difficulties in continuous casting production caused by excessively high Al content, especially the stringent requirements for continuous casting mold slag, and also avoiding the formation of brittle ferrite structure in the product structure. A trace amount of V alloy is added to the chemical composition to improve the yield strength of the hot-rolled coil, thereby overcoming the excessively low yield strength caused by the addition of high Mn and Al alloys.

[0011] The produced 980MPa-grade lightweight steel has high yield strength, tensile strength, elongation and cold bending performance, which reduces the production difficulty and production cost of the industrialization of high-manganese and high-aluminum lightweight steel. The product is suitable for the manufacture of ultra-high-strength lightweight components with complex deformation, which is conducive to promoting the transformation and practical application of lightweight steel research results. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0013] Figure 1This is a process flow chart for producing 980MPa grade lightweight steel suitable for industrial production; Figure 2 This is the metallographic structure diagram of the 980MPa grade lightweight steel prepared in Example 1; Figure 3 This is the cross-sectional shape of the torsion beam obtained in Application Example 1. DETAILED DESCRIPTION

[0014] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0015] Existing technologies have many laboratory research results, but relatively few have been tested in actual production. Most of these results require complex heat treatment processes and cold rolling to produce laboratory products with excellent mechanical properties. The post-rolling heat treatment in the present invention includes laminar cooling with water spray cooling, slow cooling, and tempering. The relatively demanding tempering heat treatment is also low-temperature tempering, which is relatively easy to implement.

[0016] The present invention aims to provide an industrialized production method for a high-manganese aluminum alloy with a Mn content of 2.0% or greater and an Al content of 0.8% or greater, resulting in a 980 MPa-grade lightweight steel with excellent performance suitable for industrial production, meeting the manufacturing requirements for high-strength, complex-shaped components, such as those used in new energy vehicles. The high-manganese aluminum alloy referred to in the present invention has a manganese content of 2.50% to 4.0% and an aluminum content of 1.40% to 2.5%. The details are as follows: In a first aspect, an embodiment of the present invention provides a 980 MPa grade lightweight steel suitable for industrial production. The chemical composition of the 980 MPa grade lightweight steel is as follows, calculated by weight percentage: C: 0.20%-0.30%, Si: 0.30%-0.50%, Mn: 2.50%-4.0%, Al: 1.40%-2.5%, V: 0.010%-0.050%, P: ≤0.020%, S: ≤0.005%, the rest are Fe and unavoidable residual elements and impurities.

[0017] It should be noted that the embodiments of the present invention adopt an optimized ratio of Mn and Al alloy contents and rationally design the Al content, which not only avoids the difficulties in continuous casting production caused by excessively high Al content, especially the stringent requirements for continuous casting protection slag, but also avoids the formation of brittle ferrite structure in the product structure; a trace amount of V alloy is added to the chemical composition to improve the yield strength of the hot-rolled coil, overcoming the excessively low yield strength caused by the addition of high Mn and Al alloys.

[0018] In the 980MPa grade lightweight steel of the present invention, the functions of the various chemical components are as follows: Carbon: Carbon is the most important strengthening element in this invention. According to the application range of this steel forming process, the material is required to meet the strength requirements while having good cold forming properties and maintaining the proportion and stability of retained austenite. Therefore, the limited range of carbon element is 0.20%. <C<0.30%。

[0019] Silicon: Silicon is one of the important strengthening elements in this invention. A part of the solid solution silicon element can improve the solid solution strength of the steel, and the other part of the silicon can control the precipitation of second phase particles and improve the precipitation strength. According to the application range of this steel forming processing, the material is required to meet the strength requirements while having good product surface quality. Therefore, the limited range of silicon element is 0.30% <Si<0.50%。

[0020] Manganese: Manganese is one of the important strengthening and toughening elements in the present invention. It can reduce the density of steel and increase the hardenability of steel. It is easy to obtain bainite and martensite structures during the quenching and tempering heat treatment process. According to the application scope of forming processing of this steel, the material is required to meet the strength requirements while having obvious lightweight advantages. Therefore, the limited range of manganese element is 2.5%. <Mn<4.0%。

[0021] Aluminum: Aluminum is the most important lightweight element in this invention. When 1% Al is added to steel, the density decreases by 0.101 g / cm 3 , which can reduce weight by about 1.3%. According to the application scope of forming and processing of this type of steel, the material is required to have obvious lightweight advantages while meeting the strength requirements. If the aluminum content is less than 1.40%, the lightweight effect is significantly reduced, but the density of the material does not meet the standard requirements; if the aluminum content is greater than 2.50%, brittle structure is easily produced, which requires cold rolling and complex heat treatment to eliminate, increasing the difficulty of industrial production and process costs. In addition, the aluminum element is also very easy to promote ferrite phase transformation, reduce yield strength, and reduce the welding performance of steel. Therefore, the limited range of aluminum elements is 1.40% <Al<2.50%。

[0022] Vanadium: Vanadium is one of the important strong carbonitride (C, N) forming elements in this invention. V, which is easily precipitated in low temperature processes, plays a significant role in precipitation. According to the application scope of this steel forming process, the material is required to meet the lightweight requirements while having a high yield strength. Therefore, the limited range of vanadium element is 0.010% <V<0.050%。

[0023] Phosphorus and sulfur: Phosphorus and sulfur are major harmful elements in steel, degrading product performance and requiring cost control. Therefore, sulfur is limited to 0.006% or less, and phosphorus is limited to 0.020% or less.

[0024] In a second aspect, an embodiment of the present invention provides a method for producing the aforementioned 980 MPa grade lightweight steel, comprising the following steps: Smelting and continuous casting are carried out according to the chemical composition of 980MPa grade lightweight steel to produce slabs; Heating the slab; The slab after the heating treatment is rolled to obtain strip steel; The strip steel is cooled and coiled to obtain a hot rolled coil; The hot rolled coil is heat treated to produce 980MPa grade lightweight steel.

[0025] It should be noted that the present invention optimizes the ratio of Mn and Al alloy contents, adds a trace amount of V to the chemical composition, combines cold rolling and complex heat treatment in the production method, reduces the production cost of lightweight steel hot-rolled coils, and achieves a density reduction of about 3%; after rolling, a simple slow cooling and low-temperature tempering heat treatment process is adopted, especially a hood annealing furnace is used for low-temperature tempering process, the heat treatment is simple, suitable for mass production, and the heat treatment cost is low, which not only eliminates the complex heat treatment and cold rolling processes, but also greatly reduces the process cost.

[0026] The embodiment of the present invention produces qualified slabs through continuous casting of about 215 tons of qualified molten steel in one furnace. The slabs are then heated, rolled and heat treated to produce slabs of 230 mm × 1050 mm. For the first time, six rolls of lightweight steel hot-rolled coils with the thinnest thickness of 3.4 mm were successfully produced, realizing the industrial production of lightweight steel.

[0027] In an optional embodiment, the smelting process includes molten iron pretreatment, oxygen converter smelting, argon station treatment, LF furnace refining and RH furnace refining.

[0028] It should be noted that in the embodiment of the present invention, the continuous casting process needs to use special protective slag suitable for the production of high Mn and high Al steel, and pay attention to the denaturation of the protective slag during the casting process. If necessary, slag replacement operations are performed to ensure normal continuous casting production.

[0029] In an optional embodiment, the slab heat treatment includes at least one of the following features: Feature 1: The tapping temperature of the slab during heating treatment is 1220℃-1270℃, and the slab's time in the furnace is not less than 170 minutes; Feature 2: When the slab is hot charged and delivered, the heating treatment time is 170min-220min; Feature 3: When the slab is cold loaded, the heating treatment time is 200min-300min; Feature 4: During the slab heating process, the soaking time is 25min-40min.

[0030] It should be noted that the slab heating treatment significantly improves the internal organizational state of the slab through thermal diffusion, providing a basis for the performance of the finished product after subsequent rolling; the heating temperature setting of the slab in the present invention is conducive to fully dissolving the alloy elements in the austenite, forming uniform austenite grains, and avoiding local performance differences caused by component segregation; it is conducive to balancing surface quality and rolling stability, and achieving efficient production and low energy consumption.

[0031] In an optional embodiment, the rolling process includes a rough rolling process and a finish rolling process; wherein the rough rolling process includes 5-7 rough rolling passes, and the thickness of the slab after rough rolling is 35mm-45mm.

[0032] It should be noted that the implementation of a rough rolling process helps ensure temperature uniformity of the intermediate bar, control the internal structure of the intermediate bar, and match the mill's capacity and cadence. Excessive rough rolling cycles can extend the rolling cycle, leading to rolling line blockage and reduced production capacity. Excessive rough rolling cycles can cause excessive rolling forces (e.g., due to thick slabs) exceeding the mill's rated load, leading to equipment failure. In an optional embodiment, the rough rolling process is performed in a total of six passes.

[0033] and / or, the rough rolling process includes 5-7 descaling passes, each descaling pass being performed independently using high-pressure water descaling; Furthermore, the water pressure of high-pressure water descaling is 18MPa-25MPa.

[0034] It's important to note that descaling involves removing the oxide scale (primarily FeO, Fe3O4, and Fe2O3) that forms on the steel surface during rolling using high-pressure water or mechanical impact. If descaling passes are too few (e.g., just one), the oxide scale (especially the secondary oxide scale, which is hard and adheres strongly) will be difficult to completely remove, significantly increasing the risk of residual scale. If the oxide scale isn't completely removed, it will be pressed into the steel matrix during subsequent rolling, forming "oxide scale indentation" defects (manifested as surface pitting and pockmarking), seriously affecting the surface quality of the finished product (for example, automotive and appliance steel sheets have a zero tolerance for surface defects).

[0035] If there are too many descaling passes, the strip temperature will drop, even below the lower limit of the process temperature, increasing energy consumption and cost investment.

[0036] In an optional embodiment, a total of five descaling passes are performed during the rough rolling process.

[0037] The water pressure of high-pressure water descaling is selected from any one of 18 MPa, 20 MPa, 22 MPa and 25 MPa, or other values ​​within the range of 18 MPa-25 MPa according to actual needs.

[0038] In an optional embodiment, the finishing rolling process adopts 5-8 stand continuous rolling; the starting rolling temperature of the finishing rolling process is 980°C-1080°C, and the final rolling temperature is 860°C-950°C.

[0039] It should be noted that the finishing rolling process facilitates precise control of strip dimensions, improving strip properties and optimizing surface quality. Multi-stand continuous rolling improves production efficiency, ensures product consistency, and achieves efficient, high-quality, and low-cost rolling production.

[0040] In an optional embodiment, the finishing rolling process adopts 7-stand continuous rolling and performs a high-pressure water descaling. The water pressure of the high-pressure water descaling is selected from any one of 18MPa, 20MPa, 22MPa and 25MPa, or other values ​​within the range of 18MPa-25MPa according to actual needs.

[0041] In an optional embodiment, when producing a lightweight steel hot rolled coil, at least one of the following features is included: Feature 1: When the strip cooling treatment is laminar cooling, rear cooling is adopted; Feature 2: The average cooling rate of the strip is not less than 25℃ / s; Feature 3: The coiling temperature is 380℃-470℃.

[0042] It should be noted that if laminar cooling adopts the front-end processing method, the performance of the strip produced will fluctuate greatly, the yield strength is generally low, the elongation fluctuates greatly, and it fails the 180° bending test when D=4a.

[0043] Specifically, the cooling process utilizes post-cooling with water spray, which facilitates precise control of the coiling temperature, which directly influences the phase transformation type and microstructure. Controlling the appropriate cooling rate promotes the formation of a bainite + ferrite + retained austenite microstructure, while avoiding excessive martensite formation from excessive cooling and excessive ferrite and pearlite formation from excessive cooling.

[0044] In an optional embodiment, the heat treatment of the hot-rolled coil includes a slow cooling process and a tempering process; wherein the slow cooling process is carried out in a slow cooling pit, and the slow cooling time is not less than 36 hours.

[0045] It should be noted that the slow cooling process of hot-rolled coils creates certain conditions for bainite transformation. However, due to the limited conditions of the slow cooling pit, the hot-rolled coils have uneven performance, high internal stress, uneven mechanical properties, and unsatisfactory process performance. Therefore, the hot-rolled coils exiting the slow cooling pit need to undergo a tempering process.

[0046] In an optional embodiment, when performing the tempering process in a bell-type furnace, the hot-rolled coil is heated to 280° C.-350° C. and kept at this temperature for 150 min-240 min.

[0047] It should be noted that this tempering process reduces internal stress in the material and enhances the stability of retained austenite, which is key to the TRIP effect, significantly improving the material's processing properties. After tempering, the strip's tensile strength slightly decreases compared to the original hot-rolled coil, while its yield strength slightly increases. Cold bending performance also improves significantly, from failing at D=4a to passing at D=2a, with some samples also passing at D=0a.

[0048] The researchers sampled hot-rolled coils and, after multiple heat treatment tests, ultimately developed a tempering heat treatment process suitable for 980MPa-grade lightweight steel. When the tempering temperature is below 280°C and the holding time is less than 150 minutes, the tensile strength does not decrease significantly, and mechanical and processing properties do not improve significantly, with the elongation remaining essentially the same as the hot-rolled coil. However, when the tempering temperature exceeds 350°C and the holding time is longer than 240 minutes, the elongation increases significantly to over 20%, and cold bending performance improves significantly, but the tensile strength drops significantly to below 800MPa.

[0049] In an optional embodiment, the density of 980 MPa grade lightweight steel is 7.50 g / cm 3 -7.70g / cm 3 , tensile strength is ≥980MPa, yield strength is 650MPa-900MPa, elongation is ≥18%, and 180° bending test D=2a is qualified.

[0050] The present invention independently develops a production process and combines it with an optimized alloy composition to ensure that the lightweight texture of the product meets the lightweight requirements, greatly reducing the difficulty of continuous casting production and the generation of brittle structure caused by high aluminum. It not only reduces the cost of industrial production, but also makes up for the shortcomings of the simple slow cooling pit of hot rolling, and achieves uniform performance of the entire coil.

[0051] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0052] Example 1 This embodiment provides a production method for 980 MPa grade lightweight steel suitable for industrial production, wherein the nominal capacity of the converter of the practical steel plant is 180 t, the thickness of the continuous casting billet is 230 mm, and its chemical composition is shown in Table 1.

[0053] Table 1 Chemical composition of 980MPa grade lightweight steel (wt%)

[0054] The process flow chart for producing 980MPa grade lightweight steel in this embodiment is shown in FIG. Figure 1 , specifically including the following steps: 1.1. Preparation of slab The raw materials are sequentially smelted and continuously cast to obtain slabs; Among them, smelting includes molten iron pretreatment, converter smelting, argon station treatment, LF furnace refining and RH furnace refining.

[0055] Special protective slag is used for continuous casting, the continuous casting speed is 0.9m / min, the slab thickness is 230mm and the width is 1050mm.

[0056] 1.2. Heat treatment Take 6 slabs prepared in step 1.1 and load them into the heating furnace. The loading temperatures are 280℃, 272℃, 265℃, 270℃, 276℃ and 275℃ respectively. The residence time of the slabs in the heating furnace is 270min-300min. The temperatures of the slabs out of the heating furnace are 1236℃, 1248℃, 1254℃, 1250℃, 1259℃ and 1251℃.

[0057] 1.3 Rolling treatment The slab after heat treatment is descaled and subjected to 6 passes of rough rolling and 5 passes of high-pressure water descaling at a pressure of 20 MPa to obtain an intermediate billet with a thickness of 36 mm. The intermediate billet is descaled in one pass with high-pressure water at a pressure of 22 MPa and then rolled in a 7-stand hot continuous finishing mill to produce strip steel. The starting rolling temperature of the finishing rolling is 1035-1060°C, and the final rolling temperature of the finishing rolling is 896-906°C. Among the strip steel, 2 coils have a thickness of 4.0 mm and 4 coils have a thickness of 3.4 mm.

[0058] 1.4 Cooling treatment Two coils of 3.4 mm thick strip steel obtained in step 1.3 were subjected to online laminar cooling in the back-end cooling mode; the coiling temperature during the back-end cooling was 430°C, and the average cooling rate was 47°C / s, to obtain hot-rolled coils.

[0059] 1.5 Heat treatment The hot-rolled coil obtained in step 1.4 was subjected to slow cooling treatment in a slow cooling pit for 38 hours to obtain hot-rolled original coils, which were recorded as sample 1 and sample 2 respectively.

[0060] After slow cooling, the hot rolled coils were tempered in a bell furnace. The tempering process was as follows: the hot rolled coils were heated to 300±10°C and kept at this temperature for 200 minutes to produce 980MPa grade lightweight steel, which were recorded as Samples 3 and 4 respectively.

[0061] The specific process parameters are shown in Table 2.

[0062] Table 2 Process parameters

[0063] Comparative Example 1 This comparative example provides a method for producing 980 MPa grade lightweight steel suitable for industrial production, which differs from Example 1 only in that: 1.4 Cooling treatment Another two coils of 3.4 mm thick strip steel obtained in step 1.3 were subjected to online laminar cooling using the front-end cooling mode; the coiling temperature target during the front-end cooling was 600°C, and the average cooling rate was 32°C / s to produce hot-rolled coils.

[0064] 1.5 Heat treatment The hot-rolled coil obtained in step 1.4 was subjected to slow cooling treatment in a slow cooling pit for 38 hours to obtain hot-rolled original coils, which were recorded as Sample 5 and Sample 6 respectively.

[0065] After slow cooling, the hot rolled coils were tempered in a bell furnace. The tempering process was as follows: the hot rolled coils were heated to 300±10°C and kept at this temperature for 200 minutes to produce 980MPa grade lightweight steel, which were recorded as Samples 7 and 8 respectively.

[0066] The specific process parameters are shown in Table 2.

[0067] Test Example 1 In this test example, the hot-rolled coil produced in Example 1 (Sample 1) and the hot-rolled coil produced in Comparative Example 1 (Sample 5) were tested for mechanical properties and process performance. Samples were collected and prepared according to "GB / T 2975 Sampling Location and Preparation for Mechanical Properties of Steel and Steel Products." Mechanical properties were tested according to "GB / T 228.1 Metallic Materials - Tensile Tests - Part 1: Room Temperature Test Methods." Bend testing was conducted according to "GB / T 232 Metallic Materials - Bend Test Methods." The test results are shown in Table 3.

[0068] Table 3 Mechanical properties and process properties of hot-rolled original coils

[0069] The data in Table 3 show that the properties of the hot-rolled steel sheets produced by the processes of Example 1 and Comparative Example 1 fluctuate greatly. Specifically, the yield strength is generally lower than 700 MPa, the elongation is lower than 20%, and the steel sheets fail the 180° bending test at D=4a.

[0070] Test Example 2 In this test example, the mechanical properties and process performance of the 980 MPa grade lightweight steel (Sample 3) prepared in Example 1 and the 980 MPa grade lightweight steel (Sample 7) prepared in Comparative Example 1 were tested, respectively. The tests were conducted with reference to Test Example 1. The test results are shown in Table 4.

[0071] Table 4 Mechanical properties and processing properties of 980MPa grade lightweight steel

[0072] It can be seen from the data in Table 4 that the 980 MPa grade lightweight steel obtained by tempering and partitioning heat treatment has greatly improved the bending test of Example 1 (Sample 3) and Comparative Example 1 (Sample 7), and both are qualified in the 180° bending test when D=2a.

[0073] Example 1 (Samples 3-4) exhibited superior overall performance, with a tensile strength exceeding 1000 MPa, improved yield strength, and an elongation exceeding 18.5%. Comparative Example 1 (Samples 7-8) exhibited a significant decrease in tensile strength, falling to around or below 800 MPa, while elongation improved significantly, reaching over 24%.

[0074] Test Example 3 This test example tests the metallographic structure of the 980MPa grade lightweight steel (sample 3) obtained in Example 1. Figure 2 .

[0075] from Figure 2 It can be seen that the matrix structure is bainite + ferrite + a small amount of retained austenite.

[0076] Test Example 4 According to GB / T3850-2015 standard, the density of the lightweight steel of the present invention was tested and the test value was 7.64 g / cm 3 .

[0077] Application Example 1 In this application example, the 980MPa-grade lightweight steel (sample 3) obtained in Example 1 is used to produce downstream automotive parts, specifically the torsion beam of new energy vehicles. The torsion beam has stringent requirements on the cold deformation ability of the product. The position of the maximum deformation not only requires the material to have excellent bending properties, but also the shape after deformation must be symmetrical. The preparation process path is: 980MPa-grade lightweight steel - pickling - striping - pipe making (laser welding) - preforming - internal high-pressure forming - stress relief annealing (if necessary) - finishing and painting - finished torsion beam.

[0078] This application example captures the cross-sectional shape of the torsion beam, as shown in the following example: Figure 3 shown.

[0079] from Figure 3 It can be seen that the cross-sectional shape of the component is "V"-shaped and needs to be symmetrical. The two "water drop-shaped" parts have large bending deformations, and the forces and deformations at different parts are different, which places extremely high demands on the uniformity of the product's bending performance and mechanical properties.

[0080] In summary, the present invention fully utilizes the synergistic effect of various elements and reduces production costs in chemical composition design and process control, accurately controls the microstructure of the finished product, and forms a matrix structure dominated by bainite. The focus is on increasing the proportion of retained austenite to 5%-15%, so as to facilitate the phase transformation induced plasticity, that is, the TRIP effect. Through the independently developed heat treatment process, the microstructure morphology is optimized, and the mechanical properties and process performance of the product are improved. In particular, the design of the Al content not only meets certain lightweight needs, but also greatly reduces the difficulty of continuous casting production caused by high aluminum, and avoids the low yield strength problem caused by Al alloys; it avoids the generation of brittle structure due to excessive Al content, thereby avoiding problems such as edge cracking and complex subsequent processing during hot rolling. The independently developed tempering heat treatment process can not only achieve low-cost industrial production, but also make up for the shortcomings of the simple slow cooling pit of hot rolling, and realize lightweight steel with uniform performance, high cost performance and excellent performance throughout the roll, which is conducive to promoting the manufacture and application of torsion beams for new energy vehicles.

[0081] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A 980MPa grade lightweight steel suitable for industrial production, characterized in that: The chemical composition of the 980MPa grade lightweight steel is as follows, by weight percentage: C: 0.20%-0.30%, Si: 0.30%-0.50%, Mn: 2.50%-4.0%, Al: 1.40%-2.5%, V: 0.010%-0.050%, P: ≤0.020%, S: ≤0.005%, the rest are Fe and unavoidable residual elements and impurities.

2. A method for producing 980 MPa grade lightweight steel according to claim 1, characterized in that: The steps include: Smelting and continuous casting are carried out according to the chemical composition of 980MPa grade lightweight steel to produce slabs; heating the slab; The slab after the heating treatment is rolled to obtain strip steel; Cooling the steel strip and coiling it into a coil to obtain a hot-rolled coil; The hot rolled coil is heat treated to produce 980 MPa grade lightweight steel.

3. The production method according to claim 2, characterized in that The smelting process includes molten iron pretreatment, oxygen converter smelting, argon station treatment, LF furnace refining and RH furnace refining.

4. The production method according to claim 2, characterized in that During the slab heating treatment, at least one of the following characteristics is included: Feature 1: The tapping temperature of the slab during heating treatment is 1220℃-1270℃, and the slab's time in the furnace is not less than 170 minutes; Feature 2: When the slab is hot charged and delivered, the heating treatment time is 170min-220min; Feature 3: When the slab is cold loaded, the heating treatment time is 200min-300min; Feature 4: During the slab heating process, the soaking time is 25min-40min.

5. The production method according to claim 2, characterized in that The rolling process includes a rough rolling process and a finish rolling process; wherein the rough rolling process includes 5-7 rough rolling passes, and the thickness of the slab after rough rolling is 35mm-45mm; And / or, the rough rolling process includes 5-7 descaling passes, each descaling pass is performed independently using high-pressure water; Preferably, the water pressure of the high-pressure water descaling is 18 MPa-25 MPa.

6. The production method according to claim 5, characterized in that The finishing rolling process adopts 5-8 stand continuous rolling; the starting rolling temperature of the finishing rolling process is 980°C-1080°C, and the final rolling temperature is 860°C-950°C.

7. The production method according to claim 2, characterized in that When producing lightweight steel hot rolled coils, at least one of the following features is included: Feature 1: When the strip cooling treatment is laminar cooling, rear cooling is adopted; Feature 2: The average cooling rate of the strip is not less than 25℃ / s; Feature 3: The coiling temperature is 380℃-470℃.

8. The production method according to claim 2, characterized in that The heat treatment of the hot-rolled coil includes a slow cooling process and a tempering process; wherein the slow cooling process is carried out in a slow cooling pit, and the slow cooling time is not less than 36 hours.

9. The production method according to claim 8, characterized in that When the tempering process is carried out in a bell-type furnace, the hot-rolled coil is heated to 280° C.-350° C. and kept at this temperature for 150 min-240 min.

10. The production method according to claim 2, characterized in that The density of the 980MPa grade lightweight steel is 7.50g / cm 3 -7.70g / cm 3 , tensile strength is ≥980MPa, yield strength is 650MPa-900MPa, elongation is ≥18%, and 180° bending test D=2a is qualified.

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