A low-density warm-rolled steel sheet and a method of manufacturing the same

Abstract

The application discloses a kind of low-density warm-rolled steel plate and its preparation method, belong to cold-rolled plate strip production field.Preparation method includes: smelting process: according to chemical composition requirement smelting and cast into ingot;Forging process: after ingot is heated to 1210~1250 DEG C and keeps for 3~5 hours, ingot is forged into slab, final forging temperature is greater than or equal to 850 DEG C;Hot rolling process: after slab is heated to 1230~1270 DEG C and keeps for 3~7 hours, hot rolling is carried out to the first set thickness specification strip steel, final rolling temperature is 880~940 DEG C, and the strip steel after rolling is air-cooled to room temperature;Warm rolling process: after the strip steel after hot rolling is heated to 630~670 DEG C, warm rolling is carried out, and the strip steel of second set thickness specification is rolled, and the reduction of each pass is 12%~18%;Heat treatment process: after the strip steel after warm rolling is heated to 730~780 DEG C and keeps for 10~20min, it is cooled to room temperature, and low-density warm-rolled steel plate is obtained.The low-density warm-rolled steel plate prepared by the application reaches 580~670 MPa, and the tensile strength reaches 760~850 MPa, and the elongation A 50 Reaches 26.0~33%, and the yield strength ratio reaches 0.70~0.88.

Description

Technical Field

[0001] This invention belongs to the field of cold-rolled sheet and strip production, specifically relating to a low-density warm-rolled steel sheet and its preparation method. Background Technology

[0002] The automotive manufacturing industry is a pillar industry of the national economy. Coupled with energy shortages and environmental pollution, lightweighting has become a key focus for carbon emission reduction in the automotive sector. Currently, there are three main approaches to achieving automotive lightweighting: First, using lightweight raw materials such as aluminum alloys, magnesium alloys, and carbon fiber composites. While these materials can reduce the overall weight of a vehicle, they suffer from drawbacks such as complex forming processes, poor welding performance, low impact energy absorption, high cost, and low production volume, limiting their market application. Second, using ultra-high-strength steel to replace traditional automotive steel, reducing steel sheet thickness to lighten the vehicle. However, as steel sheet strength increases, forming capacity decreases, leading to problems such as cracking, wrinkling, and excessive springback. Furthermore, as steel sheet thickness decreases, the stiffness of some vehicle components is excessively reduced, and acoustic problems causing discomfort for passengers arise, limiting their widespread adoption. Third, developing a steel grade that combines high strength, high elongation, and low density. Low-density steel possesses excellent mechanical and physical properties and significant weight reduction effects, making it a hot research topic for the automotive industry and its suppliers.

[0003] Existing patents similar to low-density warm-rolled steel sheets and their preparation methods include:

[0004] CN 108998734 A discloses an ultra-high strength and ductility cold-rolled Mn-Al TRIP steel sheet and its rapid annealing preparation method. The chemical composition, by weight percentage, is: C: 0.18–0.22%, Si: 0.30–0.70%, Mn: 6.0–7.5%, Al: 2.0–3.0%, V: 0.08–0.12%, P≤0.005%, S≤0.005%, N≤0.006%, O≤0.003%, with the remainder being Fe and unavoidable impurities. The melting temperature is 1550–1600℃, and a resistance-type continuous annealing furnace is used to rapidly heat the cold-rolled steel sheet at a heating rate of 100–400℃ / s, with a cooling rate to room temperature greater than 10℃ / s. The slab is homogenized and heated to 1170–1230℃, isothermal for 2–2.5 hours. The initial rolling temperature is 1080–1160℃, and the final rolling temperature is 900–950℃. The reduction per pass is 30%, with a cumulative reduction ≥95%. After hot rolling, the slab is coiled at 680–740℃. Intermediate annealing is performed at 720–780℃ for 0.5–2 hours. Cold rolling is then carried out with a reduction of 0.1–0.2 mm per pass, resulting in a cumulative total reduction of 75–90%, yielding a cold-rolled steel sheet with a thickness of 0.5–1.2 mm. Two-phase annealing involves heating the cold-rolled steel sheet to 820–850℃, isothermal for 20–40 seconds, and then cooling to room temperature. The patented process involves intermediate annealing before cold rolling, which is quite lengthy. Each cold rolling pass reduces the material by 0.1 to 0.2 mm, with a cumulative reduction rate of 75 to 90%. This requires repeated cold rolling passes, making the process quite complex.

[0005] CN 108396244 A discloses a cold-rolled medium-manganese high-aluminum low-density steel and its preparation method. Its chemical composition by weight percentage is C: 1.20-1.30%, Mn: 10.7-11.3%, Al: 9.5-10%, P≤0.005%, S≤0.003%, and the balance is Fe and unavoidable impurities. The ingot is heated to 1170–1230℃ and held for 2–4 hours; then forged at 950–1050℃ to obtain a steel billet; the steel billet is heated to 1180–1250℃ and held for 1–5 hours; the held steel billet is then subjected to multiple hot rolling passes, with an initial rolling temperature of 1120–1160℃ and a final rolling temperature of ≥950℃, and a total cumulative reduction of 80–90%; it is then water-cooled to room temperature to obtain a hot-rolled plate; the hot-rolled plate is held at 990–1010℃ for 1–2 hours and directly water-quenched to room temperature; the solution-treated plate is then subjected to multiple cold rolling passes, with a total cumulative reduction of 70–80% to obtain a cold-rolled plate; the cold-rolled plate is held at 980–1020℃ for 5–15 minutes and water-quenched to room temperature to obtain cold-rolled medium-manganese high-aluminum low-density steel. The high C content (1.20-1.30%) of this patent is not conducive to obtaining good welding performance. Direct water quenching to room temperature is performed before cold rolling, and the solution-treated plate is subjected to multiple cold rolling passes with a total cumulative reduction of 70-80%. Due to the strong hardenability of medium manganese steel, although some martensite will transform into austenite after solution treatment to improve cold rolling performance, the transformation of austenite into martensite during cold rolling causes the strip to become hard and brittle, which limits the reduction of each pass and is not conducive to smooth production.

[0006] CN 115323275 B discloses a high-strength, high-toughness rare-earth warm-rolled low-carbon, low-manganese TRIP steel and its preparation method. Its chemical composition by weight percentage is: C: 0.18–0.21%, Mn: 1.69–1.83%, Si: 0.23–0.41%, Al: 1.44–1.65%, Mo: 0.02–0.04%, Cu≤0.01%, Ni≤0.01%, Ti≤0.01%, N≤0.01%, Ce: 0.03–0.5%, with the balance being Fe and unavoidable impurities. After homogenization treatment by heating the ingot to 1200±20℃ and holding for 2–4 hours, it is rolled in 7 passes starting at 1200±20℃, with the final rolling temperature controlled at 850±50℃. The total deformation is 92.5%. The resulting hot-rolled plate is then air-cooled to room temperature. During warm rolling, the steel plate is first held at 750±20℃ for 30 minutes, then rolled in 8 passes to 1.5 mm, with a reduction of 10-20% per pass. Between passes, the steel plate is held at 750±20℃ for 5 minutes and then air-cooled to room temperature. The warm-rolled steel plate undergoes bainitic isothermal treatment, specifically: first, it is held at 800±50℃ for 120 seconds, then rapidly quenched to 400±20℃ and held for 300 seconds, and finally air-cooled to room temperature. This patented method involves holding the steel plate at 750±20℃ for 5 minutes between passes before rolling, a cumbersome and inefficient process involving repeated heating.

[0007] Therefore, existing technologies need to be improved. Summary of the Invention

[0008] In view of at least one of the problems in the prior art, the present invention aims to provide a low-density warm-rolled steel sheet and a method for preparing the same.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] According to a first aspect of the present invention, a method for preparing a low-density warm-rolled steel sheet is provided, comprising the following steps:

[0011] (a) Smelting process: Smelting and casting into ingots according to the chemical composition of low-density warm-rolled steel sheet, wherein the chemical composition of low-density warm-rolled steel sheet by weight percentage includes: C: 0.25% to 0.35%, Si: 0.30% to 0.60%, Mn: 4.5% to 5.8%, P≤0.018%, S≤0.008%, Als: 2.5% to 3.8%, with the balance being Fe and unavoidable impurities;

[0012] (b) Forging process: After heating the ingot to 1210℃~1250℃ and holding it for 3~5 hours, the ingot is forged into a slab with a final forging temperature ≥850℃;

[0013] (c) Hot rolling process: After heating the slab to 1230℃~1270℃ and holding it for 3~7 hours, hot roll it to the first set thickness specification of strip steel. The final rolling temperature is 880℃~940℃. After rolling, the strip steel is air cooled to room temperature.

[0014] (d) Warm rolling process: The hot-rolled strip is heated to 630℃~670℃ and then warm rolled to form a strip of the second set thickness specification. The reduction rate per pass is 12%~18%.

[0015] (e) Heat treatment process: The warm-rolled strip is heated to 730℃~780℃ and held for 10~20 minutes, then cooled to room temperature to obtain low-density warm-rolled steel sheet.

[0016] Preferably, the chemical composition of the low-density warm-rolled steel sheet, by weight percentage, includes: C: 0.28%–0.32%, Si: 0.40%–0.55%, Mn: 4.8%–5.5%, P≤0.012%, S≤0.005%, Als: 2.8%–3.3%, with the balance being Fe and unavoidable impurities.

[0017] According to some embodiments of the present invention, in step (c), after the slab is heated and held at a certain temperature and before hot rolling, the iron oxide scale on the surface of the slab is removed.

[0018] According to some embodiments of the present invention, in step (d), the strip is pickled before warm rolling.

[0019] According to some embodiments of the present invention, the first set thickness specification is 4.0 to 7.0 mm, and the second set thickness specification is 0.8 to 2.0 mm.

[0020] According to some embodiments of the present invention, in step (b), the ingot is heated to 1210°C to 1250°C at a rate of 7 to 9°C / min.

[0021] According to some embodiments of the present invention, in step (e), the strip is air-cooled to room temperature.

[0022] According to a second aspect of the present invention, a low-density warm-rolled steel sheet is provided, which is prepared by means of the method according to the first aspect of the present invention.

[0023] According to some embodiments of the present invention, the low-density warm-rolled steel sheet has a yield strength of 580–670 MPa, a tensile strength of 760–850 MPa, and an elongation A. 50 The strength ranges from 26.0% to 33%, and the yield strength ratio is 0.70 to 0.88.

[0024] According to some embodiments of the present invention, the microstructure of the low-density warm-rolled steel sheet is composed of banded δ-ferrite, acicular α-ferrite, lath martensite and retained austenite, wherein banded δ-ferrite accounts for 15%-20%, acicular α-ferrite accounts for 30%-35%, lath martensite accounts for 35%-40%, and retained austenite accounts for 10%-15%.

[0025] By adopting the above technical solution, the present invention has the following beneficial effects:

[0026] This invention, through compositional design of alloying elements and their contents, combined with a process flow of smelting, forging, hot rolling, warm rolling, and heat treatment, produces low-density warm-rolled steel sheets with a yield strength of 580–670 MPa, a tensile strength of 760–850 MPa, and an elongation of A. 50 The yield strength reaches 26.0-33%, and the yield strength ratio reaches 0.70-0.88. The preparation method used in this invention is simple and efficient.

[0027] This invention reduces the density of steel by adding the lightweight element Al, thereby reducing the material density while maintaining high plasticity. Al is a typical ferrite-forming element that expands the ferrite phase region (δ-F, α-F). During warm rolling, this invention rapidly heats the strip to the two-phase region, causing some martensite to transform into austenite, which is beneficial for plastic deformation. At the same time, the relatively low heating temperature inhibits recrystallization, which is conducive to further refining the grains. This provides raw materials with good shape / thickness accuracy for subsequent heat treatment and provides technical support for the development of high-strength, low-density automotive steel. Attached Figure Description

[0028] Figure 1 A flowchart of the method for preparing low-density warm-rolled steel sheet provided by the present invention;

[0029] Figure 2 Metallographic micrographs of low-density warm-rolled steel sheets prepared by the method of the present invention;

[0030] Figure 3 Scanning electron microscope (SEM) images of low-density warm-rolled steel sheets prepared using the method of this invention;

[0031] Figure 4 The stress-strain curve of the low-density warm-rolled steel sheet prepared by the method of the present invention is shown. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] Specific embodiments of the invention are disclosed herein as needed; however, it should be understood that the embodiments disclosed herein are merely examples of the invention that may be implemented in various alternative forms. In the following description, various operating parameters and components are described in several contemplated embodiments. These specific parameters and components are provided as examples only and are not intended to be limiting.

[0034] According to a first aspect of the present invention, a method for preparing a low-density warm-rolled steel sheet is provided. For example... Figure 1 As shown, the method includes the following steps:

[0035] (a) Smelting process: Smelting and casting into ingots according to the chemical composition of low-density warm-rolled steel sheet, wherein the chemical composition of low-density warm-rolled steel sheet by weight percentage includes: C: 0.25% to 0.35%, Si: 0.30% to 0.60%, Mn: 4.5% to 5.8%, P≤0.018%, S≤0.008%, Als: 2.5% to 3.8%, with the balance being Fe and unavoidable impurities;

[0036] (b) Forging process: After heating the ingot to 1210℃~1250℃ and holding it for 3~5 hours, the ingot is forged into a slab with a final forging temperature ≥850℃;

[0037] (c) Hot rolling process: After heating the slab to 1230℃~1270℃ and holding it for 3~7 hours, hot roll it to the first set thickness specification of strip steel. The final rolling temperature is 880℃~940℃. After rolling, the strip steel is air cooled to room temperature.

[0038] (d) Warm rolling process: The hot-rolled strip is heated to 630℃~670℃ and then warm rolled to form a strip of the second set thickness specification. The reduction rate per pass is 12%~18%.

[0039] (e) Heat treatment process: The warm-rolled strip is heated to 730℃~780℃ and held for 10~20 minutes, then cooled to room temperature to obtain low-density warm-rolled steel sheet.

[0040] Preferably, the chemical composition of the low-density warm-rolled steel sheet by weight percentage includes: C: 0.28% to 0.32%, Si: 0.40% to 0.55%, Mn: 4.8% to 5.5%, P ≤ 0.012%, S ≤ 0.005%, Als: 2.8% to 3.3%, with the balance being Fe and unavoidable impurities.

[0041] The roles of alloying elements in this low-density warm-rolled steel sheet are as follows:

[0042] Carbon: C is an important austenitic element in steel, which can stabilize the austenitic structure and promote density reduction. Simultaneously, C can react with microalloying elements in steel to form nanoscale carbides, and with Mn and Al to form κ-carbides ((Fe,Mn)3AlC). The combined effect of these two elements produces precipitation strengthening, increasing the strength of the steel. Too low a C content will cause instability in the austenitic structure of the steel, reduce the amount of carbide precipitation, and decrease the strength and toughness of low-density steel. However, too high a C content will promote coarse austenitic grain boundaries and the formation of κ-carbides, impairing the elongation of low-density steel. Therefore, the C content in this invention is 0.25%–0.35%, preferably 0.28%–0.32%.

[0043] Silicon (Si) can dissolve in ferrite and austenite to improve the strength of steel, second only to C and P, and stronger than elements such as Mn, Cr, Ti, and Ni. Si can also inhibit the precipitation of carbides in ferrite, allowing dissolved C atoms to fully accumulate in austenite, thereby improving its stability. Too low a Si content makes it difficult to obtain retained austenite at room temperature. However, when the Si content is too high, the surface oxide scale formed by Si in the heating furnace is difficult to remove, increasing the difficulty of dephosphorization; simultaneously, it easily accumulates on the surface during annealing to form SiO2, leading to surface defects such as incomplete plating. Therefore, the Si content of this invention is 0.30–0.60%, preferably 0.40–0.55%.

[0044] Manganese (Mn) is an austenitizing element. Adding Mn can expand the austenite phase region and increase the austenite content, thereby increasing the stacking fault energy of steel, suppressing martensitic transformation, generating dense twins during deformation, and effectively improving the elongation of steel. However, a significant increase in manganese content will lead to increased costs and severe segregation. Therefore, in this invention, the Mn content is 4.5% to 5.8%, preferably 4.8% to 5.5%.

[0045] Aluminum: The density of Al is 2.7 g / cm³. 3 It is far below 7.85 g / cm³ 3 The Fe density can be significantly reduced, thus lowering the material density. A certain Al content can also significantly improve the hot deformation resistance and corrosion resistance of steel, delay dynamic cracking, and significantly increase the stacking fault energy of steel, altering the deformation mechanism. Al-containing medium-manganese steel can provide some buffering effect during violent collisions. However, considering that Al is a strong ferritinizing element, excessively high Al content can easily promote the formation of the ferrite phase and reduce the austenite phase content. Therefore, the Al content in this invention is 2.5%–3.8%, preferably 2.8%–3.3%.

[0046] Phosphorus: In steel, phosphorus (P) is generally dissolved in ferrite and has a strong solid solution strengthening effect. However, during the solidification of the slab, phosphorus (P) segregates along columnar or equiaxed grain boundaries, making the slab brittle at both high and room temperatures and potentially causing cracks. Furthermore, after processing, P increases the ductility-brittle transition temperature of steel and makes it more susceptible to hydrogen embrittlement. Therefore, the P content, expressed as a weight percentage, is preferably ≤0.018%, and more preferably ≤0.012%.

[0047] Sulfur: S is an impurity element in steel, which tends to segregate at grain boundaries and forms low-melting-point FeS with Fe in the steel, reducing the toughness of the steel. In addition, S can also form inclusions such as MnS, which can cause cracking when the steel is hot-rolled or cold-rolled. Therefore, the S content is set in the range of ≤0.008% by weight, preferably ≤0.005%.

[0048] In this invention, by designing the composition of alloying elements and their contents in conjunction with corresponding process steps, a steel plate combining high strength, high elongation, and low density is obtained. In particular, by adding the lightweight element Al, the density of the steel is reduced, thereby reducing the material density while ensuring strength and plasticity. Al is a typical ferrite forming element that expands the ferrite phase region (δ-F, α-F). In order to obtain good strength and plasticity, austenite forming elements (usually C and Mn) need to be added, which will improve the hardenability of the strip steel.

[0049] In step (b), the ingot heating temperature is typically, but not limited to, for example, 1210°C, 1220°C, 1230°C, 1240°C, or 1250°C; the ingot holding time is typically, but not limited to, for example, 3 hours, 4 hours, or 5 hours; and the final forging temperature is typically, but not limited to, for example, 850°C, 860°C, 870°C, 880°C, 890°C, or 900°C.

[0050] Preferably, in step (b), after slowly heating the ingot to 1230°C and holding it at that temperature for 4 hours, the ingot is forged into a slab with a final forging temperature ≥850°C. Here, the reason for slow heating is to make the composition and structure of each micro-zone of the strip more uniform. Slow heating means heating at a rate of 7-9°C / min.

[0051] In step (c), the slab heating temperature is typically, but not limited to, for example, 1230°C, 1240°C, 1250°C, 1260°C, or 1270°C; the slab holding time is typically, but not limited to, for example, 3 hours, 4 hours, 5 hours, 6 hours, or 7 hours; and the final rolling temperature is typically, but not limited to, for example, 880°C, 890°C, 900°C, 910°C, 920°C, 930°C, or 940°C.

[0052] In some embodiments, in step (c), after the slab is heated and held at a certain temperature and before hot rolling, the iron oxide scale on the surface of the slab is removed using a descaling device.

[0053] Preferably, in some embodiments, in step (c), the slab is heated to 1250±20℃ and held for 4 hours to remove iron oxide scale, rolled in 7 passes to 4.0~7.0mm, and finished at a final rolling temperature of 880~940℃. The rolled strip is then air-cooled to room temperature.

[0054] In step (d), the warm rolling temperature is typically, but not limited to, 630°C, 640°C, 650°C, 660°C or 670°C, and the reduction rate per pass is typically, but not limited to, 12%, 13%, 14%, 15%, 16%, 17% or 18%.

[0055] In some embodiments, in step (d), the strip is pickled before warm rolling.

[0056] Preferably, in some embodiments, in step (d), after pickling, the hot-rolled coil is warm-rolled to 0.8–2.0 mm using a four-roll cold / warm rolling mill. The strip is then heated to 650°C using the clamping end before warm rolling, with a reduction rate of 12–18% per pass. By heating to the two-phase region, some martensite transforms into austenite, ensuring the quality of the strip's edges. Simultaneously, the warm rolling temperature is biased towards the lower limit of the reverse transformation temperature, ensuring smooth rolling while suppressing recrystallization and refining the grains. Ferritic low-density steels (such as medium-manganese steel) generate martensite under air cooling conditions; direct cold rolling would risk edge cracking or even strip breakage. Therefore, intermediate annealing (reverse transformation annealing) is usually performed before cold rolling. During cold rolling, austenite is subjected to plastic deformation and transforms back into martensite, resulting in a lower reduction rate per pass and more complex processes for ferritic low-density steels (such as medium-manganese steel). Some researchers employ a method of warm rolling by heating the strip to the two-phase region and performing an intermediate annealing before each rolling pass, which is detrimental to smooth production. This invention provides a warm rolling method that utilizes electric heating at the strip clamping end to rapidly heat the strip to the two-phase region, causing some martensite to transform into austenite, which facilitates plastic deformation. Simultaneously, the relatively low heating temperature inhibits recrystallization, further refining the grain size and providing raw materials with good shape / thickness accuracy for subsequent heat treatment. This provides technical support for the development of high-strength, low-density automotive steel.

[0057] In step (e), the strip heating temperature is typically, but not limited to, 730°C, 740°C, 750°C, 760°C, 770°C, or 780°C, and the strip holding time is typically, but not limited to, 10 min, 11 min, 12 min, 13 min, 14 min, 15 min, 16 min, 17 min, 18 min, 19 min, or 20 min.

[0058] In some embodiments, in step (e), the strip is heated to 730–780°C and held for 10–20 minutes, then slowly cooled to room temperature in air to obtain good microstructure properties.

[0059] According to a second aspect of the present invention, a low-density warm-rolled steel sheet is provided, which is prepared by means of the method described in the first aspect of the present invention.

[0060] The yield strength of low-density warm-rolled steel sheet is 580–670 MPa, the tensile strength is 760–850 MPa, and the elongation is A. 50 The strength ranges from 26.0% to 33%, and the yield strength ratio is 0.70 to 0.88.

[0061] The microstructure of low-density warm-rolled steel sheet consists of banded δ-ferrite, acicular α-ferrite, lath martensite, and retained austenite. Among them, banded δ-ferrite accounts for 15%-20%, acicular α-ferrite accounts for 30%-35%, lath martensite accounts for 35%-40%, and retained austenite accounts for 10%-15%.

[0062] The present invention will now be described in detail through specific embodiments.

[0063] Example

[0064] The following provides seven groups of low-density warm-rolled steel sheets, whose chemical compositions are shown in Table 1.

[0065] Table 1 Chemical composition (wt.%) of low-density warm-rolled steel sheet

[0066] serial number C Si Mn P S Als Example 1 0.29 0.48 5.3 0.010 0.004 3.0 Example 2 0.30 0.45 5.1 0.008 0.003 3.2 Example 3 0.32 0.42 4.9 0.007 0.003 2.9 Example 4 0.25 0.30 4.5 0.018 0.008 3.8 Example 5 0.35 0.60 5.8 0.015 0.007 2.5 Example 6 0.32 0.40 5.5 0.010 0.007 3.3 Example 7 0.28 0.55 4.8 0.012 0.005 2.8

[0067] The specific process for preparing the aforementioned low-density warm-rolled steel sheet is as follows:

[0068] A. Smelting process: Through smelting process, low-density steel slabs with the chemical composition shown in Table 1 are prepared;

[0069] B. Forging and hot rolling process: The ingot is heated to 1230℃ and held for 4 hours to be forged into a slab. The slab is then heated, hot rolled and air-cooled to obtain a hot-rolled coil. The specific hot rolling process parameters are shown in Table 2.

[0070] Table 2 Main process parameters for forging and hot rolling of low-density warm-rolled steel plates

[0071] serial number Final forging temperature / °C Hot rolling heating temperature / ℃ Rolling temperature / ℃ Final rolling temperature / ℃ Hot-rolled thickness / mm Example 1 890 1266 1115 918 5.0 Example 2 880 1258 1131 906 4.5 Example 3 900 1243 1144 925 6.0 Example 4 850 1230 1118 880 4.0 Example 5 860 1250 1120 940 7.0 Example 6 880 1260 1135 900 5.0 Example 7 870 1270 1140 920 6.0

[0072] C. Warm rolling process: After pickling the hot-rolled coil, the strip is warm rolled to 1.0-2.5mm using a four-roll cold / warm rolling mill at a temperature of 650℃. The thickness of the warm-rolled strip and the cold rolling reduction rate are shown in Table 3.

[0073] Table 3. Raw materials, finished products, and reduction rates for cold rolling of low-density warm-rolled steel sheets

[0074] serial number Hot-rolled plate thickness / mm Cold-rolled sheet thickness / mm Total reduction rate in warm rolling / % Example 1 5.0 1.5 70 Example 2 4.5 1.0 78 Example 3 6.0 1.8 70 Example 4 4.0 0.8 80 Example 5 7.0 2.0 68 Example 6 5.0 1.5 70 Example 7 6.0 1.8 70

[0075] D. Heat treatment process: Heat the strip steel to 730-780℃ and hold for 10-30 minutes, then slowly cool it to room temperature. The specific process parameters are shown in Table 4.

[0076] Table 4 Main process parameters for low-density warm-rolled steel sheets

[0077] serial number Heating rate / ℃ / s Heating temperature / ℃ Insulation time / min Example 1 1.2 750 15 Example 2 1.5 735 12 Example 3 1.0 765 18 Example 4 1.0 730 10 Example 5 1.0 780 15 Example 6 1.0 750 20 Example 7 1.0 760 18

[0078] Comparative Example

[0079] The corresponding products were prepared as comparative examples according to the processes disclosed in documents CN 108998734 A, CN 108396244 A, and CN 115323275 B.

[0080] The preparation process of CN 108998734 A is as follows: (1) Hot rolling: The slab is subjected to homogenization treatment, heated to 1170~1230℃, and isothermal for 2~2.5h; the initial rolling temperature is 1080~1160℃, the final rolling temperature is 900~950℃, the reduction rate per pass is 30%, the cumulative reduction rate is ≥95%, and after hot rolling, it is cooled to 680~740℃ for coiling; (2) Intermediate annealing: The annealing temperature is 720~780℃, and the annealing time is 0.5~2h; (3) Cold rolling: The reduction per pass is 0.1~0.2mm, the cumulative total reduction rate is 75~90%, and a cold-rolled steel plate with a thickness of 0.5~1.2mm is obtained. (4) Two-phase annealing: The cold-rolled steel plate is heated to 820~850℃, isothermal for 20~40s, and then cooled to room temperature. (The case study only focuses on performance, not organizational proportions)

[0081] The preparation process of CN 108396244 A is as follows: (1) Forging: heating the ingot to 1170~1230℃ and holding for 2~4h; then forging at 950~1050℃ to obtain a steel billet; (2) Hot rolling: heating the steel billet to 1180~1250℃ and holding for 1~5h; the steel billet after holding is subjected to multiple hot rolling passes, with an initial rolling temperature of 1120~1160℃ and a final rolling temperature of ≥950℃, and a total cumulative reduction rate of 80~90%. (2) Solution treatment: The hot-rolled plate is kept at 990-1010℃ for 1-2 hours and then directly quenched in water to room temperature; (3) Cold rolling: The plate after solution treatment is subjected to multiple cold rolling passes with a total cumulative reduction of 70-80% to obtain a cold-rolled plate; (4) Heat treatment: The cold-rolled plate is kept at 980-1020℃ for 5-15 minutes and then quenched in water to room temperature to obtain a cold-rolled medium-manganese high-aluminum low-density steel. (No specific microstructure ratio is specified in the case study)

[0082] The preparation process of CN 115323275 B is as follows: (1) Hot rolling: After heating the ingot to 1200±20℃ and holding it for 2~4h for homogenization treatment, it is rolled in 7 passes starting at a starting temperature of 1200±20℃. The final rolling temperature is controlled at 850±50℃, and the total deformation is 92.5%. The resulting hot-rolled plate is air-cooled to room temperature. (2) Warm rolling: First, it is held at 750±20℃ for 30min, and then rolled in 8 passes to 1.5mm. The reduction in each pass is 10~20%. Between passes, the steel plate is held at 750±20℃ for 5min. The warm-rolled steel plate is air-cooled to room temperature. (3) Bainitic isothermal treatment: First, it is held at 800±50℃ for 120s, and then rapidly quenched to 400±20℃ and held for 300s. Finally, it is air-cooled to room temperature. The microstructure of the experimental steel consisted of ferrite, bainite, and retained austenite, with volume fractions of 36±1%, 45±1%, and 20±1%, respectively.

[0083] Performance Characterization

[0084] The microstructure of the low-density warm-rolled steel sheet (corresponding to Example 1) prepared by the process of the present invention is as follows: Figures 2 to 3 As shown, the stress-strain curves are... Figure 4 As shown. From Figure 2 and Figure 3 As can be seen from the data, the microstructure of the low-density warm-rolled steel plate prepared by this invention consists of banded δ-ferrite, acicular α-ferrite, lath martensite, and retained austenite, and the microstructure is uniform.

[0085] The properties of the above-mentioned low-density warm-rolled steel plates were tested according to GB / T228-2010 "Metallic materials, tensile test at room temperature", as shown in Table 5.

[0086] Table 5 Mechanical Properties of Low-Density Warm-Rolled Steel Sheets

[0087]

[0088] As shown in Table 5, the low-density warm-rolled steel sheet prepared by the process of the present invention, compared with the prior art, has excellent elongation and density while ensuring high yield strength and tensile strength.

Claims

1. A method for preparing low-density warm-rolled steel sheet, characterized in that, Includes the following steps: (a) Smelting process: Smelting and casting into ingots according to the chemical composition of low-density warm-rolled steel plate, wherein the chemical composition of low-density warm-rolled steel plate includes, by weight percentage: C: 0.25%~0.35%, Si: 0.30%~0.60%, Mn: 4.5%~5.8%, P≤0.018%, S≤0.008%, Als: 3.2%~3.8%, with the balance being Fe and unavoidable impurities; (b) Forging process: After heating the ingot to 1210℃~1250℃ and holding it for 3~5 hours, the ingot is forged into a slab with a final forging temperature ≥850℃; (c) Hot rolling process: After heating the slab to 1230℃~1270℃ and holding it for 3~7 hours, hot roll it to the first set thickness specification of strip steel. The final rolling temperature is 880℃~940℃. After rolling, the strip steel is air cooled to room temperature. (d) Warm rolling process: The hot-rolled strip is heated to 660℃~670℃ and then warm rolled to produce a second strip of a set thickness specification. The reduction rate per pass is 12%~18%. (e) Heat treatment process: The warm-rolled strip is heated to 730℃~780℃ and held for 10~20 minutes, then cooled to room temperature to obtain low-density warm-rolled steel sheet.

2. The method for preparing low-density warm-rolled steel sheet according to claim 1, characterized in that, The chemical composition of the low-density warm-rolled steel sheet, by weight percentage, includes: C: 0.28%~0.32%, Si: 0.40%~0.55%, Mn: 4.8%~5.5%, P≤0.012%, S≤0.005%, Als: 3.2%~3.3%, with the balance being Fe and unavoidable impurities.

3. The method for preparing low-density warm-rolled steel sheet according to claim 1, characterized in that, In step (c), after the slab is heated and held at a constant temperature and before hot rolling, the iron oxide scale on the surface of the slab is removed.

4. The method for preparing low-density warm-rolled steel sheet according to claim 1, characterized in that, In step (d), the strip is pickled before warm rolling.

5. The method for preparing low-density warm-rolled steel sheet according to claim 1, characterized in that, The first set thickness specification is 4.0~7.0mm, and the second set thickness specification is 0.8~2.0mm.

6. The method for preparing low-density warm-rolled steel sheet according to claim 1, characterized in that, In step (b), the ingot is heated to 1210°C to 1250°C at a rate of 7 to 9°C / min.

7. The method for preparing low-density warm-rolled steel sheet according to claim 1, characterized in that, In step (e), the air is cooled to room temperature.

8. A low-density warm-rolled steel sheet, characterized in that, It is prepared using the method described in any one of claims 1-6.

9. The low-density warm-rolled steel sheet according to claim 8, characterized in that, The low-density warm-rolled steel sheet has a yield strength of 580~670MPa, a tensile strength of 760~850MPa, and an elongation A. 50 The strength ranges from 26.0% to 33%, and the yield strength ratio is 0.70 to 0.

88.

10. The low-density warm-rolled steel sheet according to claim 8, characterized in that, The microstructure of the low-density warm-rolled steel sheet consists of banded δ-ferrite, acicular α-ferrite, lath martensite, and retained austenite, wherein banded δ-ferrite accounts for 15%-20%, acicular α-ferrite accounts for 30%-35%, lath martensite accounts for 35%-40%, and retained austenite accounts for 10%-15%.

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