A vanadium microalloyed lightweight high-strength steel and its preparation method

Abstract

The present invention relates to a vanadium microalloyed lightweight high-strength steel and a preparation method thereof. The vanadium microalloyed lightweight high-strength steel comprises the following components by mass percentage: C: 0.15% to 0.30%, Si: 0.25% to 0.50%, Mn: 3.5% to 4.8%, P≤0.015%, S≤0.010%, Als: 3.0% to 4.2%, V: 0.03% to 0.07%, and the remaining elements are Fe and inevitable impurities. The present invention overcomes the problem of edge cracking and even band breakage easily occurring during cold rolling of ferrite-based lightweight steel (such as medium manganese steel) through warm rolling, effectively ensuring smooth production. At the same time, while V microalloying refines the grains and precipitates a second phase to strengthen the matrix, some V-containing second phases undergo "re-dissolution" during heating to improve the stability of austenite, thereby obtaining good strength and plasticity, and contributing positively to the achievement of the "dual carbon goals" of the automotive industry.

Description

Technical Field

[0001] The present invention belongs to the technical field of cold-rolled plate and strip production, and particularly relates to a vanadium microalloyed lightweight high-strength steel plate suitable for production by a warm rolling / cold rolling mill and a preparation method thereof. Background Art

[0002] The automotive manufacturing industry is a pillar of the national economy. With energy shortages and environmental pollution becoming increasingly prominent, lightweighting has become a key focus for carbon emission reduction in the automotive industry. Currently, there are three main approaches to achieving vehicle lightweighting: First, the use of lightweight raw materials, such as aluminum alloys, magnesium alloys, and carbon fiber composites. While the use of lightweight raw materials can reduce the overall weight of a vehicle, these materials suffer from limitations such as complex forming processes, poor welding performance, low collision absorption, high costs, and low production volumes, limiting their market adoption. Second, the use of ultra-high-strength steel in place of conventional automotive steel reduces the thickness of steel sheets to reduce vehicle weight. However, as steel sheet strength increases, forming capacity decreases, leading to cracking, wrinkling, and excessive springback. Furthermore, as steel sheet thickness decreases, the stiffness of certain vehicle components decreases excessively, leading to acoustic issues that create uncomfortable conditions for passengers, limiting their widespread adoption. Third, the development of a steel grade that combines high strength, high elongation, and low density. These lightweight steels offer excellent mechanical and physical properties and significant weight reduction, making them a hot research topic for automakers and their suppliers.

[0003] By adding the lightweight element Al, the density of the steel can be reduced while maintaining strong ductility. Al is a typical ferrite-forming element, expanding the ferrite phase region (δ-F, α-F). To achieve good strong ductility, austenite-forming elements (usually C and Mn) need to be added, which improves the hardenability of the strip. Ferritic lightweight steels (such as medium-manganese steel) form martensite under air cooling conditions. Direct cold rolling poses risks such as edge cracking and even strip breakage. Intermediate annealing (reverse phase transformation annealing) is usually performed before cold rolling. During the cold rolling process, austenite is affected by plastic deformation and transforms back into martensite, resulting in low pass reduction ratios and complex processes for ferritic lightweight steels (such as medium-manganese steel). Some researchers use the method of heating the strip to the two-phase region before warm rolling, and performing an intermediate annealing before each rolling pass, which is not conducive to smooth production.

[0004] Through relevant literature search, patents similar to low-density warm-rolled steel plates and their preparation methods include:

[0005] CN 107841691 B discloses a 750MPa ultra-high strength Fe-Mn-Al-C lightweight cast steel and its preparation method, wherein the chemical composition by weight is as follows: C: 0.60-1.50%, Mn: 10.00-14.00%, Al: 6.50-12.00%, Si: 0.50-1.50%, Ni: 0.15-0.55%, Cr: 0.15-0.45%, Cu: 0.01-0.06%, Mo: 0.10 ~0.60%, V: 0.05~0.25%, Ti: 0.05~0.35%, Nb: 0.01~0.08%, Ce: 0.001~0.010%, P≤0.008%, S≤0.008%, of which the weight percentages of Mn and Al meet the requirement of 18%≤Mn+Al≤22%; the weight percentages of Nb, V and Ti meet the requirement of 0.25%≤Nb+V+Ti≤0.50%, and the remainder is Fe and unavoidable impurities. The heat treatment of the casting is carried out in a box-type heat treatment furnace. First, solution treatment is carried out at a solution temperature of 900~1100℃, holding for 0.5~3 hours, and water cooling to room temperature; then aging treatment is carried out within 4 hours at an aging temperature of 350~600℃, holding for 30min~15 hours, and air cooling to room temperature. Finally, ultra-high strength Fe-Mn-Al-C low-density cast steel is obtained. This patent adds more precious alloy elements (Ni, Mo, Nb), which increases the alloy cost. The lightweight steel has a high yield strength ratio (>0.94) and low plasticity (elongation of about 20%), which makes it difficult to meet the needs of complex parts.

[0006] CN 111926264 A discloses a lightweight steel and its manufacturing method. Its chemical composition, by weight, is as follows: 0.8-1.6% C, 6.0-9.5% Al, the sum of Mn, Nb, V, Mo, and Ti ≤ 8%, with the remainder being Fe and unavoidable impurities. The hot-rolled steel is heated to a temperature above Ac1 and 20-130°C below the critical temperature Ac3, and held at that temperature for 1-60 minutes. The steel is then cooled to 0-50°C below the critical temperature Ac1 at a cooling rate of 0.1-200°C / h, and then cooled to room temperature. The patent's excessively high C content results in poor weldability, and the rolling process is ambiguous and not a reference. Alloys with high precious metal content (Mn+Nb+V+Mo+Ti ≤ 8%) are also costly.

[0007] CN 108998734 A discloses an ultra-high-strength, ductile, cold-rolled Mn-Al TRIP steel sheet and a rapid annealing method for its preparation. The steel sheet's chemical composition, by weight, is as follows: 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 cold-rolled steel sheet is rapidly heated in a resistance continuous annealing furnace at a melting temperature of 1550-1600°C, with a heating rate of 100-400°C / s and a cooling rate to room temperature exceeding 10°C / s. The slab undergoes microstructure homogenization treatment, is heated to 1170-1230°C, and is isothermaled for 2-2.5 hours. The starting rolling temperature is 1080-1160°C, the finishing rolling temperature is 900-950°C, the reduction rate per pass is 30%, and the cumulative reduction rate is ≥95%. After hot rolling, the slab is heated to 680-740°C for coiling. Intermediate annealing: The annealing temperature is 720-780°C, and the annealing time is 0.5-2 hours. Cold rolling: The reduction per pass is 0.1-0.2mm, and the cumulative total reduction rate is 75-90%, resulting in a cold-rolled steel sheet with a thickness of 0.5-1.2mm. Two-phase zone annealing: The cold-rolled steel sheet is heated to 820-850°C, is isothermaled for 20-40 seconds, and then cooled to room temperature. This patented method performs intermediate annealing before cold rolling, which is a relatively long process; the reduction in each cold rolling pass is 0.1~0.2mm, and the cumulative reduction rate is 75~90%. It requires repeated cold rolling passes, and the process is relatively complicated.

[0008] CN 108396244 A discloses a cold-rolled medium-manganese high-aluminum lightweight steel and a preparation method thereof. The chemical composition of the cold-rolled medium-manganese high-aluminum lightweight steel is as follows by weight: 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°C and kept warm for 2-4 hours; then forged at 950-1050°C to obtain a billet; the billet is heated to 1180-1250°C and kept warm for 1-5 hours; the billet after being kept warm is subjected to multiple hot rolling, with a starting rolling temperature of 1120-1160°C, a finishing rolling temperature of ≥950°C, a total cumulative reduction rate of 80-90%, and water cooled to room temperature to obtain a hot-rolled plate; the hot-rolled plate is kept warm at 990-1010°C for 1-2 hours, and directly water quenched to room temperature; the plate after solution treatment is subjected to multiple cold rolling, with a total cumulative reduction rate of 70-80%, to obtain a cold-rolled plate; the cold-rolled plate is kept warm at 980-1020°C for 5-15 minutes, and water quenched to room temperature to obtain a cold-rolled medium manganese high aluminum lightweight steel. The higher C content (1.20-1.30%) of this patent is not conducive to obtaining good welding performance. Before cold rolling, "the plate after solution treatment is directly water quenched to room temperature" and multiple cold rollings are performed with a total cumulative reduction rate of 70-80%. Since the hardenability of medium manganese steel is relatively strong, although part of the martensite will be reversely transformed into austenite after solution treatment to improve the cold rolling performance, the transformation of austenite into martensite during the cold rolling process causes the strip to become hard and brittle, limiting the reduction amount of each pass, etc., which is not conducive to smooth production.

[0009] CN 115323275 B discloses a high-strength, high-toughness rare earth warm-rolled low-carbon, low-manganese TRIP steel and its preparation method. The steel's chemical composition, by weight percentage, is as follows: 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 remainder being Fe and unavoidable impurities. The steel is heated to 1200±20°C and held for 2-4 hours for homogenization. The steel is then rolled in seven passes starting at a starting temperature of 1200±20°C, with the final rolling temperature controlled at 850±50°C. The steel has a total deformation of 92.5%, and the resulting hot-rolled sheet is air-cooled to room temperature. During warm rolling, the steel is first held at 750±20℃ for 30 minutes, then rolled to 1.5mm in 8 passes, with a reduction of 10-20% per pass. Between passes, the steel plate is held at 750±20℃ for 5 minutes, and the warm-rolled steel plate is air-cooled to room temperature. The warm-rolled steel plate is then subjected to bainite isothermal treatment, with the specific process being: first, holding at 800±50℃ for 120 seconds, then rapidly quenching to 400±20℃ for 300 seconds, and finally air-cooling to room temperature. During warm rolling, the patented steel plate is held at 750±20℃ for 5 minutes between passes before rolling. This repeated heating process is cumbersome and inefficient.

[0010] Based on this, the existing technology still needs to be improved. Summary of the Invention

[0011] In response to the shortcomings of the existing technology, the present invention proposes a warm rolling method, which uses electric heating at the clamping end of the strip to quickly heat the strip to the two-phase region, causing part of the martensite to transform into austenite, which is conducive to plastic deformation. At the same time, the relatively low heating temperature inhibits recrystallization, which is conducive to further refinement, providing raw materials with good plate shape / thickness accuracy for subsequent heat treatment, and providing technical support for the development of high-strength, low-density automotive steel.

[0012] Specifically, according to one aspect of the present invention, a vanadium microalloyed lightweight high-strength steel is provided, which includes the following components in mass percentage: C: 0.15%~0.30%, Si: 0.25~0.50%, Mn: 3.5%~4.8%, P≤0.015%, S≤0.010%, Als: 3.0%~4.2%, V: 0.03~0.07%, and the remaining elements are Fe and unavoidable impurities.

[0013] In an embodiment of the present invention, the vanadium microalloyed lightweight high-strength steel includes the following components in mass percentage: C: 0.20%-0.28%, Si: 0.28%-0.46%, Mn: 3.8%-4.5%, P≤0.010%, S≤0.008%, Als: 3.2%-4.0%, V: 0.035%-0.060%, and the remaining elements are Fe and unavoidable impurities.

[0014] In the embodiment of the present invention, the yield strength of the vanadium microalloyed lightweight high-strength steel is 610-700 MPa, the tensile strength is 790-870 MPa, and the elongation A 50 It is 28.0~36.0%, and the yield strength ratio is 0.71-0.86.

[0015] In an embodiment of the present invention, the microstructure of the vanadium microalloyed lightweight high-strength steel consists of banded delta ferrite (20%-25%), ferrite (25%-30%), lath martensite (30%-35%), and retained austenite (15%-20%).

[0016] According to another aspect of the present invention, a method for preparing vanadium microalloyed lightweight high-strength steel is provided, comprising the following steps: a. smelting a vanadium microalloyed lightweight high-strength steel plate comprising the following components in percentage by mass: C: 0.15% to 0.30%, Si: 0.25% to 0.50%, Mn: 3.5% to 4.8%, P ≤ 0.015%, S ≤ 0.010%, Als: 3.0% to 4.2%, V: 0.03% to 0.07%, with the remaining elements being Fe and unavoidable impurities; b. forging the ingot: slowly heating the ingot to a first predetermined temperature, holding the temperature for a first predetermined time, and then forging the ingot into a slab; c. hot rolling: heating the slab to a second predetermined temperature, holding the temperature for a second predetermined time, removing iron oxide scale, and rolling the slab in multiple passes to a first predetermined thickness. The rolled strip is then air-cooled to room temperature to obtain a hot-rolled coil; d. Warm rolling process: After the hot-rolled coil is pickled, the strip is heated to a third predetermined temperature and then warm rolled; e. Heat treatment process: The strip is heated to a fourth predetermined temperature, kept at this temperature for a third predetermined time, and then slowly cooled to room temperature to obtain vanadium microalloyed lightweight high-strength steel.

[0017] In an embodiment of the present invention, in step b, the first predetermined temperature is 1200-1280° C., the first predetermined time is 3.5-4.5 hours, and the final forging temperature is ≥850° C.

[0018] In an embodiment of the present invention, in step c, the second predetermined temperature is 1240±20°C, the second predetermined time is 3.5~4.5h, the first predetermined thickness is 4.0~6.0mm, and the finishing temperature is 890~950°C.

[0019] In an embodiment of the present invention, step d includes: after pickling the hot-rolled coil, warm-rolling the strip to a second predetermined thickness using a four-roll cold / warm rolling mill, heating the strip to a third predetermined temperature using a clamping end, and then warm-rolling.

[0020] In an embodiment of the present invention, in step d, the second predetermined thickness is 0.8-2.0 mm, the third predetermined temperature is 600-730° C., and the reduction rate per pass is ≥10%.

[0021] In an embodiment of the present invention, in step e, the fourth predetermined temperature is 750-800° C., and the third predetermined time is 10-20 minutes.

[0022] This invention reduces the density of the steel by adding the lightweight element Al, maintaining high strength and ductility. V forms secondary phases such as VC, VN, and V(C, N), which precipitate and refine the grains, strengthening the matrix. Furthermore, some vanadium-containing secondary phases undergo "re-dissolution" at relatively low heating temperatures (above 700°C), enhancing the stability of the austenite. Al is a typical ferrite-forming element, expanding the ferrite phase region (δ-F and α-F). To achieve excellent strength and ductility, austenite-forming elements (typically C and Mn) are added, which improves the hardenability of the strip. Ferritic lightweight steels (such as medium-manganese steels) form martensite during air cooling. Direct cold rolling risks edge cracking and even strip breakage, so intermediate annealing (reverse transformation annealing) is typically performed before cold rolling. During the cold rolling process, austenite undergoes plastic deformation and transforms back into martensite, resulting in low reduction ratios and complex production processes for ferritic lightweight steels (such as medium-manganese steels). Some researchers heat the strip to the two-phase region before warm rolling, and perform an intermediate annealing before each rolling pass, which is detrimental to smooth production. Furthermore, the present invention provides a warm rolling method that utilizes electrical heating at the strip clamping end to rapidly heat the strip to the two-phase region, partially transforming martensite into austenite, facilitating plastic deformation. Simultaneously, the relatively low heating temperature inhibits recrystallization, facilitating further refinement. This provides raw material with excellent shape and thickness accuracy for subsequent heat treatment, providing technical support for the development of high-strength, low-density automotive steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic flow chart of a method for preparing vanadium microalloyed lightweight high-strength steel provided by the present invention is shown;

[0024] Figure 2 An optical microscope image of the vanadium microalloyed lightweight high-strength steel provided by the present invention is shown; and

[0025] Figure 3 The scanning electron microscope image of the vanadium microalloyed lightweight high-strength steel provided by the present invention is shown. DETAILED DESCRIPTION

[0026] It should be understood that the embodiments of the present invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in the present invention, it is readily apparent to those skilled in the art that various modifications are feasible without departing substantially from the teachings of the subject matter of the present invention. Accordingly, all such modifications should be included within the scope of the present invention. Without departing from the gist of the present invention, other replacements, modifications, variations, and deletions may be made to the design, operating conditions, and parameters of the following exemplary embodiments.

[0027] According to one aspect of the present invention, there is provided a vanadium microalloyed lightweight high-strength steel comprising the following components in mass percentage: C: 0.15%-0.30%, Si: 0.25%-0.50%, Mn: 3.5%-4.8%, P≤0.015%, S≤0.010%, Als: 3.0%-4.2%, V: 0.03%-0.07%, and the remaining elements are Fe and unavoidable impurities.

[0028] In an embodiment of the present invention, the vanadium microalloyed lightweight high-strength steel includes the following composition by mass percentage: C: 0.20%-0.28%, Si: 0.28%-0.46%, Mn: 3.8%-4.5%, P ≤ 0.010%, S ≤ 0.008%, Als: 3.2%-4.0%, V: 0.035%-0.060%. The remaining elements are Fe and unavoidable impurities. Als refers to the effective Al dissolved in the iron matrix.

[0029] The above-mentioned vanadium microalloyed lightweight high-strength steel plate has a yield strength of 610~700MPa, a tensile strength of 790~870MPa, and an elongation of A 50 The tensile strength of the steel is 28.0-36.0%, and the yield strength ratio is 0.71-0.86. Its structure is composed of banded delta ferrite (20%-25%), ferrite (25%-30%), lath martensite (30%-35%), and retained austenite (15%-20%).

[0030] According to another aspect of the present invention, a method for preparing vanadium microalloyed lightweight high-strength steel is provided, such as Figure 1 Said, it comprises the following steps:

[0031] a. Smelting process: Smelting a vanadium microalloyed lightweight high-strength steel plate having the following mass percentages and casting it into an ingot: C: 0.15% to 0.30%, Si: 0.25% to 0.50%, Mn: 3.5% to 4.8%, P ≤ 0.015%, S ≤ 0.010%, Als: 3.0% to 4.2%, V: 0.03% to 0.07%, and the remaining elements are Fe and unavoidable impurities;

[0032] b. Forging step: slowly heating the ingot to a first predetermined temperature and holding it for a first predetermined time, and then forging the ingot into a slab;

[0033] c. Hot rolling process: heating the slab to a second predetermined temperature and holding it for a second predetermined time, removing the scale by multiple rolling passes to a first predetermined thickness, and air-cooling the rolled strip to room temperature to obtain a hot-rolled coil;

[0034] d. Warm rolling process: After the hot-rolled coil is pickled, the strip is heated to a third predetermined temperature and then warm-rolled;

[0035] e. Heat treatment process: heating the steel strip to a fourth predetermined temperature, holding the temperature for a third predetermined time, and then slowly cooling the steel strip to room temperature to obtain vanadium microalloyed lightweight high-strength steel.

[0036] In an embodiment of the present invention, in step b, the first predetermined temperature is 1200-1280°C, the first predetermined time is 3.5-4.5 hours, and the final forging temperature is ≥850°C. Furthermore, step b specifically includes slowly heating the ingot to 1240°C and holding the temperature for 4 hours, then forging the ingot into a slab, with the final forging temperature being ≥850°C.

[0037] In an embodiment of the present invention, in step c, the second predetermined temperature is 1240±20°C, the second predetermined time is 3.5-4.5 hours, the first predetermined thickness is 4.0-6.0 mm, and the finishing rolling temperature is 890-950°C. Step c specifically includes heating the slab to 1240±20°C and holding the temperature for 4 hours, removing the iron oxide scale, and rolling the slab in multiple passes to a thickness of 4.0-6.0 mm. The finishing rolling temperature is 890-950°C, and the rolled strip is air-cooled to room temperature.

[0038] In an embodiment of the present invention, step d includes: after pickling the hot-rolled coil, warm-rolling the strip to a second predetermined thickness using a four-roll cold / warm rolling mill, heating the strip to a third predetermined temperature using a clamping end before warm rolling. The second predetermined thickness is 0.8-2.0 mm, the third predetermined temperature is 600-730°C, and a reduction ratio of 10% or more per pass is used. Step d specifically includes: after pickling the hot-rolled coil, warm-rolling the strip to a second predetermined thickness using a four-roll cold / warm rolling mill, heating the strip to a temperature of 600-730°C using a clamping end before warm rolling, with a reduction ratio of 10% or more per pass. By heating the strip until a portion of the martensite in the two-phase region transforms to austenite, the edge quality of the strip is ensured. Furthermore, the warm rolling temperature is biased towards the lower limit of the reverse transformation temperature, which not only ensures smooth rolling but also suppresses recrystallization and refines the grain size.

[0039] In an embodiment of the present invention, in step e, the fourth predetermined temperature is 750-800°C, the third predetermined time is 10-20 minutes, and step e specifically includes heating the steel strip to 750-800°C, holding the temperature for 10-20 minutes, and then slowly cooling the steel strip to room temperature.

[0040] By the above-mentioned method of the present invention, a vanadium microalloyed lightweight high-strength steel plate with excellent performance can be obtained, and each alloying element plays a different role in the vanadium microalloyed lightweight high-strength steel plate, specifically:

[0041] Carbon: C is an important austenitic element in steel. It can stabilize the austenitic structure and also promote density reduction. At the same time, C can react with the microalloying elements in the steel to generate nano-scale carbides, and react with Mn and Al elements to generate κ-carbides ((Fe, Mn) 3AlC). The two act together to produce precipitation strengthening and improve the strength of the steel. If the C content is too low, the austenitic structure in the steel will be unstable, the amount of carbide precipitation in the steel will be reduced, and the strength and toughness of the lightweight steel will be reduced. However, if the C content is too high, it will promote the formation of coarse κ-carbides at the austenite grain boundaries and destroy the elongation of the lightweight steel. Therefore, the C content of the present invention is 0.15% to 0.30%, preferably 0.20 to 0.28%.

[0042] Silicon: Si can be dissolved in ferrite and austenite to increase the strength of steel. Its effect is second only to C and P, and is stronger than elements such as Mn, Cr, Ti, and Ni. Si can also inhibit the precipitation of carbides in ferrite, allowing the dissolved C atoms to fully enrich 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. At the same time, it is easy to enrich on the surface to form SiO2 during annealing, resulting in surface defects such as plating leakage. Therefore, the Si content of the present invention is 0.25-0.50%, preferably 0.28-0.46%.

[0043] Manganese: Mn is an austenitizing element. Adding Mn can expand the austenite phase and increase the austenite content, improve the stacking fault energy of the steel, inhibit martensitic transformation, produce dense twins during deformation, and effectively increase the elongation of the steel. However, a significant increase in Mn content increases costs and also causes severe segregation. Therefore, in the present invention, the Mn content is 3.5% to 4.8%, preferably 3.8 to 4.5%.

[0044] Aluminum: The density of Al is 2.7g / cm3, which is much lower than the density of Fe of 7.85g / cm3, and can significantly reduce the density of the material. A certain Al content can also significantly improve the thermal deformation resistance of steel, improve the corrosion resistance of steel, and delay dynamic cracking. Al can also significantly increase the stacking fault energy of steel and change the deformation mechanism. Medium manganese steel containing Al can have a certain buffering effect in the event of a violent collision. However, considering that Al is a strong ferritizing element, an excessively high Al content can easily promote the formation of ferrite phase and reduce the austenite phase content. Therefore, the Al content in the present invention is 3.0%~4.2%, preferably 3.2~4.0%.

[0045] Phosphorus: Phosphorus (P) in steel is generally dissolved in ferrite, providing a strong solid-solution strengthening effect. However, during slab solidification, phosphorus (P) segregates along columnar or equiaxed grain boundaries, making the slab brittle at high temperatures and room temperature and potentially causing cracks. Furthermore, after processing, P increases the steel's ductile-brittle transition temperature and makes it susceptible to hydrogen embrittlement. Therefore, the P content is set within a range of ≤ 0.015% by mass, preferably ≤ 0.010%.

[0046] Sulfur: Sulfur (S) is an impurity element in steel. It tends to segregate at grain boundaries and, combined with Fe in the steel, form low-melting-point FeS, reducing the steel's toughness. Furthermore, S forms inclusions such as MnS, which can cause cracking during hot or cold rolling. Therefore, the S content is set within a range of ≤0.010% by mass, preferably ≤0.008%.

[0047] Vanadium: V: Mainly utilizes the fine grain strengthening and precipitation strengthening effects of V. V can be fully dissolved in austenite and precipitate fine V (C, N) particles in proeutectoid ferrite. This precipitation can significantly improve the strength of the steel. In addition, the high V (C, N) solubility in austenite allows the use of lower reheating temperatures, which means lower production costs. The deformation mechanism of the steel of the present invention is mainly dislocation slip. The addition of V can precipitate fine precipitates in the steel. On the one hand, these precipitates can increase the nucleation rate and hinder grain growth to refine the grains; on the other hand, they can hinder dislocation movement to increase strength, so that good comprehensive mechanical properties are finally obtained. Therefore, the V content is set in the range of 0.030~0.070% by mass, preferably 0.035~0.060%.

[0048] The present invention is further described below by means of specific examples:

[0049] This embodiment provides three groups of vanadium microalloyed lightweight high-strength steels, the chemical compositions of which are shown in Table 1;

[0050] Table 1 Chemical composition of vanadium microalloyed lightweight high-strength steel plate (wt.%)

[0051]

[0052] The specific process of the preparation method of the above-mentioned vanadium microalloyed lightweight high-strength steel plate is as follows:

[0053] (a) Smelting process: Smelting the vanadium microalloyed lightweight high-strength steel plate according to the chemical composition shown in Table 1 and casting it into an ingot;

[0054] (b) Forging process: slowly heat the ingot to 1240℃ and keep it for 4 hours, then forge it into a slab. The final forging temperature is ≥850℃

[0055] (c) Hot rolling process: The slab is heated to 1240±20℃ and kept warm for 4 hours. The iron oxide scale is removed and the slab is rolled to 4.0~6.0mm in multiple passes. The final rolling temperature is 890~950℃. The strip is air-cooled to room temperature after rolling.

[0056] (d) Warm Rolling: After pickling the hot-rolled coil, the strip is warm-rolled to a thickness of 0.8-2.0 mm using a four-roll cold / warm rolling mill. The strip is heated to 600-730°C using the clamping end before warm rolling, with a reduction of 10% or more per pass. Heating allows for partial transformation of martensite in the two-phase region to austenite, ensuring strip edge quality. Furthermore, the warm rolling temperature is kept near the lower limit of the reverse transformation temperature, ensuring smooth rolling while suppressing recrystallization and refining the grain size.

[0057] (e) Heat treatment process: Heat the strip to 750~800℃, keep it at this temperature for 10~20min, and then slowly cool it to room temperature.

[0058] The specific process parameters of steps (c) to (e) are shown in Table 2.

[0059] Table 2 Main process parameters of vanadium microalloyed lightweight high-strength steel

[0060]

[0061] The microstructure of the vanadium microalloyed lightweight high strength steel prepared by the above process is as follows Figures 2 to 3 The properties of the vanadium microalloyed lightweight high-strength steel plate were tested in accordance with GB / T228-2010 "Metallic Materials Room Temperature Tensile Test Methods", as shown in Table 3:

[0062] Table 3 Mechanical properties of vanadium microalloyed lightweight high-strength steel plates

[0063]

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of implementation of the present invention. If the present invention is modified or replaced with equivalents without departing from the spirit and scope of the present invention, it should be included in the scope of protection of the claims of the present invention.

Claims

1. A vanadium microalloyed lightweight high-strength steel, characterized in that: The vanadium microalloyed lightweight high-strength steel comprises the following components in percentage by mass: C: 0.15% to 0.30%, Si: 0.25% to 0.50%, Mn: 3.5% to 4.8%, P≤0.015%, S≤0.010%, Als: 3.0% to 4.2%, V: 0.03% to 0.07%, and the remaining elements are Fe and unavoidable impurities. The vanadium microalloyed lightweight high-strength steel is prepared by a method comprising the following steps: a. Smelting process: Smelting a vanadium microalloyed lightweight high-strength steel plate having the following mass percentages and casting it into an ingot: C: 0.15% to 0.30%, Si: 0.25% to 0.50%, Mn: 3.5% to 4.8%, P ≤ 0.015%, S ≤ 0.010%, Als: 3.0% to 4.2%, V: 0.03% to 0.07%, and the remaining elements are Fe and unavoidable impurities; b. Forging step: slowly heating the ingot to a first predetermined temperature and holding it for a first predetermined time, and then forging the ingot into a slab; c. Hot rolling process: heating the slab to a second predetermined temperature and holding it for a second predetermined time, removing the scale by multiple rolling passes to a first predetermined thickness, and air-cooling the rolled strip to room temperature to obtain a hot-rolled coil; d. Warm rolling process: After the hot-rolled coil is pickled, the strip is heated to a third predetermined temperature and then warm-rolled; e. Heat treatment step: heating the strip to a fourth predetermined temperature and then slowly cooling to room temperature for a third predetermined time to obtain a vanadium microalloyed lightweight high-strength steel. Wherein, step d comprises: after pickling the hot-rolled coil, warm-rolling the strip to a second predetermined thickness using a four-roll cold / warm rolling mill, heating the strip to a third predetermined temperature using a clamping end and then warm-rolling, wherein in step d, the second predetermined thickness is 0.8-2.0 mm, the third predetermined temperature is 600-730° C., and the reduction rate of each pass is ≥10%; In step e, the fourth predetermined temperature is 750-800° C., and the third predetermined time is 10-20 minutes.

2. The vanadium microalloyed lightweight high-strength steel according to claim 1, characterized in that: It includes the following components in mass percentage: C: 0.20%~0.28%, Si: 0.28~0.46%, Mn: 3.8%~4.5%, P≤0.010%, S≤0.008%, Als: 3.2%~4.0%, V: 0.035~0.060%. The remaining elements are Fe and inevitable impurities.

3. The vanadium microalloyed lightweight high-strength steel according to claim 1, characterized in that: Its yield strength is 610~700MPa, tensile strength is 790~870MPa, elongation A 50 It is 28.0~36.0%, and the yield strength ratio is 0.71-0.

86.

4. The vanadium microalloyed lightweight high-strength steel according to claim 3, characterized in that: Its structure is composed of 20%-25% of strip-shaped delta ferrite, 25%-30% of ferrite, 30%-35% of lath martensite, and 15%-20% of retained austenite.

5. The vanadium microalloyed lightweight high-strength steel according to claim 1, characterized in that: In step b, the first predetermined temperature is 1200-1280° C., the first predetermined time is 3.5-4.5 hours, and the final forging temperature is ≥850° C.

6. The vanadium microalloyed lightweight high-strength steel according to claim 1, characterized in that: In step c, the second predetermined temperature is 1240±20° C., the second predetermined time is 3.5-4.5 hours, the first predetermined thickness is 4.0-6.0 mm, and the finishing temperature is 890-950° C.

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