Nb microalloyed light weight high strength steel prepared by warm rolling

Abstract

The application discloses a niobium micro-alloyed light high-strength steel prepared by a warm rolling processing mode, and the niobium micro-alloyed light high-strength steel is a low-density warm-rolled steel plate containing chemical components in the following mass percentages: C: 0.20%-0.32%, Si: 0.20-0.40%, Mn: 5.3%-6.5%, P: less than or equal to 0.015%, S: less than or equal to 0.006%, Als: 3.3%-4.2%, Nb: 0.010-0.035%, B: 0.0010-0.0035%, N: less than or equal to 0.006%, and the rest of the elements are Fe and inevitable impurities. The preparation method of the niobium micro-alloyed light high-strength steel comprises the following steps: a smelting process, a hot rolling process, a warm rolling process and a heat treatment process. The application provides the niobium micro-alloyed light high-strength steel with relatively good processing performance, and the production method is simple in process, beneficial to plastic deformation and capable of inhibiting recrystallization.

Description

Technical Field

[0001] This invention belongs to the field of warm-rolled strip (a low-temperature heating + rolling mode) production technology, specifically relating to a niobium microalloyed lightweight high-strength steel prepared by warm rolling and its preparation method. Background Technology

[0002] The automotive manufacturing industry is a pillar industry of the national economy. With the increasing prominence of energy shortages and environmental pollution, lightweighting has become a key focus for carbon emission reduction in the automotive industry. 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 the use of lightweight raw materials can reduce the overall weight of a car, 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 reduce vehicle weight. 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 its 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] Prior art 1 (CN 108998734 A) discloses an ultra-high strength and ductility cold-rolled Mn-Al TRIP steel sheet and its rapid annealing preparation method. Its 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 impurity elements. 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 existing technology 1 involves intermediate annealing before cold rolling, which is a long process; the reduction per cold rolling pass is 0.1 to 0.2 mm, and the cumulative reduction rate is 75 to 90%, requiring repeated cold rolling passes, which is a relatively complex process.

[0004] Prior art 2 (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%, with the balance being 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 then 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 then water-quenched to room temperature to obtain cold-rolled medium-manganese high-aluminum low-density steel. The existing technology 2 has a high carbon content (1.20-1.30%), which is not conducive to obtaining good welding performance. Before cold rolling, the plate is directly water-quenched to room temperature to improve the cold rolling performance, and then subjected to multiple cold rolling passes with a total cumulative reduction of 70-80%. Since medium manganese steel has strong hardenability, 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.

[0005] Prior art 3 (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. In the existing technology 3, the steel plate is first held at 750±20℃ for 30 minutes, then rolled to 1.5 mm 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 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. The existing technology involves holding the steel plate at 750±20℃ for 5 minutes between passes before rolling, a cumbersome and inefficient process involving repeated heating.

[0006] Based on the above, it is necessary to improve the preparation method and properties of lightweight high-strength steel. Summary of the Invention

[0007] Based on this, in order to make up for the shortcomings of existing high-strength steel, a method for preparing niobium microalloyed lightweight high-strength steel and its preparation is provided by warm rolling. This method can achieve simple process and facilitate plastic deformation while inhibiting recrystallization, and provides raw materials with good shape / thickness accuracy for subsequent heat treatment.

[0008] To achieve the above objectives, the following technical solution is adopted:

[0009] This invention provides a niobium microalloyed lightweight high-strength steel prepared by warm rolling. This niobium microalloyed lightweight high-strength steel is a low-density warm-rolled steel sheet containing the following chemical composition by mass percentage: C: 0.20%–0.32%, Si: 0.20–0.40%, Mn: 5.3%–6.5%, P≤0.015%, S≤0.006%, Als: 3.3%–4.2%, Nb: 0.010–0.035%, B: 0.0010–0.0035%, N≤0.006%, with the remainder being Fe and unavoidable impurities.

[0010] In some embodiments, the yield strength of niobium microalloyed lightweight high-strength steel is 650–720 MPa, the tensile strength is 880–960 MPa, the elongation A50 is 36.0–48.0%, and the yield strength ratio is 0.66–0.83; its microstructure consists of (22%–28%) banded δ-ferrite + (10%–15%) α-ferrite + (27%–43%) lath martensite + (25%–30%) retained austenite.

[0011] In some embodiments, the chemical composition by mass percentage is: C: 0.23%–0.28%, Si: 0.25%–0.35%, Mn: 5.5%–6.3%, P≤0.010%, S≤0.004%, Als: 3.5%–4.0%, Nb: 0.015%–0.030%, B: 0.0015%–0.0030%, N≤0.003%, with the remainder being Fe and unavoidable impurities.

[0012] The present invention also provides a method for preparing niobium microalloyed lightweight high-strength steel as described above, the method comprising: a smelting process, a hot rolling process, a warm rolling process, and a heat treatment process;

[0013] Among them, the smelting process involves smelting the steel according to the chemical composition of niobium microalloyed lightweight high-strength steel and casting it into ingots.

[0014] Hot rolling process: The ingot is heated to 1250±20℃ and held for a certain time before being forged into a slab. After being taken out of the furnace, the iron oxide scale on the surface is removed. The slab is then heated, hot rolled and air cooled to obtain hot rolled strip steel.

[0015] Warm rolling process: After pickling, the hot-rolled strip steel is heated to 650-680℃ using a rolling mill and then warm-rolled to obtain warm-rolled plates of the corresponding thickness specifications.

[0016] Heat treatment process: The warm-rolled plate that has undergone the warm rolling process is slowly heated to 760-820℃ at a rate of 3-6℃ / min, held at the temperature, and then slowly cooled to room temperature to obtain niobium microalloyed lightweight high-strength steel.

[0017] In some embodiments, the ingot thickness obtained in the smelting process is 60-80 mm.

[0018] In some embodiments, the step "obtaining hot-rolled strip steel by heating, hot rolling and air cooling the slab" includes the following parameters: the heating temperature of the hot rolling process is 1240-1260℃, the final rolling temperature is 880-940℃, and the hot-rolled strip steel thickness is 3.5-7.0mm.

[0019] In some embodiments, the ingot is heated to 1250±20°C and held at that temperature for 4 to 5 hours during the hot rolling process.

[0020] In some embodiments, after pickling the hot-rolled strip steel in the warm rolling process, the hot-rolled strip steel is warm-rolled into a warm-rolled plate with a thickness of 1.0 to 2.5 mm using a four-roll cold / warm rolling mill.

[0021] In some embodiments, during the heat treatment process, the warm-rolled plate that has undergone the warm rolling process is slowly heated to 760-820°C at a rate of 3-6°C / min, held at that temperature for 30-50 minutes, and then slowly cooled to room temperature in the furnace.

[0022] The present invention has the following beneficial technical effects:

[0023] This invention relates to a method for preparing niobium microalloyed lightweight high-strength steel using a warm rolling process. The method utilizes electric heating at the clamping end of the strip to rapidly heat 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 refinement. 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.

[0024] This invention reduces the density of steel by adding the lightweight element Al, thereby lowering the material density while maintaining strength and plasticity. Al is a typical ferrite-forming element that expands the ferrite phase region. To obtain good strength and plasticity, austenite-forming elements (usually C and Mn) need to be added, which improves the hardenability of the strip. The Nb element's atomic dragging effect and the action of Nb-containing second-phase nailed grain boundaries refine the grains and precipitate, effectively improving strength and plasticity. The warm rolling method of this invention 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 is beneficial for plastic deformation. At the same time, the relatively low heating temperature inhibits recrystallization, which is conducive to further refinement and provides raw materials with good shape / thickness accuracy for subsequent heat treatment. The niobium microalloyed lightweight high-strength steel of the present invention has a yield strength of 650-720 MPa, a tensile strength of 880-960 MPa, an elongation A50 of 36.0-48.0%, and a yield strength ratio of 0.66-0.83. Its microstructure consists of 22%-28% banded δ-ferrite + 10%-15% α-ferrite + 27%-43% lath martensite + 25%-30% retained austenite. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1Metallographic photographs of the strip steel involved in the niobium microalloyed lightweight high-strength steel of this invention;

[0027] Figure 2 This is a scanning electron microscope image of the strip steel involved in the niobium microalloyed lightweight high-strength steel of this invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the embodiments of this invention will be further described in detail below with reference to specific examples and the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0029] The density of steel is reduced by adding the lightweight element Al, thus lowering the material density while maintaining strength and plasticity. Al is a typical ferrite-forming element, expanding the ferrite phase region. To obtain good strength and plasticity, austenite-forming elements (usually C and Mn) need to be added, which improves the hardenability of the strip. The Nb element's atomic dragging effect and the action of Nb-containing second-phase nailed grain boundaries refine the grains and precipitate, effectively improving strength and plasticity. Ferrite-based low-density steels (such as medium-manganese steel) form martensite under air cooling conditions. Direct cold rolling can lead to risks such as edge cracking or even strip breakage. Intermediate annealing (reverse phase transformation annealing) is usually performed before cold rolling. During cold rolling, austenite is affected by plastic deformation and will transform back into martensite, resulting in a low pass reduction rate and complex processes for ferrite-based low-density steels (such as medium-manganese steel). Some existing technologies use warm rolling after heating the strip to the two-phase region and performing an intermediate annealing before each rolling pass, which is not conducive to smooth production. This invention provides a warm rolling method that utilizes the clamping end of the strip for electric heating, rapidly heating 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 refinement. 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.

[0030] like Figure 1 As shown in Figure 2, a niobium microalloyed lightweight high-strength steel prepared by warm rolling is a low-density warm-rolled steel plate containing the following chemical composition by mass percentage: C: 0.20%–0.32%, Si: 0.20–0.40%, Mn: 5.3%–6.5%, P≤0.015%, S≤0.006%, Als: 3.3%–4.2%, Nb: 0.010–0.035%, B: 0.0010–0.0035%, N≤0.006%, with the remaining elements being Fe and unavoidable impurities.

[0031] The yield strength of niobium microalloyed lightweight high-strength steel is 650-720 MPa, the tensile strength is 880-960 MPa, the elongation A50 is 36.0-48.0%, and the yield strength ratio is 0.66-0.83. Its microstructure consists of 22%-28% banded δ-ferrite + 10%-15% α-ferrite + 27%-43% lath martensite + 25%-30% retained austenite.

[0032] Preferably, the chemical composition by mass percentage is: C: 0.23%–0.28%, Si: 0.25%–0.35%, Mn: 5.5%–6.3%, P≤0.010%, S≤0.004%, Als: 3.5%–4.0%, Nb: 0.015%–0.030%, B: 0.0015%–0.0030%, N≤0.003%, with the remainder being Fe and unavoidable impurities.

[0033] The following embodiments generally follow the steps below to prepare niobium microalloyed lightweight high-strength steel:

[0034] The method for preparing the niobium microalloyed lightweight high-strength steel as described above includes: a smelting process, a hot rolling process, a warm rolling process, and a heat treatment process.

[0035] Among them, the smelting process involves smelting the steel according to the chemical composition of niobium microalloyed lightweight high-strength steel and casting it into ingots.

[0036] Hot rolling process: The ingot is heated to 1250±20℃ and held forged into a slab. After being taken out of the furnace, the iron oxide scale on the surface is removed. The slab is then heated, hot rolled and air cooled to obtain hot rolled strip steel.

[0037] Warm rolling process: After pickling, the hot-rolled strip steel is heated to 650-680℃ using a rolling mill and then warm-rolled to obtain warm-rolled plates of the corresponding thickness specifications. The strip steel is heated to 650-680℃ using the clamping end for warm rolling, causing some martensite to undergo austenite inverse phase transformation, significantly reducing deformation resistance and the risk of edge cracking and strip breakage due to martensite stress, thus improving surface quality. The reduction rate for the first five passes of warm rolling is ≥15%, and the reduction rate for the last six passes is 6-12%. Simultaneously, the warm rolling temperature is biased towards the lower limit of the inverse transformation temperature, ensuring smooth rolling while inhibiting recrystallization and refining the grains, which is beneficial for obtaining good strength and plasticity.

[0038] Heat treatment process: The warm-rolled plate that has undergone the warm rolling process is slowly heated to 760-820℃ at a rate of 3-6℃ / min, held at the temperature, and then slowly cooled to room temperature to obtain niobium microalloyed lightweight high-strength steel.

[0039] Preferably, the ingot thickness obtained in the smelting process is 60-80 mm.

[0040] The step "obtaining hot-rolled strip steel by heating, hot rolling and air cooling of slab" includes the following parameters: hot rolling heating temperature of 1240-1260℃, final rolling temperature of 880-940℃, and hot-rolled strip steel thickness of 3.5-7.0mm, that is, after removing iron oxide scale, it is rolled to 3.5-7.0mm in 14 passes.

[0041] Preferably, in the hot rolling process, the slab is heated to 1250±20℃ and then held at that temperature for 4 to 5 hours.

[0042] Preferably, in the warm rolling process, after the hot-rolled strip steel is pickled, it is warm-rolled into a warm-rolled plate with a thickness of 1.0 to 2.5 mm using a four-roll cold / warm rolling mill.

[0043] Preferably, in the heat treatment process, the warm-rolled plate that has undergone the warm rolling process is slowly heated to 760-820°C at a rate of 3-6°C / min, held at that temperature for 30-50 minutes, and then slowly cooled to room temperature in the furnace.

[0044] The role of alloying elements in low-density warm-rolled steel sheets:

[0045] Carbon (C): 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 the formation of coarse κ-carbides at the austenitic grain boundaries, impairing the elongation of low-density steel. Therefore, the C content in this invention is 0.20%–0.32%, preferably 0.23%–0.28%.

[0046] Silicon (Si): 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 descaling; at the same time, 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 0.20–0.40%, preferably 0.25–0.35%.

[0047] Manganese (Mn): 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, causing dense twins to form 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 5.3% to 6.5%, preferably 5.5% to 6.3%.

[0048] Aluminum (Al): 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 impacts. 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 3.3%–4.2%, preferably 3.5%–4.0%.

[0049] Phosphorus (P): In steel, P is generally dissolved in ferrite and has a strong solid solution strengthening effect. However, during the solidification of the slab, 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 ductile-brittle transition temperature of steel and makes it more susceptible to hydrogen embrittlement. Therefore, in this invention, the P content, expressed as a percentage by mass, is specified to be ≤0.015%, preferably ≤0.010%.

[0050] Sulfur (S): 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 present invention specifies that the S content, expressed as a percentage by mass, is ≤0.006%, preferably ≤0.004%.

[0051] Nitrogen (N): N is an impurity element in steel. During the solidification stage, the hot rolling stage, and the annealing stage after cold rolling, N readily combines with Ti, Al, and Nb in the steel to form TiN, AlN, and NbN. TiN and AlN present in molten steel pose a risk of inclusion defects, and large amounts of TiN and AlN can promote crack formation in cast slabs. Liquid-precipitated TiN significantly deteriorates the plasticity and toughness of steel. Therefore, this invention specifies the N content, by mass percentage, to be ≤0.006%, preferably ≤0.003%.

[0052] Niobium (Nb): As a strong carbide-forming element, Nb combines with carbon (C) in steel to form the NbC second phase, hindering dislocation movement and resulting in precipitation strengthening. NbC precipitation also acts as a pinning agent at grain boundaries, preventing recrystallization and inhibiting austenite grain growth, thus refining the grain size. However, to save costs and reduce the impact on the specific gravity of low-density steel, the Nb content should not be too high. Therefore, in this invention, the Nb content is 0.010–0.035%, preferably 0.015–0.030%.

[0053] Boron (B): B significantly improves the hardenability of austenite and inhibits the formation of α-ferrite; at the same time, it readily combines with nitrogen in steel to form BN, which plays a role in precipitation strengthening. However, excessive B content will reduce the toughness of the steel and increase the manufacturing cost. Therefore, the B content in this invention is selected as 0.0010–0.0035%, preferably 0.0015–0.0030%.

[0054] Examples: Examples 1-3 provide three groups of niobium microalloyed lightweight high-strength steels, the chemical compositions of which are shown in Table 1;

[0055] Table 1. Chemical composition (wt.%) of niobium microalloyed lightweight high-strength steel

[0056] Example 1 0.24 0.34 5.85 0.008 0.004 3.8 0.022 0.0022 Example 2 0.27 0.30 5.92 0.006 0.002 3.6 0.018 0.0026 Example 3 0.26 0.33 6.08 0.007 0.003 3.9 0.024 0.0020

[0057] The specific process for preparing the aforementioned low-niobium microalloyed lightweight high-strength steel plate is as follows:

[0058] A. Smelting process: Niobium microalloyed lightweight high-strength steel ingots with the chemical composition shown in Table 1 are prepared through smelting process.

[0059] B. Hot rolling process: The ingot is heated to 1230℃ and held for 4 hours before being forged into a slab. After being taken out of the furnace, the iron oxide scale on the surface is removed. The slab is then heated, hot rolled and air cooled to obtain hot rolled strip steel. The specific hot rolling process parameters are shown in Table 2.

[0060] Table 2. Main process parameters for forging and hot rolling of low-density warm-rolled steel plates

[0061]

[0062] C. Warm rolling process: After pickling the hot-rolled strip steel, the strip steel 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 steel and the warm rolling reduction rate are shown in Table 3.

[0063] Table 3. Relevant parameters for the warm rolling process

[0064]

[0065] D. Heat treatment process: The strip steel is slowly heated to 760-820℃ at a rate of 3-6℃ / min, held at that temperature for 30-50min, and then slowly cooled to room temperature in the furnace to obtain niobium microalloyed lightweight high-strength steel. The specific process parameters are shown in Table 4.

[0066] Table 4. Main process parameters for heat treatment

[0067]

[0068] The microstructure of the niobium microalloyed lightweight high-strength steel prepared by the above process is as follows: Figures 1 to 2 As shown in Table 5, the properties of the above-mentioned niobium microalloyed lightweight high-strength steel were tested according to GB / T228-2010 "Metallic Materials - Tensile Testing at Room Temperature".

[0069] Table 5 Mechanical properties of niobium microalloyed lightweight high-strength steel

[0070]

[0071] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. Although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.

[0072] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of different aspects of the invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A method for preparing niobium microalloyed lightweight high-strength steel, characterized in that, The preparation method includes: a smelting process, a hot rolling process, a warm rolling process, and a heat treatment process; Among them, the smelting process involves smelting the steel according to the chemical composition of niobium microalloyed lightweight high-strength steel and casting it into ingots. Hot rolling process: The ingot is heated to 1250±20℃ and held for a certain time before being forged into a slab. After being taken out of the furnace, the iron oxide scale on the surface is removed. The slab is then heated, hot rolled and air cooled to obtain hot rolled strip steel. Warm rolling process: After pickling, the hot-rolled strip steel is heated to 650~680℃ using a rolling mill and then warm-rolled to obtain warm-rolled plates of the corresponding thickness specifications. Heat treatment process: The warm-rolled plate that has undergone the warm rolling process is slowly heated to 760~820℃ at a rate of 3~6℃ / min, held at the temperature and then slowly cooled to room temperature to obtain niobium microalloyed lightweight high-strength steel. The niobium microalloyed lightweight high-strength steel is a low-density warm-rolled steel plate containing the following chemical composition by mass percentage: C: 0.20%~0.32%, Si: 0.20~0.40%, Mn: 5.3%~6.5%, P≤0.015%, S≤0.006%, Als: 3.3%~4.2%, Nb: 0.010~0.035%, B: 0.0010~0.0035%, N≤0.006%, with the remaining elements being Fe and unavoidable impurities; its microstructure consists of 22%-28% banded δ-ferrite + 10%-15% α-ferrite + 27%-43% lath martensite + 25%-30% retained austenite.

2. The preparation method according to claim 1, characterized in that, The niobium microalloyed lightweight high-strength steel has a yield strength of 650~720MPa, a tensile strength of 880~960MPa, an elongation A50 of 36.0~48.0%, and a yield strength ratio of 0.66-0.

83.

3. The preparation method according to claim 1, characterized in that, The chemical composition of the mass percentage is as follows: C: 0.23%~0.28%, Si: 0.25~0.35%, Mn: 5.5%~6.3%, P≤0.010%, S≤0.004%, Als: 3.5%~4.0%, Nb: 0.015~0.030%, B: 0.0015~0.0030%, N≤0.003%, and the remaining elements are Fe and unavoidable impurities.

4. The preparation method according to claim 1, characterized in that, The ingots obtained in the smelting process are 60-80 mm thick.

5. The preparation method according to claim 1, characterized in that, The step "obtaining hot-rolled strip steel by heating, hot rolling and air cooling of slab" includes the following parameters: the heating temperature of the hot rolling process is 1240-1260℃, and the final rolling temperature is 880~940℃.

6. The preparation method according to claim 5, characterized in that, The step "obtaining hot-rolled strip steel by heating, hot rolling and air cooling of slab" includes the following parameters: the thickness of the hot-rolled strip steel is 3.5~7.0mm.

7. The preparation method according to claim 1, characterized in that, In the hot rolling process, the slab is heated to 1250±20℃ and then held at that temperature for 4 to 5 hours.

8. The preparation method according to claim 1, characterized in that, In the warm rolling process, after the hot-rolled strip steel is pickled, it is warm-rolled into a warm-rolled plate with a thickness of 1.0~2.5mm using a four-roll cold / warm rolling mill.

9. The preparation method according to claim 1, characterized in that, In the heat treatment process, the warm-rolled plate that has undergone the warm rolling process is slowly heated to 760-820°C at a rate of 3-6°C / min, held at that temperature for 30-50 minutes, and then slowly cooled to room temperature in the furnace.

Images