1800mpa grade hot formed steel based on commercial vehicle applications and method of manufacture

Abstract

The application discloses a 1800MPa grade hot forming steel based on commercial vehicle application and a preparation method, breaks through the alloy component system of traditional boron-containing steel, i.e., 22MnB5, and provides a brand-new chemical component design of thick-specification hot forming steel for commercial vehicles. The size and quantity of inclusions of the hot-rolled steel plate are improved through a suitable smelting process, the fatigue resistance of the steel plate is improved, the hot forming steel plate has uniformity and high hardenability, the edge and core structures are both full martensite, the qualified rate of stamping parts is improved, the yield strength is greater than or equal to 1100MPa, the tensile strength is greater than or equal to 1800MPa, and the elongation is greater than or equal to 7%, the hot forming steel plate has the characteristics of high strength and high elongation, the strength and plasticity are well matched, the safety of the commercial vehicle can be improved, and the high-strength and light-weight requirements of the commercial vehicle can be met; the process flow is simple, the processes of conventional acid rolling, continuous annealing, cover annealing and galvanizing are omitted, the production cost is saved, and industrialization is easy to realize.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel technology, specifically relating to an 1800MPa grade hot-formed steel for commercial vehicle applications and its preparation method. Background Technology

[0002] With the advent of the concept of automotive lightweighting, the demand for advanced high-strength steel sheets in automobiles has been increasing year by year. Hot-formed steel, as one of the representatives of advanced high-strength steel, aligns with the development direction of automotive lightweighting and is a product with continuously increasing applications. There are currently over 200 production lines for hot-formed steel in China, and this number continues to grow. Currently, hot-formed steel is mainly used in passenger cars, with thicknesses ranging from 1.0mm to 2.5mm. With the emergence of the concept of lightweighting for commercial vehicles, the application of hot-formed steel in commercial vehicles has received significant attention, with thicknesses gradually increasing from 3.0mm to 10.0mm. Its application in commercial vehicles can achieve weight reductions of 30% to 50%. The application and promotion of hot-formed steel in the new field of commercial vehicles, further achieving lightweighting through weight reduction, is an effective way to reduce carbon emissions. Major advanced steel companies in China have also developed related products. The main challenges they face are increased heat removal during the stamping process due to increased thickness, insufficient hardenability of the core, and inconsistent microstructure between the edges and core of hot-formed parts, leading to reduced strength, decreased toughness, and reduced fatigue life, ultimately causing stamped parts to fail. Meanwhile, thick-gauge products are mainly supplied in hot-rolled form, replacing cold-rolled products with hot-rolled products, as users place higher demands on the strength, toughness, and fatigue resistance of the steel.

[0003] Patent CN 109136759 B discloses a 1300MPa grade hot-formed steel for wheel spokes and its preparation method. This method gives the steel good formability and weldability, but the use of a bell-type annealing process increases production costs, and the tensile strength after hot forming is only 1300MPa. Patent CN 111575602A discloses a 1500MPa grade hot-formed steel plate for wheels and its production method. Wheels made from this hot-formed steel plate undergo fatigue life testing on a wheel test bench, showing a fatigue life greater than 1 million cycles, indicating good fatigue resistance. However, the heating temperature in its hot-forming process is 960℃, increasing energy consumption in the hot-forming production line and raising production costs. It also requires a stamping die cooling rate greater than 50℃ / s, placing high demands on the die's cooling capacity and increasing the difficulty of die design. Summary of the Invention

[0004] To overcome the shortcomings of the existing technology, this invention breaks through the traditional alloy composition system of boron-containing steel, namely 22MnB5, and provides a brand-new chemical composition design for thick-gauge hot-formed steel for commercial vehicles. It adopts a suitable smelting process to improve the size and quantity of inclusions in hot-rolled plates, thereby improving the fatigue resistance of the steel plates. Furthermore, combined with the hot stamping process design, this composition design has good hardenability, resulting in a uniform microstructure after hot stamping of the steel plates, with both the edges and the core being full martensite, thus improving the yield rate of stamped parts.

[0005] To achieve the above-mentioned objectives, this invention provides an 1800MPa grade hot-formed steel for commercial vehicle applications. The alloy composition of the hot-formed steel is: C: 0.30–0.38%, Si: 0.15–0.27%, Mn: 1.45–1.65%, Al: 0.06–0.10%, Ti: 0.01–0.05%, P≤0.02%, S≤0.01%, Nb: 0.20–0.30%, Mo: 0.15–0.35%, B: 0.002–0.005%, RE: 0.04–0.10%, with the remainder being Fe and unavoidable impurities.

[0006] The rationale for the alloy composition design of the hot-formed steel is as follows: Increasing the carbon (C) content improves the hardenability of the steel sheet; adding appropriate amounts of nitrogen (Nb) and molybdenum (Mo) replaces the chromium (Cr) in traditional boron-containing steel. Nb refines the high-temperature austenite grains, ensuring the steel's strength; Mo significantly improves the hardenability of the steel sheet, delays the high-temperature ferrite transformation, and ensures that ferrite transformation does not occur within a wider hot stamping process window, allowing for lower heating and stamping temperatures and a wider stamping process window. Adding appropriate amounts of rare earth elements and employing suitable smelting processes improves the size and quantity of inclusions in the hot-rolled sheet, thereby enhancing the steel sheet's fatigue resistance.

[0007] C: Carbon is an essential element for ensuring high hardenability and strength, and it is an austenite stabilizing element. If the carbon content is too low, the strength will decrease and the performance will not meet the target requirements; if the carbon content is too high, it will increase the carbon equivalent, making it difficult to weld the steel strip during pickling and rolling, and affecting pickling and rolling production. Therefore, its content is limited to between 0.30% and 0.38%.

[0008] Si: Silicon is an essential alloying element that plays a role in solid solution strengthening. It can also inhibit the precipitation of cementite and improve the plasticity of materials. However, excessive Si can cause brittleness and affect the overall performance of materials. Therefore, its content is limited to between 0.15% and 0.27%.

[0009] Mn: Mn can improve hardenability and delay the transformation of pearlite and bainite. However, excessive Mn content can cause banded defects in the microstructure. Therefore, its content is limited to 1.45% to 1.65%.

[0010] P and S: Phosphorus and sulfur are usually considered harmful elements in steel. P element easily causes segregation in the center of the billet, and S element combines with Mn to form MnS, which reduces the performance of steel. Therefore, the P content is limited to below 0.02% and the S content is limited to below 0.01%.

[0011] Al: Aluminum is a deoxidizer in steel. A certain amount of acid-soluble aluminum should be ensured in the steel. However, if the aluminum content is too high, aluminum inclusions will be formed in the steel. Therefore, its content is limited to 0.06-0.10%.

[0012] B: B element can improve the hardenability of steel. At the same time, a small amount of B added to hot-formed steel will accumulate at the austenite grain boundaries, delaying the formation of ferrite, thus providing favorable conditions for the ferrite-free transformation in the subsequent hot stamping process. However, too much B element will destroy the toughness of the material, so its content is limited to 0.002-0.005%.

[0013] Ti: A small amount of Ti added to steel will fix with the N element in the steel to form a precipitate phase, while refining the original austenite grains. However, too much Ti will combine with C and reduce the strength of the martensite in the final steel microstructure, so its content is limited to 0.01-0.05%.

[0014] Nitrogen (Nb): Nitrogen can partially dissolve in solid solutions, acting as a solid solution strengthening agent. It combines with nitrogen in steel to form a second phase and precipitates in the matrix, hindering austenite grain growth and refining the austenite grains. A content below 0.2% has little grain-refining effect, while a content above 0.3% increases production costs; therefore, its content is limited to 0.20–0.30%.

[0015] Mo: Mo can significantly improve the hardenability of steel, ensuring that the steel plate forms martensite faster during quenching, and can delay the high-temperature ferrite transformation, reducing the heating temperature during hot stamping and saving production and processing costs. Too little Mo results in poor hardenability of the steel plate, while too much Mo reduces its plasticity; therefore, its content is controlled between 0.15% and 0.35%.

[0016] RE: It inhibits hydrogen embrittlement of steel, improves the purity of steel plates, improves the quality of modified inclusions, and enhances fatigue performance. If the amount added is too low, the effect of rare earth elements is not obvious, and if the amount added is too high, it will cause inclusions to accumulate and reduce the plasticity of the material. Therefore, its content is limited to 0.04-0.10%.

[0017] Furthermore, the thickness of the hot-formed steel is not less than 3 mm.

[0018] Furthermore, the original austenite grain size of the hot-formed steel is ≥12, and both the edge and core microstructures are composed of full martensite.

[0019] Furthermore, the hot-formed steel has a yield strength ≥1100MPa, a tensile strength ≥1800MPa, and an elongation ≥7%.

[0020] A method for preparing the above-mentioned 1800MPa grade hot-formed steel for commercial vehicle applications, the method comprising the following technical solutions:

[0021] ① Hot-rolled plates and coils are obtained through smelting, casting, and hot continuous rolling processes;

[0022] ② The hot-rolled coil is leveled to improve the strip shape, and the leveling elongation is controlled at 1.2-1.4% to obtain a hot-rolled leveled coil;

[0023] ③ The hot-rolled flattened coil is treated with EPS to remove the iron oxide scale on the surface of the steel plate. The process speed of EPS treatment is 15-60m / min to obtain hot-rolled steel plate.

[0024] ④ The hot-rolled steel sheet is uncoiled and blanked for hot stamping. The heating temperature is 875-935℃ and the holding time is 3-8 minutes. The steel sheet transfer time is appropriately controlled. The stamping die temperature is greater than 700℃ and the die cooling rate is greater than 10℃ / s. The furnace should have a protective atmosphere such as inert gas nitrogen.

[0025] Furthermore, the smelting and casting process is as follows: the components are mixed evenly according to the required percentages to form molten iron, which is then refined outside the furnace after being smelted in a converter, and cast into a billet.

[0026] Furthermore, the smelting and casting process is as follows: molten iron is formed by uniformly mixing according to the required composition, with a temperature T ≥ 1300℃. High-quality scrap steel is used. After converter smelting, the molten iron undergoes LF ladle refining. Deep desulfurization is performed during the LF process, requiring [S] ≤ 0.0030% after LF. Protective casting is implemented throughout the continuous casting process. The target superheat for continuous casting is 15–25℃, and the target casting speed is 1.0–1.2 m / min.

[0027] Furthermore, the hot continuous rolling process is as follows: the billet heating temperature is 1200–1300℃, and the slab exit temperature is 1230–1270℃, ensuring uniform heating temperature and sufficient heating time, with a total furnace time ≥150 min. The furnace atmosphere is controlled to reduce the formation of iron oxide scale on the billet. The heated billet is then rolled in sections, with the roughing rolling ending temperature at 1110–1150℃ and the finishing rolling ending temperature at 870–900℃. Front-end cooling is used, with a coiling temperature of 680–720℃, followed by air cooling to room temperature. The resulting steel plate has a microstructure of ferrite and pearlite, with a grain size grade ≥12. The hot-rolled steel plate is slowly cooled using a surrounding cooling method, i.e., placed inside other high-temperature coils, away from the warehouse, to ensure sufficient and slow cooling, guaranteeing the plate shape and the release of internal stress.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] ① The steel plate composition system of this invention is novel, breaking away from the traditional boron-containing steel, namely the 22MnB5 alloy system. The original austenite grain size in the hot stamping heating process of traditional boron-containing steel is grade 9-10. This invention increases the C content to improve the strength of the steel based on the traditional boron-containing steel, and refines the original austenite grain size by adding an appropriate amount of Nb element to replace the Cr element in the traditional boron-containing steel. Compared with the traditional boron-containing steel 22MnB5, the original austenite grain size is ≥12 grade, ensuring the strength of the steel. By adding an appropriate amount of Mo element, the high-temperature ferrite transformation is suppressed, the heating temperature and stamping temperature in the hot stamping forming process are reduced, energy consumption is saved, and the stamping process window is expanded. At the same time, an appropriate amount of rare earth elements are added, and the size and quantity of inclusions in the hot-rolled steel plate are improved through appropriate smelting processes, thereby improving the fatigue resistance of the steel plate.

[0030] ② The hot-formed steel sheet designed in this invention not only ensures the thickness required for commercial vehicle use, but also, combined with the hot stamping process design, results in a hot-formed steel sheet with uniform microstructure and high hardenability. The microstructure of both the edges and the core is fully martensitic, improving the yield rate of stamped parts. In terms of performance, the sheet exhibits a yield strength ≥1100MPa, tensile strength ≥1800MPa, and elongation ≥7%, demonstrating high strength and high elongation. It also shows good strength-plasticity matching, which can improve the safety of commercial vehicle use and meet the high-strength and lightweight requirements of commercial vehicles. This is of great significance for achieving lightweighting in commercial vehicles.

[0031] ③ The hot-formed steel for commercial vehicles produced by this process has a simple manufacturing process, eliminating the need for conventional pickling, continuous annealing, bell-type annealing and galvanizing processes, which greatly saves production costs and makes it easy to industrialize. Attached Figure Description

[0032] Figure 1 The image shows the metallographic structure (ferrite + pearlite, grain size grade ≥12) of the hot-formed steel sheet before hot stamping in Embodiment 1 of the present invention.

[0033] Figure 2 The image shows the original austenite grain size (grain size grade: 12.5) of the hot-formed steel sheet after hot stamping in Embodiment 1 of the present invention.

[0034] Figure 3 This is a metallographic diagram (martensite) of the edge of the hot-formed steel sheet after hot stamping in Embodiment 1 of the present invention.

[0035] Figure 4 This is a metallographic diagram (martensite) of the core of the hot-formed steel sheet after hot stamping in Embodiment 1 of the present invention.

[0036] Figure 5 This is a diagram showing the distribution of inclusions (level B0.5, D2.0) of the hot-formed steel sheet before hot stamping in Embodiment 1 of the present invention.

[0037] Figure 6 The original austenite grain size diagram (grain size grade: 11.5) of the hot-formed steel sheet of Comparative Example 1 of the present invention after hot stamping.

[0038] Figure 7 This is a diagram showing the inclusion distribution of the hot-formed steel sheet of Comparative Example 3 of the present invention before hot stamping (levels B1.5, D3.0). Detailed Implementation

[0039] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified.

[0040] Example

[0041] A 1800MPa grade hot-formed steel for commercial vehicle applications, wherein the alloy composition of the hot-formed steel is C: 0.30-0.38%, Si: 0.15-0.27%, Mn: 1.45-1.65%, Al: 0.06-0.10%, Ti: 0.01-0.05%, P≤0.02%, S≤0.01%, Nb: 0.20-0.30%, Mo: 0.15-0.35%, B: 0.002-0.005%, RE: 0.04-0.10%, with the remainder being Fe and unavoidable impurities. The specific compositions of the steels in the embodiments and comparative examples of this invention are shown in Table 1.

[0042] Table 1. Composition of steels in the embodiments and comparative examples of the present invention.

[0043] C Si Mn P S Al Nb Mo B Ti RE Example 1 0.30 0.20 1.53 0.006 0.006 0.066 0.25 0.3 0.002 0.029 0.05 Example 2 0.35 0.15 1.50 0.007 0.008 0.073 0.23 0.35 0.004 0.025 0.09 Example 3 0.36 0.27 1.45 0.012 0.005 0.09 0.26 0.15 0.002 0.038 0.08 Example 4 0.33 0.23 1.65 0.009 0.007 0.06 0.28 0.2 0.003 0.049 0.06 Example 5 0.38 0.17 1.48 0.015 0.006 0.08 0.29 0.26 0.005 0.01 0.08 Comparative Example 1 0.32 0.21 1.60 0.007 0.007 0.069 -- 0.33 0.003 0.032 0.04 Comparative Example 2 0.35 0.25 1.57 0.017 0.005 0.085 0.28 -- 0.002 0.043 0.06 Comparative Example 3 0.36 0.18 1.62 0.009 0.009 0.071 0.27 0.32 0.003 0.027 --

[0044] A method for preparing 1800MPa grade hot-formed steel for commercial vehicle applications, the method comprising the following technical solutions:

[0045] ① Hot-rolled plates and coils are obtained through smelting, casting, and hot continuous rolling processes;

[0046] Smelting and casting: Molten iron is formed by uniformly mixing the components according to the stated percentage requirements. The molten iron temperature T ≥ 1300℃. High-quality scrap steel is used. After converter smelting, the molten iron undergoes LF ladle refining. Deep desulfurization is performed during the LF process, requiring [S] ≤ 0.0030% after LF. Protective casting is implemented throughout the continuous casting process. The target superheat for continuous casting is 15–25℃, and the target casting speed is 1.0–1.2 m / min.

[0047] Hot continuous rolling: Heating temperature 1200-1300℃, slab exit temperature 1230-1270℃, ensuring uniform heating temperature and sufficient heating time, with a total furnace time ≥150min. Controlling the furnace atmosphere to reduce iron oxide scale formation on the slab. The heated slab is rolled in sections, with roughing finishing at 1110-1150℃ and finishing at 870-900℃. Front-end cooling is used, and the slab is coiled at 680-720℃. The resulting steel plate has a microstructure of ferrite and pearlite with a grain size grade ≥12. The hot-rolled steel plate is slowly cooled using a surrounding cooling method, placing it inside other high-temperature coils away from the warehouse for thorough and slow cooling, ensuring proper plate shape and stress release. The main hot-rolling process parameters for the embodiments and comparative steels of this invention are shown in Table 2.

[0048] Table 2 Main hot rolling process parameters of the embodiments and comparative steels of the present invention.

[0049]

[0050] ② The hot-rolled coil is leveled to improve the strip shape, and the leveling elongation is controlled at 1.2-1.4% to obtain a hot-rolled leveled coil;

[0051] ③ The hot-rolled flattened coil is treated with EPS to remove the iron oxide scale on the surface of the steel plate. The process speed of EPS treatment is 15-60m / min to obtain hot-rolled steel plate.

[0052] ④ The hot-rolled steel sheet is uncoiled and blanked for hot stamping. The heating temperature is 875–935℃, and the holding time is 3–8 minutes. The steel sheet transfer time is appropriately controlled, the stamping die closing temperature is greater than 700℃, and the die cooling rate is greater than 10℃ / s. An inert gas is used as the protective atmosphere in the furnace. The main process parameters for hot forming of the steel in the embodiments and comparative examples of this invention are shown in Table 4. Among them, Example 5 in Publication No. CN 111575602 A is used as Comparative Example 4 for comparison.

[0053] According to the component ratio of the technical solution, the hot-formed steel produced by the embodiment undergoes processes such as smelting, continuous casting, hot rolling, hot rolling leveling, EPS treatment, and hot forming. The resulting hot-formed steel has a yield strength ≥1100 MPa, tensile strength ≥1800 MPa, and elongation ≥7%. The microstructure consists entirely of martensite at both the edges and the core, with an original austenite grain size ≥12. The properties of the steel in the embodiment and comparative examples before hot stamping are shown in Table 3. The microstructure and properties of the steel in the embodiment and comparative examples after hot forming are shown in Table 5.

[0054] Table 3. Performance and inclusion rating results of the steels in the embodiments and comparative examples of the present invention before hot forming.

[0055]

[0056] Table 4. Main process parameters for hot stamping forming of steel in the embodiments and comparative examples of the present invention.

[0057]

[0058] Table 5. Microstructure and properties of the hot-formed steels of the embodiments and comparative examples of the present invention.

[0059]

[0060]

[0061] In this invention, Comparative Example 1 is without the addition of alloying element Nb, Comparative Example 2 is without the addition of alloying element Mo, and Comparative Example 3 is without the addition of rare earth element RE. This invention also includes a comparison between the hot-formed steel sheet obtained by this invention and the steel sheet described in Example 5 of application publication number CN111575602 A, with the steel sheet described in Example 5 of CN111575602 A serving as Comparative Example 4. From Table 4, it is clear that the heating temperature during the hot stamping process of the embodiments and comparative examples is lower in this invention. From Table 5, it is clear that the comprehensive performance of the embodiments in the final quenched state is significantly higher than that of Comparative Example 4, meeting the high-strength and lightweight requirements of commercial vehicles. The inclusion level of the hot-rolled steel sheet in this invention is significantly lower than that of the hot-rolled steel sheet without rare earth elements in Comparative Example 3.

[0062] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A method for the production of a 1800 MPa grade hot formed steel based on commercial vehicle applications, characterized in that, The alloying components of the hot forming steel are C: 0.30%-0.38%, Si: 0.17%-0.27%, Mn: 1.45%-1.65%, Al: 0.06%-0.10%, Ti: 0.029%-0.05%, P≤0.02%, S≤0.01%, Nb: 0.25%-0.30%, Mo: 0.30%-0.35%, B: 0.002%-0.005%, RE: 0.04%-0.09%, and the rest is Fe and inevitable impurities; the thickness of the hot forming steel is not less than 3mm; The preparation method of the 1800MPa-grade hot forming steel based on commercial vehicle application comprises the following technical solutions: ①a hot-rolled plate coil is obtained through smelting, casting and hot continuous rolling process; ②the shape of the strip steel is improved in a flattening manner, the flattening elongation is controlled at 1.2%-1.4%, and a hot-rolled flattened plate coil is obtained; ③the hot-rolled flattened plate coil is treated by EPS to remove the iron oxide skin on the surface of the steel plate, the process speed of the EPS treatment is 15-60m / min, and a hot-rolled steel plate is obtained; ④the hot-rolled steel plate is uncoiled and cut for hot stamping forming, the heating temperature is 875℃-935℃, the holding time is 3-8min; the steel plate transfer time is controlled, the stamping die closing temperature is greater than 700℃, the die cooling rate is greater than 10℃ / s, and the inert gas is used as the protective atmosphere in the furnace; the smelting and casting process is that: the molten iron is prepared according to the component requirements, the molten iron is subjected to external refining after converter smelting, and is cast into a casting blank; the hot continuous rolling process is that: the casting blank heating temperature is 1200℃-1300℃, the slab discharge temperature is 1230℃-1270℃, the heating temperature is uniform, the heating time is sufficient, and the total furnace time is greater than or equal to 150min; the casting blank after heating is subjected to sectional rolling, the rough rolling end temperature is 1110℃-1150℃, the finish rolling final rolling temperature is 870℃-900℃, the front-stage cooling mode is used for cooling, the coiling temperature is 680℃-720℃, and then air cooling is performed to room temperature; the original austenite grain size of the hot forming steel is greater than or equal to 12 grades, and the edge and core structures are both composed of full martensite; the yield strength of the hot forming steel is greater than or equal to 1100MPa, the tensile strength is greater than or equal to 1800MPa, and the elongation is greater than or equal to 7%.

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