1000mpa grade ultra-high strength steel and a method for manufacturing the same

Abstract

The application discloses a 1000MPa-grade ultra-high-strength steel and a preparation method thereof, and relates to the technical field of steel smelting and rolling, wherein the chemical components of the ultra-high-strength steel include, in mass fraction, C: 0.10-0.20%, Si: 0.05-0.26%, Mn: 2.1-2.6%, P: ≤0.008%, S: ≤0.005%, Alt: 0.020-0.040%, Ni: 0.04-0.10%, Cr: 0.70-1.5%, Sb: 0.02-0.045%, V: 0.03-0.06%, and the balance of Fe and inevitable impurities, and the control condition is [Ni] / [Sb]≥2.0. The method comprises the following steps: obtaining molten steel after smelting; performing continuous casting on the molten steel to obtain a slab; heating and insulating the slab, and then performing hot rolling, coiling, pickling, continuous annealing and skin passing to obtain a 1000MPa-grade ultra-high-strength steel for resisting sulfuric acid corrosion; and the ultra-high-strength steel prepared by the method has a yield strength of ≥850MPa, a tensile strength of ≥1000MPa, and an elongation A50 of ≥7%, and the microstructure of the ultra-high-strength steel is tempered bainite, a small amount of tempered martensite and ferrite, the plasticity and toughness of the ultra-high-strength steel are good, and the ultra-high-strength steel meets 180-degree d=a forming without cracking.

Description

Technical Field

[0001] This application relates to the field of steel smelting and rolling technology, and in particular to a 1000MPa grade ultra-high strength steel and its preparation method. Background Technology

[0002] Currently, the automotive, construction machinery, and building industries are accelerating the adoption of lightweight materials, with increasingly sophisticated applications and higher demands placed on the comprehensive mechanical properties of high-strength steel plates. However, as the strength of steel increases, its hydrogen embrittlement sensitivity also increases significantly. Especially when the tensile strength reaches 1000 MPa, hydrogen from both the production process and the environment severely impacts the steel's service performance, particularly when the material needs to operate in acidic environments. On one hand, acid corrosion thins the steel, leading to a decrease in structural stiffness; on the other hand, hydrogen ions from the acid can enter the steel, causing brittle cracking. Many factors influence the hydrogen embrittlement sensitivity of steel, including its strength, microstructure, and hydrogen trapping. Generally speaking, higher strength leads to higher hydrogen embrittlement sensitivity, with martensitic structures exhibiting particularly high sensitivity.

[0003] In the field of steel materials, the development of ultra-high strength steel has been relatively mature. However, ultra-high strength steel is generally not acid-resistant. In addition, the strength of acid-resistant steel is also relatively low. Overall, the current ultra-high strength steel technology can be summarized as high strength but not acid-resistant, and acid-resistant but not high strength. Summary of the Invention

[0004] This application provides a 1000MPa grade ultra-high strength steel and its preparation method to solve the technical problems of existing ultra-high strength steel being not acid-resistant and acid-resistant steel having low strength.

[0005] In a first aspect, this application provides a 1000MPa grade ultra-high strength steel, wherein the chemical composition of the ultra-high strength steel, by mass fraction, includes: C: 0.10%–0.20%, Si: 0.05%–0.26%, Mn: 2.1%–2.6%, P≤0.008%, S≤0.005%, Alt: 0.020%–0.040%, Ni: 0.04%–0.10%, Cr: 0.70%–1.5%, Sb: 0.02%–0.045%, V: 0.03%–0.06%, with the balance being Fe and unavoidable impurities, and the [Ni] / [Sb] ratio is controlled to be ≥2.0, where [Ni] represents the mass fraction of Ni and [Sb] represents the mass fraction of Sb.

[0006] Optionally, the microstructure of the ultra-high strength steel, by volume fraction, includes:

[0007] Tempered bainite: 85%–95%, tempered martensite: 0%–5%, ferrite: 5%–10%.

[0008] Secondly, this application provides a method for preparing the ultra-high strength steel described in the first aspect, the method comprising:

[0009] The molten steel obtained after smelting has a chemical composition comprising: C: 0.10%–0.20%, Si: 0.05%–0.26%, Mn: 2.1%–2.6%, P ≤ 0.008%, S ≤ 0.005%, Alt: 0.020%–0.040%, Ni: 0.04%–0.10%, Cr: 0.70%–1.5%, Sb: 0.02%–0.045%, V: 0.03%–0.06%, with the balance being Fe and unavoidable impurities;

[0010] The molten steel is continuously cast to obtain a slab;

[0011] The slab is heated and held at a constant temperature, then hot-rolled, coiled, pickled, continuously annealed and leveled to obtain high-strength ultra-high-strength steel.

[0012] Optionally, the slab is a chamfered slab, produced by using a chamfered crystallizer during continuous casting. The four corners of the chamfered slab are not sharp, which can suppress the occurrence of cracks in the continuously cast slab.

[0013] Optionally, the heating and heat preservation adopts a high-temperature rapid heating process, specifically including: rapid heating to 15°C to 25°C above the uniform temperature in the first and second heating stages, followed by cooling to the uniform temperature of 1150°C to 1180°C, and heat preservation at the uniform temperature for 30 to 40 minutes.

[0014] Optionally, the hot rolling includes roughing and finishing rolling. The total compression ratio of the roughing rolling is controlled between 80% and 88%, the single-pass compression ratio is ≥25%, and the final rolling temperature of the roughing rolling is between 1000℃ and 1150℃. The compression ratio of the finishing rolling gradually decreases, the compression ratio of the F7 stand is ≤10%, and the final rolling temperature of the finishing rolling is between 880℃ and 920℃.

[0015] Optionally, the winding temperature is 660℃~700℃.

[0016] Optionally, the pickling process includes cold rolling, wherein the cold rolling reduction rate is 40% to 50%.

[0017] Optionally, the continuous annealing adopts an air-cooling method, including annealing heating, slow cooling, rapid cooling, and over-aging treatment.

[0018] Optionally, the annealing heating temperature is 780℃~840℃, the slow cooling endpoint temperature is 740℃~780℃, the rapid cooling rate is 10℃ / s~15℃ / s, the rapid cooling endpoint temperature is 370℃~430℃, and the over-aging treatment temperature is 300℃~350℃.

[0019] Optionally, the elongation of the flattened surface is 0.10% to 0.15%.

[0020] The technical solution provided by this invention has the following advantages compared with the prior art:

[0021] This invention provides a 1000MPa grade ultra-high strength steel and its preparation method. By limiting the contents of C, Si, Mn, Ni, Cr, and Ti, while simultaneously limiting the contents of harmful elements P and S, and adding Sb at a content of 0.02%–0.045%, the microstructure is controlled to be predominantly tempered bainite with small amounts of martensite and ferrite. Through the control of C, Mn, Cr, and Ti elements combined with the annealing process, a microstructure dominated by tempered bainite is obtained. The strengthening mechanism combines solid solution strengthening, microstructure strengthening, and precipitation strengthening to achieve a high strength of 1000MPa. The combined effect of Sb, Cr, and Ni improves its resistance to sulfuric acid corrosion, and the control of the tempered bainite microstructure type and the precipitation of V carbonitrides enhances its resistance to hydrogen embrittlement. Finally, a 1000MPa grade ultra-high strength steel resistant to sulfuric acid corrosion, predominantly tempered bainite, is obtained. It exhibits good ductility and toughness, meeting the technical requirement of no cracking during 180° forming (d=a). Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the preparation method provided in this application.

[0025] Figure 2 SEM images of microstructures provided in embodiments of this application.

[0026] Figure 3 The tensile fracture morphology of the embodiment of this application after hydrogen purging of NACE solution is shown.

[0027] Figure 4 This is a comparison diagram of corrosion weight loss at different times in sulfuric acid solutions of different concentrations according to embodiments of this application. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0029] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0030] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0031] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0032] In a first aspect, this application provides a 1000MPa grade ultra-high strength steel. The chemical composition of the ultra-high strength steel, by mass fraction, includes: C: 0.10%–0.20%, Si: 0.05%–0.26%, Mn: 2.1%–2.6%, P≤0.008%, S≤0.005%, Alt: 0.020%–0.040%, Ni: 0.04%–0.10%, Cr: 0.70%–1.5%, Sb: 0.02%–0.045%, V: 0.03%–0.06%, with the balance being Fe and unavoidable impurities. The ratio of [Ni] / [Sb] is controlled to be ≥2.0, where [Ni] represents the mass fraction of Ni and [Sb] represents the mass fraction of Sb.

[0033] In this application, carbon (C) is one of the most economical strengthening elements in steel. It effectively improves the hardenability of steel products and determines the bainite content in steel after continuous annealing. Therefore, an appropriate C content ensures that the steel obtains sufficient bainite during cooling to guarantee its strength. However, if the mass fraction exceeds the maximum value of the range, the C content will be too high, leading to deterioration in the weldability and formability of the steel. Conversely, if the mass fraction is below the minimum value of the range, the C content will be too low, requiring the addition of other elements to improve hardenability and obtain sufficient bainite, thus compromising the strength of the steel. Therefore, this application designs the C content to be 0.10%–0.20%, which can be selected as 0.10%, 0.13%, 0.15%, 0.18%, or 0.20%.

[0034] Si is a solid solution strengthening element in steel, improving its hardenability and strength. When the mass fraction exceeds the maximum value of this range, it affects the surface quality of the steel, easily causing red streaks that are difficult to remove in subsequent pickling processes. Conversely, when the mass fraction is below the minimum value of this range, the excessively low Si content fails to improve the hardenability of austenite. Therefore, this application designs the Si mass fraction to be 0.05%–0.26%, which can be selected as 0.05%, 0.10%, 0.15%, 0.20%, or 0.26%.

[0035] Mn is a solid solution strengthening element. When heated in the critical region, it dissolves in austenite, which is beneficial for improving the hardenability of austenite, obtaining bainitic structure, and increasing the strength of bainite. When the mass fraction is greater than the maximum value at the end of the range, the Mn content will be too high, affecting the weldability and cold formability of the steel product. When the mass fraction is less than the minimum value at the end of the range, the Mn content will be insufficient, failing to effectively play the role of solid solution strengthening, which is not conducive to obtaining a tensile strength of over 1000 MPa. Therefore, the mass fraction of Mn in this invention is 2.1% to 2.6%, and can be selected as 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, or 2.6%.

[0036] Because phosphorus (P) easily causes center segregation in steel, it deteriorates the weldability and toughness of steel products. Sulfur (S) is a harmful element that combines with manganese (Mn) to produce MnS, reducing the toughness of steel products. Therefore, this application controls P to below 0.008% and S to below 0.005%.

[0037] Al primarily functions as a deoxidizer while ensuring the cold forming performance and strength of the steel plate. When the mass fraction exceeds the maximum value of this range, excessive AlN inclusions will appear in the steel, reducing the elongation of the steel product. When the mass fraction is less than the minimum value of this range, insufficient Al content will prevent complete deoxidation of the steel, thus affecting its cold forming performance. Therefore, in this invention, the mass fraction of Al is 0.020% to 0.040%, and can be selected as 0.020%, 0.025%, 0.030%, 0.035%, or 0.040%.

[0038] Cr is a solid solution strengthening element. Adding a certain amount of Cr to steel can increase its hardenability, which is beneficial for obtaining bainitic structure under air cooling. On the other hand, Cr is an important element for improving the weather resistance of steel plates. In the sulfuric acid corrosion environment, it promotes the formation of a dense rust layer on the steel surface, which physically blocks the corrosive medium and changes the corrosion environment at the matrix location. As the corrosion depth increases, the corrosion gradually slows down. When the mass fraction is greater than the maximum value at the end of the range, excessively high Cr will damage the weldability and cold forming properties of the steel. When the mass fraction is less than the minimum value at the end of the range, excessively low Cr cannot guarantee the obtaining of bainitic structure under the low cooling rate of air cooling and cannot obtain the resistance to sulfuric acid corrosion. The mass fraction of Cr in this invention is 0.70% to 1.5%, and can be selected as 0.70%, 0.90%, 1.10%, 1.30%, and 1.50%.

[0039] Sb can suppress anodic reactions and has a good inhibitory effect on sulfuric acid corrosion, especially when used in combination with Cr to achieve good resistance to sulfuric acid corrosion. When the mass fraction is greater than the maximum value at the end of the range, higher Sb content easily liquefies during heating, weakening grain boundary properties and leading to hot brittleness of the material. When the mass fraction is less than the minimum value at the end of the range, excessively low Sb content results in low resistance to sulfuric acid corrosion. Therefore, in this invention, the mass fraction of Sb is 0.02% to 0.045%, and can be selected as 0.02%, 0.025%, 0.03%, 0.035%, 0.040%, or 0.045%.

[0040] Ni can improve the corrosion resistance of steel and also alleviate the surface brittleness caused by Sb by increasing the melting point of the Sb-rich phase. When the mass fraction is less than the minimum value at the end of the range, it cannot suppress the brittleness of Sb. However, when the mass fraction is greater than the maximum value at the end of the range, Ni, as an important strategic material, is expensive, and excessive addition will significantly increase the alloy cost of the material. Therefore, the mass fraction of Ni in this application is in the range of 0.04% to 0.10%, and can be selected as 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, or 0.10%. At the same time, based on extensive research, the ratio of the mass fraction of Ni to the mass fraction of Sb is controlled above 2.0, that is, [Ni] / [Sb]≥2.0.

[0041] V is a strong carbide-forming element that can produce a strong precipitation strengthening effect. VC has a low precipitation temperature and precipitates during annealing heating and subsequent cooling and over-aging processes, forming a nanoscale second phase, thus increasing its contribution to precipitation strengthening. On the other hand, the nanoscale VC second phase can act as an irreversible hydrogen trap, improving the material's resistance to hydrogen embrittlement. When the mass fraction is less than the minimum value at the end of the range, it cannot suppress hydrogen embrittlement; however, when the mass fraction is greater than the maximum value at the end of the range, it damages the toughness of the steel and is detrimental to cost control. In this invention, the mass fraction of VC is 0.03% to 0.06%, and can be selected as 0.03%, 0.04%, 0.05%, or 0.06%.

[0042] In some embodiments, the microstructure of the ultra-high strength steel, by volume fraction, comprises:

[0043] Tempered bainite: 85%–95%, tempered martensite: 0%–5%, ferrite: 5%–10%.

[0044] In this application, when the volume fraction of tempered bainite is between 85% and 95%, such as 85%, 87%, 90%, 93%, and 95%, it not only ensures that the strength of the steel is within a suitable range, but also ensures that the steel has a certain toughness and strength, thereby ensuring the overall performance of the steel. When the volume fraction is greater than or less than the endpoint of this range, it will lead to poor overall strength and toughness matching of the steel.

[0045] Retaining a certain amount of ferrite is beneficial to improving the plasticity of steel and the cold forming performance. Therefore, the volume fraction of ferrite in the ultra-high strength steel provided in this invention is 5% to 10%, and can be selected as 5%, 6%, 7%, 8%, 9%, or 10%.

[0046] Regarding the content of tempered martensite, up to 5% tempered martensite can be selectively obtained, which can further improve the strength of the steel.

[0047] Secondly, this application provides a method for preparing the ultra-high strength steel described in the first aspect, such as... Figure 1 As shown, the method includes:

[0048] The molten steel obtained after smelting has a chemical composition comprising: C: 0.10%–0.20%, Si: 0.05%–0.26%, Mn: 2.1%–2.6%, P ≤ 0.008%, S ≤ 0.005%, Alt: 0.020%–0.040%, Ni: 0.04%–0.10%, Cr: 0.70%–1.5%, Sb: 0.02%–0.045%, V: 0.03%–0.06%, with the balance being Fe and unavoidable impurities;

[0049] The molten steel is continuously cast to obtain a slab;

[0050] The slab is heated and held at a constant temperature, then hot-rolled, coiled, pickled, continuously annealed and leveled to obtain high-strength ultra-high-strength steel.

[0051] In some embodiments, the slab is a chamfered slab, produced using a chamfered crystallizer during continuous casting. The four corners of the chamfered slab are not sharp. The positive effect of using a chamfered crystallizer is to suppress the occurrence of cracks in the continuously cast slab easily caused by the low-melting-point element Sb. When a chamfered crystallizer is not used during continuous casting, the adverse effect is that micro-cracks easily appear at the corners of the slab, leading to hot-rolled edge cracks during subsequent rolling.

[0052] In some embodiments, the heating and holding processes employ a high-temperature rapid heating process, specifically including: rapid heating to 15°C–25°C above the homogenization temperature, preferably 20°C, during the first and second heating stages, followed by cooling to the homogenization temperature of 1150°C–1180°C, and holding at the homogenization temperature for 30–40 minutes. For example, if the final homogenization temperature is 1150°C–1180°C, rapid heating to 1170–1200°C in the first and second heating stages, followed by holding at the homogenization temperature for 30–40 minutes, can ensure rolling stability and suppress Sb enrichment at the interface between the sheet metal and the matrix due to severe burn-off, especially when its concentration exceeds its solubility and precipitates at grain boundaries, leading to edge cracking during hot rolling. When the values ​​of temperature and holding time are greater than or less than the endpoints of this range, the adverse effects will be poor rolling stability or hot rolling edge cracking caused by Sb grain boundary precipitation.

[0053] In some embodiments, the hot rolling includes roughing and finishing rolling. The roughing rolling controls the total compression ratio between 80% and 88%, for example, values ​​of 80%, 82%, 84%, 86%, and 88%, with a single-pass compression ratio ≥25%. The finishing rolling temperature of the roughing rolling is between 1000℃ and 1150℃. At this stage, the material exhibits good high-temperature plasticity, preventing cracking during rolling due to the precipitation of small amounts of Sb at grain boundaries. When the roughing rolling compression ratio and the finishing rolling temperature are greater than or less than the extreme values ​​of this range, the adverse effect will be the occurrence of hot-rolled edge cracks.

[0054] The finishing mill compression ratio gradually decreases, with the F7 stand compression ratio ≤10%, and the finishing mill final rolling temperature ranging from 880℃ to 920℃. As the temperature decreases, the material's plasticity gradually decreases, and this gradual decrease in compression ratio helps ensure rolling stability and avoid edge cracking. When the finishing mill compression ratio and final rolling temperature are greater than or less than the extreme values ​​within this range, the adverse effects include poor rolling stability and the occurrence of edge cracking.

[0055] In some embodiments, the winding temperature is 660℃ to 700℃, for example, values ​​of 660℃, 670℃, 680℃, 690℃, and 700℃. This range of winding temperatures helps prevent flattening and maintains a coarse and uniform original microstructure, which is beneficial for rolling stability during the cold rolling stage and the mechanical properties of the final product. When the winding temperature is greater than or less than the extreme values ​​of this range, the adverse effects will be the occurrence of flattening or insufficient strength in the finished product.

[0056] In some embodiments, the pickling process includes cold rolling, with a reduction rate of 40% to 50%, such as 40%, 42%, 44%, 46%, 48%, or 50%. This range of cold rolling compression ratios ensures the strength requirements of the finished product while avoiding edge cracks and surface microcracks caused by excessive compression ratios, which would affect the final cold forming performance. When the cold rolling compression ratio is greater than or less than the extreme values ​​of this range, the adverse effects will include edge cracks, surface microcracks, or insufficient strength.

[0057] In some embodiments, the continuous annealing is performed using an air-cooling method, including annealing heating, slow cooling, rapid cooling, and over-aging treatment.

[0058] In some embodiments, the annealing heating temperature is 780℃~840℃, for example, 780℃, 800℃, 820℃, or 840℃; the slow cooling endpoint temperature is 740℃~780℃, for example, 740℃, 750℃, 760℃, 770℃, or 780℃; and the rapid cooling rate is 10℃ / s~15℃ / s, for example, 10℃ / s or 11℃ / s. The annealing speeds are 12℃ / s, 13℃ / s, 14℃ / s, and 15℃ / s; the final temperature of the rapid cooling is 370℃~430℃, for example, 370℃, 380℃, 390℃, 400℃, 410℃, 420℃, and 430℃; the over-aging treatment temperature is 300℃~350℃, for example, 300℃, 310℃, 320℃, 330℃, 340℃, and 350℃. The positive effect of the annealing process design in this application embodiment is to ensure that the finished product microstructure is mainly tempered bainite, with a small amount of ferrite or ferrite + martensite microstructure, improving the material's formability and reducing hydrogen embrittlement sensitivity. The adverse effect of an unreasonable annealing process design is the appearance of martensite, affecting cold forming performance, hydrogen embrittlement resistance, strength, and other indicators.

[0059] In some embodiments, the flattening elongation is 0.10% to 0.15%, for example, it can be selected as 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, or 0.15%. This range of elongation can maximize the quality of the finished product's shape. When the flattening elongation is greater than or less than the endpoints of this range, it will lead to waviness defects or affect the performance of the finished product.

[0060] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0061] This application provides Examples 1-4 and Comparative Examples 1-3. The chemical composition of the ultra-high strength steel in each example and comparative example is shown in Table 1, and the various process parameters are shown in Table 2.

[0062] Table 1. Chemical composition of various ultra-high strength steel products.

[0063]

[0064] Table 2 shows the details of each production process parameter.

[0065]

[0066] The performance parameters and cracking conditions of the ultra-high strength steel products obtained in Examples 1-4 and Comparative Examples 1-3 were tested, and the results are shown in Table 3:

[0067] Table 3 shows the performance parameters and cracking conditions of the ultra-high strength steel products prepared in Examples 1-4 and Comparative Examples 1-3.

[0068]

[0069]

[0070] Regarding the table above, the following explanation is required:

[0071] Yield strength refers to the yield limit of a steel plate when it undergoes yielding, which is the stress that resists slight plastic deformation. The greater the yield strength, the higher the yield limit of the steel plate.

[0072] Tensile strength refers to the maximum stress that a prepared steel plate can withstand before it breaks. The greater the tensile strength, the greater the maximum stress that the steel plate can withstand before it breaks.

[0073] Elongation after fracture refers to the percentage by which the gauge length of a steel plate extends beyond its original gauge length after it breaks. The higher the elongation after fracture, the better the toughness of the steel plate.

[0074] 180 degrees d=a refers to the cold bending deformation of the sample. The better the cold forming performance of the material, the less likely microcracks will appear after forming, indicating that the material has better cold forming performance.

[0075] Hydrogen embrittlement sensitivity index I δ The hydrogen embrittlement sensitivity index refers to the slow tensile test of a material in a NACE solution. The ratio of plasticity loss in air to that in NACE solution is used as an index. The higher the hydrogen embrittlement sensitivity index, the weaker its resistance to hydrogen embrittlement cracking.

[0076] From the data in Examples 1 to 4, it can be seen that:

[0077] The composition and preparation method provided in this application, through the composite design of C-Mn-Cr-Ni-Sb-V, by controlling the cold pressing compression ratio and strictly controlling the temperature of each stage during annealing, can obtain a microstructure dominated by tempered bainite and carbides dispersed on the bainite matrix. This not only produces ultra-high strength steel products with tensile strength exceeding 1000 MPa, but also products with low corrosion rate and low hydrogen embrittlement sensitivity in sulfuric acid corrosion environment.

[0078] The ultra-high strength steel obtained in this application has a yield strength greater than 850 MPa, a tensile strength greater than 1000 MPa, and an elongation (A50) greater than 7.0%, with a maximum of 9.8%; it passes the 180°d=a cold bending test and has a hydrogen embrittlement sensitivity index of less than 14%.

[0079] From the data in comparative examples 1 to 3, we can see that:

[0080] Insufficient chemical composition or improper process control can lead to problems such as insufficient product strength, cold bending cracking, edge cracking during production, and excessively high hydrogen embrittlement sensitivity index.

[0081] Furthermore, the present invention also performed electron microscopy scanning of the microstructure of the ultra-high strength steel provided in Example 1, such as... Figure 2 As shown in the figure, the microstructure consists of tempered bainite and a small amount of tempered martensite and ferrite. The design of the multiphase structure has a good match between strength and plasticity, which can meet the requirement of forming 180° without cracking.

[0082] The ultra-high strength steel from Example 1 was placed in a NACE solution and then hydrogen-charged. The tensile fracture morphology of the steel was then photographed. Figure 3 As shown in the figure, after hydrogen purging in NACE solution, a large number of dimple morphologies are present on the tensile fracture surface, indicating that the material still exhibits ductile fracture after hydrogen purging and has good resistance to hydrogen embrittlement.

[0083] To further verify the corrosion resistance, the ultra-high strength steel in Example 1 was placed in sulfuric acid solutions of different concentrations for different periods of time. The same experiment was also performed on the ultra-high strength steel in Comparative Example 1. The comparison of corrosion weight loss is shown in the figure below. Figure 4 As shown in the figure, under the same immersion concentration and time, the corrosion weight loss of the ultra-high strength steel provided by this patent is less than that of the steel in the comparative example, especially at a concentration of 15%, where the difference is the greatest. It can be seen that the higher the concentration, the more obvious the corrosion resistance of the ultra-high strength steel provided by this invention is compared with that of the comparative example.

[0084] One or more technical solutions in the embodiments of this application have at least the following technical effects or advantages:

[0085] By controlling the contents of C, Si, Mn, Ni, Cr, and Ti, while also limiting the contents of harmful elements P and S, and adding Sb at a content of 0.02%–0.045%, the microstructure was controlled to be predominantly tempered bainite with small amounts of martensite and ferrite. Through the combined control of C, Mn, Cr, and Ti elements and the annealing process, a microstructure dominated by tempered bainite was obtained. The strengthening mechanism consisted of solid solution strengthening, microstructure strengthening, and precipitation strengthening, achieving a high strength of 1000 MPa. The combined effect of Sb, Cr, and Ni improved its resistance to sulfuric acid corrosion, and the control of the tempered bainite microstructure and the precipitation of V carbonitrides improved its resistance to hydrogen embrittlement. Finally, a 1000 MPa grade ultra-high strength steel for sulfuric acid corrosion resistance, dominated by tempered bainite, was obtained. It exhibits good ductility and toughness, meeting the technical requirement of no cracking during 180° forming (d=a).

[0086] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A 1000MPa grade ultra-high strength steel, characterized in that, The chemical composition of the ultra-high strength steel, by mass fraction, includes: C: 0.10%–0.20%, Si: 0.05%–0.26%, Mn: 2.1%–2.6%, P≤0.008%, S≤0.005%, Alt: 0.020%–0.040%, Ni: 0.04%–0.10%, Cr: 0.70%–1.5%, Sb: 0.02%–0.045%, V: 0.03%–0.06%, with the balance being Fe and unavoidable impurities, and the [Ni] / [Sb] ratio is controlled to be ≥2.0, where [Ni] represents the mass fraction of Ni and [Sb] represents the mass fraction of Sb. The preparation method of the 1000MPa grade ultra-high strength steel includes the following steps: The molten steel obtained after smelting has the following chemical composition: C: 0.10%–0.20%, Si: 0.05%–0.26%, Mn: 2.1%–2.6%, P ≤ 0.008%, S ≤ 0.005%, Alt: 0.020%–0.040%, Ni: 0.04%–0.10%, Cr: 0.70%–1.5%, Sb: 0.02%–0.045%, V: 0.03%–0.06%, with the balance being Fe and unavoidable impurities; The molten steel is continuously cast to obtain a slab; The slab is heated and held at a certain temperature, and then hot rolled, coiled, pickled, continuously annealed and leveled to obtain high-strength ultra-high-strength steel. The continuous annealing adopts an air cooling method, including annealing heating, slow cooling, rapid cooling and over-aging treatment processes; The annealing heating temperature is 780℃~840℃, the slow cooling endpoint temperature is 740℃~780℃, the rapid cooling rate is 10℃ / s ~15℃ / s, the rapid cooling endpoint temperature is 370℃~430℃, and the over-aging treatment temperature is 300℃~350℃. The winding temperature is 660℃~700℃; the pickling includes cold rolling, and the reduction rate of the cold rolling is 40%~50%.

2. The ultra-high strength steel according to claim 1, characterized in that, The microstructure of the ultra-high strength steel, by volume fraction, comprises: Tempered bainite: 85%–95%, tempered martensite: 0%–5%, ferrite: 5%–10%.

3. The ultra-high strength steel according to claim 1, characterized in that, The slab is a chamfered slab, produced using a chamfered crystallizer during continuous casting, and the four corners of the slab are non-sharp.

4. The ultra-high strength steel according to claim 1, characterized in that, The heating and heat preservation adopts a high-temperature rapid heating process, which specifically includes: rapid heating to 15°C to 25°C above the uniform heat temperature through a first heating and a second heating stage, followed by cooling to the uniform heat temperature of 1150°C to 1180°C, and holding at the uniform heat temperature for 30 to 40 minutes.

5. The ultra-high strength steel according to claim 1, characterized in that, The hot rolling includes roughing and finishing. The total compression ratio of the roughing is controlled between 80% and 88%, the single-pass compression ratio is ≥25%, and the final rolling temperature of the roughing is 1000℃ to 1150℃. The compression ratio of the finishing is gradually reduced, the compression ratio of the F7 stand is ≤10%, and the final rolling temperature of the finishing is 880℃ to 920℃.

6. The ultra-high strength steel according to claim 1, characterized in that, The elongation of the flattened surface is 0.10% to 0.15%.

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