A thin-gauge high-strength bridge steel plate and its production method

Abstract

The present invention provides a thin-gauge, high-strength bridge steel plate and its production method. The chemical composition of the steel plate includes C, Si, Mn, Cr, Cu, Ni, Nb, Ti, Al, trace amounts of S, P, and H, with the remainder being Fe and other unavoidable impurities. The production method is simplified to a single heating step followed by rolling. Through composition optimization and process innovation, the present invention overcomes the bottlenecks of the prior art, such as low efficiency, uneven performance, and insufficient corrosion resistance caused by two rolling steps and reliance on coiling furnaces. This achieves efficient production of thin-gauge bridge steel plate and comprehensively improves its overall performance.

Description

Technical Field

[0001] The present invention relates to the technical field of steel processing, and in particular to a thin-gauge high-strength bridge steel plate and a production method thereof. Background Art

[0002] With the rapid development of long-span bridge construction, steel box girders have become the mainstream structural form for sea-crossing bridges, suspension bridges, and other structures due to their advantages such as light weight, excellent seismic performance, and convenient construction. The U-shaped stiffening ribs (U-ribs) in steel box girders require the extensive use of thin, high-strength bridge steel plates with a thickness of 6-10 mm. Design specifications explicitly require that these plates be made from unrolled flat plates, prohibiting the use of hot-rolled coils. However, existing wide and heavy plate production lines face the following technical bottlenecks when manufacturing thin-gauge bridge steel:

[0003] 1. The production process is complex and costly: The traditional process requires the continuous casting slab to be heated twice (with a soaking temperature ≥ 1200°C) and rolled twice (slab opening + finishing rolling). This results in a 30%-40% extension of the production cycle, an increase in energy consumption of more than 25%, a reduction in the yield rate, and a significant increase in production costs.

[0004] 2. The problem of plate shape control is prominent: Thin-gauge steel plates (thickness ≤ 10mm) are prone to defects such as warping, edge waves, and center waves during the rolling process. The unevenness is generally greater than 5mm / m, which seriously affects the subsequent welding and assembly accuracy.

[0005] 3. The contradiction between strength and toughness matching is exacerbated: To meet the design strength requirements of bridges (yield strength ≥ 420 MPa), the existing process strengthens steel by increasing the carbon equivalent. However, this causes the yield strength ratio to rise to above 0.92, the impact energy at -40°C to drop to below 60J, and the cold bending qualification rate to be less than 80%, which seriously threatens the structural safety.

[0006] 4. Insufficient corrosion resistance and service life: Traditional bridge steel has poor resistance to chloride ion corrosion in the marine atmospheric environment. The corrosion rate at the U-rib welds of cross-sea bridges can reach 0.3mm / year, and the service life is shortened by 30%-40% compared to the design requirements, resulting in a surge in subsequent maintenance costs.

[0007] Patent CN114951267A proposes a method for directly rolling narrow, thin steel plates from 220mm billets using a single-stand Steckel mill. While this method eliminates one rolling step, it relies on repeated heating in a coiling furnace (coiling temperature ≥ 850°C), resulting in a 1.2% increase in oxidation and burnout, and is unable to produce steel plates wider than 3000mm. High-temperature rapid rolling reduces temperature drop, but the finish rolling temperature fluctuates widely (750°C-850°C), which can lead to uneven microstructure and difficulty balancing yield strength and low-temperature toughness (impact energy ≤ 60J at -40°C). Furthermore, the coiling process is complex and requires high maintenance costs, while only increasing the yield rate by 10%. Patent CN111889512B describes a method for producing thin steel plates using a single-stand mill. This method uses a two-pass rolling process, which reduces unevenness to less than 3mm / m. However, the billet requires secondary heating after slitting, resulting in significant grain coarsening and a 5%-8% decrease in elongation. It can be seen that the existing technology still finds it difficult to take into account the comprehensive requirements of efficient production of thin-gauge bridge steel, excellent plate shape, strength and toughness balance, and long-term corrosion resistance. Summary of the Invention

[0008] In order to solve the above-mentioned technical problems, the present invention provides a thin-gauge high-strength bridge steel plate and a production method thereof. Through composition optimization and process innovation, it breaks through the bottlenecks of low efficiency, uneven performance, insufficient corrosion resistance and other problems caused by double rolling and reliance on coiling furnaces in the existing technology, and realizes the efficient production of thin-gauge bridge steel plates and comprehensive improvement of their comprehensive performance.

[0009] To achieve the above objectives, the present invention proposes the following technical solutions:

[0010] A thin-gauge high-strength bridge steel plate, wherein the chemical composition of the steel plate comprises, by mass percentage:

[0011] C: 0.055%-0.085%, Si: 0.20%-0.25%, Mn: 0.55%-0.65%, Cr: 0.88%-1.00%, Cu: 0.30%-0.40%, Ni: 0.10%-0.18%, Nb: 0.035%-0.045%, Ti: 0.010%-0.018%, Al: 0.03%-0.04%, S≤0.002%, P: 0.012%-0.019%, H≤0.00015%, the rest are Fe and other unavoidable impurities;

[0012] Among them, Si, Cr, Cu, Ni, and P satisfy the following corrosion resistance index formula:

[0013] [26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu)2 ]≥6.5.

[0014] As a preferred technical solution of the present invention, the structural organization of the steel plate is a ferrite+bainite duplex structure, wherein the ferrite accounts for 25%-35% and the bainite accounts for 65%-75%.

[0015] The present invention also provides a method for producing thin-gauge high-strength bridge steel plates, comprising the following steps:

[0016] (1) Smelting and continuous casting: The molten iron is subjected to pre-desulfurization treatment, converter smelting to adjust the elemental composition, ladle refining, and RH vacuum refining, and then continuously cast into billets, which are stacked and cooled. The billets have a thickness of 220±10mm, a width of 1400±100mm, and a length of 2300-3800mm.

[0017] (2) Rolling: The billet is heated to 1230-1255°C, kept warm for 220-280 minutes, and quickly sent to the feed end of the rolling mill, and directly rolled into a target steel plate with a thickness of 6-10 mm and a length of ≤40,000 mm;

[0018] The starting rolling temperature of the rolling mill is ≥1200°C, and the finishing rolling temperature is 830-870°C. During the rolling process, the reduction of the rolling mill rolls is dynamically controlled according to temperature changes.

[0019] (3) Cooling after rolling: The target steel plate after rolling is cooled to 480-530℃ at a cooling rate of 15-25℃ / s, and then placed in a holding pit for insulation.

[0020] As a preferred technical solution of the present invention, the RH vacuum refining specifically includes: degassing the ladle at a vacuum degree of ≤2 mBar for at least 15 minutes, and simultaneously performing soft stirring for at least 15 minutes.

[0021] As a preferred technical solution of the present invention, in step (2), before the billet is quickly delivered to the feed end of the rolling mill, roll scheduling is first performed, including:

[0022] The billet is placed at the 40th to 80th block of the rolling mill's roll production sequence as the target billet to be rolled. Before rolling the target billet to be rolled, 6 to 10 blocks of the billet are first rolled into steel plate transition material with a thickness specification of 8 to 14 mm, and the steel plate transition material is rolled from thick to thin.

[0023] As a preferred technical solution of the present invention, in step (2), during the rolling process of the billet, the total rolling passes are ≤13 passes, wherein the billet is kept warm after the 6th pass, the starting rolling temperature of the 7th pass is 1050-1060°C, and the last pass is a leveling pass.

[0024] As a preferred technical solution of the present invention, in step (2), during the rolling process of the billet, the rolling mill first bites into the billet at an initial speed of 2.0 m / s, and after rolling for 1 m, linearly accelerates to 8.0 m / s for constant speed rolling, and finally linearly decelerates for rolling and decelerates to 4.0 m / s before throwing the steel.

[0025] As a preferred technical solution of the present invention, in step (2), during the rolling process of the billet, the cooling water of the rolling mill rolls is turned off before the billet is bitten, the cooling water of the rolling mill rolls is turned on after rolling for 1 m, and the cooling water of the rolling mill rolls is turned off when the remaining 2 m of rolling is completed.

[0026] As a preferred technical solution of the present invention, in step (2), dynamically controlling the reduction amount of the rolling mill rolls according to temperature changes during the rolling process includes:

[0027] When the rolling temperature is ≥1060℃, the reduction is dynamically controlled by the total torque of the twin rolls of 5000-6000 kN·m;

[0028] When the rolling temperature is 960-1060℃, the reduction is regulated by the rolling force of 55000-66000kN and the bending roll force of 3000-4000kN;

[0029] When the rolling temperature is less than 960℃, the rolling force and bending roll force are gradually reduced to reduce the reduction.

[0030] As a preferred technical solution of the present invention, in step (2), during the billet rolling process, when the intermediate billet thickness is ≤32 mm, the roller cooling water is turned off, and the single-pass reduction meets the following requirements:

[0031] When the thickness of the intermediate blank is 10-16mm, the single-pass reduction is greater than 3mm;

[0032] When the thickness of the intermediate blank is less than 10mm, the single-pass reduction is greater than 1.5mm.

[0033] As can be seen from the above technical solutions, the technical solution of the present invention provides a thin-gauge high-strength bridge steel plate and a production method thereof, which has the following advantages over traditional production methods:

[0034] (1) Without adding new equipment, it is possible to directly roll 220±10mm thick billets into 6-10mm thick bridge steel plates on the wide and thick plate production line. The original process of two heating and two rolling steps is reduced to one heating and one rolling step, eliminating the intermediate billet opening process, shortening the production cycle, reducing energy consumption, improving the yield rate, and reducing production costs.

[0035] (2) Improve the welding performance of steel plates through low-carbon composition design, and improve the corrosion resistance of steel plates by adding elements such as Cr, Cu, Ni, and Si, thereby increasing the service life of steel plates and reducing maintenance costs.

[0036] (3) Improve the quality and flatness of the steel plate shape by scheduling the roll period and matching the parameters such as billet heating temperature, reduction, rolling speed, rolling temperature, rolling thickness, rolling force, torque, and bending roll force.

[0037] (4) By controlling the upper and lower surface temperatures of the billet when it leaves the furnace, the cooling water of the rolling rollers, the cooling water of the roller table (the conveyor roller used to transport the billet), rapid cooling after rolling, and insulation in the insulation pit, the temperature uniformity of the entire plate is improved and the performance difference of the same plate is reduced.

[0038] (5) During the RH vacuum refining process, the degassing time is guaranteed to be ≥15 min and the soft stirring time is ≥15 min, which can improve the purity of the steel plate and thus improve its comprehensive mechanical properties; the billet is stacked and cooled, the stacking cooling time is ≥48 h, and the billet is placed in a heat preservation pit for heat preservation after rolling, which can enable the billet and steel plate to fully release hydrogen and improve the low-temperature toughness and cold forming performance of the steel plate.

[0039] (6) During the billet heating process, the furnace temperature should be ≥1230℃. After the billet is taken out of the furnace, the transportation speed should be no less than 6m / s, and the rolling temperature should be ≥1200℃. This can effectively avoid the warping problem during the steel transfer stage and ensure that the plate shape quality meets the standard during the rolling process. At the same time, it helps to improve the rolling temperature of the steel plate. In addition, the total number of rolling passes is limited to ensure that the steel plate can reach the expected final rolling temperature. The heating temperature of the billet should be controlled at ≤1255℃ to avoid abnormal growth of the austenite structure. The billet holding time is controlled at 220-280min. When the billet is taken out of the furnace, the temperature difference between the upper and lower surfaces of the billet is controlled at ≤20℃ to ensure that the billet reaches a fully "burned through" state. This not only improves the uniformity of the billet temperature, but also promotes the full solid solution of alloying elements. The uniformity of the billet temperature is an important prerequisite for ensuring uniform deformation of the steel plate, and improving the deformation uniformity is of great significance to improving the shape quality of the steel plate. In addition, the full solid solution of alloying elements is an important prerequisite for ensuring the strength of the steel plate.

[0040] (7) Low-speed biting and throwing can ensure the shape of the steel plate at the head and tail, and prevent warping or buckling. High-speed rolling can prevent the final rolling temperature of thin gauges from being too low, ensuring the final rolling temperature of the steel plate; secondly, it can improve production efficiency; thirdly, through the inertial traction of the steel plate's rapid movement, it can prevent the steel plate from running off and avoid the occurrence of bevel angles, thus ensuring the shape of the steel plate.

[0041] (8) The control of the roller cooling water is intended to maintain the uniformity of the rolling temperature of the entire plate. The temperature drop at the head and tail is fast, and the temperature of the head and tail of the steel plate can be compensated by controlling the roller cooling water. If the final rolling temperature is too high, the microstructure grains are too coarse and the low-temperature toughness is poor; if the final rolling temperature is too low, the microstructure grains are too fine and the yield strength ratio is likely to exceed the standard. Therefore, in order to ensure that the bridge steel plate has both good low-temperature toughness and a low yield strength ratio, the final rolling temperature must be strictly controlled. Therefore, the temperature uniformity of the entire plate is also a prerequisite for ensuring the uniformity of the steel plate performance.

[0042] (9) The rolling process adopts a torque-force synergistic control strategy based on deformation resistance zoning: in the high temperature zone (≥1060℃), the low deformation resistance characteristics of the material are preferentially utilized, and the ultimate reduction is achieved by setting the upper limit of the total torque of the double rolls (5000-6000kNm); when the temperature drops to the medium and low temperature zone (960℃-1060℃), it switches to the rolling force-bending roll force coupling control mode (rolling force 55000-66000kN, bending roll force 3000-4000kN), and actively increases the roll gap convexity by increasing the bending roll force to 3000-4000kN, forming an asymmetric reduction distribution of thinning in the middle and thickening at the edge, so as to suppress the deviation of the rolled piece and enhance the lateral stability; when the temperature is less than 960℃, a progressive load reduction procedure is implemented according to the work hardening effect, and the rolling force and bending roll force are progressively reduced. After rolling, a leveling pass is added to make the plate shape flatter, which is also the prerequisite for ensuring uniform cooling of the steel plate.

[0043] (10) The smaller the single-pass reduction, the more significant the temperature drop. For example, when the intermediate billet is 9 mm, a single-pass reduction of 0.7 mm results in a temperature drop of approximately 70°C; a single-pass reduction of 2 mm results in a temperature drop of approximately 30°C. The faster the temperature drop of the steel plate, the more difficult it is to maintain the final rolling temperature; and the lower the final rolling temperature, the more difficult it is to control the plate shape. Therefore, when producing thin-gauge steel plates, the single-pass reduction in the subsequent passes must be ensured to avoid excessive temperature drop.

[0044] (11) When the intermediate billet thickness is ≤32mm, turning off the roller cooling water can reduce the heat loss of the rolled piece. When the intermediate billet thickness is ≤14mm, a low-temperature black band along the width direction is likely to appear when the intermediate billet stays on the roller. Therefore, turning off the roller cooling water can also ensure the temperature uniformity of the steel plate.

[0045] (12) The lower the rolling temperature, the greater the deformation resistance of the steel plate and the more difficult it is to control the plate shape. If the rolling temperature is too low, the grain size of the structure will be too fine, which is not good for the yield strength ratio of the steel plate. If the final rolling temperature is too high, it will be not good for the low-temperature toughness. Therefore, the final rolling temperature is controlled at 830-870℃.

[0046] (13) Rapid cooling after rolling can refine the phase transformation structure and improve the strength, plasticity and toughness of the steel plate.

[0047] (14) Insulation in the insulation pit can fully utilize the residual heat of the steel plate, so that the steel plate undergoes uniform self-tempering in the insulation pit, which can not only ensure the plate shape of the steel plate, but also fully eliminate the residual stress caused by rolling and cooling of the steel plate, and can eliminate the hard phase structure, thereby improving the low-temperature toughness of the steel plate.

[0048] (15) Ferrite + bainite dual phase structure design, bainite provides higher strength for the steel plate, and the evenly distributed ferrite structure provides lower yield strength and higher elongation for the steel plate, making the steel plate have good seismic resistance and cold forming performance.

[0049] It should be appreciated that all combinations of the foregoing concepts, as well as additional concepts described in greater detail below, to the extent such concepts are not mutually inconsistent, can be considered to be part of the inventive subject matter of this disclosure.

[0050] The foregoing and other aspects, embodiments, and features of the present invention will be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the present invention, such as features and / or beneficial effects of the exemplary embodiments, will become apparent from the following description or through practice of specific embodiments according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The accompanying drawings are not drawn to scale. In the accompanying drawings, each identical or nearly identical component shown in various figures may be represented by the same reference numeral. For clarity, not every component is labeled in every figure. Embodiments of various aspects of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0052] Figure 1 This is a 500-fold magnified microstructural photograph of 1 / 2 of the longitudinal section of a bridge steel plate produced in an embodiment of the present invention. DETAILED DESCRIPTION

[0053] To further clarify the objectives, technical solutions, and advantages of the embodiments of the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only a portion of the embodiments of the present invention, not all of them. Based on the described embodiments of the present invention, all other embodiments derived by persons of ordinary skill in the art without requiring creative effort are within the scope of protection of the present invention. Unless otherwise defined, technical or scientific terms used herein shall have the ordinary meanings understood by persons of ordinary skill in the field to which the present invention pertains.

[0054] The terms "first," "second," and similar terms used in the patent application specification and claims of this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "a," "an," or "the" and similar terms do not indicate a limitation on quantity, but rather indicate the presence of at least one. Terms such as "include" or "comprising" indicate that the elements or objects preceding the word "include" or "comprising" encompass the features, integers, steps, operations, elements, and / or components listed after the word "include" or "comprising," and do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or combinations thereof.

[0055] In order to solve the technical problem that the existing technology is still difficult to take into account the comprehensive requirements of efficient production, excellent plate shape, strength and toughness balance and long-term corrosion resistance of thin-gauge bridge steel, the embodiment of the present invention provides a thin-gauge high-strength bridge steel plate and its production method. Through composition optimization and process innovation, it breaks through the bottlenecks of low efficiency, uneven performance, insufficient corrosion resistance and other problems caused by two rolling and reliance on coiling furnaces in the existing technology, and realizes the efficient production of thin-gauge bridge steel plates and comprehensive improvement of comprehensive performance.

[0056] The embodiment of the present invention provides a thin-gauge high-strength bridge steel plate, whose chemical composition, calculated by mass percentage, includes:

[0057] C: 0.055%-0.085%, Si: 0.20%-0.25%, Mn: 0.55%-0.65%, Cr: 0.88%-1.00%, Cu: 0.30%-0.40%, Ni: 0.10%-0.18%, Nb: 0.035%-0.045%, Ti: 0.010%-0.018%, Al: 0.03%-0.04%, S≤0.002%, P: 0.012%-0.019%, H≤0.00015%, the rest are Fe and other unavoidable impurities; Among them, Si, Cr, Cu, Ni, and P meet the following corrosion resistance index formula:

[0058] [26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu) 2 ]≥6.5. Wherein, (%Cu) represents the mass percentage of copper element, and the same applies to other elements.

[0059] The steel plate produced in the embodiment of the present invention has a thickness of 6-10 mm and has the following mechanical properties: yield strength ≥500 MPa, tensile strength ≥660 MPa, yield ratio ≤0.78, elongation ≥25%, -60°C impact energy KV2 ≥250 J; strength difference of the same plate ≤30 MPa, yield ratio difference of the same plate ≤0.04; internal stress distribution at different positions of the steel plate is uniform, and the maximum value of residual internal stress is <15 MPa; the unevenness of the steel plate is ≤2 mm / m; in addition, in a marine atmospheric environment, the annual average corrosion rate of the steel plate is less than 0.15 mm / a; Figure 1 As shown in FIG, the structural organization of the steel plate is a duplex organization of ferrite + bainite, in which ferrite accounts for 25%-35% and bainite accounts for 65%-75%.

[0060] The following is a detailed analysis and explanation of the effects of the components contained in the steel plate and the selection of their amounts, in conjunction with the corrosion resistance index formula provided in the embodiment of the present invention.

[0061] C: Element C is a solid solution strengthening element that effectively increases the strength of steel plates with minimal impact on the yield strength ratio. However, carbon has an adverse effect on the corrosion resistance, weldability, and low-temperature toughness of steel plates. Furthermore, when the carbon content exceeds 0.085%, the steel enters the peritectic region, where surface cracks are more likely to form in the billet during the continuous casting process. Therefore, in the embodiments of the present invention, the carbon content is controlled within a range of 0.055% to 0.085%.

[0062] Si: Si is a solid solution strengthening element that increases the strength of steel plates and enhances their corrosion resistance in natural environments, particularly localized corrosion resistance. However, steel plates with high silicon contents exhibit poor scale stripping performance, which can easily leave residual scale and negatively impact surface quality. Furthermore, high silicon contents can negatively impact weldability and can easily lead to cold brittleness. Therefore, the silicon content is constrained by the corrosion resistance index formula to [1.49(%Si)], keeping it within a range of 0.20%-0.25%.

[0063] Mn: Mn is a solid solution strengthening element that can improve the strength and hardness of the steel plate. Manganese, as an austenite stabilizing element, can expand the austenite region and inhibit the formation of ferrite. However, excessive manganese content has an adverse effect on the welding performance of the steel plate. Therefore, the manganese content is controlled at 0.55%-0.65%.

[0064] Cr: Cr is also a solid solution strengthening element and can effectively improve the strength of steel plates. Chromium is a stabilizing element in the ferrite zone and can inhibit pearlite transformation. Controlled cooling can produce a ferrite + bainite duplex structure, which improves the strength of the steel plate while maintaining a low yield strength ratio and good plasticity. In addition, in corrosive environments, chromium can form a stable passivation film on the surface of the steel plate, preventing further corrosion. However, excessive chromium content is detrimental to the weldability of the steel plate. Therefore, the chromium content is constrained by the corrosion resistance index formula [1.20 (%Cr)], and the chromium content is controlled within 0.88%-1.00%.

[0065] Cu: The addition of Cu element can not only effectively improve the strength and impact toughness of the steel plate, but also enhance the corrosion resistance of the steel plate. When copper element is added, in the corrosive environment of the marine atmosphere, the copper in the steel plate will be segregated and gradually enriched as the iron dissolves, and a layer of oxide intermediate film will be formed between the corrosion interface and the copper-enriched layer, which will be adsorbed on the surface of the steel material, preventing the corrosive medium from corroding into the interior of the steel plate, and ultimately achieving the effect of corrosion resistance. However, the price of copper is relatively high, and when its content is high, it is not conducive to hot deformation processing. Therefore, the content of copper element is limited by the corrosion resistance index formula [26.01 (%Cu), 7.29 (%Cu) (%Ni), 33.39 (%Cu) 2 ], and control the copper content at 0.30%-0.40%.

[0066] Ni: Ni is an austenite-stabilizing element that significantly expands the austenite phase of steel and refines the grain size, thereby improving the material's toughness and hardenability. In terms of mechanical properties, Ni enhances the strength of steel through a solid solution strengthening mechanism, while also improving low-temperature impact toughness and reducing the material's susceptibility to stress concentration and fatigue crack growth. Furthermore, the addition of Ni optimizes the corrosion resistance of steel. After accumulation at the rust layer interface, Ni forms a selective ion barrier, inhibiting chloride ion penetration and promoting the formation of a protective rust layer (γ-FeOOH), making it particularly suitable for harsh environments such as marine splash zones. When Ni is added simultaneously with Cu and Cr, the synergistic effect creates a denser corrosion-resistant layer, significantly enhancing and optimizing the corrosion resistance of the steel plate. However, since Ni is a strategically scarce resource, practical applications prioritize multi-element synergistic substitution solutions to reduce costs. Therefore, the Ni content is constrained by the corrosion resistance index formula [3.88 (%Ni), 7.29 (%Cu) (%Ni), 9.10 (%Ni) (%P)], and the Ni content is controlled within 0.10%-0.18%.

[0067] Nb and Ti: When the billet is heated to high temperatures, Nb and Ti solid solution atoms tend to segregate at austenite grain boundaries, dragging them and inhibiting grain growth during heating. The Nb and Ti carbonitrides that precipitate during rolling also inhibit austenite recrystallization and subsequent grain growth. Therefore, trace amounts of Nb and Ti have a significant grain refinement strengthening effect. Grain refinement can improve the strength of the steel plate without compromising its ductility and toughness. During the holding pit, Nb and Ti compounds are fully precipitated, achieving optimal precipitation strengthening, further improving the steel plate's strength and low-temperature toughness. Even trace amounts of Nb can have a significant effect. Further additions yield minimal improvement, but increase alloy cost. Excessive Nb levels can also lead to the formation of large impurities and brittle phases. Therefore, the Nb content is controlled between 0.035% and 0.045%. However, if the Ti content is too high, large inclusions will be easily formed, which is not conducive to low-temperature impact toughness. Therefore, the Ti content is controlled within 0.010%-0.018%.

[0068] Al: During steelmaking, Al effectively reduces the oxygen content in steel, providing a strong deoxidizing effect. Aluminum also inhibits surface corrosion and internal oxidation, extending the service life of the steel. Aluminum refines the original austenite grains in steel and increases the temperature at which grains coarsen. However, when the dissolved aluminum content in steel exceeds a certain value, the austenite grains tend to grow and coarsen. Therefore, the aluminum content is controlled within a range of 0.03%-0.04%.

[0069] Phosphorus (P): Elemental phosphorus also offers corrosion resistance and can improve steel plate strength. However, phosphorus increases low-temperature brittleness, can cause severe segregation, and can increase weld sensitivity, negatively impacting weldability. Therefore, the phosphorus content is constrained by the corrosion resistance index formula [17.28%P, 9.10%Ni, %P], maintaining a phosphorus content within the range of 0.012%-0.019%.

[0070] S: S is a harmful element in steel. It exists in the form of FeS, which forms a low-melting-point (985°C) compound with Fe. Since billet heating temperatures are generally above 1100°C, premature melting of the FeS compound during heating can cause cracking during forging and rolling. S also negatively impacts the weldability of steel plates and reduces corrosion resistance. Therefore, the S content is controlled to ≤ 0.002%.

[0071] H: H is also a harmful element in steel. Dissolved H in steel can cause defects such as hydrogen embrittlement and white spots. Like O and N, H has extremely low solubility in solid steel. When dissolved at high temperatures, H dissolves in the molten steel and, during cooling, has no time to escape, accumulating in the steel structure, forming high-pressure micropores. This drastically reduces the steel's plasticity, toughness, and fracture toughness, and in severe cases, can cause cracks and brittle fracture. Therefore, the H content is controlled to ≤ 0.00015%.

[0072] The corrosion resistance index formula disclosed in the embodiment of the present invention reflects that the technical means of the present invention maximizes the corrosion resistance of steel through the synergistic effect of multiple elements, while limiting the negative effects of excessive elements to achieve performance balance. The formula requires ≥6.5, and the balance of component ratios is achieved through mathematical optimization, and elements such as Cu, Ni, Cr, Si, and P that have both improved corrosion resistance and adverse effects on steel plates are constrained. The embodiment of the present invention has formulated a performance quantification standard. The formula transforms the complex multi-element interaction into a quantifiable index, provides a clear goal for component design, and avoids the inefficiency of traditional empirical trial and error methods. The range of the content of each element is constrained by the formula to ensure that the corrosion resistance of different batches of steel plates is consistent and meets the requirements of engineering specifications. Through regression analysis of experimental data, the nonlinear relationship between elements is revealed, such as the synergistic / antagonistic effect of Cu and Ni elements [7.29 (%Cu) (%Ni)], and the ratio is optimized. The nonlinear terms in the formula include quadratic terms such as 33.39 (%Cu) 2 The presence of cross-terms such as 7.29 (%Cu)(%Ni) and 9.10 (%Ni)(%P) indicates that the interactions between elements are not simply linear superpositions, but rather complex, nonlinear interactions. This formula quantifies the synergistic and antagonistic effects of multiple elements through nonlinear terms, addressing the limitations of traditional single-element limit methods and ensuring comprehensive optimization of corrosion resistance, strength, and ductility. Furthermore, the significant differences in the coefficients of each element (for example, Cu's coefficient of 26.01 is much higher than Ni's 3.88) reflect the primary and secondary contributions to corrosion resistance.

[0073] The embodiment of the present invention further provides a method for producing thin-gauge high-strength bridge steel plates, comprising the following steps:

[0074] (1) Smelting and continuous casting: The molten iron is pre-desulfurized to remove sulfur from the molten iron (too much sulfur will cause cracking of the steel plate); then the molten iron is mixed with other alloying elements (such as Cr, Cu, Ni, etc.) for smelting in a converter to adjust the elemental composition of the molten iron; then the molten steel is further purified by ladle refining to remove impurities; further RH vacuum refining is carried out, specifically degassing for at least 15 minutes at a vacuum degree of ≤2mBar, and soft stirring is carried out for at least 15 minutes at the same time to reduce bubbles in the steel, improve the purity, and thus improve the comprehensive mechanical properties of the steel plate; finally, the billet is continuously cast into a billet with a size of 220±10mm in thickness, 1400±100mm in width, and 2300-3800mm in length (cut according to the size of the final steel plate), and stacked for cooling, with a stacking cooling time of ≥48h to slowly release internal stress and avoid deformation.

[0075] (2) Rolling: The size of the billet remains the same throughout the rolling process. However, in order to ensure the rolling quality of the target steel plate, a scientific production sequence arrangement strategy needs to be formulated. Therefore, the embodiment of the present invention performs the following roll scheduling before rolling.

[0076] Roll Scheduling: Billets are placed in the 40th to 80th positions of the rolling mill's roll production sequence as the target billet to be rolled. At this point, roll wear is moderate to ensure plate shape accuracy. Before rolling the target billet, 6-10 billets are rolled into transitional steel plates with a thickness of 8-14mm (i.e., transitional steel plates with a thickness of 8-14mm are rolled starting with billets 30-34, while non-target steel plates with less stringent thickness and / or shape requirements can be rolled before this). The transitional steel plates are rolled from thick to thin, with a gradual thickness reduction buffer to gradually converge the roll gap to the required accuracy range for the target steel plate. The mill is then gradually adjusted to a stable state, and then mill parameters are adjusted to roll the target steel plate with the desired thickness (i.e., 6-10mm). The mill uses rolls with an out-of-roundness of ≤0.4mm.

[0077] Billet heating: The billet is heated to 1230-1255°C and kept warm for 220-280 minutes to ensure uniform temperature inside the billet and a temperature difference of ≤20°C between the upper and lower surfaces of the billet when it is taken out of the furnace to avoid uneven deformation during rolling. After the billet is taken out of the furnace, the surface of the billet is deoxidized with high-pressure water (dephosphorization treatment) to prevent it from being pressed into the steel plate during rolling.

[0078] High-temperature rolling: Dephosphorized billets are rapidly delivered to the mill's feed end at a speed of ≥6 m / s. Without opening the billet, the billet is directly rolled into target steel plates with a thickness of 6-10 mm and a length of ≤40,000 mm. The mill's starting rolling temperature is ≥1200°C (to reduce deformation resistance through high-temperature rolling), and the final rolling temperature is 830-870°C (to ensure grain refinement and enhance the steel's strength and toughness). During the rolling process, the mill's roll reduction is dynamically controlled based on temperature changes.

[0079] During the billet rolling process, the total number of rolling passes is ≤ 13. The fewer the rolling passes, the smaller the temperature drop. After the sixth rolling pass, the billet is allowed to warm up, and the seventh rolling pass begins at a temperature of 1050-1060°C. The final pass is a leveling pass to eliminate residual stress and improve flatness.

[0080] For the speed control of the rolling process: at the beginning of rolling, the rolling mill bites the billet at an initial speed of 2.0 m / s, and after rolling for 1 m, it starts linear acceleration (acceleration 2.0 m / s 2 ), after 3s, it is accelerated to 8.0m / s for constant speed rolling; when the remaining 14m is reached, it is switched to linear deceleration (deceleration 2.0m / s 2 ) for rolling, and the steel is thrown after being decelerated to 4.0m / s after 2s.

[0081] For the cooling water control during the rolling process: the cooling water of the rolling mill rolls is turned off before the billet is bitten, the cooling water of the rolling mill rolls is turned on after rolling for 1m, and the cooling water of the rolling mill rolls is turned off when the remaining 2m is rolled. The amount of cooling water used during rolling is the original water volume × 3 times the thickness of the finished steel, where the original water volume is the cooling water consumption of the traditional process.

[0082] When the rolling temperature is ≥1060℃, the reduction is dynamically controlled by the total torque of the twin rolls of 5000-6000 kN·m;

[0083] When the rolling temperature is 960-1060℃, the reduction is coordinated by the rolling force of 55000-66000kN and the bending roll force of 3000-4000kN; when the rolling temperature is less than 960℃, the rolling force, bending roll force and reduction are gradually reduced.

[0084] At the same time, when the thickness of the intermediate blank is ≤32mm, the roller cooling water is turned off, and the single-pass reduction meets the following requirements: when the thickness of the intermediate blank is 10-16mm, the single-pass reduction is greater than 3mm; when the thickness of the intermediate blank is less than 10mm, the single-pass reduction is greater than 1.5mm.

[0085] (3) Post-rolling cooling: After rolling, the steel plate is quickly put into water with a water temperature of ≥780℃. After entering the water, it is cooled to 480-530℃ at a cooling rate of 15-25℃ / s, and then quickly lowered from the cooling bed and put into the holding pit with a pit temperature of ≥420℃ and a holding time of ≥45min.

[0086] The steel plate production method of the embodiment of the present invention, through appropriate design of the chemical composition and heating, rolling, and cooling processes, produces steel plates with a thickness of 6-10 mm. The steel plates have a yield strength of 500 MPa or greater, a tensile strength of 660 MPa or greater, a yield ratio of 0.78 or less, an elongation of 25% or greater, and a -60°C impact energy (KV2) of 250 J or greater. The strength difference within the same plate is 30 MPa or less, and the yield ratio difference within the same plate is 0.04 or less. The internal stress distribution at different locations on the steel plate is uniform, with a maximum residual internal stress of less than 15 MPa. The steel plate unevenness is 2 mm / m or less. In a marine atmosphere, the steel plate has an average annual corrosion rate of less than 0.15 mm / a.

[0087] While the present invention has been disclosed above with reference to preferred embodiments, this is not intended to limit the present invention. Persons skilled in the art will readily appreciate that various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A thin gauge high strength bridge steel plate, characterized in that: The chemical composition of the steel plate includes, by mass percentage: C: 0.055%-0.085%, Si: 0.20%-0.25%, Mn: 0.55%-0.65%, Cr: 0.88%-1.00%, Cu: 0.30%-0.40%, Ni: 0.10%-0.18%, Nb: 0.035%-0.045%, Ti: 0.010%-0.018%, Al: 0.03%-0.04%, S≤0.002%, P: 0.012%-0.019%, H≤0.00015%, the rest are Fe and other unavoidable impurities; Among them, Si, Cr, Cu, Ni, and P satisfy the following corrosion resistance index formula: [26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu) 2 ]≥6.5; The mechanical properties of the steel plate are as follows: yield strength ≥500MPa, tensile strength ≥660MPa, yield ratio ≤0.78, elongation ≥25%, -60°C impact energy KV2 ≥250J; strength difference of the same plate ≤30MPa, yield ratio difference of the same plate ≤0.04; maximum residual internal stress <15MPa; unevenness of the steel plate ≤2mm / m; in a marine atmospheric environment, the average annual corrosion rate of the steel plate is less than 0.15mm / a.

2. The thin gauge high strength bridge steel plate according to claim 1, characterized in that: The structural organization of the steel plate is a ferrite+bainite complex phase organization, wherein the ferrite accounts for 25%-35% and the bainite accounts for 65%-75%.

3. A method for producing thin gauge high strength bridge steel plate according to claim 1, characterized in that: The steps include: (1) Smelting and continuous casting: The molten iron is subjected to pre-desulfurization treatment, converter smelting to adjust the elemental composition, ladle refining, and RH vacuum refining, and then continuously cast into billets, which are stacked and cooled. The billets have a thickness of 220±10mm, a width of 1400±100mm, and a length of 2300-3800mm. (2) Rolling: The billet is heated to 1230-1255°C, kept warm for 220-280 minutes, and quickly sent to the feed end of the rolling mill, and directly rolled into a target steel plate with a thickness of 6-10 mm and a length of ≤40,000 mm; The starting rolling temperature of the rolling mill is ≥1200°C, and the finishing rolling temperature is 830-870°C. During the rolling process, the reduction of the rolling mill rolls is dynamically controlled according to temperature changes. (3) Cooling after rolling: The target steel plate after rolling is cooled to 480-530℃ at a cooling rate of 15-25℃ / s, and then placed in a holding pit for insulation.

4. The method for producing thin-gauge high-strength bridge steel plates according to claim 3, characterized in that: The RH vacuum refining specifically includes: degassing the ladle at a vacuum degree of ≤2 mBar for at least 15 minutes, and simultaneously performing soft stirring for at least 15 minutes.

5. The method for producing thin-gauge high-strength bridge steel plates according to claim 3, characterized in that: In the step (2), before the billet is quickly delivered to the feed end of the rolling mill, roll scheduling is first performed, including: The billet is placed at the 40th to 80th block of the rolling mill's roll production sequence as the target billet to be rolled. Before rolling the target billet to be rolled, 6 to 10 blocks of the billet are first rolled into steel plate transition material with a thickness specification of 8 to 14 mm, and the steel plate transition material is rolled from thick to thin.

6. The method for producing thin-gauge high-strength bridge steel plates according to claim 3, characterized in that: In the step (2), during the rolling process of the billet, the total rolling passes are ≤13 passes, wherein the billet is kept warm after the 6th pass, the starting rolling temperature of the 7th pass is 1050-1060°C, and the last pass is a leveling pass.

7. The method for producing thin-gauge high-strength bridge steel plates according to claim 6, characterized in that: In the step (2), during the rolling process of the billet, the rolling mill first bites into the billet at an initial speed of 2.0 m / s, and after rolling for 1 m, linearly accelerates to 8.0 m / s for constant speed rolling, and finally linearly decelerates for rolling and decelerates to 4.0 m / s before throwing the steel.

8. The method for producing thin-gauge high-strength bridge steel plates according to claim 7, characterized in that: In the step (2), during the rolling process of the billet, the cooling water of the rolls of the rolling mill is turned off before the billet is bitten, the cooling water of the rolls is turned on after rolling for 1 m, and the cooling water of the rolls is turned off when the remaining 2 m of rolling is completed.

9. The method for producing thin-gauge high-strength bridge steel plates according to claim 8, characterized in that: In the step (2), dynamically controlling the reduction amount of the rolling mill rolls according to temperature changes during the rolling process includes: When the rolling temperature is ≥1060℃, the reduction is dynamically controlled by the total torque of the twin rolls at 5000-6000 kN·m; When the rolling temperature is 960-1060℃, the reduction is regulated by the rolling force of 55000-66000kN and the bending roll force of 3000-4000kN; When the rolling temperature is less than 960℃, the rolling force and bending roll force are gradually reduced to reduce the reduction.

10. The method for producing thin-gauge high-strength bridge steel plates according to claim 9, characterized in that: In the step (2), during the billet rolling process, when the intermediate billet thickness is ≤32 mm, the roller cooling water is turned off, and the single-pass reduction meets the following requirements: When the thickness of the intermediate blank is 10-16mm, the single-pass reduction is greater than 3mm; When the thickness of the intermediate blank is less than 10mm, the single-pass reduction is greater than 1.5mm.

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