An isotropic high-toughness bridge steel with low yield ratio and a method for manufacturing the same

Abstract

This invention discloses an isotropic bridge steel with low yield strength ratio and high toughness, and its manufacturing method. Its chemical composition by weight percentage is: C: 0.02–0.05%, Si: 0.25–0.45%, Mn: 1.00–1.40%, P≤0.01%, S≤0.003%, Nb: 0.02–0.05%, Ni: 0.40–0.70%, Cu: 0.25–0.45%, Mo: 0.10–0.20%, Cr: 0.50–0.80%, Ti: 0.005–0.02%, B: 0.001–0.0025%, Al: 0.01–0.04%, CEV: 0.36–0.52%, Pcm: 0.13–0.20%, I≥6.30, with the balance being iron and unavoidable impurities. The steel of this invention has excellent weather resistance and weldability, uniform overall plate performance, and minimal differences in transverse and longitudinal properties.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to an isotropic bridge steel with low yield strength ratio and high toughness and its manufacturing method. Background Technology

[0002] As large steel bridges develop towards fully welded structures and high-parameter designs, the requirements for the safety and reliability of bridge structures are becoming increasingly stringent. This not only places higher demands on designers but also on the quality of steel plates. Steel plates must not only possess high strength to meet lightweight structural requirements but also exhibit excellent low-temperature toughness, weldability, and corrosion resistance to meet the requirements for safety, reliability, and longevity of steel structures.

[0003] Traditional high-strength bridge steel not only has poor impact toughness, weldability, yield strength ratio, and isotropy, but also cannot resist atmospheric corrosion. At the same time, different bridge structure types require a large amount of cutting and welding of steel plates with equal cross-sections, which seriously affects construction efficiency and quality. Therefore, materials scientists at home and abroad have put forward higher requirements for the new generation of bridge steel. In addition to having high strength, the weldability, low-temperature toughness, seismic resistance, machinability, corrosion resistance, and performance uniformity of the steel have been greatly improved.

[0004] Currently, existing bridge steel can meet most of the market demand in the bridge industry. However, high-strength, high-performance special steels remain the development goal for countries worldwide. Ultra-high-strength steel plates with high service safety present significant research challenges, require stringent production processes, demanding equipment, and are difficult to develop. The complexity and diversity of technical characteristics and indicators significantly increase the difficulty of developing isotropic, low yield strength ratio, high toughness bridge steel. First, increasing the thickness of the steel plate leads to a large difference in deformation from the surface to the core during rolling, reducing cooling uniformity in the thickness direction and resulting in poor isotropy. Second, for variable cross-section bridge steel, the different rolling temperatures and deformation amounts at the thin and thick ends make it difficult to control the uniformity of strength and toughness. Furthermore, bridge steel needs good corrosion resistance and weldability, requiring further optimization of its composition.

[0005] Currently, there is some research on high-strength, high-toughness, and weather-resistant bridge steel plates both domestically and internationally. A search revealed some patents and literature, but the content described therein differs significantly from the technical solution of this invention in terms of composition, production method, performance, and product category.

[0006] Patent CN 109797342 A discloses a high-strength, high-toughness, atmospheric corrosion-resistant steel plate for steel structure fabrication and its manufacturing method. The chemical composition, by mass fraction, includes: C: 0.03%–0.10%, Si: 0.30%–0.50%, Mn: 1.10%–1.50%, P < 0.010%, S < 0.003%, Cr: 0.45%–0.70%, Cu: 0.25%–0.40%, Ni: 0.30%–0.40%, Alt: ≥ 0.030%. The composition is as follows: Ti: 0.006%–0.030%, V: 0.040%–0.080%, Mo: 0.02%–0.08%, Ca: 0.0010%–0.0030%, N: 0.0020%–0.0080%, B: 0.0002%–0.0030%, Ce: 0.001%–0.010%, atmospheric corrosion resistance index I>6.5, CEV<0.54, Pcm<0.27, with the balance being Fe and unavoidable impurities. This patent utilizes chemical composition design and steel plate tempering processes to obtain high-performance steel plates with bainitic structure, suitable for use in steel structures such as bridges and high-rise buildings. However, the steel plates obtained by this patent do not have a low yield strength ratio, resulting in poor safety performance. Furthermore, the addition of rare elements such as Ca, B, and Ce makes smelting difficult, increases production costs, and results in excessively high carbon equivalent, leading to poor weldability. Additionally, tempering treatment is required, complicating the process.

[0007] Patent CN 107557665 A discloses a rare earth weathering bridge steel plate with a yield strength of 345MPa and its production method. The chemical composition, by mass fraction, includes: C: 0.04%–0.06%, Si: 0.20%–0.30%, Mn: 1.10%–1.30%, P<0.015%, S<0.008%, Cr: 0.40%–0.50%, Cu: 0.25%–0.35%, Ni: 0.30%–0.40%, Als: 0.024%–0.034%, Nb: 0.01%–0.02%, Ce: 0.0005%–0.0030%, with the balance being Fe and unavoidable impurities. This steel plate has the advantages of low yield strength ratio, good corrosion resistance, low temperature toughness, cold forming performance, weldability, and can be used without painting; however, its strength is relatively low and cannot meet the requirements of long-span steel structure bridges. In addition, the addition of rare earth elements makes smelting difficult and increases production costs.

[0008] Patent CN 110541117 A discloses a 620MPa grade high-performance bridge steel welded at a low preheating temperature and its preparation method. The chemical composition, by mass fraction, includes: C: 0.051%–0.091%, Si: 0.41%–0.50%, Mn: 1.65%–2.15%, P<0.010%, S<0.003%, Nb: 0.06%–0.09%, Ti: 0.015%–0.03%, Cr: 0.35%–0.65%, Ca: 0.0019%–0.0045%, Ce: 0.02%–0.04%, with the balance being Fe and unavoidable impurities. Through the above composition control and steel plate production process control, the obtained steel plate exhibits excellent strength, low-temperature toughness, and weldability. The welding preheating temperature is less than or equal to 30℃, meeting the requirements for highway bridges, railway bridges, and dual-purpose road-rail bridges. However, this steel plate does not have a low yield strength ratio, low elongation, poor seismic performance, and lacks weather resistance, resulting in poor overall performance.

[0009] Patent CN 108396245 A discloses a wedge-shaped steel plate with one thin end and one thick end, rated at 345MPa. It is produced by casting a continuously cast billet with chemical composition varying along the length using two ladles of molten steel with different compositions, followed by rolling. This process makes the mechanical properties of the rolled wedge-shaped steel plate more uniform. However, the process is complex and difficult to implement in actual production. It is only suitable for wedge-shaped steel plates with one thin end and one thick end, and the strength is only at the 345MPa level. It does not have weather resistance and cannot meet the application requirements of long-span bridge structures.

[0010] In summary, current research on steel plates for bridges is insufficient, and the main problems with bridge steel plates are as follows:

[0011] 1. The addition of a large number of rare elements makes smelting difficult and the cost high.

[0012] 2. High-strength steel plates require quenching and tempering heat treatment, which involves complex production processes and long production cycles.

[0013] 3. The steel plate is a uniform cross-section steel plate, which cannot meet the user's personalized actual needs.

[0014] 4. The steel plate has low strength and poor toughness, which cannot meet the requirements of bridge engineering in cold regions.

[0015] 5. The high yield strength ratio and poor isotropic properties of the steel plate have a certain impact on the seismic safety of the bridge.

[0016] 6. Steel plates do not have weather resistance, require painting, and are not environmentally friendly. Summary of the Invention

[0017] The purpose of this invention is to overcome the shortcomings of existing technologies and provide an isotropic, low yield strength ratio, high toughness bridge steel and its manufacturing method. Products manufactured according to the chemical composition and production process requirements of the steel of this invention possess ultra-high strength, high toughness, low yield strength ratio, high plasticity, and excellent weather resistance and weldability. The overall plate exhibits uniform performance with minimal differences in transverse and longitudinal properties.

[0018] This invention improves the weldability and toughness of materials through low C and low Mn composition. Nb and Ti elements are used to inhibit austenite grain growth and promote nucleation during austenite transformation, thus refining the grain size, controlling the core microstructure of the steel plate, and improving microstructure uniformity. Nb's inhibitory effect on austenite recrystallization increases the rolling temperature, further refining the grain size and ensuring the correct plate shape. Simultaneously, Cr, Mi, and Cu elements enhance the weather resistance of the steel plate, and the combined addition of Cr and Mo increases hardenability, improving the cooling rate during air or water cooling, refining the grain size, and achieving increased strength and thickness-direction microstructure uniformity. Combined with appropriate smelting, heating, rolling, and heat treatment processes, this results in a variety of shapes, large thicknesses, high strength and toughness, low yield strength ratio, excellent weather resistance, weldability, isotropy, and other comprehensive properties, along with an ideal microstructure.

[0019] To achieve the above objectives, the technical solution of the present invention is as follows:

[0020] This invention provides an isotropic bridge steel with a low yield strength ratio and high toughness, the chemical composition by weight percentage being:

[0021] C: 0.02%–0.05%, Si: 0.25%–0.45%, Mn: 1.00%–1.40%, P≤0.010%, S≤0.003%, Nb: 0.02%–0.05%, Ni: 0.40%–0.70%, Cu: 0.25%–0.45%, Mo: 0.10%–0.20%, Cr: 0.50–0.80%, Ti: 0.005%–0.020%, B: 0.0010%–0.0025%, Al: 0.01%–0.04%, CEV: 0.36%–0.52%, Pcm: 0.13%–0.20%, atmospheric corrosion resistance index I≥6.30, balance being iron and unavoidable impurities.

[0022] In the above technical solution, the final microstructure of the steel plate is composed of ferrite, pearlite and bainite; wherein, ferrite accounts for 11-28%, pearlite accounts for 3-7%, and bainite accounts for 69-82%.

[0023] In the above technical solution, the steel plate further has the following characteristics: yield strength ≥ 500 MPa, tensile strength ≥ 630 MPa, elongation after fracture ≥ 21%, yield strength ratio ≤ 0.83, and impact energy at -40℃ ≥ 200 J; the difference in yield strength between the thin and thick ends of the same plate is ≤ 25 MPa, the difference in tensile strength is ≤ 25 MPa, the difference in elongation after fracture is ≤ 3%, the difference in yield strength ratio is ≤ 0.02, and the difference in impact energy at -40℃ is ≤ 30 J; the ratio of transverse to longitudinal tensile strength at the same location is ≤ 5%, and the ratio of impact energy is ≤ 5%.

[0024] The selection principles and content design rationale for each chemical component in this invention are as follows:

[0025] C: In this invention, C can exert a strengthening effect through interstitial solid solution and can also react with alloying elements such as Nb to form fine carbide precipitates. These precipitates occur before rolling deformation or austenitic phase transformation, hindering grain growth, increasing the nucleation rate, and refining the microstructure. However, excessive C content will cause the steel plate to form obvious banded structures during rolling, seriously affecting the consistency of the steel plate's properties in all directions. Moreover, excessive C content will increase the carbon equivalent of the steel, deteriorating the weldability and low-temperature toughness of the steel plate. Therefore, the C content should not be too high. In this invention, the C content is best controlled at 0.02% to 0.05%.

[0026] Si: Si is one of the deoxidizing elements in steel. Si also has a strong solid solution strengthening effect, which can purify ferrite, reduce the content of pearlite, and help reduce the Bauschinger effect in the matrix material, thus lowering the yield strength ratio of the steel plate. However, excessive Si will reduce the toughness of the weld heat-affected zone of the base metal, worsening the toughness and weldability of the steel. Therefore, in this invention, the Si content is best controlled between 0.25% and 0.45%.

[0027] Mn: Mn can be dissolved in large quantities in the Fe matrix. It is an element that expands the austenite phase region, improves the stability of austenite, inhibits the growth of phase transformation grains before accelerated cooling of steel plates, plays a role in refining grains, and improves the strength of steel plates. When the Mn content is less than 1.0%, it has little effect on the strength of steel plates. However, if the Mn content is too high, it will easily inhibit the ferrite transformation, affect the yield strength of steel, and is not conducive to reducing the yield strength ratio. Excessive Mn content will induce segregation, resulting in more banded structures, which will worsen the uniformity of steel plate structure and the thickness direction lamellar tearing performance. At the same time, it will affect the uniformity of properties in all directions and is not conducive to welding. This invention believes that it is more appropriate to control the Mn content between 1.00% and 1.40%.

[0028] P and S: In this invention, P and S are considered harmful impurity elements, and the lower the content, the better. Excessive P content can lead to microstructure segregation, significantly negatively impacting isotropy and low-temperature toughness. Therefore, this invention controls P to ≤0.010%. Increased S content promotes the formation and growth of inclusions, deteriorating low-temperature performance and thickness-direction properties; therefore, S ≤0.003%.

[0029] Nb: The role of Nb in this invention includes: (1) precipitation during rolling, pinning grain boundaries, promoting nucleation, effectively refining grains, thereby improving strength and toughness; (2) lowering the austenite phase transformation temperature, which can increase the recrystallization temperature, refine grains under high-temperature rolling, improve the strength of steel plates, and facilitate plate rolling control; (3) some Nb C and N compounds are formed during tempering, ensuring the strength of steel plates. However, excessive Nb content will deteriorate the toughness of welds and heat-affected zones, and also increase costs. This invention believes that controlling the Nb content at 0.02% to 0.05% is more appropriate.

[0030] Cr: Cr is the main element in this invention for improving the weather resistance of steel plates. Cr accumulates within the rust layer and can also promote the growth of Fe. 3+ The reduction reaction forms iron-chromium-copper polyalloy oxides, which refine the rust particles and hinder the reduction of Cl. - Cr can also improve the hardenability of steel plates, refine grains, increase tensile strength, and reduce yield strength ratio. However, excessive Cr can easily lead to carbide precipitation, affecting the toughness of the steel. Considering economic efficiency, weldability, and toughness, this invention suggests that controlling the Cr content between 0.50% and 0.80% is more suitable.

[0031] Mo (Mo): Mo can improve the hardenability of steel, inhibit the segregation of impurity elements such as P and S at grain boundaries, and reduce temper brittleness. Mo also plays a unique role in stabilizing rust layers and improving weather resistance. Appropriate amounts of Mo can also reduce the yield strength ratio and improve the seismic performance of steel plates. When Mo is added simultaneously with Nb, it can promote the precipitation of fine carbonitriding compounds of Nb, which have high high-temperature stability, thereby further improving the strength of the steel plate. However, excessive Mo will combine with C, reducing the plasticity and low-temperature toughness of the steel plate, and the cost is also high. This invention suggests that controlling the Mo content between 0.10% and 0.20% is more suitable.

[0032] Cu: Cu improves the hardenability of steel, significantly increases the core strength of thick steel plates, and is also an important element for improving weather resistance. During the slow cooling process of thick steel plates, an appropriate amount of Cu can precipitate ε-Cu through self-tempering, increasing the strength of the steel plate. Excessive Cu content can cause hot cracks in the steel plate and reduce its plasticity. This invention suggests that controlling the Cu content between 0.25% and 0.45% is more suitable.

[0033] Ni: Ni can effectively improve the low-temperature toughness of steel, inhibit the corrosion of steel by chloride ions, and improve the hot brittleness caused by copper in steel. Ni can also improve the isotropy of steel plates after heat treatment. However, nickel is a precious metal, and adding a large amount will drastically increase the cost and affect the welding performance. According to the performance requirements, an appropriate amount is added. In this invention, the Ni content is controlled at 0.40% to 0.70%.

[0034] Ti: Ti can perform nitrogen fixation, forming a TiN-dominant precipitate phase, which can inhibit austenite grain growth under high-temperature conditions and improve the toughness of the heat-affected zone after welding. During welding, TiN particles prevent grain growth in the coarse-grained zone of the heat-affected zone, improving the low-temperature toughness of the weld joint. Furthermore, due to its low solid solubility, Ti easily appears as interphase precipitation during the austenite-ferrite transformation process, increasing strength. However, excessive Ti will reduce the toughness of the steel. This invention suggests that controlling the Ti content between 0.005% and 0.020% is more suitable.

[0035] B: Element B can improve the hardenability of steel plates. Even a trace amount of element B can have a significant effect on improving hardenability and ensuring the strength of the steel plate. However, when element B is excessive, the brittleness of the steel plate increases and the tendency for welding cracks increases. Therefore, this invention controls element B at 0.001% to 0.0025%.

[0036] Al: Al is a strong deoxidizing element and can also combine with N to form AlN, which can refine grains, improve low-temperature impact toughness, and lower the brittle transition temperature of steel. If AlN is formed, it also refines the microstructure. When the Al content exceeds 0.040%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. This invention suggests that the Al content should be controlled between 0.010% and 0.040%.

[0037] Another aspect of the present invention provides a method for manufacturing the above-mentioned isotropic low yield strength ratio and high toughness bridge steel, comprising: rolling, cooling, straightening, and heat treatment, the specific steps of which are as follows:

[0038] (1) Rolling: The continuously cast billet is placed into the heating furnace at a furnace temperature of 650-750℃ to ensure that the internal and external temperatures of the billet are consistent during the low-temperature stage, preparing for a uniform microstructure in the high-temperature stage; the heating temperature of the continuously cast billet is 1120-1230℃, the soaking temperature is 1110-1220℃, and the soaking time is 3.7-5.3h to allow C and N compounds to dissolve fully, especially Nb and Mo elements, and to prevent abnormal growth of the as-cast microstructure; at the same time, it ensures that the austenite grains grow fully, providing sufficient deformation dynamics for austenite deformation; in addition, the billet size design should ensure that the width of the billet after transfer does not exceed the length of the rolling mill roll body; the roughing stage adopts a longitudinal-transverse rolling process, and the roughing rolling start temperature is 1000-1100℃. The cumulative reduction rate in longitudinal rolling is 20-30%, and the cumulative reduction rate in transverse rolling is 20-40%. The final rolling temperature in roughing rolling is 950-1050℃, and the cumulative reduction rate in the roughing stage is ≥60%. The purpose of using longitudinal-transverse rolling is to improve the banded structure and texture orientation generated inside the steel plate during rolling, weaken the influence of the rolling structure on the transverse and longitudinal mechanical properties, and prepare the structure for subsequent heat treatment. Transverse rolling in the high-temperature section can improve the as-cast structure of the slab. The rolling temperature and deformation process in the roughing stage cause the austenite grains to recrystallize and inhibit grain growth. The superposition effect of multi-pass large reduction rate deformation promotes the recrystallization of austenite and achieves the grain refinement target, which is suitable for the production of the isotropic low yield strength ratio and high toughness bridge steel plate of this invention.

[0039] The finishing rolling stage employs longitudinal variable thickness rolling. The relationship between the finishing rolling parameters and the thickness of the thick end of the finished steel plate is as follows:

[0040] The thickness of the finished steel plate at the thick end is ≤20mm, the thickness of the intermediate billet is 5~9t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 920~970℃, the final rolling temperature of the finishing rolling is 850~890℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 15%.

[0041] The thickness of the finished steel plate is 20mm < and 30mm, the thickness of the intermediate billet is 3 to 6t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 890 to 960℃, the final rolling temperature of the finishing rolling is 840 to 880℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 13%.

[0042] The thickness of the finished steel plate is 30mm < and 50mm, the thickness of the intermediate billet is 2.5 to 4.0t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 880 to 940℃, the final rolling temperature of the finishing rolling is 820 to 860℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 12%.

[0043] The thickness of the finished steel plate is 50mm < the thickness of the thick end and ≤ 60mm. The thickness of the intermediate billet is 2.0~3.5t, where t is the thickness of the thick end of the finished steel plate. The initial rolling temperature of the finishing rolling is 860~900℃, the final rolling temperature of the finishing rolling is 800~840℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 10%.

[0044] A suitable intermediate billet thickness can satisfy the austenite deformation and deformation energy accumulation in the non-recrystallization zone, and also ensure sufficient deformation rate in the rough rolling stage to achieve grain refinement under a certain original billet thickness. The large single-pass reduction in the non-recrystallization zone further promotes austenite refinement, improves the small defects inside the billet, and enhances the isotropy and toughness of the steel plate. Higher rolling and red-heat temperatures can reduce the mill load during the rolling of longitudinally variable thickness steel plates, ensuring precise control of the steel plate shape.

[0045] (2) Cooling: After rolling, water cooling is used for cooling. The cooling parameters are related to the thickness of the thick end of the finished steel plate as follows:

[0046] The thickness of the finished steel plate at the thick end is ≤20mm, the initial cooling temperature is 750~780℃, and the red-heating temperature is 580~650℃.

[0047] For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the initial cooling temperature is 740~770℃, and the reheating temperature is 500~600℃.

[0048] For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the initial cooling temperature is 710~740℃, and the reheating temperature is 480~550℃.

[0049] For finished steel plates with a thickness of 50mm < thick end and ≤ 60mm, the initial cooling temperature is 690~720℃;

[0050] Controlling the initial cooling temperature allows the steel plate to generate a large amount of ferrite within this temperature range, ensuring high plasticity and toughness. Accelerated cooling can further refine the austenite grain structure and improve the strength of the steel plate. Controlling the reheating temperature allows the steel plate to form some bainite within this temperature range, improving the matrix strength. A suitable reheating temperature provides a suitable temperature for subsequent straightening, avoiding excessive straightening force during straightening and resulting in poor plate shape.

[0051] (3) Straightening: Straightening is performed using a hot straightening machine. The relationship between the straightening temperature and the thickness of the thick end of the finished steel plate is as follows:

[0052] The thickness of the thick end of the finished steel plate is ≤20mm, and the straightening temperature is ≥550℃;

[0053] 20mm < Thickness of the finished steel plate at the thick end ≤ 30mm, straightening temperature ≥ 480℃;

[0054] Thickness of finished steel plate at thick end ≤ 50mm, straightening temperature ≥ 450℃;

[0055] 50mm < Thickness of the finished steel plate at the thick end ≤ 60mm, straightening temperature ≥ 400℃;

[0056] (4) Heat treatment: The tempering temperature is 550-650℃ and the tempering holding time is 3-5 min / mm. The purpose is to further improve the isotropic properties of the steel plate, reduce the residual stress of the steel plate, and thus improve the uniformity of the steel plate structure.

[0057] The steel plate produced by the above manufacturing method has a final microstructure consisting of fine ferrite, pearlite, and bainite, with a uniform structure. The small amount of bainite can further improve the tensile strength of the steel plate and reduce the yield strength ratio. The steel plate has high strength, excellent plasticity, low yield strength ratio, low-temperature toughness, good weather resistance, weldability, and isotropic properties, meeting the needs of various bridge projects and reducing the total amount of steel used in bridge engineering.

[0058] In the above technical solution, further, in step (1), the method for preparing the continuous casting billet includes the following steps:

[0059] a. After pretreatment of molten iron, it is smelted in a converter and refined outside the ladle. In the LF refining process, the net argon blowing time is ≥5min, the RH vacuum degassing is performed, and the molten steel is treated with Ca and micro-Ti with a net circulation time of ≥5min. The refining and degassing processes can effectively reduce impurity elements.

[0060] b. The target superheat of the continuous casting tundish is ≤30℃. The entire casting process is protected and lightly compressed. The thickness of the continuous casting billet / the maximum thickness of the finished steel plate is controlled to be ≥6.0. Controlling the superheat of the casting and lightly compressing can effectively reduce the quality defects of the billet. Increasing the compression ratio from the continuous casting billet to the finished steel plate can effectively control the grain size.

[0061] In the above technical solution, further, in step (1), the continuously cast billet is heated at a rate of 4 to 6 °C / min to avoid the billet being heated too quickly and causing uneven heating inside the billet.

[0062] In the above technical solution, further, in step (1), the relationship between the rolling speed in the finishing rolling stage and the thickness of the thick end of the finished steel plate is as follows:

[0063] The thickness of the finished steel plate at the thick end is ≤20mm, and the rolling speed is 1.3~2.7m / s;

[0064] For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the rolling speed is 1.2~2.6m / s;

[0065] For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the rolling speed is 1.1~2.4m / s;

[0066] For finished steel plates with a thickness of 50mm < thick end and ≤ 60mm, the rolling speed is 0.9~1.8m / s;

[0067] In the above technical solution, further, in step (2), the relationship between the cooling rate and the thickness of the thick end of the finished steel plate is as follows:

[0068] The thickness of the thick end of the finished steel plate is ≤20mm, and the cooling rate is 3~10℃ / s;

[0069] For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the cooling rate is 6~14℃ / s;

[0070] For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the cooling rate is 10~18℃ / s;

[0071] For finished steel plates with a thickness of 50mm < thick end and ≤ 60mm, the cooling rate is 12~20℃ / s.

[0072] In the above technical solution, further, in step (3), the straightening speed of the parallel section is 0.2-0.4 m / s, and the straightening speed of the variable thickness section is 0.1-0.20 m / s; according to the variable thickness specification of the steel plate, when straightening enters the variable thickness section, the straightening roller gap is increased / decreased at a uniform speed, and the upper straightening roller rises / falls in linkage, wherein the calculation formula for the rising / falling roller speed is:

[0073] Rising / falling roller speed = Variable thickness × Straightening speed / Horizontal length of variable thickness section

[0074] The thickness variation of the steel plate is ≤50mm to ensure the uniformity of temperature and good plate shape of the entire plate; where, the thickness variation of the steel plate = the thickness of the thick end of the steel plate - the thickness of the thin end of the steel plate.

[0075] Figure 1 The diagram shows three types of LP steel plates. As illustrated, the meanings of the thick end, thin end, parallel section, and variable thickness section of the steel plate in this invention are as follows:

[0076] In the 01 steel plate, h1 is the thin end, h2 is the thick end, and L1 is the variable thickness section;

[0077] In the 02 steel plate, h'1 is the thin end, h'2 is the thick end, L'1 is the variable thickness section, and L'2 is the parallel section;

[0078] In the 03 steel plate, h”1 is the thin end, h”2 is the thick end, L”1 is the parallel section, L”2 is the variable thickness section, and L”3 is the parallel section.

[0079] The beneficial effects of this invention are as follows:

[0080] (1) The composition of this invention improves the toughness of the material through low C and low Mn design, refines the grains by using Nb and Ti elements to inhibit the growth of austenite grains and promote nucleation during the austenite transformation process, increases the recrystallization temperature, ensures the uniformity of the steel plate structure at high temperature, and improves the hardenability of the steel plate by adding inexpensive Cr element, thereby further improving the strength of the steel plate.

[0081] (2) By adding appropriate amounts of weather-resistant elements such as Ni, Cu, and Mo, the atmospheric corrosion resistance index I≥6.3 of this invention improves the hardenability of steel plates and greatly enhances their weather resistance, reducing or even eliminating the need for painting in bridge engineering, thus achieving resource conservation and environmental protection effects.

[0082] (3) The steel plate of the present invention has a CEV of 0.36 to 0.52 and a Pcm of 0.13 to 0.20, and has excellent welding performance, which can improve the processing efficiency of components.

[0083] (4) The present invention, combined with a unique production process, solves the comprehensive problems of poor shape control and poor weather resistance of high-strength bridge steel plates; the use of TMCP+tempering rolling process can further improve production efficiency, produce LP steel plates of various shapes, reduce the overall structural weight of bridge projects, and improve the construction efficiency and overall safety of bridge structural projects.

[0084] (5) The present invention improves the banded structure and texture orientation generated inside the steel plate during the rolling process by alternating longitudinal-transverse-longitudinal rolling process, and further eliminates the influence of banded austenite grains on the transverse and longitudinal impact toughness of the steel plate by subsequent tempering treatment.

[0085] (6) The steel plate produced by this invention has a maximum thickness of 60mm at the thick end and a maximum variable thickness of 50mm. It can produce a variety of variable thickness specifications and sizes to meet the needs of steel for longitudinally variable thickness bridges of various shapes.

[0086] (7) The steel plate has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation after fracture ≥21%, yield strength ratio ≤0.83, impact energy at -40℃ ≥200J, and uniform performance of the same plate: the difference in strength between the thin and thick ends of the same plate is ≤25MPa, the difference in elongation after fracture is ≤3%, the difference in yield strength ratio is ≤0.02, the difference in impact energy is ≤30J, the difference in strength and impact energy between the transverse and longitudinal ends at the same position is ≤5%, and the steel plate structure is fine ferrite + pearlite + bainite. Attached Figure Description

[0087] Figure 1 This is a schematic diagram of the types of LP steel plates;

[0088] Figure 2 This is the metallographic structure of Example 5 of the present invention. Detailed Implementation

[0089] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0090] Examples 1-10

[0091] The chemical composition of the steels in Examples 1-10 of this invention is shown in Table 1;

[0092] Table 1. Chemical composition (wt%) of steels in Examples 1-10

[0093]

[0094]

[0095] CEV(%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15

[0096] Pcm(%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B

[0097] Atmospheric corrosion resistance index I = 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

[0098] The CEV, Pcm, and I values ​​of the steels in Examples 1-10 of this invention are shown in Table 2.

[0099] Table 2. Values ​​of CEV, Pcm, and I for steels in Examples 1-10

[0100] Example CEV (%) Pcm (%) I value 1 0.37 0.13 6.32 2 0.39 0.15 6.96 3 0.50 0.19 7.06 4 0.45 0.18 6.87 5 0.49 0.19 6.66 6 0.48 0.19 7.15 7 0.51 0.19 7.17 8 0.45 0.18 6.92 9 0.40 0.16 6.79 10 0.52 0.20 6.95

[0101] The manufacturing method of the aforementioned isotropic, low yield strength ratio, high toughness bridge steel includes: smelting, continuous casting, rolling, cooling, straightening, and heat treatment, with the specific steps as follows:

[0102] (1) Smelting and continuous casting: After pretreatment of molten iron, it is smelted in a converter and refined in a ladle. In the LF refining process, the net argon blowing time is ≥5min, the RH vacuum degassing is performed, and the molten steel is treated with Ca and micro-Ti with a net circulation time of ≥5min. The refining and degassing treatment can effectively reduce impurity elements.

[0103] (2) Continuous casting and billet heating: The target superheat of the continuous casting tundish is ≤30℃. Full-process protective casting is performed, and light pressure is applied to control the billet thickness / maximum finished steel plate thickness to be ≥6.0. Controlling the casting superheat and applying light pressure can effectively reduce billet quality defects. Increasing the compression ratio from the billet to the finished steel plate can effectively control grain size. The billet is placed into the heating furnace at a furnace temperature of 650~750℃ to ensure consistent internal and external temperatures during the low-temperature stage, preparing for uniform microstructure in the high-temperature section. The subsequent heating rate is 4... The heating speed is ~6℃ / min to avoid uneven heating of the billet due to excessively rapid heating. The continuous casting billet heating section temperature is 1120~1230℃, the soaking section temperature is 1110~1220℃, and the soaking section holding time is 3.7~5.3h to ensure sufficient dissolution of C and N compounds, especially the solid solution of Nb and Mo elements, and to prevent abnormal growth of the as-cast structure. At the same time, it ensures sufficient growth of austenite grains to provide sufficient deformation dynamics for austenite deformation. In addition, the billet size design should ensure that the width of the billet after transfer does not exceed the length of the rolling mill roll body.

[0104] Table 3. Continuous casting and billet heating process parameters for steels in Examples 1-10

[0105]

[0106] (3) Rolling: The roughing stage adopts a longitudinal-transverse rolling process. The initial rolling temperature of the roughing stage is 1000-1100℃, the cumulative reduction rate of longitudinal rolling is 20-30%, the cumulative reduction rate of transverse rolling is 20-40%, the final rolling temperature of the roughing stage is 950-1050℃, and the cumulative reduction rate of the roughing stage is ≥60%. The purpose of adopting longitudinal-transverse rolling is to improve the banded structure and texture orientation generated inside the steel plate during the rolling process, weaken the influence of the rolling structure on the transverse and longitudinal mechanical properties, and prepare the structure for subsequent heat treatment. The transverse rolling in the high-temperature section can improve the as-cast structure of the slab. The rolling temperature and deformation process in the roughing stage cause the austenite grains to recrystallize and inhibit grain growth. The superposition effect of the deformation with multiple large reduction rates promotes the recrystallization of austenite and achieves the grain refinement target, which is suitable for the production of the isotropic low yield strength ratio and high toughness bridge steel plate of this invention.

[0107] Table 4. Roughing process parameters for steels in Examples 1-10

[0108]

[0109]

[0110] The finishing rolling stage employs longitudinal variable thickness rolling. The relationship between the finishing rolling parameters and the thickness of the thick end of the finished steel plate is as follows:

[0111] The thickness of the finished steel plate at the thick end is ≤20mm, the thickness of the intermediate billet is 5~9t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of finishing rolling is 920~970℃, the final rolling temperature of finishing rolling is 850~890℃, the single-pass deformation rate of the thick end of the steel plate is not less than 15%, and the rolling speed is 1.3~2.7m / s.

[0112] The thickness of the finished steel plate is 20mm < and 30mm at the thick end, the thickness of the intermediate billet is 3 to 6t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 890 to 960℃, the final rolling temperature of the finishing rolling is 840 to 880℃, the single-pass deformation rate of the thick end of the steel plate is not less than 13%, and the rolling speed is 1.2 to 2.6m / s.

[0113] The thickness of the finished steel plate is 30mm < thick end and 50mm ≤, the thickness of the intermediate billet is 2.5~4.0t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 880~940℃, the final rolling temperature of the finishing rolling is 820~860℃, the single-pass deformation rate of the thick end of the steel plate is not less than 12%, and the rolling speed is 1.1~2.4m / s;

[0114] The thickness of the finished steel plate is 50mm < thick end and 60mm ≤, the thickness of the intermediate billet is 2.0~3.5t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 860~900℃, the final rolling temperature of the finishing rolling is 800~840℃, the single-pass deformation rate of the thick end of the steel plate is not less than 10%, and the rolling speed is 0.9~1.8m / s;

[0115] A suitable intermediate billet thickness can satisfy the austenite deformation and deformation energy accumulation in the non-recrystallization zone, and also ensure sufficient deformation rate in the rough rolling stage to achieve grain refinement under a certain original billet thickness. The large single-pass reduction in the non-recrystallization zone further promotes austenite refinement, improves the small defects inside the billet, and enhances the isotropy and toughness of the steel plate. Higher rolling and red-heat temperatures can reduce the mill load during the rolling of longitudinally variable thickness steel plates, ensuring precise control of the steel plate shape.

[0116] Table 5. Steel finishing process parameters for Examples 1-10

[0117]

[0118]

[0119] (4) Cooling: After rolling, water cooling is used for cooling. The cooling parameters are related to the thickness of the thick end of the finished steel plate as follows:

[0120] The thickness of the finished steel plate at the thick end is ≤20mm, the initial cooling temperature is 750~780℃, the red-hot temperature is 580~650℃, and the cooling rate is 3~10℃ / s;

[0121] For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the initial cooling temperature is 740~770℃, the red-hot temperature is 500~600℃, and the cooling rate is 6~14℃ / s.

[0122] For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the initial cooling temperature is 710~740℃, the red-hot temperature is 480~550℃, and the cooling rate is 10~18℃ / s.

[0123] For finished steel plates with a thickness of 50mm < thick end and 60mm ≤ thick end, the initial cooling temperature is 690~720℃ and the cooling rate is 12~20℃ / s.

[0124] Controlling the initial cooling temperature allows the steel plate to generate a large amount of ferrite within this temperature range, ensuring high plasticity and toughness. Accelerated cooling can further refine the austenite grain structure and improve the strength of the steel plate. Controlling the reheating temperature allows the steel plate to form some bainite within this temperature range, improving the matrix strength. A suitable reheating temperature provides a suitable temperature for subsequent straightening, avoiding excessive straightening force during straightening and resulting in poor plate shape.

[0125] Table 6 Cooling process parameters for steels in Examples 1-10

[0126]

[0127]

[0128] (5) Straightening: Straightening is performed using a hot straightening machine. The relationship between the straightening temperature and the thickness of the thick end of the finished steel plate is as follows:

[0129] The thickness of the thick end of the finished steel plate is ≤20mm, and the straightening temperature is ≥550℃;

[0130] 20mm < Thickness of the finished steel plate at the thick end ≤ 30mm, straightening temperature ≥ 480℃;

[0131] Thickness of finished steel plate at thick end ≤ 50mm, straightening temperature ≥ 450℃;

[0132] 50mm < Thickness of the finished steel plate at the thick end ≤ 60mm, straightening temperature ≥ 400℃;

[0133] The straightening speed in the parallel section is 0.2–0.4 m / s, and the straightening speed in the variable thickness section is 0.1–0.20 m / s. According to the variable thickness specifications of the steel plate, the straightening roll gap is increased / decreased uniformly when entering the variable thickness section, and the upper straightening roll rises / falls in tandem. The formula for calculating the rising / falling roll speed is as follows:

[0134] Rising / falling roller speed = Variable thickness × Straightening speed / Horizontal length of variable thickness section

[0135] The thickness variation of the steel plate is ≤50mm to ensure the temperature uniformity and good plate shape of the entire steel plate; where, the thickness variation of the steel plate = thickness at the thick end of the steel plate - thickness at the thin end of the steel plate;

[0136] Table 7 Straightening process parameters for steels in Examples 1-10

[0137]

[0138] (6) Heat treatment: The tempering temperature is 550-650℃ and the tempering holding time is 3-5 min / mm. The purpose is to further improve the isotropic properties of the steel plate, reduce the residual stress of the steel plate, and thus improve the uniformity of the steel plate structure.

[0139] Table 8 Heat treatment process parameters for steels in Examples 1-10

[0140]

[0141]

[0142] The mechanical properties of the thick end of the steel in Examples 1-10 of this invention are shown in Table 9, and the mechanical properties of the thin end are shown in Table 10.

[0143] Table 9 Mechanical properties of the thick end of steels in Examples 1-10

[0144]

[0145] Table 10 Mechanical properties of thin-end steels in Examples 1-10

[0146]

[0147]

[0148] The final microstructure of the steel in Examples 1-10 of this invention consists of ferrite, pearlite, and bainite, with proportions shown in Table 11.

[0149] Table 11 Microstructure proportions of steels in Examples 1-10

[0150] Example Ferrite (%) Pearlite (%) Bainite (%) 1 15 5 80 2 22 7 71 3 18 4 78 4 25 6 69 5 28 3 69 6 21 5 74 7 16 6 78 8 19 5 76 9 11 7 82 10 14 5 81

[0151] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. An isotropic bridge steel with low yield strength ratio and high toughness, characterized in that, Its chemical composition by weight percentage is: C: 0.02%~0.05%, Si: 0.25%~0.45%, Mn: 1.00%~1.40%, P≤0.010%, S≤0.003%, Nb: 0.02%~0.05%, Ni: 0.40%~0.70%, Cu: 0.25%~0.45%, Mo: 0.10%~0.20%, Cr: 0.50%~0.80%, Ti: 0.005%~0.020%, B: 0.0010%~0.0025%, Al: 0.01%~0.04%, with the balance being iron and unavoidable impurities; CEV: 0.36%~0.52%, Pcm: 0.13%~0.20%, atmospheric corrosion resistance index I≥6.30; Its manufacturing method includes: rolling, cooling, straightening, and heat treatment, with the specific steps as follows: (1) Rolling: The continuously cast billet is placed into the heating furnace at a furnace temperature of 650~750℃. The temperature of the heating section of the continuously cast billet is 1120~1230℃, the temperature of the soaking section is 1110~1220℃, and the soaking section is held for 3.7~5.3h. The roughing stage adopts the longitudinal-transverse rolling process. The roughing rolling start temperature is 1000~1100℃, the longitudinal rolling cumulative reduction rate is 20%~30%, the transverse rolling cumulative reduction rate is 20%~40%, the roughing rolling finish temperature is 950~1050℃, and the cumulative reduction rate of the roughing stage is ≥60%. The finishing rolling stage employs longitudinal variable thickness rolling. The relationship between the finishing rolling parameters and the thickness of the thick end of the finished steel plate is as follows: The thickness of the finished steel plate at the thick end is ≤20mm, the thickness of the intermediate billet is 5~9t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 920~970℃, the final rolling temperature of the finishing rolling is 850~890℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 15%. The thickness of the finished steel plate is 20mm < thick end and 30mm ≤, the thickness of the intermediate billet is 3~6t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 890~960℃, the final rolling temperature of the finishing rolling is 840~880℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 13%. The thickness of the finished steel plate is 30mm < thick end and 50mm ≤, the thickness of the intermediate billet is 2.5~4.0t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 880~940℃, the final rolling temperature of the finishing rolling is 820~860℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 12%. The thickness of the finished steel plate is 50mm < thick end and 60mm ≤, the thickness of the intermediate billet is 2.0~3.5t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 860~900℃, the final rolling temperature of the finishing rolling is 800~840℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 10%. (2) Cooling: After rolling, water cooling is used for cooling. The cooling parameters are related to the thickness of the thick end of the finished steel plate as follows: The thickness of the finished steel plate at the thick end is ≤20mm, the initial cooling temperature is 750~780℃, and the red-heating temperature is 580~650℃. For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the initial cooling temperature is 740~770℃, and the reheating temperature is 500~600℃. For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the initial cooling temperature is 710~740℃, and the reheating temperature is 480~550℃. For finished steel plates with a thickness of 50mm < thick end and ≤ 60mm, the initial cooling temperature is 690~720℃, and the reheating temperature is 450~500℃. (3) Straightening: Straightening is performed using a hot straightening machine. The relationship between the straightening temperature and the thickness of the thick end of the finished steel plate is as follows: The thickness of the thick end of the finished steel plate is ≤20mm, and the straightening temperature is ≥550℃; 20mm < Thickness of the finished steel plate at the thick end ≤ 30mm, straightening temperature ≥ 480℃; Thickness of finished steel plate at thick end ≤ 50mm, straightening temperature ≥ 450℃; 50mm < Thickness of the finished steel plate at the thick end ≤ 60mm, straightening temperature ≥ 400℃; (4) Heat treatment: The tempering temperature is 550~650℃ and the tempering holding time is 3~5min / mm.

2. The isotropic low yield strength ratio and high toughness bridge steel according to claim 1, characterized in that, The microstructure of the isotropic low yield strength ratio and high toughness bridge steel consists of ferrite, pearlite and bainite; wherein, ferrite accounts for 11~28%, pearlite accounts for 3~7%, and bainite accounts for 69~82%.

3. The isotropic low yield strength ratio and high toughness bridge steel according to claim 1, characterized in that, The isotropic, low yield-to-tensile ratio, high-toughness bridge steel has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation after fracture ≥21%, yield-to-tensile ratio ≤0.83, and impact energy at -40℃ ≥200J; the difference in yield strength between the thin and thick ends of the same plate is ≤25MPa, the difference in tensile strength is ≤25MPa, the difference in elongation after fracture is ≤3%, the difference in yield-to-tensile ratio is ≤0.02, and the difference in impact energy at -40℃ is ≤30J; the difference in yield strength between the transverse and longitudinal ends at the same location is ≤5%, the difference in tensile strength between the transverse and longitudinal ends is ≤5%, and the difference in impact energy between the transverse and longitudinal ends is ≤5%.

4. A method for manufacturing isotropic low yield strength ratio and high toughness bridge steel according to any one of claims 1-3, characterized in that, include: The specific steps of rolling, cooling, straightening, and heat treatment are as follows: (1) Rolling: The continuously cast billet is placed into the heating furnace at a furnace temperature of 650~750℃. The temperature of the heating section of the continuously cast billet is 1120~1230℃, the temperature of the soaking section is 1110~1220℃, and the soaking section is held for 3.7~5.3h. The roughing stage adopts the longitudinal-transverse rolling process. The roughing rolling start temperature is 1000~1100℃, the longitudinal rolling cumulative reduction rate is 20%~30%, the transverse rolling cumulative reduction rate is 20%~40%, the roughing rolling finish temperature is 950~1050℃, and the cumulative reduction rate of the roughing stage is ≥60%. The finishing rolling stage employs longitudinal variable thickness rolling. The relationship between the finishing rolling parameters and the thickness of the thick end of the finished steel plate is as follows: The thickness of the finished steel plate at the thick end is ≤20mm, the thickness of the intermediate billet is 5~9t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 920~970℃, the final rolling temperature of the finishing rolling is 850~890℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 15%. The thickness of the finished steel plate is 20mm < thick end and 30mm ≤, the thickness of the intermediate billet is 3~6t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 890~960℃, the final rolling temperature of the finishing rolling is 840~880℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 13%. The thickness of the finished steel plate is 30mm < thick end and 50mm ≤, the thickness of the intermediate billet is 2.5~4.0t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 880~940℃, the final rolling temperature of the finishing rolling is 820~860℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 12%. The thickness of the finished steel plate is 50mm < thick end and 60mm ≤, the thickness of the intermediate billet is 2.0~3.5t, where t is the thickness of the finished steel plate at the thick end, the initial rolling temperature of the finishing rolling is 860~900℃, the final rolling temperature of the finishing rolling is 800~840℃, and the single-pass deformation rate of the thick end of the steel plate is not less than 10%. (2) Cooling: After rolling, water cooling is used for cooling. The cooling parameters are related to the thickness of the thick end of the finished steel plate as follows: The thickness of the finished steel plate at the thick end is ≤20mm, the initial cooling temperature is 750~780℃, and the red-heating temperature is 580~650℃. For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the initial cooling temperature is 740~770℃, and the reheating temperature is 500~600℃. For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the initial cooling temperature is 710~740℃, and the reheating temperature is 480~550℃. For finished steel plates with a thickness of 50mm < thick end and ≤ 60mm, the initial cooling temperature is 690~720℃, and the reheating temperature is 450~500℃. (3) Straightening: Straightening is performed using a hot straightening machine. The relationship between the straightening temperature and the thickness of the thick end of the finished steel plate is as follows: The thickness of the thick end of the finished steel plate is ≤20mm, and the straightening temperature is ≥550℃; 20mm < Thickness of the finished steel plate at the thick end ≤ 30mm, straightening temperature ≥ 480℃; Thickness of finished steel plate at thick end ≤ 50mm, straightening temperature ≥ 450℃; 50mm < Thickness of the finished steel plate at the thick end ≤ 60mm, straightening temperature ≥ 400℃; (4) Heat treatment: The tempering temperature is 550~650℃ and the tempering holding time is 3~5min / mm.

5. The manufacturing method according to claim 4, characterized in that, In step (1), the method for preparing the continuously cast billet includes the following steps: a. Smelting: After pretreatment of molten iron, it is smelted in a converter and refined outside the ladle. Among them, the net argon blowing time in the LF refining treatment is ≥5min, the RH vacuum degassing time is ≥5min. b. Continuous casting: The target superheat of the tundish in continuous casting is ≤30℃. The casting process is protected throughout and light pressure is applied to control the thickness of the continuous casting billet / maximum thickness of the finished steel plate to be ≥6.

0.

6. The manufacturing method according to claim 4, characterized in that, In step (1), the continuous casting billet is heated at a rate of 4~6℃ / min.

7. The manufacturing method according to claim 4, characterized in that, In step (1), the relationship between the rolling speed in the finishing rolling stage and the thickness of the thick end of the finished steel plate is as follows: The thickness of the finished steel plate at the thick end is ≤20mm, and the rolling speed is 1.3~2.7m / s; For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the rolling speed is 1.2~2.6 m / s. For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the rolling speed is 1.1~2.4m / s. For finished steel plates with a thickness of 50mm < thick end and ≤ 60mm, the rolling speed is 0.9~1.8m / s.

8. The manufacturing method according to claim 4, characterized in that, In step (2), the relationship between the cooling rate and the thickness of the thick end of the finished steel plate is as follows: The thickness of the thick end of the finished steel plate is ≤20mm, and the cooling rate is 3~10℃ / s; For finished steel plates with a thickness of 20mm < thick end and ≤ 30mm, the cooling rate is 6~14℃ / s; For finished steel plates with a thickness of 30mm < thick end and ≤ 50mm, the cooling rate is 10~18℃ / s; For finished steel plates with a thickness of 50mm < thick end and ≤ 60mm, the cooling rate is 12~20℃ / s.

9. The manufacturing method according to claim 4, characterized in that, In step (3), the straightening speed of the parallel section is 0.2~0.4 m / s, and the straightening speed of the variable thickness section is 0.1~0.20 m / s; The thickness of the steel plate is ≤50mm, where the thickness of the steel plate = thickness at the thick end of the steel plate - thickness at the thin end of the steel plate.

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