Low-cost high-performance bridge steel and method for producing the same

Abstract

The application relates to the technical field of steel smelting, in particular to a low-cost high-performance bridge steel and a preparation method thereof. The bridge steel comprises the following components: C 0.06%-0.08%, Si 0.10%-0.20%, Mn 1.65%-1.80%, P<=0.012%, S<=0.003%, Als 0.010%-0.030%, Cr 0.45%-0.65%, Nb 0.030%-0.040%, Ti 0.010%-0.020%, B 0.0012%-0.0018%, and the rest is Fe and inevitable impurities. The application adopts a lower carbon component to improve the welding performance of the steel plate, and meanwhile, micro-alloy elements are used in combination with two-stage rolling to ensure the high performance of the steel plate and reduce the cost.

Description

Technical Field

[0001] This invention relates to the field of iron and steel smelting technology, specifically to a low-cost, high-performance bridge steel and its preparation method. Background Technology

[0002] Bridge steel is a type of steel plate specifically designed for constructing railway or highway bridges. It requires high strength and toughness to withstand the loads and impacts of locomotives and rolling stock, as well as good fatigue resistance and a certain level of low-temperature toughness. Bridge steel plays a crucial role in bridge construction, and its quality directly affects the durability and safety of the bridge.

[0003] 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 high strength, excellent low-temperature toughness, and weldability to meet the requirements for safety, reliability, and longevity of steel structures.

[0004] CN108330399A discloses a low-alloy high-performance bridge steel and its production method. The finished steel plate has a thickness of 12-60mm, tensile strength ≥560MPa, yield strength ≥430MPa, elongation A ≥20%, V-notch impact energy ≥200J at -40℃, and low C content of 0.04%-0.09%. When the thickness is 35-60mm, the amount of Mo used is 0.10%-0.20%, which adds expensive microalloying elements, increasing the cost. At the same time, the mechanical properties of the bridge steel produced are difficult to meet the requirements.

[0005] CN106811704A discloses a low yield strength ratio bridge steel with a yield strength of 500MPa and its manufacturing method. The steel has a C content of 0.05% to 0.10% and contains microalloying components with the following percentages: Ni 0.05% to 0.20%, Cu 0.10% to 0.5%, and Mo 0.08% to 0.25%. The production cost is relatively high. Summary of the Invention

[0006] To address the technical problems of high cost and poor performance in bridge steel manufacturing, this invention provides a low-cost, high-performance bridge steel and its manufacturing method. It uses a lower carbon content to improve the weldability of the steel plate, and utilizes microalloying elements in conjunction with two-stage rolling to ensure high performance and low cost.

[0007] In a first aspect, the present invention provides a low-cost, high-performance bridge steel, comprising the following chemical composition by weight percentage: C 0.06%–0.08%, Si 0.10%–0.20%, Mn 1.65%–1.80%, P ≤0.012%, S ≤0.003%, Als 0.010%–0.030%, Cr 0.45%–0.65%, Nb 0.030%–0.040%, Ti 0.010%–0.020%, B 0.0012%–0.0018%, with the remainder being Fe and unavoidable impurities; and the chemical composition content satisfies the following relationship:

[0008] 9≤[(T+30℃)×ξ1] / H≤75, 7≤[(T-3*H)×ξ2] / H≤75, where,

[0009] T = 970 + 5668*(%Nb) - 302*(%C) - 78*(%Mn) - 26*(%Cr) - 5896*(%B) - 0.5*(H-6), where ξ1 is the cumulative reduction rate of controlled rolling in the austenite single-phase region without recrystallization, in %; ξ2 is the cumulative reduction rate of controlled rolling in the ferrite / austenite two-phase region, in %; and H is the thickness of the steel plate, in mm.

[0010] Furthermore, the finished steel plate has a thickness of 10-60mm, a yield strength ≥500MPa, a tensile strength ≥650MPa, an elongation after fracture ≥25%, a yield strength ratio ≤0.85, and a KV2 ≥200J at -60℃.

[0011] Secondly, the present invention provides a method for preparing low-cost, high-performance bridge steel, including smelting and continuously casting raw materials to obtain a billet, and slowly cooling, heating, rolling and cooling the billet to obtain a finished product; the rolling step includes two-stage rolling, with the first stage rolling temperature being (T+30)℃ and the second stage rolling temperature being (T-3*H)℃.

[0012] Furthermore, the smelting steps include: using a converter for smelting, top and bottom blowing, decarburization and dephosphorization, vacuum degree <133Pa, holding time ≥12min, pure degassing time ≥8min, soft blowing time ≥12min; calcium treatment is carried out during the smelting process, and the calcium treatment meets the requirements of (%Ca) × (%S). 0.25 ≤2.2×10 -3 .

[0013] Furthermore, the continuous casting process includes: full-process protective casting, electromagnetic stirring, and heavy reduction technology at the end of casting, with a reduction ratio of 12% to 14%.

[0014] Furthermore, the starting temperature for slow cooling of the billet is ≥500℃, and the ending temperature is ≤150℃.

[0015] Furthermore, the temperature at which the cast billet is heated and removed from the furnace is 1040~1060℃.

[0016] Furthermore, the thickness of the intermediate billet in the rolling process is 2 to 4.5 times the thickness of the finished steel.

[0017] Furthermore, in the cooling process, the initial cooling temperature is 740~760℃, and the final cooling temperature is 200~300℃.

[0018] Chemical composition is one of the important factors affecting the overall performance of a product. The chemical composition of this invention is limited as follows.

[0019] Carbon (C) is a major solid solution strengthening element in steel, which can significantly improve the strength of steel plates. However, a high carbon content is detrimental to the welding, toughness, and plasticity of steel plates. To effectively reduce segregation, improve the uniformity of microstructure, avoid galvanic corrosion caused by potential differences between heterogeneous phases, and improve the corrosion resistance of steel, while also considering economic efficiency, its mass percentage content is limited to 0.06%–0.08%.

[0020] Si: Silicon is one of the effective deoxidizing and exothermic elements in the steelmaking process. It has a certain solid solution strengthening effect, which is conducive to the formation of a dense rust layer and can improve the atmospheric corrosion resistance of steel. However, excessive silicon content will reduce the surface quality, weldability and low temperature toughness of steel. The silicon content in this invention is controlled at 0.10% to 0.20%.

[0021] Mn: Manganese has a strong solid solution strengthening effect, which can significantly reduce the phase transformation temperature of steel, refine the microstructure of steel, and is inexpensive. It is an effective element for improving strength. In order to achieve sufficiently high tensile strength and obtain a low yield strength ratio, the content should not be less than 0.80%. However, if the content is too high, it will easily cause segregation of the billet, which will form a banded structure, reduce the weldability and toughness of the weld heat-affected zone of the steel plate, as well as the corrosion resistance. In this invention, the manganese content is controlled at 1.65% to 1.80%.

[0022] P: Phosphorus can improve the weather resistance of steel, increase its cold brittleness, and worsen its weldability; therefore, this invention does not use high phosphorus content to increase weather resistance, but achieves high corrosion resistance through a reasonable combination of other alloying elements; the phosphorus content of this invention is controlled at ≤0.012%.

[0023] S: Sulfur causes hot brittleness in steel, reduces the ductility and toughness of steel, promotes the anisotropy of steel plates, and sulfide inclusions also significantly reduce the corrosion resistance of steel. The sulfur content in steel should be strictly controlled; the sulfur content in this invention is controlled at ≤0.003%.

[0024] Al: Aluminum is one of the effective deoxidizing elements in the steelmaking process. It can effectively reduce the content of inclusions in steel and refine the grains. However, if the content is too high, it can easily cause cracks on the surface of the billet. In this invention, the acid-soluble aluminum content is controlled at 0.01% to 0.03%.

[0025] Cr: Chromium is not only an element that improves the hardenability of steel, but also one of the effective elements for improving the weather resistance of steel. This invention does not use high phosphorus content to improve the weather resistance of steel, so the chromium content cannot be too low, and its lower limit is controlled at 0.45%. On the other hand, too high chromium content will affect the toughness of steel and the weldability will also be worse, so the upper limit of the content is 0.65%.

[0026] Niobium (Nb) is one of the important elements for grain refinement and strengthening. It increases the recrystallization temperature of austenite, prevents austenite recrystallization and inhibits grain growth, thus refining austenite grains. Its carbonitrides precipitate on dislocations and segregate at austenite grain boundaries, improving strength and toughness. However, if the niobium content is too high, the billet is prone to surface cracks and the welding performance is also deteriorated. In this invention, the niobium content is controlled at 0.030% to 0.040%.

[0027] Ti: Produces strong precipitation strengthening and moderate grain refinement. Adding trace amounts of titanium to steel aims to combine with nitrogen in the steel to generate highly stable titanium nitride particles, inhibiting austenite grain growth in the weld HAZ and altering secondary phase transformation products, thus improving the low-temperature toughness of high heat input welding. The titanium content added to the steel must match the nitrogen content. If the titanium content is too low, the number of titanium nitride particles formed will be insufficient to inhibit austenite grain growth in the HAZ and alter secondary phase transformation products, thereby improving the low-temperature toughness of the HAZ in high heat input welding. If the titanium content is too high, large-sized titanium nitride particles will precipitate during the solidification process of the molten steel. These large-sized particles not only fail to inhibit austenite grain growth in the HAZ but also become the initiation point for crack initiation. In this invention, the titanium content is controlled at 0.010%–0.020%.

[0028] B: It can significantly improve the hardenability and quenchability of steel plates, thereby increasing their strength and hardness. Its effect weakens and eventually disappears as the carbon content in the steel increases. It slightly promotes the temper brittleness of steel. However, its segregation at grain boundaries can affect the toughness of the steel plate; the boron content in this invention is controlled at 0.0012% to 0.0018%.

[0029] The beneficial effects of this invention are as follows: It has low production costs, uses a lower carbon content to improve the weldability of the steel plate, and utilizes microalloying elements in conjunction with a two-stage rolling process to ensure the high performance of the steel plate. The high-performance bridge steel plate prepared by this invention has a yield strength ≥500MPa, tensile strength ≥650MPa, elongation after fracture ≥25%, yield strength ratio ≤0.85, and KV2 ≥200J at -60℃, meeting the requirements of modern bridge engineering construction for high-performance bridge steel plates. Detailed Implementation

[0030] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0031] In the following examples, "T" is a temperature point summarized based on the characteristics of steel composition, finished product thickness, and rolling temperature, with the unit being °C. The formula for calculating "T" is:

[0032] T = 970 + 5668*(%Nb) - 302*(%C) - 78*(%Mn) - 26*(%Cr) - 5896*(%B) - 0.5*(H-6), where %Nb, %C, %Mn, %Cr, and %B represent the percentage content of Nb, C, Mn, Cr, and B in the finished steel, respectively, and H represents the thickness of the finished steel in mm.

[0033] Example 1

[0034] This embodiment provides a low-cost, high-performance bridge steel with a finished thickness of 10 mm. Its chemical composition is as follows: C 0.07%, Si 0.15%, Mn 1.68%, P 0.012%, S 0.002%, Als 0.026%, Nb 0.032%, Ti 0.015%, B 0.0013%, with the remainder being Fe and unavoidable impurities.

[0035] The preparation method includes smelting the raw materials sequentially and continuously casting them to obtain a billet with the above chemical composition. The billet is then slowly cooled, heated, rolled, and cooled to obtain the finished product.

[0036] Specifically, the steps include the following:

[0037] (1) Smelting: Converter smelting is adopted, and the hot metal is pretreated to achieve a final S content of 0.0025%; the converter adopts top and bottom blowing to fully dephosphorize and decarburize, with a final phosphorus content of 0.004%. Argon is blown throughout the process, with weak stirring and slight turning of the slag surface to prevent exposure. Lime and fluorite are used to make white slag; RH vacuum degree is 100pa, vacuum holding time is 12min, and pure degassing time is 13min; the molten steel is calmed for 28min before tapping; calcium treatment is carried out during the smelting process, and the amount of calcium added meets the requirement of (%Ca) × (%S). 0.25 =2.1×10 -3 The endpoint [H] × [N] = 3.0 × 10 -11 ,

[0038] (2) Continuous casting: full-process protective casting, electromagnetic stirring, heavy pressure reduction technology is adopted at the end of casting, with a reduction ratio of 12.4%.

[0039] (3) Heating: The temperature of the billet after heating is 1046℃. It is then slowly cooled in the pit at 680℃. The temperature of the billet after exiting the pit is 102℃ and the thickness is 150mm.

[0040] (4) Rolling: The intermediate billet thickness is 45mm. After the billet is removed from the furnace, it is descaled by 21MPa high-pressure water and then rolled. The rolling process includes two stages. The calculated temperature of the rolled piece is T=976℃. The first stage rolling temperature is (T+30)=1006℃, [(T+30℃)×ξ1] / H=73, ξ1=73%, which is the cumulative reduction rate of controlled rolling in the austenite single-phase region without recrystallization. The second stage rolling temperature is (T-3*H)=946℃, [(T-3*H)×ξ2] / H=71, ξ2=75%, which is the cumulative reduction rate of controlled rolling in the ferrite / austenite two-phase region.

[0041] (5) Cooling: The rolled steel plate is cooled in a controlled manner, with an initial cooling temperature of 746℃ and a final cooling temperature of 283℃.

[0042] Example 2

[0043] This embodiment provides a low-cost, high-performance bridge steel with a finished thickness of 30mm. Its chemical composition is as follows: C 0.07%, Si 0.19%, Mn 1.70%, P 0.012%, S 0.002%, Als 0.023%, Nb 0.035%, Ti 0.017%, B 0.0014%, with the remainder being Fe and unavoidable impurities.

[0044] The preparation method includes smelting the raw materials sequentially and continuously casting them to obtain a billet with the above chemical composition. The billet is then slowly cooled, heated, rolled, and cooled to obtain the finished product.

[0045] Specifically, the steps include the following:

[0046] (1) Smelting: Converter smelting was adopted, and the molten iron was pretreated to achieve a final S content of 0.0020%. The converter adopted top and bottom blowing to fully dephosphorize and decarburize, with a final phosphorus content of 0.004%. Argon was blown throughout the process, with weak stirring and slight turning of the slag surface to prevent exposure. Lime and fluorite were used to make white slag. The RH vacuum degree was 100pa, the vacuum holding time was 13min, and the pure degassing time was 13min. The molten steel was calmed for 29min before tapping. Calcium treatment was carried out during the smelting process, and the amount of calcium added met the requirements of (%Ca) × (%S). 0.25 =2.0×10 -3 The endpoint [H] × [N] = 3.6 × 10 -11 ,

[0047] (2) Continuous casting: full-process protective casting, electromagnetic stirring, heavy pressure reduction technology is adopted at the end of casting, with a reduction ratio of 12%.

[0048] (3) Heating: The temperature of the billet after heating is 1052℃. It is then placed in the pit at 680℃ for slow cooling. The temperature of the billet after exiting the pit is 112℃, and the thickness is 300mm.

[0049] (4) Rolling: The intermediate billet thickness is 90mm. After the billet is removed from the furnace, it is descaled by 21MPa high-pressure water and then rolled. The rolling process includes two stages. The calculated temperature of the rolled piece is T=981℃. The first stage rolling temperature is (T+30)=1011℃, [(T+30℃)×ξ1] / H=24, ξ1=70%, which is the cumulative reduction rate of controlled rolling in the austenite single-phase region without recrystallization. The second stage rolling temperature is (T-3*H)=891℃, [(T-3*H)×ξ2] / H=20, ξ2=67%, which is the cumulative reduction rate of controlled rolling in the ferrite / austenite two-phase region.

[0050] (5) Cooling: The rolled steel plate is cooled in a controlled manner, with an initial cooling temperature of 752℃ and a final cooling temperature of 260℃.

[0051] Example 3

[0052] This embodiment provides a low-cost, high-performance bridge steel with a finished thickness of 60mm. Its chemical composition is as follows: C 0.06%, Si 0.18%, Mn 1.72%, P 0.011%, S 0.001%, Als 0.022%, Nb 0.038%, Ti 0.018%, B 0.0014%, with the remainder being Fe and unavoidable impurities.

[0053] The preparation method includes smelting the raw materials sequentially and continuously casting them to obtain a billet with the above chemical composition. The billet is then slowly cooled, heated, rolled, and cooled to obtain the finished product.

[0054] Specifically, the steps include the following:

[0055] (1) Smelting: Converter smelting is adopted, and the hot metal is pretreated to achieve a final S content of 0.0020%. The converter adopts top and bottom blowing to fully dephosphorize and decarburize, with a final phosphorus content of 0.004%. Argon is blown throughout the process, with weak stirring and slight turning of the slag surface to prevent exposure. Lime and fluorite are used to make white slag. The RH vacuum degree is 100pa, the vacuum holding time is 12min, and the pure degassing time is 15min. The molten steel is calmed for 36min before tapping. Calcium treatment is carried out during the smelting process, and the amount of calcium added meets the requirement of (%Ca) × (%S). 0.25 =1.9×10 -3 The endpoint [H] × [N] = 3.4 × 10 -11 ,

[0056] (2) Continuous casting: full-process protective casting, electromagnetic stirring, and heavy pressure reduction technology at the end of casting with a reduction ratio of 12.6%.

[0057] (3) Heating: The temperature of the billet after heating is 1056℃. It is then placed in the pit at 680℃ for slow cooling. The temperature of the billet after exiting the pit is 122℃, and the thickness is 300mm.

[0058] (4) Rolling: The intermediate billet thickness is 130mm. After the billet is removed from the furnace, it is descaled by 21MPa high-pressure water and then rolled. The rolling process includes two stages. The calculated temperature of the rolled piece is T=982℃. The first stage rolling temperature is (T+30)=1012℃, [(T+30℃)×ξ1] / H=10, ξ1=57%, which is the cumulative reduction rate of controlled rolling in the austenite single-phase region without recrystallization. The second stage rolling temperature is (T-3*H)=802℃, [(T-3*H)×ξ2] / H=7.2, ξ2=54%, which is the cumulative reduction rate of controlled rolling in the ferrite / austenite two-phase region.

[0059] (5) Cooling: The rolled steel plate is cooled in a controlled manner, with an initial cooling temperature of 758℃ and a final cooling temperature of 272℃.

[0060] The tensile properties and low-temperature toughness mechanical properties of Examples 1-3 of the present invention were tested, and the specific results are shown in Table 1 and Table 2.

[0061] Table 1 Tensile property data of steel plates obtained in the embodiments of the present invention

[0062]

[0063] Table 2 Low-temperature toughness data of steel plates obtained in the embodiments of the present invention

[0064]

[0065] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing low-cost, high-performance bridge steel, characterized in that, The low-cost, high-performance bridge steel comprises the following chemical composition by weight percentage: C 0.06%–0.08%, Si 0.10%–0.20%, Mn 1.65%–1.80%, P≤0.012%, S≤0.003%, Als 0.010%–0.030%, Cr 0.45%–0.65%, Nb 0.030%–0.040%, Ti 0.010%–0.020%, B 0.0012%–0.0018%, with the remainder being Fe and unavoidable impurities; and the chemical composition content satisfies the following relationship: 9≤[(T+30℃)×ξ1] / H≤75, 7≤[(T-3*H)×ξ2] / H≤75, where, T = 970 + 5668*(%Nb) - 302*(%C) - 78*(%Mn) - 26*(%Cr) - 5896*(%B) - 0.5*(H-6), where ξ1 is the cumulative reduction rate of controlled rolling in the austenite single-phase region without recrystallization, in %; ξ2 is the cumulative reduction rate of controlled rolling in the ferrite / austenite two-phase region, in %; and H is the thickness of the steel plate, in mm. The method for preparing low-cost, high-performance bridge steel includes smelting raw materials sequentially and continuously casting them to obtain a billet. The billet is then slowly cooled, heated, rolled, and cooled to obtain a finished steel plate. The rolling process includes two-stage rolling, with the first stage starting temperature at (T+30)℃ and the second stage starting temperature at (T-3*H)℃. The finished steel plate has a thickness of 10-60mm, a yield strength ≥500MPa, a tensile strength ≥650MPa, an elongation after fracture ≥25%, a yield strength ratio ≤0.85, and a KV2 ≥200J at -60℃.

2. The preparation method according to claim 1, characterized in that, The smelting steps include: smelting in a converter, top and bottom blowing, decarburization and dephosphorization, vacuum degree <133Pa, holding time ≥12min, pure degassing time ≥8min, soft blowing time ≥12min; calcium treatment is carried out during the smelting process, and the calcium treatment meets the requirements of (%Ca) × (%S). 0.25 ≤2.2×10 -3 .

3. The preparation method according to claim 1, characterized in that, The continuous casting process includes: full-process protective casting, electromagnetic stirring, and heavy reduction technology at the end of casting, with a reduction ratio of 12% to 14%.

4. The preparation method according to claim 1, characterized in that, The billet should be slowly cooled starting at a temperature ≥500℃ and ending at a temperature ≤150℃.

5. The preparation method according to claim 1, characterized in that, The temperature at which the cast billet is heated and removed from the furnace is 1040~1060℃.

6. The preparation method according to claim 1, characterized in that, The thickness of the intermediate billet during the rolling process is 2 to 4.5 times the thickness of the finished steel plate.

7. The preparation method according to claim 1, characterized in that, During the cooling process, the initial cooling temperature is 740~760℃, and the final cooling temperature is 200~300℃.

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