Micro-niobium low-manganese q355b low alloy angle steel and high-efficiency short-process production method thereof

Abstract

This invention discloses a low-alloy angle steel (Q355B) with micro-niobium and low-manganese content, and its efficient short-process production method. The chemical composition of the angle steel is: C 0.14%~0.21%, Si 0.20%~0.45%, Mn 1.00%~1.25%, Nb 0.005%~0.014%, P≤0.025%, S≤0.020%, N≤0.009%, with the balance being Fe and unavoidable impurities. This method uses scrap steel as raw material, sequentially processing it through electric arc furnace smelting, LF refining, and continuous casting to obtain a high-temperature billet. The high-temperature billet is then directly conveyed to the heating furnace, achieving hot charging and hot delivery. Compared with existing technologies, this invention omits the billet cooling and secondary heating processes, significantly reducing energy consumption, shortening the production cycle, and reducing the formation of iron oxide scale. By substituting a small amount of Mn with Nb, and utilizing the dispersed precipitation of Nb(C,N) to simultaneously achieve grain refinement and precipitation strengthening, a ferrite + pearlite microstructure with an average grain size of 15~23μm is obtained, resulting in angle steel with excellent mechanical properties. This invention, while ensuring the strength and toughness of angle steel, also possesses the comprehensive advantages of energy saving, cost reduction, and efficiency control, and has promising prospects for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy and steel processing technology, specifically relating to a micro-niobium, low-manganese Q355B low-alloy angle steel and its efficient short-process production method. This method achieves energy saving, consumption reduction, and high-performance preparation of angle steel by shortening the traditional production process, reducing secondary heating, and incorporating micro-alloying control. Background Technology

[0002] Q355 low-alloy high-strength angle steel is a key material in large steel structures such as buildings, bridges, towers, and ships. According to the national standard GB / T 1591—2018, Q355 steel must meet the requirement of a yield strength of not less than 355 MPa, and possess good plasticity, weldability, and stable comprehensive mechanical properties. Therefore, high requirements are placed on the stability and economy of its production process.

[0003] The steel industry faces the dual pressures of energy conservation and cost reduction. Specifically, this manifests in two ways: First, in traditional angle steel production processes, continuously cast billets typically need to be cooled to room temperature before being reheated to the rolling temperature, a process that consumes a lot of energy and emits large amounts of carbon dioxide. Second, to ensure the strength and toughness of the material, conventional processes often rely on increasing the manganese (Mn) content to achieve solid solution strengthening. This not only increases the cost of the alloy but may also lead to an increase in carbon equivalent and exacerbate central segregation, affecting the weldability and microstructure uniformity of the material.

[0004] Current technological improvements mainly revolve around two directions: First, using micro-alloying elements (such as Nb and Ti) to achieve fine grain strengthening and precipitation strengthening. For example, the micro-niobium alloying scheme proposed in CN114959461A helps to improve strength and toughness, but this scheme is mainly applicable to steel plate production and still retains the billet cooling and secondary heating process, resulting in limited energy-saving effects. Second, optimizing the heating regime of continuously cast billets to reduce energy consumption, but most processes still follow the traditional path of "cooling-reheating-rolling", failing to fundamentally break through the bottleneck of high energy consumption and long cycle time.

[0005] Traditional cooling-reheating processes have significant drawbacks: the cooling and reheating processes are time-consuming, leading to extended production cycles and a significant increase in energy consumption and carbon emissions; the steel billet undergoes severe surface oxidation during repeated heating, forming a thick iron oxide scale, which not only increases cleaning costs but also easily causes rolling defects; in addition, temperature fluctuations during the reheating process can easily cause grain coarsening and exacerbate the tendency for central segregation, which is not conducive to the stable control of microstructure uniformity and mechanical properties.

[0006] In recent years, the hot-charging-hot-delivery high-efficiency short-process (also known as "hot-charging direct rolling of continuously cast billets") technology has attracted widespread attention due to its significant energy-saving potential. This process utilizes the residual heat of the continuously cast billet, directly feeding it into a heating furnace for homogenization and rolling at high temperatures. This effectively reduces secondary heating energy consumption, shortens the production cycle, and reduces the formation of iron oxide scale. However, the application of this technology in angle steel production is still immature, lacking a matching chemical composition system and rolling process regime, which easily leads to problems such as coarse grains and performance fluctuations, hindering its industrialization.

[0007] Therefore, developing a high-efficiency, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel, which can achieve energy saving, consumption reduction, alloy cost reduction, and production efficiency improvement while ensuring mechanical and weldability properties, has significant industrial significance and application value. Summary of the Invention

[0008] The traditional production process for Q355B low-alloy angle steel generally adopts a long process of "continuous casting billet cooling - secondary heating - rolling". This process not only leads to increased energy consumption, increased iron oxide scale, and extended production cycle due to repeated heating, but also easily causes problems such as grain coarsening and center segregation. In addition, to meet mechanical property requirements, traditional processes usually rely on increasing the manganese content to enhance the solid solution effect. This not only drives up raw material costs, but also leads to an increase in carbon equivalent, which in turn affects weldability and increases the risk of fluctuations in mechanical properties.

[0009] To address the aforementioned problems, this invention proposes a low-alloy angle steel with micro-niobium and low-manganese content (Q355B) and its efficient, short-process production method. This method significantly reduces production energy consumption and alloy costs while ensuring the strength, toughness, and weldability of the angle steel, providing a feasible path for efficient, stable, and green industrialized angle steel production.

[0010] To achieve this technical objective, the present invention adopts the following solution: In a first aspect, the present invention provides a low-alloy angle steel of micro-niobium and low-manganese Q355B, which, by mass percentage, comprises the following chemical composition: C 0.14%~0.21%, Si 0.20%~0.45%, Mn 1.00%~1.25%, Nb 0.005%~0.014%, P ≤0.025%, S ≤0.020%, N ≤0.009%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, the microstructure of hot-rolled angle steel is ferrite + pearlite with an average grain size of 15~23μm, and its mechanical properties meet the following requirements: upper yield strength ≥355 MPa, tensile strength ≥490 MPa, elongation after fracture ≥25%, and longitudinal V-shaped impact energy Akv ≥75J at 20 ℃.

[0012] Secondly, this invention provides a highly efficient, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel, the production process of which is as follows: (1) Smelting process: Scrap steel is smelted in an electric furnace, and the temperature of the molten pool is controlled at 1655~1670℃; then LF refining and continuous casting are carried out to obtain high temperature billets, and the temperature of the billets exiting the continuous casting machine is ≥1100℃. (2) Hot charging and hot delivery: After the billet exits the continuous casting machine, the roller conveyor is used for high-speed heat preservation and conveying (with heat preservation cover). Within 13 to 16 minutes after the billet exits the continuous casting machine, when the billet temperature is still maintained at 870 to 900 ℃, the billet is directly sent into the heating furnace to avoid the brittle zone temperature of 700 to 860 ℃ of the niobium-containing steel of this invention. (3) Heating process: The high-temperature billet is further heated to 1220~1260 ℃ and held for 60~85 min; (4) Hot rolling process: The initial rolling temperature is controlled at 1160~1200 ℃, and the final rolling temperature is controlled at 900 ℃ or above; (5) Cooling process: air cool to room temperature after rolling.

[0013] The design basis for the chemical composition of the micro-niobium, low-manganese Q355B low-alloy angle steel of this invention is as follows: C (0.14%~0.21%): As the main strengthening element in steel, carbon content is controlled within a reasonable range to ensure the strength of the matrix, so as to balance the toughness and weldability of the material.

[0014] Si (0.20%~0.45%) plays a role in deoxidation and solid solution strengthening, but too high a content can easily lead to thicker iron oxide scale and reduced toughness, so its upper limit is set at 0.45%.

[0015] Mn (1.00%~1.25%): In traditional Q355B steel, the Mn content is usually not less than 1.4%. This invention reduces the Mn content to 1.00%~1.25% by introducing trace amounts of Nb, effectively reducing alloy cost and carbon equivalent while maintaining strength.

[0016] P≤0.025%, S≤0.020%: As harmful elements, their content must be strictly controlled to avoid adverse effects on the toughness and weldability of the material.

[0017] N (≤0.009%): Excessive nitrogen content can easily lead to porosity or pore defects in steel, so it must be strictly limited.

[0018] Nb (0.005%~0.014%): Combines with C and N to form a stable Nb(C,N) dispersed precipitate phase, which plays a role in grain refinement and precipitation strengthening, while inhibiting austenite grain growth and ensuring the stability of the microstructure during high-efficiency short-process rolling.

[0019] By introducing trace amounts of Nb, the Mn content can be significantly reduced by about 0.3% while ensuring strength and toughness, thereby saving alloy costs. Combined with the high-efficiency, short-process hot charging-hot delivery, the stability of the final product performance is ensured.

[0020] Compared with traditional long-process technology, this invention has the following outstanding advantages: ① Energy saving and consumption reduction: The hot charging-hot delivery process avoids the complete cooling and secondary reheating of the billet, reducing overall energy consumption by about 10% to 15% and effectively reducing CO2 emissions.

[0021] ② Shorter production cycle: Eliminating the secondary heating step reduces the overall process time from continuous casting to rolling by more than 30%, significantly improving the production line capacity utilization rate.

[0022] ③ Improved surface quality: Reduce repeated high-temperature oxidation, control the oxide scale thickness on the billet surface to within 0.3 mm, and reduce the risk of rolling defects.

[0023] ④ Stable microstructure and properties: The synergistic effect of trace Nb and low Mn content, through the dispersed precipitation of Nb(C,N) and the grain refinement strengthening mechanism, maintains excellent strength, toughness and weldability while reducing the amount of Mn by about 0.3%, and reduces the carbon equivalent.

[0024] ⑤ Wide range of applications: The mechanical properties of the hot-rolled angle steel produced fully meet the requirements of Q355 low alloy angle steel grade B standard, and can meet the needs of various applications such as buildings, bridges and high-rise steel structures.

[0025] In summary, this invention, through the synergistic design of a high-efficiency, short-process hot charging-hot delivery system and a micro-niobium, low-manganese alloy system, achieves energy conservation, reduced consumption, shorter process, and lower alloy costs while ensuring that the angle steel possesses excellent and stable mechanical properties, resulting in significant economic and social benefits. Attached Figure Description

[0026] Figure 1 The image shows the microstructure of the angle steel produced in Example 1.

[0027] Figure 2 The image shows the microstructure of the angle steel produced in Example 2.

[0028] Figure 3 The image shows the microstructure of the angle steel produced in Example 3.

[0029] Figure 4 The image shows the microstructure of the angle steel produced in Example 4.

[0030] Figure 5 The image shows the microstructure of the angle steel produced in Example 5.

[0031] Figure 6 The image shows the microstructure of the angle steel produced in Comparative Example 1.

[0032] Figure 7 The image shows the microstructure of the angle steel produced in Comparative Example 2. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Example 1

[0034] A high-efficiency, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel is disclosed. The chemical composition and mass percentage of each element are shown in Table 1. Scrap steel is smelted in an electric furnace, with the molten pool temperature controlled at 1660℃ and the billet exiting the continuous casting machine at 1125℃. Within 15 minutes of exiting the continuous casting machine, while the billet temperature remains at 892℃, it is directly fed into a heating furnace. The high-temperature billet is further homogenized to 1230℃ and held for 80 minutes. The initial rolling temperature is controlled at 1170℃, and the final rolling temperature is controlled at 910℃. After rolling, the billet is air-cooled to room temperature.

[0035] The microstructure of the angle steel in this embodiment is shown below. Figure 1 The main components are ferrite and pearlite, with pearlite exhibiting a uniform free state distribution and an average grain size of 20.4 μm. The typical mechanical properties of the manufactured angle steel are shown in Table 2, with an upper yield strength of 378 MPa, tensile strength of 522 MPa, elongation after fracture of 28.7%, and longitudinal V-shaped impact energy of 97.3 J at 20 ℃. Example 2

[0036] A high-efficiency, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel is disclosed. The chemical composition and mass percentage of each element are shown in Table 1. Scrap steel is smelted in an electric furnace, with the molten pool temperature controlled at 1670℃ and the billet exiting the continuous casting machine at 1150℃. Within 14 minutes of exiting the continuous casting machine, while the billet temperature is still maintained at 900℃, it is directly fed into a heating furnace. The high-temperature billet is further homogenized to 1260℃ and held for 60 minutes. The initial rolling temperature is controlled at 1200℃, and the final rolling temperature is controlled at 945℃. After rolling, the billet is air-cooled to room temperature.

[0037] The microstructure of the angle steel in this embodiment is shown below. Figure 2 The main components are ferrite and pearlite, with pearlite exhibiting a uniform free state distribution and an average grain size of 15 μm. Typical mechanical properties of the manufactured angle steel are shown in Table 2: upper yield strength 402 MPa, tensile strength 538 MPa, elongation after fracture 29.2%, and longitudinal V-shaped impact energy at 20 ℃ 116.6 J. Example 3

[0038] A high-efficiency, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel is disclosed. The chemical composition and mass percentage of each element are shown in Table 1. Scrap steel is smelted in an electric furnace, with the molten pool temperature controlled at 1663℃ and the billet exiting the continuous casting machine at 1127℃. Within 15 minutes of exiting the continuous casting machine, while the billet temperature remains at 894℃, it is directly fed into a heating furnace. The high-temperature billet is further homogenized to 1235℃ and held for 82 minutes. The initial rolling temperature is controlled at 1175℃, and the final rolling temperature is controlled at 915℃. After rolling, the billet is air-cooled to room temperature.

[0039] The microstructure of the angle steel in this embodiment is shown below. Figure 3 The main components are ferrite and pearlite, with pearlite exhibiting a uniform free state distribution and an average grain size of 21.2 μm. The typical mechanical properties of the manufactured angle steel are shown in Table 2, with an upper yield strength of 380 MPa, tensile strength of 525 MPa, elongation after fracture of 30.3%, and longitudinal V-shaped impact energy of 102.3 J at 20 ℃. Example 4

[0040] A high-efficiency, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel is disclosed. The chemical composition and mass percentage of each element are shown in Table 1. Scrap steel is smelted in an electric furnace, with the molten pool temperature controlled at 1665℃ and the billet exiting the continuous casting machine at 1145℃. Within 16 minutes of exiting the continuous casting machine, while the billet temperature is still maintained at 878℃, it is directly fed into a heating furnace. The high-temperature billet is further homogenized to 1240℃ and held for 75 minutes. The initial rolling temperature is controlled at 1190℃, and the final rolling temperature is controlled at 935℃. After rolling, the billet is air-cooled to room temperature.

[0041] The microstructure of the angle steel in this embodiment is shown below. Figure 4 The main components are ferrite and pearlite, with pearlite exhibiting a uniform free state distribution and an average grain size of 16.2 μm. Typical mechanical properties of the manufactured angle steel are shown in Table 2: upper yield strength 388 MPa, tensile strength 532 MPa, elongation after fracture 29.6%, and longitudinal V-shaped impact energy at 20 ℃ 122.3 J. Example 5

[0042] A high-efficiency, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel is disclosed. The chemical composition and mass percentage of each element are shown in Table 1. Scrap steel is smelted in an electric furnace, with the molten pool temperature controlled at 1655℃ and the billet exiting the continuous casting machine at 1100℃. Within 13 minutes of exiting the continuous casting machine, while the billet temperature is still maintained at 870℃, it is directly fed into a heating furnace. The high-temperature billet is further homogenized to 1220℃ and held for 85 minutes. The initial rolling temperature is controlled at 1160℃, and the final rolling temperature is controlled at 900℃. After rolling, the billet is air-cooled to room temperature.

[0043] The microstructure of the angle steel in this embodiment is shown below. Figure 5 The main components are ferrite and pearlite, with pearlite exhibiting a uniform free state distribution and an average grain size of 23 μm. Typical mechanical properties of the manufactured angle steel are shown in Table 2: upper yield strength 355 MPa, tensile strength 490 MPa, elongation after fracture 25.0%, and longitudinal V-shaped impact energy at 20 ℃ 75 J. Comparative Example 1

[0044] This comparative example did not employ micro-Nb alloying. To maintain steel performance, the Mn content was increased, while the C and Si contents were similar to those in Example 1. Specific chemical compositions and their mass fractions are shown in Table 1. The production process was essentially the same as in Example 1 of this invention, specifically utilizing scrap steel for electric furnace smelting. The molten pool temperature was controlled at 1662°C, and the billet exiting the continuous casting machine was at 1126°C. Within 15 minutes of the billet exiting the continuous casting machine, while the billet temperature remained at 893°C, the billet was directly fed into the heating furnace. The high-temperature billet was further homogenized to 1230°C and held for 85 minutes. The initial rolling temperature was controlled at 1170°C, and the final rolling temperature was controlled at 910°C. After rolling, the billet was air-cooled to room temperature.

[0045] The microstructure of the angle steel in this comparative example is shown below. Figure 6 The main components are ferrite and pearlite, with the pearlite exhibiting a uniform free distribution and an average grain size of 27.4 μm. Typical mechanical properties of the manufactured angle steel are shown in Table 2, meeting the requirements of upper yield strength 370 MPa, tensile strength 512 MPa, elongation after fracture 26.0%, and longitudinal V-shaped impact energy ≥77.3 J at 20 °C. Although the mechanical properties of this angle steel are similar to those of Example 1 of this invention, the significantly higher Mn content leads to increased alloy cost, a higher carbon equivalent, a slight decrease in impact toughness, and a tendency to produce central segregation and banded pearlite. Comparative Example 2

[0046] To further compare and verify, this comparative example deviates from the core high-efficiency short-process design of this invention in the production process, but still uses a chemical composition (mass percentage) similar to that of Example 1 of this invention, as shown in Table 1. However, the hot charging-hot delivery stage is changed to conventional cold charging and secondary heating. The specific process is as follows: electric furnace smelting → LF refining → billet protective casting → continuous casting billet is completely cooled to room temperature and then charged into the heating furnace → heated to 1230 ℃ and held for 85 min → hot rolling (initial rolling temperature 1170 ℃, final rolling temperature 910 ℃) → air cooling to room temperature after rolling.

[0047] The microstructure of the angle steel in this comparative example is shown below. Figure 7 The steel mainly consists of ferrite and pearlite, with the pearlite exhibiting a uniform free distribution and an average grain size of 21.2 μm. Typical mechanical properties of the manufactured angle steel are shown in Table 2: upper yield strength 372 MPa, tensile strength 518 MPa, elongation after fracture 27.5%, and longitudinal V-shaped impact energy at 20 °C 92.8 J. Test results indicate that the upper yield strength, tensile strength, and impact toughness of this angle steel are similar to those of Example 1 of this invention, but its energy consumption and production efficiency are significantly inferior to the hot-charging-hot-delivery process. Furthermore, the cold-charging process leads to secondary oxidation of the billet surface, increasing the difficulty of surface quality control and resulting in a higher rate of surface defects in the finished product.

[0048] Table 1 Chemical composition of the examples and comparative examples

[0049] Table 2. Mechanical property test results of the examples and comparative examples

[0050] This invention provides a highly efficient, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel. This method effectively overcomes the problems of high energy consumption, increased iron oxide scale, prolonged production cycle, grain coarsening, and center segregation caused by repeated heating in traditional production processes by employing a hot-charging and hot-delivery process. Simultaneously, through the micro-niobium, low-manganese alloying design, the manganese content is significantly reduced by approximately 0.3% while maintaining the material's strength and toughness. This not only saves alloy costs but also mitigates the drawbacks caused by excessive manganese content, such as increased carbon equivalent, decreased weldability, and fluctuations in mechanical properties.

[0051] The micro-niobium low-manganese angle steel prepared by this invention has an upper yield strength of not less than 355 MPa, a tensile strength of not less than 490 MPa, an elongation after fracture of not less than 25%, and a longitudinal V-shaped impact energy Akv of not less than 75 J at 20℃. All of its properties are superior to the standard requirements of Q355B low alloy angle steel, making it particularly suitable for engineering scenarios such as power transmission towers where both safety and economy are highly demanding.

[0052] In summary, the production method proposed in this invention integrates two key processes: hot charging and hot delivery, and micro-niobium alloying. The continuously cast billet is directly fed into a heating furnace for homogenization and rolling at a high temperature, fully utilizing residual heat, significantly reducing secondary heating energy consumption, shortening the process flow, and minimizing the generation of iron oxide scale. This method achieves dual control of energy consumption and alloy cost while ensuring the strength, toughness, and weldability of angle steel, providing an efficient, stable, and green feasible technical path for industrialized angle steel production.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; if these modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should all be considered to be within the protection scope of the present invention.

Claims

1. A low-alloy angle steel with micro-niobium and low-manganese content, Q355B, characterized in that, By mass percentage, it includes the following chemical components: C 0.14%~0.21%, Si 0.20%~0.45%, Mn 1.00%~1.25%, Nb 0.005%~0.014%, P ≤0.025%, S ≤0.020%, N ≤0.009%, with the balance being Fe and unavoidable impurities.

2. The micro-niobium, low-manganese Q355B low-alloy angle steel according to claim 1, characterized in that, The microstructure is ferrite + pearlite with an average grain size of 15~23μm. Its mechanical properties meet the following requirements: upper yield strength ≥355 MPa, tensile strength ≥490 MPa, elongation after fracture ≥25%, and longitudinal V-shaped impact energy Akv ≥75J at 20 ℃.

3. A highly efficient, short-process production method for micro-niobium, low-manganese Q355B low-alloy angle steel according to any one of claims 1-2, characterized in that, The production process is as follows: (1) Smelting process: Scrap steel is smelted in an electric furnace, and the temperature of the molten pool is controlled at 1655~1670℃; then LF refining and continuous casting are carried out to obtain high temperature billets, and the temperature of the billets exiting the continuous casting machine is ≥1100℃. (2) Hot charging and hot delivery: Within 13 to 16 minutes after the billet leaves the continuous casting machine, when the billet temperature is still maintained at 870 to 900 ℃, the billet is directly sent into the heating furnace; (3) Heating process: The high-temperature billet is further heated to 1220~1260 ℃ and held for 60~85 min; (4) Hot rolling process: The initial rolling temperature is controlled at 1160~1200 ℃, and the final rolling temperature is controlled at 900 ℃ or above; (5) Cooling process: air cool to room temperature after rolling.

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