A method for producing weather-resistant structural steel Q550NH based on an ESP headless rolling production line

Abstract

The application belongs to the technical field of steel production, and provides a method for producing weather-resistant structural steel Q550NH based on an ESP endless rolling production line, which comprises smelting, continuous casting, rough rolling, finish rolling, cooling and coiling; wherein the smelting process forms molten steel, and the molten steel comprises, in terms of mass percentage, 0.025-0.045% of C, 0.10-0.20% of Si, 1.4-1.5% of Mn, ≤0.015% of P, ≤0.003% of S, 0.15-0.25% of Ni, 0.05-0.08% of Ti, 0.40-0.57% of Cr, 0.23-0.33% of Cu, and the rest is iron elements and inevitable impurities. The method can stably produce 1.2-4.0 mm specifications of Q550NH weather-resistant steel with high strength and no cracks in the 180° bending test.

Description

Technical Field

[0001] This invention belongs to the field of steel production technology and relates to a method for producing weathering structural steel Q550NH based on an ESP headless rolling production line. Background Technology

[0002] Q550NH is a high-strength weathering structural steel widely used in railway, marine, and highway transportation. It is a type of weathering steel. Weathering steel is a low-alloy high-strength steel made by adding small amounts of alloying elements such as Cu, P, Cr, and Ni to plain carbon steel, allowing it to form a dense and stable rust layer when exposed to the natural atmosphere. This rust layer (called a "protective rust layer") effectively blocks further corrosion of the substrate by oxygen and moisture, thus significantly slowing down the corrosion rate and achieving long-term weather resistance.

[0003] Weathering steel, as a product that combines functionality, economy, and a wide range of applications, will continue to develop in the future towards high strength, ultra-thinness, and "weathering with heat resistance and cooling resistance".

[0004] Currently, high-efficiency, energy-saving, and green production lines, represented by the ESP (Electronic Stability Program) endless rolling mill production line, are booming. Research on various process technologies and product development based on the ESP production line has always been a research hotspot. To date, there has been considerable research on the development and production of weathering steel, but most of it is limited to traditional hot rolling and CSP (Continuous Processing) production lines, which suffer from issues such as thicker product specifications (2.0mm) and higher energy consumption. However, research on producing weathering steel using the ESP production line is relatively scarce. For example, Chinese patent CN 116479309 A discloses a process for producing 400MPa grade weathering steel based on endless rolling of thin slabs in an electric arc furnace. It adopts a Cu-P-Cr-Ni elemental design scheme, utilizing the inherent Cu, Cr, and Ni content in the molten steel produced by the electric arc furnace, which can save on alloy additions. Furthermore, VOD (Vacuum Degassing) can reduce the nitrogen content in the steel to below 0.005%, enabling stable production of steel with a thickness of 1.2–3 mm. The method produces 400MPa grade weathering steel with a thickness of 0.5mm, but the strength of the weathering steel prepared by this method needs further improvement. Chinese Patent Publication No. CN106367686 A discloses a method for producing weathering steel based on the ESP thin slab continuous casting and rolling process. The raw materials used in this method, by mass percentage, include: 0.01–0.06% C, 0.30–0.60% Si, 0.30–0.60% Mn, 0.05–0.12% P, 0.25–0.45% Cu, 0.30–0.80% Cr, ≤0.30% Ni, ≤0. The raw materials contain 0.01% sulfur, with the remainder being iron. The raw materials are sequentially smelted in a converter and then in an LF furnace. The molten steel from the LF furnace is then processed through an ESP production line to produce hot-rolled strips of varying thicknesses. In the ESP line, the molten steel from the LF furnace enters a continuous casting machine and is cast at a speed of at least 4.5 m / min, with the finishing mill exit temperature at at least 820°C. The hot-rolled strip is then laminar-cooled to 500–600°C and coiled into coils for storage. This method addresses the problems of large thickness and high cost associated with traditional hot rolling and thin-plate continuous casting and rolling processes like CSP for weathering steel. However, the strength of the weathering steel produced by this method needs further improvement. This method does not include optimization studies on the material's plasticity and toughness; that is, a 180° bending test was not conducted, and it is unclear whether the steel possesses good formability.

[0005] In summary, while the ESP production line possesses unique advantages in producing ultra-thin, low-cost products, its application potential in the field of high-strength weather-resistant structural steel (such as Q550NH) has not yet been fully explored. It is of great significance to find ways to obtain high-strength weather-resistant structural steel that passes the 180° bending test without cracking, while achieving thinner thickness and lower cost. Summary of the Invention

[0006] This invention proposes a method for producing weathering structural steel Q550NH based on an ESP headless rolling production line. This method optimizes the steel composition for the ESP production line, and stably produces Q550NH weathering steel with a specification of 1.2 to 4.0 mm, high strength, and no cracks after passing a 180° bending test.

[0007] The technical solution of this invention is implemented as follows:

[0008] A method for producing weathering structural steel Q550NH based on an ESP headless rolling production line, the key aspects of which include smelting, continuous casting, rough rolling, finish rolling, cooling and coiling;

[0009] The smelting process described above produces molten steel, which, by mass percentage, comprises 0.025–0.045% C, 0.10–0.20% Si, 1.4–1.5% Mn, ≤0.015% P, ≤0.003% S, 0.15–0.25% Ni, 0.05–0.08% Ti, 0.40–0.57% Cr, and 0.23–0.33% Cu, with the remainder being iron and unavoidable impurities.

[0010] The carbon mass percentage is between 0.025% and 0.045%, and a low-carbon, low-phosphorus approach is adopted. This is mainly because weather-resistant structural steel has high requirements for weldability, and higher carbon content is detrimental to subsequent processing and welding.

[0011] The improvement in material strength mainly relies on the solid solution strengthening of Mn and the grain refinement and precipitation strengthening of Ti. Mn accounts for 1.4-1.5% of the raw materials and has the effect of solid solution strengthening, which can expand the γ region, reduce the γ→α phase transformation temperature, refine the grains, and facilitate the formation of strengthening phases. Ti accounts for 0.05-0.08% of the raw materials. Ti can refine grains, improve phase transformation behavior, and combine with C and N to form fine carbonitrides, which can also delay recrystallization and prevent ferrite grain growth. Therefore, it has a strong grain refinement strengthening effect and a relatively strong precipitation strengthening effect.

[0012] The low-P+Cu-Ni-Cr composition system can significantly improve the atmospheric corrosion resistance of steel while ensuring mechanical properties. ① The key to the low-P design is to avoid cold brittleness. Although P (0.07%~0.15%) in traditional weathering steel can improve corrosion resistance, it will deteriorate low-temperature toughness. Low P (≤0.015%) can reduce grain boundary segregation and ensure safety in extreme climates or environments. ② Improve weldability. Low P reduces the tendency of the heat-affected zone of the weld to become embrittled and reduces the risk of cracking.

[0013] Preferably, the finishing rolling process includes a five-stand finishing mill unit, wherein the upper and lower work rolls of the first to second stands are concave rolls with a concavity of 0.30 to 0.35 mm, and the upper and lower work rolls of the third to fifth stands are tapered rolls with a concavity of 0.25 to 0.35 mm and a cone height of 0.8 to 1.0 mm.

[0014] Preferably, the continuous casting process involves casting a continuous casting slab using a continuous casting machine, and the thickness of the continuous casting slab is 95–125 mm.

[0015] Preferably, the outlet slab temperature of the continuous casting machine is ≥1030℃.

[0016] Preferably, the casting speed of the continuous casting machine is not less than 4.1 m / min.

[0017] Preferably, the roughing process is carried out using a high-reduction mill, in which the continuously cast billet is rolled into an intermediate plate with a thickness of 8-18 mm. The intermediate plate is then heated in an induction heating furnace at a temperature of 1130-1160°C, and then descaled to obtain rough-rolled strip steel.

[0018] Preferably, the descaling process is performed using a pressure of 350–370 bar.

[0019] Preferably, the finishing rolling process involves rolling the rough-rolled strip into a finished strip at a final rolling temperature of 810–850°C; the thickness of the finished strip is 1.2–4.0 mm.

[0020] Preferably, the cooling process cools the temperature to 640–660°C.

[0021] Preferably, the cooling process uses a front-end cooling mode with a cooling rate of 25-50°C / s.

[0022] The beneficial effects of the present invention using the above technical solution are as follows:

[0023] 1. The method for producing weathering structural steel Q550NH based on an ESP endless rolling production line provided by this invention, through steel composition optimization, can produce 1.2-4.0mm thick weathering steel Q550NH, further expanding the product range of the ESP production line and solving the problems of high cost and thicker specifications compared to traditional production lines. Compared to traditional hot continuous rolling products, the thickness of the specifications has been expanded from 2.0mm to 1.2mm, enabling the production of thinner specifications and broadening downstream application scenarios. Production verification shows that the ESP endless rolling technology can stably produce weathering structural steel Q550NH. We produce 550MPa grade weathering structural steel with a yield strength of 562-613MPa, a tensile strength of 652-707MPa, and an elongation after fracture of 17.5-22%. All of our products passed the 180° bending test without cracking, which proves that the material has both high strength and good formability. The products have stable performance, small dimensional tolerances, and corrosion resistance that meets the standard requirements. Furthermore, through the synergistic optimization of steel composition, finishing roll shape, and reduction rate, we have achieved significant results in ensuring higher strength and better plate shape and dimensional quality.

[0024] 2. In the finishing rolling process of this invention, the upper and lower work rolls of the first to second stands are concave rolls with a concavity of 0.30–0.35 mm, and the upper and lower work rolls of the third to fifth stands are tapered rolls with a concavity of 0.25–0.35 mm and a cone height of 0.8–1.0 mm. The middle part of the roll body is designed as a concave roll, and the ends are designed as tapered rolls, which effectively compensates for the thermal crown generated when the rolls are heated, improves the uniformity of roll wear, enhances the strip deviation control capability, and stably controls the strip crown. At the same time, it maintains the straightness of the strip as needed, thereby giving the finishing mill a larger range and capability for shape adjustment. This results in stable control of the strip crown C40 within 15–25 μm, and the wedge shape within ±10 μm, further reducing shape defects. The crown hit rate is increased from 92.3–93.0% to 97.6–98.5%, achieving stable rolling of thin specifications.

[0025] 3. The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line provided by this invention adopts a thin slab continuous casting machine with a continuous casting outlet slab temperature ≥1030℃. It makes full use of the heat stored inside the slab and directly rolls it into intermediate plates in a large reduction roughing mill, reducing the load on the roughing mill and further saving the energy consumption of the roughing mill. It achieves the goal of energy saving and consumption reduction by "replacing cold with heat", which can solve the problems of high cost and high energy consumption of traditional hot rolling production lines, realize the substitution of similar products, and has high social and economic benefits. Attached Figure Description

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0027] Figure 1This is a metallographic microstructure diagram of Example 3 of the method for producing weathering structural steel Q550NH based on the ESP headless rolling production line of the present invention.

[0028] Figure 2 This is a schematic diagram of the work roll shape of a method for producing weathering structural steel Q550NH based on an ESP headless rolling production line according to the present invention.

[0029] In the diagram, A: a schematic diagram of a conical roller; B: a schematic diagram of a concave roller. In Figure A, segment ab represents a straight segment, segment bc represents a concave segment (also called a parabolic segment), segment cd represents a conical segment, H1 represents concavity, H2 represents cone height (i.e., the horizontal height between points cd), L1 represents the length of the straight segment, L2 represents the distance between points bc in the concave segment (also called the concave segment length), and L3 represents the horizontal distance between points cd in the conical segment (also called the conical segment length). In Figure B, segment ab represents a straight segment, segment bc represents a concave segment, segment cd represents a straight segment, H1 represents concavity, L1 represents the length of the straight segment, L2 represents the distance between points bc in the concave segment (also called the concave segment length), and L3 represents the horizontal distance between points cd in the conical segment. Detailed Implementation

[0030] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0031] It should be noted that ESP (Endless Strip Production) is a type of thin slab continuous casting and rolling production line that can produce a whole strip of steel in one casting cycle without frequent strip threading, thus making it a fully continuous endless strip production line.

[0032] This invention discloses a method for producing weathering structural steel Q550NH based on an ESP endless rolling mill production line, comprising smelting, continuous casting of slabs, rough rolling, finish rolling, cooling, and coiling. Specifically, it includes the following steps:

[0033] A. Smelting: Smelting produces molten steel, which, by mass percentage, consists of 0.025–0.045% C, 0.10–0.20% Si, 1.4–1.5% Mn, ≤0.015% P, ≤0.003% S, 0.15–0.25% Ni, 0.05–0.08% Ti, 0.40–0.57% Cr, and 0.23–0.33% Cu, with the remainder being iron and unavoidable impurities.

[0034] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 95-125mm. The slab temperature at the outlet of the continuous casting machine is ≥1030℃ and the casting speed of the continuous casting machine is not less than 4.1m / min.

[0035] C. Rough Rolling: The billet then proceeds directly to a three-stand high-reduction rolling mill for rough rolling. The rough rolling zone employs a high reduction rate, with the cumulative reduction across the three stands set at 84-92%. This ensures grain elongation, crushing, and rapid nucleation in the austenite region, refining the austenite microstructure. In the high-reduction mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 8-18 mm. A shearing platform is located after the high-reduction mill for removing the sprue rod and for cutting off the intermediate plate in case of problems with subsequent equipment. Heated in an induction heating furnace at 1130–1160℃, the surface iron oxide scale is removed by high-pressure water descaling, which maintains a pressure of 350–370 bar throughout the descaling process, resulting in rough-rolled strip steel. It should be noted that the ESP production line is a rigid connection between the continuous casting machine and the rolling mill. The production line makes full use of the residual heat of the continuously cast slab for rough rolling, eliminating the need for the gas-fired heating furnace of the traditional rolling line, thus making full use of the slab's residual heat for rolling and saving energy consumption in the rough rolling mill.

[0036] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip; the thickness of the finished strip obtained in the finishing rolling process is 1.2-4.0 mm, and the final rolling temperature is 810-850℃; the finishing rolling process includes a five-stand finishing mill unit, the upper and lower work rolls of the first to second stands are concave rolls with a concavity of 0.30-0.35 mm, and the upper and lower rolls of the work rolls of the third to fifth stands are tapered rolls with a concavity of 0.25-0.35 mm and a cone height of 0.8-1.0 mm;

[0037] It should be noted that the middle part of the roll body is designed as a concave roll, and the end is designed as a tapered roll. The main advantages are that it effectively compensates for the thermal crown generated when the roll is heated, improves the uniformity of roll wear, enhances the strip deviation control capability, and stably controls the strip crown. At the same time, it maintains the straightness of the strip as needed, thereby giving the finishing mill a larger range and capability for shape adjustment. This allows for stable control of the strip crown C40 within 15 to 25 μm, and the wedge shape within ±10 μm, further reducing shape defects. The crown hit rate is increased from 92.3 to 93.0% to 97.6 to 98.5%, achieving stable rolling of thin specifications.

[0038] It should be noted that, to ensure stability during the finishing rolling process and reduce vibration and steel piling accidents caused by excessive reduction rates or overload in individual stands, maximum reduction rates are set for each stand: F1≤60%, F2≤45%, F3≤41%, F4≤35%, and F5≤25%. In this invention, composition design is fundamental. The low-carbon and low-phosphorus composition system fundamentally guarantees the excellent weldability and low-temperature toughness of the steel, reducing the risk of cracking due to compositional segregation, and laying the foundation for subsequent stable rolling and high yield. The concave rolls of the front stand (concavity 0.30–0.35 mm) resist roll bending caused by the huge rolling force and ensure stable strip threading. The tapered rolls of the rear stand (concavity 0.25–0.35 mm, cone height 0.8–1.0 mm) provide dynamic shape control capabilities. Combined with the decreasing reduction rate design of stands F1–F5, they jointly ensure the extreme stability of the rolling process. Ultimately, the combination of these three factors effectively prevents production accidents such as steel piling, rolling defects, and deviation, reducing unplanned downtime. Simultaneously, it ensures consistent thickness and shape accuracy throughout the entire product length. The low reduction rate of the final stand, combined with the tapered rolls (F5≤25%), avoids rolling vibration in the last stand, eliminating periodic vibration marks and surface quality defects. The excellent shape control capability of the tapered rolls (stabilizing the strip crown C40 at 15~25μm) ensures strip flatness, preventing secondary defects such as creases and indentations caused by waviness and warping during cooling and coiling. This invention, through the synergy of composition, roll shape, and reduction rate, stably produces Q550NH weathering steel with an ultra-thin 1.2mm specification and higher strength and better dimensional quality, resulting in significant overall economic benefits.

[0039] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to a coiling temperature of 640–660°C using a laminar flow cooling pre-cooling mode, and finally coiled into a hot-rolled coil by an underground coiler. It should be noted that this invention, combined with a low-carbon steelmaking design, sets a relatively low coiling temperature of 640–660°C and a relatively fast cooling rate of 25–50°C / s. Furthermore, by combining the laminar flow cooling model, a uniform and fine microstructure can be obtained at a lower carbon content, thereby achieving the strength standards required by national standards.

[0040] Example 1

[0041] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0042] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0043] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 115mm. The slab temperature at the outlet of the continuous casting machine is 1040℃ and the casting speed of the continuous casting machine is 4.1m / min.

[0044] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 12mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1150℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 350 bar to obtain the rough-rolled strip.

[0045] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip; the thickness of the finished strip obtained in the finishing rolling process is 2.0 mm, and the final rolling temperature is 826℃; the finishing rolling process includes a five-stand finishing mill unit, the upper and lower work rolls of the first and second stands are concave rolls with a concavity of 0.30 mm, and the upper and lower rolls of the work rolls of the third to fifth stands are tapered rolls with a concavity of 0.25 mm and a cone height of 0.8 mm; the reduction rates of each stand are set as follows: F1: 43%, F2: 33%, F3: 29%, F4: 25%, F5: 18%;

[0046] Specifically, such as Figure 2 -A represents a schematic diagram of a conical roller, where segment ab represents the straight section, segment bc represents the concave section (also called the parabolic segment), segment cd represents the conical section, H1 represents the concavity, H2 represents the cone height (the horizontal height between points cd), L1 represents the length of the straight section, L2 represents the distance between points bc in the concave section (also called the concave section length), and L3 represents the horizontal distance between points cd in the conical section (also called the conical section length). The length of the straight section L1 is 210 mm, the length of the concave section L2 is 1330 mm, and the length of the conical section L3 is 660 mm.

[0047] like Figure 2 -B represents a schematic diagram of the concave roll shape, where segment ab represents the straight section, segment bc represents the concave section, segment cd represents the straight section, H1 represents the concavity, L1 represents the length of the straight section, L2 represents the distance between the concave sections bc (also called the concave section length), and L3 represents the horizontal distance between the cd points of the conical section. The concave section length L2 is 1330mm, and the straight section length L1=L2=310mm. It should be noted that the diagram is for illustrative purposes only, and the actual dimensions can be adjusted by those skilled in the art based on the mill width. Preferably, the lengths of the straight section, concave section, and conical section of the concave and conical rolls are optimized according to the frame dimensions, with the straight section length being 200-220mm, the concave section length being 1300-1350mm, and the conical section length being 650-670mm.

[0048] E. When the strip steel exits the last finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 640℃ through laminar flow cooling, with a front-end cooling mode and a cooling rate of 25℃ / s. Finally, it enters the coiler to be coiled and then the product is put into storage.

[0049] Product performance is shown in Table 3.

[0050] Example 2

[0051] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0052] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0053] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 95mm. The slab temperature at the outlet of the continuous casting machine is 1045℃ and the casting speed of the continuous casting machine is 4.1m / min.

[0054] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 10mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1130℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 350 bar to obtain the rough-rolled strip.

[0055] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip. The thickness of the finished strip obtained in the finishing rolling process is 1.5 mm, and the final rolling temperature is 820℃. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first to second stands are concave rolls with a concavity of 0.30 mm. The upper and lower work rolls of the third to fifth stands are conical rolls with a concavity of 0.25 mm and a cone height of 0.9 mm. Specifically, the straight section length L1 of the conical roll is 210 mm, the concave section length L2 is 1330 mm, and the cone section length L3 is 660 mm. The concave section length L2 of the concave roll is 1330 mm, and the straight section length L1 = L2 = 310 mm. The reduction rates for each stand are set as follows: F1: 45%, F2: 37%, F3: 28%, F4: 26%, F5: 19%.

[0056] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 650℃ using laminar flow cooling, with a front-end cooling mode and a cooling rate of 30℃ / s. Finally, it enters the coiler to form a coil, and the product is then stored in the warehouse. Product performance is shown in Table 3.

[0057] Example 3

[0058] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0059] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0060] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 115mm. The slab temperature at the outlet of the continuous casting machine is 1050℃ and the casting speed of the continuous casting machine is 4.2m / min.

[0061] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 8mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1150℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 360 bar to obtain the rough-rolled strip.

[0062] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip. The finished strip obtained in the finishing rolling process has a thickness of 1.2 mm and a final rolling temperature of 830℃. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first to second stands are concave rolls with a concavity of 0.30 mm. The upper and lower work rolls of the third to fifth stands are conical rolls with a concavity of 0.30 mm and a cone height of 0.8 mm. Specifically, the straight section length L1 of the conical roll is 210 mm, the concave section length L2 is 1330 mm, and the cone section length L3 is 660 mm. The concave section length L2 of the concave roll is 1330 mm, and the straight section length L1 = L2 = 310 mm. The reduction rates for each stand are set as follows: F1: 46%, F2: 37%, F3: 29%, F4: 24%, F5: 18%.

[0063] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 640℃ using laminar flow cooling, with a front-stage cooling mode at a cooling rate of 50℃ / s. Finally, it enters the coiler for coiling, and the product is then stored. Product performance is shown in Table 3. Product metallographic structure is shown in Table 4. Figure 1 .

[0064] Example 4

[0065] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0066] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0067] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 115mm. The slab temperature at the outlet of the continuous casting machine is 1030℃ and the casting speed of the continuous casting machine is 4.2m / min.

[0068] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 8mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1150℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 360 bar to obtain the rough-rolled strip.

[0069] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip. The thickness of the finished strip obtained in the finishing rolling process is 1.2 mm, and the final rolling temperature is 820℃. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first and second stands are concave rolls with a concavity of 0.35 mm. The upper and lower work rolls of the third to fifth stands are conical rolls with a concavity of 0.35 mm and a cone height of 1.0 mm. Specifically, the straight section length L1 of the conical roll is 210 mm, the concave section length L2 is 1330 mm, and the cone section length L3 is 660 mm. The concave section length L2 of the concave roll is 1330 mm, and the straight section length L1 = L2 = 310 mm. The reduction rates for each stand are set as follows: F1: 47%, F2: 36%, F3: 28%, F4: 26%, F5: 17%.

[0070] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 640℃ using laminar flow cooling, with a front-end cooling mode and a cooling rate of 50℃ / s. Finally, it enters the coiler to form a coil, and the product is then stored in the warehouse. Product performance is shown in Table 3.

[0071] Example 5

[0072] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0073] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0074] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 105mm. The slab temperature at the outlet of the continuous casting machine is 1040℃ and the casting speed of the continuous casting machine is 4.2m / min.

[0075] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 13mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1160℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 350 bar to obtain the rough-rolled strip.

[0076] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip. The thickness of the finished strip obtained in the finishing rolling process is 2.1 mm, and the final rolling temperature is 825℃. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first and second stands are concave rolls with a concavity of 0.30 mm. The upper and lower work rolls of the third to fifth stands are conical rolls with a concavity of 0.25 mm and a cone height of 0.9 mm. Specifically, the straight section length L1 of the conical roll is 210 mm, the concave section length L2 is 1330 mm, and the cone section length L3 is 660 mm. The concave section length L2 of the concave roll is 1330 mm, and the straight section length L1 = L2 = 310 mm. The reduction rates for each stand are set as follows: F1: 43%, F2: 37%, F3: 29%, F4: 25%, and F5: 15.5%.

[0077] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 650℃ using laminar flow cooling, with a front-end cooling mode and a cooling rate of 25℃ / s. Finally, it enters the coiler to form a coil, and the product is then stored in the warehouse. Product performance is shown in Table 3.

[0078] Example 6

[0079] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0080] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0081] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 115mm. The slab temperature at the outlet of the continuous casting machine is 1040℃ and the casting speed of the continuous casting machine is 4.2m / min.

[0082] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 18mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1160℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 370 bar to obtain the rough-rolled strip.

[0083] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip. The thickness of the finished strip obtained in the finishing rolling process is 3.0 mm, and the final rolling temperature is 810℃. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first to second stands are concave rolls with a concavity of 0.35 mm. The upper and lower rolls of the work rolls of the third to fifth stands are conical rolls with a concavity of 0.35 mm and a cone height of 0.8 mm. Specifically, the straight section length L1 of the conical roll is 210 mm, the concave section length L2 is 1330 mm, and the cone section length L3 is 660 mm. The concave section length L2 of the concave roll is 1330 mm, and the straight section length L1 = L2 = 310 mm. The reduction rates of each stand are set as follows: F1: 42%, F2: 35%, F3: 28%, F4: 25%, F5: 18%.

[0084] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 660℃ using laminar flow cooling, with a front-end cooling mode and a cooling rate of 26℃ / s. Finally, it enters the coiler to form a coil, and the product is then stored in the warehouse. Product performance is shown in Table 3.

[0085] Example 7

[0086] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0087] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0088] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 115mm. The slab temperature at the outlet of the continuous casting machine is 1050℃ and the casting speed of the continuous casting machine is 4.2m / min.

[0089] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 14mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1160℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 360bar throughout the descaling process to obtain the rough-rolled strip.

[0090] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip. The thickness of the finished strip obtained in the finishing rolling process is 2.3 mm, and the final rolling temperature is 850℃. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first to second stands are concave rolls with a concavity of 0.30 mm. The upper and lower work rolls of the third to fifth stands are conical rolls with a concavity of 0.30 mm and a cone height of 0.9 mm. Specifically, the straight section length L1 of the conical roll is 210 mm, the concave section length L2 is 1330 mm, and the cone section length L3 is 660 mm. The concave section length L2 of the concave roll is 1330 mm, and the straight section length L1 = L2 = 310 mm. The reduction rates for each stand are set as follows: F1: 42%, F2: 35%, F3: 30%, F4: 26%, F5: 16%.

[0091] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 640℃ using laminar flow cooling, with a front-end cooling mode and a cooling rate of 27℃ / s. Finally, it enters the coiler to form a coil, and the product is then stored in the warehouse. Product performance is shown in Table 3.

[0092] Example 8

[0093] The method for producing weathering structural steel Q550NH based on the ESP headless rolling production line specifically includes the following steps:

[0094] A. Smelting: Smelting to form molten steel, the composition of which, by mass percentage, includes the elements listed in Table 1 below, with the remainder being iron and unavoidable impurities;

[0095] B. Continuous casting: The continuous casting slab is cast by a high-speed continuous casting machine. The thickness of the continuous casting slab is 115mm. The slab temperature at the outlet of the continuous casting machine is 1050℃ and the casting speed of the continuous casting machine is 4.2m / min.

[0096] C. Rough Rolling: The strip then goes directly to a three-stand high-reduction rolling mill for rough rolling. In the high-reduction rolling mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 15mm. A sway shear scrap pusher is arranged after the high-reduction rolling mill for removing the dummy bar and cutting off the intermediate plate when problems occur with subsequent equipment. The intermediate plate is then heated in an induction heating furnace at a temperature of 1160℃. After removing the surface iron oxide scale by high-pressure water descaling, the high-pressure water descaling is maintained at a pressure of 360 bar to obtain the rough-rolled strip.

[0097] D. Finishing Rolling: The rough-rolled strip is finished by finishing rolling to obtain finished strip. The finished strip obtained in the finishing rolling process has a thickness of 2.5 mm and a final rolling temperature of 816℃. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first and second stands are concave rolls with a concavity of 0.30 mm. The upper and lower work rolls of the third to fifth stands are conical rolls with a concavity of 0.30 mm and a cone height of 1.0 mm. Specifically, the straight section length L1 of the conical roll is 210 mm, the concave section length L2 is 1330 mm, and the cone section length L3 is 660 mm. The concave section length L2 of the concave roll is 1330 mm, and the straight section length L1 = L2 = 310 mm. The reduction rates for each stand are set as follows: F1: 44%, F2: 36%, F3: 28%, F4: 22%, F5: 17%.

[0098] E. When the strip exits the final finishing mill at the required final rolling temperature, it is cooled to the coiling temperature of 650℃ using laminar flow cooling, with a front-end cooling mode and a cooling rate of 40℃ / s. Finally, it enters the coiler to form a coil, and the product is then stored in the warehouse. Product performance is shown in Table 3.

[0099] Table 1

[0100]

[0101] Comparative Example 1

[0102] Compared with Example 3, the only difference is that the composition of the molten steel is different. By mass percentage, the composition of the molten steel includes the elements listed in Table 2 below, with the remainder being iron and unavoidable impurities.

[0103] Comparative Example 2

[0104] Compared with Example 3, the only difference is that the composition of the molten steel is different. By mass percentage, the composition of the molten steel includes the elements listed in Table 2 below, with the remainder being iron and unavoidable impurities.

[0105] Comparative Example 3

[0106] Compared with Example 3, the only difference is that the composition of the molten steel is different. By mass percentage, the composition of the molten steel includes the elements listed in Table 2 below, with the remainder being iron and unavoidable impurities.

[0107] Comparative Example 4

[0108] Compared with Example 3, the only difference is that the composition of the molten steel is different. By mass percentage, the composition of the molten steel includes the elements listed in Table 2 below, with the remainder being iron and unavoidable impurities.

[0109] Comparative Example 5

[0110] Compared with Example 3, the only difference is that the concavity and cone height of the conical roller are different, with a concavity of 0.15 mm and a cone height of 0.7 mm.

[0111] Comparative Example 6

[0112] Compared with Example 3, the only difference is that the concavity and cone height of the conical roller are different, with a concavity of 0.45 mm and a cone height of 0.6 mm.

[0113] Table 2

[0114]

[0115] Table 3

[0116]

[0117] As can be seen from the above embodiments and comparative examples, the mechanical properties of the weathering steel produced according to the composition and parameters of this invention meet the requirements of GB / T 4171-2008 "Weathering Structural Steel". Verification shows that the weathering steel produced by this invention has a yield strength of 562-613 MPa, a tensile strength of 652-707 MPa, and an elongation of 17.5-22%. The bending test and plate crown all meet the requirements. However, the comparative examples, due to differences in composition and roll shape parameters, exhibited unusual strength, elongation, and crown, failing to meet product requirements. Furthermore, testing shows that the weathering steel produced in Examples 1-8 of this invention has corrosion resistance that meets the requirements of GB / T 4171-2008 "Weathering Structural Steel".

[0118] It should be noted that the yield strength, tensile strength, and elongation (elongation after fracture) in this invention are tested in accordance with GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature", and the 180° bending test is tested in accordance with GB / T232-2010 "Metallic materials - Bending test method".

[0119] The crown hit rate described in this invention refers to the percentage of all crown sampling points whose C40 values ​​fall within the target tolerance range along the strip length direction during the statistical period. The calculation formula is: Crown Hit Rate = (Number of hit points in total strip length / Number of total strip length inspection points) × 100%. C40 represents the difference between the thickness at the center of the strip width and the thickness at a distance of 40mm from the edge. For example, in Example 1, the crown hit rate is 98.4%, indicating that 98.4% of the inspection points along the entire rolling length of a strip have a C40 value falling within the 15-25μm range. Specifically, the sampling frequency is: longitudinal: once every 10m. Single-point C40 calculation: for each set of cross-sectional data, C40 = h... center -h 40 , where h center The average thickness within ±8mm from the center; h 40 The average thickness is 40mm from both sides.

[0120] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing weathering structural steel Q550NH based on an ESP headless rolling production line, characterized in that, This includes smelting, continuous casting, roughing, finishing, cooling, and coiling; The smelting process described above produces molten steel, which, by mass percentage, comprises 0.025–0.045% C, 0.10–0.20% Si, 1.4–1.5% Mn, ≤0.015% P, ≤0.003% S, 0.15–0.25% Ni, 0.05–0.08% Ti, 0.40–0.57% Cr, and 0.23–0.33% Cu, with the remainder being iron and unavoidable impurities. The finishing rolling process includes a five-stand finishing mill unit. The upper and lower work rolls of the first and second stands are concave rolls with a concavity of 0.30–0.35 mm. The upper and lower work rolls of the third to fifth stands are tapered rolls with a concavity of 0.25–0.35 mm and a cone height of 0.8–1.0 mm. The maximum reduction rate for each stand is set as follows: F1≤60%, F2≤45%, F3≤41%, F4≤35%, F5≤25%. The continuous casting process involves casting a continuous casting slab using a continuous casting machine. The thickness of the continuous casting slab is 115 mm. The cooling process uses a front-end cooling mode with a cooling rate of 25–50 °C / s. The roughing process is carried out using a high-reduction mill. In the high-reduction mill, the continuously cast billet is rolled into an intermediate plate with a thickness of 8-18 mm. Then, the intermediate plate is heated in an induction heating furnace at a temperature of 1130-1160°C. After descaling, the rough-rolled strip is obtained. The finishing rolling process involves rolling the rough-rolled strip into a finished strip at a final rolling temperature of 810–850°C; the thickness of the finished strip is 1.2–4.0 mm.

2. The method for producing weathering structural steel Q550NH based on an ESP headless rolling production line according to claim 1, characterized in that, The outlet slab temperature of the continuous casting machine is ≥1030℃.

3. The method for producing weathering structural steel Q550NH based on an ESP headless rolling production line according to claim 1, characterized in that, The casting speed of the continuous casting machine shall not be less than 4.1 m / min.

4. The method for producing weathering structural steel Q550NH based on an ESP headless rolling production line according to claim 1, characterized in that, The descaling process is performed using a pressure of 350–370 bar.

5. A method for producing weathering structural steel Q550NH based on an ESP headless rolling production line according to claim 1, characterized in that, The cooling process cools the temperature to 640–660°C.

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