Hot continuous rolling weather-resistant steel plate with yield strength of 550MPa and manufacturing method of hot continuous rolling weather-resistant steel plate

Abstract

The invention discloses a hot continuous rolling weather-resistant steel plate with yield strength of 550 MPa and a manufacturing method thereof, and belongs to the technical field of weather-resistant steel plates, the weather-resistant steel plate comprises the following chemical components: C, Si, Mn, Cr, Ni, Cu, Alt, Sb, Nb and Ca; the thickness specification of the hot continuous rolling weather-resistant steel plate is 16-23 mm, the metallographic structures of the surface, the 1 / 4 thickness and the core of the hot continuous rolling weather-resistant steel plate are ferrite, granular bainite and pearlite, the problems of high strengthening of the hot continuous rolling weather-resistant steel plate and toughness control at the extremely low temperature of-60 DEG C are solved, and meanwhile the hot continuous rolling weather-resistant steel plate has excellent corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the technical field of weathering steel plates, and relates to a hot-rolled weathering steel plate with a yield strength of 550MPa and its manufacturing method. Specifically, it relates to a thick-gauge high-toughness hot-rolled weathering steel plate with excellent corrosion resistance and its manufacturing method. Background Technology

[0002] Weathering steel, also known as atmospheric corrosion resistant steel, has excellent resistance to atmospheric corrosion, slows down the corrosion rate of steel materials in atmospheric environments, and extends their service life. It is widely used in the manufacture of railway vehicles, containers, bridges, buildings, outdoor towers, photovoltaic supports, etc.

[0003] In recent years, with the continuous expansion of applications for weathering steel, the demand for its comprehensive properties, such as high strength, high toughness, and high plasticity, has been constantly upgrading. However, as the thickness of the steel plate increases, the cooling rate during laminar cooling after rolling is relatively low, easily forming coarse microstructures, which restricts the improvement of the steel plate's strength. Furthermore, the cooling rate is uneven along the thickness direction of the steel plate, with a faster cooling rate on the surface and a slower cooling rate in the core, resulting in uneven microstructure along the thickness direction of thick-gauge weathering steel plates, poor low-temperature toughness, and large fluctuations in impact absorption energy. Especially when using hot continuous rolling processes to produce thick-gauge steel plates, the limited compression ratio of the cast billet makes it even more difficult to produce thick-gauge weathering steel plates that combine high strength and high toughness. Developing thick-gauge high-strength weathering steel with excellent low-temperature toughness at extremely low temperatures of -60℃ presents an even greater challenge.

[0004] Chinese patent CN110565024A discloses a niobium-titanium 550MPa grade thick weathering steel and its production method. Its chemical composition by weight percentage is: C: 0.05–0.07%, Si: 0.25–0.35%, Mn: 1.65–1.80%, P≤0.018%, S≤0.007%, Cu: 0.20–0.30%, Cr: 0.32–0.42%, Ni: 0.12–0.20%, Nb: 0.065–0.075%, Ti: 0.020–0.035%, N≤0.006%, Als: 0.010–0.045%, with the balance being Fe and unavoidable impurities. The patented method is applicable to thicknesses of 12–16 mm, employs sparse cooling (slow cooling rate), and has a coiling temperature of 560–600℃. Strength is improved by adding relatively high amounts of Mn and Nb, resulting in high manufacturing costs.

[0005] Chinese patent CN110616375A discloses a 550MPa grade thick weathering steel containing niobium and vanadium and its production method. The chemical composition by weight percentage is: C: 0.07–0.10%, Si: 0.35–0.45%, Mn: 1.45–1.60%, P≤0.018%, S≤0.007%, Cu: 0.20–0.30%, Cr: 0.32–0.42%, Ni: 0.12–0.20%, V: 0.04–0.06%, Nb: 0.045–0.060%, Als: 0.015–0.050%, with the balance being Fe and unavoidable impurities. The scheme described in this patent is suitable for thicknesses of 7–10 mm, and the addition of relatively high contents of Mn, Nb, and V results in higher manufacturing costs.

[0006] Chinese patent CN110592486A discloses a niobium-containing 550MPa grade thick weathering steel and its production method. Its chemical composition by weight percentage is: C: 0.06–0.08%, Si: 0.25–0.35%, Mn: 1.55–1.70%, P≤0.018%, S≤0.007%, Cu: 0.20–0.30%, Cr: 0.32–0.42%, Ni: 0.12–0.20%, Nb: 0.055–0.065%, Als: 0.015–0.050%, with the balance being Fe and unavoidable impurities. The patented method is applicable to thicknesses of 12–16 mm, employs sparse cooling (slow cooling rate), and has a coiling temperature of 560–600℃. The addition of relatively high amounts of Mn and Nb increases strength, resulting in higher manufacturing costs. Summary of the Invention

[0007] To solve the above-mentioned technical problems, the present invention provides a hot-rolled weathering steel plate with a yield strength of 550MPa and its manufacturing method. The thickness of the hot-rolled weathering steel plate is 16~23mm, and its surface, 1 / 4 thickness and core microstructure are all ferrite, granular bainite and pearlite. This solves the problem of high strength and toughness control at extremely low temperatures of -60℃ for hot-rolled thick weathering steel plates. At the same time, the hot-rolled weathering steel plate has excellent corrosion resistance.

[0008] The technical solution adopted in this invention is as follows:

[0009] This invention provides a hot-rolled weathering steel plate with a yield strength of 550 MPa. The hot-rolled weathering steel plate with a yield strength of 550 MPa comprises the following chemical composition by weight percentage: C: 0.045%–0.065%, Si: 0.30%–0.50%, Mn: 0.95%–1.25%, P: ≤0.012%, S: ≤0.0020%, Cr: 0.80%–1.00%, Ni: 0.07%–0.15%, Cu: 0.25%–0.40%, Alt: 0.015%–0.030%, Sb: 0.015%–0.030%, Nb: 0.030%–0.050%, Ca: 0.0010%–0.0025%, O ≤50 ppm, N ≤50 ppm, with the remainder being Fe and unavoidable impurity elements.

[0010] The surface, quarter-thickness, and core microstructure of the hot-rolled weathering steel plate with a yield strength of 550 MPa consist of ferrite, granular bainite, and pearlite. The pearlite is uniformly fine and dispersed around the ferrite matrix, without pearlite bands, and its proportion is low, less than 5% by volume. The microstructure is uniform in the thickness direction, with uniformly distributed Mao island particles in the granular bainite, maintaining high strength while also ensuring good plasticity and toughness. Pearlite consists of alternating lamellar ferrite and cementite. Because cementite is hard and brittle, lamellar cementite distribution leads to rapid crack propagation, reducing the material's toughness. Therefore, this invention patent controls the uniform distribution of pearlite and its small size (less than 5% by volume) through chemical composition, smelting process, and controlled rolling and cooling processes, improving the material's toughness and avoiding the formation of banded structures that reduce toughness.

[0011] The thickness of the hot-rolled weathering steel plate with a yield strength of 550MPa is 16~23mm; the yield strength of the hot-rolled weathering steel plate with a yield strength of 550MPa ≥ 550MPa, tensile strength ≥ 650MPa, and A 50 ≥40%, KV2≥270J at -30℃, KV2≥200J at -60℃, and no cracks when cold-bent at 180° with d=0.5a.

[0012] The present invention also provides a method for manufacturing the hot-rolled weathering steel plate with a yield strength of 550MPa, the method comprising the following processes: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → hot rolling → laminar flow cooling → coiling.

[0013] In the converter smelting step, the converter tapping temperature is 1660-1680℃.

[0014] In the LF refining step, calcium wire is fed in to promote the spheroidization of non-metallic inclusions. The weak stirring time before and after feeding calcium wire is ≥13min, which promotes the non-metallic inclusions to float to the steel slag and improves the cleanliness of molten steel.

[0015] In the continuous casting process, the temperature of the molten steel in the tundish is 1530~1550℃, and argon gas is blown through the stopper rod to prevent air from contacting the molten steel. The argon gas flow rate is controlled at 5~8 L / min. The molten steel calming time is ≥13min, and the liquid surface fluctuation is ≤±3mm to prevent non-metallic inclusions from being drawn into the molten steel. The continuous casting adopts dynamic light pressure and electromagnetic stirring, and the secondary cooling section adopts intermediate cooling.

[0016] After the billet obtained from continuous casting is cut, it is stacked in the heat preservation pit and slowly cooled to ≤200℃, with a cooling rate of ≤7℃ / h, in order to promote the uniform diffusion of alloying elements inside the billet and improve the internal quality of the billet. The billet thickness is 230~250mm.

[0017] In the slab heating step, the temperature of the soaking zone in the heating furnace is 1160℃~1220℃, the holding time in the soaking zone is 30~40min, and the slab is in the furnace for 150~180min. This promotes the full solid solution of alloying elements and the homogenization of austenite. Excessively high soaking zone temperature or excessively long furnace time will lead to coarsening of the original austenite grains during heating. Since the microstructure is hereditary, this is not conducive to refining the grain size after phase transformation, thus reducing the low-temperature toughness of the material. Conversely, excessively low soaking zone temperature or excessively short holding time will result in insufficient solid solution of microalloying elements within the slab, reducing the precipitation strengthening and grain refinement effects of microalloying elements, and ultimately decreasing the strength and toughness of the material.

[0018] In the hot continuous rolling process, after the slab exits the heating furnace, it first undergoes high-pressure water descaling and width setting via a side press, followed by rolling. The roughing stage employs two reversible mills, each with three passes, reducing the number of roughing passes, increasing the single-pass reduction rate, and promoting the recovery and recrystallization of deformed austenite grains, thereby refining the austenite grain size. The roughing stage rolling temperature is ≥1050℃, and the pass reduction rate is ≥20%; when the steel plate thickness is 16mm ≤ ≤ 18mm, the intermediate slab thickness is 60mm; when the steel plate thickness is 18mm ≤ ≤ 20mm, the intermediate slab thickness is 64mm; and when the steel plate thickness is 20mm ≤ ≤ 23mm, the intermediate slab thickness is 68mm.

[0019] In the hot continuous rolling process, the finishing stage employs a seven-stand four-high mill for continuous rolling. The initial rolling temperature is ≤1000℃, and the rolling is performed in the non-recrystallized austenite region. Through cumulative large deformation, the deformation bands and dislocation density within the austenite grains are increased, thereby increasing the phase deformation nuclei and refining the ferrite grains. The final rolling temperature is 800–850℃. If the final rolling temperature is higher than 850℃, the austenite grains coarsen, reducing the material's strength and low-temperature toughness. If the final rolling temperature is lower than 800℃, rolling occurs in the two-phase region of austenite and ferrite, which easily leads to mixed crystal formation and reduces the material's low-temperature toughness.

[0020] In the laminar flow cooling step, the steel plate is rapidly cooled at a cooling rate of 20-35℃ / s after exiting the finishing mill.

[0021] In the coiling step, when the steel plate thickness is 16mm ≤ thickness < 19mm, the coiling temperature is 450~500℃; when the steel plate thickness is 19mm ≤ thickness < 23mm, the coiling temperature is 400~450℃. Coiling temperatures above 500℃ coarsen the ferrite grain size and precipitated two-phase particles, resulting in insufficient material strength and low-temperature toughness. Coiling temperatures below 400℃ generate a large amount of granular bainite, leading to excessively high material strength and reduced steel plasticity. Moreover, for thick-gauge high-strength weather-resistant steel plates, excessively low coiling temperatures result in excessively high steel plate strength, high deformation resistance, difficulty in coiling by the coiling machine, and easy unwinding of the hot-rolled coil.

[0022] The roles and controls of each chemical component in the hot-rolled weathering steel plate with a yield strength of 550MPa provided by this invention are as follows:

[0023] Carbon (C): C is the most economical element for improving the strength of steel, and it has a significant interstitial solid solution strengthening effect. However, it also causes strong lattice distortion, significantly deteriorating the low-temperature toughness of steel. If the C content is too low, other alloying elements need to be added to improve the strength of the material, increasing manufacturing costs. Therefore, this invention designs the C content to be 0.045~0.065%.

[0024] Silicon (Si): Si is a commonly used deoxidizing element in steelmaking; it also promotes ferrite formation and increases ferrite strength; it helps refine corrosion products and promotes the formation of a dense protective rust layer on the steel surface, thereby improving atmospheric corrosion resistance. However, for thick steel plates, due to the slow rolling speed, the steel plate surface is easily oxidized at high temperatures, and excessive Si content will significantly deteriorate the surface quality of thick steel plates. Therefore, this invention designs the Si content to be 0.30~0.50%.

[0025] Manganese (Mn): Mn is an important solid solution strengthening element in steel, improving its strength. However, excessively high Mn content not only increases the manufacturing cost of weathering steel but also increases the stability of supercooled austenite, expands the austenite phase region, and increases the inhomogeneity of the microstructure along the thickness direction of thick steel plates. Mn also readily reacts with sulfur (S) to form MnS inclusions, which form elongated MnS inclusions along the rolling direction during rolling. During cooling, proeutectoid ferrite preferentially nucleates on the MnS inclusions, promoting the formation of banded structures and significantly reducing the low-temperature toughness of thick weathering steel. Therefore, this invention designs the Mn content to be 0.95~1.25%.

[0026] Alt (aluminum): Al is a major deoxidizing element in steelmaking and has a strong affinity for oxygen. Al can also combine with nitrogen atoms in molten steel to form AlN particles, reducing the harmful effects of nitrogen on low-temperature toughness, inhibiting austenite grain coarsening during heating, and refining grain size. However, excessively high Al content not only increases alloy costs but also leads to an increase in hard and brittle Al2O3 oxide inclusions in the steel, reducing its low-temperature toughness. Therefore, this invention designs the Alt content to be 0.015~0.030%.

[0027] Cr (chromium): Cr is a common alloying element in weathering steel that improves atmospheric corrosion resistance. When enriched in the rust layer, it promotes the formation of a dense, protective rust layer that is tightly bonded to the matrix, thus promoting anodic passivation and slowing down matrix corrosion. Cr also improves the hardenability of steel, especially enhancing the uniformity of the microstructure along the thickness direction of thick weathering steel plates. However, excessively high Cr content can worsen the weldability of the steel and increase manufacturing costs. Therefore, this invention designs the Cr content to be 0.80~1.00%.

[0028] Cu (copper): The combined addition of Cu and Cr can significantly improve the atmospheric corrosion resistance of steel. Cu easily accumulates within the rust layer, increasing its density and thus inhibiting the damage of corrosive media to the steel matrix. During the cooling process of hot-rolled coils, Cu can also precipitate fine second-phase particles, improving the strength of the steel. However, Cu has a melting point of only 1083℃, and excessive content can easily lead to surface copper embrittlement defects during hot rolling. Therefore, the Cu content is designed to be 0.25~0.40%.

[0029] Ni (Ni): Ni can increase the self-corrosion potential of steel, reduce its corrosion tendency, and improve its resistance to atmospheric corrosion. It can also react with Cu to form a high-melting-point Cu-Ni binary alloy, effectively preventing copper embrittlement caused by Cu. Furthermore, it can increase the stacking fault energy of ferrite, suppressing the cleavage fracture tendency of ferrite at low temperatures and improving the low-temperature toughness of the material. However, Ni is a precious metal element, and excessively high content will significantly increase manufacturing costs. Therefore, the Ni content is designed to be 0.07~0.15%.

[0030] Antimony (Sb): Sb can significantly improve the corrosion resistance of steel in acidic environments. Industrial atmospheres are often rich in acidic media such as SO2, SO3, and CO2. Adding an appropriate amount of Sb can significantly improve the corrosion resistance of steel in industrial atmospheric environments. However, excessive Sb content will increase manufacturing costs, and Sb is a low-melting-point alloying element, which increases the susceptibility of the cast billet to corner cracks. Therefore, this invention controls the Sb content to be between 0.015% and 0.030%.

[0031] Niobium (Nb): Niobium is a strong carbonitride forming element, exhibiting significant grain refinement and precipitation strengthening effects. At high temperatures, precipitated niobium carbonitride pins dislocations, inhibits the recrystallization of deformed austenite, expands the non-recrystallized region, and enhances the grain refinement effect during rolling. During cooling, nanoscale NbC two-phase particles precipitate, improving the material's strength and toughness. However, excessively high Nb content significantly increases the manufacturing cost of steel. Therefore, the Nb content is controlled between 0.030% and 0.050%.

[0032] Ca (calcium): Ca promotes the spheroidization of non-metallic inclusions, preventing stress concentration at the sharp corners of these inclusions, which can lead to crack initiation and become a rapid pathway for crack propagation, thus improving low-temperature toughness and atmospheric corrosion resistance. Its content should be controlled between 0.0010% and 0.0025%.

[0033] Phosphorus (P): While phosphorus can improve the atmospheric corrosion resistance of weathering steel, it tends to segregate at grain boundaries, significantly reducing the low-temperature toughness of the steel, especially the low-temperature toughness of thick-gauge weathering steel, which is difficult to control. Therefore, this invention strictly controls the content of phosphorus. However, if the phosphorus content is too low, the smelting cost of molten steel will increase. Therefore, this invention designs its content to be ≤0.012%, preferably P≤0.010%.

[0034] Sulfur (S): S is a harmful residual element in steel, easily forming sulfide inclusions that deteriorate the low-temperature toughness and atmospheric corrosion resistance of the steel. However, excessively low S content increases the smelting cost of molten steel. Therefore, this invention designs the S content to be ≤0.0020%, preferably S≤0.0015%.

[0035] O (Oxygen): O is a harmful gaseous element in steel. It easily forms brittle alumina inclusions in steel, disrupting the continuity of the matrix, causing stress concentration, becoming a rapid channel for crack initiation and propagation, and deteriorating the low-temperature toughness of the steel. The difference in corrosion potential between non-metallic inclusions and the steel matrix easily forms micro-cells, inducing localized corrosion and reducing the material's resistance to atmospheric corrosion. Therefore, this invention designs the O content to be ≤50ppm, preferably O≤40ppm.

[0036] Nitrogen (N): N is a harmful gaseous element in steel. The small diameter of N atoms and their interstitial solid solution cause severe lattice distortion, deteriorating the low-temperature toughness of the steel. Therefore, this invention designs the N content to be ≤50ppm, preferably N≤40ppm.

[0037] To improve the strength and toughness of thick hot-rolled weathering steel plates at extremely low temperatures (-60°C) and achieve excellent corrosion resistance, the present invention employs the following methods:

[0038] 1) The low C-low Mn-Nb-Cu multi-component microalloying composition design is adopted, and the grain size is precisely controlled by the controlled rolling and cooling process to promote the uniform precipitation of two-phase particles of NbC, Nb(C,N) and Cu. This fully leverages the synergistic effect of grain refinement strengthening, precipitation strengthening and solid solution strengthening, thereby reducing the amount of Nb and Mn elements added and lowering the manufacturing cost.

[0039] 2) By controlling P, S, O, and N, calcium feeding promotes the spheroidization of non-metallic inclusions, improves the cleanliness of molten steel, and reduces the adverse effects of residual elements and non-metallic inclusions on corrosion resistance and low-temperature toughness. Continuous casting employs dynamic light reduction, electromagnetic stirring, secondary cooling rate control, and slow cooling of the billet to improve the internal quality of the billet and enhance low-temperature toughness.

[0040] 3) By improving the hardenability of the material and using different reduction amounts and coiling temperatures in the finishing rolling process according to the thickness variation of the steel plate, the microstructure of the entire thickness of the steel plate can be precisely controlled, so that the microstructure of the steel plate surface, 1 / 4 thickness and core is ferrite, granular bainite and a small amount of pearlite. This solves the problem of uneven microstructure control from the surface to the core due to slow and uneven cooling of thick steel plates, avoids the formation of banded structures and improves the low-temperature toughness of the material.

[0041] 4) By using Cu-Cr-Ni-Sb small-volume multi-element alloy synergistic design, the atmospheric corrosion resistance of steel is improved, especially in harsh industrial atmospheric environments, where it exhibits excellent corrosion resistance.

[0042] This invention addresses the challenges of controlling the high strength and low-temperature toughness of hot-rolled thick weathering steel plates by designing a multi-component composite micro-alloying system, designing smelting and continuous casting processes, and designing controlled rolling and cooling processes.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] 1. This invention regulates chemical composition, smelting process, and controlled rolling and cooling processes to produce thick-gauge weathering steel plates with a thickness of 16-23mm using a hot continuous rolling strip process. The plates have a yield strength ≥550MPa, tensile strength ≥650MPa, and an A... 50 With a strength of ≥40%, KV2≥270J at -30℃ and KV2≥200J at -60℃, and no cracks after 180° cold bending (d=0.5a), thick-gauge weathering steel achieves a combination of high strength, excellent low-temperature toughness, and formability.

[0045] 2. Through the Cr-Cu-Sb-Ni synergistic design, it has good atmospheric corrosion resistance, especially in industrial atmospheric environments, with a corrosion rate of ≤40% relative to Q345B.

[0046] 3. By precisely controlling the grain size through multi-component micro-alloying composition design and controlled rolling and cooling process, the microstructure of the steel plate surface, 1 / 4 thickness and core is ferrite, granular bainite and a small amount of pearlite, reducing the content of Mn, Nb and Ni alloys and lowering the alloy cost. Attached Figure Description

[0047] Figure 1 Metallographic images of the weathering steel plate in Example 1 at the surface (a), 1 / 4 thickness (b), and core (c) locations;

[0048] Figure 2 Metallographic images of the weathering steel plate in Comparative Example 2 at the surface (a), 1 / 4 thickness (b), and core (c) locations;

[0049] Figure 3 The image shows the SEM morphology of the -60℃ impact fracture surface of the weathering steel plate in Example 1.

[0050] Figure 4 The image shows the SEM morphology of the -60℃ impact fracture surface of the weathering steel plate in Comparative Example 2. Detailed Implementation

[0051] This invention provides a hot-rolled weathering steel plate with a thickness of 16-23 mm and a yield strength of 550 MPa, comprising the following chemical composition by weight percentage: C: 0.045%-0.065%, Si: 0.30%-0.50%, Mn: 0.95%-1.25%, P: ≤0.012%, S: ≤0.0020%, Cr: 0.80%-1.00%, Ni: 0.07%-0.15%, Cu: 0.25%-0.40%, Alt: 0.015%-0.030%, Sb: 0.015%-0.030%, Nb: 0.030%-0.050%, Ca: 0.0010%-0.0025%, O ≤50 ppm, N ≤50 ppm, with the remainder being Fe and unavoidable impurity elements.

[0052] The manufacturing method of the hot-rolled weathering steel plate with a yield strength of 550MPa includes the following processes: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → hot rolling → laminar flow cooling → coiling.

[0053] In the converter smelting step, the converter tapping temperature is 1660-1680℃.

[0054] In the LF refining step, calcium wire is fed in to promote the spheroidization of non-metallic inclusions. The weak stirring time before and after feeding calcium wire is ≥13min, which promotes the non-metallic inclusions to float to the steel slag and improves the cleanliness of molten steel.

[0055] In the continuous casting process, the temperature of the molten steel in the tundish is 1530~1550℃, and argon gas is blown through the stopper rod to prevent air from contacting the molten steel. The argon gas flow rate is controlled at 5~8 L / min. The molten steel calming time is ≥13min, and the liquid surface fluctuation is ≤±3mm to prevent non-metallic inclusions from being drawn into the molten steel. The continuous casting adopts dynamic light pressure and electromagnetic stirring, and the secondary cooling section adopts intermediate cooling.

[0056] After the billet obtained from continuous casting is cut, it is stacked in the heat preservation pit and slowly cooled to ≤200℃, with a cooling rate of ≤7℃ / h, in order to promote the uniform diffusion of alloying elements inside the billet and improve the internal quality of the billet. The billet thickness is 230~250mm.

[0057] In the slab heating step, the temperature of the soaking zone of the heating furnace is 1160℃~1220℃, the soaking zone holding time is 30~40min, and the slab time in the furnace is 150~180min.

[0058] In the hot continuous rolling process, after the slab exits the heating furnace, it first undergoes high-pressure water descaling and width setting using a side press, and then is rolled. The roughing stage uses two reversible mills, each with three passes. The rolling temperature in the roughing stage is ≥1050℃, and the pass reduction rate is ≥20%. When the steel plate thickness is 16mm ≤ 18mm, the intermediate slab thickness is 60mm; when the steel plate thickness is 18mm ≤ 20mm, the intermediate slab thickness is 64mm; and when the steel plate thickness is 20mm ≤ 23mm, the intermediate slab thickness is 68mm.

[0059] In the hot continuous rolling step, the finishing rolling stage uses a 7-stand four-high rolling mill for continuous rolling, with an initial rolling temperature of ≤1000℃ and a final rolling temperature of 800~850℃.

[0060] In the laminar flow cooling step, the steel plate is rapidly cooled at a cooling rate of 20-35℃ / s after exiting the finishing mill.

[0061] In the winding step, when the steel plate thickness is 16mm ≤ steel plate thickness < 19mm, the winding temperature is 450~500℃; when the steel plate thickness is 19mm ≤ steel plate thickness < 23mm, the winding temperature is 400~450℃.

[0062] The present invention will now be described in detail with reference to the embodiments.

[0063] The specific chemical composition and weight percentage of the weathering steel plates in each embodiment and comparative example are shown in Table 1. The remainder consists of iron and unavoidable impurities.

[0064] Table 1 Chemical composition of various embodiments and comparative examples of the present invention

[0065]

[0066] The manufacturing process of weathering steel plates in each embodiment and comparative example is as follows: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → controlled rolling → controlled cooling → coiling. The main production process parameters are shown in Tables 2 and 3.

[0067] Table 2. Main process parameters of each embodiment and comparative example

[0068]

[0069] Table 3. Main process parameters of each embodiment and comparative example

[0070]

[0071] The properties of the weathering steel plates in each embodiment and comparative example are shown in Table 4. Tensile performance tests were conducted according to GB / T 228.1 "Metallic materials, tensile testing—Part 1: Test at room temperature," and cold bending performance tests were conducted according to GB / T 232 "Metallic materials, bending test." Impact performance tests were conducted according to GB-T 229 "Metallic materials, Charpy impact test."

[0072] Table 4. Main process parameters and performance of each embodiment and comparative example

[0073]

[0074] The atmospheric corrosion resistance of each embodiment was tested by cyclic immersion corrosion testing. The test solution was 0.01 mol / L NaHSO3 solution, the temperature was 45℃, the humidity was 70%RH, the test period was 72 hours, and the corrosion sample size was 40mm×60mm×4mm. Q345B low alloy steel was used as the control corrosion sample. The test results are shown in Table 5. The atmospheric corrosion resistance of each embodiment was more than twice that of Q345B low alloy steel, exhibiting excellent atmospheric corrosion resistance.

[0075] Table 5 Atmospheric corrosion resistance of each embodiment and comparative example

[0076]

[0077] Figure 1The images show the metallographic structures of the weathering steel plate in Example 1 at the surface (a), 1 / 4 thickness (b), and core (c). As can be seen, the microstructure along the thickness direction is uniform, consisting of ferrite, granular bainite, and a small amount of pearlite. The pearlite is uniform and fine, dispersed around the ferrite matrix, without pearlite streaks, and its proportion is low, less than 5% by volume. The average grain sizes of the ferrite at the surface, 1 / 4 thickness, and core are 5.6 μm, 6.4 μm, and 7.3 μm, respectively. The microstructure is uniform along the thickness direction, with uniformly distributed Mao island particles in the granular bainite, maintaining high strength while also exhibiting good plasticity and toughness. The pearlite consists of alternating lamellar ferrite and cementite. Because cementite is hard and brittle, the lamellar distribution of cementite leads to rapid crack propagation, reducing the material's toughness. Therefore, this invention patent uses chemical composition, smelting process and controlled rolling and cooling process to control the chemical composition, smelting process and controlled rolling and cooling process to make the steel plate have a uniform and fine microstructure throughout its thickness, control the uniform distribution of pearlite and make it small in size with a volume fraction of less than 5%, avoid the formation of banded structure, which helps to prevent crack propagation and improve low temperature toughness.

[0078] Figure 2 The images show the metallographic structure of the weathering steel plate in Comparative Example 2 at the surface (a), 1 / 4 thickness (b), and core (c). As can be seen from the images, pearlite bands are present from the surface to the core. In particular, the pearlite bands are more obvious at the 1 / 4 thickness and core of the steel plate due to the slower cooling rate, and the ferrite grains are coarse.

[0079] Figure 3 The image shows the SEM morphology of the impact fracture surface of the weathering steel plate in Example 1 at -60℃. The impact fracture surface is a typical dimple fracture. The weathering steel plate in this example has an impact absorption energy of 247~251J at -60℃ and good low-temperature toughness.

[0080] Figure 4 The SEM morphology of the weathering steel plate in Comparative Example 2 is shown at -60℃ impact fracture. The impact fracture is a typical cleavage fracture. The impact absorption energy at -60℃ is 20~26J, and the low-temperature toughness is poor.

[0081] In summary, the weathering steel plates provided in the various embodiments of the present invention can meet the requirements of yield strength ≥550MPa, tensile strength ≥650MPa, elongation ≥40%, KV2 ≥270J at -30℃, KV2 ≥200J at -60℃, and corrosion rate ≤40% relative to Q345B. They have high strength, high toughness and excellent corrosion resistance, and are especially suitable for manufacturing corrosion-resistant, long-life equipment and components in industrial atmospheric environments.

[0082] In Comparative Example 1, the excessively high C and Mn content resulted in a high pearlite volume fraction and a tendency to form banded structures, reducing the material's toughness. The excessively high final rolling temperature during finishing milling led to grain coarsening, further reducing toughness.

[0083] In Comparative Example 2, the lack of strict control over the content of P, S, O, and N resulted in a high content of non-metallic inclusions in the steel matrix, which became a rapid channel for crack initiation and propagation, reducing the toughness of the material.

[0084] In Comparative Example 3, although Cr, Ni, Cu and Nb were added, the low Cr-Ni-Cu content resulted in insufficient corrosion resistance and reduced the effect of Ni on improving low-temperature toughness; the low Nb content led to insufficient grain refinement and precipitation strengthening, thus reducing the strength and toughness of the material.

[0085] In Comparative Example 4, the lack of a heat-insulating pit for slow cooling of the billet increased the inhomogeneity of the microstructure in the thickness direction of the steel plate; in addition, the insufficient thickness of the intermediate billet, the low reduction rate of the finishing rolling, the low rate of laminar cooling, and the excessively high coiling temperature led to grain coarsening and reduced the strength and toughness of the material.

[0086] In Comparative Example 5, the insufficient argon flow rate of the stopper rod during continuous casting and the excessive fluctuation of the molten steel surface led to an increase in the content of non-metallic inclusions in the molten steel, insufficient intermediate billet thickness, low finishing rolling reduction, grain coarsening, and reduced material strength and low-temperature toughness. Furthermore, due to the absence of Sb element in the composition, the material's resistance to industrial atmospheric corrosion was insufficient.

[0087] The above detailed description of the hot-rolled weathering steel plate with a yield strength of 550MPa and its manufacturing method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A hot-rolled weathering steel plate with a yield strength of 550 MPa, characterized in that, The hot-rolled weathering steel plate with a yield strength of 550 MPa comprises the following chemical composition by weight percentage: C: 0.045%–0.065%, Si: 0.30%–0.50%, Mn: 0.95%–1.25%, P: ≤0.012%, S: ≤0.0020%, Cr: 0.80%–1.00%, Ni: 0.07%–0.15%, Cu: 0.25%–0.40%, Alt: 0.015%–0.030%, Sb: 0.015%–0.030%, Nb: 0.030%–0.050%, Ca: 0.0010%–0.0025%, O ≤50 ppm, N ≤50 ppm, with the remainder being Fe and unavoidable impurity elements.

2. The hot-rolled weathering steel plate with a yield strength of 550 MPa according to claim 1, characterized in that, The surface, 1 / 4 thickness and core microstructure of the hot-rolled weathering steel plate with a yield strength of 550MPa are all ferrite, granular bainite and pearlite.

3. The hot-rolled weathering steel plate with a yield strength of 550 MPa according to claim 1, characterized in that, The thickness of the hot-rolled weathering steel plate with a yield strength of 550MPa is 16~23mm; the yield strength of the hot-rolled weathering steel plate with a yield strength of 550MPa ≥ 550MPa, tensile strength ≥ 650MPa, and A 50 ≥40%, KV2≥270J at -30℃, KV2≥200J at -60℃, and no cracks when cold-bent at 180° with d=0.5a.

4. The method for manufacturing hot-rolled weathering steel plate with a yield strength of 550 MPa as described in any one of claims 1-3, characterized in that, The manufacturing method includes the following processes: molten iron pretreatment → converter smelting → LF refining → continuous casting → slab heating → hot continuous rolling → laminar flow cooling → coiling.

5. The manufacturing method according to claim 4, characterized in that, In the continuous casting process, the argon flow rate of the stopper rod is controlled at 5~8 L / min; the molten steel calming time is ≥13min, and the liquid level fluctuation is ≤±3mm; after the billet obtained from continuous casting is cut, it is stacked in the heat preservation pit and slowly cooled to ≤200℃.

6. The manufacturing method according to claim 4, characterized in that, In the slab heating step, the temperature of the soaking zone of the heating furnace is 1160℃~1220℃, the soaking zone holding time is 30~40min, and the slab time in the furnace is 150~180min.

7. The manufacturing method according to claim 4, characterized in that, In the hot continuous rolling step, the roughing stage uses two reversible mills, each rolling 3 passes. The rolling temperature in the roughing stage is ≥1050℃, and the pass reduction rate is ≥20%. When the steel plate thickness is 16mm≤18mm, the intermediate billet thickness is 60mm; when the steel plate thickness is 18mm≤20mm, the intermediate billet thickness is 64mm; and when the steel plate thickness is 20mm≤23mm, the intermediate billet thickness is 68mm.

8. The manufacturing method according to claim 4, characterized in that, In the hot continuous rolling step, the finishing rolling stage uses a 7-stand four-high rolling mill for continuous rolling, with an initial rolling temperature of ≤1000℃ and a final rolling temperature of 800~850℃.

9. The manufacturing method according to claim 4, characterized in that, In the laminar flow cooling step, the steel plate is rapidly cooled at a cooling rate of 20-35℃ / s after exiting the finishing mill.

10. The manufacturing method according to claim 4, characterized in that, In the winding step, when the steel plate thickness is 16mm ≤ steel plate thickness < 19mm, the winding temperature is 450~500℃; when the steel plate thickness is 19mm ≤ steel plate thickness < 23mm, the winding temperature is 400~450℃.

Images