A weather resistant steel for railway freight cars having excellent fatigue resistance and a method of manufacturing the same

Abstract

The application discloses a weather-resistant steel with excellent fatigue resistance for railway freight cars and a manufacturing method thereof. The weather-resistant steel has the following chemical components in percentage by weight: C: 0.04%-0.07%, Si: 0.1%-0.3%, Mn: 0.8%-1.2%, P: 0.015% or less, S: 0.005% or less, Cr: 0.30%-0.50%, Cu: 0.20%-0.28%, Nb: 0.05%-0.07%, Ti: 0.03%-0.05%, Ti / S: 10 or more, Ce: 70-150ppm, Ce / Nb: 0.1-0.3, and the balance of Fe and inevitable impurities. The application adopts a Cu-Ce-Nb-Cr quaternary corrosion-resistant system and a preparation process suitable for the chemical components, does not need heat treatment, reduces the cost, is environment-friendly without nickel, has excellent fatigue resistance and corrosion resistance, is suitable for extremely cold areas, and ensures stable performance of the steel for railway freight cars and prolongs the service life.

Description

Technical Field

[0001] This invention belongs to the technical field of railway freight car steel, specifically a nickel-free, heat-treatment-free, corrosion-resistant, low-temperature toughness-excellent, and fatigue-resistant weathering steel for railway freight cars and its manufacturing method. Background Technology

[0002] Railway transportation is a major mode of transport in my country, with advantages including low cost, large capacity, and high stability. The "road-to-rail" policy has further promoted the shift of heavy-duty freight from road to rail. With the development of railways, the railway transportation environment has become more complex. Heavy-duty and suitable for extremely cold regions are the future development trends of my country's railway vehicles, and the market has placed higher demands on the strength and ductility of steel used in railway freight cars. Currently, the steel used in railway freight cars is mainly 345MPa and 450MPa grade weathering steel. However, it has been found that under the combined effects of alternating loads during service, insufficient impact toughness and fatigue limit can lead to sudden fractures of components without significant deformation warnings, such as cracking of the car body welds. Furthermore, in extremely cold, humid, and salt spray environments, SO4 levels in the environment... 2- Cl - The penetration of acidic particulate ions leads to a significant cumulative effect of corrosion fatigue, severely impacting the service life of trucks. While hot-dip galvanizing can indeed extend service life, it generates dust pollution during surface treatment, and the coating metal and related media cause serious pollution to soil and water resources, drawing significant attention from relevant national departments. Consequently, its application is limited and costly.

[0003] Chinese invention patent CN115491604A discloses a rare-earth weathering bridge steel with a yield strength of 500 MPa and its manufacturing method. Its chemical composition by weight percentage is: C: 0.075~0.095%, Si: 0.2~0.35%, Mn: 1.35~1.50%, Ni: 0.35~0.45%, Cu: 0.35~0.45%, Mo: 0.15~0.25%, Nb: 0.045~0.05%. The rare earth weathering steel obtained has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation ≥18%, yield strength ratio ≤0.85, and low-temperature impact energy of -40℃ ≥120J. However, this invention contains low nickel and rare earth content, requires tempering heat treatment, and has a complicated process. The specific fatigue properties of the steel are not described. Chinese invention patent CN115679212 A discloses a novel high-strength corrosion-resistant steel for railway open wagons and its preparation method. The chemical composition and mass percentage of the high-strength corrosion-resistant steel are as follows: C: ≤0.10%, Si: 0.02~0.50%, Mn: 0.8~2.0%, P: ≤0.03%, S: ≤0.005%, Cu: 0.3~1.0%, Zr: 0.01~0.20%, Als: 0.010~0.050%, with the balance being Fe and unavoidable impurities. This invention adopts a Cu-Zr-Si system, requires tempering heat treatment, and has a complicated process. The specific fatigue properties of the steel are not described.

[0004] In summary, in order to meet the development needs of steel for railway vehicles in my country, there is an urgent need to develop a weathering steel for railway freight cars that is suitable for extremely cold regions, is nickel-free and environmentally friendly, corrosion-resistant, has excellent low-temperature toughness, and excellent fatigue resistance. Summary of the Invention

[0005] To address the problems of insufficient fatigue resistance and corrosion resistance, high maintenance costs, short service life, and inability to meet market demands associated with traditional weathering steel, the present invention aims to provide a weathering steel for railway freight cars with excellent fatigue resistance and its manufacturing method. This invention employs a Cu-Ce-Nb-Cr quaternary corrosion-resistant system and a preparation process adapted to the chemical composition, eliminating the need for heat treatment, reducing costs, being nickel-free and environmentally friendly, and exhibiting excellent fatigue resistance and corrosion resistance. It is suitable for extremely cold regions, ensuring stable performance of railway freight car steel and extending its service life.

[0006] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a weathering steel for railway freight cars with excellent fatigue resistance, the chemical composition of which, by weight percentage, is: C: 0.04%~0.07%, Si: 0.1%~0.3%, Mn: 0.8%~1.2%, P≤0.015%, S≤0.005%, Cr: 0.30%~0.50%, Cu: 0.20%~0.28%, Nb: 0.05%~0.07%, Ti: 0.03%~0.05%, Ti / S≥10, Ce: 70~150ppm, Ce / Nb=0.1~0.3, with the balance being Fe and unavoidable impurities.

[0007] The following details the mechanism of action of each alloy component in the weathering steel of this invention, where the percentage symbol % represents a weight percentage: Carbon (C) enhances the strength of steel through solid solution strengthening. However, excessive carbon can lead to microscopic galvanic corrosion with ferrite, which, especially in corrosive environments such as humidity and salt spray, accelerates pitting corrosion and forms fatigue crack initiation sites. The steel described in this invention employs a low-carbon design; to ensure the strength of the steel plate, the C content is selected to be between 0.04% and 0.07%.

[0008] Si strengthens steel through solid solution, increases the density of rust layers, slows down the erosion of corrosive media, and thus improves the corrosion resistance of materials. As a deoxidizer, it can reduce weld porosity and improve weld quality. Therefore, the Si content in this invention is not less than 0.1%. When the Si content is higher than 0.3%, brittle fracture is likely to occur under cyclic loading. Therefore, the Si content in this invention is controlled at 0.1% to 0.3%.

[0009] Mn can expand the austenite phase region, lower the phase transformation temperature, refine ferrite grains, improve the strength and toughness of steel, and delay the initiation of fatigue cracks. The Mn content described in this invention is not less than 0.8%. If the Mn content is too high, it will increase the hardenability of steel, worsen the weldability of steel, and increase the risk of cold cracking. The Mn content described in this invention is not higher than 1.2%. At the same time, by increasing Ce, the generation of MnS is inhibited, and the fatigue resistance is improved. The Mn content is controlled between 0.8% and 1.2%.

[0010] Excessive phosphorus (P) content can form low-melting-point eutectic with carbon, increasing the susceptibility to hot cracking, and can also form a brittle phase with iron, exacerbating the risk of cold cracking. Therefore, the upper limit of P content is strictly controlled and set at 0.015%.

[0011] S reacts with iron to form a low-melting-point eutectic FeS, which can lead to hot cracking and deteriorate the toughness and plasticity of the weld metal. Therefore, the upper limit of S content is strictly controlled and set at 0.005%.

[0012] The proper matching of Cr and Cu forms a dense Cr-Cu compound rust layer, which can improve the corrosion resistance of steel. The CeO2-Cr2O3 composite film of this invention is less prone to cracking under alternating stress and has self-healing ability, which can slow down the chemical erosion of crack tips. Therefore, the Cr content is not less than 0.3%. Excessive Cr content will increase the hardenability of steel, easily form brittle σ phase, and cause cold cracks. Therefore, the Cr content is controlled at 0.3%~0.5%.

[0013] Cu, along with P in steel, affects the formation and distribution of rust layers. The oxides formed can prevent further oxidation and corrosion of steel, significantly improving its corrosion resistance, thereby extending its service life and reducing maintenance costs. Excessive Cu will produce low-melting-point eutectics, leading to hot cracking. Therefore, the Cu content should be controlled between 0.20% and 0.28%.

[0014] Niobium (Nb) strengthens through precipitation and grain boundary processes. The NbC precipitate hinders dislocation movement, impedes crack propagation, and improves resistance to hydrogen embrittlement. Nb (C, N) and rare earth compounds jointly optimize the distribution of the second phase, reducing localized corrosion and fatigue crack initiation. With an Nb content of not less than 0.05% and a Ce / Nb ratio of 0.1~0.3, this ratio ensures the effect of Nb fine-grain strengthening, uniform niobium compound precipitation distribution, and reduced niobium compound size, thus passivating crack tips and significantly improving low-temperature toughness. At this Nb content, the α-FeOOH phase increases, leading to increased rust layer density. Simultaneously, the rust layer's self-corrosion potential shifts positively, slowing down electrochemical reactions and collectively improving weather resistance. However, exceeding this content does not significantly improve corrosion resistance and increases costs; therefore, the Nb content in this invention is controlled at 0.05%~0.07%.

[0015] Ti plays a role in refining grains, precipitation strengthening, and preventing embrittlement in steel. TiN pins austenite grain boundaries and refines austenite grains. Simultaneously, Ti preferentially fixes S and N, reducing Nb consumption. TiN provides nucleation sites for NbC, ensuring more Nb exerts its strengthening effect as NbC, replacing Ni's austenite stabilizing effect and hindering intergranular penetration of corrosive media. However, excessive Ti content leads to oversized TiN particles that become crack initiations. Therefore, this invention controls the Ti content within the range of 0.03% to 0.05%. This invention controls the Ti / S ratio to be ≥10, resulting in the formation of Ti₄C₂S₂ which inhibits MnS precipitation, reduces ineffective Ce purification, and generates more Ce₂O₃ for grain boundary strengthening.

[0016] Ce combines with S and P to reduce segregation and avoid grain boundary embrittlement. It forms Ce₂S₃ and Ce₂O₃ with S and O, effectively reducing MnS and Al₂O₃ inclusions, spheroidizing residual inclusions, and reducing stress concentration effects. Simultaneously, it increases melt fluidity, promotes convection and diffusion, effectively suppresses Cu grain boundary segregation, improves homogeneity and high-temperature stability, effectively reduces cracks, promotes TiN nanoforming, and the Nb-Ce composite precipitate NbCeN significantly reduces the corrosion rate. Solid-solution rare earth elements, rare earth intermetallic compounds, and rare earth sulfides in the corrosive medium... Rare earth cations, due to their reactive chemical properties, precipitate in alkaline regions with high pH values, hindering electrochemical corrosion and slowing down corrosion. The combined effect of rare earth elements and phosphorus also inhibits electrochemical corrosion, reduces the pH value of the material surface, promotes the formation of dense α-FeOOH, slows down corrosion, and replaces the passivation effect of Ni. When the rare earth Ce content is below 70 ppm, performance improvement is not significant; when Ce exceeds 150 ppm, inclusion coarsening and clustering occur, becoming sources of fatigue cracks. Therefore, the Ce content is controlled between 70 ppm and 150 ppm. At this content, rare earth oxides can effectively promote grain refinement. The nanoparticles and matrix have specific lattice interfaces and crystallographic orientation relationships, serving as effective strengthening phases for weathering steel, effectively improving the material's low-temperature toughness and fatigue resistance. When using a Cu-Ce-Nb-Cr quaternary corrosion-resistant system with a rare earth content of 70-150 ppm, the low-temperature impact absorption energy of weathering steel for railway freight cars reaches its maximum value.

[0017] Based on the above technical solution, the weathering steel for railway freight cars has a thickness of 2mm~8mm, a yield strength ≥500MPa, a tensile strength ≥610MPa, an elongation ≥25%, qualified cold bending performance, KV2 ≥100J at -50℃ (tested with the minimum impact standard sample), and a 72-hour cyclic immersion corrosion rate ≤1.5g / (m²) under industrial atmospheric conditions. 2 The corrosion rate of Q345B is only 41%~45%, and the fatigue life is ≥2 million cycles under room temperature and air environment and maximum stress of 420MPa.

[0018] This invention employs a Cu-Ce-Nb-Cr quaternary corrosion-resistant system and controls the Ti / S ratio to reduce the ineffective consumption of Ce purification. It also controls the Ce / Nb ratio to achieve uniform niobium precipitation distribution and reduce niobium size. The Ce-Cu composite film is not easily broken and has self-healing ability, significantly improving the material's low-temperature toughness, fatigue resistance, and corrosion resistance, enabling the material to be used in extremely cold regions.

[0019] Secondly, the present invention provides a method for manufacturing the above-mentioned weathering steel for railway freight cars with excellent fatigue resistance. The manufacturing method involves hot metal desulfurization pretreatment, smelting, LF ladle refining, RH refining, continuous casting, billet heating, hot continuous rolling, laminar flow cooling and coiling to produce weathering steel for railway freight cars. After RH refining, the oxygen content in the molten steel is no higher than 6 ppm. The ultrasonic generator is activated, rare earth cerium-iron alloy is added, and the mixture is homogeneous. The continuous casting process uses ultrasound to reduce the adsorption of rare earth elements on the inner wall of the steel ladle nozzle. As the amount of remaining molten steel decreases, the ultrasonic power and frequency are gradually increased to promote the shedding of nodules on the inner wall of the steel ladle nozzle, thereby improving the yield and production efficiency of rare earth elements in the steel smelting process. The heating temperature for the billet heating process is controlled at 1200~1250℃; The hot rolling process includes roughing and finishing. The roughing start temperature is controlled at 1090~1120℃ and the roughing end temperature is controlled at 1060~1090℃; the intermediate billet thickness is controlled at 35~50mm; the finishing start temperature is controlled at 1040~1060℃ and the finishing end temperature is controlled at 840~880℃. The cooling rate of the laminar flow cooling process is ≥15℃ / s.

[0020] Based on the above technical solution, further, after the desulfurization pretreatment of molten iron, the S content in the molten iron is ≤0.008%.

[0021] Based on the above technical solution, the smelting process is further carried out in a top-and-bottom combined blowing converter, with the converter's final temperature controlled at 1650~1670℃; when the steel is tapped 1 / 3 to 2 / 3, calcium-magnesium alloy is added to deoxidize the molten steel, controlling the oxygen content to be no higher than 0.045%, spheroidizing inclusions, and reducing stress concentration.

[0022] Based on the above technical solution, further, the argon blowing time of molten steel in the LF ladle refining process is ≥10min, and the calming time is ≥5min, to ensure that inclusions float to the surface, improve the purity of molten steel, and ensure that the sulfur content is not higher than 0.003%, which is beneficial to the weldability of steel.

[0023] Based on the above technical solution, the amount of rare earth cerium-iron alloy added is further controlled at 0.2~0.8 kg / t, the Ce content in the rare earth cerium-iron alloy is 30%~50%, and the oxygen content in the alloy is not higher than 100 ppm.

[0024] Based on the above technical solution, furthermore, the continuous casting process employs electromagnetic stirring to reduce billet segregation, controls the superheat at 15~25℃, controls the continuous casting speed at 0.45~0.66m / min, and controls the coolant flow rate in the crystallizer at 100~120m³ / min. 3 / h, the secondary cooling water flow rate is controlled at 0.35~0.65L / kg, the electromagnetic stirring current intensity of the crystallizer is controlled at 320~350A, the operating frequency is controlled at 3.5~4.0Hz; the billet thickness is controlled at 210~250mm.

[0025] Based on the above technical solution, further, during the continuous casting process, when the molten steel is poured to a remaining 50%~85%, the working frequency of the ultrasonic generator is controlled at 30~60KHz and the power is controlled at 300~600W; when the molten steel is poured to a remaining 30%~49%, the working frequency of the ultrasonic generator is controlled at 70~110KHz and the power is controlled at 900W~1200W; when the molten steel is poured to a remaining 10%~29%, the working frequency of the ultrasonic generator is controlled at 100~130KHz and the power is controlled at 1100W~1300W.

[0026] Based on the above technical solution, the heating temperature of the billet heating process is further controlled at 1200~1230℃, and the furnace time is controlled at 120min~160min.

[0027] Based on the above technical solution, furthermore, the roughing mill adopts 4 to 6 passes of rolling, and the roughing mill adopts large reduction rolling, with the single-pass reduction rate controlled between 25% and 40%; the finishing mill adopts 5 to 7 passes of rolling, and the reduction rate is controlled, with the reduction rate of F1 mill not exceeding 70%, and the remaining single-pass reduction rate gradually decreasing with each pass, with the single-pass reduction rate controlled between 10% and 50%.

[0028] Based on the above technical solution, the cooling rate of the laminar flow cooling process is further controlled at 15~20℃ / s to suppress Ce2S3 coarsening: as the cooling rate increases, although the temperature difference of Ar3 between the alloy element depletion zone and the enrichment zone increases, the suppression effect on carbon diffusion distance is more obvious, thereby suppressing the formation of C region and C-rich region, and reducing or eliminating the banded structure level.

[0029] Based on the above technical solution, the winding temperature of the winding process is further controlled at 560~620℃.

[0030] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention employs a specific composition ratio of the Cu-Ce-Nb-Cr quaternary corrosion-resistant system, combined with appropriate smelting, refining, continuous casting, and hot rolling processes. It controls the oxygen content during smelting, the method, timing, and amount of rare earth alloy addition, the temperature and cooling regimes during hot rolling, controls the Ti / S ratio, reduces the ineffective consumption of Ce purification, controls the Ce / Nb ratio, achieves uniform niobium precipitation distribution, reduces niobium size, and significantly improves the low-temperature toughness, fatigue resistance, and corrosion resistance of the material through inclusion modification, suppression of segregation, and fine grain strengthening. It is suitable for extremely cold regions.

[0031] 2. The weathering steel for railway freight cars with a thickness of 2mm~8mm manufactured by this invention is suitable for extremely cold regions. It is nickel-free, requires no heat treatment, has a yield strength ≥500MPa, tensile strength ≥610MPa, elongation ≥26%, and meets cold bending performance requirements. At -50℃, KV2 ≥100J (tested with the minimum impact standard sample). The 72-hour cyclic immersion corrosion rate under industrial atmospheric conditions is ≤1.5g / (m³). 2 The corrosion rate of Q345B is only 41%~45% compared to that of Q345B. Under room temperature and air environment and maximum stress of 420MPa, the fatigue life is ≥2 million cycles. It has excellent fatigue resistance and corrosion resistance and has a very good application prospect. Attached Figure Description

[0032] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0033] Figure 1 Metallographic micrograph of the steel plate prepared in Example 1. Detailed Implementation

[0034] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0035] In a first aspect, the present invention provides a weathering steel for railway freight cars with excellent fatigue resistance, the chemical composition of which, by weight percentage, is: C: 0.04%~0.07%, Si: 0.1%~0.3%, Mn: 0.8%~1.2%, P≤0.015%, S≤0.005%, Cr: 0.30%~0.50%, Cu: 0.20%~0.28%, Nb: 0.05%~0.07%, Ti: 0.03%~0.05%, Ti / S≥10, Ce: 70~150ppm, Ce / Nb=0.1~0.3, with the balance being Fe and unavoidable impurities.

[0036] Based on the above technical solution, the weathering steel for railway freight cars has a thickness of 2mm~8mm, a yield strength ≥500MPa, a tensile strength ≥610MPa, an elongation ≥25%, qualified cold bending performance, KV2 ≥100J at -50℃ (tested with the minimum impact standard sample), and a 72-hour cyclic immersion corrosion rate ≤1.5g / (m²) under industrial atmospheric conditions. 2 The corrosion rate of Q345B is only 41%~45%, and the fatigue life is ≥2 million cycles under room temperature and air environment and maximum stress of 420MPa.

[0037] Secondly, the present invention provides a method for manufacturing the above-mentioned weathering steel for railway freight cars with excellent fatigue resistance. The manufacturing method involves hot metal desulfurization pretreatment, smelting, LF ladle refining, RH refining, continuous casting, billet heating, hot continuous rolling, laminar flow cooling and coiling to produce weathering steel for railway freight cars. After RH refining, the oxygen content in the molten steel is no higher than 6 ppm. The ultrasonic generator is activated, rare earth cerium-iron alloy is added, and the mixture is homogeneous. The continuous casting process uses ultrasound to reduce the adsorption of rare earth elements on the inner wall of the steel ladle nozzle. As the amount of remaining molten steel decreases, the ultrasonic power and frequency are gradually increased to promote the shedding of nodules on the inner wall of the steel ladle nozzle, thereby improving the yield and production efficiency of rare earth elements in the steel smelting process. The heating temperature for the billet heating process is controlled at 1200~1250℃; The hot rolling process includes roughing and finishing. The roughing start temperature is controlled at 1090~1120℃ and the roughing end temperature is controlled at 1060~1090℃; the intermediate billet thickness is controlled at 35~50mm; the finishing start temperature is controlled at 1040~1060℃ and the finishing end temperature is controlled at 840~880℃. The cooling rate of the laminar flow cooling process is ≥15℃ / s.

[0038] Based on the above technical solution, further, after the desulfurization pretreatment of molten iron, the S content in the molten iron is ≤0.008%.

[0039] Based on the above technical solution, the smelting process is further carried out in a top-and-bottom combined blowing converter, with the converter's final temperature controlled at 1650~1670℃; when the steel is tapped 1 / 3 to 2 / 3, calcium-magnesium alloy is added to deoxidize the molten steel, controlling the oxygen content to be no higher than 0.045%, spheroidizing inclusions, and reducing stress concentration.

[0040] Based on the above technical solution, further, the argon blowing time of molten steel in the LF ladle refining process is ≥10min, and the calming time is ≥5min, to ensure that inclusions float to the surface, improve the purity of molten steel, and ensure that the sulfur content is not higher than 0.003%, which is beneficial to the weldability of steel.

[0041] Based on the above technical solution, the amount of rare earth cerium-iron alloy added is further controlled at 0.2~0.8 kg / t, the Ce content in the rare earth cerium-iron alloy is 30%~50%, and the oxygen content in the alloy is not higher than 100 ppm.

[0042] Based on the above technical solution, furthermore, the continuous casting process employs electromagnetic stirring to reduce billet segregation, controls the superheat at 15~25℃, controls the continuous casting speed at 0.45~0.66m / min, and controls the coolant flow rate in the crystallizer at 100~120m³ / min. 3 / h, the secondary cooling water flow rate is controlled at 0.35~0.65L / kg, the electromagnetic stirring current intensity of the crystallizer is controlled at 320~350A, the operating frequency is controlled at 3.5~4.0Hz; the billet thickness is controlled at 210~250mm.

[0043] Based on the above technical solution, further, during the continuous casting process, when the molten steel is poured to a remaining 50%~85%, the working frequency of the ultrasonic generator is controlled at 30~60KHz and the power is controlled at 300~600W; when the molten steel is poured to a remaining 30%~49%, the working frequency of the ultrasonic generator is controlled at 70~110KHz and the power is controlled at 900W~1200W; when the molten steel is poured to a remaining 10%~29%, the working frequency of the ultrasonic generator is controlled at 100~130KHz and the power is controlled at 1100W~1300W.

[0044] Based on the above technical solution, the heating temperature of the billet heating process is further controlled at 1200~1230℃, and the furnace time is controlled at 120min~160min.

[0045] Based on the above technical solution, furthermore, the roughing mill adopts 4 to 6 passes of rolling, and the roughing mill adopts large reduction rolling, with the single-pass reduction rate controlled between 25% and 40%; the finishing mill adopts 5 to 7 passes of rolling, and the reduction rate is controlled, with the reduction rate of F1 mill not exceeding 70%, and the remaining single-pass reduction rate gradually decreasing with each pass, with the single-pass reduction rate controlled between 10% and 50%.

[0046] Based on the above technical solution, the cooling rate of the laminar flow cooling process is further controlled at 15~20℃ / s to suppress Ce2S3 coarsening: as the cooling rate increases, although the temperature difference of Ar3 between the alloy element depletion zone and the enrichment zone increases, the suppression effect on carbon diffusion distance is more obvious, thereby suppressing the formation of C region and C-rich region, and reducing or eliminating the banded structure level.

[0047] Based on the above technical solution, the winding temperature of the winding process is further controlled at 560~620℃.

[0048] Examples 1-10 This embodiment provides a method for manufacturing weathering steel for railway freight cars with excellent fatigue resistance. Taking a 200t converter as an example, the chemical composition and weight percentage of the weathering steel are shown in Table 1. Table 1. Chemical composition and weight percentage (%) of weathering steel in Examples 1-10

[0049] Includes the following steps: (1) Smelting: First, the molten iron is pretreated for desulfurization to ensure that the S content after desulfurization is ≤0.008%; after smelting in a top and bottom blowing converter, the final temperature of the converter is not lower than 1650℃. When the steel is tapped 1 / 2, calcium magnesium alloy is added to deoxidize the molten steel and control the oxygen content to be no higher than 0.045%, spheroidizing inclusions and reducing stress concentration; then, LF furnace refining is carried out. The argon blowing time and the calming time of the molten steel are shown in Table 2 to ensure that the inclusions float to the surface, improve the purity of the molten steel, and ensure that the sulfur content is no higher than 0.003%, which is beneficial to the weldability of the steel; the RH process is vacuum treated. After the vacuum is completed, the oxygen content of the molten steel is determined. The oxygen content control of the steel after RH is shown in Table 2. The ultrasonic generator is started, and then rare earth cerium iron alloy (Ce content 40%, oxygen content in the alloy is no higher than 100ppm) is added. The specific amount added is shown in Table 2. After holding for 1 minute, continuous casting is carried out. (2) Continuous casting: When the molten steel is poured to 50%~85% remaining, the ultrasonic generator is set to a frequency of 30~60KHz and a power of 300~600W; when the molten steel is poured to 30%~49% remaining, the ultrasonic generator is set to a frequency of 70~110KHz and a power of 900W~1200W; when the molten steel is poured to 10%~29% remaining, the ultrasonic generator is set to a frequency of 100~130KHz and a power of 1100W~1300W; the rare earth elements in the molten steel are reduced by ultrasound. Adsorption on the inner wall of the ladle nozzle is addressed by gradually increasing power and frequency, which promotes the shedding of nodules on the inner wall of the ladle nozzle, thereby improving the rare earth recovery rate and production efficiency in the steelmaking process. The continuous casting speed, superheat, crystallizer cooling water flow rate, secondary cooling water flow rate, crystallizer electromagnetic stirring current intensity and operating frequency are shown in Table 2. Electromagnetic stirring is used to reduce billet segregation, and the rare earth Ce detection amount is 70~150ppm. The rare earth recovery rate reaches 60% through this method, which can reduce the rare earth consumption per ton of steel, reduce costs, and improve the fatigue life and low-temperature toughness of corrosion-resistant steel.

[0050] 3. Heating: The heating temperature and furnace time are shown in Table 4.

[0051] 4. Rolling: This includes roughing and finishing rolling. Descaling is performed before roughing. Roughing is done in 5 passes with a large reduction. The reduction rate per pass is controlled between 25% and 40%. As the deformation increases, the area defect density of the original austenite grain boundaries increases, thereby increasing the number of phase deformation nucleation points. Fine grain strengthening is beneficial to improving the fatigue performance of steel. The start and end temperatures of roughing are shown in Table 4. Finishing is done in 7 passes with controlled reduction. To avoid slippage on the F1 mill, the reduction rate is not higher than 70%. The specific reduction rate of finishing rolling is shown in Table 5. The start and end temperatures of finishing rolling are shown in Table 4.

[0052] 5. Cooling: The laminar flow cooling section adopts the front-end cooling method, cooling to the coiling temperature, increasing the water flow rate of the upper and lower manifolds, and the cooling rate is shown in Table 4, which inhibits Ce2S3 coarsening; as the cooling rate increases, although the temperature difference of Ar3 between the alloy element depletion zone and the enrichment zone increases, the inhibition effect on carbon diffusion distance is more obvious, thereby inhibiting the formation of C region and C-rich region, reducing or eliminating the banded structure level.

[0053] 6. Coiling: After cooling, the steel is coiled. The coiling temperature is controlled as shown in Table 4 to obtain nickel-free, environmentally friendly, low-cost weathering steel with excellent fatigue resistance and suitable for railway freight cars in extremely cold regions with a thickness of 2mm~8mm.

[0054] Table 2. Smelting and continuous casting process parameters of weathering steel in Examples 1-10

[0055] Table 3. Ultrasonic process parameters for weathering steel in Examples 1-10

[0056] Table 4. Heating, rolling, cooling, and coiling process parameters for weathering steel in Examples 1-10

[0057] Table 5. Billet thickness, roughing reduction rate, intermediate billet thickness, and finishing reduction rate of weathering steel in Examples 1-10

[0058] Table 6. Test results of various performance indicators of weathering steel in Examples 1-10

[0059] As shown in Table 6, the weathering steel prepared in each embodiment has a yield strength ≥500MPa, tensile strength ≥610MPa, elongation ≥26%, qualified cold bending performance, and KV2 ≥100J at -50℃ (tested with minimum impact standard specimen).

[0060] To simulate corrosion under industrial atmospheric conditions, the test method specified in TB / T 2375-1993 was followed. The cyclic test solution was a 0.01 mol / L NaHSO3 solution with a pH of 4.4-4.8, a temperature of 38.5℃, an ambient relative humidity of 70% RH, and an air temperature of 70℃. The single wet-dry cycle corrosion period was set to 60 minutes, including 12 minutes of immersion and 48 minutes of exposure to air. The total test period was 72 hours. After the sample was removed, it was cleaned at room temperature for 10 minutes in a solution of 500 ml hydrochloric acid + 500 ml deionized water + 10 g hexamethylenetetramine, then degreased in acetone for 5 minutes, dehydrated in anhydrous ethanol for 3 minutes, dried, and weighed. The corrosion rate was calculated by the weight loss method.

[0061] A 72-hour cyclic immersion rapid corrosion evaluation test was conducted on the weathering steel of Examples 1-10 of the present invention and the comparative steel Q345B. Table 7 shows the comparison results of the corrosion resistance of the weathering steel of Examples 1-10 of the present invention and the comparative steel. It can be seen that the weathering steel of Examples 1-10 of the present invention has high atmospheric corrosion resistance.

[0062] Table 7 Comparison of rapid corrosion test results between weathering steel and control steel in Examples 1-10

[0063] Fatigue tests were conducted at room temperature and in air. On the fatigue testing machine, the cyclic stress ratio (R ratio = minimum stress / maximum stress) was set to 0.1, and the loading frequency was 30 Hz. The specimens were subjected to the specified stress until fracture or 10... 7 The experiment was terminated after one week.

[0064] Table 8. Fatigue test results of weathering steels in Examples 1-10

[0065] As shown in Table 8, the fatigue life of the weathering steel prepared in each embodiment is greater than 2 million cycles, and the fatigue performance is stable.

[0066] In summary, the weathering steel for railway freight cars prepared by this invention has a yield strength ≥500MPa, tensile strength ≥610MPa, elongation ≥26%, qualified cold bending performance, KV2 ≥100J at -50℃ (tested with the minimum impact standard sample), and a 72-hour cyclic immersion corrosion rate ≤1.5g / (m²) under industrial atmospheric conditions. 2 The corrosion rate of this material is only 41%~45% compared to Q345B. It has no nickel segregation and excellent fatigue resistance and corrosion resistance.

[0067] Comparative Examples 1-2 The chemical composition, weight percentage, and manufacturing method of the weathering steels in Comparative Examples 1-2 are basically the same as those in Example 1. The difference lies in the different contents of Cr, Cu, Nb, and Ce, as shown in Table 9. The test results of various performance indicators of the prepared weathering steels are shown in Table 10.

[0068] Table 9 Chemical composition and weight percentage (%) of weathering steel in Comparative Examples 1-2

[0069] Table 10 Test results of various performance indicators of weathering steel in Comparative Examples 1-2

[0070] As shown in Table 10, the low-temperature toughness, fatigue resistance and corrosion resistance of the weathering steel in Comparative Examples 1-2 are significantly lower than those of the weathering steel in Example 1.

[0071] Comparative Examples 3-4 The chemical composition, weight percentage, and manufacturing method of the weathering steels in Comparative Examples 3-4 are basically the same as those in Example 1, except that Nb and Ce are not added. The specific chemical composition and weight percentage are shown in Table 11, and the test results of various performance indicators of the prepared weathering steels are shown in Table 12.

[0072] Table 11 Chemical composition and weight percentage (%) of weathering steel in Comparative Examples 3-4

[0073] Table 12 Test results of various performance indicators of weathering steel in Comparative Examples 3-4

[0074] As shown in Table 12, the yield strength, tensile strength, low-temperature toughness, fatigue resistance and corrosion resistance of the weathering steel in Comparative Examples 3-4 are significantly lower than those of the weathering steel in Example 1.

[0075] Comparative Examples 5-6 The chemical composition, weight percentage, and manufacturing method of the weathering steels in Comparative Examples 5-6 are basically the same as those in Example 1. The difference lies in the smelting and rolling process parameters in the preparation process, as shown in Table 13. The test results of various performance indicators of the prepared weathering steels are shown in Table 14.

[0076] Table 13 Smelting and rolling process parameters of weathering steel in Comparative Examples 5-6

[0077] Table 14. Rare earth yield and test results of various performance indicators of weathering steel in Comparative Examples 5-6

[0078] In Comparative Example 5, the rare earth addition method was different from that in Example 1, resulting in a lower rare earth yield and significantly lower low-temperature toughness and fatigue resistance compared to Example 1. In Comparative Example 6, the RH oxygen content was significantly higher than that in Example 1, resulting in a slightly lower rare earth yield and slightly lower low-temperature toughness and fatigue resistance compared to Example 1.

[0079] In summary, this application adopts a Cu-Ce-Nb-Cr quaternary corrosion-resistant system. During the preparation process, a series of measures, such as reducing the oxygen content after RH smelting and controlling the Ce content at 70~150ppm, are taken to significantly improve the corrosion resistance, low-temperature toughness and fatigue resistance of weathering steel materials.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A weathering steel for railway freight cars with excellent fatigue resistance, characterized in that, Its chemical composition by weight percentage is as follows: C: 0.04%~0.07%, Si: 0.1%~0.3%, Mn: 0.8%~1.2%, P≤0.015%, S≤0.005%, Cr: 0.30%~0.50%, Cu: 0.20%~0.28%, Nb: 0.054%~0.064%, Ti: 0.03%~0.05%, Ti / S≥10, Ce: 70~150ppm, Ce / Nb=0.1~0.3, with the balance being Fe and unavoidable impurities; The method for manufacturing the weathering steel for railway freight cars with excellent fatigue resistance involves hot metal desulfurization pretreatment, smelting, LF ladle refining, RH refining, continuous casting, billet heating, hot continuous rolling, laminar flow cooling and coiling to produce the weathering steel for railway freight cars. After RH refining, the oxygen content in the molten steel is no higher than 6 ppm. The ultrasonic generator is activated, rare earth cerium-iron alloy is added, and the mixture is homogeneous. The continuous casting process uses ultrasound to reduce the adsorption of rare earth elements on the inner wall of the steel ladle nozzle; The heating temperature for the billet heating process is controlled at 1200~1250℃; The hot rolling process includes roughing and finishing. The roughing start temperature is controlled at 1090~1120℃ and the roughing end temperature is controlled at 1060~1090℃; the intermediate billet thickness is controlled at 35~50mm; the finishing start temperature is controlled at 1040~1060℃ and the finishing end temperature is controlled at 840~880℃. The cooling rate of the laminar flow cooling process is ≥15℃ / s; During continuous casting, when the molten steel is poured to a remaining 50%~85%, the ultrasonic generator operates at a frequency of 30~60KHz and a power of 300~600W; when the molten steel is poured to a remaining 30%~49%, the ultrasonic generator operates at a frequency of 70~110KHz and a power of 900W~1200W; when the molten steel is poured to a remaining 10%~29%, the ultrasonic generator operates at a frequency of 100~130KHz and a power of 1100W~1300W. Roughing is carried out in 4 to 6 passes with a large reduction, and the single-pass reduction rate is controlled between 25% and 40%. Finishing is carried out in 5 to 7 passes with the reduction rate controlled. The reduction rate of F1 mill is not higher than 70%, and the remaining single-pass reduction rate gradually decreases with each pass, and the single-pass reduction rate is controlled between 10% and 50%.

2. The weathering steel for railway freight cars according to claim 1, characterized in that, The aforementioned weathering steel for railway freight cars has a thickness of 2mm~8mm, a yield strength ≥500MPa, a tensile strength ≥610MPa, an elongation ≥25%, qualified cold bending performance, KV2 ≥100J at -50℃, and a 72-hour cyclic immersion corrosion rate ≤1.5g / (m²) under industrial atmospheric conditions. 2 ·h), under room temperature and air environment, and a maximum stress of 420MPa, the fatigue life is ≥2 million cycles.

3. The method for manufacturing the weathering steel for railway freight cars with excellent fatigue resistance as described in claim 1 or 2, characterized in that, The manufacturing method involves hot metal desulfurization pretreatment, smelting, LF ladle refining, RH refining, continuous casting, billet heating, hot continuous rolling, laminar flow cooling and coiling to produce weathering steel for railway freight cars. After RH refining, the oxygen content in the molten steel is no higher than 6 ppm. The ultrasonic generator is activated, rare earth cerium-iron alloy is added, and the mixture is homogeneous. The continuous casting process uses ultrasound to reduce the adsorption of rare earth elements on the inner wall of the steel ladle nozzle; The heating temperature for the billet heating process is controlled at 1200~1250℃; The hot rolling process includes roughing and finishing. The roughing start temperature is controlled at 1090~1120℃ and the roughing end temperature is controlled at 1060~1090℃; the intermediate billet thickness is controlled at 35~50mm; the finishing start temperature is controlled at 1040~1060℃ and the finishing end temperature is controlled at 840~880℃. The cooling rate of the laminar flow cooling process is ≥15℃ / s; During continuous casting, when the molten steel is poured to a remaining 50%~85%, the ultrasonic generator operates at a frequency of 30~60KHz and a power of 300~600W; when the molten steel is poured to a remaining 30%~49%, the ultrasonic generator operates at a frequency of 70~110KHz and a power of 900W~1200W; when the molten steel is poured to a remaining 10%~29%, the ultrasonic generator operates at a frequency of 100~130KHz and a power of 1100W~1300W. Roughing is carried out in 4 to 6 passes with a large reduction, and the single-pass reduction rate is controlled between 25% and 40%. Finishing is carried out in 5 to 7 passes with the reduction rate controlled. The reduction rate of F1 mill is not higher than 70%, and the remaining single-pass reduction rate gradually decreases with each pass, and the single-pass reduction rate is controlled between 10% and 50%.

4. The manufacturing method according to claim 3, characterized in that, After desulfurization pretreatment, the sulfur content in the molten iron is ≤0.008%. The smelting process is carried out in a top-and-bottom blown converter, with the final converter temperature controlled at 1650~1670℃; when the steel is 1 / 3 to 2 / 3 tapped, calcium-magnesium alloy is added to deoxidize the molten steel, and the oxygen content is controlled to be no higher than 0.045%. The argon blowing time for molten steel in the LF ladle refining process is ≥10 min, the calming time is ≥5 min, and the sulfur content is guaranteed to be no higher than 0.003%.

5. The manufacturing method according to claim 3, characterized in that, The rare earth cerium-iron alloy is added at a rate of 0.2~0.8 kg / t, the Ce content in the rare earth cerium-iron alloy is 30%~50%, and the oxygen content in the alloy is not higher than 100 ppm.

6. The manufacturing method according to claim 3, characterized in that, Electromagnetic stirring is used in the continuous casting process to reduce billet segregation. Superheat is controlled at 15~25℃, casting speed is controlled at 0.45~0.66 m / min, and coolant flow rate in the crystallizer is controlled at 100~120 m³ / min. 3 / h, the secondary cooling water flow rate is controlled at 0.35~0.65L / kg, the electromagnetic stirring current intensity of the crystallizer is controlled at 320~350A, the operating frequency is controlled at 3.5~4.0Hz; the billet thickness is controlled at 210~250mm.

7. The manufacturing method according to claim 3, characterized in that, The heating temperature of the billet heating process is controlled at 1200~1230℃, and the furnace time is controlled at 120min~160min.

8. The manufacturing method according to claim 3, characterized in that, The cooling rate of the laminar flow cooling process is controlled at 15~20℃ / s; the winding temperature of the winding process is controlled at 560~620℃.

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