A weathering-resistant steel for railway vehicles and a method for manufacturing the same

Abstract

A weathering steel for railway vehicle and a manufacturing method thereof, the steel having a composition in weight percentage of C 0.04-0.09%, Si 0.12-0.24%, Mn 0.6-0.9%, P ≤0.015%, S ≤0.006%, Al 0.02-0.04%, Cu 0.15-0.35%, Cr 0.60-0.95%, Ni 0.05-0.12%, Mo 0.03-0.12%, Ti 0.01-0.02%, Nb 0.01-0.02%, N ≤0.006%, 5 ≤Cr / Mo ≤25, balance Fe and inevitable impurities, the steel having a yield strength of 450-620 MPa, a tensile strength ≥600 MPa, an elongation A ≥18%, and an impact energy value at -40 ℃ ≥100 J, and the steel plate having a corrosion resistance under a simulated coal train environment of a corrosion rate ≤0.8 g / m 2 2 per unit area, a corrosion rate ≤55% relative to plain carbon steel under an industrial atmospheric environment, and being particularly suitable for cold forming processes in the production of railway vehicles, and being also suitable for use in equipment such as coal gas pipelines, boiler preheating pipelines and the like which are subjected to acidic medium corrosion.

Description

Technical Field

[0001] This invention relates to the field of low alloy steel manufacturing, and in particular to a weathering steel for railway vehicles and its manufacturing method. Background Technology

[0002] Atmospheric corrosion resistant steel, also known as weathering steel, is mainly used in the fabrication of steel structures requiring corrosion resistance, such as containers, railway vehicles, bridges, and outdoor towers. Railway vehicles made of weathering steel can effectively extend the service life of the car body, extend maintenance cycles, and thus reduce costs. To further extend the car body's lifespan and withstand high-speed, heavy-load loads, the steel used in railway vehicles requires high strength and resistance to atmospheric corrosion. This is especially true for coal transport vehicles, which, in addition to high strength, high toughness, and ease of forming, also require resistance to acidic media corrosion from coal leachate. Under 24 hours of uniform corrosion in a 10% H₂SO₄ + 3.5% NaCl solution, the corrosion weight loss must be ≤0.8 g / m³. 2 ·h.

[0003] Currently, weathering steel used in railway vehicles has evolved from the early 345MPa level 09CuP to 450MPa level high-strength weathering steel and high corrosion-resistant steel with even better corrosion resistance. This has also led to the development of many related steel patents. For example:

[0004] Chinese patent CN101033520A discloses "An AlSi type economical weathering steel," which is based on C-Si-Mn and adds up to 8% Al to achieve corrosion resistance. This patent does not mention the strength of the steel, only publishing the electrochemical test results, and does not provide more specific data on its corrosion resistance performance. Furthermore, the addition of rare earth elements to ensure corrosion resistance not only increases production difficulty but also raises costs due to the 8% Al content. Additionally, the 0.3-0.8% Si content is detrimental to impact toughness.

[0005] Chinese patent CN101660099B discloses "High-strength low-alloy hot-rolled ferritic bainitic weathering steel and its production method", which has a yield strength of 450MPa, adopts a high Mn content design, and has corrosion resistance at the level of conventional weathering steel.

[0006] Chinese patents CN1986864 ("A High-Strength Low-Alloy Atmospheric Corrosion Resistant Steel and Its Production Method"), CN102168229B ("Weathering Steel Plate and Its Manufacturing Method"), and CN107779740A ("Hot-Rolled Steel Strip with Yield Strength of 700MPa and Its Manufacturing Method") disclose the same.

[0007] While the steel plates involved in the aforementioned patents achieve higher strengths of over 450 MPa, they all use higher levels of Mn in their composition and are further strengthened by the addition of reinforcing elements such as Mo, Nb, V, and Ti, resulting in higher costs. Their weather resistance is comparable to traditional weathering steel, with a relative corrosion rate of ≤55%. However, they do not meet the corrosion resistance requirements of coal transport vehicles for acidic media.

[0008] To meet the application requirements under more diverse working conditions, weathering steel, in addition to high strength, is developing towards high corrosion resistance and high toughness, while also requiring good machinability and lower cost. Examples include: Japanese patent JP10025550A's "CORROSION RESISTANT STEEL", Japanese patent JP2002363704's "CORROSION RESISTENTSTEEL HAVING EXCELLENT TOUGHNESS IN BASE MATERIAL AND HEAT AFFECTED ZONE", and Chinese patent CN102127717A's "High Corrosion Resistance Cr-Containing Weathering Steel with Excellent Toughness".

[0009] The steel grades involved in the above patents all have better atmospheric corrosion resistance and lower relative corrosion rates, but do not involve the acidic corrosion resistance of coal transport vehicles; in addition, the steels contain high levels of elements such as Cr, Al, and Ni, making steelmaking difficult and manufacturing costs high.

[0010] Existing studies have found severe corrosion in the middle and bottom of coal-carrying railway freight cars. This corrosion is electrochemical, rather than the conventional alternating wet and dry atmospheric corrosion pattern. It is primarily caused by chloride and sulfate ions in the coal leachate, classifying it as acidic media corrosion. This type of acidic media corrosion is common in power plant boilers and various boiler combustion exhaust pipes in factories. These exhaust gases contain SO2 and SO3 generated from sulfur combustion, as well as water vapor (5-18%). When the exhaust gas temperature drops below the dew point or the flue gas comes into contact with the cooler flue pipe walls or metal equipment, sulfuric acid dew point corrosion occurs. Early attempts to improve the corrosion resistance of flue gas pipes in this environment using chromium-copper steel have yielded unsatisfactory results. To address this, Sb-containing alloys such as S-TEN3 steel were employed, thereby achieving further improvements in the overall resistance to dew point corrosion (see Huang Zhenzhong et al., "Resistance to Sulfuric Acid Dew Point Corrosion of S-TEN3 and CRIA Series Steels," Journal of Beijing University of Science and Technology, 1996, 16: 87-91); Several related patents have also been published, such as:

[0011] Japanese invention JP2001164335A discloses "HIGH WORKABILITY AND GOOD WELDABILITYSULFURIC ACID DEW POINT CORROSION RESISTANT STEEL SHEET".

[0012] Chinese patent CN102268613A discloses "A hot-rolled steel plate resistant to atmospheric corrosion for railway vehicles and its manufacturing method".

[0013] Both patents mentioned above address acid-resistant steels designed for such corrosive environments. The former contains 0.01-0.15% Sb and is primarily used in equipment and power plants that burn heavy oil. The latter, in addition to containing 0.01-0.04% P, also contains 1.0-2.15% Cr, 0.15-0.65% Ni, and 0.25-0.65% Si, and requires the addition of appropriate amounts of Ca, Mg, Ce, and Sb. The higher Si and P content negatively impacts low-temperature toughness and formability, while the higher Cr and Ni content increases production costs, and the addition of Mg and Ce increases production difficulty.

[0014] Ansteel's S450AW weathering steel for railway freight cars, launched in 2012, also contains approximately 0.1% Sb to ensure corrosion resistance under acidic corrosive media (see "Research and Development History of Ansteel Railway Freight Car Body Steel", Ansteel Technology, 2018, (4): 9-14). Sb combined with Cu can form a Cu2Sb protective film on the surface, thereby improving sulfuric acid dew point corrosion performance. However, the addition of Sb is obviously detrimental to the environment and human health. Sb is a typical toxic and harmful heavy metal element that has chronic toxicity and potential carcinogenicity to humans and animals. With the increasing environmental awareness of the whole society and the tightening of environmental protection policies, the production and application of Sb-containing steel will inevitably be subject to more restrictions.

[0015] A comparison with existing patents reveals that current weathering steels, whether conventional or highly corrosion-resistant, are designed for atmospheric corrosion environments and are not suitable for the acidic corrosive environment of coal trucks. Existing weathering steels that can improve this acidic corrosive environment generally have higher alloy costs or contain sulfur (Sb), which is detrimental to the environment and human health. Summary of the Invention

[0016] The purpose of this invention is to provide a weathering steel for railway vehicles and its manufacturing method. This weathering steel is environmentally friendly, abandoning the existing Sb-containing system design of weathering steels. Its yield strength is 450–620 MPa, tensile strength ≥600 MPa, elongation A ≥18%, and impact energy at -40℃ ≥100 J. Under simulated coal car corrosion conditions, the corrosion resistance of the steel plate is comparable to or even better than existing Sb-containing sulfuric acid dew point resistant steels, meeting the requirement of weight loss per unit area ≤0.8 g / m². 2 It meets the requirements of h; it also has good atmospheric corrosion resistance, with a corrosion rate of ≤55% compared to ordinary carbon steel in industrial atmospheric environments, thus satisfying the corrosion resistance requirements in various environments. In addition to high strength, high toughness, and good corrosion resistance, this steel also has good welding and cold bending processing properties, excellent elongation, and is particularly suitable for various cold forming processes in railway vehicle production. It can also be used in equipment with acidic media corrosion, such as gas pipelines and boiler preheating pipelines. Furthermore, the steel plate is produced by controlled rolling and controlled cooling, which has a wider process window and a simple production method; it does not require heat treatment, has a short production cycle, and lower steel costs.

[0017] To achieve the above objectives, the technical solution of the present invention is as follows:

[0018] The weathering steel for railway vehicles described in this invention is environmentally friendly. It abandons the existing Sb-containing system design of weathering steel and adopts a method that strengthens high strength and good low-temperature toughness by precipitation of trace amounts of Ti and Nb on a low C-Si-Mn basis. Furthermore, it achieves good corrosion resistance in acidic media corrosion environments through Cr-Mo composition design, thus realizing a combination of corrosion resistance, high strength, and high toughness at a low cost.

[0019] Specifically, the weathering steel for railway vehicles described in this invention has the following composition by weight percentage: C: 0.04-0.09%, Si: 0.12-0.24%, Mn: 0.6-0.9%, P≤0.015%, S≤0.006%, Al: 0.02-0.04%, Cu: 0.15-0.35%, Cr: 0.60-0.95%, Ni: 0.05-0.12%, Mo: 0.03-0.12%, Ti: 0.01-0.02%, Nb: 0.01-0.02%, N≤0.006%, with the balance including Fe and other unavoidable impurities; and it must simultaneously satisfy: 5≤Cr / Mo≤25.

[0020] Furthermore, the weathering steel for railway vehicles described in this invention may also include one or more of Sn≤0.15%, RE≤0.15%, V≤0.05%, and Ca≤0.005%.

[0021] In the composition design of the weathering steel described in this invention:

[0022] C is an effective strengthening element in steel. When dissolved in the matrix, it has a solid solution strengthening effect. At the same time, it exists in the steel in the form of carbides, and combines with alloying elements to play a role in precipitation strengthening and grain refinement. Therefore, the amount added should not be less than 0.04%. However, excessive C forms more carbides in the steel, which acts as a galvanic cell, promotes corrosion, and reduces the corrosion resistance of the steel. It is also not conducive to welding. Therefore, the C content is limited to not exceeding 0.09%.

[0023] Si is generally added to deoxidize steel. It is also a corrosion-resistant element and has a solid solution strengthening effect. Therefore, the lower limit of its content is controlled at 0.12%. Higher Si content will lead to deterioration of weldability and toughness of the weld heat-affected zone. Therefore, its upper limit is specified as 0.24%.

[0024] Manganese (Mn) is an important strengthening and toughening element, playing a role in solid solution strengthening and improving the strength and toughness of steel. Mn also expands austenite formation, lowering the transformation temperature of supercooled austenite and promoting the transformation of low- and medium-temperature strengthening structures in steel, which is beneficial to improving steel strength. However, excessive Mn content increases hardenability, leading to a deterioration in weldability and the toughness of the weld heat-affected zone. Higher Mn content also increases costs. Therefore, the Mn content is specified to be limited to 0.6%–0.9%.

[0025] Phosphorus (P) is a major corrosion-resistant element in traditional atmospheric corrosion-resistant steels, promoting the formation of a protective rust layer and effectively improving the steel's atmospheric corrosion resistance. However, P tends to segregate at grain boundaries, reducing grain boundary bonding energy and the steel's toughness and plasticity. Furthermore, the coexistence of P and manganese (Mn) exacerbates temper brittleness, and segregated P makes the steel plate prone to intergranular fracture, reducing its impact toughness. P also negatively impacts weldability. This invention requires high toughness in its steel grade; therefore, P is controlled as an impurity element to minimize its content. The P content is limited to no more than 0.015%.

[0026] Sulfur (S) in steel is controlled as a harmful impurity element. S not only reduces the low-temperature toughness of steel but also promotes anisotropy in steel sheets, which is detrimental to cold forming performance. Furthermore, sulfide inclusions significantly reduce the weather resistance of steel. Therefore, the steel grade of this invention is designed with an extremely low S content, controlled below 0.006%.

[0027] Al is an element added to steel for deoxidation. Adding an appropriate amount of Al helps to refine the grains and improve the strength and toughness of the steel. However, a high amount of Al is not conducive to the casting of billets during continuous casting. Therefore, the control range is 0.02 to 0.04%.

[0028] Cr is a valuable alloying element and an effective element for improving the corrosion resistance of steel plates. Cr forms a continuous solid solution with Fe in steel, resulting in solid solution strengthening. Cr has a significant effect on improving the passivation ability of steel, promoting the formation of a dense passivation film or protective rust layer on the steel surface, while simultaneously increasing the self-corrosion potential and improving the steel's resistance to atmospheric corrosion. In this invention, Cr works in combination with Mo and Cu to improve the steel's resistance to sulfuric acid dew point corrosion. However, adding more Cr would increase manufacturing costs; therefore, considering cost reduction, this invention limits the Cr content to 0.60–0.95%.

[0029] In steel, Cu primarily functions as a solid solution and precipitation strengthener. Simultaneously, Cu's electrochemical potential is higher than Fe's, promoting the formation of a dense rust layer on the steel surface. Its interaction with Cr promotes passivation, improving corrosion resistance. Furthermore, a suitable amount of Cu combines with residual S in the steel to form a Cu₂S protective film, mitigating corrosion in acidic environments. However, excessive Cu content not only impairs the toughness of the weld heat-affected zone but also easily leads to network cracking during hot rolling, deteriorating the surface properties of the steel plate and increasing costs. Therefore, the Cu content is limited to 0.15–0.35%.

[0030] Ni is an austenite-forming element. Ni improves low-temperature impact toughness by refining grains and reducing stacking fault energy; grain refinement also has a grain-refining strengthening effect. Furthermore, Ni is an important element for improving the corrosion resistance of steel, promoting rust layer stability and mitigating hot-working brittleness caused by Cu. However, Ni is a valuable element, and to reduce costs, its content is limited to 0.05–0.12%.

[0031] Ti is a strong ferrite-forming element and a carbonitride-forming element, readily forming compounds with C, N, O, S, etc. In steel, Ti mainly exists in the form of TiC or Ti(C,N). In this invention, the addition of Ti primarily utilizes TiN to inhibit austenite grain growth, thus refining the microstructure; simultaneously, it produces precipitation strengthening during cooling. Furthermore, Ti inhibits the recrystallization of deformed austenite and promotes the formation of granular bainite. The precipitated Ti carbonitride particles can prevent grain coarsening in the weld heat-affected zone, improving weldability. However, excessively high Ti content leads to the easy growth and agglomeration of titanium nitride particles at high temperatures, impairing the steel's plasticity and toughness. Therefore, the Ti content is limited to 0.01–0.02%.

[0032] Nitrogen (Nb) is a strong nitride-carbide-forming element. During post-rolling cooling, it can combine with carbon and nitrogen in steel to form intermediate phases such as NbC, Nb(CN), and NbN. The resulting fine carbide particles refine the microstructure, producing fine-grain strengthening and precipitation strengthening effects, significantly improving the strength of the steel plate. Simultaneously, the refined microstructure contributes to improved toughness. Furthermore, Nb can inhibit the expansion of austenite interfaces, increasing the recrystallization temperature of steel and allowing for rolling in the non-recrystallization zone at higher temperatures. Therefore, adding an appropriate amount of Nb to steel is beneficial for strength improvement. However, higher Nb content leads to the formation of coarse carbonitride particles at grain boundaries, deteriorating impact toughness. Nb is also a precious alloying element, with its content limited to 0.01–0.02%.

[0033] Mo can exist in steel in solid solution, resulting in solid solution strengthening. During quenching heat treatment, Mo promotes the formation of bainite and martensite structures. It has phase transformation strengthening and dislocation strengthening effects. At the same time, Mo can increase the solubility of Nb, V and Ti, promoting precipitation strengthening. In low-alloy high-strength steel, the strength of the steel increases significantly with the increase of Mo content. The addition of Mo can improve the corrosion product film structure, improve the steel's resistance to pitting corrosion, crevice corrosion and other localized corrosion, and in particular, increase the adhesion of the protective rust layer on the surface, further improving corrosion resistance. In this invention, the combination of Cr and Mo is used to significantly improve the corrosion resistance in acidic media. However, higher Mo content is detrimental to weldability and has a higher cost, so the Mo content is limited to 0.03-0.12%, and the Cr and Mo content is limited to satisfy the relationship: 5 ≤ Cr / Mo ≤ 25.

[0034] Nitrogen (N) in steel can form nitrides with Al and Ti. These fine precipitates act as grain boundaries, refining austenite grains. Higher N content in steel readily combines with Al to form AlN, significantly increasing the amount of nitrides. When AlN exists independently as a non-metallic inclusion in steel, it disrupts the continuity of the steel matrix. This is especially true when the Al content is high, resulting in a larger quantity of AlN that is aggregated, thus exacerbating the problem. Furthermore, higher N content tends to accumulate at defects, worsening low-temperature impact toughness. Therefore, the N content must be controlled below 0.0060%.

[0035] In addition to the elements mentioned above, to further improve performance, the weathering steel composition of the present invention may further include one or more of Sn, RE, V and Ca.

[0036] Sn exhibits good corrosion inhibition in steel. Sn ions can dissolve in the anode, thus inhibiting the anodic reaction and reducing the formation of β-FeOOH, which is detrimental to corrosion resistance. Its content is limited to ≤0.12%. Rare earth elements (RE) form RE compounds, RE / Fe intermetallic compounds, and dissolved rare earth elements in steel. These hydrolyze in the thin corrosion film and precipitate at the cathode with a higher pH, thus inhibiting corrosion. Its content is limited to ≤0.12%. Volatile metals (V) are strong carbonitride forming elements and can precipitate during phase transformation. In steel, they provide solid solution strengthening and carbonitride precipitation strengthening, and increase tempering stability, thereby improving strength. Their content is limited to ≤0.05%. Adding calcium (Ca) to steel can alter the shape of sulfides, inhibit the hot brittleness of sulfur (S), and improve toughness. Its addition amount is limited to ≤0.005%.

[0037] As described above, this invention abandons the existing Sb-containing composition system design for weathering steel. Based on an appropriate amount of Cu-Ni, it achieves a significant improvement in corrosion resistance under acidic media by matching Cr and Mo. Cu in steel can form a Cu2S protective film with residual S, preventing anodic and cathodic reactions, thereby improving the steel's resistance to atmospheric corrosion and sulfuric acid dew point corrosion. However, S is detrimental to low-temperature toughness and formability. Therefore, in this invention, S is controlled as an impurity element, supplementing the acidic media corrosion resistance only through the formation of a small amount of Cu2S from Cu and residual S. The superior acidic media corrosion resistance is mainly achieved through the Cr-Mo combination. The addition of Cr increases the steel's self-corrosion potential, promotes the formation of a dense passivation film or protective rust layer on the steel surface, and forms FeCr2O4 with good anti-corrosion function within the rust layer, while significantly improving the steel's passivation ability. Under acidic media corrosion, Cr enables the rapid formation of a CrO passivation film on the steel surface, while also increasing the polarization potential of the passivation zone. The passivation film is a reactant of sulfuric acid corrosion. As the reaction products accumulate, the anodic potential gradually increases, causing anodic polarization and reducing corrosion, thereby slowing down the corrosion rate. The addition of Cu further promotes the passivation effect of the steel. The addition of Mo can improve the structure of the corrosion product film, enhance the steel's resistance to pitting corrosion, crevice corrosion, and other localized corrosion, and in particular, increase the adhesion of the protective rust layer on the surface, thus contributing to improved corrosion resistance in acidic media. In this invention, Mo is selected in combination with Cr to further improve corrosion resistance in acidic media, and the Cr and Mo contents are limited to satisfy the relationship: 5 ≤ Cr / Mo ≤ 25.

[0038] Based on Cu-Ni, and through the corrosion-resistant composition design of Cr-Mo, the steel plate of this invention exhibits excellent corrosion resistance, with a unit area weight loss of ≤0.8 g / m² under uniform corrosion test conditions of 10% H₂SO₄ + 3.5% NaCl solution. 2The requirements for corrosion resistance are comparable to or even better than existing Sb-containing sulfuric acid dew point steels; and it possesses atmospheric corrosion resistance comparable to conventional weathering steels, with a corrosion rate ≤55% compared to ordinary carbon steel, thus meeting corrosion resistance requirements in various environments. The new corrosion-resistant composition system avoids the toxic pollution of Sb to the environment and human body, while achieving excellent corrosion resistance, making it an environmentally friendly product.

[0039] Meanwhile, this invention utilizes the solid solution strengthening, precipitation strengthening, and grain refinement strengthening effects of Mo, Nb, and Ti to achieve a yield strength of over 450-620 MPa, while also possessing excellent low-temperature impact toughness. At -40℃, the low-temperature impact energy exceeds 100 J, achieving a balance between high strength and high toughness, meeting the requirements for high-strength and lightweight steel in railway vehicles. The steel plate also exhibits good formability, satisfying the processing requirements for steel used in railway vehicles.

[0040] The method for manufacturing weathering steel for railway vehicles according to the present invention includes the following steps:

[0041] 1) Smelting and casting

[0042] The above-mentioned components are smelted and cast into billets;

[0043] 2) Heating of the billet

[0044] The heating furnace is in a reducing atmosphere, and the billet exit temperature is controlled to be ≥1230℃.

[0045] 3) Rolling

[0046] The billet roughing end temperature is ≥1050℃, and the cumulative reduction rate during the roughing stage is ≥80%; the finishing rolling start temperature is ≥980℃, and the finishing rolling end temperature is 880±30℃.

[0047] 4) Cooling

[0048] The cooling process employs laminar flow cooling, with a cooling rate of ≥8℃ / s to 460~580℃ for winding.

[0049] Preferably, in step 1), the LF refining process is used during the smelting process.

[0050] Preferably, the winding temperature in step 3) is 480–580°C.

[0051] Preferably, step 4) cooling rate ≥ 20℃ / s.

[0052] In the method for manufacturing weathering steel for railway vehicles according to the present invention:

[0053] Step 2) Heating the billet: The heating furnace must be in a reducing atmosphere, and the billet exit temperature must be controlled above 1230°C. Cu has a low melting point and is prone to copper embrittlement during high-temperature heating. Adding 0.05–0.12% Ni to the steel helps mitigate this problem. Therefore, the billet heating of the steel grade of this invention does not need to consider copper embrittlement, and the temperature and time control range is wider, facilitating production organization.

[0054] Step 3) Rolling is performed using a hot continuous rolling process. In this invention, the addition of trace amounts of Nb ensures that the austenitizing temperature Ac3 of the steel is above 915℃. From Figure 1 It can be seen that the ferrite begins to form at approximately 849℃ under continuous cooling conditions. Considering the refinement of the microstructure after water cooling, the water cooling start temperature is required to be as high as possible. However, an excessively high finishing rolling end temperature also requires a correspondingly higher billet exit temperature, which will increase production energy consumption and production costs. Meanwhile, to avoid abrupt changes in rolling force caused by rolling in the two-phase region, the finishing rolling end temperature is set at 880±30℃, the billet roughing rolling end temperature is above 1050℃, the cumulative reduction rate during roughing rolling is ≥80%, and the finishing rolling start temperature is ≥980℃.

[0055] To ensure mechanical properties, the steel described in this invention requires a bainitic + ferrite matrix structure. To achieve fine-grained strengthening, the highest possible finishing rolling temperature is required, followed by immediate water cooling. Pearlite in the matrix is ​​a high-carbon component, readily forming galvanic cells and promoting corrosion. To improve the steel's corrosion resistance, the formation of pearlite in the matrix should be minimized. Figure 1 According to the CCT curve, controlling the cooling rate after rolling to above 8℃ / s (preferably ≥20℃ / s) can avoid the formation of pearlite during the cooling process. This reduces the difficulty of controlling laminar cooling during strip steel production and facilitates on-site production.

[0056] To achieve high yield strength, a greater amount of bainite strengthening phase is required in the matrix. This invention employs a Cr-Mo corrosion-resistant composition system, which, on the one hand, achieves better corrosion resistance, and on the other hand, shrinks the austenite region by both Cr and Mo. Cr reduces the diffusion rate of C in austenite, lowers the critical cooling rate, and improves the hardenability of the steel plate. The addition of 0.60–0.95% Cr can separate the pearlite and bainite transformation curves, shifting the proeutectoid ferrite and pearlite phase transformation curves to the right, thus lowering the critical cooling rate; it also results in a greater reduction in Bs (base saturation) and a smaller reduction in Ms (marginal saturation), which is beneficial for reducing the tendency for weld cracking, while refining the microstructure and lowering the ductile-brittle transition temperature. 0.03–0.12% Mo inhibits the formation of polygonal ferrite and pearlite in the steel, promoting the formation of bainite with a large number of dislocations within the crystal over a wider cooling rate range, thereby improving strength. See also... Figure 2According to the TTT temperature curve of the steel, around 542℃ is the temperature at which bainite forms most rapidly. To ensure sufficient bainite formation in the matrix, the coiling temperature needs to be controlled between 460 and 580℃, preferably between 480 and 580℃. The weathering steel described in this invention does not require post-rolling heat treatment, thus shortening the production cycle and reducing production costs.

[0057] The present invention has the following advantages:

[0058] The weathering steel described in this invention abandons the existing Sb composition design, avoiding harm to the environment and human health, and belongs to environmentally friendly products.

[0059] The steel of this invention exhibits excellent corrosion resistance, meeting the corrosion resistance requirements of coal freight cars. By replacing Sb with a Cr-Mo composition, its corrosion resistance in acidic media is comparable to or even better than existing Sb-containing steels. Under uniform corrosion testing conditions of 10% H₂SO₄ + 3.5% NaCl solution, the weight loss per unit area is ≤0.8 g / m². 2 The requirements for atmospheric corrosion resistance are met; the corrosion rate is also at the level of conventional weathering steel, and the corrosion rate is 55% lower than that of Q345B, thus meeting the corrosion resistance requirements in various environments.

[0060] The steel of this invention possesses excellent mechanical properties, with a yield strength of 450–620 MPa or higher, a tensile strength ≥600 MPa, an elongation A ≥18%, and an impact energy value of over 100 J at -40℃. It also exhibits good cold working properties, meeting the cold bending requirements of D=1a and 180°, and is easy to weld. These are characteristics not found in any existing weathering steels.

[0061] This invention employs controlled rolling and cooling production, delivering the rolled product without heat treatment; the steel plate has a low critical cooling rate for bainite, the steel grade has a larger process control window, the production process is simple, the production cycle is short, and it can be implemented using existing rolling mill equipment.

[0062] The difference between this invention and the prior art is that:

[0063] Compared to Patent 1 (Chinese Patent CN102127717A "High Corrosion Resistance Weathering Steel with Excellent Toughness"), which is designed with a high Cr-Ni composition, the Cr content is 2.5-7.0% and the Ni content is 0.2-1.2%, which is much higher than this steel grade.

[0064] The steel grade of this invention has a significantly different chemical composition from that of comparative patent 1. The steel grade of this invention achieves excellent corrosion resistance by adding Cr and Mo to Cu-Ni.

[0065] Compared to Patent 2 (Chinese Patent CN102268613A, "A Hot-Rolled Steel Plate Resistant to Atmospheric Corrosion for Railway Vehicles and Its Manufacturing Method"), which not only contains Sb but also high levels of Si, P, Cr, Ni, and Ti, V, Ca, Mg, and Ce are also required. The higher Si and P content negatively impacts low-temperature toughness and formability, while the higher Cr and Ni content increases production costs, and the addition of Mg and Ce increases production difficulty. In particular, Sb is a typical toxic and harmful heavy metal element, exhibiting chronic toxicity and potential carcinogenicity in humans and animals.

[0066] Compared to patent 3 (Japanese invention JP2001164335A "HIGH WORKABILITY AND GOOD WELDABILITYSULFURIC ACID DEW POINT CORROSION RESISTANT STEEL SHEET"), in addition to Sb, the content of Si and Ni is also very high.

[0067] The performance requirements of the steel of this invention are completely different from those of comparative patents 1 to 3. The steel of this invention has a yield strength of 450-620 MPa or more, an elongation of more than 18%, and an impact energy of more than 100 J at -40℃. It has good corrosion resistance in acidic corrosive media and industrial atmospheric environments.

[0068] In contrast, the yield strength of the steel grades in Comparative Patents 1-3 is generally lower than that of this invention, and their low-temperature impact toughness is also inferior. Comparative Patent 1 only exhibits good corrosion resistance in atmospheric environments and does not meet the requirements for corrosion resistance in acidic media.

[0069] This invention uses a hot continuous rolling process. During the post-rolling cooling process, only the cooling rate needs to be controlled above 8°C / s. This provides a wider process window, reduces production difficulty, and facilitates on-site production.

[0070] In contrast, Patent 1 requires controlling the post-rolling cooling rate to 5-20℃ / s, specifying both an upper and lower limit, which obviously increases the difficulty of controlling water cooling. Patent 2, on the other hand, requires final rolling at 880-950℃ followed by 1-35 seconds of air cooling, then cooling at a rate of over 10℃ / s to 550-690℃ for coiling. This post-rolling air cooling followed by water cooling process clearly increases production difficulty, especially since air cooling prolongs production time and affects production rhythm. Furthermore, its 880-950℃ final rolling temperature, compared to higher billet heating temperatures, increases energy consumption and production costs. Attached Figure Description

[0071] Figure 1 The phase transformation temperature curve (CCT) of the weathering steel described in this invention;

[0072] Figure 2The phase transformation temperature curve (TTT) of the weathering steel described in this invention;

[0073] Figure 3 This is a micrograph of the weathering steel of Example 2 of the present invention. Detailed Implementation

[0074] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0075] The chemical composition of the weathering steel described in this invention is shown in Table 1, the production process parameters are shown in Table 2, and the mechanical properties and corrosion resistance of the weathering steel described in this invention are shown in Table 3.

[0076] Example A

[0077] According to the chemical composition requirements of the steel of this invention, converter smelting is adopted, followed by post-furnace refining to obtain the chemical composition shown in Table 1. The continuous casting billet is cut and sent to the hot rolling production line. The billet heating temperature is 1234℃, the final rolling temperature is 904℃, and the coiling temperature is 579℃.

[0078] Example B

[0079] According to the composition requirements of this invention, the steel of this invention was smelted in a 500kg vacuum induction furnace in a laboratory. The chemical composition is shown in Table 2. The billet heating temperature was 1237℃, the final rolling temperature was 909℃, and after rolling, it was accelerated cooled to 563℃ for coiling, followed by air cooling to room temperature. The microstructure of the prepared steel plate was bainite + ferrite (see Table 2). Figure 3 ).

[0080] According to TB / T2375 "Cyclic Immersion Corrosion Test Method for Weathering Steel for Railways", the corrosion rate is less than 55% compared to Q345B. Under acidic media corrosion conditions, the full immersion test was conducted according to GB 10124-1988 "Laboratory Uniform Corrosion Full Immersion Test Method for Metallic Materials", with a test solution of 10.0% H2SO4 + 3.5% NaCl, a test time of 24 hours, and a test temperature of 23±2℃. The corrosion resistance is comparable to or even better than existing materials such as S450AW, and the weight loss per unit area under the uniform corrosion test conditions of 10% H2SO4 + 3.5% NaCl solution meets the requirement of ≤0.8 g / m². 2 The requirements for atmospheric corrosion resistance are met; the corrosion rate is also at the level of conventional weathering steel, and the corrosion rate is 55% lower than that of Q345B, thus meeting the corrosion resistance requirements in various environments.

[0081] The steels obtained according to the steel composition design range and rolling process control technology of this invention exhibit good mechanical properties within a thickness range of 1.5-10 mm (but not limited to this range). Their yield strength is 450-620 MPa, tensile strength ≥600 MPa, elongation ≥18%, and impact energy at -40℃ exceeds 100 J (for a standard 10*10*55 mm specimen). This indicates that the steel plates described in this invention have a good process window, reducing on-site production difficulty and facilitating production organization.

[0082] The weathering steel described in this invention, in addition to possessing high strength and high toughness, also exhibits excellent corrosion resistance under atmospheric and acidic corrosive media conditions, meeting the harsh corrosive environment of coal freight cars. Simultaneously, it boasts excellent elongation and good formability, making it particularly suitable for various cold forming processes in railway freight car production. Besides its use in coal transportation, the weathering steel described in this invention demonstrates corrosion resistance in acidic media environments comparable to or even better than existing materials such as S450AW, with a unit area weight loss of ≤0.8 g / m² under uniform corrosion testing conditions of 10% H₂SO₄ + 3.5% NaCl solution. 2 It meets the requirements of ·h; its atmospheric corrosion resistance also reaches the level of conventional weathering steel, with a corrosion rate of less than 55% compared to Q345B, thus meeting the corrosion resistance requirements in various environments; it can also be used in equipment with acidic media corrosion such as gas pipelines and boiler preheating pipelines, as well as in the fabrication of steel structures with conventional requirements for industrial atmospheric corrosion resistance.

[0083]

[0084]

[0085]

Claims

1. A weathering steel for railway vehicles, comprising the following composition by weight percentage: C: 0.04–0.09%, Si: 0.12–0.24%, Mn: 0.6–0.9%, P≤0.015%, S≤0.006%, Al: 0.02–0.04%, Cu: 0.15–0.35%, Cr: 0.60–0.95%, Ni: 0.05–0.12%, Mo: 0.03–0.12%, Ti: 0.0 1~0.02%, Nb: 0.01~0.02%, N≤0.006%, balance includes Fe and unavoidable impurity elements; and must simultaneously satisfy: 5≤Cr / Mo≤25; the yield strength of the weathering steel plate is 450~620MPa, tensile strength exceeds ≥600MPa, elongation A≥18%, impact energy value at -40℃≥100J; under the corrosive environment of a simulated coal transport vehicle, the weight loss per unit area of ​​the steel plate is ≤0.8g / m². 2 • h; Corrosion rate relative to ordinary carbon steel under industrial atmospheric conditions ≤ 55%.

2. The weathering steel for railway vehicles as described in claim 1, characterized in that, It also includes one or more of Sn≤0.15%, RE≤0.15%, V≤0.05%, and Ca≤0.005%.

3. The method for manufacturing weathering steel for railway vehicles as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Smelting and casting Smelting and casting into billets according to the composition described in claim 1 or 2; 2) Heating of the billet The heating furnace is in a reducing atmosphere, and the billet exit temperature is controlled to be ≥1230℃. 3) Rolling The billet roughing end temperature is ≥1050℃, and the cumulative reduction rate during the roughing stage is ≥80%; the finishing rolling start temperature is ≥980℃, and the finishing rolling end temperature is 880±30℃. 4) Cooling The cooling process employs laminar flow cooling, with a cooling rate of ≥8℃ / s to 460~580℃ for winding.

4. The method for manufacturing weathering steel for railway vehicles as described in claim 3, characterized in that, Step 1) The LF refining process is adopted in the smelting process.

5. The method for manufacturing weathering steel for railway vehicles as described in claim 3, characterized in that, Step 3) Winding temperature 480~580℃.

6. The method for manufacturing weathering steel for railway vehicles as described in claim 3, characterized in that, Step 4) Cooling rate ≥ 20℃ / s.

Images