Hardness 500hbw steel plate for erosion and delayed cracking resistant slurry pipeline and method for producing the same

Abstract

Hardness 500HBW slurry dredging pipe with anti-delayed cracking and erosion resistant steel plate and its production method, its component weight percentage is: C 0.23~0.26%, Si 0.6~1.2%, Mn 1.0~1.4%, P≤0.015%, S≤0.005%, Al 0.2~0.8%, Ni 0.05~0.15%, B 0.001~0.003%, N≤0.005%; contain one or both of Nb 0.01~0.03%, Ti 0.01~0.03%, the rest is Fe and inevitable impurities, and meet: 6.65N < Nb+Ti≤0.04.Its yield strength≥1200MPa, tensile strength≥1500MPa, elongation≥10%, hardness 500±30HBW, -40℃ impact energy value≥60J, erosion resistance reaches 2.5 times of current ordinary steel plate; at the same time, cracking time under U-bending 0.1mol / L hydrochloric acid solution immersion test condition is more than 580h, which embodies excellent anti-delayed cracking performance; suitable for reclamation, channel dredging and other fields of dredging pipeline production.

Description

Technical Field

[0001] This invention relates to the field of low alloy steel manufacturing, and particularly to a 500HBW hardness resistant, wear-resistant steel plate for slurry dredging pipes with delayed cracking resistance and abrasion resistance, and its production method. Background Technology

[0002] In land reclamation, channel dredging, and dike maintenance, large quantities of solid particles such as silt and gravel are transported long distances in the form of slurry through dredging pipelines. The pipelines are simultaneously subjected to electrochemical corrosion from the slurry medium and abrasion from the solid particles, as well as the interaction between the two. This abrasion is particularly severe when seawater slurry contains weathered rock, coral reefs, and medium-coarse sand. Existing dredging pipelines are mostly made of ordinary Q235B and Q345B steel, which have a short service life under harsh operating conditions, sometimes failing within a year. Due to the interaction of corrosion and abrasion during failure, the material failure caused by abrasion is far greater than the sum of corrosion and abrasion alone. Therefore, the steel used for dredging pipes must not only have wear resistance but also corrosion resistance, thus possessing excellent abrasion and corrosion resistance. To reduce dredging costs, higher-strength, abrasion-resistant steel plates are required for dredging pipes to extend pipeline life. Studies show that high-strength steel plates suffer from delayed cracking in corrosive dredging operations, so the problem of delayed cracking must be solved in high-strength wear-resistant steel for dredging pipes.

[0003] Numerous patented technologies have been disclosed regarding improving the wear resistance of steel materials. For example, Chinese patent CN102517509A discloses "HB500 grade wear-resistant steel plate and its preparation method," and Chinese patent CN103397275A discloses "a martensitic series wear-resistant steel and its preparation method." Both patents involve adding large amounts of alloying elements such as Cu, Mo, and Ni to the C-Mn base, resulting in higher alloy costs. The steel grades involved in these patents achieve a hardness of approximately 500 HBWW and exhibit good wear resistance, primarily used in engineering machinery and mining equipment. However, because the composition design of these steel grades does not consider corrosion inhibition, they cannot meet the requirements under corrosion and wear conditions, and there are no requirements for their resistance to delayed cracking.

[0004] Numerous patents have been filed and published abroad for wear-resistant applications, primarily for use in engineering machinery manufacturing, but not in the field of slurry transportation, which has abrasive properties. For example, Japanese patents JP2007231321A and JP2008169443A disclose "wear-resistant steel sheet" and "wear-resistant steel sheet superior in workability and manufacturing method therefor," respectively. These patents disclose methods to improve wear resistance through the precipitation of Ti and W carbide particles. The composition also contains valuable alloying elements such as Cu, Ni, Cr, and Mo. Although the steel grades have high hardness and wear resistance, the former's hardness is generally between 396-431 HBW, while the latter is less than 300 HBW, failing to reach the 500 HBW hardness level. At the same time, the large number of carbide particles in the matrix act as "cathodes" under corrosive wear conditions, promoting electrochemical corrosion, reducing corrosion resistance, and worsening the material's wear resistance. Furthermore, they do not address resistance to delayed cracking, making it difficult to meet the abrasive requirements of large particle size and high slurry content in slurry dredging environments.

[0005] Another example is the "Corrosion-resistant and wear-resistant steel and its preparation method" disclosed in Chinese patent CN101886225A. This patent involves a steel grade with up to 0.4-0.9% C and 14-16% Mn, and the contents of Mo and Cr are both 5-10%. In addition, it also contains a certain amount of rare elements such as Pr, Nd and Gd, which belongs to high alloy steel and has a very high cost.

[0006] Chinese patents CN102776445A and CN10893002A disclose "A lower bainitic wear-resistant steel pipe for slurry transportation and its manufacturing method" and "Abrasion-resistant steel plate for slurry dredging pipe with a hardness of 500HB and its production method." The former has a bainitic or bainitic + acicular ferrite structure with low matrix hardness and a tensile strength of only 600-800MPa. It is mainly used in environments with slight wear, such as the transportation of fine-particle (tens of μm) mineral slurry or crude oil, and is not suitable for the transportation of large-particle, high-density seawater slurry. The latter is a wear-resistant steel plate with a hardness of 500HBW, which has certain wear and corrosion resistance and is used for the processing of dredging pipes. However, as an ultra-high-strength wear-resistant steel with a hardness of 500HBW, its composition design and performance requirements do not consider the problem of delayed cracking. During dredging operations, steel plates are prone to crack initiation when subjected to impacts or scratches from hard objects. In particular, delayed cracking can occur in corrosive environments, leading to leakage or even cracking of the pipe body during dredging, which affects the smooth progress of the dredging operation.

[0007] During dredging operations, dredging pipelines, as a crucial component, face corrosion both internally and externally during use. Simultaneously, the outer wall of the pipeline inevitably endures impacts and scratches from hard objects. When the strength of the pipe steel plate is low, such as Q235B or 3Q345B, the low yield strength of the steel plate allows it to absorb impact energy through deformation, thus ensuring the safety of the pipeline. However, for high-strength steel plates, especially ultra-high-strength steel plates with a yield strength exceeding 1000 MPa, the stress borne by such damage is unlikely to exceed the yield strength, preventing deformation and leading to the initiation and propagation of cracks at the damaged site. In corrosive environments, crack initiation and propagation promote hydrogen penetration and diffusion, particularly electrochemical corrosion, which also promotes hydrogen precipitation and accumulation. Hydrogen atoms penetrate into the internal crystal lattice of the steel, increasing the vacancy concentration and forming micropores of vacancy clusters. This further promotes the initiation of microcracks, leading to brittle fracture, or delayed cracking, in the steel plate. This will significantly impact the normal progress of dredging operations, shorten the service life of dredged pipelines, and increase dredging costs. The higher the strength of the steel plate, the more susceptible it is to hydrogen permeation. Therefore, under dredging conditions, even without surface damage, hydrogen-induced delayed cracking can occur. High-strength steel plates used in dredging pipelines must be designed with resistance to delayed cracking in mind.

[0008] Existing patents reveal that current wear-resistant steels either do not consider corrosion resistance or delayed cracking under high stress, making them unsuitable for manufacturing dredging pipes. Summary of the Invention

[0009] The purpose of this invention is to provide a delayed cracking and wear-resistant steel plate for slurry dredging pipes with a hardness of 500HBW and its production method. The steel plate has a yield strength ≥1200MPa, tensile strength ≥1500MPa, elongation ≥10%, hardness 500±30HBW, impact energy value at -40℃ ≥60J, and wear resistance 2.5 times that of ordinary steel plates. Furthermore, under U-bending immersion in 0.1mol / L hydrochloric acid solution, the cracking time is over 580 hours, demonstrating excellent delayed cracking resistance. It is suitable for dredging pipe fabrication in land reclamation, waterway dredging, and other fields. Even under corrosive environments, the surface is not susceptible to cracking or leakage when subjected to impacts or scratches, thus significantly improving dredging efficiency and reducing operating costs.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] The steel plate for slurry dredging pipes with a hardness of 500HBW, resistant to delayed cracking and wear, has the following composition by weight percentage: C: 0.23-0.26%, Si: 0.6-1.2%, Mn: 1.0-1.4%, P≤0.015%, S≤0.005%, Al: 0.2-0.8%, Ni: 0.05-0.15%, B: 0.001-0.003%, N≤0.005%; it contains one or both of Nb: 0.01-0.03% and Ti: 0.01-0.03%, with the balance being Fe and other unavoidable impurity elements, and must satisfy: 6.65N<Nb+Ti≤0.04.

[0012] Furthermore, the wear-resistant steel plate of the present invention also contains one or more of the following: Cr≤1.5%, Cu≤0.3%, W0.01~0.5%, Mo0.01~0.5%, Sb0.01~0.2%, RE0.01~0.2%, V0.01~0.2%, and Ca0.001~0.01%.

[0013] In the composition design of the anti-delayed cracking and wear-resistant steel plate described in this invention:

[0014] Carbon (C) is the cheapest strengthening element in steel and can significantly improve the strength of steel plates. However, excessive C is detrimental to the weldability, toughness, and plasticity of steel plates. The C content is limited to 0.23-0.26% while meeting performance requirements.

[0015] Si is a deoxidizing element and a solid solution strengthening element, as well as a commonly used corrosion-resistant element in atmospheric corrosion-resistant steels. In steel, Si replaces Fe atoms through substitution, hindering dislocation movement and thus achieving solid solution strengthening. Simultaneously, Si can reduce the diffusion coefficient of C in ferrite, inhibiting carbide formation and suppressing the precipitation of coarse carbides at defects, thereby improving toughness. As a corrosion-resistant element, Si can form complex oxides with other elements such as Ca and Al. SiO2 is an acidic oxide, which can weaken the alkalization caused by Ca, thus making the pH value of the corrosion interface suitable for the formation of a protective rust layer. Si is enriched in the surface layer, and the distribution of Si and Cr in the passivation film is almost synchronous, increasing the inhibition of anodic dissolution while decreasing the inhibition of cathodic oxygen absorption. With appropriate content, Si has a relatively large inhibitory effect on both anodic and cathodic reactions, thus achieving the best corrosion resistance at the uniform corrosion rate. Adding silicon to iron-based alloys can form a dense SiO2 oxide film (passivation film). The coexistence of Si and Cr promotes the formation of a stable and dense silicate film on the surface, which is beneficial for improving corrosion resistance in polluted seawater and alternating wet and dry seawater environments. This invention achieves corrosion inhibition through the addition of Si and its combination with other corrosion-resistant elements. However, excessive Si promotes the graphitization of C, which is detrimental to toughness; it also negatively impacts surface quality and weldability. Therefore, the Si content is limited to 0.6-1.2%.

[0016] Mn is also a common strengthening element in steel. It improves yield strength through solid solution strengthening, reduces elongation, significantly lowers the phase transformation temperature of steel, and refines the microstructure of steel. It is an important strengthening and toughening element. However, excessive Mn content increases hardenability, which leads to deterioration of weldability and toughness of the weld heat-affected zone. Therefore, this invention controls the Mn content between 1.0-1.4%.

[0017] P is a major corrosion-resistant element in traditional atmospheric corrosion-resistant steel, promoting the formation of a protective rust layer on the surface and effectively improving the steel's atmospheric corrosion resistance. However, during the abrasion process, the formation of the surface rust layer will accelerate the abrasion weight loss of the material, reduce its abrasion resistance, and at the same time, the presence of P is prone to segregation, reducing the steel's toughness and plasticity, and making the steel plate brittle and affecting its toughness. Therefore, the P content in steel should be reduced as much as possible. In this invention, the P content is required to be controlled below 0.015%.

[0018] Sulfur (S) can improve the yield strength of steel, but its presence will worsen the steel's resistance to atmospheric corrosion and make the steel plate brittle, reducing the steel's low-temperature toughness. Therefore, the S content should be controlled below 0.005%.

[0019] Al is a ferrite-forming element, typically added to steel as a deoxidizer during the steelmaking process. Trace amounts of Al precipitate fine AlN during steelmaking, which refines austenite grains during subsequent cooling, improving the steel's strength and toughness. However, the addition amount is generally below 0.06%. Al is also a corrosion-resistant element; it increases the corrosion potential of steel, which helps inhibit corrosion during the abrasion process and improves corrosion resistance. The formation and aggregation of complex nanoscale oxides containing Al and Si in the inner rust layer can increase the charge-mass resistance, thereby inhibiting the corrosion process. This invention improves the corrosion resistance of steel through the combined application of Al and Si. However, excessive Al reduces the strength of the steel plate and increases the brittleness of ferrite, leading to a decrease in steel toughness. Simultaneously, when AlN exists independently as a non-metallic inclusion in the steel, it disrupts the continuity of the steel matrix. This is especially true when the Al content is high, resulting in a large number of AlN particles that are aggregated, exacerbating the problem and simultaneously forming oxides with poor plasticity. Therefore, its content is limited to 0.2-0.8%.

[0020] B has good hardenability, which improves the hardness of steel plates, but too high a B content is detrimental to welding, so its content is controlled at 0.001-0.003%.

[0021] Ni exists in steel in solid solution form and does not form carbides, thus contributing to austenite formation. The addition of Ni refines the grain size and improves low-temperature impact toughness by refining the grains and reducing stacking fault energy. In particular, Ni promotes rust layer stability and mitigates hot-working brittleness caused by Cu. In high-strength steels, nickel can also homogenize the microstructure, inhibit hydrogen diffusion, and reduce the content of irreversible hydrogen traps, thereby improving resistance to delayed cracking. However, Ni is a precious element, and its content is limited to 0.05-0.15%.

[0022] Nitrogen (Nb) is a strong nitrogen-carbide-forming element, capable of combining 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 and produce precipitation strengthening, significantly improving the strength of the steel plate. Furthermore, Nb can inhibit the expansion of austenite interfaces, increasing the recrystallization temperature of steel and enabling rolling in the non-recrystallization zone at higher temperatures. Therefore, adding an appropriate amount of Nb to steel is beneficial for strength improvement. The carbonitrides formed by Nb can pin austenite grain boundaries during austenitization, inhibiting abnormal austenite grain growth and improving the toughness of the steel plate after quenching. However, excessive Nb is detrimental to welding. Additionally, it readily forms brittle metallic hydrides with hydrogen, exhibiting significant differences in ductility and toughness compared to the matrix, and also exhibiting poor adhesion to the matrix, leading to delayed cracking. Therefore, this invention controls the Nb content to 0.01-0.03%.

[0023] Adding 0.01-0.03% Ti serves two purposes: firstly, it inhibits austenite grain growth during slab reheating, and secondly, it suppresses ferrite grain growth during recrystallization controlled rolling, thus improving the steel's toughness. Furthermore, Ti preferentially combines with nitrogen (N) in the steel, reducing the amount of AlN. However, excessive Ti content is detrimental to low-temperature impact toughness, and like Nb, it readily forms brittle hydrides with hydrogen, negatively impacting resistance to delayed cracking.

[0024] Nitrogen (N) in steel can form nitrides with Nb, V, and Ti. These fine precipitates act as grain boundaries, refining austenite grains. The precipitated nitrides also provide precipitation strengthening. However, higher N levels in steel readily combine with Al to form AlN, significantly increasing the amount of nitrides in the steel. 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 aggregates, exacerbating its harmful effects. Furthermore, higher N levels tend to accumulate at defects, worsening low-temperature impact toughness. Similar to carbon (C), N readily agglomerates at dislocations, forming Cotillard atmospheres and leading to strain concentration. This invention adds a higher Al content to improve corrosion resistance and delayed cracking resistance. To reduce AlN formation in the steel, N is consumed through the formation of Nb and Ti nitrides; therefore, the N content must be controlled below 0.0050%. To minimize the adverse effects of N, the contents of the three components must satisfy the following relationship: 6.65N<Nb+Ti≤0.04.

[0025] In addition to the elements mentioned above, to further improve performance, one or more of Cr, Cu, W, Mo, Sb, RE, V, and Ca can be added to the steel. The specified ranges for each element are: Cr ≤ 1.5%, Cu ≤ 0.3%, W and Mo: 0.01-0.5%, Sb, RE, and V: 0.01-0.2%, and Ca: 0.001-0.01%.

[0026] Cr is an important corrosion-resistant element with solid solution strengthening effects. The addition of Cr effectively increases the self-corrosion potential of steel, inhibiting corrosion and thus reducing the promoting effect of corrosion on material failure during abrasion, thereby improving abrasion resistance. In particular, the improved corrosion resistance reduces hydrogen evolution during corrosion, further enhancing resistance to delayed cracking. However, Cr is a valuable alloying element, and a high Cr content promotes the formation of a protective rust layer on the steel surface. Under abrasive conditions, this rust layer rapidly detaches from the surface, accelerating material abrasion failure. Therefore, Cr is added, with its content limited to an upper limit of 1.5%.

[0027] Cu has solid solution and precipitation strengthening effects. At higher content, it exhibits a secondary hardening effect during tempering at appropriate temperatures, thereby increasing strength. Cu is also an element that improves corrosion resistance; its electrochemical potential is higher than that of Fe, promoting the densification and stabilization of the rust layer on the steel surface. Furthermore, adding copper to steel can inhibit hydrogen diffusion and reduce sensitivity to hydrogen-induced cracking. However, excessive Cu can cause cracks in the steel billet during heating and hot rolling, deteriorating surface properties. Therefore, in this invention, Cu can be selectively added, with an upper limit of ≤0.3%.

[0028] Mo (Mo) exhibits phase transformation strengthening and dislocation strengthening effects, improving the tempering stability of steel, mitigating temper softening, suppressing high-temperature temper brittleness, and enhancing the low-temperature impact toughness of steel plates. W (W) forms carbides in steel, generating secondary strengthening and solid solution strengthening effects, and during over-aging, it inhibits the segregation of impurity atoms and non-metallic inclusions at grain boundaries, thus improving fracture toughness. The addition of rare earth elements (RE) improves corrosion resistance; it forms RE compounds, RE / Fe intermetallic compounds, and solid-solution rare earth elements in steel, which hydrolyze in the thin corrosion film and precipitate at the cathodic site with a high pH value, thereby acting as a corrosion inhibitor. Sb can combine with Cu in steel to form a Cu2Sb film on the surface, further improving corrosion resistance. V (V) is also a strong carbonitride forming element, precipitating during phase transformation. In steel, it has solid solution strengthening and carbonitride precipitation strengthening effects, and increases tempering stability, thereby improving strength. The addition of Ca (Ca) to steel can change the shape of sulfides, suppress the hot brittleness of S, and improve toughness.

[0029] The steel grade designed with the above-mentioned composition achieves a high-strength martensitic structure with a yield strength ≥1200MPa, tensile strength ≥1500MPa, elongation ≥10%, hardness 500±30, and impact energy ≥60J at -40℃, thus exhibiting excellent wear resistance. Simultaneously, the addition of an appropriate amount of corrosion-resistant alloy increases the self-corrosion potential of the matrix, effectively slowing down and inhibiting corrosion, thereby improving the material's corrosion resistance and resulting in excellent abrasion resistance, reaching 2.5 times that of existing ordinary carbon steel materials such as Q235B. Furthermore, through compositional design and performance optimization, it achieves excellent resistance to delayed cracking. High-strength dredging pipes made from this material are particularly suitable for transporting large-particle, high-density slurries, and are less prone to cracking and leakage during use.

[0030] The method for producing the 500HBW hardness resistant and wear-resistant steel plate for slurry dredging pipes according to the present invention includes the following steps:

[0031] 1) Smelting and casting

[0032] The above-mentioned components are smelted and cast into billets;

[0033] 2) Heating of the billet

[0034] The heating temperature is above 1230℃, and the total heating time in the heating furnace is no less than 2 hours.

[0035] The heat preservation time should be no less than 40 minutes.

[0036] 3) Rolling

[0037] The roughing stage adopts a large reduction rolling process, controlling the reduction rate per pass to be above 15% or the reduction per pass to be above 25mm. At the same time, the deformation ratio in the roughing stage is greater than 80%, and the reduction rate of the last pass in the finishing stage is controlled to be not less than 16%. The finishing temperature is ≥850℃, preferably ≥880℃.

[0038] 4) Cooling

[0039] Laminar flow cooling is used, and the coil is wound up after cooling to 550-680℃.

[0040] 5) Heat treatment

[0041] The steel plate is subjected to quenching and tempering treatment, including:

[0042] The quenching heating temperature is 810~835℃. The quenching holding time T1 starts from the core of the steel plate when it reaches the temperature. T1=(1~2)×H, where T is in min and H is the plate thickness in mm. After the steel plate is taken out of the furnace, it is directly water quenched to room temperature. The cooling rate is required to be ≥50℃ / s.

[0043] The tempering temperature is 180~240℃, and the tempering holding time T2 starts from the core of the steel plate when it reaches the temperature. T2=(2~3)×H, where T is in min and H is the plate thickness in mm. T2≥12min.

[0044] Finally, the tempered steel plate undergoes finishing treatment.

[0045] Preferably, in step 1), the billet is hot-charged into the furnace after casting, that is, after confirming that there are no quality problems on the surface of the billet, it is directly transported from the casting area to the heating furnace through the roller conveyor for heating and heat preservation, thereby reducing energy consumption; if it cannot be hot-charged, the cast billet must be placed in the heat preservation pit for slow cooling after casting, and the heat preservation pit can be removed for air cooling after the temperature drops below 200°C.

[0046] Preferably, in step 5), the steel coil cooled to room temperature is uncoiled and straightened before being cut into plates, and then the steel plates are quenched and tempered.

[0047] Preferably, the thickness of the obtained wear-resistant steel plate is 8 to 20 mm.

[0048] In the production method of the 500HBW hardness slurry dredging pipe anti-delayed cracking and wear-resistant steel plate of the present invention:

[0049] Rolling is divided into two stages: roughing and finishing. To obtain a fine original austenite grain size, the billet is rolled with a large reduction in the roughing stage, controlling the reduction per pass to be above 15% or the reduction per pass to be above 25mm, within the allowable conditions of the mill load. To obtain a fine grain size and good plate shape, the deformation ratio in the roughing stage is greater than 80%, and the reduction in the final pass of finishing is controlled to be no less than 16%.

[0050] Since this invention relates to offline heat treatment of steel grades after rolling, there are no special requirements for the rolling temperature of the cast billet. However, in order to reduce the rolling load, the highest possible finishing rolling and coiling temperatures are used. Figure 1 Based on the continuous transformation curve, the α→γ transformation point of the steel grade is approximately 771℃. Therefore, a final rolling temperature above 880℃ is recommended to ensure rolling in the complete austenitic region, thereby achieving low and stable rolling loads, which is beneficial for obtaining high-quality sheet shape. For thicker steel plates, the final rolling temperature can be appropriately reduced, but it should not be lower than 850℃. After rolling, the steel coil is cooled to between 550-680℃ by laminar flow cooling and then coiled. If the temperature is too high, the cooling rate will be too low, resulting in coarse grains in the steel coil, and it will also be detrimental to the coiling machine. If the temperature is too low, bainite structure is easily formed, which increases the strength of the steel plate and increases the difficulty of subsequent uncoiling and straightening.

[0051] After cooling to room temperature, the steel coils are uncoiled, straightened, and then cut into plates. These plates are then quenched and tempered. The quenching temperature directly affects the grain size of the subsequent martensite structure, thus influencing the toughness of the steel plate. To ensure sufficient austenitization of the matrix, a heating temperature of 30-50°C above the Ac3 point is generally used. Excessively high heating temperatures can coarsen the austenite grains, resulting in a coarse martensite structure after quenching and deteriorating toughness; conversely, insufficient heating temperatures lead to incomplete austenitization, preventing the formation of a fully martensitic structure after quenching and negatively impacting toughness. The holding time also has a similar effect on quenching performance. Excessive holding time can lead to coarse grains, increased energy consumption, and higher costs; insufficient holding time results in incomplete austenitization, causing the hardness and strength to fail to meet requirements after quenching. To achieve outstanding low-temperature toughness, this invention specifically employs a critical zone quenching process for the steel plate. The quenched microstructure in the critical zone contains undissolved acicular ferrite. Although this undissolved acicular ferrite reduces strength, it reaches the strength limit before martensite under external force, causing cracks to initiate and propagate first within it, absorbing energy and thus improving toughness. The quenching temperature should be controlled between -5℃ and +20℃ above Ac3, i.e., 810-835℃, to obtain better low-temperature toughness. The quenching holding time T1 is calculated from the core of the steel plate and is 1-2 times (min) the steel plate thickness H (mm). After the steel plate is removed from the furnace, it should be directly water-quenched to room temperature, requiring a cooling rate ≥50℃ / s.

[0052] Tempering primarily reduces and eliminates quenching stress, improving toughness and resilience. Higher tempering temperatures can lead to excessive reductions in the strength and hardness of the steel plate, failing to meet design requirements and increasing costs. Therefore, the tempering process parameters for steel plates should be limited. In this invention, the steel plate is tempered within the range of 180-240℃. The tempering holding time T2 is started from the core of the steel plate reaching the desired temperature and is 2-3 times (min) the plate thickness H (mm), but not less than 12 min. Finally, the quenched and tempered steel plate undergoes finishing treatment (straightening and edge trimming), and is released from the factory after passing performance tests.

[0053] The present invention has the following advantages:

[0054] This invention employs a simple and economical C-Mn composition design, supplemented with small amounts of Nb and Ti microalloying elements, to achieve high hardness in the steel. Simultaneously, the addition of Al and Si, along with the selective addition of corrosion-resistant elements such as Cr, Cu, and Ni, increases the matrix potential, inhibiting corrosion during the abrasion process and improving the corrosion resistance of the steel plate. This results in excellent abrasion resistance in corrosive environments, particularly under conditions of transporting large-particle, high-density seawater slurry, where its abrasion resistance is more than 2.5 times that of ordinary pipes.

[0055] This invention relates to steel grades with good low-temperature impact toughness and cold bending performance, which meet the pipe manufacturing requirements of subsequent dredging pipelines, and can easily manufacture pipes from high-hardness steel plates based on existing equipment.

[0056] The steel grade involved in this invention significantly improves the steel plate's resistance to delayed cracking by enhancing its toughness and corrosion resistance, thereby reducing the risk of cracking and leakage in dredging pipes during use, improving dredging efficiency, and reducing maintenance costs.

[0057] This invention relates to a steel grade with a simple production process and low content of precious alloying elements, which reduces production difficulty and cost, and facilitates the widespread promotion of the steel grade.

[0058] The present invention differs significantly from existing comparative patents in terms of steel composition and performance.

[0059] In terms of composition, compared with Patent 1 (Chinese Patent CN102776445A "A bainitic wear-resistant steel pipe for slurry transportation and its manufacturing method"), it is required to add 0.01-1.0% of Mo, Ca and RE, and also requires N content of 0.01-0.1% to improve strength through N. At the same time, the composition has a high Mn content, with the upper limit of Mn content reaching 5%, which is close to the composition of medium manganese steel.

[0060] Compared to patent 2 (Chinese patent CN101886225A "A corrosion-resistant and wear-resistant steel and its preparation method"), the C, Mn and Cr contents are as high as 0.4-0.9%, 14-16% and 5-10% respectively, and it requires the addition of a variety of rare elements such as Pr, Dy, Gd and Nd.

[0061] Compared with Patent 3 (Chinese Patent CN10893002A "500HBW Hardness Slurry Dredging Pipe Wear-resistant Steel Plate and Its Production Method"), which uses a Cr-Al corrosion-resistant system, the Al-Si composition system of this invention is different and has a higher cost.

[0062] Furthermore, the mechanical property requirements of the steel in this invention are different from those in the prior art.

[0063] The steel of this invention requires a yield strength of ≥1200MPa, an elongation of ≥10%, and a low-temperature impact energy of ≥60J at -40℃, and clearly possesses good resistance to delayed cracking, which is not found in the three comparative patents 1-3. Among them, comparative patent 1 has a relatively wide yield strength range, from 300MPa to 2500MPa. Although it can achieve very high strength, it sacrifices plasticity, and the elongation cannot be guaranteed, limiting its cold working application range. Comparative patent 2 can achieve a hardness of over 50HRC through a high content of strengthening elements, resulting in excessively high costs and the problem of unreliable elongation affecting processing performance. Moreover, the steel grades of comparative patents 1 and 2 do not possess good low-temperature impact toughness. Attached Figure Description

[0064] Figure 1 This is the CCT curve of the anti-delayed cracking and wear-resistant steel plate described in this invention. Detailed Implementation

[0065] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0066] The composition of the steel in the embodiments of the present invention is shown in Table 1, the manufacturing process parameters of the steel in the embodiments are shown in Table 2, and the performance parameters of the steel in the embodiments are shown in Table 3.

[0067] The process path of this invention embodiment is as follows: deep desulfurization of molten iron (to ensure low sulfur content in steel) → converter top and bottom combined blowing (to control carbon content) → ladle refining → continuous casting (machine cleaning) → slab reheating → controlled rolling → controlled cooling → coiling → uncoiling → straightening → plate cutting → heat treatment (quenching + tempering) → finishing → delivery.

[0068] Example 1

[0069] According to the chemical composition requirements of this invention, steel was smelted in a 500kg vacuum induction furnace, and the chemical composition is shown in Table 1. The steel was then cast into 100kg ingots, heated to a temperature above 1230℃, rolled to a final temperature of 892℃, and coiled at a temperature of 678℃. The steel plate was quenched at 812℃ and tempered at 180℃.

[0070] The U-shaped bending immersion test was used to evaluate the delayed cracking resistance of steel plates. The sample size was 2*20*90mm. The sample was bent into a U-shape with a radius of 10mm, and a clamp was used to load the sample so that both sides were parallel. The sample was then immersed in a 0.1mol / L hydrochloric acid solution, with the solution changed every 24 hours. Observations were made twice daily during the test, and the specific cracking time was confirmed by video playback and recorded. A shorter cracking time indicates poorer delayed cracking resistance and a higher risk of cracking under corrosive conditions. Generally, no cracking after more than 300 hours is considered to indicate good delayed cracking resistance.

[0071] As shown in Table 3, the steel plates involved in this invention all achieve a hardness level of 500 HBW, and their tensile properties also meet the design requirements. They exhibit excellent low-temperature impact toughness. Simultaneously, the addition of elements such as Si, Cr, Cu, and Ni increases the self-corrosion potential of the matrix, inhibiting corrosion and thus providing excellent wear resistance, approximately 2.5 times that of ordinary carbon steel. In particular, the delayed cracking time is almost always above 600 hours, demonstrating exceptionally good resistance to delayed cracking.

[0072] A comparison was made between the existing BW500 wear-resistant steel and the steel grade of this invention. Comparative Examples 1-4 adopted a C-Si-Mn composition design, with C and Mn contents comparable to those of this invention; they also contained small amounts of Nb and Ti, and Cr content of 0.67-1.12%. Their finishing rolling temperature was 880℃, significantly higher than the 820-845℃ of this invention. Therefore, their composition design and processing did not consider resistance to delayed cracking. Although Comparative Examples 1-4 also underwent quenching-tempering treatment and were comparable to this invention in tensile strength and hardness, their low-temperature impact energy was lower, with a maximum of only 26J, far lower than that of this invention. Poor low-temperature toughness will lead to brittle cracking when the steel plate is subjected to impact. Furthermore, the cracking time of all comparative examples in the U-bending immersion test did not exceed 60 hours, lower than that of the steel grade of this invention. This indicates that the comparative steel plates do not possess resistance to delayed cracking and cannot meet the performance requirements of dredging pipelines.

[0073] In summary, this invention provides a high-hardness, delayed-cracking, wear-resistant steel plate with a thickness of 8-20mm, designed for the service conditions of dredged pipelines. After heat treatment, the steel plate forms a high-hardness martensitic structure with a yield strength exceeding 1200MPa, a tensile strength exceeding 1500MPa, an elongation ≥8%, and a hardness of 500±30HBW. Simultaneously, its impact energy at -40℃ exceeds 60J. It exhibits excellent wear resistance, and with improved corrosion resistance, its wear resistance reaches 2.5 times that of existing materials. Furthermore, it possesses good resistance to delayed cracking, is easy to weld and cold-bend, and significantly reduces the risk of pipe cracking and leakage during dredging. These features are not found in other known patented steel grades.

[0074] The wear-resistant steel plate described in this invention can be used to manufacture slurry dredging pipes, and is widely used in land reclamation, waterway dredging, inland river dredging and slurry transportation, etc., replacing the current ordinary dredging pipelines of Q235 and Q345 grade, thereby improving production efficiency and reducing operating costs.

[0075]

[0076]

[0077]

[0078]

[0079]

Claims

1. A 500HBW hardness slurry dredging pipe anti-delay cracking and wear-resistant steel plate with the following composition by weight percentage: C: 0.23-0.26%, Si: 0.6-1.2%, Mn: 1.0-1.4%, P≤0.015%, S≤0.005%, Al: 0.2-0.8%, Ni: 0.05-0.15%, B: 0.001-0.003%, N≤0.005%; containing one or both of Nb: 0.01-0.03% and Ti: 0.01-0.03%, with the balance containing Fe and other unavoidable impurity elements, and satisfying the following condition: 6.65N<Nb+Ti≤0.04; The wear-resistant steel has a yield strength ≥1200MPa, tensile strength ≥1500MPa, elongation ≥10%, hardness 500±30HBW, and impact energy value ≥60J at -40℃. Its wear resistance is 2.5 times that of ordinary steel plates. At the same time, the time for cracking under the U-bending immersion test in 0.1mol / L hydrochloric acid solution is more than 580h.

2. The 500HBW hardness slurry dredging pipe anti-delayed cracking and wear-resistant steel plate as described in claim 1, characterized in that, It also contains one or more of the following: Cr ≤ 1.5%, Cu ≤ 0.3%, W 0.01–0.5%, Mo 0.01–0.5%, Sb 0.01–0.2%, RE 0.01–0.2%, V 0.01–0.2%, and Ca 0.001–0.01%.

3. The method for producing the 500HBW hardness resistant and wear-resistant steel plate for slurry dredging pipes 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 temperature is above 1230℃, and the total heating time in the heating furnace is not less than 2 hours, of which the heat soaking and heat preservation time is not less than 40 minutes. 3) Rolling The roughing stage adopts large reduction rolling, controlling the reduction rate per pass to be above 15% or the reduction per pass to be above 25mm. At the same time, the deformation ratio in the roughing stage is greater than 80%, and the reduction rate of the last pass in the finishing rolling is controlled to be not less than 16%; the finishing rolling temperature is ≥850℃. 4) Cooling Laminar flow cooling is used, and the coil is wound up after cooling to 550-680℃. 5) Heat treatment The steel plate is subjected to quenching and tempering treatment, including: The quenching heating temperature is 810~835℃. The quenching holding time T1 starts from the core of the steel plate when it reaches the temperature. T1=(1~2)×H, where T is in min and H is the plate thickness in mm. After the steel plate is taken out of the furnace, it is directly water quenched to room temperature. The cooling rate is required to be ≥50℃ / s. The tempering temperature is 180–240℃, and the tempering holding time T2 is started from the core of the steel plate when it reaches the desired temperature. T2 = (2–3) × H, where T is in minutes and H is the plate thickness in mm. T2 ≥ 12 min; Finally, the tempered steel plate undergoes finishing treatment.

4. The method for producing the 500HBW hardness resistant and wear-resistant steel plate for slurry dredging pipes as described in claim 3, characterized in that, In step 1), the billet is hot-charged into the furnace after casting. That is, after confirming that there are no quality problems on the surface of the billet, it is directly transported from the casting area to the heating furnace through the roller conveyor for heating and heat preservation. If it cannot be hot-charged, the cast billet must be placed in the heat preservation pit for slow cooling after casting. The heat preservation pit can be removed and air-cooled after the temperature drops below 200℃.

5. The method for producing the 500HBW hardness resistant and wear-resistant steel plate for slurry dredging pipes as described in claim 3, characterized in that, Step 5) After the steel coil cooled to room temperature is uncoiled and straightened, it is cut into plates, and then the steel plates are quenched and tempered.

6. The method for producing the 500HBW hardness resistant and wear-resistant steel plate for slurry dredging pipes as described in claim 3, characterized in that, The thickness of the obtained wear-resistant steel plate is 8-20 mm.

7. The method for producing the 500HBW hardness resistant and wear-resistant steel plate for slurry dredging pipes according to claim 3, characterized in that, Step 3), the finishing rolling temperature is ≥880℃.

Images