Hardened 600 hbw abrasion resistant delayed cracking resistant steel plate for slurry pipeline and method of production

Abstract

Hardness 600HBW slurry dredging pipe anti-delayed cracking abrasion-resistant steel plate and its production method, its component weight percentage is: C 0.34~0.40%, Si 0.1~0.3%, Mn 1.4~1.8%, P ≤0.015%, S ≤0.005%, Al 0.02~0.04%, Cu 0.15~0.35%, Ni 0.1~0.3%, B 0.001~0.003%, N ≤0.005%, Nb 0.05~0.2%, Ti ≤0.03%, the rest Fe and inevitable impurities.Its hardness reaches 600HBW level, the impact energy value at -40 DEG C is more than 40J, has good anti-delayed cracking characteristics, the abrasion resistance performance reaches 5 times of common carbon steel material such as Q235B;Meanwhile, the cracking time under the condition of U-bending 0.1mol / L hydrochloric acid solution immersion test is more than 400h, reflects excellent anti-delayed cracking performance, is suitable for the dredging pipeline production in the fields such as reclamation, channel dredging, etc., has no cracking leakage risk when the surface is collided, scratched under the corrosion environment.

Description

Technical Field

[0001] This invention relates to the field of low alloy steel manufacturing, and particularly to a 600HBW 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 over 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 resistance. To reduce dredging costs, higher-strength, corrosion-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 CN103060689A discloses a "Preparation Method of Wear-Resistant Steel Plate for Mining Machinery." This patent adds a large amount of alloying elements such as Ti, V, and La to a C-Si-Mn base, while also containing as much as 0.7-0.9% C and 0.6-0.8% Si. Although the steel's hardness meets the 600 HBW requirement, the 0.7-0.9% C content results in a large number of carbide particles in the matrix. Under abrasive conditions, these carbide particles act as cathodes, promoting electrochemical corrosion. While Si is a common corrosion-resistant element in steel, excessive Si promotes the graphitization of C, which is detrimental to toughness and weldability. All these factors contribute to the poor wear resistance of the steel in this patent, limiting its application to engineering machinery and mining equipment. Furthermore, this patented steel does not consider the issue of delayed cracking, thus failing to meet the requirements for dredging pipes.

[0004] Foreign countries have also applied for and published a large number of patents in the field of wear resistance, mainly for engineering machinery manufacturing, but not in the field of slurry transportation with abrasive properties.

[0005] 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 introduce methods to improve wear resistance by precipitating Ti and W carbide particles, and the composition also contains precious 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 600 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 deteriorating the material's wear resistance. Furthermore, they do not address resistance to delayed cracking, making it difficult to meet the requirements of large particle size and high slurry content abrasion in slurry dredging environments.

[0006] Chinese patent CN101886225A discloses "a corrosion-resistant and wear-resistant steel and its preparation method". The steel involved in this patent contains 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. The hardness can reach more than 52HRC. However, the addition of a large number of precious alloys increases the manufacturing cost, and it does not involve the characteristics of delayed cracking resistance.

[0007] Chinese patents CN102776445A and CN108950421A 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 600HB and its production method." The former has a bainitic or bainitic + acicular ferrite structure, with a tensile strength of only 600-800MPa and low matrix hardness. 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 600HBW, which has certain wear and corrosion resistance. However, as an ultra-high-strength wear-resistant steel with a hardness of 600HBW, 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.

[0008] 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 impact 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.

[0009] A comparison with existing patents reveals that current wear-resistant steels either do not consider corrosion resistance or the problem of delayed cracking under high stress, making them unsuitable for the manufacture of dredging pipes. Summary of the Invention

[0010] The purpose of this invention is to provide a 600HBW hardness, anti-delayed cracking, wear-resistant steel plate for slurry dredging pipes and its production method. This wear-resistant steel plate achieves a hardness of 600HBW, has an impact energy exceeding 40J at -40℃, and exhibits excellent anti-delayed cracking characteristics. Its wear resistance is five times that of existing ordinary carbon steel materials such as Q235B. Furthermore, under U-bending immersion in 0.1mol / L hydrochloric acid solution, the cracking time is over 400 hours, demonstrating superior anti-delayed cracking performance. It is suitable for the fabrication of dredging pipelines in land reclamation, waterway dredging, and other fields. Even under corrosive environments, when the surface is subjected to impact or scratches, there is no risk of cracking or leakage, thus significantly improving dredging efficiency and reducing operating costs.

[0011] To achieve the above objectives, the technical solution of the present invention is as follows:

[0012] The 600HBW hardness slurry dredging pipe anti-delay cracking and wear-resistant steel plate has the following composition by weight percentage: C: 0.34-0.40%, Si: 0.1-0.3%, Mn: 1.4-1.8%, P≤0.015%, S≤0.005%, Al: 0.02-0.04%, Cu: 0.15-0.35%, Ni: 0.1-0.3%, B: 0.001-0.003%, N≤0.005%, Nb: 0.05-0.2%, Ti≤0.03%, with the balance including Fe and other unavoidable impurities.

[0013] Preferably, it also contains one or more of the following: Cr ≤ 2.0%, 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%.

[0014] In the composition design of the wear-resistant steel plate described in this invention:

[0015] Carbon (C) is the cheapest strengthening element in steel. After heat treatment, it produces a high-hardness martensitic structure, which can significantly improve the strength of the steel plate. However, excessive C is detrimental to the weldability, toughness, and plasticity of the steel plate. It also easily forms a large number of carbide particles in the matrix, promoting electrochemical corrosion. The C content is limited to 0.34-0.40% while meeting performance requirements.

[0016] 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, increase carbon activity, inhibit carbide formation, and suppress the precipitation of coarse carbides at defects, thereby improving toughness. However, excessive Si promotes C graphitization, which is detrimental to toughness; it also negatively impacts surface quality and weldability. Therefore, the Si content is limited to 0.1-0.3%.

[0017] Mn is also a common strengthening element in steel. It increases 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, the Mn content should be controlled between 1.4% and 1.8%.

[0018] 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%.

[0019] S can improve the yield strength of steel, but the presence of S will worsen the atmospheric corrosion resistance of steel and make the steel plate brittle, and reduce the low temperature toughness of steel. It is required to control its content to below 0.005%.

[0020] Al is typically added to steel during the steelmaking process as a deoxidizer. Trace amounts of Al can help refine grains and improve the strength and toughness of the steel. However, as a ferrite-forming element, excessive Al can reduce the strength of the steel plate and increase the brittleness of ferrite, thus reducing the toughness of the steel. Therefore, the Al content is limited to 0.02-0.04%.

[0021] Boron (B) has good hardenability, which improves the hardness of steel plates. However, excessive B content is detrimental to welding. Therefore, this invention controls the B content to be 0.001-0.03%.

[0022] Cu (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, especially when combined with Cr to improve resistance to delayed cracking. To ensure the effectiveness of Cu, its content should not be less than 0.15%. Excessive Cu content causes cracks in the steel billet during heating and hot rolling, deteriorating surface properties; the upper limit is limited to 0.35%.

[0023] 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 the hot-working brittleness caused by Cu. In high-strength steel, nickel can also homogenize the steel's microstructure, inhibit hydrogen diffusion, and reduce the content of irreversible hydrogen traps, thereby improving resistance to delayed cracking. However, Ni is a valuable element; therefore, this invention limits the Ni content to 0.05-0.15%.

[0024] 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 raises the recrystallization temperature of steel, allowing for rolling in the non-recrystallization zone at higher temperatures. Adding an appropriate amount of Nb to steel is beneficial for strength improvement, but levels exceeding 0.06% no longer have a further strengthening effect. 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. In addition, Nb also improves corrosion resistance. Niobium-containing steel can form stable Nb₂O₅ oxide in corrosive media, which is a dense protective oxide film. The resulting corrosion products are also denser, increasing corrosion resistance during corrosion and thus improving the corrosion resistance of the alloy steel. Simultaneously, the grain-refining effect of Nb can mitigate localized grain boundary corrosion. Furthermore, the strong affinity of Nb for C can inhibit the formation of CrC, thereby increasing the Cr content in the iron matrix and indirectly improving the corrosion resistance of the alloy. Moreover, Nb can increase the stability of the passivation film by increasing the Cr content, thus raising the pitting potential of the alloy. With increasing niobium content, the rust layer on the surface of weathering steel becomes denser, the number of cracks and pores in the rust layer decreases, and the α-FeOOH and Fe2SiO4 in the rust layer products, which promote rust layer stability, increase, improving the protective performance of the rust layer. Therefore, this invention uses a higher Nb content to improve strength and toughness, and combines it with other corrosion-resistant elements such as Cu, Ni, and Si to further enhance corrosion resistance. However, a higher Nb content is detrimental to welding, and it easily forms brittle metal hydrides with hydrogen, resulting in a significant difference in ductility and toughness compared to the matrix, as well as poor adhesion to the matrix, leading to delayed cracking. Furthermore, Nb is a valuable alloying element; therefore, this invention controls the Nb content to be 0.05-0.2%.

[0025] Adding no more than 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 metallic hydrides with hydrogen, negatively impacting resistance to delayed cracking.

[0026] In steel, nitrogen (N) 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. 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, it forms oxides with poor plasticity. Higher N levels also 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. Therefore, the N content must be controlled below 0.0050%. The addition of Ti and Nb allows N to form nitrides, mitigating its adverse effects.

[0027] In addition to the elements mentioned above, to further improve performance, steel grades may also selectively add one or more of Cr, W, Mo, Sb, RE, V, and Ca. The specified ranges for each element are: Cr ≤ 2.0%, W and Mo: 0.01-0.5%, Sb, RE, and V: 0.01-0.2%, and Ca: 0.001-0.01%.

[0028] Among them, Cr is an important corrosion-resistant element with solid solution strengthening effect. The addition of Cr can effectively increase the self-corrosion potential of steel, inhibiting corrosion and thus effectively reducing the promoting effect of corrosion on material failure during abrasion, thereby improving abrasion resistance. In particular, with the improvement of corrosion resistance, hydrogen evolution during corrosion can be reduced, thereby improving resistance to delayed cracking. However, Cr is a precious alloying element, and a high Cr content promotes the formation of a protective rust layer on the steel surface. Under abrasive conditions, these rust layers quickly detach from the surface, promoting abrasion failure of the material. Therefore, it is chosen to be added, with its content limited to an upper limit of 2.0%.

[0029] 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.

[0030] The steel grade designed with the above composition, after heat treatment, obtains a high-strength martensitic structure with a hardness of 600±30HB and an impact energy value of ≥40J at -40℃, 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, thus improving the material's corrosion resistance and resulting in excellent abrasion resistance. Furthermore, through compositional design and performance optimization, it achieves good 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.

[0031] The method for manufacturing the 600HBW hardness slurry dredging anti-delayed cracking and wear-resistant steel plate of the present invention includes the following steps:

[0032] 1) Smelting and casting

[0033] The above-mentioned components are smelted and cast into billets;

[0034] 2) Heating of the billet

[0035] The heating temperature is above 1230℃, and the total heating time in the heating furnace is no less than 2 hours.

[0036] The heat preservation time should be no less than 40 minutes.

[0037] 3) Rolling

[0038] 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 rolling temperature is ≥830℃; preferably ≥850℃.

[0039] 4) Cooling

[0040] Laminar flow cooling is used, and the coil is wound up after cooling to 540–680°C.

[0041] 5) Heat treatment

[0042] The steel plate is subjected to quenching and tempering treatment, including:

[0043] The quenching heating temperature is 785~810℃, and the quenching holding time T1 starts from the core of the steel plate when it reaches the desired temperature. T1=(1.5~2.5)×H, where T is in min and H is the plate thickness in mm.

[0044] After the steel plate comes out of the furnace, it is directly water-quenched to room temperature, requiring a cooling rate of ≥50℃ / s;

[0045] The tempering temperature is 140~200℃, 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.

[0046] Finally, the tempered steel plate undergoes finishing treatment.

[0047] 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 to below 150°C.

[0048] 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.

[0049] Preferably, the thickness of the obtained wear-resistant steel plate is 8 to 18 mm.

[0050] In the production process of the wear-resistant steel plate described in this invention:

[0051] Before rolling, the billet is heated and held at a temperature above 1230℃ for 2 hours, including a soaking time of no less than 40 minutes. Alternatively, the billet can be hot-charged into the furnace after casting, meaning that after confirming there are no quality issues on the billet surface, it can be directly transported from the casting area to the heating furnace via roller conveyor for heating and holding, thus reducing energy consumption. If hot charging is not possible, the cast billet must be placed in a holding pit for slow cooling until the temperature drops below 150℃ before being removed for air cooling.

[0052] 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%.

[0053] 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 740℃. Therefore, a final rolling temperature above 850℃ 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 830℃. After rolling, the steel coil is cooled to between 540-680℃ 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.

[0054] 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 there first, absorbing energy and thus improving toughness. Therefore, the quenching temperature should be controlled between -5℃ and +20℃ above Ac3, i.e., 785~810℃, to obtain better low-temperature toughness. The holding time should be calculated from the core of the steel plate to the point of reaching the desired temperature, and should be 1.5-2.5 times the thickness of the steel plate (mm) (min). After the steel plate is removed from the furnace, it should be directly water-quenched to room temperature, requiring a cooling rate ≥50℃ / s.

[0055] 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 140-200℃. The tempering holding time begins from the core of the steel plate reaching the desired temperature and is 2-3 times the plate thickness (mm) in minutes, but not less than 12 minutes. Finally, the quenched and tempered steel plate undergoes finishing treatment (straightening and edge trimming), and is released from the factory after passing performance tests.

[0056] The method of this invention can produce high-hardness, wear-resistant steel plates with a thickness of 8-18mm. The steel plate has a hardness of 600±30HBW and an impact energy value exceeding 40J at -40℃. Combined with the corrosion-resistant design of the steel grade, the steel plate has excellent wear and corrosion resistance. In environments where large particles and high density seawater slurry are transported, its wear and corrosion resistance can reach more than 5 times that of ordinary Q235B pipes.

[0057] The present invention has the following advantages:

[0058] This invention employs a simple and economical C-Mn composition design, supplemented with appropriate Nb and Ti microalloying elements, to achieve high hardness in the steel. At the same time, by combining Nb with corrosion-resistant elements such as Cu, Ni, and Si, the matrix potential is increased, thereby improving the corrosion resistance of the steel plate. As a result, the steel exhibits excellent wear resistance in corrosive and abrasive environments, especially under conditions of transporting large-particle, high-density seawater slurry, where its wear resistance is more than 5 times that of ordinary pipes.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] This invention relates to steel grades that differ significantly from existing technologies in terms of composition and properties:

[0063] In terms of composition, compared with Patent 1 (Chinese Patent CN102776445A "A bainitic wear-resistant steel pipe for slurry conveying and its manufacturing method"), it is required to add 0.01-1.0% of Mo, Ca and RE, and also explicitly requires the addition of N, with an N content of 0.01-0.1%, to improve strength through N. At the same time, the Mn content is relatively high, with the upper limit of Mn content reaching 5%, which is close to the composition of medium manganese steel.

[0064] 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 various rare elements such as Pr, Dy, Gd, and Nd, resulting in high production costs.

[0065] Compared with the high Al content in the composition of patent 3 (Chinese patent CN108950421A "wear-resistant steel plate for slurry dredging pipe with hardness of 600HB and its production method"), which is unfavorable to low-temperature impact toughness and does not have the ability to resist delayed cracking.

[0066] Furthermore, the mechanical property requirements of the steel of this invention are different from those of the prior art.

[0067] The steel of this invention does not require specific tensile properties, but it must achieve a hardness of 600±30HB, a low-temperature impact energy of ≥40J at -40℃, and clearly possess excellent resistance to delayed cracking, which is not found in the steel grades of comparative patents 1 to 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 exceeding 50HRC through a high content of strengthening elements, resulting in excessively high costs and the problem of unreliable elongation affecting processing performance. Moreover, neither comparative patent 1 nor 2 steel grades possess good low-temperature impact toughness. Attached Figure Description

[0068] Figure 1 This invention relates to the calculation of CCT curves for steel grades. Detailed Implementation

[0069] The present invention will be further described below with reference to the embodiments.

[0070] 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.

[0071] The production process of this invention is as follows: deep desulfurization of molten iron (to ensure low sulfur content in steel) → top and bottom composite blowing in converter (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.

[0072] Example 1

[0073] 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℃, finished at a rolling temperature of 850℃, and coiled at a coiling temperature of 544℃. The steel plate was quenched at 785℃ and tempered at 200℃.

[0074] Example 2

[0075] According to the chemical composition requirements of the wear-resistant steel plate involved in this invention, steel plates of different thicknesses were prepared. The chemical composition is shown in Table 1. The billet heating temperature was 1230℃, the finishing rolling temperature was 854℃, and the steel was water-cooled to 632℃ after rolling before coiling.

[0076] The performance of the steel grade of the present invention was compared with that of an existing 600HBW hardness wear-resistant steel as a comparative example.

[0077] 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. The sample condition was observed 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.

[0078] As shown in Table 3, the steel plates involved in this invention all achieve a hardness of 600 HBW, exhibiting excellent wear resistance. Simultaneously, the addition of Nb, Si, Cu, Ni, Cr, etc., effectively increases the self-corrosion potential of the matrix, inhibiting corrosion and thus providing excellent wear and corrosion resistance. In particular, the delayed cracking time is over 400 hours, demonstrating exceptionally good resistance to delayed cracking.

[0079] The performance of the steel grade of this invention was compared with that of existing 600HBW hardness wear-resistant steel as a comparative example. Comparative examples 1-4 adopted a C-Mn-Mo-B composition system with the addition of 0.4-1.2% Cr, and the final rolling temperature was above 860℃, significantly higher than the 785-810℃ of this invention. Although this composition design and process achieved a hardness of 600HBW, the impact energy value at -40℃ was relatively low, and the longest cracking time of the U-shaped bending immersion sample was only 32 hours, which is far lower than the cracking time of over 400 hours of this invention. This indicates that the comparative examples are inferior to the steel plate of this invention in terms of low-temperature impact toughness and resistance to delayed cracking, and do not meet the performance requirements of dredging pipelines.

[0080] 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, replacing the current ordinary dredging pipelines of Q235 and Q345 grades, thereby improving production efficiency and reducing operating costs.

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Claims

1. A wear-resistant steel plate for slurry dredging pipes with a hardness of 600 HBW, comprising the following components by weight percentage: C: 0.34–0.40%, Si: 0.1–0.3%, Mn: 1.4–1.8%, P≤0.015%, S≤0.005%, Al: 0.02–0.04%, Cu: 0.15–0.35%, Ni: 0.1–0.3%, B: 0.001–0.003%, N≤0.005%, Nb: 0.05–0.2%, Ti≤0.03%, with the balance including Fe and other unavoidable impurities; the wear-resistant steel has a hardness of 600±30 HBW, an impact energy value of ≥40 J at -40℃, and wear resistance that is more than 5 times that of ordinary steel plate Q235B; and the time for cracking under U-bending immersion in 0.1 mol / L hydrochloric acid solution is more than 400 h.

2. The 600HBW hardness slurry dredging 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 ≤ 2.0%, 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 manufacturing a 600HBW hardness slurry dredging anti-delayed cracking and wear-resistant steel plate 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 stage is controlled to be not less than 16%; the finishing rolling temperature is ≥830℃. 4) Cooling Laminar flow cooling is used, and the coil is wound up after cooling to 540–680°C. 5) Heat treatment The steel plate is subjected to quenching and tempering treatment, including: The quenching heating temperature is 785~810℃, and the quenching holding time T1 starts from the core of the steel plate when it reaches the temperature. T1=(1.5~2.5)×H, where T is in min and H is the plate thickness in mm. After the steel plate comes out of the furnace, it is directly water quenched to room temperature, and the cooling rate is required to be ≥50℃ / s. The tempering temperature is 140–200℃, 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 manufacturing the 600HBW hardness slurry dredging anti-delayed cracking and wear-resistant steel plate 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, thereby reducing energy consumption. If it cannot be hot-charged, the billet after casting must be placed in the heat preservation pit for slow cooling. The heat preservation pit can be removed and air-cooled after the temperature drops below 150℃.

5. The method for manufacturing the 600HBW hardness slurry dredging anti-delayed cracking and wear-resistant steel plate 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 manufacturing the 600HBW hardness slurry dredging anti-delayed cracking and wear-resistant steel plate as described in claim 3, characterized in that, The thickness of the obtained wear-resistant steel plate is 8-18 mm.

7. The method for manufacturing a 600HBW hardness slurry dredging anti-delayed cracking and wear-resistant steel plate as described in claim 3, characterized in that, In step 3), the finishing rolling temperature is ≥850℃.

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