A high-hardness slurry dredging delayed cracking resistant and abrasion resistant steel and a method of manufacturing the same

Abstract

A high-hardness, slurry dredging-resistant, delayed-cracking, and wear-resistant steel and its manufacturing method are disclosed. The steel comprises the following components by weight percentage: C 0.17–0.22%, Si 0.1–0.3%, Mn 1.0–1.4%, P ≤0.015%, S ≤0.005%, Al 0.02–0.04%, Cu 0.15–0.60%, Ni 0.1–0.3%, 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 remainder being Fe and unavoidable impurities, and satisfying the following conditions: 6.65N < Nb + Ti ≤ 0.04 and Cu / Ni ≤ 2. Its yield strength is ≥1100MPa, tensile strength is ≥1300MPa, elongation is ≥12%, hardness is 450±30HBW, impact energy at -40℃ is ≥60J, and its wear resistance is twice that of ordinary steel plates. At the same time, the cracking time under the U-bending immersion test in 0.1mol / L hydrochloric acid solution is more than 600h, which shows excellent resistance to delayed cracking. It is suitable for the fabrication of dredging pipelines in the fields of land reclamation and waterway dredging.

Description

Technical Field

[0001] This invention belongs to the field of low alloy steel manufacturing, specifically relating to a high-hardness slurry dredging anti-delay cracking and wear-resistant steel and its manufacturing 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 the 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 in less than 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, corrosion-resistant steel plates are required to manufacture 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. Examples include Chinese patent CN103397272A, which discloses "A wear-resistant steel plate with low crack sensitivity index and high strength and its preparation method," and Chinese patent CN103103448A, which discloses "A low-alloy high-strength and high-toughness wear-resistant steel plate." These two patents involve steel grades with a hardness reaching 450 HBW, primarily used in engineering machinery, mining equipment, and other fields, exhibiting good wear resistance. Both patents involve the addition of high levels of Mo alloying element to a C-Mn base, resulting in higher alloy costs. Simultaneously, the steel contains a high level of the corrosion-resistant element Si, which negatively impacts toughness. Furthermore, these patented steel grades do not employ measures to control corrosion, failing to meet usage requirements under corrosion and wear conditions.

[0004] Numerous patents have been filed and published abroad for wear-resistant applications, primarily in engineering machinery manufacturing, but not in the field of slurry conveying, which exhibits abrasive properties. For example:

[0005] US Patent 5284529A discloses "abrasion-resistant steel". This patent involves a steel containing up to 0.05-1.5% Ti and 0.1-3.0% Mo. The alloy has a high cost and a maximum hardness of 420 HBW.

[0006] 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 through the precipitation of Ti and W carbide particles. However, the former's hardness is generally between 396-431 HBW, while the latter is less than 300 HBW, failing to reach the 450 HBW hardness level. The large number of carbide particles in the matrix act as cathodes in the abrasive environment, promoting electrochemical corrosion and increasing the material's abrasive weight loss. Therefore, although the steel sheet has good wear resistance, its corrosion resistance is poor, and the issue of delayed cracking is not considered.

[0007] Another example is the "Corrosion-resistant and wear-resistant steel and its preparation method" disclosed in Chinese patent CN101886225A. This patent involves a steel with a hardness of 52HRC or higher. The matrix 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. It belongs to a high alloy steel and has a very high cost.

[0008] Chinese patents CN102776445A and CN108930001A disclose "A lower bainitic wear-resistant steel pipe for slurry transportation and its manufacturing method" and "A high-hardness wear-resistant steel plate for slurry dredging and its production method." The former involves steel grades with bainitic or bainitic + acicular ferrite structures, resulting in low matrix hardness and tensile strength of only 600-800 MPa. It is mainly used for transporting fine-particle (tens of μm) mineral slurries or crude oil, and is unsuitable for transporting large-particle, high-density seawater slurries. The latter is a 450HBW ultra-high-strength wear-resistant steel plate, but its composition design and performance requirements do not consider the problem of delayed cracking. During dredging operations, the steel plate is prone to crack initiation when subjected to impacts or scratches from hard objects, especially in corrosive environments where delayed cracking is likely to occur. This can lead to pipe leakage or even cracking during dredging, affecting the smooth progress of the dredging operation.

[0009] 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.

[0010] Existing patents reveal that current wear-resistant steels either fail to 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

[0011] The purpose of this invention is to provide a high-hardness, slurry dredging-resistant, delayed-cracking, and wear-resistant steel and its manufacturing method. The steel has a yield strength ≥1100MPa, tensile strength ≥1300MPa, elongation ≥12%, hardness 450±30HBW, and impact energy value at -40℃ ≥60J. Its wear resistance is twice that of ordinary steel plates. Furthermore, under U-bending immersion in 0.1mol / L hydrochloric acid solution, the cracking time is over 600 hours, demonstrating excellent delayed-cracking resistance. It is suitable for dredging pipeline 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.

[0012] To achieve the above objectives, the technical solution of the present invention is as follows:

[0013] A high-hardness, slurry dredging steel with delayed cracking and wear resistance, comprising the following components by weight percentage: C: 0.17–0.22%, Si: 0.1–0.3%, Mn: 1.0–1.4%, P≤0.015%, S≤0.005%, Al: 0.02–0.04%, Cu: 0.15–0.60%, Ni: 0.1–0.3%, 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 being Fe and other unavoidable impurities, and satisfying: 6.65N<Nb+Ti≤0.04 and Cu / Ni≤2.

[0014] Furthermore, 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%.

[0015] In the composition design of the wear-resistant steel plate described in this invention:

[0016] 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. Its range is limited to 0.17–0.22% while meeting performance requirements.

[0017] 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, its content is limited to 0.1%–0.3%.

[0018] 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 in the weld heat-affected zone, and increases costs. Therefore, it should be controlled between 1.0% and 1.4%.

[0019] 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 atmospheric corrosion resistance of steel. However, during the abrasion process, the formation of the surface rust layer will accelerate the abrasion weight loss of the material, reduce the abrasion resistance, and at the same time, the presence of P is prone to segregation, reducing the toughness and plasticity of steel, and making the steel plate brittle and affecting toughness. Therefore, the content of P in steel should be reduced as much as possible. In this invention, its content is required to be controlled below 0.015%.

[0020] 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.

[0021] 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, leading to a decrease in steel toughness. Therefore, its content is limited to 0.02–0.04%.

[0022] B has good hardenability, thus improving the hardness of steel plates. However, excessive B content is detrimental to welding, so its range is controlled to 0.001-0.03%.

[0023] 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. Adding an appropriate amount of Cu helps increase the self-corrosion potential of the steel plate, reducing the corrosion rate; it also promotes the densification and stabilization of the rust layer on the steel surface, thus improving corrosion resistance. Furthermore, the improved corrosion resistance reduces hydrogen evolution during corrosion, improving resistance to delayed cracking. Adding copper to steel can inhibit hydrogen diffusion, reducing 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 can cause cracks in the steel billet during heating and hot rolling, deteriorating surface properties; the upper limit is limited to 0.60%.

[0024] 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 high-strength steel, Ni can also homogenize the steel's microstructure, inhibit hydrogen diffusion, and reduce the content of irreversible hydrogen traps, thereby improving resistance to delayed cracking. Ni also accumulates in rust layers as an important corrosion-resistant element, refining the rust grains and promoting the formation of nano-phase, superparamagnetic α-FeOOH particles in the inner rust layer. The resulting α-FeOOH particles are smaller than 15 nm, increasing the density of the inner rust layer and making it difficult for chloride ions to penetrate and contact the steel matrix, thus reducing the corrosion rate. In particular, Ni promotes rust layer stability and improves the hot-working brittleness problem caused by Cu. Considering the effect of Cu on increasing potential and the inhibitory effects of Cu and Ni on hydrogen diffusion, this invention uses Cu and Ni as important elements to improve resistance to delayed cracking. To achieve optimal matching and to suppress copper embrittlement, restrictions are placed on the matching of Cu and Ni contents, requiring Cu / Ni ≤ 2.0. However, Ni is a valuable element, and its content is limited to 0.1–0.3%.

[0025] Nitrogen (Nb) is a strong nitride-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 inhibits the expansion of austenite interfaces, raising 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, and it readily forms brittle metallic hydrides with hydrogen. These hydrides exhibit significant differences in ductility and toughness compared to the matrix, and their bonding strength with the matrix is ​​also poor, leading to delayed cracking. A content of 0.01–0.03% is recommended.

[0026] Adding 0.01–0.03% Ti inhibits austenite grain growth during slab reheating and ferrite grain growth during recrystallization controlled rolling, thus improving steel 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.

[0027] 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 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. Therefore, in this invention, N is controlled as an impurity element, with its content limited to below 0.0050%. The addition of Ti and Nb allows N to form nitrides, mitigating its adverse effects. To minimize the negative effects of N, the content of the three elements must satisfy the following relationship: 6.65N < Nb + Ti ≤ 0.04.

[0028] In addition to the elements mentioned above, to further improve performance, the wear-resistant steel plate of the present invention may also selectively contain one or more of Cr, W, Mo, Sb, RE, V and Ca, wherein 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%.

[0029] 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 2.0%.

[0030] 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.

[0031] The steel grade designed with the above-mentioned composition not only possesses high strength and hardness but also a high self-corrosion potential, inhibiting corrosion and improving corrosion resistance. After heat treatment, it obtains a high-strength martensitic structure with a yield strength ≥1100MPa, tensile strength ≥1300MPa, elongation ≥12%, hardness 450±30HBW, and impact energy ≥60J at -40℃. It exhibits excellent wear resistance, and combined with improved corrosion resistance, the steel grade possesses good abrasion resistance. Furthermore, through compositional design and performance optimization, it achieves excellent resistance to delayed cracking. High-strength dredging pipes made from this steel are particularly suitable for transporting large-particle, high-density slurries, and are less prone to cracking and leakage during use.

[0032] The method for manufacturing high-hardness, slurry dredging-resistant, anti-delayed cracking, and corrosion-resistant steel according to the present invention includes the following steps:

[0033] 1) Smelting and casting

[0034] The above-mentioned components are smelted and cast into billets;

[0035] 2) Heating of the billet

[0036] The heating temperature is above 1230℃, and the total heating time of the billet in the heating furnace is required to be no less than 2 hours, of which the holding time in the soaking section is no less than 40 minutes.

[0037] 3) Rolling

[0038] 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 ≥880℃.

[0039] 4) Cooling

[0040] Laminar flow cooling is used, and the coil is wound up after cooling to 550-680℃.

[0041] 5) Heat treatment

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

[0043] The quenching heating temperature is 820~845℃, and the quenching holding time T1 starts from the core of the steel plate when it reaches the temperature. T1=(1.5~2)×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.

[0044] The tempering temperature is 200-240℃, and the tempering holding time T2 is started 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 ≥ 12 min.

[0045] Finally, the tempered steel plate undergoes finishing treatment.

[0046] 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.

[0047] 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.

[0048] Preferably, the thickness of the obtained wear-resistant steel plate is 8 to 20 mm.

[0049] In the method for manufacturing the wear-resistant steel plate of the present invention:

[0050] Before rolling, the billet is heated and held at a temperature above 1230℃. The heating and holding of the billet in the heating furnace is divided into a preheating section, a heating section, and a soaking section. This invention requires that the total heating time of the billet in the heating furnace be no less than 2 hours, of which the holding time in the soaking section be no less than 40 minutes. In addition, the billet can be hot-charged into the furnace after casting, that is, after confirming that there are no quality problems on the surface of the billet, it can be directly transported from the casting area to the heating furnace for heating and holding via roller conveyor, thereby reducing energy consumption; if it cannot be hot-charged, the cast billet must be placed in a holding pit for slow cooling after casting, and can only be removed from the holding pit for air cooling after the temperature drops below 200℃.

[0051] Rolling is divided into two stages: roughing and finishing. To obtain a fine initial 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, provided the mill load allows. To obtain a fine grain size and good plate shape, the deformation ratio in the roughing stage is required to be greater than 80%, and the reduction in the final pass of finishing is controlled to be no less than 16%.

[0052] 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 780℃. Therefore, a finishing 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 finishing rolling temperature can be appropriately reduced, but it should not be lower than 850℃. After rolling, the steel coil is cooled to between 550 and 680℃ by laminar flow cooling before coiling. 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.

[0053] After the steel coil is cooled to room temperature, it is uncoiled and straightened and then cut into plates. The steel plates are then quenched and tempered to obtain high strength and hardness, and to ensure wear resistance.

[0054] The quenching temperature directly affects the grain size of the subsequent martensitic structure, thus affecting 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 martensitic 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, leading to insufficient hardness and strength after quenching. To achieve outstanding low-temperature toughness, this invention specifically employs a critical zone quenching process for the steel plate. The critical zone quenched structure contains undissolved acicular ferrite. Although this undissolved acicular ferrite reduces strength to some extent, it reaches its strength limit before martensite under external force, causing cracks to initiate and propagate within it first, absorbing energy and thus improving toughness. Therefore, the quenching heating temperature should be controlled between -5℃ and +20℃ above point Ac3, i.e., between 820℃ and 845℃, to obtain better low-temperature toughness. The quenching holding time T1 is calculated from the core of the steel plate to the point of reaching the desired temperature, and is 1.5 to 2 times (min) the plate thickness H (mm). After the steel plate is taken out of 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 plasticity and toughness. 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 in the range of 200–240°C. 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.

[0056] The process of this invention can produce high-hardness, wear-resistant steel plates with a thickness of 8–20 mm. The steel plates have a yield strength exceeding 1100 MPa, a tensile strength exceeding 1300 MPa, an elongation ≥12%, a hardness of 450±30 HBW, and an impact energy exceeding 60 J at -40℃. Combined with the corrosion-resistant design of the steel grade, the steel plates exhibit excellent wear resistance and resistance to delayed cracking. In environments where large particles and high density seawater slurry are transported, the wear resistance can be more than twice 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 small amounts of Nb and Ti microalloying elements, to achieve high hardness in the steel. Simultaneously, by using corrosion-resistant elements such as Cu, Ni, and Cr, the matrix potential is increased, inhibiting corrosion 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 twice 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 has excellent low-temperature toughness and corrosion resistance, which significantly improves the steel plate's resistance to delayed cracking, reduces the risk of cracking and leakage of dredging pipes during service, improves dredging efficiency, and reduces 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 addresses the operational conditions of dredging pipelines by providing a high-hardness, wear-resistant steel plate. After heat treatment, the steel plate forms a high-hardness martensitic structure with a yield strength ≥1100MPa, tensile strength ≥1300MPa, elongation ≥12%, hardness 450±30HBW, and impact energy ≥60J at -40℃. It exhibits excellent wear resistance, and with enhanced corrosion resistance, its wear resistance is twice that of existing ordinary carbon steel materials. It also possesses good resistance to delayed cracking and is easy to weld and cold-bend. High-strength dredging pipes made from this steel are particularly suitable for transporting large-particle, high-density slurries, and are less prone to cracking and leakage during use—features not found in other known patented steel grades.

[0063] Compared with the prior art, the steel grade involved in this invention differs significantly from the comparative patent in terms of composition and properties:

[0064] In terms of composition, compared with Patent 1 (Chinese Patent CN102776445A), it is required to add 0.01 to 1.0% of Mo, Ca and RE, and also requires N content of 0.01 to 0.1% to improve strength through N, while the upper limit of Mn content reaches 5%, which is close to the composition of medium manganese steel.

[0065] Compared to patent 2 (Chinese patent CN101886225A), 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.

[0066] Compared to patent 3 (Chinese patent CN10893001A), which has a lower Cr content but a higher Al content, the steel of this invention improves corrosion resistance through the addition of Si, Cr, Cu, and Ni, and the content of these elements differs from that of patent 3.

[0067] Furthermore, the performance requirements of the steel of this invention are different from those of comparative patents 1 to 3.

[0068] The steel of this invention requires a yield strength of ≥1100MPa, elongation ≥12%, and a low-temperature impact energy of ≥60J at -40℃, and clearly possesses good resistance to delayed cracking, which is not found in the steel grades of comparative patents 1-3. Among them, the yield strength range of comparative patent 1 is relatively wide, from 300MPa to 2500MPa. Although it can achieve very high strength, it sacrifices plasticity, and the elongation cannot be guaranteed, limiting its application range. Comparative patent 2 can achieve a hardness of over 50HRC through 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

[0069] Figure 1 This is the CCT curve of the steel described in this invention. Detailed Implementation

[0070] The present invention will be further described below with reference to the embodiments.

[0071] 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.

[0072] 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.

[0073] Example 1

[0074] According to the chemical composition requirements of the wear-resistant steel plate described in this invention, steel is smelted in a 500kg vacuum induction furnace. The specific chemical composition is shown in Table 1. The steel is then cast into 100kg steel ingots. The heating temperature is above 1230℃, the finishing rolling temperature is 892℃, and the coiling temperature is 680℃. After straightening, the steel coils are cut into plates, and the steel plates are quenched and tempered. The quenching temperature of the steel plates is 820℃, and the tempering temperature is 210℃.

[0075] 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 resistance to delayed cracking and a higher risk of delayed cracking under corrosive conditions. Generally, no cracking after more than 300 hours is considered to indicate good resistance to delayed cracking.

[0076] As shown in Table 3, the steel plates involved in this invention all achieve a hardness of 450 HBW and their tensile properties also meet the design requirements, thus exhibiting excellent wear and corrosion resistance. In particular, the delayed cracking time is generally above 600 hours, demonstrating extremely excellent resistance to delayed cracking.

[0077] This invention is compared with the conventional 450HBW grade wear-resistant steel as a comparative example.

[0078] Comparative Examples 1-4 were designed with a C-Si-Mn composition, with a Mn content of approximately 1.6% and a Cr content of 0.4-1.2%, without the addition of Cu and Ni. Comparative Example 1 used a finishing rolling temperature of 820℃, but its impact energy value at -40℃ was only 33J, and it cracked after 48 hours in a U-bending immersion test. Its low-temperature toughness and resistance to delayed cracking were far inferior to the steel grade of this invention. Comparative Examples 2-4 used finishing rolling temperatures of 880-900℃, and their low-temperature impact energy value at -40℃ was 23-33J. The cracking time in the U-bending immersion test was a maximum of only 57 hours, far lower than that of the steel grade of this invention. Therefore, the comparative examples do not possess the resistance to delayed cracking required for dredging operations and are not suitable for the fabrication of dredged pipelines.

[0079] The wear-resistant steel plate involved in this invention can be used in the manufacture of slurry dredging pipes and is widely used in land reclamation, waterway dredging, inland river dredging and slurry transportation, etc., to replace the current ordinary dredging pipelines of Q235 and Q345 grade, thereby improving production efficiency and reducing operating costs.

[0080]

[0081]

[0082]

[0083]

[0084]

Claims

1. A high-hardness, slurry dredging steel with delayed cracking resistance and wear resistance, comprising the following composition by weight percentage: C: 0.17–0.22%, Si: 0.1–0.3%, Mn: 1.0–1.4%, P≤0.015%, S≤0.005%, Al: 0.02–0.04%, Cu: 0.15–0.60%, Ni: 0.1–0.3%, B: 0.001–0.003%, N≤0.005%; containing Nb: 0.01–0.03% and Ti: 0.01–0.03%. One or two, with the balance containing Fe and other unavoidable impurities, and satisfying: 6.65N<Nb+Ti≤0.04 and Cu / Ni≤2; the wear-resistant steel has a yield strength ≥1100MPa, tensile strength ≥1300MPa, elongation ≥12%, hardness 450±30HBW, impact energy value at -40℃ ≥60J, and wear resistance performance more than twice that of ordinary steel plate Q235B; at the same time, the time for cracking under U-bending 0.1mol / L hydrochloric acid solution immersion test is more than 600h.

2. The high-hardness, slurry dredging-resistant, anti-delayed cracking, and corrosion-resistant steel 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 high-hardness, slurry dredging-resistant, anti-delayed cracking, and wear-resistant steel 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 ≥880℃. 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 820~845℃, and the quenching holding time T1 starts from the core of the steel plate when it reaches the temperature. T1=(1.5~2)×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 200–240℃, 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 manufacturing high-hardness, slurry dredging-resistant, anti-delayed cracking, and wear-resistant steel 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 200℃.

5. The method for manufacturing the high-hardness, slurry dredging-resistant, anti-delayed cracking, and corrosion-resistant steel 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 high-hardness, slurry dredging-resistant, anti-delayed cracking, and wear-resistant steel as described in claim 3, characterized in that, The thickness of the obtained wear-resistant steel plate is 8-20 mm.

Images