Steel plate with resistance to delayed cracking and wear resistance and method for producing the same

Abstract

Steel plate with resistance to delayed fracture and wear resistance, and method for producing the same. The steel plate contains the following chemical elements by wt%: C: 0.17 - 0.22%, Si: 0.1 - 0.3%, Mn: 1.0 - 1.4%, P ≤ 0.015%, S ≤ 0.005%, Al: 0.018 - 0.04%, Cu: 0.15 - 0.60%, Ni: 0.1 - 0.31%, B: 0.001 - 0.003%, N ≤ 0.005%, and one or two of Nb: 0.01 - 0.03% and Ti: 0.01 - 0.03%, with the balance being Fe and inevitable impurities; and the steel plate satisfies: 5.68N ≤ Nb + Ti ≤ 0.044 and Cu / Ni ≤ 2.0. The steel plate of the present application exhibits excellent resistance to delayed fracture characteristics and is suitable for manufacturing pipelines in areas such as landfills and waterway dredging.

Description

Technical Field

[0001] Technical Field The present invention relates to the field of alloys, in particular to an erosion-corrosion resistant steel plate having resistance to delayed cracking and suitable for slurry dredging, and a method for manufacturing the same.

Background Art

[0002] Background In operations such as landfilling, channel dredging, and levee maintenance management, a large amount of solid particles such as mud and gravel are transported over long distances in the form of slurry through a dredging pipe. The pipe body is subjected to electrochemical corrosion from the slurry medium and abrasion from solid particles, as well as the interaction between both. This abrasion is particularly severe when the seawater slurry contains weathered rock, coral reef, and medium to coarse sand, which causes significant erosion-corrosion to the inner wall of the pipe body. Most existing dredging pipelines are made of ordinary steels such as Q235B and Q345B, have a short service life under harsh operating conditions, and are often scrapped within less than one year. Due to the interaction between corrosion and abrasion during the failure process, the material failure caused by erosion-corrosion is much higher than the combined effect of pure corrosion and abrasion. Therefore, the steel for dredging pipes requires not only abrasion resistance but also corrosion resistance to achieve good erosion-corrosion resistance. In order to reduce the dredging cost, it is usually necessary to manufacture dredging pipes using higher strength erosion-corrosion resistant steel plates, thereby increasing the service life of the pipes. Research shows that high-strength steel plates tend to have delayed cracking in corrosive dredging operations, and thus high-strength dredging pipes made of erosion-corrosion resistant steel plates must address the problem of delayed cracking.

[0003] Regarding the improvement of the wear resistance of steel materials, many related existing technologies have already been disclosed. For example, CN103397272A discloses "wear-resistant steel plates with low crack sensitivity index and high strength and their manufacturing methods", and CN103103448A discloses "low-alloy high-strength and high-toughness wear-resistant steel plates". The two disclosures are related to steel grades with a hardness reaching 450 HBW. The steel plates are mainly used in fields such as construction machinery and mining equipment, which show good wear resistance. However, regarding the composition design, both are based on C-Mn and add a large amount of Mo alloying elements, resulting in a higher alloy cost. And the steel contains a large amount of corrosion-resistant element Si, which has an adverse effect on toughness. These patents do not take measures to suppress corrosion and cannot meet the performance requirements under operating conditions including both corrosion and wear.

[0004] Several foreign patents have also been filed and are mainly disclosed for wear-resistant steels used in the manufacture of construction machinery, but are not related to the field of slurry transportation with polishing characteristics. For example:

[0005] US5284529A discloses "wear-resistant steel", which is related to a steel grade containing Ti from 0.05% to 1.5% up to a maximum and Mo from 0.1% to 3.0%. The alloy cost is relatively high, and the maximum hardness is 420 HBW.

[0006] JP2007231321A and JP2008169443A respectively disclose "wear-resistant steel plates" and "wear-resistant steel plates with excellent workability and their manufacturing methods", and introduce methods to improve wear resistance by carbide precipitation of Ti and W. However, the hardness of the former is generally 396 - 431 HBW, while the latter is less than 300 HBW and does not reach the hardness level of 450 HBW. A large number of carbide particles in the matrix act as cathodes in an erosive environment, which promotes the occurrence of electrochemical corrosion and increases the material loss due to erosion. Therefore, the steel plates show good wear resistance but are poor in erosion-corrosion resistance. These patents do not address delayed cracking.

[0007] CN101886225A discloses "Corrosion- and Wear-Resistant Steel and Its Manufacturing Method", and this patent relates to a steel grade with a hardness of 52 HRC or more. The matrix contains C up to 0.4 - 0.9% and 14 - 16% Mn, and the contents of both Mo and Cr are in the range of 5 - 10%. Further, it contains rare elements such as Pr, Nd, and Gd. This steel plate belongs to the high-alloy steel grade and is associated with high costs.

[0008] CN102776445A and CN108930001A disclose "Bainite Wear-Resistant Steel Pipe for Slurry Transportation and Its Manufacturing Method" and "High-Hardness Erosion-Corrosion-Resistant Steel Plate for Slurry Dredging and Its Manufacturing Method" respectively. The steel grades contained in the former all have a bainite or bainite + acicular ferrite microstructure, which has a relatively low matrix hardness and a tensile strength of only 600 - 800 MPa. These steel pipes are mainly used for the transportation of slurries (tens of microns) or crude oil, and are not suitable for the transportation of seawater slurries with large particles and high density. The latter is an ultra-high-strength erosion-corrosion-resistant steel plate with 450 HBW, which does not consider delayed cracking in its composition design and performance requirements. During the dredging operation, the steel plate tends to crack when it is impacted and damaged by hard objects, especially in a corrosive environment. The steel plate is susceptible to the influence of delayed cracking, which can even cause leakage and cracking of the pipe body, and affect the dredging operation.

[0009] In dredging operations, as an important component, the dredging pipeline faces problems of corrosion on both the inner and outer sides of the pipe body during use, as well as mechanical damage such as impact and scratches on its outer wall. For low-strength steel plates such as Q235B and Q345B, their low yield strength enables deformation and energy absorption, ensuring safety. However, especially for high-strength steel plates with a yield strength exceeding 1000 MPa, the damage stresses they experience often exceed the steel's yield strength with difficulty. As a result, when subjected to such damage, the steel plates cannot deform. This leads to the occurrence and propagation of cracks at the damaged locations. In a corrosive environment, especially due to electrochemical corrosion, cracks promote the penetration and diffusion of hydrogen. Hydrogen atoms penetrate the steel lattice, increase the vacancy concentration, form microvoids, and further promote the occurrence of microcracks, leading to brittle cracks, i.e., delayed cracks. This has a great impact on dredging operations, shortens the lifespan of the pipeline, and increases costs. The higher the strength of the steel plate, the more it is affected by hydrogen penetration. Therefore, even when the surface of the pipeline is not damaged, high-strength steel plates used in dredging must consider the resistance to delayed cracking.

[0010] From the prior art, wear-resistant steels do not consider either their corrosion resistance or the problem of delayed cracking under high stress, making them unsuitable for dredging pipes.

Summary of the Invention

Means for Solving the Problems

[0011] Abstract An object of the present invention is to provide an erosion-corrosion resistant steel sheet having excellent delayed fracture resistance, the steel sheet having a yield strength of ≧1100 MPa, a tensile strength of ≧1300 MPa, an elongation of ≧12%, a hardness of 450±30 HBW, and an impact energy at -40°C of ≧60 J. The erosion-corrosion resistance of the steel sheet is more than twice that of a normal steel sheet, and in a U-bending test, the steel sheet does not crack after being immersed in a 0.1 mol / L hydrochloric acid solution for 600 hours or more (i.e., the cracking time is 600 hours or more). The steel sheet of the present application exhibits excellent delayed fracture resistance, is suitable for pipeline manufacturing in landfills and channel dredging, and has no cracks or leaks when subjected to impact and scratches on its surface in a corrosive environment, thereby significantly improving the dredging efficiency and reducing the working cost.

[0012] To achieve the above object, the present invention provides an erosion-corrosion resistant steel sheet having excellent delayed fracture resistance, the steel sheet containing the following chemical elements by wt%: C: 0.17~0.22%, Si: 0.1~0.3%, Mn: 1.0~1.4%, P≦0.015%, S≦0.005%, Al: 0.018~0.04%, Cu: 0.15~0.60%, Ni: 0.1~0.31%, B: 0.001~0.003%, N≦0.005%, and one or both of Nb: 0.01~0.03% and Ti: 0.01~0.03%, with the balance being Fe and unavoidable impurities.

[0013] Preferably, the contents of the elements N, Nb, and Ti satisfy the following inequality: 5.68N≦Nb+Ti≦0.044. In some embodiments, the lower limit of Nb+Ti in the inequality can be, for example, 6.15N, 6.37N, or 6.65N. In some embodiments, the upper limit of Nb+Ti can be, for example, 0.044, 0.04, 0.039, or 0.034. For example, the contents of the elements N, Nb, and Ti satisfy the following inequality: 6.65N≦Nb+Ti≦0.04.

[0014] Preferably, the elements Cu and Ni satisfy the following inequality: Cu / Ni ≤ 2.0. The lower limit of Cu / Ni is not limited and can be 0, 0.7, or 1.1. The upper limit of Cu / Ni is also not limited and can be 2, 1.9, 1.8, 1.6, or 1.5. For example, the elements Cu and Ni satisfy the following inequality: 0.7 ≤ Cu / Ni ≤ 2.0.

[0015] Preferably, the steel sheet further 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%, REM: 0.01 - 0.2%, V: 0.01 - 0.2%, and Ca: 0.001 - 0.01%.

[0016] Preferably, the Cu content in the steel sheet is 0.29 - 0.60%.

[0017] Preferably, the thickness of the steel sheet is 8 - 20 mm.

[0018] In the composition design of the steel sheet in the present invention: C is the most cost - effective strengthening element that can significantly improve the strength of the steel sheet. However, excessive C has an adverse effect on the weldability, toughness, and plasticity of the steel sheet. Therefore, the C content is limited to 0.17 - 0.22% to meet the performance requirements.

[0019] Si acts as a deoxidizing element and a solid - solution strengthening element. It is also a common corrosion - resistant element in weathering steels. Si in the steel replaces Fe atoms by substitution, inhibits dislocation movement, and thus achieves solid - solution strengthening. Si reduces the diffusion coefficient of C in ferrite, increases the activity of carbon, suppresses the formation of carbides, and inhibits the precipitation of coarse carbides at defects, thereby improving toughness. However, excessive Si promotes the graphitization of C, which has an adverse effect on toughness, surface quality, and weldability. Therefore, the content of Si is limited to 0.1 - 0.3%.

[0020] Mn is also a common strengthening element in steel. It improves the yield strength through solid-solution strengthening, which reduces elongation, significantly lowers the phase transformation temperature of the steel, and refines the microstructure of the steel. Mn is an important strengthening and toughening element. However, an excessive Mn content leads to an increase in hardenability, poor weldability, a decrease in the toughness of the heat-affected zone of the weld, and an increase in cost. Therefore, the Mn content is controlled within 1.0 - 1.4%.

[0021] P is the main corrosion-resistant element in weathering steel. It promotes the formation of a protective rust layer on the surface and effectively improves the atmospheric corrosion resistance. However, during erosion and corrosion, the formation of the surface rust layer accelerates the material loss due to erosion and corrosion and reduces the erosion-corrosion resistance. The presence of P easily causes segregation, reduces the toughness and plasticity of the steel, makes the steel brittle, and affects its toughness. Therefore, the P content in the steel should be minimized. In the present invention, the P content is controlled to be 0.015% or less.

[0022] S improves the yield strength of the steel, but the presence of S reduces the atmospheric corrosion resistance of the steel, makes the steel brittle, and reduces the low-temperature toughness of the steel. Therefore, the S content is controlled to be 0.005% or less.

[0023] Al is usually added to steel as a deoxidizer during the steelmaking process. A trace amount of Al is beneficial for refining particles and improving the strength and toughness of the steel. As a ferrite-forming element, excessive Al reduces the steel strength and increases the brittleness of ferrite, thereby reducing toughness. Therefore, the Al content is limited to 0.018 - 0.04%, preferably 0.02 - 0.04%.

[0024] Cu plays a role in solid-solution strengthening and precipitation strengthening. When the Cu content is relatively high, the steel plate shows a secondary hardening effect when annealed at an appropriate temperature, thereby increasing the strength of the steel plate. Cu is also one of the elements that improve corrosion resistance. The electrochemical potential of Cu is higher than that of Fe. On the one hand, the addition of an appropriate amount of Cu helps to increase the self-corrosion potential of the steel plate and reduce the corrosion rate. On the other hand, it promotes the densification and stabilization of the surface rust layer, thereby improving the corrosion resistance. When the corrosion resistance of the steel plate is improved, the generation of hydrogen during the corrosion process decreases, which improves the resistance to delayed cracking. The addition of Cu inhibits the diffusion of hydrogen, reduces the sensitivity to hydrogen-induced cracking, and especially when combined with Cr, jointly improves the resistance to delayed cracking. To ensure the effect of Cu, its content is maintained at 0.15% or more. However, excessive Cu causes cracks during the heating and hot rolling of steel billets, deteriorates the surface properties, and therefore the upper limit of the copper content is set at 0.60%.

[0025] Ni exists in a solid solution state in steel and does not form carbides, acting as an austenite-forming element. The addition of Ni to steel has a particle refinement effect, improving the low-temperature impact toughness by refining particles and reducing the stacking fault energy. In high-strength steel, Ni homogenizes the microstructure of the steel, inhibits the diffusion behavior of hydrogen, and reduces the content of irreversible hydrogen traps, thereby improving the stress corrosion cracking resistance. Ni is also an important corrosion-resistant element, being abundant in the rust layer, refining the particles in the rust layer, and promoting the formation of nano-phase superparamagnetic α-FeOOH in the internal rust layer. The particle size of the formed α-FeOOH is less than 15 nm, which increases the density of the internal rust layer, making it difficult for chloride ions to penetrate the rust layer and contact the steel substrate, thereby reducing the corrosion rate. In particular, Ni stabilizes the rust layer and alleviates the hot working brittleness caused by Cu. The present invention regards Cu and Ni as important elements for improving stress corrosion cracking resistance in consideration of the effect of Cu on the rising potential and the inhibition of hydrogen diffusion by Cu and Ni. To achieve the optimal matching effect and suppress the brittleness of copper, the ratio of Cu to Ni is limited, requiring Cu / Ni ≦ 2.0. However, since Ni is a precious element, the content of Ni is limited to 0.1 - 0.31%, preferably 0.1 - 0.30%.

[0026] B accumulates in dislocations and defects in steel, reduces the grain boundary energy, suppresses the ferrite transformation, thereby improving the hardenability and increasing the hardness of the steel plate. Furthermore, trace amounts of B have a strong tendency to accumulate at the austenite grain boundaries, forming Fe2B, which can form a good coherent interface with austenite, reduce the interfacial energy at the grain boundaries, thus delaying ferrite nucleation and stabilizing austenite. The addition of B improves the low-temperature impact toughness of the steel plate after low-temperature tempering and reduces the transition temperature from ductility to brittleness. B-containing steel tempered at around 300 °C has higher impact toughness than steel without B, but when tempered above 500 °C, the impact toughness is low. When the B content is low, B accumulates on the half-atomic plane of the edge dislocation due to the hydrostatic pressure field from the edge dislocations in austenite, minimizing the influence on the grain boundaries and having no significant effect on the hardenability. Therefore, the B content is required to be 0.001% or more. However, excessive B reduces the grain boundary strength, causes grain boundary fracture and cleavage under stress, resulting in the "boron embrittlement" phenomenon. Furthermore, too much B has an adverse effect on weldability, and the strengthening effect does not increase further, while promoting grain boundary segregation, leading to embrittlement and a decrease in stamping performance. Therefore, the B content is controlled to be 0.003% or less.

[0027] N forms nitrides with Nb, V, and Ti in steel. These fine precipitates fix the grain boundaries and refine the austenite grains. The precipitated nitrides also provide precipitation strengthening. However, a high content of N in steel tends to combine with Al to form AlN, which significantly increases the amount of nitrides in the steel. When AlN exists as a non-metallic inclusion, it disrupts the continuity of the steel matrix. Especially when the Al content is high, a large amount of AlN is formed and clustered, causing more harm and forming oxides with poor plasticity. Furthermore, a high N content tends to accumulate in defects, deteriorating the low-temperature impact toughness. Similar to C, N has a tendency to segregate to dislocations, forming a Cottrell atmosphere, leading to strain concentration. Therefore, in the present invention, N is controlled as an impurity element, and the N content is limited to 0.0050% or less.

[0028] Ti and Nb are added to form N and nitrides, which reduces the adverse effects of N. To further eliminate the adverse effects of N, the contents of N, Nb, and Ti preferably satisfy the relationship of 5.68N ≤ Nb + Ti ≤ 0.044, preferably 6.65N ≤ Nb + Ti ≤ 0.04.

[0029] Cu / Ni: Since the melting point of Cu is only about 1083 °C lower than that of the steel matrix, excessive Cu causes cracking during heating and hot rolling, leading to embrittlement of copper and deterioration of surface properties. The addition of an appropriate amount of Ni can suppress the embrittlement of copper caused by Cu and can improve the low-temperature impact toughness. However, since Ni is an expensive alloying element, excessive Ni increases the manufacturing cost. Research shows that maintaining the Cu / Ni ratio below 2.0 is sufficient to address the problem of copper embrittlement caused by the addition of Cu. Therefore, this ratio is limited to Cu / Ni ≤ 2.0.

[0030] Nb is a strong carbide and nitride forming element and can combine with carbon and nitrogen in steel to form intermediate phases such as NbC, Nb(CN), and NbN. The formed fine carbide particles refine the microstructure and enhance the precipitation strengthening effect, which can significantly increase the strength of the steel plate. Furthermore, Nb inhibits the expansion of the austenite interface, raises the recrystallization temperature of the steel, and thus enables rolling in the non-recrystallized region at a higher temperature. Therefore, adding an appropriate amount of Nb to the steel is beneficial for improving strength. The carbonitrides formed by Nb can fix the austenite grain boundaries during austenitization and inhibit abnormal grain growth of austenite, which is beneficial for improving the toughness of the steel plate after quenching. However, a high content of Nb is harmful for welding because it easily forms brittle metal hydrides with hydrogen. These hydrides have significantly different plasticity and toughness compared to the matrix and have poor bonding with the matrix, resulting in delayed cracking. The recommended content of Nb is 0.01 - 0.03%.

[0031] Ti plays a role in inhibiting the growth of austenite particles during the reheating of the slab and suppressing the growth of ferrite particles during the recrystallization controlled rolling process, thereby improving the toughness of the steel. Further, Ti preferentially binds with N in the steel, reducing the amount of AlN. However, an excessive Ti content is harmful to low-temperature impact toughness, and like Nb, it easily forms brittle hydrides with hydrogen, which is not favorable for the resistance to delayed cracking. Therefore, the addition of Ti is 0.01 - 0.03%.

[0032] Preferably, to further perfect the performance, the steel plate of the present invention may add one or more of Cr, W, Mo, Sb, REM, V, and Ca in the following amounts as required: Cr ≤ 2.0%, W: 0.01 - 0.5%, Mo: 0.01 - 0.5%, Sb: 0.01 - 0.2%, REM: 0.01 - 0.2%, V: 0.01 - 0.2%, and Ca: 0.001 - 0.01%.

[0033] Cr is an important corrosion-resistant element and has a solid-solution strengthening effect. The addition of Cr can effectively increase the self-corrosion potential of the steel and inhibit the occurrence of corrosion, thereby significantly reducing the promotion of material destruction corrosion during wear and improving the erosion-corrosion resistance. In particular, with the improvement of corrosion resistance, the amount of hydrogen released during the corrosion process decreases, thereby improving the resistance to delayed cracking. However, Cr is an expensive alloying element, and a high Cr content promotes the formation of a protective rust layer on the steel surface, which peels off immediately in a wear environment and accelerates material destruction by wear. Therefore, the addition is selected and its content is limited to a maximum of 2.0%.

[0034] W in the steel forms carbides, which provide a secondary hardening effect and a solid-solution strengthening effect. During overaging, W also inhibits the segregation of impurity atoms and non-metallic inclusions at grain boundaries, thereby improving the fracture toughness. Mo has a phase transformation strengthening effect and a dislocation strengthening effect, which improves the tempering stability of the steel, reduces temper softening, inhibits high-temperature temper brittleness, and improves the low-temperature impact toughness of the steel plate. Sb combines with Cu in the steel to form a Cu2Sb film on the surface, thereby improving the corrosion resistance. The addition of REM is beneficial for improving the corrosion resistance. In the steel, REM forms compounds, intermetallic compounds with iron (REM / Fe), and solid solutions of rare earths, which hydrolyze in thin corrosion liquid films and precipitate on the cathode under high pH conditions, thereby providing a corrosion inhibition effect. V is also a strong carbide and nitride forming element. It precipitates during phase transformation and provides both solid-solution strengthening and precipitation strengthening through the formation of carbides and nitrides. V also improves the tempering stability, thereby improving the strength. Ca added to the steel changes the shape of sulfides, suppresses S-induced hot brittleness, and improves the toughness.

[0035] The steel grade developed based on the above composition not only has high strength and high hardness, but also has a high self-corrosion potential, which inhibits corrosion and improves the erosion-corrosion resistance (the erosion-corrosion resistance is more than twice that of the ordinary Q235B steel plate). After heat treatment, the steel achieves a high-strength martensite structure with a yield strength of ≥1100 MPa, a tensile strength of ≥1300 MPa, an elongation of ≥12%, a hardness of 450±30 HBW, and an impact energy of ≥60 J at -40°C. This steel grade shows excellent wear resistance and, with the improvement of corrosion resistance, good erosion-corrosion resistance. Through composition design and performance optimization, the steel grade achieves good resistance to delayed cracking (cracking occurrence time of more than 600 hours). The high-strength dredging pipe made of this steel is particularly suitable for transporting large particles and high-density slurries and is resistant to cracking and leakage during use.

[0036] Furthermore, the present application provides a method for manufacturing the aforementioned steel plate, including the following steps: 1) Steps of smelting and casting Smelt and cast the molten steel to obtain a cast slab; 2) Step of heating the cast slab The heating temperature is 1230 °C or higher, and the total heating time in the heating furnace is 2 hours or longer, where the holding time in the soaking zone is 40 minutes or longer; 3) Steps of rough rolling and finish rolling In the rough rolling stage, the reduction ratio per pass is 15% or higher, and / or the reduction amount per pass is 25 mm or higher, and / or the total pass deformation ratio is greater than 80%; In the finish rolling stage, at a finish rolling temperature of ≧880 °C, preferably 880 - 898 °C, the reduction ratio of the final pass is 16% or higher; 4) Steps of cooling and coiling Use laminar cooling to cool to 550 - 680 °C, and then coil; 5) Steps of quenching and tempering The quenching temperature is 820 - 845 °C, and the quenching holding time T1 is 1.5H - 2H in minutes, where H represents the plate thickness in mm; after coming out of the furnace, the steel plate is water-cooled to room temperature at a cooling rate of ≧50 °C / s; The tempering temperature is 200 - 240 °C with a tempering holding time T2 of 2H - 3H in minutes, where H represents the plate thickness in mm, and T2 ≧ 12 minutes; 6) Step of finish processing Perform straightening and shear cutting.

[0037] Preferably, in step 1), after casting is completed, the cast slab is hot charged into the furnace. That is, after confirming that there are no problems with the surface quality, the cast slab is directly transported from the casting area to the heating furnace by rollers for heating and holding, thereby reducing energy consumption. If hot charging is not possible, the cast slab must be placed in a heat-insulating pit for slow cooling, and can only be taken out for air cooling after the temperature drops below 200 °C.

[0038] Preferably, in step 4), the steel is cooled to 560 - 680 °C and then coiled.

[0039] Preferably, in step 5), the quenching temperature is 828 - 845 °C.

[0040] Preferably, in step 5), the tempering temperature is 210 - 240 °C, more preferably 220 - 240 °C.

[0041] Preferably, in step 5), the steel coil cooled to room temperature is unwound, strain - corrected, then cut into sheets, and subsequently subjected to quenching and tempering treatments.

[0042] Preferably, the thickness of the obtained corrosion - erosion resistant steel sheet is 8 - 20 mm.

[0043] In the method for manufacturing the steel sheet of the present invention: Before rolling, the casting slab is heated and held at a temperature of 1230 °C or higher. The heating and holding of the casting slab in the heating furnace are divided into a pre - heating stage, a heating stage, and an immersion stage. The present invention requires that the total heating time of the casting slab in the furnace is 2 hours or more, and the immersion stage lasts for 40 minutes or more. Further, after casting is completed, the casting slab can be directly hot - charged into the furnace. That is, after confirming that there are no problems with the surface quality, the casting slab is directly transported from the casting area to the heating furnace by rollers for heating and holding, thereby reducing energy consumption. If hot - charging is not possible, the casting slab must be placed in a heat - insulating pit for slow cooling, and can only be taken out for air - cooling after the temperature drops below 200 °C.

[0044] The rolling is divided into two stages: rough rolling and finish rolling. In order to obtain a fine original austenite grain size, the casting slab is rolled at a large reduction ratio during the rough rolling stage. Under the condition that the load of the rolling mill is allowed, the reduction per pass is controlled to be 15% or more, and / or the reduction per pass is 25 mm or more. In order to achieve a fine grain size and a good sheet shape, the total deformation ratio in the rough rolling stage can be controlled to be greater than 80%, and / or the reduction ratio of the final pass in the finish rolling is 16% or more. In the present invention, the "deformation ratio" refers to the ratio (percentage) of the reduction in the thickness after rolling to the initial thickness, that is, deformation ratio = (initial thickness - current thickness) / initial thickness × 100%.

[0045] Since the present invention includes offline heat treatment after rolling, there are no special requirements for the rolling temperature of the casting slab. However, in order to reduce the rolling load, the highest possible finish rolling and coiling temperature are used. According to the continuous transformation curve shown in Figure 1, the α→γ transformation point of the steel grade is about 780°C. Therefore, it is recommended to use a finish rolling temperature of 880°C or higher. This ensures complete austenitization during rolling, resulting in a lower and more stable rolling load. This is beneficial for obtaining a high-quality sheet shape in the subsequent process. For thicker steel plates, the finish rolling temperature can be appropriately lowered, but it should not be lower than 850°C. After rolling, the steel coil is cooled to a temperature between 550 and 680°C by laminar cooling before coiling. If the temperature is too high, the cooling rate becomes too slow, causing coarse particles in the steel coil, which is harmful to the coiling machine. If the temperature is too low, a bainite structure is likely to be formed, increasing the strength of the steel plate and making subsequent uncoiling and straightening difficult.

[0046] After cooling to room temperature, the steel coil is uncoiled, straightened, and cut into sheets. The steel sheet then undergoes quenching and tempering treatments to achieve high strength and hardness and ensure good wear resistance.

[0047] The quenching temperature directly affects the grain size of the subsequent martensite structure, thereby affecting the toughness of the steel plate. To ensure complete austenitization of the matrix, a heating temperature 30 - 50 °C higher than the Ac3 point is usually used. If the heating temperature is too high, the austenite grains coarsen, resulting in a coarse martensite structure and a decrease in toughness after quenching. If the heating temperature is too low, it causes insufficient austenitization, resulting in incomplete martensite formation after quenching and adversely affecting the toughness. The holding time also follows the same pattern as quenching. If the holding time is too long, it can lead to grain coarsening, increased energy consumption, and higher costs. Conversely, if the holding time is too short, it results in insufficient austenitization and the hardness and strength of the steel after quenching do not meet the required standards. To achieve excellent low-temperature toughness, the present invention particularly employs an intercritical quenching process for the heat treatment of the steel plate. In the intercritical quenching structure, undissolved acicular ferrite exists. These undissolved acicular ferrite can slightly reduce the strength of the steel plate, but they reach the strength limit earlier than martensite under external force, first generate and propagate cracks within them, absorb energy, and thereby improve the toughness. Therefore, the quenching temperature is controlled to be a temperature - 5 °C higher than the Ac3 point and 20 °C higher than the Ac3 point, that is, 820 - 845 °C, thereby obtaining better low-temperature toughness. The quenching holding time T1 is calculated from the moment the center of the plate reaches that temperature, and the time (minutes) is 1.5 - 2 times the plate thickness H (mm). After the steel plate exits the furnace, the steel plate is directly water-cooled to room temperature at a cooling rate of ≧ 50 °C / s.

[0048] The tempering process mainly relaxes and removes the quenching stress and improves plasticity and toughness. However, a high tempering temperature can excessively reduce the strength and hardness of the steel plate, fail to meet the design requirements, and increase the production cost. Therefore, it is essential to control the tempering process parameters for the steel plate. In the present invention, the steel plate is tempered in the temperature range of 200 - 240 °C. The tempering holding time T2 is calculated from the moment when the center of the plate reaches that temperature, and the time T2 (minutes) is 2 - 3 times the plate thickness H (mm), but not less than 12 minutes. Finally, the quenched and tempered steel plate undergoes finishing treatments (straightening and trimming). After passing the performance test, the steel plate is shipped.

[0049] In an exemplary embodiment, the process route is as follows: deep desulfurization of molten iron (to ensure a low S content in the steel) → combination of top blowing and bottom blowing in a converter (to control the C content) → secondary refining → continuous casting (mechanical cleaning) → reheating of the slab → controlled rolling → controlled cooling → coiling → uncoiling → straightening → plate cutting → heat treatment (quenching and tempering) → finishing → delivery.

[0050] The process of the present invention can achieve the production of a high - hardness erosion - corrosion - resistant steel plate with a thickness of 8 - 20 mm. The yield strength of the steel plate is 1100 MPa or more, the tensile strength is 1300 MPa or more, the elongation is ≧ 12%, the hardness is 450 ± 30 HBW, and the impact energy at - 40 °C is ≧ 60 J. Combined with the corrosion - resistant design of the steel grade, the steel plate exhibits erosion - corrosion resistance and resistance to stress corrosion cracking. In an environment where large particles and high - density seawater slurry are transported, the erosion - corrosion resistance can be more than twice that of a normal Q235B steel pipe.

[0051] The advantages of the present invention are as follows: The present invention adopts a simple and economical C-Mn composition design, supplemented with a small amount of Nb and Ti microalloying elements to obtain a high-hardness steel grade. The addition of corrosion-resistant elements such as Cu, Ni, and Cr increases the matrix potential, inhibits corrosion, and improves the corrosion resistance of the steel plate. Therefore, the steel exhibits excellent erosion-corrosion resistance in corrosive-abrasive environments, especially in environments where large particles and high-density seawater slurries are transported, and the erosion-corrosion resistance is more than twice that of ordinary steel pipes.

[0052] The present invention relates to a steel grade having excellent low-temperature impact toughness and cold bendability, satisfying the requirements for subsequent pipe manufacturing. The steel can be easily formed into high-hardness steel plates using existing equipment.

[0053] The present invention relates to a steel grade having good low-temperature impact toughness and corrosion resistance, significantly improving the stress corrosion cracking resistance, and reducing the risk of cracking and leakage during the use of dredging pipes, thereby improving the dredging efficiency and reducing the maintenance cost.

[0054] The manufacturing process of the steel grade related to the present invention is simple and has a low content of expensive alloying elements, reducing the manufacturing difficulty and cost, which is beneficial for the wide adoption of the steel grade.

[0055] The present invention provides a high-hardness erosion-corrosion resistant steel plate specially designed according to the operating conditions of dredging pipelines. After heat treatment, the steel plate forms a high-hardness martensite structure with the following properties: yield strength ≥ 1100 MPa, tensile strength ≥ 1300 MPa, elongation ≥ 12%, hardness of 450 ± 30 HBW, and impact energy ≥ 60 J at -40°C. The steel plate exhibits excellent wear resistance, and with the improvement of corrosion resistance, the erosion-corrosion resistance reaches more than twice that of conventional carbon steel materials. Furthermore, the steel plate has good resistance to delayed cracking and is easy to weld and cold bend. The high-strength dredging pipes made from this steel plate are particularly suitable for transporting large particles and high-density slurries, and they are less likely to crack and leak during use. These properties are not found in other patented steel grades.

[0056] Compared with the prior art, the steel grade related to the present invention has significant differences in composition and properties compared with the comparative patents: Regarding the composition, Comparative Patent 1 (CN102776445A) requires the addition of 0.01 - 1.0% of Mo, Ca, and REM to improve strength and specifies a nitrogen (N) content of 0.01 - 0.1%. The upper limit of the manganese (Mn) content is 5%, approaching the composition of medium manganese steel.

[0057] Comparative Patent 2 (CN101886225A) specifies high contents of C, Mn, and Cr in the ranges of 0.4 - 0.9%, 14 - 16%, and 5 - 10% respectively. Furthermore, it requires the addition of multiple rare elements such as Pr, Dy, Gd, and Nd.

[0058] Comparative Patent 3 (CN10893001A) has a low Cr content but a high Al content, which is harmful to toughness. In contrast, the steel grade of the present invention improves corrosion resistance by adding Si, Cr, Cu, and Ni, and the contents of these elements are different from those of Comparative Patent 3.

[0059] Furthermore, the performance requirements for the steel in the present invention also differ from those disclosed in Comparative Patents 1 to 3.

[0060] The steel in the present invention requires a yield strength of 1100 MPa or more, an elongation of ≧12%, and a low-temperature impact energy at -40°C of ≧60 J, and has an obvious resistance to delayed cracking, which are characteristics not found in the steel grades of Comparative Patents 1 to 3. Here, the yield strength range of Comparative Patent 1 is relatively wide from 300 MPa to 2500 MPa. Although it can achieve very high strength, this is accompanied by the cost of reduced plasticity, and the elongation cannot be guaranteed, limiting the scope of application. Comparative Patent 2 achieves a hardness exceeding 50 HRC due to a high content of strengthening elements, but the cost is too high and the elongation cannot be guaranteed, affecting its workability. Furthermore, Comparative Patents 1 and 2 lack good low-temperature impact toughness.

Brief Description of the Drawings

[0061]

Figure 1

Embodiments for Carrying out the Invention

[0062] Detailed Description The present invention will be further described in connection with the following embodiments, but the present invention is not limited to these embodiments.

[0063] Examples 1 to 22 1) Smelting and Casting The molten steel was smelted in a 500 kg vacuum induction furnace and cast into 100 kg casting slabs. 2) Heating of Casting Slabs The heating temperature was 1230°C or higher, and the total heating time in the heating furnace was 2 hours or more. Here, the holding time in the immersion zone was 40 minutes or more. 3) Rough Rolling and Finish Rolling In the rough rolling stage, the reduction ratio per pass was controlled to be 15% or more, and / or the reduction per pass was 25 mm or more, and / or the total pass deformation ratio was greater than 80%; In the finish rolling stage, the reduction ratio of the final pass was 16% or more, and the finish rolling temperature was ≧880 °C. 4) Cooling and coiling Cooling was carried out to 550 - 680 °C, followed by coiling. 5) Quenching and tempering The quenching heating temperature was 820 - 845 °C, and the quenching holding time T1 (minutes) was 1.5H - 2H. Here, H represents the plate thickness in mm; after coming out of the furnace, the steel plate was water-cooled to room temperature at a cooling rate of ≧50 °C / s; The tempering temperature was 200 - 240 °C, and the tempering holding time T2 (minutes) was 2H - 3H. Here, H represents the plate thickness in mm, and T2 ≧ 12 minutes. 6) Finishing treatment This included strain rectification and slitting.

[0064] Comparative Examples 1 to 4 Comparative Examples 1 - 4 were manufactured using the same method as in the Examples. However, one or more of the elemental compositions and manufacturing processes in Comparative Examples 1 - 4 were not within the scope of the present invention.

[0065] Table 1 shows the compositions of the steel plates in the Examples and Comparative Examples. Table 2 shows some of the process parameters for the Examples and Comparative Examples, and Table 3 shows the performance parameters of the Examples and Comparative Examples.

[0066] The cracking time was measured as follows: The resistance of the steel plate to delayed cracking was evaluated using a U-bending immersion test. A 2*20*90 mm sample was bent into a U-shape with a radius of 10 mm. The sample was fixed with a fixture until both sides of the sample were parallel. The sample was then immersed in a 0.1 mol / L hydrochloric acid solution, and the solution was changed every 24 hours. During the test, the sample was observed twice a day, and the video was played back to confirm the exact cracking time, and the cracking time of the sample was recorded. The shorter the cracking time of the sample, the lower the resistance to delayed cracking, which means a higher risk of delayed cracking under corrosion conditions. Generally, if there is no cracking after more than 300 hours, it is considered to show good resistance to delayed cracking.

[0067] As shown in Table 3, the present invention relates to steel plates all reaching a hardness level of 450 HBW, and their tensile properties also meet the design requirements and show excellent erosion-corrosion resistance (the erosion-corrosion resistance of the steel plates in the present invention is more than twice that of ordinary Q235B steel plates). In particular, the delayed cracking time of the steel plates in the present application is 600 hours or more, indicating their excellent resistance to delayed cracking.

[0068] The present invention was compared with a conventional 450 HBW-level wear-resistant steel as a comparative example.

[0069] Comparative Examples 1 to 4 were designed using a C-Si-Mn composition design with a Mn content of about 1.6% and a Cr content ranging from 0.4% to 1.2% without adding Cu and Ni. Comparative Example 1 adopted a final rolling temperature of 820 °C, but the impact energy at -40 °C was only 33 J, and cracking occurred within 48 hours during the U-bending immersion test, showing significantly lower low-temperature toughness and resistance to delayed cracking compared with the steel of the present invention. Comparative Examples 2 to 4 used a final rolling temperature of 880 to 900 °C, had a low-temperature impact energy at -40 °C of 23 to 33 J, and the maximum cracking time in the U-bending immersion test was only 57 hours. This is much lower than the maximum cracking time of the steel in the present invention. Therefore, the steel plates of the comparative examples do not have the resistance to delayed cracking required for dredging conditions and are not suitable for the manufacture of dredging pipelines.

[0070] The present invention relates to erosion-corrosion resistant steel plates that can be used for the manufacture of slurry dredging pipes. They can be applied to various fields such as landfill, waterway dredging, sediment removal from inland rivers, and mineral slurry transportation. This steel plate can replace the currently used ordinary dredging pipeline made of Q235B and Q345B grade steel, thereby improving production efficiency and reducing operating costs.

[0071]

Table 1

[0072]

Table 2

[0073]

Table 3

[0074] Note: *For Examples 1 and 3, and Comparative Example 1, the -40°C AKV values in parentheses are corrected values. The steel plate thicknesses of Examples 1 and 3, and Comparative Example 1 are 8 mm with an impact test sample size of 7.5 x 10 x 55 mm, while the full-size impact test sample size is 10 x 10 x 55 mm. Therefore, when converting the samples to full-size impact test samples, the corrected values for Examples 1 and 3, and Comparative Example 1 are as follows: 49*(10 / 7.5)=65J, 52*(10 / 7.5)=69J, and 33*(10 / 7.5)=44J, respectively.

Claims

1. A steel plate containing the following chemical elements by wt%: C: 0.17 - 0.22%, Si: 0.1 - 0.3%, Mn: 1.0 - 1.4%, P ≤ 0.015%, S ≤ 0.005%, Al: 0.018 - 0.04%, Cu: 0.15 - 0.60%, Ni: 0.1 - 0.31%, B: 0.001 - 0.003%, N ≤ 0.005%, and one or both of Nb: 0.01 - 0.03% and Ti: 0.01 - 0.03%, the balance being Fe and unavoidable impurities; The contents of elements N, Nb, and Ti satisfy the following inequality: 5.68N ≤ Nb + Ti ≤ 0.044; The contents of elements Cu and Ni satisfy the following inequality: Cu / Ni ≤ 2.

0.

2. The steel plate according to Claim 1, further containing one or more of the following: Cr ≤ 2.0%, W: 0.01 - 0.5%, Mo: 0.01 - 0.5%, Sb: 0.01 - 0.2%, REM: 0.01 - 0.2%, V: 0.01 - 0.2%, and Ca: 0.001 - 0.01%.

3. The steel plate according to Claim 1 or 2, satisfying one or more of the following: Cu: 0.29 - 0.60%; 6.65N ≤ Nb + Ti ≤ 0.04; 0.7 ≤ Cu / Ni ≤ 2.

0.

4. The steel plate according to Claim 1 or 2, wherein the thickness of the steel plate is 8 - 20 mm.

5. The steel plate according to Claim 1 or 2, satisfying one or more of the following characteristics: Yield strength ≥ 1100 MPa; Tensile strength ≥ 1300 MPa; Elongation ≥ 12%; Hardness of 450 ± 30 HBW; Impact energy at -40°C ≥ 60 J; Crack time ≥ 600 hours.

6. The steel plate according to Claim 5, wherein the corrosion - erosion resistance of the steel plate is at least twice that of a normal Q235B steel plate.

7. A method for manufacturing the steel plate according to any one of Claims 1 - 6, comprising the following steps: 1) The step of smelting and casting Smelting and casting the molten steel to obtain a casting slab; 2) The step of heating the casting slab The heating temperature is 1230°C or higher, and the total heating time in the heating furnace is 2 hours or more, where the holding time in the immersion zone is 40 minutes or more; 3) The step of rough rolling and finish rolling In the rough rolling stage, the reduction ratio per pass is 15% or more, and / or the reduction amount per pass is 25 mm or more, and / or the total pass deformation ratio is greater than 80%; In the finish rolling stage, at a finish rolling temperature of ≥880°C, preferably 880 - 898°C, the reduction ratio of the final pass is 16% or more; 4) Cooling and coiling process Cool to 550 - 680°C and then coil; 5) Quenching and tempering process The quenching temperature is 820 - 845°C, the quenching holding time T1 is 1.5H - 2H in minutes, where H represents the plate thickness in mm; after exiting the furnace, the steel plate is water-cooled to room temperature at a cooling rate of ≥50°C / s; The tempering temperature is 200 - 240°C with a tempering holding time T2 of 2H - 3H in minutes, where H represents the plate thickness in mm, and T2 ≥ 12 minutes; 6) Finishing process Straighten and cut the head.

8. The method according to claim 7, wherein in step 4), it is cooled to 560 - 680°C and then coiled.

9. The method according to claim 7 or 8, wherein in step 5), the quenching temperature is 828 - 845°C; and / or the tempering temperature is 210 - 240°C, preferably 220 - 240°C.

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