A special steel plate for crane jib and a method for manufacturing the same

Abstract

The application discloses a special steel plate for a crane boom and a preparation method thereof. By properly adding micro-alloy elements such as W and V, and the synergistic effect between C and N, the steel plate reaches good strength and toughness matching through the dispersion and precipitation of carbides, nitrides and carbonitrides of the micro-alloy elements. In order to solve the problem of high content of inclusions in the steel, calcium iron wire and magnesium wire are fed after refining, so that Ca in the steel is 0.0030% to 0.0040%, Mg is 0.0022% to 0.0035%, and Mg / S is greater than or equal to 1.5. A new preparation process matched with the component design is adopted to promote the more sufficient and uniform dissolution of each alloy element in the austenite grain, which is beneficial to the uniformity and strengthening effect of austenitization. Further, the requirements of the forming property, welding property and fatigue property of the crane boom are met.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and in particular relates to a special steel plate for a crane boom and a preparation method thereof. Background Art

[0002] At present, seamless steel pipes are generally used for crane booms in my country. Only a few steel pipe factories such as Baosteel and Hengyang Steel Pipe produce seamless steel pipes. In addition, there are a series of problems such as poor formability, high scrap rate, excessive gas and inclusion content in the steel, poor low-temperature impact resistance and poor welding performance. Therefore, it is necessary to study the use of welded steel pipes to replace seamless steel pipes. By controlling the precipitation and dispersion of carbides, nitrides and carbonitrides of micro-alloying elements, the steel plate can meet the strength and toughness requirements of 800MPa steel grade, and the surface of the steel plate has good welding performance, excellent low-temperature toughness, high fatigue resistance and surface quality.

[0003] As fixed tower cranes, rail-structured bridge cranes and self-propelled cranes used in high-rise building construction, they have the characteristics of high formability, high technical content and complex production process. When the crane is working, the entire boom structure bears the forces of lifting, luffing, extension, rotation, control and support. In addition, it has the advantages of being able to travel while lifting heavy loads, small equipment turning radius, large lifting load, safe and stable operation, low requirements for the operating environment, and free replacement of truss combinations. It is widely used in wind turbines, oilfield equipment installation, offshore drilling platform construction, high-rise (super-high-rise) building hoisting and bridge construction. The crane boom is a key component of lifting equipment. Its use conditions require high strength-plasticity, good formability, low inclusion content in the steel, excellent welding performance and certain corrosion resistance.

[0004] Some countries have gradually adopted welded high-strength structural steels for crane booms. These steels incorporate significant amounts of microalloying elements such as Nb, V, Ti, Cr, Ni, and Mo. However, these steels suffer from high alloy costs, poor formability, high inclusion content, and poor weldability. Therefore, the challenge of developing new steel compositions specifically designed for crane booms, reducing gas and inclusion content, and meeting user requirements for 90° and 180° bends, has become a technical bottleneck. The development of new steels specifically designed for crane booms is urgently needed.

[0005] In the prior art, patent CN108251747A discloses a "steel pipe for crane boom and its manufacturing method". The alloy formula uses C0.10-0.20%, Si0.10-0.90%, Al0.008-0.070%, Cr0.80-3.60%, Mo0.40-1.80%, V0.03-0.16%, Nb0.03-0.35%, W0.10-1.50% and N0-0.005%. The alloy ratio uses high content of precious alloys such as Cr, Mo, V, Nb and W. Due to the high alloy content, it leads to high superheat, which easily causes serious surface quality and center segregation. In addition, the alloy price is high and the production cost is high. Patent CN108774710A discloses "A high-strength steel for crane booms and its production method." The steel contains the following elements by weight: C 0.050-0.080%, Si 0.15-0.25%, Mn 1.70-1.90%, P ≤ 0.020%, S ≤ 0.010%, Als 0.020-0.060%, Cr 0.20-0.24%, Nb 0.06-0.08%, Mo 0.10-0.12%, and Ti 0.11-0.13%. Its composition is designed to include low C, Cr, Nb, Mo, and Ti elements, resulting in high alloy cost, and a high Mn content that is detrimental to welding performance. The process flow is molten iron pretreatment → converter smelting → alloy fine-tuning station → LF refining → continuous casting → ingot heat treatment → controlled rolling and controlled cooling → flattening → cross-cutting → finished product. The production cycle is long, the process is relatively complex, and it is not easy to implement. Summary of the Invention

[0006] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to address the defects of existing crane boom steel such as low strength-ductility product, poor formability, high inclusion content in steel, poor welding performance and poor fatigue performance. By controlling the precipitation and dispersion precipitation of carbides, nitrides and carbonitrides of microalloying elements, the steel plate can meet the strength and toughness requirements of 800MPa steel grade, and the surface of the steel plate has good welding performance, excellent low-temperature toughness, high fatigue resistance and surface quality.

[0007] The present invention provides a steel plate specifically designed for crane booms. By appropriately adding microalloying elements such as W and V, and utilizing the synergistic effect of C and N, and utilizing the dispersion and precipitation of carbides, nitrides, and carbonitrides, the steel plate achieves an excellent balance of strength and toughness, meeting the requirements for formability, weldability, and fatigue performance of crane booms. This product is primarily used in the manufacture of high-grade crane booms, such as those for bridge cranes, tower cranes, and mobile crane boom structures. It fills a gap in the domestic market for specialized steel for crane booms produced through welded steel pipe production.

[0008] In order to achieve the above-mentioned object of the invention, the present invention provides a special steel plate for crane boom, wherein the chemical composition mass fraction of the steel plate is C: 0.08% to 0.12%, Si≤0.03%, Mn: 0.60% to 0.80%, P≤0.010%, S≤0.0020%, W: 1.5% to 1.80%, V: 0.16% to 0.20%, H≤0.0002%, O≤0.0010%, N≤0.0050%. In addition, the steel is required to contain Ca: 0.0030% to 0.0040% and Mg: 0.0022% to 0.0035%, Mg / S≥1.5, Ceq≤0.40, and the rest is Fe and other inevitable impurities.

[0009] The design principle of the chemical composition of the special steel plate for crane boom is as follows:

[0010] The [C] element is the most effective element for improving strength, but a high C content causes serious segregation, which is detrimental to cold forming performance. In addition, a high C content can easily produce banded structure, which is detrimental to welding performance. Therefore, the C content is controlled at 0.08% to 0.12%.

[0011] Excessive Si content can easily form fayalite on the steel plate surface, which is difficult to remove during the production process and affects the surface quality of the steel plate. In addition, Si can reduce the low-temperature toughness of the welded joint. Therefore, the Si content is controlled at ≤0.03%.

[0012] [Mn] element is the most common strengthening element. In steel, Mn combines with S to form MnS inclusions. When the Mn content in steel exceeds 1.20%, it is easy to form center segregation in the continuous casting billet, resulting in stamping cracking. Therefore, the Mn content is controlled at 0.60% to 0.80%.

[0013] The element [W] forms a special carbide with carbon. It is a strong carbide and a high melting point forming element. Therefore, it can increase the tempering stability and thermal strength of steel and reduce the crack sensitivity of steel. Moreover, when W is in the solid solution state, it can significantly improve the tempering stability of steel after quenching. During tempering, the precipitation and dispersion of the special carbide W2C formed by W will promote significant secondary hardening of the steel, reduce the banded structure of the steel plate, and inhibit the formation of pearlite. The special carbide W2C forms a passivation film on the surface of the steel plate, inhibiting the entry of air and harmful gases and improving the corrosion resistance of the steel plate. In addition, the addition of W to steel can also increase the fatigue life of the crane boom; therefore, the W content is controlled at 1.50% to 1.80%.

[0014] The [V] element forms extremely stable nitrides and carbides with C and N in steel, and exists in the steel in a fine, dispersed form. It can significantly refine the steel's structure and grains, increase the grain coarsening temperature, reduce the steel's overheating sensitivity, thereby increasing the steel's strength and toughness, and lowering the steel's brittle transition temperature and improving weldability. Therefore, the V content is controlled at 0.16% to 0.20%.

[0015] [Mg], [Ca]: In order to precisely control the type and size of inclusions and ensure that the size of inclusions is below 8μm during the solidification process of molten steel, a composite treatment of magnesium and calcium can be used. The principle is that low-melting-point compounds composed of Mg and Ca can be produced during the solidification process of molten steel, which is beneficial to corrosion resistance and fatigue performance. Therefore, the Mg content in the steel is controlled at 0.0022% to 0.0035% and the Ca content is controlled at 0.0030% to 0.0040%.

[0016] [P], [S]: When the sulfur content in steel exceeds 0.005%, as the sulfur content increases, sulfur easily combines with manganese to form MnS inclusions. The presence of sulfide inclusions in the steel increases fatigue susceptibility and reduces fatigue life. When the sulfur content in steel is less than 0.002%, the number and size of sulfide inclusions in the steel decrease significantly, reducing fatigue susceptibility and improving fatigue performance. Phosphorus is a readily segregating element in steel, and the hardenability of the segregated zone is approximately twice that of carbon. Inclusions formed by phosphorus can cause red brittleness and reduce ductility in the steel. Reducing the phosphorus content can significantly improve the steel's HIC resistance. Control S ≤ 0.002% and P ≤ 0.010%.

[0017] [H], [O]: H is the main cause of white spots and cracking. The higher the mass fraction of hydrogen, the greater the probability of HIC generation, the higher the corrosion rate, and the more significant the increase in average crack length. In addition, it is also necessary to avoid moisture in slag-forming agents, modifiers, alloying agents, protective slag, covering agents, etc. added in subsequent processes. If the oxygen content in steel is too high, oxide inclusions and macro inclusions will increase, which will endanger the comprehensive performance of the steel. In order to prevent the appearance of oxide inclusions with a diameter greater than 8μm in the steel and reduce the number of oxide inclusions, the oxygen content in the steel is generally controlled to be less than 0.0015%. The methods of controlling oxygen often use precipitation deoxidation, diffusion deoxidation and comprehensive deoxidation. Therefore, [O] in the steel is controlled to be ≤0.0010% and [H] ≤0.0002%.

[0018] Furthermore, the continuous casting billet obtained by continuous casting according to the above composition is subjected to heating, rough rolling, finish rolling, laminar cooling and slow cooling treatment to obtain a special steel plate for crane boom, whose yield strength is ≥800MPa, tensile strength is ≥850MPa, and elongation after fracture is A 50 ≥26%, -50℃KV2≥150J, -70℃KV2≥120J.

[0019] A preparation method of the special steel plate for crane jib, the method comprises the following steps:

[0020] ① The raw materials of each component are weighed according to the formula proportion, and then smelting is carried out to obtain molten steel; the molten steel comprises the following mass fractions of chemical components: C: 0.08%-0.12%, Si≤0.03%, Mn: 0.60%-0.80%, P≤0.010%, S≤0.0020%, W: 1.5%-1.80%, V: 0.16%-0.20%, H≤0.0002%, O≤0.0010%, N≤0.0050%, in addition, Ca: 0.0030%-0.0040% and Mg: 0.0022%-0.0035% in the steel are required, Mg / S≥1.5, Ceq≤0.40, and the rest is Fe and other inevitable impurities.

[0021] Further, the smelting step comprises hot metal desulfurization, converter smelting, ladle LF refining, RH vacuum refining, and calcium and magnesium composite treatment to obtain the molten steel.

[0022] Specifically, to solve the problem of high content of inclusions in the steel, after the RH vacuum refining is finished, 400-500 m of calcium feeding wire is fed, and 500-550 m of magnesium wire is added, so that Ca: 0.0030%-0.0040% and Mg: 0.0022%-0.0035% in the steel, and Mg / S≥1.5, on the one hand, it is beneficial to the smooth casting of the steel, and prevents the nozzle from being blocked; on the other hand, it is beneficial to the spheroidization treatment of inclusions, prevents the generation of micro-cracks during pipe forming and flattening experiments, and improves the fatigue life.

[0023] ② The molten steel is cast into a continuous casting billet, and the continuous casting billet is subjected to heating, rough rolling, finish rolling, laminar cooling and slow cooling treatment to obtain the new type of special steel plate for crane jib;

[0024] Further, the molten steel is obtained by continuous casting, and the continuous casting billet is heated in a heating furnace, and the heating temperature is 1200-1280°C, and the heating time is 90-120 min, including four stages of preheating, first heating, second heating and soaking, and the heating furnace combustion system mainly comprises a burner, a combustion air system, a mixed gas system, a nitrogen purging and diffusion system and a smoke exhaust system.

[0025] Specifically, the preheating temperature is 1050-1100°C, and the preheating time is 20-40 min; the first heating temperature is 1100-1200°C, and the heating time is 20-40 min; the second heating temperature is 1200-1280°C, and the heating time is 20-40 min; the soaking temperature is 1220-1260°C, and the soaking time is 20-40 min.

[0026] Furthermore, the thickness of the continuous casting slab is 230 mm, and high-pressure water descaling is performed before entering rough rolling. The target thickness of the intermediate slab of the coil is 65 mm, and the rough rolling unit adopts a "3+3" reduction mode.

[0027] The starting temperature of rough rolling is 1130℃~1160℃. In order to fully refine the austenite structure of the steel plate and accumulate phase transformation, all 7 finishing rolling stands are put into use. The finishing rolling reductions of F1 and F2 are ≥15% each, the F7 reduction is 10%, and the rest are distributed to F3~F6 stands in sequence. The compression ratio is ≥5.0, the starting target temperature of finishing rolling is 950℃~980℃, the target temperature of final rolling is 785℃~815℃, the cooling rate in the cooling step is 15~20℃ / S, the target temperature of coiling is 450℃~480℃, and the low-temperature final rolling and low-temperature coiling processes are used to make the steel plate structure fine and uniform. The coiled steel coil adopts a slow cooling process in the warehouse and can be moved after 48 hours. Finally, a steel plate with good strength and toughness is obtained.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] ① The present invention achieves a good match between strength and toughness of the steel plate by appropriately adding microalloying elements such as W and V, as well as the synergistic effect between C and N, and by the dispersion and precipitation of carbides, nitrides and carbonitrides of the microalloying elements, thereby meeting the requirements of the forming performance, welding performance and fatigue performance of the crane boom.

[0030] ② A new heating process is adopted with a heating temperature of 1200℃~1280℃ and a heating time of 90~120min, including four stages: preheating, first heating, second heating and soaking. The heating furnace combustion system mainly includes burner, combustion air system, mixed gas system, nitrogen purge and release system and smoke exhaust system. This is conducive to promoting the more sufficient and uniform dissolution of various alloy elements (especially the insoluble tungsten carbide particles) in the austenite grains, which is beneficial to the uniformity and strengthening effect of austenitization. The low-temperature final rolling and low-temperature curling processes are adopted during cooling to ensure the sufficiency and uniformity of martensitic transformation.

[0031] ③ In order to fully refine the austenite structure of the steel plate and accumulate phase transformation, all 7 finishing rolling stands are put into use, the finishing reduction of F1 and F2 is ≥15% each, the F7 reduction is 10%, and the rest are distributed to F3~F6 stands in turn, the compression ratio is ≥5.0, the finishing rolling target temperature is 950℃~980℃, the final rolling target temperature is 785~815℃, the cooling rate in the cooling step is 15~20℃ / S, the coiling target temperature is 450℃~480℃, and the low-temperature final rolling and low-temperature coiling processes are used to make the steel plate structure fine and uniform. The coiled steel coil adopts a slow cooling process in the warehouse and can be moved after 48 hours.

[0032] ④ In order to solve the problem of high inclusion content in the steel, after refining, 400-500m of calcium iron wire and 500-550m of magnesium wire are fed to make the Ca content in the steel 0.0030%-0.0040%, Mg content 0.0022%-0.0035%, and Mg / S ≥ 1.5. On the one hand, this is beneficial to the smooth casting of the steel and prevents nozzle blockage; on the other hand, it is beneficial to the spheroidization treatment of inclusions, prevents the generation of microcracks during pipe forming and flattening tests, and improves fatigue life. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The metallographic structure diagram of the steel plate for crane boom prepared in Example 1;

[0034] Figure 2 This is a microstructure diagram of inclusion inspection of the steel plate for crane boom prepared in Example 1;

[0035] Figure 3 This is a photo of the positive bending test of the steel plate for crane boom prepared in Example 1;

[0036] Figure 4 This is a photo of the reverse bending experiment of the steel plate for crane boom prepared in Example 1;

[0037] Figure 5 This is a macrostructure diagram of the weld area of ​​the crane boom steel plate obtained in Example 1;

[0038] Figure 6 This is the microstructure diagram of the fusion zone of the weld of the special steel plate prepared in Example 1. DETAILED DESCRIPTION

[0039] The present invention will be further described below with reference to specific examples, but the present invention is not limited in any way. To avoid redundancy, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0040] A special steel plate for crane booms, wherein the chemical composition of the steel plate is as follows: C: 0.08%-0.12%, Si≤0.03%, Mn: 0.60%-0.80%, P≤0.010%, S≤0.0020%, W: 1.5%-1.80%, V: 0.16%-0.20%, H≤0.0002%, O≤0.0010%, N≤0.0050%. In addition, the steel is required to contain Ca: 0.0030%-0.0040% and Mg: 0.0022%-0.0035%, Mg / S≥1.5, Ceq≤0.40, and the remainder is Fe and other inevitable impurities.

[0041] A method for preparing the above-mentioned crane boom special steel plate, the method comprising the following steps:

[0042] ① Weigh the raw materials of each component according to the formula ratio, and then smelt them to obtain molten steel;

[0043] ② Cast the molten steel into continuous casting billets, and perform heating, rough rolling, finish rolling, laminar cooling and slow cooling treatment on the continuous casting billets to obtain special steel plates for crane booms.

[0044] Any matters not described in the following embodiments are the same as those described in the above specific implementation manner.

[0045] Example

[0046] A special steel plate for crane boom and its preparation method, the specific process flow is as follows:

[0047] The new steel plate specially used for crane booms contains the following chemical components in mass fractions. The specific chemical compositions of Examples 1 to 5 and Comparative Examples 1 to 2 are shown in Table 1; the remainder is Fe and unavoidable impurities.

[0048] Table 1 Chemical composition of Examples 1 to 5 and Comparative Examples 1 to 2

[0049]

[0050] The technical solutions of the above-mentioned embodiments 1 to 5 are designed with low silicon and low carbon components, and appropriate amounts of tungsten and vanadium alloy elements are added. The addition of tungsten maintains the hardness of the steel, improves the fatigue life of the steel, and at the same time improves the strength of the steel, increases the resistance to deformation and cracking, and eliminates the residual stress of the steel plate to a certain extent. The vanadium element has the effect of improving the plasticity and toughness of the steel plate, adjusting the tendency of grain growth of the steel at high temperature, and improving the welding sensitivity and low-temperature toughness of the steel. In order to improve the morphology and quantity of inclusions, calcium and magnesium are intentionally added, and Mg / S is required to be ≥1.5, to improve the formability and fatigue resistance of the steel. The reasonable ratio of the above alloy elements effectively exerts the performance advantages of this steel in the crane boom structure.

[0051] The method for preparing the above-mentioned crane boom special steel plate comprises the following steps:

[0052] ① Weighing each component raw material according to the formula ratio and then smelting to obtain molten steel (the composition of the molten steel is shown in Table 1); the smelting steps include molten iron desulfurization, converter smelting, ladle LF refining, RH vacuum refining, and calcium and magnesium composite treatment; wherein, after the refining is completed, 400-500m of calcium iron wire and 500-550m of magnesium wire are added to adjust the steel's Ca content to 0.0030%-0.0040%, Mg content to 0.0022%-0.0035%, and Mg / S ratio to 1.5 or higher.

[0053] In Examples 1 to 5, in order to further mass-produce steel plates for crane booms economically and to control the content of impurity elements, preferably, the ingredients include high-quality scrap steel, molten iron, ferroalloys and aluminum particles, and the amount of scrap steel added is ensured to be ≥20%.

[0054] In a preferred embodiment, the raw materials are mixed once and subjected to combined blowing converter smelting, LF ladle furnace refining, RH vacuum treatment and continuous casting to obtain a continuous casting billet. Specifically, the smelting steps are as follows:

[0055] The raw materials are mixed and sequentially smelted in a double-blown converter, using top-blowing oxygen for decarburization and bottom-blowing N / Ar switching; LF refining includes producing white slag in the LF refining process to make the [FeO] in the steel ≤1%, and adding lime and aluminum to the steel for deep desulfurization and deoxidation alloying; the RH treatment steps include vacuuming and decarburization to remove gases (hydrogen, oxygen and nitrogen content) in the steel under vacuum conditions; the continuous casting step uses a straight arc continuous casting machine with a casting speed of 1.0 to 1.2 m / min and an overheating degree of 25°C to 30°C.

[0056] ② Cast the molten steel into continuous casting billets, and perform heating, rough rolling, finish rolling, laminar cooling and slow cooling treatment on the continuous casting billets to obtain special steel plates for crane booms.

[0057] The continuous casting billet is heated in a heating furnace at a heating temperature of 1200°C to 1280°C for a heating time of 90 to 120 minutes, including four stages: preheating, first heating, second heating and soaking; wherein the temperature of the preheating section is 1050°C to 1100°C and the preheating time is 20 to 40 minutes; the temperature of the first heating section is 1100°C to 1200°C and the heating time is 20 to 40 minutes; the temperature of the second heating section is 1200°C to 1280°C and the heating time is 20 to 40 minutes; the soaking temperature is 1220°C to 1260°C and the soaking time is 20 to 40 minutes.

[0058] In a preferred embodiment, the heating temperature is 1210°C to 1230°C, the heating time is 90 to 110 minutes, and the heating process includes four stages: preheating, primary heating, secondary heating, and soaking. The heating furnace combustion system mainly includes a burner, a combustion-supporting air system, a mixed gas system, a nitrogen purge and release system, and a smoke exhaust system. This is conducive to promoting the more complete and uniform dissolution of various alloying elements (especially insoluble tungsten carbide particles) in the austenite grains, which is beneficial to the uniformity and strengthening effect of austenitization. During subsequent cooling, low-temperature final rolling and low-temperature curling processes are used to ensure the sufficiency and uniformity of the martensitic transformation.

[0059] In a preferred embodiment, the rolling process of the continuous casting billet includes: the thickness of the continuous casting billet is 230 mm, high-pressure water descaling is performed before entering the rough rolling, the rough rolling start temperature is 1130°C to 1160°C, the target thickness of the plate and coil intermediate billet is 65 mm, and the rough rolling unit adopts a "3+3" reduction mode to obtain the intermediate billet.

[0060] In a preferred embodiment, the finishing rolling process includes: in order to fully refine the austenite structure of the steel plate and accumulate phase transformation, all 7 finishing rolling stands are put into use, the finishing reduction of F1 and F2 is ≥15% each, the finishing reduction of F7 is 10%, and the rest is distributed to F3 to F6 stands in sequence, the compression ratio is ≥5.0, the starting target temperature of finishing rolling is 950℃~980℃, and the final rolling target temperature is 785℃~815℃.

[0061] In a preferred embodiment, the laminar cooling process includes: a cooling rate of 15 to 20°C / s in the cooling step, a coiling target temperature of 450 to 480°C, and low-temperature final rolling and low-temperature coiling processes to make the steel plate structure fine and uniform.

[0062] Preferably, the coiled steel coil is subjected to a slow cooling process in the warehouse and can be moved after 48 hours.

[0063] The properties of the hot-rolled steel sheets obtained in Examples 1 to 5 and Comparative Examples 1 to 2 were characterized, and the test results are shown in Table 2.

[0064] Table 2 Performance test results of hot-rolled steel sheets obtained from Examples 1 to 5 and Comparative Examples 1 to 2

[0065] Rel / MPa Rm / MPa A / ％ <![CDATA[-50℃KV2]]> <![CDATA[-70℃KV2]]> Example 1 850 900 32 255 195 Example 2 862 915 30 247 180 Example 3 870 920 29.5 230 178 Example 4 875 942 28 225 165 Example 5 890 965 26.5 220 154 Comparative Example 1 625 670 24.5 135 68 Comparative Example 2 630 665 24 140 70

[0066] Judging from the performance of the embodiments, the yield strength reaches more than 850MPa, the tensile strength reaches more than 900MPa, the elongation after break reaches more than 26%, the impact energy at -50℃ reaches more than 220J, and the impact energy at -70℃ reaches more than 150J, achieving a good strength-toughness match and good low-temperature impact performance.

[0067] Figure 1 The metallographic structure diagram of the crane boom special steel plate obtained in Example 1 is as follows: Figure 1 It can be seen that the metallographic structure is B+F+(MA). The presence of the Maho Island component increases the hardness of the material and improves its resistance to deformation, cracking and fatigue; the acicular ferrite can meet the organization of large line energy welding, effectively improve the low-temperature toughness, reduce the cold crack sensitivity, and ensure that the steel plate has good pipe-making performance and weldability; the bainite structure is a mixed structure of ferrite and carbide, which improves the strength and toughness of the steel plate, and the uniform and fine grains improve the comprehensive performance of the steel plate. Figure 2 The inclusion inspection microstructure diagram of the crane boom steel plate obtained in Example 1 is as follows: Figure 2According to the inclusion inspection results, by adding calcium wire and magnesium wire, on the one hand, the number of inclusions and modification treatment of the steel plate were effectively reduced, and only 0.5-level Class D inclusions were found, which were spherical, avoiding the occurrence of cracking during the forming and welding process and improving the fatigue resistance of the steel plate; on the other hand, low-melting-point compounds were formed in the steel, which effectively prevented the occurrence of nozzle blockage and made the casting process smooth.

[0068] Figure 3 This is a photo of the positive bending test of the steel plate for crane boom prepared in Example 1. Figure 4 This is a photo of the reverse bending experiment of the special steel plate for crane boom obtained in Example 1; Figure 3 and Figure 4 The results of the forward and reverse bending tests of the steel plate show that no cracks appear at the bending parts of the steel plate in both the forward and reverse bending tests, and the results are completely qualified.

[0069] Figure 5 The macrostructure of the weld zone of the crane boom steel plate obtained in Example 1 is as follows: Figure 5 The low-magnification test results show that in the weld area, the weld cross-section cracks, root cracks, and surface cracks are all zero; Figure 6 The microstructure of the weld fusion zone of the special steel plate obtained in Example 1 is as follows: Figure 6 It can be seen from the high-magnification inspection results of the weld fusion zone that the fusion transition effect between the weld zone and the base material is good, and the different metallographic structures penetrate each other and transition smoothly, indicating that the material has good weldability.

[0070] As illustrated in Examples 1 to 5 above, the novel steel plate for crane booms of the present invention has stable chemical composition control and a yield strength of 850 MPa or above. Furthermore, by appropriately adding tungsten and vanadium alloy elements, as well as the synergistic effect between C and N elements, and by the dispersion, precipitation, and precipitation of carbides, nitrides, and carbonitrides of the microalloying elements, the steel plate achieves high strength while also having good toughness, meeting the requirements for formability and fatigue performance of crane booms, and in particular, exhibiting excellent low-temperature impact toughness.

[0071] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A special steel plate for crane boom, characterized in that: The chemical composition of the steel plate is as follows: C: 0.08%-0.12%, Si≤0.03%, Mn: 0.60%-0.80%, P≤0.010%, S≤0.0020%, W: 1.52%-1.80%, V: 0.18%-0.20%, H≤0.0002%, O≤0.0010%, N≤0.0050%. In addition, the steel is required to contain Ca: 0.0030%-0.0040% and Mg: 0.0022%-0.0035%, Mg / S≥1.5, Ceq≤0.40, and the rest is Fe and other unavoidable impurities. The method for preparing the special steel plate for crane boom comprises the following steps: (1) Weighing the raw materials of each component according to the formula ratio, and then smelting to obtain molten steel; (2) Casting the molten steel into a continuous casting billet, heating, rough rolling, finish rolling, laminar cooling, coiling and slow cooling the continuous casting billet to obtain a special steel plate for the crane boom; In step (2), all the 7 finishing rolling stands are put into operation, the finishing rolling reductions of F1 and F2 are ≥15% each, the finishing rolling reduction of F7 is 10%, and the rest are distributed to F3 to F6 stands in sequence, the compression ratio is ≥5.0, the finishing rolling start target temperature is 950°C to 980°C, and the final rolling target temperature is 785°C to 815°C; The cooling rate in the laminar cooling step of step (2) is 15-20°C / s, and the target coiling temperature is 450-480°C; the coiled steel coil adopts a slow cooling process and can be moved after 48 hours.

2. The special steel plate for crane boom according to claim 1, characterized in that: The yield strength of the special steel plate is ≥800MPa, the tensile strength is ≥850MPa, and the elongation after fracture is A 50 ≥26%, -50℃KV2≥150J, -70℃KV2≥120J.

3. A method for preparing a special steel plate for a crane boom as claimed in claim 1, characterized in that: The method comprises the following steps: (1) Weighing the raw materials of each component according to the formula ratio, and then smelting to obtain molten steel; (2) Casting the molten steel into a continuous casting billet, heating, rough rolling, finish rolling, laminar cooling, coiling and slow cooling the continuous casting billet to obtain a special steel plate for the crane boom; In step (2), all the 7 finishing rolling stands are put into operation, the finishing rolling reductions of F1 and F2 are ≥15% each, the finishing rolling reduction of F7 is 10%, and the rest are distributed to F3 to F6 stands in sequence, the compression ratio is ≥5.0, the finishing rolling start target temperature is 950°C to 980°C, and the final rolling target temperature is 785°C to 815°C; The cooling rate in the laminar cooling step of step (2) is 15-20°C / s, and the target coiling temperature is 450-480°C; the coiled steel coil adopts a slow cooling process and can be moved after 48 hours.

4. The preparation method according to claim 3, characterized in that The molten steel in step (1) includes the following chemical components by mass fraction: C: 0.08%-0.12%, Si≤0.03%, Mn: 0.60%-0.80%, P≤0.010%, S≤0.0020%, W: 1.52%-1.80%, V: 0.18%-0.20%, H≤0.0002%, O≤0.0010%, N≤0.0050%. In addition, the steel is required to contain Ca: 0.0030%-0.0040% and Mg: 0.0022%-0.0035%, Mg / S≥1.5, Ceq≤0.40, and the rest is Fe and other inevitable impurities.

5. The preparation method according to claim 3, characterized in that The smelting steps in step (1) include molten iron desulfurization, converter smelting, ladle LF refining, RH vacuum refining, and calcium and magnesium composite treatment; Among them, after RH refining, 400~500m of calcium iron wire is fed and 500~550m of magnesium wire is added to make the Ca content in the steel 0.0030%~0.0040%, Mg: 0.0022%~0.0035%, and Mg / S ≥1.

5.

6. The preparation method according to claim 3, characterized in that The continuous casting billet in step (2) is heated in a heating furnace, including four stages: a preheating stage, a first heating stage, a second heating stage, and a soaking stage; Among them, the temperature of the preheating section is 1050℃~1100℃, and the preheating time is 20~40min; the temperature of the first heating section is 1100℃~1200℃, and the heating time is 20~40min; the temperature of the second heating section is 1200℃~1280℃, and the heating time is 20~40min; the temperature of the soaking section is 1220℃~1260℃, and the soaking time is 20~40min.

7. The preparation method according to claim 3, characterized in that The thickness of the continuous casting slab in step (2) is 230 mm, and high-pressure water descaling is performed before entering rough rolling. The target thickness of the intermediate slab is 65 mm, and the rough rolling unit adopts a "3+3" reduction mode.

8. The preparation method according to claim 3, characterized in that The rough rolling start temperature in step (2) is 1130°C~1160°C.

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