Steel plate special for crane boom and preparation method thereof

Abstract

According to the special steel plate for the crane boom and the preparation method of the special steel plate, microalloy elements such as W and V are properly added, the synergistic effect of C and N is achieved, and carbide, nitride and carbonitride of the microalloy elements are dispersed and separated out, and precipitate is separated out, so that the steel plate achieves good obdurability matching; in order to solve the problem that the content of inclusions in steel is high, calcium iron wires are fed and magnesium wires are added after refining is finished, so that the content of Ca in the steel is 0.0030%-0.0040%, the content of Mg in the steel is 0.0022%-0.0035%, and Mg / S is larger than or equal to 1.5; a new preparation process matched with component design is adopted, so that all alloy elements are more sufficiently and uniformly dissolved in austenite grains, and the uniformity and the strengthening effect of austenitizing are facilitated; and further, the requirements of forming performance, welding performance and fatigue performance 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 them. Moreover, there are a series of problems such as poor formability, high scrap rate, excessive gas and inclusion content in steel, poor low-temperature impact performance and welding performance. Therefore, it is necessary to study the use of welded steel pipes to replace seamless steel pipes, and to control the precipitation and dispersion of carbides, nitrides and carbonitrides of micro-alloying elements so that the steel plate can reach 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, amplitude change, extension, rotation, control and support. In addition, it has the advantages of being able to travel with heavy loads, small turning radius of equipment, 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 part of lifting equipment. Its use conditions require high strength-plasticity product, good formability, low inclusion content in steel, excellent welding performance and certain corrosion resistance.

[0004] Some countries have gradually adopted welded high-strength structural steel to produce crane booms. These steels are added with a large amount of micro-alloy elements such as Nb, V, Ti, Cr, Ni and Mo. The disadvantages are high alloy cost, poor forming performance, high inclusion content in steel, poor welding performance, etc. Therefore, how to adopt new composition design to produce special steel for crane booms, reduce the gas and inclusion content in steel, and meet the technical requirements of users for 90° positive bend and 180° reverse bend has become the current technical bottleneck, and it is urgent to develop new special steel for crane booms.

[0005] In the prior art, patent CN108251747A discloses "steel pipe for crane boom and its manufacturing method", the alloy formula of which adopts 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 adopts high content of Cr, Mo, V, Nb and W and other precious alloys. Due to the high alloy content, the overheating degree is high, which is easy to cause 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 boom and its production method", which contains the following elements by weight: C0.050-0.080%, Si0.15-0.25%, Mn1.70-1.90%, P≤0.020%, S≤0.010%, Als0.020-0.060%, Cr0.20-0.24%, Nb0.06-0.08%, Mo0.10-0.12%, Ti 0.11-0.13%. Its composition design adds low C, Cr, Nb, Mo and Ti elements, the alloy cost is relatively high, and the high Mn content is not conducive 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 achieve. 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 the existing crane boom steel, such as low strength-plasticity product, poor formability, high inclusion content in the steel, poor welding performance and poor fatigue performance. By controlling the precipitation and dispersion precipitation of carbides, nitrides and carbonitrides of micro-alloying elements, the steel plate can reach 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 invention provides a special steel plate for crane boom, which achieves good strength and toughness matching of the steel plate by appropriately adding microalloy elements such as W and V, and the synergistic effect between C and N, and utilizing the dispersion and precipitation of carbides, nitrides and carbonitrides, and meets the requirements of the forming performance, welding performance and fatigue performance of the crane boom. The product is mainly used to manufacture high-grade crane booms, such as bridge cranes, tower cranes and movable crane vehicle boom structures, etc., filling the gap of domestic welded steel pipe production of special steel for crane booms.

[0008] In order to achieve the above-mentioned purpose of the invention, the present invention provides a special steel plate for crane boom, the chemical composition mass fraction of the steel plate is 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, it is also required that the steel contains 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.

[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 tends to produce banded structure, which is detrimental to welding performance. Therefore, the C content is controlled at 0.08% to 0.12%.

[0011] [Si] If the content of the element is too high, it is easy to form fayalite on the surface of the steel plate, which is difficult to remove during the production process and affects the surface quality of the steel plate. In addition, the Si element will 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. Mn combines with S in steel 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] [W] element forms special carbide with carbon, which is a strong carbide and 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, W in the solid solution state can significantly improve the tempering stability of steel after quenching. During tempering, the precipitation and dispersion of special carbide W2C formed by W will promote the significant secondary hardening of 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, adding 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 the steel, and exists in the steel in a fine, dispersed form. It can significantly refine the structure and grains of the steel, increase the grain coarsening temperature, reduce the overheating sensitivity of the steel, thereby improving the strength and toughness of the steel, and reducing the brittle transition temperature of the steel and improving the welding performance; therefore, the V content is controlled at 0.16% to 0.20%.

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

[0016] [P], [S]: When the sulfur content in steel is greater than 0.005%, as the sulfur content in steel increases, sulfur is easy to combine with manganese to form MnS inclusions. The presence of sulfide inclusions in steel increases the sensitivity to fatigue damage and reduces fatigue life; when the sulfur content in steel is less than 0.002%, the number and size of sulfide inclusions in steel are significantly reduced, the sensitivity to fatigue damage is reduced, and it is beneficial to improve fatigue performance. Phosphorus is an element that easily segregates in steel. The hardenability of the segregation zone is about twice that of carbon. The inclusions formed by P can cause the red brittleness and plasticity of steel to decrease. Reducing the P content can significantly improve the HIC resistance of steel. 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-making 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 harm the comprehensive performance of steel. In order to prevent the appearance of oxide inclusions with a diameter greater than 8μm in steel and reduce the number of oxide inclusions, the oxygen content in steel is generally controlled to be less than 0.0015%. The method of controlling oxygen often uses precipitation deoxidation, diffusion deoxidation and comprehensive deoxidation. Therefore, [O] in 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 method for preparing the above-mentioned crane boom special steel plate, the method comprising the following steps:

[0020] ① Weigh the raw materials of each component according to the formula ratio, and then smelt to obtain molten steel; the molten steel 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: 1.5%-1.80%, V: 0.16%-0.20%, H≤0.0002%, O≤0.0010%, N≤0.0050%, in addition, it is also required that the steel contains 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.

[0021] Furthermore, the smelting steps include molten iron desulfurization, converter smelting, ladle LF refining, RH vacuum refining, and calcium and magnesium composite treatment to obtain molten steel;

[0022] Specifically, in order to solve the problem of high inclusion content in steel, after RH vacuum refining, 400-500m of calcium iron wire and 500-550m of magnesium wire are added to make the Ca: 0.0030%-0.0040%, Mg: 0.0022%-0.0035%, and Mg / S ≥1.5 in the steel. On the one hand, it 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 experiments, and improves fatigue life.

[0023] ② Casting the molten steel into continuous casting billets, heating, rough rolling, finishing rolling, laminar cooling and slow cooling treatment of the continuous casting billets to obtain a new type of special steel plate for crane booms;

[0024] Furthermore, the molten steel is continuously cast to obtain continuous casting billets, which are heated in a heating furnace at a temperature of 1200°C to 1280°C for a time of 90 to 120 minutes, including four stages: preheating, primary heating, secondary heating and soaking. The combustion system of the heating furnace mainly includes a burner, a combustion-supporting air system, a mixed coal gas system, a nitrogen purge and release system and a smoke exhaust system.

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

[0026] Furthermore, the continuous casting billet has a thickness of 230 mm, and high-pressure water descaling is performed before entering rough rolling. The target thickness of the intermediate billet 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 reduction of finishing rolling F1 and F2 is ≥15% each, the reduction of F7 is 10%, and the rest are distributed to F3~F6 stands in turn. 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 48h; 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 good strength-toughness matching 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, so as to meet 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, one heating, two 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. It is beneficial to make the alloy elements (especially the insoluble tungsten carbide particles) dissolve more fully and evenly in the austenite grains, which is beneficial to the uniformity and strengthening effect of austenitization. The low-temperature final rolling and low-temperature curling process 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 rolling F1 and F2 reductions are ≥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 48h.

[0032] ④ In order to solve the problem of high inclusion content in steel, after refining, 400-500m of calcium iron wire and 500-550m of magnesium wire are added to make the Ca: 0.0030%-0.0040%, Mg: 0.0022%-0.0035%, Mg / S ≥1.5 in the steel. On the one hand, it is beneficial to the smooth casting of the steel and prevent the clogging of the water nozzle; on the other hand, it is beneficial to the spheroidization treatment of inclusions, prevent the generation of microcracks during pipe forming and flattening test, and improve 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 The inclusion inspection microstructure diagram of the steel plate for crane boom prepared in Example 1;

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

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

[0039] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, 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 boom, the chemical composition mass fraction of the steel plate is 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, it is also required that the steel contains 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.

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

[0042] ① Weigh each component raw material according to the formula ratio, and then smelt it 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] The matters not described in the following embodiments are the same as the description of the above specific implementation methods.

[0045] Example

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

[0047] The novel steel plate specially used for crane boom contains the following chemical components in mass fractions. The specific chemical components of Examples 1 to 5 and Comparative Examples 1 to 2 are shown in Table 1; the rest are 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 at high temperature of the steel, 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, so as to improve the forming performance 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 Weigh the raw materials of each component according to the formula ratio, and then smelt 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 is fed, and 500-550m of magnesium wire is added to make Ca: 0.0030%-0.0040%, Mg: 0.0022%-0.0035%, and Mg / S≥1.5 in the steel.

[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 ≥20%.

[0054] In a preferred embodiment, the above raw materials are mixed once for double-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 successively smelted in a double-blown converter, with top-blowing oxygen decarburization and bottom-blowing N / Ar switching; LF refining includes making white slag in the LF refining process to make [FeO] ≤1% in the steel, adding lime and aluminum to the steel for deep desulfurization and deoxidation alloying; the RH treatment step includes 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 billet drawing speed of 1.0 to 1.2 m / min and a superheat 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 with a heating temperature of 1200°C to 1280°C and a heating time of 90 to 120 minutes, including four stages of 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, the heating process includes four stages: preheating, first heating, second heating and soaking, and the heating furnace combustion system mainly includes: burner, combustion air system, mixed gas system, nitrogen purge and release system and smoke exhaust system; it is beneficial to promote the alloy elements (especially the insoluble tungsten carbide particles) to be more fully and evenly dissolved in the austenite grains, which is beneficial to the uniformity and strengthening effect of austenitization; low-temperature final rolling and low-temperature curling processes are adopted during cooling to ensure the sufficiency and uniformity of 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 carried out before entering the rough rolling, the starting temperature of the rough rolling is 1130°C to 1160°C, the target thickness of the intermediate billet of the coil 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 make the austenite structure of the steel plate fully refined and accumulate phase transformation, all 7 finishing rolling stands are put into use, the finishing rolling F1 and F2 reductions are ≥15% each, the F7 reduction is 10%, and the rest are distributed to the F3~F6 stands in sequence, the compression ratio is ≥5.0, the finishing rolling start target temperature 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 15-20°C / S in the cooling step, a coiling target temperature of 450°C-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 the above-mentioned 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 match between strength and toughness and good low-temperature impact performance.

[0067] Figure 1 The metallographic structure diagram of the steel plate for crane boom prepared in Example 1 is as follows: Figure 1 It can be seen that the metallographic structure is B+F+(MA). The presence of the MAO 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 sensitivity of cold cracks, 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 2From the results of inclusion inspection, 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 grade D inclusions were found in spherical shape, which avoided the occurrence of cracking during forming and welding and improved 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 The reverse bending test photo of the steel plate for crane boom prepared 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 appeared at the bending parts of the steel plate in both the forward bending test and the reverse bending test, and the results are completely qualified.

[0069] Figure 5 The macrostructure diagram of the weld area of ​​the crane boom special steel plate obtained in Example 1 is as follows: Figure 5 The low-magnification test results show that in the weld area, the cross-section cracks, root cracks, and surface cracks of the weld 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 that the high-magnification inspection results of the weld fusion zone show that the fusion transition effect between the welding 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 described in the above Examples 1 to 5, the new steel plate for crane boom 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, and the synergistic effect between C and N elements, and by the dispersion and precipitation of carbides, nitrides and carbonitrides of microalloying elements, the steel plate has high strength and good toughness, meeting the requirements of the forming performance and fatigue performance of the crane boom, especially excellent low-temperature impact toughness.

[0071] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should 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 mass fractions of the steel plate are 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 are Fe and other inevitable impurities.

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 Weigh each component raw material according to the formula ratio, and then smelt it to obtain molten steel; 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.

4. The preparation method according to claim 3, characterized in that: Step ① The molten steel 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: 1.5%-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 rest is Fe and other inevitable impurities.

5. The preparation method according to claim 3, characterized in that: Step ① The smelting steps 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 Ca: 0.0030%-0.0040%, Mg: 0.0022%-0.0035%, and Mg / S ≥1.5 in the steel.

6. The preparation method according to claim 3, characterized in that: Step ② The continuous casting billet is heated in a heating furnace at a temperature of 1200° C. to 1280° C. for a heating time of 90 to 120 minutes, 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: In step ②, the continuous casting billet has a thickness of 230 mm and is descaled by high-pressure water before entering rough rolling. The target thickness of the intermediate billet of the coil 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 temperature in step ② is 1130°C to 1160°C.

9. The preparation method according to claim 3, characterized in that: 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~F6 stands in sequence, the compression ratio is ≥5.0, the finishing rolling start target temperature is 950℃~980℃, and the final rolling target temperature is 785℃~815℃.

10. The preparation method according to claim 3, characterized in that: In step ②, the cooling rate in the cooling step is 15-20°C / S, and the coiling target temperature is 450°C-480°C; the coiled steel coil adopts a slow cooling process and can be moved after 48 hours.

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