Round billet continuous casting and rolling micro-alloying steel for shield tunneling machine cutter hub and manufacturing method of round billet continuous casting and rolling micro-alloying steel

Abstract

The invention relates to steel for a microalloying shield tunneling machine cutter hub. The steel comprises the following chemical components in percentage by mass: 0.25 to 0.45 percent of C, 0.05 to 0.37 percent of Si, 0.40 to 1.00 percent of Mn, 1.00 to 1.80 percent of Cr, 0.05 to 0.50 percent of Ni, 0.10 to 0.35 percent of Mo, 0.0010 to 0.0030 percent of B, 0.010 to 0.050 percent of Al, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.15 percent of Cu, 0.010 to 0.040 percent of Ti, less than or equal to 0.030 percent of Nb, less than or equal to 0.00050 percent of Ca, less than or equal to 0.040 percent of As, less than or equal to 0.030 percent of Sn, less than or equal to 0.0050 percent of Sb, less than or equal to The manufacturing process comprises the steps of full molten iron treatment, BOF converter treatment, LF refining furnace treatment, RH vacuum degassing, CCM round billet continuous casting (the size of phi 700 mm or above), warm feeding / slow cooling, heating rolling (round steel with the diameter of phi 200 mm or above), saw cutting, slow cooling, flaw detection, finished product inspection and packaging. According to the steel for the shield tunneling machine cutter hub, round billets with the specification of phi 700 mm or above are rolled into bar round steel, and the macroscopic quality, the mechanical property, the impact quality and the internal quality of the steel for the shield tunneling machine cutter hub are guaranteed.

Description

Technical Field

[0002] The invention belongs to the technical field of special steel smelting, and specifically relates to a round billet continuous casting and rolling micro-alloyed steel for a shield machine hub and a manufacturing method thereof. Background Art

[0004] As people gradually realize the impact of cities and humans on the environment, the utilization of underground space has become a trend. As a national infrastructure heavyweight, the shield machine is a special tunnel boring equipment, mainly used in subway rail transit tunnel boring, water conservancy projects, and underground culvert construction. Therefore, the shield machine has a very broad application prospect in the construction and utilization of underground space.

[0005] The basic working principle of the shield machine is that a cylindrical steel component moves forward along the axis of the tunnel. The front end is a cutterhead with various cutters installed on it. It rotates and moves forward to excavate the soil. The excavated residual soil passes through the soil discharge hole on the cutterhead and enters the mud and water tank in the middle of the shield machine, and then is transported to the ground through the conveyor. When the shield machine is working, the huge cutterhead rotates, driving the various cutters on it to complete the excavation of the soil. Due to the extremely complex underground environment, when the cutter rotates forward, it directly contacts the rock shoulder and rock, and bears great positive and lateral forces. Each cutter is assembled in the cutter hub, which transmits these forces to the cutter hub for bearing; at the same time, due to the rotation of the cutterhead, the gravel in the mud and water tank will constantly wear the cutter hub. If the cutter hub is broken or excessively worn, it will directly affect the normal operation of the cutter. This also requires that the cutter hub must have sufficient strength, toughness, impact resistance, and extremely high wear resistance.

[0006] In addition, during the forming process of the shield machine hub, the raw steel materials need to go through processes such as forging, punching, turning, heat treatment, and surface treatment. The center hole of the hub is used to fit with the end of the tool. During the operation of the shield machine, the inner wall of the center hole should be evenly worn. Traditional billet rolling will inevitably produce frame-shaped segregation, resulting in uneven hole wall structure and anisotropy. When the tool and the hub are assembled and used, when the tool rotates, it will cause uneven wear on the hole wall of the hub, and even peeling, which will seriously reduce its service life. Therefore, it is necessary to ensure the density and uniformity of the internal structure of the steel and eliminate frame-shaped segregation.

[0007] Because shield machines work underground for a long time, once the cutter hub fails or is damaged, it is difficult to repair and replace, and there are also great safety risks. Therefore, the selection of cutter hub materials is extremely strict, and the strength, toughness, impact resistance, wear resistance, and internal quality of the steel mentioned above must be guaranteed.

[0008] At present, the steel used for shield machine hubs in China is mainly medium-carbon chromium-molybdenum steel, which is produced by continuous casting and rolling of square billets, with specifications below φ200mm. This is because C and Cr in the steel can improve the hardenability and ensure sufficient strength and surface wear resistance after heat treatment; Mo can make parts with larger cross-sections harden deeply and thoroughly, improve the tempering stability of steel, and enable parts to be tempered at higher temperatures, effectively eliminating residual stress and improving plastic toughness. However, the medium-carbon chromium-molybdenum steel grades commonly used in the market now have limited hardenability. When the round steel specifications exceed φ200mm, it will be difficult to obtain sufficient hardening depth. Rolling round steel from square billets will inevitably produce frame-shaped segregation, which greatly limits the service life of the hub.

[0009] As the specifications of shield machines are becoming larger and larger, the diameter of the steel used for the hub has long exceeded φ200mm, and the service life requirements are getting higher and higher. The traditional square billet continuous casting and rolling of medium-carbon chromium-molybdenum round steel can no longer meet the requirements for shield machine hub steel. Summary of the invention

[0011] The technical problem to be solved by the present invention is to provide a new round billet continuous casting and rolling micro-alloyed round steel for shield machine cutter hub and its manufacturing method in view of the above-mentioned prior art. By reasonably designing the chemical composition of the steel and the manufacturing process, the manufacturing diameter is φ200mm and above.

[0012] The technical solution adopted by the present invention to solve the above problems is: a round billet continuous casting and rolling micro-alloyed shield machine hub steel, the chemical composition of the steel is C: 0.25-0.45%, Si: 0.05-0.37%, Mn: 0.40-1.00%, Cr: 1.00-1.80%, Ni: 0.05-0.50%, Mo: 0.10-0.35%, B: 0.0010~0.0030%, Al: 0.010~0.050%, P≤0.015%, S: ≤0.010%, Cu≤0.15%, Ti: 0.010-0.040%, Nb≤0.030%, Ca: ≤0.00050%, As≤0.040%, Sn≤0.030%, Sb≤0.0050%, Pb≤0.0020%, the balance is Fe and unavoidable impurities. Select round billets of φ700mm or above, and use continuous casting and rolling technology to produce round steel bars with a diameter of φ200mm and above.

[0013] The chemical composition design basis of the shield machine hub steel of the present invention is as follows:

[0014] 1) Determination of C content

[0015] C is the most basic element in steel and also the most economical strengthening element. It improves the strength of steel through solid solution strengthening and precipitation strengthening of carbide-forming elements. However, if the carbon content of C is too high, the plasticity and toughness of the steel will deteriorate, and the cold brittleness and aging sensitivity of the steel will also increase. At the same time, B element is added as a strengthening element in this project. Too high carbon content will make B element lose its effect. Therefore, the range of C content in this project is determined to be 0.25~0.45%.

[0016] 2) Determination of Si content

[0017] Si can be used as a deoxidizing element, and is also a basic solid solution strengthening element that improves hardenability. However, excessive Si content increases the brittleness of ferrite, increasing the overheat sensitivity and crack sensitivity in the steel. Since multiple high-temperature forgings are required during hub processing, if the Si content is too high, it is easy to cause forging cracks, so too much Si element cannot be added. Therefore, the range of Si content in this project is determined to be 0.05~0.37%.

[0018] 3) Determination of Mn content

[0019] Mn is a solid solution strengthening element of steel, which can significantly improve the hardenability of steel. However, if the Mn content in steel is too high, it will produce obvious temper brittleness phenomenon, and Mn promotes grain growth, resulting in increased overheating sensitivity and crack tendency of steel. Since multiple high-temperature forgings are required during the processing of the hub, too much Mn element cannot be added. The Mn content range of this project is determined to be 0.40~1.00%.

[0020] 4) Determination of Cr content

[0021] Cr is a medium carbide-forming element that can improve the hardenability and wear resistance of steel. It can also prevent decarburization during the heating process of steel. Therefore, the Cr element is determined as the main alloying component of this project, and the Cr content range is determined to be 1.00-1.80%.

[0022] 5) Determination of Ni content

[0023] Ni can improve the strength of steel while maintaining good plasticity and toughness. Ni-containing steel is generally not easy to overheat, so it can prevent the growth of grains at high temperatures and still maintain fine grain structure. At the same time, the addition of Cr and Ni can improve the impact resistance of steel to a certain extent. However, Ni is a scarce resource with a high cost. Therefore, a small amount of Ni is added in this project, and the range of Ni content is determined to be 0.05-0.50%.

[0024] 6) Determination of Mo content

[0025] Mo is a carbide-forming element. When combined with Cr, it can improve hardenability, refine grains, and increase toughness. For heat-treated alloy structural steel, it can greatly reduce temper brittleness and improve the toughness of steel. Therefore, Mo is determined as the main alloying element of this project, and its content range is determined to be 0.10-0.35%.

[0026] 7) Determination of B content

[0027] Element B can greatly improve the hardenability of steel, and it is low-cost. After heat treatment, the hub can obtain a deeper hardenability layer. However, since the effect of B weakens or even disappears with the increase of C content in steel, it needs to be matched with a suitable carbon content. Considering the cost factor, element B is used as the main alloying element in this project, and the content range is determined to be 0.0010~0.0030%.

[0028] 8) Determination of S and P content

[0029] S and P are inevitable impurity elements in steel, which are easy to form defects such as segregation and inclusions. P dissolves in ferrite to make the grains twisted and coarsened, and increase cold brittleness. S makes steel hot brittle, reducing the ductility and toughness of steel. Since the hub processing is high-temperature forging, in order to avoid hot working cracking, the S content should not be too high. This project controls P≤0.015%, S ≤0.010%.

[0030] 9) Determination of Al content

[0031] Al is added as a deoxidizing element in steel. In addition to reducing the dissolved oxygen in molten steel, Al and N form dispersed and fine aluminum nitride particles to refine the grains. However, the high Al content can easily form brittle inclusions such as Al2O3 during the molten steel smelting process, reducing the purity of the molten steel. In this project, B element is also added as a strengthening element, so a certain amount of Al element needs to be added to deoxidize the molten steel. Therefore, the Al content in this project is determined to be in the range of 0.010-0.050%.

[0032] 10) Determination of Nb content

[0033] It is an element that plays a significant role in grain refinement during rolling. In the recrystallization rolling stage, Nb precipitates through strain induction to hinder the recovery and recrystallization of deformed austenite, thereby refining the grains. However, due to the limitation of C content and the influence of heating temperature, too high Nb cannot be dissolved, and it also has no effect and increases the cost. In addition, too high Nb content has an adverse effect on performance. The present invention controls its content to ≤0.03%.

[0034] 11) Determination of Cu content

[0035] Cu can improve the hardenability and atmospheric corrosion resistance of steel and reduce the hydrogen-induced crack sensitivity of steel. However, excessive Cu content is prone to copper brittleness and deteriorates the surface properties of steel. The present invention controls the Cu content to ≤0.15%.

[0036] 12) Determination of Ti content

[0037] Ti has a strong binding force with N and O, and can effectively remove N and O in steel. Because the B element is added in this project, N and O will make it lose its strengthening effect, so a certain amount of Ti needs to be added to the steel to remove N and O in the steel, so the Ti content is controlled to 0.010-0.040% in the present invention.

[0038] 13) Determination of As, Sn, Sb and Pb content

[0039] Trace elements such as As, Sn, Sb, and Pb are all low-melting-point nonferrous metals. Their presence in steel will cause soft spots on the surface of parts and uneven hardness. Therefore, they are regarded as harmful elements in steel. The content range of these elements in the present invention is determined to be As≤0.04%, Sn≤0.03%, Sb≤0.005%, and Pb≤0.002%.

[0040] 14) Determination of Ca content and Ca / S ratio

[0041] Ca is mainly used for inclusion modification, making inclusions in molten steel spheroidized and evenly distributed, and also improving the fluidity of molten steel and the problem of nozzle nodules. However, on the other hand, the addition of Ca will form hard particles in the steel, which will deteriorate the subsequent machining performance of the hub. Therefore, in the present invention, it is not advisable to add too much Ca, and the Ca content is controlled to be ≤0.00050%.

[0042] The manufacturing process of the above-mentioned micro-alloyed steel for automobile airbag tubes is: molten iron + scrap steel → BOF converter → LF refining furnace → RH vacuum degassing → CCM round billet continuous casting → warm conveying / slow cooling → hot rolling → sawing → flaw detection → finished product inspection → packaging.

[0043] The main production process features are as follows:

[0044] 1. Use high-purity molten iron for smelting. Avoid impurities brought in by scrap steel and reduce the content of harmful elements in molten steel.

[0045] 2. Boron-containing steel smelting technology. Aluminum is added to deoxidize the steel when it is tapped from the converter. Aluminum powder and aluminum wire are added in the early stage of the refining process, and the good dynamic conditions in the molten steel are used to carry out centralized early deoxidation; after deoxidation, titanium wire is fed in for denitrification, and finally B iron is fed in to ensure the effective B content.

[0046] 3. Production mode of rolling round steel from round billets. Using round billets with a specification of φ700mm or above as raw materials for steel rolling, compared with traditional square billets, the internal low-power quality of round billets is better, and the anisotropy caused by frame segregation can be effectively avoided.

[0047] Compared with the prior art, the advantages of the present invention are:

[0048] 1) Reasonable design of chemical composition improves the strength and toughness of steel, meets the processing performance of the hub, and also ensures the impact resistance and wear resistance requirements of the final use;

[0049] 2) Use round billets with a cross-sectional size of φ700mm or above to roll round steel with a specification of φ200mm or above to ensure the uniformity of the internal structure of the steel and avoid the anisotropy caused by frame segregation. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is a macro-segregation picture of hot-rolled round steel for micro-alloyed shield machine hub produced by continuous casting and rolling of round billets in an embodiment of the present invention.

[0052] Figure 2 This is a macro-segregation picture of the hot-rolled round steel for the shield machine hub produced by continuous casting and rolling of round billets in the comparative example. DETAILED DESCRIPTION

[0054] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0055] The chemical compositions (wt%) of the shield machine hub steels according to the embodiments of the present invention and (for comparison) the shield machine hub steels currently used in the market are shown in Table 1.

[0056] Table 1 (wt%)

[0057] Example C Si Mn P S Cr Mo Ni Al B 1 0.42 0.28 0.64 0.008 0.001 1.43 0.29 0.18 0.036 0.0022 2 0.42 0.29 0.64 0.008 0.001 1.46 0.28 0.18 0.038 0.0025 3 0.41 0.28 0.66 0.009 0.001 1.45 0.29 0.17 0.038 0.0023 Comparative Example 1 0.42 0.33 0.74 0.013 0.002 1.13 0.18 0.02 0.025 0.0003 Comparative Example 2 0.42 0.35 0.76 0.012 0.002 1.12 0.19 0.02 0.023 0.0002

[0058] Table 1 (continued)

[0059] Example Cu As Sn Sb Pb Ca Ti Nb 1 0.008 0.001 0.001 0.001 0.001 0.0003 0.021 0.0011 2 0.007 0.001 0.001 0.001 0.001 0.0003 0.023 0.0012 3 0.008 0.001 0.001 0.001 0.001 0.0003 0.021 0.0010 Comparative Example 1 0.01 0.003 0.002 0.003 0.003 0.001 0.007 0.0018 Comparative Example 2 0.01 0.003 0.002 0.003 0.002 0.002 0.008 0.0020

[0060] The manufacturing process of the shield machine hub steel in each embodiment of the present invention is: full molten iron → BOF converter → LF refining furnace → RH vacuum degassing → CCM round billet continuous casting → warm delivery / slow cooling → hot rolling → sawing → slow cooling → flaw detection → finished product inspection → packaging.

[0061] Further, the above steelmaking and continuous casting process is as follows: first, smelting is carried out in a BOF converter, and aluminum is added to the steel from the converter for deoxidation. Aluminum powder and aluminum wire are added in the early stage of the LF refining process, and the good dynamic conditions in the molten steel are used for centralized early deoxidation, and the white slag time is maintained for more than 15 minutes; after deoxidation, titanium wire is fed for denitrification, and finally B iron is fed to ensure the effective B content. After refining, the ladle is transferred to the RH furnace for high vacuum degassing. At a vacuum of 1.33mbar, the circulation time should be more than 20 minutes. After all the alloys are added, the ladle is softly blown with argon for more than 15 minutes to ensure that the inclusions can fully float. During continuous casting, the target superheat of the molten steel is controlled at 15-35℃, and the casting is protected throughout the process. The appropriate pulling speed and secondary cooling water volume are matched. The pulling speed of the continuous casting machine is 0.5-1.0 m / min, and the secondary cooling water volume is 2-5L / min. According to the change of pulling speed, the water volume is adjusted accordingly. At the same time, the secondary cooling zone is divided into three sections. The first section adopts the direct cooling method of spraying columnar water; the second and third sections adopt steam mist cooling, using compressed air to atomize the cooling water into fine droplets to form a gas-liquid two-phase fluid, which is sprayed onto the surface of the billet to achieve uniform and rapid cooling of the billet, and the compressed air pressure is not less than 0.5MPa. During the solidification process of the continuous casting square billet, when the billet shell thickness reaches 1 / 4 of the continuous casting billet thickness, the electromagnetic stirring technology of the secondary cooling zone is adopted. The frequency of electromagnetic stirring is controlled at 2~4Hz and the current intensity is 300~400A. Through electromagnetic stirring, the dendrite bridge in the liquid phase hole can be broken, and the formation of equiaxed crystals can be promoted, thereby improving the central equiaxed crystal area of ​​the continuous casting billet, and effectively improving and reducing the internal organization and segregation of the continuous casting billet. A circular cross-section continuous casting billet with a diameter of φ700mm or above is obtained.

[0062] The specific process of the billet heating and rolling stage is: the continuous casting billet is heated in a heating furnace with residual oxygen ≤1.5%, through the preheating section, heating section, and soaking section. The preheating section needs to slowly heat the cold billet to 700-800℃ to avoid the billet from cracking due to rapid phase change and volume change. The heating rate should not exceed 15℃ / min, and the furnace gas temperature is controlled at 350-850℃; when the continuous casting billet enters the heating section, the internal microstructure phase change has been completed, and the temperature can be quickly increased to save heating time, but it should not be too fast to cause overheating and overburning of the billet surface, so the heating rate is controlled at 20-40℃ / min, and the furnace gas temperature is controlled at 1000-1150℃; the soaking section is mainly to ensure that the temperature of each section of the continuous casting billet is uniform, so that it can be uniformly deformed during the rolling process, so the furnace gas temperature of the soaking section is controlled at 1000-1150℃, and the soaking and heat preservation time is 10-30min. The heat preservation time should not be too long, which will lead to excessive decarburization.

[0063] After leaving the furnace, it is sent to the descaling box through a wedge roller. Through multi-angle high-pressure water impact, the descaling water pressure is above 19MPa, and the furnace-born oxide scale is completely removed. The descaled round billet is aligned with the rolling groove through a special steel pushing equipment and enters the continuous rolling process. The starting rolling temperature is 950℃-1050℃, and the rough rolling is 8 frames, alternating horizontal and vertical; the first four passes adopt a large reduction process: the first two rough rolling reductions are more than 65%, and the next two reductions should be more than 40%, so that the deformation is penetrated to the core to ensure the core welding and dense organization. At the same time, controlled cooling is required after rolling, the final rolling temperature is 800℃-950℃, the cooling bed is covered with an insulation cover, the steel is passed quickly, and the material is put into the pit for slow cooling after gathering. The pit temperature is 600-800℃, and the slow cooling time is ≥48 hours. After the rolled material temperature is cooled to below 200℃, it is hoisted out of the pit. The material is subjected to Class A flaw detection of GB / T4162. Finally, round steel products with a diameter of φ220mm and above that meet the requirements are produced.

[0064] The mechanical properties of the steel materials in each embodiment are compared in Table 2.

[0065] Table 2

[0066] Example Tensile strength MPa Yield strength MPa Elongation% Sectional shrinkage% Impact energy J Rockwell hardness HRC at 15mm from the surface 1 1378 1253 14 52 68 45.3 2 1384 1254 14 51 65 45.6 3 1376 1253 13 53 64 44.9 Comparative Example 1 1172 985 11 44 53 37.6 Comparative Example 2 1165 987 11 46 54 38.2

[0067] *The mechanical property tests in Examples 1-3 and Comparative Examples 1-2 are to evaluate the strength and toughness of the materials. Samples were taken directly from the center of the raw materials of the rolled products and processed into blanks with a cross section of φ100mm in diameter and a length of ≥200mm. After heat treatment, standard tensile specimens were made according to the requirements of GB / T 228.1. The heat treatment system was: quenching temperature 850±5℃ oil cooling, tempering 500±20℃ air cooling. The mechanical properties of the steel were measured according to GB / T 228.1.

[0068] **The Charpy impact test in Examples 1-3 and Comparative Examples 1-2 is to evaluate the impact resistance of the material. Samples were taken directly from the center of the raw material of the rolled material, processed into blanks with a cross section of φ100mm, and heat-treated to make standard U-notch impact specimens of 10mm×10mm×55mm with an opening depth of 2mm. The heat treatment system was: quenching temperature 850±5℃ oil cooling, tempering 500±20℃ air cooling. The impact energy of the steel was measured according to GB / T 229.1.

[0069] **The Rockwell hardness index at 15mm from the surface in Examples 1-3 and Comparative Examples 1-2 is to assess whether the steel has the ability to obtain a hardened layer of sufficient depth to meet the wear resistance of the hub. The higher the hardness value, the more wear-resistant it is. The test is to directly sample from the center of the raw material of the rolled material, process it into a blank with a diameter of φ100mm and a length of ≥200mm, and after heat treatment, cut a hardness test piece with a thickness of 2mm at the middle length position to detect the Rockwell hardness value at 15mm from the surface.

[0070] As shown in Table 1, compared with the shield machine hub steel commonly used in the market in Comparative Example 1-2, in terms of chemical composition, the elements such as Si and Mn that increase hot brittleness are lowered in Examples 1-3 of the present invention to avoid cracking during high-temperature forging of the material; in order to make up for the insufficient hardenability and strength caused by the reduction of Si and Mn elements, the addition amount of Cr and Mo elements is increased; considering that the final round steel specification is to exceed 200mm, the strong hardenability element B is deliberately added to ensure that the material can be hardened in the subsequent heat treatment process; the addition of Al and Ti elements is to remove oxygen and nitrogen in the molten steel to ensure the effective collection of B; in order to avoid the reduction of toughness and impact resistance due to increased strength, Ni elements are added to refine the grains and improve the toughness and impact resistance of the material. In addition, Examples 1-3 of the present invention adopt all-iron smelting, which effectively avoids the introduction of impurities, and the content of other residual elements is significantly lower than that of Comparative Example 1-2.

[0071] It can be seen from Table 2 that the shield machine hub steel in the above embodiments of the present invention is superior to the existing similar steel in terms of tensile strength, yield strength, elongation, cross-sectional shrinkage, impact energy, Rockwell hardness at 15 mm from the surface and other indicators.

[0072] In addition, if Figure 1 As shown in the low-magnification photos of the embodiments of the present invention, the present invention uses an ultra-large cross-section φ700mm round billet to roll into round steel, and there is no frame-shaped segregation at low magnification. Figure 2 The frame segregation is obvious in the low-magnification photo of the comparative example. The frame segregation of the embodiment of the present invention is obviously superior to the existing similar steel materials, and can effectively eliminate the anisotropy of the center hole wall of the hub, and can greatly increase the service life of the center hole wall of the hub.

[0073] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A round billet continuous casting and rolling micro-alloyed shield machine hub steel, characterized by: The chemical composition of the steel is, by mass percentage, C: 0.25-0.45%, Si: 0.05-0.37%, Mn: 0.40-1.00%, Cr: 1.00-1.80%, Ni: 0.05-0.50%, Mo: 0.10-0.35%, B: 0.0010-0.0030%, Al: 0.010-0.050%, P≤0.015%, S: ≤0.010%, Cu≤0.15%, Ti: 0.010-0.040%, Nb≤0.030%, Ca: ≤0.00050%, As≤0.040%, Sn≤0.030%, Sb≤0.0050%, Pb≤0.0020%, and the balance is Fe and unavoidable impurities.

2. The round billet continuous casting and rolling micro-alloyed shield machine hub steel according to claim 1, characterized in that: The diameter of the steel is 200 mm or above; the internal quality meets the A-level flaw detection requirements of GB / T4162; there is no frame-shaped segregation; the mechanical properties after heat treatment meet the following requirements: tensile strength ≥1350MPa, yield strength ≥1200MPa, elongation ≥10%, cross-sectional shrinkage ≥40%, impact energy ≥58J, and Rockwell hardness ≥42HRC at 15mm from the surface.

3. A method for manufacturing a microalloyed shield machine hub steel by continuous casting and rolling of round billets as claimed in claim 1, characterized in that: The method comprises the following steps: 1) First, the steel is smelted in BOF converter, aluminum is added to the steel out of the converter for deoxidation, and LF is refined. After the refining is completed, the ladle is transferred to the RH furnace for high vacuum degassing treatment; 2) During continuous casting, the target superheat of molten steel is controlled at 15-35℃, the casting is protected throughout the whole process, the appropriate casting speed and secondary cooling water volume are matched, and electromagnetic stirring technology is used to produce circular cross-section continuous casting billets with a diameter of φ700mm or above; 3) The continuous casting billet is heated in a heating furnace with residual oxygen ≤1.5% in three stages, with the temperature increasing step by step, and heated to 1100℃-1200℃ for 2 hours, with a total furnace time of 5-5.5 hours; 4) After coming out of the furnace, it is sent to the descaling box through the wedge roller. After multi-angle high-pressure water impact, the descaled round billet enters the continuous rolling process; 5) The starting rolling temperature is 950℃-1050℃, the rough rolling is carried out in 8 stands, alternating between horizontal and vertical rolling, the first four passes adopt a large reduction process, the final rolling temperature is 800℃-950℃, the cooling bed is covered with an insulation cover, the steel is passed quickly, and the material is put into the pit for slow cooling after aggregation. The temperature entering the pit is 600-800℃, and the slow cooling time is ≥48 hours. After the rolled material temperature is cooled to below 200℃, it is hoisted out of the pit and subjected to Class A flaw detection of GB / T4162, finally the round steel products with a diameter of φ200mm and above that meet the requirements are obtained.

4. The method for manufacturing a round billet continuous casting and rolling microalloyed shield machine hub steel according to claim 3, characterized in that: In step (1), aluminum powder and aluminum wire are added at the early stage of the LF refining process, and the favorable kinetic conditions in the molten steel are utilized to carry out centralized early deoxidation, and the white slag time is maintained for more than 15 minutes. After the deoxidation is completed, titanium wire is fed to carry out denitrification, and finally B iron is fed to ensure the effective B content.

5. The method for manufacturing a round billet continuous casting and rolling microalloyed shield machine hub steel according to claim 3, characterized in that: In step (1), the RH furnace is subjected to high vacuum degassing treatment at a vacuum degree of 1.33 mbar, and the circulation time is more than 20 minutes. After all the alloys are added, the ladle is softly blown with argon for more than 15 minutes.

6. The method for manufacturing a round billet continuous casting and rolling microalloyed shield machine hub steel according to claim 3, characterized in that: In step (2), the casting speed of the continuous casting machine is 0.5-1.0 m / min, and the secondary cooling water volume is 2-5 L / min. The water volume is adjusted accordingly according to the change of the casting speed. At the same time, the secondary cooling area is divided into three sections for management. The first section adopts a direct cooling method of spraying columnar water; the second and third sections adopt steam mist cooling, and compressed air is used to atomize the cooling water into fine droplets, wherein the compressed air pressure is not less than 0.5 MPa.

7. The method for manufacturing a round billet continuous casting and rolling microalloyed shield machine hub steel according to claim 3, characterized in that: In step (2), during the solidification process of the continuous casting square billet, when the billet shell thickness reaches 1 / 4 of the continuous casting billet thickness, the secondary cooling zone electromagnetic stirring technology is adopted, and the frequency of the electromagnetic stirring is controlled at 2-4 Hz and the current intensity is 300-400A.

8. The method for manufacturing a round billet continuous casting and rolling microalloyed shield machine hub steel according to claim 3, characterized in that: In step (3), the preheating section needs to slowly heat the cold billet to 700-800°C, the heating rate should not exceed 15°C / min, and the furnace gas temperature is controlled at 350-850°C; when the continuous casting billet enters the heating section, the heating rate is controlled at 20-40°C / min, and the furnace gas temperature is controlled at 1000-1150°C; the furnace gas temperature in the soaking section is controlled at 1000-1150°C, and the soaking and heat preservation time is 10-30min.

9. The method for manufacturing a round billet continuous casting and rolling microalloyed shield machine hub steel according to claim 3, characterized in that: In step (5), the first two rough rolling passes have a reduction of more than 65%, and the next two rough rolling passes have a reduction of more than 40%.

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