Method for controlling structure uniformity of large-size ultralow-expansion alloy bar

Abstract

A method for controlling the structure uniformity of a large-size ultralow-expansion alloy bar comprises the following steps that (1) an ultralow-expansion alloy electroslag ingot with the diameter larger than that of the bar is heated to 1100-1130 DEG C, and heat preservation is conducted for 3-4 hours; (2) the heated electroslag ingot is subjected to multi-pass drawing-out and anvil forging rounding, and a cogging bar with the diameter being phi 330-phi 370 mm is obtained; (3) the surface of the cogging rod is subjected to sanding treatment, and then the cogging rod is fully coated with heat preservation paint; (4) the treated cogging rod is heated to 1050-1100 DEG C, and heat preservation is conducted for 2-3 hours; the upsetting ratio and the drawing-out ratio of the first time are controlled to be 1.3-1.6, the upsetting ratio of the second time is controlled to be 1.3-1.6, the drawing-out ratio is larger than 2.4, then rounding is conducted, a finished bar is obtained through water cooling, and (6) the radial thickness of the finished bar is removed by 10-20 mm through turning, and the finished large-specification ultralow-expansion alloy bar with the diameter being 230-250 mm is obtained.

Description

Technical Field

[0001] This invention relates to the field of ultra-low expansion alloy processing technology, specifically to a method for controlling the microstructure uniformity of large-size ultra-low expansion alloy bars. Background Technology

[0002] Ultra-low expansion alloys, with Fe, Ni, and Co as the main components, have become core materials for manufacturing key structural components in high-end fields such as aerospace and precision instruments due to their excellent dimensional stability. Their microstructure uniformity directly affects the local dimensional accuracy and long-term reliability of the components, and has a significant impact on the imaging or detection accuracy in actual use conditions. Therefore, related industries have put forward high requirements for the microstructure quality of this type of alloy bar.

[0003] For small and medium-sized ultra-low expansion alloy bars with a diameter ≤200mm, a larger forging ratio can be achieved through multiple deformation cycles, resulting in uniform and refined microstructure. However, when the bar diameter exceeds 200mm, due to the difference in deformation resistance between the core and the edge, coarse grains (i.e., mixed grain phenomenon) are prone to appear at 1 / 2 radius and the edge, and after corrosion, their low-magnification microstructure will show obvious contrast differences, which seriously affects the consistency of material properties.

[0004] Regarding the optimization of the uniformity of the microstructure of alloy bars, those skilled in the art have some solutions for other materials. For example, the invention patent with publication number CN119411047A, "A process for optimizing the grain size difference of large-size nickel-based high-temperature alloy bars", uses nickel-based high-temperature alloys to reduce the grain size difference through four-stage diffusion annealing, multiple heat treatments, and ambient temperature control. However, this process relies on the pinning effect of the second phase in the alloy and requires long-term heat holding (110 to 115 hours in the high-temperature stage alone), resulting in high energy consumption and unsuitability for ultra-low expansion alloys without a second phase. The invention patent with publication number CN114769478A, "A forging method to improve the axial microstructure uniformity of large-size titanium alloy bars", uses titanium alloys to improve uniformity by adjusting the height-to-diameter ratio through multiple heating processes in single-phase / two-phase regions. However, relying on phase transformation characteristics, multiple heating processes can actually exacerbate grain coarsening in ultra-low expansion alloys.

[0005] Currently, large-size ultra-low expansion alloy bars suffer from poor microstructure uniformity and a large number of coarse grains, making it difficult to meet the requirement of no more than 50% bottom loss in Class A ultrasonic testing according to GJB1580A. Existing technologies have not yet formed an effective solution for the material's lack of a second phase and its tendency to coarsen. However, with increasingly stringent material performance requirements in high-end fields such as aerospace and precision instruments, and the manufacturing industry's pursuit of efficient and low-consumption production, how to effectively control the microstructure uniformity of large-size ultra-low expansion alloy bars to reduce coarse grains and meet stringent testing requirements while simplifying processes to improve production efficiency has become a key problem urgently needing to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for controlling the microstructure uniformity of large-size ultra-low expansion alloy bars. This method solves the problems of uneven microstructure and severe coarse grain formation in large-size ultra-low expansion alloy bars (φ230~φ250mm), effectively improving the uniform forging capability of the alloy bars, producing large-size ultra-low expansion alloy bars that meet flaw detection quality requirements, and offering advantages such as high microstructure uniformity and strong process operability.

[0007] The objective of this invention is achieved through the following approach: A method for controlling the microstructure uniformity of large-size ultra-low expansion alloy bars includes the following steps: 1) Electroslag ingot heating Heat ultra-low expansion alloy electroslag ingots larger than the diameter of the bar to 1100~1130℃ and hold for 3~4 hours; 2) Forging billet The heated electroslag ingot is drawn in multiple passes in an octagonal shape. The first pass reduces the ingot by 10-20 mm per anvil, the middle pass reduces it by 20-30 mm per anvil, and the last pass reduces it by 10-20 mm per anvil. Then, the ingot is rounded by anvil forging to obtain a billet with a billet size of φ330-φ370 mm. 3) Surface treatment After surface sanding to remove surface defects, the billet is then coated with thermal insulation coating. 4) Heating of the billet bar Heat the processed billet to 1050~1100℃ and hold for 2~3 hours; 5) Two upsettings and two pullings The heated billet is forged in one pass using two upsetting and two drawing processes. The upsetting ratio (cross-sectional area after upsetting / initial cross-sectional area) is controlled at 1.3~1.6, and the drawing ratio (cross-sectional area before drawing / cross-sectional area after drawing) is controlled at 1.3~1.6. The upsetting ratio is controlled at 1.3~1.6, and the drawing ratio is >2.4. The billet is then rounded and water-cooled for more than 10 minutes to obtain the finished billet. 6) Remove 10~20mm of radial thickness from the finished bar by turning to obtain a large-size ultra-low expansion alloy bar with diameters of φ230~φ250mm.

[0008] Preferably, in step 1), the heating rate of the electroslag ingot is ≤120℃ / h, and after heating to 600~700℃, it is held for 1 hour and then heated to 1100~1130℃.

[0009] Preferably, in step 2), the anvil forging and rounding process is performed by using an arc-shaped anvil for rounding.

[0010] Preferably, in step 3), the surface sanding treatment is performed by grinding with a grinding wheel.

[0011] Preferably, in step 3), the thermal insulation coating has the following weight percentages: water glass 30wt%~40wt%, kaolin 20wt%~30wt%, expanded perlite 15wt%~25wt%, graphite 5wt%~10wt%, and borax 2wt%~5wt%.

[0012] Preferably, in step 4), the heating rate of the billet rod is ≤150℃ / h.

[0013] Preferably, in step 5), the rounding is performed by clamping both ends of the forging bar with the jaws of the high-speed forging machine and rotating the entire bar 360° to round it.

[0014] Preferably, in step 5), the water cooling is performed within 10 seconds after forging, using circulating water cooling.

[0015] The beneficial effects of this invention include the following: The present invention uses the above method to precisely control the heating temperature of the electroslag ingot at 1100~1130℃ and the holding time at 3~4 hours, so that the steel ingot is easier to plastically deform during billet making, but avoids overheating due to thermal deformation, and promotes element homogenization. A multi-pass, octagonal elongation forging method is employed, controlling the reduction per anvil in the first pass to 10-20 mm, the reduction per anvil in the middle passes to 20-30 mm, and the reduction per anvil in the final pass to 10-20 mm. This specific reduction setting for each pass ensures the smooth progress of the entire forging process, effectively breaking down the as-cast structure and promoting grain refinement and uniform distribution. The billet is then rounded through anvil forging to obtain a billet with a size of φ330-φ370 mm. Surface treatment removes defects and an insulating coating is applied, resulting in a good surface quality for subsequent finished forging, significantly reducing oxide scale formation to achieve near-net-shape forming, and slowing down the temperature drop during forging. Simultaneously, heating the billet to 1050-1100℃ and holding it for 2-3 hours shortens the holding time, improves production efficiency, and inhibits… The microstructure is coarsened; then, a two-upsetting and two-drawing process is adopted in a single heat treatment. The upsetting ratio of both upsetting processes is controlled at 1.3~1.6, the drawing ratio of the first upsetting process is controlled at 1.3~1.6, and the drawing ratio of the second drawing process is greater than 2.4. Through multiple upsetting and drawing stages of deformation, the strain path is optimized, which makes it easier to uniformly transfer strain energy than a single large deformation, and maximizes the uniform deformation effect of the microstructure. Combined with water cooling treatment of more than 10 minutes, the high-temperature austenitic microstructure can be fixed to avoid the precipitation of harmful phases, while inhibiting the continued growth of static recrystallization of grains. Finally, 10~20mm of radial thickness is removed by turning to remove the surface fine grain area and microcracks. The resulting φ230~φ250mm large-size ultra-low expansion alloy bars have excellent microstructure uniformity, effectively ensuring the dimensional accuracy and performance stability of the material, and meeting the high performance requirements of large-size products.

[0016] In other words, through the synergistic effect of each step, the present invention not only significantly improves the uniformity of the microstructure of the large-size ultra-low expansion alloy bar, but also stably maintains the ultra-low expansion performance, effectively solving the problems of uneven microstructure and performance fluctuation that are prone to occur in large-size bars, and improving the dimensional accuracy and reliability of the product.

[0017] Preferably, in step 1), the heating rate of the electroslag ingot is ≤120℃ / h, and after heating to 600~700℃, it is held for 1 hour and then heated to 1100~1130℃.

[0018] In step 1) of this invention, by limiting the heating rate of the electroslag ingot to ≤120℃ / h, and holding it at 600~700℃ for 1 hour before continuing heating to the target temperature, thermal stress can be reduced, the temperature of the core and surface of the ingot can be uniformized, and the influence of magnetic transformation can be eliminated. Specifically, ultra-low expansion alloys have poor thermal conductivity, with a thermal conductivity of only 13W / m·K. Under the large cross-section of a φ500mm ingot, the temperature difference between the inside and outside of the ingot is prone to exceed the limit. Slow heating can avoid thermal stress cracking. Holding it at the critical temperature range of 600~700℃ for 1 hour can compensate for the temperature lag caused by the difference in heating rates between the inside and outside, relax the generated thermal stress, stabilize the magnetic domain structure, and avoid dimensional fluctuations caused by magnetostriction during subsequent heating.

[0019] In other words, by strictly controlling the heating rate of the electroslag ingot to ≤120℃ / h and holding it at 600~700℃ for 1 hour, the present invention can effectively avoid the problems of high thermal stress and structural instability caused by the inherent characteristics of the alloy and large-size interfaces during the heating process, thus achieving safe and controllable heating.

[0020] Preferably, in step 2), the anvil forging and rounding process is performed by using an arc-shaped anvil for rounding.

[0021] In step 2) of this invention, the preferred method of using an arc-shaped anvil for rounding as the anvil forging rounding method can significantly improve the size and surface quality of the steel bar, reduce the risk of surface cracking, increase rounding efficiency, and save energy. Specifically, the curved surface design of the arc-shaped anvil is more compatible with the arc shape of the bar, allowing for more uniform stress distribution on all parts of the bar surface during the rounding process. This avoids the localized stress concentration that may occur with methods such as using a flat anvil, reducing structural deformation or defects and ineffective deformation caused by uneven stress, and achieving rapid rounding. At the same time, uniform stress ensures smoother metal flow during the rounding process, promotes more uniform internal grain distribution, and provides good microstructure conditions for subsequent finished product forging processes, avoiding the inheritance of coarse grain structure.

[0022] Preferably, in step 3), the surface sanding treatment is performed by grinding with a grinding wheel.

[0023] In step 3) of this invention, the preferred method of surface grinding using a grinding wheel can efficiently and thoroughly remove surface defects from the billet, reducing the risk of surface cracking during subsequent finished product forging. Specifically, grinding wheel grinding has high grinding efficiency and strong grinding capability, which can quickly remove defects such as cracks, oxide scale, and folds generated on the surface of the billet due to forging and other processes. Furthermore, the grinding wheel speed and grinding force can be adjusted during the grinding process to ensure more precise and thorough removal of surface defects, preventing residual defects from becoming sources of crack propagation during subsequent finished product forging.

[0024] Preferably, in step 3), the thermal insulation coating has the following weight percentages: water glass 30wt%~40wt%, kaolin 20wt%~30wt%, expanded perlite 15wt%~25wt%, graphite 5wt%~10wt%, and borax 2wt%~5wt%.

[0025] In step 3) of this invention, by limiting the weight percentage of each component of the thermal insulation coating, namely water glass 30wt%~40wt%, kaolin 20wt%~30wt%, expanded perlite 15wt%~25wt%, graphite 5wt%~10wt%, and borax 2wt%~5wt%, this technical feature can fully leverage the synergistic effect of each component, significantly improving thermal insulation performance and near-net-shape forming effect. Specifically, water glass, as a binder, can ensure the integrity and adhesion of the coating, preventing the coating from peeling off during heating; kaolin can enhance the density of the coating and reduce heat loss; expanded perlite has excellent thermal insulation performance and can effectively maintain the uniformity of the internal temperature of the billet; graphite can improve the high-temperature resistance of the coating and avoid coating failure at high temperatures; borax can lower the sintering temperature of the coating and promote a tighter bond between the components. This specially formulated thermal insulation coating can slow down the heat radiation and convection heat loss on the surface of the steel bar during the forging process, keep the overall temperature of the billet uniform, make the metal's plastic deformation capacity more consistent, avoid sudden changes in deformation resistance caused by local temperature changes, and at the same time reduce the formation of surface oxide scale and improve surface quality.

[0026] Preferably, in step 4), the heating rate of the billet rod is ≤150℃ / h.

[0027] In step 4) of this invention, by limiting the heating rate of the billet to ≤150℃ / h, this technical feature shortens the holding time, improves production efficiency, and simultaneously suppresses microstructure coarsening. Specifically, after billet preparation, the surface area / volume ratio of the billet increases, improving heat transfer efficiency and allowing for rapid heating; at the same time, the reduced cross-section (improved heat penetration) reduces the risk of thermal stress, thus balancing efficiency and plasticity; furthermore, billet preparation breaks up the original cast dendrites, forming a refined dynamic recrystallized structure, and rapid heating can suppress excessive grain growth.

[0028] Preferably, in step 5), the rounding is performed by clamping both ends of the forging bar with the jaws of the high-speed forging machine and rotating the entire bar 360° to round it.

[0029] In step 5) of this invention, the forging bar is rotated 360° to achieve roundness by clamping both ends with the jaws of a high-speed forging machine. This is an efficient rounding method when a curved anvil cannot be used, which can significantly improve the shape accuracy and near-surface microstructure uniformity of the forging bar. Specifically, the rigid clamping of the high-speed forging machine jaws ensures stable force on the forging bar during rotation, avoiding local deformation deviations caused by unstable clamping. Meanwhile, the 360° rotation of the entire forging bar ensures uniform stress distribution along the axial, radial, and circumferential directions, resulting in more consistent metal flow and avoiding excessive local deformation such as ellipses and sharp edges caused by traditional unidirectional hammering. Dynamic adjustment of the stress area during rotation reduces the risk of tensile stress in the core and prevents core loosening or cracking caused by repeated pressure on a fixed area. The rotation and rounding process ensures more continuous stress distribution on the billet surface, avoiding the uneven "step-like" transitions seen in flat anvil forging, reducing surface defects such as folds and indentations, and achieving high-precision forging and rapid rounding. It also reduces uneven machining allowance distribution caused by irregular shapes in subsequent turning processes, significantly improving the yield rate of machining operations, achieving efficient utilization of alloy materials, and ultimately reducing costs and increasing efficiency.

[0030] Preferably, in step 5), the water cooling is performed within 10 seconds after forging, using circulating water cooling.

[0031] In step 5) of this invention, by limiting water cooling to within 10 seconds after forging and using circulating water cooling, the grains can be refined, the high-temperature austenite structure can be fixed, the precipitation of coarse second-phase carbides can be suppressed, and the uniformity of the forged bar structure can be improved. Furthermore, production efficiency can be increased. Specifically, timely water cooling within 10 seconds after forging can quickly reduce the temperature of the forged bar to below the critical temperature for grain growth, avoiding coarse or uneven microstructure and coarsening of grain boundary carbides caused by continuous grain growth at high temperatures, and firmly locking in the fine and uniform microstructure formed by the two upsetting, two drawing, and rounding processes. Furthermore, circulating water cooling ensures the continuity and uniformity of the cooling process, allowing all parts of the forged bar to obtain similar cooling rates within the same time, avoiding stress concentration caused by uneven cooling and reducing the risk of cracking. Compared to air cooling or furnace cooling, water cooling can significantly shorten the cooling time, enabling continuous processing production.

[0032] In other words, the cooling method of post-forging circulating water cooling can be well coordinated with the previous forging and rounding processes, which can further consolidate the uniform microstructure of the forged bar and maximize the control over the microstructure uniformity of large-size ultra-low expansion alloy bars.

[0033] In summary, the advantages of this invention are as follows: ① This invention precisely focuses on the characteristics of large-size ultra-low expansion alloy rods, which have no second phase and are easy to coarsen. It breaks through the dependence of existing technologies on alloys containing second phases or materials with phase transformation characteristics to achieve microstructure refinement. It specifically solves the core problems of φ230~φ250mm large-size ultra-low expansion alloy rods, such as 1 / 2 radius and edge coarse grains and mixed crystal phenomena caused by size factors. The prepared rods can meet the stringent requirement of Class A bottom loss not exceeding 50% when ultrasonically tested according to GJB 1580A.

[0034] ② This invention constructs a precise control process chain with full-process collaboration: First, the electroslag ingot heating adopts a slow heating rate of ≤120℃ / h and a holding time of 600~700℃ for 1 hour. The slow heating and holding time within the key temperature range eliminate the influence of internal and external temperature differences and magnetic transformation. Combined with gradient pressing and octagonal elongation + rounding processes, the as-cast structure is broken up, the recrystallization process is optimized, and uniform grain distribution is promoted, avoiding the inheritance of coarse-grained structures from the source. Second, sand grinding can specifically remove surface defects such as microcracks, eliminating forging risks. Then, By holding the material at 1050~1100℃ for 2~3 hours and using a suitable aspect ratio, a progressive, staged deformation process involving two upsetting and two drawing operations is completed in one forging cycle. This avoids the impact of tempering on the uniformity of the microstructure, effectively optimizes the strain path, and maximizes the uniform deformation effect. Finally, rapid water cooling for more than 10 minutes within 10 seconds after forging effectively fixes and refines the microstructure, inhibits grain growth, and results in large-size ultra-low expansion alloy bars with good microstructure uniformity, with a grain size deviation of ≤1 grade from the center to the edge of the bar.

[0035] ③ This invention significantly improves the microstructure uniformity of large-size ultra-low expansion alloy bars through the synergistic effect of multi-directional forging deformation and precise temperature control, avoids the lag in expansion curves caused by local microstructure coarsening and deviation of expansion coefficient under long-term service conditions, significantly improves the stability of expansion coefficient, solves the problem of performance fluctuation of large-size bars, and the prepared large-size ultra-low expansion alloy bars can meet the performance stability requirements of large structural components in industries such as aerospace and heavy machinery.

[0036] ④ Compared with the existing technology, which often involves heat preservation treatment or multiple heating processes, this invention significantly shortens the production cycle and reduces energy consumption and oxidation decarburization risk through single-fire forging, optimized heating rate and heat preservation time, and post-forging water cooling. It balances high-efficiency production with low consumption and cost, which is in line with the development trend of high efficiency and low consumption in the manufacturing industry. Attached Figure Description

[0037] Figure 1 This is a process flow diagram of the present invention; Figure 2 A low-magnification photograph of the φ230mm ultra-low expansion alloy of Example 1, showing a uniform microstructure; Figure 3 for Figure 2 High-magnification photographs showing uniform tissue; Figure 4 A low-magnification photograph of the φ230mm ultra-low expansion alloy of Example 2, showing a uniform microstructure; Figure 5 for Figure 4 High-magnification photographs showing uniform tissue; Figure 6 A low-magnification photograph of a φ230mm ultra-low expansion alloy with a mixed-grain structure, as described in the prior art. Figure 7 Figure 6 shows a high-magnification photograph of the mixed crystal structure. Detailed Implementation

[0038] like Figure 1 As shown, a method for controlling the microstructure uniformity of large-size ultra-low expansion alloy bars includes the following steps: 1) Electroslag ingot heating The mass percentage composition of the ultra-low expansion alloy electroslag ingot is C: ≤0.05wt%, Si: ≤0.20wt%, Mn: 0.20~0.60 wt%, Cu: 0.40~0.80 wt%, Ni: 31.50~33.0 wt%, Co: 3.20~4.20 wt%, Fe: balance. The ultra-low expansion alloy electroslag ingot, which is larger than the diameter of the bar, is heated to 1100~1130℃ and held for 3~4 hours. Setting the heating temperature of the ultra-low expansion alloy electroslag ingot to 1100~1130℃ can effectively improve the plasticity of the metal and reduce its deformation resistance, providing suitable plasticity conditions for the subsequent multi-pass forging of the electroslag ingot, and ensuring that the metal can achieve uniform and sufficient deformation during the forging process.

[0039] 2) Forging billet The heated electroslag ingot is elongated using a multi-pass octagonal drawing process. The first pass reduces the ingot by 10-20 mm per anvil, the intermediate passes by 20-30 mm per anvil, and the final pass by 10-20 mm per anvil. The smaller reduction in the first pass compacts the surface structure, preventing surface roughness and crack initiation during subsequent large reductions. The larger reduction in the intermediate passes effectively breaks up the as-cast structure, promoting dynamic recrystallization, refining grains, and increasing material density. The smaller reduction in the final pass prevents excessive deformation and cracking due to decreased plasticity caused by temperature reduction. It also refines the surface shape and dimensions to ensure the finished product meets specifications. Then, by anvil forging and rounding, a billet bar with a blank size of φ330~φ370mm is obtained. Using anvil forging and rounding can give the billet bar a regular shape that meets the requirements of the subsequent upsetting and drawing process, thereby ensuring uniform stress and consistent deformation of the metal during the upsetting process, effectively reducing local stress concentration caused by irregular shape, and thus reducing the risk of cracking.

[0040] 3) Surface treatment The billet is sand-ground to remove surface defects, and then coated with a thermal insulation coating. The thermal insulation coating comprises the following weight percentages: water glass 30wt%~40wt%, kaolin 20wt%~30wt%, expanded perlite 15wt%~25wt%, graphite 5wt%~10wt%, and borax 2wt%~5wt%. Due to its porous structure and low thermal conductivity, this coating significantly reduces the heat dissipation rate of the billet surface, thereby slowing down the surface temperature drop during forging. This allows the billet to maintain a uniform and suitable temperature field during subsequent heating and forging, providing favorable thermal deformation conditions for the full fragmentation of the coarse as-cast structure, and thus promoting the refinement and homogenization of the microstructure.

[0041] 4) Heating of the billet bar The treated billet is heated to 1050~1100℃ and held for 2~3 hours. Heating the treated billet to the suitable hot working temperature of the alloy, 1050~1100℃, can ensure the plasticity of the metal and avoid abnormal grain growth, providing good material properties for subsequent forging. Then, holding it for 2~3 hours can make the internal temperature of the billet uniform, the softening degree and deformation capacity of each part of the structure are synchronized, and it can also eliminate the internal stress of the early forging, lay the foundation for two upsetting and two drawing, ensure uniform metal flow and deformation, reduce the risk of structural defects and cracking, and improve the uniformity of the billet structure.

[0042] 5) Two upsettings and two pullings The heated billet is forged in one heat with two upsetting and two drawing processes. The upsetting ratio (cross-sectional area after upsetting / initial cross-sectional area) is controlled at 1.3~1.6, and the drawing ratio (cross-sectional area before drawing / cross-sectional area after drawing) is controlled at 1.3~1.6. The upsetting ratio is controlled at 1.3~1.6, and the drawing ratio is >2.4. Then, the billet is rounded and water-cooled for more than 10 minutes to obtain the finished billet. The heated billet is forged in a single pass at a relatively low forging temperature of 1050~1100℃. This process ensures sufficient recrystallization while preventing grain growth. The first upsetting and drawing process has an upsetting ratio and a drawing length ratio of 1.3~1.6, which initially breaks up large grains and promotes preliminary microstructure reconstruction and homogenization, laying the foundation for subsequent deformation. The second upsetting process, with an upsetting ratio of 1.3~1.6, repairs internal defects, while the large deformation with a drawing length ratio >2.4 further refines the grains and eliminates segregation and porosity. The subsequent rounding process ensures a regular shape of the billet, and the water cooling treatment of >10 minutes fixes the uniform microstructure, ultimately obtaining a finished billet with a uniform microstructure and stable performance, providing a core guarantee for subsequent turning processes.

[0043] 6) The finished bar is machined to remove 10-20mm of radial surface thickness, resulting in a large-size ultra-low expansion alloy bar with a diameter of φ230-φ250mm. Removing 10-20mm of radial thickness precisely removes any oxide or decarburized layers that may be present on the surface of the finished bar, as well as surface defects such as microcracks and uneven microstructure caused by forging and cooling processes, ensuring the final product meets surface quality standards. Simultaneously, removing a certain radial thickness allows the bar to achieve a precise target size of φ230-φ250mm, meeting the dimensional accuracy requirements of large-size products, while preserving the core area with uniform internal structure and stable performance. This further guarantees the mechanical properties and ultra-low expansion characteristics of the finished bar, enabling it to meet the stringent requirements of practical applications.

[0044] The following is an example of implementing the method described above: Example 1

[0045] A φ500mm electroslag ingot weighing 2570kg was placed in the center of the furnace of a natural gas trolley heater. The heating temperature was set to 1100℃ and the heating rate was 100℃ / h. When the temperature reached 600℃, it was held for 1 hour, and then the temperature was raised to 1100℃ and held for 4 hours.

[0046] A high-speed forging mill with a nominal pressure of 2500 tons is used to forge the electroslag ingot after heating and holding. The first pass reduction is 15 mm per anvil, the intermediate passes are 25 mm per anvil, and the final pass is 15 mm per anvil. When the surface temperature is below 900℃, it needs to be tempered immediately to 1100℃, and the tempering holding time is 1 hour. The entire forging and billet opening process is completed in one heating cycle (i.e., within the same heating cycle), using a multi-pass drawing process. In each pass, the cross-section of the billet is gradually deformed into a regular octagon through an octagonal drawing method. When the billet is drawn to a regular octagonal cross-section and the distance between opposite sides reaches 350 mm, the process is switched to an arc-shaped anvil for rounding, so that the cross-section of the billet gradually transitions from octagonal to circular, finally forming a billet with a circular cross-section of φ350 mm.

[0047] Subsequently, the billets were sawn and cut into blanks, with each blank having a blank length of 620mm (denoted as L=620mm). After blanking, the surface of the billets was sanded using a grinding wheel to remove macroscopic defects such as micro-cracks, flash, and folds. After sanding, a thermal insulation coating was uniformly applied to the surface of the billets (the weight percentage of the thermal insulation coating used was: water glass 38wt%, kaolin 27wt%, expanded perlite 23wt%, graphite 8wt%, and borax 4wt%). Then, the treated billets were placed in the center of the furnace of a natural gas trolley heater, and the heating process parameters were set as follows: heating temperature 1070℃, heating rate 120℃ / h, and holding at 1070℃ for 2 hours.

[0048] Next, the billet is forged into a finished product through two upsetting and two drawing processes: In the first upsetting and drawing process, the billet is first upset from φ350mm×620mm to φ420mm×405mm, and then drawn to φ340mm×620mm. The upsetting and drawing ratios for both deformations are 1.52. In the second upsetting and drawing process, the billet is first upset from φ340mm×620mm to φ410mm×425mm, and then drawn to φ242mm×1220mm. The corresponding upsetting and drawing ratios are 1.42 and 2.87, respectively. After forging, the two ends of the forging bar are held by the jaws of a high-speed forging machine and the entire bar is rotated 360° to achieve rounding.

[0049] The forged steel bar is immediately placed in circulating water for cooling within 6 seconds, and the cooling time is 12 minutes.

[0050] Finally, the forged bar stock is machined to a diameter of φ230. +2 mm finished bar specifications.

[0051] From the middle section of the finished bar, cut a circular slice with a length of L=20mm for low-magnification microstructure observation (e.g. Figure 2As shown), then take five φ10mm samples at equal intervals from the center to the edge (center, 1 / 4R, 1 / 2R, 3 / 4R, R) for grain size testing (as shown). Figure 3 As shown in Table 1, the grain size rating results are as follows.

[0052] Table 1. Grain size rating results of finished bars at different radial positions obtained in Example 1

[0053] The large-size ultra-low expansion alloy rods prepared by the above control methods have uniform microstructure at low magnification and no contrast differences. The grain size from the center to the edge is stable at grade 5.5~6.5, and the grain size deviation is ≤1 grade. Ultrasonic testing according to GJB1580A meets the quality grade A, and the bottom loss does not exceed 50%.

[0054] Example 2 A φ500mm electroslag ingot weighing 2570kg was placed in the center of the furnace of a natural gas trolley heater. The heating temperature was set to 1110℃ and the heating rate was 100℃ / h. When the temperature reached 600℃, it was held for 1 hour, and then the temperature was raised to 1110℃ and held for 4 hours. A high-speed forging mill with a nominal pressure of 2500 tons is used to forge the electroslag ingot after heating and holding. The first pass reduction is 15 mm per anvil, the intermediate passes are 25 mm per anvil, and the final pass is 15 mm per anvil. When the surface temperature is below 900℃, it needs to be tempered immediately to 1110℃, and the tempering holding time is 1 hour. The entire forging and billet opening process is completed in one heat (i.e., within the same heating cycle), using a multi-pass drawing process. In each pass, the cross-section of the billet is gradually deformed into a regular octagon through an octagonal drawing method. When the billet is drawn to a regular octagonal cross-section and the distance between opposite sides reaches 350 mm, the process is switched to an arc-shaped anvil for rounding, so that the cross-section of the billet gradually transitions from octagonal to circular, finally forming a billet with a circular cross-section of φ350 mm.

[0055] Subsequently, the billets were sawn and cut into blanks, with each blank having a blank length of 650mm (denoted as L=650mm). After blanking, the surface of the billets was sanded using a grinding wheel to remove macroscopic defects such as micro-cracks, flash, and folds. After sanding, a thermal insulation coating was uniformly applied to the surface of the billets (the weight percentage of the thermal insulation coating used was: water glass 38wt%, kaolin 27wt%, expanded perlite 24wt%, graphite 7wt%, and borax 4wt%). Then, the treated billets were placed in the center of the furnace of a natural gas trolley heater, and the heating process parameters were set as follows: heating temperature 1070℃, heating rate 120℃ / h, and holding at 1070℃ for 2 hours.

[0056] Next, the billet is forged into a finished product through two upsetting and two drawing processes: In the first upsetting and drawing process, the billet is first upset from φ350mm×650mm to φ420mm×450mm, and then drawn to φ350mm×650mm. The upsetting and drawing ratios for both deformations are 1.44. In the second upsetting and drawing process, the billet is first upset from φ350mm×650mm to φ420mm×450mm, and then drawn to φ242mm×1030mm. The corresponding upsetting and drawing ratios are 1.44 and 3.0, respectively. After forging, the two ends of the forging bar are held by the jaws of a high-speed forging machine and the entire bar is rotated 360° to achieve rounding.

[0057] The forged steel bar is immediately placed in circulating water for cooling within 6 seconds, and the cooling time is 12 minutes. Finally, the forged bar stock is machined to a diameter of φ230. +2 mm finished bar specifications. From the middle section of the finished bar, cut a circular slice with a length of L=20mm for low-magnification microstructure observation (e.g. Figure 4 As shown), then take five φ10mm samples at equal intervals from the center to the edge (center, 1 / 4R, 1 / 2R, 3 / 4R, R) for grain size testing (as shown). Figure 5 As shown in the figure), the grain size rating results are shown in Table 2.

[0058] Table 2 Grain size rating results of finished bars at different radial positions obtained in Example 2

[0059] The large-size ultra-low expansion alloy rods prepared by the above control methods have uniform microstructure at low magnification and no contrast differences. The grain size from the center to the edge is stable at grade 5.5~6.5, and the grain size deviation is ≤1 grade. Ultrasonic testing according to GJB1580A meets the quality grade A, and the bottom loss does not exceed 50%.

[0060] Example 3 A φ500mm electroslag ingot weighing 2570kg was placed in the center of the furnace of a natural gas trolley heater. The heating temperature was set to 1110℃ and the heating rate was 100℃ / h. When the temperature reached 600~700℃, it was held for 1 hour, and then the temperature was raised to 1110℃ and held for 4 hours. The electroslag ingot, after being heated and held at a nominal pressure of 2500 tons, is forged into a billet using a high-speed forging mill. The initial reduction is 18 mm per anvil, the intermediate reduction is 25 mm per anvil, and the final reduction is 15 mm per anvil. When the surface temperature is below 900℃, it needs to be immediately tempered to 1110℃ and held for 1 hour. The entire forging process is completed in one heating cycle, using a multi-pass drawing process. In each pass, the billet cross-section is gradually deformed into a regular octagon through an octagonal drawing method. When the billet is drawn to a regular octagonal cross-section with a side-to-side distance of 360 mm, the process is switched to an arc-shaped anvil for rounding, gradually transitioning the cross-section from octagonal to circular, ultimately forming a billet with a circular cross-section of φ360 mm. Subsequently, the billets were sawn and cut into blanks, with each blank having a blank length of 670mm (denoted as L=670mm). The blanks were then subjected to targeted surface sanding using a grinding wheel to remove macroscopic defects such as micro-cracks, burrs, and folds. After sanding, a thermal insulation coating was uniformly applied to the surface of the blanks (the weight percentage of the thermal insulation coating was: water glass 37wt%, kaolin 28wt%, expanded perlite 23wt%, graphite 8wt%, and borax 4wt%). The treated billets were then placed in the center of the natural gas trolley furnace, with the heating temperature set at 1080℃ and the heating rate at 120℃ / h. After reaching 1080℃, the temperature was held for 2 hours. Next, the billet is forged into a finished product through two upsetting and two drawing processes: In the first upsetting and drawing process, the billet is first upset from φ360mm×670mm to φ440mm×450mm, and then drawn to φ360mm×670mm. The upsetting and drawing ratios for both deformations are 1.49. In the second upsetting and drawing process, the billet is first upset from φ360mm×670mm to φ430mm×470mm, and then drawn to φ262mm×1260mm. The corresponding upsetting and drawing ratios are 1.42 and 2.82, respectively. After forging, the two ends of the forging bar are held by the jaws of a high-speed forging machine and the entire bar is rotated 360° to achieve rounding.

[0061] The forged steel bar is immediately placed in circulating water for cooling within 5 seconds, and the cooling time is 15 minutes.

[0062] Finally, the forged bar stock is machined to a diameter of φ250. +2mm finished bar specifications.

[0063] In the middle section of the finished bar, a circular piece with a length of L=20mm was cut off for low-magnification microstructure observation. Then, five φ10mm samples were taken at equal intervals from the center to the edge (center, 1 / 4R, 1 / 2R, 3 / 4R, R) for grain size testing. The grain size rating results are shown in Table 3.

[0064] Table 3. Grain size rating results of finished bars at different radial positions obtained in Example 3

[0065] The large-size ultra-low expansion alloy rods prepared by the above control methods have uniform microstructure at low magnification and no contrast differences. The grain size from the center to the edge is stable at grade 5.0~6.0, with a grain size deviation of ≤1 grade. Ultrasonic testing according to GJB1580A meets the quality grade A, with a bottom loss of no more than 50%.

[0066] In summary, the large-size ultra-low expansion alloy prepared by the control method described in this invention effectively solves the problems of poor microstructure uniformity, excessive coarse grains, and inability to meet the quality requirements of ultrasonic testing caused by large-size bars. The grain size deviation of the prepared steel bars from the center to the edge is controlled to ≤1 grade. Ultrasonic testing according to GJB 1580A can meet the requirement of Class A bottom loss not exceeding 50%.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications made to the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for controlling microstructure uniformity of large-size ultra-low expansion alloy bar, characterized by, The method comprises the following steps: 1) ESR ingot heating The ultra-low expansion alloy ESR ingot with a diameter greater than that of the bar is heated to 1100-1130℃ and kept for 3-4 hours; 2) Forging breakdown The heated ESR ingot is elongated by eight passes, wherein the first pass reduction is controlled to be 10-20 mm per anvil, the intermediate pass reduction is controlled to be 20-30 mm per anvil, and the last pass reduction is controlled to be 10-20 mm per anvil, and then the anvil forging is wrapped around to obtain a breakdown bar with a size of φ330-φ370 mm; 3) Surface treatment The breakdown bar is subjected to surface sanding treatment, and after the surface defects are removed, the breakdown bar is coated with heat preservation paint; 4) Breakdown bar heating The treated breakdown bar is heated to 1050-1100℃ and kept for 2-3 hours; 5) Two upsetting and two drawing The heated breakdown bar is subjected to two upsetting and two drawing in one fire, wherein the first upsetting ratio is controlled to be 1.3-1.6, the first drawing ratio is controlled to be 1.3-1.6, the second upsetting ratio is controlled to be 1.3-1.6, and the second drawing ratio is greater than 2.4, and then the bar is wrapped around and cooled by water for more than 10 minutes to obtain a finished bar; 6) The finished bar is subjected to turning to remove 10-20 mm of radial thickness to obtain a φ230-φ250 mm large-size ultra-low expansion alloy bar product.

2. The control method according to claim 1, characterized by, In step 1), the heating rate of the ESR ingot heating is ≤120℃ / h, and when heated to 600-700℃, the temperature is kept for 1 hour before being continuously heated to 1100-1130℃.

3. The control method according to claim 1, characterized by, In step 2), the anvil forging is wrapped around by using an arc anvil to perform a round processing.

4. The control method according to claim 1, characterized by, In step 3), the surface sanding treatment is performed by using a grinding machine.

5. The control method according to claim 1, characterized by, In step 3), the weight percentage of the heat preservation paint is: water glass 30wt%-40wt%, kaolin 20wt%-30wt%, expanded perlite 15wt%-25wt%, graphite 5wt%-10wt%, and borax 2wt%-5wt%.

6. The control method according to claim 1, characterized by In step 4), the heating rate of the breakdown bar heating is ≤150℃ / h.

7. The control method according to claim 1, characterized by, In step 5), the wrapping around is performed by clamping the ends of the forged bar by the jaws of the fast forging machine to rotate the whole bar by 360°.

8. The control method according to claim 1, characterized by, In step 5), the water cooling is performed within 10 seconds after the forging is completed, and circulating water cooling is used.

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