A cold rotary swaging erw oil cylinder steel pipe and a manufacturing method thereof

Abstract

The application discloses a kind of cold rotary swaging ERW oil cylinder steel pipe and manufacturing method thereof, belong to steel pipe manufacturing technical field.The chemical composition of the application is as follows: C:0.05%~0.07%, Si:0.05%~0.15%, Mn:1.50%~1.70%, P≤0.010%, S≤0.0050%, Als:0.025%~0.055%, Nb:0.035%~0.045%, Ti:0.060%~0.070%, Cr:0.10%~0.20%, N:≤0.0040%, H:≤0.0002%, Ceq≤0.39, the rest is Fe and inevitable impurities.The application adopts the new process of steel plate→striping→high-frequency resistance welding (ERW)→straightening→flaw detection→saw cutting→cold rotary swaging→finished product, avoids high-temperature heating, piercing, normalizing heat treatment and pickling before cold rolling (pulling) and other processes, greatly shortens production flow, reduces production cost and improves production efficiency.

Description

Technical Field

[0001] This invention belongs to the field of steel pipe manufacturing technology, and more specifically, relates to a cold-forged steel pipe for ERW hydraulic cylinders and its manufacturing method. Background Technology

[0002] With the sustained, healthy, and rapid development of the economy, the construction of various major infrastructure projects has entered a new development boom, leading to a significant increase in demand for construction machinery. Hydraulic cylinders, as crucial components of mainframe machinery, have a very promising future; the annual domestic demand for excavators alone is expected to reach tens of thousands of units. In recent years, my country has continuously increased its support for various industries and introduced a series of favorable policies to encourage the development of advanced manufacturing and strengthen technological innovation, focusing on the research and development and production of new types of hydraulic cylinders.

[0003] Because hydraulic cylinders need to withstand high pressure reciprocating motion, the cylinder barrel needs to have high strength and toughness, as well as high dimensional accuracy and surface quality. Therefore, there are high requirements for the steel pipes used for cylinder barrels.

[0004] A search revealed the following patents related to steel pipes for hydraulic cylinder barrels and their manufacturing processes.

[0005] 1) Chinese patent CN 107746916 A discloses a method for manufacturing seamless steel pipes for hydraulic cylinders;

[0006] 2) Chinese patent CN 112095049 A discloses a new alternative material for seamless steel pipes used in hydraulic cylinders;

[0007] 3) Chinese patent CN 114836681 A discloses a high-strength seamless steel pipe with good fatigue resistance and its manufacturing method;

[0008] 4) Chinese patent CN 102527767 A discloses a manufacturing process for cold-drawn welded pipes for high-precision hydraulic cylinders;

[0009] 5) Chinese patent CN 104646449 A discloses a method for manufacturing a cold-drawn welded pipe for a hydraulic cylinder;

[0010] 6) Chinese patent CN 110052792 A discloses a method for manufacturing a cylinder barrel for a hydraulic cylinder.

[0011] The first three related patents use seamless steel pipes, and their main processes are: bar heating → piercing → rough tube → rolling → blank tube → rolling (diameter reduction) → clean tube → cooling → sawing → normalizing → straightening (flaw detection) → cold drawing (first pickling and saponification to remove iron oxide scale, then cold drawing to reduce diameter and wall thickness; generally, multiple cold drawing passes are used to improve the dimensional accuracy and surface quality of the steel pipe) → stress-relieving tempering → flaw detection → finished product. The disadvantage is that the entire process is complex and the manufacturing cost is high.

[0012] The latter three related patents use welded pipes instead of seamless steel pipes, which simplifies the process to some extent. However, patents CN 102527767A and CN 104646449A both include a cold drawing process, and these processes require additional heat treatment and pickling steps before and after the cold drawing. Furthermore, CN 110052792A adds a tempering treatment (quenching + tempering) to the welded pipe process, further extending the process and hindering cost control. Therefore, the process flow still needs further optimization. Summary of the Invention

[0013] 1. The problem to be solved

[0014] In view of at least some of the problems existing in the prior art, the present invention proposes a cold-forged ERW hydraulic cylinder steel pipe and its manufacturing method, the purpose of which is to solve the problems of long process flow and high cost of existing hydraulic cylinder finished pipe.

[0015] 2. Technical Solution

[0016] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0017] The present invention discloses a cold-forged ERW hydraulic cylinder steel pipe, wherein the chemical composition of the steel pipe by weight percentage is as follows:

[0018] C: 0.05%–0.07%, Si: 0.05%–0.15%, Mn: 1.50%–1.70%, P≤0.010%, S≤0.0050%, Als: 0.025%–0.055%, Nb: 0.035%–0.045%, Ti: 0.060%–0.070%, Cr: 0.10%–0.20%, N: ≤0.0040%, H: ≤0.0002%, Ceq≤0.39, with the remainder being Fe and unavoidable impurities.

[0019] Furthermore, the cold-forged ERW cylinder steel pipe has a yield strength ≥600MPa, tensile strength ≥650MPa, elongation ≥15%, and the finished product absorbs ≥170J of energy in a 10mm*10mm*55mm V-shaped impact test specimen under room temperature and -20℃ conditions.

[0020] Furthermore, the metallographic structure of the cold-forged ERW cylinder steel pipe is 25-55% ferrite and 45-75% pearlite by volume, with a grain size of 12.5-15.0.

[0021] The present invention discloses a method for manufacturing a cold-forged ERW hydraulic cylinder steel pipe, comprising the following steps: molten iron pretreatment → converter smelting → alloy fine-tuning station → LF → RH → continuous casting → hot rolling → hot-rolled steel coil → slitting → high-frequency resistance welding → flaw detection → straightening → sawing → cold forging → finished product; wherein, during ERW welding, online weld heat treatment is added to reduce the weld cooling rate.

[0022] Furthermore, the billet is heated in a heating furnace for 1.5 to 3.5 hours, and the furnace exit temperature is controlled at 1240℃ to 1270℃.

[0023] Furthermore, the roughing process employs a 3+3 pass rolling method with full descaling water flow; the finishing rolling inlet temperature is 1010~1050℃, and the final rolling temperature is controlled at 850℃~890℃.

[0024] Furthermore, laminar flow cooling is adopted after finishing rolling, with a cooling rate ≥40℃ / s; the coiling temperature of the hot rolling process is 500~650℃.

[0025] Furthermore, the slitting roll width W = 3.14 * D0 - 2 * t0 + 2.0 mm, where W is the slitting roll width, D0 is the outer diameter of the welded pipe, and t0 is the thickness of the raw material.

[0026] Furthermore, the post-weld heating temperature at the weld location is 650–750℃.

[0027] 3. Beneficial effects

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) The present invention discloses a new manufacturing method for cold-forged ERW hydraulic cylinder steel pipes, which adopts a new process of steel plate → slitting → high-frequency resistance welding (ERW) → straightening → flaw detection → sawing → cold forging → finished product. This avoids the high-temperature heating, piercing, normalizing heat treatment before cold rolling (drawing), and pickling processes, greatly shortening the production process, reducing production costs, and improving production efficiency. At the same time, cold forging can effectively avoid the occurrence of vibration marks on the inner wall of the finished product during grinding, ensuring the quality of the inner wall. In addition, by adding online weld heat treatment when using ERW welded pipes, the cooling rate of the weld is reduced, which can ensure that the microstructure and properties of the weld position are basically consistent with those of the base material.

[0030] (2) The present invention discloses a method for manufacturing cold-forged ERW hydraulic cylinder steel pipes. Through cold forging, the pipe blank can be reduced in diameter and increased in wall thickness, and the dimensional accuracy and surface quality of the pipe blank can be improved. The dimensional accuracy and surface quality of the pipe blank are comparable to those of cold-drawn pipe blanks, and the cold-forged pipe blank does not require further annealing. In addition, different pipe diameters can be obtained by changing the slitting width, and the final finished product size can be obtained by controlling the pipe diameter and wall thickness during subsequent cold forging processes. Therefore, the manufacturing method of cold-forged ERW hydraulic cylinder steel pipes is more flexible and efficient, thereby helping to further reduce inventory.

[0031] (3) The present invention discloses a method for manufacturing a cold-forged steel pipe for an ERW hydraulic cylinder. This method employs a low-carbon (C) composition design, avoiding the composition range of peritectic steel, thus preventing defects such as cracks at the corners of the cast billet, improving product quality, and eliminating the need for corner cleaning of the cast billet. Simultaneously, the contents of N, H, and S are strictly controlled to ensure that the billet is free of defects such as porosity and large-sized inclusions. Furthermore, the low C content further reduces Ceq, ensuring that Ceq ≤ 0.39, thereby further improving the weldability of the material and guaranteeing the welding quality of the welded pipe seam and the bottom and port of the finished hydraulic cylinder. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the tissue structure in Example 1;

[0033] Figure 2 This is a schematic diagram of the corner of the cast billet in Example 1.

[0034] Figure 3 This is a schematic diagram of the corner of the cast billet in Comparative Example 2;

[0035] Figure 4 This is a schematic diagram of the surface of a hot-rolled sheet with three components, as shown in the comparative example.

[0036] Figure 5 This is a schematic diagram of the pitting defect on the inner wall after ERW welding of component 3 in the comparative example.

[0037] Figure 6 The surface pits were removed from the red rust area of ​​the original board in Comparative Example 3 after grinding. Detailed Implementation

[0038] To further understand the content of this invention, a detailed description of the invention will be provided in conjunction with the accompanying drawings.

[0039] The present invention discloses a cold-forged steel pipe for ERW hydraulic cylinders, the chemical composition and weight percentage of which are as follows: C: 0.05%~0.07%, Si: 0.05%~0.15%, Mn: 1.50%~1.70%, P≤0.010%, S≤0.0050%, Als: 0.025%~0.055%, Nb: 0.035%~0.045%, Ti: 0.060%~0.070%, Cr: 0.10%~0.20%, N: ≤0.0040%, H: ≤0.0002%, Ceq≤0.39, with the remainder being Fe and unavoidable impurities. Wherein, Ceq = [C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15] * 100%.

[0040] This invention employs a C-Si-Mn+Nb-Ti-Cr composition system. In the specific design of the alloy elements, to ensure that the cast billet meets the requirements for hot charging during production, and to achieve high strength and high toughness in the material, while also considering alloy cost, cold bending performance, and weldability, the C content is controlled between 0.05% and 0.07%, avoiding the composition range of peritectic steel. Furthermore, the low C content further reduces Ceq, thereby further improving the weldability of the material and ensuring the welding quality of the welded pipe seam and the bottom and port of the finished oil cylinder.

[0041] To obtain higher surface quality and give the product surface a certain degree of oxidation resistance, and to reduce the large amount of iron oxide powder and decarburization generated on the surface of the product during the heating process, the steel uses low Si (0.05% to 0.15%) and adds an appropriate amount of Cr (0.10% to 0.20%).

[0042] Mn can enhance the strength of steel through solid solution strengthening. Simultaneously, it promotes the dissolution of carbonitride precipitates during heating and inhibits their precipitation during rolling. This helps maintain a higher concentration of precipitating elements within the ferrite structure during post-rolling cooling, thus strengthening precipitation hardening. Furthermore, Mn can expand the austenite phase region and lower the transformation temperature of the supercooled austenite phase, which is beneficial for refining the phase transformation microstructure.

[0043] The main function of Nb and Ti microalloying is to form carbonitride compounds in steel. These compounds, through dissolution and precipitation mechanisms under different conditions, refine the grain size and provide precipitation strengthening, while also improving the weldability of the material. However, considering overall cost, only a small amount of Nb (0.035%–0.045%) is added to achieve grain refinement and strengthening while reducing costs.

[0044] Al is a deoxidizing element in steel, which can reduce oxide inclusions and refine grains. However, if the concentration is too high, this refining effect is weakened, casting becomes difficult during production, and sulfide inclusions in the steel increase, which is detrimental to the fatigue performance of the finished stabilizer bar.

[0045] To reduce the inclusion content in steel and ensure the weldability and good fatigue performance of the finished product, the contents of elements such as P, S, N, and H are strictly controlled. Furthermore, Ceq=[C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15]*100%≤0.39, resulting in good weldability of the material.

[0046] The cold-forged ERW cylinder steel pipe of the present invention is prepared by the following steps: molten iron pretreatment → converter smelting → alloy fine-tuning station → LF → RH → continuous casting → hot rolling → hot-rolled steel coil → slitting → high-frequency resistance welding (ERW) → flaw detection → straightening → sawing → cold forging → finished product.

[0047] Specifically, after steelmaking, continuous casting, and refining in the LF+RH furnace, calcium treatment is performed. In addition to controlling the chemical composition of C, Si, Mn, Nb, Ti, and Cr, the content of P, S, H, and N elements is kept low, and the content and morphology of steel inclusions are controlled to ensure high strength, high plasticity, and high fatigue performance of the finished product. Furthermore, a stable casting speed method with controlled molten steel surface fluctuations within ±3mm and a uniform cooling process are employed. Dynamic light pressing is used, and electromagnetic rollers are used to agitate impurities to the surface, resulting in good internal quality of the cast billet.

[0048] To further ensure the quality of the cast billets and save energy, a hot charging method for billet assembly is adopted. The billet temperature entering the furnace is ≥500℃, and it is heated in the furnace for 1.5 to 3.5 hours. The exit temperature is controlled at 1240℃ to 1270℃ to ensure the precipitation strengthening effect of Ti.

[0049] Rough rolling is performed in 3+3 passes with full descaling water flow; finish rolling is carried out on a 2250mm hot continuous rolling mill with an inlet temperature of 1010~1050℃; in order to ensure the surface quality of the hot-rolled product, the rolls need to be replaced before finish rolling; at the same time, in order to obtain a fine and uniform microstructure and reduce the generation of iron oxide scale, the final rolling temperature is controlled at 850℃~890℃.

[0050] After finishing rolling, laminar flow cooling is used for rapid cooling in the front-end cooling mode, with a cooling rate ≥40℃ / s. In order to ensure the microstructure and surface quality of the steel plate raw material, the coiling temperature is controlled at 500~650℃, preferably 550~600℃.

[0051] The steel plates manufactured using the above method are slitted, with the slitting coil width W = 3.14*D0 - 2*t0 + 2.0mm (W is the target width of the slitting coil, D0 is the outer diameter of the welded pipe, and t0 is the thickness of the raw material). The width accuracy of the slitting coil is controlled within ±1.0mm. A slitting coil that is too narrow will cause inclusions or incomplete welds during ERW welding, while a slitting coil that is too wide will increase the weld extrusion and weld reinforcement during ERW welding, thus increasing ERW welding energy consumption and reinforcement scraping, while reducing material utilization. The slitting coil undergoes high-frequency resistance welding (ERW) → online weld induction heat treatment → flaw detection → straightening → sawing → welded pipe.

[0052] Preferably, the hot-rolled steel strip material corresponding to the cold-forged ERW steel pipe for hydraulic cylinders has a thickness of 4.0–8.0 mm and a width of 1000–1500 mm. Furthermore, the metallographic structure of the hot-rolled steel strip for the cold-forged ERW steel pipe consists of 25–55% ferrite and 45–75% pearlite by volume, with a grain size of 12.5–15.0.

[0053] To ensure the microstructure and properties of the weld are similar to those of the base metal, ERW pipe welding incorporates online high-frequency induction heating to reduce the post-weld cooling rate, resulting in a weld with the same Fiber-Plastic (F+P) microstructure as the base metal. The post-weld heating temperature at the weld is 650–750℃ to eliminate residual stress and further ensure the weld achieves an F+P microstructure, thus maintaining properties closely resembling those of the base metal. The final dimensions obtained are… (Outer diameter * wall thickness) tube blank.

[0054] The welded pipe obtained above is then subjected to cold rotary forging to reduce the pipe blank's diameter and increase its wall thickness. Because ERW welded pipe blanks have high dimensional accuracy, the diameter reduction is controlled within 5-10 mm, and the wall thickness increase within 0.2-1.0 mm. Cold rotary forging further improves the dimensional accuracy of the pipe blank, especially ensuring the dimensional accuracy of the inner wall at the weld seam, and ensuring that vibration marks appear on the inner wall of the finished product during grinding. This results in a pipe with dimensions of... Precision steel tubing for cold-forged ERW hydraulic cylinders (outer diameter * wall thickness).

[0055] The cold-forged ERW cylinder steel pipe obtained by the above method has a yield strength ≥600MPa, tensile strength ≥650MPa, elongation ≥15%, and the finished pipe meets the requirements after H=1 / 2D flattening test. The energy absorbed by the 10mm*10mm*55mm V-type impact test specimen is ≥170J under room temperature and -20℃ conditions.

[0056] The key technical indicators of the above-described embodiments and comparative examples are as follows:

[0057] Table 1. Chemical composition (mass percentage %) of the embodiments and comparative examples of the present invention

[0058] type C Si Mn P S Als Nb Cr Ti N H Ceq Example 1 0.051 0.12 1.68 0.007 0.0020 0.046 0.035 0.19 0.061 0.0035 0.0002 0.37 Example 2 0.070 0.10 1.51 0.010 0.0049 0.038 0.044 0.12 0.069 0.0020 0.0001 0.35 Example 3 0.062 0.09 1.62 0.006 0.0016 0.035 0.039 0.15 0.065 0.0026 0.0001 0.36 Example 4 0.059 0.07 1.64 0.005 0.0018 0.029 0.036 0.16 0.063 0.0028 0.0002 0.36 Comparative Example 1 0.062 0.09 1.62 0.006 0.0016 0.035 0.039 0.15 0.065 0.0026 0.0001 0.36 Comparative Example 2 0.101 0.10 1.6 0.009 0.0048 0.038 0.040 0.15 0.02 0.0026 0.0002 0.41 Comparative Example 3 0.041 0.72 1.78 0.010 0.0020 0.035 0.026 / / 0.0045 / 0.34

[0059] Table 2 Hot-rolling process parameters and steel plate properties

[0060]

[0061] Table 3. Slitting width, billet size, and properties of the embodiments and comparative examples.

[0062]

[0063] As can be seen from Tables 1-3, raw materials are selected according to the weight percentage of the chemical composition given in this invention, and then production is carried out according to the process given in this invention. This avoids processes such as high-temperature heating, piercing, normalizing heat treatment before cold rolling (drawing), and pickling, greatly shortening the production process, reducing production costs, and improving production efficiency; at the same time, it ensures that the microstructure and properties of the weld seam are basically consistent with those of the base material. The final cold-forged ERW hydraulic cylinder steel pipe has a yield strength ≥600MPa, tensile strength ≥650MPa, elongation ≥15%, and the finished pipe meets the requirements after H=1 / 2D flattening test. The absorbed energy of a 10mm*10mm*55mm V-type impact specimen is ≥170J under room temperature and -20℃ conditions.

[0064] Comparative Example 1: Although the chemical composition range of the steel pipe meets the requirements of this invention, the hot rolling coiling temperature is too high, which leads to a decrease in the strength of the hot-rolled steel plate and fails to meet the strength requirements of the finished product.

[0065] Comparative Example 2: The C composition of the material does not meet the requirements of this invention because its C content is high, and cracks exist at the corners of the cast billet within the peritectic steel composition range (e.g., Figure 3 This makes hot charging production impossible; it requires slow cooling and corner cleaning of the billet, which reduces production efficiency. At the same time, reheating the billet after cooling increases energy consumption, which is not conducive to energy conservation and environmental protection.

[0066] Comparative Example 3: The material composition does not meet the requirements of this invention, especially the high Si content. During the high-temperature heating process of hot rolling, it easily forms a siliceous olivine phase (Fe2SiO4), which accumulates at the interface between the iron oxide scale and the steel plate substrate, as well as within the iron oxide scale. This increases the adhesion of the iron oxide scale, making hot rolling descaling more difficult and causing surface oxide pressing or red rust defects, such as… Figure 4 During ERW welding, the increased thickness of the iron oxide scale and the peeling of red rust caused by the pressing during the forming process resulted in pitting defects on the inner wall of the tube blank, making it impossible to meet the dimensional accuracy requirements of the cylinder tube. Figure 5 and Figure 6 .

[0067] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A method for manufacturing a cold-forged steel pipe for an ERW hydraulic cylinder, characterized in that: The process includes the following steps: hot metal pretreatment → converter smelting → alloy fine-tuning station → LF → RH → continuous casting → hot rolling → hot-rolled steel coil → slitting → high-frequency resistance welding → flaw detection → straightening → sawing → cold forging → finished product; wherein, the molten steel is smelted according to the following chemical composition by weight percentage: C: 0.05%~0.07%, Si: 0.05%~0.15%, Mn: 1.50%~1.70%, P≤0.010%, S≤0.0050%, Als: 0.025%~0.055%, Nb: 0.035%~0.045%, Ti: 0.060%~0.070%, Cr: 0.10%~0.20%, N: ≤0.0040%, H: ≤0.0002%, Ceq≤0.39, with the remainder being Fe and unavoidable impurities; During ERW pipe welding, online weld heat treatment is added to reduce the weld cooling rate; In the aforementioned cold forging step, the diameter reduction is controlled at 5~10mm, and the wall thickness increase is controlled at 0.2~1.0mm; The coiling temperature in the hot rolling process is 500~650℃.

2. The manufacturing method of a cold-forged ERW hydraulic cylinder steel pipe according to claim 1, characterized in that: The billet is heated in the heating furnace for 1.5 to 3.5 hours, and the furnace exit temperature is controlled at 1240℃ to 1270℃.

3. The manufacturing method of a cold-forged steel pipe for an ERW hydraulic cylinder according to claim 2, characterized in that: The roughing process uses a 3+3 pass rolling method with full descaling water flow; the finishing rolling inlet temperature is 1010~1050℃, and the final rolling temperature is controlled at 850℃~890℃.

4. The manufacturing method of a cold-forged ERW hydraulic cylinder steel pipe according to claim 3, characterized in that: After finishing rolling, laminar flow cooling is adopted, with a cooling rate of ≥40℃ / s.

5. A method for manufacturing a cold-forged ERW hydraulic cylinder steel pipe according to claim 4, characterized in that: Strip volume W=3.14 D0-2 t0+2.0mm, where W is the width of the slitting roll, D0 is the outer diameter of the welded pipe, and t0 is the thickness of the raw material.

6. A method for manufacturing a cold-forged steel pipe for an ERW hydraulic cylinder according to claim 5, characterized in that: The post-weld heating temperature at the weld location is 650~750℃.

7. A cold-forged steel pipe for ERW hydraulic cylinders, characterized in that: The product is obtained by the manufacturing method described in claims 1-6, and comprises the following components in weight percentages. C: 0.05%~0.07%, Si: 0.05%~0.15%, Mn: 1.50%~1.70%, P≤0.010%, S≤0.0050%, Als: 0.025%~0.055%, Nb: 0.035%~0.045%, Ti: 0.060%~0.070%, Cr: 0.10%~0.20%, N: ≤0.0040%, H: ≤0.0002%, Ceq≤0.39, with the remainder being Fe and unavoidable impurities.

8. The cold-forged steel pipe for an ERW hydraulic cylinder according to claim 7, characterized in that: The cold-forged ERW hydraulic cylinder steel pipe has a yield strength ≥600MPa, tensile strength ≥650MPa, elongation ≥15%, and the finished product has a thickness of 10mm under room temperature and -20℃ conditions. 10mm The energy absorbed by a 55mm V-shaped impact specimen is ≥170J.

9. A cold-forged steel pipe for an ERW hydraulic cylinder according to claim 8, characterized in that: The metallographic structure of the cold-forged ERW cylinder steel pipe consists of 25-55% ferrite and 45-75% pearlite by volume, with a grain size of 12.5-15.0.

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