Seamless steel tube with excellent drawing performance for oil cylinder and banded structure control method thereof

Abstract

The invention relates to the technical field of seamless steel tubes, in particular to an oil cylinder seamless steel tube with excellent drawing performance and a banded structure control method thereof. The steel is composed of the following chemical components in percentage by weight: 0.24%-0.34% of C, 1.00%-1.40% of Si, 1.00%-1.40% of Mn, 0.20%-0.50% of Cr, 0.0005%-0.0015% of B, less than or equal to 0.015% of P, less than or equal to 0.010% of S, less than or equal to 1.57% of C and Mn, and the balance of Fe and inevitable impurities. According to the seamless steel pipe and the preparation method thereof, through cooperative control of the whole process of component design, continuous casting, heating, rolling and cooling, the industrial problem of the banded structure of the high-strength oil cylinder pipe is successfully solved, the prepared seamless steel pipe has the excellent cold-drawing machining performance and the excellent final mechanical performance at the same time, the comprehensive technical level is high, and the industrial application value is remarkable.

Description

Technical Field

[0001] This invention relates to the field of seamless steel pipe technology, specifically to a seamless steel pipe for hydraulic cylinders with excellent drawing performance and a method for controlling its strip structure. Background Technology

[0002] As a core actuator in engineering machinery, the performance of hydraulic cylinders directly determines the working efficiency, load-bearing capacity, and reliability of the main equipment. Seamless steel tubing for high-strength cylinders is the most critical structural material in the manufacture of hydraulic cylinders; its quality directly affects the dimensional accuracy, mechanical strength, and service life of the finished cylinder.

[0003] In recent years, with the rapid development of my country's construction machinery industry towards lightweighting and large-scale production, more stringent requirements have been placed on hydraulic cylinders. On the one hand, to adapt to larger equipment, the volume and wall thickness of cylinders are constantly increasing; on the other hand, to meet the demand for lightweighting, it is necessary to significantly improve the strength of materials while reducing the size of structural components. Furthermore, construction machinery often operates in harsh environments such as frigid regions, which requires the steel pipes used in cylinders to possess excellent low-temperature impact toughness. Currently, the industry generally requires that the final performance of cylinder steel pipes after quenching and tempering heat treatment meet high standards, including a yield strength ≥835MPa, tensile strength ≥980MPa, and impact energy CVN ≥39J.

[0004] To achieve the aforementioned high-performance goals, the typical manufacturing process for hydraulic cylinders is "hot-rolled tube → pickling → saponification → cold drawing (wall and diameter reduction) → quenching and tempering heat treatment." Among these, cold drawing is a crucial step, precisely controlling the steel tube dimensions, improving surface finish, and pre-strengthening the material. However, this process also places extremely high demands on the original state of the hot-rolled seamless steel tube, particularly requiring users to ensure that the elongation after fracture of the hot-rolled steel tube is not less than 18% to guarantee sufficient plasticity reserve to withstand cold drawing deformation without cracking.

[0005] Currently, a prominent technical challenge restricting the quality and cold-drawing yield of high-strength hydraulic cylinder tubes is the presence of banded microstructure. Banded microstructure is a chemically uneven structural defect, characterized by alternating bands of ferrite and pearlite in the steel. During cold drawing, the softer ferrite bands undergo plastic flow first, while the harder pearlite bands are difficult to deform. This uneven deformation leads to significant internal stress concentration. These stress concentration points easily become initiation sites for microcracks, directly causing the steel tube to crack during drawing or subsequent use. Even if cracking does not occur immediately, this uneven deformation can still cause "herringbone" or "bamboo-like" surface defects in the cold-drawn steel tube, resulting in product scrap and a low yield.

[0006] Therefore, how to fundamentally improve the internal structure of hot-rolled seamless steel pipes and effectively suppress or eliminate harmful banded structures, thereby simultaneously ensuring high strength, high toughness, and excellent cold-drawing performance, has become a pressing technical challenge in this field. Existing technologies attempt to address this issue either by adjusting the composition or by optimizing the rolling process, but these methods often fail to perfectly resolve the banded structure problem caused by Mn element segregation, and the resulting deficiencies in cold-drawing performance and low-temperature toughness, while maintaining strength. Summary of the Invention

[0007] In order to overcome the shortcomings of the prior art, the present invention provides a seamless steel pipe for hydraulic cylinders with excellent drawing performance and a method for controlling the banded structure thereon, which suppresses or reduces the banded structure on the inner wall of the steel pipe and improves the drawing performance of hot-rolled seamless steel pipe.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A seamless steel tube for hydraulic cylinders with excellent drawing properties is composed of the following chemical composition by weight percentage:

[0010] C: 0.24%~0.34%, Si: 1.00%~1.40%, Mn: 1.00%~1.40%, Cr: 0.20%~0.50%, B: 0.0005%~0.0015%, P≤0.015%, S≤0.010%, C+Mn≤1.57%, with the remainder being Fe and unavoidable impurities.

[0011] The effect of selecting the above alloying elements and their contents:

[0012] 1. Carbon (C), as a fundamental strengthening element in steel, is used in this invention to ensure the strength of the steel pipe. Excessive C content reduces the elongation and low-temperature impact toughness of the steel. Simultaneously, C lowers the solidus temperature of the steel, expands the liquid-solid two-phase region, creating conditions for Mn segregation and indirectly exacerbating the banded structure. Therefore, the C+Mn content should be ≤1.57%. Thus, this invention precisely controls the C content to 0.24%~0.34%, with C+Mn ≤1.57%.

[0013] 2. Si in steel increases its hardness and strength, while refining the grain size and mitigating the hardening tendency that Mn may cause, giving the steel both strength and toughness. However, excessive Si reduces the toughness and plasticity of the steel pipe. Therefore, this invention precisely controls the Si content to 1.00%~1.40%.

[0014] 3. Mn can improve the strength of steel pipes, compensating for the strength loss due to the reduced carbon content. Mn lowers the critical cooling temperature of steel, refines pearlite, and improves the plasticity of steel. However, during solidification, Mn tends to accumulate in unsolidified areas, forming Mn-rich zones, which, when combined with C, exacerbate the banded structure. Therefore, this invention precisely controls the Mn content to 1.00%~1.40%, and C+Mn≤1.57%.

[0015] 4. Cr can inhibit grain growth rate, improve hardenability, and help obtain a finer grain structure after heat treatment. However, excessive Cr reduces the toughness of hot-rolled seamless steel pipes, which is detrimental to the pipe's drawing performance. Therefore, this invention precisely controls the Cr content to 0.20%~0.50%.

[0016] 5. Boron (B) can significantly improve hardenability, but excessive B content or improper processing can lead to the formation of coarse borides at grain boundaries, severely deteriorating the toughness of the steel. Therefore, this invention precisely controls the B content to be 0.0005%~0.0015%.

[0017] 6. P and S are segregating elements. They readily accumulate at grain boundaries, exacerbating segregation in the microstructure. Therefore, this invention controls P ≤ 0.015% and S ≤ 0.010%.

[0018] The above-mentioned method for controlling the strip structure of seamless steel tubes for hydraulic cylinders with excellent drawing performance specifically includes the following steps:

[0019] 1) Preparation of continuously cast round billets:

[0020] The steel is smelted and vacuum treated, wherein the holding time of the vacuum treatment is ≥8min; the vacuum-treated steel is then continuously cast to obtain a continuously cast round billet.

[0021] 2) Heating of tube blank:

[0022] The continuously cast round billet is heated at 1220~1260℃ for a total heating time of 150~300min.

[0023] 3) Hot rolling:

[0024] The heated tube blank is pierced, rolled and sized in sequence, and the final rolling temperature is controlled at 870℃~940℃.

[0025] 4) Cooling:

[0026] The rolled steel pipe is cooled at a cooling rate of 0.5~1.0℃ / s to obtain a microstructure in which ferrite is distributed in an equiaxed manner.

[0027] Furthermore, prior to continuous casting in step 1), molten steel is prepared by converter smelting and sequential refining in an LF furnace and an RH furnace.

[0028] Furthermore, prior to continuous casting in step 1), molten steel is prepared by converter smelting and sequential refining in an LF furnace and a VD vacuum refining process.

[0029] Further, the chemical composition of the continuously cast round billet in step 1) by weight percentage is as follows: C: 0.24%~0.34%, Si: 1.00%~1.40%, Mn: 1.00%~1.40%, Cr: 0.20%~0.50%, B: 0.0005%~0.0015%, P≤0.015%, S≤0.010%, C+Mn≤1.57%, with the remainder being Fe and unavoidable impurities.

[0030] Furthermore, in step 1), the superheat of the molten steel is controlled at 15~25℃, the billet pulling speed is controlled at 1.0~1.3m / min, and the end electromagnetic stirring current is 100~250A.

[0031] Furthermore, in step 2), the tube blank heating is carried out in an annular furnace.

[0032] Furthermore, in step 4), the cooling rate is controlled to be 0.5~0.8℃ / s.

[0033] Seamless steel pipes produced using the above control method have a microstructure of granular ferrite and pearlite in the hot-rolled state, with a grain size grade of 7-10 and an elongation at break ≥25%. After quenching and tempering heat treatment, the seamless steel pipe has a yield strength ≥835MPa, a tensile strength ≥980MPa, and an impact energy CVN ≥39J.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. This invention employs converter smelting, combined with a refining process of LF and RH / VD, to achieve deep desulfurization and dephosphorization of molten steel, effectively reducing the content of harmful elements and non-metallic inclusions. In particular, by controlling the vacuum holding time in the continuous casting process to ≥8 minutes, it significantly promotes the full flotation and removal of inclusions, resulting in steel purity superior to conventional processes.

[0036] 2. This invention effectively promotes the formation of equiaxed crystal zones by precisely controlling the superheat of molten steel and the continuous casting speed, and controls the end electromagnetic stirring current to 100~250A. This series of synergistic measures significantly reduce the segregation tendency of Mn elements between dendrites from the solidification source, fundamentally suppressing the formation of banded structures during subsequent heat treatment, and ensuring the uniformity of the steel pipe's microstructure.

[0037] 3. This invention synergistically optimizes the heating temperature and total heating time of the annular furnace. By controlling the billet heating temperature within a reasonable range of 1220~1260℃, the necessary conditions for the full diffusion and homogenization of Mn in austenite are provided, while effectively preventing abnormal coarsening of austenite grains. Based on this, the rolling process stabilizes the final rolling temperature at 870~940℃, ensuring that the rolled austenite can complete sufficient static recrystallization in a short time, thereby obtaining a fine and uniform grain structure.

[0038] 4. The core of this invention lies in the precise control of the post-rolling cooling rate, strictly limiting it to the range of 0.5~1.0℃ / s. This rapid cooling regime, on the one hand, significantly reduces the ferrite-pearlite banded structure commonly found in traditional processes by suppressing the preferential precipitation of proeutectoid ferrite in the Mn segregation zone; on the other hand, it promotes the direct decomposition of supercooled austenite into uniform and fine equiaxed ferrite and pearlite structures, while avoiding the formation of acicular ferrite that is detrimental to toughness due to excessively rapid cooling, ultimately achieving a good balance between strength and toughness. Attached Figure Description

[0039] Figure 1 This is a metallographic image of the microstructure of the seamless steel pipe for hydraulic cylinders obtained in Embodiment 1 of the present invention.

[0040] Figure 2 This is a metallographic image of the microstructure of the seamless steel pipe for hydraulic cylinders obtained in Comparative Example 1. Detailed Implementation

[0041] This invention discloses a seamless steel tube for hydraulic cylinders with excellent drawing performance and a method for controlling its strip-like microstructure. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0042] A method for controlling the strip-like microstructure of seamless steel tubes for hydraulic cylinders with excellent drawing performance, specifically including the following steps:

[0043] 1. Continuous casting process control:

[0044] The continuously cast round billets are produced, and the vacuum treatment holding time is ≥8min to promote the flotation of inclusions. Inclusion control ensures that the steel pipe flaw detection meets the requirements and that safety is ensured during application. The molten steel is superheated to 15~25℃ to promote the formation of equiaxed crystals and reduce Mn segregation in dendrites. Depending on the cross-sectional size, the cross-sectional size of the continuously cast round billets is generally small, and the casting speed is controlled at 1.0~1.3m / min. The end electromagnetic stirring current is 100~250A.

[0045] The continuously cast round billet is composed of the following chemical composition by weight percentage:

[0046] C: 0.24%~0.34%, Si: 1.00%~1.40%, Mn: 1.00%~1.40%, Cr: 0.20%~0.50%, B: 0.0005%~0.0015%, P≤0.015%, S≤0.010%, C+Mn≤1.57%, with the remainder being Fe and unavoidable impurities.

[0047] 2. Billet heating:

[0048] The billet heating temperature is 1220~1260℃. The billet homogenization temperature affects the grain size and properties of the final product. Lowering the homogenization temperature helps control the final rolling temperature and prevents excessive grain growth. Temperatures below 1220℃ can easily lead to excessive wear and tear on equipment and dies. The total heating time for the billet is 150~300 minutes, providing sufficient time for Mn to homogenize in austenite. After piercing, rolling, and sizing, the steel pipe temperature can reach 870~940℃.

[0049] 3. Cooling control:

[0050] After final rolling, the cooling rate of the steel pipe must be controlled at 0.5~1.0℃ / s. Increasing the cooling rate of the inner wall of the steel pipe inhibits the pre-precipitation of ferrite, reduces banded structure, and directly decomposes the supercooled austenite into ferrite + pearlite. At a cooling rate of 0.5~1.0℃ / s, the ferrite is distributed in an equiaxed pattern. When the cooling rate reaches 1.0℃ / s, acicular ferrite begins to appear in the microstructure with increasing cooling rate. Acicular ferrite increases the strength of the hot-rolled steel pipe, reduces toughness, and is detrimental to drawing performance.

[0051] 4. Performance control:

[0052] The final result is a granular ferrite + pearlite structure. Depending on the wall thickness of the steel pipe, the grain size of the hot-rolled steel pipe reaches grade 7 to 10, the elongation after fracture is ≥25%, and after quenching and tempering heat treatment, the yield strength of the steel pipe is ≥835MPa, the tensile strength is ≥980MPa, and the impact energy CVN is ≥39J.

[0053] The chemical composition of the steel pipe in this embodiment of the invention is shown in Table 1, the key process parameters in this embodiment of the invention are shown in Table 2, and the main mechanical properties in this embodiment of the invention are shown in Table 3.

[0054] Table 1. Composition (wt%) of the inner tube of the present invention in an embodiment.

[0055]

[0056] Table 2 Key process parameters of the embodiments of the present invention

[0057]

[0058] Table 3 Mechanical properties of products according to embodiments of the present invention

[0059]

[0060] Note: The banded tissue grades in Table 3 are rated according to the national standard GB / T34474.1.

[0061] Figure 1 This is a metallographic image of the microstructure of the seamless steel tube for hydraulic cylinders obtained in Embodiment 1 of the present invention. Figure 1 It is clearly demonstrated that the present invention successfully obtained a uniform, equiaxed, slightly banded ferrite + pearlite microstructure. Comparative Example 1 used the same continuously cast round billet composition as Example 1 of the present invention, but changed the key process: the billet heating temperature was 1270℃, and no controlled cooling measures were used after final rolling; the air cooling rate was approximately 0.2℃ / s. The remaining steps were the same as in Example 1. Figure 2 The image shows the microstructure of the seamless steel pipe for hydraulic cylinders obtained in Comparative Example 1, revealing a distinct ferrite-pearlite banded structure (indicated by the arrows in the image). Its elongation after fracture in the hot-rolled state is only 20%, and "bamboo-like" surface defects appeared in subsequent cold-drawing experiments.

[0062] The performance data of the embodiments shown in Table 3 strongly demonstrate the success and excellent effects of the present invention:

[0063] 1. Excellent drawing performance: The elongation after fracture (28.0%~32.5%) of all embodiments in the hot-rolled state is far higher than the target of ≥25% set by the technical solution, and significantly better than the user requirements (≥18%) mentioned in the background art. This indicates that the hot-rolled tube provided by the present invention has extremely excellent plasticity reserve, providing a solid guarantee for the subsequent cold drawing process and fundamentally avoiding the risk of drawing cracks.

[0064] 2. Successful microstructure control: Based on the revised "banded microstructure level" and "grain size level 7-10," this invention, through comprehensive process control, successfully achieved a microstructure with uniform microstructure, fine grains, and slight banded defects. This is the fundamental reason for achieving high elongation and excellent overall performance.

[0065] 3. Final performance fully meets standards with ample margin: After quenching and tempering heat treatment, the strength (yield strength ≥ 835 MPa, tensile strength ≥ 980 MPa) and impact energy (CVN ≥ 39 J) of all embodiments fully meet and generally significantly exceed the highest industry standards. This indicates that the present invention not only solves the problems of banded structure and pull-out performance, but also ensures that the finished hydraulic cylinder tube can easily meet the stringent requirements of lightweighting, large-scale production, and harsh working conditions.

[0066] In summary, this invention successfully solves the industry problem of strip-shaped structure in high-strength hydraulic cylinder tubes through the coordinated control of the entire process of "composition design-continuous casting-heating-rolling-cooling". The seamless steel pipes produced have both excellent cold drawing performance and outstanding final mechanical properties, with a high overall technical level and significant industrial application value.

[0067] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A seamless steel tube for hydraulic cylinders with excellent drawing properties, characterized in that, consists of the following chemical components by weight percentage: C: 0.24%~0.34%, Si: 1.00%~1.40%, Mn: 1.00%~1.40%, Cr: 0.20%~0.50%, B: 0.0005%~0.0015%, P≤0.015%, S≤0.010%, C+Mn≤1.57%, the rest being Fe and unavoidable impurities; And the microstructure of the seamless steel pipe in hot-rolled state is granular ferrite and pearlite, the grain size level is 7~10, and the elongation after fracture is ≥25%. After quenching and tempering heat treatment, the yield strength of the seamless steel pipe is ≥835MPa, the tensile strength is ≥980MPa, and the impact energy CVN is ≥39J.

2. The oil cylinder seamless steel pipe having excellent drawing property according to claim 1, characterized by Comprising the following steps:

3. A method of controlling banded structure of a seamless steel tube for a cylinder as claimed in claim 1 or 2, characterized in that, 1) Continuous casting round billet preparation: Carrying out molten steel smelting and vacuum treatment, wherein the holding time of the vacuum treatment is ≥8min; carrying out continuous casting on the molten steel after vacuum treatment to obtain a continuous casting round billet; 2) Tube blank heating: Heating the continuous casting round billet at 1220~1260℃, and the total heating time is 150~300min; 3) Hot rolling: Carrying out perforation, pipe rolling and sizing on the heated tube blank in sequence, and controlling the finish rolling temperature to be 870℃~940℃; 4) Cooling: Cooling the steel pipe after finish rolling at a cooling speed of 0.5~1.0℃ / s, so as to obtain a microstructure in which the ferrite is equiaxed distributed. Before the continuous casting of step 1), adopting converter smelting and sequentially refining in a LF furnace and refining in a RH to prepare the molten steel.

4. The method of controlling the banded structure of a seamless steel pipe for a cylinder with excellent drawing property according to claim 3, characterized by Before the continuous casting of step 1), adopting converter smelting and sequentially refining in a LF furnace and refining in a VD to prepare the molten steel.

5. The method of controlling the banded structure of a seamless steel pipe for a cylinder with excellent drawing property according to claim 3, characterized by The chemical components of the continuous casting round billet in step 1) are as follows by weight percentage: C: 0.24%~0.34%, Si: 1.00%~1.40%, Mn: 1.00%~1.40%, Cr: 0.20%~0.50%, B: 0.0005%~0.0015%, P≤0.015%, S≤0.010%, C+Mn≤1.57%, the rest being Fe and unavoidable impurities.

6. The method of controlling the banded structure of a seamless steel pipe for a cylinder with excellent drawing properties according to claim 3, characterized by In step 1), the superheat of the molten steel is controlled to be 15~25℃, the breakout speed is controlled to be 1.0~1.3m / min, and the current of the end electromagnetic stirring is 100~250A.

7. The method of controlling a banded structure of a seamless steel pipe for a cylinder with excellent drawing property according to claim 3, characterized by In step 2), the tube blank heating is carried out in a ring furnace.

8. The method of controlling a banded structure of a seamless steel pipe for a cylinder with excellent drawing property according to claim 3, characterized by In step 4), the cooling speed is controlled to be 0.5~0.8℃ / s.

9. The method of controlling a banded structure of a seamless steel pipe for a cylinder with excellent drawing property according to claim 3, characterized by ​

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