Die and process for preventing large-diameter steel pipe from reducing and empty drawing

Abstract

The mold comprises an outer mold and an inner mold, the outer mold is provided with an outer bevel angle, a slope surface of an inner bevel angle, the inner bevel angle and an outer mold sizing belt, a fillet II is arranged between the slope surface of the outer bevel angle and the slope surface of the inner bevel angle, and a fillet I is arranged between the inner bevel angle and the outer mold sizing belt; the outer die is provided with a chamfer, an inner die inlet slope and an inner die sizing belt length; the length of the outer die sizing belt and the length of the inner die sizing belt are overlapped, and the tail of the outer die sizing belt exceeds the tail of the length of the inner die sizing belt and has indentation. The invention mainly aims at a cold-drawing hole shrinkage process and is used for producing a high-quality large-diameter hydraulic cylinder steel pipe, the inner die is set to be a straight die, and the relative position of the inner die and the outer die is better controlled through asymmetric die design and die position control, so that the problems of empty drawing, out-of-tolerance of roundness and non-uniform wall thickness during single-pass large hole shrinkage of the large-diameter steel pipe are solved; the cold-drawing forming of the next pass under large hole shrinkage is realized, the problem of empty drawing is avoided, and the operability of production is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of hydraulic equipment technology, specifically a mold and process for preventing the empty pulling of large-diameter steel pipes due to diameter reduction. Background Technology

[0002] Hydraulic cylinder steel pipes are one of the commonly used high-precision structures in engineering. To achieve characteristics such as smooth inner and outer surfaces, good straightness, high roundness, and high strength, the production of hydraulic cylinder steel pipes currently mostly adopts a drawing method with a mandrel. Because the production of large-diameter hydraulic cylinder steel pipes requires very large drawing forces, to reduce these forces, a fixed short mandrel drawing process is often used. This involves using a short-sized inner mold, which is fixed by a mandrel.

[0003] Traditional processes consider the ideal die design to be when the inlet slopes of the inner and outer dies are the same, and the sizing bands of the inner and outer dies are the same length (i.e., the inner and outer dies are symmetrical along the pipe wall thickness midline). However, in actual production, it has been found that the drawing process using the traditional die design is unstable, making it difficult to produce products with ideal quality. For hydraulic cylinder steel pipes with small diameters and small diameter-to-thickness ratios, after using traditional cold drawing dies and processes, further straightening and cutting treatments are usually required to obtain products that meet quality requirements. However, for large-diameter hydraulic cylinder steel pipes, especially those with a diameter-to-thickness ratio (D / S) ≥ 22, the use of traditional drawing dies and processes often leads to various process and product quality problems. For example, if the outer diameter reduction in a single pass is greater than 10 mm, it is easy to draw empty, requiring secondary straightening, phosphating, and saponification before drawing again. This not only affects production efficiency but also causes uncontrollable changes in wall thickness and a decrease in inner surface quality, dimensional accuracy, and straightness due to empty drawing. This results in severe uneven deformation of the inner and outer surfaces of the steel pipe during the second drawing process, excessive residual stress inside the pipe, and poor straightness and roundness. Even with subsequent straightening and cutting processes, it is difficult to obtain the final ideal product.

[0004] In actual production, it has been found that many factors affect the quality of cold drawing, including the entry slope of the inner and outer dies, the length of the sizing zone, and the relative position of the inner and outer dies. These factors are also interconnected, and their effects vary depending on the cold drawing process. Accurately grasping the relationship between die dimensions, production process, and product quality is quite difficult in actual production.

[0005] Meanwhile, the relative position of the inner and outer molds has a significant impact on the forming result; even slight changes in their relative position can cause substantial stress and deformation. Mandrel deformation, equipment tolerances, and equipment clearances can all lead to changes in the relative position of the inner and outer molds. Therefore, it is difficult to accurately control the relative position of the inner and outer molds in actual production. According to feedback from on-site testing, current production equipment can only guarantee a relative positional error of approximately 3–5 mm between the inner and outer molds. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a process and method for coordinating internal and external mold positioning, which can prevent the mold and process of large-diameter steel pipe from being pulled out in a reduced diameter.

[0007] This invention is achieved through the following technical solution: a mold for preventing the shrinkage and empty pulling of large-diameter steel pipes, comprising an outer mold and an inner mold. The outer mold is provided with an outer bevel, an inner bevel, an inner bevel, and an outer mold sizing band in sequence from the feeding direction. A fillet II is provided between the outer bevel and the inner bevel, and a fillet I is provided between the inner bevel and the outer mold sizing band. The outer mold is provided with a chamfer, an inner mold inlet slope, and an inner mold sizing band length in sequence from the feeding direction. The outer mold sizing band and the inner mold sizing band have an overlapping portion, and the tail of the outer mold sizing band extends beyond the tail of the inner mold sizing band and has a retraction amount.

[0008] Furthermore, the outer oblique angle is a cone angle of 15°-20°.

[0009] The inner oblique angle is 5°-6°.

[0010] The length of the outer mold sizing belt is 40mm-70mm.

[0011] The radius of fillet I is not less than 15mm, and the radius of fillet II is not less than 15mm.

[0012] The chamfer R1 of the inner mold inlet is ≤3mm.

[0013] The inlet slope of the inner mold is 0°-1°.

[0014] The indentation is ≥2mm, and the overlap is ≥40mm.

[0015] A process for using a mold to prevent large-diameter steel pipes from being drawn without diameter reduction involves adjusting the position of the inner mold during actual production based on the elongation of the mandrel during the drawing process. The formula for the elongation of the mandrel is as follows: l = (σ / E)L Where σ=f / S, f=F / 2, S=3.14 / 4, σ is the cross-sectional stress, E is the elastic modulus, L is the length of the core rod, f is the tensile force of the core rod, S is the cross-sectional area of ​​the core rod, F is the pull-out force, D is the outer diameter of the core rod, and d is the inner diameter of the core rod.

[0016] The present invention has the following advantages: The mold and process for preventing empty drawing of large-diameter steel pipes during diameter reduction are mainly aimed at the cold drawing diameter reduction process and are used to produce high-quality large-diameter hydraulic cylinder steel pipes. By setting the inner mold as a straight mold and using asymmetric mold design and mold position control, the relative positions of the inner and outer molds are better controlled, which solves the problems of empty drawing, out-of-roundness deviation and uneven wall thickness when large-diameter steel pipes are reduced in a single pass. It enables cold drawing forming in the next pass after large diameter reduction without empty drawing problems, which greatly improves the operability of production. Attached Figure Description

[0017] The accompanying drawings, as part of this invention, are provided to further illustrate the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation thereof. Clearly, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0018] In the attached diagram: Figure 1 This is a schematic diagram of the structure of the inner and outer molds of the present invention; Figure 2 This is a schematic diagram showing the uneven deformation of the material mesh lines during cold drawing with a traditional mold in the process of diameter reduction; Figure 3 This is a schematic diagram showing the uniform deformation of the material mesh lines during cold drawing with the mold of this invention.

[0019] In the diagram: W1, outer bevel angle; W2, inner bevel angle; W3, slope of the inner bevel angle; W4, outer mold sizing zone; W5, fillet I; W6, fillet II; N1, inner mold inlet slope; N4, length of inner mold sizing zone; A, indentation amount; B, overlapping portion.

[0020] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0022] In the description of this invention, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0024] like Figure 1 The mold shown is for preventing the shrinkage and empty pulling of large-diameter steel pipes. It includes an outer mold and an inner mold. The outer mold is provided with an outer bevel angle W1, an inner bevel angle slope W3, an inner bevel angle W2, and an outer mold sizing band W4 in sequence from the feeding direction. A fillet II W6 is provided between the outer bevel angle W1 and the inner bevel angle slope W3, and a fillet I W5 is provided between the inner bevel angle W2 and the outer mold sizing band W4. The outer mold is provided with a chamfer, an inner mold inlet slope N1, and an inner mold sizing band length N4 in sequence from the feeding direction. The outer mold sizing band W4 and the inner mold sizing band length N4 have an overlapping portion B, and the tail of the outer mold sizing band W4 extends beyond the tail of the inner mold sizing band length N4 and has a retraction amount A. The mold for preventing large-diameter steel pipe shrinkage during empty drawing of the present invention has an outer mold and an inner mold. The outer mold has six features in sequence from the feeding direction: an outer bevel angle W1, an inner bevel angle W2, a slope surface of the inner bevel angle W3, an outer mold sizing zone W4, a fillet I W5, and a fillet II W6. Among them, the outer mold has a large outer bevel angle. Due to the limitations of large-diameter hot rolling mills, large shrinkage is usually required to obtain the required product specifications. In order to reduce the number of cold drawing passes, reduce production costs, and improve production efficiency, the outer mold is designed with a large cone angle at the inlet to facilitate pipe extrusion. In addition, by adjusting the outer mold inlet angle, the mold inlet state will not change due to misalignment of the inner and outer molds in the drawing direction, which greatly improves the operability of production. At the same time, the inner mold has three structural features in sequence from the feeding direction: chamfer, inner mold inlet slope N1, and inner mold sizing zone length N4. The inner mold is a straight mold. Since shrinkage drawing is prone to empty drawing, the inner mold sizing front end is designed with a small chamfer and a large rounded corner W6 working together to ensure that the inner diameter of the steel pipe and the inner mold are engaged during shrinkage, and there is no empty drawing phenomenon during shrinkage drawing, which ensures normal production efficiency.

[0025] like Figure 1The mold shown is for preventing the empty pulling of large-diameter steel pipes with reduced diameter. The outer mold of this invention mainly has five shape features: 1. The outer bevel angle W1 is a cone angle of 15°-20°. The outer bevel angle W1 of this invention is relatively large, with a designed inlet cone angle of 15-20°, mainly used before drawing to facilitate steel pipe alignment and insertion into the die. Due to the limitations of large-diameter hot-rolled pipe mills, large diameter reduction is usually required. To reduce the number of cold drawing passes, lower production costs, and improve production efficiency, a large inlet angle facilitates pipe extrusion.

[0026] 2. The inner bevel angle W2 is 5°-6°. The inner bevel angle W2 is set to 5°-6°, mainly to achieve uniform extrusion deformation of the steel pipe.

[0027] 3. The slope length W3 of the inner bevel should be long enough to ensure that the steel pipe will not come into contact with the inlet bevel W1 during stable drawing. W3 should not be too long either, as an excessively long W3 will increase the manufacturing cost of the outer mold.

[0028] 4. The length of the outer mold sizing strip W4 is 40mm-70mm. The outer mold sizing strip W4 is mainly used to correct the final outer diameter of the product. A sizing strip that is too short will not achieve the desired size correction effect, while a sizing strip that is too long will result in excessive pulling force. The ideal length of the outer mold sizing strip designed in this invention is 40mm-70mm.

[0029] 5. The radius of fillet I W5 shall not be less than 15mm, and the radius of fillet II W6 shall not be less than 15mm. The inner bevel angle W2 and the sizing band W4 should have a large fillet W5 transition, with the radius of W5 not less than 15mm. The outer bevel angle W1 and the inner bevel angle W2 should have a large fillet W6 transition, with the radius of W6 not less than 15mm. When the fillet radius is less than 15mm, finite element simulation shows a sharp increase of 23% in the surface stress of the pipe.

[0030] like Figure 1 The mold shown is for preventing the empty pulling of large-diameter steel pipes with reduced diameter. The inner mold of this invention mainly has three shape features: 1. The chamfer R1 at the inner mold inlet shall be ≤3mm. Otherwise, the inner mold may be unable to close properly under force during mold closing, and can only be pulled out empty.

[0031] 2. The inner mold inlet ramp N1 is 0°-1°. The inner mold inlet is designed with a ramp N1 of 0°-1°, which is mainly used to facilitate the alignment and pushing of the steel pipe into the mold before drawing.

[0032] 3. The length N4 of the inner die sizing band should be long enough (for a straight inner die) to ensure that the steel pipe does not come into contact with the inner die inlet slope N1 during a stable drawing process. That is, before the steel pipe comes into contact with the outer die, N4 and the steel pipe should have a certain overlap length.

[0033] like Figure 1 The diagram illustrates a mold for preventing the empty drawing of large-diameter steel pipes with reduced diameter. The reduction amount A is ≥ 2mm, and the overlap portion B is ≥ 40mm. In actual production, at the mold exit position, the inner mold should always have a certain reduction amount A relative to the outer mold. This ensures better roundness of the product; the ideal reduction amount is A ≥ 2mm. After determining the value of A, the overlap portion B of the sizing bands N4 and W4 of the inner and outer molds must not be too short; the ideal B value is ≥ 40mm.

[0034] A process for using a die to prevent large-diameter steel pipe from shrinking during empty drawing. The relative positions of the inner and outer dies refer to the relative positions of the pipe during the drawing process. In actual production, attention should be paid to the influence of factors such as the deformation of the inner die core rod itself and the bending of the pipe tail due to its own weight on the change of the die position. Based on the actual drawing force, the theoretical elongation of the core rod during the drawing process is calculated using empirical formulas. The position of the inner die is adjusted according to the elongation of the core rod during the drawing process. The formula for the elongation of the core rod is as follows: l = (σ / EL) Where σ=f / S, f=F / 2, S=3.14D²-d² / 4, σ is the cross-sectional stress, E is the elastic modulus, L is the length of the core rod, f is the tension of the core rod, S is the cross-sectional area of ​​the core rod, F is the pull-out force, D is the outer diameter of the core rod, and d is the inner diameter of the core rod. Example

[0035] The cold-drawn masterbatch specification is φ273×16, and the finished cold-drawn tube specification is φ264×φ240 (-1.0-1.5), with a diameter reduction rate of 2.2%. Traditionally, symmetrical molds are used with an entry angle of 6°, resulting in an average roundness of 0.19mm after cold drawing. This invention employs external mold parameters W1=15°, W2=5°, W4=40mm, and fillet radius W5=W6=15mm; internal mold parameters: R1=2mm, N1=0°, and straight internal mold N4=140mm; the relative position of the internal and external molds is A=2mm. The average roundness after cold drawing is 0.14mm, a reduction of 26.3%. The small chamfer R1=2mm achieves proper engagement, eliminating the problem of abnormal empty drawing, avoiding wall thickness fluctuations caused by empty drawing, and ensuring normal production efficiency. Example

[0036] The cold-drawn masterbatch specification is φ290×15, and the finished cold-drawn tube specification is φ275×φ255 (-1.0 -1.5), with a diameter reduction rate of 6% and an outer diameter reduction of 15mm. Previously, this required two cold-drawing passes. The new invention uses the following external die parameters: W1=20°, W2=5°, W4=60mm, and corner radius W5=W6=15mm; internal die parameters: R1=3mm, N1=0°, and straight inner die N4=140mm; relative position of the inner and outer dies: A=5mm. This allows for single-pass cold drawing without any empty drawing issues, achieving an average roundness of 0.25mm after cold drawing. It eliminates the need for inter-pass straightening and phosphating / saponification processes, improving cold-drawing production efficiency by over 50%. When R1=5mm, empty drawing occurs during the first pass, requiring a second straightening before further drawing. This empty drawing results in uneven wall thickness, causing the final average roundness to deteriorate to 0.4mm, failing to meet process requirements.

[0037] For cold-drawn tubes with an inner diameter φ≤180mm, the recommended inner die inlet chamfer R1=1mm; for 180<φ≤250mm, the recommended inner die inlet chamfer R1=2mm; for 250<φ≤320mm, the recommended inner die inlet chamfer R1=3mm. From cold-drawn masterbatch to finished cold-drawn tube specifications, for outer diameter reduction ≤10mm, the recommended outer die inlet bevel angle W1=15°-17°; for 10<outer diameter reduction ≤15mm, the recommended outer die inlet bevel angle W1=18°-20°.

[0038] By adopting the mold shape and production process requirements of this invention, the produced hydraulic cylinder steel pipes exhibit uniform deformation, good straightness and roundness, and require less post-processing work, thus greatly improving product quality and reducing product production cycle and cost. Figure 2 and Figure 3 The figures show the changes in the material grid lines of the steel pipe during the drawing process when using a traditional mold and an optimized mold, respectively. It can be seen that when the mold shape designed in this invention is used for drawing, the changes in the material grid lines are more uniform.

[0039] The mold and process for preventing empty drawing of large-diameter steel pipes with reduced diameter in this invention have an optimal design scheme. Through repeated calculations, induction, comparison and verification using finite element simulation technology, the influence of various dimensional parameters on product quality was summarized. Finally, a relatively ideal internal and external mold collaborative positioning process and mold design method were derived. The optimal values ​​for each mold dimension and precautions during cold drawing are comprehensively listed. The core is asymmetric mold design + mold position control, achieving cold drawing forming in a single pass with large diameter reduction without empty drawing problems. It is specifically used for producing high-quality large-diameter hydraulic cylinder steel pipes, solving the problems of empty drawing, out-of-roundness deviation, and uneven wall thickness when producing large-diameter steel pipes (D / S≥22) with single-pass large diameter reduction (reduction rate>5%). Specifically, it has the following advantages: 1. Straight inner mold design: The inner mold is a cylinder with a constant diameter throughout (no taper in section N4), which eliminates the risk of mold misalignment, ensures the stability of the inner and outer mold positioning, and facilitates dynamic positioning control.

[0040] 2. Inner mold inlet constraint structure: The small chamfer design R1≤3mm constrains the inner wall of the steel pipe during mold closing, preventing the inner mold from retracting and failing to engage, thus avoiding stress concentration caused by traditional stepped structures. The inlet ramp N1=0°-1° is a near-horizontal design, only assisting in centering.

[0041] 3. External mold dual-angle collaborative structure: The inlet cone angle W1 = 15°-20°, the relatively large inlet cone angle is conducive to the alignment and biting of the steel pipe when the diameter is large; the transition cone angle W2 = 5°-6°, the small angle realizes uniform extrusion deformation of the steel pipe. The two angles are transitioned by a large radius of R≥15mm to prevent stress concentration.

[0042] 4. Relative position parameters of inner and outer molds: Diameter reduction A ≥ 2mm to ensure the roundness of the steel pipe exit after drawing; sizing zone overlap B ≥ 40mm to maintain the stability of the deformation zone. The synergistic relationship between A and B: When A increases, the length of N4 needs to be increased simultaneously to ensure B ≥ 40mm.

[0043] 5. Core rod deformation compensation mechanism: An empirical formula for the elongation of the core rod during the drawing process is used to pre-adjust the initial position of the inner mold based on the empirical formula, providing a theoretical basis for adjusting the drawing parameters and offsetting the positioning offset caused by the elongation of the core rod during the drawing process.

[0044] Numerous specific details are set forth in the specification provided herein. However, it will be understood that embodiments of the invention may be practiced without these specific details. In some instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.

[0045] Furthermore, those skilled in the art will understand that although some embodiments described herein include certain features found in other embodiments but not others, combinations of features from different embodiments are also within the scope of protection of this invention and form different embodiments. For example, in the embodiments described above, those skilled in the art can use them in combination based on known technical solutions and the technical problems to be solved by this application.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A mold for preventing the empty pulling of large-diameter steel pipes with reduced diameter, characterized in that: The device includes an outer mold and an inner mold. The outer mold is provided with an outer bevel (W1), an inner bevel slope (W3), an inner bevel (W2), and an outer mold sizing band (W4) in sequence from the feeding direction. A fillet II (W6) is provided between the outer bevel (W1) and the inner bevel slope (W3), and a fillet I (W5) is provided between the inner bevel (W2) and the outer mold sizing band (W4). The outer mold is provided with a chamfer, an inner mold inlet slope (N1), and an inner mold sizing band length (N4) in sequence from the feeding direction. The outer mold sizing band (W4) and the inner mold sizing band length (N4) have an overlapping portion (B). The tail of the outer mold sizing band (W4) extends beyond the tail of the inner mold sizing band length (N4) and has a retraction amount (A).

2. The mold for preventing large-diameter steel pipe from being pulled out during diameter reduction as described in claim 1, characterized in that: The external oblique angle (W1) is a cone angle of 15°-20°.

3. The mold for preventing large-diameter steel pipe from being pulled out during diameter reduction as described in claim 1, characterized in that: The inner oblique angle (W2) is 5°-6°.

4. The mold for preventing large-diameter steel pipe from being pulled out during diameter reduction as described in claim 1, characterized in that: The length of the outer mold sizing belt (W4) is 40mm-70mm.

5. The mold for preventing large-diameter steel pipe from being pulled out during diameter reduction as described in claim 1, characterized in that: The radius of fillet I (W5) is not less than 15mm, and the radius of fillet II (W6) is not less than 15mm.

6. The mold for preventing large-diameter steel pipe from being pulled out during diameter reduction as described in claim 1, characterized in that: The chamfer R1 of the inner mold inlet is ≤3mm.

7. The mold for preventing large-diameter steel pipe from being pulled out during diameter reduction as described in claim 1, characterized in that: The inlet ramp (N1) of the inner mold is 0°-1°.

8. The mold for preventing large-diameter steel pipe from being pulled out during diameter reduction as described in claim 1, characterized in that: The indentation (A) is ≥2mm, and the value of the overlapping portion (B) is ≥40mm.

9. A process for using the mold for preventing large-diameter steel pipe shrinkage during empty pulling as described in claim 1, characterized in that: In actual production, the position of the inner mold is adjusted according to the elongation of the mandrel during the drawing process. The formula for the elongation of the mandrel is as follows: l = (σ / E)L Where σ=f / S, f=F / 2, S=3.14(D²-d²) / 4, σ is the cross-sectional stress, E is the elastic modulus, L is the length of the core rod, f is the tensile force of the core rod, S is the cross-sectional area of ​​the core rod, F is the pull-out force, D is the outer diameter of the core rod, and d is the inner diameter of the core rod.

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