Method for controlling straightness of ends of hot-rolled seamless steel pipes

Abstract

The present application belongs to the technical field of hot rolling manufacturing of seamless steel pipes, in particular to a method for controlling straightness of pipe ends of hot-rolled seamless steel pipes. The method is characterized in that a longitudinal straightening rack is connected in series after the working rack and the finished product rack of the stretch-reducing machine, and the longitudinal straightening rack is arranged closely with the last finished product rack; the rotational speed of the longitudinal straightening rack is set to be 1%~10% higher than that of the last finished product rack; the rack adopts an elliptical pass, the length of the major axis is 0.2~0.5mm larger than that of the minor axis, and the length of the minor axis is equal to that of the last finished product rack. Through the synergistic effect of the rack layout, micro-tension control and specific elliptical pass, the present application realizes online hot straightening of the pipe ends, the straightness of the pipe ends can be stably controlled to be ≤1.8mm / 1.5m, the offline secondary straightening process is completely eliminated, the production cost is reduced, and the product quality and the subsequent processing efficiency of the customers are significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of hot rolling production technology of seamless steel pipes, and in particular, it is a method for controlling the straightness of the pipe ends of hot-rolled seamless steel pipes. Background Technology

[0002] The tension reducing mill is the core finishing rolling equipment in a hot-rolled seamless steel pipe production line. It employs a multi-stand continuous mandrel-less rolling process, relying on the speed difference between stands to establish stable axial tension, simultaneously completing the pipe diameter reduction and wall reduction forming. It is currently the most advanced technology available. This is a key core equipment for the large-scale, high-efficiency production of small-diameter seamless steel pipes below 114.3mm. This process boasts significant advantages such as high production efficiency, strong diameter reduction capability, and a wide range of product specifications. It features a large rolling elongation coefficient, enabling continuous rolling of ultra-long steel pipes and effectively increasing the overall production capacity of the rolling mill. Simultaneously, the equipment has a large total diameter reduction rate, allowing a single specification of rough pipe to be adapted to the production of various small-diameter finished pipes. This simplifies the production process, reduces the need for tooling and spare parts, and significantly improves economic efficiency compared to sizing rolling processes. It is the mainstream process equipment for the mass production of small-diameter hot-rolled seamless steel pipes.

[0003] In actual production processes, tension reducing machines are mostly used for... The hot rolling production of seamless steel pipes with a diameter of 114.3mm or less places extremely high demands on the precision of the tension reducing mill stand, the transmission system, the stand assembly, and the roll profile turning. Small-diameter seamless steel pipes commonly exhibit poor uniformity of end wall thickness, uneven temperature drop at the pipe ends, and unstable end shapes, leading to high sensitivity to end deformation. During the unsteady rolling stage at the pipe end, axial tension cannot be fully established, and the pipe is subjected to asymmetrical stress. Combined with minor stand assembly deviations, equipment precision deviations, and uneven circumferential cooling during hot rolling, this easily triggers end bending deformation, resulting in quality problems such as unstable straightness and out-of-tolerance dimensions. (See attached diagram) Figure 1 .

[0004] While pipe body bending can be corrected using a straightening machine, pipe end bending is much more difficult to improve due to the blind spots in the straightening machine. Pipe end straightness is a core indicator of customer concern, directly impacting the production efficiency of subsequent machining processes such as threading, upsetting, welding, and cold drawing, and increasingly influencing customer purchasing intentions. According to the API SPEC 5CT standard, pipe end straightness should be ≤3.18mm / 1.5m. In reality, customer orders typically require ≤2.54mm / 1.5m for ordinary steel pipes, while special products, such as those for customer-processed steel pipes, usually require ≤1.8mm / 1.5m or even stricter requirements. Currently, pipe end straightness has become a key indicator for evaluating the product quality of seamless steel pipe manufacturers. Only by achieving high-precision and high-stability control of pipe end straightness can companies win customer trust and recognition, and solidify long-term cooperative relationships.

[0005] Because the billet length for seamless steel pipes is usually optimally calculated based on the finished pipe length before rolling, regardless of fluctuations in pipe end straightness, process engineers, in order to ensure yield and control production costs, typically only consider whether the pipe end straightness meets the order agreement, neglecting the customer's experience. For steel pipes whose end straightness does not meet the order agreement after cutting, process engineers organize them to be removed from the production line, and then re-straightened during the hot rolling line's production gap. This results in increased rework costs, reduced production efficiency, and delayed contract fulfillment. How to consistently achieve qualified pipe end straightness control for hot-rolled seamless steel pipes is currently a key challenge. Technical challenges that urgently need to be addressed in the production of small-diameter hot-rolled seamless steel pipes with a diameter of 114.3mm or less. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention provides a method for controlling the straightness of hot-rolled seamless steel pipe ends. By innovating the tension reducing mill stand combination and formulating special rolling process parameters, it effectively improves the unsteady rolling process at the pipe ends, alleviates the production defects of unstable pipe end curvature, accurately controls the forming quality of pipe ends, ensures the stable and qualified straightness index of hot-rolled seamless steel pipe ends, and improves product quality and production efficiency.

[0007] outer diameter The main rolling deformation process for small-diameter hot-rolled seamless steel pipes below 114.3mm is: piercing → rolling → tension reduction. The purpose of tension reduction is to obtain finished steel pipes with qualified outer diameter geometric dimensions. The processing stand for tension reduction is divided into a working stand (WS) and a finishing stand (FS). The working stand (WS) mainly performs the significant reduction of the outer diameter geometric dimensions of the steel pipe, while the finishing stand (FS) mainly ensures the final accuracy of the outer diameter dimensions. The number of finishing stands (FS) is generally 3-4, and its design concept is to "gradually reduce the reduction rate and ellipticity of a single stand in each stand to achieve the goal of gradually rounding the steel pipe."

[0008] The technical solution of this invention patent is:

[0009] Following the existing specification frame sequence: working frame WS + finished product frame FS, a longitudinal straightening frame SS is connected in series to control the straightness index of the pipe end.

[0010] A special-hole-shaped "longitudinal straightening frame SS" is connected in series behind the finished product frame FS, forming a new process mode layout of "working frame WS + finished product frame FS + longitudinal straightening frame SS". The longitudinal straightening frame SS and the finished product frame FS are arranged closely together.

[0011] Set the rotational speed V of the "longitudinal straightening frame SS" SS Compared to the speed V of the last finished rack FS 1-10% higher, i.e., V SS =(1+A%)V FS , 1%≤A%≤10%.

[0012] The added longitudinal straightening frame SS is selected with an elliptical bore, and the b value of the straightening frame SS is b. SS Take the b value of the last rack in the finished rack FS. FS That is, b SS =b FS The a value of the straightening frame SS SS Compared to b SS The value is 0.2~0.5mm larger, i.e., a SS =b SS +η, 0.2mm≤η≤0.5mm.

[0013] Furthermore, the number of work frames is 10 to 21, and the number of finished product frames is 3 or 4.

[0014] Furthermore, the specific value of A% is 5%.

[0015] Furthermore, the specific value of η is 0.2mm, 0.3mm, or 0.5mm.

[0016] Furthermore, the method is used to produce products with an outer diameter ≤ 114.3mm small diameter seamless steel pipe.

[0017] Advantages and beneficial effects of the present invention:

[0018] The technical solution of this invention provides excellent control over the straightness of pipe ends when producing small-diameter seamless steel pipes, consistently ensuring a straightness of ≤1.8mm / 1.5m. This straightness accuracy far exceeds the API standard, completely eliminating the efficiency and cost losses associated with secondary straightening in the manufacturing plant caused by excessive pipe end straightness. Attached Figure Description

[0019] Figure 1 This is a diagram showing the bending of the end of a conventional hot-rolled small-diameter seamless steel pipe.

[0020] Figure 2 This is a layout diagram of a tension reduction mill for conventional hot-rolled small-diameter seamless steel pipes, where WS represents the working mill stand and FS represents the finishing mill stand.

[0021] Figure 3 This is a layout diagram of the tension reduction mill for hot-rolling small-diameter seamless steel pipes using this method, where: WS is the working mill stand; FS is the finishing mill stand; and SS is the straightening mill stand.

[0022] Figure 4 A diagram showing the bending of the end of a hot-rolled small-diameter seamless steel pipe using this method;

[0023] Figure 5 This is a schematic diagram of a tension reduction bore, where: a is the long axis of the bore; b is the short axis of the bore.

[0024] Figure 6 This is a diagram showing the outward bending and wire drawing at the pipe end during the production of small-diameter seamless steel pipes, as shown in Comparative Example 1.

[0025] Figure 7 This is a linear outward folding diagram of the entire surface of the steel pipe that appears during the production of small-diameter seamless steel pipes, as shown in Comparative Example 3. Detailed Implementation

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0027] It should be noted that the longitudinal straightening frame (SS) is a conventional tension reducing machine frame structure in the art. Its internal structure (such as the arch, drive shaft, bearings, etc.) is not an improvement of this invention. Those skilled in the art can choose any known mature tension reducing machine frame to construct it. This invention endows this conventional frame with new straightening functions through a specific combination of frame arrangement sequence, speed difference, and bore size.

[0028] Example 1

[0029] A seamless steel pipe hot rolling production line, hot rolling production 60x4mm small-diameter seamless steel pipe, with accessories Figure 2 The conventional tensioning machine layout shown is 17 working frames (WS) + 4 finishing frames (FS). During production, the steel pipe ends bend, and the straightness exceeds the standard. Figure 1 As shown.

[0030] To improve the pipe end curvature and ensure stable control of pipe end straightness, the method provided in this patented invention is used in production. This involves connecting a longitudinal straightening frame SS in series after the existing frame sequence: working frame WS + finished product frame FS, to control the pipe end straightness index. Specifically, this involves three steps:

[0031] 1. A special-shaped longitudinal straightening frame SS is connected in series behind the finished product frame FS, forming a new process layout of "17 working frames WS + 4 finished product frames FS + 1 longitudinal straightening frame SS". The longitudinal straightening frame SS and the finished product frame FS are arranged closely together, such as... Figure 3 As shown;

[0032] 2. Set the rotational speed V of the "longitudinal straightening frame SS". SS Compared to the speed V of the last finished rack FS 5% higher, i.e., V SS =(1+5%)V FS The rotational speed V of the last finished rack FS =400rpm, calculate V SS =420rpm;

[0033] 3. The added longitudinal straightening frame SS is selected with an elliptical bore type, and the b value of the straightening frame SS is b. SS Take the b value of the last rack in the finished rack FS. FS That is, b SS =b FS =60.3mm, value a SS Compared to b SS The value is greater than 0.3, that is, a SS =b SS +0.3=60.3+0.3=60.6mm;

[0034] This technical solution is used in production The 60x4mm small-diameter seamless steel pipe product exhibits good control over pipe end straightness, consistently ensuring a straightness of ≤1.8mm / 1.5m. (See attached image) Figure 4 .

[0035] Example 2

[0036] A seamless steel pipe hot rolling production line, hot rolling production The 57x4mm small-diameter seamless steel pipes were produced using a conventional tension reduction machine setup, consisting of 18 working frames (WS) and 3 finished product frames (FS). During production, the pipe ends bent, resulting in straightness exceeding the standard.

[0037] To improve the pipe end curvature and ensure stable control of pipe end straightness, the method provided in this patented invention is used in production. This involves connecting a longitudinal straightening frame SS in series after the existing frame sequence: working frame WS + finished product frame FS, to control the pipe end straightness index. Specifically, this involves three steps:

[0038] 1. A special-hole-shaped "longitudinal straightening frame SS" is connected in series behind the finished product frame FS to form a new process mode layout of "18 working frames WS + 3 finished product frames FS + 1 longitudinal straightening frame SS". The longitudinal straightening frame SS and the finished product frame FS are arranged closely together.

[0039] 2. Set the rotational speed V of the "longitudinal straightening frame SS". SS Compared to the speed V of the last finished rack FS 5% higher, i.e., V SS =(1+5%)V FS The rotational speed V of the last finished rack FS =402rpm, calculate V SS =422rpm;

[0040] 3. The added longitudinal straightening frame SS is selected with an elliptical bore type, and the b value of the straightening frame SS is b. SS Take the b value of the last rack in the finished rack FS. FS That is, b SS =b FS =57.3mm, value a SS Compared to b SS The value is greater than 0.2, that is, a SS =b SS +0.2=57.3 +0.2=57.5mm;

[0041] This technical solution is used in production The 57x3.5mm small-diameter seamless steel pipe has good control over the straightness of the pipe ends, and can stably guarantee that the straightness of the pipe ends is ≤1.8mm / 1.5m.

[0042] Comparative Example 1

[0043] The method described in Example 1 is adopted, wherein there is a 2-space gap between the longitudinal straightening frame SS and the finished product frame.

[0044] Hot rolling was performed using the process described in Comparative Example 1. Compared to Example 1, the 60x4mm small-diameter seamless steel pipe exhibits wire-like outward bending at the pipe end during production. Figure 6 As shown.

[0045] Comparative Example 2

[0046] Using the method described in Example 1, the rotational speed V of the longitudinal straightening frame SS is... SS With the rotational speed V of the last finished rack FS equal.

[0047] Hot rolling was performed using the process described in Comparative Example 2. Compared to Example 1, the 60x4mm small-diameter seamless steel pipe showed no effect on improving the pipe end curvature during production; the pipe end curvature remained unchanged. Figure 1 As shown.

[0048] Comparative Example 3

[0049] Using the method described in Example 1, the rotational speed V of the longitudinal straightening frame SS is... SS Compared to the speed V of the last finished rack FS 20% higher, i.e., V SS =(1+20%)V FS =300rpm.

[0050] Hot rolling was performed using process 3 of Comparative Example. Compared to Example 1, the 60x4mm small-diameter seamless steel pipe produced has a linear outward fold throughout its entire surface, as shown in Example 2. Figure 7 As shown.

[0051] Comparative Example 4

[0052] Using the method of Example 1, the longitudinal straightening frame SS is selected as a circular hole type, that is, the b value b of the straightening frame SS. SS Take the b value of the last rack in the finished rack FS. FS That is, b SS =b FS =60.5mm, value a SS With b SS The values ​​are equal, that is, a SS =b SS =60.5mm.

[0053] Hot rolling was performed using process 4 (Comparative Example 4). Compared with Example 1, the outer diameter of the 60x4mm small-diameter seamless steel pipe produced was 0.2mm smaller, resulting in the outer diameter geometric dimension exceeding the standard and failing to meet the requirements.

[0054] Effect evaluation

[0055] The test subject was hot-rolled small-diameter seamless steel pipes. The production process was carried out using the example and comparative processes, and the comparison results of key process indicators are shown in Table 1.

[0056] Table 1

[0057]

[0058] Comparing the embodiments and comparative examples, the embodiments of small-diameter seamless steel pipes produced by hot rolling using the method of the present invention are as follows: Compared with comparative example 1, the quality of the pipe end of the finished steel pipe is good, with no wire drawing or external bending; compared with comparative example 2, the straightness of the pipe end is stable and qualified; compared with comparative example 3, the quality of the pipe end and the quality of the pipe body are good, with no linear external bending throughout the surface; compared with comparative example 4, the outer diameter geometric dimensions are qualified and do not exceed the lower limit of the standard.

[0059] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the inventive concept, and these all fall within the protection scope of the present invention.

Claims

1. A method for controlling the straightness of the pipe ends of a hot-rolled seamless steel pipe, characterized in that, Includes the following steps: A longitudinal straightening frame is connected in series behind the working frame and the finished product frame of the tension reducing mill, forming a process layout of working frame, finished product frame and longitudinal straightening frame, and the longitudinal straightening frame is closely arranged with the last finished product frame. Set the rotational speed V of the longitudinal straightening frame. SS Compared to the speed V of the last finished rack FS 1-10% higher, V SS =(1+A%)V FS , 1%≤A%≤10%; The longitudinal straightening frame adopts an elliptical bore type, with a minor axis dimension b. SS Equal to the short axis dimension b of the last finished frame FS That is, b SS =b FS Its major axis dimension a SS Compared to b SS Value is 0.2~0.5mm larger, a SS =b SS +η, 0.2mm≤η≤0.5mm.

2. The method for controlling the straightness of the pipe ends of hot-rolled seamless steel pipes according to claim 1, characterized in that, The number of working frames is 10 to 21, and the number of finished product frames is 3 or 4.

3. The method for controlling the straightness of the pipe ends of hot-rolled seamless steel pipes according to claim 1, characterized in that, The specific value of A% is 5%.

4. The method for controlling the straightness of the pipe ends of hot-rolled seamless steel pipes according to claim 1, characterized in that, The specific value of η is 0.2mm, 0.3mm or 0.5mm.

5. The method for controlling the straightness of the pipe ends of hot-rolled seamless steel pipes according to claim 1, characterized in that, The method is used to produce products with an outer diameter ≤ 114.3mm small diameter seamless steel pipe.

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