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Analysis of Inner Folding Defects of Hot-rolled Seamless Steel Tubes

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Description: The built-in hot-rolled seamless steel pipe refers to the spiral, semi-spiral or irregularly distributed zigzag folds on the inner surface of the steel pipe. The inner folds mainly exist at the head of the steel pipe, or they are distributed regularly or irregularly. . Part of the hot-rolled seamless steel pipes can be used through simple grinding after the inward folding defect occurs, and the severe ones need to be scrapped directly. In current social production, inward folding defects have always been an important factor affecting the performance of hot-rolled seamless steel pipes. How to effectively avoid inward folding defects of hot-rolled seamless steel pipes has always been a key issue of concern to related scholars.

Keywords: Hot-rolled seamless steel pipe; inward folding defect; piercing method

Related product: rolling mill rolls, Tungsten carbide roller rolls

  • Hot-rolled seamless steel internal folding morphology and its causes

1.1 Inner fold morphology of hot-rolled seamless pipe

The internal folding morphology of hot-rolled seamless steel pipes is not fixed. The common morphology is fine flakes, but also large flakes, straight lines, and fish scales.

1.2 Reasons for inward folding of hot-rolled seamless steel pipes

When the steel pipe is pierced and pierced, under the influence of tensile stress, the core of the continuous casting pipe blank will have the problem of metal center rupture, resulting in the formation of cavities. Due to the uncertainty in the generation of the cavity, the internal metal surface is rough and will be oxidized by the cooling water and air of the head, and then form a fold during the subsequent rolling, and adhere to the inside of the steel pipe. Therefore, in order to achieve the purpose of effectively avoiding the occurrence of inward folding, it is necessary to avoid the formation of cavities, which is also the key issue of the whole research.

2. Research on the quality of continuous casting tube blanks and inward folding defects

At present, in the production of continuous casting tube blanks, if one wants to obtain capillary tubes without internal folds and no internal and external surface cracks, then finished tubes that meet relevant standards can be produced. Under the influence of this thought, the level of smelting technology has become the key to the entire production. According to the results of current continuous casting billet smelting research, in order to ensure that the smelting process level can meet expectations, relevant personnel should carry out quality control in accordance with the relevant content in Table 1.

  • Perforation process and internal folding defects

3.1 Perforation temperature and internal folding defects

In the production of seamless steel pipes, the piercing process is an important factor that affects inward folding. In the implementation of continuous casting billet piercing and heating, if the temperature is too high or the heating time is long, the piercing temperature will rise rapidly, and the problem of inward folding will occur. At this time, fish scale-like inward folds will appear inside the continuous casting billet. If the temperature is too low, it will affect the heating time, which is not conducive to ensuring the metal plasticity of the steel pipe. This result shows that it is necessary to correctly recognize the relationship between the perforation temperature and the internal folding defect to avoid the occurrence of internal folding.

3.2 The rotation speed of the perforated roller and the defect of inward folding

When designing the rotation speed of the perforating roll, the relevant personnel must fully consider the influence of many factors such as the material properties of the continuous casting tube blank, the size of the roll diameter, and so on. The speed of the continuous casting tube blank of different specifications also has obvious differences. Judging from the current actual situation, there are two main types of common speeds, including higher than standard speed and lower than standard speed. Under different production conditions, there are obvious differences in the ratio of inflection between these two speeds. , Relevant studies have shown that compared with low speeds, high speeds are more likely to cause inward folding. The reason for this phenomenon is considered: at high drilling speeds, increasing the roll speed will increase the amount of slip produced by the steel pipe. , Resulting in a decrease in axial efficiency, which in turn increases the risk of inward folding. At the same time, with the support of this result, it can be considered that in order to effectively avoid the occurrence of inward folding, it is necessary to adjust the direction of the punch in time and adopt the maximum bite angle for production. When the bite angle is increased, the slip of the continuous casting tube blank is improved, which helps to reduce the number of rotations of the continuous casting tube blank during the biting process.

3.3 Defects of the top and inward folds

As we all know, in the case of severe wear of the plug, its surface is uneven, which will scratch the inner wall during the perforation process, resulting in a spiral inward fold in the tube. At the same time, the shape of the steel head also has a certain impact on the quality of the steel pipe: (1) If the plug is tapered, the reduction of the plug from the tip to the hole will increase. This phenomenon is not conducive to ensuring the quality of the waste pipe; ( 2) When using the spherical plug, it helps to ensure that the waste pipe is fed in the predetermined direction, and the reduction amount is reduced accordingly to ensure the quality of the waste pipe. Of course, the best plug shape recognized in the current production is a shape that guarantees the same amount of pressure along the length of the deformation zone. Under the screw action of the plug, the bite performance of the continuous casting tube is guaranteed, and the inward folding control effect reaches the expected .

  • Research on preventive measures

In order to effectively avoid the occurrence of infolding defects, relevant personnel must focus on the following tasks:

(1) Strictly control the properties of continuous casting tube blanks, and in accordance with relevant production requirements, expand the ratio of equiaxed crystals in continuous casting tube blanks, or shorten the length of columnar crystals correspondingly to reduce the probability of inflection.

(2) Develop a strict continuous casting billet heating system in accordance with production requirements. According to the billet diameter, length, material production, etc., determine the corresponding continuous casting tube billet adding temperature, and implement the waiting-rolling stop rolling cooling system. In temperature control, the continuous casting tube billet heating temperature = penetration temperature-piercing temperature + discharge temperature.

(3) Use high-quality piercing and rolling tools. High-quality piercing and rolling tools play an important role in preventing inward folding defects, so high-quality rolling and piercing equipment must be used. For example, it is necessary to actively replace the hot centering punch, and correct it in time after routine inspection of the centering hole to avoid affecting production quality. For new perforated rolls, larger head extension and bite angle parameters are used, and quality control is carried out in accordance with the low roll speed.

Conclusion: This article mainly studies the related problems of inward fold defects of hot-rolled seamless steel pipes, and analyzes the control methods of their inward fold defects. Judging from the results of this study, in order to effectively avoid inward folding defects of hot-rolled seamless steel pipes, relevant personnel must start with material quality control, heating temperature, use high-quality appliances, etc., and control them to ensure that inward folding can be effectively avoided. Occurs to ensure the performance of hot-rolled seamless steel pipes.

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